N-type Carrier Density Research Articles

Graphite/h-BN van der Waals heterostructure as a gate stack for HgTe quantum wells

Two-dimensional topological insulators have attracted much interest due to their potential applications in spintronics and quantum computing. To access the exotic physical phenomena, a gate electric field is required to tune the Fermi level into the bulk band gap. Hexagonal boron nitride (h-BN) is a promising alternative gate dielectric due to its unique advantages such as flat and charge-free surface. Here we present a h-BN/graphite van der Waals heterostructure as a top gate on HgTe heterostructure-based Hall bar devices. We compare our results to devices with h-BN/Ti/Au and HfO2/Ti/Au gates. Devices with a h-BN/graphite gate show no charge carrier density shift compared to as-grown structures, in contrast to a significant n-type carrier density increase for HfO2/Ti/Au. We attribute this observation mainly to the comparable work function of HgTe and graphite. In addition, devices with h-BN gate dielectric show slightly higher electron mobility compared to HfO2-based devices. Our results demonstrate the compatibility between layered materials transfer and wet-etched structures and provide a strategy to solve the issue of significant shifts of the carrier density in gated HgTe heterostructures.

Thermodynamics and robust n-type charge carrier density in Co-doped FeTe2: strain strategy

Herein, we investigated the combined effect of Co-doping and strain (biaxial [110] and hydrostatic [111]) on the thermodynamics and electronic structure of the FeTe2 motif using ab-initio calculations by considering the strong correlation effects. The pristine one has a non-magnetic semiconducting nature with an indirect band gap (E g ) of 0.384 eV. Interestingly, our results revealed that the Co-doping at the Fe site induced an n-type conductivity (i.e. few states are crossing the Fermi level from the valence to conduction band) in the system having a substantial charge carrier density magnitude of 0.14 × 1021 cm−3. The metallicity mainly comprises the Co-3d orbitals along with a significant contribution from Fe-3d states. Thermodynamic, mechanical, and dynamical stability of the Co-doped FeTe2 structure is confirmed by computing the formation energetic, elastic constants, and phonon band structure, respectively. Generally, an increasing and decreasing trend in E g value is evident against the applied compressive and tensile strains having ranged from −5% to +5% for the case of the undoped system, respectively. On the other hand, the Co-doped structure maintained its n-type conduction against considered both types of strains. Moreover, it is demonstrated that compressive strains strengthen the charge carrier density amplitude, while tensile strains show a negative impact. Hence, the present work displays that robust n-type conductivity and stable structure of Co-doped FeTe2 system, makes it a desirable candidate for device applications.

Effect of Threading Dislocations on the Electronic Structure of La-Doped BaSnO3 Thin Films.

In spite of great application potential as transparent n-type oxides with high electrical mobility at room temperature, threading dislocations (TDs) often found in the (Ba,La)SnO3 (BLSO) films can limit their intrinsic properties so that their role in the physical properties of BLSO films need to be properly understood. The electrical properties and electronic structure of BLSO films grown on SrTiO3 (001) (STO) and BaSnO3 (001) (BSO) substrates are comparatively studied to investigate the effect of the TDs. In the BLSO/STO films with TD density of ~1.32 × 1011 cm−2, n-type carrier density ne and electron mobility are significantly reduced, as compared with the BLSO/BSO films with nearly no TDs. This indicates that TDs play the role of scattering-centers as well as acceptor-centers to reduce n-type carriers. Moreover, in the BLSO/STO films, both binding energies of an Sn 3d core level and a valence band maximum are reduced, being qualitatively consistent with the Fermi level shift with the reduced n-type carriers. However, the reduced binding energies of the Sn 3d core level and the valence band maximum are clearly different as 0.39 and 0.19 eV, respectively, suggesting that the band gap renormalization preexisting in proportion to ne is further suppressed to restore the band gap in the BLSO/STO films with the TDs.

