Modulation-doped Field-effect Transistors Research Articles

Characteristics of β-(AlxGa1−x)2O3/Ga2O3 dual-metal gate modulation-doped field-effect transistors simulated by TCAD

β-(AlxGa1−x)2O3/Ga2O3 modulation-doped field-effect transistors (MODFETs) with a dual-metal gate (DMG) architecture are designed, and the electrical characteristics of the DMG device are investigated in comparison with the single-metal gate (SMG) device by the Technology Computer-Aided Design (TCAD) simulation. The results demonstrate that the DMG MODFETs possess a superior transconductance (gm), current gain cut-off frequency (fT), and power gain cut-off frequency (fMAX) than those of SMG transistors, which is attributed to the regulated channel electric field by a DMG structure. With a gate length of 0.1 μm, the peak values of fT/fMAX of the designed DMG MODFET are obtained as 48.6/50.6 GHz, respectively. Moreover, a comprehensive thermal analysis is conducted between the SMG and DMG devices under steady-state and transient conditions. The DMG MODFET exhibits a lower maximum temperature than the SMG counterpart due to the reduced channel electric field, each subjected to the same power dissipation. This finding underscores the potential of the β-(AlxGa1−x)2O3/Ga2O3 MODFET with the DMG architecture as a promising approach for high-power radio frequency operations.

N2O grown high Al composition nitrogen doped β-(AlGa)2O3/β-Ga2O3 using MOCVD

We report on the MOCVD growth of smooth (010) (AlxGa1–x)2O3 and (100) (AlyGa1–y)2O3 epitaxial films on β-Ga2O3 substrates with (010) and (100) orientations, respectively, using N2O for oxidation. High resolution x-ray diffraction was used to evaluate the phase purity and strain characteristics of the (AlGa)2O3 layers and estimate the Al composition. The incorporation efficiency of Al into the (AlGa)2O3 films depends on process conditions, including chamber pressure, growth temperature, and gas phase Al concentration. Layers grown at lower reactor pressure and substrate temperature and higher gas phase Al concentration showed higher Al incorporation. Pure beta phase (AlGa)2O3 films with a record high Al composition of x = 30% for a film grown on a (010) β-Ga2O3 substrate and with an Al composition of up to y = 45% on the (100) β-Ga2O3 substrate was realized by introducing ∼18% Al mole fraction into the reactor. N2O grown β-(AlGa)2O3/β-Ga2O3 superlattice structures with an Al composition of 5% were also demonstrated on both substrate orientations. When higher gas phase Al concentration is introduced into the reactor, pure γ-phase (AlxGa1–x)2O3 is grown on (010) β-Ga2O3 substrates. In contrast, on the (100) β-Ga2O3 substrate, the (AlyGa1–y)2O3 layers are β-phase, but with two separate Al compositions owing to the local Al segregation. The nitrogen doping of (010) β-(AlxGa1–x)2O3 with [N] ranging 6 × 1017–2 × 1019 cm−3 was achieved using N2O. Higher Al composition and lower substrate temperature lead to higher N incorporation. The results show that using N2O as an oxygen source can lead to the growth of high Al content β-(AlGa)2O3, which paves the way for the realization of efficient power devices, such as modulation-doped field effect transistors.

Normally-Off β-(AlxGa1-x)2O3/Ga2O3 Modulation-Doped Field-Effect Transistors With p-GaN Gate: Proposal and Investigation

A novel <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -(AlxGa1-x)2O3/gallium oxide (Ga2O3) modulation-doped field-effect transistor (MODFET) with p-GaN gate is proposed and investigated for the first time. TCAD device simulator calibrated with experimental results is used to develop the physical insight of device operation and to evaluate the proposed device’s performance. Besides normally- OFF operation, the proposed MODFET also shows a higher driving capability and lower ON-resistance compared to conventional Ga2O3-MODFETs of similar device dimensions. The effect of individual device parameters on the electrical characteristics of the proposed device is also investigated in detail. Besides, an analytical model is formulated to estimate the threshold voltage of these normally- OFF devices. This model is rigorously validated with numerical results for a wide range of device parameters.

