Optical Properties Of Semiconductor Nanostructures Research Articles

Colloidal Atomic Layer Deposition with Stationary Reactant Phases Enables Precise Synthesis of "Digital" II-VI Nano-heterostructures with Exquisite Control of Confinement and Strain.

In contrast to molecular systems, which are defined with atomic precision, nanomaterials generally show some heterogeneity in size, shape, and composition. The sample inhomogeneity translates into a distribution of energy levels, band gaps, work functions, and other characteristics, which detrimentally affect practically every property of functional nanomaterials. We discuss a novel synthetic strategy, colloidal atomic layer deposition (c-ALD) with stationary reactant phases, which largely circumvents the limitations of traditional colloidal syntheses of nano-heterostructures with atomic precision. This approach allows for significant reduction of inhomogeneity in nanomaterials in complex nanostructures without compromising their structural perfection and enables the synthesis of epitaxial nano-heterostructures of unprecedented complexity. The improved synthetic control ultimately enables bandgap and strain engineering in colloidal nanomaterials with close to atomic accuracy. To demonstrate the power of the new c-ALD method, we synthesize a library of complex II-VI semiconductor nanoplatelet heterostructures. By combining spectroscopic and computational studies, we elucidate the subtle interplay between quantum confinement and strain effects on the optical properties of semiconductor nanostructures.

Tuning Electrical and Raman Scattering Properties of Cadmium Sulfide Nanoribbons via Surface Charge Transfer Doping

Surface charge transfer doping (SCTD) has emerged as an efficient approach to tune the electrical and optical properties of semiconductor nanostructures. As an important II–VI semiconductor, cadmium sulfide (CdS) nanostructures possess superior optoelectronic properties. However, the SCTD on the CdS nanostructures remains a challenge, and the underlying mechanism for optical property modulation is unclear. Herein, we demonstrate that both the electrical and Raman scattering properties of CdS nanoribbons (NRs) can be tuned by using MoO3 and benzyl viologen (BV) as surface dopants. The MoO3 (or BV) molecules, which have a high work function (or strong reducing capability), can extract (or inject) electrons from (or into) the CdS NRs because of the large difference in energy levels, leading to electron depletion (or accumulation) within the CdS NRs. By controlling the amount of surface dopants, the electron concentration as well as resistivity of the CdS NRs can be readily modulated. Significantly, Raman sca...

Control of quantum dot laser emission by coherent phonon wave packets

Travelling coherent phonons can be actively used to manipulate the optical properties of semiconductor nanostructures on the picosecond time scale. Phonon wave packets that interact with a quantum dot (QD) ensemble can significantly vary the output intensity of a laser, which uses the QDs as active medium. Based on a recently developed theoretical model to describe this coupled phonon-QD-photon system, we here study how the laser response on phonon wave packets depends on several parameters, for example phonon pulse properties and laser pump rate.

Manipulating Optical Properties of ZnO/Ga:ZnO Core–Shell Nanorods Via Spatially Tailoring Electronic Bandgap

However, there exists a trade-off between the electrical and optical properties of semiconductor nanostructures in this approach. Specifi cally, although dopants are eligible to modulate carrier concentration, high level of doping would typically introduce extra structural defects in the near defect-free nanorods.

The linear and nonlinear optical properties of a bulged GaAs nanowire

It is well-known that the optical properties of semiconductor nanostructures are so sensitive to alteration of confinement while the nanostructure’s morphology often determines the confinement. Accordingly existence of a bulge on the wire surface will affect the specifications of the nanowire. However, bulge existence effects are significant for ultrafine nanowires and negligible for wires with large radius. In the present work we consider a GaAs ultrafine nanowire with a bulge on the middle of wire axis. The linear, third-order nonlinear and total absorption coefficient and refractive index changes variations are investigated as functions of bulge and nanowire radius. Also the effects of bulge and wire radius on saturation are studied. It has been found that increment in wire (bulge) radius leads to a red shift (blue shift) in absorption coefficient and refractive index changes curves. Additionally, alteration of wire and bulge radii, changes the corresponding incident photon intensity that saturation occurs at it.

