Native Defects Research Articles

Enhancement of the near-band-edge electroluminescence from the active ZnO layer in the ZnO/GaN-based light emitting diodes using AlN-ZnO/ZnO/AlN-ZnO double heterojunction structure

In this work, an AlN-ZnO/ZnO/AlN-ZnO double heterojunction (DH) structure prepared using the cosputtering technology was deposited onto the p-type GaN epitaxial layer. The indiffusion of the oxygen atoms to the p-GaN epilayer was obstructed as the cosputtered AlN-ZnO film inset between n-ZnO/p-GaN interface. The near-ultraviolet (UV) emission from this ZnO/GaN-based light emitting diode (LED) was greatly improved as compared to an n-type ZnO film directly deposited onto the p-GaN epilayer. Meanwhile, the native defects in the n-ZnO layer associated with the green luminescence was less likely to form while it was sandwiched by the cosputtered AlN-ZnO film. As the thickness of the active n-ZnO layer in the DH structure reached 10 nm, the near-band-edge (NBE) emission became the predominated luminescence over the resulting LED spectrum.

Collision-Free Motion Planning of a Six-Link Manipulator Used in a Citrus Picking Robot

This paper presents the results of a motion planning algorithm that has been used in an intelligent citrus-picking robot consisting of a six-link manipulator. The real-time performance of a motion planning algorithm is urgently required by the picking robot. Within the artificial potential field (APF) method, the motion planning of the picking manipulator was basically solved. However, the real-time requirement of the picking robot had not been totally satisfied by APF because of some native defects, such as the large number of calculations used to map forces into torques by the Jacobian matrix, local minimum trap, and target not reachable problem, which greatly reduce motion planning efficiency and real-time performance of citrus-picking robots. To circumvent those problems, this paper proposed some novel methods that improved the mathematical models of APF and directly calculates the attractive torques in the joint space. By using the latter approach, the calculation time and the total joint error were separately reduced by 54.89% and 45.41% compared with APF. Finally, the novel algorithm is presented and demonstrated with some illustrative examples of the citrus picking robot, both offline during the design phase as well as online during a realistic picking test.

Intense cold-white emission due to native defects in Zn2GeO4 nanocrystals

Broad emission bands in semiconducting oxides due to native defects could readily be an efficient source of white light if they are adjusted properly. In this work, Zn2GeO4 nanoparticles of 10 and 16 nm sizes, obtained by a precipitation method, are presented as potential white light emitter. Temperature dependence measurements have been performed to discuss the Zn2GeO4 luminescence, which consist of emission bands in the ultraviolet, blue and green regions. The analysis of these results provided some clues about the interplay between defect-related centres and the surface states in nanocrystalline Zn2GeO4. Finally, a highly efficient cold-white emission at room temperature has been achieved from this nanomaterial.

Impact of defects on the structural and electrical transport properties of Sb2Te3 thin films by SHI irradiation

Implantation of defects is an attractive trend for the modification of important properties of thin films for practical applications. In the present attempt, an effective approach for tuning the native defects in thin films is reported. The films were sequentially irradiated at five different fluences (5 × 1011, 1 × 1012, 3 × 1012, 5 × 1012, and 1 × 1013 ions/cm2). The key role of high-energy Ni ion irradiation-induced native defects was observed by the modifications in the physical properties of samples. However, irradiation induces a structural phase transition through strain. The average crystallite size as determined from X-ray diffraction (XRD) peak broadening was found to be 19.3 ± 5 (nm). A monotonic increase in the lattice strain was observed up to a fluence of 5 × 1012 ions/cm2. The resistivity v/s temperature measurements revealed an increase in resistivity up to the fluence 3 × 1012 ions/cm2 indicating an increase in grain boundaries effect. Our findings provide an insight into the correlation of tuned properties with the defects induced by irradiation and also a route to have the defects in a precise and controlled way.

