CHARACTERISTICS OF CdTe FILMS OF DIFFERENT COMPOSITIONS

**FABRICATED BY CMBD**

> T.M. Razykov^1^, R. Acher^2^, O.D. Crisalle^2^, V.Craciun^3^, T.J.
> Anderson^2^, K. Kouchkarov^1^, S.S. Li^4^, D.Y. Goswami^5^, S.
> Vijayaraghavan^5^
>
> ^1^ Physical-Technical Institute, Scientific Association
> "Physics-Sun",
>
> Uzbek Academy of Sciences, G. Mavlyanov Street 2 B, Tashkent 700084,
> Uzbekistan
>
> ^2^ Department of Chemical Engineering, University of Florida
>
> ^3^ Department of Materials Science and Engineering, University of
> Florida
>
> ^4^ Department of Electrical and Computer Engineering, University of
> Florida
>
> ^5^ Department of Aerospace and Mechanical Engineering, University of
> Florida
>
> Gainesville, FL 32611, USA

**ABSTRACT**

CdTe films with near stoichiometric compositions and cadmium-rich
compositions were fabricated by a novel low-cost chemical molecular beam
deposition (CMBD) technique at atmospheric pressure under hydrogen or
nitrogen gas flow. X-ray diffraction analysis and scanning electron
microscopy studies show different grain growth by CMBD depending on the
composition of samples. Those grown with cadmium rich compositions were
of poor quality with large discontinuous island structures. On the other
hand, samples grown with near stoichiometric compositions were
polycrystalline with large grain size and uniform thickness.
Additionally, films grown under nitrogen flow contained a cadmium
tellurium oxide phase while those grown under hydrogen did not.

**INTRODUCTION**

Thin film solar cells using CdTe absorber layers are one of the primary
contenders for large scale commercialization of photovoltaics. Thin film
solar cells based on CdTe have been the subject of a great deal of
research and development in recent years. Substantial progress has been
made thus far in the area of materials research, device fabrication, and
technology development. Numerous applications based on CdTe have been
deployed worldwide. A world record efficiency of 16.5% had been achieved
for thin film CdTe solar cells using modified device structure \[1\].

Many methods have been used for the fabrication of CdTe layers, for
example close-spaced sublimation, electro deposition, magnetron
sputtering, chemical vapor deposition, and metal-organic chemical vapor
deposition. The 16.5% efficiency was obtained for CdTe films deposited
by close-spaced sublimation. The efficiency of thin film CdS-CdTe solar
cells can be increased to 20-25%. To reach such high efficiencies, it is
necessary to have a detailed understanding of the basic properties of
the materials and processes involved in fabricating the photovoltaic
devices. One of the critical stages in device fabrication is the
deposition of CdTe films with controllable composition, since a thin
Te-rich layer on the film surface is necessary for the formation of high
quality ohmic contacts.

We have previously reported a novel and low-cost chemical molecular beam
deposition (CMBD) method for fabrication of II-VI binary and multinary
films at atmospheric pressure under gas flow \[2\]. X-ray analysis of
the CdTe films have shown predominantly (111) orientation of the grains.
A linear dependence between the molecular beam intensity ratio in the
vapor phase and the film composition has also been shown \[3\]. This
paper focuses on the properties of CdTe films grown by CMBD with Cd-rich
and near stoichiometric compositions.

**EXPERIMENTAL**

The CMBD method was used to grow CdTe films of various compositions.
Effusion sources with elemental Cd and Te with purity of 99.9% and
99.999% respectively were used. The films were deposited on glass and
ceramic substrates held at a temperature of 600°C. Samples were grown at
atmospheric pressure under hydrogen or nitrogen flows. The thickness of
the resulting films ranged from 2 to 4μm. The fabrication of CdTe films
grown by CMBD has previously been described \[2\]. CdTe films with
different compositions were fabricated at various molecular beam
intensity (MBI) ratios. This ratio is defined as the intensity of the
cadmium flux divided by the intensity of the tellurium flux during
growth. There was a good correlation between the composition of the CdTe
films grown and the MBI ratio the films \[3\]. This allows for control
of the composition of the CdTe films on a molecular level during the
growth process, and consequently, their phy­sical properties, such as
intrinsic point defects \[4\].

This paper includes the results of morphology studies carried out with a
JEOL 6400 scanning electron microscope (SEM). Energy dispersive
spectroscopy was preformed using the JEOL 6400 to determine the film
composition. Finally, X-ray diffraction (XRD) analysis was performed
using a Philips APD 3720.

**RESULTS AND DISCUSSION**

Three different CdTe samples grown by CMBD are discussed in this paper.
The first sample has a Cd/Te composition ratio of 1.22 and was grown
under nitrogen flow on a glass substrate. The second sample has a Cd/Te
composition ratio of 1.029 and was grown under hydrogen flow on a
ceramic substrate. The third sample has a Cd/Te composition ratio of
1.015 and was grown under nitrogen flow on a glass substrate.

Figure 1 shows XRD spectra for CdTe films with three different
compositions. Figure 1(a) is the XRD spectrum for a cadmium rich film
grown under nitrogen flow with a Cd/Te composition of ratio 1.22. The
peaks for CdTe are labeled with circle markers above them. The
corresponding crystal plane orientations are also listed above the
peaks. Peak for cadmium tellurium oxide (CdTeO~3~) are seen in the XRD
spectrum, and are labeled with square markers.

