In Press
In Press articles are edited and published online ahead of issue. When the final article is assigned to volumes/issues, the Article in Press version will be removed and the final version will appear in the associated published volumes/issues.
+ show detail
• ## Reducing the power consumption of two-dimensional logic transistors

, Available online

Abstract Full Text PDF

The growing demand for high-performance logic transistors has driven the exponential rise in chip integration, while the transistors have been rapidly scaling down to sub-10 nm. The increasing leakage current and subthreshold slope (SS) induced by short channel effect (SCE) result in extra heat dissipation during device operation. The performance of electronic devices based on two-dimensional semiconductors such as the transition metal dichalcogenides (TMDC) can significantly reduce power consumption, benefiting from atomically thin thickness. Here, we discuss the progress of dielectric integration of 2D metal–oxide–semiconductor field effect transistors (MOSFET) and 2D negative capacitance field effect transistors (NCFET), outlining their potential in low-power applications as a technological option beyond scaled logic switches. Above all, we show our perspective at 2D low-power logic transistors, including the ultra-thin equivalent oxide thickness (EOT), reducing density of interface trap, reliability, operation speed etc. of 2D MOSFET and NCFET.

• ## Applications of Huang–Rhys theory in semiconductor optical spectroscopy

, Available online

Abstract Full Text PDF

A brief review of Huang–Rhys theory and Albrechtos theory is provided, and their connection and applications are discussed. The former is a first order perturbative theory on optical transitions intended for applications such as absorption and emission involving localized defect or impurity centers, emphasizing lattice relaxation or mixing of vibrational states due to electron–phonon coupling. The coupling strength is described by the Huang–Rhys factor. The latter theory is a second order perturbative theory on optical transitions intended for Raman scattering, and can in-principle include electron–phonon coupling in both electronic states and vibrational states. These two theories can potentially be connected through the common effect of lattice relaxation – non-orthonormal vibrational states of associated with different electronic states. Because of this perceived connection, the latter theory is often used to explain resonant Raman scattering of LO phonons in bulk semiconductors and further used to describe the size dependence of electron–phonon coupling or Huang–Rhys factor in semiconductor nanostructures. Specifically, the A term in Albrechtos theory is often invoked to describe the multi-LO-phonon resonant Raman peaks in both bulk and nanostructured semiconductors in the literature, due to the misconception that a free-exciton could have a strong lattice relaxation. Without lattice relaxation, the A term will give rise to Rayleigh or elastic scattering. Lattice relaxation is only significant for highly localized defect or impurity states, and should be practically zero for either single particle states or exciton states in a bulk semiconductor or a semiconductor nanostructure that is not extremely small.

• ## Revisit of the band gaps of rutile SnO2 and TiO2: a first-principles study

, Available online

Abstract Full Text PDF

From the recent experimentally observed conduction band offset and previously reported band gaps, one may deduce that the valence band offset between rutile SnO2 and TiO2 is around 1 eV, with TiO2 having a higher valence band maximum. This implication sharply contradicts the fact that the two compounds have the same rutile structure and the Γ3+ VBM state is mostly an oxygen p state with a small amount of cation d character, thus one would expect that SnO2 and TiO2 should have small valence band offset. If the valence band offset between SnO2 and TiO2 is indeed small, one may question the correctness of the previously reported band gaps of SnO2 and TiO2. In this paper, using first-principles calculations with different levels of computational methods and functionals within the density functional theory, we reinvestigate the long-standing band gap problem for SnO2. Our analysis suggests that the fundamental band gap of SnO2 should be similar to that of TiO2, i.e., around 3.0 eV. This value is significantly smaller than the previously reported value of about 3.6 eV, which can be attributed as the optical band gap of this material. Similar to what has been found in In2O3, the discrepancy between the fundamental and optical gaps of SnO2 can be ascribed to the inversion symmetry of its crystal structure and the resultant dipole-forbidden transitions between its band edges. Our results are consistent with most of the optical and electrical measurements of the band gaps and band offset between SnO2 and TiO2, thus provide new understanding of the band structure and optical properties of SnO2. Experimental tests of our predictions are called for.

