Single Crystal Diamond Substrates Research Articles

145-MW/cm2 Heteroepitaxial Diamond MOSFETs With NO2 p-Type Doping and an Al2O3 Passivation Layer

In this study, we investigated diamond metal oxide semiconductor field effect transistors (MOSFETs) with NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> p-type doping and Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> passivation layer fabricated on a high-quality heteroepitaxial single crystal (001) diamond substrate called Kenzan diamond <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">®</sup> . MOSFETs with a gate length of 1.4 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> and nearly zero source-gate spacing exhibited a high drain current density of −776 mA/mm with a negligible gate leakage current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$&lt; 0.1~\mu \text{A}$ </tex-math></inline-formula> /mm). MOSFETs with a gate-to-drain length of 4.8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> delivered a high off-state breakdown voltage (−618 V) at an average breakdown field of 1.2 MV/cm and a specific on-resistance of 2.63 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}\Omega \cdot $ </tex-math></inline-formula> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Baliga’s Figure-Of-Merits was calculated as 145 MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and the anticipated maximum power density was 12 W/mm. The diamond MOSFET was improved with high crystal quality.

High-Q diamond microresonators in the long-wave infrared.

High quality factor (Q) photonic devices in the room temperature thermal infrared region, corresponding to deeper long-wave infrared with wavelengths beyond 9 microns, have been demonstrated for the first time. Whispering gallery mode diamond microresonators were fabricated using single crystal diamond substrates and oxygen-based inductively coupled plasma (ICP) reactive ion etching (RIE) at high angles. The spectral characteristics of the devices were probed at room temperature using a tunable quantum cascade laser that was free space-coupled into the resonators. Light was extracted via an arsenic selenide (As2Se3) chalcogenide infrared fiber and directed to a cryogenically cooled mercury cadmium telluride (HgCdTe) detector. The quality factors were tested in multiple microresonators across a wide spectral range from 9 to 9.7 microns with similar performance. One example resonance (of many comparables) was found to reach 3648 at 9.601 µm. Fourier analysis of the many resonances of each device showed free spectral ranges slightly greater than 40 GHz, matching theoretical expectations for the microresonator diameter and the overlap of the whispering gallery mode with the diamond.

Integration of polycrystalline Ga2O3 on diamond for thermal management

Gallium oxide (Ga2O3) has attracted great attention for electronic device applications due to its ultra-wide bandgap, high breakdown electric field, and large-area affordable substrates grown from the melt. However, its thermal conductivity is significantly lower than that of other wide bandgap semiconductors such as SiC, AlN, and GaN, which will impact its ability to be used in high power density applications. Thermal management in Ga2O3 electronics will be the key for device reliability, especially for high power and high frequency devices. Similar to the method of cooling GaN-based high electron mobility transistors by integrating it with high thermal conductivity diamond substrates, this work studies the possibility of heterogeneous integration of Ga2O3 with diamond for the thermal management of Ga2O3 devices. In this work, Ga2O3 was deposited onto single crystal diamond substrates by atomic layer deposition (ALD), and the thermal properties of ALD-Ga2O3 thin films and Ga2O3–diamond interfaces with different interface pretreatments were measured by Time-domain Thermoreflectance. We observed a very low thermal conductivity of these Ga2O3 thin films (about 1.5 W/m K) due to the extensive phonon grain boundary scattering resulting from the nanocrystalline nature of the Ga2O3 film. However, the measured thermal boundary conductance (TBC) of the Ga2O3–diamond interfaces is about ten times larger than that of the van der Waals bonded Ga2O3–diamond interfaces, which indicates the significant impact of interface bonding on TBC. Furthermore, the TBC of the Ga-rich and O-rich Ga2O3–diamond interfaces is about 20% smaller than that of the clean interface, indicating that interface chemistry affects the interfacial thermal transport. Overall, this study shows that a high TBC can be obtained from strong interfacial bonds across Ga2O3–diamond interfaces, providing a promising route to improving the heat dissipation from Ga2O3 devices with lateral architectures.

