Growth of High-Quality CuGaSe2 Thin Films using Ionized Ga Precursors

GROWTH OF HIGH-QUALITY CuGaSe 2 THIN FILMS USING

IONIZED Ga PRECURSORS

Akira Yamada 1, Hisashi Miyazaki 2, Takahiro Miyake 3, Yoshiyuki Chiba 2and Makoto Konagai 21

Quantum Nanoelectronics Research Center,Tokyo Institute of Technology, Tokyo 152-8552, Japan

2

Department of Physical Electronics, Tokyo Institute of Technology, Tokyo 152-8552, Japan

3

Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan

ABSTRACT

We have newly proposed a novel growth method for high quality CuGaSe 2(CGS) thin films in which ionized Ga was used as new source material.The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. The grain size enlargement was successfully demonstrated by using ionized Ga precursors. The experiments revealed that the local heating caused by energy released from high energy Ga ions was one of possible reasons of this grain size enlargement and that the migration energy enhancement of Ga precursors played an important role in the crystal growth of high quality CGS films.

INTRODUCTION

Cu-based chalcopyrite and related semiconductors are promising for high-efficiency thin-film solar cells. Up to now, the conversion efficiency of over 19% has been achieved for Cu(InGa)Se 2(CIGS)-based solar cells using a multi-source vacuum evaporation system by NREL group [1, 2]. However, the band gap of highly efficient CIGS solar cells was about 1.15 eV and an increasing of efficiency can be expected by increasing its band gap. The band gap of CIGS can be controlled by addition of column III atom or S atom. For example, the CIGS film with a Ga composition of 0.7 shows a band gap of about 1.4 eV, and the value matched to the solar spectrum. Therefore, many researchers have focused attention on developing Cu(In 0.3Ga 0.7)Se 2thin-film solar cells. H owever, the open circuit voltage (V oc ) of CIGS solar cells doesn’t show linear dependence on the band gap, and it is the major challenging issue for researchers.Additionally, CIGS material is one of the leading candidates as a top-cell absorber of tandem-type solar cell and the theoretical efficiency of the Cu(InGa)Se 2/CuGaSe 2tandem solar cells are expected to be as high as 25% [3]. Therefore, the demand of high-quality wide band gap chalcopyrite material is rapidly increasing in these years.

However, the efficiency of Cu(In 0.3Ga 0.7)Se 2solar cell is still low, and the small grain size of CuGaSe 2(CGS)and CIGS thin films with high Ga composition is one of the reasons of this poor solar cell performance (~9%) [4, 5] due to low diffusivity of Ga atoms on growing surface [6]. The increase of the migration energy of Ga atoms during the growth is though to be a one of the methods to grow high quality films, and the increase of the substrate temperature is a possible method in order to enhance the migration energy. However, soda-lime glass (SLG) used as a substrate of CIGS solar cells has low melting and/or strained point. Therefore, it is difficult to increase the substrate

temperature of over 600o

C. Consequently, another method for increasing the migration energy of Ga precursors is necessary for improvement of film qualities of CIGS films.

The pioneer works to enhance activity of source atoms during evaporation were carried out by using an

ionized cluster beam (ICB) deposition technique [7, 8], and CuInSe 2films were grown at a low substrate temperature of 300 o C. In this paper, we have grown CuGaSe 2(CGS) films by a co-evaporation system with ionized Ga precursors. Structural properties of CGS films were evaluated by X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy. EXPERIMENTAL CGS thin films were deposited on Mo/SLG substrates using an evaporation technique with an ionized K-cell for Ga precursors and two normal evaporation cells for Cu and Se precursors. In the experiment, we grew CGS films and employed a co-evaporation method as a deposition technique in order to clarify effect of Ga ionization on the film properties. The chamber was evacuated by a diffusion pump and the base pressure was around 10-8Torr. The ionized Ga K-cell consisted of three parts; a heated crucible for Ga source, an ionization system and an acceleration system.The schematic diagram of the K-cell is shown in Fig. 1.The growth conditions are summarized in Table I.

The typical thickness of the films was about 1.3 m m.

