High Internal Electric Fields Research Articles

High conductivity coherently strained quantum well XHEMT heterostructures on AlN substrates with delta doping

Polarization-induced two-dimensional electron gases (2DEGs) in AlN/GaN/AlN quantum well high-electron-mobility transistors on ultrawide bandgap AlN substrates offer a promising route to advance microwave and power electronics with nitride semiconductors. The electron mobility in thin GaN quantum wells embedded in AlN is limited by high internal electric field and the presence of undesired polarization-induced two-dimensional hole gases (2DHGs). To enhance the electron mobility in such heterostructures on AlN, previous efforts have resorted to thick, relaxed GaN channels with dislocations. In this work, we introduce n-type compensation δ-doping in a coherently strained single-crystal (Xtal) AlN/GaN/AlN heterostructure to counter the 2DHG formation at the GaN/AlN interface, and simultaneously lower the internal electric field in the well. This approach yields a δ-doped XHEMT structure with a high 2DEG density of ∼3.2×1013 cm−2 and a room temperature (RT) mobility of ∼855 cm2/Vs, resulting in the lowest RT sheet resistance 226.7 Ω/□ reported to date in coherently strained AlN/GaN/AlN HEMT heterostructures on the AlN platform.

Strong interface interaction and internal electric field promote electron transfer of Bi2O2S/NiFe2O4 heterojunction for photocatalytic antibiotic degradation

Heterojunction photocatalysts have shown considerable activities for organic pollutants degradation. However, the faint connection interface and inferior charge shift efficiency critically block the property of heterojunction photocatalysis. Herein, Bi2O2S/NiFe2O4 nanosheets heterojunction with ultrastrong interface interaction and high internal electric field are designed by an in-situ growth method. Tentative and theoretical consequences prove that the interfacial interaction and internal electric field not only act as the electron flow bridge but also decrease the electrons shift energy obstacle, thus speeding up electrons transfer and achieving effective spatial electron-hole separation. Therefore, a large amount of ·O2– and holes as active species were generated. Remarkably, Bi2O2S/NiFe2O4 establishes a considerably boosted photocatalytic performance for tetracycline degradation (0.032 min–1), which is about 14.2-fold and 7.8-fold of the pristine BOS and NFO, respectively. This work provides a promising motivation for modulating charge transfer by interface control and internal electric field to boost photocatalytic performance.

Charging of Piezoelectric Cellular Polypropylene Film by Means of a Series Dielectric Layer.

Piezoelectric polymer cellular films have been developed and improved in the past decades. These piezoelectric materials are based on the polarization of the internal cells by means of induced discharges in the gas inside the cells. Internal discharges are driven by an external applied electric field. With this polarization method, cellular polypropylene (PP) polymers exhibit a high piezoelectric coefficient d33 and have been investigated because of their low dielectric polarization, high resistivity, and flexibility. Charging polymers foams is normally obtained by applying a corona discharge to the surface with a single tip electrode-plane arrangement or a triode electrode, which consists of a tip electrode-plane structure with a controlled potential intermediate mesh. Corona charging allows the surface potential of the sample to rise without breakdown or surface flashover. A charging method has been developed without corona discharge, and this has provided good results. In our work, a method has been developed to polarize polypropylene foams by applying an insulated high-voltage electrode on the surface of the sample. The dielectric layer in series with the sample allows for a high internal electric field to be reached in the sample but avoids dielectric breakdown of the sample. The distribution of the electric field between the sample and the dielectric barrier has been calculated. Experimental results with three different electrodes present good outcome in agreement with the calculations. High d33 constants of about 880 pC/N have been obtained. Mapping of the d33 constant on the surface has also been carried out showing good homogeneity on the area under the electrode.

