Depletion Region Width Research Articles

Operation of BaSi2 homojunction solar cells on p+-Si(111) substrates and the effect of structure parameters on their performance

Operation of n+-BaSi2/p-BaSi2(500 nm)/p+-BaSi2 homojunction solar cells on p+-Si(111) is demonstrated, showing a saturation current density of 9.4 mA cm−2 and an open-circuit voltage of 0.11 V under AM1.5 illumination. Photogenerated electrons deep in the p-BaSi2 light absorber layer are likely to be transferred to the p+-Si side, leading to negative values of internal quantum efficiency (IQE) at longer wavelengths. The negative IQE can be solved by extending the width of depletion region in the p-BaSi2 light absorber layer by decreasing its hole concentration. The importance of Si substrate surface morphology is also discussed.

High voltage endurance and switchable resistance effect observed in nanoscale copper groove structure

A switchable lateral resistance effect with high reverse voltage has been observed in nanoscale copper groove structure. With the stimulation of electric pulse and local illumination of laser, the lateral resistance of the structure can be modulated in a non-volatile manner. We attribute this phenomenon to the different width of depletion region and the Schottky barrier change caused by the nanoscale charge trapping effect. This work may inspire a new approach of resistance modulation and help the development of laser-assisted electric pulse merged devices.

17.78% efficient low-temperature carbon-based planar perovskite solar cells using Zn-doped SnO2 electron transport layer

Organic-inorganic lead halide perovskite solar cells have garnered enormous interest in scientific communities due to comparable power conversion efficiencies and solution-processed fabrication techniques. Among them, carbon-based perovskite solar cells seem to be one of the most promising candidates for addressing the stability issue while suffering from inferior efficiency or high processing temperature. Herein, we introduce low-temperature processed Zn-doped SnO2 (below 200 °C) as an effective electron transport layer in perovskite solar cells for the first time, and demonstrate a low-temperature carbon-based device with Cu-phthalocyanine as hole transport layer. We find that Zn doping contributes to a more suitable energy level alignment and an improved conductivity for SnO2 films. As a result, the electron transfer and extraction are enhanced and the charge recombination is suppressed. In addition, devices with Zn-doped SnO2 have a wider depletion region, leading to an enhanced photovoltaic performance. An optimal efficiency of 17.78% can be obtained after 2 mM Zn doping. Furthermore, these devices, introducing highly stable and hydrophobic Cu-phthalocyanine and carbon, maintain almost 100% of their initial efficiencies over 1200 h in ambient air. Our study paves the way for developing highly efficient and flexible carbon-based perovskite solar cells.

Local structural changes due to the electric field‐induced migration of oxygen vacancies at Fe‐doped SrTiO3 interfaces

AbstractWe report on our study of dc voltage‐induced structural changes at reduced and oxidized Fe‐doped SrTiO3 (Fe:STO) electrode interfaces using second harmonic generation (SHG) together with photoluminescence (PL) method. We show that oxygen vacancy defects play a critical role in determining the local electrical and structural properties of interfacial depletion regions at Schottky junctions. The SHG results show that the dc electric field causes oxygen ions and vacancies to displace toward the anode and cathode in the low field regime, respectively. This process forms electrostrictive distortions within local interfacial depletion regions which are described by Fe:Ti‐O bond stretching and bending. Differences in the EFISHG responses from the oxidized and reduced crystal interfaces are explained according to local oxygen vacancy concentrations and dynamics and their effects on the Schottky barrier heights and depletion region widths at each interface. These results are further supported by our PL measurements. Oxygen ion migration toward the Fe:STO surface leads to enhanced fluorescence intensities from in‐gap acceptor states. We demonstrate that SHG and PL measurements are well‐suited for understanding and resolving the underlying causes of dielectric breakdown processes and device failure brought on by dc electric field and ionic defect migrations in perovskite‐type electroceramics.

Significant Carrier Extraction Enhancement at the Interface of an InN/p-GaN Heterojunction under Reverse Bias Voltage.

In this paper, a superior-quality InN/p-GaN interface grown using pulsed metalorganic vapor-phase epitaxy (MOVPE) is demonstrated. The InN/p-GaN heterojunction interface based on high-quality InN (electron concentration 5.19 × 1018 cm−3 and mobility 980 cm2/(V s)) showed good rectifying behavior. The heterojunction depletion region width was estimated to be 22.8 nm and showed the ability for charge carrier extraction without external electrical field (unbiased). Under reverse bias, the external quantum efficiency (EQE) in the blue spectral region (300–550 nm) can be enhanced significantly and exceeds unity. Avalanche and carrier multiplication phenomena were used to interpret the exclusive photoelectric features of the InN/p-GaN heterojunction behavior.

