Nanoporous TiO2-Based Distributed Bragg Reflectors for Near-Infrared Wavelength Applications.

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).

Full-angle distributed Bragg reflectors (DBRs) consisting of numerous sub-DBRs with discrete central wavelengths have been developed to enhance performance of GaN-based flip-chip mini light-emitting diodes (FC mini-LEDs). However, relatively low reflectivity of full-angle DBRs at large angle incidence restricts further enhancement in performance of FC mini-LEDs. Here, we introduce a reflectivity optimization strategy for constructing high-reflectivity multiple-stack DBRs by rationally engineering the number of sub-DBRs and adjusting central wavelength distribution of sub-DBRs. Based on the reflectivity optimization strategy, we devise a Ti3O5/SiO2 quintuple-stack DBR which is composed of five sub-DBRs. Our quintuple-stack DBR maintains a high reflectivity (>97.5%) over a wide range of incident angles of light. Notably, compared with the full-angle DBR, our quintuple-stack DBR exhibits higher reflectivity at large angle incidence and thinner multilayer thickness. Furthermore, we demonstrate two types of GaN-based blue FC mini-LEDs with indium-tin oxide (ITO)/quintuple-stack DBR and ITO/full-angle DBR p-type ohmic contacts. Benefiting from superior reflection performance, blue FC mini-LED with ITO/quintuple-stack DBR achieves an enhancement of ∼5.8% in light output power at 10 mA, in comparison with blue FC mini-LED with ITO/full-angle DBR. Our work signifies an advancement towards high-reflectivity DBRs, which enables higher-performance FC mini-LEDs.

The refractive index contrast in dielectric multilayer structures, optical resonators and photonic crystals is an important figure of merit, which creates a strong demand for high quality thin films with a very low refractive index. SiO2 nano-rod layers with low refractive indices n= 1.08, the lowest ever reported in thin-film materials, is grown by oblique-angle e-beam deposition of nano-rod layer with vapor incident angle thetas = 85deg. Scanning electron micrographs reveal a highly porous columnar structure of the low-refractive-index (low-n) film. The gap between the SiO2 nano-rods is les 50 nm, i.e. much smaller than the wavelength of visible light, and thus sufficiently small to make scattering negligibly small. Optical micrographs of the low-n film deposited on a Si substrate reveal a uniform specular film with no apparent scattering. The unprecedented low index of the SiO2 nano-rod layer is confirmed by both ellipsometry measurements and thin film interference measurements. A single-pair distributed Bragg reflector (DBR) employing the SiO2 nano-rod layer is demonstrated to have enhanced reflectivity, showing the great potential of low-n films for applications in photonic structures and devices

