Diamond-based Electronic Devices Research Articles

Boosting surface charge-transfer doping efficiency and robustness of diamond with WO3 and ReO3

An advanced charge-transfer yield is demonstrated by employing single monolayers of transition-metal oxides—tungsten trioxide (WO3) and rhenium trioxide (ReO3)—deposited on the hydrogenated diamond surface, resulting in improved p-type sheet conductivity and thermal stability. Surface conductivities, as determined by Hall effect measurements as a function of temperature for WO3, yield a record sheet hole carrier concentration value of up to 2.52 × 1014 cm−2 at room temperature for only a few monolayers of coverage. Transfer doping with ReO3 exhibits a consistent narrow sheet carrier concentration value of around 3 × 1013 cm−2, exhibiting a thermal stability of up to 450 °C. This enhanced conductivity and temperature robustness exceed those reported for previously exposed surface electron acceptor materials used so far on a diamond surface. X-ray photoelectron spectroscopy measurements of the C1s core level shift as a function of WO3 and ReO3 layer thicknesses are used to determine the respective increase in surface band bending of the accumulation layers, leading to a different sub-surface two-dimensional hole gas formation efficiency in both cases. This substantial difference in charge-exchange efficiency is unexpected since both surface acceptors have very close work functions. Consequently, these results lead us to consider additional factors influencing the transfer doping mechanism. Transfer doping with WO3 reveals the highest yet reported transfer doping efficiency per minimal surface acceptor coverage. This improved surface conductivity performance and thermal stability will promote the realization of 2D diamond-based electronic devices facing process fabrication challenges.

Demonstration of Diamond-Based Schottky p-i-n Diode With Blocking Voltage > 500 V

Diamond is considered to be the ultimate semiconductor for power devices due to its high breakdown electric field, high carrier mobility, and superior thermal properties. The success of diamond-based electronic devices has been difficult due to critical challenges involved with poor doping efficiency and achievement of ohmic contacts. Achieving n-type diamond has proved to be more difficult over p-type so far. In this letter, we report the achievement of n-type doping in diamond, verified using Hall measurements, which was then used to fabricate Schottky p-i-n diodes measuring a forward current density greater than 300 A/cm2 at 4 V and breakdown voltage of over 500 V with a 3.5- $\mu \text{m}$ -thick drift layer. A Silvaco simulation was performed which agreed well with the experimental data showing turn-ON voltage of 1 V and an ideality factor of 1.04, consistent with the model of a p-i-n diode with a fully depleted n-type contact.

Heterojunction Diodes Comprising Nitrogen-Doped Ultrananocrystalline Diamond/Hydrogentad Amorphous Carbon and p-Type Silicon

Ultrananocrystalline diamond/hydrogenated amorphous carbon composite (UNCD/a-C:H) films, wherein diamond crystallites with diameters of less than 10 nm are embedded in an a-C:H matrix, have a large absorption coefficients of 105 - 106 cm-1 in the photon energy range between 3 and 6 eV that make them applicable to optoelectronics devices. Moreover, the realizations of p-type and n-type diamonds are desirable for the applications of diamond-based electronic devices. In this work, the heterojunction diodes comprising nitrogen-doped UNCD/a-C:H and p-type Si were fabricated by coaxial arc plasma deposition (CAPD) and the heterojunction parameters were evaluated from the current–voltage (IV) and capacitance–voltage (CV) measurements carried out in dark at room temperature. Furthermore, the photodetection performance was studied.Experimentally, n-type UNCD/a-C:H films were deposited on p-type silicon substrate at a substrate temperature of 550 ºC with ambient pressure of 53.3 Pa. the Pd electrode is deposited on both sides by using of Radio Frequency (RF) sputtering. The (I–V) characteristics of the diodes measured in the dark . I-V curve exhibited a rectification ratio of more than 104, which evidently confirms that nitrogen-doped UNCD/a-C:H possesses n-type conduction and easily forms junction with p-type semiconductor. C–V measurements of the diode confirm junction formation, in which capacitance non-linearly decreases with increasing reverse bias voltage. The built-in potential of the heterojunction was estimated to be 0.68 eV from the extended linear part of 1/C2 plot. From the slop of 1/C2, carrier concentration of the UNCD/a-C:H film was estimated to be 6×1015 cm-3. Here, carrier concentration of Si substrate was 1×1015 cm-3 for the fitting analysis. The value of depletion region width \U0001d44aUNCD in the UNCD/a-C:H film was estimated to be 200 nm at the reverse voltages of ±1 V, which is close to that value generally measured for heterojunctions with closed ranges of carrier concentrations. More details will be explained in presentation during the conference.

