Gallium Nitride Epitaxial Layers Research Articles

High-performance GaN ultraviolet polarization-sensitive photodetector based on ferroelectric polarization LiNbO3

High responsivity ultraviolet (UV) photodetectors (PDs) are essential for abundant civilian and military applications. Gallium nitride (GaN) has emerged as an ideal material for UV PD fabrication due to its favorable properties. However, the quality of GaN epitaxial layers significantly impacts device performance and reliability. Sapphire-based GaN epitaxial growth technology enables the realization of high-quality GaN epitaxial layers, making it the preferred choice for GaN substrates. Nonetheless, the thermal expansion coefficient mismatch between sapphire and GaN can lead to crystal mismatch and stress accumulation at high temperatures, affecting device performance and reliability. In contrast, lithium niobate (LiNbO3) exhibits similar coefficients of thermal expansion to GaN, mitigating crystal mismatch and stress accumulation issues. Here, we report the realization of a GaN UV PD by laminating GaN membrane onto ferroelectric LiNbO3 through selective electrochemical etching of the sapphire-based GaN epitaxial film. The LiNbO3-based GaN PD achieves a specific high on/off ratio of 107. At a 5 V bias voltage, the device exhibits a high peak responsivity of 1.712 × 103 A/W under 325 nm laser illumination. Furthermore, the device demonstrates excellent performance for polarization light detection, with a polarization ratio of approximately 54.95. Exploiting the local ferroelectric polarization of x-cut LiNbO3, the photogenerated electron–hole pairs in GaN are efficiently separated by the electrostatic field from the polarization of ferroelectric LiNbO3, resulting in enhanced light-to-electric conversion efficiency. Our work presents a method for fabricating high responsivity GaN-based UV PD, showcasing the potential of integrating ferroelectric LiNbO3 to enhance device performance.

Impact of oxygen plasma power on the performance of Ga2O3 passivated GaN ultraviolet photodetectors

The role of oxygen plasma power on the extent of surface passivation of Gallium Nitride (GaN) epitaxial layers and the photo-response of Au/Ni/GaN Schottky detectors is investigated. The surface morphology is found to be preserved in oxide passivated GaN at intermediate plasma power which significantly deteriorates at higher power. The photoluminescence spectrum shows a fall in the intensity of band edge and defects related yellow luminescence at higher power indicating the generation of non-radiative recombination centres. The chemical and electronic structures of the passivated layer investigated by x-ray photoelectron spectroscopy revealed the formation of stoichiometric Ga2O3 up to intermediate plasma power and partial decomposition of Ga2O3 along with formation of gallium sub-oxides at higher power. This leads to an enhancement in the responsivity of the photodetector after passivation at an intermediate plasma power followed by a considerable reduction at higher power. From the electrical transport measurement across the Au/Ni/GaN Schottky diode, it is observed that the diode manifests more Schottky (Ohmic) character after oxygen plasma treatment at intermediate (higher) power. The present investigations provide a clear guideline on the selection of oxygen plasma power for surface passivation on GaN, which is highly beneficial in optimizing the performance of GaN-based optoelectronic and power devices.

Wafer level quasi-van der Waals epitaxy of AlGaN/GaN heterojunctions on sp2-bonded BN controlled by AlN nucleation layer

Two dimensional materials, such as sp2-bonded boron nitride (BN) and graphene, have been proven to be excellent templates for the growth and fabrication of independent group III-nitride materials. Herein, wafer level sp2-bonded BN film was successfully grown as a release layer on two-inch sapphire substrate using MOCVD. The different nucleation mechanisms of gallium nitride (GaN) and aluminum nitride (AlN) grown on the BN surface were investigated. We discovered that AlN has a higher nucleation island density compared to GaN on the surface of BN which facilitates better merging of the GaN epitaxial layer. In addition, the quality of the GaN epitaxial layer was optimized by adjusting the growth temperature of AlN. A complete AlGaN/GaN heterojunction structure was subsequently grown on the best quality of GaN, and its average mobility and interlayer carrier concentration was about 1600 cm2 V−1 s−1 and 1 × 1013 cm−2. Finally, the AlGaN/GaN heterojunction was easily transferred from the substrate using tape, which proved the good 2D characteristics of sp2-bonded BN and can undergo non-destructive van der Waals transfer for the upper layer device.

