Strain modification of AlGaN layers using swift heavy ions

Epitaxial AlGaN/GaN layers grown by molecular beam epitaxy (MBE) on SiC substrates were irradiated with 150 MeV Ag ions at a fluence of 5×10¹² ions/cm². The samples used in this study are 50 nm Al_0.2Ga_0.8N/1 nm AlN/1 μ m GaN/0.1 μ m AlN grown on SI 4H-SiC. Rutherford backscattering spectrometry/channeling strain measurements were carried out on off-normal axis of irradiated and unirradiated samples. In an as-grown sample, AlGaN layer is partially relaxed with a small tensile strain. After irradiation, this strain increases by 0.22% in AlGaN layer. Incident ion energy dependence of dechanneling parameter shows E ^1/2 dependence, which corresponds to the dislocations. Defect densities were calculated from the E ^1/2 graph. As a result of irradiation, the defect density increased on both GaN and AlGaN layers. The effect of irradiation induced-damages are analyzed as a function of material properties. Observed results from different characterization techniques such as RBS/channeling, high-resolution XRD and AFM are compared and complemented with each other to deduce the information. Possible mechanisms responsible for the observations have been discussed in detail.     

Growth of quaternary AlInGaN/GaN heterostructures by plasma-induced molecular beam epitaxy

Epitaxial growth of AlInGaN/GaN heterostructures on sapphire substrates was achieved by plasma-induced molecular beam epitaxy. Different alloy compositions were obtained by varying the growth temperature with constant Al, In, Ga and N fluxes. The In content in the alloy, measured by Rutherford backscattering spectroscopy, increased from 0.4% to 14.5% when the substrate temperature was decreased from 775°C to 665°C. X-ray reciprocal space maps of asymmetric AlInGaN (2.05) reflexes were used to measure the lattice constants and to verify the lattice match between the quaternary alloy and the GaN buffer layers.     

First-principles calculations to investigate optical properties of ByAlxIn1−x−yN alloys for optoelectronic devices

First-principles density-functional theory of Full-Potential Linear Augmented Plane Wave (FP-LAPW) within local density approximation (LDA) of the optical properties of B_yAl_xIn_1−x−yN systems (with x = 0.187 and y = 0.062, 0.125 and 0.187) has been performed. Substitutional atoms of Boron induced in small amounts into the (Al_xIn_1−x)-cationic sublattice of AlInN affects the energy gap of BAlInN. The higher band gap of Al_0.375In_0.625N alloy can form a useful quantum well (QW) laser structure. A best choice of B-content, B_yAl_xIn_1−x−yN could be an alternative to Al_xIn_1−xN. The results of accurate calculations of the band structures and optical properties show the better performance characteristics belong to the structure containing B-content (y) of 12.5%. The NaCl metallic B_yAl_0.1875In_0.8125−yN has a direct character for y = 12.5%. The imaginary part of dielectric function, reflectivity, refractive index, absorption coefficient and optical conductivity are investigated well and provide reasonable results for optoelectronic devices applications.     

Properties of Si-doped GaN and AlGaN/GaN heterostructures grown by RF-MBE on high resistivity Fe-doped GaN

Silicon-doped GaN epilayers and AlGaN/GaN heterostructures were developed by nitrogen plasma-assisted molecular beam epitaxy on high resistivity iron-doped GaN (0001) templates and their properties were investigated by atomic force microscopy, x-ray diffraction and Hall effect measurements. In the case of high electron mobility transistors heterostructures, the AlN mole fraction and the thickness of the AlGaN barrier employed were in the range of from 0.17 to 0.36 and from 7.5 to 30 nm, respectively. All structures were capped with a 2 nm GaN layer.Despite the absence of Ga droplets formation on the surface, growth of both GaN and AlGaN by RF-MBE on the GaN (0001) surfaces followed a step-flow growth mode resulting in low surface roughness and very abrupt heterointerfaces, as revealed by XRD. Reciprocal space maps around the reciprocal space point reveal that the AlGaN barriers are fully coherent with the GaN layer.GaN layers, n-doped with silicon in the range from 10¹⁵ to 10¹⁹ cm⁻³ exhibited state of the art electrical properties, consistent with a low unintentional background doping level and low compensation ratio. The carrier concentration versus silicon cell temperatures followed an Arhenius behaviour in the whole investigated doping range. The degenerate 2DEG, at the AlGaN/GaN heteroiterfaces, exhibited high Hall mobilities reaching 1860 cm²/V s at 300 K and 10 220 cm²/V s at 77 K for a sheet carrier density of 9.6E12 cm⁻².The two dimensional degenerate electron gas concentration in the GaN capped AlGaN/GaN structures was also calculated by self-consistent solving the Schrödinger–Poisson equations. Comparison with the experimental measured values reveals a Fermi level pinning of the GaN (0001) surface at about 0.8 eV below the GaN conduction band.     

Homoepitaxial growth and electrical characterization of GaN-based Schottky and light-emitting diodes

Homoepitaxial growth of GaN epilayers on free-standing hydride vapor phase epitaxy (HVPE) GaN substrates offered a better control over surface morphology, defect density, and doping concentration compared to conventional heteroepitaxial growth. The FWHM of the (0 0 0 2) X-ray diffraction (XRD) rocking curve from homoepitaxial GaN was measured to be as low as 79 arcsec, much smaller than 230 arcsec for GaN grown on sapphire. Schottky diodes grown on GaN substrates exhibited sharper breakdown characteristics and much lower reverse leakage than diodes on sapphire. However, the homoepitaxial devices had poor scalability due to the presence of yield-killing defects originating from the substrate surface. Vertical InGaN/GaN light-emitting diodes (LEDs) on GaN substrates showed reduced series resistance and reverse leakage compared to lateral LEDs on sapphire. Wafer mapping demonstrated that the distribution of leaky homoepitaxial devices correlated well with that of macroscopic defects in the GaN substrates.