Search

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.

Zinc oxide is a very promising material for short-wavelength light-emitting devices due to its large band gap and high exciton binding energy. Although great progress has been made in recent years, p-type doping and control over native defects introduced during or after material growth are still significant problems that hinder the development of efficient ZnO based optoelectronic devices. Here we demonstrate a versatile method for the growth or p-type or n-type ZnO nanorods from the same growth solution at temperature as low as 90 °C, where the conductivity type is controlled by the preparation of the seed layer for nanorod growth. The differences in the conductivity type can be attributed to dependency of native defect concentrations and hydrogen incorporation on the seed layer preparation method. Room temperature electroluminescence has been demonstrated from homojunction and heterojunction light emitting diodes containing p-ZnO nanorods.

The Richardson-Lucy, Maximum Entropy and Huber regularization methods are popularly used in solving ill-posed inverse problems. This paper considers the use of these three methods in the deconvoluting DBARS (Doppler Broadening of Annihilation Radiation Spectroscopy) data. As DBARS data have a constant background on the high-energy side and a long exponential tail on the low-energy side, we check the different deconvolution schemes paying specific attention to the quality of the deconvolution at the peak and tail positions. Comparison of the three methods is made by testing on Monte-Carlo simulated data both in terms of the deconvoluted quality and computational resources required. Finally, we apply these methods to experimental DBARS data taken on polycrystalline metal samples. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Positron annihilation lifetime spectroscopy has been used to study the formation of positronium in high density polyethylene as a function of temperature over the range 30 K-350 K. It is observed that the thermal history of the sample, while having no influence on the positronium lifetime, has a strong effect on the formation of positronium. A hysteresis is seen in the positronium formation probability in cooling and heating cycles. This is explained on a two channel formation model, the first channel being through “blob” formation and the second through the pick-up of shallow trapped electrons. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Deconvoluted Coincidence Doppler Broadening Spectroscopy (CDBS) measurements have been made on 300 keV and 1.7 MeV electron irradiated SiC. The lower energy irradiation produces only carbon vacancies while the higher energy produces both carbon and silicon vacancies. This distinction is easily seen in the high (20-35 mrad) momentum range where a clear atomic signal of Si is seen for the carbon vacancy. In addition to the higher momentum region the higher resolution of the deconvoluted CDBS spectra show structural information relating to the crystal lattice. The autocorrelation function obtained for positrons trapped at carbon vacancies is found to show a stronger lattice signal indicative of a more extended positron wave function and a less strongly bound state. Conversely that positron trapped at the silicon vacancy shows a more damped autocorrelation function characteristic of a more spatially confined positron state. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Positron lifetime and coincidence Doppler broadening spectroscopic (CDBS) measurements were carried out to study the defects in two hydrothermal (HT) grown ZnO single crystal samples (HT1 and HT2) obtained from two companies. Single component model could offer good fittings to the room temperature spectra of HT1 and HT2, with the positron lifetimes equal to 199 ps and 181 ps respectively. These two lifetime components were associated with saturated positron trapping into two VZn-related defects with different microstructures. The positron lifetimes of HT1 was found to be temperature independent. For the HT2 sample, the positron lifetime remained unchanged with T > 200 K and decreased with decreasing temperature as T

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.

Indium tin oxide (ITO) thin films were deposited on glass substrates using the e-beam evaporating technique. The influence of deposition rate and post-deposition annealing on the optical properties of the films was investigated in detail. It is found that the deposition rate and annealing conditions strongly affect the optical properties of the films. The transmittance of films greatly increases with increasing annealing temperature below 300°C but drastically drops at 400°C when they are annealed in forming gas (mixed N2 and H2 gas). An interesting phenomenon observed is that the transmittance of the darkened film can recover under further 400°C annealing in air. Atomic force microscopy, x-ray diffraction and x-ray photoemission spectroscopy were employed to obtain information on the chemical state and crystallization of the films. Analysis of these data suggests that the loss and re-incorporating of oxygen are responsible for the reversible behaviour of the ITO thin films.

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.

Obtaining a lateral S-parameter image scan from positrons implanted into semiconductor devices can be a helpful research tool both for localizing device structures and in diagnozing defect patterns that could help interpret function. S-parameter images can be obtained by electromagnetically rastering a variable energy positron beam of small spot size across the sample. Here we describe a general hardware and software architecture of relatively low cost that has recently been developed in our laboratory which allows the whole sub-surface S-parameter image of a sample or device to be obtained in real time. This system has the advantage over more conventional sequential scanning techniques of allowing the operator to terminate data collection once the quality of the image is deemed su‐cient. As an example of the usefulness of this type of imaging architecture, S-parameter images of a representative sample are presented at two difierent positron implantation energies.

