My research regarded the fabrication of InSb and InAsSb large diameter nanowires for infrared applications. / InSb and InAsSb pillars, which are large diameter nanowires (NWs), were investigated as an alternative infrared (IR) detector technology to HgCdTe (MCT) for tunable multispectral IR detection with optical properties manipulated by pillar diameter and pitch. Undoped InSb and InAsSb thin films were grown on undoped Si (100) substrates by molecular beam epitaxy (MBE) with a thin AlSb buffer layer. A top-down etching method was used to fabricate pillars of diameters ranging from 300 nm to 1500 nm for InSb, and 1700 nm to 4000 nm for InAsSb. Pillar arrays were analyzed optically by Fourier transform IR spectroscopy (FTIR). The InSb and InAsSb pillars produced narrow absorption peaks with wavelength ranging from 1.61 μm to 6.86 μm for InSb and 8.1 μm to 16.2 μm for InAsSb. A 100 nm increase in pillar diameter corresponded to a 0.495 μm increase in peak absorption wavelength. InSb thin films were also grown on n-type (As doped, ≤ 0.005 Ω cm) Si (100) substrates to create a p-i-n junction, with an initial 2 μm thick undoped InSb region grown directly on the substrate, and a 0.5 μm thick p-type (Be doped, 2x1019 cm-3) InSb top layer. These films were used to create two devices; an interdigitated contact photoconductor with varying finger geometry, and a photovoltaic device with square top contacts of varying area. I-V characterization demonstrated trends in current with varying finger geometry. Photocurrent measurements were obtained for both the photoconductor and photovoltaic devices under IR and solar illumination. The photocurrent values were orders of magnitude higher for the photoconductive device compared to the photovoltaic device, indicative of potential photoconductive gain. Photocurrent generation in the InSb p-i-n structure introduces the possibility of diameter-dependent photocurrent generation in etched pillar devices. / Thesis / Master of Applied Science (MASc) / Infrared light (IR) falls in the wavelength range of 0.75 μm to 1000 μm, with IR based technology having numerous applications in society. With uses in the sciences, research, medicine, and general everyday technology, common IR ranges for material analysis range from 1.4 to 3 μm in the short-wavelength IR (SWIR), 3-5 μm in the mid-wavelength IR (MWIR), and 8-15 μm in the long-wavelength IR (LWIR). These ranges include IR absorption due to molecular vibrations, and includes wavelengths corresponding to the bandgaps of relevant semiconductor materials for IR detectors. To aid in light absorption in semiconductor materials, nanometer scale cylindrical structures called nanowires or pillars can be used on the detector surface, enhancing light absorption, and allowing for absorption wavelength manipulation by adjusting nanowire diameter. This work focuses on developing IR detectors with wavelength absorption in the 1-16 μm range, dependent on nanowire geometry.

Approved for public release, distribution is unlimited / In this thesis, the performance of a step quantum well infrared photodetector (QWIP), which was designed to detect a laser spot at 1.05 m æ and IR radiation between 9.5 m æ to 10.5 m æ, was evaluated. In the far infrared (FIR), the maximum responsivity of the test detector at 10 K was measured at 1.02 A/W with a peak wavelength of 10.3 æm under a negative bias of 0.83 V. The D* at background limited infrared performance (BLIP) was measured at 9 8.0 10 cm Hz /W with 180o field of view. The BLIP temperature was found to be about 55 K. In addition, the FIR detection wavelength found shift with the amount of bias across the device. The amount of shift observed was 0.21 meV/KVcm-1 which is due to linear Stark effect associated with the step quantum well. The quantum well infrared detectors made of square quantum wells found to have an order of magnitude lower Stark shift originated from second order effects. This suggests that the step well infrared detector can have applications in tunable wavelength infrared detectors. / Major, Republic of Singapore Navy

Semiconductor quantum dots and metal nanoparticles feature very strong light-matter interactions, which has led to their use in many photonic applications such as photodetectors, biosensors, components for telecommunications etc.Under illumination both structures exhibit collective electron-photon resonances, described in the frameworks of quasiparticles as exciton-polaritons for semiconductors and surface plasmon-polaritons for metals.To date these two approaches to controlling light interactions have usually been treated separately, with just a few simple attempts to consider exciton-plasmon interactions in a system consisting of both semiconductor and metal nanostructures.In this work, the exciton-polaritons and surface \\plasmon-polaritons are first considered separately, and then combined using the Finite Difference Time Domain numerical method coupled with a master equation for the exciton-polariton population dynamics.To better understand the properties of excitons and plasmons, each quasiparticle is used to investigate two open questions - the source of the Stokes shift between the absorption and luminescence peaks in quantum dots, and the source of the photocurrent increase in quantum dot infrared photodetectors coated by a thin metal film with holes. The combined numerical method is then used to study a system consisting of multiple metal nanoparticles close to a quantum dot, a system which has been predicted to exhibit quantum dot-induced transparency, but is demonstrated to just have a weak dip in the absorption. / <p>QC 20120417</p>

