AlGaN Interface Research Articles

Interface state density in mist chemical vapor deposited Al2O3/AlGaN/GaN structure

Uniform thickness Al2O3 thin films have been deposited by eco-friendly mist CVD. The obtained Al2O3 film has an optical band gap value of more than 6.5 eV and a refractive index of 1.64 at 633 nm. The combination of capacitance–voltage (C–V) fitting method with non-linear least-squares algorithm, frequency dispersion, photo-assisted, and proposed reverse bias-assisted C–V methods revealed interface state densities ranging from 1 × 1012 to 3 × 1013 cm−2eV−1 along the mist-Al2O3/AlGaN interface. These values are comparable to those reported for atomic layer deposited Al2O3 thin films.

In-situ S/TEM DC biasing of p-GaN/AlGaN/GaN heterostructure for E-mode GaN HEMT devices

This work describes an in-situ electrical DC bias study of the E-mode GaN high electron mobility transistor (HEMT) device. A single transistor structure is biased and studied in real-time. The sample was made from an E-mode GaN HEMT device using Focused Ion Beam (FIB) milling and upright lift-off. The device lamella is subjected to forward gate bias to understand the device operation and physical changes under the bias. Active device area and micron level changes due to biasing were studied and identified as crucial factors affecting device reliability during continuous operation. Electric bias-induced physical changes are observed at the p-GaN layer and AlGaN interface on the p-GaN and GaN sides. Localized damage and defect formation, along with elemental diffusion, is observed. The formation of new defects over existing growth defects was seen in the p-GaN/AlGaN/GaN heterostructure. The study helped us identify the exact location of the failure, the region affected under bias, and the occurrence of physical changes due to the electrical bias on the in-situ device. Based on the study, gate breakdown failure and its location at the metal/p-GaN interface are understood to result from physical changes activated by electrical bias.

Modeling the processes of formation of defects that form deep levels in SiON/AlGaN/GaN

The effect on the electrical parameters of the SiON/AlGaN/GaN structures of treatment of different durations of low-energy nitrogen plasma was studied. The AlGaN surface was subjected to plasma treatment in the working chamber of the plasma-chemical deposition unit before starting the monosilane to form the SiON film. Changes in the transport properties (conductivity and mobility) of the canal and capacitive properties of the structures were evaluated. It has been experimentally shown that such treatment leads to a change in the magnitude of polarization charges both at the insulator-AlGaN interface and at the AlGaN/GaN interface. With the help of C–V measurements-in the hysteresis mode, it is shown that at the control voltage U > +4 — +5 V ), some of the channel electrons are captured at deep centers at the SiON-AlGaN interface, and with an increase in the duration of exposure to plasma time, a sharp increase is observed charge Qit, formed by electronic boundary states. The use of additional treatment with nitrogen plasma transfers work for nitride structures from the D-mode (Vth = –4 V) to the E-mode (Vth = +0.9 V).Using Auger-measurements, it was shown that plasma treatment leads to a change in the amount of oxygen in the SiON layer and in nano-regions of the barrier layer, and with an increase in the duration of plasma exposure, a sharp decrease in the amount of oxygen in these layers is observed. Also, when using plasma treatment, the redistribution of Ga and Al at the AlGaN–GaN interface i.e. in the channel area. Using Auger measurements near the SiON–AlGaN interface from the side of the insulator, the localization of nitrogen atoms chemically bonded with silicon N(Si) with the formation of a peak at the interface, the size of which increases with increasing duration of plasma exposure.

Control of Metal‐Rich Growth for GaN/AlN Superlattice Fabrication on Face‐to‐Face‐Annealed Sputter‐Deposited AlN Templates

