Metal Oxide Semiconductor Heterostructure Research Articles

Constructing of n‐Type Semiconductor Heterostructures for Efficient Hydrazine‐Assisted Hydrogen Production

AbstractHydrazine‐assisted water electrolysis presents a promising approach toward energy‐efficient hydrogen production. However, the progress of this technology is hindered by the limited availability of affordable, efficient, and durable catalysts. In this study, a feasible strategy is proposed for interface modulation that enables efficient hydrogen evolution and hydrazine oxidation through the construction of n‐type semiconductor heterostructures. The metal–semiconductor contacts are rationally designed using ruthenium nanoclusters and a range of metal oxide (M–O) semiconductor heterostructures, including p‐type semiconductor substrates (NiO, Co3O4, NiCo2O4, CuCo2O4, ZnCo2O4) and n‐type semiconductor substrate (Fe2O3). Intriguingly, Ru nanoclusters supported on p‐type M–O substrates induce a transition from p‐type M–O to n‐type M‐O/Ru. The design of n‐type semiconductor heterostructures can significantly reduce space‐charge regions and increase charge carrier concentration, thereby improving the electrical conductivity of electrocatalysts. Moreover, Ru atoms can serve as highly efficient active sites for hydrogen evolution reaction and hydrazine oxidation reaction. The NiO/Ru heterostructure can drive current densities of 10 and 100 mA cm−2 with only 0.021 and 0.22 V cell voltages for hydrazine‐assisted water electrolysis. This work provides new insights for the development of highly efficient semiconductor catalysts, enabling energy‐saving hydrogen production.

Metal-organic gel derived ZnO/α-Fe2O3 heterostructures for sensitive NO2 detection

Fabrication of metal oxide semiconductor heterostructures is an important means to improve sensor performance. However, the design of efficient heterostructures to reduce the operating temperature of metal oxides and obtain high sensitivity and selectivity remains a challenge. In this work, ZnO/α-Fe2O3 heterostructures were prepared by pyrolysis of Fe-based organic gel on ZnO nanorods, and their NO2 sensing properties were systematically studied. The α-Fe2O3 nanoparticles transformed from Fe-organic gel are evenly dispersed on the surface of ZnO nanorods, endowing the materials with a large specific surface area and remarkable charge modulation at the ZnO/α-Fe2O3 interface. The ZnO/α-Fe2O3 sensor shows superior selectivity and repeatability for NO2 detection at 175 °C, as well as short response-recovery time (24/32 s), low detection limit (79 ppb) and reliable stability. This study provides a useful strategy for the construction of core-shell heterostructure sensing materials.

Engineering sensitive gas sensor based on MOF-derived hollow metal-oxide semiconductor heterostructures

Metal-organic frameworks (MOFs) derived hollow heterostructured metal oxide semiconductors (MOSs) are a class of functional porous materials exhibiting distinctive physiochemical properties. Owing to the unique advantages, including large specific surface, high intrinsic catalytic performance, abundant channels for facilitating electron transfer and mass transport, and strong synergistic effect between different components, MOF-derived hollow MOSs heterostructures can work as promising candidates for gas sensing, which have thus attracted increasing attention. Aiming to provide a deep understanding on the design strategy and MOSs heterostructure, this review presents a comprehensive overview on the advantages and applications of MOF-derived hollow MOSs heterostructures when they used n for the detection of toxic gases. In addition, a deep discussion about the perspective and challenge of this interesting field is also well organized, hoping to provide guidance for the future design and development of more accurate gas sensors.

Threshold voltage control with high-temperature gate-oxide annealing in ultrawide bandgap AlGaN-channel MOSHFETs

We report threshold voltage (V TH) control in ultrawide bandgap Al0.4Ga0.6N-channel metal oxide semiconductor heterostructure field-effect transistors using a high-temperature (300 °C) anneal of the high-k ZrO2 gate-insulator. Annealing switched the polarity of the fixed charges at the ZrO2/AlGaN interface from +5.5 × 1013 cm−2 to −4.2 × 1013 cm−2, pinning V TH at ∼ (−12 V), reducing gate leakage by ∼103, and improving subthreshold swing 2× (116 mV decade−1). It also enabled the gate to repeatedly withstand voltages from −40 to +18 V, allowing the channel to be overdriven doubling the peak currents to ∼0.5 A mm−1.

