Bias-enhanced Nucleation Research Articles

Coalescence as a key process in wafer-scale diamond heteroepitaxy

Due to fascinating physical properties powered by remarkable progress in chemical vapor deposition of high-quality epilayers, diamond thin films attract great attention for fabrication of nitrogen-vacancy-based solid-state spin systems capable of operating in ambient conditions. To date, diamond heteroepitaxy via bias-enhanced nucleation is an unavoidable method for reliable wafer-scale film manufacturing. In this work, we analyze the coalescence phenomena in nitrogen doped, heteroepitaxial diamond epilayers, with a particular focus on their specific role in the annihilation of macroscopic crystal irregularities such as grain boundaries, non-oriented grains, and twinned segments. Here, we also report on the growth mechanism for the “primary” crystal orientation along with a predominant formation of two different types of boundaries highlighting the {011}-type as a main source of the crystal lattice irregularities.

Effect of bias-enhanced nucleation on the microstructure and thermal boundary resistance of GaN/SiNx/diamond multilayer composites

The low effective thermal boundary resistance (TBReff) at the GaN/diamond interface is very important for the development of high-power, high-frequency and high-temperature GaN-on-diamond devices. The nucleation and growth of diamond are key processes for modulating the TBReff. This work proposed the bias enhanced nucleation technique to adjust the nucleation of GaN/SiNx/diamond multilayer composites in the MPCVD process at different bias voltages (400–700 V), thereby adjusting TBReff. Pulse bias is beneficial to a stable plasma environment and to obtain a complete GaN/diamond interface structure. The transient thermoreflectance characterization indicated that the GaN/SiNx/diamond multilayer composite prepared under 700 V bias nucleation condition had the lowest TBReff (26 ± 10 m2K/GW), whereas the GaN/SiNx/diamond multilayer composite at 600 V bias had the highest TBReff (83 ± 18 m2K/GW). The role of bias enhanced nucleation process in the TBReff was systematically analyzed by electron microscopies (TEM and SEM) and Raman spectroscopy. The GaN/SiNx/diamond multilayer composite prepared at 600 V showed a thick mixed transition layer containing multiphase structures and rough interfaces due to efficient subsurface ion implantation, resulting in high TBReff. In contrast, at 700 V, a thinner nucleation zone and smoother interface result in the lowest TBReff. This work demonstrated the potential of adjusting TBReff at the GaN/diamond interface by using bias enhanced nucleation technique to modulate the diamond nucleation and growth processes.

Diamond nucleation in carbon films on Si wafer during microwave plasma enhanced chemical vapor deposition for quantum applications

Nucleation is important in processing of good quality diamond crystals and textured thin films by microwave plasma enhanced chemical vapor deposition (MPECVD) for applications in quantum devices and systems. Bias-enhanced nucleation (BEN) is one approach for diamond nucleation in situ during MPECVD. However, the mechanism of diamond nucleation by BEN is not well understood. This paper describes results on the nucleation of diamond within a carbon film upon application of electric field during the BEN-facilitated MPECVD process. The nature of the diamond film and nuclei formed is characterized by SEM (scanning electron microscopy), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM). The HRTEM images and associated diffraction patterns of the nucleation layer show that the diamond nuclei are formed within the carbon film close to the Si (100) substrate surface under the influence of microwaves and electric fields that lead to formation of the textured diamond film and crystal upon further growth. These results are expected to develop diamond films of optimum quality containing a nitrogen-vacancy center for application in quantum systems.

