Nanograting Layer Research Articles

Optical properties of periodically and aperiodically nanostructured p-n junctions

Recently, semiconductor nanograting layers have been introduced and their optical properties have been studied. Spectroscopic ellipsometry has shown that nanograting significantly modifies the dielectric function of c-Si layers. Photoluminescence spectroscopy reveals the emergence of an emission band with a remarkable peak structure. It has been observed that nanograting also alters the electronic and magnetic properties. In this study, we investigate the quantum efficiency and spectral response of Si p-n junctions fabricated using subwavelength grating layers and aperiodically nanostructured layers. Our findings indicate that the quantum efficiency and spectral response are enhanced in the case of nanograting p-n junctions compared to plain reference junctions. Aperiodically nanostructured junctions exhibit similar results to nanograting junctions. However, aperiodic nanostructuring is a more straightforward fabrication method and, consequently, more appealing for the solar cell industry.

Drive Type Virtual Reality Image on a Head-Mounted Display

We fabricate a head-mounted display (HMD) providing a gradation VR image generated at the surface by using the excitation of the guided mode in a TiO2-slanted nanograting layer. In the TiO2 layer, the beam is waveguided along the short axis direction of the grating at a specific injection angle Θ. On the surface of the layer, a beam consisting of leakage generated during the waveguide is emitted. It generates an intensity gradient image for the waveguide direction. In this work, we convert this gradation image to VR by using another coupler. Finally, the obtained gradation VR image is also discussed.

Direct laser writing regimes for bulk inscription of polarization-based spectral microfilters and fabrication of microfluidic bio/chemosensor in bulk fused silica

Laser inscription of form-birefringent nanograting layers inside fused silica by ultrashort (femto-picosecond) laser pulses at 515 nm was studied as a function of laser, average power and pulsewidth in terms of fabrication regimes. Retardance magnitudes were acquired for single birefringent layers inscribed at different laser parameters, enabling fabrication of polarization-based spectral micro-filters in the form of few-layer microdevices and of microfluidic multi-color refractive-index chemo/biosensor.

Fermi-Level Tuning of G-Doped Layers.

Recently, geometry-induced quantum effects were observed in periodic nanostructures. Nanograting (NG) geometry significantly affects the electronic, magnetic, and optical properties of semiconductor layers. Silicon NG layers exhibit geometry-induced doping. In this study, G-doped junctions were fabricated and characterized and the Fermi-level tuning of the G-doped layers by changing the NG depth was investigated. Samples with various indent depths were fabricated using laser interference lithography and a consecutive series of reactive ion etching. Four adjacent areas with NG depths of 10, 20, 30, and 40 nm were prepared on the same chip. A Kelvin probe was used to map the work function and determine the Fermi level of the samples. The G-doping-induced Fermi-level increase was recorded for eight sample sets cut separately from p-, n-, p+-, and n+-type silicon substrates. The maximum increase in the Fermi level was observed at a10 nm depth, and this decreased with increasing indent depth in the p- and n-type substrates. Particularly, this reduction was more pronounced in the p-type substrates. However, the Fermi-level increase in the n+- and p+-type substrates was negligible. The obtained results are explained using the G-doping theory and G-doped layer formation mechanism introduced in previous works.

G-doping junction-formation mechanism

Recently, geometry-induced quantum effects in periodic low-dimensional structures were introduced and observed. Geometry-induced doping or G-doping has been shown to originate from nanograting. The resistivity, Hall coefficient, and magnetoresistance temperature dependences of the nanograting layers were recorded. G-doping junctions, termed p–p(v) junctions, have been fabricated in p-Si substrates and studied experimentally. Here, a p–p(v) junction-formation mechanism, based on previously obtained experimental data, is reported. The G-doping depth, carrier concentration, depletion layer thickness, and junction current are calculated and compared with experimental values. A voltage-dependent doping level, p(v), and corresponding physical mechanism is introduced, which explains the low values of the built-in potential barrier (approximately KT), and low normalized reverse currents. High saturation currents are explained by the thermionic emission of majority carriers. The calculated I(V) dependence is in good agreement with the recorded I–V curves.

Nanograting layers of Si

The paper presents research regarding the complex behavior of materials based on Si and SiO2, geometrically processed at nano-scale. The geometry, which induces the doping effect (G-doping), occurred when it was possible to fabricate nanograting structures. Studies on the influence of nanograting structures on the properties of materials have shown that this process may lead to effects similar to those created by doping with donors. The resistivity values measured in Si-based nanograting layers, for example, were approximately 10−2 Ω cm, similar to those of Si semiconductors doped with phosphorus ‘impurities’ having a volume concentration of 1018 cm−3. This increase in electronic properties, as a result of the nanograting structure, seems to appear due to the fact that the electrons rejected in the process are placed in the structure vacancies. It has been experimentally proven that, in the case of semiconductors, the nanograting structuring makes the rejected electrons from the valence band to be placed in the conduction band. In the paper, after samples fabrication with nanograting structures on Si films placed on SiO2 support, the I–V curves of the obtained layers were drawn, both by measuring in four points and also in two points. The resistivity measurements were made in two directions: along and perpendicular to the strips of the structure and showed the existence of an anisotropy.

