G. J. Jan

Temperature dependence of photoreflectance in InAs/GaAs quantum dots

Temperature dependent photoreflectance (PR) and photoluminescence experiments of the InAs/GaAs quantum dot (QD) structures were performed. At 20 K, effective band-gap transitions due to the InAs QDs, wetting layers, and GaAs buffer and cap layers were identified. Transition energies of the ground state and four excited states with nearly equal interlevel spacings (75–80 meV) were observed. The linewidth of the ground-state transition decreased as the temperature increased from 20 K to 100 K while the linewidth became broader at temperatures above 100 K. Energy features of the PR spectra originating from QDs and relating to the in-plane parabolic potentials were discussed.

CHARACTERIZATION OF PIEZOELECTRIC (111)B INGAAS/GAAS P-I-N QUANTUM WELL STRUCTURES USING PHOTOREFLECTANCE SPECTROSCOPY

Strained-layer (111)B In0.2Ga0.8As/GaAs p-i-n quantum well structures grown with exciton transitions well resolved at room temperature have been studied by photoreflectance spectroscopy. Using the reduced mass deduced from experiments, the built-in electric field in the barrier region is obtained from the above band-gap Franz–Keldysh oscillations. The strain-induced piezoelectric field is then determined directly from a comparison of the periods of Franz–Keldysh oscillations in different samples. Numerical solutions for the exciton transitions from the derived potential profiles are in good agreement with the experimental results. The piezoelectric constant is also determined using the piezoelectric field.

Photoreflectance characterization of an InAlAs/InGaAs heterostructure bipolar transistor

We have measured the photoreflectance spectrum at 300 K from a lattice‐matched InAlAs/InGaAs heterostructure bipolar transistor grown by molecular beam epitaxy. The energy features of photoreflectance spectra have been identified and the built‐in dc electric fields and associated doping profiles have been evaluated in the n‐InAlAs emitter from the observed Franz–Keldysh oscillations. The undoped InGaAs spacer between emitter and base was added on to change the built‐in electric field. The results showed that the energy features above the InGaAs band gap are the transitions from the valence band to the quantized state of the conduction band. The quantum well of the conduction band is in the interface of the InAlAs and InGaAs heterojunction. The interface charge densities in the spacer channel are determined to be 3.54×1011 cm−2 and 4.22×1011 cm−2, corresponding to the samples with spacer thicknesses of 300 and 500 A, respectively. A triangular potential profile model was used to calculate the microstructur...

Energy-band diagrams of p-i-n heterostructures for single quantum-well lasers

The variations of energy-band diagrams with forward bias voltages for step separate-confinement-heterostructure single quantum-well lasers operated below threshold are investigated by numerically solving the semiconductor device equations with two band parameters described by the generalized Einstein relation and with various position-dependent parameters for the materials. The effects of band-gap shrinkage are included. By examining band diagrams, it is found that the potential profiles of the quantum wells with respect to the centres of the wells are symmetric at high injection, tilted at low injection, or almost flat with reasonable injection. In the intrinsic regions, the difference of the quasi-Fermi potentials can be well approximated by the applied voltage. At high injection, the current density of the I-V characteristic and the current injection efficiency of the device may be overestimated by using the Boltzmann statistics when compared with the Fermi-Dirac statistics. >

Photoreflectance spectroscopy of strained-layer (111)B InGaAs/GaAs quantum well diodes

InGaAs/GaAs (111)B quantum well p-i-n structures grown by gas source molecular beam epitaxy have been investigated with a photoreflectance technique. Using the reduced mass deduced from experiments, the built-in electric field is obtained from the above band-gap Franz–Keldysh oscillations (FKOs). The strain-induced piezoelectric field is then determined directly from the comparison of the periods of FKOs in different samples. Numerical solutions for exciton transition energies with the experimentally derived potentials are in good agreement with experimental results. Hence, the piezoelectric constant can be determined using the piezoelectric field. The temperature dependences of the quantized energy levels indicate that the influence of temperature on exciton transitions is essentially the same as that of the gaps of the relevant bulk constituent materials.

