Diamond surface studies

The structure, composition, and chemical reactivity of the diamond surface are of key importance both during CVD growth and for post - growth applications. M uch of the current understanding of CVD grown diamond is inferred from UHV science and theoretical studies. More recently, atomic scale images have provided useful information. Diamond shares the same 100 % covalent bonding structure as the semiconductors silicon and germanium when highly pure (Pate [1986]). Unlike the latter two, diamond is highly insulating due to its large band gap (« 5.5 eV).

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4.1.2.1 The (111) diamond surface

The most studied surface of diamond is the (111) cleavage surface of natural diamond . A summary of the atomic and electronic structure of this surface has been given by Pate [1986]. Truncation of the diamond bulk to expose the C ( l l l ) face leaves a surface with one dangling bond per surface carbon atom. Polished C(111) surfaces heated in vacuum to 500°C exhibit sharp ( l x l ) LEED patterns. The surface structure is known to be a truncation of the bulk geometry, where the dangling bonds are terminated by hydrogen adatoms as shown in figure 4.1(a). Heating the surface to 950°C in-vacuo under UHV pressures, induces the desorption of hydrogen and reconstruction of the surface in a way that the dangling bonds are eliminated to create an energetically stable surface, this time without hydrogen adatom termination. H alf order LEED diffraction spots have been observed, indicative of a ( 2 x 2), or three rotated domains of ( 2 x 1) surface reconstruction.

Several models have been proposed for the structure of this (2 x 2)/(2 x 1) - reconstructed (111) surface of diamond. However, theoretical calculations (Pandey [1982], Vanderbilt [1984], Dovesi [1987]) have been performed and favoured Pandey's p - bonded chain model (Pandey [1982]) where the dangling bonds form zig-zag chains as shown in figure 4.1(b). Likewise, experimental data of photo emission from occupied states (Himpsel [1981]), X-ray adsorption (Morar [1986]), and ion scattering (Derry [1986]) experiments all support Pandey's model for reconstruction.

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(a) Diamond C (1 11)-(1 x 1) O H ^ C

(b) Diamond C (111)-(2 x 1) Pandey

F ig u re 4 .1 (a) C (111) (1 x 1) diam ond cleavage surface term inated by hydrogen atom s; (b) Pandey 7t-bonded surface reconstruction o f clean diam ond C (111) surface.

4 . 1 . 2 . 2 T h e ( 1 0 0 ) d i a m o n d s u r f a c e

T runcation o f the bulk diam ond to expose the C(100) surface leaves the surface with two dangling bonds per surface carbon atom. Lurie and W ilson [1977a] have obse rv e d ( l x l ) L E E D patterns from as-polished C(100) surfaces. O ther researchers e stablished that this surface is hydrogen term inated (Pate [1986], Derry [1983]). T hus, the dihydride structure s how n in figure 4.2(a) is the best candidate for the C ( 1 0 0 ) -( l x 1) surface. A s with the d iam ond (111) surface, heating in UE1V to 1000°C induces surface reconstruction, and two rotated d o m ain s o f (2 x 1) L E E D sym m etry are o b se rv e d (Lurie [1977a]). T h eoretical calculations (V erw oerd [1981], Takai [1985], Bechstedt [1988]) and experimental evidence ( H a m z a [1990]) p o s tu la te s y m m e tr ic d im e rs, sh o w n in fig u re 4 .2 ( b ), w ith e ith e r m o n o h y d rid e (one hydrogen adatom per surface carbon atom in the d im e r pair) or C-C b o n d in g as the possible structures for the ( 2 x 1 ) reconstruction. A ltern a tiv e ly , Y ang [1992] has also proposed a (3 x 1) reconstruction consisting o f alternating carbon dim ers

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and carbon dihydride-term inated carbon atom s as show n in figure 4.2(c). Both the ( 2 x 1 ) and ( 3 x 1 ) reconstruction can relax the severe steric constraints o f the fully h y d rogena ted surface and hence are thermodynamically more stable.

Diamond C (100)-(2 x 1):H Diamond C (100)-(2 x 1)

Diamond C (100)-(3 x 1)

F ig u re 4.2 P ossible surface bond term ination configurations for the diam ond (100) surface with hydrogen.

