Glancing angle deposited ITO films for efficiency enhancement of a-Si:H/μc-Si:H tandem thin film solar cells

Visible transparent mid-infrared broadband absorbers based on gradient refractive indexes and multi-size cavity resonances

We proposed a multi-layered nanorod structure with the same tilt angle and different diameters, which has high visible transmittance and strong 3-5 µm absorption based on the principles of the gradient of the refractive index and the multi-size cavity resonances. The indium tin oxide (ITO) was selected as the target material to fabricate the structure by using a glancing angle deposition method. The experimental results show that when the deposition angle θ is 80°, swing deposition is successively done with the rotation angle φ of ±8°, ± 5°, ± 3°, and 0° on the surface of the substrate, and the quartz crystal microbalance thicknesses of ITO nanorods are 220 nm for each deposition, the average transmittance is 80.5% in the range of 400-800 nm and the integrated absorption is 86% in the 3-5 µm band. Such a simple, low-cost, and easy-to-fabricate device has potential applications in window stealth materials and other related fields.

3-5 µm mid-infrared broadband absorbers composed of layered ITO nanorod arrays with high visible light transmittance

A mid-infrared broadband absorber with high visible light transmittance is proposed in this paper. The absorber is composed of layered ITO nanorod arrays with increasing angles fabricated by oblique angle deposition technique. The experimental results show that the average transmittance of the absorber reaches 80% in the 400-800 nm band and the integrated absorption reaches 82.9% in the 3-5 µm band, when the QCM thickness of the first layer of film is 100 nm and the deposition angle θ is 10°, the QCM heights of the second to fifth layers of nanorods are all 330 nm, and their deposition angles are 55°, 68°, 80°, and 87°, respectively. The high transmittance in the visible band is attributed to the gradient of the refractive index. The broadband absorption in the mid-infrared band results from different resonances in the empty cavities with different sizes. Such a simple and large-area absorber has potential applications in window materials and infrared cloaking.

A quantum optical study of thresholdless lasing features in high-β nitride nanobeam cavities

Exploring the limits of spontaneous emission coupling is not only one of the central goals in the development of nanolasers, it is also highly relevant regarding future large-scale photonic integration requiring energy-efficient coherent light sources with a small footprint. Recent studies in this field have triggered a vivid debate on how to prove and interpret lasing in the high-β regime. We investigate close-to-ideal spontaneous emission coupling in GaN nanobeam lasers grown on silicon. Such nanobeam cavities allow for efficient funneling of spontaneous emission from the quantum well gain material into the laser mode. By performing a comprehensive optical and quantum-optical characterization, supported by microscopic modeling of the nanolasers, we identify high-β lasing at room temperature and show a lasing transition in the absence of a threshold nonlinearity at 156 K. This peculiar characteristic is explained in terms of a temperature and excitation power-dependent interplay between zero-dimensional and two-dimensional gain contributions.

Structural control in porous/compact multilayer systems grown by magnetron sputtering

In this work we analyze a phenomenon that takes place when growing magnetron sputtered porous/compact multilayer systems by alternating the oblique angle and the classical configuration geometries. We show that the compact layers develop numerous fissures rooted in the porous structures of the film below, in a phenomenon that amplifies when increasing the number of stacked layers. We demonstrate that these fissures emerge during growth due to the high roughness of the porous layers and the coarsening of a discontinuous interfacial region. To minimize this phenomenon, we have grown thin interlayers between porous and compact films under the impingement of energetic plasma ions, responsible for smoothing out the interfaces and inhibiting the formation of structural fissures. This method has been tested in practical situations for compact TiO₂/porous SiO₂ multilayer systems, although it can be extrapolated to other materials and conditions.

