Type GaAs Research Articles

Polarization charges at spontaneously ordered (In, Ga)P/GaAs interfaces

The depth-resolved electrical characteristics of n- and p-type GaAs/(In, Ga)P/GaAs heterojunctions are examined by capacitance–voltage measurements. Different epitaxial growth conditions are chosen to produce heterointerfaces with (In, Ga)P layers of various degrees of order. Irrespective of the conduction type of the heterojunction studied, we find positive [negative] sheet charges at the (In, Ga)P-on-GaAs [GaAs-on-(In, Ga)P] interfaces. The density of both interfacial charges increases with increasing degree of (In, Ga)P order. The experimental results can be completely explained by taking into account the spontaneous polarization of ordered (In, Ga)P. The polarization difference between ordered (In, Ga)P and GaAs (no polarization) results in opposite sheet charges at the two complementary interfaces with GaAs, in accordance with theoretical predictions.

Dose dependence of proton-isolated n -type GaAs layers implanted at room temperature and 200°C

Hydrogen has been implanted into n-type GaAs layers to obtain inter-device isolation. The effects of variable doses and target temperature during implantation on the degree of isolation have been investigated. Possible identification of antisite defects responsible for isolation and their sensitivity to enhanced dynamic annealing is discussed. The role of threshold doses for an effective isolation scheme is investigated. It is found that hot implants provide better optimisation of the isolation process.

Hopping conductivity in heavily dopedn-type GaAs layers in the quantum Hall effect regime

We investigate the magnetoresistance of epitaxially grown, heavily doped n-type GaAs layers with thickness (40-50 nm) larger than the electronic mean free path (23 nm). The temperature dependence of the dissipative resistance R_{xx} in the quantum Hall effect regime can be well described by a hopping law (R_{xx} \propto exp{-(T_0/T)^p}) with p=0.6. We discuss this result in terms of variable range hopping in a Coulomb gap together with a dependence of the electron localization length on the energy in the gap. The value of the exponent p>0.5 shows that electron-electron interactions have to be taken into account in order to explain the occurrence of the quantum Hall effect in these samples, which have a three-dimensional single electron density of states.

Defect engineering in annealed n-type GaAs epilayers using SiO2/Si3N4 stacking layers

We have used SiO2/Si3N4 stacking layers to control the creation of defects in rapid thermally annealed epitaxial GaAs layers. Annealing at 900 °C introduces three electron traps S1 (Ec−0.23 eV), S2 (Ec−0.53 eV), and S4 (Ec−0.74 eV) in SiO2/n-GaAs. The concentrations of S1 and S4 decreased by factors of ∼28 and ∼19, respectively, in Si3N4/SiO2/n-GaAs. The overlap of a hole trap with the S2 peak in Si3N4/SiO2/n-GaAs results in an apparent decrease in the concentration of S2 by over two orders of magnitude. The lower concentration of defects in the region probed by deep level transient spectroscopy is explained by the tensile stress which the Si3N4 layer imposes on the structure during annealing. In addition to S1 and S4, hole traps H1 (Ev+0.28 eV) and H2 (Ev+0.42 eV) are observed in Si3N4/n-GaAs and SiO2/Si3N4/n-GaAs, respectively. The concentration of defects is larger by ∼1.5 times in the latter structure. SiO2/Si3N4 stacking layers can, therefore, be used to achieve spatially selective modification of GaAs-based structures using defect engineering.

Full band calculation of doping-induced band-gap narrowing inp-type GaAs

We have calculated the energy shift of the fundamental band gap as a function of dopant concentration in p-type GaAs, using a perturbation theory based on the zero-temperature Green's function formalism within the random-phase approximation. The electronic structure of the intrinsic crystal was obtained from a relativistic, full-potential band-structure calculation, with the exchange potential of Engel and Vosko [Phys. Rev. B 47, 13 164 (1993)]. We have found that this potential gives accurate band curvatures at the band edges. The nonparabolicity of the uppermost valence bands has a reducing effect on the energy shifts of both the conduction and valence bands for high acceptor concentrations. The resulting narrowing of the band gap is in good agreement with photoluminescence measurements. Thus, it is crucial to take into account the full band structure to calculate the band-gap narrowing accurately for high acceptor concentrations.

