Defect study of proton-irradiated liquid-encapsulated Czochralski GaAs using the positron-annihilation technique

The positron lifetime of undoped Liquid-Encapsulated Czochralski (LEC)-GaAs and Si-doped (1.3×10¹⁸ cm^–3) LEC-GaAs was measured before and after irradiation with protons (dose 1×10¹⁵/cm², 15 MeV). In Si-doped GaAs, the decrease of positron lifetime at temperatures between 10 and 300 K are due to the decrease of the positron-diffusion length and the increase of the effective shallow traps such as antisite Ga_As. The annealing stage of the proton-irradiation-induced defects which show the different behavior from that of electron-irradiation-induced defects suggests that proton irradiation creates more complicated defect complexes, containing vacancies rather than isolated vacancy-type defects or simple complexes which have been observed during electron-irradiation processes. Above 700 K, proton-irradiation-induced defects such as vacancy-type defects and simple vacancy complexes are almost annealed out, while Si-induced defects such as Si_Ga-V_Ga complexes cannot be annealed out above 973 K.     

Analysis of semi-insulating GaAs and the role of positron annihilation

The uses of Positron Annihilation (PA) techniques for the analysis of native defects in bulk GaAs are reported. PA has allowed the structure of the metastable state of the important EL2 defect to be examined and has demonstrated that a vacancy is present in the complex. PA has also provided strong evidence that the centre responsible for minority-carrier lifetime degradation in this material is the arsenic vacancy.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Defect structure and dielectric properties of Bi-based pyrochlores probed by positron annihilation

Positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques have been performed to identify structural defects of the bismuth based pyrochlore systems with generic formula (Bi_1.5Zn_0.5)(Zn_0.5−x/3Ti_xNb_1.5−2x/3)O₇ (x = 0, 0.25, 0.5,1.0, 1.5). We found that all studied compounds contain substantial amount of the lattice vacancy defects, the variation of the annihilation lifetime suggests that the defects structure undergoes significant changes. The complex defects could be produced with increasing content of Ti, resulting in a drop in the intensity I₂ in the Ti-rich sample. At 1 MHz their dielectric constant (?′) varies from 150 for Ti-poor system to 210 for Ti-rich system and loss tangent (tan δ) remains rather low level. The high dielectric constant response of the BZTN ceramics is attributed to loosening state of cations located in the center of octahedral, so favor off-center displacement. The occurrence of complex defects help to enhance the dielectric constant.     

Comment on “Amorphous hydrogenated silicon studied by positron lifetime spectroscopy”

H.-E. Schaefer, R. Wurschum, R. Schwarz, D. Slobodin, and S. Wagner [Appl. Phys. A40, 145 (1986)] have recently assigned positron lifetimes to various vacancy sizes. In this comment we will show that their discussions are ill founded.     

Amorphous hydrogenated silicon studied by positron lifetime spectroscopy

Hydrogenated amorphous silicon (a-Si:H) prepared by dc glow discharge in silane was investigated by positron lifetime measurements at room temperature. The lifetime spectrum shows considerably longer lifetimes than in simultaneously measured Si single crystals. The dominant component with the time constant ₂=402 ps is discussed thoroughly in conjunction with positron trapping at microvoids containing more than 10 to 15 vacancies. Positron trapping at H-saturated dangling bonds cannot be ruled out. The long-lived component with ₃=1800 ps (I ₃=0.06) indicates positronium formation at larger voids.     

Microwave waveguide method for the measurement of electron mobility and conductivity in GaAs/AlGaAs heterostructures

The microwave waveguide method for contactless determination of the electron mobility and conductivity of thin active layers is reported. The method is based on relative measurements of the magnetic field dependences of the derivative of the reflection coefficient with respect to the magnetic field from a semiconductor wafer bridging the waveguide.Experiments are performed on GaAs/AlGaAs heterostructures at microwave frequency = 36.4 GHz and liquid nitrogen temperature. For the analysis of the experimental data the theoretical basis for arbitrary frequencies is developed. The main advantage of the proposed method is that this method enables one to determine material parameters - mobility and conductivity - without careful calibration of the microwave system and does not require the accurate measurements of the absolute values of the reflection coefficient and phase of the reflected wave.     

A study of defects in deformed FeSi alloys using positron annihilation techniques

Steels with high amounts of silicon are used in electrical applications due to their low magnetostriction, high electrical resistivity and reduced energy losses, but they exhibit poor formability. The slow positron beam of Gent is used to investigate defects in different deformed FeSi alloys. It was found that the concentration of defects for the alloys deformed at high temperatures are different from the ones related to the alloys deformed at room temperature. These results are correlated to the results of positron annihilation lifetime spectroscopy (PALS).     

New evaluation of the angular correlation of positron annihilation spectra for noble and d-transition metals

A new fit to the long-range angular correlation of annihilation radiation data of the sum of a parabola, so-called Ferrell function, and a Gaussian for noble and d-transition metals is presented. The three functions are considered to describe the positron annihilation with three different electron groups: nearly free, d-like and rare-gas core electrons, respectively.     

Anisotropy of the electron momentum distribution in Bi2Sr2CaCu2O8+δ superconductor studied by positron annihilation

The temperature dependent (30-300 K) Doppler broadening of the positron annihilated γ-radiation measurement has been investigated on single crystalline Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) high T_c superconducting samples along two different crystallographic orientations. It has been observed that throughout the temperature range the electron momentum distribution has a larger value along the crystallographic c-axis than in the a-b plane. The temperature dependent Doppler broadened positron annihilation γ-radiation lineshape analysis shows a step like increase of S-parameter at the temperature region 92-116 K.     

