Conduction Band Discontinuity Research Articles

Dependence of diode behaviour and photoresponse of Ga-doped ZnO (GZO)/p-Si junction on the carrier concentration of GZO layer

Ga-doped ZnO (GZO)/p-Si junctions were fabricated by reactive co-sputtering of Zn-GaAs target in Ar–O2 atmosphere. Highly degenerate GZO (carrier concentration ∼1021 cm−3) deposited at 4 % O2 forms non-rectifying contact with p-Si. GZO/p-Si diodes fabricated with moderately doped GZO layers deposited at 5 % O2, having carrier concentration ∼1019 cm−3 display low turn-on voltage (≲ 0.5 V) and ideality factor ≈ 2. Measurement of barrier height by temperature dependent I–V and built-in potential by C–V reveal respective values of (0.74 ± 0.07 eV) and (0.44 ± 0.04 eV), suggesting formation of Schottky type diodes. These diodes display nearly linear I–V characteristics under UV illumination, yielding photoresponsivity ∼1 A/W at ± 1 V. In contrast, GZO/p-Si diodes fabricated with lightly doped (non-degenerate) GZO layers deposited at 6–8 % O2, having carrier concentration ∼1018 cm−3, show higher turn-on voltages (4.5–6.5 V) and ideality factors of 2.5–2.7. I–V and C–V measurements reveal nearly equal values of barrier height and built-in potential, indicating formation of n-p heterojunctions, with near-elimination of conduction band discontinuity. Under UV illumination, I–V characteristics of these GZO/p-Si diodes display non-linear behaviour in forward bias and yield photoresponsivity of ∼0.1 A/W at ±1 V.

Investigation on effect of AlN barrier thickness and lateral scalability of Fe-doped recessed T-gate AlN/GaN/SiC HEMT with polarization-graded back barrier for future RF electronic applications

In this study, the influence of AlN barrier thickness (tb) on the RF & DC performances of 50 nm recessed T-gate Fe-doped AlN/GaN/SiC HEMT is investigated. The proposed HEMT incorporates a graded back-barrier (GBB) feature that raises the conduction band (CB) discontinuity at GaN/AlGaN junction to increase carrier confinement. With excellent electron confinement, the electrical characteristics for a 6 nm AlN barrier thickness show the highest transconductance of 468.9 mS/mm, a maximum ID of 1.546 A/mm, and a peak fT of 355 GHz at VDS = 2.5 V due to better epitaxial quality, and a reduction in parasitic channel generation due to GBB. When tb increases, the Vth decreases or becomes more negative i.e., moves left, which causes the ID to fall and degrades performance. This study also examines the influence of gate metals on the electrical performance of the proposed HEMT. According to the TCAD results, Pt-gate HEMTs had better RF performance. Furthermore, the simulation performed to examine the effects of scaling source-drain spacing (LSD) showed that the key to improving RF/DC performance is to reduce gate-to-drain and gate-to-source distances. It has been observed that the electron velocity of the HEMT is significantly influenced by the barrier thickness, LGS, and LGD. It comes as no surprise that this technology will be at the frontlines of future RF power applications.

High-Performance Solar-Blind p-NiO/n-ZnO/p-Si Ultraviolet Heterojunction Bipolar Phototransistors With High Optical Gain

Ultraviolet (UV) photodetectors (PDs) have attracted significant attention for civil and military applications. Zinc-oxide (ZnO) and nickel-oxide (NiO) have been widely applied in UV-PDs owing to their wide bandgap (3.2–3.8 eV), transparency, excellent optical and electrical properties, and good chemical stability. However, UV signals are generally weak; hence, UV-PDs with high optical gain are essential. In this work, high-performance solar-blind p-NiO/n-ZnO/p-Si heterojunction bipolar phototransistors (HBPTs) were fabricated. The fabricated HBPTs exhibited a high responsivity of 9.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> A/W at a wavelength of 280 nm with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V_CE</i> = –7 V, a high optical gain of 3.96 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> , and a large detectivity of 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> Jones. In addition, the UV/visible rejection ratio was as high as 880. These behaviors indicate that the prepared HBPTs are good solar-blind photodetectors and suitable for the detection of weak UV signals. However, for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V_CE</i> value below –7 V, (| <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V_CE</i> | > 7V), the optical gain decreased owing to the punch-through effect. Furthermore, band-diagrams showed that the photogenerated electrons were blocked by the potential barrier at the NiO/ZnO interface (base–emitter junction) owing to a large conduction-band discontinuity (Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E_C</i> ) of 2.7 eV, which resulted in a large optical gain in the prepared HBPTs.

