Carrier Tunneling Research Articles

Ultrafast photoresponse in n-MoS2/AlN/p-Si (SIS) heterojunction based visible to NIR photodetectors

In the present study, n-MoS2/p-Si and n-MoS2/AlN/p-Si Semiconductor-Insulator-Semiconductor (SIS) heterojunctions based photodetectors have been fabricated using dc magnetron sputtering technique. The recorded value of responsivities for both n-MoS2/p-Si and n-MoS2/AlN/p-Si heterojunctions are found to be 0.3378 AW-1 and 1.865 AW-1 for visible radiations (532 nm) and 0.1245 AW-1 and 1.4743 AW-1 for NIR radiations (1064 nm), respectively. The response time (rise time and decay time) of the fabricated heterojunction decreased from 47.30 μs and 56.58 μs to 3.32 μs and 4.73 μs with the insertion of an ultrathin insulating AlN layer. The higher current flows in SIS heterojunction can be ascribed to the carrier tunneling mechanism through the ultrathin insulating layer. Moreover, the thickness effect of the insulating layer on the photodetection performance of the fabricated heterostructure has been investigated. The obtained results demonstrate that SIS heterojunctions with an excellent carrier transport mechanism are proved to be potential candidates in next-generation ultrafast optoelectronics applications.

Surface passivation in c-Si solar cells via a double-barrier quantum-well structure for ameliorated performance

Modern solar cell technology suffers low surface passivation, high recombination losses at the interface, and dopant diffusion losses that limit solar cell’s efficiency. Therefore, a double-barrier quantum-well (DBQW) structure-based surface passivation contact is introduced here to resolve these shortcomings. In this regard, DBQWs of different thicknesses with stacks of SiOx/ nc-SiOx (3, 5, 8, and 10 nm)/SiOx layers were explored as surface passivation layers and dopant diffusion barriers. Multiple characterizations were utilized to examine DBQW thickness-dependent properties, such as contact resistance, passivation, and recombination current density; the best result was for the 5-nm-thickness QW passivation layer. To justify the obtained result, theoretical calculations were carried out based on the experimental results, which suggested the resonance tunneling of charge carriers across the 5 nm DBQW structure. Furthermore, SIMS was explored to examine the diffusion barrier property of these QWs, which revealed that dopant diffusion was suppressed by the QW with the double SiOx layer. Finally, the nc-SiOx(n) /5-nm QW/c-Si surface passivation structure showed substantial enhancement in a lifetime (τeff) of 3560 μs, an implied open-circuit voltage (iVoc) of 735 mV, and reduced recombination current density (Jo) = 1.5 fA/cm2.

High detectivity graphene/si heterostructure photodetector with a single hydrogenated graphene atomic interlayer for passivation and carrier tunneling

Hydrogenated graphene is easy to prepare and chemically stable. Besides, hydrogenation of graphene can open the band gap, which is vital for electronic and optoelectronic applications. Graphene/Si photodetector (PD) has been widely studied in imaging, telecommunications, and other fields. The direct contact between graphene and Si can form a Schottky junction. However, it suffers from poor interface state, where the carrier recombination at the interface causes serious leakage current, which in turn leads to a decrease in the detectivity. Hence, in this study, hydrogenated graphene is used as an interfacial layer, which passivates the interface of graphene/Si (Gr/Si) heterostructure. Besides, the single atomic layer thickness of hydrogenated graphene is also crucial for the tunneling transport of charge carriers and its suitable energy band position reduces the recombination of carrier. The fabricated graphene/hydrogenated-graphene/Si (Gr/H–Gr/Si) heterostructure PD showed an extremely low dark current about 10−7 A. As a result, it had low noise current and exhibited a high specific detectivity of ∼2.3 × 1011 Jones at 0 V bias with 532 nm laser illumination. Moreover, the responsivity of the fabricated PD was found to be 0.245 A W−1 at 532 nm illumination with 10 μW power. These promising results show a great potential of hydrogenated graphene to be used as an interface passivation and carrier tunneling layer for the fabrication of high-performance Gr/Si heterostructure PDs.

