Growth of CuInTe2 polycrystalline thin films

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ELSEVIER

CRYSTAL GROWTH

Journal of Crystal Growth 146 (1995) 251-255

Growth of CulnTe 2 polycrystalline thin films V. Nadenau

*, T. W a l t e r , H . W . S c h o c k

Institut fiir Physikalische Elektronik, Universit& Stuttgart, Pfaffenwaldring 47, D-70569 Stuttgart, Germany

Abstract

The coevaporation of elements is one of the most successful techniques to deposit polycrystalline chalcopyrite thin films for photovoltaic applications. The morphology of CuInTe 2 thin films and different growth mechanisms depending on the chalcogen rates during deposition are presented. The tellurium rate turned out to be the most important parameter influencing the morphology of CuInTe 2 thin films. A low tellurium rate has several effects on the film morphology in In-rich areas: For slightly In-rich films the grain size increases and the crystallites are well (112) oriented, for strongly In-rich films the morphology changes to oriented cuboids with tetragonal symmetry. Energy dispersive spectroscopy (EDS) measurements proved, that the film composition follows the pseudobinary tie line i.e. Cu2Te-In2Te 3 for a wide range of copper to indium ratios. X-ray diffraction (XRD) measurements showed the presence of Cu2Te as secondary phase in Cu-rich films and the defect chatcopyrite CuInsTe 8 in strongly In-rich films.

I. I n t r o d u c t i o n

The family of I - I I I - V I 2 chalcopyrite semiconductors has proved to be one of the most successful candidates to be used as an absorber layer in thin film heterojunction solar cells. Cells with efficiencies exceeding 16% have been produced using the penternary compound Cu(In,Ga)(S,Se) 2 as an absorber layer in laboratory scale [1]. All the m e m b e r s of this semiconductor family are direct semiconductors and have a very high absorption coefficient, so that the thickness of the film needed for total absorption of the sunlight is only 1 ~ m . The bandgap of single crystal

* Corresponding author.

C u I n T e 2 was measured in the range 0.96-1.06 eV [2,31. Even though it is possible to manufacture good solar cells some of the physical processes concerning the crystal growth during preparation and electrical processes during operation of the solar cells under illumination are not understood. One of the techniques to deposit the polyerystalline thin film absorber layer is coevaporation of the elements. High cell efficiencies have been achieved using this relatively simple technology, which is important for future upscaling and commercial production. Normally selenides or sulfides are used for absorber fabrication. It is difficult to control the partial pressure of selenium and sulphur during evaporation. Therefore deposition of sulphur and selenium is usually carried out with an excess of

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V. Nadenau et al. /Journal of Crystal Growth 146 (1995) 251-255

factor 2 - 3 with respect to the metal rates. The partial pressure of tellurium is relatively low compared to the other chalcogenides, so it is suitable to study the influence of the chalcogen rate on film formation during preparation. There is little information about single crystalline CulnTe2 (carrier concentrations in the range 1 0 1 6 - 5 )< 1019, mobilities in the range 50-150 c m 2 / V • s) [4] and even less about thin films of this material [5]. This work contains some elementary information about CulnTe 2 thin films, but concerns itself mainly with the influence of tellurium partial pressure during film formation.

2. Experimental procedure The experimental setup for the deposition of the C u l n T e 2 films on molybdenum-coated glass consists of a high vacuum chamber with four graphite crucibles and a heated substrate holder. The crucibles with heaters are mounted in a water-cooled copper box to ensure good thermal decoupling of the sources. The source temperatures are controlled by a high precision process controller with a temperature resolution of 0.1°C. For all experiments described in the following only elementary constituents were evaporated. The substrate temperature ranges between 500 and 600°C. Due to the spatial separation of the copper and indium sources, a lateral gradient is obtained which allows the investigation of growth due to compositional changes in one deposition run. Typical deposition times for a final film thickness of 2 /xm are 60-70 minutes. The total pressure in the chamber during deposition is in the range of 10-6-10 -7 mbar.