High mobility Ge 2DHG based MODFETs for low-temperature applications

Modulation-doped field-effect transistors (MODFET) that are typically based on either a two-dimensional electron gas or a two-dimensional hole gas (2DHG) are highly suitable for low-temperature applications due to the high mobilities attained by the charge carriers in the device channel. The Ge 2DHG is particularly interesting, since Ge exhibits the highest hole mobility among all semiconductors. In this paper, we report on the temperature-dependent direct current-characteristics of normally-on MODFETs based on a high mobility Ge 2DHG. Here, we specifically investigate device characteristics down to cryogenic temperatures and analyze the impact of different MOD-doping on device performance. We find that the largest On-Off ratio of and lowest sub-threshold swing (SS) of at a temperature of are attained for a MOD-doping of , while the highest effective mobility is obtained from a device with a MOD-doping of . Furthermore, while effective mobilities in the devices are strongly reduced compared to Hall mobilities measured in the unstructured Ge 2DHGs, both quantities can be seen to follow a similar trend. Our results motivate further investigations of Ge 2DHG MODFETs for applications in cryogenic low-noise amplifiers, besides their well-established potential for spintronic applications.

Study of β-Ga2O3-Based Thin-Channel MODFET Devices Using a Coupled Drift-Diffusion/Multisubband BTE Solver

Gallium-oxide-based modulation-doped field-effect transistors (MODFETs) are theoretically investigated. In order to properly consider the electrical characteristics of the two-dimensional electron gases (2DEGs) in thin-channel devices, a coupled drift-diffusion (DD)/multisubband Boltzmann transport equation (MS-BTE) solver is implemented. Parameters adopted in the simulation are determined by referring to the experimental results of existing long-channel devices. The impact of the channel length, the channel thickness, the ungated region length, and contact resistance on the cutoff frequency is rigorously calculated. It is found that the cutoff frequency can have about 90 GHz with a contact resistance of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.4~\Omega ~\cdot $ </tex-math></inline-formula> mm and a channel whose length and thickness are 30 and 5 nm, respectively.

β‐(Al0.17Ga0.83)2O3/Ga2O3 Delta‐Doped Heterostructure MODFETs with an Ultrathin Spacer Layer and a Back‐Barrier Layer: A Comprehensive Technology Computer‐Aided Design Analysis

The output characteristics of β‐(Al0.17Ga0.83)2O3/β‐Ga2O3‐based heterostructure modulation‐doped field‐effect transistors (MODFETs) with an ultrathin spacer layer and a back‐barrier layer are fitted with experimental measurements using a Silvaco ATLAS technology computer‐aided design (TCAD) simulation environment, and the calibration of the physical model and material parameters is realized. The effects of spacer layer thickness, barrier layer thickness, Si‐δ doping density, and insertion of a β‐Ga2O3 cap layer on the transfer and transconductance characteristics are examined. It is found that a β‐Ga2O3 cap layer on the top of the heterostructure can increase the sheet carrier density in the heterostructure. A breakdown analysis is also carried out to reveal the effects of several layers on the off‐state characteristics. A range of channel layer thicknesses from 15 to 25 nm is found to be the optimum range to avoid a high off‐state leakage current and earlier breakdown voltage.