Behavior of p-Type GaAs in an Aerated Boric Acid Solution at the Open-Circuit Potential. Influence of the Presence of Co(II) Ions

The electroless dissolution of p-GaAs in aerated 0.5 M boric acid solutions, in the absence and in the presence of Co(II) ions in the solution, is reported. At the pH of these solutions, the anodic dissolution of GaAs occurs in oxide-forming conditions. The oxide film formed during GaAs dissolution is composed of Ga2O3. The evolutions of the morphology and of the thickness of the oxide layer are followed using scanning electron microscopy (SEM), energy-dispersive X-ray analyses (EDX), and X-ray photoelectron spectroscopy (XPS). All of the results show that the oxidation rate of p-GaAs in an aerated boric acid solution is enhanced in the presence of Co(II) in the solution; the increase is larger for higher Co(II) concentration. This leads to an increase of the oxide layer thickness. From the position of the redox potentials of the various species present in the electrolyte, with respect to the GaAs valence bandedge, it appears that dioxygen is responsible for the GaAs anodic dissolution. At the GaAs/solution interface, O2 reduction occurs, leading to GaAs oxidation. The mechanism of O2 reduction at the GaAs surface is discussed regarding the energetic situation at the interface. The influence of the Co(II) on the kinetics of O2 reduction is discussed, taking into account the results of the literature.

Electronic and isochronal annealing properties of electron traps in rapid thermally annealed SiO2-capped n-type GaAs epitaxial layers

We have recently shown [P. N. K. Deenapanray et al., Appl. Phys. Lett. 77, 626 (2000)] that four electron traps S1(Ec−0.23 eV), S2(Ec−0.46 eV), S3(Ec−0.72 eV), and S4(Ec−0.74 eV) are introduced in rapid thermally-annealed (RTA) SiO2-capped n-type GaAs epitaxial layers. In the present study, we have used deep level transient spectroscopy to investigate the electronic and annealing properties of these deep levels. The electron emission kinetics of S1 is enhanced by an electric field, and the activation energy of S1 decreases linearly from ∼233 to ∼199 meV when the field is increased from 7.5×104 to 13.4×104 V cm−1. The intensities of S1, S2, and S4 show Arrhenius-like dependencies on the RTA temperature, which relate to the outdiffusion of Ga atoms into the SiO2 layer. The intensity of S2(VGa–SiGa) also increases exponentially with the square of the annealing time for RTA at 800 °C. Isochronal annealing experiments show that S1 and S2 are thermally stable below 500 and 400 °C, respectively. S4, which is a member of the EL2 family, is stable up to 600 °C. Secondary defects are introduced during isochronal annealing above 400 °C, and some of these defects are thermally stable at 600 °C.

Properties of metastable hydrogen-related defects in n-type GaAs studied by isothermal deep-level transient spectroscopy

Metastable hydrogen-related defects (M3/M4) in n-GaAs were studied in detail by using isothermal deep-level transient spectroscopy. In order to clarify the electric-field dependence of the electron-emission process, the double-correlation technique was applied to both M3 and M4 defects. It was clearly shown that with increasing electric field, the M4 level observed around 140 K split into two discrete components labeled M4(1) and M4(2). The result supports the recent finding that the M4 defect consists of two different configurations. On the other hand, the M3 defect gave a single peak in the double-correlation spectra around 230 K. The electric field dependence of the emission rates suggests that the M3 and M4(1) defects have a donor-like nature, while the M4(2) defect is acceptor-like. It was speculated that the metastable defect coupling with the M3 level is only M4(2) out of two components of the M4 defect.

