abstract: Recently a new materials platform consisting of semiconductors grown on GaSb and InAs substrates with lattice constants close to 6.1 A was proposed by our group for various electronic and optoelectronic applications. This materials platform consists of both II-VI (MgZnCdHg)(SeTe) and III-V (InGaAl)(AsSb) compound semiconductors, which have direct bandgaps spanning the entire energy spectrum from far-IR (~0 eV) up to UV (~3.4 eV). The broad range of bandgaps and material properties make it very attractive for a wide range of applications in optoelectronics, such as solar cells, laser diodes, light emitting diodes, and photodetectors. Moreover, this novel materials system potentially offers unlimited degrees of freedom for integration of electronic and optoelectronic devices onto a single substrate while keeping the best possible materials quality with very low densities of misfit dislocations. This capability is not achievable with any other known lattice-matched semiconductors on any available substrate. In the 6.1-A materials system, the semiconductors ZnTe and GaSb are almost perfectly lattice-matched with a lattice mismatch of only 0.13%. Correspondingly, it is expected that high quality ZnTe/GaSb and GaSb/ZnTe heterostructures can be achieved with very few dislocations generated during growth. To fulfill the task, their MBE growth and material properties are carefully investigated. High quality ZnTe layers grown on various III-V substrates and GaSb grown on ZnTe are successfully achieved using MBE. It is also noticed that ZnTe and GaSb have a type-I band-edge alignment with large band offsets (delta_Ec=0.934 eV, delta_Ev=0.6 eV), which provides strong confinement for both electrons and holes. Furthermore, a large difference in refractive index is found between ZnTe and GaSb (2.7 and 3.9, respectively, at 0.7 eV), leading to excellent optical confinement of the guided optical modes in planar semiconductor lasers or distributed Bragg reflectors (DBR) for vertical-cavity surface-emitting lasers. Therefore, GaSb/ZnTe double-heterostructure and ZnTe/GaSb DBR structure are suitable for use in light emitting devices. In this thesis work, experimental demonstration of these structures with excellent structural and optical properties is reported. During the exploration on the properties of various ZnTe heterostructures, it is found that residual tensile strains exist in the thick ZnTe epilayers when they are grown on GaAs, InP, InAs and GaSb substrates. The presence of tensile strains is due to the difference in thermal expansion coefficients between the epilayers and the substrates. The defect densities in these ZnTe layers become lower as the ZnTe layer thickness increases. Growth of high quality GaSb on ZnTe can be achieved using a temperature ramp during growth. The influence of temperature ramps with different ramping rates in the optical properties of GaSb layer is studied, and the samples grown with a temperature ramp from 360 to 470 C at a rate of 33 C/min show the narrowest bound exciton emission peak with a full width at half maximum of 15 meV. ZnTe/GaSb DBR structures show excellent reflectivity properties in the mid-infrared range. A peak reflectance of 99% with a wide stopband of 480 nm centered at 2.5 um is measured from a ZnTe/GaSb DBR sample of only 7 quarter-wavelength pairs. / Dissertation/Thesis / Ph.D. Physics 2012

No description available.

