AlGaN Research Articles

Light extraction efficiency of AlGaN nanowire deep ultraviolet light-emitting diodes on Si with different photonic structures

The low light extraction efficiency (LEE) stemming from the transverse-magnetic dominant emission is one of the major factors limiting the performance of aluminum gallium nitride (AlGaN)-based deep ultraviolet (UV) light-emitting diodes (LEDs) when the light emission wavelengths are close to 200 nm. These wavelengths, nonetheless, are pivotal to applications including sensing and sterilization and are considered human safe. We investigate the LEE of AlGaN/AlN nanowire (NW) deep UV LEDs emitting at 225 nm, with a focus on the top-surface LEE by considering different NW arrangements, typical NW spacings and radii from AlGaN NWs grown by selective area epitaxy, the influence of Si on the LEE in comparison to an Al reflector, and graphene as the top electrode. Our results show that given the selected range of design parameters, the top-surface LEE for honeycomb, square, and hexagonal lattices can be up to around 42% for complete devices on Si with the graphene top electrode; and compared to using an Al reflector, the Si substrate does not reduce LEE considerably. In the end, the light extraction mechanism is discussed by simulating the two-dimensional photonic crystal band structures.

Molecular beam epitaxy growth and characterization of AlGaN epilayers in nitrogen-rich condition on Si substrate

In this work, we investigate the molecular beam epitaxy growth and characterization of aluminum gallium nitride (AlGaN) epilayers in nitrogen (N)-rich condition on silicon using an aluminum nitride (AlN) buffer layer on a nanowire template. It is found that the growth condition of the AlN buffer layer plays an important role on the surface morphology of the AlGaN epilayers. Under the optimized growth temperature of the AlN buffer layer, a relatively smooth AlGaN epilayer can be formed, with a root-mean-square roughness of around 1.8 nm, comparable to the typical III-nitride thin films grown in N-rich conditions by molecular beam epitaxy. The epilayer is further confirmed to be N-polar from both the reflection high-energy electron diffraction 3 × 3 reconstruction and potassium hydroxide etching experiments.

UV-Based Technologies for SARS-CoV2 Inactivation: Status and Perspectives

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the etiologic agent of COVID-19, which has affected the international healthcare systems since the beginning of 2020. Among sanitizing approaches, UV irradiation is a well-known technology often used in different environments to reduce the microbial contamination and the viral transmission. In particular, several works have demonstrated that UVC radiation is able to inactivate SARS-CoV-2 compromising its viral genome and virion integrity. With this work we review and analyze the current status of the pandemic and the state of the art of the UV technology. With traditional UVC discharge lamps having a serious environmental issue, due to their working principle based on mercury, a primary focus is shifted on the aluminum gallium nitride based deep-ultraviolet light emitting diodes. These devices are exploited for compact and environmentally friendly disinfection systems, but efficiency and reliability still play a limiting role into their mass market adoption and system efficacy. In this work we then analyze the latest reports on the effects of dose and wavelength on viral inactivation, thus providing two key pillars for the development of UVC based disinfection systems: the status of the technology and a quantitative evaluation of the dose required to achieve an effective coronavirus inactivation.

High Internal Quantum Efficiency AlGaN Epilayer Grown by Molecular Beam Epitaxy on Si Substrate

We report the molecular beam epitaxy growth and characterization of aluminum gallium nitride (AlGaN) epilayer on an Si substrate. The AlGaN epilayer was grown on an AlN buffer layer through a coalescence process on a nanowire template. The AlGaN epilayer possessed a relatively smooth surface. The room-temperature internal quantum efficiency (IQE) was investigated using power-dependent photoluminescence experiments and theoretical analysis. A maximum IQE of around 50% is derived, with an estimated carrier density of 3 × 1018 cm−3. This IQE is significantly improved compared to the previously reported bulk AlGaN epilayers grown on sapphire and/or AlN-on-sapphire templates. The efficiency droop mechanism is also discussed, which could presumably be attributed to the carrier delocalization.