Improvement of AlSiO/GaN interface by a novel post deposition annealing using ultra high pressure

In this study, a novel post-deposition annealing (PDA) technique employing ultra-high pressure was demonstrated for the first time. A 40 nm thick AlSiO gate insulator was deposited using atomic layer deposition (ALD) on n-type gallium nitride (GaN) epitaxial layers grown on free-standing GaN substrates. These PDA techniques were performed at 600 °C in a nitrogen ambient under 400 MPa, with normal pressure conditions used as the references. The annealing duration varied within the range of 10, 30, 60, and 120 min. For normal pressure annealing, the flat-band voltage of capacitance-voltage curves exhibited a shift towards the positive bias direction as the annealing time increased. Conversely, for the 400 MPa annealing, the flat-band voltage approached the ideal curve as the annealing time extended. For 400 MPa and 120 min, low interface state density of ∼5 × 1011 cm−2 eV−1 or less at E c −0.20 eV was obtained. These results suggest that post-deposition annealing under ultra-high pressure could be a viable method for improving the interfacial characteristics of AlSiO/GaN.

Research Progress in Capping Diamond Growth on GaN HEMT: A Review

With the increased power density of gallium nitride (GaN) high electron mobility transistors (HEMTs), effective cooling is required to eliminate the self-heating effect. Incorporating diamond into GaN HEMT is an alternative way to dissipate the heat generated from the active region. In this review, the four main approaches for the integration of diamond and GaN are briefly reviewed, including bonding the GaN wafer and diamond wafer together, depositing diamond as a heat-dissipation layer on the GaN epitaxial layer or HEMTs, and the epitaxial growth of GaN on the diamond substrate. Due to the large lattice mismatch and thermal mismatch, as well as the crystal structure differences between diamond and GaN, all above works face some problems and challenges. Moreover, the review is focused on the state-of-art of polycrystalline or nanocrystalline diamond (NCD) passivation layers on the topside of GaN HEMTs, including the nucleation and growth of the diamond on GaN HEMTs, structure and interface analysis, and thermal characterization, as well as electrical performance of GaN HEMTs after diamond film growth. Upon comparing three different nucleation methods of diamond on GaN, electrostatic seeding is the most commonly used pretreatment method to enhance the nucleation density. NCDs are usually grown at lower temperatures (600–800 °C) on GaN HEMTs, and the methods of “gate after growth” and selective area growth are emphasized. The influence of interface quality on the heat dissipation of capped diamond on GaN is analyzed. We consider that effectively reducing the thermal boundary resistance, improving the regional quality at the interface, and optimizing the stress–strain state are needed to improve the heat-spreading performance and stability of GaN HEMTs. NCD-capped GaN HEMTs exhibit more than a 20% lower operating temperature, and the current density is also improved, which shows good application potential. Furthermore, the existing problems and challenges have also been discussed. The nucleation and growth characteristics of diamond itself and the integration of diamond and GaN HEMT are discussed together, which can more completely explain the thermal diffusion effect of diamond for GaN HEMT and the corresponding technical problems.

Homoepitaxial GaN terahertz planar Schottky barrier diodes

In this work, a new type of terahertz Schottky barrier diode (SBD) based on homoepitaxial gallium nitride (GaN) was fabricated for high-power and high-frequency multiplier applications. The measured full width at half maximum of x-ray diffraction peaks for homoepitaxial GaN (002) and (102) plane is only one third of the heteroepitaxial GaN on sapphire substrate. Additionally, the increased electron mobility in both n−/n+ GaN epitaxial layers, induced by improved material quality, can effectively reduce the epitaxial resistance (R epi) and the spreading resistance. As a result, the total series resistance of the fabricated GaN SBD is 13.7 Ω, which is only 65% of that of heteroepitaxial GaN on a sapphire substrate. Additionally, the cut-off frequency (f c) is improved to 1.61 THz at zero bias voltage and the measured breakdown voltage is 16.17 V at −1 μA.

