Engineered quantum dots for infrared photodetectors

Quantum Dot Infrared Photodetector (QDIP) Focal Plane Arrays (FPAs) have been proposed as an alternative technology for the 3rd generation FPAs. QDIPs are emerging as a competitive technology for infrared detection and imaging especially in the midwave infrared (MWIR) and longwave infrared (LWIR) regime. These detectors are based on intersubband transitions in self-assembled InAs quantum dots (QDs) and offer several advantages such as normal incidence detection, low dark currents and high operating temperatures, while enjoying all the benefits of a mature GaAs fabrication technology. However, due to Stranski-Krastanov (SK) growth mode and the subsequent capping growth, the conventional SK QDs are pancake shaped\u27 with small height to base ratio due to interface diffusion. Thus they cannot fully exploit the 3D \u27artificial atom\u27 properties. This dissertation work investigates two approaches for shape engineered QDs: (1) Selective capping techniques of Stranski-Krastanov QDs, and (2) Growth of Sub-Monolayer (SML) QDs. Using Molecular Beam Epitaxy (MBE) growth, engineered QDs have been demonstrated with improved dot geometry and 3D quantum confinement to more closely resemble the 3D \u27artificial atom\u27. In SK-QDs, the results have demonstrated an increased dot height to base aspect ratio of 0.67 compared with 0.23 for conventional SK-QD using Transmission Electron Microscope (TEM) images, enhanced s-to-p polarized spectral response ratio of 37% compared with 10% for conventional SK-QD, and improved SK-QDIP characterization such as: high operating temperature of 150K under background-limited infrared photodetection (BLIP) condition, photodetectivity of 1x109 cmHz1/2/W at 77K for a peak wavelength of 4.8 μm, and photoconductive gain of 100 (Vb=12V) at 77 K. In SML-QDs, we have demonstrated dots with a small base width of 4~6 nm, height of 8 nm, absence of wetting layer and advantage optical property than the SK-QDs. SML-QD shows adjustable dot height to base aspect ratio of 8nm/6nm, increased s-to-p polarized spectral response ratio of 33%, and a narrower full width at half maximum (FWHM), long wavelength 10.5 μm bound-to-bound intersubband transition, and higher responsivity of 1.2 A/W at -2.2 V at 77K and detectivity of 4x109 cmHz1/2/W at 0.4 V 77K.\u2

Semiconductor Quantum Structures for Ultraviolet-to-Infrared Multi-Band Radiation Detection

In this work, multi-band (multi-color) detector structures considering different semiconductor device concepts and architectures are presented. Results on detectors operating in ultraviolet-to-infrared regions (UV-to-IR) are discussed. Multi-band detectors are based on quantum dot (QD) structures; which include quantum-dots-in-a-well (DWELL), tunneling quantum dot infrared photodetectors (T-QDIPs), and bi-layer quantum dot infrared photodetectors (Bi-QDIPs); and homo-/heterojunction interfacial workfunction internal photoemission (HIWIP/HEIWIP) structures. QD-based detectors show multi-color characteristics in mid- and far-infrared (MIR/FIR) regions, where as HIWIP/HEIWIP detectors show responses in UV or near-infrared (NIR) regions, and MIR-to-FIR regions. In DWELL structures, InAs QDs are placed in an InGaAs/GaAs quantum well (QW) to introduce photon induced electronic transitions from energy states in the QD to that in QW, leading to multi-color response peaks. One of the DWELL detectors shows response peaks at ∼ 6.25, ∼ 10.5 and ∼ 23.3 µm. In T-QDIP structures, photoexcited carriers are selectively collected from InGaAs QDs through resonant tunneling, while the dark current is blocked using AlGaAs/InGaAsAlGaAs/ blocking barriers placed in the structure. A two-color T-QDIP with photoresponse peaks at 6 and 17 µm operating at room temperature and a 6 THz detector operating at 150 K are presented. Bi-QDIPs consist of two layers of InAs QDs with different QD sizes. The detector exhibits three distinct peaks at 5.6, 8.0, and 23.0 µm. A typical HIWIP/HEIWIP detector structure consists of a single (or series of) doped emitter(s) and undoped barrier(s), which are placed between two highly doped contact layers. The dual-band response arises from interband transitions of carriers in the undoped barrier and intraband transitions in the doped emitter. Two HIWIP detectors, p-GaAs/GaAs and p-Si/Si, showing interband responses with wavelength thresholds at 0.82 and 1.05 µm, and intraband responses with zero response thresholds at 70 and 32 µm, respectively, are presented. HEIWIP detectors based on n-GaN/AlGaN show an interband response in the UV region and intraband response in the 2-14 µm region. A GaN/AlGaN detector structure consisting of three electrical contacts for separate UV and IR active regions is proposed for simultaneous measurements of the two components of the photocurrent generated by UV and IR radiation

