Thick GaN Films Research Articles

Wafer-scale bonded GaN–AlN with high interface thermal conductance

Wide and ultrawide bandgap semiconductors, such as GaN, play a crucial role in high-power applications, yet their performance is often constrained by thermal management challenges. In this work, we introduce a high-quality interface between GaN and AlN, prepared through wafer-scale bonding and verified via high-resolution transmission electron microscopy and transport experiments. We experimentally measured the thermal boundary conductance of the GaN–AlN interface, achieving up to 320 MW/m2K at room temperature using an ultrafast optical technique and sensitivity examinations. Non-equilibrium atomistic Green's functions and density functional theory simulations were conducted to model the interface phonon modes and their contributions to thermal transport, demonstrating good agreement with the experimental results from 80 to 300 K. Additionally, we observed a size-dependent effect on the thermal boundary conductance related to the GaN film thickness from 180 to 450 nm, which we attributed to quasi-ballistic thermal transport through molecular dynamics simulations. Our study has demonstrated a scalable processing route for wafer-sized chip packaging and provides fundamental insights to mitigate near-junction thermal resistance. Further exploration of interface engineering could facilitate co-design strategies to advanced thermal management technologies.

Heterogeneous integration of thick GaN and polycrystalline diamond at room temperature through dynamic plasma polishing and surface-activated bonding

Direct heterogeneous integration of GaN on diamond enhances device performance and reliability for high-power applications, while the implementation on thick GaN films and polycrystalline diamond (p-diamond) substrates remains challenging. In this study, the direct bonding between thick GaN films (∼370 μm) and p-diamond substrates was achieved. A dynamic plasma polishing (DPP) technique was adopted on the diamond to flatten surficial spikes from maximum 15 nm to 1.2 nm, obtaining a smooth surface with 0.29 nm Ra, achieving a robust GaN/diamond bonding with high bonding rate of ∼92% at room temperature combining the surface-activated bonding (SAB) method. The chemical status, thermal stress, and interfacial microstructures of GaN/diamond heterostructures were analyzed, revealing a residual stress of ∼200 MPa at the GaN/diamond interface, and the asymmetric increase of interfacial stress at rising temperature demonstrates the effectiveness of amorphous interlayer to release the stress.

High growth rate magnetron sputter epitaxy of GaN using a solid Ga target

Magnetron sputter epitaxy (MSE) is a promising processing route for group-III nitride semiconductors, with the potential to enable high-quality and low cost GaN growth for widespread use. However, fundamental technological hurdles must be overcome to enable the adoption of MSE in industrial production. Here, we present a new UHV-compatible magnetron design with high-performance cooling, enabling high GaN growth rates at high growth temperatures using a solid Ga target. The magnetron is tested with a wide range of process parameters and a stable process is feasible while maintaining the solid state of the Ga target. High GaN growth rates of up to 5 μm/h are achieved at room temperature and a growth rate of 4 μm/h at high temperature, which is one order of magnitude higher compared to MSE with a liquid target. We grow GaN on c-plane sapphire substrates and show the impact of partial pressure ratio and target-to-substrate distance (TSD) on growth rate, film morphology and crystal quality of GaN films with scanning electron microscopy and X-ray diffraction. While the growth rate and film morphology are strongly impacted by the process parameter variation, the crystal quality is further impacted by the overall film thickness. For a 2 μm thick GaN film a full width at half maximum of X-ray rocking curve (ω-FWHM) of GaN 10 1‾ 1 reflection of 0.32° is achieved. We demonstrate a process window for growth of dense and smooth GaN films with high crystal quality using low N2 flow rates and high TSD. By introducing a 20 nm AlN nucleation layer prior to the growth of 390 nm GaN, the ω-FWHM of GaN 0002 reflection of 0.19° is achieved. The epitaxially grown crystalline structure is precisely examined by transmission electron microscopy.

