AlN Passivation Layer Research Articles

Recess-Free E-Mode AlGaN/GaN MIS-HFET with Crystalline PEALD AlN Passivation Process

We utilized a plasma-enhanced atomic layer deposition (PEALD) process to deposit an AlN passivation layer on AlGaN/GaN surface to enhance the polarization effects, which enabled the fabrication of an enhancement-mode (E-mode) AlGaN/GaN metal-insulator-semiconductor heterojunction field-effect transistor (MIS-HFET) without the need for a gate recess process. The AlN film deposited by PEALD exhibited a crystalline structure, not an amorphous one. The enhanced polarization effect of introducing the PEALD AlN film on a thin AlGaN barrier was confirmed through electrical analysis. To fabricate the E-mode AlGaN/GaN MIS-HFET, the PEALD AlN film was deposited on a 4.5 nm AlGaN barrier layer and then a damage-free wet etching process was used to open the gate region. The MIS-gate structure was formed by depositing a 15 nm plasma-enhanced chemical vapor deposition (PECVD) silicon dioxide (SiO2) film. The fabricated thin-AlGaN/GaN MIS-HFET demonstrated successful E-mode operation, with a threshold voltage of 0.45 V, an on/off ratio of approximately 109, a specific on-resistance of 7.1 mΩ·cm2, and an off-state breakdown voltage exceeding 1100 V.

Improved electro-optical and photoelectric performance of GaN-based micro-LEDs with an atomic layer deposited AlN passivation layer.

The quantum efficiency of GaN-based micro-light-emitting diodes (micro-LEDs) is of great significance for their luminescence and detection applications. Optimized passivation process can alleviate the trapping of carriers by sidewall defects, such as dangling bonds, and is regarded as an effective way to improve the quantum efficiency of micro-LEDs. In this work, an AlN passivation layer was prepared by atomic layer deposition to improve the electro-optical and photoelectric conversion efficiency in GaN-based micro-LEDs. Compared to conventional Al2O3 passivation, the AlN passivation process has a stronger ability to eliminate the sidewall defects of micro-LEDs due to the homogeneous passivation interface. Our experiments show that the AlN-passivated device exhibits two orders of magnitude lower forward leakage and a smaller ideality factor, which leads to significantly enhanced external quantum efficiency (EQE). For 25*25 μm2 micro-LEDs, the EQE of the AlN-passivated device was 18.3% and 57.7% higher than that of the Al2O3-passivated device in luminescence application and detection application, respectively.

In Situ Crystalline AlN Passivation for Reduced RF Dispersion in Strained‐Channel AlN/GaN/AlN High‐Electron‐Mobility Transistors

The recent demonstration of W mm−1 output power at 94 GHz in AlN/GaN/AlN high‐electron‐mobility transistors (HEMTs) has established AlN as a promising platform for millimeter‐wave electronics. The current state‐of‐art AlN HEMTs using ex situ‐deposited silicon nitride (SiN) passivation layers suffer from soft gain compression due to trapping of carriers by surface states. Reducing surface state dispersion in these devices is thus desired to access higher output powers. Herein, a potential solution using a novel in situ crystalline AlN passivation layer is provided. A thick, 30+ nm‐top AlN passivation layer moves the as‐grown surface away from the 2D electron gas (2DEG) channel and reduces its effect on the device. Through a series of metal‐polar AlN/GaN/AlN heterostructure growths, it is found that pseudomorphically strained 15 nm thin GaN channels are crucial to be able to grow thick AlN barriers without cracking. The fabricated recessed‐gate HEMTs on an optimized heterostructure with 50 nm AlN barrier layer and 15 nm GaN channel layer show reduction in dispersion down to compared with in current state‐of‐art ex situ SiN‐passivated HEMTs. These results demonstrate the efficacy of this unique in situ crystalline AlN passivation technique and should unlock higher mm‐wave powers in next‐generation AlN HEMTs.

