Free Carrier Density Research Articles

Free carrier density effects on DyN optical spectra

The optical energy gap is a parameter of fundamental importance in semiconductors. However, concentration of free carriers may conceal the true absorption edge. In this paper, we show that the density of the free carriers can affect the onset of the direct absorption. The optical reflectance and transmittance spectra were obtained in the photon energy range of 0.5–5.0 eV thin films of the nitride compound of Dy metal. The films with the greater number of carriers exhibit a greater blue shift in the absorption edge.

Determining the Free Carrier Density in Cd x Hg1–xTe Solid Solutions from Far-Infrared Reflection Spectra

A new contactless nondestructive technique for determining the free carrier density in single-crystal samples of Cd x Hg1–x Te solid solutions and multilayer epitaxial heterostructures based on them from farinfrared reflection spectra is proposed. The characteristic point and corresponding wavenumber in the room-temperature spectral dependence of the reflectance are determined. The heavy hole density is established using calculated calibration curves. It is shown that in constructing the calibration curves, it is necessary to take into account the interaction of plasma oscillations with longitudinal optical phonons.

Out-of-plane ionicity versus in-plane covalency interplay in high-T c cuprates

It may be argued that the remarkable properties of the high-temperature superconducting cuprates such as the insulator–metal transition (IMT) and the metal–superconductor transition (MST) originate from competition and interplay between the interlayer ionic interaction and the intralayer covalent bonds in these materials. It is proposed here that the microscopic order parameter is the local field estimated from the ionic polarization at the sub-unit cell level, and it is demonstrated that it shows a strong temperature as well as chemical doping dependence. The out-of-plane ionicity induces an interlayer electron transfer that reduces the ionicity of the layers and leads to IMT, while the in-plane covalency induces in-plane intersite hole transfer that increases the out-of-plane ionicity. It is suggested that this competition leads to a local field catastrophe at a critical temperature T c that drives the compound to MST. The asymmetry of the free charge carrier density breaks locally the mirror reflection symmetry of the order parameter, leading to a pairing between the real current and the polarization current.

Size-Induced Enhancement of Carrier Density, LSPR Quality Factor, and Carrier Mobility in Cr–Sn Doped In2O3 Nanocrystals

Heterovalent dopant ions, such as Sn4+, in In2O3 nanocrystals (NCs) provide free electrons for localized surface plasmon resonance (LSPR). But the same heterovalent dopants act as electron scattering centers, both independently and by forming complexes with interstitial oxygen, thereby increasing LSPR line width. Also, such complexes decrease free carrier density. These detrimental effects diminish the figure-of-merit of LSPR known as the quality factor (Q-factor). Herein, we designed colloidal Cr–Sn codoped In2O3 NCs, where both high carrier density and low carrier scattering can be achieved simultaneously, yielding a high LSPR Q-factor of 7.2, which is a record high number compared to prior reports of doped In2O3 NCs. Q-factors increase systematically from 3.2 for 6.6% Sn doped In2O3 NCs to 7.2 for 23.8% Cr–6.6% Sn codoped In2O3 NCs by increasing the Cr codoping concentration, which is also accompanied by an increase in NC size from 6.7 to 22.1 nm. Detailed characterization and analysis of LSPR spectra ...

Effect of temperature on the properties of SnO2 layer fabricated via AACVD and its application in photoelectrochemical cells and organic photovoltaic devices

