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Ksendzov, A.

Publications and source records attributed to Ksendzov, A..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

The Mars oxidant experiment (MOx) for Mars '96

The MOx instrument was developed to characterize the reactive nature of the martian soil. The objectives of MOx were: (1) to measure the rate of degradation of organics in the martian environment; (2) to determine if the reactions seen by the Viking biology experiments were caused by a soil oxidant and measure the reactivity of the soil and atmosphere: (3) to monitor the degradation, when exposed to the martian environment, of materials of potential use in future missions; and, finally, (4) to develop technologies and approaches that can be part of future soil analysis instrumentation. The basic approach taken in the MOx instrument was to place a variety of materials composed as thin films in contact with the soil and monitor the physical and chemical changes that result. The optical reflectance of the thin films was the primary sensing-mode. Thin films of organic materials, metals, and semiconductors were prepared. Laboratory simulations demonstrated the response of thin films to active oxidants.

Flight Experiment↗
NASA NTRS1998

Multi-Bandwidth Frequency Selective Surfaces for Near Infrared Filtering: Design and Optimization

Frequency selective surfaces are widely used in the microwave and millimeter wave regions of the spectrum for filtering signals. They are used in telecommunication systems for multi-frequency operation or in instrument detectors for spectroscopy. The frequency selective surface operation depends on a periodic array of elements resonating at prescribed wavelengths producing a filter response. The size of the elements is on the order of half the electrical wavelength, and the array period is typically less than a wavelength for efficient operation. When operating in the optical region, diffraction gratings are used for filtering. In this regime the period of the grating may be several wavelengths producing multiple orders of light in reflection or transmission. In regions between these bands (specifically in the infrared band) frequency selective filters consisting of patterned metal layers fabricated using electron beam lithography are beginning to be developed. The operation is completely analogous to surfaces made in the microwave and millimeter wave region except for the choice of materials used and the fabrication process. In addition, the lithography process allows an arbitrary distribution of patterns corresponding to resonances at various wavelengths to be produced. The design of sub-millimeter filters follows the design methods used in the microwave region. Exacting modal matching, integral equation or finite element methods can be used for design. A major difference though is the introduction of material parameters and thicknesses that may not be important in longer wavelength designs. This paper describes the design of multi- bandwidth filters operating in the 1-5 micrometer wavelength range. This work follows on a previous design. In this paper extensions based on further optimization and an examination of the specific shape of the element in the periodic cell will be reported. Results from the design, manufacture and test of linear wedge filters built using microlithographic techniques and used in spectral imaging applications will be presented.

Cwik, Tom↗
NASA NTRS1993

Visible photoluminescence of porous Si(1-x)Ge(x) obtained by stain etching

We have investigated visible photoluminescence (PL) from thin porous Si(1-x)Ge(x) alloy layers prepared by stain etching of molecular-beam-epitaxy-grown material. Seven samples with nominal Ge fraction x varying from 0.04 to 0.41 were studied at room temperature and 80 K. Samples of bulk stain etched Si and Ge were also investigated. The composition of the porous material was determined using X-ray photoemission spectroscopy and Rutherford backscattering techniques to be considerably more Ge-rich than the starting epitaxial layers. While the luminescence intensity drops significantly with the increasing Ge fraction, we observe no significant variation in the PL wavelength at room temperature. This is clearly in contradiction to the popular model based on quantum confinement in crystalline silicon which predicts that the PL energy should follow the bandgap variation of the starting material. However, our data are consistent with small active units containing only a few Si atoms that are responsible for the light emission. Such units are present in many compounds proposed in the literature as the cause of the visible PL in porous Si.

Ksendzov, A.↗
NASA NTRS1993

Infrared photodetectors with tailorable response due to resonant plasmon absorption in epitaxial silicide particles embedded in silicon

Tailorable infrared photoresponse in the 1-2 micron range are demonstrated in a device incorporating electrically floating metal silicide particles. Photons absorbed by excitation of the metallic-particle surface plasmon are shown to contribute to the photoresponse. Quantum efficiencies of roughly 0.2 percent are measured at 77 K, with dark currents of less than 2 nA/sq cm at a reverse bias of 1 V and detectivities of 4 x 10 exp 9 - 8 x 10 exp 9 cm sq rt Hz/W are obtained.