<p>Heteroepitaxial films of group III-antimonide-based semiconductor compounds on Si are amongst the most appealing candidates for solar applications because of the well-established Si platform and also for offering band-gap energies beyond the silicon road map. Nonetheless, high lattice mismatch between GaSb and Si as well as ambiguous nucleation of GaSb on Si are major drawbacks in manufacturing of heteroepitaxial GaSb/Si films because they can generate various defects in films. Atomic-level detection of these defects and delving into their origin, orientation, distribution, propagation, and interaction with each other will therefore provide an insight into inhibiting their formation or reducing their severity. State-of-the-art aberration-corrected transmission electron microscopes have marked a new era in the investigation of interfaces and defects. With sub-angstrom electron probes in scanning transmission electron microscopes, it is possible to pinpoint the individual atomic columns at interfaces and defects.</p> <p>In this thesis, GaSb epilayers grown with molecular beam epitaxy on Si substrates were studied through aberration-corrected scanning transmission electron microscopy. The strain-relief mechanism of the epitaxial GaSb through formation of interfacial misfit dislocations was investigated and the strain distribution in the vicinity of dislocation cores as well as epitaxial layer was analyzed. The specific atomic-number dependent contrast mechanism of the high-angle annular dark-field technique enabled the unprecedented direct observation of anti-phase boundaries, the extended defects of highest interest in polar-on-nonpolar growths. This observation unraveled the ambiguity of nucleation of GaSb at interface regardless of preferential deposition of atomic species during growth procedure. The growth of GaSb at the initial stage of deposition was further investigated to understand the role of an AlSb buffer layer and growth mechanism of GaSb precisely. This investigation showed that AlSb and GaSb epilayers occur by Volmer-Weber growth mode and AlSb islands provide energetically favorable nucleation sites for GaSb film. Furthermore, taking advantage of atomic-resolution detection capability of high-angle annular dark-field in scanning transmission electron microscopy a novel mechanism of strain relief through multiple twining resulting in a lattice-registered growth of GaSb on Si(211) was elucidated. This contribution demonstrates that aberration-corrected scanning transmission electron microscopy provides profound insight into the polar-on-nonpolar growth which can be exploited to suppress the formation of structural defects.</p> / Doctor of Philosophy (PhD)

All-optical time-resolved measurement techniques provide a powerful tool for investigating critical parameters that determine the performance of infrared photodetector and emitter semiconductor materials. Narrow-bandgap InAs/GaSb type-II superlattices (T2SLs) have shown great promise as a next generation source of these materials, due to superior intrinsic properties and versatility. Unfortunately, InAs/GaSb T2SLs are plagued by parasitic Shockley-Read-Hall recombination centers that shorten the carrier lifetime and limit device performance. Ultrafast pump-probe techniques and time-resolved differential transmission measurements are used here to demonstrate that Ga-free InAs/InAsSb T2SLs and InAsSb alloys do not have this same limitation and thus have significantly longer carrier lifetimes. Measurements at 77 K provided minority carrier lifetimes of 9 μs and 3 μs for an unintentionally doped mid-wave infrared (MWIR) InAs/InAsSb T2SL and InAsSb alloy, respectively; a two order of magnitude increase compared to the 90 ns minority carrier lifetime measured in a comparable MWIR InAs/GaSb T2SL. Through temperature-dependent lifetime measurements, the various carrier recombination processes are differentiated and the dominant mechanisms identified for each material. These results demonstrate that these Ga-free materials are viable options over InAs/GaSb T2SLs for potentially improved infrared photodetectors. In addition to carrier lifetimes, the drift and diffusion of excited charge carriers through the superlattice growth layers (i.e. vertical transport) directly affects the performance of photodetectors and emitters. Unfortunately, there is a lack of information pertaining to vertical transport, primarily due to difficulties in making measurements on thin growth layers and the need for non-standard measurement techniques. However, all-optical ultrafast techniques are successfully used here to directly measure vertical diffusion in MWIR InAs/GaSb T2SLs. By optically generating excess carriers near one end of a MWIR T2SL and measuring the transit time to a thin, 2 lower-bandgap superlattice placed at the other end, the time-of-flight of vertically diffusing carriers is determined. Through investigation of both unintentionally doped and p-type superlattices at 77 K, the vertical hole and electron diffusion coefficients are determined to be 0.04±0.03 cm2/s and 4.7±0.5 cm2/s, corresponding to vertical mobilities of 6±5 cm2/Vs and 700±80 cm2/Vs, respectively. These measurements are, to my knowledge, the first direct measurements of vertical transport properties in narrow-bandgap superlattices. Lastly, the widely tunable two-color ultrafast laser system used in this research allowed for the investigation of nonlinear optical properties in narrow-bandgap semiconductors. Time-resolved measurements taken at 77 K of the nondegenerate two-photon absorption spectrum of bulk n-type GaSb have provided new information about the nonresonant change in absorption and two-photon absorption coefficients in this material. Furthermore, as the nondegenerate spectrum was measured over a wide range of optical frequencies, a Kramers-Kronig transformation allowed the dispersion of the nondegenerate nonlinear refractive index to be calculated.

