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Investigation on structural properties of ZnSe, CdTe, and ZnTe films grown by ionized cluster beam epitaxy
Journal of Crystal Growth 187 (1998) 387—390
Investigation on structural properties of ZnSe, CdTe, and ZnTe films grown by ionized cluster beam epitaxy J.Y. Feng*, J.Q. Xie Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People+s Republic of China Received 15 November 1997
Abstract Epitaxial films of ZnSe, CdTe, and ZnTe were grown, on GaAs(1 0 0) substrates by ionized cluster beam epitaxy (ICBE). Streaky reflective high energy electron diffraction (RHEED) patterns indicated good crystallinity and surface flatness of the epitaxial ZnSe, CdTe, and ZnTe films. It was found that the crystalline quality of the epilayers depended on the kinetic energy of the clusters and the substrate temperature. The minimum X-ray rocking curve peak widths of the ZnSe, CdTe, and ZnTe epilayers were determined to be 160, 630, and 110 arc sec, respectively. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 81.15.!z; 73.61.Ga Keywords: Ionized cluster beam epitaxy; ZnSe; CdTe; ZnTe
1. Introduction The recent progress in blue—green laser diodes and light emitting diodes with wide band-gap II—VI semiconductors [1,2] has attracted much attention for the development of future optoelectronics. ZnSe is a promising candidate material for fabricating laser diodes which operates in the blue spectral region, and many research papers on ZnSe and ZnSe-based II—VI semiconductors have been published [3—5]. CdTe is considered the ideal substrate for the growth of high quality HgCdTe-based structures for infrared detector application. However,
* Corresponding author.
bulk CdTe single crystal of sufficiently good crystalline quality and large dimensions are costly and difficult to grow. Hence, the growth of high quality and low cost CdTe/GaAs and CdTe/Si hybrid substrates is desirable as an alternative to bulk CdTe substrate for the HgCdTe epitaxy. High-quality ZnTe epilayer may be used as the substrate material for various heterostructures and superlattices, for optoelectronic materials such as CdTe [6] and for semimagnetic HgSe(Hg,Fe)Se superlattices [7]. It has been well understood that low temperature is one of the most important requirements for the growth of high quality wide-gap II—VI semiconductors, such as ZnSe, CdTe, and ZnTe. Atomic layer epitaxy (ALE), molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) have
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 8 5 6 - 7
388
J.Y. Feng, J.Q. Xie / Journal of Crystal Growth 187 (1998) 387—390
been considered as promising techniques for this purpose. However, the epitaxy of MBE and MOVPE is usually conducted under very strict conditions with expensive facilities which need an ultra-high vacuum system. Ionized cluster beam epitaxy (ICBE) [8] is a new and effective technique for the growth of atomically flat and dense films, as well as for the epitaxy of metal [9], organic and semiconductor films [10]. For ICBE, II—VI compounds such as ZnSe, CdTe, or ZnTe can be used as evaporation materials in a single crucible. Unlike in many other deposition techniques, in ICBE, materials evaporate in a state of clusters, instead of atoms or moleculae. During the ICBE growth of films, the kinetic energy and ionic charge of the clusters assist positively the migration and nucleation of the deposited atoms, and thus help to improve the crystalline quality of the grown films. Compared to MBE and MOVPE, the ICBE technique is simple in structure and operation, inexpensive, and more compatible to industrial production. This paper reports the growth of epitaxial ZnSe, CdTe, and ZnTe films on GaAs(1 0 0) substrates by ICBE. The structural properties of epitaxial ZnSe, CdTe, and ZnTe films are investigated, and the characteristics of the ICBE growth are discussed.