Transition Metal Dichalcogenide Resonators for Second Harmonic Signal Enhancement

In addition to their strong nonlinear optical response, transition metal dichalcogenides (TMDCs) possess a high refractive index in the visible and infrared regime. Therefore, by patterning those TMDCs into dielectric nanoresonators, one can generate highly confined electromagnetic modes. Controlled fabrication of TMDC nanoresonators does not only enhance the material’s intrinsic nonlinear response, but also allows for spatially shaping the emission via nanoresonator arrays. Here we fabricate patterned WS2 disks that support a high internal resonant electric field and show strong enhancement of second harmonic (SH) generation in the visible regime. In addition, we assemble the WS2 disks in arrays to spatially direct the coherent SH emission, in analogy to phased array antennas. Finally, we investigate and discuss drastic differences in the areal emission origin and intensity of the measured SH signals, which we find to depend on material variations of the used bulk WS2.

Review—Defect-Tolerant Luminescent Properties of Low InN Mole Fraction InxGa1-xN Quantum Wells under the Presence of Polarization Fields

Different from the case of GaN or AlGaN alloys, the near-band-edge (NBE) emission of quantum wells (QWs) and even epilayers of InxGa1-xN alloys of low InN mole fractions (x) exhibits high quantum-efficiency (QE) against the presence of threading dislocations (TDs) as high as 109 cm−2. Accordingly InxGa1-xN alloys are exclusively used as an active region of green to ultraviolet light-emitting diodes (LEDs) and laser diodes, as well as the heart of white LEDs. Here, current understandings on the emission mechanisms of InxGa1-xN QWs, especially the defect-tolerant emission probability of localized excitons, are reviewed. There exist three disadvantages in obtaining high QE, namely a high density of TDs that cause the nonradiative recombination, kinetic immiscibility of In-containing alloys that introduces high-concentration Shockley-Read-Hall nonradiative recombination centers (NRCs) consisting of vacancy complexes, and high internal electric-fields across the QWs induced by the polarization discontinuities at heterointerfaces along the c-axis, which decrease the radiative recombination rate. The use of InxGa1-xN alloys of low x overcomes such disadvantages with the aid of In-originated localization effects that prevent excitons from trapping by TDs or NRCs. Simultaneous observation of short diffusion length and sufficiently long nonradiative recombination lifetime at room temperature indicates strong localization of holes or excitons.

Atomic scale depletion region at one dimensional MoSe2-WSe2 heterointerface

Lateral heterojunctions based on two dimensional (2D) transition metal dichalcogenides (TMDCs) potentially realize monolayer devices exploiting 2D electronic structures and the functions introduced by the presence of 1D heterointerfaces. Electronic structures of a lateral MoSe2-WSe2 junction have been unveiled using scanning tunneling microscopy and spectroscopy. A smooth and narrow depletion region exists despite a defect-rich heterointerface deviating from the preferred zigzag orientations of the TMDC lattice. From the characteristics of the depletion region, a high carrier concentration and high internal electric fields are inferred, offering to benefit designs of lateral TMDC devices.

Low temperature characteristics of SiPMs after very high neutron irradiation

Abstract The design of the CMS phase II upgrade for the HL-LHC uses SiPMs for the Barrel Timing Layer (BTL) and the Behind HCAL detector (BH or CEH). In both sub-detectors the SiPMs will see a 1 MeV equivalent dose of around 1 0 14 n/cm 2 . To lower the noise in the SiPMs the design is to keep the SiPMs at a low temperature of −30 ° C. Different samples from two manufactures of SiPMs were irradiated to a total dose of resp. 2 × 1 0 12 , 5 × 1 0 13 at the TRIGA reactor at the JSI in Slovenia. The noise in SiPMs is dominated by trap assisted tunneling which is a result of the high internal electric field in SiPMs. We therefore studied the noise behavior from ＋10 ° C to −40 ° C from standard high internal field and specially designed low field SiPMs from FBK-irst and Hamamatsu. After the initial characterization before annealing the noise decrease in SiPMs was also studied using accelerated annealing.

