Low-temperature Electron Mobility Research Articles

Finite-temperature Hubbard local field corrections on electron mobility in strictly 2D electron gas

Low-temperature electron mobility of a strictly two-dimensional electron gas (2DEG) in a modulation delta-doped AlxGa1-xAs/GaAs/AlxGa1-xAs heterostructure has been obtained by using Boltzmann equation and relaxation time approximation. At low temperatures, the electron mobility is governed by the electron-ionized impurity scattering in which the electrons screen electron-impurity interaction through the dielectric function of the system. We have investigated the effect of ionized impurities on the low-temperature electron mobility beyond the random phase approximation by including the local field corrections. We have used Hubbard and finite-temperature Hubbard local field corrections to calculate the electron mobility as a function of temperature. We have found that the Hubbard local field corrections enhance the electron mobility at low temperatures in comparison with RPA results and less enhancement is obtained by using temperature dependent Hubbard approximation.

High mobility metamorphic AlSb/InAs heterostructures grown on InP substrates

AlSb/InAs heterostructures show promising characteristics for the realisation of very high electron mobility devices. This report focuses on the metamorphic growth of these structures on InP substrate with GaSb and AlSb buffers. We observe poorer low-temperature electron mobility and room temperature (RT) densities using an AlSb buffer. The photoluminescence (PL) of a GaSb quantum well (QW) grown on the two kinds of buffers suggests that this difference does not result from a larger density of dislocations with an AlSb buffer. The hypothesis of a strain relaxation in the InAs channel on AlSb is also denied since a heterostructure with a thinner InAs QW does not show a higher electron mobility. This leads us to discuss the influence of the roughness of the buffer layer on the temperature mobility and the density of the two-dimensional electron gas.

Low temperature electron mobility and concentration under the gate of AlGaN∕GaN field effect transistors

High electron mobility field effect transistors were fabricated on AlGaN∕GaN heterostructures and their magnetoresistance was measured at 4.2K up to 10T with simultaneous modulation of the gate potential. Low and high magnetic field data were used to determine the electron mobility (μ) and concentration (n), respectively, in the gated part of the transistor channel. With these measurements we present a method to determine μ and n under the gate of a transistor, which does not require knowledge of the transistor gate length, access resistance, threshold voltage, or capacitance. We discuss applications of this method for nanometer and ballistic transistors.

Limitation of electron mobility in modulation-doped In0.53Ga0.47As/InP quantum wells at low temperatures

The low-temperature electron mobility is investigated here for electrons confined in modulation-doped In0.53Ga0.47As/InP single symmetric quantum wells. The subband structure calculation is developed via variational method, both Schrödinger and Poisson equations being solved simultaneously with adequate heterointerface matching conditions. With this in hands, the main electron scattering rates are computed, namely alloy disorder, remote ionized impurity, and interface roughness. As a result, interesting interchanges in these scattering rates were found by varying the well width and the spacer width, which show that some scattering mechanisms can surpass the alloy disorder scattering rate and come to limit the electron mobility, a behavior not reported in the literature.

Sub-meV photoluminescence linewidth and >106cm2∕Vs electron mobility in AlGaAs∕GaAs quantum wells grown by metalorganic vapor phase epitaxy on slightly misoriented substrates

We report sub-meV (as low as 0.6meV) low-temperature photoluminescence linewidth and high low-temperature electron mobility (μ∼1−1.5×106cm2∕Vs) of GaAs quantum wells in AlGaAs barriers grown by standard metalorganic vapor phase epitaxy. These records values are achieved by epitaxial growth on (100) slightly misoriented substrates [⩽0.6 degrees off-(100) GaAs substrates] in combination with a high V/III ratio for AlGaAs growth. Such small misorientations are sufficient to drastically modify the optical and transport properties as well as the growth mode and surface morphologies of both GaAs and AlGaAs epitaxial layers, allowing greater interface quality and reduced impurity incorporation. The quantum wells so obtained show optical properties comparable to high-quality samples grown by molecular beam epitaxy. In addition, the slight misorientation considerably reduces the impact of substrate temperature on electron mobility, which allows achieving high values of μ within a much broader range of growth temperatures.