Effects of surface oxidation on the pH-dependent surface charge of oxidized aluminum gallium nitride

Hypothesis: The properties of the oxidized surface for common materials, such as silicon and titanium, are known to be markedly different from the reduced surface. We hypothesize that surface-oxidized aluminum gallium nitride ((oxidized-AlGaN)/GaN) surface charge behavior is different to unoxidized AlGaN (with ultrathin native oxide only), which can be validated via surfactant adsorption. Understanding these differences will explain why (oxidized-AlGaN)/GaN-based sensors are better performing than AlGaN ones, which has been previously demonstrated but not understood.Experiments: The surface of an AlGaN/GaN structure was oxidized with hot piranha solution and oxygen plasma. AFM force measurements and imaging were performed to probe the charge properties of the surface in aqueous solutions of varying pH containing only an acid or base, or with an added ionic surfactant: cationic cetyltrimethylammonium bromide (CTAB) or anionic sodium dodecylsulfate (SDS).Findings: The (oxidized-AlGaN)/GaN surface is positively charged at pH 4 and pH 5.5, although pH 5.5 should be close to the isoelectric point of the surface. The surface is negatively charged at pH 10 and pH 12, and sufficiently charged to attract cooperative adsorption of CTAB aggregates at pH 12. At pH 2, the evidence is inconclusive, but the surface is most likely positively charged. Compared to unoxidized AlGaN, the (oxidized-AlGaN)/GaN surface shows a wider range of surface charge magnitude over pH values between 2 and 12. This suggests that the (oxidized-AlGaN)/GaN surface has a higher surface hydroxyl group density than unoxidized AlGaN, which explains the higher sensitivity for pH sensors based on (oxidized-AlGaN)/GaN structures.

A Schottky-Type Metal-Semiconductor-Metal Al0.24Ga0.76N UV Sensor Prepared by Using Selective Annealing.

Asymmetric metal-semiconductor-metal (MSM) aluminum gallium nitride (AlGaN) UV sensors with 24% Al were fabricated using a selective annealing technique that dramatically reduced the dark current density and improved the ohmic behavior and performance compared to a non-annealed sensor. Its dark current density at a bias of −2.0 V and UV-to-visible rejection ratio (UVRR) at a bias of −7.0 V were 8.5 × 10−10 A/cm2 and 672, respectively, which are significant improvements over a non-annealed sensor with a dark current density of 1.3 × 10−7 A/cm2 and UVRR of 84, respectively. The results of a transmission electron microscopy analysis demonstrate that the annealing process caused interdiffusion between the metal layers; the contact behavior between Ti/Al/Ni/Au and AlGaN changed from rectifying to ohmic behavior. The findings from an X-ray photoelectron spectroscopy analysis revealed that the O 1s binding energy peak intensity associated with Ga oxide, which causes current leakage from the AlGaN surface, decreased from around 846 to 598 counts/s after selective annealing.

Well-suppressed interface states and improved transport properties of AlGaN/GaN MIS-HEMTs with PEALD SiN gate dielectric

SiN deposited by plasma-enhanced atomic layer deposition (PEALD) is investigated as gate dielectric layer for GaN metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs). The X-ray photoelectron spectroscopy (XPS) confirms low oxygen content in SiN-layer and the interface of SiN/(Al)GaN. The fabricated MIS-HEMTs exhibit significant reduction in gate leakage current (two or three orders of magnitude) and improvement in drain current (ID) characteristics, compared with the controlled HEMTs. A remarkable small threshold voltage (VTH) hysteresis (0.12 ± 0.05 V) and an enhanced 2DEG channel mobility are also achieved for PEALD SiN/(Al)GaN MIS-HEMTs. Simultaneously, a distribution of the interface states is quantitatively described by constant-capacitance deep-level transient spectroscopy (CC-DLTS), displaying a low density of 1.6 × 1013 cm−2 eV−1 at level depth of 0.03 eV and below 2.1 × 1012 cm−2 eV−1 at deep level depth of EC - ET > 0.4 eV. The suppressed interface traps density results in an improved performance for MIS-HEMTs. The dependence of the characteristics of MIS-HEMTs on SiN thickness (tSiN) was also studied in detail.