Structural Colors Based on Diamond Metasurface for Information Encryption

AbstractStructural colors based on dielectric metasurfaces are attractive because of their potential application in next‐generation color printing. However, the commonly used dielectric materials suffer from issues of high light absorption (e.g., Si at blue wavelength) and low resistance to corrosion (e.g., titanium oxide), which greatly limit their application as structural color for information encryption. Here, dielectric‐metasurface‐based structural color constructed on nanocrystalline diamond (NCD) film by using bias enhanced nucleation (BEN) technique is demonstrated. Owing to the formation of highly crystalline diamond by the BEN process, the synthesized NCD film with a thickness of ~500 nm exhibits excellent optical quality with a relatively high refractive index that produces a strong magnetic dipole mode by Mie resonance. To realize the dynamic functionalities of diamond‐metasurface color with high resolution, a periodic array of asymmetric diamond cuboids with high aspect ratio (≈3.1) is fabricated, which produces polarization‐dependent response due to the enhanced Mie resonance. By optimizing the configuration of the diamond‐metasurface system, the color performance is improved with high brightness and a relatively wide gamut. This work demonstrates for the first time that NCD can serve as a robust and highly tunable dielectric platform for application in information encryption.

Bias-assisted epitaxial Ir/YSZ (1 0 0) substrate for diamond nucleation and growth

Iridium (Ir) films were deposited on yttrium stabilized zirconia (YSZ) substrates by bias-assisted RF magnetron sputtering. The orientation of Ir films can be orientally modulated by negative substrate bias, and then used for diamond nucleation and growth by microwave plasma chemical vapor deposition technology combined with the bias-enhanced nucleation process. The morphology, microstructure and composition of Ir and diamond films were investigated by scanning electron microscope (SEM), atomic force microscope, X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy. The morphological characterizations of Ir films showed that the grain shape was more uniform and regular symmetry with the increase of substrate negative bias voltage. XRD showed that the crystal orientation of the Ir films changed from (111) texture to (200) texture, and the optimal (200) preferred orientation is achieved under a substrate bias of −150 V. SEM images of diamond films exhibited that the diamond islands gradually merge into continuous diamond films and present a high degree of epitaxy. The crystalline quality of the central region of the epitaxial diamond films is higher than that of the edge region due to the substrate temperature gradient. The Raman full width at half maximum of the central region was 5.75 cm−1.

Initial growth mechanism of high-quality CVD diamond on Ir/sapphire substrate compared with Ir/MgO substrate

We demonstrate the growth of the highest-quality 2-in-diameter (001) diamond on a (112¯0) sapphire substrate. To elucidate the mechanism of the formation of this diamond, we observe the surfaces of the Ir buffer layer, bias-enhanced nucleation (BEN)-treated Ir surfaces, and diamond surfaces grown at the initial stage with different diamond growth times on both the (112¯0) sapphire and (001) MgO substrates. The (001) Ir buffer surface exhibits atomically flat terraces with atomic steps. However, after the BEN treatment, a ridge surface appears and diamond island growth begins from the bottom of the ridges during the diamond chemical vapor deposition growth. On Ir/sapphire, quadrangular-pyramid diamond three-dimensional (3D) islands with {111} sidewalls grow first. Subsequently, the sidewall facet changes from {111} to {011} and preferentially coalesced in the 〈010〉 direction. In contrast, on Ir/MgO, quadrangular-pyramid diamond 3D islands with {111} sidewalls grow first. Subsequently, the columnar growth proceeds. Consequently, the coalescence of diamond 3D islands on Ir/MgO requires a longer time than that on Ir/sapphire. We detect Ir on the top and sidewalls of the diamond 3D islands. This suggests that the Ir buffer can promote diamond growth at the initial stage as a catalytic effect. The X-ray rocking curve full width at half maximum for the 311 Bragg reflection is roughly proportional to the etch pit density. This is in accord with the transmission electron microscopy observation that the etch pits are of point-bottom-type and originated from a pure edge-type dislocation.

Growth and characterization of heteroepitaxial (001) and (111) diamond on Ir/sapphire structures

