Sheet Charge Research Articles

Theoretical Evaluation of the Effects of Isolation-Feature Size and Geometry on the Built-In Strain and 2-D Electron Gas Density of AlGaN/GaN Heterostructures

Using a commercial self-consistent Poisson–Schrodinger solver with the built-in possibility of allowing elastic energy minimization, the strain and the sheet charge density induced at the pseudomorphically grown Ga-face Wurtzite AlGaN/GaN heterojunctions are evaluated in the context of 3-D simulation of heterostructure field-effect transistor (HFET) epilayers etched into a variety of isolation-feature sizes and geometries. Through these studies and in the presence of surface states, the extent of the relevance of strain minimization in the vicinity of the unconstrained boundaries of isolation features of different degrees of roundness and perimeter-to-area ratio to threshold-voltage engineering is assessed. Although it is demonstrated that threshold-voltage shift caused by this induced strain minimization is smaller than the amount of shift levied by the depleting effect of the negatively charged states on the sidewall facets, it is shown that the reduction of the isolation-feature size is capable of substantially reducing the average trace of the stress tensor across such heterointerfaces. Considering the importance of this factor to the long-term reliability of AlGaN/GaN HFETs, especially when undergoing self-heating at high-power levels, use of alternative isolation features such as small islands of a lateral area less than 1000 nm2 is proposed as a solution.

Nanoscale enhancement of photoconductivity by localized charge traps in the grain structures of monolayer MoS2

We report a method for mapping the nanoscale anomalous enhancement of photoconductivity by localized charge traps in the grain structures of a molybdenum disulfide (MoS2) monolayer. In this work, a monolayer MoS2 film was laterally scanned by a nanoscale conducting probe that was used to make direct contact with the MoS2 surface. Electrical currents and noise maps were measured through the probe. By analyzing the data, we obtained maps for the sheet resistance and charge trap density for the MoS2 grain structures. The maps clearly show grains for which sheet resistance and charge trap density were lower than those of the grain boundaries. Interestingly, we found an unusual inverse proportionality between the sheet resistance and charge trap density in the grains, which originated from the unique role of sulfur vacancies acting as both charge hopping sites and traps in monolayer MoS2. In addition, under light illumination, the larger the trap density of a region was, the larger the photocurrent of the region was, indicating anomalous enhancement of the photocurrent by traps. Since our method provides valuable insights to understand the nanoscale effects of traps on photoconductive charge transport, it can be a powerful tool for noise studies and the practical application of two-dimensional materials.

Ultra-wide band gap AlGaN polarization-doped field effect transistor

Polarization-doped field effect transistors (PolFETs) were realized with an unintentionally doped AlxGa1−xN channel layer graded over Al compositions 0.70 ≤ x ≤ 0.85 and ohmic contact formation to the Al0.85Ga0.15N surface. Current density of 24 mA/mm was attained for a modest threshold voltage of −3.95 V owing to the high sheet charge concentration (ns) of 5.4 × 1012 cm−2 and average electron mobility (μ) of 210 cm2 V−1 s−1 for the channel region along with ohmic contacts with a specific contact resistivity of 1.1 mΩ cm2 and a large Schottky barrier height of 3.0 eV for the metal–semiconductor gate. The combination of ultra-wide band gap energy, high μ, and high ns, linear ohmic contacts, and large Schottky barrier for AlGaN PolFETs makes them attractive for both high voltage switches and high power rf devices.

Alleviation of efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes with trapezoidal quantum barriers

We numerically investigated the effects of trapezoidal quantum barriers (QBs) on efficiency droop in InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs). Simulations showed that the electrostatic field in QWs of LEDs with trapezoidal barriers is reduced because of the reduced sheet charge density at the QW-QB interface caused by the thin GaN layer in trapezoidal QBs. Additionally, the InGaN grading region in trapezoidal QBs suppresses hot carrier transport and this enhances efficient carrier injection into the QWs. The electroluminescence intensity of an LED with trapezoidal QBs is increased by 10.2% and 6.7% at 245 A cm−2 when compared with the intensities of LEDs with square-type GaN barriers and multilayer barriers, respectively. The internal quantum efficiency (IQE) droop of an LED with trapezoidal QBs is 16% at 300 A cm−2, while LEDs with square-type GaN barriers and multilayer barriers have IQE droop of 31% and 24%, respectively. This IQE droop alleviation in LEDs with trapezoidal QBs is attributed to the reduced energy band bending, efficient hole injection, and more uniform hole distribution in the MQWs that results from reduction of the piezoelectric field by the trapezoidal QBs. These results indicate that the trapezoidal QB in MQWs is promising for enhanced efficiency in high-power GaN-based LEDs.

