Nanocrystalline Diamond Layers Research Articles

Nanodiamond resonators fabricated on 8″ Si substrates using adhesive wafer bonding

In this work, the adhesive wafer bonding of diamond thin films onto 8″ silicon substrates is reported. In order to characterize bonded nano-crystalline diamond layers, vibrometry and interferometry studies of micro-fabricated flexural beam and disk resonators were carried out. In particular, surface topology along with resonant frequencies, eigenmodes and mechanical quality factors were recorded and analyzed in order to obtain physical parameters of the transferred films. The vibration properties of the bonded resonators were compared to those fabricated directly on 3″ silicon substrates.

Influence of grain boundaries on elasticity and thermal conductivity of nanocrystalline diamond films

Diamond combines several outstanding material properties such as the highest thermal conductivity and highest elastic moduli of all materials. This makes diamond an interesting candidate for a multitude of applications. Nonetheless, nanocrystalline diamond films, layers and coatings, usually show properties different to those of single crystalline diamond. This is usually attributed to the larger volume fraction of the grain boundaries with atomic structure different from the single crystal. In this work we measured Young's modulus and thermal conductivity of nanocrystalline diamond films with average grain sizes ranging from 6 to 15 nm. The measured thermal conductivities are modeled considering the thermal boundary conductance between grains as well as a grain size effect on the phonon mean free path. We make a comparison between elastic modulus and thermal boundary conductance of the grain boundaries Gk for different nanocrystalline diamond films. We conclude that the grain boundaries thermal boundary conductance Gk is a measure of the cohesive energy of the grain boundaries and therefore also of the elastic modulus of the nanocrystalline diamond films.

Wirelessly Powered High-Temperature Strain Measuring Probe Based on Piezoresistive Nanocrystalline Diamond Layers

AbstractA high-temperature piezo-resistive nano-crystalline diamond strain sensor and wireless powering are presented in this paper. High-temperature sensors and electronic devices are required in harsh environments where the use of conventional electronic circuits is impractical or impossible. Piezo-resistive sensors based on nano-crystalline diamond layers were successfully designed, fabricated and tested. The fabricated sensors are able to operate at temperatures of up to 250°C with a reasonable sensitivity. The basic principles and applicability of wireless powering using the near magnetic field are also presented. The system is intended mainly for circuits demanding energy consumption, such as resistive sensors or devices that consist of discrete components. The paper is focused on the practical aspect and implementation of the wireless powering. The presented equations enable to fit the frequency to the optimal range and to maximize the energy and voltage transfer with respect to the coils’ properties, expected load and given geometry. The developed system uses both high-temperature active devices based on CMOS-SOI technology and strain sensors which can be wirelessly powered from a distance of up to several centimetres with the power consumption reaching hundreds of milliwatts at 200°C. The theoretical calculations are based on the general circuit theory and were performed in the software package Maple. The results were simulated in the Spice software and verified on a real sample of the measuring probe.

