Carrier Concentration Gradient Research Articles

Gradient Doping Enables an Extraordinary Efficiency in Thermoelectric PbTe1-xIx.

The conversion efficiency of thermoelectric generators that have to be operated under a temperature difference (ΔT) is mainly determined by material's dimensionless figure of merit (zT). However, maximization of zT at each temperature requires an optimization of carrier concentration (nopt) which strongly depends on the temperature and band parameters. Commonly utilized strategy of chemical doping usually enables a homogeneous carrier concentration throughout the material, leading the maximal zT to be achievable only within a narrow temperature range. In this work, a gradiently doping is successfully realized in PbTe1-xIx using a vertical gradient solidification technique, enabling a spatial gradient in carrier concentration that correspondingly optimizes zT at each portion of the material under its operating temperature. Such a gradient doping results in an extraordinary device efficiency of ≈14% at a ΔT of ≈500K, corresponding to a ≈40% improvement as compared to that of homogeneous doping. Since directional solidification technique commonly enables gradient dopant concentrations in semiconductors, the resultant gradient carrier concentration is illustrated here as an effective approach for advancing thermoelectrics.

Heat-Annealed Zinc Oxide on Flexible Carbon Nanotube Paper and Exposed to Gradient Light to Enhance Its Photoelectric Response.

Buckypaper (BP), a flexible and porous material, exhibits photovoltaic properties when exposed to light. In this study, we employed radio frequency (RF) sputtering of zinc oxide (ZnO) followed by rapid thermal annealing to enhance the photovoltaic response of BP. We investigated the impact of various sputtering parameters, such as the gas flow ratio of argon to oxygen and deposition time, on the morphology, composition, resistivity, and photovoltaic characteristics of ZnO-modified BP. Additionally, the photovoltaic performance of the samples under different illumination modes and wavelengths was compared. It was found that optimal sputtering conditions-argon to oxygen flow ratio of 1:2, deposition time of 20 min, and power of 100 watts-resulted in a ZnO film thickness of approximately 45 nanometers. After annealing at 400 °C for 10 min, the ZnO-modified BP demonstrated a significant increase in photocurrent and photovoltage, along with a reduction in resistivity, compared to unmodified BP. Moreover, under gradient illumination, the ZnO-modified BP exhibited a photovoltage enhancement of 14.70-fold and a photocurrent increase of 13.86-fold, compared to uniform illumination. Under blue light, it showed a higher photovoltaic response than under other colors. The enhancement in photovoltaic response is attributed to the formation of a Schottky junction between ZnO and BP, an increased carrier concentration gradient, and an expanded light absorption spectrum. Our results validate that ZnO sputtering followed by annealing is an effective method for modifying BP for photovoltaic applications such as solar cells and photodetectors.

Fermi level induced band edge alignment and band bending in Ag3PO4/Cu2O p-n heterojunction for proficient photocatalytic applications

Fabrication of binary p-n heterojunction and the band alignment at the interface of the individual semiconductor photocatalyst has been extensively studied to verify superior charge separation and migration capability in photocatalytic applications. Here, a simple wet chemical followed by a hydrothermal fabrication strategy was introduced for the development of the binary p-n Ag3PO4/Cu2O heterostructures. The tunable band structure of individual semiconductors with the work function (φ), witnessed a band bending at the space charge region. The bending at the interface induces a carrier concentration gradient and manifests a rectifying current transport diode. The fabricated p-n heterostructures and carrier migration between n-type Ag3PO4 and p-type Cu2O were verified by different morphological and physicochemical methods. The band banding at the interface, leading to a narrow depletion region, favors the tunneling of electron-hole pairs through a Z-scheme carrier transport mechanism. The electron-hole pair movement has further been confirmed by considering the band edge position after contact, photocatalytic scavengers, and the radical trapping experiment. The Ag3PO4/Cu2O p-n heterojunction photocatalyst manifested a 737.4 μmol g−1h−1 of H2 generation and 91% endosulfan degradation efficiency, these are 12 and 9 times higher than that of pure Ag3PO4. The p-n heterojunction photocatalyst displayed a higher current density with electron-hole migration efficiency, synergistically enhancing the catalytic activity through the interfacial space charge junction.

Efficient Doping Induced by Charge Transfer at the Hetero-Interface to Enhance Photocatalytic Performance.

