Growth of SiGe Virtual Substrates and Nanostructures by Ultra-High Vacuum Chemical Vapor Deposition

• Main Authors: Sheng-Wei Lee, 李勝偉
• Other Authors: Lih-Juann Chen
• Format: Others
• Language: en_US
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/14891719237257569987

博士 === 國立清華大學 === 材料科學工程學系 === 93 === The Si-based metal-oxide-silicon field-effect-transistor (MOSFET) is currently the device of choice for state-of-the-art digital electronics. Historically, the performance improvements of MOSFETs have been attained by shrinking device dimensions such as gate length and gate oxide thickness. However, the practical benefit of scaling is declining as physical limits are approached. Therefore, the incorporation of new materials, from the interconnect level (low-k), to the gate stack (high-k), and even the substrate [silicon-on-insulator (SOI)] is emerging as an important way to continue to improve circuit performance. Si1-xGex/Si heterostructures have been under extensive study because they can provide adjustable bandgaps and improved carrier mobilities compared with Si homostructures. In this dissertation, the growth of high-quality SiGe virtual substrates and novel SiGe nanostructures by ultra-high vacuum chemical vapor deposition (UHV/CVD) was demonstrated. An intermediate Si or Si1-yCy layer in the Si1-xGex film, replacing the conventionally graded buffer layer, was used to form the high-quality relaxed SiGe substrates. The results indicate that the Si (Si1-yCy) layer serves as preferential nucleation sites for misfit dislocation array to relax the mismatch strain during the SiGe overgrowth. Threading dislocation density and the residual strain remained in those SiGe uniform epilayers were found to be reduced effectively. The shallow pits on the surface related to strain relief were also found to be suppressed. The 800-nm-thick Si1-xGex/Si1-yCy/Si1-xGex (x=0.2, y=0.014) heterostructure was demonstrated to have a threading dislocation density of 5.4��105 cm-2 with a residual strain of only 2 %. Effective electron mobility for the strained-Si device with this novel substrate technology was found to be 95% higher than that of Si control device [Chapter 5 and 6]. High-quality SiGe films with a buffer layer containing Ge quantum dots have also been grown by UHV/CVD. Threading dislocation density and the residual strain remained in the SiGe uniform epilayers were found to reduce drastically with the increasing period of Ge dots/Si bilayers up to 10 periods. The Si0.8Ge0.2 film grown on a 10-period Ge dots/Si bilayers was demonstrated to have a threading dislocation density of 2.0��105 cm-2 with a residual strain of only 11%. The resulting SiGe films with multiple Ge quantum dots layers also exhibit distinct characteristics compared with the modified Frank-Reed (MFR) mechanism observed in compositionally graded layers structures. Effective electron mobility for the strained-Si device with the multiple Ge quantum dots buffer layer was found to be 90% higher than that of Si control device. These experimental results demonstrate the great promise of strained-Si devices and suggest a path for future investigations [Chapter 7]. Epitaxial deposition of Ge/Si(001) heterostructures can be regarded as a well understood model system for self-assembly techniques. Nano-rings with an average height and diameter of 1.2 and 65 nm, respectively, were observed to from in Si-capped Ge quantum dots grown at 600 ℃ by UHV/CVD. The nano-rings were captured with the rapid cooling of the samples with appropriate amount of Si capping. Based on the results of transmission electron microscopy and Raman spectroscopy, the formation of nano-rings is attributed to alloying and strain relief in the Si/Ge/(001)Si system. The self-assembly of nano-rings provides a useful scheme to form ultrasmall ring-like structure and facilitates the characterization of the physical properties of unconventional quantum structures. In addition, field emission characteristics of self-assembled Si-capped Ge quantum dots with different Si coverages have also been investigated. With an appropriate amount of Si capping to form the truncated pyramids, the field-emission behaviors of Si-capped Ge quantum dots were found to be improved significantly. Based on transmission electron microscope examinations, this improvement can be attributed to the sharper apex and a higher density of the truncated pyramids as compared to the uncapped domes. However, further Si capping could degrade the field emission properties owing to the flattening of Ge islands features [Chapter 8]. The SiCH6-mediations were used to modify the Stranski-Krastanow (S-K) growth mode of Ge dots on Si(001) at 550 ℃ by UHV/CVD. With the appropriate SiCH6-mediation, the elongated Ge hut clusters can be transformed to highly uniform multi-faceted domes with a high Ge composition at the core. The modified growth mode for the formation of SiCH6-mediated Ge dots can be attributed to (i) an almost hydrogen-passivated Si surface to limit the nucleation sites for dot formation, and (ii) the incorporation of Ge atoms, repelled by the C-rich areas, into the existing Ge dots. These SiCH6-mediated dots were found to exhibit the improved field emission characteristics compared to shallow Ge huts. These experimental results demonstrate a useful scheme to utilize self-assembled Ge QDs as field-emitter arrays [Chapter 9]. The pre-intermixing treatments of Ge quantum dots were demonstrated to be effective in improving the size uniformity and preventing the formation of cone-shaped defects (CSDs) in the Ge-dot multilayers. The rapid decrease in density of Ge dots with the number of layers is also alleviated. The strain relaxation of Si/Ge multifold layers is characterized by high-resolution rocking curves. The pre-intermixed Ge dots have stronger photoluminescence (PL) intensity due to a higher Ge dots density and a lower cone-shaped defect density. The results indicate that pre-intermixing treatment of Ge quantum dots is a promising technique for the fabrication of emitters and detectors in Si-based optoelectronic devices [Chapter 10]. The growth and evolution of the Ge islands on Si(113) during the high-temperature annealing have been investigated by in-situ TEM examinations. Most of Ge islands on Si(113) were found to form at the edges of surface terraces. The analysis of Moiré pattern reveals that the Ge islands undergo the serious Si-Ge intermixing during the annealing. In addition, the decomposition of the severely intermixed Ge islands with further annealing was first observed [Chapter 11].