Enhanced Responsivity of ZnSe‐Based Metal–Semiconductor–Metal Near‐Ultraviolet Photodetector via Impact Ionization

We report on high‐responsivity, fast near‐ultraviolet photodetectors based on bulk ZnSe employing a metal–semiconductor–metal structure with and without interdigital contacts. A very high responsivity of 2.42 and 4.44 A W−1 at 20 V bias voltage and high rejection rate of 7900 and 4810 for the light with a wavelength of 325 nm is obtained for photodetectors without and with interdigital contacts, which indicates an internal gain. The mechanism of internal gain is attributed to the impact ionization of ZnSe atoms under high internal electric field strength of 133 kV cm−1. Also a low dark current of ≈3.4 nA and high detectivity of ≈1.4 × 1011 cm Hz1/2 W−1 at a voltage of 20 V is achieved for the device with interdigital contacts at room temperature.

Direct evaluation of influence of electron damage on the subcell performance in triple-junction solar cells using photoluminescence decays

Tandem solar cells are suited for space applications due to their high performance, but also have to be designed in such a way to minimize influence of degradation by the high energy particle flux in space. The analysis of the subcell performance is crucial to understand the device physics and achieve optimized designs of tandem solar cells. Here, the radiation-induced damage of inverted grown InGaP/GaAs/InGaAs triple-junction solar cells for various electron fluences are characterized using conventional current-voltage (I–V) measurements and time-resolved photoluminescence (PL). The conversion efficiencies of the entire device before and after damage are measured with I–V curves and compared with the efficiencies predicted from the time-resolved method. Using the time-resolved data the change in the carrier dynamics in the subcells can be discussed. Our optical method allows to predict the absolute electrical conversion efficiency of the device with an accuracy of better than 5%. While both InGaP and GaAs subcells suffered from significant material degradation, the performance loss of the total device can be completely ascribed to the damage in the GaAs subcell. This points out the importance of high internal electric fields at the operating point.

Relationship between surface potential and d33 constant in cellular piezoelectric polymers

The development of cellular piezoelectric polymers has shown very promising results thanks to their high d33 piezoelectric constants which make them candidates for many applications. Cellular piezoelectric polymers, known as ferroelectrets, are obtained by means of an activation process which consists in generating an internal dipole with electrostatic charges produced by internal electric discharges. The most common system for this activation process is the application of a corona discharge on the surface of the sample in order to produce a high internal electric field. The theoretical electrostatic model of the process which is widely used is the Sessler model which relates the internal surface charge density, the air and polymer layers thickness, the dielectric permittivity of the polymer and the Young's Modulus of the cellular material to the d33 piezoelectric constant. In our work, we relate the internal charges of the material with the d33 piezoelectric constant by means of a surface potential scanning of cellular polypropylene biaxially stretched samples. Samples were charged by a corona discharge controlled with a triode electrode. Surface potentials were high enough to generate internal discharges and obtain measurable d33 piezoelectric constants but low enough to be measured with spatial resolution by means of a 3 kV electrostatic probe. Surface potential profiles showed some deviations from the expected bell-shape profile due to the internal electric field generated by the internal static charge. These deviations can be numerically related to the measured d33 piezoelectric constant with the electrostatic Sessler model.

Source‐gating effect in hydrothermally grown ZnO nanowire transistors

Nanowire source‐gated field‐effect transistors (NW SGT) are demonstrated using hydrothermally grown ZnO NWs. Device quality ZnO NWs with moderate n‐type doping are achieved by thermal annealing in ambient air at ∼550 °C. A single ZnO NW device with Au source‐drain contacts (s/d) is found to operate under source‐gating mode, with characteristics markedly different from a reference device with ohmic contacts. The NW SGT shows exceptionally early drain current–voltage saturation (IDSAT–VDSAT) below 1 V. The change in saturation with the gate voltage (VG) is over 80 times lower than a reference device with ohmic contacts. This device behavior is attributed to the source‐gate overlap, enabling gate field penetration inside the depleted source. Current modulation is obtained by a combination of gate‐induced image force barrier lowering and the high internal electric fields at source pinch‐off. Effective Schottky barrier heights are extracted from activation energy measurements, revealing systematic barrier lowering with increasing VG. These features of the device lead us to conclude that the single NW field‐effect transistor (FET) with Schottky contacts operated under SGT mode.

