High Free Carrier Concentration Research Articles

Delafossite NaYTe2 as a transparent conductive material with bipolar conductivity: A first-principles prediction

Doping asymmetry is a long-standing issue for the progress of wide-bandgap semiconductors, including also several transparent conductors. However, a few compounds exhibit bipolar conductivity, implying a desired band gap in combination with proper energy positions of the band edges according to the empirical doping limit rule. The present first-principles study of delafossite NaYTe2 reveals that the compound is thermodynamic stable with an optical gap energy of ∼3.0 eV and an ionization potential of 6.2 eV, and the electronic structure is thus in the range for realizing bipolar character as a transparent conductive material. In addition, the analysis of the defect properties strengthens this prediction, especially for high free carrier concentration in NaYTe2, obtained by either extrinsic doping or intrinsic defects under suitable growth conditions.

Time-resolved pump–probe spectroscopic ellipsometry of cubic GaN. I. Determination of the dielectric function

An ultra-fast change of the absorption onset for zincblende gallium-nitride (zb-GaN) (fundamental bandgap: 3.23 eV) is observed by investigating the imaginary part of the dielectric function using time-dependent femtosecond pump–probe spectroscopic ellipsometry between 2.9 and 3.7 eV. The 266 nm (4.66 eV) pump pulses induce a large electron–hole pair concentration up to 4×1020cm−3, which shift the transition energy between conduction and valence bands due to many-body effects up to ≈500 meV. Here, the absorption onset increases due to band filling while the bandgap renormalization at the same time decreases the bandgap. Additionally, the absorption of the pump-beam creates a free-carrier profile within the 605 nm zb-GaN layer with high free-carrier concentrations at the surface, and low concentrations at the interface to the substrate. This leads to varying optical properties from the sample surface (high transition energy) to substrate (low transition energy), which are taken into account by grading analysis for an accurate description of the experimental data. For this, a model describing the time- and position-dependent free-carrier concentration is formulated by considering the relaxation, recombination, and diffusion of those carriers. We provide a quantitative analysis of optical experimental data (ellipsometric angles Ψ and Δ) as well as a plot for the time-dependent change of the imaginary part of the dielectric function.

Characterization of the defect in CIGSe solar cell by admittance spectroscopy

In this work, we used admittance spectroscopy to accurately characterize the defect energy distribution of defects in the absorption layer of a CuIn1−xGaxSe2 (CIGSe) solar cell. We found that the dark current of the CIGSe cell decreased by an order of magnitude, and its ideality factor dropped from 2.16 to 1.85 after annealing. The capacitance of the CIGSe solar cell before annealing was higher than that after annealing under reverse bias. Moreover, a higher free carrier concentration was found in the absorption layer of the cell before annealing. The results of our tests show that the built-in electric voltages of the CIGSe cell before and after annealing were 0.52 and 0.64 V, respectively. After annealing, the activation energy of defects in the absorption layer decreased, and their concentration remained almost constant, as measured by admittance spectroscopy. The decrease of the defect activation energy indicated a corresponding decrease in the Shockley–Read–Hall recombination probability of defects in the present cell; hence, the increase of the open circuit voltage and parallel resistance of the CIGSe solar cells after annealing resulted in improved performance.

Controlled plasmon-induced nonlinear absorption and optical limiting in Al/PVP composite nanofibers.