We report the comparative study on the growth and characterization of heteroepitaxial (001) and (111) diamond on Ir/(112¯0) and (0001) sapphire using direct current plasma chemical vapor deposition, in order to understand differences in heteroepitaxial behavior in both crystal directions as a preliminary step for large-sized high quality heteroepitaxial (111) diamond growth. An Ir deposition and bias-enhanced nucleation treatment were followed by 60, 190, and 1500 s diamond growth, under the same process conditions. The growth behaviors, morphological, and structural properties of the two crystal directions were greatly different. The growth rates of (001) and (111) diamond were 500 and 60 nm/min, respectively. The epitaxial relationships of (001) and (111) diamond were confirmed to be (001) diamond//(001) Ir//(112¯0) sapphire and (111) diamond//(111) Ir//(0001) sapphire. Further, in-plane relationship of (001) diamond was [1¯10] diamond//[1¯10] Ir//[11¯00] sapphire. In the case of (111) diamond growth, 180°-rotated twins and penetration twins in diamond were observed. The atomic force microscopy and X-ray rocking curve measurements exhibited that the surface roughness and the crystalline quality of (111) diamond were inferior to those of (001) diamond. Consequently, the growth conditions of Ir and diamond in [111] direction need to be considerably modified for suppressing twins and surface roughening and obtaining a high growth rate and high crystalline quality.

Growth defects in heteroepitaxial diamond

In focus of this report are the mechanisms of formation, propagation, and interaction of growth defects in heteroepitaxial diamond films along with their impact on the optical emission properties of N- and Si-vacancy (NV and SiV) color centers. Here, we analyze and discuss the properties of incoherent grain boundaries (IGBs) and extended defects in a nitrogen- and boron-doped heterodiamond nucleated and grown on Ir(001) via bias-enhanced nucleation and chemical vapor deposition techniques. We show that the low-angle IGBs alter the structural and optical emission properties of NV and SiV complexes by subduing NV emission and supporting the formation of interstitial Si-vacancy complexes dominating in the faulted IGB regions. We also demonstrate that the IGB-confined threading dislocations are responsible for the vertical transport and incorporation of Si impurities in thick layers, leading to an enhanced SiV emission from the IGBs.

Virtues of Ir(1 0 0) substrate on diamond epitaxial growth: First-principle calculation and XPS study

The heteroepitaxial growth of single crystal diamond requires substrate of high quality, and Ir(100) is an ideal substrate which can meet the standard perfectly. However, the reason of its outstanding performance still remains unclear. In this paper, the interaction mechanism between carbon atoms and Ir (100) surface at the initial stage of nucleation and its uniqueness caused by distinctive energy changes compared with (100) faces of Cu and Ni are explained by first-principle calculation, which acts as a dominating factor in the transformation from epitaxial films to single crystal wafers. The energy driving force of the dissolution-precipitation process is described, which can completely accord with bias-enhanced nucleation(BEN) method to illustrate the effect of voltage on diamond nucleation. The precipitation of carbon atoms and appearance of primary nuclei with the increase of carbon concentration are discussed, which can effectively explain the special phenomenon of the “domain” formation. A carbon-rich pre-treatment before primary nucleation on Ir (100) surface shows that the initial nucleation process can form supersaturated solid solution of carbon in Ir (100), which can greatly accelerate the subsequent dissolution-precipitation circulation.

Spectroscopic ellipsometry: A sensitive tool to monitor domains formation during the bias enhanced nucleation of heteroepitaxial diamond

Surface morphology and surface chemistry of epitaxial Ir/SrTiO3/Si(001) pseudo-substrates were systematically characterized by Field Emission Scanning Electron Microscopy (FE-SEM), Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS) before and after the Bias Enhanced Nucleation (BEN) of diamond. Based on these characterizations, an ellipsometric model was built taking into account both the roughening of the iridium surface and the formation of domains occurring during the BEN process. Sequential spectroscopic ellipsometry (SE) characterizations allowed estimating the roughening of the iridium surface, the thickness of the amorphous carbon layer and the coverage ratio of domains. By annealing the sample in air after BEN, domains were successfully removed without changing the iridium surface morphology. From these analyses, we demonstrate that SE is enough sensitive and discriminative to monitor both the formation of domains and the roughening of iridium surface, distinctly. SE can then be viewed as a powerful in situ tool of critical importance to study, monitor and finally enhance the diamond nucleation and growth on iridium for various advanced applications.

