High Hall Mobility Research Articles

Fabrication of high resistivity cold-implanted InGaAsP photoconductors for efficient pulsed terahertz devices

A multiple-energy, high fluence, MeV Fe ion implantation process was applied at 83 K to heavily damage a low band gap (0.79 eV) epitaxial InGaAsP layer. Optimal rapid thermal annealing conditions were found and produced a fast photoconductor with high resistivity (up to 2500 Ωcm) and Hall mobility around 400 cm2V−1s−1. Short photocarrier trapping times (0.3 ps – 3 ps) were observed via transient differential reflectivity measurements. Furthermore, photoconductive terahertz devices with coplanar electrodes were fabricated and validated. Under pulsed excitation with a 1550 nm femtosecond fiber laser source, antennas based on Fe-implanted InGaAsP are able to emit broadband radiation exceeding 2 THz. Given such specifications, this new material qualifies as a worthy candidate for an integration into optical terahertz spectrometer designs.

Effects of In Situ Boron Doping in Si Epitaxial Growth on a VIC Processed Poly-Si Seed Layer Using Hot-Wire CVD

We investigated the epitaxial growth of large-grained poly-Si film on vapor induced crystallization (VIC) seed layer using hot-wire chemical vapor deposition(HWCVD) for hetero-junction Si solar cells. In this study, p-type poly-Si films were prepared by changing in-situ boron doping time. After epitaxial growth on VIC seed layer, average grain size of about 20μm, are obtained and the crystallographic defects in epitaxial poly-Si layer on VIC seed layer are mainly low angle grain boundaries (LAGB < 2°) and coincident site lattice boundaries (CSL), which are special boundaries of less electrical activity. Moreover, with decreasing in-situ boron doping time, mis-orientation angle and in-grain defects in epitaxial Si decrease. Using in-situ boron doping, highly doped p+ back surface field layer was fabricated, and it had high Hall mobility.

220-GHz Quaternary Barrier InAlGaN/AlN/GaN HEMTs

Depletion-mode high-electron mobility transistors (HEMTs) based on a quaternary barrier <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{In}_{0.13}\hbox{Al}_{0.83}\hbox{Ga}_{0.04}\break\hbox{N/AlN/GaN}$</tex> </formula> heterostructure on SiC substrate were fabricated. The 66-nm-long gate device shows a dc drain current density of 2.1 A/mm, a peak extrinsic transconductance of 548 mS/mm, and a record current gain cutoff frequency <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$f_{T}$</tex></formula> of 220 GHz for quaternary barrier GaN-based HEMTs, which is also among the highest <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$f_{T}$</tex></formula> for all GaN-based HEMTs. The large <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$L_{g} \cdot f_{T}$</tex></formula> product of 14.5 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{GHz} \cdot \mu\hbox{m}$</tex></formula> with a gate-length-to-barrier-thickness aspect ratio of 5.8 indicates a high effective electron velocity of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{0.9} \times \hbox{10}^{7}\ \hbox{cm/s}$</tex></formula> , attributed to a high electron Hall mobility (1790 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{cm}^{2}/\hbox{V} \cdot \hbox{s}$</tex> </formula> at an <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$n_{s}$</tex></formula> of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$ \hbox{1.8} \times \hbox{10}^{13}\ \hbox{cm}^{-2}$</tex></formula> )—the highest reported in GaN-channel HEMTs with In-containing barriers. An intrinsic electron velocity of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{1.7} \times \hbox{10}^{7}\ \hbox{cm/s}$</tex></formula> , extracted from conventional Moll delay-time analysis, is comparable to that reported in the state-of-art AlGaN/GaN HEMTs.

Self-Limiting Film Growth of Transparent Conducting In2O3 by Atomic Layer Deposition using Trimethylindium and Water Vapor

Conformal and self-limiting film growth of transparent conducting In2O3 is demonstrated with atomic layer deposition (ALD) using trimethylindium (TMIn) and water vapor. Various parameters, including pulsing times of TMIn and H2O and deposition temperature, are varied to confirm the ALD processing window of In2O3. It is found that an extremely large Langmuir exposure of H2O (∼2 Torr s) is required to fully react the surface-adsorbed In-(CH3)* groups with H2O. Self-limiting film growth is obtainable at the deposition temperatures between 200 and 251 °C where the film growth is controlled by surface adsorption of TMIn molecules. Within the ALD window, ALD-In2O3 exhibits a film growth rate of ∼0.039 nm/cycle at 217 °C. In addition, the film exhibits an excellent step coverage of ∼97% on a high-aspect-ratio (∼11) nanostructure. The resistivity of the ALD-In2O3 film deposited at 200 °C is about 2.8 × 10–3 Ω cm, and this is attributed to the high Hall mobility of 84 cm2/(V s). Finally, the reduction of the Hall mobility of the films with respect to increases in the deposition temperature is correlated to the evolution of microstructures.

