Reduction Of Carrier Density Research Articles

Gas Detection with Vertical InAs Nanowire Arrays

Nanowire-based devices show great promise for next generation (bio)chemical sensors as evidenced by the large volume of high-quality publications. Here, a nanoscale gas sensing device is presented, based on gold-free grown vertical InAs nanowire arrays. The nanowires are contacted Ohmically in their as-grown locations using an air bridge construction, leaving the nanowire surface free for gas adsorption. Noise measurements were performed to determine the measurement resolution for gas detection. These devices are sensitive to NO(2) concentrations well below 100 ppb at room temperature. NO(2) exposure leads to both a reduction in carrier density and electron mobility.

Top gated graphene transistors with different gate insulators

AbstractTop gated graphene field effect transistors with AlN and Si3N4 gate insulator materials were fabricated by plasma assisted deposition. Transistors with an AlN top gate insulator showed a reduced mobility and channel carrier density, whereas for the transistors with Si3N4 dielectric an increase of the mobility and a decrease of the channel carrier density was observed. The reduced carrier density can be explained by bond formation between the insulator and the graphene sheets. The different behaviour of the carrier mobility is due to defect passivation by H2 in case of Si3N4 and defect formation in the graphene channel for the AlN gate insulator. A reduced transconductance in top gate device configurations compared to the back gate configurations was observed, which can be caused by the Klein paradox or differences in the trap densities in the top gate insulators. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermoelectric properties of Bi–Sb–Te materials prepared by electric current stressing

Abstract Bi 0.5 Sb 1.5 Te 3 compounds were prepared by cold-press and thermal/electrical sintering process. A high-density electric current (∼100 A/cm 2 ) was introduced through the cold-pressed Bi 0.5 Sb 1.5 Te 3 samples during pressure-free sintering at a temperature ∼200 °C. The electrically sintered Bi 0.5 Sb 1.5 Te 3 shows a 6-fold increase in carrier mobility and 60% reduction in carrier density as compared to the thermally sintered sample. The change in carrier transport properties is suggested to be associated with the elimination of charged defects in Bi 0.5 Sb 1.5 Te 3 crystal during electric current stressing. An electric current induced defect restoration mechanism is proposed to explain the enhanced carrier transport property of the electrically sintered Bi 0.5 Sb 1.5 Te 3 .

Suppressed surface‐recombination structure and surface passivation for improving current gain of 4H‐SiC BJTs

Abstract4H‐SiC bipolar junction transistors (BJTs) are one of the promising candidates for next‐generation power devices. 4H‐SiC BJTs have the advantages of low on‐resistance and high temperature capability. On the other hand, high common emitter current gain is required from a practical use point of view because BJTs are current‐controlled devices. In order to improve the current gain of 4H‐SiC BJTs, we have concentrated on suppressing surface recombination on the SiC surface, which is a critical limiting factor of the current gain and can be suppressed by reducing carrier density and/or trap density. We have proposed novel 4H‐SiC bipolar junction transistors with suppressed surface‐recombination structure: SSR‐BJTs that have the SiC surface with low carrier density, characterized by a lightly doped n‐type layer (LDN layer) and a highly resistive p‐type region (HRP region). In addition, surface passivation suitable for reducing surface traps has been investigated by applying a new characterization method of surface‐recombination current in which the sp · Ls value, a product of a surface‐recombination velocity (sp) and a surface‐diffusion length (Ls), is derived from an analysis of forward I–V characteristics of SiC pn diodes. We have experimentally demonstrated that the sp · Ls value is a practical indicator to evaluate surface passivation. By using the characterization, an effective passivation method with the combination of a pyrogenic oxidation, a post‐oxidation anneal in H2 ambient, and an anneal in NH3 ambient has been developed. SSR‐BJTs with a variety of device structures and process conditions have been fabricated to investigate their characteristics. A fabricated SSR‐BJT showed a recorded maximum current gain of 134 at room temperature with a specific on‐resistance of 3.2 mΩ cm2 and a blocking voltage VCEO of 950 V. The SSR‐BJT kept a current gain of 60 °C at 250 °C with a specific on‐resistance of 8 mΩ cm2. These results have demonstrated the outstanding current‐gain capability of the SSR‐BJTs compared to conventional BJTs. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhancement of Pairing Interaction and Magnetic Fluctuations toward a Band Insulator in an Electron-DopedLixZrNClSuperconductor

The doping dependence of specific heat and magnetic susceptibility has been investigated for Li(x)ZrNCl superconductors derived from a band insulator. As the carrier concentration is decreased, the anisotropy of superconducting gap changes from highly anisotropic to almost isotropic. It was also found that, upon reducing carrier density, the superconducting coupling strength and the magnetic susceptibility are concomitantly enhanced in parallel with T(c), while the density of states at the Fermi level is kept almost constant. Theoretical calculations taking into account the on-site Coulomb interaction reproduced the experimental results, suggesting a possible pairing mediated by magnetic fluctuations, even in the doped band insulators.