Self-biased boron-10 coated high-purity epitaxial GaAs thermal neutron detectors

Semiconductor thermal neutron detection devices based on /sup 10/B-coated high-purity GaAs films were investigated. The fundamental device consisted of high-purity /spl nu/-type epitaxial GaAs films grown onto n-type GaAs substrates. Two blocking contact adaptations were applied to the high-purity /spl nu/-type GaAs regions: 2000 /spl Aring/ thick p+GaAs blocking contacts and 200 /spl Aring/ thick Schottky blocking contacts. The /spl nu/-type GaAs active layers ranged between 1 micron and 5 microns in thickness. The device sensitive areas were 3 mm/spl times/3 mm, each of which was coated with a 1.5 mm diameter film of 98% enriched high-purity /sup 10/B. The built-in potential of the blocking contact interface was sufficient to operate the devices, and no external voltage bias was necessary to operate the detectors. Preliminary calculations on intrinsic detection efficiency indicate values between 1.6% and 2.6%.

Effect of strain on the chemical bonds in InAs nanocrystals self-organized on GaAs and Se-terminated GaAs surfaces

We employed surface sensitive photoelectron spectroscopy (PES) using synchrotron radiation to investigate the strain of both deposited InAs and GaAs substrates with and without Se-termination. The two distinct chemical components comprising GaAs bonding and InAs bonding states in the As 3d spectrum are clearly observed for the first time, which indicates that the strain in both the deposited InAs and GaAs substrates can be separately evaluated using the core-level energy difference between the respective core-level chemical components. This difference between the chemical components of Ga 3d and As 3d levels for both two kinds of samples is found to be independent of InAs deposition time, and to be almost the same as that of a bulk GaAs, implying that no strain is generated in both two type GaAs substrates. On the other hand, in the case of the deposited InAs, this value increases with an increase in the layer thickness of InAs for the InAs/GaAs system and approaches to a bulk InAs value, whereas, there exits only a slight change for the InAs/Se/GaAs system at the very early stages of InAs growth and is almost the same as a bulk InAs value. These results suggest that the driving force for the formation of InAs nanocrystals in the InAs/GaAs system is the elastic strain, whereas, in the InAs/Se/GaAs system, passivation-induced self-organizing mechanism is crucial.

Spin-dependent electron transport in NiFe/GaAs Schottky barrier structures

Photoexcitation at the Schottky barrier formed between 5 nm thick Ni80Fe20 films and both n+- and p−-type GaAs(100) substrates with doping density in the range 1023⩽n(p)⩽1025 m−3 was investigated using circularly polarized laser light. A helicity-dependent photocurrent dependent upon the magnetization configuration of the film and the Schottky barrier height was detected. The results provide evidence of spin-dependent electron transport through the NiFe/GaAs interface and show that the Schottky barrier height controls the spin-dependent current passing from the semiconductor to the ferromagnet.

Comparison of the annealing characteristics of resistivity and vacancy defects for implant isolated n-type GaAs

Gallium arsenide layers, Si-doped at concentrations of 2×1019, 1×1019, and 5×1018 cm−3, grown on SI substrates were implanted using multiple-energy regimes, with O+, He+, and H+, respectively, to produce resistive structures. Sample resistivity was measured following annealing in the temperature range 400–800 °C. Maximum resistivity values were achieved after annealing at 600 °C for the O+ and He+ and 500 °C for the H+ implanted sample. Equivalently implanted and annealed semi-insulating GaAs samples were analyzed using positron annihilation spectroscopy in the gamma-ray Doppler-broadening mode, a technique which is predominantly sensitive to negatively charged, or neutral, vacancy-type defects. The annealing behavior of the resistivity is in good agreement with previous reports. Vacancy defects to which the positron is sensitive are found to be removed from all semi-insulating samples at a temperature which is 100 °C below that at which maximum resistivity is achieved. Therefore, such vacancy types can be eliminated as the defect responsible for optimum electrical isolation of GaAs following implantation, and the source of vacancies necessary to annihilate such defects.

