Nonstoichiometry and defects in III–V compounds

The deviation from stoichiometry or nonstoichiometry characterises a homogeneity range of chemical compounds. It is determined as the difference in the nonmetal- to metal atoms ratio between a real A_nB_m+δ (δ><0) and stoichiometric A_nB_m composition. Nonstoichiometry creates defects which have an effect on all properties of a crystal. The classification of defect is given. The enthalpies and entropies of quasi-chemical reactions describing the defects composition of III–V compounds are presented and give the possibility to estimate the concentration of the major defects for the solubility limits of III–V compounds.     

Influence of ionizing radiation in electronic and optoelectronic properties of III–V semiconductor compounds

The interaction between radiation and matter is very important in the study of materials used in the aerospace industry. The improvement of the resistance of various devices is crucial. In the present work we have produced simulation results of damages induced in electronic devices of III–V semiconductor compounds, using SRIM-TRIM, CASINO and GEANT4 programs. The energies used for ⁺ particles, SRIM-TRIM, were from 500 keV to 4 MeV for both FETs and HEMTs and the energies used for β⁻ particles, CASINO, were from 1 to 500 keV.     

III–V semiconductor waveguiding devices using adiabatic tapers

In the past few years much effort has been put into the fabrication and optimization of III–V semiconductor waveguiding devices with integrated adiabatic mode size converters (tapers). By integrating a taper with a waveguide device, one wants to reduce the coupling losses and the packaging cost of OEICs in future optical communication systems. This paper gives an overview of different taper designs, their performance and the technological approaches used in realizing such tapered devices.     

The junction depth of concentration-dependent diffusion. Zinc in III–V compounds

The dependence of the junction depth on various parameters in a concentration-dependent diffusion is considered. The result is applicable to any diffusion in which the diffusion coefficient is expressible in terms of the ratio of the concentration to the surface concentration, and decreases at low concentrations. In particular, the process of interstitial-substitutional diffusion is investigated. The derived expression of the junction depth is compared and found in agreement with the experimental results of zinc diffusion in some III–V compound semiconductors. This result enables one to predict the junction depth under various experimental conditions. It also supports an interstitial-substitutional mechanism for zinc diffusion in all the semiconductors under consideration.     

Reliability issues in III–V compound semiconductor devices: optical devices and GaAs-based HBTs

This paper reviews the current status of reliability issues in III–V optical devices, semiconductor lasers and light emitting diodes, and GaAs-based heterojunction bipolar transistors (HBTs). First, material issues in III–V alloy semiconductors and our current understanding of degradation in III–V semiconductor lasers and light emitting diodes are systematically presented. Generation of defects and thermal instability are among these issues for these systems. Defects introduced during crystal growth are classified into two types: interface defects and bulk defects. Defects belonging to the former type are stacking faults, V-shaped dislocations, dislocation clusters, microtwins, inclusions, and misfit dislocations, and the latter group includes precipitates and dislocation loops. Defects in the substrate can also be propagated into the epi-layer. Structural imperfections due to thermal instability are also found. They are quasi-periodic modulated structures due to spinodal decomposition of the crystal either at the liquid–solid interface or growth surface, and atomic ordering which also occurs on the growth surface through migration and reconstruction of the deposited atoms. Three major degradation modes of optical devices, rapid degradation, gradual degradation, and catastrophic failure, are discussed. For rapid degradation, recombination-enhanced dislocation climb and glide are responsible for degradation. Differences in the ease with which these phenomena occur in different heterostructures are presented. Based on the results, dominant parameters involved in the phenomena are discussed. Gradual degradation takes place presumably due to recombination enhanced point defect reaction in GaAlAs/GaAs-based optical devices. This mode is also enhanced by the internal stress due to lattice mismatch. However, we do not observe this mode in InGaAsP/InP-based optical devices. Catastrophic failure is found to be due to catastrophic optical damage at a mirror or at a defect in GaAlAs/GaAs double-heterostructure (DH) lasers, but not in InGaAsP/InP DH lasers. In each degradation mode, the role of defects in the degradation and methods of elimination of degradation are discussed. Secondly, we review the current status of two major reliability issues in GaAs-based HBTs, particularly InGaP/GaAs HBTs: degradation in current gain (β) and variation of turn-on voltage (V_be). In the case of AlGaAs/GaAs HBTs, the β gradually decreased, then drastically degraded. After degradation, the device exhibits an increase in base current I_b, which has an ideality factor n≈2 in the Gummel plot. The activation energy for the degradation was estimated to be 0.6 ± 0.1 eV. On the other hand, in InGaP/GaAs HBTs, much higher reliability than in AlGaAs/GaAs HBTs was achieved although the degradation mode is similar. The estimated E_a and time to failure for InGaP/GaAs HBTs are 2.0 ± 0.2 eV and 10⁶ h at T_j = 200°C, respectively, which are the highest values ever reported. We also review previously proposed degradation mechanisms for GaAs-based HBTs: hydrogen reactivation, microtwin-like defect formation, dark defect formation and carbon precipitation. TEM observation of a degraded InGaP/GaAs HBT indicated that there are at least two possible degradation mechanisms: formation of carbon precipitates in the base region and migration of metallic impurities from the base electrode to the base region. The second issue is concerned with the exponential increase in V_be with operating time. The mechanism for the increase in V_be has been clarified based on reactivation of passivated carbon acceptors in the base region during operation. If the device suffers from H⁺ isolation, V_be rapidly decreases at the initial stage, then exponentially increases. The first stage of V_be variation can be explained by the fact that a high density of hydrogen atoms migrating from the isolation region to the intrinsic base region, passivate the carbon atoms at the initial stage. From these results, one can expect that the use of He⁺ as an implant instead of H⁺ can solve this problem.     