Delta‐Doped β‐(Al0.17Ga0.83)2O3/Ga2O3 Double‐Channel Heterostructure MODFETs

Herein, in delta‐doped β‐(AlxGa1–x)2O3/β‐Ga2O3 heterostructures, the 2D electron gas (2DEG) density is mainly limited by the low conduction‐band offset at the heterointerface. Double‐channel heterostructures may be a candidate solution for enhancing the 2DEG density in these heterostructures. In this study, the output characteristics, transfer, transconductance, and off‐state current characteristics of delta‐doped β‐(Al0.17Ga0.83)2O3/β‐Ga2O3 double‐channel heterostructure modulation‐doped field‐effect transistors (MODFETs), including the effects of an ultrathin spacer layer and a back‐barrier layer, are numerically evaluated. The electrical characteristics of the proposed double‐channel heterostructure MODFET are compared with those of a single‐channel heterostructure MODFET. The proposed double‐channel heterostructure MODFET shows a maximum drain current of 133.7 mA mm−1 and a g m peak of 40 mS mm−1, which are higher than those in the single‐channel case. The calculated threshold voltage is −3.45 V. The dependence of the Si δ‐doping density in the second barrier and channel layer thickness on the electrical characteristics of the proposed device is evaluated and discussed. A β‐(Al0.17Ga0.83)2O3/β‐Ga2O3 superlattice back‐barrier structure is also implemented in delta‐doped β‐Ga2O3‐based heterostructure MODFETs to improve their off‐state characteristics for the first time.

Al0.30Ga0.70N /GaN MODFET with triple-teeth metal for RF and high-power applications

A modulation-doped field-effect transistor (MODFET) has been investigated in this paper. It is also called HEMT (high electron mobility transistor). The proposed MODFET is made up of Al0.30Ga0.70N as supply layer and GaN as channel or buffer layer, in which floating metal is embedded. Its T-gate is recessed to obtain higher gm, which results in improved RF characteristics. T-gate is used to minimize the gate resistance which reduces the power consumption of the proposed HEMT. A floating metal having triple teeth (TT), which resembles a comb is developed in GaN channel/buffer layer between the gate and drain electrodes to improve the device performance without increasing the dimensions of the device. The transition or cutoff frequency (f T) of 125 GHz and unity power gain (also called) maximum oscillation) frequency (f MAX) of 530 GHz at V DS = 10 V with V GS = 0.4 V have been reported in this paper. Analysis of power consumption of the proposed FET such as power gain (A P), output power (P OUT), and power-added efficiency (PAE) have been analyzed at 10 GHz in continuous wave mode and V DS = 35 V have been analyzed. The value obtained for P OUT, A P, and PAE is 67.5 dBmW, 11.6 dB, and 24.6%, respectively. All the obtained results from the Silvaco TCAD software have been verified with the mathematical model.

Fabrication of diamond modulation-doped FETs by NO2 delta doping in an Al2O3 gate layer

We demonstrated modulation doping in diamond and fabricated diamond field-effect transistors (FETs) by NO2 p-type delta doping in an Al2O3 gate layer. We confirmed modulation doping effects: a spatial separation between the NO2 acceptors in the Al2O3 gate insulator and the hole channel on the diamond surface increased the hole mobility, and the high hole sheet concentration was maintained till high temperatures. The diamond FETs showed maximum drain current density of −627 mA mm−1, and transconductance of 131 mS mm−1. The mobility increased to 2465 cm2 V−1 · s−1 near the threshold voltage, and the Baliga’s figure-of-merit was 179 MW cm−2.

Design of Ga2O3 modulation doped field effect transistors

The design of β-Ga2O3-based modulation-doped field effect transistors is discussed with a focus on the role of self-heating and resultant modification of the electron mobility profile. Temperature- and doping-dependent model of the electron mobility as well as temperature- and orientation-dependent approximations of the thermal conductivity of β-Ga2O3 are presented. A decrease in drain current was attributed to a position-dependent mobility reduction caused by a coupled self-heating mechanism and a high electric-field mobility reduction mechanism. A simple thermal management solution is presented where heat is extracted through the source contact metal. Additionally, it is shown that an undesired secondary channel can form at the modulation-doped layer that is distinguished by an inflection in the transconductance curve.

Modulation Doping for Threshold Voltage Control in Organic Field-Effect Transistors.

