GaAs Region Research Articles

Photoreflectance and the electric fields in a GaAs depletion region

We present results which may resolve the recently reported discrepancy between the experimental and the calculated electric fields in the depletion region of undoped GaAs. The photoreflectance theory reportedly underestimates the electric field by nearly a factor of 2. We have found that changes in photoreflectance with laser pump penetration reveal the full character of the electric field over the entire depletion zone. It is often assumed that the built-in surface potential produces a uniform electric field throughout a thin (100 nm) undoped layer of GaAs grown on top of a heavily doped energy pinning underlayer. Instead, it appears that the heavily doped underlayer provides a potential step at the interface. The step is separated from the surface depletion zone by a region of low electric field which is characteristic of the low fields found in thick, undoped GaAs with (2–4)×1014/cm3 of unintentional impurities.

Controlled Modifications in the Electrical Properties of Metal/GaAs Junctions

ABSTRACTControlled modifications in the electrical properties of metal/GaAs junctions were obtaind by a few different approaches. The first approach is based on modifications induced by solid state reactions occurring between the metal and GaAs substrate, resulting in compound formation and component redistribution. The characteristics of such contacts can further be modified when the contact metal is alloyed with another metal or with a dopant. The second approach is based on modifying the doping level of the near surface region of the GaAs. Here an enhancement of the barrier height was obtained by heavily counter doping the top GaAs region by recoil implantation of Mg from a Mg thin film irradiated by As− ions. The correlations between the electrical properties of the junctions and the physical processes taking place using the above mentioned approaches are discussed.

Elevated temperature reactive ion etching of GaAs and AlGaAs in C2H6/H2

The etch rate, surface morphology and atomic composition, and depth of hydrogen passivation of Si dopants in n-type GaAs and AlGaAs have been examined as a function of the temperature of the sample during reactive ion etching in C2H6/H2. While there is no temperature dependence of the etch rate of AlGaAs over the range 50–350 °C, the etching of GaAs shows an increase of a factor of 2 between 150 and 250 °C, decreasing at higher temperatures. The As-to-Ga ratio in the near-surface region of GaAs remains unchanged over the whole temperature range investigated and there is no polymer deposition. The etched surface morphology is smooth for both GaAs and AlGaAs for all temperatures while the depth of Si dopant passivation by hydrogen shows an increase with increasing substrate temperature during the etching treatment.

Characterization of n-type regions in GaAs formed by silicon fluoride molecular ion implantations

Implantations of silicon and silicon fluoride ions into semi-insulating GaAs wafers were performed to compare and evaluate the quality of the n-type layers that were produced. The use of SiFx (x=1,2,3) molecular ion implantations creates n-type active regions and n+ ohmic contact layers employing a higher implant energy than that needed to achieve a similar projected range for silicon ions. The higher energy allows the implanter to operate with a more stable and reproducible ion beam for shallow implant applications. Variables affecting the net electrically active ion distributions, such as the ion beam incidence angle, and the use of furnace or rapid thermal annealing cycles for implantation activation are addressed. Capacitance-voltage profiles were obtained for the n-type regions, and the free carrier distributions for the n+ implantations were obtained by polaron electrolytic profiling. The implanted regions were characterized by Hall-effect measurements, cathodoluminescence, and secondary ion mass spectroscopy.

Direct Formation of Three-Dimensional Structures in GaAs by Excimer Laser Doping

Direct formation of three-dimensional structures of Si in GaAs by excimer laser doping has been demonstrated. The doped Si is limited in a very shallow region (30∼110 nm) of GaAs with very high concentration (>1×1020 cm-3) that might be controlled easily by laser irradiation conditions. By using a thin solid Si dopant source, linewidths as narrow as 0.3 µm are achieved. On the other hand, the linewidths of 0.76 µm are obtained by projection patterned doping in SiH4 gas ambient. The linewidths of the doped regions are discussed relating to the lateral profiles of the calculated transient temperature rise of the substrates.

