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Epitaxial growth of AlN and GaN on Si(1 1 1) by plasma-assisted molecular beam epitaxy
Journal of Crystal Growth 201/202 (1999) 359}364
Epitaxial growth of AlN and GaN on Si(1 1 1) by plasma-assisted molecular beam epitaxy H.P.D. Schenk * , G.D. Kipshidze , V.B. Lebedev , S. Shokhovets, R. Goldhahn, J. KraK ublich, A. Fissel , Wo. Richter Institut fu( r Festko( rperphysik, Friedrich-Schiller-Universita( t Jena, Max-Wien-Platz 1, 07743 Jena, Germany Iowe Physico-Technical Insitute, Russian Academy of Science, 26 Polytechnicheskaya, St. Petersburg 194021, Russia Institut fu( r Physik, Technische Universita( t Ilmenau, PF 10 05 65, 98684 Ilmenau, Germany Institut fu( r Optik und Quantenelektronik, Friedrich-Schiller-Universita( t Jena, Max-Wien-Platz 1, 07743 Jena, Germany
Abstract Epitaxial wurtzite aluminum nitride (AlN) and gallium nitride (GaN) "lms have been grown on Si(1 1 1) by plasma-assisted molecular beam epitaxy (PA-MBE). Two-dimensional growth (2DG) of single-crystalline AlN "lms is achieved on Si(1 1 1) near stoichiometric supply of aluminum and atomic nitrogen. Two distinct categories of AlNsurface reconstructions have been observed. GaN "lms exhibit 1;1 high-energy electron di!raction (RHEED) pattern if grown between 6503C and 7703C substrate temperature on AlN bu!ers. Stable Ga- and N-adlayer-induced surface reconstructions have been studied below 6003C after the growth. The X-ray di!raction (XRD) pattern show sharp and well separated (0 0 0 l) re#ections of wurtzite GaN and AlN indicating complete texture with GaN[0 0 0 1]#AlN[0 0 01]#Si[1 1 1]. From the determined GaN lattice constant complete strain relaxation can be concluded which is further con"rmed by the temperature-dependent photoluminescence (PL) investigations. 1999 Elsevier Science B.V. All rights reserved. PACS: 61.10.Nz; 61.14.Hg; 78.55.Cr Keywords: AlN; GaN; Nitrides; RHEED; Adlayer; Surface reconstruction
1. Introduction The wurtzite polytypes of (In, Ga, Al)N form a continuous alloy system with band gaps ranging from 1.9 to 6.2 eV. They are key materials for the * Corresponding author. Present address: CRHEA-CNRS, Rue Bernard GreH gory, Sophia Antipolis, F-06560 Valbonne, France. Fax: #33-493958361; e-mail: [email protected].
production of short wavelength light emitting [1}3] and light detecting devices [4,5]. AlN, a material with high surface acoustic wave (SAW) velocity, is feasible for application in SAW devices [6]. Furthermore (Al, Ga)N and AlN are good materials for the fabrication of high-temperature and high-power transistors [7]. So far, sapphire remains the most widely used substrate material for the heteroepitaxial growth of nitrides. However, the nitrides and the silicon
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microelectronics technology would both bene"t from the availability of nitride structures on silicon instead. In most attempts to grow nitrides on silicon, intermediate layers such as SiC [8,9], GaAs [10,11], oxidized AlAs [12] or c-Al O [13] were used. After early studies by Meng et al. [14] the best results have been achieved on AlN bu!er layers by using plasma-assisted molecular beam epitaxy (PA-MBE) [2,4,15,16] and gas-source MBE [5,17]. The AlN/Si(1 1 1) interface is very sharp. Nearly complete relaxation is observed to occur within one bilayer [18] by the formation of a coincidence lattice [19,20]. It is well established that GaN grown directly on Si has a poor "lm morphology [16,21,22]. Yang et al. found that the nucleation of GaN is obstacled by the formation of amorphous Si N in the GaN/Si V W interfacial region [11]. Two-dimensional growth (2DG) of N-face GaN(0 0 0 1 ), indicated by 1;1 RHEED pattern during the growth, as well as the growth of Ga-face GaN(0 0 0 1), indicated by 1;1 and 2;2 RHEED pattern, has been thoroughly investigated on Al O , SiC and GaN. See Smith et al. [23] for a recent review. 2DG of AlN, as evident from streaky RHEED pattern, has been reported on Si (1 1 1) [5,19,20]. Surface reconstructions, namely the 2;2- [16,24] and the 3;3-reconstruction [18], have been observed on AlN grown on Si(1 1 1). However, no details about the corresponding surface stoichiometry have been given. In this work, we study the 2DG growth of AlN on Si(1 1 1) and of GaN on AlN bu!ers by PAMBE. We focus on occurring surface reconstructions as a function of growth initiation, surface stoichiometry, "lm temperature and adlayer coverage. The "lms have been characterized by RHEED, X-ray di!raction (XRD) and photoluminescence (PL).