Low-temperature electron mobility in a strictly 2D layer

Theoretical low-temperature electron mobility of a two-dimensional electron gas (2DEG) confined near the interface of a modulation δ-doped Al x Ga 1− x As/GaAs heterostructure with a very thin width GaAs (delta well) is calculated by using Boltzmann equation and relaxation time approximation. In low temperatures the electrons are scattered from ionized impurities. Screening of the charged impurities by electrons is properly taken into account by using random phase approximation method. At last the mobility is plotted as a function of donor concentration at different temperatures and with various spacer widths.

Formula omitted]-Factor tuning of 2D electrons in double-gated Si/SiGe quantum wells

We report on the design and first experiments of Si/SiGe heterostructures that allow gate-operated shifting of a 2D electron gas between two channels with different Landé g -factors. This allows gate-operated moving of electrons in and out of resonance in an electron spin resonance (ESR) experiment, which can act as a building block of a proposed solid-state quantum computer. We use MBE-grown modulation-doped quantum-wells (QWs) on SiGe pseudosubstrates with up to 30% Ge and low-temperature electron mobilities up to 250 000 cm 2 / Vs . A double QW structure with two different Ge contents separated by a thin barrier was optimized for this purpose with self-consistent simulations. The band structure simulations show that by applying gate voltages one can completely shift the wave function from one well to the other. First experiments on pure Si channels show the working of the gate setup. Both carrier density and mobility can be increased by using the back gate which corresponds to shifting the wave function in the channel.

Phonon Confinement Effects in Hybrid Virus-Inorganic Nanotubes for Nanoelectronic Applications

Genetically modified viruses have been proposed recently as templates for the assembly of nanometer-scale components of electronic circuits. Here we show that, in addition to their role as nanotemplates, viruses can actually improve the electron transport properties in semiconductor nanotubes grown on them. In the considered hybrid virus-inorganic nanostructures, which consist of silica or silicon nanotubes deposited on tobacco mosaic viruses, the confined acoustic phonons are found to be redistributed between the nanotube shell and the acoustically soft virus enclosure. As a result, the low-temperature electron mobility in the hybrid virus-silicon nanotube can increase by a factor of 4 compared to that of an empty silicon nanotube. Our estimates also indicate an enhancement of the low-temperature thermal conductivity in the virus-silicon nanotube, which can lead to improvements in heat removal from the hybrid nanostructure-based nanocircuits.

Strain induced effects on the transport properties of metamorphic InAlAs/InGaAs quantum wells

The relationship between structural and low-temperature transport properties is explored for In x Al 1 − x As/In x Ga 1 − x As metamorphic quantum wells with x > 0.7 grown on GaAs by molecular beam epitaxy. Different step-graded buffer layers are used to gradually adapt the in-plane lattice parameter from the GaAs towards the InGaAs value. We show that using buffer layers with a suitable maximum In content the residual compressive strain in the quantum well region can be strongly reduced. Samples with virtually no residual strain in the quantum well region show a low-temperature electron mobility up to 29 m 2/V s while for samples with higher residual compressive strain the low-temperature mobility is reduced. Furthermore, for samples with buffers inducing a tensile strain in the quantum well region, deep grooves are observed on the surface, and in correspondence we notice a strong deterioration of the low-temperature transport properties.

Scattering mechanisms in undoped In 0.75Ga 0.25As/In 0.75Al 0.25As two-dimensional electron gases

We have investigated the scattering mechanisms limiting the low-temperature electron mobility in unintentionally doped In 0.75Al 0.25As/In 0.75Ga 0.25As metamorphic quantum wells grown on GaAs (0 0 1) substrates. We found that the mobility is limited by background impurity scattering for densities lower than 2.0×10 11 cm −2, and by the combination of this mechanism and alloy disorder scattering for higher densities. From such analysis we estimate an alloy disorder scattering potential of about 0.5±0.1 eV. Moreover we show that when a strained InAs layer is located in the centre of the In 0.75Ga 0.25As well, the alloy disorder scattering can be reduced, yielding mobilities up to 320 000 cm 2/Vs at a carrier concentration of 3.1×10 11 cm −2.

