A. Mogilatenko

Modulated Epitaxial Lateral Overgrowth of AlN for Efficient UV LEDs

A reduction of the threading dislocation density in AlN layers on a sapphire from 1010 cm-2 to 109 cm-2 was achieved by applying epitaxial lateral overgrowth (ELO) of patterned AlN and sapphire templates. By varying the growth temperature, it is possible to influence the lateral growth rate and modulate the thickness before coalescence. With a two-step growth at two different temperatures, up to 11-μm thick crackfree layers were achieved. Using these ELO AlN templates, the light emitting diode (LED) output power was >;1 mW dc at 295 nm and ~ 4mW at 324 nm which is a significant increase compared to planar templates. The usefulness of modulated ELO AlN templates for ultraviolet LEDs has thus been validated.

Electrical properties and microstructure of vanadium-based contacts on ICP plasma etched n-type AlGaN:Si and GaN:Si surfaces

Light-emitting diodes emitting in the UV-B spectral range usually contain an n-type AlxGa1?xN contact layer with x of about 0.4 and require a low specific contact resistivity in the range of 10?6?? cm2?to operate at their optimal capacities. We compare the Ti/Al/Mo/Au metal system commonly used on GaN with the V/Al//V/Au metal system on n-Al0.4Ga0.6N:Si and GaN:Si surfaces. On Al0.4Ga0.6N, the lowest specific contact resistivity, (2.3 ? 0.2)??10?6?? cm2, was reached with V/Al/V/Au whereas for Ti/Al/Mo/Au the lowest resistivity was (1.7 ? 0.7)??10?3?? cm2. Independently of the metal system, lower contact resistivities are obtained on GaN where Ti/Al/Mo/Au is still performing at least one order of magnitude better than V/Al/V/Au. The contact resistivity of V/Al/V/Au on Al0.4Ga0.6N was found to be sensitive to surface treatments (wet chemical etching or sputtering) applied prior to metal deposition, to the thickness of the first vanadium layer and to that of the aluminum layer above. Structural and compositional investigations showed that a low contact resistivity is accompanied by the formation of an interfacial layer between AlGaN and V/Al/V/Au on the nm-scale, which contains aluminum, in the form of aluminum nitride, and separately vanadium as metal or metal nitride.

High-power UV-B LEDs with long lifetime

UV light emitters in the UV-B spectral range between 280 nm and 320 nm are of great interest for applications such as phototherapy, gas sensing, plant growth lighting, and UV curing. In this paper we present high power UV-B LEDs grown by MOVPE on sapphire substrates. By optimizing the heterostructure design, growth parameters and processing technologies, significant progress was achieved with respect to internal efficiency, injection efficiency and light extraction. LED chips emitting at 310 nm with maximum output powers of up to 18 mW have been realized. Lifetime measurements show approximately 20% decrease in emission power after 1,000 operating hours at 100 mA and 5 mW output power and less than 30% after 3,500 hours of operation, thus indicating an L50 lifetime beyond 10,000 hours.

Temperature-Dependent Thermoelectric Properties of Individual Silver Nanowires

The thermoelectric properties of the Ag NWs are discussed in comparison to the bulk: SAg;Pt(T) was measured with respect to platinum and is in agreement with the bulk, (T) and (T) showed reduced values with respect to the bulk. The latter are both notably dominated by surface scattering caused by an increased surface-to-volume ratio. By lowering T the electron mean free path strongly exceeds the NW’s diameter of 150 nm so that the transition from diusive transport to quasi ballistic one dimensional transport is observed. An important result of this work is that the Lorenz numberL(T) turns out to be independent of surface scattering. Instead the characteristic ofL(T) is determined by the material’s purity. Moreover, (T) and L(T) can be described by the bulk Debye temperature of silver. A detailed discussion of the temperature dependence of L(T) and the scattering mechanisms is given.

Structure and magneto-optic Kerr measurements of epitaxial MnSi films on Si(111)

MnSi films are grown by evaporation of Mn onto Si(111) substrates under ultrahigh-vacuum conditions. Films are characterized with real- and reciprocal-space surface-science techniques such as scanning tunneling microscopy and low-energy electron diffraction. The bulk structure is determined ex situ by transmission electron microscopy (TEM). Thin (<60A Mn-deposited) MnSi films show a regularly modulated surface due to a 3% lattice mismatch of the interfaces. TEM confirms epitaxial growth and demonstrates smooth, atomically flat interfaces. Temperature-dependent ex situ measurements of the magneto-optic Kerr effect show ferromagnetism with an in-plane easy axis magnetic anisotropy for 60- and 100-A-thick Mn films.