Strain mapping of GaN substrates and epitaxial layers used for power electronic devices by synchrotron X-ray rocking curve topography

Strain and tilt maps of gallium nitride (GaN) materials including substrates, ion-implanted and annealed epilayers, and etched and regrown epilayers by different etching methods have been generated from synchrotron X-ray rocking curve topography. The strain map of the ammonothermal grown GaN substrate shows uniform low strain values, while that of the patterned hydride vapor phase epitaxy (HVPE) grown substrate shows high strain centers as well as uniformly distributed low strain between them. Strain variations within the as-implanted wafers and the annealed wafers are similar, while the tilt levels are significantly decreased for the annealed wafers. The inductively coupled plasma (ICP) etched and then regrown sample gives rise to the significant variations in strain values within the imaged region, while using tertiary butyl chloride (TBCl) to etch the sample does not affect the strain distribution compared to the continuously grown sample without etching and can remove some damage from a preceding ICP etching process.

Optical Transitions and Magnetism in Mn-Implanted Gallium Nitride for Three-Level Magnetooptic Devices

A method of preparing passive materials suitable for magnetooptic interactions is shown using manganese implantation into gallium nitride (GaN) epitaxial layers, which establishes both dilute ferromagnetism and a three-level optical system with persistence to over 300 K. A sweep of thermal anneal parameters for a high implant dose into Mg-doped p-type GaN films is presented, and the materials are tested for both their magnetization and photoluminescence (PL). The optimal anneal process at 825 °C for 5 min maintains ferromagnetism with $T_{C}$ > 305 K, confirming magnetic alignment at room temperature with a coercivity of ~100 Oe. PL and spectrophotometry of the optimally prepared materials show the effects of the mid-gap defect state on the material's optical characteristics. The anneal process returns the real part index to its baseline dispersion while retaining an onset of absorption starting at the defect level $E_{A}$ = 1.8 eV, signifying a stable mid-gap energy transition with a measured state lifetime of $τ _{PL}$ = 2.7 ns. The scalability of this process for producing three-level transition magnetic materials suggests passive optical or magnetooptic devices can be constructed that interconnect photonic and spintronic effects for emerging system designs and potential applications in quantum information.

Properties of carbon-doped GaN using isobutane as a dopant

Carbon doping is an effective method to obtain semi-insulating GaN buffer, which is a necessity to prevent current leakage, in the high-electron-mobility-transistor device structure. The properties of intentionally carbon-doped GaN using isobutane gas as a dopant has been studied in detail. The carbon incorporation efficiency has been measured by secondary ion mass spectrometry. It is found that the carbon concentration could be directly controlled by the flow rate of isobutane precursor. The surface morphology of carbon-doped gallium nitride epitaxial layers has been investigated by optical microscopy and atomic force microscopy. The growth mode of GaN layers changes from step-flow to island growth, when the incorporated carbon concentration is higher than 1×1019 cm-3. In order to evaluate the structural quality of intentionally carbon-doped GaN, the full-width-at-half-maximum values are extracted from the rocking curves in six different reflections measured by high resolution X-ray diffraction. Raman spectroscopy is utilized to evaluate the physical properties of the carbon-doped GaN epitaxial layer.

Interfacial stress characterization of GaN epitaxial layer with sapphire substrate by confocal Raman spectroscopy

As an important wide-bandgap semiconductor, gallium nitride (GaN) has attracted considerable attention. This paper describes the use of confocal Raman spectroscopy to characterize undoped GaN, n-type GaN, and p-type GaN through depth profiling using 405-, 532-, and 638-nm wavelength lasers. The Raman signal intensity of the sapphire substrate at different focal depths is studied to analyze the depth resolution. Based on the shift of the E2H mode of the GaN epitaxial layer, the interfacial stress for different types of GaN is characterized and calculated. The results show that the maximum interfacial stress appears approximately at the junction of the GaN and the sapphire substrate. Local interfacial stress analysis between the GaN epitaxial layer and the substrate will be very helpful in furthering the applications of GaN devices.

