Band Discontinuity Research Articles

HgCdTe surface passivation with low-temperature plasma-enhanced atomic layer deposited HfO2

Mercury cadmium telluride (Hg1−xCdxTe) is widely utilized for infrared detection applications. However, the quality of the surface and the presence of interface defects significantly impact device performance, so further improvements in surface passivation are still necessary. In this study, we explore the use of plasma-enhanced atomic layer deposition (PE-ALD) to deposit HfO2 as a passivation layer for HgCdTe in the temperature range of 80–160 °C. The insulating film, semiconductor surface, and their interface are characterized using spectral ellipsometry, X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (EELS), atomic force microscopy (AFM), electron microscopy, and capacitance–voltage (C–V) methods. XPS analysis reveals that the fraction of impurities in the HfO2 films significantly decreases at higher deposition temperatures. Simultaneously, changes in the surface composition of HgCdTe become more pronounced. The bandgap width of HfO2 and the insulator–semiconductor interfacial band discontinuity are estimated using EELS and XPS. The surface morphology of the deposited coatings closely resembles that of the underlying HgCdTe. The permittivity of the insulator and the effective fixed charge strongly depend on the deposition temperature. Our findings demonstrate that low-temperature PE-ALD HfO2 effectively passivates the HgCdTe surface, and we propose an optimal deposition temperature for the further development of HgCdTe-based devices.

Design of long‐wavelength infrared InAs/InAsSb type‐II superlattice avalanche photodetector with stepped grading layer

AbstractWeak response in long‐wavelength infrared (LWIR) detection has long been a perennial concern, significantly limiting the reliability of applications. Avalanche photodetectors (APDs) offer excellent responsivity but are plagued by high dark current during the multiplication process. Here, we propose a high‐performance type‐II superlattices (T2SLs) LWIR APD to address these issues. The low Auger recombination rate of the InAs/InAsSb T2SLs absorption layer is exploited to reduce the dark current initially. AlAsSb with a low k value is employed as the multiplication layer to suppress device noise while maintaining sufficient gain. To facilitate carrier transport, the conduction band discontinuity is optimized by inserting an InAs/AlSb T2SLs stepped grading layer between the absorption and multiplication layers. As a result, the device exhibits excellent photoresponse at 8.4 μm at 100 K and maintains a low dark current density of 5.48 × 10−2 A/cm2. Specifically, it achieves a maximum gain of 366, a responsivity of 650 A/W, and a quantum efficiency of 26.28% under breakdown voltage. This design offers a promising solution for the advancement of LWIR detection.

Enhanced Electroluminescence Quantum Efficiency via Tunable 2D Built‐In Electric Fields

AbstractThe built‐in electric field (BIEF) is of utmost importance for 2D optoelectronic devices, which are typically realized in heterostructures created through interfacial engineering. However, the limitations of the sharp field distribution, inevitable energy band discontinuity, and inflexible construction of 2D heterostructures restrict their application. In this work, a large‐scale, continuous, and controllable BIEF using monolayer WxMo1−xS2 alloys with tunable composition gradients is constructed. Density functional simulations demonstrate that a BIEF can be formed within the whole alloy due to the charge distribution resulting from the composition gradients. Accordingly, a monolayer WxMo1−xS2 alloy with a controllable composition gradient via two‐step chemical vapor deposition and confirmed the continuous tunability of the band structure and the variation in the composition gradient within the alloy is synthesized. Using the BIEF, 2D light‐emitting diodes (LEDs) based on WxMo1−xS2 alloys and achieve strong electroluminescence emission are constructed. A large BIEF facilitates charge carrier migration and recombination, and the external quantum efficiency (over 0.62%) is more than 5 times that of small BIEF LEDs. The study not only provides a novel approach for the on‐demand design of a BIEF but also provides an opportunity for potential applications in optoelectronic devices.

Carrier transport mechanisms of titanium nitride and titanium oxynitride electron-selective contact in silicon heterojunction solar cells

It is widely accepted that an effective carrier-selective contact is indispensable for high performance crystalline silicon (c-Si) solar cells. However, the properties of these carrier-selective contact materials significantly differ from c-Si in terms of band gap, work function, lattice constant. Consequently, this gives rise to challenges such as band discontinuity and suspended bonds at the interface, which subsequently impact the specific carrier transport process and potentially lead to a reduction primarily in the fill factor at the device level. Titanium nitride (TiN) and titanium oxynitride (TiOxNy) have been employed as an electron-selective contact in both c-Si and perovskite solar cells, demonstrating their effectiveness in enhancing the performance of these devices. Based on the detailed characterizations of the band alignment, the carrier transport mechanisms are analyzed using multiple models, and the theoretical results are basically self-consistent through the verification of variable temperature experiments. These analytical methods can also provide solutions for analyzing the band structure and transport mechanism of diverse heterojunctions, ultimately contributing to the design and optimization of semiconductor heterojunction devices.

