High-quality AlN Research Articles

4 inch 11 μm high-quality AlN thick films grown on nanopatterned sapphire substrates

A high-quality AlN thick film with 11 μm thickness and low defect density was grown on a 4 inch hexagonal hole nano-patterned sapphire substrate by metal oxide chemical vapor deposition. The density of dislocation etch pits in the AlN heteroepitaxial film reached 1.1 × 107 cm−2. The surface and microstrcutres of the AlN thick film were characterized in detail. The dislocation evolution mechanism and stress evolution of AlN were investigated. Dislocations were mainly generated at the interface between the sapphire and AlN, and the voids above the patterned region were generated by the undesirable grain boundaries caused by the lateral epitaxy of AlN and the substrate during the growth process, which prompted a large number of screw dislocations to bend and merge, and while some mixed dislocations to merge and extend upwards, resulting in a high-quality AlN thick film.

Optimizing crucible geometry to improve the quality of AlN crystals by the physical vapor transport method

In the conventional crucible structure for AlN crystal growth by physical vapor transport, owing to the long molecular transport path of Al vapor and the disruption of the gas flow by the presence of a deflector, the Al vapor easily forms polycrystals in the growth domain. The result is increased internal stress in the crystals and increased difficulty in growing large-sized crystals. On this basis, with the help of finite element simulations, a novel crucible structure is designed. This crucible not only optimizes the gas transport but also increases the radial gradient of the AlN crystal surface, making the enhanced growth rate in the central region more obvious. The thermal stresses between the deflector and the crystal are also reduced. High-quality AlN crystals with an FWHM of 79 arcsec were successfully grown with this structure, verifying the accuracy of finite element simulation of the growth of AlN crystals. Our work has important guiding significance for the growth of high-quality AlN crystals.

Improved DC and RF Characteristics of GaN-Based Double-Channel HEMTs by Ultra-Thin AlN Back Barrier Layer.

In order to improve the off-state and breakdown characteristics of double-channel GaN HEMTs, an ultra-thin barrier layer was chosen as the second barrier layer. The strongly polarized and ultra-thin AlN sub-barrier and the InAlN sub-barrier are great candidates. In this article, the two epitaxial structures, AlGaN/GaN/AlN/GaN (sub-AlN) HEMTs and AlGaN/GaN/InAlN/GaN (sub-InAlN) HEMTs, were compared to select a more suitable sub-barrier layer. Through TEM images of the InAlN barrier layer, the segregation of In components can be seen, which decreases the mobility of the second channel. Thus, the sub-AlN HEMTs have a higher output current density and transconductance than those of the sub-InAlN HEMTs. Because the high-quality AlN barrier layer shields the gate leakage current, a 294 V breakdown voltage was achieved by the sub-AlN HEMTs, which is higher than the 121 V of the sub-InAlN HEMTs. The current gain cut-off frequency (fT) and maximum oscillation frequency (fmax) of the sub-AlN HEMTs are higher than that of the sub-InAlN HEMTs from low to high bias voltage. The power-added efficiency (PAE) and output power density (Pout) of the sub-AlN HEMTs are 57% and 11.3 W/mm at 3.6 GHz and 50 V of drain voltage (Vd), respectively. For the sub-InAlN HEMTs, the PAE and Pout are 41.4% and 8.69 W/mm, because of the worse drain lag ratio. Thus, the Pout of the sub-AlN HEMTs is higher than that of the sub-InAlN HEMTs.

Comparison of AlN/GaN heterojunctions grown by molecular beam epitaxy with Al and Ga assistance

Although conventional III-nitride molecular-beam-epitaxy growth theories indicate that high-quality AlN should be prepared in Al-assisted regimes, AlN/GaN heterojunctions grown in Ga-assisted regimes actually demonstrate superior performances. In this letter, we compared Al-assisted and Ga-assisted grown AlN/GaN heterojunctions and explored the underlying mechanisms. Aberration-corrected transmission electron microscope, energy dispersive spectrometer, and atomic force microscopy measurements indicate that the Al-assisted growth results in partly-relaxed AlN of high-density defects, deteriorating AlN/GaN interfaces, and surface morphology consisting of irregular and truncated atomic steps. The Ga-assisted growth produces high-quality pseudomorphic AlN with excellent AlN/GaN interfaces, surface morphology, and electrical properties. It is suggested that the better crystalline quality and higher performances of the Ga-assisted grown samples are attributed to a lower N subsurface diffusion barrier resulting from a higher energy of the fcc adsorption positions in the adlayer-enhanced-lateral-diffusion model and intense thermal motions of Ga adatoms. It helps to understand the kinetics of metal-adlayer-enhanced growth of AlN and AlN/GaN heterojunctions at atomic scale and optimize the growth processes. Meanwhile, it is demonstrated that amorphization and Al diffusion near the AlN surfaces occur due to Pt bombardment on highly tensile-strained AlN during focused-ion-beam fabrications. It implies that large strain in AlN/GaN heterojunctions might be an important issue to be considered for device reliability.

