Diamond-based Electronic Devices Research Articles

Achieving ultrahigh hole mobility in hydrogen-terminated diamond via boron nitride modifications

Wide-bandgap semiconductors with high carrier mobility are in great demand for high-power radio-frequency applications. In the past decades, extensive efforts have been devoted to investigating the carrier transport properties of hydrogen-terminated diamond (H-diamond), however, achieving its high hole mobility remains a challenge, thereby limiting the development of diamond electronic devices. Herein, we propose a novel strategy to increase the hole mobility of H-diamond by boron nitride (BN) clusters modifications. Amorphous BN clusters were deposited on high-quality H-diamond surfaces using magnetron sputtering. The modified H-diamond exhibits an ultrahigh hole mobility of 1100 cm2 V−1 s−1, over 10-fold higher than that of H-diamond prior to BN modification. Moreover, the BN-modified H-diamond also exhibits outstanding high-temperature tolerance and excellent thermal stability benefitting from the passivation effect of BN. At 380 K, it still maintains a hole mobility of 385 cm2 V−1 s−1. Even after annealing at 350 K for over 8 h, there are no noticeable variations in its carrier transport properties. The hole mobility enhancing mechanism and the factors influencing carrier transport properties of the BN-modified H-diamond are discussed using first principles calculations analysis. The developed BN-modified H-diamond opens up new possibilities for diamond-based radio-frequency electronic devices operating in challenging temperature environments.

First-principles prediction of n-type diamond: novel co-doped structure using N and Be impurities

To address the limitation of diamond-based electronic devices, a comprehensive study on n-type diamond is crucial. A novel dopant structure, nitrogen–beryllium co-doping, is proposed for achieving n-type doping. The dopant structure, electronic property, synthesis route and internal strain are analyzed using first-principles density functional theory. The formation energy and ionization energy of xN–Be (x = 1–4) are compared to elucidate the distinct doping effects associated with varying numbers of N atoms. The formation energy initially decreases and then gradually increases with an incensing number of N atoms. Notably, 3N–Be and 4N–Be exhibit appealing n-type diamond properties, with low ionization energies of 0.30–0.41 eV (3N–Be) and 0.23–0.37 eV (4N–Be). To facilitate the preparation of 3/4N–Be co-doped diamond, a non-molecular synthetic route is proposed, involving the prior generation of the 3N-V and 4N-V. The 3/4N–Be co-doped diamond exhibits relatively small strain energy. Finally, this investigation highlights the potential of 3/4N–Be co-doping as excellent choices for n-type dopants, providing a greater variety of n-type structures with similar ionization energy compared to 4N–Li co-doping.

High-rate growth of single-crystal diamond with an atomically flat surface by microwave plasma chemical vapor deposition

High quality single crystal diamond builds the foundation for the applications in high-power, high-frequency electronic devices. In this paper, high-quality (100) single-crystal diamond film grown by microwave plasma chemical vapor deposition (MPCVD) was studied. By optimizing the pre-etching process before growth, where the distance between the substrate and plasma plays an important role, high-quality chemical vapor deposition (CVD) diamond layer with a smooth surface can be acquired. The root-mean-square roughness of CVD diamond surface was reduced to 0.2 nm ∼ 0.5 nm within 5 μm × 5 μm area while maintaining the growth at a high rate of 3 μm/h ∼ 30 μm/h. The sheet resistance of the CVD diamond layer was 6.1 kΩ/sq by transmission line method, indicating a high potential to fabricate high-performance hydrogen-terminal diamond-based electronic devices.

Overcoming the impact of post-annealing on uniformity of diamond (100) Schottky barrier diodes through corrosion-resistant nanocarbon ohmic contacts

Diamond-based Schottky barrier diodes (SBDs) are involved in many technological applications. In a conventional SBD fabrication process that involves interface carbide forming ohmic contacts, a post-annealing step is necessary for ohmic contacts to achieve their operational efficiency. However, this step deteriorates the essential oxygen coverage at the diamond surface which in turn affects SBDs uniformity. So, an additional oxygen termination step is necessary prior to Schottky metal deposition. In this study, a non-conventional fabrication method is introduced using corrosion-resistant nanocarbon ohmic contacts fabricated by coaxial arc plasma deposition. As a result, The SBD parameters including ideality factors and barrier heights exhibited high uniformity with a very small standard deviation for the proposed fabrication process flow when compared with process flow including a post-annealing step. Furthermore, the contact behavior of nanocarbon ohmic electrodes is investigated on a heavily boron-doped diamond film using circular transmission line model theory and a specific contact resistance of ∼10−5 Ωcm2 is obtained, suggesting the practical application of nanocarbon ohmic contacts for diamond-based electronic devices.

