Chapter 3 - GaAs/AIGaAs based quantum well infrared photodetector focal plane arrays

Abstract

This chapter opens with a discussion of using a quantum well to detect light that can be understood by the basic principles of quantum mechanics. When a photon strikes the well, it can impart its energy to the ground state electron and excite it to the next allowed energy level, a process called intersubband absorption. The simplest quantum well infrared photodetector (QWIP) design uses a simple square quantum well designed to hold just two states: a ground state deep inside the well, and the first excited state near the well top. The major advantage of the bound-to-continuum QWIP is that the photoelectron can escape from the quantum well to the continuum transport states without being required to tunnel through the barrier. In addition to the photocurrent, all detectors including QWIPs produce a dark current that must be minimized to achieve high performance. In order to be absorbed by the electrons in the quantum wells, the incoming light should have an electric field component in the quantum well direction, that is, in the growth direction, normal to the layers. One unique feature of this spatially separated four-band focal plane array is that the four infrared bands are independently readable on a single imaging array. This feature's benefits are explained in the chapter. This four-band device structure was achieved by the growth of multi-stack QWIP structures separated by heavily doped contact layers, on a GaAs substrate. QWIP is able to operate at low temperatures, with a higher sensitivity under high background irradiance levels. The advantage of the Blocked Intersubband Detector (BID) detector is that its infrared sensing photoemitter is a GaAs/A1GaAs based superlattice, thus its cutoff wavelength can be easily tuned by band gap engineering.