Excitation power-independent photoluminescence of inverted quantum hut structures embedded in SiGe superlattice

Optical properties of semiconductor nanostructures can be quite different from the bulk semiconductors. We present here results of a photoluminescence study of MBE-grown SiGe superlattice structures having inverted quantum huts of Ge formed below Ge-on-Si interfaces. As these structures form by continuous alloying with the lower Si layers, the interfacial strain is greatly reduced, leading to a band-alignment, which is different from that obtained for conventional quantum dots formed above Ge-on-Si interfaces. Thus, unlike conventional Ge quantum dots, the presented quantum structures exhibit excitation power-independent photoluminescence emission. Temperature dependence of photoluminescence energy follows typical relation expected in a semiconductor, but at higher temperature when dominating contribution comes from the tip of the quantum hut, change in the photoluminescence energy becomes weakly dependent on temperature. These results may be influential in the development of silicon-based optical materials.

Negative thermal quenching of the 3.338 eV emission in ZnO nanorods

We investigate the negative thermal quenching behavior of the 3.338eV emission in ZnO nanorods. A correlation between the 3.338eV and the 3.368eV (surface exciton) emissions is determined from temperature-dependent photoluminescence. The activation energies of the 3.338eV emission, obtained using an approximated multi-level model, indicate an trap state between the two surface exciton emissions. The present study demonstrates a nondestructive and easy method to understand the surface effects on the optical properties of semiconductor nanostructures.

Atomic layer deposition for nanofabrication and interface engineering

Atomic layer deposition (ALD) provides a tool for conformal coating on high aspect-ratio nanostructures with excellent uniformity. It has become a technique for both template-directed nanofabrications and engineering of surface properties. This Feature Article highlights the application of ALD in selected fields including photonics, SERS and energy materials. Specifically, the topics include fabrication of plasmonic nanostructures for the SERS applications, fabrication of 3-D nanoarchitectured photoanodes for solar energy conversions (dye-sensitized solar cells and photoelectrochemical cells), and coating of electrodes to enhance the cyclic stability and thus device life span of batteries. Dielectric coating for tailoring optical properties of semiconductor nanostructures is also discussed as exemplified by ZnO nanowires. Future direction of ALD in these applications is discussed at the end.

Semiconductor nanostructures modified by the UV laser radiation

The shortening of the absorption recovery time by a factor of more than 50 is observed for the semiconductor nanostructure consisting of ten GaAs/In x Ga1-x As/GaAs quantum wells irradiated with the nanosecond pulses of the XeCl laser.A possible reason for such a significant variation in the optical properties lies in the generation of point defects,which are responsible for recombination of charge carriers.The result can be employed in the UV photomodification of optical properties of semiconductor nanostructures.

The effects of the focus ion beam milling process on the optical properties of semiconductornanostructures

In this work, the effects of the focus ion beam (FIB) milling process on the opticalproperties of semiconductor nanostructures were investigated. With this aim, a sensitivematerials system based on InGaAs/GaAs quantum dots with well known and excellentoptical properties was selected for the FIB treatment. The FIB technique was used tolocally remove a metallic mask deposited on top of the quantum dot sample. Thephotoluminescence (PL) signal, collected from the circular openings, was used toinfer the possible damage effects of the ion beam on the properties of the dots.

Nextnano: General Purpose 3-D Simulations

nextnano is a semiconductor nanodevice simulation tool that has been developed for predicting and understanding a wide range of electronic and optical properties of semiconductor nanostructures. The underlying idea is to provide a robust and generic framework for modeling device applications in the field of nanosized semiconductor heterostructures. The simulator deals with realistic geometries and almost any relevant combination of materials in one, two, and three spatial dimensions. It focuses on an accurate and reliable treatment of quantum mechanical effects and provides a self-consistent solution of the Schrodinger, Poisson, and current equations. Exchange-correlation effects are taken into account in terms of the local density scheme. The electronic structure is represented within the single-band or multiband kldrp envelope function approximation, including strain. The code is not intended to be a ldquoblack boxrdquo tool. It requires a good understanding of quantum mechanics. The input language provides a number of tools that simplify setting up device geometry or running repetitive tasks. In this paper, we present a brief overview of nextnano and present four examples that demonstrate the wide range of possible applications for this software in the fields of solid-state quantum computation, nanoelectronics, and optoelectronics, namely, 1) a realization of a qubit based on coupled quantum wires in a magnetic field, 2) and 3) carrier transport in two different nano-MOSFET devices, and 4) a quantum cascade laser.