Characterization of diode-connected heterojunction bipolar transistors for near-infrared detecting applications

The characterization of SiGe diode-connected heterojunction bipolar transistors (HBTs) through measurements of two-circuit configurations is presented. Characterization is done to understand the behavior of these diodes for near-infrared detecting applications at room temperature and 77 K. The two configurations that are considered differ; the first is a base-emitter shorted HBT and the second is a base-collector shorted HBT. The parameters measured are current density-voltage, capacitance–voltage, and noise. The two configurations are implemented using the austriamicrosystems AG 0.35-μm process. The base-emitter shorted configuration exhibits a flatter JC versus V curve when in reverse bias compared with the base-collector configuration. The C − V curves are the same for both configurations. The noise voltage of the base-emitter configuration is 36 and 14.48 μV / Hz at 102.5 Hz for 293 and 77 K temperature points, respectively, to 14.48 and 12.42 μV / Hz at 50 kHz for 293 and 77 K, respectively. The noise voltage for the base-collector configuration is 12.6 and 7.56 μV / Hz at 102.5 Hz for 293 and 77 K, respectively, to 2.228 and 5.981 μV / Hz at 50 kHz for 293 and 77 K, respectively. This work is done using a standard Si-based technology, where a detector array with readout circuitry can be prototyped as a single chip. The floating base transistor topology is analyzed and used as a foundation for this work. The characteristics of a floating base configuration result in a wide depletion region, large-series resistance, and small-series capacitance. When shorting the base with the emitter and collector, respectively, compared with a floating base configuration, a smaller depletion region, reduced series resistance, and larger series capacitance are observed.

Large area MoS2/Si heterojunction-based solar cell through sol-gel method

Large area MoS2/Si heterojunction-based solar cell was prepared in which MoS2 thin film was directly grown on p-Si wafer using sol-gel method. Chemical bonding and microstructure analysis show that the MoS2 film was in the initial region of transition from amorphous to microcrystalline phase. Compared with the ITO/p-Si/Ag structure without MoS2, the open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF) of the ITO/MoS2/p-Si/Ag solar cell were improved dramatically, and the conversion efficiency (CE) increased from 1.1% to 4.6%. The insertion of MoS2 film reduced the interface defects concentration and increased the depletion region width of the solar cell, showing its enhancement effect on both chemical passivation and field passivation.

High-Performance Back-Illuminated Three-Dimensional Stacked Single-Photon Avalanche Diode Implemented in 45-nm CMOS Technology

We present a high-performance back-illuminated three-dimensional stacked single-photon avalanche diode (SPAD), which is implemented in 45-nm CMOS technology for the first time. The SPAD is based on a P+/Deep N-well junction with a circular shape, for which N-well is intentionally excluded to achieve a wide depletion region, thus enabling lower tunneling noise and better timing jitter as well as a higher photon detection efficiency and a wider spectrum. In order to prevent premature edge breakdown, a P-type guard ring is formed at the edge of the junction, and it is optimized to achieve a wider photon-sensitive area. In addition, metal-1 is used as a light reflector to improve the detection efficiency further in backside illumination. With the optimized 3-D stacked 45-nm CMOS technology for back-illuminated image sensors, the proposed SPAD achieves a dark count rate of 55.4 cps/μm2 and a photon detection probability of 31.8% at 600 nm and over 5% in the 420–920 nm wavelength range. The jitter is 107.7 ps full width at half-maximum with negligible exponential diffusion tail at 2.5 V excess bias voltage at room temperature. To the best of our knowledge, these are the best results ever reported for any back-illuminated 3-D stacked SPAD technologies.

