Photoreflectance of GaAs doping superlattices

We have performed room-temperature photoreflectance measurements on two GaAs doping superlattices having considerably different built-in potentials (1.2 eV and 85 meV). The first sample exhibits Franz-Keldysh oscillations, the period of the oscillations corresponding to the . A second dc pump beam has been used to change the electron-hole concentration and hence the built-in field. The spectrum of the second sample displays a number of features corresponding to quantized electron and hole states. There is qualitative agreement between experiment and theoretical calculation based on a two-band tight-binding model. In both samples the dependence of the amplitude of the photoreflectance signal on pump chopping frequency yields the minority carrier lifetime.     

Photoreflectance spectroscopy of GaAs doping superlattices

Room temperature photoreflectance of molecular beam epitaxy GaAs doping superlattices was measured. Additional structures corresponding to forbidden transitions (Δn≠0) were observed at very low pump beam intensity. Experimental results show that the dominant modulation mechanism of photoreflectance of doping superlattices is different from that of bulk materials. We suggest that the photoreflectance spectrum of doping superlattices have mainly first derivative functional lineshapes, which is caused by the subband shift in doping superlattices. The experiments are well explained by this mechanism.     

Photoreflectance study of InAs quantum dots on GaAs(n 1 1) substrates

InAs self-assembling quantum dots (SAQDs) were grown on GaAs(n 1 1) substrates (n=2,3,4,5) by molecular beam epitaxy. Their structural and optical properties were studied by reflection high-energy electron diffraction, atomic force microscopy (AFM) and photoreflectance spectroscopy (PR). The PR spectra from 0.7 to 1.3 eV presented transitions associated to the SAQDs. The energy transitions were obtained by fitting the PR spectra employing the third derivative line-shape model. For n=2,4,5, two functions were required to fit the spectra. For n=3 only one function was required, in agreement with the more uniform SAQDs size distribution observed by AFM on GaAs(3 1 1)A. Franz–Keldysh oscillations (FKO) were observed in the PR spectra at energies higher than the GaAs band gap. From the FKO analysis we obtained the GaAs built-in internal electric field strength (F_int) at the InAs/GaAs(n 1 1) heterointerface. From F_int we made an estimation of the GaAs strain at the heterointerface.     

FTIR-spectroscopy of (GaAS)n/(AlAs)m superlattices

The optical properties of (GaAs)_n/(AlAs)_m superlattices in the infra-red spectral region have been studied. The confinement of optical phonons has been observed in both GaAs and AlAs layers of superlattices under investigation. The superlattice modes caused by the coupling between LO phonons and collective intersubband excitations have been found in doped superlattices. Macroscopic and microscopic calculations have been used for the analysis of experimental results. Good agreement with experiment has been obtained.     

Origin of antimony segregation in GaInSb/InAs strained-layer superlattices

We show how cross-sectional scanning tunneling microscopy may be used to reconstruct the Sb segregation profiles in GaInSb /InAs strained-layer superlattices. These profiles are accurately described by a one-dimensional model parametrizing the spatial evolution of an Sb seed at the InAs-on-GaInSb interface in terms of two-anion-layer exchange. We argue that the segregation seed, which decreases from 2 / 3 to 1 / 2 monolayer when growth conditions are made less anion rich, has its origin in the Sb-bilayer reconstruction maintained during GaInSb epitaxy.     