High-Q film bulk acoustic resonator with high quality AlN film based on transfer method

Thin film bulk acoustic resonator (FBAR) with high quality factor (Q) is preferred for many communication applications. AlN film deposited in conventional FBAR fabrication process is poor, resulting in unsatisfactory device performance. This study demonstrated the comparison of two Si-substrate FBARs, adopting the same AlN deposition process by physical vapor deposition (PVD) method, however using different device fabrication method. Instead of depositing AlN film on Mo/SiO2/Si substrate in existing fabrication process, we deposited AlN directly on Si substrate by PVD to get better crystal quality piezoelectric layer of the FBAR. The full width at half maximum (FWHM) of rocking curve and surface roughness of AlN directly deposited on Si substrate are 1.4° and 1.96 nm, while those of AlN deposited on Mo/SiO2/Si substrate are 8.5° and 5.54 nm, respectively, indicating AlN deposited directly on Si substrate has a better crystal quality than that on Mo/SiO2/Si substrate. The dielectric loss of FBAR using AlN developed on Si substrate decreases from 0.4 Ω to 0.11 Ω leading to the increase of Q m from 470 to 830. By preserving the high quality AlN deposited on Si substate with film transfer method, the FBAR device enjoys an increasing of Q-values as high as 76%.

Localized Surface Plasmonic Resonance of an Al Nanorod Array on High-Quality AlN Templates into the Far-UVC Spectral Region

Localized Surface Plasmonic Resonance of an Al Nanorod Array on High-Quality AlN Templates into the Far-UVC Spectral Region

A Theoretical Approach to Study the Frequency Dependent Dielectric behavior of Breast Cancer Cells using AlN buffer-based AlGaN/GaN HEMT

Abstract This article investigates the applicability of open gated high electron mobility transistor (HEMT) as a biosensing platform for Breast cancer detection. The present study reports the detection of healthy (MDA-10) and malignant cells (MDA-MB-231) associated with breast cancer using high-quality AlN buffer AlGaN/GaN HEMT with dual cavity structure formed by etching out the gate metal from source and drain sides under the gate region. The AlN buffer AlGaN/GaN HEMT provides better 2DEG confinement and large conduction band offset than GaN buffer. The proposed design is analyzed at frequencies of 900 MHz and 10 GHz, as breast cells have distinct dielectric properties at different microwave frequencies. The variation in dielectric constant of the cavity region corresponding to biomolecular species will change the device’s electrical characteristics and hence can be used to detect breast cancer cells. In this work, the device sensing parameters considered for analysis are shift in threshold voltage and drain current sensitivity. The device performance has also been analyzed by optimizing cavity thickness and length to select the best design for the sensing. The effect of non-ideal conditions such as steric hindrance and probing is also studied by emulating these real time effects in simulation. The device shows a maximum drain current sensitivity of 29.94% for 20 nm of cavity thickness and 1µm of the cavity length. The results depict that the proposed device design exhibits a highly sensitive response and can be promising alternate for future biosensing applications.

Improving the structural performance of low-temperature sputtered AlN on silicon substrate

Preparing high-quality AlN films at low temperatures is always highly demanded in plenty of application-fields, despite the high temperature is always necessary to enable the AlN crystallization. Therefore, improving the structural properties at low temperature is still challenging in the field. In the present work, a metal organic chemical vapor deposition (MOCVD) grown AlN nucleation layer is employed to improve the crystallinity and morphology of sputtered AlN on 6-inch Si (111) substrate at temperatures even as low as 100 °C where it is not principally possible to achieve crystallization of sputtered AlN. When compared with the as-sputtered AlN prepared even at 650 °C, it is found that the full width at half maximum of AlN (002) x-ray rocking curves is intensively decreased to 0.96° from 1.61°, and such a reduction indicates that the screw dislocation density is decreased from 7.31 × 1010 down to 2.14 × 1010 cm−2. In addition to the crystallinity, the morphology is also obviously improved that the root-mean-square roughness is reduced from 4.99 nm down to 0.83 nm in a scanned area of 3 × 3 μm2 through introducing AlN nucleation. Therefore, such a combination highlights the contribution of AlN nucleation grown by MOCVD in improving the structural properties of low-temperature sputtered AlN layer which is compatible with the manufacturing of Si integrated circuits.

