Semiconductor Samples Research Articles

Mn and Al co-doped CdS:Cr nanoparticles for spintronic applications

Nowadays, diluted magnetic semiconductors with improved ferromagnetic feature is utterly seeking for use in modern spintronic applications.We describe the optical as well as magnetic properties of CdS, CdS:Cr, CdS:(Cr + Al), and CdS:(Cr + Mn) co-doped samples. The substitution of Cr dopant and co-dopant ions (Al and Mn) in the host sites were confirmed through structural analysis. Declining of energy band gap with the incorporation of Cr dopant and co-dopant ions (Al and Mn) into the CdS host lattice was estimated through optical studies. Room-temperature magnetic hysteresis curves revealed that pure CdS exhibited paramagnetic nature due to its surface defects, whereas CdS:Cr, CdS:(Cr + Al), and CdS:(Cr + Mn) co-doped nanoparticles showed better ferromagnetic characteristics. Among all synthesized nanoparticles, CdS:(Cr + Al) semiconductor sample exhibited enhanced ferromagnetic nature and may benficial for spintronic applications.

Experimental Validation of Formula for Calculation Thermal Diffusivity in Superlattices Performed Using a Combination of Two Frequency-Domain Methods: Photothermal Infrared Radiometry and Thermoreflectance

In this paper, we validate two theoretical formula used to characterize thermal transport of superlattices at different temperatures. These formulas are used to measure cross-plane thermal conductivity and thermal boundary resistance, when it is not possible to obtain heat capacity or thermal diffusivity and in-plane thermal conductivity. We find that the most common formula for calculating thermal diffusivity and heat capacity (and density) can be used in a temperature range of −50 °C to 50 °C. This confirms that the heat capacity in the very thin silicon membranes is the same as in bulk silicon, as was preliminary investigated using an elastic continuum model. Based on the obtained thermal parameters, we can fully characterize the sample using a new procedure for characterization of the in-plane and cross-plane thermal transport properties of thin-layer and superlattice semiconductor samples.

Comparative Study of Al2O3 and HfO2 for Surface Passivation of Cu(In,Ga)Se2 Thin Films: An Innovative Al2O3/HfO2 Multistack Design

In Cu(In,Ga)Se2 (CIGS) thin‐film solar cells, interface recombination is one of the most important limiting factors with respect to device performance. Herein, metal–insulator–semiconductor samples are used to investigate and compare the passivation effects of Al2O3 and HfO2 at the interface with CIGS. Capacitance–voltage–frequency measurements allow to qualitatively and quantitatively assess the existence of high negative charge density (Q f ≈ −1012 cm−2) and low interface‐trap density (D it ≈ 1011 cm−2 eV−1). At the rear interface of CIGS solar cells, these, respectively, induce field‐effect and chemical passivation. A trade‐off is highlighted between stronger field‐effect for HfO2 and lower interface‐trap density for Al2O3. This motivates the usage of Al2O3 to induce chemical passivation at the front interface of CIGS solar cells but raises the issue of its processing compatibility with the buffer layer. Therefore, an innovative Al2O3/HfO2 multistack design is proposed and investigated for the first time. Effective chemical passivation is similarly demonstrated for this novel design, suggesting potential decrease in recombination rate at the front interface in CIGS solar cells and increased efficiency. 300 °C annealing in N2 environment enable to enhance passivation effectiveness by reducing D it while surface cleaning may reveal useful for alternative CIGS processing methods.

Defect Distribution in Doped Silicon Nanostructures Characterized By Means of Scanning Spreading Resistance Microscopy