Depth profiled Doppler broadening of positron annihilation spectroscopy (DBPAS), which is also called the variable energy positron annihilation spectroscopy (VEPAS), is used in characterization of GaN grown on sapphire substrates with metal-organic chemical vapour deposition (MOCVD). The GaN film and the film/substrate interface are investigated. The VEPFIT (variable energy positron fit) software was used for analysing the data, and the positron diffusion length of the sapphire is obtained. The results suggest that there is a highly defected region near the GaN/sapphire interface. This thin dislocated region is generated at the film/substrate interface to relieve the strain. Effects of implantation dose on defect formation, for the GaN/Sapphire samples, which implanted by Al+ ions, are also investigated. Studies on Al+ implanted GaN films (not including the interface and sapphire) have revealed that there are two different regions of implantation damage. For the low Al+ implantation dose samples, in the region close to the surface, defects are mainly composed of vacancy pairs with small amount of vacancy clusters, and in the interior region of the film the positron traps are vacancy clusters without micro-voids. For the highest dose sample, however, some positron trap centres are in the form of micro-voids in the second region.

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 positron lifetime technique was employed to study vacancy-type defects in 8 MeV electron-irradiated n-type 6H silicon carbide. A long-lifetime component having a characteristic lifetime of 223–232 ps was observed in the irradiated sample and was attributed to the VCVSi divacancy. Other positron traps, which dominated at low temperatures, were observed to compete with the VCVSi for trapping positrons. A positron trapping model involving a positron shallow trap, a negatively charged monovacancy and the VCVSi divacancy was found to give a good description of the temperature-dependent positron lifetime data of the 1200 ◦ C annealed sample. The identity of the monovacancy could not be unambiguously determined, but its lifetime was found to be in the range 160–172 ps.

Micro-Raman spectroscopy and capacitance–voltage (C–V) measurements have been used to investigate 2 in GaN epitaxial wafers grown by hydride vapour phase epitaxy on sapphire substrates. The position and line shape of the A1 longitudinal optical (LO) phonon mode were used to determine the carrier concentration at different locations across the wafer. The line-shape fitting of the Raman A1 (LO) coupled modes taken from horizontal lateral-different positions on the wafer yielded a rudimentary spatial map of the carrier concentration. These data compare well with a carrier density map of the wafer obtained by C–V measurements, confirming the non-uniform distribution of carrier concentration in the GaN epitaxial film and that Raman spectroscopy of the LO phonon–plasmon mode can be used as a reliable and production friendly wafer quality test for GaN wafer manufacturing processes.

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.

Electron irradiated undoped liquid encapsulated Czochralski (LEC) grown GaSb samples were studied by positron lifetime spectroscopy (PLS) and photoluminescence (PL). In addition t ot he 315 ps component reported in the previous studies, another defect with a lifetime of 280 ps was also identified in the present electron irradiated samples. The bulk lifetime of the GaSb material was found to be 258 ps. The VGa,280 ps and the VGa,315 ps defects were associated with two independent Ga vacancy related defects having different microstructures. The well known 777 meV PL signal (usually band A) was also observed in the electron irradiated undoped GaSb samples. The band A intensity decreases with increasing electron irradiation dosage and it disappears after the 300 ◦ Ca nnealing regardless of the irradiation dosage. The origin of the band A signal is also discussed.

Temperature-dependent Hall (TDH) measurements and confocal micro-Raman spectroscopy have been used to study the free carrier spatial distribution and scattering mechanism in unintentionally doped GaN film grown on the sapphire substrate with the method of metalorganic chemical vapor deposition. Both the TDH data and the depth-profiled Raman spectra agreed with the existence of a nonuniform spatial distribution of free carriers in the GaN film with a highly conductive layer of ∼1 μm thickness near the GaN sapphire boundary. With the consideration of this parallel conduction channel adjacent to GaN sapphire boundary, detailed analysis of the TDH mobility data suggests that a relatively high concentration of nitrogen vacancies exists and nitrogen vacancy scattering has an important influence on limiting the electron mobility in the bulk film of the present GaN sample.

