Polycrystalline Diamond Film Growth on Gallium Nitride with Low Boundary Thermal Resistance

Abstract

As the demand for high-frequency and high-power electronic devices has increased, gallium nitride (GaN), particularly in the context of high-electron mobility transistors (HEMTs), has attracted considerable attention. However, the ‘self-heating effect’ of GaN HEMTs represents a significant limitation regarding both performance and reliability. Diamond, renowned for its exceptional thermal conductivity, represents an optimal material for thermal management in HEMTs. This paper proposes a novel method for directly depositing diamond films onto N-polar GaN (NP-GaN) epitaxial layers. This eliminates the complexities of the traditional diamond growth process and the need for temporary substrate steps. Given the relative lag in the development of N-polar material growth technologies, which are marked by surface roughness issues, and the recognition that the thermal boundary resistance (TBR_GaN/diamond) represents a critical factor constraining efficient heat transfer, our study has introduced a series of optimizations to enhance the quality of the diamond nucleation layer while ensuring that the integrity of the GaN buffer layer remains intact. Moreover, chemical mechanical polishing (CMP) technology was employed to effectively reduce the surface roughness of the NP-GaN base, thereby providing a more favorable foundation for diamond growth. The optimization trends observed in the thermal performance test results are encouraging. Integrating diamond films onto highly smooth NP-GaN epitaxial layers demonstrates a reduction in TBR_GaN/diamond compared to that of diamond layers deposited onto NP-GaN with higher surface roughness that had undergone no prior process treatment.

1. Introduction

In recent years, there has been a notable increase in the demand for electronic devices capable of operating at high frequencies and power. GaN, a third-generation semiconductor material that follows silicon (Si) and gallium arsenide (GaAs), has attracted considerable research interest due to its exceptional properties, including a wide bandgap and high breakdown electric field strength [1,2]. It is noteworthy that GaN HEMTs, which utilize the polarization effect within the AlGaN/GaN heterojunction to generate a two-dimensional electron gas (2DEG), exhibit remarkable carrier densities approaching 10¹³ cm⁻² and mobilities spanning 1800–2300 cm²/(V·s). These attributes serve to enhance both the operating voltage and frequency [3,4]. However, this performance advancement is accompanied by a significant ‘self-heating effect’. The primary cause of this effect lies in the excessive power dissipation under high bias voltage operating conditions. This results in a substantial increase in device temperature, which hinders the diffusion of heat into the surrounding environment. Consequently, phonon scattering is enhanced, leading to a reduction in carrier mobility within the potential wells. Ultimately, this causes a degradation in the static I-V characteristics of the device, thereby adversely affecting both its performance and its reliability [5,6,7,8]. To address this challenge and fully realize the potential of GaN HEMTs, effective thermal management strategies have become a crucial consideration. While traditional passive methods offer certain benefits, they are constrained by size, weight, and power consumption (SWaP) considerations. In contrast, active thermal management techniques use high-κ materials to enhance the transfer of heat within the near-junction region of the device, thereby improving the efficiency of heat dissipation. This approach allows for the attainment of higher power densities while simultaneously reducing the dimensions of the device. Among the numerous materials that have been investigated, diamond is an exceptionally promising candidate for enhancing the thermal performance of HEMTs. Its unparalleled κ, exceeding 1800 W/m·K even in a polycrystalline form, coupled with its excellent insulating properties, renders it markedly superior to conventional materials, such as sapphire, Si, and silicon carbide (SiC) [9].

