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Metal-organic chemical vapor deposition growth of GaN
MATERIALS SCIENCE & ENGINEERING ELSEVIER
Materials Science and Engineering B29 (1995) 58-60
B
Metal-organic chemical vapor deposition growth of GaN Da-cheng LW', Du Wang a, Xiaohui Wang ~, Xianglin Liu ~',Jianrong Dong', Weibin Gao b, Chengji Li ~', Yunyan Lib ~'Laborato~y of Semiconductor Material Science, Institute of Semiconductors, Chinese Academy of Sciences, Beifing 100083, People's Republic of China ~'lnstitute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People ~"Rel)ublic of China
Abslract Single-crystal GaN films have been deposited on (01i 2)sapphire substrates using trimethylgallium (TMGa) and NI-I~ as sources. The morphological, crystalline, electrical and optical characterizations of GaN film are investigated. The carrier concentration of undoped GaN increases with decreasing input NH3-to-TMGa molar flow ratio.
Keywords:Gallium qitride: Epitaxy of thin films: MOCVD: Chemical vapor deposition
1. Introduction G a N is a promising material for fabricating bluelight optoelectronical devices, especially light-emitting diodes, laser diodes and detectors because it has a large direct band gap energy of 3.39 eV. Several methods, such as hydride chemical vapor deposition [1~, metal-organic chemical vapor deposition ( M O C V D ) [2] and molecular beam epitaxy [3], have been employed to grow GaN. M O C V D has, by far, proven its suitability for the epitaxial growth of high quality G a N [4,5]. Because of the lack of an ideal substrate, G a N has been usually grown on sapphire despite the fact that there is large mismatch of lattice constants and thermal expansion coefficients. Sapphire has a high thermal stability and can be obtained with high quality and large size at low cost. T h e r e f o r e it is difficult to obtain a G a N film of good crystalline quality. Moreover, the high background electron concentration, usually attributed to the high density of nitrogen vacancies, and the difficulty of achieving p-type material hinder the progress of GaN-related device research. Recently, with a thin buffer layer of A1N [5] or GaN [4j, the electrical and optical properties as well as the crystalline quality of G a N grown by M O C V D have been improved. Khan et al. [6] found that the electron concentration of GaN grown at 1050°C was lower than that of GaN grown at 775 °C. This result contra-
dicts the nitrogen vacancy theory. We report our preliminary results on the growth and characterization of GaN epitaxial layers on sapphire grown by atmospheric pressure M O C V D . T h e effect of the input NH~to-trimethylgallium (TMGa) molar flow ratio on the carrier concentration of GaN is investigated.
2. Experimental procedure GaN thin films were grown in a horizontal M O C V D reactor. T M G a and ammonia (NH3) were used as sources. T h e T M G a was utilized by bubbling hydrogen through the liquid; then they were diluted by another flow of H 2 ( 4 5 0 0 - 1 0 0 0 standard cm ~ min ~). T h e flow rate of NH~ was varied from 500 to 4500 standard cm ~ min ~. T h e sources were mixed at the entrance of the reactor in order to suppress the parasitic reaction. T h e (0112)-oriented sapphire was first degreased with organic solutions and etched in a hot H : S O 4 : H s P O 4 = 3 : I mixture; it was then rinsed in deionized water and dried naturally before loading. Prior to growth, the substrate was heated to 1150 °C in an H~ stream for I 0 rain. T h e typical growth temperature is 1050°C. Several analytical techniques were employed to characterize the grown layers. T h e surface morphology was observed by Nomarski interference contrast microscopy and scanning electron microscopy
~)921-5107/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved A\S'I)I0921-5107(94)04036-2
D.-A. Lu et al.
/
Materials Science and Engineering B29 (1995) 58-60
(SEM). The crystalline quality was measured by X-ray diffraction (XRD) and double-crystal X-ray diffraction (DXRD). The carrier concentration and mobility were characterized by the Van der Pauw technique. Optical absorption measurements were performed with a double-beam UV-visible spectrophotometer. Cathodoluminescence was investigated using a refitted electron probe microanalyzer.
