Diamond Heterostructures Research Articles

Direct-bonded diamond membranes for heterogeneous quantum and electronic technologies

Diamond has superlative material properties for a broad range of quantum and electronic technologies. However, heteroepitaxial growth of single crystal diamond remains limited, impeding integration and evolution of diamond-based technologies. Here, we directly bond single-crystal diamond membranes to a wide variety of materials including silicon, fused silica, sapphire, thermal oxide, and lithium niobate. Our bonding process combines customized membrane synthesis, transfer, and dry surface functionalization, allowing for minimal contamination while providing pathways for near unity yield and scalability. We generate bonded crystalline membranes with thickness as low as 10 nm, sub-nm interfacial regions, and nanometer-scale thickness variability over 200 by 200 μm2 areas. We measure spin coherence times T2 for nitrogen vacancy centers in 150 nm-thick bonded membranes of up to 623 ± 21 μs, suitable for advanced quantum applications. We demonstrate multiple methods for integrating high quality factor nanophotonic cavities with the diamond heterostructures, highlighting the platform versatility in quantum photonic applications. Furthermore, we show that our ultra-thin diamond membranes are compatible with total internal reflection fluorescence (TIRF) microscopy, which enables interfacing coherent diamond quantum sensors with living cells while rejecting unwanted background luminescence. The processes demonstrated herein provide a full toolkit to synthesize heterogeneous diamond-based hybrid systems for quantum and electronic technologies.

Electronic Band Offset in a Diamond|cBN|Diamond System for Modulation Doping

AbstractDiamond is a material with promising electronics applications but with challenges in effective doping. Cubic boron nitride (cBN), due to its similar lattice structure, is ideal for forming heterostructures with diamond, to modify electronic properties and especially shift the band positions. Here it is demonstrated how integrating cBN layers of finite thickness in diamond can induce an electronic band offset near the polarized diamond‐cBN interface. Beyond the direct density functional theory computations, an analytical model is developed for rapid examination of the band offset, for a broad number of crystallographic orientations, in excellent agreement with ab initio calculations. Results show that near [100] and [111] crystal directions, the diamond|cBN|diamond heterostructure displays a 3–4 eV band offset which is expected to facilitate effective modulation doping of diamond.

Insight into Interfacial Heat Transfer of β-Ga2O3/Diamond Heterostructures via the Machine Learning Potential.

β-Ga2O3 is an ultrawide-band gap semiconductor with excellent potential for high-power and ultraviolet optoelectronic device applications. Low thermal conductivity is one of the major obstacles to enable the full performance of β-Ga2O3-based devices. A promising solution for this problem is to integrate β-Ga2O3 with a diamond heat sink. However, the thermal properties of the β-Ga2O3/diamond heterostructures after the interfacial bonding have not been studied extensively, which are influenced by the crystal orientations and interfacial atoms for the β-Ga2O3 and diamond interfaces. In this work, molecular dynamics simulations based on machine learning potential have been adopted to investigate the crystal-orientation-dependent and interfacial-atom-dependent thermal boundary resistance (TBR) of the β-Ga2O3/diamond heterostructure after interfacial bonding. The differences in TBR at different interfaces are explained in detail through the explorations of thermal conductivity value, thermal conductivity spectra, vibration density of states, and interfacial structures. Based on the above explorations, a further understanding of the influence of different crystal orientations and interfacial atoms on the β-Ga2O3/diamond heterostructure was achieved. Finally, insightful optimization strategies have been proposed in the study, which could pave the way for better thermal design and management of β-Ga2O3/diamond heterostructures according to guidance in the selection of the crystal orientations and interfacial atoms of the β-Ga2O3 and diamond interfaces.

Surface Transfer Doping in MoO3-x/Hydrogenated Diamond Heterostructure.

Surface transfer doping is proposed to be a potential solution for doping diamond, which is hard to dope for applications in high-power electronics. While MoO3 is found to be an effective surface electron acceptor for hydrogen-terminated diamond with a negative electron affinity, the effects of commonly existing oxygen vacancies remain elusive. We have performed reactive molecular dynamics simulations to study the deposition of MoO3-x on a hydrogenated diamond (111) surface and used first-principles calculations based on density functional theory to investigate the electronic structures and charge transfer mechanisms. We find that MoO3-x is an effective surface electron acceptor and the spatial extent of doped holes in hydrogenated diamond is extended, promoting excellent transport properties. Charge transfer is found to monotonically decrease with the level of oxygen vacancy, providing guidance for engineering of the surface transfer doping process.

