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Positronium (Ps) formation in high-density polyethylene (HDPE) has been studied below the glass transition temperature. The formation probability increases with positron irradiation time due to an increasing number of inter-track trapped electrons becoming available for positron capture. The temperature variation of the saturated Ps level is discussed in different models. The quenching of trapped electrons by light has been studied and the optical de-trapping cross-section for different photon energies has been estimated over the visible region.

Coincidence doppler broadening measurements on electron-irradiated polyurethanes were performed in the presence of air. It is shown that, after a certain electron irradiation, the momentum density distributions of annihilation electrons have obvious changes for the high crosslinking polyurethane, but no significant changes have been observed for the low crosslinking polyurethane. The results were performed to analyse by irradiation crosslinking and degradation principles.

Coincident Doppler Broadening Spectroscopy (CDBS) measurements are popular in positron solid-state studies of materials. By utilizing the instrumental resolution function obtained from a gamma line close in energy to the 511 keV annihilation line, it is possible to significantly enhance the quality of the CDBS spectra using deconvolution algorithms. In this paper, we compare two algorithms, namely the Non-Negativity Least Squares (NNLS) regularized method and the Richardson–Lucy (RL) algorithm. The latter, which is based on the method of maximum likelihood, is found to give superior results to the regularized least-squares algorithm and with significantly less computer processing time.

Vacancy-sensing positron deep level transient spectroscopy (PDLTS) is a positron beam-based technique that seeks to provide information on the electronic ionization levels of vacancy defects probed by the positron through the monitoring of thermal transients. The experimental discoveries leading to the concept of vacancy-sensing PDLTS are first reviewed. The major problem associated with this technique is discussed, namely the strong electric fields establish in the near surface region of the sample during the thermal transient which tend to sweep positrons into the contact with negligible defect trapping. New simulations are presented which suggest that under certain conditions a sufficient fraction of positrons may be trapped into ionizing defects rendering PDLTS technique workable. Some suggestions are made for techniques that might avoid the problematic electric field problem, such as optical-PDLTS where deep levels are populated using light and the use of high forward bias currents for trap filling.

There are many advantages in being able to perform positron annihilation lifetime spectroscopy (PALS) using a variable energy positron beam, the most obvious being the easy identification of different defect types at different depths. The difficulty in conducting variable energy (VE) PALS studies lies in the fact that a “start” signal is required to signal the entry of the positron into the target. Two methods have been used to overcome this problem, namely the bunching technique, which employs radio frequency (RF) cavities and choppers, and secondly the use of secondary electrons emitted from the target. The latter technique is in terms of experimental complexity much simpler, but has in the past suffered from poor time resolution (typically ∼500 ps). In this work, we present a series of computer simulations of a design based on the secondary electron emission from thin C-foils in transmission mode which shows that significant improvements in time resolution can be made with resolutions ∼200 ps being in principle possible.

The University of Hong Kong positron beam employs conventional magnetic field transport to the target, but has a special hybrid lens design around the positron moderator that allows the beam to be focused to millimeter spot sizes at the target. The good focusing capabilities of the beam are made possible by extracting work-function positrons from the moderator in a magnetic field free region using a conventional Soa lens thus minimizing beam canonical angular momentum. An Einzel lens is used to focus the positrons into the magnetic funnel at the end of transportation magnetic field while at the same time bringing up the beam energy to the intermediate value of 7.5 keV. The beam is E × B filtered at this intermediate energy. The final beam energy is obtained by floating the Soa–Einzel system, E × B filter and flight tube, and accelerating the positrons just before the target. External beam steering saddle coils fine tune the position, and the magnetic field around the target chamber is adjusted so as to keep one of the beam foci always on the target. The system is fully computer controlled. Variable energy-Doppler broadened annihilation radiation (VEDBAR) data for a GaN sample are shown which demonstrate the performance of the positron beam system.