3. Results and discussion
Fig. 1 is an SEM photograph that shows threedimensional island growth of GaN on (0112) sapphire, which frequently appears on large mismatched substrates. First, island-like GaN nucleation deposits on the (0112) sapphire; then the density and dimension of these islands increases as shown in Fig. l(a). Finally the islands coalesce together, exhibiting a rough surface with ridge-like facets, as shown in Fig. l(b). The (2 i ]0) plane of GaN is parallel to the (01i2) plane of sapphire. The facets are parallel to the {01]0} plane. The ridge is in the (00()1'~ direction. Similar surface
59
morphology was observed by Sasaki and Zembutsu [7] for GaN grown on (01 ]2) sapphire. Hiramatsu et al. [8] reported a similar mode of growth of GaN grown on (0001 ) sapphire. The XRD from (2110) diffraction of GaN grown on (0112) sapphire is shown in Fig. 2. All other peaks in Fig. 2 are attributed to the substrate. The wide full width at half-maximum (FWHM) of the GaN peak indicates the high density of crystalline imperfections in the epitaxial layer. It has been reported that crystalline imperfections were still observed by transmission electron microscopy even when an A1N buffer layer was used [8]. We made further measurements of the same sample in Fig. 2 by DXRD. The F W H M of the (2i10) peak of the DXRD rocking curve is 24 min. The narrowest F W H M obtained by us is 16 min. Room-temperature optical transmission measurements on undoped GaN revealed a very sharp absorption edge, as shown in Fig. 3. The absorption edge is the energy of the band gap. The cathodoluminescence is a blue-violet color to the naked eye at room temperature. In Fig. 4, a near-band gap emission is clearly seen. The main peak is at 360 nm. Another peak at 420 nm and a broad peak around 570 nm which may be due to deep-level impurities and/or lattice defects are also observed. The Hall electrical data at room temperature show that the unintentionally doped GaN films grown at 1050°C demonstrate n-type conduction with a mobility of 50-70 cm 2 V- ~ s ~. The n-type conduction is generally attributed to nitrogen vacancies [9]. The
GaN (2110) cc - AI203 (02"24)
r
El --
I c¢- A I 2 0 3 ( 0 1 1 2 )
~li
I !
[I~ 10
20
~ AI2O3(03~6) 30
40
e (Degree)
Fig. 2. X R D profile obtained from the GaN film growtn on
Fig. 1. Scanning electron micrographs of GaN layers.
(0112) sapphire.
60
D.-A. Lu et al. GaN/c~ -
i
AI203
ii •
/
Materials Science and Engineering B29 (1995) 58-00
(01 ]2)-
/
/
/
¸•
•f
/ /
/
/
/
3000
4000 5000 Wavelength (/~)
6000
Fig. 3. Room-temperature optical transmission spectrum for the GaN layer.
2.0 i
Energy (eV) 2.5 3.0
3.5
i
4.0
partial pressure and decomposition ratio of N H s result in a higher concentration of nitrogen vacancies and therefore a higher electron concentration. T h e GaN that we obtained appears to have a mosaic structure by XRD. T h e high background carrier concentration in our samples may be partially due to poor crystalline quality as reported by Akasaki et al. [5]. T h e r e f o r e both nitrogen vacancies and p o o r crystalline completeness contribute to the high electron concentration in our samples. Further study of the origin of the high electron concentration in undoped GaN is under way. In summary, the heteroepitaxial growth of G a N was achieved on (0112) sapphire by M O C V D . T h e morphological, crystalline, optical and electrical properties were investigated. T h e carrier concentration of an undoped GaN epitaxial layer was a function of the input NH3-to-TMGa molar flow ratio.