Thin-film tin oxide (SnO2) thin film was prepared at different temperatures via aerosol-assisted chemical vapour deposition (AACVD) technique and the morphological, optical as well as electrical properties was investigated using X-ray diffraction, atomic force microscopy, UV–Vis spectroscopy and Mott-Schottky plot. The SnO2 thin film deposited at high temperature exhibited higher crystallinity, rougher surface, higher light absorption and greater free-carrier density in comparison to low temperature processed counterpart. The prepared thin films were employed as photoanode in photoelectrochemical (PEC) water splitting and as electron transporting layer (ETL) in P3HT:PC70BM based-organic solar cell. Interestingly, it was discovered that the photocurrent density of PEC cells based on high temperature (450°C) processed SnO2 layer was about 27μA/cm2 at 1.0V, approximately five times higher than that of SnO2 layer prepared at low temperature (180°C). On the other hand, low temperature-based SnO2 ETL was found to enhance the efficiency of organic solar cells by about six times in comparison to their high temperature based counterparts. The reasons of such phenomenon lay in the morphological, optical and electrical properties of the fabricated SnO2 layer which is thoroughly explained in this paper.

Thermally ruggedized ITO transparent electrode films for high power optoelectronics.

We present two strategies to minimize laser damage in transparent conductive films. The first consists of improving heat dissipation by selection of substrates with high thermal diffusivity or by addition of capping layer heatsinks. The second is reduction of bulk energy absorption by lowering free carrier density and increasing mobility, while maintaining film conductance with thicker films. Multi-pulse laser damage tests were performed on tin-doped indium oxide (ITO) films configured to improve optical lifetime damage performance. Conditions where improvements were not observed are also described. When bulk heating is not the dominant damage process, discrete defect-induced damage limits damage behavior.

Electric Field Reconstruction and Transport Parameter Evaluation in CZT X-Ray Detectors

The laser-induced current technique has been successful used to reconstruct the spatial profile of the electric field along the thickness of a set of CdZnTe spectroscopic X-ray detectors. Current transient profiles for electrons at different applied voltages have been analyzed by means of a minimization procedure demonstrating its applicability to samples with thickness ranging from 250 $\mu \text{m}$ to 4 mm. Mobility and lifetime of electrons have also been deduced and compared with the mobility-lifetime products, as evaluated by fitting the charge collection efficiency curves under a suitable electric field profile model. Comparison between results from both techniques gives a good agreement and confirms the validity of the procedure. This method results applicable each time that carrier transit times can be evaluated from the laser-induced current transients. It could be suitable for many other devices provided that they are made of materials with sufficiently high resistivity, i.e., with a sufficiently low density of free carriers in dark conditions.

Diindenoperylene (DIP) concentration dependent photovoltaic performance and dielectric properties for mixed heterojunctions

To find some intrinsic relevance between the photovoltaic performance and dielectric properties of diindenoperylene (DIP) mixed heterojunctions (MHJs), this work demonstrates both the photovoltaic performance and dielectric characteristic dependence on the DIP concentration. 5wt%-DIP MHJ shows relatively higher performance compared to other MHJs with higher DIP ratio over the range of 5wt%–90wt%. The dielectric constant of blend layers decreases with the DIP ratio, thus results in larger Coulomb capture radius and exciton binding energy, which suppresses the exciton dissociation in MHJs with high DIP ratio. Then, the dielectric characteristic of capacitance-voltage (C-V) in dark and under illumination as well as of capacitance-frequency (C-f) at dc bias of 1.6V for the DIP MHJs were measured. The geometric capacitance of the MHJs extracted from the C-V in dark indicates a positive dependence on the DIP concentration. C-V peak height decreases with the DIP concentration from 0 to 60wt% implies that the hole drift becomes dominated at higher DIP content. The open circuit voltage (VOC) of all the MHJs shows an identical trend with the normalized built-in potential. In dark condition, depletion width (W) gradually decreases with the DIP concentration from 5wt% to 60wt%.W becomes narrower under illumination, ascribed to the increased capacitance from photo-generated charge. The free carrier density (ND) imposes a negative influence on fill factor of MHJs. The extracted electron mobility by C-f characteristic gradually decreases with the DIP ratio.

