Surface Dimer Engineering and Properties of GaAs(N)(Bi) Alloys

Due to the significant bandgap narrowing induced by dilute fractions of N and Bi in III-V semiconductors, emerging dilute nitride-bismide semiconductor alloys are of significant interest for long-wavelength applications ranging from temperature-insensitive laser diodes to ultra-high efficiency multijunction photovoltaic cells. However, both dilute nitride and dilute bismide devices have exhibited significant sensitivity to the local atomic environments of N or Bi solute atoms, while their incorporation mechanisms are not well understood. In this work, we investigate the role of the surface reconstruction on doping, alloy formation, and electronic and optical properties of GaAs(N)(Bi) alloys. For GaAs(Bi), we examine the influence of surface reconstruction on silicon dopant incorporation and electronic properties. Si incorporation into GaAs(Bi) with an (nx3) surface reconstruction leads to n-type conductivity, while growth with a (2x1) reconstruction leads to p-type conductivity. We hypothesize that the presence or absence of surface arsenic dimers prevents or enables dopant incorporation into arsenic lattice sites. We consider the influence of bismuth anions on arsenic-dimer mediated dopant incorporation and the resulting electronic transport properties, demonstrating the applicability of this mechanism to mixed anion semiconductor alloys. For GaAsNBi alloys, we examine the influence of Bi and N fluxes on N and Bi incorporation. The incorporation of Bi is found to be independent of N flux, while the total N incorporation and the fraction of N atoms occupying non-substitutional lattice sites increase with increasing Bi flux. A comparison of channeling nuclear reaction analysis with Monte Carlo – molecular dynamics simulations indicates that the non-substitutional N primarily incorporate as (N-As) interstitial complexes. We discuss the influence of Bi adatoms on the formation of arsenic-terminated [110]-oriented step edges with a (1x3) surface reconstruction and the resulting enhancement in total N incorporation via the formation of additional (N-As). We also consider the influence of Bi as an incorporating surfactant on chemical ordering in GaAsN:Bi alloys. While epitaxy with a (2x1) reconstruction leads to random GaAsN formation, the introduction of a Bi flux induces long-range chemical ordering of the {111} planes. We propose a mechanism in which Bi enhances the formation of dimer rows aligned along the [110] direction in the (2x1) surface reconstruction, facilitating N incorporation beneath surface dimers and Bi incorporation between dimer rows to form alternating N-rich and Bi-rich {111} planes. These findings suggest a route to tailoring the local atomic environment of N and Bi atoms in a wide range of emerging dilute nitride-bismide alloys. Finally, we have examined the alloy composition dependence of the energy bandgap and electronic states in GaAsNBi alloys. Using direct measurements of N and Bi mole fractions, via ion beam analysis, in conjunction with direct measurements of the out-of-plane misfit via x-ray rocking curves, we determine a new "magic ratio" for lattice-matching of GaAsNBi alloys with GaAs substrates. In addition, using a combination of photoreflectance and photoluminescence spectroscopy, we determine a new map of the composition- and misfit-dependence of the energy bandgaps, along with revealing the energetic position of Bi-related states at approximately 0.18 eV above the valence band maximum.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147570/1/joccena_1.pd

