Chaffra A. Affouda

Quantum wells and superlattices for III-V photovoltaics and photodetectors

Semiconductor quantum wells and superlattices have found numerous applications in optoelectronic devices, such as lasers, LEDs and SOAs, and are an increasingly common feature of high efficiency solar cells and photodetectors. In this paper we will highlight some of the recent developments in the use of low-dimensional III-V semiconductors to improve the performance of photovoltaics by tailoring the bandgap of the junction. We also discuss novel structures designed to maximize photo-generated carrier escape and the application of quantum confinement to other components of the solar cell, such as tunnel junctions. Recent developments in type-II superlattices for photodetectors will also be discussed, including the graded-gap LWIR device based on the W-structured superlattices demonstrated at the Naval Research Laboratory. Modeled results will be presented using the NRL BANDSTM integrated 8-band kp and Poisson solver, which was developed for computing the bandstructures of superlattice and multi-quantum well photodiodes

Analysis and performance of type-II superlattice infrared detectors

We discuss the current performance of long-wavelength infrared photodetectors based on type-II superlattices, and the projected characteristics for diffusion-limited operation. For optimized architectures such as graded-gap and abrupt-heterojunction designs, the dark currents are strongly dominated by Shockley-Read (SR) rather than Auger processes. A factor of 10 improvement over the demonstrated SR lifetimes would lead to a factor of 4 lower dark current than state-of-the-art HgCdTe devices.

Recent developments in type-II superlattice-based infrared detectors

Much has been accomplished in the last few years in advancing the performance of type-II superlattice (T2SL) based infrared photodiodes, largely by focusing on device and heterostructure design. Quantum efficiency (QE) has increased to 50% and higher by using thicker absorbing layers and making use of internal reflections, and dark currents have been reduced by over a factor of ten by using bandstructure engineering to suppress tunneling and generation-recombination (G-R) currents associated with the junction. With performance levels of LWIR T2SL photodiodes now within an order of magnitude of that of HgCdTe (MCT) based technology, however, there is renewed interest in understanding fundamental materials issues. This is needed both to move performance toward the theoretical Auger limit, and to facilitate the task of transitioning T2SL growth from laboratories to commercial institutions. Here we discuss recent continuing efforts at NRL to develop new device structures for enhanced detector performance, and to further our understanding of this material system using advanced structural and electronic probes. Results from electron beam induced current (EBIC) imaging and analysis of point defects in T2SL photodiodes will be presented, showing differentiated behavior of bulk defect structures. We will also describe a study comparing intended vs. as-grown T2SL photodiode structures by crosssectional scanning microscopy (XSTM). Using parameters extracted from the XSTM images, we obtain detailed knowledge of the composition and layer structures through simulation of the x-ray diffraction spectra.

Mitigation of BPD by Pre-Epigrowth High Temperature Substrate Annealing

Basal Plane Dislocations (BPD) intersecting the SiC substrate surface were converted to threading edge dislocations (TED) by high temperature annealing of the substrates in the temperature range of 1750 °C – 1950 °C. Successively, epitaxial growth on annealed as well as non-annealed samples was performed, concurrently, to investigate the effect of the substrate annealing on BPD mitigation in the epilayers. For the 1950 °C/10min anneal, a 3x reduction in BPD density was observed. Additionally, surface roughness measured using atomic force microscopy revealed no degradation in surface morphology of the grown epilayers after annealing.

Degradation modeling of InGaP/GaAs/Ge triple-junction solar cells irradiated by protons

Experimental results on triple-junction solar cells irradiated by 3 MeV proton irradiation to very high damage levels are presented. The minority carrier transport properties were obtained through quantum efficiency and EBIC measurements and an analytical drift-diffusion solver was used in understanding the results for different degradation levels where multiple damage mechanisms are evident.