Performance analysis of gallium nitride-based DH-HEMT with polarization-graded AlGaN back-barrier layer

In this paper, polarization-graded AlGaN back-barrier nanolayer has been introduced to improve the DC and RF parameters of gallium nitride-based high electron mobility transistors (HEMT). To explore the characteristics, both graded and non-graded double heterojunction high electron mobility transistor (DH-HEMT) structures are optimized using SILVACO-ATLAS physical simulator. Enhanced DC and RF parameters have been observed in the optimized graded DH-HEMT. In this paper, we have also studied the development of the quantum wells at the AlGaN/GaN interfaces due to the conduction band discontinuity in both structures.

Simulation investigation of double-heterostructure T-gate HEMT with graded back-barrier engineering for improved RF performance

This article proposes a 70 nm T-gate In0.17Al0.83N/AlN HEMT. The RF and DC characteristics of the proposed structure are investigated using the Silvaco Atlas tool. The proposed device has the graded back-barrier feature, which improves the carrier confinement at the GaN/AlGaN junction by increasing the conduction band discontinuity. The reduction of Al composition in the back-barrier layer led to the lattice’s complete relaxation. The presence of a graded back-barrier diminished the piezoelectric polarization, which significantly reduced the 2DEG concentration of the parasitic channel (1.0 × 1018 cm3) about two times and enhanced the transfer characteristics compared to the Al0.1Ga0.9N back-barrier. Moreover, the surface of the device is also passivated with the SiN layer, source/drain regions are highly doped, and the shape of the gate is taken as a T shape. These features improve the transconductance (740 mS/mm), drain current density (2.5 A/mm), cut-off frequency (295 GHz), and maximum oscillation frequency (323 GHz).

Conduction Band Discontinuity in n-type Si/TiO2 Heterojunction Interfaces

The conduction band discontinuity between n-type Si substrates and anatase TiO2 films has been investigated. n-type Si substrates with three different dopant concentrations were used as a substrate for TiO2 thin-films: ND = 1015–16 cm−3 (as n-Si); ND = 1017–18 cm−3 (as n+-Si); ND = 1020–21 cm−3 (as n++-Si). The translation of X-ray photoelectron spectroscopy (XPS) results to an energy band diagram through the valence band offset (VBO) enables us to evaluate the conduction band discontinuities accurately: n-Si/TiO2—−0.22 eV, n+-Si/TiO2—−0.06 eV, and n++-Si/TiO2—+0.07 eV. Temperature–dependent current–voltage (I–V) characteristics were measured to evaluate the Fermi energy level (EF) of the TiO2 thin-films. Light transmittance was measured to evaluate the energy bandgap of the TiO2 thin-films. The band diagram of the n-type Si/TiO2 heterojunction was proposed. Deep-insight analysis of n-type Si/TiO2 was carried out on the basis of measured I–V characteristics.

Trapping and Detrapping Mechanisms in β-Ga₂O₃ Vertical FinFETs Investigated by Electro-Optical Measurements

We present a detailed investigation of the trapping and detrapping mechanisms that take place in the gate region of $\beta $ -Ga2O3 vertical finFETs and describe the related processes. This analysis is based on combined pulsed characterization, transient measurements, and tests carried out under monochromatic light, with photon energies between 1.5 and 5 eV. The original results presented in this article demonstrate that: (i) when submitted to positive gate stress with ${V}_{\text{GS}} > 3$ V, the devices show a significant threshold voltage variation; (ii) this effect is not recoverable in 10 000 s in rest condition (zero bias, dark condition). (iii) ${V}_{\text{TH}}$ can quickly recover its initial value when the device is illuminated with UV-C light at 280 nm. (iv) Stress-recovery experiments carried out at different photon energies allowed us to estimate the threshold energy for the release of carriers from the Al2O3/Ga2O3 interface, and for the injection of electrons from metal to the Al2O3 insulator (conduction band discontinuity at the metal/Al2O3 interface).

