AlGaN-based LEDs for UV-C-light emission still suffer from relatively poor efficiency. Besides problems with carrier injection and light extraction, strong piezoelectric fields in the optically active region originating from lattice mismatch between quantum wells and barrier material are a major issue. Mixing only few percent of boron into the AlGaN active region may be sufficient to achieve lattice matched conditions, thus decreasing the influence of the quantum-confined Stark effect on the radiative recombination efficiency. However, the epitaxial growth of AlBGaN layers with sufficient crystalline quality is still a challenge, particularly due to the low solubility of boron in AlGaN and the low mobility of boron ad-atoms on the surface. Consequently, only extremely weak luminescence has been reported on layers containing few percents of boron. By thoroughly optimizing the metalorganic vapor phase epitaxial growth of AlBGaN layers with a boron content of some percent, we could achieve similar luminescence intensities as for reference AlGaN layers along with smooth hetero-interfaces and low surface roughness as measured by TEM and AFM. Besides studying the influence of basic growth parameters like temperature, V-III ratio etc., we investigate possible improvements by an optimized pulsed precursor supply sequence. To reduce the unintentional doping with impurities like oxygen or carbon, typically attributed to the standard boron precursor tri-ethyl boron (TEB), we investigate the novel metalorganic precursor tri-isopropyl-boron (TiPB). Its lower vapor pressure as compared to TEB facilitates a controlled incorporation of small B amounts. First PL spectra of AlBGaN layers grown with TiPB show promising data.
Improving the crystal quality of AlGaN epitaxial layers is essential for the realization of efficient III-nitride-based light
emitting diodes (LEDs) with emission wavelengths below 365 nm. Here, we report on two different approaches to
improve the material quality of AlGaN buffer layers for such UV-LEDs, which are known to be effective for the
MOVPE growth of GaN layers. Firstly, we grew AlGaN on thin GaN nucleation islands which exhibit a threedimensional
facetted structure (3D GaN nucleation). Lateral overgrowth of these islands results in a lateral bending of
dislocation lines at the growing facets. Secondly, in-situ deposited SiNx interlayers have been used as nano-masks
reducing the dislocation density above the SiNx layers. Both approaches result in reduced asymmetric HRXRD ω-scan
peak widths, indicating a reduced edge-type dislocation density. They can be applied to the growth of AlGaN layers with
an Al concentration of at least 20%, thus suitable for LEDs emitting around 350 nm. On-wafer electroluminescence
measurements at 20 mA show an increase in output power by a factor of 7 and 25 for LED structures grown on 3D GaN
nucleation and SiNx interlayer, respectively, compared to structures grown on a purely 2D grown low Al-content AlGaN
nucleation layer. Mesa-LEDs fabricated from the LED layer sequences grown on buffers with SiNx interlayer exhibit a
low forward voltage of 3.8 V at 20 mA and a maximum continuous wave (cw) output power of 12.2 mW at 300 mA.
Bluish-green semipolar GaInN/GaN light emitting diodes (LEDs) were investigated as possible candidates for
high-brightness devices even in the long wavelength visible regime. To combine the high material quality known
from c-GaN and the advantages of a reduced piezoelectric field, the LED structures were realized on the {1¯101} side facets of selectively grown GaN stripes with triangular cross section. Structural investigations using transmission
electron microscopy, scanning electron microscopy, high resolution x-ray diffraction, and atomic force
microscopy have been performed and could be related to the luminescence properties in photoluminescence and
cathodoluminescence. The defect-related luminescence peaks at 3.3 eV and 3.42 eV typically observed in planar
non- and semipolar GaN structures as fingerprints of prismatic and basal plane stacking faults, respectively,
could be eliminated in our facet LED structures by optimized growth conditions.
Furthermore, an indium incorporation efficiency for these {1¯101} facets is found to be about 50% higher
as compared to c-plane growth, what helps significantly to achieve longer wavelength emission in spite of the
reduced quantum confined Stark effect in such non- and semipolar materials.
Combining these findings, we could realize a bluish-green semipolar light emitting diode on the side facets of
our GaN stripes. Continuous wave on-wafer optical output powers as high as 240 µW@20mA could be achieved
for about 500nm emission wavelength in electroluminescence measurements. The external efficiency was nearly
constant for the investigated current range. Furthermore, the relatively small wavelength shift of about 3 nm for
currents between 10mA and 100mA confirmed the reduced piezoelectric field in our LED structures.
In order to understand and control the fabrication of nanomaterials, it is essential that they be characterized at close to the atomic scale. The atomic structure of interfaces, defects and nanostructures can be investigated by atomic resolution transmission electron microscopy (TEM), using either high resolution TEM accompanied by simulation or high angle annular dark-field scanning TEM (HAADF-STEM), usually referred to as Z-contrast imaging. Just as atomic force microscopy, scanning tunneling microscopy and the atom probe have become the primary tool for studying surfaces, so TEM has become the method of choice for studying defects and nanostructures within materials. Many analytical signals are available on modern small-probe-forming TEMs. These techniques include convergent beam electron diffraction (CBED), electron energy loss spectroscopy (EELS) and energy dispersive X-ray (EDX) microanalysis. Atomic scale information can be obtained about defects, strain, chemical content, site occupancy, crystallographic and electronic structure. The combination of reciprocal, real space and analytical information with atomic resolution Z-contrast imaging and EELS spectroscopy offers great potential for unraveling structure-property relationships in nanostructures.