Epitaxial growth of high mobility Bi2Se3 thin films on CdS

We report the experiment of high quality epitaxial growth of Bi2Se3 thin films on hexagonal CdS (0001) substrates using a solid source molecular-beam epitaxy system. Layer-by-layer growth of single crystal Bi2Se3 has been observed from the first quintuple layer. The size of surface triangular terraces has exceeded 1 μm. Angle-resolved photoemission spectroscopy clearly reveals the presence of Dirac-cone-shape surface states. Magneto-transport measurements demonstrate a high Hall mobility of ∼6000 cm2/V s for the as-grown Bi2Se3 thin films at temperatures below 30 K. These characteristics of Bi2Se3 thin films promise a variety of potential applications in ultrafast, low-power dissipation devices.

Optimum packing density and crystal structure of tin-doped indium oxide thin films for high-temperature annealing processes

The influence of high-temperature annealing on the electrical properties and microstructure of tin-doped indium oxide (ITO) thin films was investigated as a function of oxygen gas flow ratio to argon gas during the sputtering deposition. The ITO thin films were annealed at 500 °C in air after the deposition. It was found that the ITO thin films, which were deposited in relatively low oxygen gas flow ratio, exhibited high Hall mobility and low-resistivity after the annealing. Furthermore, the X-ray reflectivity and diffraction measurement revealed that the ITO thin film with low-resistivity after annealing exhibited high packing density, smooth surface and low crystallization degree. It can be considered that the carrier electron scattering was suppressed with increasing in the packing density of the ITO thin film; as a result, the Hall mobility and resistivity were improved.

Growth of large-sized graphene thin-films by liquid precursor-based chemical vapor deposition under atmospheric pressure

Large-sized thin-films composed of single- and few-layered graphene have been synthesized by chemical vapor deposition (CVD) on copper foils under atmospheric pressure using ethanol or pentane as the precursor. Confocal Raman measurements, transmission electron microscopy and scanning tunneling microscopy show that the majority part of the obtained films exhibit hexagonal graphene lattice. Optical microscopy and electrical measurements confirm the continuity of these films. It is also found that the CVD-grown graphene films with ethanol as the precursor exhibit lower defect density, higher electrical conductivity, and higher hall mobility than those grown with pentane as the precursor. This liquid-precursor-based atmospheric pressure CVD synthesis provides a new route for simple, inexpensive and safe growth of graphene thin-films.

Microstructural Evidence of Hall Mobility Anisotropy in c-Axis Textured Al-Doped ZnO

The high electrical conductivity, 1150 S/cm at room temperature, in the ab-plane of c-axis textured Al-doped ZnO is attributed to its high Hall mobility that is almost double the mobility in the c-axis direction. Temperature-independent mobility in the ab-plane below 200 K suggests that ionized impurity dominates the scattering of electron transport, which reasonably agrees with a modified Brooks–Herring–Dingle model taking into account nonparabolic E–k dispersion. However, the pronounced anisotropy between ab-plane and c-axis cannot be expected based on the model. Detailed observations of the grain boundary (GB) by means of high-resolution transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy revealed the existence of an Al-enriched, Zn-deficient layer near the GB traversing the c-axis direction. In contrast, the highly conductive direction encompasses a tilt grain boundary, in which coincident sites were observed and Al segregation was barely evident. We conclude that such a preferential segregation in the GB and/or GB structure itself are responsible for the anisotropy of mobility in the textured Al-doped ZnO.

Low temperature silicon epitaxy from trichlorosilane via mesoplasma chemical vapor deposition

Epitaxial silicon thick films have been deposited at around 400 °C by mesoplasma chemical vapor deposition with trichlorosilane (TCS) as source gas. The deposition rate of the Si films increases linearly with the TCS flow rate and reaches 30 nm/s at 15 sccm of TCS. These films have exhibited relatively high hall mobility (~ 200 cm 2/V-s) independently of the deposition rate.

Thermomagnetic Effects in High-Mobility PbTe Films

The narrow-gap compound semiconductor PbTe has high Hall mobility. The Fermi surface at the L-point in the Brillouin zone has large anisotropy. In this work, we measured thermomagnetic effects in PbTe thin films to confirm anisotropy of the Nernst coefficient A Ne and show Nernst mobility from the ratio of A Ne and the Seebeck coefficient S: μ Ne = A Ne/S. Angular dependences of the Nernst voltage show that A Ne is independent of the angle between the temperature gradient and the magnetic field, because of the high L-point symmetry. The calculated Nernst mobility was compared with the Hall mobility. Because the former is smaller, the Mott equation cannot explain the Seebeck coefficient at room temperature.