Plasmonic chirality of L-shaped nanostructure composed of two slices with different thickness

A concise method is proposed to fabricate L-shaped Ag nanostructures (LSANs) for generating chirality. Prepared by glancing angle deposition, the LSAN composed of two slices with different thickness is stacked on self-assembled monolayer polystyrene nanosphere arrays by controlling substrate azimuth and deposition time. The strong optical chirality of LSANs is achieved in visible and near-IR regions by measurement. For the circular dichroism spectrum of LSANs, the intensity is enlarged, and its peaks red-shift with increasing thickness difference. When LSANs are stacked on polystyrene spheres of different diameters, enlargement and red-shift are also observed in their circular dichroism spectra with increasing thickness difference. The numerical calculations of finite element method show that the two slices composing LSAN provide cross-electric dipoles and their thickness difference provides phase difference for generating optical chirality. This study not only provides a concise and scalable method for fabricating chiral plasmonic nanostructures but also contributes to understand the knowledge of the mechanism of circular dichroism.

Anisotropic In-Plane Conductivity and Dichroic Gold Plasmon Resonance in Plasma-Assisted ITO Thin Films e-Beam-Evaporated at Oblique Angles

ITO thin films have been prepared by electron beam evaporation at oblique angles (OA), directly and while assisting their growth with a downstream plasma. The films microstructure, characterized by scanning electron microscopy, atomic force microscopy, and glancing incidence small-angle X-ray scattering, consisted of tilted and separated nanostructures. In the plasma assisted films, the tilting angle decreased and the nanocolumns became associated in the form of bundles along the direction perpendicular to the flux of evaporated material. The annealed films presented different in-depth and sheet resistivity as confirmed by scanning conductivity measurements taken for the individual nanocolumns. In addition, for the plasma-assisted thin films, two different sheet resistance values were determined by measuring along the nanocolumn bundles or the perpendicular to it. This in-plane anisotropy induces the electrochemical deposition of elongated gold nanostructures. The obtained Au-ITO composite thin films were characterized by anisotropic plasmon resonance absorption and a dichroic behavior when examined with linearly polarized light.

Broadband high-reflective distributed Bragg reflectors based on amorphous silicon films for semiconductor laser facet coatings

We fabricated amorphous silicon (a-Si)-based distributed Bragg reflectors (DBRs) consisting of alternating dense/porous films (i.e., pair) for a center wavelength (λ(c)) of 0.96 μm by oblique angle deposition (OAD) technique using an electron-beam evaporation system. The dense (high refractive index, i.e., high-n) and porous (low-n) a-Si films were deposited at two incident vapor flux angles of 0° and 80° in the OAD, respectively. Their optical reflectance characteristics were investigated in the wavelength range of 0.6-1.5 μm, including theoretical comparison using a rigorous coupled-wave analysis method. Above three pairs, the reflectivity (R) of a-Si DBRs was almost saturated at wavelengths around 0.96 μm, exhibiting R values of >97%. For the a-Si DBR with only three pairs, a broad normalized stop bandwidth (Δλ/λ(c)) of ∼22.5% was obtained at wavelengths of ∼0.87-1.085  μm, keeping high R values of >95%. To simply demonstrate the feasibility of device applications, the a-Si DBR with three pairs was coated as a high-reflection layer at the rear facet of GaAs/InGaAs quantum-well laser diodes (LDs) operating at λ=0.96  μm. For the LDs coated with three-pair a-Si DBR, external differential quantum efficiency (η(d)) was nearly doubled compared to the uncoated LDs, indicating the η(d) value of ∼50.6% (i.e., η(d)∼25.5% for the uncoated LDs).

Tunable distributed Bragg reflectors with wide-angle and broadband high-reflectivity using nanoporous/dense titanium dioxide film stacks for visible wavelength applications

Highly-tolerant distributed Bragg reflectors (DBRs) based on the same materials consisting of nanoporous/dense titanium dioxide (TiO2) film pair structures with wide-angle and broadband highly-reflective properties at visible wavelengths are reported. For a high refractive index contrast, the two dense and nanoporous TiO2 film stacks are alternatingly deposited on silicon (Si) substrates by a oblique angle deposition (OAD) method at two vapor flux angles (θα) of 0 and 80° for high and low refractive indices, respectively. For the TiO2 DBRs at a center wavelength (λ(c)) of 540 nm, the maximum level in reflectance (R) band is increased with increasing the number of pairs, exhibiting high R values of > 90% for 5 pairs, and the normalized stop bandwidth (∆λ/λ(c)) of ~17.8% is obtained. At λ(c) = 540 nm, the patterned TiO2 DBR with 5 pairs shows an uniform relative reflectivity over a whole surface of 3 inch-sized Si wafer and a large-scalable fabrication capability with any features. The angle-dependent reflectance characteristics of TiO2 DBR at λ(c) = 540 nm are also studied at incident angles (θ(inc)) of 20-70° for p-, s-, and non-polarized lights in the wavelength region of 350-750 nm, yielding high R values of > 70.4% at θ(inc) values of 20-70° for non-polarized light. By adjusting the λ(c)/4 thicknesses of nanoporous and dense films, for λ(c) = 450, 540, and 680 nm, tunable broadband TiO2 DBRs with high R values of > 90% at wavelengths of 400-800 nm are realized.