Isolation properties and failure mechanisms of vertical Pt / n-GaN SBDs

This paper reports on the isolation properties and failure mechanism of n-type vertical Pt / n-GaN Schottky barrier diodes and the dependence on the drift layer doping concentration. The results indicate that (i) independently on doping density, the Schottky barrier height is lower than the theoretical expectation, and this limits the blocking properties of the junction; (ii) barrier lowering was associated to an injection mechanism that involves deep levels in the semiconductor layer, near the junction, favouring injection and tunneling of carriers. By performing a detailed analysis of the breakdown mechanism, we also demonstrated that (iii) the failure of the devices in reverse bias condition is related to a power-related mechanism associated to current flowing along the mesa edges. We thus conclude that (iv) a good edge termination and passivation of the surfaces is fundamental to exploit the full blocking capability of the semiconductor.

Novel approach for augmented carrier transfer and reduced Fermi level pinning effect in InAs surface quantum dots

The present work introduces an in-situ technique to tune the inter-dot coupling between vertically aligned InAs surface and buried quantum dots (SQD and BQD). Here, we utilize the self-assembly growth kinetics to get the most stable SQDs with lowest possible surface states. We vary the monolayer (ML) coverage of top SQD layer (from 2.2 ML to 1.6 ML), keeping monolayer coverage of BQD constant (2.7 ML). The reduced Fermi level pinning effect is observed through the monolayer coverage minimization, and the upshifting of energy level in the conduction band is obtained. The degeneracy between BQD and SQD energy levels is maximized in case of the 1.6 ML SQD and an enhanced coupling between the two QD families is observed through the photoluminescence (PL) result analysis. The type-II carrier distribution with effect of the Fermi level pinning is reduced in the 1.6 ML SQD structure, which claims the highest level of Coulomb coupling between the two QD families. Also, AFM images show lower dispersion in size and shape of the surface dot with 1.6 ML coverage. The proposed optimum QD heterostructure would promote a QD based sensor with high efficiency in effect with higher level of inter- dot carrier tunneling.

Heterointerface-engineered type Ⅱ SnO2/boron-doped diamond heterojunction photodiodes with diverse diode characteristics and binary photoresponse

As an inevitable existence in semiconductor heterostructures, interfacial states have a non-negligible impact on the performance of heterojunction-based optoelectronic devices. Here, we develop high-performance photodiodes based on heterointerface-engineered type II SnO2/boron-doped diamond (BDD) heterojunctions. We modulate the type of the interfacial states of SnO2/BDD heterojunctions by changing the partial pressure of oxygen during the process of SnO2 deposition by RF magnetron sputtering. As a result, backward rectifying, Zener, and forward rectifying diodes are obtained. The diversity of the diode characteristics is related to the carrier tunneling and avalanche multiplication effects. In addition, the I–V curve of the Zener diode has a negative differential resistance precursor under UV light irradiation. The photogenerated holes in the forward rectifier diode are easily trapped at the heterointerface during transmission. Zener and forward rectifier diodes can output positive and negative photocurrents (i.e., binary photoresponse) under on/off periodic light illumination owing to photovoltaic and pyro-phototronic effects. These results reveal the potential of SnO2/BDD heterojunctions in the field of optical logic computing devices.

Nanosheet-Compatible Complementary-Field Effect Transistor Logic Non-Volatile Memory Device

This study proposes a logic non-volatile memory (NVM) device based on the state-of-the-art gate-all-around (GAA) nanosheet process. By adopting a complementary FET structure, we not only reduce the layout footprint area but also demonstrate the compatibility with sub-3nm nanosheet CMOS technology. We simulated the device in TCAD for program and erase operations, considering both Fowler Nordheim tunneling and hot carrier injection mechanisms. The memory window is optimized by adjusting the device structure, such as tunneling and blocking oxide thicknesses, top- and bottom-channel width, and the floating gate length, resulting in a significant enhancement in the memory window. Finally, NAND/NOR type CFET logic NVM are demonstrated to reveal the possible applications of gate-all-around stacked junctionless nanosheet transistor towards embedded flash.

