New negative differential resistance device based on resonant interband tunneling

We propose and demonstrate a novel negative differential resistance device based on resonant interband tunneling. Electrons in the InAs/AlSb/GaSb/AlSb/InAs structure tunnel from the InAs conduction band into a quantized state in the GaSb valence band, giving rise to a peak in the current-voltage characteristic. This heterostructure design virtually eliminates many of the competing transport mechanisms which limit the performance of conventional double-barrier structures. Peak-to-valley current ratios as high as 20 and 88 are observed at room temperature and liquid-nitrogen temperature, respectively. These are the highest values reported for any tunnel structure

Observation of large peak-to-valley current ratios and large peak current densities in AlSb/InAs/AlSb double-barrier tunnel structures

We report improved peak-to-valley current ratios and peak current densities in InAs/AlSb double-barrier, negative differential resistance tunnel structures. Our peak-to-valley current ratios are 2.9 at room temperature and 10 at liquid-nitrogen temperatures. Furthermore, we have observed peak current densities of 1.7×10^5 A/cm^2. These figures of merit are substantially better than previously reported values. The improvements are obtained by adding spacer layers near the barriers, thinner well regions, and thinner barriers

Demonstration of large peak-to-valley current ratios in InAs/AlGaSb/InAs single-barrier heterostructures

We report large peak-to-valley current ratios in InAs/AlxGa1−xSb/InAs single-barrier tunnel structures. The mechanism for single-barrier negative differential resistance (NDR) has been proposed and demonstrated recently. A peak-to-valley current ratio of 3.4 (1.2) at 77 K (295 K), which is substantially larger than what has been previously reported, was observed in a 200-Å-thick Al0.42Ga0.58Sb barrier. A comparison with a calculated current-voltage curve yields good agreement in terms of peak current and the slope of the NDR region. The single-barrier structure is a candidate for high-speed devices because of expected short tunneling times and a wide NDR region

Experimental observation of negative differential resistance from an InAs/GaSb interface

We have observed negative differential resistance at room temperature from devices consisting of a single interface between n-type InAs and p-type GaSb. InAs and GaSb have a type II staggered band alignment; hence, the negative differential resistance arises from the same mechanism as in a p+-n+ tunnel diode. Room-temperature peak current densities of 8.2×10^4 A/cm^2 and 4.2×10^4 A/cm^2 were measured for structures with and without undoped spacer layers at the heterointerface, respectively

Growth and characterization of ZnTe films grown on GaAs, InAs, GaSb, and ZnTe

We report the successful growth of ZnTe on nearly lattice-matched III-V buffer layers of InAs (0.75%), GaSb (0.15%), and on GaAs and ZnTe by molecular beam epitaxy. In situ reflection high-energy electron diffraction measurements showed the characteristic streak patterns indicative of two-dimensional growth. Photoluminescence measurements on these films show strong and sharp features near the band edge with no detectable luminescence at longer wavelengths. The integrated photoluminescence intensity from the ZnTe layers increased with better lattice match to the buffer layer. The ZnTe epilayers grown on high-purity ZnTe substrates exhibited stronger luminescence than the substrates. We observe narrow luminescence linewidths (full width at half maximum ~ 1–2 Å) indicative of uniform high quality growth. Secondary-ion mass spectroscopy and electron microprobe measurements, however, reveal substantial outdiffusion of Ga and In for growths on the III-V buffer layers

City Avoidance in the Early Phase of Psychosis: A Neglected Domain of Assessment and a Potential Target for Recovery Strategies.

Background: A considerable amount of research has explored the link between living in an urban environment during childhood and the increased risk to develop psychosis. However, the urban milieu is more than a risk factor as it is also a place for socialization and enrichment. The aims of the current study were to explore, in a large sample of early psychosis (EP) patients, their pattern of use of the city, their perception when exposed to various critical stressors, and their sensitivity to diverse forms of stimuli. Methods: We sent a questionnaire (based on previous work conducted in a group of patients, including video-recorded walk-along in the city and a literature review) to 305 EP patients and to 220 medical students. Results: Response rate in patients was low (38%). City avoidance and negative perceptions towards the urban environment increased in patients after onset of psychosis. Patients' tendency to avoid city center correlates with both problematic social interactions and stimuli perceived as unpleasant. Patients seemed less likely to enjoy urban spaces considered as relaxing, suggesting a lower capacity to benefit from positive aspects of this environment. Conclusions: The development of psychosis influences the way EP patients perceive the city and their capacity to feel at ease in the urban environment, leading to a high rate of city avoidance. Considering the possible influence of city avoidance on social relations and the recovery process, the development of strategies to help patients in this regard may have a significant effect on their recovery process

Light trapping effects in thin film silicon solar cells

We present advanced light trapping concepts for thin film silicon solar cells. When an amorphous and a microcrystalline absorber layers are combined into a micromorph tandem cell, light trapping becomes a challenge because it should combine the spectral region from 600 to 750 nm for the amorphous top cell and from 800 to 1100 for the microcrystalline bottom cell. Because light trapping is typically achieved by growing on textured substrates, the effect of interface textures on the material and electric properties has to be taken into account, and importantly, how the surface textures evolve with the thickness of the overgrowing layers. We present different scenarios for the n-i-p configuration on flexible polymer substrates and p-i-n cells on glass substrate, and we present our latest stabilized efficiencies of 9.8% and 11.1%, respectively