M. Zacharias

Thermal crystallization of amorphous Si/SiO2 superlattices

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

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 ...

Formation of Ge nanocrystals in amorphous GeOx, and SiGeOx alloy films

Abstract We present a systematic analysis of the conditions for the formation of Ge nanocrystals in GeO x and SiGeO x alloy films during annealing. Amorphous SiO x , GeO x and SiGeO x alloy films prepared by d.c. magnetron sputtering were subsequently annealed for times between 10 min and 60 min and at annealing temperatures ranging from 350 °C up to 800 °C. The formation of nanocrystals is analysed by X-ray scattering under grazing incidence. No crystallisation is found for the SiO x films under any annealing conditions. On the other hand, clear evidence for the formation of Ge nanocrystals is found in the GeO x as well as in the SiGeO x films. For the GeO x films the particle diameter d s linearly decreases with increasing oxygen content yielding values of d s = 40 nm down to 10 nm. The as-prepared amorphous SiGeO x films were found to always consist of nanoamorphous Ge clusters in an amorphous SiO x matrix determining the size of the resulting Ge nanocrystal after annealing. A Ge nanocrystal diameter of around d s = 6.0 nm is found for all SiGeO x alloy films independent of the annealing time and temperature.

Er doping of nanocrystalline-Si/SiO2 superlattices

Abstract We report here on Er doping in the vicinity of ordered arrays of Si nanocrystals. Using a Si/SiO2 superlattice structure allows control of both the size and arrangement of the nanocrystals. Amorphous Si/SiO2 superlattices with 15 periods and an additional 30 nm of top oxide are deposited by RF sputtering and plasma-enhanced chemical vapor deposition (PECVD). Erbium is implanted into the as-prepared samples with a dose ranging from 1×1015 to 5×1016 cm−2. The superlattices are investigated using wide-angle X-ray scattering, Rutherford backscattering spectroscopy and transmission electron microscopy. It is shown that annealing the implanted samples at 800°C for various times results in a controlled crystallization and in activation of the Er dopand. Damage to the superlattice structure due to the implantation process and the re-distribution of the Er dopand after annealing is discussed. Preliminary luminescence results are shown.

Extraordinary crystallization of amorphous Si/SiO2 superlattices

Abstract Ordered Si crystals with nanometer sizes are prepared in a Si/SiO 2 superlattice structure using rf sputtering and plasma oxidation. Decreasing the a-Si layer thickness to 1.9 nm increases the inhomogeneous strain by one order of magnitude. An exponential increase of the crystallization temperature with decreasing thickness is found. A new model using the melting temperature and the interior amorphous crystallization temperature is reported. The extension of the model to Ge/SiO 2 and Si/SiO x superlattices is demonstrated.

Visible luminescence from Ge nanocrystals embedded in a-Si1−xOx films: correlation of optical properties and size distribution

Abstract Ge nanocrystals embedded in a-Si 1− x O x films with a small size distribution were investigated using cathodoluminescence and photoluminescence. Luminescence bands around 2.3 and 1.93 eV were observed by cathodoluminescence. A photoluminescence band was found at 2.4 eV. It was not possible to confirm a peak shift correlated to the crystal size. With incipient crystallization the 2.4 eV peak increases about four orders of magnitude in intensity demonstrating the correlation of the band to the Ge nanocrystals.

Room temperature luminescence of Er doped nc-Si/SiO2 superlattices

Abstract Amorphous Si/SiO2 superlattices with a 80 nm top oxide are implanted with various erbium doses (1.2×1015– 5.2×10 16 cm −2 ) . The effect of Si nanocrystals in the vicinity of the Er ions is investigated. We found that the luminescence intensity is increased compared to an a-SiO2 film implanted with the same erbium dose. The Si layers are completely crystallized with an average crystal size of 5.7 nm after annealing at 800°C. Room temperature luminescence is found at 1.54 μm, and the intensity scales with the annealing time. Increasing the implantation dose decreases the room temperature luminescence intensity. Over the whole range of 7–300 K the luminescence quenches below one order of magnitude. Luminescence at 2, 2.55 and 3.0 eV is assigned to interface defects at the nanocrystal surface and to implantation damage in the SiO2.

Ge nanocrystals with a sharp size distribution: A detailed study of the crystallization of a-Si1-xOxGey alloy films

Abstract Thin amorphous Si 1− x O x Ge y alloy films were prepared by dc magnetron sputtering. We found that the as-prepared films contain amorphous Ge clusters with small size distributions. A low Ge content of the films is connected with very small Ge clusters in the as-prepared films and with regions where Ge atoms are densely spread in the matrix. Films with a larger Ge content show an increasing number of larger clusters. These clusters are closed together, thus producing dimeres and trimeres. Different annealing procedures resulted in different modes of crystallization. A crystallization of the as-prepared clusters is found after an annealing process with crystal diameters slightly greater than the cluster size. With annealing step by step inside the diffraction system we demonstrated the growth process including the final recrystallization of the aggregates.

Crystallization in the limit of ultra thin layers- A new crystallization model

Annealing of amorphous Si/SiO2 multilayer produces nanocrystals embedded between oxide interfaces with the size of the nanocrystals depending on the Si layer thickness. It is found that the crystallization temperature is strongly enhanced by the presence of the oxide interface and follows an exponential law. A model is presented, which derives the exponential dependence taking into account the interface energies, the thickness of the layers, the melting point of the system, and the bulk amorphous crystallization temperature. The critical crystallization radius and the critical thickness of the Si layer are discussed.

Spatially Resolved Imaging of the Spectral Emission Characteristic of an InGaN/GaN-Multi Quantum Well-Light-Emitting Diode by Scanning Electroluminescence Microscopy.

The microscopic spectral emission characteristic of an InGaN/GaN multi quantum well light-emitting diode is directly imaged by highly spectrally- and spatially-resolved scanning electroluminescence microscopy under operation as a function of injection current density from both, contact and substrate side. The mono- and panchromatic 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 ?m spatial resolution. Indium concentrations varying from around 0.09 to 0.11 are found in the active InGaN-region of the near ultraviolet diode emitting at 404 nm with a FWHM of 20 nm. At high current density a 4 nm redshift of emission wavelength due to ohmic heating is observed. Whereas the mesa electroluminescence intensity mapping of the front side becomes uniform for higher current densities, the backside electroluminescence still shows electroluminescence intensity fluctuations.