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Nanocrystals of doped semiconductors, such as Cu_(2-x)S, hold much promise for near infrared active devices because, unlike in noble metals, their tunable infrared plasmon can exist in nanocrystals with diameters

The Berreman effect, by which thin films of polar dielectric materials exhibit strong, narrow resonances near their longitudinal optic (LO) phonon frequency, results in strong material interactions...

Hexagonal boron nitride (hBN) is a wide, indirect bandgap semiconductor that holds great promise for optoelectronic devices in the ultraviolet and mid-infrared spectral regimes. The efficiency of o...

In this contribution we compare size-dependent optical properties of PbTe and SnTe nanocrystals (NCs). We demonstrate that the size dependence of the band edge absorption line and the photoluminescence (PL) of PbTe NCs can be quantitatively described by optical transitions between the lowest quantum confined states of conduction and valence bands. In contrast, the optical properties of SnTe NCs are strongly influenced by intrinsic gap states that arise from the inverted band structure that can be described by introducing a negative energy gap Eg < 0. In principle, these intrinsic gap states could be observed directly in PL and absorption of small size SnTe NCs, where the wave functions of these states are spread over the entire NC volume. In our samples, however, these transitions are not observed due to self-doping of SnTe NCs by holes created from negatively charged Sn vacancies. As a result, the absorption is blue-shifted due to filling of the gap states and confined valence band levels by holes (pheno...

The mid-infrared spectra of many polar materials are dominated by highly reflective reststrahlen bands that occur between the transverse and longitudinal optical phonons. Within the reststrahlen bands, light can couple with optical phonons to support phonon-polariton modes. These modes enhance light-matter interactions through the concentration of light to nanoscale dimensions, and therefore, are particularly promising for mid-infrared nanophotonic applications. Here, we discuss our work on expanding the spectral range over which phonon-polaritons are supported by using new material systems, as well as active tuning of the modes via carrier photoinjection. In particular, we report on the confinement of hyperbolic phonon-polaritons in calcite, a ubiquitous polar material. We also report the use of the LO-phonon-plasmon-coupling (LOPC) effect to actively tune the Berreman mode of a GaN thin film.

Self-assembled superlattices of nanocrystals offer exceptional control over the coupling between nanocrystals, similar to how solid-state crystals tailor the bonding between atoms. By assembling nanocrystals of different properties (e.g., plasmonic, excitonic, dielectric, or magnetic), we can form a wealth of binary superlattice metamaterials with new functionalities. Here, we introduce infrared plasmonic Cu

We demonstrate a synthetic method to add a ZnS shell, with controlled thickness, to PbS nanocrystals using Zn oleate and thioacetamide as Zn and S precursors. The ZnS shell reaction is self-limiting and deposits approximately a monolayer of ZnS per shell reaction without causing the PbS nanocrystals to Ostwald ripen. The reaction is self-limiting because the sulfur precursor, thioacetamide, is less reactive toward the PbS/ZnS core/shell nanocrystal surface as compared to the Zn oleate precursor present in the reaction solution. To increase the ZnS shell thickness beyond a monolayer, subsequent ZnS shell reactions are modified by adding the thioacetamide 10 minutes before the Zn oleate. This gives the thioacetamide time to react at the PbS/ZnS core/shell nanocrystal surface before the Zn oleate is added. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) shows most ZnS shells lack crystalline order. However, select core/shell nanocrystals have epitaxial crystalline (zinc-b...

Surface phonon polaritons (SPhPs) are attractive alternatives to infrared plasmonics for subdiffractional confinement of infrared light. Localized SPhP resonances in semiconductor nanoresonators are narrow, but that linewidth and the limited extent of the Reststrahlen band limit spectral coverage. To address this limitation, we report active tuning of SPhP resonances in InP and 4H-SiC by photoinjecting free carriers into nanoresonators, taking advantage of the coupling between the carrier plasma and optic phonons to blueshift SPhP resonances. We demonstrate state-of-the-art tuning figures of merit upon continuous-wave excitation (in InP) or pulsed excitation (in 4H-SiC). Lifetime effects cause the tuning to saturate in InP, and carrier redistribution leads to rapid (

Naturally occurring materials with hyperbolic optical properties are attracting considerable interest due to their ability to confine light in small volumes and their resulting potential for applications in photonics. This paper uses a first-principles theoretical approach without adjustable parameters to investigate the hyperbolic optical properties of bulk calcite (CaCO3). This material exhibits natural hyperbolic behavior within its Reststrahlen bands at 1403–1552 and 864–887 cm−1. The calculated results are shown to be in good agreement with our reflectance data obtained from frequency- and polarization-dependent measurements. These results show that calcite is an attractive natural hyperbolic material; in addition, it has the advantage of low losses and is available commercially in a variety of crystal orientations.