Scanning Spreading Resistance Microscopy (SSRM) is a well-known technique for electrical characterization of dopant distributions in semiconductors. The resistance between a conductive atomic force microscope (AFM) tip and a large area contact on a semiconductor sample gives direct hint of sample resistivity underneath the tip. The sample surface has to be polished to achieve a smooth surface which is crucial for a low ohmic electrical contact between tip and sample. In theory, the formula of spreading resistance Rspr = ρ/4a with tip radius a connects SSRM raw data to resistivity data which can be transformed to concentration data of corresponding dopants. In experiment, additional series resistances and electrical defects have an impact on the SSRM analysis so that the formula does not hold any more. A SSRM scan on a calibration sample with regions of different well-known dopant concentrations enables an empirical relationship between R and ρ. This relationship is only valid for the specific defect distribution and tip-sample contact of the calibration sample. Both the defect distribution and the tip sample contact, respectively, depend strongly on sample preparation. Reliable calibration of unknown dopant distributions in semiconductor samples is only possible in the case of a perfectly reproducible sample preparation which is however hard to attain. Therefore, an accurate quantification of SSRM data remains difficult.One first step towards SSRM quantification requires an improved understanding and control of electrical defects on the sample surface which affect the SSRM measurement. In this work, we apply SSRM on well characterized n- and p-type dopant profiles in silicon bulk- and nanostructures. The deviation of the experimental SSRM data from simulated spreading resistance profiles provides direct information on type, quantity and origin of electrical defects in the sample.Our analysis of bulk samples reveals two type of preparation induced defects: n-type surface defects and hydrogen traps which deactivate boron atoms. The number of surface defects is proportional to the surface roughness. An improved sample preparation with additional polishing steps reduces the impact of this defect on the SSRM analysis. Hydrogen traps are driven into the sample during water based chemical mechanical polishing steps. An annealing of the sample at 250°C for 20 minutes reactivates most of the boron atoms.A third type of defect was noticed in nanostructured samples. Silicon nano pillars with diameters between 500 and 600 nm are manufactured in a top-down approach out of a silicon wafer with a 600 nm deep boron diffusion profile. Figure 1 shows the SSRM data of the bulk sample (a), the cross-sectional prepared nano pillar (b), and the depth profiles of SSRM analysis (c). The SSRM signal at the top of the pillar is dominated by the boron profile (b). The side walls are covered with gold as second contact for SSRM analysis. Obviously, the SSRM depth profile at the shell region (#2, circles) of the pillar is deeper than at the core (#3, triangles) and in the bulk sample (#1, squares) (c). The dopant distribution itself cannot vary across the pillar as the nanofabrication is done by dry-etching at cryogenic temperatures. We explain the variance in SSRM signal by acceptor defect states at the pillar shell which increase the number of p-type charge carriers in the shell region.COMSOL simulations of the SSRM analysis, including the impact of defects on the resistance profile, is used to quantify the defect distributions. The calculated resistance profiles accurately describe the experimental data (black dotted lines in (c)) and support our assumption on the distribution of electrical defects in silicon nanostructures. Figure 1

Electric current noise in mesoscopic organic semiconductors induced by nuclear spin fluctuations

We demonstrate that nuclear spin fluctuations lead to the electric current noise in the mesoscopic samples of organic semiconductors showing the pronounced magnetoresistance in weak fields. For the bipolaron and electron-hole mechanisms of organic magnetoresistance, the current noise spectrum consists of the high frequency peak related to the nuclear spin precession in the Knight field of the charge carriers and the low frequency peak related to the nuclear spin relaxation. The shape of the spectrum depends on the external magnetic and radiofrequency fields, which allows one to prove the role of nuclei in magnetoresistance experimentally.

Doped or Quantum-Dot Layers as In Situ Etch-Stop Indicators for III/V Semiconductor Reactive Ion Etching (RIE) Using Reflectance Anisotropy Spectroscopy (RAS).

Reflectance anisotropy spectroscopy (RAS), which was originally invented to monitor epitaxial growth, can—as we have previously shown—also be used to monitor the reactive ion etching of III/V semiconductor samples in situ and in real time, as long as the etching rate is not too high and the abrasion at the etch front is not totally chaotic. Moreover, we have proven that—using RAS equipment and optical Fabry‒Perot oscillations due to the ever-shrinking thickness of the uppermost etched layer—the in situ etch-depth resolution can be as good as ±0.8 nm, employing a Vernier-scale type measurement and evaluation procedure. Nominally, this amounts to ±1.3 lattice constants in our exemplary material system, AlGaAsSb, on a GaAs or GaSb substrate. In this contribution, we show that resolutions of about ±5.6 nm can be reliably achieved without a Vernier scale protocol by employing thin doped layers or sharp interfaces between differently doped layers or quantum-dot (QD) layers as etch-stop indicators. These indicator layers can either be added to the device layer design on purpose or be part of it incidentally due to the functionality of the device. For typical etch rates in the range of 0.7 to 1.3 nm/s (that is, about 40 to 80 nm/min), the RAS spectrum will show a distinct change even for very thin indicator layers, which allows for the precise termination of the etch run.