Near-field radiative heat transfer between nanoporous GaN films

Photon tunneling effects give rise to surface waves, amplifying radiative heat transfer in the near-field regime. Recent research has highlighted that the introduction of nanopores into materials creates additional pathways for heat transfer, leading to a substantial enhancement of near-field radiative heat transfer (NFRHT). Being a direct bandgap semiconductor, GaN has high thermal conductivity and stable resistance at high temperatures, and holds significant potential for applications in optoelectronic devices. Indeed, study of NFRHT between nanoporous GaN films is currently lacking, hence the physical mechanism for adding nanopores to GaN films remains to be discussed in the field of NFRHT. In this work, we delve into the NFRHT of GaN nanoporous films in terms of gap distance, GaN film thickness and the vacuum filling ratio. The results demonstrate a 27.2% increase in heat flux for a 10 nm gap when the nanoporous filling ratio is 0.5. Moreover, the spectral heat flux exhibits redshift with increase in the vacuum filling ratio. To be more precise, the peak of spectral heat flux moves from ω = 1.31 × 1014 rad⋅s−1 to ω = 1.23 × 1014 rad⋅s−1 when the vacuum filling ratio changes from f = 0.1 to f = 0.5; this can be attributed to the excitation of surface phonon polaritons. The introduction of graphene into these configurations can highly enhance the NFRHT, and the spectral heat flux exhibits a blueshift with increase in the vacuum filling ratio, which can be explained by the excitation of surface plasmon polaritons. These findings offer theoretical insights that can guide the extensive utilization of porous structures in thermal control, management and thermal modulation.

Growth of thick GaN films on 2-inch ScAlMgO4 substrates by halide vapor phase epitaxy

Thick GaN bulks up to 5.2 mm were grown on 2-inch ScAlMgO4 (SAM) substrates by halide vapor phase epitaxy. Free-standing GaN substrates were fabricated by spontaneous separation during the cooling process. The crystalline quality and orientation of GaN for each film thickness were evaluated by X-ray Rocking Curve (XRC) measurements on the (0002) and (10–12) planes. The FWHM of the XRC (0002) and (10–12) diffraction peaks for the 2.9-mm-thick GaN free-standing bulk were 39 and 51 arcsec, respectively, and the dark spot density measured by cathodoluminescence mapping was 1.5 × 106 cm−2. In addition, the radius of curvature of its free-standing GaN bulk was 5.0 m. The impurity concentrations originating from the SAM almost reached detection limits of the SIMS measurements at a GaN film thickness of around 10 µm and more. The crystalline quality of GaN grown on the SAM substrates was comparable to that of GaN grown on sapphire substrates, indicating that the SAM substrate can replace conventional sapphire substrate because of the ease of fabrication of free-standing GaN bulk.

First-principles study for self-limiting growth of GaN layers on AlN(0001) surface

The GaN thickness dependence of surface structural stability and adsorption behavior of Ga adatom in GaN layers on a AlN(0001) surface are investigated on the basis of first-principles calculations to clarify the self-limiting growth on AlN(0001) surface during metal-organic vapor phase epitaxy. The calculations demonstrate that the stability of reconstructed GaN layers on a AlN(0001) surface is similar to that of a GaN(0001) surface irrespective of the GaN film thickness. Furthermore, we find that the adsorption of a Ga adatom on the AlN(0001) surface easily occurs compared with that on AlN(0001) surface with GaN layers. The difference in the adsorption behavior implies that the growth of GaN layers on a AlN(0001) surface is suppressed. The calculated results provide theoretical guidance for understanding the self-limiting growth of GaN layers, resulting in the formation mechanism of GaN quantum wells.

Growth of bulk single crystal ScAlMgO4 boules and GaN films on ScAlMgO4 substrates for GaN-based optical devices, high-power and high-frequency transistors

Single crystal growth of ScAlMgO4 boules with 10 mm up to 4 in. (c.a. 101.6 mm) in diameter by Czochralski technique was demonstrated. Some high quality ScAlMgO4 single crystal boules with 2-inches (c.a. 50.8 mm) in diameter were successfully grown without dislocation, which was revealed by X-ray rocking curve and X-ray topography measurements for the c-plane wafers. After a GaN template was formed on a substrate cut out from a grown ScAlMgO4 boule by using metalorganic vapor phase epitaxy, a thick GaN film was epitaxially grown on the template using hydride vapor phase epitaxy (HVPE). At the cooling stage after the growth in the HVPE process, the thick grown GaN film was separated by itself from the ScAlMgO4 substrate due to high cleavability of ScAlMgO4, which makes it possible the reuse of the substrate. The free-standing GaN grown on the reused ScAlMgO4 substrate showed the same quality as a GaN fabricated on a fresh ScAlMgO4 substrate. The reuse of ScAlMgO4 substrates is beneficial for saving the GaN films fabrication cost. The fabrication of a free-standing GaN substrate with 2 in. in diameter and 1 mm in thickness on ScAlMgO4 substrates was demonstrated to be more high-yield than the fabrication on a sapphire substrate. The growth method shown in this study is very promising, and it opens an efficient way to obtain GaN free-standing wafers and to reduce the production cost of GaN wafers.

