We report on the chemical, structural and morphological modifications introduced in nanothick Fe80B20 films by the use of Ar assistance (at different pressures) during their pulsed laser ablation deposition process. As a reference, a film deposited in absence of Ar and under ultra-high vacuum conditions was also prepared. Samples were characterized from the measurement of X-ray photoelectron spectroscopy depth profiles, atomic force microscopy, and transmission electron microscopy as well as from the standpoint of their dc angular dependence of the hysteresis, high applied field magnetization and tens of GHz dynamics. Our results include i) the induction of surface roughening that increases with the assisting gas pressure, ii) the amorphousness of the ultra-high vacuum film, iii) the induction, by the Ar assistance, of partial crystallization, yielding Fe-rich columns whose transverse dimension increased with the Ar pressure, iv) the progressive isotropization of the in-plane hysteresis parameters associated to the increase of Ar pressure, v) the occurrence on the samples deposited with the largest Ar pressures of spin waves stiffness constant having a magnitude 20 times larger than the value measured in the reference sample, and vi) the decrease of the ferromagnetic resonance damping parameter with regard to the reference sample.

The recent addition of Bi to GaAs has been demonstrated to be an effective method for reducing excess noise in avalanche photodiodes (APDs), owing to the significant decrease in the hole ionization coefficient. It has been proposed that the incorporation of group III elements in GaAsBi alloys, such as Al, could facilitate the development of a novel class of ultra-low noise APDs utilizing GaAs substrates. However, the structural attributes of these novel alloys have not yet been comprehensively investigated, and the inclusion of Al can potentially exacerbate the significant challenges associated with the integration of Bi into GaAs. The influence of different parameters in AlGaAsBi alloys, such as the Bi flux and III/As ratio, was examined using transmission scanning electron microscopy, Nomarsky microscopy, and photoluminescence techniques. In addition to analyzing the quality of the interfaces, particular attention has been devoted to the occurrence of volcano-shaped surface nanomotifs. Two distinct types of nano-motifs associated with Bi-Al and Ga-Al-Bi droplet formation during AlGaAsBi deposition under different conditions were characterized. The growth regimes that govern the formation of surface droplets on AlGaAsBi were significantly altered by the concomitant presence of Al.

Reverse biasing triple-junction GaInP/Ga(In)As/Ge solar cells may affect their performance by the formation of permanent shunts even if the reverse breakdown voltage is not reached. In previous works, it was observed that, amid the three components, GaInP subcells are more prone to degrade when reverse biased suffering permanent damage, although they present an initial good performance. The aim of this work is, firstly, to study the characteristics of the defects that cause the catastrophic failure of the devices. For this, GaInP isotype solar cells were analysed by visual inspection and electroluminescence maps and submitted to reverse bias stress test. We find that specific growth defects (i.e. hillocks), when covered with metal, cause the degradation in the cells. SEM cross-section imaging and EDX compositional analysis of these defects reveal their complex structures, which in essence consist of material abnormally grown on and around particles present on the wafer surface before growth. The reverse bias stress test is proposed as a screening method to spot defects hidden under the metal that may not be detected by conventional screening methods. By applying a quick reverse bias stress test, we can detect those defects that cause the degradation of devices at voltages below the breakdown voltage and that may also affect their long-term reliability.

The study investigated the impact of introducing bismuth into the GaAs capping layer (CL) on InAs quantum dots (QDs) to enhance their QD properties. Three different time-temperature routes (TTRs) were examined, as growth interruption (GI) stages are necessary due to the temperature requirements for the growth processes of QDs (510 degrees C) and GaAsBi CL (370 degrees C). Two of the TTRs revealed defective regions with bismuth-free nanotracks in the GaAsBi CL, which are linked to the formation of bismuth-rich droplets on the surface. Interestingly, in one of the TTRs, novel icosahedral-type nanoparticles appeared embedded at the first interface, leaving trails behind them. Upon detailed characterization, it was found that these nanoparticles consist of three distinct phases containing rhombohedral Bi, pure Ga, and a new In4Bi phase that had not been experimentally described before. The long particle trajectories and low temperatures suggest that the NPs remained liquid throughout the growth process, solidifying upon final cooling to room temperature. This work presents a new technique for incorporating plasmonic nanoparticle arrays made of non-noble metals into buried semiconductor layered interfaces, which offers greater flexibility in device design.