Advances in AlGaN/GaN HEMT Surface Passivation

The high efficiency and power density of AlGaN/GaN high electron mobility transistors (HEMTs) can be limited by parasitic charge trapping at sites on the semiconductor surface. Trapped electrons at the surface deplete the two-dimensional electron gas (2DEG), resulting in effects known as current collapse, increased dynamic on-resistance, DC-RF dispersion, among others. A SiN layer is conventionally implemented as surface passivation for GaN HEMTs, however the properties of this SiN layer and the impact of this layer on current collapse depend drastically on the deposition conditions of this film. Although passivation of GaN HEMTs has been studied extensively, the specific details behind the SiN passivation deposition are not frequently provided in literature. In this work, we investigate several passivation techniques and study their performance under dynamic switching conditions, under off-state stress, and implement hyperspectral electroluminescence imaging to identify electric field nonunifornities. We investigate plasma enhanced chemical vapor deposition (PECVD) SiN, a common GaN HEMT passivation method, and demonstrate how to optimize the deposition conditions to effectively passivate the HEMT. Generally, a mixed frequency PECVD SiN process is used, where a combination of low frequency (LF) e.g. 100-360 kHz and high frequency (HF) 13.56 MHz plasmas are used to create the SiN film. However, the LF step could create undesirable damage to the surface of the AlGaN barrier from N+ ion bombardment, degrading device performance. We developed a bilayer PECVD process, where a 10 nm thick layer is deposited using HF plasma only in order to protect the AlGaN surface, followed by a HF/LF mixed frequency SiN deposition. Annealing the 10 nm thick HF layer densified the film and resulted in a HEMT with significantly improved current collapse. On the other hand, SiN layers grown in situ during MOCVD growth of the GaN/AlGaN layers have also been demonstrated to serve as an effective protective layer for subsequent HF/LF PECVD SiN deposition, yielding HEMTs with low current collapse. Atomic layer epitaxy (ALE) deposition of a thin (10 nm) AlN layer is evaluated, and is shown to serve well as a passivation layer. This is particularly advantageous, since the thin layer is beneficial for reducing gate capacitance for high frequency operation. Finally, a nanocrystalline diamond (NCD) layer is incorporated on the surface of the GaN HEMT, primarily to improve thermal management, but this also is effective as a surface passivation layer, mitigating surface charge trapping. Effective passivation of the AlGaN/GaN HEMT surface is essential for mitigating current collapse, and we demonstrate recent advancements in passivation technology.

Development and growth time optimization of boron-doped micro-crystalline, undoped micro-crystalline and undoped nano-crystalline composite diamond film

Based on a well-designed growth procedure, a tri-material, namely, a three-layer boron-doped micro-crystalline, undoped micro-crystalline and undoped nano-crystalline composite diamond film, is deposited on the pretreated WC–6 wt% Co substrate, the basic characters of which are systematically studied and compared with some other commonly used diamond films. Besides, the growth times for three respective layers are accordingly determined. It is further clarified that the underlying boron-doped micro-crystalline diamond layer can well adhere to the WC–Co substrate due to either the reduction in the residual stress or the formation of B–Co compounds. There is no doubt that the surface undoped nano-crystalline diamond layer with relatively lower hardness and initial surface roughness is more convenient to be polished to the required surface roughness. Moreover, when the growth times for the middle undoped micro-crystalline diamond layer and the surface undoped nano-crystalline diamond layer are both appropriate, the undoped micro-crystalline diamond layer with extremely high diamond quality and hardness can effectively reinforce the surface hardness of the whole composite film. Based on the discussions on the influences of the growth times for the different layers on the performance of the composite diamond film, the growth times for the boron-doped micro-crystalline diamond, undoped micro-crystalline diamond and undoped nano-crystalline diamond layers are, respectively, determined as 4, 4 and 2 h. Under such conditions, the reinforcement effect of the middle layer on the surface hardness can be guaranteed, and the undoped nano-crystalline diamond grains have totally covered the undoped micro-crystalline diamond layer.

Patterned hydrophobic and hydrophilic surfaces of ultra-smooth nanocrystalline diamond layers

In this work, we show that ultra nanocrystalline diamond (UNCD) surfaces have been modified to add them hydrophobic and hydrophilic properties. The nanocrystalline diamond films were deposited using the hot filament chemical vapor deposition (HFCVD) technique. This allows growing diamond on different substrates which can be even 3D or structured. Silicon and, for optical applications, transparent quartz glass are the preferred substrates for UNCD layers growth. Fluorine termination leads to strong hydrophobic properties as indicated by a high contact angle for water of more than 100°. Hydrogen termination shows lesser hydrophobic behavior. Hydrophilic characteristics has been realised with oxygen termination. X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) measurements confirm the oxygen and fluorine- termination on the nanocrystalline diamond surface. Further, by micropatterning using photolithography, multi-terminated layers have been created with both hydrophobic and hydrophilic areas. In addition, we have shown that retermination is achieved, and the properties of the surface have been changed from hydrophobic to hydrophilic and vice versa. Micro- roughness and stress in the grown film influences slightly the wetting angle as well. The opportunity to realize local differences in hydrophobicity on nanocrystalline diamond layers, in any size or geometry, offers interesting applications for example in microbiological investigations. Multi-terminated arrays show identical surface roughness and at the same time differences in hydrophobicity. These arrays have been visualized with scanning electron microscopy (SEM) and lateral force microscopy (LFM).