Photo-Assisted Wet-Etched III–V Heterostructure Laser Mirrors

We demonstrate the first photo-assisted wet-etched flat-surface mirrors in III–V laser heterostructure having an average reflectance of 0.87 of the ideal cleaved facet reflectance $R _{ c}$ independent of crystal orientation. The largest observed reflectance was $0.93R _{ c}$. These lasers were fabricated using a 532-nm laser source, sulfuric acid, and exposed-metal mask. The lateral waveguiding formed from air–semiconductor interfaces produced an average internal loss of 28/cm (smallest of 14/cm) compared with similar cleaved-mirror dark-etched lasers having 46/cm.      

Explanation for the dark I–V curve of III–V concentrator solar cells

The measurement of the dark I–V curve is one of the most straightforward methods for characterizing solar cells. Consequently, an accurate knowledge of its meaning is of high relevance for the comprehension and technological feedback of these devices. In this paper, an explanation of the dark I–V curve for concentrator III–V solar cells is presented using a 3D (three‐dimensional) model in order to provide a proper data fit that provides meaningful physical parameters that are also compatible and coherent with a data fit from illumination curves. The influence on the dark I–V curve of the most significant series resistance components of concentrator solar cells is also analysed concluding that only the vertical component as well as the front contact‐specific resistance can be assessable by means of this characterization method while both emitter and metal sheet resistances cannot be detected. For comparison purposes, the same experimental data have been fitted by means of a traditional two‐diode model showing that, although an accurate dark I–V curve fitting can be achieved, the extracted parameters are unable to reproduce illumination data since lumped models assume the same ohmic losses distribution for both dark and illumination conditions. Copyright © 2007 John Wiley & Sons, Ltd.     

Designing III–V multijunction solar cells on silicon

Single junction Si solar cells dominate photovoltaics but are close to their efficiency limits. This paper presents ideal limiting efficiencies for tandem and triple junction multijunction solar cells featuring a Si subcell also serving as substrate. Subject to this Si bandgap constraint, we design optimum cell structures that we show depart from the unconstrained ideal. In order to progress to manufacturable designs, the use of III–V materials is considered, using a novel growth method capable of yielding low defect density III–V layers on Si. In order to evaluate the real potential of these proposed multijunction designs, a quantitative model is presented, the strength of which is the joint modelling of external quantum efficiency and current–voltage characteristics using the same parameters. The method yields a single‐parameter fit in terms of the Shockley–Read–Hall lifetime. This model is validated by fitting experimental data of external quantum efficiency, dark current and conversion efficiency of world record tandem and triple junction cells under terrestrial solar spectra without concentration. We apply this quantitative model to the design of tandem and triple junction solar cells, yielding cell designs capable of reaching efficiencies without concentration of 32% for the best tandem cell and 36% for the best triple junction cell. This demonstrates that efficiencies within a few per cent of world records are realistically achievable without the use of concentrating optics, with growth methods being developed for multijunction cells combining III–V and Si materials. Copyright © 2014 John Wiley & Sons, Ltd.     

Degradation study of III–V solar cells for concentrator applications

AlGaAs/GaAs heteroface solar cells with a high aluminium content tend to degrade. The degradation mechanism has been examined and appropriate accelerated ageing procedures have been established. They effectively test the ruggedness of the device against oxidation. Changing the window layer material to (Al_xGa_1−x)_0.51In_0.49P with x = 0, 0.5 or 1 leads to stable devices. In addition, III–V tandem solar cells for concentrator applications were subjected to accelerated ageing tests. They proved to be robust against oxidation. The potential degradation due to the high current density involved in concentrator solar cells was assessed in preliminary experiments. Copyright © 2005 John Wiley & Sons, Ltd.     

