Low-temperature Hall Mobility Research Articles

High electron mobility in metamorphic epitaxial InAs0.7Sb0.3 compound and its p-i-n photodetector

Our study explores growth techniques and the electrical/optical behavior of InAs0.7Sb0.3 compounds on GaAs substrates, utilizing various metamorphic buffer layers. The InAlSb graded-buffer layer emerged as the most promising, displaying a remarkable electron mobility of 26,370 cm/V∙s, a record high. It signifies excellent lateral crystal quality for room-temperature carrier transport. Meanwhile, the presence of micro-twins, evident in cross-sectional TEM images, degrades vertical crystal quality. We further assess crystal quality through low-temperature Hall mobility and photoresponses, featuring reduced bowing parameters. These findings offer valuable insights for optimizing metamorphic growth platforms for ternary InAsSb compounds.

High Mobility and Quantum Oscillations in Semiconducting Bi2O2Te Nanosheets Grown by Chemical Vapor Deposition.

Bi2O2Te has the smallest effective mass and preferable carrier mobility in the Bi2O2X (X = S, Se, Te) family. However, compared to the widely explored Bi2O2Se, the studies on Bi2O2Te are very rare, probably attributed to the lack of efficient ways to achieve the growth of ultrathin films. Herein, ultrathin Bi2O2Te crystals were successfully synthesized by a trace amount of O2-assisted chemical vapor deposition (CVD) method, enabling the observation of ultrahigh low-temperature Hall mobility of >20 000 cm2 V-1 s-1, pronounced Shubnikov-de Haas quantum oscillations, and small effective mass of ∼0.10 m0. Furthermore, few nm thick CVD-grown Bi2O2Te crystals showed high room-temperature Hall mobility (up to 500 cm2 V-1 s-1) both in nonencapsulated and top-gated device configurations and preserved the intrinsic semiconducting behavior with Ion/Ioff ∼ 103 at 300 K and >106 at 80 K. Our work uncovers the veil of semiconducting Bi2O2Te with high mobility and brings new blood into Bi2O2X family.

Revealing high room and low temperatures mobilities of 2D holes in a strained Ge quantum well heterostructures grown on a standard Si(0 0 1) substrate

Carrier mobility is one of the most important parameters of any semiconductor material, determining its suitability for applications in a large variety of electronic devices including field effect transistors (FETs). Today the capabilities of modern planar Si FET devices are almost exhausted and researchers are seeking either new device architectures or new materials. Here we report an extremely high room temperature (at 293K) 2D hole gas (2DHG) drift mobility of 4500cm2V−1s−1 at a carrier density of 1.2×1011cm−2 obtained in a compressively strained Ge quantum well (QW) heterostructure, grown by an industrial type chemical vapor deposition system on a standard Si(001) substrate. The low-temperature Hall mobility and carrier density of this structure, measured at 333mK, are 777,000cm2V−1s−1 and 1.9×1011cm−2, respectively. These hole mobilities are the highest not only among the group-IV Si and Ge based semiconductors, but also among p-type III–V and II–VI materials. The obtained room temperature mobility is substantially higher than those reported so far in strained Ge QW heterostructures and reveals a huge potential for further applications of this material in a wide variety of electronic devices.

Invited: In Situ Characterization of Plasma Assisted Hydrogenation/Fluorination of Graphene and CFx Battery Applications