Enhanced Sheet Charge Density in DIBS Grown CdO Alloyed ZnO Buffer Based Heterostructure

In this letter, we report on achieving significantly high (~ $6{\times}$ ) sheet charge density (ns) in MgZnO/CdZnO heterostructure, as compared with that in MgZnO/ZnO, at lower Mg ( ${\le} 0.15$ ) compositions in barrier MgZnO layer, with both heterostructures grown by dual ion beam sputtering technique. Buffer CdZnO and barrier MgZnO layers are probed separately to investigate carrier density in defect prone sputtered layers. Capacitance-voltage measurement and temperature-dependent Hall measurement confirm the presence of quantum confined carrier density at barrier-buffer interface, suggesting that the enhancement in carrier densities in the heterostructure over individual layers is probably due to the formation of 2-D electron gas (2-DEG).

Modeling and simulation of DC and microwave characteristics of AlInN(AlGaN)/AlN/GaN MOSHEMTs with different gate lengths

AbstractIn this paper, direct current (DC) and microwave characteristics are modeled and analyzed for AlInN(AlGaN)/AlN/GaN metal‐oxide‐semiconductor high electron mobility transistors with different gate lengths. The 2‐dimensional electron gas sheet charge density (ns) model is developed by considering polarization effect and flat‐band voltage with lowest occupied energy level (E0) in the quantum well. The drain current, transconductance, and gate charge model is developed by incorporating approximation of ns as proposed in this paper. Then, the cut‐off frequency is calculated from the transconductance and gate capacitance model. The analytical model results are in good agreement with Technology Computer Aided Design simulation and experimental results from literatures for DC and microwave characteristics for the proposed gate lengths (0.1, 0.2, and 0.3 μm). From these characteristics, it is also observed that AlInN/AlN/GaN MOSHEMT shows superior DC and radio frequency performance as compared with AlGaN/AlN/GaN MOSHEMT. AlInN/AlN/GaN MOSHEMT produces high drain current (2.34 A/mm) and high cut‐off frequency (124 GHz) at smaller gate length.

Epitaxial engineering of polar ε-Ga2O3 for tunable two-dimensional electron gas at the heterointerface

We predict the formation of a polarization-induced two-dimensional electron gas (2DEG) at the interface of ε-Ga2O3 and CaCO3, wherein the density of the 2DEG can be tuned by reversing the spontaneous polarization in ε-Ga2O3, for example, with an applied electric field. ε-Ga2O3 is a polar and metastable ultra-wide band-gap semiconductor. We use density-functional theory (DFT) calculations and coincidence-site lattice model to predict the region of epitaxial strain under which ε-Ga2O3 can be stabilized over its other competing polymorphs and suggest promising substrates. Using group-theoretical methods and DFT calculations, we show that ε-Ga2O3 is a ferroelectric material where the spontaneous polarization can be reversed through a non-polar phase by using an electric field. Based on the calculated band alignment of ε-Ga2O3 with various substrates, we show the formation of a 2DEG with a high sheet charge density of 1014 cm−2 at the interface with CaCO3 due to the spontaneous and piezoelectric polarization in ε-Ga2O3, which makes the system attractive for high-power and high-frequency applications.