Deformation potential extraction and computationally efficient mobility calculations in silicon from first principles

We present a first-principles framework to extract deformation potentials in Silicon based on density-functional theory (DFT) and density-functional perturbation theory (DFPT). We compute the electronic band structures, phonon dispersion relations, and electron-phonon matrix elements to extract deformation potentials for acoustic and optical phonons for all possible processes. The matrix elements clearly show the separation between intra- and inter-valley scattering in the conduction band, and quantify the strength of the scattering events in the degenerate bands of the valence band. We then use an advanced numerical Boltzmann transport equation (BTE) simulator that couples DFT electronic structures and energy/momentum-dependent scattering rates to compute the transport properties for electrons and holes. By incorporating ionized impurity scattering as well, we calculate the n-type and p-type mobility versus carrier density and make comparisons to experiments, indicating excellent agreement. The fact that the method we present uses well-established theoretical tools and requires the extraction of only a limited number of matrix elements, makes it generally computationally very attractive, especially for semiconductors with a large unit cell and lower symmetry.

Quantitative Assessment of Carrier Density by Cathodoluminescence. I. GaAs Thin Films and Modeling

Doping is a fundamental property of semiconductors and constitutes the basis of modern microelectronic and optoelectronic devices. Their miniaturization requires contactless characterization of doping with nanometer scale resolution. Here, we use low- and room-temperature cathodoluminescence (CL) measurements to analyze p-type and n-type GaAs thin films over a wide range of carrier densities ($2\times 10^{17}$ to $1\times 10^{19}$ cm$^{-3}$). The spectral shift and broadening of CL spectra induced by shallow dopant states and band filling are the signature doping. We fit the whole spectral lineshapes with the generalized Planck's law and refined absorption models to extract the bandgap narrowing (BGN) and the band tail for both doping types, and the electron Fermi level for n doping. This work provides a rigorous method for the quantitative assessment of p-type and n-type carrier density using CL. Taking advantage of the high spatial resolution of CL, it can be used to map the doping in GaAs nanostructures, and it could be extended to other semiconductor materials.

Spectroscopic signatures of native charge compensation in Mg doped GaN nanorods

We study the native charge compensation effect in Mg doped GaN nanorods (NRs), grown by Plasma Assisted Molecular Beam Epitaxy (PAMBE), using Raman, photoluminescence (PL) and x-ray photoelectron spectroscopies (XPS). The XPS valence band analysis shows that upon Mg incorporation the EF − EVBM reduces, suggesting the compensation of the native n-type character of GaN NRs. Raman spectroscopic studies of these samples reveal that the line shape of the longitudinal phonon plasmon (LPP) coupled mode is sensitive to Mg concentration and hence to the background n-type carrier density. We estimate a two order of native charge compensation in GaN NRs upon Mg-doping with a concentration of 1019–1020 atoms cm−3. The atomistic origin of this compensation effect in these GaN nanorod samples is discussed.

Halide vapor phase epitaxy of Si doped β-Ga2O3 and its electrical properties

Silicon doped homoepitaxial films were grown on beta‑gallium oxide (001) substrates by halide vapor phase epitaxy using gallium monochloride, oxygen and silicon tetrachloride gases as precursors. It was confirmed that the n-type carrier density at room temperature was almost equal to the doped silicon concentration, which was controlled in the range of 1015 to 1018 cm−3. In the doped film with the carrier density of 1 × 1016 cm−3, the activation energy and the mobility at room temperature were 45.6 meV and 145 cm2/V⋅s, respectively. The carrier scattering mechanism in the low carrier density film was dominated by optical phonon scattering with the phonon energy of 33 meV. These results suggest that the doped homoepitaxial film grown by halide vapor phase epitaxy is a high quality film with good crystallinity comparable to bulk crystals.

Thermoelectric properties and chemical potential tuning by Cu-doping in n-type ionic conductors CuxAg2−xSe0.5Te0.5