The formation and evolution of vacancies and precipitates created by implantation of 60 keV, 2 × 10 16 cm −2 Sb + in pre-amorphized (0 0 1) Cz–Si is studied using the Doppler broadening (DB) and two-dimensional angular correlation of annihilation radiation (2D-ACAR) positron beam techniques. After implantation, samples were laser annealed (LTA) and subsequently thermal annealed at temperatures ranging from 400 to 1000 °C. Implantation-induced vacancy-type defects were detected up to a depth of 280 nm. After LTA, positron annihilation related to both Sb and remaining defects is observed in the first 100 nm below the surface. The deeper region only shows positron trapping at vacancy-type defects with strong reduced concentration. Complete removal is obtained after 600 °C anneal. At this temperature, the positron data for the upper region reveals trapping at Sb and Si sites only. With increasing annealing time (at 600 °C) or increasing temperature (up to 1000 °C) positron annihilation at Sb-sites associated with neighboring vacancies becomes apparent. Results are correlated with the observed Sb electrical deactivation above 600 °C, the shift from small Sb aggregates to precipitates and out-diffusion of Sb from the implantation region at higher temperatures.

Two least squares minimization methods for the deconvolution of 1D Doppler Broadening Annihilation Radiation Spectroscopy (DBARS) spectra have been tested with spectra generated by Monte Carlo simulation according to the following functional forms: inverted triangle, inverted parabola, Laplace, Lorentz and a model DBARS spectrum for a metal composed of an inverted parabola and a Gaussian function. These reference spectra were firstly convoluted with a Gaussian broadening factor and then restored to its original form with the algorithms. The method with Tikhonov regularizer and non-negativity constraint still failed to restore the sharp features of these spectral functions although the negative signal found in an earlier study was removed. On the other hand, the method with the Huber regularizer was successful. Optimization of the deconvolution in terms of regularization parameters is necessary to achieve good deconvolution. The optimization of the deconvolution was checked with visual matching and a quality factor which takes into account the number of counts in the spectrum.

Electrical characteristics of Au/n-GaN Schottky contacts with different Au film thicknesses up to 1300 A, have been investigated using current–voltage (I–V) and capacitance–voltage (C–V) techniques. Results show a steady decrease in the quality of the Schottky diodes for increasing Au film thickness. I–V measurements indicate that thin ( 500 A). Depth profiling Auger electron spectroscopy (AES) shows that the width of the Au/GaN junction interface increases with increasing Au thickness, suggesting considerable inter-mixing of Au, Ga and N. The results have been interpreted in terms of Ga out-diffusion from the GaN giving rise to gallium vacancies that in turn act as sites for electron–hole pair generation within the depletion region. The study supports the recent suggestion that gallium vacancies associated with threaded dislocations are playing an important role in junction breakdown.

The slow positron yield has been measured for various tungsten (W) moderator samples from a 22 Na radioactive source. Multi-folded W mesh, W(1 0 0) single crystal foil and W polycrystalline foil samples have been investigated. It is found that the fast to slow conversion efficiency of the W mesh moderator depends on: (1) the annealing pretreatments, (2) the chemical etching duration and (3) the number of the folding layers. With the raw W mesh material having a wire diameter of 20 lm and transmission efficiency of 92.5%, an optimal efficiency of 1.2 · 10 � 3 was achieved with 5 min etching duration and a folding number of 12 layers.

Recently, a room temperature electrically induced metastability in semi-insulating (SI)-GaAs has been reported in which the normally high resistance state of SI-GaAs converts into a low resistance state when breakdown electric fields are applied to the metal/SI-GaAs/metal system. The low resistance state continues to persist when the electric field is lowered below the breakdown bias and as such may be treated as metastable state of the material. This phenomenon is believed to be closely related to the ‘lock-on’ effect utilized in high power photoconductive semiconductor switches made from SI-GaAs. The present study seeks to understand the mechanism for this electrically induced metastability by studying the influence of the injection current on the metastable phase. The data favor an interpretation of the high current state of the SI-GaAs in terms of double carrier injection and the sustaining of an internal electron–hole plasma in the material.