Optimization design of betavoltaic battery based on titanium tritide and silicon using Monte Carlo code

This article presents the optimization design and simulation of a betavoltaic battery composed of a silicon p-n junction converter and a titanium tritide film as an isotope source. The self-absorption of β particles emitted from the tritium radioisotope in the titanium tritide film and the energy deposition of β particles in the silicon converter are investigated by the Monte Carlo simulation with the Geant4 radiation transport toolkit. The relationships between doping concentrations and basic parameters such as depletion region width, minority carrier diffusion length and leakage current of the PN junction are discussed through the calculation formulas. By optimizing the doping concentrations in the P-type and N-type regions, the optimized betavoltaic battery can maximize the output power and the conversion efficiency based on the energy deposition in the silicon. The results show that the optimal thickness of the titanium tritide film is about 0.7 µm and the optimal doping concentrations of the battery with a PN junction depth of 50 nm are Na=5.75×1019cm−3,Nd=2.95×1018cm−3. Under these parameters, the size 1mm×1mm proposed battery with 2.9 mCi/mm2 3H can achieve the output power 0.902 nW and the conversion efficiency 0.91%. The open circuit voltage, short circuit current and fill factor of the battery are 0.389 V, 3.03 nA and 0.766, respectively.

Effects of the unintentional background concentration, indium composition and defect density on the performance of InGaN p-i-n homojunction solar cells

We theoretically investigate the effects of the unintentional background concentration, indium composition and defect density of intrinsic layer (i-layer) on the photovoltaic performance of InGaN p-i-n homojunction solar cells by solving the Poisson and steady-state continuity equations. The built-in electric field and carrier generation rate depend on the position within the i-layer. The collection efficiency, short circuit current density, open circuit voltage, fill factor, and conversion efficiency are found to depend strongly on the background concentration, thickness, indium composition, and defect density of the i-layer. With increasing the background concentration, the maximum thickness of field-bearing i-layer decreases, and the width of depletion region may become even too small to cover the whole i-layer, resulting in a serious decrease of the carrier collection. Some oscillations as a function of indium composition are found in the short circuit current density and conversion efficiency at high indium composition and low defect density due to the interference between the absorbance and the generation rate of carriers. The defect density degrades seriously the overall photovoltaic performance, and its effect on the photovoltaic performance is roughly seven orders of magnitude higher than the previously reported values [Feng et al., J. Appl. Phys. 108 (2010) 093118]. As a result, the high crystalline quality InGaN with high indium composition is a key factor in the device performance of III-nitride based solar cells.

Design and electrical performance of CdS/Sb2Te3 tunneling heterojunction devices

In the current work, a tunneling barrier device made of 20 nm thick Sb2Te3 layer deposited onto 500 nm thick CdS is designed and characterized. The design included a Yb metallic substrate and Ag point contact of area of 10−3 cm2. The heterojunction properties are investigated by means of x-ray diffraction and impedance spectroscopy techniques. It is observed that the coating of the Sb2Te3 onto the surface of CdS causes a further deformation to the already strained structure of hexagonal CdS. The designed energy band diagram for the CdS/Sb2Te3 suggests a straddling type of heterojunction with an estimated conduction and valence band offsets of 0.35 and 1.74 eV, respectively. In addition, the analysis of the capacitance-voltage characteristic curve revealed a depletion region width of 14 nm. On the other hand, the capacitance and conductivity spectra which are analyzed in the frequency domain of 0.001–1.80 GHz indicated that the conduction in the device is dominated by the quantum mechanical tunneling in the region below 0.26 GHz and by the correlated barrier hopping in the remaining region. While the modeling of the conductivity spectra allowed investigation of the density of states near Fermi levels and an average scattering time of 1.0 ns, the capacitance spectra exhibited resonance at 0.26 GHz followed by negative differential capacitance effect in the frequency domain of 0.26–1.8 GHz. Furthermore, the evaluation of the impedance and reflection coefficient spectra indicated the usability of these devices as wide range low pass filters with ideal values of voltage standing wave ratios.

Bending effect on the resistive switching behavior of a NiO/TiO2 p-n heterojunction.

Herein, NiO/TiO2 heterojunctions were fabricated by sol–gel spin coating on plastic substrates to investigate the effects of bending on resistive switching. The switching mechanism is well explained by the formation and rupture of oxygen-vacancy conducting filaments modulated by the p–n interface. Compared with that of the unbent film, the device ON/OFF ratio is slightly improved after 5000 bending repetitions. Finite element calculations indicate that the tensile stress of 0.79% can lead to the formation of channel cracks. Further charge transport analysis shows that the conducting filaments may cause an incomplete rupture because the bending-induced channel crack permeates through the p–n interface and reduces the local depletion-region width.

