Development of Superlattice Infrared Photodetectors

Abstract

Infrared (IR) detectors play a critical role in both military and civilian applications and have been widely researched in recent decade. Because the atmospheric transparent windows for the IR radiation exist within the spectral ranges of 3−5 and 8−12 μm, and the spectrum of the black-body radiation at room temperature has a peak at 10 μm, the detectors with the 8-12 μm detection spectra are helpful to identify the heat radiation from a target at room temperature. Such detectors are the research focus in this chapter. The employment of intersubband transitions for the infrared radiation detection has drawn much attention. The transition is completed by the electrons which absorb photons with the appropriate energy equal to the suband energy difference to transit from the low subband to the high one. The intersubband photodetectors can be made from semiconductor heterostructures of multiple quantum wells (West & Eglash, 1985) (Harwit & Harris Jr., 1987) ( Levine et al., 1987) or superlattices as shown in Fig. 1. Infrared detection will be done by the intersubband transitions between two quantum states in the multiple quantum wells or two minibands in the superlattices. The wells are sandwiched by thick barriers in the multiple quantum well structure. Therefore electron wavefunctions in the wells would not interact with each others and discrete quantum states are formed. Contrarily, the adjacent wells in the superlattice are separated by thin barriers. Minibands are formed in the superlattice region by the coupling of electron wavefunctions. As shown in Fig. 1, in comparison with the quantum well infrared photodetectors (QWIPs), superlattice infrared photodetectors (SLIPs) have three different characteristics. The first one is the low operational bias. The electrons in the miniband of the superlattice (SL) are conductive while those in the quantum states of the multiple quantum wells (MQWs) are confined. The SL hence becomes a low resistance structure and thus no externally applied bias drops on the SL under low bias range. Therefore, the current blocking layer is needed to decrease the dark current in SLIPs and can determine the operational bias range. 6