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1-4244-0016-3/06/$20.00 ?2006 IEEE

Fig. 1 Schematic of ionized K-Cell

The grid voltage was changed from 0 V to 80 V. The growth temperature was varied from 550 o C to 750 o C. We employed 650 o C as a standard temperature. The temperature was measured by a thermocouple located on the back of the substrate holder, therefore, the surface temperature was about 100 o C lower than the indicated temperature. The beam fluxes of all elements were kept at constant through the deposition process. The beam fluxes of ionized and unionized Ga were fixed at the same value to compare effect of Ga ionization on the film properties. The crystallinity of CGS films was evaluated by XRD using Cu K a-line and Raman spectroscopy. Raman scattering spectra were

Table I.Growth conditions

Growth method Co-evaporation Substrate temperature (o C)550-750

Ga+(Ga) 2.0x10-6

Cu0.4x10-6

Beam Flux (Torr)

Se 2.0x10-5 Filament current (A)60

Grid voltage (V)0, 20, 40, 60, 80measured at a room temperature using 532 nm emission line. The surface morphology was measured by scanning electron microscopy (SEM).

RESULTS AND DISCUSSION

Figures2(a) and (b) show surface morphology and cross-sectional image of the CGS film deposited by ionized Ga precursors, respectively. Figures2(c) and (d) indicate SEM observations for the film deposited by unionized Ga precursors. The samples were grown with a grid voltage of 60 V. The grain size of the CGS film grown by ionized Ga precursors was about 1-2m m.The size was comparable to that of Cu(In0.7Ga0.3)Se2films. Furthermore, we observed clear columnar structure in the film as shown in Fig. 2(b). In our previous experiments, we have found that the diffusivity of Ga atoms was lower than that of In atoms which hindered the grain growth of CIGS films with high Ga composition [6]. The above results suggested that the ionized Ga precursors released their energy on the growing surface which resulted in the local heating. Therefore, the migration energy and/or chemical reactions of Ga atoms were enhanced by this heat energy, thus we have successfully obtained the CGS films with a large grain size. On the contrary, the grain size of CGS films using unionized Ga precursors was about 0.5-1m m, which was a typical grain size of CGS films grown by a conventional method.

Figure 3shows surface morphologies of the CGS films grown by ionized Ga precursors for various growth temperatures. The grid voltage was fixed at 60 V. We observed deformation of the glass substrate at a growth temperature of 750 o C. Thus the surface temperature during growth was about 100 o C lower than the indication of the controller as mentioned in previous section. The gain size of the CGS film grown at a temperature of 650 o C was about1m m, while that of the film grown at 750 o C was larger than 1m m, which clearly indicated that the grain growth occurred by increasing the growth temperature. The result strongly suggested that the effect of Ga ionization on CGS growth was similar to substrate heating and that the local heating by high energy source would occur on the growing surface

(a)(c)

(b)

1m m (d)

1m m

From the above results, it was found that the employment of ionized Ga as a new Ga source was useful for improvement of CGS film quality. Thus, we checked the film quality as a function of grid voltage. Figure 4shows XRD spectra of the CGS films as a function of grid voltage. The grid voltage was varied from 20 V to 80 V. The peak of (112)-plane of chalcopyrite structure was clearly observed in the films grown at a grid voltage higher than 40 V, and the peak intensity increased with increasing the grid voltage.In contrast, a weak peak of (112) diffraction was detected for the film grown at a grid voltage of 20 V. Furthermore, the FWHM (full width at half maximum)also decreased with increasing the grid voltage, suggesting the improvement of the film quality.The direct observation of grain size was carried out by SEM. The grain size of the sample grown at 20 V was approximately100 nm, while the larger gain size was obtained for the sample grown at 80 V. These results showed that the minimum grid voltage to enhance grain growth existed and that the higher grid voltage was favorable to improve the film quality. The possible reason of this tendency is a migration enhancement of Ga on the growing surface, and the migration energy can be increased by increase of the grid voltage. The grid voltage higher than 40 V was necessary for improvement of CGS film quality in our experiments.