Influence of current conduction paths and native defects on gas sensing properties of polar and non-polar GaN

The CO gas sensing characteristics of polar GaN (P–GaN) and non-polar GaN (NP–GaN) thin-films grown by RF–plasma assisted molecular beam epitaxy on c–Al2O3 and r–Al2O3 substrate is analyzed. The temperature-dependent (27–300 °C) current-voltage (I–V) measurements were performed for both (P–GaN & NP–GaN) Schottky barrier diodes having identical device dimensions with Au as the metal contact. The Schottky barrier height increases with the increment in temperature, while vice versa was perceived for the ideality factor and series resistance. The I–V characteristics dictated that the terminal current for P–GaN is 25 times that of NP-GaN, further corroborated by simulation results. The I-V curve fitting suggests the initial emission of charge carrier from a trapped state to a continuum of electronic state, following the Frenkel-Poole emission model for P-GaN. The NP–GaN demonstrated a higher surface-to-volume ratio and native (shallow and deep level) defects corresponding to VGa and ON as compared to the P–GaN. The sensing response obtained for the NP–GaN for 100 ppm CO gas at 300 °C is ~ 33%, which is about seven times the response in the case of P–GaN. The role of native defects and the potentiality of the fabricated NP–GaN over P–GaN films in providing a better sensing characteristic by reducing the current conduction paths are elaborated.

Elucidating the influence of native defects on electrical and optical properties in semiconducting oxides: An experimental and theoretical investigation

Defect engineering using self-doping or creating vacancies in polycrystalline oxide based materials has profound influence on optical absorption, UV photo detection, and electrical switching. However, defects induced semiconducting oxide devices show enhancement in photo detection and photosensitivity, and hence still remain interesting in the field of optoelectronics. In this study, defect engineering in semiconducting oxide materials is discussed along with their roles in optoelectronic device-based applications. Theoretical investigations have been done for identifying defect states by performing first-principles electronic structure calculations, employing the density-functional theory. Particularly, in this work we have focused on probing the defect-induced changes in optical and electrical processes by means of experimental as well as computational investigations. Hence, systematic experimental measurements of optical absorption, electrical switching, charge density, and photocurrent in defect-rich semiconducting oxide samples have been performed. Our results suggest that defects can lead to enhancement of optoelectronic properties such as photocurrent, switching speed, optical absorption etc. for semiconducting oxide based devices.

Doping limitation due to self-compensation by native defects in In-doped rocksalt Cd x Zn1−x O

Cadmium oxide (CdO)–ZnO alloys (Cd x Zn1−x O) exhibit a transformation from the wurtzite to the rocksalt (RS) phase at a CdO composition of ∼70% with a drastic change in the band gap and electrical properties. RS–Cd x Zn1−x O alloys (x > 0.7) are particularly interesting for transparent conductor applications due to their wide band gap and high electron mobility. In this work, we synthesized RS–Cd x Zn1−x O alloys doped with different concentrations of In dopants and evaluated their electrical and optical properties. Experimental results are analyzed in terms of the amphoteric native defect model and compared directly to defect formation energies obtained by hybrid density functional theory (DFT) calculations. A saturation in electron concentration of ∼7 × 1020 cm−3 accompanied by a rapid drop in electron mobility is observed for the RS–Cd x Zn1−x O films with 0.7 ⩽ x < 1 when the In dopant concentration [In] is larger than 3%. Hybrid DFT calculations confirm that the formation energy of metal vacancy acceptor defects is significantly lower in RS–Cd x Zn1−x O than in CdO, and hence limits the free carrier concentration. Mobility calculations reveal that due to the strong compensation by native defects, RS–Cd x Zn1−x O alloys exhibit a compensation ratio of >0.7 for films with x < 0.8. As a consequence of the compensation by native defects, in heavily doped RS–Cd x Zn1−x O carrier-induced band filling effect is limited. Furthermore, the much lower mobility of the RS–Cd x Zn1−x O alloys also results in a higher resistivity and reduced transmittance in the near infra-red region (λ > 1100 nm), making the material not suitable as transparent conductors for full spectrum photovoltaics.

