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.     

Photoreflectance spectroscopy of low-dimensional GaAs/AlGaAs structures

Results of room-temperature photoreflectance measurements on three GaAs/Al_0.33Ga_0.67As multiquantum well (MQW) structures with three different widths of wells and on two GaAs/Al_0.33Ga_0.67As high-electron-mobility transistor (HEMT) structures are presented. Energy-gap-related transitions in GaAs and AlGaAs were observed. The Al content in AlGaAs was determined. MQW transition energies were determined using the first derivative of a Gaussian profile of the measured resonances. In order to identify the transitions in the MQS, the experimentally observed energies were compared with results of the envelope function calculation method for a rectangular quantum well. The Franz–Keldysh oscillation (FKO) model was also used to determine the built-in electric field in various parts of the investigated structures. The values of the electric fields allow us to hypothesise about the origin of these fields. © 1997 John Wiley & Sons, Ltd.     

Subthreshold current in undoped AlGaAs/GaAs MODFET structures

The subthreshold current in undoped AlGaAs/GaAs MODFET structures can have a negative gradient with respect to the gate voltage, whereas in doped structures it has normally either a positive gradient or is constant. A two-dimensional finite-difference analysis is used to clarify this difference. It is shown that the behavior of the threshold current in an undoped structure is due to the effect of gate bias on the leakage between the drain and the gate. This leakage occurs near the top surface of the AlGaAs layer, and is strongly affected by the lateral straggle of the n⁺ implants and by the surface states in AlGaAs. The charge-control model, which is conventionally applied to the doped case, does not describe the behavior of the subthreshold current in the undoped case because it neglects leakage currents     

Photoelectrical memory in GaAs/AlGaAs multilayer quantum-well structures

An increase in the dark current (by 2–3 orders of magnitude) in GaAs/Al_xGa_1−x As multilayer quantum-well structures with x⋍0.4 is observed after illumination of the structures with optical light (λ<1.3 μm). This increase is sustained for an extended time (more than 10³ s) at low temperatures. It then decreases to its initial value upon heating of the sample. A model of the barrier with local sag of the conduction band facilitating tunneling is proposed. The conduction band sag and the magnitude of the current grow due to optical ionization of uncontrolled deep level clusters present in the barrier and decrease due to subsequent capture of electrons from the conduction band by the deep levels upon heating. Fiz. Tekh. Poluprovodn. 32, 209–214 (February 1998)     

Investigation of GaAs/AlGaAs quantum cascade structures by optical methods based on hot luminescence in the near-infrared range

The time-resolved photoluminescence of GaAs/AlGaAs quantum-cascade structures under intense pulse excitation is studied. Aside from optical transitions between the ground electron and hole states of a system of two tunnel-coupled quantum wells, the photoluminescence spectrum at short times after the excitation pulse exhibits features corresponding to transitions between the excited states of these wells, which are not observed in time-integrated photoluminescence spectra. It is shown that, due to a high pump level, the electron gas is initially strongly heated, which makes it possible to observe band-to-band transitions between both the ground and excited states. Nonequilibrium carriers cool down with a characteristic relaxation time of ～125 ps.     

A two-dimensional analytical subthreshold behavior model for short-channel AlGaAs/GaAs HFETs

Based on the minimum heteroface potential through an evanescent-mode analysis of two-dimensional potential distribution, the comprehensive and accurate expressions for the short-channel threshold voltage and subthreshold swing for self-aligned gated AlGaAs/GaAs HFETs are developed. It is found that the 2-D electron gas is strongly affected by the DIBL effect which will significantly influence the subthreshold behavior of the AlGaAs/GaAs HFETs. Besides the doping density, the thickness of the spacer and doped body can aggressively affect the short-channel subthreshold behavior comprising threshold voltage shift and subthreshold swing degradation. This model not only gives physical insights into the short-channel effects in HFETs but also offers basic designing guideline for the small-geometry AlGaAs/GaAs HFETs.     

Electroluminescence from AlGaAs/GaAs quantum-cascade structures in the terahertz range

Electroluminescence from a quantum-cascade structure comprising 40 periods of GaAs/Al_0.15Ga_0.85As tunnel-coupled quantum wells (QW) was studied. A terahertz emission band in the range 1.0–1.8 THz is observed under bias exceeding 1.5–2.0 V. The emission band peak shifts linearly to higher frequency with the increasing bias. The effect is accounted for by spatially indirect electron transitions between states in the neighboring QWs.     

