Graded-Band-Gap Zinc-Tin Oxide Thin-Film Transistors with a Vertically Stacked Structure for Wavelength-Selective Photodetection.

Filter-free wavelength-selective photodetectors have garnered significant attention due to the growing demand for smart sensors, artificial intelligence, the Internet of Everything, and so forth. However, the challenges associated with large-scale preparation and compatibility with complementary metal-oxide-semiconductor (CMOS) technology limit their wide-ranging applications. In this work, we address the challenges by constructing vertically stacked graded-band-gap zinc-tin oxide (ZTO) thin-film transistors (TFTs) specifically designed for wavelength-selective photodetection. The ZTO thin films with various band gaps are fabricated via atomic layer deposition (ALD) by varying the ALD cycle ratios of zinc oxide (ZnO) and SnO2. The ZTO film with a small Sn ratio exhibits a decreased band gap, and the resultant TFT shows a degraded performance, which can be attributed to the Sn4+ dopant introducing a series of deep-state energy levels in the ZnO band gap. As the ratio of Sn increases further, the band gap of the ZTO also increases, and the mobility of the ZTO TFT increases up to 30 cm2/V s, with a positive shift of the threshold voltage. The photodetectors employing ZTO thin films with distinct band gaps show different spectral responsivities. Then, vertically stacked ZTO (S-ZTO) thin films, with gradient band gaps increasing from the bottom to the top, have been successfully deposited using consecutive ALD technology. The S-ZTO TFT shows decent performance with a mobility of 18.4 cm2/V s, a threshold voltage of 0.5 V, an on-off current ratio higher than 107, and excellent stability under ambient conditions. The resultant S-ZTO TFT also exhibits obviously distinct photoresponses to light at different wavelength ranges. Furthermore, a device array of S-ZTO TFTs demonstrates color imaging by precisely reconstructing patterned illuminations with different wavelengths. Therefore, this work provides CMOS-compatible and structure-compact wavelength-selective photodetectors for advanced and integrable optoelectronic applications.

High Performance On-Chip Energy Storage Capacitors with Plasma-Enhanced Atomic Layer-Deposited Hf0.5Zr0.5O2/Al-Doped Hf0.25Zr0.75O2 Nanofilms as Dielectrics.

Concurrently achieving high energy storage density (ESD) and efficiency has always been a big challenge for electrostatic energy storage capacitors. In this study, we successfully fabricate high-performance energy storage capacitors by using antiferroelectric (AFE) Al-doped Hf0.25Zr0.75O2 (HfZrO:Al) dielectrics together with an ultrathin (1 nm) Hf0.5Zr0.5O2 underlying layer. By optimizing the Al concentration in the AFE layer with the help of accurate controllability of the atomic layer deposition technique, an ultrahigh ESD of 81.4 J cm-3 and a perfect energy storage efficiency (ESE) of 82.9% are simultaneously achieved for the first time in the case of the Al/(Hf + Zr) ratio of 1/16. Meanwhile, both the ESD and ESE exhibit excellent electric field cycling endurance within 109 cycles under 5~5.5 MV cm-1, and robust thermal stability up to 200 °C. Thus, the fabricated capacitor is very promising for on-chip energy storage applications due to favorable integratability with the standard complementary metal-oxide-semiconductor (CMOS) process.

Performance improvement of Hf0.45Zr0.55Oxferroelectric field effect transistor memory with ultrathin Al-O bonds-modified InOxchannels.

Ferroelectric field effect transistor (FeFET) memories with hafnium zirconium oxide (HZO) ferroelectric gate dielectric and ultrathin InOxchannel exhibit promising applicability in monolithic three-dimensional (M3D) integrated chips. However, the inferior stability of the devices severely limits their applications. In this work, we studied the effect of single cycle of atomic-layer-deposited Al-O bonds repeatedly embedded into an ultrathin InOxchannel (∼2.8 nm) on the Hf0.45Zr0.55OxFeFET memory performance. Compared to the pure InOxchannel, three cycles of Al-O bonds modified InOxchannel (IAO-3) generates a much larger memory window (i.e. drain current ratio between the programmed and erased devices) under the same program conditions (+5.5 V/500 ns), especially after post-annealing at 325 °C for 180 s in O2(1238 versus 317). Meanwhile, the annealed IAO-3 FeFET memory also shows quite stable data retention up to 104s, and much more robust program/erase stabilities till 105cycles. This is because the modification of strong Al-O bonds stabilizes the oxygen vacancies and reduces the bulk trap density in the channel. Furthermore, it is indicated that the program and erase efficiencies increase gradually with reducing the channel length of the memory device. By demonstrating markedly improved performance of the HZO FeFET memory with the ultrathin IAO-3 channel, this work provides a promising device for M3D integratable logic and memory convergent systems.

