Sheet Electron Density Research Articles

Electron velocity decline in Si nanoscales MOSFETs with the shortening of gate length

The ensemble Monte Carlo device simulations are employed to obtain I-V characteristics for the 25 nm gate length template Si MOSFETs designed by the SiNANO consortium. The simulated ID-VG characteristics are compared against previous results from various Monte Carlo device codes [Fiegna C et al., in Proc. SISPAD 2007, pp. 57-60 (Springer Vienna, 2007)]. After this verification, we have scaled the transistor laterally only from a gate length of 25 nm to gate lengths of 20, 15, 10 and 5 nm. We have then monitored the average electron velocity and sheet density along the channel at a supply voltage of 1.0 V in order to gain an insight into the degradation of the injection velocity experimentally observed in various Si transistor architectures when the gate length is scaled into deep sub-50 nm dimensions. We have found a substantial decrease of the overall velocity profile along the channel including a decrease of the peak velocity when the channel is smaller than 15 nm while the drive current is able to sustain its increase thanks to the increasing velocity at the drain side.

Double pulse doped InGaAs/AlGaAs/GaAs pseudomorphic high-electron-mobility transistor heterostructures

Double pulse doped (δ-doped) InGaAs/AlGaAs/GaAs pseudomorphic high-electron-mobility transistor (HEMT) heterostructures were grown by molecular-beam epitaxy using a multiwafer technological system. The room-temperature electron mobility was determined by the Hall method as 6550 and 6000 cm2/(V s) at sheet electron densities of 3.00 × 1012 and 3.36 × 1012 cm−2, respectively. HEMT heterostructures fabricated in a single process feature high uniformity of structural and electrical characteristics over the entire area of wafers 76.2 mm in diameter and high reproducibility of characteristics from process to process.

Design of near lattice‐matched AlGaInN‐barriers for highly‐scalable GaN‐based transistor structures

AbstractAlGaInN layers at compositions near lattice‐matched to GaN have been grown by molecular beam epitaxy. X‐ray diffraction and atomic force microscopy show single‐phase quaternary layers with smooth morphology. Heterostructures with thin nearly strain‐free AlGaInN‐barriers exhibit high values for sheet electron density and mobility ranging up to 2.1×1013 cm‐2 and 1240 cm2/Vs, respectively. The epitaxial design of the structures comprises a multi‐layer spacer which enables a wider separation between channel and barrier compared to a single AlN interlayer and serves as a protection during growth. A series of wafers with varied composition in the barrier reveals that the electron mobility increases with Ga‐content in AlGaInN as expected considering the miscibility of quaternary nitrides. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Compositional variation of nearly lattice-matched InAlGaN alloys for high electron mobility transistors

Quaternary InAlGaN semiconductors with AlN mole fractions between 40% and 81% and respective InN contents of 7% and 19% including InAlN as a ternary border case have been grown by plasma-assisted molecular beam epitaxy. The electron mobility in InAlGaN-based heterostructures increases with Ga concentration in the barrier up to 1460 cm2/V s at a sheet electron density of 1.9×1013 cm−2. An advanced spacer comprising an AlN/GaN/AlN triple-layer sequence is inserted between the GaN-buffer and the InAlGaN-barrier. Transistors with thin quaternary barrier show a large current density of 2.3 A/mm and an excellent transconductance of 675 mS/mm.

High temperature electron transport properties of AlGaN/GaN heterostructures based on unintentionally doped and semi-insulating GaN buffer layer

High temperature electron transport properties of Al0.35Ga0.65N/GaN heterostructures grown on unintentionally doped (UID) and semi-insulating (SI) GaN buffer layer (BL) have been investigated. Electron sheet density and electron mobility dependence on temperature have been systematically measured by Hall effect measurement. It is found that the electron mobility of AlGaN/GaN heterostructure is mainly limited by the longitudinal optical (LO) phonon scattering at high temperature. The sheet carrier density of AlGaN/GaN heterostructure based on UID-BL increases gradually with the increase of temperature, while it decreases firstly, and then increases for the sample based on SI-BL. It is indicated by the analysis of the high temperature resistance properties for the corresponding UID-GaN and SI-GaN films, that the effect of the background carrier-limited electron mobility increases gradually for the UID-BL sample, while for the SI-BL sample the electron mobility is mainly limited by the additional dislocation scattering around room temperature but the background carrier limited effect increases after 600 K. This is meaningful for investigating the high temperature property of the devices based on AlGaN/GaN heterostructure. Moreover, the theoretical calculation indicates that, the two dimensional gas (2DEG) has expanded to the AlGaN barrier layer and the deeper position of the GaN buffer layer, and the electron occupation in the first subband decreases to 81% at 700 K.