Organic electronics is the technology enabling truly flexible electronic devices. However, despite continuous improvements in the charge-carrier mobility, devices used for digital circuits based on organic field-effect transistors (OFETs) have still not achieved a commercial breakthrough. A substantial hurdle to the realization of effective digital circuitry is the proper control of the threshold voltage Vth. Previous approaches include doping or self-assembled monolayers to provide the threshold voltage control. However, while self-assembled monolayers-modified OFETs often do not show the level of reproducibility which is required in digital circuit engineering, direct doping of the channel material results in a poor on/off ratio leading to unfavorable power dissipation. Furthermore, direct doping of the channel material in organic semiconductors could cause the formation of trap states impeding the charge-carrier transport. Employing the concept of modulation-doped field-effect transistors (MODFETs), which is well established in inorganic electronics, the semiconductor-dopant interaction is significantly reduced, thereby solving the above-described problems. Here, we present the concept of an organic semiconductor MODFET which is composed of an organic-organic heterostructure between a highly doped wide-energy-gap material and an undoped narrow-energy-gap material. The effectiveness of charge transfer across the interface is controlled by the doping concentration and thickness of an undoped buffer layer. A complete picture of the energy landscape of this heterostructure is drawn using impedance spectroscopy and ultraviolet photoelectron spectroscopy. Furthermore, we analyze the effect of the dopant density on the charge-carrier transport properties. The incorporation of these heterostructures into OFETs enables a precise adjustment of the threshold voltage by using the modulation doping concept.

Numerical Study of the Coupling of Sub-Terahertz Radiation to n-Channel Strained-Silicon MODFETs.

This paper reports on a study of the response of a T-gate strained-Si MODFETs (modulation-doped field-effect transistor) under continuous-wave sub-THz excitation. The sub-THz response was measured using a two-tones solid-state source at 0.15 and 0.30 THz. The device response in the photovoltaic mode was non-resonant, in agreement with the Dyakonov and Shur model for plasma waves detectors. The maximum of the photoresponse was clearly higher under THz illumination at 0.15 THz than at 0.3 THz. A numerical study was conducted using three-dimensional (3D) electromagnetic simulations to delve into the coupling of THz radiation to the channel of the transistor. 3D simulations solving the Maxwell equations using a time-domain solver were performed. Simulations considering the full transistor structure, but without taking into account the bonding wires used to contact the transistor pads in experiments, showed an irrelevant role of the gate length in the coupling of the radiation to the device channel. Simulations, in contradiction with measurements, pointed to a better response at 0.3 THz than under 0.15 THz excitation in terms of the normalized electric field inside the channel. When including four 0.25 mm long bonding wires connected to the contact pads on the transistor, the normalized internal electric field induced along the transistor channel by the 0.15 THz beam was increased in 25 dB, revealing, therefore, the important role played by the bonding wires at this frequency. As a result, the more intense response of the transistor at 0.15 THz than at 0.3 THz experimentally found, must be attributed to the bonding wires.

Orientation-dependent band offsets between (AlxGa1−x)2O3 and Ga2O3

Band alignments between Al2O3 and Ga2O3 are investigated for four different orientations [(100), (010), (001), and (2¯01)] using density functional theory with a hybrid functional. By comparing with explicit interface calculations, we confirm that alignments can be reliably extracted from slab calculations that take atomic relaxation and strain into account. The presence of pseudomorphic strain shifts the average electrostatic potential as well as the band edges. Tensile strain in Al2O3 is found to lower the conduction band; the absolute deformation potential is calculated to be −9.5 eV for Al2O3 and −9.1 eV for Ga2O3. Our results show that the largest conduction-band offset will occur at a pseudomorphic (AlxGa1−x)2O3/Ga2O3 interface with the (100) orientation, allowing us to estimate the density of the two-dimensional electron gas in a modulation-doped field-effect transistor.

Electromechanical coupling and motion transduction in β -Ga2O3 vibrating channel transistors

We report on device physics modeling, assisted by experimental measurement results, for all-electronic transduction of resonant electromechanical motion of nanoscale β-Ga2O3 vibrating channel transistors (VCTs), including both direct readout of high frequency transmission current (without frequency down-conversion) and frequency modulation (FM) down-conversion techniques. The β-Ga2O3 VCTs under consideration have fundamental mode resonance frequencies at ∼25 MHz–1 GHz, with quality factors (Qs) of ∼100–5000 and transconductance gm at ∼0.2 nS–5 μS. In analysis of signal transduction with varying device parameters, the transistor's gate trench depth z0 and transconductance gm play key roles in improving the electromechanical coupling and device performance. Reduction of trench depth z0 can engender a major enhancement in readout current. Reducing channel thickness h and increasing channel length L can improve the readout current while downshifting the resonance frequency at the same time. We design β-Ga2O3 VCT with a suspended modulation doped field effect transistor structure for efficient operation in the GHz range. This study paves the way for future engineering of all-electronic transduction and integration of high frequency β-Ga2O3 resonators on chip with β-Ga2O3 electronics and optoelectronics.

