Deep Band Research Articles

Radiative recombination properties of near-stoichiometric CuInSe2 thin films

The properties of electronic defects and their relation to structural defects are of high relevance for $\mathrm{CuInS}{\mathrm{e}}_{2}$ photovoltaic absorbers. Here, we use Raman scattering and steady-state photoluminescence to study the intrinsic optoelectronic properties of near-stoichiometric $\mathrm{CuInS}{\mathrm{e}}_{2}$ samples with a lateral composition gradient around the Cu saturation point. Apart from a well-known shallow defect band at 0.97 eV, we also observe a deep defect band at 0.8 eV, which is not discernable in photoluminescence spectra at lower temperatures. The preparation of a laterally graded sample with a very precise relative composition range by in situ process control allows for a measurement of a significant decrease of the photoluminescence emission yield at the Cu-poor/Cu-rich transition on a very narrow composition scale. Possible assignments of the bands to structural point defects are discussed.

Band tail state related photoluminescence and photoresponse of ZnMgO solid solution nanostructured films.

A series of Zn1−xMgxO thin films with the composition range x = 0.00–0.40 has been prepared by sol–gel spin coating on Si substrates with a post-deposition thermal treatment in the temperature range of 400–650 °C. The morphology of the films was investigated by scanning electron microscopy and atomic force microscopy while their light emission properties were studied by photoluminescence spectroscopy under excitation at 325 nm. It was found that annealing at 500 °C leads to the production of macroscopically homogeneous wurtzite phase films, while thermal treatment at higher or lower temperature results in the degradation of the morphology, or in the formation of ZnO particles embedded into the ZnMgO matrix, respectively. Local compositional fluctuations leading to the formation of deep band tails in the gap were deduced from photoluminescence spectra. A model for the band tail distribution in the bandgap is proposed as a function of the alloy composition. Thin films were also prepared by aerosol spray pyrolysis deposition using the same sol–gel precursors for the purpose of comparison. The prepared films were tested for photodetector applications.

Ultrathin conjugated polymer nanosheets as highly efficient photocatalyst for visible light driven oxygen activation

Visible light induced photochemical synthesis represents a promising approach towards green and sustainable organic transformations. However, this protocol is greatly limited because of overly dependence on the noble metal complex based photosensitizers. In this work, we firstly reported newly designed imide based conjugated polymer nanosheets as metal-free heterogeneous photocatalysts for efficient visible light driven oxygen activation. Upon integrating perylene core with triazine- and heptazine- blocks, ultrathin polyimide nanosheets with broad light response, deep valence band level, and excellent oxygen activation property were synthesized. The polymer photocatalysts exhibited superior photocatalytic activity for amine oxidation at room temperature under visible light irradiation. The conversion yield reached up to 99 % with nearly 100 % selectivity as N-benzyl benzaldimine was found the exclusive product. The conversion rates of 250 mmol g−1 h−1 was two orders of magnitude higher than that of representative polymer photocatalysts and metal oxide based semiconductors.

P‐Type Transparent Quadruple Perovskite Halide Conductors: Fact or Fiction?

AbstractHigh‐performance p‐type transparent conductors (TCs) can be used for a variety of optoelectronic applications, particularly transparent electronics with complementary metal–oxide–semiconductor circuits. Recently, several quadruple perovskite halides, such as Cs4CdSb2Cl12 and Cs4CdBi2Cl12, have been theoretically predicted to be excellent p‐type TCs that surpass all the conventional p‐type TCs and hence attracted a lot of attention. In this work, experiments and hybrid density functional theory (DFT) calculations are combined to evaluate the p‐type dopability in these quadruple perovskite halides. The experimental results reveal that these materials are optically transparent but electrically insulating, which contradicts the previous prediction. The hybrid DFT calculations show that the difficulty in forming p‐type doping in these compounds is because of the easy formation of the compensating n‐type intrinsic defects in the p‐type region, which is resulted from too deep valence band maximum levels. It is further demonstrated that the bandgap underestimation associated with the semilocal DFT and the unintentional chemical boundary overestimation should be the origins of the p‐type conductivity predicted previously. The results highlight the importance of reasonable bandgap description and chemical boundary determination in predicting defect thermodynamics.