Detection of mid-wavelength infrared radiation is crucial for many industrial, military and biomedical applications. Photon detectors in the market can operate at only low temperature which increases weight, power consumption and total cost. Type-II InAs/GaSb superlattice infrared detectors are expected to have a major role in the infrared detector market with providing high quality detection characteristics at higher temperatures. Therefore, in the past decade, there has been an increasing interest in infrared detectors based on type-II InAs/GaSb superlattice technology due to their long range adjustable bandgap, low tunneling current and Auger recombination rates which bring potential of high temperature operation. Characterization of this photodiodes requires detailed investigations on different aspects. This study focuses on various optical and electrical characterization techniques for single pixel infrared detectors: responsivity characterization using FTIR and blackbody source, dark I-V and R-V characterizations, response time measurement. Characterizations of detector noise with respect to frequency and bias voltage are studied in detail. These characterization techniques are carried out in order to observe the effects of design with three different &ldquo / standard&rdquo / and a new &ldquo / N&rdquo / structure designs and also to understand the effects of surface passivation with atomic layer deposited Al2O3 layer and ordinary PECVD deposited Si3N4 and SiO2 layers. When standard photodiodes are compared, we observed that the one with the thickest active absorber region has the highest response and dark current density values. &ldquo / N&rdquo / structure design photodiode has very low dark current density while its optical performance is not as high as the standard designs. Si3N4 passivation degrades both optical and electrical performances. SiO2 and Al2O3 passivation layers improve optical and electrical characteristics of photodiodes. Theoretical and experimental dark current noise values of SiO2 passivated sample in agreement up to 0.18V reverse bias while those values of unpassivated and Si3N4 passivated samples agree only at zero bias. Temperature dependent R-V characteristics of photodiodes are analyzed and the surface limited activation energy is calculated in order to investigate the additional noise. At the end, surface recombination noise is proposed to cover the deficit on the noise calculation.

In this thesis the InSb(111), InAs(111) and GaSb(001) surfaces have been studied by means of time- and angle-resolved photoemission spectroscopy based upon the femtosecond laser system. The pump-and-probe technique allows to analyse both electron states in the valence band and normally unpopulated electron states above the valence band, which can be occupied by transiently excited carriers at the optically pumped surface. The life time of excited carriers is analysed by controlling over the time delay between pump and probe pulses. Experimental studies of the InSb(111) surface and comparison with a previously studied InSb(110) surface show electron excitations in the bulk region with a minor surface contribution. Time-resolved experiments of carrier dynamics at the polar InAs(111)A and InAs(111)B surfaces show about the same life time of excited carriers, while no populated states above the valence band maximum have been found at the InAs(111)A due to the charge removal. Surface intergap electron states have been found at the GaSb(001) surface located at ~250 meV above the valence band maximum. Angle-resolved experiments showed a strong confinement of this state at the centre of the surface Brillouin zone. A new two dimensional angle-resolved multi-anode analyser for the femtosecond laser photoemission setup has been constructed. The analyser can resolve a cone opening angle of ~1º at a drift distance of ~0.5 m with an energy resolution of ~125 meV. A continuous series of binary system SrTiO3–PbZr0.52Ti0.48O3 has been grown by pulsed laser deposition (PLD) on sapphire substrate with crystalline quality control by x-ray diffraction (XRD). The maximum tunability has been tailored to room temperature, where STO�PZT (71/29) composition shows superior performance. A PbZr0.52Ti0.48O3 thin film pressure sensor has been fabricated by PLD and characterised by XRD and electrical measurements. The piezoelectric constant was found to be ~20 % higher compared to the bulk ceramics. A ferroelectric thin film electro-optical cell Na0.5K0.5NbO3/La0.5Sr0.5CoO3 (NKN/LSCO) on sapphire has been fabricated by PLD. Refractive indices and electro-optical coefficient of the cell were characterised by prism coupling refractometry. The tunability of the PLD fabricated 2 μm slot NKN thin film interdigital capacitor has been found ~23 % at 40 V bias voltage and frequency 1 MHz. / QC 20101015

