![](media/image1.png){width="0.5798611111111112in"
height="0.5798611111111112in"}

**Plasma-Assisted Coevaporation of S and Se for Wide Band Gap
Chalcopyrite Photovoltaics**

**Final Report**

**December 2001 -- April 2005**

**Thin Film Partnership Subcontract #NDJ-2-30630-11**

**Principal Investigator: I. Repins, ITN Energy Systems, Inc.**

**CSM Technical Lead: C. Wolden, Colorado School of Mines**

Table of Contents

1 Introduction, Goals, and Approach
[6](#introduction-goals-and-approach)

2 Plasma Source Development [7](#plasma-source-development)

2.1 Source Hardware [7](#source-hardware)

2.2 ICP Source Diagnostics [10](#icp-source-diagnostics)

2.3 ICP Source Adjustments [13](#icp-source-adjustments)

2.3.1 RF Leakage [13](#rf-leakage)

2.3.2 Pressure and Orifice Size [14](#pressure-and-orifice-size)

2.3.3 Chalcogen Delivery [14](#chalcogen-delivery)

2.3.4 Uniformity [19](#uniformity)

2.3.5 Source Length [20](#source-length)

3 Plasma-Assisted Film Kinetics [22](#plasma-assisted-film-kinetics)

3.1 Selenization and Sulphurization of Precursor Films
[22](#selenization-and-sulphurization-of-precursor-films)

3.1.1 β-In~2~S~3~ Formation [22](#β-in2s3-formation)

3.1.2 CuInS~2~ Formation [23](#cuins2-formation)

3.1.3 CuIn~x~Ga~1-x~S~2~ Formation [24](#cuinxga1-xs2-formation)

3.1.4 CuInSe~2~ and CuIn~x~Ga~1-x~Se~2~ Formation
[25](#cuinse2-and-cuinxga1-xse2-formation)

3.2 Co-evaporated Binary Compounds [27](#co-evaporated-binary-compounds)

3.2.1 Optimizing Source Operating Conditions
[27](#optimizing-source-operating-conditions)

3.2.2 Measuring the Dissociation Fraction
[30](#measuring-the-dissociation-fraction)

3.2.3 Flux Distributions and Resulting Film Properties
[33](#flux-distributions-and-resulting-film-properties)

3.2.4 Film Properties [40](#film-properties)

4 Application of PACE Sources to CIGS Co-Evaporation
[45](#application-of-pace-sources-to-cigs-co-evaporation)

4.1 Baseline Process [45](#baseline-process)

4.2 Source Installation and Modification
[46](#source-installation-and-modification)

4.3 Film Fabrication [52](#film-fabrication)

4.4 Device Analysis [52](#device-analysis)

4.5 Discussion [53](#discussion)

5 Team Activities [55](#team-activities)

6 Conclusions and Future Directions
[55](#conclusions-and-future-directions)

7 Publications [56](#publications)

8 References [56](#references)

List of Figures

Figure 1 : Thermodynamics of CuInSe~2~ and CuInS~2~ formation at 300 K
using various chalcogen sources. [6](#__RefHeading___Toc100572864)

Figure 2 : ICP source in a) diagnostic chamber, b) benchtop
configuration, and c) PACE chamber. [8](#__RefHeading___Toc100572865)

Figure 3 : PACE source operating in glass bell jar at CSM. Photos show
a) entire bell jar, and b) close up of reactor tube therein.
[9](#__RefHeading___Toc100572866)

Figure 4 : Photographs of co-evaporation baseline bell jar a) with
cut-out showing deposition zone and positioning of components with lid
down, and b ) showing EIES head position relative to bell jar wall.
[10](#__RefHeading___Toc100572867)

Figure 5 : OES spectra from ICP sources with flow of a) O~2~, b) N~2~,
and c) both O~2~ and N~2~ gases. [11](#__RefHeading___Toc100572868)

Figure 6 : MS signals for an O~2~ Plasma.
[12](#__RefHeading___Toc100572869)

Figure 7 : MS signals for a N~2~ Plasma.
[12](#__RefHeading___Toc100572870)

Figure 8 : Actinometers vs. control temperature for S source
[13](#__RefHeading___Toc100572871)

Figure 9 : Relationship between ICP line pressure and PACE chamber
pressure. [14](#__RefHeading___Toc100572872)

Figure 10 : ICP source in operation with both a) Ar plasma, and b) Ar/S
plasma. [15](#__RefHeading___Toc100572873)

Figure 11 : OES spectra of a) an Ar plasma and b) and Ar/S plasma.
[15](#__RefHeading___Toc100572874)

Figure 12 : Se spectra versus power. [16](#__RefHeading___Toc100572875)

Figure 13 : Schematic comparison of two approaches for chalcogen
delivery. [17](#__RefHeading___Toc100572876)

Figure 14 : Se rate from PACE effusion source a) without and b) with
quartz reactor tube. Effusion source temperature is also shown.
[17](#__RefHeading___Toc100572877)

Figure 15 : Changes in relative density of sulfur-containing species as
a function of initial H~2~S content. [19](#__RefHeading___Toc100572878)

Figure 16 : Configuration for measuring flux distribution of PACE
source. [19](#__RefHeading___Toc100572879)

Figure 17 : Measurements of PACE source flux profile under different
operating conditions. [20](#__RefHeading___Toc100572880)

Figure 18 : Photos of first (left) and more compact (right) source
designs shown 50% of actual size. Inset is magnification of reactor tube
orifice. [21](#__RefHeading___Toc100572881)

Figure 19: XRD patterns of indium films exposed to the 1.5% H~2~S as a
function of substrate temperature, with and without plasma activation.
[23](#__RefHeading___Toc100572882)

Figure 20: Plots of (αε)^2^ vs. ε of plasma--treated indium films at
different temperatures. Band gaps were estimated by the intercept of the
linear portion of the curves with the x axis.
[23](#__RefHeading___Toc100572883)

Figure 21: XRD patterns of Cu/In films exposed to 1.5% H~2~S as a
function of substrate temperature, with and without plasma activation.
All films shown employed Cu poor precursors (Cu/In =0.8), with exception
(\*) of the top pattern which was stoichiometric.
[24](#__RefHeading___Toc100572884)

Figure 22: XRD patterns of Cu/In/Ga films exposed to the 1.5% H~2~S as a
function of substrate temperature, with and without plasma activation.
[25](#__RefHeading___Toc100572885)

Figure 23: XRD patterns of Cu/In films exposed to the selenium with
plasma activation at 320^o^C and without plasma activation at 350^o^C.
[26](#__RefHeading___Toc100572886)

Figure 24**:** XRD patterns and corresponding ESEM micrographs of
samples (A)-(C). [27](#__RefHeading___Toc100572887)

Figure 25 : Flux of excited Ar\* as a function of ICP operating pressure
[28](#__RefHeading___Toc100572888)

Figure 26: Schematic map of In flux(blue), S flux (yellow) across a 4"
wafer, with the symbols indicating the crystal quality obtained as
function of operating pressure. [29](#__RefHeading___Toc100572889)

Figure 27 : Three XRD patterns exemplifying the 3 phases observed in
Figure 26 [30](#__RefHeading___Toc100572890)

Figure 28 : Schematic of diagnostic setup
[31](#__RefHeading___Toc100572891)

Figure 29 : %H~2~S dissociation as a function of (a) power and (b) ICP
pressure [31](#__RefHeading___Toc100572892)

Figure 30 : . OES densities of \[H\] and \[S\] as a function of RF
power. [32](#__RefHeading___Toc100572893)

Figure 31 : Cross sectional view of substrate, Indium evaporation source
and ICP source. The positions are given in Cartesian co-ordinates.
[33](#__RefHeading___Toc100572894)

Figure 32 : Modeled In flux (top) compared with photograph of a 4"
silicon sample. Note the distortion in the upper left quadrant is due to
the presence of the quartz crystal microbalance.
[36](#__RefHeading___Toc100572895)

Figure 33 : Models of the sulfur distribution (top) and S/In ratio
(bottom). [37](#__RefHeading___Toc100572896)

Figure 34 : Comparison of measured (points) and modeled (line) film
thickness as a function of radial position from the origin.
[38](#__RefHeading___Toc100572897)

Figure 35 : Optical properties as a function of position across regions
L and M as shown in Figure 32 [39](#__RefHeading___Toc100572898)

Figure 36 : S/In ratio as determined by EDS across region A.
[39](#__RefHeading___Toc100572899)

Figure 37 : Quantitative distributions of the H~2~S/In flux ratios
across a 4" diameter for the films described in this report.
[40](#__RefHeading___Toc100572900)

Figure 38 : Photograph of an In~x~S~y~ film deposited by PACE, shown for
comparison with a small sample of β-In~2~S~3~ that was produced in the
bench top system. The green boxes denote the approximate positions of
the indium and ICP sources. The samples were divided into 9 pieces for
analysis as shown. [41](#__RefHeading___Toc100572901)

Figure 39 : XRD patterns from selected regions of co-evaporated films on
glass (above) and silicon (right). [42](#__RefHeading___Toc100572902)

Figure 40 : XRD patterns as a function of H~2~S/In obtained from a
single sample deposited at T~s~ = 200 ºC\>
[43](#__RefHeading___Toc100572903)

Figure 41 : XRD patterns as a function of substrate temperature for
films deposited with S/In ratio of 7. [44](#__RefHeading___Toc100572904)

Figure 42 : AFM morphology as a function of substrate temperature.
[44](#__RefHeading___Toc100572905)

Figure 43 : Time, temperature, and deposition flux profiles for CIGS
deposition. [45](#__RefHeading___Toc100572906)

Figure 44 : JV curve of small area device from bell jar. No AR coating
was applied. Cell parameters shown in inset.
[46](#__RefHeading___Toc100572907)

Figure 45 : a) Predicted and b) measured flux distributions from PACE
source in CIGS three-stage bell jar. [47](#__RefHeading___Toc100572908)

Figure 46 : Flux versus position from PACE source testing in three-stage
bell jar. [48](#__RefHeading___Toc100572909)

Figure 47 : Rates and temperatures from fully-automated CIGS run
incorporating PACE source. [48](#__RefHeading___Toc100572910)

Figure 48 : Example portion of recipe used to program the deposition
with the PACE source. [49](#__RefHeading___Toc100572911)

Figure 49 : Apparent flux as a function of time for three-stage CIGS
deposition, with effect of background pressure increase due to
outgassing circled in red. [50](#__RefHeading___Toc100572912)

Figure 50 : PACE source with plasma operating in three-stage CIGS
co-evaporation chamber. [51](#__RefHeading___Toc100572913)

Figure 51: Device performance as a function of maximum deposition
temperature, recipe, and plasma-activation (solid points).
[53](#__RefHeading___Toc100572914)

Figure 52:Potential Energy diagram for copper chalcopyrite formation
from different sources. [55](#__RefHeading___Toc100572915)

List of Tables

.

Table 1 : Summary of change in mass spectrometer signals for O2 and N2
plasmas. [13](#__RefHeading___Toc100572916)

Table 2: Summary of CIGS deposition conditions, composition, and device
performance. [53](#__RefHeading___Toc100572917)

# Introduction, Goals, and Approach

In terms of small-area device efficiency and module stability
CuIn~x~Ga~1-x~Se~2~ (CIGS) devices provide a benchmark for all thin film
photovoltaics. Nonetheless, there is significant opportunity for
improvement in manufacture of CIGS devices. First, high-quality CIGS
deposition requires high substrate temperatures (\>500°C), limiting the
selection of substrate materials, and increasing deposition cycle times
due to heat-up and cool-down periods. Furthermore, current
co-evaporation technology requires overpressure of chalcogens (Se~2~ or
S~2~) during deposition, resulting in low material utilization and high
equipment maintenance costs. Furthermore, the useful available energy
bandgap expansion via alloying is limited to less than 1.3eV.

In this work, ITN Energy Systems (ITN) and lower-tier subcontractor
Colorado School of Mines (CSM) explore the replacement of the molecular
chalcogen precursors during deposition (e.g. Se~2~ or H~2~Se) with more
reactive chalcogen monomers or radicals (e.g. Se). Molecular species are
converted to atomic species in a low-pressure inductively-coupled plasma
(ICP). The thermodynamics that motivate this approach are illustrated in
Figure 1. This diagram shows the changes in Gibbs free energy for
CuInSe~2~ (left) and CuInS~2~ (right) synthesis from various chalcogen
sources.[^1]^,^[^2] The formation energy of copper and indium are zero,
so the reactant energy simply reflects the chalcogen source. Sulfur and
selenium both sublime as dimers that may readily oligomerize into ring
structures of S~n~ or Se~n~, where n varies from 2 \< n \< 8. However,
the variation of n between 2 and 8 has relatively little effect on the
reactant energy. For example, the energetics of a sublimated S source
would be bound between the S~2~ and S~8~ values shown in the right half
of Figure 1.

