Number Of Free Carriers Research Articles

Influence of Generated Defects by Ar Implantation on the Thermoelectric Properties of ScN.

Nowadays, making thermoelectric materials more efficient in energy conversion is still a challenge. In this work, to reduce the thermal conductivity and thus improve the overall thermoelectric performances, point and extended defects were generated in epitaxial 111-ScN thin films by implantation using argon ions. The films were investigated by structural, optical, electrical, and thermoelectric characterization methods. The results demonstrated that argon implantation leads to the formation of stable defects (up to 750 K operating temperature). These were identified as interstitial-type defect clusters and argon vacancy complexes. The insertion of these specific defects induces acceptor-type deep levels in the band gap, yielding a reduction in the free-carrier mobility. With a reduced electrical conductivity, the irradiated sample exhibited a higher Seebeck coefficient while maintaining the power factor of the film. The thermal conductivity is strongly reduced from 12 to 3 W·m–1·K–1 at 300 K, showing the influence of defects in increasing phonon scattering. Subsequent high-temperature annealing at 1573 K leads to the progressive evolution of these defects: the initial clusters of interstitials evolved to the benefit of smaller clusters and the formation of bubbles. Thus, the number of free carriers, the resistivity, and the Seebeck coefficient are almost restored but the mobility of the carriers remains low and a 30% drop in thermal conductivity is still effective (ktotal ∼ 8.5 W·m–1·K–1). This study shows that control defect engineering with defects introduced by irradiation using noble gases in a thermoelectric coating can be an attractive method to enhance the figure of merit of thermoelectric materials.

Hydrogen Behavior at Crystalline/Amorphous Interface of Transparent Oxide Semiconductor and Its Effects on Carrier Transport and Crystallization.

The role of disorder and particularly of the interfacial region between crystalline and amorphous phases of indium oxide in the formation of hydrogen defects with covalent (In-OH) or ionic (In-H-In) bonding are investigated using ab initio molecular dynamics and hybrid density-functional approaches. The results reveal that disorder stabilizes In-H-In defects even in the stoichiometric amorphous oxide and also promotes the formation of deep electron traps adjacent to In-OH defects. Furthermore, below-room-temperature fluctuations help switch interfacial In-H-In into In-OH, creating a new deep state in the process. This H-defect transformation limits not only the number of free carriers but also the grain size, as observed experimentally in heavily H-doped sputtered In2Ox. On the other hand, the presence of In-OH helps break O2 defects, abundant in the disordered indium oxide, and thus contributes to faster crystallization rates. The divergent electronic properties of the ionic vs covalent H defects─passivation of undercoordinated In atoms vs creation of new deep electron traps, respectively─and the different behavior of the two types of H defects during crystallization suggest that the resulting macroscopic properties of H-doped indium oxide are governed by the relative concentrations of the In-H-In and In-OH defects. The microscopic understanding of the H defect formation and properties developed in this work serves as a foundation for future research efforts to find ways to control H species during deposition.

Millimeter wave difference-frequency mixing by mobile carriers in gated monolayer graphene with broken centrosymmetry

Motivated by the device application potential of microwave difference-frequency generation (DFG) in graphene, we develop a theory of this second-order non-linear phenomenon in individual graphene layers lacking center-of-inversion symmetry due to their interaction with a substrate such as SiC or h-BN. Using the Boltzmann equation techniques, we calculate the microwave DFG conductivity of the graphene layer where the number of free carriers is controlled by an external gate voltage. The results obtained shed light on how the doping level of graphene, as well as a substrate-induced bandgap in the graphene’s electronic spectrum, affects the efficiency of the graphene-based frequency downconversion from the millimeter wave band to the microwave one. In particular, it is shown that there are some possibilities to maximize the output power of the microwave DFG by selecting graphene/substrate pairs with smaller substrate-induced bandgap values and by tuning, in the proper way, the Fermi energy of charge carriers via electrostatic gating. The gate controllable DFG enhancement can be achieved in gapped and doped graphene samples at the electron concentration level of the order of cm−2, thus paving the way to electrically tunable frequency downconvertors for millimeter wave applications in high data rate communication systems and signal processing.

