Measurement of Depositing and Bombarding Species Involved in the Plasma
Production of Amorphous Silicon and Silicon/Germanium Solar Cells

Quarterly Technical Report

June 1, 2004 -- Aug. 31, 2004

DOE Interagency Agreement De-AC36-02G010244

Principal investigator: Alan Gallagher

Contributors: Peter Horvath, Damir Kujundcik

DOE Program Officer: Ms. Lizana Pierce

NREL Technical monitor: Dr. Bolko Von Roedern

The technical approach to this project is to utilize mass spectrometry
to measure radical and ion species that arrive at the substrate of a
plasma-enhanced-chemical-vapor-deposition (PECVD) reactor. Threshold
ionization mass spectrometry (TIMS) is utilized for the radical
detection. A small-scale reactor mimics those used to produce
hydrogenated amorphous (a-Si:H) and microcrystalline (μc-Si) silicon and
silicon/germanium (a-Si:Ge:H) solar cells. Radio frequency (RF) and high
frequency (HF) discharges will normally be studied, and the reactor
utilizes a similar electrode gap (2-3 cm), substrate temperature (20-250
°C), gas pressures (0.1-5 Torr), gas mixtures and discharge power
density to that used in an industrial reactor.

In the last report, we discussed how the designed ion-collection and
mass-spectrometer behavior was achieved. Here we describe film-growth
measurements in our film-deposition reactor. Most industrial and
experimental reactors measure the ratio (R) of H~2~ versus SiH~4~ flows,
the reactor chamber pressure (P~Ch~) and the power (P~Tot~) delivered to
a matching network that is connected to the discharge RF electrode.
Since only 10-20% of P~Tot~ is typically dissipated in the discharge,
and this fraction is generally not known or controlled, it is difficult
to relate the conditions of different reactors. The film growth rate (G)
is also measured by device manufacturers and developers, and as G is a
monotonic function of power delivered to the discharge we use it to
characterize the power conditions. The RF voltage (V~RF~) applied to the
discharge is easy to measure, and is also monotonically related to the
discharge power, so we measure G versus R, P~Ch~ and V~RF~ to relate our
conditions to those of device makers. **In essence, similar conditions
exist in different reactors when the values of G, R and P~Ch~ are the
same.** There are actually several additional variables that these
parameters do not provide. One is the silane depletion in the reactor.
The actual R value of the gas mixture depends on this depletion, and can
become much higher than the inlet H~2~/SiH~4~ flow ratio. Furthermore,
disilane builds up in the chamber, and can be important. Sometimes it is
possible to estimate this depletion from G, if the gas flows are well
confined to the reactor volume. A second variable is the electrode gap,
so we utilize a 2.0 cm gap, which is in the middle of the range used for
in many reactors. Two additional variables are the temperatures of the
substrate (T~Sub~) and the RF electrode (T~RF~). While T~Sub~ is
normally measured and reported, T~Sub~ -T~RF~ can also be important and
is normally not known.

An overview of the experimental arrangement is provided in Fig. 1. The
film-thickness measurement is done with the laser setup shown in the
lower left corner of this diagram. In essence, reflection fringes
provide the film thickness, and the rate of change in a fringe yields
the film growth rate. The film-growth rate data, for a range of
conditions, is shown in Figs. 2 and 3. These measurements were carried
out with the entire discharge chamber at 300K. Primarily the vapor
densities influence the discharge behavior, so for an isothermal chamber
at the typical operating temperature of 500 K, the pressures should be
multiplied by 5/3.

![](media/image1.wmf){width="6.299305555555556in"
height="4.877083333333333in"}Fig.1. Overview of the apparatus used to
study radicals at the discharge substrate, and the film growth rate. The
RF-to-substrate electrode gap is 2.0 cm.

![](media/image2.wmf)

Fig.2. Film growth rate (G), versus V~RF~ and P~Tot~, for R = 16 and 36.

![](media/image3.wmf){width="6.429166666666666in"
height="5.384027777777778in"}

Fig.3. Film growth rate with pure silane inlet flow, versus pressure and
V~RF~. The silane depletion is 15% at 0.35 nm/s film growth rate.

Comparing Figs. 2 and 3, one can see that diluting the silane to R = 16
does not have a major influence on G, at least for P~Tot~ = 1.5 and 2.0
Torr. The silane pressure is \~0.1 Torr for these cases, which are "on
the edge" of producing microcrystalline films. However, diluting further
to R = 36 reduces G by a factor of 5-10. Here the hydrogen etching is
seriously competing with silicon deposition, and microcrystalline films
are being produced. This reduction in G can be partly compensated by
increasing V~RF~, but this is accompanied by an increase in ion
bombardment energies.