![](media/image3.wmf){width="6.750694444444444in"
height="3.1638888888888888in"}

[]{#__RefHeading___Toc100572864 .anchor}Figure 1 : Thermodynamics of
CuInSe~2~ and CuInS~2~ formation at 300 K using various chalcogen
sources.

The details of the CIS reaction pathway are still somewhat of an open
question. The formation of CuInSe~2~ may occur directly, or it may
proceed through the formation of the sulfide intermediates Cu~2~Se and
In~2~Se~3~. ***In either case significant activation barrier(s) must be
present since the reaction does not proceed spontaneously at ambient
conditions.*** The premise of this work is to overcome these barriers by
activating the chalcogen source. This premise can be illustrated by
considering, for example, the energy of S in its various states. At 300
K the free energy of sublimated S is near zero. The alternative hydrogen
sulfide source is significantly more stable at this temperature. Our
approach is to activate these chalcogen precursors using an inductively
coupled plasma (ICP) source. In the plasma, high-energy electrons
dissociate these feed gasses into atoms, excited metastables (S\*), and
ions (S^+^). Atomic sulfur provides a tremendous amount of energy, as
was illustrated in Figure 1. The Gibbs heat of reaction using atomic
sulfur would be very exothermic, -185 kcal/mol of CuInS~2~. Metastable
and ionic sulfur would provide even more energy to the system. The
selenium system behaves very similarly, but the energy differences are
slightly less dramatic. The dissociation of the selenium dimer requires
\~88 kcal/mol versus \~95 kcal/mol for the sulfur dimer.Error: Reference
source not found ICP devices have proven very efficient for dissociation
of the dimers (H~2~, O~2~, Cl~2~) used in chemical vapor deposition and
etching processes.[^3]^,^[^4] The plasma kinetics of chalcogens have not
been studied. However in the case of oxygen, a molecule that has nearly
the same bond strength as S~2~, nearly complete dissociation is achieved
in these devices.^Error:\ Reference\ source\ not\ found^

Tasks of the proposed program centered on development and validation of
monatomic chalcogen chemistry, tuning of low-pressure monomer chalcogen
sources, and evaluation of plasma-assisted co-evaporation (PACE) for
CIGS co-evaporation. Likely advantages of deposition by plasma-enhanced
co-evaporation include:

-   Provides potential for lower deposition temperature and/or for
    better film quality at higher deposition temperature.

-   Provide potential for decreased deposition times.

-   Provides high material utilization efficiency (\~90%) that results
    in less deposition on other parts of the reactor leading to lower
    clean up and maintenance costs, as well as longer equipment
    lifetime. High material utilization efficiency also reduces the
    total operating pressure, which is beneficial for the design and
    control of metal co-evaporation. Advantages include minimal
    metal-vapor beam spread and lower source operating temperatures.

-   Enables deposition of wide bandgap copper indium gallium
    sulfur-selenide (CIGSS) films with controlled stoichiometry.

University researchers at CSM have developed and tested the fundamental
chemistry and engineering principles, while industrial researchers at
ITN adapted PACE technology to CIGSS co-evaporation. The effect of
plasma activation was first examined for binary films (In~x~Se~y~ and
Cu~x~Se~y~) and gradually extended to ternary and quaternary compounds
and the complete three-stage CIGS co-evaporation process. Work under
this contract has proceeded in three areas: development of the plasma
source hardware (section 2), examination of the film kinetics (section
3), and application of the PACE sources to CIGS fabrication by
co-evaporation (section 4).

# Plasma Source Development

## Source Hardware

ICP sources for chalcogen delivery were operated and developed in four
different configurations and over a wide range of operation conditions
during this program. Three configurations used for ICP source operation
are shown in Figure 2. Figure 2a shows an ICP source operating in the
CSM diagnostic chamber. The diagnostic chamber is used to examine the
source characteristics for low chalcogen flows, and is fitted with
advanced instrumentation such as an optical emission spectrometer (OES),
a quadrupole mass spectrometer (QMS), and various gauging. The ICP
source, common to each configuration, is the glowing tube seen Figure
2a. The plasma is generated in a one-inch diameter quartz tube using a
five turn, water- cooled copper coil powered by a 13.56 MHz RF supply
and associated match network. Chalcogen vapors and Ar are fed into the
tube outside the chamber. The RF plasma converts the stable vapors into
highly active monomers and ions. The reactive species effuse from a
small orifice at the end of the ICP device into the collision less high
vacuum environment (10-5 Torr).

![](media/image4.wmf){width="5.997222222222222in"
height="3.7666666666666666in"}

[]{#__RefHeading___Toc100572865 .anchor}Figure 2 : ICP source in a)
diagnostic chamber, b) benchtop configuration, and c) PACE chamber.

Figure 2b shows a schematic of an ICP source operating in benchtop mode.
This mode is useful for tests requiring significant chalcogen flux that
would contaminate the advanced instrumentation in the diagnostic
chamber. The plasma is contained in a 1" diameter pyrex tube. Argon is
supplied through a needle valve, while an on/off valve isolates the
chalcogen effusion source. A butterfly valve on the exhaust line is used
to control the pressure. The effusion source is maintained at the
desired temperature through PID control. This benchtop unit is operated
at the similar conditions as will be used for CIGS film depositions. The
only difference is that this system is exhausted directly to a pump,
instead of effusing into a high vacuum environment. The benchtop plasma
source allows extensive quantification of sulfur/selenium flux and
optimization ICP operation as a function of pressure, power, and
composition without contamination of other equipment.

The first simultaneous exposure of a substrate to the plasma-activated
chalcogens, heat, and In was performed in the PACE chamber. The PACE
chamber is shown in Figure 2c. This chamber is the next step in the
transition of ICP sources to full CIGSS coevaporation. It allows the
study of plasma-activated binary film formation and source uniformity
without the stringent requirements on temperature and multi-source flux
rate monitoring imposed by CIGSS co-evaporation. The PACE chamber
includes a glass bell jar with implosion shield and hoist, a cabinet for
housing pumps and instrumentation, a liquid trap, diffusion pump, and
pneumatic gate valve. The baseplate was modified to accommodate the ICP
sources and In evaporation. Figure 3 shows a PACE source operating in
the glass bell jar at CSM. On the left is shown the entire bell jar,
with a close up of reactor tube shown on the right.

![](media/image5.png)![](media/image6.png)

[]{#__RefHeading___Toc100572866 .anchor}Figure 3 : PACE source operating
in glass bell jar at CSM. Photos show a) entire bell jar, and b) close
up of reactor tube therein.

The fourth environment used for ICP source operation was a CIGS bell
jar. The bell jar design is based on that of the NREL CIGS
co-evaporators, and was assembled at ITN. Co-evaporation of the elements
is performed through resistive heating of small Cu, In, Ga, and Se
boats, each with a capacity of several grams. Se rate is monitored by
quartz crystal microbalance (QCM), and metals rates are monitored by
electron impact emission spectrometry (EIES). CIGS emission, as measured
by thermopile, is used for end-point detection.[^5] The substrate is
heated radiatively, and its temperature is monitored by thermocouple.
Figure 4 shows two views of the bell jar. Figure 4a uses a cut-out view
of the bell jar exterior to show the positioning of the EIES emission
head relative to the deposition zone. Figure 4a utilizes a cut-out view
to show the deposition zone - and the position of the EIES relative to
it - with the bell jar down. The evaporation boats are located near the
bottom of the cut-out. The substrate heaters and thermocouple are
visible at the top of the cut-out. A copper-colored shutter is seen
shielding the substrate in the top 1/3 of the cut-out. The location of
the EIES emission head is level with the black photomultiplier tubes at
the right of the photo. Figure 4b shows the EIES emission head affixed
to the inside of the bell jar, as the bell jar descends from the open
position into its deposition position on the base plate.

![](media/image7.wmf){width="5.998611111111111in"
height="2.7597222222222224in"}

[]{#__RefHeading___Toc100572867 .anchor}Figure 4 : Photographs of
co-evaporation baseline bell jar a) with cut-out showing deposition zone
and positioning of components with lid down, and b ) showing EIES head
position relative to bell jar wall.

## ICP Source Diagnostics

An important aspect of the ICP source development is the ability to
monitor deposition rate, chalcogen dissociation, and plasma
characteristics such as plasma density and electron temperature. To
achieve these functions, the interpretation of optical emission
spectroscopy (OES) signals from the ICP sources was developed
simultaneously with the sources and compared with mass spectrometry
data.

The use of OES on the ICP sources was first tested on the diagnostic
chamber using O~2~ and N~2~ gases. Figure 5 shows OES spectra from the
ICP source on the diagnostic chamber for different gas flows. Figure 5a
shows the spectrum with O~2~ flow only, and the source operated at 60W.
Figure 5b shows the spectrum with N~2~ flow only, and the source
operated at 40W and 60W. In the spectra of Figure 5c, both gases are
flowing through the source, and the characteristic peaks of each gas are
evident.

![](media/image8.wmf){width="3.7375in" height="6.749305555555556in"}

[]{#__RefHeading___Toc100572868 .anchor}Figure 5 : OES spectra from ICP
sources with flow of a) O~2~, b) N~2~, and c) both O~2~ and N~2~ gases.

The O~2~ and N~2~ OES spectra were then correlated with mass
spectrometer data. The mass spectrometer (MS) was attached to the
chamber and dual MS/ OES experiments were performed. MS signals have
been cross-referenced with OES signals so that OES may be used as a real
time sensor. Efforts were directed at quantifying the mass spectrometry.
These included the use of laser to align the MS and the ICP unit. A beam
stop was purchased, modified and installed to distinguish ICP from
background signal. The mass spectrometry data demonstrate the efficacy
of the ICP sources in cracking gas species such as N~2~ and O~2~. Figure
6 and Figure 7 show MS data demonstrating the dissociation of oxygen and
nitrogen dimers. When the plasma is ignited, the parent peaks - 32 and
28 respectively - decrease while the atomic peaks (16 and 14) increase.
Table 1 shows a quantitative summary of the signal changes of the
previous two figures. The relatively long transients associated with the
data in Figure 6 and Figure 7 is attributed to heating of the ICP
devices. This has been corroborated by simultaneous optical emission
spectroscopy (OES) measurements.

![](media/image9.wmf){width="5.3069444444444445in"
height="3.704861111111111in"}

[]{#__RefHeading___Toc100572869 .anchor}Figure 6 : MS signals for an
O~2~ Plasma.

![](media/image10.wmf){width="5.429861111111111in"
height="3.8652777777777776in"}

[]{#__RefHeading___Toc100572870 .anchor}Figure 7 : MS signals for a N~2~
Plasma.

+-----------------------+----------------------+----------------------+
| **Signal**            | **Change for a**     | **% Change for a**   |
|                       |                      |                      |
|                       | **80 W plasma**      | **100 W plasma**     |
+-----------------------+----------------------+----------------------+
| O~2~: m/e = 32        | -42%                 | -46%                 |
+-----------------------+----------------------+----------------------+
| O: m/e = 16           | +25%                 | +42%                 |
+-----------------------+----------------------+----------------------+
| N~2~: m/e = 28        | -16.5%               | -20%                 |
+-----------------------+----------------------+----------------------+
| N: m/e = 14           | +19%                 | +29%                 |
+-----------------------+----------------------+----------------------+

[]{#__RefHeading___Toc100572916 .anchor}Table 1 : Summary of change in
mass spectrometer signals for O2 and N2 plasmas.

Quantification of the S flux also requires some careful consideration.
The S source temperature measurement is indirect. It is achieved through
a thermocouple mounted outside the S source. As such, the absolute
temperature of the sulfur source can vary from run to run. Similarly the
absolute OES signal can vary with fiber optic placement and integration
time. Actinometry was therefore developed, relying on ratio of S
emission to Ar emission as the control metric for S and Se flux. Figure
8 shows plots of actinometers versus control temperature. The S lines
are at 450 and 468.5 nm, while the Ar actinometer line is 737 nm.

![](media/image11.wmf){width="4.978472222222222in"
height="4.115277777777778in"}

[]{#__RefHeading___Toc100572871 .anchor}Figure 8 : Actinometers vs.
control temperature for S source

## ICP Source Adjustments

### RF Leakage

A major improvement in ICP source operation in the PACE source
configuration was made with the elimination of RF leakage. The leakage
had adverse implications for plasma tuning and the operation of other
components. A prototype Faraday cage was designed, fabricated out of
copper mesh, and implemented. The cage proved successful in eliminating
RF noise to an undetectable level, an allowed tuning with near-zero
reflected power. A permanent faraday cage was then constructed. In
addition, improved RF connectors were obtained for both sides of the RF
vacuum feedthrough. These improvements resulted in elimination of RF
noise and reproducible ICP operation.