Novel Multilayer SAW Temperature Sensor for Ultra-High Temperature Environments

Performing high-temperature measurements on the rotating parts of aero-engine systems requires wireless passive sensors. Surface acoustic wave (SAW) sensors can measure high temperatures wirelessly, making them ideal for extreme situations where wired sensors are not applicable. This study reports a new SAW temperature sensor based on a langasite (LGS) substrate that can perform measurements in environments with temperatures as high as 1300 °C. The Pt electrode and LGS substrate were protected by an AlN passivation layer deposited via a pulsed laser, thereby improving the crystallization quality of the Pt film, with the function and stability of the SAW device guaranteed at 1100 °C. The linear relationship between the resonant frequency and temperature is verified by various high-temperature radio-frequency (RF) tests. Changes in sample microstructure before and after high-temperature exposure are analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The analysis confirms that the proposed AlN/Pt/Cr thin-film electrode has great application potential in high-temperature SAW sensors.

Performance of thin AlxOy, SixNy and AlN passivation layers for high temperature SAW device applications

This paper investigates the potential of passivation coatings for surface acoustic wave devices (SAW) based on aluminum nitride as a piezoelectric layer to operate at temperatures up to 1000 °C and in pure oxygen atmosphere. To increase the durability of SAW devices, a thin passivation layer was deposited on top of the structure, consisting of either aluminum nitride, alumina or silicon nitride. To investigate the effectivity of the passivation layer as oxygen diffusion barrier, time resolved X-ray diffraction (XRD) measurements were performed at 1000 °C in pure oxygen atmosphere for up to 24 h. Furthermore, the influence of an additional layer on top of the structure on the SAW performance were examined. Therefore, samples were measured before and after the deposition process. Afterwards, the samples were placed in a furnace in pure oxygen atmosphere to test the robustness of the different layers.

Improving Defect-Based Quantum Emitters in Silicon Carbide via Inorganic Passivation.

Defect-based color centers in wide-bandgap crystalline solids are actively being explored for quantum information science, sensing, and imaging. Unfortunately, the luminescent properties of these emitters are frequently degraded by blinking and photobleaching that arise from poorly passivated host crystal surfaces. Here, a new method for stabilizing the photoluminescence and charge state of color centers based on epitaxial growth of an inorganic passivation layer is presented. Specifically, carbon antisite-vacancy pairs (CAV centers) in 4H-SiC, which serve as single-photon emitters at visible wavelengths, are used as a model system to demonstrate the power of this inorganic passivation scheme. Analysis of CAV centers with scanning confocal microscopy indicates a dramatic improvement in photostability and an enhancement in emission after growth of an epitaxial AlN passivation layer. Permanent, spatially selective control of the defect charge state can also be achieved by exploiting the mismatch in spontaneous polarization at the AlN/SiC interface. These results demonstrate that epitaxial inorganic passivation of defect-based quantum emitters provides a new method for enhancing photostability, emission, and charge state stability of these color centers.

Effect of Surface Passivation and Substrate on Proton Irradiated AlGaN/GaN HEMT Transport Properties

In this work, we fabricate Van der Pauw structures on AlGaN/GaN high electron mobility transistors (HEMTs) with both SiN and AlN passivation layers and both SiC and GaN substrates to study temperature dependent electron transport properties of 2 dimensional electron gases (2DEG) in as-fabricated and proton irradiated devices. We confirm that using a GaN substrate or AlN passivation layer enhances the mobility particularly at low temperatures, though this enhancement is greatly reduced upon irradiating the samples with high proton fluences. Our results suggest that the sheet density may be affected by Al diffusion in the sample creating electron donating impurities, which freeze out around 200 K and are more prominent in AlN passivated samples and minimized by using a GaN over a SiC substrate. Additionally, we show that a large dose of radiation forms traps in the AlGaN layer, which drop the sheet density at high proton fluences.

Passivation of InGaAs interface states by thin AlN interface layers for metal-insulator-semiconductor applications

The passivation of InGaAs by thin AlN layers allows a significant reduction of the interface state density compared to that of the widely used Al2O3/InGaAs structure. The influence of the AlN layer thickness on the interface electrical properties, as well as the role of the post-deposition annealing, was carefully examined. Ultrathin AlN layers (∼1 nm) provide high quality interfacial electrical properties after a mild anneal (400 °C). Thick AlN passivation layers require annealing at higher temperature (500 °C) to achieve low interface states density. Possible explanations of the observed trend are suggested.