Strain Engineering and Two-Dimensional Electron Gas in Polar ε-Ga2O3

As a wide gap semiconductor, Ga2O3 is rapidly emerging as a promising candidate for power electronics applications. While most of the studies have focused on its stable β-phase, there are a handful of reports on its metastable polar ε-phase having a spontaneous polarization. Numerous experimental groups have recently attempted to stabilize ε-Ga2O3 using epitaxial strain using substrates such as Al2O3(0001)1, GaN(0001)1, AlN(0001)1, MgO(111)2 and SiC(0001)3. However, due to the lack of an understanding of the stability of various Ga2O3 phases under epitaxial strain, these trial-and-error based attempts have been of limited success. All the films are observed to be of inherently poor quality. There are also diverging reports on the structure and properties of the deposited thin films. A recent experimental report of ε-Ga2O3 grown on Al2O3 substrate has even suggested the film to be ferroelectric, where the direction of the spontaneous polarization could be switched with an external electric field. It implies that the stabilization of ε-Ga2O3 will open new avenue of polarization engineering in Ga2O3 power electronics4. We have used first-principles density-functional theory (DFT) calculations in combination with coincidence site lattice theory to develop a phase-diagram of Ga2O3 under epitaxial strain. We show that all the previously used substrates impose an epitaxial strain over 3% on ε-Ga2O3, which explains the poor structural quality of the deposited thin films. In this presentation, we will discuss promising commercially available substrates that can stabilize ε-Ga2O3 with epitaxial strain < 1 %. We will discuss the electronic structure of ε-Ga2O3 under epitaxial strain, including properties such as the band gap, polarization constants and its ferroelectric nature. We will theoretically demonstrate a way to achieve two-dimensional electron gas (2DEG) in ε-Ga2O3 heterostructure simply by using polarization engineering. The formation of 2DEG is due to the polar catastrophe, and the formation and sheet charge density 2DEG can be controlled by thickness of ε-Ga2O3 and its polarization direction. Our study highlights the potential of the polar behavior of ε-Ga2O3 and suggest pathway to realize its stabilize epitaxial growth. Y. Oshima, E.G. Víllora, Y. Matsushita, S. Yamamoto and K. Shimamura: Journal of Applied Physics 118, 085301 (2015).2. H. Nishinaka, D. Tahara and M. Yoshimoto: Japanese Journal of Applied Physics 55, 1202BC (2016).X. Xia, Y. Chen, Q. Feng, H. Liang, P. Tao, M. Xu and G. Du: Applied Physics Letters 108, 202103 (2016).F. Mezzadri, G. Calestani, F. Boschi, D. Delmonte, M. Bosi and R. Fornari: Inorganic Chemistry 55, 12079 (2016).

Anomalous Electron Accumulation at Al2O3/in2O3 Interface Via Short-Range Ordered Two-Dimensional Electron Gas

Transparent conducting oxide (TCO) films, which is one of the widely used as window electrodes at the various devices, fulfil high electrical conductivity and optical transparency in the visible region of spectrum. Conventional TCO films are formed by the tuning charge transport of wide band gap oxides like indium gallium zinc oxide (IGZO), zinc oxide (ZnO), tin dioxide (SnO2), and indium oxide (In2O3). Among the various TCO candidates, In2O3 is an attractive TCO candidate since In2O3 can satisfied the visible transparency and high carrier density via doping processes. To achieve the high mobility In2O3 films, doping processes with metal dopants such as Ti, Zr, Mo, and W have been suggested by many previous study. Intrinsic In2O3, however, has n-type semiconductor due to its native oxygen vacancy act as n-type donor and indium interstitial. As another approach to gain highly conductivity TCOs, two-dimensional electron gas (2DEG) channel can be suggested since 2DEG channel can be applied to increase the electrical conductivity of oxide semiconductors up to the metallic conduction level. The tuning of electrical and optical properties in oxides via surface and interfacial 2DEG channels is of great interest, as they reveal the extraordinary transition from insulating or semiconducting characteristics to metallic conduction or superconductivity enabled by the ballistic transport of spatially-confined electrons. Although most previous studies have been carried out in crystalline phase like single crystal SrTiO3 or III-V compound semiconductor, realizing practical aspects of this exotic phenomenon toward short-range ordered and air-stable 2DEG channels remains a great challenge. In this study, we fabricated Al2O3/In2O3 heterojunction system on soda-lime glass consisting of In2O3 deposited by RF reactive sputter with a thickness from 8 to 100 nm and Al2O3 deposited by ALD with a thickness below 100 nm. We suggest that Al2O3/In2O3 heterojunction acts as short-range ordered 2DEG channel with a low temperature (~200 ℃). After deposition of Al2O3 layer, the heterojunction two-dimensional metallic channel was fabricated between Al2O3 and In2O3 interface as showing the metallic conduction with sheet charge density > 10 14/cm2, sheet resistance of 850 Ω/cm2, room temperature Hall mobility of 20.5cm2 V-1 sec-1 depending on the In2O3 thickness. To prove existence of 2DEG channel at the interface, we performed the I-V measurement, time of flight secondary ion mass spectrometry, ultraviolet-visible spectrophotometer, tunneling electron microscope–electron energy loss spectroscopy, X-ray photoelectron spectroscopy, and Hall measurement.