• ## Improvement of tunnel compensated quantum well infrared detector

, Available online

Abstract Full Text PDF

To reduce the difficulty of the epitaxy caused by multiple quantum well infrared photodetector (QWIP) with tunnel compensation structure, an improved structure is proposed. In the new structure, the superlattices are located between the tunnel junction and the barrier as the infrared absorption region, eliminating the effect of doping concentration on the well width in the original structure. Theoretical analysis and experimental verification of the new structure are carried out. The experimental sample is a two-cycle device, each cycle contains a tunnel junction, a superlattice infrared absorption region and a thick barrier. The photosurface of the detector is 200 × 200 μm2 and the light is optically coupled by 45° oblique incidence. The results show that the optimal operating voltage of the sample is –1.1 V, the dark current is 2.99 × 10–8 A, and the blackbody detectivity is 1.352 × 108 cm·Hz1/2·W–1 at 77 K. Our experiments show that the new structure can work normally.

• ## A gate-free MoS2 phototransistor assisted by ferroelectrics

, Available online

Abstract Full Text PDF

During the past decades, transition metal dichalcogenides (TMDs) have received special focus for their unique properties in photoelectric detection. As one important member of TMDs, MoS2 has been made into photodetector purely or combined with other materials, such as graphene, ionic liquid, and ferroelectric materials. Here, we report a gate-free MoS2 phototransistor combined with organic ferroelectric material poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)). In this device, the remnant polarization field in P(VDF-TrFE) is obtained from the piezoelectric force microscope (PFM) probe with a positive or negative bias, which can turn the dipoles from disorder to be the same direction. Then, the MoS2 channel can be maintained at an accumulated state with downward polarization field modulation and a depleted state with upward polarization field modulation. Moreover, the P(VDF-TrFE) segregates MoS2 from oxygen and water molecules around surroundings, which enables a cleaner surface state. As a photodetector, an ultra-low dark current of 10–11 A, on/off ration of more than 104 and a fast photoresponse time of 120 μs are achieved. This work provides a new method to make high-performance phototransistors assisted by the ferroelectric domain which can operate without a gate electrode and demonstrates great potential for ultra-low power consumption applications.

• ## Recent advances in lithographic fabrication of micro-/nanostructured polydimethylsiloxanes and their soft electronic applications

, Available online

Abstract Full Text PDF

The intensive development of micro-/nanotechnologies offers a new route to construct sophisticated architectures of emerging soft electronics. Among the many classes of stretchable materials, micro-/nanostructured poly(dimethylsiloxane) (PDMS) has emerged as a vital building block based on its merits of flexibility, stretchability, simple processing, and, more importantly, high degrees of freedom of incorporation with other functional materials, including metals and semiconductors. The artificially designed geometries play important roles in achieving the desired mechanical and electrical performances of devices and thus show great potential for applications in the fields of stretchable displays, sensors and actuators as well as in health-monitoring device platforms. Meanwhile, novel lithographic methods to produce stretchable platforms with superb reliability have recently attracted research interest. The aim of this review is to comprehensively summarize the progress regarding micro-/nanostructured PDMS and their promising soft electronic applications. This review is concluded with a brief outlook and further research directions.

• ## A compact two-dimensional analytical model of the electrical characteristics of a triple-material double-gate tunneling FET structure

, Available online

Abstract Full Text PDF

This paper presents a compact two-dimensional analytical device model of surface potential, in addition to electric field of triple-material double-gate (TMDG) tunnel FET. The TMDG TFET device model is developed using a parabolic approximation method in the channel depletion space and a boundary state of affairs across the drain and source. The TMDG TFET device is used to analyze the electrical performance of the TMDG structure in terms of changes in potential voltage, lateral and vertical electric field. Because the TMDG TFET has a simple compact structure, the surface potential is computationally efficient and, therefore, may be utilized to analyze and characterize the gate-controlled devices. Furthermore, using Kane's model, the current across the drain can be modeled. The graph results achieved from this device model are close to the data collected from the technology computer aided design (TCAD) simulation.

• ## Analytical model for the effects of the variation of ferrolectric material parameters on the minimum subthreshold swing in negative capacitance capacitor

, Available online

Abstract Full Text PDF

In this paper, we analytically study the relationship between the coercive field, remnant polarization and the thickness of a ferroelectric material, required for the minimum subthreshold swing in a negative capacitance capacitor. The interdependence of the ferroelectric material properties shown in this study is defined by the capacitance matching conditions in the subthreshold region in an NC capacitor. In this paper, we propose an analytical model to find the optimal ferroelectric thickness and channel doping to achieve a minimum subthreshold swing, due to a particular ferroelectric material. Our results have been validated against the numerical and experimental results already available in the literature. Furthermore, we obtain the minimum possible subthreshold swing for different ferroelectric materials used in the gate stack of an NC-FET in the context of a manufacturable semiconductor technology. Our results are presented in the form of a table, which shows the calculated channel doping, ferroelectric thickness and minimum subthreshold for five different ferroelectric materials.