(Invited) Surface Passivation of High-k Dielectric Materials on Diamond Thin Films

Single-crystalline CVD diamond films have excellent electrical and material properties with potential applications in high power, high voltage and high frequency applications that are out of reach for conventional semiconductor materials. For realization of efficient devices (e.g. MOSFET), finding a suitable dielectric is essential to improve the reliability and electrical performance of devices. In the current study, we present results from surface passivation studies by high-k dielectric materials such as aluminum oxide and hafnium oxide deposited by ALD on intrinsic and boron doped diamond substrates. The hole transport properties in the intrinsic diamond films were evaluated and compared to unpassivated films using the lateral time-of-flight technique. Also, for the advancement of diamond-based electronic devices, the fabrication of MOSFET is crucial as this device finds applications in numerous fields of power electronics and high frequency systems. The MOS capacitor structure which forms the basic building block of the MOSFET is discussed. Inversion phenomenon crucial for the operation of MOSFET observed for the first time in boron doped diamond will be discussed.

Reduction of dislocation densities in single crystal CVD diamond by using self-assembled metallic masks

The development of diamond-based electronic devices designed to operate at high power is strongly hampered by the lack of low dislocation single crystal material. Dislocations in Chemically Vapor Deposited (CVD) diamond are indeed generally responsible for leakage current, seriously deteriorating the performance of the devices. They can be due to defects such as polishing damage or contamination found at the substrate's surface, or they can directly originate from existing bulk defects that extend into the homoepitaxial layer. Although significant improvements have been achieved by using adapted surface treatments, dislocations found in CVD diamond grown on standard quality single crystal substrates are still typically in the range 105–106cm−2. In this work, we report on a new growth strategy aiming at preventing threading dislocations from propagating into CVD diamond layers. It is based on the selective masking of existing defects revealed at the surface of the substrates by Pt nanoparticles. The interaction of dislocations with such embedded particles has been assessed and critical remarks are given as to the use of this technique in order to reduce dislocation densities in synthetic diamond.

Inversion in Metal–Oxide–Semiconductor Capacitors on Boron-Doped Diamond

For the advancement of diamond-based electronic devices, the fabrication of metal-oxide-semiconductor field-effect transistors (MOSFETs) is crucial, as this device finds applications in numerous fields of power electronics and high-frequency systems. The MOS capacitor forms the basic building block of the MOSFET. In this letter, we describe planar MOS capacitor structures fabricated with atomic layer deposited aluminum oxide as the dielectric on oxygen-terminated boron-doped diamond substrates with different doping levels. Using capacitance-voltage measurements, we have, for the first time, observed inversion behavior in MOS structures on boron-doped diamond, with a doping concentration of 4.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> .

Spectroscopic ellipsometry of homoepitaxial diamond multilayers and delta-doped structures

The optimization of diamond-based unipolar electronic devices such as pseudo-vertical Schottky diodes or delta-doped field effect transistors relies in part on the sequential growth of nominally undoped (p–) and heavily boron doped (p++) layers with well-controlled thicknesses and steep interfaces. Optical ellipsometry offers a swift and contactless method to characterize the thickness, roughness, and electronic properties of semiconducting and metallic diamond layers. We report ellipsometric studies carried out on delta-doped structures and other epitaxial multilayers with various boron concentrations and thicknesses (down to the nanometer range). The results are compared with Secondary Ion Mass Spectroscopy and transport measurements.