The effect of aluminum (Al) concentration on the surface plasmon resonance (SPR) band position of aluminum/polyvinylpyrrolidone (Al/PVP) composite nanofibers was investigated to strengthen nonlinear absorption (NLA) and widen its spectral range. With increasing Al content in PVP nanofibers, the SPR band was shifted towards excitation wavelength and an improved NLA response was achieved. The NLA response was examined both experimentally, by conducting Z-scan experiments, and theoretically, using two models. In the first model, the contributions of one-photon absorption (OPA), two-photon absorption (TPA), excited state absorption (ESA) and saturated absorption (SA) are considered. The second model, on the other hand, is a model that is widely used in the literature, and while taking into account the contributions of OPA and TPA, it neglects the ESA. The first model provides more accurate results due to the high concentration of free carriers in the samples examined. In order to reveal the contribution of Al to the nonlinear absorption, a laser excitation wavelength of 532 nm was chosen to minimize both the defect-assisted sequential and genuine two-photon absorption contributions of PVP. While the nonlinear absorption of pure PVP is quite weak, the NLA performance of Al/PVP nanofibers significantly improved as the Al content increased. As the amount of Al increased, the aggregation effect increased and a broadening and red shift in the SPR band were observed in the plasmonic behavior. This indicates a decreasing interparticle distance in Al particles. The sample with the highest amount of Al is anticipated as a potential candidate for optical limiting (OL) applications due to its superior NLA performance and SPR band furthest towards the near infrared (NIR) region, allowing a wider range of wavelength set to be used in OL applications.

Over 130 cm2/Vs Hall mobility of flexible transparent conductive In2O3 films by excimer-laser solid-phase crystallization

Flexible transparent electrodes on flexible plastic sheets are in significant demand for use in flexible perovskite solar cells (f-PSCs). However, the combination of the broadband high optical transparency and low electrical resistivity required for the tandemization of f-PSCs sets a stringent requirement on flexible transparent electrodes that are based on traditional Sn-doped In2O3 (ITO) films, owing to the high free-carrier concentration needed to reduce the electrical resistivity. Herein, we used excimer laser irradiation to achieve a Ce and H codoped In2O3 (ICO:H) film on flexible polyethylene terephthalate (PET) that had ultrahigh electron mobility of 133 cm2/Vs, which is the highest among those reported for flexible transparent electrodes, and low sheet resistance of 14.2 Ω/□, which is approximately three times lower than the 40 Ω/□ sheet resistance of commercially available ITO/PET. Furthermore, compared to ITO, this ICO:H film had higher infrared transparency. These nontrivial performances were achieved by an optimized excimer-laser solid-phase crystallization process guided by the correlation between laser pulse counts and the volume fractions of the amorphous and crystalline phases in the films. These high performances resolved the problems faced by ITO films, thus facilitating the performance of flexible solar cells and optoelectronic devices.

Approaching Theoretical Band Gap of ZnSnN2 Films via Bias Magnetron Cosputtering at Room Temperature

Zinc tin nitride (ZnSnN2) is a promising semiconductor candidate for solar cell applications and optoelectronic devices. However, the practical use of this material has been limited by several issues as, among others, oxygen contamination, a very high concentration of free carriers, and the difficulty to reach the theoretically predicted band gap. Here, we deposit thin films of ZnSnN2 by reactive DC magnetron cosputtering at room temperature with an RF bias power (Pb) in the range of 0–50 W. Using first principles calculations, we explore the structural and optoelectronic properties that are favorably compared to experimental results. We demonstrate that the optical band gap energy can be decreased from 1.7 to 1.34 eV, close to the predicted value of 1.37 eV. The free electron concentration is decreased down to 1017 cm–3 which results in the reduction of the absorption by free electrons in the IR range. In addition, in a given range of applied bias powers, we observe a densification of the films and a significant decrease of their oxygen contamination from 6.7 down to 2.0 at. %. The study underlines that a value of 20 W power bias leads to the optimal structural, optical, and electrical properties. Our results provide an interesting method to obtain a potential candidate for photovoltaic applications, in an environmentally friendly way, for low-cost industrialization.