A room temperature electrically induced metastability in semi-insulating (SI)-GaAs has recently been reported in which the normally high resistivity state of SI-GaAs converts into a low resistance state when breakdown electric fields are applied to the metal/SI-GaAs/metal system. The present study finds that this post-breakdown metastable state can be classified into two metastable phases, namely a metastable high current phase and a metastable low current phase. This effect resembles the poorly understood ‘lock-on’ effect utilized in high power photoconductive semiconductor switches. It is argued that instead of a short pulse of light photo-ionizing the SI-GaAs and causing a carrier avalanche current that does not ‘switch off’ at low biases, the same avalanche current and effects are being brought about by the electrical breakdown of the sample.

The slow positron beam facility at the University of Hong Kong has been used to study the Cu/6H–SiC(0 0 0 1) system. The S–E data show the presence of the Cu/SiC interface buried at a depth of 30 nm. Keeping the beam energy fixed and sweeping the sample temperature, sharp discontinuities are noted in the S-parameter at both ∼17 and ∼250 K. The S-parameter transitions, which are in opposite directions, are indicative of sharp free volume changes that come as a result of the sudden changes in the structure at the Cu/SiC interface accompanying some phase transition. Energy dispersive X-ray spectroscopy (EDXS) room temperature scans reveal the presence of O in addition to Cu, C, Si at the interface, and thus copper oxide phases should be considered in interpreting this new phenomenon. It is suggested that TEM investigation together with temperature dependent X-ray diffraction spectroscopy may be able to shed further light on the nature of this interesting bi-directional phase transition.

This paper will review progress being made on developing more defect selective forms of positron annihilation spectroscopy (PAS) at the University of Hong Kong. The first of these, positron deep-level transient spectroscopy (PDLTS), parallels conventional deep-level transient spectroscopy (DLTS) except that the positron is used as the probe, either to tell if the defects have vacancies attached to their microstructure (type I PDLTS) or as a simple electric field probe (type II PDLTS). It is shown the more important type I PDLTS has an intrinsic problem brought about by the high donor densities required to operate electrical trap filling. The problem—namely fast positron drift out of the active deep-level region into the reverse biased junction—is suggested as having two solutions. The first is to move to higher positron beam intensities and take spectra of 10 9 events. The second is that by using lower dopant densities ( 15 cm −3 ) deep levels may be filled by inter-band optical excitation thus forming the workable technique positron optical (PO)-DLTS. Other techniques briefly considered in this paper are deconvoluted-coincidence Doppler broadening spectroscopy (CDBS) and Fourier transform (FT)-CDBS. Such are seen from a different perspective than most contemporary works, which tend to concentrate on the high momentum region. It is pointed out that the additional root-of-two improvement in hardware resolution and the factor of three improvement gained through deconvolution, can produce final effective resolutions similar to ACAR. Moreover, since in deconvolution, the natural space for regularized solutions is “real crystal space”—it is suggested that the autocorrelation function B 2 γ ( r ) be taken as the experimental CDBS data—not just because it provides easier visualization,—but because data in this form lies directly on the crystal lattice.

A compact mono-energetic positron beam system has been constructed at The University of Hong Kong in order to perform positron re-emission measurements, after fast positrons from a radioactive 22Na source moderated by a variety of samples. The positron re-emission measurements are performed at a vacuum of 10−8 Torr. Re-emitted slow positrons are guided by longitudinal magnetic fields provided by three field coils located outside the vacuum tank and are detected by a gamma photon insensitive channel electron multiplier (CEM). The radioactive source, the sample, the focusing grid, the retarding-field energy analyzer (RFA) and the CEM detector were put into a compact assembly having a length of 33 cm and a diameter of 6 cm. Positron re-emission from W(1 1 1), GaN/SiC, Ga/GaN/SiC, 4H and 6H–SiC surfaces have been studied. The moderator conversion efficiencies of these samples are reported.