Monolayer graphene synthesized by chemical vapor deposition was subjected to controlled and sequential hydrogenation using RF plasma while monitoring its electrical properties in-situ. Low temperature transport properties, viz., electrical resistance (R), thermopower (S), Hall mobility (m), and magneto-resistance (MR) were measured for each sample and correlated with ex-situ Raman scattering and X-ray photoemission (XPS) characteristics. For weak-hydrogenation, the transport is seen to be governed by electron diffusion and low temperature transport properties show metallic behavior (conductance G remains non-zero as 0). For strong-hydrogenation, the transport is found to be describable by variable range hopping (VRH) and the low T conductance shows insulating behavior (0 as 0). Weak localization (WL) behavior is seen with a negative MR for weakly-hydrogenated graphene and this WL effects are seen to diminish as the hydrogenation progresses. A clear transition to strong localization (SL) is evident with the emergence of pronounced negative MR for strongly-hydrogenated graphene. Multi-walled carbon nanotubes synthesized using fluidized bed chemical vapor deposition technique were fluorinated sequentially to prepare a series of CFx battery electrodes. Primary battery performance was tested using CFx as a cathode against Li. Fully fluorinated MWNTs showed capacity exceeding 815 mAh/g while partially fluorinated samples showed systematically lowered capacity with decreasing x (in CFx). However, fully fluorinated MWCNTs showed distinctly low rechargeable capacity compared to the subfluorinated samples when used as an anode against Li. Mildly fluorinated MWNTs show high capacity and better stability during charge/discharge cycles. High concentrations of fluorine seem to affect capacity retention due to the increased defect densities and reduced electronic conduction. These defects of nanotubes will provide additional pathways for lithium ions to diffuse within the core of the fluorinated structure and to access the electrochemically active C-F sites. XRD, XPS, and Raman spectroscopy were utilized to characterize the samples. Finally the electrochemical performance of fluorinated MWNTs was compared with that of Natural Chinese Graphite (NCG).

Enhanced thermoelectric performance in Cd doped CuInTe2 compounds

CuIn1−xCdxTe2 materials (x = 0, 0.02, 0.05, and 0.1) are prepared using melting-annealing method and the highly densified bulk samples are obtained through Spark Plasma Sintering. The X-ray diffraction data confirm that nearly pure chalcopyrite structures are obtained in all the samples. Due to the substitution of Cd at In sites, the carrier concentration is greatly increased, leading to much enhanced electrical conductivity and power factor. The single parabolic band model is used to describe the electrical transport properties of CuInTe2 and the low temperature Hall mobility is also modeled. By combing theoretical model and experiment data, the optimum carrier concentration in CuInTe2 is proposed to explain the greatly enhanced power factors in the Cd doped CuInTe2. In addition, the thermal conductivity is reduced by extra phonon scattering due to the atomic mass and radius fluctuations between Cd and In atoms. The maximum zTs are observed in CuIn0.98Cd0.02Te2 and CuIn0.9Cd0.1Te2 samples, which are improved by over 100% at room temperature and around 20% at 600 K.

An extremely high room temperature mobility of two-dimensional holes in a strained Ge quantum well heterostructure grown by reduced pressure chemical vapor deposition

An extremely high room temperature two-dimensional hole gas (2DHG) drift mobility of 4230 cm2 V−1 s−1 in a compressively strained Ge quantum well (QW) heterostructure grown by an industrial type RP-CVD technique on a Si(001) substrate is reported. The low-temperature Hall mobility and carrier density of this structure, measured at 333 mK, are 777000 cm2 V−1 s−1 and 1.9 × 1011 cm−2, respectively. These hole mobilities are the highest not only among the group-IV Si based semiconductors, but also among p-type III–V and II–VI ones. The obtained room temperature mobility is substantially higher than those reported so far for the Ge QW heterostructures and reveals a huge potential for further application of strained Ge QW in a wide variety of electronic and spintronic devices.

In Situ Study of Hydrogenation of Graphene and New Phases of Localization between Metal–Insulator Transitions

Monolayer graphene synthesized by chemical vapor deposition was subjected to controlled and sequential hydrogenation using RF plasma while monitoring its electrical properties in situ. Low-temperature transport properties, namely, electrical resistance (R), thermopower (S), Hall mobility (μ), and magnetoresistance (MR), were measured for each sample and correlated with ex situ Raman scattering and X-ray photoemission (XPS) characteristics. For weak hydrogenation, the transport is seen to be governed by electron diffusion, and low-temperature transport properties show metallic behavior (conductance G remains nonzero as T → 0). For strong hydrogenation, the transport is found to be describable by variable range hopping (VRH) and the low T conductance shows insulating behavior (G → 0 as T → 0). Weak localization (WL) behavior is seen with a negative MR for weakly hydrogenated graphene, and these WL effects are seen to diminish as the hydrogenation progresses. A clear transition to strong localization (SL) is evident with the emergence of pronounced negative MR for strongly hydrogenated graphene.