Impact of rapid thermal annealing and hydrogenation on the doping concentration and carrier mobility in solid phase crystallized poly-Si thin films

ABSTRACTThe effect of the rapid thermal annealing (RTA) and hydrogenation step on the electronic properties of the n+ and p+ solid phase crystallized (SPC) poly-crystalline silicon (poly-Si) thin films was investigated using Hall effect measurements and four-point-probe measurements. Both the RTA and hydrogenation step were found to affect the electronic properties of doped poly-Si thin films. The RTA step was found to have the largest impact on the dopant activation and majority carrier mobility of the p+ SPC poly-Si thin films. A very high Hall mobility of 71 cm2/Vs for n+ poly-Si and 35 cm2/Vs for p+ poly-Si at the carrier concentration of 2×1019 cm-3 and 4.5×1019 cm-3, respectively, were obtained.

Two-dimensional electron gas in Zn-polar ZnMgO/ZnO heterostructure grown by metal-organic vapor phase epitaxy

We report the formation of two-dimensional electron gas (2DEG) at the Zn1−xMgxO/ZnO interface grown by metal-organic vapor phase epitaxy on sapphire substrates. The existence of the 2DEG is confirmed by the observation of Shubnikov–de Haas oscillations and the integer quantum Hall effect. In particular, the Zn0.8Mg0.2O/ZnO heterostructure shows a high Hall mobility of 2138 cm2/V s with a carrier sheet density of 3.51×1012 cm−2 at 1.4 K. We attribute the origin of 2DEG to be the donor states on ZnMgO surface. The dependence of carrier sheet density of 2DEG on ZnMgO layer thickness and Mg composition (x) are also investigated.

High rate reactive magnetron sputtering of ZnO:Al films from rotating metallic targets

Aluminum doped zinc oxide (ZnO:Al) films were reactively sputtered at a high discharge power from dual rotating metallic targets (Zn:Al = 99.5:0.5 wt.%). Deposition conditions like substrate temperature and working points were varied in order to prepare high quality ZnO:Al films. The influences on electrical and optical ZnO:Al thin film properties and surface texture before and after chemical etching in diluted HCl were studied in order to achieve light scattering films as front contact for solar cells. High dynamic deposition rate close to 90 nm m/min and high Hall mobility of up to 47 cm 2/Vs were obtained. Transmission of more than 85% in the visible spectral range is obtained for all ZnO:Al films in this study. In addition, the absorption in near infrared region is low due to low doping. Surface texture after etching is usually much rougher than before. However, some films reveal after etching small surface features that are similar to initial surface features. We propose a relationship between initial and post-etched surface textures.

Effect of substrate temperature on structural and optoelectrical properties of silver doped zinc oxide thin films

Abstract Silver doped zinc oxide (ZnAgO) thin films were grown on quartz substrate using pulsed laser deposition technique. The effect of substrate temperature on structural, optical, and electrical properties was studied. The films grown at low temperatures were amorphous in nature, while films grown at high temperatures were crystalline. It is observed that the films are highly oriented along c -axis. The surface roughness of these films is observed to increase with substrate temperature; on the other hand optical transmittance is almost independent of substrate temperature. The mobility of the films increases with an increase in substrate temperature. The highest Hall mobility of 62 cm 2 /V s is observed for the film grown at 600 °C. These highly transparent and high mobility films could be used as bottom electrodes in optoelectronic devices.

High mobility amorphous zinc oxynitride semiconductor material for thin film transistors

Zinc oxynitride semiconductor material is produced through a reactive sputtering process in which competition between reactions responsible for the growth of hexagonal zinc oxide (ZnO) and for the growth of cubic zinc nitride (Zn3N2) is promoted. In contrast to processes in which the reaction for either the oxide or the nitride is dominant, the multireaction process yields a substantially amorphous or a highly disordered nanocrystalline film with higher Hall mobility, 47 cm2 V−1 s−1 for the as-deposited film produced at 50 °C and 110 cm2 V−1 s−1 after annealing at 400 °C. In addition, it has been observed that the Hall mobility of the material increases as the carrier concentration decreases in a carrier concentration range where a multicomponent metal oxide semiconductor, indium–gallium–zinc oxide, follows the opposite trend. This indicates that the carrier transports in the single-metal compound and the multimetal compound are probably dominated by different mechanisms. Film stability and thin film transistor performance of the material have also been tested, and results are presented herein.