Local structures of phosphorus atoms implanted in crystalline diamond

Effective impurity doping into diamond by an ion implantation technique has been one of the crucial issues for realizing diamond-based high-power electronic devices. Especially for n-type impurity doping, the electrical activation has not been accomplished yet in a practically available level. In this study, local structures and depth profiles of implanted phosphorus atoms were studied by x-ray absorption spectroscopy, secondary ion mass spectroscopy, and first-principles calculations. P ion implantations were performed at two extreme substrate temperatures of room temperature and 900°C at multiple incident energies from 10 to 150 keV for flat doping and a single energy of 200 keV for δ-doping followed by activation annealing at 1300°C. The x-ray absorption spectra and the theoretical calculation showed that most of the implanted phosphorus atoms implanted with a flat doping concentration are existent in the substitutional site; however, they seem to bond with hydroxyl or vacancy complexes, probably resulting in electrical inactivation. Indeed, secondary ion mass analysis showed that a large number of O and H atoms are distributed in the P-doped layer, probably diffused from the surface through a damaged network. On the other hand, impurity diffusion was not observed in the P δ-doped sample followed by high-temperature annealing with a cap layer. It is clearly suggested that a damaged layer by ion irradiation near the surface acts as diffusion channels and trap sites of various impurities. High-temperature annealing with a cap layer is also quite effective in suppressing the formation of the defective layer that becomes a diffusion path of O and H.

A single-crystalline diamond X-ray detector based on direct sp3-to-sp2 conversed graphene electrodes

Diamond is an ultrawide bandgap semiconductor with excellent electronic and photonic properties, which has great potential applications in microelectronic and optoelectronic devices. As an allotrope of diamond, graphene also has many fantastic properties like diamond, which caught much attention in combing them together. In this work, a direct sp3-to-sp2 conversion method was proposed to fabricate graphene layers on single crystal diamond by thermal treatment with Ni film catalyst. By optimizing the conversing conditions, a thin graphene layer with low sheet resistance was obtained on diamond. Based on this, an all-carbon sandwich structural graphene-diamond-graphene (GDG) detector was fabricated, which shows low dark current of 0.45 nA at 0.5 V μm−1 applied electric field. The maximum sensitivity of this detector is obtained when the incident X-ray is 12 keV, with the value of 2.88 × 10−8 C Gy−1. Moreover, the rise time and delay time of the GDG detector is about 1.2 and 22.8 ns, respectively, which are very close to that of diamond detector with Ti/Au electrode. The realization of the direct in-situ sp3-to-sp2 conversion on diamond shows a promising approach for fabricating diamond-based all-carbon electronic devices.

Nanocarbon ohmic electrodes fabricated by coaxial arc plasma deposition for phosphorus-doped diamond electronics application

n-Type (phosphorus-doped) diamond is a promising material for diamond-based electronic devices. However, realizing good ohmic contacts for phosphorus-doped diamonds limits their applications. Thus, the search for non-conventional ohmic contacts has become a hot topic for many researchers. In this work, nanocarbon ohmic electrodes with enhanced carrier collection efficiency were deposited by coaxial arc plasma deposition. The fabricated nanocarbon ohmic electrodes were extensively examined in terms of specific contact resistance and corrosion resistance. The circular transmission line model theory was used to estimate the charge collection efficiency of the nanocarbon ohmic electrodes in terms of specific contact resistance at a specific voltage range (5–10 V); they exhibited a specific contact resistance of 1 × 10−3 Ωcm2. The result revealed one order reduction in the specific contact resistance and, consequently, a potential drop at the diamond/electrode interface compared to the conventional Ti electrodes. Moreover, the fabricated nanocarbon electrodes exhibited high mechanical adhesion and chemical inertness over repeated acid treatments. In device applications, the nanocarbon electrodes were evaluated for Ni/n-type diamond Schottky diodes, and they exhibited nearly one order enhancement in the rectification ratio and a fast charge collection at lower biasing voltages.

Monoatomic tantalum induces ordinary-pressure phase transition from graphite to n-type diamond

It is a key challenge to synthesize large-area n-type diamond, which significantly obstacles its application in electronic devices. Here we find that monoatomic tantalum (Ta) lets graphite transform to diamond under normal pressure, subverting the common sense that this transition needs high temperature and high pressure, opening an era for preparing large-area diamond based on large-sized graphite. This phase transition removes non-diamond component in grain boundaries forming closely-packed nanocrystalline diamond films and terminates the grains by Ta, producing high mobility n-type conductivity. First principle calculations reveal that Ta supplies electrons to the adjacent graphene layer, transforming sp2 electronic configuration to sp3 state with energy barrier of −5.38 eV/unit cell. Ta terminated diatomic layer diamond forms a shallow donor energy level of ∼0.4 eV. This supplies important significance in the fabrication of diamond-based electronic devices and gives a way for the sp2/sp3 hybridized matter to realize the phase transition under ordinary pressure.

