Birefringence of disordered AlGaAs/GaAs quantum wells

Birefringence at room temperature is analysed for interdiffusion induced (disordered) Al/sub 0.3/Ga/sub 0.7/As/GaAs single quantum well structures in the wavelength range 0.5 mu m to 1.0 mu m. The confinement profile for the disordered QW is modelled by an error function and the refractive index model includes excitonic effects and contributions from the Gamma , X and L Brillouin zones. Results show that at longer wavelengths the birefringence is small and varies from positive to negative before reducing to zero as interdiffusion proceeds. For wavelengths between the QW and barrier band-edges, the birefringence is large and reduces with increasing interdiffusion.<>     

Photoluminescence and electroreflectance studies of modulation-doped pseudomorphic AlGaAs/InGaAs/GaAs quantum wells

In this study, we describe the correlations between the photoluminescence (PL) spectra and electrical properties of pseudomorphic modulation-doped AlGaAs/InGaAs/GaAs quantum wells (MDQWs) grown by molecular beam epitaxy. In MDQWs, the presence of a large sheet carrier density contributes significantly to the PL linewidth. At low temperatures (4.2 K), free carrier induced broadening of the PL linewidth is influenced by the material quality of the structure. At higher temperatures (77 K), differences in the material quality do not affect the linewidth significantly, and under these conditions the PL linewidth is a good measure of the sheet carrier density. The ratio of the 77 K to 4.2 K PL linewidths provides useful information about the crystalline quality of the MDQW structures as illustrated by the correlation with 77 K Hall mobility data and a simple model. We present results of Electron Beam Electroreflectance (EBER) to characterize MDQWs and undoped quantum wells in the AlGaAs/InGaAs/GaAs material system. Several transitions have been observed and fitted to excitonic Lorentzian lineshapes, providing accurate estimates of transition energy and broadening parameter at temperatures of 96 K and 300 K.     

Shallow impurity levels in AlGaAs/GaAs semiconductor quantum wells

This paper reviews current knowledge of shallow impurity states (donors and acceptors) in AlGaAs/GaAs multiple-quantum-well structures. Calculations of these levels have been performed by a number of groups, most of which solved an effective-mass Schrödinger equation using the variational method. These results are generally consistent with each other, any differences being related to different approximations made in each calculation. The ground states of some of these shallow impurity levels have also been measured in several laboratories using low-temperature photoluminescence, Raman and far-infrared absorption techniques. These methods yield similar results which are consistent with the calculations. Both theoretical and experimental methods and results are discussed.     

Intervalley scattering in GaAs/AlGaAs quantum wells and quantum cascade lasers

The results of simulations of Γ−X scattering in GaAs/AlGaAs quantum wells are presented, discussing the importance of the mole fraction, doping density, and lattice and electron temperatures in determining the scattering rates. A systematic study of Γ−X scattering in GaAs/Al_xGa_1−xAs heterostructures, using a single quantum well to determine the importance of well width, molar concentration x, lattice temperature, and doping density, has been performed. After this we consider a double quantum well to determine the role of intervalley scattering in the transport through single-layer heterostructures, i.e. Γ−X−Γ scattering compared with Γ−Γ scattering. Finally, we estimate the relative importance of intervalley scattering in a GaAs-based quantum-cascade laser device and compare it with other relevant scattering mechanisms important to describe carrier dynamics in the structure. Our simulations suggest that Γ−X scattering can be significant at room temperature but falls off rapidly at lower temperatures.     

AlGaAs/InGaAs/GaAs quantum well doped channel heterostructure fieldeffect transistors

The results of experimental and theoretical studies of pseudomorphic AlGaAs/InGaAs/GaAs quantum-well doped-channel heterostructure field effect transistors (QW-DCHFETs) are presented. The channel doping was introduced in two ways: during growth by molecular beam epitaxy or by direct ion implantation. The latter technique may be advantageous for fabrication of complementary DCHFET circuits. Peak transconductances of 471 mS/mm and peak drain currents of 660 mA/mm in 0.6-μm-gate doped-channel devices were measured. The results show the advantages of the DCHFET over standard heterostructure FET structures and their potential for high-speed IC applications. Self-consisted calculations of the subband structure show that the potential barrier between the quasi-Fermi level in the channel and the bottom of the conduction band in the barrier layer is considerably larger for the doped channel structure than for the structure with an undoped channel. This lowers the thermionic emission gate current of the doped channel device compared to the undoped channel device     

Photoluminescence study of AlGaAs/GaAs/AlGaAs double quantum wells separated by a thin AlAs layer

