Abstract
Five InAs/GaAs quantum dot infrared photodetector (QDIP) samples with different device structures are discussed in the thesis. The QDIP samples are grown on semi-insulating (100) GaAs substrate by Riber Epineat solid source molecular beam epitaxy (MBE). High dot density of 1 x 1011 cm-2 and short average distance between neighboring quantum dots of 7.4 nm are observed from the uniform quantum dot distribution atomic force microscopy (AFM) image. Three different energy levels are distinguished from the photoluminescence (PL) spectra. The phenomenon is attributed to the state filling effect. The wetting layer formed and mediated the electronics interaction between the barrier states and the localized quantum dot states. Transitions from the occupied quantum dot states to the wetting layer or to the continuum states will result in infrared detection, such that the response signals of a fabricated QDIP devices exhibit the same energy position as the shifted PL spectra relative to the energy of wetting layer. 10- and 30-period InAs/GaAs QDIPs are investigated in the thesis. Lower dark current for 30-period device is attributed to the increase of quantum dot period such that the total barrier thickness is increased. The dark current of the QDIP increases rapidly with temperature, which is due to its exponential dependence on temperature. Such that the background-limited performance (BLIP) temperature can be observed from the temperature varying current-voltage measurement. Since the dark current at higher temperature can be inhibited effectually by higher activation energy in 30-period device, higher BLIP temperature of 60 K for 30-period device can be observed. As a result, 30-period device can still detect the spectral response at 100K while 10-period device can only detect the spectral response at about 50 K. The depression of dark current and the enhancement of responsivity are observed for sample with higher period numbers. In order to investigate the effect of doping density within quantum dots to the performance of QDIP, wafers with InAs quantum dot doping densities varied from 2 x 1018 cm-3, 1 x 1018 cm-3, 5 x 1017 cm-3 to undoped are also investigated. In all silicon doped samples, the PL spectra emitted from quantum dots show multi-peaks emission base on the state filling effect. However undoped sample exhibits only a unique peak. The activation energy increases with decreasing quantum dot density. Such that the dark current can be inhibited effectually with lower quantum dot doping density. The undoped device has the highest BLIP temperature because of its high energy barrier, such that the best operation temperature of 200 K is observed for undoped device. Since higher energy barrier with lower quantum dot doping density not only inhibit the dark current effectually, but also inhibit the photocurrent. There must be an optimum quantum dot doping density of QDIP to design.