Synthesis of highly oriented CVD diamond films by ultra short bias enhanced nucleation step

Highly oriented diamond films have been deposited on silicon substrate by the MPCVD technique (microwave plasma assisted chemical vapour deposition) using an ultra short bias enhanced nucleation process (so called USBEN). We focus our attention on two points: the homogeneity of the deposit in order to perform a precise characterisation whatever surface location (on 1×1 cm² of single silicon substrate); and the simplification of the successive steps usually performed in the BEN process. This is carried out by optimising the microwave cavity and the d.c. discharge extension and by keeping the pretreatments just necessary to obtain high nucleation density with an acceptable epitaxial ratio and a good homogeneity. This leads to a drastic reduction of the bias time of only 30 s for low bias voltage. As we obtain a highly oriented diamond film with a short bias pretreatment without preliminary carburation step, we discuss the substrate transformation under a weak bombardment duration of ions having a quite low energy. We think that the bias step probably consists to a slight modification of the substrate surface.     

Towards homogeneous and reproducible highly oriented diamond films

Diamond films have been deposited by the microwave plasma assisted chemical vapor deposition technique using an ultra short bias enhanced nucleation step to synthesize highly oriented diamond films on single silicon substrate. Firstly in this paper, we focus on the bias enhanced nucleation process to obtain homogeneous and reproducible diamond deposits. By optimizing the process, we obtained a crystal density value of 10⁹ cm⁻² on the whole substrate surface for a reduced polarization time of 60 s. Then, using scanning electron microscopy and image analysis, we report cartographies of crystal densities, covering rate and average radius on the whole sample surface. Next, we analyze a local area of the surface to produce a size distribution of the particles versus their type. Lastly, we present a discussion on the ratio of epitaxial crystals.     

Effect of oxygen on the bias-enhanced nucleation of diamond on silicon

The influence of traces of oxygen in the process gas on the bias-enhanced nucleation (BEN) of diamond on silicon has been studied in the present work. CO₂ in concentrations ranging from 0 to 3000 ppm was added during the nucleation procedure at U_bias=−200 V in microwave plasma chemical vapour deposition (MPCVD). A significant influence of CO₂ could already be detected for a concentration of 75 ppm, corresponding to a C:O ratio of 600:1. It resulted in a continuous reduction of the biasing current and a delay in the nucleation process accompanied by a decrease of the final diamond-covered substrate surface area with increasing CO₂ concentration. At 3000 ppm, the nucleation was completely suppressed. An etching of diamond nuclei by the oxygen could be excluded from in-situ growth rate measurements under bias. Instead, optical emission spectra of the H_β Balmer line indicated a decrease in electrical field strength in the plasma above the substrate. For all gas compositions allowing diamond nucleation, epitaxially aligned films could be obtained, provided that the duration of the biasing step was chosen appropriately. Thus, traces of oxygen do not completely suppress epitaxy. However, the in-plane orientation of the films as determined by X-ray diffraction measurements deteriorates with increasing oxygen concentration.     

Bias-enhanced nucleation of oriented diamonds on cone-like Si

Uniform distribution of bias-enhanced nucleation of diamond has been improved on Si substrate of an area of 1 × 1 cm² by using a dome-shaped Mo counter electrode in a microwave plasma chemical vapor deposition reactor. A nucleation density of 10⁹ cm² can be reached within a few minutes when the bias voltage of − 100 V is applied on the substrates. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that a single-crystalline diamond in a few nanometered size can be deposited on a volcano-shaped cubic SiC which is epitaxially formed on a Si cone. Examination reveals a large fraction of diamond nuclei are oriented along with one side of SiC on each Si cone. The Si cone formed on the Si substrate is due to plasma etching. The diamond nuclei have a shape close to rhombus in TEM. With further growth, secondary nucleation of diamond occurs on top of diamond nuclei and SiC which grows with Si cones. As a result, polycrystalline diamonds are deposited on each Si cone.     

Effect of bias enhanced nucleation on the nucleation density of diamond in microwave plasma CVD

The variation of diamond nucleation density as a function of the conditions of bias enhanced nucleation (BEN) were studied. The nucleation density increased with microwave power, but decreased with the substrate temperature. The nucleation density also increased with bias voltage above 60 V, and had a maximum around 100 V. The crystal growth of diamond took place when either the bias voltage was high or the deposition time was long. The shift of C_1s energy measured by X-ray photoelectron spectroscopy indicated that the ratio of carbon sp³ bonds in the amorphous carbon and/or SiC phases formed before the nucleation of diamond, increased around the bias voltage of 100 V, which seemed to be the reason for enhancement of diamond nucleation by bias voltage. A simple computer simulation was performed in order to understand the effect of BEN conditions on the nucleation of diamond. The simulation reproduced the experimentally observed changes of nucleation density and particle size.     

