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An investigation of structural, optical and electrical properties of GaN thin films grown on Si(111 ) by reaction evaporation
Solid-State Electronics 46 (2002) 301–306
An investigation of structural, optical and electrical properties of GaN thin films grown on Si(1 1 1) by reaction evaporation Haoxiang Zhang *, Zhizhen Ye, Binghui Zhao State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China Received 24 August 1999; received in revised form 23 April 2001
Abstract In this paper, gallium nitride (GaN) grown on Si(1 1 1) is reported using a GaN buffer layer by a simple reactive evaporation method. X-ray diffraction and transmission electron microscopy results indicated that the single crystalline wurtzite GaN was successfully grown on Si(1 1 1) substrate. The results of photoluminescence (PL) characterization showed that a band edge photoluminescence peak at 365 nm without yellow luminescence present was observed in the PL spectra at room temperature. The comparative analysis of PL spectra of GaN grown at different temperatures indicated both positive and negative impacts of substrate temperature on the photoluminescence of GaN, the GaN epilayer grown at 1050°C exhibited the strongest PL and annealing could heighten the PL. It was demonstrated in second ion mass spectroscopy that both of gallium and nitrogen distributed uniformly with the epilayer, while gallium seggregated on the surface of epilayer. The unintentionally doped films were n type with a carrier concentration of 5:27 1017 cm3 and an electron mobility of 238 cm2 /V s. The high carrier concentration was associated with the impurities of silicon and oxygen and native defect existed in the epilayer. Ó 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Gallium nitride (GaN) has been considered as a promising material for photoelectric devices such as blue and near ultraviolet and violet light emitting diodes (LEDs) [1–3] and laser diodes [4] since it has a direct band gap of 3.4 eV at room temperature. GaN single-crystal films are conventionally obtained using molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) on sapphire. However, since the sapphire substrate is not conductive, the electrodes for the devices are not readily fabricated. Accordingly, Si is regarded as one the most promising substrate for GaN epitaxy, since it is available in large sizes and high quality at a relatively low cost, Furthermore, the GaN epitaxy on Si will facilitate the integra-
*
Corresponding author. Tel.: +86-571-795-2124; fax: +86571-795-1954. E-mail address: [email protected] (H. Zhang).
tion of the microelectronics and optoelectronics. However, it is difficult to grow single crystalline GaN films directly on the Si substrate because of the large lattice mismatch and the largest difference in the thermal expansion coefficient between GaN (in wurtzite structure) and Si. Takeuchi et al. [5] and Watanabe et al. [6] have reported the GaN epitaxy on Si(1 1 1) by MOCVD using buffer layers of SiC and AlN respectively. Guha and Bojarczuk [7] reported the fabrication of GaN LEDs grown by MBE on Si(1 1 1) substrates which was used as a bottom contact for electron injection into the device structure through a thin AlN growth initiation layer. Yang et al. [8] have grown cubic GaN on Si(0 0 1) by plasma-assisted MBE. Yoshinobu et al. [9] have grown wurtzite GaN on Si3 N4 buffer layer formed on Si(1 1 1) surfaces by plasma-assisted MBE. Up to date, it is still difficult to prepare single-crystal GaN film on Si substrate. In this paper, we report the single crystalline GaN epitaxy on Si(111) using a GaN buffer layer by the vaccum reactive evaporation which is much simpler
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compared with MOCVD and MBE. The X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL), Hall measurement and secondary ion mass spectroscopy (SIMS) were performed to characterize the structural, optical and electronic properties of the GaN films.
