Ex-situ spectroscopic ellipsometry studies of micron thick CVD diamond films

Micron thick diamond films have been studied by spectroscopic ellipsometry (SE). The films were grown, on previously prepared Si(100) substrates, by the plasma enhanced chemical vapor deposition (PECVD) technique. Ex situ SE measurements were carried out on samples grown under different conditions, such as substrate temperature and methane fraction in the gas mixture. An optical model consisting of five layers was constructed in order to explain the SE spectra and to provide the optical and structural parameters of the films. This model was deduced from results of various measurements performed by other characterization techniques (Raman spectroscopy, scanning electron microscopy, atomic force microscopy and positron annihilation spectroscopy) which have revealed the optical and structural parameters of the samples. Its sensitivity to the surface and interface roughness as well as to the absorption of the nondiamond phase of the film is demonstrated. Several values of the percentage of the nondiamond phase can be obtained, with the same fit quality, however, depending on the amorphous carbon reference used in the model. These references were obtained by performing SE measurements on various amorphous carbon films. Finally, our SE analysis has allowed us to monitor the lateral homogeneity of the thickness, surface and interface roughness and nondiamond phase concentration over the diamond film.     

Surface electronic properties on boron doped (111) CVD homoepitaxial diamond films after oxidation treatments

Surface electronic properties on oxidized boron (B) doped (111) homoepitaxial diamond films are investigated by Hall effect measurements and Schottky junction characterizations. Surface electronic properties on (111) diamond strongly depend on annealing treatments after wet-chemical oxidation, whereas for those on (001) diamond no change due to annealing can be detected. Hall effect results show that a p-type surface conductive layer (SCL) exists on (111) diamond surface in air after wet-chemical oxidation followed by annealing in Ar atmosphere (WO–AN) above 300 °C, but does not if only wet-chemical oxidation or air-oxidation is applied. This SCL disappears at annealing temperature above 350 °C in air. Schottky junction characteristics suggest that the Fermi level is unpinned at the (111) surface after WO–AN. Surface electronic characteristics on (111) diamond after WO–AN are similar to those generated by hydrogen termination.     

Electrical contacts to nitrogen incorporated nanocrystalline diamond films

The effect of surface plasma treatment on the nature of the electrical contact to the nitrogen incorporated nanocrystalline diamond (n-NCD) films is reported. Nitrogen incorporated NCD films were grown in a microwave plasma enhanced chemical vapor deposition (MPECVD) reactor using CH₄ (1%)/N₂ (20%)/Ar (79%) gas chemistry. Raman spectra of the films showed features at ∼ 1140 cm^− 1, 1350 cm^− 1(D-band) and 1560 cm^− 1(G-band) respectively with changes in the bonding configuration of G-band after the plasma treatment. Electrical contacts to both untreated and surface plasma treated films are formed by sputtering and patterning Ti/Au metal electrodes. Ohmic nature of these contacts on the untreated films has changed to non-ohmic type after the hydrogen plasma treatment. The linear current–voltage characteristics could not be obtained even after annealing the contacts. The nature of the electrical contacts to these films depends on the surface conditions and the presence of defects and sp² carbon.     

Photoconductive properties of lightly N-doped single crystal CVD diamond films

In the preparation of high power diamond photoswitches, thick (more than 100 μm) lightly nitrogen-doped single crystals were grown at LIMHP, for which Differential Interference Contrast Microscopy, Raman spectroscopy, photoluminescence, and cathodoluminescence have confirmed good morphology and very low but well-controlled impurity doping level. In order to evaluate the effect of nitrogen incorporation on the electronic properties of these films, photoconductivity measurements have been carried out. In an initial study, I–V and transient photocurrent measurements were conducted on several films with N-doping from 0 to 20 ppm intentionally added to the gas phase during growth, resulting into nitrogen concentrations lower than 100 ppb in the film. The results of these measurements are presented showing typical semiconductor behavior in terms of gain versus settling time, relatively high external quantum efficiency (EQE) and corresponding derived μτ (mobility × lifetime) product. In particular, samples with no nitrogen showed EQEs of several hundreds while their settling time was quite long (tens of seconds). However, samples with small nitrogen addition were observed to have settling times decreasing below a few seconds while EQEs close to 10 showed that a compromise could be found between efficiency and response time.     