Silicon substrate preparation for epitaxial diamond crystals

Using the microwave plasma-assisted chemical-vapour deposition technique (MPCVD), we study the role of the (100) silicon substrate preparation prior to the ultra short bias enhanced nucleation (USBEN) step via various pre-treatments. We study the effect of the silicon HF cleaning coupled with the hydrogen plasma exposure in order to determine the efficiency of these pre-treatments to eliminate the native oxide layer on the silicon surface prior to the bias step. We show that the residual oxygen content in the gas phase during the hydrogen plasma exposure can strongly affect the nucleation density because of the formation of an oxide layer which is detrimental for the synthesis of highly oriented diamond (HOD) films. Moreover, we study the effect of the carburation step, often used for the synthesis of HOD films and show that it raises the percentage of epitaxial crystals and the crystal density. Thus, we show that the achievement of high epitaxial crystal density is not only due to the BEN parameters but also to the silicon substrate preparation. As a conclusion, it appears that the silicon substrate preparation prior to BEN is fundamental for controlling the quality of epitaxial diamond films.     

In situ study of the initial stages of diamond deposition on 3C–SiC (100) surfaces: Towards the mechanisms of diamond nucleation

The mechanisms involved in the diamond nucleation on 3C–SiC surfaces have been investigated using a sequential in situ approach using electron spectroscopies (XPS, XAES and ELS). Moreover, diamond crystals have been studied by HRSEM. The in situ nucleation treatment allows a high diamond nucleation density close to 4 × 10¹⁰ cm^− 2. During the in situ enhanced nucleation treatment under plasma, a negative bias was applied to the sample. The formation of an amorphous carbon phase and the roughening of the 3C–SiC surface have been observed. The part of these competing mechanisms in diamond nucleation is discussed.     

Highly oriented diamond growth on SixGe1−x (100) thin films

Highly oriented (100) diamond films have been successfully grown on Si_xGe_1−x (100) thin films by bias enhanced nucleation (BEN) in microwave plasma chemical vapor deposition (MPCVD) system. Raman spectra show the 1332 cm⁻¹ peak which proves the formation of diamond. Diamond nucleation density on Si_xGe_1−x substrate estimated by scanning electron microscopy is higher than 10⁹ cm⁻². The interface between diamond and Si_xGe_1−x substrate was characterized by transmission electron microscopy (TEM). About 20 nm decrease in thickness of the Si_xGe_1−x film was observed after bias enhanced nucleation step. TEM shows the existence of silicon carbide and heteroepitaxial diamond grains grown on Si_xGe_1−x substrate. Characterization from high-resolution TEM on the specimen of short time deposition reveals that a number of epitaxial diamond grains were directly nucleated on Si_xGe_1−x with {111} interplanar spacing ratio of diamond and Si_xGe_1−x of 2:3. The diamond nucleation is found to be preferred on the ridge position of the rough substrate surface. Diamond {100} facets were quickly developed in the early stage of growth.     

Mechanism of diamond nucleation enhancement by electron emission via hot filament chemical vapor deposition

The mechanism of diamond nucleation enhancement by electron emission in the hot filament chemical vapor deposition process has been investigated by scanning electron microscopy, Raman spectroscopy and infrared (IR) absorption spectroscopy. The maximum value of the nucleation density was found to be 10¹¹ cm⁻² with a −300 V and 250 mA bias. The electron emission from the diamond coating on the electrode excites a plasma, and greatly increases the chemical species, as we have seen by in situ IR absorption. The experimental studies showed that the diamond and chemical species were transported and scattered from the diamond coating on the electrode and through the plasma towards the substrate surface, where they caused enhanced nucleation.     

Oxygen at the interface of CVD diamond films on silicon

The oxygen incorporation at the interface between the silicon substrate and chemical vapour deposited (CVD) diamond films nucleated by the bias-enhanced nucleation (BEN) procedure has been studied by heavy-ion elastic recoil detection (ERD). Using standard process conditions for the realisation of heteroepitaxial films, oxygen with a concentration equivalent to about 1 nm SiO₂ has been found, which was mainly incorporated during textured growth with a certain CO₂ admixture to the process gas. By completely omitting CO₂ during nucleation and growth, the oxygen at the interface can be reduced by nearly one order of magnitude to 6.3×10¹⁵ at cm⁻², corresponding to 0.14 nm SiO₂. Intentional addition of highly enriched C¹⁸O₂ to the gas phase shows that the oxygen incorporation is strongly enhanced during BEN with hydrocarbon in the gas phase. The results indicate that roughening of the surface, the deposition of Si_xO_yC_z phases and strong lateral inhomogeneities at the silicon interface may explain the coexistence of epitaxial crystallites and amorphous phases. It is suggested that a further reduction of the oxygen concentration at the interface may have consequences for an improved heteroepitaxy of diamond on silicon.     