An Integral Equation Approach to Model the Drastic Change in Depletion Width From Bulk to Nanoscale Junctions

The space-charge region (SCR) in semiconductor bulk and nanoscale junctions is mostly analyzed using the differential equation, namely, Poisson’s equation. We explore an alternate analysis using the integral equation, namely, Coulomb’s law, and show its superiority to the differential equation approach in revealing the drastic change in SCR electrostatics from bulk to single nanofilm (NF) to single nanowire/nanotube (NW/NT) junction. The advantages of the integral equation approach are as follows. First, it helps us to view the junction SCR in bulk as a series of sheet charges, that in thin NFs as a series of line charges, and that in thin NWs/NTs as a series of point charges; such a view logically yields the different functional forms of the depletion width dependence on dielectric constant, potential drop, and inverse doping, namely, squareroot in bulk, linear in NFs, and exponential in NWs/NTs. Second, it shows that the partially depleted space-charge tails in NFs/NWs/NTs must extend to infinity, and that these tails rather than the completely depleted regions set up the required potential difference across the junction; it also reveals that for large distance, ${z}$ away from the junction, the field varies as $1/{z}^{2}$ in NF and $1/{z}^{3}$ in NW/NT, and the potential/charge vary as $1/{z}$ in NF and $1/{z}^{2}$ in NW/NT. To ensure accuracy of the depletion width model, we capture the effect of space-charge tails in NFs/NWs/NTs by employing a periodic charge distribution. We also show how prior works overestimated the depletion width by orders of magnitude.

Effects of surface passivation dielectrics on carrier transport in AlGaN/GaN heterostructure field-effect transistors

Dielectric layers prepared by different deposition methods were used for the surface passivation of AlGaN/GaN heterostructure field-effect transistors (HFETs) and the corresponding electrical characteristics were examined. Increases in the sheet charge density and the maximum drain current by approximately 45% and 28%, respectively, were observed after the deposition of a 100 nm-thick SiO2 layer by plasma-enhanced chemical vapor deposition (PECVD) on the top of the AlGaN/GaN HFETs. However, SiO2 deposited by a radio frequency (rf) sputter system had the opposite effect. As the strain applied to AlGaN was influenced by the deposition methods used for the dielectric layers, the carrier transport in the two-dimensional electron gas formed at the interface between AlGaN and GaN was affected accordingly.

An analytical drain current model for symmetric double-gate MOSFETs

An analytical surface-potential-based drain current model of symmetric double-gate (sDG) MOSFETs is described as a SPICE compatible model in this paper. The continuous surface and central potentials from the accumulation to the strong inversion regions are solved from the 1-D Poisson’s equation in sDG MOSFETs. Furthermore, the drain current is derived from the charge sheet model as a function of the surface potential. Over a wide range of terminal voltages, doping concentrations, and device geometries, the surface potential calculation scheme and drain current model are verified by solving the 1-D Poisson’s equation based on the least square method and using the Silvaco Atlas simulation results and experimental data, respectively. Such a model can be adopted as a useful platform to develop the circuit simulator and provide the clear understanding of sDG MOSFET device physics.

Intercalation of imidazolium cations into graphite via ion exchange

New graphite intercalation compounds (GICs) containing 1-alkyl-2,3-dimethyl imidazolium cations (Im1-1-a, a = alkyl length) are obtained from [Na(ethylenediamine)1.0]C15 by cation exchange. Exchange reactions occur rapidly, but require higher temperature than for substituted alkylammoniums. Powder X-ray diffraction, thermogravimetry and structural modeling indicate that [Im1-1-4]C47 is a stage-2 GIC with 0.36 nm monolayer galleries and cations oriented parallel to graphene sheets. [Im1-1-12]C44 is a stage-1 GIC with a gallery expansion of 0.40 nm. The lower sheet charge density for [Im1-1-12]C44 is commensurate with its larger intercalate. Imidazolium cations without alkyl substitution at the imidazolium ring C2 do not form stable GICs by this route.