Wafer reuse for repeated growth of III–V solar cells

The epitaxial lift‐off (ELO) technique can be used to separate a III–V solar cell structure from its underlying GaAs or Ge substrate. ELO from 4‐inch Ge wafers is shown and 2‐inch GaAs wafer reuse after lift‐off is demonstrated without degradation in performance of the subsequent thin‐film GaAs solar cells that were retrieved from it. Since a basic wet chemical smoothing etch procedure appeared insufficient to remove all the surface contamination, wafer re‐preparation is done by a chemo‐mechanical polishing procedure. Copyright © 2010 John Wiley & Sons, Ltd.     

Limiting factors on the semiconductor structure of III–V multijunction solar cells for ultra‐high concentration (1000–5000 suns)

The main limiting factors of multijunction solar cells operating under ultra‐high concentration (>1000 suns) are examined by means of 2D physically based numerical modelling. The validation of the model is carried out by fitting calibrated light concentration measurements. Because the series resistance is the most important constraint in the electrical performance of the solar cell under ultra‐high irradiance, it is analysed and quantified detailing different contributions such as: (i) the electrical properties of the emitter; (ii) window layer of the top cell; and (iii) the band discontinuities formed at heterojunctions. We found the role of window layer to be important at very high concentrations (above 700 suns), while at ultra‐high concentrations, (above 1000 suns) a gain in efficiency (~ 1% absolute) can be obtained by a proper structural design of the window layer. In the case of the heterojunctions included in the multijunction solar cell, the impact of a high‐band offset can be mitigated by increasing the doping level density thus favouring the tunnelling effect. Moreover, the influence of different recombination mechanisms and high‐injection effects at ultra‐high irradiance is discussed. Finally, an optimisation of the complete solar cell taking into account the ohmic contacts to work under ultra‐high irradiances (from 1000 to 5000 suns) is presented as well as the implications on the use of ultra‐high irradiance in different multijunction solar cell architectures. Copyright © 2016 John Wiley & Sons, Ltd.     

Band-Structure Effects on the Performance of III–V Ultrathin-Body SOI MOSFETs

This paper examines the impact of band structure on deeply scaled III-V devices by using a self-consistent 20-band -SO semiempirical atomistic tight-binding model. The density of states and the ballistic transport for both GaAs and InAs ultrathin-body n-MOSFETs are calculated and compared with the commonly used bulk effective mass approximation, including all the valleys (, , and ). Our results show that for III-V semiconductors under strong quantum confinement, the conduction band nonparabolicity affects the confinement effective masses and, therefore, changes the relative importance of different valleys. A parabolic effective mass model with bulk effective masses fails to capture these effects and leads to significant errors, and therefore, a rigorous treatment of the full band structure is required.     

Analysis of temperature coefficients for III–V multi‐junction concentrator cells

The temperature dependence of the I–V parameters of different III–V multi‐junction concentrator cells at several concentration levels was investigated. Moreover, the influence of spectral changes on the temperature coefficients of multi‐junction solar cells was examined. Complete sets of temperature coefficients of a metamorphic Ga_0.35In_0.65P/Ga_0.83In_0.17As dual‐junction cell, a metamorphic Ga_0.35In_0.65P/Ga_0.83In_0.17As/Ge triple‐junction cell and a lattice‐matched Ga_0.50In_0.50P/Ga_0.99In_0.01As/Ge triple‐junction cell determined under well‐controlled laboratory conditions are reported. Copyright © 2012 John Wiley & Sons, Ltd.     

Facet formation and characterization of III–V structures grown on patterned surfaces

In selective area growth there is a lateral transition from growth to non-growth areas. At this point the growth is determined by the lowest growing crystal planes. This review summarizes the mechanisms during the facet formation in the InP/GaInAsP material system with respect to the growth conditions in metalorganic molecular beam epitaxy. The effect of interfacet diffusion and the anisotropic surface diffusion process as well as the molecular beam flux density at the facets is discussed. Planar selective area epitaxy (SAE), where the facets can evolve freely, is selected as the starting point. Low lateral growth rates at side wall (01 ) planes of the structure are achieved under perpendicular molecular beam geometry. The results are transferred to embedded SAE for the lateral coupling of heterostructures having constant material compositions up to the lateral contact. Applications for SAE-grown waveguides and laser-waveguide integration are presented.     