The construction of heterojunction photocatalysts is an effective method to improve photocatalytic efficiency since the potential gradient and built-in electron field established at the junction could enhance the efficiency of charge separation and interfacial charge transfer. Nevertheless, heterojunction photocatalysts with strong built-in electron fields remain difficult to build since the two adjacent constitutes must be satisfied with an appropriate band alignment, redox potential, and carrier concentration gradient. Here, an efficient charge transfer-induced doping strategy is proposed to enhance the heterojunction built-in electron field for stable and efficient photocatalytic performance. Carrier transfer tests show that the rectification ratio of the n-TiO2-X/n-BiOI heterojunction is significantly enhanced after being coated with graphene oxide (GO). Consequently, both the hydrogen production and photodegradation performance of the GO composite heterojunction are considerably enhanced compared with pure TiO2-X, BiOI, and n-TiO2-X/n-BiOI. This work provides a facile method to prepare heterojunction photocatalysts with a high catalytic activity.

Sign reversal of anisotropic magnetoresistance and anomalous thickness-dependent resistivity in Sr2CrWO6/SrTiO3 films

High-quality Sr2CrWO6 (SCWO) films have been grown on SrTiO3 (STO) substrate by pulsed laser deposition under low oxygen pressure. With decrease of the film thickness, a drastic conductivity increase is observed. The Hall measurements show that the thicker the film, the lower the carrier density. An extrinsic mechanism of charge doping due to the dominance of oxygen vacancies at SCWO/STO interfaces is proposed. The distribution and gradient of carrier concentration in SCWO films are considered to be related to this phenomenon. Resistivity behavior observed in these films is found to follow the variable range hopping model. It is revealed that with increase of the film thickness, the extent of disorder in the lattice increases, which gives a clear evidence of disorder-induced localization charge carriers in these films. Magnetoresistance measurements show that there is a negative magnetoresistance in SCWO films, which is considered to be caused by the magnetic scattering of magnetic elements Cr3+ and W5+. In addition, a sign reversal of anisotropic magnetoresistance (AMR) in SCWO film is observed for the first time, when the temperature varies across a characteristic value, T M. Magnetization–temperature measurements demonstrate that this AMR sign reversal is caused by the direction transition of easy axis of magnetization from the in-plane ferromagnetic order at T > T M to the out-of-plane at T < T M.

Microstructural Gradational Properties of Sn-Doped Gallium Oxide Heteroepitaxial Layers Grown Using Mist Chemical Vapor Deposition.

This study examined the microstructural gradation in Sn-doped, n-type Ga2O3 epitaxial layers grown on a two-inch sapphire substrate using horizontal hot-wall mist chemical vapor deposition (mist CVD). The results revealed that, compared to a single Ga2O3 layer grown using a conventional single-step growth, the double Ga2O3 layers grown using a two-step growth process exhibited excellent thickness uniformity, surface roughness, and crystal quality. In addition, the spatial gradient of carrier concentration in the upper layer of the double layers was significantly affected by the mist flow velocity at the surface, regardless of the dopant concentration distribution of the underlying layer. Furthermore, the electrical properties of the single Ga2O3 layer could be attributed to various scattering mechanisms, whereas the carrier mobility of the double Ga2O3 layers could be attributed to Coulomb scattering owing to the heavily doped condition. It strongly suggests the two-step-grown, lightly-Sn-doped Ga2O3 layer is feasible for high power electronic devices.

Improving performance of Cu2ZnSnS4 solar cell via back contact interface engineering

Although kesterite Cu2ZnSnS4 (CZTS) has emerged as a potential absorber material for development of low cost thin film solar cells, the performance of the CZTS devices has been limited by a large deficit in open circuit voltage caused by recombinations of charge carriers primarily at the back contact interface. Recently, field passivation through engineering of the back contact interface has been shown to control the interface recombination and thereby minimise the open circuit voltage deficit. In this work, we have numerically established the limiting role of defect densities in the bulk of CZTS and at interfaces (CZTS-CdS and CZTS-Mo) on performance metrics of the champion CZTS cell (∼11%). Furthermore, we have shown that field passivation by using back surface field (BSF) layers in the form of very thin p+ type Cu2O and SnS layers, sandwiched between the bottom electrode Mo and CZTS, greatly improved the device efficiency (to about 15%) despite significant high bulk and interface defect densities. Numerical calculations have revealed critical dependence of back contact interface recombination on charge carrier concentration gradient and band offset at the interface, and thickness of the BSF layer. Based on the results, a mechanism for suppressing the recombinations at back contact interface has been illustrated.

Piezoresistive Effect with a Gauge Factor of 18 000 in a Semiconductor Heterojunction Modulated by Bonded Light-Emitting Diodes.