Impact ionization coefficients and excess noise in Al0.55Ga0.45As0.56Sb0.44 lattice matched to InP

The avalanche multiplication and noise characteristics of Al0.55Ga0.45As0.56Sb0.44p–i–n and n–i–p structures grown lattice matched on InP have been investigated. From measurements undertaken using 530 nm illumination on several devices, the electron (α) and hole (β) impact ionization coefficients have been determined. While α only shows a relatively small increase compared to the higher Al composition alloys of AlxGa1−xAsSb, β is found to increase significantly. Although the β/α ratio is increased to ∼0.125–0.2, higher than the ∼0.003–0.02 seen in the higher-Al alloys, a relatively low excess noise factor of 2.2 was measured in the p-i-n with electron-initiated multiplication of 20. This noise performance is significantly lower than that predicted using a local-field model and comparable to some commercial silicon APDs. This avalanching material with a bandgap of ∼1.24 eV will have the advantages of a smaller band discontinuity with the absorber region and should also operate at a lower voltage.

Computation of the near-infrared electro-absorption in GeSn/SiGeSn step quantum wells

In this study, we propose a theoretical simulation of the type-I step quantum well obtained from GeSn/SiGeSn to scan a wide range of telecommunication wavelengths and obtain near-infrared optical modulators. At T = 300 K, the band discontinuities and energy gap between stretched Ge1−xSnx and relaxed Si0.1Ge0.9−ySny due to the acquisition of the heterostructure were calculated.Then, optimization of this heterostructure based on (Si) GeSn was performed using the solid theory model to balance out the composition y of Si0.1Ge0.9-ySny relaxed and thickness of Ge0.91Sn0.09 QWs. The eigenenergies and their related wavefunctions are computed by solving the Schrödinger equation using the finite difference method under the framework of the effective mass approximation. Depending on the y concentration, the energy levels of the electron and the heavy hole, the change of transition energies and oscillator strength were examined for different well widths. Additionally, the absorption coefficient with y concentration and structure parameters were examined.From the findings obtained, it was determined that this material group is very important to obtain high efficiency from electro-absorption modulators covering the 1.55 μm wavelength range.

InP Crystal Phase Heterojunction Transistor with a Vertical Gate-All-Around Structure.

Crystal phase transitions can form a new type of heterojunction with different atomic arrangements in the same material: crystal phase heterojunction (CPHJ). The CPHJ has an inherently strong impact on band engineering without concerns over critical thicknesses with misfit dislocations and a semiconductor-metal transition. In-plane CPHJ was recently demonstrated in two-dimensional (2D) transition-metal dichalcogenide (TMD) materials and utilized for conventional planar field-effect transistor applications. However, scalability such as gate electrostatic control, miniaturization, and multigate structure have been limited because of the geometrical issue. Here, we demonstrated a transistor using the CPHJ with a vertical gate-all-around structure by forming a CPHJ in conventional III-V semiconductors. The CPHJ, composed of wurtzite InP nanowires with zincblende InP substrates, showed an atomically flat heterojunction without dislocations and indicated a Type-II band discontinuity across the junction. The CPHJ transistor had moderate to good gate electrostatic controllability with high on-state currents and transconductance. The CPHJ offer will provide a new switching mechanism and add a new junction and device design choice to the long history of transistors.

Effect of emitter-base spacer design on the performance of InP/GaAsSb/InP DHBTs grown by MOCVD

Effect of spacer layer design between InP emitter and GaAsSb base in InP/GaAsSb/InP double heterojunction bipolar transistors (DHBTs) grown by MOCVD was investigated. A very thin tensile-strained GaAs layer, or a thin GaInP layer, or combination of both was inserted between InP emitter and GaAsSb base to mitigate Sb segregation and/or eliminate electron pile-up between emitter and base. With a base sheet resistances of ∼1800 ohm sq−1 for all devices, DHBTs with a GaAs spacer, a GaInP spacer and the combination demonstrate a current gain of 24, 49 and 64, respectively. The conduct band discontinuity ΔEc at InP/GaAsSb interfaces and the current blocking effect are effectively eliminated by employing the combination of GaAs and GaInP layers.