Polarization of Bi2Se3 thin film toward non-volatile memory applications

In recent years, topological insulators have drawn growing interest as a unique electronic state of matter toward quantum information technology. Despite the logic devices with magnetization switching through spin–orbit torque or the topological magneto-electric effect, realizing memory devices based on topological insulators has been urged in quantum computing applications. In this work, we report the design and fabrication of a non-volatile memory device that employs polarization of Bi2Se3 thin films achieving fast memory speed, sufficient memory window, and good reliability. The Bi2Se3 film polarizes under an external electrical field with charges accumulated on the top and bottom surfaces separating the electrons and holes. Such polarization is much faster than the carrier tunneling in conventional floating-gate flash memory and ferroelectric-based memory devices. In addition, good memory retention and endurance properties have also been obtained, showing great potential in high-performance memory application in future topological insulator-involved information technology.

Single- Versus Multi-Step Trap Assisted Tunneling Currents—Part I: Theory

Leakage currents through dielectrics in modern logic, memory, and power devices, and back-end interlevel layers can severely increase the time-zero power dissipation and shorten the lifetime of the material structure. Depending on thickness, material properties, and fabrication quality, different conduction mechanisms through the insulating layer, such as band-to-band and trap-assisted tunneling (TAT) are observed. These leakage currents can be distinguished by their dependence on electric field strength, temperature, and their transient characteristics. The identification of the underlying mechanisms is of utmost importance for the selection of suitable dielectrics and optimization of device processing. In this work, we present a nonradiative multiphonon model which extends our compact physics (Comphy) reliability framework to also account for TAT. The model is based on a single physical defect parameter set from which charge transfer kinetics between charge carrier reservoirs and oxide defects (single-TAT) as well as between the defects (multi-TAT) can be derived. We thereby demonstrate that the thermal barrier for inelastic defect–defect carrier tunneling is approximately twice as large in comparison to reservoir-defect interaction and further show in which parameter regime multi-TAT plays a role. Our results show that multi-TAT currents are a less important contribution to the overall tunnel current than previously assumed.

Two-dimensional Ruddlesden-Popper halide perovskite solar absorbers with short-chain interlayer spacers

Two-dimensional Ruddlesden-Popper (2DRP) halide perovskites have emerged as promising solar absorbers due to their much-enhanced stability compared with their three-dimensional counterparts, but the light conversion efficiency is limited by their intrinsic optoelectronic properties such as increased exciton binding energy and poor cross-layer carrier transport properties. We herein demonstrate, through first-principles calculations, that the efficiency of 2DRP halide perovskites can be enhanced by adopting the short-chain interlayer spacers among perovskite layers. We adopted short-chain alkylmethylammonium (MA) and formamidinium (FA) organic cations to exchange the regular long-chain butylammonium (BA) and phenethylammonium (PEA) cations in 2DRP halide perovskites with the formula ${A}_{2}B{\mathrm{Pb}}_{2}{\mathrm{I}}_{7}$ ($A=\mathrm{BA}$, PEA, MA, and FA; $B=\mathrm{MA}$ and FA). We find that varying the interlayer spacers results in changed distortion of ${\mathrm{PbI}}_{6}$ octahedra of the perovskite framework, which in turn influences the stability and electronic structure of 2DRP perovskites. Compared with the long-chain BA/PEA intercalated 2DRP perovskites, the short-chain MA/FA intercalated ones have slightly reduced thermodynamic stability, but their calculated bandgaps are closer to the ideal value for solar cells (1.3--1.6 eV), accompanied with comparable carrier effective masses and optical absorption. More importantly, they demonstrate lower exciton binding energies and two orders of magnitude increased out-of-plane carrier tunneling probability. These factors are expected to further enhance the power conversion efficiency of 2DRP-based solar cells.

