AlN Interlayer Research Articles

A strategy for enhancing interfacial thermal transport in Ga2O3-diamond composite structure by introducing an AlN interlayer

Heat dissipation issues have emerged in power devices due to miniaturization and high power density, particularly for materials like low-thermal-conductivity gallium oxide (Ga2O3). Increasing interfacial heat transfer has been identified as a critical strategy for tackling these issues. This study first explored the thermal transport of Ga2O3-diamond interfaces in composite structures containing an AlN interlayer. First-principles calculations revealed that the AlN interlayer improved interfacial bonding between Ga2O3 and diamond. Subsequently, Ga2O3 membranes were deposited on diamond substrates with and without interlayers using pulsed laser deposition (PLD), and the structural and thermal characteristics were examined. The interlayer strategy was shown to be effective in improving the quality of Ga2O3 thin films, including improved crystallinity, a smoother surface, and fewer oxygen vacancies. The thermal characteristics were accordingly improved: the thermal conductivity of Ga2O3 increased from 5.5±0.3–6.0±0.3 W/m·K, and the thermal boundary conductance of Ga2O3-diamond interface (TBCGaO-dia) increased from 46.1±2.3–60.9±3.0 MW/m2·K. Molecular dynamics (MD) analysis further revealed that the enhancement in phonon transmission was due to the increase in the low-frequency phonon participation rate. Additionally, the electro-thermal simulation using COMSOL confirmed the effectiveness of the AlN interlayer in mitigating the self-heating effect. These findings offer a new route for improving interface heat transport and pave the way for the optimization and design of reliable Ga2O3-based devices.

AlN as interlayer for effective thermal dissipation from gallium nitride to CVD diamond using nanocrystalline diamond seeding

AbstractWith the increasing power density achieved in gallium nitride (GaN) electronic devices, the thermal dissipation becomes a key issue that restricts their ultimate performances. However, the effective thermal boundary resistance (TBReff) between GaN and their heat spreader usually dominates the heat concentration. Here we introduce a super‐thin AlN interlayer with nanocrystalline diamond (NCD) seeding as the nucleation for the polycrystalline diamond (PCD) film growth on the GaN films. A thermal conductivity approaching 250 W/mK for the 1.2 μm‐thick PCD film is obtained. The TBReff between GaN and PCD films is estimated to be 5 m2K/GW, which is much smaller than that of the typical SiNx interlayer. Since AlN can be deposited simultaneously with the device structure, this work is promising to achieve the full potential of using diamond as the heat spreader for GaN‐based transistors.

Relation of V/III ratio of AlN interlayer with the polarity of nitride

Abstract N-polar GaN film was obtained by using a high-temperature AlN buffer layer. It was found that the polarity could be inverted by a thin low-temperature AlN interlayer with the same V/III ratio as that of the high-temperature AlN layer. Continuing to increase the V/III ratio of the low-temperature AlN interlayer, the Ga-polarity of GaN film was inverted to N-polarity again but the crystal quality and surface roughness of GaN film greatly deteriorated. Finally, we analyzed the chemical environment of the AlN layer by x-ray photoelectron spectroscopy (XPS), which provides a new direction for the control of GaN polarity.

Tracking Charge Carrier Paths in Freestanding GaN/AlN Nanowires on Si(111).

Functional and abundant substrate materials are relevant for applying all sophisticated semiconductor-based device components such as nanowire arrays. In the case of GaN nanowires grown by metalorganic vapor phase epitaxy, Si(111) substrates are widely used, together with an AlN interlayer to suppress the well-known Ga-based melt-back-etching. However, the AlN interlayer can degrade the interfacial conductivity of the Si(111) substrate. To reveal the possible impact of this interlayer on the overall electrical performance, an advanced analysis of the electrical behavior with suitable spatial resolution is essential. For the electrical investigation of the nanowire-to-substrate junction, we used a four-point probe measurement setup with sufficiently high spatial resolution. The charge separation behavior of the junction is also demonstrated by an electron beam-induced current mode, while the n-GaN nanowire (NW) core exhibits good electrical conductivity. The charge carrier-selective transport at the NW-to-substrate junction can be attributed to different, local material compositions by two main effects: the reduction of Ga adatoms by shadowing of the lower part of the NW structure by the top part during growth, i.e. the protection of the pedestal footprint from Ga adsorption. Our combination of investigation methods provides direct insight into the nanowire-to-substrate junction and leads to a model of the conductivity channels at the nanowire base. This knowledge is crucial for all future GaN bottom-up grown nanowire structure devices on conductive Si(111) substrates.

