Carrier Concentration Dependence Research Articles

Investigation of group 13 elements as potential candidates for p-type dopants in the narrow-gap thermoelectric semiconductor α-SrSi2

AbstractTo investigate the possibility of p-type doping of α-SrSi2, a promising as an eco-friendly thermoelectric material, the energy changes of substitutions of the Si site of α-SrSi2 by group 13 elements were evaluated using first-principles calculations. It is found that Ga doping was the most energetically favorable dopant while In is the most unfavorable. We examined the synthesis of Ga- and In-doped α-SrSi2 using the vertical Bridgeman method and investigated their thermoelectric properties. The Ga atoms were doped to α-SrSi2 successfully up to 1.0 at. %, while In atoms could not be doped as suggested by calculations. For experimental prepared Ga-doped samples, the carrier density was observed to increase with Ga doping, from 3.58 × 1019 cm−3 for undoped α-SrSi2 to 4.49 × 1020 cm−3 for a 1.0 at. % Ga-doped sample at 300 K. The temperature dependence of carrier concentrations was observed to change from negative to positive with increasing Ga content. In addition, the temperature dependence of the Seebeck coefficient was also observed to change from negative to positive with increasing Ga content. The results indicate that α-SrSi2 undergoes a semiconductor–metal transition with Ga doping. The power factor for the undoped sample was quite high, at 2.5 mW/mK2, while the sample with 0.3 at. % Ga had a value of 1.1 mW/mK2 at room temperature.

Study of the free carrier characteristics and surface morphology of AlGaAs/GaAs thin films deposited using MOCVD

Ultra-low loss optical thin films find broad applications in fields such as vertical-cavity surface-emitting lasers and optical atomic clocks. The main optical losses in AlGaAs/GaAs distributed Bragg reflectors (DBRs) prepared using metal-organic chemical vapor deposition (MOCVD) arise from absorption loss caused by free carriers within the layers and scattering loss caused by surface roughness. In this study, we fabricated AlGaAs and GaAs single-layer thin films with varying Al compositions on substrates of three crystal orientations and under different V/III ratios. The dependence of carrier concentration and surface morphology on different substrates and growth conditions was investigated. Thin films grown on substrates with three different crystal orientations exhibited three distinct growth modes (step-flow mode, SK mode, and FM mode). The impact of the V/III ratio on the growth mode was found to be complex. Higher V/III ratios resulted in poorer morphology for films grown on (100) substrates, while better morphology was observed on (211) B substrates. Furthermore, the surface morphology of films grown on (100) 15° off substrates showed less sensitivity to changes in the V/III ratio. With increasing Al composition, the carrier concentration of the films significantly increased. Elevating the V/III ratio proved effective in suppressing the incorporation of carbon, thereby reducing the carrier concentration of AlGaAs films. GaAs films exhibited a low carrier concentration at an appropriate V/III ratio. Additionally, the distinct abilities of different substrates to adsorb impurities exerted a significant impact on the carrier concentration of the films. This study demonstrates that, under optimal conditions, it is feasible to fabricate AlGaAs/GaAs Bragg mirrors with low carrier concentration and relatively small roughness on (100) 15° off substrates.

Localization effect in single crystal of RuAs2

We report the magnetotransport and thermal properties of RuAs2 single crystal. RuAs2 exhibits semiconductor behavior and localization effect. The crossover from normal state to diffusive transport in the weak localization (WL) state and then to variable range hopping (VRH) transport in the strong localization state has been observed. The transitions can be reflected in the measurement of resistivity and Seebeck coefficient. Negative magnetoresistance (NMR) emerges with the appearance of localization effect and is gradually suppressed in high magnetic field. The temperature dependent phase coherence length extracted from the fittings of NMR also indicates the transition from WL to VRH. The measurement of Hall effect reveals an anomaly of temperature dependent carrier concentration caused by localization effect. Our findings show that RuAs2 is a suitable platform to study the localized state.

