- Email: [email protected]
Growth of cubic GaN on Si(001) by plasma-assisted MBE
applie~ surface s c i e n c e ELSEVIER
Applied Surface Science 123/124 (1998) 1-6
Growth of cubic GaN on Si(001) by plasma-assisted MBE B. Yang *, O. Brandt, A. Trampert, B. Jenichen, K.H. Ploog Paul-Drude-lnstitut fiir FestkiSrperelektronik, Hauscogteiplatz 5 - 7, D- 10117 Berlin, Germany
Abstract
GaN films grown directly on Si(001) by plasma-assisted molecular beam epitaxy are found to undergo a structural phase transformation from epitaxial growth of the cubic (/3) phase towards textured growth of the hexagonal (c~) phase. The origin of this phase transformation is investigated and identified to be due to the formation of amorphous SixN , material at the GaN/Si interface during the nucleation stage. A GaAs buffer layer overcomes this problem as evidenced by the phase purity of the resulting epitaxial /3-GaN films on GaAs/Si(001). © 1998 Elsevier Science B.V. PACS: 81.15.Hi; 68.35.Ct; 61.16.Bg; 78.55.Cr Keywords: Cubic GaN; Epitaxy; Coincidence lattice
1. Introduction
Investigation of the growth of /3-GaN on Si(001) is motivated by several interesting issues. First, Si has obvious advantages over any other substrate from a technological point of view [1,2]. Second, the high thermal stability of Si allows the exploration of high growth temperatures which cannot be reached for substrates such as GaAs. Third, the possible occurrence of a coincidence lattice between /3-GaN and Si (a6aN : asi = 5 : 6 ) , which has been found in previous studies [3] to promote epitaxial growth of highly mismatched systems, makes growth o f / 3 - G a N on Si(001) to be also of fundamental interest. However, the use of Si(001) substrates for GaN growth results in several problems, such as the formation of
* Corresponding author. Tel.: +49-30-20377366; fax: +49-3020377201; e-mail: [email protected].
antiphase domains [4] characteristic for polar-onnonpolar epitaxy. In fact, growth of GaN directly on Si(001) has been found to generally result in phase mixtured [5,6] or polycrystalline films [7]. As a consequence, both the structural and the optical quality of GaN films grown on Si(001) substrates reported so far were poor. It is thus necessary to study the origin of the phase mixture in order to develop procedures to improve the quality of the /3-GaN films. In this paper, we study the direct nucleation of GaN on Si(001) as well as the use of GaAs buffer layers. The j 3 - a phase transformation occurring in the former case during growth is investigated in detail and is traced back to the formation of amorphous SixN,. in the nucleation stage. The positive effect of the GaAs buffer layer is highlighted by the phase purity of the /3-GaN films grown on GaAs/Si(001).
0169-4332/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0169-4332(97)00463-7
2
B. Yang et al./Applied Sulfate Science 1 2 3 / 1 2 4 (1998) 1 6
2. Experiment The samples studied in this paper are grown in a home-made molecular beam epitaxy (MBE) system equipped with a DC glow discharge N plasma source operating with a power of 30 W [8]. Si(001) wafers with miscut of 4 ° toward (110) are used as substrates. Prior to loading into the MBE chamber, the substrates are degreased in solvents and etched in a buffered HF solution. In the growth chamber, the Si substrates are annealed at 850°C for half an hour prior to growth. GaN deposition is initiated by opening both Ga and N shutters and igniting the N plasma simultaneously. The GaN growth rate is set to 0.06 monolayers (ML)/s. In situ reflection high energy electron diffraction (RHEED), ex situ double-crystal X-ray diffractometry (DCXRD) and atomic force microscopy (AFM) are used to investigate the crystal structure and the morphology of the GaN films. Both conventional (dark-field) and highresolution (HR) transmission electron microscopy (TEM) observations are carried out in JEOL JEM 4000FX/EX microscopes operating at 400 kV.
