AlGaN Surface Research Articles

(Invited) Material Considerations for the Development of III-Nitride Power Devices

AlGaN is of high interest for high-power electronics based on vertical structures due to its high breakdown field and electron mobility. Recent studies in GaN based p-n junctions indicate a dielectric strength in the range of 3.3–3.75 MVcm-1, resulting in a Baliga’s figure of merit several times larger than that of SiC. However, high breakdown voltages (>4kV) are currently confined to GaN p-n junctions indicating GaN and AlGaN surface instabilities. Consequently, defect-free metal-nitride interface and passivation of Al/GaN are necessary for breakthroughs in high power vertical Al/GaN Schottky diodes. Accordingly, we report defect-free behavior in Ni/GaN Schottky diodes with unity ideality factor and fabricated by photolithography. They also exhibit high temperature stability (>600 oC) and low leakage with forward and reverse I-V-T characteristics successfully modeled by a single homogeneous barrier (0.7 eV) by ATLAS simulations. It is significant that the forward and reverse characteristics could be modeled by a common set of parameters without a defect based second diode to model the reverse leakage. XPS chemical and electronic analysis was employed to determine the surface treatment necessary to obtain such a Schottky interface. Consequently, we demonstrate the suitability of GaN surface and interface with metals for high power electronics. Although a near-ideal diode may be obtained on GaN, the relatively low barrier height limits the GaN Schottky diode to operating voltages <1000V. Increasing the barrier by controlling the Fermi level pinning and producing a camel junction by surface Mg doping is possible. Passivation is the next challenge to implementing Al/GaN based high power electronics. Silicon nitride has emerged as the standard passivation material for GaN and low Al composition AlxGa1-xN (x<0.3). However, the bandgap of silicon nitride is lower than AlGaN for x>0.75 and the feasibility of silicon nitride functioning as an insulating dielectric (by providing appropriate electron/hole barriers) in Al rich AlGaN and AlN is uncertain. Similarly, reduction of surface states by silicon nitride passivation has not been reported for Al rich AlGaN (x>0.3). In this work, we have employed X-ray photoelectron spectroscopy to determine the band offsets and interface Fermi level at the hetero-junction formed by stoichiometric silicon nitride deposited on metal polar AlxGa1-xN (of varying Al composition ‘x’) via high temperature low pressure chemical vapor deposition. Silicon nitride is found to form a type II staggered band alignment with AlGaN for all Al compositions (0<x<1) and presents an electron barrier into AlGaN even at higher Al compositions where Eg(AlGaN)>Eg(Si3N4). No band bending is observed in AlGaN for x<0.6 and indicating excellent passivation beyond x=0.3. Further a reduced band bending (by 1 eV relative to free surface) is observed for x>0.6. The Fermi level in silicon nitride is found to be at 3 eV with respect to its valence band and is likely due to silicon (≡Si0/-1) dangling bonds. The presence of band bending for x>0.6 is seen as a likely consequence of Fermi level alignment at Si3N4/AlGaN hetero-structure and not due to interface states indicating likely passivation on AlGaN beyond x=0.6. Photoelectron spectroscopy is corroborated by capacitance-voltage measurements. A shift in the interface Fermi level from the conduction band in Si3N4/n-GaN to the valence band in Si3N4/p-GaN is observed which strongly indicates no or greatly reduced mid-gap interface states. Further, required compensation of spontaneous polarization charge is typically performed by oppositely charged surface states/defects on free surface is hypothesized to be replaced by oppositely charged silicon dangling bonds (≡Si0/-1) after passivation. This hypothesis is supported by characterization of silicon nitride on N-polar GaN. In conclusion, HT LPCVD silicon nitride is found to be a suitable passivation for Al rich AlGaN.