Raman Analysis of E2 (High) and A1 (LO) Phonon to the Stress-Free GaN Grown on Sputtered AlN/Graphene Buffer Layer

The realization of high-speed and high-power gallium nitride (GaN)-based devices using high-quality GaN/Aluminum nitride (AlN) materials has become a hot topic. Raman spectroscopy has proven to be very useful in analyzing the characteristics of wide band gap materials, which reveals the information interaction of sample and phonon dynamics. Four GaN samples grown on different types of buffer layers were fabricated and the influence of graphene and sputtered AlN on GaN epitaxial layers were analyzed through the E2 (high) and A1 (LO) phonon. The relationship between the frequency shift of E2 (high) phonons and the biaxial stress indicated that the GaN grown on the graphene/sputtered AlN buffer layer was stress-free. Furthermore, the phonon lifetimes of A1 (LO) mode in GaN grown on graphene/sputtered AlN buffer layer suggested that carrier migration of GaN received minimal interference. Finally, the Raman spectra of graphene with the sputtered AlN interlayer has more disorder and the monolayer graphene was also more conducive to nucleation of GaN films. These results will have significant impact on the heteroepitaxy of high-quality thin GaN films embedded with a graphene/sputtered AlN buffer, and will facilitate the preparation of high-speed GaN-based optoelectronic devices.

Strain-enhanced high Q-factor GaN micro-electromechanical resonator

ABSTRACT We report on a highly sensitive gallium nitride (GaN) micro-electromechanical (MEMS) resonator with a record quality factor (Q) exceeding 105 at the high resonant frequency (f) of 911 kHz by the strain engineering for the GaN-on-Si structure. The f of the double-clamped GaN beam bridge is increased from 139 to 911 kHz when the tensile stress is increased to 640 MPa. Although it is usually regarded that the energy dissipation increases with increasing resonant frequency, an ultra-high Q-factor which is more than two orders of magnitude higher than those of the other reported GaN-based MEMS is obtained. The high Q-factor results from the large tensile stress which can be intentionally introduced and engineered in the GaN epitaxial layer by utilizing the lattice mismatch between GaN and Si, leading to the stored elastic energy and drastically decreasing the energy dissipation. The developed GaN MEMS is further demonstrated as a highly sensitive mass sensor with a resolution of 10−12 g/s through detecting the microdroplet evaporation process. This work provides an avenue to improve the f × Q product of the MEMS through an internally strained structure.

Enhanced production rates of hydrogen generation and carbon dioxide reduction using aluminum gallium nitride/gallium nitride heteroepitaxial films as photoelectrodes in seawater

In this study, aluminum gallium nitride/gallium nitride heterostructure grown on the sapphire substrate is used as the photoanodes for the generation of hydrogen gas and formic acid through water photoelectrolysis and carbon dioxide reduction, respectively, in photoelectrochemical reaction. These processes are conducted using carbon dioxide-containing sodium chloride aqueous solution and seawater, as the electrolytes. The typical production rates of formic acid and hydrogen gas produced from photoanodes made of aluminum gallium nitride/gallium nitride heterostructure are significantly higher than those of photoanodes made of a single gallium nitride epitaxial layer. This enhancement in production rate is attributed to facilitated transport of photogenerated electrons by two-dimensional electron gas at the aluminum gallium nitride/gallium nitride interface from the working area to the ohmic metal electrode on the photoelectrodes.

Processing of GaN/Si(111) Epitaxial Structures for MEMS Applications

Two different approaches for the etching of the gallium nitride epitaxial layers grown on Si(111) substrates by plasma-assisted molecular beam epitaxy have been elaborated. It is demonstrated that anisotropic etch profiles can be achieved by both photoenhanced wet chemical etching and reactive plasma-chemical etching methods. Moreover, it is shown, that the photoenhanced wet chemical etching allows to remove GaN layer without damaging silicon substrate.

Quality-Improved GaN Epitaxial Layers Grown on Striped Patterned Sapphire Substrates Ablated by Femtosecond Laser

In this work, we propose a new approach to create striped patterned sapphire substrate (PSS) under the circumstance that grooved patterned sapphire substrate technology exhibits more potential to reduce dislocation density in GaN (gallium nitride) epilayers. The striped grooves of patterned sapphire substrate are ablated by femtosecond laser. After the process of metal-organic chemical vapor deposition (MOCVD) method, the c-plane GaN epitaxial layers grown on striped PSS have larger crystallite size, which brings much less crystal boundary. There is much less compressive stress between the GaN crystals which improves the smoothness and compactness of GaN epilayers. This result demonstrates a significant improvement in the crystallinity of the c-plane GaN epitaxial layers grown on striped PSS.

