Room Temperature Electron Mobility Research Articles

High electron mobility in InSb epilayers and quantum wells grown with AlSb nucleation on Ge-on-insulator substrates

InSb epilayers and InSb/Al0.20In0.80Sb quantum wells (QWs) were grown on 4°-off-axis Ge-on-insulator (GeOI) substrates by molecular beam epitaxy. An initial AlSb nucleation was found to be important for achieving good crystalline quality. For a 4.0-μm-thick InSb epilayer and 25-nm-thick InSb QW, the room-temperature (RT) electron mobility was increased by 25% and 60% [58 000 cm2/(V-s) for the epilayer and 24 000 cm2/(V-s) for the QW], respectively, by using an off-axis GeOI substrate instead of an on-axis GeOI (001) substrate. This significant improvement may be attributed to the reduction of antiphase domains, microtwins, and threading dislocations. A modified QW structure on a 4°-off-axis GeOI substrate showed a further 25% improvement in RT electron mobility with a value 32 000 cm2/(V-s). This is the highest RT electron mobility in an InSb QW grown on a Ge-based substrate to date.

Closed-cycle cooling of cryopanels in molecular beam epitaxy

Closed-cycle cooling of the cryoshroud in a molecular beam epitaxy (MBE) system with a dimethyl polysiloxane heat transfer fluid has reduced liquid nitrogen consumption by an order of magnitude, significantly lowering operating costs. The temperature dependence of cryopanel pumping efficacy in the MBE system has been investigated. H2O, CO, CO2, and As4 are all pumped effectively by liquid nitrogen cooled cryopanels (−196 °C) in the MBE. At −78 °C, the operating temperature of the closed-cycle chiller, H2O and As4 are pumped effectively, while CO and CO2 are not. The pumping speed for H2O is found to increase exponentially with decreasing temperature. Below ∼−40 °C and ∼−95 °C, the pumping speeds for As4 and H2O saturate, respectively. AlGaAs layers grown with the closed-cycle-cooled shroud show strong photoluminescence, expected room temperature electron mobility, and background doping levels less than 4 × 1015 cm−3.

Effect of substrate miscut on the electron mobility in InSb(001) structures on Ge and Ge-on-insulator substrates

InSb epilayers and InSb/Al0.20In0.80Sb quantum wells were grown on Ge(001) substrates and Ge-on-insulator (GeOI)-on-Si(001) substrates by molecular beam epitaxy. Growth on both on-axis and 4°-off-axis substrate orientations was studied. Anti-phase domains were formed when InSb films were grown on on-axis substrates, but suppressed significantly by the use of 4°-off-axis substrates. Such off-axis substrates also reduced the densities of micro-twin defects and threading dislocations. The defect reduction resulted in an increase in the room-temperature electron mobility from 37,000 to 59,000 cm2/Vs in 4.0-μm-thick InSb epilayers and from 10,000 to 20,000 cm2/Vs in 25-nm-thick InSb quantum wells on Ge(001) and GeOI-on-Si(001) substrates.

Environmentally Responsive Graphene Systems

Graphene materials have been attracting significant research interest in the past few years, with the recent focuses on graphene-based electronic devices and smart stimulus-responsive systems that have a certain degree of automatism. Owing to its huge specific surface area, large room-temperature electron mobility, excellent mechanical flexibility, exceptionally high thermal conductivity and environmental stability, graphene is identified as a beneficial additive or an effective responding component by itself to improve the conductivity, flexibility, mechanical strength and/or the overall responsive performance of smart systems. In this review article, we aim to present the recent advances in graphene systems that are of spontaneous responses to external stimulations, such as environmental variation in pH, temperature, electric current, light, moisture and even gas ambient. These smart stimulus-responsive graphene systems are believed to have great theoretical and practical interests to a wide range of device applications including actuators, switches, robots, sensors, drug/gene deliveries, etc.

Strained InGaAs/InAlAs quantum wells for complementary III–V transistors

Quantum wells of InGaAs clad by InAlAs were grown on AlGaAsSb buffer layers by molecular beam epitaxy. The buffer layer lattice parameters were near 6.0Å, yielding tensile strains up to 2% in the InGaAs and InAlAs. Room-temperature electron mobilities of 9000–11,000cm2/Vs were achieved. Field-effect transistors (FETs) were fabricated and exhibited good DC and RF characteristics. Previous work demonstrated compressively-strained GaSb quantum wells on similar buffer layers with high hole mobilities and good transistor performance. Hence, a single buffer layer of AlGaAsSb should be suitable for complementary circuits comprised of n-channel FETs based on the mature InGaAs/InAlAs technology and p-channel FETs based on high-mobility antimonides.