Carrier trapping and activation at short-period wurtzite/zinc-blende stacking sequences in polytypic InAs nanowires

We explore the effects of random and short-period crystal-phase intermixing in InAs nanowires (NW) on the carrier trapping and thermal activation behavior using correlated optical and electrical transport spectroscopy. The polytypic InAs NWs are grown by catalyst-free molecular beam epitaxy under different temperatures, resulting in different fractions of wurtzite (WZ) and zincblende (ZB) and variable short-period (\ensuremath{\sim}1--4 nm) WZ/ZB stacking sequences. Temperature-dependent microphotoluminescence $(\ensuremath{\mu}\text{PL})$ studies reveal that variations in the WZ/ZB stacking govern the emission energy and carrier confinement properties. The optical transition energies are modeled for a wide range of WZ/ZB stacking sequences using a Kronig-Penney type effective mass approximation, while comparison with experimental results suggests that polarization sheet charges on the order of \ensuremath{\sim}0.0016--0.08 C/m along the WZ/ZB interfaces need to be considered to best describe the data. The thermal activation characteristics of carriers trapped inside the short-period WZ/ZB structure are directly reproduced in the temperature-dependent carrier density evolution (4--300 K) probed by four-terminal (4T) NW-field effect transistor measurements. In particular, we find that activation of carriers in-between $\ensuremath{\sim}{10}^{16}\ensuremath{-}{10}^{17}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ follows a two-step process, with activation at low temperature attributed to WZ/ZB traps and activation at high temperature being linked to surface states and electron accumulation at the InAs NW surface.

Two‐dimensional models for quantum effects on short channel electrostatics of lightly doped symmetric double‐gate MOSFETs

Analytical Verilog-A compatible 2D model including quantum short channel effects and confinement for the potential, threshold voltage and the carrier charge sheet density for symmetrical lightly doped double-gate metal-oxide-semiconductor field effect transistors (MOSFETs) is developed. The proposed models are not only applicable to ultra-scaled devices but they have also been derived from 2D Poisson and 1D Schrodinger equations including 2D electrostatics, in order to incorporate quantum mechanical effects. Electron and hole quasi-Fermi potential effects were considered. The models are continuous and have been verified by comparison with COMSOL and BALMOS numerical simulations for channel lengths down to 7 nm at 1 nm oxide thicknesses; very good agreement within ±5% has been observed for silicon thicknesses down to 3 nm.

A closed-form trapped-charge-included drain current compact model for amorphous oxide semiconductor thin-film transistors

A closed-form drain current compact model for amorphous oxide semiconductor (AOS) thin-film transistors (TFTs), including the influence from trapped charges, is presented in this paper. Accounting for both channel and interface trapped charges in this model, we explicitly solve the inherent closed-form surface potential by improving the computational efficiency of the effective charge density approach. Furthermore, based on the explicit solution of the surface potential, the expressions of the trapped and inversion charges in the channel film are derived analytically, and the drain current is integrated from the charge sheet model. Then, for the cases of the different operational voltages, the accuracy and practicability of our model are verified by numerical results of the surface potential and experimental data of the drain current in amorphous In-Ga-Zn-O TFTs, respectively. Finally, we give a discussion about the influence of the interface trapped charges on the device reliability. As a result, the model can be easily to explore the drain current behavior of the AOS TFTs for next-generation display circuit application.

Enhanced mobility in vertically scaled N-polar high-electron-mobility transistors using GaN/InGaN composite channels

A GaN/InGaN composite channel design for vertically scaled N-polar high-electron-mobility transistor (HEMT) structures is proposed and demonstrated by metal-organic chemical vapor deposition. In a conventional N-polar HEMT structure, as the channel thickness (tch) decreases, the sheet charge density (ns) decreases, the electric field in the channel increases, and the centroid of the two-dimensional electron gas (2DEG) moves towards the back-barrier/channel interface, resulting in stronger scattering and lower electron mobility (μ). In this study, a thin InGaN layer was introduced in-between the channel and the AlGaN cap to increase the 2DEG density and reduce the electric field in the channel and therefore increase the electron mobility. The dependence of μ on the InGaN thickness (tInGaN) and the indium composition (xIn) was investigated for different channel thicknesses. With optimized tInGaN and xIn, significant improvements in electron mobility were observed. For a 6 nm channel HEMT structure, the electron mobility increased from 606 to 1141 cm2/(V·s) when the 6 nm thick pure GaN channel was replaced by the 4 nm GaN/2 nm In0.1Ga0.9N composite channel.

