Browsing by Subject "Molecular beam epitaxy"

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Characterization of as-grown and annealed narrow band gap nitrides grown by molecular beam epitaxy
(2003-05) Reifsnider, Jason Miles, 1967-; Holmes, Archie L.
The application of narrow band gap nitrides to optoelectronic devices is a rapidly growing field, with the promise to supplant existing material systems used for optical fiber communications. While the field is relatively new, significant device advancements have occurred. These improvements have been driven by experimental advances in growing high quality material. The rush for device improvement has left some gaps in the understanding behind the growth improvements. This work examines several fundamental growth parameters in detail, to improve the understanding of their relationship to optical quality of the films. First, the plasma source operation is examined for nitrogen production and ion production. Samples with and without ion removal are examined. Inconclusive results were found for the as-grown films with and without ion removal. Next, the dependence on growth temperature is examined, with strong improvement in optical quality occurring over the small range of temperatures examined. The RF power level and the growth system pressure is also examined, to look for independent and compounding effects. A critical RF power level was identified, between 175 W and 300 W, where damage to the crystal begins to occur. For the system pressure, a monotonic improvement in optical quality is seen as the system pressure is lowered. To more fully characterize the relationships found herein, annealing was performed on all of the samples examined. The growths with ions removed by deflector plates showed an interdependence between the deflector plate voltage and the gas flow rate: at low gas flow rates, high deflector plate voltage could be used to remove the ions and improve optical quality. At high flow rates, degradation occurred when the deflector voltage was too high. The temperature study samples retained their dependence after anneal, with higher temperature growths being superior optical quality. The critical RF power level was also supported by the post-anneal data, occurring near 300W. Finally, after anneal, the optical quality improvement continued to be seen as the system pressure is lowered. Understanding these observed relationships provide a set of recommendations for further improving the growth of narrow band gap nitride material.
Emerging epitaxial materials for coherent III-V (opto)electronic heterostructure devices
(2019-05-06) McNicholas, Kyle Marshall; Bank, Seth Robert; Shi, Li; Wang, Yaguo; Wasserman, Daniel; Yu, Ed T
The boundaries of semiconductor heterostructure device design are often defined by the epitaxial incompatibility of semiconductor materials. One avenue to expand these boundaries is the exploration of new materials that can be expitaxially incorporated with conventional semiconductors to enhance or extend device performance. Here we present our work on two emerging epitaxial materials, the highly-mismatch alloy system B [subscript x] Ga [subscript 1-x] As and quaternary alloys of the rare-earth pnictides. The boron pnictides (B-V) remain one of the least explored III-V materials systems, with many fundamental properties still awaiting experimental investigation. The small lattice constants of the B-V compounds result in greater than 3.4% tensile mismatch with conventional semiconductor substrates, offering unique opportunities for strain engineering in III-V heterostructures via mixed boron-III-V alloys. With sufficient boron concentrations, these materials can even be lattice-matched to Si, potentially providing a new avenue for monolithic integration of strain-free, direct-bandgap layers with silicon. The B concentrations reported in these alloys have thus far been restricted by solid solubility limits to dilute amounts. We demonstrate that a highly-kinetically limited growth regime enables near unity substitution boron incorporation outside the dilute limit, with measured boron concentrations greater than 2X those reported in the literature, large enough for the growth of coherent layers on Si. The optimized growth regime presented also facilitated the first characterization of these compounds outside the dilute regime, including composition dependent changes in the energy gap, dopant activation and transport, and prototype photodetector devices. The rare-earth pnictides (RE-V) are excellent candidates for the epitaxial integration of metallic layers in heterostructure devices. Many of these compounds are rock-salt structured semimetals that form abrupt and thermodynamically stable epitaxial interfaces with conventional III-V semiconductors. Furthermore, the potential to tailor the properties of these compounds to specific applications has been demonstrated through alloying, with broad tunability of the optical/electrical properties observed in alloys of LaLuAs. We present our efforts to extend these tunable metal layers to arbitrary substrates through the growth of quaternary RE-V alloys. We first demonstrate the epitaxial growth of GdAs, then use GdAs to demonstrate complete miscibility of the quaternary alloy LaGdLuAs while remaining lattice-matched to InP. Our observations indicate that the tunable properties observed in LaLuAs persist in these quaternary alloys. We also investigate the effects of growth conditions on the surface morphology of RE-V layers and demonstrate that under non-ideal conditions excess group-III elements can accumulate as nanostructures at the surface of these layers. We identified ideal growth conditions to mitigate the formation of these nanostructures.
