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Maskless selective epitaxy of InxGa1−xAs using low-energy In0.15Ga0.85-FIB and As4 molecular beam
Journal of Crystal Growth 201/202 (1999) 610}613
Maskless selective epitaxy of In Ga As using low-energy V \V In Ga -FIB and As molecular beam D.H. Cho*, M. Hachiro, Y. Abe, K. Pak Department of Electrical and Electronic Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8122, Japan .
Abstract In this study, we have demonstrated maskless selective epitaxy (MLSE) of In Ga As on GaAs substrate using V \V a low-energy In Ga -FIB and an As molecular beam. We used an In Ga liquid alloy ion source (LAIS) as a source of the group III. Investigations of the In composition on the In Ga As maskless selective epitaxial layers V \V using AES showed that as the ¹ was raised, the In composition of the "lms increased. At ¹ of &3503C, it was *' 1 *' 1 similar to the original source composition (&In ). Single crystal In Ga As "lms were obtained above ¹ of 5503C V \V at E "50 eV and below E of 200 eV at ¹ "6003C. 1999 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ji; 68.55.Bd; 79.70.#q; 81.15.Ef Keywords: Maskless selective epitaxy; Low-energy FIB; LAIS; k-RHEED; In Ga As V \V
1. Introduction Maskless selective epitaxy (MLSE) of III}V compound semiconductors could become an important technique in the preparation of complex integrated device structures such as opto-electronic integrated circuits (OEICs). Focused ion beam (FIB) technology is useful for various semiconductor maskless processes such as selective doping, ion beam assisted etching and deposition. In particular, in situ MLSE using low-energy FIB technology is desirable to eliminate surface contamination which degrades the device quality [1]. We have reported in situ MLSE of GaAs using a focused low-energy Ga> ion beam and an As molecular [2,3]. Liquid * Corresponding author. Tel.: #81-532-446747; fax: #81532-446757; e-mail: [email protected].
metal ion source (LMIS) have attracted increasing interest due to their potential advantages in FIB technology [4,5]. The variety of ion species has been increased by using alloys as well as pure elements as the source materials of LMIS. A eutectic alloy of Ga and In with 15 at% of In melts at only about 183C. By using this alloy as a liquid alloy ion source (LAIS) [6,7], In> and Ga> ion beams may be conveniently produced for ion beam processing. In this paper, we present the in situ MLSE of In Ga As using a low-energy In Ga -FIB V \V and an As molecular beam for the "rst time. 2. Experiment The apparatus which was designed for the MLSE of III}V compound semiconductors and the
0022-0248/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 4 2 1 - 3
D.H. Cho et al. / Journal of Crystal Growth 201/202 (1999) 610}613
ion beam characteristics for various incident energy obtained with this system have been described elsewhere [2,3]. The beam diameter of FIB employed in this experiment was &20 lm. The system consists of a LAIS, an optical column and a growth chamber. The LAIS is a thin tantalum tube "lled with the indium}gallium alloy (15 at% In and 85 at% Ga: In Ga ). GaAs (1 0 0) wafers were used as substrates which were initially cleaned with standard cleaning procedure. Prior to growth, in order to remove the natural oxides on the GaAs surface, the substrate was heated to 6203C for 20 min in an ambient As pressure of &1;10\ Torr. During the growth, the growth chamber pressure was maintained at &5;10\ Torr. Nondoped In Ga As "lms were grown at the various V \V ¹ using 100 eV FIB and the In composition of * '1 the grown "lms was measured by Auger electron spectroscopy (AES). To calculate the In composition ratio of the grown "lms, we used a MBE grown In Ga As reference sample. To investigate the e!ect of substrate temperature (¹ ) and ion incident energy (E ) on crystallization, ¹ and E were varied from 2503C to 6003C at E "50, 100 eV and from 30 to 200 eV at ¹ "6003C, respectively. The "lm qualities are investigated with micro-probe re#ection high-energy electron di!raction (k-RHEED) and Normarski microscope.
