Rengarajan Sudharsanan

Next-generation, high-efficiency III-V multijunction solar cells

Next-generation solar cell approaches such as AlGaInP/GaAs/GaInNAs/Ge 4-junction cells, lattice-mismatched GaInP/GaInAs/Ge, concentrator cells, and improved 3-junction device structures hold the promise of greater efficiency than even todays highly successful multijunction cells. Wide-bandgap tunnel junctions, improved heterointerfaces, and other device structure improvements have resulted in several record-efficiency GaInP/GaAs/Ge cell results. Triple-junction (3J) cells grown in this work have demonstrated 29.3% efficiency for space (AMO, 1 sun). Space concentrator 3J cells have efficiency up to 30.0% at low concentration (AMO, 7.6 suns), and terrestrial concentrator cells grown at Spectrolab and processed at NREL have reached 32.3% (AM1.5D, 440 suns).

Origin of dark counts in In0.53Ga0.47As∕In0.52Al0.48As avalanche photodiodes operated in Geiger mode

A dark count rate in InP-based single photon counting avalanche photodiodes is a limiting factor to their efficacy. The temperature dependence of the dark count rate was studied to understand its origin in In0.53Ga0.47As∕In0.52Al0.48As separate-absorption-charge-multiplication avalanche photodiodes. The dark count rate was observed to be a very weak function of temperature in the range from 77Kto300K. Various mechanisms for dark count generation were considered. Simulations of band-to-band tunneling in the In0.52Al0.48As multiplication layer were found to agree well with the experimental temperature dependence of dark count rate at various excess biases. To reduce tunneling-induced dark counts, a suitable design change to the detector structure is proposed.

Low-cost compact MEMS scanning ladar system for robotic applications

Future robots and autonomous vehicles require compact low-cost Laser Detection and Ranging (LADAR) systems for autonomous navigation. Army Research Laboratory (ARL) had recently demonstrated a brass-board short-range eye-safe MEMS scanning LADAR system for robotic applications. Boeing Spectrolab is doing a tech-transfer (CRADA) of this system and has built a compact MEMS scanning LADAR system with additional improvements in receiver sensitivity, laser system, and data processing system. Improved system sensitivity, low-cost, miniaturization, and low power consumption are the main goals for the commercialization of this LADAR system. The receiver sensitivity has been improved by 2x using large-area InGaAs PIN detectors with low-noise amplifiers. The FPGA code has been updated to extend the range to 50 meters and detect up to 3 targets per pixel. Range accuracy has been improved through the implementation of an optical T-Zero input line. A compact commercially available erbium fiber laser operating at 1550 nm wavelength is used as a transmitter, thus reducing the size of the LADAR system considerably from the ARL brassboard system. The computer interface has been consolidated to allow image data and configuration data (configuration settings and system status) to pass through a single Ethernet port. In this presentation we will discuss the system architecture and future improvements to receiver sensitivity using avalanche photodiodes.

Single photon counting Geiger mode InGaAs(P)/InP avalanche photodiode arrays for 3D imaging

We have designed, fabricated and characterized InGaAs/InP Geiger-mode avalanche photodiode (APD) 32 x 32 arrays optimized for operation at both 1.06 and 1.55 μm wavelengths Single element devices with a thick multiplication layer thickness showed dark count rate as low as 60 kHz at a 3 V overbias, while photon detection efficiencies at a wavelength of 1.55 μm exceed 30% at 2 V overbias. Back illuminated 32 x 32 detector arrays exhibited breakdown uniformity of greater than 97% and excellent dark current uniformity. Detector arrays were integrated with low-noise read-out integrated circuits for an imaging demonstration. 3D imaging was demonstrated using 1.06 micron detector arrays.

Monolithic multi-cell GaAs laser power converter with very high current density

A very high current density 2-Volt Laser Power Photovoltaic GaAs converter has been fabricated to produce over 360 mW of output power with monochromatic illumination of one optical Watt at 810 nm. To build-up the voltage, the converter consists of two interconnected N/P GaAs elements integrated in series on the semi-insulating GaAs substrate. Several technological issues, including, conductive losses through the sheet for the series interconnection of elements, have been successfully addressed. This device operates at a nominal optical power density of 57W/cm/sup 2/, which is equivalent to over 700 suns of AM1.5D illumination, demonstrating feasibility of fabricating high voltage integrated cells for concentrator applications.

