Band Gap Of Diamond Research Articles

Picometer-Scale Atomic Shifts Governing Subdisordered Structures in Diamond.

Diamond is considered the most promising next-generation semiconductor material due to its excellent physical characteristics. It has been more than three decades since the discovery of a special structure named n-diamond. However, despite extensive efforts, its crystallographic structure and properties are still unclear. Here, we show that subdisordered structures in diamond provide an explanation for the structural feature of n-diamond. Monocrystalline diamond with subdisordered structures is synthesized via the chemical vapor deposition method. Atomic-resolution scanning transmission electron microscopy characterizations combined with the picometer-precision peak finder technology and diffraction simulations reveal that picometer-scale shifts of atoms within cells of diamond govern the subdisordered structures. First-principles calculations indicate that the bandgap of diamond decreases rapidly with increasing shifting distance, in accordance with experimental results. These findings clarify the crystallographic structure and electronic properties of n-diamond and provide new insights into the bandgap adjustment in diamond.

Depth profiling of microwave nitrogen-terminated polycrystalline diamond surfaces by energy-dependent X-ray photoelectron spectroscopy

Nitrogen-terminated diamonds hold promise for stabilizing near-surface NV− centers, which is essential for reliable quantum sensing. Among various surface preparation methods, microwave (MW) nitrogen plasma, known for its minimal surface damage, appears as the most effective choice. In this investigation, we explore the nature of nitrogen bonding of polycrystalline diamond (PCD) surfaces exposed to MW nitrogen plasma using X-ray Photoelectron Spectroscopy (XPS) depth profiling and Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy at the C K- and N K-edges. XPS depth profiling with atomic resolution, achieved by varying the probing photon energy using synchrotron radiation and supported by a physical model considering the associated inelastic mean free path, suggests a surface of almost fully saturated nitrogen in two main bonding configurations of similar contribution and a low coverage of ∼5 % of graphene-like islands residing atop the nitrogen-terminated diamond surface. The thermal stability of these surface groups is monitored by in situ annealing up to 700 °C. The depth profiles reveal that nitrogen atoms do not diffuse in the diamond crystal, resulting in excellent diamond crystallinity in the first atomic planes below the surface. C K-edge NEXAFS analysis reveals the position of unoccupied surface states within the diamond bandgap, opening new perspective on the stabilization of near-surface NV− centers.

Phonon stability boundary and deep elastic strain engineering of lattice thermal conductivity

Recent studies have reported the experimental discovery that nanoscale specimens of even a natural material, such as diamond, can be deformed elastically to as much as 10% tensile elastic strain at room temperature without the onset of permanent damage or fracture. Computational work combining ab initio calculations and machine learning (ML) algorithms has further demonstrated that the bandgap of diamond can be altered significantly purely by reversible elastic straining. These findings open up unprecedented possibilities for designing materials and devices with extreme physical properties and performance characteristics for a variety of technological applications. However, a general scientific framework to guide the design of engineering materials through such elastic strain engineering (ESE) has not yet been developed. By combining first-principles calculations with ML, we present here a general approach to map out the entire phonon stability boundary in six-dimensional strain space, which can guide the ESE of a material without phase transitions. We focus on ESE of vibrational properties, including harmonic phonon dispersions, nonlinear phonon scattering, and thermal conductivity. While the framework presented here can be applied to any material, we show as an example demonstration that the room-temperature lattice thermal conductivity of diamond can be increased by more than 100% or reduced by more than 95% purely by ESE, without triggering phonon instabilities. Such a framework opens the door for tailoring of thermal-barrier, thermoelectric, and electro-optical properties of materials and devices through the purposeful design of homogeneous or inhomogeneous strains.

Quantum Vibronic Effects on the Excitation Energies of the Nitrogen-Vacancy Center in Diamond.

We investigated the impact of quantum vibronic coupling on the electronic properties of solid-state spin defects using stochastic methods and first-principles molecular dynamics with a quantum thermostat. Focusing on the negatively charged nitrogen-vacancy center in diamond as an exemplary case, we found a significant dynamic Jahn-Teller splitting of the doubly degenerate single-particle levels within the diamond's band gap, even at 0 K, with a magnitude exceeding 180 meV. This pronounced splitting leads to substantial renormalizations of these levels and, subsequently, of the vertical excitation energies of the doubly degenerate singlet and triplet excited states. Our findings underscore the pressing need to incorporate quantum vibronic effects into first-principles calculations, particularly when comparing computed vertical excitation energies with experimental data. Our study also reveals the efficiency of stochastic thermal line sampling for studying phonon renormalizations of solid-state spin defects.

