Structural, Compositional and Optical Properties of Self-Organised Ge Quantum Dots

Abstract

In this paper we present a study of the structure, composition and optical properties of SiGe quantum dots, grown by gas-source molecular beam epitaxy on Si (001). Atomic force microscopy (AFM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy dispersive X-ray analysis (EDX), photoluminescence (PL) spectroscopy and decay time measurements of the quantum dots suggest that there are two distinct sizes of quantum dot, contributing two distinct emission bands in the PL spectra. IntroductionThe growth of self-organised Ge quantum dots on Si has attracted a great deal of attention because of their potential application in Si based opto-electronic devices. Confinement of carriers in the quantum dots is expected to increase the luminescence efficiency [1, 2], due to the discrete nature of the confined states. Development of an efficient Si based light emitting material would make it possible to combine both optical and electronic components on the same Si substrate. Various methods of fabricating Ge quantum dots on Si substrates have been studied [3] but here we are concerned only with self organised quantum dots grown by molecular beam epitaxy (MBE). Two types of quantum dots have been commonly reported [4]: small pyramidal dots and larger domes. A broad photoluminescence (PL) band centred at % 0.8 eV, from Ge quantum dots has been reported [1, 5], but the nature of the recombination processes occurring therein is not completely understood. In order to accurately model the recombination processes detailed information about the composition and structure of the quantum dots is required.

Compositional mapping of semiconductor quantum dots and rings

“…26) [87]. It was shown that the cap layer's material was pure Si, while it was Si 0.75 Ge 0.25 in the QD center and Si 0.83 Ge 0.17 in the WL.…”

“…The effects of Si overgrowth of SiGe islands are a dilution of the QD material and Ge segregation into the cap layer [83,87,107], which become more pronounced with increasing temperature [116] (see Table 4). …”

Hole confinement and dynamics in δ-doped Ge quantum dots

“…AFM studies [8] of our dot structures indicate dome-or pyramidal-shaped structures with base diameters in the range 80-120 nm, heights in the range 10-20 nm (above the wetting layer) and a surface density of around 2 Â 10 9 /cm 2 . Electron microscopy studies of the structure of similar Si/Ge dots to those studied here [9], have shown them to be composed of a Si 1Àx Ge x alloy with an average concentration of around x ¼ 0:5: The wetting layer is essentially a 4 nm thick quantum well composed of a Si 1Àx Ge x alloy with xE0:23: In terms of the hole confinement in the system, we therefore expect a valence-band offset between the dot and the surrounding silicon/ wetting layer of around 0.35 eV.…”