Currently, the mainstream integration methods for diamond and GaN can be broadly classified into two categories, as follows: using diamond as a bottom heat sink substrate/quasi-substrate and using diamond as a top heat-dissipating passivation layer. The first method can be further subdivided into the following three major types: the deposition of diamond on GaN, the epitaxy of GaN on diamond, and the bonding of diamond with GaN [5,8,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32]. Among these, the chemical vapor deposition (CVD) of polycrystalline diamond (PCD) on GaN represents a relatively mature technology for integrating GaN and diamond. Its distinctive advantage lies in its ability to deposit materials on large bases. However, the prevailing diamond-on-GaN growth process frequently necessitates intricate pre-processing stages, particularly the prerequisite of bonding the GaN front side to a temporary substrate. This step provides structural support during the deposition of diamond films on the back side, subsequent to removing the original substrate [33]. This methodology inevitably gives rise to technical challenges in terms of bonding and stripping, which may result in the introduction of additional stresses and residual surface defects on the GaN surface. This, in turn, may compromise the stability and reliability of the final device, particularly given that the HEMT is essentially a surface device. An alternative methodology entails the direct utilization of a back-side-up epitaxial wafer (NP-GaN) as the growth base. This eliminates the necessity for a temporary substrate, simplifies the process flow (as illustrated in Figure 1), and enables the direct deposition of diamond on NP-GaN. However, the technological capabilities for the growth of N-polar materials are significantly less advanced than those for Ga-polar materials, which are more prevalent. The primary challenge arises from the generally inferior quality of N-polar Group III nitride materials produced under the current technological conditions, particularly surface roughness issues, such as the proliferation of hexagonal defects on the surface of NP-GaN films heteroepitaxially grown on orthorhombic sapphire or C-face SiC (C-SiC) substrates [34,35,36]. These defects persist even during GaN homoepitaxy, severely limiting their potential applications [37]. Moreover, irrespective of whether diamond is grown on Ga-polar GaN (GaP-GaN) or NP-GaN bases, TBR_GaN/diamond represents a significant obstacle to efficient heat conduction from the active region to the diamond layer [38]. The TBR_GaN/diamond is predominantly contingent upon the low-κ dielectric layer, which protects the GaN layer [39]. Furthermore, the issue is compounded by the low quality of the diamond nucleation layer and the poor interface quality [17]. Therefore, although the direct deposition of diamond on NP-GaN considerably simplifies the process, the compromised surface quality of NP-GaN is likely to have a significant negative impact on TBR_GaN/diamond, ultimately limiting the overall thermal management performance.

In response to the aforementioned issues, we conducted experimental investigations with the objective of minimizing TBR_GaN/diamond. This was achieved by employing microwave-plasma chemical vapor deposition (MPCVD) for PCD growth on NP-GaN bases that were initially fabricated through metal–organic chemical vapor deposition (MOCVD). Our primary focus was twofold, as follows: First, we aimed to enhance the material quality at the GaN/diamond interface and its adjacent regions. This was achieved by meticulously refining the diamond seeding process and the operational parameters employed in the MPCVD. This entailed enhancing the quality of the diamond nucleation layer to promote greater densification and ensuring the integrity of the GaN buffer layer under harsh diamond growth conditions, thereby averting any detrimental structural alterations that could compromise the electrical performance of the resulting device. Second, to overcome the technological challenges associated with achieving smooth NP-GaN layers with low defect densities following epitaxial growth, we conducted experiments utilizing CMP technology to further reduce the surface roughness of the NP-GaN bases, building upon the already optimized NP-GaN growth conditions. To validate the feasibility of our proposed methodology and gauge the efficacy of our optimizations, we conducted comparative analyses to assess the influence of varying the surface roughness of NP-GaN bases on seed crystal seeding, evaluate the material’s structural quality on both sides of the interface post diamond growth, and quantify the specific impact on TBR_GaN/diamond.