Heterostructures of materials with dramatically different properties are exciting for a variety of devices. In particular, the epitaxial integration of metals with semiconductors is promising for low-loss tunnel junctions, embedded Ohmic contacts, high-conductivity spreading layers, as well as optical devices based on the surface plasmons at metal/semiconductor interfaces. This thesis investigates the structural, electrical, and optical properties of compound (III-V) semiconductors employing rare-earth monopnictide (RE-V) nanostructures. Tunnel junctions employing RE-V nanoparticles are developed to enhance current optical devices, and the epitaxial incorporation of RE-V films is discussed for embedded electrical and plasmonic devices. Leveraging the favorable band alignments of RE-V materials in GaAs and GaSb semiconductors, nanoparticle-enhanced tunnel junctions are investigated for applications of wide-bandgap tunnel junctions and lightly-doped tunnel junctions in optical devices. Through optimization of the growth space, ErAs nanoparticle-enhanced GaAs tunnel junctions exhibit conductivity similar to the best reports on the material system. Additionally, GaSb-based tunnel junctions are developed with low p-type doping that could reduce optical loss in the cladding of a 4 μm laser by ~75%. These tunnel junctions have several advantages over competing approaches, including improved thermal stability, precise control over nanoparticle location, and incorporation of a manifold of states at the tunnel junction interface. Investigating the integration of RE-V nanostructures into optical devices revealed important details of the RE-V growth, allowing for quantum wells to be grown within 15nm of an ErAs nanoparticle layer with minimal degradation (i.e. 95% of the peak photoluminescence intensity). This investigation into the MBE growth of ErAs provides the foundation for enhancing optical devices with RE-V nanostructures. Additionally, the improved understanding of ErAs growth leads to development of a method to grow full films of RE-V embedded in III-V materials. The growth method overcomes the mismatch in rotational symmetry of RE-V and III-V materials by seeding film growth with epitaxial nanoparticles, and growing the film through a thin III-V spacer. The growth of RE-V films is promising for both embedded electrical devices as well as a potential path towards realization of plasmonic devices with epitaxially integrated metallic films. / text

Ce travail de thèse porte sur l'élaboration, la croissance par épitaxie par jets moléculaires, et la caractérisation de superréseaux InAs/GaSb (SR) pour la réalisation de photodétecteurs infrarouges opérant dans la gamme de longueur d'onde 3-5 μm à température ambiante (RT). La première partie de ce mémoire présente les particularités de la détection infrarouge, ainsi qu'un état de l'art des différentes filières de détecteurs. Nous mettons également en exergue les propriétés des photodétecteurs infrarouges à SR (SLIPs) faisant de ce système de matériaux, un candidat très prometteur pour s'imposer dans la prochaine génération de caméras infrarouges. La seconde partie expose la croissance par EJM des SR sur substrat GaSb. La compensation de la contrainte par insertion d'une couche d'InSb à l'interface GaSb sur InAs a été étudiée, ainsi que l'influence de divers paramètres de croissance (température de croissance, pression équivalente des éléments V, ...). Les échantillons ont été caractérisés , et les mesures ont confirmé une grande qualité cristalline, et des SR épais (jusqu'à 2.5 μm) ont été réalisés. Enfin, nous avons élaboré des SLIPs p-i-n avec une longueur d'onde de coupure de 5.6 μm dont les caractérisations sont présentées dans la dernière partie. Ces composants à géométrie mesa ont fonctionné à RT avec un R0A~2-4.10-3 Ω.cm² , une sensibilité de 80 mA/W à 0 V donnant une détectivité spécifique calculée à 4 μm de 4.107 cmHz0.5/W. Une légère amélioration a été obtenue en insérant une couche d' Al0.4GaSb entre la couche buffer et le SR: un R0A~6.10-3 Ω.cm² , une sensibilité de 300 mA/W à -0.4 V donnant une détectivité spécifique calculée à 4 μm de 7.107 cmHz0.5/W.