ダイヤモンドを用いた光·電子デバイスの開発

ダイヤモンドを用いた光·電子デバイスの開発

Annealing dependence of diamond-metal Schottky barrier heights probed by hard x-ray photoelectron spectroscopy

Hard x-ray photoelectron spectroscopy was applied to investigate the diamond-metal Schottky barrier heights for several metals and diamond surface terminations. The position of the diamond valence-band maximum was determined by theoretically calculating the diamond density of states and applying cross section corrections. The diamond-platinum Schottky barrier height was lowered by 0.2 eV after thermal annealing, indicating annealing may increase carrier injection in diamond devices leading to photoconductive gain. The platinum contacts on oxygen-terminated diamond was found to provide a higher Schottky barrier and therefore a better blocking contact than that of the silver contact in diamond-based electronic devices.

Development of AlN/diamond heterojunction field effect transistors

AlN/diamond heterojunction field effect transistors with p-channel and normally-on depletion mode are developed, and the device performance is improved by reducing the contact resistances at source and drain contacts reported previously. The heterojunction structure is constructed from a c-axis-oriented AlN epilayer grown on oxygen-terminated (111) diamond substrate by metal-organic vapor phase epitaxy at temperatures as high as 1250°C. Thermal treatment in the mixed hydrogen (H2) and ammonia atmosphere just before AlN growth improves the AlN adhesion to diamond surface. In addition, this treatment simultaneously produces a much larger surface hole-carrier density than that obtained by conventional H2-plasma treatment. X-ray photoelectron spectroscopy reveals the existence of carbon-nitrogen bonds at the diamond surface, and these may be responsible for such a large hole density. These results are promising in relation to new opportunities for developing diamond-based power electronic devices.

Shape-controlled synthesis of diamond crystal by epitaxial growth under high pressure and high temperature conditions

In this paper, the diamond epitaxial growth mechanism has been studied in detail by employing several types of diamond as a seed in a catalyst—graphite system under high pressure and high temperature (HPHT) conditions. We find that the diamond nucleation, growth rate, crystal orientation, and morphology are significantly influenced by the original seeds. The smooth surfaces of seeds are beneficial for the fabrication of high-quality diamond. Our results reveal that the diamond morphology is mainly determined by the original shape of seeds in the early growth stage, but it has an adjustment process during the growth and leads to well symmetry. Additionally, we have also established the growth model for the twinned diamond grown on several seeds, and proposed the possible growth processes by tracking the particular shapes of seeds before and after treatment under HPHT conditions. These results suggest that the shape-controlled synthesis of diamond with well morphology can be realized by employing certain suitable diamond seeds. This work is expected to play an important role in the preparation of trustworthy diamond-based electronic and photonic devices.

Dislocations and impurities introduced from etch-pits at the epitaxial growth resumption of diamond

The fabrication of diamond-based electronic devices requires that several active layers with different doping concentrations are grown in different reactors. In this paper, we have investigated the effect of interrupting and resuming the epitaxial growth of a homoepitaxial diamond film using high-power plasma CVD. It was found that long lifetime blue phosphorescence which is localised on regions with a high dislocation density is generated. Such phosphorescence is related to a higher uptake of impurities at the interface between two subsequent films, due to an increased surface roughness from etching at the epitaxial growth resumption. Etching was found to occur preferentially on threading dislocations leaving typical etch-pits. Cathodoluminescence helps identify boron as the main background impurity. Besides, the formation of new dislocations was observed on the facets of these etch-pits. The continuation of epitaxy on a roughened surface thus comes with a substantial decrease in crystal quality.