Fabrication of 2-Inch Free-Standing GaN Substrate on Sapphire With a Combined Buffer Layer by HVPE.

Free-standing GaN substrates are urgently needed to fabricate high-power GaN-based devices. In this study, 2-inch free-standing GaN substrates with a thickness of ~250 μm were successfully fabricated on double-polished sapphire substrates, by taking advantage of a combined buffer layer using hydride vapor phase epitaxy (HVPE) and the laser lift-off technique. Such combined buffer layer intentionally introduced a thin AlN layer, using a mix of physical and chemical vapor deposition at a relatively low temperature, a 3-dimensional GaN interlayer grown under excess ambient H2, and a coalescent GaN layer. It was found that the cracks in the epitaxial GaN layer could be effectively suppressed due to the large size and orderly orientation of the AlN nucleus caused by pre-annealing treatment. With the addition of a 3D GaN interlayer, the crystal quality of the GaN epitaxial films was further improved. The 250-μm thick GaN film showed an improved crystalline quality. The full width at half-maximums for GaN (002) and GaN (102), respectively dropped from 245 and 412 to 123 and 151 arcsec, relative to those without the 3D GaN interlayer. The underlying mechanisms for the improvement of crystal quality were assessed. This method may provide a practical route for fabricating free-standing GaN substrates at low cost with HVPE.

Fe-doped GaN grown on stirring-assisted Na-flux process GaN thick film and its application on Ohmic contact UV detector

Abstract Na-flux method is the most promising approach to grow GaN thick film substrate for GaN homoepitaxy. Here, the influence of solution stirring on grown GaN crystal morphology, crystal quality, dislocation density and impurity concentration was assessed for GaN crystal growth by Na-flux method. It is proved that the crystallinity, surface roughness and dominated orientation of the GaN thick layers are greatly influenced by stirring process, revealing a direct and correlated growth mechanism for the growth of GaN using Na-flux method. Notably, the surface roughness was reduced by three orders of magnitude under stirring (compared to un-stirred sample). This stirring-assisted GaN thick film was then used as seed layer to grow high resistance Fe-doped GaN epitaxial layer by applying HVPE method. On which Ohmic contact was successfully fabricated and an UV-detector with a high detectivity of ~1013 Jones, which was among the highest values, and fast response (rise time of 100 ms, decay time of 160 ms) was obtained.

2 in. Free-standing GaN grown by HVPE with sputtered AlN buffer on GaAs substrate

A magnetron sputtered AlN buffer layer is applied for preparing 2 in. free-standing (FS) GaN on the GaAs substrate by hydride vapor phase epitaxy (HVPE). The AlN buffer layer with excellent compactness can prevent As atoms from diffusing to the GaN growth surface. Thus it serves as an effective protective layer for GaAs substrate and also a seed layer for the deposition of GaN in the early stage of the high-temperature GaN growth. Benefiting from the decomposition property of GaAs at temperature higher than 900 °C, thick GaN film self-separated completely from GaAs substrate during growth. By this way, a 2 in. FS-GaN layer with the thickness of 1.3 mm is finally obtained after the long growth time about 20 h.

Comparison study of GaN films grown on porous and planar GaN templates**Project supported by the National Key R&D Program of China (Grant No. 2017YFB0404201), the State Key R&D Program of Jiangsu Province, China (Grant No. BE2019103), the Six-Talent Peaks Project of Jiangsu Province, China (Grant No. XCL-107), the Fund from the Solid-state Lighting and Energy-saving Electronics Collaborative Innovation Center, PAPD,

The GaN thick films have been grown on porous GaN template and planar metal-organic chemical vapor deposition (MOCVD)-GaN template by halide vapor phase epitaxy (HVPE). The analysis results indicated that the GaN films grown on porous and planar GaN templates under the same growth conditions have similar structural, optical, and electrical properties. But the porous GaN templates could significantly reduce the stress in the HVPE-GaN epilayer and enhance the photoluminescence (PL) intensity. The voids in the porous template were critical for the strain relaxation in the GaN films and the increase of the PL intensity. Thus, the porous GaN converted from β-Ga2O3 film as a novel promising template is suitable for the growth of stress-free GaN films.