Self-Induced Growth of GaN Nanowall Structure on Si (111) by Laser Molecular Beam Epitaxy.

Evolution of GaN nanostructure grown on Si (111) substrate has been studied systematically using laser molecular beam epitaxial process. The in-situ reflection high energy electron diffraction and ex-situ high resolution X-ray diffraction studies reveal that the GaN nanostructures have a hexagonal-wurtzite phase and grow along c-axis. The GaN morphology changes from compact granular layer to faceted pyramids to nanowall structure as a function of laser ablation frequency of the KrF excimer laser and radio frequency nitrogen plasma condition. It is observed that GaN nanowall structure is formed on Si (111) when grown under strong nitrogen-rich flux at a higher growth rate and growth temperature. The crystalline and optical quality of the GaN nanostructures significantly improved with increase of laser ablation frequency. The nanowall structure shows good optical emission properties with an intense, sharp near-band edge emission and a negligible deep band luminescence.

Characteristics and mechanism of all-optical switching based on one-dimensional periodic two-segment-connected tetrahedral optical waveguide network

In this work, we design all-optical switching based on a one-dimensional (1D) periodic two-segment-connected tetrahedron optical waveguide network. The network equation and generalized eigenfunction method are used for studying the switching characteristics and mechanism. We found that the all-optical switch designed using a tetrahedral optical waveguide network constructed by polystyrene had an ultrahigh switching efficiency of about 7 × 1017, an ultrafast switching time of about 34 fs, an ultralow threshold control energy of about 11 zJ, and the switching volume was about 0.05μm3. The tetrahedral primitive cells contain a high density of equilateral triangle primitives that are capable of generating a strong anti-resonance state, which leads to a deep and wide photonic band gap. Moreover, the integer ratio of the waveguide length of the tetrahedral optical waveguide network is broken, which generates an extremely narrow gap in the passband, and strong photonic localization is generated near the nonlinear material. This research presents a new method for the design of all-optical switching with higher performance indicators, provides the possibility for further practical use of all-optical switching, and also deepens people’s understanding of optical waveguide networks.

Groups-dependent phosphines as the organic redox for point defects elimination in hybrid perovskite solar cells

Abstract Lead (Pb)0 and iodine (I)0 point defects generated during perovskite solar cell (PSC) fabrication and photoconversion form deep band energy levels as the carriers’ recombination centers. These defects not only deteriorate device efficiency, but also facilitate chemical degradation with ion migration, resulting in restricted device lifetime. Herein, we present a novel type of phosphines as the point defects stabilizer for hybrid perovskite solar cells with enhanced performances. Three phosphines with varied side groups of tributyl, trioctyl and triphenyl are exampled as the dopants in perovskite films. The group dependent redox properties were observed in the perovskite film, dependent on their molecular weights and steric hinderances of phosphines. The partially oxidized tributyl phosphine (TBUP) with additional tributyl phosphine oxides (TBPO) is efficient in reduction of lead (Pb)0 and iodine (I)0 concentrations during the device fabrication and operation. The device with TBUP-TBPO pair showed enhanced power conversion efficiency (PCE) to 20.48% and maintain 91.7% of their initial PCEs after 500 h at 65 °C thermal annealing. Thus, this work presents an efficient route of utilize the phosphine species to reduce point defects in the perovskite film, which promoting further development of novel phosphorous additives with defects stabilization, interface passivation and encapsulation for low-cost solution processed PSCs.