Low-temperature electron mobilities due to ionized-impurity scattering in multisubband two-dimensional semiconductor systems

We present a theoretical and experimental study on the low-temperature electron mobilities due to ionized impurity scattering in a multisubband quasi-two-dimensional semiconductor system. The scattering rate is obtained from the solution the Lippmann-Schwinger equation in momentum space and the screening is considered within the random-phase approximation. A quantitative agreement is reached between the theoretical and experimental results for both the quantum and transport mobilities.

Influence of manganese contamination on high-mobility GaAs∕AlGaAs heterostructures

Photoluminescence and magnetotransport measurements have been performed to assess the quality of modulation-doped GaAs∕AlGaAs heterostructures. The temporal evolution of the low-temperature electron mobility of samples prepared in a molecular-beam-epitaxy chamber containing manganese as a source material was studied. Mn contamination was identified to be responsible for the reduction of the electron mobility to 1×106cm2∕Vs and the appearance of a distinct photoluminescence band. In contrast, structures in which this signal is absent reach mobility values of 5.4×106cm2∕Vs. This directly proves that the epitaxy of high-mobility electron systems and structures containing GaMnAs layers, in principle, can be combined in one growth chamber.

Characterization of undoped ZnO layers grown by molecular beam epitaxy towards biosensing devices

AbstractUndoped ZnO layers grown by molecular beam epitaxy (MBE) were investigated aiming at biosensing devices based on organic/inorganic hybrid structures. Clear observation of free exciton emission in low temperature photoluminescence (PL) spectra and electron mobility as high as 114 cm2/Vs at room temperature indicate that the quality of the ZnO layers is applicable for biosensors. Surface bonding of the ZnO the layers, which is one of the key issues for hybridization, was examined by means of xray photoelectron spectroscopy (XPS). The XPS spectra of O 1s revealed that formation and desorption of hydroxyl (OH) bonds can be controlled by air exposure and thermal treatments. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Roughness-induced piezoelectric scattering in lattice-mismatched semiconductor quantum wells

We present a theory of the mobility of electrons in real semiconductor quantum wells (QW's) made from lattice-mismatched epitaxial layers. In the case of zinc-blende structure QW's, we prove that besides the conventional scattering mechanisms, e.g., impurity doping, surface roughness, and alloy disorder there exists an ad hoc scattering source, which is due to a large fluctuating density of roughness-induced piezoelectric charges. Scattering by their piezoelectric field is found to be a new important scattering mechanism limiting the electron mobility of real strained QW's, especially those with a well thickness of the order of or greater than 50 \AA{}. By incorporating this scattering into the theory, we are able to provide a perfect explanation for the low-temperature electron mobility measured in lattice-mismatched InGaAs-based QW's, which has not been understood starting from the so far-known scattering sources. The possibility of applying our theory to other lattice-mismatched systems such as Si/SiGe heterostructures and nitride-based QW's is outlined.

Low-temperature electron mobility in Ga0.5In0.5P/GaAsquantum wells

The electron mobility is calculated for Ga0.5In0.5P/GaAsquantum wells at 77 K. Deformation potential acoustic phonon and alloyscattering are assumed to be effective. Account has been taken of the energy bandnonparabolicity and extension of the wavefunction into the Ga0.5In0.5Pbarrier layer. Calculated results for Al0.3Ga0.7As/GaAsquantum wells are also presented for comparison.

Effect of residual acceptors on electron mobility in single asymmetric quantum wells

We present the results of the calculation of the low-temperature electron mobility in one-side modulation-doped single asymmetric quantum wells, taking into account the presence of residual impurities. The effects of ionized acceptors were included in the variational calculation within the depletion approximation. All relevant scattering rates were taken into account, namely acoustic phonons, ionized impurities (remote and background), alloy disorder and interface roughness. The mobility dependencies on the electron gas density and residual acceptor concentration were found and we show that the background impurity scattering rate is the main scattering mechanism for acceptor concentrations above ∼3×10 15 cm −3 , in good agreement to recent experimental works involving high-quality GaAs/AlGaAs modulation-doped quantum wells.