Examples of two types of nanostructures: quantum well and quantum dot structures in SiC are given as well as of optical multiplayer structures for the deep UV. The strain state in SiC-quantum well structures will be determined and atomic-resolution and EELS spectroscopy illustrations are given for the chemically dissimilar cases of nanocrystals formed after erbium and germanium implantation in silicon carbide. The crystallographic and electronic structure of single nanocrystals will be determined and finally the longstanding question of whether the cluster nucleation is defect-mediated or spontaneous will be addressed.
The structure evolution characteristics of single and stratified NdF3 optical thin films on amorphous quartz substrates have been investigated by cross sectional transmission electron microscopy, x-ray diffractometer measurements and atomic force microscopy. The films were deposited by physical vapor deposition under ultra high vacuum conditions. The morphology changes with substrate temperature from V shaped columnar (Zone T) to a columnar morphology corresponding to zone II. The zone boundary falls in the range above 300 °C. The surface roughness of the single films is lowest at 300 °C substrate temperature. The main texture components are <111<, <1 13< and <001< respectively for the three single films of increasing substrate temperature. The surface roughness is decreased by the stratification. The grain size is increased by stratification with CaF2 and decreased by stratification with MgF2 compared to the grain size ofthe single film at the same substrate temperature. Keywords: thin film, stratification, morphology, texture, surface roughness, extinction
The structure evolution characteristics of MgF2 and NdF3 optical thin films on CaF2 (111) substrates have been investigated by cross sectional transmission electron microscopy and atomic force microscopy and were related to their extinction coefficient. The films were deposited by physical vapor deposition under ultra high vacuum conditions. A series of experiments have been carried out at various substrate temperatures at 300 nm thickness and by the stratification of the two fluoride materials. The structure evolution of single and stratified films was investigated and discussed in relation to the structure zone models. The effect of stratification on the structure and extinction coefficients is discussed in comparison to the effect of segregating impurities. The development of morphology of stratified films is analyzed in relation to the thickness of the interlayers and temperature. The optimum conditions for preparing films of low extinction coefficients are found at parameters corresponding both to the transition between zone T and zone II growth and to the formation of nanocrystalline structures.
Optical coatings for the ultraviolet spectral region consisting of fluorides have been made by molecular beam deposition in ultrahigh vacuum. The laser induced damage threshold at (lambda) equals 248 nm of the NdF3 single layers has been improved by means of reactive deposition. It was found that the threshold of fluoride antireflection coatings for 248 nm is significantly lower than that of single layers and does not show significant dependence on reactive gas conditions. A surface smoothing has been obtained due to an interrupted growth of very thin sublayers crystallizing in different crystal lattices. The damage threshold of interrupted grown antireflection coatings is somewhat higher than that of a conventional one.
A new technique for the fabrication of optical interference layers is being introduced. MgF2 and NdF3 have been evaporated by molecular beam deposition (MBD) in an ultrahigh vacuum system. The fluoride films are used in optical interference coatings for optical components of high power excimer lasers. The growth conditions were varied by changing the remaining gas composition, variation of the substrate temperature from 50 degree(s)C to 300 degree(s)C, and interruption of grain growth. The optical losses of the films were measured by transmission spectroscopy and laser calorimetry, and the film morphology was investigated by transmission electron microscopy. It was shown, that a stratification of very thin (1 nm - 10 nm) layers of two different fluorides can avoid the growth of large grains and wide columns, and gives rise to smoother surfaces. The packing density of MBD-films is higher in comparison with fluoride films deposited under conventional high vacuum conditions.
In the EUREKA EU205 project the target products are industrial excimer lasers in the average power range of one kilowatt or more. The high power optical components and dielectric coatings have to be developed in close adaption to cavity design (optics), beam relay optics, mask imaging optics, and masks. Therefore, we used ultra low loss conventional e-beam evaporation for Al2O3/SiO2 dielectric multilayers. Based on a fundamental coating technique, both multilayer mean background absorption and absorption at localized spikes have been reduced drastically. The resulting KrF laser damage threshold of HR coatings is 16 J/cm2 (1-on-1, 30 ns, EMG-202-MSC). Measurements have been performed with an automated damage testing facility, being part of the EUREKA program. Multilayers have been characterized by Atomic Force Microscopy, Photothermal Microscopy, absorption measurements, and Spectroscopy of Sputtered Neutrals.
In the thickness range 50 nm - 500 nm at substrate temperatures of 300 K, 375 K, and 575 K the structure of physical-vapor-deposited magnesium, lanthanum, calcium, and lithium fluoride films were investigated using TEM microfractographical replication technique. Two growth groups, columnar and granular growth, were found and main structure building elements with their characteristic medium size have been determined. Using SIMS, SNMS, RBS, and partially spectroscopical and gravimetrical measurements, the O and C contamination of the films has been investigated. The quantitative results were related to the characteristic structure building elements.
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