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Characterization of GaN layers grown on porous silicon
Materials Science and Engineering B82 (2001) 98 – 101 www.elsevier.com/locate/mseb
Characterization of GaN layers grown on porous silicon A. Missaoui a,b, M. Saadoun b, T. Boufaden a, B. Bessaı¨s b,*, A. Rebey a, H. Ezzaouia b, B. El Jani a b
a Laboratoire de Physique des Mate´riaux, Faculte´ des Sciences de Monastir, Monastir 5000, Tunisia Laboratoire des Applications Solaires, Institut National de Recherche Scientifique et Technique, B.P. 95, 2050 Hammam-Lif, Tunisia
Abstract This work reports the first successful results of the growth of GaN on a porous silicon (PS) substrate. GaN layers have been grown on PS substrates by metalorganic vapour phase epitaxy (MOVPE) at atmospheric pressure. The growth rate (measured by laser reflectometry) was found to be dependent on the growth temperature. The surface morphology and crystallinity of the GaN films were characterized by atomic force microscopy (AFM), and X-ray diffraction (XRD). I–V and C –V characteristics of the GaN/PS structure measured at room temperature are reported. We found that the GaN/PS/Si heterojunction forms a diode-like structure with a rather good rectification behaviour. © 2001 Elsevier Science B.V. All rights reserved. Keywords: AFM; GaN; MOVPE; Porous silicon; X-ray diffraction
1. Introduction
2. Experimental procedure
In recent years, there has been a considerable interest and an extensive work on the epitaxial growth of GaN films on Si substrates for optoelectronic applications. A good crystalline quality with an appreciable surface morphology was the main goal of all workers [1–3]. It is well known that the differences in lattice constant and thermal expansion coefficients between GaN and Si lead to numerous defects in GaN films. Several techniques [4,5] have been used to reduce defect density in GaN layers. PS may be used as a host substrate for the epitaxy of the GaN layers due to its elastic properties resulting from the existence of small pores ranging from 30 to 1000A, [6,7]. Hence, by using PS as an intermediate layer, we expect to reduce the stresses due to the large lattice misfit and the difference in thermal expansion coefficients between GaN and Si. In this paper, we report structural and electrical studies of the initial growth stages of GaN on PS substrate by MOVPE.
The growth of the GaN layer on a PS substrate were carried out in a horizontal-type atmospheric pressure metalorganic vapor phase epitaxy (MOVPE) reactor. Trimethygallium (TMG) and ammonia (NH3) were used as the Ga and N sources, respectively. Hydrogen (H2) and nitrogen (N2) were used as carrier gases. TMG and NH3 were introduced separately and were mixed just in front of the PS substrate. The direct growth of the GaN layers was performed at a substrate temperature ranging from 400 to 800°C. PS was formed by the vapour-etching (VE) technique [8], using a (100)-oriented p-type Si substrate with a mirror-polished surface. The VE method involves exposing Si substrates to acid vapours issued from a mixture of HNO3 and HF. The main parameters to be controlled are the HNO3/HF volume ratio, the temperature of the acid solution and the exposure time of the Si substrate to the acid vapours These parameters allow us to control the etch kinetic, hence the morphology and the thickness of as-formed PS layers. To form PS layers in such a manner, an incubation period is needed before obtaining a highly luminescent PS. The temperature of the acid solution may accelerate the acid particles toward the Si substrate and hence play a catalytic role in the starting of the PS layer formation. This
* Corresponding author. Tel.: + 216-1-430053; fax: +216-1430934. E-mail address: [email protected] (B. Bessaı¨s).
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0921-5107(00)00752-2
A. Missaoui et al. / Materials Science and Engineering B82 (2001) 98–101
important parameter was optimised and fixed at 40°C. Homogeneous layers were obtained by controlling the HNO3/HF volume ratio, the temperature of the acid solution and the exposure time. Prior to GaN growth, the PS substrate was etched in a 10% HF for 5 min in order to remove the eventual oxide layer that may form during storage, then rinsed in deionised water and dried with filtered N2. The reactor was equipped with an in-situ growth controlled by laser reflectometry.
3. Results and discussion Fig. 1 shows the reflectivity signal of a GaN layer after 1 h of growth at a temperature of 600°C. Laser reflectometry is known to be very useful for determining, in real time, the growth rate and the thickness of the GaN layer [9,10]. From the interference oscillations (Fig. 1), the growth rate and thickness at 600°C can be estimated to be about 4.63 A, s − 1 and 1.6 mm, respectively. The growth rate increases rapidly as the substrate temperature rises from 450 to 700°C, but it does not exhibit any noticeable change in the 400–450°C and the 700–800°C temperature ranges. In Fig. 2, AFM images show the surface morphology of GaN layer grown at 500°C. The growth of GaN on PS is found to be pyramidal (Fig. 2). One may observe (Fig. 2) that the GaN grains are randomly distributed and are of approximately the same size (180 nm in length, 80 nm in width). The surface roughness rms (root mean square) and grain size were found to be dependent on growth temperature. The grain size increases as the roughness (rms) increases.