Flexible Microspectrometers Based on Printed Perovskite Pixels with Graded Bandgaps.

Miniaturized spectrometers have attracted much attention due to their capability to detect spectral information within a small size. However, such technology still faces challenges including large-scale preparation and performance repeatability. In this work, we overcome these challenges by demonstrating a microspectrometer constructed with a series of pixelized graded-bandgap perovskite photodetectors fabricated with inkjet printing. High-quality perovskite films with minimal pinholes and large grains are deposited by optimizing printing conditions including substrate temperature and surface modification. The resulting perovskite photodetectors show decent photosensing performance, and the different photodetectors based on perovskite films with different bandgaps exhibit various spectral responsivities with different cutoff wavelength edges. Microspectrometers are then constructed with the array of the pixelized graded-bandgap perovskite photodetectors, and incident spectra are algorithmically reconstructed by combining their output currents. The reconstruction performance of the miniaturized spectrometer is evaluated by comparing the results to the spectral curve measured with a commercial bulky spectrometer, indicating a reliable spectral reconstruction with a resolution of around 10 nm. More significantly, the miniaturized spectrometers are successfully fabricated on polymer substrates, and they demonstrate excellent mechanical flexibility. Therefore, this work provides a flexible miniaturized spectrometer with large-scale fabricability, which is promising for emerging applications including wearable devices, hyperspectral imaging, and internet of things.

Superhigh energy storage density on-chip capacitors with ferroelectric Hf0.5Zr0.5O2/antiferroelectric Hf0.25Zr0.75O2 bilayer nanofilms fabricated by plasma-enhanced atomic layer deposition.

Thanks to their excellent compatibility with the complementary metal-oxide-semiconductor (CMOS) process, antiferroelectric (AFE) HfO2/ZrO2-based thin films have emerged as potential candidates for high-performance on-chip energy storage capacitors of miniaturized energy-autonomous systems. However, increasing the energy storage density (ESD) of capacitors has been a great challenge. In this work, we propose the fabrication of ferroelectric (FE) Hf0.5Zr0.5O2/AFE Hf0.25Zr0.75O2 bilayer nanofilms by plasma-enhanced atomic layer deposition for high ESD capacitors with TiN electrodes. The effects of the FE/AFE thickness composition and annealing conditions are investigated, revealing that the Hf0.5Zr0.5O2 (1 nm)/Hf0.25Zr0.75O2 (9 nm) bilayer can generate the optimal ESD after optimized annealing at 450 °C for 30 min. This is mainly ascribed to the factor that the introduction of a 1 nm Hf0.5Zr0.5O2 layer enhances the formation of the tetragonal (T) phase with antiferroelectricity in the AFE Hf0.25Zr0.75O2 layer as well as the breakdown electric field of the bilayer while fixing the FE/AFE bilayer thickness at 10 nm. As a result, a ESD as high as 71.95 J cm-3 can be obtained together with an energy storage efficiency (ESE) of 57.8%. Meanwhile, with increasing the measurement temperature from 300 and 425 K, the capacitor also demonstrates excellent stabilities of ESD and ESE. In addition, superior electrical cycling endurance is also demonstrated. Further, by integrating the capacitor into deep silicon trenches, a superhigh ESD of 364.1 J cm-3 is achieved together with an ESE of 56.5%. This work provides an effective way for developing CMOS process-compatible, eco-friendly and superhigh ESD three-dimensional capacitors for on-chip energy storage applications.

Spectrum Reconstruction with Filter-Free Photodetectors Based on Graded-Band-Gap Perovskite Quantum Dot Heterojunctions.