2. Materials and Methods

In this study, we employed an MOCVD system incorporating a distinctive planetary air-cushioned, horizontally rotating reaction chamber to epitaxially synthesize approximately 1.6-μm-thick NP-GaN buffer layers onto C-SiC substrates with a 4° off-axis crystal orientation. Subsequently, the surface of the NP-GaN layer was refined through the implementation of a CMP process. The deliberate choice of bias-crystalline-oriented SiC as the underlying substrate stems from two key rationales. First, its characteristic narrow width and elevated density of steps facilitate the efficient transportation of Ga species towards the twisting sites, thereby significantly mitigating the formation of detrimental hexagonal defects [40]. This, in turn, results in the enhanced stability and overall quality of the epitaxial NP-GaN layers. Second, there is a significant disparity in the coefficients of thermal expansion (CTE) between diamond and GaN-on-sapphire, whereas GaN-on-SiC exhibits a lower, more compatible CTE. This characteristic makes GaN-on-SiC an ideal platform for the growth of large-area and thicker diamond films [41]. To prevent the NP-GaN base from being susceptible to plasma etching during the subsequent diamond growth process, we employed low-pressure chemical vapor deposition (LPCVD) to deposit a 30-nm-thick silicon nitride (SiN_x) thin film onto the NP-GaN epitaxial layer. In contrast to plasma-enhanced chemical vapor deposition (PECVD), the LPCVD approach, facilitated by its elevated deposition temperatures, enhances surface mobility, thereby imparting exceptional uniformity, densification, and step coverage to the SiN_x films. This ensures effective protection of the NP-GaN layer and addresses the challenges posed by its intricate surface morphology [42]. In addition, prior to each process step, we incorporated an additional sample cleaning procedure to minimize the introduction of surface impurities and their potential impact on the subsequent experimental results. This was particularly crucial considering that the CMP process could introduce a significant amount of abrasive particulate impurities.

The experimental setup for diamond growth utilized the SSDR 400 MPCVD system manufactured by PLASSYS. This system features a maximum power of 36 kW and an advanced water-cooling system, which ensures that the sample maintains an optimal growth temperature under high-power microwave radiation. With an incident power range of 6–36 kW and a chamber pressure of 40–280 mbar, the reflected microwave power can be adjusted to be below 1000 W, allowing the growth of up to 6-inch PCD films. Prior to MPCVD growth of PCD, the base material must be pretreated to increase the number of nucleation sites. Conventional surface damage techniques can compromise the protection of the base and the smoothness of the GaN/diamond interface, so a seed crystal spin-coating method was employed in this study. This approach not only simplifies the process, but also effectively increases the density of the nucleation sites, providing a foundation for subsequent diamond growth. The details of the seed crystal seeding process and the specific parameters for MPCVD diamond growth are given in Section 3. In particular, when simulating large-area 6-inch diamond growth, achieving the desired growth temperature of 700 °C and above requires increasing the power to at least 24 kW. The introduction of methane (CH₄) below this temperature threshold results in reduced etching rates of non-diamond carbon and increased sp² carbon incorporation. This, in turn, reduces the purity of the diamond phase and leads to the formation of small, weakly bonded soot carbons. This ultimately degrades the thermal performance of the PCD film and can even lead to material delamination [43]. However, the potential damage to the base from the high-power plasma during the heating phase (prior to CH₄ introduction) must also be considered. Therefore, this study adopted a strategy of rapid power and pressure ramp-up to achieve optimal diamond growth conditions in the shortest possible time, balancing growth quality with base protection.

Several advanced techniques were used to characterize the experimental results. Optical microscopy (OM, SOPTOP MX8R, Zhejiang, China) was used to observe the surface morphology and seed crystal density of NP-GaN. Atomic force microscopy (AFM, SEMILAB, Budapest, Hungary), in tapping mode, was used to measure the surface roughness of NP-GaN, while scanning electron microscopy (SEM, Hitachi S-4700, Tokyo, Japan) was used to analyze the etching of the SiN_x/GaN layer and the density of the diamond nucleation layer. The constituent phases of the samples and their respective contents were determined by high-resolution X-ray diffraction (HRXRD, Bruker D8 Discover, Billerica, MA, USA). The scanning speed was 0.02°/s, and the scanning range was 30–90°. The carbon configuration of the diamond layers was studied using laser confocal Raman spectroscopy (LabRAM HR Evolution, Palaiseau, France) with a 532 nm laser excitation wavelength, focusing on assessing the residual stress in diamond crystals and evaluating the effect of various NP-GaN surface morphologies on diamond crystal quality. In addition, transient thermal reflectance (TTR) measurements were performed to evaluate the thermal properties of diamond and GaN. During the TTR tests, a metallic aluminum (Al) film was deposited on the diamond surface by thermal evaporation. This, in combination with a 785 nm continuous wave probe laser and a 355 nm pump laser, allowed the precise measurement of key parameters such as TBR and κ [44].