(Invited) Electrothermal Performance Optimization of III-Nitride HEMTs Capped with Nanocrystalline Diamond

AlGaN/GaN high electron mobility transistors (HEMT) capped with nanocrystalline diamond (NCD) have been demonstrated in the past to outperform electrically and thermally their SiN-passivated counterparts [1, 2]. However, a major process limitation for the integration of a diamond heat spreader has been the O2-plasma damage in the gate opening associated with etching the diamond cap. To reduce plasma damage, Anderson et al. have demonstrated a two-step diamond etch process, where the plasma power is reduced towards the end of the etch to minimize damage to the AlGaN surface [3]. While this process has enabled improved electrical performance of NCD-capped HEMTs, optimal passive heat spreading requires NCD films thicker than 0.5 µm which challenges the two-step etch approach [4]. Further complications such as on-wafer variations in NCD thickness have motivated the search for a more robust gate process. Recently, we have proposed the use of a sacrificial gate (SG) prior to diamond growth [5, 6]. The SG stack consisted of a 10 nm thick ALD-grown Al2O3 layer followed by a 100 nm thick PECVD-grown SiNx layer. The SiNx layer served as an etch stop for the NCD layer, and the Al2O3 served as an etch stop for the SiNx layer. The Al2O3/SiNx stack was patterned with the dimensions of the gate and a blanket 500 nm thick CVD NCD film was conformally-grown over the feature. The gate feature was then realigned and a high power O2-plasma was used to clear the NCD film in the gate region. This resulted in a much more reliable gate opening since the window for NCD over-etch was much larger. The sacrificial SiNx finger was subsequently etched away using a SF6-ICP process. The Al2O3 layer was removed in dilute HF solution with no measureable degradation to the rest of the device. Finally, the Ni/Au gate metal stack was lifted off. On HEMTs with a SG, the addition of a NCD cap did not cause any significant degradation in mobility, carrier density, or sheet resistance. By contrast, when a SG process was not used to protect the AlGaN surface, the NCD-capped HEMT experienced a decrease in mobility and an increase in sheet resistance. For the SG NCD-capped HEMT, the off-state current increased by less than an order of magnitude and threshold voltage shifted by about 1 V. However, on-state drain current levels remained comparable and degradation in dynamic on resistance (RON,DYN) upon an off-state drain bias stress was improved, indicating potential improvement in AlGaN surface passivation by the NCD film [2, 3]. Such improvement is also corroborated by the improvement in breakdown voltage in NCD-capped HEMTs with a conventional gate opening design. This presentation will review progress in simulation, process integration, channel temperature mapping, and electrical characterization of diamond-capped devices. We will also review several novel NCD concepts, such as in-plane control of NCD thermal conductivity, as well as a diamond-gated AlGaN/GaN HEMT [7, 8]. [1] M.J. Tadjer, et al., IEEE Electron Dev. Lett., vol. 33, no. 1, pp. 23-25, 2012. [2] D.J. Meyer, et al., IEEE Electr. Dev. Lett., vol. 35, no. 10, pp. 1013, 2014. [3] T.J. Anderson et al., 70th Dev. Research Conf. Proc., pp. 155-156, 2012. [4] A. Wang, M.J. Tadjer, and F. Calle, Semicond. Sci. Technol., vol. 28, pp. 055010 (2013). [5] T.J. Anderson et al., CS Mantech Conf. Digest, pp. 205-208, 2013. [6] M.J. Tadjer, et al., Phys. Status Solidi A, 2015, in press. [7] T.J. Anderson, et al., IEEE Electr. Dev. Lett., vol. 34, no. 11, pp. 1382, 2013. [8] J. Anaya et al., Acta Materialia 103 (2016) 141-152.