32 x 32 Geiger-mode ladar camera

For the wide applications of LAser Detection and Ranging (LADAR) imaging with large format Geiger-mode (GM) avalanche photodiode (APD) arrays, it is critical and challenging to develop a LADAR camera suitable to volume production with enough component tolerance and stable performance. Recently Spectrolab and Black Forest Engineering developed a new 32x32 Read-Out Integrated Circuit (ROIC) for LADAR applications. With a specially designed high voltage input protection circuit, the ROIC can work properly even with more than 1 % of pixels shorted in the APD array; this feature will greatly improve the camera long-term stability and manufacturing throughput. The Non-uniform Bias circuit provides bias voltage tunability over a 2.5 V range individually for each pixel and greatly reduces the impact of the non-uniformity of an APD array. A SMIA high speed serial digital interface streamlines data download and supports frame rates up to 30 kHz. The ROIC can operate with a 0.5 ns time resolution without vernier bits; 14 bits of dynamic range provides 8 μs of range gate width. At the meeting we will demonstrate more performance of this newly developed 32x32 Geiger-mode LADAR camera.

Development of terrestrial concentrator modules incorporating high-efficiency multi-junction cells

This paper presents key results of an on-going program to develop terrestrial concentrator modules incorporating high-efficiency multi-junction (MJ) photovoltaic (PV) cell technology. This program evolved from a successful space concentrator module development. Indeed, space mini-concentrators, comprising a stretched-membrane line-focus Fresnel lens and a prism-covered triple-junction solar cell, have been found to perform exceptionally well in the terrestrial environment. In outdoor tests at both ENTECH and NREL, space mini-concentrators have achieved overall module (combined lens and cell) operational solar-to-electric conversion efficiency levels from 25 to 29% under a variety of conditions. The long-term goal of the present program is to incorporate MJ cell technology into existing field-proven modules and two-axis sun-tracking arrays. With twice the efficiency of present silicon-based concentrators, these next-generation MJ concentrators should offer unprecedented economics.

Geiger-mode ladar cameras

The performance of Geiger-mode LAser Detection and Ranging (LADAR) cameras is primarily defined by individual pixel attributes, such as dark count rate (DCR), photon detection efficiency (PDE), jitter, and crosstalk. However, for the expanding LADAR imaging applications, other factors, such as image uniformity, component tolerance, manufacturability, reliability, and operational features, have to be considered. Recently we have developed new 32×32 and 32×128 Read-Out Integrated Circuits (ROIC) for LADAR applications. With multiple filter and absorber structures, the 50-μm-pitch arrays demonstrate pixel crosstalk less than 100 ppm level, while maintaining a PDE greater than 40% at 4 V overbias. Besides the improved epitaxial and process uniformity of the APD arrays, the new ROICs implement a Non-uniform Bias (NUB) circuit providing 4-bit bias voltage tunability over a 2.5 V range to individually bias each pixel. All these features greatly increase the performance uniformity of the LADAR camera. Cameras based on these ROICs were integrated with a data acquisition system developed by Boeing DES. The 32×32 version has a range gate of up to 7 μs and can cover a range window of about 1 km with 14-bit and 0.5 ns timing resolution. The 32×128 camera can be operated at a frame rate of up to 20 kHz with 0.3 ns and 14-bit time resolution through a full CameraLink. The performance of the 32×32 LADAR camera has been demonstrated in a series of field tests on various vehicles.

Performance of very low dark current SWIR PIN arrays

Boeing Spectrolab has grown, fabricated and tested InGaAs PIN arrays with less than 1 nA/cm2 dark current density at 280 °K. The PIN diodes display greater than 1 A/W responsivity at -100 mV reverse bias with about 50 fF of diode capacitance.

High-performance InP Geiger-mode SWIR avalanche photodiodes

LAser Detection And Ranging (LADAR) is a promising tool for precise 3D-imaging, which enables field surveillance and target identification under low-light-level conditions in many military applications. For the time resolution and sensitivity requirements of LADAR applications, InGaAsP/InP Geiger-mode (GM) avalanche photodiodes (APDs) excel in the spectrum band between 1.0~1.6 μm. Previously MIT Lincoln Laboratory has demonstrated 3D LADAR imaging in the visible and near infrared (1.06 μm) wavelengths with InP/InGaAsP GM-APD arrays. In order to relieve the design tradeoffs among dark count rate (DCR), photo detection efficiency (PDE), afterpulsing, and operating temperature, it is essential to reduce the DCR while maintaining a high PDE. In this paper we will report the progress of GM-APD detectors and arrays with low DCR and high PDE at 1.06 μm. In order to improve both DCR and PDE, we optimized the multiplication layer thickness, substrate, and epitaxial growth quality. With an optimized InP multiplier thickness, a DCR as low as 100 kHz has been demonstrated at 4V overbias at 300 °C. and at 240 K, less than 1 kHz DCR is measured. A nearly 40% PDE can be achieved at a DCR of 10 kHz at the reduced temperature.