Spin-polarized current in wide bandgap hexagonal boron nitrides containing 4|8 line defects

Line defects (LDs) which are commonly present in two-dimensional materials play an important role in spintronic applications for generating and transporting spin current. We theoretically predict of the electronic and magnetic properties of n-type doped monolayers of hexagonal boron nitride (h-BN) with 4|8 LDs. The LDs can create two deep narrow bands in the band gap of the h-BN sheet with an energy separation about 2.75 eV at Γ point. Interestingly, at a certain concentration of electrons introduced by n-type doping, a tantalizing spontaneous one-dimensional ferromagnetic ordering emerges due to the partially-occupied narrow conduction band. Moreover, it is found that this electron doping induced spin polarization (of charge current) is up to 90% in the bias voltage range from 0.1 V to 0.4 V. Our findings indicate that the h-BN monolayer can functionalize a semiconductor for spintronic devices applications, which is beyond a common sense of h-BN tunneling barrier.

Anisotropy and in-plane polarization of low-symmetrical β-Ga2O3 single crystal in the deep ultraviolet band

As a deep ultraviolet transparent conducting semiconductor, β-Ga2O3 has been actively studied for short wavelength optical and optoelectronic applications. This study investigated the anisotropy of optical properties, valence band structures, and the in-plane angle-resolved transmittance spectra of the three main crystal planes. The optical bandgaps of unintentionally doped β-Ga2O3 (100), (010), and (001) planes were 4.70, 4.55, and 4.70 eV, respectively. The valence band maximum of the (100), (010), and (001) planes obtained by VB-XPS were 3.4, 3.3, and 3.4 eV, respectively. A 0.05 eV larger surface bending band was found in the (100) and (001) planes than in the (010) plane. The transmitted intensity was established as a function of the in-plane polarization angles. The polarized optical bandgaps of E//a, E//b, and E//c were fit to 4.58, 4.73, and 4.48 eV, respectively, indicating that β-Ga2O3 can be used in polarization filtering and spectrally selective photodetectors in the UVC band, which was also demonstrated using two vertically stacked (100) single crystal wafers.

Study of point defects in wide-bandgap Cu2CdGeS4 microcrystals by temperature and laser power dependent photoluminescence spectroscopy

We present temperature and laser power dependent photoluminescence (PL) study of high quality wide-bandgap Cu2CdGeS4 microcrystals. At T = 10 K three PL bands were detected at about 1.919 eV (#1), 1.855 eV (#2) and 1.748 eV (#3). The temperature and laser power dependencies indicate that the properties of PL bands can be explained by donor- acceptor pair model, where the #1 and #2 bands result from a recombination between distant pairs involving the same shallow acceptor VCu with EA ≈ 30 meV and different deep donor defects. The #3 PL band originates from the deep donor-deep acceptor pair recombination where the depth of deep acceptor defect is more than 157 meV. Detailed analysis of the PL spectra show the absence of deep potential or band gap fluctuations in this material making it suitable for photovoltaic applications.

The physics of extreme rainfall event: An investigation with multisatellite observations and numerical simulations

An extreme rainfall event that made major calamity over the Indian peninsula region has been investigated with a focus on the physics of the cloud system as indicated from the multi-satellite observations, ground-based radar, and model forecast experiments. The vertical structure of clouds showed deep convective cores embedded in the stratiform and is dominated by warm and mixed-phase clouds. Model output indicates that moisture convergence was present in two episodes before both heavy rainfall events, and can be used as a precursor for the imminent heavy rainfall. It is the sustenance of high winds and availability of moisture content that contributed to the development of deep convective cloud bands which propagated inland, perpendicular to the coastline. These cloud systems produced deep convective cores with a deep supersaturated layer throughout the middle and upper atmosphere, introduced a significant amount of supercooled liquid water, which facilitated mixed-phase clouds. These cloud liquid drops at supercooled temperatures facilitated the production of more ice hydrometeors on freezing. The rapid growth of hydrometeors through riming and aggregation in the mixed-phase region lead to heavy and sustained precipitation. The sustenance of the system is also due to the enhanced latent heating over Western Ghats that supported low-level moisture convergence in a strongly sheared environment. Documented evidence from this study suggests that the heavy precipitation was a result of the convective cluster as illustrated through satellite observations and numerical simulations.