Effect of plasma composition on nanocrystalline diamond layers deposited by a microwave linear antenna plasma‐enhanced chemical vapour deposition system

The addition of CO2 into the process gas has a significant impact on the quality and the incorporation of boron in CVD diamond layers. In this report we study the effect of CO2 addition in the gas phase on the properties of boron doped nano‐crystalline diamond (BNCD) layers grown at low substrate temperatures (450–500 °C) using a microwave linear antenna plasma‐enhanced chemical vapour deposition apparatus (MW‐LA‐PECVD). Experimental results show an increase in the layers' conductivity with a reduction in CO2 concentration, which is consistent with the variation in the atomic boron emission line intensity measured by optical emission spectroscopy (OES). At CO2 concentrations close to zero, we observed the formation of a smooth, transparent and highly resistive layer on unseeded substrates. This layer has been identified as silicon carbide (SiC) by transmission electron microscopy and X‐ray photoelectron microscopy. The presence of silicon in the plasma is confirmed by OES and it is attributed to quartz tube etching. In this specific deposition condition, diamond growth is in competition with SiC growth, which affects the diamond layer properties.SEM image of layer growth on a silicon substrate at low CO2 concentration on unseeded area (left) with a smooth, highly resistive and transparent SiC layer and seeded area (right) with a boron doped nano‐crystalline diamond layer.

Photoemission properties of nanocrystalline diamond thin films on silicon

The authors have built up a dedicated ultrahigh vacuum setup to measure ultraviolet (266 nm photons) photoemission properties of nanocrystalline diamond thin films obtained by chemical vapor deposition on silicon substrates. The authors validated their setup by measuring polycrystalline copper quantum efficiency of ∼10−6, which is in good agreement with literature. The authors also measured quantum efficiency of bare silicon (highly p and n doped) and demonstrate strong influence of doping type. The authors then measured quantum efficiency of silicon samples coated with submicron (50 and 100 nm thick) nanocrystalline diamond layers. This coating reveals to have major influence on the photoemission properties when deposited on highly n-doped silicon samples. The authors obtain quantum yield as high as 1.60 × 10−5. The relatively high quantum efficiency of such structure associated with its high stability in air and easy processing make it a good candidate as fast electron source for electron gun based systems such as scanning/transmission electron microscopes or x-ray sources.

Diamond-coated silicon wires for supercapacitor applications in ionic liquids

For a long time sp2 carbon has been the dominating material for supercapacitor applications. In this paper a new concept of using boron-doped diamond for supercapacitors is proposed. Diamond surface enlargement is realized via bottom-up template-growth. In this method, silicon nanowire electrodes are coated with a thin (~100nm) layer of nanocrystalline diamond (NCD) by microwave enhanced chemical vapor deposition (MWCVD). The quality of overgrowth is characterized by high resolution scanning electron microscopy which reveals a homogeneous coverage of diamond on Si nanowire surface. To enhance the potential window to 4V, a room temperature ionic liquid is used as electrolyte. The dilution of the ionic liquid is investigated in terms of conductivity and specific capacitance. The capacitance as measured via cyclic voltammetry reaches 105μF/cm2. An energy density of 84μJ/cm2 and a high power density of 0.94mW/cm2 are obtained in combination with good stability of over 10,000charging/dischargingcycles.

Advances in Diamond Integration for Thermal Management in GaN Power HEMTs

GaN high electron mobility transistors (HEMTs) performance is limited by self-heating during high power operation. Topside nanocrystalline diamond (NCD) layers have been integrated on AlGaN/GaN (HEMTs) to improve thermal management, exhibiting a 20%. decrease in peak channel temperature compared to reference HEMTs in a scalable process. Processing improvements, such as eliminating the SiN x passivation interlayer and developing a sacrificial gate process are being actively pursued. Also, boron doped p+-NCD films were implemented as gate electrodes for the AlGaN/GaN HEMT for a thermally stable heat-spreading gate contact.

