Optical control of the mobility of a MODFET with a layer of self-assembled quantum dots

We report an experimental study of GaAs/Al_0.33Ga_0.67As modulation doped field effect (MODFET) transistors, in which an InAs layer of self-assembled quantum dots is placed in one of the Al_0.33Ga_0.67As barrier layers close to the two-dimensional electron gas (2DEG). We find the source–drain resistance is bistable with the two states controlled by illumination and applied gate bias. Brief illumination induces a large, persistent drop in the resistance, which can be recovered by applying a positive gate bias. Magneto-transport measurements show that while illumination causes only a relatively small change in the 2DEG density, it can greatly enhance its mobility. We suggest this is because the 2DEG mobility is limited by percolation of the electrons through the rough electrostatic potential induced by the charged dots. Illumination reduces the negative charge trapped in the dots, thus smoothing the conduction band potential, which produces a large increase in the mobility.     

Investigation of various optical transitions in GaAs/Al0.3Ga0.7As double quantum ring grown by droplet epitaxy

This work examines the optical transitions of a GaAs double quantum ring (DQR) embedded in Al_0.3Ga_0.7As matrix by photoreflectance spectroscopy (PR). The GaAs DQR was grown by droplet epitaxy (DE). The optical properties of the DQR were investigated by excitation‐intensity and temperature‐dependent PR. The various optical transitions were observed in PR spectra, whereas the photoluminescence (PL) spectrum shows only the DQR and GaAs band emissions. The various optical transitions were identified for the GaAs near‐band‐edge transition, surface confined state (SCS), DQR confined state, wetting layer (WL), spin–orbital split (E_GaAs + Δ_o), and AlGaAs band transition. PR spectroscopy can identify various optical transitions that are invisible in PL. The PR results show that the GaAs/AlGaAs DQR has complex electronic structures due to the various interfaces resulting from DE.     

Photoluminescence studies of confined states in AlGaAs/GaAs asymmetric quantum well

In the recombination spectra of AlGaAs/GaAs heterostructures, a peculiar and asymmetric photoluminescence (PL) band F has previously been reported [Aloulou et al., Mater. Sci. Eng. B 96 (2002) 14] to be due to recombinations of confined electrons from the two-dimensional electron gas (2DEG) formed at AlGaAs/GaAs interface in asymmetric quantum well (AQW). Detailed experiments are reported here on GaAs/Al_0.31Ga_0.69As/GaAs:δSi/Al_0.31Ga_0.69As/GaAs samples with different spacer layer thicknesses. We show that the band F is the superposition of two PL bands F′ and F″ associated, respectively, to AQW and a symmetric quantum well (SQW). In the low excitation regime, the F′ band present a blue shift (4.4 meV) followed by important red shift (16.5 meV) when increasing optical excitation intensity. The blue shift in energy is interpreted in terms of optical control of the 2DEG density in the AQW while the red shift is due to the narrowing of the band gaps caused by the local heating of the sample and band bending modification for relatively high-optical excitation intensity. Calculation performed using self-consistent resolution of the coupled Schrödinger–Poisson equations are included to support the interpretation of the experimental data.     

Band offsets in In0.15Ga0.85As/ GaAs and In0.15Ga0.85As/Al0.15Ga0.85As studied by photoluminescence and cathodoluminescence

Photoluminescence and cathodoluminescence measurements of strained undoped In_0.15Ga_0.85As/GaAs and In_0.15Ga_0.85As/Al_0.15Ga_0.85As quantum well structures with emission lines attributed to the first electron–first heavy hole and first electron–first light hole excitonic transitions have been analysed theoretically within the eight-band effective mass approximation. For In_0.15Ga_0.85As/GaAs the results are consistent with either type I or type II alignment of the light hole band. In the case of In_0.15Ga_0.85As/Al_0.15Ga_0.85As our results indicate type II alignment for the light hole band and offset ratio ofQ = 0.83.     