Computational Analysis of Joule Heating Effect in Triple Material Gate AlGaN/GaN High Electron Mobility Transistor

Aluminum gallium nitride (AlGaN)/gallium nitride (GaN) high electron mobility transistor (HEMT) has shown great potential and superiority over other devices in high power and high speed-based systems [1-4]. Recently, a novel AlGaN/GaN HEMT structure named triple material gate (TMG) HEMT has been proposed in [5] in which the gate contact is composed of three metals with different work functions. TMG HEMT ensures high carrier velocity and better suppression of short channel effects. In this work, a two-dimensional electro-thermal model has been developed to investigate the joule heating effect in TMG AlGaN/GaN HEMT using the software COMSOL [6]. The current continuity and heat transport equations are solved self-consistently to obtain the temperature distribution profile in various layers of the device. Thermal efficiency of TMG HEMT is compared with dual material gate and single material gate HEMT. It has been found that, besides providing better current carrying capability, TMG HEMT shows optimum thermal performance. Moreover, the variation of peak temperature in the channel with drain and gate voltages is analyzed to study the bias voltage dependence of the device temperature rise. Effects of several device parameters like total channel length, spontaneous and piezoelectric polarization induced charges and ratio of lengths of control and screen gates, on the thermal performance have also been explored. Simulations are carried out for the device lying on different substrate materials (Sapphire, GaN, Si and SiC) with varying thermal conductivity to demonstrate their performance in heat removal capacity. This simulation model can serve as a very useful tool to comprehend the thermal behavior of the device for high power and high frequency applications.

Recent Progress on Aluminum Gallium Nitride Deep Ultraviolet Lasers by Molecular Beam Epitaxy

Over the past decades, the aluminum gallium nitride (AlGaN) alloy system has received wide interest for the development of semiconductor deep ultraviolet (DUV) lasers due to its direct, tunable, and ultrawide bandgap energies (3.4–6.2 eV). The progress, nonetheless, has been remained slow, which is ascribed to a few major challenges, including large dislocation and defect densities, difficulty in obtaining p‐type high‐Al‐content AlGaN layers with a sufficient p‐type conduction, the large electric polarization fields, and the unfavorable optical polarization. In recent years, with AlGaN alloys grown by molecular beam epitaxy (MBE), including both thin films and nanowire structures, remarkable advancements have been made, such as highly conductive p‐type high‐Al‐content AlGaN epilayers with resistivities as low as 0.7 Ω cm and DUV lasing down to 239 nm with nanowire structures under a direct current injection. Herein, the recent progress on the DUV lasers by the MBE‐grown AlGaN is reviewed. The challenges and prospects of the MBE‐grown AlGaN for DUV lasers are also discussed.

Computational Analysis of Joule Heating Effect in Triple Material Gate AlGaN/GaN High Electron Mobility Transistor

A two-dimensional electro-thermal model has been developed to investigate the Joule heating effect in triple material gate (TMG) aluminum gallium nitride (AlGaN) / gallium nitride (GaN) high electron mobility transistor (HEMT) using the software COMSOL. The current continuity and heat transport equations are solved self-consistently to obtain the temperature distribution profile in various layers of the device. By comparing with dual material gate and single material gate HEMTs, it has been found that, besides providing better current carrying capability, TMG HEMT shows optimum thermal performance. Effects of several device parameters like total channel length, spontaneous and piezoelectric polarization induced charges and ratio of lengths of control and screen gates, on the thermal efficiency have been explored. Bias voltage and underlying substrate material dependence of the device operation have also been investigated.