Improved interface properties of GaN metal-oxide-semiconductor device with non-polar plane and AlN passivation layer

Utilizing a non-polar plane substrate and an ultra-thin AlN passivation layer results in significantly improved interface properties of a GaN metal-oxide-semiconductor (MOS) device. After depositing an Al2O3 gate dielectric layer on GaN substrates with polar c-plane and non-polar m-plane surfaces, it is found that the devices on the non-polar surface show much better interface properties than those on the polar surface. To further improve the interface properties, an amorphous ultra-thin AlN layer is deposited on the substrate before the Al2O3 deposition. The interface properties of both devices on the c-plane and m-plane are dramatically improved by the AlN passivation layer. The interface trap density of the Al/Al2O3/AlN/GaN MOS capacitor on the non-polar surface is reduced by two orders of magnitude compared to that on the polar surface.

Impacts of Thermal Atomic Layer-Deposited AlN Passivation Layer on GaN-on-Si High Electron Mobility Transistors.

Thermal atomic layer deposition (ALD)-grown AlN passivation layer is applied on AlGaN/GaN-on-Si HEMT, and the impacts on drive current and leakage current are investigated. The thermal ALD-grown 30-nm amorphous AlN results in a suppressed off-state leakage; however, its drive current is unchanged. It was also observed by nano-beam diffraction method that thermal ALD-amorphous AlN layer barely enhanced the polarization. On the other hand, the plasma-enhanced chemical vapor deposition (PECVD)-deposited SiN layer enhanced the polarization and resulted in an improved drive current. The capacitance-voltage (C-V) measurement also indicates that thermal ALD passivation results in a better interface quality compared with the SiN passivation.

Electrical properties of GaAs metal–oxide–semiconductor structure comprising Al2O3 gate oxide and AlN passivation layer fabricated in situ using a metal–organic vapor deposition/atomic layer deposition hybrid system

This paper presents a compressive study on the fabrication and optimization of GaAs metal–oxide–semiconductor (MOS) structures comprising a Al2O3 gate oxide, deposited via atomic layer deposition (ALD), with an AlN interfacial passivation layer prepared in situ via metal–organic chemical vapor deposition (MOCVD). The established protocol afforded self-limiting growth of Al2O3 in the atmospheric MOCVD reactor. Consequently, this enabled successive growth of MOCVD-formed AlN and ALD-formed Al2O3 layers on the GaAs substrate. The effects of AlN thickness, post-deposition anneal (PDA) conditions, and crystal orientation of the GaAs substrate on the electrical properties of the resulting MOS capacitors were investigated. Thin AlN passivation layers afforded incorporation of optimum amounts of nitrogen, leading to good capacitance–voltage (C–V) characteristics with reduced frequency dispersion. In contrast, excessively thick AlN passivation layers degraded the interface, thereby increasing the interfacial density of states (Dit) near the midgap and reducing the conduction band offset. To further improve the interface with the thin AlN passivation layers, the PDA conditions were optimized. Using wet nitrogen at 600 °C was effective to reduce Dit to below 2 × 1012 cm−2 eV−1. Using a (111)A substrate was also effective in reducing the frequency dispersion of accumulation capacitance, thus suggesting the suppression of traps in GaAs located near the dielectric/GaAs interface. The current findings suggest that using an atmosphere ALD process with in situ AlN passivation using the current MOCVD system could be an efficient solution to improving GaAs MOS interfaces.

High breakdown voltage in AlN/GaN metal–insulator–semiconductor high-electron-mobility transistors

The breakdown characteristics of AlGaN/GaN based metal–insulator–semiconductor high-electron-mobility transistors (MISHEMTs) using a 10 nm thick AlN gate insulator and passivation layer deposited plasma enhanced atomic layer deposition. The AlN was effective in significantly reducing gate leakage current relative to Schottky gate devices and showed only small decreases in drain current during gate lag measurements. The devices exhibited a strong dependence of gate breakdown voltage on source–drain distance, reaching a value of 2000 V for a source–drain distance of 40 μm limited by the measurement instrument. The specific on-state resistance was 1.3 and 10.9 mΩ cm2 for the devices with the gate–drain distance of 7.5 and 37.5 μm, respectively. The saturation drain current was inversely dependent on source–drain distance and the on–off ratios were in excess of 108 due to the low gate leakage current in the MISHEMTs.