In the last experiment, several films were grown by different growth methods, and the film qualities were compared. Figure 5shows XRD spectra of the CGS films grown by ionized and unionized Ga precursors. The XRD spectrum of CGS films grown by a three-stage process is also shown in Fig. 5. In order to compare the film quality, these films had the same thickness, and it was 1.3 m m.The XRD patterns showed a typical pattern of chalcopyrite CGS film. The lattice parameters, a and c, determined by the spectra were a=0.551 nm and c=1.09 nm with a c/a ratio of 1.98. These results were in good agreement with the values of powder diffraction.Furthermore, the films showed a preferential orientation toward the (112) direction.The peak intensities of (220)/(204) and (312)/(116) diffractions were weak for the sample grown by unionized Ga precursors compared with the other two samples. In order to evaluate the crystallinity of the films, the FWHM of the (112) peak was measured. The FWHM of the films grown by both ionized Ga and the three-stage growth method was 0.7o, while the value of the film grown by unionized Ga was about 1.0o. The results showed that the crystallinity of the films grown by the three-stage method and grown by ionized Ga precursors was equivalent.

Figures6(a) and (b) show SEM images of the CGS films grown by ionized Ga and grown by the three-stage method, respectively.The magnification of the samples was different. These samples showed the similar XRD spectra and (112)-diffraction peak intensity. However, the grain size of the films was quite different. The grain size of CGS grown by the three-stage method was approximately 0.2-0.5 m m, while that of CGS grown by ionized Ga was approximately1-2 m m, which was four times larger than the grain size of the film grown by the three-stage method. It was concluded

1m m

(a) Tsub. 650o C

1m m

(b) Tsub. 750o C

Fig. 3 SEM surface images of CIGS films grown at various temperatures. The films were grown with

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ionized Ga is suitable to grow high quality CIGS film with high Ga composition.

Al/ZnO:B/CdS/CGS/Mo/SLG were fabricated, and we successfully confirmed the photovoltaic effect. However, the conversion efficiency was low. It has been reported that the existence of low-resistivity Cu-Se compounds on the CGS surface deteriorates the solar cell performance [9]. Thus, we measured the Raman spectra of the films grown by ionized Ga precursors.

Figure 7shows the Raman spectra of the samples grown by ionized and unionized Ga precursors. The peak at 240 cm-1corresponds to B2or E-vibration mode of chalcopyrite CGS [10], and this peak was observed in both samples. On the contrary, the peak at 210 cm-1 which corresponds to the B2or E-vibration was only observed in the film grown with ionized Ga precursor. The B2or E-vibration mode of CGS structure is the other evaluation factor of the film quality. The appearance of this peak indicated that the quality of the CGS film was improved by using ionized Ga precursors. Raman peak at approximately 270 cm-1were observed in these samples. The peak corresponds to the low-resistivity Cu-Se compound. The Cu-Se compound observed on the CGS surface layer was one of the main causes of degradation of solar cell performance in our experiments. Therefore, it is important to reduce the formation of Cu-Se compound on the CGS surface. It is known that these binary compounds can be removed by etching in KCN solution or rapid thermal annealing using hydrogen and nitrogen mixture gas [9].

CONCLUSIONS

High quality CuGaSe2(CGS) thin films have been successfully fabricated by using a molecular beam deposition system with ionized Ga as a new precursor. The CGS films were characterized by SEM, XRD and Raman spectroscopy.The films grown by ionized Ga were compared with the films grown by other methods, and both the grain size enlargement and the improvement of film crystallinity were confirmed. The possible cause of these improvements was migration enhancement during growth by high energy Ga precursors. Thus, it can be concluded that the ionization of Ga is very effective to improve the CGS film quality.

ACNOWLEDGEMENTS

The authors would like to thank Dr. K. Kushiya at Showa Shell Sekiyu K.K. for providing Mo/SLG substrates. This study was supported by New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade of Industry of Japan. This study was also supported by Japan Society for the Promotion of Science (JSPS) Research Fellowships for Young Scientists, No. 08427.

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