Ge–GaAs–Ge Heterojunction MOSFETs for Mixed-Signal Applications

A lattice matched heterojunction intraband tunnel (HJIBT) FET is proposed. The performance dependence of the device on conduction band (CB) discontinuity at source-channel and drain-channel interface is addressed using numerical simulation. Various mechanisms governing transport phenomena in the HJIBT FET are investigated in detail for different CB offsets (CBOs). For low gate to source voltage ( ${V}_{\text {GS}}$ ), thermionic emission is found to be the most significant transport mechanism. For moderate ${V}_{\text {GS}}$ , intraband tunneling phenomenon dominates over thermionic emission and continues to remain so. At high ${V}_{\text {GS}}$ , band-to-band tunneling occurs in HJIBT FETs. The proposed device shows improved figures of merit such as drain-induced barrier lowering (DIBL), ON-current ( ${I}_{ \mathrm{\scriptscriptstyle ON}}$ ) to OFF-current ( ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ ) ratio ( ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ), subthreshold slope (SS), gate capacitance ( ${C}_{\text {G}}$ ), ${g}_{m}$ (transconductance), and ${f}_{T}$ (cut-off frequency), with respect to conventional MOSFET. Also, the design of a high-performance hybrid 6T-static random access memory (SRAM) is proposed.

Engineering the Interfacial Electronic Structure of Epitaxial Ge/AlAs(001) Heterointerfaces via Substitutional Boron Incorporation: The Roles of Doping and Interface Stoichiometry

A joint experimental and theoretical framework for the decoupling of boron (B) doping and stoichiometric-induced modifications to the structural properties and electronic band structure of germanium (Ge)/AlAs(001) heterostructures is presented. The effect of B-induced stress on nearest-neighbor Ge bonds is quantified via X-ray diffractometry and Raman spectroscopic analysis and subsequently interpreted through the lens of density functional perturbation theory. Similarly, experimental determination of the energy band alignment at the p-type Ge:B/AlAs heterointerface is understood using a density functional theory approach to model the influence of heterointerface stoichiometry and interatomic bonding between group IV and III–V interfacial atoms on the valence and conduction band discontinuities. The modeled two monolayer interatomic diffusion at the Ge/AlAs heterointerface is confirmed via atom probe tomography analysis, demonstrating a ∼6 A interfacial width. These results present a unified picture of th...

Theoretical Study of InAlN/GaN High Electron Mobility Transistor (HEMT) with a Polarization-Graded AlGaN Back-Barrier Layer

An inserted novel polarization-graded AlGaN back barrier structure is designed to enhance performances of In0.17Al0.83N/GaN high electron mobility transistor (HEMT), which is investigated by the two-dimensional drift-diffusion simulations. The results indicate that carrier confinement of the graded AlGaN back-barrier HEMT is significantly improved due to the conduction band discontinuity of about 0.46 eV at interface of GaN/AlGaN heterojunction. Meanwhile, the two-dimensional electron gas (2DEG) concentration of parasitic electron channel can be reduced by a gradient Al composition that leads to the complete lattice relaxation without piezoelectric polarization, which is compared with the conventional Al0.1Ga0.9N back-barrier HEMT. Furthermore, compared to the conventional back-barrier HEMT with a fixed Al-content, a higher transconductance, a higher current and a better radio-frequency performance can be created by a graded AlGaN back barrier.