High performance 1300 nm photodetectors grown by molecular beam epitaxy
(2002) Sun, Xiaoguang; Holmes, Archie L.
Photodetectors operating at 1.3 µm on GaAs substrates with low noise, low dark current, high quantum efficiency and high bandwidth are very attractive for applications in long haul, fiber optic communication systems. GaAsSb is a promising material for 1.3 µm emission and absorption on GaAs substrates. However, GaAsSb is not lattice-matched on GaAs substrate and As and Sb have different sticking coefficients with Ga, which makes both the growth and device realization difficult. This dissertation discusses the molecular beam epitaxy (MBE) growth of compressively strained GaAsSb layers on GaAs substrates. We found that the optical properties and alloy composition of GaAsSb highly depend on the growth parameters, such as growth temperature, Ga growth rate, Sb and As fluxes. We also reported two resonant-cavity-enhanced (RCE) avalanche photodiode (APD) structures with GaAsSb/GaAs multiple quantum well absorption regions. The RCE GaAsSb p-i-n photodiode exhibited a peak external quantum efficiency of 54% at the wavelength of 1.3 µm with a full-width-at-halfmaximum of 8 nm. In the RCE GaAsSb APD with separate absorption, charge and multiplication regions (SACM), the high electric field multiplication region was separated from the intrinsic absorption region. As a result, the RCE GaAsSb SACM APD exhibited very low dark current (~5 nA) at 90% of the breakdown. By utilizing thin undoped Al0.9Ga0.1As layer as the multiplication region, very low multiplication noise with k value ~ 0.1 was obtained in the SACM APD. This is the lowest noise reported to date for APDs operating at 1.3 µm.
III-V optoelectronic devices in the BGaInAs material system
(2022-07-01) El-Jaroudi, Rasha Huntoon; Bank, Seth Robert; Dodabalapur, Ananth; Hutter, Tanya; Wasserman, Daniel; Yu, Edward T
In this work, we investigate the potential of direct-bandgap, III-V materials for lattice-matched absorbers and emitters on silicon. Leveraging the small lattice constant of the boron pnictides, BGaInAs can be grown lattice-matched or nearly lattice-matched to Si; however, the difficult growth mechanics of elemental boron and the highly mismatched nature of BGaInAs have limited prior synthesis of this alloy to compositions outside those required for lattice-matched growth on Si and outside of technologically interesting wavelengths. Therefore, this work focuses on BGaInAs growth on GaAs. Focusing on GaAs reduces the simultaneous amount of boron and indium required to maintain lattice-matching or near lattice-matching as compared to Si. Here, we exploit the non-equilibrium growth properties of molecular beam epitaxy to simultaneously increase the incorporation of boron and indium - enabling the longest wavelength emission from BGaInAs, the first boron-containing active region electrically injected emitter, and all-BGaInAs photodetectors. Our approach is the optimization of BGaInAs on GaAs, focusing on further extending emission wavelengths and increasing B, In concentrations in lattice-matched detectors in order to develop understanding of how increasing B concentrations (and requisite In concentrations) affect BGaInAs-based devices, thus providing a path towards lattice-matched optoelectronic devices on Si.
Low index channels embedded in III-V semiconductors
(2018-08) Skipper, Alec Mason; Bank, Seth Robert
Low index materials integrated with high-quality epitaxial semiconductors have wide ranging potential applications, including high-contrast photonics, gas sensing, and opto-fluidics. For example, encapsulated air gratings can be exploited to replace Bragg reflectors in VCSELs, create high-Q resonators, create lab-on-a-chip sensing systems, or utilize Fano resonance for all-optical switching. However, it is difficult to integrate these structures with standard III-V optoelectronic devices without compromising material quality as conventional III-V growth techniques offer limited lateral control and other techniques, such as wafer bonding of patterned structures, can introduce interfacial defects. Furthermore, previous attempts to create air gaps in zincblende III-V semiconductors have been limited in shape and size due to low III-adatom mobility. We have shown that tailored molecular beam epitaxy (MBE) can be used to encapsulate patterned silicon dioxide structures into a high-quality crystalline III-V matrix, yielding monolithically integrated high contrast photonic structures. Here, we increase the achievable refractive index contrast by post-growth selective etching of the silicon dioxide. Specifically, mesas were defined that extend through the silicon dioxide layers, followed by highly-selective lateral wet etching of the silicon dioxide with buffered oxide etch (BOE). This approach is capable of yielding interconnected air cavities of arbitrary shape and size, embedded in single-crystal III-V materials. We confirmed the successful fabrication of nanometer-scale air cavities in GaAs with scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Complete etching was confirmed by monitoring the silicon dioxide phonon resonance at 1063 cm-1. In addition, we confirm the high quality integration of a quantum emitter above the low index channels using photoluminescence spectroscopy. This work establishes a basis for improved high-contrast photonics and opens up new potential applications, including lab-on-a-chip sensing via seamless integration of microfluidics with III-V semiconductor devices.