Due to its 1 eV band gap and GaAs-matched lattice constant, GaInNAs has long been considered for use in four-junction multi-junction solar cells; but, material quality issues have impeded its use in highly efficient devices. Here, we present an analysis of GaInNAs samples partially hydrogenated via a UV-activated process in which nitrogen-related alloy fluctuations, impurities, and defects have been passivated; remarkably, removing completely the ‘s-shape’ dependence of the photoluminescence while keeping intact the effects of nitrogen substitution, i.e., the band gap of the alloy prior to passivation. Hydrogenation of the optical samples by a UV-activated process has resulted in GaInNAs photoluminescence dominated by the free-excitonic band gap transition, rather than radiative recombination processes from the shallow localized centers that result due to unavoidable alloy fluctuations. This behavior is unique since these centers dominate the low temperature photoluminescence even in the highest quality dilute nitrides. Density functional theory calculations show that the hydrogenation of the N and Ga atoms eliminates the defect levels from the band gap through the formation of H–N centers that act as donors; while at high concentration of hydrogen, Ga–H2–N complexes reside within the continuum. The formation of these hydrogen complexes, along with the incumbent change of the band structure, explains the reduction of emission from the localized centers upon hydrogenation.

We present direct visualization of the dynamics of oleic-acid-capped PbTe nanoparticles suspended in different organic solvents using liquid cell transmission electron microscopy. Liquid cell transmission electron microscopy is a powerful tool to directly observe the behavior of a variety of nanoparticles in liquids, but requires careful consideration and quantification of how the electron beam affects the systems being investigated. We find that etching and dissolution of PbTe nanoparticles occurs with a strong dependence on electron dose rate ranging from no perceivable effect on the nanoparticles with lower dose rates (50 e− A−2 s−1) to complete dissolution within seconds or minutes at higher dose rates (100 and 200 e− A−2 s−1). We propose that oxidative etching, resulting from the radiolysis of small amounts of water, causes the PbTe nanoparticles to dissolve after exposure to a threshold electron dose rate of 50 e− A−2 s−1.

Surface phonon polaritons (SPhPs), the surface-bound electromagnetic modes of a polar material resulting from the coupling of light with optic phonons, offer immense technological opportunities for nanophotonics in the infrared (IR) spectral region. However, once a particular material is chosen, the SPhP characteristics are fixed by the spectral positions of the optic phonon frequencies. Here, we provide a demonstration of how the frequency of these optic phonons can be altered by employing atomic-scale superlattices (SLs) of polar semiconductors using AlN/GaN SLs as an example. Using second harmonic generation (SHG) spectroscopy, we show that the optic phonon frequencies of the SLs exhibit a strong dependence on the layer thicknesses of the constituent materials. Furthermore, new vibrational modes emerge that are confined to the layers, while others are centered at the AlN/GaN interfaces. As the IR dielectric function is governed by the optic phonon behavior in polar materials, controlling the optic phonons provides a means to induce and potentially design a dielectric function distinct from the constituent materials and from the effective-medium approximation of the SL. We show that atomic-scale AlN/GaN SLs instead have multiple Reststrahlen bands featuring spectral regions that exhibit either normal or extreme hyperbolic dispersion with both positive and negative permittivities dispersing rapidly with frequency. Apart from the ability to engineer the SPhP properties, SL structures may also lead to multifunctional devices that combine the mechanical, electrical, thermal, or optoelectronic functionality of the constituent layers. We propose that this effort is another step toward realizing user-defined, actively tunable IR optics and sources.

We experimentally observe a significant improvements in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics, with resolution greater than $\lambda/125$ observed in a preliminary hyperlens design. Our results provide the foundation for a materials-growth-directed approach aimed at realizing the loss control necessary for the development of next generation PhP-based nanophotonic devices.

The size of a quantum-confined nanocrystal determines the energies of its excitonic transitions. Previous work has correlated the diameters of PbS nanocrystals to their excitonic absorption; however, we observe that PbS quantum dots synthesized in saturated dispersions of PbCl2 can deviate from the previous 1Sh-1Se energy vs diameter curve by 0.8 nm. In addition, their surface differs chemically from that of PbS quantum dots produced via other syntheses. We find that these nanocrystals are coated in a shell that is measurable in transmission electron micrographs and contains lead and chlorine, beyond the monatomic chlorine termination previously proposed. This finding has implications for understanding the growth mechanism of this reaction, the line width of these quantum dots' photoluminescence, and electronic transport within films of these nanocrystals. Such fundamental knowledge is critical to applications of PbS quantum dots such as single-photon sources, photodetectors, solar cells, light-emitting diodes, lasers, and biological labels.