2. Experimental procedure The "lm growth has been performed in a homemade MBE-system using standard e!usion sources for evaporation of Al (6 N) and Ga (7 N) and using
a RF-source to supply activated nitrogen. For details see Ref. [19,20]. Growth mode and surface superstructures have been monitored by a 10 kV RHEED system. The Si(1 1 1) wafers ('5 ), p-type) have been prepared by a modi"ed Shiraki procedure [19]. They were degassed in ultra-high vacuum above growth temperature. The 7;7-surface reconstruction which appears at 7803C transfers into 1;1surface above 8503C. The 7;7-reconstruction is recovered when the substrate temperature is lowered to 8503C and persists at any temperature below. The AlN growth process has been started at 7003. At "rst we induce a (3;(3-reconstructed Si(1 1 1) surface by the deposition of monolayer of aluminium [25]. A nitridation step is carried out after ignition of the RF-plasma. The growth experiment is started when the transition to the 1;1 RHEED pattern of unreconstructed AlN is observed. Subsequently, the growth temperature is raised up to its "nal value. Alternatively, the Si(1 1 1)-7;7 surface is exposed to activated nitrogen for few seconds until a 3;3-surface reconstruction appears, presumably induced by an ordered N adlayer. Then the growth is started by opening the Al-shutter. The growth of GaN directly on Si(1 1 1) is started at 5003C with monolayer of gallium forming a (3;(3-reconstructed Si(1 1 1) surface as well [25]. The procedure is otherwise identical to the Al-initiation of the AlN growth. The GaN growth on AlN bu!er layers is started at 6503C. The growth temperature is then gradually raised up to 7703C. The growth is insensitive to the mode employed during the initial stage.
3. Results and discussion The 2DG growth of single-crystalline AlN "lms is achieved within the range of substrate temperatures from 8503C to 9003C with a III : V ratio of 91 : 1 and the plasma source working under optimized conditions. We "nd two distinct categories of AlN surface reconstructions. In the "rst category, we observe 2DG of AlN under N-rich, stoichiometric and Al-rich growth
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conditions corresponding to streaky (3;(3, 1;1 and 2;6 RHEED pattern, respectively [20]. In the second category, only streaky 1;1 RHEED pattern of the unreconstructed AlN surface occur during the growth. The 3;3- and 3;6-surface reconstructions can be induced by exposition of the AlN surface to the #ux of Al-atoms at 6003C after the growth. Also we observe a N-rich (3;(3-reconstruction by exposition to activated nitrogen. The "rst category of surface reconstructions mostly occurs if the growth is started with aluminum, the second if started with nitrogen instead. Furthermore, thermodynamical conditions such as Al supersaturation play a role in the initial stage. By now we can only suppose, that both categories of AlN-surface reconstructions can be assigned to di!erent AlN polarities. Investigations by electron beam channelling and by transmission electron microscopy are underway. GaN "lms have been grown directly on the Si(1 1 1)-face as well as on AlN bu!er layers. GaN invariably grows three-dimensionally on silicon as indicated by spotty wurtzite RHEED pattern but grows two-dimensionally on AlN bu!ers. In this work we describe GaN growth experiments on AlN bu!er layers. The AlN bu!ers have been grown under conditions corresponding to the occurrence of surface reconstructions of the second category as 1;1 or further (3;(3, 3;3 and 3;6. The GaN growth has been carried out under growth conditions controlled by the appearance of bright, streaky 1;1 RHEED pattern. No other RHEED pattern are visible during the growth. However, the 2DG of GaN requires high gallium supersaturation and is very sensitive to changes of the growth temperature and surface stoichiometry. More Ga-rich conditions are recognized by a decreasing intensity of the RHEED pattern due to the formation of Ga-droplets. More N-rich conditions lead to spotty wurtzite RHEED pattern indicating 3DG. N- or Al-adlayer surface reconstructions have been obtained by exposing the GaN surface either to activated nitrogen or to the #ux of Ga atoms after "nishing the growth process. The unreconstructed 1;1-surface persists if the RF-nitrogen source is switched o! above 6503C. An
