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Register a new userFour wave mixing study on coalescence overgrowth of GaN nanocolumns on sapphire
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Coalescence overgrowth of pattern-grown GaN nanocolumns (NC) on c-plane sapphire substrate with metal organic chemical vapour deposition is demonstrated. The subsequent coalescence overgrowth opens a possibility for dislocation reduction due to the lateral strain relaxation in columnar geometry. We present further growth optimization and innovative characterization of MOCVD layers, overgrown on the columnar structure with varying diameter of colums. Nanoimprint lithography was applied to open circular holes of 250, 300, 450, 600 nm in diameter on the SiO2 layer, deposited on the GaN layer on c-plane sapphire template.
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V-pit-defects in GaN-based light-emitting diodes induced by dislocations are considered beneficial to electroluminescence because they relax the strain in InGaN quantum wells and also enhance the hole lateral injection through sidewall of V-pits. In this paper, regularly arranged V-pits are formed on c-plane GaN grown by metal organic vapor phase epitaxy on conventional c-plane cone-patterned sapphire substrates. The size of V-pits and area of flat GaN can be adjusted by changing growth temperature. Five pairs of InGaN/GaN multi-quantumwell and also a light-emitting diode structure are grown on this V-pit-shaped GaN. Two peaks around 410 nm and 450 nm appearing in both photoluminescence and cathodeluminescence spectra are from the semipolar InGaN/GaN multi-quantum-well on sidewalls of V-pits and cplane InGaN/GaN multi-quantum-well, respectively. In addition, dense bright spots can be observed on the surface of light-emitting diode when it works under small injection current, which are believed owing to the enhanced hole injection around V-pits.
Single GaN nanowires formed spontaneously on a given substrate represent nanoscopic single crystals free of any extended defects. However, due to the high area density of thus formed GaN nanowire ensembles, individual nanowires coalesce with others in their immediate vicinity. This coalescence process may introduce strain and structural defects, foiling the idea of defect-free material due to the nanowire geometry. To investigate the consequences of this process, a quantitative measure of the coalescence of nanowire ensembles is required. We derive objective criteria to determine the coalescence degree of GaN nanowire ensembles. These criteria are based on the area-perimeter relationship of the cross-sectional shapes observed, and in particular on their circularity. Employing these criteria, we distinguish single nanowires from coalesced aggregates in an ensemble, determine the diameter distribution of both, and finally analyze the coalescence degree of nanowire ensembles with increasing fill factor.
Magnetic anisotropy, spin wave (SW) excitation and exchange stiffness constant of amorphous FeTaC ($d$ = 20-200 nm) films were studied as a function of thickness using micro-strip ferromagnetic resonance (MS-FMR) technique. The MS-FMR spectra for in-plane applied magnetic field show the presence of uniform precessional mode ($n$ = 0) along with first perpendicular standing spin wave (PSSW) mode ($n$ = 1) especially for $d$ = 50, 100 and 200 nm films. The angular ($varphi_{H}$) dependence of resonance field ($H_{r}$) and magnetic field dependence of resonance frequencies ($f_{r}$) in planar configuration for the uniform and PSSW modes were modeled successfully by using dispersion relation which arises from a combination of exchange and dipolar interactions. The relevant parameters such as saturation magnetization ($4pi M_{S}$), uniaxial anisotropic constant ($K_{u}$), $g$-factor, and exchange stiffness constants ($A_{ex}$) are estimated for different FeTaC film thickness. $A_{ex}$ is found to increase from 1.52(4)$times$10$^{-7}$ to 5.0(5)$times$10$^{-6}$ erg/cm as the thickness of film increases from 50 to 200 nm, possibly due to surface pinning effect or significant inhomogeneity especially at higher thickness films.
In recent years the physics of two-dimensional semiconductors was revived by the discovery of the class of transition metal dichalcogenides. In these systems excitons dominate the optical response in the visible range and open many perspectives for nonlinear spectroscopy. To describe the coherence and polarization dynamics of excitons after ultrafast excitation in these systems, we employ the Bloch equation model of a two-level system extended by a local field describing the exciton-exciton interaction. We calculate four-wave mixing signals and analyze the dependence of the temporal and spectral signals as a function of the delay between the exciting pulses. Exact analytical results obtained for the case of ultrafast ($delta$-shaped) pulses are compared to numerical solutions obtained for finite pulse durations. If two pulses are used to generate the nonlinear signal, characteristic spectral line splittings are restricted to short delays. When considering a three-pulse excitation the line splittings, induced by the local field effect, persist for long delays. All of the found features are instructively explained within the Bloch vector picture and we show how the exciton occupation dynamics govern the different four-wave mixing signals.
In metal organic vapor phase epitaxy of GaN, the growth mode is sensitive to reactor temperature. In this study, V-pit-shaped GaN has been grown on normal c-plane cone-patterned sapphire substrate by decreasing the growth temperature of high-temperature-GaN to around 950 oC, which leads to the 3-dimensional growth of GaN. The so-called WM well describes the shape that the bottom of GaN V-pit is just right over the top of sapphire cone, and the regular arrangement of V-pits follows the patterns of sapphire substrate strictly. Two types of semipolar facets (1101) and (1122) expose on sidewalls of V-pits. Furthermore, by raising the growth temperature to 1000 oC, the growth mode of GaN can be transferred to 2-demonsional growth. Accordingly, the size of V-pits becomes smaller and the area of c-plane GaN becomes larger, while the total thickness of GaN keeps almost unchanged during this process. As long as the 2-demonsional growth lasts, the V-pits will disappear and only flat c-plane GaN remains. This means the area ratio of c-plane and semipolar plane GaN can be controlled by the duration time of 2-demonsional growth.
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