We report a giant enhancement of the mid-infrared (MIR) magneto-optical complex Kerr angle (polarization change of reflected light) in a variety of materials grown on SiC. In epitaxially-grown multilayer graphene, the Kerr angle is enhanced by a factor of 68, which is in good agreement with Kerr signal modeling. Strong Kerr enhancement is also observed in Fe films grown on SiC and Al-doped bulk SiC. Our experiments and modelling indicate that the enhancement occurs at the high-energy edge of the SiC reststrahlen band where the real component of the complex refractive index n passes through unity. Furthermore, since the signal is greatly enhanced when n=1, the enhancement is predicted to exist over the entire visible/infrared (IR) spectrum for a free-standing film. We also predict similar giant enhancement in both Faraday (transmission) and Kerr rotation for thin films on a metamaterial substrate with refractive index n=-1. This work demonstrates that the substrate used in MOKE measurements must be carefully chosen when investigating magneto-optical materials with weak MOKE signals or when designing MOKE-based optoelectronic devices., 15 pages, 4 figures

Dielectric and semiconductor structures at the nanoscale are increasingly being applied in nanophotonic applications such as enhanced sensing, magnetic field enhancements, and metasurfaces. In contrast to their traditional metal plasmon counterparts -- such as Au and Ag -- dielectric materials benefit from low losses and CMOS compatibility. Here we explore of 3D dielectric structures on the nanometer and micron scales via a new patterning method, which employs both 3D, direct laser write (DLW) and reactive ion etching (RIE). Polymer structures, which are controlled down to the submicron scale both laterally and in height are printed using DLW onto various dielectric materials and are sub sequentially, etched using RIE. By tuning the etch ratio of the dielectric and polymer, the 3D printed pattern is transferred into the dielectric. By patterning a range of different 3D geometries onto Si, SiC, and hBN, we show that this method is applicable to a range of dielectric and semiconductor materials and to a range of different microstructures and nanostructures. Further, we show the possibility of selectively removing the polymer mask without damaging the underlying dielectric material, which enables the possibility of additional fabrication methods, such as for etching thin films.

Conventional optical components are limited to size-scales much larger than the wavelength of light, as changes to the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultrathin, so-called “flat” optical components that beget abrupt changes in these properties over distances significantly shorter than the free space wavelength. While high optical losses still plague many approaches, phonon polariton materials have demonstrated long lifetimes for localized modes in comparison to plasmon-polariton based nanophotonics. Our work predicts a further 14-fold increase in the optic phonon lifetime and we experimentally report a ~3-fold improvement through isotopic enrichment of hexagonal boron nitride (hBN). We establish commensurate increases in the phonon polariton propagation length via direct imaging of polaritonic standing waves by means of infrared nano-optics. Our results provide the foundation for a materials-growth-directed approach towards realizing the loss control necessary for the development of phonon polariton based nanophotonic devices.

Localized surface phonon-polariton (SPhP) resonances in polar semiconductor nanostructures can provide highly sub-diffractional electromagnetic fields. Furthermore, SPhP resonances offer enhanced resonant quality factors when compared to plasmon-polariton based systems. The various material platforms and nanostructure geometries achievable in polar semiconductors suggest they would be ideal platforms for tunable, long-wavelength photonics applications. Moreover, the constituent atomic basis defines the operating frequency regime for SPhP resonances; tunable from the mid-infrared to THz. Here, we investigate Raman active aspects of SPhP modes in GaN nanowire arrays that are grown via selective area molecular beam epitaxy. We detect strong Raman peaks within the Reststrahlen band of GaN that are not found in the bulk GaN Raman spectrum. These SPhP modes occur around 700 cm^-1 (~ 14.3 microns), offering a spectral region for device applications which is currently not accessible by plasmonic based systems or other SPhP enabled materials. Utilizing selective area epitaxy, we created GaN nanowire arrays with various diameters and pitches, from which the Raman spectra showed tuning of the apparent SPhP resonances. Infrared reflectance measurements were also performed with an FTIR microscope to further establish the physical properties of the resonances. Finally, computational studies of the structures’ reflectance were used to solidify our understanding of the geometry/SPhP-resonance-tuning relationship.

Hot carrier solar cells (HCSCs) have been proposed as potential systems to increase the conversion efficiency of single gap solar cells beyond the Shockley-Queisser limit. Despite a great deal of recent progress in HCSCs, designing an effective and efficient hot carrier absorber remains challenging. To evaluate the efficiency of any proposed absorber accurate determination of the carrier temperature is required. This can be non-trivial, particularly in the quantum wells (QW) where state-filling effects can complicate the simple extraction of carrier temperature from photoluminescence (PL) spectra. Specifically, the PL may be distorted if there are other linewidth broadening mechanisms prevalent in addition to non-equilibrium carriers. These may include; in addition to state filling, the effects of phonons and system inhomogeneities, all of which serve to perturb the PL linewidth. In this study, an InGaAsP QW with a type-II band alignment is investigated using continuous wave power and temperature dependent PL to evaluate the effects of broadening on the extraction of the true carrier temperature in the system [1]. Since there is a relatively small energy separation between the ground and first excited state transitions in the QW studied, state filling effects can be controlled and their contribution to the linewidth evaluated, using various conditions and combinations of excitation power and lattice temperature. The role of phonons is also presented, and supported within the framework of a theoretical model that includes the various phononic processes and their temperature dependent contribution to the PL. This work is supported by the National Science Foundation Grant ECCS #1610062 [1] Esmaielpour, Hamidreza, Vincent R. Whiteside, Louise C. Hirst, Joseph G. Tischler, Chase T. Ellis, Matthew P. Lumb, David V. Forbes, Robert J. Walters, and Ian R. Sellers. Progress in Photovoltaics: Research and Applications (2017).

