New developments in InAs/InGaAs quantum dot-in-a-well infrared photodetectors

This thesis presents experimental studies of InAs/InGaAs/GaAs quantum dot-in-awell
infrared photodetectors (DWELL QDIPs) grown by molecular beam epitaxy
(MBE). Detailed studies were carried out to investigate the effects of design
parameters on the performance of DWELL photodetectors, along with fundamental
studies to determine the intraband optical and electronic properties of such structures.
Using the results of these studies, an optimised structure was designed. In addition,
the observation of a strong bias dependent spectral photoresponse demonstrated the
capability of post growth spectral tunability within the long wavelength IR (LWIR)
atmospheric window.
Various approaches were investigated for enhancing the performance of quantum dot
(QD) based devices. The main shortcoming of QDIPs versus quantum well infrared
photodetectors (QWIPs) has been addressed i.e. the low dot density, which prohibits
the high doping of these structures. The use of an antimonide surfactant to enhance
the dot density in DWELL QDIPs is presented here for the first time. Also a method
for decreasing the dark current in QDIPs was investigated, via the use of wide band
gap AIGaAs barriers. Another technique using GaP strain balancing layers to reduce
the strain in multilayer structures and allow the growth of >20 layer QD devices was
illustrated.
The effects of intermixing via thermal annealing are also reported in for DWELL
QDIPs. As part of this study, the possibility of using such a technique to shift the
spectral photoresponse across the 8-12J..lm LWIR window is demonstrated, and it is
shown that the performance still remains in a competitive range within the LWIR
range.
Non-linear two photon absorption in QDIPs was demonstrated and studied. As a
result of this study, the capability of QDIPs to operate as quadratic detectors in the
far-infrared was established, which could prove very significant, since detector
availability is reduced in that range.
Finally, a novel approach to photovoltaic QDIPs was investigated experimentally,
using a purpose built design in order to provide an internal electric field, which
preferentially drives carriers in one direction.