Design and control of interface reaction between Al-based dielectrics and AlGaN layer in AlGaN/GaN metal-oxide-semiconductor structures

Important clues for achieving well-behaved AlGaN/GaN metal-oxide-semiconductor (MOS) devices with Al-based gate dielectrics were systematically investigated on the basis of electrical and physical characterizations. We found that low-temperature deposition of alumina insulators on AlGaN surfaces is crucial to improve the interface quality, thermal stability, and variability of MOS devices by suppressing Ga diffusion into the gate oxides. Moreover, aluminum oxynitride grown in a reactive nitric atmosphere was proven to expand the optimal process window that would improve the interface quality and to enhance immunity against charge injection into the gate dielectrics. The results constitute common guidelines for achieving high-performance and reliable AlGaN/GaN MOS devices.

Achieving Low-Recovery Time in AlGaN/GaN HEMTs With AlN Interlayer Under Low- Noise Amplifiers Operation

Three transistors with different AlGaN/GaN interface designs (sharp interface, standard interface, and an extra AlN interlayer) were studied in-depth under conditions mimicking low-noise amplifiers (LNAs) operation. A new measurement setup, analog to LNAs operation condition, is established to measure recovery time on device level. For the first time, a direct relationship between the recovery time and the design of AlGaN/GaN interface is revealed in devices with Carbon doping buffer in this letter. An extremely low-recovery time is demonstrated in the transistor with an AlN interlayer. Both transistors without an AlN interlayer exhibit severe gain and drain current degradation after pulsed input stress. The transistor with a sharp interface shows a recovery time around 10 ms, whereas the transistorwith a standard interface shows even much longer recovery time. These results imply that AlN interlayer, which can effectively block the injection of hot electrons to AlGaN bulk or surface traps, is highly preferred in systems where LNAs need to function promptly after an input overdrive.

Effective Passivation With High-Density Positive Fixed Charges for GaN MIS-HEMTs

An effective passivation with high-density positive fixed charges was demonstrated on GaN MIS-HEMTs. The positive fixed charges at the interface between passivation and AlGaN surface can reduce the surface potential and expand the quantum well under Fermi level. Besides, to satisfy charge balance, the net charge density at the AlGaN surface must equal to the 2DEG carrier density. Thus, the positive fixed charges passivation can increase the 2DEG carrier density and improve the switching performance of GaN MIS-HEMTs. In this paper, we demonstrated a high-density positive fixed charges $(\sim 2.71 \times 10^{13} e/cm^{-2})$ passivation using SiON for GaN MIS-HEMTs. The device with SiON passivation exhibits significant improvements in $I$ – $V$ characteristics and dynamic RON compared to the conventional SiN passivated device.

Effect of thermal oxidation treatment on pH sensitivity of AlGaN/GaN heterostructure ion-sensitive field-effect transistors

In this article, AlGaN/GaN heterostructure ion-sensitive field-effect transistors (ISFETs) were prepared and evaluated by thermal oxidation treatment on the AlGaN surface. The ISFETs were fabricated on the AlGaN/GaN heterostructure and then thermally oxidized with dry oxygen in 600, 700, and 800°C, respectively. It indicates that the performance of the AlGaN/GaN heterostructure ISFETs, such as noise and sensitivity, has been improved owing to the thermal oxidation treatment process at different temperatures. The X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) results indicate that after thermal oxidation treatment at different temperatures, hydroxide who possesses high surface state density will transfer to oxide owing to the higher chemical stability of the latter. Moreover, a crystalline α-Al2O3 phase generated at 700°C can not only provide a relatively smooth surface, but also improve the sensitivity to 57.7mV/pH for the AlGaN/GaN heterostructure ISFETs, which is very close to the Nernstian limit.

Electronic structure and chemical state analysis of nanoflowers decorated GaN and AlGaN/GaN heterostructure

Abstract The present article reports electronic structure, chemical and defect states analysis of Quasi-continuous GaN film, nanoflowers decorated nanostructured GaN and nanoflowers decorated AlGaN/GaN heterostructure. The nanostructured GaN and AlGaN surfaces were decorated with nanoflowers having a size variation between 200 and 400 nm. Extensive photoemission analysis was performed to analyse surface chemistry and electronic structure and their correlation with surface morphology. Indication of free electron accumulation was perceived by the observed downwards band bending at the interface of AlGaN/GaN heterostructure. The optical response inveterate defects minimization in nanoflower decorated GaN and AlGaN/GaN heterostructure and the presence of minimum residual stress.