Diamond C(100)-(1 x 1):2H

4 . 1 . 2 . 3 T h e ( 1 1 0 ) d i a m o n d s u r f a c e

T he (110) surface o f diam ond is the least studied of the low index planes. T he as-polished (110) surface also exhibits a (1 x 1) L E E D pattern c onsistent with the structure o f the truncated bulk as show n in figure 4.3. How ever, unlike the ( 111) and (100) surfaces, the picture o f a high tem perature, hydrogen-free (110) surface is far m ore speculative as no reconstruction is observed after annealing to over 1000°C (Lurie [1977a]).

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4.1.2.4 CVD diamond surfaces

As thin film diamond growth on non-diamond substrates is being recognised now as a comparatively inexpensive and controllable technique to produce semiconducting diamond, its use in various device applications such as radiation detectors, rectifying contacts, and field effect transistors has been evaluated (Gildenblat [1991]). A critical aspect of device fabrication is reliability and reproducibility. Many of the techniques used rely on clean and/or smooth surfaces. The polycrystalline nature of CVD diamond does not initially seem a good starting point for reliable devices from such material. Therefore the CVD diamond substrate has to be suitably modified to obtained the desired reliability and reproducibility. There is a wide range of techniques and deposition conditions available to synthesise thin film diamond, which is a polycrystalline material containing non-diamond carbon in low levels (Grot [1990], Plano [1994]). The roughness of CVD diamond films has been found to be dependent on their thickness (Singh [1996], W ild [1994]). The surface roughness to film thickness ratio is proportionately greater for thin CVD diamond layers of a few micrometres, than for layers on the order of 10's to 100's of (im thick. For thinly grown films, individual crystallites are still at the stage of coalescing together as they grow to form a continuous layer: if the film is allowed to grow, the coalescence will become more complete resulting in a smoother surface. Treating CVD diamond films with laser irradiation has been adopted as a useful technique for planarising rough surfaces thereby rendering them more efficacious for a subsequent lithographic process (Singh [1996], Cremades [1996], Tosin [1995]). Indeed, Tosin and co-workers [1995] reported the average surface roughness of a laser irradiated CVD diamond film at 15 nm. Therefore this planarisation is highly valuable if device structures of micrometre accuracy are to be established upon diamond. Polycrystalline diamond can be grown using many different CVD techniques (Noda [1994], Sakiyama [1994], Ravi [1995]) which give rise to bulk and surface defects such as surface/bulk grain boundaries and inclusions, stacking faults and twinning, and areas of spontaneously formed non-diamond carbon. Along with these defects, the conductivity of the diamond may only be due to its surface: a 100°C anneal in a nitrogen or air ambient reduced the electrical conductivity of the film. The conductivity was restored when the film was subsequently treated in a hydrogen plasma indicating that hydrogen may play a role in the passivation of surface traps (Landstrass [1989a], [1989b]).

Post growth cooling techniques prove to be as effective in affecting the conductivity as the growth parameters are. Mori et al. [1992] observed that cooling polycrystalline or single crystal diamond in an oxygen ambient increased the films' resistivity. Films cooled in- vacuo within a nitrogen or hydrogen atmosphere increased their conductivity by 6 orders

of magnitude. The conductivity of the non-oxygen cooled films was believed to arise from

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surface defects detected by electron spin resonance (ESR), and an amorphous carbon phase which was judged not to be a complete overlayer by transmission electron microscopy (TEM).

Therefore, alongside the issue of surface roughness which may introduce problems in the photolithographic stage of device fabrication (Plano [1995]), the overall role of hydrogen in the electrical conductivity of the film and of non-diamond carbonaceous layers upon the surface of as-deposited films must not be overlooked. "Wet" methods have been employed to remove the surface graphitic layer. Geis and co-workers [1987] used a mixture of heated sulphuric acid with chromic acid or ammonium persulphate, the latter two both being strong oxidants. Grot [1990] and co-workers observed significant differences in the device characteristics after the substrate had been "wet" treated: a series of sheet resistance measurements did not reflect the trend of the films' doping level until chemical cleaning had been carried out.

The aim of this chapter is to systematically analyse various surface cleaning techniques which have been used previously, assess their effectiveness, and observe if they chemically or physically alter the surface. Their results are contrasted against spectra from laser irradiated samples which clearly indicate the presence of surface graphite. Auger electron spectroscopy is the surface sensitive analysis tool used to characterise the results from the various cleaning procedures.