### Pressure and Orifice Size

Satisfactory base pressures with Ar flow to the PACE source were
demonstrated. Figure 9 plots the relationship between the ICP pressure
as measured by a convection gauge upstream of the actual device and the
PACE chamber pressure as measured by a cold cathode gauge. Since the ICP
gauge is upstream of the ICP device, the true pressure in the ICP device
is estimated to be \~25% of the measured pressure due to friction drop
and expansion in the line. Figure 9 shows that the two pressure regimes
are effectively decoupled. The chamber pressure is approximately 4
orders of magnitude less than the ICP source. Best ICP device operations
occurs around 100 mtorr, which means chamber pressure will be around
10^-5^ torr for depositions. It should be noted that the relationship
between ICP and chamber pressure is very sensitive to the size of the
reactor tube orifice. Furthermore, the Ar pressure required for a stable
plasma depends on the reactor tube geometry. Thus, as the reactor tubes
are custom-made glassware, small variations from one reactor tube to the
next may cause a significant modification to the relationship graphed in
Figure 9.

![](media/image12.wmf){width="3.64375in" height="3.1034722222222224in"}

[]{#__RefHeading___Toc100572872 .anchor}Figure 9 : Relationship between
ICP line pressure and PACE chamber pressure.

### Chalcogen Delivery

During initial tests, ICP sources were operated in the benchtop
configuration to examine methods of supplying S and Se to the plasma
reactor tube.

Figure 10 shows photographs of the source in operation with both an Ar
plasma and an Ar/S plasma. A heated sparger filled with S shot was used
to supply S vapor to the plasma. Stable sulfur/argon plasmas were
obtained over a range of powers (30 -- 100 W) with minimal reflected
power (\< 3 W), and the plasmas were stable over a large pressure range
(20 -- 200 mTorr). For most conditions the Pyrex wall remained clear (no
S condenstion) throughout the majority of experiments. The exception was
extremely high sulfur effusion temperatures.

![](media/image13.wmf){width="3.7083333333333335in"
height="2.6069444444444443in"}

[]{#__RefHeading___Toc100572873 .anchor}Figure 10 : ICP source in
operation with both a) Ar plasma, and b) Ar/S plasma.

A fiber optic-based optical emission spectroscopy (OES) system was then
used to characterize the plasmas and relate chalcogen rates to OES
signal, allowing use of OES as a rate monitor. Representative OES
spectra are shown in Figure 11. The S-containing plasma clearly shows
the bands from 290-540 nm associated with the B^3^Σ^-^ transitions of
electronically excited sulfur. The pyrex tube absorbs signal for
wavelengths less than 350 nm.

![](media/image14.wmf){width="5.998611111111111in"
height="2.910416666666667in"}

[]{#__RefHeading___Toc100572874 .anchor}Figure 11 : OES spectra of a) an
Ar plasma and b) and Ar/S plasma.

Similar data were taken for the examination of Se delivery from the ICP
source. A source similar to the S source was fabricated for Se
vaporization and the first Ar/Se plasma spectra was obtained. Figure 12
shows a representative spectrum. The main peaks of interest are in the
region around 325-355 nm and a distinct peak at 376 nm. It is important
to note that these peaks are out of the range of the S spectra, whose
main peaks are located 400 \< λ \< 600 nm. Thus, if both S and Se were
activated in the same ICP source, the contributions of each chalcogen to
the OES spectra could be deconvolved.

![](media/image15.wmf){width="4.582638888888889in"
height="3.247916666666667in"}

[]{#__RefHeading___Toc100572875 .anchor}Figure 12 : Se spectra versus
power.

The sparger method of chalcogen delivery poses several difficulties.
First, since the gas temperature is always less than the source
temperature (*T~g~ \< T~s~*), it is inherently susceptible to nucleation
and condensation. Second, due to the thermal mass of the effusion source
and the heating arrangements the time constants are large, causing
frequent overshoot of the desired rate set point.

One possible solution is to reverse the heated elements, utilizing a
cold chalcogen source and a hot carrier gas. Two advantages result.
First, the gas temperature is always greater than the source (*T~g~ \>
T~s~*), eliminating the possibility of gas-phase nucleation and
condensation. Second, the time constants are much shorter. Gas heating
was achieved using immersion heaters that work by transporting the gas
across resistively heated wires. The rapid response of both the heating
elements and the low thermal mass of the gas allow for rapid turn on,
turn off, and enable real time control. The source tubes were prepared
by loading a known amount of chalcogen in a tube, purging it with argon,
and sealing it. It was then put into a furnace where it will be slowly
heated and cooled, vaporizing the chalcogen and allowing it to
recondense as a uniform film on the inside of the tube. This is
analogous to the preparation of source plates used for CdTe deposition
or CdCl~2~ treatment in close space sublimation. These approaches are
illustrated in Figure 13. In the first approach, chalcogen flux is
controlled mainly by the effusion source temperature. Advantages of this
first approach include large source capacity and capabilities for high
Se pressures. In the second approach, chalcogen flux is controlled
mainly by the hot carrier gas temperature.

![](media/image16.wmf){width="5.55in" height="1.9513888888888888in"}

[]{#__RefHeading___Toc100572876 .anchor}Figure 13 : Schematic comparison
of two approaches for chalcogen delivery.

For implementation into CIGS co-evaporation, a hybrid of the two
approaches described was used. Ar was heated prior to introduction into
the source, to prevent condensation. A sparger, instead of coated
plates, was used to allow adequate capacity for a full deposition. A
tightly-sealed stainless steel source utilizing high voltage and low
current heating rods was fabricated. Use of molecular beam epitaxy and
valved thermal cracker sources was also investigated but not pursued due
to high cost. Acceptable rate control was achieved by minimizing sparger
thermal mass, avoiding Se condensation via heating vapor tubing and the
Ar gas prior to its entry to the source, and tuning proportional
integral derivative (PID) control loops. Figure 14 shows Se rate as a
function of time for the source, both with and without the quartz plasma
tube in place. Rate was measured by a QCM placed 1" above the source
opening. Se rate control is not as tight as that achieved using the
coated-plate sources in the benchtop system, but is still inside an
acceptable envelope for CIGS co-evaporation, with the added benefit of
adequate capacity. Source temperatures at the Se melt and at the source
nozzle are also shown in Figure 14.

![](media/image17.wmf){width="4.34375in" height="3.0479166666666666in"}

![](media/image18.wmf){width="4.152777777777778in"
height="2.717361111111111in"}

[]{#__RefHeading___Toc100572877 .anchor}Figure 14 : Se rate from PACE
effusion source a) without and b) with quartz reactor tube. Effusion
source temperature is also shown.

[]{#H2Sdelivery .anchor}Additional experiments were attempted in the
benchtop source at high Se rates, commensurate with those required
during industrial three-stage CIGS deposition. However the flux was
difficult to control, and the ICP tube became coated with Se. Under
these conditions, a plasma cannot be maintained. For more precise
control of the plasma and the chalcogen flux at high rates, plumbing was
installed to the benchtop source for utilization of H~2~S. This H~2~S
equipment is capable of delivering H~2~S/Ar mixtures with precise
control over arrange of H~2~S concentrations ranging from 0.5--10% using
calibrated metering valves. Preliminary experiments indicated that the
plasma is highly capable of full dissociating H~2~S. Using a 1% mixture,
the quartz tube downstream of the ICP coil was coated with yellow solid
sulfur after just minutes of operation. It was also observed that there
was significant substrate heating when H~2~S is introduced into the
plasma. This is attributed to the exothermic recombination of atomic
hydrogen on the susceptor.

The use of H~2~S raises safety concerns, which were addressed using the
following safety measures:

-   The cylinder containing the 10% H~2~S/Ar mixture is mounted in a
    ventilated fume hood.

-   An emergency shut off valve was installed, which is position in a
    ventilated box.

-   Two portable H~2~S monitors with ppm sensitivity are mounted on the
    apparatus

-   The entire setup was rigorously leak tested.

-   A standard operating procedure (SOP) was established to ensure that
    all lines are completely vented of H~2~S both before and after
    operation.

-   The Experimental setup and SOP were reviewed and approved by the CSM
    Environmental Health and Safety (EHS) office.

The CSM EHS would not approve the utilization of the more toxic
Se-containing gas, H~2~Se, in any configuration.

Stable plasma operation was demonstrated over many hours. There was no
evidence of sulfur condensation on either the inside or outside of the
ICP device. This lack of coating is in sharp contrast to previous
experiments with elemental S vapor, where S condensation was always a
concern. One reason for the difference may be heating associated with
atomic hydrogen recombination. It was observed that the substrate was
heated to temperatures \~ 250 ºC when H~2~S was added to argon plasma.
Such a degree of heating would be sufficient to ensure that the sulfur
is completely volatile.

Actinometry was employed to study the changes in the relative density
excited sulfur species as a function of composition. All signals were
normalized by the emission intensity and density of the argon line at λ
= 706 nm. Figure 15 plots the relative density as a function of the
%H~2~S in the ICP device. All of the signals increase monotonically with
%H~2~S, and all display the same functionality. This is shown by the
plot of \[S\]/sum of all S-containing species. It remains essentially
constant. This indicates that the ditribtuion of species (S vs. S~2~ vs.
S~3~, etc.) stays essentially constant with % H~2~S in Ar. The highest
fraction of H~2~S that has been successful operated in the plasma is
studied was 5 %.

![](media/image19.wmf){width="4.000694444444444in"
height="2.7291666666666665in"}

[]{#__RefHeading___Toc100572878 .anchor}Figure 15 : Changes in relative
density of sulfur-containing species as a function of initial H~2~S
content.

### Uniformity

An understanding of the flux profile from the PACE sources is very
important to determine acceptable geometries for installation of the
PACE sources in the CIGS three-stage bell jar. A thorough
characterization of the flux profile from the PACE source indicates that
flux follows the expected 1/r^2^ fall-off with distance from the source,
as well as the expected[^6] cos^3^θ dependence of flux per solid angle
on azimuthal angle, independent of Se rate and Ar pressure.

Experiments leading to this conclusion were performed at a variety of Se
rates and Ar pressures, with witness slides placed around the source in
the configuration of Figure 16. Se thickness at several points on each
witness slide was measured by mechanical profilometer. In Figure 16, z
is the vertical distance from the source aperture to the plane of a
given witness slide. R is the distance from the aperture to any
measurement point, and θ is the angle
between$\overset{\rightarrow}{R}$and the source axis. If flux per solid
follows a cos^n^θ dependence and is constant with increasing R (i.e.
neither Se-Se nor Se-Ar collisions cause significant spreading of the
flux after it leaves the aperture), then it can be derived that

$$\frac{\text{Flux}\ \text{at}\ \text{measurement}\ \text{point}\ (z,R)}{\text{Flux}\ \text{on}\ \text{axis}\ \text{at}\ \text{distance}\ z_{0}} = \left( \frac{z}{R} \right)^{3 + n}\left( \frac{z_{0}}{z} \right)^{2}$$

\(1\)

This expression includes the necessary factors for dependence of flux on
θ, distance to slide, and angle between slide and direction of flux. A
similar expression can be derived for a source axis that is not normal
to the substrate plane.

![](media/image20.wmf){width="2.7402777777777776in"
height="2.8965277777777776in"}

[]{#__RefHeading___Toc100572879 .anchor}Figure 16 : Configuration for
measuring flux distribution of PACE source.

Experimental verification of the expected flux profile (1) for multiple
source operating conditions is shown in Figure 17. In Figure 17a,
discrete points show thickness measured on multiple witness slides (as
per Figure 16) graphed against R. Lines show a least squares fit to the
flux profile, with n as the adjustable parameter. Figure 17b lists the
test conditions and best fit n for each deposition. The distribution is
invariant over the range explored: 3x variations in Ar pressure, 2x
variations in rate, and 2x variations in distance. The data of Figure 17
enable prediction of flux to the substrate and its uniformity for any
geometric configuration of the PACE source into the three-stage CIGS
bell jar.

![](media/image21.wmf){width="5.997222222222222in"
height="2.3340277777777776in"}

[]{#__RefHeading___Toc100572880 .anchor}Figure 17 : Measurements of PACE
source flux profile under different operating conditions.

### Source Length

Improvements were also made to the PACE source length. The first source
designed, shown on the left side of Figure 18, was more than 14" in
length. This original dimension makes incorporation into CIGS
co-evaporation impractical: It is longer than the distance from the
baseplate to the substrate in ITN's bell jar. Several steps were taken
to decrease source length. First, a gooseneck was built into the source
interconnects. However, decreased rate controllability, most likely due
to limited flow of Se vapor through the gooseneck, was observed. Next,
all unnecessary length was trimmed from each component. The resulting
design, shown on the right side of Figure 18, is approximately 5.5"
tall. In Figure 18, most of each reactor tube is covered by metal
shielding. Thus, the inset shows a magnification of the reactor tube
orifice to indicate where Se exits the source.

# Plasma-Assisted Film Kinetics

The film kinetics essential to the use of plasma-assisted processing
were examined via two techniques. In the first technique, metallic
precursors were introduced into the benchtop source (Figure 2b) and
their reaction was studied as a function of source operating conditions,
such as power, chalcogen rate, sample temperature, etc. In the second
technique, the PACE chamber (Figure 2c) was used to co-expose a
substrate to the chalcogen and one metals flux, and the resulting films
were characterized as a function of operating conditions. Results and
conclusions from the benchtop source technique are given below in
section 3.1, while those from the PACE chamber are given in section 3.2.