Noise and electron diffusion in dopedn-type GaAs at heating electric fields

Experimental results on microwave noise in $n$-type GaAs $(n=3\ifmmode\times\else\texttimes\fi{}{10}^{17}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3},$ 80 K) at electric fields below a few hundred $\mathrm{V}/\mathrm{c}\mathrm{m}$ are interpreted combining the analytic approach and Monte Carlo simulation. The analytic approach is based on applicability, due to high frequency of interelectron collisions, of the electron temperature approximation. Monte Carlo simulation shows a rather low sensitivity of the conductance to the electron heating, accompanied by small deviations from the Ohm law, these circumstances favoring a nearly isotropic noise temperature and an approximate validity of the Price fluctuation-diffusion relation. The degree of violation of this relation caused by the interelectron-collision-born correlation is found to depend on the applied electric field and the donor compensation by acceptors, remaining below 0.01 in typical partially compensated $n$-type GaAs channels. This enables one to obtain the field-dependent electron diffusion coefficients from calculated and/or measured noise and current characteristics in a wide range of electric fields including those where the interelectron collisions are essential.

Ballistic transport in p-type GaAs

The ballistic transport of electrons in the lightly p-type GaAs base of ballistic transistors has been measured at temperatures from 4.2 to 80 K and base lengths of 0.2, 1.7, and 5.7 μm. The transistors have n-type emitters, p-type base and collector with an undoped superlattice as the energy analyzer, and the electron energy is below that of the longitudinal optical phonon. Ballistic transport through the 5.7 μm base device is seen at up to 25 K while in the 0.2 μm transistor ballistic effects persist to liquid nitrogen temperature. A mean free path about 3 μm at 5 K is deduced from the experiments with little change up to 15 K. Calculation shows that neutral impurities dominate the scattering in this temperature range, and the calculated ballistic range of 3.2 μm is in excellent agreement with the experimental results.

Electrostatic force characterization on InAs dot-covered n-type (001) GaAs surfaces by contact-mode atomic force microscopy with a conductive tip

We performed atomic force microscopy in contact mode while applying an ac bias voltage between a conductive tip and a sample to characterize near surface band structures of InAs-covered n-type (001) GaAs, where self-assembled dot structures were formed. Electrostatic force of less than 10 pN was detectable, and clear electrostatic force images and topographic images were simultaneously obtained with lateral resolution higher than 20 nm. The electrostatic force images from single and double frequency components reveal that the gap width between the tip and the conductive region under the dot-covered area of the sample is smaller and is less modulated by the bias voltage than under the wetting layer. The results indicate that surface depletion is more suppressed beneath the dots.

Substrate misorientation effects on Zn 1− xMg xSe layers grown on GaAs(1 1 1) by molecular beam epitaxy

The microstructural and electronic characteristics of both undoped Zn 1− x Mg x Se epilayers and Schottky diodes grown by the molecular beam epitaxy method on different misoriented n +-type GaAs(1 1 1) substrates were systematically investigated via three measurements: transmission electron microscopy, current–voltage ( I– V), and deep level transient spectroscopy. The density of V-shaped stacking faults, originating at or near the ZnMgSe (epilayer)/GaAs (substrate) interface and extending into the whole ZnMgSe epilayer, were discovered to be affected by the angles of miscut of n +-type GaAs substrates. Evidence also revealed that more stacking faults existed in the samples with 3 and 15° angles of miscut of substrates. In these cases there were also more interface state densities created at the Au/ZnMgSe Schottky junction. Such results are suspected to be the main factors contributing to the undesirable I– V characteristics of Au/ZnMgSe Schottky diodes, including less forward conduction current density, bigger turn-on voltage, and larger reverse leakage current followed by a smaller reverse breakdown voltage.

Radiative tunneling recombination and luminescence of trapezoidal δ-doped superlattices

Radiative recombination is investigated in the authors’ previously proposed δ-doped superlattice, which can be grown from one of several well-known single-crystal semiconductors of the type InSb, InAs, or GaAs. The energy diagram of the superlattice consists of alternating trapezoidal n-type and p-type potential wells for electrons and holes. An equation for the radiative recombination rate is derived for such a trapezoidal superlattice, and it is shown that, owing to the spatial separation of electrons and holes, the radiative lifetime can attain values of the order of 1 ms, and that it depends weakly on the temperature. This result is attributable to the fact that radiative recombination in a trapezoidal superlattice is governed by optical tunneling transitions of electrons from states near the bottom of the n-type wells to states near the bottom of the p-type wells. An expression is obtained for the luminescence spectrum of the superlattice, where the spectral maximum corresponds to a photon energy much smaller than the width of the semiconductor band gap and can be situated in the far-infrared range. It is noted that such a trapezoidal superlattice can be an efficient converter of thermal radiation into very long-wavelength radiation.