2. Experimental details The system of growing GaN films is similar to the system of hot wall epitaxy (HWE). An elemental gallium (Ga) as a Ga-atom source put in a quartz crucible is heated by the tungsten filaments in the center of the chamber. NH3 and N2 as N-atom source are introduced near the substrate, in order to have a high N pressure on the surface of the substrate. The substrate was heated from the backside by thermal radiation of coiled tungsten filament in closely arranged parallel quartz tubes. The evaporating Ga atoms from the quartz crucible to the substrate are all within the heat field. Therefore, the technique and growth system are different from MBE and HWE. We call this the case vacuum reactive evaporation. The p-type Si(1 1 1) substrates (off-orient 4°) with a resistivity of 8–12 X cm were cleaned using RCA1 solution NH4 OH:H2 O2 :H2 O ¼ 1:1:6, followed by RCA2 solution HCl:H2 O2 :H2 O ¼ 1:1:6, each for 15 min at 85°C. Then they were dipped into 10% HF solution of 10 s to remove native SiO2 on the Si surface. The substrate was immediately transferred into the chamber from the solution and the chamber was pumped down. When the base pressure was pumped to 1 105 Pa, the substrate was heated to 950°C and maintained for 20 min to remove the passivation layer on the substrate. NH3 gas was subsequently inlet for in situ cleaning of the substrate to form a clean surface. Prior to the growth of the GaN epilayer, the buffer layer was grown under the following growth conditions. First, Ga was pre-deposited at a Tsub of 700°C without NH3 gas flow for 5 min, and then raising the Tsub from Si 700°C to 800°C by supplying NH3 gas to grow a GaN buffer layer for 10 min. Lastly, the substrate temperature was then raised above 1000°C for the GaN epitaxy. After the epitaxial growth, NH3 flow continued to supply for 15 min. The average growth rate of the buffer layer was 1–5 nm/min and the average growth rate of the epilayer varied from 0.4 to 2 lm/h depending on the experimental conditions such as the growth temperature, the temperature of Ga source and the gas flow rates.
Fig. 1. XRD of the GaN films with a thickness of 5 lm grown on Si(1 1 1) at 1050°C.
XRD pattern of the GaN film deposited at 1050°C on Si(1 1 1). Besides a (1 1 1) peak of Si 2h ¼ 28:5°, there is a pronounced (0 0 0 2) peak of the GaN and the plane spacing of a (0 0 0 2) strong peak is 0.2590 nm. Therefore the c-axis lattice constant is 0.5180 nm which is consistent with the wurtzite theoretical value (0.5185 nm). By the way, there was a deviation of 4° between the surface and the Si(1 1 1). Therefore, in Fig. 1, the Si(1 1 1) was far away from the Bragg diffraction condition and the Si(1 1 1) peak was wide and weak. When the samples were rotated to certain degree, the Si(1 1 1) peak would be very strong and the GaN(0 0 0 2) peak weak, but the positions of these peaks did not shift. The double crystal XRD was performed to investigate the crystalline quality of Si(1 1 1), the result revealed it had a full-width at half-maximum (FWHM) of 10 arcsec, which meant the Si substrate still had a very high quality. The double-crystal X-ray rocking curve (DCXRC) of the (0 0 0 2) diffraction of the GaN film is found to have a FWHM of 12 arcmin as shown in Fig. 2. For almost
3. Results and discussion XRD and TEM were performed to investigate the crystalline quality of the GaN films. Fig. 1 shows the
Fig. 2. DCXRC for the (0 0 0 2) diffraction of the GaN epilayer grown on Si(1 1 1), the FWHM ¼ 12 arcmin.
H. Zhang et al. / Solid-State Electronics 46 (2002) 301–306
all the samples, the FWHM of GaN(0 0 0 2) diffraction is between 24 and 12 arcmin. The big FWHM in the DCXRC indicates many defects in the GaN epilayer. It should be pointed out that the FWHM depends on the thickness of the epitaxial films. It has been demonstrated in the literature that the density of defects was remarkably reduced far from the film/substrate interface [10, 11], that is, the quality of the GaN film had been improved. It was found in our samples, under the same experimental conditions, with the thickness increasing, the FWHM of GaN(0 0 0 2) diffraction is 24 arcmin with a 1.5 lm thickness, and 12 arcmin with a 5 lm thickness. On the other hand, the GaN films has a tilt of crystal from the Si substrate surface, which combined with an original 4° deviation of the Si substrate surface from the actual Si(1 1 1) plane, results in a tilt of crystal about 5° between the GaNh0 0 0 1i and the Sih1 1 1i. TEM was employed to characterize the microstructure of the GaN films. Fig. 3 illustrates the HRTEM of the GaN film on the substrate. It is obvious that a 10 nm buffer layer exists between the silicon substrate and the epilayer. The inserted figure in Fig. 3 shows the selected area electron diffraction (SAED) pattern of the buffer layer and the substrate. With the exception of the strong regular patterns pertaining to the silicon substrate, the weak elongated patterns are believed to be ascribed to the buffer layer in a microcrystalline nature. The Figs. 4 and 5 were the SAED image of the interfacial zone of GaN thin films and the GaN epilayer, which suggested GaN thin films was in the single crystalline form. GaN
