Yash R. Puri

Development of low dark current SiGe-detector arrays for visible-NIR imaging sensor

SiGe based Focal Plane Arrays offer a low cost alternative for developing visible- NIR focal plane arrays that will cover the spectral band from 0.4 to 1.6 microns. The attractive features of SiGe based IRFPAs will take advantage of Silicon based technology, that promises small feature size, low dark current and compatibility with the low power silicon CMOS circuits for signal processing. This paper discusses performance comparison for the SiGe based VIS-NIR Sensor with performance characteristics of InGaAs, InSb, and HgCdTe based IRFPAs. Various approaches including device designs are discussed for reducing the dark current in SiGe detector arrays; these include Superlattice, Quantum dot and Buried junction designs that have the potential of reducing the dark current by several orders of magnitude. The paper also discusses approaches to reduce the leakage current for small detector size and fabrication techniques. In addition several innovative approaches that have the potential of increasing the spectral response to 1.8 microns and beyond.

Multispectral EO/IR Sensor Model for Evaluating UV, Visible, SWIR, MWIR and LWIR System Performance

Next Generation EO/IR Sensors using Nanostructures are being developed for a variety of Defense Applications. In addition, large area IRFPAs are being developed on low cost substrates. In this paper, we will discuss the capabilities of a EO/IR Sensor Model to provide a robust means for comparing performance of infrared FPAs and Sensors that can operate in the visible and infrared spectral bands that coincide with the atmospheric windows - UV, Visible-NIR (0.4-1.8μ), SWIR (2.0-2.5μ), MWIR (3-5μ), and LWIR (8-14μ). The model will be able to predict sensor performance and also functions as an assessment tool for single-color and for multi-color imaging. The detector model can also characterize ZnO, Si, SiGe, InGaAs, InSb, HgCdTe and Nanostructure based Sensors. The model can predict performance by also placing the specific FPA into an optical system, evaluates system performance (NEI, NETD, MRTD, and SNR). This model has been used as a tool for predicting performance of state-of-the-art detector arrays and nanostructure arrays under development. Results of the analysis can be presented for various targets for each of the focal plane technologies for a variety of missions.

American depository receipts and calendar anomalies

This is the first study to examine the presence of calendar anomalies in American Depository Receipts (ADR) returns. Existing literature has documented several calendar anomalies in US and foreign markets. ADRs, however, represent a unique class of securities because they represent the ownership of stock of a foreign firm, but they are traded on US markets. We use the Standard & Poors (S&P) ADR index returns for the period 1998–2004 to look for the presence of four important anomalies: the January effect, the day-of-the-week effect, the Turn-Of-The-Month (TOTM) effect and the holiday effect. For comparison, we do the same analysis on S&P 500 index returns. While we do not find evidence of any anomalies for S&P 500 index returns, we do find evidence to support the TOTM anomaly in the S&P ADR index returns. These results suggest that the market for ADRs may not be as efficient as the broader US stock market.

Development of high performance radiation hardened antireflection coatings for LWIR and multicolor IR focal plane arrays

High Performance Radiation Hardened LWIR and Multicolor Focal Plane Arrays are critical for many space applications. Reliable focal plane arrays are needed for these applications that can operate in space environment without any degradation. In this paper, we will present various LWIR and Multicolor Focal Plane architectures currently being evaluated for LWIR and Multicolor applications that include focal plane materials such as HgCdTe, PbSnTe, QWIP and other Superlattice device structures. We also present AR Coating models and experimental results on several promising multi-layer AR coatings that includes CdTe, Si3N4 and diamond like Carbon, that have the necessary spectral response in the 2-25 microns and are hard materials with excellent bond strength. A combination of these materials offers the potential of developing anti-reflection coatings with high optical quality with controlled physical properties.

EO/IR sensors development using zinc oxide and carbon nanostructures

EO/IR Sensors have been developed for a variety of Military Systems Applications. These include UV, Visible, SWIR, MWIR and LWIR Sensors. The conventional SWIR Sensors using InGaAs Focal Plane Array (FPA) can operate in 0.4 - 1.8 micron region. Similarly, MWIR Sensors use InSb and HgCdTe based FPAs that are sensitive in 3-5 and 8-14 micron region. DOD investments in the last 10 years have provided the necessary building blocks for the IR Sensors that are being deployed in the field. In this paper, we discuss recent developments and work under way to develop Next Generation nanostructure based EO/IR detectors that can potentially cover UV, Visible and IR regions of interest. The critical technologies being developed include ZnO nanostructures with wide band gap for UV detection and Carbon Nanostructures that have shown the feasibility for IR detection. Experimental results on ZnO based nanostructures demonstrate enhanced UV sensitivity and path forward for larger arrays. Similarly, recent works on carbon nanostructures have shown the feasibility of IR detection. Combining the two technologies in a sensor can provide multispectral capability.