Tunable stimulated emission of radiation in GaAs doping superlattices

Tunable stimualted emission of radiation is achieved in AlxGa1−xAs/GaAs double heterostructures, in which the waveguiding GaAs region consists of a delta-doped doping superlattice. The low-temperature emission energy is 45 meV below the bulk band gap of GaAs for homogeneous optical excitation of the Fabry–Perot cavity. The emission energy is continuously tunable over 35 Å by inhomogeneous excitation of the cavity.

GaAs/Ga 1- xAl xAs Bragg reflectors at absorption wavelengths

Classical Bragg reflection computation is extended from the transparent wavelength region of GaAs to the 0.6–0.9 μm wavelength range. Our model includes (i) absorption effects, (ii) refractive index changes around the bandgap of both materials, and (iii) air Ga 1- x Al x As interface reflection. Reflection measures are made and compared with the results of the model. Actual thickness and Al atomic fractions of layers are being determined by double X-ray diffraction (DXD) technique. Good agreement between theory and experiment is observed.

Interface electronic structures in an InAs/GaAs lattice-mismatched system.

Interface electronic structures in an InAs/GaAs lattice-mismatched system are studied, and strain effects are investigated. The electronic structures are examined under three different strain conditions characterized by the lattice constant parallel to the interface. Investigations of strain effects on the interface electronic properties give new information on band discontinuity and interface-state properties. The electronic structures are calculated by use of the self-consistent pseudopotential method. The valence-force-field approach is employed in determining the atom positions. Deep interface states are found about 10 eV below the top of the valence bands, independent of the interface-strain conditions. It is found that the strain conditions and the symmetry change at the interface are not important factors in forming interface states. The important factor is charge modification near the interface. Band discontinuities and carrier confinements greatly depend on strain. Interface-strain relaxation influences charge transfer from the GaAs region to the InAs region. It also affects the band lineup. A correlation is found between band lineup and interface dipole.

Photoelectrochemical studies of a Ga(As,P)/GaAs/Ga(As,P)/n+-GaAs multijunction electrode

A multijunction structure, Ga(As,P)/GaAs/Ga(As,P)/n+-GaAs, and a conventional Ga(As,P)/n+-GaAs structure were compared using a variety of photoelectrochemical (PEC) techniques. The samples were grown, using metalorganic chemical vapor deposition, on n+-GaAs substrates with an initial graded region with gradually varying composition. The conventional sample had a terminating top layer with constant composition GaAs0.45 P0.55. In the multijunction sample, a GaAs region was introduced between the graded region and the top layer, resulting in an interface with an abrupt heterojunction. The multijunction electrode showed several unusual PEC features, including a very low dark current over a wide potential region, both cathodic and anodic photoeffects, and a pronounced decrease in photoactivity when going to shorter wavelengths. A plausible interpretation is presented, also providing insights into the effects of graded regions versus heterojunctions, and illustrating the value of PEC studies in probing the interior of complex semiconductor materials.

Elevated Temperature Reactive Ion Etching of GaAs and AIGaAs in C2H6 / H2

ABSTRACTThe temperature dependence of etch rate, surface morphology and atomic composition, and depth of hydrogen passivation of Si dopants in n-type GaAs and AIGaAs has been measured for reactive ion etching in C2H6 /H2. The etching of GaAs shows an increase of a factor of two between 150 and 250°C, decreasing at higher temperatures, while there is no temperature dependence for the etch rate of AlGaAs over the range 50-350°C. The As-to-Ga ratio in the nearsurface region of GaAs remains unchanged over the whole temperature range investigated and there is no polymer deposition. The etched surface morphology is smooth for both GaAs and AIGaAs for all temperatures while the depth of Si dopant passivation by hydrogen shows an increase with increasing substrate temperature during the etching treatment.

