N-type Carrier Concentration Research Articles

Epitaxial Graphene on 4H-SiC(0001) Grown under Nitrogen Flux: Evidence of Low Nitrogen Doping and High Charge Transfer

Nitrogen doping of graphene is of great interest for both fundamental research to explore the effect of dopants on a 2D electrical conductor and applications such as lithium storage, composites, and nanoelectronic devices. Here, we report on the modifications of the electronic properties of epitaxial graphene thanks to the introduction, during the growth, of nitrogen-atom substitution in the carbon honeycomb lattice. High-resolution transmission microscopy and low-energy electron microscopy investigations indicate that the nitrogen-doped graphene is uniform at large scale. The substitution of nitrogen atoms in the graphene planes was confirmed by high-resolution X-ray photoelectron spectroscopy, which reveals several atomic configurations for the nitrogen atoms: graphitic-like, pyridine-like, and pyrrolic-like. Angle-resolved photoemission measurements show that the N-doped graphene exhibits large n-type carrier concentrations of 2.6 × 10(13) cm(-2), about 4 times more than what is found for pristine graphene, grown under similar pressure conditions. Our experiments demonstrate that a small amount of dopants (<1%) can significantly tune the electronic properties of graphene by shifting the Dirac cone about 0.3 eV toward higher binding energies with respect to the π band of pristine graphene, which is a key feature for envisioning applications in nanoelectronics.

Carrier Concentration Effect of Cu-Doped ZnO Films for Room Temperature Ferromagnetism

Recent reports on the observation of room temperature ferromagnetism in Cu doped ZnO have been highly regarded by the scientific community. There are several contradicting reports where some authors have confirmed the occurrence of ferromagnetism in ZnO while others have ruled it out. Even in studies where room temperature ferromagnetism is reported, the effect of carrier type with the ferromagnetic properties is still unclear. To study this problem, the observation of room temperature ferromagnetism in Cu-doped (5%) single crystalline ZnO films grown on Si substrates was observed. ZnO films were prepared by pulsed laser deposition. Hall measurements showed that the Zn0.95Cu0.05O film was n-type with carrier density around 4.3×1017 cm-3. This film exhibits room-temperature ferromagnetism with a saturation magnetization of 0.31 µB/Cu atom. The predominant valence state of the doped Cu atoms is monovalent. When additional electrons were introduced into the films, the ferromagnetism was weaken and vanished. Our results confirm that the p-type nature of the film is not essential for realizing ferromagnetic characteristics, if only the concentration of n-type carriers were not very high.

Development of MBE II–VI Epilayers on GaAs(211)B

Large-area, low-cost substrates are envisioned for next-generation HgCdTe infrared focal-plane arrays (IRFPA). Si, GaAs, Ge, and InSb have been previously examined as potential candidates. Fabrication of IRFPAs based on these substrates is limited by fundamental materials properties that potentially lead to lower detector performance and operability. Lattice and thermal mismatch between the substrate and epilayer are just two of several material factors that must be considered. We have reviewed these factors in the context of more recent data, and determined it worthwhile to revisit the use of GaAs substrates for epitaxial growth of HgCdTe. Our study starts with an evaluation of the surface quality (epireadiness) of commercially available (211) B-oriented GaAs substrates. Molecular beam epitaxial growth of CdTe buffer layers and subsequent HgCdTe absorber layers are performed in separate vacuum-interconnected chambers. The importance of optimization of the CdTe buffer layer growth for high-quality HgCdTe is detailed through surface morphology and x-ray studies. x-Ray diffraction rocking-curve full-width at half-maximum values as low as 52 arcsec have been obtained. Long-wave infrared Hg1−xCdxTe (x = 0.23) has been grown on these buffer layers, producing cross-hatch-dominated surface morphologies, with dislocation densities as low as ∼3 × 106 cm−2. We have also obtained (for optimized layers), 80-K Hall-effect n-type carrier concentration and electron mobility of approximately ~1.5 × 1015 cm−3 and 1 × 105 cm2 V−1 s−1, respectively. Finally, we briefly compare GaAs and Si in light of our preliminary investigation.

High phosphorous doped germanium: Dopant diffusion and modeling

The in situ n-type doping of Ge thin films epitaxial grown on Si substrates is limited to 1 × 1019 cm−3 by the phosphorous out-diffusion during growth at 600 °C. By studying the phosphorous diffusion in Ge with different background doping, we find that the diffusion coefficient is extrinsic and is enhanced 100 times in Ge doped at 1 × 1019 cm−3 compared to intrinsic diffusivity. To achieve higher phosphorous concentration, delta-doped layers are used as a dopant source for phosphorous in-diffusion. We show that the doping level is a result of the competition between in-diffusion and dopant loss. The high diffusivity at high n-type carrier concentration leads to a uniform distribution of phosphorous in Ge with the concentration above 3 × 1019 cm−3.