Acknowledgments
i
GaN 80k
T h e authors would like to thank Lanying Lin and Qimimg Wang, Chinese Academy of Sciences, for support in this work, Professor Zhanguo Wang for valuable discussions, and Dr. Wannian Wang and Xueming Xue for their electrical and optical measurements.
g >. .O
E
E
References
d
k
I
680
600
520 440 Wavelength (nm)
360
280
Fig. 4. Cathodoluminescence spectrum of GaN layer on the (012) sapphire at 80 K.
concentration of nitrogen vacancies is influenced by the partial pressure of nitrogen and the growth temperature. While the input N H s - t o - T M G a molar flow ratio varies from 9230 to 990 and the T M G a molar flow rate and total flow rate are kept constant, the electron concentration increases from 1 x 10 ]9 to 1 x 102o cm -~. This may be interpreted as follows: the lower
[1] H.R Maraska and J.J. Tetjen, Appl. Phys. Lett., 16 (1969) 337. [2] S. Nakamura, T. Mukai and M. Senoh, Appl. Phys. Lett., 64 (1994) 1687. [3] M.J. Paisley, Z. Sitar, J.B. Posthill and R.E Davis, J. Vac. Sci. Technol. A, 7(1989) 701. [4] S. Nakamura, Jpn. Z AppL Phys., 30 (1991) L1705. [5] I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu and N. Sawaki, J. Cryst, Growth, 98 (1989) 209. [6] M.A. Khan, J.N. Kuznia, J.M. Van Hove, D.T. Olson, S. Krishnankutty and R.M. Kolbas, Appl. Phys. Lett., 58 ( 1991 ) 526. [7] T. Sasaki and S. Zembutsu, J. Appl. Phys., 61 (1987) 2533. [8] K. Hiramatsu, S. Itoh, H. Amano, I. Akasaki, N. Kuwano, T. Shiraishi and K. Oki, J. Crvst. Growth, 115 ( 1991 ) 628. [9] M. llezems and H.C. Montgomery, J. Phys. Chem. Solids, 34 (1973)885.
Materials Science and Engineering B29 (1995) 58-60
B
Metal-organic chemical vapor deposition growth of GaN Da-cheng LW', Du Wang a, Xiaohui Wang ~, Xianglin Liu ~',Jianrong Dong', Weibin Gao b, Chengji Li ~', Yunyan Lib ~'Laborato~y of Semiconductor Material Science, Institute of Semiconductors, Chinese Academy of Sciences, Beifing 100083, People's Republic of China ~'lnstitute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People ~"Rel)ublic of China
Abslract Single-crystal GaN films have been deposited on (01i 2)sapphire substrates using trimethylgallium (TMGa) and NI-I~ as sources. The morphological, crystalline, electrical and optical characterizations of GaN film are investigated. The carrier concentration of undoped GaN increases with decreasing input NH3-to-TMGa molar flow ratio.
Keywords:Gallium qitride: Epitaxy of thin films: MOCVD: Chemical vapor deposition
1. Introduction G a N is a promising material for fabricating bluelight optoelectronical devices, especially light-emitting diodes, laser diodes and detectors because it has a large direct band gap energy of 3.39 eV. Several methods, such as hydride chemical vapor deposition [1~, metal-organic chemical vapor deposition ( M O C V D ) [2] and molecular beam epitaxy [3], have been employed to grow GaN. M O C V D has, by far, proven its suitability for the epitaxial growth of high quality G a N [4,5]. Because of the lack of an ideal substrate, G a N has been usually grown on sapphire despite the fact that there is large mismatch of lattice constants and thermal expansion coefficients. Sapphire has a high thermal stability and can be obtained with high quality and large size at low cost. T h e r e f o r e it is difficult to obtain a G a N film of good crystalline quality. Moreover, the high background electron concentration, usually attributed to the high density of nitrogen vacancies, and the difficulty of achieving p-type material hinder the progress of GaN-related device research. Recently, with a thin buffer layer of A1N [5] or GaN [4j, the electrical and optical properties as well as the crystalline quality of G a N grown by M O C V D have been improved. Khan et al. [6] found that the electron concentration of GaN grown at 1050°C was lower than that of GaN grown at 775 °C. This result contra-
dicts the nitrogen vacancy theory. We report our preliminary results on the growth and characterization of GaN epitaxial layers on sapphire grown by atmospheric pressure M O C V D . T h e effect of the input NH~to-trimethylgallium (TMGa) molar flow ratio on the carrier concentration of GaN is investigated.