2. Experimental details The ICBE build-up has been described elsewhere [11]. High purity ingots of ZnSe, CdTe, or ZnTe were used as source materials in a quasi-closed graphite crucible with a nozzle of 1 mm in diameter and 1.5 mm in length on the top. The crucible was heated by a tungsten filament and electron bombardment. The bombardment current and voltage were 0.1—0.2 A and 400 V, respectively. The growth rate depended mainly on the vapor pressure of the source material, which could be adjusted by controlling the temperature of the crucible. ZnSe, CdTe, or ZnTe polycrystals changed into the vapor phase in the crucible and emitted from the nozzle in a state of clusters. The clusters are positively ionized by electron beam and then are accelerated towards the substrate by a static electron field to form a film.
Cr-doped semi-insulating GaAs substrates with orientation (1 0 0) off 2° toward [1 1 0] were used in the experiment. First, the substrate was chemically etched in a 5 : 1 : 1 (5H SO : H O : H O) mixed 2 4 2 2 2 solution at room temperature. Then it was thermally cleaned in the vacuum chamber at 500°C for 12 min prior to the epitaxial growth. To investigate the factors influencing the film quality, the growth parameters such as substrate temperature (¹ ) and acceleration voltage (» ) were 4 ! systematically adjusted. In the experiment, the growth rate (R) was about 1.2 nm/s and the film thickness (¹ ) varied from 1 to 2 lm. The crystallo) graphic properties of the ZnSe, CdTe, and ZnTe epilayers were investigated by RHEED and X-ray double crystal diffraction. RHEED was performed at a JEM200CX electron microscope with camera length of 312 mm. The X-ray double crystal diffraction was carried out in the h—2h scanning mode with Cu K radiation and with Si(4 2 2) as the first a crystal.
3. Results and discussion RHEED measurements were performed to investigate the surface structure of the ICB deposited II—VI semiconductor films. The epitaxial growth of ZnSe, CdTe, and ZnTe on GaAs(1 0 0) was observed when the growth conditions were optimized. Typical RHEED patterns of ZnSe, CdTe, and ZnTe films on GaAs are shown in Fig. 1. The streaky RHEED patterns in Fig. 1a (¹ "1.2 lm, ) R"1 nm/s), Fig. 1b (¹ "2 lm, R"1.6 nm/s), ) and Fig. 1c (¹ "1 lm, R"1.2 nm/s) indicate ) a fine crystallinity and atomic flatness of the ZnSe, CdTe, and ZnTe epilayers. Many reports [8—10] on ICB deposited films also support that ICB deposition is an advantageous epitaxial technique due to its unique cluster ions whose ionic charge and kinetic energy are beneficial to the structure perfection of the grown films. We evaluated the crystallographic quality of ZnSe, CdTe, and ZnTe films by X-ray double crystal diffraction. Several effects of the ionized cluster beam on film formation were studied. Fig. 2 shows the dependence of the crystallinity of the ZnSe(1 0 0) and CdTe(1 0 0) epilayers on the
J.Y. Feng, J.Q. Xie / Journal of Crystal Growth 187 (1998) 387—390
389
Fig. 2. Dependence of the crystallinity of the ZnSe and CdTe films on the acceleration voltage. The full-width at half-maximum (FWHM) of the X-ray double crystal rocking curve (DCRC) of ZnSe(1 0 0) and CdTe(1 0 0) determines the crystalline quality.
Fig. 1. RHEED patterns of ZnSe, CdTe, and ZnTe films grown, respectively, by ICBE under the experimental conditions (» : ! acceleration voltage; I : ionization current; ¹ : substrate temper% 4 ature): (a) ZnSe films: » "1.6 kV, I "100 mA, ¹ "250°C; ! % 4 (b) CdTe films: » "4 kv, I "150 mA, ¹ "250°C; (c) ZnTe ! % 4 films: » "0.8 kV, I "50 mA, ¹ "300°C. ! % 4
acceleration voltage. The substrate temperature was kept at 230°C for ZnSe and 300°C for CdTe, and the acceleration voltage was changed from 0 to 5 KV. The minimum FWHM of the X-ray rocking curve of the ZnSe(1 0 0) and CdTe(1 0 0) was determined to be 160 and 630 arc sec and was realized at 1.6 KV and 2.0 KV, respectively. We attribute this phenomenon to the following reasons. An important requirement in growing epitaxial films from cluster deposition is that each impinging cluster should dissociate and spread uniformly on the substrate. If the cluster energy is low, the spreading of the cluster is small and therefore atom diffusivity is inhibited. Low atom diffusivity is not convenient to epitaxial growth because the atoms may not have enough energy to migrate to their lowest energy sites. However, with the increase of the cluster energy, the distorted collision region becomes wider and deeper. Although the impact region can reconstruct in the following relaxation time, the damage may not be completely eliminated. So epitaxial growth is also prohibited when the cluster energy is much higher.