It was about 125 years ago that the light bulb was commercialized by Thomas Edison. No doubt a brilliant invention at the time, today its low power conversion efficiency is one of the reasons why lighting in the western world has such high energy consumption. Thus, the potential for saving energy is enormous in this area. The introduction of halogen, discharge and fluorescent lamps has lead to certain efficiency improvements, however more than half of the energy is still lost as heat. Light-emitting diodes (LEDs) are very promising candidates for high efficiency light sources, with modern devices showing internal quantum efficiencies of virtually 100 %. However, due to the high refractive index of the commonly used semiconductor materials it is very difficult to have a large extraction efficiency; in a standard cubic geometry most of the internally emitted light is trapped inside the device due to total internal reflection. Several methods have been developed in order to circumvent this problem, either by optimizing the device geometry in order to increase the escape cone or by incorporating a resonant structure in order to force the internal emission into the existing escape cone. The latter approach is called microcavity LED (MCLED) or resonant cavity LED (RCLED). In a MCLED the spontaneous internal emission is controlled by placing the emitter inside an optical cavity with a thickness of the order of its emitting wavelength. The resulting interference effects increase the part of the emission that can be extracted. Contrary to the other approaches this is possible without changing the device geometry and thus without additional costly back-end processing steps. The control of the farfield radiation pattern makes these devices particularly interesting for high brightness applications, which demand highly directional emitters, such as for printing, bar code reading, large area displays and optical communication. The extraction efficiency of a MCLED is inversely proportional to the effective cavity length. An ideal cavity, allowing an extraction efficiency close to unity, consists of a low refractive index material and has an optical length of λ/2. In contrast to this, to obtain high internal quantum efficiencies it is necessary to use high index cavities with an optical length of at least λ. It should be noted, that the large penetration depth of the optical field in the semiconductor-based distributed Bragg reflectors (DBRs) leads to a significant increase of the effective cavity length and thus further reduces the achievable extraction efficiencies. In this thesis novel concepts to reduce effective cavity lengths and therefore increase extraction efficiencies are implemented into standard MCLED structures. The phaseshift cavity principle whilst maintaining the electrical properties of a standard A cavity achieves optical properties approaching that of a λ/2 cavity. The use of AlOx instead of AlAs as the, low refractive index component in the DBRs leads to smaller penetration depths and a concomitant reduction of the effective cavity length. A similar effect can be obtained by combining a resonant cavity with a thin-film structure. Thanks to these design improvements, the external quantum efficiency of different types of MCLEDs was increased. Near infrared emitting InGaAs/GaAs MCLEDs including a phase-shift cavity were realized, as both bottom and top emitting structures. The external quantum efficiencies achieved for emission into air were 18 and 19 %, respectively. With the additional incorporation of an oxide based bottom DBR, the efficiency of top emitting near infrared MCLEDs was further increased to 28 %. Red emitting AlGaInP-based structures are not compatible with the phase-shift cavity principle. However the beneficial effect of the implementation of an oxide DBR is greater at their wavelength rather than in the near infrared. Thus with preliminary red emitting GaInP/AlGaInP MCLEDs containing a bottom oxide DBR external quantum efficiencies of 12% could be achieved. Unfortunately, the incorporation of an oxide DBR significantly complicates the device design and the device fabrication. These problems can be avoided by combining the resonant cavity with a thin-film structure instead. Initial non-optimized red emitting thin-film MCLEDs were realized by OSRAM Opto Semiconductors and characterized in this work. They show external quantum efficiencies of 23 % and 18 % with and without encapsulation, respectively. It is assumed that a significant fraction of the high external quantum efficiency is due to a strong photon recycling effect in these devices. Simulations presented in this thesis show that the theoretical limits for the MCLEDs discussed above are slightly higher than the values obtained, encouraging further device optimization. The thin-film MCLEDs seem to hold the biggest potential for high efficiency emission from MCLEDs, independent of the wavelength range of emission.

We describe the design, fabrication, and characterization of mechanically stable, reproducible, and highly reflecting distributed Bragg reflectors (DBR) composed of thermally evaporated thin films of calcium fluoride (CaF2) and zinc sulfide (ZnS). CaF2 and ZnS were chosen as the low and high refractive index components of the multilayer DBR structures, with n = 1.43 and n = 2.38 respectively, because neither material requires substrate heating during the deposition process in order to produce optical quality thin films. DBRs consisting of seven pairs of CaF2 and ZnS layers, were fabricated with thicknesses of 96 and 58 nm, respectively, as characterized by high-resolution scanning electron microscopy (HR-SEM), and exhibited a center wavelength of λc = 550 nm and peak reflectance exceeding 99%. The layers showed good adhesion to each other and to the glass substrate, resulting in mechanically stable DBR coatings. Complete optical microcavities consisting of two such DBR coatings and a CaF2 spacer layer between them could be fabricated in a single deposition run. Optically, these structures exhibited a resonator quality factor of Q > 160. When a CaF2/ZnS DBR was grown, without heating the substrate during deposition, on top of a thin film containing the fluorescent dye Rhodamine 6G, the fluorescence intensity showed no degradation compared to an uncoated film, in contrast to a MgF2/ZnS DBR coating grown with substrate heating which showed a 92% reduction in signal. The ability to fabricate optical quality CaF2/ZnS DBRs without substrate heating, as introduced here, can therefore enable formation of low-loss high-reflectivity coatings on top of more delicate heat-sensitive materials such as organics and other nanostructured emitters, and hence facilitate the development of nanoemitter-based microcavity device applications.

