Carrier Concentration Measurements Research Articles

Low lattice thermal conductivity of novel NiVSn with 19 valence electron count contrasted to 18 of NiVAl and NiAlSb Half-Heusler compounds

In the current study, the novel pristine compounds of both VEC 18 (NiVAl & NiAlSb) and VEC 19 (NiVSn) materials have been synthesized employing arc-melting and hot-pressed at 1073 K. The measured carrier concentration for all the compounds was found ∿ 1020-1021 cm−3. Further, NiAlSb, NiVAl, and NiVSn half-Heusler (HH) compounds exhibited n-type semiconductor behaviour, with maximum Seebeck coefficients (S) of −50.2 μV/K, −40.5 μV/K, and −14.5 μV/K at 723 K. As a result of the measured S and electrical conductivity, the estimated power factor for NiVSn (0.20 mWm−1K−2), NiAlSb (0.41 mWm−1K−2), and NiVAl (0.65 mWm−1K−2) at 723 K. The NiVSn lattice thermal conductivity (κl) was reduced to ∿ 79.80 % and 76.71 % as compared to NiAlSb and NiVAl compounds. Despite having reduced κl, low zT exhibited for NiVSn compound mainly due to nominal S. The reduced κl was mainly attributed to weak chemical bonding results from lower group velocity of phonon, leading to a higher anharmonicity effect in the lattice. Therefore, NiVSn of VEC 19-based compound can be considered as a potential candidate material for TE applications other than traditional 18-VEC HH compounds. The thermoelectric performance of the novel HH compounds is suitably corroborated with the structural and microstructural investigations.

Thermoelectric and mechanical performance of NiTiAl Heusler compound

In the current study, the NiTiAl exhibited the combination of full Heusler (f-Heusler) and Half-Huesler (h-Heusler) phases. The NiTiAl sample was prepared by arc-melting followed and spark plasma sintering at 1073K. The synthesized sample exhibited p-type semiconductor behaviour having a Seebeck coefficient of 50 μV/K at 790K. Further, the resulting power factor is 1.15mWm−1K−2 at 790 K, which is 115% higher than the TiCoSb. The enhancement in power factor is good in agreement with the increased electrical conductivity and measured carrier concentration (nH) ∼ 1020 cm−3. On the other hand, NiTiAl exhibited a hardness value of 7.8 GPa and the estimated fracture toughness (Kc) is 12% higher than the conventional 18 VEC of TiCoSb. Further, the NiTiAl exhibited a low value of κl ∿1.15 Wm−1K−1 at 473K, due to the phonon-phonon scattering of the h-Heusler/f-Heusler interfaces with increasing temperature. However, due to the presence of a larger fraction of metallic f-Heusler phase, the κe is increased significantly and that led to a higher κ, resulting in lower zT∿0.007 at 473 K. Finally, the acquired results of NiTiAl Heusler compound with 17 VEC will open a door for unexplored compounds than the conventional 18 VEC, which could help the thermoelectric community.

Multicrystalline, Highly Oriented Thick‐Film Silicon from Reduction of Soda‐Lime Glass

AbstractThe study describes synthesis and characterization of > 10 µm thick multicrystalline (mc), highly oriented, p‐doped silicon layers by aluminothermic reduction of low‐cost soda‐lime glass. X‐ray diffraction shows a highly preferred (111)‐orientation and excellent crystallinity. Low compressive stress and very good crystallinity are confirmed by the peak position and width of the Raman LO‐phonon line, approaching the one of bulk single‐crystalline wafer material. Due to strong bonding to the glass substrate layer, spalling is not observed. A conductive aluminum‐rich oxide layer is formed underneath the silicon, serving as an electrical back‐contact for electronic devices. Using secondary ion mass spectrometry very low concentrations of 1014–1015 cm−3 of impurities are found originating from the soda‐lime glass with an iron content below the detection limit. Furthermore, a plateau‐like, very homogenous Al concentration of ≈4 × 1018 cm−3 over a thickness of ≈10 µm is found, which corresponds to the solubility of Al in Si at the process temperature. Complete electronic activation within the plateau region is confirmed by carrier concentration measurements using electrochemical capacitance–voltage profiling and Raman spectroscopy. Hole concentrations in the range of few 1018 cm−3 are beneficial for the p‐type base material of full‐emitter cell mc‐silicon photovoltaic devices.