390
J.Y. Feng, J.Q. Xie / Journal of Crystal Growth 187 (1998) 387—390
disadvantageous to the layer by layer growth. It is also observed that for a variety of substrates, such as Si and glass, the optimum growth temperature was the same 300°C, which indicates that the optimum growth temperature may be independent of the substrate. However, the mechanism of this effect needs further investigation.
4. Conclusions
Fig. 3. Dependence of the full-width at half-maximum (FWHM) of the X-ray double crystal rocking curve of the ZnTe(1 0 0) films on the substrate temperature.
Epitaxial films of ZnSe(1 0 0), CdTe(1 0 0), and ZnTe(1 0 0) have been successfully grown, respectively, on GaAs(1 0 0) substrates by means of the ICBE technique. The crystalline quality of the epilayers depended on the kinetic energy of the clusters and the substrate temperature. The minimum X-ray rocking curve peak widths of the ZnSe, CdTe, and ZnTe epilayers were determined to be 160, 630, and 110 arc sec, respectively.
Acknowledgements It was found that the substrate temperature also has a significant effect on the ZnTe(1 0 0) epitaxial layers. As illustrated in Fig. 3, the crystalline quality of the ZnTe epilayers, which is represented by the FWHM of the X-ray double crystal rocking curve, reaches its optimum, 110 arc sec, at a certain temperature. This result indicates that the substrate temperature has an effect on the film crystallinity similar to the acceleration voltage in ICBE. The optimum substrate temperature for epitaxial growth depends on many factors such as the sticking coefficient, the atom migration, the difference of expansion coefficient between the substrate and the film material, the crystallographic unit cell mismatch at the given temperature, and the remaining stress in the films. In the low temperature range, an increase of the substrate temperature is beneficial to promote uniform orientation of the nucleation and enhances the migration and diffusion of the adatoms to grow layer by layer. When the substrate temperature is too high, however, thermal defects in the films will increase substantially, and the deposited material tends to agglomerate, which is
This work is supported by the National Natural Science Foundation of China. References [1] A. Ishibashi, IEEE J. Selected Topics in Quantum Electronics 1 (1995) 741. [2] D. Eason, J. Ren, Z. Yu, C. Huges, N.A. El-Masry, J.W. Cook Jr., J.F. Schetzina, J. Crystal Growth 150 (1995) 718. [3] M.Y. Yeh, C.C. Hu, G.L. Lin, M.K. Lee, Thin Solid Films 215 (1992) 142. [4] R. Kinura, M. Konagai, K. Takahashi, J. Crystal Growth 116 (1992) 283. [5] Sz. Fujita, Y. Kawakami, Sg. Fujita, J. Crystal Growth 164 (1996) 196. [6] H. Nishino, T. Saito, Y. Nishijima, J. Crystal Growth 165 (1996) 227. [7] M.V. Ortenberg, Adv. Solid State Phys. 31 (1991) 261. [8] I. Yamada, G.H. Takaoka, Jpn. J. Appl. Phys. 32 (1993) 2121. [9] I. Yamada, H. Inokawa, T. Takagi, J. Appl. Phys. 56 (1984) 2746. [10] M. Shinohara, F. Ohtani, O. Ishiyama, M. Asari, J. Saraie, Nucl. Instr. and Meth. B 99 (1995) 576. [11] J.Q. Xie, Y. Zheng, J.Y. Feng, Nucl. Instr. and Meth. B 122 (1997) 239.