In Situ and Real-Time Monitoring of Doping Levels by Reflectance Anisotropy Spectroscopy (RAS) during Molecular Beam Epitaxial (MBE) Growth of III/V Semiconductors

Reflectance anisotropy/difference spectroscopy (RAS/RDS) had been developed for monitoring epitaxial semiconductor growth, especially for the metal-organic chemical vapor deposition (MOCVD) of III/V semiconductors. But RAS is also well suited for the control of III/V growth with molecular beam epitaxy (MBE). Although the work on RAS has already started at least three decades ago, the potential of this in situ and real-time monitoring technique, especially for doping control, is not well known yet. Experimental results are given here on the identification of doping types and concentration during MBE growth, exemplarily for GaAs and AlGaAs. Especially, the dependence of the majority charge carrier concentration (i.e., the doping concentration) on the RAS signal difference between the nondoping and doping cases is addressed here.

Surface recombination velocities for 4H–SiC: Dependence of excited carrier concentration and surface passivation

Carrier lifetime plays a crucial role in optimizing the performance and reliability of SiC devices. Surface recombination is one of the factors affecting carrier lifetime. However, the reported values for surface recombination velocity (S) have been inconsistent. This study aimed to elucidate the discrepancy in S observed in various reports by conducting microwave photoconductivity decay measurements on n-type 4H–SiC freestanding epitaxial layers at different excitation carrier concentrations. Additionally, we investigated the impact of passivation by examining the surfaces subjected to oxidation and post-oxidation annealing. We observed an increase in S with increasing excited carrier concentration for the surfaces, irrespective of the crystal faces (S- and C-faces) and passivation. This suggests the presence of a potential barrier or shallow recombination centers on the surfaces. Furthermore, we observed that S remained nearly constant with increasing temperature, contrary to the behavior observed under low-injection conditions. This difference also suggests the existence of a potential barrier or shallow recombination centers on both Si- and C-faces. The comprehensive insights gained from our study have significant implications for the design of SiC devices with enhanced performance and reliability.

First-principles analysis of the structural, thermodynamic, elastic and thermoelectric properties of LuXCo2Sb2 (X = V, Nb and Ta) double half Heusler alloys

We used the pseudopotential plane wave approach, as implemented in the Quantum Espresso program, to investigate the impact of X atoms (V, Nb, and Ta) on the physical properties of LuXCo2Sb2 double half Heusler alloys. We determined the equilibrium structural parameters, including the crystal lattice parameters, atomic position coordinates, and bulk modulus, with and without including the spin-orbit effects. The predicted single-crystal elastic constants (Cij) show that the title compounds are mechanically stable with a pronounced elastic anisotropy. The bulk modulus, shear modulus, Young's modulus, Poisson coefficient, Debye temperature, and Vickers hardness coefficient were deduced from Cij via the Voigt-Reuss-Hill approximations. We also determined the variations of some macroscopic physical parameters as functions of temperature and pressure, namely the thermal expansion coefficient, lattice thermal conductivity, heat capacity at constant volume, Debye temperature and entropy. The considered alloys demonstrate special thermal properties under pressure and heat conditions, specifically, their low thermal expansion coefficient and lattice thermal conductivity; their thermal expansion coefficient is lower than 4.5 × 10−5 K−1 at 1000 K, and lattice thermal conductivity don’t exceed 1 W.m−1 K−1 for temperatures higher than 300 K. Furthermore, we investigated the temperature and charge carrier concentration dependencies of some thermoelectric coefficients. The results of this study reveal the potential of the considered compounds for achieving a figure of merit greater than 0.5 at a temperature of 500 K and a doping concentration of 1020 cm−3.