Fe2O3-sensitized SnO2 nanosheets via atomic layer deposition for sensitive formaldehyde detection

Metal oxide semiconductor (MOS) nanostructures have been widely explored for formaldehyde sensors. The low surface chemical and electronic properties of pure MOS, however, greatly limits the sensor functions. In this work, Fe2O3-sensitized SnO2 nanosheets are designed via atomic layer deposition (ALD) to realize high performance formaldehyde detection. By varying the ALD cycles, the influence of different Fe2O3 loading on the sensing performance of SnO2 nanosheets is revealed. It is found that Fe2O3 ALD can greatly boost the sensing performance and the SnO2 nanosheets with 20 Fe2O3 ALD cycles exhibits the best response (Ra/Rg = 4.5) and fast response and recovery dynamics (9 and 34 s) to 20 ppm formaldehyde at a relative low temperature of 220 °C. The sensor based on SnO2/Fe2O3 also displays good selectivity to formaldehyde as well as the reliable stability and low limit of detection (LOD). This work will shed some light to design efficient MOS heterostructures for detection of formaldehyde.

Construction of Zn/Ni Bimetallic Organic Framework Derived ZnO/NiO Heterostructure with Superior N-Propanol Sensing Performance.

Bimetallic organic frameworks (Bi-MOFs) have been recognized as one of the most ideal precursors to construct metal oxide semiconductor (MOS) composites, owing to their high surface area, various chemical structures, and easy removal of the sacrificial MOF scaffolds through calcination. Herein, we synthesized Zn/Ni Bi-MOF for the first time via a facile ion exchange postsynthetic strategy, formed a three-dimensional framework consisting of infinite one-dimensional chains that is unattainable through the direct solvothermal approach, and then transformed the Zn/Ni Bi-MOF into a unique ZnO/NiO heterostructure through calcination. Notably, the obtained sensor based on a ZnO/NiO heterostructure exhibits an ultrahigh response of 280.2 toward 500 ppm n-propanol at 275 °C (17.2-fold enhancement compared with that of ZnO), remarkable selectivity, and a limit of detection of 200 ppb with a notable response (2.51), which outperforms state-of-the-art n-propanol sensors. The enhanced n-propanol sensing properties may be attributed to the synergistic effects of several points including the heterojunction at the interface between the NiO and ZnO nanoparticles, especially a one-dimensional chain MOF template structure as well as the chemical sensitization effect of NiO. This work provides a promising strategy for the development of a novel Bi-MOF-derived MOS heterostructure or homostructure with well-defined morphology and composition that can be applied to the fields of gas sensing, energy storage, and catalysis.

Interface property and band offset investigation of GaN based MOS heterostructures with diffusion-controlled interface oxidation technique

This paper presents the interface analysis and band offset of Al2O3/AlGaN/GaN metal-oxide-semiconductor (MOS) heterostructures with diffusion-controlled interface oxidation (DCIO) treatment. After conventional surface pre-treatment with wet cleaning and nitridation plasma, 1 nm Al2O3 was prepared with atomic layer deposition, followed by in situ plasma-assisted interface oxidation for 30 minutes. The interface oxidation process is limited by oxidant diffusion through the pre-deposited Al2O3 layer, contributing to the formation of a high quality crystalline interfacial oxide layer. For MOS heterostructures with 24.1 nm Al2O3, a positive threshold voltage shift by 1.8 V was obtained by using the DCIO technique. The energy band structures and band offset at the Al2O3/AlGaN interface was investigated with x-ray photoelectron spectroscopy (XPS). XPS results show that DCIO treatment causes an increase in conduction band offset from 2.29 ± 0.37 eV to 2.92 ± 0.36 eV. There is also a decrease in Al2O3 band slope, indicating a decrease in internal electrical field strength and interface charges. The Al2O3 energy band for these two cases may also cross with each other at a certain point, defining the critical thickness of gate oxide. Generally, the decrease in interface charges by DCIO causes a positive voltage shift, while the voltage shift will change sign with Al2O3 thickness smaller than the critical value. The MOS heterostructures with and without DCIO exhibit very nice relations between threshold voltage and Al2O3 thickness, giving a critical oxide thickness of about 1 nm. DCIO results in a decrease in the slope of linear function by about 0.08 V nm−1, indicating the reduced interface charges by 3.96 × 1012 cm−2.