Local initial heteroepitaxial growth of diamond (111) on Ru (0001)/c-sapphire by antenna-edge-type microwave plasma chemical vapor deposition

Heteroepitaxial growth is critical for large-scale synthesis of diamond (111) substrates. In this study, the local initial epitaxial growth of diamond (111) on Ru/c-sapphire was investigated. As the economic viability of ruthenium (Ru) is more than that of iridium (Ir), a 150-nm Ru (0001) thin film was sputter-deposited on an Al2O3 (0001) substrate using a RF/DC magnetron sputtering system. X-ray diffraction analyses of the Ru film revealed the (0001) phase orientation with high crystalline quality. Both bias-enhanced nucleation and initial heteroepitaxial growth of diamond were realized via antenna-edge-type microwave plasma chemical vapor deposition. After 30 min of heteroepitaxial growth, the crystallite (diameter ∼500 nm) with a smooth surface was observed through scanning electron microscopy. Electron backscattering diffraction (EBSD) orientation mapping indicated the presence of the highly oriented diamond (111) crystallite. The epitaxial orientations between diamond (111) and Ru were determined as [111]diamond//[0001]Ru and [112¯]diamond//[101¯0]Ru. The EBSD pole-figure pattern represented the formation of a double positioning defect. This study demonstrated the feasibility of heteroepitaxial growth of diamond (111) on Ru, which provides a more economically viable approach to fabricating large-size diamond substrates.

Graphene Induced Diamond Nucleation on Tungsten

Chemical vapor deposition (CVD) of a diamond film on a non-diamond substrate begins with the insertion of diamond seeds or the formation of diamond nuclei on the substrate. For the deposition of a smooth, large-area and pin-hole free diamond film that adheres well to the substrate, diamond seeds or nuclei need to be of high density, uniformly distributed and adhere well to the substrate. Diamond seeding is not a diamond nucleation process. Bias enhanced nucleation (BEN) is the most effective means of heterogeneous nucleation of diamond for CVD diamond. It is based on a negative biasing voltage between the substrate and the diamond CVD plasma to accelerate positive ions from the plasma to bombard the substrate. Both direct diamond seeding and BEN have technical barriers in practical applications. New diamond nucleation techniques are desired. This paper reports novel heterogenous diamond nucleation along edge line of graphene on tungsten leading to the deposition of continuous diamond films. Based on experimental observation, a diamond nucleation mechanism assisted by sp3 C-W bonds at graphene edge is proposed. It is wished that scientists will become interested in revealing the precise diamond nucleation mechanism. With that, further optimization of this invention may lead to a new, complementary diamond nucleation process for practical deposition of diamond films.

Influence of substrate holder configurations on bias enhanced nucleation area for diamond heteroepitaxy: Toward wafer-scale single-crystalline diamond synthesis

A simple and effective method to extend the area of bias enhanced nucleation (BEN) for heteroepitaxial diamond growth is introduced. Two-inch substrates are placed on a flat and smooth surface of approximately 3-in. Mo substrate holder and then treated via BEN with or without metal-covered Si plates located right outside of the substrates. It is clarified that not only the plates themselves but also their thickness has a great impact on the BEN area, or in other words, the homogeneity of nucleation density on the substrates. As a result, the epitaxial diamond nucleation is successfully performed on nearly the whole area of a 2-in. Ir/YSZ/Si(001) substrate using the 1 mm thick half-ring plates. For a proof of this concept, finite element method simulations are also performed to investigate the influence of such plates on plasma (electron) density distributions above the substrates. Throughout this study, the significance of substrate holder configurations for the widely accessible wafer-scale diamond heteroepitaxy is revealed.

In situ bias current monitoring of nucleation for epitaxial diamonds on 3C-SiC/Si substrates

Heteroepitaxial diamond on Si substrates can offer large-area, low-cost diamond wafers for many practical applications. Bias-enhanced nucleation (BEN) has been shown to be one of the best process methods for diamond heteroepitaxy. However, the precise control of the nucleation is difficult because the optimized nucleation time is sensitive to both plasma and substrate conditions. In this study, we performed in situ monitoring for epitaxial diamond nucleation on 3C-SiC/Si substrates that detected the bias current during the BEN process. The bias current was found to decrease at the beginning of the BEN process and then increase during diamond formation. An increase in the bias current of 10% led to epitaxial diamond nucleation with a nucleation density of 1010 cm−2. We found from an analytical model that the nucleation occurs at a constant rate in this region. Similar characteristics were obtained on both (001)- and (111)-oriented substrates. This technique was based on the difference between the secondary electron emission coefficient of diamond and that of the material underneath. Thus, this method would be applicable to optimizing the epitaxial diamond nucleation on other substrates or buffer layers in addition to the 3C-SiC/Si studies here.