Electro-thermal co-design of β -(AlxGa1-x)2O3/Ga2O3 modulation doped field effect transistors

Ultra-wide bandgap β-gallium oxide (Ga2O3) devices are of considerable interest with potential applications in both power electronics and radio frequency devices. However, current Ga2O3 device technologies are limited by the material's low intrinsic electron mobility and thermal conductivity. The former problem can be addressed by employing modulation-doped β-(AlxGa1−x)2O3/Ga2O3 heterostructures in the device architecture. In this work, (AlxGa1−x)2O3/Ga2O3 modulation-doped field effect transistors (MODFETs) have been investigated from a thermal perspective. Thermoreflectance thermal imaging was used to characterize the self-heating of the MODFETs. The (Al0.18Ga0.82)2O3 thermal conductivity (3.1–3.6 W/mK) was determined using a frequency domain thermoreflectance technique. Electro-thermal modeling was used to discern the effect of design parameters such as substrate orientation and channel length on the device self-heating behavior. Various thermal management schemes were evaluated using the electro-thermal device model. From an electro-thermal co-design perspective, the improvement in electrical performance followed by the mitigation of self-heating was also studied. For example, by employing a Ga2O3-on-SiC composite wafer, which was fabricated in this work, a 50% increase in power handling capability can be achieved as compared to a homoepitaxial device. Furthermore, flip-chip heterointegration and double-sided cooling approaches can lead to more than 2× improvement in the power handling capability. Using an augmented double-sided cooling design that includes nanocrystalline diamond passivation, a 5× improvement in the power handling capability can be accomplished, indicating the potential of the technology upon implementation of a suitable thermal management scheme.

Effect of the Front and Back Illumination on Sub-Terahertz Detection Using n-Channel Strained-Silicon MODFETs

Plasma waves in semiconductor gated 2-D systems can be used to efficiently detect Terahertz (THz) electromagnetic radiation. This work reports on the response of a strained-Si Modulation-doped Field-Effect Transistor (MODFET) under front and back sub-THz illumination. The response of the MODFET has been characterized using a two-tones solid-state continuous wave source at 0.15 and 0.30 THz. The DC drain-to-source voltage of 500-nm gate length transistors transducing the sub-THz radiation (photovoltaic mode) exhibited a non-resonant response in agreement with literature results. Two configurations of the illumination were investigated: (i) front side illumination in which the transistor was shined on its top side, and (ii) back illumination side where the device received the sub-THz radiation on its bottom side, i.e., on the Si substrate. Under excitation at 0.15 THz clear evidence of the coupling of terahertz radiation by the bonding wires was found, this coupling leads to a stronger response under front illumination than under back illumination. When the radiation is shifted to 0.3 THz, as a result of a lesser efficient coupling of the EM radiation through the bonding wires, the response under front illumination was considerably weakened while it was strengthened under back illumination. Electromagnetic simulations explained this behavior as the magnitude of the induced electric field in the channel of the MODFET was considerably stronger under back illumination.

Breakdown Characteristics of $\beta$ -(Al0.22Ga0.78)2O3/Ga2O3 Field-Plated Modulation-Doped Field-Effect Transistors

We investigate voltage breakdown of a $\beta $ -(Al0.22Ga0.78)2O3/Ga2O3 modulation-doped field-effect transistor with a gate-connected field-plate and silicon nitride (SiNx) as the passivation layer. Breakdown characteristics were found to be limited by the Schottky gate. We demonstrate a high breakdown voltage of 1.37 kV for a gate-to-drain separation ( ${L} _{\text {GD}}$ ) of $16~\mu \text{m}$ with a specific ON-resistance $\vphantom {\int ^{AAAAA^{AAA}}}$ of 120.1 $\text{m}\Omega \cdot \text {cm}^{{2}}$ . A high average electric field of 3.9 MV/cm is extracted for ${L} _{\text {GD}}={320}$ nm. The reported results suggest SiNx as a potential passivation dielectric for Ga2O3.

