Growth, electronic properties and applications of nanodiamond

by O.A. Williams ^a,b,*, M. Nesladek ^a,b,c, M. Daenen ^a, S. Michaelson ^d, A. Hoffman ^d, E. Osawa ^e, K. Haenen ^a,b, R.B. Jackman ^f

Diamond & Related Materials 17 (2008) 1080-1088

Purpose

To differentiate nanocrystalline diamond films into two categories, those that are grown conventionally with the suppression of re-nucleation, termed nanocrystalline diamond (NCD), and those grown with the enhancement of re-nucleation, termed ultrananocrystalline diamond (UNCD). It is generally accepted that nanocrystalline diamond consists of facets less than 100 nm is size, whereas a second term “ultrananocrystalline diamond” has been coined to describe diamond material with grain sizes less than 10 nm.

Methods

NCD and UNCD films were grown on silicon or quartz wafers in Microwave Plasma Enhanced Chemical Vapor Deposition (MWPECVD) reactors with frequencies of 2.45 Ghz. For UNCD growth, a hydrogen poor gas mixture of 99% Ar and 1% CH₄ was used to grow intrinsic material. NCD growth was performed with a conventional H₂/CH₄ plasma with methane concentrations below 3%. Scanning Electron Microscopy (SEM) images were taken with and FEI Quanta 200F. Raman spectra of NCD films were taken in the backscattering configuration using a 514.5 nm line of an Ar-ion laser and a DILOR triple monochromator/liquid nitrogen CCD detector. Raman spectra of UNCD films were taken with Renishaw bench top system at 632 nm.

Key Findings

(Nanocrystalline Diamond Film)

1. Thin film grown with a very high initial nucleation density.

2. Is grown in the van der Drift regime, its grain size and roughness increase with film thickness.

3. Little or no re-nucleation above a thickness of approximately 1μm.

4. Can be doped with boron but mobilities are low.

5. Transparency is high when undoped.

(Ultrananocrystalline Diamond film)

1. Very fine grain material grown with a high re-nucleation rate.

2. Does not grow in the standard van der Drift regime.

3. 3-5 nm grains with abrupt grain boundaries.

4. No columnar structure and surface roughness is independent of film thickness.

5. Can be made conductive by increasing sp² content.

6. Transparency is intrinsically low, and becomes significantly worse with nitrogen addition in the gas phase.

Keywords: Nanocrystalline; Microwave Plasma Enhanced Chemical Vapor Deposition

Nanodiamond lateral field emitter devices on thick insulator substrates for reliable high power applications

by K. Subramanian, W.P. Kang*, J.L. Davidson, M. Howell

Diamond & Related Materials 17 (2008) 786-789

Purpose

To fabricate nanocrystalline diamond field emitter devices on thick insulator substrates for high power applications. These monolithic lateral field emitter diodes will be fabricated in comb arrays on 640 μm-thick aluminum nitride insulating substrates that have been integrated with nanodiamond for device electrode isolation.

Fabrication

A 1 μm-thick layer of polysilicon was deposited on the aluminum nitride (ALN) substrates by low-pressure chemical vapor deposition (LPCVD). This layer is needed to enhance adhesion between the AlN substrate and CVD diamond film. The silicon/aluminum nitride sandwich substrates were then pretreated by ultra-sonicating with a 5-20 nm nanodiamond powder/acetone solution to increase diamond nucleation. Microwave plasma enhanced chemical vapor deposition (MPECVD) was used to grow the nanodiamond film on the polysilicon side of the Si/AlN substrates. The nanodiamond film was then lithographically micro-patterned in the lateral emitter diode structure, with aluminum as the mask for diamond etch using oxygen plasma by ICP-RIE (Inductive coupled plasma reactive ion etch) process technique. The polysilicon layer was then etched by SF₆ plasma RIE, isolating the nanodiamond electrodes on the ALN, and yielding the lateral device.

