Unimplanted GaAs Research Articles

Influence of Post-Implantation Annealing Parameters on the Focused Ion Beam Directed Nucleation of InAs Quantum Dots

We present the effect of post-implantation annealing conditions on the structural and optical quality of InAs quantum dots (QDs) grown by combination of focused ion beam (FIB) and molecular beam epitaxy (MBE) approach. A FIB of Ga + ion was employed to pattern a homogeneously GaAs buffer layers and then, an in situ annealing step followed by InAs deposition was performed. Three different post-implantation annealing conditions were tested and under well-optimized conditions, a dislocation and defect-free InAs QDs growth on FIB patterned surface was successfully achieved. Furthermore, using photoluminescence (PL) study, we demonstrate that our best sample shows almost similar optical quality as MBE grown QDs on unimplanted GaAs surface. The patterning technique described here can presumably be applied to systems other than InAs / GaAs and highly interesting for site-controlled nucleation of QDs that finds its potential applications in nanooptoelectronic devices.

Investigation of indium diffusion process in In + ion implanted GaAs

Rutherford backscattering (RBS) has been used to determine diffusion coefficients of indium atoms in semi-insulating (1 0 0) GaAs implanted with 250 keV In + ions at a fluence of 3×10 16 cm −2 and isobarically annealed at 600 and 800 °C temperature. Computer modeling of the indium depth profiles based on Ga vacancy-mediated diffusion has shown good agreement with the RBS experimental results. Self-annealing effect was confirmed to play an important role due to high defect mobility at room temperature ion implantation. Large values of the In diffusion coefficients obtained in the present study, in comparison with the literature data for unimplanted GaAs, confirms strong enhancement of the diffusion process by radiation damage.

High-energy ion implantation: an alternative technology for micromachining three-dimensional GaAs structures

We present an alternative technology to micromachine gallium arsenide (GaAs) using deep ion implantation for MEMS applications. Energetic low dose nitrogen ions were used to implant deeply into n-type GaAs substrate. After annealing at 600 °C the nitrogen implanted n-GaAs top layer was converted to semi-insulating GaAs. A pulsed electrochemical etching process was used to selectively remove n-GaAs and leave the top patterned semi-insulating GaAs layer as mechanical membrane structures. Various GaAs microstructures, such as cross-bridge, coiled, and corrugated membranes have been successfully fabricated using this unique micromachining technology. By increasing the ion doses, the GaAs will be heavily damaged. The damaged GaAs shows much higher etch rate compared to the un-implanted GaAs using proper etch solutions and can be, therefore, used as sacrificial layer. A unique capacitive pressure sensor fabrication technology using AlGaAs/GaAs wafer is proposed, where the air gap between two electrodes is formed by releasing the locally damaged GaAs.

Structural, electrical, and optical studies of GaAs implanted with MeV As or Ga Ions

The structural properties of GaAs implanted with high doses of 2 MeV arsenic or gallium ions with subsequent annealing at different temperatures were studied by transmission electron microscopy, Rutherford backscattering spectrometry-channeling, double crystal x-ray diffraction. Optical absorption, electrical conductivity, Hall effect and time-resolved photoluminescence were applied to monitor changes in electrical and optical characteristics of the material. An important conclusion from this investigation is that there was hardly any difference between materials implanted with gallium or arsenic. For implantation of either species, a large number of point defects was introduced and for a high enough dose a buried amorphous layer was formed. Hopping conduction and high absorption below band-to-band transition were observed for both cases. After low temperature annealing of the amorphous material, a high density of stacking faults and microtwins were found. Regrowth rates at the front and back amorphous-crystalline interfaces showed a significant difference. This was attributed to differences in local nonstoichiometry of the material at the upper and lower amorphous-crystalline interfaces. Structural studies showed the presence of some residual damage (a band of polycrystalline material in the center of the regrown area) with some associated strain even after annealing at high temperatures. Recovery to the conduction band transport in annealed samples was observed but mobilities, of the order of 2000 cmWs, were still smaller than in unimplanted GaAs. These results show that, in as-implanted material and even after annealing at lower temperatures, the point defects introduced by the implantation are responsible for the very short photocarrier lifetime.

Nonlinear optical response of As+-ion implanted GaAs studied using time resolved reflectivity

The transient reflectivity response of GaAs with a 2% excess As concentration (GaAs:As), prepared by As+ ion implantation, has been measured at photon energies near the band gap. Results are compared with similar measurements on implanted GaAs with a 0.01% excess As concentration, and unimplanted GaAs. For GaAs:As, the transient refractive index change Δn, is larger than, but of the opposite sign to that of unimplanted GaAs. The measured carrier lifetime of 1±0.1 ps is identical to that of low-temperature GaAs. The wavelength dependence of Δn indicates the presence of an induced absorption peak at photon energies near the band gap, which is attributed to band-gap renormalization.

Formation of GaAsP by ion implantation and annealing

Surface layers of GaAsP have been formed by a high dose, shallow implantation of phosphorus into GaAs. Al Schottky contacts on these layers have a higher barrier height (1.04 eV) than similar contacts on unimplanted GaAs. Electron channeling patterns in the scanning electron microscope, infrared absorption measurements, and Auger electron spectroscopy show that a single crystal with a surface composition of GaAs0.83P0.17 is formed.

Optical determination of exciton sizes in semiconductors

We have studied the gradual disappearance of Ga3d core-exciton-related features in energy derivative reflectance spectra of GaAs crystals ion-implanted to increasing fluences. Our data yield radii rL = 23 Å and rx = 28 Å for the L and X excitons. In addition, a lineshape fit for unimplanted GaAs yields core exciton binding energies EL = 0.172 eV and Ex = 0.129 eV. These radii and binding energies are not related through an effective mass approximation description of the core exciton.

Effect of electric fields on Cr redistribution at GaAs surfaces

During the annealing of ion-implanted Cr-doped GaAs, Cr often redistributes and accumulates at the surface. Although this behavior has been attributed to strain fields and other mechanisms, the widths of these accumulation regions suggest that electric fields due to surface states are a limiting factor in Cr redistribution. For this reason we have developed a thermodynamic model for Cr redistribution which takes into account the electric field due to surface states. A qualitative fit to SIMS data on annealed unimplanted GaAs samples can be obtained with this model. We have also used applied voltages during annealing to modify the amount of band bending and Cr buildup at the surface. This experiment indicates that the accumulated ions are positively charged. We conclude from these experiments that electric fields play a significant role in the redistribution of Cr at GaAs surfaces.

Electrical Defect Analysis Following Pulsed Laserirradiation of Unimplanted GaAs

ABSTRACTCurrent-voltage, capacitance-voltage and defect spectroscopy techniques are used to characterize the electrical properties of GaAs crystals after pulsed laser irradiation with either a Nd-YAG or a Ruby laser. I(V) and C(V) measurements performed in conjunction on Au/GaAs Schottky structures after laser irradiation at low energy density show an important barrier lowering, of the order of 300mV. Carrier compensation up to 6×lO16/cm3 is observed in a subsurface layer whose thickness increases with deposited laser energy density. D.L.T.S. is used to study the tail of laser induced defects behind the heavily compensated layer. Finally the results are compared to those obtained following conventional thermal treatment.