Single crystal diamond gain mirrors for high performance vertical external cavity surface emitting lasers

We report on the design, fabrication and optical performance of gain mirrors in single crystal diamond substrates for vertical external cavity surface emitting lasers (VECSELs). VECSELs have gained attention recently due to their potential for high emission power in single mode with low beam divergence, yet their maximum output power remains typically limited due to thermal roll-over resulting from insufficient heat dissipation. In order to increase the heat transfer, we exploit the excellent thermal conductivity of single crystal diamond, which is assembled in direct contact with the active structure. The optical cavity is hereby defined by an output coupler and a high reflection grating structure etched into the diamond surface. We here present the design and microfabrication of a diffraction grating that was optimized to reflect light into the 0th order, therefore combining the role of a gain mirror and a heatsink at the same time. Our process involved metal mask deposition onto the diamond surface, e-beam lithography and reactive ion etching. Characterization showed reflection above 95% at a center wavelength of 1550 nm, potentially allowing the integration of the diamond mirror into a vertical external cavity surface emitting laser.

Модификация соотношения sp^2/sp^3-гибридного углерода в PECVD пленках DLC

Abstract It is known that diamond-like carbon layers consist of carbon components with sp ^2 (graphite) and sp ^3 (diamond) hybridizations of electron orbitals. The quantitative ratio between sp ^2 and sp ^3 components has a profound effect on the structural, morphological, optical, electrical, and mechanical properties of the films. In this study, the possibility of controlling the fractions of sp ^2- and sp ^3-hybridized carbon in diamond-like films produced by plasma-enhanced chemical-vapor deposition onto single-crystal silicon and diamond substrates is analyzed. In-situ methods of controlling the fraction of the sp ^3 component by varying the power of the capacitive and inductively coupled discharges directly during production of the film and ex-situ methods, in which use is made of thermal annealing and the application of an electric field, are demonstrated.

Surface Morphology of the Interface Junction of CVD Mosaic Single-Crystal Diamond.

The diamond mosaic grown on the single-crystal diamond substrates by the microwave plasma chemical vapor deposition (MPCVD) method has been studied. The average growth rate was about 16–17 μm/h during 48 hours’ growth. The surface morphologies of the as-grown diamond layer were observed. It was found that the step flow was able to move across the substrates and cover the junction interface. Raman spectroscopic mapping in the central area of the junction revealed the high stress region movement across the junction interface from one substrate to the other for about 200–400 μm. High-resolution X-ray diffractometry (HRXRD) results proved that the surface step flow movement direction had nothing to do with the off-axis directions of the original substrates. It was found that the surface height difference of substrate was the main driving force for the step flow movement, junction combination and surface morphology changing. The mechanism of the mosaic interface junction combination and step flow transformation on the mosaic surface was proposed.

High-precision mechanical polishing method for diamond substrate using micron-sized diamond abrasive grains

To remove subsurface damage and surface irregularities, we demonstrated a high-precision mechanical polishing technique with a polishing plate charged with micron-sized diamond abrasive grains. First, we prepared a polishing plate with a uniform protrusion height of the diamond abrasive grains and evaluated its geometrical structure with a scanning laser microscope. Subsequently, to determine the influence of the protrusion height of the diamond abrasive grains on polishing performance, we performed the mechanical polishing of a single-crystal diamond (100) substrate using two types of polishing plates. The obtained results showed that, using our mechanical polishing technique, the polishing plate with a uniform protrusion height of the grains could be fabricated and a high surface quality of the diamond substrate could be achieved. Moreover, the polishing plate significantly influenced the surface roughness of the polished diamond surfaces. During mechanical polishing, an average roughness (Ra) ranging from ~0.1 nm to 0.3 nm was obtained easily by controlling the process parameters.