At the University of Hong Kong’s positron beam two related positron defect spectroscopies are being developed, namely, positron deep level transient spectroscopy (PDLTS), and coincidence Doppler broadening spectroscopy. Both of these spectroscopies require the use of high purity Ge detectors and the standard high gain nuclear instrumentation amplifiers. In this paper we review some of the difficulties that we have encountered in making such measurements satisfactorily in one of the busiest cities of the world. Attempts, such as using battery power supply, opto-isolating from local mains, connecting to an independent ground and adding a spectrum stabilizer, have been made to overcome the noise problem. However, the results are not particularly promising. The reasons why are discussed.

The slow positron beam facility Hong Kong submillimeter electron annihilator (HOSEA) at the University of Hong Kong has now been in operation for 6 years with a beam diameter of ∼6 mm and intensity ∼10 4 e + s −1 . It is now planned to improve the facility by narrowing the beam diameter to less than 1 mm, while increasing the intensity to ∼5×10 5 e + s −1 . The motivation behind the submillimeter beam is to make it a more effective tool in the study of MOS and MS systems that are hard to fabricate in larger than millimeter dimensions. In particular the development of positron deep level transient (thermal) spectroscopy (PDLTS) could also benefit from a beam of submillimeter dimensions. While consisting of a hybrid electric/magnetic lens structure close to the moderator to produce low canonical momentum phase space, the guidance to the target is purely magnetic. The beam structure is however unusual, consisting as it does of a recurrent re-focusing of the first primary positron focus produced by the hybrid focusing lens close to the moderator. The simulation program, SIMION, is employed to simulate the trajectories of positrons through possible hybrid lens structures and optimize the working parameters of the facility. The simulations show that an outstanding focusing diameter and a repetitive focusing can be obtained all the way to the target above beam energies of 1 keV.

Beryllium has been implanted into both n- and p-type 6H–SiC with post-implantation annealing at 1600 °C. Photoluminescence (PL) measurements have been performed, and PL lines at 420.5, 431 nm, and a broad band at around 505 nm have been observed. The line at 420.5 nm is attributed to an intrinsic defect D II -center induced by beryllium implantation. The effects of excitation intensity and temperature during the PL experiments are investigated. Based on the excitation laser dependence PL result, the new doublet lines at around 431 nm are thought to be associated with beryllium related bound excitons. The broad band corresponding to the green luminescence at room temperature has been attributed to the recombination of free carriers to beryllium bound levels.

Beryllium has been implanted into both n- and p-type 6H-silicon carbide (SiC) with high and low doses. Upon subsequent annealing at 1600°C, Beryllium implantation induced deep levels have been investigated by deep level transient spectroscopy. Five deep level centers labeled as BE1–BE5 were detected from high dose beryllium implantation produced pn junctions. A comparative study of low dose beryllium implanted n-type 6H-SiC sample proved that the BE1–BE3 centers were electron traps located at 0.34, 0.44, and 0.53 eV, respectively below the conduction band edge. At the same time, the BE4 and BE5 centers were found to be hole traps situated at 0.64 and 0.73 eV, respectively, above the valence band edge. In the case of beryllium implanted p-type 6H-SiC, four hole traps labeled as BEP1, BEP2, BEP3, and BEP4 have been observed. The observed levels of the hole traps BEP1 and BEP2 at 0.41 and 0.60 eV, respectively, above the valence band agree well with those from the Hall effect data from material with beryllium acting as doubly charged acceptor. The other hole traps BEP3 and BEP4 at 0.76 and 0.88 eV, above the valence band, respectively, are thought to be due to beryllium implantation induced defects or complexes.

Beryllium was implanted into both n- and p-type 6H-SiC and the samples were subsequently annealed at 1600°C. Photoluminescence (PL) measurements were performed and PL lines at 420 and 472 nm were observed. The PL lines at around 420nm have been detected from various ion implanted SiC samples and have been attributed to transitions involving some implantation induced intrinsic defect labeled as D II . The present observation of the PL lines at 420 nm from Be implanted 6H-SiC supports the intrinsic model that D II might be a carbon-di-interstitial defect. The lines at 472 nm labeled as D 1 , in the PL spectra have previously been identified as divacancy defect (V Si - V c ). We note that it was suggested that the electron traps labeled Z 1 /Z 2 , observed in deep level transient spectroscopy (DLTS) were due to the same divacancy defect. In our experiments, while the D 1 series PL lines are prominent, DLTS results from the same samples show no Z 1 /Z 2 related peaks. The PL and DLTS results seem to be against the possibility that Z 1 /Z 2 arise from the same defect.