Preparation and characterization of optical and electrical properties of copper selenide sulfide polycrystalline thin films

The structural, optical and electrical properties of Cu1.8Se1-xSx (0.25 ≤ x ≤ 0.75) polycrystalline thin films deposited at room temperature by vacuum evaporation were studied as a function of composition. Energy dispersive X -ray analysis was used to determine the elemental composition of different films. Results indicate the incorporation of sulfur at the expense of selenium. X-ray diffraction analysis reveal that the (220) diffraction peak of the Cu1.8Se1-xSx films shifts to a higher 2θ with an increase in the sulfur content. Atomic force microscopy images of the deposited films show dense and well-defined grains. Analysis of the absorption coefficient data show the existence of two optical transition mechanisms - direct transitions with Eg-dir ranging from 2.2 to 2.5 eV, and indirect transitions with Eg-indir ranging from 1.23 to 1.52 eV as the sulfur content was increased from 0.25 to 0.75, respectively. The Hall coefficient, Hall mobility, and electrical conductivity were measured at room temperature for different film compositions. All investigated films are highly degenerated p-type semiconductors with the number of free carriers up to 1020 cm−3. Combining the results of the optical and electrical measurements, the effective mass for the highly degenerate compositions is found to be 1.718 to 0.587 as the sulfur content varies from 0.25 to 0.75.

Anisotropic Electric Field Effect on the Photoluminescence of CH3NH3PbI3 Perovskite Sandwiched between Conducting and Insulating Films

Photoluminescence (PL) of a nanocrystalline film of methylammonium lead iodide perovskite (MAPbI3) sandwiched between an electrode of a fluorine-doped tin oxide (FTO) layer and an insulating film of poly(methyl methacrylate) is found to increase and decrease significantly with the application of an external electric field (Fext), depending on the direction of the applied field, based on the measurements of electrophotoluminescence (E-PL) spectra, i.e., field-induced change in PL spectra. The field-induced change in PL intensity is confirmed to originate from the field-induced change in the number of free carriers which induce radiative recombination, based on temporally resolved E-PL measurements. We propose that an internal field (Fint) exists even without application of Fext. The anisotropic behavior of the effect of Fext on PL is interpreted in terms of a synergy effect of Fint and Fext; both fields are additive with the applied field direction from Ag to FTO electrode (positive direction) or subtracti...

Size‐Controlled Synthesis of Cu2‐xE (E = S, Se) Nanocrystals with Strong Tunable Near‐Infrared Localized Surface Plasmon Resonance and High Conductivity in Thin Films

AbstractA facile method for preparing highly self‐doped Cu2‐xE (E = S, Se) nanocrystals (NCs) with controlled size in the range of 2.8–13.5 nm and 7.2–16.5 nm, for Cu2‐xS and Cu2‐xSe, respectively, is demonstrated. Strong near‐infrared localized surface plasmon resonance absorption is observed in the NCs, indicating that the as‐prepared particles are heavily p‐doped. The NIR plasmonic absorption is tuned by varying the amount of oleic acid used in synthesis. This effect is attributed to a reduction in the number of free carriers through surface interaction of the deprotonated carboxyl functional group of oleic acid with the NCs. This approach provides a new pathway to control both the size and the cationic deficiency of Cu2‐xSe and Cu2‐xS NCs. The high electrical conductivity exhibited by these NPs in metal‐semiconductor‐metal thin film devices shows promise for applications in printable field‐effect transistors and microelectronic devices.

Effect of Ga substitution on the optical properties of La-Sr manganites

In a metallic manganite, the substitution of ${\mathrm{Mn}}^{+3}$ by ${\mathrm{Ga}}^{+3}$ locally destroys both the ferromagnetic order and the Jahn--Teller distortion, without heavily affecting the crystal structure. One can thus observe an unusual metal-to-insulator transition, which is induced neither by the temperature nor by changes in the divalent-ion doping. This transition is studied here by comparing the infrared reflectivity of five samples of ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}{\mathrm{O}}_{3}$, with $x$ increasing from 0 to 0.30. The dramatic effect of Ga on the transport properties is monitored through the decrease in the number of free carriers and the increase in their effective mass, until a good metal such as ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_{3}$ is turned into an insulator at all temperatures. A simple model which links the measured optical parameters to the magnetization $M(x,T)$ well describes the behavior of the plasma frequency, the scattering rate, and the midinfrared absorption, throughout the Ga-induced metal-to-insulator transition.

Fourier transform infrared spectroscopy and scanning tunneling spectroscopy of porous silicon in the presence of methanol

Abstract Porous silicon samples were obtained from p+- and n-type silicon wafers. Gas sensing measurements showed that the electrical conductivity of porous Si on p+- and n-type wafers increases strongly and decreases weakly in the presence of methanol gas, respectively. Scanning tunneling spectroscopy (STS) indicates that the adsorption of methanol on the surface of n-porous silicon decreases the average density of states especially in the band gap. Fourier transform infrared (FTIR) spectroscopy reveals that after methanol exposure partial surface oxidation occurs which produces electron traps as well as methanol adsorption on the porous surfaces. These observations imply that the number of free-carriers in p+-type (n-type) porous silicon increases (decreases) in methanol environment. These results explain the change in electrical conductivity of porous Si in the presence of methanol.

