Type GaAs Research Articles

Relaxation dynamics of electrons between Landau levels in GaAs.

The relaxation dynamics of free electrons between Landau levels is traced on a picosecond time scale via the time evolution of the band-to-acceptor luminescence in $p$-type GaAs after resonant excitation of the $n=1$ Landau level. For electron densities $N>{10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, relaxation occurs via electron-electron scattering with a $\frac{1}{e}$ decay time of 7.5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ps ${\mathrm{cm}}^{\ensuremath{-}3}$/N. For lower electron densities, relaxation becomes independent of density due to elastic impurity scattering.

Laser irradiation effects on tunneling properties of n-type GaAs and InAs by scanning tunneling microscopy

We have studied the effect of laser irradiation on the tunneling properties of n-type GaAs and InAs by scanning tunneling microscopy. We measured the changes of tip height between on and off cycles of the laser light at different wavelengths and laser power. The laser-induced tip-sample separation is observed to be very large in GaAs compared with InAs when above-band-gap laser light is used. This result can be explained in terms of the shrinkage of the surface depletion layers by generated photocarriers.

Novel in situ cleavage technique for cross-sectional scanning tunneling microscopy sample preparation

A novel in situ sample cleavage technique has been developed for fabricating specimens for cross-sectional scanning tunneling microscopy applications. This technique can be easily adapted to any ultrahigh vacuum scanning tunneling microscope that has coarse motion and tip ex- change capabilities. A 90° bent diamond tip attached to a tip holder is used to make micron long scratches on GaAs(001) surfaces along a 〈110〉 direction. The sample is then fractured and the cross-sectional surface is scanned in the conventional way. Atomic resolution images of {110}-type GaAs surfaces are readily and reproducibly obtained.

Generation-recombination noise analysis in heavily doped p-type GaAs transmission line models

Low-frequency noise measurements are performed on heavily doped p-type GaAs transmission line models. Excess noise exhibits 1/f noise and generation-recombination (GR) noise components. A study of the GR components vs device geometry shows the spectral densities due to contact resistances to be negligible. Thus the noise sources due to the volume resistances are predominant, and have to be located in the bulk layer or in the space-charge region of the devices. These two possibilities concerning the location of the GR noise sources are investigated. For both cases, expressions for the variance and the relaxation time associated to fluctuations in the charge carriers are given. The comparison between the experimental data with the theoretical results shows that the GR noise sources are located in all probability in the space-charge region.

A new structure of In-based ohmic contacts to n -type GaAs

A new structure of In-based ohmic contacts to n -type GaAs

Two-band analysis of hole mobility and Hall factor for heavily carbon-doped p-type GaAs

We solve a pair of Boltzmann transport equations based on an interacting two-isotropic-band model in a general way first to get transport parameters corresponding to the relaxation time. We present a simple method to calculate effective relaxation times, separately for each band, which compensate for the inherent deficiencies in using the relaxation time concept for polar optical–phonon scattering. Formulas for calculating momentum relaxation times in the two-band model are presented for all the major scattering mechanisms of p-type GaAs for simple, practical mobility calculations. In the newly proposed theoretical framework, first-principles calculations for the Hall mobility and Hall factor of p-type GaAs at room temperature are carried out with no adjustable parameters in order to obtain direct comparisons between the theory and recently available experimental results. In the calculations, the light-hole-band nonparabolicity is taken into account on the average by the use of energy-dependent effective mass obtained from the k⋅p method and valence-band anisotropy is taken partly into account by the use the Wiley’s overlap function.. The calculated Hall mobilities show a good agreement with our experimental data for carbon-doped p-GaAs samples in the range of degenerate hole densities. The calculated Hall factors show rH=1.25–1.75 over hole densities of 2×1017–1×1020 cm−3.

Band structure of Al/Si/n-type GaAs with a strained Si interfacial layer.

The band structure of a coherently strained Si layer (15 \AA{}) on GaAs has been calculated using the empirical pseudopotential method. The pseudopotential from factor of the strained Si layer is derived by considering that the pseudopotential of the strained layer is slightly modified by a factor which is proportional to the volume change of the unit cell. The band-structure calculation indicates that the band gap of the strained Si layer is 0.7 eV, which is substantially smaller than that of the bulk Si. A revised band-structure model for $\mathrm{AI}/\mathrm{Si}/n$-type GaAs based on the above calculation is proposed, which agrees very well with the experimental results. This model implies that for the coherently strained Si interfacial layer, the Fermi level is unpinned not only at the Si-GaAs interface but also at the Al-Si interface.