## Selenization and Sulphurization of Precursor Films

Experiments examining film kinetics via exposure of metallic precursors
to the benchtop source (Figure 2a) are described below. The major
findings are the following:

-   Films treated at the highest temperature thermally showed no or
    little chalcogen conversion.

-   The binary b-In~2~S~3~ phase was formed at 250 ºC, while ternary and
    quaternary phases were formed by plasma processing at 350 ºC in less
    than one hour.

-   Films with well-defined XRD patterns were also characterized by
    proper optical band gaps and a well-defined polycrystalline
    morphology.

Three material systems (In, Cu-In, Cu-In-Ga) were successfully converted
to sulfides and selenides using plasma processing.

![](media/image22.png){width="1.2070866141732284in"
height="1.4082677165354331in"}![](media/image23.jpeg){width="3.0875in"
height="8.129861111111111in"}![](media/image24.jpeg){width="1.5625in"
height="3.0625in"}

[]{#__RefHeading___Toc100572881 .anchor}Figure 18 : Photos of first
(left) and more compact (right) source designs shown 50% of actual size.
Inset is magnification of reactor tube orifice.

### β-In~2~S~3~ Formation

Indium films of \~ 300 nm thickness were deposited on glass substrates
by thermal evaporation. These samples were exposed to a mixture of 1.5%
H~2~S in argon as a function of substrate temperature, with and without
plasma treatment. For all experiments described here, the plasma source
was operated at a total pressure = 70 mTorr, RF power = 100W, and an
exposure time of 35 minutes. It was found that exposure to an argon
plasma results in a substrate temperature rise of \~100-110 ºC without
external heating due to ion bombardment. Exposure to the H~2~S/Ar plasma
mixture resulted in significantly more heating, with a minimum substrate
temperature \~210 ºC for RF power 100W. This difference was attributed
to surface recombination of atomic hydrogen,[^7] an exothermic process
which shows that H~2~S was effectively dissociated under these
conditions. Sulfur condensation was also observed on the quartz tube
downstream of the plasma coil, which is additional evidence that the
plasma effectively dissociates H~2~S into sulfur and atomic hydrogen.
Indium films were exposed to H~2~S at substrate temperatures from
210-350 ºC, with and without plasma activation. The PID- controlled
heater was employed for experiments performed without plasma, or those
using temperatures greater then 210 ºC.

Films were analyzed by XRD and spectrophotometry. Figure 19 shows the
resulting XRD patterns. The as-deposited film was polycrystalline indium
with preferential orientation in the (100) direction. For the film
treated thermally at the maximum temperature of 350 ºC, little
conversion occurred. The indium diffraction peak remains strong, and
only a weak In~6~S~7~ peak is observed. In contrast, all the films
exposed to plasma formed new diffraction peaks that were positively
identified as β-In~2~S~3~. At T~s~ = 210 the film was partially
converted, as both In and In~2~S~3~ peaks are present. Plasma-treated
films at temperatures ≥ 250^o^C contain only β-In~2~S~3~ peaks. The
optical characterization confirms the XRD results. Wavelength dependent
absorption coefficients (α) were extracted from transmission spectra and
film thickness. Figure 20 shows plots of (αε)^2^ vs. ε,  where ε is the
photon energy. The optical band gap is determined by extrapolating the
linear portion of the curves to the x-intercept.[^8] The films at T~s~ ≥
250^o^C show a band gap of \~ 2.5 eV, which is in good agreement with
literature values for β-In~2~S~3~.[^9] The film at T~s~ \~210^o^C
exhibits a lower value of 2.15 eV, consistent with the partial
conversion observed by XRD. All thermally treated samples remained
opaque. Results show that β-In~2~S~3~ may be readily formed with plasma
activation of H~2~S at temperatures of 250ºC.

![](media/image25.wmf){width="3.5in" height="3.104861111111111in"}

[]{#__RefHeading___Toc100572882 .anchor}Figure 19: XRD patterns of
indium films exposed to the 1.5% H~2~S as a function of substrate
temperature, with and without plasma activation.

![](media/image26.wmf){width="2.5944444444444446in"
height="2.0930555555555554in"}

[]{#__RefHeading___Toc100572883 .anchor}Figure 20: Plots of (αε)^2^ vs.
ε of plasma--treated indium films at different temperatures. Band gaps
were estimated by the intercept of the linear portion of the curves with
the x axis.

### CuInS~2~ Formation

Co-evaporated Cu/In films deposited were exposed to H~2~S plasma for 40
minutes as a function of temperature. The plasma conditions were the
same as the indium sulphurisation experiments described above. Two sets
of Cu/In films were used. The first set were deposited on
molybdenum-coated stainless steel and were copper poor, with a Cu/In
atomic ratio of 0.8 and a total thickness of 730 nm prior to
sulphurisation. The second set of films were stoichiometric with Cu/In
ratio of unity and were deposited on glass. Films were plasma treated at
substrate temperatures of 230 -350 ºC, with and without plasma
activation.

Figure 21 shows the resulting XRD patterns. Again, films treated
thermally up to the maximum temperature of 350 ºC showed no evidence of
conversion. In contrast, chalcogen conversion was observed in the case
of all plasma treated films. The copper-poor samples exposed to plasma
all resulted in a mixture of binary and ternary phases at all
temperatures. The presence of the binary indium sulfide phases was
attributed to the stoichiometry of the precursor films. The copper is
fully consumed in the chalcopyrite phase, while excess indium is left in
binary phases. In the case of stoichiometric sample only the ternary
CuInS~2~ was observed at T~s~ =350 ^o^C. The optical band gap of this
film was 1.5 eV, as expected for the ternary semiconductor.

![](media/image27.wmf){width="2.470138888888889in"
height="2.3534722222222224in"}

[]{#__RefHeading___Toc100572884 .anchor}Figure 21: XRD patterns of Cu/In
films exposed to 1.5% H~2~S as a function of substrate temperature, with
and without plasma activation. All films shown employed Cu poor
precursors (Cu/In =0.8), with exception (\*) of the top pattern which
was stoichiometric.

### CuIn~x~Ga~1-x~S~2~ Formation

The third material system examined used sputtered Cu/In/Ga films
deposited on molybdenum-coated stainless steel. The precursors were
again copper poor with a Cu/(In+Ga) ratio of 0.83. The gallium fraction
Ga/(In+Ga) was 0.28, and the total thickness was 648 nm prior to
sulphurisation. A small amount of unintentional selenium, resulting in a
Se fraction Se/(Cu+In+Ga) of 0.016 was also present in the films, due to
contamination during sputtering. These films were treated at
temperatures of 230 - 350 ºC, with and without plasma activation.

Figure 22 shows the resulting XRD patterns. There is almost no change in
the XRD pattern of the film treated thermally at 350 ºC. The plasma
treated films all showed formation of the ternary and/or quaternary
chalcopyrite (i.e. CuInS~2~ or CuIn~x~Ga~1-x~S~2~) phases. The film
exposed at 230ºC was only partially converted, as there is a significant
amount of polycrystalline indium remaining in the film. For temperatures
at 300 and 350ºC, the films appeared to be fully converted to
chalcopyrite phases. The films appear to be primarily a mixture of
ternary CIS/CGS phases. The (112) peak at 2θ \~26.7º is very broad
indicating the presence of both phases instead of a single quaternary.
The 300 ºC sample was also subjected to 400ºC temperature for 2 minutes
at the end of the experiment due to heater malfunction. This explains
why the XRD pattern of the 300ºC film was somewhat superior to that from
the 350 ºC sample. Interestingly, there is very little evidence of
binary phases as was observed with the Cu/In samples (Fig. 6). There are
a couple of possible reasons. These Cu-In-Gas films were sputtered, and
perhaps are more reactive than the co-evaporated Cu-In films. Another
possibility is that the small amount of unintentional selenium present
in the films enhanced the reactivity. The window of Cu/In compositions
that can be converted to the ternary chalcopyrite formation appears to
be larger in the case of selenium. Films with Cu/In ratios as low as 0.8
routinely form ternary CuInSe~2~, without the presence of binary phases.
As shown above, in the case of the sulfur both ternary and binary phases
are present. The presence of the unintentional selenium may explain the
absence of the binary phases for the Cu-In-Ga samples.

![](media/image28.wmf){width="3.377083333333333in"
height="3.017361111111111in"}

[]{#__RefHeading___Toc100572885 .anchor}Figure 22: XRD patterns of
Cu/In/Ga films exposed to the 1.5% H~2~S as a function of substrate
temperature, with and without plasma activation.

### CuInSe~2~ and CuIn~x~Ga~1-x~Se~2~ Formation

The copper-poor Cu/In samples on molybdenum were also exposed to
selenium vapor plasma. Figure 23 shows the results from an experiment
with a substrate temperature T~s~ = 320ºC, 80 W, a selenium source
temperature of T~se~ = 232 ºC, and a process time of 55 minutes. The
Figure 23 also shows the XRD pattern of thermal selenization at 350 ºC
without plasma activation performed with a higher selenium flux using
T~se~ = 248 ºC. With plasma activation the film was converted into the
ternary chalcopyrite, and no crystalline binary phases (i.e. In~2~Se~3~,
CuSe) were observed. The primary peaks observed for CuInSe~2~ were
(112), (200), (204/220) and (116/312) with strong 112 orientation. This
was somewhat interesting as copper--poor alloys such as these typically
prefer the (204/220) orientation in conventional thermal selenization
experiments.[^10] In the case of sample without plasma activation little
evidence of ternary chalcopyrite phase formation was observed.

![](media/image29.wmf){width="3.373611111111111in"
height="2.6493055555555554in"}

[]{#__RefHeading___Toc100572886 .anchor}Figure 23: XRD patterns of Cu/In
films exposed to the selenium with plasma activation at 320^o^C and
without plasma activation at 350^o^C.

Up to this point we have used XRD as the primary metric of evidence for
sulphurisation /selenization. As shown earlier, XRD correlated well with
the optical bandgap in the case of β-In~2~S~3~ and CuInS~2~. Here we
show that XRD also correlates well with surface structure and
morphology. Figure 24 compares the XRD patterns and morphology from
three Cu/In/Ga films subjected to different degrees of selenization at
substrate temperature 350 ^o^C. The first film was thermally selenized
at high selenium flux (T~se~ \~248ºC). The resulting XRD pattern (A)
shows only minimal conversion. The corresponding micrograph shows a
filamentary morphology, consistent with presence of condensed selenium.
The second film (B) has an XRD pattern that includes both distinct
CuInSe~2~ and CuGaSe~2~ phases in the XRD pattern. This film was exposed
to plasma, but with a low selenium flux (T~se~ \~205ºC). The
corresponding SEM shows a cauliflower-like or nanocrystalline
morphology. Finally, the third sample (C) is from a film that exhibits
the quaternary CuIn~x~Ga~1-x~Se~2~ phase. This sample was exposed to
plasma with a higher flux of selenium (T~se~ = 232 ^o^C). The quaternary
phase was readily identified by the peak shifts that occur due to the
changing lattice constants.[^11] The (112) peak position is centered at
26.9º, as opposed to 26.65^o^ in the case of CuInSe~2~. The film has
distinct polycrystalline morphology with uniform grains of about \~1 μm
in size.

![](media/image30.png){width="3.0in"
height="2.7598425196850394in"}![](media/image31.png){width="3.0in"
height="2.763779527559055in"}![](media/image32.png){width="3.1251968503937007in"
height="2.75in"}![](media/image33.wmf){width="3.5in" height="3.0in"}

[]{#__RefHeading___Toc100572887 .anchor}Figure 24**:** XRD patterns and
corresponding ESEM micrographs of samples (A)-(C).

## Co-evaporated Binary Compounds

Experiments examining film kinetics using the PACE chamber (Figure 2c)
to co-expose a substrate to the chalcogen and one metals flux are
described below. The major findings are the following:

-   The rate is controlled to first order by indium flux, and not
    strongly impacted by either temperature of S/In ratio.

-   Plasma activation infinitely increases the reactivity of H~2~S as
    only pure indium is formed without plasma activation, while complete
    β-In~2~S~3~ is formed with plasma activation.

-   The film composition is a v*ery* strong dependence of the H~2~S/In
    ratio. For H~2~S/In ratios \< 6.5 film are essentially indium, while
    for H~2~S/In \> 6.5 the films are β-In~2~S~3~.

-   Mass spectrometer measurements indicate that at least 70% of H~2~S
    is dissociated under typical plasma conditions.

-   Film composition was largely independent of substrate temperature,
    however crystal orientation and quality varied significantly with
    temperature. We note that β-In~2~S~3~ was formed as low as 100ºC.

