Abrupt Heterojunction Research Articles

P-GaAs/n-Ga2O3 heterojunction diode with breakdown voltage of ∼800 V

As an attractive next generation ultrawide bandgap material, Ga2O3 has been demonstrated to be capable of high voltage operation. However, the lack of shallow p-type dopant in Ga2O3 leads to difficulties in developing Ga2O3 based bipolar devices. The semiconductor grafting technology, by bonding a single crystalline p-type semiconductor nanomembrane (NM) to n-Ga2O3 to from abrupt heterojunction, offers a way to expand the application of Ga2O3 in bipolar areas. In this Letter, we report a p-GaAs/(001)-Ga2O3 heterojunction diode fabricated using semiconductor grafting technology. The single crystalline GaAs NM was prepared and transferred onto a Ga2O3 substrate, showing good integrity. Diodes of a mesa diameter of 25 μm were fabricated, showing a high breakdown voltage of ∼800 V, a power figure of merit of 126.5 MW/cm2, and a calculated 1D peak electric field of 1.60 MV/cm. The turn on voltage of the diode was determined to be around 1.91 V with an on–off ratio to be 6 × 106 at −4/4 V and the specific on resistance to be 5.06 mΩ cm2. The ideality factor was extracted to be approximately 1.34, indicating reasonable interface quality. The interfacial defects and the self-passivation effects of Ga2O3 are discussed, and the theoretical band diagram was analyzed for the p-GaAs/n-Ga2O3 heterojunction. By managing the peak electric field with suitable edge termination techniques, higher breakdown voltage can be expected. In addition, by taking advantage of the InGaAsP alloy system, a wide variety of GaAs-Ga2O3 based high voltage HBT can be further developed.

Non-Hermitian fermions with effective mass

In this work, we readdress the Dirac equation in the position-dependent mass (PDM) scenario. Here, one investigates the quantum dynamics of non-Hermitian fermionic particles with effective mass assuming a $(1+1)$-dimension flat spacetime. In seeking a Schr\"{o}dinger-like theory with PT symmetry is appropriate to assume a complex potential. This imaginary interaction produces an effective potential purely dependent on mass distribution. Furthermore, we study the non-relativistic limit by adopting the Foldy-Wouthuysen transformation. As a result, that limit leads to an ordering equivalent to the ordering that describes abrupt heterojunctions. Posteriorly, particular cases of mass distribution also were analyzed. Thus, interesting results arise for the case of a linear PDM, which produces an effective harmonic oscillator and induces the emergence of bound states. For a hyperbolic PDM, an effective potential barrier emerges. However, in this case, the fermions behave as free particles with positive-defined energy.

Color-tunable transparent photovoltaics for onsite power production under sunlight and indoor light

Transparent photovoltaics (TPVs) are energy-generating devices made of transparent material and incorporating solar cell technology. The ultimate goal of using such devices is to reach a level of sustainability by harnessing energy from common structures such as windows on buildings and vehicles. Applications of this concept can be widened by incorporating a color-tuning scheme into TPVs. In this sense, light can be managed; here, we introduce a method for this using a ZnO-based back reflection method. Large-scale (16 cm2) TPVs are achieved by solid-state sputtering of metal oxide layers. They exhibited high power conversion efficiency (η) (3.8% for indoor light-emitting diode lamp) and long-term stability (>100 days). The abrupt heterojunction between n-type ZnO and p-type NiO spontaneously forms a space charge region with a strong electric field. An additional thin Si film in the heterojunction effectively improves solar cell performance, making it useful for power generation, even under indoor illumination. Substantial efficiency of 3.02% was achieved by an Al-doped ZnO for the Sun spectrum. This implies the ability to produce TPV power from the Sun or artificial light in versatile ways. TPVs and color-tuned photovoltaics can be used in windows and other such objects to realize an onsite power supply.

The barrier capacitance of n-SnS2/n-CdIn2Te4 heterojunction

The results of the study of the electrical characteristics of the n-SnS2/n-CdIn2Te4, obtained by a method of deposition over optical contact, have been presented. It is shown that the current-voltage characteristics and the capacitance-voltage (C-V) characteristics are typical for an abrupt heterojunction. The frequency dependence of the C-V characteristic is established, namely, with increasing frequency of alternating voltage, the barrier capacitance decreases, which is due to the presence of intrinsic structural defects and related localized donor states in the band gap of CdIn2Te4. The obtained results are explained by the frequency-dependent recharging processes of deep donor centers.