Novel broadband light sources and pulse generation techniques at 1.5 [mu]m

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2009.In title on title page, [mu] appear as lower case Greek letter. Cataloged from PDF version of thesis.Includes bibliographical references.A wide diversity of applications, in both fundamental science and practical technology, has come to rely on broadband optical light sources as key enabling tools. In this thesis, we investigate three devices that contribute to the generation of broadband light at 1.5 [mu]m. The first two fall into the same broader class of saturable absorber mirrors - one device was developed for low-repetition-rate sub-100-fs ultrafast lasers and the other for high-repetition- rate femtosecond lasers. The third device bypasses generating broadband light directly from a laser altogether through the use of extra-cavity spectral broadening in a novel highly nonlinear glass fiber. In the first category, ultra-broadband saturable absorber mirrors based on III/V and Si material systems were developed for ultrafast lasers. The III/V-based mirrors were designed, fabricated, characterized and implemented in a wide range of wavelengths, spanning the visible to the near-infrared. These mirrors exhibited high-reflectivity ranges of >300 nm. Implementation of these mirrors in Ti:sapphire, Cr4+:forsterite, Cr4+:YAG, and erbium-doped bismuth-oxide lasers demonstrated self-starting and stable modelocked operation. Saturable absorber mirrors were also developed for high-repetition-rate short-cavity femtosecond lasers, with modulation depths ranging from 1.7% to 11%. Post-growth proton bombardment was used to improve recovery times, and preliminary laser testing has yielded promising results, with all structures demonstrating modelocking in an erbium-doped fiber laser.(cont.) Saturable absorber mirrors with integrated dispersion compensation were also designed with 750 fs2 of anomalous group delay dispersion at 1.5 [mu]m. Finally, a novel highly nonlinear bismuth-oxide glass fiber was used to generate smooth, controlled supercontinuum spanning 1200 to 1800 nm. With a 2-cm length and a grating pair for dispersion compensation, compression of 150-fs pulses down to 25-fs was also demonstrated.by Hanfei M. Shen.Ph.D

Bipolar cascade lasers

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2000.Includes bibliographical references.This thesis addresses issues of the design and modeling of the Bipolar Cascade Laser (BCL), a new type of quantum well laser. BCLs consist of multiple single stage lasers electrically coupled via tunnel junctions. The BCL ideally operates by having each injected electron participate in a recombination event in the topmost active region, then tunnel from the valence band of the first active region into the conduction band of the next active region, participate in another recombination event, and so on through each stage of the cascade. As each electron may produce more than one photon the quantum efficiency of the device can, in theory, exceed 100%. This work resulted in the first room temperature, continuous-wave operation of a BCL, with a record 99.3% differential slope efficiency. The device was fully characterized and modeled to include light output and voltage versus current bias, modulation response and thermal properties. A new singlemode bipolar cascade laser, the bipolar cascade antiresonant reflecting optical waveguide laser, was proposed and modeled.by Steven G. Patterson.Ph.D

