Oleg V. Sulima

Strong broadband optical absorption in silicon nanowire films

The broadband optical absorption properties of silicon nanowire (SiNW) films fabricated on glass substrates by wet etching and chemical vapor deposition (CVD) have been measured and found to be higher than solid thin films of equivalent thickness. The observed behavior is adequately explained by light scattering and light trapping though some of the observed absorption is due to a high density of surface states in the nanowires films, as evidenced by the partial reduction in high residual sub-bandgap absorption after hydrogen passivation. Finite difference time domain simulations show strong resonance within and between the nanowires in a vertically oriented array and describe the experimental absorption data well. These structures may be of interest in optical films and optoelectronic device applications.

AlGaAsSb/InGaAsSb phototransistors for 2-μm remote sensing applications

Two-micron detectors are critical for atmospheric CO2 profiling using the lidar technique. InGaAs and HgCdTe detectors are commercially available for this wavelength but they lack sufficient gain, which limits their detectivity. The characterization results of a novel AlGaAsSb/InGaAsSb phototransistor for 2-μm application are reported. The device was developed by AstroPower, Inc. for NASA Langley Research Center. Spectral response measurements showed the highest responsivity in a 1.9- to 2.1-μm region with a maximum value of 2650 A/W at 2 μm. A 2-μm detectivity of 3.9×10 11 cm Hz 1/2 /W was obtained, which corresponds to noise equivalent power of 4.6×10 –14 W/Hz 1/2 .

Passivation of AlGaAsSb∕InGaAsSb∕GaSb photodiodes using aqueous (NH4)2S solution and polyimide encapsulation

We studied passivation effects of treatment in aqueous (NH4)2S (ammonium sulfide) solution and polyimide encapsulation on AlGaAsSb∕InGaAsSb∕GaSb mesa photodiodes. X-ray photoelectron spectroscopy on InGaAsSb material revealed that (NH4)2S passivation reduces oxide bonds, increases III-V bonds, and produces sulfide bonds. Auger electron spectroscopy depth profiling shows a 1:1 replacement of oxygen by sulfur. The percentage of oxygen replaced by sulfur is about 30% at the surface and decays with the depth. Polyimide encapsulation of the photodiode mesa sidewall not preceded by (NH4)2S passivation reduces the dark current of the photodiodes at −0.5V bias by 3.6 times while additional 9.2 times reduction is obtained when it is preceded by (NH4)2S passivation. The dark current performance of devices did not degrade for 240 days after the combined passivation, indicating a promising long-term stability. The capacitance of devices treated with the combined passivation exhibits a standard deviation three times s...

Diffusion of Zn in TPV materials: GaSb, InGaSb, InGaAsSb and InAsSbP

This paper reviews recent results of the study of Zn diffusion from the vapor phase in important thermophotovoltaic (TPV) materials such as GaSb‐, InGaAsSb‐, InGaSb‐ and InAsSbP. Peculiarities of Zn diffusion in each of these materials and different ways of tailoring the Zn diffusion profile for fabrication of optimized emitters in TPV cells are discussed.

Characterization and analysis of InGaAsSb detectors

Profiling of atmospheric CO2 at 2 μm wavelength using the LIDAR technique, has recently gained interest. Although several detectors might be suitable for this application, an ideal device would have high gain, low noise and narrow spectral response peaking around the wavelength of interest. This increases the detector signal-to-noise ratio and minimizes the background signal, thereby increasing the device sensitivity and dynamic range. Detectors meeting the above idealized criteria are commercially unavailable for this particular wavelength. In this paper, the characterization and analysis of Sb-based detectors for 2 μm lidar applications are presented. The detectors were manufactured by AstroPower, Inc., with an InGaAsSb absorbing layer and AlGaAsSb passivating layer. The characterization experiments included spectral response, current versus voltage and noise measurements. The effect of the detectors bias voltage and temperature on its performance, have been investigated as well. The detectors peak responsivity is located at the 2 μm wavelength. Comparing three detector samples, an optimization of the spectral response around the 2 μm wavelength, through a narrower spectral period was observed. Increasing the detector bias voltage enhances the device gain at the narrow spectral range, while cooling the device reduces the cut-off wavelength and lowers its noise. Noise-equivalent-power analysis results in a value as low as 4x10-12 W/Hz1/2 corresponding to D* of 1x1010 cmHz1/2/W, at -1 V and 20°C. Discussions also include device operational physics and optimization guidelines, taking into account peculiarity of the Type II heterointerface and transport mechanisms under these conditions.