Quantum information entropies for a soliton at hyperbolic well

In this work, the probability uncertainties related to a stationary quantum system with solitonic mass distribution when subjected to deformable hyperbolic potentials are studied. Shannon’s entropy and Fisher’s information of a position-dependent mass are calculated. To investigate the concept of Shannon and Fisher entropies of the solitonic mass distribution subject to the hyperbolic potential, it is necessary to obtain the analytic solutions at position and momentum space. For the Hamiltonian operator to be Hermitian, we consider the stationary Schrödinger equation ordered by Zhu–Kroemer. This ordering is known to describe abrupt heterojunctions in semiconductor materials.

Г-L intervalley separation and electron mobility in GaAsSb grown on InP: Transport comparison with the GaInAs and GaInAsSb alloys

GaAs0.51Sb0.49 is lattice-matched to InP and finds electron transport applications in base or absorber layers in high-speed heterostructure bipolar transistors or photodiodes, because its staggered (“type-II”) band alignment with InP favors electron injection across abrupt heterojunctions. Little remains known about electron transport properties and band structure details of GaAsxSb1−x near x = 0.5. Particularly, based on the Γ-L intervalley separation in binary constituents (ΔΓL = 84 meV in GaSb and 290 meV in GaAs at 300 K), interpolation suggests a low Γ-L separation in GaAs0.51Sb0.49 before considering energy gap bowing effects. To gain insight into electron transport in GaAs0.51Sb0.49, we characterized experimental Hall electron mobilities vs carrier concentration at 300 and 77 K in n-type GaAs0.51Sb0.49, Ga0.47In0.53As, and Ga0.76In0.24As0.67Sb0.33 alloys nearly matched to InP. In marked contrast to the other two alloys, GaAs0.51Sb0.49 exhibits a sharp rise in 77 K electron mobility, which evidences L-valley de-population for lower electron concentrations. A two-band transport analysis reveals a Γ-L valley separation ΔΓL = 91 meV at 77 K, significantly lower than values recommended in the literature. Based on the reported temperature variations of ΔΓL in GaAs and GaSb, 84 < ΔΓL < 95 meV is expected at 300 K. The corresponding GaAsxSb1−x L-valley bowing parameter is cL = 1.63 eV, significantly higher than the 1.1–1.2 eV recommended in the literature. In contrast to GaAsxSb1−x, GaInAsSb grown on InP displays a strong alloy scattering, which limits its low-temperature electron mobility.

On-wire axial perovskite heterostructures for monolithic dual-wavelength laser

All-inorganic lead halide perovskites have attracted tremendous attention for their tunable bandgaps, excellent photoluminescence efficiency and robust stability. Here, we report on a direct vapor-phase growth of high-quality CsPbCl3/CsPbI3 axial perovskite heterostructure and multi-heterojunction nanowires using a newly developed magnetic-pulling chemical vapor deposition approach. Microstructural characterization and optical investigations reveal that these structures are crystalline with abrupt heterojunctions. Micro-photoluminescence spectra and mapping at the heterojunctions exhibit dual-wavelength emissions at 417 nm and 698 nm, from the adjacent two disparate perovskites, respectively, further demonstrating the formation of unique heterostructures. Additionally, under a focused laser illumination, asymmetrical waveguide behavior along a single CsPbCl3/CsPbI3 wire is clearly observed. Taking a step further, we fabricated a monolithic dual-wavelength laser using an on-wire axial perovskite heterostructure and successfully realized blue and red emissions (425.5 nm and 687.4 nm). The capability to synthesize on-wire heterostructures represents a major step toward high-integration optoelectronic circuits and nanophotonics.

