Static quantum ab initio and quantum semiempirical molecular dynamics calculations were employed to study reconstructions of (100) diamond surfaces in presence of hydrogen and oxygen. The results indicate that the energetically most favorable structures of oxygenated surfaces are those with chemisorbed hydroxyl groups. It was found that hydrogen bonds are formed among chemisorbed oxygenated species. The formation of these hydrogen bonds is shown to be an important factor in stabilization of adlayers. A number of important vibrational modes characteristic of oxygenated diamond surfaces were identified. The analysis of surface vibrational spectra demonstrates the influence of the local environment on the position of vibrational modes and can be useful for interpretation of experimental data.

An energetic and structural comparison has been made concerning adsorption processes on a diamond (111) surface from a hydrogen (H)-, halogen (X)- or H/X-carrying carbon precursors. The halogens X examined were F, Cl and Br. The quantum mechanical methods used in the investigation were the ab initio MO method as well as the LDA method. F, Cl, and H, were all shown to be able to sustain the sp3 structural configuration of the surface carbon atoms. For Br large sterical hindrances were induced and the diamond (111) surface could not be stabilized. The calculated adsorption energy of CF3 to a H-terminated diamond (111) surface was found to be numerically the largest one (489 kJ/mol on the MP2 level of theory) in comparison to the other adsorption processes studied in the present investigation. It was, in particular, much larger than the corresponding adsorption energy of CH3 (418 kJ/mol). The calculated adsorption energies do support the initial step in the growth process (adsorption) to be energetically more favourable for systems incorporating a mixture of halogen and hydrogen in the growth vapor than for corresponding systems involving unmixed halogen or hydrogen in the carbon containing precursor.

The photodecomposition of CF3I and C4F9I overlayers at 119 K on diamond (100) surfaces has been shown to be an efficient route to fluorination of the diamond surface. X-ray photoelectron spectroscopy (XPS) has been used for photoactivation as well as for studies of the following processes: the photodecomposition of the fluoroalkyl iodide molecules: the attachment of the photofragments to the diamond surface; and the thermal decomposition of the fluoroalkyl ligands. Chemisorbed CF3 groups on diamond (100) decompose by 300 K whereas C4F9 groups decompose over the temperature range of 300 K to ∼ 700 K. Both of these thermal decomposition processes produce surface C-F bonds on the diamond surface which thermally decompose over a wide temperature range extending up to 1500 K. Hydrogen passivation of the diamond surface is ineffective in preventing free radical attack from the photodissociated products of the fluoroalkyl iodides. The use of photoactivation of fluoroalkyl iodides for the fluorination of diamond surfaces provides a convenient route compared to other methods involving the direct production of atomic F, molecular fluorine, XeF2, and F containing plasmas.

As-polished and homoepitaxial diamond (100)- and (111)-oriented single crystals, natural diamond powders of 0.12-6 μm diameter, and high-pressure/hightemperature (HPHT) diamond powders of 50-100 μm diameter were treated in a microwave hydrogen plasma under four sets of conditions. Negligible changes in the weights (±10 μg) of the as-polished and homoepitaxial diamonds were observed.Post-treatment atomic force microscopy (AFM) showed a combination of smoothing and pit formation on (100) surfaces, the details of which were sensitive to the structure of the starting surface. Asymmetric pits formed on an as-polished (111) surface, but no qualitative changes in the structure of highly defective (100) and (111) films were observed. Natural diamond powders, which were quite irregular and rough prior to treatment, became markedly smoother and well-faceted, as observed by scanning electron microscopy (SEM). The degree of faceting was sensitive to plasma power level but was independent of H2 flow rate. The size and degree of faceting appeared to be the same after plasma treatment for isolated and closelypacked particles; however, the latter fused into a quasi-continuous film. “Regrown” crystallites were observed on the surfaces of synthetic type Ib diamond particles following plasma treatment. We argue that surface diffusion is the dominant mechanism for the observed morphological changes.