This paper investigates the effect of GaAsBi strain reduction layers (SRLs) on InAs QDs with different Bi fluxes to achieve nanostructures with improved temperature stability. The SRLs are grown at a lower temperature (370 °C) than the usual capping temperature for InAs QDs (510 °C). The study finds that GaAs capping at low temperatures reduces QD decomposition and leads to larger pyramidal dots but also increases the threading dislocation (TD) density. When adding Bi to the capping layer, a significant reduction in TD density is observed, but unexpected structural changes also occur. Increasing the Bi flux does not increase the Bi content but rather the layer thickness. The maximum Bi content for all layers is 2.4%. A higher Bi flux causes earlier Bi incorporation, along with the formation of an additional InGaAs layer above the GaAsBi layer due to In segregation from QD erosion. Additionally, the implementation of GaAsBi SRLs results in smaller dots due to enhanced QD decomposition, which is contrary to the expected function of an SRL. No droplets were detected on the surface of any sample, but we did observe regions of horizontal nanowires within the epilayers for the Bi-rich samples, indicating nanoparticle formation.

We present a quantitative analysis of Sb segregation in ultrathin GaAsSb films (1-20 ML) grown with soaking/ desorption methods. Sb soaking advances Sb incorporation almost to the beginning of the growth, increasing the compositional gradient for all thicknesses. Application of the desorption step alone increases the compositional gradient at the upper interface by removing a fixed amount of Sb but erases layers thinner than 10 ML. When both methods are combined, the Sb balance changes with layer thickness. Thinner layers lose Sb while thicker layers gain Sb. The amount of Sb desorbed is related to the floating layer content at the time of desorption, which varies with thickness. Segregation simulations show that the segregation energy changes during layer growth, being initially higher but stabilizing around the same value for all cases. Sb-soaked samples reach the segregation steady state at a distance of 5 ML from the origin, much earlier than non-soaked samples (10-15 ML). The effect of desorption produces a large perturbation of the segregation steady state, which is much greater in the desorption-only sample than when combined with soaking. The combined effect of soaking and desorption gives the best GaAsSb films with steeper interfaces and no delay in onset.

Type-II GaAsSb/GaAsN structures as 1eV bandgap materials for photovoltaics suffer from low carrier collection efficiency. To evaluate recombination and transport, in this work, a Mott-Schottky analysis and JV measurements under AM1.5G conditions are performed, both as a function of temperature (150K-330K). N-related unintentional doping is found to be thermally activated, and a short-circuit current decay with temperature is observed for GaAs(Sb)(N) materials, which is attributed to a decline in mobility. By means of drift-diffusion modelling, the N-related temperature dependence of mobility is estimated for the different structures.

The development of devices relying on spin phenomena requires of an ideal spin polarized electron source. This can be achieved by taking advantage of half-metallic full Heusler alloy thin films. However, their implementation requires a controlled growth of stoichiometric films with large activation volumes. In this work, we report on the growth of epitaxial Fe3Si ultra-thin films by pulsed laser deposition on SrTiO3(001) substrates, analyzing the effect of deposition temperature in the structural, morphological and magnetic properties of the deposited films. We conclude that optimal compromise between phase purity and interface quality is obtained at 200 ºC, obtaining the best magnetic response under this condition.

For optoelectronic devices from the near to the far infrared, the advantages of using ultrathin III-Sb layers as quantum wells or in superlattices are well known. However, these alloys suffer from severe surface segregation problems, so that the actual profiles are very different from the nominal ones. Here, by inserting AlAs markers within the structure, state-of-the-art transmission electron microscopy techniques were used to precisely monitor the incorporation/segregation of Sb in ultrathin GaAsSb films (from 1 to 20 monolayers (MLs)). Our rigorous analysis allows us to apply the most successful model for describing the segregation of III-Sb alloys (three-layer kinetic model) in an unprecedented way, limiting the number of parameters to be fitted. The simulation results show that the segregation energy is not constant throughout the growth (which is not considered in any segregation model) but has an exponential decay from 0.18 eV to converge asymptotically towards 0.05 eV. This explains why the Sb profiles follow a sigmoidal growth model curve with an initial lag in Sb incorporation of 5 MLs and would be consistent with a progressive change in surface reconstruction as the floating layer is enriched.

