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Recrystallization of PVD CdTe 

Thin Films Induced by CdCl2

Treatment – A Comparison 

Between Vapor and Solution 

Processes 

Preprint 

H.R. Moutinho, R.G. Dhere, M.J. Romero, 

C.S. Jiang, B. To, and M.M. Al-Jassim 

National Renewable Energy Laboratory 

Presented at the 33rd IEEE Photovoltaic Specialists Conference 

San Diego, California 

May 11–16, 2008 

Conference Paper 

NREL/CP-520-42512 

May 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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RECRYSTALLIZATION OF PVD CdTe THIN FILMS INDUCED BY CdCl2 TREATMENT – 

A COMPARISON BETWEEN VAPOR AND SOLUTION PROCESSES 

 H.R. Moutinho, R.G. Dhere, M.J. Romero, C.-S. Jiang, B. To, and M.M. Al-Jassim 

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80127, USA 

ABSTRACT 

The untreated physical-vapor-deposited CdTe films 

had small grains, with <111> texture, and a large 

concentration of deep levels. For CdCl2 treatments at 

moderate temperature/time, there was partial 

recrystallization, and a moderate decrease in the 

concentration of deep levels. For the optimal treatment 

conditions, there was complete recrystallization and grain 

growth, and further decrease in stress. Electron 

backscatter diffraction showed that these films had a 

mixture of small and large grains, many larger than 30 μm. 

The larger grains had <111> orientation, whereas the 

smaller grains were randomly oriented, which provided 

insight on the grain-growth mechanism after 

recrystallization. There was a significant increase in the 

cathodoluminescence signal, indicating a decrease in the 

concentration of deep levels. When performed at the 

same conditions, the CdCl2 vapor treatment is more 

effective in the recrystallization and grain-growth 

processes than the solution CdCl2 treatment. All the films 

had a large concentration of twin boundaries. 

INTRODUCTION 

It is well known that heat treatment with CdCl2 is a 

major step in preparing high-quality CdTe/CdS solar cells 

[1,2]. This treatment may or may not induce 

recrystallization of the CdTe films, depending on the initial 

stress state of the material, and the type and conditions of 

the treatment [3]. However, even in films that do not 

recrystallize, this treatment is important, because it 

decreases the density of deep levels inside the bandgap 

and changes the defect structure, resulting in better 

devices [4]. We have deposited close-spaced sublimation 

(CSS) CdTe films for many years that do not show 

significant structural changes after CdCl2 treatment, 

because they have a relatively low initial stress state and 

large grains. Recently, we began depositing CdTe films 

by physical vapor deposition (PVD), at much lower 

temperatures, which readily recrystallize after CdCl2

treatment. In this work, we study the effects of the CdCl2

treatment performed by two different methods—vapor and 

solution—at different times and temperatures, to 

determine the best treatment parameters and the 

advantages of one method over the other. 

One of the main analytical techniques used in this 

study is electron backscatter diffraction (EBSD) [5,6]. 

Although this technique was developed a few decades 

ago, it has gained more attention only recently, with the 

development of fast personal computers and appropriate 

software. In EBSD, the electron beam of a scanning 

electron microscope (SEM) is used to generate diffracted 

electrons, which are collected by a detector placed close 

to the sample surface. Because of the small area of the 

electron beam, this technique gives information on the 

crystalline orientation of the sample with nanometer 

resolution, providing orientation maps of the surface, 

where grains with different orientations can be 

distinguished, as well as defects, such as twin boundaries. 

The samples were also analyzed by cathodoluminescence 

(CL), to study changes in the concentration of deep levels 

inside the bandgap with the different stages of 

recrystallization; X-ray diffraction (XRD), to accurately 

determine lattice parameter; and scanning electron 

microscopy (SEM) and atomic force microscopy (AFM), to 

determine surface topography and roughness. 

EXPERIMENTAL PROCEDURE 

The PVD CdTe films were grown at 325°C on 

glass/SnO2/CdS substrate, forming a solar-cell structure. 

The CdCl2 treatment was performed using two methods: 

vapor and solution. The vapor treatment was performed in 

vacuum, by maintaining a constant vapor pressure of 

CdCl2 over the surface of the film, in a mixture of oxygen 

and helium. The solution treatment consisted of applying 

a few drops of saturated CdCl2 solution on the sample 

surface, then heating it in air. The samples were treated 

at 350°, 400°, or 420°C, for 5 or 30 min. 