Spectrum reconstruction with filter-free microspectrometers has attracted much attention owing to their promising potential in in situ analysis systems, on-chip spectroscopy characterizations, hyperspectral imaging, etc. Further efforts in this field can be devoted to improving the performance of microspectrometers by employing high-performance photosensitive materials and optimizing the reconstruction algorithms. In this work, we demonstrate spectrum reconstruction with a set of photodetectors based on graded-band-gap perovskite quantum dot (PQD) heterojunctions using both calculation and machine learning algorithms. The photodetectors exhibit good photosensitivities with nonlinear current-voltage curves, and the devices with different PQD band gaps show various spectral responsivities with different cutoff wavelength edges covering the entire visible range. Reconstruction performances of monochromatic spectra with the set of PQD photodetectors using two different algorithms are compared, and the machine learning method achieves relatively better accuracy. Moreover, the nonlinear current-voltage variation of the photodetectors can provide increased data diversity without redundancy, thus further improving the accuracy of the reconstructed spectra for the machine learning algorithm. A spectral resolution of 10 nm and reconstruction of multipeak spectra are also demonstrated with the filter-free photodetectors. Therefore, this study provides PQD photodetectors with the corresponding optimized algorithms for emerging flexible microspectrometer systems.

Power-Efficient Gas-Sensing and Synaptic Diodes Based on Lateral Pentacene/a-IGZO PN Junctions.

Function convergence of gas sensing and neuromorphic computing is attracting much research attention due to the promising potential in electronic olfactory, artificial intelligence, and internet of everything systems. However, the current neuromorphic gas-sensing systems are either realized via integration of gas detectors and neuromorphic devices or operating with three-terminal synaptic transistors at high voltages, leading to a rather high system complexity or power consumption. Herein, gas-modulated synaptic diodes with lateral structures are developed to converge sensing, processing, and storage functions into a single device. The lateral synaptic diode is based on a p-n junction of an organic semiconductor (OSC) and amorphous In-Ga-Zn-O, in which the upper OSC layer can directly interact with the gas molecules in the atmosphere. Typical synaptic behaviors triggered by ammonia, including inhibitory postsynaptic current and paired-pulse depression, are successfully demonstrated. Meanwhile, a low power consumption of 6.3 pJ per synaptic event has been achieved, which benefits from the simple device structure, the decent chemosensitivity of the OSC, and the low operation voltage. A simulated ammonia analysis in human exhaled breath is further conducted to explore the practical application of the synaptic diode. Therefore, this work provides a gas-modulated synaptic diode for circuit-compact and power-efficient artificial olfactory systems.

Photoelectric Logic and In Situ Memory Transistors with Stepped Floating Gates of Perovskite Quantum Dots.

Electronic-Photonic integrated systems have attracted intensive attention in addressing the explosively increasing data-processing issue in the post-Moore era. However, the tremendous size difference between basic electronic and photonic units poses challenges for the further deep convergence of optoelectronic microprocessors. Here, we report a floating-gate transistor fabricated with complementary metal-oxide-semiconductor compatible technologies, which can realize multilevel photoelectric logic computing and in situ memory simultaneously. The transistor presents stepped floating gates of perovskite quantum dots with different bandgaps and exhibits nonvolatile multilevel memory states written/erased by electrical and high-bandwidth optical signals. Meanwhile, the device can also realize logic functions such as an optoelectronic AND gate by separably programming the states of the stepped floating gates with bias and optical wavelength. A convergence of multilevel logic computing and storage is further achieved on the transistor. By demonstrating such multifunctionality in a single device, the photoelectric transistors, even with a rather large size to match photonic cells, can provide the optoelectronic microprocessors with substantially improved performances.

Flexible boron nitride-based memristor for in situ digital and analogue neuromorphic computing applications.

The data processing efficiency of traditional computers is suffering from the intrinsic limitation of physically separated processing and memory units. Logic-in-memory and brain-inspired neuromorphic computing are promising in-memory computing paradigms for improving the computing efficiency and avoiding high power consumption caused by extra data movement. However, memristors that can conduct digital memcomputing and neuromorphic computing simultaneously are limited by the difference in the information form between digital data and analogue data. In order to solve this problem, this paper proposes a flexible low-dimensional memristor based on boron nitride (BN), which has ultralow-power non-volatile memory characteristic, reliable digital memcomputing capabilities, and integrated ultrafast neuromorphic computing capabilities in a single in situ computing system. The logic-in-memory basis, including FALSE, material implication (IMP), and NAND, are implemented successfully. The power consumption of the proposed memristor per synaptic event (198 fJ) can be as low as biology (fJ level) and the response time (1 µs) of the neuromorphic computing is four orders of magnitude shorter than that of the human brain (10 ms), paving the way for wearable ultrahigh efficient next-generation in-memory computing architectures.

Flexible and Filter-Free Color-Imaging Sensors with Multicomponent Perovskites Deposited Using Enhanced Vapor Technology.