Recently room temperature electrically induced metastability in semi-insulating (SI)-GaAS has been reported in which the normally high resistivity state of SI-GaAs converts into a low resistivity state when breakdown electric fields are applied to the metal/SI-GaAs/metal systems. In this brief report, the methods of photo-quenching and positron annihilation lifetime spectroscopy have been employed to see whether the high field breakdown and subsequent metastability has its origins in some configurational change of a native defect in the sample. Our results indicate that rather than being due to an atomic configurational change the observed metastability is most likely of purely electronic origin. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Two-dimensional angular correlation of annihilation radiation (2D-ACAR) spectra have been taken for 1019 cm−3 phosphorus-doped Si in the as-grown state after having been subjected to 1.8 MeV electron fluences of 1×1018 and 2×1018 cm−2. Positron annihilation lifetime spectroscopy confirms, in accordance with previous works, that positrons are saturation trapping into (VSi:P) pair defect (E-center) monovacancy sites in the electron irradiated samples. In the as-grown case, the positron–electron autocorrelation functions along the [111] and [1-10] directions, obtained through Fourier transformation of the 2D-ACAR data, reveal zero-crossings that deviate only slightly from the lattice points, in a manner consistent with positron–electron correlation effects. Conversely, in the spectra of the irradiated samples, the zero-crossing points are observed to move outward further by between 0.15 and 0.50 A. This displacement is associated with positron annihilation with electrons in localized orbitals at the defect s...

Current–voltage (I–V) characteristics of macroscopic Schottky diodes fabricated on different GaN templates grown by metalogranic chemical vapor deposition on sapphire substrates were investigated. The number of dislocations under the Au Schottky contact was determined by atomic force microscopy combined with hot H3PO4 etching and the screw dislocations in the GaN films were found to have a strong influence on the reverse leakage current of the Au/n-GaN Schottky diodes. The leakage current is increased when high-density screw dislocations exist under the Au Schottky contact. A model based upon the presence of dislocations at the Au/GaN interface has been used to explain this behavior. It has been proposed that these dislocations result in the lowering of the barrier height in the localized regions, and thus significantly affect the reverse I–V characteristics of the Schottky diodes.

A generalized least-square method with Tikonov–Miller regularization and non-negativity constraints has been developed for deconvoluting two-dimensional coincidence Doppler broadening spectroscopy (CDBS) spectra. A projected Newton algorithm is employed to solve the generalized least-square problem. The algorithm has been tested on Monte Carlo generated spectra to find the best regularization parameters for different simulated experimental conditions. Good retrieval of the underlying positron–electron momentum distributions in the low momentum region is demonstrated. The algorithm has been successfully used to deconvolute experimental CDBS data from aluminum.

Deep level transient spectroscopy has been employed to study the deep level defects introduced in n-type 6H-SiC after neutron irradiation. Deep levels situated at E-C-0.23, E-C-0.36/0.44, E-C-0.50, and E-C-0.62/0.68 eV have been detected in the temperature range of 100-450 K, which have been identified with the previously reported deep levels ED1, E-1/E-2, E-i, and Z(1)/Z(2), respectively. Thermal annealing studies of these deep levels reveal that ED1 and E-i anneal at a temperature below 350degreesC, the Z(1)/Z(2) levels anneal out at 900degreesC, while the intensity of the E-1/E-2 peaks is increased with annealing temperature, reaching a maximum at about 500-750degreesC, and finally annealing out at 1400degreesC. The possible nature of the deep levels ED1, E-1/E-2, E-i, and Z(1)/Z(2) are discussed in the context of their annealing behavior. Upon further annealing at 1600degreesC, four deep levels labeled NE1 at E-C-0.44 eV, NE2 E-C-0.53 eV, NE3 E-C-0.64 eV, and NE4 E-C-0.68 eV are produced. Evidence is given that these levels are different in their origin to E-1/E-2 and Z(1)/Z(2). (C) 2003 American Institute of Physics.

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.

Slow-positron emission from the surfaces of as-grown n-type 4H-SiC and 6H-SiC (silicon carbide) with a conversion efficiency of ~ 10−4 has been observed. After 30 min of 1000 oC annealing in forming gas, the conversion efficiency of the n-type 6H-SiC sample was observed to be enhanced by 75% to 1.9 × 10−4, but it then dropped to ~ 10−5 upon a further 30 min annealing at 1400 oC. The positron work function of the n-type 6H-SiC was found to increase by 29% upon 1000 oC annealing. For both p-type 4H-SiC and p-type 6H-SiC materials, the conversion efficiency was of the order of ~ 10−5, some ten times lower than that for the n-type materials. This was attributed to the band bending at the p-type material surface which caused positrons to drift away from the positron emitting surface.

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.

Positrons from a radioactive source are implanted into a reverse-biased metal–semiconductor contact and are drifted back towards the contact by the internal electric field where they trap into voids and annihilate. The electric field dependent interface annihilation fraction is monitored by way of the intensity of the long (∼400–500 ps) void lifetime component using positron-lifetime spectroscopy. Unlike previous analyses of such systems a numerical model involving positron drift, annihilation and trapping into the interfacial state has been constructed to describe the positron dynamics in the presence of the non-uniform junction electric field. The use of the positron-lifetime technique in probing the internal electric field at buried contacts is thus demonstrated. Results obtained using the numerical method for the Au, Al and Ni/Semi-Insulating (SI)-GaAs contact systems are found to be consistent with the findings of previous studies on the Au/SI-GaAs system.