Influence of He-ion irradiation on the characteristics of Pd/n-Si0.90Ge0.10/Si Schottky contacts

Current–voltage (I–V) and capacitance–voltage (C–V) characteristics of He-ion irradiated Pd/n-Si09Ge0.10 Schottky contacts have been measured in the temperature range from 100 to 300 K. Schottky barrier properties such as the Schottky barrier height (Φbn) and ideality factor (n) have been studied as a function of temperature. The degree to which their characteristics deviated from the ideal case increased as the temperature decreased. A decrease in Φbn and an increase in n with decreasing temperature are observed. Additionally, linear dependence between the so-called temperature factor T0 and temperature as well as between Φbn and n are shown. This type of strong temperature dependence indicates the presence of a large degree of lateral inhomogeneities of the barrier height, resulting from the He-ion irradiation induced defects and traps which produce a variation in the number of free carriers. The presence of electrically active defects introduced by He-ion irradiation at and below the Si0.90Ge0.10 surface support this interpretation.

Scanning tunneling spectroscopy of porous silicon in presence of methanol

In this research, we used the scanning tunneling spectroscopy (STS) technique to probe the local electrical properties of the surface of meso-porous silicon and its substrate, including local density of states (DOS) in air and in methanol environment to increase our knowledge of sensing phenomena. Meso-porous silicon was prepared on p +-type Si which has high sensitivity toward methanol. Observations revealed that while the surface electrical properties of p +-type Si have not sensible change toward methanol, average local density of state of the porous layer increases after the exposure to methanol especially in the E < E F region. Moreover, large number of surface states is produced in band gap which implies that the number of free-carriers in porous silicon increases in methanol environment. This phenomenon explains the change in electrical resistivity of meso-porous Si in the presence of methanol which is the base of organic gas sensitivity of this structure.

Production of native donors in ZnO by annealing at high temperature in Zn vapor

Zinc oxide crystals grown by the seeded chemical vapor transport method have been annealed in zinc vapor at 1100 °C for 30 min. These thermochemical reduction treatments produce a deep red coloration in the crystals and increase their n-type electrical conductivity. Electron paramagnetic resonance (EPR), optical absorption, and Hall measurements were used to monitor changes in the crystals. After an anneal, an intense optical absorption band is present that extends from the band edge out to approximately 550 nm, and the EPR signal near g=1.96 (due to shallow donors and/or conduction-band electrons), the free-carrier absorption, and the Hall electron concentration are all larger. Hydrogen was not present during these anneals, thus leaving oxygen vacancies and zinc interstitials as candidates for the added donors. Neutral oxygen vacancies are produced at high temperature by the additive-coloration mechanism, and are responsible for the broad near-edge absorption band. The observed increase in the number of free carriers is a result of either (1) the formation of zinc interstitials or (2) having the ground state of the neutral oxygen vacancy near the conduction band.

Stress evolution in Ti-C:H films

Time evolution of stress in diamond-like films with small quantities(10-20 at%) of titanium (Ti-C:H) was investigated together with theirstructure, electrical resistivity, optical constants, Raman spectra, andcomposition. The compressive stresses for all films were found to increase,regardless of their structure (either amorphous or a-C:H with TiCnanocrystalline inclusions) and composition (10-20 at% Ti) over a period ofseveral months. The conductivity and optical constants (n and k) decreasedwith time, and Raman spectra indicated no change of the film structure. Theobservations can be explained by a migration of interstitial atoms (probablyhydrogen) resulting in a saturation of dangling bonds and, consequently,leading to a reduction in the number of free carriers. Vacancy clustering andeffects of hydrogen atoms on stress increase are discussed.

The effect of polysilicon grain boundaries on MOS based devices

Analysis is provided of the flow of current through undoped polysilicon. Charge is assumed to be trapped in grain boundaries giving potential barriers across which the free electrons and holes pass. Barriers height depends on gate and drain voltage, as does charge in the grain. At low current the electrostatic effect of the free charge is much less than trapped. There is, therefore, no gradual channel and flux of charge is controlled by the differences in concentration in adjacent grains(quasi-diffusion) as in the subtreshold region of MOSFETs. When the number of free carriers becomes comparable with that on the gate, there is an ohmic change in potential, and the channel current is drift limited (quasi-drift) by analogy with the gradual channel condition in MOSFETs. Variation field effect current with gate and drain voltages is compared with measurements at various temperatures

Low frequency noise measurements of p-channel Si/sub 1-x/Ge/sub x/ MOSFET's

In this brief, we report an investigation of the low frequency noise in p-channel SiGe MOSFET's. At low gate bias the noise spectrum consists of several trap related generation-recombination (g-r) noise components. At higher gate bias, the noise spectrum is dominated by 1/f noise. The 1/f noise is attributed to a fluctuation in the number of free carriers and the effective slow state trap density at the Fermi energy calculated.

