Defect structure studies using positron annihilation spectroscopy

The positron annihilation method is a new addition to the range of sensitive complementary nuclear techniques available for materials’ research. The preferential sensitivity of positrons towards micro-defect domains which are not assessable by other techniques makes it an attractive tool for many materials science problems. The present paper is intended as a brief introduction on the principle of measurements and its potential is exemplified with the help of results on some metallic and ceramic systems.     

Irradiation-induced atomic defects in SiC studied by positron annihilation

The present paper reports on positron lifetime measurements on atomic defects in SiC after low-temperature (80 K) electron irradiation of low (0.47 MeV) and high (2.5 MeV) electron energies and doses from 1.8×10¹⁷ to 1.9×10¹⁹ e/cm² as well as after subsequent isochronal annealing up to 1900 K. For these studies the single crystals of nitrogen doped (2–3×10¹⁸ cm^–3) SiC grown by a modified Lely technique with hexagonal structure (6H polytype) were used.According to the positron lifetime measurements, very different types of vacancy-like positron traps are introducted after irradiation with electrons of either low or high energy. The formation of defect agglomerates and their decay at high temperatures is studied during isochronal annealing and related to earlier studies.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Structural and optical characterization of GaN heteroepitaxial films on SiC substrates

We have estimated the threading dislocation density and type via X-ray diffraction and Williamson-Hall analysis to elicit qualitative information directly related to the electrical and optical quality of GaN epitaxial layers grown by PAMBE on 4H- and 6H-SiC substrates. The substrate surface preparation and buffer choice, specifically: Ga flashing for SiC oxide removal, controlled nitridation of SiC, and use of AlN buffer layers all impact the resultant screw dislocation density, but do not significantly influence the edge dislocation density. We show that modification of the substrate surface strongly affects the screw dislocation density, presumably due to impact on nucleation during the initial stages of heteroepitaxy.     

Novel attributes of AlGaN/AlN/GaN/SiC HEMTs with the multiple indented channel

In this paper, a high performance AlGaN/AlN/GaN/SiC High Electron Mobility Transistor (HEMT) with the multiple indented channel (MIC-HEMT) is proposed. The main focus of the proposed structure is based on reduction of the space around the gate, stop of the spread of the depletion region around the source–drain, and decrement of the thickness of the channel between the gate and drain. Therefore, the breakdown voltage increases, meanwhile the elimination of the gate depletion layer extension to source/drain decreases the gate–source and gate–drain capacitances. The optimized results reveal that the breakdown voltage and the drain saturation current increase about 178% and 46% compared with a conventional HEMT (C-HEMT), respectively. Therefore, the maximum output power density is improved by factor 4.1 in comparison with conventional one. Also, the cut-off frequency of 25.2 GHz and the maximum oscillation frequency of 92.1 GHz for the MIC-HEMT are obtained compared to 13 GHz and 43 GHz for that of the C-HEMT and the minimum figure noise decreased consequently of reducing the gate–drain and gate–source capacitances by about 42% and 40%, respectively. The proposed MIC-HEMT shows a maximum stable gain (MSG) exceeding 24.1 dB at 3.1 GHz which the greatest gain is yet reported for HEMTs, showing the potential of this device for high power RF applications.     

Hydrogen-induced buckling of Pd films studied by positron annihilation

Hydrogen loading of thin films introduces very high compressive stresses which grow in magnitude with increasing hydrogen concentration. When the hydrogen-induced stresses exceed a certain critical in-plane stress value, the loaded film starts to detach from the substrate. This results in the formation of buckles of various morphologies in the film layer. Defect studies of a hydrogen loaded Pd film which undergoes a buckling process are presented, using slow positron implantation spectroscopy, in situ acoustic emission, and direct observations of the film structure by transmission electron and optical microscopies. It is found that buckling of the film occurs at hydrogen concentrations x_H ≥ 0.1 and causes a significant increase of the dislocation density in the film.     

Enhancement-mode AlGaN/GaN high electronic mobility transistors with thin barrier

An enhancement-mode (E-mode) AlGaN/GaN high electron mobility transistor (HEMTs) was fabricated with 15-nm AlGaN barrier layer. E-mode operation was achieved by using fluorine plasma treatment and post-gate rapid thermal annealing. The thin barrier depletion-HEMTs with a threshold voltage typically around --1.7 V, which is higher than that of the 22-nm barrier depletion-mode HEMTs (--3.5 V). Therefore, the thin barrier is emerging as an excellent candidate to realize the enhancement-mode operation. With 0.6-μ m gate length, the devices treated by fluorine plasma for 150-W RF power at 150 s exhibited a threshold voltage of 1.3 V. The maximum drain current and maximum transconductance are 300 mA/mm, and 177 mS/mm, respectively. Compared with the 22-nm barrier E-mode devices, V_T of the thin barrier HEMTs is much more stable under the gate step-stress.     

