GaAs Emitter Research Articles

A theoretical explanation for the red energy shift of a newly discovered, exclusively acceptor-associated emission in GaAs

Radiative mechanism of a newly discovered near-band-edge emission, [g-g], exclusively associated with acceptor impurities in GaAs, was theoretically discussed. Since the emission energy of [g-g] for the lowest acceptor concentration limit is nearly identical to that of the excited state of the acceptor impurities and it shows a noticeable shift towards lower-energy levels with increasing acceptor concentration, it was suggested that the energy of [g-g] corresponds to the 2pσ bond state of the acceptor-acceptor pair formed by the overlapping of the 2p excited state. The calculated energy of the 2pσ bond as a function of the distance between the acceptor-acceptor pair shows good agreement with the experimentally obtained value of [g-g] in Mg-doped GaAs grown by molecular-beam epitaxy (MBE). From a comparison of binding energies in the acceptor-doped GaAs samples prepared by ion implantation (Zn+ or Mg+) and by MBE, the activation ratio of the implanted atoms was estimated.

Degradation of band-gap photoluminescence in GaAs

Evidence is presented to show that the degradation of near-band-gap photoluminescence emission in GaAs with time of exposure to low power, cw laser excitation can be attributed to a surface mechanism that is independent of surface oxidation and may involve an interaction between crystal point defects and the photogenerated electron-hole plasma near the surface.

Laser-induced damage and ion emission of GaAs at 106 μm

This study focused on the multipulse laser damage and the subdamage threshold ion emission of GaAs. The initial goals were to determine the pulse-dependent damage threshold and to correlate ion emission with surface damage. A Q-switched Nd:YAG laser was used to irradiate the (100) GaAs samples. Using values of N from 1 to 100, we obtained accumulation curves based on 50% damage probability. Corresponding damage threshold fluences were 0.4-0.8 J/cm2 for N > 1 and 1.5 J/cm2 for N = 1. We observed large site-to-site fluctuations in ion emission and found the onset of emission at 0.2 J/cm2 for all cases. Once surface damage occurred, ion emission increased greatly. The observed behavior supports a surface cleaning model for the ion emission which precedes surface damage. Measurements of linear and nonlinear free carrier absorption were made, but no anomalous absorption was observed.

Valence band mixing and optical emission in modulation-doped GaAs-(AlGa)As heterostructures

Unexpected optical emission spectra from electrons confined in GaAs quantum layers reveal a strong component with polarization normal to the plane of the layers. For elementary electron-hole recombination processes, this suggests surprisingly large band mixing in the ground valence subband. Effective mass theories that include conventional symmetry breaking mechanisms do not satisfactorily account for this phenomenon. The stress dependence of the spectra confirm that many body shake-up processes in the Fermi sea are important in describing emission associated with the 2D electron plasma.

A tunneling emitter bipolar transistor

We propose a new device-a Tunneling Emitter Bipolar Transistor (TEBT)-where the enhancement of the emitter injection efficiency is achieved by utilizing a very large difference in the tunneling probabilities for electrons and holes in a thin doped graded AlGaAs layer. This layer is inserted between the n-type GaAs emitter and p-type GaAs base. This device should have a high emitter efficiency and low parasitic resistances.

Cryogenic-pressure response of optical transitions in quantum well and bulk GaAs: A direct comparative study

Cryobaric (11–100 K and 300 K, 0–65 kbar) measurements of photoluminescence in GaAs/AlxGa1−xAs multiquantum well structures are described. Results on a narrow-well (38 Å wide) structure exhibiting both quantum well and bulk GaAs emission (from a thick buffer region) allow direct comparison of the two for the first time. We find that transitions between Γ-derived n=1 confined levels have the same pressure coefficient, within ±0.2 meV/kbar, as the bulk E0 gap, 11.4 meV/kbar. An interaction between n=1 electron states and a state 35–40 meV below the X-conduction minima is observed within the pressure-induced Γ-X crossover region for our narrow-well sample. The proposed origin of the latter state is residual interface impurities.