Neutron radiation hardness of aluminum gallium nitride UV LEDs at various wavelengths

Radiation hard diagnostics are critical to the success of nuclear fusion at National Ignition Facility, Z, ITER, and prolonged space explorations. We have first demonstrated the exceptional proton radiation hardness of initial GaN devices and qualified their space flight and deployment for missions such as the Laser Interferometer Space Antenna and International Space Station. We have further conducted neutron radiation hardness experiments at Los Alamos Neutron Science Center by opening a new high fluence beam station. During 2014-2016, we irradiated multiple Aluminum Gallium Nitride (AlGaN) LEDs with a maximum fluence of 2.4 × 1013 neutrons/cm2 in 3 years and generated 54 161 current-voltage (I-V) scan traces. Our data processing program analyzes each and all I-V traces. In addition, we retrieved local temperature records to analyze and remove temperature effects in the outdoor environment. The I-V curve families of AlGaN UV LEDs with emitting wavelengths of 265, 275, and 310nm were compared. The I-V curves of 265nm AlGaN UV LEDs have the smallest deviations from the average value, while the I-V curves for 310nm AlGaN LEDs showed the largest deviations from the average value. We have reached another important recommendation for the optimal use of multiple AlGaN optoelectronic devices or imaging arrays for inertially confined fusion diagnostics: Shorter wavelength devices at 265nm exhibit more consistent radiation hardness performance than the 310nm devices. Higher aluminum content LEDs or AlxGa1-xN devices with higher mole fraction x for generating shorter wavelengths have better radiation hardness for fusion diagnostics.

Bright room temperature near-infrared single-photon emission from single point defects in the AlGaN film

Near-infrared (NIR) single-photon source plays a key role in a wide range of applications in quantum technology. In particular, in quantum communication, the NIR wavelength operation perfectly matches the relatively low-attenuation transmission window of the optical fiber, which attracts more and more research interest. Here, we report the room temperature single-photon emission from single point defects in the aluminum gallium nitride (AlGaN) film. The obtained single-photon emission covers from 720 to 930 nm and exhibits highly linear polarization and high photon brightness. This may provide a platform for future integrated on-chip quantum photonic devices.

An ultrathin-barrier AlGaN/GaN heterostructure: a recess-free technology for the fabrication and integration of GaN-based power devices and power-driven circuits

An AlGaN-recess-free, ultrathin-barrier (UTB) AlGaN (<6 nm)/GaN heterostructure is presented for the fabrication and integration of AlGaN/GaN enhancement/depletion-mode (E/D-mode) heterojunction field-effect transistors (HFETs), and metal–insulator-semiconductor HFETs (MIS-HFETs). The 2D electron gas in the access region of the UTB-AlGaN/GaN (MIS)HFETs can effectively be recovered by a low-pressure chemical-vapor-deposited SiNx passivation layer (LPCVD-SiNx), which is capable of introducing about 2.75 × 1013 cm−2 positive fixed charges at the LPCVD-SiNx/(Al)GaN interface. LPCVD-SiNx can also serve as a good gate insulator for D-mode MIS-HFETs. Using the self-terminating etching of LPCVD-SiNx on III-nitride as well as a low-damage remote plasma pretreatment, high uniformity E-mode HFETs and low-hysteresis E-mode MIS-HFETs have been fabricated using the GaN-on-Si platform. E/D-mode MIS-HFET inverters with a large logic swing have also been demonstrated on this platform. The UTB-AlGaN/GaN heterostructure is an attractive technology platform for the on-chip integration of power and RF devices with power-driven circuits for GaN-based smart power integrated circuits.