N-type liquid encapsulated Czochralski (LEC) undoped InP wafers are annealed between 700 and 900 C for different durations. From a large quantity of Hall measurement results, it is found that there is a Hall mobility distribution dependent on the carrier concentration of the annealed material. The lowest mobility is observed in the samples with concentration around ∼ 10 10 cm -3 . In these samples carrier mobility increases with increasing temperature which is caused by nonuniformity. Combined with results of defect investigation in the annealed material, formation of defects and their nonuniform distribution are found to correlate with this mobility distribution. The formation of defect complexes is interpreted as a reason for the high mobility observed in annealed InP.

A micron-sized water bubble can be held fixed in a glass flask using an ultrasonic pressure field, and can be made to emit picosecond pulses of light that contains all colors. This phenomenon is called single bubble sonoluminescence (SBSL). The late Prof. Julian Schwinger had put forth a possible cause for SBSL, and postulated that the SBSL light consisted of photons that are emitted in pairs. Prior to testing for Schwinger's idea of photon pairs, we conducted experiments to measure the intensity fluctuations in SBSL using single-photon counting. By controlling the relative phase between the drive and feedback signals, through a PC computer, a light-emitting water bubble in an acoustically resonant flask could be maintained, while the fluctuations in the single-photon counts were limited to the square root of the number of detected photons. We demonstrate the possibility of maintaining a long-term stabilized sonoluminescence bubble, while simultaneously keeping the bubble light intensity at the level of single-photon sensitivity. Our results explicitly show that sonoluminescence light can be stabilized for experimental investigations that test for possible photon pairs from SBSL.

A hydrogen indium vacancy complex V In H 4 in undoped and Fe-doped liquid encapsulated Czochralski (LEC) InP is measured by infrared absorption spectroscopy in wafers sliced from the seed-end, middle and tail of an ingot. The concentration of V In H 4 is found much lower in wafers sliced from the ingot tail. The concentration of V In H 4 in Fe-doped InP is higher than that of the undoped InP. The concentration change of V In H 4 in an InP ingot is qualitatively in agreement with the mass action law expectation based on defect reactions. The influence of this complex on the electrical properties of n-type LEC undoped and Fe-doped InP is discussed. The high concentration of V In H 4 in the seed-end of an InP ingot correlates with two facts. The first is the high threshold concentrations of Fe and Zn required to get semi-insulating and p-type material, respectively. The second is that there is a large thermally induced reduction of carrier concentration in seed-end InP wafers than that of wafers from the ingot tail. The results reveal the influence of V In H 4 on the thermal stability of InP material due to the fact that the bond of hydrogen complex is weak and dissociates easily upon annealing. This dissociation has a relationship with the defects formed in high-temperature annealed InP, which are involved in the electrical compensation.

We report on the use of a slow-positron beam to examine aluminium delta layers in GaAs grown at low substrate temperature (250°C) by molecular beam epitaxy (LTMBE). It is now established that LTMBE-GaAs contains an excess of arsenic which causes an increase in the lattice parameter. After annealing, this arsenic is redistributed and forms precipitates resulting in the relaxation of the lattice. Previous positron beam studies have shown that the as-grown material has a large concentration of gallium vacancies, and after annealing the S -parameter increases above the as-grown value indicating that vacancy clusters have formed. A region depleted of arsenic precipitates has been shown to form near to aluminium delta layers, and this work is the first to study the vacancy distribution associated with this depletion region. We observe that the as-grown material has a peak value of the normalized S -parameter that is ∼3.5% higher than the substrate, which is much larger than that for a single layer LT-GaAs structure (0.8–1.5%). After annealing in the range 600–800°C the S -parameter collapses down to the substrate value, which again is opposite to the LT-GaAs case and indicates that few precipitates have formed. We correlate these findings with SIMS and TEM data and propose a mechanism involving compositional disordering due to the aluminium layers.