Tetramethysilane-assisted enhancement of diamond nucleation on silicon substrate

This paper describes the effect of tetramethylsilane (TMS) on diamond nucleation on Si substrate under an applied bias voltage of 120 V, performed in a modified microwave plasma chemical vapor deposition (MPCVD) reactor. The introduction of TMS in the CH4/H2 plasma leads to a significant enhancement of nucleation density of diamond nuclei, namely from (3.7 ± 0.2) × 107/cm2 without the introduction of TMS to (4.7 ± 0.5) × 1010/cm2 at the TMS flow rate of 4 sccm. On the contrary, further increasing in the TMS flow rate to 8 sccm slightly reduces the nucleation density of diamond nuclei to (2.1 ± 0.3) × 1010/cm2, along with the increase in average size of diamond nuclei. High-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) allow us to study the microstructural and chemical composition evolution of Si/diamond interface. The obtained results suggest that bias-enhanced nucleation of diamond on Si substrate exhibits a direct relationship with the TMS flow rate.

Highly oriented diamond (111) films synthesized by pulse bias-enhanced nucleation and epitaxial grain selection on a 3C-SiC/Si (111) substrate

We report the synthesis of highly oriented diamond (HOD) (111) films on 3C-SiC/Si (111) substrates. Bias-enhanced nucleation (BEN) is a key process for the heteroepitaxial growth of HOD films. Conventional long nucleation periods have been found to lead to a polycrystalline diamond film on the 3C-SiC (111) surface. Here, we propose a method that combines brief BEN (<30 s), called pulse BEN, and epitaxial grain selection by oxidative etching. Smaller diamond nuclei with a higher spatial density on the substrate were formed by pulse BEN with a pulse duration of <30 s. We found that precisely controlling the pulse duration is important for obtaining a nucleation density that is sufficiently high to obtain the HOD films. By adding oxygen gas to the subsequent growth process, non-epitaxial nuclei were removed and epitaxial diamond grains selectively remained. There was no notable difference in the relative growth rate of [111] to [100] with and without oxygen, and the orientation improvement was observed on both the (100) and (111) substrates. This suggests that the mechanism of oxidative removal was not evolutionary selective growth, but etching of the non-epitaxial interfaces between the nuclei and the (111) substrate. Finally, the HOD (111) films covering the entire 3C-SiC surface were synthesized, and they exhibited distinct diffraction spots, indicating the formation of the oriented diamond.

Nanocrystalline diamond films growth by microwave ECR CVD: Studies of structural and photoconduction properties

We report studies on growth of nanocrystalline diamond films by electron cyclotron resonance chemical vapour deposition (ECR CVD) and their structural and photoconduction properties. Bias enhanced nucleation (BEN) under higher methane partial pressure has been used to grow diamond nanocrystals with sp2 carbon clusters at grain boundaries in an amorphous carbon matrix. Structural studies indicate the size of diamond nanocrystals decreased and sp2 content increased in the films with increase in methane partial pressure, which is explained using subplantation model. Current- Voltage (I-V) measurements under dark and deep ultraviolet (DUV) light illumination conditions showed higher photocurrent in films with smaller sized diamond nanocrystals (3 nm–8 nm) compared to films having diamond nanocrystals of larger size (∼15 nm). The results indicates that the photocurrent generated in the films has major contributions from photoexcited electrons in diamond nanocrystals and sp2 carbon nanoclusters, with minor contribution from amorphous carbon phase.