Effect of the oxide-semiconductor interface on the passivation of hybrid type-II superlattice long-wave infrared photodiodes

In order to be commercially viable, the type-II superlattice (T2SL) LWIR focal plane array technology will require the development of effective passivation of exposed surfaces. Here we investigate the relationship between the thickness and composition of the native oxide at the T2SL-SiO2 interface and the diode performance in terms of sidewall resistivity. Device performance is compared between samples with untreated surfaces, those for which the native oxides have been removed at various intervals prior to SiO2 deposition, and samples for which oxide growth was promoted by ozone exposure with and without a prior oxide strip. InAs- and GaSb-capped pieces were processed in an identical manner and studied using X-ray photoelectron spectroscopy (XPS). From these spectra, the compositions and thicknesses of the surface oxides just prior to SiO2 deposition were determined, complementing the electrical characterization of devices. Correlation of the performance and surface composition is presented.

Threading and Near-Surface Dislocations in InGaSb/AlSb Films with Blocking and Anti-Blocking Layers

In order to mitigate the formation of threading dislocations in an In0.1Ga0.9Sb heteroepitaxial buffer layer grown on a (001) GaSb substrate, an AlSb blocking layer was grown within the buffer layer. Transmission electron microscopy measurements revealed that the film with the AlSb blocking layer had a significantly lower threading dislocation density than a buffer layer of equivalent thickness in the absence of the blocking layer. Because the AlSb blocking layer is of greater mechanical stiffness than the In0.1Ga0.9Sb buffer layer, attractive image forces that act to draw dislocations to the film surface are countered by repulsive image forces traceable to the In0.1Ga0.9Sb/AlSb interface. These experimental results are consistent with calculations based on anisotropic elasticity theory for repulsive image forces acting on a dislocation beneath the blocking layer.

High absorption long wave infrared superlattices using metamorphic buffers

The narrow band gap and staggered band alignment of InAsSb alloys make it possible to engineer type-II superlattices (T2SLs) for mid-wave and long-wave (LW) infrared sensors operating in the 3–12 μm range. However, InAs/InAsSb T2SLs that are strain balanced to the underlying GaSb substrate have much lower absorption coefficients for LWIR operation because of the larger superlattice (SL) period, leading to reduced electron-hole overlap. The absorption coefficient of T2SLs can be greatly improved by growing on metamorphic buffers (MBs) with reduced lattice mismatch to the InAsSb layers, which allows the SL period to be reduced. For this study, MBs were capped with InAs/InAsSb T2SLs to assess the suitability of the materials for detector applications by X-ray diffraction and photoluminescence lifetime measurements. We show that the absorption of T2SLs can be significantly increased with no apparent degradation in the minority-carrier lifetime.

Wide bandgap, strain-balanced quantum well tunnel junctions on InP substrates

In this work, the electrical performance of strain-balanced quantum well tunnel junctions with varying designs is presented. Strain-balanced quantum well tunnel junctions comprising compressively strained InAlAs wells and tensile-strained InAlAs barriers were grown on InP substrates using solid-source molecular beam epitaxy. The use of InAlAs enables InP-based tunnel junction devices to be produced using wide bandgap layers, enabling high electrical performance with low absorption. The impact of well and barrier thickness on the electrical performance was investigated, in addition to the impact of Si and Be doping concentration. Finally, the impact of an InGaAs quantum well at the junction interface is presented, enabling a peak tunnel current density of 47.6 A/cm2 to be realized.

Effect of carrier confinement and doping in quantum well tunnel junctions

Quantum well tunnel junctions have attractive properties for the design of multi-junction solar cells and represent a good alternative to homo-junction tunnel diodes. Tunnel junctions based on InAlAs quantum wells were grown strain balanced on InP with varying well and barrier thicknesses. We characterized their current voltage characteristics and found that devices with thicker well and thinner tunnel barrier performed best provided the doping profile across the structure is accurately controlled. We explain the effects of barrier and well thicknesses due to quantum confinement as well as doping using a Poisson-Schrodinger model coupled to a non-local tunneling model that were implemented in a numerical drift-diffusion solver.