Les antimoniures sont des semi-conducteurs III-V prometteurs pour le développement de dispositifs optoélectroniques puisqu'ils ont une grande mobilité d'électrons, une large gamme spectrale d'émission ou de détection et offrent la possibilité de former des hétérostructures confinées dont la recombinaison est de type I, II ou III. Bien qu'il existe plusieurs publications sur la fabrication de dispositifs utilisant un alliage d'In(x)Ga(1-x)As(y)Sb(1-y) qui émet ou détecte à une certaine longueur d'onde, les détails, à savoir comment sont déterminés les compositions et surtout les alignements de bande, sont rarement explicites. Très peu d'études fondamentales sur l'incorporation d'indium et d'arsenic sous forme de tétramères lors de l'épitaxie par jets moléculaires existent, et les méthodes afin de déterminer l'alignement des bandes des binaires qui composent ces alliages donnent des résultats variables. Un modèle a été construit et a permis de prédire l'alignement des bandes énergétiques des alliages d'In(x)Ga(1-x)As(y)Sb(1-y) avec celles du GaSb pour l'ensemble des compositions possibles. Ce modèle tient compte des effets thermiques, des contraintes élastiques et peut aussi inclure le confinement pour des puits quantiques. De cette manière, il est possible de prédire la transition de type de recombinaison en fonction de la composition. Il est aussi montré que l'indium ségrègue en surface lors de la croissance par épitaxie par jets moléculaires d'In(x)Ga(1-x)Sb sur GaSb, ce qui avait déjà été observé pour ce type de matériau. Il est possible d'éliminer le gradient de composition à cette interface en mouillant la surface d'indium avant la croissance de l'alliage. L'épaisseur d'indium en surface dépend de la température et peut être évaluée par un modèle simple simulant la ségrégation. Dans le cas d'un puits quantique, il y aura une seconde interface GaSb sur In(x)Ga(1-x)Sb où l'indium de surface ira s'incorporer. La croissance de quelques monocouches de GaSb à basse température immédiatement après la croissance de l'alliage permet d'incorporer rapidement ces atomes d'indium et de garder la seconde interface abrupte. Lorsque la composition d'indium ne change plus dans la couche, cette composition correspond au rapport de flux d'atomes d'indium sur celui des éléments III. L'arsenic, dont la source fournit principalement des tétramères, ne s'incorpore pas de la même manière. Les tétramères occupent deux sites en surface et doivent interagir par paire afin de créer des dimères d'arsenic. Ces derniers pourront alors être incorporés dans l'alliage. Un modèle de cinétique de surface a été élaboré afin de rendre compte de la diminution d'incorporation d'arsenic en augmentant le rapport V/III pour une composition nominale d'arsenic fixe dans l'In(x)Ga(1-x)As(y)Sb(1-y). Ce résultat s'explique par le fait que les réactions de deuxième ordre dans la décomposition des tétramères d'arsenic ralentissent considérablement la réaction d'incorporation et permettent à l'antimoine d'occuper majoritairement la surface. Cette observation montre qu'il est préférable d'utiliser une source de dimères d'arsenic, plutôt que de tétramères, afin de mieux contrôler la composition d'arsenic dans la couche. Des puits quantiques d'In(x)Ga(1-x)As(y)Sb(1-y) sur GaSb ont été fabriqués et caractérisés optiquement afin d'observer le passage de recombinaison de type I à type II. Cependant, celui-ci n'a pas pu être observé puisque les spectres étaient dominés par un niveau énergétique dans le GaSb dont la source n'a pu être identifiée. Un problème dans la source de gallium pourrait être à l'origine de ce défaut et la résolution de ce problème est essentielle à la continuité de ces travaux. / Antimonide-based semiconductors are promising in the development of optoelectronic devices considering that the high electron mobility, the possibility to emit or absorb light for a large number of wavelengths in the infrared region and the change in recombination type for confined heterostructure make them a prime subject of research. A good number of publications are aimed at developing devices based on In(x)Ga(1-x)As(y)Sb(1-y) alloys to emit or detect a specific wavelength without giving much information about the composition determination or the band alignment. There are only a few fundamental studies about the incorporation of indium and none about the incorporation of arsenic tetramers by molecular beam epitaxy. Also, the values of the band offsets between binary compounds forming the In(x)Ga(1-x)As(y)Sb(1-y) alloys diverge and the methods used to do so are sometimes arbitrary. A model was constructed and predicts the band alignment between In(x)Ga(1-x)As(y)Sb(1-y) alloys and GaSb