For EBSD analysis, some CdTe films received one or 

more of the following treatments: mechanical polishing 

with 0.05-μm alumina suspension, ion-beam milling with 

Ar, and etching with a bromine-methanol solution for few 

seconds. 

The samples were analyzed by SEM, using a FEI FIB 

NanoSEM 200 SEM; EBSD, using an HKL Technology 

EBSD system, with Nordlys detectors and electronics, 

mounted in the NanoSEM 200 SEM; CL, using a spectrum 

imaging system with a cryostage, installed in a JEOL 5800 

SEM; AFM, using a Veeco Metrology Dimension 3100 

SPM with Nanoscope V controller; and XRD, using a 

Scintag S1 diffractometer, with Bragg-Brentano 

configuration. 

RESULTS AND DISCUSSION 

Surface Morphology 

The untreated samples had grain sizes smaller than 1 

μm. Treatment with solution CdCl2 at 350°C for 5 and 30 

minutes did not affect the morphology significantly, except 

1
for softening the sharp features in the film. However, the 

same treatment conditions for vapor CdCl2 resulted in 

small grains appearing mostly at grain boundaries, with 

the sample treated for 30 minutes showing a higher 

concentration of these grains, which, in general, were also 

larger and more elongated. These small grains have been 

observed before [3,4] and clearly indicate ongoing 

recrystallization, as discussed in the next section. The 

small grains were also observed on the sample treated by 

solution CdCl2 at 400°C for 5 minutes. These results are 

the first indication that the vapor treatment, which keeps 

an infinite source of CdCl2 on the CdTe surface during the 

treatment, is more efficient in the recrystallization process 

than the solution process, which uses a finite amount of 

CdCl2. 

There was a striking difference on the surface 

morphology of the CdTe films after the vapor treatment at 

400° and 420°C. The recrystallization process was 

complete and there was grain growth. The microstructure 

showed a large variation of grain sizes, from 1 μm to over 

30 μm. These films had the largest grains sizes observed 

for CdTe produced in our laboratory, using any growth 

method. Yet, they were extremely smooth when 

compared to CSS CdTe films. As-deposited CSS CdTe 

films have grain sizes of a few micrometers and roughness 

of about 290 nm. The untreated PVD CdTe films have a 

roughness of about 13 nm, and the roughest films, 

produced after CdCl2 vapor treatment at 420°C for 30 

minutes, have a roughness of about 29 nm. This feature 

is very important for EBSD analysis, as discussed in the 

next section. The topographies of different CdTe films are 

shown in Fig. 1. As observed in Fig. 1(d), the grain 

morphology was not uniform. We observed three types of 

topographies in most recrystallized films: grains with a 

smooth surface [top of Fig. 1(d)], grains with terraces at an 

angle with the surface normal [bottom of Fig. 1(d)], and 

grains with terraces parallel to the film surface. Also, there 

were some grains with more than one topography. It was 

noticed that the small grains had a smooth surface, 

whereas the large grains had terraces. This characteristic 

was compared with EBSD data and will be discussed later. 

There were no major differences on the topography and 

grain size between films treated in vapor at 400° and 

420°C, and for 5 and 30 minutes. 

Fig. 1. SEM micrographs of CdTe films treated under different conditions: (a) solution 350°C/5 min; (b) vapor 350°C/5 min; (c) 

vapor 420°C/5 min; (d) vapor 400°C/5 min. 

For the solution-treated samples, we observed that 

recrystallization occurred for higher temperature/time 

values. For instance, the sample treated at 400°C for 5 

minutes was still in the early stages of the recrystallization 

process. The sample treated at 400°C for 30 minutes and 

the samples treated at 420°C were completely 

recrystallized. However, the grain size was much smaller 

than in equivalent vapor-treated samples—in general, not 

2
larger than 5 μm. Also, although grains with terraces were 

observed, they were present in a smaller scale. This was 

another indication that the solution treatment is much less 

effective in the recrystallization process than the vapor 

treatment. 