GaN/AlN superlattices consisting of few‐monolayer GaN wells have attracted considerable attention for use in deep‐ultraviolet (DUV) light‐emitting devices. To avoid the formation of droplets and AlGaN interface layers, precise growth control is essential for fabricating superlattices with flat and abrupt interfaces. Herein, GaN/AlN superlattice structures are grown on face‐to‐face‐annealed sputter‐deposited AlN (FFA Sp‐AlN) template substrates using radio‐frequency plasma‐excited molecular beam epitaxy (RF‐MBE) utilizing in situ reflection high‐energy electron diffraction (RHEED) monitoring. Both AlN and GaN are grown under metal‐rich conditions, and subsequently, the droplets are eliminated by droplet elimination by radical beam irradiation (DERI) method for AlN and by growth interruption for GaN. Furthermore, the dependence of AlN thickness on the properties of superlattices is investigated. The AlN thickness changes linearly with the supply time of the Al metal; thus, the AlN thickness is easily controllable. A total of 20‐period GaN/AlN superlattices with flat and abrupt interfaces is fabricated, as confirmed using atomic force microscopy and X‐ray diffraction. Cathodoluminescence with a peak wavelength of 230–260 nm at room temperature is obtained from the fabricated superlattices. Moreover, the emission wavelength shifts with an increase in AlN thickness.

Current transport dynamics and stability characteristics of the NiO x based gate structure for normally-off GaN HEMTs

The application of p-type oxide typified with NiO x as cap layer in AlGaN/GaN high electron mobility transistor for normally-off operation has specific benefits, including etching free fabrication process, high hole density and elimination of Mg dopants diffusion effects. This work presents a device configuration exploration combining the p-NiO x gate cap layer and a thin AlGaN barrier layer. Calculation method for the threshold voltage has been discussed, which obtains good consistence with the experimental measurements. In addition, a nitrogen based post-annealing process was developed to improve the film stoichiometry for elevated gate controllability, realizing normally-off operation with enhanced channel conduction capability. The current transport dynamics in the gate stack as coupled with the NiO x /AlGaN interface states have also been studied, where a deep level trap was recognized in dominating the gate current characteristics and the gate stability performance under different forward gate bias conditions.

Status of Aluminum Oxide Gate Dielectric Technology for Insulated-Gate GaN-Based Devices.

Insulated-gate GaN-based transistors can fulfill the emerging demands for the future generation of highly efficient electronics for high-frequency, high-power and high-temperature applications. However, in contrast to Si-based devices, the introduction of an insulator on (Al)GaN is complicated by the absence of a high-quality native oxide for GaN. Trap states located at the insulator/(Al)GaN interface and within the dielectric can strongly affect the device performance. In particular, although AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs) provide superior properties in terms of gate leakage currents compared to Schottky-gate HEMTs, the presence of an additional dielectric can induce threshold voltage instabilities. Similarly, the presence of trap states can be detrimental for the operational stability and reliability of other architectures of GaN devices employing a dielectric layer, such as hybrid MIS-FETs, trench MIS-FETs and vertical FinFETs. In this regard, the minimization of trap states is of critical importance to the advent of different insulated-gate GaN-based devices. Among the various dielectrics, aluminum oxide (Al2O3) is very attractive as a gate dielectric due to its large bandgap and band offsets to (Al)GaN, relatively high dielectric constant, high breakdown electric field as well as thermal and chemical stability against (Al)GaN. Additionally, although significant amounts of trap states are still present in the bulk Al2O3 and at the Al2O3/(Al)GaN interface, the current technological progress in the atomic layer deposition (ALD) process has already enabled the deposition of promising high-quality, uniform and conformal Al2O3 films to gate structures in GaN transistors. In this context, this paper first reviews the current status of gate dielectric technology using Al2O3 for GaN-based devices, focusing on the recent progress in engineering high-quality ALD-Al2O3/(Al)GaN interfaces and on the performance of Al2O3-gated GaN-based MIS-HEMTs for power switching applications. Afterwards, novel emerging concepts using the Al2O3-based gate dielectric technology are introduced. Finally, the recent status of nitride-based materials emerging as other gate dielectrics is briefly reviewed.

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.