Electrical Characterization of In Schottky Contacts to Epitaxial n-In0.46Ga0.54P Grown on n+-GaAs by Mocvd

AbstractIn/n-In0.46Ga0.54P Schottky diode was fabricated by thermal evaporation of In on chemically etched surface of In0.45Ga0.54P:Si epitaxial layer grown on highly doped n type GaAs. The In metal formed a high quality rectifying contact to In0.46Ga0.54P:Si with a rectification ratio of 500. The direct current-voltage/temperature (I-V/T) characteristics were non-ideal with the values of the ideality factor (n) between 1.26-1.78 for 400>T>260 K. The forward I-V data strongly indicated that the current was controlled by the generation-recombination (GR) and thermionic emission (TE) mechanisms for temperature in the range 260-400 K. From the temperature variation of the TE reverse saturation current, the values of (0.75±0.05)V and the (4.5±0.5)×10-5 Acm-2K-2 for the zero bias zero temperature barrier height (φoo) and modified effective Richardson constant were obtained. The 1 MHz capacitance-voltage (C-V) data for 260 K < T < 400 K was analyzed in terms of the C-2-V relation including the effect of interface layer to obtain more realistic values of the barrier height (φbo). The temperature dependence of φbo was described the relation φbo =(0.86±10.03) - (8.4±0.7)×l0-4T. The values of φoo, obtained by the I-V and C-V techniques agreed well.

InN-Based Ohmic Contacts to AlInN

ABSTRACTIn order to maximize the performance of III-Nitride devices, it is necessary to develop thermally stable low resistance Ohmic contacts to III-N based electronic structures. This paper reports on the utility of InN as an aid to contact formation on widegap materials such as InAIN. For n-type materials, several questions relating to the growth conditions have been explored. Specifically, the impact of substrate type (GaAs vs. Sapphire), cap layer growth temperature and V/III ratio on contact resistance has been investigated. It was found that the use of sapphire substrates combined with high growth temperatures (575°C) and high V/III ratios produced acceptable contact resistances (∼10−6Ohm-cm2) to InAIN.

Electrical and microstructural investigation of Au/Pd/ri ohmic contacts for AIGaAs/GaAs heterojunction bipolar transistors

AbstractThe microstructure and electrical properties of as deposited and annealed Au (400 nm)/Pd (75 nm)/Ti (10 nm) contact structures to p type GaAs, C doped with a concentration of 5 × 1018 and 5 × 1019 cm−3, have been investigated using transmission electron microscopy, and current-voltage measurements as afunction of temperature in the range 198–348 K. The specific contact resistivities have also been measured using the transmission line method. It was found that increasing the epilayer doping level by an order of magnitude, from 5 × 1018 to 5 × 1019 cm−3, caused the dominant current transport mechanism to change from thermionic field emission to field emission. For the lower level doped epilayers generationrecombination within the depletion region was found to be the dominant current transport mechanism for temperatures below 289 K. The contacts to the more highly doped epilayers (C doped, 5 × 1019 cm−3) had specific contact resistivities of 0·08 ± 0·03 Ωmm and 0·05 ± 0·06 Ωmm, respectively. These va...

Calculation and photoresponse measurement of the bound-to-continuum infrared absorption in p-type GaAs/AlxGa1-xAs quantum wells.

The bound-to-continuum linear absorption coefficient in p-type GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As quantum wells is calculated based on the electronic structure, wave functions, and optical matrix elements obtained from an 8\ifmmode\times\else\texttimes\fi{}8 envelope-function approximation calculation and then compared to experimental photoresponse measurements on 30-, 35-, and 40-\AA{}-wide wells. The calculation for the bound and continuum states proceeds independently, owing to the different boundary conditions satisfied by these states. We eschew the use of an artificial large box to confine the continuum wave functions. Good agreement is obtained between experiment and theory with respect to the shape, relative magnitude, and long-wavelength threshold of the bound-to-continuum spectrum for the three measured samples. For an aluminum content of 30%, the calculated linear absorption coefficient for bound-to-continuum absorption in the 8--12-\ensuremath{\mu}m window is optimized for the well width of 45 \AA{} due to the presence of a resonant LH2 (second light-hole) state at the top of the well.

