Doped AlGaAs Layer Research Articles

Identification of photoluminescence bands in AlGaAs/InGaAs/GaAs PHEMT heterostructures with donor-acceptor-doped barriers

The photoluminescence of AlGaAs/InGaAs/GaAs pseudomorphic high-electron mobility transistor heterostructures with donor-acceptor-doped AlGaAs barriers is studied. It is found that the introduction of additional p +-doped AlGaAs layers into the design brings about the appearance of new bands in the photoluminescence spectra. These bands are identified as resulting from transitions (i) in donor-acceptor pairs in doped AlGaAs layers and (ii) between the conduction subband and acceptor levels in the undoped InGaAs quantum well.

Effects of bias cooling on charge states in heterostructures embedding self-assembled quantum dots

Carrier transfer behavior has been investigated in selectively doped InGaAs/AlGaAs heterojunctions, in which a layer of self-assembled InAs quantum dots is embedded in close vicinity to a two-dimensional electron gas (2DEG). We applied the bias-cooling technique to derive information on the electron exchange properties from the bias dependence of the capacitance-voltage (CV) spectroscopy. Noise is observed in the CV results at large reverse biases after sufficiently negative bias cooling. The noise is temperature sensitive and can be eliminated when raising the temperature to 130 K. In combination with the experimental results in the control sample, we find that the noise induced by the bias cooling is most likely associated with the hot-electron trapping and detrapping in DX centers in the selectively doped AlGaAs layer. The effect of light illumination on the CV results supports our speculation on the noise origin. Furthermore, the variation in the 2DEG carrier concentration and mobility with bias-cooling processes has been discussed, which provides a direct insight into the vertical charge-transfer mechanism among the quantum dot related electronic states, DX centers, and 2DEG channel.

Steady states of a microwave-irradiated modulation-doped heterostructure under perpendicular magnetic field

We present a theoretical model and study dynamics of the modulation-doped GaAs/AlxGa1−xAs heterostructure under a perpendicular magnetic field and microwave radiation. We propose a mechanism, which is based on the nonlinear dynamics of real-space electron transfer and delayed dielectric relaxation of the interface potential barrier resulting from the space charge in the doped AlGaAs layer. Static analysis specifies the negative differential conductance region when the dc bias voltage varies. The self-sustained oscillations and bistability between oscillationary and stationary states are predicted. Varying with the microwave irradiation amplitude and the perpendicular magnetic field, the routes from period-doubling to chaos, quasiperiodicity, and frequency-locking are found. In addition varying with the microwave irradiation frequency can lead to time-independent homogeneous steady states spatially and result in a longitudinal resistance oscillation with period tuned by the ratio of microwave radiation frequency ω to the cyclotron frequency ωc and local minima at ω/ωc=integer+1/4.

Electron–impurity scattering in free-standing quantum wires: Effect of dielectric confinement

We calculate the impurity-scattering limited mobility of the one-dimensional electron gas in a rectangular GaAs quantum wire confined in the vertical (growth) direction by n-modulation doped AlGaAs layers and free-standing along the transverse direction. The Fourier-transformed scattering potential of the ionized impurity is obtained analytically by solving the Poisson equation with z-dependent electrostatic permittivity. An abrupt permittivity change at the GaAs(AlGaAs)/air interfaces gives rise to the image charge effect which strongly modifies the unperturbed scattering potential. We show that the “image impurity” scattering tends to drastically reduce the electron mobility for sufficiently small (∼10 nm) transverse wire widths.

Impurity-Scattering Limited Electron Mobility in Free Standing Quantum Wires: Image Charge Effect

We calculate the impurity-scattering limited mobility of the one-dimensional electron gas in a rectangular GaAs quantum wire confined in the vertical (growth) direction by n-modulation doped AlGaAs layers and free standing along the transverse direction. The scattering potential of the ionized is obtained by solving the Poisson equation with z-dependent electrostatic permittivity in order to take into account the charge effect due to the abrupt permittivity change at the GaAs/air interfaces. We show that the image impurity scattering tends to drastically reduce the electron mobility for sufficiently small ( 10 nm) transverse wire widths.

Self organized growth of doped vertical quantum wells for normal incidence intersubband transitions

The self-organized growth of N-doped vertical AlGaAs quantum wells by metalorganic vapor phase epitaxy of a single doped AlGaAs layer on a submicron grating is described. Intersubband absorption at normal incidence is demonstrated in those vertical quantum wells. This opens new possibilities for infrared quantum well devices using intersubband transitions, including normal incidence infrared modulators.

Deep level transient spectroscopy assessment of silicon contamination in AlGaAs layers grown by metalorganic vapor phase epitaxy

A systematic silicon contamination has been detected by deep level transient spectroscopy in undoped and n-type doped (Te, Se, Sn) AlGaAs layers, grown in two different metalorganic vapor phase epitaxy reactors. DX center generation by substitutional donors, with very specific capture and emission thermal barriers (fingerprints), is the key to unambiguously identifying their presence, with detection limits well below the standard secondary ion mass spectroscopy capability. We comment on the potential sources of Si contamination (most common in this epitaxial technique), and on the relevance of such contamination to interpreting correctly experimental data related to the microscopic structure of DX centers.