Free-volume changes at nanoscale in doped polyacrylic acid studied by positron annihilation spectroscopy

Polyacrylic acid (PAA) doped with carbon black (CB), chromium oxide (Cr₂O₃) and cupferron with different wt% (0.25%, 0.50%, 0.75%, and 1%) was studied using positron annihilation lifetime (PAL) technique and Doppler broadening of annihilation radiation (DBAR). Ortho-positronium lifetime components (τ₃ and I₃) were used to estimate the nanoscale free-volume hole sizes (V_f) and its fractions (f). It was found that the hole size V_f and its fractions f as well as S-parameters decreased at high value of doping concentration due to dopants-polymer formation. These results are supported by a significant narrowing in the nanoscale free-volume hole size distributions.

The correlation between positron annihilation parameters and electric conductivity are discussed.     

Polydimethylsiloxane rubber gamma radiation effect studies by positron annihilation lifetime spectroscopy

Polydimethylsiloxane rubber was irradiated at various radiation doses up to 800 kGy in air. The lifetime and intensity of the long component τ₃ were obtained by positron annihilation lifetime spectroscopy (PALS). The crystallization property and the molecular flexibility were estimated using the differential scanning calorimetry and dynamic thermomechanic analysis. Thermal volatile property was determined by thermogravimetry analysis. It was proved that the cross-linking reaction made the chain flexibility of the rubber to reduce gradually during gamma radiation, which resulted in the reduction of the o-Ps intensity observed by PALS. Although the degradation effect existed during gamma radiation, it was still less significant than the cross-linking effect when the radiation dose was up to 800 kGy.     

Operational improvement of AlGaN/GaN HEMT on SiC substrate with the amended depletion region

In this paper, a high performance AlGaN/GaN High Electron Mobility Transistor (HEMT) on SiC substrates is presented to improve the electrical operation with the amended depletion region using a multiple recessed gate (MRG–HEMT). The basic idea is to change the gate depletion region and a better distribution of the electric field in the channel and improve the device breakdown voltage. The proposed gate consists of lower and upper gate to control the channel thickness. Also, the charge of the depletion region will change due to the optimized gate. In addition, a metal between the gate and drain including the horizontal and vertical parts is used to better control the thickness of the channel. The breakdown voltage, maximum output power density, cut-off frequency, maximum oscillation frequency, minimum noise figure, maximum available gain (MAG), and maximum stable gain (MSG) are some parameters for designers which are considered and are improved in this paper.     

Characterization of a SiC/SiC composite by X-ray diffraction, atomic force microscopy and positron spectroscopies

A SiC/SiC composite is characterized by X-ray diffraction, atomic force microscopy and various positron spectroscopies (slow positron implantation, positron lifetime and re-emission). It is found that besides its main constituent 3C-SiC the composite still must contain some graphite. In order to better interpret the experimental findings of the composite, a pyrolytic graphite sample was also investigated by slow positron implantation and positron lifetime spectroscopies. In addition, theoretical calculations of positron properties of graphite are presented.     

Interface defects in GaN/sapphire studied using Rutherford backscattering spectroscopy and channeling

GaN on sapphire was grown by MOCVD technique. Rutherford backscattering spectra together with channeling along [0 0 0 1] axis were recorded to study the defects at the interface. Detailed calculation shows that the defects at GaN/sapphire interface are due to dislocations which are distributed into the whole thickness of the film and are mainly aligned on the growth direction.     

Helium bubbles in neutron-irradiated copper-boron studied by positron annihilation

The influence of helium, introduced by the ¹⁰B(n, α)⁷Li reaction, on the evolution of defect structure in copper containing a few hundred ppm boron has been studied by detailed positron lifetime and two-photon angular correlation measurements, supplemented by TEM studies. In the as-irradiated state of Cu-B, two lifetime components have been resolved. The shorter lifetime, τ₁, = 167 ps of 97% intensity, has been understood as due to positron trapping at small helium-vacancy complexes, while the longer lifetime τ₂ = 450 ps of 3% intensity is explained as due to helium-free voids. Marked changes in the annihilation characteristics observed at 670 K are interpreted in terms of the nucleation of microbubbles, controlled by thermally activated helium migration to vacancy traps. Corroborative evidence for the onset of helium clustering is obtained from the change in the average size of positron traps as deduced from the smearing of the measured angular correlation spectra. Helium bubbles and helium-free voids coexisting in the system have been distinguished by a three-component analysis of the lifetime spectra. Bubbles are found to be stable beyond the temperature of dissociation of voids. The size and concentration of bubbles, determined independently by TEM measurements, are in accordance with the positron annihilation results in the growth stage. The observed positron lifetime at higher annealing temperatures has been analysed by relating the annihilation rate to helium atom density and helium pressures in bubbles evaluated. These pressures are in satisfactory agreement with the estimates of equilibrium pressures, leading to the conclusion that bubble relaxation occurs by the mechanism of thermal vacancy condensation.     

Efficiency enhancement of an InGaN light-emitting diode with a p-AIGaN/GaN superlattice last quantum barrier

In this study, the efficiency droop of an InGaN light-emitting diode （LED） is reduced slgnlncanUy oy using a p-AlGaN/GaN superlattice last quantum barrier. The reduction in efficiency droop is mainly caused by the decrease of electron current leakage and the increase of hole injection efficiency, which is revealed by investigating the light currents, internal quantum efficiencies, energy band diagrams, carrier concentrations, carrier current densities, and radiative recombination efficiencies of three LED structures with the advanced physical model of semiconductor device （APSYS）.     