Design Considerations using APD Detectors for High-Resolution UV Imaging Applications

High resolution imaging in UV band has a lot of applications in Defense and Commercial systems. The shortest wavelength is desired for spatial resolution which allows for small pixels and large formats. UVAPDs have been demonstrated as discrete devices demonstrating gain. The next frontier is to develop UV APD arrays with high gain to demonstrate high resolution imaging. We will discuss an analytical model that can predict sensor performance in the UV band using p-i-n or APD detectors with and without gain and other detector and sensor parameters for a desired UV band of interest. SNRs can be modeled from illuminated targets at various distances with high resolution under standard MODTRAN atmospheres in the UV band and the solar blind region using detector arrays with unity gain and with high gain APD along with continuous or pulsed UV lasers. The performance can be determined by the signal level which results from the UV laser return energy (laser power, beam divergence, target reflectance and atmospheric transmittance), the optics f/number, the response of the detector (collection area, quantum efficiency, fill factor and gain), and the total noise which will be the sum of the dark current noise, the scene noise, and the amplifier noise. We also discuss trades as a function of detector response, dark current noise and the 1/f noise. We also present various approaches and device designs that are being evaluated for developing APDs in wide band gap semiconductors. The paper also discusses current state of the art in UV APD and the future directions for small unit cell size and gain in the APDs.

ZnO nanostructures for optoelectronic applications

In this Paper we present growth and characterization of ZnO nanowires on wideband gap substrates, such as SiC and GaN. Experimental results on the ZnO nanowires grown on p-SiC and p-GaN are presented with growth morphology, structure analysis, and dimensionality control. We also present experimental results on individual nanowires such as I-V measurements and UV sensitivity measurements with use of polymer coating on ZnO nanowires. The ZnO nanowires can be used for a variety of nanoscale optical and electronics applications.

Development of large area nanostructure antireflection coatings for EO/IR sensor applications

Electro-optical/infrared sensors are being developed for a variety of defense and commercial systems applications. One of the critical technologies that will enhance EO/IR sensor performance is the development of advanced antireflection coatings with both broadband and omnidirectional characteristics. In this paper, we review our latest work on high quality nanostructure-based antireflection structures, including recent efforts to deposit nanostructured antireflection coatings on large area substrates. Nanostructured antireflection coatings fabricated via oblique angle deposition are shown to enhance the optical transmission through transparent windows by minimizing broadband reflection losses to less than one percent, a substantial improvement over conventional thin-film antireflection coating technologies. Step-graded antireflection structures also exhibit excellent omnidirectional performance, and have recently been demonstrated on 6-inch diameter substrates.

Characterization of SiGe-detector arrays for visible-NIR imaging sensor applications

SiGe based focal plane arrays offer a low cost alternative for developing visible- near-infrared focal plane arrays that will cover the spectral band from 0.4 to 1.6 microns. The attractive features of SiGe based foal plane arrays take advantage of silicon based technology that promises small feature size, low dark current and compatibility with the low power silicon CMOS circuits for signal processing. This paper discusses performance characteristics for the SiGe based VIS-NIR Sensors for a variety of defense and commercial applications using small unit cell size and compare performance with InGaAs, InSb, and HgCdTe IRFPAs. We present results on the approach and device design for reducing the dark current in SiGe detector arrays. The electrical and optical properties of SiGe arrays at room temperature are discussed. We also discuss future integration path for SiGe devices with Si-MEMS Bolometers.

Design and development of wafer-level short wave infrared micro-camera

Low cost IR Sensors are needed for a variety of Defense and Commercial Applications as low cost imagers for various Army and Marine missions. SiGe based IR Focal Planes offers a low cost alternative for developing wafer-level shortwave infrared micro-camera that will not require any cooling and can operate in the Visible-NIR band. The attractive features of SiGe based IRFPA’s will take advantage of Silicon based technology, that promises small feature size and compatibility with the low power silicon CMOS circuits for signal processing. SiGe technology offers a low cost alternative for developing Visible-NIR sensors that will not require any cooling and can operate from 0.4- 1.7 microns. The attractive features of SiGe based IRFPA’s will take advantage of Silicon based technology that can be processed on 12-inch silicon substrates, that can promise small feature size and compatibility with the Silicon CMOS circuit for signal processing. In this paper, we will discuss the design and development of Wafer-Level Short Wave Infrared (SWIR) Micro-Camera. We will discuss manufacturing approaches and sensor configurations for short wave infrared (SWIR) focal plane arrays (FPAs) that significantly reduce the cost of SWIR FPA packaging, optics and integration into micro-systems.