- Email: [email protected]
sapphire interface
Physica B 324 (2002) 223–228
Thermal stability of GaN epitaxial layer and GaN/sapphire interface M. Senthil Kumara,*, G. Soniaa, V. Ramakrishnanb, R. Dhanasekarana, J. Kumara b
a Crystal Growth Centre, Anna University, Chennai 600025, India Laser Laboratory, Department of Physics, Kamaraj University, Madurai, India
Received 10 June 2002; accepted 19 June 2002
Abstract Thermal stabilities of GaN epitaxial layer and of GaN/sapphire interface has been investigated using Raman scattering, photoluminescence (PL), optical transmission and SEM analysis. GaN epitaxial layers were annealed up to temperatures as high as 11001C in nitrogen ambient for a period of 20 min. Raman scattering identifies two new additional vibrational modes in GaN between E2 and Al(LO) modes for the samples annealed beyond 9001C. Annealing up to the temperature of 9001C enhances the PL intensity and it is decreased drastically for the high-temperature annealed samples. Optical transmission and SEM studies exhibit a clear indication of reaction between GaN/sapphire interface at higher temperatures above 10001C. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 78.30.Fs; 78.55.Cr; 78.40.Fy Keywords: Gallium nitride; Interface; Raman scattering; Photoluminescence; Optical transmission
1. Introduction The family of group III nitrides has made the dream of realising high-efficiency blue/violet light emitting diodes (LEDs) and laser diodes (LDs) a reality in practice [1]. Due to their wide band gap and high thermal conductivity, these materials have also been employed for high-power and hightemperature applications [2]. Among group III nitrides, gallium nitride (GaN) received considerable attention because of its significant band gap *Corresponding author. Tel./fax: +91-44-235-2774. E-mail address: [email protected] (M. Senthil Kumar).
value of 3.43 eV at room temperature. The establishment of p-type conductivity in GaN has been a key factor behind all these flourishing achievements in a short period [3,4]. The structural, optical and electrical properties are critical for device performance and of course all these properties are temperature dependent [5,6]. High-temperature treatment of nitrides is an essential processing part in GaN device fabrication such as to activate Mg acceptors, to achieve p-GaN and to perform very good ohmic contacts for the GaN devices. There are several reports of using thermal annealing at higher temperatures up to 11501C to activate Mg dopant in GaN layer and devices [7–9]. Also, the thermal-annealing processing of
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 3 1 6 - 9
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M. Senthil Kumar et al. / Physica B 324 (2002) 223–228
GaN is one of the successful methods to reduce the defect density [6] and enhance the photoluminescence (PL) signal [10]. Therefore, the study of thermal stability of GaN due to high-temperature annealing is of great interest in recent years. Raman scattering is a non-invasive and nondestructive technique for probing crystalline disorder in epitaxial layers. Kuball et al. have investigated the thermal stability of GaN epilayers using Raman scattering [11]. PL technique has also been exclusively used to study the thermal behaviour of GaN layers. Lin et al. have reported that the GaN PL intensity degrades when annealing temperature reaches 9001C [4]. Considerable improvements in surface morphology and PL intensity have been achieved by high-temperature rapid thermal annealing of GaN [5]. There are several contradictory reports on the critical temperature below which the GaN epitaxial layers are stable. However, it has been identified from most of the reports that GaN properties are affected when the processing temperature crosses 9001C. In this report, the thermal behaviours of metal organic chemical vapour deposition (MOCVD)- and molecular beam epitaxy (MBE)grown GaN epitaxial layers and GaN/sapphire interface are discussed using Raman scattering, PL, optical transmission and scanning electron microscopy (SEM).