Interband and free charge carrier absorption in silicon at 800 nm: experiments and model calculations

The average transmitted power of a Ti:sapphire femtosecond (fs) laser beam through thin, oxidized silicon (Si) membranes in the 10–30 \(\, \upmu \text{m}\) thickness range at \(45{^\circ }\) external angle of incidence at first increases with the incident average laser power and then decreases considerably. This stationary state behavior is quantitatively reproduced by a recently derived model using an effective absorption coefficient \(\alpha _{\text {eff}} = \alpha _{1} + \alpha _{\text {FCA}}\), adding the coefficients of two independent linear absorption processes, \(\alpha _{1}\) of the indirect interband transition and \(\alpha _{\text {FCA}}\) of free charge carrier absorption (FCA). The relations between the empirical parameter \(\alpha _{\text {FCA}}\) and the model parameters \(n_{\text {eh}}\) (density of free charge carriers) and \(\sigma _{\text {eh}}\) (absorption cross section of free charge carriers) are found to depend on the Si membrane thickness and are briefly discussed. Using different sample thicknesses, experimental evidence has been found for a nonlinear intensity dependence of \(\alpha _{\text {FCA}}\)∼\(I_{0}^{n}\), where \(1\le n \le 3\) shows systematic variation. The FCA model in combination with its empirical fit parameters allowed to fully calculate the transmission behavior of the Si membranes within the investigated sample thickness and laser power regimes.

Simultaneous drop in mean free path and carrier density at the pseudogap onset in high- cuprates

High-temperature superconducting cuprates are distinguished by an enigmatic pseudogap which opens near optimal doping where the superconducting transition temperature is highest. Key questions concern its origin and whether it is essential in any way to superconductivity. Recent field-induced normal-state transport experiments on hole-doped cuprates have measured abrupt changes in the doping-dependent Hall number and resistivity, consistent with a drop in carrier density from to p holes per copper atom, on entering the pseudogap phase. In this work the change in resistivity is analyzed in terms of an antiferromagnetic-order-induced Fermi surface reconstruction model that has already successfully described the Hall number. In order for this model to describe the resistivity we find that the zero-temperature mean free path must also drop abruptly in proportion to the size of the Fermi surface. This suggests that intrapocket scattering underlies the observed upturn in resistivity in the pseudogap state.

Impact of defect distribution on IrOx/ZnO interface doping and Schottky barriers

We used depth-resolved cathodoluminescence spectroscopy (DRCLS) to measure the nature and spatial distribution of native point defects at Zn- and O-polar ZnO interfaces with iridium oxide (IrOx) and their impact on Schottky barrier formation. IrOx and other metal oxides exhibit higher Schottky barriers than their pure metal counterparts, consistent with wider depletion regions and potentially useful for ohmic contacts to p-type semiconductors. DRCLS with I-V and 1/C2-V barrier height and carrier profile measurements showed high zinc vacancy VZn and CuZn defect densities that compensate free carrier densities, increase depletion widths, and form higher effective barriers than Ir/ZnO contacts. Zn-polar versus O-polar ZnO interfaces with IrOx exhibit 40% higher VZn + CuZn interface segregation and lower carrier densities within a wider depletion region, accounting for the significantly higher (0.89 vs. 0.67 eV) barrier heights. Both the depth of VZn density segregation and the Zn-deficient layer thickness measured microscopically match the depletion width and applied electric fields comparable to spontaneous polarization fields across similar layers displaying analogous defect segregation. These results account for the difference in polarity-dependent segregation due to the electric field-driven diffusion of native defects near ZnO interfaces.