Giant piezoresistive effect enables the development of ultrasensitive sensing devices to address the increasing demands from hi-tech applications such as space exploration and self-driving cars. The discovery of the giant piezoresistive effect by optoelectronic coupling leads to a new strategy for enhancing the sensitivity of mechanical sensors, particularly with light from light-emitting diodes (LEDs). This paper reports on the piezoresistive effect in a 3C-SiC/Si heterostructure with a bonded LED that can reach a gauge factor (GF) as high as 18 000. This value represents an approximately 1000 times improvement compared to the configuration without a bonded LED. This GF is one of the highest GFs reported to date for the piezoresistive effect in semiconductors. The generation of carrier concentration gradient in the top thin 3C-SiC film under illumination from the LED coupling with the tuning current contributes to the modulation of the piezoresistive effect in a 3C-SiC/Si heterojunction. In addition, the feasibility of using different types of LEDs as the tools for modulating the piezoresistive effect is investigated by evaluating lateral photovoltage and photocurrent under LED's illumination. The generated lateral photovoltage and photocurrent are as high as 14 mV and 47.2 μA, respectively. Recent technologies for direct bonding of micro-LEDs on a Si-based device and the discovery reported here may have a significant impact on mechanical sensors.

An Improved Fourier-Series-Based IGBT Model by Mitigating the Effect of Gibbs Phenomenon at Turn on

The Fourier-series-based insulated-gate bipolar transistor (IGBT) model has been very effective in simulating its switching behaviors and meanwhile, reflecting carrier dynamics. However, in turn-on process, such a model has difficulty in distinguishing the boundary between undepleted area and depletion layer of the drift region due to Gibbs phenomenon and therefore, cannot accurately describe the carrier behavior of carrier storage region (CSR). In this study, the Fourier-series-based model is modified to determine the more precise depletion layer edge for turn on. This is achieved by utilizing the actual characteristics of the carrier concentration gradient at the edge of CSR in turn-on process. Such a modified model is implemented in MATLAB/Simulink and it can better reflect the depletion layer and simulate the distribution of both the carrier concentration and electric field during turn-on process. It corrects the unreasonable voltage tail caused by the error of drift region voltage calculation in previous models. The results of the proposed model under both the resistive load and inductive load are in good agreement with TCAD. Such a modified model can more accurately simulate both carrier dynamics and switching behaviors in the turn-on process.

Porous SiC electroluminescence from p–i–n junction and a lateral carrier diffusion model

Electroluminescence of porous silicon carbide is achieved in a forward-biased SiC p–i–n junction. A broad green spectral feature centered at ∼510 nm is shown to arise from porous SiC. A large SiC surface area in the vicinity of the junction is created by diamond cutting followed by an electrochemically enhanced hydrogen fluoride etch that produces a layer of porous SiC. Photoluminescence is shown not to be responsible for the green emission. This supports the model of carrier recombination at the porous region via lateral bipolar diffusion of carriers. A lateral bipolar diffusion model is presented in which mobile carriers diffuse laterally from the junction toward the porous SiC surface region driven by a lateral carrier concentration gradient. Lateral bipolar diffusion in conjunction with suitable radiative recombination centers provides a possible pathway to achieve high quantum efficiencies in future SiC p–n homojunction or double heterojunction light-emitting diodes. Competing recombination processes and associated ideality factors in 4H-SiC diodes are also examined.

Self-powered monolithic accelerometer using a photonic gate

Abstract Harvesting sustainable energy resources from surrounding environments to power small electronic devices and systems has attracted massive research attention. Herein, we develop a novel technology to harvest light energy to self-power and simultaneously sense mechanical acceleration in a monolithic structure. When the photonic gate is illuminated the operation mode of the device changes from conventional mode to light harvesting and self-powered operation. The light illumination provides a gradient of majority carrier concentration on the top semiconductor layer, generating a lateral photovoltage, which is the output voltage of the sensor. Under acceleration, the mechanical inertial force induces stress in the sensor material leading to the change of mobility of the charge carriers, which shifts their diffusion rate, and hence changes the gradient of the majority carriers and the lateral photovoltage. The sensitivity at 480 lx light illumination was measured to be 107 μ V / g , while it was approximately 30 μ V / g under the ambient light illumination without any electrical power source. In addition, the acceleration sensitivity is tunable by controlling parameters of the photonic gate such as light power, light spot position and light wavelength. The integration of sensing and powering functions into a monolithic platform proposed in this work eliminates the requirement of external power sources and offers potential solutions for wireless, independent, remote, and battery-free sensing devices and systems.