### Optimizing Source Operating Conditions

Source operating conditions are considered optimized when the source can
be operated with good control over time periods and rates consistent
with CIGS co-evaporation, and when the chalcogen dissociation fraction
is maximized.

At deposition rates similar to those typically used for CIGS
co-evaporation, use of H~2~S as a chalcogen source provided the tightest
control of the plasma and chalcogen rate, as previously discussed on
page [17](#H2Sdelivery). Thus, H~2~S was used as the chalcogen source
for examinations of co-evaporated binary compounds.

For maximum S disocciation, source power was operated at the maximum of
150 W. The limitation to 150 W is due to heating of the ICP tube.
Despite using a water-cooled coil, the heat transfer away from the tube
in the high vacuum PACE chamber is, of course, much less than on the
benchtop. At powers above 150, the ICP tube begin to glow and eventually
softens.

Determining the optimum pressure is more complex. The total amount of
exited species exiting the ICP source is given by the following
expression:

Total = Total Flow x Fraction Excited

With the choked flow geometry, the total flow rate exiting the ICP tube
is proportional to the square root of the ICP pressure. The fraction
excited is a complex function of the plasma. It is well-known that the
electron temperature decreases with pressure. This is the amount of
energy that the electrons have to dissociate and excite
chalcogen-containing species. Because of the opposite trends in flow vs.
fraction excited, we expect an optimum operating pressure. To determine
this we pressure, the emission intensity at 704 nm from an excited Ar\*
state was measured as a surrogate of plasma intensity, since it is much
more well-defined than any of the chalcogens bands or lines. The flux of
excited species should then be given by:

$\text{Flux} \propto \frac{I_{\text{Ar} \ast}}{\sqrt{P_{\text{ICP}}}}$

\(2\)

where I~Ar\*~ is the measured emission intensity

This quantity is plotted in Figure 25 using OES measurements. The flux
initially increases steeply with pressure, maximizes around 150 --170
mtorr, and starts to decrease slowly. The trend indicates an optimum as
expected and suggests that the optimal ICP pressure should be around
\~160 mtorr.

![](media/image34.wmf){width="3.0743055555555556in"
height="2.890277777777778in"}

[]{#__RefHeading___Toc100572888 .anchor}Figure 25 : Flux of excited Ar\*
as a function of ICP operating pressure

To test this hypothesis, a series of PACE experiments as a function of
ICP pressure were performed. All experiments had in common following
conditions: Indium flux: 0.3 Å/s, Gas Source: 10% H~2~S in argon, ICP
power = 140 W, T~s~ = 350 ºC. Films were deposited on 4" silicon wafers
for XRD analysis and glass for optical transmission analysis. The
spatial dependence of both indium flux and H~2~S flux across the subject
were also quantified. This arrangment allows a probing of a continuum of
In: S ratios in a single deposition experiment. The qualitative results
are shown in Figure 26, along with film quality distribution. The films
were sectioned into 12 pieces and analyzed. For each piece there is a
cluster of 3 symbols, which indicates the crystal quality as a function
of 3 different operating pressures: 125, 155, and 215 mtorr. The indium
evaporation boat is located directly below the edge of the sample, and
its distribution follows a 1/r^2^ dependence as illustrated by the blue
triangle. The ICP source has a "bulls eye" pattern, and it is directed
at the center of the sample as shown in yellow.

![](media/image35.wmf){width="5.2972222222222225in"
height="3.363888888888889in"}

[]{#__RefHeading___Toc100572889 .anchor}Figure 26: Schematic map of In
flux(blue), S flux (yellow) across a 4" wafer, with the symbols
indicating the crystal quality obtained as function of operating
pressure.

Examples of the 3 different crystal phases are shown in Figure 27. The
circles indicate that only polycrystalline indium was observed. The
triangles indicate that only β-In~2~S~3~ was observed. The cross
indicates a mixed phase, which was highly variable but usually contained
both phases. In some cases peaks associated with In~6~S~7~ were also
observed in the mixed regime. Thus Figure 27 contains a great deal of
information. First, in the row of samples closest to the In source, only
polycrystalline indium was observed in all experiments. Like wise in the
row farthest away, a mixed phase was observed in all experiments. This
is where the indium flux is the smallest. In the center two rows the
results were strongly dependent on the ICP operating pressure. For the
suggested optimum pressure, 155 mtorr, resulted in β-In~2~S~3~ formation
throughout this region. In contrast, mostly polycrystalline phase was
observed both at higher (215) and lower (125) pressures. The 215 mtorr
experiment was slightly more effective than the 125 mtorr experiment, as
some regions with mixed phase were observed. These results are in
excellent agreement with the assessment of ICP performance that were
shown in Figure 25.

![](media/image36.wmf){width="2.8847222222222224in"
height="2.8868055555555556in"}

[]{#__RefHeading___Toc100572890 .anchor}Figure 27 : Three XRD patterns
exemplifying the 3 phases observed in Figure 26

Optical transmission and variable angle spectroscopic ellipsometry were
also performed on samples deposited on glass and silicon, respectively.
Regions with crystalline β-In~2~S~3~ were observed to have the
appropriate band gap of \~ 2.5 eV. Likewise the refractive indices of
these films were in good agreement with literature values.

A control sample was also deposited at our optimum pressure without
turning on the plasma. As expected the film was essentially indium. No
evidence of conversion was observed in XRD. The film had the lustrous
grey color, and could be easily wiped off the substrate, and generally
resembled indium films evaporated in vacuum.

### Measuring the Dissociation Fraction

The dissociation of H~2~S in the ICP source was quanitified. The
schematic of the setup is shown in Figure 28. A residual gas analyzer
(RGA) was connected to the main PACE chamber. The pressure at the RGA
was further reduced using a small turbo pump to improve resolution and
minimize transients in the system. It is important to note that this is
not a line of site technique. The products effusing from the ICP source
collide with the substrate and walls of the chamber several time before
making it to the RGA. So the RGA measure what exits the ICP source plus
any gas-surface reactions that follow. So we are only sensitive to
stable species H~2~S or S. Radicals such as atomic H and S recombine or
condense on walls before arriving at the RGA. The OES was used in-situ
to make relative measurements of H and S using actinometry. Experiments
were performed as a function of rf power and pressure, which is the ICP
pressure measured upstream. In all experiments the PACE chamber itself
was order 10^-5^ torr. A 10% H~2~S/Ar mixture was used in all
experiments. When the plasma was ignited we observed two things. Signals
associated with H~2~S (MW = 34) dropped and H~2~ (MW = 2) increased.
There was no change in the argon and no other peaks were observed.

![](media/image37.wmf){width="3.5770833333333334in"
height="2.829861111111111in"}

[]{#__RefHeading___Toc100572891 .anchor}Figure 28 : Schematic of
diagnostic setup

Dissociation was quantified using the following relationship

%Dissociation =
$\text{100} \ast \frac{(H_{2}S_{\text{off}} - H_{2}S_{\text{on}})}{H_{2}S_{\text{off}}}$

\(3\)

where off and on designate the status of the plasma. The results for
power and pressure are shown in Figure 29a and b below. The results did
not make sense at first. As shown in Figure 29a the H~2~S dissociation
starts at 60-70% for rf = 60 W, but then steadily decreases as power is
increased. This is counterintuitive as one expects more dissociation at
higher powers. The apparent dissociation was also slightly greater at
higher pressure, which is also counter intuitive. In general pressure
does not have a strong impact as shown in Figure 29b.

![](media/image38.wmf){width="5.750694444444444in"
height="2.7006944444444443in"}

[]{#__RefHeading___Toc100572892 .anchor}Figure 29 : %H~2~S dissociation
as a function of (a) power and (b) ICP pressure

The OES data clears up the picture a bit. Figure 30 plots the atomic
\[H\] and \[S\] as a function of power. The atomic sulfur density
remains essentially constant while the atomic hydrogen density increases
linearly with power.

![](media/image39.wmf){width="2.707638888888889in"
height="2.547222222222222in"}

[]{#__RefHeading___Toc100572893 .anchor}Figure 30 : . OES densities of
\[H\] and \[S\] as a function of RF power.

We interpret the data in the following manner. There are two electron
impact dissociation pathways:

\(1\) H~2~S + e- →  H~2~ + S

\(2\) H~2~S + e- →  2H + S

\(3\) H~2~ + e- →  2H

\(4\)

The former dominates at low power. At higher power either the second
pathway becomes dominant or the hydrogen dissociation reaction becomes
important. This has important consequences on the interpretation of the
RGA experiments. When S exits the ICP source, we expect (and observe) it
to simply condense on the cold substrate and reactor walls. If molecular
hydrogen exits the ICP source, it will be unreactive and will exit the
reactor. In contrast, atomic hydrogen exiting the source is very
reactive and it will react on the walls, either with itself to form H~2~
or with absorbed S to remake H~2~S. So in Figure 29a when we observe and
apparent decrease in % dissociation with power, what we are really
observing is an increase of atomic hydrogen production and related
scavenging. Thus, the 70% dissociation recorded in Figure 29a may be
understood to be a minimum. The true dissociation is higher, perhaps
approaching 100%.

These results are interesting when compared with our deposition
experience. We observe that crystallinity improves with rf power. Films
at 140 W are crystalline while those at 60W contain substantial
amorphous components. As shown in Figure 30, the atomic S flux is
essentially unchanged under these conditions. This indicates that atomic
H may play a positive role in the deposition process. This has been
observed in several CVD systems. In diamond CVD atomic H improves
quality by preferentially etching sp2-bonded carbon and leaving behind
sp3-bonded diamond. A similar phenomenon may be occurring here. Atomic H
may preferentially etch weakly bound, amorphously bonded S and leave
behind crystalline β-In~2~S~3~. Another possibility is that atomic
hydrogen recombination heats the substrate, facilitating enhanced
mobility and improved film quality. We note that this may represent a
significant advantage for the chalcogen hydrides over elemental
chalcogens with respect to the PACE process.

### Flux Distributions and Resulting Film Properties

The distribution of In and S inside the PACE chamber were calculated.
The predicted distributions were verified by several film property
measurements.

First, detailed models were developed to quantify the flux of indium and
sulfur in the PACE chamber. The geometric representation of the
substrate, the indium evaporation source, and ICP source are shown in
Figure 31. The relative positions are given in Cartesian co-ordinates
with the edge of the substrate above indium source as origin. Evaporated
indium and ICP cracked gas impinge on the substrate at different fluxes
depending on distance and angle from respective sources. Due to this
variation of flux the indium to chalcogen ratio varies on the substrate
as a function of position. By understanding this distribution the
dependence of the In/S ratio can be extracted from a sinfle deposition
experiment.

Physical vapor deposition from boat sources is studied extensively and
the flux distribution is well established. The distribution of
evaporated indium will be representative of a point source as given by
equation;

$$\frac{\text{dM}_{\text{In}}}{\text{dA}}\alpha\frac{1}{r^{2}}\text{         }$$

\(5\)

where M~In~ is indium flux, A is the differential area and r the
relative distance from indium boat.

![](media/image40.wmf){width="6.125694444444444in"
height="4.250694444444444in"}

[]{#__RefHeading___Toc100572894 .anchor}Figure 31 : Cross sectional view
of substrate, Indium evaporation source and ICP source. The positions
are given in Cartesian co-ordinates.

The distribution of the chalcogen source is more complicated, due to the
sonic expansion as the gas exits the ICP source. The flux distribution
in such sources will be representative of supersonic free jet expansion
of gases in vacuum. The angular variation of density field (ρr^2^)
estimated from data using method of characteristics calculations is only
a function of θ. The approximate relation for density variation is;

$$\rho r^{2}\text{   }\alpha\text{   }\text{cos}^{2}(\frac{\text{πθ}}{2\phi})\text{    }$$

\(6\)

where ρ is the density of expanding gas, r - distance from jet source,
θ - angle from the normal to jet source. The constant φ depends on
specific heats ratio. For γ = 1.67 (argon) the value for φ is 1.365.
Along a given gas stream line the speed of the gas (u) is essentially
constant. The flux is give by the equation;

$$\frac{\text{dM}_{\text{chalcogen}}}{\text{dA}} = \rho\text{u  }$$

\(7\)

Combining equations (6) and (7);

$$\frac{\text{dM}_{\text{chalcogen}}}{\text{dA}}\text{     }\alpha\text{     }\frac{\text{cos}^{2}(\frac{\text{πθ}}{2\phi})}{r^{2}}\text{  }$$

\(8\)

In Cartesian coordinates the distance of a given point B (x, y, 0) on
substrate from ICP source A (72, 0, -178) will be;

$$r = \sqrt{(\text{x-72})^{2} + (\text{y-0})^{2} + (\text{0-}(\text{-178}))^{2}}$$

\(9\)

The angle θ is estimated using the dot product between vectors AC (axial
line of ICP source) (178tan(δ), 0, 178) and AB ((x-72-178tan(δ), y,
178);

$$\theta\text{  } = \text{cos}^{- 1}\left( \frac{- (\text{x-72-178tan}(\delta))\text{*178tan}(\delta) + 0 + (\text{178})^{2}}{\sqrt{\left\lbrack \text{178tan}(\delta) \right\rbrack^{2} + \left\lbrack \text{178} \right\rbrack^{2}}\sqrt{\left\lbrack x - \text{72} - \text{178}\text{tan}(\delta) \right\rbrack^{2} + y^{2} + \left\lbrack \text{178} \right\rbrack^{2}}} \right)\text{     }$$

\(10\)

Substituting (9) and (10) in (8) the chalcogen flux at point B from ICP
source A can be estimated as;

$$\frac{\text{dM}_{\text{chalcogen}}}{\text{dA}}\text{   } = \text{   C}_{2}\frac{\text{cos}^{2}\left( \frac{\pi\text{cos}^{- 1}\left( \frac{- (\text{x-72-178tan}(\delta))\text{*178tan}(\delta) + 0 + (\text{178})^{2}}{\sqrt{\left\lbrack \text{178tan}(\delta) \right\rbrack^{2} + \left\lbrack \text{178} \right\rbrack^{2}}\sqrt{\left\lbrack x - \text{72} - \text{178}\text{tan}(\delta) \right\rbrack^{2} + y^{2} + \left\lbrack \text{178} \right\rbrack^{2}}} \right)\text{  }}{2\phi} \right)}{(x - \text{72})^{2} + y^{2} + (\text{178})^{2}}\text{         }$$

\(11\)

Model predictions were compared with measurements of thickness,
composition and optical properties. The film discussed was deposited at
T~s~ = 200 ºC. First it was observed that film thickness was most
closely associated with the indium flux. Figure 32 compares a photograph
of the sample with model predictions of the In flux variation.

