Tensilely strained germanium nanomembranes for infrared light emitting devices

Thesis (Ph.D.)--Boston Universitydevelopment of group-IV semiconductor lasers has attracted significant attention in recent years, since it represents the key missing ingredient for the large-scale monolithic integration of electronics and photonics in a CMOS-compatible fashion. The main challenge is to convert the indirect-bandgap group-IV materials into efficient light emitters. Many researchers have focused on improving the light emission efficiency of these materials in the near-infrared (NIR) spectral region, to replace the existing chip-to- chip communication technology with optical links. At the same time, group-IV lasers operating at mid-infrared (MIR) wavelengths also possess many important applications, mainly in the area of chemical and biological sensing, such as trace-gas detection, environmental monitoring, medical diagnostics, and industrial process control. Motivated by these applications, here I focus on improving the light emission efficiency of germanium (Ge). The small energy difference between its direct and indirect bandgaps can be further decreased with the introduction of tensile strain, leading to significantly improved radiative efficiency. At the same time, the bandgap energy shifts into the technologically important 2.1-2.5 µm MIR atmospheric transmission window. At 1.9% tensile strain, Ge even becomes a direct-bandgap semiconductor. In this work, tensile strain is introduced in Ge nanomembranes (NMs), i.e., single-crystal sheets with nanoscale thicknesses, through the application of mechanical stress. Our strain-resolved photoluminescence (PL) measurements performed on these NMs demonstrate a significant red-shift and enhancement in the emission spectra with increasing strain. PL measurement results obtained with a 24-nm-thick NM also reveal that the membrane is converted into direct-bandgap Ge with the application of 2% tensile strain. Furthermore, theoretical analysis of the high-strain PL spectra shows that population inversion can be achieved in these ultrathin NMs with gain values as high as 300 cm−1. Two-dimensional periodic structures fabricated on the top surface of such membranes result in further enhanced light collection through first-order diffraction of the in-plane emitted luminescence. Furthermore, the cavity modes of these periodic structures are also resolved in the strain-dependent PL spectra. These results are promising for the demonstration of Ge NM lasers operating in the technologically important 2.1-2.5 µm spectral region for potential applications in biochemical sensing and spectroscopy

Combined Optical-Electrical Optimization of Cd1−xZnxTe/Silicon Tandem Solar Cells

Although the fundamental limits have been established for the single junction solar cells, tandem configurations are one of the promising approaches to surpass these limits. One of the candidates for the top cell absorber is CdTe, as the CdTe photovoltaic technology has significant advantages: stability, high performance, and relatively inexpensive. In addition, it is possible to tune the CdTe bandgap by introducing, for example, Zn into the composition, forming Cd1−xZnxTe alloys, which can fulfill the Shockley–Queisser limit design criteria for tandem devices. The interdigitated back contact (IBC) silicon solar cells presented record high efficiencies recently, making them an attractive candidate for the rear cell. In this work, we present a combined optical and electrical optimization of Cd1−xZnxTe/IBC Si tandem configurations. Optical and electrical loss mechanisms are addressed, and individual layers are optimized. Alternative electron transport layers and transparent conductive electrodes are discussed for maximizing the top cell and tandem efficiency

Coupled fiber taper extraction of 1.53 um photoluminescence from erbium doped silicon nitride photonic crystal cavities

Optical fiber tapers are used to collect photoluminescence emission at ~1.5 um from photonic crystal cavities fabricated in erbium doped silicon nitride on silicon. Photoluminescence collection via fiber taper is enhanced 2.5 times relative to free space, with a total taper collection efficiency of 53%. By varying the fiber taper offset from the cavity, a broad tuning range of coupling strength is obtained. This material system combined with fiber taper collection is promising for building on-chip optical amplifiers.Comment: 10 pages, 7 figure

Enhanced Light Emission from Erbium Doped Silicon Nitride in Plasmonic Metal-Insulator-Metal Structures

Plasmonic gratings and nano-particle arrays in a metal-insulator-metal structures are fabricated on an erbium doped silicon nitride layer. This material system enables simple fabrication of the structure, since the active nitride layer can be directly grown on metal. Enhancement of collected emission of up to 12 is observed on resonance, while broad off-resonant enhancement is also present. The output polarization behavior of the gratings and nano-particle arrays is investigated and matched to plasmonic resonances, and the behavior of coupled modes as a function of inter-particle distance is also discussed.Comment: 9 pages, 6 figures updated because pdf was still non-functiona

Optical and electrical design guidelines for ZnO/CdS nanorod-based CdTe solar cells

An alternative structure to planar CdTe solar cells is realized by coating ZnO/CdS nanorods (NRs) with a CdTe layer. These structures are expected to achieve high-powered conversion efficiencies through enhanced light absorption and charge carrier collection. ZnO NR-based CdTe solar cell efficiencies; however, they have remained well below their planar counterparts, thus hindering NRs in CdTe solar cells' advantages. Here, we analyze the light trapping and carrier collection efficiencies in two types of ZnO NR-based CdTe solar cells through optical and electrical simulations. The buried CdTe solar cells are formed by completely filling the gaps in between ZnO/CdS NRs. This produces a maximum achievable photo-current of 27.4 mA/cm(2) when 2000 nm-tall and 20 degrees-angularly-deviated NRs are used. A short-circuit current density of 27.3 mA/cm(2) is achievable with the same geometry for 5 rods/mu m(2)-dense NRs when a moderate CdTe doping density and a CdS/CdTe surface velocity of 10(16) cm(-3) and 10(4) cm/s are used, respectively. We reveal the potential of buried CdTe solar cell for high-charge carrier collection and provide a design guideline in order to achieve high short-circuit current densities with ZnO NR-based CdTe solar cells. (c) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen

Observation of Transparency of Erbium-doped Silicon nitride in photonic crystal nanobeam cavities

One-dimensional nanobeam photonic crystal cavities are fabricated in an Er-doped amorphous silicon nitride layer. Photoluminescence from the cavities around 1.54 um is studied at cryogenic and room temperatures at different optical pump powers. The resonators demonstrate Purcell enhanced absorption and emission rates, also confirmed by time-resolved measurements. Resonances exhibit linewidth narrowing with pump power, signifying absorption bleaching and the onset of stimulated emission in the material at both 5.5 K and room temperature. We estimate from the cavity linewidths that Er has been pumped to transparency at the cavity resonance wavelength.Comment: 10 pages, 7 figure

Guideline for Optical Optimization of Planar Perovskite Solar Cells

Organometallic halide perovskite solar cells have emerged as a versatile photovoltaic technology with soaring efficiencies. Planar configuration in particular, has been a structure of choice thanks to its lower temperature processing, compatibility with tandem solar cells and potential in commercialization. Despite all the breakthroughs in the field, the optical mechanisms leading to highly efficient perovskite solar cells lack profound insight. In this paper, a comprehensive guideline is introduced involving semi-analytical equations for thickness optimization of the front and rear transport layers, perovskite, and transparent conductive oxides to improve the antireflection and light trapping properties, and therefore to maximize the photocurrent of perovskite solar cells. It is shown that a photocurrent enhancement above 2 mA/cm2 can be achieved by altering - reducing or increasing - the thicknesses of the layers constituting a CH3NH3PbI3 (MAPI) type perovskite solar cell. The proposed guideline is tested against experiments as well as previously published experimental and simulation results for MAPI. Additionally, the provided guideline for various types of perovskites can be extended to other direct band gap absorber-based solar cells in superstrate configuration

EN KÜÇÜK KARELERLE SPEKTRAL ANALİZ VE FOURIER TEKNİĞİNİN KARŞILAŞTIRILMASI

İstatistik biliminin en önemli amaçlarından birisi, deneysel zaman dizilerinin analizi yapılarak fiziksel süreçler hakkında bilgiler çıkarılmasıdır. Bu amaca uygun olarak, literatürde birçok matematiksel araç geliştirilmiştir. Fourier tekniği ve En Küçük Karelerle Spektral Analiz (EKKSA) bunlardan sadece iki tanesidir. Deneysel zaman dizilerinin büyük bir kısmı içerisinde trend, kısa boşluklar, datum kayıklıkları ve eşit olmayan veri aralıkları ile ağırlıklarını bulundurur. Çoğu kez bunlar analiz aşamasında zorlaştırıcı etki yapar. Fourier tekniğinde trend ön analizle kaldırılırken, kısa boşluklar ve eşit olmayan veri aralıkları enterpolasyon teknikleri kullanılarak doldurulur. Söz konusu prosedürler, hem dizilerin içindeki gerekli bilginin yok sayılmasına hem de yapay sinyallerin elde edilmesine neden olur. Bu durum analiz işlemi için istenmeyen sonuçlar doğması anlamına gelir. Bu tür zorlukların üstesinden gelebilecek alternatif yöntemlerden birisi de EKKSA’dır. Bu çalışmada sözü edilen zorlukları içeren deniz düzeyi gözlemleri hem EKKSA hem de Fourier tekniği kullanılarak analiz edilmiş, sayısal sonuçlar karşılaştırılarak EKKSA’nın üstün yönlerine dikkat çekilmiştir. Sonuçlar, deneysel zaman dizilerinin spektral analizinde EKKSA’nın Fourier tekniğinden daha güçlü bir matematiksel araç olduğunu göstermektedir

Preparation and Characterization of Mixed Halide MAPbI3−xClx Perovskite Thin Films by Three‐Source Vacuum Deposition

Chloride has been extensively used in the preparation of metal halide perovskites such as methylammonium lead iodide (MAPbI3-xClx), but its persistence and role in solution-processed materials has not yet been rationalized. Multiple-source vacuum deposition of perovskites enables a fine control over the thin-film stoichiometry, and allows to incorporate chemical species irrespectively of their solubility. In this communication, we present the first example of mixed MAPbI3-xClx thin films prepared by three-source vacuum deposition using MAI, PbI2 and PbCl2 as precursors. The optoelectronic properties of the material are evaluated through photovoltaic and electro-/photo-luminescent characterizations. Besides the very similar structural and optical properties of MAPbI3 and MAPbI3-xClx, we observed an increased electroluminescence efficiency, longer photoluminescence lifetimes, as well as larger photovoltage in the presence of chloride, suggesting a reduction of the non-radiative charge recombination