Photoluminescence and electronic structure of α'-Ag8GeS6 crystals

The Ag8GeS6 crystal was synthesized by the direct melting of the elementary Ag, Ge, and S high-purity grade stoichiometric mixture in a sealed quartz ampoule. This argyrodite crystallizes in orthorhombic structure [Pna21 space group (No. 33)] below ≈483 K. The theoretical first-principle calculations of the electronic band structure and density of states of an α'-Ag8GeS6 crystal are estimated by the generalized gradient approximation. The Perdew–Burke–Ernzerhof functional was utilized. All of the calculated parameters correlate well with the known experimental data. Based on the electronic band structure, the effective mass of the electrons and holes was calculated. The anisotropic behavior of electronic band structure is discussed. Temperature and carrier concentration dependence of the Seebeck coefficient is calculated. The temperature behavior of α'-Ag8GeS6 photoluminescence spectra is presented. The emission spectra are studied in the temperature range between 17 and 130 K and laser power excitation between 4 and 512 mW and in the spectral range of 750–1650 nm. Two main emission bands at ≈1.35 and ≈1.57 eV are observed and discussed.

Charge Carrier Statistics-Based Analytical Model and Simulation of Optical Mechanisms in White Graphene for High-Quality FETs and Optoelectronic Applications

The aim of the present paper is to investigate the scaling behaviors of charge carriers and optical mechanisms in white graphene. The approach in this work is to provide analytical models for carrier velocity, carrier mobility, relaxation time and optical mechanisms of white graphene such as optical conductivity, absorption, transmittance, reflectivity, extinction coefficients and electron energy loss function. For doing so, one starts with identifying the analytical modeling of carrier concentration in the degenerate and nondegenerate regions. The computational models of carrier velocity, mobility and relaxation time with numerical solutions are analytically derived, in which the normalized Fermi energy, carrier concentration and temperature characteristics dependence are highlighted. Moreover, the optical mechanisms of white graphene are analytically modeled based on degenerate conductance. The proposed analytical models demonstrate a rational agreement with our simulation results and previous experiments in terms of trend and value. The remarkable properties of white graphene mentioned in this paper and obtained results bring new hopes for using of white graphene as a good substrate for nanomaterials such as graphene, germanene, stanene and silicene in electronics and optoelectronic applications.

Temperature and illumination dependence of silicon heterojunction solar cells with a wide range of wafer resistivities

AbstractRecently, the significant improvements in the surface and contact passivation of silicon (Si) solar cells as well as their bulk quality have shifted their operating point to higher injections. Hence, they are less dependent on wafer doping. This shift opens an opportunity of using high‐resistivity wafers for practical photovoltaic applications, introducing a promising approach to push the cell efficiency towards the intrinsic limit and to improve the module reliability by increasing the cell breakdown voltage. Therefore, insights into the performance of Si solar cells using high‐resistivity wafers at various operating temperatures are of significant interest. In this study, we investigate the temperature‐ and illumination‐dependent performance of Si heterojunction (SHJ) solar cells using a wide range of wafer resistivities (between 3 and 1000 Ω⋅cm). Although a reduction in the passivation quality of the passivating contacts is observed at elevated temperature, the impact on the temperature coefficient of the open‐circuit voltage (TCVoc)—the dominant contributor to the temperature coefficient (TC) of the cell efficiency—is very limited. Their TCVoc are still dominated by the temperature dependence of the effective intrinsic carrier concentration. Furthermore, we also find that the investigated cells are more sensitive to temperature variation at lower illumination intensities. It is noteworthy that the efficiency of the cells fabricated using high‐resistivity wafers is comparable to that of the reference cells at any given temperature, highlighting the potential of using high‐resistivity wafers for solar cells.

First-principles investigation on the novel half-Heusler VXTe (X=Cr, Mn, Fe, and Co) alloys for spintronic and thermoelectric applications