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epitaxy started with nucleus formed on the silicon substrate, then followed by a GaN buffer layer growth at low temperature. This polycrystalline buffer layer recrystallized to highly oriented GaN in a microcrystalline form in subsequent high temperature annealing, with the orientation relationships GaNh0 0 0 1ikSih1 1 1 i and GaNh1 1 2 0ikSih1 1; 1 i. Considering the characterizations of XRD and TEM together, we think the single crystalline GaN thin films were epitaxially grown on Si(1 1 1) substrate. At the temperature above 900°C, the GaN single crystal films could be successfully grown on Si substrates. However, as stated above, the control of the buffer layer thickness was very important. If the buffer layer were too thin (<10 nm), the buffer action would not be very effective and the GaN film is polycrystalline, as shown in Fig. 6. On the contrary, if the buffer layer was too thick (>100 nm), the orientation relationship between the GaN and the Si substrate could not be established. At the same time because the GaN buffer layer was in the form of polycrystal, a thick buffer layer could not serve as a smooth and planar template for the GaN ‘‘homoepitaxy’’, resulting in a rough surface, as shown in Fig. 7. With the surface morphology and crystal quality of GaN considered together, the optimum growth conditions of the buffer layer were as follows. The time of the Ga pre-deposition was 5 min at 700°C. Below 5 min, the GaN films were polycrystalline. The flow rate of NH3 and N2 was 10.2 and 5 sccm respectively. The optimum thickness of the buffer layer
Fig. 3. Cross-sectional HRTEM image of the GaN epilayer with the electron beam parallel to the h0 1 1i zone axis of Si.
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Fig. 4. SEAD pattern of the GaN thin films with the electron beam parallel to the h1 1 1i zone axis of Si, (a) GaN/Si interfacial zone and (b) GaN epilayer.
Fig. 5. SEAD pattern of the GaN thin films with the electron beam parallel to the h1 1 0i zone axis of Si, (a) interfacial zone and (b) GaN epilayer.
Fig. 6. XRD of the GaN films with a 5 nm GaN buffer layer grown on Si(1 1 1) at 1050°C.
was around 10 nm, while the growth temperature of the buffer layer was 800°C. In this case, the growth rate of the buffer layer should be kept relatively slow. In order to study the effect of the growth temperature on the luminescence of the GaN epilayer, the PL spectra with the different temperature are conducted while other experimental parameters are kept unchanged. As shown in Fig. 8, no shift of the luminescence peak observed, stable in 365 nm (3.4 eV). When the growth temperature varies from 900°C to 1050°C, the intensity increases a little with the strongest at the 1050°C. However, a little decrease is observed as the temperature rises above 1050°C. It is assumed that this phenomenon is responsible for the GaN epilayers of good crystal quality. When the temperature exceeds 1050°C, some by-reactions and the intensifying diffusion of impurities from
H. Zhang et al. / Solid-State Electronics 46 (2002) 301–306
Fig. 7. SEM image of GaN thin films grown on Si(1 1 1) with a 100 nm buffer layer.
the substrate to epilayer result in a decrease in crystal quality. This respond to the rise–fall rule of the GaN luminescence peak in the PL spectra. In Fig. 9, graph 2 and graph 1 were the PL of a GaN sample after 1 h annealing treatment at 1000°C respectively, with Si(1 1 1) substrate and the growth temperature of 1050°C. It revealed that after annealing, there were no shift in peak position and FWHM of luminescence peak (365 and 8 nm respectively). While the intensity increased about four times. Therefore, it is concluded that the annealing treatment could affect the luminescence of GaN epilayers greatly. The possible reason was that the high annealing temperature relaxes the lattice mismatch of the GaN epilayers. The DCXRC
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of the GaN(0 0 0 2) of films grown at 1050°C had a FWHM of 18 arcmin and it became 15 arcmin after annealing. This resulted in an increase in crystal quality of the GaN epilayers and luminescence intensity accordingly. Fig. 10 is the depth profile of SIMS of the GaN films, with the etching time as the abscissa and the signal intensity as the ordinate. Owing to the variety of sensitivity to different elements, the intensity is marked relatively, only reflecting the longitude changes of different elements without the exact percentage. The beginning section only indicates an unstable state and cannot demonstrate the actual conditions. As is shown in the figure, Ga, N, Si and O are co-existing in the epilayer, with the former two and the latter two similar to each other respectively. In the whole epilayer, the distribution of the Ga and N indicates that the GaN epilayer grows with steady steps. At the same time, the Ga graph in the SIMS has a rising ending. The following may be the main reasons. First, the incomplete reaction between the Ga and the N may be result in some N vacancies and lattice defects in the growing process. These defects in the epilayer can diffuse to the surface due to the high temperature maintained during the growth; secondly, although the growth comes to an end with the supply of Ga cut and the substrate heater closed, there are still a few Ga atoms existent in the chamber. The remaining Ga and N react incompletely and form some sediments in the upper epilayer due to the lowering growth temperature; thirdly, the increased Ga signals near the ambient interface almost certainly resulted from the presence of oxide and enhance positive secondly ion yields.