Suppressing the efficiency droop in AlGaN-based UVB LEDs

The optoelectronic properties of semiconducting aluminum gallium nitride (AlGaN)-based ultraviolet-B (UVB) light-emitting diodes (LEDs) are crucial for real-world medical applications such as cancer therapy and immunotherapy. However, the performance of AlGaN-based UVB LED devices is still poor due to the low hole injection efficiency. Therefore, we have numerically investigated the performance of AlGaN-based UVB LEDs for the suppression of efficiency droop as well as for the enhancement of hole injection in the multiquantum wells (MQWs). The influence of the undoped (ud)-AlGaN final quantum barrier (FQB), as well as the Mg-doped multiquantum barrier electron blocking layer (p-MQB EBL), on the efficiency droop has been focused on specifically. To evaluate the performance of the proposed device, we have compared its internal quantum efficiency (IQE), carrier concentration, energy band diagram, and radiative recombination rate with the conventional device structure. Furthermore, the influence of Al composition in the Al-graded p-AlGaN hole source layer (HSL) on the operating voltages of the proposed UVB LEDs was considered. The simulation results suggest that our proposed structure has a high peak efficiency and much lower efficiency droop as compared to the reference structure (conventional). Ultimately, the radiative recombination rate in the MQWs of the proposed UVB LED-N structure has increased up to ∼73%, which is attributed to the enhanced level of electron and hole concentrations by ∼64% and 13%, respectively, in the active region. Finally, a high efficiency droop of up to ∼42% in RLED has been successfully suppressed, to ∼7%, by using the optimized ud-AlGaN FQB and the p-MQB EBL, as well as introducing Al-graded p-AlGaN HSL in the proposed UVB LED-N structure.

Low-Damage and Self-Limiting (Al)GaN Etching Process through Atomic Layer Etching Using O2 and BCl3 Plasma

This paper reports on the use of low-damage atomic layer etching (ALE) performed using O2 and BCl3 plasma for etching (Al)GaN. The proposed ALE process led to excellent self-limiting etch characteristics with a low direct current (DC) self-bias, which resulted in a high linearity between the etching depth and number of cycles. The etching damage was evaluated using several methods, including atomic force microscopy, photoluminescence (PL), and X-ray photoelectron spectroscopy, and the I–V properties of the recessed Schottky diodes were compared with those of digital etching performed using O2 plasma and HCl solution. The electrical characteristics of the recessed Schottky diode fabricated using the proposed ALE process were superior to those of the diodes fabricated using the conventional digital etching process. Moreover, the ALE process yielded a higher PL intensity and N/(Al + Ga) ratio of the etched AlGaN surface, along with a smoother etched surface.

Intrinsic excitation-dependent room-temperature internal quantum efficiency of AlGaN nanowires with varying Al contents

Aluminum gallium nitride (AlGaN) nanowires have become an emerging approach for semiconductor deep ultraviolet light-emitting devices. To further improve the device performance, it is critical to understand the optical quality of AlGaN nanowires. However, today, the room-temperature internal quantum efficiency (IQE) of AlGaN nanowires is predominantly analyzed by the temperature-dependent photoluminescence (PL) approach under one excitation power or taking the PL intensity ratio at the room temperature and low temperature with different excitation powers. In both cases, one needs to assume the low temperature IQE to be 100%, which is not always valid, in particular when the excitation power changes at the low temperature. In this work, we study the room-temperature IQE of AlGaN nanowires through the detailed excitation power-dependent PL experiments and theoretical analysis. This allows us to derive the intrinsic room-temperature IQE of AlGaN nanowires as a function of the excitation power. It is found that for an Al content in the range of 22%–54%, the IQE of all samples increases as the excitation increases, followed by an efficiency droop. Moreover, comparing different samples, the IQE at low excitations increases as the Al content increases, whereas the peak IQE reduces from 73% to 56% as the Al content increases. The underlying mechanisms are also discussed in this paper.

Determination of carbon-related trap energy level in (Al)GaN buffers for high electron mobility transistors through a room-temperature approach

A room-temperature method to determine the trap energy levels in the carbon-doped (Al)GaN buffers is developed via a transient current measurement on the AlGaN/GaN high electron mobility transistors under back-gate voltages combined with a measurement of the buffer vertical leakage. Under high back-gate voltages, a linear relationship is obtained between the trap energy levels and the square roots of electric field strength, suggesting that the vertical conduction in the C-doped buffer follows the Poole–Frenkel law. The trap energy level in C-doped Al0.07Ga0.93N is finally determined to be 1.1 eV through the established room-temperature approach, while that in C-doped GaN is extracted to be 0.9 eV, both of which are related to the carbon impurities.

AlGaN/GaN on SiC Devices without a GaN Buffer Layer: Electrical and Noise Characteristics.