Thermally Controlled Charge-Carrier Transitions in Disordered PbSbTe Chalcogenides.

Binary and ternary chalcogenides have recently attracted much attention due to their wide range of applications including phase-change memory materials, topological insulators, photonic switches, and thermoelectrics. These applications require a precise control of the number and mobility of charge carriers. Here, an unexpected charge-carrier transition in ternary compounds from the PbTe-Sb2 Te3 pseudo-binary line is reported. Upon thermal annealing, sputtered thin films of PbSb2 Te4 and Pb2 Sb2 Te5 undergo a transition in the temperature coefficient of resistance and in the type of the majority charge carriers from n-type to p-type. These transitions are observed upon increasing structural order within one crystallographic phase. To account for this striking observation, it is proposed that the Fermi energy shifts from the tail of the conduction band to the valence band because different levels of overall structural disorder lead to different predominant types of native point defects. This view is confirmed by an extensive computational study, revealing a transition from excess cations and SbPb for high levels of disorder to PbSb prevailing for low disorder. The findings will help fine-tune transport properties in certain chalcogenides via proper thermal treatment, with potential benefits for memories, thermoelectric material optimization, and neuromorphic devices.

Native Atomic Defects Manipulation for Enhancing the Electronic Transport Properties of Epitaxial SnTe Films.

P-type SnTe-based compounds have attracted extensive attention because of their high thermoelectric performance. Previous studies have made tremendous efforts to investigate native atomic defects in SnTe-based compounds, but there has been no direct experimental evidence so far. On the basis of MBE, STM, ARPES, DFT calculations, and transport measurements, this work directly visualizes the dominant native atomic defects and clarifies an alternative optimization mechanism of electronic transport properties via defect engineering in epitaxially grown SnTe (111) films. Our findings prove that positively charged Sn vacancies (VSn) and negatively charged Sn interstitials (Sni) are the leading native atomic defects that dominate electronic transport in SnTe, in contrast to previous studies that only considered VSn. Increasing the substrate temperature (Tsub) and decreasing the Te/Sn flux ratio during film growth reduces the density of VSn while increasing the density of Sni. A high Tsub results in a low hole density and high carrier mobility in SnTe films. The SnTe film grown at Tsub = 593 K and Te/Sn = 2/1 achieves its highest power factor of 1.73 mW m-1 K-2 at 673 K, which is attributed to the optimized hole density of 2.27 × 1020 cm-3 and the increased carrier mobility of 85.6 cm2 V-1 s-1. Our experimental studies on the manipulation of native atomic defects can contribute to an increased understanding of the electronic transport properties of SnTe-based compounds.

Photo-oxidation Dynamics in GaSe Flakes Probed through Temporal Evolution of Raman Spectroscopy

The effect of native defect density on structure evolution during oxidation has not been systematically studied for two-dimensional (2D) materials. Photo-oxidation provides a way to fasten oxidatio...

Carrier transport mechanism in bottom gate thin‐film transistor with SnO as active layer for CMOS displays

AbstractIn this work, we report on four tin monoxide (SnO) thin‐film transistor (TFT) grain boundary (GB) models of carrier transport considering the native defects in the thin film, interface traps, and GB deep/tail states. The changes in the activation energy and the GB barrier potential on the application of gate electric field are thoroughly investigated. The shift in Fermi level and the charge carrier transport mechanisms are examined for the two‐channel model by the application of external potential. Four models are developed to study the impact of phase transformation of SnO material on the TFT characteristics. Among the four developed models which are considered as four different cases, Case (iv) shows excellent performance and the simulation results revealed that the location of Fermi level closer to the mid gap are suggested to favor the ambipolar behavior. Also, the influence of SnO material thickness and the effect of different dielectrics on the ambipolar device characteristics are examined aiming at optimized performance of the device. The developed optimized model will help the process engineers in tuning the SnO material parameters for achieving better performance in both p‐type and n‐type TFTs when employed in CMOS based displays.