3. Results and discussion 3.1. GaN fihns grown directly on Si(O01) Based on our experience in the growth of /3-GaN on GaAs(001) [8], GaN films are deposited on Si(001) in two steps which include a low-temperature (500°C-550°C) nucleation layer followed by hightemperature growth (650°C) for the rest of the film. In the nucleation stage, we focus on studying growth conditions under which (i) epitaxial growth of/3-GaN is obtained and (ii) already a thin (10 ML) GaN layer achieves complete coverage of the Si(001) surface. Such an epitaxial 'wetting' layer is expected to facilitate further growth of a single-phase /3-GaN layer with a smooth surface morphology [8]. In situ RHEED patterns exhibit a well-defined cubic symmetry after growth of 10 ML GaN when nucleation takes place on a single-domain (1 × 2)-Si(001) surface (which requires use of 4 ° misoriented Si(001) substrates) at a temperature of about 500°C-550°C and an effective N / G a flux ratio of N / G a =-2.0. AFM micrographs of such samples reveal a smooth
surface morphology, in contrast to layers grown under smaller or larger N / G a ratios where comparatively large, isolated GaN islands are found by AFM. However, a more detailed analysis of the layers grown under optimized conditions reveals that epitaxy is only partially achieved. Fig. l(a) displays a large scale HRTEM micrograph of one of these layers. Up- and downward arrows indicate regions directly at the interface and within the layer, respectively, which exhibit a contrast different from that of /3-GaN. Some portions of this micrograph are shown in higher magnification in Fig. l(b), (c) and (d) and reveal the following interesting phenomena. The nucleation layer is actually not a homogeneous and continuous GaN layer, but is interrupted by randomly distributed amorphous domains (as indicated by the downward arrow in Fig. l(b)) in-between the /3-GaN grains. Detailed studies [9] of the interaction between the active N plasma and the Si(001) surface demonstrate that this amorphous material at the G a N / S i interface is SigN,. At some G a N / S i interfacial regions (indicated by the upward arrow in Fig. l(b)), the /3-GaN grains are heavily misoriented and contain a very high density of stacking faults, c~-GaN grains whose basal plane is parallel to Si(001) surface are observed to grow on top of the amorphous patches at the interface (Fig. l(c)). For those G a N / S i interfacial regions where a good epitaxial relation between GaN and Si is established (Fig. l(d)), the large lattice mismatch ( = 17%) between /3-GaN and Si is accommodated by periodic misfit dislocations formed every 5 Si lattice planes (6 GaN lattice planes) as expected from the coincidence lattice model. Subsequent growth of GaN on top of the 'wetting' layer is found to lead to a clear transition from the /3- towards the o~-phase as observed by RHEED. This transition is found to be independent of both the thickness of the GaN nucleation layer and the conditions used for subsequent growth. The phase transformation is further confirmed by X-ray diffractometry of a 700 nm thick GaN film, where the dominant peak stems from the GaN(0002) reflection. Azimuthal scans across asymmetric reflections show the in-plane orientation of the hexagonal GaN to be close to random. The cross-sectional TEM dark-field micrograph of this sample is shown in Fig. 2. The GaN film
Fig. I. (110) cross-sectional HRTEM micrographs of a 10 ML thick GaN nucleation layer. (a) Overview, (b) higher magnification, showing the presence of amorphous material at the interface, (c) higher magnification, showing an u-GaN grain nucleating on top of the amorphous matcrial, (d) higher magnification, showing an area where a well-defined epitaxial relation between GaN and Si is achieved. In (a) and (b), the downward and upward arrows point to the locations of the amorphous material both in the layer and at the interface: in (d) the arrows indicate the edge-type dislocations formed every 5 Si-Si interatomic distances.