Three-photon annihilation of the electron-positron pairs

Three-photon annihilation of the electron-positron pairs (=(e⁻,e⁺)-pairs) is considered in the electron rest frame. The energy of the incident positron can be arbitrary. The analytical expression for the cross-section of three-photon annihilation of the (e⁻,e⁺)-pair has been derived and investigated.     

On the sensitivity limit of positron annihilation: detection of vacancies in as-grown silicon

14 cm^-3 in FZ-Si was obtained. Received: 2 June 1998 / Accepted: 20 November 1998 / Published online: 24 February 1999     

The recovery of electron irradiated zinc and cadmium by positron annihilation spectroscopy

Positron lifetime and Doppler-broadening measurements have been performed in Zn and Cd specimens irradiated with 3 MeV electrons at 20 K. Isochronal annealing of the irradiation induced defects between 20 and 280 K has been studied. No evidence of three-dimensional vacancy agglomerates was found.     

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.     

Application of positron annihilation spectroscopy on the ion implantation damaged Fe-Cr alloys

Four different Fe-Cr binary alloys with Cr content 2.5-11 wt% were studied in details using various methods. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were applied to obtain basic information, required for standard positron annihilation lifetime spectroscopy (PALS) spectra analysis. Additionally, PALS measurements were performed on as-received state as well as on helium implanted specimens. The He implantation was proposed for simulation of radiation damage and obtain high doses even in near surface areas (up to 1 μm). The implantation was based on the SRIM code simulation and next DPA calculations. Final concentration of vacancy type defects were calculated for 250 keV He²⁺ beam and the maximum was determined in 600 μm depth. Such specimens are very suitable for positron beam study of vacancy type defect mobility as a result of thermal treatment, which will be performed simultaneously in the future.     

Simulation of positron annihilation response to mechanical deformation of nanostructured Ni3Al

Simulations of the positron response to the mechanical deformation of Ni₃Al nanowires are performed on modelled samples obtained using molecular dynamics. Particular attention is paid to the evolution of the various open volume defects and their interaction during deformation. Positron simulations are done in conjunction with a general, geometrical analysis of open volumes in the studied samples, which brings complementary information to the positron response results.     

Positron annihilation spectroscopy and specific heat study of Neon ion irradiated MgB2

Specific heat studies under magnetic field and positron annihilation spectroscopy were carried out on 160 MeV Ne ion irradiated polycrystalline MgB₂ samples. There is an unusual decrease in positron lifetime in the irradiated sample which may be due to neon ion implantation. This was also indicated by change in cell volume. Coincidence Doppler Broadening Spectra of Mg, B, irradiated and unirradiated MgB₂ show that positrons primarily annihilate in boron sublattice in the unirradiated sample whereas there is some similarity of the spectrum of the irradiated sample with that of Mg. There is Mg deficiency in the unirradiated sample whereas predominantly boron vacancies exist in Ne ion irradiated MgB₂ sample. Specific heat measurements show that there is a small increase in electronic part of the specific heat and electron-phonon coupling constant.     

Determination of positron beam parameters by various diagnostic techniques

Various diagnostic techniques have been applied at the neutron-induced positron source Munich NEPOMUC in order to determine the positron beam parameters such as intensity, beam shape and energy distribution. The positron beam intensity is determined by the detection of the annihilation radiation of positrons, which annihilate in a movable target. The use of a micro-channel plate (MCP) detector with a CCD-camera allows a direct measurement of the positron beam shape and the lateral resolved intensity distribution. At NEPOMUC a movable MCP-assembly inside the evacuated beam line enables a quick examination of the beam shape during operation. A retarding grid was mounted inside the homogeneous magnetic guiding field in order to determine the distribution of the longitudinal positron momentum, and hence estimate the energy distribution of the positrons.     

Study of the surface contamination of copper with the improved positron annihilation-induced Auger electron spectrometer at NEPOMUC

The high intensity positron source NEPOMUC at the FRM-II in Munich enables measurement times for positron annihilation-induced Auger electron spectroscopy (PAES) of only 2.4 h/spectrum, in contrast to usual lab beams with measurement times up to several days. The high electron background due to surrounding experiments in the experimental hall of the FRM-II has been eliminated and hence background free experiments have become possible. Due to this, the signal to noise ratio has been enhanced to 4.5:1, compared to 1:3 with EAES. In addition, a long-term measurement has been performed in order to observe the contamination of a polycrystalline copper foil at 150 °C.     

Slow positron annihilation studies of vacancy-type defects in the near-surface region of Cu and Nb before and after wear

Received: 22 June 1998 / Accepted: 9 November 1998     

Positron annihilation in the quenched and electron-irradiated NiSb intermetallic compound

Samples of the near equiatomic NiSb compound were irradiated by 3 MeV electrons at 20 K or quenched from 1103 K and 1333 K and subsequently annealed isochronally. The behaviour of defects created by quench or irradiation were studied by the positron annihilation technique. Only one recovery stage was found around 425 K for quenched specimens, but two distinct stages (100 K and 425 K) were observed after irradiation. The 425 K stage is ascribed to the migration of Ni vacancies giving dislocation loops. The recombination of mobile interstitials with vacancies after irradiation is assumed to occur between 100 K and 250 K. Doppler broadening and lifetime variations of positrons as a function of the measuring temperature in these irradiation samples are discussed.