AlGaAs Samples Research Articles

Observation of the zero-magnetic-field exciton spin splitting in high quality bulk GaAs and AlGaAs

We present an experimental study of the zero-magnetic-field exciton spin splitting measured by time-resolved photoluminescence in high purity bulk GaAs and AlGaAs samples. The dynamics of the splitting differs from that observed in two dimensions. Initially, the splitting increases during 100–250 ps and then decays. This initial increase is attributed to the fast rise of the density of excitons formed from noncorrelated e-h pairs. The splitting dynamics is used to determine the exciton formation time, which is found to vary with excitation density from 70 to 360 ps.

Investigation of effective-medium approximation, alloy, average-composition, and graded-composition models for interface analysis by spectroscopic ellipsometry

We critically test the capabilities of the effective-medium approximation (EMA) and alloy models to describe multilayer samples with gradual interfaces by analyzing spectroscopic ellipsometric (SE) data of two AlGaAs samples grown expressly for this purpose. The dielectric functions ε of the interfaces are calculated in the EMA and alloy models, and the interfaces themselves simulated either as a single layer of Al0.5Ga0.5As or a stack of layers of AlxGa1−xAs with x increasing or decreasing between 0.1 and 0.9 in increments of 0.1. The EMA essentially fails completely for either interface representation. For the alloy model the stepwise-graded representation is significantly better, not only simulating the data more accurately but also yielding thicknesses in essential agreement with those obtained by cross-sectional transmission electron microscopy. The results highlight the types of errors that are encountered with the different models, and show that the analysis of SE data can provide information about these interfaces.

Reduction of band-gap energy in GaNAs and AlGaNAs synthesized by N+ implantation

We have studied the optical properties of nitrogen implanted GaAs and AlGaAs samples. The fundamental band-gap energy has been found to decrease with the increasing N+ implantation dose in a manner similar to that commonly observed in GaNAs and GaInNAs alloys grown by molecular beam epitaxy or metal organic chemical vapor deposition. Our results indicate that GaNxAs1−x and AlxGa1−xNyAs1−y alloys can be formed by implantation of nitrogen followed by appropriate postimplantation annealing treatments. As inferred from the magnitude of the band gap shift, the percentage of the implanted N atoms incorporated on the substitutional As sites is estimated to be around 12%.

Modeling of SIMS profiles at low energy: Application to algaas

This work proposes an original solution to model Secondary Ions Mass Spectrometry. This modeling allows the accurate determination of both the thickness and the composition of AlGaAs samples.

Optical Investigations on Microcavities: Mode Splitting by an Effective Anisotropy

Optical investigations at the stop-band of semiconductor microcavities are presented. The observed polarization dependent splitting of the resonator mode is due to an effective anisotropy. The measured spectral shift of the resonator mode as function of the angle of incidence is used to determine the refractive index of the cavity. The experimental results on AlGaAs samples are compared with calculations using the transfer-matrix method.

Effects of electron beam irradiation on transient photoluminescence measurements of GaAs and AlGaAs double heterostructures

We have performed transient photoluminescence measurements on Se doped AlGaAs and GaAs double heterostructure materials (similar to those found in advanced solar cells) as a function of radiation fluence from 1 MeV electrons for total fluences between 1/spl times/10/sup 13/ and 1/spl times/10/sup 15/ e/sup -//cm/sup 2/. We extract the effective minority carrier lifetime from the transient photoluminescence measurements and use these results to obtain lifetime damage coefficients. Carrier lifetimes in the un-irradiated samples range from a few nsec for heavily doped samples to almost 100 nsec for the more lightly doped samples. Bombardment with energetic electrons causes the carrier lifetime to decrease to values in the range of 100-200 psec. Lifetime damage coefficients are obtained for Al concentrations of 0, 10, and 20% and for Se donor concentrations between 3/spl times/10/sup 16/ and 1.5/spl times/10/sup 18/ cm/sup -3/. To our knowledge, these are the first direct measurements of lifetime damage coefficients in GaAs and AlGaAs double heterostructures. The values extracted for lifetime damage coefficients suggest: (1) greater damage than the values obtained from previous measurements using indirect techniques; (2) that the AlGaAs samples are more susceptible to reduction of carrier lifetime than are the GaAs samples; and (3) that the damage depends upon dopant concentration. We discuss the implications of these measurements for solar cell degradation in the space radiation environment.