Fig. 8. PL spectra of GaN epilayers grown on Si(111) at different temperature: (a) 900°C, (b) 950°C, (c) 1000°C, (d) 1050°C and (e) 1100°C.
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in the Si substrate, indicating a relatively low content of O in the GaN films. At room temperature, Hall effect measurement is performed on the GaN films grown on Si. The unintentionally doped films are normally n type with a typical carrier concentration of 5:27 1017 cm3 and an electron mobility of 238 cm2 /V s. In these experiments, the background electron concentration could not be reduced less than 1017 atom/cm3 . The same results have also been reported in Ref. [12].
4. Conclusions
Fig. 9. PL spectra of GaN epilayer grown on Si(1 1 1): (1)postannealing and (2) as-grown.
In summary, we succeeded in growing the single crystalline GaN films on Si(1 1 1) substrates by reactive evaporation. The XRD, TEM, PL, SIMS and Hall measurement were performed to characterize the structural, optical and electronic properties of the GaN films. These results showed that the reactive evaporation method is a simple but very effective technique to grow single crystalline GaN films on Si(1 1 1) substrates.
Acknowledgements This project was supported by Special Funds for Major State Basic Research Project under grant no. G20000683-06 and National Natural Science Foundation of China under grant no. 69890230. References
Fig. 10. The depth profile of SIMS of the GaN films.
The Si signal in the SIMS is strong, we think the following reason was responsible for this. Under the high growth temperature and the low growth pressure, the Si would have been evaporated from the highly heated Ga quartz crucible and quartz tubes. The O curve in Fig. 10 has a rising start during the early stage of etching due to the oxygen adsorbed by the films surface. The graph is flat in the interface between the substrate and the epilayer, revealing that we have removed the oxygen effectively and gotten a clean growth surface by RCA cleaning and in situ cleaning. In our experiments, in the SIMS of samples without in situ cleaning, the O graph always gets small peaks in the interface. It is also shown that the content of O in the epilayer is lower than
[1] Nakamura S, Tietian J. Appl Phys Lett 1994;74:1687. [2] Yang QK, Li AZ, Zhang YG, Yang B, Brandt O, Ploog K. J Cryst Growth 1998;192:28. [3] Brandt O, Wuensche H-J, Yang H, Klann R, Muellhaeuser JR, Ploog KH. J Cryst Growth 1998;189–190:790. [4] Nakamura S, Senoh M, Nagahama S. Jpn J Appl Phys 1996;35:74. [5] Takeuchi T, Amano H, Hiramatsu K, Sawaki N, Akasaki I. J Cryst Growth 1991;115:634. [6] Watanabe A, Takeuchi T, Hirosawa K, Amano H, Hiramatsu K, Akasaki I. J Cryst Growth 1993;128:391. [7] Guha S, Bojarczuk N. Appl Phys Lett 1998;72:415. [8] Yang B, Brandt O, Trampert A, Jenichen B, Ploog KH. Appl Surf Sci 1998;123–124:1. [9] Nakada Y, Aksenov L, Okumura H. Appl Phys Lett 1998;73:827. [10] Ishida A, Yamamoto E, Ishino K, Ito K, Fujiyasu H. Appl Phys Lett 1995;67:665. [11] Qian W, Skowronski M, De Graef M. Appl Phys Lett 1995;66(10):1252. [12] Tanoue F, Sakakibara S, Ohbora M, Ishino K, Ishida A, Fujiyasu H. J Cryst Growth 1998;188–190:47.