Ab initioinvestigation of lithium on the diamond C(100) surface

We have performed ab initio calculations to investigate the adsorption of Li onto the clean and oxygenated diamond C(100) surface. Despite a large amount of interest in alkali-metal absorption on clean and oxidized semiconductor surfaces for both fundamental and technological applications, lithium adsorption on the diamond surface has not been reported. We find that Li adopts structures on the clean C(100) surface similar to those reported for Na, K, and Rb on diamond, though Li exhibits significantly higher binding energies in the range 2.7--3.1 eV per Li adsorbate. For the oxygenated $\text{C}(100)\text{\ensuremath{-}}(1\ifmmode\times\else\texttimes\fi{}1):\text{O}$ surface, the lowest energy involving a full Li monolayer structure shows an exceptionally large work-function shift of $\ensuremath{-}4.52\text{ }\text{eV}$ relative to the clean surface, an effect similar to that seen for $\text{Cs}\text{O}$ on diamond, but with a higher binding energy of 4.7 eV per Li atom. We propose that such a system, if verified by experiment, is suitable for the surface coating of diamond-based vacuum electronic devices, as it should exhibit higher thermal stability than the commonly used $\text{Cs}\text{O}$ surface while retaining the advantage of a large lowering of the work function.

Tuning the Electron Affinity and Secondary Electron Emission of Diamond (100) Surfaces by Diels−Alder Reaction

The tuning of electron affinity and secondary electron emission on diamond (100) surfaces due to cycloaddition with 1,3-butadiene is investigated by photoemission experiments and density functional theory (DFT) calculations. A significant reduction in electron affinity up to 0.7 eV and enhancement of secondary electron emission were observed after 1,3-butadiene adsorption. The lowering of vacuum level via 1,3-butadiene adsorption is supported by DFT calculations. The C-H bonds in the covalently bonded organics on diamond contribute to the enhanced secondary electron emission and reduced electron affinity in a mechanism similar to that of C-H bonds on hydrogenated diamond surfaces. This combination of strong secondary emission and low electron affinity by the organic functionalization of diamond has potential applications in diamond-based molecular electronic devices.

Diamond power devices. Concepts and limits

The prospects and limits of diamond power devices incorporating boron as dopant are discussed using the results of a theoretical/empirical analysis coupled to 2D-numerical simulation. To obtain full activation of the boron acceptor at room temperature, the concept of delta-doping is introduced and used as basic element. Two device concepts were chosen for this analysis: (i) RF power FET incorporating a monolayer boron delta-doped channel and gate recess and gate filed plate structure; and (ii) various types of vertical power diodes including a novel stacked delta-doped structure, which is proposed here for the first time. The analysis was performed considering the limiting material parameters known for single crystal diamond including recent data on the carrier mobility, the phenomena related to thermal activation of boron acceptor and carrier scattering along and across delta-doped layers, thermal and electrical breakdown limits. The results of this study reveal that the optimised delta-FET with the field plate of 1 μm may be capable to produce approximately 34 W/mm continues-wave RF power at 20 °C operation temperature. The projected RF power of a similar FET structure with the field plate of 2 μm is about 75 W/mm, which may be beyond the thermal limit. The proposed stacked delta-doped diode could have the best on-resistance to blocking voltage ratio than any other type of diamond and n-type SiC-based diodes up to 20 kV blocking voltage. Below 3 kV the projected characteristics of the stacked delta diode on diamond are also favourable in comparison to those of an ideal n-type GaN diode. These examples demonstrate that boron delta-doped layer is a very promising configuration for various diamond-based power electronic devices.

Charge collection distance measurements in single and polycrystalline CVD diamond

Diamond is one of the most promising materials for extreme performance electronic applications, such as high power, high temperature, and high frequency devices. Recent advances in the synthesis of device-quality, single crystal diamond by chemical vapour deposition (CVD) have opened up the possibility to make efficient diamond-based electronic devices. Two important material parameters, which should be maximized, that characterise the performance of diamond in applications of this type are the carrier mobility and the free carrier lifetime. We report an experimental study comparing photocurrent mobility×lifetime ( μτ) products in polycrystalline and single crystal CVD diamond. It is shown that the μτ products of the single crystal samples are 2–3 orders of magnitude higher than those of the polycrystalline samples.