Interface studies by simulation on methylammonium lead iodide based planar perovskite solar cells for high efficiency

Perovskite solar cells gained importance in the recent years due to high power conversion efficiency and low-cost fabrication methods. Suitable solar cell properties can be obtained by fine tuning process parameters. Charge transport, optical band gap of perovskite absorber and energy band discontinuity on either side of absorber are an important factor in determining cell efficiency. In this work, theoretical studies on cell efficiency were performed as a function of absorber band gap since it determines band discontinuity. Band alignment investigation was performed to minimize band discontinuity between interfaces. Our results show that, valance band discontinuity at HTL/absorber interface (ΔEv1) increases from 0.01 eV to 0.42 eV as the absorber band gap increases from 1.20 eV to 1.65 eV. On the other hand, valance band discontinuity at absorber/ETL interface (ΔEv2) decreases from 2.07 eV to 1.62 eV. Conduction band discontinuity on both sides of absorber interface is almost constant and independent of absorber band gap. The increase in ΔEv1 restricts holes collection which in turn affects cell efficiency. Depending upon ΔEv1, cell efficiency increased from 24.96% to 29.40% and then decreased to 26.27%. This is due to the fact that maximum acceptable limit for ΔEv1 is 0.20 eV and holes can reach back electrode when ΔEv1 ≤ 0.20 eV. For the absorber band gap of 1.40 eV with ΔEv1 of 0.17 eV, a high efficiency of 29.40% has been obtained with Jsc of 28.70 mA/cm2, Voc of 1.24 V, and FF of 82.60%.

Study of GaN layer crystallization on GaAs(100) using electron cyclotron resonance or glow discharge N2 plasma sources for the nitriding process

Two kinds of N2 plasma source, ECR (electron cyclotron resonance) and GDS (glow discharge source) generating mostly N-radical atoms and N-cationic species respectively, were used to grow a thin nitride layer on a GaAs(100) substrate. It was found that this nitridation followed by annealing at 620 °C permits the crystallization of the nitride layer. Pyramidal Zinc Blende GaN nanostructures (zb-GaN) with four facets were obtained using GDS plasma. Surprisingly, a planar and pure wurtzite structure (w-GaN) was obtained using the ECR source. This w-GaN structure shows low photoluminescence intensity and a biaxial tensile strain due to lattice mismatch. Accordingly, the operator can select which phase is formed, simply by switching plasma source. The valence band discontinuity ΔEv has been determined to be 1.74 eV for the zb-GaN/GaAs and w-GaN/GaAs junctions by X-ray photoelectron spectroscopy. As a consequence the conduction bands of the GaAs substrate and the elaborated GaN thin layer are aligned for a zb-GaN/GaAs junction giving efficient electron transport at the zb-GaN/GaAs interface. For w-GaN/GaAs junction, the conduction band discontinuity ΔEc is 0.23 eV inducing an electron confinement in the GaAs(100) which can be an effective way to improve the electronic or optical properties of GaAs devices.

A New Expression for the Gain-Noise Relation of Single-Carrier Avalanche Photodiodes With Arbitrary Staircase Multiplication Regions

We propose a simple expression to relate the total excess noise factor of a single-carrier multiplication staircase avalanche photodiode (APD) to the excess noise factor and gain given by the individual conduction band discontinuities. The formula is valid when electron impact ionization dominates hole impact ionization; hence, it is especially suited for staircase APDs with In-rich multiplication regions, as opposed, for example, to GaAs/AlGaAs systems where hole ionization plays an important role. The formula has been verified by accurate means of numerical simulations based on a newly developed nonlocal history dependent impact ionization model.

Investigation of band alignment between InAlAs and atomic-layer-deposited HfO2/Al2O3 by X-ray photoelectron spectroscopy

Band alignments between atomic layer deposition (ALD) HfO2/Al2O3 double layers and InAlAs were investigated by angle-resolved X-ray photoelectron spectroscopy (AR-XPS). The relationship of As and Hf 4f peaks reducing with the decreasing take-off angle θ indicated upward band bending on the InAlAs surface and a potential gradient in the HfO2/Al2O3 layer. AR-XPS measurements were combined with numerical calculations. The valence band and conduction band discontinuity between HfO2/Al2O3 and InAlAs were 2.49 ± 0.2 eV and 1.86 ± 0.2 eV, respectively. A low leakage current density of 7.01 × 10−7 A cm−2 at 1 V and oxide trapped charge densities of 9.925 × 1011 cm−2 were obtained.