Transport Properties in GaN Metal–Oxide–Semiconductor Field‐Effect Transistor Almost Free of Interface Traps with AlSiO/AlN/p‐Type GaN Gate Stack

The factors limiting channel mobility in AlSiO/p‐type GaN metal–oxide–semiconductor (MOS) field‐effect transistors are examined by performing Hall‐effect measurements in conjunction with a gate bias, with and without a thin AlN interlayer. In the absence of this interlayer, the free carrier concentration associated with the Hall effect is significantly reduced compared with the net gate charge density estimated from capacitance–voltage data, indicating that electrons are trapped to a significant extent at the MOS interface. These interface traps are found to have an energy ≈20 meV above the Fermi level in strong inversion based on temperature‐dependent Hall effect data. The insertion of a 0.8 nm‐thick AlN interlayer eliminates charge trapping such that almost all gate charges are mobile. The mobility components could be divided into types based on their effect on the effective electric field perpendicular to the channel. Coulomb scattering centers resulting from interface states are evidently reduced by inserting the AlN interlayer, which also enhances the channel mobility to over 150 cm2 V−1s−1.

Epitaxial Growth of the Large-Scale, Highly-Ordered 3D GaN-Truncated Pyramid Array Toward an Ultrahigh Rejection Ratio and Responsivity Visible-Blind Ultraviolet Photodetection.

The micro- and nanostructures of III-nitride semiconductors captivate strong interest owing to their distinctive properties and myriad potential applications. Nevertheless, challenges endure in managing the damage inflicted on crystals through top-down processes or achieving extensive control over the large-area growth of these microstructures via bottom-up methods, thereby impacting their optical and electronic properties. Here, we present novel epitaxially grown 3D GaN truncated pyramid arrays (TPAs) on patterned Si substrates, devoid of any catalyst. These GaN TPAs feature highly ordered, large-scale structures, attributed to the utilization of 3D Si substrates and thin AlN interlayers to alleviate epitaxial strains and limit dislocation formation. Comprehensive characterization via scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and cathodoluminescence attests to the superior structural and optical attributes of these crystals. Furthermore, photoluminescence and ultraviolet (UV)-visible diffuse reflectance spectroscopy reveal sharp band-edge emission and significant light trapping in the UV bands. Employing these GaN TPAs, we constructed metal-semiconductor-metal visible-blind UV photodetectors (PDs) incorporating Ti3C2 MXene as Schottky electrodes. These PDs display exceptional responsivity, achieving 5.32 × 103 mA/W at 255 nm and an ultrahigh UV/visible rejection ratio (R255nm/R450nm) approaching 106, which are 1-2 orders of magnitude higher than most recently reported works. This exploration showcases novel GaN-based microstructures characterized by uniformity, ordered geometry, and exemplary crystalline integrity, paving the way for developing optoelectronic applications.

III-Nitride Magnetron Sputter Epitaxy on Si: Controlling Morphology, Crystal Quality, and Polarity Using Al Seed Layers.

Group III-nitride semiconductors have been subject of intensive research, resulting in the maturing of the material system and adoption of III-nitrides in modern optoelectronics and power electronic devices. Defined film polarity is an important aspect of III-nitride epitaxy as the polarity affects the design of electronic devices. Magnetron sputtering is a novel approach for cost-effective epitaxy of III-nitrides nearing the technological maturity needed for device production; therefore, control of film polarity is an important technological milestone. In this study, we show the impact of Al seeding on the AlN/Si interface and resulting changes in crystal quality, film morphology, and polarity of GaN/AlN stacks grown by magnetron sputter epitaxy. X-ray diffraction measurements demonstrate the improvement of the crystal quality of the AlN and subsequently the GaN film by the Al seeding. Nanoscale structural and chemical investigations using scanning transmission electron microscopy reveal the inversion of the AlN film polarity. It is proposed that N-polar growth induced by Al seeding is related to the formation of a polycrystalline oxygen-rich AlN interlayer partially capped by an atomically thin Si-rich layer at the AlN/Si interface. Complementary aqueous KOH etch studies of GaN/AlN stacks demonstrate that purely metal-polar and N-polar layers can be grown on a macroscopic scale by controlling the amount of Al seeding.

