Thin Ga Research Articles

Thin Ga(Sb,P)/GaP quantum wells with indirect band gap: Crystal structure, energy spectrum, exciton recombination and spin dynamics

Heterostructures with thin GaSb layers embedded in a GaP matrix are studied by transmission electron microscopy as well as steady-state and transient photoluminescence. For a single, one monolayer thick deposition with subsequent overgrowth, nonmonotonic Sb segregation results in self-organized Ga(Sb,P) double-quantum well (QW) formation. One QW is positioned deep in GaP at the designated place according to the heterostructure design, and the other QW is in the near surface region after 40 monolayers of GaP overgrowth. Both QWs are characterized by an indirect band gap. The band alignment in the QWs is identified as type I. In spite of the long (up to milliseconds) exciton lifetimes in the QWs in combination with the electron g factor of +2, the emission of the QWs demonstrates a low circular polarization degree in longitudinal magnetic fields as strong as 10 T. We demonstrate that the electron spin polarization in the Ga(Sb,P)/GaP heterostructure subject to a magnetic field occurs in the GaP layer, and spin-polarized charge carriers captured in the QWs store their spin orientation up to the millisecond time range, indicating very long spin relaxation times for the strongly localized electrons in the Ga(Sb,P)/GaP QWs.

Facile sulfur-assisted synthesis of GaAs nanowires /si heterojunctions for broadband self-powered photodetector

Nanowires (NWs)-based heterojunction photodetectors (PDs) have been widely applied in the fields of remote sensing, night vision, environmental monitoring, and optical communications with high light absorption coefficient and photoelectric conversion efficiency. Here, S-GaAs NWs/Si heterojunction was developed to form a broadband self-powered PDs by growing GaAs NWs on Si substrate through the sulfur-catalyzed assisted chemical vapor deposition (CVD) technology. Compared with the pure GaAs NWs, the sulfur-assisted catalysts could not only inhibit Ostwald ripening of Au seeds to improve the crystalline quality of NWs but also form a thin Ga–S bond passivation layer to reduce surface defects and enhance the optoelectronic performance of the PDs. The fabricated GaAs NWs/Si heterojunction PDs exhibit excellent performances with the large responsivity of 10.3 mA W−1 and high detectivity of 9.59 × 1012 cm Hz1/2 W−1. Additionally, the designed PDs have the broadband wavelength photoresponse from visible to near infrared and fast response speed (rise/fall time of 3.8/21.8 ms). Importantly, the develop strategy of hybrid heterojunction PDs can be extended to other semiconductor materials for facilitating the practical application of high-performance optoelectronic devices.

Effect of pulsed laser annealing on the properties of (Ga,Mn)As layers

• Ferromagnetic (Ga,Mn)As layers were obtained by pulsed laser deposition and annealing. • Results of X-ray diffraction, TEM and the magnetization study are presented. • Decomposition of MnAs inclusions is observed after laser annealing of thin (Ga,Mn)As layer. • The Curie temperature increased to 105–110 K in the annealed thin (Ga,Mn)As layer. We have investigated the layers of the (Ga,Mn)As diluted magnetic semiconductor with the thickness in the range of 150 – 400 nm, which have been formed by the combination of the methods of a pulsed laser deposition and a post-growth pulsed laser annealing. It has been found that the laser annealing has a significant effect on the structural, magnetic, and electrical properties of the thinnest (∼150 nm) (Ga,Mn)As layer. For this layer the additional activation of the Mn impurity, the Curie temperature increase and the dissolution of the second phase MnAs clusters have been observed. The results of model calculations of heat spread in (Ga,Mn)As layer have shown that in thicker layers a MnAs melting point temperature is not achievable (for the chosen conditions of pulsed laser annealing) in the entire layer.

Redescription of Neocypridella fossulata (Daday, 1910) (Crustacea: Ostracoda) and description of a new subfamily.