Half-Heusler (HH) alloys are a well-known and extensively researched family of thermoelectric (TE), magnetic, and spintronic materials. Doping may significantly increase the thermoelectric conversion efficiency of these materials; nevertheless, practical applications remain far. As a result, the hunt for superior parent TE alloys is critical. Using extensive first-principles density functional calculations, we predicted a novel class of vanadium-based four HH VXTe alloys, where X is one of the four elements: Cr, Mn, Fe, and Co. Their TE properties, as well as their mechanical, magnetic, electrical, and structural stability, have been studied in depth. Their mechanical and thermodynamic stability is confirmed using the predicted elastic constants, formation and cohesive energies, and phonon spectra. A comprehensive analysis of elastic constants and moduli demonstrates that HH VXTe alloys possess elastic anisotropy with reasonably good machinability, higher melting and Debye temperatures, mixed bonding characteristics with ionic and covalent contributions, and brittle nature, except for VCoTe, which is ductile. We find that the ground state of HH VCrTe and VFeTe is ferrimagnetic, while HH VCoTe is ferromagnetic and HH VMnTe is non-magnetic. Three VXTe (X = Cr, Fe, & Co) alloys demonstrated half-metallicity with 100% spin-polarization, whereas HH VMnTe is an 18-electron indirect semiconductor. In the studied HH VXTe alloys, most of the heat flow is caused by the phonon-group velocity of the acoustic phonons. Further studies on the relationship between carrier concentration and temperature dependence of TE properties reveal that the high ZT ∼1.2 at 1000 K of the pristine HH VMnTe alloy is obtained due to its high-power factor of 249.4×1012Wm−1K−2 and this value is greater than the values of some known pristine and doped HH TE materials. Our findings pave the way for further investigation into the HH VXTe alloys in the quest for improved TE and spintronic materials for use in domains that necessitate high thermoelectricity and spintronic performance.

Hall effect test bench for temperature dependence of carrier concentration

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Random band-edge model description of thermoelectricity in high-mobility disordered semiconductors: Application to the amorphous oxide In-Ga-Zn-O

Unraveling the dominant charge transport mechanism in high-mobility amorphous oxide semiconductors is still a matter of controversy. In the present study we extended the random band-edge model suggested before for the charge transport and Hall-effect mobility in such disordered materials [Fishchuk et al., Phys. Rev. B 93, 195204 (2016)], and also describe the field-effect-modulated thermoelectricity in amorphous In-Ga-Zn-O ($a$-IGZO) films under the same premises. The model is based on the concept of charge transport through the extended states and assumes that the transport is limited by the spatial variation of the position of the band edge due to the disorder potential, rather than by localized states. The theoretical model is formulated using the effective medium approximation framework and describes well basic features of the Seebeck coefficient in disordered materials as a function of energy disorder, carrier concentration, and temperature. Carrier concentration dependencies of power factor and thermoelectric figure of merit have been also considered for such systems. Besides, our calculations reveal a remarkable turnover effect from a negative to a positive temperature dependence of Seebeck coefficient upon increasing carrier concentration. The suggested unified model provides a good quantitative description of available experimental data on the Seebeck coefficient and the charge mobilities measured in the same $a$-IGZO transistor as a function of the gate voltage and temperature by considering the same charge transport mechanisms. This promotes a deeper understanding and a more credible and accurate description of the transport process in $a$-IGZO films.

Electrical and galvanomagnetic properties of black phosphorus single crystals

Black phosphorus (b-P) single crystals having the n-type electrical conductivity produced in a high pressure set-up (~1 GPa) with six diamond anvils at 800 °C for 12 h have been studied. The electrical conductivity σ(Т,В) and the Hall constant Rh(Т,В) have been analyzed within one-band and two-band models as functions of temperature in the 2 < Т < 300 K range and magnetic field in the 0 < В < 8 T range. Fitting of the experimental σ(Т,В) and Rh(Т,В) curves suggests the following key properties of the crystals: (1) intrinsic conductivity type, (2) approximately equal electron and hole concentrations and mobilities, (3) anisotropic behavior of electron and hole conductivities, concentrations and mobilities and (4) combination of negative and positive contributions to magnetoresistance (magnetoresistive effect, MR). In a zero magnetic field the anisotropy coefficient α = [σа(Т) – σс(Т)]/σс(Т) below 50—70 K is positive whereas above 220 K its sign changes to negative due to a specific combination of the temperature dependences of carrier concentration and mobility. It has been shown that the negative sign of relative MR (negative magnetoresistive effect) dominates at T < 25 K and B < 6 T and is presumably caused by the effects of strong localization resulting from structural disorder. The positive MR sign (positive magnetoresistive effect) is associated with the Lorentz mechanism of carrier movement and exhibits itself above 25 K in 6–8 T magnetic fields.