While plasmonics have a broad range of technological applications including infrared photovoltaics and photodetectors, plasmonic metals are subject to high optical losses in the long-wave infrared spectral regime. In order to reduce optical losses in the infrared, alternatives to plasmonic metals are being explored. One promising alternative employs polar dielectric materials, which exhibit a highly-reflective, optically-metallic spectral band (Reststrahlen band), bounded by the LO and TO optical phonons, and are capable of supporting plasmonic-like resonance in the infrared. In polar dielectrics, plasmonic-like resonances, known as surface phonon polariton (SPhP) resonances, arise from a coupling between incident light and collective oscillations of bound lattice charges, which are mediated by the optical phonons. In this study, we have examined the SPhP resonances of SiC nanopillars with constant height of 950 nm and width in the range of 200–400 nm, as a function of their aspect ratio (AR=Length/Width=0.5–16). As the nanopillar width is decreased, we have found that localized SPhP resonances redshift towards the zone folded LO (ZFLO) phonon that is normally not infrared active. However, as localized SPhP resonances are spectrally tuned through the ZFLO mode, we have found that the latter mode becomes infrared active. Furthermore, reflectance measurements have revealed strong coupling between the ZFLO and both the monopolar and dipolar localized SPhP resonances.

The synthesis of PbSe nanorods with low branching (

Inorganic CdSe/CdTe nanocrystals for solid-state photovoltaic devices are typically sintered into a bulk-like material after annealing in the presence of solid cadmium chloride. As in commercial CdTe devices, this salt exposure is a key component to improve device performance by promoting grain growth. However, in contrast to vapor depositions, we demonstrate that the role of the salt treatment also involves crucial ligand removal reactions, which are a unique challenge facing nanocrystal ink depositions. After testing other salts such as CdF2, CdCl2, CdBr2, CdI2 and Cd(NO3)2 for oleate ligand removal as determined by FTIR, SEM imaging of CdTe grain growth revealed the largest grains were observed from reactions with CdCl2 (142 ± 26 nm) and, to a lesser extent, CdBr2 (131 ± 19 nm). These results were used to identify cadmium-free alternatives. Trimethylsilyl chloride (28.0 ± 5.1 nm), NH4Br (75.5 ± 31 nm) and NH4Cl (136 ± 39 nm) were also tested, demonstrating comparable ligand removal and grain growth to the cadmium halides. In order to validate these observations, heterojunction photovoltaic devices were fabricated from CdSe/CdTe nanocrystals treated with non-toxic NH4Cl in place of the conventional CdCl2. Under AM 1.5G illumination, open circuit voltages (Voc), short circuit currents (Jsc) and efficiencies (η) of solar cells processed with evaporated Au and commercial ITO were found to be Voc = 0.46 ± 0.02 V, Jsc = 9.27 ± 0.6 mA cm−2, and η = 1.73 ± 0.24 demonstrating minimal differences in film morphology and device performance compared to those fabricated using cadmium chloride. Specific properties of the salts (solubility, reactivity, melting point and the identity of both the cation and the anion) were found to have a profound impact on grain growth and consequently device performance, suggesting the need for further investigation of additional non-toxic metal halide salts for this reaction.

A response is provided to comments by Krell concerning the validity of the Hall–Petch relationship and the optical transmission in nanocrystalline ceramics discussed in a recent Acta Materialia paper.

Hot-carrier solar cells require absorber materials with restricted carrier thermalization pathways, in order to slow the rate of heat energy dissipation from the carrier population to the lattice, relative to the rate of carrier extraction. Absorber suitability can be characterized in terms of carrier thermalization coefficient (Q). Materials with lower Q generate steady-state hot-carrier populations at lower levels of incident solar power and, therefore, are better able to perform as hot-carrier absorbers. In this study, we evaluate Q = 2.5±0.2 W·K -1 · cm -2 for a In 0.52 AlAs/In 0.53 GaAs single-quantum-well(QW) heterostructure using photoluminescence spectroscopy. This is the lowest experimentally determined Q value for any material system studied to date. Hot-carrier solar cell simulations, using this material as an absorber yield efficiency ~39% at 2000X, which corresponds to a >5% enhancement over an equivalent single-junction thermal equilibrium device.

An InGaAsP quantum well with a type-II band alignment is studied using continuous wave power and temperature dependent photoluminescence (PL) spectroscopy. The small energy separation between the ground state and first excited state results in significant thermal carrier redistribution and excited state occupation, particularly, with increasing excitation power and temperature. This state filling is evident as a high-energy shoulder in the PL spectra, the same energy region where in the simplest Planck-description the gradient is considered inversely proportional to carrier temperature. The outcome of an excited state occupation in broadening the high-energy PL tail is to perturb the temperature extracted using this analysis; therefore, the true temperature of carriers is not properly evaluated when significant state filling occurs. In addition, broadening of the PL due to phonons at higher temperatures also distorts (or falsely increases) the non-equilibrium “hot” carrier temperature and as such should be considered when using Planck's relation. The role of these two effects is considered and their mutual effect on the analysis of the extracted hot carrier temperature discussed. Copyright © 2017 John Wiley & Sons, Ltd.