Investigation on structural properties of ZnSe, CdTe, and ZnTe films grown by ionized cluster beam epitaxy J.Y. Feng*, J.Q. Xie Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People+s Republic of China Received 15 November 1997
Abstract Epitaxial films of ZnSe, CdTe, and ZnTe were grown, on GaAs(1 0 0) substrates by ionized cluster beam epitaxy (ICBE). Streaky reflective high energy electron diffraction (RHEED) patterns indicated good crystallinity and surface flatness of the epitaxial ZnSe, CdTe, and ZnTe films. It was found that the crystalline quality of the epilayers depended on the kinetic energy of the clusters and the substrate temperature. The minimum X-ray rocking curve peak widths of the ZnSe, CdTe, and ZnTe epilayers were determined to be 160, 630, and 110 arc sec, respectively. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 81.15.!z; 73.61.Ga Keywords: Ionized cluster beam epitaxy; ZnSe; CdTe; ZnTe
1. Introduction The recent progress in blue—green laser diodes and light emitting diodes with wide band-gap II—VI semiconductors [1,2] has attracted much attention for the development of future optoelectronics. ZnSe is a promising candidate material for fabricating laser diodes which operates in the blue spectral region, and many research papers on ZnSe and ZnSe-based II—VI semiconductors have been published [3—5]. CdTe is considered the ideal substrate for the growth of high quality HgCdTe-based structures for infrared detector application. However,
* Corresponding author.
bulk CdTe single crystal of sufficiently good crystalline quality and large dimensions are costly and difficult to grow. Hence, the growth of high quality and low cost CdTe/GaAs and CdTe/Si hybrid substrates is desirable as an alternative to bulk CdTe substrate for the HgCdTe epitaxy. High-quality ZnTe epilayer may be used as the substrate material for various heterostructures and superlattices, for optoelectronic materials such as CdTe [6] and for semimagnetic HgSe(Hg,Fe)Se superlattices [7]. It has been well understood that low temperature is one of the most important requirements for the growth of high quality wide-gap II—VI semiconductors, such as ZnSe, CdTe, and ZnTe. Atomic layer epitaxy (ALE), molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) have
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 8 5 6 - 7
388
J.Y. Feng, J.Q. Xie / Journal of Crystal Growth 187 (1998) 387—390
been considered as promising techniques for this purpose. However, the epitaxy of MBE and MOVPE is usually conducted under very strict conditions with expensive facilities which need an ultra-high vacuum system. Ionized cluster beam epitaxy (ICBE) [8] is a new and effective technique for the growth of atomically flat and dense films, as well as for the epitaxy of metal [9], organic and semiconductor films [10]. For ICBE, II—VI compounds such as ZnSe, CdTe, or ZnTe can be used as evaporation materials in a single crucible. Unlike in many other deposition techniques, in ICBE, materials evaporate in a state of clusters, instead of atoms or moleculae. During the ICBE growth of films, the kinetic energy and ionic charge of the clusters assist positively the migration and nucleation of the deposited atoms, and thus help to improve the crystalline quality of the grown films. Compared to MBE and MOVPE, the ICBE technique is simple in structure and operation, inexpensive, and more compatible to industrial production. This paper reports the growth of epitaxial ZnSe, CdTe, and ZnTe films on GaAs(1 0 0) substrates by ICBE. The structural properties of epitaxial ZnSe, CdTe, and ZnTe films are investigated, and the characteristics of the ICBE growth are discussed.
2. Experimental details The ICBE build-up has been described elsewhere [11]. High purity ingots of ZnSe, CdTe, or ZnTe were used as source materials in a quasi-closed graphite crucible with a nozzle of 1 mm in diameter and 1.5 mm in length on the top. The crucible was heated by a tungsten filament and electron bombardment. The bombardment current and voltage were 0.1—0.2 A and 400 V, respectively. The growth rate depended mainly on the vapor pressure of the source material, which could be adjusted by controlling the temperature of the crucible. ZnSe, CdTe, or ZnTe polycrystals changed into the vapor phase in the crucible and emitted from the nozzle in a state of clusters. The clusters are positively ionized by electron beam and then are accelerated towards the substrate by a static electron field to form a film.
