M. Albrecht

Defect structure of epitaxial GaN films determined by transmission electron microscopy and triple-axis X-ray diffractometry

Abstract Important structural characteristics (correlation lengths of columnar crystallites, dislocation densities, angles of rotational disorder) of hexagonal GaN grown by metallorganic chemical vapour deposition on c-plane sapphire are determined by transmission electron microscopy and triple-axis X-ray diffractometry. GaN films exhibit an edge dislocation density in the range of 1011 cm−2, a tilt and twist angle of 0.1° and 1.3° and a columnar structure with a lateral and vertical correlation length of 150 and 1000 nm respectively. The determination of correlation lengths and dislocation densites from X-ray patterns was undertaken using two independent evaluation methods which are discussed in detail. It is also shown that triple-axis X-ray diffractometry is a highly suitable technique for the separation of different kinds of structural defects such as edge and screw dislocations that lead to a characteristic broadening of symmetric and asymmetric Bragg reflections. The correlation lengths and dislocat...

Effect of annealing on the In and N distribution in InGaAsN quantum wells

We analyze the influence of annealing on compositional fluctuations in InGaAsN quantum wells by means of composition-sensitive high-resolution transmission electron microscopy and photoluminescence. In as-grown samples, we find In-concentration fluctuations of ±5% on a length scale of 20 nm in a two-dimensional grown quantum well. No indications for N concentration fluctuations are found within the limits of resolution. Annealing homogenizes the In distribution within the well and causes diffusion of N out of the quantum well. According to our compositional analysis, the blueshift in the photoluminescence can in part be attributed to reduction in N concentration inside the well. The more homogeneous In distribution leads to a reduction in linewidth and Stokes shift.

Solid-phase crystallized Si films on glass substrates for thin film solar cells

Abstract We investigate the potential of solid-phase crystallized Si films on glass for use in polycrystalline Si thin film solar cells. Low-pressure chemical vapour deposition serves to form amorphous Si films on borosilicate, SiO2-coated borosilicate, aluminosilicate glass and fused silica substrates. The films are crystallized at temperatures of around 600°C. Using transmission electron microscopy we determine the grain size in the crystallized films. The average grain size strongly depends on the substrate type, increases with the deposition rate of the amorphous film and is independent of the film thickness. The grain size distribution in our films is log-normal. Films crystallized on SiO2-coated borosilicate glass have an average grain size up to 2.3 μm, while the area weighted average grain size peaks at 4 μm. Since thin crystalline Si solar cells only require a film thickness of several micron, our films seem to be suitable for application to such devices.

Study of individual grown-in and indentation-induced dislocations in GaN by defect-selective etching and transmission electron microscopy

Abstract Vickers diamond indentation at 370°C has been employed to introduce dislocations into the plate-like (000-1) N-polar GaN single crystals. It has been established that using standard Vickers diamond indenter, well-defined ‘rosettes’ of defects are formed under 1.5–2 N load applied for 10 min. The resolved patterns of dislocation-related etch pits are formed using molten KOH–NaOH eutectic (E) at 200°C for 1.5–2 min. Individual grown-in dislocations are revealed by this E etch in the GaN matrix. Transmission Electron Microscopy confirmed the correlation of etch pits to individual dislocations emerging at the surface. Nano-crystalline material was found in the highly deformed central region of the indentation rosette. The structure of these nano-crystals was analyzed using electron diffraction. Speculative explanation on a phase transition induced by high local pressure is briefly discussed.

Dynamics of lateral grain growth during the laser interference crystallization of a-Si

Laser interference crystallization of amorphous silicon (a-Si) thin films, a technique that combines pulsed laser crystallization with holography, enables the fabrication of periodic arrays of polycrystalline silicon (poly-Si) lines with lateral dimensions between 0.5 and 20 μm. The lines consist of grains with well-defined grain boundary locations and lateral dimensions that are appreciably larger than the thickness of the initial a-Si:H film (up to 2 μm for a 300 nm thick film). We investigated the dynamics of the crystallization process by two-dimensional finite element computer simulations of the heat transport and phase transitions during laser crystallization. The theoretical results were compared to: (i) measurements of the crystallization kinetics, determined by recording the transient changes of the reflectance during laser exposure, and to (ii) the structural properties of the crystallized films, determined by scanning force and transmission electron microscopy. The simulations indicate that the...

