P-type GaAs Layers Research Articles

Raman study of InAs/GaAs quantum dot solar cells

We report polarized and resonant Raman study of InAs/GaAs quantum dot solar cell (QDSC) structures. Raman spectra obtained from the top surfaces of the samples suggested that the formation of InAs QDs induced tensile strain in the overgrown GaAs layers. Furthermore, a longitudinal optical phonon-plasmon (LPP) coupled modes were observed in the p-type GaAs layers. The tensile strain was increased with an increase in the QD size. The hole concentrations estimated by fitting the individual LPP coupled modes were in the range of 2.4–3.5 × 1018 cm−3. Resonant Raman spectra obtained from the cleaved sides, where the QDs were located, showed a 225 cm−1 mode in parallel polarization configurations. Based on accurate analysis, this mode was identified as the LA(X) phonon of GaAs.

A GaAs-based plasmonic source employing a nanoscale vertical cavity

In this paper, a new structure of a vertical cavity plasmonic source is presented. The structure, which is adopted from a metal–semiconductor–metal plasmonic photodetector, contains a gold substrate, n-type GaAs layer, active GaAs layer and p-type GaAs layer, respectively from bottom to up. A subwavelength aperture and metal gratings on both sides of the aperture are placed on the top of the device and act both as a gold clad and as the contact for applying DC bias. The metal gratings are improved by adding SiO2 between metal grooves and semiconductor. Applying voltage bias to p-and n-regions leads to photons generation in the active region. The generated photons in the active region excite a resonant surface plasmonic mode in the nanoscale cavity just below the aperture. One can use light outflowing from the nanoscale aperture, with wavelength of 882 nm, as a source to excite an arbitrary plasmonic guiding structure. The output beam has a suitable concentration so that in a 1000 nm distance from the aperture, still 37% of the power is concentrated in a width of 50 nm which is almost a unique specification among plasmonic sources, so far. The function of the device is validated using finite-difference time-domain numerical algorithm.

Spin-polarized injection into a p-type GaAs layer from a Co2MnAl injector

Electric luminescence and its circular polarization in a Co2MnAl injector-based light emitting diode (LED) has been detected at the transition of e−A0C, where injected spin-polarized electrons recombine with bound holes at carbon acceptors. A spin polarization degree of 24.6% is obtained at 77 K after spin-polarized electrons traverse a distance of 300 nm before they recombine with holes bound at neutral carbon acceptors in a p+-GaAs layer. The large volume of the p+-GaAs layer can facilitate the detection of weak electric luminescence (EL) from e−A0C emission without being quenched at higher bias as in quantum wells. Moreover, unlike the interband electric luminescence in the p+-GaAs layer, where the spin polarization of injected electrons is destroyed by a very effective electron—hole exchange scattering (BAP mechanism), the spin polarization of injected electrons seems to survive during their recombination with holes bound at carbon acceptors.

Selective-Area Growth and Electrical Characterization of Hybrid Structures between Semiconducting GaAs Nanowires and Ferromagnetic MnAs Nanoclusters

We report on the structural and electrical characterizations of MnAs/GaAs hybrid nanowires fabricated by combining selective-area metal–organic vapor phase epitaxy of undoped GaAs nanowires and endotaxial nanoclustering of MnAs. As a result of endotaxy, MnAs nanoclusters are typically embedded in the six ridges of hexagonal GaAs nanowires. However, the MnAs nanoclusters are formed not only at the six ridges, but on the surfaces of six 01̄1 facets of hexagonal GaAs nanowires, when we decrease the growth temperature from 600 to 400 °C. From the cross-sectional characterizations by transmission electron microscopy, the size and density of the MnAs nanoclusters formed at the top part of the GaAs nanowires are much larger than those formed at the bottom part of the nanowires. Current and voltage characteristics of MnAs/GaAs hybrid nanowires are investigated using two-terminal device structures of free-standing hybrid nanowires. The hybrid nanowires formed on Zn-doped p-type GaAs (111)B substrates show ohmic characteristics, while those formed on Si-doped n-type GaAs (111)B substrates show clear rectifying characteristics. The hybrid nanowires show p-type conductivity possibly owing to the formation of p-type GaAs layers near the nanowire surfaces caused by the thermal diffusion of Mn atoms into GaAs nanowires during the endotaxial nanoclustering of MnAs.