Tuneable in-situ nanoCT workflow using FIB/SEM

Inspired by the standard computed tomography, a new method of 3D X-ray imaging embedded in FIB-SEM microscope is proposed. The unique combination of TEM-like specimen stage enabling in lens STEM detection (referred to as CompuStage), nanomanipulator (referred to as EasyLift) facilitating in-situ sample transfer from bulk sample to TEM-like stage and pixelated in-situ Timepix X-ray detector in Helios G4 FX FIB-SEM system offers an unprecedented workflow. Motivated by common circular CT scan known from microCT world, the object under study is placed on CompuStage rod which enables two possible rotation (in TEM/SEM terminology called tilt) movements – α-tilt – rotation of the CompuStage rod around its axis, and β-tilt – rotation around axis perpendicular to CompuStage rod. β-tilt rotation enables a circular movement of the sample while α-tilt sets the correct position of sample with respect to target and detector. Thin metal lamella of suitable material welded to EasyLift manipulator needle is used as an X-ray target. The final target-sample geometry – position, distance – can be fine-tuned using position control of CompuStage and EasyLift and in-situ monitored by SEM. Both sample and target can also be easily prepared in-situ. Radiographs are recorded by Timepix detector with inherent noise-free operation and energy filtration. For the 3D reconstruction standard microCT reconstruction algorithm is used with the procedure adjusted for the format and quality of nanoCT images.The experiments were carried out on Helios G4 FX DualBeam using titanium and tungsten targets and various semiconductor samples. The ultimate resolution of the proposed method in orders of tens of nanometers was achieved both by the possibility of close target to sample positioning and of adjustment of primary beam energy down to low energies reducing the interaction volume in the target. Since the lower energy radiation is well suited for life-science, the method was also tested on several bio-samples using silver target. The silver target, thanks to its massive low energy Lα line, allowed to distinguish subtle structures in the resin embedded stained mouse brain and also to observe and reconstruct canaliculi in the mouse bone (earlier reported by Dierolf et al. 2010, Nature 467 436).

Invariance of the r2-ln(F) relationship and attainable precision in ultrafast laser ablation experiments.

Pursuing ever-smaller feature size in laser-based lithography is a research topic of vital importance to keep this technique competitive with other micro-/nano-fabrication methods. Features smaller than the diffraction-limited spot size can be obtained by "thresholding", which utilizes the deterministic nature of damage threshold with ultrashort laser pulses and is achieved by precisely tuning pulse energies so that only the central portion of the focal spot produces permanent modification. In this paper, we examine the formulation commonly used to describe thresholding and show that the relationship between feature size (r) and laser fluence (F) is invariant with respect to the nature of laser absorption. Verified by our experiments performed on metal, semiconductor, and dielectric samples, such invariance is used to predict the smallest feature size that can be achieved for different materials in a real-world system.

Analysis and Growth Modeling of CuInSe2 Films by Electrodeposition for Photocell Applications

CuInSe2 (CIS) thin films were deposited onto FTO/glass substrates by continuous current potentiostatic electrodeposition method. The aqueous deposition bath was prepared from a pH = 3.0 NaBF4 supporting electrolyte, containing CuSO4, In2(SO4)3, and SeO2 precursors. The characterization of these samples was done by scanning electron microscopy, energy-dispersive X-ray spectrometry, X-ray diffractometry, Raman spectroscopy, and optical spectroscopy. The experimental techniques have allowed the understanding of the atomic concentration dynamics of samples, and its relationship with the concentration of precursors. It was obtained p-type polycrystalline semiconductor samples, with 0.95 eV gap energy; the crystalline quality of these samples was also verified. Besides this, the nucleation and growth of CIS were studied during its electrochemical deposition on top of FTO, and vitreous carbon electrodes. Chronoamperometry I-t plots permitted the study of current transients, through classic nucleation models, which were used to understand the nucleation mechanism which occurred during the electrodeposition process, besides predicting some of its characteristic properties, as its diffusion coefficient, density of active sites, and nucleation rate constant per active site. Atomic force microscopy measurements were used to investigate the first stages of growth of these CIS films, deposited at different overpotentials. Preliminary devices CIS/In2Se3/FTO were mounted, with In2Se3 (IS) film also deposited by continuous current electrodeposition. Its electrical properties were analyzed, revealing a Schottky diode like behavior; there were observed light dependent open circuit voltage generation and resistance drop, disclosing promising characteristics for photovoltaic production.