Defects signature in VOC characterization of thin-film solar cells

Abnormal features of VOC dependence on temperature and illumination are probed for p-n+-n++ heterojunction family of solar cell devices. Thin-film solar cells of CIGS, CZTS, CdTe, SnS have p-n+-n++ configuration. VOC plots associated with the device configuration/defect scenarios give identifiable features in a one-sided abrupt heterojunction. These features could be assigned to their generating causes akin to high surface recombination velocity, hole mobility, conduction band offset, resistance, Schottky barrier height and bulk defects. This analysis helps make a general prediction of defects or shortcomings present in the device by observing the VOC-temperature and VOC-illumination plots. An individual defect/shortcoming is introduced at a time in the simulated device model and a comparative study for temperature-VOC and illumination-VOC is performed. Features associated with various device shortcomings (high SRV, hole mobility, conduction band offset, resistance, Schottky barrier height, and bulk defects) are summarized. Finally based on the simulation of VOC-temperature and VOC-illumination plots characterization could diagnose and distinctly identify the defect present in the device.

Theoretical modeling and simulation-based assessment of graded-bandgap AlGaAs/GaAs electron-injection cathode

The electron emission model of a negative electron affinity graded-bandgap AlGaAs/GaAs electron-injection cathode was developed from two-dimensional continuity equations. The emission current was obtained from a simulation of the model, and the emission current efficiency and emission current per unit length were calculated. Based on the simulation results and preparation conditions, the range of optimum parameters for the cathode structure were determined. The ranges of optimum thickness for the p-AlGaAs and the graded-bandgap p-AlGaAs layers were 0.05–0.15 μm and 0.1–0.3 μm, respectively. The optimum width of the base electrode ranged from 1 to 4 μm, and the optimum molar ratios of Al in the p-AlGaAs and the n-AlGaAs layers were 0.2–0.3 and 0.4–0.5, respectively. This abrupt PN heterojunction inhibited the hole current and increased the emission current efficiency, with a maximum value of 25.3%. According to the emission current per unit length, the optimum range of width of emission unit surface was 6 to 10 μm, and the peak emission current per unit length reached 43.2 μA/μm.

Electronic and optoelectronic performance of nano-Vanadyl 2,3-naphthalocyanine/n-Si (organic/inorganic) solar cells: Temperature dependence

Vanadyl 2,3-naphthalocyanine (VONc) /n-Si heterojunction was prepared by vacuum deposition of VONc compound onto n-Si single crystals. The analysis of the X-ray diffraction pattern and the scanning electron microscopy images reveal the nanostructure nature of the deposited VONc film. At low voltage, the temperature dependence of current density-voltage characteristics corroborates that the multi-tunneling mechanism is the dominant current mechanism for the dark forward bias. In the case of the higher forward bias (0.4 < V < 2 V), the conduction mechanism is a space charge limited current dominated by a single trap level. The capacitance-voltage characteristics imply that the junction has an abrupt heterojunction description and the value of built-in voltages, at room temperature, was estimated to be 0.67 eV. The values of the open-circuit voltage, the short-circuit current, fill factor and power conversion efficiency under illumination (100 mW/cm2) and at room temperatures were calculated to be 0.70 V, 11.30 mA/cm2, 0.394 and 3.12%, respectively. These results indicate that VONc/n-Si heterojunction is characterized by remarkable considerable photovoltaic properties comparing with the other organic/inorganic heterojunctions.