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unstable 2;2-reconstruction is observed by exposing the 1;1-surface to activated nitrogen again. It held only as long as the N-shutter is opened (Fig. 1(a)). A transition from the 1;1-surface to a 3;1reconstruction is observed at 6503C if the RFsource is switched o! below 6503C. The unstable 2;2-reconstruction can be demonstrated on this surface by opening the N-shutter as well (Fig. 1(a)). Later the samples were cooled down to below 6003C. We "nd stable N- and Ga-adatom-induced surface reconstructions in this temperature range. They are in the order of increasing N-coverage 3;1, 2;2 and 3;3 and in the order of increasing Ga-coverage 2;2, 3;3, 3;6, 6;6 and c(6;12) as can be seen in Fig. 1(a) and (b). Probably, we observe the growth of N-face GaN [23] on N-face AlN. The GaN "lms grown on AlN-bu!ered Si(1 1 1) have been characterized by XRD. Besides the AlN(0 0 0 l ) and the Si(1 1 1) re#ections only the narrow and well separated (0 0 0 2) GaN re#ection at 20"34.603 and the (0 0 0 4) re#ection are observable (Fig. 2). They indicate complete texture with GaN[0 0 0 1]#AlN[0 0 0 1]#Si[1 1 1]. Thus, the growth procedure described above results in single-crystalline wurtzite GaN "lms, consistent with the observed RHEED patterns. The FWHM of the GaN(0 0 0 2) re#ection of a 250 nm GaN "lm is 0.0763 in u/20 and 0.623 in u, which means an improved "lm quality achieved under our experimental conditions compared with results reported by other groups [13}16]. For all investigated samples a GaN lattice constant c"(5.185$0.001)As was determined emphasising that the layers are nearly strain free which is expected if thick AlN bu!er layers are used [26]. Thus, our results are markedly di!erent to studies of the GaN/AlN/Si(1 1 1) system reported in Ref. [16] where c values indicated strong biaxial tensile strain. The good quality of the layers is con"rmed by the PL results shown in Fig. 3. The investigated 880 nm thick GaN "lm on a 125 nm AlN bu!er shows no yellow but intense near-band-edge luminescence. For completely relaxed GaN (as in our case) a neutral donor-bound excitonic transition (D3X, sometimes referred also as I line)
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Fig. 1. RHEED pattern of GaN surface reconstructions observed in post-growth experiments below 6003C taken along the [2 1 1 0](left) and the [1 1 0 0]-azimuth (right). (a) N-rich reconstructions, in the order of decreasing N-coverage: 3;3 (top), 2;2 (center) and 3;1. (b) Ga-rich reconstructions, in the order of increasing Ga-coverage: 3;3 (top), 3;6 (center) and 6;6.
the main emission is found at 3.453 eV which dominates up to room temperature. Performing lowtemperature time-resolved PL studies Godlewski et al. [17] identi"ed it as a further D3X line, however, no explanation concerning the origin of the donor state was given. The temperature dependence makes also a donor-to-band contribution likely, which involves a common donor state (E &55 meV) and the C valence band. It would " T explain the small shift to higher energies found at 40 K. Further work, including electrical and structural characterization is under progress to clarify the nature of this transition. Fig. 2. X-ray di!raction pattern of a 250 nm GaN "lm grown on a 90 nm AlN bu!er on Si(1 1 1) at 7703C (N -#ow rate 0.75 sccm, RF-power 500 W). The pattern have been recorded using Cu K and open detector. Inset: FWHM of the a GaN(0 0 0 2) re#ection 0.0763"5 in u/20 and 0.623 in u (recorded using a 0.22 mm slit aperture).
rapidly diminishing with temperature is normally observed at 3.471 eV. At this energy a weak shoulder in the 5 K spectrum is seen (Fig. 3). However,
4. Summary AlN grown on Si(1 1 1) exhibits two distinct categories of surface reconstructions. In the order of Al-coverage we observe the (3;(3!, 1;1- and 2;6- or the (3;(3!, 1;1-, 3;3- and the 3;6-reconstructions as function of the growth initiation.