2. Experimental Unintentionally doped n-GaN epitaxial layers of 2 mm thickness grown on sapphire (0 0 0 1) substrates by MBE and MOCVD techniques were taken for the experiment. The measured carrier concentration and mobility of MBE-grown GaN layers were 1.2 1017 cm3 and 153 cm2/V s respectively and the respective values of MOCVD grown layers were 1–2 1017 cm2 and 550 cm2/V s. The samples were properly cleaned by degreasing and etching them before annealing. All samples were annealed in nitrogen ambient in a conventional heating furnace for a period of 20 min for various temperatures. The annealing temperature was raised up to 11001C. The
characterisation studies were carried out after cooling the samples to room temperature. Raman spectra of GaN layers were recorded at 300 K in the backscattering geometry using a Spectra Physics (2020-045) Ar ion laser (l¼ 496 nm). The laser power was maintained at 75 mW. The output of the picometer was interfaced with a computer through a data acquisition ADD on CARD and the Raman data were collected in the computer. The spectral resolution is 1–3 cm1. The optical absorption spectra of as-grown and high-temperature annealed GaN samples were recorded using UV-VIS-NIR spectrometer model UV-3101 PC (Shimadazu, Japan) in the wavelength range 200–900 nm. The PL spectra were recorded at room temperature using He–Cd laser of 325 nm excitation wavelength with a maximum input power of 13 mW. The GaN/ sapphire interface annealed at various temperatures was analysed through scanning electron microscope (Leica Stereoscan 440).
3. Results 3.1. Raman scattering Room temperature Raman spectra of MBEgrown GaN layers after annealing at various temperatures of 20 min duration are shown in Fig. 1. An as-grown GaN layer shows two Raman modes E2 (high) and Al(LO) at frequencies 568 and 734 cm1 respectively along with sapphire substrate peaks. Raman spectrum was recorded on sapphire substrate also in order to identify the sapphire Raman modes. Two peaks at 575 and 749 cm1 corresponding to Eg Raman modes of sapphire were observed as shown in Fig. 1 (lowest spectrum). The Raman modes of GaN layers remain unchanged for annealing temperatures up to 9001C while the peak intensity of sapphire Raman modes increases subsequently with increase in annealing temperature up to 10001C. Heat treatments at temperatures higher than 9001C result in the appearance of two distinct additional peaks at frequencies 629 and 654 cm1 in the spectrum. There is no considerable change in E2 (high) and A1(LO) Raman modes of GaN
M. Senthil Kumar et al. / Physica B 324 (2002) 223–228
225
900 ˚C
PL intensity (a.u.)
Un-annealed
1000 ˚C
1100 ˚C
3.0
3.1
3.2
3.3
3.4
3.5
Energy (eV) Fig. 2. PL spectra of thermally annealed MOCVD-grown GaN epitaxial layers. Fig. 1. Raman spectra of thermally annealed MBE-grown GaN epitaxial layers.
3.3. Optical transmission layers observed for annealing temperatures up to 10001C while their full-width at half-maximum increases above this annealing temperature. The GaN layer remained visually mirror-like up to 10001C, but which may have microscopic crystalline disorder. No shift in Raman modes of either GaN layer or sapphire substrate was observed in all cases beyond the experimental resolution. 3.2. Photoluminescence The PL spectrum of MOCVD GaN layer (before and after annealing) recorded at room temperature is shown in Fig. 2. Emission of band to band transition has been observed at an energy of 3.42 eV, which is exactly the band gap of GaN. Annealing improves the PL intensity of the as-grown GaN layer. However, further annealing beyond the temperature of 9001C decreases the PL intensity. The emission peak almost disappears for 11001C annealed GaN samples.
The optical transmission spectrum of the MBEgrown GaN layer along with thermal treated samples at 10001C and 11001C is shown Fig. 3. A very sharp absorption edge was observed in all cases at a wavelength of 363 nm. The well-defined interference pattern observed in the transmission spectrum is due to the difference in refractive index between GaN layer and sapphire substrate, which indicates the formation of a very good interface between the two materials. For the sample annealed at 11001C, however, the interference peaks are severely affected in the transmission spectra and also the percentage of transmission decreases for this sample. 3.4. Scanning electron microscopy SEM photographs of MOCVD-grown GaN/ sapphire interface of as-grown and annealed (10001C, 10501C and 11001C) samples are shown in Fig. 4. The as-grown sample shows a clear interface while the sample annealed at 11001C shows a severely damaged GaN/sapphire interface.