Influence of a surface electric field on the line shape of the excitonic emission in GaAs.

We report luminescence experiments on excitons in a GaAs-AlGaAs heterostructure with an applied electric field perpendicular to the surface. In the absence of any surface electric field we observe a single emission line centered at the excitonic eigenenergy. With increasing electric field the line shape changes subsequently to a camel-back structure. We explain the field-induced change of the excitonic-emission line shape by absorption in the depletion layer due to field-ionized excitons.

Elastic scattering of exciton polaritons by neutral impurities.

A model based on the elastic scattering of exciton polaritons from residual neutral impurities is proposed to account for the line shape of the exciton-polariton photoluminescence emission in GaAs at low temperatures. The good agreement obtained with experimental spectra emphasizes the importance of impurity scattering on exciton-polariton transport for modes in the vicinity of the exciton energy. This model is applicable to any solid possessing dipole-active excitons.

Luminescence from hot electrons relaxing by LO phonon emission in p-GaAs and GaAs doping superlattices

Laser excited hot electrons in GaAs relax by LO phonon emission within a few hundred femtoseconds, leading to a series of peaks in the distribution of hot electrons in the conduction band, which we observe in luminescence. We find that the luminescence peaks shift according to the acceptor binding energy for C−, Ge−, Zn−, and Be-p-doped GaAs layers grown by MBE and LPE. Thus we prove that recombination is between hot electrons and neutral acceptors. The series of peaks due to electrons from the heavy hole band agree well with k. p band structure, while peaks due to those from light holes are about 15 meV lower than expected from the band structure. We show that the discrepancy is not due to heating or surface fields. The peak separation in the luminescence ladder is about 6% larger than the LO energy suggesting emission of renormalised LO phonons. We find thermalisation by LO emission also in GaAs nipi doping superlattices. In nipi crystals the emission is shifted to higher energies (by 12 meV for light and by 6 meV for heavy holes) due to a change in band structure caused by the space charge fields.

High-magnetic-field Zeeman spectroscopy of the 0.84-eV Cr-related emission and absorption line in GaAs(Cr): Experiment and theory

The results of a series of high-magnetic-field (up to 22 T) Zeeman-spectroscopy measurements are reported on the Cr-related 0.84-eV luminescence line in GaAs. The observed Zeeman splitting and oscillator strengths can be satisfactorily explained in terms of a model in which both the excited and ground state have ${C}_{3v}$ symmetry and a twofold-orbital and fivefold-spin ($S=2$) nature. In addition the model provides an excellent fit to the oscillator strengths and energies of the fine structure observed in the no-phonon line seen at zero magnetic field in both absorption and luminescence. Our result conclusively demonstrates that a recently proposed model based on a ground-state effective Hamiltonian containing terms of only cubic symmetry is incorrect.

Photoionization and thermal activation of compound semiconductor MOS interfaces and origin of interface states

Dynamic properties of GaAs and InP anodic MOS interfaces are studied, using photocapacitance transient spectroscopy (PCTS) and deep level transient spectroscopy (DLTS) techniques. Measured PCTS spectra possess a gate bias dependent portion corresponding to U-shaped continuous distribution of interface states and a bias-independent portion with a large photoionization rate. The photoionization cross section of a state continuum increases monotonically with photon energy, similarly to Si MOS, but very differently from bulk deep traps. Anomalously reduced DLTS activation energies of interface states are observed for electron emission in GaAs and for hole emission in InP, and they are correlated to a bias-independent portion of PCTS spectra. Measured results are explained by coexistence of an energetically and spatially distributed localized state continuum region and a partially delocalized state region. Surface disorder is proposed to be the origin of such composite interface state structure rather than the surface-vacancy based model.

