B.K. Panda

Semi-insulating GaAs: A possible substrate for a field-assisted positron moderator

Positron lifetime spectroscopy measurements have been carried out for semi-insulating GaAs with applied electric fields in the samples directed towards, and away from the positron injecting contact. The lifetime spectra have been decomposed into two components, the longer of which (≈400 ps) is characteristic of open volume defects at the metal-semiconductor interface through which positrons are injected. The interesting feature of these experiments is the large increase in the intensity of this interface component as the field is directed towards the contact. We show that this increase is caused by a significant fraction of implanted positrons drifting under the influence of a strong electric field produced by a layer of space-charge formed adjacent to the positron injecting contact. The general trend of the intensity variation is well explained by the proposed model. Experiments involving the application of an ac bias to the samples strengthen the suggestion that the space charge region is largely formed from ionized EL2 donors. The results of the present work indicate that semi-insulating GaAs possesses properties that make it a suitable material for the fabrication of a high efficiency (≈10%) room-temperature field-assisted positron moderator. The extraction of positrons from the GaAs substrate into the vacuum through a thin metalization is discussed based upon available positron affinities for the GaAs and various elemental metals. These data suggest that a few monolayers of a strongly electronegative metal such as Au or Pd may allow vacuum emission through quantum tunneling.

On the possible identification of defects using the autocorrelation function approach in double Doppler broadening of annihilation radiation spectroscopy

The recent revived interest in the use of double-Doppler broadening of annihilation radiation (D-DBAR) spectroscopy, which employs two Ge detectors in back-to-back geometry, has stemmed mainly from its potential in defect identification as a result of its elemental sensitivity through core annihilations in atoms at the defect site. Emphasis has thus largely concentrated on the high momentum spectral range. In contrast the present work emphasizes the need to also focus attention on the low momentum region of the D-DBAR spectra. It is argued that the improved resolving power of D-DBAR, in conjunction with spectral deconvolution, should give future 1D (one dimensional) momentum data approaching in quality those obtainable using 1D-ACAR (angular correlation of annihilation radiation), thus forming an alternative technique for observing the structure containing diffraction patterns that originate from annihilations with localized electron states at positron trapping defects. Rotation of the sample about a specified crystal axis, and the binning of events by angle, is suggested as a means of extending the technique to form a 2D- (two dimensional) DBAR counterpart to 2D-ACAR. The advantages of considering the real space positron electron wavefunction product AF (autocorrelation function), obtained by simple manipulation of the D-DBAR data in Fourier space, are outlined. In particular the possible visualization offered in real space of a defects physical geometry, with the prospect of building up a library of contour patterns for future defect identification, is discussed, taking the silicon monovacancy in Si and the negative As vacancy in GaAs as examples.

Application of Fourier series methods for studying tunnelling of electrons out of quantum wells in an electric field

The electron energy shifts and mean tunnelling lifetimes in a quantum well with different potential profiles under an applied electric field are calculated using Fourier series methods. In the present work, single-rectangular, symmetric double, and diffusion-modified quantum wells have been taken into account. The mean lifetime at high field strength obtained from the time evolution of the wave packets is found to be lower than that obtained from the conventional standing-wave approach.

Positron deep level transient spectroscopy — a new application of positron annihilation to semiconductor physics

Abstract Recent positron mobility and lifetime measurements made on ac-biased metal on semi-insulating GaAs junctions, which have identified the native EL2 defect through a determination of the characteristic ionization energy of the donor level, are reviewed. It is shown that these measurements point towards a new spectroscopy, tentatively named positron-DLTS (deep level transient spectroscopy), that is the direct complement to conventional DLTS in that it monitors transients in the electric field of the depletion region rather than the inversely related depletion width, as deep levels undergo ionization. In this new spectroscopy, which may be applied to doped material by use of a suitable positron beam, electric field transients are monitored through the Doppler shift of the annihilation radiation resulting from the drift velocity of the positron in the depletion region. Two useful extensions of the new spectroscopy beyond conventional capacitance-DLTS are suggested. The first is that in some instances information on the microstructure of the defect causing the deep level may be inferred from the sensitivity of the positron to vacancy defects of negative and neutral charge states. The second is that the positron annihilation technique is intrinsically much faster than conventional DLTS with the capability of observing transients some 10 6 times faster, thus allowing deep levels (and even shallow levels) to be investigated without problems associated with carrier freeze-out.