White light upconversion in NdOHCO3 to Nd2O3 nanocrystals: Structural and optical transition

In a previous report, some morphological, structural, and optical properties of NdOHCO3 to Nd2O3 transition were presented. This paper continues investigating this interesting inorganic nanomaterial, proposing the creation of the coordination complex [Nd(NH3)6]3+ ion during crystalline growth as a crucial step for the growth of NdOHCO3 nanocrystals. A concise experimental-theoretical model is detailed in order to understand the spherical-like crystalline growth observed. The orthorhombic and cubic phases for NdOHCO3 and Nd2O3 nanocrystal samples were identified from the X-Ray diffraction results. In addition, the average grain size was estimated for each sample: NdOHCO3 ~15.31–18.82 nm and Nd2O3 ~45.10–59.01 nm. Other optical parameters like absorbance, transmittance, and reflectance are examined. The Urbach energy (Eu) model allows us to investigate the degree of absorption. The Maxwell-Boltzmann statistical (MBS) model results fit well with the experimental photoluminescence (PL) spectroscopy data registered, implying that native intrinsic crystalline defects behave as free particles and the emission bands overlap 4f4s2→4f4s2 electronic intra-transitions. The PL spectra show two emission bands: Yellow ~3.10–1.55 eV and Red > 1.99 eV. Finally, the trap densities of NdOHCO3 (~1.08 × 1014 cm−3) and Nd2O3 (~9.64 × 1013 cm−3) were estimated.

The effect of SrFeO2.5 native point defects on its electrical properties: First principles investigations

Determining microscopic origin of electronic conductivity in strontium ferrites is a pivotal step toward accomplishing control of a reversible topotactic phase transformation. This functionality is essential for use of these oxides in resistive switching non-volatile memory devices and future neuromorphic computing. Here, we use first principles computations for in-depth study of native defects and their effect on electronic structure in SrFeO2.5. We elucidate the role native defects play and show that an uptake of oxygen in diluted concentrations leads to a p-type conductivity in the SrFeO2.5. We identify two acceptor defect transition states at 0.25 eV for atomic and 0.1 eV for molecular oxygen interstitials.

(Invited) Developments in Vertical GaN p-n Junction Structures Via p-Type Ion Implantation

Vertical GaN power devices have emerged to become promising candidates for next-generation high power applications due to superior material properties such as high breakdown voltage, low on-resistance, and high mobility compared to devices based on Si and SiC. Silicon is now used for most power switches, but silicon-based devices lose efficiency under high power demands. GaN-based p-n junction switching devices enable higher voltage power with significantly higher efficiencies with added advantages of reduced size and weight systems. A technological limitation of GaN, however, has been the inability to achieve high p-type doping in a planar, vertical device. Here, we will focus on recent developments to achieve high p-type efficiency though ion implantation, novel high temperature annealing schemes, and the importance of defects and morphology in native substrates and epitaxial layers with an emphasis on x-ray scattering and electron microscopy measurements.GaN epitaxial layers for subsequent p-type ion implantation can be grown on sapphire substrates but vertical devices are optimal when using GaN homoepitaxial structures. Unlike most other semiconductor substrates, GaN substrates are not grown from the melt. Hydride vapor phase epitaxy or ammonothermal growth are predominantly used to produce substrates and these substrates will experience higher temperatures during subsequent epitaxy and ion implant activation annealing than occurs during substrate formation. We have developed x-ray topography techniques and combined these with electron microscopy and x-ray diffraction to understand how strain and defects in the substrates and epitaxial layers evolve during subsequent high temperature processing steps.The activation of the implanted p-type dopants requires high temperature annealing. For most semiconductors, an annealing temperature of ~ 2/3 the melting point leads to a high activation fraction (> 95%) of the implanted species. The implantation process introduces large concentrations of native defects as well as the p-type implant (typically Mg), so it is necessary to remove the defects and for the dopants to locate on substitutional sites (Ga sites for Mg). We demonstrate how high resolution x-ray scattering and transmission electron microscopy are used to understand the defect-induced strain recovery process and to mitigate the formation of defects such as dislocation loops, stacking faults, and other non-equilibrium defect structures that cause a degradation in dopant activation efficiency.These recent developments, partly through the ARPA-E PNDIODES program as well as international efforts, have brought understanding of the key processing steps and substrate requirements to achieve high activation efficiency p-type doping for planar, vertical device structures in a scalable framework.

