Electrical characterization of p-GaAs epilayers disordered by doped spin-on-glass

Impurity-free disordering (IFD) of uniformly dopedp‐GaAsepitaxial layers was achieved using either undoped or doped (Ga or P) spin-on-glass (SOG) in conjunction with rapid thermal annealing in the temperature range from 800to925°C. Capacitance-voltage measurements showed a pronounced increase in the doping concentration (NA) in the near-surface region of the layers disordered using both undoped and P:SOG. The increase in NA showed an Arrhenius-like dependence on the inverse of annealing temperature. On the other hand, NA did not change significantly for Ga-doped SOG. These changes can be explained by the relative injection of excess gallium vacancies (VGa) during IFD of p‐GaAs by the different SOG layers. Deep-level transient spectroscopy showed a corresponding increase in the concentration of a defect HA (EV+0.39eV), which can be attributed to Cu, in the undoped and P:SOG disordered p‐GaAs layers, but not in the epilayers disordered by Ga:SOG. We have explained the increase in free carrier concentration by the segregation of Zn atoms towards the surface during the injection of VGa. The redistribution of Zn during disordering of buried marker layers in GaAs and Al₀.₆Ga₀.₄As using either undoped or Ga-doped SOG was verified by secondary-ion mass spectrometry.One of the authors P. N. K. D.d acknowledges the financial support of the Australian Research Council. A second sF. D. A.d is grateful to the National Research Foundation, South Africa, for its financial support

The human resource implications of improving financial risk protection for mothers and newborns in Zimbabwe

Peer reviewedPublisher PD

Do exchange rates follow random walks? A variance ratio test of the Zambian foreignexchange market

The random-walk hypothesis in foreign-exchange rates market is one of the most researched areas, particularly in developed economies. However, emerging markets in sub-Saharan Africa have received little attention in this regard. This study applies Lo and MacKinlay’s (1988) conventional variance ratio test and Wright’s (2000) non-parametric ranks- and signs-based variance ratio tests to examine the validity of the random-walk hypothesis in the Zambian foreign-exchange market. The study utilises daily nominal United States dollar/Zambian kwacha (USD/ZMK) exchange-rate returns for data from August 2003 to December 2012. Both types of variance ratio tests reject the random-walk hypothesis over the data span. The implication is that technical and fundamental analysis can help traders and other investors to earn higher-than-average market returns.Key words: variance ratio tests, random-walk hypothesis, exchange rates, marketefficienc

Distinction between the Poole-Frenkel and tunneling models of electric field-stimulated carrier emission from deep levels in semiconductors

The enhancement of the emission rate of charge carriers from deep-level defects in electric field is routinely used to determine the charge state of the defects. However, only a limited number of defects can be satisfactorily described by the Poole-Frenkel theory. An electric field dependence different from that expected from the Poole-Frenkel theory has been repeatedly reported in the literature, and no unambiguous identification of the charge state of the defect could be made. In this article, the electric field dependencies of emission of carriers from DX centers in AlxGa1-xAs:Te, Cu pairs in silicon, and Ge:Hg have been studied applying static and terahertz electric fields, and analyzed by using the models of Poole-Frenkel and phonon assisted tunneling. It is shown that phonon assisted tunneling and Poole-Frenkel emission are two competitive mechanisms of enhancement of emission of carriers, and their relative contribution is determined by the charge state of the defect and by the electric-field strength. At high-electric field strengths carrier emission is dominated by tunneling independently of the charge state of the impurity. For neutral impurities, where Poole-Frenkel lowering of the emission barrier does not occur, the phonon assisted tunneling model describes well the experimental data also in the low-field region. For charged impurities the transition from phonon assisted tunneling at high fields to Poole-Frenkel effect at low fields can be traced back. It is suggested that the Poole-Frenkel and tunneling models can be distinguished by plotting logarithm of the emission rate against the square root or against the square of the electric field, respectively. This analysis enables one to unambiguously determine the charge state of a deep-level defect