3. Results and Discussion

3.1. Optimizing GaN/Diamond Interface and Material Quality on Both Sides

In 1993, Goodwin [45] proposed a simplified model for CVD diamond growth, considering only hydrogen atoms ([H]) and methyl radicals ([CH₃]). The growth rate (G_R) is expressed as follows:

$${\mathrm{G}}_{\mathrm{R}}={\displaystyle \frac{{\mathrm{k}}_{\left({\mathrm{T}}_{\mathrm{s}}\right)}{\left[\mathrm{C}{\mathrm{H}}_3\right]}_{\mathrm{s}}{\left[\mathrm{H}\right]}_{\mathrm{s}}}{5\times {10}^{-9}+{\left[\mathrm{H}\right]}_{\mathrm{s}}}}$$

(1)

In an MPCVD deposition system, the primary gases are hydrogen (H₂) and CH₄, which decompose into a number of reactive species under microwave energy. Here, [H] plays a critical role in the diamond surface. They not only provide the necessary active sites for [CH₃] radicals, but also facilitate high-quality diamond growth through a selective etching mechanism, where the etching rates of amorphous carbon and graphite are significantly higher than those of diamond. Consequently, the GR and crystal quality of diamond depend on the surface densities of [H] and [CH₃]. Diamond growth can be divided into two stages, as follows: initially, it follows the Volmer–Weber island growth mode, forming a lateral nucleation layer; and, later, it transitions to the van der Drift columnar growth mechanism as the islands expand and merge, resulting in vertical crystal growth. Thus, when growing diamond on a GaN base, it is imperative to consider the etching effect of [H] on the exposed base material during the nucleation phase. To achieve a high-quality GaN/diamond interface, especially when the thickness of the SiN_x dielectric layer is limited, an appropriate seed crystal size and uniform nucleation density must be combined with optimized MPCVD process parameters (including power, pressure, CH₄ concentration, and temperature). This optimization allows for the rapid and continuous growth of seed crystals, minimizing the time that the base material is exposed to the plasma environment. This in turn protects the NP-GaN buffer layer from damaging [H] etching, thereby preserving the intrinsic electrical properties of the device. In addition, it is critical to reduce the formation of small grain stacks and voids in the nucleation layer, thereby densifying and thinning the nucleation layer and lowering the TBR_GaN/diamond [46].

As illustrated in Figure 2a, when the seed crystal size is insufficiently large, the particles exhibit a considerable specific surface area and elevated surface energy. The surfaces of these particles are devoid of adjacent coordinating atoms, which results in a large number of unsaturated bonds and, consequently, a notable degree of surface reactivity. This results in a thermodynamically unstable state, rendering the particles susceptible to spontaneous aggregation. Furthermore, the siphon effect results in a highly nonuniform distribution of seed crystals, with large clusters of crystals leaving almost no seed crystals between them. Conversely, using larger diamond seeds as nucleation sites can mitigate the particles’ aggregation to a certain extent, yet simultaneously introduce new growth challenges. The preferential interaction with active radicals at the top of the seeds, in comparison to the bottom, results in a significantly higher growth rate at the apices than at the bottom. This ultimately leads to the formation of sealed-off tops and the emergence of voids at the bottom. The presence of numerous voids not only undermines the compactness of the diamond film nucleation layer, thereby weakening the adhesion strength between the diamond and the base material, but also increases the interfacial thermal resistance, thereby hindering the efficient dissipation of heat from the active region into the diamond. On the other hand, the mass fraction of the diamond nanoparticle suspension utilized during spin-coating exerts a pivotal influence on the seeding density and its uniformity, as illustrated in Figure 2b. The use of diamond suspensions with a particle size of 100 nm and a mass fraction of approximately 0.3% (diamond seed crystals in acetone) allows for the optimal balance between the uniformity and density of the seed distribution to be achieved. An inadequate seeding density, whether insufficient or excessive, has specific disadvantages. A low seeding density prolongs the exposure area and time from the base to the plasma, which may result in the undesirable outcome of the underlying GaN being etched away between diamond grains after growth. However, excessive seeding densities promote the agglomeration and overlapping of seeds, impeding the growth of underlying particles that fail to access carbon-containing radicals and thus remain undeveloped. This results in a dense yet small-grained diamond nucleation layer with poor κ.