Direct observation of trapped charges under field-plate in p-GaN gate AlGaN/GaN high electron mobility transistors by electric field-induced optical second-harmonic generation

Trapped charges underneath the field-plate (FP) in a p-gallium nitride (GaN) gate AlGaN/ GaN high electron mobility transistor device were visualized by using electric field-induced optical second-harmonic generation imaging. Second-harmonic (SH) signals in the off-state of the device with FP indicated that the electric field decreased at the p-GaN gate edge and concentrated at the FP edge. Nevertheless, SH signals originating from trapped charges were slightly observed at the p-GaN gate edge and were not observed at the FP edge in the on-state. Compared with the device without FP, reduction of trapped charges at the p-GaN gate edge of the device with FP is attributed to attenuation of the electric field with the aid of the FP. Negligible trapped charges at the FP edge is owing to lower trap density of the SiO2/AlGaN interface at the FP edge compared with that of the SiO2/p-GaN sidewall interface at the p-GaN gate edge and attenuated electric field by the thickness of the SiO2 passivation layer on the AlGaN surface.

The effects of tetramethylammonium hydroxide treatment on the performance of recessed-gate AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated using a gate recess process and a surface treatment with tetramethylammonium hydroxide (TMAH) prior to gate metal deposition. Electrical characterizations show improved extrinsic transconductance and saturation current, as well as more uniform off-state behavior with reduced off-current by a factor of 3.5 and gate leakage current by a factor of 4.2 in the devices with TMAH treatment. The analyses based on atomic force microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy show that the TMAH treatment effectively reduces the roughness of the recess-etched AlGaN surface and removes the native oxide layer on the AlGaN surface, suggesting a simple and viable route towards the fabrication of gate-recessed HEMTs based on AlGaN/GaN heterostructure with improved controllability and uniformity.

Influence of AlN buffer layer on growth of AlGaN by HVPE

To the end of improvement of layer morphology and crystalline perfection of thick AlGaN layers grown by hydride vapor phase epitaxy (HVPE) as well as for the suppression of crystallite formation during growth, the impact of AlN buffer layer properties on AlGaN growth was investigated. While the surface morphology of 500 nm thick AlN layer improves with higher V/III ratio toward 50 and lower growth temperatures of 1020 °C, its use as a buffer layer for thick AlGaN layers leads to strong degradation of AlGaN surface and even to formation of different crystallites during growth. Best surface morphology and crystalline perfection of thick AlGaN layers was observed for the growth on an AlN buffer layer with a higher crystal quality disregarding its 3D morphology and high surface roughness. Best results were achieved at medium V/III ratio of about 10–20 and AlN buffer growth temperature of 1060 °C.

Hydrazine (N2H4)-Based Surface Treatment for Interface Quality Improvement in Al2O3/AlGaN/GaN MIS-HEMT

This paper highlights N2H4 treatment on AlGaN surface prior to Al2O3 deposition for insulator/III-nitride interface quality improvement. AlGaN surface with and without N2H4 treatment was systematically analyzed through X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), which show an effective removal of the native oxide and improved roughness on AlGaN surface. In order to investigate the effects of N2H4 treatment on the electrical properties of Al2O3/AlGaN/GaN MIS-HEMT, capacitance-voltage (C(V)) characteristics and transfer characteristics after gate bias stress were measured. Compared to the devices without N2H4 treatment featuring a large hysteresis of 0.3 V and a high subthreshold slope (SS) = 119 mV/dec, N2H4-treated devices exhibit an almost negligible hysteresis of 50 mV and an improved SS of 105 mV/dec.