Quantum confined Stark shift in arbitrarily shaped single quantum wells by Fourier series method

A new technique for obtaining energies and wavefunctions in the Fourier series method in the as-grown and annealing induced diffusion modified single quantum wells under applied d.c. electric field perpendicular to the growth direction is presented. The ground state and the first excited energies and wavefunctions are compared with those obtained in the inverse power method. Both methods give energies and wavefunctions in complete agreement with each other.

Depolarization and exciton-like shifts in modulation-doped single rectangular and diffusion-modified quantum wells under uniform electric fields

Energy levels in the modulation-doped single rectangular and diffusion-modified quantum wells A10.3Ga0.7As/GaAs are obtained using the Kohn and Sham density functional theory (DFT). The generalized DFT which uses a screened exchange potential in the local density approximation has been used to improve the accuracy of the energies and wavefunctions in these wells. The energy difference between the first excited state and the ground state has been studied in the presence of combined photon and electric fields for two wells with well widths of 70 A and 100 A. The energy difference is found to increase as a result of the depolarization and exciton-like shifts when there is no external bias. Under the electric field the shift in the energy difference decreases due to the vanishing of the exciton-like effect. The results obtained within the conventional Kohn and Sham DFT and within the generalized DFT are found to be the same for the well with a width of 100 A. However, the results obtained from these two schemes are found to be different when the well width is 70 A.

Positron lifetime analysis using the matrix inverse Laplace transformation method

Abstract The generalized least squares matrix formalism method with non-negativity constraint is developed to invert positron annihilation lifetime spectra to positron annihilation rate (PAR) spectra. The method, which employs an instrumental resolution function (IRF) obtained from a single component positron lifetime spectrum, has the advantage of not requiring any initial guess of non-linear fitting parameters. The present code is tested using computer simulated positron lifetime data and some experimental spectra.

Calculation of the momentum distributions of positron annihilation radiation in Ge

One- and two-dimensional momentum distributions of positron annihilation radiation in Ge are calculated using the independent-particle model (IPM), the local density approximation (LDA) and the generalized gradient approximation (GGA). It is found that these three approximations are all capable of reproducing the main structures of the momentum distributions observed in the one- and two-dimensional angular correlation of positron annihilation radiation measurements. The positron annihilation rate is, however, found to be overestimated in the LDA in the low-momentum regime. While the GGA alone gives the correct total annihilation rate, it is found that the GGA is essentially just scaled with the LDA, and introduces no new momentum dependence to eliminate the overestimation of the LDA in the low-momentum regime.

Positron lifetime analysis from generalized constrained least-squares technique

The generalized least-squares method with non-negativity constraint is developed to analyze positron annihilation lifetime (PAL) spectra to obtain lifetimes and intensities. The method for obtaining the instrumental function from a single-component PAL spectrum is demonstrated. The present code is tested using computer simulated lifetime data.

Doppler-broadening measurements of microvoids at the Au/GaAs interface

Recent positron lifetime studies made on the Au/GaAs interface with an applied electric field returning a significant fraction of bulk implanted positrons to the interface have revealed the presence of microvoids(≈ 1 nm diameter) at the interface. In this work an attempt has been made to study these microvoids by observing the Doppler broadening on the annihilation radiation coming from them. This is done both by observing theS-parameter as a function of applied bias and by applying the generalized least-squares method to the deconvolution of the annihilation radiation lineshape. The general conclusion is that the Doppler-broadened data are consistent with the majority of positrons trapping into microvoids, probably associated with grain boundaries. The data suggest that these open volume defects are more associated with the Au film rather than the Au-Ga alloyed interfacial region.