Magneto-transport in MoS2: Phase Coherence, Spin–Orbit Scattering, and the Hall Factor

We have characterized phase coherence length, spin-orbit scattering length, and the Hall factor in n-type MoS2 2D crystals via weak localization measurements and Hall-effect measurements. Weak localization measurements reveal a phase coherence length of ~50 nm at T = 400 mK for a few-layer MoS2 film, decreasing as T(-1/2) with increased temperatures. Weak localization measurements also allow us, for the first time without optical techniques, to estimate the spin-orbit scattering length to be 430 nm, pointing to the potential of MoS2 for spintronics applications. Via Hall-effect measurements, we observe a low-temperature Hall mobility of 311 cm(2)/(V s) at T = 1 K, which decreases as a power law with a characteristic exponent γ = 1.5 from 10 to 60 K. At room temperature, we observe Hall mobility of 24 cm(2)/(V s). By determining the Hall factor for MoS2 to be 1.35 at T = 1 K and 2.4 at room temperature, we observe drift mobility of 420 and 56 cm(2)/(V s) at T = 1 K and room temperature, respectively.

Thermoelectric power, Hall effect, and mobility ofn-typeBaTiO3

The resistivity and the Hall and Seebeck effects of polycrystalline n-type ${\mathrm{Ba}}_{0.996}{\mathrm{Y}}_{0.004}{\mathrm{TiO}}_{3}$ and undoped, reduced single crystal ${\mathrm{BaTiO}}_{3}$ were studied in the 100--300 K interval. Both Hall coefficient and resistivity showed similar temperature activated dependence for the single crystal in the rhombohedral phase and for the polycrystal in the rhombohedral and orthorhombic phases. Low-temperature Hall mobility in the ${\mathrm{BaTiO}}_{3}$ single crystal decreased from $12{\mathrm{cm}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1}$ at 120 K to $2{\mathrm{cm}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1}$ at 220 K, implying a phonon scattering of charge carriers. The temperature dependence of the Hall mobility of the polycrystal showed a weak temperature dependence in the 100--220 K range with a maximum of $1.2\ifmmode\pm\else\textpm\fi{}0.3{\mathrm{cm}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1}$ at 170 K, probably indicating a transition from ionized impurity scattering at low temperature to a phonon scattering at higher temperatures. Temperature dependence of thermopower and Hall coefficient confirmed that the increase in conductivity of the single crystal below 200 K was caused by an increase in the concentration of the charge carriers. The results suggest that below 300 K, electron transport in ${\mathrm{BaTiO}}_{3}$ occurs via the conduction band rather than by a small polaron hopping.

GaN/AlGaN Two-Dimensional Electron Gas Grown by Ammonia-MBE on MOCVD GaN Template

The growth of high quality GaN/AlGaN two-dimensional electron gas on MOCVD GaN template substrates by ammonia-MBE is reported. The homoepitaxial growth required a significantly lower growth temperature than that needed for growth on SiC or sapphire substrates, and yielded significantly improved surface smoothness, and consequently improved electrical characteristics of the two-dimensional electron gas. A low temperature Hall mobility of 14300 cm 2 /V s and quantum mobility of 2000 cm 2 /V s were obtained, representing the highest values observed in the GaN/AlGaN structures grown by the ammonia-MBE technique. A correlation between the surface roughness and increased Hall mobility to quantum mobility ratio was observed, suggesting that the surface roughness could be a source of small-angle scattering in these structures.

Heteroepitaxial PbTe-on-Si pn-junction IR-sensors: correlations between material and device properties

Epitaxial IV–VI narrow band gap semiconductors on Si(1 1 1) substrates exhibit high structural quality despite the large lattice and thermal expansion mismatch. Test arrays with photovoltaic n +–p PbTe infrared sensors (cut-off wavelength 5.5 μm at 80 K ) of different sizes were fabricated and analyzed. The sensitivities are generation–recombination (g–r)-limited in the 90– 200 K range. The g–r carrier lifetimes in the depletion region are determined from the R 0 A-products (inverse noise current densities). The corresponding carrier diffusion lengths are correlated with the material properties, namely low-temperature saturation Hall mobilities and X-ray rocking curve line widths. It turns out that all these parameters are determined by the density of the threading dislocations, and each dislocation crossing the active area gives rise to a shunt resistance. At lower temperatures, the R 0 A-products saturate and the ideality factors increase above a value of 2. This behaviour suggests that, as in the case of Schottky barriers of metal–semiconductor junctions of IV–VIs, fluctuations of the built-in electric field occur near the dislocation cores which cross the active areas of the devices.