Fig. 1. SEM micrographs of C u l n T e 2 thin films ( × = 20000). (a) Morphology of a Cu-rich CuInTe 2 thin film. (b) Morphology of an In-rich C u I n T e 2 thin film.

faceted. Models for the different crystal growth mechanisms, symmetrical polyhedrons on the Cu-rich side and crystallites with threefold symmetry on the In-rich side are presented elsewhere [6,7]. The synthesized compound CulnTe 2 was measured by X R D to be single phase for stoichiometric composition. The typical reflections of the chalcopyrite structure can be seen clearly in the XRD-pattern of a CulnTe 2 film (Fig. 2a). Films with a wide range of C u / I n compositional ratios were deposited. The final film composition, measured by energy dispersive spectroscopy (EDS) fitted very well to the calculated pseudobinary tie line Cu2Te-In2Te 3. The optical bandgap shifts from 0.95 eV for a C u / I n ratio of ,,2 , 1 . ,

,

In/Cu,-3

3. Results Fig. 1 shows the morphology of a C u l n T e 2 thin film. The left part (a) is slightly copper rich, the right part (b) slightly indium rich. The deviation from stoichiometry is very small in both cases. A similar morphology is well known from the CulnSe 2 thin films, but at same preparation conditions CulnTe 2 crystallites are larger and more

In/Cu-1

211

101~ 20.0

~03004 30.0

40.0 2O (d~)

50.0

BO.0

Fig. 2. XRD pattern (FeKa radiation). (a) CulnTe 2 thin film. (b) l n / C u = 3. (c) I n / C u = 5. Peaks marked by an arrow a r e the additional reflections of the defect chalcopyrite phase.

V. Nadenau et al. / Journal of Crystal Growth 146 (1995) 251-255

1 (Cu = 24.53 at%, In = 24.61 at%, Te = 50.86 at%) to 1.05 eV for a C u / I n ratio of 3.3 (Cu = 9.89 at%, In = 33.23 at%, Te = 56.88 at%). For In-rich material the phase diagram [8] predicts the defect chalcopyrite CulnsTe 8 as secondary phase for strongly In-rich films. The additional peaks in the X R D spectra are caused by the ordered copper vacancies which lower the degree of symmetry (Figs. 2b and 2c). Even for a I n / C u ratio ~ 5 (Cu = 7.7 at%, In = 34.6 at%, Te = 57.7 at%) there are no other additional peaks. X R D spectra of Cu-rich material exhibit additional peaks of the Cu2Te binary compound (Fig. 3). From the material systems C u - I n - S e and C u - I n - S it is known that the Cu-secondary phases Cu2Se or CuS, respectively, segregate on the film surface [9]. Even though Cu2Te is observed in XRD, beam energy dependent wavelength dispersive spectroscopy measurements (WDS depth profile) do not indicate the existence of a copper-rich surface layer on the film. Cu2Te segregation could not be localized; the Cu-contents of the films is constant versus the film thickness. In the following a "high tellurium rate" means a T e / ( C u + In) ratio of about three, a "low tellurium rate" a ratio of about 1.5 calculated from the evaporated masses. In both cases the evapo-

Cu2Te

i

;'11

301

213

Cu-dc'n C u l n T k 30.0

33.5

37,0

40.5

44.0

47.5 51.0 2 0 (aeg)

54.5

58.0

61.5

65.0

Fig. 3. X R D pattern (FeKc~ radiation). (a) Cu2Te thin film. (b) Cu-rich CulnTe 2 thin film. Peaks marked with an arrow or magnified are reflections of the secondary phase Cu2Te. Miller indices denote chalcopyrite peaks.

253

Fig. 4. SEM micrograph of CulnTe 2 thin film ( × = 5000). (a) Prepared with high tellurium rate. ( I n / C u = 1.25). (b) Prepared with low tellurium rate. ( I n / C u = 1.11).

rated tellurium amount is larger than needed for stoichiometry. In the final compositions no difference in the tellurium content could be detected by EDS. The tellurium partial pressure turned out to be an important parameter influencing the film morphology and crystal quality. The films shown in Fig. 1 were deposited with a high tellurium partial pressure. The morphology is as described above and known from CulnSe 2. There is no difference in the morphology of the films with I n / C u ratios ranging between unity and five. Films deposited with low tellurium rates display several effects. The crystal size increased by a factor of ten or more. Fig. 4 shows both morphologies at the same magnification. The crystallites grown with low tellurium rates are well (112) oriented, which coincides with the threefold symmetry shown in Fig. 4b, i.e. the (112) planes are parallel to the substrate. Another important effect found with X R D is the improvement of crystal quality. For samples grown under high tellurium partial pressure, there is no peak splitting between the 116 and the 321 peak. However, XRD-spectra of films grown under low tellurium rates exhibit a clear splitting of this peak, which reflects the small F W H M of the peaks. A peak profile fit of this double-peak was performed to evaluate the exact position of the 116 and the 312 peak. From this data a c/a ratio of 2.004 was calculated for the unit cell.