GaN-based MIS-HEMTs with Al2O3 dielectric deposited by low-cost and environmental-friendly mist-CVD technique

We report on the fabrication and characterization of AlGaN/GaN metal-insulator-semiconductor (MIS) capacitors and high-electron-mobility transistors (MIS-HEMTs) using a 5 nm thick Al2O3 dielectric deposited by cost-effective and environmental-friendly mist chemical vapor deposition (mist-CVD) technique. Practically hysteresis-free capacitance–voltage profiles were obtained from the fabricated two-terminal MIS-capacitors indicating high quality of the mist-Al2O3/AlGaN interface. Compared with reference Schottky-gate HEMTs, mist MIS-HEMTs exhibited much improved performance including higher drain current on-to-off ratio, much lower gate leakage current in both forward and reverse directions and lower subthreshold swing. These results demonstrate the potential and viability of non-vacuum mist-CVD Al2O3 in the development of high-performance GaN-based MIS-HEMTs.

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.

Novel Vertical GAA-AlGaN/GaN Dopingless MIS-HEMT: Proposal and Investigation

This paper presents a gate all around (GAA) AlGaN/GaN HEMT (GAA-MIS-HEMT) with AlN as an interfacial passivation layer. Gate all around technique is used to improve the performance of device such as carrier concentration, electric field and current density at the interface of AlGaN & GaN. The enhanced control over the 2DEG due to gate all around structure helped in attaining superior performance. Al2O3 is used as a dielectric. The results of GAA-MIS-HEMT are compared with planar-MIS-HEMT, which shows that the GAA-MIS-HEMT provides better ON-state current, OFF-state current, transconductance, cutoff frequency (11 GHz) and ON-state to OFF-state current ratio (1011), ON-resistance (0.9Ω-cm2) and subthreshold slope (63 mV/dec). All the layers of proposed structure are dopingless.

Growth characteristics of Fe-doped GaN epilayers on SiC (001) substrates and their effects on high breakdown voltage devices

The growth characteristics of Fe-doped GaN epitaxial layers on semi-insulating SiC (001) substrates were studied using metalorganic chemical vapor deposition for high breakdown voltage device applications. A smooth Fe-doped GaN epilayer surface can be realized by changing the ferrocene flow, while higher Fe concentrations in the GaN epilayer affect the surface morphology. To reduce the Fe trapping carrier and the sheet resistances of the two-dimension electron gas generated from the interface of AlGaN and GaN, the thickness ratio of Fe-doped and undoped GaN bi-epilayers was also optimized. AlGaN/GaN high electron mobility transistors with the optimum doping concentration of Fe-doped GaN and suitable thickness of undoped GaN have been successfully developed. The achieved breakdown voltage of the Fe-doped GaN epitaxial layer can be as high as 2457 V, which is attributed to the Fe-doped GaN epitaxial layer with higher resistance, which can sustain the high breakdown voltage. The details of the correlation between the surface morphology, Fe concentration, and thickness of Fe-doped GaN epitaxial layers used for high breakdown voltage devices will be also discussed in this paper.

High-performance dual-gate-charge-plasma-AlGaN/GaN MIS-HEMT

This paper presents a novel structure of dual-gate-charge-plasma (DG-CP) AlGaN/GaN metal–insulator–semiconductor high electron mobility transistor (MIS-DG-CP-HEMT) with gate oxides employing Al2O3. AlN as an interfacial layer between dielectric and semiconductor. Dual-gate technique for both CP and doped HEMT is used to improve the transport characteristics in the channel. CP-HEMT is used to enhance the conductance charge density and electron concentration at the interface of AlGaN and GaN. The DG-CP-HEMT is compared with DG-doped HEMT. The dual-gate structure helps in incorporating the two different layers of 2-dimensional electron gas (2DEG) for higher current density. Results showed a better performance of DG-CP-HEMT in term of various analog and RF parameters. HEMT gives better transconductance (29 mS), Cutoff frequency (11 GHz) and ON-state-to-OFF-state current ratio (1011), ON-resistance (6 Ω-cm2) and subthreshold slope (63 mV/dec). The proposed HEMT architecture can be used for RF applications due to its exemplary characteristics.

