8. Overview

 

This chapter is intended to provide an overview of the main results and implications of the work presented within this thesis.  The low temperature growth of diamond using 50%CH₄/50%CO₂ gas mixtures will be dealt with first, followed by a discussion concerning sulfur doping of diamond films using H/C/S and H/C/O/S containing gas mixtures.

 

8.1. Growth of Diamond from 50%CH₄/50%CO₂ gas mixtures

 

The use of 50%CH₄/50%CO₂ CVD gas mixtures has been shown to allow the growth of diamond films at lower T_sub than is possible using 1%CH₄/H₂ gas mixtures [[1]].  It was argued that the growth surface of the diamond film could be (at least partially) terminated by CO, in the case of growth from CO₂/CH₄ gas mixtures.  This surface termination was predicted to be less stable than the H-termination that dominates in the case of growth using 1%CH₄/H₂ gas mixtures.  Thus, a more dynamic surface was envisaged with CO (or HCO) species frequently attaching and detaching from the diamond surface.  This has the effect of increasing the chance of a CH₃ radical finding a hanging bond to attack and thus add a carbon to the lattice.  In other words the CO (or HCO) diamond surface termination is less stable than H surface termination with the result that growth is possible at lower T_sub for 50%CH₄/50%CO₂ versus 1%CH₄/H₂ gas mixtures.

 

However, as the results in Chapter 5 demonstrate, there is still a fall observed in both the growth rate and quality of the deposited films with reduced T_sub.  This can be attributed to two separate effects.  Firstly, reducing T_sub has the effect of increasing the gradient in T_gas between the microwave plasma and the substrate, thus leading to increased quenching of CH₃ (e.g. to CH₄) as the species diffuses towards the substrate.  Secondly, surface diffusion of species to a step edge is generally considered important for the regular crystal growth of diamond films [[2]].  The lowering of T_sub hampers such surface diffusion and therefore leads to a reduction in crystal and film quality. 

 

In conclusion, although compared to 1%CH₄/H₂ systems CO₂/CH₄ gas mixtures provide a better chance for an incoming CH₃ radical to find a hanging bond (and therefore incorporate into the lattice) reducing T_sub has the effect of both reducing the flux of CH₃ to the surface and also impeding the surface diffusion of CH₃ (or more probably CH₂, as discussed in References 2 and [3]).  It is therefore reasonable to conclude that the growth of well faceted, high quality, diamond films by CVD at reduced T_sub is probably an impossible goal.

 

8.2. Sulfur Doping of Diamond Films

 

This section will discuss the major differences observed during the investigation of hot filament (HF) and microwave (MW) activated H/C/S containing gas mixtures, as discussed in Chapter 6.  A more general discussion of the major conclusions, and wider implications, of investigations concerning the attempted sulfur doping of diamond films using H₂S addition to 1%CH₄/H₂ and 51%CH₄/49%CO₂ gas mixtures will follow.

 

8.2.1. Hot Filament Versus Microwave Activation for H/C/S Gas Mixtures

 

The major contrast between the gas phase environment in the vicinity of a HF, as opposed to a MW plasma, is the spatial profile of T_gas.  The temperature gradient in the region of the HF is relatively steep as T_gas reduces from ~ 1800 K within 1 mm of the filament, to ~1100 K at a distance of 5 mm from the filament [[4]].  On the other hand, the temperature gradient within a MW plasma is much more shallow, falling from ~ 2000 K in the plasma centre to ~ 1600 K at its visible edge [[5],[6]]. 

 

The chemistry of a HF activated gas mixture is initiated by H atoms that are produced (from H₂) on the filament surface.  These H atoms then diffuse away from the filament and initiate gas phase reactions, thus activating the input gas mixture.  However, the steep gradient in T_gas means that the volume of gas that is within the temperature range for which this activation occurs is relatively small.  This has the effect (as discussed in Section 6.8) of making CS production unfavourable in the case of HF activated H₂S/1%CH₄/H₂ and CS₂/H₂ gas mixtures.  In contrast, a MW plasma consists of a much larger volume of gas, which is at a temperature high enough to allow activation of the input gas mixture.  This has the effect of making CS production favourable for MW activated H₂S/1%CH₄/H₂ gas mixtures. 

 

8.2.2. Growth of Diamond films using H₂S addition to 1%CH₄/H₂ and 51%CH₄/49%CO₂ gas mixtures

 

Chapter 6 highlighted the possible role of CS in the incorporation of sulfur into CVD diamond films.  Although sulfur was seen to be incorporated (with film S/C ratios of up to 0.22%) into the films deposited using H₂S/1%CH₄/H₂ gas mixtures, little effect on film resistivities was observed.  This suggests that sulfur was incorporated into the grain boundaries in preference to the diamond lattice.  This has similarities to the findings of Carlisle et al. in which N₂ addition to 1%CH₄/Ar gas mixtures was found to yield conductive nanocrystalline diamond films [[7]].  They concluded that nitrogen incorporation was via CN, which they suggested moves to the film surface and then into the grain boundaries [[8]]. 

 

These findings relating to CN could be analogous to the situation relating to sulfur incorporation into diamond films via CS.  This means that the lack of success with the S-doping of homoepitaxial diamond films could be due to such a movement of CS to the surface where the S could be either retained in a thin layer, or etched away, thus resulting in a low incorporation of sulfur (i.e. the film S/C ratio versus the input gas S/C ratio).  On the other hand, the situation is different during the growth of polycrystalline films.  Now the CS can be trapped within grain boundaries, thus leading to the relatively high degree of sulfur incorporation into films, as shown in Chapter 6.

 

The use of H₂S/51%CH₄/49%CO₂ gas mixtures was found to increase the degree of S incorporation into the resulting films by about an order of magnitude, when compared to growth using H₂S/1%CH₄/H₂ gas mixtures.  However, as with the investigation of H₂S/1%CH₄/H₂ grown samples, it proved impossible to deconvolute the effects of surface termination and grain boundaries from the four point probe resistivity measurements.  Thus, the effect of using oxygen-containing gases on the S-doping of diamond films was not clearly demonstrated.

 

It is reasonable to conclude that H₂S addition to either 1%CH₄/H₂, or 51%CH₄/49%CO₂ gas mixtures does not provide a satisfactory route to the incorporation of sulfur into the diamond lattice (rather than the grain boundaries).  A more profitable direction for future investigation may be that of co-doping using both S and B in the input gas mixture.  Such an approach was first suggested by Angus et al. who showed that n‑type conductivity could be obtained for films grown with an input B/S ratio <0.23 [[9]].  They concluded that the formation of HBS, BS2 or BS in the gas phase could provide an alternative route to the incorporation of S into diamond films.  Therefore, it is clear that there is scope for future investigations into both the solid state and gas phase chemistry of H/C/B/S containing gas mixtures in relation to diamond CVD.

 

8.3. References