for any values of x and y. This model considers thermal effects, strain and confinement for quantum wells. Therefore, it is possible to predict the type of recombination for any composition. Indium atoms tend to segregate on the surface while the growth of In(x)Ga(1-x)Sb on GaSb is taking place by molecular beam epitaxy. This behavior has already been seen before and the work presented here corroborates this observation. It is possible to build up a thin layer of indium on the surface prior to the growth of the alloy to avoid a change of composition in the layer. The thickness of this layer is dependent on the temperature of the substrate and can be evaluated with a simple model of segregation. In the case of a quantum well, there will be another interface where the indium floating on the surface will incorporate. To avoid the formation of a long gradient of composition at this interface, it is recommended to grow a few monolayers of GaSb at low temperature without a growth interruption. This way, the indium will incorporate rapidly and leave a sharp interface. The ratio between the indium beam equivalent pressure and the beam equivalent pressure of indium and gallium gives the nominal composition and is the same as the measured composition by XRD in the alloy. The incorporation of arsenic tetramers is not as straightforward in In(x)Ga(1-x)As(y)Sb(1-y) alloys and is shown to decrease when the V/III ratio is increased as measured by XRD. A simple kinetic model explained that this behavior is caused by antimony occupying a large fraction of the surface. The dissociation of tetramers into dimers is a reaction of second order and the tetramers occupy two sites on the surface and makes the incorporation a slower process. Therefore, the use of arsenic tetramers is not the best choice for a good control on the arsenic composition in the layer. In(x)Ga(1-x)As(y)Sb(1-y) quantum wells were grown on GaSb and were optically characterized to observe the transition of type I recombination to type II. This transition could not be corroborated because all the measurements showed an unknown transition related to the GaSb buffer layer. The origin of this optical signature could not be identified, but may be related to a contaminant in the gallium cell. Identifying the source of this problem and solving it will be essential to go further and observe the transition of type I to type II.

This work describes infrared (IR) photon detector techniques based on novel semiconductor device concepts and detector designs. The aim of the investigation was to examine alternative IR detection concepts with a view to resolve some of the issues of existing IR detectors such as operating temperature and response range. Systems were fabricated to demonstrate the following IR detection concepts and determine detector parameters: (i) Near-infrared (NIR) detection based on dye-sensitization of nanostructured semiconductors, (ii) Displacement currents in semiconductor quantum dots (QDs) embedded dielectric media, (iii) Split-off band transitions in GaAs/AlGaAs heterojunction interfacial workfunction internal photoemission (HEIWIP) detectors. A far-infrared detector based on GaSb homojunction interfacial workfunction internal photoemission (HIWIP) structure is also discussed. Device concepts, detector structures, and experimental results discussed in the text are summarized below. Dye-sensitized (DS) detector structures consisting of n-TiO2/Dye/p-CuSCN heterostructures with several IR-sensitive dyes showed response peaks at 808, 812, 858, 866, 876, and 1056 nm at room temperature. The peak specific detectivity (D*) was 9.5E+10 Jones at 812 nm at room temperature. Radiation induced carrier generation alters the electronic polarizability of QDs provided the quenching of excitation is suppressed by separation of the QDs. A device constructed to illustrate this concept by embedding PbS QDs in paraffin wax showed a peak D* of 3E+8 Jones at ~540 nm at ambient temperature. A typical HEIWIP/HIWIP detector structures consist of single (or multiple) period(s) of doped emitter(s) and undoped barrier(s) which are sandwiched between two highly doped contact layers. A p-GaAs/AlGaAs HEIWIP structure showed enhanced absorption in NIR range due to heavy/light-hole band to split-off band transitions and leading to the development of GaAs based uncooled sensors for IR detection in the 2 5 μm wavelength range with a peak D* of 6.8E+5 Jones. A HIWIP detector based on p-GaSb/GaSb showed a free carrier response threshold wavelength at 97 µm (~3 THz)with a peak D* of 5.7E+11 Jones at 36 μm and 4.9 K. In this detector, a bolometric type response in the 97 - 200 µm (3-1.5 THz) range was also observed.