Advances in Diamond Integration for Thermal Management in GaN Power HEMTs

The gallium nitride (GaN) materials system is an attractive wide-bandgap material for next-generation power devices, including RF amplifiers, high voltage power switches, and high breakdown voltage diodes. Self-heating effects, however, have limited the device capabilities. Attempts to mitigate thermal impairment have been limited due to the difficulties involved with placing high thermal conductivity materials close to heat sources in the device. Heat spreading schemes have been proposed involving growth of AlGaN/GaN on single crystal or CVD diamond, or capping of fully-processed HEMTs using nanocrystalline diamond (NCD). All approaches have suffered from reduced HEMT performance or limited substrate size.Recently, we have successfully demonstrated a “gate after diamond” approach, which enables scalable, large-area diamond integration and improves the thermal budget of the process by depositing NCD before the thermally sensitive Schottky metal gate [1,2]. The diamond-capped device appeared to demonstrate improved DC I-V characteristics in most key performance areas, notably improved on-resistance, saturation current, and transconductance, and reduced off-state current and gate leakage, and a 20% reduction in channel temperature as measured by Raman thermography. Thermal mapping has also indicated a change in the temperature profile through the source-drain region by pulling the hot spot away from the gate edge and broadening the temperature distribution. This process has recently been improved to enable the deposition of diamond directly on the GaN surface by using a sacrificial layer to protect the gate region in the diamond recess etch step. As the diamond has been shown to mitigate current collapse as much as SiNx, the need for a thin passivation layer, which is an additional thermal boundary in the layer structure, has been effectively eliminated. To further address thermal limitations, a back-side diamond via process has also been developed. High aspect ratio vias are fabricated in Si substrates, followed by a conformal diamond coating.Finaly, taking advantage of the ability to control doping in the NCD layer, we have demonstrated the use of diamond as a gate electrode. The boron-doped p-NCD is both transparent, enabling optical probing under the gate, and thermally stable, enabling operation in extreme environments and facilitating integration with other high-temperature processing steps, such as top-side and back-side undoped diamond heat spreading films. An additional advantage of this device structure is that the gate electrode is governed by the NCD-AlGaN heterojunction, which is inherently lower leakage than the conventional Schottky diode. Furthermore, if high doping levels can be achieved at the interface, the p-n junction can deplete the 2DEG, presenting a novel path to enhancement-mode devices.This presentation will review progress in simulation, process integration, channel temperature mapping, and electrical characterization of diamond-capped devices.1. M.J. Tadjer, T.J. Anderson, K.D. Hobart, T.I. Feygelson, J.D. Caldwell, C.R. Eddy, Jr. F.J. Kub, J.E. Butler, B.B. Pate, J. Melngailis. IEEE Electron Dev. Lett. 33, 23-25 (2012)2. T.J. Anderson, A.D. Koehler, M.J. Tadjer, K.D. Hobart, T.I. Feygelson, J.K. Hite, B.B. Pate, C.R. Eddy, Jr, F.J. Kub. 2013 CS Mantech Technical Digest

Investigation of diamond electrodes for photo‐electrochemistry

Boron doped diamond (BDD) shows high chemical stability in contact with a liquid phase even under extreme pH conditions, high electrochemical activity, and allows very stable covalent functionalization on O‐ or N‐terminated surfaces. It is therefore an excellent electrode material for many applications such as analytics, biosensors, and purification. Furthermore, the electrodes can be covalently functionalized with photo‐sensitive dyes to generate photo currents. However, the energetic conditions and charge transfer mechanisms are not fully known and therefore have been investigated in this work. The focus was set on the energy band diagrams of nanocrystalline diamond (NCD) layers, especially on the flat‐band potential VF and the band‐bending ϕB in the space charge layer, for both O‐ and N‐terminated surfaces, and the charge transfer mechanisms at the semiconductor/electrolyte phase‐boundary, with or without electro‐active organic dyes, such as manganese phthalocyanine. For this study NCD samples were grown by hot filament chemical vapor deposition (HFCVD) with a boron doping density NA ranging between 5.8 and 9.6 × 1020 cm−3. The pristine surface was then processed by oxygen or ammonia plasma to modify the termination to O and NH2, respectively. After that a series of Mott–Schottky plots was measured for both terminations at several frequencies spanning from 3 Hz to 10 kHz and in two different electrolytes, namely 0.1 M H2SO4 and 0.1 M KCl. The results showed nearly the same flat‐band potential for both surface terminations, but a remarkable difference in the band‐bending. The latter is most probably responsible for the better photoelectrical conversion observed in N‐terminated NCD samples. The open‐circuit potential (OCP) and chronoamperometric measurements performed after covalent functionalization of the samples with the color dyes revealed that only the N‐terminated material delivered a measurable photocurrent. Although the global efficiency of the photoelectric conversion was low, these initial results show that by optimized selection of materials and modification technologies providing a proper match of the energy levels at all charge transfer steps, a higher energy output could be achieved.