Breakdown characteristics of AlGaN/GaN Schottky barrier diodes fabricated on a silicon substrate

In this work, the breakdown characteristics of AlGaN/GaN planar Schottky barrier diodes(SBDs) fabricated on the silicon substrate are investigated. The breakdown voltage(BV) of the SBDs first increases as a function of the anodeto-cathode distance and then tends to saturate at larger inter-electrode spacing. The saturation behavior of the BV is likely caused by the vertical breakdown through the intrinsic GaN buffer layer on silicon, which is supported by the postbreakdown primary leakage path analysis with the emission microscopy. Surface passivation and field plate termination are found effective to suppress the leakage current and enhance the BV of the SBDs. A high BV of 601 V is obtained with a low on-resistance of 3.15 mΩ·cm2.     

Breakdown characteristics of AIGaN/GaN Schottky barrier diodes fabricated on a silicon substrate

In this work, the breakdown characteristics of AlGaN/GaN planar Schottky barrier diodes （SBDs） fabricated on the silicon substrate are investigated. The breakdown voltage （BV） of the SBDs first increases as a function of the anodeto-cathode distance and then tends to saturate at larger inter-electrode spacing. The saturation behavior of the BV is likely caused by the vertical breakdown through the intrinsic GaN buffer layer on silicon, which is supported by the post-breakdown primary leakage path analysis with the emission microscopy. Surface passivation and field plate termination are found effective to suppress the leakage current and enhance the BV of the SBDs. A high BV of 601 V is obtained with a low on-resistance of 3.15 mΩ·cm^2.     

Investigation of current transport parameters of Ti/4H-SiC MPS diode with inhomogeneous barrier

The current transport parameters of 4H-SiC merged PiN Schottky(MPS) diode are investigated in a temperature range of 300-520 K.Evaluation of the experimental current-voltage(I-V) data reveals the decrease in Schottky barrier height Φ b but an increase in ideality factor n,with temperature decreasing,which suggests the presence of an inhomogeneous Schottky barrier.The current transport behaviours are analysed in detail using the Tung’s model and the effective area of the low barrier patches is extracted.It is found that small low barrier patches,making only 4.3% of the total contact,may significantly influence the device electrical characteristics due to the fact that a barrier height of 0.968 eV is much lower than the average barrier height 1.39 eV.This shows that ion implantation in the Schottky contact region of MPS structure may result in a poor Ti/4H-SiC interface quality.In addition,the temperature dependence of the specific on-resistance(R on sp),T 2.14,is determined between 300 K and 520 K,which is similar to that predicted by a reduction in electron mobility.     

Determination of the relative permittivity of the AlGaN barrier layer in strained AlGaN/GaN heterostructures

Using the measured capacitance--voltage curves and the photocurrent spectrum obtained from the Ni Schottky contact on a strained Al_0.3Ga_0.7N/GaN heterostructure, the value of the relative permittivity of the AlGaN barrier layer was analysed and calculated by self-consistently solving Schr?dinger's and Poisson's equations. It is shown that the calculated values of the relative permittivity are different from those formerly reported, and reverse biasing the Ni Schottky contact has an influence on the value of the relative permittivity. As the reverse bias increases from 0 V to --3~V, the value of the relative permittivity decreases from 7.184 to 7.093.     

Defect characterization of slow-cooled and quenched samples of calcium-copper-titanate through positron annihilation spectroscopy

Abstract

Positron annihilation lifetime and Doppler-broadened gamma-ray spectra have been analyzed for slow-cooled and thermally quenched polycrystalline samples of calcium-copper-titanate. Two positron lifetimes revealing the characteristic defects in the respective samples were carefully analyzed to compare and contrast the significance of their origin and implication. A third component arising from positronium formation at the powdered particle surfaces has been considered in the analysis although its significance is lost in its very small intensity (～ 1.1–1.2%). In the quenched sample, the defect-specific long positron lifetime (τ₂) is found to larger and the mean lifetime smaller while its intensity I₂ is found drastically smaller and the concentration of defects less by an order of magnitude as compared to the slow-cooled sample. The observed changes in electrical parameters of slow-cooled and quenched samples were found to have correlations with the positron annihilation lifetime and Doppler-broadened lineshape parameters.     

X-ray topographic imaging of (Al, Ga)N/GaN based electronic device structures on SiC

Structural defects and their impact on the performance, lifetime and reliability of electronic devices are of permanent interest for crystal growers and device manufacturers. This is especially true for epitaxial (Al, Ga)N/GaN based high electron mobility transistor (HEMT) structures on 4H-SiC (0 0 0 1) substrates. This work points out how micropipes, dislocations and grain boundaries present in a 4H-SiC (0 0 0 1) wafer and subsequently overgrown with an (Al, Ga)N-GaN-HEMT layer sequence show up in X-ray topographic images and two-dimensional XRD maps. Using X-ray topography in transmission geometry, micropipes and other structural defects are localized non-destructively below structured metallization layers with a spatial resolution of a few tens of micrometers.