Surface Activated Bonding of LiNbO3 and GaN at Room Temperature

Lithium niobate (LiNbO3: LN) is a unique ferroelectric material with excellent electro-optical (EO), non-linear optical (NLO), acousto-optical, piezo-electrical, and pyro-electrical characteristics, in addition to a wide transmission curve ranging from the ultraviolet to the mid-infrared. Thus, it is widely utilized for numerous applications, including ultra-fast optical modulators, wavelength convertors, surface acoustic wave filter, and pyro-imaging devices. At the same time, Gallium Nitride (GaN) film is a direct widegap semiconductor commonly used for light emitting diode light, power devices, and radio frequency components. Therefore, wafer bonding of LN and GaN has the potential for creating a new platform capable of realizing multi-functional micodevices that can fully exploit the unique properties mentioned above. Generally, a GaN sample is prepared as a structure of an as-grown GaN film on a sapphire. The coefficient thermal expansion (CTE) of LN is 14.4 (a-, b-axes)-7.5 (c-axis) × 10-6/K at room temperature, whereas that of GaN is 5.59 (a-axis)-3,17 (c-axis) × 10-6/K and that of sapphire is 5 × 10-6/K. Therefore, it is desirable for low-temperature bonding of LN and as-grown GaN film for overcoming the large CTE mismatch. Conventional plasma activated bonding requires relatively high temperature annealing process at above 200 °C to achieve a high bond strength. The high-temperature process causes a large thermal stress, which may often generate cracks. Surface activated bonding (SAB) is a powerful method for low temperature bonding of dissimilar materials. By a surface activation process with argon (Ar) fast atom beam (FAB) or ion beam bombardment, a high bond strength can be achieved at room temperature due to the strong attractive forces between atomically cleaned surfaces. Recently, room-temperature bonding of a GaN and a Si has been demonstrated using ion beam bombardment [F. Mu et al., Applied Surface Science, 2017]. It is interesting to apply the SAB to bond a LN wafer and a GaN. To date, little work has focused on the application of the SAB method to the direct bonding of LN and GaN. In this paper, we report surface activated bonding of a LN and a GaN at room temperature. In this experiment, we prepared a commercially available 3-inch LN (Z-cut) wafers and a 2 inch as-grown GaN wafer which is a 2-μm-thick GaN epitaxial layer grown on a 430-μm-thick sapphire wafer. In direct bonding, sub-nm RMS roughness of the bonding surface is typically required to achieve strong bonding at room temperature. Using atomic force microscopy, the measured RMS roughness of LN and GaN epitaxial layer was approximately 0.1 nm and 0.3 nm, respectively. For surface activation process, Ar FAB bombardment on each bonding surface was performed. Figure 1 (a) shows a schematic diagram of our SAB process. When the background pressure fell below 1×10-5 Pa, the two wafers were activated by Ar FAB bombardment. After that, the surfaces of LN and GaN epitaxial layer were brought into contact with an applied load at room temperature. The Ar FAB bombardment time was 60 s, and the applied load was approximately 10 MPa. Figure 1 (b) shows the photograph of the bonded LN and GaN wafers at room temperature. As shown, LN/GaN hybrid structure without generating serious cracks was demonstrated in wafer scale. The void was formed by particle contaminations and non-bonding at the peripheral area may be responsible for the thickness distribution of the GaN epitaxial layer. In this presentation, we will discuss the bond quality and the bonding interface state in details. The authors would like to acknowledge Mitsubishi Heavy Industries Machine Tool Co., Ltd. for assisting with surface-activated bonding experiment. This study was supported in part by JSPS KAKENHI Grant Number JP17H04925. Figure 1