<p>Quantum Dot Infrared Photodetectors (QDIPs) are important alternatives to conventional infrared photodetectors with high potential to provide required detector performance, such as higher temperature operation and multispectral response, due to the 3-D quantum confinement of electrons, discrete energy levels, and intrinsic response to perpendicular incident light due to selection rules. However, excessive dark current density, which causes QDIPs to underperform theoretical predictions, is a limiting factor for the advancement of QDIP technologies. The purpose of this dissertation research is to achieve a better understanding of dopant incorporation into the active region of QDIPs, which is directly related to dark current control and spectral response. From this dissertation research, doping related dipole fields are found to be responsible for excessive dark current in QDIPs. </p><p>InAs/GaAs QDIPs were grown using solid source molecular beam epitaxy (MBE) with different doping conditions. The QDIPs were optically characterized using photoluminescence and Fourier transform infrared (FT-IR) spectroscopy. Devices were fabricated using standard cleanroom fabrication procedures. Dark current and capacitance measurements were performed under different temperature to reveal electronic properties of the materials and devices. A novel scanning capacitance microscopy (SCM) technique was used to study the band structure and carrier concentration on the cross section of a quantum dot (QD) heterostructure. In addition, dark current modeling and bandstructure calculations were performed to verify and better understand experimental results.</p><p>Two widely used QDIP doping methods with different doping concentrations have been studied in this dissertation research, namely direct doping in InAs QD layer, and modulation doping in the GaAs barrier above InAs QD layer. In the SCM experiment, electron redistribution has been observed due to band-bending in the modulation-doping region, while there is no band-bending observed in directly doped samples. A good agreement between the calculated bandstructure and experimental results leads to better understanding of doping in QD structures. The charge filling process in QDs has been observed by an innovative polarization-dependent FT-IR spectroscopy. The red-shift of QD absorbance peaks with increasing electron occupation supports a miniband electronic configuration for high-density QD ensembles. In addition, the FT-IR measurement indicates the existence of donor-complex (DX) defect centers in Si-doped QDIPs. The existence of DX centers and related dipole fields have been confirmed by dark current measurements to extract activation energies and by photocapacitance quenching measurements. </p><p>With the understanding achieved from experimental results, a further improved dark current model has been developed based on the previous model originally established by Ryzhii and improved by Stiff-Roberts. In the model described in this dissertation, two new factors have been considered. The inclusion of background drift current originating from Si shallow donors in the low bias region results in excellent agreement between calculated and measured dark currents at different temperatures, which has not been achieved by previous models. A very significant effect has been observed in that dark current leakage occurs due to the dipole field caused by doping induced charge distribution and impact-ionized DX centers. </p><p>Last but not least, QDIPs featuring the dipole interface doping (DID) method have been designed to reduce the dark current density without changing the activation energy (thus detection wavelength) of QDIPs. The DID samples involve an InAs QD layer directly-doped by Si, as well as Be doping in the GaAs barrier on both sides of the QD layer. The experimental result shows the dark current density has been significantly reduced by 104 times without any significant change to the corresponding activation energy. However, the high p-type doping in the GaAs barrier poses a challenge in that the Fermi level is reduced to be well below the QD energy states. High p-type doping is reported to reduce the dark current, photocurrent and the responsivity of the devices. </p><p>To conclude, it is significant to identify to effect of Si-induced defect centers on QDIP dark currents. The subsequent study reveals doping induced dipole fields can have significant effects on QDIP device performance, for example, causing charge leakage from QDs and reducing activation energy, thereby increasing dark current density. The DID approach developed in this work is a promising approach that could help address these issues by using controlled dipole fields to reduce dark current density without changing the minimum detectable energy of QDIPs.</p> / Dissertation

The objective of this work was to study the noise in semiconductors and relate the transport mechanisms in the devices with the noise from the devices. The main part of the work was to set up a system for the measurement of noise in semiconductor devices. To establish the sensitivity of the system, it was calibrated at different temperatures. Some of the results from GaAs pn-junction showed some anomaly from that available in the literature. But certain points are yet to be clarified. This requires certain developments in the measurement system. In the case of QWIPS structures, studies on some samples with varying number of wells are required and in order to study the GR noise spectra and other activated processes, we need to study the temperature dependence of the noise and a larger bias variation for studying the low frequency current noise.