Effect of C-doped GaN film thickness on the structural and electrical properties of AlGaN/GaN-based high electron mobility transistors

High-resistivity carbon doped GaN (C-GaN) is highly desirable for high electron mobility transistors (HEMTs) application to reduce the leakage current at high electric fields. Herein, we investigate the structural and electrical properties of AlGaN/GaN-based HEMTs with different thicknesses of the underlying C-GaN layer. Reciprocal space mapping analysis indicates more lattice defects within GaN layer of the AlGaN/GaN heterostructure with thicker underlying C-GaN layer. Finite element method simulations reveal that the stress distribution over the AlGaN/GaN heterostructure thickness can be tuned by increasing the underlying C-GaN layer thickness from 2 to 7 μm. Consequently, a significant decrease of two-dimensional electrons gas mobility from 1188 to 653 cm2 V−1 s−1 measured by Hall effect method is observed at a higher underlying C-GaN layer thickness (7 μm). The change of electrical properties for HEMT structures with different underlying C-GaN thicknesses is not only affected by structural quality of the epilayers but also by thermal stress over HEMT structure thickness. Our results pave the avenue for improving the electrical properties of HEMT with a suitable thickness of underlying C-GaN layer.

Wafer-scale crack-free 10 µm-thick GaN with a dislocation density of 5.8 × 107 cm−2 grown on Si

The increasing demand for complementary metal-oxide-semiconductor integrated GaN-based optoelectronics and power electronics calls for wafer-scale crack-free and high quality GaN thick film grown on Si. However, the huge mismatch in both lattice constant and coefficient of thermal expansion between GaN and Si often causes a high density of threading dislocations (TDD, typically 109–1010 cm−2) and the formation of micro-crack networks, producing a poor-quality GaN film with a limited thickness (<6 µm). By virtue of a well-designed Al-composition step-graded AlN/AlGaN buffer, a wafer-scale crack-free 10 µm-thick successive GaN with a low TDD of 5.8 × 107 cm−2 grown on Si was achieved for the first time. Detailed study on the depth-dependent strain and dislocation evolution shed light on the complex interplay between strain relaxation and defect reduction, as well as their influences on the GaN optical properties along the growth direction. This work paves the way for building high-performance GaN-based optoelectronics and power electronics on the Si platform.

Critical thickness of GaN film in controllable stress-induced self-separation for preparing native GaN substrates

Stress-induced self-separation is one of the most efficient process for preparing native GaN substrate. The control of GaN film thickness is the key point for GaN film separating from substrate completely. Considering the bowing of bilayer, we studied the radial stress in GaN film before separation. A shrunken circular delamination front model was proposed to derive the unrelaxed stress after separation, and the energy release rate for GaN/sapphire systems of different thicknesses was investigated during the whole separation process. A critical thickness about 500–700 μm was determined for separating 2-inch (5.08 cm) GaN film from a sapphire substrate. By precisely controlling the GaN film thickness around such critical thickness, the complete separation rate could be increased greatly to 74%, which is of great importance in realizing the industrialization of GaN substrate.

Reuse of ScAlMgO4 substrates utilized for halide vapor phase epitaxy of GaN

ScAlMgO4 (SCAM) substrates with a small lattice-mismatch to GaN and c-plane cleavability are promising for fabricating high-quality free-standing GaN wafers. To reduce the cost in the fabrication of free-standing GaN wafers, the reuse of a SCAM substrate is demonstrated. By cleaving a SCAM substrate which has been already utilized for the growth of a thick GaN film by halide vapor phase epitaxy, the atomically flat surface can be obtained. The threading dislocation density of a 320 μm thick GaN film grown on this cleaved SCAM substrate is 2.4 × 107 cm−2, which is almost the same as that on a new SCAM substrate. This result indicates that a SCAM substrate can be reused for GaN growth.

Theoretical investigations on the growth mode of GaN thin films on an AlN(0001) substrate

The growth modes of GaN thin films on an AlN(0001) substrate are systematically investigated using our macroscopic theory with the aid of empirical interatomic potential and ab initio calculations. Using empirical interatomic potential calculations, we demonstrate that a GaN/AlN(0001) system with misfit dislocation (MD) is stabilized compared with a coherently grown system when the GaN film thickness exceeds six monolayers. According to the calculated results including the surface energy of GaN, the macroscopic free energy calculations for the growth mode imply that the growth of GaN on AlN(0001) proceeds along the lower-energy path from two-dimensional coherent (2D-coherent) to 2D-MD under Ga-rich conditions but from 2D-coherent to three-dimensional coherent with truncated hexagonal pyramid islands consisting of {} facets under N-rich conditions owing to surface energy anisotropy, which depends strongly on the growth conditions. These results suggest that surface energy anisotropy is crucial for the growth of GaN on an AlN(0001) substrate.

