The decade of the 1990's is an opportune time for scientists and engineers to create cost-effective silicon "superchips" that merge silicon photonics with advanced silicon electronics on a silicon substrate. We can expect significant electrooptical devices from Column IV materials (Si, Ge, C and Sn) for a host of applications. The best devices will use strained-layer epitaxy, doped heterostructures, and bandgap engineering of quantum-confined structures. This paper reviews Si-based photonic components and optoelectronic integration techniques, both hybrid and monolithic. >

Silicon-based optoelectronics

Atomically thin two-dimensional materials have emerged as promising candidates for flexible and transparent electronic applications. Here we show non-volatile memory devices, based on field-effect transistors with large hysteresis, consisting entirely of stacked two-dimensional materials. Graphene and molybdenum disulphide were employed as both channel and charge-trapping layers, whereas hexagonal boron nitride was used as a tunnel barrier. In these ultrathin heterostructured memory devices, the atomically thin molybdenum disulphide or graphene-trapping layer stores charge tunnelled through hexagonal boron nitride, serving as a floating gate to control the charge transport in the graphene or molybdenum disulphide channel. By varying the thicknesses of two-dimensional materials and modifying the stacking order, the hysteresis and conductance polarity of the field-effect transistor can be controlled. These devices show high mobility, high on/off current ratio, large memory window and stable retention, providing a promising route towards flexible and transparent memory devices utilizing atomically thin two-dimensional materials.

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Controlled charge trapping by molybdenum disulphide and graphene in ultrathin heterostructured memory devices

Defects in three different types of ZnO nanostructures before and after annealing under different conditions were studied. The annealing atmosphere and temperature were found to strongly affect the yellow and orange-red defect emissions, while green emission was not significantly affected by annealing. The defect emissions exhibited a strong dependence on the temperature and excitation wavelength, with some defect emissions observable only at low temperatures and for certain excitation wavelengths. The yellow emission in samples prepared by a hydrothermal method is likely due to the presence of OH groups, instead of the commonly assumed interstitial oxygen defect. The green and orange-red emissions are likely due to donor acceptor transitions involving defect complexes, which likely include zinc vacancy complexes in the case of orange-red emissions.

http://iopscience.iop.org/article/10.1088/0957-4484/18/9/095702/pdf

Defect emissions in ZnO nanostructures

This paper presents a novel and facile method for the fabrication of ZnO hollow spheres. In this approach, zinc ions were first adsorbed onto the surfaces of sulfonated polystyrene core-shell template spheres, and then reacted with NaOH to form a ZnO crystal nucleus, which was followed by a growth step to form ZnO nanoshells. During the formation of ZnO nanoshells or later on, the template spheres were "dissolved" in the same media to obtain ZnO hollow spheres directly. Neither additional dissolution nor calcination process was needed in this method to remove the templates, and the reaction conditions were very mild: neither high temperature nor long time was needed. Transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Brunauer-Emmett-Teller analysis were used to investigate the morphology, surface composition, crystalline structure, specific surface area, and porosity of the ZnO hollow spheres, respectively. UV-visible spectra show that these ZnO hollow spheres had very good photocatalytic activity.

A Facile Method to Fabricate ZnO Hollow Spheres and Their Photocatalytic Property

Oxygen vacancies (VOs) in ZnO are well-known to enhance photocatalytic activity (PCA) despite various other intrinsic crystal defects. In this study, we aim to elucidate the effect of zinc interstitials (Zni) and VOs on PCA, which has applied as well as fundamental interest. To achieve this, the major hurdle of fabricating ZnO with controlled defect density requires to be overcome, where it is acknowledged that defect level control in ZnO is significantly difficult. In the present context, we fabricated nanostructures and thoroughly characterized their morphological (SEM, TEM), structural (XRD, TEM), chemical (XPS) and optical (photoluminescence, PL) properties. To fabricate the nanostructures, we adopted atomic layer deposition (ALD), which is a powerful bottom-up approach. However, to control defects, we chose polysulfone electrospun nanofibers as a substrate on which the non-uniform adsorption of ALD precursors is inevitable because of the differences in the hydrophilic nature of the functional groups. For the first 100 cycles, Znis were predominant in ZnO quantum dots (QDs), while the presence of VOs was negligible. As the ALD cycle number increased, VOs were introduced, whereas the density of Zni remained unchanged. We employed PL spectra to identify and quantify the density of each defect for all the samples. PCA was performed on all the samples, and the percent change in the decay constant for each sample was juxtaposed with the relative densities of Znis and VOs. A logical comparison of the relative defect densities of Znis and VOs suggested that the former are less efficient than the latter because of the differences in the intrinsic nature and the physical accessibility of the defects. Other reasons for the efficiency differences were elaborated.

