•The initial growth of heteroepitaxial diamond on Ir(001)/MgO(001) was investigated.•Diamond grains gradually grew larger in diameter and thickness in a three-dimensional manner.•In-plane orientation ...of diamond grains had improved significantly before coalescence.•The proportion of diamond component gradually increased as diamond grain grew.•Initial growth of heteroepitaxial diamonds from nucleation-treated surfaces to film formation through coalescence was modelled.
It is important to clarify the mechanism of heteroepitaxial diamond growth, focusing on the change from three-dimensional to two-dimensional growth through coalescence, including dislocation reduction, impurity doping, etc. A detailed investigation was made of the relationship between the surface morphology and the crystalline quality of diamonds during the initial and early stages of their growth. An Ir (001) layer formed on a MgO (001) substrate was treated by ion irradiation through direct-current discharge.
Diamond was then grown on the ion-irradiated surface for a short time by using a microwave plasma CVD (MPCVD) process. Surface and cross-sectional scanning electron microscopy (SEM) and atomic-force microscopy (AFM) were used to investigate how the diamond grains grew to form a film. The resulting diamond grains and film were also characterized by means of X-ray diffraction (XRD) and Raman spectroscopy. The diamond grains randomly united to form a film, their in-plane orientation improved until coalescence was complete, and the proportion of diamond component gradually increased. Finally, we developed a model for growth of heteroepitaxial diamond grains based on our detailed observations.
A quantitative analysis of the surface and subsurface damage to gallium nitride (GaN) substrate from precise mechanical polishing with diamond abrasives and characterizations of their removal by the ...chemical mechanical polishing (CMP) process with colloidal silica slurry was performed. The cross-sectional transmission electron microscopy showed that the highly-defected regions were approximately 200 nm from the surface. The cathodoluminescence (CL) imaging showed that the subsurface damage layers were a cluster of extremely high-density networks of line-shaped defects. The depth-resolved and CMP-process-time-resolved CL imaging sensitively detected that while the high defect density subsurface damage layers were approximately 200 nm from the surface some of the subsurface damage reached over 1 μm in depth. Using the high-resolution x-ray diffraction, the CMP process was shown to decrease the dislocation densities, enlarge the correlation length, and release the compressive stress in the crystals induced by the mechanical polishing process. Improvement in these crystal properties was found to occur by removing the highly-defected surface regions.
•Subsurface damage (SSD) depth in GaN substrate was quantified.•Cross-sectional transmission electron microscopy showed 200 nm SSD layers.•Cathodoluminescence imaging indicated cluster of line-shaped defects in SSD regions.•Dislocation densities were decreased by chemical mechanical polishing (CMP) process.•CMP process was shown to release compressive stress.
Chemical mechanical polishing (CMP) of sapphire, GaN, and SiC substrates, which are categorized as hard-to-process materials, is demonstrated with a colloidal silica slurry under acidic and alkaline ...slurry pH conditions. Atomic level surface flatness was achieved by CMP and was confirmed to be equivalent to an almost ideally minimized surface roughness. By comparing the Preston coefficients under different slurry conditions, differences in the CMP properties among the three substrate materials and difficulties in the CMP of the GaN and SiC substrates are presented. The difference in CMP properties between the (0001) and (000-1) planes of GaN and SiC due to their non-revers crystallographical symmetry is also presented. Oxidation processes that occur during CMP of GaN and SiC are also discussed. By comparing the removal rate among GaN, SiC, and their oxides, it was found that the rate-limiting step in the total CMP process for GaN and SiC was surface oxidation reaction of GaN and SiC.
In-situ reflectance interferometry was utilized in this study to understand the characteristics of the reflectance profile for the growth of heteroepitaxial diamonds. We observed the time-resolved ...Fabry-Pérot oscillation of the growing heteroepitaxial diamond. Oscillations in the reflectance profile provided information on the grown thickness and provided the growth rate in real-time. The obtained in-situ profile was experimentally compared with the actual growth status via ex-situ observations and the strong correlation between the in-situ profile and the actual growth status was suggested. The analysis of the profiles provided a reasonable prediction to whether there would be any unexpected growth behavior in real-time for heteroepitaxial diamond growth.
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•In-situ reflectance interferometry was demonstrated for heteroepitaxial diamonds.•Time-resolved Fabry-Pérot oscillation was observed in the reflectometry.•Growth rate estimation of heteroepitaxial diamonds was possible in real-time.•Important growth event such as layer coalescence was observed in real-time.
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.
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•New concept for heteroepitaxial diamond substrate was proposed.•Fabrication of high-aspect-ratio diamond microneedles was demonstrated.•Air gaps remained between microneedles after overgrowth of diamond on 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.
•Detailed study of the microneedle growth method for diamond substrate fabrication.•Stress state changes from compressive to nearly stress-free during growth.•Strong feasibility of the microneedle growth method for the fabrication of high-quality freestanding SCD substrate is shown.
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Processing by a laser beam focused within the substrate is used to control the initial bowing of sapphire substrates for III-nitride epitaxy. The process modifies the sapphire crystallinity at and ...near the focal area from single crystal to an amorphous phase. As volume expansion occurs inside the sapphire, strain is generated and, consequently, changes in the bowing. By controlling the focal depth and process pitch, we demonstrate a ∼250μm pre-bowed sapphire substrate while only ±15μm of bowing control is possible with a regular wafering process. We also demonstrate epitaxial growth of III-nitride on the pre-bowed sapphire substrates by metal organic chemical vapor deposition (MOCVD), which suggests an enlargement for the process window for III-nitride epitaxy on sapphire substrate. It is also shown that the pre-bowing by laser treatment functions to improve the crystal quality of grown III-nitride films.
► New approach to control the initial bow of sapphire substrate was developed. ► Laser processing focused within the substrate was used for bowing control. ► Controllable bowing range of substrate is widened from ±15 to ±250μm. ► Pre-bowed substrate successfully enlarged the MOCVD process window. ► Accommodation of substrate bowing to any targeted values became possible for the MOCVD process.
A 600µm thick GaN layer was successfully grown by hydride vapor phase epitaxy by replacing the standard sapphire substrate with that processed by a focused laser beam within the substrate. The ...effects of the laser processing on the curvature and cracking of the GaN layer were investigated. Microscopic observations of the interior of the thick GaN layer revealed that the laser-processed substrate suppressed the generation of microcracks in the GaN layer. In addition, the laser processing was also found to reduce the change in the curvature during the GaN layer growth in comparison to that on the standard substrate. It is shown that the overlapping microcracks observed in the GaN layer on the standard sapphire substrate lead to serious cracking after thick GaN layer growth.