Mobility enhancement and temperature dependence in top-gated single-layer MoS2

The deposition of a high-$\ensuremath{\kappa}$ oxide overlayer is known to significantly enhance the room-temperature electron mobility in single-layer MoS${}_{2}$ (SLM) but not in single-layer graphene. We give a quantitative account of how this mobility enhancement is due to the nondegeneracy of the two-dimensional electron gas system in SLM at accessible temperatures. Using our charged impurity scattering model [Ong and Fischetti, Phys. Rev. B 86, 121409 (2012)] and temperature-dependent polarizability, we calculate the charged impurity-limited mobility (${\ensuremath{\mu}}_{\mathrm{imp}}$) in SLM with and without a high-$\ensuremath{\kappa}$ (HfO${}_{2}$) top-gate oxide at different electron densities and temperatures. We find that the mobility enhancement is larger at low electron densities and high temperatures because of finite-temperature screening, thus explaining the enhancement of the mobility observed at room temperature. ${\ensuremath{\mu}}_{\mathrm{imp}}$ is shown to decrease significantly with increasing temperature, suggesting that the strong temperature dependence of measured mobilities should not be interpreted as being solely due to inelastic scattering with phonons. We also reproduce the recently seen experimental trend in which the temperature scaling exponent ($\ensuremath{\gamma}$) of ${\ensuremath{\mu}}_{\mathrm{imp}}\ensuremath{\propto}{T}^{\ensuremath{-}\ensuremath{\gamma}}$ is smaller in top-gated SLM than in bare SLM. Finally, we show that $\ensuremath{\sim}37%$ mobility enhancement can be achieved by reducing the HfO${}_{2}$ thickness from 20 to 2 nm.

AlGaN/GaN Three-Terminal Junction Devices for Rectification and Transistor Applications on 3C-SiC/Si Pseudosubstrates

AlGaN/GaN heterosystems with their two dimensional electron gas at the interface enable high-mobility transistor devices. We present a technology for high-mobility three-terminal junction (TTJ) devices on 3C-SiC(111)/Si(111) pseudosubstrates. Those devices that allow multipurpose applications are determined by only slight variations in their geometrical properties such as side gate transistor behavior besides positive and negative type rectifications on a single chip based on AlGaN/GaN heterostructures grown on silicon substrates using a SiC transition layer. In this paper, we present the technological process and the nonlinear electrical properties of TTJ devices using Si basic substrates with room temperature electron mobility up to 1680 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V s. Narrow and wide bar types of rectifying devices with T and Y type active regions are shown as well as side gate transistors with a transconductance of 20 mS/mm and an output conductance of 600 mS/mm. The TTJ active region withstands fields .

空气稳定的氮杂蒄电子传输材料分子设计及三氮杂蒄衍生物迁移率计算

The successive replacement of CH moieties by nitrogen atoms in coronene was studied computationally using the density functional theory (DFT). And the effects of different N-substituted patterns on geometries, electronic properties, and charge transport parameters were discussed in detail. The results show that the frontier molecular orbital energies decrease linearly with the increase of the number of nitrogen atoms, which is not only ideal for electron injection but also contribute to improved air stability of these semiconductors. Relatively, the ortho nitrogen-substitution is the most efficient strategy stabilizing the HOMO and LUMO levels compared with the peri -and sym -substitutions. Particularly, dodecazacoronene ( 12ac ) with a unique bowl-shape structure may be a potential building block as air-stable n-channel materials due to its large electron affinity (3.45 eV). In addition, the predicted room-temperature electron mobility for 2,6,10-tri(4-methoxyphenyl)-3,4,7,8,11,12-hexamethoxy-1,5,9-triazacoronene ( 3b ) single crystal reaches to 0.242 cm2/V s, which indicates that 3b is a promising n-channel OFET material.