The design, fabrication, and characterization of abrupt interface and graded interface distributed Bragg reflector (DBR) multilayers is reported. Electron-beam deposited zinc selenide (ZnSe) and Barium Fluoride (BaF2) thin films were chosen as the high and low refractive index dielectric materials respectively, for the DBRs in the mid-wavelength infrared (MWIR) region. In this region, DBRs comprised of 3 stacks of abrupt interface ZnSe and BaF2 thin films with central wavelength of 4 µm were found to suffer from defects and absorption loss that later affect the optical transmission and spectral resolution of assembled Fabry-Perot optical filters. Interfacial grading of the DBRs with a thin layer of amorphous silicon monoxide (SiO) ~ 80 nm, reduced the absorption loss and the interdiffusion of defect atoms at the interface between alternating layers. Fabry-Perot filters assembled from abrupt interface DBRs showed 22% optical transmission, whereas those assembled from graded interface DBRs acheived 26.5% optical transmission. The Full-width at half maximum (FWHM) of the transmission peak for graded interface Fabry-Perot optical filter decreased significantly compared to the abrupt interface optical filter, improving its spectral resolution.

AlGaN/AlGaN distributed Bragg reflectors (DBRs) designed for the ultraviolet spectral region have been attained. The crack-free structures were grown on <i>c</i>-plane sapphire by plasma assisted molecular beam epitaxy (MBE). To minimize the built-in strain in DBRs, a thin buffer layer was used directly on <i>c</i>-plane sapphire. A peak reflectivity of 95% at 381 nm with a 21 nm stop band width was obtained at room temperature (RT) using a 32.5 pairs Al_0.7Ga_0.3N/Al_0.15Ga_0.85N DBR. With a driving force for DBRs and emitting regions in wide band gap semiconductor microcavities, such as those based on GaN and ZnO, is the quest for cavity polariton which is the coupled mode between the exciton and photon modes. Moreover, the exploitation of cavity polaritons could be expected in the course of the development of extremely low-threshold optoelectronics devices.

In this project, we explored how different oxygen flow rates influenced the creation of TiO2 nanostructures in thin films using magnetron sputtering preparation with oblique angle deposition (OAD) technique. The experiment involved oxygen flow rates of 20, 40, 60, 80 and 100 sccm along with a constant argon flow at 20 sccm. Morphological analysis by FE-SEM was performed to determine film thickness and deposition rate. In a subsequent stage, thin films thickness was confirmed to be 100 nm. After coating, specimens were annealed at 300°C for 2 h using various O2 flow rates. FE-SEM revealed morphological changes, GIXRD to explore crystal structures, and UV-Vis spectrophotometry to measure light transmittance. The results showed an optimal oxygen flow rate at 60 sccm, with annealed specimens exhibiting anatase structure and promising light transmittance of 73-83%.

A 13.5-pair Al0.98In0.02N/Al0.77Ga0.23N distributed Bragg reflector (DBR) was grown by molecular beam epitaxy on an AlN template substrate. The DBR exhibited a peak reflectivity of 83.9% at 246 nm within the deep solar-blind ultraviolet region and a stopband width of 18 nm. The average refractive index contrast for the DBR was 9.25% at 246 nm, which is relatively high compared with that of a traditional Al(Ga)N/(Al)GaN DBR. Simulated results indicated that a 25.5-pair Al0.98In0.02N/Al0.77Ga0.23N DBR will provide a reflectivity higher than 99% and a 19 nm wide stopband with a center wavelength of 246 nm.