Precision sub-monolayer manipulation of diamond surface chemistry using laser direct write oxidation in air

Manipulation and patterning of diamond surface chemistry is of interest for a wide range of diamond-based technologies. We report the patterned oxidation of hydrogen-terminated diamond surfaces with sub-monolayer (ML) precision by a deep-UV two-photon process performed in air. Using focused laser pulses of photon energy 4.66 eV (266 nm; below the diamond bandgap of 5.47 eV), hydrogen-terminated (001) surfaces were exposed with calibrated doses to remove carbon with a precision of 0.02 ML. The measurement of the electrical properties of the laser-exposed zone between ohmic electrodes enabled monitoring of the transition from a conducting H-terminated surface to insulating O-terminated. The surface resistance increases by more than 7 orders of magnitude for doses corresponding to 0.5 ML, and the I–V characteristics show a transition from linear to nonlinear for doses above 0.30 ML. We show that this behavior agrees well with a surface percolation model for carrier diffusion in which the laser etch rate for the H-terminated top layer is the same as for O-terminated. Hence, this work reveals an ultra-precise method for modifying the sub-monolayer surface chemistry with the practical advantages of a laser-induced mechanism compared to conventional plasma or chemical processing methods.

Rare isotope-containing diamond colour centres for fundamental symmetry tests.

Detecting a non-zero electric dipole moment in a particle would unambiguously signify physics beyond the Standard Model. A potential pathway towards this is the detection of a nuclear Schiff moment, the magnitude of which is enhanced by the presence of nuclear octupole deformation. However, due to the low production rate of isotopes featuring such 'pear-shaped' nuclei, capturing, detecting and manipulating them efficiently is a crucial prerequisite. Incorporating them into synthetic diamond optical crystals can produce defects with defined, molecule-like structures and isolated electronic states within the diamond band gap, increasing capture efficiency, enabling repeated probing of even a single atom and producing narrow optical linewidths. In this study, we used density functional theory to investigate the formation, structure and electronic properties of crystal defects in diamond containing [Formula: see text], a rare isotope that is predicted to have an exceptionally strong nuclear octupole deformation. In addition, we identified and studied stable lanthanide-containing defects with similar electronic structures as non-radioactive proxies to aid in experimental methods. Our findings hold promise for the existence of such defects and can contribute to the development of a quantum information processing-inspired toolbox of techniques for studying rare isotopes. This article is part of the Theo Murphy meeting issue 'Diamond for quantum applications'.

Computational design of a reliable intermediate-band photovoltaic absorber based on diamond.

To reduce the wide bandgap of diamond and expand its applications in the photovoltaic fields, a diamond-based intermediate-band (IB) material C-Ge-V alloy was designed by first-principles calculations. By replacing some C with Ge and V in the diamond, the wide bandgap of the diamond can be reduced sharply and a reliable IB, which is mainly formed by the d states of V, can be formed in the bandgap. With the increase of Ge content, the total bandgap of the C-Ge-V alloy will be reduced and close to the optimal value of an IB material. At a relatively low atomic concentration of Ge (below 6.25%), the IB formed in the bandgap is partially filled and varies little with the concentration of Ge. When further increasing the content of Ge, the IB moves close to the conduction band and the electron filling in the IB increases. The 18.75% content of Ge might be the limitation to form an IB material, and the optimal content of Ge should be between 12.5% and 18.75%. Compared with the content of Ge, the distribution of Ge has a minor effect on the band structure of the material. The C-Ge-V alloy shows strong absorption for the sub-bandgap energy photons, and the absorption band generates a red-shift with the increase of Ge. This work will further expand the applications of diamond and be helpful to develop an appropriate IB material.

Improving the Performance of HPHT-Diamond Detectors for Pulsed X-Ray Dosimetry Using the Synchronous Detection Technique