M. Senthil Kumar et al. / Physica B 324 (2002) 223–228
226 100 1000 ˚C
80
Transmission (%)
Un-annealed 60
1100 ˚C
40
20
0
400
600
800
Wavelength (nm)
Fig. 3. Optical transmission spectra of thermally annealed MBE-grown GaN epitaxial layers.
Fig. 4. SEM pictures of MOCVD-grown GaN/sapphire interface: (a) as-grown, (b) 10001C, (c) 10501C and (d) 11001C annealed.
4. Discussion Two regions of thermal stability/degradation of GaN layers have been identified as (1) below and (2) above 10001C. Below 10001C, no structural modification of disorder of the GaN layer is observed from Raman measurements, which is the normally used range of temperatures for hightemperature processing of GaN devices. Barker et al. have observed forbidden Raman modes of sapphire at frequencies 633, 652 and 770 cm1 on
surface-damaged sapphire [12]. The forbidden mode of sapphire at 770 cm1 has been observed for all samples. It was also observed for the virgin sapphire annealed at 11001C but not for unannealed sapphire. The activation of this sapphire mode annealed above 10001C indicates the modification of the GaN/sapphire interface at these temperatures. The Raman spectrum of GaN layer annealed at temperatures higher than 10001C also exhibits two distinct peaks at 629 and 654 cm1. These Raman modes may arise due to lattice disorder induced Raman scattering from the GaN layer. It is also possible that these peaks might arise from sapphire since it has forbidden modes at these frequency ranges. However, Kuball et al. have clearly reported that they mainly arise from the GaN/sapphire interface region, suggesting a larger contribution from the forbidden sapphire modes [10]. The full-width at half-maximum and the area under the peak of E2 Raman mode of GaN layer for annealing above 10001C increases due to the degradation of crystalline quality and also of layer surface. Raman experiments were performed on the MOCVD-grown GaN layers also. The observed changes in their Raman modes are very similar to the results obtained for MBE-grown samples. But, the appearance of two intermediate peaks with good intensity was observed for the annealing temperature of 10001C whereas the same peaks were observed for MBE-grown samples with less intensity. The GaN layer surface was visually smooth up to 10001C; however, the pitting of layer surface could be observed above this temperature. These results conclude that annealing of GaN layer at higher temperatures affects both the crystalline/surface quality of GaN epitaxial layer as well as GaN/sapphire interface. No considerable change in the peak position of PL spectra with respect to annealing temperature could be observed. The only change obtained is the variation in PL intensity. Annealing of as-grown GaN layers up to 9001C increases the PL intensity considerably. Normally, PL intensity is sensitive to surface recombination rate at both film surface and film/substrate interfaces and also carrier diffusion length [4]. Therefore, the increase in band-edge luminescence intensity up to 9001C may
M. Senthil Kumar et al. / Physica B 324 (2002) 223–228
be due to the improvements in surface quality of GaN layer upon annealing. At the same time, severe degradation in the PL intensity above the annealing temperature 10001C may be considered as massive damage of GaN layer surface for high-temperature annealing. The strong reaction between GaN/sapphire at such elevated temperatures is also responsible for the decrease in PL signal. No change in transmission spectra for the samples annealed up to 10001C indicates that any degradation at the GaN/sapphire interface has not taken place during annealing at these temperatures. The transparency of the GaN layer decreases when annealing temperature is increased above 10001C, which is even visible to the human eye and is reflected in the recorded transmission spectrum also. The disappearance of the interference pattern in the spectrum for samples annealed above 10001C clearly indicates the heavy reaction at the GaN/sapphire interface at such high temperatures. Annealing even at high temperatures does not have any effect on the absorption edge of GaN layers. The same nature of transparency and interface stability has been observed for MOCVD-grown samples also except that the decrease in percentage of optical transparency is more when compared to MBE-grown GaN layers. A clear interface between the GaN and sapphire substrate was observed for the as-grown sample from SEM analysis. Also, there is no reaction at the interface for samples annealed up to 10001C. It is clearly seen from SEM pictures that a reaction takes place at the GaN/sapphire interface when annealed above 10001C and the interface is heavily damaged once the processing temperature reaches 11001C. The SEM results substantiate the results obtained from optical transmission studies.