A high-temperature oxidation-resistant diffusion barrier for PbZrxTi1−xO3 coating on nanocrystalline diamond film

Abstract A buffer layer for PbZr x Ti 1− x O 3 (PZT) coating on nanocrystalline diamond (NCD) film was investigated to prevent the oxidation damage of NCD layer during ambient air annealing at high-temperature. As for the phase of buffer layer, metal nitride is more effective than pure metal for enhancing adhesion of PZT coating on NCD film. For the metal nitride-based buffer layer, the incorporation of Al and Si increases further the adhesive strength of PZT coating on NCD film. As a microstructural point of view, nanoscale multilayered structure was observed to contribute to the increase of adhesive strength between PZT and NCD. As a consequence, introducing thin (∼70 nm) composite buffer composed of Ti(Al)N/SiN x nanoscale multilayer with bilayer period of ∼5 nm as an intermediate layer between PZT coating and NCD film improved the high-temperature oxidation resistance of the NCD film and relevant adhesive strength of PZT coating even after the 900 °C air-annealing. The improved adhesion was attributed to the suppressed oxygen diffusion to the NCD film through the PZT layer at high temperature by the intervening nanoscale multilayer buffer.

Large area deposition of boron doped nano-crystalline diamond films at low temperatures using microwave plasma enhanced chemical vapour deposition with linear antenna delivery

We report on the preparation and characterisation of boron (B) doped nano-crystalline diamond (B-NCD) layers grown over large areas (up to 50 cm × 30 cm) and at low substrate temperatures (< 650 °C) using microwave plasma enhanced linear antenna chemical vapour deposition apparatus (MW-LA-PECVD). B-NCD layers were grown in H2/CH4/CO2 and H2/CH4 gas mixtures with added trimethylboron (TMB). Layers with thicknesses of 150 nm to 1 μm have been prepared with B/C ratios up to 15000 ppm over a range of CO2/CH4 ratios to study the effect of oxygen (O) on the incorporation rate of B into the solid phase and the effect on the quality of the B-NCD with respect to sp3/sp2 ratio. Experimental results show the reduction of boron acceptor concentration with increasing CO2 concentration. Higher sp3/sp2 ratios were measured by Raman spectroscopy with increasing TMB concentration in the gas phase without CO2. Incorporation of high concentrations of B (up to 1.75 × 1021 cm3) in the solid is demonstrated as measured by neutron depth profiling, Hall effect and spectroscopic ellipsometry.

Topside Nanocrystalline Diamond Integration on AlGaN/GaN HEMTs for High Temperature Operation

GaN high electron mobility transistors (HEMTs) performance is limited by increased channel temperature, particularly resulting from self-heating during high power operation. Topside nanocrystalline diamond (NCD) layers have been integrated on AlGaN/GaN (HEMTs) to improve thermal management. HEMTs with NCD heat-spreading layers exhibit a 20% decrease in peak channel temperature compared to reference HEMTs, measured by Raman thermography, as well as improved sheet carrier density, transconductance, sheet resistance, Hall mobility, on-state resistance, and breakdown voltage. A “gate after diamond” approach is implemented to improve the thermal budget of the deposition process while maintaining the integrity of the Schottky gate electrode in a scalable process. Processing improvements for integrating NCD-capping with the HEMT are being pursued, such as eliminating the SiNx passivation interlayer, such that the NCD film is directly on the AlGaN barrier, as well as a sacrificial gate process. Also, boron doped p+-NCD films were implemented as gate electrodes for the AlGaN/GaN HEMT to place the heat-spreading layer in direct contact with the heat source, for a thermally stable heat-spreading gate contact.