Structural and electronic properties of nitrogen-terminated diamond (100) surfaces

Three types of nitrogen (N) coverage configurations of 1 ML-α, 1 ML-β and 1 ML-γ (ML: monolayer, 1 ML: one N atom per carbon atom) on diamond (100) surface are dynamically and thermally stable predicted by first-principles calculation. The electron affinities of the nitrogen-terminated diamond surfaces are positive of 3.03 eV, 2.87 eV and 2.64 eV for the cases of 1 ML-α, 1 ML-β and 1 ML-γ, in turn. The energy levels contributed from N-termination are presented in the bandgap of bulk diamond. This work provides theoretical guidance for the design and fabrication of N-terminated diamond-based electronic devices.

Nitrogen modulation of boron doping behavior for accessible n-type diamond

The n-type doping of diamond is quite difficult, hindering the development of diamond-based electronic devices for decades. In this work, we have designed a boron–nitrogen co-doping technique to realize n-type diamonds. Basically, the activation energy of the donors has been greatly reduced by around 50%, thanks to the successful synthesis of the boron–nitrogen related donor-like complex by a fine control of the synthesis condition. Compared to the sole nitrogen doping scheme, it is found that the co-incorporation of boron elements is beneficial to a lot of aspects, including better crystalline quality, faster growth, higher nitrogen solubility, and stability. With the technique, a p-i-n diamond homojunction has been fabricated. A clear rectification behavior has been recorded, demonstrating that the current co-doping technique we proposed is a feasible path to the accessible n-type diamond.

Dislocation density reduction using overgrowth on hole arrays made in heteroepitaxial diamond substrates

The growth of large-area diamond films with low dislocation density is a landmark in the fabrication of diamond-based power electronic devices or high-energy particle detectors. Here, we report the development of a growth strategy based on the use of micrometric laser-pierced hole arrays to reduce dislocation densities in heteroepitaxial chemical vapor deposition diamond. We show that, under optimal growth conditions, this strategy leads to a reduction in dislocation density by two orders of magnitude to reach an average value of 6 × 105 cm−2 in the region where lateral growth occurred, which is equivalent to that typically measured for commercial type Ib single crystal diamonds.

Selectively boron doped homoepitaxial diamond growth for power device applications

Diamond lateral growth is a powerful technique for the design and fabrication of diamond-based power electronic devices. Growth orientation affects the diamond deposition in terms of growth rate, surface roughness, and impurity incorporation. It has been shown that the finally grown surface of a patterned substrate can be predesigned based on the growth conditions. Thus, simultaneous growth along different surface orientations yields regions with different properties. In line with this, the incorporation of boron in a microwave plasma enhanced chemical vapor deposition laterally deposited epilayer over a mesa patterned {100}-oriented diamond substrate was studied by cathodoluminescence. It was observed that laterally oriented facets were highly boron doped in contrast to the {100}-oriented surfaces, which did not show any bound exciton emission, related to the doping. This study shows that, by designing the initial pattern and tuning the conditions, it is possible to drive a selective incorporation of boron into the grown layer.

Hydrogenation of diamond nanowire surfaces for effective electrostatic charge storage.

We report a novel versatile method for writing charged areas on diamond nanowire (DNW) surfaces using an atomic force microscopy (AFM) tip. Transmission electron microscopy (TEM) investigations revealed the existence of abundant plate-like diamond aggregates, which were encased in layers of graphite, forming nano-sized diamond-graphite composites (DGCs) on DNW surfaces. These DGCs are the main feature, acting as charge-trapping centers and storing electrostatic charge. A hydrogenation process has been observed effectively enhancing the charge-trapping properties of these DNW materials. The effective charge trapping properties with hydrogenation are ascribed to the disintegration of the DGCs into smaller pieces, with an overall increase in the metallic nanographitic phase fractions in a dielectric diamond matrix. Moreover, the written charge on the surface can be easily modified, re-written, or completely erased, enabling application in diamond-based re-writable electronic devices. However, excessive hydrogenation degrades the charge-trapping properties, which is attributed to the etching of the DGCs from the surface. This study demonstrates the potential importance of a simple hydrogenation process in effective electrostatic charge trapping and storage for diamond related nanocarbon materials and the role of DGCs to further enhance it.