5. Conclusions Thermal stability of MOCVD- as well as MBEgrown GaN epitaxial layers has been investigated using Raman scattering, PL and optical transmission techniques. Raman indicates disorder in
227
crystalline quality for GaN layers treated beyond 10001C and also a reaction between GaN/sapphire interface at the elevated temperatures. PL intensity increases for samples annealed up to 9001C due to the improvement in surface quality of the GaN layer. And it is severely affected for the hightemperature (above 10001C) treated GaN layers, which may possibly be due to the degradation of the surface/crystalline quality of the GaN layer and also the GaN/sapphire interface. Optical transmission and SEM studies confirm the reaction between GaN layer and sapphire substrate at the interface when annealing temperature crosses 10001C. Therefore, annealing up to 9001C does not show any degradation in GaN layer properties in all the cases; however, it is clearly evident that a heavy reaction takes place at the GaN/sapphire interface for the higher temperature annealing. It has been observed through all characterisation techniques that MBE-grown GaN layers taken for the experiment are somewhat thermally stable than MOCVD-grown layers.
Acknowledgements The authors are extremely thankful to Dr. K. Jeganathan for the kind help in recording PL spectra. Mr. M. Senthil Kumar acknowledges the Council of Scientific and Industrial Research (CSIR), Govt. of India for the award of Senior Research Fellowship. The authors are also thankful to the Department of Science and Technology for financial assistance.
References [1] S. Nakamura, Frasol, the Blue Laser Diode, Springer, Berlin, 1999. [2] H. Morkoc, S. Strite, G.B. Gao, M.E. Lin, B. Sverdlov, M. Burns, J. Appl. Phys. 76 (1994) 1363. [3] S. Nakamura, M. Senoh, T. Mukai, Jpn. J. Appl. Phys. 30 (1991) 1708. [4] M.E. Lin, G. Xue, G.L. Zhou, J.E. Greene, H. Morkoc, Appl. Phys. Lett. 63 (1993) 932. [5] J.C. Zolper, M. Hagerott Crawford, A.J. Howard, Appl. Phys. Lett. 68 (1996) 200.
228
M. Senthil Kumar et al. / Physica B 324 (2002) 223–228
[6] H. Siegle, G. Kaczmarczyk, L. Filippidis, A.P. Litvinchuk, A. Hoffmann, C. Thomsen, Phys. Rev. B 55 (1997) 7000. [7] S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, Jpn. J. Appl. Phys. 31 (1992) L139. [8] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 28 (1989) L2112. [9] M.A. Khan, Q. Chen, R.A. Skogman, J.N. Kuznia, Appl. Phys. Lett. 66 (1995) 2040.
[10] J. Hong, J.W. Lee, J.D. MacKenzie, S.M. Donovan, C.R. Abernathy, S.J. Pearton, J.C. Zolper, Semicond. Sci. Technol. 12 (1997) 1310. [11] M. Kuball, F. Demangeot, J. Frandon, M.A. Renucci, J. Massies, N. Grandjean, R.L. Aulombard, O. Briot, Appl. Phys. Lett. 73 (1998) 960. [12] A.S. Barker, Phys. Rev. 132 (1963) 1474.