![](media/image41.wmf){width="4.500694444444444in"
height="7.329861111111111in"}

[]{#__RefHeading___Toc100572895 .anchor}Figure 32 : Modeled In flux
(top) compared with photograph of a 4" silicon sample. Note the
distortion in the upper left quadrant is due to the presence of the
quartz crystal microbalance.

The next figure compares the predicted sulfur distribution and the In/S
ratio. The ICP source was directed to the left side of the sample as
shown in the top of Figure 32. Combining this calculation with the In
flux one obtains the S/In ratio. Notice the max is shifted to the right,
and the gradients are steeper than in the individual elemental
distributions.

![](media/image42.wmf){width="5.750694444444444in"
height="7.339583333333334in"}

[]{#__RefHeading___Toc100572896 .anchor}Figure 33 : Models of the sulfur
distribution (top) and S/In ratio (bottom).

The thickness comparison that was shown schematically in Figure 33 was
evaluated quantitatively. Figure 34 shows the measured thickness as a
function of radial position across the substrate. The line is model
predictions based on just the indium flux. The data contains two types
of points. The red triangles were taken from the region of high sulfur
flux as indicated by the purple circle in Figure 33a. The remaining data
points were taken from other regions. The agreement is generally very
good across the wafer. The measured thickness in the high sulfur region
is slightly higher than the indium model prediction, as one would
expect.

![](media/image43.png){width="6.004166666666666in"
height="5.000694444444444in"}

[]{#__RefHeading___Toc100572897 .anchor}Figure 34 : Comparison of
measured (points) and modeled (line) film thickness as a function of
radial position from the origin.

Next we compare the properties of the as-deposited films using energy
dispersive spectroscopy (EDS) for composition and spectroscopic
ellipsometry (SE) for refractive index. The refractive index (n) and
extinction coefficient (k) were measured across regions L and M as shown
in Figure 32. The results are shown in Figure 35. As one transverses the
wafer the refractive index has a sharp increase, plateaus for 2 --3 cm,
and then decreases sharply. The plateau region corresponds nicely to the
maxima in the S/In ratio as shown in Figure 32. We also note that the
crystal structure of the films with the high refractive index display
only β-In~2~S~3~ peaks. We also note that films outside this region
displayed either In peaks or a mixture of materials.

![](media/image44.png){width="5.179166666666666in"
height="3.5409722222222224in"}

[]{#__RefHeading___Toc100572898 .anchor}Figure 35 : Optical properties
as a function of position across regions L and M as shown in Figure 32

Finally we surveyed the S/In ratio across region A using EDS. The
results are shown in Figure 36. Again there is very good quantitative
agreement between the data and the profiles predicted in Figure 33. The
S/In ratio does not change dramatically, \~20% across the wafer, but the
structural and optical properties change much more dramatically.

![](media/image2.wmf)

[]{#__RefHeading___Toc100572899 .anchor}Figure 36 : S/In ratio as
determined by EDS across region A.

Using the detailed models presented plus quantitative measurements of
the indium deposition rates and H~2~S flow rates, we established a
quantitative measure of the S/In ratio. This distribution is shown in
Figure 37 for a specific set of operating conditions. The data presented
are all from films deposited at this condition. We also note that the
ratios presented in Figure 37 represent the upper limit, assuming that
all of the chalcogen is activated. In reality only a fraction of the
H~2~S is cracked by the ICP source. A quadrupole mass spectrometer was
mounted on the on the high vacuum PACE chamber to measure this variable.
The dissociation of H~2~S was monitored by watching the change in the
primary cracking peaks at m/z = 33 and 34. When the ICP source was
turned on the H~2~S signals dropped by \~ 50%, and were also accompanied
by a concomitant increase in the H2 signal. Nothing else significant was
observed. As discussed previously, H~2~S is inactive in the PACE chamber
as films deposited without plasma activation yield simply
polycrystalline indium. So the ratios of active chalcogen to indium are
expected to be about ½ of the values shown in Figure 37. From Figure 37
we see that the ratio can vary by approximately a factor of \~ 40% over
the 4"diameter wafers.

![](media/image3.wmf){width="5.995138888888889in" height="4.1125in"}

[]{#__RefHeading___Toc100572900 .anchor}Figure 37 : Quantitative
distributions of the H~2~S/In flux ratios across a 4" diameter for the
films described in this report.

### Film Properties

After initial optimization of conditions for ICP operation and indium
evaporation, the following conditions were used to make a sample set for
further study: Indium flux: 0.3 Å/s, Gas Source: 10% H~2~S in argon, ICP
power = 140 W, ICP pressure = 110 mtorr, T~s~ = 350 ºC. Films were
deposited on both silicon and glass. Figure 38 shows a photograph of a
PACE film deposited on glass. The relative positions of the ICP source
and indium evaporation boat are shown as well. The small sample in the
picture is β-In~2~S~3~ that was fabricated in the benchtop system. The
two samples are similar in color. The gradient in film color across the
sample reflects the gradient in indium flux. The sample was sectioned
into 9 pieces as shown for detailed analysis.

![](media/image4.wmf){width="5.184722222222222in"
height="5.0368055555555555in"}

[]{#__RefHeading___Toc100572901 .anchor}Figure 38 : Photograph of an
In~x~S~y~ film deposited by PACE, shown for comparison with a small
sample of β-In~2~S~3~ that was produced in the bench top system. The
green boxes denote the approximate positions of the indium and ICP
sources. The samples were divided into 9 pieces for analysis as shown.

Figure 39 shows XRD patterns obtained from selected regions for samples
deposited on glass and silicon, respectively. In both cases the crystal
quality improves as the S:In ratio increases (mobbing from region 3 to
region 1). In the case of glass, the patterns are dominated by the
amorphous signature of the underlying glass substrate. No crystalline
phases are observed at position B3, but they begin to appear in B2 and
are clearly identified at B1. These two peaks are both β-In~2~S~3~. In
the case of the silicon substrate, the XRD patterns are much more clear
since there is little background. The trend is the same. In the
indium-rich regions (3) there is a mixture of β-In~2~S~3~ and indium. As
one moves to region (1) the indium peak is attenuated, and the
-In~2~S~3~ phase becomes dominant. The sample from region C1 contained
only β-In~2~S~3~ phases.

![](media/image5.wmf){width="6.36875in" height="3.5743055555555556in"}

[]{#__RefHeading___Toc100572902 .anchor}Figure 39 : XRD patterns from
selected regions of co-evaporated films on glass (above) and silicon
(right).

Figure 40 shows XRD patterns obtained from a single sample that was
deposited at T~s~ = 200 ºC. The sample was broken into several pieces
and characterized by XRD. In this figure we show XRD patterns from 2
different positions that had nominally identical S/In ratios to both
validate the contour plot shown in Figure 37 and demonstrate the
reproducibility of the experiment. Films deposited at S/In = 5.6 were
dominated by the sharp indium peak at 33 ºC. As the ratio was increased
to 6.5 strong β-In~2~S~3~ peaks are formed, but the In peak remains.
With just a slight increase to S/In = 6.9 the indium peaks completely
disappeared, and an additional β-In~2~S~3~ peak at 27º is observed.
Films at the highest ratio of S/In = 8.3 were of course completely
β-In~2~S~3~. The film structure is extremely sensitive to the S/In
ratio, as evidenced by sharp the differences between films at 6.5 and
6.9. This was seen in other experiments as well. Two samples were grown
at nominally identical conditions with the exception that the indium
evaporation rate was varied 33%. The sample with the higher indium flux
was almost completely indium, while the other was completely
β-In~2~S~3~. We also note that the S/In did not have a significant
impact on film thickness, which to first order is governed by the indium
evaporation rate. Films in high S/In regions were 10-20 % thicker.

![](media/image6.wmf){width="3.6930555555555555in"
height="3.557638888888889in"}

[]{#__RefHeading___Toc100572903 .anchor}Figure 40 : XRD patterns as a
function of H~2~S/In obtained from a single sample deposited at T~s~ =
200 ºC\>

A series of films were also deposited as a function of substrate
temperature. The indium and chalcogen sources were operated identically
in all experiments. Figure 41 compares films deposited with a S/In ratio
of \~7 as a function of temperature. In all cases the peaks displayed
correspond to β-In~2~S~3~. Temperature was observed to have a negligible
influence over the S/In ratio needed to effect the transition from
indium to β-In~2~S~3~, however it does have an influence on crystal
quality and orientation. The film orientation changes with temperature
as shown in Figure 41. In addition, one observes that the peak widths
and background signal both decreases with temperature, indicating higher
crystal quality and a smaller amorphous component, respectively. Again,
film thickness was found to be independent of temperature.

![](media/image7.wmf){width="3.3944444444444444in" height="3.075in"}

[]{#__RefHeading___Toc100572904 .anchor}Figure 41 : XRD patterns as a
function of substrate temperature for films deposited with S/In ratio of
7.

Figure 42 shows the AFMs at 4 substrate temperatures. As expected grain
size increases with temperature, going from nanocrystalline to
microcrystalline.

![](media/image8.wmf){width="5.502083333333333in"
height="3.4743055555555555in"}

[]{#__RefHeading___Toc100572905 .anchor}Figure 42 : AFM morphology as a
function of substrate temperature.

In contrast, when depositions were performed *without* plasma
activation, no S incorporation was detected by EDS or XRD. In all
respects, samples fabricated without the plasma appeared to consist of
metallic In.

# Application of PACE Sources to CIGS Co-Evaporation

The PACE sources, whose development was described in previous sections,
were installed in standard CIGS deposition equipment and evaluated for
effect on CIGS properties.

## Baseline Process

An important aspect of evaluating the effect of PACE sources is the
demonstration of a reproducible and high quality baseline (no plasma)
process for CIGS device fabrication. The bell jar used for baseline
processing was described earlier (Figure 4). For baseline devices, a
three-stage recipe is used to deposit CIGS onto Mo-coated glass
substrates. Flux and temperature profiles from a typical deposition are
shown in Figure 43. Reproducibility was improved by insuring a
consistent Cu-rich excursion of the film growth, using an infrared
sensor to monitor the film emissivity change as it crosses in and out of
the Cu-rich regime.Error: Reference source not found This method is
similar to those described elsewhere using a contact thermocouple to
sense the temperature change resulting from the emissivity
change.[^12]^,^[^13] Devices were completed by application of chemical
bath deposited CdS, sputtered resistive ZnO, sputtered indium tin oxide,
and metallic grids. Efficiencies of completed devices are typically
around 10%. Figure 44 shows an example of a CIGS device from the bell
jar, with all layers deposited at ITN. The device is 1 cm^2^, with no
anti-reflective coating. Quoted characterization is under AM1.5,
total-area conditions. Throughout the contract, a number of adjustments
to the baseline process, such as improved calibration procedures, were
made to maximize reproducibility and overall efficiency.

![](media/image9.wmf){width="3.6368055555555556in"
height="2.4881944444444444in"}

[]{#__RefHeading___Toc100572906 .anchor}Figure 43 : Time, temperature,
and deposition flux profiles for CIGS deposition.

![](media/image10.wmf){width="3.6166666666666667in"
height="2.810416666666667in"}

[]{#__RefHeading___Toc100572907 .anchor}Figure 44 : JV curve of small
area device from bell jar. No AR coating was applied. Cell parameters
shown in inset.