Neocypridella fossulata (Daday, 1910) is redescribed based on type material from the Daday collection (Hungarian Natural History Museum, Budapest). The major diagnostic characters of the genus are the ovate shape of the carapace in lateral view, with the left valve overlapping the right valve, the absence of marginal septa on the valve margins, the right valve anterior selvage not inwardly displaced, the presence of a Wouters organ on the first antenna, the divided penultimate segment of the second antenna, the markedly large -seta on the mandibular palp, the subquadrate terminal segment of the maxillula palp, the divided penultimate segment of the second thoracopod with setae d1 and d2 both long, the slender caudal ramus and the slender caudal ramus attachment with thin dorsal and ventral branches. Neocypridella fossulata can be further characterized by the oblique carapace in frontal view, the two smooth bristles on the maxillula third endite and the thin and long Ga claw of the caudal ramus, which is c. 3/5 of the length of the ramus. The morphology of Neocypridella is compared with related taxa and the taxonomic position of the genus is discussed. This revealed that Neocypridella cannot be assigned to any of the extant subfamilies in the Cyprididae. Therefore, a new subfamily, Neocypridellinae subfam. nov., is erected.

Recombination and spin dynamics of excitons in thin (Ga,Al)(Sb,As)/AlAs quantum wells with an indirect band gap and type-I band alignment

The dynamics of exciton recombination and spin relaxation in thin (Ga,Al)(Sb,As)/AlAs quantum wells (QWs) with indirect band gap are studied. The band alignment in these QWs is identified as type I. The exciton recombination time exceeds hundreds of microseconds, while the spin relaxation times of the electron and the heavy hole in an exciton do not exceed hundreds of nanoseconds. The heavy-hole longitudinal $g$ factor is determined to be $+2.5$. Despite the long exciton lifetimes, the photoluminescence circular polarization degree induced by a magnetic field is unexpectedly small and does not exceed $25%$.

Ultrasonic cavitation at liquid/solid interface in a thin Ga–In liquid layer with free surface

Cavitation in thin layer of liquid metal has potential applications in chemical reaction, soldering, extraction, and therapeutic equipment. In this work, the cavitation characteristics and acoustic pressure of a thin liquid Ga–In alloy were studied by high speed photography, numerical simulation, and bubble dynamics calculation. A self-made ultrasonic system with a TC4 sonotrode, was operated at a frequency of 20 kHz and a max output power of 1000 W during the cavitation recording experiment. The pressure field characteristic inside the thin liquid layer and its influence on the intensity, types, dimensions, and life cycles of cavitation bubbles and on the cavitation evolution process against experimental parameters were systematically studied. The results showed that acoustic pressure inside the thin liquid layer presented alternating positive and negative characteristics within 1 acoustic period (T). Cavitation bubbles nucleated and grew during the negative-pressure stage and shrank and collapsed during the positive-pressure stage. A high bubble growth speed of 16.8 m/s was obtained and evidenced by bubble dynamics calculation. The maximum absolute pressure was obtained at the bottom of the thin liquid layer and resulted in the strongest cavitation. Cavitation was divided into violent and weak stages. The violent cavitation stage lasted several hundreds of acoustic periods and had higher bubble intensity than the weak cavitation stage. Cavitation cloud preferentially appeared during the violent cavitation stage and had a life of several acoustic periods. Tiny cavitation bubbles with life cycles shorter than 1 T dominated the cavitation field. High cavitation intensities were observed at high ultrasonication power and when Q235B alloy was used because such conditions lead to high amplitudes on the substrate and further high acoustic pressure inside the liquid.

Ultrafast Modulation of Magnetization Dynamics in Ferromagnetic (Ga, Mn)As Thin Films

Magnetization precession induced by linearly polarized optical excitation in ferromagnetic (Ga,Mn)As was studied by time-resolved magneto-optical Kerr effect measurements. The superposition of thermal and non-thermal effects arising from the laser pulses complicates the analysis of magnetization precession in terms of magnetic anisotropy fields. To obtain insight into these processes, we investigated compressively-strained thin (Ga,Mn)As films using ultrafast optical excitation above the band gap as a function of pulse intensity. Data analyses with the gyromagnetic calculation based on Landau-Lifshitz-Gilbert equation combined with two different magneto-optical effects shows the non-equivalent effects of in-plane and out-of-plane magnetic anisotropy fields on both the amplitude and the frequency of magnetization precession, thus providing a handle for separating the effects of non-thermal and thermal processes in this context. Our results show that the effect of photo-generated carriers on magnetic anisotropy constitutes a particularly effective mechanism for controlling both the frequency and amplitude of magnetization precession, thus suggesting the possibility of non-thermal manipulation of spin dynamics through pulsed laser excitations.