Electrical and ultraviolet characterization of 4H-SiC Schottky photodiodes

Fabrication and electrical and optical characterization of 4H-SiC Schottky UV photodetectors with nickel silicide interdigitated contacts is reported. Dark capacitance and current measurements as a function of applied voltage over the temperature range 20 °C - 120 °C are presented. The results show consistent performance among devices. Their leakage current density, at the highest investigated temperature (120 °C), is in the range of nA/cm(2) at high internal electric field. Properties such as barrier height and ideality factor are also computed as a function of temperature. The responsivities of the diodes as functions of applied voltage were measured using a UV spectrophotometer in the wavelength range 200 nm - 380 nm and compared with theoretically calculated values. The devices had a mean peak responsivity of 0.093 A/W at 270 nm and -15 V reverse bias.

Experimental observation of RF avalanche gain in GaN‐based PN junction diodes

Radio-frequency (RF) reflection gain in GaN homojunction p-n diode structures has been observed experimentally. A vertical p+/n structure grown on a native GaN substrate was used to achieve the high internal electric fields necessary to induce impact ionisation in GaN while minimising the effects of dislocations on the device performance. Amplitude and phase signatures in the measured RF reflection coefficient indicate the onset of impact ionisation within the device, with reflection gain observed at frequencies above 350 MHz. The magnitude of the measured gain is consistent with theoretical estimates of impact ionisation parameters, and the bias dependence of the measured reflection phase response is consistent with avalanche as the gain mechanism.

Light induced structural changes in CH3NH3PbI3 Perovskite Solar Cells

Perovskite solar cells have recently become one of the most promising alternative to the current solar cell technologies by achieving efficiencies superior to 20%. Owing to their distortable structure these devices have proven to show unusually slow relaxation processes upon illumination. We have identified this phenomenon in the frequency domain by impedance spectroscopy (IS) and in the time domain by transient photovoltage decay (TPD) and open circuit voltage decay (OCVD). However the origin of this process is still unclear although three main hypotheses have been proposed in the literature: i) high internal electric field, ii) ferroelectric domains and iii) ion/defect diffusion. In this conference paper we first present our works on light induced structural changes in perovskite and then discuss the main hypotheses previously mentioned with the insight of first principle calculations.

Contact‐Induced Mechanisms in Organic Photovoltaics: A Steady‐State and Transient Study

The role of the contacts in thin‐film, blended heterojunctions (<100 nm thick) organic photovoltaics is explored, specifically considering concepts of carrier selectivity, injection, and extraction efficiency, relative to recombination. Contact effects are investigated by comparing two hole‐collecting interlayers: a phosphonic acid monolayer on indium tin oxide (ITO) and a nickel oxide thin film. The interlayers have equivalent work functions (≈5.4 eV) but widely variant energy band offsets relative to the lowest unoccupied molecular orbital of the acceptor (electron blocking versus not), which are coupled to large differences in carrier density. Trends in open‐circuit voltages (VOC) as a function of light intensity and temperature are compared and it is concluded that the dominant mechanism limiting VOC for high density of states contacts is free carrier injection, not surface recombination or extraction barriers. Transient photocurrent decay measurements confirm excess reinjected carriers decrease the extraction efficiency via increased recombination and decrease free carrier lifetime, even at high internal electric fields, due to space charge accumulation. These results demonstrate that the energetics and injection dynamics of the interface between interlayers and high carrier density electrodes (typically ITO and metals) must be considered with fabrication and processing of interlayers, in addition to possible carrier selectivity and the interface with the active layer.