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The GaN "lms show no yellow but intense nearband-edge luminescence.
Acknowledgements This work was partially "nanced by the Bundesministerium fuK r Bildung, Wissenschaft, Forschung und Technologie on contract 13N6809/7. G.D.K. and V.B.L. acknowledge "nancial support from the RFBR project No. 97-02-18199, R.G. and S.S. acknowledge support from the INTAS-94-2608 and the ThMWFK B 401-96033 grant. The authors wish to thank S. Klohr and H. SchoK nau for technical assistance.
References
Fig. 3. Normalized near-band-edge photoluminescence taken from 5 K up to room temperature of a 880 nm thick GaN "lm, grown on a 125 nm AlN bu!er on Si(1 1 1) at 7703C (N -#ow rate 1 sccm, 500 W RF-power). FWHM at 5 K is 22 meV.
The usage of AlN bu!ers promotes the 2DG of single-crystalline wurtzite GaN "lms on Si(1 1 1). The XRD pattern show sharp (0 0 0 2) and (0 0 0 4) re#ections. Thus the GaN "lms are epitaxial and single crystalline in agreement with the observed RHEED pattern. Streaky 1;1 RHEED patterns have been observed on GaN grown on AlN bu!ers which exhibit surface reconstructions of the second category. The RHEED patterns indicate 2DG of probably N-face GaN. Stable N- and Ga-rich surface reconstructions are observed below 6003C after the growth. GaN "lms show 3;1-, 2;2- and 3;3-surface reconstructions in the order of N-coverage and 2;2-, 3;3-, 3;6-, 6;6- and c(6;12)-surface reconstructions in the order of Ga-coverage.
[1] S. Nakamura, M. Senoh, S.-i. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, K. Chocho, Appl. Phys. Lett. 72 (1998) 211. [2] S. Guha, N.A. Bojarczuk, Appl. Phys. Lett. 73 (1998) 1487. [3] N. Grandjean, J. Massies, M. Leroux, P. Lorenzini, Appl. Phys. Lett. 72 (1998) 82. [4] K.S. Stevens, M. Kinniburgh, R. Beresford, Appl. Phys. Lett. 66 (1995) 3518. [5] A. Osinsky, S. Gangopadhyay, J.W. Yang, R. Gaska, D. Kuksenkov, H. Temkin, I.K. Shmagin, Y.C. Chang, J.F. Muth, R.M. Kolbas, Appl. Phys. Lett. 72 (1998) 551. [6] H.P.D. Schenk, G.D. Kipshidze, M. Weihnacht, R. Kunze, U. Kaiser, J. Schulze, Wo. Richter, in: Mater. Res. Soc. Symp. Proc., Vol. 535, Materials Research Society, Pittsburg, PA, 1999, and references therein. [7] J. Burm, W.J. Scha!, L.F. Eastman, H. Amano, I. Akasaki, Appl. Phys. Lett. 68 (1996) 2849. [8] A. Barski, U. RoK ssner, J.L. Rouvie`re, M. Arlery, MRS Internet J. Nitride Semicond. Res. 1 (1996) 21. [9] Z. Yang, F. Guarin, I.W. Tao, W.I. Wang, S.S. Iyer, J. Vac. Sci. Technol. B 13 (1995) 789. [10] J.W. Yang, C.J. Sun, Q. Shen, M.Z. Anwar, M.A. Khan, S.A. Nikishin, G.A. Seryogin, A.V. Osinsky, L. Chernyak, H. Temkin, C. Hu, S. Mahajan, Appl. Phys. Lett. 69 (1996) 3566. [11] B. Yang, A. Trampert, O. Brandt, B. Jenichen, K.H. Ploog, J. Appl. Phys. 83 (1998) 3800. [12] 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. [13] L. Wang, X. Liu, Y. Zan, J. Wang, D. Wang, D.-c. Lu, Z. Wang, Appl. Phys. Lett. 72 (1998) 109.