Photolytic Water Splitting By Surface-Conditioned n-Gallium Phosphide(100) Photoanodes

The production of hydrogen via photoelectrochemical (PEC) water splitting using III-V semiconductors as photoelectrodes is a field of current research and challenges in materials science. Under PEC conditions relevant to water splitting, III-V semiconductors are prone to corrosion and suffer from corrosion-related decrease of efficiency, which so far impedes long-term usage of III-V semiconductor-based photoelectrodes. Gallium phosphide (GaP) has an indirect band gap of 2.26 eV which covers both the hydrogen evolution potential (HEP) and the oxygen evolution potential (OEP). Thus, in principal, GaP can be used both as photocathode and photoanode. Notwithstanding the favorable band gap energy, the use of GaP photoelectrodes for the photolysis of water has so far not been successful without applying an additional bias potential. In this contribution we demonstrate that, by specific surface conditioning, we are able to control and optimize the photoelectrochemical surface properties of photoanodes prepared from n-doped GaP(100) for PEC water splitting. We have studied the structural and chemical surface modifications of such photoanodes before and after extended PEC processes by scanning electron microscopy (SEM), atomic force microscopy (AFM), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). An approximately (2±1) nm thin oxide film is produced at the surface of n-GaP(100) via oxidizing the surface at 0.8 V vs RHE (reversible hydrogen electrode) and subsequent hydrogen evolution at very cathodic potentials. The latter step is crucial for a substantial reduction of the potential for water splitting, presumably because the evolved hydrogen passivates electrically active defects in the oxide film. After appropriate surface conditioning, photolytic water splitting was observed without applied voltage or addition of catalysts, as confirmed by detection of hydrogen gas evolving at the Pt counter cathode with a gas spectrometer. The n-GaP(100) photoanode was exposed to high-intensity illumination (120 mW/cm2) at 0 V vs RHE in 0.02 M HCl electrolyte for a duration of 3 days while yielding stable photocurrents. Ex-situ surface characterization of the n-GaP(100) photoanodes was carried out after extended PEC water splitting. AFM revealed a very flat surface, and no signs of corrosion were observed even after extended periods of water splitting. AES depth profiles showed that phosphorous was depleted near the surface of the n-GaP(100) photoanode. Using AES spectra, the thickness of the oxide film was estimated. XPS measurements revealed that the surface oxide consists mainly of Ga2O3, while small concentrations of GaPO4, P2O5 and also metal-like Ga are also present at the topmost surface layers of the n-GaP(100) photoanode. Electrochemical impedance spectroscopy (EIS) was carried out to understand the charge transfer processes across the semiconductor/electrolyte interface. Our EIS results suggest that defect states in the surface oxide and metal-like Ga at the very surface in contact with the electrolyte play crucial roles in the effective charge transfer across the semiconductor-electrolyte interface. Our surface conditioning processes lead to the formation of a nonporous, thin and stable Ga surface oxide which inhibits PEC surface corrosion and, at the same time, subserves the water splitting process.

Magneto-optical Kerr spectroscopy of (Ga,Mn)(As,P) ferromagnetic layers: Experiments and k.p theory

We present a theoretical and experimental study of the magneto-optical properties of thin (Ga,Mn)(As,P) layers on GaAs substrates with varying phosphorus fractions. Using a 40-band k.p model as well as an antiferromagnetic (s, p)-d exchange interaction, we calculate the dielectric tensor, the Kerr rotation angle, and the ellipticity in the interband transition region. Our calculations are validated through a set of accurate comparisons with experimental results. The Kerr ellipticity peak is found to be 2 to 3 times larger than the Kerr rotation angle both experimentally and theoretically. This work will enable to optimize the magneto-optical effects in these layers for maximum sensitivity in ultra-fast magnetization dynamics and domain microscopy experiments.