Halide perovskites are promising photoactive materials for filter-free color-imaging sensors owing to their outstanding optoelectronic properties, tunable bandgaps, and suitability for large-scale fabrication. However, producing patterned perovskite films of sufficiently high quality for such applications poses a challenge for existing fabrication methods: using solution processes to prepare patterned perovskite films is complicated, while evaporation methods often result in perovskite photodetectors with limited performance. In this paper, the authors report the development of an improved evaporation method in which substrates are treated with a brominated (3-aminopropyl) triethoxysilane self-assembled monolayer to improve the properties of the patterned perovskite films. The resulting perovskite photodetectors exhibit significantly enhanced photosensitivity and long-term stability (exceeding 100 days). Additionally, the polymer substrates facilitate device flexibility. Finally, perovskites comprising three different halide components, each with a different bandgap, are integrated into a device array using the developed evaporation technology, yielding sensors that enable the discrimination of red, green, and blue colors. Thus, the flexible photosensor arrays can generate colorful images closely resembling perceived patterns, demonstrating reliable color imaging. Therefore, this study successfully demonstrates filter-free color-imaging by integrating high-performance patterned and multicomponent perovskite photodetectors, highlighting the potential of such detectors for advanced optoelectronic applications, including hyperspectral imaging.

High-bandwidth light inputting multilevel photoelectric memory based on thin-film transistor with a floating gate of CsPbBr3/CsPbI3 blend quantum dots.

The electronic-photonic convergent systems can overcome the data transmission bottleneck for microchips by enabling processor and memory chips with high-bandwidth optical input/output. However, current silicon-based electronic-photonic systems require various functional devices/components to convert high-bandwidth optical signals into electrical ones, thus making further integrations of sophisticated systems rather difficult. Here, we demonstrate thin-film transistor-based photoelectric memories employing CsPbBr3/CsPbI3 blend perovskite quantum dots (PQDs) as a floating gate, and multilevel memory cells are achieved under programming and erasing modes, respectively, by imputing high-bandwidth optical signals. For different bandwidth light input (i.e. 500-550, 575-650 and 675-750 nm) with the same intensity, three levels of programming window (i.e. 3.7, 1.9 and 0.8 V) and erasing window (i.e. -1.9, -0.6 and -0.1 V) are obtained under electrical pulses, respectively. This is because the blend PQDs have two different bandgaps, and different amounts of photo-generated carriers can be produced for different wavelength optical inputs. It is noticed that the 675-750 nm light inputs have no effects on both programming and erasing windows because of no photo-carriers generation. Four memory states are demonstrated, showing enough large gaps (1.12-5.61 V) between each other, good data retention and programming/erasing endurance. By inputting different optical signals, different memory states can be switched easily. Therefore, this work directly demonstrates high-bandwidth light inputting multilevel memory cells for novel electronic-photonic systems.

Spectrum projection with a bandgap-gradient perovskite cell for colour perception.

Optoelectronic devices for light or spectral signal detection are desired for use in a wide range of applications, including sensing, imaging, optical communications, and in situ characterization. However, existing photodetectors indicate only light intensities, whereas multiphotosensor spectrometers require at least a chip-level assembly and can generate redundant signals for applications that do not need detailed spectral information. Inspired by human visual and psychological light perceptions, the compression of spectral information into representative intensities and colours may simplify spectrum processing at the device level. Here, we propose a concept of spectrum projection using a bandgap-gradient semiconductor cell for intensity and colour perception. Bandgap-gradient perovskites, prepared by a halide-exchanging method via dipping in a solution, are developed as the photoactive layer of the cell. The fabricated cell produces two output signals: one shows linear responses to both photon energy and flux, while the other depends on only photon flux. Thus, by combining the two signals, the single device can project the monochromatic and broadband spectra into the total photon fluxes and average photon energies (i.e., intensities and hues), which are in good agreement with those obtained from a commercial photodetector and spectrometer. Under changing illumination in real time, the prepared device can instantaneously provide intensity and hue results. In addition, the flexibility and chemical/bio-sensing of the device via colour comparison are demonstrated. Therefore, this work shows a human visual-like method of spectrum projection and colour perception based on a single device, providing a paradigm for high-efficiency spectrum-processing applications.

Ultralow Power Wearable Heterosynapse with Photoelectric Synergistic Modulation.

