### RF SPUTTER ETCH AS A SURFACE CLEANING PROCESS FOR CdTe SOLAR CELLS

V. Viswanathan, D. L. Morel and C. S. Ferekides

Clean Energy Research Center, Department of Electrical Engineering

University of South Florida, Tampa, FL 33620

**ABSTRACT**

Thin film CdTe/CdS solar cells benefit from the use of a wet chemistry
step prior to the application of the back contact electrode. A
nitric-phosphoric acid mixture and a bromine solution are the most
commonly used etchants. Following the CdCl~2~ heat treatment the
CdTe/CdS structures are briefly etched in one of the above solutions in
order to remove native oxides and leave the CdTe surface Te-rich. A
higher carrier concentration at the surface of CdTe is believed to be a
key factor in facilitating the formation of an effective back contact.
The primary objective of this work is to investigate the potential of an
alternative dry, vacuum-based process, as a surface modification step
for CdTe prior to the application of the back contact electrode.
RF-sputter etch has been used as an alternative surface cleaning
process. The process was optimized for the type of etchant gas,
pressure, time, and sputter power. Solar cells exhibited V~OC~'s greater
than 830 mV and FF's in the high 60's. This performance demonstrates the
potential of this process for the development of an all-dry vacuum based
CdTe technology.

**INTRODUCTION**

The formation of an effective back contact to CdTe solar cells has been
in general accomplished using a wet chemistry step prior to the
application of the back contact electrode. The two most commonly
utilized solutions are based on a nitric-phosphoric acid mixture and
bromine diluted in methanol. In both cases it has been reported that
after treatment the CdTe surface becomes Te-rich (or p+)\[1,2\]. A
Te-rich surface is one of the necessary conditions for fabricating
effective ohmic contacts to CdTe. In addition to the Te-rich surface,
the chemical treatment is important in that it removes native surface
oxides that can be present on the CdTe surface as a result of the
CdCl~2~ heat treatment which is carried out in the presence of O~2~
(air). In addition to these wet chemistry methods, ion milling has also
been utilized as a surface modification/cleaning step, in conjunction
with sputter deposited ZnTe:Cu \[3\]. An alternative dry, vacuum based
surface cleaning process is the main focus of this work. Instead of the
typical Br~2~/methanol etch, the CdTe surface was exposed to an rf
plasma (N~2~ or Ar), following the CdCl~2~ heat treatment and
immediately prior to the application of the back electrode.

**EXPERIMENTAL**

Solar cell fabrication up to the formation of the back contact was based
on previously developed processes \[4\]. Borosilicate glass substrates
were coated with SnO~2~ bi-layers by chemical vapor deposition (CVD).
The CdS films were prepared using chemical bath deposition (CBD), and
the CdTe films using close spaced sublimation (CSS). The CdCl~2~ heat
treatment was carried at 400°C in the presence of O~2~ (20%) For this
work the excess CdCl~2~ following the CdCl~2~ heat treatment was rinsed
ultrasonically using methanol. Ultimately, for a completely dry
fabrication process, a vapor based CdCl~2~ process would be desirable,
since such a technique leaves no CdCl~2~ residue on the CdTe surface
\[5\]. After rinsing the excess CdCl~2~, the devices were exposed to an
rf plasma using a MARCH Plasmod sputter etcher. Gases utilized included
N~2~, Ar, and N~2~/O~2~; the main reason for including O~2~ in the
sputter gas mixture, was to simply determine the tolerance of the
process to this gas. The devices were sputter-etched using plasma power
ranging from 30-100W, and gas pressures ranging from 150-400 mTorr.
Following the sputter etch process a back electrode of graphite doped
with HgTe:Cu was applied to the surface of CdTe to complete the
fabrication of the back contact. The geometry of the plasma etcher was
such that allowed for uniform etching of samples with approximate area
of 2 x 2 cm^2^. Solar cells were characterized using standard J-V
measurements.

**RESULTS AND DISCUSSION**

The rf sputter etch process was optimized empirically for optimum solar
cell performance. The parameters studied included: (a) sputter power,
(b) etch time, (c) ambient, and (c) pressure. The following sections
summarize the results obtained during this study.

**Plasma Power -- Etch Time**

The V~OC~ and FF for a series of cells for which the CdTe surface was
sputter etched using different power levels and for different times are
shown in Fig. 1; the sputter gas was N~2~, and the pressure was 250
mTorr. From these data it is evident that a plasma power of 50W provides
the highest V~OC~ which progressively decreases as the power is
increased. The V~OC~ is also influenced by the etch time, in that at
lower power it improves with time reaching a maximum for a 10-15 minute
etch time. At higher powers (above 50W) it deteriorates with the time of
![](media/image1.wmf){width="3.248031496062992in"
height="2.936220472440945in"}~OC~, suggesting that the observed behavior
is a reflection of the back contact properties. This behavior is
illustrated by comparing the effect of etching time at 50W
![](media/image2.wmf){width="3.248031496062992in"
height="2.936220472440945in"}on the J-V characteristics of the solar
cells (see Fig. 2). The improvement in FF initially with the etch time
is due to improved contact characteristics with time. The reduction in
the FF is due to the onset of rectification observed in the J-V curves
of Fig. 2 as one moves away from the optimum time of 10-5 min. The onset
of rectification at the back contact is better illustrated in Fig. 3,
where the dynamic resistance (i.e. dV/dJ \[Ω-cm^2^\]) is shown in the
range of 0.5 -- 1.25 Volts. The resistance of the devices shown,
initially decreases with time (from 5 to 10 minutes), but increases
monotonically for the two longest times of 15 and 20 minutes. The
inflection point in the dynamic resistance of the device etched for 20
minutes (see arrows on Fig. 3) clearly indicates the presence of a
non-ohmic (presence of a barrier) back contact. To a lesser extent this
behavior is present for the device etched for 15 minutes. This behavior
is similar to what was obtained for the devices sputter-etched at 75 W;
in that case however (as indicated in Fig. 1) the FF's were overall
lower than the devices sputter-etched at 50W.

