Study The Electrical Properties Of Nanoscale Metal - Semiconductor Contact

Metal-semiconductor contacts including Ohmic contact and Schottky contact are the essential parts of virtually all semiconductor electronic and optoelectronic devices. The effect of downscaling the dimensions of a semiconductor device on its electrical properties is an important topic today, especially when the dimensions of a semiconductor device is scaled down to namometer region, called nano-device. Furthermore, metal-semiconductor nanocontacts prepared by metallic nanodots on semiconductor surface have received much attention due to their potential application in nano-devices in the future, as well as their compatibility with the present microelectronic technology. Meanwhile, rare earth silicides also have been studied extensively because of their high conductivity, low Schottky barrier height on n-Si and high Schottky barrier height on p-Si. Therefore, it is very meaningful to study the electrical properties of rare earth silicide nanocontacts on Si surface.This thesis is focused on the preparation and electrical properties of erbium silicide nanocontacts on silicon, which includes the following four sections:1. Section I is on the investigation of the morphologies of erbium silicides nanostructures on Si(001) and Si(111) in situ by ultrahigh vaccum scanning tunneling microscopy (UHV-STM). By means of adjusting the growth conditions such as Er coverage, anneling temperature and annealing time, it is realized to control the morphology and crystalline structure of the erbium silicide nanostructures. ErSi2 nanoislands with tetragonal structure and high-quality interface can be formed by depositing Er on Si(001) surface at room temperature and then annealing at a high temperature of 750℃. These nanoislands with a pre-pyramid shape have the length from several to several handreds of nanometers, the width from several to tens of nanometers and the height of 3-9 nm. Furthermore, the highly-resolved STM images show that the substrate surface among ErSi2 islands consists mostly of Si dimmer rows and Er-induced reconstructions. For those erbium silicide nanostructures on Si(111), the nanowires with the length of 200-500 nm are formed at low Er coverage and repeating low temperature annealing. The growth mechanism of these nanowires is found to be different from that of ErSi2 nanowires on Si(001) which is attributed to the lattice mismatch between ErSi2 nanowires and Si substrate. We suggest that it is due to the revolution and coalescence of small 2DErSi2 islands with the triangular shape. The larger 3D islands and the smaller 2D islands are formed at the higher Er coverage and the higher annealing temperature. However, the sizes of these 3D islands usually are greater than 100 nm while those 2D islands are metastable. So the studies of the electrical properties of erbium silicide nanocontacts on silicon are focused on the ErSi2 islands on Si(001).2. Section 2 deals with electrical properties of nano-Schottky contact between ErSi2 nanoislands and p-Si(001). The I-V measurements are performed by contacting STM tip on ErSi2 islands with the feedback loop switched off and then the bias voltage ramped. The I-V characteristics show that all nano-Schottky contacts have a rectifying behavior, but their current densities are at least five orders of magnitude larger than that of the macroscopic ErSi2/p-Si diodes, and the current densities increase with the decrease of contact area. Furthermore, their effective Schottky barrier heights are found to be 0.28-0.32 eV, much smaller than that of the macroscopic ones (0.73 eV). Tunneling and image force lowering are the possible reasons of the low barrier heights due to their small size. However, it also has been found that the I-V properties of ErSi2 nano-Schottky contacts are sensitive to surface absorption of residual gases in UHV chamber. After 24 hours exposures in UHV chamber, the effective barrier heights can be raised about 0.1 eV and the zero-bias conductivities can be lowered at least one order in magnitude. It indicates that the I-V properties of ErSi2 nano-Schottky contacts have a strong dependence on electronic properties of Si surface states surrounding ErSi2 islands.3. Surface state effects on electrical properties of ErSi2 nano-Schottky contacts are investigated in section 3 by using surface absorption of O2 and NH3, respectively. By gradually modifying the electronic properties of substrate surface states around ErSi2 islands, I-V properties of ErSi2 nano-Schottky contacts show an asymmetric change. That is, the leakage current shows a sharp decrease while the forward current decreases slowly with the increase of the dose of O2 and NH3, respectively. Further analysis of the experimental results indicates that there is a conducting path between ErSi2 islands and Si substrate, which is dependent on electronic properties of substrate surface states. Its behavior is similar to an ohmic contact at the backward bias, but a Schottky contact at the forward bias. Based on the theory of energy bands bending, we suggest that there are three possible conductive channels between ErSi2 islands and Si substrate:surface states band, surface space charge layer and interface space charge layer. The channels of surface states band and surface space charge layer are controlled by the electronic properties of substrate surface states. At the backward bias, the current prefer flow along the channel of surface states band because the other two channels show a high barrier resistance. At the forward bias the three channels are no obvious defference for carriers because the channels of surface space charge layer and interface space charge layer show a low barrier resistance.4. Section 4 deals with the negative effect of the low chemical stability of ErSi2 nanoislands. In experiments of surface absorption, we noticed that ErSi2 islands are "oxidized" easily when they are exposed in atmosphere of O2 and NH3 or exposed in UHV chamber for more than 26 hours, respectively. The "oxidation" always results in the sharp increase of the contact resistance between STM tip and the islands so that the obtained I-V data cannot reflect completely the electrical properties of the nano-Schottky contacts. So it is very important to eliminate surface states effect by other alternative methords or to choose the appropriate metal materials with high chemical stability.