Defect and Threading Dislocations in Single Crystal Diamond: A Focus on Boron and Nitrogen Codoping

For the fabrication of vertical power electronic components, the use of a freestanding highly boron‐doped single crystal diamond substrate is mandatory, on top of which a thin low‐doped active layer is overgrown. The crystalline quality of this substrate is of prime importance as the high doping level required is also likely to lead to the generation of extended defects in the active layer. Herein, the improvement of the quality of thick highly boron‐doped diamond films grown by chemical vapor deposition (CVD) is investigated. Codoping with a low amount of nitrogen is explored as a way to limit the formation and propagation of extended defects. It is shown that the growth rate and boron concentration are not impacted by the addition of nitrogen with the parameters used in this study. Surface morphology and defect density estimated for each amount of nitrogen are presented; and Raman spectroscopy confirms that nitrogen addition compensates the strain induced by boron incorporation in single crystal diamond. Codoping with nitrogen is thus demonstrated as a useful strategy to obtain high quality boron‐doped diamond substrates potentially suitable for the development of power devices.

Fabrication of hundreds of microns three-dimensional single crystal diamond channel along with high aspect ratio by two-step process

Abstract A hundreds of microns three-dimensional single crystal diamond (SCD) channel along with high aspect ratio has been fabricated successfully by two-step process. Firstly, laser machining technique was used to produce trenches in high-temperature high-pressure SCD substrate. Then, microchannels were achieved by epitaxial lateral overgrowth of diamond layer on patterned substrate by microwave plasma chemical vapor deposition system. Dimensional variations and crystalline quality around the microchannel cross section were analyzed using scanning electron microscopy and Raman spectroscopy. The hollowness and continuity of microchannels were also be tested. It is a significant step towards fabrication of hundreds of microns SCD channel along with controlled high aspect ratio by a simple and controllable way.

Precision Polishing of Single Crystal Diamond (111) Substrates Using a Sol-Gel (SG) Polishing Pad

In recent years, the application prospect of single crystal diamond (SCD) in the field of electronics is remarkable, and material processing technologies are the basis for the application of semiconductor materials. In this paper, a novel polishing method for SCD substrates was developed utilizing a sol-gel (SG) polishing pad, which is a semi-fixed abrasive polishing tool fabricated by SG technique. The surface morphology of SCD substrates was observed with a 3-D optical profiler and scanning electron microscopy (SEM). The results showed that the surface roughness was 1.32 nm Ra and the material removal rate of SCD (111) was about 120 nm/h. The polished diamond surfaces and wear debris were investigated using Raman spectroscopy and transmission electron microscopy (TEM), respectively. The results showed the removal mechanism of SCD using an SG polishing pad was mainly dominated by the mechanical removal and phase transformation, including the mechanical cleavage, amorphization, and graphitization.

Micro graphite-patterned diamond sensors: Towards the simultaneous in vitro detection of molecular release and action potentials generation from excitable cells

In neuroscience, a deep understanding of communication mechanisms at the cellular level is of paramount importance, since their dysfunction determines the onset of several diseases. The development of innovative sensors devoted to the investigation of both chemical and electrical signals it is therefore essential to improve the outcome of standard trials and to define novel methodologies. Here we report on the fabrication and the characterization of multi-functional micrographite patterned diamond multi-electrode arrays. These sensors are obtained by means of a three-dimensional patterning process of single-crystal diamond substrates by means of MeV ion-beam-based lithography, which allows the direct fabrication of graphitic micro-channels embedded within the bulk of the electrically insulating diamond matrix.Proof-of-concept in vitro experiments on cultured neurons and cardiac tissue were performed, in which quantal secretory events were amperometrically recorded from dopaminergic neurons, while potentiometric measurements of action potential generation were collected from both hippocampal neuronal networks and intact sinoatrial nodes. These achievements represent the demonstration of the applicability of an all-carbon hybrid graphite/diamond device for the multi parametric detection of chemical and electrical signals, thus representing a fundamental step for the simultaneous in vitro measurement of the two types of signals from the same biological sample.

Operando Analysis of Electron Devices Using Nanodiamond Thin Films Containing Nitrogen-Vacancy Centers.