Prospects for n-type conductivity in cubic boron nitride

Cubic boron nitride is an ultrawide-bandgap semiconductor with potential applications in high-power electronics, ultraviolet optoelectronics, and quantum information science. Many of those applications are predicated on the ability to control doping. Using hybrid-functional first-principles calculations, we systematically explore potential donors including group-IV (C, Si, and Ge) and group-VI (O, S, and Se) elements, as well as Li and F. We also address the role of compensation either by substitution on the wrong site or due to native point defects. We identify SiB and ON as promising dopants, as they have the lowest formation energies and do not suffer from self-compensation. However, compensation by boron vacancies, which act as deep acceptors, poses a challenge. We discuss strategies to mitigate these effects.

Theoretical investigation of the effect of native defects on spin polarization and antiferromagnetic state in Co3O4

The effect of the native defects on the spin polarization and the antiferromagnetic state of Co3O4 was investigated using the first-principles calculations. The results reveal that the AFM ground state of Co3O4 is changed to a ferromagnetic state by introducing one OCo antisite defect at the A/B site, while O vacancy can induce the highest spin polarization and most stable AFM state. The interactions between two O vacancies with different distances are all AFM interactions and the strength of AFM interaction is strongest at the distance of 2.85 Å. The theoretical investigation suggests an effective route to improve the spin polarization and stability of AFM state by defect engineering, promoting the applications of Co3O4 in antiferromagnetic spintronics.

Optical and structural analysis of the charge transfer of Ce3++ e- →Ce4+ ion in the cerium oxide (CeO2)

Cerium oxide (CeO2(s)) is prepared by Chemical Bath Deposition (green chemistry) at ~23.0 °C and subsequent thermal annealing treated (TAT) at ~1000 °C. This manuscript continues with previous research examining the spectra situated at UV–Vis-region to investigate the 4fds→4fds electronic intra-transitions of CeO2. The in-plane and out-of-plane strains are located at εc ~1.1 × 10−3, ~− 3.0 × 10−4 and εa ~− 1.2 ×10−3, ~4.0 × 10−4, dislocation density were ~3.87×1014 lines/m2 and ~4.31×1016 lines/m2 for the As-grown and the CeO2-TAT nanocrystals, respectively. As a first approximation, we consider that the inorganic nanomaterial has intrinsic native crystalline defects. It is supposed that the strain is caused by native point defects (vacancies and interstices). According to the d(DO)/dE equation, absorption band associated at Ce3+ + e-→Ce4+, assigned to 5d→4f electronic intra-transitions (~2.48 eV) at Vis-region. Deconvolution of the observed absorbance spectra shows that the emissions bands originating from the F0 centers prevail over those of F+ centers of V0. For CeO2-TAT nanocrystals, the band gap energy was at ~289 nm (~4.29 eV), whereas the As-grown was found at ~304 nm (~4.07 eV). In As-grown nanocrystals, three optical signals can be seen ~277 nm (~4.47 eV), ~330 nm (~3.75 eV), and ~393 nm (~3.15 eV). The absorption band at ~3.15 eV is relatively narrow and assigned to Ce3+ + e-→Ce4+ charge transfer. Calculations of Urbach energy were Eu~1.54 eV for As-grown and Eu~0.52 eV for CeO2-TAT nanocrystals, respectively.