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[14] W.J. Meng, T.A. Perry, J. Appl. Phys. 76 (1994) 7824. [15] A. Ohtani, K.S. Stevens, R. Beresford, Appl. Phys. Lett. 65 (1994) 61. [16] E. Calleja, M.A. SaH nchez-GarcmH a, D. Basak, F.J. SaH nchez, F. Calle, P. Youinou, E. Mun oz, J.J. Serrano, J.M. Blanco, C. Villar, T. Laine, J. Oila, K. Saarinen, P. HautojaK rvi, C.H. Molloy, D.J. Somerford, I. Harrison, Phys. Rev. B 58 (1998) 1550, and references therein. [17] M. Godlewski, J.P. Bergman, B. Monemar, U. RoK ssner, A. Barski, Appl. Phys. Lett. 69 (1996) 2089. [18] A. Bourret, A. Barski, J.L. Rouvie`re, G. Renaud, A. Barbier, J. Appl. Phys. 83 (1998) 2003. [19] H.P.D. Schenk, U. Kaiser, G.D. Kipshidze, A. Fissel, J. KraK ublich, H. Hobert, J. Schulze, Wo. Richter, Mat. Sci. Eng. B (in print). [20] H.P.D. Schenk, G.D. Kipshidze, U. Kaiser, A. Fissel, J. KraK ublich, J. Schulze, Wo. Richter, J. Crystal Growth 200 (1999) 45.
[21] T. Lei, M. Fanciulli, R.J. Molnar, T.D. Moustakas, R.J. Graham, J. Scanlon, Appl. Phys. Lett. 59 (1991) 944. [22] P. Kung, A. Saxler, X. Zhang, D. Walker, T.C. Wang, I. Ferguson, M. Razeghi, Appl. Phys. Lett. 66 (1995) 2958. [23] A.R. Smith, R.M. Feenstra, D.W. Greve, M.-S. Shin, M. Skrowonski, J. Neugebauer, J.E. Northrup, Appl. Phys. Lett. 72 (1998) 2114, and references therein. [24] M.A.L. Johnson, S. Fujita, W.H. Rowland Jr., K.A. Bowers, W.C. Hughes, Y.W. He, N.A. El-Masry, J.W. Cook Jr., J.F. Schetzina, J. Ren, J.A. Edmond, J. Vac. Sci. Technol. B 13 (1996) 2349. [25] W. MoK nch, Semiconductor Surfaces and Interfaces, Springer, Berlin, 1993, p. 257. [26] W. Rieger, T. Metzger, H. Angerer, R. Dimitrov, O. Ambacher, M. Stutzmann, Appl. Phys. Lett. 68 (1996) 970.
Epitaxial growth of AlN and GaN on Si(1 1 1) by plasma-assisted molecular beam epitaxy H.P.D. Schenk * , G.D. Kipshidze , V.B. Lebedev , S. Shokhovets, R. Goldhahn, J. KraK ublich, A. Fissel , Wo. Richter Institut fu( r Festko( rperphysik, Friedrich-Schiller-Universita( t Jena, Max-Wien-Platz 1, 07743 Jena, Germany Iowe Physico-Technical Insitute, Russian Academy of Science, 26 Polytechnicheskaya, St. Petersburg 194021, Russia Institut fu( r Physik, Technische Universita( t Ilmenau, PF 10 05 65, 98684 Ilmenau, Germany Institut fu( r Optik und Quantenelektronik, Friedrich-Schiller-Universita( t Jena, Max-Wien-Platz 1, 07743 Jena, Germany
Abstract Epitaxial wurtzite aluminum nitride (AlN) and gallium nitride (GaN) "lms have been grown on Si(1 1 1) by plasma-assisted molecular beam epitaxy (PA-MBE). Two-dimensional growth (2DG) of single-crystalline AlN "lms is achieved on Si(1 1 1) near stoichiometric supply of aluminum and atomic nitrogen. Two distinct categories of AlNsurface reconstructions have been observed. GaN "lms exhibit 1;1 high-energy electron di!raction (RHEED) pattern if grown between 6503C and 7703C substrate temperature on AlN bu!ers. Stable Ga- and N-adlayer-induced surface reconstructions have been studied below 6003C after the growth. The X-ray di!raction (XRD) pattern show sharp and well separated (0 0 0 l) re#ections of wurtzite GaN and AlN indicating complete texture with GaN[0 0 0 1]#AlN[0 0 01]#Si[1 1 1]. From the determined GaN lattice constant complete strain relaxation can be concluded which is further con"rmed by the temperature-dependent photoluminescence (PL) investigations. 1999 Elsevier Science B.V. All rights reserved. PACS: 61.10.Nz; 61.14.Hg; 78.55.Cr Keywords: AlN; GaN; Nitrides; RHEED; Adlayer; Surface reconstruction
1. Introduction The wurtzite polytypes of (In, Ga, Al)N form a continuous alloy system with band gaps ranging from 1.9 to 6.2 eV. They are key materials for the * Corresponding author. Present address: CRHEA-CNRS, Rue Bernard GreH gory, Sophia Antipolis, F-06560 Valbonne, France. Fax: #33-493958361; e-mail: [email protected].
