Hole Hall Mobility Research Articles

Accurate Prediction of Hall Mobilities in Two-Dimensional Materials through Gauge-Covariant Quadrupolar Contributions.

Despite considerable efforts, accurate computations of electron-phonon and carrier transport properties of low-dimensional materials from first principles have remained elusive. By building on recent advances in the description of long-range electrostatics, we develop a general approach to the calculation of electron-phonon couplings in two-dimensional materials. We show that the nonanalytic behavior of the electron-phonon matrix elements depends on the Wannier gauge, but that a missing Berry connection restores invariance to quadrupolar order. We showcase these contributions in a MoS_{2} monolayer, calculating intrinsic drift and Hall mobilities with precise Wannier interpolations. We also find that the contributions of dynamical quadrupoles to the scattering potential are essential, and that their neglect leads to errors of 23% and 76% in the room-temperature electron and hole Hall mobilities, respectively.

Long-range electrostatic contribution to electron-phonon couplings and mobilities of two-dimensional and bulk materials

Charge transport plays a crucial role in manifold potential applications of two-dimensional materials, including field-effect transistors, solar cells, and transparent conductors. At most operating temperatures, charge transport is hindered by scattering of carriers by lattice vibrations. Assessing the intrinsic phonon-limited carrier mobility is thus of paramount importance to identify promising candidates for next-generation devices. Here we provide a framework to efficiently compute the drift and Hall carrier mobility of two-dimensional materials through the Boltzmann transport equation by relying on a Fourier-Wannier interpolation. Building on a recent formulation of long-range contributions to dynamical matrices and phonon dispersions [Phys. Rev. X 11, 041027 (2021)], we extend the approach to electron-phonon coupling including the effect of dynamical dipoles and quadrupoles. We identify an unprecedented contribution associated with the Berry connection that is crucial to preserve the Wannier-gauge covariance of the theory. This contribution is not specific to two-dimensional crystals, but also concerns the three-dimensional case, as we demonstrate via an application to bulk SrO. We showcase our method on a wide selection of relevant monolayers ranging from ${\mathrm{SnS}}_{2}$ to ${\mathrm{MoS}}_{2}$, graphene, BN, InSe, and phosphorene. We also discover a nontrivial temperature evolution of the Hall hole mobility in InSe whereby the mobility increases with temperature above 150 K due to the Mexican-hat electronic structure of the InSe valence bands. Overall, we find that dynamical quadrupoles are essential and can impact the carrier mobility in excess of 75%.

CVD-deposited Cu2O thin films with a record Hall hole mobility of 263 cm2 V−1 s−1 and field-effect mobility of 0.99 cm2 V−1 s−1

Using a combination of chemical vapor deposition, controlled nucleation, and surface energy optimization, we demonstrate record hole mobility in Cu2O thin-films and thin-film transistors.

Compositions and properties of high-conductivity nitrogen-doped p-type β-Ga2O3 films prepared by the thermal oxidation of GaN in N2O ambient

A remarkable improvement in the conductivity of grown nitrogen-doped p-type β-Ga2O3 films was successfully achieved via the thermal oxidation of GaN in N2O ambient at 1100 °C. In the vacuum test environment, its room temperature Hall hole concentration is 1.64 × 1017 cm−3, the Hall resistivity is 7.66 Ω cm, the Hall hole mobility is 4.98 cm2 V−1 s−1, and the acceptor ionization energy is 0.087 eV. In addition, the compositions and properties were also extensively investigated. When the oxidation temperature was raised from 900 to 1100 °C, the FWHM of the {2¯ 01}-oriented β-Ga2O3 diffraction peak from the X-ray diffraction (XRD) pattern decreased to 0.237°, confirming an improvement in the crystalline quality of the β-Ga2O3 nanocrystal structure. From both the cross-sectional view of the field emission scanning electron microscopy (FESEM) image and the cross-sectional lamellae prepared by focused ion beam (FIB) milling, it was observed that the generated β-Ga2O3 film transformed into a two-layer morphology when oxidized at 1100 °C. The top layer is a rather loose film composed of nanorods, and the selected area electron diffraction (SAED) pattern of a nanorod confirmed it as high-quality (6¯03)-dominated monocrystal β-Ga2O3. The second layer is a comparatively compact β-Ga2O3 film. This approach to the efficient growth of p-type β-Ga2O3 can broaden the applicability of these two-layered structures. The randomly distributed monocrystal nanorods on the top layer are an appropriate candidate for the photocatalyst, while the relatively more compact second layer with high p-type conductivity can be employed in high-performance solar-blind UV photodetectors or high-power electronic devices.