The need to obtain more intense positron beams is an ever-pressing concern in experimental positron physics. The present paper explores one route of obtaining better conversion efficiencies, namely by use of the field assisted positron technique. The basic principles of field assisted moderation are first reviewed. Theoretical estimates of possible expected positron yields from various semiconductor based field assisted moderators are then presented, the general aim being to give some guidance to experimentalists seeking to test such potential moderator systems. The calculations are based on positron affinities calculated using density functional ab initio pseudopotential theory and are found to be in good accord with experimental data where such exist. The work suggests that wide-gap materials such as GaN, C and SiC tend to compress the positron wavefunction leading to higher energy positron band states that favor positron emission. However, these materials also suffer from short positron lifetimes and low densities. In contrast, materials such as GaP, GaAs and ZnSe are also favored as positron emitters albeit with lower emission energy, but these materials have the advantage of longer lifetimes and higher densities that favor more efficient moderation.

We have grown a multilayer structure of GaAs and Al x Ga 1− x As ( x =0.25) by molecular beam epitaxy at low substrate temperature (250°C) in order to investigate the effects of annealing on arsenic precipitation and vacancy-defect formation. The as-grown heterostructure contained ∼5×10 17 cm −3 gallium vacancies, but information about the individual layers was lost because the layer width (∼45 nm) was smaller than the average positron diffusion length (∼70 nm). Annealing at 500 and 600°C showed increases in the S -parameter (above the bulk value) of ∼2.5% and ∼1.5%, respectively, smaller than for a single LT-GaAs layer (∼3–4%). We explain this phenomenon by suggesting that the excess arsenic which migrates from the material with the higher precipitate/matrix energy (LT-Al x Ga 1− x As) reduces the gallium vacancy concentration in the LT-GaAs and hence the S -parameter. This hypothesis is supported by SIMS data which shows a build-up of As in the LT-GaAs layers, and TEM images which indicate that arsenic precipitation occurs to a greater extent in the LT-GaAs than in the LT-Al x Ga 1− x As.

We present computer simulations of a new design of a variable energy positron lifetime beam that uses for a start signal the secondary electron emission from a 25-nm thick carbon foil (C-foil) located in front of the sample. A needle of ∼30 μm diameter is positioned on-axis right behind the foil, creating a radial electric field that deflects the secondary electrons radially outward so as to miss the sample and to hit the micro-channel plate (MCP) detector placed down beam. The MCP signal provides the start signal for the positron lifetime spectrometer. A grid can be further introduced between the sample holder and the MCP to yield a cleaner signal by preventing the positrons with large transmitted scattering angle from hitting the MCP. The cylindrical symmetry of this design reduces the experimental complexity and offers good timing resolution. We show that the design is robust against the transmitted energy and angle of the secondary electrons and positrons.

An electrically induced metastable state of the Al/SI-GaAs/Al back-to-back system is reported for the first time. The state is measured by placing the system under reverse bias breakdown conditions and its transient decay observed through conductivity measurements at room temperature. Similar, but not identical metastabilities were also found on Ti/SI-GaAs/Ti and Au/SI-GaAs/Au systems. The objective of this paper is to report on these findings.

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.

Positron annihilation measurements have been carried out on a-Si:H thin films deposited by plasma enhanced chemical vapour deposition at a high rate and on pin/pin double-junction diodes prepared conventionally by means of the variable-energy positron beam Doppler-broadening technique. The depth profiles of microvoids in films grown under different conditions have been determined. We found a smaller void fraction in the surface region of all films compared to the interior. The depth profiles of microvoid-like defects in the a-Si:H films were extracted by use of the VEPFIT programme. Variable-energy positron-annihilation spectroscopy results on the diode show that the interfaces are of good quality except for the Au/n interface, consistent with the diode characteristic.

