Development of SiGe arrays for visible-near IR applications

Abstract

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 will discuss 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 IRFPA's. We will present results on the approach and device design for reducing the dark current in SiGe detector arrays. We will discuss electrical and optical properties of SiGe arrays at room temperature and as a function of temperature. We will also discuss future integration path for SiGe devices with other Silicon-based technology for defense and Commercial Applications. TECHNICAL DISCUSSIONThere is significant interest in developing low cost IR Sensors for a variety of applications such as low cost thermal imagers and the ability to detect and defeat incoming threats. There are several other technologies such as InGaAs, InSb and HgCdTe, which cover different part of the IR Spectrum. HgCdTe IR focal plane arrays are being developed for 3-5 and 8-14 micron applications [1]. InSb is being used for 3-5 micron applications [2]. Similarly InGaAs offers an attractive approach for Visible-NIR sensors that can cover spectral band up to 1.8 microns [3]. SiGe offers a low cost alternative for developing Vis-NIR sensors that will not require any cooling and can operate from 0.4 to 1.6 micron [4,5]. The attractive features of SiGe based IRFPA's will take advantage of Silicon based technology with much larger substrates with up to 10 inch, that can promise very small feature size and compatibility with the Silicon CMOS circuit for signal processing. Large area silicon substrates have the potential of bring the cost down substantially compared with InGaAs. In addition, we will discuss new approaches that can further increase the spectral response to 2 microns and beyond. *

SiGe Based Visible-NIR Photodetector Technology for Optoelectronic Applications

“…Seed layer doping has been found to scale linearly with boron doping levels up to 10 20 cm -3 [74]. It has also been determined to reduce series resistance under forward bias and lower dark current under reverse bias [63].…”

“…While some early attempts to develop SiGe IR detectors concentrated on potential LWIR applications [62,63], in this chapter we focus solely the development of devices for applications involving detection in the NIR band (up to ~1700 nm). The most straightforward method by which to adjust the cutoff wavelength of a SiGe photodetector in order to tune its range of response is to modify the Si 1-x Ge x alloy composition.…”

“…For Si 1-x Ge x alloys, the lattice constant does not exactly follow Vegard's law. The relative change of the lattice constant is given by [63]:…”

“…Various approaches have been proposed to further reduce the dark current in SiGe detectors by several orders of magnitude, including superlattice structures [24], incorporation of quantum dots [63], use of buried junctions [69], and graded compositional layer designs [68]. Dark current generally scales with device area, so reducing the overall size of SiGe detector devices is one means of limiting leakage current for a given photodetector design.…”

“…The splitting of the valence bands in Ge due to the presence of tensile strain also lowers the density of states for holes, leading to reduction of intrinsic carrier density that can likewise contribute to reduced reverse dark current in devices [65]. Further methods to effect reductions in dark current include improving surface passivation and/or utilizing smaller selective growth regions during device fabrication [63].…”