Temperature dependence of the positronium work function in aluminum

The electron and positron chemical potentials and their volume derivatives are calculated in Al using the first-principles pseudopotential technique and the frozen-core approximation method, respectively. The positron deformation potential of -8.03 eV obtained by this method agrees very well with that found in other first-principles calculations. The variation of the positronium work function with temperature is calculated by including the thermal lattice vibrations into the electron and positron potentials. It is observed that the temperature dependence of the positronium work function does not arise entirely from the volume expansion of the lattice as has been suggested in the past. Proper inclusion of the nonzero-temperature derivative of the positronium work function at constant volume leads to good agreement between the positron deformation potential as obtained from positronium work function data and present theoretical values. © 1994 The American Physical Society.published_or_final_versio

Isochronal annealing studies of n-type 6H-SiC with positron lifetime spectroscopy

n-type 6H silicon carbide has been studied using positron lifetime spectroscopy with isochronal annealing temperatures of 400, 650, 900, 1200, and 1400 °C. In the as-grown sample, we have identified the VSi vacancy, the VCVSi divacancy, and probably the VC vacancy. The silicon vacancy and the carbon vacancy were found to anneal out in the temperature range 400-650 °C. The VCVSi divacancy was found to persist at an annealing temperature of 1400 °C.published_or_final_versio

Design and implementation of a real-time positron imager

In this paper we are going to present the first real-time S-parameter positron imager. This is a useful tool in solid state technology for mapping the lateral defect types and concentrations on a material sample. This technology has been developed for two major categories of researchers, the first being those that have a focused low energy positron beam and second those that do not. Here we describe the design and implementation of a real-time automated scanning system that rasters a sample surface with a 0.5mm diameter positron source (or beam focus) so as to give an S-parameter image of a sample. The source (or beam) rasters across a region of a semiconductor sample in rectilinear motion while gamma ray energies E γ are processed using a standard HP Ge spectroscopy system and a 14 bit nuclear ADC. Two other ADCs are used to obtain the x, y coordinate data corresponding to each event by storing voltage pulses from the x & y stepper motor drives (or saddle coil currents) gated with the event pulses. Using these event data triplets (x, y, E γ) the S-parameter is computed in real time for each pixel region and is used to refresh a color image display on the screen coordinates. Optimal use is made of processing time and the system resources. This user-friendly system is efficient for producing high resolution S-parameter images of the sample, (patent pending 2003).published_or_final_versio

Nature of the bulk defects in GaAs through high-temperature quenching studies

Deep-level transient spectroscopy has been applied to n-type horizontal gradient freeze grown GaAs that has been subjected to thermal stressing (quenching) and varying degrees of arsenic outdiffusion during rapid thermal annealing. The concentrations and activation energies of the various deep donor levels have been monitored. As a result of the external excitations in the lattice due to the thermal stress (quenching), dramatic effects occur in the defect level structure that could be of importance to device technology. It is found that the native EL6 group of defects is nearly absent in rapid thermally annealed material, while the levels EL5 and EL8 appear with EL3 becoming a dominant level that could act as a recombination center. With the lengthening of annealing time and significant As outdiffusion, there is a general reduction of the EL2, EL3, and EL5 defect concentrations together with a complete removal of EL8. Moreover, the EL2 activation energy may be varied from 0.827 to 0.922 eV by controlling the level of As out-diffusion. These observations are discussed in terms of the As Ga-As i model of the EL2 defect and the V As-V Ga divacancy model for the EL6 group of defects. The EL3, EL5, EL8, and EL15 defect levels seen in samples subjected to rapid thermally quenching are attributed to the breakup of V As-As i Frenkel pair defects known to be present in the as-grown material.published_or_final_versio

A simple and inexpensive circuit for emission and capture deep level transient spectroscopy

A simple and inexpensive circuit for deep level transient spectroscopy is described, which allows rapid characterization of emission as well as capture activation energies of deep levels. This flexibility of making capture activation studies affords more information on defect morphology than the more standard emission activation studies. This is demonstrated by making a representative capture activation energy measurement on the EL6 level in undoped n-type GaAs of 0.484±0.005 eV. Also the spectrometer has shown better performance than earlier reported systems by its ability to resolve the side peaks of the EL6 level, for which emission activation energies of 0.29 and 0.4 eV are assigned. Constructed around a commercially available capacitance meter and pulse generator, the control circuitry is designed and developed using inexpensive and off-the-shelf integrated circuits. © 1996 American Institute of Physics.published_or_final_versio