The Schottky barrier heights in Au/semi-insulating (SI)-GaAs (1 0 0) and Ni SI-GaAs (1 0 0) contacts at room temperature have been determined from photovoltage and internal photoemission measurements. Both techniques give excellent agreement with each other. A small upward surface band bending of 0.141 eV for Au SI-GaAs (1 0 0) and of 0.062 eV for Ni SI-GaAs (1 0 0) has been determined. The interfacial Fermi level is found to lie at 0.772 eV and 0.690 eV below the conduction band minimum (CBM) at the Au SI-GaAs (1 0 0) and Ni SI-GaAs (1 0 0) interfaces, respectively. The accuracy of both photoelectric techniques suggests their beneficial use in future systematic studies on semi-insulating substrates in the study of Schottky barrier formation.

Properties of metal contacts on III–V semiconductors are dependent on the electronegativity of the metal. Generally metals have been classified into three categories according to their behaviour upon heat treatment. Au, Ni and W are the three metals belonging to different categories with the descending order of electronegativity. In this study a low energy positron beam has been used to investigate as-grown Au GaAs (SI), Ni GaAs (SI) and W GaAs (SI) samples with different thicknesses of metal overlayers. The method that has been employed is that of monitoring the Doppler broadening of annihilation radiation through S parameter measurements as a function of the beam energy. The S - E data were analysed by the program VEPFIT with different models and different implantation profiles. It is found that the interfacial information able to be extracted from the fitting is limited by present uncertainties in the implantation profile.

The 22 Na nuclide, having a relatively long half-life (2.6 year) and high branching ratio for positron emission (90%), is often the preferred nuclide for producing low-energy positron beams. Suitable commercial 22 Na sources are, however, relatively expensive and are sometimes subject to delay in supply. To circumvent such problems in the long-term maintenance of our positron beam a radioactive source deposition apparatus, capable of preparing up to 20 mCi 22 Na sources from aqueous solution, has been designed and constructed. The 22 NaCl solution is deposited onto a platinum substrate, dried and remotely capped with 3 μm of titanium. The apparatus has been safely employed to produce both 10 mCi and 20 mCi 22 Na sources both of which are now in use in our magnetically guided positron beam.

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.

The reverse I-V characteristics of the Au/semi-insulating (SI)GaAs(1 0 0) contact structure have been studied in the temperature range 240–300 K. The temperature dependence of the resistance determined from the low bias (0–15 V) linear portion of the I-V curves gives a Fermi-level position for the bulk GaAs 0.68 ± 0.05 eV below the conduction band, in good agreement with that determined from other techniques, indicating that the reverse current transport in this regime is predominantly determined by the bulk resistance of the SiGaAs. At higher reverse biases a current limitation establishes which is explained in terms of thermionic and thermionic field emission across the Au SI-GaAs Schottky-like barrier. The electric fields that occur at the interface cause a larger than expected saturated emission current due to the barrier lowering effect and because the effective Richardson constant increases by about two orders of magnitude as electrons are transferred into the L band minima of the GaAs. The onset of the Gunn effect also explains the saturation of the reverse current for applied biases greater than 50 V where the electric field close to the interface self limits through the faster EL2+ electron capture from the L band.

Step quenching from the solid solution temperature is a common treatment to enhance the shape memory properties in many copper-based shape memory alloys (SMAs). However, the martensite stabilisation problem is more serious in step quenched Cu Al Ni Mn Ti (CANTiM) SMAs. After step quenching, martensite in CANTiM cannot reverse transform to its parent phase. In contrast, fast martensite stabilisation is still present. However, martensite stabilisation can be removed by post-quench ageing at 100°C. In rapid quenching from high temperature, a large number of defects such as vacancies are usually produced. The martensite stabilisation due to fast quenching is attributed to the pinning effect of these excessive vacancies. After post-quench ageing, the concentration of vacancies is lowered and thus the pinning effect is reduced. This paper reports the behaviour of vacancies at different cooling rates in different quenching medium, and the effect of post-quench ageing. The evolution of vacancies in the alloy is studied using positron annihilation techniques. In contrast to the martensite stabilisation due to ageing at higher temperatures, the martensite stabilisation due to fast quenching rate can be removed by a carefully chosen low temperature ageing programme.