Low-noise AlxIn₁₋xAsySb₁₋y digital alloy staircase avalanche photodiodes grown by molecular beam epitaxy
(2021-06-14) March, Stephen Daniel; Bank, Seth Robert; Campbell, Joe C; Tutuc, Emanuel; Wang, Yaguo; Wasserman, Daniel M; Yu, Edward T
We present AlInAsSb digital alloy staircase avalanche photodiodes (APDs) grown on GaSb substrates with low-noise behavior for near- to mid-infrared optical sensing applications. Detectors that rely on electron ionization, such as traditional APDs or photomultiplier tubes, suffer from non-deterministic gain, which manifests itself in the form of noise. In this work, we leverage the widely tunable AlInAsSb band parameters afforded by the digital alloy growth technique to design multi-step staircase APDs that incorporate deterministic spatial localization of impact ionization events. Using these methods, we demonstrate the first implementation of staircase APDs with deterministic 2 [superscript N] gain, extremely low excess noise (F(M) ~ 1.1), and linear noise power scaling with gain, unlike the quadratic relationship observed in conventional APD noise. We further explore the staircase APD design with electrostatic and Monte Carlo simulation analyses to study low-noise APD carrier dynamics, namely: thermionic emission, tunneling, and impact ionization. Furthermore, we use the knowledge gained from those parameterization studies to produce 4- and 5-step staircase APDs. Future work will focus on the pursuing higher step counts to reap the benefits of the low-noise, deterministic gain characteristics of the staircase APD architecture. Additional work will focus on wavelength-flexible staircase APDs for low-noise mid-IR sensing applications
MBE growth of GaSb-based alloys for mid-infrared semiconductor diode lasers
(2013-05) Nair, Hari Parameswaran; Bank, Seth Robert
Mid-infrared lasers in the 3-5 µm range are important for wide variety of applications including trace gas sensing, infrared counter measures, free space optical communications, etc. GaSb-based type-I quantum well (QW) diode lasers are an attractive choice due to their relatively simple design and growth tolerances, as compared with quantum cascade lasers and interband cascade lasers. Excellent diode lasers have been demonstrated for wavelengths up to ~3.0 µm, employing GaInAsSb/AlGaAsSb QW active regions. But, device performance tends to degrade at longer wavelengths, due to Auger recombination and decreasing QW valence band offsets. In this work we look into the feasibility of using highly strained GaInAsSb/GaSb QWs as active regions for diode lasers operating at wavelengths beyond 3.0 µm. Heavy strain in the QW can improve valence band offset and also increase the splitting between the heavy and light hole bands which can help minimize Auger recombination. Through optimized molecular beam epitaxy (MBE) growth conditions we were able to incorporate up to 2.45 % compressive strain in these QWs enabling laser operation up to 3.4 µm at room temperature. An alternate path to extend the emission wavelength is to incorporate dilute quantities of nitrogen into the QW. Incorporating dilute quantities of substitutional nitrogen into traditional III-V’s strongly reduces the bandgap of the alloy. The advantage for the case of GaSb based dilute-nitrides is that the bandgap reduction is almost exclusively due to the lowering of the conduction band leaving the valence band offsets unaffected; thus providing a path to mitigating hole leakage while extending the emission wavelength. Although GaSb-based dilute-nitrides are a potentially elegant solution for extending the operating wavelength of GaSb-based type-I QW diode lasers, the luminescence efficiency of this material system has been relatively poor. This is most likely due to the presence of a high concentration of point defects, like nitrogen substitutional clusters. Through careful optimization of MBE growth conditions and post growth annealing, we demonstrate improved luminescence efficiency. With further optimization this material system can potentially extend the emission wavelength of GaSb-based type-I QW diode lasers even further into the mid-infrared spectrum.