(a)
4~
7"
4
B. Yang et al./Applied Su~,iTce Science 123 / 124 (1998) 1-6
displays the characteristic columnar structure of aGaN. The columns are separated by straight boundaries and have well-defined surface facets. Within these a-GaN grains, a high density of planar defects is detected, which are identified to be stacking faults lying in the close-packed basal plane. These stacking faults are running parallel to the G a N / S i interface, showing that the out-of-plane orientation spread of the columns is small, resulting in a highly textured, fiber-like microstructure. The change of contrast close to the G a N / S i interface originates from grains of fl-GaN, as demonstrated by means of high-resolution TEM (see the inset of Fig. 2) and selected area electron diffraction (SAD) patterns. These small /3GaN domains are separated from each other by the c~-GaN columns which extend from the amorphous SixNy patches at the interface. It is thus apparent that the nucleation and growth of GaN on Si(001) takes place via two different
parallel paths. /3-GaN with an epitaxial relationship to Si(001) is formed on the bare Si surface, but complete coverage is inhibited by the simultaneous formation of amorphous SixN,.. GaN condensing on top of this SixNy-covered Si does not experience an epitaxial constraint and grows thus in its ce-modification with random in-plane orientation. Further growth leads to largely columnar growth of a-GaN which rapidly overgrows the initial /3-GaN grains. This latter fact seems to imply a higher growth rate of the basal plane a n d / o r a lower surface energy, as compared with the (001) plane of /3-GaN. In any case, the phase transformation seems inevitable unless the Si surface is protected against impinging N. 3.2. GaN layers grown on GaAs-buffered Si(O01)
In order to inhibit the formation of SixN ,, at the nucleation stage, a GaAs buffer layer is employed in
Fig. 2. (110) cross-sectional TEM dark-field image of a 0.7 /xm thick GaN film grown directly on Si(001). The inset displays a high-resolution image of a fl-GaN grain at the interface.
B. Yang et al. / Applied Surface Science 123/124 (1998) 1-6
our experiments. Optimized growth conditions are used to grow both the GaAs buffer layer and the /3-GaN epilayers [10,11]. The X-ray diffraction profile of a 0.7 /~m thick GaN film grown on GaAsbuffered Si(001) shown in Fig. 3 is dominated by the (002) reflection of /3-GaN. The phase content and crystallinity of the GaN film are also characterized by taking SAD patterns from a (110) cross-sectional sample of the G a N / G a A s heterostructure (see the inset of Fig. 3). The superposition of the diffraction patterns of the GaAs buffer layer and the GaN film along their (110) zone axes visualizes the epitaxial orientation relationship. The lattice mismatch of 20% measured from the distance of the corresponding GaN and GaAs diffraction spots is in agreement with the result obtained by DCXRD. The streaks along the (111) directions reveal that the major structural defects in this film are stacking faults. Fig. 4 displays the cross-sectional HRTEM micro-
5
graphs of the / 3 - G a N / G a A s / S i heterostructure. The micrographs taken from both the G a A s / S i (Fig. 4(a)) and G a N / G a A s (Fig. 4(b)) interfaces demonstrate the well-defined epitaxial relationships. The large lattice mismatches at both interfaces are primarily accommodated by misfit dislocations. Furthermore, stacking faults and microtwins penetrate into the epilayers. These and the above results are essentially identical to those obtained for /3-GaN layers grown directly on GaAs(001) [12]. However, TEM micrographs covering a larger interfacial length (not shown here) reveal the additional presence of bundles of threading dislocations originating at the G a N / G a A s interface which are n o t observed for /3-GaN layers on on-axis GaAs(001) substrates. We speculate that the origin of these threading dislocations is related to the vertical mismatch between GaN and GaAs at the steps on the off-axis GaAs surface. The presence of these additional defects
40 GaAs (002)
00 c-
30
..d v
>- 20
GaN (002)
03 Z 10 IJJ
0
I
16
,
1
20
,
I
24
~
I
28
ANGLE (degree) Fig. 3. X-ray ~o-20 profile of a 0.7 ~m thick GaN film grown on GaAs/Si(001). The inset shows a SAD pattern taken from a (110) cross-sectional sample of the GaN/GaAs/Si heterostructure.
6
B. Yang et al./Applied Sucface Science 123 / 124 (1998) 1-6
tially smaller than that obtained from layers grown directly on GaAs(001).
4. Summary In conclusion, we have demonstrated that, though the large lattice mismatch ( ~ 17%) between /3-GaN and Si is effectively relieved by an array of misfit dislocations determined by a coincidence lattice ( a s i ' a c , ~ N = 6:5), epitaxial nucleation of ,8-GaN directly on Si(001) is obstacled by the formation of amorphous S i , N , material at the G a N / S i interface. Therefore, buffer layers are necessary to improve the quality of the GaN films grown on Si(001). A GaAs buffer layer employed in our experiment is demonstrated to be useful for the growth of /3-GaN epilayers on Si(001) substrates.