Effects of Various Seeding Methods on Multi-Layered Diamond Coatings Deposited via Hot Filament Chemical Vapor Deposition

Multi-layer alternating nanocrystalline diamond (NCD) layer and polycrystalline diamond (PCD) layer was successfully deposited on pretreated tungsten carbide (WC) substrates with various seeding sizes (&lt;0.1μm synthetic, &lt;0.5μm synthetic, &lt;0.25μm natural, &lt;0.5μm natural, and &lt;1μm natural) diamond with and without hammering by silicon carbide. X-rays penetrate through the coating to the substrate from XRD method was able to show strong peaks of diamond relative to WC despite the diamond film being 4μm thick only. It is found that substrates with no hammering produce stronger signals. The coating was cross sectioned and analysed using field emission scanning electron microscopy showing the multi-layer with NCD grains that has coalesced and columnar structure for PCD. None of the diamond coating delaminated during cross sectioning showing good adhesion. Raman was able to capture data from the 1-1.6μm thick NCD layer only while AFM measured the extreme low roughness of the NCD surface.

Novel Aspect in Grain Size Control of Nanocrystalline Diamond Film for Thin Film Waveguide Mode Resonance Sensor Application

Nanocrystalline diamond (NCD) thin film growth was systematically investigated for application for the thin film waveguide mode resonance sensor. The NCD thin film was grown on the Si wafer or on the SiO2-coated sapphire substrate using the hot filament chemical vapor deposition (HFCVD). The structural/optical properties of the samples were characterized by the high-resolution scanning electron microscopy (HRSEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDS), near edge X-ray absorption fine structure (NEXAFS), X-ray diffraction (XRD), and ultraviolet-visible (UV-vis) spectroscopy. The waveguide modes of the NCD layer were studied by prism coupler technique using laser (wavelength: 632.8 nm) with varying incident angle. A novel aspect was disclosed in the grain size dependence on the growth temperature at the relatively low methane concentration in the precursor gas, which was important for optical property: the grain size increased with decreasing growth temperature, which was contrary to the conventional knowledge prevailing in the microcrystalline diamond (MCD) domain. We have provided discussions to reconcile such observation. An optical waveguide mode resonance was demonstrated in the visible region using the microstructure-controlled transparent NCD thin film waveguide, which provided a strong potential for the waveguide mode resonance sensor applications.

Metallic Seed Nanolayers for Enhanced Nucleation of Nanocrystalline Diamond Thin Films

The enhancement of the nucleation and subsequent growth of nanocrystalline diamond (NCD) films with a submicrometer thickness control on silicon substrates is demonstrated by using a sputter deposition of six different metallic (Cr, Mo, Nb, Ti, V and W) seed nanolayers. The effectiveness of altered surface morphology and surface chemistry is discussed. We show that the number density of nanodiamond particles embedded on the nanorough metallic surfaces after an ultrasonic seeding step together with the dynamic surface chemistry during hot-filament chemical vapor deposition of diamond determine the nucleation kinetics, microstructure and surface topography of the NCD films. Overall, the smoothest NCD layer (root-mean-square roughness 10 nm) was obtained with the highest seed density of diamond nanoparticles anchored to the metallic (W) surface. In particular, the rapid carbide-forming metals Mo, Nb and W showed the highest number density of diamond crystallites formed during the NCD nucleation stage, which ...

Sensing of phosgene by a porous-like nanocrystalline diamond layer with buried metallic electrodes

Nanocrystalline diamond with a porous-like morphology was used as the functional part of a semiconductor gas sensor. The device function is based on the two-dimensional p-type surface conductivity of intrinsic diamond with a H-terminated surface. Metallic electrodes are buried beneath the diamond film. Therefore, these electrodes are protected from harmful substances, and the electronic connection is facilitated by grain boundaries. The gas sensing properties of the sensor structure were examined using oxidising gases (i.e., phosgene, humid air) at various operating temperatures. A pronounced and selective increase by two orders of magnitude was found in the surface conductivity after sensor exposure to phosgene gas (20ppm) at 140°C. Density functional theory calculations indicated no direct charge transfer between the phosgene molecule and diamond. We present a model in which phosgene indirectly yet efficiently increases the H3O+ concentration, which consequently leads to multiplied electron transfer and a pronounced sensor response.