Microstructural changes in MBE growth of LT-GaAs observed by in- situ ellipsometry
(1995-05) Eyink, Kurt G. (Kurt Gerard), 1960-; Not available
Mid-infrared type-I diode laser design using molecular beam epitaxy
(2020-08-14) Sifferman, Scott Daniel; Bank, Seth Robert; Belkin, Mikhail A; Wang, Yaguo; Wasserman, Daniel M; Yu, Edward T
The mid-infrared region of the electromagnetic spectrum, particularly in the wavelength range between 3 and 5 µm, is important for a number of applications in spectroscopy, gas sensing, infrared countermeasures, and communications. Despite these motivations, mid-infrared laser development has lagged behind that of visible and near-infrared technology. This is in part because semiconductor laser sources, while they exist across the mid-infrared, suffer from one or several drawbacks such as high power consumption, high threshold currents, low characteristic temperatures, limited wallplug efficiency, parasitic non-radiative recombination processes, or reduced carrier confinement. The latter impediment, specifically reduced carrier confinement of holes, is endemic to the active regions of GaSb-based type-I quantum-well diode lasers as the optical emission wavelength is extended past 3 µm. In this work, we present our efforts toward enhancing mid-infrared active regions to extend the emission wavelength of type-I emitters. Through the use of highly-strained, high indium-content quantum wells we demonstrate type-I diode laser operation from aluminum-free active regions up to 3.62 µm, and photoluminescence emission from type-I quantum wells out past 4 µm. Additional studies focused on the effect of using bismuth during the growth of these materials. While increased compressive strain in the quantum well alloy enables greater hole confinement at longer emission wavelengths, it also leads to material roughening and defect formation that restrict the number of and thickness of strained regions that can be grown before material quality irreparably degrades. We observed that by using bismuth as a surfactant during the growth of highly-strained GaIn(As)Sb alloys, material degradation was suppressed as these materials were grown well beyond classical critical thickness limits. We were also able to leverage the epitaxial growth conditions used for highly-strained, high indium-content quantum wells to incorporate dilute amounts of bismuth, up to 3%, into the quantum well materials. The addition of bismuth to the quantum well alloys modifies the valence band to provide additional hole confinement, leading to brighter emitters with up to 34% higher peak intensity. It also resulted in overall lower materials strain without reducing the emission wavelength or performance. This opens a promising approach to overcome strain-related limitations to laser performance and emission wavelength, allowing for device designs with increased numbers of quantum wells and potentially reducing the effects of gain saturation. An additional path toward improved mid-infrared devices is to switch the quantum well barrier material from GaSb to a lattice-matched AlGaAsSb alloy. This is the same strategy employed for many other mid-infrared type-I diode lasers, albeit for emission wavelengths less than 3.1 µm. By changing the barrier alloy, the quantum well valence band offset is increased, providing stronger hole confinement. Coupling these barriers with the highly-strained, high indium-content quantum wells results in a 3× improvement in peak photoluminescence and a >30% reduction in emission linewidth for quantum wells operating up to 4.2 µm. Using this coupled approach, we propose a laser diode device designed to operate at 4.1 µm.
Molecular beam epitaxial growth of rare-earth compounds for semimetal/semiconductor heterostructure optical devices
(2012-05) Crook, Adam Michael; Bank, Seth Robert; Yu, Edward; Cheng, Julien; Zhang, John; Belkin, Mikhail
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.
Molecular beam epitaxy of topological insulator Bi₂Se₃
(2012-05) Chen, Yuxuan, 1986-; Shih, Chih-Kang; de Lozane, Alejandro L.
In this thesis, I show my effort in growing atomically flat Bi₂Se₃ thin films using molecular beam epitaxy (MBE) method. Bi₂Se₃ is a kind of topological insulator, whose exotic surface states have been found in the samples that I grew.
Molecular-beam epitaxial growth of low-dark-current avalanche photodiodes
(2007-12) Hurst, Jeffrey Byron, 1977-; Holmes, Archie L.
The quaternary material system In[subscript x]Ga[subscript 1-x]As[subscript y]P[subscript 1-y] is an important material system for optoelectronic devices, specifically covering optimum fiber optic wavelengths. Among the limitations of using this material system concerning photodetector performance is generation of carriers due to material defects and impurities. This dissertation reports on the growth optimization of InGaAs using molecular-beam epitaxy for low-dark-current avalanche photodiodes through the study of the effects of the growth conditions on dark current. An optimum growth temperature of 545°C and arsenic beam equivalent pressure of 2x10⁻⁵ Torr was found for producing the lowest dark current density. Avalanche photodiodes were implemented with a dark current density 80 mA/cm² at 90% of the breakdown voltage.