Photoluminescence (PL) spectra of Al_0.21Ga_0.79As/GaAs/Al_0.21Ga_0.79As double quantum wells (DQWs) separated by a thin AlAs barrier have been studied in the temperature range 77–300 K. The well width was varied from 65 to 175 Å, and the thickness of the AlAs barrier was 5, 10, or 20 Å. In the case of a sufficiently thin (5, 10 Å) AlAs barrier, the energy spectrum of QW states is considerably modified by coupling between the QWs. This effect shifts the main spectral peak of PL, and specific features associated with the splitting of the ground state into symmetric and asymmetric states are observed in the spectra at higher temperatures. The DQW structure with a 20-Å-thick AlAs barrier is a system of two uncoupled asymmetric Al_0.21Ga_0.79As/GaAs/AlAs QWs. The energy levels in double coupled QWs were calculated as functions of the well width and AlAs barrier thickness, and good correlation with the experimentally observed energies of optical transitions was obtained.     

Persistent photoeffects in p-i-n GaAs/AlGaAs heterostructures with double quantum wells

Abrupt changes in the capacitance between the p and n regions were observed in a planar p-i-n GaAs/AlGaAs heterostructure with two tunneling-coupled quantum wells exposed to laser irradiation (λ=633 nm). These changes can be caused by variations in both temperature (in the vicinity of T～2 K) and the dc voltage applied to the structure. A memory effect was detected; this effect manifested itself in the long lifetime of anomalies observed after the illumination had been turned off. Self-consistent calculations of distributions of charge and electric field were performed for the structures that contained a donor impurity in AlGaAs layers; this impurity is responsible for origination of DX centers that give rise to persistent photoconductivity. It is demonstrated that abrupt changes in capacitance can occur in such a structure, and the values of the parameters required for origination of these jumps are determined.     

Extremely strong and sharp photoluminescence lines from nitrogen atomic-layer-doped GaAs,AlGaAs and AlGaAs/GaAs single quantum wells

We have performed nitrogen atomic-layer doping into GaAs, AlGaAs, and AlGaAs/GaAs single quantum wells using atomic nitrogen cracked by a hot tungsten filament. While the atomic-layer-doped GaAs layers show a series of sharp and strong photoluminescence lines relating to excitons bound to nitrogen atoms at 8K, atomic-layer-doped AlGaAs layers show several broad nitrogen-related lines. For the atomic-layer-doped single quantum well at the center of the GaAs layer, the quantum well luminescence itself disappears and a dominant and sharp luminescence is observed at a longer wavelength. It is found that the As pressure during the atomic-layer doping greatly affects the luminescence characteristics.     

First demonstration of room temperature intersubband-interbanddouble-resonance spectroscopy of GaAs/AlGaAs quantum wells

Intersubband-interband double-resonance experiments in undoped GaAs/Al_0.33Ga_0.67As multiple quantum well (MQW) structures at room temperature are discussed. The well width is 78 Å. A Ti:sapphire laser is used to pump the interband transitions, while the first intersubband transition is probed with a CO₂ laser. The intersubband absorption is found to peak at 10.6 μm, and a 10-meV linewidth is measured. The absorption signal is also recorded at a fixed CO₂ tuning while varying the pump laser wavelength from 700 to 850 nm. A high-resolution spectrum is obtained, reflecting the steplike density of states with sharp peaks at the exciton resonances     

Single-pulse pump-probe measurement of optical nonlinear propertiesin GaAs/AlGaAs multiple quantum wells

Single-pulse ps-pump and ns-probe nonlinear transmission measurements provide carrier-density-dependent optical nonlinear spectra in GaAs/AlGaAs multiple quantum wells grown by metalorganic chemical vapor deposition. The use of the ps pump eliminates the need to know carrier lifetime to determine carrier density. The saturation behavior of changes in absorption coefficient and refractive index are modeled by a simple saturation equation to obtain saturation carrier density. The saturation spectra for different well thicknesses are obtained. The minimum saturation carrier density appears around 150 Å     

Intersubband absorption of GaAs/AlGaAs quantum wells in MBE grownmid-infrared slab waveguides

A structure to enhance the absorbance due to intersubband transitions in GaAs/AlGaAs quantum wells is discussed. Mid-infrared slab waveguides including 30 quantum wells were grown using molecular-beam epitaxy (MBE). Photoluminescence experiments revealed an excellent uniformity of the samples. Absorption measurements over the whole 9-13.4-μm spectral range were performed for the first time using the combination of CO₂ and NH₃ lasers. Effective absorbance due to intersubband transitions as high as 14 dB were measured for 3-mm-long waveguides. The waveguide structure is expected to be a good candidate for optoelectronic devices in the 10-μm region     

Energy relaxation in GaInAs/AlInAs heterojunctions and GaAs/AlGaAs multiple quantum wells

We have studied electron energy relaxation in GaInAs/AlInAs heterojunctions and GaAs/AlGaAs multiple quantum wells using mobility measurements as a function of electric field and temperature, in the range 3K to 300K. The results in the range 3 to 20K show a power loss rate which is dependent on (T_e − T_l), suggesting that the energy relaxation occurs through acoustic phonon scattering. At electron temperatures greater than 20K, the experimental results are modelled using a standard expression for polar optical phonons. This modelling yields 30meV and 31meV for the polar optical phonon energy in GaAs and InGaAs respectively.     