## Source Installation and Modification

Integration of plasma sources into full CIGS co-evaporation was
performed. Incorporation of the plasma sources into three-stage CIGS
co-evaporation is nontrivial, as the deposition environment exacts
rigorous requirements on rate control, geometry, uniformity, resistance
to heat and deposition product, and compatibility with the existing and
complex sensors in the bell jar (e.g. EIES, QCM, IR).

First, necessary hardware was added to the three-stage CIGS bell jar.
Use of a low-current, high-voltage Se effusion source provides
flexibility in placement of the current feedthroughs relative. Effective
solutions for grounding and shielding issues were implemented as
demonstrated in the CSM PACE chamber. RF, Ar, and power feedthroughs
were installed. To accommodate feedthroughs for RF power and the source
placement, several chamber components had to be moved, including the
quartz crystal microbalance, the In source bus bar, the infrared
substrate sensor, and the traditional tantalum Se boat. Argon was piped
into the source area. The RF power supply and network were mounted in
the chamber electrical cabinet, including the necessary 280V supply and
increased cabinet cooling. Several components on the chamber that would
physically obstruct the RF were relocated, including vacuum gauges,
Eurotherms, and the emergency stop button. Bracketing to affix the PACE
source in the chamber was fabricated and tested. A power controller for
the Se pot nozzle heater was obtained and tested.

In section 2.3.4, Se from the PACE source in a test chamber was observed
to follow the expected cos^3^θ flux distribution. The same flux
distribution was confirmed with the PACE source installed in the
three-stage bell jar. Expected flux profile was generated from the
cos^3^θ flux distribution, in combination with chamber and source
dimensions and orientation. This prediction is shown in Figure 45a. For
experimental verification, Se from the PACE source was deposited on
glass witness slides placed across the area used for substrates in CIGS
depositions. As expected from earlier tests, PACE source Se rate control
was acceptable in the bell jar. Multiple thickness measurements were
performed across the witness slides via mechanical profilometry. The
measured flux distribution is shown in Figure 45b. Two thickness
measurements near some Se-coated fixturing were omitted, as some coating
appears to have originated from inadvertent heating of the fixturing.
There is reasonable agreement between Figure 45a and Figure 45b,
considering that there are several degrees of uncertainty in the angles
between the source axis and substrate. Agreement between measured and
predicted PACE source flux profiles is significant because it indicates
that the sources continue to function as expected, and because it
provides guidelines for adjustments to sources.

![](media/image11.png) ![](media/image12.png)

[]{#__RefHeading___Toc100572908 .anchor}Figure 45 : a) Predicted and b)
measured flux distributions from PACE source in CIGS three-stage bell
jar.

Source angle was adjusted so that flux distribution is centered at the
substrate center. Because the PACE source axis is not perpendicular to
the substrate, and the flux falls off both with distance from the nozzle
and angle off-axis, the source axis must be directed at a spot slightly
beyond the substrate to center the flux distribution. The required angle
for flux centering agrees well with calculations of the flux
distribution.

Figure 46 shows relative flux as function of position over the
substrate, during a source test. Uniformity is ± 20%, and has been
demonstrated to be reproducible over several depositions. This
uniformity is within limits needed to test chalcopyrite utilization in
the CIGS process with and without plasma-assist. The uniformity is
better than that obtained with the traditional tantalum baffled-box
evaporation boat.

![](media/image13.png){width="3.946527777777778in"
height="3.2583333333333333in"}

[]{#__RefHeading___Toc100572909 .anchor}Figure 46 : Flux versus position
from PACE source testing in three-stage bell jar.

The new PACE source was integrated into the fully-automated CIGS
control. The automated control in the bell jar maximizes reproducibility
between different depositions and operators, since all depositions are
run from pre-programmed recipes. Furthermore, all measured quantities
are logged to the computer for later reference. Incorporating the new
PACE source into this control strategy required adding hardware and
software to communicate with the PACE source temperature controllers,
heat the PACE source according to computer-specified setpoints, log PACE
source rates and temperatures, and allow recipe programming of these
quantities. Figure 47 shows rates and temperatures from a
fully-automated CIGS run incorporating a PACE source. Because the PACE
source has a much larger thermal mass than the traditional Ta boat, some
minor oscillations in Se rate are unavoidable. However, these \~15%
oscillations are much smaller than the recommended 2 to 10x Se-to-metals
ratio envelope that is considered acceptable for high-quality CIGS
formation. Figure 48 shows a sample of the recipe program used to run
the deposition.

![](media/image14.wmf){width="3.8229166666666665in"
height="2.7215277777777778in"}

[]{#__RefHeading___Toc100572910 .anchor}Figure 47 : Rates and
temperatures from fully-automated CIGS run incorporating PACE source.

![](media/image15.png){width="2.8222222222222224in"
height="2.8916666666666666in"}

[]{#__RefHeading___Toc100572911 .anchor}Figure 48 : Example portion of
recipe used to program the deposition with the PACE source.

The PACE sources are not compatible with the EIES rate monitor. The data
shown earlier in Figure 18 predict a 10^-5^ Torr background pressure of
Ar during PACE source operation. Such pressures will add significant
background noise to the already weak EIES signal. The interference is
particularly severe for Ga, which is the weakest emitter of the three
metals monitored by EIES. For illustration, Figure 49 shows apparent
flux profiles from a CIGS deposition in which background pressure
reached 8 x 10^-6^ Torr by the end of the second stage, due to an
outgassing problem. Cu, In, and Ga rates graphed are those indicated by
the EIES. Se rate is that indicated by the QCM. During the second stage
(where the Cu rate is nonzero) the Ga source is turned off. However,
significant rate is indicated, as circled in red, due to the increasing
background pressure. The false rate is nearly equal in magnitude to the
desired Ga rate, resulting, no doubt, in a significant deviation between
the actual and intended Ga rates during the third stage. Ar emits
efficiently at a multitude of wavelengths upon electron excitation, and
is therefore exhibits a similar effect on the signal.

![](media/image16.png){width="4.676388888888889in"
height="3.201388888888889in"}

[]{#__RefHeading___Toc100572912 .anchor}Figure 49 : Apparent flux as a
function of time for three-stage CIGS deposition, with effect of
background pressure increase due to outgassing circled in red.

Therefore, in order to avoid interference between the EIES and Ar from
the PACE sources, an alternate deposition procedure was implemented.
This procedure involves setting metals source rates using the EIES
(without Ar flowing to the PACE source), holding the metals sources at
constant deposition power[]{#constantpower .anchor} while the PACE
source is operating, and judging the end of the second and third stages
utilizing the film emissivity. This procedure is similar to that
described by Kessler et alError: Reference source not found to maintain
accurate control of maximum and final Cu ratio without the use of a
multi-species rate monitor. The new procedure must be flexible enough to
allow for some variation in background Ar pressures due to
source-to-source differences in reactor tubes. Device results on control
films (without plasma) confirm that acceptable device quality is
obtained using this technique.

Operation of the PACE source in the CIGS chamber is shown in Figure 47.
In this photo, the PACE source is viewed by looking just under the
substrate holder (black item at top of photo), and around the open
substrate shutter (black item at left of photo). The bolted enclosure
beneath the plasma is the Se sparger.

![](media/image17.png){width="3.754166666666667in" height="3.55625in"}

[]{#__RefHeading___Toc100572913 .anchor}Figure 50 : PACE source with
plasma operating in three-stage CIGS co-evaporation chamber.

As a final step in the source installation, plasma-activated Se was
delivered to the substrate at rates commensurate with co-evaporation, in
the three-stage CIGS co-evaporation chamber. Plasma-activated Se was
delivered at a plasma power of 100 W and rate of 25 Å/sec. This rate was
maintained for approximately the time of a CIGS deposition without
evidence of Se condensation in the plasma tube or difficulty maintaining
the plasma. Several iterations of the PACE source installation were
necessary before the desired rate was obtained. It was found that

-   Higher tube pressures ([\>]{.underline}500 mT Ar) and plasma powers
    ([\>]{.underline}60 W) are helpful in preventing coating of the
    quartz plasma tube with Se and preventing the plasma from vanishing
    into the neck of the tube.

-   Chiller water temperature of 50, rather than 20, ^o^C supplied to
    the RF coil also helps prevent tube coating.

-   The tube should be wrapped with several layers of fiber glass to
    keep Ar or secondary electron generation away from the RF coil,
    thereby keeping the plasma confined to the inside of the reactor
    tube. The fiber glass also prevents coating of the RF coil with Cu,
    In, Ga, and Se.

-   Metallic shielding should not be included, due to severe heating and
    subsequent shorting through the fiberglass to the RF coil.

```{=html}
<!-- -->
```
-   The IR sensor usually used for endpoint detection is responding
    either to RF or light from the plasma. Shielding reduced the
    magnitude of the noise, but not sufficiently. The resulting films
    were therefore off-composition. Since the system lamps and
    thermocouples have not been affected by the RF, in future
    depositions a lamp-power method of endpoint detectionError:
    Reference source not found can be used. Endpoint detection is
    particularly important for these depositions, as background Ar from
    the plasma causes false emission in the EIES filament, thus
    preventing use of the EIES rate monitor when the plasma is ignited.

-   After re-filling the Se sparger, Se pellets should be melted and
    then cooled before attempting deposition. Otherwise, spitting Se
    into the quartz ICP tube causes the tube to coat.

-   Slight variations in the dimensions of the hand-blown quartz tubing
    can make a difference in how easily the plasma is ignited. A shorter
    neck and body on the tube is preferable.

## Film Fabrication

A key anticipated benefit of PACE is the reduction of CIGS deposition
temperatures. To demonstrate this advantage, CIGS was deposited at
reduced temperatures known[^14]^,^[^15] to be detrimental to device
efficiency. Some depositions were performed with plasma-activation of
the chalcogen, and others without, and device performance was compared.

Operation of the PACE source during CIGS co-evaporation caused several
difficulties beyond those encountered during source development. First,
in order to apply the Se rates utilized during baseline CIGS processing
(around 50 Å/sec), and maintain a plasma ICP pressure had to be
increased to near 1 Torr. Se is more difficult to ionize than Ar, and
this pressure indicates that an approximate 10:1 ratio of Ar to Se is
necessary to maintain the plasma. He was also introduced into the PACE
source in hopes of achieving more efficient ionization, but no benefit
was realized. Second, the resulting elevated Ar pressure in the chamber
(\~0.5 mTorr) caused some decrease in the deposition rate and uniformity
of Cu, In, and Ga fluxes. This Ar-dependent rate increase is
particularly problematic since the EIES rate sensor cannot be used
during PACE source operation, and depositions with plasma therefore rely
on a calibrated constant source power (as described on page
[48](#constantpower)). Third, the IR sensor used for endpoint detection
responds significantly to light from the ICP tube. Thus, the operator's
only indications of film emissivity during a PACE deposition are changes
in lamp power required to maintain a given substrate temperature. Such
changes are often difficult to distinguish from thermal transients
related to chamber and source heating. Finally, although coating of the
RF coil was mitigated by wrapping it in fiberglass, the coil's
occasional degradation required re-building the source with a fresh Cu
coil when plasma ignition failed.

These operational difficulties caused composition control to be
difficult for the PACE films. Composition control is important both for
creating acceptable final film properties, and insuring that the film
experiences the Cu-rich growth period that has been demonstrated to have
an important effect on CIGS films grown at low temperatures.Error:
Reference source not found Many PACE depositions failed because the
final film composition was not within the desired Cu/(In+Ga) atomic
ratio envelope of 0.8 to 0.95.

Difficulties in controlling composition when operating the PACE source
prompted the use of three different CIGS recipes: three-stage,[^16]
two-stage,[^17] and "Cu-first". The standard three-stage CIGS recipe
requires endpoint detection for the second stage, when the film is
mildly Cu-rich, and for the third stage, when the film returns to
Cu-poor. A two-stage recipe, in which Cu and In are deposited
simultaneously during the initial portion of the deposition, requires
only one instance of endpoint detection,Error: Reference source not
found and therefore may allow better composition control under difficult
circumstances. However, if the high Ar pressure causes significant
decrease in Cu rate from that intended, even the two-stage film may not
experience Cu-rich growth. To avoid this lack of Cu-rich growth, a
modified two-stage deposition was also performed where all Cu was
deposited before the In. This recipe is referred to in this report as
"Cu-first".

CIGS films were formed into devices using the standard baseline window
layers described in section 4.1.

## Device Analysis

PACE and control (no plasma) films were deposited as a function of
maximum substrate temperature and CIGS recipe. Table 2 shows a summary
of these deposition conditions, film composition, and the resulting
device performance. The same information is presented graphically in
Figure 51. In Figure 51, hollow points show devices made without plasma
activation, and solid points show those made with the plasma. Circles
show films made using the three-stage recipe, while squares denote the
two-stage recipe, and triangles the Cu-first recipe. No benefit is
detected from the addition of plasma activation. In fact, in its current
implementation, equipment difficulties are a significant deterrent.