Band bending, electronic affinity and density of states at several (100) surfaces of boron-doped homoepitaxial diamond thin films

Using ultraviolet and x-ray photoelectron spectroscopies, we investigate four types of the (100) surface of homoepitaxial diamond films, either doped with boron or undoped. We measure the position of the valence band maximum at the surface, the electronic affinity and the density of states. In p-type doped films, conductivity measurements indicate that the bulk Fermi level is pinned close to 0.4 eV above the valence band edge and subtracting this value from the valence band maximum at the surface permits us to derive the band bending. The different (100) surfaces of the p-type diamond samples are (i) hydrogenated with the 2 × 1 reconstruction (H-surface), (ii) free with the 2 × 1 reconstruction, and either (iii) post-oxygenated or (iv) oxygenated during the growth, both with the 1 × 1 reconstruction (O-surface). All these surfaces show a downward band bending, which implies hole depletion, ranging from 0.3 eV for the H-surface to 1.2 eV for the O-surface. Only the H-surface exhibits a negative electronic affinity (−0.9 eV) whereas the others display positive electronic affinities in the range 0.9–2.2 eV. Oxygen passivates the diamond surface, giving both very low conductivity and density of states in the bandgap, and increasing the band bending. These results, obtained in homoepitaxial diamond thin films, are shown to be relevant for implementing diamond-based electronic devices.

Characterization of capacitance–voltage features of Ni/diamond Schottky diodes on oxidized boron-doped homoepitaxial diamond film

The electrical properties of Ni/diamond Schottky diodes fabricated on oxidized boron-doped homoepitaxial diamond film have been studied in order to investigate the electrical behavior of diamond-based electronic devices. The current–voltage ( I– V) characteristics of the Ni–Schottky contacts to the boron-doped homoepitaxial diamond film show excellent rectification properties. The capacitance–voltage ( C– V) features of the Ni/diamond Schottky diodes were characterized in the frequency range from 10 −3 to 2×10 5 Hz. The C– V measurements indicate that the space charge density ( N I) and built-in potential ( V d) values are approximately 6.0×10 16 cm −3 and 1.25 V, respectively, and show weak frequency dependence in the range from 10 −3 to 10 4 Hz. Capacitance–frequency measurement at zero bias indicated that the degrading capacitance at high frequency (>10 4 Hz) is primarily due to the high series resistance of the homoepitaxial diamond film.

Electrical properties of graphite/homoepitaxial diamond contact

We have studied the electrical properties of the graphitic electrodes formed in sulfur- and boron-doped homoepitaxial diamond films in order to make reliable ohmic contacts in diamond-based electronic devices. Sulfur-doped diamond films were achieved by sulfur ion implantation in undoped homoepitaxial diamond films. Boron-doped homoepitaxial diamond films were synthesized by gas-phase doping of boron during the chemical vapor deposition growth. The graphitization of diamond by Ar ion implantation was utilized for the graphitic electrodes. The electrical properties of graphite/diamond contacts were investigated by using the current–voltage measurement. The specific contact resistances were characterized by applying the linear transmission line model (TLM) and circular TLM extrapolation methods, and were determined to be 5.2×103 and 1.18 Ω·cm2, respectively.

MeV Ion Implantation Doping of Diamond.

AbstractDiamond-based semiconductor devices offer the promise of operation at high temperatures and under extreme radiation conditions. An essential step in the drive towards operational diamond-based electronic devices is the ability to controllably and reproducibly dope the diamond. Ion implantation is the method of choice for such doping because it offers precise control of the dopant concentration and spatially selective doping is achievable using standard masking techniques. However, compared to silicon, the doping of diamond is complicated by the tendency of the diamond to relax to graphite upon thermal annealing. Furthermore, even if graphitization can be avoided, the compensation of dopants by residual defects has proved in the past to be a limiting factor in obtaining very high mobility material. In this paper, we present a scheme for the effective doping of diamond using MeV ion-implantation. For MeV ion- implantation the doped layer is deeply buried under a cap of undamaged diamond, and so the scheme includes a method using pulsed laser irradiation for making electrical contact to the buried layer. We show that a boron doped layer fabricated by the MeV implantation scheme has, after suitable annealing and removal of these compensating/trapping defects, very high mobility and low compensation ratio. In fact, its electrical properties are quite similar to those of natural boron-doped type IIb diamond.