Enhancement of carrier transport properties of BixSb2−xTe3 compounds by electrical sintering process

Bi x Sb 2 − x Te 3 (x=0.4 and 0.5) powders were pressed and sintered by passing a high-density electric current under nitrogen ambient. The electrically sintered BixSb2−xTe3 was found to have a marked increase in Hall mobility but a moderate reduction in carrier density. We propose that electric-current induced atomic diffusion plays an important role in modulating crystal defects in BixSb2−xTe3 compounds. The hot-pressed Bi0.4Sb1.6Te3 shows a threefold increase in ZT after electrical sintering process. With the electrical sintering approach we can preserve fine-grain microstructure, and hence low thermal conductivity of BixSb2−xTe3 with reasonable electrical resistivity and Seebeck coefficient.

Luminescence properties of thick InGaN quantum‐wells

AbstractIn this paper, we discuss the physics of recombination in thick InGaN quantum‐well (QW) based structures. Thick InGaN QWs have been suggested as one concept to reduce the typical decrease of internal efficiency of InGaN based light emitters towards high current densities. We show that at typical operation current densities, recombination in such thick QWs mainly originates from excited QW‐states, which exhibit good electron hole overlap and large spatial extent, enabling a reduction of carrier density. We identify these states by comparing current dependent electroluminescence spectra to band structure simulations. The reduction of carrier density is verified by measuring current dependent carrier lifetimes. We find that saturation of efficiency is reduced for increasedQWthickness. However, the same effect can also be achieved using an MQW structure optimized for real MQW emission. We conclude that regardless of the employed concept, a decrease in carrier density is central to improve the high current efficiency of InGaN based light emitters (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optimizing residual carriers in undoped InAs/GaSb superlattices for high operating temperature mid-infrared detectors

The mid-infrared 21 Å InAs/24 Å GaSb superlattices (SLs) designed for the 4 μm cutoff wavelength were grown by molecular beam epitaxy at growth temperatures between 370 and 430 °C in order to reduce residual background carriers. The lowest density of 1.8×10 11 cm −2 was obtained from the SLs grown at 400 °C. With increasing growth temperature, in-plane hole mobility decreased from 8740 to 1400 cm 2/V s due to increased interfacial roughness, while the photoluminescence (PL) intensity increased due to a decrease in the number of nonstoichiometric nonradiative defects. Further reduction of carrier density to 1×10 11 cm −2 was achieved by increasing barrier width. As GaSb layer width increases from 24 to 48 Å, the cutoff wavelength decreased from 4.1 to 3.4 μm, which is still in the mid-infrared detection window. More importantly, a dramatic improvement on the PL intensity and the full width at half maximum was achieved from the SL samples with the wider GaSb widths. All mid-infrared SL samples investigated in our studies were residually p-type.

Degradation Mechanism of AlGaAs/InGaAs Power Pseudomorphic High-Electron-Mobility Transistors under Large-Signal Operation

We have comprehensively investigated the degradation mechanism of output power in AlGaAs/InGaAs pseudomorphic high-electron-mobility transistors (PHEMTs) under large-signal operation. The degradation of output power is caused by the decrease in maximum drain current (Imax) around the knee voltage (Vk) with an increase in drain resistance (Rd). The decrease in Imax originates from a degradation layer containing a significant amount of oxygen formed at the drain recess surface region. This layer causes a reduced carrier density in the drain region underneath, resulting in a decreased Imax and an increased Rd. The decrease in Imax is accelerated with increasing drain voltage, temperature, and humidity. These results suggest that the degradation of output power in the PHEMTs is mainly determined by the growth of a corrosion layer owing to an electrochemical reaction between O2 and/or H2O and the semiconductor surface. We successfully suppress the output power degradation due to this electrochemical corrosion reaction with the help of a special surface treatment prior to the deposition of a passivation film on the recess surface. We demonstrate a highly reliable RF operation of PHEMTs in air even without a hermetic package.

Progress in semiconducting oxide-based thin-film transistors for displays

Recent progress in the development of transparent thin-film transistors for integration with flexible displays is discussed. Specifically, the fabrication and properties of ZnO-based thin-film transistors on glass are described. Top-gate-type thin-film transistors with transparent n-type ZnO as the active channel layer have been fabricated via wet photolithography processing. The ZnO layers were deposited using pulsed laser deposition. A low leakage current of 10−7 A cm−2 was realized with amorphous HfO2 or (Ce, Tb)MgA11O19 as the gate dielectric. N-channel depletion-mode operation was shown for the undoped ZnO thin-film transistors. Phosphorus-doped ZnO and (Zn, Mg)O were also utilized as channel materials in order to realize a reduction in carrier density. The current–voltage measurements demonstrate an enhancement-mode device operation for the thin-film transistors with P-doped (Zn, Mg)O as the active channel layer and HfO2 serving as the gate dielectric.

