Free-standing Diamond Substrates Research Articles

Study on the Performance Impact of Introducing an InN Buffer Layer at Various Deposition Temperatures on InN Film Grown by ECR-PEMOCVD on Free-Standing Diamond Substrate

In this study, InN films are grown at a relatively low temperature by electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition (ECR-PEMOCVD) on free-standing diamond substrates. Due to the high lattice mismatch rate between InN film and the free-standing diamond substrate, the function of a buffer layer is to build a bridge between the substrate and film to reduce the lattice mismatch between them. Therefore, here, we study the performance impact of introducing an InN buffer layer at various deposition temperatures and explore the optimal buffer layer deposition temperature used to grow relatively high-quality InN films. The experimental results show that when an InN buffer layer is introduced at a deposition temperature of 100 °C, the growth direction of the InN film is perpendicular to the substrate with a high c-axis preferred orientation, the roughness of the surface is minimal, and the particle sizes are consistent with growth in the same direction. Additionally, the carrier mobility is highest, and the carrier concentration is lowest compared with other conditions.

Diamond Schottky barrier diodes fabricated on sapphire-based freestanding heteroepitaxial diamond substrate

In this study, diamond Schottky barrier diodes (SBDs) fabricated on a sapphire-based heteroepitaxial diamond substrate were demonstrated. For commercializing diamond-based power electronics, a large-sized heteroepitaxial diamond substrate is essential. Sapphire is one of the most promising candidates among the diamond heteroepitaxial substrates because it can be produced on a large scale and its thermal expansion coefficient is similar to that of diamond, compared to that of any other non-diamond hetero substrate. The current-voltage characteristics of heteroepitaxial diamond SBDs grown under different growth rates were analyzed, and good rectification properties of SBDs were observed at the device grown with low growth rate. The ideality factor was 1.4, and the maximum breakdown field was approximately 1.1 MV/cm. The value of built-in potential derived from the capacitance-voltage characteristics was in good agreement with the approximated barrier heights. Some ball-shaped features on the SBD along the threading dislocation were directly observed after performing the breakdown test under reverse bias condition using transmission electron microscope (TEM). Threading dislocation which induces breakdown or large leakage current under reverse bias in diamond SBD was clarified and this will lead the research direction of a heteroepitaxial diamond electronic device. Prime novelty statementsRecently, the diamond SBD grown on large-size heteroepitaxial diamond substrates becoming alternative to conventional high temperature high pressure (HPHT) diamond-based diamond SBD. In this study, diamond SBD grown on sapphire-based freestanding heteroepitaxial diamond substrate was successfully demonstrated. The sapphire substrate is one of the most promising material as substrate for heteroepitaxial diamond which is low cost and large sized. The heteroepitaxial diamond SBD structures with different growth rate were grown using microwave plasma CVD (MPCVD) and its structural properties were characterized. After device fabrication process, the detailed analysis of electrical properties of SBD were carried out. The direct observation of breakdown and leakage current features on SBD was obtained before and after electrical test under reverse bias condition.

Boron‐Doped Single‐Crystal Diamond Growth on Heteroepitaxial Diamond Substrate Using Microwave Plasma Chemical Vapor Deposition

Boron‐doped diamond layers are grown on freestanding heteroepitaxial diamond substrates by microwave plasma chemical vapor deposition (MPCVD) to verify the high potential of large‐size heteroepitaxial diamond as an ultimate semiconductor material. Due to the high crystallinity and atomically flat surface morphology of the substrate, the MPCVD‐grown boron‐doped diamond layer exhibit excellent surface properties and crystallinity, as measured by X‐ray diffraction and atomic force microscopy. The temperature‐dependent Hall effect measurements are conducted at temperature ranges between 300–800 K with cloverleaf‐shaped van der Pauw geometry. The hole concentration of boron‐doped diamond samples is between 1.1 × 1015 and 5 × 1019 cm−3 at room temperature, and the resistivity is controlled between 10−1 and 20 Ω cm by changing boron to carbon ratio. A specific contact resistance as low as 1.41 × 10−4 Ω cm2 is obtained via annealing at 500 °C. The activation energy of the boron‐doped diamond layers is reduced from 0.35 to 0.12 eV as the amount of boron dopant increases, which is attributed to the formation of impurity band. Finally, the change in the carrier mobility of boron‐doped heteroepitaxial diamond is discussed based on the scattering mechanism.