Simultaneous measurement of charge carrier concentration, mobility, and lifetime

Simultaneous measurement of charge carrier concentration, mobility, and lifetime

Tuning and interpretation of electronic transport properties with

Electronic transport coefficients such as the electrical conductivity, the termo-power, and the charge carrier concentration are routinely measured in a variety of application areas such as electronics or thermoelectric power-generation and cooling. Using fitting procedures, those measurements are used to infer microscopic features of the samples. The code ▪ facilitates the estimation of electronic structure parameters such as the effective masses and band energies improving the current approach by increasing the complexity of the band structure representation. The software is designed with a server-client architecture to enhance performance and scalability. The server is implemented in the Julia programming language. We illustrate the design and the efficiency of the code with selected applications, which are relevant to the optimization of thermoelectric materials. Program summaryProgram Title:▪CPC Library link to program files:https://doi.org/10.17632/494nj6ftbv.1Developer's repository link:https://github.com/marcofornari/etransport.gitLicensing provisions: GPLv3Programming language: Julia, Python.Nature of problem: Electronic transport measurements are routinely used to link the functional properties exploited in electronics and energy sciences applications with the underlying electronic structure of the material under investigation. Experimentalists collect data of derived and integrated physical quantities, from which they estimate properties and parameters, e.g., nature of the conduction mechanisms and effective mass. The interpretation is based on simplistic models that rarely represent the complexity of the band structure. The code ▪ improves the current state of the art and facilitates the estimation of effective masses and energies from experimental measurements of electrical conductivity, Seebeck coefficients, and carrier concentration using a multi-valley anisotropic parabolic band structure as a model for the unknown real band structure. Experimental data can be imported and visualized to facilitate the comparison with theoretical analysis.Solution method: The Boltzmann equation within the relaxation-time approximation is used to compute the electronic transport tensors from the electronic density of states characterized in terms of a set of effective masses tensors mi⁎ and critical energies ϵi. Numerical and analytical techniques are adopted to integrate the Boltzmann equation efficiently. By tuning the features of the density of states, temperature, Fermi level, and the functional expression for the relaxation time, the software streamlines the reconstruction of the underlying electronic properties from experimental data.Additional comments including restrictions and unusual features: The software is designed with a server-client architecture to enhance performance and scalability. The server is implemented in the Julia programming language as a stand-alone library. It performs all the numerical operations to compute the transport tensors from a set of multi-valley parabolic band structures. The client is implemented either as a command-line user interface (CLI) or a graphical user interface (GUI) written in the Python programming language. The communications between the server and clients are handled by a REST API, where the data are exchanged using JSON payloads.

Experimental and computational study of the role of defects and secondary phases on the thermoelectric properties of TiNi1+x Sn (0 ≤ x ≤ 0.12) half Heusler compounds

The half Heusler TiNiSn compound is a model system for understanding the relationship among structural, electronic, microstructural and thermoelectric properties. However, the role of defects that deviate from the ideal crystal structure is far from being fully described. In this work, TiNi1+x Sn alloys (x = 0, 0.03, 0.06, 0.12) were synthesized by arc melting elemental metals and annealed to achieve equilibrium conditions. Experimental values of the Seebeck coefficient and electrical resistivity, obtained from this work and from the literature, scale with the measured carrier concentration, due to different amounts of secondary phases and interstitial nickel. Density functional theory calculations showed that the presence of both interstitial Ni defects and composition conserving defects narrows the band gap with respect to the defect free structure, affecting the transport properties. Accordingly, results of experimental investigations have been explained confirming that interstitial Ni defects, as well as secondary phases, promote a metallic behavior, raising the electrical conductivity and lowering the absolute values of the Seebeck coefficient.