An investigation of structural, optical and electrical properties of GaN thin films grown on Si(1 1 1) by reaction evaporation Haoxiang Zhang *, Zhizhen Ye, Binghui Zhao State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China Received 24 August 1999; received in revised form 23 April 2001
Abstract In this paper, gallium nitride (GaN) grown on Si(1 1 1) is reported using a GaN buffer layer by a simple reactive evaporation method. X-ray diffraction and transmission electron microscopy results indicated that the single crystalline wurtzite GaN was successfully grown on Si(1 1 1) substrate. The results of photoluminescence (PL) characterization showed that a band edge photoluminescence peak at 365 nm without yellow luminescence present was observed in the PL spectra at room temperature. The comparative analysis of PL spectra of GaN grown at different temperatures indicated both positive and negative impacts of substrate temperature on the photoluminescence of GaN, the GaN epilayer grown at 1050°C exhibited the strongest PL and annealing could heighten the PL. It was demonstrated in second ion mass spectroscopy that both of gallium and nitrogen distributed uniformly with the epilayer, while gallium seggregated on the surface of epilayer. The unintentionally doped films were n type with a carrier concentration of 5:27 1017 cm3 and an electron mobility of 238 cm2 /V s. The high carrier concentration was associated with the impurities of silicon and oxygen and native defect existed in the epilayer. Ó 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Gallium nitride (GaN) has been considered as a promising material for photoelectric devices such as blue and near ultraviolet and violet light emitting diodes (LEDs) [1–3] and laser diodes [4] since it has a direct band gap of 3.4 eV at room temperature. GaN single-crystal films are conventionally obtained using molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) on sapphire. However, since the sapphire substrate is not conductive, the electrodes for the devices are not readily fabricated. Accordingly, Si is regarded as one the most promising substrate for GaN epitaxy, since it is available in large sizes and high quality at a relatively low cost, Furthermore, the GaN epitaxy on Si will facilitate the integra-
*
Corresponding author. Tel.: +86-571-795-2124; fax: +86571-795-1954. E-mail address: [email protected] (H. Zhang).
tion of the microelectronics and optoelectronics. However, it is difficult to grow single crystalline GaN films directly on the Si substrate because of the large lattice mismatch and the largest difference in the thermal expansion coefficient between GaN (in wurtzite structure) and Si. Takeuchi et al. [5] and Watanabe et al. [6] have reported the GaN epitaxy on Si(1 1 1) by MOCVD using buffer layers of SiC and AlN respectively. Guha and Bojarczuk [7] reported the fabrication of GaN LEDs grown by MBE on Si(1 1 1) substrates which was used as a bottom contact for electron injection into the device structure through a thin AlN growth initiation layer. Yang et al. [8] have grown cubic GaN on Si(0 0 1) by plasma-assisted MBE. Yoshinobu et al. [9] have grown wurtzite GaN on Si3 N4 buffer layer formed on Si(1 1 1) surfaces by plasma-assisted MBE. Up to date, it is still difficult to prepare single-crystal GaN film on Si substrate. In this paper, we report the single crystalline GaN epitaxy on Si(111) using a GaN buffer layer by the vaccum reactive evaporation which is much simpler
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compared with MOCVD and MBE. The X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL), Hall measurement and secondary ion mass spectroscopy (SIMS) were performed to characterize the structural, optical and electronic properties of the GaN films.