We propose a novel methodology for III-V substrate cost reduction that does not rely on chemical-mechanical polishing (CMP) or low-throughput release layer etching. The details of this process are provided, and results of an economic model for GaAs and InP virtual substrates dictate 20x and 34x reductions to substrate cost, respectively. The impact of modeled assumptions are quantified in order to better understand the potential range of end-goal virtual substrate cost.

A significant improvement in the quality of dilute nitrides has recently led to the ability to reveal depletion widths in excess of 1 μm at 1 eV [1]. The real viability of dilute nitrides for PV has been recently demonstrated with the reporting of a record efficiency of 43.5% from a 4J MJSC including GaInNAs(Sb) [2]. Despite the progress made, these materials remain poorly understood and work continues to improve their lifetime and reproducibility. We have investigated the possibility of improving the functionality of GaInNAs using hydrogenation to selectively passivate mid-gap defects, while preserving the substitutional nitrogen. Temperature dependent photoluminescence measurements of the intrinsic region of a GaInNAs p-i-n solar cell show a classic “s-shape” associated with localization prior to hydrogenation. No sign of this “s-shape” is evident after hydrogenation, despite the retention of substitutional nitrogen as evidenced by the band absorption of 1 eV. The absence of an “s-shape” at low-temperature in hydrogenated GaInNAs is rather curious since, even in high quality nitrides this behavior is due to the emission of isoelectronic centers created via N-As substitution [3]. The potential origins of this behavior will be discussed. The promise of this process for GaInNAs solar cells was demonstrated by a three-fold improvement in the performance of a hydrogenated device with respect to an as-grown reference [4]. [1] “Wide-depletion width GaInNAs solar cells by thermal annealing,” I. R. Sellers, W-S. Tan, K. Smith, S. Hooper, S. Day and M. Kauer, Applied Physics Letters 99, 151111 (2011) [2] “43.5% efficient lattice matched solar cells,” M. Wiemer, V. Sabnis, and H. Yuen, Proc. SPIE 8108, 810804 (2011) [3]“Probing the nature of carrier localization in GaInNAs, epilayers using optical methods,” T. Ysai, B. Barman, T. Scarce, G. Lindberg, M. Fukuda, V. R. Whiteside, J. C. Keay, M. B. Johnson, I. R. Sellers, M. Al Khalfioui, M. Leroux, B. A. Weinstein and A. Petrou. Applied Physics Letters 103, 012104 (2013) [4] “Improved performance in GaInNAs solar cells by hydrogen passivation by hydrogen passivation,” M. Fukuda, V. R. Whiteside, J. C. Keay, A. Meleco, I. R. Sellers, K. Hossain, T. D. Golding, M. Leroux, and M. Al Khalfioui, Applied Physics Letters 106, 141904 (2015)

Plasmonics provides great promise for nanophotonic applications. However, the high optical losses inherent in metal-based plasmonic systems have limited progress. Thus, it is critical to identify alternative low-loss materials. One alternative is polar dielectrics that support surface phonon polariton (SPhP) modes, where the confinement of infrared light is aided by optical phonons. SiC nanopillar arrays support such modes, exhibiting a dipolar resonance transverse to the nanopillar axis and a monopolar resonance associated with the longitudinal axis dependent upon the SiC substrate. Both exhibit exceptionally narrow linewidths (7–24 cm−1), with quality factors of 40–135, which exceed the theoretical limit of plasmonic systems, with extreme subwavelength confinement of (λres3/Veff)1/3 = 50–200. These observations promise to reinvigorate research in SPhP phenomena and their use for nanophotonic applications. Another approach is the use of hyperbolic materials, which have been a focus of the nanophotonics community for their potential to realize sub-diffractional imaging and focusing of light, and novel optical properties, such as a negative index of refraction. The recent observation that hexagonal boron nitride (hBN) is a natural, high efficiency hyperbolic material has led to a surge in research within this field. Due to the low-loss nature, van der Waals bonding and extreme crystal anisotropy, the hyperbolic polaritons within hBN are not only promising for novel applications within the mid-infrared, but is also extremely well suited for fundamental investigations into their resonant behaviors. We have used scattering near-field optical microscopy (s-SNOM) to directly probe the local surface electromagnetic fields of three-dimensionally confined nanostructures of hBN, reporting the first experimental observation of frequency dependent internal angular reflection within a hyperbolic nanostructure, a phenomenon previously theoretically predicted.