Picosecond GaAs photoconductors on silicon substrates for local integration with silicon devices and circuits

A fabrication procedure for local integration of GaAs photoconductive devices with processed silicon circuits is discussed. The process allows isolated regions of GaAs to be epitaxially grown by MBE at temperatures which are compatible with already processed silicon circuits with first-level metallization. GaAs photoconductors with 15- mu m gap lengths fabricated on silicon substrates have exhibited >16-mA sampling-oscilloscope-limited responses, with electrical pulse widths less than 20 ps as determined by autocorrelation measurements. >

Migrations of Interstitial Atoms in Semiconductors (Surface Diffusion and Kick-Out Mechanism)

AbstractGe and Zn atoms were introduced into the unirradiated regions of Si at 150°C and GaAs wafers at 50°C, respectively by using the electron-beam doping method. The surfaces of Si and GaAs substrates were covered partially by the overlayers of Ge and Zn sheets, respectively. The only surfaces of the Ge and Zn sheets were irradiated locally. with high energy electrons at 7MeV with the fluences of 5×1017 – 1×1018 electrons cm-2 . Even at a distance of ~10mm from the irradiated overlayers in the Si and GaAs substrates, Ge and Zn atoms respectively, whose interstitials may migrate the unirradiated regions, were detected by SIMS measurements. PL signal due to band-to-acceptor (ZnGa ) transition at 1.48eV becomes observable after annealing at 800°C for 20mm in the unirradiated GaAs region, which is separated from Zn sheet by nearly 10mm.

Microscopic calculation of electric field effects in GaAs/AlxGa1-xAs/GaAs tunnel structures.

The combined effects of an applied electric field and the inherent crystal band structure on the tunneling of electrons through ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As barriers between field-free regions of GaAs are presented. A microscopic pseudopotential-based calculation is used. It includes the effect of all the higher conduction-band minima. We find that a finite field changes the electron tunneling probabilities and produces intervalley transfers if the energy of the tunneling particle is near a conduction-band minimum. The net result for direct-band-gap barriers is small, but for indirect-band-gap barriers a field-dependent transition region between a \ensuremath{\Gamma} behavior and an X behavior is found, with the electron transferred from an evanescent \ensuremath{\Gamma} state to a propagating X state in the barrier. The exact rate of transfer and proportion of the \ensuremath{\Gamma} and the X states' contributions to the total wave function is found to be strongly dependent on the exact composition of the barrier material.

Negative differential resistance in AlAs/NiAl/AlAs metal base quantum wells: toward a resonant tunneling transistor

The authors report the observation of a negative differential resistance region (NDRR) in the axial current-voltage characteristics of a nominally 13-monolayer epitaxial NiAl film sandwiched between two 12-monolayer AlAs layers. The active area is separated by two 100-AA undoped GaAs regions from the heavily doped n-type GaAs electrodes on both sides. The NDRR is observable at room temperature in many of the devices but varies in strength across the wafer. The highest documented peak-to-valley ratio at room temperature is 2.0, but the majority of the devices exhibit lower ratios, typically around 1.2. The onset of the NDRR occurs around 800 meV. This value, however, contains the voltage drop at the top contact which is believed to be substantial. The origin of this effect is believed to be resonant tunneling through quantized electronic levels is the thin metal layer. This appears to be the first direct observation of size quantization in semiconductor-metal heterostructures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effects of N2, O2, and H2O on GaAs passivated by photowashing or coating with Na2S⋅9H2O

The photoluminescence (PL) from the surface region of GaAs passivated by photowashing or coating with Na2S⋅9H2O is shown to be sensitive to the gas ambient. Both water vapor and oxygen must be present in order to obtain a large PL signal. The effects are activated by the measuring laser light.

Effect of iso-electronic dopants on the dislocation density of GaAs

The addition of 1% In to LEC GaAs has been reported to reduce the dislocation density in this material; similar data exists for Sb doping. Several effects have been inferred to explain these phenomena, the most prevailing one stating that the solid stoichiometry is affected by an as yet unknown mechanism. Similar postulations have been made to explain the growth of semi-insulating GaAs. A thermodynamic model is described, based on earlier work, that shows a broadening of the existence region of GaAs when In or Sb are added to GaAs. Comparing the solidus phase diagrams of In- or Sb-doped GaAs to undoped GaAs shows that addition of either one of these two iso-electronic dopants has a similar effect on the solid stoichiometry as adding more As to the melt. However, the increased pressure problems in LEC growth of GaAs, normally associated with adding As, are circumvented if instead In or Sb are added to the melt. From our calculations it is also shown that the addition of the iso-electronic dopants Al or P to GaAs would not result in the same effect on the solid stoichiometry. Published experimental evidence supports this and shows that no dislocation reduction and semi-insulating GaAs is obtained with the use of these dopants. The model described in this paper explains the postulation that iso-electronic doping is of critical significance in controlling the solid stoichiometry and thereby obtaining zero dislocation density LEC GaAs and semi-insulating GaAs.