机械法抛光加工金刚石膜研究

在平面研磨机上使用金刚石抛光盘对金刚石膜进行了抛光实验。通过观察金刚石膜机械抛光过程中表面形貌的变化，并结合拉曼光谱分析，提出了金刚石抛光盘抛光金刚石膜的抛光机理主要是微切削和压力破碎作用，同时还可能有少量金刚石转变为非晶碳的作用。通过单因素实验研究，发现金刚石盘的粒度对抛光金刚石膜材料去除率的影响最为显著。转速越高，表面粗糙度越小，压力和粒度存在最优值，超过该值后，表面粗糙度并不会随之降低。     

热丝CVD法制备大面积高质量自支撑金刚石厚膜

采用热丝CVD方法,在Ф110mm的钼基体上,以丙酮为碳源,在高纯氢的作用下,成功地合成了高质量自支撑金刚石厚膜.X射线、SEM和拉曼光谱分析表明,所合成金刚石厚膜质量均匀,晶体单一、完好、纯度高,生长速度快,已接近制备实用化的金刚石膜的要求.     

砷化镓上生长金刚石薄膜的研究

以CH4和H2为气源,用微波辅助等离子体装置,在10.0 mm×7.0 mm的砷化镓基底上沉积了CVD金刚石薄膜,用扫描电子显微镜观察沉积效果,拉曼光谱表征沉积质量,分析薄膜附着力与砷化镓材料性能的关系。结果表明,当基体温度为600℃,气压为5 kPa,甲烷浓度为2.0%时,在砷化镓片表面上沉积出了CVD金刚石薄膜,晶粒尺寸均匀,晶形完整、规则,晶界非常清晰。     

Vibrational properties of nitrogen-doped ultrananocrystalline diamond films grown by microwave plasma CVD

Ultrananocrystalline diamond (UNCD) films grown in an argon-rich Ar/CH₄/H₂ microwave plasma with nitrogen gas added in amounts of 0%–20% were studied by Raman spectroscopy with multiple excitation wavelengths in the range of 244–647 nm and by optical absorption in UV–visible. The Raman spectra have demonstrated the presence of diamond, amorphous carbon and polyacetylene in the UNCD films. Analysis of vibrational and optical properties of amorphous carbon phase proves that nitrogen stimulates the transition from amorphous carbon into an ordered graphite-like structure with narrowed optical band gap, which is supposed to be responsible for the high electrical conductivity of the N-doped UNCD.     

Highly and heavily boron doped diamond films

From the high ionization energy E_i = 0.368 eV and high solid solubility ≥ 1.4 × 10²² cm^− 3 of boron in diamond, metallic conductivity is expected on the boron impurity band within the band gap (Mott model). On the contrary, the numerical models used to describe the superconductivity of metallic diamond mainly use the Bardeen model with a Fermi level within the valence band. Taking into account the decrease to zero of E_i through the high [B] range and the band gap narrowing through the high and heavy [B] ranges, both specific of the Bardeen model, we discuss the validity of the Mott and Bardeen models from the literature and cathodoluminescence and Raman experiments. They agree with the Mott rather than the Bardeen model. Several experiments independently show the coupling of boron related levels with the zone centre optical phonons which soften for heavy [B]. The Mott model might explain the similar range of the superconductivity temperature of homoepitaxial and polycrystalline films from the similarity of their boron impurity band.     

Secondary electron emission from CVD diamond films

Secondary electron emission from boron doped diamond polycrystalline membranes (hole concentration 5×10¹⁸ cm⁻³), prepared by microwave plasma assisted CVD, was investigated in both the reflection and transmission configurations. The model of secondary electrons behavior taking into account the distribution and diffusion mechanism of secondary electrons is proposed to explain the yield dependencies on primary electron energy in both configurations. The model predicts the SEE yield K=19 at the primary electron energy E₀ close to 1 keV for reflection configuration and K=3–7 at E₀=15–30 keV for transmission configuration for polycrystalline films used in the study. Experimental measurements of the SEE yield vs. primary electron energy (18 at E₀=950 eV for the reflection scheme and 3.5–4 at E₀=25 keV for the transmission one) are found to accord well with the theoretical results. Estimations, which were made using the model, show that SEE yield in transmission configuration can be increased up to 60 for the primary electron energy of about 10 keV. Since such high yields in transmission scheme may be obtained in monocrystalline membrane, another approach using porous polycrystalline diamond membranes is considered. Porous diamond membranes having SEE yield in transmission scheme of more than 10 at the primary electron energy E₀=1 keV were fabricated.     

Superconducting properties of homoepitaxial CVD diamond

Superconductivity was achieved above 10 K in heavily boron-doped diamond thin films deposited by the microwave plasma-assisted chemical vapor deposition (CVD) method. Advantages of the CVD method are the controllability of boron concentration in a wide range, and a high boron concentration, compared to those obtained using the high-pressure high-temperature method. The superconducting transition temperatures of homoepitaxial (111) films are determined to be 11.4 K for T_C onset and 8.4 K for zero resistance from transport measurements. In contrast, the superconducting transition temperatures of (100) films T_C onset = 6.3 K and T_C zero = 3.2 K were significantly suppressed.     

