Abstract

Intersubband transitions using normal incidence in the conduction band of a direct gap semiconductor quantum well structure is very unconventional, yet very useful in quantum well infrared photodetectors (QWIPs). On the other hand, normal incidence absorption is allowed in both p-type QWIPs and indirect bandgap n-QWIPs. However, the low electron effective mass and high electron mobility in the Г valley of the conduction band made n-type QWIPs more desirable for the infrared detection considering the sensitivity and speed. Normal incidence n-type QWIPs without grating are especially attractive and important for large format focal plane array applications. In this paper, experimental results of n-type QWIPs with InGaAs/AlGaAs and GaAs/AlGaAs structures are given which show obvious evidence of intersubband transition under normal incidence condition. The cause of this phenomenon can be either from the sample processing and testing conditions in which the normal incidence light has been directed to the in-plane direction, or due to intrinsic normal incidence absorption. Three QWIP samples with different device structures, processing procedures, and experimental conditions are examined to study the effects of the mesa edge, the substrate edge, and the metal-semiconductor interface diffusion. Theoretical models are also examined to understand the intrinsic mechanisms of normal incidence absorption.

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