Thermal stability of GaN epitaxial layer and GaN/sapphire interface M. Senthil Kumara,*, G. Soniaa, V. Ramakrishnanb, R. Dhanasekarana, J. Kumara b
a Crystal Growth Centre, Anna University, Chennai 600025, India Laser Laboratory, Department of Physics, Kamaraj University, Madurai, India
Received 10 June 2002; accepted 19 June 2002
Abstract Thermal stabilities of GaN epitaxial layer and of GaN/sapphire interface has been investigated using Raman scattering, photoluminescence (PL), optical transmission and SEM analysis. GaN epitaxial layers were annealed up to temperatures as high as 11001C in nitrogen ambient for a period of 20 min. Raman scattering identifies two new additional vibrational modes in GaN between E2 and Al(LO) modes for the samples annealed beyond 9001C. Annealing up to the temperature of 9001C enhances the PL intensity and it is decreased drastically for the high-temperature annealed samples. Optical transmission and SEM studies exhibit a clear indication of reaction between GaN/sapphire interface at higher temperatures above 10001C. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 78.30.Fs; 78.55.Cr; 78.40.Fy Keywords: Gallium nitride; Interface; Raman scattering; Photoluminescence; Optical transmission
1. Introduction The family of group III nitrides has made the dream of realising high-efficiency blue/violet light emitting diodes (LEDs) and laser diodes (LDs) a reality in practice [1]. Due to their wide band gap and high thermal conductivity, these materials have also been employed for high-power and hightemperature applications [2]. Among group III nitrides, gallium nitride (GaN) received considerable attention because of its significant band gap *Corresponding author. Tel./fax: +91-44-235-2774. E-mail address: [email protected] (M. Senthil Kumar).
value of 3.43 eV at room temperature. The establishment of p-type conductivity in GaN has been a key factor behind all these flourishing achievements in a short period [3,4]. The structural, optical and electrical properties are critical for device performance and of course all these properties are temperature dependent [5,6]. High-temperature treatment of nitrides is an essential processing part in GaN device fabrication such as to activate Mg acceptors, to achieve p-GaN and to perform very good ohmic contacts for the GaN devices. There are several reports of using thermal annealing at higher temperatures up to 11501C to activate Mg dopant in GaN layer and devices [7–9]. Also, the thermal-annealing processing of
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 3 1 6 - 9
224
M. Senthil Kumar et al. / Physica B 324 (2002) 223–228
GaN is one of the successful methods to reduce the defect density [6] and enhance the photoluminescence (PL) signal [10]. Therefore, the study of thermal stability of GaN due to high-temperature annealing is of great interest in recent years. Raman scattering is a non-invasive and nondestructive technique for probing crystalline disorder in epitaxial layers. Kuball et al. have investigated the thermal stability of GaN epilayers using Raman scattering [11]. PL technique has also been exclusively used to study the thermal behaviour of GaN layers. Lin et al. have reported that the GaN PL intensity degrades when annealing temperature reaches 9001C [4]. Considerable improvements in surface morphology and PL intensity have been achieved by high-temperature rapid thermal annealing of GaN [5]. There are several contradictory reports on the critical temperature below which the GaN epitaxial layers are stable. However, it has been identified from most of the reports that GaN properties are affected when the processing temperature crosses 9001C. In this report, the thermal behaviours of metal organic chemical vapour deposition (MOCVD)- and molecular beam epitaxy (MBE)grown GaN epitaxial layers and GaN/sapphire interface are discussed using Raman scattering, PL, optical transmission and scanning electron microscopy (SEM).