Microneedle growth method as an innovative approach for growing freestanding single crystal diamond substrate: Detailed study on the growth scheme of continuous diamond layers on diamond microneedles

A detailed study of diamond growth on diamond microneedles was conducted using micro-Raman spectroscopy of the microneedle and coalescence regions to approach the production of freestanding diamond substrate by heteroepitaxy with the microneedle growth method. The high-density non-diamond-phase carbon is contaminated in the initial stage of overgrowth of diamond on diamond microneedles, but this completely disappears through the quick recovery of the crystallinity of the overgrown diamond layers during the coalescence with the lateral direction growth. We also point out the possibility that a strong driving force is applied to the dislocations generated at the regrowth point and at the coalescence front to enhance the mutual annihilation of dislocations. In addition, we reveal that the stress state changes from compressive stress in the initial diamond layers to a nearly stress-free state in the bulk layers on the microneedles through a momentary tensile stress state at the regrowth point at the tip of the microneedle. Overall results indicate the strong feasibility of producing freestanding, stress-free, single-crystal diamond substrate by heteroepitaxy.

Overgrowth of diamond layers on diamond microneedles: New concept for freestanding diamond substrate by heteroepitaxy

A new concept for producing freestanding diamond substrate by heteroepitaxy is proposed. Thick diamond growth by heteroepitaxy is often prevented by heteroepitaxial-strain-related substrate bowing as it leads to substrate cracking. The possibility of using diamond microneedles as a mechanism to neglect and/or exploit substrate bowing is discussed in this new concept; the self-breaking effect of the microneedles is proposed as an application to prevent the main bulk diamond layers from cracking. With an aim toward the realization of this concept, the present study shows the first two important experimental verifications through a homoepitaxial experiment with a high-pressure high-temperature diamond substrate: (1) fabrication of high-aspect-ratio diamond microneedles (2 and 100μm in diameter and length, respectively) by a thermo-chemical etching reaction and (2) overgrowth of diamond on the microneedles with air gaps remaining between microneedles after continuous diamond film overgrowth. We also detail a growth scheme that illustrates how continuous diamond films are created from the microneedles. The strong feasibility for applying the concept in actual heteroepitaxy is suggested through the present study.

Characterization and thermal shock behavior of Y2O3 films deposited on freestanding CVD diamond substrates

Y2O3 anti-reflection films were deposited on freestanding CVD diamond substrates by radio frequency magnetron sputtering. The thermal shock resistance and oxidation resistance of Y2O3/diamond/Y2O3 samples at 727°C and 800°C in atmospheric air were investigated. No delamination of the Y2O3 films occurred after thermal shock, thereby demonstrating extreme adhesion to freestanding diamond substrates. After thermal shock, Y atoms in the films were almost fully oxidized. Moreover, the majority of monoclinic phase in the Y2O3 films transformed into cubic phase, crystallinity was enhanced, and the average grain size significantly increased. The maximum transmittance in the 8–12μm long-wave IR range of the Y2O3/diamond/Y2O3 samples increased from 81.3%±0.3% to 84.7%±0.2% and 83.6%±0.4%. These findings indicated that the Y2O3 anti-reflection films displayed good resistance to thermal shock and provided sufficient protection for diamond substrates against elevated temperature oxidation.

Fabrication of freestanding heteroepitaxial diamond substrate via micropatterns and microneedles

The fabrication of a high-quality freestanding diamond substrate was successfully demonstrated via heteroepitaxy by introducing diamond micropatterns and microneedles in the early stage of growth. Micropatterns contributed to a marked reduction in the number of dislocations induced by epitaxial lateral overgrowth, and microneedles relaxed heteroepitaxial strain. Raman spectroscopy indicated the absence of nondiamond carbon inclusions in the obtained freestanding substrate. The full width at half maximum of the X-ray rocking curve for diamond (004) reflections was 0.07°, the lowest value for heteroepitaxial diamond that has been reported so far. The results provide novel insights toward realizing large-diameter single-crystalline diamond substrates.

The diamond radiation detector with an Ohmic contact using diamond-like carbon interlayer

A diamond-like carbon thin layer was used as an interlayer between a free-standing diamond substrate grown by microwave plasma enhanced chemical vapour deposition and a Pt/Au layer. The dark-current of about 5·0 pA cm−2 was found for this detector at an electric field of 200 V cm−1. Ohmic contact was achieved by this diamond-like carbon/Pt/Au trilayer structure as evidenced by a linear dark I-V curve. The ratios of photocurrent to dark-current were about 26 and 35 for diamond detectors with Ti/Pt/Au and diamond-like carbon/Pt/Au trilayer electrodes, respectively. The energy resolution for the diamond detector with diamond-like carbon/Pt/Au electrode was about 3·30%, better than 4·56% for the detector with Ti/Pt/Au electrode.