High carrier mobility in transparent Ba1−xLaxSnO3 crystals with a wide band gap

We discovered that perovskite (Ba,La)SnO3 can have excellent carrier mobility even though its band gap is large. The Hall mobility of Ba0.98La0.02SnO3 crystals with the n-type carrier concentration of ∼8-10 × 1019 cm−3 is found to be ∼103 cm2 V−1 s−1 at room temperature, and the precise measurement of the band gap Δ of a BaSnO3 crystal shows Δ = 4.05 eV, which is significantly larger than those of other transparent conductive oxides. The high mobility with a wide band gap indicates that (Ba,La)SnO3 is a promising candidate for transparent conductor applications and also epitaxial all-perovskite multilayer devices.

Rectifying characteristics of Te-doped GaAs nanowires

We report on the electrical properties of Te-doped GaAs nanowires (NW) grown via the vapor-liquid-solid mechanism. Gold nanoparticles were used as growth catalysts and contacts for electrical measurements using a nanoprobe technique. Semi-log I-V curves show 6-8 decades of forward-voltage linearity giving an ideality factor of 1.25 ± 0.06 and barrier height of 0.78 ± 0.04 eV. When normalized to NW cross-sectional area, all curves overlapped consistent with an n-type carrier concentration (9 ± 1) × 1017 cm−3 and a constant NW resistivity (2.5 ± 0.5) × 10−3 Ω cm. Raman spectroscopy indicated a small surface depletion width of less than 5 nm.

Impedometric anion sensing behaviour of In xGa 1 − xN films grown by modified activated reactive evaporation

In the present work, In x Ga 1 − x N films with different indium compositions ( x = 0.88, 0.63, 0.36 and 0.18) were prepared on glass substrates using a commercially viable technique known as modified activated reactive evaporation. Electrochemical impedance was used to investigate the anion sensing properties of these films for KCl, KI and KNO 3 salt solutions of different molar concentrations. The anion sensing behaviour of InGaN films is attributed to the presence of high n-type background carrier concentration and positively charged surface donor states. The InGaN based anion sensors were found to have good sensitivity with faster response and recovery time. The film with x = 0.63 was found to have the highest sensitivity for all the anions due to the presence of more active surface area together with large number of surface donor states.

Modification of electrical properties of zinc oxide by continuous wave ytterbium fiber laser irradiation

The polycrystalline plate-like ZnO samples were irradiated by a continuous wave Yb fiber laser and electrical properties of modified layer were investigated. The laser beam of spot size of 16 μm in diameter was scanned on the surface at a velocity of 5mm/s. There was a threshold for the laser modification. The laser etched grooves were formed above laser power of 20 W. The laser etched depth increased in relation to the laser power, 0.46 mm at 20 W and 5.0 mm at 126 W. The surface layers of laser etched grooves were modified in color and electrical property. The color changed from light yellow to black, and the electrical resistivity drastically decreased from initial value of 1.1×105 Ωcm to 3.2×10−1 Ωcm at 56 W, 2.8×10−1 Ωcm at 91 W, and 2.0×10−1 Ωcm at 126 W. The Hall measurement showed that the modified surface layer was an n-type semiconductor and carrier concentration of the layer was 1.5×1017 cm−3 at 56 W, 7.2×1017 cm−3 at 91 W, and 1.9×1018 cm−3 at 126 W.

Epitaxial samarium disilicide films on silicon (0 0 1) substrates: growth, structural and electrical properties

In this paper the effect of the growth temperature on the structural and electrical properties of samarium silicide films is investigated. The growth of the epitaxial films is performed under ultrahigh vacuum by reactive-deposition epitaxy on silicon (0 0 1) substrates. The structural properties are assessed by reflection high-energy electron diffraction and x-ray diffractometry. Random and channelling Rutherford backscattering experiments show that the films have the correct stoichiometry, i.e. Sm/Si ratio = 1 : 2, with channelling yields as low as 20% for the best samples. The electrical properties of these films are studied by Hall effect and resistivity measurements. The films have a metallic character, with a high concentration of n-type charge carriers (>1022 cm−3) and a resistivity lower than 200 µΩ cm at room temperature. The metallic character is confirmed by the experimental optical conductivity deduced from ellipsometry experiments. Finally, evidence is presented showing the potential of SmSi2/n-type Si junctions for electronic application with a Schottky barrier height of about 0.32 eV.

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.