A polished diamond (110) surface was observed to undergo surface rearrangement above ˜700 °C to a micro faceted diamond surface when in contact with a nickel film and in the presence of hydrogen. Formation of a graphitic layer at the diamond-nickel interface above 700 °C was observed in the absence of hydrogen. The presence of 0.1 Torr of hydrogen was shown to be sufficient to prevent graphite formation at the nickel-diamond interface.

The effects of surface damage on the secondary electron emission characteristics of a natural diamond (100) surface have been investigated using ultraviolet photoelectron spectroscopy and scanning electron microscopy. Surface damage was intentionally induced by abrading the (100) diamond face with diamond paste. Removal of the damage was achieved by a sequence of ion implantation, graphitization, electrochemical etching and oxygen/argon plasma etching. Prior to characterization performed between steps in the sequence, the surface was hydrogenated by exposure to a hydrogen plasma in attempts to create a negative electron affinity surface condition. Upon removal of the surface damage, the secondary electron yield from the negative electron affinity surface was enhanced by a factor of ˜20 over that from the damaged negative electron affinity surface.

The semiconducting properties of diamond make it a material of interest for the fabrication of active electronic devices. Device performance depends on the properties of the host diamond film and the device structure. Improvements in the electronic properties of chemical vapor deposited (CVD) diamond films and in device design are required for development of commercial diamond devices. Hall-effect and capacitance-voltage measurements have been used to assess the electronic quality of diamond films and to evaluate potential device materials and structures. Homoepitaxial, highly oriented and polycrystalline CVD diamond films have been deposited and characterized by variable-temperature Hall-effect measurements. The highest room temperature hole mobility measured in a homoepitaxial film was 1590 cm2 /V•s where the compensation was ∼2 × 1015 cm-3. These properties are now beginning to approach those of the best type IIb natural diamonds. In a comparison of homoepitaxial, highly oriented diamond film, and polycrystalline diamond films, the room temperature hole mobility was 1470, 229 and 70 cm2/V•s, respectively. This data provides further evidence that the alignment of the grains improves the transport properties of diamond films. Capacitance-voltage measurements have been performed on single crystal type lib diamond with metal-insulating diamond-semiconducting diamond and metal-oxi de-semiconducting diamond structures. Using a conductance versus frequency technique, the density of interface states was estimated to be ∼1012 cm-2eV-1 for both structures. Although optimization of these structures must still be performed, the ability to measure the interface trap density offers the opportunity to provide direct feedback to the device fabrication process so that the structure may be optimized.

We have investigated the electrical properties of undoped polycrystalline diamond thin films deposited on (100)-oriented n-type and p-type silicon substrates. The films, intended for electronic applications, were manufactured using hot filament chemical vapour deposition (HFCVD). To a large extent the capacitance-voltage characteristics are influenced by traps located close to the interface between the diamond layer and the silicon substrate. These traps play an important role for voltage sharing between the diamond layer and the silicon space charge region. The DC current density through the diamond film has the same functional dependence on the electric field for films deposited on both n- and p-Si. The field dependency agrees with a Frenkel-Poole transport model. Further, although the DC current transport is thermally activated, it does not follow an Arrhenius relation. A possible reason is that traps within a broad range of energy levels are involved in the charge transport. Finally, current transients resulting from stepwise changes in the applied voltage follow a power law time dependence where the kinetics depend only weakly on temperature.

Nominally undoped, heteroepitaxially nucleated diamond films have been grown on Si(001). For different temperatures ranging between room temperature and 490°C the DCconductivity through the films has been measured for field strengths up to 600kV/cm. In the framework of the Poole-Frenkel theory a fit of the curves is possible for the model of isolated as well as for the model of overlapping coulombic potentials yielding an activation energy of 1.03eV.