(Invited) Vertical Tunnel FET Technologies Using III-V/Si Heterojunction

Conventional field-effect transistors (FETs) have inherent issues for lowering supply voltage because there is physical limitation of subthreshold slope (SS = 2.3 k BT/q = 60 mV/dec), which determine supply voltage of FETs. This limitation will stop further scaling of the power consumption even if a multi-gate architecture and III-V/Ge channels are implemented. Thus, non-thermionic switches achieving steep SS have attracted much attentions as a building blocks for future low-power integrated circuits. Among the switches, tunneling FET (TFETs) are promising alternative FETs because their good Si-CMOS compatibility and very small off-state leakage current due to band offset across the tunnel junction. There are, however, still challenges in achieving steep SS with wide range of current, high conductance, and complementary operation. In this talk we reported on recent technologies for vertical TFET using III-V/Si tunnel junctions [1,2], which is formed by selective-area growth of III-V nanowires (NWs) on Si [3-5], and demonstrated vertical TFETs with steeper SS below physical limitation. Next, we investigate current-boosting technologies by using core-shell structure induced strain and modulation-doping of the NW-channels. Moreover, we demonstrate complementary switching operation with very steep SS and very high transconductance efficiency by using same device architecture. This new tunnel junctions can inherently forms abrupt heterojunction regardless of precise doping because the band discontinuity is determined only by the offset of each III-V and Si. Thus, good gate-electrostatic control and depletion-width control for the tunnel transport is defined only by the III-V channel region regardless of degeneration of source materials. As for integration of III-V NWs on Si, selective-area growth has no thick buffer layer, thus the III-V NWs/Si interface is able to show unique band structures. The narrow gap III-Vs such as In0.7Ga0.3As NWs, for instance, exhibits staggered-Type II band structure when the n-type NWs are formed on p-Si substrates regardless precise doping. Thus, we integrated the InGaAs NWs on Si substrate by utilizing specific growth sequence to align vertical NWs [6]. Sn was used for n-type dopant, and Zn-pulse doping technique [7] was used to make pseudo intrinsic layer as channel region. The formation of intrinsic layer in such small NW-volume would be very important to induce large internal electrical field at the InGaAs/Si heterojunction. The length of the Zn-pulse doped region correspond to channel-length (200 nm-long in this case). AS for high-performance TFETs, we demonstrated a high-performance switch using a vertical nanowire-channel consisting of InGaAs/Si tunnel junction with two-dimensional electron gas. The device exhibit rapid enhancement in conductance with steep slope over four magnitude of decades, and complementary switching operation by inverting the ground terminal (minimum slope of 25 mV/decade as n-channel, or 6 mV/decade as p-channel at 0.25 V). Furthermore, the transconductance efficiency, meaning the current efficiency of integrated circuits, exceeds the theoretical limit in conventional circuits which was around 1900 /V (physical limitation of conventional MOSFET is 38.5 /V).

Understanding the open circuit voltage in organic solar cells on the basis of a donor-acceptor abrupt (p-n++) heterojunction

By using electrical characterization and classical solid state semiconductor device theory, we demonstrate that the open circuit voltage (Voc) in organic solar cells based on non-intentional doped semiconductors is fundamentally limited by the built-in potential (Vbi) originated at a donor-acceptor abrupt (p-n++) heterojunction in case of selective contacts. Our analysis is validated using P3HT:PCBM devices fabricated in our research group. We also demonstrate that such a result can be generalized using data already reported in literature for fullerene-based solar cells. Finally, we show that the dependence of Voc on the device contacts can be understood in terms of the potential barriers formed by the Fermi level alignment of semiconductors at the heterojunction and at the Schottky junctions.

1D ZnSSe‐ZnSe Axial Heterostructure and its Application for Photodetectors

AbstractSemiconductor heterostructure nanowires (HNWs) are excellent candidates for application in compact optoelectronics devices with high performance due to the heterojunction interface effect. However, the controllable fabrication of high‐quality nano‐heterostructures is elusive. In this paper, the controllable growth and optoelectronics device application of high‐quality ZnS0.49Se0.51/ZnSe axial HNWs are reported. The as‐synthesized HNWs are straight with uniform diameter distribution of 50–100 nm. Microstructural characterization reveals single crystal and abrupt heterojunction interface in these HNWs. The micro‐photoluminescence of single component region and heterojunction zone show one and integrated two near bandgap emission of ZnS0.49Se0.51 and ZnSe, respectively, further demonstrating the abrupt heterojunction interfaces nature in the as‐grown nanowires. Moreover, photodetectors based on single HNW are fabricated and the responsivity, external quantum efficiency, response speed, and detection wavelength range are superior to that of single component nanowire photodetectors at room temperature. These results show that the ZnS0.49Se0.51/ZnSe HNWs and corresponding high‐performance photodetectors are prospective candidates for photonics and optoelectronics applications.

