We report absorption measurements on two types of long-wave infrared detector structures. Both types were grown by ultrahigh vacuum chemical vapor deposition, and were characterized by multiple analytic techniques. In both multiple quantum well (MQW) and heterojunction internal photoemission (HIP) structures, it is found that free-carrier absorption is dominant for normally incident radiation. The measured absorption is fit well by the classical expression for free-carrier absorption, with scattering times of about 10−14 s (MQW) and 5×10−15 s (HIP). The measured absorption is used to evaluate the responsivity that results when all carriers energetically able to surmount the barrier are collected. Based on this analysis, higher responsivity is predicted for HIP detectors, largely because of the greater density of initial states. The responsivity obtained in practice depends upon the photoconductive gain (MQW detectors) or the escape probability (HIP detectors). The escape probability for HIP detectors is measured in Part II.
Germanium–silicon epitaxial growth by chemical vapor deposition (CVD) has proven to be suitable for growth of many heterojunction devices. We report here on recent work with one CVD technique (UHV/CVD, or ultrahigh vacuum chemical vapor deposition) which is capable of multiwafer deposition of advanced device structures. First, the physics and chemistry of the growth process are outlined and the factors which influence layer uniformity and heterojunction abruptness are discussed. We then present recent results from the characterization of doped multiple quantum well structures suitable for far-infrared detectors.
The absorption characteristics of GexSi1−x quantum well infrared photodetector (QWIP) structures have been studied in samples over a range of germanium compositions and doping levels. In all these samples, quantum well intersubband transitions are either very weak or nonexistent for normally incident light. However, free carrier absorption in GexSi1−x quantum wells is a strong absorbing mechanism in the long wavelength infrared regime, and has been found to be stronger than in silicon for similar doping levels. Therefore, detectors relying upon free carrier absorption in GexSi1−x quantum wells may offer superior responsivity and quantum efficiency.
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