Carbon-Induced Ge Islands on Si(001) Grown by LPCVD
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ABSTRACT The formation of carbon-induced Ge islands has been studied using low-pressure chemical vapour deposition (LPCVD) of Ge on Si(001) at temperatures between 600'C and 700°C. Propane (C3 H 8 ) diluted in He was used as a carbon source. The experiments show that the influence of carbon was most significant for deposition at low growth temperature of the Ge island layer. Small-sized islands with a narrow size distribution could be achieved using a carbon adsorption layer. Compared to a sample grown without this layer, the size distribution was significantly smaller. An enhancement of the growth rate of Ge, as seen from Rutherford backscattering spectroscopy (RBS) will be discussed.
INTRODUCTION Devices based on quantum effects e.g. cellular automata, have attracted considerable interest during the last few years. Such devices require nanometer-scale structures, which are usually difficult to fabricate using conventional semiconductor technology. Selforganized structures such as quantum wires or quantum dots offer an interesting possibility to reach dimensions in the nanometer regime. To remain compatible with standard silicon technology, the group - IV elements are the materials of choice. Pure Ge islands which form selforganized structures could act as semiconductor quantum dots, but they are still too large in size, i.e. they have typical lateral dimensions of 50 nm and bigger. The use of a sub-monolayer of carbon deposited prior to Ge growth in MBE has shown promising results: the lateral island
dimension was reduced down to 10 nm while the areal density was increased. Triggered by the results presented by Schmidt et al. [1, 2] using molecular beam epitaxy and by Seal et al. [3] using UHVCVD, our aim was to transfer the carbon predeposition technology to a low pressure chemical vapour phase (LPCVD) deposition system. Propane (C3 H8 ) diluted in He was used as a carbon source. Because of the frequent use of propane in SiC CVD [4, 5] the carbon formation process has already been studied in detail [6]. Compared with methylsilane or methylgermane propane offers the possibility to deposit carbon independently from Si or Ge. Depending on the growth conditions (base pressure of the system, partial pressure of C3 H8 ), it is possible to achieve a low but defined carbon deposition rate which enables a submonolayer control of the layer thickness. The novelty of our study is the carbon deposition from propane at relatively low temperature. Usually, carbon deposition is carried out at temperatures beyond 1200'C [5] to achieve a homogeneous SiC template layer for further SiC deposition. As we are interested in carbon deposition on the Si surface rather than in SiC formation, a lower deposition temperature is necessary. Thus we prepared SiGeC layers with an identical amount of Ge but a different amount of carbon to test the possibility of carbon deposition at 700'C. The amount of carbon incorporated was determined by measuring the degree of elastic relaxation of these layers by X-ray scattering and by photoluminescence.
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