Near infrared to visible intersubband transitions in all-oxide quantum wells using BaSnO₃
(2023-12) Jang, Suyeong; Lai, Keji, 1978-; Demkov, Alexander A.
Quantum wells (QWs) are made up of a larger bandgap “barrier” material sandwiching the smaller bandgap “well” material. Recently, quantum well structures using metal oxides such as SrTiO₃ (STO)/LaAlO₃ have been reported. They boast a rather large conduction band offset of ~2.3eV but suffer from large effective mass of STO (used as their “well” material). On the other hand, BaSnO₃ has a small effective mass of m* [subscript BSO] = 0.2m [subscript e] that allows for larger spacing of the QW energy levels. BaSnO₃ also, has very high mobility (150cm²(Vs)⁻¹) and a wide bandgap (~3.1eV) making it a great candidate in applications calling for transparent conductive films. We will discuss MBE-grown Al₂O₃/BaSnO₃ quantum wells demonstrating a high level of confinement. We have grown different thickness of our BSO layer and performed many characterizations on our quantum well such as XRD, XRR, RHEED, XPS, and ellipsometry measurement. The quantum wells in this thesis can allow intersubband transitions from visible to infrared range enabling many applications.
Oxide materials at the two-dimensional limit
(2017-05-05) Kormondy, Kristy Joy; Demkov, Alexander A.; Lai, Keji; De Lozanne, Alejandro L; Tsoi, Maxim; Ekerdt, John G
Emergent phenomena in transition metal oxide films are receiving considerable attention with the development of techniques for the preparation of well-controlled oxide surfaces. On the macroscopic scale, such display novel physics phenomena including superconductivity, magnetism, ferroelectricity, and more. On the nanometer scale, the properties of epitaxial interfaces are further impacted by strain, band alignment, and crystal imperfections that may affect the long-range as well as the short-range order. Furthermore, symmetry lowering at the interface creates entirely new environments that are not accessible in the bulk environment. Thus, thin-film oxide materials are increasingly important in many applications. My work focuses on epitaxial oxides of the perovskite, spinel, and rocksalt structure and covers two main phenomena: (1) the two-dimensional electron gas at epitaxial oxide interfaces, and (2) thin epitaxial electro-optic oxides. Because polar oxides are of prominent interest as a mechanism for the formation of the two-dimensional electron gas, I start with a study of polar semiconductor Co₃O₄. Ellipsometry reveals a direct band gap of 0.75 eV, and magnetic measurements show the signature of antiferromagnetic ordering at 49 K, higher than the typical bulk value. Next, I look closer at the role of defects by studying the highly conducting layer at the crystalline [gamma]-alumina/SrTiO₃ (STO) interface which is attributed to oxygen vacancies. Annealing in oxygen is found to reduce the carrier density and turn a conductive sample into an insulator. Building upon these results, I show that even at room temperature, out-diffusion of oxygen from SrTiO₃ during epitaxy of highly spin-split semiconductor EuO epitaxy creates a highly conductive layer of oxygen vacancies on the SrTiO₃ side of the interface. The films are ferromagnetic with a Curie temperature of 70 K and display giant magnetoresistance below the transition temperature. Leveraging this approach offers an as-yet unexplored route to seamlessly integrate ferromagnetism and the oxide two-dimensional electron gas for the development of novel nano-oxide spintronic devices. The large effective Pockels coefficient for high-quality epitaxial BaTiO₃ (BTO) films on Si distinguishes BaTiO₃ as a highly promising material for integrated silicon nanophotonics. However, the linear electro-optic effect in BaTiO₃ thin films determined in previous experiments clearly shows deteriorated properties compared to bulk BTO crystals. First, I study BaTiO₃ films of varied thickness in order to quantify the Pockels coefficient with respect to crystalline orientation. As a next step, I report on the strong dependence of the Pockels effect in BaTiO₃ thin films on their microstructure, and provide guidelines on how to engineer thin films with strong electro-optic response. The 25× enhancement of the Pockels coefficient indicates a promising route to increase the performance of nonlinear oxides in the two-dimensional limit for the development of novel hybrid silicon photonic platform.