In addition the breakdown voltage of the films has been measured as a function of the temperature. A drastic linear decrease with temperature is observed. Starting at a dielectric strength of about 1 MV/cm at room temperature the breakdown field decreases by an order of magnitude for 400°C. This observed behaviour is consistent with thermal breakdown based on Poole-Frenkel conduction in the pre-breakdown region.

A high-quality, low-stress 200 gim epitaxial diamond film has been grown on a 400 μm thick high-temperature-high-pressure Ila diamond. X-ray diffraction images of the film indicate that a large region of the film is fairly defect free and individual dislocations have been imaged in this region. Depth-resolved Raman results indicate that the region of the film with a low density of defects also has lower stress than in the higher defect density region. Transient photoconductivity measurements were performed on the high and low line defect density regions of the homoepitaxial diamond film to determine the effects of the stress and defect density on the combined electron-hole mobility and carrier lifetime. The correlation between the electrical properties and the x-ray diffraction imaging suggests that line defects may not be the limiting factor in the carrier transport at the present film quality

The evolution of hydrogen and oxygen from a 0.5 M H2SO4 solution on heavily borondoped diamond electrodes was studied. A very wide potential range without water decomposition, from approximately -1.5 to +2.75 V relative to the standard hydrogen electrode, was observed on high quality diamond. A much smaller range, from -0.7 to 1.7 V, and higher background currents were observed on diamond electrodes with significant sp2 content. The Ce(III)/Ce(IV) redox couple was observable on diamond electrodes, but was highly irreversible.

High resolution electron microscopy and electron energy-loss spectrometry were used to investigate both atomic and electronic structures of grain boundaries in diamond thin films grown by chemical vapor deposition. The atomic structures of {112}σ3 and {114}σ9 <110= tilt boundaries in diamond show different features from those in other diamond structure semiconductors. The electron energy-loss spectra recorded from the grain boundary regions show extra intensity near the energy-loss corresponding to carbon 1s-to-π*; transition, as compared to the spectra recorded from neighboring crystalline regions. This gives the evidence that the dangling bonds are not fully reconstructed along <110= direction in the boundary structure. Atomic models are constructed for these boundaries with the presence of non-tetracoordinated atoms. The stability of the boundary structure is explained by the π-like bonding between the nontetracoordinated atoms.

The hydrogen content of free-standing polycrystalline CVD diamond samples was determined by Fourier Transform Infrared (FTIR) measurements and was quantified by a notched neutron spectrum technique. The latter measures the total bonded and unbonded hydrogen. The concentration of total hydrogen in these samples was of the same order of magnitude. The FTIR measurements, which are sensitive to the bonded hydrogen, indicated different hydrogen to diamond peak ratios.

Chemical incorporation of impurities into diamond films is important for both electronic and optoelectronic applications. The study reveals the effect of incorporated dopants in diamond films using electron paramagnetic resonance (EPR), a non-destructive highly sensitive spectroscopy. Both the stress and damage associated with dopants is assessed using the center line of the EPR spectra. The films studied were grown by chemical vapordeposition and doped with phosphorus by two different methods, fast ion doping and chemical incorporation. The results suggest that damage increases with increased dopant density and saturates with dopant densities about l×1020 cm-3. Although electrical measurements did not reveal n-type conductivity, at low fluences the films grown using fast ion doping exhibited a density of 1017 cm-3 shallow substitutional phosphorus sites. Comparison of different dopants in the films doped by chemical incorporation show that phosphorus produces ten times greater damage than boron. Overall, the study illustrates that EPR may be used as a quick nondestructive characterization tool for diamond films.

Both passive and active electronic applications of CVD diamond have been proposed since the earliest stages of its development, largely based on an extrapolation of the superlative properties of single crystal diamond. Consequently, CVD diamond research has striven hard to match up to this expectation and significant advances have been made.

CVD diamond compares favourably with natural or high pressure synthetic single crystal material for passive electronic applications. The development of CVD diamond deposition technology for thermal management has led producers to address issues such as production cost, yield and quality. CVD polycrystalline diamond is becoming a commodity material and commercial applications in thermal management are emerging. Many of these developments are expected to feed into active electronic applications and will act as a springboard for diamond into commercial technology.