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The crystallinity of the GaN layers was characterized by XRD. A representative XRD spectrum is shown in Fig. 3. The XRD pattern shows that the GaN layer has a polycrystalline structure with the presence of both a hexagonal and a cubic phase. GaN exhibits two dominants peaks that were identified as the (00.2) hexagonal plane and the (200) cubic plane, respectively. The full width at half maximum (FWHM) of the (00.2) line was found to be dependent on the growth temperature. GaN layers with a good crystalline quality are achieved as the growth temperature rises, while a poor crystalline quality is obtained at low temperatures. The room-temperature current–voltage (I–V) characteristics of the GaN/PS structure are shown in Fig. 4. The ohmic contacts on the n-GaN layers and the Si substrate were achieved by In/Au and Al, respectively. The metallic contact area is 125× 10 − 3 cm2. The I–V characteristic clearly demonstrates that a Schottky diode has been formed. The diode shows a rather good rectification behaviour with a reasonably low leakage current. The forward ‘‘turn-on’’ voltage is typically about 0.8 V, and the leakage current is about 4.9 mA under a 3 V bias. Fig. 5 shows room-temperature capacitance–voltage curves (C − 2 –V) of the junction, recorded at high frequencies (100 kHz–10 MHz). The C − 2 –V plot is linear in a range of about 3V, corresponding to an almost perfect Mott–Schottky behaviour. Extrapolation of the linear part of the C − 2 –V curve gives a flat band potential, Vfb, of about − 3.9 V. Further investigations are needed to determine the band gap structure of the Si/PS/GaN heterojunction The carrier density determined from the linear part of the C − 2 –V curve is about 2.3× 1012 cm − 3.
Fig. 1. Reflectivity data of a GaN layer on PS substrate obtained after 1 h at 600°C.
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A. Missaoui et al. / Materials Science and Engineering B82 (2001) 98–101
Fig. 4. I– V characteristic of the GaN/porous Si heterojunction diode at room temperature.
Fig. 2. AFM images of GaN grown on PS: (a) plan-view image and (b) three-dimensional view image.
Fig. 5. High-frequency C − 2 – voltage characteristic of a GaN/PS junction.
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Fig. 3. XRD patterns of GaN grown on PS substrate at 800°C.
A. Missaoui et al. / Materials Science and Engineering B82 (2001) 98–101
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4. Conclusion
References
We report for the first time the growth of GaN layers on PS substrate by the MOVPE technique. The AFM images provide direct evidence of the surface topography of GaN and particularly the surface roughness. Investigations by XRD confirm the polycrystalline nature of the GaN layers. These first results show that PS may be a cheap candidate for obtaining GaN films for large-area applications.
[1] T. Takeuchi, H. Amano, K. Hiramatsu, N. Sawaki, I. Akasaki, J. Cryst. Growth 115 (1991) 634. [2] T. Lei, T.D. Moustakas, R.J. Graham, Y. He, S.J. Berkowitz, J. Appl. Phys. 71 (1992) 4933. [3] A. Ohtani, K.S. Stevens, R. Beresford, Appl. Phys. Lett. 65 (1994) 61. [4] N.P. Kobayashi, J.T. Kobayashi, P.D. Dapkus, W.J. Choi, A.E. Bond, X. Zhang, D.H. Rich, Appl. Phys. Lett. 71 (1997) 3569. [5] F. Calle, F.J. Sanchez, J.M.G. Tijero, M.A. Sanchez, E. Calleja, R. Beresford, Semicond. Sci. Technol. 12 (1997) 1396. [6] R. Herino, A. Perio, K. Barla, G. Bomchil, Mater. Lett. 2 (1984) 519. [7] R. Herino, G. Bomchil, K. Barla, C. Bertrand, J.L. Ginoux, J. Electrochem. Soc. 134 (1994) 1987. [8] M. Saadoun, N. Mliki, K. Daoudi, H. Kaabi, B. Bessaı¨s, H. Ezzaouia, M. Oueslati, R. Bennaceur, to be published. [9] A. Rebey, A. Bchetnia, B. El Jani, J. Cryst. Growth 194 (1998) 286. [10] A. Rebey, T. Boufaden, B. El Jani, J. Cryst. Growth 203 (1999) 12.
Acknowledgements This work was supported by the Tunisian Secretary of State for Research Science and Technology (PEN9601 and PRC (98) SERST-INRST).