Surface Oxygen Adsorption and Electric Property of Hydrogen-Terminated Single Crystal Diamonds by UV/ozone Treatment**Supported by the National Natural Science Foundation of China under Grant Nos. 51672102 and 51972135.

Surface terminations of diamond play an important role in determining the electric properties of diamond-based electronic devices. We report an ultraviolet/ozone (UV/ozone) treatment process on hydrogen-terminated single crystal diamond (H-diamond) to modulate the carrier behavior related to varying oxygen adsorption on surfaces. By UV/ozone treatments, the induced oxygen radicals are chemically adsorbed on the H-terminated diamond and replace the original adsorbed H, which is analyzed by x-ray photoelectron spectroscopy. The concentration of oxygen adsorbed on surface increases from ∼3% to ∼8% with increasing the ozone treatment time from 20 s to 600 s. It is further confirmed by examining the wettability properties of the varying diamond surfaces, where the hydrophobic for H-termination transfers to hydrophilic for partly O-termination. Hall effect measurements show that the resistance (hole mobility) of the UV/ozone-treated H-diamond continuously increases (decrease) by two orders of magnitude with increasing UV/ozone treatment time from 20 s to 600 s. The results reveal that UV/ozone treatment becomes an efficient method to modulate the surface electrical properties of H-diamonds for further investigating the oxygenation effect on two-dimensional hole gas based diamond devices applied in some extreme environments.

H-Terminated Diamond Surface Band Bending Characterization by Angle-Resolved XPS

Concerning diamond-based electronic devices, the H-terminated diamond surface is one of the most used terminations as it can be obtained directly by using H2 plasma, which also is a key step for diamond growth by chemical vapour deposition (CVD). The resultant surfaces present a p-type surface conductive layer with interest in power electronic applications. However, the mechanism for this behavior is still under discussion. Upward band bending due to surface transfer doping is the most accepted model, but has not been experimentally probed as of yet. Recently, a downward band bending very near the surface due to shallow acceptors has been proposed to coexist with surface transfer doping, explaining most of the observed phenomena. In this work, a new approach to the measurement of band bending by angle-resolved X-ray photoelectron spectroscopy (ARXPS) is proposed. Based on this new interpretation, a downward band bending of 0.67 eV extended over 0.5 nm was evidenced on a (100) H-terminated diamond surface.

Structural and electronic properties of two-dimensional atomically thick (100) diamond nanofilms by first-principles calculations

In this work, the structural and electronic properties of two-dimensional (2D) atomically thick (100) diamond nanofilms are investigated by first-principles calculations. The results of phonon dispersion curves and ab initio molecular dynamics simulations indicate that the nanofilms are dynamically and thermally stable with three types of reconstructed surfaces (named 5-MR, 5-7-MR, and 5/5-7-MR). The bandgaps (Eg) of the nanofilms with 5-MR and 5-7-MR patterns are in regions of 1.02–1.40 eV and 0.32–0.55 eV, respectively, and an oscillatory phenomenon of Eg appears related to parity of the layer number. The nanofilms with the 5/5-7-MR pattern show a metallic feature. The variations in Eg are determined by surface states with different configurations. These novel diamond-based structures may be useful for applications such as 2D semiconductors in diamond-based electronic devices.

Boron–oxygen complex yields n-type surface layer in semiconducting diamond

Diamond is a wide-bandgap semiconductor possessing exceptional physical and chemical properties with the potential to miniaturize high-power electronics. Whereas boron-doped diamond (BDD) is a well-known p-type semiconductor, fabrication of practical diamond-based electronic devices awaits development of an effective n-type dopant with satisfactory electrical properties. Here we report the synthesis of n-type diamond, containing boron (B) and oxygen (O) complex defects. We obtain high carrier concentration (∼0.778 × 1021 cm-3) several orders of magnitude greater than previously obtained with sulfur or phosphorous, accompanied by high electrical conductivity. In high-pressure high-temperature (HPHT) boron-doped diamond single crystal we formed a boron-rich layer ∼1-1.5 μm thick in the {111} surface containing up to 1.4 atomic % B. We show that under certain HPHT conditions the boron dopants combine with oxygen defects to form B-O complexes that can be tuned by controlling the experimental parameters for diamond crystallization, thus giving rise to n-type conduction. First-principles calculations indicate that B3O and B4O complexes with low formation energies exhibit shallow donor levels, elucidating the mechanism of the n-type semiconducting behavior.

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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.