Copper and silver chalcogenides with superionic conduction behavior have shown impressively high ZT values, but there has been no intensive effort to optimize their carrier density to further improve their ZT values. Here, we prepared polycrystalline CuxAg2−xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) samples using high temperature melting followed by hot-press sintering, and characterized their thermoelectric properties. We demonstrated that Cu substitution for Ag was achieved with <10% Cu content for CuxAg2−xSe0.5Te0.5 and the Cu doping was quite effective and significantly enhanced the compound's n-type carrier density, which was one order of magnitude higher than the pristine Ag2Se0.5Te0.5 (4.10 × 1018 cm−3). Impressively, the enhancement in electrical conductivity with increasing Cu content was greater than the decrease in absolute value of the Seebeck coefficient in the superionic conduction state. This led to relatively high power factors for Cu0.1Ag1.99Se0.5Te0.5, ranging between 1.10 and 1.30 mW m−1 K−2 over the broad temperature range of 400–560 K, and resulted in the highest ZT of 0.85 at 560 K. Furthermore, ZT values approached >0.7 over a wide temperature range of 460–560 K for x > 0.05. We suggest that the unusual Cu doping effect in Ag2Se0.5Te0.5 can be attributed to the creation of Cu ion conduction in addition to Ag ion conduction, and the optimization of the compound's n-type carrier density.

Structural, optical and magnetic investigation of Gd implanted CeO2 nanocrystals

Gadolinium implanted cerium oxide (Gd-CeO2) nanocomposites is an important candidate which have unique hexagonal structure and high K- dielectric constant. Gd-CeO2 nanoparticles were synthesized using hydrothermal method. X-ray diffraction (XRD) results showed that the peaks are consistent with pure phase cubic structure the XRD pattern also confirmed crystallinity and phase purity of the sample. Nanocrystals sizes were found to be up to 25nm as revealed by XRD and SEM. It is suggested that Gd gives an affirmative effect on the ion influence behavior of Gd-CeO2. XRD patterns showed formation of new phases and SEM micrographs revealed hexagonal structure. Photoluminescence measurement (PL) reveals the systematic shift of the emission band towards lower wavelength thereby ascertaining the quantum confinement effect (QCE). The PL spectrum has wider broad peak ranging from 390nm to 770nm and a sharp one centered on at 451.30nm which is in tune with Gd ions. In the Raman spectra showed intense band observed between 460cm−1 and 470cm−1 which is attributed to oxygen ions into CeO2. Room temperature ferromagnetism was observed in un-doped and Gd implanted and annealed CeO2 nanocrystals. In the recent studies, ceria based materials have been considered as one of the most promising electrolytes for reduced temperature SOFC (solid oxide fuel cell) system due to their high ionic conductivities allowing its use in stainless steel supported fuel cells. CeO2 having an optical bandgap 3.3eV and n-type carrier density which make it a promising candidate for various technological application such as buffer layer on silicon on insulator devices.

Effective Doping of Monolayer Phosphorene by Surface Adsorption of Atoms for Electronic and Spintronic Applications

ABSTRACTWe study the effect of surface adsorption of 27 different adatoms on the electronic and magnetic properties of monolayer black phosphorus using density functional theory. Choosing a few representative elements from each group, ranging from alkali metals (group I) to halogens (group VII), we calculate the band structure, density of states, magnetic moment and effective mass for the energetically most stable location of the adatom on monolayer phosphorene. We predict that group I metals (Li, Na, K), and group III adatoms (Al, Ga, In) are effective in enhancing the n-type mobile carrier density, with group III adatoms resulting in lower effective mass of the electrons, and thus higher mobilities. Furthermore, we find that the adatoms of transition metals Ti and Fe produce a finite magnetic moment (1.87 and 2.31 μB) in monolayer phosphorene, with different band gap and electronic effective masses (and thus mobilities), which approximately differ by a factor of 10 for spin-up and spin-down electrons, opening up the possibility for exploring spintronic applications.

Doping Strategies for Monolayer MoS2 via Surface Adsorption: A Systematic Study

Using density functional theory calculations, we have systematically explored the effect of surface adsorption of different atoms on the electronic properties of monolayer molybdenum disulfide (MoS2). We have chosen a few representative members from each group in the periodic table, ranging from alkali metals (group I) to halogens (group VII), and calculated the electronic band structure of the adatom–MoS2 system for the most energetically stable location of the adatom adsorbed on MoS2. The calculated value of charge transfer from the adsorbed adatom to MoS2 and resultant shifting of the Fermi level to the conduction band suggest that the group I (Li, Na, K) and group II metals (Mg, Ca) are the most effective adatoms to enhance the n-type mobile carrier density in MoS2. Our calculation is in good agreement with the experimental observation for K [Nano Lett. 2013, 13, 1991].

