Studies of photocurrent and dark current in quantum dots-in-a-well infrared photodetectors

In this thesis, results are presented from the development of quantum dot (QD) based infrared photodetectors (IPs). This includes epitaxial growth of QDs, investigations of the structural, optical and electronic properties of the QD based material as well as characterisation of the resulting components. Metal organic vapour phase epitaxy is used for growth of selfassembled indium arsenide (lnAs) QDs on gallium arsenide (GaAs) substrates. Through characterisation by atomic force microscopy, the correlation between size distribution and density of quantum dots and different growth parameters, such as temperature, InAs deposition time and V/III-ratio (ratio between group Vand group III species) is achieved. The V/111ratio is identified as the most important parameter, in finding the right growth conditions for QDs. A route towards optimisation of the dot size distribution through successive variations of the growth parameters is presented. The QD layers are inserted in Ino.15Gao.85As/GaAs quantum wells (QWs), forming so-called dots-in-a-well (DWELL) structures. These structures are used to fabricate IPs, primarily for detection in the long wavelength infrared region (LWIR, 8-12 µm). The electron energy level schemes of the DWELL structures are deduced by means of Fourier transform photoluminescence (FTPL) and FTPL excitation (FTPLE) measurements. Further characterisation of the IPs, through interband and intersubband photocurrent (pe) measurements as weIl as dark current measurements, is performed. By comparisons of the deduced energy level scheme of the DWELL structure and the results of the intersubband PC measurements, the origin of the PC is determined. The main intersubband transition contributing to the PC is identified as the QD ground state to the QW excited state transition. Significant variations of PC and dark current are observed, when voltage and temperature are used as variable parameters. A possible explanation to this could be the strong variation of the escape probability from different energy states in the DWELL structure, as revealed by interband PC measurements. These results are important for the further optimisation of the DWELL lP.