3. Results and discussion 3.1. In composition control of the maskless selective epitaxial layer The typical surface morphology of the selectively grown "lm is shown in Fig. 1(a) which had a #at surface and Fig. 1(b) shows the In surface composition of the grown "lms measured with the AES peak intensity at various ¹ . The In surface * '1 composition of the In Ga As "lms deposited at V \V high ¹ (&3503C) almost corresponds to the * '1 source composition ratio (In Ga ). However, at lower ¹ , the In surface composition ratio * '1 decreases with the decrease of the ¹ and be* '1 comes In Ga As at 403C. We consider that these In composition di!erences are due to the di!erence of "eld evaporation at a jet-like protru-
611
Fig. 1. (a) Normarski micrograph of a maskless selective epilayer using a 50 eV In Ga -FIB and an As molecular beam. (b) In composition variations of the selectively grown In Ga As "lms as a function of liquid alloy ion source V \V temperature (¹ ) at E "100 eV. * '1
sion on the end of Taylor cone shape [8]. At low source temperature, "eld evaporation is the dominant ion formation mechanism [8]. Because of the "eld evaporation di!erence between In> and Ga>, that is, the evaporating "eld for In> is lower than that for Ga>, angular distributions of In> ions from the InGa-LAIS have side maxima [7]. As the ¹ is raised, thermal evaporation of neutral * '1 atoms and clusters occurs from the sides of the protrusion, giving the second most important ion formation mechanism of "eld ionization [8]. Consequently, there is a pronounced change in the angular distributions of In> and Ga> ions at higher source temperature. Detailed studies of angular distributions of Ga> and In> ions are shown in Ref. [7]. Because of 200 lm aperture used in this
612
D.H. Cho et al. / Journal of Crystal Growth 201/202 (1999) 610}613
experiment, particles contributed to the selective growth are contained within a cone of &0.63 halfangle ion beam, resulting in low In composition "lms at low ¹ , while it increases at higher * '1 source temperature. These results agree best with the result of Ref. [7], according to which the In> ion component increases in the central part of the In>}Ga> ion beams from the InGa-LAIS at higher ¹ . * '1 In this experiment, we found that In composition of the In Ga As maskless selective epitaxial V \V layer could be controlled by varying ¹ . * '1 3.2. In Ga1 As xlm quality V \V Fig. 2 shows k-RHEED patterns taken of about 600 As thick "lms for an expected growth rate of &0.2 ML/s grown at 250}6003C by a 50 eV In Ga -FIB and an As molecular beam. It is found that the growth temperature gradually changes the crystallization. When the "lm is grown at
5503C or higher the spotty patterns are obtained, showing that the "lms are single crystalline. On other hand, the k-RHEED patterns change from spotty to the ring patterns which show that the "lm is polycrystalline through twinning patterns with the decrease of ¹ from 6003C to 2503C. According to ion beam deposition (IBD) and/or combined ion and molecular deposition (CIMD) model [9], crystallization can be accounted for by the interstitial and/or vacancy motion. A great majority of Frenkel-type defects produced in InGaAs "lms by collision of In> and Ga> ions with growth surface are In, Ga and As interstitial atoms, vacancies and their complexes. These vacancies recombine with the nearby interstitials, leading to net "lm growth. At high ¹ , the interstitial atoms migrate su$ ciently in the growing "lms, resulting in a good crystal quality, but the thermal energy supplied to the interstitial atoms is not su$cient for atoms to di!use in the growing "lm at low ¹ , leading to polycrystalline or amorphous "lms. In the twinning
Fig. 2. k-RHEED patterns along the [0 1 1] azimuth after the selective epitaxial growth of In Ga As on GaAs (1 0 0) at E "50 eV. V \V
D.H. Cho et al. / Journal of Crystal Growth 201/202 (1999) 610}613
formation there are uncertainties regarding the motion of the interstitials and/or vacancies in the crystallization zone. However, at E "100 eV single crystalline In Ga As "lms were obtained V \V through 250}6003C of ¹ with weak twinning pat terns at 450, 5003C. Although the di!erence between the results of 50 and 100 eV of E is not understood, it may be related to the thermal enhancement of atomic mobility and the subsequent modi"cation of growth kinetics. In the FIB growth mechanism, In segregation and evaporation on surface at high ¹ are not clear. The e!ect of E on crystallization showed that below 200 eV at 6003C, crystallization is independent of the incident ion energy, in agreement with our previous experimental work [2]. Thus, it may be concluded that the "lm quality of the selectively grown In Ga As on GaAs (1 0 0) depends on ¹ . V \V 4. Conclusion MLSE of In Ga As using a low-energy V \V In Ga -FIB and an As molecular beam was studied for the "rst time. In this experiment, we have been able to grow In Ga As epitaxial "lms V \V on GaAs using this growth technology. In composition on the epilayers showed that as the ¹ was * '1 raised, In composition of the "lms increased. The crystal quality of the selectively grown In Ga As V \V
613
"lms was found to depend upon ¹ , not to E below E of 200 eV. Single crystal In Ga As "lms were V \V obtained above ¹ of 5503C at E "50 eV and below E of 200 eV at ¹ "6003C. Acknowledgements The authors would like to thank Professor T. Nishinaga and Dr. A. Yamashiki of Tokyo University for the k-RHEED measurement. This work was partially supported by the MURATA Academic Promotion Foundation.