2. Experimental details The system of growing GaN films is similar to the system of hot wall epitaxy (HWE). An elemental gallium (Ga) as a Ga-atom source put in a quartz crucible is heated by the tungsten filaments in the center of the chamber. NH3 and N2 as N-atom source are introduced near the substrate, in order to have a high N pressure on the surface of the substrate. The substrate was heated from the backside by thermal radiation of coiled tungsten filament in closely arranged parallel quartz tubes. The evaporating Ga atoms from the quartz crucible to the substrate are all within the heat field. Therefore, the technique and growth system are different from MBE and HWE. We call this the case vacuum reactive evaporation. The p-type Si(1 1 1) substrates (off-orient 4°) with a resistivity of 8–12 X cm were cleaned using RCA1 solution NH4 OH:H2 O2 :H2 O ¼ 1:1:6, followed by RCA2 solution HCl:H2 O2 :H2 O ¼ 1:1:6, each for 15 min at 85°C. Then they were dipped into 10% HF solution of 10 s to remove native SiO2 on the Si surface. The substrate was immediately transferred into the chamber from the solution and the chamber was pumped down. When the base pressure was pumped to 1 105 Pa, the substrate was heated to 950°C and maintained for 20 min to remove the passivation layer on the substrate. NH3 gas was subsequently inlet for in situ cleaning of the substrate to form a clean surface. Prior to the growth of the GaN epilayer, the buffer layer was grown under the following growth conditions. First, Ga was pre-deposited at a Tsub of 700°C without NH3 gas flow for 5 min, and then raising the Tsub from Si 700°C to 800°C by supplying NH3 gas to grow a GaN buffer layer for 10 min. Lastly, the substrate temperature was then raised above 1000°C for the GaN epitaxy. After the epitaxial growth, NH3 flow continued to supply for 15 min. The average growth rate of the buffer layer was 1–5 nm/min and the average growth rate of the epilayer varied from 0.4 to 2 lm/h depending on the experimental conditions such as the growth temperature, the temperature of Ga source and the gas flow rates.
Fig. 1. XRD of the GaN films with a thickness of 5 lm grown on Si(1 1 1) at 1050°C.
XRD pattern of the GaN film deposited at 1050°C on Si(1 1 1). Besides a (1 1 1) peak of Si 2h ¼ 28:5°, there is a pronounced (0 0 0 2) peak of the GaN and the plane spacing of a (0 0 0 2) strong peak is 0.2590 nm. Therefore the c-axis lattice constant is 0.5180 nm which is consistent with the wurtzite theoretical value (0.5185 nm). By the way, there was a deviation of 4° between the surface and the Si(1 1 1). Therefore, in Fig. 1, the Si(1 1 1) was far away from the Bragg diffraction condition and the Si(1 1 1) peak was wide and weak. When the samples were rotated to certain degree, the Si(1 1 1) peak would be very strong and the GaN(0 0 0 2) peak weak, but the positions of these peaks did not shift. The double crystal XRD was performed to investigate the crystalline quality of Si(1 1 1), the result revealed it had a full-width at half-maximum (FWHM) of 10 arcsec, which meant the Si substrate still had a very high quality. The double-crystal X-ray rocking curve (DCXRC) of the (0 0 0 2) diffraction of the GaN film is found to have a FWHM of 12 arcmin as shown in Fig. 2. For almost
3. Results and discussion XRD and TEM were performed to investigate the crystalline quality of the GaN films. Fig. 1 shows the
Fig. 2. DCXRC for the (0 0 0 2) diffraction of the GaN epilayer grown on Si(1 1 1), the FWHM ¼ 12 arcmin.
H. Zhang et al. / Solid-State Electronics 46 (2002) 301–306
all the samples, the FWHM of GaN(0 0 0 2) diffraction is between 24 and 12 arcmin. The big FWHM in the DCXRC indicates many defects in the GaN epilayer. It should be pointed out that the FWHM depends on the thickness of the epitaxial films. It has been demonstrated in the literature that the density of defects was remarkably reduced far from the film/substrate interface [10, 11], that is, the quality of the GaN film had been improved. It was found in our samples, under the same experimental conditions, with the thickness increasing, the FWHM of GaN(0 0 0 2) diffraction is 24 arcmin with a 1.5 lm thickness, and 12 arcmin with a 5 lm thickness. On the other hand, the GaN films has a tilt of crystal from the Si substrate surface, which combined with an original 4° deviation of the Si substrate surface from the actual Si(1 1 1) plane, results in a tilt of crystal about 5° between the GaNh0 0 0 1i and the Sih1 1 1i. TEM was employed to characterize the microstructure of the GaN films. Fig. 3 illustrates the HRTEM of the GaN film on the substrate. It is obvious that a 10 nm buffer layer exists between the silicon substrate and the epilayer. The inserted figure in Fig. 3 shows the selected area electron diffraction (SAED) pattern of the buffer layer and the substrate. With the exception of the strong regular patterns pertaining to the silicon substrate, the weak elongated patterns are believed to be ascribed to the buffer layer in a microcrystalline nature. The Figs. 4 and 5 were the SAED image of the interfacial zone of GaN thin films and the GaN epilayer, which suggested GaN thin films was in the single crystalline form. GaN