Current transport mechanism at metal-semiconductor nanoscale interfaces based on ultrahigh density arrays of p-type NiO nano-pillars

The present work focuses on a qualitative analysis of localised I-V characteristics based on the nanostructure morphology of highly dense arrays of p-type NiO nano-pillars (NiO-NPs). Vertically aligned NiO-NPs have been grown on different substrates by using a glancing angle deposition (GLAD) technique. The preferred orientation of as grown NiO-NPs was controlled by the deposition pressure. The NiO-NPs displayed a polar surface with a microscopic dipole moment along the (111) plane (Tasker's type III). Consequently, the crystal plane dependent surface electron accumulation layer and the lattice disorder at the grain boundary interface showed a non-uniform current distribution throughout the sample surface, demonstrated by a conducting AFM technique (c-AFM). The variation in I-V for different points in a single current distribution grain (CD-grain) has been attributed to the variation of Schottky barrier height (SBH) at the metal-semiconductor (M-S) interface. Furthermore, we observed that the strain produced during the NiO-NPs growth can modulate the SBH. Inbound strain acts as an external field to influence the local electric field at the M-S interface causing a variation in SBH with the NPs orientation. This paper shows that vertical arrays of NiO-NPs are potential candidates for nanoscale devices because they have a great impact on the local current transport mechanism due to its nanostructure morphology.

Single-material zinc sulfide bi-layer antireflection coatings for GaAs solar cells

We demonstrated the efficiency improvement of GaAs single-junction (SJ) solar cells with the single-material zinc sulfide (ZnS) bi-layer based on the porous/dense film structure, which was fabricated by the glancing angle deposition (GLAD) method, as an antireflection (AR) coating layer. The porous ZnS film with a low refractive index was formed at a high incident vapor flux angle of 80° in the GLAD. Each optimum thickness of ZnS bi-layer was determined by achieving the lowest solar weighted reflectance (SWR) using a rigorous coupled-wave analysis method in the wavelength region of 350-900 nm, extracting the thicknesses of 20 and 50 nm for dense and porous films, respectively. The ZnS bi-layer with a low SWR of ~5.8% considerably increased the short circuit current density (J(sc)) of the GaAs SJ solar cell to 25.57 mA/cm(2), which leads to a larger conversion efficiency (η) of 20.61% compared to the conventional one without AR layer (i.e., SWR~31%, J(sc) = 18.81 mA/cm(2), and η = 14.82%). Furthermore, after the encapsulation, its J(sc) and η values were slightly increased to 25.67 mA/cm(2) and 20.71%, respectively. For the fabricated solar cells, angle-dependent reflectance properties and external quantum efficiency were also studied.

Broadband terahertz conductivity and optical transmission of indium-tin-oxide (ITO) nanomaterials

