Optical properties of thin semiconductor device structures with reflective back-surface layers

Ultrathin semiconductor device structures incorporating reflective internal or back surface layers have been investigated recently as a means of improving photon recuperation, eliminating losses associated with free carrier absorption in conductive substrates and increasing the above bandgap optical thickness of thermophotovoltaic device structures. However, optical losses in the form of resonance absorptions in these ultrathin devices have been observed. This behavior in cells incorporating epitaxially grown FeAl layers and in devices that lack a substrate but have a back-surface reflector (BSR) at the rear of the active layers has been studied experimentally and modeled effectively. For thermophotovoltaic devices, these resonances represent a significant loss mechanism since the wavelengths at which they occur are defined by the active TPV cell thickness of {approximately} 2--5 microns and are in a spectral range of significant energy content for thermal radiators. This study demonstrates that ultrathin semiconductor structures that are clad by such highly reflective layers or by films with largely different indices of refraction display resonance absorptions that can only be overcome through the implementation of some external spectral control strategy. Effective broadband, below-bandgap spectral control using a back-surface reflector is only achievable using a large separation between the TPV active layers and the back-surface reflector

Nanoscale characterization of metal/semiconductor nanocontacts

Ballistic Electron Emission Microscopy (BEEM) and finite-element electrostatic modeling were used to quantify how "small-size" effects modify the energy barrier at metal/semiconductor nanostructure nanocontacts, formed by making Schottky contacts to cleaved edges of GaAs quantum wells (QWs). The Schottky barrier height over the QWs was found to systematically increase with decreasing QW width, by up to ???140 meV for a 1nm QW. This is mostly due to a large quantum-confinement increase (???200 meV for a 1nm QW), modified by smaller decreases due to "environmental" electric field effects. Our modeling gives excellent quantitative agreement with measurements for a wide range of QW widths when both quantum confinement and environmental electric fields are considered.close1

Growth and properties of InGaAs/FeAl/InAlAs/InP heterostructures for buried reflector/interconnect applications in InGaAs thermophotovoltaic devices

Thermophotovoltaic cells consisting of InGaAs active layers are of extreme promise for high efficiency, low bandgap TPV conversion. In the monolithic interconnected module configuration, the presence of the InGaAs lateral conduction layer (LCL) necessary for the series connection between TPV cells results in undesirable free carrier absorption, causing a tradeoff between series resistance and optical absorption losses in the infrared. A potential alternative is to replace the LCL with an epitaxial metal layer that would provide a low-resistance interconnect while not suffering from free carrier absorption. The internal metal layer would also serve as an efficient, panchromatic back surface reflector, providing the additional advantage of increased effective optical thickness of the InGaAs cell. In this paper, the authors present the first results on the growth and development of buried epitaxial metal layers for TPV applications. High quality, single crystal, epitaxial Fe{sub x}Al{sub 1{minus}x} layers were grown on InAlAs/InP substrates, having compositions in the range x = 0.40--0.80. Epitaxial metal layers up to 1,000 {angstrom} in thickness were achieved, with excellent uniformity over large areas and atomically smooth surfaces. X-ray diffraction studies indicate that all FeAl layers are strained with respect to the substrate, for the entire composition range studied and for all thicknesses. The FeAl layers exhibit excellent resistance characteristics, with resistivities from 60 {micro}ohm-cm to 100 {micro}ohm-cm, indicating that interface scattering has a negligible effect on lateral conductivity. Reflectance measurements show that the FeAl thickness must be at least 1,000 {angstrom} to achieve > 90% reflection in the infrared

Optical properties of thin semiconductor device structures with reflective back-surface layers

Ultrathin semiconductor device structures incorporating reflective internal or back surface layers have been investigated recently as a means of improving photon recuperation, eliminating losses associated with free carrier absorption in conductive substrates and increasing the above bandgap optical thickness of thermophotovoltaic device structures. However, optical losses in the form of resonance absorptions in these ultrathin devices have been observed. This behavior in cells incorporating epitaxially grown FeAl layers and in devices that lack a substrate but have a back-surface reflector (BSR) at the rear of the active layers has been studied experimentally and modeled effectively. For thermophotovoltaic devices, these resonances represent a significant loss mechanism since the wavelengths at which they occur are defined by the active TPV cell thickness of {approximately} 2--5 microns and are in a spectral range of significant energy content for thermal radiators. This study demonstrates that ultrathin semiconductor structures that are clad by such highly reflective layers or by films with largely different indices of refraction display resonance absorptions that can only be overcome through the implementation of some external spectral control strategy. Effective broadband, below-bandgap spectral control using a back-surface reflector is only achievable using a large separation between the TPV active layers and the back-surface reflector