An Intensity Modulator for Terahertz Electromagnetic Waves Utilizing Two-Dimensional Plasmon Resonance in a Dual-Grating-Gate High-Electron-Mobility Transistor

We propose an intensity modulator utilizing two-dimensional plasmons (2DPs) in a dual-grating-gate high-electron-mobility transistor. The device primarily functions as a plasmon-resonant emitter in which the seeds of non-radiative longitudinal modes of 2DPs are efficiently converted into radiative transverse modes of terahertz (THz) electromagnetic waves. In this work, we numerically study the behavior of the mode conversion between 2DPs and THz electromagnetic waves as a function of the 2DP dispersion. The finite difference time domain analysis demonstrates that the coupling of THz electromagnetic waves and 2DPs changes with the electron drift velocity and with the sheet electron density in 2DPs. The analysis also reveals that the intensity of transmitted waves can be modulated over a wide THz range with an extinction ratio beyond 60% by optimizing the sheet electron density and the drift velocity under nominal area-factor condition (ratio of the 2DP area over the total active channel area) up to 0.6.

2H-AlGaN/GaN HEMTs on 3C-SiC(111)/Si(111) Substrates

We present the realization of high electron mobility transistors (HEMTs) based on AlGaN/GaN heterostructures grown on silicon substrates using a SiC transition layer. The growth of AlGaN/GaN heterostructures on Si (111) was performed using metalorganic chemical vapour deposition (MOCVD). The (111) SiC transition layer was realized by low pressure CVD and prevented Ga-induced meltback etching and Si-outdiffusion in the subsequent MOCVD growth. The two-dimensional electron gas (2DEG) formed at the AlGaN/GaN interface showed an electron sheet density of 1.5x1013 cm-3 and a mobility of 870 cm²/Vs proving the high structural quality of the heterostructure. Device processing was done using electron beam lithography. DC and RF characteristics were analysed and showed a peak cut-off frequency as high as 6 GHz for a 1.2 µm gate HEMT.

Control of intersubband quantum well transitions with chirped electromagnetic pulses

We study the interaction of chirped electromagnetic pulses with intersubband transitions of a double semiconductor quantum well. We consider the ground and first excited subbands and give emphasis to controlled intersubband population transfer. The system dynamics is described by the nonlinear density matrix equations that include the effects of electron-electron interactions. These equations are solved numerically for various values of the electron sheet density for a realistic double GaAs/AlGaAs quantum well, and the efficiency of population transfer is discussed.

Strain dependent electron drift velocity in Al1–xInxN/AlN/GaN

AbstractWe have measured the room‐temperature velocity–field characteristics in Al1–x Inx N/AlN/GaN strained structures (x = 0.12, 0.15, 0.22) with two‐dimensional electron gas channels biased up to 200 kV/cm using nanosecond‐pulsed current–voltage technique. The drift velocity was estimated under assumptions of uniform electric field and no change in sheet electron density. No velocity saturation was observed. The lowest velocity was obtained for the heterostructure with an In content of x = 0.22. The experimental results are compared with those for lattice‐matched Al1–x Inx N/AlN/GaN, AlGaN/GaN, AlGaN/GaN/AlN/GaN, and silicon‐doped GaN. The conduction band profile, electron wave functions were calculated for the investigated nitride heterostructures through a self‐consistent solution of Schrödinger–Poisson equations. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron accumulation at nonpolar and semipolar surfaces of wurtzite InN from generalized infrared ellipsometry

The free electron properties of nonpolar (112¯0)-oriented and semipolar (101¯1)-oriented wurtzite InN films are studied by generalized infrared ellipsometry (GIRSE). We demonstrate the sensitivity of GIRSE to the surface charge accumulation layer and find a distinct surface electron accumulation to occur at all surfaces. The obtained surface electron sheet densities are found to vary from 0.9×1013 to 2.3×1014 cm−2 depending on the surface orientation and bulk electron concentration. The upper limits of the surface electron mobility parameters of 417–644 cm2/V s are determined and discussed in the light of electron confinement at the surface.