A study of the ultrafast optical response of magnetized GaAs/AlGaAs quantum wire structures

We have studied analytically the ultrafast optical response of GaAs/AlGaAs quantum wire subjected to a moderately strong transverse magnetic field. The energy dispersion relations have been numerically calculated and show a significant deviation from parabolic behaviour as the magnetic field is increased. The effective semiconductor Bloch equation technique is used to calculate the induced polarization and differential transmission spectra in the quantum wire. The calculated induced polarization is used to study the optical coherent transient phenomenon of optical nutation. The analysis demonstrates that the magnetic field effectively alters the optical response of the semiconductor quantum wire nanostructures. It is observed that the nutating signal frequency enhances with an increasing magnetic field. The results are useful to explain magnetic field effects on the transient optical properties of semiconductor nanostructures.

Development of Scanning Near-Field Optical Microscope Working under Cryogenic Temperature and Strong Magnetic Field

We report on the development of a cantilever-based scanning near-field optical microscope (SNOM) working in an extreme environment, at cryogenic temperature around 10 K and under strong magnetic field up to 7 T. We designed a new optical system based on an infinite conjugate microscope, which extracts the near-field signal from a small aperture through a narrow chamber into free space as collimated light. Using this system, we successfully measured near-field and topographical images of a metal-hole sample simultaneously. Combining the local optical accessing technique with the external control of the electronic state, this SNOM system will be a powerful tool to study optical properties of semiconductor nanostructures.

Near-Field Spectroscopy of Disordered Nanostructures

For many years, Roland Zimmermann and his research group have devoted much of their attention to the effects of disorder and exciton localization on the optical properties of semiconductor nanostructures. With the recent development of spectroscopic techniques providing subwavelength spatial resolution, new experiments became possible to reveal some of the rich physics linked to exciton localization. This paper briefly reviews recent work of the authors' group using near-field nano-spectroscopy. Specifically, it is shown that near-field autocorrelation spectra give strong evidence for quantum mechanical level repulsion of localized exciton states and allow for a quantitative estimate of the underlying microscopic disorder features. Femtosecond non-linear near-field spectroscopy allows to probe the transient optical nonlinearity from a single localized exciton in a thin quantum film on ultrashort time scales.

The Application of the Single-Band Envelope-Function Model in the Entire Brillouin Zone for Electronic Structure and Optical Properties Calculations

The object of this paper is to analyze the electronic and optical properties of semiconductor nanostructures made of semiconductors with more than one significant minimum in the conduction band by applying the single-band envelope-function model in the entire Brillouin zone. We have developed a method of calculating the eigenvalues and eigenfunctions of the corresponding Hamiltonian and explored the possibility of forming a single-zone model for the entire Brillouin zone by taking the exact dependence of the conduction band energy on the wave vector E(k). Since the crystalline potential is periodic, an expansion of the electron energy band can be made in terms of vectors of the direct lattice. This method is applied to GaAs and Si quantum wells and quantum wires with square and rectangular cross sections of [001] and [110] orientation, and its accuracy is examined by comparing it with other numerically more complex and demanding methods.

Progress Made in Near-Field Imaging with Light from a Sharp Tip

R esearchers are striving to image objects optically at smaller and smaller scales. For example, they'd like to determine the chemical identity of individual molecules and examine the optical properties of semiconductor nanostructures. The required resolution is finer than that allowed by diffraction, which sets a lower limit of half the wavelength of light (hundreds of nanometers for visible light). But much higher resolution can be achieved by viewing objects at distances closer than the wavelength of light. Such near-field imaging has been hotly pursued since the early 1990s, when researchers demonstrated its promise for imaging single molecules (see PHYSICS Today. May 1994, page 17 and November 1997, page 67). As Dieter Pohl (University of Basel) puts it, “Imagine the potential of microscopy that combines the resolving power of an electron microscope with the enormous spectral resolving power of light.”

Self-consistent tight-binding calculations of electronic and optical properties of semiconductor nanostructures

Optical properties and electronic states of semiconductor nanostructures are calculated by using tight-binding models which account for valence band mixing, strain and external applied potentials in a self-consistent fashion. An appropriate formulation of the optical susceptibility in the tight-binding basis is given without introducing any additional parameters. Results for strained and unstrained systems are given.