Proton and deuteron induced reactions on natGa: Experimental and calculated excitation functions

Cross-sections for reactions on natGa, induced by protons (up to 65MeV) and deuterons (up to 50MeV), producing γ-emitting radionuclides with half-lives longer than 1h were measured in a stacked-foil irradiation using thin Ga–Ni alloy (70–30%) targets electroplated on Cu or Au backings. Excitation functions for generation of 68,69Ge, 66,67,68,72Ga and 65,69mZn on natGa are discussed, relative to the monitor reactions natAl(d,x)24,22Na, natAl(p,x)24,22Na, natCu(p,x)62Zn and natNi(p,x)57Ni. The results are compared to our earlier measurements, the scarce literature values and to the results of the code TALYS 1.6 (online database TENDL-2014).

Electronic Coupling in Nanoscale InAs/GaAs Quantum Dot Pairs Separated by a Thin Ga(Al)As Spacer.

The electronic coupling in vertically aligned InAs/GaAs quantum dot (QD) pairs is investigated by photoluminescence (PL) measurements. A thin Al0.5Ga0.5As barrier greatly changes the energy transfer process and the optical performance of the QD pairs. As a result, the QD PL intensity ratio shows different dependence on the intensity and wavelength of the excitation laser. Time-resolved PL measurements give a carrier tunneling time of 380 ps from the seed layer QDs to the top layer QDs while it elongates to 780 ps after inserting the thin Al0.5Ga0.5As barrier. These results provide useful information for fabrication and investigation of artificial QD molecules for implementing quantum computation applications.

Double-island Coulomb blockade in (Ga,Mn)As nanoconstrictions

We report on a systematic study of the Coulomb blockade effects in nanofabricated narrow constrictions in thin (Ga,Mn)As films. Different low-temperature transport regimes have been observed for decreasing constriction sizes: the ohmic, the single electron tunnelling (SET) and a completely insulating regime. In the SET, complex stability diagrams with nested Coulomb diamonds and anomalous conductance suppression in the vicinity of charge degeneracy points have been observed. We rationalize these observations in the SET with a double ferromagnetic island model coupled to ferromagnetic leads. Its transport characteristics are analyzed in terms of a modified orthodox theory of Coulomb blockade which takes into account the energy dependence of the density of states in the metallic islands.

Scanning transmission electron microscopy measurement of bismuth segregation in thin Ga(As,Bi) layers grown by molecular beam epitaxy

Annular dark‐field (ADF) imaging in a scanning transmission electron microscope (STEM) has been used to measure concentration profiles across thin Ga(As,Bi) layers grown by molecular beam epitaxy, from which the segregation lengths for bismuth surface segregation have been calculated. Performing this for layers grown at two different temperatures, the activation energies for Bi surface segregation have been determined for the lower (GaAsBi‐on‐GaAs) and for the upper (GaAs‐on‐GaAsBi) interface. The inequivalence observed is attributed to strain driving the larger Bi atoms preferably towards the free surface during growth.

Electric-field effects on magnetic properties of molecular beam epitaxially grown thin (Ga,Mn)Sb layers

We report molecular beam epitaxy of a ferromagnetic semiconductor (Ga,Mn)Sb, which is a single crystal without detectable second phases. We report also the details of magnetotransport properties of (Ga,Mn)Sb and the effects of electric fields on them. The difference between the properties observed here and properties of those reported earlier for a ferromagnetic semiconductor (Ga,Mn)As, provides information critical for further understanding of fundamental and device physics of ferromagnetic semiconductors.

Hole concentration dependence of the Curie temperature of (Ga,Mn)Sb in a field-effect structure

We investigate magnetotransport properties of thin (Ga,Mn)Sb layers in a field-effect structure. By changing the hole concentration p in the channel by applied electric fields, we establish the relationship between the Curie temperature TC and p, which shows γ of 1.3–1.6 in TC ∝ pγ. The exponent γ is several times larger than γ ∼ 0.2 reported previously for (Ga,Mn)As. Analyses based on the p-d Zener model taking into account of non-uniform hole distribution in the channel shows that the lager γ is explained by the presence of hole accumulation at the interface of (Ga,Mn)Sb and the gate insulator.