Epitaxy of iridium on SrTiO3/Si (001): A promising scalable substrate for diamond heteroepitaxy

Iridium epitaxy on SrTiO3/Si (001) was investigated using field emission scanning electron microscopy (FE-SEM), spectroscopic ellipsometry, X-ray diffraction (XRD) and atomic force microscopy (AFM). Thermal stability of SrTiO3 buffer layers (12–40nm thick) was first investigated by annealing at different temperatures (620°C–920°C) under vacuum to optimize iridium epitaxy conditions. The surface morphology was monitored by FE-SEM and optical constants by spectroscopic ellipsometry. Iridium films were then deposited and their morphology and crystalline quality were evaluated by FE-SEM and XRD. It was found that iridium epitaxy is optimized at 660°C on SrTiO3 films thicker than ~30nm. The polar and azimuthal mosaicities of the iridium films on SrTiO3/Si (001) were 0.3° and 0.1°, respectively. These epitaxial iridium films were further used for diamond heteroepitaxy. The bias enhanced nucleation (BEN) treatment resulted in highly homogeneous and dense diamond domains. Heteroepitaxial diamond films were further grown by microwave plasma enhanced chemical vapor deposition (MPCVD) on 7×7mm2 Ir/SrTiO3/Si (001) substrates and characterized by XRD.

Highly Conductive Diamond-Graphite Nanohybrid Films with Enhanced Electron Field Emission and Microplasma Illumination Properties.

Bias-enhanced nucleation and growth of diamond-graphite nanohybrid (DGH) films on silicon substrates by microwave plasma enhanced chemical vapor deposition using CH4/N2 gas mixture is reported herein. It is observed that by controlling the growth time, the microstructure of the DGH films and, thus, the electrical conductivity and the electron field emission (EFE) properties of the films can be manipulated. The films grown for 30 min (DGHB30) possess needle-like geometry, which comprised of a diamond core encased in a sheath of sp(2)-bonded graphitic phase. These films achieved high conductivity of σ = 900 S/cm and superior EFE properties, namely, low turn-on field of 2.9 V/μm and high EFE current density of 3.8 mA/cm(2) at an applied field of 6.0 V/μm. On increasing the growth time to 60 min (the DGHB60), the acicular grain growth ceased and formed nanographite clusters or defective diamond clusters (n-diamond). Even though DGHB60 films possess higher electrical conductivity (σ = 1549 S/cm) than the DGHB30 films, the EFE properties degraded. The implication of this result is that higher conductivity by itself does not guarantee better EFE properties. The nanosized diamond grains with needle-like geometry are the most promising ones for the electron emission, exclusively when they are encased in graphene-like layers. The salient feature of such materials with unique granular structure is that their conductivity and EFE properties can be tuned in a wide range, which makes them especially useful in practical applications.

Bias-Enhanced Nucleation and Growth Processes for Ultrananocrystalline Diamond Films in Ar/CH4 Plasma and Their Enhanced Plasma Illumination Properties

Microstructural evolution of ultrananocrystalline diamond (UNCD) films in the bias-enhanced nucleation and growth (BEN-BEG) process in CH4/Ar plasma is systematically investigated. The BEN-BEG UNCD films possess higher growth rate and better electron field emission (EFE) and plasma illumination (PI) properties than those of the films grown without bias. Transmission electron microscopy investigation reveals that the diamond grains are formed at the beginning of growth for films grown by applying the bias voltage, whereas the amorphous carbon forms first and needs more than 30 min for the formation of diamond grains for the films grown without bias. Moreover, the application of bias voltage stimulates the formation of the nanographite phases in the grain boundaries of the UNCD films such that the electrons can be transported easily along the graphite phases to the emitting surface, resulting in superior EFE properties and thus leading to better PI behavior. Interestingly, the 10 min grown UNCD films under bias offer the lowest turn-on field of 4.2 V/μm with the highest EFE current density of 2.6 mA/cm(2) at an applied field of 7.85 V/μm. Such superior EFE properties attained for 10 min bias grown UNCD films leads to better plasma illumination (PI) properties, i.e., they show the smallest threshold field of 3300 V/cm with largest PI current density of 2.10 mA/cm(2) at an applied field of 5750 V/cm.