T. Torunski

Anisotropy of the magnetotransport in (Ga,Mn)As/MnAs paramagnetic-ferromagnetic hybrid structures

We investigated the temperature-dependent magnetoresistance of granular (Ga,Mn)As/MnAs hybrids grown on (100) GaAs in different transport geometries. The observed magnetoresistance effects are much bigger than for a corresponding (Ga,Mn)As reference sample without MnAs nanoclusters. We find that the magnetoresistance effects depend strongly on the chosen transport geometry. When the external field is perpendicular to the sample plane the effects are largest. The smallest effects occur when the external field is in the sample plane and parallel to the current. Furthermore, we have established by ferromagnetic resonance studies that the magnetic properties of the ensemble of ferromagnetic MnAs nanoclusters is similar for the magnetic field orientations studied. Therefore, the observed anisotropy of the magnetoresistance mainly reflects the difference in current path through the sample which leads to a variation of the degree of interaction between the free carriers in the matrix and nanoclusters.

Detection of nanometer-sized strain fields in (GaIn)(NAs) alloys by specific dark field transmission electron microscopic imaging

The N-induced large strain fields in metal organic vapor phase epitaxy grown (GaIn)(NAs) quantum wells are imaged using dark field imaging in a transmission electron microscope with two different reflections, from which one is sensitive to the chemical composition and the other one to the strain in the material. By comparing the images of the (GaIn)(NAs) to those of ternary (GaIn)As alloys, which have identical macroscopic compressive strain as the quaternary alloys, as well as to those of ternary Ga(NAs) with identical N content than the quaternary alloys, it can be shown that by using the presented technique, one indeed images the N-induced strain fields in the material. The density of the strain fields increases with increasing N content to a critical value above which the crystal undergoes a morphological transition. From the density of the strain fields one could speculate that they might be originated by N-III-N next-nearest neighbors or by a N-induced N-III-N ordering with a longer chain length.

Columnar [001]-oriented nitrogen order in Ga(NAs) and (GaIn)(NAs) alloys

By calculations in the framework of the valence force field method, we show that nitrogen atoms in diluted GaAs1−xNx tend to align along the [001] direction. In quaternary alloys Ga1−yInyAs1−xNx this tendency is observed only in “as-grown” samples, while in the annealed samples nitrogen atoms build more energetically favorable bonds with indium. Experimentally observed inhomogeneous strain profiles in these material systems, as well as their dissolution upon annealing, agree qualitatively with results of the calculations.

Overgrowth experiments of ferromagnetic (MnGa)As-cluster layers by MOVPE

We present the results of the epitaxial overgrowth of magnetic (MnGa)As-cluster structures with GaAs, (AlGa)As and AlAs using metal organic vapor phase epitaxy (MOVPE). The structural differences in the overgrowth are investigated by means of atomic force microscopy (AFM) combined with transmission electron microscopy (TEM), in particular, to proof the successful overgrowth of the cluster layers with AlAs. Out of these experiments a first model for the overgrowth is developed. Measurements using a SQUID-magnetometer confirm the existence of ferromagnetism above room temperature in the cluster layers after overgrowth; however, other magnetic properties as the coercitive field are influenced by the overgrowth process.

Nanoanalytical quantification of the nitrogen content in Ga(NAs)∕GaAs by using transmission electron microscopy in combination with refined structure factor calculation

We have studied systematically the nitrogen content in Ga(NAs)∕GaAs quantum wells by (002) dark-field transmission electron microscopy (TEM). The nitrogen contents derived from this analysis, when assuming that all the atoms occupy their unperturbed positions in a virtual crystal, deviate significantly from the nitrogen contents we derive for the same samples by other methods; for example, high-resolution x-ray diffraction (XRD) and dynamical simulation of those XRD patterns. The nitrogen causes a significant local strain in the crystal and can accordingly displace the neighboring atoms dramatically. We show that, if the structure factor of the crystals is recalculated, taking these static displacements of the Ga atoms into account, the composition derived from the TEM analysis with that from XRD is in perfect agreement. It is hence necessary for tetragonally distorted crystals that have mixed sublattices containing atoms with different covalent radii to take these static displacements into account when q...