Photoreflectance study of InAs ultrathin layer embedded in Si-delta-doped GaAs/AlGaAs quantum wells

Photoreflectance and photoluminescence studies were performed to characterize InAs ultrathin layer embedded in Si-delta-doped GaAs/AlGaAs high electron mobility transistors. These structures were grown by Molecular Beam Epitaxy on (1 0 0) oriented GaAs substrates with different silicon-delta-doped layer densities. Interband energy transitions in the InAs ultrathin layer quantum well were observed below the GaAs band gap in the photoreflectance spectra, and assigned to electron-heavy-hole (E_e-hh) and electron-light-hole (E_e-lh) fundamental transitions. These transitions were shifted to lower energy with increasing silicon-δ-doping density. This effect is in good agreement with our theoretical results based on a self-consistent solution of the coupled Schrödinger and Poisson equations and was explained by increased escape of photogenerated carriers and enhanced Quantum Confined Stark Effect in the Si-delta-doped InAs/GaAs QW. In the photoreflectance spectra, not only the channel well interband energy transitions were observed, but also features associated with the GaAs and AlGaAs bulk layers located at about 1.427 and 1.8 eV, respectively. By analyzing the Franz-Keldysh Oscillations observed in the spectral characteristics of Si-δ-doped samples, we have determined the internal electric field introduced by ionized Si-δ-doped centers. We have observed an increase in the electric field in the InAs ultrathin layer with increasing silicon content. The results are explained in terms of doping dependent ionized impurities densities and surface charges.     

Photoreflectance study on the photovoltaic effect in InAs/GaAs quantum dot solar cell

To investigate the effect of quantum dot (QD) layers on the photovoltaic process of InAs/GaAs QD solar cell (QDSC), QD layers were embedded in conventional GaAs p-n junction SC (GaAs SC) structures. The photoreflectance (PR) was examined at different temperatures (T) and excitation light intensities (I_ex) to investigate the photovoltaic effects through observation of the Franz-Keldysh oscillations (FKOs) in the PR spectra. The evaluated the p-n junction electric fields (F_pn) of the InAs QDSC was different from that of the GaAs SC. Moreover, InAs QDSC show that the different photovoltaic behaviors compared with GaAs SC by varying I_ex and T. From these considerations, we suggest that the different photovoltaic behaviors are caused by the effect of the additional photo-carrier generation in InAs QD layers resulting in enhancement of the field screening effect in F_pn.     

Layer ordering and faulting in (GaAs)n/(AlAs)n ultrashort-period superlattices

We report studies of (GaAs)(n)/(AlAs)(n) ultrashort-period superlattices using synchrotron x-ray scattering. In particular, we demonstrate that interfaces of these superlattices contain features on two different length scales: namely, random atomic mixture and ordered mesoscopic domains. Both features are asymmetric on the two interfaces (AlAs-on-GaAs and GaAs-on-AlAs) for n>2. Periodic compositional stacking faults, arising from the intrinsic nature of molecular-beam epitaxy, are found in the superlattices. In addition, the effect of growth interruption on the interfacial structure is discussed. The relevant scattering theory is developed to give excellent fits to the data.     

Photoreflectance and photoluminescence study of short period (GaAs)n/(AlAs)n superlattices with n = 1 – 15

Photoreflectance and photoluminescence measurements were performed in (GaAs)_n/(AlAs)_n (n = 1 – 15) short-period superlattices in the temperature range from 25 K to 275 K. Weak signals of photoreflectance associated with the critical point of the pseudodirect transition, weakly allowed direct transition arising from the zone-folding effect, have been observed as well as main signals associated with the direct allowed transitions. This assignment is supported by the measurement and analysis of the temperature dependence of the photoluminescence intensity.     

Photoreflectance study of energy level structure of self-assembled InAs/GaAs quantum dots emitting at 1.3 μm

Photoreflectance and photoluminescence measurements were performed on the ensemble of self assembled InAs/GaAs quantum dots designed to emit at 1.3 μm. As many as six QDs-related optical transitions were observed in PR spectra, the energies of which were confirmed by high-excitation PL results. Numerical calculations allowed estimating the average size of the dots, which is larger than for standard InAs/GaAs QDs. This result is in agreement with structural data. Additionally, the energy level structure for such QDs was derived and compared with the electronic structure of standard InAs/GaAs dots. It was shown that the energy level structure of such large dots qualifies them for the active region of a laser emitting at 1.3 μm.