Crystalline Structure 

The XRD analyses corroborated the interpretation of 

the SEM and AFM results, as inferred from Table I. The 

untreated sample had <111> texture and a larger lattice 

parameter than a powder sample (6.481 Å), indicating a 

compressed homogeneous stress on the horizontal plane. 

This stress may be caused by differences in the thermal 

expansion coefficients between CdTe and the underneath 

layers. After moderate CdCl2 treatment, two lattice 

parameters are observed in the analysis (see Fig. 2 for an 

example): the largest corresponds to the original <111>-

oriented lattice, for which the homogeneous stress has 

been partially released (decrease in lattice parameter), 

and the smaller is for the new recrystallized lattice [small 

grains observed in Fig. 1(b)]. This new material is 

randomly oriented and has a lattice parameter similar to a 

powder sample. For higher treatment temperature/time 

values, there is only one value of lattice parameter—close 

to 6.481 Å—and the samples are randomly oriented, 

indicating that the recrystallization process is complete. 

For these samples, there is also a reduction in the 

inhomogeneous stress, characterized by a decrease in the 

full width at half maximum of the diffraction peaks. 

From Table I, we observe the slower degree of 

recrystallization for the solution-treated samples. For 

instance, the sample treated at 350°C for 5 minutes does 

not show any sign of ongoing recrystallization, whereas 

the one treated at 400°C for 5 minutes is not completely 

recrystallized yet. This is equivalent to what was observed 

in the SEM and AFM analyses. 

For the untreated samples, there were no Kikuchi 

patterns, and, consequently, no EBSD signal coming from 

grain-boundary regions. In general, this is caused by 

topographic features that prevent diffracted electrons from 

reaching the detector. This effect has been observed 

before, in CSS CdTe samples, and a solution for the 

problem is to flatten the sample surface. In the past, we 

successfully used the following methods for this purpose 

[7]: ion-beam milling; or mechanical polishing followed by 

light ion-beam milling or etching in bromine methanol 

solution. We applied these methods to several samples, 

but the EBSD results did not change significantly. This 

was not surprising, because these samples had a 

roughness about one order of magnitude less than CSS 

samples, and it was unlikely that surface features were the 

problem. Another possibility is that regions around grain 

boundaries have very poor crystallinity, and do not 

generate Kikuchi patterns. An example of typical EBSD 

data from these films is shown in Fig. 3(a). The images in 

Fig. 3 are formed by a combination of band contrast, which 

is a measure of the intensity of the Kikuchi patterns and is 

related to crystallinity quality, grain boundaries (black 

lines), coincidental site lattice (CSL) boundaries (red 

lines), and inverse pole-figure coloring. The CSL Σ3 

boundaries are twin planes related by a rotation of 60° 

around the <111> direction between neighboring material. 

The <111> texture of the untreated films is characterized 

by the blue coloration of the grains in this representation, 

and by the central region and the ring on the {111} pole 

figure inset. Note that there is a spread of several degrees 

on the <111> preferential orientation. Similar EBSD 

results were obtained for the partially recrystallized 

samples concerning the poor signal at grains boundaries 

and the <111> texture [Fig. 3(c)]. Unfortunately, EBSD did 

not have enough spatial resolution to detect the randomly 

oriented small recrystallized grains. 

Unlike CSS CdTe films, the PVD recrystallized films, 

which have good crystallinity and are still reasonably flat, 

produced excellent EBSD data without any surface 

preparation, as observed in Figs. 3(b) and 3(d). The 

problems with regions close to grain boundaries were not 

observed in these films. Notice that there is still a large 

Table I. Lattice parameter for CdTe films before and 

after treatment with vapor (V) or solution (S) CdCl2. 

Sample Lattice 

Parameter 1 

(Å) 

(111) Texture 

Lattice 

Parameter 2 

(Å) 

No Texture 

Untreated 6.489 - 

V350°C/5 min 6.484 6.482 

V400°C/5 min - 6.480 

V420°C/5 min - 6.480 

V350°C/30 min 6.483 6.481 

V400°C/30 min - 6.480 

V420°C/30 min - 6.481 

S350°C/5 min 6.491 - 

S400°C/5 min 6.486 6.483 

S420°C/5 min - 6.480 

S350°C/30 min 6.485 6.481 

S400°C/30 min - 6.480 

S420°C/30 min - 6.480 

Fig. 2. Lattice parameter determination for a CdTe film 

after vapor CdCl2 treatment. 