The effect of a Bi supply on the development of InAs/GaAs (001) QDs has been structurally explored in two growth temperature regimes, one for Bi alloying (380 degrees C) and another for conventional formation of InAs QDs (510 degrees C). At high growth temperature (HT), there is no trace of Bi incorporation into the film and Bi acts as a surfactant limiting the diffusion length of In. All HT QDs are coherent and free of defects with an inverted coneshaped In distribution. Increasing the Bi supply increases both the QD size and the In content (up to 40 %), while decreasing their areal density. At low temperature (LT), the addition of Bi goes beyond the surfactant role, promoting the transformation of the growth mode from 2D to 3D, resulting in an exponential increase in the QD size with Bi supply. Based on size, these LT QDs are divided into two populations in which the larger QDs relax plastically through the formation of misfit dislocations. LT QDs grown under a Bi flux reach a higher In content (80 %) with a homogeneous distribution within them. Remarkably, incorporation of Bi into larger QDs has been detected but it is not homogeneously distributed.

GaAsSb/GaAsN superlattices (SL) outperform GaAsSbN bulk material in solar cells but still suffer from material imperfections. This work studies the presence of unintentional doping in different structures by means of a Mott-Schottky analysis. The SL structure is found to reduce unintentional doping by 40% with respect to the bulk material, correlating with the increased performance.

Recently, thin AlAs capping layers (CLs) on InAs quantum dot solar cells (QDSCs) have been shown to yield better photovoltaic efficiency compared to traditional QDSCs. Although it has been proposed that this improvement is due to the suppression of the capture of photogenerated carriers through the wetting layer (WL) states by a de-wetting process, the mechanisms that operate during this process are not clear. In this work, a structural analysis of the WL characteristics in the AlAs/InAs QD system with different CL-thickness has been made by scanning transmission electron microscopy techniques. First, an exponential decline of the amount of InAs in the WL with the CL thickness increase has been found, far from a complete elimination of the WL. Instead, this reduction is linked to a higher shield effect against QD decomposition. Second, there is no compositional separation between the WL and CL, but rather single layer with a variable content of InAlGaAs. Both effects, the high intermixing and WL reduction cause a drastic change in electronic levels, with the CL making up of 1–2 monolayers being the most effective configuration to reduce the radiative-recombination and minimize the potential barriers for carrier transport.

The use of thin AlA capping layers (CLs) on InAs quantum dots (QDs) has recently received considerable attention due to improved photovoltaic performance in QD solar cells. However, there is little data on the structural changes that occur during capping and their relation to different growth conditions. In this work, we studied the effect of AlA capping growth rate (CGR) on the structural features of InAs QDs in terms of shape, size, density, and average content. As will be shown, there are notable differences in the characteristics of the QDs upon changing CGR. The Al distribution analysis in the CL around the QDs was revealed to be the key. On the one hand, for the lowest CGR, Al has a homogeneous distribution over the entire surface, but there is a large thickening of the CL on the sides of the QD. As a result, the QDs are lower, lenticular in shape, but richer in In. On the other hand, for the higher CGRs, Al accumulates preferentially around the QD but with a more uniform thickness, resulting in taller QDs, which progressively adopt a truncated pyramidal shape. Surprisingly, intermediate CGRs do not improve either of these behaviors, resulting in less enriched QDs.

Recently, very thin AlAs capping layers (CLs) have been proposed as a useful tool to increase the performance of InAs/GaAs quantum dot (QDs) devices. However, the structure of QDs after AlAs deposition remains poorly understood and the mechanisms to explain it are often contradictory. In this work, the structural and compositional changes of InAs QDs using different AlAs CL thicknesses have been studied by state-of-the-art STEMrelated techniques. First, the heights and In contents of InAs QDs progressively increase with the CL thickness, demonstrating that the AlAs capping produces a strong shielding effect against the decomposition of QDs. However, QD populations for CL thicknesses above 5 ML split into a bimodal distribution in which smaller lenticular QDs cohabit with bigger truncated pyramids. Second, the actual Al contents around the QDs are well below the nominal design, but increasing for thicker CLs. Its distribution is initially non-uniform, tending to accumulate on the flanks of the QDs to the detriment of the apex. Only for thicknesses above 2 ML the Al contents around the QDs start to be similar to those in the regions between the QDs, behaving as a continuous film without irregularities from 5 ML onwards.