Optoelectronic properties of R-F magnetron sputtered Cadmium Tin Oxide (Cd2SnO4) thin films for CdS/CdTe thin film solar cell applications

The influence of substrate temperature on the microstructural behavior, optical, electrical properties and on the scattering mechanism of charge carriers were studied for the as-prepared radio-frequency (R-F) magnetron sputtered Cadmium Tin Oxide (Cd2SnO4) thin films. Films prepared at the substrate temperature of 300°C were found to be polycrystalline in nature with preferential orientation along (311) plane. Well pronounced Moss–Burstein shift, in the transmittance spectra with dispersions in the optical band gap from 3.07 to 3.30eV, was observed at substrate temperatures between 25 and 300°C. Optical property of high visible transmittance was retained by the films. Analysis of the electrical properties on the prepared crystalline Cd2SnO4 films showed a calculated resistivity of 10−3–10−4Ωcm, with n-type carrier density in the range of 1019–1020cm−3 and the charge carrier mobility in the range of 63–30cm2/Vs. The effects of structural, compositional and optical properties on the scattering mechanism of charge carrier are elaborated and reported to be an experimental evidence for the theoretical predictions. The results revealed the essential DC electrical conduction behavior, which is ideal for the fabrication of Cd2SnO4-based CdS/CdTe thin film solar cells.

High-performance few-layer Mo-doped ReSe₂ nanosheet photodetectors.

Transition metal dichalcogenides (TMDCs) have recently been the focus of extensive research activity owing to their fascinating physical properties. As a new member of TMDCs, Mo doped ReSe2 (Mo:ReSe2) is an octahedral structure semiconductor being optically biaxial and highly anisotropic, different from most of hexagonal layered TMDCs with optically uniaxial and relatively high crystal symmetry. We investigated the effects of physisorption of gas molecule on the few-layer Mo:ReSe2 nanosheet based photodetectors. We compared the photoresponse of the as-exfoliated device with annealed device both in air or ammonia (NH3) environment. After annealing at sub-decomposition temperatures, the Mo:ReSe2 photodetectors show a better photoresponsivity (~55.5 A/W) and higher EQE (10893%) in NH3 than in air. By theoretical investigation, we conclude that the physisorption of NH3 molecule on Mo:ReSe2 monolayer can cause the charge transfer between NH3 molecule and Mo:ReSe2 monolayer, increasing the n-type carrier density of Mo:ReSe2 monolayer. The prompt photoswitching, high photoresponsivity and different sensitivity to surrounding environment from the few-layer anisotropic Mo:ReSe2 can be used to design multifunctional optoelectronic and sensing devices.

Erratum to: Controlling n-Type Carrier Density from Er Doping of InGaAs with MBE Growth Temperature

Erratum to: Controlling n-Type Carrier Density from Er Doping of InGaAs with MBE Growth Temperature

Ann-Type Transparent Conducting Oxide: Nb12O29

A Nb12O29 film is found to show transparent conducting properties with lowest resistivity of 3.3 × 10–3 Ω·cm at room temperature. High transmittance (∼70% at wavelength of 435 nm and ∼50% at wavelength of 730 nm, 120 nm in thickness) in the visible is maintained with an optical band gap of ∼4.0 eV. The optical dielectric constant and effective mass were both larger than those for conventional transparent conducting oxides, leading Nb12O29 to exhibit high transparency. The n-type carrier density and Hall mobility at room temperature were 1.8 × 1021 cm–3 and 0.8 cm2 V–1 s–1, respectively. The origin of the charge carriers lies in the mixed-valent character of Nb, as revealed by photoemission measurements. This intrinsically doped d-electron-based transparent conducting oxide shows unique characteristics such as high refractive index and high-temperature stability, that is, maintaining high conductivity after annealing at 1000 °C in vacuum.

Hydrogen-related n-type conductivity in hydrothermally grown epitaxial ZnO films

Epitaxial ZnO films were grown on single crystal spinel substrates at 90 °C in water. Hall measurements showed that the n-type carrier concentration and conductivity of the ZnO films decreased as postgrowth annealing temperature increases or pH of the growth solution decreases. At the same time, x-ray photoelectron spectroscopy, room temperature and low-temperature photoluminescence, and secondary ion mass spectroscopy observations revealed that H incorporation was enhanced at a higher pH value but reduced with increasing annealing temperature. All these observations show that the unintentionally-incorporated hydrogen in solution grown ZnO acts as a shallow donor and enhances the n-type carrier density.