Heteroepitaxial growth of β-Ga2O3 thin films on single crystalline diamond (111) substrates by radio frequency magnetron sputtering

In this work, we demonstrate the first achievement in heteroepitaxial growth of β-Ga2O3 thin films on single crystalline diamond (111) wafers using RF magnetron sputtering. A single monoclinic (β-phase) structure with a monofamily {01} plane was obtained. XRD pole figure shows (02) and (002) textures of the (01) β-Ga2O3 plane parallel to (111) diamond with six distinct rotational domains, confirming successful epitaxial growth. Collectively, this research provides valuable insights into the epitaxial growth of β-Ga2O3 on diamond via sputtering, paving the way for scalable β-Ga2O3/diamond heterostructures for future electronic and optoelectronic applications with not only high performance but also effective self-thermal management.

Investigation of gallium oxide thin film hetero-integrated with bulk diamond via atomic layer deposition

In this work, the characteristics of the gallium oxide (Ga2O3)/diamond heterostructure were thoroughly examined after the preparation of Ga2O3 thin film on bulk diamond via atomic layer deposition. The X-ray diffraction (XRD) analysis revealed the Ga2O3 film amorphous and diamond polycrystalline. The atomic force microscopy (AFM) mapping displayed remarkably smooth surfaces of the Ga2O3 film and diamond substrate (RMS of 0.184 and 0.508 nm, respectively). The scanning electron microscopy (SEM) images showed conspicuous grains formed on the diamond, and small crystallites on the surface of the film. The optical characteristics were investigated via spectroscopic ellipsometry (SE) and UV/Vis/NIR spectrophotometer. Raman spectroscopy suggested a sharp diamond-related (sp3) peak and an extremely weak bulge band around 1350–1620 cm−1. X-ray photoelectron spectroscopy (XPS) analysis indicated Ga2O3/diamond heterojunction to be staggered (type II) band alignment with valence band and conduction band offsets of around 1.18 eV and 2.09 eV, respectively. Moreover, according to time-domain thermoreflectance (TDTR) measurements, the thermal conductivity of Ga2O3 and the thermal boundary conductivity of the heterointerface were 5.13 W/(m·K) and 19.22 MW/(m2·K), respectively. These findings not only demonstrate the feasibility of Ga2O3-on-diamond hetero-integration but open up new prospects for the design and physical analysis of Ga2O3/diamond-based devices in the future.

Improved Gas Sensing Capabilities of MoS2/Diamond Heterostructures at Room Temperature.

Molybdenum disulfide (MoS2) and nanocrystalline diamond (NCD) have attracted considerable attention due to their unique electronic structure and extraordinary physical and chemical properties in many applications, including sensor devices in gas sensing applications. Combining MoS2 and H-terminated NCD (H-NCD) in a heterostructure design can improve the sensing performance due to their mutual advantages. In this study, the synthesis of MoS2 and H-NCD thin films using appropriate physical/chemical deposition methods and their analysis in terms of gas sensing properties in their individual and combined forms are demonstrated. The sensitivity and time domain characteristics of the sensors were investigated for three gases: oxidizing NO2, reducing NH3, and neutral synthetic air. It was observed that the MoS2/H-NCD heterostructure-based gas sensor exhibits improved sensitivity to oxidizing NO2 (0.157%·ppm-1) and reducing NH3 (0.188%·ppm-1) gases compared to pure active materials (pure MoS2 achieves responses of 0.018%·ppm-1 for NO2 and -0.0072%·ppm-1 for NH3, respectively, and almost no response for pure H-NCD at room temperature). Different gas interaction model pathways were developed to describe the current flow mechanism through the sensing area with/without the heterostructure. The gas interaction model independently considers the influence of each material (chemisorption for MoS2 and surface doping mechanism for H-NCD) as well as the current flow mechanism through the formed P-N heterojunction.

Surface-Dependent Adhesion Properties of Graphene on Diamonds for the Fabrication of Nanodevices: A Molecular Dynamics Investigation