Graphitic carbon-encapsulated V2O5 nanocomposites as a superb photocatalyst for crystal violet degradation

To materialize the excellent photocatalyst for crystal violet dye-degradation, the graphitic carbon-encapsulated vanadium pentoxide (GC-V2O5) nanocomposites were synthesized through the simple sonication method by using the green tea waste-derived GC nanoflakes and the sonochemically synthesized V2O5 nanorods. The nanocomposites were confirmed to comprise an aggregated morphology, in which the orthorhombic V2O5 nanorods were well anchored with the intertwingled GC nanoflakes. Owing to the encapsulation of defective V2O5 by conductive GC, the GC-V2O5 nanocomposites exhibited the enhanced photocatalytic dye-degradation efficiency up to 98.4% within 105 min. Namely, the encapsulated GC nanosheets might compensate the native defects (i.e., charge traps) on the V2O5 surface; hence, the charge transport could be enhanced during the dye-degradation process while the photocarrier recombination could be suppressed. The results suggest the conducting layer-encapsulated semiconducting oxide nanocomposites (e.g., GC-V2O5) to be of good use for future green environmental technology, particularly, as a superb photocatalyst for dye degradation.

Reliability and failure analysis in power GaN-HEMTs during S-band pulsed-RF operating

This paper reports a reliability study on two technologies of AlGaN/GaN high-electron mobility transistors (AlGaN/GaN HEMTs) (Device “A” and Device “B”). A failure analysis study is conducted on devices stressed under real operating conditions for radar applications. The devices underwent pulsed-RF long ageing tests and after 11,000 h show a degradation in RF and DC performances (Drop of drain current and RF output power, pinch-off shift, decrease of the maximum of transconductance, lateral translation of transconductance, and increase of gate-lag and drain-lag). Hot electron effects are supposed to be the origin of the observed degradations and trapping phenomena within the passivation or GaN layers. Photon emission microscopy (PEM), Optical Beam Induced Resistance Change (OBIRCH), Electron Beam Induced Current (EBIC) measurements concur with this hypothesis. The three techniques reveal a non-uniform response and an inhomogeneous distribution along the gate fingers, in addition to the presence of some localized spots localized on the gate edge either on the drain side or on the source side. Spectral PEM analysis of these spots identifies a native defect that could be related to crystallographic defects such as dislocations or impurities. Atom probe tomography (APT) analysis on the two technologies of AlGaN/GaN HEMTs supports this hypothesis. APT results show the presence of some chemical impurities like carbon and oxygen. These impurities are in relatively significant concentrations in device “A” which could explain the high level of gate-lag and drain-lag in this device compared to device “B”.

Rippling Ferroic Phase Transition and Domain Switching In 2D Materials.

Ripples are a class of native structural defects widely existing in 2D materials. They originate from the out-of-plane flexibility of 2D materials introducing spatially evolving electronic structure and friction behavior. However, the effect of ripples on 2D ferroics has not been reported. Here a molecular dynamics study of the effect of ripples on the temperature-induced ferroic phase transition and stress-induced ferroic domain switching in ferroelastic-ferroelectric monolayer GeSe is presented. Ripples stabilize the short-range ferroic orders in the high-temperature phase with stronger ferroicity and longer lifetime, thereby increasing the transition temperature upon cooling. In addition, ripples significantly affect the domain switching upon loading, changing it from a highly correlated process into a ripple-driven localized one where ripples act as source of dynamical random stress. These results reveal the fundamental role of ripples on 2D ferroicity and provide theoretical guidance for ripple engineering of controlled phase transition and domain switching with potential applications in flexible 2D electronics.