In‐plane photocurrent spectroscopy in GaAs-AlAs superlattices

The in‐plane photoconductivity of GaAs‐AlAs superlattices on GaAs substrates is experimentally studied as a function of the incident photon energy at different temperatures and light intensities. Superlattice and substrate are electrically isolated by a thick  Al0.3Ga0.7As barrier but connected through penetrating contacts. Depending on the transport properties of the two subsystems pseudo‐negative photoconductivity can be observed, i.e., at the absorption maximum of the superlattice the photocurrent exhibits a minimum

Electro‐optical multistability in GaAs/AlAs superlattices at room temperature

We have studied the optical absorption properties of a GaAs/AlAs short‐period superlattice at room temperature in an electric field perpendicular to the layers. Several pronounced optical transitions related to Wannier–Stark localization are observed indicating a coherence length of at least five superlattice periods. These transitions produce multiple regions of negative differential photoconductivity which are used to realize a multistable self‐electro‐optic effect device

Bipolar charge redistribution in resonant-tunneling light-emitting-diodes

Resonant-tunneling light-emitting diodes contain three regions where charges accumulate during device operation: the electron and hole accumulation layer in the emitter and collector, respectively, and the quantum well of the structure for bipolar accumulation. It is shown that charges redistribute over these regions with increasing current through the device and that the amount of redistribution depends on tunneling escape rate and hence on the thickness of the tunneling barriers. This charge redistribution is particularly pronounced at the resonances in the current-voltage characteristics. The process is clarified by electroluminescence spectroscopy on a set of resonant-tunneling light-emitting diodes with different barrier thickness. Signal intensities from the three spectral regions are compared and yield the relative majority carrier occupation of each region. A tradeoff in quantum-well versus accumulation-layer emission is observed as the tunneling escape rate increases with decreasing barrier thickness. Line-shape analysis yields free-carrier electron and hole broadening in the quantum well. Peak position analysis allows the determination of the electric field across the well region from which an accurate picture of the band bending during tunneling is obtained. © 1995 The American Physical Society.status: publishe

Polarized photoreflectance spectroscopy of strained A-plane GaN films on R-plane sapphire

We have investigated a [11 (2) over bar0]-oriented A-plane GaN film on R-plane sapphire, where the c axis of GaN lies in the film plane, by polarized photoreflectance (PR) spectroscopy. Near the fundamental energy gap of GaN, the PR spectrum with the probe light polarized perpendicular to the c axis exhibits one feature corresponding to a single transition labeled T-1. For a polarization parallel to c axis, two different features labeled T-2 and T-3 are observed at higher energies than the transition T-1. In order to explain the origin of these three features, we compare the measured energies with calculations of the transition energies and oscillator strengths of the three band-to-band transitions of GaN near its fundamental gap for an anisotropic in-plane strain in the A plane. The analysis shows that the observed transition energies and polarization properties of the three transitions can be explained by the presence of an overall compressive, anisotropic in-plane strain in the film. (c) 2005 American Institute of Physics

Polarization anisotropy in GaN films for different nonpolar orientations studied by polarized photoreflectance spectroscopy

We use photoreflectance (PR) spectroscopy to study the electronic band structure modification of GaN films grown along different nonpolar orientations due to biaxial, anisotropic in-plane strain. The exciton transition energies of an unstrained, high-quality C-plane GaN film are used to accurately determine the crystal-field and spin-orbit splitting energies. For films with a nonpolar orientation, the resonant features observed in the PR spectra exhibit a strong in-plane polarization anisotropy and different transition energies from the ones measured in the C-plane GaN film. The deformation potential D-5 is accurately determined from four GaN films with a nonpolar orientation using the measured energies together with the polarization properties and out-of-plane strain. (c) 2006 American Institute of Physics

Spatially resolved investigation of strain and composition variations in (In,Ga)N/GaN epilayers

The strain state and composition of a 400nm thick (In,Ga)N layer grown by metal-organic chemical vapor deposition on a GaN template are investigated by spatially integrated x-ray diffraction and cathodoluminescence (CL) spectroscopy as well as by spatially resolved CL and energy dispersive x-ray analysis. The CL investigations confirm a process of strain relaxation accompanied by an increasing indium content toward the surface of the (In,Ga)N layer, which is known as the compositional pulling effect. Moreover, we identify the strained bottom, unstrained top, and gradually relaxed intermediate region of the (In,Ga)N layer. In addition to an increase of the indium content along the growth direction, the strain relaxation leads to an enhancement of the lateral variations of the indium distribution toward the surfac

Spontaneous quasi-periodic current self-oscillations in a weakly coupled GaAs/(Al,Ga)As superlattice at room temperature

We have experimentally observed spontaneous quasi-periodic current self-oscillations at room temperature in a doped, weakly coupled GaAs/(Al,Ga)As superlattice (SL) with 50 periods, 7 nm well width, and 4 nm barrier width. The mole fraction of the aluminum in the barrier has been chosen to be 0.45 so that the direct barrier at the Gamma point is as high as possible and thermal carrier leakage through the X valley is as small as possible. A spectral analysis of the current self-oscillations, which are observed under DC voltage bias alone, demonstrates that spontaneous quasi-periodic oscillation modes coexist with periodic ones