Recently, GaAsSb/GaAsN type II short-period superlattices (SLs) have been proposed as suitable structures to be implemented in the optimal design of monolithic multi-junction solar cells. However, due to strong surface Sb segregation, experimental Sb composition profiles differ greatly from the nominal square-wave design. In this work, the improvement of the interface quality of these SLs in terms of compositional abruptness and surface roughness has been evaluated by implementing different growth interruption times under Sb4/As4 (soaking) and As4 (desorption) overpressure conditions before and after the growth of GaAsSb layers, respectively. The com-bined effects of both processes enhance Sb distribution, achieving squarer compositional profiles with reduced surface roughness interfaces. It has been found that the improvement in compositional abruptness is quantita-tively much higher at the lower interface, during soaking, than at the upper interface during desorption. Conversely, a larger decrease in surface roughness is achieved at the upper interface than at the lower interface. Fitting of the Sb segregation profiles using the 3-layer kinetic fluid model has shown that the increase in Sb incorporation rate is due to the decrease in segregation energy, presumably to changes in the surface recon-struction of the floating layer at the surface.

The implementation of GaAs0.8Sb0.2 as CL to obtain type-II strain-coupled InAs MQD structures has been examined and compared to similar structures without Sb or without strain coupling. First, it has been demonstrated that capping with GaAsSb prevents the formation of In-rich agglomerations that hampered the QD formation as it has been observed in the sample without Sb. Instead, it promotes the vertical alignment (VA) of almost all QDs with a high density of QD columns. Second, there is a preferential Sb accumulation over the dots together with an undulation of the growth front, contrary to the observed in the uncoupled structure. In case of a deficient covering of GaAsSb, as occurs for giant QDs, In-rich agglomerations may develop. Each VAQD column consists of a sequence of alternating quantum blocks of pyramid-shaped In(Ga)As separated by GaAsSb blocks that rest over them. These Sb-rich blocks are not homogeneous accumulating around the pyramidal apex like a collar. Between the columns, there is an impoverishment of In and Sb compared to the uncoupled sample. These columns can behave as self-aligned nanowires with type II band alignment between self-assembled InAs and GaAsSb quantum blocks that opens new opportunities for novel devices.

We demonstrate type-II GaAsSb/GaAsN superlattices (SL) as a suitable structure to form the lattice-matched 1.0-1.15 eV subcell that would allow the implementation of the optimum monolithic multi-junction solar cell design. The separation of Sb and N atoms during growth leads to an improved composition homogeneity and a lower defect density than in the bulk GaAsSbN counterparts. The type-II band alignment SLs provide long radiative lifetimes that facilitate carrier collection as compared to equivalent type-I SLs. Moreover, the radiative lifetime can be controllably tuned through the period thickness, which is not possible in type-I SLs. A reduced period thickness results in enhanced absorption due to increased wavefunction overlap, as well as in a change in the transport regime from diffusive to quasiballistic, providing improved carrier extraction efficiency. As a result, the short period SL single junction solar cells show an enhanced power conversion efficiency of 134% over the equivalent bulk devices.

The study explores phenomena that occur during the growth of multi-stacked quantum dots (MQD) InAs layers using thin GaAs spacers. An arrangement of plastically relaxed agglomerations that extend along the MQD structure with diameters of about 70-120 nm and separated every 200-400 nm are observed. These agglomerations hinder the regular development of vertically aligned QD columns, leaving only a regular density of QDs in the first layer. The generation of these agglomerations has been modelled based on two extreme cases: (i) conical-like, it presents a more contrasted base and its nucleation is related to the presence of two coalesced QDs. The progression of these agglomerations can be deactivated in the upper layers, probably to a decrease in surface stress as consequence of misfit dislocation formation in the lower layers; (ii) volcano-like, it presents a crater shape due to the collapse of the upper layers with a higher accumulation of In. It is proposed that the origin of this type of agglomerations is due to the formation of a quantum ring (QR) in the first layer. The initial concavity of the QR increases during successive GaAs/InAs deposition cycles as tendency of Ga to out-diffuse rises.

In this work we investigate the effect of rapid thermal annealing (RTA) on the performance of solar cells consisting of different GaAsSbN-based structures and correlate the device results with modifications of the optical and structural properties of the alloy. In particular, bulk layers grown at different growth rates and type-II GaAsSb/GaAsN superlattices with different period thickness are analyzed. We find evidences of material quality improvement after the annealing process such as a reduction of N-related radiative defects and Sb clusters. These RTA-induced changes lead to a notable enhancement of the open circuit voltage (V-OC), which results in values of the bandgap-voltage offset (W-OC = E-G/q-V-OC) comparable to that of a non-optimized reference GaAs solar cell with the same device structure (W-OC similar to 0.63 eV). The decrease in W-OC after annealing shows a correlation with the reduced radiative recombination at low energy N-related sub-bandgap states. These results suggest that radiative recombination in a broad band of deep defect states is a source of V-OC degradation in GaAsSbN solar cells.