Examination of the Feasibility of Transferred Electron Devices for Optoelectronic Interaction: Theory and Experiment

The engineering feasibility of Transferred Electron Devices (TEDs), configured as modulators, was explored. Vertical and planar implementations of the same device were pursued concurrently. An existing "side-wall bonding pad" vertical device was replaced by a revised structure, incorporating dual anode contacts and dielectric isolation. Natural self-resonant frequency inhibition with device capacitance was assessed for both of these vertical devices. The reappraisal was shown to provide benefits of reduced capacitance, reduced current capacity and greater symmetry of the electric field distribution in the active region. Modelling of the optical confinement attributes of rib waveguides for device designs was performed using the FWAVE III and LWAVE application programs. Confinement afforded by the rib-substrate interface and 33% AlGaAs layers, in vertical and planar devices respectively, was determined for multi-moded propagation. Efficiently confmed propagation at infrared wavelengths was observed. Theoretical predictions for modulation depths were calculated for rib waveguide devices. The modulation potential of mechanisms such as Pockels effect, Franz-Keldysh electro-absorption and free carrier influences were analysed. These evaluations were made employing very idealised conditions, leading to consistent over-estimates of modulation indices. The requirements for deep modulation were identified in the context of the engineering designs of real devices. The prospect for voltage-controlled frequency modulation in planar devices, required the development of analytical theory for tapered (graded area) devices. The functional variation of the pre threshold electric field distribution was obtained. This permitted the derivation of a predictive current-voltage function for a general transferred electron device with defined semiconductor and physical parameters. The time constant governing growth / decay of nucleated disturbances in tapered devices was sought, verifying the possibility of transit mode operation, A fully calculable function, encapsulating the influences of inter-valley transfer and dipole formation, was developed and quoted. Both vertical and planar devices were fabricated and assessed experimentally. The theoretical current voltage function was used as a fulcrum for the analysis of the influence of the cathode boundary conditions in planar devices. The behaviour of graded and non-graded elements was modified by varying the annealing conditions in a standard contact metal recipe. A mechanism was conjectured to explain these variations in the context of the Kroemer hypothesis. The resulting empirical functions were used to assess the inhibition of injected electrons at the cathode. Endfire analysis was performed on waveguide devices, with electrical biassing applied in-situ. In response to theoretical thermal modelling, in conjunction with practical observations of real devices, a circular mesa form of waveguide device was proposed and fabricated. Estimates of maximum permissible pulse width were made, assisted by Laplace transform analysis of a thermal - electrical analogue circuit. A simple formula for rib-substrate thermal resistance was derived for this analysis. Permissible pulse widths were demonstrated to be incompatible with the notion of a viable modulator operating near to CW

Cutting Edge Nanotechnology

The main purpose of this book is to describe important issues in various types of devices ranging from conventional transistors (opening chapters of the book) to molecular electronic devices whose fabrication and operation is discussed in the last few chapters of the book. As such, this book can serve as a guide for identifications of important areas of research in micro, nano and molecular electronics. We deeply acknowledge valuable contributions that each of the authors made in writing these excellent chapters

A Comprehensive Review on Raman Spectroscopy Applications

Raman spectroscopy is a very powerful tool for material analysis, allowing for exploring the properties of a wide range of different materials. Since its discovery, Raman spectroscopy has been used to investigate several features of materials such carbonaceous and inorganic properties, providing useful information on their phases, functions, and defects. Furthermore, techniques such as surface and tip enhanced Raman spectroscopy have extended the field of application of Raman analysis to biological and analytical fields. Additionally, the robustness and versatility of Raman instrumentations represent a promising solution for performing on-field analysis for a wide range of materials. Recognizing the many hot applications of Raman spectroscopy, we herein overview the main and more recent applications for the investigation of a wide range of materials, such as carbonaceous and biological materials. We also provide a brief but exhaustive theoretical background of Raman spectroscopy, also providing deep insight into the analytical achievements