Monte Carlo modeling of high-field transport in III-V heterostructures

A Monte Carlo model of parallel high-field transport in III-V heterostructures is presented. Special features of the model are the following: only two-dimensional electron states are considered, the possible existence of secondary wells inside the barriers is accounted for, and nonparabolicity effect and quantization of satellite valleys are included. The wave functions and eigenenergies are calculated by self-consistent resolution of Poisson and Schrödinger equations. The effect of nonparabolicity on dispersion relations is determined at first order by a perturbation method. First, the simple case of an infinite GaAs square well is investigated as a test for the model, then more realistic heterostructures are considered. A study of a modulation-doped pseudomorphic AlxGa1−xAs/In0.15Ga0.85As structure shows that the electric field induces a significant repopulation of the doped AlGaAs layer. When x=0.32, this real-space transfer is strongly correlated with the intervalley transitions toward X valley states. For In0.52Al0.48As/In0.53Ga0.47As the situation is quite different and a good confinement in the InGaAs well is preserved even at high fields owing to the large band offset in the L valley. This study demonstrates a complicated influence of band structure on electron transport in heterostructures.

Quasi-one-dimensional electron gas and its magnetic depopulation in a quantum wire prepared by overgrowth on a cleaved edge of AlGaAs/GaAs multiple quantum wells

We have succeeded in the formation of a quasi-one-dimensional electron gas on an edge surface of an AlGaAs/GaAs multiple quantum well (MQW) structure with well width of 100 nm. The sample was prepared by the exposure of a cleaved surface of an MQW substrate and subsequent overgrowth of a doped AlGaAs layer on the edge. The existence of electrons on the edge surface and their one dimensionality are, respectively, evidenced by the angular dependence of magnetoresistance and the magnetic depopulation effect.

On factors influencing Si segregation and its effect on the two-dimensional electron gas mobility in inverted GaAs/n-AlGaAs heterostructures

The electrical properties of inverted GaAs/n-AlGaAs heterostructures grown at varying As-to-Ga (group V:III) flux ratios and Si doping concentrations in the AlGaAs layer were investigated. In these heterostructures, it was found that the two-dimensional electron gas (2DEG) mobility, as well as being affected by the Si doping level, is also strongly influenced by the group V:III flux ratio at which the structures are grown. The results indicate a significant contribution from a probable diffused profile of doped Si impurities in the AlGaAs spacer and GaAs channel layers resulting from Si surface segregation in the doped AlGaAs layer. This effect of Si segregation, which dominates at high Si doping levels, is greatly enhanced at lower group V:III flux ratios. At higher group V:III flux ratios of about 5.66 and low Si doping concentration ( N rmd = 3 × 10 17 cm -3 ) , a high 2DEG mobility of 5 × 10 4 cm 2 V -1 s -1 was achieved at 77 K, with an interface carrier density N rms ≈ 1.7 × 10 11 cm -2 .

Molecular-beam epitaxial growth and characterization of inverted, pulse-doped AlGaAs/InGaAs/GaAs transistor structures

Inverted, pulse-doped AlGaAs/InGaAs pseudomorphic high electron mobility transistor structures were grown by molecular-beam epitaxy. Growth conditions were optimized to improve the quality of the selectively doped AlGaAs layer and to minimize dopant diffusion into the InGaAs channel. The sheet densities and mobilities of the inverted structure were found to be essentially equivalent to those obtained with the normal structure. Shubnikov–de Haas measurements exhibited strong oscillations in the magnetoresistance and plateaus in the Hall resistance. Four optical transitions from the lowest bound electron and hole quantum well states were observed in room-temperature photoluminescence spectra.

Surface acoustic wave attenuation by localized electrons in a 2DEG at a GaAs/AlGaAs heterojunction

The surface acoustic wave (SAW) transmission has been measured at 4.2 K, as a function of magnetic field up to 5 T, for a GaAs wafer on which a heterojunction has been grown by MBE. Frequencies up to 1920 MHz were generated by interdigital SAW transducers deposited directly on the GaAs surface. At all frequencies, oscillations in the SAW attenuation, analogous to the Shubnikov-de Haas oscillations in sigma xx, can be observed due to piezoelectric coupling to the non-localized electrons of the two-dimensional electron gas (2DEG) of the heterojunction. The authors find, at frequencies of about 900 MHz and above, an additional attenuation at all magnetic fields, which increases with magnetic field. At half-integer filling factor, the increase in attenuation is initially linear with increase of field and then, at higher fields, the attenuation increases as the square root of the field. They interpret this as due to deformation potential coupling to the localized electrons of the 2DEG. A small SAW attenuation is observed at zero magnetic field which may be due to a very low density of three-dimensional electrons in the GaAs buffer layer or remaining in the doped AlGaAs layer.