Electron-positron momentum density in diamond, Si, and Ge

The electron-positron momentum densities in diamond. Si, and Ge are calculated using a first-principles method. Comparison of the theoretical momentum densities with the experiment shows that the electronpositron correlation effects are very important in Si and Ge, while this effect is negligible in diamond because the electrons are tightly bound. Our analysis shows that only the upper two bands, which consist of the sp3 hybridized orbitals, contribute to the structures at the low-momentum region of the momentum density. Diamond does not show these structures at the low-momentum region is due to its smaller lattice constant and weak electron-positron correlation effects.published_or_final_versio

Inexpensive circuit for the measurement of capture cross section of deep level defects in semiconductors

A simple and inexpensive circuit to facilitate the direct measurement of capture cross section, when synchronized with a deep level transient spectroscopy system, is described. It avoids the most commonly encountered problem of loading and distortion of the bias (trap filling) pulses of nanosecond duration in the capture cross-section measurement. The capacitance meter, whose internal circuitry is responsible for the distortion, is connected and disconnected from the rest of the apparatus with the help of simple and low-cost reed relay switches featuring high operating speed and low contact resistance. Sharp bias pulses as small as 30 ns can successfully be applied to the sample with no observable distortion. Finally, a representative measurement is shown to demonstrate the simplicity and high performance of the circuit. © 1996 American Institute of Physics.published_or_final_versio

Stark shift and field induced tunneling in doped quantum wells with arbitrary potential profiles

The energies and resonance widths of single doped quantum wells consisting of AlGaAs/GaAs with rectangular and annealing induced diffusion modified shapes are calculated under an uniform electric field using the stabilization method. The electronic structure is calculated without an electric field in the finite temperature density functional theory with exchange-correlation potential treated in the local density approximation. Our scheme for solving the Schrodinger and Poisson equations is based on the Fourier series method. The electric field is added to the self-consistent potential and energies are obtained as a function of the combined width of the well and barriers. This yields us the stabilization graph from which the energies and resonance widths at different field strengths are extracted using the Fermi Golden rule.published_or_final_versio

Electric field effect on the diffusion modified AlGaAs/GaAs single quantum well

The electron subband energies and wave functions in an interdiffusion-induced Al xGa 1-xAs/GaAs/Al xGa 1-xAs single quantum well are calculated in the presence of the dc electric field using the finite difference method. The mean lifetimes are obtained from the time-dependent probability of tunneling of the wave packet out of the well by the applied electric field. The effect of the applied electric field on the subband energies in the well is the same as in the as-grown square quantum well when the interdiffusion length is below 20 Å. In the well with higher diffusion length the barrier height reduces so that the wave function tunnels out of the well. The linear and nonlinear intersubband absorption coefficients and the change in the real part of the index of refraction are calculated with the applied electric field at 100 kV/cm and without the field in both the as-grown square well and the diffusion modified well with the interdiffusion length at 20 Å. © 1996 American Institute of Physics.published_or_final_versio

Saturated electric field effect at semi-insulating GaAs-metal junctions studied with a low energy positron beam

The interfacial electric field established under different reverse bias conditions in Au and Ni on semi-insulating GaAs junctions has been studied by means of a low energy positron beam. The technique used is that of monitoring the positron drift to the interface through changes in the annihilation radiation lineshape as a function of incident positron beam energy at different reverse biases The data show a small but clear electric field drift of positrons towards the interface that increases more rapidly at low voltages (less than 50 V) which at higher biases tends towards saturation This confirmation of electric field saturation adds further weight to the picture of an electric field enhanced electron capture cross section for the ionized EL2 defect. Electric field values extracted from the data are compared with results from other techniques and suggest that enhanced electron capture is already occurring at the relatively low built-in fields (∼1 kV cm-1) found at the unbiased junction, with a rapid increase of EL2+ neutralization occurring for biases above 10 V. At still higher fields ∼10 kV cm-1 (biases>50 V), there appears to be an additional threshold for more complete EL2+ neutralization adjacent to the contact. The present study clearly demonstrates the often overlooked necessity of catering for built-in electric fields in positron diffusivity studies of III-V semiconductors where surface midgap Fermi-level pinning is common. © 1997 American Institute of Physics.published_or_final_versio