Single-crystalline GaN porous template prepared by a simple top-down nitridation technique

We have developed a simple top-down technique for preparing GaN porous templates with high single-crystallinity by high-temperature nitridation of β-Ga2O3 films. The GaN template converted from β-Ga2O3 film contains many voids and exhibits good-crystallinity with (0 0 2) orientation. The results confirm that the GaN porous template is stress-free, and the re-nitridation can effectively improve the crystal quality and luminescence properties of the GaN template. A ∼100 μm GaN thick film has been grown on the GaN porous template by halide vapor phase epitaxy, and self-separation of the GaN thick film from the sapphire substrate has been successfully achieved.

Self-Powered Ultraviolet Photodetector with Superhigh Photoresponsivity (3.05 A/W) Based on the GaN/Sn:Ga2O3 pn Junction.

Ultraviolet (UV) radiation has a variety of impacts including the health of humans, the production of crops, and the lifetime of buildings. Based on the photovoltaic effect, self-powered UV photodetectors can measure and monitor UV radiation without any power consumption. However, the current low photoelectric performance of these detectors has hindered their practical use. In our study, a super-high-performance self-powered UV photodetector based on a GaN/Sn:Ga2O3 pn junction was generated by depositing a Sn-doped n-type Ga2O3 thin film onto a p-type GaN thick film. The responsivity at 254 nm reached up to 3.05 A/W without a power supply and had a high UV/visible rejection ratio of R254nm/ R400nm = 5.9 × 103 and an ideal detectivity at 1.69 × 1013 cm·Hz1/2·W-1, which is well beyond the level of previous self-powered UV photodetectors. Moreover, our device also has a low dark current (1.8 × 10-11A), a high Iphoto/ Idark ratio (∼104), and a fast photoresponse time of 18 ms without bias. These outstanding performance results are attributed to the rapid separation of photogenerated electron-hole pairs driven by a high built-in electric field in the interface depletion region of the GaN/Sn:Ga2O3 pn junction. Our results provide an improved and easy route to constructing high-performance self-powered UV photodetectors that can potentially replace traditional high-energy-consuming UV detection systems.

Homoepitaxial HVPE GaN: A potential substrate for high performance devices

We investigated the structural, optical, and electrical properties of thick freestanding HVPE GaN films deposited on and then removed from substrates consisting of thick HVPE-GaN deposited on ammonothemal-substrates. High-resolution XR diffraction of both Ga- and N-polar faces and XR topography studies confirmed high crystalline quality. Raman scattering measurements carried on both polar faces of these samples confirm that they are free of biaxial stress, and also confirm that the Si concentration increases with growth, as verified by SIMS analysis. This variation in the incorporation of the Si shallow impurity was also observed in the photoluminescence of these samples. Electrical transport measurements are consistent with good electrical properties. The results strongly suggest that this type of substrate could be conveniently tailored for the fabrication of high performance devices.

Characterization of the ScAlMgO4 cleaving layer by X-ray crystal truncation rod scattering

ScAlMgO4—easily cleaved in c-plane—forms a natural superlattice structure of a ScO2 layer and two Al0.5Mg0.5O layers stacking along c-axis. ScAlMgO4 is one of the RAMO4-type layered multicomponent oxides and a promising lattice-matching substrate material for InGaN and ZnO. Identification of the topmost layer and the surface atomic structure of the cleaved ScAlMgO4 (0001) are investigated by the X-ray crystal truncation rod scattering method. It is confirmed that ScAlMgO4 is cleaved between the two Al0.5Mg0.5O layers. The two parts separated at this interlayer are inversion symmetric to each other and without surface charge. This prevents parallel-plate-capacitor-like electrostatic force during the cleavage. Two different mechanisms are proposed for the two types of cleavage caused by the impact of a wedge and by the in-plane stress due to an overgrown thick GaN film. It is also revealed that about 10%–20% of the topmost O atoms are desorbed during a surface cleaning at 600 °C in ultra-high vacuum. Surface observations using reflection high-energy electron diffraction are possible only after the high-temperature cleaning because the electrical conduction caused by the oxygen deficiency prevents the charge-up of the insulating sample.