Study of the heavily p-type doping of cubic GaN with Mg

We have studied the Mg doping of cubic GaN grown by plasma-assisted Molecular Beam Epitaxy (PA-MBE) over GaAs (001) substrates. In particular, we concentrated on conditions to obtain heavy p-type doping to achieve low resistance films which can be used in bipolar devices. We simulated the Mg-doped GaN transport properties by density functional theory (DFT) to compare with the experimental data. Mg-doped GaN cubic epitaxial layers grown under optimized conditions show a free hole carrier concentration with a maximum value of 6 × 1019 cm−3 and mobility of 3 cm2/Vs. Deep level transient spectroscopy shows the presence of a trap with an activation energy of 114 meV presumably associated with nitrogen vacancies, which could be the cause for the observed self-compensation behavior in heavily Mg-doped GaN involving Mg-VN complexes. Furthermore, valence band analysis by X-ray photoelectron spectroscopy and photoluminescence spectroscopy revealed an Mg ionization energy of about 100 meV, which agrees quite well with the value of 99.6 meV obtained by DFT. Our results show that the cubic phase is a suitable alternative to generate a high free hole carrier concentration for GaN.

Design strategy and interface chemistry of ageing stable AZO films as high quality transparent conducting oxide

In this work, an attempt has been made to produce high quality Al-doped ZnO (AZO) transparent conductors by pulsed direct current (DC) magnetron sputtering under varied process conditions, namely sputtering power, sputtering pressure, doping level, and deposition time. The idea is to develop stable AZO films with excellent electro-optical properties, which can replace indium tin oxide (ITO) as transparent conducting oxide. The films are deposited on glass and polyethylene terephthalate (PET) substrates and characterized by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDS) for material properties. The electronic and optical properties are investigated using UV–Vis-NIR absorption spectroscopy, photoluminescence spectroscopy, four probe resistivity and Hall measurements, which facilitate the design strategy to achieve promising electro-optical properties. AZO films under optimized process conditions possess sheet resistance below 5 Ω/sq. and maximum visible light transmittance close to 90%. The lowest value of electrical resistivity is found to be 2.1 × 10−4 Ω·cm. The combination of low electrical resistivity and high optical transparency leading to the figure of merit of the order of 10−2 Ω−1 is critically dependent on the process conditions of sputtering as well as doping contents. High free carrier concentration in AZO films leads to the Burstein-Moss shift and this observed blue shift of the absorption spectrum as compared to that of ZnO with native defects depends on the doping contents and the process conditions. This work further explores the role of interface chemistry to achieve high stability of AZO films under ambient oxidizing conditions. The adsorption kinetics of cationic rhodamine 6G and anionic methyl orange at AZO surface with varying adsorption sites and ageing studies on the sheet resistance reveal that the excellent stability of AZO films is delicately linked to the process conditions as well as the content of the dopant material used in the preparation of the AZO sputtering targets. The findings of the study confirm that the sheet resistances of selective AZO films undergo no change and fixed at 4.5 Ω/sq. even for ageing period of over six months.

Superior optoelectrical properties of magnetron sputter-deposited cerium-doped indium oxide thin films for solar cell applications

Indium tin oxide (ITO) is the most commonly used front contact material for a variety of photovoltaic technologies. However, the presence of a high free carrier concentration in ITO thin films results in the well-known phenomenon of free carrier absorption in the near-infrared (NIR) region of the solar spectrum. This causes optical losses especially in those solar cells where the active layer is designed to preferentially absorb NIR photons. Therefore, a combination of high carrier mobility and high NIR transparency is desired for advanced transparent conductive oxides for substituting ITO in solar cells. In this work, cerium-doped indium oxide (ICeO) thin films are deposited by pulsed DC magnetron sputtering, giving a remarkable 137% improvement of the mobility (71 cm2 V−1 s−1) compared to the previous record value of 30 cm2 V−1 s−1 for DC magnetron sputtered cerium-doped ITO films on glass. When compared to conventional ITO films prepared in this work, the highest mobility of ICeO is found to be almost four times higher and also the NIR transmission is substantially enhanced. Theoretical modelling of the experimental results indicates that neutral impurity scattering limits the carrier mobility in our films. With the recent advancements in single and multi-junction organic and perovskite solar cells, the development of ICeO/glass substrates (as possible replacements for the commonly used ITO/glass substrates) demonstrates significant potential in minimizing optical losses in the NIR region.