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.

In our recent report, [Xu , Appl. Phys. Lett. 76, 152 (2000)], profile distributions of five elements in the GaN/sapphire system have been obtained using secondary ion-mass spectroscopy. The results suggested that a thin degenerate n(+) layer at the interface is the main source of the n-type conductivity for the whole film. The further studies in this article show that this n(+) conductivity is not only from the contribution of nitride-site oxygen (O-N), but also from the gallium-site silicon (Si-Ga) donors, with activation energies 2 meV (for O-N) and 42 meV (for Si-Ga), respectively. On the other hand, Al incorporated on the Ga sublattice reduces the concentration of compensating Ga-vacancy acceptors. The two-donor two-layer conduction, including Hall carrier concentration and mobility, has been modeled by separating the GaN film into a thin interface layer and a main bulk layer of the GaN film. The bulk layer conductivity is to be found mainly from a near-surface thin layer and is temperature dependent. Si-Ga and O-N should also be shallow donors and V-Ga-O or V-Ga-Al should be compensation sites in the bulk layer. The best fits for the Hall mobility and the Hall concentration in the bulk layer were obtained by taking the acceptor concentration N-A=1.8x10(17) cm(-3), the second donor concentration N-D2=1.0x10(18) cm(-3), and the compensation ratio C=N-A/N-D1=0.6, which is consistent with Rode's theory. Saturation of carriers and the low value of carrier mobility at low temperature can also be well explained. (C) 2001 American Institute of Physics.

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.

Undoped and Fe-doped liquid encapsulated Czochralski (LEC) InP has been studied by Hall effect, current–voltage (I–V), and infrared absorption (IR) spectroscopy. The results indicate that a native hydrogen vacancy complex donor defect exists in as-grown LEC InP. By studying the IR results, it is found that the concentration of this donor defect in Fe-doped InP is much higher than that in undoped InP. This result is consistent with the observation that a much higher concentration of Fe2+ than the apparent net donor concentration is needed to achieve the semi-insulating (SI) property in InP. By studying the I–V and IR results of Fe-doped InP wafers sliced from different positions on an ingot, the high concentration of Fe2+ is found to correlate with the existence of this hydrogen complex. The concentration of this donor defect is high in wafers from the top of an ingot. Correspondingly, a higher concentration of Fe2+ can be detected in these wafers. These results reveal the influence of the complex defect o...

Variable-temperature current–voltage has been used to study the conduction properties of Fe-doped semi-insulating (SI) InP in the as-grown and annealed states. It is found that the trap-filling (TF) process disappears gradually with lengthening of annealing time. This phenomenon is explained by the decrease of the concentration of the empty Fe deep level (Fe3+) that is caused by the thermally induced donor defect formation. The TF process cannot be observed in annealed undoped and long-time annealed Fe-doped SI InP material. The breakdown field of annealed undoped and Fe-doped SI InP is much lower than that of as-grown Fe-doped InP material. The breakdown field decreases with decreasing of temperature indicating an impact ionization process. This breakdown behavior is also in agreement with the fact that the concentration of the empty deep level in annealed InP is lowered.

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 changes in variable-energy positron annihilation spectroscopy measurements on undoped hydrogenated amorphous silicon films after light soaking. The change, seen predominantly in the high momentum band of the annihilation radiation, is not reversed by thermal annealing. We suggest, following recent models of the Staebler-Wronski effect, that light exposure induces hydrogen trapped in vacancylike defects to become mobile in the Si network. The observations place constraints on models of hydrogen motion fitting macroscopic Staebler-Wronski effect kinetics and may help to achieve a definitive description of metastability in a-Si:H.

Positron lifetime spectroscopy has been employed to study the as-grown n-type 1.2×1018 cm-3 N-doped and p-type 1.8×1018 cm-3 Al-doped 6H-silicon carbide in the temperature range 10 K–300 K. For the p-type material, a positron trapping site, which has a lifetime of 225±11 ps, was found and is attributed to positron annihilating from the VSiVC divacancy-related defect having a neutral charge. For the case of the n-type material, a positron trapping centre having a lifetime of 200±9 ps, attributed to a VSi-related defect, and a shallow trap were observed. The shallow trap, having binding energy of l8 meV was attributed to Rydberg-like states resulting from positron binding with a negative ion.