The initial stage of surface reactions between monomethylgermane (MMGe: GeH3CH3) and Si(001) surfaces has been analyzed using reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The reactions were similar to those using monomethylsilane. That is, a c(4x4) structure, which may be caused by incorporation of carbon into a Si substrate, appeared after the supply of MMGe molecules, followed by SiC island formation as the c(4x4) disappeared. At the same time, Ge-related spots were observed near SiC twin spots. XPS revealed that Ge atoms existed in the form of Ge or SiGe on the surface layer. Weak photoluminescence (PL) peaks at approximately 0.85 eV and 0.95 eV, which are considered to originate from Ge nanodots, were observed.
Keywords

Ge, SiC, Si(001)-c(4x4) Structure, Monomethylgermane
Introduction

Recently, fabrications of quantum dots made from four-column semiconductors, which can be used as active layers of quantum dot lasers and single-electron devices, have attracted much attention due to their applicability in silicon (Si) integrated circuits [1-4]. For the formation of these dots, the control of the density and size of the dots is essential to apply these a high-efficiency light-emitting device. It has been reported that a dot density of 1011cm-2 was achieved using a pregrowth of sub-monolayer carbon [4]. In this case, carbon atoms were incorporated into a Si(001) substrate and surface strain was induced leading to the formation of Si(001)-c(4x4) at the surface, which changed the germanium (Ge) growth mode from the Stranski-Krastanov (SK) mode to the Volmer-Weber (VW) mode, leading to high-density dot formation. The dots at the surface arranged themselves randomly in this experiment. The formation of regularly arranged dots, which were formed using only the self-assembling technique, has never been reported so far. We have recently found that cubic silicon carbide (3C-SiC) nanodots were grown with lateral ordering after annealing a c(4x4) surface formed on a Si(001)-2˚off substrate. In this case, it is thought that step bunching occurred during c(4x4) formation and the alignment of SiC along <110> was achieved due to the effect of the step bunching [5]. This surface inducing the SiC dot alignment is expected as a template substrate for the formation of aligned Ge dots.

It has been reported that the Ge dots embedded in a SiC layer are expected to strongly confine carriers due to the large difference between their band gaps [6-8]. However, in these studies, a complicated fabrication procedure is used, that is, Ge ion implantation in 4H-SiC followed by thermal annealing. So far, the Ge-dot array structure embedded in a 3C-SiC crystal has not been able to be fabricated using simple methods such as chemical vapor deposition (CVD). Monomethylgerman (MMGe) is a type of alkylgermane compound and contains both carbon and Ge atoms in its molecule. It has been reported that the interaction between carbon and Ge is repulsive on a Si(001) substrate [9-11]. This source gas, therefore, is anticipated to form SiC dots and Ge dots separately on a Si surface and is considered to be suitable for the formation of nanostructures such as a Ge nano dot array embedded in SiC. To confirm this fact, it is important to understand reaction processes between MMGe and Si(001) substrates. In this study, we investigated the surface reaction of MMGe using reflection high-energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS). After that, we also attempted to form Ge dots embedded in SiC layers and analyzed their optical properties by photoluminescence (PL) measurement.