The physical performance of a heterostructure is strongly influenced by the adhesion properties. The study of adhesion properties between graphene and diamond lattice is an inescapable issue in the development of diamond-based graphene multifunctional nanodevices. Herein, a series of adhesion intensities have been theoretically examined, and the optimum facet of diamond and theoretically recommended orientation angle were obtained, respectively. Moreover, the atomistic peeling behavior and the effect of three typical graphene topological defects on adhesion intensity were explored. The study demonstrated that the presence of double-vacancy defects impairs the adhesion strength due to the reduction of the contact area. In contrast, the presence of Stone-Wales defects is conducive to enhancing the adhesion strength. Interestingly, the effect of single-vacancy defects mainly depends on the delicate competition between single-atom removal and single-atom attraction enhancement. Meanwhile, the effects of diamond surface morphology on graphene adhesion were systematically elaborated by the modeling of one-dimensional and two-dimensional surfaces, and randomly rough surfaces. The adhesion details of graphene on regularly tunable diamonds were explored, and the relations of adhesion intensity and graphene morphology with the random roughness of a diamond surface were further revealed in depth. Since the mechanical and electrical performance of a graphene–diamond heterostructure is sensitively influenced by the adhesion intensity, our findings provide insight into the substrate design of graphene–diamond hybrid devices.

Aluminium nitride thin films directly grown on conducting boron-doped nanocrystalline diamond films without using buffer layer for high frequency applications

Aluminium Nitride (AlN)-diamond hetero-structures are promising candidates for high frequency acoustic wave resonator applications. This paper reports the electrical, piezoelectric and mechanical properties of AlN films directly grown on boron-doped nanocrystalline diamond (B-NCD) films. X-ray diffraction (XRD) and Raman spectroscopy were used to determine the crystallinity of B-NCD and AlN films. The resistivity (∼8 ×107Ω.m) and dielectric constant (∼14) were obtained from the electrical characterisation of the fabricated Al/AlN/B-NCD capacitors. Piezoelectric coefficient (d33, 4.3 pmV−1) of AlN films was measured by piezo force microscopy (PFM) technique. Nanoindentation technique was used to determine the hardness (∼14 GPa) and elastic modulus (∼202 GPa) of AlN films. These different properties of AlN/B-NCD/Si (100) hetero-structure show that it will provide a high-performance platform to realize high frequency resonators for 5G applications.

Hybrid density functional theory study on zinc blende GaN and diamond surfaces and interfaces: Effects of size, hydrogen passivation, and dipole corrections

GaN-diamond heterojunctions show promise in high electron mobility transistor applications. This prompts the need for accurate models of structural and electronic properties. We detail here a hybrid density functional theory study of zinc blende (ZB) GaN and diamond heterostructures using slab supercell models. The dependence of electronic properties on supercell size, pseudo-hydrogen passivation, and dipole corrections is detailed. The larger bulk modulus of diamond is shown to provide a templating structure for GaN to grow upon, where the large lattice mismatch is accounted for through the inclusion of a cationic Ga adlayer. Looking at both type I and II surfaces and interfaces of GaN shows the instability of zb GaN without an adlayer (type II), where increased size, pseudo-hydrogen passivation and dipole corrections do not remove the spurious interaction between the top and bottom layers in type II GaN. Layer dependent density of states, local potential differences, and charge density differences show that the type I interface (with a Ga adlayer) is stable with an adhesion energy of 0.704 eV/Å2 (4.346 J/m2); interestingly, the diamond charge density intercalates into the first layer of GaN, which was seen experimentally for wurtzite GaN grown over diamond. The type II interface is shown to be unstable which implies that, to form a stable, thin-film zb interface between GaN and diamond, the partial pressure of trimethylgallium must be controlled to ensure a Ga layer exists both on the top and bottom layer of the GaN thin film atop the diamond. We believe our results lead to a better understanding of the GaN/diamond multifaceted interface present in the GaN overgrowth on diamond heterojunction used in electronic device applications.

Simulation research on nucleation mechanism of graphene deposition assisted by diamond grain boundary

The growth of high-quality graphene is always a focused issue in the field of two-dimensional materials, and the growth of graphene on brand new substrates has received considerable attention from scholars especially. The research on the nucleation mechanism of graphene deposited on a polycrystalline diamond substrate is of significance in the large-scale preparation of graphene in practice. Here in this work, the direct growth without transfer process of graphene on a diamond substrate is used to obtain the high-quality graphene. The reactive molecular dynamics simulation technology is adopted to imitate the process of graphene deposition and growth on bi-crystal diamond assisted by nickel catalyzed at an atomic level. The effect of the bi-crystal diamond grain boundary on the dynamic behavior of graphene nucleation and growth process is studied. The results demonstrate that the grain boundary carbon atoms can be used as a supplementary carbon source to diffuse into the nickel free surface and participate in the nucleation and growth of graphene. Furthermore, the effect of temperature on the diffusion behavior of carbon atoms is explored, finding that high temperature facilitates the dissociation of atoms in the grain boundary. When the deposition temperature equals 1700 K, it is most conducive to the diffusion of grain boundary carbon atoms in the nickel lattice, which effectively enhances the nucleation density of graphene. Besides, the effect of the deposition carbon source flow rate on the surface quality of graphene is explored, finding that the high-quality graphene surface can be obtained by adopting a lower carbon deposit rate of 1 ps<sup>–1</sup> at 1700 K. In brief, the research results obtained not only provide an effective theoretical model and analysis of the mechanism for diamond grain boundary assisted graphene deposition and growth, but also reveal the regular pattern of influence of deposition temperature and deposition carbon source flow rate on the surface quality of synthesized graphene. The present study can lay a theoretical foundation for the fabrication and application of new functional graphene-polycrystalline diamond heterostructures in the fields of ultra-precision manufacturing and microelectronics.