Rare-earth arsenide III-V nanocomposites for heterodyne terahertz generation

Room-temperature, broadly tunable, continuous-wave, terahertz sources operating in the range from 0.3 to 3 THz are required for applications such as high-resolution spectroscopy and medical imaging. Optical heterodyne conversion, or photomixing, is a technique for generating terahertz frequencies by modulating the photoconductance of a material via the interference of two incident lasers. For efficient terahertz generation, the photoconductive material must simultaneously possess high dark resistivity, high carrier mobilities, and short carrier lifetimes. Superlattice based nanocomposites of rare-earth arsenide nanoparticles epitaxially embedded into In [subscript 0.53] Ga [subscript 0.47] As are attractive candidate materials for terahertz generation via photomixing. In addition to the superior charge transport properties of InGaAs relative to GaAs, InGaAs-based devices can leverage the well-developed fiber-optic technology infrastructure available at 1550 nm, enabling low-cost, compact, room-temperature terahertz sources. Unfortunately, due to the alignment of the Fermi level near the InGaAs conduction band in previously investigated ErAs-InGaAs nanocomposites, photomixers based on these materials currently exhibit high leakage currents resulting in poor device performance. High depositions of ErAs (up to 1.75 monolayers per superlattice period) and beryllium counter-doping can partially overcome this limitation by adjusting the Fermi level closer to the midgap. However, carrier mobilities suffer, reaching a high of only 202 cm²/V-s, as a result of carrier scattering from the high counter doping required. Through an exploration of other rare-earth species, we were able to discern the importance of the structural quality of the III-V host matrix overgrowth. By improving the material overgrowth through surfactant-mediated epitaxy and nanoparticle morphology optimizations, we were able to increase the total amount of deposited rare-earth arsenide to 3.2 monolayers per superlattice period, while maintaining good structural quality of the overall nanocomposite. The properties of the resulting photoconductive material were also significantly improved, achieving Fermi level alignments near 240 meV below the conduction band and high dark resistivities of 4.2 Ω-cm, a record for a 1550 nm absorbing nanocomposite material without beryllium counter doping. Mobilities remained above 2700 cm²/V-s, however, the carrier lifetimes of these enhanced nanocomposites leveled off at 1.45 ps, above the terahertz threshold of 1 ps. An alternate path to improve the properties of nanocomposites is to change the Fermi level alignment of the nanoparticle/matrix interface by using alloy compositions of In [subscript 0.53] Ga [subscript 0.47] As and GaAs [subscript 0.5] Sb [subscript 0.5]. Since the Fermi level aligns closer to the valence band in GaAsSb and to the conduction band in InGaAs, the Fermi level can be tailored toward the midgap. We demonstrate rare-earth arsenide nanocomposite materials based on these quaternary alloys that achieve 2× improvement in carrier lifetimes over previous nanocomposites without any counter-doping, 0.73 ps, while maintaining carrier mobilities at least 4× higher than any other material based on counter-doping, over 4000 cm²/V-s, and modest dark resistivities of 0.8 Ω-cm. Although the InGaAsSb alloy matrix is a potentially elegant solution, the growth of these materials is very challenging due to the miscibility gap that exists for this quaternary alloy. With further optimization, RE-As-InGaAsSb nanocomposite systems show great potential for the development of 1550 nm based photomixing devices.Electrical and Computer Engineerin

Manipulating Quantum Dot Nanostructures for Photonic and Photovoltaic Applications.

Semiconductor quantum dots are of recent interest for use in various optoelectronic devices such as solar cells, lasers, and quantum computing. For example, embedding quantum dots within an optical nano-cavity is expected to greatly enhance performance of micro-lasers and quantum gates due to their non-linear optical response. For solar cells, quantum dots can be used to create an intermediate energy state within the band gap of the bulk material as originally proposed by Luque and Marti, increasing the thermodynamic efficiency limit to >63%, well beyond that of current devices. These device applications require selecting an appropriate material system, properly preparing the starting growth surface prior to quantum dot growth, and understanding the resulting structural, compositional, and optoelectronic properties of the dots. This work is presented in two parts, each containing multiple related studies on quantum dot nanostructures and the background information necessary for understanding the analysis presented. Part I describes the effects of lateral patterning on the size and composition of InAs quantum dots and advances the current understanding of the effects of lateral separation on dot size and composition. Increasing the pattern spacing results in an increase in quantum dot dimensions, even doubling their size, an increase in wetting layer thickness, and increased dissolution during capping. The In diffusion length during quantum dot nucleation and dissolution upon capping can be determined via patterning to be approximately 0.5 μm and >1.0 μm, respectively. Part II describes the effects of growth conditions and GaAs capping on size, shape, and segregation of Sb in type-II band offset GaSb quantum dots using various analysis techniques capable of analyzing the morphology, composition, and optical properties of uncapped and buried nanostructures. In particular, three-dimensional analysis of the morphology and composition of buried structures shows that approximately 70% of GaSb dots disintegrate into clusters of small islands with about 1/3 the diameter of their precursor, significantly altering their optoelectronic properties. A detailed analysis of the quantum dot nanostructures is presented in both parts, and examples of devices fabricated through collaborations provide a better understanding of how quantum dots can be properly tailored for specific device applications.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/100032/1/andymar_1.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/100032/2/andymar_2.pd

Formation of ordered III-V semiconductor nanostructures by different technological approaches

Tesis doctoral inédita. Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 15-06-0

Material Properties of MBE Grown ZnTe, GaSb and Their Heterostructures for Optoelectronic Device Applications

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/ThesisPh.D. Physics 201

Growth and Characterization of Magnetic MNSB Nanostructures

Master'sMASTER OF SCIENC

Self-Assembly and Characterization of Germanium Quantum Dots on Silicon by Pulsed Laser Deposition

