Vapor-Liquid-Solid Growth of Group IV (Si, Ge, SiGe) Single and Heterostructured Nanowires
Open Access
- Author:
- Minassian, Sharis
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 05, 2011
- Committee Members:
- Joan Marie Redwing, Dissertation Advisor/Co-Advisor
Joan Marie Redwing, Committee Chair/Co-Chair
Ali Borhan, Committee Member
Kristen Ann Fichthorn, Committee Member
Theresa Stellwag Mayer, Committee Member - Keywords:
- Nanowire
VLS growth
Disilane
Kinetic model - Abstract:
- Several potential applications of semiconducting nanowires (NWs) from nanoscale electronic and photonic devices to biosensors have made them interesting materials and have motivated a vast amount of studies on the synthesis of nanowires and their electrical characterization. Group IV single and heterostructured nanowires, in particular, have received increasing attention due to their compatibility with the current microelectronic industry and numerous research has been going on to optimize their growth process. For this purpose, the majority of reports have primarily used silane (SiH4) and germane (GeH4) as their Si and Ge containing gas sources. In this thesis, an alternative Si source, disilane (Si2H6) has been investigated which is of interest since it is more reactive than SiH4 and therefore may enable higher growth rates at lower temperature and lower partial pressures. The lower thermal stability of Si2H6 could also be an advantage to enable the growth of Si1-xGex nanowires over the entire composition range at lower temperatures which are more compatible with the range of conditions typically used for Ge nanowire growth and in turn may enable the fabrication of different types of heterostructures. To fulfill the objective of this research,a systematic study has been developed to explore the growth of group IV (Si, Ge, and Si1-xGex alloy) single and heterostructured nanowires from Si2H6 and GeH4 precursors. First, the growth kinetics of individual SiNWs from Si2H6 was investigated by examining the effects of growth parameters on their growth rate. The results were compared to that obtained with SiH4. In addition, to gain a better insight into the SiNW growth process, the results were also compared with Si films deposited under similar conditions inside the same reactor. Overall compared to SiH4, the use of Si2H6 enabled higher growth rates for both SiNWs and Si films. For both gases, a nonlinearity was observed in the growth rate of nanowire as a function of gas partial pressure which was explained by a simple decomposition mechanism including the adsorption, desorption and incorporation of precursor molecule on the Au droplet surface. The apparent activation energy of the process was found to be identical for both gases under the conditions examined in the present study, suggesting similar rate-determining step in the nanowire growth process from the two precursors. Upon completion of studies on SiNW growth, the synthesis parameter space was then determined for undoped GeNWs and the influence of growth conditions on their morphology as well as their growth rate was examined. It was found that the fabrication of GeNWs strongly depends on the growth temperature and is achievable only for a very narrow range. To get nanowires with uniform diameter, growth temperature should be kept low otherwise, uncatalyzed radial growth on nanowire sidewalls results in tapered nanowires. The growth rate as a function of temperature revealed an Arrhenius behavior with a small apparent activation energy and was observed to increase linearly with germane partial pressure in the range examined here. Besides the undoped GeNWs, the fabrication of radial p-n junction GeNWs, which are of interest for photovoltaic cells, was also studied and it was found that the morphology of both the p-type core and the ntype shell is influenced by temperature and dopant ratios. The outcomes of the studies on undoped GeNWs combined with the results on SiNWs were next used to explore the growth of Si1-xGex nanowires from Si2H6 and GeH4 gases. The variations in Ge fraction as well as growth rate of Si1-xGex nanowires with growth parameters were studied. In general, the higher reactivity of Si2H6 compared to SiH4 enabled the growth of Si1-xGex nanowires with Ge composition over a wide range from x ~ 20 – 80 % which was controllable by varying the inlet GeH4/Si2H6 ratio at a constant temperature. It was observed that at a given inlet gas ratio, the growth rate increases exponentially with increasing temperature and the apparent activation energy of the process reduces as the Ge content of the nanowire is enhanced indicating a change in the rate-limiting step of the process. For different growth temperatures, the growth rate of Si1-xGex nanowire was found to have a non-linear dependence on variations in either GeH4 or Si2H6 partial pressures which increases with both parameters when the other is fixed. The observed results were explained through a kinetic model that describes the reactions of precursors on the Au droplet surface. Finally, the information gathered from single nanowire synthesis was used to fabricate Ge/SiGe and SiGe/Si axial heterostructures. The SiGe/Si nanowires were further processed as a possible approach to form SiNWs with more uniform length by selectively etching the SiGe segment away and releasing the SiNW segment. Producing monodisperse silicon nanowires (SiNWs) with uniform length has been a critical difficulty in the bottom-up assembly of nanowire devices. Currently, ultra-sonic agitation is frequently used to break the wires off of the substrate; however, they break at differing lengths, degrading the yield of successful wire integration and device fabrication. It was shown here that this problem can be mitigated through the use of SiGe/Si axial-heterostructure nanowire. After the fabrication of these structures, the SiGe segment can be selectively removed using NH4OH:H2O2:H2O solution. The process releases the SiNW segments with narrower length distribution.