• ## Hydride vapor phase epitaxy for gallium nitride substrate

, Available online

Abstract Full Text PDF

Due to the remarkable growth rate compared to another growth methods for gallium nitride (GaN) growth, hydride vapor phase epitaxy (HVPE) is now the only method for mass product GaN substrates. In this review, commercial HVPE systems and the GaN crystals grown by them are demonstrated. This article also illustrates some innovative attempts to develop homebuilt HVPE systems. Finally, the prospects for the further development of HVPE for GaN crystal growth in the future are also discussed.

• ## Giant modulation of magnetism in (Ga,Mn)As ultrathin films via electric field

, Available online

Abstract Full Text PDF

Taking the advantages of semiconducting properties and carrier-mediated ferromagnetism in (Ga,Mn)As, a giant modulation of magnetism via electric field in (Ga,Mn)As ultrathin film has been demonstrated. Specifically, huge interfacial electric field is obtained by using ionic liquid as the gate dielectric. Both magnetization and transport measurements are employed to characterize the samples, while the transport data are used to analyze the electric filed effect on magnetism. Complete demagnetization of (Ga,Mn)As film is then realized by thinning its thickness down to ~2 nm, during which the degradation of ferromagnetism of (Ga,Mn)As ultrathin film induced by quantum confinement effect is suppressed by inserting a heavily-doped p-type GaAs buffer layer. The variation of the Curie temperature is more than 100 K, which is nearly 5-times larger than previous results. Our results provide a new pathway on the efficient electrical control of magnetism.

• ## Broadband polarized photodetector based on p-BP/n-ReS2 herterojunction

, Available online

Abstract Full Text PDF

Two-dimensional (2D) atomic crystals, such as graphene, black phosphorus (BP) and transition metal dichalcogenides (TMDCs) are attractive for use in optoelectronic devices, due to their unique crystal structures and optical absorption properties. In this study, we fabricated BP/ReS2 van der Waals (vdWs) heterojunction devices. The devices realized broadband photoresponse from visible to near infrared (NIR) (400–1800 nm) with stable and repeatable photoswitch characteristics, and the photoresponsivity reached 1.8 mA/W at 1550 nm. In addition, the polarization sensitive detection in the visible to NIR spectrum (532–1750 nm) was demonstrated, and the photodetector showed a highly polarization sensitive photocurrent with an anisotropy ratio as high as 6.44 at 1064 nm. Our study shows that van der Waals heterojunction is an effective way to realize the broadband polarization sensitive photodetection, which is of great significance to the realization and application of multi-functional devices based on 2D vdWs heterostructures.

• ## Deep-ultraviolet integrated photonic and optoelectronic devices: A prospect of the hybridization of group III-nitrides, III-oxides, and two-dimensional materials

, Available online

Abstract Full Text PDF

Progress in the design and fabrication of ultraviolet and deep-ultraviolet group III–nitride optoelectronic devices, based on aluminum gallium nitride and boron nitride and their alloys, and the heterogeneous integration with two-dimensional and oxide-based materials is reviewed. We emphasize wide-bandgap nitride compound semiconductors (i.e., (B, Al, Ga)N) as the deep-ultraviolet materials of interest, and two-dimensional materials, namely graphene, two-dimensional boron nitride, and two-dimensional transition metal dichalcogenides, along with gallium oxide, as the hybrid integrated materials. We examine their crystallographic properties and elaborate on the challenges that hinder the realization of efficient and reliable ultraviolet and deep-ultraviolet devices. In this article we provide an overview of aluminum nitride, sapphire, and gallium oxide as platforms for deep-ultraviolet optoelectronic devices, in which we criticize the status of sapphire as a platform for efficient deep-ultraviolet devices and detail advancements in device growth and fabrication on aluminum nitride and gallium oxide substrates. A critical review of the current status of deep-ultraviolet light emission and detection materials and devices is provided.

• ## Study of the morphology evolution of AlN grown on nano-patterned sapphire substrate

, Available online

Abstract Full Text PDF

This study focused on the evolution of growth front about AlN growth on nano-patterned sapphire substrate by metal-organic chemical vapor deposition. The substrate with concave cones was fabricated by nano-imprint lithography and wet etching. Two samples with different epitaxy procedures were fabricated, manifesting as two-dimensional growth mode and three-dimensional growth mode, respectively. The results showed that growth temperature deeply influenced the growth modes and thus played a critical role in the coalescence of AlN. At a relatively high temperature, the AlN epilayer was progressively coalescence and the growth mode was two-dimensional. In this case, we found that the inclined semi-polar facets arising in the process of coalescence were \begin{document}$\left\{ {11\bar 21} \right\}$\end{document} type. But when decreasing the temperature, the \begin{document}$\left\{ {11\bar 22} \right\}$\end{document} semi-polar facets arose, leading to inverse pyramid morphology and obtaining the three-dimensional growth mode. The 3D inverse pyramid AlN structure could be used for realizing 3D semi-polar UV-LED or facet-controlled epitaxial lateral overgrowth of AlN.

