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A schematic process flow for epitaxial growth of AlN by ALE.
(a) As synthesized GaN/a-sapphire AFM image, (b) AFM image of the GaN surface after in situ and ex situ cleaning. (c) and (d) AFM image of ALE grown AlN on GaN/a-sapphire for the TMA pulse length of 0.03 and 0.06 s, respectively. The AlN nucleation is non-uniform for an unsaturated GaN surface with TMA molecules.
(a) The Variation of GR with TMA pulse length (dose) for the purge time 10 s. The error bars are at ±1.5% level. Fig 3(b) shows the effect of precursors purge time on GR. Dependence of impurity concentrations on the purge time is inset of the Fig. 3(b) . 10 s purge resulted saturated growth with lowest carbon concentration.
The GR dependence on Tg with error bars are at ±1.5% level is shown in Fig. 4(a) . For Tg = 200–400 °C, GR is almost constant. The variation of carbon and oxygen impurities and Al/N ratio with Tg is shown in Fig. 4(b) . Figure 4(c) shows the variation of imaginary part of dielectric constant with energy. Band edge related features are visible for Tg ≥ 500 °C.
XRD peaks from the AlN/GaN/a-sapphire stack. Insets show the rocking curve of ALE grown AlN (0002) reflection. The rocking curve FWHM is 630 arc sec for growth conditions: Tg = 500 °C, 10 s purge, and TMA pulse length = 0.06 s. Imaginary part of the dielectric constant measured by spectroscopic ellipsometry is shown in Fig 5(b) . The band gap of 6.2 eV was measured for ALE AlN/Si(111).
Summary of studied growth parameters range and established values.
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