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V. Nadenau et al. /Journal o f Crystal Growth 146 (1995) 251-255

Fig. 5. SEM micrographs of big fiat crystalsin a CulnTe2 thin film prepared with low tellurium rate, (a) x = 500, (b) x = 30000. A further effect of low tellurium rates during deposition, is the formation of very large and fiat crystallites, as shown in Fig. 5. The diameters of these fiat crystals are typically 50/xm. These are spread over the film, embedded in the fine grained matrix. Solidified droplets stick to the surface. All droplets are located at the end of elevated traces. This suggest that these fiat crystals grow from a liquid phase. During the deposition process the droplets move around on the surface depositing the crystal material. The typical diameter of these droplets is 1 /xm. The assumption that the drops are a liquid indium/tellurium could not be confirmed by WDS measurements. The WDS measurements were performed at different accelerating voltages (5, 15, 25 kV), which results in penetration depths of 150, 700 and 1500 nm. Both the depth profile of the background crystal and of the drop are very similar. The surface is strongly In-rich, but the integral composition is near stoichiometric C u l n T e 2. A possible explanation for the formation of the plates could be a growth from a In + Te liquid phase with subsequently indiffusion of copper. The melting point of In2Te (460°C) is lower than the substrate temperature

age sticking time of the tellurium atoms on the substrate decreases with increasing temperature and therefore the effective tellurium rate decreases. Due to the very low partial pressure of In and Cu at substrate temperatures, a loss of metals from the substrate is impossible. The effects of different tellurium rates on film morphology are negligible on the Cu-rich part of the film. The explanation for this phenomenon is the film formation mechanism mentioned above. On the Cu-rich part of the film, the incorporation of tellurium occurs due the supply of tellurium from the binary compound. This fast reaction ensures the tellurium supply out of the liquid secondary phase [3]. Similar investigations concerning the crystal growth under different tellurium rates were carried out on samples with an I n / C u ratio of approximately three. Films with high I n / C u ratios prepared with low tellurium rates exhibit a totally different morphology. The symmetry of the crystallites changes from a threefold symmetry to a tetragonal symmetry. Fig. 6b displays the morphology of these films. Cuboid shaped crystallites with a typical size of 2-3 tzm are embedded in a fine crystalline matrix. Most of theses cuboids are oriented. On top of the cuboids the growth of some crystallites with a threefold symmetry started. To clarify the origin of the crystallites with the tetragonal symmetry InTe was prepared as a polycrystalline thin film on a glass substrate. Fig. 6a shows the mor-

[101. The threshold of the tellurium rates which determines the growth mechanism is strongly dependent on the substrate temperature. The samples shown above were prepared at a substrate temperature of 500°C. At high substrate temperatures very large amounts of tellurium are needed to get the fine grained growth because the aver-

Fig. 6. SEM micrographs showing film morphologyof (a) InTe (x = 20000), and (b) Culn3Te5 prepared with low tellurium rate (x = 10000).

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Table 1 Summary of CuInTe2 growth features Cu-rich High tellurium rates

- Similar growth as CulnSe2

Low tellurium rates

- Cu2Te does not segregate at the surface - See above - Not affected by low tellurium rate

Slightly In-rich

Strongly In-rich

- Similar growth as CulnSe2 from the vapour phase - Not oriented

- Same morphology as slightly In-rich films

- Growth from liquid phase, most likely from In 2Te - Strongly (112) orien ted

- Crystallites are oriented cuboids - Tetragonal InTe acts as nucleation seed

phology of this film. I n T e crystallizes in a tetragonal s y m m e t r y as well a n d the macroscopic shape is similar to the strongly I n - r i c h c o m p o u n d . T h e m a t e r i a l system Cu, In, T e is for strongly In-rich c o m p o s i t i o n s very sensitive to t e l l u r i u m deficiencies a n d forms I n T e as secondary p h a s e [11]. As shown in Ref. [12] i n d i u m s e l e n i d e can act as seed for n u c l e a t i o n of C u l n S e 2, in a similar way the growth of the t e t r a g o n a l C u l n 3 T e 5 phase can h a p p e n with I n T e as n u c l e a t i o n seed. X R D spectra of this m a t e r i a l exhibit a strong (112) orientation.

4. Conclusions

T h e f o r m a t i o n of C u l n T e 2 thin films by coevapor a t i o n of e l e m e n t s is possible w i t h o u t secondary phases. T h e growth f e a t u r e s of C u l n T e 2 u n d e r different d e p o s i t i o n c o n d i t i o n s are s u m m a r i z e d in T a b l e 1.

Acknowledgements

This work has b e e n s u p p o r t e d by the G e r m a n Ministry for Science a n d T e c h n o l o g y ( B M F T ) , c o n t r a c t No. 0 3 2 8 0 5 9 D / E a n d the C o m m i s s i o n

of the E u r o p e a n C o m m u n i t i e s , c o n t r a c t JOUR/CT92/0141.

No.

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