Indium-tin-oxide (ITO) nanorods (NRs) and nanowhiskers (NWhs) were fabricated by an electron-beam glancing-angle deposition (GLAD) system. These nanomaterials are of interests as transparent conducting electrodes in various devices. Two terahertz (THz) time-domain spectrometers (TDS) with combined spectral coverage from 0.15 to 9.00 THz were used. These allow accurate determination of the optical and electrical properties of such ITO nanomaterials in the frequency range from 0.20 to 4.00 THz. Together with Fourier transform infrared spectroscopic (FTIR) measurements, we found that the THz and far-infrared transmittance of these nanomaterials can be as high as 70% up to 15 THz, as opposed to about 9% for sputtered ITO thin films. The complex conductivities of ITO NRs, NWhs as well films are well fitted by the Drude-Smith model. Taking into account that the volume filling factors of both type of nanomaterials are nearly same, mobilities, and DC conductivities of ITO NWhs are higher than those of NRs due to less severe carrier localization effects in the former. On the other hand, mobilities of sputtered ITO thin films are poorer than ITO nanomaterials because of larger concentration of dopant ions in films, which causes stronger carrier scattering. We note further that consideration of the extreme values of Re{σ} and Im{σ} as well the inflection points, which are functions of the carrier scattering time (τ) and the expectation value of cosine of the scattering angle (γ), provide additional criteria for accessing the accuracy of the extraction of electrical parameters of non-Drude-like materials using THz-TDS. Our studies so far indicate ITO NWhs with heights of ~1000 nm show outstanding transmittance and good electrical characteristics for applications such as transparent conducting electrodes of THz Devices.

Multi-functional antireflective surface-relief structures based on nanoscale porous germanium with graded refractive index profiles

We fabricated the multi-stacked graded refractive index (GRIN) porous amorphous germanium (a-Ge) films with linear, quintic, and Gaussian index profiles for their refractive indices at a wavelength of 1.33 μm by e-beam evaporation with a glancing angle deposition technique. The porous a-Ge films with inclined nanocolumnar structures were deposited on Ge substrates at incident vapor flux angles of 40, 55, and 75°. The surface wetting behaviour of the porous a-Ge films was explored, exhibiting the hydrophilic surfaces with water contact angles below 78°. Their optical reflectance properties were investigated, together with the theoretical analysis using the rigorous coupled-wave analysis simulation. For the GRIN a-Ge films with a quintic index profile, a low reflectance band of <2% was shifted towards the longer wavelength with increasing the total thickness (ttotal) of GRIN a-Ge layers. For the GRIN a-Ge films with optimized ttotal values of 350, 333, and 225 nm for linear, quintic, and Gaussian index profiles, respectively, the low reflectance spectra of <10% were obtained at wavelengths of 1.1-1.7 μm, which yielded the lowest average reflectance value (Ravg) of ∼2.2% for Gaussian index profile (compared to the Ravg ∼ 3.7 and 4% for linear and quintic ones, respectively). The angle-dependent reflectance characteristics were also studied at incident angles of 15-85° for s- and p-polarized lights at a wavelength of 1.33 μm. The calculated results showed similar trends to the experimental data.

Broadband and wide-angle distributed Bragg reflectors based on amorphous germanium films by glancing angle deposition

We fabricated the distributed Bragg reflectors (DBRs) with amorphous germanium (a-Ge) films consisted of the same materials at a center wavelength (λc) of 1.33 μm by the glancing angle deposition. Their optical reflectance properties were investigated in the infrared wavelength region of 1-1.9 μm at incident light angles (θ inc) of 8-70°, together with the theoretical analysis using a rigorous coupled-wave analysis simulation. The two alternating a-Ge films at the incident vapor flux angles of 0 and 75° were formed as the high and low refractive index materials, respectively. The a-Ge DBR with only 5 periods exhibited a normalized stop bandwidth (∆λ/λ c) of ~24.1%, maintaining high reflectance (R) values of > 99%. Even at a high θ inc of 70°, the ∆λ/λ c was ~21.9%, maintaining R values of > 85%. The a-Ge DBR with good uniformity was obtained over the area of a 2 inch Si wafer. The calculated reflectance results showed a similar tendency to the measured data.

THz conductivities of indium-tin-oxide nanowhiskers as a graded-refractive-index structure

Indium-tin-oxide (ITO) nanowhiskers with attractive electrical and anti-reflection properties were prepared by the glancing-angle electron-beam evaporation technique. Structural and crystalline properties of such nanostructures were examined by scanning transmission electron microscopy and X-ray diffraction. Their frequency-dependent complex conductivities, refractive indices and absorption coefficients have been characterized with terahertz time-domain spectroscopy (THz-TDS), in which the nanowhiskers were considered as a graded-refractive-index (GRIN) structure instead of the usual thin film model. The electrical properties of ITO GRIN structures are analyzed and fitted well with Drude-Smith model in the 0.2~2.0 THz band. Our results indicate that the ITO nanowhiskers and its bottom layer atop the substrate exhibit longer carrier scattering times than ITO thin films. This signifies that ITO nanowhiskers have an excellent crystallinity with large grain size, consistent with X-ray data. Besides, we show a strong backscattering effect and fully carrier localization in the ITO nanowhiskers.

