Non-Destructive X-ray Characterization of Wide-Bandgap Semiconductor Materials and Device Structures

Abstract

In this work non-destructive x-ray characterization techniques have been used to study undoped and intentionally doped bulk and epitaxial layers, and device structures of wide bandgap semiconductor materials, GaN and SiC. Novel non-destructive x-ray characterization methods were developed to evaluate the uniformity of strain in AlGaN/GaN device structures across the wafer and the results were correlated with device electrical characteristics. In-situ bias induced strain measurements were also carried out for the first time on the AlGaN/GaN Schottky diodes to estimate change in piezoelectric polarization charge at the heterojunction interface with the gate bias voltage. A variety of high resolution x-ray measurements were performed on freestanding Gallium Nitride (GaN) films grown by three different laboratories using hydride phase vapor epitaxy (HVPE) technique. The lattice parameters of the quasi-bulk films were obtained using high-resolution x-ray diffraction spectra. The crystalline quality of the films was determined by measuring the x-ray rocking curves and by ^71 Ga nuclear magnetic resonance (NMR) technique. The anisotropic in-plane strain was determined using a novel grazing incidence x-ray diffraction technique (GID) and conventional x-ray diffraction measurements. Based on these measurements the best free standing films have surface strain anisotropy of 4.0791 x 10^-3 up to a depth of 0.3 μm and the dislocation density is in the range of 10^5-10^7 /cm^2. High resolution x-ray topography (HRXT) measurements were also performed on the freestanding GaN films. Complete mapping of defects for the entire surface of the GaN films was obtained in a non-destructive way. From these measurements, the lateral dimensions of crystallites and cavities in the films are in the range, 200-500 nm, and 0.5-400 μm, respectively. The GaN films were found to be warped with a radius of curvature of about 0.5 m. The warpage is attributed to thermal mismatch between GaN and the sapphire substrate during growth. The characteristics of freestanding GaN films measured in this work are detrimental to the fabrication of high-speed devices such as high electron mobility transistors (HEMT) because their performance is highly dependent on the surface and interface quality. High resolution x-ray measurements were also performed on Al+ ion-implanted 4H-Silicon Carbide (SiC) epitaxial layers, before and after 30s ultra-fast microwave annealing in the temperature range 1750-1900 °C, to examine the crystalline quality of the material. Based on the FWHM values of the rocking curves, an improvement in the crystalline quality of the microwave annealed samples was observed compared to the conventional furnace annealed sample. The sample annealed at 1900 °C showed the best rocking curve FWHM of 9 ± 2 arcsecs, which not only confirmed annihilation of the defects introduced during the Al+ ion-implantation process, but also an improvement in crystalline quality over the as-grown virgin 4H-SiC sample that had a rocking curve FWHM of 18.7 ± 2 arcsecs. The theoretical and measured rocking curve FWHM values were obtained and correlated with the depth dependent microwave absorption in the SiC epilayer. These results are very significant for optimizing the annealing parameters to achieve the highest possible implant activation, carrier mobility and crystal quality. Magnesium ion-implantation doped GaN films were also characterized using x-ray diffraction measurements after microwave annealing in the temperature range of 1300 °C – 1500 °C for 5 – 15 s. The FWHM values of the in-situ Mg-doped samples did not change with the microwave annealing for 5 s anneals. The electrical measurements on these samples also showed poor electrical activation of the Mg-implant in the GaN films. These results may be due to the presence of a high concentration of implant generated defects still remaining in the material, even after high temperature annealing for 5 s. From the FWHM values, the 15 s annealing showed an improvement in the crystalline quality of the GaN samples. Also the x-ray diffraction measurements show activation of the Mg implant. Electrical conductivity was observed in these samples, which is due to significant improvement in the crystalline quality and sufficient activation of the Mg implant. In this work, x-ray measurements were also performed on AlGaN/GaN device structures to study the effect of localized strain on the transport measurements across the wafer. The map of in-plane strain of the AlGaN/GaN HEMT wafer showed a one-to-one correspondence with the variation in electrical resistivity. The in-plane strain variation is in the range of 2.295x10^-4 – 3.539x10^-4 resulting in a sheet resistance variation of 345 - 411 Ω/ . The in-situ high resolution x-ray diffraction measurements, performed on the AlGaN/GaN device structures under variable bias conditions, showed in-plane tensile strain for forward bias conditions, and compressive strain for reverse bias. A linear variation in the strain was observed with the bias voltage, which results in a change in the piezoelectric charge at the AlGaN/GaN interface with bias. This variation needs to be considered for the correct modeling of the device transport characteristics.