Spectral and dynamical study of III–V triple junction solar cells and the application to multiflash I–V measurement

This paper presents a method to predict the I–V characteristic of triple junction InGaP/GaAs/Ge solar cells when different illumination spectra are used, and it is based on the measurement of a set of commercial isotype cells together with numerical simulations. The study includes the utilization of continuous and pulsed light sources. Several spectra were considered for the continuous light sources, where different subcells limit the current of the tandem cell. For pulsed light sources, a dynamical analysis was carried out by simulating a triple junction cell through PSpice software. For the simulations, the subcell capacitances were estimated and introduced into an electrical model of the triple junction. When a fast pulse in the multiflash I–V measurement technique is used, dynamical effects associated with this kind of source were observed and accounted for by the simulations. It was established that the dynamical effects did not affect the I–V characteristics measured with this technique. Finally, the proposed method and analysis were successfully applied to the electrical characterization of the Aquarius/SAC‐D Argentine satellite solar array modules. Copyright © 2014 John Wiley & Sons, Ltd.     

A New Understanding of Au‐Assisted Growth of III–V Semiconductor Nanowires

Semiconductor nanowires of III–V materials have generated much interest in recent years. However, the growth mechanisms by which these structures form are not well understood. The so‐called vapor–liquid–solid (VLS) mechanism has often been proposed for III–V systems, with a chemically inert, liquid metal particle (typically Au) acting as a physical catalyst. We assert here that Au is, in fact, not inert with respect to the semiconductor material but rather interacts with it to form a variety of intermetallic compounds. Moreover, we suggest that III–V nanowire growth can best be understood if the metallic particle is not a liquid, but a solid‐phase solution or compound containing Au and the group III material. The four materials GaP, GaAs, InP, and InAs will be considered, and growth behavior related to their particular temperature‐dependent interaction with Au.     

An accurate and compact large signal model for III–V HBT devices

An accurate and compact large signal model is proposed for modeling heterojunction bipolar transistors (HBTs) based on III–V materials. In DC mode, the model includes self-heating, Kirk and Early effects, as well as the temperature dependence of the model parameters. In small signal mode, the model captures the variation of various AC parameters with bias. The procedure of extracting the model parameters uses DC and multiple bias S-parameter measurements. The model is compiled in the HP–ADS circuit simulator as user-compiled model and is verified by comparing its simulations to measurements in all modes of operation for an AlGaAs/GaAs transistor with an emitter area of 2 × 25 μm².     

Four‐junction solar cells using lattice‐matched II–VI and III–V semiconductors

Four‐junction solar cells are designed using lattice‐matched II–VI (ZnCdSeTe) and III–V (AlGaAsSb) semiconductors grown on GaSb substrates. These materials have a zinc blende crystal structure, similar thermal expansion coefficients, and bandgaps that cover the entire solar spectrum. Numerical simulations of the energy conversion efficiencies of various designs for both the AM0 and AM1.5D spectra are performed using published material parameters. These results indicate that the achievable 1 sun AM0 efficiency is 43% for an optimal design and 40% for a more practical design; for comparison the ideal limit provided by Henry's model is 49%. While for the AM1.5D spectrum an optimal design can reach 46% under 1 sun and 55% under 1000 suns while a more practical design can reach 44 and 54%, respectively; for comparison Henry's model gives 51 and 62%, respectively. Copyright © 2010 John Wiley & Sons, Ltd.     

Photolithographic Route to the Fabrication of Micro/Nanowires of III–V Semiconductors

Nano/microwires of semiconducting materials (e.g., GaAs and InP) with triangular cross‐sections can be fabricated by “top–down” approaches that combine lithography of high‐quality bulk wafers (using either traditional photolithography or phase‐shift optical lithography) with anisotropic chemical etching. This method gives good control over the lateral dimensions, lengths, and morphologies of free‐standing wires. The behaviors of many different resist layers and etching chemistries are presented. It is shown how wire arrays with highly ordered alignments can be transfer printed onto plastic substrates. This “top–down” approach provides a simple, effective, and versatile way of generating high‐quality single‐crystalline wires of various compound semiconductors. The resultant wires and wire arrays have potential applications in electronics, optics, optoelectronics, and sensing.     

Ultra‐thin,high performance tunnel junctions for III–V multijunction cells

Tunnel junctions (TJ) made of p‐Al_0.1 Ga_0.9As/n‐GaAs are used because of their high peak current and low series resistance, but are not fully transparent. The influence of reducing the thickness of these tunnel junctions on the characteristics of InGaP/GaAs tandem cells was investigated. It was found that ultra‐thin TJs with excellent performance can be realized. Even for a 7.5/6‐nm thick TJ, which is the thinnest possible in our growth reactor, the peak current density is at least 600 A/cm². The series resistance of the TJs was found to be at a constant level of 0.6 ± 0.2 mΩ cm² for all total thicknesses of the TJ in the 13.5–40 nm range. Because of a lower absorption in the TJ, a tandem cell with a 7.5/6‐nm thick TJ, compared with a cell with a 20/20‐nm thick TJ, gained 0.53 ± 0.05 mA/cm² in short circuit current to a value of 14.8 mA/cm². Copyright © 2012 John Wiley & Sons, Ltd.