Indium tin oxide subwavelength nanostructures with surface antireflection and superhydrophilicity for high-efficiency Si-based thin film solar cells

We fabricated the parabola-shaped subwavelength grating (SWG) nanostructures on indium tin oxide (ITO) films/Si and glass substrates using laser interference lithography, dry etching, and subsequent re-sputtering processes. The efficiency enhancement of an a-Si:H/μc-Si:H tandem thin film solar cell was demonstrated theoretically by applying the experimentally measured data of the fabricated samples to the simulation parameters. Their wetting behaviors and effective electrical properties as well as optical reflectance properties of ITO SWGs, together with theoretical prediction using a rigorous coupled-wave analysis method, were investigated. For the parabola-shaped ITO SWG/ITO film, the solar weighted reflectance (SWR) value was ~10.2% which was much lower than that (i.e., SWR~20%) of the conventional ITO film, maintaining the SWR values less than 19% up to a high incident angle of 70° over a wide wavelength range of 300-1100 nm. Also, the ITO SWG with a superhydrophilic surface property (i.e., water contact angle of 6.2°) exhibited an effective resistivity of 2.07 × 10(-3) Ω-cm. For the a-Si:H/μc-Si:H tandem thin film solar cell structure incorporated with the parabola-shaped ITO SWG/ITO film as an antireflective electrode layer, the conversion efficiency (η) of 13.7% was theoretically obtained under AM1.5g illumination, indicating an increased efficiency by 1.4% compared to the device with the conventional ITO film (i.e., η = 12.3%).

Use of two-dimensional nanorod arrays with slanted ITO film to enhance optical absorption for photovoltaic applications

Two-dimensional (2D) Si-nanorod arrays offer a promising architecture that has been widely recognized as attractive devices for photovoltaic applications. To further reduce the Fresnel reflection that occurs at the interface between the air and the 2D Si-nanorod array because of the large difference in their effective refractive indices, we propose and adopt a slanted ITO film as an intermediate layer by using oblique-angle sputtering deposition. The nearly continuous surface of the slanted ITO film is lossless and has high electrical conductivity; therefore, it could serve as an electrode layer for solar cells. As a result, the combination of the above-mentioned nanostructures exhibits high optical absorption over a broad range of wavelengths and incident angles, along with a calculated short-circuit current density of JSC = 32.81 mA/cm2 and a power generation efficiency of η = 22.70%, which corresponds to an improvement of approximately 42% over that of its bare single-crystalline Si counterpart.

Broadband wide-angle antireflection enhancement in AZO/Si shell/core subwavelength grating structures with hydrophobic surface for Si-based solar cells

Broadband wide-angle antireflection characteristics of aluminum-doped zinc oxide (AZO)/silicon (Si) shell/core subwavelength grating (SWG) structures with a hydrophobic surface, together with theoretical prediction using a rigorous coupled-wave analysis simulation, were investigated for Si-based solar cells. The AZO films with different thicknesses were deposited on Si SWGs by rf magnetron sputtering method, which forms a shell/core structure. The AZO/Si shell/core SWGs reduced significantly the surface reflection compared to the AZO films/Si substrate. The coverage of AZO films on Si SWGs improved the antireflective property over a wider incident angle. The AZO/Si shell/core SWG structure with a 200 nm-thick AZO layer deposited at an rf power of 200 W exhibited a water contact angle of 123°. This structure also exhibited a low average reflectance of ~2% over a wide wavelength range of 300-2100 nm with a solar weighted reflectance of 2.8%, maintaining a reflectance of < 9.2% at wavelengths of 300-2100 nm up to the incident angle of θ(i) = 70°. The effective electrical properties of AZO films in AZO/Si shell/core SWGs were also analyzed.