2. Experimental procedure GaN thin films were grown in a horizontal M O C V D reactor. T M G a and ammonia (NH3) were used as sources. T h e T M G a was utilized by bubbling hydrogen through the liquid; then they were diluted by another flow of H 2 ( 4 5 0 0 - 1 0 0 0 standard cm ~ min ~). T h e flow rate of NH~ was varied from 500 to 4500 standard cm ~ min ~. T h e sources were mixed at the entrance of the reactor in order to suppress the parasitic reaction. T h e (0112)-oriented sapphire was first degreased with organic solutions and etched in a hot H : S O 4 : H s P O 4 = 3 : I mixture; it was then rinsed in deionized water and dried naturally before loading. Prior to growth, the substrate was heated to 1150 °C in an H~ stream for I 0 rain. T h e typical growth temperature is 1050°C. Several analytical techniques were employed to characterize the grown layers. T h e surface morphology was observed by Nomarski interference contrast microscopy and scanning electron microscopy
~)921-5107/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved A\S'I)I0921-5107(94)04036-2
D.-A. Lu et al.
/
Materials Science and Engineering B29 (1995) 58-60
(SEM). The crystalline quality was measured by X-ray diffraction (XRD) and double-crystal X-ray diffraction (DXRD). The carrier concentration and mobility were characterized by the Van der Pauw technique. Optical absorption measurements were performed with a double-beam UV-visible spectrophotometer. Cathodoluminescence was investigated using a refitted electron probe microanalyzer.
3. Results and discussion
Fig. 1 is an SEM photograph that shows threedimensional island growth of GaN on (0112) sapphire, which frequently appears on large mismatched substrates. First, island-like GaN nucleation deposits on the (0112) sapphire; then the density and dimension of these islands increases as shown in Fig. l(a). Finally the islands coalesce together, exhibiting a rough surface with ridge-like facets, as shown in Fig. l(b). The (2 i ]0) plane of GaN is parallel to the (01i2) plane of sapphire. The facets are parallel to the {01]0} plane. The ridge is in the (00()1'~ direction. Similar surface
59
morphology was observed by Sasaki and Zembutsu [7] for GaN grown on (01 ]2) sapphire. Hiramatsu et al. [8] reported a similar mode of growth of GaN grown on (0001 ) sapphire. The XRD from (2110) diffraction of GaN grown on (0112) sapphire is shown in Fig. 2. All other peaks in Fig. 2 are attributed to the substrate. The wide full width at half-maximum (FWHM) of the GaN peak indicates the high density of crystalline imperfections in the epitaxial layer. It has been reported that crystalline imperfections were still observed by transmission electron microscopy even when an A1N buffer layer was used [8]. We made further measurements of the same sample in Fig. 2 by DXRD. The F W H M of the (2i10) peak of the DXRD rocking curve is 24 min. The narrowest F W H M obtained by us is 16 min. Room-temperature optical transmission measurements on undoped GaN revealed a very sharp absorption edge, as shown in Fig. 3. The absorption edge is the energy of the band gap. The cathodoluminescence is a blue-violet color to the naked eye at room temperature. In Fig. 4, a near-band gap emission is clearly seen. The main peak is at 360 nm. Another peak at 420 nm and a broad peak around 570 nm which may be due to deep-level impurities and/or lattice defects are also observed. The Hall electrical data at room temperature show that the unintentionally doped GaN films grown at 1050°C demonstrate n-type conduction with a mobility of 50-70 cm 2 V- ~ s ~. The n-type conduction is generally attributed to nitrogen vacancies [9]. The
GaN (2110) cc - AI203 (02"24)
r
El --
I c¢- A I 2 0 3 ( 0 1 1 2 )
~li
I !