Bulk properties of InN are determined by combining experimental and theoretical studies. In this work, we produced high quality InN film deposited on GaN templates by a modified ion beam assisted deposition technique confirmed by low temperature photoluminescence and absorption. The density of states, real and imaginary parts of the complex dielectric function and the absorption coefficient are calculated by means of first-principles beyond density-functional theory. The quasi-particle aspect is described in the framework of a quasi-particle method (the GW approximation). The calculated band-gap energy is ~0.8 eV whereas significance in the optical absorption occurs at ~1.2 eV, which are consistent with both luminescence and absorption results. The Bethe–Salpeter equation is utilized to model the two-particle exciton interactions, revealing a strong excitonic peak just below the absorption edge of InN.

We report on a two step buffer layer preparation for the growth of InSb quantum wells on a (110) GaAs surface. At each buffer layer step, layer conditions were optimized to produce smooth surfaces compatible with InSb quantum wells. Through varying growth rate, group V/III flux ratio, substrate temperature, and the addition of in situ annealing, we are able to grow In0.85Al0.15Sb on a GaAs substrate with an RMS surface roughness of approximately 2 nm. Surface morphology and cross-sectional transmission electron microscopy (TEM) were analyzed to understand the formation of threading dislocations, inclusions and dislocation filtering. This work presents an initial study for the growth of large lattice mismatched III-V materials on the (110) surface.

Solution-synthesized inorganic cadmium telluride nanocrystals (∼4 nm; 1.45 eV band gap) are attractive elements for the fabrication of thin-film-based low-cost photovoltaic (PV) devices. Their encapsulating organic ligand shell enables them to be easily dissolved in organic solvents, and the resulting solutions can be spray-cast onto indium-tin oxide (ITO)-coated glass under ambient conditions to produce photoactive thin films of CdTe. Following annealing at 380 °C in the presence of CdCl2(s) and evaporation of metal electrode contacts (glass/ITO/CdTe/Ca/Al), Schottky-junction PV devices were tested under simulated 1 sun conditions. An improved PV performance was found to be directly tied to control over the film morphology obtained by the adjustment of spray parameters such as the solution concentration, delivery pressure, substrate distance, and surface temperature. Higher spray pressures produced thinner layers (60 nm) with lower surface roughness (200 nm), leading to devices with improved open-circuit voltages (Voc) due to decreased surface roughness and higher short-circuit current (Jsc) as a result of enhanced annealing conditions. After process optimization, spray-cast Schottky devices rivaled those prepared by conventional spin-coating, showing Jsc = 14.6 ± 2.7 mA cm(-2), Voc = 428 ± 11 mV, FF = 42.8 ± 1.4%, and Eff. = 2.7 ± 0.5% under 1 sun illumination. This optimized condition of CdTe spray deposition was then applied to heterojunction devices (ITO/CdTe/ZnO/Al) to reach 3.0% efficiency after light soaking under forward bias. The film thickness, surface morphology, and light absorption were examined with scanning electron microscopy, optical profilometry, and UV/vis spectroscopy.

Mechanical strengthening by grain refinement is a method whereby a material’s strength and hardness can be increased by decreasing the average crystallite grain size. The empirical Hall–Petch relationship mathematically describes grain boundary strengthening and provides guidance for a straightforward way to produce stronger materials. While the phenomenon has been widely explored in nanocrystalline metals, the difficulty associated with fabricating high-quality dense nanocrystalline ceramics has left unanswered the question of the validity and extent of the relationship in ceramics. Prior studies suggest the occurrence of an inverse Hall–Petch response in ceramics with grain sizes

In this paper, we report a novel heterojunction solar cell based on sintered CdTe and CdSe nanocrystal thin films using solution-based deposition. For the absorber layer, CdTe thin films were made using a layer-by-layer deposition process consisting of spin coating colloidal CdTe nanocrystals followed by a sintering step. The optical constants for these sintered CdTe films were accurately determined through a combination of optical modeling and measurements. For the all-solution processed p–n heterojunction, we focus on CdSe nanocrystal thin films due to their excellent compatibility with solution processing. For the optimized inverted structure (glass/ITO/CdSe/CdTe/Cr/Au), a high open-circuit voltage $(V_{\rm oc})$ of 593 ± 32 mV with an efficiency of 1.9 ± 0.2% was obtained under simulated one sun illumination. These preliminary results demonstrate that this novel inverted heterojunction structure has the potential to produce a high-quality CdSe/CdTe junction for utilization in heterojunction solar cells.

PbSe nanocrystal thin-film transistors (TFTs) were passivated using remote plasma atomic layer deposition (ALD) of a ~10 nm thick Al2O3 film at 150 °C. By using a highly reactive remote oxygen plasma source, the time for one complete ALD cycle was about 15 s with growth rates of ~1.1 A/cycle. The effective mobilities measured under atmospheric condition from AlO-passivated PbSe nanocrystal TFTs were comparable to the values reported previously for air-free PbSe nanocrystal TFTs, demonstrating that ALD Al2O3 layers prevent oxidation and degradation of nanocrystal films from air exposure. The variation in the effective mobility of passivated devices was also found to be negligible under ambient conditions over a period of 30 days. The results show that remote plasma ALD processing of Al2O3 is capable of producing an effective passivation layer on air-sensitive nanocrystals with high deposition rates at reduced temperature.