Tunneling resonances in structures with a two-step barrier

Electron transport through an asymmetric heterostructure with a two-step barrier N⁺GaAs/N⁻GaAs/Al_0.4Ga_0.6As/Al_0.03Ga_0.97As/N⁻GaAs/N⁺GaAs was investigated. Features due to resonance tunneling both through a size-quantization level in a triangular quantum well, induced by an external electric field in the region of the bottom step of the barrier (Al_0.03Ga_0.97As layer), and through virtual levels in two quantum pseudowells of different width are observed in the tunneling current. The virtual levels form above the bottom step or above one of the spacers (N⁻GaAs layer) as a result of interference of electrons, in the first case on account of reflection from the Al_0.4Ga_0.6As barrier and a potential jump at the Al_0.03Ga_0.97As/N⁻GaAs interface and in the second case — from the Al_0.4Ga_0.6As barrier and the potential gradient at the N⁻GaAs/N⁺GaAs junction, reflection from which is likewise coherent. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 814–819 (25 May 1998)     

The Investigation of Plasmon-Longitudinal Optical Phonon Coupling and Surface Polariton Modes in Donor Doped GaAs-Al0.33Ga0.67As Multi Quantum Wells

Infrared Fourier transform spectroscopy has been used to investigate phonon, plasmon, surface polariton and plasma-longitudinal optical phonon coupling in highly donor doped multi quantum wells (GaAs/Al_0.33Ga_0.67As) and direct band gap n- type Al_XGa_1-XAs thin layer on GaAs substrate. Using different samples with different concentration of free carriers. The dispersion equation of coupling modes have been calculated by using the condition which the dielectric functions of samples are zero for longitudinal coupled modes and experimental papameters which have been obtained from the best fit p-polarized oblique incidence far infrared reflection spectra. In MQW samples, the free carriers confined to the well and carriers are quasi two dimensional. So, plasmon- LO phonon coupling occur in the well (GaAs). In n- type Al_XGa_1-XAs thin layer, the coupled modes consist of three branches of the high, intermediate and low frequency modes. Their frequencies depend on both concentration and alloy composition. To analyses the surface polariton modes we carry out attenuated total reflection (ATR) measurements. In order to support our assignment the magnetic field profiles and surface polariton dispersion curves have been calculated.     

Investigation of blue InGaN light-emitting diodes with step-like quantum well

The concept of a step-like quantum well is proposed with the purpose to reduce the influence of electrostatic field resulting from the piezoelectric effect on the optical performance of blue InGaN light-emitting diodes. Particularly, the optical properties of the LED structures with the In_0.23Ga_0.77N single quantum well, In_0.20Ga_0.80N/In_0.26Ga_0.74N step-like quantum well, and In_0.26Ga_0.74N/In_0.20Ga_0.80N step-like quantum well are numerically investigated in detail. Simulation results show that the In_0.20Ga_0.80N/In_0.26Ga_0.74N step-like-quantum-well LED structure has the best optical performance in virtue of the improvement in spatial overlap of electrons and holes in the quantum well.     

Absence of nonradiative recombination centers in modulation-doped quantum wells revealed by two-wavelength excited photoluminescence

Absence of nonradiative recombination (NRR) centers inside GaAs wells and at GaAs/Al_0.2Ga_0.8As hetero-interfaces in a Si modulation-doped GaAs/Al_0.2Ga_0.8As multiple quantum well (MQW) structure became clear for the first time by an improved two-wavelength excited photoluminescence (PL). The NRR parameters of modulation and uniform-doped MQWs were determined self-consistently by combining the analysis of the PL intensity change due to the below-gap excitation with the internal quantum efficiency and the recombination lifetime. These results showed the superiority of modulation-doping scheme over that of uniform-doping for light emitting devices.     

A digital-alloy AlGaAs/GaAs distributed Bragg reflector for application to 1.3μm surface emitting laser diodes

A distributed Bragg reflector (DBR) utilizing the digital alloy Al_0.9Ga_0.1As was grown by using the molecular beam epitaxy (MBE) method for application in 1.3 μm optical communications and compared to the analog-alloy AlGaAs/GaAs DBR. The transmission electron microscopic (TEM) image showed a highly abrupt boundary of AlAs and GaAs, which supports the formation of a digital-alloy Al_0.9Ga_0.1As layer. The measurement showed that the digital-alloy AlGaAs/GaAs DBR had similar reflection spectra with enhanced uniform distribution over the whole substrate surface compared to the analog-alloy one. In the digital-alloy Al_0.9Ga_0.1As/GaAs DBR cavity, the reflection dip position was measured at around 1273 nm and the standard deviation of the distribution of the reflection dip was 1.31 nm in wavelength over 1/4 of a three-inch wafer.     

