R. K. Yuan

Enhanced ultraviolet photoluminescence from SiO2/Ge:SiO2/SiO2 sandwiched structure

SiO2/Ge:SiO2/SiO2 sandwiched structure was fabricated for exploring efficient light emission. After annealed in N2 (O2<1%), this structure shows three photoluminescence (PL) bands at 293, 395, and 780 nm. The intensity of the 395 nm band is largely enhanced in comparison with that from the monolayered Ge:SiO2 film. Spectral analyses suggest that the three PL bands originate from S1→S0, T∑(T∏)→S0, and T∏′→S0 optical transitions in GeO color centers, respectively. The improvement of the GeO density resulting from the confinement on Ge diffusion is responsible for the enhanced ultraviolet PL. This structure is expected to have important applications in optoelectronics.

Capacitance of semiconductor-electrolyte junction and its frequency dependence

The frequency dependent capacitance of semiconductor-electrolyte junction and its relationship to the surface roughness of the semiconductor and the ions in the electrolyte are discussed. Due to very low mobility of the ions, the observed capacitance can be dominated by the Helmholtz double-layer of the electrolyte rather than the space charge layer of the semiconductor. The capacitance will also depend on the frequency. This, often observed power-law frequency dependence of capacitance is ascribed to the contribution of constant phase angle impedance. The power-law exponent can easily be related to the fractal dimension if the semiconductor surface can be described by fractal geometry.

Electrically induced light emission and novel photocurrent response of a porous silicon device

An electrically induced visible light emitting porous silicon (PS) device was fabricated by laterally anodizing an n‐type single‐crystal silicon (Si) wafer. Al/PS Schottky junctions exhibited rectifying I‐V characteristics with an ideality factor of 7. The intensity of current‐induced light emission increased with applied electrical current. Novel photocurrent spectra of the device under different reverse biases were measured at room temperature and reported for the first time, which showed three peaks of light absorption pertinent to the quantized energy levels of the PS. Interpretation is given based on the quantum confinement model.

Optical emission from nano-poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] arrays

Using an anodic alumina membrane with an ordered nanopore array as a template, we have fabricated a two-dimensional light-emitting nanopolymer array by embedding the luminescence polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) into the nanopores. It is revealed experimentally that the conformation of the MEH-PPV chains in the nanopores is in the form of a bunch of polymer chains. The number of the polymer chains in a bunch depends on the concentration of the polymer solution and the diameters of nanopores in the template. Investigation on the photoluminescence spectra of the nano-MEH-PPV array annealed under different temperatures shows that the nanopores can effectively hinder the relaxation of MEH-PPV, which leads to its high thermal stability.

Correlation of electroluminescence with Ge nanocrystal sizes in Ge-SiO2 co-sputtered films

Abstract Ge-doped SiO 2 films were deposited on p -Si substrates by magnetron sputtering and metal–insulator–semiconductor (MIS) structures were fabricated with semitransparent Au layers on SiO 2 films. The MIS structures exhibit electroluminescence (EL) peaked at 590 nm. The positions of the EL peaks from the samples with different Ge nanocrystal sizes keep unchanged. The maximal peak intensity appears in the sample annealed at 600°C in N 2 atmosphere, in which the average size of Ge nanocrystals is about 4 nm. X-ray diffraction results show that the EL intensities are closely related to Ge nanocrystal sizes. Theoretical analyses suggest that the energy gaps of Ge nanocrystals relate with the discrepancy of EL intensities by influencing transport properties of carriers. Our experiments provide a possible avenue for improving the EL intensity, which will be more useful in optoelectronics.

Strong violet emission from Ge–SiO2 co-sputtered films annealed in O2 atmosphere

Abstract Photoluminescence (PL) spectra of Ge–SiO 2 co-sputtered films annealed under O 2 , N 2 , and air were examined using the 250 nm excitation line of Xe lamp. Violet and ultraviolet PL peaks were observed at ∼400 and ∼300 nm. The two peaks were found to have a similar behavior with annealing temperature. Their maximal intensities appear in the sample annealed at 800°C and in O 2 atmosphere. Fourier transform infrared absorption results suggest that the two PL peaks are closely related to Ge oxide in the samples. The PL excitation spectral examinations indicate that they arise from optical transitions in GeO color centers. The existence of oxygen during annealing improves the GeO density in the samples and makes the PL intensities enhanced largely. Our experiments provide a way for improving the violet and ultraviolet PL intensities, which will be more useful in device applications.

Optical emission from the aggregated state in poly [2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene]

We report the photoluminescence characteristics of solid solutions of poly[2-methox-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) with different concentrations in polystyrene. Reduction in the distances between the MEH-PPV chains with increased MEH-PPV concentrations makes the conjugation segments aggregated in the solid solution. Absorption by the conjugation segments in the aggregated state leads to a redshift absorption edge. By comparing the photoluminescence and absorption spectra of the solid solutions with those of the 100% MEH-PPV film, it is shown that the emission from the 100% MEH-PPV film originates from the radiative recombination of excitons in the aggregated state.

Temperature-dependent photoluminescence and Raman spectra from porous GeSi/Si heterostructures

Porous GeSi/Si heterostructures were fabricated by laterally anodization in HF-based solutions. Photoluminescence spectra have been investigated as a function of temperature (77–300 K), showing that porous GeSi has a quite different temperature dependence from that of porous silicon. Raman spectra indicated that the sample structure changed after anodization. Phonon participation and direct recombination of excitons are proposed to be responsible in the light emission processes of porous GeSi and Si, respectively.