Perpendicular And Parallel Field Magnetoresistance In Molecular Beam Epitaxy Grown Bi2Te3
(2014-08) Dey, Rik; Banerjee, Sanjay
The topological insulator Bi2Te3 has been grown on Si(111)-(7 × 7) surface by molecular beam epitaxy. Reflection high energy electron diffraction, in situ scanning tunnelling microscopy, x-ray photoelectron spectroscopy and ex situ x-ray diffraction studies have been performed to analyze the quality of the growth. These analyses suggest a very good layer-by-layer epitaxial growth of Bi2Te3 on the atomically at Si surface. The magnetoresistance of the samples has been studied with magnetic field perpendicular and parallel to the sample surface, up to 9 T, over a temperature range of 2 K to 20 K. A sharp dip at low fields (0 T - 1 T) and near-linear behavior for high fields (> 4 T) have been observed in the perpendicular field magnetoresistance. The low field dip is due to weak antilocalization that agrees well with the simplified Hikami-Larkin-Nagaoka model. It has been demonstrated that both the low field dip and the high field near-linear behavior can be explained by the original Hikami-Larkin-Nagaoka formula alone in a system with strong spin-orbit coupling. From the fitting of the perpendicular field magnetoresistance the phase coherence length, the mean free path and the spin-orbit relaxation time have been estimated. The phase coherence length shows power law dependence with temperature indicating two dimensional nature of the transport. The power law also suggests electron electron interaction as the prominent dephasing mechanism. The out-of-plane spin-orbit relaxation time is determined to be small and the in-plane spin-orbit relaxation time is found to be comparable to the momentum relaxation time. The estimation of these charge and spin transport parameters is useful for topological insulator based magneto electric device applications. It also has been shown that the strong spin-orbit coupling suppresses the Zeeman contribution in perpendicular field magnetoresistance. The logarithmic divergence of perpendicular field magnetoresistance with temperature for low temperature range (2 K - 20 K) at high fields shows the presence of Coulomb interaction in the spin singlet channel. For magnetoresistance with the field parallel to the sample surface, the observed magnetoresistance has parabolic dependence for small fields (0 T - 0.6 T) and logarithmic dependence for large fields (> 3 T), which is due to the Zeeman effect. It is found that the data are inconsistent with only the Maekawa and Fukuyama theory of non interacting electrons with Zeeman contributions to the transport, but are consistent with theory if one also takes into account the electron electron interaction and the Zeeman splitting term in the electron electron interaction theory of Lee and Ramakrishnan. The Zeeman g-factor and the strength of Coulomb scattering due to electron electron interaction have been estimated from fitting of the parallel field magnetoresistance. The magnetoresistance also shows anisotropy with respect to the field directions. The angle dependent anisotropic magnetoresistance can be fitted well by the original HLN theory alone. The anisotropy can have potential application in anisotropic magnetic sensors.
Phase transitions, transfer and nanoscale growth of epitaxial Bi and Bi1-xSbx thin films
(2018-06-22) Walker, Emily Susan; Bank, Seth Robert; Akinwande, Deji; Liechti, Kenneth M; Tutuc, Emanuel; Yu, Edward T
Bismuth (Bi) and Bismuth-Antimony (Bi [subscript 1-x] Sb [subscript x]) alloys are considered very promising for emerging spintronic devices due to their large spin-orbit coupling, high mobility, and conductive, spin-split surface states, which are topologically non-trivial in Bi [subscript 1-x] Sb [subscript x]. Due to the long mean free path in Bi, quantum confinement effects become significant in relatively thick (~100 nm) films, resulting in the opening of a small, indirect band gap and enabling tuning of the electronic properties through the film thickness. Quantum confinement effects are expected to occur in Bi [subscript 1-x] Sb [subscript x] films at a similar length scale, which may enlarge the bulk band gap and extend the topologically insulating composition regime. When the film thickness of epitaxial Bi on Si(111) is reduced below a few nanometers, a puckered-layer allotropic structure similar to black phosphorus is stable. This puckered-layer structure is expected to exhibit unique properties, including a larger band gap and increased spin splitting, which may be useful for 2-D spintronics; however, the tendency of this structure to grow in small islands inhibits characterization. This dissertation explores the growth of both bulk-like and puckered-layer Bi and Bi [subscript 1-x] Sb [subscript x] on Si(111), and discusses how the unique properties of this system may be controlled through the growth parameters, film thickness, and composition. We find that while alloying bulk-like Bi with Sb in the quantum confinement thickness regime may increase the band gap, the crystalline orientation changes with increasing concentrations of Sb. This effect has not been observed in epitaxial Bi [subscript 1-x] Sb [subscript x] on other substrates, and significantly impacts the electronic properties of the films. In contrast, alloying Sb with nanoscale puckered-layer Bi improves the crystallinity and continuity, suggesting a promising route towards tuning the band structure of puckered-layer Bi and producing large-area films for electrical measurements. Finally, we demonstrate that epitaxial Bi and Bi [subscript 1-x] Sb [subscript x] films exhibit surprisingly weak adhesion to the Si(111) growth substrate, which may originate from the early allotropic transition. This weak adhesion enables the straightforward transfer of these films, opening a route toward the integration of epitaxial-quality Bi and Bi [subscript 1-x] Sb [subscript x] films with arbitrary substrates for novel heterostructures.