Geometry‐induced quantum effects in periodic nanostructures

Recently, geometry‐induced quantum effects in periodic nanostructures were introduced and observed. Nanograting has been shown to dramatically improve thermoelectric and electron emission properties, and originate a geometry induced doping or G‐doping. Here, we concentrate on experimental investigation of G‐doping. We fabricate nanograting (NG) layers and measure their electron transport properties. The grating was fabricated on the surface of a silicon on insulator (SOI) wafer device layer using laser interference lithography followed by reactive ion etching. Next, large square islands were shaped in the device layer. The characteristics of NG and plain islands were compared to investigate G‐doping. Resistivity temperature dependences were recorded in the range of 4–300 K. For all 21 samples, the NG layers show a 2–3 order of magnitude reduction in resistivity with respect to the plain layer. Hall coefficient and thermopower measurements demonstrate that the NG layers are n‐type. Obtained G‐doping level corresponded to an “effective impurity” concentration of 1018 cm−3. The dependence of the resistivity and Hall coefficient on temperature and magnetic field were recorded in the ranges of 2–300 K and 0–3 T, respectively.

Broadband ultrathin circular polarizer at visible and near-infrared wavelengths using a non-resonant characteristic in helically stacked nano-gratings.

Modern imaging and spectroscopy systems require to implement diverse functionalities with thin thickness and wide wavelength ranges. In order to meet this demand, polarization-resolved imaging has been widely investigated with integrated circular polarizers. However, the circular polarizers which operate at the entire visible wavelengths and have a thickness of several tens of nanometers have not been developed yet. Here, a circular polarizer, operating at the entire visible wavelength range, is demonstrated using helically stacked aluminum nano-grating layers. High extinction ratio and broad operation bandwidth are simultaneously achieved by using non-resonant anisotropic characteristics of the nano-grating. It is theoretically verified that the averaged extinction ratio becomes up to 8 over the entire visible wavelength range while having a thickness of 390 nm. Also, the feasibility of the proposed structure and circular polarization selectivity at the visible wavelength range are experimentally verified. It is expected that the proposed structure will lead to extreme miniaturization of a circular polarizer and contribute greatly to the development of mobile/wearable imaging systems such as virtual reality and augmented reality displays.

Observation of Geometry Induced Doping in Thin Si Nano-grating Layers

We fabricate Si nano-grating layers and measure their electrical characteristics to monitor geometry induced doping. SOI device layer was thinned down by thermal oxidizing and subsequent wet etching of the oxide. Grating was fabricated using laser interference lithography (375nm laser) followed by reactive ion etching of Si. Next, large square island (0.5x0.5mm) was shaped in the device layer and four Si\\Ti\\Ag ohmic contacts were formed to measure electrical characteristics. The I-V characteristics were recorded using both 4 wire and 2 wire methods. Resistance-temperature dependences (T= 4-300K) were recorded as well. For all 12 samples, nano-grating layers show 2-3 order reduction of resistivity. Resistivity anisotropy was in the range 0.2-1 at 300K. Obtained geometry induced doping level corresponds to “effective impurity” concentration of 3x1018 cm-3. The dependence is in agreement with G-doping theory only below T=150K.

Density of quantum states in quasi-1D layers

Recently, new quantum effects have been studied in thin nanograting layers. Nanograting on the surface imposes additional boundary conditions on the electron wave function and reduces the density of states (DOS). When the nanograting dimensions are close to the de Broglie wavelength, the DOS reduction is considerable and leads to changes in the layer properties. DOS calculations are challenging to perform and are related to the quantum billiard problem. Performing such calculations requires finding the solutions for the time-independent Schrödinger equation with Dirichlet boundary conditions. Here, we use a numerical method, namely the Method of Auxiliary Sources, which offers significant computational cost reduction relative to other numerical methods. We found the first five eigenfunctions for the nanograting layer and compared them with the corresponding eigenfunctions for a plain layer by calculating the correlation coefficients. Furthermore, the numerical data were used to analyze the DOS reduction. The nanograting is shown to reduce the probability of occupation of a particular quantum state, reducing the integrated DOS by as much as 4.1-fold. This reduction in the DOS leads to considerable changes in the electronic properties.

