GaN Decomposition in Ammonia
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Fig 1. Plot of the GaN decomposition rate as a function of the NH3 flow rate at three different pressures. For this plot the GaN was heated to a temperature of 992 ºC using H2 and NH3 for a total flow of 6 SLM.
H2 at pressures ranging from 40 to 300 torr. GaN decomposition was studied under similar flow conditions as the growth. The measured weights were converted to growth and decomposition rates per surface area (i.e. cm-2s-1) following Ref. 10. Expressed this way, a rate of 1.14x1015 cm-2s-1 corresponds to a thickness of 1 µm per hour. Temperature was calibrated by observing the melting point of 0.005” diameter Au wire on sapphire and correlating it to a thermocouple in direct contact with the susceptor underside [8]. After 2 years of use the set point temperature needed to melt the Au wire was reproducible to within 10 °C. RESULTS The change in the GaN decomposition rate, kGaN, at a temperature of 992 °C as NH3 is substituted for the H2 flow is shown in Fig. 1. In Fig. 1, kGaN is plotted as the NH3 flow is increased from 0 to 2 SLM for pressures of 40, 76, and 150 torr. The total flow rate was kept constant at 6 SLM with the balance being H2. Note that kGaN decreases from § 1x1016 cm-2s-1 to §[14 cm-2s-1 as the NH3 flow increases. At 150 torr, the minimum kGaN occurs at a flow of 0.75 SLM of NH3. At 76 torr, the kGaN minimum occurs between 1.25 and 2.0 SLM of NH3. At 40 torr no minimum in kGaN is observed. At 150 torr and a flow of 2 SLM of NH3, kGaN is §[14 cm-2s-1. In Fig. 2, the data from Fig. 1 are replotted as a function of the NH3 gas density, which depends on pressure. In Fig. 2, it appears that the kGaN measured at different pressures have a common minimum at a NH3 density of §[19 cm-3. This NH3 density is the same order of magnitude as the calculated N desorption rate from GaN, which should be 9x1019 cm-2s-1 at 992 °C [10]. Two different dependencies of kGaN on the NH3 density are evident in Fig. 2. At lower NH3 density, the kGaN drops steeply as the NH3 density increases from 3x1017 cm-3 to a value of near 1x1019 cm-3. For NH3 densities greater than 1x1019 cm-3, kGaN increases. This differs from the behavior observed by Grandjean et al., where kGaN only decreased for increasing NH3 flow [7]. To determine the dependence of kGaN on the NH3 density, [NH3], separate fits were calculated for [NH3] both less than and greater than 1x1019 cm-3. For [NH3] < 1x1019 cm3 , a functional form of kGaN = c[NH3]x was used and the data were fit by
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Fig. 2. Plot of the GaN decomposition rate measured as a function of the NH
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