References [1] I. Hayashi, Japan J. Appl. Phys. 32 (1993) 266. [2] K. Pak, I. Saitoh, N. Ohshima, H. Yonezu, J. Crystal Growth 140 (1994) 244. [3] I. Saitoh, K. Pak, Y. Inoue, N. Ohshima, H. Yonezu, J. Crystal Growth 146 (1995) 359. [4] R.L. Seliger, R.L. Kubena, V. Wang, Japan J. Appl. Phys. 21 (Suppl. 1) (1982) 3. [5] J. Melngailis, J. Vac. Sci. Technol. B 5 (1987) 469. [6] D.S. Swatik, C.D. Hendricks, AIAA J. 6 (1968) 1596. [7] V.K. Medvedev, V.I. Chernyi, N.N. Popovich, J. Vac. Sci. Technol. B 11 (2) (1993) 523. [8] L.W. Swanson, D.R. Kingham, Appl. Phys. A 41 (1986) 223. [9] O. Vancauwenberghe, N. Herbots, O.C. Hellman, J. Vac. Sci. Technol. B 9 (4) (1991) 2027.
Maskless selective epitaxy of In Ga As using low-energy V \V In Ga -FIB and As molecular beam D.H. Cho*, M. Hachiro, Y. Abe, K. Pak Department of Electrical and Electronic Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8122, Japan .
Abstract In this study, we have demonstrated maskless selective epitaxy (MLSE) of In Ga As on GaAs substrate using V \V a low-energy In Ga -FIB and an As molecular beam. We used an In Ga liquid alloy ion source (LAIS) as a source of the group III. Investigations of the In composition on the In Ga As maskless selective epitaxial layers V \V using AES showed that as the ¹ was raised, the In composition of the "lms increased. At ¹ of &3503C, it was *' 1 *' 1 similar to the original source composition (&In ). Single crystal In Ga As "lms were obtained above ¹ of 5503C V \V at E "50 eV and below E of 200 eV at ¹ "6003C. 1999 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ji; 68.55.Bd; 79.70.#q; 81.15.Ef Keywords: Maskless selective epitaxy; Low-energy FIB; LAIS; k-RHEED; In Ga As V \V
1. Introduction Maskless selective epitaxy (MLSE) of III}V compound semiconductors could become an important technique in the preparation of complex integrated device structures such as opto-electronic integrated circuits (OEICs). Focused ion beam (FIB) technology is useful for various semiconductor maskless processes such as selective doping, ion beam assisted etching and deposition. In particular, in situ MLSE using low-energy FIB technology is desirable to eliminate surface contamination which degrades the device quality [1]. We have reported in situ MLSE of GaAs using a focused low-energy Ga> ion beam and an As molecular [2,3]. Liquid * Corresponding author. Tel.: #81-532-446747; fax: #81532-446757; e-mail: [email protected].