production of short wavelength light emitting [1}3] and light detecting devices [4,5]. AlN, a material with high surface acoustic wave (SAW) velocity, is feasible for application in SAW devices [6]. Furthermore (Al, Ga)N and AlN are good materials for the fabrication of high-temperature and high-power transistors [7]. So far, sapphire remains the most widely used substrate material for the heteroepitaxial growth of nitrides. However, the nitrides and the silicon
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microelectronics technology would both bene"t from the availability of nitride structures on silicon instead. In most attempts to grow nitrides on silicon, intermediate layers such as SiC [8,9], GaAs [10,11], oxidized AlAs [12] or c-Al O [13] were used. After early studies by Meng et al. [14] the best results have been achieved on AlN bu!er layers by using plasma-assisted molecular beam epitaxy (PA-MBE) [2,4,15,16] and gas-source MBE [5,17]. The AlN/Si(1 1 1) interface is very sharp. Nearly complete relaxation is observed to occur within one bilayer [18] by the formation of a coincidence lattice [19,20]. It is well established that GaN grown directly on Si has a poor "lm morphology [16,21,22]. Yang et al. found that the nucleation of GaN is obstacled by the formation of amorphous Si N in the GaN/Si V W interfacial region [11]. Two-dimensional growth (2DG) of N-face GaN(0 0 0 1 ), indicated by 1;1 RHEED pattern during the growth, as well as the growth of Ga-face GaN(0 0 0 1), indicated by 1;1 and 2;2 RHEED pattern, has been thoroughly investigated on Al O , SiC and GaN. See Smith et al. [23] for a recent review. 2DG of AlN, as evident from streaky RHEED pattern, has been reported on Si (1 1 1) [5,19,20]. Surface reconstructions, namely the 2;2- [16,24] and the 3;3-reconstruction [18], have been observed on AlN grown on Si(1 1 1). However, no details about the corresponding surface stoichiometry have been given. In this work, we study the 2DG growth of AlN on Si(1 1 1) and of GaN on AlN bu!ers by PAMBE. We focus on occurring surface reconstructions as a function of growth initiation, surface stoichiometry, "lm temperature and adlayer coverage. The "lms have been characterized by RHEED, X-ray di!raction (XRD) and photoluminescence (PL).
2. Experimental procedure The "lm growth has been performed in a homemade MBE-system using standard e!usion sources for evaporation of Al (6 N) and Ga (7 N) and using
a RF-source to supply activated nitrogen. For details see Ref. [19,20]. Growth mode and surface superstructures have been monitored by a 10 kV RHEED system. The Si(1 1 1) wafers ('5 ), p-type) have been prepared by a modi"ed Shiraki procedure [19]. They were degassed in ultra-high vacuum above growth temperature. The 7;7-surface reconstruction which appears at 7803C transfers into 1;1surface above 8503C. The 7;7-reconstruction is recovered when the substrate temperature is lowered to 8503C and persists at any temperature below. The AlN growth process has been started at 7003. At "rst we induce a (3;(3-reconstructed Si(1 1 1) surface by the deposition of monolayer of aluminium [25]. A nitridation step is carried out after ignition of the RF-plasma. The growth experiment is started when the transition to the 1;1 RHEED pattern of unreconstructed AlN is observed. Subsequently, the growth temperature is raised up to its "nal value. Alternatively, the Si(1 1 1)-7;7 surface is exposed to activated nitrogen for few seconds until a 3;3-surface reconstruction appears, presumably induced by an ordered N adlayer. Then the growth is started by opening the Al-shutter. The growth of GaN directly on Si(1 1 1) is started at 5003C with monolayer of gallium forming a (3;(3-reconstructed Si(1 1 1) surface as well [25]. The procedure is otherwise identical to the Al-initiation of the AlN growth. The GaN growth on AlN bu!er layers is started at 6503C. The growth temperature is then gradually raised up to 7703C. The growth is insensitive to the mode employed during the initial stage.