Effect of Temperature on GaN Films Deposited on Diamond Substrate Using an ECR-PEMOCVD

Highly-quality GaN films were deposited on diamond substrate using an electron cyclotron resonance plasma enhanced metal organic chemical vapor deposition system (ECR-PEMOCVD) at the proper temperature. The source of gallium is Trimethyl gallium (TMGa) and N2, and the influence of temperature on the properties of GaN films was investigated systematically by X-ray diffraction analysis (XRD), atomic force microscopy (AFM), room temperature photoluminescence (PL), respectively. The dense and uniformed GaN films with highly c-axis preferred orientation were successfully achieved on free-standing diamond substrates under optimized deposition temperature of 400 °C.

Growth of High Quality Znmgo Films on Diamond Substrates

Abstract ZnMgO films were prepared at room temperature on freestanding diamond (FSD) substrates by co-sputtering. The Mg content was controlled by varying RF sputtering power of MgO and the effects of Mg contents on the properties of ZnMgO films were investigated. The results showed that the (0002) peak of ZnMgO shifted from 34.5° to 35.6° with the increasing sputtering power of MgO target. The UV-visible and PL spetra of ZnMgO films revealed that the bandgap of ZnMgO was approximately linear related to the sputtering power of MgO target.

Effects of Annealing Treatment and Buffer Layer on Properties of ZnMgO Films Prepared on Diamond Substrates

ZnMgO films were prepared at room temperature on freestanding diamond (FSD) substrates with and without ZnO buffer layers by radio-frequency (RF) reactive magnetron sputtering method. The effects of annealing treatment and ZnO buffer layers on the structural, optical, and electrical properties of the ZnMgO films were studied by X-ray diffraction (XRD), UV-visible spectrophotometer, and electrical measurements. The experimental results suggested that the annealing treatment and buffer layer were helpful to improve the crystalline quality of ZnMgO/diamond heteroepitaxial films.

ZnO Films Deposited on Various Diamond Film Substrates with Different Surface Roughness

ZnO thin films were deposited by radio frequency (R. F.) magnetron sputtering on various diamond film substrates with different surface roughness. The influence of surface roughness on structural properties and surface morphology of ZnO thin films was investigated by X-ray diffraction (XRD) and atom force microscopy (AFM), respectively. Only on the nanocrystalline and free-standing diamond substrates, ZnO films with preferential c-axis orientation and smooth surface were obtained.

Growth and Application of High Quality ZnO Films Grown on Diamond Substrates

Highly c-axis oriented ZnO films were grown on the nucleation sides of freestanding diamond (FSD) substrates with homobuffer layer by radio-frequency (RF) magnetron sputtering method. The effect of oxygen partial pressure (O2/Ar) on the properties of ZnO films was studied. ZnO-based UV photoconductive detector with planar interdigital (IDT) electrodes was fabricated by the lift off technique. Current-voltage (I-V) characteristics of the detector under ultraviolet (UV) light were measured and discussed.

Highly c-axis oriented GaN films grown on free-standing diamond substrates for high-power devices

GaN films with highly c-axis preferred orientation are deposited on free-standing thick diamond films by low temperature electron cyclotron resonance plasma enhanced metal organic chemical vapor deposition (ECR-PEMOCVD). The TMGa and N 2 are applied as precursors of Ga and N, respectively. The quality of as-grown GaN films are systematically investigated as a function of deposition temperature by means of X-ray diffraction (XRD) analysis, Hall Effect measurement (HL), room temperature photoluminescence (PL) and atomic force microscopy (AFM). The results show that the dense and uniformed GaN films with highly c-axis preferred orientation are successfully achieved on free-standing diamond substrates under optimized deposition temperature of 400 °C, and the room temperature PL spectra of the optimized GaN film show a intense ultraviolet near band edge emission and a weak yellow luminescence. The obtained GaN/diamond structure has great potential for the development of high-power semiconductor devices due to its excellent heat dissipation nature.