Complementary Tunneling Behaviors in van der Waals Vertical Heterostructures

Vertical tunneling transistors are of interest in the development of next-generation electronics. However, due to nonideal metal-semiconductor interfaces, the majority tunneling carrier type and device polarity are largely fixed, posing a key limitation for practical application. The authors demonstrate complementary tunneling behaviors in vertical heterostructures using ultrathin BN as a tunneling barrier and damage-free van der Waals metal as the top contact, in which the majority carrier type can be switched from electrons to holes by a bias voltage. This work provides proof of principle and comprehensive understanding of the tunneling mechanism in these high-performance devices.

Steering coherence in quantum dots by carriers injection via tunneling

Abstract Coherent control is a key experimental technique for quantum optics and quantum information processing. We demonstrate a new degree of freedom in coherent control of semiconductor quantum dot (QD) ensembles operating at room temperature using the tunneling injection (TI) processes in which charge carriers tunnel directly from a quantum well reservoir to QD confined states. The TI scheme was originally proposed and implemented to improve QD lasers and optical amplifiers, by providing a direct injection path of cold carriers thereby eliminating the hot carrier injection problem which enhances gain nonlinearity. The impact of the TI processes on the coherent time of the QDs was never considered, however. We show here that since the cold carriers that tunnel to the oscillating QD state are incoherent, the rate of injection determines the coherent time of the QDs thereby controlling coherent light–matter interactions. Coherent interactions by means of Rabi oscillations were demonstrated in absorption and for weak excitation pulses in the gain regime. However, Rabi oscillations are totally diminished under strong excitation pulses which increase the rate of stimulated emission, causing the tunneling processes to dominate what shortens the coherence time significantly. Since the tunneling rate, and hence, the coherence time, were controlled by the optical excitation and electrical bias, our finding paves the way for TI-based coherence switching on a sub-picosecond time scale in room-temperature semiconductor nanometric structures.

N-type polysilicon passivating contact combined with hydrogen-containing TCO as the interconnected structure for perovskite/silicon tandem solar cells

An interconnected structure of perovskite/silicon tandem solar cell is the key point to accomplishing high efficiency over 30% due to its high performance of carrier recombination and tunneling. In general, an interconnected structure is designed by a polysilicon (poly-Si) passivating contact structure and a hydrogen-containing transparent conductive oxides (TCO). When the bottom cell structure was fabricated in mass production, poly-Si passivating contact inevitably suffered from a degradation of the passivation quality associated with the metallization firing process. In this work, we investigated the effect of hydrogen-containing indium tin oxide (ITO:H) and Al-doped ZnO (AZO:H) prepared by dc magnetron sputtering on poly-Si passivating contacts passivation degradation. The passivation quality of the poly-Si passivating contacts can be significantly improved by sputtering and annealing at 350 °C after firing of both the hydrogen-containing TCOs. The improvement on poly-Si passivating contacts passivation quality increased with the increased of annealing dwell time. When at the optimal annealing time of 30 min, the iVoc value of ITO:H increased to 725 mV while that of AZO:H increased to 710 mV, respectively. In addition, the optical and electrical properties of ITO films can be improved by annealing process, including transmittance, optical bandgap, Hall mobility and resistivity. When the annealing time is less than 30min, the contact resistivity between Ag/ITO and doped poly-Si increases slightly with the increase of annealing time. However, the optical and electrical properties of AZO films degraded, which may be due to the instability of the prepared AZO:H films. Eventually, based on this interconnected structure, a perovskite/Si tandem cell structure was designed, and silicon substrate cells could be fabricated in mass production.