(Invited) Ultra-Wide-Bandgap Aluminum Gallium Nitride Power Switching Devices

SiC- and GaN-based power semiconductor devices have in the past few years enabled great improvements in the efficiency and power density of switching power converters. A wide variety of SiC devices (e.g. MOSFETs, JFETs, BJTs, thyristors, and PiN/Schottky/JBS/MPS diodes) are now available from a number of manufacturers, and the same is true for GaN HEMTs. Thus, while vertical GaN devices are not yet mature, new research in the field is increasingly turning to the “ultra” wide-bandgap (UWBG) semiconductors, including diamond, gallium oxide, and aluminum gallium nitride (AlGaN), due to the expected scaling of the critical electric field as the bandgap to the 2.0-2.5 power. Notably, AlGaN is an alloy system, so that heterostructures are available, and it is also a polar material, which enables polarization doping. Both of these benefits significantly expand the range of device architectures that may be considered, compared to materials that do not have these properties; this is especially significant for UWBG materials, all of which have energetically deep impurity dopants that do not fully ionize at room temperature. This talk will report on both vertical PiN diodes and lateral HEMTs composed of Al-rich Al x Ga1-x N (x ≥ 0.7, EG ≥ 5.2 eV) designed as prototype power switching devices. The talk will begin with an overview of the properties of AlGaN that motivate its application to power switching, as well as an analysis of conduction and switching loss mechanisms as a function of AlGaN bandgap for various device types (PiN diodes, Schottky diodes, etc.) These results will be compared to similar metrics for SiC, GaN, and UWBG materials other than AlGaN. The talk will then present results on several PiN diode structures. All of the structures to be presented have thick (5-8 mm) Al0.7Ga0.3N drift regions doped in the 1-3×1016 cm-3 range, and were grown on thick (1.3 mm) sapphire substrates; due to the insulating nature of the substrate, the cathode contact is to a heavily-doped n-layer on the front side of the wafer in a so-called “quasi-vertical” configuration. The diodes differ in the design of the p-type anode. Homojunction diodes suffer from low free carrier density in the p-Al0.7Ga0.3N due to the low fraction of Mg than is ionized in material of such a wide bandgap (EA > 400 meV). Thus, two alternate approaches to achieve appreciable hole density were examined: heterojunction diodes in with the anode is composed of Al0.3Ga0.7N, in which reasonable Mg activation can be achieved; and diodes utilizing anodes in which the composition is graded from Al0.7Ga0.3N down to GaN. In order to avoid premature edge and/or surface breakdown, all diodes studied utilized a junction termination extension formed by nitrogen implantation into the p-region. The talk will conclude with a presentation of a second class of device, which is a power switching HEMT utilizing an Al0.85Ga0.15N/Al0.70Ga0.30N heterostructure. This heterostructure, which as a 25 nm thick barrier, is characterized by a pinch-off voltage of -4.5 V, a channel mobility of 250 cm2/Vs, and a channel sheet charge density of 6×1012 cm-2. The key challenge for Al-rich HEMTs has been the formation of Ohmic source and drain contacts; the current density in previous HEMTs reported by our group has been limited by the quasi-rectifying nature of these contacts. The HEMTs in the present work utilize planar source and drain contacts, as opposed to the etched-and-regrown contacts that we have reported previously; for those previous contacts, the regrown material was heavily-doped n-type GaN. The planar contacts in this study are composed of a Ti/Al/Ni/Au stack deposited directly on the Al0.85Ga0.15N barrier. While the current-voltage characteristics of these contacts are still quasi-rectifying, they have enabled a current density of 46 mA/mm, which is more than ten times greater than that achieved in our previous-generation AlGaN-channel HEMT, which had comparable channel sheet resistance. An effective breakdown field of ~170 V/um was achieved in a field-plated device with 4 um gate-to-drain spacing. This effective breakdown field exceeds that typically reported for GaN-channel HEMTs (~100 V/um), demonstrating the potential of AlGaN-channel HEMTs for high-performance power switching devices. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Electrical Characteristics of Fluorine-Doped Zinc Oxynitride Thin-Film Transistors