References
Fig. 4. {110) cross-sectional HRTEM images of the 0.7/zm thick GaN film grown on GaAs/Si(001) showing (a) the GaAs/Si interface and (b) the G a N / G a A s interface.
might also explain that, while band-edge photoluminescence of/3-GaN is observed for these layers up to room temperature, its integrated intensity is substan-
[1] T. Lei, M. Fanciulli, R.J. Molnar, T.D. Moustakas, R.J. Graham, J. Scanlon, Appl. Phys. Lett. 59 (1991) 944. [2] T. Lei, T.D. Moustakas, Mater. Res. Soc, Syrup. Proc. 242 (1992) 433. [3] A. Trampcrt, O. Brandt, H. Yang. K.H. Ploog, Appl. Phys. Lctt. 70 (1997) 583. [4] S.N. Basu, T. Li, T.D. Moustakas, J. Mater. Res. 9 (1994) 2370, [5] T. Lei, T.D. Moustakas, R.J. Graham, Y. He, S.J. Berkowitz, J. Appl. Phys. 71 (1992) 4933. [6] T. Lei, K.F. Ludwig Jr., T.D. Moustakas, J. Appl. Phys. 74 (1993) 4430. [7] Z. Sitar. M.J. Paiseley, B. Yan, R.F. Davis. MRS Symp. Proc. 162 (1990) 537. [8] H. Yang, O. Brandt, A. Trampert, K.H. Ploog, Appl. Surf. Sci. 104 (1996) 461. [9] B. Yang, A. Trampert, O. Brandt, B. Jenichen, K.H. Ploog, unpublished. [10] W.1. Wang, Appl. Phys. Lett. 44 (1984) 1149. [1 l] O. Brandt, H. Yang, B. Jenichen, Y. Suzuki, L. D~iweritz, K.H. Ploog, Phys. Rev. B 52 (1995) R2253. [12] O. Brandt, H. Yang, K.H. Ploog, Mater. Sci. Eng. B 43 (1997) 214.
Applied Surface Science 123/124 (1998) 1-6
Growth of cubic GaN on Si(001) by plasma-assisted MBE B. Yang *, O. Brandt, A. Trampert, B. Jenichen, K.H. Ploog Paul-Drude-lnstitut fiir FestkiSrperelektronik, Hauscogteiplatz 5 - 7, D- 10117 Berlin, Germany
Abstract
GaN films grown directly on Si(001) by plasma-assisted molecular beam epitaxy are found to undergo a structural phase transformation from epitaxial growth of the cubic (/3) phase towards textured growth of the hexagonal (c~) phase. The origin of this phase transformation is investigated and identified to be due to the formation of amorphous SixN , material at the GaN/Si interface during the nucleation stage. A GaAs buffer layer overcomes this problem as evidenced by the phase purity of the resulting epitaxial /3-GaN films on GaAs/Si(001). © 1998 Elsevier Science B.V. PACS: 81.15.Hi; 68.35.Ct; 61.16.Bg; 78.55.Cr Keywords: Cubic GaN; Epitaxy; Coincidence lattice
1. Introduction
Investigation of the growth of /3-GaN on Si(001) is motivated by several interesting issues. First, Si has obvious advantages over any other substrate from a technological point of view [1,2]. Second, the high thermal stability of Si allows the exploration of high growth temperatures which cannot be reached for substrates such as GaAs. Third, the possible occurrence of a coincidence lattice between /3-GaN and Si (a6aN : asi = 5 : 6 ) , which has been found in previous studies [3] to promote epitaxial growth of highly mismatched systems, makes growth o f / 3 - G a N on Si(001) to be also of fundamental interest. However, the use of Si(001) substrates for GaN growth results in several problems, such as the formation of
* Corresponding author. Tel.: +49-30-20377366; fax: +49-3020377201; e-mail: [email protected].
antiphase domains [4] characteristic for polar-onnonpolar epitaxy. In fact, growth of GaN directly on Si(001) has been found to generally result in phase mixtured [5,6] or polycrystalline films [7]. As a consequence, both the structural and the optical quality of GaN films grown on Si(001) substrates reported so far were poor. It is thus necessary to study the origin of the phase mixture in order to develop procedures to improve the quality of the /3-GaN films. In this paper, we study the direct nucleation of GaN on Si(001) as well as the use of GaAs buffer layers. The j 3 - a phase transformation occurring in the former case during growth is investigated in detail and is traced back to the formation of amorphous SixN,. in the nucleation stage. The positive effect of the GaAs buffer layer is highlighted by the phase purity of the /3-GaN films grown on GaAs/Si(001).
