Showing papers by "Margit Zacharias published in 1999"

Thermal crystallization of amorphous Si/SiO2 superlattices

Abstract: Annealing of amorphous Si/SiO2 superlattices produces Si nanocrystals. The crystallization has been studied by transmission electron microscopy and x-ray analysis. For a Si layer thinner than 7 nm, nearly perfect nanocrystals are found. For thicker layers, growth faults and dislocations exist. Decreasing the a-Si layer thickness increases the inhomogeneous strain by one order of magnitude. The origin of the strain in the crystallized structure is discussed. The crystallization temperature increases rapidly with decreasing a-Si layer thickness. An empirical model that takes into account the Si layer thickness, the Si/SiO2 interface range, and a material specific constant has been developed.

Direct imaging of the spectral emission characteristic of an InGaN/GaN-ultraviolet light-emitting diode by highly spectrally and spatially resolved electroluminescence and photoluminescence microscopy

Abstract: The microscopic spectral emission characteristic of an InGaN/GaN double-heterostructure light-emitting diode is directly imaged by highly spectrally and spatially resolved scanning electroluminescence microscopy under operation as a function of injection current density. The luminescence intensity maps and especially the peak-wavelength scanning images provide access to the optical quality of the final device and yield direct images of the In fluctuations with 1 μm spatial resolution. Indium concentrations varying from 6% to 9% are found in the active InGaN region of the ultraviolet diode emitting at 400 nm. While for low injection current densities the electroluminescence is dominated by emission from the p GaN originating from the whole accessible area, the emission from the InGaN active layer increases and takes over for higher injection conditions showing a strongly localized spatial emission characteristic. Correlation of the results with low-temperature scanning photoluminescence microscopy enables ...

Spatially Resolved Electroluminescence of InGaN-MQW-LEDs

Abstract: Electroluminescence (EL) is the most significant measure for light-emitting diodes since it probes the most relevant properties of the fully processed device during operation. In addition to the information gained by conventional spectrally resolved EL, scanning micro-EL provides spatially resolved information. The devices under investigation are InGaN/GaN-LEDs with single peak band-band emission at about 400 nm grown by MOVPE on sapphire substrates. The μ-EL-characterization is performed as a function of injection current densities and the emission is investigated from the epitaxial layer as well as from substrate side. Spatially resolved wavelength images reveal emission peaks between 406 nm and 417 nm, corresponding either to In fluctuations of 1%–1.5% or local fluctuations of piezo electric fields. Beside the information on the emission wavelength fluctuations ν-EL is used to determine the temperature distribution in the LEDs and to investigate transparent contacts.

Ge and Si nanocrystals—New ways to luminescence structures

Abstract: Ge nanocrystals with a radius from 25 down to 1.5 nm are formed in a SiOx matrix. A luminescence is observed around 3.1 eV with a subnanosecond photoluminescence (PL) dynamic. The strongest PL is found for films containing the largest nanocrystals. These results are a clear indication that the blue luminescence is not produced by radiative recombination of excitons confined in the Ge nanocrystals. The investigations on Ge nanocrystals are summarized and the PL mechanisms are clarified. Ordered Si nanocrystals are prepared in a Si/SiO2 superlattice structure. Decreasing the a-Si layer thickness down to 1.9 nm increases the inhomogeneous strain by one order of magnitude. The exponential increase of the crystallization temperature with decreased thickness is described with a semi-empirical model using the melting point and the bulk amorphous crystallization temperature. The validity of the model to Ge/SiO2 superlattices is demonstrated. Enhancement in band gap luminescence is discussed for films with nanocrystal sizes near the Bohr radius. First results of investigations on Er doped superlattices are presented.