Spin dependent carrier capture processes observed by ODMR on the 0.84 eV luminescence in SI-GaAs:Cr

Optically detected magnetic resonance (ODMR) observed on the 0.84eV Cr-related emission in semi-insulating GaAs:Cr implies that the excited state associated with this internal transition of the axial Cr 2+-X centre is formed by the spin dependent free carrier capture process: Cr 3+-X + e CB a ̊ Cr 2+-X ∗ The results imply that the luminescence is not due to the presence of Cr 2+-X ground state centres in the material as previously assumed. A competing charge transfer process, possibly involving Cr 2+-X , is observed as a quenching ODMR signal on the 0.84eV luminescence.

Generation of tunable picosecond pulses from a bulk GaAs laser

Tunable picosecond pulses have been generated from a bulk GaAs crystal which supports a gain column 1 cm in length. Using two-photon absorption as the means of excitation, deep penetration of the pump intensity further increases the volume of the gain medium and enables the generation of pulses with peak power in the megawatt range. The output wavelength of the GaAs emission is tunable by changing the temperature of the crystal. A tuning range covering 840-885 nm has been achieved over a temperature range from 97 to 260 K. The variation is approximately linear with a slope of ∼0.26 nm/K. It has also been observed that the intensity of the output is highest when the temperature of the crystal is at its lowest, 97 K in our experiment.

Generation of tunable picosecond pulses from a bulk GaAs laser

Tunable picosecond pulses have been generated from a bulk GaAs crystal which supports a gain column 1 cm in length. Using two-photon absorption as the means of excitation, deep penetration of the pump intensity further increases the volume of the gain medium and enables the generation of pulses with peak power in the megawatt range. The output wavelength of the GaAs emission is tunable by changing the temperature of the crystal. A tuning range covering 840-885 nm has been achieved over a temperature range from 97 to 260 K. The variation is approximately linear with a slope of ∼0.26 nm/K. It has also been observed that the intensity of the output is highest when the temperature of the crystal is at its lowest, 97 K in our experiment.

High-energy (Visible-red) stimulated emission in GaAs

The high-energy (visible-red) photpumped laser operation (6345 Å at 77 K, 6785 Å at 300 K) of AlxGa1−xAs-AlAs-GaAs quantum-well heterostructures (QWH) grown by metalorganic chemical vapor deposition (MO-CVD) is described. The QWH active regions are alloy-free and consist of GaAs quantum wells and AlAs barrier layers. The effect of the AlAs barrier-layer thickness on the energy banding of the confined-carrier states and transitions is demonstrated. The laser operation of the coupled GaAs quantum wells is observed as high as 400–445 meV above the bulk-GaAs band edge, which agrees with the calculated locations (Lz∼30 Å) of the lowest (n = 1) electron, heavy-hole, and light-hole confined-particle states or energy bands.

Recombination centers and the spatial distribution of spontaneous emission in n-type GaAs at high excitation levels

We have investigated the excitation intensity dependence of the spatial distribution of the emission of photoluminescence from heavilly doped n-type GaAs. It was found that above a certain threshold of excitation, maximum emission originated from an anular zone surrounding the point of excitation rather than from that point. This new effect is explained in terms of the quasi Fermi level dependence of the rate of recombination through recombination centers.

Thermalization of the Electron—Hole Plasma in GaAs.

AbstractThe band‐to‐band luminescence lineshape of the electron—hole plasma emission in GaAs is analyzed as a function of the excess energy of the photocreated electrons and holes. This yields experimental data for the dependence of the effective carrier temperature of the electron—hole plasma on the excess excitation energy for high electron and hole densities. Carrier temperatures as low as 5 K can be obtained in the case of resonant excitation. A discussion of these data in terms of the possible energy dissipation processes shows that hole—phonon interaction gives an important contribution to the cooling of the carrier system.