Effects of post‐annealing on (110) Cu2O epitaxial films and origin of low mobility in Cu2O thin‐film transistor

AbstractWe grew epitaxial (110) Cu2O films on (110) MgO substrates toward high‐mobility p‐channel oxide thin‐film transistors (TFTs). The (110) Cu2O films exhibited high Hall mobilities ∼90 cm2(Vs)−1 comparable to those of high‐quality single‐crystals, which were obtained in a narrow growth condition for 650 nm‐thick films. TFTs using the epitaxial (110) Cu2O channels exhibited p‐channel operation, but the field‐effect mobilities and the on‐to‐off drain current ratio were far from satisfaction (∼0.04 cm2(Vs)−1 and ∼2, respectively). In order to investigate the origin of the poor mobility, the films were subjected to post‐deposition annealing under various oxygen partial pressures (PO2−A = 0.65–10−3 Pa). Optical measurements revealed that subgap states exist in all the films and their amounts were increased by post‐deposition annealing irrespective of PO2−A. The subgap density of states estimated by the optical analyses are consistent roughly with that estimated from the TFT mobility.

Preparation and Characterization of Ga&lt;sub&gt;2&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;In&lt;sub&gt;2(1-&lt;i&gt;x&lt;/i&gt;)&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Films Deposited on α-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; (0001) by MOCVD

Ga2xIn2(1-x)O3 thin films with different gallium content x [x = Ga/(Ga+In) atomic ratio] have been prepared on -Al2O3 (0001) substrates at 650°C by metalorganic chemical vapor deposition (MOCVD). Structural, electrical and optical properties of these films have been investigated in detail. The XRD analysis revealed that, as the gallium content increased, the crystalline quality of the films decreased. The highest Hall mobility of the films was 41.32 cm2v−1s−1. The absolute average transmittance of the Ga2xIn2(1-x)O3 thin films in the visible range exceeded 91%. The band gap could be tuned from 3.59 to 4.87 eV as gallium content increased.

The grain boundary related p-type conductivity in ZnO films prepared by ultrasonic spray pyrolysis

Intrinsic p-type ZnO thin film was fabricated on sapphire substrate by ultrasonic spray pyrolysis, the p-type ZnO film is achieved using O2 as the carrier gas, with a resistivity of 2.18 Ω cm−1, a carrier concentration of 1.10×1016 cm−3, and a high Hall mobility of 261 cm2/V s. The scanning capacitance microscopy images and annealing the p-type ZnO indicate that the absorbed oxygen in the grain boundary (GB) aroused the p-type conductivity, and the high Hall mobility of the p-type ZnO film own to the quasi-two-dimensional hole gas, which was induced by the negatively charged interface states in the GBs.

High quality of InAsSb epilayer with cutoff wavelength longer than 10 μm grown on GaAs by the modified LPE technique

The InAsSb epilayers with a cutoff wavelength of 11.5 μm were successfully grown on highly lattice-mismatched semi-insulating (1 0 0) GaAs substrate by the modified liquid phase epitaxy (LPE) technique. Fourier transform infrared (FTIR) transmission spectrum revealed a strong band gap narrowing for this alloy. The electrical properties were investigated by the Van der Pauw measurements at 300 and 77 K. InAsSb epilayers showed high Hall mobilities being 11,800 cm 2/V s at room temperature (RT). After an annealing treament for 10 h, the electron mobility at 77 K were improved from 1730 cm 2/V s (prior to annealing) to 13,470 cm 2/V s. Wet etching was used to display the surface etch pits prior to and after annealing treatment, showing that the mobility improvement was due to the reduction of the etch pits density.

High mobility holes in a strained Ge quantum well grown on a thin and relaxed Si0.4Ge0.6/LT-Si0.4Ge0.6/Si(001) virtual substrate

Epitaxial growth of a compressively strained Ge quantum well (QW) on an ultrathin, 345 nm thick, Si0.4Ge0.6/LT-Si0.4Ge0.6/Si(001) virtual substrate (VS) has been demonstrated. The VS, grown with a low temperature Si0.4Ge0.6 seed layer on a Si(001) substrate, is found to be fully relaxed and the Ge QW is fully strained. The temperature dependence of Hall mobility and carrier density clearly indicates a two-dimensional hole gas in the Ge QW. At room temperature, which is more relevant for electronic devices applications, the samples show a very high Hall mobility of 1235 cm2 V−1 s−1 at a carrier density of 2.36×1012 cm−2.