0169-4332/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0169-4332(97)00463-7
2
B. Yang et al./Applied Sulfate Science 1 2 3 / 1 2 4 (1998) 1 6
2. Experiment The samples studied in this paper are grown in a home-made molecular beam epitaxy (MBE) system equipped with a DC glow discharge N plasma source operating with a power of 30 W [8]. Si(001) wafers with miscut of 4 ° toward (110) are used as substrates. Prior to loading into the MBE chamber, the substrates are degreased in solvents and etched in a buffered HF solution. In the growth chamber, the Si substrates are annealed at 850°C for half an hour prior to growth. GaN deposition is initiated by opening both Ga and N shutters and igniting the N plasma simultaneously. The GaN growth rate is set to 0.06 monolayers (ML)/s. In situ reflection high energy electron diffraction (RHEED), ex situ double-crystal X-ray diffractometry (DCXRD) and atomic force microscopy (AFM) are used to investigate the crystal structure and the morphology of the GaN films. Both conventional (dark-field) and highresolution (HR) transmission electron microscopy (TEM) observations are carried out in JEOL JEM 4000FX/EX microscopes operating at 400 kV.
3. Results and discussion 3.1. GaN fihns grown directly on Si(O01) Based on our experience in the growth of /3-GaN on GaAs(001) [8], GaN films are deposited on Si(001) in two steps which include a low-temperature (500°C-550°C) nucleation layer followed by hightemperature growth (650°C) for the rest of the film. In the nucleation stage, we focus on studying growth conditions under which (i) epitaxial growth of/3-GaN is obtained and (ii) already a thin (10 ML) GaN layer achieves complete coverage of the Si(001) surface. Such an epitaxial 'wetting' layer is expected to facilitate further growth of a single-phase /3-GaN layer with a smooth surface morphology [8]. In situ RHEED patterns exhibit a well-defined cubic symmetry after growth of 10 ML GaN when nucleation takes place on a single-domain (1 × 2)-Si(001) surface (which requires use of 4 ° misoriented Si(001) substrates) at a temperature of about 500°C-550°C and an effective N / G a flux ratio of N / G a =-2.0. AFM micrographs of such samples reveal a smooth
surface morphology, in contrast to layers grown under smaller or larger N / G a ratios where comparatively large, isolated GaN islands are found by AFM. However, a more detailed analysis of the layers grown under optimized conditions reveals that epitaxy is only partially achieved. Fig. l(a) displays a large scale HRTEM micrograph of one of these layers. Up- and downward arrows indicate regions directly at the interface and within the layer, respectively, which exhibit a contrast different from that of /3-GaN. Some portions of this micrograph are shown in higher magnification in Fig. l(b), (c) and (d) and reveal the following interesting phenomena. The nucleation layer is actually not a homogeneous and continuous GaN layer, but is interrupted by randomly distributed amorphous domains (as indicated by the downward arrow in Fig. l(b)) in-between the /3-GaN grains. Detailed studies [9] of the interaction between the active N plasma and the Si(001) surface demonstrate that this amorphous material at the G a N / S i interface is SigN,. At some G a N / S i interfacial regions (indicated by the upward arrow in Fig. l(b)), the /3-GaN grains are heavily misoriented and contain a very high density of stacking faults, c~-GaN grains whose basal plane is parallel to Si(001) surface are observed to grow on top of the amorphous patches at the interface (Fig. l(c)). For those G a N / S i interfacial regions where a good epitaxial