Origin of anisotropy and compositional dependence of phonon and electron transport in ZnO based natural superlattices and role of atomic layer interfaces

Reaction of ZnO with trivalent ions can form natural superlattice (SL) compounds, which possess unusual structural characteristics and strong anisotropic physical transport properties. In this work, by synthesizing strongly textured bulk polycrystals of pure SL phases and performing characterization and measurements for both in-plane and cross-sectional directions, we revealed the strongly crystal orientation dependent transport properties. The observed compositional or SL interface spacing dependent electrical conductivity is largely attributed to the overall doping of ZnO wurtzite regions. The atomic layer SL interfaces are phonon barriers, and the interfacial thermal (Kapitza) resistance depends on SL interface spacing. The transport direction perpendicular to these SL interfaces are electron-conductive paths with remarkably higher electron mobility. There are electron potential barriers associated with these atomic layer SL interfaces. The effective and absolute potential barrier heights are determined, which are proportional to the natural conduction band discontinuities. The current study provides new findings and knowledge about the role of SL interfaces in phonon and electron transport process in these ZnO based natural SLs. The present work can be useful and inspiring to design and modify complex layer-structured compounds, including synthetic superlattice systems, for a variety of applications where energy carriers transport is involved.

Modelling, simulation, optimization of Si/ZnO and Si/ZnMgO heterojunction solar cells

Currently, crystalline and polycrystalline silicon wafer based solar cells dominate the photovoltaic market, which employs expensive manufacturing processes. Zinc oxide is one among the inexpensive alternatives for acting as emitter/anti-reflection coating layer which can be used in combination with widely available p-type silicon. Hence it becomes essential to gain in-depth knowledge about the charge transport mechanisms involved in the n-type ZnO/p-type silicon (Si) heterojunction devices. Therefore, Si/ZnO solar cell is modeled and the carrier transport mechanisms are carefully examined by analyzing the band edge discontinuities, electric field distributions at the Si/ZnO interface, carrier generation-recombination profile for varying ZnO thickness, carrier density and affinity values. The photo- generation rate degrades with the increase in ZnO emitter thickness owing to increased absorption in the blue region and is independent of ZnO affinity and donor concentration. The cliff like configuration with high conduction band discontinuity at the Si/ZnO interface for higher values of ZnO affinity has resulted in the formation of poor electric field and the lattice mismatch resulting in high defects formation (recombination centers) at Si/ZnO interface are the predominant causes for deficit in open circuit voltage and fill factor. Hence the conduction band offset alignment is further engineered by designing Si/MgZnO solar cell model and Mg doped ZnO emitter has significantly improved the band offset alignment at the p-n heterojunction when compared to ZnO emitter. Moreover, the simulation results revealed that the stronger electric field strength developed at the heterojunction is suggested to favor the carrier separation process by drift motion thereby improving the open circuit voltage and fill factor of the device. As a next step, both the models are optimized for varying emitter parameters like thickness, carrier density etc using Silvaco ATLAS simulator. Based on this investigation, the optimized Si/ZnO and Si/MgZnO designs anticipate improved conversion efficiencies of 11.57% and 14.46% respectively.