Free and bound charge carriers dependent Raman susceptibilities in weakly-polar magnetoactive semiconductors

In this paper, an analytical investigation of free and bound charge carriers dependent Raman susceptibilities has been made in weakly-polar magnetoactive semiconductors. The coupled mode approach has been used to obtain expressions for real and imaginary parts of Raman susceptibilities. The semiconductor sample is assumed to be illuminated by an off-resonant pulsed laser under Voigt geometry. For numerical estimations, we choose n-type doped InSb crystal at liquid nitrogen temperature and illuminated by a pulsed CO2 laser at 10.6 μm wavelength. Efforts have been paid to optimize the doping concentration, external magnetic field and pump intensity to obtain enhanced values of Raman susceptibilities and change of their sign. The change of sign of enhanced Raman susceptibilities, in weakly-polar semiconductors under appropriate selection of n-type doping and external magnetic field, validates the possibility of selected sample as a potential candidate material for the fabrication of stimulated Raman scattering dependent widely tunable optoelectronic devices such as optical switches and frequency converters.

Градиентометр напряженности магнитного поля

Measurements of magnetic field parameters are the basis of all magnetic measurements. Most methods of measuring magnetic field parameters are based on force (energy) interaction of magnetic field with macro- or microscopic currents and electromagnetic induction. The paper considers the possibility of constructing a magnetic field strength gradient meter based on the absolute helical instability of electron-hole plasma of a semiconductor sample. The functional scheme of the gradiometer and the results of experimental and theoretical studies of the sensitive element of the gradiometer are presented

Gamma-ray attenuation, fast neutron removal cross-section and build up factor of Cu2MnGe[S, Se, Te]4 semiconductor compounds: Novel approach

This study is original work on semiconductor materials for gamma-ray detectors and fast neutron removal cross-section. This work is focused on the investigating of γ-ray interactions using Cu2MnGeS4, Cu2MnGeSe4, and Cu2MnGeTe4 semiconductor compounds. The photon interaction parameters (MAC, LAC, HVL, TVL, MEF, ACS, ECS, Zeff, Neff, Ceff, Zeq, EBF, and EABF) of the three different semiconductor compounds were graphically presented within 0.015–15 MeV energy range. A useful comparison with regular radiation shielding with standard glass materials with these samples has been developed. Cu2MnGeSe4 and Cu2MnGeTe4 semiconductor compounds have higher values of MAC and LAC compared with ordinary and steel magnetite concrete at the 0.1, 10, and 15 MeV. Interestingly, the checked semiconductor samples named Cu2MnGeSe4 andCu2MnGeTe4 have lower values for HVL and MFP than some commercial glasses used in radiation safety applications. At 15 KeV, Zeff values are 27.13, 32.11 and 39.21 and Neff values are 3.41, 2.71 and 2.47 × 1023 electron cm−3 for Cu2MnGeS4, Cu2MnGeSe4, and Cu2MnGeTe4 semiconductor compounds, respectively. Semiconductor compounds have better neutron protection features compared to RS-520, RS-253-G18, and RS-360 standard glassy materials. In a large energy range, the third sample, Cu2MnGeTe4, showed high sensing for gamma-rays and neutrons. These studies are very important for the manufacture of detectors for semiconductors used for X and gamma-ray measurements (detection of radiation).