Electric field effects on electron mobility in n-AlGaAs/GaAs/AlGaAs single asymmetric quantum wells

We calculated low-temperature electron mobility in n-AlGaAs/GaAs/AlGaAs single asymmetric quantum wells in the presence of a uniform electric field directed perpendicularly to the interfaces. The quantum well asymmetry is due to the doping profile (one-side modulation doping). Following a variational scheme, we solved both Schrödinger and Poisson equations simultaneously and the results were used to calculate the low-temperature (quasielastic) scattering rates. Only relevant scattering mechanisms were taken into account, namely ionized impurity, interface roughness, alloy disorder, and acoustic phonons (deformation potential and piezoelectric coupling). Our results show that both interface roughness and alloy disorder scattering rates are strongly dependent upon the electric field strength. We also show that there are interesting changes in the dominance of the mobility among different scattering processes, which leads to the formation of a maximum in the mobility dependence on the electric field strength. In general, this maximum occurs for some positive electric field (antiparallel to growth direction) and it indicates that longitudinal electric fields can be used to dislocate the electron wave function, enhancing in turn the two-dimensional electron mobility in such quantum wells.

Characterization of ultrafast devices using near-field optical heterodyning

We demonstrate a novel technique for highly localized injection of millimeter waves in ultrafast devices that combines optical heterodyning and near-field optics. The technique relies on evanescent coupling of two interfering lasers to a submicron area of a device by means of a near-field fiber optic probe. Scanning measurements show the dc and ac photoresponses of two ultrafast device structures, namely low-temperature GaAs photoconductive switches and InP-based high electron mobility transistors. The response characteristics were rich in structures that revealed important details of device dynamics.

AlN/GaN and (Al,Ga)N/AlN/GaN two-dimensional electron gas structures grown by plasma-assisted molecular-beam epitaxy

We report on an extensive study of the two-dimensional electron gas (2DEG) structures containing AlN layers. It is shown that the presence of large polarization fields in the AlN barrier layer in AlN/GaN heterostructures results in high values of the 2DEG sheet density of up to 3.6×1013 cm−2. Room-temperature sheet resistance of 180 Ω/□ is demonstrated in the AlN/GaN structure with a 35 Å AlN barrier. As a result of reduced alloy disorder scattering, low-temperature electron mobility is significantly enhanced in AlN/GaN heterostructures in comparison to AlGaN/GaN structures with similar values of the 2DEG sheet density. The growth of GaN cap layers on top of AlN/GaN structures with relatively thick (∼35 Å) AlN barriers is found to lead to a significant decrease in the 2DEG sheet density. However, inserting a thin (∼10 Å) AlN layer between AlxGa1−xN and GaN in the AlxGa1−xN/GaN (x∼0.2–0.45) 2DEG structures does not affect the 2DEG sheet density and results in an increase of the low-temperature electron mobility in comparison to standard AlGaN/GaN structures. At room temperature, a combination of the high 2DEG sheet density of 2.15×1013 cm−2 and high electron mobility of 1500 cm2/V s in Al0.37Ga0.63N/AlN/GaN yielded a low sheet resistance value of 194 Ω/□.

Improved purity of long-wavelength InAsSb epilayers grown by melt epitaxy in fused silica boats

In this study, we first present the process of the melt epitaxial (ME) growth method, and the improvement of low-temperature electron mobility of the long-wavelength InAsSb epilayers grown by ME in a fused silica boat. The electrical properties were investigated by van der Pauw measurement at 300 and 77 K. It is seen that the electron mobility of the InAsSb samples grown by graphite boat decreased from 55,700 to 26,600 cm 2/V s when the temperature was reduced from 300 to 77 K, while for the samples grown by fused silica boat, the electron mobility increased from 52,600 at 300 K to 54,400 cm 2/V s at 77 K. The electron mobility of 54,400 cm 2/V s is the best result, so far, for the InAsSb materials with cutoff wavelength of 8–12 μm at 77 K. This may be attributed to the reduction of the carbon contamination by using a fused silica boat instead of a graphite boat.