Spectroscopic ellipsometric monitoring of electron cyclotron resonance plasma etching of GaAs and AlGaAs

In situ real time spectroscopic ellipsometry measurements were made during electron cyclotron resonance plasma etching of radio frequency biased GaAs and AlGaAs samples. Gas mixtures used were CH4/H2/Ar, pure H2, and pure Ar. Ellipsometry provided information about damage to the surface region and AlGaAs epilayer thickness. For the methane mixture GaAs etch, damage appeared in the form of redshifted and broadened E1 and E1+Δ1 critical point features in a surface layer several tens of nm thick. The damage layer began forming within a few seconds after the start of etching, and stabilized within 1 min. Hydrogen etching caused a thicker damage layer with greater redshifting and broadening, while argon caused relatively little damage. Possible mechanisms for the redshifting are discussed. During etching of an AlGaAs/GaAs heterostructure with the methane mixture, the same redshifting and broadening effects were seen in the AlGaAs critical point structure. The AlGaAs thickness was determined from the real time data, from which an etch rate of about 20 nm/min was derived.

Electron transport in shallow heterostructures with AlGaAs and AlAs barriers

Two series of shallow GaAs heterostructures, with AlGaAs and AlAs barriers respectively and both delta -doped with around 4*1016 m-2 Si donors, have been studied using low-temperature magnetotransport techniques. The electrons in these structures were confined against interfaces 28 nm from the surface. The AlGaAs barrier samples depleted at a uniform rate with bias under a Schottky gate, but the carrier mobility was considerably greater at all biases than predicted assuming randomly positioned donors. The depletion and mobility data for the AlAs barrier samples could only be explained by postulating the existence of a pool of electrons around the doping plane, which screened the donors to produce high mobilities in ungated samples but which could be removed by the application of gate bias. Bias cooling experiments on the AlGaAs samples showed that a proportion of the donor centres were occupied when samples with as few as 4*1016 m-2 donors were cooled without bias. The mobility data from such samples are discussed assuming correlations between the positions of these occupied donors.

Minority carrier recombination in post-growth hydrogenated AlGaAs

The authors have studied AlGaAs grown by molecular beam epitaxy (MBE) before and after post-growth hydrogen plasma treatment by means of temperature-dependent photoluminescence (PL), capacitance-voltage (C/V), and electron-beam-induced current (EBIC) analysis. The excitonic and extrinsic luminescence features of n-type AlGaAs samples subjected to a hydrogen plasma reveal the neutralization effect of vacancies and shallow acceptors. The hydrogen incorporation provides an increase of the non-radiative lifetime of the minority carriers. This behavior was checked by investigations of the minority carrier diffusion and of the quantum yield, which increase due to the hydrogen incorporation. The increase of the non-radiative lifetime is probably caused by the passivation of non-radiative recombination channels involving vacancies.

Te-related DX centre of GaAs and AlGaAs

The pressure dependences of the electrical conductivity and Hall coefficient have been studied for GaAs and AlxGa1-xAs (x=0.20 and 0.25) heavily doped with Te. The results obtained show that for a GaAs crystal the resonant donor level related to Te is located approximately 0.45 eV above the conduction band minimum. The effect of persistent photoconductivity observed at high pressures proves the DX-like character of this donor state. Comparison of the data on the energetic position of the DX level illustrates its strong chemical energetic dependence (for Si, Sn and S dopants an EDX value of around 0.3 eV has been reported previously). It appears that Te represents an impurity which is very suitable for testing the microscopic models of the DX centre. Studies of processes of the thermal recovery after high pressure freeze-out of electrons on the metastable states of the DX centres clearly demonstrate the multicomponent structure of the DX state in AlGaAs samples.

Hall measurements under weak persistent photoexcitation in Si-doped AlxGa1-xAs

The low-temperature Hall mobility of photoexcited electrons has been measured in Si-doped MBE AlGaAs samples. Different fractions of occupied DX centres were obtained by selecting different free-electron densities: possible systematic errors in Hall measurements due to the method of photoexcitation are demonstrated and critically analysed. Using suitable values for the acceptor density and the alloy scattering potential a fair fitting of the experimental data was achieved within both negative-U and positive-U models for the DX centre. Discrepancies between calculated and experimental mobility versus temperature curves are observed, which are more evident the lower the free-electron densities. They are tentatively explained as being due to electrons in an impurity band originated by the shallow effective-mass state related to the Gamma minimum of the conduction band.