Electrical Characterization of ALD ZnO and HfO2 Thin Films

ALD ZnO was deposited at 150oC and HfO2 was deposited at 250oC. The deposition rate for ZnO was calculated to be 2 Aå/cycle and that of HfO2 was found to be 1 Aå/cycle. A 4nm thick film of HfO2 was deposited on 400 nm of ALD ZnO as well as on a reference bulk single crystal ZnO. The Hall measurements revealed an n-type carrier concentration of~1.7 x1019cm-3 and a mobility of 24cm2/V-s for the 400nm as-deposited ALD ZnO. Four-point was used to measure the electrical properties of ALD ZnO.

Optical characterization and determination of carrier density of ultrasonically sprayed CdS:Cu films

In this work, CdS and Cu doped CdS films (at the Cu percentages of 1, 3 and 5) have been deposited onto glass substrates at 350 ± 5 °C by ultrasonic spray pyrolysis technique and their application potential for photovoltaic solar cells have been investigated. Optical properties and thicknesses of the films have been investigated by spectroscopic ellipsometry (SE). Ellipsometric angle ψ was used as the source point for optical characterizations. The optical constants ( n and k) and the thicknesses of the films have been fitted according to Cauchy model. Also, optical properties of the produced films have been analyzed by transmittance and reflectance spectra. Refractive index ( n), extinction coefficient ( k) and reflectance ( R) spectra have been taken by spectroscopic ellipsometer, while transmittance spectra have been taken by UV/vis spectrophotometer. The optical method has been used to determine the band gap type and value of the films. Mott-Schottky (M-S) measurements have been made to determine the conductivity type and carrier concentration of the films. Samples showed n-type conductivity and carrier concentration of undoped CdS sample was found to be 1.19 × 10 19 cm −3. Also, it was concluded that Cu doping has an acceptor effect in CdS samples. From the results of these investigations, the application potential of CdS:Cu films for photovoltaic solar cells as window layer was searched.

Properties of In-Doped ZnO Films Grown by Metalorganic Chemical Vapor Deposition on GaN(0001) Templates

We investigated the properties of indium-doped zinc oxide layers grown by metalorganic chemical vapor deposition on semi-insulating GaN(0001) templates. Specular and transparent films were grown with n-type carrier concentrations up to 1.82 × 1019 cm−3 as determined by Hall measurements, and all In-doped films had carrier concentrations significantly higher than that of a comparable undoped film. For low In flows, the carrier concentration increased accordingly with trimethyl-indium (TMIn) flow until a maximum carrier concentration of 1.82 × 1019 cm−3 was realized. For higher In flows, the carrier concentration decreased with increasing TMIn flow rate. Sheet resistance as low as 185 Ω/sq was achieved for the In-doped films, which is a significant decrease from that of a comparable undoped ZnO film. Our n-type doping studies show that In is an effective dopant for controlling the n-type conductivity of ZnO.

Temperature-dependent Hall and photoluminescence evidence for conduction-band edge shift induced by alloying ZnO with magnesium

This work discusses the effect of conduction-band edge shift induced by alloying ZnO with magnesium. Temperature-dependent Hall and temperature-dependent photoluminescence measurements are used to characterize the epitaxial Zn1−xMgxO thin films grown on (111) Si using intervening epitaxial Lu2O3 buffer layers, which prove that the addition of Mg in ZnO shifts the conduction-band edge to higher energy, thus increasing the activation energy of the defect donor states and reducing the n-type background carrier concentration.

The influence of Sn doping on the growth of In2O3 on Y-stabilized ZrO2(100) by oxygen plasma assisted molecular beam epitaxy

The influence of Sn doping on the growth of In2O3 on Y-stabilized ZrO2(100) by oxygen plasma assisted molecular beam epitaxy has been investigated over a range of substrate temperatures between 650 and 900 °C. The extent of dopant incorporation under a constant Sn flux decreases monotonically with increasing substrate temperature, although the n-type carrier concentration in “overdoped” films grown at 650 °C is lower than in films with a lower Sn concentration grown at 750 °C. The small increase in lattice parameter associated with Sn doping leads to improved matching with the substrate and suppresses breakup of the films into square islands observed in high temperature growth of undoped In2O3 on Y-stabilized ZrO2(100). Plasmon energies derived from infrared reflection spectra of Sn-doped films are found to be close to satellite energies in core level photoemission spectroscopy, but for a nominally undoped reference sample there is evidence for carrier accumulation at the surface. This influences both the In 3d core line shape and the intensity of a peak close to the Fermi energy associated with photoemission from the conduction band.