2. Experimental Unintentionally doped n-GaN epitaxial layers of 2 mm thickness grown on sapphire (0 0 0 1) substrates by MBE and MOCVD techniques were taken for the experiment. The measured carrier concentration and mobility of MBE-grown GaN layers were 1.2 1017 cm3 and 153 cm2/V s respectively and the respective values of MOCVD grown layers were 1–2 1017 cm2 and 550 cm2/V s. The samples were properly cleaned by degreasing and etching them before annealing. All samples were annealed in nitrogen ambient in a conventional heating furnace for a period of 20 min for various temperatures. The annealing temperature was raised up to 11001C. The
characterisation studies were carried out after cooling the samples to room temperature. Raman spectra of GaN layers were recorded at 300 K in the backscattering geometry using a Spectra Physics (2020-045) Ar ion laser (l¼ 496 nm). The laser power was maintained at 75 mW. The output of the picometer was interfaced with a computer through a data acquisition ADD on CARD and the Raman data were collected in the computer. The spectral resolution is 1–3 cm1. The optical absorption spectra of as-grown and high-temperature annealed GaN samples were recorded using UV-VIS-NIR spectrometer model UV-3101 PC (Shimadazu, Japan) in the wavelength range 200–900 nm. The PL spectra were recorded at room temperature using He–Cd laser of 325 nm excitation wavelength with a maximum input power of 13 mW. The GaN/ sapphire interface annealed at various temperatures was analysed through scanning electron microscope (Leica Stereoscan 440).
3. Results 3.1. Raman scattering Room temperature Raman spectra of MBEgrown GaN layers after annealing at various temperatures of 20 min duration are shown in Fig. 1. An as-grown GaN layer shows two Raman modes E2 (high) and Al(LO) at frequencies 568 and 734 cm1 respectively along with sapphire substrate peaks. Raman spectrum was recorded on sapphire substrate also in order to identify the sapphire Raman modes. Two peaks at 575 and 749 cm1 corresponding to Eg Raman modes of sapphire were observed as shown in Fig. 1 (lowest spectrum). The Raman modes of GaN layers remain unchanged for annealing temperatures up to 9001C while the peak intensity of sapphire Raman modes increases subsequently with increase in annealing temperature up to 10001C. Heat treatments at temperatures higher than 9001C result in the appearance of two distinct additional peaks at frequencies 629 and 654 cm1 in the spectrum. There is no considerable change in E2 (high) and A1(LO) Raman modes of GaN
M. Senthil Kumar et al. / Physica B 324 (2002) 223–228
225
900 ˚C
PL intensity (a.u.)
Un-annealed
1000 ˚C
1100 ˚C
3.0
3.1
3.2
3.3
3.4
3.5
Energy (eV) Fig. 2. PL spectra of thermally annealed MOCVD-grown GaN epitaxial layers. Fig. 1. Raman spectra of thermally annealed MBE-grown GaN epitaxial layers.
3.3. Optical transmission layers observed for annealing temperatures up to 10001C while their full-width at half-maximum increases above this annealing temperature. The GaN layer remained visually mirror-like up to 10001C, but which may have microscopic crystalline disorder. No shift in Raman modes of either GaN layer or sapphire substrate was observed in all cases beyond the experimental resolution. 3.2. Photoluminescence The PL spectrum of MOCVD GaN layer (before and after annealing) recorded at room temperature is shown in Fig. 2. Emission of band to band transition has been observed at an energy of 3.42 eV, which is exactly the band gap of GaN. Annealing improves the PL intensity of the as-grown GaN layer. However, further annealing beyond the temperature of 9001C decreases the PL intensity. The emission peak almost disappears for 11001C annealed GaN samples.
The optical transmission spectrum of the MBEgrown GaN layer along with thermal treated samples at 10001C and 11001C is shown Fig. 3. A very sharp absorption edge was observed in all cases at a wavelength of 363 nm. The well-defined interference pattern observed in the transmission spectrum is due to the difference in refractive index between GaN layer and sapphire substrate, which indicates the formation of a very good interface between the two materials. For the sample annealed at 11001C, however, the interference peaks are severely affected in the transmission spectra and also the percentage of transmission decreases for this sample. 3.4. Scanning electron microscopy SEM photographs of MOCVD-grown GaN/ sapphire interface of as-grown and annealed (10001C, 10501C and 11001C) samples are shown in Fig. 4. The as-grown sample shows a clear interface while the sample annealed at 11001C shows a severely damaged GaN/sapphire interface.