.
Characterization of GaN layers grown on porous silicon A. Missaoui a,b, M. Saadoun b, T. Boufaden a, B. Bessaı¨s b,*, A. Rebey a, H. Ezzaouia b, B. El Jani a b
a Laboratoire de Physique des Mate´riaux, Faculte´ des Sciences de Monastir, Monastir 5000, Tunisia Laboratoire des Applications Solaires, Institut National de Recherche Scientifique et Technique, B.P. 95, 2050 Hammam-Lif, Tunisia
Abstract This work reports the first successful results of the growth of GaN on a porous silicon (PS) substrate. GaN layers have been grown on PS substrates by metalorganic vapour phase epitaxy (MOVPE) at atmospheric pressure. The growth rate (measured by laser reflectometry) was found to be dependent on the growth temperature. The surface morphology and crystallinity of the GaN films were characterized by atomic force microscopy (AFM), and X-ray diffraction (XRD). I–V and C –V characteristics of the GaN/PS structure measured at room temperature are reported. We found that the GaN/PS/Si heterojunction forms a diode-like structure with a rather good rectification behaviour. © 2001 Elsevier Science B.V. All rights reserved. Keywords: AFM; GaN; MOVPE; Porous silicon; X-ray diffraction
1. Introduction
2. Experimental procedure
In recent years, there has been a considerable interest and an extensive work on the epitaxial growth of GaN films on Si substrates for optoelectronic applications. A good crystalline quality with an appreciable surface morphology was the main goal of all workers [1–3]. It is well known that the differences in lattice constant and thermal expansion coefficients between GaN and Si lead to numerous defects in GaN films. Several techniques [4,5] have been used to reduce defect density in GaN layers. PS may be used as a host substrate for the epitaxy of the GaN layers due to its elastic properties resulting from the existence of small pores ranging from 30 to 1000A, [6,7]. Hence, by using PS as an intermediate layer, we expect to reduce the stresses due to the large lattice misfit and the difference in thermal expansion coefficients between GaN and Si. In this paper, we report structural and electrical studies of the initial growth stages of GaN on PS substrate by MOVPE.
The growth of the GaN layer on a PS substrate were carried out in a horizontal-type atmospheric pressure metalorganic vapor phase epitaxy (MOVPE) reactor. Trimethygallium (TMG) and ammonia (NH3) were used as the Ga and N sources, respectively. Hydrogen (H2) and nitrogen (N2) were used as carrier gases. TMG and NH3 were introduced separately and were mixed just in front of the PS substrate. The direct growth of the GaN layers was performed at a substrate temperature ranging from 400 to 800°C. PS was formed by the vapour-etching (VE) technique [8], using a (100)-oriented p-type Si substrate with a mirror-polished surface. The VE method involves exposing Si substrates to acid vapours issued from a mixture of HNO3 and HF. The main parameters to be controlled are the HNO3/HF volume ratio, the temperature of the acid solution and the exposure time of the Si substrate to the acid vapours These parameters allow us to control the etch kinetic, hence the morphology and the thickness of as-formed PS layers. To form PS layers in such a manner, an incubation period is needed before obtaining a highly luminescent PS. The temperature of the acid solution may accelerate the acid particles toward the Si substrate and hence play a catalytic role in the starting of the PS layer formation. This
* Corresponding author. Tel.: + 216-1-430053; fax: +216-1430934. E-mail address: [email protected] (B. Bessaı¨s).
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0921-5107(00)00752-2
A. Missaoui et al. / Materials Science and Engineering B82 (2001) 98–101
important parameter was optimised and fixed at 40°C. Homogeneous layers were obtained by controlling the HNO3/HF volume ratio, the temperature of the acid solution and the exposure time. Prior to GaN growth, the PS substrate was etched in a 10% HF for 5 min in order to remove the eventual oxide layer that may form during storage, then rinsed in deionised water and dried with filtered N2. The reactor was equipped with an in-situ growth controlled by laser reflectometry.
3. Results and discussion Fig. 1 shows the reflectivity signal of a GaN layer after 1 h of growth at a temperature of 600°C. Laser reflectometry is known to be very useful for determining, in real time, the growth rate and the thickness of the GaN layer [9,10]. From the interference oscillations (Fig. 1), the growth rate and thickness at 600°C can be estimated to be about 4.63 A, s − 1 and 1.6 mm, respectively. The growth rate increases rapidly as the substrate temperature rises from 450 to 700°C, but it does not exhibit any noticeable change in the 400–450°C and the 700–800°C temperature ranges. In Fig. 2, AFM images show the surface morphology of GaN layer grown at 500°C. The growth of GaN on PS is found to be pyramidal (Fig. 2). One may observe (Fig. 2) that the GaN grains are randomly distributed and are of approximately the same size (180 nm in length, 80 nm in width). The surface roughness rms (root mean square) and grain size were found to be dependent on growth temperature. The grain size increases as the roughness (rms) increases.