3. Results and discussion The 2DG growth of single-crystalline AlN "lms is achieved within the range of substrate temperatures from 8503C to 9003C with a III : V ratio of 91 : 1 and the plasma source working under optimized conditions. We "nd two distinct categories of AlN surface reconstructions. In the "rst category, we observe 2DG of AlN under N-rich, stoichiometric and Al-rich growth
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conditions corresponding to streaky (3;(3, 1;1 and 2;6 RHEED pattern, respectively [20]. In the second category, only streaky 1;1 RHEED pattern of the unreconstructed AlN surface occur during the growth. The 3;3- and 3;6-surface reconstructions can be induced by exposition of the AlN surface to the #ux of Al-atoms at 6003C after the growth. Also we observe a N-rich (3;(3-reconstruction by exposition to activated nitrogen. The "rst category of surface reconstructions mostly occurs if the growth is started with aluminum, the second if started with nitrogen instead. Furthermore, thermodynamical conditions such as Al supersaturation play a role in the initial stage. By now we can only suppose, that both categories of AlN-surface reconstructions can be assigned to di!erent AlN polarities. Investigations by electron beam channelling and by transmission electron microscopy are underway. GaN "lms have been grown directly on the Si(1 1 1)-face as well as on AlN bu!er layers. GaN invariably grows three-dimensionally on silicon as indicated by spotty wurtzite RHEED pattern but grows two-dimensionally on AlN bu!ers. In this work we describe GaN growth experiments on AlN bu!er layers. The AlN bu!ers have been grown under conditions corresponding to the occurrence of surface reconstructions of the second category as 1;1 or further (3;(3, 3;3 and 3;6. The GaN growth has been carried out under growth conditions controlled by the appearance of bright, streaky 1;1 RHEED pattern. No other RHEED pattern are visible during the growth. However, the 2DG of GaN requires high gallium supersaturation and is very sensitive to changes of the growth temperature and surface stoichiometry. More Ga-rich conditions are recognized by a decreasing intensity of the RHEED pattern due to the formation of Ga-droplets. More N-rich conditions lead to spotty wurtzite RHEED pattern indicating 3DG. N- or Al-adlayer surface reconstructions have been obtained by exposing the GaN surface either to activated nitrogen or to the #ux of Ga atoms after "nishing the growth process. The unreconstructed 1;1-surface persists if the RF-nitrogen source is switched o! above 6503C. An
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unstable 2;2-reconstruction is observed by exposing the 1;1-surface to activated nitrogen again. It held only as long as the N-shutter is opened (Fig. 1(a)). A transition from the 1;1-surface to a 3;1reconstruction is observed at 6503C if the RFsource is switched o! below 6503C. The unstable 2;2-reconstruction can be demonstrated on this surface by opening the N-shutter as well (Fig. 1(a)). Later the samples were cooled down to below 6003C. We "nd stable N- and Ga-adatom-induced surface reconstructions in this temperature range. They are in the order of increasing N-coverage 3;1, 2;2 and 3;3 and in the order of increasing Ga-coverage 2;2, 3;3, 3;6, 6;6 and c(6;12) as can be seen in Fig. 1(a) and (b). Probably, we observe the growth of N-face GaN [23] on N-face AlN. The GaN "lms grown on AlN-bu!ered Si(1 1 1) have been characterized by XRD. Besides the AlN(0 0 0 l ) and the Si(1 1 1) re#ections only the narrow and well separated (0 0 0 2) GaN re#ection at 20"34.603 and the (0 0 0 4) re#ection are observable (Fig. 2). They indicate complete texture with GaN[0 0 0 1]#AlN[0 0 0 1]#Si[1 1 1]. Thus, the growth procedure described above results in single-crystalline wurtzite GaN "lms, consistent with the observed RHEED patterns. The FWHM of the GaN(0 0 0 2) re#ection of a 250 nm GaN "lm is 0.0763 in u/20 and 0.623 in u, which means an improved "lm quality achieved under our