Effect of atomic layer deposited Al2O3:ZnO alloys on thin-film silicon photovoltaic devices

In this work, we present the effects of the Al2O3:ZnO ratio on the optical and electrical properties of aluminum doped ZnO (AZO) layers deposited by atomic layer deposition, along with AZO application as the anti-reflective coating (ARC) layer and in heterojunction configurations. Here, we report complex refractive indices for AZO layers with different numbers of aluminum atomic cycles (ZnO:Al2O3 = 1:0, 39:1, 19:1, and 9:1) and we confirm their validity by fitting models to experimental data. Furthermore, the most conductive layer (ZnO:Al2O3 = 19:1, conductivity ∼4.6 mΩ cm) is used to fabricate AZO/n+/p-Si thin film solar cells and AZO/p-Si heterojunction devices. The impact of the AZO layer on the photovoltaic properties of these devices is studied by different characterization techniques, resulting in the extraction of recombination and energy band parameters related to the AZO layer. Our results confirm that AZO 19:1 can be used as a low cost and effective conductive ARC layer for solar cells. However, AZO/p-Si heterojunctions suffer from an insufficient depletion region width (∼100 nm) and recombination at the interface states, with an estimated potential barrier of ∼0.6–0.62 eV. The work function of AZO (ZnO:Al2O3 = 19:1) is estimated to be in the range between 4.36 and 4.57 eV. These material properties limit the use of AZO as an emitter in Si solar cells. However, the results imply that AZO based heterojunctions could have applications as low-cost photodetectors or photodiodes, operating under relatively low reverse bias.

Impact of defect distribution on IrOx/ZnO interface doping and Schottky barriers

We used depth-resolved cathodoluminescence spectroscopy (DRCLS) to measure the nature and spatial distribution of native point defects at Zn- and O-polar ZnO interfaces with iridium oxide (IrOx) and their impact on Schottky barrier formation. IrOx and other metal oxides exhibit higher Schottky barriers than their pure metal counterparts, consistent with wider depletion regions and potentially useful for ohmic contacts to p-type semiconductors. DRCLS with I-V and 1/C2-V barrier height and carrier profile measurements showed high zinc vacancy VZn and CuZn defect densities that compensate free carrier densities, increase depletion widths, and form higher effective barriers than Ir/ZnO contacts. Zn-polar versus O-polar ZnO interfaces with IrOx exhibit 40% higher VZn + CuZn interface segregation and lower carrier densities within a wider depletion region, accounting for the significantly higher (0.89 vs. 0.67 eV) barrier heights. Both the depth of VZn density segregation and the Zn-deficient layer thickness measured microscopically match the depletion width and applied electric fields comparable to spontaneous polarization fields across similar layers displaying analogous defect segregation. These results account for the difference in polarity-dependent segregation due to the electric field-driven diffusion of native defects near ZnO interfaces.

Controllable Electrical Contact Resistance between Cu and Oriented-Bi2Te3 Film via Interface Tuning.

The contact resistance between metals and semiconductors has become critical for the design of thin-film thermoelectric devices with their continuous miniaturization. Herein, we report a novel interface tuning method to regulate the contact resistance at the Bi2Te3-Cu interface, and three Bi2Te3 films with different oriented microstructures are obtained. The lowest contact resistivity (∼10-7 Ω cm2) is observed between highly (00l) oriented Bi2Te3 and Cu film, nearly an order of magnitude lower than other orientations. This significant decrease of contact resistivity is attributed to the denser film connections, lower lattice misfit, larger effective conducting contact area, and smaller width of the surface depletion region. Meanwhile, our results show that the reduction of contact resistance has little dependence on the interfacial diffusion based on the little change in contact resistivity after the introduction of an effective Ti barrier layer. Our work provides a new idea for the mitigation of contact resistivity in thin-film thermoelectric devices and also gives certain guidance for the size design of the next-level miniaturized devices.

Influence of Indium Content on the Unintentional Background Doping and Device Performance of InGaN/GaN Multiple-Quantum-Well Solar Cells

We experimentally investigate the influence of indium composition on the photovoltaic performance of InGaN/GaN multiple-quantum-well (MQW) solar cells. Three MQW structures with different In content are grown via metal-organic chemical vapor deposition, and then fabricated into solar cell devices. The solar cells with lower In content show stronger photovoltaic response, which may be ascribed to both smaller piezoelectric fields in InGaN well layers and better material quality of MQW active region. In addition, based on the capacitance–voltage measurements by which the profiles of apparent carrier concentration can be determined, we find that in the higher-In-content solar cells the background electron concentration in unintentionally doped InGaN QWs is higher. Consequently, the effective volume of active region, or the width of depletion region, may become smaller at zero bias and correspondingly the light absorption by InGaN active layers is reduced. This may be attributed to the increased nitrogen vacancy-related defects which are easier induced into In-rich InGaN alloys as a source of shallow donors.