Demonstration of controllable Si doping in N-polar AlN using plasma-assisted molecular beam epitaxy

In this study, we present the demonstration of controllable Si doping in N-polar AlN films grown on single-crystal AlN substrates by plasma-assisted molecular beam epitaxy. Through optimization of growth conditions, we obtained high-quality N-polar AlN films at 950 °C. However, our studies revealed that Si incorporation dramatically decreases at such high growth temperature. To enable higher Si incorporation, a hybrid low-temperature and high-temperature growth condition was developed by using Ga as a surfactant at low-temperature growth. By lowering the growth temperature of AlN to 750 °C, we were able to incorporate Si with concentrations as high as 2×1020 cm−3 and demonstrated an electron concentration as high as 1.25×1019 cm−3 at room temperature. The secondary ion mass spectrometry analysis revealed that, <0.2% Ga is incorporated in the AlN films grown with Ga as a surfactant at low temperature.

Ion implantation induced nucleation and epitaxial growth of high-quality AlN

AlN materials have a wide range of applications in the fields of optoelectronic, power electronic, and radio frequency. However, the significant lattice mismatch and thermal mismatch between heteroepitaxial AlN and its substrate lead to a high threading dislocation (TD) density, thereby degrading the performance of device. In this work, we introduce a novel, cost-effective, and stable approach to epitaxially growing AlN. We inject different doses of nitrogen ions into nano patterned sapphire substrates, and then deposit the AlN layers by using metal-organic chemical vapor deposition. Ultraviolet light-emitting diode (UV-LED) with a luminescence wavelength of 395 nm is fabricated on it, and the optoelectronic properties are evaluated. Compared with the sample prepared by the traditional method, the sample injected with N ions at a dose of 1×10<sup>13</sup> cm<sup>–2</sup> exhibits an 82% reduction in screw TD density, the lowest surface roughness, and a 52% increase in photoluminescence intensity. It can be seen that appropriate dose of N ion implantation can promote the lateral growth and merging process in AlN heteroepitaxy. This is due to the fact that the process of implantation of N ions can suppress the tilt and twist of the nucleation islands, effectively reducing the density of TDs in AlN. Furthermore, in comparison with the controlled LED, the LED prepared on the high quality AlN template increases 63.8% and 61.7% in light output power and wall plug efficiency, respectively. The observed enhancement in device performance is attributed to the TD density of the epitaxial layer decreasing, which effectively reduces the nonradiative recombination centers. In summary, this study indicates that the ion implantation can significantly improve the quality of epitaxial AlN, thereby facilitating the development of high-performance AlN-based UV-LEDs.

The AlN lattice-polarity inversion in a high-temperature-annealed c-oriented AlN/sapphire originated from the diffusion of Al and O atoms from sapphire.

AlN films are widely used owing to their superior characteristics, including an ultra-wide bandgap, high breakdown field, and radiation resistance. High-temperature annealing (HTA) makes it easy to obtain high-quality AlN films, with the advantages of a simple process, good repeatability, and low cost. However, it is always found that there is a lattice-polarity inversion from a N-polarity near the sapphire to an Al-polarity in the HTA c-oriented AlN/sapphire. Currently, the formation mechanism is still unclear, which hinders its further wide applications. Therefore, the formation mechanism of the polarity inversion and its impacts on the quality and stress profile of the upper AlN in the HTA c-oriented AlN/sapphire were investigated. The results imply that the inversion originated from the diffusion of the Al and O atoms from the sapphire. Due to the presence of abundant Al vacancies (VAl) in the upper AlN, Al atoms in the sapphire diffuse into the upper AlN during the annealing to fill the VAl, resulting in the O-terminated sapphire, leading to the N-polar AlN. Meanwhile, O atoms in the sapphire also diffuse into the upper AlN during the annealing, forming an AlxOyNz layer and causing the inversion from N- to Al-polarity. The inversion has insignificant impacts on the quality and stress distribution of the upper AlN. Besides, this study predicts the presence of a two-dimensional electron gas at the inversion interface. However, the measured electron concentration is much lower than that predicted, which may be due to the defect compensation, low polarization level, and strong impurity scattering.

