Mat. Res. Soc. Symp. Proc. Vol. 764 2003 Materials Research Society C7.7.1 GaN Growth on Si Using ZnO Buffer Layer K.C. Kim, S.W. Kang, O. Kryliouk and T.J. Anderson Department of Chemical Engineering, P.O. Box 116005, Gainesville, FL 32611-6005 D. Craciun National Institute for Laser, Plasma and Radiation, Romania V. Craciun, R.K. Singh Materials Sciences and Engineering, P.O. Box 116400, Gainesville, FL 32611-6400 ABSTRACT ZnO films were deposited by Pulsed Laser Deposition (PLD) onto silicon substrates to serve as a buffer layer for GaN films grown by MOCVD. A ZnO buffer layer was found to improve the quality of GaN grown on Si. The thermal stability of ZnO as a buffer layer was also examined. It was determined that exposure of ZnO/Si to NH 3 at high temperature (> 600 o C) results in the decomposition of ZnO and subsequent poor nucleation of GaN. The ZnO layer thickness on GaN quality was found to be important. INTRODUCTION It is well documented that the column III-nitride semiconductors have considerable potential for use in displays, optical data storages, reprographics, high power, high frequency electronic devices, UV detectors, and related technologies [1,2]. The lack of a suitable substrate, however, has hindered the growth of high quality GaN films. Although the favored substrate for GaN growth is (0001) sapphire, it has limitations including a large lattice mismatch with GaN and the accompanying significant bowing at large diameters, high electrical resistivity, and lack of suitable cleavage planes. The Si substrates for GaN growth is attractive given its low cost, large diameter, high crystal and surface quality, controllable electrical conductivity, and high thermal conductivity. In addition, the use of Si wafers promises easy integration of GaN based optoelectronic devices with Si based electronic devices [3]. Indeed, GaN-based devices have been demonstrated on Si [4]. The direct growth of GaN on Si, however, resulted in substantial diffusion of Si into the GaN film, relatively high dislocation density (~10 10 cm -2 ) [5] and cracking of the GaN film [6]. GaN is also known to poorly nucleate on Si substrate, leading to an island-like GaN structure and poor surface morphology [6]. In this study, a ZnO buffer layer was evaluated as a buffer layer for GaN growth on Si. ZnO has previously been tested as a buffer layer for Hydride Vapor Phase Epitaxy (HVPE) growth of GaN on sapphire [7-10]. GaN growth on ZnO/Si structures has also been reported [11-12]. In general, the use of a ZnO buffer layer produced good quality GaN on both Si and sapphire substrates [7-12], even though ZnO is known to be thermally unstable at the high growth temperature of GaN. For ZnO/Si, no continuous two-dimensional GaN layer could be obtained without first growing a low temperature GaN buffer layer to prevent the thermal decomposition of ZnO [11]. HVPE grown GaN films on ZnO/sapphire without this low temperature GaN buffer layer exhibited cracks and peeling when thick (200 nm) ZnO buffer layer were grown [7]. It was suggested that the thermal decomposition of ZnO led to the
C7.7.2 growth of poor quality GaN. In this study, the thermal stability of ZnO as a buffer layer for GaN growth was examined. The effect of ZnO thickness on the quality of GaN has been reported for sapphire [7,8,13], but not when deposited on Si wafers. The effect of ZnO thickness on GaN quality on Si was investigated in this research. GaN films were grown on ZnO/Si substrates with variable ZnO thickness and characterized by LRXRD, AFM, and SIMS. EXPERIMENTAL DETAILS ZnO buffer layers were deposited on Si(100) and Si(111) by Pulsed Laser Deposition (PLD). The Si substrates were first degreased with trichloroethylene (TCE), acetone, methanol and warm water for 3 min each and then treated with a