Light-Bias-Dependent External Quantum Efficiency of Kesterite Cu2ZnSnS4 Solar Cells

The light-bias-dependent behavior of external quantum efficiency (EQE) in kesterite Cu2ZnSnS4 solar cells utilizing different buffers has been reported. Under red light bias, the blue EQE can be enhanced significantly, exceeding unity in magnitude due to photoconductivity of buffers increasing depletion region width and thereby increasing collection efficiency. Under blue light bias, the red EQE increases only in devices with a hybrid buffer. It stays constant with a CdS buffer due to saturated photoconductivity and even decreases with an In2S3 buffer due to optical injection of blue photons through the more transparent In2S3 into the absorber. Under white illumination, the enhancement can be observed only with unsaturated buffers such as In2S3 and the hybrid buffer, while the red EQE reduces due to optical injection. This light-bias-dependent behavior in EQE (including EQE exceeding unity) can be attributed to two key factors: photoconductivity of the buffer layers combined with low minority carrier life...

Ohmic contact behavior of aluminum-doped zinc oxide with carbon-doped p-GaP epilayer for AlGaInP LEDs applications

Aluminum-doped zinc oxide (AZO) thin films used for ohmic contact layers on carbon-doped GaP window layers (p-GaP:C) of AlGaInP light-emitting diodes were fabricated and characterized. AZO thin films with different Zn:Al cycle ratios (15:1, 20:1, and 25:1) were deposited on p-GaP:C window layers through atomic layer deposition. The contact characteristics of the AZO thin films on p-GaP:C were considerably changed from Schottky contact to ohmic contact after rapid thermal annealing (RTA) at 350 °C for 1 min. The most favorable specific contact resistance of AZO/p-GaP:C was evaluated using a circular transmission line model as 6.3 × 10−3 Ω/cm2. Angle-resolved X-ray photoelectron spectroscopy was employed to understand the ohmic contact behavior of AZO/p-GaP:C. After RTA, Zn atoms in the AZO thin films notably diffused into the p-GaP:C layers and Ga atoms diffused out of the p-GaP:C layer. Therefore, the Ga vacancies were occupied by Zn atoms, which increased the doping concentration in the near-surface region of p-GaP:C and reduced the depletion region width of the semiconductor region. Thus, numerous carriers were able to tunnel through the reduced Schottky barrier and those carriers produced the ohmic contact behavior between the AZO and p-GaP:C.

Ultra-thin Cu2ZnSnS4 solar cell by pulsed laser deposition

We report on the fabrication of a 5.2% efficiency Cu2ZnSnS4 (CZTS) solar cell made by pulsed laser deposition (PLD) featuring an ultra-thin absorber layer (less than 450nm). Solutions to the issues of reproducibility and micro-particulate ejection often encountered with PLD are proposed. At the optimal laser fluence, amorphous CZTS precursors with optimal stoichiometry for solar cells are deposited from a single target. Such precursors do not result in detectable segregation of secondary phases after the subsequent annealing step. In the analysis of the solar cell device, we focus on the effects of the finite thickness of the absorber layer. Depletion region width, carrier diffusion length, and optical losses due to incomplete light absorption and back contact reflection are quantified. We conclude that material- and junction quality is comparable to that of thicker state-of-the-art CZTS devices, even though the efficiency is lower due to optical losses.

Impulse voltage control of continuously tunable bipolar resistive switching in Pt/Bi0.9Eu0.1FeO3/Nb-doped SrTiO3 heterostructures

Epitaxial Bi0.9Eu0.1FeO3 (BEFO) thin films are deposited on Nb-doped SrTiO3 (NSTO) substrates by pulsed laser deposition to fabricate the Pt/BEFO/NSTO (001) heterostructures. These heterostructures possess bipolar resistive switching, where the resistances versus writing voltage exhibits a distinct hysteresis loop and a memristive behavior with good retention and anti-fatigue characteristics. The local resistive switching is confirmed by the conductive atomic force microscopy (C-AFM), suggesting the possibility to scale down the memory cell size. The observed memristive behavior could be attributed to the ferroelectric polarization effect, which modulates the height of potential barrier and width of depletion region at the BEFO/NSTO interface. The continuously tunable resistive switching behavior could be useful to achieve non-volatile, high-density, multilevel random access memory with low energy consumption.