Selective-Area Growth and Electrical Characterization of Hybrid Structures between Semiconducting GaAs Nanowires and Ferromagnetic MnAs Nanoclusters

We report on the structural and electrical characterizations of MnAs/GaAs hybrid nanowires fabricated by combining selective-area metal–organic vapor phase epitaxy of undoped GaAs nanowires and endotaxial nanoclustering of MnAs. As a result of endotaxy, MnAs nanoclusters are typically embedded in the six ridges of hexagonal GaAs nanowires. However, the MnAs nanoclusters are formed not only at the six ridges, but on the surfaces of six 01 facets of hexagonal GaAs nanowires, when we decrease the growth temperature from 600 to 400 °C. From the cross-sectional characterizations by transmission electron microscopy, the size and density of the MnAs nanoclusters formed at the top part of the GaAs nanowires are much larger than those formed at the bottom part of the nanowires. Current and voltage characteristics of MnAs/GaAs hybrid nanowires are investigated using two-terminal device structures of free-standing hybrid nanowires. The hybrid nanowires formed on Zn-doped p-type GaAs (111)B substrates show ohmic characteristics, while those formed on Si-doped n-type GaAs (111)B substrates show clear rectifying characteristics. The hybrid nanowires show p-type conductivity possibly owing to the formation of p-type GaAs layers near the nanowire surfaces caused by the thermal diffusion of Mn atoms into GaAs nanowires during the endotaxial nanoclustering of MnAs.

Single-Period InAs–GaAs Quantum-Dot Infrared Photodetectors

In this letter, we investigate the performance of single-period InAs-GaAs quantum-dot (QD) infrared photodetectors, in which the single-period QD is sandwiched by different thicknesses of the undoped GaAs confinement layers. By using a 5-nm p-type GaAs layer as a current blocking barrier, the investigated three devices exhibit no response, the highest response, and the medium response, respectively. It is attributed to three different electron occupancy situations in the QD region resulted from the various locations of the Fermi level. A higher barrier for the thermionic emission current is observed for the device with a lower Fermi level in the QD structure. It is attributed to the acceptor-like behavior of the depleted QD such that a higher barrier height would be observed.

Gallium–Arsenide-Based Bipolar Cascade Lasers With Deep Quantum-Well Tunnel Junctions

Edge-emitting GaAs-based lasers were fabricated from metal-organic chemical vapor deposition-grown material containing two diode laser structures separated by a quantum-well tunnel junction (QWTJ). The QWTJ was comprised of a thin, high indium content InGaAs layer sandwiched between relatively low-doped p-type and n-type GaAs layers. Comparison of near-field data with predictions from a one-dimensional current spreading model and comparison of current-voltage characteristics for these two-stage or double-stage lasers (DSLs) with single-stage lasers (SSLs) shows that this type of reverse-biased QWTJ has a low effective resistivity. As a consequence, current spreading perpendicular to the laser length in the plane of the layers (lateral direction) is reduced leading to a relatively low threshold current density for the second stage. In addition, the differential quantum efficiency of these DSLs is nearly twice that of SSLs

Effect of contact properties on current transport in metal/molecule/GaAs devices

Previous reports on metal/molecule/semiconductor (MMS) devices have investigated the effects of molecular species, including dipoles, but have not considered the semiconductor contact properties in detail. In this paper we report on a study of the effects of variations in the semiconductor contact on the conduction properties of MMS devices. Metal/molecule/gallium arsenide (GaAs) devices were fabricated using various semiconductor contact layers, electrically characterized versus temperature and analyzed using an electrostatic model. The various semiconductor contacts included heavily doped n-type and p-type GaAs layers, as well as n-doped and p-doped surface layers of low-temperature-grown GaAs (LTG:GaAs), which provide a high density of midgap defect states near the semiconductor surface. The impact of changing the work function of the top metal contact has also been studied. An electrostatic model that incorporates information on the molecular dipole moment, defect states in GaAs surface layers, and the work function of the metal contact, has been developed in order to understand the band diagrams corresponding to the various device types, and to explain the current-voltage behavior observed in the devices. It is shown by controlling the properties of the semiconductor contact that the device characteristics can be tuned from being dominated by the GaAs barrier (heavily doped n-type GaAs) to being dominated by the molecular states that are strongly coupled to the GaAs contact (LTG:GaAs and heavily doped p-type GaAs).

Thin film GaAs solar cells with increased quantum efficiency due to light reflection

Thin film GaAs solar cells, separated from their substrate using the weight-induced epitaxial lift-off technique, were compared with conventional cells on a substrate. The thin film cells can be illuminated from both sides using a mirror. The thickness of the p-type GaAs layer, which is the base layer for front illumination and the emitter layer for rear illumination, was varied between 0.25 and 2.5μm. For both front and rear illumination, the cell efficiency shows a maximum at a thickness of 1.5μm. The rear illuminated cell current is only 10% lower than for front illumination. Light reflection in the thin film cell enhances the external quantum efficiency and the collection efficiency in the higher wavelength region from 0.84 to 0.90 and from 0.82 to 0.95, respectively.