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.

The point-mode digital stabilization method for gamma-ray spectrum measurement over long duration is examined. The stabilizer generates a stabilizing bias to reduce the adverse effects of the electronic bias due to fluctuations in laboratory conditions or electronic instability. The stabilizing bias turns out to be a stochastic variable with a time dependent probability distribution following an exactly solvable driven diffusion equation. From the results, we calculate and discuss the optimum setting of the stabilizer for best performance.

A numerical simulation of the electrostatic properties of metal / semi - insulating GaAs contacts has been carried out for different surface potentials at different temperatures. Specific results have been obtained for the three most common compensation mechanisms within the semi - insulating GaAs material: ( a ) a deep donor and a shallow acceptor, ( b ) a shallow donor and a deep acceptor, and ( c ) a deep donor and a deep acceptor. These results are compared to those from the one - sided Schottky abrupt junction model, and some discussions on the correlation of the electrostatic properties with Schottky barrier formation are presented.

Various thicknesses of Au and Al have been evaporated onto semi-insulating GaAs(110) to form Schottky contacts, which have been subject to different annealing conditions. A variable energy positron beam has been used to probe the metal-semiconductor interfaces by observing the Doppler broadening of the annihilation radiation. The data are well fitted by a three-layer model, namely the metal layer, the interface region and the GaAs bulk. The interfacial region is attributed to the intermixing of the atoms and the formation of new phases occurring at the interface. For the Au/GaAs samples, the interfacial width was observed to increase with increasing annealing temperature and this is attributed to the increase of elemental intermixing at the interface. From the trend of the Doppler broadening signal on annealed samples, we infer that the processes of increasing interfacial order (decreasing S) (such as grain enlargement and vacancy removal) are in competition with the process of decreasing interfacial order (increasing S) (such as vacancy creation through atomic interdiffusion). For Al/GaAs, the interfacial width remained effectively constant up to the annealing temperature of 400°C. From the Doppler broadening signal, the annealing on the interfacial order was found to be metal thickness dependent. This observation has tentatively been explained by the above two competition processes and the grain boundary diffusion of atoms at the Al/GaAs interface.

Deconvolution of the Doppler-broadened annihilation radiation lineshape has been carried out using the generalised least-squares method. The nondashlinear background subtraction is examined through tail stripping and step function fitting procedures. Spurious oscillations of the residual instrumental function are found not to produce an efficient inverse transformation. For this reason least-squares solutions are subjected to a nondashnegativity constraint in order to reduce the oscillations in the deconvoluted spectrum. This procedure improves the results dramatically and shows promise as regards quantitative studies of the local momentum distributions and defect characterisation at various depths of solids in future positron beam experiments.

In this work, the decay of photovoltage at the metal-semiconductor interfaces upon cessation of illumination is investigated numerically. A simple model describing the discharging process at the interfaces is used to simulate the decay of photovoltage. The simulation gives the details of the dependence of the decay time on the temperature, the subtratee's doping concentration, the Schottky barrier height of the contacts, the leakage resistance, and the magnitude of the photo-voltage itself.

The measurements of photovoltage, internal photoemission, current-voltage (I-V) and capacitance-voltage (C-V) have been performed for Au on n-Si (111) within a temperature range of 7 to 300 K. The values of Schottky-barrier height at room temperature obtained from these four measurements are in very good agreement, but conflicting results have been obtained at low temperatures. The measurements of both the internal photoemission and the C-V show a negative temperature dependence of the barrier height which is almost identical to that of the indirect energy band gap in silicon, but the barrier height obtained from the photovoltage measurements and the I-V measurements strongly decreases as temperature decreases. In this paper, the influence of the current transport on the determination of the Schottky-barrier height is discussed, and it is demonstrated that the conflicting results may be explained well in terms of the recombination current involved in the photovoltage measurements and the I-V measurements.