The active electronic applications for diamond are more demanding in terms of materials and process technologies. For example, doping, structure delineation and contact schemes have been widely demonstrated and prototype devices are showing potential benefits in sensors, detectors, photonics and cold cathodes. The current and future status of diamond electronics is reviewed.

Diamond's unique properties are ideally suited for high power and high frequency electronic applications. Natural type Ila diamond wafers of various thicknesses and active areas were used to construct several electron beam activated diamond devices. Average voltage gradients in the diamond target on the order of a mega-volt per centimeter were obtained. Possibilities of improving the attainable average voltage gradient are discussed. Electron activation avoids the necessity of semiconducting doping of the active diamond devices. The electron bombardment on diamonds yields a current-voltage characteristics very similar to that of a bipolar transistor. In the experiments discussed here, the diamond conduction to bombarding current gain ranged from 1,000 to 3,000 depending upon the diamond thickness and the bombarding electron energy. The bombarded diamond on-state resistance is consistent with a simple carrier drift and space charge model. Maximum conduction current density achieved in diamond is 19kA/cm2. High power switching of greater than 25kW with less than 100ns risetime is demonstrated. An electron beam bombarded millimeter wave diamond device has generated more than one watt of power.

Diamond has attractive properties as an advanced electronic material. Its combination of high carrier mobility, electric breakdown, and thermal conductivity results in the largest calculated figures of merit for speed and power of any material. Recently, the discovery and development of what might be called sophisticated secondary effects in diamond, are opening interesting sensor applications. Boron doped diamond will change electrical resistance with strain (piezoresistance), meaning it can be used as a strain gauge on rugged electronic microsensors for pressure and acceleration sensing. This paper will present some critical issues of diamond for microelectromechanical sensing applications such as its rupture stress and edge stress of diamond diaphragms and the high temperature responses of a diamond pressure sensor. We will describe an all diamond pressure microsensor that measures pressure at more than 300°C. Also, we have observed that layered diamond films can behave as chemical sensors measuring hydrogen, oxygen and many other chemicals’ concentration. For example, a diamond-based chemical gas sensor using Pd/i-diamond/p+-diamond metal-insulator-semiconductor diode structure has been made and the behavior of a new diamond-based chemical gas sensor has been studied. Also, the creation and properties of diamond microtips as field emitters are discussed.

This paper will cover two diverse electronic applications for which diamond devices have shown great promise. The first application is diamond radiation sensors for high radiation environments, where the competition is mainly silicon devices. These environments arise in high energy physics experiments, and tests show diamond to be superior to silicon in many ways. The second application is vacuum microelectronics, which generally refers to field emission, where the main competitor is metal and semiconductor microtip arrays. Certain diamond and diamondlike carbon materials emit electrons readily, but the physical mechanisms for this are not well understood. Negative electron affinity and other possible explanations are discussed in this paper.

Thin film diamond has been used to fabricate a photodetector which displays high sensitivity to deep UV light, with an external quantum efficiency of greater than one, a dark current of less than 0.1nA and which is near ‘blind’ to visible light.

Recent observations are presented on the emission of electrons from n-type polycrystalline diamond and from two new families of nitride-based cold cathode films, n-type boron nitride (BN) and n-type carbon nitride (CN). The n-type polycrystalline diamond films were synthesized by MPECVD. The BN and CN n-type films (∼150 nm) were synthesized by reactive laser ablation on n-type polycrystalline diamond (∼24 μm) on (100)Si.

Emission current densities have been measured as high as 119 ma cm−2. Extraction fields have been observed to range from ∼0 V μm−1 up. The diamond films show a field activated thermionic behavior. Both the BN and the CN films show a power-law current density / applied field response implying a negative electron affinity (NEA) type behavior.