Investigation of TiO2 as a possible tunneling layer in the vertical metal insulator semiconductor tunnel transistor

Chong, Lit Ho (2006) Investigation of TiO2 as a possible tunneling layer in the vertical metal insulator semiconductor tunnel transistor. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

In this thesis, a new field effect transistor, the vertical metal insulator semiconductor tunnel transistor (VMISTT) is proposed. It is a modified version of the metal oxide tunnelling transistor (MOTT). The principle of operation of the device is based on the gate modulated Fowler Nordheim (F-N) tunnelling of carriers through an insulating layer. The VMISTT is different from the MOTT in two important aspects. Firstly, the metallic source in the MOTT is replaced by doped silicon. The body of the transistor is an oxide, which functions as a tunnel barrier. By choosing a suitable tunnel barrier and metal drain, it is possible to make both n-type and p-type devices and hence complementary devices. Secondly, the vertical structure will allow better control of material growth and device processing. The tunnel barrier can be grown by a conventional scalable process such as evaporation of thin metal film followed by thermal oxidation. This will help to reduce the leakage current and hence enhance the performance of the device.

The SILVACO device simulator ATLAS is used to study the VMISTT performance. The barrier height between the tunnel barrier and the Si substrate is a critical device parameter. A low barrier is required for a large F-N tunnelling current to occur. However, Schottky emission, which is one of the main sources of leakage current in the VMISTT, will also be huge for a low value of the barrier height. It is thus important to optimise the barrier height to minimize the effect of Schottky emission on the device performance. The simulation results show that with a barrier height of 0.6 V, an on/off current ratio of at least 4 orders of magnitude, and a subthreshold slope of 42 mV/dec can be obtained. Titanium dioxide TiO₂ is a promising candidate for the tunnel barrier due to its low barrier height to Si.

The fabrication and optimisation of the tunnel barrier of the VMISTT is the focus of the experimental chapters of this thesis. The observation of F-N tunnelling current in the tunnel barrier is essential such that the modulation of the F-N tunnelling current by the gate bias can be realised at room temperature. Electrical and structural analysis are performed on TiO₂ films grown from thermal oxidation of electron beam evaporated Ti thin film. TiO₂ MOS capacitors with different top metal electrodes (Al, Pt) and different Si substrate (n-type, p-type) were fabricated to analyse the electrical properties of the TiO₂ films. It is shown that the reactivity of Al top contact affects the electrical properties of the oxide layers. The current transport mechanism in the TiO₂ films is found to be Poole-Frenkel (P-F) emission at room temperature. At 84 K, F-N tunnelling and trap-assisted tunnelling are observed. By comparing the electrical characteristics of thermally grown TiO₂ films with the properties of those films grown by other techniques reported in the literature, it is suggested that irrespective of the deposition technique, annealing of as-deposited TiO₂ in O₂ is a similar process to thermal oxidation of Ti thin films. In conclusion, it is essential to reduce the defects density in the TiO₂ films, so that those trap-related mechanisms can be suppressed for the observation of the F-N tunnelling at room temperature.