Critical thickness of MBE-grown Ga1-xInxSb (x < 0.2) on GaSb

Several Ga1−xInxSb layers, capped with 1 μm of GaSb, were grown on GaSb(0 0 1) substrates by molecular beam epitaxy in a Varian Gen II Modular system using either the conventional sample growth position with substrate rotation, or a tilted sample position with no substrate rotation. The GaInSb layers were examined by X-ray diffraction (XRD) using both symmetrical and asymmetrical reflections. The “tilted sample method” gave a variation of ±25% in thickness of the Ga1−xInxSb layers, while the indium (In) content varied by ±10% around the nominal value. The disappearance of thickness fringes in 004 XRD scans was used to determine the onset of relaxation, as determining the in-plane lattice constant for tilted samples was found to be difficult. Determining residual strain in samples grown by the tilted method was likewise found to be very difficult. The critical thickness for several In mole fractions between 5% and 19% was determined and was found to be from 2.2 to 2.7 times higher than predicted by Matthews and Blakeslee (1974) [J. Crystal Growth 27 (1974) 118] but lower than that predicted by People and Bean (1985) [Appl. Phys. Lett. 47 (1985) 322]

Graphene-Based Transparent Conducting Substrates for GaN/AlGaN Nanocolumn Flip-Chip Ultraviolet Light-Emitting Diodes

Flip-chip ultraviolet light-emitting diodes based on self-assembled GaN/AlGaN nanocolumns have been fabricated, exploiting single-layer graphene not only as a growth substrate but also as a transparent conducting electrode. High crystalline quality of the nanocolumns is confirmed by detailed electron microscopy characterization, also showing the intrinsic GaN quantum disk in the active region of the nanocolumns. These features are further confirmed in the optical emission, where the absence of defect-related yellow emission and the presence of blue-shifted (from the usual 365 nm band gap emission of bulk wurtzite GaN) emission at ∼350 nm, ascribed to quantum confinement and strain effects, are observed. Despite a noticeable graphene damage after the nanocolumn growth that causes high sheet resistance of graphene and high turn-on voltage, the proof of concept of single-layer graphene used as the transparent conducting substrate for a nanocolumn device is demonstrated. This study offers an alternative platform for the fabrication of next-generation nano-optoelectronic and electronic devices