In the context of diamond growth, the densities of the reactive species [H] and [CH₃], along with variations in the growth temperature, are significantly governed by the manipulation of the following two crucial process parameters: plasma power density and CH₄ concentration. It is worth noting that the plasma power density is affected by the coupling of microwave power and chamber pressure. When the microwave power is maintained at a constant level while the pressure is increased gradually, the plasma sphere undergoes contraction, which indirectly enhances the plasma power density [47]. This increase results in an increase in the densities of the active radicals [H] and [CH₃] within the plasma, as well as an increase in the gas temperature. Moreover, the literature has indicated that, under elevated pressure conditions, the density of [H] increases notably faster, by a factor of 10 to 100 times, in comparison to [CH₃] [48,49]. Consequently, as depicted in Figure 3, at a CH₄ concentration of 2% (H₂: CH₄ = 980 sccm: 20 sccm) and a high plasma power density (power: pressure = 26 kW: 180 mbar), accompanied by temperatures reaching 950 °C, significantly accelerated diamond growth rates are achieved, resulting in a thickness of 9.4 μm, compared to 4.9 μm under low power density conditions within the same growth duration. However, this high-pressure environment also exacerbates the etching of the GaN layer, resulting in a large-scale contiguous loss of the GaN layer. In contrast, a reduced power density (power: pressure = 26 kW: 170 mbar, growth temperature 850 °C) serves to mitigate GaN etching but results in slower diamond growth. This ultimately leads to the GaN layer being exposed and susceptible to hole-shaped defects due to inadequate diamond film coverage. In order to address the etching challenges posed by heightened [H] densities, it is necessary to compensate for higher CH₄ concentrations. This strategy aims to accelerate diamond growth by linearly augmenting the [CH₃] density. Research indicates that the [H] concentration remains relatively stable with increasing [CH₃] at a constant pressure [48,49]. Upon elevating the CH₄ concentration to 6%, high power density and temperature conditions (26 kW, 180 mbar, 950 °C) facilitate rapid grain growth and surface coverage. However, this results in premature lateral broadening and capping of diamond grains, which leaves a considerable number of voids at the bottom and compromises the density. Conversely, a moderate reduction in power density (26 kW, 170 mbar, 850 °C) results in a notable enhancement in the bottom density of the diamond layer while simultaneously ensuring the preservation of the GaN layer’s structural integrity.

3.2. Optimizing Surface Roughness for NP-GaN-on-SiC Base Materials

In MOCVD heteroepitaxy, two primary growth modes dominate, as follows: layer-by-layer growth, which is governed by the Frank–van der Merwe (FM) model, and island growth, which adheres to the Volmer–Weber (VW) model [50]. As depicted in Figure 4a, by meticulously tuning the MOCVD growth parameters, including temperature, chamber pressure, V/III ratio, and growth rate, we successfully steered the growth mode of NP-GaN from the undesirable 3D islanding mode to the desirable 2D layer-by-layer mode. This transition significantly improves the surface morphology of the film, converting the initially irregular, island-protruding surface into a more uniform and planar structure. However, the presence of stripe-like features, potentially attributed to the step bunching effect during epitaxy on off-axis crystal orientation substrates [51] (where faster-moving steps coalesce with slower ones, forming large steps and wide terraces), persists. To further refine the surface quality of the NP-GaN films, we incorporated CMP into the processing regimen, achieving profound planarization of the base material. Following the completion of the CMP process, the initial striated morphology is successfully eliminated, resulting in an unparalleled level of smoothness and fineness on the NP-GaN film surface. To provide a quantitative assessment of this enhancement, a detailed morphological analysis was conducted using AFM (Figure 4b). Prior to CMP, the NP-GaN film surface exhibits a pronounced terrace morphology with a root mean square (RMS) roughness of approximately 12 nm, which is consistent with the observations made using OM. Following CMP, with the exception of potential residual polishing solution impurities manifesting as white highlights, the RMS value plummets to 1.2 nm, indicating a substantial improvement in surface flatness. Furthermore, the inset in the AFM image demonstrates that the peak-to-valley height across the NP-GaN film surface is less than 5 nm, reinforcing the exceptional efficacy of CMP in refining the surface quality of the film.