ALD-ZnxTiyO as Window Layer in Cu(In,Ga)Se2 Solar Cells

We report on the application of ZnxTiyO deposited by atomic layer deposition (ALD) as buffer layer in thin film Cu(In,Ga)Se2 (CIGS) solar cells to improve the photovoltaic device performance. State-of-the-art CIGS devices employ a CdS/ZnO layer stack sandwiched between the absorber layer and the front contact. Replacing the sputter deposited ZnO with ALD-ZnxTiyO allowed a reduction of the CdS layer thickness without adversely affecting open-circuit voltage (VOC). This leads to an increased photocurrent density with a device efficiency of up to 20.8% by minimizing the parasitic absorption losses commonly observed for CdS. ALD was chosen as method to deposit homogeneous layers of ZnxTiyO with varying Ti content with a [Ti]/([Ti] + [Zn]) atomic fraction up to ∼0.35 at a relatively low temperature of 373 K. The Ti content influenced the absorption behavior of the ZnxTiyO layer by increasing the optical bandgap >3.5 eV in the investigated range. Temperature-dependent current–voltage (I–V) measurements of solar cells were performed to investigate the photocurrent blocking behavior observed for high Ti content. Possible conduction band discontinuities introduced by ZnxTiyO are discussed based on X-ray photoelectron spectroscopy (XPS) measurements.

Evaluation of the Conduction Band Discontinuity of MgSe/ZnCdSe Heterojunctions on InP Substrates Using n–i–n Diodes

Conduction band discontinuity (ΔEc) of MgSe/ZnCdSe heterojunctions were evaluated using n–i–n diodes consisting of an undoped i-MgSe layer sandwiched by n-doped ZnCdSe layers. The n–i–n diodes were fabricated on InP substrates by molecular beam epitaxy. Injection current density versus applied voltage (J–V) characteristics of the n–i–n diodes were measured at 77 K and room temperature. In addition, the theoretical J–V characteristics of the n–i–n diode were calculated while varying ΔEc. By fitting the theoretical data to the experimental data, ΔEc was estimated to be 1.2 eV from the result at 77 K. This value is similar to the ΔEc estimated from the literature.

An Improved Nonlocal History-Dependent Model for Gain and Noise in Avalanche Photodiodes Based on Energy Balance Equation

We present a nonlocal history-dependent model for impact ionization gain and noise in avalanche photodiodes (APDs) especially suited for staircase APDs. The model uses a simple energy balance equation to define effective electric fields valid also in the presence of band discontinuities which are then used to express the ionization coefficients. The model parameters have been calibrated against literature data for gain and noise in GaAs and Al x Ga1− x As $({x} = {0.2}, {0.6}, {0.8})$ p-i-n diodes. Application to the experimental data for gain and noise in heterojunction and staircase separate absorption and multiplication APDs is reported to demonstrate the ability of the model in describing complex APD structures. It is found that, in spite of conduction band discontinuities being much larger than valence band ones, hole impact ionization contributes a significant degradation of the noise metrics in GaAs/AlGaAs staircase APDs. These nontrivial insights demonstrate the usefulness of the model to steer device design and optimization.

Hydrogen Sensing Characteristics of AlGaInP/InGaAs Complementary Co-Integrated Pseudomorphic Doping-Channel Field-Effect Transistors

In this article, the hydrogen sensing device and logic characteristics of the Al0.25Ga0.25In0.5P/In0.1Ga0.9As complementary co-integrated pseudomorphic doping-channel field-effect transistors are demonstrated. Due to the existence of a relatively large conduction (valence) band discontinuity at Al0.25Ga0.25In0.5P/In0.1Ga0.9As heterojunction and the employment of a small energy-gap In0.1Ga0.9As channel layer, it could provide a high gate barrier height to avoid electrons (holes) injection form channel into gate region in the studied n-channel (p-channel) device. With respect to the n-channel (p-channel) transistor under hydrogen ambient, hydrogen molecules are adsorbed and dissociated on the Pd catalytic metal surface, followed by rapid diffusion of hydrogen atoms into the MS interface where the dipoles are formed to lower (elevate) the gate barrier height and enhance (decrease) the drain current. In the n-channel (p-channel) device, the threshold voltages at drain current of 0.1 mA/mm are of 0.67 (0.05) and 0.38 (−0.26) V in air and at hydrogen concentration of 9800 ppm, respectively. As hydrogen is detected, the characteristics of inverters shift left obviously, and the VOH and VIH values are decreased.