Study of Carrier Mobilities in 4H-SiC MOSFETS Using Hall Analysis.

The channel conduction in 4H-SiC metal–oxide–semiconductor field effect transistors (MOSFETs) are highly impacted by charge trapping and scattering at the interface. Even though nitridation reduces the interface trap density, scattering still plays a crucial role in increasing the channel resistance in these transistors. In this work, the dominant scattering mechanisms are distinguished for inversion layer electrons and holes using temperature and body-bias-dependent Hall measurements on nitrided lateral 4H-SiC MOSFETs. The effect of the transverse electric field () on carrier mobility is analyzed under strong inversion condition where surface roughness scattering becomes prevalent. Power law dependencies of the electron and hole Hall mobility for surface roughness scattering are determined to be and , respectively, analogous to those of silicon MOSFETs. Moreover, for n-channel MOSFETs, the effect of phonon scattering is observed at zero body bias, whereas in p-channel MOSFETs, it is observed only under negative body biases. Along with the identification of regimes governed by different scattering mechanisms, these results highlight the importance of the selection of substrate doping and of in controlling the value of channel mobility in 4H-SiC MOSFETs.

Solid-phase crystallization of ultra-thin amorphous Ge layers on insulators

A simple method to form ultra-thin (<20 nm) semiconductor layers with a higher mobility on a 3D-structured insulating surface is required for next-generation nanoelectronics. We have investigated the solid-phase crystallization of amorphous Ge layers with thicknesses of 10−80 nm on insulators of SiO2 and Si3N4. We found that decreasing the Ge thickness reduces the grain size and increases the grain boundary barrier height, causing carrier mobility degradation. We examined two methods, known effective to enhance the grain size in the thicker Ge (>100 nm). As a result, a relatively high Hall hole mobility (59 cm2 V−1 s−1) has been achieved with a 20 nm thick polycrystalline Ge layer on Si3N4, which is the highest value among the previously reported works.

Characteristics of spark plasma sintered nanocarbon materials

We prepared binder-free monoliths of carbon nanohorns（CNHs）, hole-opened carbon nanohorns （CNHoxs）, multi-walled carbon nanotube（MWCNT）, and single-walled carbon nanotube（SWCNT）using spark plasma sintering（SPS）method at 1800℃ and 80 MPa in vacuum. The density of the SPS-treated SWCNT is 1.8 g/cm3, which is close to that of graphite, while those of CNHs, CNHoxs, and MWCNT remained 1.1-1.3 g/cm3 even after SPS treatment. We evaluated the monoliths using Raman spectroscopy and scanning electron microscopy observation, which showed a significant defect formation and graphitization of SWCNTs. Moreover, the increase of defects density in CNHs, CNHoxs, and MWCNT was moderate, and sub-micron size structures remained. We observed that the monoliths of CNHs and CNHoxs were highly conductive with a Hall mobility of positive holes of ～50 cm2/Vs and electrical conductivity of ～300 S/cm. These experimental results indicated that the SPS treatment under appropriate conditions could provide a CNHs monolith with nanostructure and good electrical conductivity.

Dilute-selenium alloying: A possible perspective for achieving p-type conductivity of β-gallium oxide

Photoconductive and heterojunction devices prepared based on n-type β-Ga2O3 have been extensively studied in the field of solar-blind ultraviolet (UV) detectors. However, for exploiting the full potential of photodetection by preparing bipolar homojunction devices, constructing p-type β-Ga2O3 through rational design remains a challenge. Herein, a p-type β-Ga2(SexO1−x)3 thin film was successfully prepared by a dilute-selenium alloying process. The bandgap was effectively regulated by varying the Se-content in the films. Meanwhile, the energy offset of valence band maximum and Fermi level is estimated to drop from 3.19 eV to 1.72 eV, indicating a transition of the semiconductor type from n-type to p-type. The β-Ga2(Se0.05O0.95)3 film prepared at the incident power of 120 W exhibited p-type conductivity, which possesses hall hole mobility of 24.3 cm2/V·s, hole concentration of 8.23 × 1014 cm−3, and resistivity of about 310.7 Ωcm at room temperature. This work clarified a possible way to realize p-type β-Ga2O3 by semiconductor alloying.