Plasma enhanced atomic layer deposition of Al2O3 gate dielectric thin films on AlGaN/GaN substrates: The role of surface predeposition treatments

Al2O3 thin films were deposited by plasma enhanced atomic layer deposition (PEALD) from trimethylaluminum precursor and oxygen plasma at 250 °C on AlGaN/GaN heterostructures. Before deposition, the sample surfaces were treated with the following solutions: (A) H2O2:H2SO4 (piranha), (B) fluoride acid (HF) + HCl, and (C) piranha + HF for 10 min each. Transmission electron microscopy analysis revealed that, independently from the surface preparation, all the films are adherent and uniform with thicknesses of about 27–28 nm. However, a different structural evolution has been observed under electron beam effect. In particular, while all the as-deposited films were found to be amorphous, the formation of polycrystalline grains was observed on the sample deposited after the A treatment. On the other hand, oriented layers were formed on the samples deposited after B and C treatments. This result is an indication that in the case of HF-based treatments, the PEALD occurred on a very clean AlGaN surface, which can act as seed layer for the formation of epitaxial films when some energy is provided. Atomic force microscopy (AFM) showed smoother Al2O3 films when grown after HF based treatments. Moreover, further AFM investigation of the initial growth stages (3 nm thick Al2O3 layers) demonstrated that the smallest three-dimensional grain nucleation resulted in deposition on C-treated surfaces. Finally, the predeposition treatments of AlGaN/GaN surfaces with the HF cleaning provided Al2O3 films having the most promising dielectric properties.

Glass-Based Transparent Conductive Electrode: Its Application to Visible-to-Ultraviolet Light-Emitting Diodes.

Nitride-based ultraviolet light-emitting diodes (UV LEDs) are promising replacements for conventional UV lamps. However, the external quantum efficiency of UV LEDs is much lower than for visible LEDs due to light absorption in the p-GaN contact and electrode layers, along with p-AlGaN growth and doping issues. To minimize such absorption, we should obtain direct ohmic contact to p-AlGaN using UV-transparent ohmic electrodes and not use p-GaN as a contact layer. Here, we propose a glass-based transparent conductive electrode (TCE) produced using electrical breakdown (EBD) of an AlN thin film, and we apply the thin film to four (Al)GaN-based visible and UV LEDs with thin buffer layers for current spreading and damage protection. Compared to LEDs with optimal ITO contacts, our LEDs with AlN TCEs exhibit a lower forward voltage, higher light output power, and brighter light emission for all samples. The ohmic transport mechanism for current injection and spreading from the metal electrode to p-(Al)GaN layer via AlN TCE is also investigated by analyzing the p-(Al)GaN surface before and after EBD.

Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area

In this paper, the ohmic contact of AlGaNGaN HEMT was optimized by introducing patterned etching in ohmic area, and the conventional structure and whole etching structure were investigated for comparison. The contact resistance decreased from 0.46Ωmm for conventional to 0.35Ωmm and 0.18Ωmm respectively for the whole etching and patterned etching structures. The current-voltage characteristics between the ohmic electrodes presented sharper slope, higher saturation current and lower knee voltage on patterned etching structures. After Cl2 plasma etching on ohmic area surface of AlGaN, the surface oxide layers and the pollutants were removed, therefore, the surface roughness of the ohmic metal reduced obviously, and the surface morphology improved. Meanwhile, the side area induced in patterned etching provided more extra contact area, which increased the tunneling current. The different apertures and the duty factor of patterned etching were investigated, and the results indicated that the quantity of side area produced in patterned etching dominated the reduction effect of ohmic contact resistance.