The structural and magnetic properties of Fe/(Ga, Mn)As heterostructures

Fe/(Ga, Mn)As heterostructures were fabricated by all molecular-beam epitaxy. Double-crystal X-ray diffraction and high-resolution cross-sectional transmission electron micrographs show that the Fe layer has a well ordered crystal orientation and an abrupt interface. The different magnetic behavior between the Fe layer and (Ga, Mn)As layer is observed by superconducting quantum interference device magnetometry. X-ray photoelectron spectroscopy measurements indicate no Fe2As and Fe—Ga—As compounds, i.e., no dead magnetic layer at the interface, which strongly affects the magnetic proximity and the polarization of the Mn ion in a thin (Ga, Mn)As region near the interface of the Fe/(Ga, Mn)As heterostructure.

Domain-wall controlled (Ga,Mn)As nanostructures for spintronic applications

Valence-band structure and magnetic properties, especially magnetic anisotropies, in the (Ga,Mn)As diluted ferromagnetic semiconductor are shortly discussed. Next, magneto-resistive, cross-like nanostructures fabricated by electron-beam lithography patterning and chemical etching from thin (Ga,Mn)As epitaxial layers are described. The nanostructures, composed of two perpendicular nanostripes crossing in the middle of their length, represent four-terminal devices, in which an electric current can be driven through any of the two nanostripes. In these devices, which make use of the patterning-induced magnetic anisotropy, a novel magneto-resistive memory effect related to a rearrangement of magnetic domain walls in the central part of the device, has been demonstrated. The effect consists in that the zero-field resistance of a nanostripe depends on the direction of previously applied magnetic field. The nanostructures can thus work as two-state devices providing basic elements of nonvolatile memory cells.

Solar Hydrogen Generation with Wide‐Band‐Gap Semiconductors: GaP(100) Photoelectrodes and Surface Modification

GaP, with its large band gap of 2.26 eV (indirect) and 2.78 eV (direct), is a very promising candidate for direct photoelectrochemical water splitting. Herein, p-GaP(100) is investigated as a photocathode for hydrogen generation. The samples are characterized after each preparation step regarding how their photoelectrochemical behavior is influenced by surface composition and structure using a combination of electrochemical and surface-science preparation and characterization techniques. The formation of an Ohmic back contact employing an annealed gold layer and the removal of the native oxides using various etchants are studied. It turns out that the latter has a pronounced effect on the surface composition and structure and therefore also on the electronic properties of the interface. The formation of a thin Ga(2)O(3) buffer layer on the p-GaP(100) surface does not lead to a clear improvement in the photoelectrochemical efficiency, neither do Pt nanocatalyst particles deposited on top of the buffer layer. This behavior can be understood by the electronic structure of these layers, which is not well suited for an efficient charge transfer from the absorber to the electrolyte. First experiments show that the efficiency can be considerably improved by employing a thin GaN layer as a buffer layer on top of the p-GaP(100) surface.

Diffusion of Mn interstitials in (Ga,Mn)As epitaxial layers

Magnetic properties of thin (Ga,Mn)As layers improve during annealing by out-diffusion of interstitial Mn ions to a free surface. Out-diffused Mn atoms participate in the growth of a Mn-rich surface layer and a saturation of this layer causes an inhibition of the out-diffusion. We combine high-resolution x-ray diffraction with x-ray absorption spectroscopy and a numerical solution of the diffusion problem for the study of the out-diffusion of Mn interstitials during a sequence of annealing steps. Our data demonstrate that the out-diffusion of the interstitials is substantially affected by the internal electric field caused by an inhomogeneous distribution of charges in the (Ga,Mn)As layer.

Anomalous Hall Effect in Field-Effect Structures of (Ga,Mn)As

The anomalous Hall effect in metal-insulator-semiconductor structures having thin (Ga,Mn)As layers as a channel has been studied in a wide range of Mn and hole densities changed by the gate electric field. Strong and unanticipated temperature dependence, including a change of sign, of the anomalous Hall conductance sigma(xy) has been found in samples with the highest Curie temperatures. For more disordered channels, the scaling relation between sigma(xy) and sigma(xx), similar to the one observed previously for thicker samples, is recovered.