3
concentration of 60° <111> twin boundaries. These 

boundaries were found in every sample analyzed in this 

work, and their density did not change significantly from 

the untreated to the recrystallized films, which is explained 

by their low formation energy, as reported in the literature 

[8]. The loss of preferential orientation is observed in the 

coloration of the grains and in the homogeneous pole 

figures. A high-magnification study of vapor-treated 

samples showed that large grains have <111> orientation 

[notice the large blue grain in Fig. 3(b)], whereas small 

grains are randomly oriented. This can be correlated with 

the differences in topography observed in the SEM and 

AFM analyses between large and small grains, i.e., in 

general, small grains are smooth and randomly oriented, 

whereas large grains have terraces and are <111> 

oriented. These results indicate that the (111) surface is 

the lowest-energy surface in CdTe, and that, after 

recrystallization, the <111>-oriented grains grow at the 

expense of grains oriented in less-favorable directions. 

Surface energy may be particularly important in the grain 

growth process of these films, which have large grain 

sizes in comparison to their thickness (about 3 μm). 

These results also indicate that, in the conditions used in 

this work, the grain growth process was not complete. 

Fig. 3. EBSD data composed of band contrast + grain boundaries (black lines) + CSL Σ3 boundaries (red lines) + 

inverse pole-figure coloring. Insets: {111} pole figure. (a) untreated CdTe; (b) CdTe after vapor CdCl2 at 400°C for 5 

minutes; (c) CdTe after solution CdCl2 at 400°C for 5 minutes; (d) CdTe after solution CdCl2 at 400°C for 30 minutes. 

As observed in the SEM and AFM analyses, there is 

no significant difference in the EBSD data for samples 

treated in vapor at 400° and 420°C, and for 5 and 30 

minutes. However, for solution-treated samples, there is a 

strong difference in the degree of recrystallization between 

samples treated at 400°C for different times, as observed 

by comparing Figs. 3(c) and 3(d). Also, the effectiveness 

of the vapor treatment is clearly demonstrated by 

comparing Figs. 3(b) and 3(c), which show samples 

treated by the two methods at the same temperature and 

time. Finally, notice the large difference in average grain 

sizes for films treated with vapor and solution CdCl2 [Figs. 

3(b) and 3(d)]. 

Electro-Optical Properties 

The untreated CdTe films produced a very weak CL 

signal, close to the system response of the equipment, 

indicating a large concentration of deep levels inside the 

bandgap. This explains the low efficiencies observed in 

4
solar cells produced with these films before CdCl2

treatment. The same was observed for the solution 

treatment at 350°C for 5 minutes, confirming the results 

from other analyses. 

In Fig. 4, we show several CL curves for CdTe films 

treated under different conditions, in vapor and solution 

CdCl2. The large central peak in each spectrum 

corresponds to donor/acceptor transitions, whereas the 

small peak, on the right, is related to excitonic transitions. 

Compared to the untreated film, there is an increase in the 

signal for samples treated in vapor CdCl2 at 350°C at 5 

and 30 min, related to a higher stage of recrystallization, 

as mentioned previously. For higher treatment 

temperatures and times, there is a significant increase in 

the CL intensity because of the complete recrystallization 

of the CdTe film. These results indicate that the new 

recrystallized material has a lower concentration of deep 

levels inside the bandgap, which increases the radiative 

recombination, and is a direct measure of the 

improvement in material quality after the CdCl2 treatment. 

We did not notice a major difference in the CL signal for 

recrystallized films treated at 400° and 420°C, and 5 and 

30 minutes. In reality, the properties of the films are not 

completely uniform, and the CL signal can vary up to 

about two times, as the beam is scanned over the surface. 

Considering the average data, the sample treated with 

vapor at 400°C for 5 minutes provided the best CL signal. 

Notice in the graph that samples treated with the solution 

CdCl2 have smaller CL signals than equivalent samples 

treated with vapor CdCl2, indicating lower quality; this is in 

complete agreement with the previous analyses. In some 

samples, the energy of the exciton peak shifts with the 

treatment, as observed in Fig. 4. This indicates a change 

in the band structure of the material. However, additional 

analyses, considering the inhomogeneity of the films, will 

be necessary before we can explain these changes. 