Improved Ultraviolet Detection and Device Performance of Al2O3-Dielectric In0.17Al0.83N/AlN/GaN MOS-HFETs

Ultraviolet (UV) detection and electrical characteristics of In0.17Al0.83N/AlN/GaN metal–oxide–semiconductor heterostructure field-effect transistors (MOS-HFETs) with Al2O3 gate-dielectric and passivation formed by using ultrasonic spray pyrolysis deposition (USPD) are studied with respect to a conventional Schottky-gate HFET. The present MOS-HFET (Schottky-gate HFET) has demonstrated superior spectral responsivity (SR) of 360 (340) A/W at 350 nm at $V_{\mathrm{ GS}} = 5$ (3) V and $V_{\mathrm{ DS}} = 6$ (7) V, maximum drain–source saturation current density ( $I_{\mathrm{ DS, max}}$ ) of 810.5 (546.6) mA/mm, maximum extrinsic transconductance of ( $g_{m, \max }$ ) of 180.4 (221.2) mS/mm, gate-voltage swing (GVS) of 2.4 (0.5) V, on/off current ratio ( $I_{\mathrm{ on}}/I_{\mathrm{ off}}$ ) of $5.5 \times \,\,10^{8}$ ( $1.7 \times \,\,10^{5}$ ), two-terminal off-state gate–drain breakdown voltage (BVGD) of −158.5 (−127) V, three-terminal drain–source breakdown voltage (BVDS) of 162 (83.4) V at $V_{\mathrm{ GS}} = -10$ V, and power-added efficiency (P.A.E.) of 26.3% (16.5%) at 2.4 GHz at 300 K. In addition to the improved device performance, this paper demonstrates, for the first time, the UV sensing based on an InAlN/AlN/GaN MOS-HFET design.

Impact of oxide/barrier charge on threshold voltage instabilities in AlGaN/GaN metal-oxide-semiconductor heterostructures

Threshold voltage instabilities in AlGaN/GaN metal-oxide-semiconductor (MOS) heterostructures were investigated by means of capacitance-voltage (CV) hysteresis measurement and simulations. We focused on the impact of net charge at oxide/semiconductor interface (Qint) on the CV hysteresis. Our simulations suggest that due to different band bending at the interface, any positive or negative Qint with lower density (in the order of 1012 cm−2) results in a presence of relatively shallow unoccupied oxide/barrier interface states in equilibrium. On the other hand, high density of negative Qint (Qint/q ≈ −1013 cm−2) results in very deep unoccupied interface states, which in turn leads to incomplete electron re-emission during backward CV sweep and thus increased CV hysteresis of the MOS heterostructures compared to previous case. Impact of Qint is illustrated experimentally on MOS heterostructures with Al2O3 gate dielectric grown by metal-organic chemical vapor deposition, showing Qint of −1.2 × 1013 and +0.5 × 1012 cm−2 for structures with and without post-deposition annealing, respectively. Our results therefore suggest that normally-off AlGaN/GaN MOS heterostructure field-effect transistors featuring high density of negative Qint can be expected to be more susceptible for threshold voltage instabilities compared to normally-on counterparts.

Synthesis of thermally stable HfOxNy as gate dielectric for AlGaN/GaN heterostructure field-effect transistors**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0403000).