metal ion source (LMIS) have attracted increasing interest due to their potential advantages in FIB technology [4,5]. The variety of ion species has been increased by using alloys as well as pure elements as the source materials of LMIS. A eutectic alloy of Ga and In with 15 at% of In melts at only about 183C. By using this alloy as a liquid alloy ion source (LAIS) [6,7], In> and Ga> ion beams may be conveniently produced for ion beam processing. In this paper, we present the in situ MLSE of In Ga As using a low-energy In Ga -FIB V \V and an As molecular beam for the "rst time. 2. Experiment The apparatus which was designed for the MLSE of III}V compound semiconductors and the
0022-0248/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 4 2 1 - 3
D.H. Cho et al. / Journal of Crystal Growth 201/202 (1999) 610}613
ion beam characteristics for various incident energy obtained with this system have been described elsewhere [2,3]. The beam diameter of FIB employed in this experiment was &20 lm. The system consists of a LAIS, an optical column and a growth chamber. The LAIS is a thin tantalum tube "lled with the indium}gallium alloy (15 at% In and 85 at% Ga: In Ga ). GaAs (1 0 0) wafers were used as substrates which were initially cleaned with standard cleaning procedure. Prior to growth, in order to remove the natural oxides on the GaAs surface, the substrate was heated to 6203C for 20 min in an ambient As pressure of &1;10\ Torr. During the growth, the growth chamber pressure was maintained at &5;10\ Torr. Nondoped In Ga As "lms were grown at the various V \V ¹ using 100 eV FIB and the In composition of * '1 the grown "lms was measured by Auger electron spectroscopy (AES). To calculate the In composition ratio of the grown "lms, we used a MBE grown In Ga As reference sample. To investigate the e!ect of substrate temperature (¹ ) and ion incident energy (E ) on crystallization, ¹ and E were varied from 2503C to 6003C at E "50, 100 eV and from 30 to 200 eV at ¹ "6003C, respectively. The "lm qualities are investigated with micro-probe re#ection high-energy electron di!raction (k-RHEED) and Normarski microscope.
3. Results and discussion 3.1. In composition control of the maskless selective epitaxial layer The typical surface morphology of the selectively grown "lm is shown in Fig. 1(a) which had a #at surface and Fig. 1(b) shows the In surface composition of the grown "lms measured with the AES peak intensity at various ¹ . The In surface * '1 composition of the In Ga As "lms deposited at V \V high ¹ (&3503C) almost corresponds to the * '1 source composition ratio (In Ga ). However, at lower ¹ , the In surface composition ratio * '1 decreases with the decrease of the ¹ and be* '1 comes In Ga As at 403C. We consider that these In composition di!erences are due to the di!erence of "eld evaporation at a jet-like protru-
611
Fig. 1. (a) Normarski micrograph of a maskless selective epilayer using a 50 eV In Ga -FIB and an As molecular beam. (b) In composition variations of the selectively grown In Ga As "lms as a function of liquid alloy ion source V \V temperature (¹ ) at E "100 eV. * '1
sion on the end of Taylor cone shape [8]. At low source temperature, "eld evaporation is the dominant ion formation mechanism [8]. Because of the "eld evaporation di!erence between In> and Ga>, that is, the evaporating "eld for In> is lower than that for Ga>, angular distributions of In> ions from the InGa-LAIS have side maxima [7]. As the ¹ is raised, thermal evaporation of neutral * '1 atoms and clusters occurs from the sides of the protrusion, giving the second most important ion formation mechanism of "eld ionization [8]. Consequently, there is a pronounced change in the angular distributions of In> and Ga> ions at higher source temperature. Detailed studies of angular distributions of Ga> and In> ions are shown in Ref. [7]. Because of 200 lm aperture used in this
612
D.H. Cho et al. / Journal of Crystal Growth 201/202 (1999) 610}613