Increasing demands for cost effective switching transistors in the flat panel display industry have led to the development of metal oxide semiconductors such as In-Ga-Zn-O (IGZO). Thin-film transistors (TFT) that incorporate such materials exhibit field effect mobility exceeding 10 cm2/Vs, and are suitable for the fabrication of large size liquid crystal display (LCD) panels with ultra-high definition (4000 ´ 2000 pixels). However, further advances in display technology aim at producing organic light-emitting diodes (OLEDs) that require relatively high driving currents. In this regard, devices with even higher field effect mobility (> 30 cm2/Vs) must be integrated in the backplane array. Recent studies of a relatively new type of semiconductor, zinc oxynitride (ZnON), have shown that high mobility semiconductor films can be obtained by reactively sputtering a Zn metal target, which involves a simple and inexpensive process. The optimization of the associated TFT properties usually consists of controlling the nitrogen to oxygen anion ratio or thermally annealing the ZnON layer, so as to obtain sufficiently low leakage current levels while preserving sufficiently high field effect mobility. It is usually reported that the nitrogen vacant sites (VNs) act as the major source of free electrons and carrier traps that may degrade the device properties over prolonged bias stress. In order to passivate such defects, fluorine doping were attempted in nitride semiconductors. The present work is a study on the effect of directly incorporating fluorine anions during the thin film growth, by co-sputtering a ZnF2 target alongside the Zn metal target. First-principles calculations indicate that fluorine may act as a carrier suppressor in the ZnON matrix by filling in the VN sites, which is an energetically favorable reaction. The properties of F-doped ZnON (ZnON:F) are investigated through various characterization methods, and the performance of the TFT devices incorporating ZnON:F active layers are studied next. In order to examine how the chemical bond properties are influenced by the presence of fluorine, X-ray photoelectron spectroscopy (XPS) analyses were carried out. The results indicate that as the ZnF2 target power increases to 6 and 10 W, the ratio of fluorine content increases up to 3.47 and 9.22 at. %, respectively. The amount of nitrogen on the other hand decreases down to 20.5 and 19.2 at. %, respectively. Note that the oxygen content remains relatively constant (Table 1.). Figures 1 (a)-(c) consist of the XPS O 1s peak spectra of the pure ZnON and ZnON:F 6, 10 W films. The O 1s peak was deconvoluted into two into two sub-peaks labelled O1 and O2, which are generally known to occur from oxygen forming metal-oxygen bonds and oxygen near oxygen vacant sites (OV) in oxide semiconductors, respectively. No apparent variation in the relative area ratios of sub-peaks O1 and O2 is observed, which implies that the incorporated fluorine does not affect the oxygen-related bonding states. Figures 1 (d)-(f) consist of XPS N 1s peak spectra of pure ZnON and the ZnON:F 6, 10 W layers. The N 1s peaks were deconvoluted into three different sub-peaks. The lowest energy sub-peak originates from nitrogen atoms in non-stoichiometric ZnxNy (including nitrogen vacancies), while the middle sub-peak arises from the nitrogen atoms in stoichiometric Zn3N2. The highest energy sub-peak represents mostly N-N bonds. The relative peak area ratios of the three sub-peaks within a single N 1s peak are represented in in table 2. Note that as the amount of incorporated fluorine increases, the relative fraction of nitrogen from stoichiometric Zn3N2 peak increases. Accordingly, the peak intensities from non-stoichiometric ZnxNy and N-N bonds diminish. The above results may be interpreted to occur from the fluorine anions passivating the vacant N sites that contribute to the sub-peak originating non-stoichiometric ZnxNy, so that the relative fraction of the latter appears to decrease with increasing F content, and consequently the sub-peak from stoichiometric Zn3N2 increases. The transfer characteristics of the devices based on ZnON and ZnON:F are shown in figure 2. Because N-rich ZnON was synthesized as the host material, a relatively large number of nitrogen vacancies is formed so as to convey a high free carrier density. ZnON device exhibits high off-state current, which diminishes as the fluorine content in the semiconductor increases. The representative transfer parameters are listed in table 3. The threshold voltages shift towards positive values with increasing F content. A field effect mobility of 151 cm2/Vs is obtained using ZnON:F 6 W as the semiconductor, with sufficiently low off-state current and threshold voltage near zero, so that the device may be used as a switching element in electronic applications. Figure 1