In this paper, we adopted thermally stable HfOxNy as gate dielectric for TiN/HfOxNy/AlGaN/GaN heterostructure field-effect transistors (HFETs) application. It demonstrated that the surface morphologies, composition, and optical properties of the HfOxNy films were dependent on oxygen flow rate in the O2/N2/Ar mixture sputtering ambient. The obtained metal–oxide–semiconductor heterostructure field-effect transistors by depositing HfO2 and HfOxNy dielectric at different oxygen flow rates possessed a small hysteresis and a low leakage current. After post deposition annealing at 900 °C, the device using HfOxNy dielectric operated normally with good pinch-off characteristics, while obvious degradation are observed for the HfO2 gated one at 600 °C. This result shows that the HfOxNy dielectric is a promising candidate for the self-aligned gate process.

Characterization of interface states in AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors with HfO2 gate dielectric grown by atomic layer deposition

The impact of oxidation agent and post-metallization annealing (PMA) on the quality of oxide-semiconductor interface in AlGaN/GaN metal-oxide semiconductor heterostructure field-effect transistors (MOS-HFETs) with HfO2 grown by atomic layer deposition (ALD) was investigated. About six orders of magnitude lower gate leakage current (∼10−7 mA/mm at −10 V) was observed for MOS-HFETs with HfO2 gate oxide grown by ALD using ozone oxidation agent (O-HfO2), compared with grown using water oxidation agent (T-HfO2). In addition, the output current frequency dispersion was effectively reduced for O-HfO2 by applying PMA performed at 400 °C in N2 ambient, where the current frequency dispersion measured using 100 ns-long pulses was reduced from 40% for as-deposited gate oxide to only 10% for devices after applied PMA. Frequency dispersion was found to be consistent with density of oxide/semiconductor interface states (Dit) determined near the semiconductor conduction and valence band edge using the capacitance-voltage curves measured at different temperatures (CV-T) and photo-assisted capacitance transient (photo-C-t) technique, respectively. MOS-HFET structures with O-HfO2 showed about one order of magnitude lower Dit near the semiconductor valence band compared with structures with T-HfO2 (2 × 1011 compared with 1012 eV−1 cm−2 at EC-E = 2.5 eV, respectively). Moreover, for devices with O-HfO2, Dit was further reduced in entire energy gap as a result of PMA performed at 400 °C. The increased Dit for T-HfO2 oxides was attributed to Hf-Hf bonds at the HfO2/GaN interface, as deduced X-ray photoelectron spectroscopy (XPS) analysis. In contrast, formation of HfHf bonds was negligible in as-deposited O-HfO2 oxide films, while PMA at 400 °C led to reduction of the hydroxyl group observed by XPS.

Modelling the threshold voltage of p‐channel enhancement‐mode GaN heterostructure field‐effect transistors

p-Channel gallium nitride (GaN) metal–oxide–semiconductor heterostructure field-effect transistors utilising a polarisation induced two-dimensional hole gas operate inherently in depletion mode. The condition for their conversion to enhancement-mode operation is examined via analytical expressions for the threshold voltage and verified via technology computer-aided design (TCAD) simulations. Between the two heterostructures: (i) conventional GaN/aluminium GaN (AlGaN)/GaN and (ii) alternate AlGaN/GaN/AlGaN/GaN examined in this work, the authors demonstrate at higher threshold voltage (), the alternate heterostructure can potentially achieve a higher on-current by a factor of 2 of (), without degradation in the on–off-current ratio, expected ideally to be of the order of .

(Invited) Nanostructured Heterojunction for Enhanced Gas-Sensing Performance

Metal oxide Semiconductor (MOS) materials based gas sensors have been developed in variety of applications such as industrial emission control, civil life, medical diagnosis, and environmental monitoring, due to their irreplaceable advantageous features, such as high sensitivity, small physical size, low cost and simplicity in fabrication. In addition to the material characteristics, heterojunction, based on a unique sensing mechanism, has already been proved to result in the improvement of the sensing performances compared to their single-component semiconductor gas sensors due to the synergetic effect from different sensing materials in the hetero-structure and the heterojunction barrier formed at the interfaces. In this talk, our recent progress on the synthesis of porous MOS heterostructures with nanostructured heterojunction in the thin film, nanowires and 2D nanomaterials will be presented. For example, highly-uniform bilayer and multilayer porous thin films were successfully fabricated using self-assembled soft template and simple sputtering deposition technique. The sensor based on the In2O3/CuO bilayer porous thin film shows obviously improved sensing performance to ethanol at the lower working temperature, compared to single layer counterpart sensors. The sensing mechanism based on p-n hetero-junction, which contributed to the enhanced sensing performance was also experimentally confirmed by a control experiment which the SiO2 insulation layer was inserted between the In2O3 and CuO layers to break the p-n junction. Besides, in this talk I would like to briefly report the progress of the electronic noise based on carbon nanostructures assembled in smart cell phone.