Mobility fluctuations in hydrogenated amorphous silicon associated with deep defects

A detailed study of noise in hydrogenated amorphous silicon (a-Si:H) shows that resistance fluctuations between 290 K and 430 K are mobility fluctuations and not fluctuations in the number of free carriers. Attempt rate and the average activation energy are determined for the characteristic rate constants. There are strong indications that population and depopulation of deep defects produce the mobility fluctuations.

Free-carrier absorption and luminescence decay of porous silicon

We present time-resolved photoluminescence (PL) and photo-induced free-carrier absorption measurements on free-standing porous silicon flakes at room temperature. The number of free carriers was monitored by absorption of infrared beams, at 3.39 μm as well as at 1.3 μm wavelength. The excellent scaling between these two wavelengths during the complete decay yields clear evidence for an absorption related to free carriers as in bulk Si. Furthermore, the total PL yield in the decay, integrated over the PL spectrum, scales with the photo-induced carrier concentration. Using the bulk-Si free-carrier absorption cross-section we find that of the order of 15% of the initially excited carriers remain after a fast initial transient, whereas only a few per cent recombine radiatively. The competition between various recombination channels is discussed.

Nonlinear far-infrared absorption in InSb due to light impact ionization

A highly nonlinear far-infrared free-carrier absorption, rising with the radiation intensity, has been observed in InSb. It is shown that the nonlinearity arises from an increase in the number of free carriers caused by the generation of electron-hole pairs by light impact ionization in the radiation field of a powerful far-infrared laser. The observed nonlinearity permits the investigation of the process of impact ionization by a contactless optical method.

High-temperature protonic conduction in mixed perovskite ceramics

Protonic conduction of both stoichiometric and nonstoichiometric mixed perovskite ceramics with the general formula A 2( B′ 1+x B″ 1−x) O 6−δ( A= Sr 2+, Ba 2+; B′= Ga 3+, Gd 3+, Nd 3+; B″= Nb 5+, Ta 5+; x=0−0.2) has been investigated. The B-site occupancies of B and B can be ordered or disordered depending on the ionic radius of the A ion and the ionic radius difference of the B and B ions. Samples are treated in H 2O vapor at 900°C to incorporate protons (or D 2O vapor for deuterons) and the amount of water uptake is measured by the weight change. None of the stoichiometric samples ( x=0) shows any proton incorporation. For nonstoichiometric samples the amount of water uptake varies from 5% to 50% of the maximum theoretical amount, depending on the material and thermal history. Protonic conductivity of the nonstoichiometric materials is measured by ac impedance analysis in a frozen-in condition following the pre-treatment in water vapor. The conductivity is found to be higher and the activation energy lower for the slowly cooled materials, with magnitudes comparable to that of Yb-doped SrCeO 3. All of the nonstoichiometric samples show non-classical isotope effect of the conductivity, in the sense that the activation energy is slightly higher (by ∼0.04 eV) for deuteronic than for protonic conduction, similarly to results reported earlier for SrCeO 3 and BaCeO 3. Finally, the conductivity shows stage II behavior with activation energy equal to the proton migration enthalpy, but the number of free carriers is smaller than the total proton uptake.

New model for the In2O3-SnO2 system

The In2O3-SnO2 (ITO) solid solution system was formulated and a model for the ITO material derived. The model starts from the formation of a solid solution — giving rise to a doping effect up to the solid solubility of tin in the In2O3 lattice, together with the formation of a new associated vacancy (VInVo)x. The mechanism is discussed by which the (VInVo)x dissociates, reducing the number of free carriers through trapping, which in turn affects the electrical characteristics.

Polarized reflection spectra of iodine-doped new-type polyacetylene

The polarized reflection spectra of lightly and moderately iodine-doped new-type polyacetylene films have been measured in the range 60 ∼ 36 000 cm −1. The optical conductivity spectra along the polyacetylene chain direction were calculated by the Kramers-Kronig transformation of reflection spectra, in which several elementary transitions are characterized by polarization measurements. In the lightly doped film ((CHI y ) x , y ⩽ 0.10), only the spectrum of the charged soliton structure is predominant. Above a critical concentration ( y ≅ 0.10) the free carrier contribution becomes evident. With a moderate doping level (0.10 ⩽ y ⩽ 0.18) the number of free carriers is increased. A fairly long lifetime for the free carriers is indicated in the far-IR spectra. The conduction mechanism of the iodine-doped new polyacetylene is suggested, based on the analysis of optical conductivity spectra.