Mechanisms of a Rectifying TiN Gate Contact for AlGaN$/$GaN HEMTs on Silicon Substrate

Rectifying Titanium Nitride (TiN) gate contact technology is developed for AlGaN $/$ GaN based micro and nanometer HEMTs. A high compressive strain occurring in thinner TiN films (ranging from 5 nm to 60 nm), deposited by sputtering, leads to a reduction in tensile strain at the surface of AlGaN barrier. The diminution in tensile strain forms a pseudo- $p$ -type layer (diode-like). This strain reduction has no effect on the bandgap of the AlGaN barrier layer, allowing the gate to withstand a reverse gate bias larger than 100 V. Characterization using the high-resolution transmission electron microscopy combined with the X-ray photoelectron spectroscopy reveals a good TiN $/$ AlGaN interface quality and no diffusion of TiN into AlGaN. The effective energy barrier of the rectifying nanoscale TiN gate contact has a relatively large height of 1.1 eV associated with an ideality factor of 1.4. A dramatic drop of the reverse-bias leakage current down to 11 pA $/$ mm is measured at −30 V. In addition, electrical measurements show very low gate and drain lag effects of 4.2 ${\%}$ and 6.7 ${\%}$ , respectively.

Investigation of the Trap States and $V_{\text{TH}}$ Instability in LPCVD Si3N4/AlGaN/GaN MIS-HEMTs with an In-Situ Si3N4 Interfacial Layer

A novel gate and passivation dielectric stack consisting of a thin metal-organic chemical vapor deposition (MOCVD) grown in-situ Si3N4 (3 nm) and a thick low-pressure chemical vapor deposition (LPCVD) grown Si3N4 (30 nm) in AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistor (MIS-HEMT) is proposed. The quality of the Si3N4/(Al)GaN interface and the effect on threshold voltage ( ${V}_{\text {TH}}$ ) instability and dynamic ${R}_{ \mathrm{\scriptscriptstyle ON}}$ in the MIS-HEMTs with/without the in-situ Si3N4 layer are investigated by high-frequency capacitance-voltage (HFCV), quasi-static (QS) ${C}$ – ${V}$ (QSCV), time-of-fly (TOF) stress/measure, and QS ${I}_{\text {D}}$ – ${V}_{\text {DS}}$ methods. It is founded that the in-situ Si3N4 interfacial layer is effective in improving the dielectric/III-N interface morphology. As a result, better ${V}_{\text {TH}}$ stability and lower ${R} _{ \mathrm{\scriptscriptstyle ON},\text {D}}/{R} _{ \mathrm{\scriptscriptstyle ON},\text {S}}$ ratio are observed in devices with the in-situ Si3N4 interfacial layer due to the reduced density of traps close to the dielectric/III-N interface. Time-dependent dielectric breakdown and Weibull performance further verified that the proposed bilayer gate dielectric stack is a promising structure for the high-reliability power transistors.

Theoretical investigation of the charges weight between interface and the oxygen traps in barrier layer on the 2DEG density of $$\hbox {Al}_{2}\hbox {O}_{3}$$ Al 2 O 3 /AlGaN/GaN HEMTs

Unintentional impurities like oxygen introduced into the undoped AlGaN barrier may present an important traps source for these devices besides $$\hbox {Al}_{2}\hbox {O}_{3}$$ /AlGaN interface traps. Here, we present a theoretical study of the charges weight between oxygen traps in barrier layer and $$\hbox {Al}_{2}\hbox {O}_{3}$$ /AlGaN interface traps on the 2DEG density for different gate bias. In order to conduct this study, a semi-analytical 1D charge model has been developed and used based on the nonlinear Poisson and Schrodinger coupled equations. The model is based on a new analytical expression of the electric field distribution under the gate including the ionized oxygen profile into the AlGaN barrier. A secondary ion mass spectrometry and deep-level transient spectroscopy measurements on a fabricated device were performed. A mean density of oxygen up to $$2\times 10^{18}\,\hbox {cm}^{-3}$$ was measured into the AlGaN barrier, associated with an activation energy of 0.44 eV and a cross section of 1.4 $$\times 10^{-14 }\,\hbox {cm}^{2}$$ . Using these values as the model inputs, the theoretical results showed that even with changing the balance between the interface states density and the barrier traps density, there is no an absolute traps domination either from the barrier or the interface traps and their impact weight on the 2DEG density changes with the gate bias.