Impact of the silicon substrate resistivity and growth condition on the deep levels in Ni-Au/AlN/Si MIS Capacitors

Deep levels formed under different growth conditions of a 200 nm AlN buffer layer on B-doped Czochralski Si(111) substrates with different resistivity were investigated by deep-level transient spectroscopy (DLTS) on metal-insulator-semiconductor capacitors. Growth-temperature-dependent Al diffusion in the Si substrate was derived from the free carrier density obtained by capacitance-voltage measurement on samples grown on p− substrates. The DLTS spectra revealed a high concentration of point and extended defects in the p− and p+ silicon substrates, respectively. This indicated a difference in the electrically active defects in the silicon substrate close to the AlN/Si interface, depending on the B doping concentration.

Study of non-equilibrium thermal transport in Ge2Sb2Te5 thin films under ultrafast laser excitation using a photo-excited carrier integrated semiconductor model

A two-temperature semiconductor model was used to investigate the non-equilibrium thermal transport in Ge2Sb2Te5 thin films caused by an ultrashort laser pulse ranging from atto- to nanoseconds. In the model, photo-excited carriers were considered based on the semiconductor absorbing mechanism. As a general rule, shorter laser pulses led to shorter equilibration time between carrier and lattice systems. However, a minimum time to reach the thermal equilibrium (about 80 ps) was obtained for both the attosecond and femtosecond laser pulses, which was mainly determined by material properties instead of the pulse width. The carrier density had a strong influence on the temperatures of both the carrier and lattice systems, and photo-excited carriers played an important role in the first 40 ps under the attosecond and femtosecond laser irradiations. Besides, the ambipolar diffusion effect significantly reduced the density of free carriers and then extended the crystallization and amorphization times of Ge2Sb2Te5.

Low-temperature spray-coating of high-performing ZnO:Al films for transparent electronics

Ultrasonic spray pyrolysis deposition of ZnO-based materials offers an attractive high-throughput low-cost route towards industrial production of high-quality transparent conductive oxide (TCO) thin-films. In this work, undoped and aluminium-doped ZnO films have been grown employing ultrasonic spray pyrolysis at relatively low-temperate (300°C), followed by a post-annealing treatment. The role of Al concentration in the starting solution, as well as the rapid thermal annealing (RTA) atmosphere, were investigated and correlated to the morphological, structural, electrical and optical properties of the films. The remarkable enhancement of electrical conductivity attained here is mainly ascribed to the combined effects of: (1) homogenous incorporation of Al3+ into the ZnO matrix, which enhances crystal quality providing higher electronic mobility; and (2) the RTA which releases the localized electrons caused by oxygen absorption and thereby increases the free carrier density. Under optimum deposition conditions, a low resistivity and a high optical transmittance around 4×10−3Ωcm and 87%, respectively, were obtained. The application of the RTA post-process after low temperature growth has several advantages relative to the direct growth at high temperature (usually 400–575°C), such as shorter growth time and lower cost associated to the spray pyrolysis equipment requirements and usage. The results suggest that the electrical and optical properties of the ZnO:Al films can be further improved for solar cell applications by controlling the temperature of the post-deposition annealing in reducing atmosphere.

Wavelength scaling of silicon laser ablation in picosecond regime

Single pulse laser ablation of silicon has been investigated at 343, 515, and 1030 nm using a laser pulse duration of 50 ps. In this large spectral range, ablation thresholds of silicon vary from 0.01 to 0.83 J/cm2, confirming a strong dependence on the wavelength. By solving the free-carrier density rate equation at threshold conditions, we show that band-to-band linear absorption dominates energy deposition at 343 and 515 nm, whereas at 1030 nm, the energy leading to ablation is primarily absorbed by the generated free-carriers. This allows us to determine the relevant criteria to derive a simple model predicting the wavelength dependence of the ablation threshold in this regime. We obtain an excellent agreement between experimental measurements and calculations by simply considering an averaged energy density required in the absorption depth for surface ablation and accounting for the laser-induced variations of the important thermophysical parameters. On the basis of this analysis, we discuss the optimal wavelength and fluence conditions for maximum removal rate, ablation efficiency, and accuracy. Despite the difference in mechanisms at the different wavelengths, we find that the maximal efficiency remains at around 7 times the ablation threshold fluence for all investigated wavelengths. This work provides guidelines for high-quality and efficient micromachining of silicon in the scarcely explored picosecond regime, while new picosecond sources offer numerous advantages for real throughput industrial applications.