Operando analysis of electron devices provides key information regarding their performance enhancement, reliability, thermal management, etc. For versatile operando analysis of devices, the nitrogen-vacancy (NV) centers in diamonds are potentially useful media owing to their excellent sensitivity to multiple physical parameters. However, in single crystal diamond substrates often used for sensing applications, placing NV centers in contiguity with the active channel is difficult. This study proposes an operando analysis method using a nanodiamond thin film that can be directly formed onto various electron devices by a simple solution-based process. The results of noise analysis of luminescence of the NV centers in nanodiamonds show that the signal-to-noise ratio in optically detected magnetic resonance can be drastically improved by excluding the large 1/f noise of nanodiamonds. Consequently, the magnetic field and increase in temperature caused by the device current could be simultaneously measured in a lithographically fabricated metal microwire as a test device. Moreover, the spatial mapping measurement is demonstrated and shows a similar profile with the numerical calculation.

Fabrication of capacitive pressure sensor using single crystal diamond cantilever beam

Fabrication of single crystal diamond capacitive pressure sensor is presented. Firstly, the single crystal diamond cantilever beam was formed on HPHT diamond substrate by using selective high-energy ion implantation, metal patterning, ICP etching and electrochemical etching techniques. Secondly, on this diamond cantilever beam, the desired electrode patterns were processed with photolithography and metal evaporation methods. Furthermore, the displacements of cantilever beam under different pressure conditions were investigated by atomic force microscopy. The capacitance-voltage curves of single crystal diamond cantilever beam and substrate under different force loading conditions were measured by using Agilent B1505A parameter analyzer. The results show that sensitivity increases with the enlargement of electrode area of cantilever beam, and decreases with the rise of measurement frequency.

A comparative study of the growth dynamics and tribological properties of nanocrystalline diamond films deposited on the (110) single crystal diamond and Si(100) substrates

Nanocrystalline diamond (NCD) films were grown on the High Pressure High Temperature (HPHT) (110) single crystal (SC) diamond substrates by Microwave Plasma Enhanced Chemical Vapor Deposition (MPCVD) in methane/hydrogen/nitrogen plasma. The thickness of the films was varied between 2.2 and 22.5 μm. The cauliflower-like surface morphology was observed by means of Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The scaling behavior of NCD films growth was investigated. The relatively high value of the roughness exponent αs = 1.5–1.6 was found indicating anomalous scaling. Therefore, shadowing and diffusional instabilities can affect the film growth. The tribological properties of the NCD films deposited on the SC(110) diamond were compared with the NCD films prepared on the Si(100) substrates. Both types of specimens were tested under similar Hertzian contact pressure. The lower wear volume losses were observed on the NCD/SC(110) specimens. Therefore, the influence of substrate and substrate/film interface properties on the tribological behavior of the NCD films grown on Si(100) can be expected to cause NCD films deflection.

Study of the Structural and Morphological Properties of HPHT Diamond Substrates

The morphological and structural properties of a series of high-pressure high-temperature (HPHT) single-crystal diamond substrates are comprehensively studied by white-light optical interference microscopy, atomic-force microscopy, and X-ray diffraction analysis. Procedures that provide a means for characterizing the substrate parameters most critical for epitaxial application with the laboratory equipment are described. It is shown that the jewelry-type characterization of diamond substrates is insufficient to assess the possibility of their use for the epitaxial growth of chemical-vapor-deposited (CVD) diamond.

Crystalline quality distributions of the type IIa diamond substrate and the CVD diamond layer processed by chemical mechanical polishing using a SiO2 wheel

We have grown a high-quality type-IIa single-crystal diamond substrate, which had a large (001) growth sector with a dislocation-free area of 3.5 × 3.5 mm2, and strong free exciton (FE) emissions were observed by cathodoluminescence (CL), while dislocations, stacking faults, and defect-related CL emissions at 340 and 400 nm were observed in the (111) growth sector. After CVD growth on the off-angled type-IIa substrate processed by SiO2 chemical mechanical polishing, the intensity of the FE emission became 3 times higher and the electronic quality became more homogeneous, indicating that a high-quality CVD layer was successfully grown and the lateral growth eliminated the evolution of point defects in the substrate. On the other hand, the CVD layer on the (111) growth sector had hillocks and defect-related CL emissions, indicating that the stacking faults have a highly negative effect on the electronic quality of the CVD layer compared with the dislocations.