The Effects of Spacer Thickness and Temperature on the Transport Properties of Modulation-Doped In0.53Ga0.47As/In0.52Al0.48As Heterojunctions

The measurements of resistivity and low-field Hall effect made in the temperature range 3.3–295 K have been used to investigate the transport properties of modulation-doped, lattice-matched In0.53Ga0.47As/In0.52Al0.48As heterojunctions as a function of the spacer thickness in the range from 0 to 400 Å. It is found that the sheet carrier density determined at temperatures below about 80 K decreases rapidly with increasing spacer thickness. The low-temperature Hall mobility increases substantially when increasing the spacer thickness from 0 to 100 Å, and decreases gradually with further increase in spacer thickness. The results suggest that, in addition to alloy scattering, remote ionized-impurity scattering is a major scattering mechanism at low temperatures in the samples with thin spacer layer and that background impurity scattering prevails in the samples with spacer thickness larger than about 100 Å. The effect of modulation doping on the mobility decreases progressively with increasing temperature: the electron mobility becomes practically independent of spacer thickness in the temperature range above about 200 K. The variation of Hall mobility with temperature in the range above 90 K has been used to determine the energy of longitudinal optical phonons that limit the mobility of electrons at high temperatures.

Space-Charge Region Scattering in Indium Monoselenide

Scattering mechanisms in undoped n-type indium monoselenide have been studied in the temperature range between 80 and 400 K. The experimental data were obtained from Hall and photo-Hall effects. It is shown that in the samples with low room temperature Hall mobility (μH = 600 to 750 cm2/Vs) the temperature dependence of μH(T) can be explained by electron scattering on charged impurity aggregates surrounded by space-charge regions. Under illumination the neutralization of the effective charge of impurity aggregates by photo-excited carriers is the reason for the predominant electron scattering on homopolar optical phonons A′1g.

A plasma process for ultrafast deposition of SiGe graded buffer layers

Low energy plasma enhanced chemical vapor deposition (LEPECVD) has been applied to the synthesis of Si-modulation doped field effect transistor structures, comprising a SiGe relaxed buffer layer and a modulation doped strained Si channel. A growth rate of at least 5 nm/s for the relaxed SiGe buffer layer is well above that obtainable by any other technique. Due to the low ion energies involved in LEPECVD, ion damage is absent, despite a huge plasma density. The structural quality of the LEPECVD grown SiGe buffer layers is comparable to that of state-of-the-art material. The electronic properties of the material were evaluated by growing modulation doped Si quantum wells on the buffer layers. We obtain a low temperature (2 K) Hall mobility of μH=2.5×104 cm2/Vs for the electrons in the Si channel at an electron sheet density of ns=8.6×1011 cm−2.

Correlations between Structural, Electrical and Optical Properties of GaN Layers Grown by Molecular Beam Epitaxy

Screw and edge dislocation densities were estimated from X-ray diffraction profiles of GaN layers grown by molecular beam epitaxy. The results were confirmed by transmission electron microscopy. The layers had varying thicknesses or were deposited on differently nitridated sapphire substrates. It was found that the low temperature Hall mobility correlates with the screw dislocation density whereas the full width at half maximum of the donor bound exciton emission line changes similarly as the edge dislocation density.

Molecular beam epitaxial growth of high quality Pb 1− xSn xTe layers with 0⩽ x⩽1

Lead tin telluride epitaxial layers with tin concentrations covering the whole compositional range have been grown by molecular beam epitaxy on cleaved BaF 2 (1 1 1) substrates, using PbTe and SnTe solid sources. The full width at half maximum of the (2 2 2) rocking curve of these samples, measured by a four crystal high resolution X-ray diffraction apparatus, was about 400 arcsec for the ternary layers, and 150 arcsec for the binary samples. Low temperature Hall mobilities decreased from 5×10 5 V cm 2/s, for PbTe, to 2×10 3 V cm 2/s for SnTe, although the crystalline quality is approximately the same. This decrease in mobility is a consequence of the increase in hole concentration from 2×10 17 cm −3 to 2×10 20 cm −3, when the Sn concentration changes from 0 to 100%. This high carrier concentration in the samples with high Sn content showed a large Burstein–Moss shift in the infrared transmission spectra measured between 77 and 300 K.