Leakage current conduction mechanisms and electrical properties of atomic-layer-deposited HfO2/Ga2O3 MOS capacitors

In this paper, current conduction mechanisms in HfO2/β-Ga2O3 metal–oxide–semiconductor (MOS) capacitors under positive and negative biases are investigated using the current–voltage (I–V) measurements conducted at temperatures from 298 K to 378 K. The Schottky emission is dominant under positively biased electric fields of 0.37–2.19 MV cm−1, and the extracted Schottky barrier height ranged from 0.88 eV to 0.91 eV at various temperatures. The Poole–Frenkel emission dominates under negatively biased fields of 1.92–4.83 MV cm−1, and the trap energy levels are from 0.71 eV to 0.77 eV at various temperatures. The conduction band offset (ΔEc) of HfO2/β-Ga2O3 is extracted to be 1.31 ± 0.05 eV via x-ray photoelectron spectroscopy, while a large negative sheet charge density of 1.04 × 1013 cm−2 is induced at the oxide layer and/or HfO2/β-Ga2O3 interface. A low C–V hysteresis of 0.76 V, low interface state density (Dit) close to 1 × 1012 eV−1 cm−2, and low leakage current density of 2.38 × 10−5 A cm−2 at a gate voltage of 7 V has been obtained, suggesting the great electrical properties of HfO2/β-Ga2O3 MOSCAP. According to the above analysis, ALD-HfO2 is an attractive candidate for high voltage β-Ga2O3 power devices.

Built-In Sheet Charge As an Alternative to Dopant Pockets in Tunnel Field-Effect Transistors`

Dopant pockets in combination with a III–V heterostructure have become a staple in simulations of tunnel field-effect transistors (TFET) to achieve acceptable on-currents ( ${I} _{\mathrm{ ON}}$ ) and to break the ${I} _{\mathrm{ ON}}$ -subthreshold swing trade-off in pTFETs. Questions on the scalability and variability of these dopant pockets remain, however. We therefore propose an alternative concept using two opposite sheet charges at the tunnel junction that are realized by exploiting different bonding types. With fully quantum mechanical 30-band k.p-based simulations, we investigate two configurations that exhibit such sheet charges both at the device level and in a ring-oscillator: a lattice-matched GaAs-Ge-GaAs heterostructure TFET and a homostructure InGaAs TFET. By comparing to pocketed broken gap GaSb-InAs TFETs, we show that built-in sheet charge is a valid alternative for both nTFET and pTFET, with the prospect of lower variability and better scalability.

Comparative Study of Heterostructure Barrier Diodes in the GaAs/AlGaAs System

A comparative study of the electron transport property and operation of the Potential Well Barrier (PWB) diode and Planar-doped Potential-well Barrier (PPB) diode has been carried out in this study. Both diodes are heterostructures in GaAs/AlGaAs system with similarities in layer design though, with a sheet charge inserted close to the well of the PPB diode. A drift-diffusion and Monte Carlo simulation models were used throughout the study to examine the behavior of electrons in terms of the electric field distribution across the diodes, electron velocities, electron energy and densities. Results of simulation has shown how the electric field varies in the left and right intrinsic regions of the device and the effect of the field on velocity. The I-V characteristics of the experimental and simulation results have shown a good agreement in the two diodes though, with little adjustment of about ± 2.5% to design parameters in order to obtain a good fit with experimental results. The I-V characteristics of the diodes reveal that the PPB diode turns on at a higher voltage than the PWB diode though, with a better asymmetry in the reverse bias operation. This is because the sheet charge in the PPB diode produces additional charge and together with the charge in the well, presents a higher potential barrier than the PWB diode whose barrier is determined by the charge in the well only. The diodes demonstrate promising RF behavior with voltage responsivity of 10900V/W and 6400V/W at 10GH for the PPB and PWB diodes respectively.