LAS ENTIDADES DE LA ADMINISTRACIÓN PÚBLICA

An alternative method of diamond synthesis is chemical vapour deposition (CVD). In CVD diamond is grown in the graphite-stable region of the phase diagram

through the chemical interaction of carbon-containing source gases with the depo- sition surface. The first example of growth using this method was patented by Ev-

ersole and Kenmore in 1958 [10, 11]. However, growth rates of only 0.1µm/h were achieved. Subsequently, Anguset al. [12] and Derjaguin et al. [13] demonstrated that the presence of hydrogen at temperatures above 1000○_{C lead to the preferen-}

tial removal of graphitic (sp2_{) material over diamond (sp}3_{) material. Therefore,}

higher growth rates of diamond are achievable from hydrocarbon-hydrogen sys- tems (e.g. H2:CH4).

Since 1958 different growth chemistries have been investigated in various at-

tempts to increase growth rates and produce doped diamond. The most popular gas mixture for CVD growth is H2 and CH4, which involves carbon-containing

radicals and atomic hydrogen. Much work has been carried out in an attempt to understand the microscopic mechanisms involved in CVD growth and has culmi-

nated in a ‘standard model’ of CVD diamond growth [14]. CVD diamond growth models are discussed by Goodwin and Butler [15], with more recent developments

Chapter 2. Literature review

Vacuum pump

Water cooled stage Substrate and stage

Excitation probe / microwave antenna Window Base Plasma Gases in Bell-jar housing Sliding short Cavity wall

Figure 2-4: Schematic of a mi- crowave plasma CVD reactor. Adapted from [18].

being discussed in [14].

For atomic hydrogen to be present in the reactor elevated temperatures are required. These temperatures can be generated by either:

1. A hot filament (∼2000○_{C) which is placed close to the growth surface. This}

involves a relatively inexpensive experimental set-up where multiple fila-

ments can allow a large deposition area. One major disadvantage is that the growth material can be contaminated with material from the filament. In ad-

dition, the growth rates are typically relatively low compared to microwave plasma CVD.

2. The microwave plasma (MP-CVD) method can offer a cleaner growth envi-

ronment and was used to produce all the samples discussed in this Thesis. With MP-CVD high quality uniform films can be produced over large sub-

strate areas. This method was first used by Kamo et al. [16] at 2.45 GHz, a frequency which has been widely adopted, although some lower frequency

reactors with larger growth areas have been reported [17]. The reactor used in the work by King et al. [17] is depicted in Figure 2-4.

Nitrogen is readily incorporated into diamond grown by MP-CVD, although at far lower concentrations than in HPHT diamond grown without a ‘getter’.

Through the minimisation of reactor leaks and the removal source gas impuri- ties, diamond containing less than 1 ppb of nitrogen defects can be produced (see

Chapter 2. Literature review

Chapter 5). However, very low concentrations of nitrogen in the source gases can

dramatically increase the growth rate of diamond [19–21]. Nitrogen is incorpo- rated into the sample in the form of single nitrogen (NS) [22, 23] as well as other

defects which can affect optical, thermal and electrical properties. The doping efficiency of nitrogen, i.e. the relative incorporation of a nitrogen atom to the in-

corporation of a carbon atom, has been determined by Samlenski et al. [24] and Tallier et al. [23] to be (0.75−6) ×10−4_{. Ultimately, a balance must be struck}

between the permissable level of nitrogen incorporation and the required growth rate. In this Thesis (Chapter 5) some very high purity samples (N0

S<1 ppb) are

studied where exhaustive efforts were taken to exclude nitrogen from the growth environment.

Any impurity introduced to the source gas could be incorporated into the

growing crystal. For example, the doping efficiency of boron is 10−1 −₁₀−2_{, signif-}

icantly higher than for nitrogen [22]. For both nitrogen and boron the incorpora-

tion efficiency is dependent upon the growth sector [22, 24], as well as the growth conditions of the diamond.

Other impurities observed in CVD diamond are phosphorous, where incorpo- ration efficiencies have been determined to be between 0.15 and 0.04 [25, 26] and

silicon, where incorporation efficiencies are lower than that of boron but greater than that of nitrogen on a {001} surface [27]. The incorporation of oxygen is considered in Chapter 9.

In 2002, Yan et al. [28] reported single crystal (SC) CVD diamond growth rates of 50–150µm/h. Comparable growth rates have been reported by several

groups subsequent to Yanet al. [29–32]. However, for CVD diamond the nitrogen impurity concentration in such material is typically high (≲10 ppm) and the mate- rial is brown in colour. The brown colour is attributed to vacancy clusters grown into the diamond [33, 34]. Methods for the production of thick SC-CVD samples

have also been patented by Element Six Ltd., as well as patents for the growth of very high purity SC-CVD diamond grown without the addition of nitrogen [35].

Recent work by Asmussenet al. [18] has discussed the use of MP-CVD reactors operating at 915 MHz [17] with a potential deposition area of 6-8 inches. Asmussen

Chapter 2. Literature review

rate of 20.8µm/h for SC-CVD diamond. Asmussen et al. [18] reported that over the deposition area there was a variation in the growth rate of 5%.

Other elements can be incorporated into the CVD growth environment for the purpose of deliberately doping the diamond (e.g. boron [22, 36]); to improve

growth rates; or inhibit the incorporation of specific impurities [27].