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epitaxy started with nucleus formed on the silicon substrate, then followed by a GaN buffer layer growth at low temperature. This polycrystalline buffer layer recrystallized to highly oriented GaN in a microcrystalline form in subsequent high temperature annealing, with the orientation relationships GaNh0 0 0 1ikSih1 1 1 i and GaNh1 1 2 0ikSih1 1; 1 i. Considering the characterizations of XRD and TEM together, we think the single crystalline GaN thin films were epitaxially grown on Si(1 1 1) substrate. At the temperature above 900°C, the GaN single crystal films could be successfully grown on Si substrates. However, as stated above, the control of the buffer layer thickness was very important. If the buffer layer were too thin (<10 nm), the buffer action would not be very effective and the GaN film is polycrystalline, as shown in Fig. 6. On the contrary, if the buffer layer was too thick (>100 nm), the orientation relationship between the GaN and the Si substrate could not be established. At the same time because the GaN buffer layer was in the form of polycrystal, a thick buffer layer could not serve as a smooth and planar template for the GaN ‘‘homoepitaxy’’, resulting in a rough surface, as shown in Fig. 7. With the surface morphology and crystal quality of GaN considered together, the optimum growth conditions of the buffer layer were as follows. The time of the Ga pre-deposition was 5 min at 700°C. Below 5 min, the GaN films were polycrystalline. The flow rate of NH3 and N2 was 10.2 and 5 sccm respectively. The optimum thickness of the buffer layer
Fig. 3. Cross-sectional HRTEM image of the GaN epilayer with the electron beam parallel to the h0 1 1i zone axis of Si.
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Fig. 4. SEAD pattern of the GaN thin films with the electron beam parallel to the h1 1 1i zone axis of Si, (a) GaN/Si interfacial zone and (b) GaN epilayer.
Fig. 5. SEAD pattern of the GaN thin films with the electron beam parallel to the h1 1 0i zone axis of Si, (a) interfacial zone and (b) GaN epilayer.
Fig. 6. XRD of the GaN films with a 5 nm GaN buffer layer grown on Si(1 1 1) at 1050°C.
was around 10 nm, while the growth temperature of the buffer layer was 800°C. In this case, the growth rate of the buffer layer should be kept relatively slow. In order to study the effect of the growth temperature on the luminescence of the GaN epilayer, the PL spectra with the different temperature are conducted while other experimental parameters are kept unchanged. As shown in Fig. 8, no shift of the luminescence peak observed, stable in 365 nm (3.4 eV). When the growth temperature varies from 900°C to 1050°C, the intensity increases a little with the strongest at the 1050°C. However, a little decrease is observed as the temperature rises above 1050°C. It is assumed that this phenomenon is responsible for the GaN epilayers of good crystal quality. When the temperature exceeds 1050°C, some by-reactions and the intensifying diffusion of impurities from
H. Zhang et al. / Solid-State Electronics 46 (2002) 301–306
Fig. 7. SEM image of GaN thin films grown on Si(1 1 1) with a 100 nm buffer layer.
the substrate to epilayer result in a decrease in crystal quality. This respond to the rise–fall rule of the GaN luminescence peak in the PL spectra. In Fig. 9, graph 2 and graph 1 were the PL of a GaN sample after 1 h annealing treatment at 1000°C respectively, with Si(1 1 1) substrate and the growth temperature of 1050°C. It revealed that after annealing, there were no shift in peak position and FWHM of luminescence peak (365 and 8 nm respectively). While the intensity increased about four times. Therefore, it is concluded that the annealing treatment could affect the luminescence of GaN epilayers greatly. The possible reason was that the high annealing temperature relaxes the lattice mismatch of the GaN epilayers. The DCXRC
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of the GaN(0 0 0 2) of films grown at 1050°C had a FWHM of 18 arcmin and it became 15 arcmin after annealing. This resulted in an increase in crystal quality of the GaN epilayers and luminescence intensity accordingly. Fig. 10 is the depth profile of SIMS of the GaN films, with the etching time as the abscissa and the signal intensity as the ordinate. Owing to the variety of sensitivity to different elements, the intensity is marked relatively, only reflecting the longitude changes of different elements without the exact percentage. The beginning section only indicates an unstable state and cannot demonstrate the actual conditions. As is shown in the figure, Ga, N, Si and O are co-existing in the epilayer, with the former two and the latter two similar to each other respectively. In the whole epilayer, the distribution of the Ga and N indicates that the GaN epilayer grows with steady steps. At the same time, the Ga graph in the SIMS has a rising ending. The following may be the main reasons. First, the incomplete reaction between the Ga and the N may be result in some N vacancies and lattice defects in the growing process. These defects in the epilayer can diffuse to the surface due to the high temperature maintained during the growth; secondly, although the growth comes to an end with the supply of Ga cut and the substrate heater closed, there are still a few Ga atoms existent in the chamber. The remaining Ga and N react incompletely and form some sediments in the upper epilayer due to the lowering growth temperature; thirdly, the increased Ga signals near the ambient interface almost certainly resulted from the presence of oxide and enhance positive secondly ion yields.
