Hole Depletion Research Articles

Improving hole injection from p-EBL down to the end of active region by simply playing with polarization effect for AlGaN based DUV light-emitting diodes

In this work, we simply take advantage of the polarization effect to efficiently improve the hole injection from the p-type electron blocking layer (p-EBL) to the end of the active region for AlGaN based deep ultraviolet light emitting diodes (DUV LEDs). By properly increasing the AlN composition of AlGaN quantum barriers, a smaller positive polarized charge density at the last quantum barrier/p-EBL interface can be obtained, which correspondingly leads to the suppressed hole depletion and the reduced hole blocking effect in the p-EBL. Meanwhile, we properly increase the quantum well thickness so that the polarized electric field can even more accelerate the holes, and this will homogenize the hole distribution more across the MQWs. Therefore, the external quantum efficiency for DUV LEDs can be enhanced.

Charge storage impact on input capacitance in p-GaN gate AlGaN/GaN power high-electron-mobility transistors

In this paper, input capacitance (CISS) of p-GaN gate AlGaN/GaN power high-electron-mobility transistors (HEMTs) is systemically investigated. CISS includes gate-to-source capacitance (CGS) and gate-to-drain capacitance (CGD). In comparison with the normally-on HEMTs, it is found that the phenomenon of CISS variation is different in the commercial p-GaN gate HEMTs. The unique charge storage effect in the typical p-GaN layer is adopted and discussed to explain the variation of CISS and establish the underlying mechanism. Owing to the depletion of holes, net negative charges are induced in the p-GaN layer under an off-state drain bias. It is demonstrated that the negative charge storage makes significant contribution to the increase of CGS before the two-dimensional-electron-gas channel under source-field-plate (SFP) pinches off. Due to the clamped electric field distributions at drain-side edge of the p-GaN layer, the charge storage stops changing CGS after the SFP pinche-off. Additionally, the storage has a minor influence on the variation of CGD. Verified by the experimentally calibrated TCAD simulation, this work reveals a novel mechanism of charge storage impact on CISS variation in p-GaN gate AlGaN/GaN power HEMTs, which is of benefit to the CISS related capacitance design and gate driver optimization of the devices.

Construction of porous LaFeO3 microspheres decorated with NiO nanosheets for high response ethanol gas sensors

Designing novel structural constructions and adopting appropriate catalyst materials are the key to overcoming inherent limitations of metal oxide gas sensors. In this paper, porous LaFeO3 microspheres decorated with NiO nanosheets were prepared by simple hydrothermal method, and the p-p heterojunction was successfully constructed. Characterization results showed that the NiO nanosheets were uniformly grown on the surface of the porous LaFeO3 microspheres and the size of nanosheets was very uniform. This kind of morphology effectively improves the contact area between metal oxides and the target gases. Gas-sensing tests revealed that the combination of porous LaFeO3 microspheres and NiO nanosheets endows superior ethanol sensing performance with high response, good selectivity and rapid responding and recovering speed. The well-defined NiO@LaFeO3 nanocomposites offered unique hierarchical structure and p-p junctions at the heterointerfaces, which could induce the large hole depletion regions and hence benefited for oxygen adsorption and gas-sensing reactions at the interfaces. This work not only demonstrates the NiO@LaFeO3 nanocomposite is a promising material for fabricating ethanol sensors but also provides a useful guidance for design well-defined core@shell p-p heterojucntions in gas-sensing applications.

Photoelectric-Responsive Extracellular Matrix for Bone Engineering.