Transient analysis of photomultiplication-type organic photodiodes

Photomultiplication-type organic photodetectors have emerged as a class of next generation solution-processed photodetectors with high gain. Despite this promising feature, the reported photodectors still suffer from relatively large dark currents at high bias voltages. To overcome this drawback, a mechanistic understanding of the photomultiplication effect in organic photodiodes is required. In this work, we advanced the performance of photomultiplication-type organic photodetectors by tuning the active layer composition and interfacial layers. The optimized devices exhibit small dark currents and flat dark current–voltage curves under the reverse bias condition up to −10 V. The optimized photodetectors also reached an ultra-high responsivity of 23.6 A/W and the specific detectivity of 1.04 × 1012 Jones at −10 V. More importantly, we investigated the photomultiplication process with multiple transient techniques and revealed that the photoconductive gain effect is a slow process, which relies on the photo-Schottky effect enabled by charge carrier tunneling and the accumulation of holes. Furthermore, we also demonstrated prototypical pulsed-light detection based on the optimized devices, which showed great potential for real applications.

Dipolar Noise in Fluorinated Molecular Wires.

We demonstrate a strategy to directly map and quantify the effects of dipole formation on electrical transports and noises in the self-assembled monolayers (SAMs) of molecular wires. In this method, the SAM patterns of fluorinated molecules with dipole moments were prepared on conducting substrates, and a conducting probe in contact-mode atomic force microscopy was utilized to map currents and noises through the probe on the molecular patterns. The maps were analyzed to extract the characteristic parameters of dipolar noises in SAMs, and the results were compared with those of hydrogenated molecular patterns without dipole moments. At rather low bias conditions, the fluorinated molecular junctions exhibited a tunneling conduction and a resistance value comparable to that of the hydrogenated molecules with a six-times-longer length, which was attributed to stronger dipoles formation in fluorinated molecules. Interestingly, conductance (G) in different regions of fluorinated molecular patterns exhibited a strong correlation with a noise power spectral density of SI/I2 like SI/I2 ∝ G−2, which can be explained by enhanced barrier fluctuations produced by the dipoles of fluorinated molecules. Furthermore, we observed that the noise power spectral density of fluorinated molecules showed an anomalous frequency (f) dependence like SI/I2 ∝ 1/f1.7, possibly due to the slowing down of the tunneling of carriers from increased barrier fluctuations. In rather high bias conditions, conductions in both hydrogenated and fluorinated molecules showed a transition from tunneling to thermionic charge transports. Our results provide important insights into the effects of dipoles on mesoscopic transport and resistance-fluctuation in molecules and could have a significant impact on the fundamental understanding and applications in this area.

High performance quantum piezotronic tunneling transistor based on edge states of MoS2 nanoribbon

High performance edge states-based quantum piezotronic tunneling transistor with MoS2 nanoribbon device architecture at room temperature is demonstrated. The edge states are identified by the tight-binding band calculations. The Fermi energy position related to carrier concentration and tunneling probability are investigated based on quantum mechanics theory. It is found that the tunneling current can be exponentially controlled by piezotronic effect, and the Schottky barrier height can also be modified. The edge states transport behavior is further elucidated by conductance and electronic density distribution with applied strains. The strain sensitivity of the quantum piezotronic transistor can reach over 103. This study is capable of advancing the design of new generation of transistor devices based on edge states, and providing prospects of realizing high performance room temperature quantum piezotronic devices.

Hysteresis and jumps in the I−V curves of disordered two-dimensional materials

The superconductor-insulator transition (SIT) in thin-film disordered superconductors is a hallmark example of a quantum phase transition. Despite being observed more than 30 years ago, its nature is still under a vivid debate. One intriguing observations concerns the insulating side of the transition, which exhibits some unusual properties. Among them is its current-voltage relation ($I-V$ curve), which includes (1) a conductance that changes abruptly by several orders of magnitude with increasing voltage, (2) hysteretic behavior, and (3) multiple (sometimes more than a hundred) smaller current jumps near the transition. Some models have been suggested before, but no model has been successful in accounting for the observed behavior in full. One commonly used approach is to model the disordered sample as a two-dimensional array of conducting islands, where charge carriers tunnel from one island to its neighbors. In those models, fast relaxation is assumed, and the system is treated as always being in electrostatic and thermal equilibrium. Those models are successful in explaining some measurement results, including the phase transition itself, but fail to reproduce hysteresis in their predicted $I-V$ curves. Here, we suggest incorporating finite relaxation time into an array model. We show that, in the slow relaxation limit, our model can reproduce hysteresis and multiple jumps in the $I-V$ curve. Based on our results we argue that a similar behavior should be also observed in two-dimensional normal (non-superconducting) arrays. This claim is supported by past observations. We analyse the role of different parameters in our model, determine the range of relevant time scale in the problem, and compare our results with selected measurements.