Photoluminescence study of single-side doped n-AlGaAs/GaAs structures with quantum wells

Photoluminescence spectra of single-side doped n-AlGaAs/GaAs structures have been studied at different quantum well widths and temperatures. The sharp growth of the photoluminescence intensity due to the resonant capture of photoexcited holes by a quantum well has been observed for the first time in such structures. A theoretical model for calculating the quantum states in single-side doped structures is proposed. The self-consistent solution of the system of Schrödinger and Poisson equations is obtained by the perturbation method.     

Intersubband absorption of light in heterostructures with double tunnel-coupled GaAs/AlGaAs quantum wells

Intersubband absorption of mid-IR light was studied in heterostructures with asymmetrical tunnel-coupled quantum wells in equilibrium conditions and under high-power pumping by picosecond pulses of light. The energy spectrum of electrons in tunnel-coupled quantum wells was found from an analysis of equilibrium and nonequilibrium intersubband absorption spectra. The dynamics of intersubband absorption under high-power optical pumping was studied using the pump-and-probe picosecond technique. The experimental data are compared with the results of calculations based on solving rate equations. The intersubband relaxation times are determined.     

Suppression of bandgap shifts in GaAs/AlGaAs quantum wells using strontium fluoride caps

Quantum well intermixing using dielectric caps has been studied using photoluminescence at 77 K. The structures which have been investigated, including shallow depth single quantum wells and multiquantum well waveguiding material, are highly sensitive to the presence of surface defects during annealing. Samples capped with either silicon nitride or silica have shown considerable energy shifts after processing in a rapid thermal annealer, and large energy shifts have also been found in uncapped material. Samples capped with strontium fluoride have shown negligible intermixing of the quantum wells.<>     

Circularly polarized photoluminescence related to A(+) centers in GaAs/AlGaAs quantum wells

Magnetic-field-induced circular polarization of the photoluminescence peak related to A(+) centers in quantum wells is measured for the first time. It is shown that, in a magnetic field of 4 T, the polarization degree is as high as 13%, while the peak splitting is virtually absent. A theory describing the behavior of the spin fine structure of A(+) centers in a magnetic field is developed. Experimental results agree well with the theoretical calculations.     

Electron mobility in a AlGaAs/GaAs/AlGaAs quantum well

An oscillatory dependence of the electron mobility on the quantum well (QW) thickness in a AlGaAs/GaAs/AlGaAs heterostructure with double-sided modulation doping has been observed experimentally. A steep decrease in mobility with increasing electron concentration in the QW is established. The conditions for an increase in mobility on introducing a thin barrier into the QW are determined. The first experimental observation of an increase in mobility by a factor of 1.3 in a QW of thickness L=26 nm upon introducing a thin (1–1.5 nm) AlAs barrier is reported.     

Modulation of optical absorption of GaAs/AlGaAs quantum wells in a transverse electric field

Results of experimental investigations of the transformation of intraband light absorption spectra by the quantum-well electrons in a transverse electric field are presented. In addition to the familiar Stark effect, absorption oscillations in the photoionization band are detected. These oscillations are caused by electron transitions from the ground state in a quantum well to quasidiscrete levels arising in the continuum of states above the well due to the linear potential of the external electric field. Fiz. Tekh. Poluprovodn. 32, 849–851 (July 1998)     

Generation of coherent terahertz radiation by polarized electron-hole pairs in GaAs/AlGaAs quantum wells

The generation of coherent terahertz radiation upon the band-to-band femtosecond laser photoexcitation of GaAs/AlGaAs multiple-quantum-well structures in a transverse electric field at room temperature is investigated. The properties of the observed terahertz radiation suggest that it is generated on account of the excitation of a time-dependent dipole moment as a result of the polarization of nonequilibrium electron-hole pairs in quantum wells by the electric field. The proposed theoretical model taking into account the dynamic screening of the electric field in the quantum wells by nonequilibrium charge carriers describes the properties of the observed terahertz signal.