Using noninvasive stimulation of cells to control cell fate and improve bone regeneration by optical stimulation can achieve the aim of precisely orchestrating biological activities. In this study, we create a fast and repeatable photoelectric-responsive microenvironment around an implant using a bismuth sulfide/hydroxyapatite (BS/HAp) film. The unexpected increase of photocurrent on the BS/HAp film under near-infrared (NIR) light is mainly due to the depletion of holes through PO43- from HAp and interfacial charge transfer by HAp compared with BS. The electrons activate the Na+ channel of mesenchymal stem cells (MSCs) and change the cell adhesion in the intermediate environment. The behavior of MSCs is tuned by changing the photoelectronic microenvironment. RNA sequencing reveals that when photoelectrons transfer to the cell membrane, sodium ions flux and the membrane potential depolarizes to change the cell shape. Meanwhile, calcium ions fluxed and FDE1 was upregulated. Furthermore, the TCF/LEF in the cell nucleus began transcription to regulate the downstream genes involved in osteogenic differentiation, which is performed through the Wnt/Ca2+ signaling pathway. This research has created a biological therapeutic strategy, which can achieve in vitro remotely, precisely, and noninvasively controlling cell differentiation behaviors by tuning the in vivo photoelectric microenvironment using NIR light.

Visible and near-infrared photodetector on chemically vapor deposited diamond

A photodetector of the visible and near infrared wavelength ranges based on chemically vapor deposited (CVD) boron-doped diamond is fabricated and tested. The principle of its operation is based on the creation of holes by incident radiation not in a CVD diamond film itself, but in an adjoining bimetal Cr/Au layer. The latter serves simultaneously as one of the Schottky contacts with an external circuit. Due to small thickness of this layer in the order of 10 nm the generated holes do not have enough time to recombine with electrons because of diffusion in the CVD diamond film. There the holes are accelerated by the electric field of a hole depletion region and create a photocurrent. For optimized boron doping profile of the CVD diamond film the photodetector Volt-Watt sensitivity in the order of 1 mV/(W/cm2) at wavelength 445 nm and 0.1 mV/(W/cm2) at 532 nm was measured. For a constant forward bias voltage 2.5 V its Ampere-Watt sensitivity at 445 nm is 0.7 A/W. For zero bias voltage this figure is 5 ⋅ 10−5 A/W at 445 nm, 3 ⋅ 10−6 A/W at 532 nm, and 1.8 ⋅ 10−7 A/W at 1.06 μm. The photodetector time response was measured in the current mode at 532 nm. It is in the order of 600 ns without a bias voltage and 200 ns with a constant reverse bias voltage 2.5 V.

On the origin of enhanced hole injection for AlGaN-based deep ultraviolet light-emitting diodes with AlN insertion layer in p-electron blocking layer.

For the [0001] oriented AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs), the holes in the p-type electron blocking layer (p-EBL) are depleted due to the polarization induced positive sheet charges at the last quantum barrier (LQB)/p-EBL interface. The hole depletion effect significantly reduces the hole injection capability across the p-EBL. In this work, we propose inserting a thin AlN layer between the LQB and the p-EBL, which can generate the hole accumulation at the AlN/p-EBL interface. Meanwhile, the holes can obtain the energy when traveling from the p-EBL into the multiple quantum wells (MQWs) by intraband tunneling through the thin AlN layer. As a result, the hole injection and the external quantum efficiency (EQE) have been remarkably enhanced. Moreover, we point out that the thick AlN insertion layer can further generate the hole accumulation in the p-EBL and increase the hole energy which helps to increase the hole injection. We also prove that the intraband tunneling for holes across the thick AlN insertion layer is facilitated by using the optimized structure.

Effects of Nb Doping on the Photocatalytic Performance of Rutile TiO2 Single Crystals

We characterized the photocatalytic performance of rutile TiO2 single crystals with Nb doping (0.01–0.05 wt%) for water splitting and then observed the effects of doping. Although Nb doping induces additional optical absorption below the bandgap energy, it does not contribute to photocatalytic performance. Moreover, Nb doping enhances carrier recombination and donor concentration, resulting in reduction of the hole diffusion length and depletion of the layer width. Furthermore, it reduces the existing holes generated by light for photocatalysis because of the difficulty of charge separation by an internal electric field. In addition, comparison of the theoretically calculated photocurrents indicates that high Nb doping has other negative effects on the photocatalytic performance. These results suggest that, although Nb doping enhances the electrical conduction by increasing the donor concentration, it has negative effects on other important characteristics for photocatalysis. Therefore, Nb doping has optimum concentrations depending on structures of the photoelectrode.