Surface Potential-Based Approach to Estimate Bias Dependent Sensitivity of 1-D Metal Oxide Resistive Gas Sensors

The current work concern a surface potential based approach to study the bias voltage dependent sensitivity of 1-D metal oxide gas sensors. Bias dependent sensitivity for granular metal oxide gas sensors has already been reported. Considering the wide applicability of 1-D nanomaterials in gas sensing application, the bias dependent sensitivity has been studied in present work by theoretical route which was supported by experimental findings. The concept was established based on the surface potential ( $\Psi _{{\mathbf {S}}}$ ) effect due to the carrier concentration gradient by surface adsorption of gases on the metal oxide. Considering the effect of both the potentials i.e. $\Psi _{{\mathbf {S}}}$ and bias voltage, the exact bias point was found to get the peak response of 1-D metal oxide gas sensor. TiO2 nanotubes array was considered as the test sensing material. In the first phase of the article, the theoretical approach has been reported to estimate the sensor response of 1-D nanotubes and in the second phase of the study, the theoretical concept has been justified with experimental results. Highly oriented TiO2 nanotube array was synthesized, characterized and used to fabricate Au/TiO2 nanotubes/Ti type vertical structured sensor device. Simulation results exhibited the highest response of the TiO2 nanotube sensor in-between 0.1-0.4 V of bias voltage which was then authenticated with the experimental ethanol sensing behavior of the sensor in the concentration range of 90 to 630 ppm at 300 K.

Constant Electricity Generation in Nanostructured Silicon by Evaporation‐Driven Water Flow

AbstractRecently, hydrovoltaic technology emerged as a novel renewable energy harvesting method, which dramatically extends the capability to harvest water energy. However, the urgent issue restricting its device performance is poor carrier transport properties of the solid surface if large charged interface is considered simultaneously. Herein, a hydrovoltaic device based on silicon nanowire arrays (SiNWs), which provide large charged surface/volume ratio and excellent carrier transport properties, yields sustained electricity by a carrier concentration gradient induced by evaporation‐induced water flow inside nanochannels. The device can yield direct current with a short‐circuit current density of over 55 μA cm−2, which is three orders larger than a previously reported analogous device (approximately 40 nA cm−2). Moreover, it exhibits a constant output power density of over 6 μW cm−2 and an open‐circuit voltage of up to 400 mV. Our finding may pave a way for developing energy‐harvesting devices from ubiquitous evaporation‐driven internal water flow in nature with semiconductor material of silicon.

Constant Electricity Generation in Nanostructured Silicon by Evaporation-Driven Water Flow.

Recently, hydrovoltaic technology emerged as a novel renewable energy harvesting method, which dramatically extends the capability to harvest water energy. However, the urgent issue restricting its device performance is poor carrier transport properties of the solid surface if large charged interface is considered simultaneously. Herein, a hydrovoltaic device based on silicon nanowire arrays (SiNWs), which provide large charged surface/volume ratio and excellent carrier transport properties, yields sustained electricity by a carrier concentration gradient induced by evaporation-induced water flow inside nanochannels. The device can yield direct current with a short-circuit current density of over 55 μA cm-2 , which is three orders larger than a previously reported analogous device (approximately 40 nA cm-2 ). Moreover, it exhibits a constant output power density of over 6 μW cm-2 and an open-circuit voltage of up to 400 mV. Our finding may pave a way for developing energy-harvesting devices from ubiquitous evaporation-driven internal water flow in nature with semiconductor material of silicon.

Degradation Mechanism in Cu(In,Ga)Se2 Material and Solar Cells Due to Moisture and Heat Treatment of the Absorber Layer

The impact of moisture and heat treatment on the microstructural, chemical, and electrical properties of Cu(In,Ga)Se2 films and their collective effect on the solar cell device performance was studied. X-ray photoelectron spectroscopy and secondary ion mass spectroscopy measurements show that water exposure causes surface modification and alters the alkali metal distribution, while no composition or structural effect was observed. Deep level transient and optical spectroscopies revealed that the trap densities (NT) for both the EV + 0.65 eV and EV + 0.98 eV traps increase after water exposure, while the majority carrier concentration (NA) decreases. Time-resolved photoluminescence (PL) and steady-state PL measurements indicated the presence of static, not dynamic, quenching. Reduction of open-circuit voltage (VOC) and fill factor (FF) was observed for the devices but was not associated with a change of recombination mechanism, which remains in the absorber space charge region. A small increase in series resistance and shunt conductance accounts for most of the FF change, while the modification in both NA and NT yield most of the change in VOC . A gradient of majority carrier concentration, related to the alkali profile, also yields a small voltage-dependent current collection after moisture and heat treatment.