Infrared Optical Properties Of Wurtzite Semiconductor Heterostructure With Arbitrary Crystal Orientations

Pergantungan orientasi hablur dengan sifat-sifat optik inframerah (IR) untuk semikonduktor heterostruktur wurtzit heksagon dan substrat yang berkaitan telah dikaji. Pengukuran spektrum pantulan IR terkutub menunjukkan bahawa tindak balas spektrum daripada hablur nilam pukal dan heterostruktur III-nitrida wurtzit yang terdiri daripada lapisan-lapisan yang bersatah kristalografi sembarangan adalah bergantung kepada orientasi sampel. Kecuali sampel yang mempunyai permukaan berorientasi satah-c, spektrum pantulan IR terkutub untuk sampel yang dikaji boleh diubahkan dengan memutar sampel pada normal permukaan. Formula pantulan yang mengambil kira kesan orientasi hablur telah digunakan bersama dengan langkah penyesuaian lengkung untuk menganalisis spektrum yang diukur. Parameter-parameter bahan yang penting seperti pemalar dielektrik, mod fonon optik, ketebalan lapisan dan orientasi kristal untuk sampel yang dikaji telah ditentukan secara tidak membinasa daripada penyesuaian yang terbaik bagi spektrum pantulan IR terkutub eksperimen dan teori. Dengan menggunakan parameter yang diperolehi, simulasi untuk spektrum penyebaran polariton fonon permukaan dan antara muka (SPhP dan IPhP) telah dijalankan dengan mengambil kira kesan-kesan parameter lembapan dan orientasi hablur. Crystal orientation dependence of the infrared (IR) optical properties of hexagonal wurtzite III-nitride heterostructures and their relevant substrates was investigated. Polarized IR reflectance measurements showed that the spectral responses of bulk sapphire crystals and wurtzite III-nitride heterostructures consisting of layers with arbitrary crystallographic planes depend on sample orientation. Except for sample with c-plane oriented surface, the polarized IR reflectance spectra of a given sample can be changed by rotating the sample about its surface normal. A reflection formula that considers the effect of crystal orientation was employed with a curve fitting procedure to analyze the measured spectra. Important materials parameters such as the dielectric constant, optical phonon modes, layer thickness and crystal orientation of the studied samples, have been non-destructively determined from the best-fit of experimental and theoretical polarized IR reflectance spectra. Using the obtained parameters, simulations of the surface and interface phonon polaritons (SPhP and IPhP) dispersion spectra have been performed by taking into account the effects of damping parameters and crystal orientation

Large scale tunneling junctions for electrically driven plasmonics

This work focuses on the fabrication of light emitting tunneling junctions in planar configuration comprised of thin-film material stacks analog to metal-insulator-semiconductor plate capacitors. Electrical and structural properties are studied by different experimental techniques (current-voltage analysis, impedance spectroscopy) and compared to existing theories. Assessment of the junction quality is done in comparison to known features of electrically-driven plasmons, such as the bias dependent cut-off frequency, the dependency of the emission intensity on the tunneling current and tuneability of the spectra by implementation of different materials. Enhanced scattering and tuneability of light emission features from tunneling junctions by adsorption of chemically-synthesized nanoparticles is demonstrated and localization of the emission hot spots by correlation with measurements in external illumination and topography scans are discussed. Operational stability is increased by decoupling of the fabrication sub-steps, i.e. deposition of high quality thin-film stacks and chemical synthesis of particles with tailored optical properties. The role of nanoparticle geometry and material as hot spots in light emitting tunneling junctions is described and distinguished to reference experiments with external illumination. Emission instabilities in low-frequency regimes from hot spots with uncorrelated phases have been observed and are discussed. Potential transferability of electrically-driven plasmons to established detection schemes is demonstrated exemplary by mimicking a study of a plasmonic nanoruler. Additionally, a first proof-of-principle study on the emission from light emitting tunneling junctions in direct water immersion is described