A single-crystal diamond sample grown by high-pressure high-temperature (HPHT) technique was used for the fabrication of a metal–semiconductor–metal (MSM) photoconductor. The sample quality was evaluated by means of spectral photocurrent measurements highlighting the presence of a significant density of defect states within the diamond bandgap, responsible for trap-mediated conduction mechanisms. The photoconductor was fully characterized under 6 MeV pulsed X-rays, sourced by a medical linear accelerator (LINAC), in the 0.05–20 Gy and 0.017–0.100 Gy/s dose (D) and dose-rate (DR) ranges, respectively. Photocurrent measurements performed with a conventional precision electrometer showed that the detector performance is strongly affected by charge-trapping phenomena, resulting in a sublinear dependence with the DR (power-law dependence with an exponent of 0.86). Measurements were repeated in the same experimental conditions by coupling the detector to a specifically developed gated integrator, allowing for a synchronous integration of photocurrent pulses (limited to 40 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <inline-formula> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> </i> s) centered on each impinging X-ray pulse. In this case, the detector photoresponse showed an excellent linearity with both the D and the DR (power-law dependence with 1.0009 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm$</tex-math> </inline-formula> 0.0004 and 1.009 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm$</tex-math> </inline-formula> 0.005 exponents, respectively). Significantly, the proposed synchronous integration technique is then able to mitigate the detrimental effect that defects have on detector performance. The proposed method paves, therefore, the way to the exploitation of HPHT diamond as a low-cost alternative to single-crystal chemical vapor deposition (CVD)-diamond for the fabrication of accurate and linear X-ray dosimeters.

High-Quality SiO2/O-Terminated Diamond Interface: Band-Gap, Band-Offset and Interfacial Chemistry.

Silicon oxide atomic layer deposition synthesis development over the last few years has open the route to its use as a dielectric within diamond electronics. Its great band-gap makes it a promising material for the fabrication of diamond-metal-oxide field effects transistor gates. Having a sufficiently high barrier both for holes and electrons is mandatory to work in accumulation and inversion regimes without leakage currents, and no other oxide can fulfil this requisite due to the wide diamond band-gap. In this work, the heterojunction of atomic-layer-deposited silicon oxide and (100)-oriented p-type oxygen-terminated diamond is studied using scanning transmission electron microscopy in its energy loss spectroscopy mode and X-ray photoelectron spectroscopy. The amorphous phase of silicon oxide was successfully synthesized with a homogeneous band-gap of 9.4 eV. The interface between the oxide and diamond consisted mainly of single- and double-carbon-oxygen bonds with a low density of interface states and a straddling band setting with a 2.0 eV valence band-offset and 1.9 eV conduction band-offset.

Tunable band gap of diamond twin boundaries by strain engineering

Diamond thin films are promising wide band gap semiconductor materials for applications in electronic and microwave devices. Revealing the mechanism of how twin boundaries impact the band gap of diamond is of critical importance since they are the most common defects in polycrystalline diamond films. Here, nanocrystalline diamond films are synthesized by microwave plasma-enhanced chemical vapor deposition. The atomic and electronic structures of diamond lamellar and fivefold twins, and their evolution behaviors with the increase of axial tensile strain, are investigated by combining aberration-corrected transmission electron microscopy with first-principles calculations. There is no intrinsic stress concentration at the lamellar twin boundary, and its band gap equals to that of the bulk diamond ( i.e. , 5.3 eV). An intrinsic in-plane compressive stress field is formed and the band gap is increased evidently ( i.e. , 0.6 eV) in the center of fivefold twins. When applying an axial tensile strain up to 15%, the band gaps of the bulk diamond, lamellar and fivefold twins reduce significantly to 2.2 eV, 2.1 eV and 2.4 eV, respectively, which are mainly due to the decrease of p z orbital energy caused by the increase of axial bond length during the tensile process. Under the same axial tensile strain, the band gap of the fivefold twin is always larger than those of the lamellar twin and the bulk diamond due to the formation of in-plane five-membered carbon rings in the center. The findings of the tunable band gap by strain engineering will benefit for the innovation and design of advanced diamond functional devices. • Atomic and electronic structures of diamond lamellar and fivefold twins under tensile strains have been investigated. • The band gap of fivefold twins is evidently larger than that of the bulk diamond. • Under axial tensile strains, the band gap of bulk diamond, lamellar and fivefold twins reduces significantly. • The reduction of band gap is due to the decrease of p z orbital energy caused by the increase of axial bond length.

A Novel Insight into the Strain Effect of T2g Mode Splitting and Bandgap in Diamond

Herein, a comprehensive analysis of the change in the bandgap and Raman frequency of diamond under the uniaxial strain along the [100], [110] and [111] directions using first‐principles calculation is presented. The results demonstrate that the triple degenerate T2g mode splitting occurs spontaneously, owing to the strain‐induced symmetry breaking. Phonon splitting into two or three different modes demonstrates its anisotropy characteristics. The tensile softening and compressive stiffening of Raman frequencies depend on the bond length, bond angle, and bond strength response to the uniaxial strain. The bandgap decreases with increasing whether the tensile or compressive strain, which is attributed to the strain‐induced change of bond identities. These findings demonstrate that the uniaxial strain engineering has potential application in future diamond electronic devices.

