X-ray Diffraction Full-width At Half-maximum Research Articles

Effects of AlN nucleation layer thickness on crystal quality of AlN grown by plasma-assisted molecular beam epitaxy

In this paper, the effects of thickness of AlN nucleation layer grown at high temperature on AlN epi-layer crystalline quality are investigated. Crack-free AlN samples with various nucleation thicknesses are grown on sapphire substrates by plasma-assisted molecular beam epitaxy. The AlN crystalline quality is analysed by transmission electron microscope and x-ray diffraction (XRD) rocking curves in both (002) and (102) planes. The surface profiles of nucleation layer with different thicknesses after in-situ annealing are also analysed by atomic force microscope. A critical nucleation thickness for realising high quality AlN films is found. When the nucleation thickness is above a certain value, the (102) XRD full width at half maximum (FWHM) of AlN bulk increases with nucleation thickness increasing, whereas the (002) XRD FWHM shows an opposite trend. These phenomena can be attributed to the characteristics of nucleation islands and the evolution of crystal grains during AlN main layer growth.

On the anisotropic wafer curvature of GaN-based heterostructures on Si(1 1 0) substrates grown by MOVPE

We highlight the challenges of GaN growth on (1 1 0)-oriented Si substrates with respect to the substrate-induced anisotropic strain state. A 5:4 lattice matching relationship, which is valid for AlN and Si(1 1 1), is also expected for AlN on Si(1 1 0) along the [1 1 –2 0]AlN/[1 −1 0]Si direction. This reduces the effective lattice mismatch for AlN on Si(1 1 0) along [1 1 –2 0] AlN/[1 −1 0] Si from 23.4% to −1.3% (compressive), while in the [1 −1 0 0] AlN/[0 0 1] Si direction, the mismatch value remains at 0.7% (tensile). The opposite signs of the two mismatch values lead to an anisotropic strain state of the deposited AlN and GaN films, which makes it difficult to balance the tensile strain induced by thermal mismatch. For a simple growth scheme using 1 μm GaN on top of a 40 nm AlN layer, this situation is reflected in an aspherical wafer curvature, anisotropic X-ray diffraction (XRD) Ω scan full width at half maximum (FWHM) values, greater surface roughness and cracking, predominantly occurring parallel to the [1 1 –2 0] direction. The anisotropic XRD FWHM may be explained by a strain-induced anisotropic nucleation mechanism for GaN on AlN. Also, a partial relaxation of the GaN film during growth caused by very high tensile strain along [1 –1 0 0] AlN/[0 0 1] Si is assumed to contribute to the XRD anisotropy. The insertion of an AlN/GaN superlattice structure in between the AlN and GaN films inhibits this relaxation process and leads to a smoother surface morphology with less visible cracks.

Effect of In and N incorporation on the properties of lattice-matched GaInNAs/GaAs grown by radio frequency plasma-assisted solid-source molecular beam epitaxy

We present the effect of nitrogen (N) and indium (In) incorporation on the structural and optical properties of Ga1−xInxNyAs1−y with low lattice mismatch to GaAs grown by solid-source molecular beam epitaxy using a radio frequency (rf) nitrogen plasma source. The results show that excessive introduction of nitrogen during the growth of GaInNAs may lead to greater incorporation of interstitial nitrogen and degradation in crystal quality. This effect is more significant in GaInNAs compared to GaNAs. A drastic 6× increase in x-ray diffraction full width at half maximum (XRD-FWHM) was observed in GaInNAs compared to 1.5× increase of the same in GaNAs when reactive nitrogen is introduced into the material. The more significant degradation in GaInNAs quality is believed to be due to greater incorporation of interstitial nitrogen in the presence of indium during growth. By changing the In content and fixing the N incorporation rate, a sample of Ga0.924In0.076N0.026As0.974/GaAs with relatively low lattice mismatch of −896 ppm (or ∼8.96×10−4) was grown. Low temperature (4 K) photoluminescence emission at 1518 nm was observed and XRD-FWHM of 118.2 arcsec was measured.

Epitaxy of Gallium Nitride by HVPE Using Low Temperature Intermediate Buffer Layers Deposited by MOVPE

We investigated the use of different GaN and AlN buffer layers pre-deposited at low temperature (LT) for the growth of thick GaN layers by Hydride Vapor Phase Epitaxy (HVPE). The layers obtained on LT buffer layers grown by Metal-Organic Vapor Phase Epitaxy (MOVPE) show an excellent homogeneity, good crystalline quality (X-Ray Diffraction (XRD) Full Width at Half Maximum (FWHM) values around 650 arcsec) and the surface morphology (characterized by optical microscopy and atomic force microscopy measurements) is significantly improved compared to layers grown directly on sapphire. The partial replacement of the carrier gas nitrogen by hydrogen leads to a further improvement of the surface morphology and crystalline quality, reaching best XRD FWHM values of 350 arcsec. In contrast, experiments using an amorphous LT-HVPE buffer layer grown in situ show rather poor results.

V/III ratio effects on the growth of In 1− x− yGa xAl yAs on InP substrates by molecular beam epitaxy

In this paper, we report the effects of arsenic pressure (V/III ratio) on the properties of undoped In 1− x− y Ga x Al y As layers grown by molecular beam epitaxy (MBE) on InP (100) substrates. The quaternary samples were analysed using double-axis X-ray diffraction (XRD) and low-temperature photoluminescence (PL). XRD analysis showed that the lattice mismatch was relatively insensitive to flux ratio variations within the range investigated (V/III from 14 to 73). PL full width at half-maximum (FWHM) as low as 12.7 meV (at 5 K), a value which is comparable to the best reported, was achieved at a V/III ratio = 33. The PL and XRD FWHMs broadened significantly as the V/III ratio was decreased below 21 or increased above 50. Within the range investigated, the optimum V/III ratio for the growth of InGaAlAss layers was found to be 21–50. The effect of the V/III ratio on the optical and crystalline properties was explained in terms of clustering, which was characterized by analysing the difference between the PL peak energy and the band-gap energy calculated from XRD compositional measurements.

The effects of arsenic pressure on the properties of In 1− x− yGa xAl yAs layers grown by molecular beam epitaxy

In this paper, we report the effects of arsenic pressure (V/III ratio) on the properties of undoped In 1− x− y Ga x Al y As layers grown by molecular beam epitaxy (MBE) on InP(1 0 0) substrates. X-ray diffraction (XRD) analysis showed that the lattice mismatch was relatively insensitive to flux ratio variations within the range investigated (V/III from 14 to 73). However, changes in the V/III flux ratio was found to have a significant effect on the XRD full-width at half-maximum (FWHM), XRD intensity, photoluminescence (PL) FWHM and PL intensity (at 5 K). PL FWHM as low as 12.7 meV (at 5 K), a value which is comparable to the best reported was achieved at V/III ratio=33. The PL and XRD FWHM broadened significantly as the V/III ratio was decreased below 21. Broadening of the PL and XRD FWHM was also observed in samples grown at high flux ratios exceeding 50, although this effect was not as drastic compared to that in samples grown at low flux ratio. Within the range of V/III flux ratio investigated, the optimum flux ratio for the growth of InGaAlAs layers was found to be within 21–50. The effect of the V/III ratio on the change in the optical and crystalline properties was explained in terms of clustering which was characterised by analysing the difference between the PL peak energy and the band gap energy calculated from XRD compositional measurement.