Strain induced exciton fine-structure splitting and shift in bent ZnO microwires

Lattice strain is a useful and economic way to tune the device performance and is commonly present in nanostructures. Here, we investigated for the first time the exciton spectra evolution in bent ZnO microwires along the radial direction via high spatial/energy resolution cathodeluminescence spectroscopy at 5.5 K. Our experiments show that the exciton peak splits into multi fine peaks towards the compressive part while retains one peak in the tensile part and the emission peak displays a continuous blue-shift from tensile to compressive edges. In combination with first-principles calculations, we show that the observed NBE emission splitting is due to the valence band splitting and the absence of peak splitting in the tensile part maybe due to the highly localized holes in the A band and the carrier density distribution across the microwire. Our studies may pave the way to design nanophotonic and electronic devices using bent ZnO nanowires

Electronic structure, linear, nonlinear optical susceptibilities and birefringence of CuInX2 (X = S, Se, Te) chalcopyrite-structure compounds

The electronic structure, linear and nonlinear optical properties have been calculated for CuInX2 (X=S, Se, Te) chalcopyrite-structure single crystals using the state-of-the-art full potential linear augmented plane wave (FP-LAPW) method. We present results for band structure, density of states, and imaginary part of the frequency-dependent linear and nonlinear optical susceptibilities. We find that these crystals are semiconductors with direct band gaps. We have calculated the birefringence of these crystals. The birefringence is negative for CuInS2 and CuInSe2 while it is positive for CuInTe2 in agreement with the experimental data. Calculations are reported for the frequency-dependent complex second-order non-linear optical susceptibilities . The intra-band and inter-band contributions to the second harmonic generation increase when we replace S by Se and decrease when we replace Se by Te. We find that smaller energy band gap compounds have larger values of in agreement with the experimental data and previous theoretical calculations.Comment: 17 pages, 6 figure

Graphitization effects on diamond surfaces and the diamond/graphite interface

Graphitic layers have previously been conjectured to play an active role in diamond nucleation by Lambrecht et al. and may also be involved in a mechanism for homoepitaxial diamond growth since the surfaces of diamond may partially graphitize under high-temperature conditions typical of growth processes. Recent molecular dynamics simulations of the diamond {111} surface, briefly reviewed and discussed here, indicate a progressive graphitization with increasing temperature which is strongly facilitated by any kind of surface perturbation or roughness such as step-like adsorbates. Here we show specifically that also twin boundaries promote graphitization. The process of debonding of the surface layer which is a simple displacive motion of the outer layer is also shown to be closely related to the delamination of the tetrahedrally bonded icosahedral C100 molecule into two concentric C20 and (fullerene-like) C80 fragments. In contrast, the tetrahedrally bonded icosahedral C300 molecule which contains one more concentric shell, does not spontaneously graphitize into a bucky onion (consisting of concentric C80 and C240 fullerenes) although the latter has lower energy. Progressive graphitization at a surface towards deeper layers before the top layer is delaminated can occur under certain conditions and then may lead to graphite/diamond prism plane interfaces similar to those previously investigated in connection with nucleation. The structural stability of the prism plane interface between graphite and diamond is re-investigated here. While the initial calculations with a classical potential underestimated the interface energy, the structural stability of the models previously presented is confirmed by the present quantum mechanical simulations

Strong hot electron reflection from subsurface 8H-SiC inclusion in 4H-SiC: Ballistic Electron Emission Microscopy (BEEM) study

The electrical properties of planar 8H stacking-fault inclusions (SFIs) formed during epilayer growth on an 8 degree miscut n-type 4H-SiC substrate were characterized using nm-resolution BEEM. A ???0.39 eV lower Schottky barrier was measured along the line where the SFI intersect a Pt metal overlayer, confirming that 8H SFIs are electron quantum wells (QWs). Interestingly, an asymmetry of the BEEM current amplitude was observed around the intersection line, which is believed to be caused by strong hot electron reflection from the subsurface 8H inclusion. We will discuss our modeling to explain this strong hot electron reflection and the lower bound of hot electron attenuation length in 4H-SiC estimated from the measured BEEM current profile. Work supported by ONR