AlGaN surfaces etched by CF4 plasma with and without the assistance of near-ultraviolet irradiation

Al0.24Ga0.76N thin film surfaces were etched with CF4 plasma, with and without the assistance of near-ultraviolet (UV) irradiation. The near-UV source was a black light lamp, which emitted wavelengths of 300–400 nm. The chemical compositions and morphologies of surfaces etched under the two plasma conditions were compared, to clarify the effect of near-UV irradiation on the properties of the AlGaN surface. Near-UV irradiation during CF4 plasma etching reduced the amounts of F atoms, AlFx fluorides, and GaFx fluorides incorporated into the surface, and enhanced the amount of CFx fluorocarbons. Near-UV irradiation did not influence the depth distribution of incorporated fluorine-related impurities, or the degree of nitrogen deficiency caused in the surface. These compositional changes in the surface occurred irrespective of the gas pressure. Near-UV irradiation caused a significant change in the morphology of surfaces etched at high gas pressure (13 Pa), as the etching time was increased to 60 and 100 min. Near-UV irradiation did not change the morphologies of surfaces etched at lower gas pressures (1.3 and 6.7 Pa), irrespective of etching time.

Three-dimensional current collapse imaging of AlGaN/GaN high electron mobility transistors by electric field-induced optical second-harmonic generation

Three-dimensional (3D) current collapse imaging of Schottky gate AlGaN/GaN high electron mobility transistor devices was achieved by a combination of two-dimensional (2D) and depth directional electric field-induced optical second-harmonic generation (EFISHG) measurements. EFISHG can detect the electric field produced by trapped carriers, which causes the current collapse. In the 2D measurement, the strong second-harmonic (SH) signals appeared within 1 μm from the gate edge on the drain side at 0.8 μs after the transition from the off- to no bias- state in both unpassivated and passivated samples. In the depth measurement, the SH signals were generated mainly from the AlGaN surface region of the unpassivated sample due to the presence of high-density trap sites in the AlGaN layer, and SH signals from bulk GaN region were also detected at 50 μs after the transition from the off- to no bias- state in the passivated sample. The origin of the traps is presumably the nitrogen vacancies in the GaN buffer layer.

Effect of oxygen plasma treatment on the performance of AlGaN/GaN ion-sensitive field-effect transistors

AlGaN/GaN ion-sensitive field-effect transistors (ISFETs) were fabricated and the sensitivity was evaluated with O2 plasma treatment for different time. For a short-time O2 plasma treatment, the sensitivity of the AlGaN/GaN ISFET improved to 55.7mV/pH, which is very close to the theoretical value of 58.7mV/pH at 23°C. However, with a long-time O2 plasma treatment, the sensitivity decreased. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and water contact angle measurement were utilized to characterize the surface conditions. It indicated that, after short-time O2 plasma treatment, the AlGaN/GaN surface with a rich aluminum oxide is the main reason to improve the sensitivity of AlGaN/GaN ISFET. The surface of the AlGaN barrier layer became much smooth and clean with the water contact angle on the AlGaN surface decreasing to approximately 5–7°. With the O2 plasma treatment time increasing, the aluminum oxide dominated AlGaN/GaN surface changes to a gallium oxide dominated surface, leading to the decreasing on the ISFET sensitivity. These results suggest that a short-time O2 plasma treatment is an appreciate technique to improve the device performance of the AlGaN/GaN ISFETs.

