J. M. Borrego

Recombination processes in doubly capped antimonide-based quaternary thin films

Recombination processes in antimonide-based materials for thermophotovoltaic (TPV) devices have been investigated using a radio-frequency (rf) photoreflectance technique, in which a Nd–YAG pulsed laser is used to excite excess carriers, and the short-pulse response and photoconductivity decay are monitored with an inductively coupled noncontacting rf probe. Both lattice-matched AlGaAsSb and GaSb have been used to double cap InGaAsSb active layers to evaluate bulk lifetime and surface recombination velocity with different active layer thicknesses. With an active layer doping of 2×1017 cm−3, effective bulk lifetimes of 95 ns and surface recombination velocities of 1900 cm/s have been obtained. As the laser intensity is increased the lifetime decreases, which is attributed to radiative recombination under these high-level injection conditions. Similar measurements have been taken on both TPV device structures and starting substrate materials for comparison purposes.

Electrical characteristics of GaAs MIS Schottky diodes

Abstract The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of GaAs metal-insulator-semiconductor (MIS) Schottky barrier diodes are investigated over a wide temperature range and compared with MS diodes. The effects of the insulating layer on barrier height and carrier transport are delineated by an activation energy analysis. Excess currents observed at low forward and reverse bias have also been analyzed and their cause identified. A capacitance anomaly consistently noticed in MIS Schottky barriers is resolved by stipulating a non-uniform interfacial layer, and a self-consistent model of the GaAs MIS Schottky barrier is developed by analyzing I-V and C-V data of both MIS and MS diodes.

InGaAsSb thermophotovoltaic diode: Physics evaluation

The hotside operating temperatures for many projected thermophotovoltaic (TPV) conversion system applications are approximately 1000 °C, which sets an upper limit on the TPV diode band gap of 0.6 eV from efficiency and power density considerations. This band gap requirement has necessitated the development of new diode material systems never previously considered for energy generation. To date, InGaAsSb quaternary diodes grown lattice matched on GaSb substrates have achieved the highest performance. In this article we relate observed diode performance to electro-optical properties such as minority carrier lifetime, diffusion length, and mobility and provide initial links to microstructural properties. This analysis has bounded potential diode performance improvements. For the 0.53 eV InGaAsSb diodes used in this analysis (active layer doping is 2×1017 cm−3) the dark current density measured is 2×10−5 A/cm2 versus a potential Auger and/or a radiative limit of 2×10−6 A/cm2 (no photon recycling), and an abso...

Non-destructive lifetime measurement in silicon wafers by microwave reflection

Abstract A non-destructive method for measuring excess carrier lifetime in semiconductor wafers is presented. The method is based on detecting the decay in the microwave reflection following the introduction of excess carriers by a pulsed light source. The theory of measurement has been tested by comparing the lifetime measured by this technique with other more conventional methods and good agreement was found among the values obtained. The method appears to be appropriate for distinguishing wafers with short Defect Free Zone wafers from those of long DFZ.

Thermodynamic analysis of thermophotovoltaic efficiency and power density tradeoffs

This report presents an assessment of the efficiency and power density limitations of thermophotovoltaic (TPV) energy conversion systems for both ideal (radiative limited) and practical (defect-limited) systems. Thermodynamics is integrated into the unique physics of TPV conversion, and used to define the intrinsic tradeoff between power density and efficiency. The results of the analysis reveal that the selection of diode band gap sets a limit on achievable efficiency well below the traditional Carnot level. In addition it is shown that filter performance dominates diode performance in any practical TPV system and determines the optimum band gap for a given radiator temperature. It is demonstrated that for a given radiator temperature, lower band-gap diodes enable both higher efficiency and power density when spectral control limitations are included. The goal of this work is to provide a better understanding of the basic system limitations that will enable successful long-term development of TPV energy ...

Ion-cleaning damage in (100) GaAs, and its effect on schottky diodes

Abstract This study investigates the effect of ion cleaning damage of (100) GaAs in the 100–1000 eV range, and also its recovery with thermal annealing to 400°C. It is shown that GaAs could be annealed to a considerable extent if the ion-damage was ⩽ 100 eV. However, full recovery was not achieved. On the other hand, samples damaged at ⩾ 400 eV became progressively worse with annealing. Measurements indicate that these samples are dominated by the effect of arsenic variances within the bulk. These remain in the bulk, but are distributed spatially upon annealing. They behave as deep donors, so that the net electron concentration in the bulk is enhanced. Aluminum-n GaAs Schottky diodes were used as a vehicle for this study.

Surface recombination velocity and lifetime in InP

Abstract Lifetime and surface recombination velocity in InP are discussed in this paper. It is shown that carrier trapping may be important in the interpretation of transient lifetime measurements. The surface recombination velocity is found to be doping dependent, indicating pinning of the surface Fermi level. The effects of the surface Fermi level on the surface recombination velocity and the surface generation velocity are used to study the Fermi level pinning at the surface of InP. The capture cross-sections of surface states are estimated to be on the order of 1 × 10−13 cm2.

The effect of hole versus electron photocurrent on microwave—Optical interactions in IMPATT oscillators

The effect of hole versus electron photocurrent on the microwave properties of IMPATT oscillators is presented and correlated with staticI-Vcharacteristics. The composition of the photocurrent was altered by fabricating both flip-chip (FC) and top-mounted (TM) devices and using an optical source with an absorption depth comparable to both the depletion-layer thickness and substrate diffusion length. The importance of the order of magnitude difference in effect of hole versus electron photocurrent is discussed.

Fabrication of GaAs tunnel junctions by a rapid thermal diffusion process

A rapid thermal diffusion process for the fabrication of GaAs tunnel junctions, utilizing a doped oxide zinc source and a protective cap layer of phosphosilicate glass, is described in this letter. It is shown that tunnel junctions fabricated by this open tube process are suited for low impedance interconnects in tandem solar cells, and also for tunnel diode field‐effect transistor logic applications. The resulting voltage‐current characteristics in both silicon and sulfur doped n+ epilayers, and the peak current as a function of effective doping concentration, are also presented. It is shown that diodes made by this process are comparable in electrical properties to those made by molecular beam epitaxy.

Interface state density in Au-nGaAs Schottky diodes☆

Abstract A method for determining the surface state density in Schottky diodes taking into account both I–V and C–V data while considering the presence of a deep donor level is presented. The model assumes that the barrier height is controlled by the energy distribution of surface states in equilibrium with the metal and the applied potential and does not include, explicitly, an interfacial layer. The model was applied to extract interface state densities of Au- n GaAs guarded Schottky diodes fabricated from bulk and VPE (100) GaAs with carrier conentrations between 3 × 10 15 and 8 × 10 16 cm −3 . These diodes exhibited ideality ( n ) factors of approximately 1.02 and room temperature saturation current densities ∼10 −8 A/cm 2 . This model is in substantial agreement with forward bias measurements over the 77–360°K temperature range investigated, in that a temperature-independent energy distribution of interface states was obtained. In reverse bias the interface state model is most valid with the higher carrier concentration material and at high temperature and low bias voltage. Typical interface state densities from 0.07 eV above the zero bias Fermi level to 0.01 eV below the Fermi level were 2 × 10 13 cm −2 eV −1 . The validity of the model under reverse bias is restricted by a non-thermionic reverse current, thought to be enhance field emission from traps.