High-$f_{{\rm MAX}}$ High Johnson's Figure-of-Merit 0.2- $\mu{\rm m}$ Gate AlGaN/GaN HEMTs on Silicon Substrate With ${\rm AlN}/{\rm SiN}_{{\rm x}}$ Passivation

This letter reports a 0.2- <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex></formula> gate AlGaN/GaN high-electron-mobility transistors (HEMTs) on an Si substrate passivated with an <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm AlN}/{\rm SiN}_{{\rm x}}$</tex> </formula> (4/20 nm) stack layer. The 4-nm-thick AlN was grown by plasma-enhanced atomic-layer-deposition. The <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm AlN}/{\rm SiN}_{{\rm x}}$</tex></formula> -passivated HEMTs exhibit a high maximum drain current of 930 mA/mm, an three-terminal OFF-state breakdown voltage <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$({BV}_{{\rm DS}})$</tex></formula> of 119 V, and a small threshold voltage shift of 130 mV in a wide drain bias range <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(V_{{\rm DS}}=3-24~{\rm V})$</tex></formula> . Owing to the additional positive polarization charge in the AlN passivation layer, the access resistance <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$R_{s}$</tex> </formula> in the GaN-on-Si HEMTs is significantly reduced while maintaining small parasitic gate-drain capacitance <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$C_{{\rm gd}}$</tex></formula> , contributing to a high power-gain cutoff frequency <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$f_{{\rm MAX}}$</tex></formula> of 182 GHz and a high Johnson's figure of merit of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${BV}_{{\rm DS}}\times f_{\,T}$</tex></formula> of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$6.43\times 10^{{12}}~{\rm V}/{\rm s}$</tex></formula> simultaneously. The accuracy of the RF performance is verified by a small signal modeling based on measured S-parameters.

Analysis of diamond surface channel field-effect transistors with AlN passivation layers

Diamond surface channel field effect transistors were passivated with thin AlN layers grown by metal-organic chemical vapor deposition in order to improve the chemical stability of the surface-near p-type channel. Electrical characterization showed that the surface-near conductivity in the diamond is preserved during AlN overgrowth if the process temperature does not exceed 800 °C. However, the sheet carrier density is decreased by a factor of about 5 compared to the unpassivated hydrogen-terminated surface. A combination of TEM and XPS analysis showed that this effect is not induced by a partial modification of the surface termination or by a polarization of the AlN passivation. The preserved, but reduced surface-near conductivity in the diamond can however be explained by a hydrogen double bond between the diamond and the AlN film. Field-effect transistor structures fabricated on the passivated diamond substrates showed stable operation up drain-source voltages to −70 V and might therefore be promising candidates for future high-voltage applications.

Atomic Layer Epitaxy AlN for Enhanced AlGaN/GaN HEMT Passivation

Enhancements in AlGaN/GaN high-electron-mobility transistor (HEMT) performance have been realized through ultrathin (4 nm) AlN passivation layers, formed by atomic layer epitaxy (ALE). A combination of ex situ and in situ surface cleans prepare the surface for deposition of ALE AlN. HEMTs passivated by high crystallinity AlN, grown at 500 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}{\rm C}$</tex></formula> , show improvements in 2-D electron gas sheet carrier density, gate leakage current, off-state drain leakage current, subthreshold slope, and breakdown voltage. In addition, degradation of dynamic on resistance during pulsed off-state voltage switching stress is suppressed by <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{50\%}$</tex></formula> compared with HEMTs passivated by conventional plasma enhanced chemical vapor deposition <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm SiN}_{x}$</tex></formula> .

High-k GaAs metal insulator semiconductor capacitors passivated by ex-situ plasma-enhanced atomic layer deposited AlN for Fermi-level unpinning

This paper examines the utilization of plasma-enhanced atomic layer deposition grown AlN in the fabrication of a high-k insulator layer on GaAs. It is shown that high-k GaAs MIS capacitors with an unpinned Fermi level can be fabricated utilizing a thin ex-situ deposited AlN passivation layer. The illumination and temperature induced changes in the inversion side capacitance, and the maximum band bending of 1.2 eV indicates that the MIS capacitor reaches inversion. Removal of surface oxide is not required in contrast to many common ex-situ approaches.