experimental conditions compared with results reported by other groups [13}16]. For all investigated samples a GaN lattice constant c"(5.185$0.001)As was determined emphasising that the layers are nearly strain free which is expected if thick AlN bu!er layers are used [26]. Thus, our results are markedly di!erent to studies of the GaN/AlN/Si(1 1 1) system reported in Ref. [16] where c values indicated strong biaxial tensile strain. The good quality of the layers is con"rmed by the PL results shown in Fig. 3. The investigated 880 nm thick GaN "lm on a 125 nm AlN bu!er shows no yellow but intense near-band-edge luminescence. For completely relaxed GaN (as in our case) a neutral donor-bound excitonic transition (D3X, sometimes referred also as I line)
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Fig. 1. RHEED pattern of GaN surface reconstructions observed in post-growth experiments below 6003C taken along the [2 1 1 0](left) and the [1 1 0 0]-azimuth (right). (a) N-rich reconstructions, in the order of decreasing N-coverage: 3;3 (top), 2;2 (center) and 3;1. (b) Ga-rich reconstructions, in the order of increasing Ga-coverage: 3;3 (top), 3;6 (center) and 6;6.
the main emission is found at 3.453 eV which dominates up to room temperature. Performing lowtemperature time-resolved PL studies Godlewski et al. [17] identi"ed it as a further D3X line, however, no explanation concerning the origin of the donor state was given. The temperature dependence makes also a donor-to-band contribution likely, which involves a common donor state (E &55 meV) and the C valence band. It would " T explain the small shift to higher energies found at 40 K. Further work, including electrical and structural characterization is under progress to clarify the nature of this transition. Fig. 2. X-ray di!raction pattern of a 250 nm GaN "lm grown on a 90 nm AlN bu!er on Si(1 1 1) at 7703C (N -#ow rate 0.75 sccm, RF-power 500 W). The pattern have been recorded using Cu K and open detector. Inset: FWHM of the a GaN(0 0 0 2) re#ection 0.0763"5 in u/20 and 0.623 in u (recorded using a 0.22 mm slit aperture).
rapidly diminishing with temperature is normally observed at 3.471 eV. At this energy a weak shoulder in the 5 K spectrum is seen (Fig. 3). However,
4. Summary AlN grown on Si(1 1 1) exhibits two distinct categories of surface reconstructions. In the order of Al-coverage we observe the (3;(3!, 1;1- and 2;6- or the (3;(3!, 1;1-, 3;3- and the 3;6-reconstructions as function of the growth initiation.
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The GaN "lms show no yellow but intense nearband-edge luminescence.
Acknowledgements This work was partially "nanced by the Bundesministerium fuK r Bildung, Wissenschaft, Forschung und Technologie on contract 13N6809/7. G.D.K. and V.B.L. acknowledge "nancial support from the RFBR project No. 97-02-18199, R.G. and S.S. acknowledge support from the INTAS-94-2608 and the ThMWFK B 401-96033 grant. The authors wish to thank S. Klohr and H. SchoK nau for technical assistance.
References
Fig. 3. Normalized near-band-edge photoluminescence taken from 5 K up to room temperature of a 880 nm thick GaN "lm, grown on a 125 nm AlN bu!er on Si(1 1 1) at 7703C (N -#ow rate 1 sccm, 500 W RF-power). FWHM at 5 K is 22 meV.
The usage of AlN bu!ers promotes the 2DG of single-crystalline wurtzite GaN "lms on Si(1 1 1). The XRD pattern show sharp (0 0 0 2) and (0 0 0 4) re#ections. Thus the GaN "lms are epitaxial and single crystalline in agreement with the observed RHEED pattern. Streaky 1;1 RHEED patterns have been observed on GaN grown on AlN bu!ers which exhibit surface reconstructions of the second category. The RHEED patterns indicate 2DG of probably N-face GaN. Stable N- and Ga-rich surface reconstructions are observed below 6003C after the growth. GaN "lms show 3;1-, 2;2- and 3;3-surface reconstructions in the order of N-coverage and 2;2-, 3;3-, 3;6-, 6;6- and c(6;12)-surface reconstructions in the order of Ga-coverage.
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