R. N. Pathak

Photoreflectance characterization of GaAs as a function of temperature, carrier concentration, and near‐surface electric field

Previous measurements of the photoreflectance line shape of GaAs at the E1 transition (2.9 eV) were extended. This study covers the combined effect of temperature and carrier concentration along with a discussion of the effect of the electric field intensity and the field inhomogeneity within a depth of 20 nm from the surface. A systematic study of changes in the line shape of the above band gap transition, E1 as a function of temperature (80–400 K) and carrier concentration (CC) (2–200×1016 cm−3) is presented and a model of the effect is discussed. It was found that as the carrier concentration increases beyond 1×1017 cm−3, the line shape changes in phase and in broadening in a characteristic way which depends on CC and temperature. A simple correlation is established between both the broadening and the line shape rotation as a function of temperature and CC. Using the Schottky relation and Fermi‐level pinning, the observed effect qualitatively was explained. Also the effect of near‐surface electric fiel...

Monoenergetic positron beam, positron lifetime, and Hall‐effect measurements in III–V epilayers grown at low temperatures by molecular beam epitaxy

A monoenergetic positron beam has been utilized to depth profile defects in GaAs and in low temperature GaAs buffer layers (LTBL) grown by molecular beam epitaxy. Doppler broadening spectra were measured as a function of positron implantation depth and analyzed in terms of positron diffusion and annihilations of free and trapped positrons. Positron lifetime spectra were measured for the LTBL samples for temperatures in the range 10–295 K. These spectra show two temperature‐dependent components resulting from the annihilations of free positrons and of positrons trapped in vacancy‐type shallow defects. We have performed Hall‐effect and low magnetic field temperature dependent resistivity measurements on the LTBL samples and have determined that the defect activation energy is =0.78 eV. The magnetic field dependence of the Hall data for the LTBL samples shows several interesting new features including sharp stepwise increases for every 700 G between 3.5 and 6 kG.

Defects in molecular beam epitaxial GaAs grown at low temperatures

We have utilized a variable energy positron beam and infrared transmission spectroscopy to study defects in GaAs epilayers grown at low temperatures (LT-GaAs) by molecular beam epitaxy. We have measured the Doppler broadening of the positron-electron annihilation gamma ray spectra as a function of positron implantation energy. From these measurements, we have obtained results for the depth profiles of Ga monovacancies in unannealed LT-GaAs and Ga monovacancies and arsenic cluster related defects in annealed LT-GaAs. We have also studied the effects of the Si impurities in annealed LT-GaAs. The infrared transmission measurements on unannealed LT-GaAs furnish a broad defect band, related to As antisites, centered at 0.370 eV below the conduction band.

Hot‐electron power loss in a doped GaAs/AlGaAs superlattice at intermediate temperature studied by infrared differential spectroscopy

The power loss of electrons in a strongly coupled n‐type GaAs/AlxGa1−xAs superlattice is studied by analyzing the temperature and electric‐field dependence of the interminiband absorption. Electrons are heated by an electric‐field pulse and the resulting change of the infrared absorption spectrum is monitored by a step‐scan Fourier transform spectrometer operated in a time‐resolved, gated mode. The measured power loss is higher than predicted by a simple three‐dimensional calculation including acoustic (deformation potential and piezoelectric) and polar‐optical phonon emission. Possible explanations for this, such as relaxation via folded acoustic phonons or coupled plasmon–phonon modes, are discussed. The energy relaxation time, which can be extracted from the power balance, decreases from 300 ps at 15 K to 20 ps at 48 K.

Electroreflectance‐photoreflectance technique for studies of built‐in electric field in layered materials

We use a new modulation technique to investigate photoreflectance from layered electronic materials. The technique can be used to determine the direction of the electric field, and separate the effects of electric field modulation from extraneous modulations due to laser generated charge carriers.

Miniband dispersion, critical points, and impurity bands in superlattices: An infrared absorption study

Abstract Infrared absorption is employed to study the structure of the minibands in GaAs/AlGaAs superlattices. The miniband dispersion is directly revealed through two absorption maxima, related to the singularities of the density of states at the center and the edge of the mini-Brillouin zone. The total lineshape is strongly asymmetric and can be quantitatively explained, when the kz-dependent transition matrix elements are taken into account. It is argued that the asymmetry follows from the oscillator sum rule. When the doping is low, so that the Fermi energy lies in the first miniband, the kz = π/d peak can be suppressed at low temperature due to thermal depopulation, which reflects the curvature of the lowest miniband. At low temperature another absorption line is observed, identified as 1s-2pz impurity transition. The existence of this transition implies that the impurity band has not merged with the conduction band despite the metallic behavior.

Intersubband Absorption in Strongly Coupled Superlattices: Miniband Dispersion, Critical Points, and Oscillator Strengths

The intersubband absorption is studied in strongly coupled GaAs/AlGaAs superlattices, which exhibit a significant dispersion along the growth direction. The miniband dispersion is directly revealed through two absorption maxima, related to the singularities of the joint density of states at the center and the edge of the superlattice Brillouin zone. The total line shape is strongly asymmetric and can be quantitatively explained, if the kz-dependent transition matrix elements are taken into account. It is argued that the asymmetry is a fundamental consequence of the oscillator sum rule. When the doping is low, so that the Fermi energy lies in the first miniband, the kz=π/d peak can be suppressed at low temperature due to thermal depopulation, which reflects the curvature of the lowest miniband. At low temperature another absorption line is observed, identified as 1s-2pz impurity transition. The existence of this transition implies that the impurity band has not merged with the conduction band despite the metallic behavior.

Slow positron beam measurements on GaAs grown by molecular beam epitaxy at low temperatures

Variable energy positron beam spectroscopy is employed to study defects and arsenic precipitates in GaAs epilayers grown at low temperatures by Molecular Beam Epitaxy (MBE). We have measured the S‐parameter as a function of positron implantation energy. We have obtained results for the depth profiles of the Ga monovacancy defects in unannealed LT‐GaAs and Ga monovacancies and arsenic clusters related defects in annealed LT‐GaAs.

Bloch and localized electrons in semiconductor superlattices

The energy spectrum of electrons in semiconductor superlattices consists of Bloch-like miniband states and localized impurity states. Optical transitions between these states are investigated by infrared spectroscopy. The inter-miniband absorption spectrum is dominated by the van Hove singularities of the quasi one-dimensional joint density of states. The detailed shape of the absorption can be understood in terms of the relevant transition matrix elements and the electron concentration, i.e. it depends on whether the first miniband is full or half-filled. We show that this has implications for intra-miniband absorption and the optical sum rules. In addition, a transition between the states localized at the lower edge of the minibands occurs, which can be traced back to the 1s-2pz hydrogenic donor transition in the low-doping limit. Finally, we discuss the relevance of our observations for the study of the metal-insulator transition in superlattices.

Photoelectrochemical depth profiling of molecular beam epitaxy grown group III‐V heterostructures

A depth profiling technique is described for molecular beam epitaxy grown (Al,Ga)Al/GaAs heterostructures which combines photoelectrochemical layer‐by‐layer removal with analyses by photocurrent spectroscopy. The technique is illustrated for two types of samples having a graded and ‘‘flat’’ Al profile in the epilayer.