Initial reaction mechanism of H-passivated Ge surface passivation by atomic layer deposition of Al 2O 3 and AlN

The authors investigated the surface passivation mechanisms of H-terminated Ge(100)(2 × 1) by atomic layer deposition of Al 2O 3, AlN through first-principles calculation method. The formation of initial Al 2O 3 and AlN passivation layer on Ge surface is investigated at atomic scale and the corresponding reaction pathways for both the reaction are also presented. The reaction barrier heights of H 2O and NH 3 half reaction are 0.53 eV/mol and 0.51 eV/mol higher than that of Al(CH 3) 3 half reaction, respectively which indicates that the overall passivation processes of the both reactions are limited by the second half reaction. Moreover, the chemisorption state and product state of the NH 3 half reaction are found to be in the same energy level, and it is possible that the second half reaction of AlN will be trapped in a chemisorption state.

RESIDUAL STRESS DEVELOPMENT IN CU THIN FILMS WITH AND WITHOUT AlN PASSIVATION BY CYCLIC PLANE BENDING

Since the thin film technology is applied to micro-machines, MEMS (micro electro-mechanical system), optical devices and others, the evaluation of mechanical properties in thin films becomes to be important. On the other hand, there are differences in mechanical properties between bulk materials and thin films, but studies in this field have not yet been made enough. The present paper reports on the evaluation of the mechanical properties of Cu thin films with and without AlN passivation layer. Specimens with different thickness of Cu film were subjected to cyclic plane bending fatigue test. Residual stresses developed in the Cu films were measured in a sequence of bending cycles using X-ray diffraction method in order to understand the effect of film thickness and passivation layer on mechanical properties of Cu thin films.

X-Ray Investigation on Strength of Thin Films

The behavior of thermal stresses and residual stresses in aluminum and copper thin films were investigated by the diffraction of X-rays and synchrotron radiations. Aluminum films were exposed in an elevated temperature in order to measure the thermal stress behavior in the films. Thermal stresses in the films in thin aluminum and copper films of 10 nm to 1000 nm thickness with and without passivation payer were measured in a sequence of thermal cycles. Stresses in thick films without passivation traced along with a hysteresis loop during thermal cycles, whereas the thin films of nano-meter size with AlN passivation layer behaved in a linear way with the temperature change. Thin copper films with and without AlN passivation layer were used to investigate strengthening mechanism of thin films. Specimens with different thickness of the films were subjected to a cyclic plane bending fatigue test. Residual stresses developed in the films were measured in a sequence of bending cycles to understand the effect of film thickness and passivation layer on mechanical properties of thin films. The film of 500 nm thickness developed less residual stresses in the film than thicker films. A passivation layer was effective to prevent residual stress development in the film.

Enhancement mode GaAs metal-oxide-semiconductor field-effect-transistor integrated with thin AlN surface passivation layer and silicon/phosphorus coimplanted source/drain

By using thin AlN film as a passivation layer between the GaAs and HfO2, GaAs metal-oxide-semiconductor (MOS) devices exhibited well-behaved accumulation and inversion capacitance-voltage characteristics with low leakage current of 6×10−5A∕cm2 at Vg=1V and interface state density of ∼4.9×1010cm−2eV−1. In order to improve the N+ source/drain activation efficiency, the Si∕P coimplantation technique was adapted to fabricate GaAs n-type metal-oxide-semiconductor field-effect-transistor (MOSFET). High quality N+∕P GaAs junction was achieved with extremely low junction leakage current of 3×10−6A∕cm2 at reverse bias of 1V and high forward current to reverse current ratio of Iforward∕Ireverse∼107 at GaAs source/drain junction. GaAs n-MOSFETs integrated with HfO2 gate dielectric and TaN gate electrode were fabricated by using AlN passivation layer and Si∕P coimplantation techniques. The results show that our GaAs n-MOSFETs operate in enhancement mode with Ion∕Ioff ratio of approximately six orders and good transfer and output characteristics.