Morphology Controlled Synthesis of Zinc Oxide Nanostructures Through Millettia pinnata (MP) Leaf Extract as Capping Agent and its Photocatalytic Degradation Efficiency of a Textile Dye

The present work reports low cost, rapid synthesis of ZnO nanostructures capped with Millettia pinnata leaf extract which produced rods, pyramids, cones and flower like morphology. Microwave irradiation method drastically reduced the reaction time. Three samples were prepared with different volumes of the extract and a control experiment was performed where no extract was used. The quantity of the extract significantly influences the size and morphology. Characterization done by X-ray diffraction established the purity and crystalline nature of the samples. The composition was further confirmed by EDS which shows the exclusive presence of elements zinc and oxygen. UV–visible spectroscopy shows absorption wavelength for the as synthesized ZnO semiconductor samples between 351 and 365 nm. FTIR spectra show a band in the region of 500–550 cm−1 corresponding to Zn–Ostr. FESEM images show rodlike morphology for uncapped ZnO which on annealing at 400 °C produced an assembled flower like morphology. The morphology varies from pyramid, to cone, to flower like as the volume of extract increases from 2.5 to 10 mL. Methylene blue was the target textile dye and its degradation was tested under UV light.

Atomistic strain and structural analysis of 120 MeV Ni ions irradiated CdSe nanocrystals through molecular dynamics simulation method

Swift heavy ion (SHI) irradiation modifies the physical and chemical properties of semiconductor sample like CdSe nanocrystals by inducing vacancies and structural change in the lattice system. Here, the atomistic strain analysis has been performed using molecular dynamics (MD) simulation on the SHI irradiated CdSe nanocrystals by approximating interatomic interaction as (ZBL) potential. MD simulation based on LAMMPS package has been employed for better understanding the time response of the sample with swift heavy ion irradiation. The primary knock-on atom (PKA) method has been used for the simulation that starts the cascade of energy transfer, which triggers the displacement of atoms. Initial parameters of PKA have been obtained from the SRIM simulation of the irradiation effect of 120 MeV Ni ions. Finally, the time evolution of the system has been studied.

Denoising of series electron holograms using tensor decomposition.

In this study, a noise-reduction technique for series low-dose electron holograms using tensor decomposition is demonstrated through simulation. We treated an entire dataset of the series holograms with Poisson noise as a third-order tensor, which is a stack of 2D holograms. The third-order tensor, which is decomposed into a core tensor and three factor matrices, is approximated as a lower-rank tensor using only noise-free principal components. This technique is applied to simulated holograms by assuming a p-n junction in a semiconductor sample. The peak signal-to-noise ratios of the holograms and the reconstructed phase maps have been improved significantly using tensor decomposition. Moreover, the proposed method was applied to a more practical situation of time-resolved in situ electron holography by considering a nonuniform fringe contrast and fringe drift relative to the sample. The accuracy and precision of the reconstructed phase maps were quantitatively evaluated to demonstrate its effectiveness for in situ experiments and low-dose experiments on beam-sensitive materials.

(Invited) Probing Semiconductor Heterostructures from the Atomic to the Micrometer Scale

Atom Probe Tomography (APT) has significantly advanced our ability to characterize buried interfaces and to map the three-dimensional distribution of atomic constituents like dopants in modern day electronic nano-devices. This precise mapping of the chemical composition and buried interfaces is also highly sought-after in the implementation and optimization of a variety of emerging photonic devices with dimensions of up to several micrometers as opposed to the few ten nanometers of modern day transistors. Herein, we show that APT is poised to contribute to the development of these devices by adapting the Focused Ion beam based preparation of APT specimen, typically used for preparing semiconductor samples. Using in-situ grown nanowires as sacrificial masks during specimen preparation, we demonstrate APT analyses of micrometer-sized volumes of semiconductor heterostructures.