Self-assembled Ge quantum dots (QD) are grown on Si(100)-(2×1) by pulsed laser deposition (PLD). In situ reflection-high energy electron diffraction (RHEED) and post-deposition atomic force microscopy (AFM) are used to study the growth dynamics and morphology of the QDs. Several films of different thicknesses were grown at a substrate temperature of 400°C using a Q-switched Nd:YAG laser (λ = 1064 mu, 40 ns pulse width, 23 J/cm2 fluence, and 10 Hz repetition rate). At low film thicknesses, but clusters that are faceted by different planes, depending on their height, are observed after the completion of the wetting layer. With increasing film thickness, the size of the clusters grows, and they gradually lose their facetation and become more rounded. With further thickness increase, the shape of these clusters becomes dome-like with some pyramids observed among the majority of domes. The effect of the laser fluence on the morphology of the grown clusters was studied. The cluster density was found to increase dramatically while the average cluster size decreased with the increase in the laser fluence. For a laser fluence of 70 J/cm2, dome-shaped clusters that are smaller than the large huts formed at 23 J/cm2 were observed. At a substrate temperature of 150°C, misoriented three-dimensional (3D) clusters formed producing only a RHEED background. At 400 and 500°C, huts and a lower density of domes formed, respectively. Above 600°C, 3D clusters formed on top of a discontinuous textured layer. As an application, pulsed laser deposition is used to fabricate multilayered Ge quantum-dot photodetector on Si(100). Forty successive Ge quantum dot layers, each covered with a thin Si layer, were deposited. Deposition and growth are monitored by in situ reflection-high energy electron diffraction and the morphology is further studied by ex situ atomic force microscopy. The difference in the current values in dark and illumination conditions was used to measure the device sensitivity to radiation. Spectral responsivity measurements reveal a peak around 2 μm, with responsivity that increases three orders of magnitude as bias increases from 0.5 to 3.5 V. The effects of laser-induced electronic excitations on the self-assembly of Ge quantum dots on Si(100)-2×1 grown by pulsed laser deposition are also studied. Electronic excitations, due to laser irradiation of the Si substrate and the Ge film during growth, are shown to decrease the roughness of films grown at a substrate temperature of ∼120°C. At this temperature, the grown films are nonepitaxial. However, electronic excitation results in the formation of an epitaxial wetting layer and crystalline Ge quantum dots at ∼260°C, a temperature at which no crystalline quantum dots form without excitation under the same deposition conditions. Finally, the very early stages of formation of Ge but clusters on Si(100) has been studied by UHV STM. Growth starts by the formation of a very low density of asymmetric huts with high aspect ratios. Further deposition results in a higher density of clusters characterized by their narrow size and height distributions. These clusters are almost of the same lateral size as those deposited at lower thicknesses

Ordering of Epitaxial Semiconductor Nanostructures Using In Situ Pulsed Laser Interference Patterning