Origin of the near infrared luminescence in n-type AlxGa1-xAs alloys

A detailed analysis of the near-infrared luminescence has been performed in undoped and n-doped (Si, Te, Sn) AlGaAs samples grown by MBE and MOVPE techniques. A wide range of Al compositions (26-74%) and donor concentrations (1016-1018 cm-3) was examined. In the spectral region from 0.9 to 2.0 mu m in all samples there is a photoluminescence (PL) peak at 1.1 mu m (IR1). MOVPE samples show an additional PL peak at 1.5 mu m (IR2), which is independent of the Al mole fraction, the doping concentration and the donor species IR2 is not present in MBE-grown samples, where a third peak IR3 appears. None of the IR peaks is related to DX deep donor centres, since the amplitudes of all IR peaks are independent of the donor doping level. By optically detected ESR of Si-doped samples it was shown that the IR2 band is due to arsenic antisite defects and not to DX centres as previously assumed. Also, deep level transient spectroscopy measurements were performed on Si-doped samples.

Magnetic freeze-out study of the DX-linked shallow level in Si-doped AlxGa1-xAs under hydrostatic pressure

Magnetic freeze-out experiments under hydrostatic pressure have been performed in the temperature range 4.2-77 K on direct-band-gap Si-doped AlGaAs samples. Successive illuminations have been used to monitor the concentrations of the metastable shallow states arising from the DX centres. With increasing concentrations of those states, evidence is given for an insulator-metal transition confirming the Gamma character of these shallow impurity states. Moreover a shallow-deep transition is shown to hold at X=0.32 as a result of the appearance in the gap at x=0.30 of a localized level also originating from the DX centre but with no potential barrier for carrier capture.

Analysis of electron mobility versus temperature after photoexcitation in Si-doped Al xGa 1− xAs

The low-temperature Hall mobility of photoexcited electrons has been measured in Si-doped MBE AlGaAs samples and compared with calculated data using the background acceptor density and the alloy scattering potential as free parameters. The possibility of discriminating between negative- or positive- U electron correlation energy for the DX centre has been investigated through a careful analysis of mobility versus temperature curves relating to different photoexcited electron densities. A crucial role of the acceptor density to explain the experimental data within the positive- U model has been evidenced.

Characterization of GaAs/AlxGa1−xAs selective reactive ion etching in SiCl4/SiF4 plasmas

Selective reactive ion etching of GaAs over AlxGa1−xAs using SiF4/SiCl4 gas mixtures is studied. Etch rates of GaAs and AlxGa1−xAs are measured versus pressure, etch gas ratio, and plasma self-bias voltage. A selectivity ratio of 500:1 has been obtained for GaAs over Al0.35Ga0.65As at a gas composition ratio of SiCl4/SiF4=0.25, a pressure of 60 mTorr, and a self-bias voltage of −60 V. The etch stop mechanism is studied using Auger electron spectroscopy. Auger surface scan spectra indicate a high concentration of fluorine at the surface of etched AlGaAs samples suggesting the formation of an AlF3 etch stop layer.1 It is also shown that the AlF3 layer is easily removed with a very short dip in HCl/H2O or NH4OH/H2O. Hall effect measurements at 300 and 77 K are made on etched GaAs/Al0.3Ga0.7As modulation doped heterostructures to determine the effect of low-power etching on two-dimensional electron gas (2-DEG) sheet concentrations and carrier mobility. Slight reduction in both 2-DEG sheet carrier concentration and mobility are observed after the RIE process. This may be an indication of ion-beam induced damage to the AlGaAs/GaAs heterostructure. Finally, the effects of extended etching on the dc and microwave device characteristics of short gate length GaAs/AlGaAs/InGaAs MODFETs are reported.

Metalorganic Chemical Vapor Deposition of High Quality GaAs and AlGaAs Using Tertiarybutylarsine

AbstractThis paper discusses recent improvements achieved in the growth of epitaxial layers of GaAs and AlGaAs using the liquid arsine substitute tertiarybutylarsine (TBA) and metal alkyls. The high purity TBA now available yields undoped GaAs with residual donor/acceptor concentrations in the low 1014 cm−3 range. Under optimized growth conditiorp the layers are either n-type and have 77*K mobilities up to 85,000 cm2 //Vs or they are compensated or p-type. For aluminum gallium arsenide, layers grown with TBA have properties similar to arsine-grown material as demonstrated by low temperature photoluminescence (PL). The PL efficiencies and line widths of the TBA-grown AlGaAs samples are comparable to those prepared with arsine.