Although the energy consumption of reported neuromorphic computing devices inspired by biological systems has become lower than traditional memory, it still remains greater than bio-synapses (≈10 fJ per spike). Herein, a flexible MoS2-based heterosynapse is designed with two modulation modes, an electronic mode and a photoexcited mode. A one-step mechanical exfoliation method on flexible substrate and low-temperature atomic layer deposition process compatible with flexible electronics are developed for fabricating wearable heterosynapses. With a pre-spike of 100 ns, the synaptic device exhibits ultralow energy consumption of 18.3 aJ per spike in long-term potentiation and 28.9 aJ per spike in long-term depression. The ultrafast speed and ultralow power consumption provide a path for a neuromorphic computing system owning more excellent processing ability than the human brain. By adding optical modulation, a modulatory synapse is constructed to dynamically control correlations between pre- and post-synapses and realize complex global neuromodulations. The novel wearable heterosynapse expands the accessible range of synaptic weights (ratio of facilitation ≈228%), providing an insight into the application of wearable 2D highly efficient neuromorphic computing architectures.

Room-temperature developed flexible biomemristor with ultralow switching voltage for array learning.

As one of the emerging neuromorphic computing devices, memristors may break through the limitation of traditional computers with a von Neumann architecture. However, the development of flexible memristors is limited by the high-temperature fabrication process, large operating voltage and non-uniform distribution of resistance. The room-temperature process has attracted great attention due to its advantages of low thermal dissipation, low cost and excellent compatibility with flexible electronics. Here, we proposed a fully physical vapour deposition (PVD) process for fabricating a memristor without additional heat treatment. The device showed excellent resistive switching characteristics with ultralow set/reset voltages (0.48 V/-0.39 V), uniform distribution (10%/15%), stable retention characteristic, multilevel storage behavior and reliable flexibility (radius of 10 mm). With continuously modulated conductance, typical synaptic plasticities were simulated by our flexible biomemristor, including excitatory post-synaptic current (EPSC), paired-pulse facilitation (PPF), long-term potentiation/depression (LTP/LTD) and learning-forgetting curve. Furthermore, the array learning behavior like that of the human brain was simulated with these trainable biomemristors. This study paves a new way for developing low-cost, wearable, neuromorphic computing electronics at room temperature and expands the applications of artificial synapse arrays.

Three-Dimensional Nanoscale Flexible Memristor Networks with Ultralow Power for Information Transmission and Processing Application.

To construct an artificial intelligence system with high efficient information integration and computing capability like the human brain, it is necessary to realize the biological neurotransmission and information processing in artificial neural network (ANN), rather than a single electronic synapse as most reports. Because the power consumption of single synaptic event is ∼10 fJ in biology, designing an intelligent memristors-based 3D ANN with energy consumption lower than femtojoule-level (e.g., attojoule-level) and faster operating speed than millisecond-level makes it possible for constructing a higher energy efficient and higher speed computing system than the human brain. In this paper, a flexible 3D crossbar memristor array is presented, exhibiting the multilevel information transmission functionality with the power consumption of 4.28 aJ and the response speed of 50 ns per synaptic event. This work is a significant step toward the development of an ultrahigh efficient and ultrahigh-speed wearable 3D neuromorphic computing system.

Light response behaviors of amorphous In-Ga-Zn-O thin-film transistors via in situ interfacial hydrogen doping modulation.

Thin-film transistors (TFTs) based on amorphous In-Ga-Zn-O (a-IGZO) channels present high mobility, large-area uniformity, mechanical flexibility and photosensitivity, and thus have extensive applicability in photodetectors, wearable devices, etc. However, pure a-IGZO based photosensors only exhibit a UV light response with limited sensitivity performance. By utilizing in situ interfacial hydrogen doping, it is demonstrated that the a-IGZO TFTs with the Al2O3 dielectric deposited by plasma-enhanced atomic layer deposition at room temperature (RT) have excellent photosensing performance, such as a photoresponsivity of over 6 × 105 A W-1 and a light to dark current ratio up to 107. This is attributed to spontaneous interfacial hydrogen doping into the a-IGZO channel during sputtering deposition of a-IGZO on hydrogen-rich Al2O3 films, thus generating subgap states in the band gap of IGZO. Further, color pattern imaging was achieved by employing an array of the color distinguishable devices, and flexibility was demonstrated by fabricating the TFTs onto polymer substrates. Moreover, it is also found that both the RT and 150 °C Al2O3 a-IGZO TFTs exhibit typical light-stimulated synaptic behaviors, including excitatory post-synaptic current and pair-pules facilitation, etc., and the memory time of the synaptic devices can be easily modulated by the degree of the interfacial hydrogen doping.