Electron transport in AlxGa1−xAs based double quantum well modulation doped field effect transistor structure: effect of non-square potential profile

The importance of non-square quantum well potential profile on the electron mobility µ of coupled double quantum well based modulation doped field effect transistor (MOD-FET) structure is studied. We consider non-square cubic structure potential, VDS (z) α z3, where z denotes the position coordinate from the center of the well. The cubic double quantum well (CD-QW) is made of AlxGa1−xAs alloy in which both the side barriers are δ-doped with Si. We consider ionized impurity (imp-) and alloy disorder (al-) scatterings for the calculation of µ. We investigate the variation of the structure parameters, such as well width, barrier width, alloy concentration and doping concentration on µ. The nonlinear behavior of µ can be achieved by properly tuning the structure parameters, mostly controlled by imp-scatterings. We show that under suitable structure parameters µ is enhanced in CD-QW as compared to that of a conventional SD-QW structure. Our analysis will help improve the channel conductivity in QW based MOD-FETs.

Evaluation of Low-Temperature Saturation Velocity in &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$\beta$ &lt;/tex-math&gt; &lt;/inline-formula&gt;-(AlxGa1–x)2O3/Ga2O3 Modulation-Doped Field-Effect Transistors

We report on the high-field transport characteristics and saturation velocity in a modulation-doped β-(Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> heterostructure. The formation of a 2-D electron gas (2DEG) in the modulation-doped structure was confirmed from the Hall measurements, and the 2DEG channel mobility increased from 143 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V·s at room temperature to 1520 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V·s at 50 K. The high electron mobility at 50 K made it feasible to achieve velocity saturation inside the channel. The saturation velocity was estimated based on both pulsed current-voltage measurements and smallsignal radio frequency (RF) measurements. The measured velocity-field profile suggested a saturation velocity above 1.1 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cm/s at 50 K. The small-signal RF characteristics were measured for the fabricated modulation-doped field-effect transistors with a Pt-based Schottky contact. The current gain cutoff frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> ) and maximum oscillation frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> ) showed significant increases from 4.0/11.8GHz at room temperature to 17.4/40.8GHz at 50 K for the device with gate length of L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</sub> = 0.61 μm. The analysis of the low temperature f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> based on device simulations indicated a peak velocity of 1.2 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cm/s. The three-terminal off-state breakdown measurement further suggested an average breakdown field of 3.22 MV/cm. The high saturation velocity and high breakdown field in β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> make it a promising candidate for high-power and high-frequency device applications.

Valence- and Conduction-Band Offsets for Atomic-Layer-Deposited Al2O3 on (010) (Al0.14Ga0.86)2O3

The wide-bandgap ternary (AlxGa1−x)2O3 forms a heterostructure system with Ga2O3 that is attracting attention for modulation-doped field-effect transistors. The options for gate dielectric on (AlxGa1−x)2O3 are limited by the need for adequate band offsets at the heterointerface. Al2O3 deposited by atomic layer deposition (ALD) is one option due to its large bandgap (6.9 eV). We measured the valence-band offset at the Al2O3/(Al0.14Ga0.86)2O3 heterointerface using x-ray photoelectron spectroscopy (XPS). Al2O3 was deposited by ALD onto single-crystal β-(Al0.14Ga0.86)2O3 (bandgap 5.0 eV) grown by molecular beam epitaxy (MBE). The valence-band offset was determined to be 0.23 ± 0.04 eV (straddling gap, type I alignment) for ALD Al2O3 on β-(Al0.14Ga0.86)2O3. The conduction-band offset was 1.67 ± 0.30 eV, providing good electron confinement.