Fathauer, R. W.↗
NASA NTRS1993

Low threshold continuous operation of InGaAs/InGaAsP quantum well lasers at about 2.0 microns

The first low threshold continuous operation of InGaAs strained layer quantum well lasers at about 2.0 microns is reported. The threshold current density of 5-micron wide and 1.5 mm long ridge waveguide lasers was less than 380 A/sq cm. The external differential quantum efficiency of 1 mm long lasers was as high as 15 percent and laser operation was observed at temperatures as high as 50 C. The lasers are characterized by T(0) = 54 C which is the highest characteristic temperature ever achieved at this wavelength in any material system.

Forouhar, S.↗
NASA NTRS1993

Interband optical transitions between confined and unconfined states in quantum wells

The interband optical absorption in single quantum wells (SQWs) due to transitions between confined states and unconfined continuum states (C-U transitions) were measured, calculated, and compared to absorption due to transitions involving pairs of confined states (C-C transiitons). An In(0.25)Ga(0.75)As/GaAs SQW with a 61-A well width was used in the experiment. A simple model describes how the slow-turnon absorption line shapes characterisic of SQWs evolve into sharp steplike features as more wells are added.

Ksendzov, A.↗
NASA NTRS1992

New class of Si-based superlattices - Alternating layers of crystalline Si and porous amorphous Si(1-x)Ge(x) alloys

Superlattices consisting of alternating layers of crystalline Si and porous amorphous Si(1-x)Ge(x) have been fabricated. This is accomplished by first growing a Si/Si(0.7)Ge(0.3) superlattice by molecular beam epitaxy, followed by Ar-ion milling to form mesa structures, and finally by immersion in HF:HNO3:H2O. This solution creates a porous structure similar to that created by anodic etching, and a high selectivity is observed for the conversion of the alloy layers relative to the Si layers. The degree of selectivity is found to depend on alloy-layer thickness and strain. Superlattices have been fabricated from 1-micron wide mesas with Si(0.7)Ge(0.3) layers fully converted to 5-nm thick porous amorphous material.

Fathauer, R. W.↗
NASA NTRS1992

InGaAs/InGaAsP/InP strained-layer quantum well lasers at about 2 microns

The first successful operation of InGaAs strained layer quantum well (Sl-QW) injection lasers at about 2 microns is reported. The threshold current density and the external differential quantum efficiency of 5 microns wide and 800 microns long ridge waveguide lasers were 2.5 kA/sq cm and 6 percent, respectively. The devices had a reverse leakage current of less than 20 micro-A at -1 V indicating epitaxial layers with low defect density.

Forouhar, S.↗
NASA NTRS1992

Room-temperature operation of MOCVD-grown GaInAs/InP strained-layer multiquantum well lasers in 1.8 micron range

The first successful room-temperature pulsed operation is reported of InGaAs strained layer multiquantum well injection lasers grown by MOVPE on InP substrates in the 1.8 micron range. The threshold current density and the external differential quantum efficiency of the 10 micron wide ridge waveguide lasers were 2.5 kA/sq cm (cavity length = 1 mm) and 5 percent (cavity length = 400 microns), respectively. Broad-area lasers, 100 microns wide and 1 mm long, had a reverse leakage current of less than 10 microamps at -1 V indicating high quality of the epitaxial layers.

Forouhar, S.↗
NASA NTRS1992

Visible luminescence from silicon wafers subjected to stain etches

Etching of Si in a variety of solutions is known to cause staining. These stain layers consist of porous material similar to that produced by anodic etching of Si in HF solutions. In this work, photoluminescence peaked in the red from stain-etched Si wafers of different dopant types, concentrations, and orientations produced in solutions of HF:HNO3:H2O was observed. Luminescence is also observed in stain films produced in solutions of NaNO2 in HF, but not in stain films produced in solutions of CrO3 in HF. The luminescence spectra are similar to those reported recently for porous Si films produced by anodic etching in HF solutions. However, stain films are much easier to produce, requiring no special equipment.

Fathauer, R. W.↗