A High Frequency Hydrogen-Terminated Diamond MISFET With ${f}_{{\text{T}}}/{f}_{\max}$ of 70/80 GHz

A high frequency hydrogen-terminated diamond metal-insulator-semiconductor field-effect transistor (MISFET) with extremely small source-drain distance of about 350 nm was realized by self-aligned process on (001)-oriented single crystal diamond substrate. To suppress the gate leakage current, a low temperature Al2O3 film was deposited as the gate insulator by inductance coupled plasma enhanced atomic layer deposition. A low ohmic contact resistance of $1.84~\Omega \cdot $ mm was measured by using transmission line method. The fabricated 100 nm gate length MISFET shows a high current density of 585 mA/mm and a high transconductance of 206 mS/mm. By minimizing the parasitic parameters of the device, a high current gain cut-off frequency ${f} _{\textsf {T}}$ of 70 GHz and maximum frequency of oscillation ${f} _{\textsf {max}}$ of 80 GHz have been realized.

Limits of single crystal diamond surface mechanical polishing

We demonstrate the process of mechanical polishing of a single crystal diamond substrate to a roughness of no >0.5 nm. We used the anisotropy of the polishing intensity depending on the crystallographic orientation of the diamond substrate relative to the rotation direction of the cast iron grinding wheel (scaife). The surface distortions and its roughness were measured by atomic force microscopy (AFM) and X-ray reflectometry (XRR). The proposed technique optimization allows preparation of ultra-smooth surfaces in a relatively short time of <10 min.

Preferentially aligned nitrogen-vacancy centers in heteroepitaxial (111) diamonds on Si substrates via 3C-SiC intermediate layers

Nitrogen-vacancy (NV) centers in diamonds are expected for high-performance quantum sensing devices. The NV centers in heteroepitaxial diamond films on Si substrates have more potential to enable low-cost and large-area sensors than typical single-crystal diamond substrates and to support the emergence of diamond/Si hybrid devices. In this paper, NV centers were formed in (111) heteroepitaxial diamond films on Si substrates with preferential atomic alignment in the [111] direction. In addition, incorporation of silicon-vacancy centers, which decrease the sensitivity of sensors, from the 3C-SiC/Si substrate was effectively suppressed using oxygen gas in the growth environment.

Vertical-substrate epitaxial growth of single-crystal diamond by microwave plasma-assisted chemical vapor deposition

Epitaxial diamond growth by microwave plasma CVD in a CH4/H2 mixture was performed on vertically standing single crystal diamond substrates with high aspect (height-to-width) ratio. This “vertical growth” mode allows simultaneous diamond deposition on two large (1 0 0) faces of the substrate, in contrast to only one working face upon conventional horizontal arrangement of the substrate plate. A high plasma density gradient in vertical direction, the characteristic feature of the geometry used (the substrate acts as an antenna), coupled to high substrate temperature gradient (ΔT ≈ 150 °C in the particular experiment), are shown to have a strong impact on the epilayer structure, that was characterized with laser scanning microscopy, Raman and photoluminescence confocal spectroscopy mapping across ≈3 mm high specimen. The single crystal film transformation to polycrystalline, and further, to nanocrystalline layer, going from top to bottom of the substrate, is described. The vertical distributions of the growth rate, intensity of photoluminescence of NV and SiV color centers imbedded in the diamond film, are measured with high spatial resolution on growth surface and in cross-section, which are also related to the film structure variation. These findings are of importance for search of optimal growth regimes of 3D diamond mosaics and large single crystals by MPCVD.