On the polarization effect of the p-EBL/p-AlGaN/p-GaN structure for AlGaN-based deep-ultraviolet light-emitting diodes

The external quantum efficiency (EQE) and light output power (LOP) for III-nitride based deep ultraviolet light-emitting diodes (DUV LEDs) are strongly influenced by the hole injection efficiency. This work shows that the hole concentration in the active region is strongly subject to the polarization level of the p-region that consists of the p-type electron blocking layer (p-EBL), the p-AlGaN and the p-GaN layers. The hole injection becomes poor once the p-EBL/p-AlGaN and p-AlGaN/p-GaN interfaces are of the [000–1] polarity. On the contrary, the improved hole injection can be obtained once the polarization level that is of the [0001] polarity for the p-EBL/p-AlGaN and p-AlGaN/p-GaN interfaces increases. The increased polarization level of the [0001] polarity can not only reduce the hole depletion within the p-AlGaN layer, increase the energy for the holes, but also reduce the valence band barrier height of the p-EBL for holes, which in turn facilitates the hole injection capability and enhances the EQE for DUV LEDs.

Threshold Voltage Instability in p-GaN Gate AlGaN/GaN HFETs

We investigate the impact of the gate contact on the threshold voltage stability in p-GaN gate AlGaN/GaN heterojunction field-effect transistors with double pulse measurements on the p-GaN gate devices and device simulations. We find that, under gate stress, in the case of high-leakage Schottky contact, a negative threshold voltage shift results from hole accumulation in the p-GaN region. Conversely, in the case of low-leakage Schottky contact, hole depletion in the p-GaN region gives rise to a positive threshold voltage shift. More generally, we show that an imbalance between the hole tunneling current through the Schottky barrier and the thermionic current across the AlGaN barrier results in a variation of the total charge stored in the p-GaN region, which in turn is responsible for the observed threshold voltage shift. Finally, we present a simplified equivalent circuit model for the p-GaN gate module.

Improved Sensitivity of Inkjet-Printed PEDOT:PSS Ammonia Sensor With “Nonideal” Morphology

Piezoelectric inkjet printing is used for fabricating poly (3,4-ethylene-dioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) ammonia (NH3) sensors; it has the advantages of drop-on-demand maskless manufacturing, great material saving, and compatibility with large area flexible substrates. Different from previous research efforts on forming line structures of smooth surface and even edge, this article shows that the inkjet-printed PEDOT:PSS line structures with nonideal morphology of scalloped edges can bring benefits of improved sensitivity. By simply controlling the drop spacing, narrower and thinner local regions are formed between adjacent ink drops in the fabricated line structures. Adsorption of NH3 gas molecules onto any of these regions, causing hole depletion, can significantly affect the carrier transport, thus improving sensitivity. Inkjet-printed PEDOT:PSS sensors present obviously improved response compared to the spin-coated ones. Compared to that of devices with smooth and straight lines, the responsivity of the devices at a drop spacing of 40m is improved by nearly threefold.

High-Efficiency Direct Ammonia Fuel Cells Based on BaZr0.1Ce0.7Y0.2O3- δ /Pd Oxide-Metal Junctions.

A direct ammonia‐type intermediate temperature fuel cell is examined by means of a hydrogen membrane fuel cell (HMFC) comprising 1‐µm‐thick BaZr0.1Ce0.7Y0.2O3− δ (BZCY) thin‐film electrolyte and Pd solid anode. It generates the maximum power density of 0.58 W cm−2 at 600 °C with ammonia fuels, and this value is found to be three times larger than the champion data of the recently reported direct ammonia‐type proton‐conducting ceramic fuel cells (PCFCs). AC impedance spectroscopy is performed to determine the interfacial polarization resistances, disclosing that the anodic overpotentials of HMFCs are at least one order of magnitude smaller than those of anode‐supported PCFC under relatively high DC outputs. The anode reactions are driven by the oxidation of monoatomic hydrogen dissolving at the BZCY/Pd solid–solid interface, mediated via proton transfer from Pd to BZCY. The electrochemical analysis reveals that the BZCY/Pd junction forms Ohmic contact without growth of wide depletion layer and thus facilitates the proton transfer reactions because the interfacial region beneath Pd electrode can accommodate amounts of protonic defects as well as the bulk of BZCY due to the small depletion of holes under hole–proton thermodynamic equilibrium.