Anisotropic dry etching of boron doped single crystal CVD diamond

Semiconducting boron doped single-crystal CVD diamond has been patterned using aluminum masks and an inductively coupled plasma (ICP) etch system. For comparison insulating HPHT diamond samples were also patterned using the same process. Diamond etch rates above 200 nm/min were obtained with an O₂/Ar discharge for a gas pressure of 2.5 mTorr using 600 W RF power. We have accomplished the fabrication of structures with a minimum feature size of 1 μm with vertical sidewalls in both CVD and HPHT diamond. The ICP etching produced smooth surfaces with a typical root-mean-square surface roughness of 3 nm. The dependence of etch rate on bias voltage was somewhat different for the two types of diamond. However, for all samples both the etch rate and anisotropy were found to improve with increasing bias voltage.     

Grain size dependent mechanical properties of nanocrystalline diamond films grown by hot-filament CVD

Nanocrystalline diamond (NCD) films with a thickness of ~ 6 µm and average grain sizes ranging from 60 to 9 nm were deposited on silicon wafers using a hot-filament chemical vapor deposition (HFCVD) process. These samples were then characterized in order to identify correlations between grain size, chemical composition and mechanical properties. The characterization reveals that our films are phase pure and exhibit a relatively smooth surface morphology. The levels of sp²-bonded carbon and hydrogen impurities are low, showing a systematic variation with the grain size. The hydrogen content increases with decreasing grain size, whereas the sp² carbon content decreases with decreasing grain size. The material is weaker than single crystalline diamond, since both stiffness and hardness decrease with the reduction in crystal size. These trends suggest gradual changes in the nature of the grain boundaries, from graphitic in case of 60 nm grain size material to hydrogen terminated sp³ carbon in 9 nm grain size material. The films exhibit low levels of internal stress and free-standing structures with a length of several centimeters could be fabricated without noticeable bending     

UV photoemission efficiency of polycrystalline CVD diamond films

The absolute quantum efficiency of polycrystalline diamond films grown on silicon substrates by chemical vapor deposition (CVD) is reported in the range of 25–200 nm. The efficiency of boron-doped and hydrogen-activated by microwave plasma reflective photocathodes peaked at 37% at 40 nm with the sensitivity cutoff observed at ∼190 nm. We confirmed that hydrogen activation is relatively stable in air: the efficiency of the photocathode degraded by less than 15% after an 18-h air exposure. It was found that diamond photocathodes can be reactivated multiple times to the same high QE values even after extended ultrasonic cleaning in both water and alcohol. The sensitivity of a diamond photocathode coated with a high dipole moment 4-nm-thick CsI layer was found to be slightly better than that of hydrogenated films, especially at longer wavelengths, while the sensitivity cutoff shifted to ∼200 nm. The observed high efficiency of thin CVD diamond films and relatively high stability of surface activation makes them a very attractive alternative to many existing UV photocathodes, especially taking into account other useful features of diamond (mechanical and chemical stability and radiation hardness).     

On the two-phonon absorption of CVD diamond films

It is well known that the absorption coefficient of diamond in the two-phonon region is constant, for example at 2000 cm^{− 1}, the absorption coefficient is 12.3 cm^{− 1}. This means that the infrared absorbance in the two-phonon region is proportional to the thickness of the samples, which is generally used as standard to normalize the infrared absorption spectra of diamond samples according to their thickness. This is true for natural and HPHT synthetic single crystal diamond. However for polycrystalline or nanocrystalline CVD diamond films, we found that the situation may be different. For high quality thick CVD diamond films of thickness > 150 μm, the infrared absorbance in the two-phonon region is proportional to its thickness. While CVD diamond films of equal thickness but of different quality show variable absorbance in the two-phonon absorption region in terms of thickness. Our investigation on this observation primarily indicates that the grain size of CVD diamond films has influence on the two-phonon absorption. In this work, we present this new result and discuss the mechanism of this phenomenon in the light of the growth mechanism of CVD diamond.     

Investigation of nitrogen addition on hydrogen incorporation in CVD diamond films from polycrystalline to nanocrystalline

The effect of nitrogen addition in the gas phase on hydrogen impurity incorporation into CVD diamond films was investigated. A series of thick diamond films of different morphology and quality ranging from large-grained polycrystalline to fine-grained nanocrystalline were deposited on silicon wafers using a 5 kW microwave plasma assisted CVD system. They were obtained only by changing the small amount of oxygen and nitrogen addition while keeping all other input parameters the same. Bonded hydrogen impurity in these diamond films was studied by using Fourier-transform infrared spectroscopy. It was found that with increasing the amount of nitrogen addition in the gas phase, the produced diamond films from large-grained polycrystalline gradually shift to fine-grained nanocrystalline and their crystalline quality is drastically degraded, while the amount of incorporated hydrogen impurity in the diamond films increases sharply. The role of nitrogen additive on diamond growth and hydrogen incorporation is discussed. These results shed light into the growth mechanism of CVD diamond films ranging from polycrystalline to nanocrystalline, and the incorporation mechanism of hydrogen impurity in CVD diamonds.     