Plasmon – Phonon interaction in ZnSnSb2 + Mn semiconductors

Semiconductors of II-IV-V2 type with chalcopyrite structure have been studied for several decades. Due to advances in materials synthesis technologies, and doping with various elements, the possibilities of their application have expanded. In this paper, polycrystalline ZnSnSb2 + Mn was examined with the aim to explain the connection of its high free carrier concentration with the material structure and influence on optical properties. Two samples of Zn1-xMnxSnSb2 with different compositions (x = 0.027 and x = 0.076) and significant difference in carrier concentrations were analyzed. Their structural properties were examined by x-ray diffraction, optical microscopy, and AFM. The existence of several different phases - ZnSnSb2, ZnSb, SnSb, and small amounts of Sn and MnSb, as well as very complex microstructures, were registered. It was found that the high free carrier concentrations are caused by a large number of defects, especially zinc vacancies. Optical properties were analyzed using IR spectroscopy at room temperature. Based on the analysis of IR reflection spectra, the presence of plasmon - phonons interaction was registered. It was determined that three ZnSnSb2 phonons of B2 symmetry interact with plasma, which then leads to the change of their positions. A detailed analysis of this interaction provides insight into the behavior of some other material parameters. Also, vibration modes of ZnSb and SnSb phases were registered on the spectra. Knowledge of phonon behavior and their interaction with plasma is important for possible applications, especially as a thermoelectric material.

Band gap engineering in ZnO based nanocomposites

Abstract In the present work, individual zinc oxide (ZnO), tin oxide (SnO2) and vanadium oxide (V2O5), their binary and ternary combinations were prepared via hydrothermal route. The structure and the morphology of all the samples were characterised using X-ray Diffractometer (XRD) and Field Emission Scanning Electron Microscopy (FE-SEM) and Raman spectroscopic tools. UV–Visible spectroscopic analysis was recorded for all the samples. The samples were also tested for ethanol sensing (0 ppm–300 ppm) at room temperature. It has been focussed on to investigate the Raman and UV spectroscopic studies on the ethanol sensing properties of these oxides. It is observed that the Raman peaks of the binary and ternary systems shifted to lower wavenumber compared to their bulk and it is attributed to the tensile stress experienced by the nanocomposite. The peculiar hierarchical nanostructures of zinc – tin – vanadium oxide (ZTV) nanocomposite with larger surface area (167.3 m2/g) provided the required active surface sites for the adsorption of ethanol molecules. The band gap of ZTV is calculated as 1.97 eV. This band gap narrowing observed in ZTV might be due to the competing effects of high free carrier concentration and Burstein-Moss shift. Hence ZTV shows pronounced sensitivity of 98% at a response time of 32 s and recovery time of 6 s. Moreover the synergistic effect of ZTV nanocomposite enhanced the sensitivity at a faster rate by overcoming the problems associated with the energy consumption, reversibility, adsorptive capacity and fabrication cost.

Bipolar Analog Memristive Switching of In2O3 Using Al Nanoparticles

Aluminum nanoparticles (AlNPs) were embedded into a sol-gel synthesized In₂O₃ thin film by using a combination of thermal evaporation and glancing Angle Deposition technique. Presence of different sizes of aluminum nanoparticles was confirmed from field emission gun scanning electron microscopy. The high-resolution X-ray diffraction confirms the formation of Al₂O₃ NPs by surface oxidation of aluminum nanoparticles. Embedded Metal-Oxide-Semiconductor like device Al/In₂O₃/AlNPs/In₂O₃/p-Si was fabricated, and its memristor behavior was analyzed. The Al/In₂O₃/AlNPs/In₂O₃/p-Si device possessed high current conduction and analog resistive switching as compared to Al/In₂O₃/p-Si device. Significant and consistent memory window up to 150 current (I)-voltage (V) loop was obtained for Al/In₂O₃/AlNPs/In₂O₃/p-Si device between ±6 V applied bias. The Al/In₂O₃/AlNPs/In₂O₃/p-Si device measured high free carrier concentration, i.e., (Nd)~1.93 × 1020 cm-3 calculated from capacitance (C)-Voltage (V) measurement. The memory was retained in accumulation and depletion regions as obtained from the C-V looping curves.