Preparation and properties of MnSi 1.7 on Si(001)

Abstract The most rich silicon silicides of manganese, the group of higher manganese silicides (HMS), have the composition MnSix with x in the range from 1.67 to 1.75. This material group with a tetragonal crystal structure shows semiconducting electronic properties, with promising application in thermoelectric devices. This paper reports the structural and morphological properties of HMS prepared by a reactive deposition process on Si(001) under UHV conditions. X-ray diffraction shows, for all samples grown at substrate temperatures ranging from 400 to 750°C, the growth of HMS only. Scanning electron microscopy and Rutherford backscattering spectrometry show a transition from film growth to island growth with increasing substrate temperature. A detailed analysis of the XRD spectra shows a change of the texture of HMS at the transition of the sample morphology. The results are discussed on the basis of anisotropic growth rates for differently oriented grains of HMS.

Analysis of doping induced wafer bow during GaN:Si growth on sapphire

In-situ curvature measurements were employed to quantify stress generation during metalorganic vapor phase epitaxy growth of Si-doped GaN sandwiched between undoped GaN layers. It is shown that the creation of tensile stress in Si-doped GaN is closely linked to the density of threading dislocations in the material. Accompanying characterization by in-situ reflectance measurements and TEM analysis supports the model that the doping induced stress formation is caused by a surface-mediated dislocation climb process. In addition, curvature measurements reveal an unusual partial relaxation in GaN:nid grown atop of highly doped GaN:Si layers. This relaxation is assigned to a notable dislocation annihilation at the upper GaN:Si+/GaN:nid interface found by TEM probably due to the formation of a thin SiNx masking layer.

Electron microscopic investigation of MnSi 1.7 layers on Si(001)

Semiconducting higher manganese silicides (HMS) with a composition near that of MnSi1.7 are of special interest due to their thermoelectric properties. We report on the growth of HMS layers deposited by MBE using the template technique. In particular the influence of the template thickness on the structure and morphology of MnSi1.7 films on (001)Si substrates was investigated. It was found that there is an optimal template thickness that causes preferred epitaxial growth of the major amount of the silicide. Three different epitaxial orientation relations of the silicide crystals to the substrate were observed. Considering the specific features of the electron diffraction patterns of MnSi1.7 the HMS phase was identified as Mn4Si7.

The effect of a distinct diameter variation on the thermoelectric properties of individual Bi0.39Te0.61 nanowires

The reduction of the thermal conductivity induced by nano-patterning is one of the major approaches for tailoring thermoelectric material properties. In particular, the role of surface roughness and morphology is under debate. Here, we choose two individual bismuth telluride nanowires (NWs), one with a strong diameter variation between 190 nm and 320 nm (NW1) and the other of 187 nm diameter with smooth sidewalls (NW2). Both serve as model systems for which bulk properties are expected if surface properties do not contribute. We investigate the role of the diameter variation by means of a combined full-thermoelectrical, structural and chemical characterization. By transmission electron microscopy the structure, chemical composition and morphology were determined after the thermoelectrical investigation. The NWs showed an oriented growth along the direction and the same composition. The Seebeck coefficients of both NWs are comparable to each other. The electrical conductivity of both NWs exceeds the bulk value indicating the presence of a topological surface state. Whereas the thermal conductivity of NW2 compares to the bulk, the thermal conductivity of NW1 is about half of NW2 which is discussed with respect to its distinct diameter variation.

2D layered transport properties from topological insulator Bi2Se3 single crystals and micro flakes.

Low-field magnetotransport measurements of topological insulators such as Bi2Se3 are important for revealing the nature of topological surface states by quantum corrections to the conductivity, such as weak-antilocalization. Recently, a rich variety of high-field magnetotransport properties in the regime of high electron densities (∼1019 cm−3) were reported, which can be related to additional two-dimensional layered conductivity, hampering the identification of the topological surface states. Here, we report that quantum corrections to the electronic conduction are dominated by the surface states for a semiconducting case, which can be analyzed by the Hikami-Larkin-Nagaoka model for two coupled surfaces in the case of strong spin-orbit interaction. However, in the metallic-like case this analysis fails and additional two-dimensional contributions need to be accounted for. Shubnikov-de Haas oscillations and quantized Hall resistance prove as strong indications for the two-dimensional layered metallic behavior. Temperature-dependent magnetotransport properties of high-quality Bi2Se3 single crystalline exfoliated macro and micro flakes are combined with high resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy, confirming the structure and stoichiometry. Angle-resolved photoemission spectroscopy proves a single-Dirac-cone surface state and a well-defined bulk band gap in topological insulating state. Spatially resolved core-level photoelectron microscopy demonstrates the surface stability.