Effect of Mg deficiency on the thermoelectric properties of Mg2(Si, Sn) solid solutions

In this study, we analyze the change in the thermoelectric properties of a Mgx(Si, Sn) system upon varying nominal Mg content x (x = 1.8, 1.9, 2.1, 2.2, 2.3). We find that Mg deficiency induces multiple point defects, such as Mg vacancies and anion anti-site defects, as well as phase separation of crystal structure, both of which alter the band structure. Systematic investigations are followed to elucidate how thermoelectric properties of a Mgx(Si, Sn) system are influeced by the degree of Mg deficiency. Temperature-dependent measurements of the Seebeck coefficient and the Hall coefficient indicate that Mg deficiency induces p-type behavior; this is confirmed by an analysis of the thermoelectric transport properties by using a single parabolic band model at low temperature. The p-type conduction in the Mg-deficient system suggests that Mg vacancies are the predominant defects. However, the efficiency of p-type doping is two to three orders of magnitude lower (∼1018 cm−3) than expected when considering only Mg vacancies as the origin of charge carriers. Density functional theory calculations suggest that the anion anti-site defect on the Mg site is attributable for the low measured carrier concentration and low doping efficiency. Moreover, the energy band structure of the Mg-deficient Mgx(Si, Sn) system is shown to undergo significant deformation and allow the formation of an impurity band at the X point due to the Mg-deficient conditions, enabling the system to have a relatively anion-rich condition. Furthermore, Mg deficiency minimizes the thermal conductivity to 1–1.5 W/m∙K at room temperature owing to an early bipolar contribution and separated structural phases. This study provides the Mg deficiency as a new control parameter that can be utilized to enhance the p-type thermoelectric performance of Mg silicides.

The Charge Distribution, Seebeck Coefficient, and Carrier Concentration of CuCr0.99Ln0.01S2 (Ln = Dy-Lu).

The atom oxidation states were determined using the binding energies of the core S2p-, Cu2p-, Cr2p-, and Ln3d-levels in CuCr0.99Ln0.01S2 (Ln = Dy-Lu) solid solutions. The charge distribution on the matrix elements (Cu, Cr, and S) remained unaffected after cationic substitution. The sulfur atoms were found to be in the S2- oxidation state, the copper-Cu+, and the chromium-Cr3+. The cationic substitution of the initial CuCrS2-matrix occurred via the isovalent mechanism. The obtained results were compared with the electrophysical properties for CuCr0.99Ln0.01S2. The measured carrier concentration was from 1017 to 1018 cm-3. The largest Seebeck coefficient value of 157 µV/K was measured for CuCr0.99Yb0.01S2 at 500 K. The cationic substitution with lanthanides allowed one to enhance the Seebeck coefficient of the initial CuCrS2-matrix.

Quantitative measurement of the charge carrier concentration using dielectric force microscopy

The charge carrier concentration profile is a critical factor that determines semiconducting material properties and device performance. Dielectric force microscopy (DFM) has been previously developed to map charge carrier concentrations with nanometer-scale spatial resolution. However, it is challenging to quantitatively obtain the charge carrier concentration, since the dielectric force is also affected by the mobility. Here, we quantitative measured the charge carrier concentration at the saturation mobility regime via the rectification effect-dependent gating ratio of DFM. By measuring a series of n-type GaAs and GaN thin films with mobility in the saturation regime, we confirmed the decreased DFM-measured gating ratio with increasing electron concentration. Combined with numerical simulation to calibrate the tip–sample geometry-induced systematic error, the quantitative correlation between the DFM-measured gating ratio and the electron concentration has been established, where the extracted electron concentration presents high accuracy in the range of 4 × 1016 – 1 × 1018 cm−3. We expect the quantitative DFM to find broad applications in characterizing the charge carrier transport properties of various semiconducting materials and devices.