Low-dimensional semiconductor nanostructures have received enormous research attention by virtue of their unique electronic structure and have shown major potential for applications in nanoelectronics, nanophotonics, and optoelectronics. In particular, III-V semiconductor quantum dots (QDs), quantum dot molecules (QDMs) and quantum rings (QRs) are deemed to be promising building blocks for quantum information processing and communications. Self-assembly during epitaxial growth has enabled the production of these structures with high crystalline and optical quality. However, self-assembly also comes with stochastic nucleation and size inhomogeneity, which can limit their potential for device integration where precise positioning and nanostructures with predictable and ideally identical electronic properties are demanded. Site-controlled growth of nanostructures using ex situ lithographic techniques presents an attractive approach; nevertheless, this involves complex fabrication processes and the resulting properties of the structures have not, in general, matched those of random self-assembled nanostructures. This dissertation seeks to develop an innovative approach to laterally align high-quality epitaxial semiconductor nanostructures using an in situ patterning process based on the direct application of optical methods. In this work, an in situ technique combining nanosecond pulsed direct laser interference patterning (DLIP) with molecular beam epitaxy (MBE) growth is introduced, which offers a fast, high-efficiency route to realise the lateral ordering of semiconductor nanostructures. In the first part, the epitaxial growth and characterisation of Stranski-Krastanov (S-K) InAs QD and QDM arrays on GaAs substrates are investigated. The nanoisland arrays induced by single-pulse four-beam DLIP are observed to act as preferential nucleation sites for InAs QDs and result in a site occupancy dependent on the growth and interference parameters. The influences of both the DLIP conditions and the epitaxial growth parameters on the ordering of InAs/GaAs QDs are discussed. Precisely ordered arrays of single InAs QDs are fabricated for the first time using this in situ and non-invasive approach. The patterned QD arrays exhibit strong photoluminescence (PL) and a narrow full width at half maxima (FWHM), indicating good size uniformity and high optical quality. The second part of the dissertation explores the fabrication of ordered GaAs/AlGaAs QD and QR arrays using the droplet epitaxy (DE) approach combined with in situ DLIP. The DE approach has emerged as an attractive method to create lattice-matched self-assembled QDs with certain advantages compared to strain-driven nucleation processes. Regular arrays of Ga droplets are initially formed on nanoisland-templated AlGaAs surfaces, which are subsequently crystallised into GaAs crystals under an arsenic flux. By optimising the growth parameters, including the deposited Ga amount, the growth temperature, and the arsenic beam equivalent pressure, highly ordered arrays of single GaAs QDs and QRs can be obtained. High optical quality and excellent size homogeneity are attained according to the low-temperature PL spectra, in which a record-narrow PL emission FWHM of ~17 meV from patterned GaAs QD arrays is observed. In the final part of the dissertation, initial studies of the selective area growth (SAG) of GaAs droplets and nanocrystals on Si (100) & (111) substrates, and the growth and characterisation of type-II GaSb QDs on GaAs substrates employing in situ DLIP are demonstrated. These initial investigations show that DLIP is able to structure a silicon substrate to create Si nanoisland arrays. These islands can serve as preferential nucleation sites for Ga droplets, which can then be crystallised under arsenic exposure. Further deposition of GaAs results in the formation of periodic GaAs nanocrystals on the surface, with the size and site occupancy depending on the interference and growth parameters. The lateral ordering of S-K GaSb QDs on GaAs substrates has also been obtained, with the QD nucleation again subject to DLIP-induced nanoisland arrays. Low-temperature PL spectra of the patterned ordered arrays of GaSb QDs exhibit a comparably narrow FWHM of ~50 meV and reveal the characteristics of type-II band alignment

InAs/GaAs Quantum Dot Solar Cells

Self-assembled III-V quantum dots (QDs) have been intensely studied for potential applications in solar cell (SC) devices in order to increase power conversion efficiency. Due to their quantum confinement of carriers, QDs have been proposed as a means of implementing the intermediate band solar cell (IBSC). The IBSC concept is characterised by in an increase in photocurrent and a preservation of output voltage, resulting from an enhanced sensitivity to the solar spectrum. The work reported in this thesis is concerned with the development of InAs QDs in GaAs p-i-n solar cell structures, with the aim of realising of an IBSC. The work involves the design, epitaxial growth by molecular beam epitaxy (MBE), device processing and characterisation of the QDSCs. This thesis first investigates InAs/InGaAs dot-in-a-well (DWELL) solar cell structures grown under different conditions. The use of a high-growth-temperature GaAs spacer layers is demonstrated to significantly enhance the performance of the multilayer DWELL solar cells. Threading dislocations were observed for a 30-layer QD structure with GaAs spacer layers grown at a low temperature (510 oC). By growing the GaAs spacer layer at a higher temperature (580 oC), the formation of threading dislocations were suppressed, resulting in enhanced optical properties. The thesis then goes on to address the main challenges facing QD IBSCs, that is, the reduction in open-circuit voltage and the lack of significant increase in short-circuit current. To eliminate the wetting layer and enhance the open-circuit voltage of the QD solar cell, an AlAs cap layer technique was used. This resulted in an enhancement of the open-circuit voltage of a 20-layer InAs/GaAs QDSC from 0.69 V to 0.79 V. Despite a slight reduction in short-circuit current, for the QDSC with AlAs cap layer, the enhancement in the open-circuit voltage was enough to ensure that its efficiency is higher than the QDSC without AlAs cap layers. In an attempt to enhance the short-circuit current, an antimony-mediated growth approach was used to grow high-density QDs. After optimisation of the growth temperature and InAs coverage, a very high in-plane QD density of 1 1011 cm-2 was achieved by applying a few monolayers of antimony prior to QD growth. Compared with a reference QDSC without the incorporation of antimony, the high-density QDSC demonstrates a distinct improvement in short-circuit current from 7.4 mA/cm2 to 8.3 mA/cm2. This result shows that a significant increase in short-circuit current could potentially compensate for the drop in open-circuit voltage observed in InAs/GaAs QD solar cells. Ongoing work on the development of QDSCs with both AlAs capping and antimony-mediated growth have resulted in the simultaneous elimination of the wetting layer and increase in QD absorption in a single device. Overall, the studies in this thesis present important implications for the design and growth of InAs/GaAs QD solar cell structures for the implementation of IBSCs