• ## A contrivance of 277 nm DUV LD with B0.313Ga0.687N/B0.40Ga0.60N QWs and AlxGa1–xN heterojunction grown on AlN substrate

, Available online

Abstract Full Text PDF

In this paper, an ultraviolet C-band laser diode lasing at 277 nm composed of B0.313Ga0.687N/B0.40Ga0.60N QW/QB heterostructure on Mg and Si-doped AlxGa1–xN layers was designed, as well as a lowest reported substitutional accepter and donor concentration up to NA = 5.0 × 1017 cm–3 and ND = 9.0 × 1016 cm–3 for deep ultraviolet lasing was achieved. The structure was assumed to be grown over bulk AlN substrate and operate under a continuous wave at room temperature. Although there is an emphasizing of the suitability for using boron nitride wide band gap in the deep ultraviolet region, there is still a shortage of investigation about the ternary BGaN in aluminum-rich AlGaN alloys. Based on the simulation, an average local gain in quantum wells of 1946 cm–1, the maximum emitted power of 2.4 W, the threshold current of 500 mA, a slope efficiency of 1.91 W/A as well as an average DC resistance for the VI curve of (0.336 Ω) had been observed. Along with an investigation regarding different EBL, designs were included with tapered and inverse tapered structure. Therefore, it had been found a good agreement with the published results for tapered EBL design, with an overweighting for a proposed inverse tapered EBL design.

• ## Hot electron effects on the operation of potential well barrier diodes

, Available online

Abstract Full Text PDF

A study has just been carried out on hot electron effects in GaAs/Al0.3Ga0.7As potential well barrier (PWB) diodes using both Monte Carlo (MC) and drift-diffusion (DD) models of charge transport. We show the operation and behaviour of the diode in terms of electric field, mean electron velocity and potential, mean energy of electrons and Γ-valley population. The MC model predicts lower currents flowing through the diode due to back scattering at anode (collector) and carrier heating at higher bias. At a bias of 1.0 V, the current density obtained from experimental result, MC and DD simulation models are 1.35, 1.12 and 1.77 μA/μm2 respectively. The reduction in current over conventional model, is compensated to a certain extent because less charge settles in the potential well and so the barrier is slightly reduced. The DD model results in higher currents under the same bias and conditions. However, at very low bias specifically, up to 0.3 V without any carrier heating effects, the DD and MC models look pretty similar as experimental results. The significant differences observed in the I–V characteristics of the DD and MC models at higher biases confirm the importance of energy transport when considering these devices.

• ## Growth properties of gallium oxide on sapphire substrate by plasma-assisted pulsed laser deposition

, Available online

Abstract Full Text PDF

Gallium oxide was deposited on a c-plane sapphire substrate by oxygen plasma-assisted pulsed laser deposition (PLD). An oxygen radical was generated by an inductive coupled plasma source and the effect of radio frequency (RF) power on growth rate was investigated. A film grown with plasma assistance showed 2.7 times faster growth rate. X-ray diffraction and Raman spectroscopy analysis showed β-Ga2O3 films grown with plasma assistance at 500 °C. The roughness of the films decreased when the RF power of plasma treatment increased. Transmittance of these films was at least 80% and showed sharp absorption edge at 250 nm which was consistent with data previously reported.

• ## Optimization of erase time degradation in 65 nm NOR flash memory chips

, Available online

Abstract Full Text PDF

Reliability issues of flash memory are becoming increasingly significant with the shrinking of technology nodes. Among them, erase time degradation is an issue that draws the attention of academic and industry researchers. In this paper, causes of the " erase time degradation” are exhaustively analyzed, with proposals for its improvement presented, including a low stress program/erase scheme with a staircase pulse and disturb-immune array bias condition. Implementation of the optimized circuit structure is verified in a 128 Mb SPI NOR Flash memory chip, which is fabricated on a SMIC 65 nm ETOX process platform. Testing results indicate a degradation of the sector erase time from 10.67 to 104.9 ms after 105 program/erase cycles, which exhibits an improvement of approximately 100 ms over conventional schemes.