Evaluation of oxygen-vacancy concentration through simulated hydrogen diffusion in amorphous In-Ga-Zn-O

A multiscale approach is presented to calculate the hydrogen diffusion in amorphous In-Ga-Zn-O (a-IGZO). Based on the fact that the presence of oxygen vacancy (VO) defects suppresses the hydrogen diffusion in a-IGZO, the simulated H diffusion depth is utilized to evaluate the VO concentration (nVo). The kinetic Monte Carlo (kMC) simulation is carried out to obtain the depth profiles of H diffusion in a-IGZO, which uses input of the H migration energy barriers (EA) calculated within the density functional theory (DFT) method. The DFT calculations showed that when H occupies the VO defects, its EA becomes higher than that of the interstitial H, which eventually suppresses H diffusion. The H diffusion function depending on nVo and temperature, fD(nVo,T), is readily obtained fitting the complementary error function to the simulated H diffusion profile. It is observed that fD(nVo,T) can closely reproduce the experimentally measured H diffusion from literature with nVo being comparable to the measured carrier concentration that can be speculated as the upper limit of nVo. The properties of H diffusion in a-IGZO are examined in detail to address the application of the presented approach for estimation of nVo.

Integration of graphene with GZO as TCO layer and its impact on solar cell performance

In this study, we investigated the impact of incorporating graphene with Ga-doped ZnO (GZO) when employing them as a TCO layer on Si-based solar cell. GZO thin films with various thicknesses (50–450 nm) were fabricated by the sputtering method using a single target. The aim here was to determine the GZO film with the optimum thickness to incorporate it with single layer graphene as TCO. This thickness was found to be 350 nm as that was the best crystalline quality found in the XRD pattern. Further, this sample had the lowest sheet resistance and highest transmission values as confirmed by electrical (sheet resistance), and optical characterizations (transmission). Topographic (SEM and AFM), electrical (resistivity and carrier concentration) measurements were also conducted on the same sample. The graphene film grown on copper in a CVD system was then transferred on top of this sample to fabricate the hybrid TCO structure. We found that graphene integrated GZO hybrid TCO film showed higher sheet resistance due to high sheet resistance of graphene and similar optical properties thanks to high optical transmission of graphene. Employing graphene-based TCO layer in the solar cell resulted in higher open-circuit voltage, consequently improving the conversion efficiency from 10.0% to 11.2%.

Investigation of electronic transport in InAs/GaAs samples. A study using the metaheuristic self-adaptive differential evolution method

This manuscript presents the study of electronic transport on a set of five multilayer molecular beam epitaxy-grown InAs/GaAs semiconductor samples. We developed an automated switch system to carry out electronic transport measurements of mobility and carrier concentration using the van der Pauw technique. Measurements were carried out as a function of temperature within the range of 260 K to 310 K. To identify which scattering mechanisms most contributed to mobility limitation, It was necessary to use the Self-adaptive Differential Evolution meta-heuristic method. This method allowed the determination of the main scattering mechanisms limiting the electronic mobility and identified as scattering by dislocations and phonons. Dislocations consist of the dominant defects in this lattice mismatch structure. Therefore, to increase carrier mobility, we propose some strategies: a change in the sample growth parameters such as substrate temperature and InAs/GaAs layer thickness. Alternatively, annealing of the samples could also be considered to improve sample mobility.

Re-evaluation of the electronic structure and thermoelectric properties of narrow-gap semiconducting α-SrSi2: A complementary experimental and first-principles hybrid-functional approach

Thermoelectric power generation in the low temperature region has attracted considerable attention as a means of the effective use of distributed energy and for sensor applications. However, it is difficult to theoretically predict the exact thermoelectric transport properties owing to the relatively narrow bandgap of low temperature thermoelectric materials. In this study, a high-purity α-SrSi2 crystal was synthesized by the vertical Bridgman (VB) method. The carrier density of the VB-grown α-SrSi2 was investigated, and, from the temperature dependence of the carrier density, it was estimated that the bandgap was 13.1 meV. First-principles calculations using the Heyd–Scuseria–Ernzerhof screened hybrid functional for α-SrSi2 predicted the bandgap to be very close to this value (13.27 meV) when assuming the mixing parameter of the Hartree–Fock contribution to the exact exchange is 18.7%. Using the calculated electronic structure and the measured carrier concentration, the predicted temperature dependence of the Seebeck coefficient was in good agreement with the experimental results.