Improved electron mobility in InSb epilayers and quantum wells on off-axis Ge (001) substrates

Two types of InSb structures, epilayers and quantum wells (QWs), have been grown on on-axis and 6°-off-axis Ge (001) substrates and examined by reflection high-energy electron diffraction, transmission electron microscopy, X-ray diffraction, atomic force microscopy, and the van der Pauw and Hall effect techniques. Anti-phase domain defects, which prevail in these InSb structures when grown on on-axis Ge (001) substrates, are significantly decreased by the use of 6° off-axis Ge (001) substrates. Such off-axis substrates also lead to reductions in the densities of micro-twins and threading dislocations. Room-temperature electron mobilities in 4.0-μm-thick InSb epilayers and 25-nm-thick InSb QWs grown on 6°-off-axis Ge (001) substrates are 59 000 and 14 000 cm2/(V s), respectively, which are ∼1.5 times higher than their counterparts grown on on-axis Ge (001) substrates. These improved mobilities are the highest among the reported values for each type of structure grown on Ge (001) substrates.

Impact of structural defects upon electron mobility in InSb quantum wells

The detrimental effects of structural defects, micro-twins (MTs) and threading dislocations (TDs), on electron mobility have been investigated for InSb quantum wells (QWs) at room temperature (RT). The constants that are necessary to calculate the electron-mobility limits of these defects were determined by a least-squares-based method that has an advantage of clear representation of the analytical results in a two-dimensional space. Based on a mathematical consideration, a general method of converting electron-mobility limits into percentage impacts upon the total electron mobility was developed. Percentage-mobility-impact analyses showed that, when InSb QWs grown on on-axis (001) GaAs substrates have a TD density of 8.7 × 108 – 3.2 × 109/cm2, 21–14 and 18–45% of electron-mobility degradation are attributed to MTs and TDs, respectively, at RT. The use of 2° off-axis (001) GaAs substrates reduces MT densities in InSb QWs, resulting in a suppression of the MT mobility impact to 3–2% and a complementary slight increase of the TD mobility impact to 22–51% in the same TD density range. This considerable TD mobility limit indicates that it should be possible to improve RT electron mobility in InSb QWs grown on 2° off-axis (001) GaAs substrates, by means of reducing TD density further (&amp;lt; 8.7 × 108/cm2). Although the mobility impacts due to phonons in InSb QW grown on-axis and 2° off-axis (001) GaAs substrates are 54–36 and 67–42%, respectively, phonon scattering is not a single dominant scattering factor: MTs and TDs have also substantial negative impacts upon RT electron mobilities in InSb QWs.

Solution processable C 60 fullerene-hydrazone dyads for optoelectronics

Ambipolar, narrow band gap (∼1.35 eV) fullerene-arylhydrazone dyads have been synthesized utilizing the Prato-Maggini-Scorrano reaction followed by the condensation with phenylhydrazines. Room temperature electron and hole mobilities, measured by the xerographic time of flight technique, exceed 10 −4 cm 2 V −1 s −1 at strong electric fields. The values of ionization potential, measured by the photoemission in air method, are in the range of 5.29–5.36 eV. Good ambipolar charge transport, photosensitivity across the whole visible light region of the spectrum, and solubility in common organic solvents makes these dyads attractive materials for various optoelectronic applications.

Electronic properties of the high electron mobility Al0.56In0.44Sb/Ga0.5In0.5Sb heterostructure

Electronic properties of the Al0.56In0.44Sb/Ga0.5In0.5Sb heterostructure grown by molecular beam epitaxy are investigated. We measure by means of x-ray photoemission spectroscopy, photoluminescence, and Hall effect measurements the key parameters involved in the achievement of a high speed and very low power consumption field effect transistor: conduction and valence band offsets, energy gaps of channel and barrier materials, electron effective mass, and density of states in the channel. We observe the influence of the quantum well thickness on the electron Hall mobility and sheet carrier density. A type I heterostructure exhibiting a room temperature electron mobility of 25 000 cm2 V−1 s−1 with a sheet carrier density of 1.5×1012 cm−2 is demonstrated for the widest channel. We show that the thickness of the channel influences both Hall density and mobility through a multisubband occupancy. We discuss about the scattering mechanisms limiting the mobility in the second subband.

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.