Low-temperature direct bonding of β-Ga2O3 and diamond substrates under atmospheric conditions

Surface-functionalized β-Ga2O3 and diamond substrates were directly bonded by annealing at 250 °C in atmospheric air. Prior to bonding, the β-Ga2O3 and diamond surfaces were OH-terminated by oxygen plasma irradiation and H2SO4/H2O2 cleaning, respectively. After contacting the OH-terminated surfaces with each other, direct bonding was formed by a thermal dehydration reaction. The annealed specimen had a shear strength of 14 MPa because of the generation of chemical bonds between β-Ga2O3 and diamond surfaces. The analysis of interface structures revealed that the β-Ga2O3 and diamond surfaces were atomically bonded without nano-voids, cracks, or any intermediate layer. The β-Ga2O3 and diamond substrates were bonded without any serious loss in crystallinity except for the outermost surfaces. We believe that the β-Ga2O3/diamond heterostructure can contribute to the high-power applications of β-Ga2O3 devices.

Investigation of Thermal Properties of β-Ga2O3 Nanomembranes on Diamond Heterostructure Using Raman Thermometry

The β-Ga2O3 nanomembrane (NM)/diamond heterostructure is one of the promising ultra-wide bandgap heterostructures that offers numerous complementary advantages from both materials. In this work, we have investigated the thermal properties of the β-Ga2O3 NM/diamond heterostructure with three different thicknesses of β-Ga2O3 nanomembranes (NMs), namely 100 nm, 1000 nm, and 4000 nm thick β-Ga2O3 NMs using Raman thermometry. The thermal property—temperature relationships of these β-Ga2O3 NM/diamond heterostructures, such as thermal conductivity and interfacial thermal boundary conductance were determined under different temperature conditions (from 100 K to 500 K with a 40 K interval). The result provides benchmark knowledge about the thermal conductivity of β-Ga2O3 NMs over a wide temperature range for the design of novel β-Ga2O3-based power electronics and optoelectronics.

Giant Reflection Coefficient on Sc0.26Al0.74N Polycrystalline Diamond Surface Acoustic Wave Resonators

Since the commercialization of surface acoustic wave (SAW) devices, the technology is steadily increasing the device performances without compromising their power handling, size and price. Herein, one‐port SAW resonators are fabricated on scandium aluminum nitride (Sc0.26Al0.74N)/polycrystalline diamond heterostructures. SAW propagation properties are studied using three different piezoelectric thin‐film thicknesses within the heterostructure. The Rayleigh and Sezawa resonance frequencies are above 1.5 and 2.5 GHz, respectively, achieving Sezawa mode reflection coefficients below −50 dB. The polycrystalline diamond substrate is synthesized by microwave plasma chemical vapor deposition (MPCVD) on top of a 500 μm‐thick Si (001) substrate. The Sc0.26Al0.74N thin films are synthesized by reactive sputtering at nominally room temperature. The thin film's composition is analyzed by Rutherford backscattering spectrometry (RBS). The full width at half maximum (FWHM) of the X‐ray diffraction (XRD) ω scans below 3° indicates that the synthesized Sc0.26Al0.74N thin films are highly c‐axis oriented. The electromechanical coupling coefficient, quality factor, and dielectric loss parameters are computed by curve fitting the device electrical measurements to the simulation results of a modified Butterworth Van Dyke (mBVD) model implemented in the advance design system (ADS) tool.

Coherent Phonon-Mode Excitation in Submicron Single-Crystal Diamond Films with a Graphitized Layer Built-In

We report the first experimental observation of high-efficient phonon-mode coherent excitation using stimulated low-frequency Raman scattering (SLFRS) in submicron diamond{graphite{diamond heterostructure films with an ion-beam-induced graphitized layer. We show that the SLFRS process in submicron diamond heterostructures has a frequency shift in the gigahertz range and estimate experimentally the SLFRS conversion efficiency and threshold.