고효율 Silicon Heterojunction Solar Cell을 위한 Indium Tin Oxide 연구

HIT (heterojunction with intrinsic thin-layer) solar cells are in the spotlight as ultra-high efficiency solar cells. While an a-Si: H (i) (intrinsic hydrogenated amorphous silicon) layer is important for high quality passivation layers, the TCO (transparent conductive oxide) has also become important. This paper describes the changes in the properties of ITO (indium tin oxide) in HIT solar cells. ITO is used as a TCO because of its high transmittance and conductivity at thin thickness. On the other hand, a relatively high free carrier concentration and low mobility are disadvantageous compared to other TCOs. To improve the properties of ITO, a method of diversifying the deposition process and subsequent heat treatment has been proposed. Alternatively, IZO (indium zinc oxide), IO: H (hydrogen doped indium oxide), IWO (indium tungsten oxide), and ITiO (indium titanium oxide), etc. have been used as TCOs to replace ITO.

Design Principles of p-Type Transparent Conductive Materials.

Transparent conductive materials (TCMs) has always been playing a significant role in electronic and photovoltaic area, due to its prominent optical and electronic properties. To render those transparent materials highly conductive, efficient n- and p- type doping is critically needed to obtain high concentration of free electron and hole carriers. Despite extensive research over the past five decades, high-quality p-type doping of wide-band-gap transparent materials remains a challenge. Here, we summarize four proposed design principles to enhance the p-type conductivity of these wide band gap materials, including (i) reducing the formation energy of the acceptors to enhance the dopant concentration; (ii) lowering the ionization energy and, hence, increasing the ionization of the acceptors to increase the concentration of the free holes; (iii) increasing the VBM of the host material to approaching the pinned Fermi level; and (iv) suppressing the compensating donors to shifting the pinning Fermi level toward the VBM. For each mechanism, we discuss in detail its underlying physics and provided some examples to illustrate the design principles. From this review, one could learn the doping principles and have a strategic mind when designing other p-type materials.

Investigation of the thermoelectric response in conducting polymers doped by solid-state diffusion

Thermoelectric effect is a physical phenomenon which intricately relates the thermal energy of charge carriers to their charge transport. Understanding the mechanism of this interaction in different systems lies at the heart of inventing novel materials which can revolutionize thermoelectric power generation technology. Despite the recent surge of interest in organic thermoelectric materials, the community has had difficulties in formulating the charge transport mechanism in the presence of a significant degree of disorder. Here, we analyze the thermoelectric properties of various conducting polymers doped by solid-state diffusion of dopant molecules based on a transport model with a power law energy dependence of transport function. A fine control of the degree of doping via postdoping annealing provides an accurate empirical evidence of a strong energy dependence of the carrier mobility in the conducting polymers. A superior thermoelectric power factor of conducting polymers doped by solid-state diffusion to that of other doping methods can be attributed to a resulting higher intrinsic mobility and higher free carrier concentration.

Drude and Kukharskii mobility of doped semiconductors extracted from Fourier-transform infrared ellipsometry spectra

The factorized plasmon-phonon polariton description of the infrared dielectric function is generalized to include an additional factor to account for the effects of interband electronic transitions. This new formalism is superior to the usual Drude–Lorentz summation of independent oscillators, especially in materials with large transverse-longitudinal optical phonon splittings, multiple infrared-active phonon modes, or high concentrations of free carriers, if a broadband description of the dielectric function from the far-infrared to the vacuum-ultraviolet spectral region is desired. After a careful comparison of both approaches, the factorized description is applied to the dielectric function of undoped and doped semiconductors (GaAs, GaSb, and InAs) and metal oxides from 0.03 to 9.0 eV. Specifically, the authors find that both descriptions of the far-infrared dielectric function yield the same carrier density and mobility, at least for a single species of carriers. To achieve valid results for moderately high doping concentrations, measurements to lower energies would be helpful.