Parameteric optimization of SiGe S/D NT JLFET using analytical modeling to improve L‐BTBT induced GIDL

AbstractIn the present work, we investigate the impact of structure dimensional parameters on the short channel effects which occurs especially below 20 nm regime particularly gate induced drain leakage (GIDL) current. Using technology computer aided design simulation (TCAD), we have examined the GIDL for SiGe as source/drain in NTJLFET. The structural dimensional parameters such as the nanotube thickness, core and outer gates thickness and gate electrode work function shows the significant impact on the band to band tunneling in lateral direction (L‐BTBT) which induced GIDL current. It is analyzed that increase in the nanotube thickness and physical oxide thickness increase the GIDL current, while increasing the gate electrode work function, core gate and outer gate thicknesses gives reduced GIDL current. The SiGe S/D NTJLFET produce a remarkable high ION/IOFF ratio ~ 1011. A compact model for GIDL current is also developed which shows the dependency of structure parameters on leakage current. The SiGe has been incorporated as source and drain in NTJLFET which creates the energy band discontinuity. Furthermore, SiGe S/D NTJLFET is fairly compared with the conventional NT JLFET and nanowire (NW) JLFET and shows an improved performance.

Electrical transport properties of ZrS2 under high pressure

The investigation of the electrical transport properties of ZrS2 is conducted up to 30.7 GPa using first-principles calculation and electrical experimental methods. The electrical parameters of ZrS2 are investigated under high pressure conditions using the thin film integrated microcircuit technology on a diamond anvil cell. The structural phase transitions occur at approximately 2.0 GPa and 6.0 GPa, as evidenced by changes in the energy band structure and discontinuities in electrical parameters. At around 12.3 GPa, the pressure-induced metallization is observed by temperature-induced resistivity and the closing of the band gap. The sudden decrease in resistivity can be attributed to the combined contribution of an increase in carrier concentration and mobility. Similarly, the abrupt decrease in Hall coefficient is attributed to the increase in carrier concentration.

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.

Development of direct current magnetron sputtered TiO[formula omitted] thin films as buffer layers for copper indium gallium diselenide based solar cells

We describe the development and characterization of direct current magnetron sputtered titanium dioxide thin films from ceramic targets mixed with metallic titanium particles. The aim of this article is to assess their suitability for the application as buffer layer in copper indium gallium diselenide (CIGSe) based solar cells. The absorber material is produced in a semi-industrial, roll-to-roll hybrid sputter co-evaporation process. A potential strategy to modify electro-optical properties of TiO2 thin films by controlling the concentration of defects, such as oxygen vacancies, is investigated by X-ray photoelectron spectroscopy (XPS), transmission/reflection spectroscopy, spectroscopic ellipsometry, and cathodoluminescence. The presence of oxygen in the plasma during the sputter deposition process has a crucial impact on the electron transport mechanism in the studied devices. The source of the changed device characteristic can be found in modified band discontinuities at the absorber–buffer interface and in an oxidized CIGSe surface, indicated by the presence of cationic selenium, as detected by XPS. We sketch a tentative band alignment diagram and outline the range of possible modifications. The harmful effect of oxygen in the plasma forces the omission of oxygen in the process and restricts the possibilities to modify the material properties of the TiO2 layers, and consequently the prospects for an application on CIGSe. Nevertheless, the results of this work depict the principal eligibility, albeit optimization on both sides of the absorber–buffer interface are inevitable for an improved cell performance.

Optical gain of CdxZn1−xTe quantum dot structures

AbstractGain spectra of undoped and doped quantum dot (QD) structures are studied under the inhomogeneous broadening assumption at four mole fractions . For the QD structure, two peaks appear due to the excited state (ES) and ground state (GS) transitions. The gain for the doped structures doubles the undoped ones. The gain increases while the wavelength is reduced with increasing Cd content due to the broader band discontinuity between the QD states. The discontinuity of the bands for each structure (mole fraction) is calculated, which is one of the merits of this work. While the structure offers the peak wavelengths 470 and 630 nm, other mole fractions offer the wavelengths between them. These visible bands are essential in different applications. The effect of QD size effect is also examined. The wavelength is extended by 20 nm for each 1 nm QD height increment.

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 electric field characteristics of GaAsSb photodiodes on InP substrates

Low noise avalanche photodiodes (APDs) detecting 1550 nm wavelength play a crucial role in optical communication and LiDAR systems. These APDs utilize a separate absorption, charge, and multiplication (SACM) architecture with an absorber for 1400–1650 nm detection and a low noise, high gain multiplier that can be independently optimized for a high signal-to-noise ratio. Recently, GaAs0.5Sb0.5/Al0.85Ga0.15As0.56Sb0.44 SACM APDs have demonstrated ultra-high gain and extremely low noise, possibly improving sensitivity over Si and InGaAs/InP commercial APDs. This accomplishment was achieved using a GaAsSb absorber instead of a conventional InGaAs absorber, mitigating band discontinuities between the absorber and the multiplier. However, further optimization is required to reduce noise due to tunneling and impact ionization from the GaAsSb absorber, which occurs at a high electric field region. This paper focuses on the study of the high-field characteristics of GaAsSb photodiodes (PDs). The tunneling phenomenon is analyzed through current density-voltage measurements, and the impact ionization behavior is evaluated by measuring the multiplication of p-i-n GaAsSb PDs. The result suggests that when designing a SACM APD with a GaAsSb absorber, the electric field in the absorber can be increased to 175 kV/cm without the detrimental effects of ionization occurring in the absorber. The findings from this investigation will assist in optimizing GaAsSb-based SACM APDs and promoting further advancements in the 1550 nm APD technology.