relation between GaN and Si is established (Fig. l(d)), the large lattice mismatch ( = 17%) between /3-GaN and Si is accommodated by periodic misfit dislocations formed every 5 Si lattice planes (6 GaN lattice planes) as expected from the coincidence lattice model. Subsequent growth of GaN on top of the 'wetting' layer is found to lead to a clear transition from the /3- towards the o~-phase as observed by RHEED. This transition is found to be independent of both the thickness of the GaN nucleation layer and the conditions used for subsequent growth. The phase transformation is further confirmed by X-ray diffractometry of a 700 nm thick GaN film, where the dominant peak stems from the GaN(0002) reflection. Azimuthal scans across asymmetric reflections show the in-plane orientation of the hexagonal GaN to be close to random. The cross-sectional TEM dark-field micrograph of this sample is shown in Fig. 2. The GaN film
Fig. I. (110) cross-sectional HRTEM micrographs of a 10 ML thick GaN nucleation layer. (a) Overview, (b) higher magnification, showing the presence of amorphous material at the interface, (c) higher magnification, showing an u-GaN grain nucleating on top of the amorphous matcrial, (d) higher magnification, showing an area where a well-defined epitaxial relation between GaN and Si is achieved. In (a) and (b), the downward and upward arrows point to the locations of the amorphous material both in the layer and at the interface: in (d) the arrows indicate the edge-type dislocations formed every 5 Si-Si interatomic distances.
(a)
4~
7"
4
B. Yang et al./Applied Su~,iTce Science 123 / 124 (1998) 1-6
displays the characteristic columnar structure of aGaN. The columns are separated by straight boundaries and have well-defined surface facets. Within these a-GaN grains, a high density of planar defects is detected, which are identified to be stacking faults lying in the close-packed basal plane. These stacking faults are running parallel to the G a N / S i interface, showing that the out-of-plane orientation spread of the columns is small, resulting in a highly textured, fiber-like microstructure. The change of contrast close to the G a N / S i interface originates from grains of fl-GaN, as demonstrated by means of high-resolution TEM (see the inset of Fig. 2) and selected area electron diffraction (SAD) patterns. These small /3GaN domains are separated from each other by the c~-GaN columns which extend from the amorphous SixNy patches at the interface. It is thus apparent that the nucleation and growth of GaN on Si(001) takes place via two different
parallel paths. /3-GaN with an epitaxial relationship to Si(001) is formed on the bare Si surface, but complete coverage is inhibited by the simultaneous formation of amorphous SixN,.. GaN condensing on top of this SixNy-covered Si does not experience an epitaxial constraint and grows thus in its ce-modification with random in-plane orientation. Further growth leads to largely columnar growth of a-GaN which rapidly overgrows the initial /3-GaN grains. This latter fact seems to imply a higher growth rate of the basal plane a n d / o r a lower surface energy, as compared with the (001) plane of /3-GaN. In any case, the phase transformation seems inevitable unless the Si surface is protected against impinging N. 3.2. GaN layers grown on GaAs-buffered Si(O01)
In order to inhibit the formation of SixN ,, at the nucleation stage, a GaAs buffer layer is employed in
Fig. 2. (110) cross-sectional TEM dark-field image of a 0.7 /xm thick GaN film grown directly on Si(001). The inset displays a high-resolution image of a fl-GaN grain at the interface.