Promoting visible-light photocatalytic activities for carbon nitride based 0D/2D/2D hybrid system: Beyond the conventional 4-electron mechanism

Photocatalysis is regarded as one of promising technology for future clean and sustaniable energy applications. Herein, we have fabricated Au-modified reduced graphene oxide coupled with carbon nitride (Au/rGO/g-C3N4) as novel 0D/2D/2D photocatalytic nanocomposites. The optimized sample 2Au/0.6rGO/g-C3N4 exhibits exceptional visible-light activity for water splititng and CO2 reduction with quantum efficiency of 3.82 % and 1.98 %, respectively. Electrochemistry and ultraviolent photoemission are combined to determine the band alignments and elaborate associated water splitting path-way mechanism. It is validated that due to intrinsic deep valence band position of g-C3N4, the obtained nanocomposites exhibit unusual two-step two-electron way of water oxidization through intermediate H2O2 catalyzed by rGO addition. The exceptional photoactivities are attributed to dual functions of enhanced charge separation and two-electron water oxidization facilitated by rGO and surface plasmon effect of decorated Au. Our work provides illuminations for low cost and high efficiency photocatalytic water splitting and CO2 reduction applications.

Ionoluminescence and photoluminescence study of annealing and ion irradiation effects on zinc oxide

The effects of annealing at 473 K and 800 K and of 2 MeV H+ irradiation on the behaviour of defects in ZnO crystals were studied via ionoluminescence (IL) and photoluminescence (PL). The PL spectra of samples annealed in vacuum at 473 K and 800 K showed stronger visible emission and weaker UV emission than virgin samples. The stronger visible emission corresponded to an increase in the concentration of point defects in the samples. The PL spectra of the sample annealed in vacuum at 473 K revealed stronger visible emission than that of the sample annealed in vacuum at 800 K; hence, the concentration of point defects in the sample annealed at 473 K was higher. The IL spectrum of ZnO annealed in vacuum at 473 K showed a weaker intensity than the spectrum of the sample annealed at 800 K, which seemed inconsistent with the PL spectra results. This inconsistency was attributed to different mechanism of PL and IL. IL and PL in some way link the ZnO DBE to different point defect related electronic transfer processes. The PL spectra of samples showed that deep band emission (DBE) is a superposition of 2.1 eV band emission and 2.4 eV band emission and that near band emission (NBE) is a superposition of 3.1 eV band emission and 3.3 eV band emission. However, the DBE of IL is a superposition of 1.8 eV band emission and 2.1 eV band emission, and the NBE exhibited overall weak intensity caused by an increase in the number of defects induced during irradiation. Comparing the effects of annealing in an oxygen atmosphere and in vacuum, the enhanced peak area ratio of 2.1 eV band emission to 2.4 eV band emission in the PL spectra and the enhanced peak area ratio of 2.1 eV band emission to 1.8 eV band emission in the IL spectra of the sample annealed in an oxygen atmosphere demonstrate a correspondence between the ~1.8 eV band and Zn interstitials (Zni), between the ~2.1 eV band and O interstitials (Oi) and between the ~2.4 eV band and O vacancies (VO). The increase in the UV emission of the sample annealed in an oxygen atmosphere suggested that the NBE is a superposition of 3.1 eV band emission and 3.3 eV band emission, which are associated with Zn vacancies (VZn) and self-trapped excitons, respectively.