The distributed Bragg reflectors (DBRs) are important optical components in a wide range of applications, such as Fabry-Perot optical resonators, hyperspectral sensors, micro-cavity structures, and waveguide lasers. In this thesis, we have developed, designed, and fabricated highly reflecting, and mechanically stable DBRs comprised of electron-beam deposited zinc selenide (ZnSe) and barium fluoride (BaF2) thin films and explored its use in optical filtering and hyperspectral sensing applications. Most published results are in the short-wavelength infrared (SWIR) and near-infrared (NIR) with limited results in the mid-wavelength infrared (MWIR). In this work, we have fabricated ZnSe/BaF2-based DBRs in the MWIR region. The fabricated DBRs showed a central wavelength of 4 µm and reflection exceeding 93%. A Fabry-Perot optical filter, that is comprised of two parallel DBR mirrors with half-wavelength optical cavity thickness was used to partially transmit and reflect the incident light at the target wavelength. The assembled fixed-cavity Fabry-Perot optical filters achieved a full-width half maximum (FWHM) of ~ 450 nm. We further showed that the FWHM of the optical filters can be reduced by ~ 50 % with the addition of nano-thick silicon monoxide (SiO) interface grading layer between every pair of the quarter-wave optical thick (QWOT) layers in the stack. The SiO interface grading layer acts as an anti-reflection coating layer to reduce reflection loss in the DBR multilayer structure. The SiO interface grading layer, ~ 80 nm thick, also has the potential to stop atomic inter-diffusion between the layers to minimize free carrier absorption loss. The principal thesis goal is to design and assemble fixed-cavity interface-graded Fabry-Perot optical filters (with SiO interfacial layers) that optimized optical transmission and spectral resolution. The thesis also proposes a low-cost micro-cavity structure that can be formed by embedding two-dimensional atomic crystals, such as black phosphorous into a DBR cavity structure. The proposed design can be used to increase absorption in 2D materials, which has the potential to increase the efficiency of next generation MWIR photodetectors and infrared sensors.

In this study, we present a floating gate memory device based on indium–gallium zinc oxide (IGZO), enhanced by applying the oblique angle deposition (OAD) technique. The IGZO floating gate with a nanostructure created using the OAD technique exhibits reduced conductivity. As a result, the IGZO floating gate memory with OAD technique demonstrates a high on/off ratio (∼106) and narrow hysteresis characteristics (0.24 V) in the transfer curve. Additionally, the photoresponse of dinaphtho[2,3 b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) channel, which has a band gap of ∼2.61 eV, enables the light-induced programming and facilitates the charge accumulation in the floating gate. The proposed memory device enables multilevel programming through both photonic (λ = 450 nm) and electrical (VG = +60 V) inputs, either independently or in combination. Under dual-mode operation, a substantial threshold voltage shift (ΔVTH = 23.1 V) is achieved, whereas single-mode inputs yield more moderate shifts of 11.8 V (VG only) and 5.9 V (light only). This dual-mode capability allows for distinct programmable memory states with enhanced control and precision. In contrast, the IGZO floating gate transistor without the OAD technique exhibits a relatively low on/off ratio (∼103), high off-current (∼10–7 A), and broad hysteresis characteristics (>30 V). These results suggest that the IGZO floating gate with the OAD technique applied is appropriate for memory operation.

Distributed Bragg reflector (DBR) structure has been widely used in optoelectronic devices during the MBE growth. Precisely controlling the growth thickness of superlattice layers is very important for successfully achieving the optical properties of DBR structure. In general, an ideal thickness controlling with better than 1% accuracy is needed to realize the devices. Both methods of optical reflection difference and dynamic optical reflectivity are usually used to measure the growth rate of epitaxial growth layer in MBE experiment. During the calculation of the growth rate, however, the refractive index at high temperature is needed. The refractive indices of semiconductor materials axe the sensitive parameters to the temperature and the wavelength of incident laser. Up to now, there are not enough parameters for the semiconductor material indices at high temperature during a relatively wide wavelength region. In this paper, we use high resolution x-ray diffraction (HRXRD) technique to decide the superlattice thickness. With these data, we calculate and extract the refractive indices of the epitaxial layers of GaAs and AlAs. The accuracy of extracted indices is certified by optical reflectivity spectrum.

