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.

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.

A novel method is presented which allows to expose and investigate interior interfaces in the Ga(NAsP)/GaP material system by a combination of highly selective etching and subsequent atomic force microscopy. We demonstrate the selectivity of the chemical etchant and prove that structural information and atomic-scale z-resolution is fully preserved when applying the method. A correlation between metal organic vapour phase epitaxy growth parameters and interior interface morphology is confirmed by subjecting compressively strained and lattice matched Ga(NAsP) material to a growth interruption. The interior interfaces of both materials smoothen during growth interruption, and for the compressively strained Ga(NAsP) we obtain a monolayer-structured morphology. The optimization of this material is very interesting from an application point of view as it has shown electrical injection lasing near room temperature and might be applied for optoelectronic integrated circuits based on Si in the future.

GaInP/GaAs/Ge triple-junction cells became commercially available in the 90ies by US manufacturers. Today these triple cells are the most common power generator in space due to their high efficiency and radiation hardness. In Europe, AZUR SPACE and the Fraunhofer ISE have teamed up in the development and industrialisation of III-V multi-junction solar cells. The 2 generation of a European III-V multi-junction cell, the AZUR 3G-ID2* 28% class, has been qualified and is now in production. It shows a state-of-the-art BOL efficiency of 28% (AM0, 1367 W/m, T=28°C) and has a very radiation hard design with a remaining factor of 88% (10 1MeV electrons/cm) resulting in an excellent EOL efficiency of 24.6%. The next generation 30%-class cell will also be based on the lattice-matched GaInP/GaInAs/Ge triple cell concept. For future generations of space cells various concepts are under investigation, such as 4J, 5J and 6J cells as well as cell concepts based on metamorphic growth. Additionally a terrestrial market for III-V multi-junction concentrator cells is emerging. AZUR SPACE is presently offering a 35% efficient lattice-matched triple-junction concentrator cell (3C-35%). This rising interest in concentrators promises exciting times for III-V and a renewed synergy between space and terrestrial.

The dilute nitride (GaIn)(NAs) material system grown lattice matched to GaAs or Ge with a 1eV band gap is an interesting material for the use in four-junction solar cells with increased efficiencies. As a result of its metastability, several challenges exist for this material system, which up to now limits the device performance. We performed nanostructural analysis in combination with photoluminescence characterization to optimize the metal organic vapor phase growth as well as the annealing conditions for the quaternary solar cell material. The optimum annealing conditions depend strongly on the In content of the quaternary material. Valence force field calculations of stable N environments in the alloy support the model that the N moves from a Ga rich environment realized during growth into an In rich environment upon annealing. Simultaneously, N induced strain fluctuations, which are detected in the N containing material upon growth, are dissolved and the device properties are improved.

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 Ga(NAs)∕GaAs as well as (GaIn)(NAs)∕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 Ga(NAs) 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 Ga(NAs) barriers for strain-compensation purposes together with (GaIn)(NAs) 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 (GaIn)(NAs) is more stable than its ternary counterpart Ga(NAs).

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.

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 quantification of the composition from dark-field or high-resolution TEM images is aimed for.

Realizing monolithic optoelectronic integrated circuits (OIECs) on silicon substrate would open up an exciting and completely new field of applications, i.e. optical interconnects at the chip level. In the past a lot of effort has been devoted to the growth of standard direct band gap III-V compound semiconductors on Si substrate, i.e. GaAs/Si or InP/Si. Due to the large lattice mismatch of these materials to the Si substrate large densities of threading dislocations are formed in the layers, preventing any long-term stable lasing operation of corresponding device structures. In this study the authors present a novel direct band gap material ( Ga(NAsP) ), which can be grown lattice-matched to GaP. Due to the similar lattice constant of GaP and Si, this novel material system might lead to the real monolithic integration of III/V-based optoelectronics and Si-based microelectronics in the near future

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.

We realized that the contribution from bond bending to the strain energy functional Estrain Ri in Eq. 1 of our letter was overestimated by a factor of two. Although it did not affect conclusions of the letter, we found it necessary to correct the corresponding values of the strain energy in Table II. The authors apologize for possible confusion to the readers that may have arisen from such an error.

Highly compressively strained (GaIn)(NP) quantum wells have been grown on (1 0 0) GaP substrates by metal organic vapour-phase epitaxy (MOVPE). We achieve a high structural quality of the grown multiple quantum well structures for this novel, metastable material system. Competition between the group-V elements on the surface determines the N incorporation in Ga(NP) as well as (GaIn)(NP). For the ternary material system Ga(NP) the N content of the deposited layers does not depend on the growth rate in contrast to the quaternary system (GaIn)(NP), where the N incorporation is enhanced with increasing growth rate. This suggests a desorption controlled N incorporation process for the In-containing material. This is in accordance with the strong dependence of the N content in the material from the growth temperature and the In content. In addition — due to the metastability of the material systems under investigation — the maximal achievable N content in Ga(NP) and (GaIn)(NP) is limited when good crystal quality is to be retained.

We have grown (GaIn)(NAs) lattice-matched bulk as well as compressively strained multi-quantum-well structures by metal-organic vapour-phase epitaxy (MOVPE) suitable for either solar cell or laser applications, respectively. By applying a specific novel TEM dark-field technique columnar strain fields, which are possibly caused by chain-like N ordering in the samples, have been detected. Valence force field calculations show that indeed these chains are energetically stable in Ga(NAs). This chain-like ordering can be dissolved in (GaIn)(NAs), however, upon appropriate annealing, as verified experimentally. On the other hand we find that device performance especially of lasers is limited by carbon impurities in the active (GaIn)(NAs) region of the lasers. The strong affinity of N–C results in an enhanced incorporation of C if the N content in the material is increased. The paper also shows the sources of C incorporation in (GaIn)(NAs) MOVPE growth and how its incorporation can possibly be avoided.

We have investigated surface morphologies and cluster formation in Mn-incorporated (GaIn)As layers grown by metal-organic vapor phase epitaxy (MOVPE) on InP (1 0 0) substrates. Whisker growth occurs on the layer surfaces under low V/III ratios and low growth temperature conditions. For temperatures above 500°C, MnAs-based cluster structures are formed near the (GaIn)As layer surfaces. The MnAs-based clusters show an in-plane anisotropy in the magnetic characteristics. During the overgrowth of the MnAs-based clusters by undoped InP, a change in cluster composition to MnP presumably occurs as indicated by the change in the observed Curie temperature.

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.

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.