Hall Study of Conductive Channels Formed in Germanium by Beams of High-Energy Light Ions

The implantation of the high-energy ions of H+ or He+ in germanium leads to the creation of buried conductive channels in its bulk with equal concentrations of acceptor centers. These centers are the structure defects of the crystal lattice which arise in the course of deceleration of high-energy particles. This method of introducing electrically active defects is similar to the doping of semiconductors by acceptor-type impurities. It has been established that the density of defects increases with the implantation dose till ≈5×10^15 cm−2. The further increase of the implantation dose does not affect the level of doping. In the range of applied doses (10^12–6×10^16) cm−2, the Hall mobility of holes in the formed conducting channels is practically independent of the implanted dose and is about (2-3)×10^4 cm2/Vs at 77 K. The doping ofthe germanium by high-energy ions of H+ or He+ to obtain conducting regions with high hole mobility can be used in the microelectronics technology.

Energy-driven multi-step structural phase transition mechanism to achieve high-quality p-type nitrogen-doped β-Ga2O3 films

It is a big challenge to grow β-Ga2O3 films of good p-type conductivity and stability due to the large acceptor ionization energy, strong hole-trapping effect, low hole mobility, and self-compensation effects. In this study, we demonstrate for the first time the success in the growth of high-quality p-type N-doped β-Ga2O3 films with an acceptor ionization energy of 0.165 eV, Hall resistivity of 17.0 Ωcm, Hall hole mobility of 23.6 cm2V−1s−1, hole concentration of 1.56 × 1016 cm−3 and good stability, and in the fabrication of high-performance p-type β-Ga2O3 films-based field effect transistors. The success paves the way to fabricate full β-Ga2O3 films based p-n diodes and will advance the applications of next-generation high-performance β-Ga2O3-based optoelectronic and electronic devices. In addition, we discover the growth mechanism of multi-step structural phase transitions from hexagonal P63mc GaN to rhombohedral R3¯c α-GaNxO3(1-x)/2 and finally to monolithic C2/m N-doped β-Ga2O3, which improves the crystalline quality, facilitates the acceptor doping, increases the acceptor activation efficiency, and thus enhances the p-type conductivity and stability of the N-doped β-Ga2O3 films. The novel p-type oxide film growth technique and mechanism open a new horizon to the other wide bandgap oxide semiconductors.

Causes of the Difference Between Hall Mobility and Field-Effect Mobility for p-Type RF Sputtered Cu₂O Thin-Film Transistors

The high Hall hole mobility (μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Hall</sub> ) of cuprous oxide (Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O) has caused great interest in using this semiconductor for p-type devices in a future complementary metal-oxide-semiconductor (CMOS) thin-film transistor (TFT) technology. However, in most studies, the field-effect mobility (μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FE</sub> ) achieved was reported to be much lower than μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Hall</sub> . To understand the large gap between μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Hall</sub> and μFE, in this work, we correlate RF sputtered p-type copper oxide (CuO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) material properties with electrical characteristics by varying the film thickness. After vacuum annealing, the different thicknesses cause differences in the oxidation state and phase. Our results show that films with a higher Cu(I) fraction that form in the Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O phase give better TFT performance than CuO films. Nonetheless, the TFTs still exhibit a much lower μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FE</sub> of 0.005-0.1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V-s than the measured μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Hall</sub> value of 12.1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V-s. We explore the reasons for this mobility gap by conducting contact resistance (RC) and interface trap (D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">it</sub> ) analysis. We find that the mobility difference can be minimized by reducing the high contact resistance (~33% of the total resistance in the ON-state) and high density of interface traps (~3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> eV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ).

Underlayer Selection to Improve the Performance of Polycrystalline Ge Thin Film Transistors

We fabricated accumulation-mode metal source/drain p-channel thin film transistors using solid-phase crystallized Ge with different thickness formed on glass substrate. By thinning the Ge layer with the largest grain size (3.7 μm), the channel region got fully-depleted, and the TFT exhibited both high on/off current ratio (~102) and field effect mobility (170 cm2 V-1 s-1). We prepared interfacial layers (GeO2 and Al2O3) between Ge and the substrate to achieve a high Hall hole mobility with a thin film, which is the key for improving TFT characteristics. As a result, inserting Al2O3 underlayer improved the electrical properties of thin Ge (50 nm) layers.