Figure 5a demonstrates that the microtopography and roughness of the base surface have a considerable influence on the distribution of seed crystals during spin-coating. Specifically, when the base surface is densely populated with island-like protrusions, centrifugal forces during the spin-coating process likely propel the seeds within the suspension towards the low potential energy regions surrounding these protrusions, culminating in pronounced seed clustering in these zones. Conversely, the apexes of the protrusions exhibit minimal seed attachment due to their elevated potential energy, resulting in a pronounced disparity in distribution. By adjusting the MOCVD process parameters to optimize the surface flatness, although optimizing the overall seed distribution, residual micro-height variations still inevitably result in the formation of localized seed agglomerations, accompanied by adjacent ‘blank zones’ with markedly reduced seed density, thereby circumscribing the achievement of uniform seed distribution. In stark contrast, the adoption of CMP technology effectively eliminates the physical barriers stemming from the surface morphology, enabling seeds to be deposited on the base in a more uniform and consistent manner, thereby establishing a robust foundation for high-quality thin-film growth. Figure 5b shows the cascading effects of the base surface morphology on the diamond seeding process and its subsequent growth under identical MPCVD conditions and durations. On the NP-GaN base, with its island-like protrusions, the absence of seeds on the protrusions results in abnormal grain enlargement and no effective meeting between the grains in these zones. Therefore, the resulting film is discontinuous. In contrast, the high density of the seeds in depressions results in the formation of small yet tightly interconnected diamond grains, which achieve complete surface coverage. The SEM image of the diamond surface reveals discernible disparities in grain size around the unconnected zones. The proximal grains exhibit significant enlargement, whereas the peripheral grains remain fine and densely packed. Notably, discontinuous diamond growth exposes the underlying NP-GaN base to the harsh energetic plasma environment, which induces severe etching and compromises the overall quality and structural integrity of the film. In contrast, the CMP-treated base exhibits superior diamond growth, with uniform and rapid lateral expansion of grains fully enveloping the base. The underlying NP-GaN layer remains well preserved and is devoid of noticeable etching. The diamond grains on these smooth bases also display remarkable uniformity in size and distribution.

Figure 6a further reinforces the impact of surface roughness on seeding and subsequent diamond growth. The prominent presence of GaN diffraction peaks directly attests to the incomplete lateral growth of diamond films over rough bases. This phenomenon is further exacerbated by the increased base roughness, as evidenced by the intensified GaN peak intensity, which indicates larger exposed GaN areas. An inverse correlation is observed between the intensities of the diamond and GaN X-ray diffraction peaks. In the case of smooth, defect-free NP-GaN layers, the PCD films exhibit complete base coverage, with the GaN diffraction peak being virtually absent. To conduct a more thorough analysis of the grown diamond layers, we employed Raman spectroscopy. As depicted in Figure 6b, under varying base surface roughness conditions, a consistent shift in the Raman peaks is observed within ~5-μm-thick diamond thin films. Specifically, compared to the standard diamond peak typically located at 1331.5 cm⁻¹, these peaks exhibit a redshift of approximately 3 cm⁻¹ towards lower frequencies, directly indicating the presence of tensile stress within the films, estimated to be around 1.7 GPa. Additionally, a series of small peaks emerge within the 1420 to 1570 cm⁻¹ range of the Raman spectrum, identified as characteristic peaks of non-diamond phases (G band), reflecting the presence of amorphous or disordered carbon structures alongside the diamond structure in the films. Notably, as the surface morphology of the NP-GaN base is progressively optimized, the intensity of these non-diamond phase peaks demonstrate a marked decrease. This change directly suggests a gradual improvement in the crystalline quality of diamond films, with a corresponding reduction in the content of amorphous or impurity phases, thereby validating the effectiveness of optimizing the base surface morphology in enhancing the purity and crystalline quality of diamond thin films.