High quality SiO2/diamond interface in O-terminated p-type diamond MOS capacitors

Metal oxide semiconductor (MOS) capacitors were fabricated based on oxygen-terminated p-type (100) oriented diamond and SiO2 grown by atomic layer deposition. A detailed electrical characterization consisting of I–V, C–V, and C–F was performed in order to analyze the electrical properties of the structure. The MOS capacitor presented no detectable leakage current in forward and very low leakage current in reverse sustaining at least 6 MV/cm without degradation. The C–V measurements showed depletion and deep depletion regimes in forward and accumulation regimes in reverse, with a low density of interface states of ∼1011 cm−2 along the diamond bandgap. The latter results were further validated by conductance and capacitance vs frequency measurements.

Current Transient Spectroscopic Study of Vacancy Complexes in Diamond Schottky p-i-n Diode

The present state-of-the-art of junction-based diamond electronic devices is limited by the challenges inherent to the synthesis of n-type layers, where the dopant itself forms defect complexes that are donor compensating centers. Recently, the fabrication of diamond Schottky p-i-n diodes (SPINDs) by chemical vapor deposition (CVD) of intrinsic diamond films followed by an n-type layer on p-type diamond substrates has been reported to demonstrate excellent diode-like behavior with high rectification factors and surface barrier height at elevated temperatures. However, due to the ultrawide bandgap of diamond and small contact area of the devices, it is challenging to characterize the defect centers that limit the performance of the SPINDs using conventional capacitance transient spectroscopy. In this article, we report the characterization of defect centers in the diamond SPINDs with a contact area of 280 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}\,\,\times \,\,280\,\,\mu \text{m}$ </tex-math></inline-formula> using current transient spectroscopy (CTS), which measures thermally induced emission rates from trap centers through change in device current instead of junction capacitance. The CTS scans in the temperature range 80–800 K revealed the presence of five trap-related peaks. Results from density functional theory (DFT) calculations were used to identify the nature of the traps. While most of the detected traps were identified to be vacancy complexes coupled to phosphorus and hydrogen impurities, one trap has been identified as single carbon vacancy. Most of the phosphorus-related defect complexes were observed to be in neutral and/or negative charge states and were identified as acceptor-like states.

Mechanism of Reverse Leakage Current in Schottky Diode Fabricated on Large Bandgap Semiconductors like Ga2O3 or Diamond Part II

The bandgap of Ga2O3 (4.5-4.9 eV) is larger than the bandgap of GaN (3.4 eV). In addition, single crystal bulk Ga2O3 wafers can be more easily manufactured than GaN wafers. Therefore, Ga2O3 has strong potential for applications in high power semiconductor devices [1]. Schottky diodes fabricated on n-type Ga2O3 have strong potential as fast high-power switching devices. Similarly, the bandgap of diamond (5.5 eV) is very large and diamond Schottky diodes have good potential.One possible mechanism of reverse leakage current in Schottky diodes is image force barrier lowering at the metal-semiconductor interface. Historically, there were 2 theories regarding the image force barrier lowering effect. In 1953, Krömer published his theory that the image force dielectric constant in the equation for Schottky emission should be equal to 1 [2]. Subsequently in 1964, Sze et al. published their theory that the image force dielectric constant in the equation for Schottky emission should be equal to n2 [3], where n is the refractive index of the semiconductor in the infrared or visible light range. In 1969, Sze published a book which has influenced many scientists [4]. Sze’s theory [3]-[4] quickly became the dominating theory whereas Krömer’s theory essentially became a forgotten theory. In 2020, the author pointed out that Krömer’s theory is quite frequently more compatible with experimental results for Ga2O3 or diamond Schottky diodes [5]. In 2021, the author attempted to propose a new theory involving the concept of electron velocity overshoot to unify Krömer’s theory and Sze’s theory, as shown in Fig. 1 [6]; Krömer’s theory is better than Sze’s theory for high reverse bias voltage.In conclusion, the author pointed out that it is necessary to resurrect an old and forgotten theory from Krömer in order to explain the experimental data on the reverse leakage current of Schottky diodes fabricated on large bandgap semiconductors like Ga2O3 and diamond, etc. A theoretical basis based on quasi-ballistic transport will be provided.