Resonant photoluminescence excitation in GaAs grown directly on Si

We describe new results observed during resonant excitation in the excitonic region of GaAs grown directly on Si by organometallic vapor phase epitaxy. Two resolved features were found in the light-hole photoluminescence (PL) region, one identified with a free-exciton process and the other with donor-related transitions. Inhomogeneous line broadening was observed for the excitonic PL lines and related to stress fluctuations. Features which track the laser excitation line were observed and identified with a process in which a donor is excited from the n=1 ground state to an n=2 state.

Electronic structure of (InAs)m(GaAs)m (m=1-7) strained superlattices.

The electronic structure of (InAs${)}_{\mathrm{m}}$(GaAs${)}_{\mathrm{m}}$ (m=1--7) strained superlattices is studied with use of the self-consistent pseudopotential method. The atom positions are determined by using the valence-force-field approach. The calculated band gaps are in good agreement with the experimental results. The weak dependence of the band gaps on layer thickness is due to the s-like character of the ${\ensuremath{\Gamma}}^{c}$ state. The conduction electron extends over the superlattice. In contrast, the hole distribution tends to be localized in the InAs regions. From the investigation of the self-consistent potential and charge-density distribution, it is found that the lattice-mismatch effect relaxes over a few atom layers near the interface. The charge is transferred from the GaAs region to the InAs region. This transferred charge is stored at the interface.

Silicon donor states in heavily doped thin GaAs-AlAs(001) superlattices.

The self-consistent pseudopotential method is used to investigate the electronic structure of silicon donor states at and near the interface of thin GaAs/AlAs(001) superlattices under high doping concentrations. The silicon impurity is substituted in the GaAs and AlAs regions and at different distances from the interface. For all cases, we find that the donor state is highly localized around the silicon donor ion. With the impurity placed at the interface, there is a shift of the donor-state charge density from AlAs to GaAs. The qualitative changes in the valence- and conduction-band discontinuities with doping are discussed. Our results indicate that doping in the GaAs region has a much larger effect on the band-edge discontinuities. In addition, the results for the zone folding of the zinc-blende X-point state onto the superlattice \ensuremath{\Gamma} point are reported. For the thin superlattices treated here, (GaAs${)}_{n}$-(AlAs${)}_{n}$ (for n=2, 3, and 4), we find that the highest occupied valence-band state is derived from the zinc-blende GaAs \ensuremath{\Gamma} point and the lowest unoccupied conduction-band state is derived from the zinc-blende AlAs X-point state. The change in the localization of the conduction-band-edge states with increasing n suggests a transition region, at which the lowest unoccupied conduction-band state becomes the zinc-blende GaAs \ensuremath{\Gamma} point. A critical layer thickness is estimated for this transition.

Cation and Anion Displacements at Hetero-Interfaces of (GaAs)28(AlAs)24 Superlattice Layers

By developing a new method of Fourier analysis the local cation and anion displacements in a superlattice, especially near the hetero-interface boundary between the GaAs and the AlAs, were determined from the X-ray integrated intensities of the satellite reflections observed around the two fundamental Bragg reflections, namely the 002 and 004 reflections. The interlayer distance of successive cation layers was found to be 2.827 Å in the GaAs region and 2.835 Å in the AlAs region, except near the hetero-interface boundary. The anion layers are, therefore, located at the mid-point between the cation layers in these regions. However, near the hetero-interface region, which consists of eight molecular layers, anion layers are not located at the mid-point between them but are slightly shifted to the GaAs side. The value of the shift at the boundary between GaAs and AlAs was 0.016 Å.