A Kinetic Model for GaN Growth
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Mat. Res. Soc. Symp. Proc. Vol. 482 01998 Materials Research Society
KINETIC MODEL FOR GaN GROWTH GaN does not melt congruently for pressures typically used for MOVPE growth. [11-15] 800 VCand Instead, it decomposes above by N2' Ga÷ into N 2 and Ga as evidenced
ions in the mass spectrometer [14]. In addition, the desorption of small GaN clusters has been + postulated [12], and GaN' and Ga2N2 ions
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D.K. Wickenden, APL A.E. Wickenden, NRL K oUNM M S.D. .
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0 E have been observed [15]. The GaN thermal -0o decomposition rate has been measured by two groups resulting in very similar values for the Le activation energy, EA, of 3.1 [12] and 3.2 eV [13]. These values are only slightly larger than ... .... the EA of 2.8 eV for Ga desorption from liquid 960 1000 1040 1080 Ga metal [16], suggesting that the rate limiting Temperature step for GaN decomposition is Ga desorption from the surface. GaN decomposition measured under H 2 flow at a total pressure of Fig. 1. T0 and V/Ill ratio used by several groups (ret. 19-22) along with the calculated kN/kc desorption line. 50 torr has a similar decomposition rate [2] to that measured in vacuum [12, 13]. Several factors influence the adsorption and subsequent decomposition of precursors on the surface, including the precursor flux, sticking coefficient, and decomposition rate [17]. It has been shown for MOVPE growth that precursors partially decompose in the gas phase boundary layer above the surface [17]. Here, it will be assumed that the Ga and N precursors completely decompose to produce adsorbed Ga and N atoms. We will also assume that the sticking coefficient for each precursor is unity and that the incident V/Ill molar ratio does not change through the boundary layer. Justification for these assumptions are discussed in detail in ref. 2, including a discussion of conditions where NH 3 decomposition is limited. Recently, Brandt, Yang and Ploog measured the kinetic rates for Ga and N desorption using RHEED diffraction on zinc-blende GaN(001) [18]. The desorption rates were obtained by dosing the surface with either N or Ga atoms to form either the (lx 1) N-terminated or c(2x2) Gaterminated reconstructions followed by measuring the recovery of the clean surface (2x2) diffraction pattern [18]. The Ga desorption rate EA (2.69 eV) is in good agreement with the Ga desorption rate from liquid Ga [16] and previous measurements of the EA for GaN decomposition [12, 13]. For Fig.l, the N desorption rate, kN, is divided by the Ga desorption rate, kGa, (i.e. kNl/kra) and plotted vs. TG. This line is described by kN/kG.a = l.921x10' 6exp(-39,560/T(K)). Note that kN/kGa is - 1000 at TG = 1020 'C and = 0.1 at TC = 725 °C. These TG and kN/kra correspond well to the TG and V/Il ratios used for MOVPE and MBE growth. For growth to occur, the growth rate must be greater than zero. The growth rate, GR, depends on incident flux, F, and desorption rate, k, according to GR = FS - kO, where S is the sticking coefficient and 0 is the surface coverage [2]. Clearly, f
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