**Effect of Pressure - Ambient**

The ![](media/image3.wmf){width="3.248031496062992in"
height="2.936220472440945in"}effect of the total gas pressure during the
sputter etch process was also studied; the sputter power for the
experimental results discussed in this section was 50 W, and the sputter
gas was N~2~. Solar cell results are tabulated in table 1. These results
suggest that the effect of the etch pressure on V~OC~ is rather
insignificant, as in all cases the V~OC~ was in the 810-830 mV range.
This is not true for the FF which is in the mid 50's for the highest
pressure and only 61% for the lowest. The cause for the observed
decrease can be deduced using the comparison of light J-V curves of
devices etched at 250, 300 and 400mT shown in Fig. 4. It can be seen
that the slight reduction in the V~OC~ and the greater reduction in FF
of the device etched at 400mT is due to a softer knee in the J-V curve;
this is believed to be caused by a voltage dependent collection;
shunting does not affect the FF and V~OC~ in this case, as the shunt
resistance based on the cells' reverse bias characteristics (around
1.5-2.0 V) is essentially the same for all cells shown.

Table 1. The Effect of Etch Pressure on Solar Cell Performance

  -------------------------------------- ---------------- ----------------
  Etch Pressure \[mTorr\]                V~OC~ \[mV\]     FF \[%\]

  150                                    831              60.6

  200                                    826              61.6

  250                                    823              67.1

  300                                    819              66.5

  400                                    814              56.8
  -------------------------------------- ---------------- ----------------

The above behavior where there exists an optimum intermediate pressure
for this process can be explained based on the energy of the ions. At
higher pressures the collision probability increases leading to lower
ion energy and therefore the CdTe surface is not effectively cleaned. At
the other extreme, at low pressures the ions can impinge the surface
with much higher energy leading to surface damage, and compensation
which can possibly damage the CdTe leading to poor collection of deeply
generated carriers.

![](media/image4.wmf){width="3.248031496062992in"
height="3.043700787401575in"}

![](media/image5.wmf){width="3.248031496062992in"
height="3.0366141732283465in"}A similar set of experiments as the ones
described above was carried out using Ar as the sputter gas; the
pressure was maintained at 250 mTorr. The light J-V for several cells
sputter-etched at different power levels are shown in Fig. 5. The
general trends observed with N~2~ exist in this case also. As the power
decreases device performance improves mainly due to improvement in the
characteristics of the back contact; at lower power levels no back
barrier forms leading to improved FF's. In this case however, ohmic
contacts form at lower power levels than the ones required for devices
sputter-etched in N~2~. This behavior as well as the results previously
described (in Fig 1 and table 1) can be explained using the same model:
there exists an optimum set of conditions where the sputtering process
results in a clean CdTe surface free of oxides. Conditions of low power
levels, short etch times, and high pressures do not result in effective
oxide removal/surface cleaning, and therefore the conditions for the
formation of an ohmic back contact are not optimum. At the other
extreme, high power levels, long etch times, and low pressures are
conditions that yield high energy ions, that can cause significant
damage to the surface of the CdTe. A damaged surface region can be
heavily compensated leading to low effective acceptor levels and
therefore non-ohmic contacts, and/or can affect collection of deeply
generated carriers. Therefore a range of intermediate optimum conditions
must be developed that will result in effective surface cleaning without
damage.

**Effect of O~2~**

![](media/image6.wmf){width="3.248031496062992in"
height="3.043700787401575in"}A series of devices were also sputter
etched in a N~2~/O~2~ mixture, to determine at what levels O~2~ can be
tolerated; the total pressure was maintained at 250 mTorr, and the power
was 50 W. The J-V characteristics for these devices are shown in Fig. 6.
It is clear that the addition of even 4% O~2~ (a smaller amount of O~2~
could not easily measured due to equipment limitations), leads to losses
in the FF, even if the back contact shows no sings of a back barrier. At
higher levels of O~2~ (20%; N~2~:O~2~ 200:50 mTorr) it is clear that a
back barrier is present, presumably due to the formation of oxides on
the surface of CdTe.

**CONCLUSION/SUMMARY**

An alternative dry surface cleaning process for thin film CdTe/CdS solar
cells has been studied. The results obtained from this work, V~OC~ and
FF in the range of 810-830 mV and \>65% respectively, clearly suggest
that an all-dry process can be developed, with manufacturing advantages
over existing wet chemistry procedures, and without significant
performance limitations. The solar cell characteristics obtained suggest
that a set of optimum conditions for power, pressure, and time, must be
developed that will result in effective oxide removal and in general
surface cleaning of the CdTe surface. Conditions that do not effectively
remove all surface oxides, or cause significant damage to the surface of
the CdTe lead to the formation of a rectifying back contact, and can
affect collection of deeply generated carriers in CdTe.

**Acknowledgements**

This work was supported by the National Renewable Energy Laboratory
(NREL) under subcontract NDJ-2-30630-18.

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