Properties of III-V digital alloys grown by molecular beam epitaxy
(2020-08-17) Rockwell, Ann Kathryn; Bank, Seth Robert; Yu, Edward T; Tutuc, Emanuel; Wasserman, Daniel M; Wang, Yaguo
Avalanche photodiodes (APDs) are widely used in industry due to their internal gain, which arises from impact ionization. Over the past 40+ years, III-V materials have been intensively studied for avalanche photodetectors, driven by applications including optical communications, imaging, quantum information processing, and autonomous vehicle navigation. Below 1.1μm, Si APDs are the current state-of-the-art, while above 4μm, HgCdTe APDs are the best option. However, the difficulties associated with growth and fabrication of these materials have motivated the search for alternatives. Impact ionization is a stochastic process that introduces noise, thereby limiting sensitivity and achievable bandwidths. Intense effort is required to mitigate this noise through the identification of different materials and device structures. The search for new materials has yielded InAs and the Al [subscript x] In [subscript 1-x] As [subscript y] Sb [subscript 1-y] alloy family, both III-V materials, as alternatives to HgCdTe and Si with very low-noise. However, the lack of a consensus on the importance of different fundamental properties for these materials and structures led to a mostly ad hoc exploration of their properties that has yielded limited success in noise mitigation. This dissertation describes an exciting step toward deterministic design of low-noise avalanche photodetector materials by alternating the composition at the monolayer scale. This represents a dramatic departure from previous approaches, which have concentrated on either unconventional compounds/alloys or nanoscale band-engineering. This dissertation will expand upon previous work on AlInAsSb digital alloys and take well-established materials, such as InGaAs and InAlAs, and improve important performance metrics, such as cutoff wavelength and excess noise, by growing them as digital alloys. Several explanations for such properties will also be proposed. Finally, it will be shown that many digital alloys exhibit favorable temperature-stable bandgaps compared with typical III-V semiconductors, offering the possibility of enhancing the temperature response in APDs
Rare-earth monopnictide alloys for tunable, epitaxial metals
(2013-08) Krivoy, Erica Michelle; Bank, Seth Robert
A variety of benefits motivate the development of epitaxial metals, among which include the ability to design fully integrated layer structures where metallic films and nanostructures can be embedded into the cores of optoelectronic devices. Applications include high-performance tunnel-junctions, epitaxial transparent Ohmic contacts, photomixer material, and thermoelectrics. Additionally, the integration of metallic nanostructures and films into optoelectronic devices has shown potential for improving device performance and functionality through sub-wavelength confinement of plasmonic modes and enhancement of light/matter interactions. The rare-earth monopnictide (RE-V) material system can be integrated epitaxially with conventional zincblende III-V substrates under normal growth conditions, resulting in high-quality, thermodynamically stable interfaces. The RE-V semimetals span a range of optical, electrical, and structural properties, making them ideal for integration into III-V-based optoelectronic devices and applications. In this dissertation, high-quality epitaxial LuAs, LaAs and La(x)Lu(1-x)As films and nanostructures were grown and characterized for their structural, electrical, optical, and plasmonic properties. Through a sweep of alloy film compositions of the RE-V alloy material La(x)Lu(1-x)As, the ability to produce tunable epitaxial metals was demonstrated, with a range of peak transmission spectra from near- to mid-infrared wavelengths, plasmonic response in the mid-infrared, moderate resistivity, and lattice-matching potential to many relevant III-V substrates. Additionally, there is a great deal of interest in developing techniques to produce optoelectronic devices that are not restricted by substrate lattice constant. Many epitaxial approaches have been tried, with moderate success; however, growing low defect-density heteroepitaxial materials with differing crystal structures and highly-mismatched lattice parameters is extremely challenging, and such structures suffer from poor thermal properties and reliability issues. A general approach is needed for thin metamorphic buffer layers with minimal threading dislocations that simultaneously have low thermal resistance for effective heat-sinking and device reliability. An investigation was conducted into the use of RE-V nanostructure superlattices towards the reduction of dislocation density in highly-mismatched III-V systems.