Photocatalytic reduction of nitrogen to ammonia on diamond thin films grown on metallic substrates

Recent studies have demonstrated that boron-doped diamond can act as a solid-state source of electrons in water when illuminated with above-bandgap light. Excitation of electrons to diamond's conduction band leads to facile emission of electrons into water; the resulting solvated electrons are potent reducing agents able to initiate many chemical reactions such as the reduction of N2 to NH3 and the reduction of CO2 to CO. Here, we report investigations of the photocatalytic activity of diamond thin films grown on Mo, Ni, and Ti substrates. Our results show that in each case, there is a maximum in the photocatalytic activity that occurs just when the diamond coalesces into a contiguous film, then decreasing as the film becomes thicker. These results suggest that electron emission arises in part from excitation of electrons in the metal substrate, followed by injection into the conduction band of the diamond film. More detailed studies on films grown on niobium show that at the open-circuit potential the films have a downward bend-bending of ~0.3V, which is favorable for barrier-free electron emission. Our result suggests that metal–diamond heterostructures may provide a scalable approach to achieving difficult photocatalytic reactions.

High-resolution wide-range dynamic neutron monochromators

This work proposes the concept of a high-resolution wide-range dynamic neutron monochromator able to operate up to a few eV, thus significantly extending the reliable operating neutron energy range of the state-of-the-art crystal-based devices, and to improve energy resolution in the epithermal range if compared to mechanical choppers. The proposed device is based on a radiation-hard aluminium nitride (AlN)/diamond heterostructure, operating as a super-high-frequency (> 3 GHz) high-speed (> 10000 m/s) acoustic resonator. The resulting surface acoustic wave (SAW) acts as a dynamic grating for the incoming neutrons: as long as neutron speed is lower than (or at least comparable to) SAW speed, diffraction angles are significantly enhanced, thus allowing for neutron beam monochromatization to be effective up to the eV energy range. SAW amplitude can be electrically tuned to increase first-order reflectivity, leading to an enhanced intensity of the monochromatized beam.

White electroluminescence of n-ZnO:Al/p-diamond heterostructure devices

An n-ZnO:Al/p-boron-doped diamond heterostructure electroluminescent device is produced, and a rectifying behavior can be observed. The electroluminescence spectrum at room temperature exhibits two visible bands centred at 450 nm–485 nm (blue emission) and 570 nm–640 nm (yellow emission). Light emission with a luminance of 15 cd/m2 is observed from the electroluminescent device at a forward applied voltage of 85 V, which is distinguished from white light by the naked eye.

Ultra-nano-crystalline/single crystal diamond heterostructure diode

A new type of highly rectifying diamond heterostructure diode is demonstrated. The p-type doped part of the diode consists of a single crystal diamond, the n-type part of a nitrogen doped ultra-nano-crystalline diamond (UNCD) layer. IV-measurements show 8 orders of magnitude of rectification (±10 V) at room temperature. The barrier behavior is rather complex and can be described by two junctions acting in parallel, reflecting the UNCD properties. This new material system displays an extraordinary thermal stability and has been tested successfully up to 1050 °C in vacuum. Thus, this novel diamond heterostructure diode belongs to the few ultrahigh temperature stable electronic devices.

Interfacial charge transfer and hole injection in α-NPD organic overlayer–CVD diamond substrate system

We have investigated interfacial charge transfers and hole injection characteristics between oxygen-terminated boron-doped homoepitaxial (100) chemical-vapor-deposited (CVD) diamond films and thermally deposited N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′biphenyl-4,4′′diamine (α-NPD) mainly using X-ray photoelectron spectroscopy (XPS), Hall effect measurements and current–voltage (J–V) characteristic measurements. On one hand, no chemical shift was observed for C 1s and N 1s XPS spectra taken from the α-NPD overlayer–CVD diamond substrate system. On the other hand, the low-temperature hole concentration of the diamond substrate decreased by ≈10–17% after the α-NPD depositions, verifying that small but substantial amounts of holes were transferred from the diamond substrates to the α-NPD films. However, compared with those obtained from the specimens before the α-NPD depositions, the sheet electrical conductivities of the specimens after the α-NPD depositions increased at T ≥ 230K while decreasing at T ≤ 200K. In addition, J–V data measured from layered α-NPD–diamond hetero-structures show evidence that the junction worked as a diode. These facts suggest that the α-NPD–CVD diamond system may be useful for a transparent anode electrode with a hole transport layer appropriate for organic light emitting diodes.