E. Koppensteiner

Investigation of strain‐symmetrized and pseudomorphic SimGen superlattices by x‐ray reciprocal space mapping

Double‐crystal and triple‐axis x‐ray diffractometry was used to characterize in detail the strain and composition of short period Si6Ge4, Si8Ge8, Si9Ge6, and Si17Ge2 strained‐layer superlattices (SLSs), grown by molecular‐beam epitaxy. Nominally strain‐symmetrized superlattices, intended to be free standing from underlying buffer layers and the substrate, grown on rather thin (20 nm thick) SiGe alloy buffers with constant Ge content (Si6Ge4 and Si8Ge8) are compared to those grown on 1.3‐μm‐thick step‐graded SiGe alloy buffers (Si6Ge4 and Si9Ge6). Due to the much higher instrumental resolution offered by triple‐axis diffractometry (Δ2Θ=12 arcsec) buffer and SLS peaks are clearly separated from each other, which overlap in corresponding double‐crystal‐diffractometry measurements (Δ2Θ in the range of 180 arcsec to 2°). The lattice constants parallel and perpendicular to the [001] growth direction are determined independently from each other and thus precise strain data of the buffers and the SLS constituting...

Evolution of strain relaxation in compositionally graded Si1−xGex films on Si(001)

High‐resolution x‐ray reciprocal space mapping was employed to determine the in‐depth strain distribution of Si1−xGex films with linear composition gradings between 4.2% and 15% Ge per μm, and thicknesses between 0.4 and 1.7 μm. The variation of grading and thickness parameters of the samples provides a complete picture of the overall relaxation behavior of linearly graded epilayers. The x‐ray data show a top layer of grading‐dependent residual strain whereas the lower parts of the films are completely and/or partly relaxed with respect to the Si substrate.

Analysis of strain and mosaicity in a short‐period Si9Ge6 superlattice by x‐ray diffraction

Triple axis x‐ray diffractometry was employed for the structural characterization of a 100 period Si9Ge6 superlattice grown by molecular beam epitaxy on a thick step‐graded SiGe alloy buffer. From the distribution of diffusely scattered intensity around reciprocal lattice points the correlation function of the deformation field due to structural defects has been calculated using kinematical theory of x‐ray diffraction. From the extension of the correlation function it turns out that on the average the entire superlattice (0.2 μm thick) scatters coherently along growth direction, whereas laterally the coherently scattering regions are extended only over about 40 nm.

X‐ray diffraction investigation of single step and step‐graded SiGe alloy buffers for the growth of short‐period SimGen superlattices using reciprocal space mapping

Double crystal and triple axis x‐ray diffractometry was used to characterize the structural properties of short period Si6Ge4 superlattices grown by molecular beam epitaxy on either a thin (20 nm) single step Si0.6Ge0.4 alloy buffer or on a thick step‐graded Si1−xGex(0<x<0.4, 700 nm thick) buffer followed by a 550 nm Si0.6Ge0.4 layer. Reciprocal space maps around the (004) and (224) reciprocal lattice points yield direct information on the strain status of the buffer and superlattice layers. For the thick step‐graded buffer indeed all layers with different Ge content are fully relaxed and thus the growth of an almost freestanding superlattice is possible.

Triple axis x-ray investigations of semiconductor surface corrugations

X‐ray reciprocal space mapping around the symmetrical (004) Bragg reflection and a kinematical x‐ray diffraction model were employed in order to determine the geometry and the structural perfection of surface corrugations or quantum wires. This method was used for the analysis of (001) Cd1−xZnxTe surface corrugations fabricated by holographic lithography and subsequently reactive ion etched with typical periods of 500 nm. Comparison of the measurement and simulation provides conclusive information on etching depth, wire period, wire width, and the inclination of the side walls. Furthermore, the analysis yields a parameter that contains information on side wall roughness, shape fluctuations and, in principle, the crystallographic damage caused by the reactive ion etching process. Due to the high resolution of triple axis diffractometry small strain gradients are observable in the damaged region.

Determination of threading dislocation density in hetero-epitaxial layers by diffuse x-ray scattering

An approach for the determination of dislocation densities in epilayers is presented, which is based on the analysis of the two-dimensional intensity distribution of diffuse X-ray scattering measured by triple-axis X-ray reciprocal-space mapping. A new formalism for the simulation of experimental iso-intensity contours is used, which assumes a defect model based on random elastic deformation due to the strain fields of dislocations. The simulation of the two-dimensional intensity contours yields the random strain field from which the energy density stored in the epilayer due to the presence of dislocations is calculated. On the other hand, the self-energy of threading dislocations is calculated assuming 60 degrees (for diamond or zincblende structure) or screw-type dislocations. The threading dislocation density is obtained by dividing the energy density by this self-energy. The method was applied for the analysis of SiGe layers grown on compositionally graded SiGe alloy buffer layers. The diffuse X-ray scattering analysis reproduces the increase in the dislocation densities in the buffer with higher Ge grading rates and yields upper limits for the threading dislocation densities.

Diffuse x‐ray scattering from p+ porous silicon by triple axis diffractometry

Strain and microstructure of porous silicon on (001) wafers with different porosity were investigated by triple axis x‐ray diffractometry using an instrumental resolution of 12 arcsec. The Bragg diffraction peaks arising from the porous Si contain information both on the mean strain (1.29–2.95×10−3) and on strain gradients (0.70–1.42×10−3) in these samples. In specimens with a porosity of 60% the pores are shown to be elongated over about 200 nm along the [001] direction, and over 50 nm in directions parallel to the growth plane. It is demonstrated that the correlation function for the pores has an extension along the [001] direction which is about a factor of 4 larger than along the [010] direction. From measurements of the intensity distribution of diffuse x‐ray scattering a crystallographical damage in the silicon skeleton can be excluded. Regions of the porous layer near the interface to air are shown to be tensilely strained both along and perpendicular to the substrate normal.

Structural investigations of GaAs/AlAs quantum wires and quantum dots by X-ray reciprocal space mapping

We have investigated periodic arrays of dry etched 150 nm and 175 nm wide, (110) oriented GaAs/AlAs quantum wires and quantum dots by means of reciprocal-space mapping using triple-axis X-ray diffractometry. From the X-ray data the lateral periodicity of wires and dots, the etch depth and the angle of misorientation of the wires with respect to the (110) direction are extracted. The reciprocal-space maps reveal that, after the fabrication process the lattice constant along the growth direction increases slightly for the wires and even more so for the dots.

An investigation of X-ray reflectivity and diffraction from electroluminescent short-period Si-Ge superlattice structures

The structural properties of short-period molecular beam epitaxy grown p-i-n doped Si17Ge2 superlattices with ten, five and two periods were investigated by X-ray triple-axis diffractometry and X-ray reflectivity at glancing incidence. The X-ray analysis of these structures shows that they are pseudomorphic and yields information in particular on the interface roughness parameter evaluated in terms of the electron density profile which follows from the analysis of the X-ray reflectivity.

X‐ray reciprocal space mapping of GaAs/AlAs quantum wires and quantum dots

Periodic arrays of 150 and 175 nm‐wide GaAs–AlAs quantum wires and quantum dots were investigated, fabricated by electron beam lithography, and SiCl4/O2 reactive ion etching, by means of reciprocal space mapping using triple axis x‐ray diffractometry. From the x‐ray data the lateral periodicity of wires and dots, and the etch depth are extracted. The reciprocal space maps reveal that after the fabrication process the lattice constant along the growth direction slightly increases for the wires and even more so for the dots.