Influence of the cap layer thickness on photoluminescence properties in strained InGaAs/GaAs single quantum wells

The influence of the GaAs cap layer thickness on the luminescence properties in strained In_0.20Ga_0.80As/GaAs single quantum well (SQW) structures has been investigated using temperature-dependent photoluminescence (PL) spectroscopy. The luminescence peak is shifted to lower energy as the GaAs cap layer thickness decreases, which demonstrates the effect of the GaAs cap layer thickness on the strain of InGaAs/GaAs single quantum wells (SQW). We find the PL quenching mechanism is the thermal activation of electron hole pairs from the wells into the GaAs cap layer for the samples with thicker GaAs cap layer, while in sample with thinner GaAs cap layer exciton trapping on misfit dislocations is dominated.     

Infrared optical characterization of GaAsAlxGa1−xAs submicron heterostructures

Infrared optical characterization of submicron, multiple layers of Al_xGa_1?xAsGaAs on a GaAs substrate is reported, demonstrating that layer resolved phonon frequencies, electron concentrations and mobilities can be obtained.     

Competition in the carrier capture between InGaAs/AlGaAs quantum dots and deep point defects

The presence of an extrinsic photoluminescence (PL) band peaked at 1.356 eV at low temperature is observed, on a large number of self-assembled InAs and In_0.5Ga_0.5As quantum dot (QD) structures, when exciting just below the GaAs absorption edge. A detailed optical characterization allows us to attribute the 1.356 eV PL band to the radiative transition between the conduction band and the doubly ionized Cu _Ga acceptor in GaAs. A striking common feature is observed in all investigated samples, namely a resonant quenching of the QD-PL when exciting on the excited level of this deep defect. Moreover, the photoluminescence excitation (PLE) spectrum of the 1.356 eV emission turns out to be almost specular to the QD PLE. This correlation between the PL efficiency of the QDs and the Cu centers evidences a competition in the carrier capture arising from a resonant coupling between the excited level of the defect and the electronic states of the wetting layer on which the QDs nucleate. The estimated Cu concentration is compatible with a contamination during the epitaxial growth. Received 13 November 2001 / Received in final form 28 May 2002 Published online 19 July 2002     

Effect of interdiffusion on the band structure and absorption coefficient of a one-dimensional semiconductor superlattice

The effect of interdiffusion of Al and Ga atoms on the confining potential, band structure and absorption coefficient of electromagnetic radiation of a one-dimensional superlattice, composed of GaAs/Ga_1?x Al _x As quantum wells with the initially rectangular potential profile, is studied within the framework of the modified Wood-Saxon potential model. It is shown that the interdiffusion leads to the widening of the energy minibands and to the blueshift of the absorption spectrum observed in experiments.     

Enlarged wells as probes to study superlattices

Within GaAs/Ga_1?xAl_x As MBE grown superlattices (SL), a few GaAs wells have been purposely enlarged. Calculations show that enlarged wells introduce localized states in the SL band gap: the distance between such a localized state and the bottom of the SL conduction band (or the top of the SL valence band) depends on the SL period and the enlarged well size. Luminescence and photoluminescence excitation results obtained on samples with different periods and enlarged well sizes are in good agreement with calculations.Once characterized, enlarged wells serve as probes in the study of SL optical and electrical properties. As examples, they were used to observe one monolayer size fluctuations and vertical transport by luminescence in GaAs/Ga_1?xAl_xAs superlattices.     

Characterization of AlxGa1−xAs/GaAs heterostructures for single quantum wells grown by a solid arsenic MOCVD system

This work presents the results of the growth and characterization of Al_xGa_1−xAs/GaAs multilayer structures obtained in a metallic-arsenic-based-MOCVD system. The main goal is to explore the ability of the growth system to grow high quality multilayer structures like quantum wells. The use of metallic arsenic could introduce important differences in the growth process due to the absence of the hydride group V precursor (AsH₃), which manifests in the electrical and optical characteristics of both GaAs and Al_xGa_1−xAs layers. The characterization of these epilayers and structures was performed using low-temperature photoluminescence, Hall effect measurements, X-ray diffraction, Raman spectroscopy, secondary ion mass spectroscopy (SIMS) and Atomic Force Microscopy (AFM).     