UV Schottky photodiode on boron-doped CVD diamond films

We report on experimental study of photosensitivity and Q-DLTS spectra of polycrystalline CVD diamond UV photodetectors. The measured characteristics of Schottky photodiode on boron-doped diamond films are compared with those obtained for planar photoconductive structures (photoresistor type) based on undoped CVD diamond. The Schottky photodiode exhibited a sharp cut-off in photoresponse with spectral discrimination ratio (between wavelengths of 190 nm and 700 nm) as high as 5 · 10⁵ at zero bias voltage (at zero dark current). The photodiode showed the maximum of photoresponse at wavelength < 190 nm, and a low density of trapping and recombination centers as evaluated with the Q-DLTS technique. The devices demonstrated the photoresponsivity at 190 nm from 0.03 to 0.1 A/W with quantum yield of 0.20 to 0.67 in closed circuit, while the photovoltage ≥ 1.6 V was measured in open circuit regime. Another type of UV detector, the planar photoconductive structures with interdigitizing ohmic electrodes fabricated on undoped diamond film and operated under a bias voltage, revealed a higher density of (surface) defect centers and the maximum photoresponse at  210 nm wavelength. A strong influence of UV light illumination on the Q-DLTS spectra of the planar photoconductive structures was observed. This effect can be used for development of new UV detectors and dosimeters based on the Q-DLTS signal measurements.     

Optical study of defects in thick undoped CVD synthetic diamond layers

The growth of thick single crystal synthetic diamonds by plasma-assisted chemical vapour deposition (PACVD) that are colourless and with a low impurity content is an important challenge to achieve gem-quality material. To this aim, advanced optical imaging and spectroscopy techniques are useful tools to optimize the growth process as well as to identify CVD-made diamond gems. In this paper, two thick synthetic diamond crystals with unique structural and spectroscopic properties were grown by PACVD without intentional addition of nitrogen and without any post-treatment to enhance their colour. While the first one, sample A, was 760 μm thick and exhibited a grey to greyish brownish colour quite unusual for undoped material, the second one, sample B, was exceptionally thick (3300 μm) and colourless. It was laser-cut and polished to obtain a high gem-quality round brilliant of 0.44 carat with G colour and VVS2 clarity grade. The amount, distribution and nature of defects in these synthetic crystals is discussed using a broad range of optical characterisation techniques towards growth optimization and getting a better understanding of the material properties (and their possible integration to the gem market).     

Coupled effect of nitrogen addition and surface temperature on the morphology and the kinetics of thick CVD diamond single crystals

In this study, homoepitaxial thick diamond films were grown by CVD at high microwave power densities for temperatures ranging from 800 °C to 950 °C and with nitrogen additions from 75 to 200 ppm relative to the total gas flow. It was observed that there is a coupled effect of these two parameters on the growth mechanisms of the CVD diamond film. For a deposition temperature close to 875 °C and for the lowest nitrogen concentration, the growth proceeded via a step flow mode identified by classical step bunching phenomena due to the presence of nitrogen and leading to the appearance of macro-steps. When nitrogen concentration was increased keeping the same temperature, the growth mode evolved from a step flow mode to a bidimensional nucleation mode, for which macro-steps are no longer observed. For higher growth temperatures (950 °C), it was found that this growth mode transition still exists but appears for much higher nitrogen concentration. These different observations, associated with the resulting growth rates, are discussed in terms of surface modification induced by the presence of nitrogen impurity. It is shown in particular that an increase of nitrogen concentration is equivalent to an increase of the surface supersaturation, this effect being compensated by an increase of the deposition temperature.     

Hydrogen passivation of boron acceptors in as-grown boron-doped CVD diamond epilayers

A homoepitaxial boron-doped diamond single layer is investigated by means of Fourier transformed infrared spectroscopy (FTIR) and cathodoluminescence (CL). Both techniques are shown to be complementary. µ-FTIR mapping allows to determine the location of active boron while CL allows discernability between passivation and compensation. Hydrogen incorporation during chemical vapour deposition (CVD) growth is revealed to passivate boron acceptors. The obtained results highlight that plasma etching can induce a dissociation of B–H centres.