Fabrication and Optical Spectroscopy of Semiconductor Quantum Structures

Fabrication of 0 and 1 dimensional structures, known as quantum dots and wires respectively, was successfully achieved in GaAs/AlGaAs, InP/InGaAs and GaAs/InGaAs semiconductor material systems. Etch masks made of either metal or resist were fabricated by electron beam lithography and the pattern then transferred to the underlying substrate using reactive ion etching with SiCl4, CH4/H2 or SiCl4/H2 and also argon ion milling. The smallest dots fabricated were 20nm in diameter and the smallest wires 60nm in width. The dominant recombination mechanism for photoexcited carriers in sub-micron GaAs/AlGaAs quantum dots was found to be radiative at 5K for lateral dimensions as small as 40nm. In contrast, in GaAs/AlGaAs wires nonradiative recombination predominates as the wire width is reduced. This is the first report of quantum GaAs/AlGaAs dots as small as 40nm in diameter luminescing as efficiently as the bulk material showing that without subsequent processing the effect of any radiation induced damage is minimal. The dominant recombination mechanism in sub-micron strained layer InGaAs/GaAs quantum dots at low temperature was also found to be nonradiative. As the temperature at which the structures were characterized was increased a size dependence was found in the relative integrated PL intensities from the dots and wires. The smaller diameter dots (75 and 150nm) and the narrower wires (80 and 150nm in width) luminesced to higher temperatures than the larger diameter dots (300 and 550nm) and the wider wires (250 and 500nm) respectively and the smaller dots luminesced to higher temperatures than the smallest wires. This was also true when comparing the larger dots to the larger wires. This temperature dependence may be due to confinement dependent on the dot diameter or wire width. Restrictions on the diffusion lengths of excitons to nonradiative sites due to the patterning of the QW into dots and wires could be the mechanism responsible for the effects seen at 5K. In wires the extra degree of freedom of the exciton contributes to the loss of luminescence at 5K as it is still possible for the excitons to diffuse to nonradiative sites within the exciton lifetime. This does not happen in the dots. This conclusion is supported by the results obtained after regrowth. Regrowth with a layer of Al0.4Ga0.6As on SiCl4 etched quantum structures increased the luminescence efficiency of the 250 and 500nm wires but reduced the efficiency of the 150nm wires. The 550 and 300nm dots showed no change in efficiency either before or after regrowth. Luminescence was completely lost in the 75 and 150nm diameter dots and 80nm wide wires. This suggests that in the wires surface states contribute significantly at 5K to the nonradiative recombination rate whereas in dots they do not. The overgrown AlGaAs layer passivates the nonradiative surface states thereby increasing the luminescence efficiency of these structures. It has been shown that shifts in exciton emission to higher energy after regrowth on samples subjected to RIE with either SiCl4 or CH4/H2 are most likely due to enhanced diffusion of A1 into the quantum well caused by impurities introduced during the dry etch process. This changes the profile of the well from a square potential profile to a compositionally graded well causing the shift to higher emission energies