Passivation of an n-type GaAs surface with an As2S3 film

This letter describes the passivation of an n-type GaAs surface using an arsenic trisulfide (As2S3) film. Metal–insulator—semiconductor diodes were fabricated with an evaporated As2S3 film as an insulator. In the inversion region, the capacitance–voltage curve shows deep depletion, that coincides with the depletion theory. In the accumulation region, surface Fermi level movement is blocked 0.25 eV below the conduction-band minimum due to a high density of interface states. The minimum interface state density is found to be about 1–2×1011 eV−1 cm−2.

Photocapacitance investigation of the two photoquenching levels in n-type GaAs crystals with excess arsenic atoms

The photocapacitance method is applied to the n-type GaAs crystal in which the excess arsenic atoms are introduced by the 67 h annealing at 900 °C under extremely high arsenic vapor pressure of 1950 Torr. The ion density photocapacitance spectra and the temperature dependencies of the photoinduced ion density measurements reveal two photoquenching levels with different level positions and thermal recovery temperatures, respectively.

The metal organic vapour phase epitaxy of ZnTe: III. Correlation of growth and layer properties

The crystalline structure, surface morphology, optical properties and purity of ZnTe layers grown by MOVPE were investigated. Various substrates, different combinations of metalorganics and various growth conditions were studied. The results of three different MOVPE growth systems and reactor cells are compared. A variety of methods were used to study the structure and morphology (e.g. TEM, HRTEM, X-ray diffraction, Nomarski microscopy, photo reflection, Raman scattering). The preparation of the GaAs and ZnTe surfaces is well advanced but problematic for GaSb. For heterostructures like ZnTe on GaAs (001) with about 7% mismatch, the crystalline structure is mainly dominated by interfacial misfit dislocations and threading dislocations penetrating about 300 nm into the ZnTe layer. With better matched substrates (GaSb or ZnTe) or different orientations (GaAs (111)), the threading dislocations can nearly be eliminated. The comparison of growth studies with MOVPE, MBE and ALE reveals that the initial growth of ZnTe on GaAs (001) is mainly determined by the misfit and its relaxation. The first ≈4 monolayers are characterized by a pseudomorphous, 2D growth mode. The following surface roughening is caused by a relaxation through a 3D growth mode with islands. Once the relaxation is completed, the surface smoothens and recovers the 2D growth mode. The initial growth stages, representing the Stranski-Krastanov mechanism, are dependent on the surface termination of the substrate and on the growth method and parameters. The surface morphology of thicker ZnTe layers (>0.1 μm on GaAs (001)) grown by MOVPE is solely determined by the conditions at the growing interface (adsorption and decomposition of the precursors, desorption of undesired species, quantities of released Zn and Te, surface stoichiometry). These interface conditions are dependent on the precursor combination, the kinetic or mass transport limitation and the reactor hydrodynamics. The studies of the layer properties were sometimes bothered by a thin native oxide with the structure ZnTe/Te/ZnO. Purity and optical quality were tested by low temperature PL. The strain induced by the cooling of the ZnTe/GaAs structures grown at 300–400°C renders the identification of the radiative transitions difficult. However, many transitions and radiative centers are now identified. Under many growth conditions, the (A 0 Asl , X1) transition which is due to As from substrate autodoping, dominates the spectra. Hence, stoichiometry and inhomogeneity of the GaAs substrates are reflected in the spectra. But also precursor combination, partial pressures and growth temperatures have a significant influence on the PL spectra. The substrate type (GaAs, GaSb, ZnTe) is reflected in the spectra by transitions due to outdiffusion and by the Y-lines which are related to the misfit dislocations. Transitions induced by layer contaminations like Cu, Li, O and N were found. With the alkyl combination DEZn/DIPTe, reproducible growth of samples is possible showing PL spectra dominated by free excitons. The frequently appearing I 1c and I' 1c transitions might be due to a V Zn vacancy or a vacancy donor complex. Complex relations to the growth stoichiometry were found. SIMS measurements helped to further identify the layer impurities. The layer purity is affected by extrinsic impurities due to substrate outdiffusion (As, Ga), contaminations from growth system (O, Cu) and precursors (Li). Impurities like H and C (and N) which are intrinsic to the MOVPE process, depend on the dissociation kinetics of the precursors which are themselves dependent on the growth parameters (e.g. temperature, carrier gas). The origins of the extrinsic impurities are identified as far as possible.