Annihilation of nonradiative recombination centers in GaAs/AlGaAs multiquantum well structures as a result of exposure to plasma

The effect of exposure to a low-energy plasma (CF₄, Ar, Kr) on the photoluminescence properties of GaAs/AlGaAs multiquantum well structures is examined. It is shown that the photoluminescence of the quantum wells in the surface region is quenched after plasma exposure and the depth of this region increases with increase of exposure time. The photoluminescence intensity from the quantum wells located beyond this region increases. We associate these changes in the photoluminescence intensity with the effect of plasma-induced nonequilibrium point defects diffusing with anomalous rapidity into the depth of the structure. Fiz. Tekh. Poluprovodn. 31, 1436–1439 (December 1997)     

Pseudo two-dimensional simulation of a three heterojunction GaAs/AlGaAs MODFET

A new model is presented for the simulation of the d.c. characteristics of three heterojunctions AlGaAs/GaAs Field Effect Transistors. The dependence of the carrier densities in the GaAs wells and in the doped AlGaAs layers is derived as a function of gate voltage by analytical resolution of Schrödinger equation and numerical resolution of Poisson's equation using Fermi-Dirac statistics. The simulated d.c. characteristics are obtained by numerical integration of the current density from source to drain and include the source and drain access resistances. The comparison between experimental measurements and calculated results for both long (L_g = 20 μm) and short (L_g = 2 μm) gate lengths MODFETs is excellent and demonstrates the validity of the model to optimize the device parameters. The influence of various parameters such as the AlGaAs layer thickness and electron velocity in the inverted well on the device performance is numerically simulated for a 1 μm gate length device. Their effect on the transistor characteristics such as transconductance and maximum drain current is discussed.     

Fabrication of 2-D photonic bandgap structures in GaAs/AlGaAs

The authors report the first realisation of a two-dimensional photonic bandgap structure with a period of λ₀/2n in the GaAs/AlGaAs material system. The structure consists of a honeycomb lattice with a wall thickness of ~30 nm and a period of 160 nm. It was found that, to realise patterns of such small size and periodicity, it is crucial to control the shape of the exposed features     

Dependence of the binding energy of <Emphasis Type="Italic">A</Emphasis>(+) centers on quantum-well width in GaAs/AlGaAs structures

The luminescence of GaAs/AlGaAs multiple-quantum-well structures with different well widths, containing A(+) centers, was studied to determine the dependence of the center binding energy on quantum-well width. It is shown that the binding energy of the A(+) centers increases markedly with decreasing well width, becoming ten times greater in 10-nm-wide wells than in the bulk material. The binding energy of A(+) centers was found to depend on their concentration.     

Field ionised impurity scattering in an AlGaAs/GaAs two-dimensional electron gas

Low field-mobility saturation has been observed in AlGaAs/GaAs heterostructures. Under illumination, the mobility decreases and the carrier concentration increases at high fields. Both effects are thought to be due to ionisation of a centre in the GaAs, probably oxygen. The experimental results are explained in terms of a simple trap model.     

Comparison of OMVPE Grown GaAs/AlGaAs and GaAs/InGaP HEMT and PHEMT structures

Symmetric δ-doped InGaP and AlGaAs PHEMT structures have been grown by organometallic vapor phase epitaxy with properties that approach those of MBE grown AlGaAs structures. The 300 and 77K carrier concentrations for the InGaP PHEMT were 2.72 and 2.56 × 10¹² cm^{2 −2} and the mobilities were 5,920 and 22,000 cm^{2 2}/V.s. These excellent values suggest that problems associated with switching the anion at the channel heterojunction have been overcome. The corresponding values for the AlGaAs PHEMT were 2.51 and 2.19 × 10¹² cm^{2 −2} and 6,500 and 20,400 cm²/V.s. The uniformity in the indium concentration in the InGaAs layer as determined by photoluminescence, photoreflection, double crystal x-ray diffraction, and Rutherford backscattering was found to be good, but the percent In in the AlGaAs pseudo-morphic high electron mobility transistor (PHEMT) was less than that in the InGaP PHEMT even though the programmed values were the same. The uniformity in the doping distribution as determined by secondary ion mass spectroscopy and electrochemical capacitance-voltage measurements was found to be good, but it decreased with distance from the center of the susceptor. Also, most of the dopants in the δ-doped InGaP and AlGaAs layers were activated.     

Study of internal electric fields in AlGaAs/GaAs two-dimensional electron gas heterostructures

Modulation-doped GaAs/AlGaAs heterostructures have been studied by photoreflectance spectroscopy. The spectra at room temperature show Franz–Keldysh oscillations associated to the substrate–buffer layer interface. The built-in electric field magnitude calculated from these oscillations is related with the two-dimensional electron gas (2DEG) mobility. In addition we observed two signals associated to the GaAs capping layer and to the 2DEG, respectively.     