Light emission from tunnelling mode GaAs static induction transistor

Vertical type GaAs static induction transistors (SIT) with 160 Å channels were fabricated with selective regrowth, using molecular layer epitaxy. The structure with a sidewall channel is suitable for electron transport investigation with electroluminescence spectra. The room temperature spectra (below bandgap, in the range from 0.6 eV to 1.4 eV) indicate tunnelling as the room temperature electron transport mechanism for this device. At sufficiently high drain–source or gate–source bias voltages, most of the electrons are injected directly from the source to the drain. The electron transit time was roughly estimated as 2 × 10–14 s.

The flip-chip bump interconnection for millimeter-wave GaAs MMIC

The flip-chip bump interconnection structure has become popular for microwave and millimeter-wave package applications. This structure is expected to provide higher performance and a low cost packaging method. This paper presents the results of an evaluation of flip-chip assembled radio frequency (RF) devices. A coplaner transmission line type GaAs monolithic microwave integrated circuit (MMIC) was mounted on an aluminum oxide (Al/sub 2/O/sub 3/) substrate using the flip-chip thermal compression method. The electrical performance (S-parameter and noise figure) was measured and the reliability of the interconnection was tested. The changing rate of the characteristic impedance (Zo) of transmission line on bare-chip caused by bare-chip surface proximity to a substrate was simulated by finite element method (FEM) analysis. Flip-chip bonding conditions were fixed to keep the gap that less than 1% of Zo changing rate and sufficient bonding strength for reliability of interconnection. The DC characteristics of a 30 GHz, 60 GHz and 77 GHz band low noise amplifier (LNA) were the same before and after mounting, and the RF performance of the assembled MMIC was the same as the bare-chip without packaging. However, the influence of underfilling was observed. When epoxy resin was injected into the gap between the bare-chip and the substrate, the frequency band of the MMIC shifted to the low side. The reliability of the bump interconnection was excellent. The interconnection resistance did not change in a temperature cycle (-55/spl deg/C to +125/spl deg/C until 1500 cycle) test.

Piezoresistive response induced by piezoelectric charges in n-type GaAs mesa resistors for application in stress transducers

The resistance change due to piezoelectric charge densities in n-GaAs mesa resistors has been studied as a function of surface stress. Those changes are verified by measuring some realized stress transducers with such resistors fabricated on the surface of micromachined thin GaAs membranes or GaInP/GaAs cantilever beams. It is shown here that the surface stresses induced by the deformation of cantilever beams can cause nonuniform stress distributions within the resistors, and the stress gradients consequently yield considerable piezoelectric charge densities that lead to an appreciable resistance change. In addition, this effect of piezoelectric charges is examined from several types of resistors that have a different doping, direction, thickness h, and width-to-thickness ratio L/h. According to those results, optimization of this mechanism is related to the doping and geometrical design of GaAs mesa resistors. In addition to the effect of piezoelectric charges, mobility change can affect the resistance as well. The resistance changes observed from the low doped resistors exhibit an opposite sign for the resistors oriented along [011] and [011̄] directions. These directional-dependent characteristics confirm that the effect of piezoelectric charges indeed exists in mesa resistors. Fitting the width-dependent sensitivities measured in experiments allows us to estimate each contribution of these two effects. For the [011̄] oriented resistors with L/h of 10, and ns of 4.8×1011, the sensitivity of relative resistance change as high as 92.7%/GPa is obtained. Results in this study demonstrate that the resistance change in n-GaAs mesa resistors is attributed to the effects of piezoelectric charges and mobility change. Moreover, GaInP/GaAs material system with its piezoresistive response originated from piezoelectric charges is highly promising in III–V compound semiconductor stress transducers.