Bias enhanced diamond nucleation on Mo and CrN coated stainless steel substrates in a HFCVD reactor

Bias enhanced nucleation was explored in order to obtain a high diamond nucleation density on Mo and CrN coated stainless steel substrates in a hot-filament chemical vapor deposition (HFCVD) reactor. Several bias geometries were tested on their nucleation enhancement for diamond on molybdenum. It was found that the nucleation properties observed for Mo were not much different for CrN coated stainless steel. The application of a bias between the filament and a cathode above the substrate at floating potential resulted in locally high diamond nucleation densities of up to 3∙10⁹ cm^− 2.     

Silicon carbide films from polycarbosilane and their usage as buffer layers for diamond deposition

Thin films of polycarbosilane (PCS) were coated on a Si (100) wafer and converted to silicon carbide (SiC) by pyrolyzing them between 800 and 1150 °C. Granular SiC films were derived between 900 and 1100 °C whereas smooth SiC films were developed at 800 and 1150 °C. Enhancement of diamond nucleation was exhibited on the Si (100) wafer with the smooth SiC layer generated at 1150 °C, and a nucleation density of 2 × 10¹¹ cm^− 2 was obtained. Nucleation density reduced to 3 × 10¹⁰ cm^− 2 when a bias voltage of − 100 V was applied on the SiC-coated Si substrate. A uniform diamond film with random orientations was deposited to the PCS-derived SiC layer. Selective growth of diamond film on top of the SiC buffer layer was demonstrated.     

Formation of nanometer-size high-density pits on epitaxial diamond (100) films

We report the formation of pits having widths of approximately 10 nm and a density of 2.5 × 10¹¹/cm² on epitaxial diamond (100) films. The pits are formed by etching the films using atomic hydrogen at a substrate temperature of approximately 500 °C. Exposure to oxygen followed by etching with atomic hydrogen forms additional pits. We propose that the high-density pits are formed due to etching that occurs both perpendicular and parallel to the surface.     

Effects of plasma treatments on the nitrogen incorporated nanocrystalline diamond films

The nitrogen incorporated nanocrystalline diamond (NCD) films were grown on n-silicon (100) substrates by microwave plasma enhanced chemical vapor deposition (MPECVD) using CH₄/Ar/N₂ gas chemistry. The effect of surface passivation on the properties of NCD films was investigated by hydrogen and nitrogen-plasma treatments. The crystallinity of the NCD films reduced due to the damage induced by the plasma treatments. From the crystallographic data, it was observed that the intensity of (111) peak of the diamond lattice reduced after the films were exposed to the nitrogen plasma. From Raman spectra, it was observed that the relative intensity of the features associated with the transpolyacetylene (TPA) states decreased after hydrogen-plasma treatment, while such change was not observed after nitrogen-plasma treatment. The hydrogen-plasma treatment has reduced the sp²/sp³ ratio due to preferential etching of the graphitic carbon, while this ratio remained same in both as-grown and nitrogen-plasma treated films. The electrical contacts of the as-grown films changed from ohmic to near Schottky after the plasma treatment. The electrical conductivity reduced from ~ 84 ohm^– 1 cm^− 1 (as-grown) to ~ 10 ohm^– 1 cm^− 1 after hydrogen-plasma treatment, while the change in the conductivity was insignificant after nitrogen-plasma treatment.     

Nucleation and growth of diamond films on single crystal and polycrystalline tungsten substrates

Silicon has been the most widely studied substrate for the nucleation and growth of CVD diamond films. However, other substrates are of interest, and in this paper, we present the results of a study of the biased nucleation and growth of diamond films on bulk single and polycrystalline tungsten. Diamond films were nucleated and grown, using a range of bias and reactor conditions, and characterized by Raman spectroscopy and scanning electron microscopy (SEM). High-quality (100) textured films (Raman FWHM<4 cm⁻¹) could be grown on both single and polycrystalline forms of the tungsten substrate. On carefully prepared substrates, by varying the bias treatment, it was possible to determine the nucleation density over a 4–5 order range, up to ∼10⁹ cm⁻². Raman measurements indicated that the diamond films grown on bulk tungsten exhibited considerable thermal stress (∼1.1 GPa), which, together with a thin carbide layer, resulted in film delamination on cooling. The results of the study show that nucleation and growth conditions can be used to control the grain size, nucleation density, morphology and quality of CVD diamond films grown on tungsten.     