Cr-doped semi-insulating GaAs substrates with orientation (1 0 0) off 2° toward [1 1 0] were used in the experiment. First, the substrate was chemically etched in a 5 : 1 : 1 (5H SO : H O : H O) mixed 2 4 2 2 2 solution at room temperature. Then it was thermally cleaned in the vacuum chamber at 500°C for 12 min prior to the epitaxial growth. To investigate the factors influencing the film quality, the growth parameters such as substrate temperature (¹ ) and acceleration voltage (» ) were 4 ! systematically adjusted. In the experiment, the growth rate (R) was about 1.2 nm/s and the film thickness (¹ ) varied from 1 to 2 lm. The crystallo) graphic properties of the ZnSe, CdTe, and ZnTe epilayers were investigated by RHEED and X-ray double crystal diffraction. RHEED was performed at a JEM200CX electron microscope with camera length of 312 mm. The X-ray double crystal diffraction was carried out in the h—2h scanning mode with Cu K radiation and with Si(4 2 2) as the first a crystal.
3. Results and discussion RHEED measurements were performed to investigate the surface structure of the ICB deposited II—VI semiconductor films. The epitaxial growth of ZnSe, CdTe, and ZnTe on GaAs(1 0 0) was observed when the growth conditions were optimized. Typical RHEED patterns of ZnSe, CdTe, and ZnTe films on GaAs are shown in Fig. 1. The streaky RHEED patterns in Fig. 1a (¹ "1.2 lm, ) R"1 nm/s), Fig. 1b (¹ "2 lm, R"1.6 nm/s), ) and Fig. 1c (¹ "1 lm, R"1.2 nm/s) indicate ) a fine crystallinity and atomic flatness of the ZnSe, CdTe, and ZnTe epilayers. Many reports [8—10] on ICB deposited films also support that ICB deposition is an advantageous epitaxial technique due to its unique cluster ions whose ionic charge and kinetic energy are beneficial to the structure perfection of the grown films. We evaluated the crystallographic quality of ZnSe, CdTe, and ZnTe films by X-ray double crystal diffraction. Several effects of the ionized cluster beam on film formation were studied. Fig. 2 shows the dependence of the crystallinity of the ZnSe(1 0 0) and CdTe(1 0 0) epilayers on the
J.Y. Feng, J.Q. Xie / Journal of Crystal Growth 187 (1998) 387—390
389
Fig. 2. Dependence of the crystallinity of the ZnSe and CdTe films on the acceleration voltage. The full-width at half-maximum (FWHM) of the X-ray double crystal rocking curve (DCRC) of ZnSe(1 0 0) and CdTe(1 0 0) determines the crystalline quality.
Fig. 1. RHEED patterns of ZnSe, CdTe, and ZnTe films grown, respectively, by ICBE under the experimental conditions (» : ! acceleration voltage; I : ionization current; ¹ : substrate temper% 4 ature): (a) ZnSe films: » "1.6 kV, I "100 mA, ¹ "250°C; ! % 4 (b) CdTe films: » "4 kv, I "150 mA, ¹ "250°C; (c) ZnTe ! % 4 films: » "0.8 kV, I "50 mA, ¹ "300°C. ! % 4
acceleration voltage. The substrate temperature was kept at 230°C for ZnSe and 300°C for CdTe, and the acceleration voltage was changed from 0 to 5 KV. The minimum FWHM of the X-ray rocking curve of the ZnSe(1 0 0) and CdTe(1 0 0) was determined to be 160 and 630 arc sec and was realized at 1.6 KV and 2.0 KV, respectively. We attribute this phenomenon to the following reasons. An important requirement in growing epitaxial films from cluster deposition is that each impinging cluster should dissociate and spread uniformly on the substrate. If the cluster energy is low, the spreading of the cluster is small and therefore atom diffusivity is inhibited. Low atom diffusivity is not convenient to epitaxial growth because the atoms may not have enough energy to migrate to their lowest energy sites. However, with the increase of the cluster energy, the distorted collision region becomes wider and deeper. Although the impact region can reconstruct in the following relaxation time, the damage may not be completely eliminated. So epitaxial growth is also prohibited when the cluster energy is much higher.