Neste trabalho, foi estudado a aplicação de novas heteroestruturas semicondutoras para detecção de radiação na região do infravermelho médio. Pontos quânticos de submonocamada, detectores de cascateamento quântico e pontos quânticos de InAlAs foram testados como opção para corrigir as deficiências em responsividade, corrente de escuro e temperatura de operação, comuns nas heteroestruturas convencionais baseadas em poços quânticos e pontos quânticos de InAs obtidos no regime de crescimento Stranski-Krastanov. Também foi projetado, fabricado e testado um circuito eletrônico de leitura de sinal misto para integração com matrizes de sensores e produção de imagens. Esse tipo de circuito possui uma série de vantagens em relação aos dispositivos convencionais que costumam ser completamente analógicos. / In this work, we studied the application of new types of semiconductor heterostructures for mid-infrared detection. Submonolayer quantum dots (SML-QDs), quantum-cascade detectors (QCDs) and InAlAs quantum dots were tested as an option to circumvent the common shortcomings of responsivity, dark current and operating temperature of the usual heterestructures based on quantum wells (QWs) and InAs Stranski-Krastanov quantum dots. We also designed, fabricated and tested a mixed-signal read-out circuit aiming the fabrication of focalplane arrays (FPAs) for applications to infrared imaging. This kind of architecture has several advantages over a fully analog design.

Neste trabalho, foi estudado a aplicação de novas heteroestruturas semicondutoras para detecção de radiação na região do infravermelho médio. Pontos quânticos de submonocamada, detectores de cascateamento quântico e pontos quânticos de InAlAs foram testados como opção para corrigir as deficiências em responsividade, corrente de escuro e temperatura de operação, comuns nas heteroestruturas convencionais baseadas em poços quânticos e pontos quânticos de InAs obtidos no regime de crescimento Stranski-Krastanov. Também foi projetado, fabricado e testado um circuito eletrônico de leitura de sinal misto para integração com matrizes de sensores e produção de imagens. Esse tipo de circuito possui uma série de vantagens em relação aos dispositivos convencionais que costumam ser completamente analógicos. / In this work, we studied the application of new types of semiconductor heterostructures for mid-infrared detection. Submonolayer quantum dots (SML-QDs), quantum-cascade detectors (QCDs) and InAlAs quantum dots were tested as an option to circumvent the common shortcomings of responsivity, dark current and operating temperature of the usual heterestructures based on quantum wells (QWs) and InAs Stranski-Krastanov quantum dots. We also designed, fabricated and tested a mixed-signal read-out circuit aiming the fabrication of focalplane arrays (FPAs) for applications to infrared imaging. This kind of architecture has several advantages over a fully analog design.

No description available.