Morphological and optical properties of p-type GaAs(001) layers doped with silicon

We report on the optimization of a new growth method to obtain p-type GaAs(001) layers using silicon as the dopant. Atomic force microscopy was used to probe the morphological quality of the surface and low-temperature photoluminescence confirmed the p-type character of the layers. This growth mode might be applied to build modulation-doped devices requiring p-type doping on GaAs(001) substrates without using beryllium.

On the morphology of films grown by droplet-assisted molecular beam epitaxy

Atomic Force microscopy was used to investigate the morphology of GaAs(001) layers deposited by molecular beam epitaxy using a non conventional growth method that involves the supply of a few monolayers of gallium (the arsenic cell is shuttered) followed by the annealing of the surface under an arsenic ux (the gallium cell is shuttered). When the silicon shutter is opened and closed together with the gallium one, this particular growth mode allows the silicon atoms to be incorporated into the arsenic sites and p-type GaAs(001) layers can be obtained. Several sets of samples were grown and analyzed in order to better understand the microscopic growth mechanisms of this kind of layers and minimize the number and size of the structural defects that are characteristic of this peculiar technique.

Electrical isolation of p-type GaAs layers by ion irradiation

The electrical isolation of p-type GaAs layers doped with acceptor impurities incorporated in the Ga sublattice (Mg) or As sublattice (C) was studied using proton bombardment. It was found that practically the same proton dose is required to reach complete isolation (isolation threshold dose, Dth) in layers doped with either Mg or C of comparable original sheet hole concentration (ps). This result is evidence that the sublattice where the acceptor dopant atoms are incorporated does not play any significant role for the isolation formation process in GaAs. The behavior of the recovery of the conductivity during subsequent thermal annealing was found very similar in Mg and C doped samples irradiated to equal proton doses. In samples irradiated to doses <Dth, the sheet resistance (Rs) increases during annealing at temperatures >100 °C, reaches a maximum at ≅200 °C, and then decreases progressively toward the original value. For proton doses ranging from Dth to 5Dth, the isolation is preserved up to the temperature of ≈500 °C. The temperature of ≈700 °C was found to be the upper limit for the thermal stability of the isolation in samples irradiated to doses of 100 Dth.

GROWTH AND CHARACTERIZATION OF p-Type GaAs LAYERS USING ATOMIC HYDROGEN AS A REAGENT

Atomic hydrogen has been found to have a great number of useful applications in the technological field of semiconducting materials. It has been used as a reagent in the epitaxial growth processes to control the incorporation of residual impurities. Atomic hydrogen can react with GaAs thus producing Ga- and As- hydrogen volatile species in controlled conditions. The atomic hydrogen can be produced in a chemical vapor deposition chamber using a hot tungsten filament. In this work we report the results of a study on GaAs layers grown using the close space vapor deposition technique with atomic hydrogen as a reagent. The conductivity type of the grown layers is closely related to the conductivity type of the GaAs source. We have grown p-type GaAs layers with l×1018 cm-3 hole concentration using GaAs sources with the same acceptor concentration. 10 K photoluminesence measurements were nlade on the source and the epitaxial GaAs layers. The PL spectra revealed that the residual impurities in the GaAs layers were originated from the source. The mirror like appearance of the grown layers as well as their electrical and optical characteristics demonstrate they can be used in the manufacture of GaAs semiconductor devices.

Influence of a Surface on the Franz-Keldysh Effect in n- and p-type GaAs Epitaxial Layers

We have performed a photoreflectance study of epitaxial n- and p-type GaAs layers having various carrier densities ranged between 3.0×10 14 and 5.8×10 17 cm -3 , in order to investigate influence of a surface on the Franz-Keldysh (FK) effect associated with the E 0 direct gap. It was found both in n- and in p-type GaAs that the observed FK oscillation periods are small by factors ranged between 1.3 and 2.1 compared to those expected from the built-in surface electric fields ( E dc ). A measurement of the surface photovoltaic energy suggests that modifying the surface Fermi energies ( E FS ) is less feasible than using the E FS values ranged between 0.7 and 0.8 eV (0.5 and 0.6 eV) for n-type(p-type) GaAs, that were studied in literature and used also in this study to estimate E dc 's. We explain the reduction in the FK oscillation periods and its carrier density variations in terms of the crystal termination effect which gives rise to an optically inactive region beneath the crystal surface.