Pressure experiments and self-consistent modelling of the transport properties in delta -doped AlGaAs layers

Magnetotransport experiments on delta -doped AlxGa1-xAs (x<or=0.22) layers are presented. In particular, the pressure dependence of the bi-dimensional carrier concentration is investigated. The DX centre is found to limit the free carrier density in the dark. Moreover, persistent photoconductivity is used to recover the donor density at 4.2 K. The objective is to study the behaviour of the persistent photoconductivity as a function of pressure and to analyse critically the possible occupancy of the L valley at high pressure. The structures are modelled by assuming a uniform doping concentration ND in a plane of a certain width a. Both parameters are determined by a fitting procedure. The Schrodinger and Poisson equations are solved self-consistently. The conclusion drawn is that in three samples the limited free-carrier concentration under pressure is a consequence of the neutralization of ionized donors and not of the L-valley occupancy.

Novel mechanism of a real-space transfer oscillator

A new mechanism for self-generated current oscillations in modulation-doped semiconductor heterostructures under parallel conduction is proposed. It is based upon the nonlinear dynamics of real-space electron transfer and delayed dielectric relaxation of the interface potential barrier resulting from the space charge in the doped AlGaAs layer. From our simulations we predict current oscillations in the 20–80 GHz range under dc bias.

Small signal nonquasistatic models for GaAs field effect transistors for implementation in SPICE. Part 1: Modulation-doped field effect transistors (MODFETs)

A general small signal nonquasistatic model for modulation doped field effect transistors (MODFETs) is presented. The model is valid for devices operating in both linear and saturation regions. The two dimensional electron gas (2DEG) current is modelled. The current flowing in the parasitic doped AlGaAs layer is considered elsewhere. In both cases, admittance parameters (Y-parameters) are derived and used to build SPICE-like equivalent circuits. The proposed model can, therefore, be easily implemented in SPICE for microwave and high frequency analogue circuit analysis. The results show that, for a 1 μm gate length MODFET, the error is less than 5% at 86 GHz.

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

Abstract 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 Schrodinger 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 (Lg = 20 μm) and short (Lg = 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.

Light scattering determination of subband structure and population of modulation-doped multiple quantum wells

We show that electronic Raman scattering measurements of the plasmon dispersion in combination with calculations of the random phase approximation dielectric response and self-consistent electronic subband calculations can determine the subband structure and populations of modulation-doped GaAs/AlxGa1−xAs multiple quantum wells with multiple subband occupancy. Thus we present a contactless optical alternative to measurements of Shubnikov–de Haas oscillations for the determination of carrier concentrations. The same technique can also be applied for the characterization of parallel conduction from doped AlGaAs layers in modulation-doped heterostructures.

Inhomogeneous broadening and the breakdown of the quantum Hall effect in GaAs/AlGaAs

The critical current for the breakdown of the quantum Hall effect at fixed filling factors has been observed to drop by more than a factor of ten when a GaAs/AlGaAs heterostructure is subjected to thermal cycling after exposure to a light pulse. Concurrent changes in the transport parameters suggest that the metastable, charged deep defect states in the doped AlGaAs layer, which are also responsible for the persistent photoconductivity effect, tend to increase inhomogeneities of the carrier density with thermal cycling and reduce the effective width for the current path in the quantum Hall regime.

One-dimensional subbands and mobility modulation in GaAs/AlGaAs quantum wires

Devices consisting of 100 parallel, modulation-doped GaAs/AlGaAs wires, each about 40 nm wide, have been fabricated and tested at 4.2 and 77 K. The wires were created by ion milling a shallow grating into the doped AlGaAs layer. The mask for etching the grating was produced by x-ray nanolithography, and lift-off of Ti/Au. The grating period was 200 nm and the linewidth about 85 nm. Backgate bias or illumination was used to modulate the charge density in the wires. Conductance measurements at 4.2 K provide clear evidence of quasi-one-dimensional density of states. A corresponding modulation of the electron mobility, above and below values in the two-dimensional system, was observed.

Modeling of MODFETs

Accurate modeling of MODFETs and of certain novel structures recently proposed requires that a number of physical phenomena occurring in these devices be considered. Among these, some electron dynamic properties of the two-dimensional gas, the influence of deep levels of the doped AlGaAs layers, and the influence of the source parasitic access impedance are reviewed. The presently available models can roughly be sorted into three classes: the particle or Monte Carlo models, the two-dimensional solving methods of semiconductor equations, and the simpler one-dimensional or analytical models. After a brief review of the physical bases on which the models rely, their main capabilities and ranges of applicability are discussed. Some conclusions are drawn as to the effort which must be developed in the near future to improve MODFET modeling. It is recommended that simulations of devices such as SISFETs, multichannel structures, and pseudomorphic AlGaAs/InGaAs transistors be undertaken.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>