Photoreflectance characterization of graded InAlAs/InGaAs heterojunction bipolar transistor layers

The photoreflectance (PR) spectroscopic technique has been used to investigate the microstructures of the graded InAlAs/InGaAs heterojunction bipolar transistor at room temperature. The energy features of the PR spectrum were fitted and identified as band‐to‐band transitions in the graded layers which were grown using pulsed molecular beam epitaxy and InGaAs as well as InAlAs layers. A linear variation relationship of band gaps with Al composition z was observed and approximated by Eg=0.737+0.759z eV. From the observed Franz‐Keldysh oscillations, we have evaluated the built‐in dc electric fields in the i‐InGaAs collector and n‐InAlAs emitter regions. These electric fields are in good agreement with the continuity condition of electric displacements in the interfaces.

Autoionization broadening of light-hole exciton in GaAs under uniaxial stress

Abstract Certain effects of uniaxial stress X(∥[100]) on the intrinsic linewidth of the ground state light-hole exciton (1L) and the exciton energies in GaAs bulk semiconductor have been investigated theoretically. The following experimental results have been interpreted: (1) when X ≈ 0.45 kbar, the level crossing between the 1L exciton and the first excited state of the heavy-hole exciton starts, and (2) when X⩾ 0.5 kbar, a Fano-like resonance (autoionization) occurs and causes a linewidth broadening of the 1L exciton line. The valence band was analyzed at the Brillouin zone center by solving perturbatively a 4×4 Luttinger-Kohn Hamiltonian in conjunction with a 4×4 strain Hamiltonian in the spin J = 3 2 basis. Reasonable comparison between theory and experiment was obtained.

Photoreflectance temperature dependence of graded InAlAs/InGaAs heterojunction bipolar transistor layers

Photoreflectance(PR) spectra of the graded InAlAs/InGaAs heterojunction bipolar transistor layers were investigated at various temperatures between 8 K and 300 K. The energy features of the PR spectra were fitted and identified as band‐to‐band transitions in the graded layers which were grown by pulsed molecular beam epitaxy (pulsed‐MBE) and InGaAs as well as InAlAs layers. The temperature variation of energy gaps can be described by the Varshni and Bose‐Einstein equations. A linear variation relationship of band gaps with Al composition (z) was observed and approximated to be E0(z)=0.809+0.769z eV at T=0 K. However, the parameters aB and ΘB derived from the Bose‐Einstein expression do not change meaningfully in the whole range of Al composition. From the observed Franz‐Keldysh oscillations (FKOs) we have evaluated the built‐in dc electric fields in the i‐InGaAs collector, i‐InGaAs spacer and n‐InAlAs emitter regions. The electric fields are in good agreement with the continuity condition of electric disp...

PHOTOREFLECTANCE STUDIES OF QUANTUM WELLS AT OBLIQUE INCIDENCE

Abstract The photoreflectance spectra of quantum well structures at room temperature measured with s-polarized or p-polarized light at oblique incidence have been theoretically and experimentally investigated. The line shapes of the spectra were constructed using Gaussian curves for the dielectric function. Exciton energies, including a modulation field, were calculated by the variational method. For s-polarization, we found that when the incident angle ϑ increases, the peaks of the spectra were nearly stationary in energy and decreased in intensity. For p-polarization, when ϑ is near the Brewster angle ϑ B , the peaks of the spectra shift suddenly in energy and extrema appear in the intensities which are amplified by two orders of magnitude over the peak intensities of the spectra for s-polarization. When ϑ is far away from ϑ B (ϑ⪢ϑ B or ϑ⪡ϑ B ), the spectra show an out-of-phase relationship. A theoretical model, used to explain these observations, is presented.

Analysis of heavily tailed size distributions of ZnTe/ZnSe quantum dot structures by using the bootstrap methodology

We have used the bootstrap methodology to analyze dot size distributions of ZnTe quantum dot (QD) structures. The photoluminescence (PL) spectrum indicates that the ZnTe QD structure belongs to a type-II band alignment. The broadness with small fluctuations in the PL represents the spatial inhomogeneity of the QD sizes. The Schrodinger equation together with the first-order perturbation correction was numerically solved to correlate the dot size and the photon energy. Using the bootstrap “loess” curve fitting method, the PL spectrum was determined to be a normal distribution with a high significance level of 46% tested by a null hypothesis H0. By examining the slope of the complementary cumulative distribution function, we found that the size distribution is heavy tailed.