P-Type doping of GaAs(0 0 1) layers grown by MBE using silicon as a dopant

For the first time, a thick p-type GaAs layer was successfully grown by molecular beam epitaxy on a GaAs(0 0 1) substrate using Si as the dopant. A hole concentration in the range of 10 18 cm −3 was achieved by a nonconventional growth method involving the supply of a few monolayers of Ga atoms together with the dopant (the As 4 cell was shuttered), followed by annealing in As 4 (the Ga and Si cells were shuttered). The repetition of this sequence led to a thick p-type layer as shown by Hall and photoluminescence measurements.

Photoluminescence in degenerate p-type GaAs layers grown by molecular beam epitaxy

The emission spectra of the heavily Be-doped GaAs layers grown by molecular beam epitaxy as a function of dopant concentration have been studied at different temperatures using photoluminescence (PL) spectroscopy. E 0 and the PL peak energy E M both change with the increase in temperature and the change can well be described by the Varshni equation. The PL full width at half-maximum was found to increase monotonically while the PL intensity decreases with temperature. The luminescence spectra of the degenerate layers are asymmetric with the maxima shifted towards lower energies as the hole concentration is increased. The spreads of the short-wavelength edges towards higher energy, however, are not as significant as that observed in Zn-doped GaAs grown by liquid phase epitaxy. The room temperature E M decreases slowly with hole concentration up to 2.25 × 10 20 cm −3 while E M at 4 K does not change significantly up to 3 × 10 18 cm −3 and drops drastically thereafter. We suggest that the main emission spectra of the heavily Be-doped layers obtained at 4 K is most likely a result of band-acceptor transitions.

Preparation of p-type GaAs layers for ohmic contact

The open-tube diffusion of zinc is used for the preparation of p-type layers in gallium arsenide. The process provides the capability of forming reproducible deep diffusions (0.4–2 Μm) over short diffusion times and in the temperature range 600–650 ‡C. TiPdAu contacts are prepared on these layers. Using cw laser annealing we have obtained specific contact resistance 6.5 × 10−7Ωcm2.

Performance comparison of metalorganic vapor phase epitaxy grown C-doped GaAs/AlGaAs heterojunction bipolar transistor wafers between AsH 3 and trimethylarsenic for the base layer growth

C-doped GaAs layers were grown with AsH 3 and As trimethylarsenic (TMAs) for the As source by a production scale metalorganic vapor phase epitaxy machine. In both cases, highly ( p = (1−4) x 10 19 cm -3) p-type C-doped GaAs layers were grown and no significant difference in their mobilities, carbon activation ratios and electron lifetimes were observed. By means of these methods, GaAs/AlGaAs heterojunction bipolar transistor (HBT) layers were grown and HBT test devices were fabricated. Current gains up to the base layer concentration of 3 x 10 19cm -3 were almost equal for these two methods, but at a concentration of p = 4 x 10 19 cm -3, the current gain of HBT whose base layer was grown with AsH 3 decreased rapidly. On the other hand, a current gain as high as 100 was demonstrated for a HBT whose base layer was grown with TMAs and was 4 x 10 19 cm -3 in concentration.

Precipitation of arsenic in doped GaAs

Precipitation processes in the p-type, n-type, and intrinsic GaAs layers grown by molecular beam epitaxy at a low temperature were studied by transmission electron microscopy. The average spacing, average diameter, and volume fraction of precipitates were measured as a function of the annealing time for the annealing temperature of 700°C. Volume fractions of precipitates are nearly constant in each layer over the period of annealing, implying that the precipitation process has reached the coarsening stage in the annealing times used in the study. The volume fraction of precipitates in the n-type layer is about a half of those in the p-type and intrinsic layers, suggesting that the incorporation rate of excess As into the n-type layer during the growth is lower than those into the p-type and intrinsic layers. Despite a large difference of amounts of excess As in as-grown n-type, p-type, and intrinsic layers, the average spacings and, hence, number densities of precipitates in three layers are nearly identical for each of the annealing conditions.

Reliability of Implant-Isolation Regions in Highly-Doped GaAs-Based Structures

ABSTRACTHighly-doped n-type (2 × 1019 cm−3) and p-type (7 × 1019 cm−3) GaAs layers were implanted with multiple energy F+ or H+ ions and subsequently annealed at ∼550°C to produce high resistance regions. The resistance of these layers at 200°C was monitored as a function of time and showed no changes for periods of several months. This is in contrast to some previously reported data, which showed complete loss of isolation after a period of several days in proton-implanted n+ GaAs. Here, we show that isolation of heavily-doped near-surface regions by protons requires implantation through cap layers which place the peak of the nuclear stopping profile at the GaAs surface. The application of these results to isolation of thick, multilayer heterostructure devices is discussed and SIMS and TEM data are used to illustrate the redistribution of the implanted ions and the defects they create, respectively.