Capacitive Properties of Metal–Insulator–Semiconductor Systems Based on an HgCdTe nBn Structure Grown by Molecular Beam Epitaxy

The capacitance–voltage (CV) curves of metal–insulator–semiconductor (MIS) systems based on an HgCdTe nBn structure grown by molecular-beam epitaxy on GaAs(013) substrates were studied for the first time in a wide range of frequencies and temperatures. The electron concentration in the near-surface film layer was determined by capacitive measurements and is close to the indium dopant concentration. It was demonstrated that the CV characteristics of MIS systems have a high-frequency behavior in a wide range of measurement conditions, and the product of the differential resistance of the space-charge region and the electrode area in the strong inversion mode is as high as 40 kΩ cm2. It was found that the capacitance of the MIS system in the accumulation mode decreases after irradiation at 0.91 μm. This effect may be attributed to the transformation of the band diagram of an abrupt heterojunction triggered by changes in the charge state of defects under irradiation.

Electromagnetic enhancement effect on the atomically abrupt heterojunction of Si/InAs heterostructured nanowires

Semiconductor nanowires (NWs) present a great number of unique optical properties associated with their reduced dimension and internal structure. NWs are suitable for the fabrication of defect free Si/III-V heterostructures, allowing the combination of the properties of both Si and III-V compounds. We present here a study of the electromagnetic (EM) resonances on the atomically abrupt heterojunction (HJ) of Si/InAs axially heterostructured NWs. We studied the electromagnetic response of Si/InAs heterojunctions sensed by means of micro-Raman spectroscopy. These measurements reveal a high enhancement of the Si Raman signal when the incident laser beam is focused right on the Si/InAs interface. The experimental Raman observations are compared to simulations of finite element methods for the interaction of the focused laser beam with the heterostructured NW. The simulations explain why the enhancement is detected on the Si signal when illuminating the HJ and also provide a physical framework to understand the interaction between the incident EM field and the heterostructured NW. The understanding of this process opens the possibility of controlling the light absorption/scattering on semiconductor NWs with the use of heterostructures while taking advantage of the properties of both Si and III-V semiconductors. This is important not only for current NW based photonic nanodevices, such as light sensors, but also for the design of new optoelectronic devices based on NWs.

Negative transconductance and negative differential resistance in asymmetric narrow bandgap 2D-3D heterostructures.

Two-dimensional layered black phosphorus is an ambipolar narrow bandgap semiconductor with excellent electronic properties. A heterostructure can be formed when black phosphorus is combined with a narrow bandgap n-type semiconductor, which can feasibly be modulated throughout the entire bandgap for both materials in momentum space, creating unique quantum tunneling devices. In this work, a black phosphorus and narrow bandgap n-type indium arsenide heterojunction is created with a broken-gap band alignment, forming two interband tunneling windows that can be modulated electrostatically. We observe simultaneous gate-tunable band-to-band tunneling induced negative differential resistance and negative transconductance in the heterointerface and the access region edge near the gate, respectively. Compared to the stringent conditions for realizing such abrupt heterojunctions with opposite doping polarities in conventional bulk semiconductors, layered two-dimensional materials provide unique opportunities for such quantum tunneling devices. Our results represent advances in the fundamental understanding of heterojunctions and will promote future applications in advanced electronics.

(Invited) Structural Properties and Carrier Transport in Axial Silicon-Germanium Nanowire Heterojunctions

Recent advances in forming semiconductor heterojunctions within spatially confined nanoscale objects, including nanowires (NWs), show that the traditional limitations in the lattice-mismatched hetero-growth can be challenged. In various III-V semiconductor NWs, abrupt heterojunctions have been successfully demonstrated using the vapor-liquid-solid (VLS) growth for GaAs/InAs (7% mismatch) and InAs/InP (3% mismatch) heterojunctions. In group IV semiconductors, the approach is complicated not only by the 4.2% lattice mismatch between Si and Ge but also because Si and Ge both have a quite high solubility in the Au-Si catalyst. During chemical vapor deposition (CVD) based VLS growth using SiH4 and GeH4 (or similar gases), a supply of Si remains effectively “on” in the catalyst, and Si effectively intermixes with the arriving Ge even if the SiH4 flow is already switched “off”. One way to address this problem is to choose a catalyst with a lower Si solubility, e.g. AlAu2 and AgAu. Another possibility is to significantly reduce growth temperature before turning a GeH4 source “on”. Using the latter technique, we fabricated Si-Ge heterojunction NWs with nearly ideal interface and only an 8 nm thick SiGe transition layer between straight and nearly micron-long Si and Ge NW segments and analyzed their structural and electrical properties. The observed changes in the photoluminescence and Raman spectra as function of temperature, non-linear and rectifying current-voltage characteristics, strong flicker noise and damped current oscillations with frequencies of 20-30 MHz are explained using the proposed SiGe heterojunction NW energy band diagram including the energy states associated with the NW surface (and near-surface) structural imperfections revealed by transmission electron microscopy and a mismatch in Si and Ge coefficients of thermal expansion.