Transient-waveguide effects and lasing time delays in electron-beam-pumped semiconductor lasers

The dynamics of electron-beam-pumped semiconductor lasers are strongly affected by transient waveguiding. We have investigated the impact of such effects on the time delay from the start of the pumping pulse to the onset of laser emission in GaAs at liquid-nitrogen temperatures. Time delays were measured as a function of pump current density for three values of the electron-beam energy (20, 30, and 40 keV). Delays up to 200 nsec were observed which are approximately two orders of magnitude larger than the minority-carrier lifetime. A theoretical model is presented for the time-dependent variations of the optical confinement to account for such long delays. The time delay to threshold is determined not by the gain buildup but by the time required to sufficiently decrease the laser-mode diffraction loss in the active layer. Initially, the presence of gain results in a negative change of the refractive index with respect to the passive material. This is opposed by a growing positive index contribution due to the rise in temperature of the pumped layer. The latter eventually dominates, thereby confining the cavity modes. Under appropriate conditions, the ensuing reduction in loss determines the onset of lasing. The importance of the waveguide effect was confirmed by measurements on devices incorporating a permanent index step at the active-passive region interface.

Nonradiative capture and recombination by multiphonon emission in GaAs and GaP

Data are presented for 9 capture cross sections of deep levels in GaAs and 4 in GaP which can be interpreted as capture by multiphonon emission (MPE). At high temperatures the cross sections have the form $\ensuremath{\sigma}={\ensuremath{\sigma}}_{\ensuremath{\infty}}{e}^{\frac{\ensuremath{-}{E}_{\ensuremath{\infty}}}{\mathrm{kT}}}$ where ${\ensuremath{\sigma}}_{\ensuremath{\infty}}={10}^{\ensuremath{-}14}\ensuremath{-}{10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ and ${E}_{\ensuremath{\infty}}=0\ensuremath{-}0.56$ eV. A simple theory of MPE capture is presented in which vibrations of a single lattice coordinate modulate the depth of the potential well binding the carrier. In this model capture results from lattice vibrations causing the crossing of free- and bound-carrier levels. The breakdown of the adiabatic approximation near the crossing is discussed. The calculated cross sections have the form $\ensuremath{\sigma}=Af(0)$ where $f(h\ensuremath{\nu})$ is the normalized line shape for radiative capture. The lattice relaxation properties of the center determine $f(0)$. The temperature dependence of $f(0)$ correctly accounts for the thermally activated behavior of the cross sections at high temperatures. Classical and quantum treatments of the lattice motion give the same expression for $\ensuremath{\sigma}$ at high temperature. A detailed calculation of $A$ is made for the capture of a carrier by an attractive neutral impurity in the case where both the free-carrier and bound-carrier wave functions are describable in a one-band effective-mass approximation. The theoretical value of $A$ leads to ${\ensuremath{\sigma}}_{\ensuremath{\infty}}\ensuremath{\approx}6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$, the same order of magnitude as the experimental values. However, many of the experimental cross sections involve complexities not accounted for in this simple model such as charged impurities and transitions between free states and bound states of different symmetry. The lattice relaxation parameters are experimentally determined for the Zn-O and O centers in GaP. Lattice relaxation is found adequate to explain the large cross sections for electron capture by the Zn-O center and hole capture by the two-electron state of O. The studies of the O and Zn-O centers also provide evidence for nonlinear changes in the impurity energy level with lattice displacement which decrease the electron capture cross sections and greatly enhance the hole recombination cross sections. The source of this nonlinearity is discussed.

Luminescence and Excitation Spectra of Exciton Emission in GaAs

AbstractLuminescence, excitation, and photoconductivity spectra of epitaxial n‐ and p‐type GaAs at 1.6 K are measured with high resolution, using very low intensity of excitation. Under this condition, it is found that the (D°, X) bound exciton emission is split into two components, and that further structure is observable between the (D°, X) and the luminescence band, recently attributed to lower branch polariton emission. In particular, two very sharp lines are seen ≈︁ 0.5 meV above the (D°, X) emission. These lines are observed in all our samples, and their intensities seem to be related to the strength of the (D°, X) line. Analysis of the excitation spectra lead to the identification of three emission lines. These are the n = 2 free exciton transition, the two‐electron transition of the (D°, X) complex with the donor left in then = 3 excited state, and most probably the n = 2 donor‐to‐valence band transition.