Fast and slow traps in Al2O3/(GaN)/AlGaN/GaN heterostructures studied by conductance technique

Influence of interface traps at Al2O3/(GaN)/AlGaN interface at low and high frequency on equivalent parallel conductance of Al2O3/(GaN)/AlGaN/GaN heterostructure capacitor was studied. By the conductance measurements two types of traps were identified in the measured structure. The traps differ in time constants by more than one order in magnitude. Fast traps with low time constants have narrow energy distribution and are probably located in the semiconductor energy gap close to the insulator semiconductor interface. Slow traps have wider distribution and are assumed to be directly at the insulator semiconductor interface. The presence of the fast traps which are able to respond to external ac signal is given into connection with the recently published increase of the structure capacitance in the plateau region with decreasing frequency.

Nonlinear analysis of vanadium- and titanium-based contacts to Al-rich n-AlGaN

In this work, we report on voltage dependent contact characteristics of Al-rich n-AlGaN due to the presence of a Schottky barrier at the metal–AlGaN interface. Current–voltage characteristics appear to be linear at low voltages, but a clear nonlinearity is evident at high voltages. Unlike Ga-rich n-AlGaN where V-based contact marginally outperforms the Ti-based contact, V-based contact to Al-rich n-AlGaN outperforms the Ti-based contact by ∼2 orders of magnitude for all voltage ranges. The improved performance of V-based contacts is likely due to a lower Schottky barrier and partly due to ease of formation of a low barrier V–AlGaN interface.

Interface traps contribution to capacitance of Al2O3/(GaN)AlGaN/GaN heterostructures at low frequencies

Influence of interface traps at Al2O3/(GaN)/AlGaN interface on low and high frequency capacitance of Al2O3/(GaN)/AlGaN/GaN heterostructure capacitor was studied. New features were observed in the capacitance curves. Obtained experimental results were modeled and simulated and accordance with the experiment has been obtained. For lower frequencies a new capacitance peak in the depletion and increase of the capacitance in a plateau region were measured. The capacitance peak in the depletion region was successfully explained by a capacitance response of the interface traps with U-shape density distribution. On the other hand the increase of the capacitance plateau was modeled by the homogeneous interface trap distribution. We assume that the traps located near the band edges having the highest density are able to respond to the low frequency measuring.

AlGaN/GaN MOS-HEMT Device Fabricated Using a High Quality PECVD Passivation Process

In this letter, an AlGaN/GaN MOS-HEMT was demonstrated using a 5-nm-thick SiOx dielectric layer deposited by plasma enhanced chemical vapor deposition (PECVD) as a gate insulator. The fabricated device exhibits a maximum ${I}_{{\text {DS}}}$ current of 570 mA/mm at ${V}_{{\text {GS}}} =+ {3}$ V, an ON-state resistance ( ${R}_{\mathrm {biosc{on}}}$ ) of $7.3~\Omega \cdot $ mm, a maximum transconductance peak of 180 mS/mm, and a gate leakage current below 1 nA/mm at a gate voltage of +/−3 V. Moreover, a very low pinchoff voltage ( ${V}_{p})$ shift was observed during the ${I}_{{\text {DS}}}$ – ${V}_{\text {GS}}$ hysteresis measurements giving an estimated interface state density ( ${D}_{\text {it}})$ as low as $3.9\times 10^{ {11}}$ cm $^{-2}$ eV $^{-1}$ . These results demonstrate the high quality of the SiO x /AlGaN interface and the efficiency of the passivation process. To the best of our knowledge, this is the lowest reported ${D}_{\text {it}}$ on an AlGaN/GaN MOS-HEMT device using a PECVD deposition technique.

GaN Based Carbon Dioxide Sensor

AlGaN/GaN high electron mobility transistor based carbon dioxide sensor with ZnO nanorods as a sensing material was fabricated. The response of the device to carbon dioxide gas was characterized at elevated measurement temperature. The transistor showed an increase in drain current upon 500 ppm CO2 due to the enhanced electron channel at the interface of AlGaN and GaN from 150°C. Reliable repeatability to cyclic exposures of various concentrations of carbon dioxide gas was observed. Also, effect of humidity on sensing behavior was investigated.