Ex situ measurement of charge carrier concentration in Nafion by Hall effect

There is no controversy regarding the nature of charge carriers in in situ Nafion processes, but still there is controversy regarding charge carriers in ex situ processes. Using Hall voltage measurements, we have shown that valence electrons rather than protons are charge carriers in ex situ processes, contrary to widely accepted assumption that protons are charge carriers in Nafion regardless of a process. Concentration of valence electrons decreases with water uptake, but a graph of valence electron concentration, as a function of water content, is steeper than a graph of inverse volume. This brings us to conclusion that the total number of valence electrons decreases with water uptake. Concentration of valence electrons in ex situ measurements is four orders of magnitude higher than proton concentration in in situ measurements. In ex situ measurements on Nafion, metallic part of the electric circuit blocks proton propagation. On the other hand, in in situ measurements electrons are blocked in Nafion due to a potential barrier of dipole layer.

Integration of electrochemical capacitance–voltage characteristics: a new procedure for obtaining free charge carrier depth distribution profiles with high resolution

A novel electrochemical profiling technique consisting in integration capacitance–voltage characteristics was proposed, developed and proven. The aim is to overcome the insufficient resolution in charge carrier concentration measurements during commonly used electrochemical capacitance–voltage profiling. This is very actual task for modern low-dimensional optoelectronics. According to the introduced “profiling window”, after each etching step the coordinate interval is selected, the concentration distribution in which is to be matched itself and by the first derivative at the edges with the concentration in adjacent intervals. To achieve these conditions the minimum dissipation factor should be maintained for each C–V characteristic at the corresponding etching depth. The method allows both to obtain general free charge carrier concentration depth distribution over unlimited structure depth and to achieve necessary resolution in key regions, even if they are lying at a high depth. Particularly, in GaAs-based heterostructure concentration profile of deep-lying quantum well and a delta layer (ultra-thin (2 nm) profile of dopant with extremely high concentration) were confidently distinguished with magnitude difference of 20% in the dip region in between. Besides, we have shown how quantum-confined objects of different origins give different concentration responses when electrochemical capacitance–voltage profiling with sequential etching. An explanation for this dissimilarity is given.

Enhancing thermoelectric performance of Sn1-xSb2x/3Te via synergistic charge balanced compensation doping

A synergy of fine-tuning the carrier concentration and implementing scatter of heat-carrying phonons is vital to the thermoelectric study of narrow bandgap semiconductors. In this work, we adopted a synergistic charge-balanced compensation-doping approach derived from the interplay between heterovalent Sb dopants and Sn vacancies in narrow band gap semiconductor SnTe, a lead-free alternative to the classic thermoelectric material PbTe. Specifically, we designed a composition series of Sn1-xSb2x/3Te (0 ≤ x ≤ 0.20 in steps of 0.02), in which every three Sn2+ are substituted by two Sb3+ and one Sn vacancy. To the first order, such chemical composition does not contribute net charge carriers in the context of electron counting and also ensures that the composition of the primary phase is not altered by the formation of secondary phase Sb2Te3, which greatly simplifies the data analysis. Interestingly, we found that the measured carrier concentration gradually reduced with increasing Sb content and in turn increased the Seebeck coefficient. Meanwhile, the coexistence of Sb dopants and Sn vacancies effectively scattered heat-carrying phonons at elevated temperatures. Consequently, a peak ZT value of ~1.1 at 873K was achieved in the x = 0.12 sample. These results offer a new avenue for thermoelectric study of narrow bandgap semiconductors.

