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This study used density functional theory and the quantum transport method to investigate the interfacial coupling and spin transport of transition metals (TMs) with a Fe, Co, and Ni/2H-VSe2 hybrid nanostructure. Because the indirect coupling of TM-Se-V led to an obvious reduction of the magnetic moment and the disappearance of the half-metal characteristics of 2H-VSe2, the expected spin-filtering effect of individual TMs and 2H-VSe2 deteriorated at the contact region. Nevertheless, all the TM/2H-VSe2-based dual-probe devices exhibited an interesting bias-dependent spin-injection efficiency with a maximum output spin-polarized current of 666 mA mm−1 in Co/2H-VSe2. The proposed TM/2H-VSe2-based spin-field-effect transistor demonstrated outstanding performance. The Ni/2H-VSe2-based transistor achieved a maximum output spin-polarized current of 3117 mA mm−1 and demonstrated a good switching characteristic of 106 mV dec−1. Importantly, all transistors achieved a widely tunable scale of spin extraction efficiency ranging consistently between 96% and −92% with gate bias. These results indicate a promising candidate for use in high-performance spintronic devices.

The effect of gamma ( $\gamma $ )-irradiation on AlGaN-based light-emitting diodes (LEDs) in the short-wavelength ultraviolet (UVC, 210–280 nm) spectral range under electrical stress is characterized by electroluminescence and current–voltage measurement. Different from previous reports that $\gamma $ -irradiation can hardly damage nitride devices, we observe that the optical power decreases and leakage current increases obviously after electrical stress under $\gamma $ -irradiation. To delve into the nature of degradation, variation of defects is studied using temperature-dependent low-frequency noise measurement. After stress, the ~0.78-eV defects attributed to N antisite are generated and lead to device degradation, which is accompanied by a reduction of the intrinsic Ga vacancy with an energy level of ~0.38 eV. The variation of defects after stress in $\gamma $ -irradiated environment is more evident than that in nonirradiated environment, which is well corresponding to the performance degradation behavior. In conclusion, $\gamma $ -irradiation is found to accelerate degradation induced by electrical stress, and the study can help improve irradiation resistance in AlGaN-based UVC LEDs.

The electrical stress induced variation of forward conduction mechanism in ultraviolet light emitting diodes in UV-A spectral range is usually attributed to the increase of trap density. The influence from trap energy level, which plays an equally significant role in the trap-assisted tunneling (TAT) theory, is nevertheless neglected in previous studies. In this work, the effect of electrical stress on the forward conduction mechanism of UV-A light emitting diodes has been studied using temperature-dependent current-voltage measurement and the variation of trap physical parameters has been monitored using deep level transient spectroscopy (DLTS) measurement. For the first time, it is demonstrated that, the rate of TAT increases after electrical stress, and has a much stronger correlation with trap energy level as compared to the trap density.

Two dimensional materials based van der waals (vdW) heterostructures have attracted much interest recent years, due to their outstanding performance in optoelectronic and electronic devices. In this work, Black phosphorus (BP)/ReS2 vdW heterostructure was proposed and systemically investigated by ab-initio methods. BP/ReS2 system overcomes the drawback of indirect band existed in most of previous reports, and naturally forms a robust direct band semiconductor with type II band offset. Moreover, the system simultaneously possesses interesting anisotropy for optical and transport properties. The anisotropically optical absorption window covers wide range from infrared to the green light, and current of armchair direction is more than 2 orders higher than that of zigzag direction. In addition, the band offset and gap value can be effectively manipulated by applying electric field and regulating layer number of BP. Our investigation suggests BP/ReS2 vdW heterostructure shows great potential in novel optoelectronic and electronic devices of high quantum efficiency.

This work provides an intensive investigation of defect evolution in the degradation process induced by electrical stress of AlGaN-based deep ultraviolet light emitting diodes. The reduced optical power and the increased leakage current are directly related to a new generated electron trap B with an energy level in the range of 0.25-0.38 eV, which is extracted from deep level transient spectroscopy (DLTS) measurement. The significantly increased "yellow" band peak in PL spectra and the linear relation between DLTS signal and pulse width indicate that, defect B corresponds to Ga vacancy along dislocation. The increase of Ga vacancy is accompanied with a decrease of hole trap A with energy level of 0.29-0.34 eV. Combining with first-principle calculation and experimental results, it is demonstrated that the generation of Ga vacancy is originated from the variation in Mg-related defect along dislocation.