Influence of magnesium doping on the structural properties of GaN layers

Abstract We investigate the incorporation of magnesium in gallium nitride (GaN) films deposited on (0 0 0 1) sapphire (c-Al 2 O 3 ) substrates by low-pressure metalorganic chemical vapor-phase deposition (MOCVD). The growth, structure and optical properties of GaN samples with a dopant concentration ranging from 10 17 to 10 21 cm −3 have been studied using transmission electron microscopy and micro-Raman spectroscopy. The growth rate and sample quality are strongly influenced by the amount of magnesium incorporated in the sample, with the inclusion of cubic GaN on the otherwise hexagonal material and a disruption of the two-dimensional growth for the highest dopant concentrations. The presence of the cubic phase is accompanied by the formation of large triangular islands due to a local increase of growth rate for a dopant concentration exceeding 10 20 cm −3 .

The transition from ripples to islands in strained heteroepitaxial growth under low driving forces

Abstract We investigate the morphological evolution during the growth of strained Ge x Si 1 − x layers from ln solution (5 ⩽ x ⩽ 15%). Surface ripples develop due to elastic stress relaxation. These ripples self-organize in a way that a defined wavelength and a two-dimensional pattern aligned along (1 0 0) directions develops. During further growth, the ripples transform into pseudomorphic islands and island growth proceeds in vertical direction at constant island width. The ripple wavelength and the island size are controlled by the germanium content and verify energetical predictions. Further, the theoretical expectation is corroborated that island development occurs even at low misfits when deposition is controlled by energetics.

Carrier Recombination at Screw Dislocations in n-Type AlGaN Layers

We analyse the recombination and scattering at dislocations in Si doped AlGaN layers by transmission electron microscopy (TEM), electron beam induced current (EBIC) and cathodoluminescence. We show that c-type screw dislocations are nonradiative recombination centres. From temperature dependent measurements of the EBIC contrast we find a shallow acceptor state 20 meV above the valence band. The level can be understood iri terms of one-dimensional dislocation bands split off by the dislocation deformation field. In addition piezoelectric potentials play a role.

High quality GexSi1-x by heteroepitaxial lateral overgrowth

Abstract We present a method for non-pseudomorphic growth of high quality Ge x Si 1− x on thermally oxidized Si (111) substrates. Liquid phase epitaxy is utilized for the Ge x Si 1− x ( x = 0.90) deposition, which starts in oxide-free seeding windows and proceeds laterally over the oxide. Low temperature cathodoluminescence imaging and transmission electron microscopy do not reveal any lattice defects in the Ge x Si 1− x , laterally grown over the SiO 2 , for seeding window lengths up to 250 μm.

Effects of phase separation and decomposition on the minority carrier diffusion length in AlxGa1−xN films

Combined electron beam induced current and transmission electron microscopy (TEM) measurements have been performed on both undoped and Si-doped AlGaN epitaxial films with aluminum contents x ranging from x=0 to x=0.79, in order to correlate the electrical and structural properties of the films. The diffusion length of holes in the films ranges between 0.3 and 15.9 μm, and the estimated lifetime of holes for doped samples varies between 0.2 ns and 16 μs. Different effects contribute to the observed increase in the diffusion length with increasing aluminum content. Among others, dislocations seem to be active as nonradiative recombination sites, and phase separation and decomposition as observed by TEM in Al-rich alloys lead to the formation of a spatially indirect recombination path due to the piezoelectric field in the films. Potential fluctuations associated with these phase irregularities could also give rise to electron induced persistent conductivity contributing to the increase of the diffusion lengt...