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Vibrational modes are observed at ambient pressure in O-doped GaN at $544{\mathrm{cm}}^{\ensuremath{-}1}$ using Raman spectroscopy. Investigation of these modes with applied hydrostatic pressure reveals the existence of three closely spaced modes that shift in relative intensity with increasing pressure. Notably, transitions between the different modes occur at previously observed electronic transitions associated with the $\mathrm{DX}$-like center behavior of substitutional O on the N site. A simple one-dimensional oscillator model is used to extract approximate force constants; these are consistent with the assignment of the $544{\mathrm{cm}}^{\ensuremath{-}1}$ mode to ${O}_{\mathrm{N}}$ and with force constants for C and B dopants in GaP. The relative intensity changes observed at 11 and 17 GPa are assigned to changes in the charge state due to the merging of the +/0 ionization level and the Fermi energy and the transition to ${\mathrm{DX}}^{\ensuremath{-}}$ that causes a previously observed drop in free electron concentration, respectively.

O and Si donors in GaN are studied by Raman spectroscopy under hydrostatic pressure p. The ground state of O is found to transfer from a shallow level to a deep gap state at p{gt}20 GPa reminiscent of {ital DX} centers in GaAs. Transferred to Al{sub x}Ga {sub 1-x}N we predict that O induces a deep gap state for x{gt}0.40. In GaN:Si no such state is induced up to the highest pressure obtained (p=25 GPa ) equivalent to x=0.56 in Al{sub x}Ga {sub 1-x}N and possibly higher. We attribute this distinction to the lattice sites of the dopants. O substituting for N is found to be the origin of high free electron concentration in bulk GaN crystals. {copyright} {ital 1997} {ital The American Physical Society}

The in-plane effective electron mass (${\mathit{m}}_{\mathrm{\ensuremath{\parallel}}}$) in narrow ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/InP quantum wells is strongly dependent on the quantization energy. Cyclotron resonance in a series of quantum wells with well widths down to 15 \AA{} reveals a mass enhancement of up to 50% (${\mathit{m}}_{\mathrm{\ensuremath{\parallel}}}$=0.065${\mathit{m}}_{0}$) over the bulk value of ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As. This effect is caused by the nonparabolicity of the conduction band and wave function penetration into the barrier material. Our experimental findings are in good agreement with calculations performed within the framework of k\ensuremath{\cdot}p theory. We obtain an easy-to-use relation between the mass and the quantization energy ${\mathit{m}}_{0}$/${\mathit{m}}_{\mathrm{\ensuremath{\parallel}}}$(\ensuremath{\epsilon})=(1-1.96\ensuremath{\epsilon}/eV)/0.044. \textcopyright{} 1996 The American Physical Society.

We investigate the absorption spectrum and the distribution of radiative decay times for two-dimensional excitons in semiconductor quantum wells, as affected by a random adiabatic in-plane excitonic potential. We limit our discussion to the case of a white-noise potential. Such a potential could arise as a result of fluctuations in the composition of an alloy semiconductor, or alternatively, fluctuations in quantum-well thickness. We find, in general, that the shortest radiative decay time is directly proportional to the inhomogeneous broadening of the exciton line in absorption, which in turn is proportional to the correlation parameter of the random potential. We calculate the magnitude of the correlation parameter for several cases of potential interest. The theory is in qualitative agreement with available experimental data.

The spin-splitting factor ${\mathit{g}}^{\mathrm{*}}$ of the electrons at the very bottom of the conduction band in strained ${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/InP type-I quantum wells is reported. Experimental proof of quantum confinement-dependent anisotropy of ${\mathit{g}}^{\mathrm{*}}$ is given. Changing the alloy composition at fixed quantization, equivalent to introducing compressive and tensile strain, changes ${\mathit{g}}^{\mathrm{*}}$. The values of ${\mathit{g}}^{\mathrm{*}}$ perpendicular to the quantum-well plane can be explained in a model calculation. Apparently however, no quantitative theory on which to base the calculation of the anisotropic spin splitting is at present available.

The electron effective mass of the conduction band in direct-band-gap ${\mathrm{GaAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$ alloys (x0.4) is reevaluated. A direct determination of ${\mathit{m}}^{\mathrm{*}}$ using the optically detected cyclotron resonance technique is presented for the low composition values. For higher x values the scattering times decrease because of alloying and it was not possible to carry out resonance experiments. Instead the diamagnetic shifts of the shallow-donor-to-acceptor recombination lines in magnetic fields up to 12 T were investigated. Within the framework of a simple perturbation approach the corresponding ${\mathit{m}}^{\mathrm{*}}$(x) values (0.17x0.44) could be deduced. The results are compared with a theoretical estimate based on the k\ensuremath{\cdot}p theory. There is good agreement between theory and experiment, resulting in a new x dependence of the conduction-band mass in the direct-band-gap ${\mathrm{GaAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$ alloys: ${\mathit{m}}^{\mathrm{*}}$=0.067+(0.06\ifmmode\pm\else\textpm\fi{}0.003)x.

Received 2 September 2009; revised manuscript received 29 January 2010; published 22 March 2010 The creation of symmetrical pairs of inclined dislocations was observed in the GaInN/GaN quantum wells QWs of c-axis grown green light-emitting diodes LEDs on low-defect density bulk GaN substrate, but not in green LEDs on sapphire substrate with high threading dislocation TD density. Pairs of dislocations start within 20 nm of the same QW and incline 18° – 23° toward two opposite 11 ¯ 00 directions or in a 120° pattern. We propose that in the absence of TDs, partial strain relaxation of the QWs drives the defect formation by removal of lattice points between the two dislocation cores. In spite of those inclined dislocation pairs, the light output power of such green LEDs on GaN is about 25% higher than in LEDs of similar wavelength on sapphire.

Experimental data obtained on undoped, ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/InP, single quantum wells using the far-infrared, optically-detected cyclotron resonance technique show a strong increase of the in-plane effective mass of electrons with increasing quantum confinement. The experimental results are compared with a model calculation in which conduction-band nonparabolicity and wave-function penetration into the barrier material have been taken into account. The roughness of the surface between the quantum well and the barrier material is proposed to be the reason for the decrease in electron scattering time from 1.1 ps (1000 \AA{}) to 120 fs (80 \AA{}).