Nucleation of diamond films by ECR-enhanced microwave plasma chemical vapor deposition

A novel nucleation technique based on electron cyclotron resonance microwave plasma was developed to enhance the nucleation of diamond. By choosing a suitable experimental condition, a nucleation density higher than 10⁸ nuclei cm⁻² was achieved on an untreated, mirror-polished silicon substrate. Uniform diamond films were obtained by combining this nucleation method with subsequent growth by the common microwave plasma chemical vapor deposition. Furthermore, the possibility of this new nucleation method to generate heteroepitaxial diamond nuclei on (001) silicon substrates was explored.     

Device scaling of pseudo-vertical diamond power Schottky barrier diodes

Device size scaling of pseudo-vertical diamond Schottky barrier diodes (SBDs) has been characterized for high-power device applications based on the control of doping concentration and thickness of the p^? CVD diamond layer. Decreasing parasitic resistance on the p⁺ layer utilising lithography and etching realises a constant specific on-resistance of less than 20 mΩ cm² with increasing device size up to 200 µm. However, the leakage current under low reverse bias conditions is increased markedly. Due to the increase in the leakage current, the reverse operation limit is decreased from 2.4 to 1.3 MV/cm when the device size is increased from 30 to 150 µm. If defects induce an increase in leakage current under the reverse conditions, the density of the defects can be estimated to be 10⁴–10⁵/cm². This value is 5–10 times larger than the density of dislocations in single crystal diamond Ib substrate.     

Pre-nucleation techniques for enhancing nucleation density and adhesion of low temperature deposited ultra-nanocrystalline diamond

Effect of pre-nucleation techniques on enhancing nucleation density and the adhesion of ultra-nanocrystalline diamond (UNCD) deposited on the Si substrates at low temperature were investigated. Four different pre-nucleation techniques were used for depositing UNCD films: (i) bias-enhanced nucleation (BEN); (ii) pre-carburized and then ultrasonicated with diamond powder solution (PC-U); (iii) ultrasonicated with diamond and Ti mixed powder solution (U-m); (iv) ultrasonicated with diamond powder solution (U). The nucleation density is lowest for UNCD/U-substrate films ( 10⁸ grains/cm²), which results in roughest surface and poorest film-to-substrate adhesion. The UNCD/PC-U-substrate films show largest nucleation density ( 1 × 10¹¹ grains/cm²) and most smooth surface (8.81 nm-rms), whereas the UNCD/BEN-substrate films exhibit the strongest adhesion to the Si substrates (critical loads =  67 mN). Such a phenomenon can be ascribed to the high kinetic energy of the carbon species, which easily form covalent bonding, Si–C, and bond strongly to both the Si and diamond.     

Diamond–carbon nanocomposites: applications for diamond film deposition and field electron emission

Diamond–carbon nanocomposites (DCN) containing diamond and graphitic particles, both a few nanometers in size, were studied as the material for field electron emission. Diamond–carbon mass ratio and grain size were varied to optimize field emission properties. The stable and uniform electron emission was observed at fields E=10 V μm⁻¹ with a negligible hysteresis of I–V curves. Treatment in microwave hydrogen plasma was found to reduce the threshold field for emission owing to preferential etching of carbon component and surface relief sharpening. Ultrathin chemical vapour deposition diamond films can be easily grown on DCN due to the very high nucleation density inherent to this composite.     

Domain formation in diamond nucleation on iridium

Domain formation in epitaxial diamond nucleation on Ir(001) surfaces using the bias-enhanced nucleation (BEN) procedure has been studied. Bright areas of up to several microns lateral size with negligible topographic contrast are observed by scanning electron microscopy (SEM) after ion bombardment. When a growth step is applied after BEN, these domains develop into islands of identical shape consisting of epitaxial diamond with a high local area density of oriented grains. Outside the domains the nucleation density is either orders of magnitude lower or the grains are completely non-oriented. The diamond nuclei or precursors which are formed during the BEN step proved to be very stable: They still yielded oriented diamond islands when the samples were stored in air for 1 year before the growth step. Electron backscatter diffraction (EBSD) patterns taken from inside and outside the domains immediately after BEN did not show any significant difference. This allows the conclusion that the modification of the iridium crystal lattice accompanied with diamond nucleation is either very faint or only restricted to a very thin layer at the surface. Kelvin probe force microscopy (KPFM) measurements indicate a reduced work function within the domains.