Non-Layered Te/In2 S3 Tunneling Heterojunctions with Ultrahigh Photoresponsivity and Fast Photoresponse.

A photodetector based on 2D non-layered materials can easily utilize the photogating effect to achieve considerable photogain, but at the cost of response speed. Here, a rationally designed tunneling heterojunction fabricated by vertical stacking of non-layered In2 S3 and Te flakes is studied systematically. The Te/In2 S3 heterojunctions possess type-II band alignment and can transfer to type-I or type-III depending on the electric field applied, allowing for tunable tunneling of the photoinduced carriers. The Te/In2 S3 tunneling heterojunction exhibits a reverse rectification ratio exceeding 104 , an ultralow forward current of 10-12 A, and a current on/off ratio over 105 . A photodetector based on the heterojunctions shows an ultrahigh photoresponsivity of 146 AW-1 in the visible range. Furthermore, the devices exhibit a response time of 5ms, which is two and four orders of magnitude faster than that of its constituent In2 S3 and Te. The simultaneously improved photocurrent and response speed are attributed to the direct tunneling of the photoinduced carriers, as well as a combined mechanism of photoconductive and photogating effects. In addition, the photodetector exhibits a clear photovoltaic effect, which can work in a self-powered mode.

Investigation of All-Oxide Thin-Film Solar Cell With p-SnOₓ as Absorber Layer

We propose an n-ZnO/p-SnO_<i>x</i> heterojunction solar cell as a novel pathway to realize all-oxide photovoltaics that offer attractive features such as stability, cost-effectiveness, and nontoxicity. TCAD device simulator was used to investigate the performance of n-ZnO/p-SnO_<i>x</i> thin-film solar cell using inputs on material parameters of SnO_<i>x</i> obtained experimentally. In this study, we considered the effects of SnO_<i>x</i> parameters, namely, bandgap and electron affinity on the performance of solar cell. Electrical transport models applied in modeling include the effects of interfacial recombination, tunneling, band discontinuity, and minority carrier lifetime. Our simulation results show that a maximum power conversion efficiency of &#x007E;15.5&#x0025; can be obtained in a window of an optimum combination of bandgap and electron affinity of SnO_<i>x</i> layer. Further enhancement in efficiency is achievable by improving the minority carrier lifetime.

Mid-IR quantum dot cascade VCSEL: feasibility study and feature extraction.

Quantum cascade lasers with quantum dot (QD) active regions have been developed to overcome the drawbacks of quantum-well-based cascade lasers. Low threshold current and enhanced bandwidth characteristics of QD lasers can be combined with promising energy-efficient features of vertical cavity surface emitting lasers (VCSELs) through introducing the novel, to the best of our knowledge, concept of a QD cascade VCSEL (QDC-VCSEL). Design steps and predicted features of a QDC-VCSEL operating at 4.5 µm wavelength are discussed in this paper, where the cavity and active region are optimized for TE mode using a high contrast grating design. Laser characteristics are investigated by developing rate equations for the cascade scheme. Threshold and L-I characteristics are obtained at different temperatures. Also, the dependency of threshold current, slope efficiency, and modulation bandwidth on design parameters, mainly the carrier tunneling time, is studied. Results exhibit output peak powers of ∼2 and ∼0.5mW at 150 and 273 K, respectively. In addition, threshold current increment and modulation bandwidth reduction are observed with the increase in tunneling time.