Theoretical design of GaN/ferroelectric heterostructure: Toward a strained semiconductor on ferroelectrics

Ferroelectric/semiconductor heterostructures are of great interest for future electronic devices. This letter examined the material parameters and carrier distributions of an AlGaN(0001)/GaN(0001)/BaTiO3(111) double heterostructure by combining first principles and charge control model. Results show that in the optimized case, there will appear two channels in GaN layer, and the sheet electron density will be doubled compared to conventional AlGaN/GaN heterojunction. A possible device structure was proposed to make the channel become switchable and reduce the source/drain resistance. This strained semiconductor on ferroelectric structure may be promising for high speed power devices.

Intrinsically limited mobility of the two-dimensional electron gas in gated AlGaN/GaN and AlGaN/AlN/GaN heterostructures

We report on the intrinsically limited low-field mobility of the two-dimensional electron gas (2DEG) in gated AlGaN/GaN and AlGaN/AlN/GaN heterostructures. Monte Carlo transport simulations are carried out to calculate the room-temperature 2DEG mobilities in dependence on the electron sheet density. The simulated 2DEG mobilities are compared to the phonon-limited mobility of bulk GaN. We estimate a maximum 2DEG mobility of about 2700 cm2 V−1 s−1 for an electron sheet density of ∼5×1012 cm−2, which remarkably exceeds the phonon-limited bulk mobility of 1520 cm2 V−1 s−1. By reducing the electron sheet density below 5×1012 cm−2, i.e., in a weak electron quantum confinement regime, the room-temperature 2DEG mobility gradually decreases and approaches the phonon-limited bulk value for vanishing quantum confinement. The insertion of a thin AlN barrier interlayer improves transport properties of the 2DEG and the mobility substantially increases due to a suppression of the alloy scattering.

Temperature mapping of Al0.85In0.15N/AlN/GaN high electron mobility transistors through micro-photoluminescence studies

Crack-free lattice-matched Al 0.85 In 0.15 N/GaN heterostructures were grown on sapphire substrates with barrier thicknesses up to 100 nm which exhibit very high polarization-induced electron sheet density ( cm −2 ) located at the heterointerface. These layers have been further processed as high electron mobility transistors (HEMTs). Optical characterization of these structures was carried out by photoluminescence and microphotoluminescence (PL) for different biased voltages. The insertion of an InGaN back-barrier unambiguously reveals that spatially direct optical recombinations occur within the AlInN alloy. Since the GaN excitonic bandgap is very sensitive to local temperature changes, the PL technique allows mapping very precisely the actual local temperature distribution in biased HEMT devices. For a gate length of 1.5 m temperatures up to 1130 K were found at a drain-source voltage of 20 V thus indicating the presence of a hot phonon bath.

Electron mobility in wurtzite nitride quantum wells limited by optical-phonons and its pressure effect

Based on the dielectric continuum phonon model, uniaxial model and force balance equation the mobility of two dimensional electron gas in wurtzite AlxGa1-xN/GaN/AlxGa1-xN quantum wells is discussed theoretically within the temperature range dominated by optical phonons. The dependences of the electron mobility on temperature, Al molar fraction and electron sheet density are presented including hydrostatic pressure effect. The built-in electric field is also taken into account. It is found that under normal pressure the main contribution to the mobility is from the scattering of interface optical phonons in narrow (for well width d 117 A and d > 65 A for finitely thick barriers and infinitely thick ones, respectively) wells, whereas that is from the scattering of confined optical phonons in a well with an intermediate width. It is shown that the electron mobility decreases with increasing Al molar fraction and temperature, whereas increases obviously with increasing electron sheet density. The theoretical calculated electron mobility is 978 cm2/V s which is higher than an available experimental data 875 cm2/V s when x equals to 0.58 at room temperature. The results under hydrostatic pressure considering the modification of strain indicate that the mobility increases slightly as hydrostatic pressure increases from 0 to 10 GPa.

Large zero-field spin splitting in AlGaN/AlN/GaN/AlN heterostructures

This work describes Shubnikov–de Haas (SdH) measurements in Al0.22Ga0.78N/AlN/GaN/AlN heterostructures. Our experiments coupled with the analysis of the Hall data at various temperatures confirm the formation of a two-dimensional electron gas (2DEG) at the AlN/GaN interface. A beating pattern in the SdH oscillations is also observed and attributed to a zero-field spin splitting of the 2DEG first energy subband. The values of the effective spin-orbit coupling parameter and zero-field spin-split energy are estimated and compared with those reported in the literature. We show that zero-field spin-split energy tends to increase with increasing sheet electron density and that our value (12.75 meV) is the largest one reported in the literature for GaN-based heterostructures.