Electronic and thermoelectric properties of nanograting layers

Recently, new quantum features have been observed and studied in the area of nanostructured layers. Nanograting (NG) on the surface of the thin layer imposes additional boundary conditions on electron wave function and forbids some quantum states. Electrons, rejected from the forbidden quantum states, have to occupy states with higher energy. In the case of semiconductor materials, electrons rejected from the valence band have to occupy empty quantum states in the conduction band. Such increase in conduction band electron concentration n can be termed as geometry-induced doping or G-doping. G-doping is equivalent to donor doping from the point of view of the increase in n. However, there are no ionized impurities. This preserves charge carrier scattering to the intrinsic semiconductor level and increases carrier mobility. G-doping involves electron confinement to NG layer. Here, we investigate the system of multiple NG layers forming a series of homo junctions. Si and GaAs homojunctions were studied and G-doping levels of 1018-1019 cm−3 were obtained. We also study a system composed of NG layer and an additional top layer forming periodic series of p-n junctions. In such system, charge depletion region develops inside the NG and its effective depth reduces, becoming a rather strong function of temperature T. Consequently, T-dependence of chemical potential magnifies and Seebeck coefficient S increases. We investigate S in the system of semiconductor NG layer having abrupt p-n junctions on the top of the grating. Analysis made on the basis of Boltzmann transport equations shows dramatic increase in S. At the same time, other transport coefficients remain unaffected by the junctions. Calculations show one order of magnitude increase in the thermoelectric figure of merit ZT relative to bulk material.

Influence of Nanostructure Geometry on Electronic Properties

Recently, new quantum features have been studied in the area of nanostructured layers. It emerges that properties of nanostructures depend not only on their size but also on their geometry. Particularly, a nanograting (NG) on the surface of the thin layer imposes additional boundary conditions on electron wave function and forbids some quantum states. Density of quantum states reduces. Unlike conventional quantum well, state density per volume, is reduced in the case of NG layer. This leads to changes in electronic properties. Electrons, rejected from forbidden quantum states, have to occupy states with higher energy. In the case of semiconductor layers, electrons rejected from the valence band have to occupy empty quantum states in the conduction band. Such increase in conduction band electron concentration can be termed as geometry-induced doping or G-doping. G-doping is equivalent to donor doping from the point of view of the increase in electron concentration. However, there are no ionized impurities. This preserves charge carrier scattering to the intrinsic semiconductor level and increases carrier mobility with respect to the donor-doped layer. As rejected electrons occupy quantum states with the higher energy, the chemical potential of NG layer increases and becomes NG size dependent. We regard a system composed of NG layer and an additional layer on the top of the NG forming periodic series of p-n junctions. In such system, charge depletion region develops inside the top of NG and its effective height reduces, becoming a rather strong function of temperature T. Consequently, T-dependence of chemical potential magnifies and Seebeck coefficient S increases. Calculations show one order of magnitude increase in the thermoelectric figure of merit ZT relative to bulk material. In the case of metal layers, electrons rejected from forbidden quantum states below Fermi energy, occupy quantum states above Fermi energy. Fermi energy moves up on energy scale and work function (WF) reduces. WF reduction was observed in thin amorphous Au films grown on thermally oxidized Si substrates. WF was measured using Kelvin Probe and PEEM microscope. WF reduction depended on film internal structure and NG sizes. Maximum reduction in WF of 0.56 eV has been obtained.

Geometry-induced electron doping in periodic semiconductor nanostructures

Recently, new quantum features have been observed and studied in the area of nanostructured layers. Nanograting on the surface of the thin layer imposes additional boundary conditions on the electron wave function and induces G-doping or geometry doping. G-doping is equivalent to donor doping from the point of view of the increase in electron concentration n. However, there are no ionized impurities. This preserves charge carrier scattering to the intrinsic semiconductor level and increases carrier mobility with respect to the donor-doped layer. G-doping involves electron confinement to the nanograting layer. Here, we investigate the system of multiple nanograting layers forming a series of hetero- or homojunctions. The system includes main and barrier layers. In the case of heterojunctions, both types of layers were G-doped. In the case of homojunctions, main layers were G-doped and barrier layers were donor-doped. In such systems, the dependence of n on layer geometry and material parameters was analysed. Si and GaAs homojunctions and GaAs/AlGaAs, Si/SiGe, GaInP/AlGAs, and InP/InAlAs heterojunctions were studied. G-doping levels of 1018–1019cm−3 were obtained in homojunctions and type II heterojunctions. High G-doping levels were attained only when the difference between band gap values was low.

Ultraviolet wave plates based on monolithic integration of two fully filled and planarized nanograting layers

We successfully fabricated a high-performance half-wave plate for the 405 nm wavelength based on monolithic integration of two nanograting layers. Each of the nanograting layers functions as a quarter-wave plate. Both of the nanograting layers were fully filled and planarized to achieve the monolithic integration. UV-nanoimprint lithography, along with thin-film deposition, high-aspect-ratio reactive ion etching, and trench-filling technologies, was used in fabrication and integration of the individual nanograting layers. High-aspect-ratio nanogratings with sub-50 nm linewidths and 100 nm spacing were fabricated to achieve good optical performance at the near-UV wavelength. The ability to integrate multiple nanostructure-based optical layers opens a path for integrated multifunction devices, as well as a new strategy for driving both miniaturization and cost.