99
The crystallinity of the GaN layers was characterized by XRD. A representative XRD spectrum is shown in Fig. 3. The XRD pattern shows that the GaN layer has a polycrystalline structure with the presence of both a hexagonal and a cubic phase. GaN exhibits two dominants peaks that were identified as the (00.2) hexagonal plane and the (200) cubic plane, respectively. The full width at half maximum (FWHM) of the (00.2) line was found to be dependent on the growth temperature. GaN layers with a good crystalline quality are achieved as the growth temperature rises, while a poor crystalline quality is obtained at low temperatures. The room-temperature current–voltage (I–V) characteristics of the GaN/PS structure are shown in Fig. 4. The ohmic contacts on the n-GaN layers and the Si substrate were achieved by In/Au and Al, respectively. The metallic contact area is 125× 10 − 3 cm2. The I–V characteristic clearly demonstrates that a Schottky diode has been formed. The diode shows a rather good rectification behaviour with a reasonably low leakage current. The forward ‘‘turn-on’’ voltage is typically about 0.8 V, and the leakage current is about 4.9 mA under a 3 V bias. Fig. 5 shows room-temperature capacitance–voltage curves (C − 2 –V) of the junction, recorded at high frequencies (100 kHz–10 MHz). The C − 2 –V plot is linear in a range of about 3V, corresponding to an almost perfect Mott–Schottky behaviour. Extrapolation of the linear part of the C − 2 –V curve gives a flat band potential, Vfb, of about − 3.9 V. Further investigations are needed to determine the band gap structure of the Si/PS/GaN heterojunction The carrier density determined from the linear part of the C − 2 –V curve is about 2.3× 1012 cm − 3.
Fig. 1. Reflectivity data of a GaN layer on PS substrate obtained after 1 h at 600°C.
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A. Missaoui et al. / Materials Science and Engineering B82 (2001) 98–101
Fig. 4. I– V characteristic of the GaN/porous Si heterojunction diode at room temperature.
Fig. 2. AFM images of GaN grown on PS: (a) plan-view image and (b) three-dimensional view image.
Fig. 5. High-frequency C − 2 – voltage characteristic of a GaN/PS junction.
.
Fig. 3. XRD patterns of GaN grown on PS substrate at 800°C.
A. Missaoui et al. / Materials Science and Engineering B82 (2001) 98–101
101
4. Conclusion
References
We report for the first time the growth of GaN layers on PS substrate by the MOVPE technique. The AFM images provide direct evidence of the surface topography of GaN and particularly the surface roughness. Investigations by XRD confirm the polycrystalline nature of the GaN layers. These first results show that PS may be a cheap candidate for obtaining GaN films for large-area applications.
[1] T. Takeuchi, H. Amano, K. Hiramatsu, N. Sawaki, I. Akasaki, J. Cryst. Growth 115 (1991) 634. [2] T. Lei, T.D. Moustakas, R.J. Graham, Y. He, S.J. Berkowitz, J. Appl. Phys. 71 (1992) 4933. [3] A. Ohtani, K.S. Stevens, R. Beresford, Appl. Phys. Lett. 65 (1994) 61. [4] N.P. Kobayashi, J.T. Kobayashi, P.D. Dapkus, W.J. Choi, A.E. Bond, X. Zhang, D.H. Rich, Appl. Phys. Lett. 71 (1997) 3569. [5] F. Calle, F.J. Sanchez, J.M.G. Tijero, M.A. Sanchez, E. Calleja, R. Beresford, Semicond. Sci. Technol. 12 (1997) 1396. [6] R. Herino, A. Perio, K. Barla, G. Bomchil, Mater. Lett. 2 (1984) 519. [7] R. Herino, G. Bomchil, K. Barla, C. Bertrand, J.L. Ginoux, J. Electrochem. Soc. 134 (1994) 1987. [8] M. Saadoun, N. Mliki, K. Daoudi, H. Kaabi, B. Bessaı¨s, H. Ezzaouia, M. Oueslati, R. Bennaceur, to be published. [9] A. Rebey, A. Bchetnia, B. El Jani, J. Cryst. Growth 194 (1998) 286. [10] A. Rebey, T. Boufaden, B. El Jani, J. Cryst. Growth 203 (1999) 12.
Acknowledgements This work was supported by the Tunisian Secretary of State for Research Science and Technology (PEN9601 and PRC (98) SERST-INRST).
.