Research on Cs/O activation process of near-infrared In0.53Ga0.47As photocathodes

Abstract We report a systematic theoretical investigation on the activation mechanism of near-infrared In0.53Ga0.47As photocathodes. Adsorption energies, dipole moments, work functions, density of states, band structures and electron affinity of Cs/O co-adsorption on (001) surface of In0.53Ga0.47As photocathodes are investigated. First-principles calculation results indicate that the structural stability is greatly enhanced when oxygen atoms adsorb on the Cs-covered surface. The incorporation of oxygen atom helps lower the work function further to achieve the true NEA state. Moreover, two adatom-induced surface dipoles, namely [Csn+-In0.53Ga0.47Asn−] and [Cs+-O2--Cs+], are also adopted to explain the relationship between the adatoms and the substrate. The HOMO and LUMO levels can further move downward and the band bending region is enlarged after Cs/O co-activation. Meanwhile, new energy bands appear in the deep valence band due to the joint effect of Cs 5s, Cs 5p, O 2s and O 2p state electrons. Finally, Cs/O activation experiments are carried out and the photocurrent curves during activation procedure are recorded. The change of photocurrent is closely related to the work function variation, which affects the photoemission of the photocathodes. In order to clearly show the electron affinity, the variation of surface barrier height of InGaAs photocathodes with Cs/O adlayer is also given.

Soil and crop response to phosphorus and potassium management under conservation tillage

AbstractIn conservation tillage systems, proper fertilizer phosphorus (P) and potassium (K) management is critical to optimize yield and minimize negative environmental impacts. The objective of this study was to assess the responses of crop yield, nutrient use efficiencies, root growth, and soil test P and K levels to tillage, fertilizer placement, and fertilizer rates in a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation. Experiments were conducted for 8 yr in Illinois with tillage/fertilizer placement as the main plot (no‐till/broadcast [NTBC], strip‐till/broadcast [STBC], and strip‐till/deep band [STDB]) and PK fertilizer rates as the subplot. The NTBC treatment consistently reduced yields in corn by 6.2 and 4.5% and in soybean by 3.1 and 6.1% relative to STBC and STDB, respectively. Also, NTBC had greater root length density at in‐row and between‐row positions relative to STBC and STDB. Nutrient use efficiency indices (partial nutrient balance, agronomic efficiency, and partial factor productivity) declined with increasing P and K rates but were not affected by tillage/fertilizer placement treatments. Deep banding significantly reduced soil P and K concentrations in surface layers while increasing them at the depth of application. These results underscore the reported potential for deep banding combined with strip‐till to improve conditions for nutrient uptake while reducing the risk for nutrient losses to the environment. Critical soil test P and K levels (21 mg P kg−1 and 217 mg K kg−1) were similar for P and greater for K than the current university recommendations, which highlights the need for continuous refinement of soil fertility recommendations to keep pace with changes in production technologies and yield levels.

Uniform and Large‐Area Cesium‐Based Quasi‐2D Perovskite Light‐Emitting Diodes Using Hot‐Casting Method

AbstractMetal halide perovskites have deep valence band maximums (VBMs). For example, the VBM of CsPbBr3 is 5.8–6.3 eV. Conjugated polymers can be a potential candidate for the hole transport layer because of their deep highest occupied molecular orbital levels, but their poor compatibility with a hydrophilic perovskite precursor results in the formation of a noncontinuous perovskite film. In addition, antisolvent dripping methods for fabricating perovskite films cause spatially inhomogeneous nucleation, which is undesirable for large‐scale applications. In this work, efficient and large‐area perovskite light‐emitting diodes (PeLEDs) are developed by introducing a poly(9‐vinylcarbazole) (PVK) interlayer and employing a hot‐casting method (substrate preheating). The PVK interlayer increases the depth of the VBM of NiOx (from 5.1 to 5.5 eV), resulting in efficient hole injection. The thermal energy of the preheated substrate facilitates not only the growth of a continuous and pinhole‐free perovskite film, but also the formation of a highly crystalline and preferentially oriented perovskite structure, resulting in improved luminescence properties. Therefore, PeLEDs fabricated using an optimal preheating temperature show an improved external quantum efficiency (from 3.08% to 8.44%) with spatially uniform electroluminescence. Finally, the development of uniform and large‐area PeLEDs (with an active area of 12.8 cm2) is demonstrated.