Anisotropic properties of MOVPE-grown m-plane GaN layers on LiAlO2 substrates

We used metal organic vapour phase epitaxy (MOVPE) to deposit m-plane GaN layers on LiAlO2 substrates and studied the influence of the V/III ratio during growth. All films exhibit a special surface pattern with pyramidal hillocks elongated in [11–20] direction of GaN and small steps perfectly aligned in the (11–20) plane. For higher V/III ratios, the roughness is reduced while holes on the surface are generated. Polarization-dependent low-temperature (LT) photoluminescence (PL) spectra show GaN excitonic peaks at ∼3.5 eV with a high degree of polarization (DoP) of ∼0.9. Another peak at 3.47 eV is much less intense compared to the excitonic peak and probably related to basal plane stacking faults (BSF). Anisotropic in-plane compressive strain which slightly relaxes for a higher V/III ratio can be detected by high-resolution X-ray diffraction (HRXRD). XRD reciprocal space maps of the symmetric (1–100) reflection show relatively weak broadening for scans taken in the [0001] direction and strong broadening for the perpendicular direction. Anisotropic room temperature electron mobilities are found with average values of 108 and 117 cm2/Vs in [11–20] and [0001] direction of GaN, respectively. We explain the higher values in c-axis direction by the presence of extended planar defects in the prism plane which may be also responsible for the observed surface steps and anisotropic broadening of the XRD RSM.

Dependence of Carrier Mobility on Nanocrystal Size and Ligand Length in PbSe Nanocrystal Solids

We measure the room-temperature electron and hole field-effect mobilities (micro(FE)) of a series of alkanedithiol-treated PbSe nanocrystal (NC) films as a function of NC size and the length of the alkane chain. We find that carrier mobilities decrease exponentially with increasing ligand length according to the scaling parameter beta = 1.08-1.10 A(-1), as expected for hopping transport in granular conductors with alkane tunnel barriers. An electronic coupling energy as large as 8 meV is calculated from the mobility data. Mobilities increase by 1-2 orders of magnitude with increasing NC diameter (up to 0.07 and 0.03 cm(2) V(-1) s(-1) for electrons and holes, respectively); the electron mobility peaks at a NC size of approximately 6 nm and then decreases for larger NCs, whereas the hole mobility shows a monotonic increase. The size-mobility trends seem to be driven primarily by the smaller number of hops required for transport through arrays of larger NCs but may also reflect a systematic decrease in the depth of trap states with decreasing NC band gap. We find that carrier mobility is independent of the polydispersity of the NC samples, which can be understood if percolation networks of the larger-diameter, smaller-band-gap NCs carry most of the current in these NC solids. Our results establish a baseline for mobility trends in PbSe NC solids, with implications for fabricating high-mobility NC-based optoelectronic devices.

Mobility limiting scattering mechanisms in nitride-based two-dimensional heterostructures with the InGaN channel

The scattering mechanisms limiting the carrier mobility in AlInN/AlN/InGaN/GaN two-dimensional electron gas (2DEG) heterostructures were investigated and compared with devices without InGaN channel. Although it is expected that InGaN will lead to relatively higher electron mobilities than GaN, Hall mobilities were measured to be much lower for samples with InGaN channels as compared to GaN. To investigate these observations the major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to interface roughness limits the electron mobility at low and intermediate temperatures for samples having InGaN channels. The room temperature electron mobilities which were determined by a combination of both optical phonon and interface roughness scattering were measured between 630 and 910 cm2 (V s)−1 with corresponding sheet carrier densities of 2.3–1.3 × 1013 cm−2. On the other hand, electron mobilities were mainly limited by intrinsic scattering processes such as acoustic and optical phonons over the whole temperature range for Al0.82In0.18N/AlN/GaN and Al0.3Ga0.7N/AlN/GaN heterostructures where the room temperature electron mobilities were found to be 1630 and 1573 cm2 (V s)−1 with corresponding sheet carrier densities of 1.3 and 1.1 × 1013 cm−2, respectively. By these analyses, it could be concluded that the interfaces of HEMT structures with the InGaN channel layer are not as good as that of a conventional GaN channel where either AlGaN or AlInN barriers are used. It could also be pointed out that as the In content in the AlInN barrier layer increases the interface becomes smoother resulted in higher electron mobility.

