Physical Vapour Deposition

Introduction

Most of the photocathode films in this thesis were grown in vacuum via a simple deposition technique called physical vapour deposition (PVD). In PVD the substances to be deposited are sublimated into gaseous form which then travels through vacuum to the sample surface where it is deposited. There are many different types of PVD, the most commonly used forms being sputtering and vacuum evaporation. The latter is primarily used in this thesis to deposit thin metal oxide films for use as UV photocathodes. Therefore vacuum evaporation will be the pri- mary focus of this section.

Vacuum evaporation is a technique that uses crucibles, tungsten boats, naked wires or electron beam heating to heat and sublimate the source material which then travels through the vacuum to the substrate. Vacuum evaporation has to take place in good vacuum conditions to avoid surface contamination and to have a long mean free path between collisions. Also care has to be taken in placing the sample far enough away from the filament and source to reduce radiant heating of the substrate or another means of cooling the sample needs to be implemented, such as water cooling. Radiant heating from the effusion cell can also deposit other contamination onto the sample surface if the chamber is not clean. In this thesis all growth occurs under vacuum pres- sures (<10−6mbar).

2.1. GROWTH TECHNIQUES

Experimental Method

Most of the photocathode layers mentioned in this thesis are deposited via a simple thermal vac- uum deposition method in a purpose built vacuum system at the University of Warwick. The chamber is separated into two main sections by a gate valve to allow for sample loading and removing without compromising the pressure and contamination in the main chamber, in par- ticular Zn has high vapour pressure at low temperatures meaning that it coats the inside of the chamber and can re-evaporate during other depositions. Figure 2.1ashows the chamber with the 2 sections labelled.

(a) (b)

Figure 2.1: a) Vacuum chamber for PVD b) bottom view of effusion cells and leak valve Both sections of the chamber have their own turbo molecular pumps and rotary pumps for back- ing pumping.

Pressure in the chambers is measured by two hot cathode pressure gauges for the two separate sections of the chamber. These gauges use a regulated electron current from a heated filament to measure the pressure in the chamber. Gas ions caused by electron collisions are attracted to a central ion collector wire where the current is measured. Over a wide range of molecular density the ion current is assumed to be directly proportional to the molecular density and therefore the pressure can be inferred from the ion current.

Each material is deposited using their own separate effusion cells, which can allow for rough control of amount deposited of each via measurement of the temperature, using thermocouples. The sublimation of the solid source material happens via thermal evaporation in a ceramic cru- cible heated by a coil of tungsten wire. After the samples are grown, oxygen is slowly released into the chamber via a leak valve and narrow capillary tube leading to the sample surface. A narrow tube is used to direct the oxygen to the sample to minimise the oxidation of the tung- sten filaments (both around crucibles and on ion gauges), whilst maximising the oxidation at the sample surface. Also on the gas line is a line to vent the side chamber with pure nitrogen for sample removal. Figure 2.1bshows a more detailed view of the bottom of the side chamber

2.1. GROWTH TECHNIQUES

showing both effusion cells and leak valve which allows oxygen into the chamber.

Evaporation Rate

The evaporation rate of a sublimated material in vacuum can be calculated using the Hertz- Knudsen vaporisation equation [71] as shown in equation 2.1,

dN

dt =Csqrt2πmKT(p ∗₋

p) (2.1)

wheredN is the number of evaporating atoms per cm2,C is a constant that depends on rota- tional degrees of freedom in vapour, p∗is the vapour pressure of the source material and p is the pressure above the surface. Maximum evaporation rate is when p=0 andC=1 however the actual vaporisation will be a third to tenth of this maximum rate due to collisions, surface contamination and other effects.

Using this equation if we know the temperature and vapour pressure of the material in question we can get an estimation on the vaporisation rate onto the substrate.

For low vaporisation rates, and ignoring collisions within the path to the sample, the flux distri- bution as from a point source can be described by a cosine distribution as shown in equation 2.2 [71],

dm dA =

E

πr2cosφcosθ (2.2)

whereEis the total mass evaporated, θ is angle from the normal to the vaporising surface and φis the angle from the source to a point on the surface.

In general, to measure the vapour pressure of various gases a Knudsen cell is used, this cell con- sists of a closed volume with a small orifice, when the container is held at constant temperature the material that escapes through the orifice depends on the pressure differential [72].