390
J.Y. Feng, J.Q. Xie / Journal of Crystal Growth 187 (1998) 387—390
disadvantageous to the layer by layer growth. It is also observed that for a variety of substrates, such as Si and glass, the optimum growth temperature was the same 300°C, which indicates that the optimum growth temperature may be independent of the substrate. However, the mechanism of this effect needs further investigation.
4. Conclusions
Fig. 3. Dependence of the full-width at half-maximum (FWHM) of the X-ray double crystal rocking curve of the ZnTe(1 0 0) films on the substrate temperature.
Epitaxial films of ZnSe(1 0 0), CdTe(1 0 0), and ZnTe(1 0 0) have been successfully grown, respectively, on GaAs(1 0 0) substrates by means of the ICBE technique. The crystalline quality of the epilayers depended on the kinetic energy of the clusters and the substrate temperature. The minimum X-ray rocking curve peak widths of the ZnSe, CdTe, and ZnTe epilayers were determined to be 160, 630, and 110 arc sec, respectively.
Acknowledgements It was found that the substrate temperature also has a significant effect on the ZnTe(1 0 0) epitaxial layers. As illustrated in Fig. 3, the crystalline quality of the ZnTe epilayers, which is represented by the FWHM of the X-ray double crystal rocking curve, reaches its optimum, 110 arc sec, at a certain temperature. This result indicates that the substrate temperature has an effect on the film crystallinity similar to the acceleration voltage in ICBE. The optimum substrate temperature for epitaxial growth depends on many factors such as the sticking coefficient, the atom migration, the difference of expansion coefficient between the substrate and the film material, the crystallographic unit cell mismatch at the given temperature, and the remaining stress in the films. In the low temperature range, an increase of the substrate temperature is beneficial to promote uniform orientation of the nucleation and enhances the migration and diffusion of the adatoms to grow layer by layer. When the substrate temperature is too high, however, thermal defects in the films will increase substantially, and the deposited material tends to agglomerate, which is
This work is supported by the National Natural Science Foundation of China. References [1] A. Ishibashi, IEEE J. Selected Topics in Quantum Electronics 1 (1995) 741. [2] D. Eason, J. Ren, Z. Yu, C. Huges, N.A. El-Masry, J.W. Cook Jr., J.F. Schetzina, J. Crystal Growth 150 (1995) 718. [3] M.Y. Yeh, C.C. Hu, G.L. Lin, M.K. Lee, Thin Solid Films 215 (1992) 142. [4] R. Kinura, M. Konagai, K. Takahashi, J. Crystal Growth 116 (1992) 283. [5] Sz. Fujita, Y. Kawakami, Sg. Fujita, J. Crystal Growth 164 (1996) 196. [6] H. Nishino, T. Saito, Y. Nishijima, J. Crystal Growth 165 (1996) 227. [7] M.V. Ortenberg, Adv. Solid State Phys. 31 (1991) 261. [8] I. Yamada, G.H. Takaoka, Jpn. J. Appl. Phys. 32 (1993) 2121. [9] I. Yamada, H. Inokawa, T. Takagi, J. Appl. Phys. 56 (1984) 2746. [10] M. Shinohara, F. Ohtani, O. Ishiyama, M. Asari, J. Saraie, Nucl. Instr. and Meth. B 99 (1995) 576. [11] J.Q. Xie, Y. Zheng, J.Y. Feng, Nucl. Instr. and Meth. B 122 (1997) 239.