[pt] Nesta tese, apresento uma investigação teórica e experimental das propriedades eletro-ópticas de um fotodetector baseado em uma superrede assimétrica InGaAs/InAlAs com um defeito estrutural. Essa heteroestrutura apresenta duas importantes características: estados parcialmente localizados no contínuo, chamados de estados eletrônicos vazantes, e um aumento virtual do band offset na banda de condução. Devido à assimetria da superrede, a função de onda do estado eletrônico vazante é localizada numa direção e estendida na outra. Em consequência dessas características, o fotodetector apresenta modo dual de operação, fotocondutivo e fotovoltaico, e temperatura de operação ambiente. O modo fotovoltaico foi alcançado devido à direção preferencial de escape do fluxo de elétrons excitados para os estados eletrônicos vazantes no contínuo. A temperatura de operação elevada ocorre devido à diminuição da corrente de escuro térmica causada pelo aumento virtual do band offset. No modo fotovoltaico, o espectro de fotocorrente apresenta dois picos estreitos de energias em 300 meV e em torno 440 meV, sendo eles relacionados às transições ópticas do estado fundamental para o primeiro e o segundo estado vazante no contínuo, respectivamente. Para o modo fotocondutivo, a largura de linha do espectro de fotocorrente é fortemente dependente da direção do bias de voltagem aplicado. Para o bias positivo, o espectro de fotocorrente apresenta um pico em 300 meV e um ombro de energia em torno de 260 meV. Para o bias negativo, o espectro de fotocorrente mostra uma banda larga com dois picos em 300 meV e 260 meV. Esse comportamento está relacionado com a população dos estados na minibanda em função da direção do bias aplicado. As figuras de mérito do fotodetector, em ambos os modos de operação, apresentam resultados similares aos melhores fotodetectores encontrados na literatura. / [en] In this thesis, I present a theoretical and experimental investigation of the electro-optical properties of a photodetector based on an InGaAs/InAlAs asymmetric superlattice with a structural defect. This heterostructure has two important characteristics: partially localized states in the continuum, called by leaky electronic states, and a virtual increase in conduction band offset. Due to the asymmetry of the superlattice, the wavefunction of the leaky electronic state is located in one direction and extended in another one. As a result of these features, the photodetector presents a dual-mode operation, photoconductive and photovoltaic modes, and room temperature operating. The photovoltaic mode has reached due to the preferential direction for the flow of excited electrons in the leaky electronic state in the continuum. The high operating temperature occurs because of the decrease in thermal dark current due to the virtual increase of band offset. In photovoltaic mode, the photocurrent spectrum has two narrow energy peaks at 300 meV and around 440 meV, which are related to optical transitions from the ground state to the first and the second leaky electronic states, respectively. For photoconductive mode, the line width of the photocurrent spectrum is strongly dependent on the direction of the applied voltage bias. For the positive bias, the photocurrent spectrum has a peak at 300 meV and a power shoulder around 260 meV. For the negative bias, the photocurrent spectrum shows broadband with two peaks at 300 meV and 260 meV. This behavior is related to the population of the mini band states as a function of the applied bias direction. The figure of merits of the photodetector, in both operation modes, present results similar to the best photodetectors found in the literature.

Quantum dots exhibit strongly size-dependent optical and electrical properties. The ability to join the dots into complex assemblies creates many opportunities for scientific discovery. This motivated our present research work on QDIPs, DWELLs, and graphene like QDs. The intention of this research was to study the size dependent achievements of QDIPs, DWELLs, and graphene like QDs with those of competitive technologies, with the emphasis on the material properties, device structure, and their impact on the device performance. In this dissertation four research studies pertaining to optical properties of quantum dot and dot-in-a-well infrared photodetectors, I-V characteristics of graphene quantum dots, and energy and spin texture of germanene quantum dots are presented. Improving self-assembled QD is a key issue in the increasing the absorption and improving the performance. In the present research work, an ideal self-assembled QD structure is analyzed theoretically with twenty-hole levels (Intraband optical transitions within the valence band) and twenty-electron energy levels (DWELL). Continuing the efforts to study self-assembled QDs we extended our work to graphene like quantum dots (graphene and germanene) to study the electronic transport properties. We study numerically the intraband optical transitions within the valence band of InxGa1-xAs/GaAs pyramidal quantum dots. We analyze the possibility of tuning of corresponding absorption spectra by varying the size and composition of the dots. Both ‘x ’ and the size of the quantum dot base are varied. We have found that the absorption spectra of such quantum dots are more sensitive to the in-plane incident light. We present numerically obtained absorption optical spectra of n-doped InAs/In0.15Ga0.85As/GaAs quantum dot-in-a-well systems. The absorption spectra are mainly determined by the size of the quantum dot and have weak dependence on the thickness of the quantum well and position of the dot in a well. The dot-in-a-well system is sensitive to both in-plane and out-of-plane polarizations of the incident light with much stronger absorption intensities for the in-plane-polarized light. We also present theoretically obtained I-V characteristics of graphene quantum dots, which are realized as a small piece of monolayer graphene. We describe graphene within the nearest-neighbor tight-binding model. The current versus the bias voltage has typical step-like shape, which is due to discrete energy spectrum of the quantum dot. The current through the dot system also depends on the position of the electrodes relative to the quantum dot. In relation to graphene quantum dots, we present our study of buckled graphene-like materials, like germanene and silicene. We consider theoretically germanene quantum dot, consisting of 13, 27, and 35 germanium atoms. Due to strong spin-orbit interaction and buckled structure of the germanene layer, the direction of the spin of an electron in the quantum dot depends on both the electron energy and external perpendicular electric field. With variation of energy, the direction of spin changes by approximately 4.50. Application of external electric field results in rotation of electron spin by approximately 0.50, where the direction of rotation depends on the electron energy.