The influence of constant current stress on the conduction mechanism of reverse leakage current in ultraviolet (UV) light emitting diodes (LEDs) in the UV-A spectral range has been investigated for the first time using temperature-dependent current-voltage measurement from 370 K to 55 K. Below 220 K, variable range hopping mechanism dominates in UV-A LEDs. While, above 220 K, the leakage current is attributed to Poole-Frenkel emission mechanism within the bias range of −3 ∼ −6 V. With the increasing of the reverse bias, the conduction mechanism transforms from Poole-Frenkel emission to space-charge-limited conduction mechanism. In particular, applying electrical stress yields an alteration of the transition voltage from −7.5 V to −6.5 V. We propose that stress could lead to a reduction of the thermal activation energy, and therefore alters the transition voltage.

The emerging mixed-dimensional heterostructures of transition metal dichalcogenides with wide bandgap semiconductors of piezoelectric characteristics have shown excellent performance with novel pressure-modulation in flexible devices. Meanwhile, the recent introduction of “twist” at the interface of van der Waals (vdW) systems brought much flexibility in the manipulation of electrical and optical properties, giving rise to new physics. In this work, twisted angles ranging from 0° to 150° were introduced to the interface of the newly MoS2/AlN(0001) vdW mixed-dimensional heterostructures. Although all systems form type-I band alignment, the interfacial coupling can be evidently modulated by the twisted angles, leading to twisted angle-dependent structural characteristics, electronic structure, and carrier transport with a period of 60°. The most intriguing is that a large manipulating scale of band offset and carrier transmission window (intensity) can be achieved by specifying the rotation, suggesting an effective route to design the electronic structure. It was further revealed that twisted angle modulated interfacial coupling gives rise to a diversity of resonant states’ distribution in the momentum space. Our study not only systemically investigated the newly MoS2/AlN(0001) vdW mixed-dimensional heterostructure, but also proposed a novel route to effectively manipulate the properties of mixed-dimensional heterostructure layer by layer.

Black phosphorus (BP)-based heterostructure with tunable band offset has been proven to be promising for rectifier diode and photoelectronic devices. However, it is usually not easy to find a suitable material to construct the heterojunction because the necessary type-II band structure and the strong unintentional p-type doping of BP should be both considered. Therefore, most of studies mainly focused on certain 2D materials, like MoS2 and WSe2. However, the low mobility of these materials greatly hinders the further promotion of device performance. For the first time, we demonstrate that HfS2, which has been proven to possess a much higher mobility of electrons and has been experimentally synthesized recently, fully satisfies conditions of heterostructure with BP. The heterojunction could be used as a tunable optoelectronic device and rectifier diode. With external normal electric field, the efficiency of photon-generated charge separation and rectification ratio could be manipulated. In addition, what i...

We investigated the influence of AlN interlayer with varied V/III ratio on the impurity incorporation of GaN films grown on sputtered AlN layer by metal organic chemical vapor deposition (MOCVD). It is found that the V/III ratio of AlN interlayer significantly influences on the growth mode and leads different impurity distribution and electron properties. In the case of AlN interlayer with an optimized V/III ratio, incorporation of oxygen impurity in GaN epitaxial films is suppressed. Leading to the GaN film shows a high sheet resistivity. However, at a lower V/III ratio, an unexpected three-dimensional growth mode plays dominant role. While at a higher V/III ratio, the GaN polarity would invert to N-polar. Both these two situations would deteriorate the electrical properties of GaN film.

Ab initio calculations are used to study the indirect doping effect in black phosphorus (BP) from defects in h-BN support layer. We find defects in substrate can strongly introduce doping to the BP layer by weak van der Waals effect. With considering various native vacancies in h-BN, we find that indirect n-type doping of BP only manifests under case of isolated nitrogen vacancy. P-type doping is presented in most cases including different divacancies, which is in consistent with published experimental reports. Besides, we find presence of BP upon h-BN can also alter the intrinsic magnetic properties of defective h-BN layer.

Ab initio calculations were performed to investigate the chemical oxidation of two-dimensional (2D) gallium nitride (GaN). The nitrogen surface was found to form a metastable configuration under oxygen adsorption, while the gallium surface could be readily transformed to a more stable configuration of HO-GaN-H with an exceptionally low energy barrier. The results also revealed that the adsorption of oxygen adatoms resulted in the reduction of work-function and induced the change from 2D GaN to a new GaNO compound. Our findings indicate that we should pay attention to the oxidation effect of 2D GaN in practical device applications.