Three-step growth of AlN films on sapphire substrates by metal nitride vapor phase epitaxy

Control of the growth process is important for growing high-quality AlN films. In this work, AlN films were grown on c-plane sapphire substrates by metal nitride vapor phase epitaxy (MNVPE). Interlayer was added between the buffer layer and the formal layer, and the effect of the Ⅴ/Ⅲ ratio of the interlayer on the crystal quality, surface morphology and stress evolution of the films were analyzed. An interlayer with proper Ⅴ/Ⅲ ratio can provide a stable transition from 3D growth mode to 2D growth mode, while high Ⅴ/Ⅲ ratio induce film separation. The optimum Ⅴ/Ⅲ ratio for the interlayer was found to be 11050, and the films obtained under this parameter had a FWHM of the (002) and (102) reflections were 641 arcsec and 840 arcsec, respectively. The maximum transmission is 88%. And the film separation phenomenon was found on the samples with high V/III ratio in the second step, which was thought to be the result of growing in 3D mode for a long time.

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> .

Parasitic AlxOyNz surface defects on high-temperature annealed AlN and their role in hillock formation

High quality AlN buffer layers on sapphire wafers are a prerequisite for further improving UV LEDs. In addition, AlN templates with low screw-dislocation density might be interesting for future power electronic devices. High-temperature annealing (HTA) has proven to be a viable route to improve the crystallinity of sputtered or thin metalorganic vapor-phase epitaxy (MOVPE) AlN layers. In this work, the influence of two different pretreatment conditions prior to the MOVPE regrowth on HTA AlN templates was analyzed. AFM studies found a hillock density of roughly 106 cm−2 in regrown AlN, whereby such hillocks could no longer be observed after introducing harsher bake conditions. The origin of the observed hillock defects was clarified by using different TEM-related measurement techniques. Based on the TEM and AFM findings, a double-spiral enhanced growth mode that emits concentric surface steps on top of γ-AlON islands is suggested as a underlying mechanism for hillock formation.

High-Quality AlN Grown by a Combination of Substrate Pretreatment and Periodic Growth Mode Control

The growth of high-quality AlN films has always been the focus of research. Combined with NH3 in situ pretreatment before growth, with optimized growth conditions, we demonstrated a periodic growth method, which alternates between three-dimensional (3D) growth mode and two-dimensional (2D) growth mode, to grow a thick crack-free AlN film with good crystal quality on the on-axis 4H-SiC substrate by metal–organic chemical vapor deposition (MOCVD). X-ray rocking curves (XRCs) indicated that full width at half-maximum (FWHM) of the (0002) and (10–12) reflections were 241.3″ and 474.4″, respectively. The film thickness was over 1.4 μm. Lattice constant and residual stress results showed residual compressive stress in the sample. The periodic two-step growth and substrate pretreatment both play an important role in controlling the residual stress of films to avoid cracking. This work can serve as an important reference for growing high-quality crack-free AlN on SiC.

Improving Performance of Al2O3/AlN/GaN MIS HEMTs via In Situ N2 Plasma Annealing.

A novel monocrystalline AlN interfacial layer formation method is proposed to improve the device performance of the fully recessed-gate Al2O3/AlN/GaN Metal-Insulator-Semiconductor High Electron Mobility Transistors (MIS-HEMTs), which is achieved by plasma-enhanced atomic layer deposition (PEALD) and in situ N2 plasma annealing (NPA). Compared with the traditional RTA method, the NPA process not only avoids the device damage caused by high temperatures but also obtains a high-quality AlN monocrystalline film that avoids natural oxidation by in situ growth. As a contrast with the conventional PELAD amorphous AlN, C-V results indicated a significantly lower interface density of states (Dit) in a MIS C-V characterization, which could be attributed to the polarization effect induced by the AlN crystal from the X-ray Diffraction (XRD) and Transmission Electron Microscope (TEM) characterizations. The proposed method could reduce the subthreshold swing, and the Al2O3/AlN/GaN MIS-HEMTs were significantly enhanced with ~38% lower on-resistance at Vg = 10 V. What is more, in situ NPA provides a more stable threshold voltage (Vth) after a long gate stress time, and ΔVth is inhibited by about 40 mV under Vg,stress = 10 V for 1000 s, showing great potential for improving Al2O3/AlN/GaN MIS-HEMT gate reliability.