Hall mobility analysis in low-temperature-grown molecular-beam epitaxial GaAs

Temperature dependent conductivity and Hall effect measurements were carried out on molecular-beam epitaxial GaAs layers grown at 200–420 °C and separated from the substrate. An analysis of experimental data with and without considering the hopping Hall mobility was made. An extremely low room temperature Hall mobility of 0.14 cm2 V−1 s−1 was measured in the 250 °C layer, which could be interpreted as the hopping Hall mobility. The room temperature band Hall mobility (μHb) increases from 500 to 6000 cm2 V−1 s−1 and the power (n) of the temperature dependence of μHb (∼T−n) increases from 0.5 to 1.2 with increasing growth temperature from 300 to 420 °C.

Anisotropy in Atomic-Scale Interface Structure and Mobility in Inas/Ga1_Xinxsb Superlattices

AbstractWe have used cross-sectional scanning tunneling microscopy to study the atomic-scale interface structure of InAs/Ga, _In.xSb superlattices grown by molecular-beam epitaxy. Detailed, quantitative analysis of interface profiles obtained from constant-current images of both (110) and (1ī0) cross-sectional planes of the superlattice indicates that interfaces in the (1ī0) plane exhibit a higher degree of interface roughness than those in the (110) plane, and that the Ga1-xln xAs interfaces are rougher than the InAs-on-Gal1-xInxSb interfaces. The roughness data are consistent with anisotropy in interface structure arising from anisotropic island formation during growth, and in addition with a growth-sequence-dependent interface asymmetry resulting from differences in interfacial bond structure between the superlattice layers. Roughness data are compared with measurements of anisotropy in low-temperature Hall mobilities of the samples.

Low temperature growth of PbTe and of PbTe/ Pb1− xEuxTe multi-quantum wells by molecular beam epitaxy

A systematic study of molecular beam epitaxy (MBE) of PbTe and Pb 1- x Eu x Te on BaF 2 (111) at substrate temperatures between 350 and 70°C is presented. Reflection high-energy electron diffraction (RHEED) intensity oscillations with more than 200 oscillation periods are observed even for substrate temperatures as low as 95°C, signifying the persistence of two-dimensional layer-by-layer growth at surprisingly low growth temperatures. In addition, for low temperature MBE growth of the ternary Pb 1- x Eu x Te, the evolution of RHEED intensity oscillations depends strongly on the beam flux composition, i.e., the Te 2 to Eu beam flux ratio, and a Te-stabilized, as well as Eu-stabilized layer-by-layer growth mode exists. A series of PbTe epitaxial layers, grown at various substrate temperatures from 350 to 120°C, was characterized by high resolution X-ray diffraction and temperature dependent Hall effect measurements. High quality epitaxial layers were obtained for substrate temperatures as low as 160°C, whereas for lower temperatures a drastic broadening of the X-ray diffraction peaks and a strong decrease of the low temperature Hall mobilities is observed. In PbTe/Pb 1- x Eu x Te multi-quantum well structures grown at various substrate temperatures, no broadening of the superlattice satellite diffraction peaks is observed when the substrate temperature is decreased down to 200°C. The high quality of the samples grown at substrate temperatures very much below the usual MBE temperatures is based on a precise control of the beam flux composition. Under such conditions, MBE of IV-VI compounds can be carried out at very low substrate temperatures, whenever technologically necessary.

DX Centers in AlGaAs and Pressurised GaAs

ABSTRACTIn this paper, we present results of our investigations on some aspects of the DX centers. It is shown from the low temperature Hall mobility measurements that the charge state of the DX center is neutral supporting the positive U model for the DX center. Hall and DLTS measurements on Al-rich Al0.8Ga0.2As:Te sample show a reversal in the ordering of the energies of the hydrogenic and deep states of the Te donor, with the DX state lying higher than the X valley related effective mass state. Thermal emission properties of the pressure induced DX states of Ge and Se donors in neutron transmutation doped (NTD) GaAs are discussed.