B. Yang et al. / Applied Surface Science 123/124 (1998) 1-6
our experiments. Optimized growth conditions are used to grow both the GaAs buffer layer and the /3-GaN epilayers [10,11]. The X-ray diffraction profile of a 0.7 /~m thick GaN film grown on GaAsbuffered Si(001) shown in Fig. 3 is dominated by the (002) reflection of /3-GaN. The phase content and crystallinity of the GaN film are also characterized by taking SAD patterns from a (110) cross-sectional sample of the G a N / G a A s heterostructure (see the inset of Fig. 3). The superposition of the diffraction patterns of the GaAs buffer layer and the GaN film along their (110) zone axes visualizes the epitaxial orientation relationship. The lattice mismatch of 20% measured from the distance of the corresponding GaN and GaAs diffraction spots is in agreement with the result obtained by DCXRD. The streaks along the (111) directions reveal that the major structural defects in this film are stacking faults. Fig. 4 displays the cross-sectional HRTEM micro-
5
graphs of the / 3 - G a N / G a A s / S i heterostructure. The micrographs taken from both the G a A s / S i (Fig. 4(a)) and G a N / G a A s (Fig. 4(b)) interfaces demonstrate the well-defined epitaxial relationships. The large lattice mismatches at both interfaces are primarily accommodated by misfit dislocations. Furthermore, stacking faults and microtwins penetrate into the epilayers. These and the above results are essentially identical to those obtained for /3-GaN layers grown directly on GaAs(001) [12]. However, TEM micrographs covering a larger interfacial length (not shown here) reveal the additional presence of bundles of threading dislocations originating at the G a N / G a A s interface which are n o t observed for /3-GaN layers on on-axis GaAs(001) substrates. We speculate that the origin of these threading dislocations is related to the vertical mismatch between GaN and GaAs at the steps on the off-axis GaAs surface. The presence of these additional defects
40 GaAs (002)
00 c-
30
..d v
>- 20
GaN (002)
03 Z 10 IJJ
0
I
16
,
1
20
,
I
24
~
I
28
ANGLE (degree) Fig. 3. X-ray ~o-20 profile of a 0.7 ~m thick GaN film grown on GaAs/Si(001). The inset shows a SAD pattern taken from a (110) cross-sectional sample of the GaN/GaAs/Si heterostructure.
6
B. Yang et al./Applied Sucface Science 123 / 124 (1998) 1-6
tially smaller than that obtained from layers grown directly on GaAs(001).
4. Summary In conclusion, we have demonstrated that, though the large lattice mismatch ( ~ 17%) between /3-GaN and Si is effectively relieved by an array of misfit dislocations determined by a coincidence lattice ( a s i ' a c , ~ N = 6:5), epitaxial nucleation of ,8-GaN directly on Si(001) is obstacled by the formation of amorphous S i , N , material at the G a N / S i interface. Therefore, buffer layers are necessary to improve the quality of the GaN films grown on Si(001). A GaAs buffer layer employed in our experiment is demonstrated to be useful for the growth of /3-GaN epilayers on Si(001) substrates.
References
Fig. 4. {110) cross-sectional HRTEM images of the 0.7/zm thick GaN film grown on GaAs/Si(001) showing (a) the GaAs/Si interface and (b) the G a N / G a A s interface.
might also explain that, while band-edge photoluminescence of/3-GaN is observed for these layers up to room temperature, its integrated intensity is substan-
[1] T. Lei, M. Fanciulli, R.J. Molnar, T.D. Moustakas, R.J. Graham, J. Scanlon, Appl. Phys. Lett. 59 (1991) 944. [2] T. Lei, T.D. Moustakas, Mater. Res. Soc, Syrup. Proc. 242 (1992) 433. [3] A. Trampcrt, O. Brandt, H. Yang. K.H. Ploog, Appl. Phys. Lctt. 70 (1997) 583. [4] S.N. Basu, T. Li, T.D. Moustakas, J. Mater. Res. 9 (1994) 2370, [5] T. Lei, T.D. Moustakas, R.J. Graham, Y. He, S.J. Berkowitz, J. Appl. Phys. 71 (1992) 4933. [6] T. Lei, K.F. Ludwig Jr., T.D. Moustakas, J. Appl. Phys. 74 (1993) 4430. [7] Z. Sitar. M.J. Paiseley, B. Yan, R.F. Davis. MRS Symp. Proc. 162 (1990) 537. [8] H. Yang, O. Brandt, A. Trampert, K.H. Ploog, Appl. Surf. Sci. 104 (1996) 461. [9] B. Yang, A. Trampert, O. Brandt, B. Jenichen, K.H. Ploog, unpublished. [10] W.1. Wang, Appl. Phys. Lett. 44 (1984) 1149. [1 l] O. Brandt, H. Yang, B. Jenichen, Y. Suzuki, L. D~iweritz, K.H. Ploog, Phys. Rev. B 52 (1995) R2253. [12] O. Brandt, H. Yang, K.H. Ploog, Mater. Sci. Eng. B 43 (1997) 214.