Polar dielectrics have garnered much attention as an alternative to plasmonic metals in the mid- to long-wave infrared spectral regime due to their low optical losses. As such, nanoscale resonators composed of these materials demonstrate figures of merit beyond those achievable in plasmonic equivalents. However, until now, only low-order, phonon-mediated, localized polariton resonances, known as surface phonon polaritons (SPhPs), have been observed in polar dielectric optical resonators. In the present work, we investigate the excitation of 16 distinct high-order, multipolar, localized surface phonon polariton resonances that are optically excited in rectangular pillars etched into a semi-insulating silicon carbide substrate. By elongating a single pillar axis we are able to significantly modify the far- and near-field properties of localized SPhP resonances, opening the door to realizing narrow-band infrared sources with tailored radiation patterns. Such control of the near-field behavior of resonances can also impact surface enhanced infrared optical sensing, which is mediated by polarization selection rules, as well as the morphology and strength of resonator hot spots. Furthermore, through the careful choice of polar dielectric material, these results can also serve as the guiding principles for the generalized design of optical devices that operate from the mid- to far-infrared.

We report the attenuance and temperature-dependent photoluminescence spectra of PbSe nanowires with diameters between 5.6 and 26.4 nm (12−23% relative standard deviation) and lengths greater than 1...

The performance of triple junction InGaP/(In)GaAs/Ge space solar cells was studied following high energy electron irradiation at low temperature. Cell characterization was carried out in situ at the irradiation temperature while using low intensity illumination, and, as such, these conditions reflect those found for deep space, solar powered missions that are far from the sun. Cell characterization consisted of I-V measurements and quantum efficiency measurements. The low temperature irradiations caused substantial degradation that differs in some ways from that seen after room temperature irradiations. The short circuit current degrades more at low temperature while the open circuit voltage degrades more at room temperature. A room temperature anneal after the low temperature irradiation produced a substantial recovery in the degradation. Following irradiation at both temperatures and an extended room temperature anneal, quantum efficiency measurement suggests that the bulk of the remaining damage is in the (In)GaAs sub-cell.

Thick c-plane unintentional doped and iron-doped GaN substrates were grown by hydride vapor phase epitaxial technique on sapphire substrates. The morphology and crystalline quality of the freestanding samples show no evident degradation due to iron doping. Low-temperature photoluminescence measurements show reduction of the exciton-bound to neutral impurities band intensities with iron doping increase. Near-infrared photoluminescence studies confirm the incorporation and activation of iron impurities. Variable temperature resistivity measurements verified that the iron-doped films are semi-insulating.

Thick freestanding GaN films were grown by hydride vapor-phase epitaxial method on both pattern-masked and unmasked GaAs substrates. Both approaches resulted on films characterized by a large excess of free carriers at room temperature. The use of a GaAs back surface NiTi substrate protective layer increased the concentration of incorporated iron impurities and yield semi-insulating films. The morphology and crystalline quality of these films show a strong dependence on nucleation layer growth approach and Fe doping concentration. Near infrared photoluminescence and Raman scattering verified the incorporation and activation of the Fe impurities, and its effect on the free carrier concentration.

Hot-carrier solar cells have high theoretical limiting efficiency however, absorber materials with slow carrier thermalization remain a development barrier for these devices. In previous studies, charge separation in core-shell colloidal quantum dots has been shown to result in slow carrier relaxation. Charge separation also occurs in III–V heterostructures with type-II band alignments. We characterize hot-carrier effects in InAlAs/InP and InAlAs/InGaAsP quantum well structures, with type-II and quasi-type-II band alignments respectively. InGaAsP is identified as a promising hot-carrier absorber candidate, with thermalization coefficient 1.77±0.12 W.K−1.cm−2, corresponding to limiting solar conversion efficiency >42%, under 2000X.

Triple-junction solar cells lattice-matched to InP have recently gained interest as an alternative to traditional GaAs-based devices. We predict a 1.74, 1.17, 0.70 eV device can attain high efficiency and can be achieved lattice-matched to InP. However, InAlAsSb, a relatively immature material, is required for the top junction. Here, we performed a systematic study of the substrate temperature and group-V overpressure of (In0.52Al0.48As)0.6(AlAs0.56Sb0.44)0.4. We found that the peak PL emission energy blue-shifted as the substrate temperature decreased and as the group-V overpressure increased, implying that low adatom mobility is key to attaining InAlAsSb with the expected bandgap energy.