Non-Maxwell–Boltzmann dependence of channel carrier concentration in a quasi one dimensional charge density wave channel in the ballistic transport regime

With the aid of a coherent transport model utilizing the non-equilibrium Green function approach, a three terminal device with metallic gate, source and drain and a quasi one dimensional charge density wave (CDW) channel is simulated focussing on the transistor behaviour brought about by a sweep of the channel potential or equivalently the chemical potential in the channel. The channel is strongly insulating only at half-filling and moving to lower and higher carrier concentrations both incur a mean field phase transition to a conducting state. With the aid of conductance calculations for a pinned CDW condensate, we present calculations for the sub-threshold slope in terms of the hopping parameter or equivalently the width of the tight-binding chain. The effects of source to drain bias and length are examined. The conductance profiles are analyed in relation to transmission profiles. The observed CDW profiles are explained in terms of filling and Fermi surface nesting. Boundary conditions, gap equations and response functions are shown to reveal the commensurability conditions and size of the transport gap. The channel carrier concentration is modulated in an athermal (non-Maxwellian–Boltzmann) fashion, thereby making it an interesting prospect for steep transistors.

Evaluation of carrier concentration dependence of phonon and electron mean free path in Si single crystals

In this study, we evaluated the carrier concentration dependence of phonon and electron mean free paths in single crystal Si. The phonon group velocity was derived using the nanoindentation method, and the thermal conductivity was derived using the laser spot periodic heating method. The phonon mean free path was derived by combining them. In addition, electron mean free paths were calculated, and the carrier concentration dependence on phonon and electron mean free paths were evaluated. As a result, both phonon and electron mean free paths were exhibited in the order of 10-8 -10-9 m, and were found to show a similar behavior depending on the carrier concentration.

Electrical and galvanomagnetic properties of black phosphorus single crystals

Black phosphorus (b-P) single crystals having the n-type electrical conductivity produced in a high pressure set-up (~1 GPa) with six diamond anvils at 800 °C for 12 h have been studied. The electrical conductivity σ(Т,В) and the Hall constant Rh(Т,В) have been analyzed within one-band and two-band models as functions of temperature in the 2 < Т < 300 K range and magnetic field in the 0 < В < 8 T range. Fitting of the experimental σ(Т,В) and Rh(Т,В) curves suggests the following key properties of the crystals: (1) intrinsic conductivity type, (2) approximately equal electron and hole concentrations and mobilities, (3) anisotropic behavior of electron and hole conductivities, concentrations and mobilities and (4) combination of negative and positive contributions to magnetoresistance (magnetoresistive effect, MR). In a zero magnetic field the anisotropy coefficient α = [σа (Т) – σс (Т)]/σс (Т) below 50–70 K is positive whereas above 220 K its sign changes to negative due to a specific combination of the temperature dependences of carrier concentration and mobility. It has been shown that the negative sign of relative MR (negative magnetoresistive effect) dominates at T < 25 K and B < 6 T and is presumably caused by the effects of strong localization resulting from structural disorder. The positive MR sign (positive magnetoresistive effect) is associated with the Lorentz mechanism of carrier movement and exhibits itself above 25 K in 6–8 T magnetic fields.

Anomalous temperature dependence of the effective mass in p -type Bi2Te3

Infrared reflectance measurements have been carried on highly $p$-type ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ crystals over the broad temperature range from 10 to 650 K and their response function investigated. An anomalous nonmonotonous temperature dependence of the plasma edge frequency is observed---with a pronounced redshift and blueshift that appear above and below room temperature, respectively. Such behavior is explained in terms of the temperature evolution of the free carrier optical weight $N/{m}^{*}$ given by particular temperature dependence of the total carrier concentration $N$ (determined by transport measurements) and characteristic (optical) mass ${m}^{*}$. The effective mass strongly varies from ${m}^{*}=0.1{m}_{0}$ (${m}_{0}$ electron rest mass) at 10 K to ${m}^{*}=0.5{m}_{0}$ respective ${m}^{*}=0.9{m}_{0}$ at 650 K depending on the evaluating procedure. The interpretation is corroborated by complementary magnetotransport and magnetoreflectivity experiments.