Advances in AlGaN/GaN HEMT Surface Passivation

The high efficiency and power density of AlGaN/GaN high electron mobility transistors (HEMTs) can be limited by parasitic charge trapping at sites on the semiconductor surface. Trapped electrons at the surface deplete the two-dimensional electron gas (2DEG), resulting in effects known as current collapse, increased dynamic on-resistance, DC-RF dispersion, among others. A SiN layer is conventionally implemented as surface passivation for GaN HEMTs, however the properties of this SiN layer and the impact of this layer on current collapse depend drastically on the deposition conditions of this film. Although passivation of GaN HEMTs has been studied extensively, the specific details behind the SiN passivation deposition are not frequently provided in literature. In this work, we investigate several passivation techniques and study their performance under dynamic switching conditions, under off-state stress, and implement hyperspectral electroluminescence imaging to identify electric field nonunifornities. We investigate plasma enhanced chemical vapor deposition (PECVD) SiN, a common GaN HEMT passivation method, and demonstrate how to optimize the deposition conditions to effectively passivate the HEMT. Generally, a mixed frequency PECVD SiN process is used, where a combination of low frequency (LF) e.g. 100-360 kHz and high frequency (HF) 13.56 MHz plasmas are used to create the SiN film. However, the LF step could create undesirable damage to the surface of the AlGaN barrier from N+ ion bombardment, degrading device performance. We developed a bilayer PECVD process, where a 10 nm thick layer is deposited using HF plasma only in order to protect the AlGaN surface, followed by a HF/LF mixed frequency SiN deposition. Annealing the 10 nm thick HF layer densified the film and resulted in a HEMT with significantly improved current collapse. On the other hand, SiN layers grown in situ during MOCVD growth of the GaN/AlGaN layers have also been demonstrated to serve as an effective protective layer for subsequent HF/LF PECVD SiN deposition, yielding HEMTs with low current collapse. Atomic layer epitaxy (ALE) deposition of a thin (10 nm) AlN layer is evaluated, and is shown to serve well as a passivation layer. This is particularly advantageous, since the thin layer is beneficial for reducing gate capacitance for high frequency operation. Finally, a nanocrystalline diamond (NCD) layer is incorporated on the surface of the GaN HEMT, primarily to improve thermal management, but this also is effective as a surface passivation layer, mitigating surface charge trapping. Effective passivation of the AlGaN/GaN HEMT surface is essential for mitigating current collapse, and we demonstrate recent advancements in passivation technology.

(Invited) Chemical Sensors Based on AlGaN/GaN Transistors

Reliable, economically accessible technology for in situ monitoring of contaminants in water has the power to transform health, industry, and society. Applications of such monitoring range from process control monitoring and optimisation for industry, to water supply quality and wastewater monitoring, to environmental monitoring for resource extraction, and beyond. Semiconductor-based technology offers high performance for in situ, real-time contaminant monitoring and can also be mass produced at low-cost with flexible functionalisation allowing for a variety of analytes. Furthermore, it offers the ability to integrate multiple sensors into one chip, along with wireless communication technology for maximum benefit of the in situ monitoring capability. AlGaN/GaN transistors have the unique combined benefits of high sensitivity to surface charge, fast response, and excellent chemical and thermal stability. AlGaN and GaN are members of the III-nitride alloy family (AlN, GaN and InN), which has become extremely technologically important for a variety of applications such as wireless base stations, solid-state lighting, automotive electronics, power conditioning and DVD information storage. The properties of AlGaN/GaN transistors are also very attractive for sensors; however, this application niche has not yet been widely explored. The strained AlGaN layer induces a region of high sheet carrier concentration, electron mobility and high saturation velocity induced by piezoelectric polarization known as a two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. The 2DEG responds to changes in surface charge, so ion selective field effect transistors (ISFETS) can be developed into robust and sensitive ion sensors. Importantly, in contrast to many other potentiometric sensors, including Si-based ISFETS, high sensitivity can be obtained without the need for a reference electrode. This is a significant advantage over electrochemical cell techniques, for which a reference electrode is a major impediment to in situ and robust operation. This paper will report on development of various AlGaN/GaN ISFETs for chemical sensing – in particular pH sensors, nitrate ion, mercury (II) ion and calcium ion sensors. The ability to fabricate different sensors from the same underlying technology paves the way for array-based multi-analyte sensing solutions. The ion sensors were all obtained by functionalising the transistors with a plasticised PVC membrane containing a dissolved ionophore selective to the ion of interest. Using transistor theory, we show that the apparent gate response for these sensors is near-Nenrstian under a variety of testing conditions. In each case, devices without a PVC membrane coating were tested under identical conditions and exhibited negligible response to ion exposure. Some other key results are summarised below. pH sensors AlGaN/GaN transistor-based pH sensors have been investigated by several research groups. When operating without a reference electrode, researchers have reported two types of pH sensor response: linear (related to pH selectivity) and U-shape (related to anion selectivity). Previously, this distinct difference in behaviour had not been well explained. We have compared the pH response of a reference electrode-free AlGaN/GaN device with and without a GaN capping layer. From our investigations, in the absence of a reference electrode, a linear response towards pH requires a GaN capping layer. Apparently the difference in surface chemistry for GaN versus AlGaN surface affects pH/anion selectivity when there is no reference electrode controlling the surface potential. Nitrate ion sensors In a 0.1 M KH2PO4 ion buffer, a detection limit of less than 10− 6M and a linear response range of 10-6-10− 3M were achieved. The detection limit remains consistently low over multiple runs/days. In addition, there was minimal change in sensor response upon addition of KOH increasing the pH from approximately 4–11 [1]. Mercury (II) ion sensors At pH 2.8 in a 10-2 M KNO3 ion buffer, a detection limit below 10−8 M in and a linear response range between 10-8 M-10−4 M were achieved. X-ray photoelectron spectroscopy (XPS) experiments confirmed that the sensing membrane is reversible after being exposed to Hg2+ solution and rinsed with deionised water. Calcium ion sensors In 0.1M KOH ion, a buffer detection limit of less than 10-8 M and a linear response range between 10-8M - 10-3M were achieved. In addition, there was negligible change in sensor response upon the addition of KOH, which increased pH level from approximately 4 to 11. [1] M. Myers, F.L.M. Khir, A. Podolska, G.A. Umana-Membreno, B. Nener, M. Baker, G. Parish, “Nitrate ion detection using AlGaN/GaN heterostructure-based devices without a reference electrode”, Sens. Actuators, B 181 301-5 (2013).

