The electrochemistry of silicon in alkali
By: by Richard James Davies.
Publisher: University of Birmingham
Published: 2000
Place of publication: Birmingham
Language: eng
Number of pages: 252
First sentence: INTRODUCTION 

This thesis is concerned with the electrochemical behaviour of silicon in potassium hydroxide. Silicon is the dominant semiconductor material in the microelectronics industry. It is commercially available in purity levels practically impossible for other materials, and intentional doping of the semiconductor enables the fabrication of microelectronic circuits, devices and sensors with unprecedented reliability. Common fabrication techniques utilise both wet (acid and alkali) and dry (reactive ion) etching with high resolution lithography. Two technologically important properties are, 1) anisotropic etching in alkali, and 2) the stability and uniformity of the silicon oxide layer.
The phenomenological understanding of wet etching enables the economical fabrication of devices. Acids are used primarily for cleaning and polishing. Alkali etching is primarily based on open circuit etching technology, relying entirely on nature of chemical alkali etching. For sub-micron device fabrication, imperfections in the pattern reproduction limit the use of wet etching to larger structures. Interest now turns to the control of chemical etching procedures by imposed bias. Advances in electrochemistry, both in the theory and available instrumentation, permit the imposition of well defined potentials at the semiconductor interface and an interpretation of the response. Various electrochemical processes have been proposed within the literature with mechanisms which both agree and conflict with the wealth of reported experimental observations. Current mechanisms tend to be focused on the surface decoration in alkali.
Advances in computer technology and the incorporation of microprocessors into instruments, enable the use of computer control systems and automated data logging in electrochemistry.
A fundamental part of this research was the design of a computer system to enable a d.c. potentiostat and a.c. impedance analyser to be controlled precisely without limitations due to the restraints imposed by a manual instrument interface. Full and flexible control of the experimental conditions enabled the generation of large quantities of statistically relevant data, captured by automated data logging procedures.
Recent advances in the theories of impedance spectroscopy and in computer software have enabled the application of complex equivalent circuit fitting routines to data sets, resulting in rational models of the electrode - electrolyte interface. Impedance spectroscopy examines the whole semiconductor - electrolyte system:- the minority and majority carrier behaviour within the semiconductor bulk and near surface; the transport of reactants and products within the solution; the semiconductor-electrolyte interface (surface states); and reactions at the interface. Understanding of processes at the interface will be facilitated by restriction of the system under test to one in which the electrolyte supplies OH and H2O as reactants. 
Although the p-type silicon-KOH system in the dark was chosen as it is a relatively simple system, with a minimal number of reacting components, the impedance response of this “simple” system is complex. The acquisition of steady state d.c. and impedance data over a large potential domain with very small potential intervals, has enabled processes within the silicon-KOH system to be discerned. We have observed both classical and non-classical depletion layer, and solution transport behaviour. Full interpretation of impedance results required the study of the effects of imposed illumination. 
The observations in this work have been married to those in the current literature to provide a unambiguous picture of the silicon-KOH system. Discrepancies within the literature concerning mechanisms, the position of the flat band potential, surface chemistry, Fermi level pinning and illumination effects have been resolved with a new electrochemical mechanism involving holes and surface states.
The work was then extended to a preliminary investigation of n-type silicon in KOH.

In brief, the objectives of this research were to determine the electrochemical processes within the p-type silicon-KOH system using d.c. and a.c. techniques; to determine the effects of illumination on those processes; correlate new measurements with observations in the literature; and to rationalise conflicting theories of the behaviour of silicon in alkali.
Description: SYNOPSIS
This thesis presents an electrochemical study of p- and n-type silicon in KOH in the dark and under variable illumination. 
Computer programs have been written for the control of d.c. and a.c. experimental work, for the acquisition of data and for their processing. Fundamental to this work was the creation of computer software to enable precise control of electrochemical work and the acquisition of steady state d.c. and impedance data over a large potential range in small potential steps. Additional software was written to enable automated cyclic potential sweeps with small increments in the sweep limits. Home written and commercial computer software routines have been employed for equivalent circuit and Mott-Schottky analyses. 
From a comparison of the information derived, the electrochemical behaviour of silicon in KOH has been interpreted for both illuminated material and for the semiconductor in the dark. It has been possible to relate detailed observations over a wide range of electrochemical conditions with the more restricted range experimental work in the literature, enabling the construction of an unambiguous picture. Discrepancies in the literature have hence been addressed with a view to formalising a consistent etching mechanism.
It is concluded that:
1.	The electrochemical behaviour of silicon is sensitive to the cell mounting technique.
2.	Localised surface states within the bandgap region enable holes to exist at the surface of p-and n-type silicon in depletion. Fermi level pinning is attributed to these states.
3.	It is necessary to correlate observations in the dark and under illumination to understand the electrochemical etching mechanism. Illumination of the semiconductor surface enables processes of the semiconductor in depletion to be studied at all frequency domains.
4.	Non-classical depletion layer behaviour results from competition between water electrolysis, recombination (under illumination) and Fermi level pinning. 
5.	Hydrogen is injected into p-type silicon at cathodic biases. The non-classical transport impedance at low anodic bias is dominated by the chemical generation of hydrogen gas. 
6.	Photoelectrons delay the onset and mode of passivation for p-type silicon, photoholes hasten passivation in n-type silicon. Positive charges are trapped beneath the illuminated n-type anodic oxide surface.
7.	Oxide formation requires the presence of surface holes.
Notes: Thesis (Ph.D) - University of Birmingham, School of Metallurgy and Materials, Faculty of Engineering,.
Table of contents: value: SEMICONDUCTORS AND SOLUTIONS; value: CHEMICAL ETCHING OF SILICON; value: ELECTROCHEMICAL ETCHING OF SILICON IN KOH; value: MECHANISMS AND MODELS; value: IMPEDANCE SPECTROSCOPY; value: ILLUMINATION EFFECTS ON SEMICONDUCTORS; value: EXPERIMENTAL; value: COMPUTER SOFTWARE CONTROL OF THE ELECTROCHEMICAL HARDWARE; value: ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY (EIS); value: DC CYCLIC VOLTAMMETRY; value: A.C. IMPEDANCE SPECTROSCOPY; value: A.C. IMPEDANCE OF P-TYPE SILICON IN THE DARK; value: THE A.C IMPEDANCE OF P-TYPE SILICON UNDER ILLUMINATION; value: THE ELECTROCHEMICAL CHARACTERISTICS OF N-TYPE SILICON AS A FUNCTION OF ILLUMINATION; value: CRITERIA FOR A SILICON-KOH REACTION MECHANISM; value: REACTION MECHANISMS OF P-TYPE SILICON IN THE DARK; value: THE ELECTROCHEMISTRY OF P-TYPE SILICON INVOLVING HOLES; value: DISCUSSION OF THE RESULTS FOR P-TYPE (100) SILICON IN THE DARK; value: DISCUSSION OF THE RESULTS FOR P-TYPE SILICON UNDER ILLUMINATION; value: DISCUSSION OF THE RESULTS FOR N-TYPE SILICON; value: BAND MODEL; value: CONCLUSIONS; value: FUTURE WORK