UVA light-emitting diode grown on Si substrate with enhanced electron and hole injections.

In this work, III-nitride based ∼370 nm UVA light-emitting diodes (LEDs) grown on Si substrates are demonstrated. We also reveal the impact of the AlN composition in the AlGaN quantum barrier on the carrier injection for the studied LEDs. We find that, by properly increasing the AlN composition, both the electron and hole concentrations in the multiple quantum wells (MQWs) are enhanced. We attribute the increased electron concentration to the better electron confinement within the MQW region when increasing the AlN composition for the AlGaN barrier. The improved hole concentration in the MQW region is ascribed to the reduced hole blocking effect by the p-type electron blocking layer (p-EBL). This is enabled by the reduced density of the polarization-induced positive charges at the AlGaN last quantum barrier (LB)/p-EBL interface, which correspondingly suppresses the hole depletion at the AlGaN LB/p-EBL interface and decreases the valence band barrier height for the p-EBL. As a result, the optical power is improved.

Self-Poling-Induced Magnetoelectric Effect in Highly Strained Epitaxial BiFeO3/La0.67Sr0.33MnO3-δ Multiferroic Heterostructures.

Highly strained epitaxial BiFeO3/La0.67Sr0.33MnO3-δ (BFO/LSMO) heterostructures were fabricated on LaAlO3 substrates by magnetron sputtering. The as-grown downward self-polarization of BFO capping layers was confirmed by piezoelectric force microscopy. Using the electrostatic field-induced charge screening, a hole depletion state was induced in ultrathin (8 nm) LSMO films. As a result of the interfacial charge coupling, appreciable saturated magnetization (MS) increase of about 500 and 100% can be observed in LSMO with BFO capping at 5 and 300 K, respectively. Besides, LSMO phase translations can be revealed by the BFO thickness-related exchange bias field (HE) and MS of the BFO/LSMO heterostructures. The results established a new approach in achieving interfacial magnetoelectric couplings with thin self-polarized multiferroic layers.

On the electric-field reservoir for III-nitride based deep ultraviolet light-emitting diodes.

The drift velocity for holes is strongly influenced by the electric field in the p-type hole injection layer for III-nitride based deep ultraviolet light-emitting diodes (DUV LEDs). In this work, we propose an electric-field reservoir (EFR) consisting of a p-AlxGa1-xN/p-GaN architecture to facilitate the hole injection and improve the internal quantum efficiency (IQE). The p-AlxGa1-xN layer in the EFR can well reserve the electric field that can moderately adjust the drift velocity and the kinetic energy for holes. As a result, we are able to enhance the thermionic emission for holes to cross over the p-EBL with a high Al composition provided that the composition in the p-AlxGa1-xN layer is properly optimized to avoid a complete hole depletion therein.

Impact of biogenic very short-lived bromine on the Antarctic ozone hole during the 21st century

Abstract. Active bromine released from the photochemical decomposition of biogenic very short-lived bromocarbons (VSLBr) enhances stratospheric ozone depletion. Based on a dual set of 1960–2100 coupled chemistry–climate simulations (i.e. with and without VSLBr), we show that the maximum Antarctic ozone hole depletion increases by up to 14 % when natural VSLBr are considered, which is in better agreement with ozone observations. The impact of the additional 5 pptv VSLBr on Antarctic ozone is most evident in the periphery of the ozone hole, producing an expansion of the ozone hole area of ∼ 5 million km2, which is equivalent in magnitude to the recently estimated Antarctic ozone healing due to the implementation of the Montreal Protocol. We find that the inclusion of VSLBr in CAM-Chem (Community Atmosphere Model with Chemistry, version 4.0) does not introduce a significant delay of the modelled ozone return date to 1980 October levels, but instead affects the depth and duration of the simulated ozone hole. Our analysis further shows that total bromine-catalysed ozone destruction in the lower stratosphere surpasses that of chlorine by the year 2070 and indicates that natural VSLBr chemistry would dominate Antarctic ozone seasonality before the end of the 21st century. This work suggests a large influence of biogenic bromine on the future Antarctic ozone layer.