Transient surface photovoltage spectroscopy of diamond

Contactless and highly sensitive probing of electronic transitions in diamond over a wide spectral range from near infrared to deep ultraviolet is still challenging. Surface photovoltage (SPV) signals depend on electronic transitions and transport phenomena leading to charge separation in space and allow for a contactless study of electronic transitions. Here, transient SPV spectroscopy in an arrangement with a charge amplifier and a laser tunable over a wide range was applied to study an undoped diamond single crystal between 0.8 and 5.9 eV at room temperature in ambient air. SPV transients were measured without and with weak visible bias light, which allowed for suppression of possible parasitic contributions in SPV signals not related to diamond and distinction of processes of charge separation that were independent of band bending. Transitions at 1.0 and 3.1 eV led to preferential separation of photogenerated holes toward the surface. In contrast, a transition at 1.8 eV caused preferential separation of photogenerated electrons toward the surface. Transitions near the indirect bandgap of diamond were observed at 5.27, 5.32, 5.48, and 5.53 eV and could be assigned to absorption assisted (i) by an indirect exciton and absorption of longitudinal optical and acoustic phonons, (ii) by absorption of transverse acoustic phonons, (iii) by emission of transverse acoustic phonons, and (iv) by emission of longitudinal optical and acoustic phonons, respectively. Charge separation under excitation at 5.27 eV was caused by directed charge transfer at/near the diamond surface after exciton diffusion followed by exciton dissociation.

Revisiting the electronic nature of nanodiamonds

Nanodiamonds, commonly described as fragments of diamond, have been theoretically found to have lower HOMO-LUMO energy splitting compared to the bandgap of bulk diamond. This apparent lack of correlation between theory and experiment is caused by the position of the LUMO, which is placed in the surface of the ND. An eventual enlargement of the ND towards a macroscopic size will turn the LUMO into the unoccupied surface states, which are not accounted if the bandgap of a bulk material is measured. Here, the electron structure of the nanodiamonds is evaluated, demonstrating that due their nature they should be described as discrete systems instead of bulk materials. Hence, the word bandgap should be avoided in the case of the nanodiamonds, using HOMO-LUMO gap instead. Additionally, our obtained ionization potentials show a satisfactory degree of correlation with the experiment, while the electron affinities are found to be positive. Although this feature fits the estimation performed from experimental data, it opposes the generally accepted idea of a negative electron affinity for hydrogenated nanodiamonds. The present article clarifies common misunderstandings regarding the electronic nature of the NDs, and provides some guidelines for the correct computation of this systems. Finally, as a helpful tool, an estimation of the content of carbon atoms and its surface to volume ratio is provided starting from the diamond unit cell.

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.

Strain could switch on diamond for optoelectronics

• Manipulation of diamond’s large bandgap by doping is a challenge. • Strain engineering is an alternative means of modulating the bandgap in single-crystal diamond. • Uniform and well-controlled straining of diamond bridge structures has been demonstrated. • Large strains up to 9.7% were achieved in diamond bridges, approaching the theoretical limit. • Strain engineering should enable modulation of diamond’s bandgap from 5 eV to 3 eV.

Effect of Deep-Defects Excitation on Mechanical Energy Dissipation of Single-Crystal Diamond.

The ultrawide band gap of diamond distinguishes it from other semiconductors, in that all known defects have deep energy levels that are less active at room temperature. Here, we present the effect of deep defects on the mechanical energy dissipation of single-crystal diamond experimentally and theoretically up to 973K. Energy dissipation is found to increase with temperature and exhibits local maxima due to the interaction between phonons and deep defects activated at specific temperatures. A two-level model with deep energies is proposed to explain well the energy dissipation at elevated temperatures. It is evident that the removal of boron impurities can substantially increase the quality factor of room-temperature diamond mechanical resonators. The deep energy nature of the defects bestows single-crystal diamond with outstanding low intrinsic energy dissipation in mechanical resonators at room temperature or above.

Novel Si-C compounds with semiconducting and metallic properties: A DFT study

A set of novel silicon-carbon alloys with chemical formula hSi6-xCx (x=1,2,⋯,6) is proposed. The structural stability of the optimized geometries was assessed using different methods, and the mechanical (elastic constants), electronic (Kohn-Sham band structures, partial densities of states, carrier effective masses) and optical properties of the stable crystals were obtained using the GGA-PBE and the hybrid HSE06 exchange–correlation functionals. In particular, the hC6 crystal exhibits a wide direct band gap of 3.29 eV (near ultraviolet), smaller than the band gap of diamond (5.5 eV) with comparable elastic constants and, together with other direct/nearly direct gap hSi6-xCx alloys covering the energy range between 0.43 and 2.42 eV (infrared to green light), are potentially useful for optoelectronic applications. Surprisingly, the theoretical calculations predict a metallic character for one of the hSi3C3 compounds. One can speculate that a combination of these materials could be used to produce light-emitting devices covering the full range between the near infrared and the near ultraviolet wavelengths, as well as for charge transport applications.