High-temperature molecular beam epitaxial growth of AlGaN/GaN on GaN templates with reduced interface impurity levels

We present combined in situ thermal cleaning and intentional doping strategies near the substrate regrowth interface to produce high-quality AlGaN/GaN high electron mobility transistors on semi-insulating (0001) GaN templates with low interfacial impurity concentrations and low buffer leakage. By exposing the GaN templates to an optimized thermal dissociation step in the plasma-assisted molecular beam epitaxy environment, oxygen, carbon, and, to lesser extent, Si impurities were effectively removed from the regrowth interface under preservation of good interface quality. Residual Si was further compensated by C-doped GaN via CBr4 to yield highly resistive GaN buffer layers. Improved N-rich growth conditions at high growth temperatures were then utilized for subsequent growth of the AlGaN/GaN device structure, yielding smooth surface morphologies and low residual oxygen concentration with large insensitivity to the (Al+Ga)N flux ratio. Room temperature electron mobilities of the two-dimensional electron gas at the AlGaN/GaN interface exceeded &amp;gt;1750 cm2/V s and the dc drain current reached ∼1.1 A/mm at a +1 V bias, demonstrating the effectiveness of the applied methods.

High electron mobility and low sheet resistance in lattice-matched AlInN/AlN/GaN/AlN/GaN double-channel heterostructure

High electron mobility and low sheet resistance were achieved in lattice-matched AlInN/AlN/GaN/AlN/GaN double-channel (DC) heterostructure. Two-dimensional electron gas (2DEG) of the DC heterostructure was divided into the double channels and the room-temperature mobility was increased to 1430 cm2/V s by reducing the 2DEG density in each channel, compared with low electron mobility (1090 cm2/V s) for lattice-matched AlInN/AlN/GaN single-channel heterostructure. It was found that the 2DEG mobility was limited by thickness of the AlN interlayer between the double channels. After the structure optimization, the room temperature electron mobility of the DC heterostructure reached 1570 cm2/V s with sheet resistance of 222 Ω/◻.

The effect of AlN interlayer thicknesses on scattering processes in lattice-matched AlInN/GaN two-dimensional electron gas heterostructures

The scattering mechanisms governing the transport properties of high mobility AlInN/AlN/GaN two-dimensional electron gas (2DEG) heterostructures with various AIN spacer layer thicknesses from zero to 2 nm were presented. The major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to alloy disorder limits the electron mobility for samples having spacer layer thicknesses up to 0.3 nm. On the other hand, alloy scattering is greatly reduced as the AlN spacer layer thickness increases further, and hence the combination of acoustic, optical and interface roughness become operative with different degrees of effectiveness over different temperature ranges. The room-temperature electron mobility was observed to increase gradually as the AlN spacer layer increases. A peak electron mobility of 1630 cm2 V−1 s−1 was realized for the sample consisting of a 1 nm AlN spacer layer. Then, the electron mobility decreased for the sample with 2 nm AlN. Moreover, the measured 2DEG densities were also compared with the theoretical predictions, which include both piezoelectric and spontaneous polarization components existing at AlN/GaN interfaces. The experimental sheet carrier densities for all AlInN/AlN/GaN HEMT structures were found to be in excellent agreement with the theoretical predictions when the parasitic (unintentional) GaN layer deposited between AlN and AlInN was taken into account. From these analyses, 1 nm AlN spacer layer thickness is found to be the optimum thickness required for high electron mobility and hence low sheet resistance once the sheet carrier density is increased to the theoretically expected value for the sample without unintentional GaN layer.

The growth of a low defect InAs HEMT structure on Si by using an AlGaSb buffer layercontaining InSb quantum dots for dislocation termination

It is found that the surface migration and nucleation behaviors of InSb quantum dots onAlSb/Si substrates, formed by molecular beam epitaxy in Stranski–Krastanov (SK) growthmode, are dependent on the substrate temperature. At relatively high temperatures above430 °C, quantum dots are migrated and preferentially assembled onto thesurface steps of high defect AlSb layers grown on Si substrates, whilethey are uniformly distributed on the surface at lower temperatures below400 °C. It is also found that quantum dots located on the defect sites lead to effective terminationof the propagation of micro-twin-induced structural defects into overlying layers, resultingin the low defect material grown on a largely mismatched substrate. The resulting 1.0 µm thickAlxGa1−xSb (x = 0.8) layer grown on the silicon substrate shows atomically flat (0.2 nm AFM mean roughness)surface and high crystal quality, represented by a narrow full width at half-maximum of 300 arc sin the x-ray rocking curve. The room-temperature electron mobility of higher than 16 000 cm2 V−1 s−1 inInAs/AlGaSb FETs on the Si substrate is obtained with a relatively thin buffer layer, when a low defectdensity (∼106 cm−2) AlGaSb buffer layer is obtained by the proposed method.