Fabrication and Electrical Characteristics of Thioindigo/Silicon Heterojunction

We demonstrate the performance of heterojunction thin films of thioindigo on Silicon which fabricated by using thermal evaporation technique under high vacuum (10− 4 Pa). The dark current–voltage (I–V) characteristics measurement at different temperatures in the range 303–383 K were analyzed in order to explain the conduction mechanism and to evaluate the important device parameters The calculated ideality factor (n) and zero-bias barrier height (φo) showed strong bias dependence. The predominant mechanism of charge transport in thioindigo on silicon was found to be thermionic emission at low voltage and space charge limited conduction, SCLC dominated by single trap distribution at the higher voltage. At reverse applied voltage, it is found that p-Si is the main source of the reverse current due to generation-recombination. The capacitance–voltage (C–V) characteristics of in these devices were measured at high frequency (1 MHz). The dependence of C− 2 vs. V for the heterojunctions for thioindigo/Si was found to be almost linear which indicates an abrupt heterojunction and (C-V) parameters was obtained. The parameter of I–V characteristics under white illumination was calculated.

Strain distributions and their influence on electronic structures of WSe2-MoS2 laterally strained heterojunctions.

Monolayer transition metal dichalcogenide heterojunctions, including vertical and lateral p-n junctions, have attracted considerable attention due to their potential applications in electronics and optoelectronics. Lattice-misfit strain in atomically abrupt lateral heterojunctions, such as WSe2-MoS2, offers a new band-engineering strategy for tailoring their electronic properties. However, this approach requires an understanding of the strain distribution and its effect on band alignment. Here, we study a WSe2-MoS2 lateral heterojunction using scanning tunnelling microscopy and image its moiré pattern to map the full two-dimensional strain tensor with high spatial resolution. Using scanning tunnelling spectroscopy, we measure both the strain and the band alignment of the WSe2-MoS2 lateral heterojunction. We find that the misfit strain induces type II to type I band alignment transformation. Scanning transmission electron microscopy reveals the dislocations at the interface that partially relieve the strain. Finally, we observe a distinctive electronic structure at the interface due to hetero-bonding.

Interfacial relaxation analysis of InGaAs/GaAsP strain-compensated multiple quantum wells and its optical property

InGaAs/GaAsP strain-compensated multiple quantum wells were prepared by metal organic chemical vapor deposition on GaAs (100) substrates with misorientation of 15° toward [111]. In order to obtain better strain-compensated abrupt heterojunction interfaces, the compressive strain and relaxation of different quantum well and the total accumulated strain were investigated by adjusting In composition and the thickness of InxGa1-xAs well and GaAs1-yPy barrier keep constant. High resolution X-ray diffraction results indicate the crystal and interfacial structures of In0.18Ga0.82As (7 nm)/GaAs1-yPy with the least relaxation and total strain mismatch are better than others. From in-situ surface reflectivity curves, we observed the slope of reflectivity curve of multiple quantum wells increases with increasing lattice relaxation. Atomic force microscopic results show surface morphologies of three samples are Volmer-Weber mode. Indium segregation at heterointerface between well and barrier were investigated by secondary ion mass spectrometry which indicate indium diffusion width increase with the increasing total strain mismatch. Finally, a shoulder peak was observed from Gaussian fitting of photoluminescence, stemming from the lattice relaxation. These results demonstrate that the relaxation process is introduced and indium segregation length widens as the relaxation increases. The experimental results will be favorable for optimizing the epitaxial growth of InGaAs/GaAsP strain-compensated quantum wells in order to obtain high quality smooth heterointerface.