M. Senthil Kumar et al. / Physica B 324 (2002) 223–228
226 100 1000 ˚C
80
Transmission (%)
Un-annealed 60
1100 ˚C
40
20
0
400
600
800
Wavelength (nm)
Fig. 3. Optical transmission spectra of thermally annealed MBE-grown GaN epitaxial layers.
Fig. 4. SEM pictures of MOCVD-grown GaN/sapphire interface: (a) as-grown, (b) 10001C, (c) 10501C and (d) 11001C annealed.
4. Discussion Two regions of thermal stability/degradation of GaN layers have been identified as (1) below and (2) above 10001C. Below 10001C, no structural modification of disorder of the GaN layer is observed from Raman measurements, which is the normally used range of temperatures for hightemperature processing of GaN devices. Barker et al. have observed forbidden Raman modes of sapphire at frequencies 633, 652 and 770 cm1 on
surface-damaged sapphire [12]. The forbidden mode of sapphire at 770 cm1 has been observed for all samples. It was also observed for the virgin sapphire annealed at 11001C but not for unannealed sapphire. The activation of this sapphire mode annealed above 10001C indicates the modification of the GaN/sapphire interface at these temperatures. The Raman spectrum of GaN layer annealed at temperatures higher than 10001C also exhibits two distinct peaks at 629 and 654 cm1. These Raman modes may arise due to lattice disorder induced Raman scattering from the GaN layer. It is also possible that these peaks might arise from sapphire since it has forbidden modes at these frequency ranges. However, Kuball et al. have clearly reported that they mainly arise from the GaN/sapphire interface region, suggesting a larger contribution from the forbidden sapphire modes [10]. The full-width at half-maximum and the area under the peak of E2 Raman mode of GaN layer for annealing above 10001C increases due to the degradation of crystalline quality and also of layer surface. Raman experiments were performed on the MOCVD-grown GaN layers also. The observed changes in their Raman modes are very similar to the results obtained for MBE-grown samples. But, the appearance of two intermediate peaks with good intensity was observed for the annealing temperature of 10001C whereas the same peaks were observed for MBE-grown samples with less intensity. The GaN layer surface was visually smooth up to 10001C; however, the pitting of layer surface could be observed above this temperature. These results conclude that annealing of GaN layer at higher temperatures affects both the crystalline/surface quality of GaN epitaxial layer as well as GaN/sapphire interface. No considerable change in the peak position of PL spectra with respect to annealing temperature could be observed. The only change obtained is the variation in PL intensity. Annealing of as-grown GaN layers up to 9001C increases the PL intensity considerably. Normally, PL intensity is sensitive to surface recombination rate at both film surface and film/substrate interfaces and also carrier diffusion length [4]. Therefore, the increase in band-edge luminescence intensity up to 9001C may
M. Senthil Kumar et al. / Physica B 324 (2002) 223–228
be due to the improvements in surface quality of GaN layer upon annealing. At the same time, severe degradation in the PL intensity above the annealing temperature 10001C may be considered as massive damage of GaN layer surface for high-temperature annealing. The strong reaction between GaN/sapphire at such elevated temperatures is also responsible for the decrease in PL signal. No change in transmission spectra for the samples annealed up to 10001C indicates that any degradation at the GaN/sapphire interface has not taken place during annealing at these temperatures. The transparency of the GaN layer decreases when annealing temperature is increased above 10001C, which is even visible to the human eye and is reflected in the recorded transmission spectrum also. The disappearance of the interference pattern in the spectrum for samples annealed above 10001C clearly indicates the heavy reaction at the GaN/sapphire interface at such high temperatures. Annealing even at high temperatures does not have any effect on the absorption edge of GaN layers. The same nature of transparency and interface stability has been observed for MOCVD-grown samples also except that the decrease in percentage of optical transparency is more when compared to MBE-grown GaN layers. A clear interface between the GaN and sapphire substrate was observed for the as-grown sample from SEM analysis. Also, there is no reaction at the interface for samples annealed up to 10001C. It is clearly seen from SEM pictures that a reaction takes place at the GaN/sapphire interface when annealed above 10001C and the interface is heavily damaged once the processing temperature reaches 11001C. The SEM results substantiate the results obtained from optical transmission studies.