Fig. 8. PL spectra of GaN epilayers grown on Si(111) at different temperature: (a) 900°C, (b) 950°C, (c) 1000°C, (d) 1050°C and (e) 1100°C.
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in the Si substrate, indicating a relatively low content of O in the GaN films. At room temperature, Hall effect measurement is performed on the GaN films grown on Si. The unintentionally doped films are normally n type with a typical carrier concentration of 5:27 1017 cm3 and an electron mobility of 238 cm2 /V s. In these experiments, the background electron concentration could not be reduced less than 1017 atom/cm3 . The same results have also been reported in Ref. [12].
4. Conclusions
Fig. 9. PL spectra of GaN epilayer grown on Si(1 1 1): (1)postannealing and (2) as-grown.
In summary, we succeeded in growing the single crystalline GaN films on Si(1 1 1) substrates by reactive evaporation. The XRD, TEM, PL, SIMS and Hall measurement were performed to characterize the structural, optical and electronic properties of the GaN films. These results showed that the reactive evaporation method is a simple but very effective technique to grow single crystalline GaN films on Si(1 1 1) substrates.
Acknowledgements This project was supported by Special Funds for Major State Basic Research Project under grant no. G20000683-06 and National Natural Science Foundation of China under grant no. 69890230. References
Fig. 10. The depth profile of SIMS of the GaN films.
The Si signal in the SIMS is strong, we think the following reason was responsible for this. Under the high growth temperature and the low growth pressure, the Si would have been evaporated from the highly heated Ga quartz crucible and quartz tubes. The O curve in Fig. 10 has a rising start during the early stage of etching due to the oxygen adsorbed by the films surface. The graph is flat in the interface between the substrate and the epilayer, revealing that we have removed the oxygen effectively and gotten a clean growth surface by RCA cleaning and in situ cleaning. In our experiments, in the SIMS of samples without in situ cleaning, the O graph always gets small peaks in the interface. It is also shown that the content of O in the epilayer is lower than
[1] Nakamura S, Tietian J. Appl Phys Lett 1994;74:1687. [2] Yang QK, Li AZ, Zhang YG, Yang B, Brandt O, Ploog K. J Cryst Growth 1998;192:28. [3] Brandt O, Wuensche H-J, Yang H, Klann R, Muellhaeuser JR, Ploog KH. J Cryst Growth 1998;189–190:790. [4] Nakamura S, Senoh M, Nagahama S. Jpn J Appl Phys 1996;35:74. [5] Takeuchi T, Amano H, Hiramatsu K, Sawaki N, Akasaki I. J Cryst Growth 1991;115:634. [6] Watanabe A, Takeuchi T, Hirosawa K, Amano H, Hiramatsu K, Akasaki I. J Cryst Growth 1993;128:391. [7] Guha S, Bojarczuk N. Appl Phys Lett 1998;72:415. [8] Yang B, Brandt O, Trampert A, Jenichen B, Ploog KH. Appl Surf Sci 1998;123–124:1. [9] Nakada Y, Aksenov L, Okumura H. Appl Phys Lett 1998;73:827. [10] Ishida A, Yamamoto E, Ishino K, Ito K, Fujiyasu H. Appl Phys Lett 1995;67:665. [11] Qian W, Skowronski M, De Graef M. Appl Phys Lett 1995;66(10):1252. [12] Tanoue F, Sakakibara S, Ohbora M, Ishino K, Ishida A, Fujiyasu H. J Cryst Growth 1998;188–190:47.