Figure 1(b) is the XRD spectrum for a CdTe film grown under hydrogen
with flow with a Cd/Te composition ratio of 1.029. The corresponding
crystal plane orientations are also listed above the peaks. The
additional peaks seen in this spectrum result from the ceramic substrate
on which the film was grown. This was confirmed by XRD analysis of the
substrate only. This spectrum matched that of the additional peaks
observed for CdTe grown on the ceramic. Peaks corresponding to an oxide
phase were not observed.

Figure 1(c) is the XRD spectrum for a CdTe film grown under nitrogen
with flow with a Cd/Te composition ratio of 1.015. The peaks for CdTe
are denoted by circles, along with their crystal orientation. The peaks
labeled with square markers correspond to a beta-CdTeO~3~ phase. Note
that, for both films grown under nitrogen flow, a cadmium tellurium
oxide phase was observed. No such oxide was seen for films grown under
hydrogen flow.

For all samples, the XRD spectra had a large peak at 2theta = 23.7^o^,
which corresponds to the (111) crystallographic plane. This shows that
the CdTe films were grown with a sphalerite structure and a
predominately (111) orientation. Peaks corresponding to the (220),
(311), (400), (331), and (422) planes were also observed (see Fig. 1(b)
and 1(c). For the cadmium-rich film, the intensity of the (220) and
(311) peaks are similar in size to the (111) peak (see Fig. 1a). The
increased relative intensity of these peaks to the (111) peak is due to
the lower crystal quality of the cadmium rich film. The (111) plane is
the predominant peak for the near-stoichiometric samples fabricated
under hydrogen (see Fig. 1b) and nitrogen (see Fig. 1c) flows. These
samples were predominantly (111) orientated CdTe with the lattice
cons­tant of a = 6.487Å.

Figure 2 shows SEM images of the three samples. Figure 2(a) is an SEM
image for the cadmium-rich sample. This sample has poor crystalline
structure, consisting of large, rounded island structures. The film has
a very rough texture and is discontinuous. Portions of the substrate
between the islands are exposed. The poor morphology properties of these
films are not suitable for photovoltaic devices.

Figure 2(b) is an SEM image of the CdTe film with a Cd/Te composition
ratio of 1.029. In addition to the well oriented polycrystalline
structure, the film also has desirable grain size of 3-5μm. Figure 2(c)
is an SEM image of the CdTe film with a Cd/Te composition of 1.015. This
film also had a well oriented polycrystalline structure with a grain
size of 3-5μm. Cross sectional SEM images were also taken for the films
(not shown). These showed a uniform film thickness of approximately 2μm.
The morphology of samples (b) and (c) is desirable for photovoltaic
devices.

  -----------------------------------------------------------------------

  ![](media/image1.png)

  Fig. 1(a). XRD of a CdTe film with Cd/Te = 1.22

  ![](media/image2.png)

  Fig. 1(b). XRD of a CdTe film with Cd/Te = 1.029.

  ![](media/image3.png)

  Fig. 1(c). XRD of a CdTe film with Cd/Te = 1.015.

  ![](media/image4.png){width="3.061111111111111in"
  height="2.303472222222222in"}

  Fig. 2(a). SEM image of a CdTe film with Cd/Te = 1.22.

  ![](media/image5.png){width="3.0395833333333333in"
  height="2.2958333333333334in"}

  Fig. 2(b). SEM image of a CdTe film with Cd/Te = 1.029.

  ![](media/image6.png)

  Fig. 2(c). SEM image of a CdTe film with Cd/Te = 1.015.
  -----------------------------------------------------------------------

**CONCLUSIONS**

CdTe films with cadmium-rich and nearly stoichiometric compositions were
prepared by a novel low-cost CMBD technique. They were grown under
hydrogen or nitrogen gas flow at atmospheric pressure. SEM and XRD
analysis of the films were performed. The cadmium-rich film was of poor
quality. It consisted of large island structures and was discontinuous.
The near stoichiometric CdTe films had a polycrystalline sphalerite
structure with a predominately (111) orientation. They had a grain size
of 3-5m and a uniform thickness of 2m. Films grown under nitrogen flow
showed a cadmium tellurium oxide phase in addition to the cadmium
telluride grown while films grown under hydrogen flow did not.
Therefore, it is possible to grow CdTe films with a desirable morphology
for photovoltaic devices using the novel CMBD technique.

**REFERENCES**

\[1\] X.Wu, J.C.Keane, C.DeHart, D.S.Albin, A.Duda, T.A.Gessert,
S.Asher, D.H.Levi and P.Scheldon, "16.5 -- Efficient CdS / CdTe
Polycrystalline Thin Film Solar -- Cell", *Proc. of the 17th European
Photovoltaic Solasr Energy Conference*, 22-26 October, 2001, Munich,
Germany, p.995-999.

\[2\] T.M. Razykov, "Chemical Molecular Beam Deposition of II-VI Binary
and Ternary Compound Films in Gas Flow", *Applied Surface Science*,
**48/49** (1991) 89-92.

\[3\] T.M. Razykov, K. Sato, T. Shimizu, N.F. Khusainova, K.M.Kuchkarov,
A.F. Troushin and A.A.Usmanov, "X- Ray Diffraction Analysis of CdTe
Films with Different Composition Fabricated by CMBD in Hydrogen Flow",
*Technical Digest of the Intern. PVSEC-12*, June 11-15, 2001, Korea, pp.
539-540.

\[4\] T.M.Razykov, K.M.Kouchkarov, M.A.Zufarov, A.N.Georgobiani, K.Sato
, T.Shimizu , T.Ishibashi and T.Nishi, "Photoluminescence of CdTe Films
With Different Compositions Fabricated by CMBD", *Technical Digest of
the Intern. PVSEC-14, 2004,* Thailand, pp. 713-714.