High quality PECVD SiO2 process for recessed MOS-gate of AlGaN/GaN-on-Si metal–oxide–semiconductor heterostructure field-effect transistors

A high quality SiO2 deposition process using a plasma enhanced chemical vapor deposition system has been developed for the gate insulator process of normally-off recessed-gate AlGaN/GaN metal–oxide–semiconductor-heterostructure field-effect transistors (MOS-HFETs). SiO2 films were deposited by using SiH4 and N2O mixtures as reactant gases. The breakdown field increased with increasing the N2O flow rate. The optimum SiH4/N2O ratio was 0.05, which resulted in a maximum breakdown field of 11MV/cm for the SiO2 film deposited on recessed GaN surface. The deposition conditions were optimized as follows; a gas flow rate of SiH4/N2O (=27/540sccm), a source RF power of 100W, a pressure of 2Torr, and a deposition temperature of 350°C. A fabricated normally-off MOS-HFET exhibited a threshold voltage of 3.2V, a specific on-resistance of 4.46mΩcm2, and a breakdown voltage of 810V.

Evaluation of a gate-first process for AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors with low ohmic annealing temperature**Project supported by the International Science and Technology Collaboration Program of China (Grant No. 2012DFG52260).

In this paper, TiN/AlOx gated AlGaN/GaN metal–oxide–semiconductor heterostructure field-effect transistors (MOS-HFETs) were fabricated for gate-first process evaluation. By employing a low temperature ohmic process, ohmic contact can be obtained by annealing at 600 °C with the contact resistance approximately 1.6 Ω·mm. The ohmic annealing process also acts as a post-deposition annealing on the oxide film, resulting in good device performance. Those results demonstrated that the TiN/AlOx gated MOS-HFETs with low temperature ohmic process can be applied for self-aligned gate AlGaN/GaN MOS-HFETs.

X-ray photoelectron spectroscopy and diffraction investigation of a metal–oxide-semiconductor heterostructure: Pt/Gd2O3/Si(111)

Platinum thin films deposited by physical vapor deposition (PVD) on Gd2O3/Si(111) templates are investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray photoelectron diffraction (XPD). Both XRD and XPD give clear evidence that Gd2O3 grows (111)-oriented and single-domain on Si(111) with mirror epitaxial relationship viz., [1¯10] Gd2O3(111)//[11¯0] Si(111). On Gd2O3/Si(111), Pt is partially crystallized with (111) orientation. There are two epitaxial domains and a slightly visible (111) fiber texture. The in-plane relationships of these Pt(111) domains with Gd2O3(111) are a direct one: [11¯0] Pt(111)//[11¯0] Gd2O3(111) and a mirror one: [1¯10] Pt(111)//[11¯0] Gd2O3(111). XPS reveals that Pt4f exhibits a metallic behavior even for small amounts of Pt but very small chemical shifts suggest that Pt environment is different at the interface with Gd2O3. These XPS chemical shifts have been correlated with features in XPD azimuth curves, which could be related with the existence of hexagonal α-PtO2(0001)layer.

High-performance GaAs-based metal–oxide–semiconductor heterostructure field-effect transistors with atomic-layer-deposited Al2O3 gate oxide and in situ AlN passivation by metalorganic chemical vapor deposition

GaAs-based metal–oxide–semiconductor heterostructure field-effect transistors (MOSHFETs) with Al2O3 gate oxide and in situ AlN passivation were investigated. Passivation with AlN improved the quality of the MOS interfaces, leading to good control of the gate. The devices had a sufficiently small subthreshold swing of 84 mV decade−1 in the drain current vs gate voltage curves, as well as negligible frequency dispersions and nearly zero hysteresis in the gate capacitance vs gate voltage curves. A maximum drain current of 630 mA/mm and a peak effective mobility of 6720 cm2 V−1 s−1 at a sheet carrier density of 3 × 1012 cm−2 were achieved.