Investigation of Band Alignment for Hybrid 2D-MoS2/3D-ß-Ga2O3 Heterojunctions with Nitridation.

Hybrid heterojunctions based on two-dimensional (2D) and conventional three-dimensional (3D) materials provide a promising way toward nanoelectronic devices with engineered features. In this work, we investigated the band alignment of a mixed-dimensional heterojunction composed of transferred MoS2 on ß-Ga2O3([Formula: see text]01) with and without nitridation. The conduction and valence band offsets for unnitrided 2D-MoS2/3D-ß-Ga2O3 heterojunction were determined to be respectively 0.43 ± 0.1 and 2.87 ± 0.1 eV. For the nitrided heterojunction, the conduction and valence band offsets were deduced to 0.68 ± 0.1 and 2.62 ± 0.1 eV, respectively. The modified band alignment could result from the dipole formed by charge transfer across the heterojunction interface. The effect of nitridation on the band alignments between group III oxides and transition metal dichalcogenides will supply feasible technical routes for designing their heterojunction-based electronic and optoelectronic devices.

Voltage-Polarity Dependent Programming Behaviors of Amorphous In-Ga-Zn-O Thin-Film Transistor Memory with an Atomic-Layer-Deposited ZnO Charge Trapping Layer.

Amorphous In-Ga-Zn-O (a-IGZO) thin-film transistor (TFT) memories are attracting many interests for future system-on-panel applications; however, they usually exhibit a poor erasing efficiency. In this article, we investigate voltage-polarity-dependent programming behaviors of an a-IGZO TFT memory with an atomic-layer-deposited ZnO charge trapping layer (CTL). The pristine devices demonstrate electrically programmable characteristics not only under positive gate biases but also under negative gate biases. In particular, the latter can generate a much higher programming efficiency than the former. Upon applying a gate bias pulse of +13 V/1 µs, the device shows a threshold voltage shift (ΔVth) of 2 V; and the ΔVth is as large as -6.5 V for a gate bias pulse of -13 V/1 µs. In the case of 12 V/1 ms programming (P) and -12 V/10 µs erasing (E), a memory window as large as 7.2 V can be achieved at 103 of P/E cycles. By comparing the ZnO CTLs annealed in O2 or N2 with the as-deposited one, it is concluded that the oxygen vacancy (VO)-related defects dominate the bipolar programming characteristics of the TFT memory devices. For programming at positive gate voltage, electrons are injected from the IGZO channel into the ZnO layer and preferentially trapped at deep levels of singly ionized oxygen vacancy (VO +) and doubly ionized oxygen vacancy (VO 2+). Regarding programming at negative gate voltage, electrons are de-trapped easily from neutral oxygen vacancies because of shallow donors and tunnel back to the channel. This thus leads to highly efficient erasing by the formation of additional ionized oxygen vacancies with positive charges.

Investigation of energy band at atomic layer deposited AZO/ß-Ga2O3 ([Formula: see text]) heterojunctions.

The Al-doped effects on the band offsets of ZnO/ß-Ga2O3 interfaces are characterized by X-ray photoelectron spectroscopy and calculated by first-principle simulations. The conduction band offsets vary from 1.39 to 1.67 eV, the valence band offsets reduce from 0.06 to - 0.42 eV, exhibiting an almost linear dependence with respect to the Al doping ratio varying from 0 to 10%. Consequently, a type-I band alignment forms at the interface of ZnO/ß-Ga2O3 heterojunction and the AZO/ß-Ga2O3 interface has a type-II band alignment. This is because incorporating Al into the ZnO would open up the band gaps due to the strong Al and O electron mixing, and the conduction and valence band edges consequently shift toward the lower level.

Investigation of Nitridation on the Band Alignment at MoS2/HfO2 Interfaces.

The effect of nitridation treatment on the band alignment between few-layer MoS2 and HfO2 has been investigated by X-ray photoelectron spectroscopy. The valence (conduction) band offsets of MoS2/HfO2 with and without nitridation treatment were determined to be 2.09 ± 0.1 (2.41 ± 0.1) and 2.34 ± 0.1 (2.16 ± 0.1) eV, respectively. The tunable band alignment could be attributed to the Mo-N bonding formation and surface band bending for HfO2 triggered by nitridation. This study on the energy band engineering of MoS2/HfO2 heterojunctions may also be extended to other high-k dielectrics for integrating with two-dimensional materials to design and optimize their electronic devices.