Tailoring nanoscale metallic heterostructures with novel quantum properties
(2013-05) Sanders, Charlotte E.; Shih, Chih-Kang; Raizen, Mark G.
Silver (Ag) is an ideal low-loss platform for plasmonic applications, but from a materials standpoint it presents challenges. Development of plasmonic devices based on Ag thin film has been hindered both by the dificulty of fabricating such film and by its fragility out of vacuum. Silver is non-wetting on semiconducting and insulating substrates, but on certain semiconductors and insulators can adopt a metastable atomically at epitaxial film morphology if it is deposited using the "two-step" growth method. This method consists of deposition at low temperature and annealing to room temperature. However, epitaxial Ag is metastable, and dewets out of vacuum. The mechanisms of dewetting in this system remain little understood. The fragility of Ag film presents a particular problem for the engineering of plasmonic devices, which are predicted to have important industrial applications if robust low-loss platforms can be developed. This dissertation presents two sets of experiments. In the first set, scanning probe techniques and low energy electron microscopy have been used to characterize Ag(111) growth and dewetting on two orientations of silicon (Si), Si(111) and Si(100). These studies reveal that multiple mechanisms contribute to Ag film dewetting. Film stability is observed to increase with thickness, and thickness to play a decisive role in determining dewetting processes. A method has been developed to cap Ag film with germanium (Ge) to stabilize it against dewetting. The second set of experiments consists of optical studies that focus on the plasmonic properties of epitaxial Ag film. Because of the problems posed until now by epitaxial Ag growth and stabilization, research and development in the area of plasmonics has been limited to devices based on rough, thermally evaporated Ag film, which is robust and simple to produce. However, plasmonic damping in such film is higher than in epitaxial film. The optical studies presented here establish that Ag film can now be stabilized sufficiently to allow optical probing and device applications out of vacuum. Furthermore, they demonstrate the superiority of epitaxial Ag film relative to thermally evaporated film as a low-loss platform for plasmonic devices spanning the visible and infrared regimes.
Wide band gap oxide-semiconductor heterostructures grown by molecular beam epitaxy
(2020-12-04) Hadamek, Tobias; Demkov, Alexander A.; De Lozanne, Alejandro L; Ekerdt, John G; Lai, Keji; Tsoi, Maxim
Wide band gap oxides and semiconductors will have tremendous impact on future energy efficient and environmentally sustainable electronics. Wide gap semiconductors like GaN and AlGaN are important in light emitting diode applications and for high-frequency telecom and radar applications like base stations for upcoming 5G networks. Further, these materials along with some wide band gap semiconducting oxides like Ga₂O₃ may prove to be invaluable for medium to high power electronics applications, starting from switching power supplies used to charge batteries in consumer devices like smartphones and laptops, to high-power supplies that can charge electric car batteries and are suitable for electric grid and power transmission line applications. Basic materials studies of these material systems are therefore in high demand. In this dissertation I will present materials studies on wide band gap oxide thin films grown by molecular beam epitaxy on crystalline semiconductor substrates. The oxide thin films are characterized with regards to epitaxial structure and electronic structure by electron and x-ray diffraction techniques, by photoelectron spectroscopy and in collaboration with researchers from UT Dallas, Arizona State University, Rutgers University and University of Turku by transmission electron microscopy, inverse photoemission spectroscopy and scanning tunneling spectroscopy. Two materials systems are discussed in detail: 1. The rare-earth sesquioxide Eu₂O₃ in regards to potential gate-dielectric applications on the wide band gap semiconductor GaN. The focus of the studies were interface quality, structural quality, and band offsets; and the electronic structure of Eu₂O₃ to determine the band gap and understand the influence of Eu 4f states on the band gap of Eu₂O₃. 2. The structural integration of ultra-wide band gap oxide semiconductor Ga₂O₃ on a standard Si semiconductor substrate. Epitaxial integration of Ga₂O₃ with the workhorse of semiconductors Si can enable cost-reduction & monolithically-integrated devices. The focus of the studies was the structural characterization of the Ga₂O₃ layers grown by plasma-assisted molecular beam epitaxy.