Enhanced Hall voltage in a gate-controlled InSb Hall device

We present an enhanced Hall voltage from the gate-controlled Hall device incorporating a micron-scaled InSb semiconductor cross junction and a single ferromagnetic element. Magnetic fringe field at an edge of the ferrogmanetic element gives rise to the Hall voltage, which shows hysteretic behavior upon magnetic-field sweep. The Hall effect is amplified by a factor of ∼40% when a gate voltage of −25V is applied. The increase is largely attributed to the reduction of carrier density affected by the gate confinement effect. The InSb Hall device controlled by gate voltage demonstrates a possible application for active nonvolatile memory cells and logic gate.

The effect of element substitution on high-temperature thermoelectric properties of Ca 3Co 2O 6 compounds

Polycrystalline Ca 3Co 1.8 M 0.2O 6 ( M=Mn, Fe, Co, Ni, Cu) and Ca 2.7Na 0.3Co 2O 6 were synthesized by solid-state reaction to evaluate the effect of substitution on the thermoelectric properties of Ca 3Co 2O 6. Substitution by Mn, Cu and Na appears to increase carrier density, given that electrical resistivity ( ρ ) and the Seebeck coefficient ( S) were simultaneously reduced. Conversely, Fe substitution seems to reduce carrier density, resulting in a simultaneous increase in S and ρ . Cu and Na substitution resulted in a significant decrease in ρ due to enhancement of grain size and grain boundary connectivity, which could have a strong impact on ρ . Not only the intrinsic substitution effect on the electronic state but also this modification of the microstructure plays an important role in improvement of the thermoelectric power factor, particularly in the case of the Na-substituted sample.

Effective carrier density in porous silicon carbide

The effective carrier density in porous silicon carbide is measured to be 40 times less than that in the starting material. An idea is proposed which relates the reduction in carrier density to some compensation effect. It is proposed that donor dopants are compensated by deep-level surface-localized, acceptor-like states appearing due to the large internal surface area developed as a result of pore formation in crystalline SiC. The estimated density of trapped electrons per unit internal surface area, 5 × 1012 cm−2, is quite reasonable for the density of deep-level states at the SiC surface.

Dimer buckling of the Si(001)2×1 surface below 10 K observed by low-temperature scanning tunneling microscopy

Using scanning tunneling microscopy (STM), we studied the dimer structure of the $\mathrm{Si}(001)2\ifmmode\times\else\texttimes\fi{}1$ surface at low temperature $(<10\mathrm{}\mathrm{K}).$ Asymmetric (buckled) dimer structure, locally forming $c(4\ifmmode\times\else\texttimes\fi{}2)$ or $p(2\ifmmode\times\else\texttimes\fi{}2)$ periodicity, was observed with positive sample bias voltages, while most of the dimers appear symmetric with negative bias voltages. Our observation indicates that actual dimer structure is asymmetric and that the apparent symmetric dimer observation is due to an artifact induced by STM imaging. Since a transition temperature between the buckled- and symmetric-dimer imaging, which is found to be $\ensuremath{\sim}40\mathrm{K},$ corresponds to the temperature where the carrier density changes dramatically from intrinsic to saturation range, the apparent symmetric-dimer imaging should be related with the reduced carrier density and ensuing charging effect.

Time-Resolved Reflection Spectroscopy of the Spatiotemporal Dynamics of Photo-Excited Carriers in Si and GaAs

We study on the sub-picosecond mid-infrared pump-probe reflection spectroscopy on photo-doped Si and GaAs. We discover experimentally that spectral profile of metallic reflection near the plasma resonance brings us the information on both of carrier density and spatial inhomogeneity. The red-shift of the plasma resonance indicates the reduction of carrier density near the surface, while the broadening of plasma resonance is caused by the steep spatial gradient of carrier density rather than carrier damping. The spectral profile of reflection in Si shows that the carrier diffusion dominates the spatial-temporal evolution of the photo-doped carriers.

Quantum wires in strained silicon quantum wells on tilted substrates

Quantum wires have been fabricated in strained silicon heterostructures grown on 2∘tilted substrates. Magnetoresistance of both wires and Hall bars have been examined at 0.3 K for slow Shubnikov–de Haas (SdH) oscillations that would indicate the formation of minigaps in the band structure of off-axis electron transport. Structures were oriented both parallel and perpendicular to the step edges. The Hall bar structures exhibit no evidence of miniband formation at 0.3 K, and little anisotropy other than slightly reduced carrier density and mobility in the perpendicular Hall bar. The quantum wires are decidedly more modulated by low-frequency oscillations, and exhibit anisotropic behavior.

