Bulk GaN Layers Research Articles

Improvement of GaN layer quality by using the bulk-GaN buffer structure grown by metalorganic chemical vapor deposition

In this article, we propose a new buffer structure to obtain the high quality GaN epitaxial layers grown on sapphire substrates by a separate-flow reactor of metalorganic chemical vapor deposition (MOCVD). This buffer structure consists of 200–300 Å GaN nucleation layer/6 μm GaN-bulk layer. The bulk-GaN layers have also been prepared by MOCVD. The GaN epitaxial layer grown on this buffer structure exhibits a full width at half maximum (FWHM) of double-crystal x-ray diffraction of 170 arcsec, a FWHM of 300 K photoluminescence of 56 meV, an electron mobility of 400 cm2/V s at 300 K and 815 cm2/V s at 140 K, and a concentration of 3.6×1017 cm−3 at 300 K. The GaN growth with this buffer structure has a wide growth window on the different nucleation-layer thicknesses. A good quality of GaN epitaxial layers can be obtained by using this buffer structure.

Maskless Lateral Epitaxial Overgrowth of GaN on Sapphire

ABSTRACTWe demonstrate a technique of lateral epitaxial overgrowth (LEO) of GaN, termed 'maskless' LEO, in which no mask is deposited prior to LEO regrowth. Instead, a bulk (> 2 μm) GaN layer on sapphire is selectively dry etched, leaving ∼5 μm-wide stripe mesas oriented in the <1010>GaN direction, with a 20 μm period. These stripes serve as seeds for LEO GaN growth, which proceeds from the tops of the stripes and expands laterally, resulting in a ‘T’, or overhang, morphology. As for LEO over an SiO2 mask, significant defect reduction (from ∼109 cm−2 to ∼106 cm−2 ) is observed in cross-sectional transmission electron microscopy (TEM). Atomic force microscopy of the top surface of the LEO GaN reveals that no threading dislocations with screw component terminate at the surfaces of laterally overgrown regions. X-ray diffraction measurements reveal that the wings exhibit a crystallographic tilt away from the seed regions in an azimuth perpendicular to the stripe direction; the tilt angle (∼0.4 – 0.5°) is relatively independent of growth temperature and wing aspect ratio.

Electron mobility in two-dimensional electron gas in AIGaN/GaN heterostructures and in bulk GaN

We report on temperature dependencies of the electron mobility in the two-dimensional electron gas (2DEG) in AIGaN/GaN heterostructures and in doped bulk GaN. Calculations and experimental data show that the polar optical scattering and ionized impurity scattering are the two dominant scattering mechanisms in bulk GaN for temperatures between 77 and 500K. In the 2DEG in AIGaN/GaN heterostructures, the piezoelectric scattering also plays an important role. Even for doped GaN, with a significant concentration of ionized impurities, a large volume electron concentration in the 2DEG significantly enhances the electron mobility, and the mobility values close to 1700 cm2/Vs may be obtained in the GaN 2DEG at room temperature. The maximum measured Hall mobility at 80K is nearly 5000 cm2/Vs compared to approximately 1200 cm2/Vs in a bulk GaN layer. With a change in temperature from 300 to 80K, the 2DEG in our samples changes from nondegenerate and weakly degenerate to degenerate. Therefore, in order to interpret the experimental data, we propose a new interpolation formula for low field mobility limited by the ionized impurity scattering. This formula is valid for an arbitrary degree of the electron gas degeneracy. Based on our theory, we show that the mobility enhancement in the 2DEG is related to a much higher volume electron concentration in the 2DEG, and, hence, to a more effective screening.