3D GaN Fins as a Versatile Platform for a‐Plane‐Based Devices

GaN fins on GaN‐on‐sapphire templates are fabricated by continuous mode selective area metalorganic vapor phase epitaxy. The fins exhibit high aspect ratios and smooth nonpolar a‐plane sidewalls with an ultra‐low threading dislocation density of a few 105 cm−2 making them ideally suited for optoelectronic to electronic applications. A detailed analysis of the inner structure of GaN fins is provided by the help of marker layer experiments and correlation of results from fins fabricated under different growth conditions, leading to the development of a growth model to explain the final geometry and optical as well as electrical properties of these high aspect ratio fins. Distinctly different material properties for the central and outer parts of the fins are detected. Whereas the outer sidewalls represent high quality GaN surfaces with very low defect densities, a strong quenching of near band edge emission (NBE) in the central part of the fins is accompanied by heavy compensation of free electrons. A possible explanation is the incorporation of excessive point defects, like intrinsic defects or carbon impurity. The sidewall regions, however, prove to be highly suitable for device applications due to their strong NBE emission, low dislocation density, and high free carrier concentration.

Resolving ZnO-based coaxial core-multishell heterostructure by electrical scanning probe microscopy

Coaxially periodic ZnO/ZnMgO core-multishell nanowire (NW) heterostructures were grown via a metal organic chemical vapor deposition method. We investigated their electrical properties via the application of two locally resolved electrical scanning probe microscopy techniques, i.e., scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM), following a planarization process. As a result, ZnO and ZnMgO layers can be unambiguously distinguished by both techniques on NWs with diameters <1 μm and the smallest layer thickness of 10 nm, where a higher free carrier concentration along with a low resistivity is revealed for the ZnO regions in comparison to ZnMgO portions, as expected. This work demonstrates the high capability of SCM/SSRM as supplementary and effective tools for probing local electrical properties within functional complex quasi-1D heterostructures.

Giant enhancement of free charge carrier concentration in boron-doped amorphous hydrogenated silicon under femtosecond laser crystallization

The modification of boron-doped amorphous hydrogenated silicon films [a-Si:H(B)] with femtosecond laser radiation is studied in this work. It is demonstrated that femtosecond laser crystallization of the a-Si:H(B) film area leads to extremely high values of the free charge carrier (hole) concentration, which is typical for degenerated semiconductors. The free charge carrier concentration is locally determined by measuring the Raman spectra in the modified areas. The shape of Raman spectra is typical for Fano resonance. It is found that the charge carrier concentration in the modified areas may exceed 1020 cm−3 depending on the femtosecond irradiation conditions. The areas with such a high concentration of free charge carriers are also characterized by high volume fraction of crystalline phase (more than 90%). Such a sharp increase in the free charge carrier concentration in the modified areas may be explained by an increase in concentration of electrically active boron atoms. The activation energy of the temperature dependence of the conductivity for laser beam treated areas is in full agreement with the data obtained from the analysis of the Raman scattering spectra.

Electrical properties modulation in spray pyrolysed Cu2SnS3 thin films through variation of copper precursor concentration for photovoltaic application

Thin film of Cu2SnS3 (CTS), a truly inexpensive photovoltaic absorber is deposited by low cost and non-vacuum spray pyrolysis technique on glass substrates. As deposited CTS films are of p-type in nature, however they are characterised with undesirably high free carrier concentration which leads to a large amount of recombination. Such a high carrier concentration is successfully reduced from 1020 cm−3 to the order of 1018 cm−3 by varying the initial molar copper concentration in the precursor solution. A combined XRD and Raman spectroscopy analysis reveals that the coexistence of secondary Cu3SnS4 phase is responsible for the higher carrier concentration of the order of 1021 cm−3, due to its semi-metallic nature. The optical study of the film also shows the reduction in the band gap from 1.73 eV to 1.29 eV with the absorption coefficient of >104 cm-1. These features of the precursor optimised CTS film enable it as a promising photovoltaic absorber.