buffer oxide (BOE) solution to remove the native oxide layer before they were loaded into PLD system. A commercially available ZnO-target was used and the samples were annealed in an oxygen atmosphere after growth. Typical growth conditions for PLD growth of ZnO on Si are listed in Table 1. Table 1. PLD growth conditions of ZnO on Si Growth Conditions KrF Excimer Laser (õ=248 nm) Post Growth Annealing Substrate temperature ( o C) 600 O 2 ambient pressure (mtorr) 3.6 Target type ZnO Laser fluence range (mj/cm 2 ) 700 Repetition rate (Hz) 5 Pulse duration (ns) 25 Cooling rate ( o C/min) -5 O 2 ambient pressure (Torr) 300 UV lamp UV off @ 300 o C GaN films were deposited on ZnO/Si in a low pressure, horizontal, cold wall Nippon Sanso MOCVD system using triethyl gallium (TEGa) and NH 3 as precursors and nitrogen as a carrier gas. The growth temperature was varied from 600 to 850 o C and the growth pressure was fixed at 100 Torr. The low growth temperature was used to prevent the thermal decomposition of ZnO buffer layer. The flow rates of TEGa and NH 3 were 50 and 1600 sccm, respectively to give a V/III ratio of 3500. RESULTS AND DISCUSSION The crystal orientation and surface morphology of PLD grown ZnO were not affected by Si substrate orientation. The LRXRD spectra shown in Figure 1 for ZnO on Si(100) and Si(111) indicate single crystal ZnO(0001) was grown. AFM images shown in Figure 2 show almost identical film roughness with a RMS surface roughness of approximately 4.7 nm.
C7.7.3 Figure 1. LRXRD spectra of ZnO on Si(100) and Si(111) substrates. RMS: 4.6 nm RMS: 4.7 nm Figure 2. AFM images of ZnO on Si(100) and Si(111) substrates. The use of a ZnO buffer layer was found to improve the structural quality and surface morphology of MOCVD GaN films. Figure 3 compares LRXRD spectra of GaN on bare Si and ZnO/Si. The intensity of GaN(0002) reflection from GaN/ZnO/Si is much higher than that of GaN/Si. The GaN surface roughness decreases significantly when a ZnO buffer layer was employed compared to bare Si substrate (Figure 4). It is noted that both samples were grown in the same run. Figure 3. LRXRD spectra of GaN on bare Si(111) and ZnO/Si(111). RMS: 114.27 nm RMS: 35.2 nm Figure 4. AFM images of GaN on bare Si(111) and ZnO/Si(111).
C7.7.4 The thermal stability of ZnO as a buffer layer for GaN growth was examined. Thermodynamically, the equilibrium oxygen partial pressure above ZnO at 850 o Cis~10-23 atm [14]. The decomposition of a ZnO film on Si was observed to be negligible when annealed at 850 o C and 100 Torr in a nitrogen atmosphere for 5 min. When the same film was exposed to NH 3 at 850 o C for 5 min, decomposition of ZnO was noticeable, and leads to poor nucleation of GaN. A SIMS depth profile of this sample showed no ZnO at GaN/Si interface (Figure 5). Figure 5. SIMS depth profile of GaN on ZnO/Si exposed to NH 3 at 850 o Cfor5min. The effect of the ZnO buffer layer thickness on the quality of the GaN was also investigated. It was reported the optimum ZnO thickness for growth on sapphire is 50 nm. A GaN film grown on a much thicker ZnO layer (200 nm) resulted in cracking and peeling [7]. Gu et al. [8] attributed the good GaN quality to the formation of a thin interfacial layer of ZnAl 2 O 4 produced by a reaction-diffusion process between the ZnO and Al 2 O 3.ZnOwas not observed to exist at the GaN/Al 2 O 3 interface after growth of the GaN in case GaN quality improved [8]. In this work, however, no cracking or peeling was observed when a ~200 nm ZnO layer was employed. The best surface morphology of GaN was obtained with a 65 nm thick ZnO layer. No evidence of a reaction product at ZnO/Si interface was detected by SIMS analysis (Figure 6). Figure 6. SIMS depth profile of GaN on ZnO/Si with LT-GaN.
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