Control of Threshold Voltage in GaN Based Metal–Oxide–Semiconductor High-Electron Mobility Transistors towards the Normally-Off Operation

The effect of defect charges on a threshold voltage (Vth) of the GaN/AlGaN/GaN metal–oxide–semiconductor heterostructures (MOS-Hs) with the gate stack thickness scaling was investigated by using capacitance–voltage measurements and a comprehensive analytical model [M. Ťapajna and J. Kuzmík: Appl. Phys. Lett. 100 (2012) 113509]. Using the MOS-Hs with Al2O3 and HfO2 dielectrics grown by plasma-enhanced atomic layer deposition, a high density (∼1013 cm-2) of negative fixed oxide charge (Nox) was extracted for Al2O3 layers, while an order of magnitude lower density (∼1012 cm-2) of positive Nox was extracted for HfO2 films. Consequently, despite similar dielectric constant of Al2O3 to that of (Al)GaN, it is advisable to attain normally-off operation by scaling the AlGaN layer thickness rather than the oxide thickness, taking advantage of the negative Nox offsetting Vth towards the positive voltages. Scaling of the AlGaN layer thickness is found to be effective also in the case of HfO2 dielectric, however, due to a positive Nox in HfO2, AlGaN layer needs to be scaled to a similar thickness (e.g., ∼3 nm) to obtain a positive Vth for the same oxide thickness of HfO2 and Al2O3 (∼5 nm). On the other hand, scaling of the GaN cap has no effect on Vth. Further, our analysis suggests that for MOS-Hs with both Al2O3 and HfO2 gate dielectric, the Fermi level position at the oxide/barrier interface in equilibrium is located within donor-like interface traps. Therefore, the oxide/GaN cap interface trap charge in MOS-Hs with the given structure parameters should lead to a negative Vth shift.

ZrO2/InAlN/GaN Metal–Oxide–Semiconductor Heterostructure Field-Effect Transistors with InAlN Barrier of Different Compositions

We report on InAlN/GaN heterostructure metal–oxide–semiconductor field-effect transistors (MOSHFETs) with an InAlN barrier layer of different compositions (xInN = 13, 17, and 21%) and ZrO2 gate-insulator/passivation. Static measurements yielded higher drain currents than those on unpassivated HFET counterparts and the currents increased with decreased xInN. Post deposition annealing of the ZrO2 insulator had less influence on the static performance but remarkable changes were observed on the capacitance–voltage characteristics. The capacitance hysteresis in both channel depletion and barrier accumulation regions was significantly suppressed after annealing. This indicates a reduction of the interfacial trap states and of fixed charge in the oxide. Pulsed current–voltage measurements confirmed this conclusion – the gate lag of only ∼80% was evaluated for 200 ns pulse width, independently on the composition of the InAlN barrier layer. These results support an application of high permittivity ZrO2 gate-insulator/passivation for the preparation of high-performance InAlN/GaN MOSHFETs.

High-Quality ICPCVD $\hbox{SiO}_{2}$ for Normally Off AlGaN/GaN-on-Si Recessed MOSHFETs

We have developed a high-quality <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{SiO}_{2}$</tex></formula> deposition process using an inductively coupled plasma chemical vapor deposition system for use as a gate oxide of AlGaN/GaN-on-Si metal–oxide–semiconductor heterostructure field-effect transistor (MOSHFET) for power switching applications. A breakdown field of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim$</tex></formula> 12 MV/cm was achieved using the optimized deposition conditions that were successfully applied in fabrication of the normally off AlGaN/GaN-on-Si MOSHFETs. The fabricated device exhibited excellent characteristics: a maximum drain current density of 375 mA/mm, a threshold voltage of 3 V, and a breakdown voltage of 820 V.