The TBR between the diamond and NP-GaN buffer layers was measured and calculated using the TTR technique. In order to comply with the sensitivity constraints of the TTR test, the thickness of the diamond layer was further controlled and reduced to approximately 2.5 μm, thus ensuring the accuracy of the measurement. Figure 7 provides a summary of the TBR_GaN/Diamond values obtained on NP-GaN bases with varying surface morphologies. The results indicate that, as the surface roughness of the NP-GaN base decreases, the TBR_GaN/Diamond generally exhibits a downward trend, with its average value significantly decreasing from 54.2 m²·K/GW to 23.5 m²·K/GW. These findings demonstrate that optimizing the surface roughness of the NP-GaN base can effectively reduce TBR_GaN/Diamond to a certain extent. Further analysis demonstrates that samples with an increased base surface roughness exhibit augmented dispersion in the TBR_GaN/diamond values among the tested points.

Table 1. Comparative analysis of the results of this study on the deposition of diamond on GaN with those of recent studies.

Year	Dielectric Layer		CVD Type/Thickness of Diamond (µm)	TBRGaN/diamond (m2·K/GW)
	Thickness (nm)	Material
This study	30	SiNx	MPCVD/~2.5	~23.5
2018 [38]	5	SiNx	MPCVD/1.0	~9.5
		AlN		~18.2
	No interlayer			~41.4
2019 [39]	100	SiNx	MPCVD/2.0	~38.5
	100	AlN		~56.4
2019 [19]	35	SiNx	MPCVD/120.0	~31.0
2019 [52]	50	SiNx	MPCVD/100.0	33.0
	36			22.0
	41			15.0
2019 [53]	36	SiNx	MPCVD/75.0	20.0
	17			13.0
2019 [54]	30	SiNx	MPCVD/100.0	18.0
2020 [20]	20	Al0.32Ga0.68N	MPCVD/35.0	~30.0

lists the relevant process parameters and experimental results (dielectric material and thickness, diamond film thickness, and TBR_GaN/diamond value) for depositing diamond on GaN reported since 2018. It can be seen that most of the GaN-on-diamond HEMTs fabricated with this method feature a ~30-nm-thick SiN_x layer and a TBR_GaN/diamond in the range of 20–30 m²·K/GW [9]. The optimized results of this study are comparable to those of recent years, indicating the feasibility of simplifying the experimental process. Furthermore, as illustrated in the inset in the top right corner of Figure 7, test points with elevated TBR_GaN/diamond values tend to correlate with regions exhibiting a higher κ of the diamond. These trends are in accordance with the SEM characterization results, which demonstrate that the inhomogeneity of the substrate surface morphology (such as island-like protrusions and stripe clustering) results in an uneven distribution of seed crystal seeding density. In regions with a sparse distribution of seed crystals, the NP-GaN base may undergo intensified local etching, resulting in elevated TBR_GaN/diamond values and increased variation in the overall TBR_GaN/diamond values. Conversely, regions with lower seed crystal density tend to form larger diamond grains during subsequent growth, and these large-grain regions exhibit higher κ due to the reduction in grain boundaries.

4. Conclusions

We have reported a new integration strategy for combining diamond with GaN, with the objective of exploiting the substantial potential of GaN for high-frequency operation in high-power density applications. This strategy involved the direct deposition of diamond films onto NP-GaN epitaxial layers, which circumvented the complexities associated with conventional integration methods. By leveraging a high-power condition of 26 kW for PCD growth, coupled with meticulous control over diamond seed seeding techniques and optimization of MPCVD process parameters, we successfully achieved a high degree of density in the PCD films while preserving the structural integrity of the NP-GaN layer. In order to address the prevalent issue of suboptimal surface quality in NP-GaN epitaxial materials, we studied the use of CMP technology. This resulted in a significant reduction in the surface roughness of NP-GaN to less than 2 nm. This improvement in surface quality provided a smoother wafer surface for subsequent diamond growth. The experimental results indicate that, with the improvement of the surface morphology of the NP-GaN buffer layer, the TBR between it and the diamond film decreases progressively. This achievement not only demonstrates the feasibility of using an alternative method for the integration of GaN and diamond materials, but also offers a potentially efficient solution to the thermal management challenges encountered in high-power electronic devices.