Ralph J. Cicerone

Ralph J. Cicerone

Spin-splitting in p-type Ge devices

Compressively strained Ge quantum well devices have a spin-splitting in applied magnetic field that is entirely consistent with a Zeeman effect in the heavy hole valence band. The spin orientation is determined by the biaxial strain in the quantum well with the relaxed SiGe buffer layers and is quantized in the growth direction perpendicular to the conducting channel. The measured spin-splitting in the resistivity ρxx agrees with the predictions of the Zeeman Hamiltonian where the Shubnikov-deHaas effect exhibits a loss of even filling factor minima in the resistivity ρxx with hole depletion from a gate field, increasing disorder or increasing temperature. There is no measurable Rashba spin-orbit coupling irrespective of the structural inversion asymmetry of the confining potential in low p-doped or undoped Ge quantum wells from a density of 6 × 1010 cm−2 in depletion mode to 1.7 × 1011 cm−2 in enhancement.

How Molecules with Dipole Moments Enhance the Selectivity of Electrodes in Organic Solar Cells – A Combined Experimental and Theoretical Approach

Simple organic molecules with permanent dipole moments – amino acids and heterocycles – have been successfully employed in bulk‐heterojunction organic solar cells as interlayer between photoactive material and electron contact. A large increase of open‐circuit voltage and fill factor can be observed for four different polymers as donor material in the photoactive layer. A combination of current–voltage curves, scanning Kelvin‐probe atomic force microscopy, ultraviolet photoelectron spectroscopy, and electroluminescence measurements as well as numerical simulations are carried out to clarify in detail the underlying mechanisms. All results fully confirm the hypothesis that the main effect is an accumulation of electrons and a depletion of holes in the photoactive layer in the vicinity of the electron contact induced by a decrease of its effective work function. Further, density functional theory calculations and literature reports of the energy levels of the dipole molecules strongly suggest that the charge carriers tunnel through the thin dipole layer which does however not limit the current. This represents a versatile, simple, and cheap method to realize highly selective contacts which may also be beneficial for other types of solar cells and devices where contact selectivity is crucial.

Gate control of ferromagnetic insulating phase in lightly-doped La0.875Sr0.125MnO3−δ film

The electric field effect on the lightly doped La0.875Sr0.125MnO3−δ (LSMO) thin film in electric double-layer transistors was investigated by measuring transport properties of the film under various gate voltages. It was found that the positive gate bias leads to an increase of the charge-orbital ordering (COO) transition temperature and a decrease of the Curie temperature TC, indicating the suppression of ferromagnetic metal (FMM) phases and preference of COO/ferromagnetic insulator (FMI) with the hole depletion by gate bias. Such different electric field effects can be ascribed to the weakening of the ferromagnetic interaction and enhancement of Jahn-Teller (JT) distortion caused by the transformation of JT inactive Mn4+-ions to JT active Mn3+-ions. Moreover, a step-like increase in the high temperature region of the ρ-T curve, which is related to the transition of cooperative JT distortion, was found to develop with increasing the positive bias, indicating that the cooperative JT distorted phase is stabilized by the depletion of holes in LSMO film. These results demonstrate that the modulation of holes via electric field strongly affects the balance between energy gains of different interactions and thus produce different effects on the competing FMI, FMM, and cooperative JT distorted phases in LSMO film.

On the Nature of the Memory Mechanism of Gated-Thyristor Dynamic-RAM Cells

With the help of numerical simulations, we revisit the operation of dynamic thin-capacitively-coupled-thyristor RAM (TRAM) and field effect diode-RAM cells and clarify the memory mechanism. The resulting carrier profiles demonstrate that the recently advanced interpretation of the physical memory (i.e., store) mechanism, as the accumulation (“1”) or depletion (“0”) of holes in the $p$ -base (under the gate), is incorrect. Instead, it turns out that it is the presence (“0”) or absence (“1”) of deeply depleted regions within the TRAM structure, associated with the two $p$ - $n$ junctions on the sides of the $p$ -base that determines the stored state of the cell.