Large cross-section edge-coupled diamond waveguides

A new approach to fabricate large cross-section edge-coupled waveguides on free-standing thin diamond substrates is reported. Combining inkjet printing of photoresist multilayers with photolithographic patterning, both edge and ‘coffee stain’ effects were successfully eliminated, allowing the fabrication of well-defined, millimetre-scale uniform photoresist micro-stripes which extend to the very edge of the diamond substrate. Subsequent transfer of these micro-stripe structures into diamond by inductively coupled plasma (ICP) etching allowed long edge-coupled waveguides in diamond to be made. Guided wave propagation in these diamond waveguides was also confirmed.

Effect of deposition temperature and quality of free-standing diamond substrates on the properties of RF sputtering ZnO films

Highly c-axis oriented ZnO film is often deposited on diamond substrates by RF magnetron sputtering and widely used for high frequency surface acoustic wave (SAW) devices. Deposition temperature is a key factor affecting the quality of the ZnO film. Different quality polished free-standing diamond films prepared by DC Arc Plasma Jet were used as the substrates to deposit ZnO films at different temperatures. Effect of the deposition temperature and the quality of the diamond films on the properties of the ZnO films were investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that highly c-axis oriented ZnO films can be much easier deposited on the optical-grade diamond films with < 111> preferred orientation than the tool-grade diamond films with < 220> preferred orientation. The optimal deposition temperature is 200 °C for highly c-axis oriented and lower roughness ZnO films. Acoustic phase velocity of more than 10,000 m/s for the SAW devices based on the ZnO/optical-grade free-standing diamond films was obtained.

Growth and characteristics of ZnO films on growth side of freestanding diamond substrate dependent on buffer layer thickness

We report the growth and properties of highly c-axis oriented ZnO films, by radio-frequency magnetron sputtering, on the growth side of freestanding chemical vapor deposited diamond film-substrate. Low-temperature ZnO buffer layer is required for the formation of continuous ZnO films. The morphology, structure, and optical properties of the ZnO films deposited are strongly dependent on the thickness of the buffer layer. The optimized thickness of ZnO buffer layer is about 10 nm to realize high-quality ZnO films having small compressive stress and high intensity ultraviolet emission. The ZnO/diamond (growth side) system is available for the applications in numerous fields, especially for high performance surface acoustic wave devices.

Growth of high-quality carbon nanotubes on free-standing diamond substrates

Carbon nanotubes (CNTs) were grown on diamond-coated Si substrates and free-standing diamond wafers to develop efficient thermal interface materials for thermal management applications. High-quality, translucent, free-standing diamond substrates were processed in a 5kW microwave plasma chemical vapor deposition (CVD) system using CH4 as precursor. Ni and Ni–9%W–1.5%Fe catalyst islands were deposited to nucleate CNTs directly onto the diamond substrates. Randomly-oriented multi-walled CNTs forming a mat of ∼5μm thickness and consisting of ∼20nm diameter tubes were observed to grow in a thermal CVD system using C2H2 as precursor. Transmission electron microscopy and Raman analyses confirmed the presence of high-quality CNTs on diamond showing a D/G peak ratio of 0.2–0.3 in Raman spectra.

Fabrication of sub-micron high aspect ratio diamond structures with nanoimprint lithography

Polycrystalline diamond with optical quality has been patterned using nanoimprint lithography. Nanoimprint lithography is a rather new method for fabrication of resist structures with features sizes down to at least 20 nm. The pattern used in this article is a grating with a period of 600 nm and a fill factor of 0.5. Using plasma etching the nanoimprinted grating is etched into a freestanding diamond substrate. We have accomplished the fabrication of 300 nm diamond features with a depth of about 2 μm, which corresponds to an aspect ratio of 7.

Preparation of ZnO Thin Films on Free-Standing Diamond Substrates

Highly c-axis-oriented ZnO films were deposited successfully on the nucleation sides of free-standing diamond (FD) films by the direct current (DC) magnetron sputtering method. The effect of the sputtering parameters, such as power, gas pressure and sputtering plasma composition of Ar-to-O2, on the properties of ZnO thin films was investigated in detail. X-ray diffraction (XRD) measurements showed that, at a sputtering power of 200 W, gas pressure of 0.5 Pa and an Ar-to-O2 composition of 1:1, a higher intensity of the (002) diffraction peak and a narrower full width at half maximum (FWHM) were detected which meant high c-axis orientation and high quality of the ZnO films. To improve the quality of the ZnO film, a thin ZnO layer was pre-grown as a homo-buffer layer. XRD measurements showed that this buffer layer had a beneficial effect on the structural and morphological properties of the post-grown ZnO film.