Anomalous electron-electron interactions in epitaxial graphene on SiC

Electron-electron interactions (EEI) play a pivotal role in the transport behavior of carriers confined in a two-dimensional system. The strength of the interaction can be tuned by changing carrier concentration in a conventional two-dimensional system, while it expectedly cannot be controlled in monolayer graphene with a linear dispersion relation since the interaction parameter is not related to carrier concentration. Here, an anomalous carrier-concentration dependence of the EEI is observed in epitaxial graphene on SiC. From quantum transport measurement, a logarithmic temperature dependence of the Hall coefficient is obtained, which is a manifestation of the EEI. From the logarithmic temperature dependence, the EEI-related factor Kee is extracted. Unexpectedly, the extracted Kee shows an increase on increasing carrier concentration, which is further confirmed by analyzing logarithmic correction to the Drude conductivity. This can be attributed to the carrier-concentration dependence of Fermi-liquid constant F0σ due to gas absorption, which offers a route to control the EEI in graphene.

Computational Search for Better Thermoelectric Performance in Nickel-Based Half-Heusler Compounds.

Half-Heusler alloys have recently received extensive attention because of their promising thermoelectric (TE) properties and great potential for applications requiring efficient thermoelectricity. Although the conversion efficiency of these materials can be greatly improved by doping, it is still far away from the real-life applications. Therefore, search for better parent TE compounds is deemed urgent. Using a high-throughput search method based on first-principles calculations in newly proposed 378 half-Heusler alloys, we identify nine nickel-based half-Heusler semiconductors as candidates and systematically study their mechanical, electronic, and transport properties. Their mechanical and dynamical stabilities are verified based on the calculated elastic constants and phonon spectra. The electronic structure calculations indicate the existence of direct energy gaps in the NiVZ (Z = Al, Ga, and In) and indirect energy gaps in the NiTiZ (Z = Si, Ge, and Sn) and NiScZ (Z = P, As, and Sb) compounds. Among them, NiVAl, NiVGa, and NiVIn exhibit a sharp slope of density of states near the Fermi level, which is predicted to be essential for a high TE performance. Further investigation on carrier concentration and temperature dependence of TE properties shows the high power factors of NiVAl, NiVGa, and NiVIn, which are responsible for their high figure of merit values. The highest maximum power factor of 5.152 mW m–1 K–2 and figure of merit of 0.309 are predicted for pristine half-Heusler NiVIn, which are larger than the values of some known pristine and doped half-Heusler TE materials. Our work opens up new avenues for rationally searching better TE materials among half-Heusler alloys for applications in fields requiring efficient thermoelectricity.

Grain-boundary segregation of magnesium in doped cuprous oxide and impact on electrical transport properties

In this study, we report the segregation of magnesium in the grain boundaries of magnesium-doped cuprous oxide (Cu2O:Mg) thin films as revealed by atom probe tomography and the consequences of the dopant presence on the temperature-dependent Hall effect properties. The incorporation of magnesium as a divalent cation was achieved by aerosol-assisted metal organic chemical vapour deposition, followed by thermal treatments under oxidizing conditions. We observe that, in comparison with intrinsic cuprous oxide, the electronic transport is improved in Cu2O:Mg with a reduction of resistivity to 13.3 ± 0.1 Ω cm, despite the reduction of hole mobility in the doped films, due to higher grain-boundary scattering. The Hall carrier concentration dependence with temperature showed the presence of an acceptor level associated with an ionization energy of 125 ± 9 meV, similar to the energy value of a large size impurity−vacancy complex. Atom probe tomography shows a magnesium incorporation of 5%, which is substantially present at the grain boundaries of the Cu2O.