Raman investigation on the surface carrier concentration of single GaN microrod grown by MOCVD

Abstract Raman measurement was applied to investigate the carrier concentration distribution along the [0001] direction of single GaN microrod. By measuring the Raman shifts of longitudinal phonon-plasmon (LPP) modes at different locations, the carrier concentration distributions along the sidewall of the microrod were obtained in the [0001] direction. It was found that the carrier concentration at the bottom of the microrod was 9.07×1016 cm−3, which was higher than that at the top. Carrier concentration gradient existed along the growth direction of the microrod. This was mainly due to the uneven distribution of some donor impurities (eg: oxygen impurities) during the growth process. Furthermore, based on the E2high mode, the distributions of residual stress and crystal qualities were characterized along the [0001] direction.

Energy Conversion Efficiency of a Piezo-Thermoelectric Material

This work presents a set of one-dimensional piezo-thermoelectricity equations for semiconductors exhibiting both piezoelectric and thermoelectric effects. Diffusion current due to the carrier concentration gradient is considered in the piezo-thermoelectricity theory. The coupled equations satisfied by the temperature, carrier concentration, and mechanical displacement are derived. The theory is subsequently applied to a thin film of a zinc oxide (ZnO) piezo-thermoelectric material subjected to a steady state temperature gradient in the thickness direction and fixed-end boundary conditions. The numerical results show that the energy conversion efficiency may be significantly increased by the coupling between the thermoelectric and piezoelectric effects.

Surface Oxidation Doping to Enhance Photogenerated Carrier Separation Efficiency for Ultrahigh Gain Indium Selenide Photodetector

This work presents an ultrahigh gain InSe-based photodetector by using a novel approach called the surface oxidation doping (SOD) technique. The carrier concentration of multilayered two-dimensional (2D) InSe semiconductor surface has been modulated by controlling the formation of a surface oxide layer. The SOD through surface charge transfer at the interface of the oxide/2D InSe semiconductor heterostructure can lead to the creation of a vertical built-in potential and band bending as a result of the carrier concentration distribution gradient. The internal electric field caused by the formation of a carrier concentration gradient in InSe layers can facilitate charge separation of photogenerated electron–hole pairs under light illumination. Consequently, the record high photoresponsivities of InSe-based photodetector with ∼5 × 106 A/W at the excitation wavelength of 365 nm and 5 × 105 A/W at the wavelength of 530 nm can be obtained, outperforming the majority of photodetectors based on other 2D materials, such as graphene, MoS2, and even highly sensitive multilayer GaTe and In2Se3 flakes. The approach based on SOD induced efficient photogenerated charge separation can be also applied to other 2D layered semiconductors.

Theoretical analysis on the sensitivity to partial pressure of oxygen of the band gap shift in CdO thin films

With the aim of providing an analytical approach (not accurate numerical results), for the first time, the sensitivity to the partial pressure of oxygen of the shift in the energy band gap experienced by cadmium-oxide thin films is evaluated for the visible region by introducing a suitable parameter. The sensitivity to the above pressure of the spatial carrier density is determined and compared with the sensitivity relative to the band gap shift. The gradient of the CdO carrier concentration as a function of the partial pressure of oxygen appears in the expressions for the two above sensitivity functions but the goal of this paper is not computing numerically this gradient so only qualitative estimations are done. In relation to the above results, the kinetics relative to the formation of CdO thin films are investigated. In addition, the sensitivity to the pressure in question of the corresponding optical-absorption shift in the visible range is calculated.

Investigation of Cu2ZnSnS4 thin-film solar cells with carrier concentration gradient

To investigate the effect of carrier concentration gradient on Cu2ZnSnS4 (CZTS) thin-film solar cells, the properties of CZTS solar cells were studied by numerical method. The photovoltaic performances of carrier concentration gradient CZTS solar cells were calculated by the solutions of Poisson's equation, continuity equation, and current density equation using AFors-Het v2.4 program. The carrier concentration gradient was changed to analyze its effect. Compared with CZTS solar cells without carrier concentration gradient, the photovoltaic performances of CZTS solar cells can be enhanced by using carrier concentration gradient absorber. The carrier concentration gradient can extend the distribution region of built-in electric field, which is beneficial to the drift of photo-generated carriers. However, the carrier concentration gradient also affects the recombination and series resistances of solar cells. When the defect density of CZTS layer is high, the photo-generated carriers are affected significantly by recombination, resulting in slight effect of carrier concentration gradient. Therefore, the defect density should be reduced to enhance the effect of carrier concentration gradient on improving conversion efficiency of CZTS thin-film solar cells.