Influence of Ga(Al)As substrates on surface morphology and critical thickness of InGaAs quantum dots

Influence of Ga(Al)As substrates on surface morphology of InGaAs quantum dots and critical thickness of In_0.5Ga_0.5As film grown by molecular beam epitaxy is investigated. The In_0.5Ga_0.5As quantum dots are grown on (001) surfaces of GaAs and Al_0.25Ga_0.75 A at 450 °C, scanning tunneling microscope images show that the size of quantum dots varied slightly for 10 ML of In_0.5Ga_0.5As grown on GaAs and Al_0.25Ga_0.75As surfaces. Reflection high energy electron diffraction (RHEED) is used to monitor the growth of 4 monolayers (ML) In_0.5Ga_0.5As on Al_0.25Ga_0.75As and GaAs surfaces during deposition. The critical thickness is theoretically calculated by adding energy caused by surface roughness and heat from substrate. The calculations show that the critical thickness of In_0.5Ga_0.5As grown on GaAs and Al_0.25Ga_0.75As are 3.2 ML and 3.8 ML, respectively. The theoretical calculation agrees with the experimental results.     

Optical properties of narrow high quality GaAs/AlGaAs quantum wells grown by MOVPE

We report optical characterization of high quality quantum well (QW) structures grown by metal-organic vapour-phase epitaxy (MOVPE). Thin QW layers of GaAs of thicknesses between 20 Å and 80 Å inserted between Al_0.36Ga_0.64 As confining layers as well as nominally 20 Å QW's in Al_xGa_1−xAs with varying x have been studied. Exciton confinement energies exceeding 250 meV and a FWHM of 6 meV for the thinnest QW have been observed. The photoluminescence (PL) data allows the observation of monolayer fluctuations in the QW widths and indicates an interface abruptness of about one atomic layer. Photoluminescence excitation spectroscopy allows electronic excited states to be seen.     

Effect of Al and Ga interdiffusion on the electronic states in GaAs/Ga<Subscript>1−x</Subscript>Al<Subscript>x</Subscript>As semiconductor superlattice

The effect of interdiffusion of Al and Ga atoms on the confining potential and band structure of a three-dimensional superlattice, composed of initially spherical GaAs/Ga_1?xAl_xAs quantum dots, is investigated in the framework of the modified Wood-Saxon potential model. It is shown that the interdiffusion leads to the disappearance of the quantum dots’ spherical symmetry and to the broadening of the superlattice energy minibands.     

The effects of hydrostatic pressure and intense laser field on the linear and nonlinear optical properties of a square quantum well

In this paper, the effects of hydrostatic pressure, temperature and intense laser field on the linear and nonlinear optical processes in the conduction band of a square quantum well are numerically investigated in the effective mass approximation. The analytical expressions of optical properties are obtained by using the compact density-matrix approach. The numerical results are presented for typical square GaAs/Al_xGa_1?xAs single quantum well system. The nonlinear optical absorption and refractive index changes depending on the hydrostatic pressure and intense laser field are investigated for two different temperature values. The results show that the intense laser field, the hydrostatic pressure and the temperature have a significant effect on the optical characteristics of these structures.     

Materials and device properties of pseudomorphic In x Ga1−x As/Al0.3Ga0.7As/GaAs high electron mobility transistors (0<x<0.5)

Modulation doped Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures for device application have been grown using molecular beam epitaxy. Initially the critical layer thickness for InAs mole fractions up to 0.5 was investigated. For InAs mole fractions up to 0.35 good agreement with theoretical considerations was observed. For higher InAs mole fractions disagreement occurred due to a strong decrease of the critical layer thickness. The carrier concentration for Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures with a constant In _x Ga_1–x As quantum well width was investigated as a function of InAs mole fraction. If the In _x Ga_1–x As quantum well width is grown at the critical layer thickness the maximum carrier concentration is obtained for an InAs mole fraction of 0.37. A considerable higher carrier concentration in comparison to single-sided -doped structures was obtained for the structures with -doping on both sides of the In _x Ga_1–x As quantum well. Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures with InAs mole fractions in the range 0–0.35 were fabricated for device application. For the presented field effect transistors best device performance was obtained for InAs mole fractions in the range 0.25–0.3. For the field effect transistors with an InAs mole fraction of 0.25 and a gate length of 0.15 m a f _T of 115 GHz was measured.Dedicated to H.-J. Queisser on the occasion of his 60th birthday