GaSb‐, InGaAsSb‐, InGaSb‐, InAsSbP‐ and Ge‐TPV cells for low‐temperature TPV applications

GaSb thermophotovoltaic (TPV) cells are the most suitable choice for modern TPV generators, both in terms of efficiency and simplicity of the diffusion technology used. Actually, TPV generators based on GaSb solar cells are the only ones available on the market. However, TPV cells with band gaps (Eg) lower than GaSb are expected to be advantageous for low‐temperature (< 1000°C) non‐wavelength‐selective TPV radiators because they provide more effective absorption of the blackbody infrared radiation. In this work, together with GaSb (Eg = 0.72 eV), semiconductors with a lower Eg ‐ Ge (Eg = 0.66 eV), InGaSb (Eg = 0.60 eV), InGaAsSb (Eg = 0.55 eV) and InAsSbP (Eg = 0.39 eV) ‐ were studied for TPV cells. InGaAsSb cells seem to be the most promising candidate to replace GaSb cells in the low‐temperature TPV generators.

RECENT DEVELOPMENT OF SB-BASED PHOTOTRANSISTORS IN THE 0.9- TO 2.2-μM WAVELENGTH RANGE FOR APPLICATIONS TO LASER REMOTE SENSING

We have investigated commercially available photodiodes and also recent developed Sb-based phototransistors in order to compare their performances for applications to laser remote sensing. A custom-designed phototransistor in the 0.9- to 2.2-μm wavelength range has been developed at AstroPower and characterized at NASA Langleys Detector Characterization Laboratory. The phototransistors performance greatly exceeds the previously reported results at this wavelength range in the literature. The detector testing included spectral response, dark current and noise measurements. Spectral response measurements were carried out to determine the responsivity at 2-μm wavelength at different bias voltages with fixed temperature; and different temperatures with fixed bias voltage. Current versus voltage characteristics were also recorded at different temperatures. Results show high responsivity of 2650 AIW corresponding to an internal gain of three orders of magnitude, and high detectivity (D*) of 3.9×1011cm.Hz1/2/W...

Novel infrared phototransistors for atmospheric CO/sub 2/ profiling at 2 /spl mu/m wavelength

Two-micron detectors are critical for atmospheric carbon dioxide profiling using the lidar technique. The characterization results of a novel infrared AlGaAsSb/ InGaAsSb phototransistor are reported. Emitter dark current variation with the collector-emitter voltage at different temperatures is acquired to examine the gain mechanism. Spectral response measurements resulted in responsivity as high as 2650 AAV at 2.05 /spl mu/m wavelength. Bias voltage and temperature effects on the device responsivity are presented. The detectivity of this device is compared to InGaAs and HgCdTe devices.

Integrated optoelectronic transmitter and receiver multi-chip modules for three-dimensional chip-level micro-optical interconnects

We demonstrate a chip-scale micro-optical interconnect. The prototype system for this demonstration consists of optoelectronic transmitter and receiver multichip modules. A diffractive optical element (DOE) is used for optically interconnecting the multichip modules and in establishing a point-to-point link. The link length, as measured from the optical source of the transmitter to the detector plane of the receiver, is only 2.332 mm. The transmitter and receiver module dimensions as well as the integrated system volume are comparable to very large scale integration (VLSI). The design, fabrication, integration of this system, and experimental results are presented.

High-performance monolithic AlGaAs/GaAs photovoltaic arrays coupled to scintillating fibers for UGS application

This paper reports progress in the development of a miniature, monolithic AlGaAs/GaAs photovoltaic solar cell array, used in combination with scintillating fibers, and its application to recharging batteries of unattended ground sensors. Improvements in the design and processing of these photovoltaic arrays resulted in significant increases in both voltage and current. Moreover, new polycarbonate scintillating fibers that are more durable and flexible than polystyrene ones were tested. Output electric power density as high as 73 mW/cm2 for total area and 105 mW/cm2 for active area was measured outdoors for a 6-cell array integrated with a 1 foot long fiber bundle.