Improvement in light output intensity of InGaN/GaN multiple-quantum-well blue light-emitting diode by SiO 2/Si 3N 4 distributed Bragg reflectors and silver back mirror

tion (AR) coatings, [7–10] and optical resonators. [11] In many cases, however, the unavailability of materials with desired refractive indices, particularly materials with very low refractive indices, prevents the implementation of optical components with very high performance. In addition, the choice of a material with desired refractive index often forces a compromise in other materials properties such as optical transmittance and electrical conductivity that are also important for most optoelectronic applications. Here, we show that oblique-angle deposition can be used to tailor the refractive index of a thinfilm material that is chosen for its desired material properties other than refractive index. The unique ability to control the refractive index of thin film materials allows one to eliminate Fresnel reflection, one of the fundamental limitations in lightextraction efficiency of light-emitting diodes (LEDs), by fabricating coatings whose refractive index gradually decreases from the refractive index of the active semiconductor layer to the refractive index of the surrounding medium. As an example of this concept, we present a six-layer graded-refractiveindex (GRIN) AR coating made entirely of a single material, indium tin oxide (ITO), chosen for its high conductivity, high optical transmittance, and low contact resistance with GaN. Each layer has a refractive index that is individually tuned to form a stack with refractive index graded from its dense ITO value down to the value close to that of air for an optimum AR performance. It is shown that GaInN LEDs with a GRIN ITOAR contact achieve a light-extraction efficiency enhancement of 24.3 % compared to the LEDs with dense ITO coating due to a strongly reduced Fresnel reflection at the ITO– air interface. Oblique-angle deposition is a method of growing porous thin films, and hence thin films with low-refractive index (lown), enabled by surface diffusion and self-shadowing effects during the deposition process. [12–16] In oblique-angle deposition, a random growth fluctuation on the substrate produces a shadow region that the incident vapor flux cannot reach, and a non-shadow region where incident flux deposits preferentially, thereby creating an oriented rodlike structure with high porosity. Figure 1 shows the cross-sectional scanning-electron microscopy (SEM) image of low-n ITO, which is electrically conductive and optically transparent in visible wavelengths, COMMUNICATION

Titanium dioxide (TiO2) has been extensively investigated for applications in photocatalysis due to its stability, nontoxicity, and robust photocatalytic properties. Although thin films of TiO2 are effective in wastewater treatment, their dense structure limits surface accessibility. The oblique angle deposition (OAD) technique can be used to create high surface area nanostructured thin films essential for photocatalytic applications. In this study, OAD was used to grow TiO2 thin films. To extend the activity to the visible light region, plasma-reduced silver (Ag) particles were coupled to the TiO2 surface. The Ag-decorated TiO2 (Ag-TiO2) nanostructured films were fabricated using a custom-built magnetron sputter deposition system with OAD technique capability. Different substrate angles of 0°, 35°, and 70° with respect to the target normal were used. The films were characterized for their structural, compositional, and optical properties. The photodegradation performance was evaluated using methylene blue (MB) as the test analyte under visible light irradiation. The results showed a significant improvement in degradation efficiency using plasma-reduced Ag-TiO2. Substrates tilted at 70° achieved the highest MB degradation efficiency of 89.84% after 5 h exposure. This tilt angle also revealed high surface roughness and high surface area. By tilting the substrate with respect to the target, high surface area thin films can be fabricated. This technique is ideal for applications that depend on the area of contact, such as photocatalysis.