experiment, particles contributed to the selective growth are contained within a cone of &0.63 halfangle ion beam, resulting in low In composition "lms at low ¹ , while it increases at higher * '1 source temperature. These results agree best with the result of Ref. [7], according to which the In> ion component increases in the central part of the In>}Ga> ion beams from the InGa-LAIS at higher ¹ . * '1 In this experiment, we found that In composition of the In Ga As maskless selective epitaxial V \V layer could be controlled by varying ¹ . * '1 3.2. In Ga1 As xlm quality V \V Fig. 2 shows k-RHEED patterns taken of about 600 As thick "lms for an expected growth rate of &0.2 ML/s grown at 250}6003C by a 50 eV In Ga -FIB and an As molecular beam. It is found that the growth temperature gradually changes the crystallization. When the "lm is grown at
5503C or higher the spotty patterns are obtained, showing that the "lms are single crystalline. On other hand, the k-RHEED patterns change from spotty to the ring patterns which show that the "lm is polycrystalline through twinning patterns with the decrease of ¹ from 6003C to 2503C. According to ion beam deposition (IBD) and/or combined ion and molecular deposition (CIMD) model [9], crystallization can be accounted for by the interstitial and/or vacancy motion. A great majority of Frenkel-type defects produced in InGaAs "lms by collision of In> and Ga> ions with growth surface are In, Ga and As interstitial atoms, vacancies and their complexes. These vacancies recombine with the nearby interstitials, leading to net "lm growth. At high ¹ , the interstitial atoms migrate su$ ciently in the growing "lms, resulting in a good crystal quality, but the thermal energy supplied to the interstitial atoms is not su$cient for atoms to di!use in the growing "lm at low ¹ , leading to polycrystalline or amorphous "lms. In the twinning
Fig. 2. k-RHEED patterns along the [0 1 1] azimuth after the selective epitaxial growth of In Ga As on GaAs (1 0 0) at E "50 eV. V \V
D.H. Cho et al. / Journal of Crystal Growth 201/202 (1999) 610}613
formation there are uncertainties regarding the motion of the interstitials and/or vacancies in the crystallization zone. However, at E "100 eV single crystalline In Ga As "lms were obtained V \V through 250}6003C of ¹ with weak twinning pat terns at 450, 5003C. Although the di!erence between the results of 50 and 100 eV of E is not understood, it may be related to the thermal enhancement of atomic mobility and the subsequent modi"cation of growth kinetics. In the FIB growth mechanism, In segregation and evaporation on surface at high ¹ are not clear. The e!ect of E on crystallization showed that below 200 eV at 6003C, crystallization is independent of the incident ion energy, in agreement with our previous experimental work [2]. Thus, it may be concluded that the "lm quality of the selectively grown In Ga As on GaAs (1 0 0) depends on ¹ . V \V 4. Conclusion MLSE of In Ga As using a low-energy V \V In Ga -FIB and an As molecular beam was studied for the "rst time. In this experiment, we have been able to grow In Ga As epitaxial "lms V \V on GaAs using this growth technology. In composition on the epilayers showed that as the ¹ was * '1 raised, In composition of the "lms increased. The crystal quality of the selectively grown In Ga As V \V
613
"lms was found to depend upon ¹ , not to E below E of 200 eV. Single crystal In Ga As "lms were V \V obtained above ¹ of 5503C at E "50 eV and below E of 200 eV at ¹ "6003C. Acknowledgements The authors would like to thank Professor T. Nishinaga and Dr. A. Yamashiki of Tokyo University for the k-RHEED measurement. This work was partially supported by the MURATA Academic Promotion Foundation.
References [1] I. Hayashi, Japan J. Appl. Phys. 32 (1993) 266. [2] K. Pak, I. Saitoh, N. Ohshima, H. Yonezu, J. Crystal Growth 140 (1994) 244. [3] I. Saitoh, K. Pak, Y. Inoue, N. Ohshima, H. Yonezu, J. Crystal Growth 146 (1995) 359. [4] R.L. Seliger, R.L. Kubena, V. Wang, Japan J. Appl. Phys. 21 (Suppl. 1) (1982) 3. [5] J. Melngailis, J. Vac. Sci. Technol. B 5 (1987) 469. [6] D.S. Swatik, C.D. Hendricks, AIAA J. 6 (1968) 1596. [7] V.K. Medvedev, V.I. Chernyi, N.N. Popovich, J. Vac. Sci. Technol. B 11 (2) (1993) 523. [8] L.W. Swanson, D.R. Kingham, Appl. Phys. A 41 (1986) 223. [9] O. Vancauwenberghe, N. Herbots, O.C. Hellman, J. Vac. Sci. Technol. B 9 (4) (1991) 2027.