We report on the high-voltage, noise, and radio frequency (RF) performances of aluminium gallium nitride/gallium nitride (AlGaN/GaN) on silicon carbide (SiC) devices without any GaN buffer. Such a GaN–SiC hybrid material was developed in order to improve thermal management and to reduce trapping effects. Fabricated Schottky barrier diodes (SBDs) demonstrated an ideality factor n at approximately 1.7 and breakdown voltages (fields) up to 780 V (approximately 0.8 MV/cm). Hall measurements revealed a thermally stable electron density at N2DEG = 1 × 1013 cm−2 of two-dimensional electron gas in the range of 77–300 K, with mobilities μ = 1.7 × 103 cm2/V∙s and μ = 1.0 × 104 cm2/V∙s at 300 K and 77 K, respectively. The maximum drain current and the transconductance were demonstrated to be as high as 0.5 A/mm and 150 mS/mm, respectively, for the transistors with gate length LG = 5 μm. Low-frequency noise measurements demonstrated an effective trap density below 1019 cm−3 eV−1. RF analysis revealed fT and fmax values up to 1.3 GHz and 6.7 GHz, respectively, demonstrating figures of merit fT × LG up to 6.7 GHz × µm. These data further confirm the high potential of a GaN–SiC hybrid material for the development of thin high electron mobility transistors (HEMTs) and SBDs with improved thermal stability for high-frequency and high-power applications.

Highly Uniform, Self‐Assembled AlGaN Nanowires for Self‐Powered Solar‐Blind Photodetector with Fast‐Response Speed and High Responsivity

AbstractSearching for power‐independent, compact, and highly environment‐sensitive photodetectors is a critical step towards the realization of next‐generation energy‐efficient and sustainable integrated optoelectronic systems. Particularly, the deep ultraviolet (UV) band, which has large photon energy, is extremely suitable for environment monitoring and invisible light communication application. Herein, the demonstration of self‐powered deep UV solar‐blind photodetectors in a photoelectrochemical (PEC) cell configuration is reported, adopting wide bandgap n‐type aluminum gallium nitride (AlGaN) nanowires as photoelectrode. After decorating nanowires with noble metal ruthenium (Ru), the constructed solar‐blind PEC photodetectors exhibited excellent responsivity of 48.8 mA W‐1, fast response speed (rise time of 83 ms and decay time of 19 ms) with large photocurrent density of 55 μA cm‐2 at 254 nm illumination. Such superior performance can be attributed to, firstly and foremost, the successful synthesis of highly uniform and defect‐free n‐type AlGaN nanowires which ensures efficient photogeneration via effective light‐harvesting, and secondly, the boosted carrier separation and collection efficiency through Ru decoration. This novel nanoarchitecture enables deep UV photodetection to work stably with low energy consumption, intriguingly, opening the possibility for the development of high‐performance PEC photodetectors based on group III‐nitride semiconductors covering the entire spectral range from infrared to deep UV.

Interface Charge Effects on 2-D Electron Gas in Vertical-Scaled Ultrathin-Barrier AlGaN/GaN Heterostructure

The combination of ultrathin-barrier (UTB) AlGaN ( x grown by low-pressure chemical vapor deposition (LPCVD) is a promising technique for development of GaN-based millimeter-wave power amplifiers and recess-free enhancement-mode (E-mode) power switches. The LPCVD-SiN x passivation is capable of inducing high density of positive charges at the SiN x /(Al)GaN interface ( $\sim 3.50 \times 10^{13}$ cm−2), ensuring efficient recovery of 2-D electron gas (2-DEG) density that is comparable with conventional AlGaN/GaN heterostructure. Temperature-dependent Hall measurements and scattering mechanism simulations confirm the positive interfacial charges as well as interface states with density below 1013 cm−2 and exert weak remote coulombic scattering of 2-DEG in metal–insulator–semiconductor heterojunction field-effect transistors (MIS-HFETs), which warrants a low ON-resistance of UTB-AlGaN/GaN-based devices. UTB-AlGaN ( x passivation is a compelling technology platform for fabrication of high-frequency power amplifiers and high-efficiency E-mode power switches.