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.

Comprehensive study of gate induced drain leakage in nanowire and nanotube junctionless FETs using Si1-xGex source/drain

We present, in this paper, optimized nanotube (NT) and nanowire (NW) junctionless field-effect transistor (JLFET) architectures of gate-induced drain leakage (GIDL). At the source/ drain–channel interface, SiGe creates a valence energy band discontinuity which alleviates lateral band-to-band tunneling (L-BTBT) of electrons in the OFF-state. It is established through 2-D TCAD simulations that inclusion of SiGe decreases L-BTBT in the NTJLFET by >5 orders, thus leading to a substantial ION/IOFF of with precision 1012 for a 20 nm gate length which is not attainable in the conventional NTJLFET (ION/IOFF ∼ 106). Furthermore, degraded OFF-state current in NWJLFET because of conventional charge-sharing effect improves with the inclusion of SiGe by ∼2 orders of magnitude. Additionally, it is effective to place SiGe layers positioned at the source and drain regions and SiGe/ Si interface inside or at the channel region. Based on the proposed design guidelines, we show that the optimized SiGe S/D NTJLFET is effective in reduction of L-BTBT GIDL when gate length is scaled to 5 nm.

Room Temperature Light Emission from Superatom-like Ge-Core/Si-Shell Quantum Dots.

We have demonstrated the high-density formation of super-atom-like Si quantum dots with Ge-core on ultrathin SiO2 with control of high-selective chemical-vapor deposition and applied them to an active layer of light-emitting diodes (LEDs). Through luminescence measurements, we have reported characteristics carrier confinement and recombination properties in the Ge-core, reflecting the type II energy band discontinuity between the Si-clad and Ge-core. Additionally, under forward bias conditions over a threshold bias for LEDs, electroluminescence becomes observable at room temperature in the near-infrared region and is attributed to radiative recombination between quantized states in the Ge-core with a deep potential well for holes caused by electron/hole simultaneous injection from the gate and substrate, respectively. The results will lead to the development of Si-based light-emitting devices that are highly compatible with Si-ultra-large-scale integration processing, which has been believed to have extreme difficulty in realizing silicon photonics.

Investigation of Gate Induced Drain Leakage in Nanotube and Nanowire: A Comprehensive Study

In this paper, a comprehensive study of gate-induced drain leakage (GIDL) in conventional silicon-nanotube (Si-NT JLFET), SiGe Source/Drain silicon-nanotube junctionless field effect transistor (S/D Si-NT JLFET) and conventional nanowire (NW) have been performed using technology computer-aided design simulations. We have also demonstrated that inclusion of SiGe S/D in Si-NT JLFET reduced the OFF-state current by order of [Formula: see text]3 from NT JLFET and by order of [Formula: see text]6 from NW JLFET. The impact of variation of core gate thickness ([Formula: see text], germanium (Ge) content [Formula: see text], and location of SiGe in source and drain regions of the S/D Si-NT JLFET have been studied from the GIDL perspective. We found that SiGe S/D Si-NT JLFET exhibits impressively high [Formula: see text]/[Formula: see text] ratio [Formula: see text]10[Formula: see text] with reduced lateral band-to-band tunneling (L-BTBT)-induced GIDL than the conventional nanowire device. The is due to SiGe S/D that creates a energy valence band discontinuity at source drain interfaces which limits the flow of electrons from channel to drain region in the OFF-state.

GaN-Based Double-Heterojunction Bipolar Transistors With a Composition Graded p-InGaN Base

The influence of energy band structure on the electrical characteristics of GaN-based double-heterojunction bipolar transistors (DHBTs) has been studied through both simulation and fabrication. According to the simulation result, a novel DHBT structure with a composition graded base was grown and fabricated. A long minority carrier lifetime of 4.08 ns has been achieved for the composition graded p-InGaN base with a greatly improved material quality. The indium composition grading of the p-InGaN base layer combined with Si doping profile tuning for the collector layer was proposed to eliminate the energy barrier usually formed at the conventional GaN/InGaN/GaN base–collector (B–C) junction interface due to the band discontinuity and polarization effect. As a result, the as-fabricated DHBT presents a high intercept voltage of 225 V extracted from the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{I}_{\textit{C}}$</tex-math> </inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{V}_{\text{CE}}$</tex-math> </inline-formula> curves and a high current gain of 49.