Free-Carrier-Induced Nonlinear Relaxation in a Silicon Waveguide

This letter reports our study on the effects of free carrier lifetimes due to surface/interface recombination and Auger recombination on the nonlinear dynamics of light propagation in a silicon waveguide at moderate to high power levels. It is shown that due to the coupling dynamics between the optical intensity and the free carrier density, the effective decay time of the optical intensity in the waveguide is not constant but becomes significantly reduced at high input powers, a result which may have important implications to nonlinear optics applications of silicon waveguides such as all-optical switching and modulation.

Poor electronic screening in lightly doped Mott insulators observed with scanning tunneling microscopy

The effective Mott gap measured by scanning tunneling microscopy (STM) in the lightly doped Mott insulator $(\rm{Sr}_{1 -x}\rm{La}_x)_2\rm{IrO}_4$ differs greatly from values reported by photoemission and optical experiments. Here, we show that this is a consequence of the poor electronic screening of the tip-induced electric field in this material. Such effects are well known from STM experiments on semiconductors, and go under the name of tip-induced band bending (TIBB). We show that this phenomenon also exists in the lightly doped Mott insulator $(\rm{Sr}_{1 -x}\rm{La}_x)_2\rm{IrO}_4$ and that, at doping concentrations of $x\leq 4 \%$, it causes the measured energy gap in the sample density of states to be bigger than the one measured with other techniques. We develop a model able to retrieve the intrinsic energy gap leading to a value which is in rough agreement with other experiments, bridging the apparent contradiction. At doping $x \approx 5 \%$ we further observe circular features in the conductance layers that point to the emergence of a significant density of free carriers in this doping range, and to the presence of a small concentration of donor atoms. We illustrate the importance of considering the presence of TIBB when doing STM experiments on correlated-electron systems and discuss the similarities and differences between STM measurements on semiconductors and lightly doped Mott insulators.

Effect of Au nanosandwiching on the structural, optical and dielectric properties of the as grown and annealed InSe thin films

In the current work, the structural, optical and dielectric properties of the InSe/Au/InSe nanosandwiched structures are investigated by means of X-ray diffraction and UV–visible light spectrophotometry techniques. The insertion of a 20 and 100nm thick Au metal slabs between two InSe layers did not alter the amorphous nature of the as grown InSe films but decreased the energy band gap and the free carrier density. It also increased; the absorption ratio and the values of dielectric constant by ~ 3 times. The insertion of 100nm Au layers as a nanosandwich enhanced the drift mobility (31.3cm2/Vs) and plasmon frequency (1.53GHz) of the InSe films. On the other hand, upon annealing, a metal induced crystallization process is observed for the InSe/Au (100nm)/InSe sandwiches. Particularly, while the samples sandwiched with a layer of 20nm thickness hardly revealed hexagonal γ−In2Se3 when annealed at 300°C, those sandwiched with 100nm Au slab, displayed well crystalline phase of hexagonal γ−In2Se3 at annealing temperature of 200°C. The further annealing at 300°C, forced the appearing of the orthorhombic In4Se3 phase. Optically, the annealing of the InSe/Au(100nm)/InSe at 200°C improved the absorption ratio by ~ 9 times and decreased the energy band gap. The nanosandwiching technique of InSe seems to be promising for the engineering of the optical properties of the InSe photovoltaic material.