Device physics towards high performance GaN-based power electronics

Gallium-nitride-based power electronic devices, with the inherent high breakdown voltage, high working frequency, high output current density and high temperature operating capability, are promising candidates for next-generation high-efficiency and compact power management systems. In this work, the scientific and technical challenges towards high-performance GaN power devices, including threshold voltage instability, dynamic ON-resistance degradation, enhancement-mode gate techniques and long-term reliability of gate dielectrics, are investigated based on in-depth analysis of the physical origin of interface states and high-voltage current collapse. Oxidation of (Al) GaN surface are suggested to be the primary cause for the surface/interface states in GaN-based power devices. State-of-the-art device technologies, including nitridation-interficial-layer to suppress dielectric/(Al) GaN interface oxidation, polarized PEALD-AlN passivation for compensation of deep interface states, E-mode techniques of fluorine plasma ion implantation and high-temperature low-damage gate-recess, high-breakdown-strength LPCVD-SiN x and O3-sourced ALD-Al2O3 gate dielectrics are introduced for fabrication of high performance and reliability GaN-based power electronic devices.

Through‐layer XPS investigations of the Si3N4/AlGaN interface

Investigations of chemical species at the dielectric/III‐N interface remain an important question in order to understand the chemical origin of the surface states, which are present at the heterostructure surface. In this work, we demonstrate a sample preparation technique to analyze the existing interface species by X‐ray photoelectron spectroscopy (XPS) through thin uniform silicon nitride (Si3N4) layers 1.4 ± 0.2 nm. We show that it is crucial that the layers are as thin as possible but as thick as necessary to avoid oxygen diffusion through the passivation. Due to the sufficient information depth provided by such layers, the photoelectrons are detected. This is an advantage over sputtering as no intermixing of atoms or cleavage of bonds occurs. We show that the deposited Si3N4 hinders oxygen diffusion through the layer to the AlGaN surface. Selective SiO2 over Si3N4 etching and XPS depth profiles indicate that ∼1 nm of the thin Si3N4 layer has formed a surface oxide. Angle‐resolved XPS measurements confirm the accumulation of oxygen at the top and bottom Si3N4 interfaces. No indication is found that interfacial oxygen is bound to any other than the group‐13 metal. In agreement with the HSAB concept, the interfacial oxygen is bound to Al rather than Ga.Scheme of the nondestructive XPS interface analysis of the dielectric/AlGaN interface.