The substructure and luminescence of low-temperature AlGaAs/GaAs(100) heterostructures

The substructure and luminescence of low-temperature epitaxial AlGaAs alloys are studied by Raman spectroscopy and photoluminescence spectroscopy. It is shown that the experimental data obtained in the study are consistent with the results of the previous structural and optical study. The assumption that, at high concentrations of carbon acceptors, the acceptor atoms concentrate at lattice defects of the AlGaAs crystal alloys to form carbon nanocrystals is confirmed.     

Charging of deep-level centers and negative persistent photoconductivity in modulationdoped AlGaAs/GaAs heterostructures

The relaxation kinetics of persistent photoconductivity in AlGaAs/GaAs modulation-doped heterostructures due to charging of EL2-and DX-centers is investigated over a wide range of temperatures and excitation photon energies. The light-induced charging of these deep centers was found to lead to accumulation of positive and negative localized charges, which give rise to positive and negative persistent photoconductivities, respectively. These positive and negative charges are accumulated in different parts of the heterostructure. Their different characteristic times, and the different temperature dependences of these times, result in nonmonotonic time and temperature dependences of the persistent photoconductivity. Charging of EL2-centers in the GaAs buffer layer leads to negative persistent conductivity in the temperature range 180–300 K, while the negative photoconductivity observed at the temperatures below 180 K is caused by excited states of DX-centers in the n ⁺-AlGaAs. Fiz. Tekh. Poluprovodn. 33, 68–74 (January 1999)     

Changes in the density of nonradiative recombination centers in GaAs/AlGaAs quantum-well structures as a result of treatment in CF4 plasma

The effect of low-energy CF₄ plasma treatment on the stationary photoluminescence (PL) spectra and PL kinetics in GaAs/AlGaAs quantum-well (QW) structures is investigated. Intensity of the PL from QWs located deeper than the surface layer damaged by plasma treatment increases. It is established that this is accompanied by an increase in the PL decay time at temperatures above 30 K. It is shown that the density of nonradiative recombination centers in the QW located below the damaged surface layer decreases by a factor of 30 after 40-s exposure to plasma.     

Analysis of AlGaAs/GaAs solar cell structures by opticalreflectance spectroscopy

The control of the thickness and composition of the GaAs cap and AlGaAs window layers is essential to the fabrication of high-efficiency AlGaAs/GaAs heteroface solar cells. The use of optical reflectance spectroscopy for the analysis of these structures is presented. The calculation of the reflectance of a system of thin films is described along with the computer program, REFIT, for the analysis of GaAs cell structures. Results are presented for structures at different stages in the solar cell fabrication process, and the method is applied to the analysis of the oxidation of high AlAs mole fraction AlGaAs layers     

MOCVD-grown InGaAs/GaAs/AlGaAs laser structures with a broad-area contact

A metal-organic chemical vapor deposition (MOCVD) technique is developed for a diode laser heterostructure in a system of InGaAs/GaAs/AlGaAs solid solutions; the optimal sizes and the doping profile of the structure are determined to minimize the internal optical losses. Mesa-strip diode lasers with a threshold density of current J _th=150–200 A/cm², internal optical loss factor α_i=1.6–1.9 cm^?1, and an internal quantum yield η_i=85–95% were fabricated. In the continuous lasing mode of a diode laser with a 100-µm-wide aperture and a wavelength of 0.98 µm, the optical power output was as high as 6.5 W and was limited by the catastrophic optical degradation of mirrors. The radiation divergence in the plane normal to the p-n junction amounts to θ_⊥. The use of wide-gap waveguide layers, which deepens the potential electron well in the active region, is shown to reduce the temperature sensitivity of the InGaAs/GaAs/AlGaAs laser heterostructures in the temperature range from 0 to 70°C.     

(N11)A GaAs: a preferable platform for high quality GaAs/AlGaAs structures

We have successfully used (311)A and (511)A GaAs for the realization of high quality two-dimensional hole gas (2DHG) and electrons gas (2DEG) structures, respectively. This study was performed mostly on a back-gated, inverted interface, GaAs/AlGaAs structure, in which a 2DHG or 2DEG is embedded. This particular structure enabled the variation of the 2D carrier concentration over two orders of magnitude in a single device, as well as measurement of extremely low carrier densities in the mid 10⁹ cm⁻² range. This remarkably low carrier concentration achieved both in a 2DHG and in a 2DEG opens new frontiers for the study of mesoscopic phenomena governed by Coulomb interactions between carriers and, in particular, the possible existence of a Wigner crystal.