CONCLUSIONS 

The vapor CdCl2 performed at moderate temperatures 

resulted in partial recrystallization; at higher temperatures, 

it resulted in recrystallized smooth films with large grains 

and a relatively low density of deep levels. These low 

values of roughness (<30 nm) are desirable for preparing 

back contacts and for cells using a substrate configuration. 

Although these films were randomly oriented, the large 

grains, some with grain sizes larger than 30 μm, had a 

<111> preferential orientation, indicating that the (111) 

surface has the lowest energy in this material. The 

solution CdCl2 treatment was not as efficient in the 

recrystallization process. Although completely 

recrystallized films could be obtained, the grain size was 

smaller and the density of deep levels was larger than in 

films treated in vapor CdCl2 at the same conditions. 

These results indicate that the vapor CdCl2 treatment is 

recommended over the solution treatment for the 

preparation of high-efficiency devices. Fig. 4. CL data for CdTe thin films after treatment with 

vapor and solution CdCl2. A large concentration of 60° <111> twin boundaries 

was observed in every CdTe film analyzed in this work, 

even after recrystallization and grain growth, confirming 

the low energy of these interfaces. 

 ACKNOWLEDGEMENTS 

This work is supported or funded under DOE Contract 

No. DE-AC36-99GO10337. 

REFERENCES 

[1] H.M. Al-Allak, A.W. Brinkman, H. Richter, and D. 

Bonnet, J. Cryst. Growth 159, 1996, p. 910. 

[2] W.N. Shafarman, R.W. Birkmire, D.A. Fardig, B.E. 

McCandless, A. Mondal, J.E. Phillips, and R.D. Varrin, Jr., 

Solar Cells 30, 1991, p. 61. 

[3] H.R. Moutinho, M.M. Al-Jassim, D.H. Levi, P.C. Dippo, 

and L.L. Kazmerski, J. Vac. Sci. Technol. A 16(3), 1998, p. 

1251. 

[4] H.R. Moutinho, R.G. Dhere, M.M. Al-Jassim, D.H. Levi, 

and L.L. Kazmerski, J. Vac. Sci. Technol. A 17(4), 1999, p. 

1793. 

[5] K.Z. Baba-Kishi and D.J. Dingley, Scanning 11, 1989, 

p. 305. 

[6] D.J. Dingley and D.P. Field, Mater. Sci. Technol. 13, 

1997, p. 69. 

[7] H.R. Moutinho, R.G. Dhere, C.-S. Jiang, B. To, and 

M.M. Al-Jassim, Mater. Res. Soc. Symp. Proc. 1012, 

1997, p. Y12-04. 

[8] Y. Yan and M.M. Al-Jassim, J. Appl. Phys. 90, 2001, p. 

3952. 

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Recrystallization of PVD CdTe Thin Films Induced by CdCl2

Treatment – A Comparison Between Vapor and Solution Processes: 

Preprint 

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14. ABSTRACT (Maximum 200 Words)

The untreated physical-vapor-deposited CdTe films had small grains, with <111> texture, and a large concentration 

of deep levels. For CdCl2 treatments at moderate temperature/time, there was partial recrystallization, and a 

moderate decrease in the concentration of deep levels. For the optimal treatment conditions, there was complete 

recrystallization and grain growth, and further decrease in stress. Electron backscatter diffraction showed that these 

films had a mixture of small and large grains, many larger than 30 μm. The larger grains had <111> orientation, 

whereas the smaller grains were randomly oriented, which provided insight on the grain-growth mechanism after 

recrystallization. There was a significant increase in the cathodoluminescence signal, indicating a decrease in the 

concentration of deep levels. When performed at the same conditions, the CdCl2 vapor treatment is more effective in 

the recrystallization and grain-growth processes than the solution CdCl2 treatment. All the films had a large 

concentration of twin boundaries. 

15. SUBJECT TERMS

CdTe; thin films; PV; recrystallization; physical vapor deposition; close-spaced sublimation; electron backscatter 

diffraction; grain-growth process; ion-beam milling; electro-optical properties; 

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