ABSTRACT
Figure 1.1 (Left) Schematic of the custom-designed sample column, part of the IBM UHV TEM. (Right) The top and bottom images show reflection mode and plan-view geometries of the samples used for nanowire growth and nucleation experiments, respectively. For CVD, precursor gases, such as disilane (Si2H6) for Si deposition, are introduced into the sample region through UHV leak valves which facilitate precise control of the total pressure in the system. In this particular UHV-TEM, the maximum attainable pressures are limited to ~10-4 Torr during the operation of the TEM. (In other systems, however, it is possible to vary the pressure over wider range, e.g.,
1 Torr and higher, with the aid of differential pumping or by using a high-pressure cell.51,52) Since the permissible pressures are lower than that typically used for nanowire growth in a CVD reactor, the nanowire nucleation and growth rates are also lower, which is ideal for real-time monitoring of the growth phenomena. Moreover, low doses of oxygen or water vapor, the common contaminants in a CVD reactor, can be controllably introduced via the leak valves and their effect investigated. In order to carry out the in situ experiments, a specially designed double-tilt holder fitted with electrical leads is used to resistively heat the samples. For the nanowire nucleation and growth experiments, two different sample geometries as shown in Fig. 1.1 are used. Plan-view samples work best for observation of nanowire nucleation and early-stage growth processes while reflection-mode geometry is best suited for the observation of the growth of nanowires.53 Chemically-thinned polished Si(111) wafers with ~80 nm-thick amorphous silicon nitride (a-SiNx) films are used as plan-view samples. The a-SiNx membranes are thermally stable, chemically inert, relatively easy to clean, and hence are ideal for the nucleation experiments. These samples are cleaned in the UHV preparation chamber, attached to the microscope by slow heating to ~600 °C, and held until the base pressure is below 5 × 10-10 Torr. For the nanowire growth experiments, rectangular pieces (4 mm × 1.5 mm × 0.5 mm) of Si(111) are cut from a Si(111) wafer, cleaned chemically by dipping in HF solution to remove the native oxide, and mounted such that one of the polished surfaces is parallel to the electron beam as shown in Fig. 1.1. The samples are cleaned in the preparation chamber by a series of heat treatments: Samples are first heated slowly to ~600 °C and held for a few hours until the base pressure in the system is below 5 × 10-10 Torr. Then, the samples are heated rapidly to ~1250 °C and held for 30-60 s after which they are cooled to ~600 °C. This procedure yields oxide-free Si surface, desirable for accurate description of the growth phenomena. For both the nucleation and growth experiments, ~2 nm-thick Au films are evaporated onto the samples in UHV. (In case of Au-Al alloys, a bilayer film composed of Au and Al was prepared by sequential evaporation with an atomic ratio of 2:1.) The samples are then transferred without breaking vacuum to the microscope and heated to the desired temperature (450 ~ 650 °C) in presence of Si2H6 gas at pressures between 1 × 10-8 and 1 × 10-5 Torr. TEM images are acquired at video rate (30
frames per second) using bright-or dark-field imaging conditions and digitized for further analysis. In order to check for and minimize the electron-beam irradiation effects, nucleation and growth kinetics data were collected with continuous as well as intermittent electron beams. In all of our studies, the electron beam does not affect the kinetics. Substrate temperatures are measured before and after the experiments using an infrared pyrometer. For Si samples, the surface can be cleaned again by heating to 1250 °C so that a series of growth experiments can be carried out on the same sample at one area. Each of the nucleation experiments, however, was carried out on new a-SiNx samples. 1.3 Silicon Nanowire Nucleation KineticsAll of the nucleation experiments are carried out using Au-covered a-SiNx plan-view samples. Upon heating the samples in presence of disilane to temperatures above, the Au-Si eutectic temperature (~363 °C),54 the solid Au crystallites alloys with the deposited Si. With increasing deposition time, Si concentration in the alloy reaches the eutectic alloy composition, at which point Au-Si alloy will exist as a liquid. Further deposition will lead to Si supersaturation in the liquid alloy followed by nucleation of solid Si (red dot in Fig. 1.2).