The low temperature thermoelectric properties of Cu xTiSe 2− yS y

The low temperature thermoelectric properties of TiSe 2, co-doped with Cu and S, are reported. Partial S substitution for Se changes the magnitude of the indirect bandgap, while the Cu-doping independently controls the n-type carrier concentration. The Seebeck coefficients are negative, in the range of −50 to −200 μV K −1, and the resistivities are 0.1–10 mΩ cm. The thermal conductivity for the sample with the largest thermoelectric power factor was found to be relatively low, 3–4 W m −1 K −1, and decreases with decreasing temperature. The thermoelectric efficiencies for the best materials found in this system, typified by Cu 0.02TiSe 1.7S 0.3, were largest at 0.07 at 300 K and decreased to 0.01 at 75 K.

Low-temperature-fabricated InGaZnO4 thin film transistors on polyimide clean-room tape

Amorphous (α-)InGaZnO4 thin film transistors (TFTs) were fabricated on polyimide clean-room tape at low temperature (&amp;lt;100 °C). The α-InGaZnO4 films with an n-type carrier concentration of ∼1016 cm−3 were deposited by rf-magnetron sputtering in a mixed ambient of Ar/O2. The bottom-gate-type TFTs showed good saturation mobility (∼5.3 cm2 V−1 s−1), drain current on-to-off ratio of approximately 105, threshold voltage of 1.1 V, and subthreshold gate-voltage swing of 0.55 V decade−1. These results were comparable to those of the same oxide TFTs that we have fabricated on either glass or polyethylene terephthalate substrates. The results demonstrate that even polyimide clean-room tape can be an appropriate substrate for inexpensive-flexible-adhesive-transparent electronic devices.

High performance indium gallium zinc oxide thin film transistors fabricated on polyethylene terephthalate substrates

High-performance amorphous (α−) InGaZnO-based thin film transistors (TFTs) were fabricated on flexible polyethylene terephthalate substrates coated with indium oxide (In2O3) films. The InGaZnO films were deposited by rf magnetron sputtering with the presence of O2 at room temperature. The n-type carrier concentration of InGaZnO film was ∼2×1017 cm−3. The bottom-gate-type TFTs with SiO2 or SiNx gate dielectric operated in enhancement mode with good electrical characteristics: saturation mobility 11.5 cm2 V−1 s−1 for SiO2 and 12.1 cm2 V−1 s−1 for SiNx gate dielectrics and drain current on-to-off ratio &amp;gt;105. TFTs with SiNx gate dielectric exhibited better performance than those with SiO2. This is attributed to the relatively high dielectric constant (i.e., high-k material) of SiNx. After more than 500 h aging time at room temperature, the saturation mobility of the TFTs with SiO2 gate dielectric was comparable to the as-fabricated value and the threshold voltage shift was 150 mV.

Structural and Electrical Properties of ZnO Thin Films Deposited by Atomic Layer Deposition at Low Temperatures

In this study, ZnO thin films were deposited by atomic layer deposition (ALD) at various process temperatures. The purpose of this paper was to investigate the controllability of the preferred orientations of ZnO thin films by varying the process temperature and to determine the effect of the preferred orientations on the electrical properties of the films. The process temperature was varied from at several increments while the other ALD process parameters were fixed. The deposition rates and uniformities, crystal structures, and electrical properties of these films were evaluated at the various process temperatures. At process temperatures of 70 and , ZnO thin films showed strong (002) preferred orientations with cylindrical, fine, columnar crystal structures, almost a 1:1 stoichiometric chemical ratio of Zn to O, and n-type carrier concentrations in the range of with resistivities of . ZnO thin films deposited at temperatures higher than had wedge-shaped crystal structures, high oxygen deficiencies, and higher n-type carrier concentrations up to than the films deposited at lower temperatures.

Effects of nitrogen vacancies induced by Mn doping in (Ga,Mn)N films grown by MOCVD

We have investigated the effects of nitrogen vacancies (VN) induced by Mn doping on the electronic structure and transport properties of (Ga,Mn)N films grown by metal organic chemical vapour deposition (MOCVD). The significant increase in n-type carrier concentration in the (Ga,Mn)N film is attributed to the additional VN induced by Mn doping. Temperature-dependent Hall data indicate that the additional VN is the dominant scattering mechanism in the (Ga,Mn)N film in higher temperature regions. The Mn L2,3 x-ray absorption spectra of the (Ga,Mn)N film shows a multiplet structure, indicating that the Mn ions are present mainly in the Mn2+(d5) states. These can be attributed to the electrons transferring from VN, which is well consistent with the electrical properties. An energy level model involving the charge transfer is proposed to explain the observed electronic structure and transport properties in the system.