Quantum confinement and coherence in a two-dimensional electron gas in a carbon-face 3C-SiC/6H-SiC polytype heterostructure

We report the observation of the quantum coherence in a two-dimensional electron gas (2DEG) at a C-face 3C-/6H-SiC polytype heterostructure. Electronic confinement and coherence were observed at 1.5 K and high magnetic fields, indicating the presence and confinement of a 2DEG. The measured mobility of the 2DEG is 2000 cm2/V s and the electron sheet density is 2.7×1012/cm2.

Effect of non-abrupt doping and interfacial profiles on the carrier sheet density in one-side modulation-doped GaN/AlGaN quantum wells

The results of an accurate theoretical study on the effects of non-abrupt doping and interfacial profiles on the electron sheet density in one-side modulation-doped wurtzite GaN/AlGaN single quantum wells at low temperatures are presented. We solve coupled Schrödinger and Poisson equations self-consistently via the finite difference method. By employing a proper discretization on a nonuniform grid and taking into account the strong piezoelectric and spontaneous polarization fields exhibited by the wurtzite III-nitride heterostructures, a substantial increase in the 2DEG density is predicted with the increase of the donor diffusion length and the reduction of the spacer thickness.

Analysis of Fringing Effect on Resonant Plasma Frequency in Plasma Wave Devices

The effect of fringing electric field due to the nonideality of the gate two-dimensional electron gas (2DEG) channel capacitance on the fundamental resonant frequency of plasma waves in a high-electron-mobility transistor (HEMT) channel was investigated. The spatial distribution of sheet electron density in the fringed region of the 2DEG channel and the expression for the fundamental resonant frequency of plasma oscillation were obtained. A cascaded transmission line (TL) equivalent circuit model was developed to represent the gated and ungated fringed regions of the HEMT 2DEG channel. Results of calculation and IsSpice simulation show that fringing effects can be the cause of the fundamental plasma frequency reduction. Thus, the developed fringing effect model improves the deviation between theoretical and experimental data.

Spectral Narrowing Effect of a Novel Super-Grating Dual-Gate Structure for Plasmon-Resonant Terahertz Emitter

We have proposed a terahertz (THz) emitter utilizing two-dimensional plasmons (2DPs) in a super-grating dual-gate (SGG) high electron mobility transistor (HEMT). The plasmon under each grating gate has a unique feature that its resonant frequency is determined by the plasma-wave velocity over the gate length. Since the drain bias voltage causes a linear potential slope from the source to drain area, the sheet electron densities in periodically distributed 2DP cavities are dispersed. As a result, all the resonant frequencies are dispersed and undesirable spectral broadening occurs. A SGG structure can compensate for the sheet electron density distribution by modulating the grating dimension. The finite difference time domain simulation confirms its spectral narrowing effect. Within a wide detuning range for the gate and drain bias voltages giving a frequency shifting of ±0.5THz from an optimum condition, the SGG structure can preserve the spectral narrowing effect.

Comparison of the transport properties of high quality AlGaN/AlN/GaN and AlInN/AlN/GaN two-dimensional electron gas heterostructures

The transport properties of high mobility AlGaN/AlN/GaN and high sheet electron density AlInN/AlN/GaN two-dimensional electron gas (2DEG) heterostructures were studied. The samples were grown by metal-organic chemical vapor deposition on c-plane sapphire substrates. The room temperature electron mobility was measured as 1700 cm2/V s along with 8.44×1012 cm−2 electron density, which resulted in a two-dimensional sheet resistance of 435 Ω/◻ for the Al0.2Ga0.8N/AlN/GaN heterostructure. The sample designed with an Al0.88In0.12N barrier exhibited very high sheet electron density of 4.23×1013 cm−2 with a corresponding electron mobility of 812 cm2/V s at room temperature. A record two-dimensional sheet resistance of 182 Ω/◻ was obtained in the respective sample. In order to understand the observed transport properties, various scattering mechanisms such as acoustic and optical phonons, interface roughness, and alloy disordering were included in the theoretical model that was applied to the temperature dependent mobility data. It was found that the interface roughness scattering in turn reduces the room temperature mobility of the Al0.88In0.12N/AlN/GaN heterostructure. The observed high 2DEG density was attributed to the larger polarization fields that exist in the sample with an Al0.88In0.12N barrier layer. From these analyses, it can be argued that the AlInN/AlN/GaN high electron mobility transistors (HEMTs), after further optimization of the growth and design parameters, could show better transistor performance compared to AlGaN/AlN/GaN based HEMTs.