In Situ Cross-Sectional Scanning Tunneling Microscopy Sample Preparation Technique

ABSTRACTA novel in situ sample cleavage technique has been developed for fabricating specimens for cross-sectional scanning tunneling microscopy (XSTM) applications. This technique can be easily adapted to any ultrahigh vacuum (UHV) STM that has a coarse motion capability. A conducting diamond STM tip is used to create micron long scratches on Ge/GaAs or GaAs {001 }-type surfaces. These {001} surfaces are imaged with STM to observe scratch characteristics, and GaAs samples are cleaved to reveal {110}-type faces. Atomic resolution images of {110}-type GaAs surfaces are readily and reproducibly obtained.

Nondestructive mapping of carrier concentration and dislocation density in n+-type GaAs

Transmission mappings (500 μm×500 μm resolution) at wavelengths of 0.9–1.5 μm on 3 in., n+-GaAs wafers (n≂1–2×1018 cm−3) correlate well with carrier concentration n, measured by the Hall effect, and dislocation density, as confirmed by KOH etch-pit patterns. The absorption for λ≳1.0 μm (below band edge) varies directly with n via free-carrier interconduction-band transitions, while the absorption for λ≲0.95 μm (near band edge) varies inversely with n because of band-filling effects. Both phenomena are highly useful for n+-GaAs wafer characterization.

Ultrafast relaxation of highly photoexcited carriers in p-type and intrinsic GaAs.

The thermalization and energy relaxation of highly photoexcited carriers in p-type and intrinsic GaAs is investigated at room temperature using an ensemble Monte Carlo approach. We present an alternative model for treating carrier-carrier scattering in Monte Carlo simulations. The model takes into account the wave vector and frequency dependence of the dielectric function by using a screened Coulomb potential in q space due to a charge moving at the velocity of the center of mass of the interacting particles. In comparison to the statically screened model, the calculated effective carrier temperatures for an excitation density of 2\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ are in good agreement with the experimental results for both p-type and intrinsic GaAs. At the lower doping and excitation density of 2.5\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, the dynamic and the static screening models result in a comparable time evolution of the carrier energies and band populations.

Schottky and ohmic contacts of Pd on p-type GaAs distinguished with hydrogen

Contacts of Pd on p-type GaAs with a native oxide layer between them reveal ohmiclike behavior in air or vacuum at room temperature. This behavior is similar to that of contacts annealed at 450 °C for 5 min. To relate the interface electrical properties of the two contacts to their interfacial structures, we propose to measure current-voltage (I-V) and capacitance-voltage (C-V) characteristics in a hydrogen ambient. We show that these two contacts can be distinguished with atomic hydrogen. This method was confirmed with I-V and C-V measurements at low temperatures.

Two-dimensional versus three-dimensional behavior of a free-carrier gas in delta -doped p-type GaAs(001).

Two-dimensional versus three-dimensional behavior of a free-carrier gas in delta -doped p-type GaAs(001).

FABRICATION TECHNIQUES FOR SELF-ALIGNED GaAs-BASED HBTs AND SUBMICRON GATE LENGTH FETs

Process technologies for self-aligned AlGaAs/GaAs and InGaP/GaAs heterojunction bipolar transistors (HBTs) as well as gate definition and dry etching fabrication schemes for submicron gate length AlGaAs/GaAs-based field effect transistors (FETs) are presented. Multiple energy F + and H + ions were used to isolate the active devices for HBTs. The resistance of test wafers at 200° C showed no change over periods of more than 50 days. Highly selective dry and wet etch techniques for InGaP/GaAs and AlGaAs/GaAs material systems were used to uniformly expose heterojunctions. Reliability of the alloyed ohmic contact and feasibility of the nonalloyed ohmic contact metallizations for both p and n type GaAs layers will be discussed. The reproducible gate recess etching is one of the critical steps for AlGaAs/GaAs-based FETs. The etching selectivity, damage, pre- and post-clean procedures were studied in terms of device performance. A simple low temperature SiN x deposition and an etch-back process with optical stepper were used to demonstrate 0.1 µm Y-shape gate feature.