![](media/image18.wmf)

[]{#__RefHeading___Toc100572917 .anchor}Table 2: Summary of CIGS
deposition conditions, composition, and device performance.

![](media/image19.wmf){width="3.5520833333333335in"
height="2.7354166666666666in"}

[]{#__RefHeading___Toc100572914 .anchor}Figure 51: Device performance as
a function of maximum deposition temperature, recipe, and
plasma-activation (solid points).

## Discussion

The lack of benefit from plasma activation is in sharp contrast with the
benefits evident from benchtop source experiments (section 3.1). The
direct incorporation of PACE sources into CIGS co-evaporation, with high
Se rates is problematic for several reasons:

-   It is difficult to establish low-pressure discharges using only
    elemental chalcogen, and significant amounts of argon must be added
    to stabilize the plasma.

-   The extra argon increases the ICP pressure, which decreases the
    electron temperature, exponentially decreasing the ability of
    electrons to dissociate chalcogen.

-   The argon increases the deposition chamber pressure, detrimentally
    impacting metal evaporation sources and impacting other equipment in
    the process.

-   Elemental chalcogens will condense in the ICP source below T\< 250
    ºC. This is especially problematic during ICP ignition and
    extinction. This requires extraordinary operational care and perhaps
    external heating to prevent this from occurring.

It is possible that these disadvantages might be overcome if one were to
modify the CIGS co-evaporation process to use H~2~(S,Se) instead of
elemental Se, analogous to the work for formation of In~2~Se~3~ (section
3.2). The use of gaseous precursors (H~2~S/ H~2~Se) offers several
benefits over the elemental chalcogen. First, H plasmas are easy to
sustain in the ICP source, so the use of gaseous precursors can
eliminate the need for argon. Second, a significant amount of atomic H
is made in the ICP source, which recombines to H~2~ on the walls. This
is an exothermic reaction which heats the ICP source to temperatures
\>200 ºC. This *in-situ* heating prevents chalcogen condensation and
keeps the ICP source clean while allowing the ICP coil to remain fully
water-cooled. Third, atomic H exiting the ICP source will also recombine
on at the substrate. The associated heating may be beneficial for film
growth. This has been seen in other CVD systems such as diamond
deposition. In the PACE chamber it has been observed that increasing the
RF power increases the atomic hydrogen density, but the atomic sulfur
density remains unchanged. Film properties improve with rf power,
suggesting that H is indeed beneficial to film growth. Finally, gaseous
precursors are expected to supply more energy to the growing film, since
elemental chalcogens sublime as long chain oligomers and likely require
multiple electron collisions to be fully reduced to chalcogen atoms. It
is clear from the emission spectroscopy data that the ICP source
produces significant quantities of diatomic chalcogens (S~2~, Se~2~).
However, the diatomic chalcogens do not supply a significant amount of
energy. As shown in Figure 52, the difference between S~8~ and S~2~ is
only about 8 kcal/atm. In contrast, the benefit in going from S~2~ to S
is about 50 kcal/atom. The energy difference between diatomic chalcogen
and gaseous chalcogen is not substantial. Elemental vapors will require
multiple electron impact reactions to decompose the oligomers, while a
single collision is required for gaseous sources.

H~2~S + e^-^ → H~2~ + S + e^-^

This decomposition has been observed experimentally. Emission spectra
using elemental vapors are dominated by diatomic chalcogen bands, and
the signal from atomic species in very weak and often cannot be resolved
from the background. In contrast, large atomic emission peaks are
clearly seen in the H~2~S plasmas, and the band emission from the
diatomic chalcogens is significantly attenuated. PACE chamber
experiments demonstrated that H~2~S is nearly complete, with \> 70%
dissociation (section 3.2.2). Since H~2~Se is less stable than H~2~S, it
is expected that complete dissociation of H~2~Se would also be achieved
in a plasma.

![](media/image20.wmf){width="3.1251968503937007in"
height="2.8244094488188978in"}![](media/image21.wmf){width="3.1251968503937007in"
height="2.8236220472440943in"}

[]{#__RefHeading___Toc100572915 .anchor}Figure 52:Potential Energy
diagram for copper chalcopyrite formation from different sources.

# Team Activities

ITN and CSM participate in team activities. This participation has
included facilitation of absorber sub-team activities, analysis and
discussion of team data in absorber sub-team transport studies,
admittance spectroscopy measurements on the same samples, attendance and
presentations at team meetings, assembly of a publication describing
absorber sub-team activities, and work with NREL scientists to sulfurize
high-efficiency CIGS material.

# Conclusions and Future Directions

This program explored the use of plasma-activated chalcogen sources in
CIGS co-evaporation to lower CIGS deposition temperature, increase
utilization, increase deposition rate, and improve S:Se stoichiometry
control.

Plasma activation sources were designed and built, then operated and
characterized over a wide range of conditions. Optical emission and mass
spectrometry data show that chalcogens are effectively dissociated in
the plasma.

The enhanced reactivity achieved by the plasma processing was
demonstrated by conversion of pre-deposited metal films to respective
chalcogen-containing phases at low temperature and low chalcogen flux.
Three material systems (In, Cu-In, Cu-In-Ga) were successfully converted
to sulfides and selenides using plasma processing. Films treated at the
highest temperature thermally showed no or little chalcogen conversion.
The binary b-In~2~S~3~ phase was formed at 250 ºC, while ternary and
quaternary phases were formed by plasma processing at 350 ºC in less
than one hour. Films with well-defined XRD patterns were also
characterized by proper optical band gaps and a well-defined
polycrystalline morphology.

The PACE sources were also implemented in CIGS co-evaporation. No
benefit from PACE was observed in device results and frequent deposition
failures occurred. Use of the PACE sources with elemental Se and the
high rates typical of co-evaporation pose a number of equipment and
physical problems. It is difficult to establish low-pressure discharges
using only elemental chalcogen, and significant amounts of argon must be
added to stabilize the plasma. High rates require high Ar pressures,
causing interference with composition control and decreasing the ability
of electrons to dissociate chalcogen. Avoiding condensation of the
chalcogen in the ICP source requires extraordinary operational care and
results in frequent deposition failures.

It is possible that these disadvantages might be overcome if one were to
modify the CIGS co-evaporation process to use H~2~(S,Se) instead of
elemental Se, analogous to the work performed to form In~2~S~3~ (section
3.2). H plasmas are easy to sustain in the ICP source, so the use of
gaseous precursors can eliminate the need for argon. A significant
amount of atomic H is made in the ICP source, which recombines to H~2~
on the walls, preventing condensation. Finally, gaseous precursors are
expected to supply much more energy to the growing film.

It is thus reasonable that future studies in this area would attempt
CIGS formation through use of gaseous precursors. Two unknowns are
involved in this alternate approach. First, while \>19% devices have
been produced using elemental Se, the process space for utilizing
gaseous precursor during co-evaporation is largely unknown. Second,
significant safety issues must be addressed for use of H~2~Se.

# Publications

1.  S. Kosuarju, I. Repins, C. A. Wolden, "Development of
    Plasma-assisted Processing for Selenization and Sulfurization of
    Absorber Layers", *Proceedings of the 2003 National Center for
    Photovoltaics Review Meeting*, 2003.

2.  S. Kosaraju, C.A. Wolden, I. Repins, "Development of Plasma-assisted
    Processing for Selenization and Sulfurization of Absorber Layers",
    Materials Research Society Symposium **763**, pp. 391-396, 2003.

3.  S. Kosaraju, I.L. Repins, C.A. Wolden, "Formation of Chalcogen
    Plasmas and their Use in Synthesis of Photovoltaic Absorbers," *DOE
    Solar Energy Technologies Program Review Meeting: Government
    Document #DOE/GO-102005-2067*, (2004).

4.  S. Kosaraju, I.L. Repins, C. A. Wolden, "Formation of chalcogen
    containing plasmas and their use in the synthesis of photovoltaic
    absorbers," *Journal of Vacuum Science and Technology*, in press,
    2005.

5.  I.L. Repins, B.J. Stanbery, D. L. Young, S.S. Li, W. K. Metzger,
    C.L. Perkins, W.N. Shafarman, M. E. Beck, L. Chen, V. K. Kapur, D.
    Tarrant, M.D. Gonzalez, D.G. Jensen, T.J. Anderson, X. Wang, L.L.
    Kerr, B. Keyes, S. Asher, A. Delahoy, B. Von Roedern, "Comparison of
    Device Performance and Measured Transport Parameters in
    Widely-Varying Cu(In~,~Ga)(Se~,~S) Solar Cells," *Progress in
    Photovoltaics*, in press, (2005).

# References

[^1]: 78^th^ Edition of the CRC Handbook of Chemistry and Physics, 78th
    Ed, Editor D. R. Lide, (CRC Press, Boca Raton) 1997.

[^2]: H. Migge and J. Grzanna, "Thermochemistry in the system Cu-In-S at
    723K," *J. Mater. Chem*., **9** (1994.) pp. 125-128.

[^3]: M. V. Malyshev and V.M. Donnelly, "Diagnostics of chlorine
    inductively coupled plasmas. Measurement of electron temperatures
    and electron energy distribution functions," *J. Appl. Phys.,*
    **87** (2000) pp. 1642-1648.

[^4]: E. Meeks, R.S. Larson, P. Ho, C. Apblett, S. M. Han, E. Edelberg,
    and E. S. Aydil, "Modeling of SiO~2~ deposition in high density
    plasma reactors and comparisons of model predictions with
    experimental measurement," *J. Vac. Sci. Technol. A*, **16** (1998)
    pp. 544-563.

[^5]: I.L. Repins, D. Fisher, W.K. Batchelor, L. Woods, M.E. Beck, "A
    Non-Contact Low-Cost Sensor for Improved Repeatability in
    Co-Evaporation of High-Quality CIGS", *Progress in Photovoltaics*,
    in press, 2005.

[^6]: R.W. Birkmire, W.N. Shafarman, E. Eser, S.S. Hegedus, B.E.
    McCandless, R. Aparicio, K. Dobson, "Optimization of Processing and
    Modeling Issues for Thin Film Solar Cell Devices Including Concepts
    for the Development of Polycrystalline Multijunctions", *Annual
    Report to NREL under Subcontract ZAK-8-17619-33*, 2001, pp. 3-4.

[^7]: C. A. Wolden, S. Mitra, and K. K. Gleason, J. Appl. Phys. **72**,
    3750 (1992).

[^8]: J. I. Pankove, *Optical Processes in Semiconductors*. (Prentice
    Hall, Englewood, NJ, 1972) p. 248.

[^9]: N. Barreau, S. Marsillac, J. C. Berne\`de, T. B. Nasrallah, and S.
    Belgacem, Phys. Stat. Sol. **184**, 179 (2001).

[^10]: T. Walter and H. W. Schock, Thin Solid Films **224**, 74 (1993).

[^11]: M. Murudachalam, R. W. Birkmire, H. Hichri, J. M. Schultz, A.
    Swartzlander, and M. M. Al-Jassim, J. Appl. Phys. **82** (1997).

[^12]: J. Kessler, J. Scholdstrom, L. Stolt, "Rapid Cu(In,Ga)Se~2~
    Growth Using "End Point Detection", *Proceedings of the 28th IEEE
    Photovoltaics Specialists Conference*, (2000), pp. 509-512.

[^13]: K. Ramanathan, M.A. Contreras, C.L. Perkins, S. Asher, F. S.
    Hasoon, J. Keane, D. Young, M. Romero, W. Metzger, R. Noufi, J.
    Ward, A. Duda, "Properties of 19.2% Efficiency CuInGaSe~2~ Thin-film
    Solar Cells", *Progress in Photovoltaics: Research and
    Applications*, Vol. **11**, (2003), pp. 225-230.

[^14]: M. Lammer, U. Klemm, M. Powalla, "Sodium co-evaporation for low
    temperature Cu(In,Ga)Se2 deposition," *Thin Solid Films* **387**,
    (2001), pp. 33-36.

[^15]: W.N. Shafarman, J. Zhu, "Effect of Grain Size, Morphology, and
    Deposition Temperature on Cu(InGa)Se~2~ Solar Cells," *Materials
    Research Society Symposium Proceedings* **668**, (2001), pp.
    H2.3.1-H2.3.6.

[^16]: A.M. Gabor, J.R. Tuttle, D.S. Albin, M.A. Contreras, R. Noufi,
    A.M. Herman, "High-efficiency CuIn~x~Ga~1-x~Se~2~ solar cells made
    from (In~x~,Ga~1-x~)~2~Se~3~ precursor films", *Applied Physics
    Letters*, **65**(2), pp. 198-200, (1994).

[^17]: Hedstrom, H. Ohlsen, M. Bodegard, A. Kylner, L. Stolt, D.
    Hariskos, M. Ruckh, H.W. Schock, *Proceedings of the 23rd IEEE
    Photovoltaics Specialists Conference*, 364-371 (1993).