Understanding Interfaces in AlScN/GaN Heterostructures

AbstractAluminum scandium nitride barrier layers increase the available sheet charge carrier density in gallium nitride‐based high‐electron‐mobility transistors and boost the output power of high‐frequency amplifiers and high voltage switches. Growth of AlScN by metal‐organic chemical vapor deposition is challenging due to the low vapor pressure of the conventional Sc precursor Cp3Sc, which induces low growth rates of AlScN and leads to thermally‐induced AlScN/GaN‐interface degradation. In this work, novel Sc precursors are employed to reduce the thermal budget by increasing the growth rate of the AlScN layer. The AlScN/GaN interfaces are investigated by high‐resolution X‐ray diffraction, high‐resolution transmission electron microscopy, time‐of‐flight secondary ion mass spectrometry, capacitance–voltage, current–voltage and temperature‐dependent Hall measurements. Linearly graded interlayers with strain‐induced stacking faults, edge, and screw dislocations form at the AlScN/GaN interface at growth rates of 0.015 nms−1. Growth rates of 0.034 nms−1 and higher allow for abrupt interfaces, but a compositional grading in the barrier remains. Homogeneous barrier layers can be achieved at growth rates of 0.067 nms−1 or by growing an AlN interlayer. The electrical properties of the heterostructures are sensitive to Sc accumulations at the cap/barrier interface, residual impurities from precursor synthesis, and surface roughness. This study paves the way for high‐performing devices.

High performance of Ag/AlN/Si Schottky diode: Study of the DC sputtering power effect on its electrical properties

The DC sputtering power (DCSP) effect on the chemical composition, structural and electrical properties of Ag/AlN/Si Schottky diode was studied. The AlN films grown on Si substrate were examined by X-ray diffraction (XRD), Fourier Transformed Infra-Red (FTIR), and Raman spectroscopy. The analyzes by XRD, FTIR and Raman show stability at the level of the structure and the qualitative chemical composition of the AlN layer for DCSP of 200 W and 400 W. However, the quantity of chemical compounds on the surface of AlN is affected by this variation. From Current-Voltage (I-V) measurements, the ideality factor (n), barrier height (φb), and series resistance (Rs) were extracted by adopting Cheung functions. Diodes with an AlN interlayer elaborated at 400 W have good rectifying behavior with n, Rs and φb of 3.11, 1930 Ω, and 0.93 eV, respectively. The enhancement of the Ag/AlN/Si diode performance was attributed to the reduction of undesirable impurities in the Ag/AlN interface as well as the morphological improvement of the AlN layer. Capacitance-Voltage (C-V) measurements showed that carrier donor concentrations are strongly affected by DC sputtering power. The elaborate structures may provide new materials with electrical properties having a wide range of applications in electronics and photovoltaic field.

Preparation and properties of anti-reflective and anti-thermal shock of Y2O3/AlN composite films on CVD diamond

Y2O3/AlN composite films with different thicknesses of AlN interlayer were prepared on CVD diamond by RF magnetron sputtering. The influence of AlN thickness on the microstructure, adhesion strength, film stress, thermal shock resistance and infrared transmittance of the films was investigated by X-ray diffractometer, scanning electron microscopy, scanning probe microscopy, UV–Vis spectrophotometer, Fourier transform infrared spectrometer, scratch tester and 3-D optical surface profiler. The results show that CVD diamond coated with Y2O3/AlN composite films with different thicknesses of AlN have different degrees of anti-reflective effects in the 8–10 μm infrared band. The adhesion strength and quality of the composite film were improved with the AlN thickness increased, and the anti-reflective and anti-thermal shock of the film showed a trend of first increasing and then decreasing. CVD diamond coated with Y2O3/AlN composite films with 700 nm AlN interlayer has an average transmittance of 77 % in the 8–10 μm band and a good adhesion between interfaces. It can withstand rapid temperature changes and effectively protect the diamond substrate with little or no degradation of infrared optical properties.

An adjustable Ar ion-beam activation strategy to achieve hydrophilic bonding of Si and diamond by deposited AlN interlayer

As a III-nitride, AlN exhibits high thermal conductivity, excellent insulation, and low thermal expansion. Therefore, AlN is an alternative to the insulating layer in conventional Si-on-insulator (SOI) systems to solve the problem of severe self-heating effects caused by the poor thermal conductivity of buried oxide (BOX, SiO2) layers. This is particularly true for high-power integrated circuit and micro-electro-mechanical system fabrication. However, heat transfer also depends on the bottom Si substrate. With the highest thermal conductivity (≥2000 W/mK) among bulk materials, diamond is a highly attractive option to replace conventional Si substrates. Herein, we describe a low-temperature vacuum-free hydrophilic bonding method based on a thin AlN interlayer to prepare a single-crystal Si layer on a polycrystalline diamond. Through suitable Ar ion-beam activation, the surface nanomorphology and chemical state can be simultaneously modified. Increasing the real contact area and removing the surface heavy oxygen layer are critical factors for achieving complete bonding. Moreover, the relatively mild deposition and bonding conditions of AlN make it compatible with the complex CMOS transistor process and could enable the more flexible integration of diamond into 3D chip packages in the future.