Low-frequency lattice phonons in halide perovskites explain high defect tolerance toward electron-hole recombination.

Low-cost solution-based synthesis of metal halide perovskites (MHPs) invariably introduces defects in the system, which could form Shockley-Read-Hall (SRH) electron-hole recombination centers detrimental to solar conversion efficiency. Here, we investigate the nonradiative recombination processes due to native point defects in methylammonium lead halide (MAPbI3) perovskites using ab initio nonadiabatic molecular dynamics within surface-hopping framework. Regardless of whether the defects introduce a shallow or deep band state, we find that charge recombination in MAPbI3 is not enhanced, contrary to predictions from SRH theory. We demonstrate that this strong tolerance against defects, and hence the breakdown of SRH, arises because the photogenerated carriers are only coupled with low-frequency phonons and electron and hole states overlap weakly. Both factors appreciably decrease the nonadiabatic coupling. We argue that the soft nature of the inorganic lattice with small bulk modulus is key for defect tolerance, and hence, the findings are general to other MHPs.

Improvement of Electrical Performance by Neutron Irradiation Treatment on IGZO Thin Film Transistors

The effects of the neutron irradiation treatment on indium-gallium-zinc oxide (IGZO) are investigated as a function of the neutron irradiation time. With an increase in neutron irradiation time, the oxygen vacancies associated the oxygen deficient states increase, and both shallow and deep band edge states below the conduction band also increase. Moreover, the conduction band offset continuously decreases because of the increase in the oxygen vacancies with increasing the neutron irradiation time. In IGZO TFTs with the neutron irradiation time for 10 s, superior device performance demonstrates such as the lower threshold voltage, higher field effect mobility, smaller sub-threshold gate swing, larger on-off current ratio, and improved bias stability, comparing those of other IGZO TFTs.

Molecular beam epitaxial growth and optical characterization of AlGaN nanowires with reduced substrate temperature

The requirement of high growth temperature for high-quality epitaxial AlGaN, which is typically around 100 °C higher than the growth temperature of GaN, is unfavorable for p-type dopant (Mg) incorporation, representing a grand challenge for AlGaN deep ultraviolet (UV) light-emitting devices. In this context, we show high-quality AlGaN nanowires emitting in the deep UV band grown at merely the growth temperature of GaN nanowires by molecular beam epitaxy. This is enabled by the discovery of a narrow GaN nanowire template growth window. We have further compared the room-temperature internal quantum efficiency of the samples emitting around 255 nm grown in the low-temperature regime and high-temperature regime. It is found that the sample grown in the low-temperature regime can possess optical quality close to the sample grown in the high-temperature regime. This study, therefore, suggests that even with a low growth temperature, using nanowire structures can still lead to AlGaN alloys with a relatively high optical quality, and the use of low substrate temperature could be beneficial for p-type doping.

BCS-BEC Crossover and Pairing Fluctuations in a Two Band Superfluid/Superconductor: A T Matrix Approach

We investigate pairing fluctuation effects in a two band fermionic system, where a shallow band in the Bardeen–Cooper–Schrieffer–Bose–Einstein condensation (BCS-BEC) crossover regime is coupled with a weakly interacting deep band. Within a diagrammatic T matrix approach, we report how thermodynamic quantities such as the critical temperature, chemical potential, and momentum distributions undergo the crossover from the BCS to BEC regime by tuning the intraband coupling in the shallow band. We also generalize the definition of Tan’s contact to a two band system and report the two contacts for different pair-exchange couplings. The present results are compared with those obtained by the simpler Nozières–Schmitt–Rink approximation. We confirm a pronounced enhancement of the critical temperature due to the multiband configuration, as well as to the pair-exchange coupling.