/pdf/role-of-zinc-interstitials-and-oxygen-vacancies-of-zno-in-konb7ejy4p.pdf

Role of zinc interstitials and oxygen vacancies of ZnO in photocatalysis: a bottom-up approach to control defect density

Photoluminescence and atomic force microscopy have been used to characterize ZnO thin films grown by metal-organic chemical vapor deposition (MOCVD) at varied growth pressures. The surface morphology with different grain structures has strong influence on the green photoluminescence of ZnO. When large discrete islands or structureless overgrowth cover the rough surface, broad green emissions around 500 nm go beyond the ultraviolet (UV) emission band; whereas, when the surface is packed closely with small grains, only weak green emission is observed with a red-shift to 528 nm. This variation of green emissions is ascribed to changes in the charge states of oxygen vacancies, which is strongly dependent on the surface morphology and grain structures. Based on the grain boundary defect model, two possible recombination processes for the green emission are proposed and discussed in detail.

Correlation between green luminescence and morphology evolution of ZnO films

The growth and annealing of single crystalline zno films by low-pressure mocvd

The ZnO films were grown by low pressure metal-organic chemical vapor deposition (LP-MOCVD) using dimethylzinc (DMZn) and oxygen. The as-grown film has strong (0002) oriented characteristic, containing the chain-like network grains in small sizes, A well-resolved shoulder emission at 3.03 eV in the photoluminescence (PL) spectra has been ascribed to the interstitial zinc in the crystal, implying a serious oxygen deficiency state of the MOCVD growth, in good agreement with the analysis of XPS results. Post-annealing of the films leads to the remarkable improvement of the structure, morphology, components and optical properties. Large hexagonal columnar grains were then observed by atomic force microscope (AFM). The luminescence efficiency was enhanced greatly due to the reduction of the nonradiative defects. Meanwhile, chemisorbed oxygen gives rise to surface acceptor states, resulting in the different radiative recombination. Photoluminescence excitation measurement shows good agreement with the above conclusions.

MOCVD growth and properties of ZnO films using dimethylzinc and oxygen

The highly c-oriented ZnO films were epitaxially grown on n-type Si (111) substrate at the temperature range of 340–460 °C using the low-pressure metalorganic chemical vapor deposition method. All films exhibit a pronounced (002) peak for ZnO, indicative of the strong c-axis oriented characteristic. The ZnO film grown at 400 °C shows the best structural quality along with the largest lateral grain size, well supported by the narrowest full width at half maximum of ZnO (002) peak about 0.19° in x-ray diffraction. However, the temperature dependence of the vibrational modes at 436 and ∼563 cm−1 in Raman spectra revealed a low density of oxygen vacancies in the films grown at low temperatures, which is supposed to determine the photoluminescence (PL) properties. At low temperatures, the narrow ultraviolet (UV) near band emission dominated the PL spectrum with a very weak low energy tail near the band. High temperature (up to 460 °C) would cause serious oxygen deficiency, resulting in the weak broad UV band w...

Liqun Hu

Papers

Correlation between green luminescence and morphology evolution of ZnO films

The growth and annealing of single crystalline zno films by low-pressure mocvd

MOCVD growth and properties of ZnO films using dimethylzinc and oxygen

Raman and photoluminescence of ZnO films deposited on Si (111) using low-pressure metalorganic chemical vapor deposition

The deposition and annealing study of MOCVD ZnMgO