The effect of rapid thermal annealing on the optical properties of InxAl1−xAsySb1−y was analyzed and compared to that for In0.52Al0.48As. Initial ellipsometry and photoluminescence experiments performed before the annealing indicate the presence of a low energy Urbach tail in the absorption spectrum. Rapid thermal annealing produces a blue-shift in the PL emission when annealed at 650°C for 60s and a decrease in the full-width-half-maximum, which originates from a reduction of the emission from the longer wavelength states. For the In0.52Al0.48As, the emission energy and the full-width-half-maximum remain constant during the annealing study. The elimination of sub-bandgap states in InxAl1−xAsySb1−y is critical for achieving a realistic path towards high efficiency multijunction cells lattice-matched to InP.

In this work, we report dark current reduction in solution-processed PbS nanocrystal-metal Schottky junction solar cells via improved LiF passivation of the interface between PbS nanocrystal films and the metal electrode. For the optimized LiF interfacial layer, the dark saturation current density (J0) is decreased by at least one order of magnitude, resulting in very high open-circuit voltage (Voc) of 692±7 mV under one sun illumination for ∼1.4 eV PbS nanocrystals. Using different size of PbS nanocrystals and therefore different bandgaps, we also demonstrate Voc (mV)=553Eg/q−59 as a function of the PbS nanocrystal bandgap (Eg). For different types of junctions employed for solution-processed PbS nanocrystal solar cells, we plot the bandgap-voltage offsets (Eg/q−Voc) under open-circuit conditions, showing strong dependence of the Voc on the Eg regardless of the types of junction used. Similar dependence is also found in solution-processed and sintered CdTe nanocrystal solar cells. These results suggest that suppressing the non-radiative recombination contributions to the dark current, such as improved passivation of nanocrystal surfaces, is more critical to improve the Voc in nanocrystal solar cells, rather than optimizing the device architecture with varying the n-type semiconducting materials.

We have performed optically detected resonance (ODR) spectroscopy on modulation-doped GaAs/AlGaAs quantum wells of different widths in which lateral fluctuations of the well width were purposely introduced by growth interruption at the interfaces. These monolayer fluctuations form quantum dots for which confinement and Coulomb correlation energies are comparable. By monitoring resonant changes of the dot ensemble photoluminescence induced by far-infrared (FIR) radiation in a magnetic field, we have observed cyclotron resonance (CR) of free electrons in the widest wells, as well as internal transitions of mobile and localized charged excitons. The latter, which are forbidden by magnetic translational invariance, have previously not been observed. For the narrower wells the effects of non-parabolicity and carrier localization on the CR and CR-like transitions have to be included for a proper interpretation of the measurements.

The electronic and optical properties of InAs/GaSb heterostructures depend on the type of bonding at the interfaces, InSb bonds or GaAs bonds. We have studied cyclotron resonance (CR) in the far-infrared on two samples, each consisting of a single 30 nm InAs quantum well surrounded by thick GaSb barriers. The only intended difference between the samples is the interface bonding type, Ga–As bonds and In–Sb bonds. The CR for the sample with Ga–As interface bonds shows two lines whose positions are determined by nonparabolicity effects, whereas the sample with In–Sb bonds shows multiple lines due to strong cross-interface coupling between the InAs conduction band Landau levels (LLs) and the valence band LLs in GaSb. We find that the CR is a sensitive probe of interface bonding type for such structures.

The effect of an InP cap on the photoluminescence (PL) spectrum of an InGaAsP/InAlAs quantum well (QW) is investigated using excitation power and temperature dependent PL. An as-grown sample with the InP cap layer shows an inverted interface created between InP and InAlAs that has a transition energy very close to the transition energy of the QW; consequently, there is an overlap between them. On the other hand, the QW sample with the cap layer etched away does not have a feature due to the inverted interface; even at very low power, the only observed feature is due to the QW transition.

In this work, we developed CH 3 NH 3 PbI 3 perovskite films using sequential vapor deposition technique for photovoltaic applications. For thin PbI 2 films vacuum-deposited on different substrates (e. g. Si, PEDOT/PSS/ITO/ glass), the complete conversion to perovskite on exposure to the CH 3 NH 3 I vapor was confirmed based on X-ray diffraction data. For perovskite films prepared under these vapor deposition conditions, planar heterojunction solar cells were fabricated using PEDOT:PSS and PCBM as the hole and electron transport layer, respectively (glass/ITO/ PEDOT:PSS/CH 3 NH 3 PbI 3 /PCBM/Ag). Under simulated one sun illumination, a high open-circuit voltage of 970 mV with an average efficiency of 4.5% was obtained. While the efficiency of the device with sequential vapor deposited perovskite films needs to be further improved through process optimization, we demonstrated that sequential vapor deposition has a potential to produce high-quality perovskite absorbers for efficient planar heterojunction solar cells.

In this paper we present the first photovoltaic devices made from the promising quaternary InAlAsSb, grown lattice matched to InP by molecular beam epitaxy. Schottky barrier solar cells using semi-transparent contacts have been fabricated, characterized and simulated using a drift-diffusion model to extract information about the barrier height, minority carrier diffusion length and optical performance of devices fabricated from this material. We have compared the performance to analogous InAlAs devices, and present a wide range of optical and electrical characterization for the materials.