State-of-the-art and prospective progress of growing AlN substrates by physical vapor transport

SiC, GaN, diamond, and AlN known as wide band-gap semiconductors, integrate the advantages of microelectronics, optoelectronics, and power electronics. The ultra-wide bandgap semiconductor AlN crystal has a direct bandgap of 6.1 eV at room temperature. AlN has led to a burgeoning research interest in the fabrication of devices due to its outstanding structure and fascinating physiochemical (microelectronics, optoelectronics and power-electronics, etc.) properties. In this review, we summarize some recent advances in growing physical vapor transport AlN substrates. First, we introduce the primary seed growth of AlN using two disparate growth strategies: on heterogeneous substrate and spontaneous nucleation. We then elaborate on their advantages and disadvantages for subsequent homogeneous growth. Second, we highlight ultramodern advances that have been made in the preparation of high-quality and large-size AlN substrates, discuss the optical properties of AlN. In the end, the current challenges and new opportunities for PVT grown AlN substrates in improving the quality are also discussed.

High quality AlN film assisted by graphene/sputtered AlN buffer layer for deep-ultraviolet-LED

The advantages of van der Waals epitaxial nitrides have become a research hot topic. It is worth noting that graphene plays an important role in the research of epitaxial AlN epitaxial layer. In this work, we demonstrate a method to obtain high-quality and low-dislocation AlN epitaxial layer by combining graphene and sputtered AlN as the nucleation layer on the C-sapphire substrate via metal organic chemical vapor deposition, and successfully fabricated a 277 nm AlGaN-based deep ultraviolet light emitting diode (DUV-LED) based on the obtained AlN epitaxial layer. The presence of graphene promotes the stress release of AlN. Compared with the AlN epitaxial layer directly grown on graphene/sapphire substrate, the exist of sputtered AlN/graphene nucleation layer facilitates most of the threading dislocations in AlN can annihilate each other in the range of about 100 nm. Thus, as grown AlN epitaxial layer shows the decreasing of the screw dislocation from 2.31 × 109 to 2.08 × 108 cm−2 significantly. We manufacture an DUV-LED with 277 nm emission wavelength by using high-quality AlN films, which shows that magnitude of the leakage current is only on the order of nanoamperes and the forward turn on voltage is 3.5 V at room temperature. This study provides a meaningful strategy to achieve high-quality AlN film and high-performance DUV-LED.

High-quality AlN epilayers prepared by atomic layer deposition and large-area rapid electron beam annealing

Low-temperature atomic layer deposition and high-temperature electron beam annealing (EBA) are used to achieve high-quality epitaxial AlN layers. Efficient energy transfer from the single exposure for only 1 min of electron irradiation with a large area contributes to substantial improvement in the film density and crystal quality of AlN thin films, as demonstrated by the X-ray reflectivity and the θ-2θ/ω-scan X-ray diffraction. High-resolution transmission electron microscopy indicates well-arranged lattice fringes in the epitaxial AlN layer on a sapphire substrate. A low surface roughness of the AlN epilayer, as revealed by the atomic force microscopy, suggests that the damage induced by the EBA treatment is insignificant. The outcomes indicate that the large-area rapid EBA is a low-damage and efficient technique for the recrystallization of thin films prepared at low temperatures.

Milliwatt-power sub-230-nm AlGaN LEDs with &amp;gt;1500 h lifetime on a single-crystal AlN substrate with many quantum wells for effective carrier injection

We fabricated sub-230-nm (far UV-C) light emitting diodes (LEDs) on a single-crystal AlN substrate. With 20 quantum well cycles implemented to enhance carrier injection into the active layers, over 1-mW output power (1.4 and 3.1 mW for 226- and 229-nm LEDs, respectively) was obtained under 100-mA operation. The maximum output power reached 21.1 mW for the single-chip 229-nm LED operating at 700 mA, without significant drooping. The forward voltage for both sub-230-nm LEDs operating at 100 mA was low (5.9 V) due to their low resistances and ideal Ohmic contacts between metal and semiconductor components. Additionally, wall plug efficiencies were 0.24% and 0.53% for the 226- and 229-nm LEDs, respectively. The lifetime of the 226-nm LED while operating at 25 °C reached over 1500 h and did not show current leakage, even after 1524 h. This long lifetime will be achieved by improving carrier injection due to many quantum wells, using a high-quality AlN substrate and achieving high wall plug efficiency.