Decoupling single nanowire mobilities limited by surface scattering and bulk impurity scattering

We demonstrate the isolation of two free carrier scattering mechanisms as a function of radial band bending in InN nanowires via universal mobility analysis, where effective carrier mobility is measured as a function of effective electric field in a nanowire field-effect transistor. Our results show that Coulomb scattering limits effective mobility at most effective fields, while surface roughness scattering only limits mobility under very high internal electric fields. High-energy α particle irradiation is used to vary the ionized donor concentration, and the observed decrease in mobility and increase in donor concentration are compared to Hall effect results of high-quality InN thin films. Our results show that for nanowires with relatively high doping and large diameters, controlling Coulomb scattering from ionized dopants should be given precedence over surface engineering when seeking to maximize nanowire mobility.

Formation and reactivity of CrII carbonyls hosted in polar and non polar supports

Organometallic complexes hosted inside porous frameworks are particularly interesting because of possible application in heterogeneous catalysis or electronic devices and optical materials. We present here a detailed spectroscopic investigation on the structure and reactivity towards simple reagents (such as CO) of chromocene molecules (Cp2Cr) encapsulated into the nanovoids of two different matrices: a non polar polystyrene (PS) and a polar NaY zeolite. We demonstrate that stable Cp2Cr(CO) complexes are formed in PS, while in NaY the presence of high internal electric fields confers to the Cp2Cr molecules a much higher reactivity towards CO. In situ EXAFS data, combined with other spectroscopic techniques and ab initio theoretical calculations, allowed the structural determination of the reaction products and afforded the full comprehension of the complex reactivity of Cp2Cr molecules inside the cavities of PS and NaY hosts.

Large-area sub-micron gap interdigitated THz emitters fabricated by interference lithography and angle evaporation

Interference-lithography and a self-aligning angle-evaporation technique are employed to fabricate interdigitated photoconductive terahertz (THz) emitters. The devices have a large active area for high directivity and submicron spaced electrodes for high internal electric fields at low bias voltages. The fabrication process offers the advantage that only one patterning step is needed to generate three isolated metallic structures. This avoids critical alignment and reduces the fabrication effort significantly. Voltage dependent THz emission is observed from 4 V upwards.

Current Equation for Hopping Ions on a Lattice under High Driving Force and Nondilute Concentration

The current density equation of hopping ions (and localized electrons) is evaluated for high driving forces originating both from high internal electric fields and high gradients in the particle concentrations. Both high concentration of the ions and the dilute limit are discussed. Interaction between the mobile ions is included. Attention is paid to details of the derivation. It requires the discussion of local thermal equilibrium, as opposed to chemical equilibrium, of local exchange current, hopping probability, and conductivity. A comparison with other equations aimed at coping with higher driving forces is given. In particular, the results are compared with those of irreversible thermodynamics and the substitution of activity instead of concentration in a chemical kinetic rate equation. The latter is shown to be inaccurate, yet to recover the correct dependence on the difference in the electrochemical potential of the ions. Additional topics that come up during the evaluation and are discussed are the interaction potential between the ions, the activity of building elements and of structure elements, and the effect of frequency on the hopping probability.

Hydrogenated Amorphous Silicon Sensor Deposited on Integrated Circuit for Radiation Detection

Radiation detectors based on the deposition of a 10 to 30 mum thick hydrogenated amorphous silicon (a-Si:H) sensor directly on top of integrated circuits have been developed. The performance of this detector technology has been assessed for the first time in the context of particle detectors. Three different circuits were designed in a quarter micron CMOS technology for these studies. The so-called TFA (Thin-Film on ASIC) detectors obtained after deposition of a-Si:H sensors on the developed circuits are presented. High internal electric fields (104 to 105 V/cm) can be built in the a-Si:H sensor and overcome the low mobility of electrons and holes in this amorphous material. However, the deposited sensor's leakage current at such fields turns out to be an important parameter which limits the performance of a TFA detector. Its detailed study is presented as well as the detector's pixel segmentation. Signal induction by generated free carrier motion in the a-Si:H sensor has been characterized using a 660 nm pulsed laser. Results obtained with a TFA detector based on an ASIC integrating 5 ns peaking time pre-amplifiers are presented. Direct detection of 10 to 50 keV electrons and 5.9 keV X-rays with the detectors are then shown to understand the potential and the limitations of this technology for radiation detection.