Intrinsic ferromagnetism in 4H-SiC single crystal induced by Al-doping

SiC single crystals with varying Al doping concentrations are grown through top-seeded solution growth method to explore the intrinsic magnetism upon non-magnetic element doping. Ferromagnetism is detected in all Al-doped crystals, where an enhancement in magnetization and magnetic anisotropy upon increasing concentrations of Al doping is observed. The coercive force and magnetic remanence enhance with increasing Al-concentration while the Curie temperatures fluctuate around 30 K. Anisotropy in magnetic hysteresis is observed when the applied magnetic field is going from perpendicular to parallel to the (0 0 0 1) plane. Based on the measured carrier concentrations and the proposed structural models of Al dopant plus Si vacancies, we claim that the extra free carriers introduced by Si vacancies around Al atoms are vital to the emergence of the ferromagnetism in SiC.

Na Doping in PbTe: Solubility, Band Convergence, Phase Boundary Mapping, and Thermoelectric Properties.

Many monumental breakthroughs in p-type PbTe thermoelectrics are driven by optimizing a Pb0.98Na0.02Te matrix. However, recent works found that x > 0.02 in Pb1-xNaxTe further improves the thermoelectric figure of merit, zT, despite being above the expected Na solubility limit. We explain the origins of improved performance from excess Na doping through computation and experiments on Pb1-xNaxTe with 0.01 ≤ x ≤ 0.04. High temperature X-ray diffraction and Hall carrier concentration measurements show enhanced Na solubility at high temperatures when x > 0.02 but no improvement in carrier concentration, indicating that Na is entering the lattice but is electrically compensated by high intrinsic defect concentrations. The higher Na concentration leads to band convergence between the light L and heavy Σ valence bands in PbTe, suppressing bipolar conduction and increasing the Seebeck coefficient. This results in a high temperature zT nearing 2 for Pb0.96Na0.04Te, ∼25% higher than traditionally reported values for pristine PbTe-Na. Further, we apply a phase diagram approach to explain the origins of increased solubility from excess Na doping and offer strategies for repeatable synthesis of high zT Na-doped materials. A starting matrix of simple, high performing Pb0.96Na0.04Te synthesized following our guidelines may be superior to Pb0.98Na0.02Te for continued zT optimization in p-type PbTe materials.

Combining theory and experiment to determine thermoelectrics of tellurium doped CoSb3

First, we solve a semi-classical electronic Boltzmann transport equation and acquire the thermoelectric coefficients for a pure unit cell using the rigid-band approximation. Then, combined with the existing experimental results, we not only reproduce the Seebeck coefficients and electrical conductivity but also yield the carrier concentrations and relaxation time, which cannot be obtained by either experiments or theories alone. Our results are in good agreement with the experiment for all doping samples. Furthermore, we show that Wiedemann-Franz law fails to connect the electronic thermal conductivity with the electrical conductivity. We also show that the Hall effect fails to measure carrier concentration in this study. We attribute these failures to the breakdown of the parabolic band. We think that our approach can be extended to other alloy or compound thermoelectric materials that utilize electronic transport property because they all have problems identifying relaxation time and carrier concentrations.

Towards standardisation of contact and contactless electrical measurements of CVD graphene at the macro-, micro- and nano-scale

Graphene has become the focus of extensive research efforts and it can now be produced in wafer-scale. For the development of next generation graphene-based electronic components, electrical characterization of graphene is imperative and requires the measurement of work function, sheet resistance, carrier concentration and mobility in both macro-, micro- and nano-scale. Moreover, commercial applications of graphene require fast and large-area mapping of electrical properties, rather than obtaining a single point value, which should be ideally achieved by a contactless measurement technique. We demonstrate a comprehensive methodology for measurements of the electrical properties of graphene that ranges from nano- to macro- scales, while balancing the acquisition time and maintaining the robust quality control and reproducibility between contact and contactless methods. The electrical characterisation is achieved by using a combination of techniques, including magneto-transport in the van der Pauw geometry, THz time-domain spectroscopy mapping and calibrated Kelvin probe force microscopy. The results exhibit excellent agreement between the different techniques. Moreover, we highlight the need for standardized electrical measurements in highly controlled environmental conditions and the application of appropriate weighting functions.