[I~ 10
20
~ AI2O3(03~6) 30
40
e (Degree)
Fig. 2. X R D profile obtained from the GaN film growtn on
Fig. 1. Scanning electron micrographs of GaN layers.
(0112) sapphire.
60
D.-A. Lu et al. GaN/c~ -
i
AI203
ii •
/
Materials Science and Engineering B29 (1995) 58-00
(01 ]2)-
/
/
/
¸•
•f
/ /
/
/
/
3000
4000 5000 Wavelength (/~)
6000
Fig. 3. Room-temperature optical transmission spectrum for the GaN layer.
2.0 i
Energy (eV) 2.5 3.0
3.5
i
4.0
partial pressure and decomposition ratio of N H s result in a higher concentration of nitrogen vacancies and therefore a higher electron concentration. T h e GaN that we obtained appears to have a mosaic structure by XRD. T h e high background carrier concentration in our samples may be partially due to poor crystalline quality as reported by Akasaki et al. [5]. T h e r e f o r e both nitrogen vacancies and p o o r crystalline completeness contribute to the high electron concentration in our samples. Further study of the origin of the high electron concentration in undoped GaN is under way. In summary, the heteroepitaxial growth of G a N was achieved on (0112) sapphire by M O C V D . T h e morphological, crystalline, optical and electrical properties were investigated. T h e carrier concentration of an undoped GaN epitaxial layer was a function of the input NH3-to-TMGa molar flow ratio.
Acknowledgments
i
GaN 80k
T h e authors would like to thank Lanying Lin and Qimimg Wang, Chinese Academy of Sciences, for support in this work, Professor Zhanguo Wang for valuable discussions, and Dr. Wannian Wang and Xueming Xue for their electrical and optical measurements.
g >. .O
E
E
References
d
k
I
680
600
520 440 Wavelength (nm)
360
280
Fig. 4. Cathodoluminescence spectrum of GaN layer on the (012) sapphire at 80 K.
concentration of nitrogen vacancies is influenced by the partial pressure of nitrogen and the growth temperature. While the input N H s - t o - T M G a molar flow ratio varies from 9230 to 990 and the T M G a molar flow rate and total flow rate are kept constant, the electron concentration increases from 1 x 10 ]9 to 1 x 102o cm -~. This may be interpreted as follows: the lower
[1] H.R Maraska and J.J. Tetjen, Appl. Phys. Lett., 16 (1969) 337. [2] S. Nakamura, T. Mukai and M. Senoh, Appl. Phys. Lett., 64 (1994) 1687. [3] M.J. Paisley, Z. Sitar, J.B. Posthill and R.E Davis, J. Vac. Sci. Technol. A, 7(1989) 701. [4] S. Nakamura, Jpn. Z AppL Phys., 30 (1991) L1705. [5] I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu and N. Sawaki, J. Cryst, Growth, 98 (1989) 209. [6] M.A. Khan, J.N. Kuznia, J.M. Van Hove, D.T. Olson, S. Krishnankutty and R.M. Kolbas, Appl. Phys. Lett., 58 ( 1991 ) 526. [7] T. Sasaki and S. Zembutsu, J. Appl. Phys., 61 (1987) 2533. [8] K. Hiramatsu, S. Itoh, H. Amano, I. Akasaki, N. Kuwano, T. Shiraishi and K. Oki, J. Crvst. Growth, 115 ( 1991 ) 628. [9] M. llezems and H.C. Montgomery, J. Phys. Chem. Solids, 34 (1973)885.