Application of GeTe phase-change films in the directly heated two-port switch design

Phase-change (PC) materials offer the property of thermally induced reversible change and exhibit different resistance values due to the structure changes between crystalline and amorphous phases. In this work, GeTe films with a thickness of 100 nm were deposited on the glass substrates by radio-frequency magnetron sputtering and annealed at different temperatures. The XRD analysis showed diffraction peaks corresponding to the rhombohedral structure of c-GeTe (021), (200), (202), and (220), respectively. Upon annealing at 300 and 340°C, the rhombohedral structure would transform into a rock salt structure. The Raman analysis revealed that a-Ge-Te covalent bond is the mixed structure of tetrahedron and defective octahedron changing into tetrahedral cells. The resistivity declined sharply from 0.72 to 3.8× 10−6 Ω-m, whereas the Hall mobility of holes sharply rose to 300 m2/Vs. Based on the above films, a two-port directly heated switch device was designed and fabricated in sandwich structure to verify its phase-change characteristics. Low-resistance state was converted from crystallization with a 20ns 9V voltage pulse, while high-resistance state was by passing a 2 μs 2V voltage pulse. The on/off resistance ratio of switch could reach five orders of magnitude, while the switch state remained stable after the process termination. The results obtained strongly suggest that GeTe is a promising material for RF switches.

Effects of hydrogen ion implantation dose on physical and electrical properties of Ge-on-insulator layers fabricated by the smart-cut process

We experimentally evaluate the influence of a hydrogen ion implantation (I/I) dose on the physical and electrical properties of Ge-on-insulator (GOI) films fabricated by the smart-cut process with the two doses of 1 × 1017 cm−2 and 4 × 1016 cm−2. It is found that thermal annealing is effective in improving the crystallinity of the GOI layers and that the defect-less GOI layers can be realized under the optimized annealing temperature of 550 °C, irrespective of the I/I dose. However, the reduction of Hall hole mobility is observed in GOI substrates fabricated with higher I/I dose condition. This mobility reduction is not observed for GOI p-channel metal-oxide-semiconductor field-effect transistors (pMOSFETs) under the back-gate operation. On the other hand, n-channel MOSFETs fabricated on the smart-cut GOI substrates with As-doped S/D junctions are found to exhibit the higher effective electron mobility for the low I/I dose than that for the high I/I dose. As a result, it can be concluded that the high H+ I/I dose of 1 × 1017 cm−2 causes the degradation in the mobility of smart-cut GOI substrates and that the choice of the hydrogen I/I dose is important in the fabrication of GOI wafers for MOSFET applications.

Remarkable p-type activation of heavily doped diamond accomplished by boron ion implantation at room temperature and subsequent annealing at relatively low temperatures of 1150 and 1300 °C

Highly efficient impurity doping in diamond by ion implantation has been a crucial issue in the field of semiconductor fabrication for several decades. We investigated the electrical properties of heavily B-doped type IIa diamond introduced by ion implantation at room temperature with a shallow and flat impurity concentration of 3.6 × 1019 cm−3 (∼200 ppm) from the surface to ∼130 nm depth, followed by thermal annealing at 1150 and 1300 °C. The activation of the implanted acceptor B was a maximum of 80% for the sample into which B ions were implanted at room temperature followed by 1150 °C annealing. The hole concentration and Hall mobility at room temperature were realized to be higher than 1 × 1014 cm−3 and 110 cm2 V−1 s−1, respectively. We confirmed p-type conductivity and typical activation energy of acceptor B at wide temperatures from −100 to 800 °C for the prepared samples. It was consequently revealed from this study that at least room temperature B-implantation followed by above 1150 °C annealing is sufficiently effective for the electrical activation of B doped in high quality diamond.

Polycrystalline thin-film transistors fabricated on high-mobility solid-phase-crystallized Ge on glass

Low-temperature formation of Ge thin-film transistors (TFTs) on insulators has been widely investigated to improve the performance of Si large-scale integrated circuits and mobile terminals. Here, we studied the relationship between the electrical properties of polycrystalline Ge and its TFT performance using high-mobility Ge formed on glass using our recently developed solid-phase crystallization technique. The field-effect mobility μFE and on/off currents of the accumulation-mode TFTs directly reflected the Hall hole mobility μHall, hole concentration, and film thickness of Ge. By thinning the 100-nm thick Ge layer with a large grain size (3.7 μm), we achieved a high μHall (190 cm2/Vs) in a 55-nm thick film that was almost thin enough to fully deplete the channel. The TFT using this Ge layer exhibited both high μFE (170 cm2/Vs) and on/off current ratios (∼102). This is the highest μFE among low-temperature (&amp;lt;500 °C) polycrystalline Ge TFTs without minimizing the channel region (&amp;lt;1 μm).