XPS of the surface chemical environment of CsMAFAPbBrI trication-mixed halide perovskite film

Perovskite-based materials have been considered as the most promising materials in several solar-driven processes, especially photovoltaics. Some features such as high sunlight harvesting and improved carrier transport have been highlighted to have an impact on the efficiency of the above topics, but limited studies have pointed out their surface composition, which mainly influence the above abilities. As a starting point to recognize the surface environment and chemical states of these materials, x-ray photoelectron spectroscopy measurements were performed. Trication-mixed halide perovskite films based on Cs0.08MA0.18FA0.78PbBr0.42I2.58 were prepared by a one step process and deposited on an indium tin oxide substrate by spin-coating. Survey spectra, Pb 4f, I 3d, Br 3d, Cs 3d, C 1s, N 1s, Pb 4d, and Pb 5d core levels, and valence band spectra were measured for the semiconductor sample. Results exhibit symmetrical peaks, indicating that a homogenous solid solution was achieved. Main chemical states of this kind of perovskites such as Cs-Br and Pb-I species are also shown. We deduced that the later signals are associated with the [PbI6]4− octahedra existing in the perovskite lattice, while the former one is related to Cs+ bounded to [PbBr6]4− units on the perovskite surface. Interestingly, the NR4+ signal was also identified, associated with the presence of methylammonium and formamidinium cations.

Measurement of thermal transport properties of selected superlattice and thin films using frequency-domain photothermal infrared radiometry

Thermal transport properties in multi-layered semiconductor samples are reported using modulated photothermal infrared radiometry (PTR). The cross-plane thermal conductivity and diffusivity of thin AlxGa(1-x)As layers and AlAs/GaAs superlattices were determined by fitting solutions of the heat equation for multi-layered systems to PTR data. The thermal conductivity of an AlxGa(1-x)As film with x = 0.5 was found to be lower than an AlxGa(1-x)As film with x = 0.33 which is expected as scattering from alloy disorder is maximized for x = 0.5. In addition, it was found that thermal conductivities of AlAs/GaAs superlattices decrease when the superlattice period is decreased for a constant total thickness which is expected since the number of AlAs/GaAs interfaces (which impede thermal transport) increases as the period decreases. The maximum PTR amplitude signal was found to occur when the diffusion length of the thermal wave is on other order of the thickness of the semiconductor layer. The accuracy and applicability of photothermal infrared radiometry to the study of semiconductor multilayer structures are further discussed in the paper.

Direct observation of charge accumulation in quantum well solar cells by cross-sectional Kelvin probe force microscopy

We report here the direct observation of charge accumulation in GaAs/AlGaAs multiple quantum well (MQW) solar cells by employing cross-sectional Kelvin probe force microscopy (KPFM). This sample is characterized by thin barrier layers that enable miniband formation. The contact potential difference, or potential between the tip and the semiconductor sample, was measured along the p–i–n junction. We observed, under illuminated conditions, a change in the potential gradient, or bending, at a position of the MQW layer, but not in the reference sample without quantum well. This clearly shows that charge is accumulated in the MQW region. We also found that electron accumulation in the MQW layer and the density measured on the surface is about 1 × 1011 cm−2. Our experimental results show that KPFM is a powerful way of understanding the device physics of nanostructure-based solar cells.

Sustainable Organic Dyes from Winemaking Lees for Photoelectrochemical Dye-Sensitized Solar Cells

During the last two decades, Dye Sensitized Solar Cells (DSSCs) have received a great deal of attention as a promising, low-cost alternative to conventional silicon photovoltaic devices. Natural dye molecules can be used as a sensitizer for their low cost, good light absorbance, easy preparation process, and biodegradability. In this study, dyes were obtained from wine lees, the last by-product of winemaking process, supplied by a venetian winery (Italy). Polyphenols, like tannins and anthocyanins, which were extracted from winemaking lees, were adsorbed on a nanostructured ordered mesoporous titanium dioxide, previously treated at different temperatures (400–600 °C). Both dyes and titania semiconductor samples were studied with different techniques. The tests were carried out on prototypes to evaluate the cell power and the photocurrent generated under simulated solar light irradiation. The obtained solar energy conversion efficiencies are comparable to those that were reported in literature by using organic dyes extracted from vegetables, fruits, and plants. It is significant that these dyes are largely available and cost effective, since recovered from a waste otherwise to be disposed of, opening up a perspective of feasibility for inexpensive and environmentally friendly dye solar cells to generate green electricity and transforming agri-food waste into a resource.