Atomic layer in situ etching and MBE regrowth

We report on atomic layer precise etching in MBE with AsBr 3 as the etching species. Selective etching of SiO 2 masked GaAs (1 0 0) substrates allows in situ preparation of extremely sharp V-grooves with almost perfect planar {1 1 0} side facets. By MBE regrowth of V-grooves we can prepare buried layers and completely embedded wires. The electrical properties of etched interfaces in modulation doped GaAs/AlGaAs heterostructures is investigated by Hall-effect measurements. The heterostructures with an etching step introduced at the GaAs/AlGaAs interface where the electron gas is located demonstrate that there is no carrier density reduction. The mobility is reduced only at low temperatures to about 40,000 cm 2/V s at 77 K compared to 100,000 cm 2/V s of non-etched reference samples. This result together with the PL line width increase indicate an enhanced interface roughness for etched samples. Atomic layer precise etching does not provide the same surface quality as epitaxial growth. A problem of the in situ etching technique is the insufficient removal of dopants as well as C and O surface contamination as determined by SIMS studies.

On-state and off-state breakdown in GaInAs/InP composite-channel HEMT's with variable GaInAs channel thickness

Short-channel Ga/sub 0.47/In/sub 0.53/As high electron mobility transistors (HEMTs) suffer from low breakdown voltages due to enhanced impact-ionization effects in the narrow bandgap channel. This could limit the application of single-channel devices to medium power millimeter-wave systems. A composite Ga/sub 0.47/In/sub 0.53/As/InP channel, which exploits the high electron mobility of Ga/sub 0.47/In/sub 0.53/As at low electric fields, and the low impact-ionization and high electron saturation velocity of InP at high electric fields can overcome this limitation. In this paper we study on-state and off-state breakdown of Ga/sub 0.47/In/sub 0.53/As/InP composite-channel HEMT's with a variable GaInAs channel thickness of 30, 50, and 100 /spl Aring/. Reduction of channel thickness leads to the improvement of both on-state and off-state breakdown voltages. In on-state conditions, the enhancement in the effective Ga/sub 0.47/In/sub 0.53/As channel bandgap that takes place when the channel thickness is reduced to the order of the de Broglie wavelength (channel quantization) effectively enhances the threshold energy for impact-ionization, which is further reduced by real space transfer of electrons from the Ga/sub 0.47/In/sub 0.53/As into the wider bandgap InP. Channel thickness reduction also causes a decrease in the sheet carrier concentration in the extrinsic gate-drain region and therefore, a reduction of the electric field beneath the gate. This, together with the adoption of an Al/sub 0.6/In/sub 0.4/As Schottky layer (increasing the gate Schottky barrier height), leads to excellent values of the gate-drain breakdown voltage. In conclusion, composite channel InAlAs/GaInAs/InP HEMTs, thanks to the combined effect of effective band-gap increase, enhanced real space transfer into InP, and sheet carrier density reduction, allow a good trade-off between current driving capability and both on-state and off-state breakdown voltage.

Raman and optical spectroscopic studies of small-to-large polaron crossover in the perovskite manganese oxides

We present an optical reflectance and Raman-scattering study of the ${A}_{1\ensuremath{-}x}{A}_{x}^{\ensuremath{'}}{\mathrm{MnO}}_{3}$ system as a function of temperature and doping $(0.2l~xl~0.5).$ The metal-semiconductor transition in the ${A}_{1\ensuremath{-}x}{A}_{x}^{\ensuremath{'}}{\mathrm{MnO}}_{3}$ system is characterized by a change from a diffusive electronic Raman-scattering response in the high-temperature paramagnetic phase, to a flat continuum scattering response in the low-temperature ferromagnetic phase. We interpret this change in the scattering response as a crossover from a small-polaron-dominated regime at high temperatures to a large-polaron-dominated low-temperature regime. Interestingly, we observe evidence for the coexistence of large and small polarons in the low-temperature ferromagnetic phase. We contrast these results with those obtained for ${\mathrm{EuB}}_{6},$ which is a low-${T}_{c}$ magnetic semiconductor with similar properties to the manganites, but with a substantially reduced carrier density and polaron energy.

Characterization of nonradiative traps by hot electron–hole plasma luminescence dynamics in polar semiconductors

Luminescence intensity dependence on excitation intensity has been studied at quasi-steady-state conditions in CdS and CdSe crystals at T=294 K and interpreted, with heating of photoelectrons taken into account. The shape of the dependence indicates a reduction of carrier density due to increased effective temperature. A method for estimating the height of the barrier for centers governing the thermally activated nonradiative capture by multiphonon emission is proposed. The barrier heights of 140 and 170 meV in CdS and CdSe, respectively, have been estimated.