Theoretical analysis of Hall factor and hole mobility in p-type 4H-SiC considering anisotropic valence band structure

The temperature dependencies of hole density and hole mobility of p-type 4H-SiC obtained by Hall effect measurement were theoretically analyzed taking account of its anisotropic valence band structure. The experimental Hall factor, which was derived from the ratio of theoretical hole density to experimental Hall hole density, was reproduced by theoretical Hall factor computed using the valence band structure and relaxation times of scattering mechanisms. The product of the theoretical Hall factor and drift mobility computed by the same transport model agreed well with the experimental Hall mobility. Based on analyses of the results, it was revealed that the temperature dependence of Hall factor can be explained by considering the anisotropic valence band structure along with consideration of anisotropic relaxation times. The contribution of each scattering mechanism was also discussed, and empirical formulas for Hall and drift mobilities are presented.

Electrical properties of pseudo-single-crystalline Ge films grown by Au-induced layer exchange crystallization at 250 °C

We study the electrical properties of pseudo-single-crystalline Ge (PSC-Ge) films grown by a Au-induced layer exchange crystallization method at 250 °C. By inserting the SiNx layer between PSC-Ge and SiO2, we initiatively suppress the influence of the Ge/SiO2 interfacial defective layers, which have been reported in our previous works, on the electrical properties of the PSC-Ge layers. As a result, we can detect the influence of the ionized Au+ donors on the temperature-dependent hole concentration and Hall mobility. To further examine their electrical properties in detail, we also fabricate p-thin-film transistors (TFTs) with the PSC-Ge layer. Although the off-state leakage currents are suppressed by inserting the SiNx layer, the value of on/off ratio remains poor (<102). Even after the post-annealing at 400 °C for the TFTs, the on/off ratio is still poor (∼102) because of the gate-induced drain leakage current although a nominal field effect mobility is enhanced up to ∼25 cm2/V s. Considering these features, we conclude that the Au contaminations into the PSC-Ge layer can affect the electrical properties and device performances despite a low-growth temperature of 250 °C. To achieve further high-performance p-TFTs, we have to suppress the Au contaminations into PSC-Ge during the Au-induced crystallization growth.

Dopant behavior in heavily doped polycrystalline Ge1−xSnx layers prepared with pulsed laser annealing in water

A low-temperature process for the formation of heavily doped polycrystalline Ge (poly-Ge) layers on insulators is required to realize next-generation electronic devices. In this study, we have systematically investigated pulsed laser annealing (PLA) in flowing water for heavily doped amorphous Ge1−xSnx layers (x ≈ 0.02) with various dopants such as B, Al, Ga, In, P, As, and Sb on SiO2. It is found that the dopant density after PLA with a high laser energy is reduced when the oxidized dopant has a lower oxygen chemical potential than H2O. As a result, for the p-type doping of B, Al, Ga, and In, we obtained a high Hall hole density of 5 × 1019 cm−3 for PLA with a low energy. Consequently, the Hall hole mobility is limited to as low as 10 cm2 V−1 s−1. In contrast, for As and Sb doping, because the density of substitutional dopants does not decrease even after PLA with a high energy, we achieved a high Hall electron density of 6 × 1019 cm−3 and a high Hall electron mobility simultaneously. These results indicate that preventing the oxidation of dopant atoms by water is an important factor for achieving heavy doping using PLA in water.

Low-cost growth of magnesium doped gallium nitride thin films by sol-gel spin coating method

Low-cost sol-gel spin coating growth of magnesium (Mg) doped gallium nitride (GaN) thin films with different concentrations of Mg was reported. The effects of the Mg concentration on the structural, surface morphology, elemental compositions, lattice vibrational, and electrical properties of the deposited films were investigated. X-ray diffraction results show that the Mg-doped samples have wurtzite structure with preferred orientation of GaN(002). The crystallite size decreases and the surface of the films with pits/pores were formed, while the crystalline quality of the films degraded as the Mg concentration increases from 2% to 6. %. All the Raman active phonon modes of the wurtzite GaN were observed while a broad peak attributed to the Mg-related lattice vibrational mode was detected at 669 cm-1. Hall effect results show that the resistivity of the thin films decreases while the hole concentration and hall mobility of thin films increases as the concentration of the Mg increases.