Damage accumulation and structural modification in a‐ and c‐plane GaN implanted with 400‐keV and 5‐MeV Au+ ions

c‐plane (0001) and a‐plane (11‐20) gallium nitride (GaN) epitaxial layers were grown by Metal organic Vapour Phase epitaxy (MOVPE) on sapphire and subsequently implanted with 400‐keV and 5‐MeV Au+ ions using fluences 5 × 1014 to 5 × 1015 cm−2. The shallow Au depth profiling was accomplished by Rutherford backscattering spectrometry. The structural changes during implantation and subsequent annealing were characterised by Rutherford backscattering spectrometry channelling and Raman spectroscopy. The interplay between nuclear and electronic stopping, influencing defect accumulation, was monitored and discussed depending on GaN orientation. Post‐implantation annealing induced a structural reorganisation of the GaN structure depending on the ion‐implantation fluence, ion energy, and on the crystallographic orientation.

Elimination of V-shaped pits in InGaN/GaN/AlN/GaN heterostructure by metal modulation growth technique

In this paper, we have demonstrated the growth process of higher Indium content indium gallium nitride (InGaN) epitaxial layer of InGaN/GaN/AlN/GaN heterostructure without any V-shaped pits formation on the growth surface. The epitaxial InGaN with In fraction of 16%, 22% and 26% has been grown by using metal modulation growth scheme. The direct growth of relaxed InGaN structures with In content of 16% and 22% on GaN/AlN/GaN periodic layer shows higher density of V-shaped pits on the surface, as the InGaN epilayer is experienced to higher lattice relaxation and threading dislocation density. But for the 26% In mole heterostructure, the V pits are mitigated by varying the Indium fraction from 16% to 26% via 22% in bottom to top direction of the structure. The FESEM images have also confirmed the absence of any pits formation in 26% In content InGaN heterostructure, which is due to the lesser lattice relaxation and threading dislocation density. The different contents of Indium are confirmed by room temperature photoluminescence and XRD measurements.

Degradation mechanisms of optoelectric properties of GaN via highly-charged 209Bi33+ ions irradiation

N-type gallium nitride (GaN) epitaxial layers were subjected to 990-keV Bi33+ ions irradiation to various fluences. Optoelectric properties of the irradiated-GaN specimens were studied by means of Raman scattering and variable temperature photoluminescence (PL) spectroscopy. Raman spectra reveal that both the free-carrier concentration and its mobility generally decrease with a successive increase in ion fluence. Electro-optic mechanisms dominated the electrical transport to a fluence of 1.061 × 1012 Bi33+/cm2. Above this fluence, electrical properties were governed by the deformation potential. The appearance of vacancy-type defects results in an abrupt degradation in electrical transports. Varying temperature photoluminescence (PL) spectra display that all emission lines of 1.061 × 1012 Bi33+/cm2-irradiated specimen present a general remarkable thermal redshift, quenching, and broadening, including donor-bound-exciton peak, yellow luminescence band, and LO-phonon replicas. Moreover, as the temperature rises, a transformation from excitons (donor-acceptor pairs’ luminescence) to band-to-band transitions (donor-acceptor combinations) was found, and the shrinkage effect of the band gap dominated the shift of the peak position gradually, especially the temperature increases above 150 K. In contrast to the un-irradiated specimen, a sensitive temperature dependence of all photoluminescence (PL) lines’ intensity obtained from 1.061 × 1012 Bi33+/cm2-irradiated specimen was found. Mechanisms underlying were discussed.

Reduction of Dislocations in GaN on Silicon Substrate Using In Situ Etching

A gallium nitride (GaN) epitaxial layer with a low density of threading dislocations is successfully grown on a silicon substrate by using in situ gas etching. Silicon nitride (SiNx) film is used as a mask, and ammonia is intermittently supplied in hydrogen ambient during the etching. After etching, high‐density deep pits appeared on the surface of a GaN template layer and corresponded to the threading dislocations in the layer. In this novel method, before growing an additional GaN layer on the template GaN layer, a second SiNx layer is deposited after the etching process, and this layer prevents GaN nuclei from growing on the upper side‐walls of the pits. By using this method, the density of threading dislocations of the GaN surface is reduced to 6.7 × 107 cm−2. This method is cost effective, completing all the necessary processes in one growth run without taking samples out from a metalorganic chemical vapor deposition (MOCVD) reactor.