5. Conclusions Thermal stability of MOCVD- as well as MBEgrown GaN epitaxial layers has been investigated using Raman scattering, PL and optical transmission techniques. Raman indicates disorder in
227
crystalline quality for GaN layers treated beyond 10001C and also a reaction between GaN/sapphire interface at the elevated temperatures. PL intensity increases for samples annealed up to 9001C due to the improvement in surface quality of the GaN layer. And it is severely affected for the hightemperature (above 10001C) treated GaN layers, which may possibly be due to the degradation of the surface/crystalline quality of the GaN layer and also the GaN/sapphire interface. Optical transmission and SEM studies confirm the reaction between GaN layer and sapphire substrate at the interface when annealing temperature crosses 10001C. Therefore, annealing up to 9001C does not show any degradation in GaN layer properties in all the cases; however, it is clearly evident that a heavy reaction takes place at the GaN/sapphire interface for the higher temperature annealing. It has been observed through all characterisation techniques that MBE-grown GaN layers taken for the experiment are somewhat thermally stable than MOCVD-grown layers.
Acknowledgements The authors are extremely thankful to Dr. K. Jeganathan for the kind help in recording PL spectra. Mr. M. Senthil Kumar acknowledges the Council of Scientific and Industrial Research (CSIR), Govt. of India for the award of Senior Research Fellowship. The authors are also thankful to the Department of Science and Technology for financial assistance.
References [1] S. Nakamura, Frasol, the Blue Laser Diode, Springer, Berlin, 1999. [2] H. Morkoc, S. Strite, G.B. Gao, M.E. Lin, B. Sverdlov, M. Burns, J. Appl. Phys. 76 (1994) 1363. [3] S. Nakamura, M. Senoh, T. Mukai, Jpn. J. Appl. Phys. 30 (1991) 1708. [4] M.E. Lin, G. Xue, G.L. Zhou, J.E. Greene, H. Morkoc, Appl. Phys. Lett. 63 (1993) 932. [5] J.C. Zolper, M. Hagerott Crawford, A.J. Howard, Appl. Phys. Lett. 68 (1996) 200.
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M. Senthil Kumar et al. / Physica B 324 (2002) 223–228
[6] H. Siegle, G. Kaczmarczyk, L. Filippidis, A.P. Litvinchuk, A. Hoffmann, C. Thomsen, Phys. Rev. B 55 (1997) 7000. [7] S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, Jpn. J. Appl. Phys. 31 (1992) L139. [8] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 28 (1989) L2112. [9] M.A. Khan, Q. Chen, R.A. Skogman, J.N. Kuznia, Appl. Phys. Lett. 66 (1995) 2040.
[10] J. Hong, J.W. Lee, J.D. MacKenzie, S.M. Donovan, C.R. Abernathy, S.J. Pearton, J.C. Zolper, Semicond. Sci. Technol. 12 (1997) 1310. [11] M. Kuball, F. Demangeot, J. Frandon, M.A. Renucci, J. Massies, N. Grandjean, R.L. Aulombard, O. Briot, Appl. Phys. Lett. 73 (1998) 960. [12] A.S. Barker, Phys. Rev. 132 (1963) 1474.