A drain engineered InGaN heterostructure tunnel field effect transistor (TFET) is proposed and investigated by Silvaco Atlas simulation. This structure uses an additional metal on the drain region to modulate the energy band near the drain/channel interface in the drain regions, and increase the tunneling barrier for the flow of holes from the conduction band of the drain to the valence band of the channel region under negative gate bias for n-TFET, which induces the ambipolar current being reduced from 1.93 &times, 10&minus, 8 to 1.46 &times, 11 A/&mu, m. In addition, polar InGaN heterostructure TFET having a polarization effect can adjust the energy band structure and achieve steep interband tunneling. The average subthreshold swing of the polar drain engineered heterostructure TFET (DE-HTFET) is reduced by 53.3% compared to that of the nonpolar DE-HTFET. Furthermore, ION increases 100% from 137 mA/mm of nonpolar DE-HTFET to 274 mA/mm of polar DE-HTFET.

Defect behaviors in the degradation of AlGaN-based UV-C light emitting diodes (LEDs) under constant current stress have been intensively investigated in this work. It is found that both the reduction of the optical power and the increase in the leakage current are derived from the newly generated Ga vacancy (VGa) along dislocation, based on the evidence of a strong “yellow” emission peak at 515 nm in the photoluminescence spectra and an energy level of 0.25–0.38 eV. More importantly, the defect evolution behind it was determined through the deep level transient spectroscopy, secondary ion mass spectrometry measurements, and density functional theory. VGa is found to be generated by the departure of the unintentionally doped Mg from MgGa along dislocation in the Si-doped region. The high activity of the unintentionally doped Mg under electrical stress can be an essential factor in the degradation of UV-C LEDs. This study not only provides an in-depth insight into the electrical stress-induced degradation in UV-C LEDs but also sheds light on the way for fabricating AlGaN-based devices with high reliability.

Temperature dependences of the polarized Raman scattering spectra in the backscattering configuration of the semipolar (11 2¯2) plane GaN thin film are analyzed in the range from 83 K to 563 K. The semipolar GaN film is cut at a tilted angle from polar GaN wafer grown by hydride vapor phase epitaxy. The spectral features of the frequency shift and linewidths of the Raman-active phonon modes Quasi-TO, E1 (TO), E2 (high), and Quasi-LO are prominently revealed, and the temperature coefficients corresponding to the lattice thermal expansion and phonon anharmonic effect, as well as defects and impurities in crystals, are well deduced by the theoretical equations. With the increasing temperature, the Raman scattering peaks would substantially shift to lower frequencies and the linewidths gradually broaden. Our studies will lead to a better understanding of the fundamental physical characteristics of the semipolar (11 2¯2) plane GaN film.

The graphene–blue phosphorus van deer Waals (vDW) heterojunction was studied by using density functional theory. Our calculations reveal that the intrinsic electronic structure of blue phosphorus and graphene is well preserved and forms an n-type schottky barrier at equilibrium state. With increasing of normal tensile strain, the n-type is well kept. With compressive strain, the Dirac cone of graphene gradually shifts from conduction band minimum to valance band maximum of blue phosphorus, leading a turning of schottky barrier from n-type to p-type, which indicates an effective way to tune the electronic structure of vDW heterojunction.

The epitaxial layers of n-type GaN were grown on both planar and patterned sapphire substrate (PSS) by metal organic chemical vapor deposition. By comparing the epitaxial layers grown on planar substrate, GaN grown on PSS exhibited many improvements both on surface morphology and crystalline quality according to the characterization of atoms force microscopy, and high resolution X-ray diffraction. Spatially resolved micro-Raman scattering results were performed for mapping the spatial variations in crystalline quality of the n-type GaN grown on PSS. According to the variations on the intensity and the full width at half maximum of GaN E2 (high) peaks, the crystalline quality improvement occurred in the lateral growth regions which correspond to center region of the pyramid patterns. We proposed that the bending of dislocations during the lateral growth plays an important role in the spatial variations of GaN crystalline quality. Cross sectional transmission electron microscope and spatial cathodoluminescence mapping results further supported the explanation of the dislocation inhibition during the growth process of GaN grown on PSS.

The epitaxial layers of InGaN/GaN MQWs structure were grown on both planar and vicinal sapphire substrates by metal organic chemical vapor deposition. By comparing the epitaxial layers grown on planar substrate, the sample grown on 4° misoriented from c-plane toward < 10 1 ̄ 0 > m-plane substrate exhibited many variations both on surface morphology and optical properties according to the scanning electronic microscopy and cathodoluminescence (CL) spectroscopy results. Many huge steps were observed in the misoriented sample and a large amount of V-shape defects located around the boundary of the steps. Atoms force microscopy images show that the steps were inclined and deep grooves were formed at the boundary of the adjacent steps. Phase separation was observed in the CL spectra. CL mapping results also indicated that the deep grooves could effectively influence the localization of Indium atoms and form an In-rich region.