Direct structural evidence of the change in N-III bonding in (GaIn)(NAs) before and after thermal annealing

The blueshift of the fundamental energy gap of (GaIn)(NAs) upon thermal treatment is well established. However, the physical reason is still controversially discussed in literature. In the present paper we give direct structural evidence using transmission electron microscopy in combination with structure factor calculation that this blueshift—for the metal organic vapor phase epitaxy grown samples investigated here—results solely from a change in the local environment of nitrogen. N is bound to Ga upon growth and moves into an In-rich environment upon annealing to minimize the strain energy of the crystal. The technique presented here can be used to unambiguously determine the reason for the blueshift of differently grown and annealed dilute nitride materials.

Direct investigation of (sub-) surface preparation artifacts in GaAs based materials by FIB sectioning

The introduction of preparation artifacts is almost inevitable when producing samples for (scanning) transmission electron microscopy ((S)TEM). These artifacts can be divided in extrinsic artifacts like damage processes and intrinsic artifacts caused by the deviations from the volume strain state in thin elastically strained material systems. The reduction and estimation of those effects is of great importance for the quantitative analysis of (S)TEM images. Thus, optimized ion beam preparation conditions are investigated for high quality samples. Therefore, the surface topology is investigated directly with atomic force microscopy (AFM) on the actual TEM samples. Additionally, the sectioning of those samples by a focused ion beam (FIB) is used to investigate the damage depth profile directly in the TEM. The AFM measurements show good quantitative agreement of sample height modulation due to strain relaxation to finite elements simulations. Strong indications of (sub-) surface damage by ion beams are observed. Their influence on high angle annular dark field (HAADF) imaging is estimated with focus on thickness determination by absolute intensity methods. Data consolidation of AFM and TEM measurements reveals a 3.5nm surface amorphization, negligible surface roughness on the scale of angstroms and a sub-surface damage profile in the range of up to 8.0nm in crystalline gallium arsenide (GaAs) and GaAs-based ternary alloys. A correction scheme for thickness evaluation of absolute HAADF intensities is proposed and applied for GaAs based materials.

Microstructural analysis of Ga(NAs)/GaP heterostructures

We have investigated the microstructure of compressively strained Ga(NAs)/GaP quantum wells (QWs) with different N contents. This material system is a promising candidate for future integration of photonics on silicon substrates. N-induced microscopic strain fields are detected applying strain sensitive transmission electron microscopy dark-field (DF-TEM) imaging. Exceeding 7% of N concentration, we find a deterioration of the upper QW interface despite a reduction of the macroscopic strain for compositions with increasing N content. These nitrogen-induced structural characteristics of the ternary alloy are presumably correlated with the optical properties as observed by photoluminescence spectroscopy.

Correlation of the physical properties and the interface morphology of AlGaAs/GaAs heterostructures

We have investigated the influence of interface roughness in GaAs/AlGaAs heterostructures on both the optical and the electronic properties by systematically varying the two growth parameters substrate temperature and growth interruption. We prove that the optimization of samples for optics and transport, respectively, requires different growth parameters. Whereas the optical properties are exclusively determined by the roughness of the two quantum well interfaces, the transport properties are additionally influenced by the ionized impurity scattering. The number of impurities increases during growth interruption and, consequently, in contrast to the optical samples, an optimization of the growth parameters is not as straightforward since it depends on the background impurities originating from the ultrahigh vacuum system. A direct correlation with information obtained from atomic force microscopy images is therefore only possible for the optical properties. At growth temperatures higher than 620 °C in ad...

Annealing effects on the nanoscale indium and nitrogen distribution in Ga"NAs… and "GaIn…"NAs… quantum wells

III/V semiconductors containing dilute amounts of nitrogen are metastable and need to be thermally treated after growth to optimize optoelectronic properties. The influence of thermal annealing on the nitrogen depth profile in metal organic vapor phase epitaxygrown GaNAs/GaAs as well as GaInNAs/GaAs heterostructures is examined on a nanometer scale by combining several high resolution transmission electron microscopy techniques, also with Rutherford backscattering spectrometry. Annealing conditions, which are optimized for quaternary alloys with respect to photoluminescence intensity, do not result in element redistribution for the In containing material. Contrary to the quaternary material, the result of annealing the ternary GaNAs is a pronounced pileup of the nitrogen profile without any out diffusion of nitrogen. These findings have important influence on device structures, which often contain GaNAs barriers for strain-compensation purposes together with GaInNAs active regions. In the light of metastability considerations for the ternary and quaternary alloy, one can conclude that the In contained in the quaternary material stabilizes the material and suppresses phase separation. Consequently GaInNAs is more stable than its ternary counterpart GaNAs .© 2007 American Institute of Physics. DOI: 10.1063/1.2794739