Field Emission Characterization

A 325-fingered nanodiamond lateral comb array was characterized for vacuum field emission. The diode had a turn-on voltage of ~ 30 V and the emission current was found to increase exponentially with the applied voltage. The device then demonstrated a high emission current of 1 mA at an anode voltage of 360 V. A Fowler-Nordheim (F-N) plot of the I-V field emission data indicates that the observed current is due to electron field emission from the nanodiamond emitter-fingers. The emission current was found to be stable over time, around 1 mA at constant applied voltage.

Keywords: Nanocrystalline diamond; Field emission; Lateral vacuum device

Early stage of diamond growth at low temperature

by A. Kromka ^*, Š. Potocký, J. Čermák, B. Rezek, J. Potměšil, J. Zemek, M. Vaněček

Diamond & Related Materials 17 (2008) 1252-1255

Purpose

To investigate the initial stages of nanocrystalline diamond (NCD) thin film growth at low substrate temperature. NCD films were grown on silicon substrates for 0-300 min at a temperature of 410 °C. Microwave plasma enhanced chemical vapor depositon (CVD) was the method used to deposit the films. The silicon substrates were ultrasonically pretreated in a suspension of detonation nanocrystalline diamond powder. The seeding density approached values of 1×10¹² cm^-2, which allows for growth of ultra-thin conformal NCD films.

Methods

The NCD films were characterized by atomic force microscopy (AFM) in the tapping mode (AFM Microscope Dimension 3100 Veeco). Silicon AFM cantilevers were used with a typical tip radius of 10 nm and resonant frequency of 230 kHz. The near-surface composition of the pretreated silicon substrates was analyzed by x-ray photoelectron spectroscopy (XPS). An ADES 400 angular-resolved photoelectron spectrometer (VG Scientific, U.K.) was used for the analysis. Peak areas were determined following the Shirley’s inelastic background subtraction method.

Key Findings

Stagnation of the AFM roughness indicates that the low temperature NCD growth is

1.) delayed due to the surface contamination of the used nanodiamond powder and

2.) possibly dominated by the growth in the lateral direction.

XPS measurements showed that the measured surface exhibits changes from a multi-phase composite (seeding layer) to single-phase one (NCD layer).

Keywords: Nanocrystalline diamond film; CVD; AFM; XPS

Nucleation, growth and characterization of nanocrystalline diamond films

by G. Cicala^a,*, P. Bruno^a,b, F. Bénédic^b, F. Silva^b, K. Hassouni^b, G. S. Senesi^a

Diamond & Related Materials 14 (2005) 421-425

Purpose

To evaluate the effects of diamond powder particle size used for ultrasonic abrasion of silicon substrates, varied from 0.125 to 45 μm, on nucleation density, growth rate and quality of deposited nanocrystalline diamond films.

Methods

Surface morphology was evaluated by scanning electron microscope (SEM) and atomic force microscope (AFM). Additionally, the AFM images were used to determine the root mean square (rms) surface roughness values, that were calculated from 5х5 and 10х10 μm² areas. Nucleation density was calculated by using a commercial image processing software (NIH image™) applied to both AFM and SEM images. UV Raman spectroscopy was performed using a 363.8 nm excitation laser, in order to identify the carbon hybridization and define the structure of the deposited films. X-ray diffraction (XRD) measurements were carried out at an incident angle of 10° using a CuK∞₁ radiation source in order to analyze the film structural properties and grain size was evaluated by the Debye-Scherrer formula.

Key Findings

1.) Ultrasonic abrasion with larger diamond powder size strongly enhances the NCD nucleation density.

2.) Larger diamond powder size reduces the surface roughness of both thick and thin NCD films.

3.) The effect of diamond powder size is negligible on the growth rate of NCD films.

4.) The effect of diamond powder size is negligible on the bulk properties of NCD films such as structure, crystallinity and phase composition.

Key Words: Nanocrystalline; Diamond film; Nucleation