Excellent reviews have been written on NCD growth, properties and applications. In what follows a summary of the nanocrystalline technology will be given following the review by Williams [35].
Diamond films and particles are interesting in many fields of fundamental and applied sciences due to their outstanding properties. Despite huge research efforts in the other allotropes and forms of carbon such as C60carbon nanotubes and graphene,
diamond remain the most successful commercially exploited allotrope to data. The advent of chemical vapor deposition (CVD) has widened the field of application and sophistication of diamond film technology since it allows the deposition of this ma- terial on foreign substrates.
Diamond films also find application where bulk diamond is inappropriatein di- verse fields, such as microelectromechanical systems or tribological coatings. In fact, it is in these fields were nanocrystalline diamond, NCD, finds its most suitable appli- cations as it exhibits many of the superior properties of diamond. Those properties of bulk diamond that are not exhibited by NCD are due to crystal size limitations. Good examples of these are electron/hole mobility and thermal conductivity, both obviously being limited by grain boundary scattering of electron/holes or phonons respectively [36, 37]. There are also mechanical properties that are affected by the grain size of nano-structured diamond. Nano-diamond particles are also of critical importance as seeds for the deposition of nano-diamond films [38].
Grain size
As already mentioned, diamond properties are significantly affected by crystalline size. High surface-to-volume fractions result in enhanced disorder, sp2 bonding, hy-
drogen content and scattering of electrons and phonons. Increased sp2 bonding re-
sults in additional disorder, a significantly more complex density of states within the bandgap, reduction of Young’s modulus, increased optical absorption, etc. Con- versely, regarding particles, when the size gets greater than 20 nm, nano-diamond particles behave like bulk diamond. This is predominantly due to the far reduced concentration of atoms at the surface with regards to the bulk.
The term “nanocrystalline diamond” was firstly used for films with poor quality. However, nanocrystalline diamond has developed into a sophisticated material with a wide variety of application and terminology [39]. The smallest grain size diamond films are called ultrananocrystalline diamond (UNCD) [40]. These films have grain sizes around 5 nm, with a considerable amount of amorphous grain boundaries which are very similar to diamond-like-carbon (DLC).
NCD films have grain sizes generally below 100 nm, but sometimes films with grains up to 500 nm are also labeled NCD [41]. Generally speaking NCD contains less sp2 and are thus more transparent than UNCD films.
Nucleation and growth
Successful growth of single crystalline diamond over larger areas and on foreign substrates is almost completely limited to growth on single crystal iridium [42]. Thus, diamond growth on the majority of foreign substrates results in polycrystalline ma- terial. Non diamond substrates require some pre-treatment in order to enhance the nucleation densities to the point where very thin (< 50 nm) coalesced films can be grown. The interested reader may find a review on several nucleation techniques at [35]. It is just worth mentioning that in the case of nanocrystalline diamond the first few tens of nanometers of the film are often a significant part of the film and certainly have profound impact on surface roughness. The CVD growth of the NCD has been covered by excellent reviews, see for example [43].
NCD properties
Thermal conductivity is a phonon scattering limited process in nanocrystalline materials and thus strongly influenced by the grain size. The thermal conductivity of films with very small grain sizes is comparable to diamond like carbon and thus is of little use for heat spreading. As the grain sizes approach 100 nm thermal conductivity rapidly approaches that of bulk diamond. The values for NCD films with grain sizes from 50 nm to 100 nm span from 20 to 200 W/mK [44], which are values still one order of magnitude lower than for bulk diamond.
Regarding the elastic properties, these are also dominated by the grain size. Young’s modulus has been measured by various techniques and the values in the literature show small values for UNCD type materials, as low as 440 GP a. When the grain size is increased above 50 nm NCD exhibits large Young’s modulus approaching the single crystal diamond value (1100 GP a) [45–47].
The optical and electrical properties of NCD films are a complex area with strong correlation between grain size and sp2 bonding effects which are deeply addressed
at the review by Williams [39]. The larger the grain size the closer the optical and electronic properties correlate with single crystal diamond. Thus the grain size is a critical determinative factor in the optical transparency as small grain sizes result in a larger grain boundary volume and thus higher sp2 content.
NCD films exhibit very high resistivities when undoped, approaching that of single crystal diamond (> 1010 Ωm). The addition of boron leads to p type conductivity
and the films behave identically to single crystal and microcrystalline diamond albeit with a lower mobility and hence lower overall maximum conductivity [37]. Thus the transport phenomena of NCD films with boron doping are easily explained by
conventional doping of diamond by boron. Despite the low mobility values NCD can be of use as high temperature stable (and UV transparent) electrodes to materials such as SiC or GaN [48, 49].
Applications
NCD has a very wide field of possible applications. The high thermal conductivity of NCD films makes it of interest for silicon on diamond applications [50, 51]. The friction coefficients, as low as single crystal diamond [52], make it a candidate for tribology applications and for optical coatings due to the NCD transparency.
Of particular interest for this thesis are the applications of NCD in MEMS and NEMS. The fabrication of such structures with single crystalline diamond is quite dif- ficult as it requires the availability of thin films of diamond on a sacrificial layer such as SiO2for most structures. With NCD films this is possible and allows the fabrication
of a diverse array of micro and nanostructures; such as resonators [47, 53–56], tun- able lenses [46], strain gauges [57], bistable membranes [58] and superconducting wires [59], among others [60–62].
High Young’s modulus of NCD can be used for fabricating high frequency res- onators with high quality factors. The fabrication with diamond is no more complex than polysilicon, in fact it can be easier due to the lack of the requirement of critical point drying due to the strength of diamond. Nevertheless, the etching of diamond is an aggressive procedure and has to be optimized to be compatible with the other fabrication processes used.
NCD allows the fabrication of unique devices. Phonon mismatch between silicon is very high and high frequency resonators (f = 1.5 GHz) have been demonstrated with a superlative frequency-quality factor product [63].
Additionally, piezoelectric driver integration of NCD microstructures has been demonstrated using several piezoelectric materials such as PZT [64], ZnO [65] and AlN [66].
Finally, NCD has been demonstrated to be an excellent electrochemical electrode [67] and a good material for field emission devices [68].