Defects in Polycrystalline Diamond

The grain boundaries in polycrystalline diamond film s can clearly be expected to modify the characteristics o f CVD material from those o f a good quality single crystal, however such regions are by their nature difficult to analyse because they consist o f inhomogeneous regions o f random disorder. The formation and hence the structure o f grain boundaries can be influenced by factors including the orientation o f the adjoining crystallites, stress and strain arising from the disordered lattice, the presence o f impurities which typically diffuse towards grain boundaries and the thermodynamic regime under which the film was formed. It is therefore expected that grain boundaries w ill contain dangling bonds, interstitials, vacancies and localised regions typical of all the possible carbon phases. Because o f the difficulty in identifying individual phases in such a region, and the question o f how many atoms in a given arrangement are necessary to constitute a region o f a particular phase, the term 'non-diamond carbon' (NDC) is widely used to describe such material.

Chapter 2: The Structure, Synthesis and Properties o f Diamond

Whilst it has been speculated that conduction in specific diamond film s may be linked to interstitial carbon donor levels and vacancy acceptor levels [2.75], the mechanisms which govern electrical conduction in poly crystalline diamond have been subject to extensive debate. In separate studies Sugino et a l [2.76] and Fiegl e t a l [2.77] have exam ined the d.c. current-voltage (I-V) characteristics o f M PA C VD film s and attributed the observed conductivity to grain boundary effects whilst an investigation by Ashok et a l [2.78] found evidence o f space charge lim ited conduction (carrier injection) (§2.7.1) within crystallites having a high density o f traps. The conclusions o f Sugino e t a l differ from those o f Fiegl e t a l in that the latter work attributes all conduction to grain boundary hopping whilst the former identifies two distinct regions o f the I-V behaviour: at fields o f up to «4xlO^Vcm‘^ an ohmic (I<^V) characteristic was observed whilst at higher fields the current was seen to exhibit a dependence on the square root o f the bias potential consistent with Poole-Frenkel conduction. This mechanism consists o f field enhanced thermal excitation o f trapped electrons into the conduction band and is entirely expected above the electric field breakdown strength o f a semiconductor [2.79] (§2.4.4).

At fields below 4xlO^Vcm‘ l Sugino et a l found different conduction mechanisms to operate according to the processing experienced by the sample. As-grown samples which had been rapidly cooled in vacuum after deposition, and which were therefore b elieved to be non-hydrogen terminated, were relatively conductive having a conductivity o^-lO '^Scm k This was attributed to surface and grain boundary conduction through non-diamond carbon material, the presence o f which was confirmed by the detection o f a broad Raman peak (§4.4.3) around 1500cm-^ which is indicative o f disordered sp^ bonded graphitic carbon. Annealing in flow ing nitrogen at 570 K and 670 K resulted in progressive reductions in the conductivity to cr^« 10‘^Scm-^ and C7c«10-i3Scm-i respectively, after which further heating yielded no additional reduction in conductivity. The greatly improved resistivity o f annealed samples was attributed to removal o f non-diamond carbon material, a postulate supported by a Raman spectrum of an annealed film which contained only the 1333cm-i diamond signature and lacked the 1500cm-^ NDC component. Further confirmation o f NDC removal was provided by etching a sample in a hot Cr0 3-H2S0 4 solution which is known to oxidise diamond and attack non-diamond material (§4.2.2); this sample generated similar I-V and Raman results to the annealed samples. Analysis o f the weak T ^^ temperature dependence o f the conductivity under this regime indicated variable range hopping to be the dominant conduction mechanism. The conductivity o f the annealed samples had a stronger temperature dependence and was found to be governed by traps with an activation energy o f ~0.93eV attributed to small levels o f residual NDC m aterial.

Chapter 2: The Structure, Synthesis and Properties o f Diamond

Pan et al. [2.80, 2.81] and Jany et al. [2.82] have id en tified the effectiv e m ean carrier collection distance d w ithin a sam ple as being a key p aram eter indicating film quality for device applications such as ionising radiation detectors. Inco rp o rated in d are the effective carrier m obility fiejf and lifetim e w hich, as a p ro d u ct w ith the applied field E = Vb/1 indicate the average distance w hich the carriers created w ithin a crystallite can be expected to travel before being lost to recom bination:

(2.8)

T he value o f J as a p aram eter is that it has both a good physical interpretation and can be determ ined em pirically by insertion o f m easured values for and r, subject to the lim itation that it is often im possible to separate the electron and hole com ponents of the input data w hich m ust therefore be expressed as 'effective' values.

S tu d ie s by Pan e t at. [2.27] and P la n o [2.83] h av e in v e s tig a te d the tra n s p o rt ch a ra c te ristic s o f c a rrie rs w ith in diam o n d film s at the g ro w th su rfa c e and in the nanocrystalline region at the nucléation surface. R eference to the electron m icrograph in figure (4.3) suggests intuitively that larger crystallite dim ensions should yield better perform ance. T his is supported by the above studies in w hich Pan e t al. w orking on 3 6 0 p m thick M P A C V D film found that m obility on the grow th (large grain) surface w as 50 tim es g rea ter than on the nucléation surface as in d ic a te d in figure (2.13(a)). Plano et al. w orking on 5 0 0 p m thick M PA C V D and flam e grow n m aterial com pared the surface collection distance ds for planar electrode structures w ith the bulk collection distance d^ for 'sandw ich' electrodes on the grow th and nucléation surfaces. T he results reproduced as figure (2.13(b)) clearly show that ds~2db for both types o f film .

10 0 s 'é> o ' V .

(a)

s u b s t r a t e s id e 10" 1 0* 1 0 " 1 0 * n (cm"*) 1 0* 1 0* 12 10 8 I 6 ■O 4 • CV D Sam ples E=10kV /cm 2 0 0 1 2 4 5 6

Figure 2.13: (a) A comparison of the carrier mobilities on the nucléation and growth surfaces of a 360pm thick MPACVD diamond film, (b) A comparison of the surface and bulk carrier collection distances in

CVD diamond films of different thicknesses. [Reproduced from 2.27, 2.83]

Chapter 2; The Structure, Synthesis and Properties o f Diamond

An observation by Pan et al. [2.27] in 1993 was that collection distances at the time of writing (dcvD<0.1[im, J//a<30|im ) were substantially less than the average crystallite size in the samples under investigation (~l|Lim diameter). This led to the conclusion that charge centre scattering within crystallites was the primary limit on mobility in such CVD films for all carrier densities in contrast to natural Ila material, wherein mobility is limited by phonon scattering at low carrier densities and by carrier scattering at high densities. More recently reported collection distances in CVD material have been in the range 60-80|im [2.84].

In document Thin film diamond devices as photon and ionising radiation detectors (Page 38-41)