Transparent p-AgCoO 2/n-ZnO diode heterojunction fabricated by pulsed laser deposition

Transparent diode heterojunction on ITO coated glass substrates was fabricated using p-type AgCoO 2 and n-type ZnO films by pulsed laser deposition (PLD). The PLD of AgCoO 2 thin films was carried out using the pelletized sintered target of AgCoO 2 powder, which was synthesized in-house by the hydrothermal process. The band gap of these thin films was found to be ∼ 3.89 eV and they had transmission of ∼ 55% in the visible spectral region. Although Hall measurements could only indicate mixed carrier type conduction but thermoelectric power measurements of Seebeck coefficient confirmed the p-type conductivity of the grown AgCoO 2 films. The PLD grown ZnO films showed a band gap of ∼ 3.28 eV, an average optical transmission of ∼ 85% and n-type carrier density of ∼ 4.6 × 10 19 cm − 3. The junction between p-AgCoO 2 and n-ZnO was found to be rectifying. The ratio of forward current to the reverse current was about 7 at 1.5 V. The diode ideality factor was much greater than 2.

Quantum Efficiency Measurements to Deduce Non-Ideal Solar-Cell Features

AbstractAppropriate interpretation of quantum-efficiency (QE) measurements made on non-ideal solar cells often reveal subtle features of the photodiodes. QE measurements on CIGS and CdTe thin-film solar cells with CdS buffer layers demonstrate some of these features, and in each case we identify the electrical processes responsible. One well-known complication in cells with CdS buffer layers is “photodoping”, where blue light exposure increases the n-type CdS carrier density. The resulting modification of the p-n junction alters the carrier collection for all wavelengths and can lead to misinterpretation of results. The use of a white, DC-bias light during QE measurements generally solves this problem, but it has not been clear what bias light level is sufficient for accurate results. Measurements with varying intensities of DC-bias light show that complications due to CdS photoconductivity are generally mitigated near 0.05 sun intensity. Other factors which can cause misinterpretation of QE measurements include the presence of secondary barriers, and photoconductivity in the absorber layer. Numerical simulations of band profiles under various light-bias conditions are consistent with the experimental data. The analysis is used to suggest a standard set of measurement conditions for reliable QE analysis.

Carrier induced ferromagnetism in Nb doped Co:TiO2 and Fe:TiO2 epitaxial thin film

We have investigated the carrier induced magnetic properties of anatase Ti1−x−yNbxMyO2 (M=Co,Fe) epitaxial films grown by the pulsed laser deposition technique. For Ti0.95−xNbxCo0.05O2, the n-type carrier density could be controlled in a wide range (4.9×1017cm−3to2.7×1021cm−3) by Nb doping (x=0–0.2). The temperature dependence of the resistivity showed metallic behavior, suggesting that Ti0.95−xNbxCo0.05O2 undergoes a semiconductor to metal transition along with a slight carrier doping less than x&amp;lt;0.03. In both Co-doped and Fe-doped films, we have confirmed hysteresis in M-H curves, and the anomalous Hall effect at room temperature. This strongly suggests that the charge carriers are spin polarized and mediate ferromagnetic interaction between local spins on transition metal ions. In the case of Ti0.94−xNbxFe0.06O2, ferromagnetism is sensitive to carrier concentration. That is, the x=0.002 film is nonmagnetic even at 3K, while room-temperature ferromagnetism appears at x=0.01.

Thin-film field-effect transistors based on La-doped SrTiO3 heterostructures

Field-effect transistors have been fabricated from epitaxial perovskite strontium titanate heterostructures. Lanthanum-doped SrTiO3 was used as the semiconducting channel while insulating SrTiO3 was used as a gate insulator. Both depletion and accumulation effects in the channel have been studied from 300 to 4 K. Hall effect measurements indicate a temperature independent n-type carrier density around 5×1019 cm−3. At 300 K, typical mobilities are 2–3 cm2 V−1 s−1 while low temperature mobilities are as high as 15 cm2 V−1 s−1.