Effect of Co-60 gamma-ray irradiation on electrical properties of Ti/Au/GaAs1-xNx Schottky diodes

Current-voltage (I-V), capacitance-voltage-frequency (C-V-f) and conductance-voltage-frequency (G/ω-V-f) measurements at room temperature are used to study 50 kGy 60Co γ-ray electrical properties irradiation dependence of Ti/Au/GaAs1−xNx Schottky diodes with 0.2%; 0.4%; 0.8% and 1.2% nitrogen dilution. This γ-ray irradiation induces a permanent damage that has increased ideality factor and series resistance for all samples. It was accompanied by a decrease in Schottky barrier height with nitrogen content up to 0.4%N and remained constant thereafter. Radiation was also found to degrade the reverse leakage current. At high frequency (1 MHz), capacitance and conductance decreased after radiation due to a decrease in net doping concentration. Interface state density and series resistance were determined from C-V-f and G/ω-V-f characteristics using Hill-Coleman methods. Interface states density exponentially decreased with increasing frequency confirming the behavior of interface traps response to ac signal. Series resistance increases after irradiation is attributed to carrier's removal effect and mobility degradation. It has two peaks in the accumulation and inversion region for some diodes (0.4%N, 0.8%N). γ-ray irradiation produced traps levels and recombination centers that reduce relaxation time. An increase in %N content can impede irradiation damage with even some compensation when the percent of diluted nitrogen is high (1.2%N)

Engaging fringe stakeholders in business and society research: applying visual participatory research methods

Business and society (B&S) researchers, as well as practitioners, have been critiqued for ignoring those with less voice and power (e.g. women, non-literate or indigenous peoples) often referred to as ‘fringe stakeholders’. Existing methods used in B&S research often fail to address issues of meaningful participation, voice and power, especially in developing countries. In this article we stress the utility of visual participatory research (VPR) methods in B&S research to fill this gap. Through a case study on engaging Ghanaian cocoa farmers on gender inequality issues we explore how VPR methods may be used by researchers to achieve more inclusive, and thus more credible, stakeholder research that can improve decision-making within businesses. Furthermore, we argue that ingrained social and environmental problems tackled by B&S research and the unique context in which they occur may open up new opportunities to develop participatory visual methods for social change

RBS investigation of annealed thin gold layers on crystalline germanium

In this work we report firstly on the behaviour of Schottky barrier diodes (SBD's) when subjected to thermal treatment after metallization. To better understand this, a systematic study of the interaction between thin gold films and crystalline germanium substrates was undertaken. Gold metal films having thicknesses of 30 and 100 nm have been prepared by means of thermal evaporation on bulk-grown (111) n -type germanium doped with Sb to a level of 2.5×10 15 cm -3 . Before metallization the samples were first degreased and then etched in a mixture of H 2 O 2 :H 2 O (1:5) for one minute. Subsequently the samples have been thermally treated in Ar-atmosphere for 10 minutes and at temperatures ranging from 300 to 600°C. Rutherford backscattering spectrometry (RBS) has been performed to estimate the composition of the as-deposited and thermally treated films. It was found, that the composition of the as-deposited film remains unchanged under thermal treatment up to 340°C. Between 340°C and 360°C a gold-rich layer containing a very small amount of germanium is formed. At 361°C this layer suddenly converts to a germanium-rich layer with a small amount of gold. This transition is accompanied by the formation of agglomerates on the surface of the substrate

Electrical Characterization of 1.8 MeV Proton-Bombarded ZnO

We report on the electrical characterization of single-crystal ZnO and Au Schottky contacts formed thereon before and after bombarding them with 1.8 MeV protons. From capacitance–voltage measurements, we found that ZnO is remarkably resistant to high-energy proton bombardment and that each incident proton removes about two orders of magnitude less carriers than in GaN. Deep level transient spectroscopy indicates a similar effect: the two electron traps detected are introduced in extremely low rates. One possible interpretation of these results is that the primary radiation-induced defects in ZnO may be unstable at room temperature and anneal out without leaving harmful defects that are responsible for carrier compensation