AlGaN-Based Self-Powered Solar-Blind UV Focal Plane Array Imaging Photosensors: Material Growth, Device Preparation, and Functional Verification

This paper reports the development of hybrid AlGaN-based self-powered solar-blind ultraviolet (UV) focal plane array imaging photosensors from material growth and array preparation to functional verification. The detailed process starts with the epitaxial growth by metal-organic chemical vapor deposition (MOCVD). A high-quality AlN template is obtained on the 2-inch double-polished c-plane sapphire substrate by introducing a mesothermal AlN (MT-AlN) interlayer and adjusting the growth rate of the high-temperature AlN epilayer (HT-AlN). Then, a polarization-enhanced p-i-n structure photodetector material is grown on the AlN template. Subsequently, based on the p-i-n structural material, a 320×256 back-illuminated solar-blind photodiode array is fabricated and hybridized to a matching Si-based CMOS readout integrated circuit (ROIC) chip to form a focal plane detector. Solar-blind UV detection is observed throughout the array in the 262-281 nm spectral region with a peak external quantum efficiency (EQE) of 65.3% at zero-bias voltage (no antireflection coating) and a nanosecond transient response time at a reverse bias of 5 V. The visible response of the ROIC is eliminated by developing a masking technology that is opaque to visible light, which enables the focal plane detector to achieve a high UV/visible rejection ratio more than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> .

Over 200 cm2 V−1 s−1 of electron inversion channel mobility for AlSiO/GaN MOSFET with nitrided interface

By controlling a metal-oxide-semiconductor interface of an AlSiO/GaN system, the electron inversion channel mobility was significantly improved to 229 cm2 V−1 s−1 in a field-effect transistor. A 3 nm thick AlN interlayer formed by atomic layer deposition effectively suppressed the oxidation of the GaN surface and reduced the border traps, resulting in high channel mobility. An additional nitrogen radical treatment before AlN deposition further improved the subthreshold slope and the channel mobility, which was consistent with the lower charged defects extracted from the mobility analysis in the low effective normal field region.

Effects of Dual Moderate-Temperature-Grown AIN Interlayers on Structural and Optical Properties of Semipolar (1122) AIN Film

High quality semipolar (1122) AlN films have been grown on (1010) m-plane sapphire substrates with the help of dual moderate-temperature-grown (MTG) AlN interlayers by using metal-organic chemical vapor deposition technology. The layer thickness of the semipolar (1122) AlN film was determined by employing relative optical transmittance spectrum measured with ultraviolet-visible spectrophotometer. The effect of the insertion of 80 nm-thick MTG AlN interlayer on structural and optical properties was investigated in detail based on the characterization results of the atomic force microscopy, high-resolution X-ray diffraction, and Raman spectroscopy. Comparing with the semipolar (1122) AlN film grown without the MTG AlN interlayer, both the surface morphology and crystalline quality of the semipolar (1122) AlN film grown with the insertion of dual 80 nm-thick MTG AlN interlayers have been improved significantly. In fact, the root mean square value of the surface roughness decreased from 3.5 to 1.4 nm, and the full width at half maximum value of X-ray rocking curve decreased from 1667 to 1174 arcsec, respectively. These facts reveal that the insertion of the dual MTG AlN interlayers is a powerful method to improve the surface morphology and crystalline quality of the semipolar (1122) AlN films owing to the formation of nanoscale patterned substrate-like structure and its blocking effect on the propagation of the dislocations.

Impact of AlN interlayer on the electronic and I-V characteristics of In0.17Al0.83N/GaN HEMTs devices

Here, we study a simulation model of In0.17Al0.83N/GaN passivated high electron mobility transistors (HEMTs) on SiC substrate. The research focused systematically on the effet of AlN interlayer on the electronic and electric characteristics using the Nextnano simulation software. The 2D–electron gas density of In0.17Al0.83N/AlN/GaN HEMTs is investigated through the dependence on various AlN layer thickness, we report calculations of I-V characteristics, with 1.5 nm AlN thickness, we find the highest maximum output current of 1.81 A/mm at Vgs 1 V, and more than 450 mS/mm as a transconductance peak. The Results are in agreement with experimental data.

Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers

Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick AlxGa1−xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schrödinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (∼ 9 × 10 12 cm−2) as the non-graded conventional structure, though the mobility drops from ∼ 2360 cm2/V s in the conventional to ∼ 960 cm2/V s in the graded structure. The transconductance gm of the linearly graded channel HEMTs is shown to be flatter with smaller g m ′ and g m ″ as compared to the conventional non-graded channel HEMT implying improved device linearity.

Quantum Efficiency Improvement of InGaN Near Ultraviolet LED Design by Genetic Algorithm

A near-ultraviolet (367-nm) InGaN light-emitting diode (LED) with 5.75 nm quantum well depth was designed and both internal/external quantum efficiency (IQE/EQE) values were optimized considering the effects of non-radiative recombination rates and possible fabrica-tion errors. Firstly, the IQE of the design was enhanced by a genetic algorithm code which was developed particularly for this study. Distributed Bragg Reflectors and optional ultra-thin 1nm AlN interlayer were also used to increase overall light extraction efficiency. Then, alloy and doping concentration effects on wavelength-dependent optical and structural parameters were analyzed via the CASTEP software package based on density functional theory to pre-sent a more detailed and realistic optimization. The relatively great values of 42.6% IQE and 90.2% LEE were achieved. The final structure with 1.00 mm × 1.00 mm surface area requires only 200 mW input power to operate at 3.75 V.

Effect of AlN and AlGaN Interlayers on AlScN/GaN Heterostructures Grown by Metal–Organic Chemical Vapor Deposition

AlScN/GaN heterostructures with their high sheet carrier density (ns) in the two-dimensional electron gas (2DEG) have a high potential for high-frequency and high-power electronics. The abruptness of the heterointerface plays a key role in the 2DEG confinement, and the presence of interlayers (AlN, AlGaN) affects ns and electron mobility (μ) and determines the sheet resistance (Rsh). AlScN/GaN heterostructures suitable for high-electron mobility transistors (HEMT) with and without a nominal AlN interlayer were grown by metal–organic chemical vapor deposition (MOCVD) and characterized electrically and structurally to gain a systematic insight into the unintentional formation and control of graded AlGaN interlayers by diffusion of atoms at the heterointerface. The AlN interlayer increases ns from 2.52 × 1013 cm–2 to 3.25 × 1013 cm–2 and, as calculated by one-dimensional Schrödinger–Poisson simulations, improves the 2DEG confinement. The barrier growth temperature was varied from 900 °C to 1200 °C to investigate the effect of the thermal budget on diffusion. Growth at 900 °C reduces the thickness of the graded AlGaN interlayer and improves the 2DEG confinement, leading to Rsh of 211 Ω/sq, ns of 2.98 × 1013 cm–2, and μ of 998 cm2/(Vs).

Atomic-column resolution quantitative composition analysis of AlN interlayer in MOCVD-grown AlGaN/AlN/GaN heterostructure using HAADF-STEM

Quantitative compositional analysis using high-angle annular dark-field scanning transmission electron microscopy was performed for a metal-organic chemical vapor deposition-grown AlGaN/AlN/GaN heterostructure at atomic-column resolution. In addition, the effects that different concentrations of Al in the AlN interlayer had on the electronic properties were evaluated. The maximum Al concentration in the AlN interlayer was 0.41, and the compositional grading at the upper and lower AlN interfaces resulted in an asymmetric broadening of the interface. Calculations show that increasing the Al concentration in the AlN interlayer leads to an approximately linear increase in the two-dimensional electron gas sheet density. The alloy scattering is also effectively suppressed when the Al concentration is greater than 0.4.

Transport properties of polarization-induced 2D electron gases in epitaxial AlScN/GaN heterojunctions

AlScN is attractive as a lattice-matched epitaxial barrier layer for incorporation in GaN high electron mobility transistors due to its large dielectric constant and polarization. The transport properties of polarization-induced two-dimensional (2D) electron gas of densities of ∼2×1013/cm2 formed at the AlScN–GaN interface is studied by Hall-effect measurements down to cryogenic temperatures. The 2D electron gas densities exhibit mobilities limited to ∼300 cm2/V s down to 10 K at AlScN/GaN heterojunctions. The insertion of a ∼2 nm AlN interlayer boosts the room temperature mobility by more than five times from ∼300 cm2/V s to ∼1573 cm2/V s, and the 10 K mobility by more than 20 times to ∼6980 cm2/V s at 10 K. These measurements provide guidelines to the limits of electron conductivities of these highly polar heterostructures.