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• Al 0.9 Ga 0.1 N alloys grown by PA-MBE show clear evidence of phase segregation in XRD. • XRD peak shows splitting: Peak separation changes with varying growth parameters. • Weighted averages of XRD peaks remain the same: Overall composition is unchanged. • Long-range atomic ordering is observed in alloys with minimal phase segregation. • Route to efficient deep-UV emitters through carrier localization at bandgap minima. AlGaN alloys with very high AlN mole fraction (∼90 %) were grown by plasma-assisted molecular beam epitaxy (PA-MBE) and their alloy properties were investigated by high-resolution X-ray diffraction (HR-XRD) measurements. Growth was carried out employing a series of group III/V ratios and for samples grown under a high group-III regime, phase-segregation in the alloy was evident from a characteristic splitting of the AlGaN (0002) peak in the HR-XRD pattern. With decreasing excess group-III conditions the separation of peak positions continuously reduced, till a single peak was observed. However, for samples where such splitting was absent, spontaneous superlattice peaks were seen at lower incident angles of the XRD patterns indicating the presence of long-range atomic ordering (LRAO). Thus, for AlGaN alloys with extremely high Al content, effects of phase-segregation, or of LRAO were observed, depending on the kinetics of growth. These results on phase-segregation effects are expected to promote the development of high-efficiency deep ultraviolet emitters for skin-safe germicidal action by mitigating the detrimental effect of dislocations and related non-radiative recombination centers through carrier localization processes at potential minima. [ABSTRACT FROM AUTHOR]

Highlights • Epitaxy of high Bi content GaSbBi alloys investigated by combinatorial MBE. • Epitaxial phases with distinct structural and Bi incorporation characteristics established. • Phases are mapped by Sb/Ga ratio and growth temperature over broad parameter space. • Phase with optimal structural quality is suppressed by increase of Bi/Ga ratio. Abstract GaSbBi alloys have recently emerged as attractive materials for mid-infrared optoelectronics owing to strong band gap reduction enabled by Bi incorporation into the GaSb matrix. The fundamental understanding of the epitaxial process required to demonstrate high quality crystals is in an early-developmental phase. From this perspective, we report on the key role played by the Sb/Ga flux ratio in controlling the structural quality and incorporation of high Bi content GaSbBi (up to 14.5%-Bi), revealing three distinct epitaxial phases. The first phase (below stoichiometric Sb/Ga) exhibits Ga-Bi compound droplets, low crystal quality, and reduced Bi content. At the second phase (above stoichiometric Sb/Ga), the crystal exhibits smooth surfaces and excellent crystallinity with efficient Bi incorporation. The last phase corresponds to exceeding a Sb/Ga threshold that leads to reduced Bi incorporation, Bi droplets and degraded crystallinity. This threshold value that defines the optimal growth window is controlled by the temperature as well as the Bi/Ga ratio. Increasing temperature increases the threshold, albeit simultaneously reducing Bi incorporation. Conversely, increasing the Bi/Ga flux ratio increases Bi incorporation, while narrowing down and ultimately closing the window. This study provides a general framework enabling development of high quality GaSbBi heterostructures for emerging mid-infrared optoelectronics. [ABSTRACT FROM AUTHOR]

Highlights • The multi-junction solar cells were grown by MBE. • A higher growth temperature is good for GaInP and InGaAsP. • Multi-junction solar cells were fabricated by wafer bonding technique. • The optimiztion process of four-junction solar cell was discussed. Abstract We report on the room-temperature wafer-bonded InGaP/GaAs//InGaAsP/InGaAs four-junction solar cell grown by all solid-source molecular beam epitaxy (MBE). The material growth and structure optimization of multi-junction solar cell were investigated. The effect of growth temperature on phosphide-related material of the solar cell was discussed. The lattice-matched InGaP/GaAs tandem cell on GaAs substrate and InGaAsP- or InGaAsP/InGaAs-tandem cell on InP substrate were grown separately by MBE. Room-temperature wafer bonding technique was used to integrate the subcells into multi-junction solar cells. The current limiting of multi-junction cells was analysed and the device structure of four-junction solar cell was optimized. The best wafer-bonded four-junction solar cell reached a conversion efficiency of 42% under concentration. [ABSTRACT FROM AUTHOR]

Highlights • A fast algorithm for the study of reflectance anisotropy (RA) spectra is developed. • Detection of two fundamental components of RA spectra during GaAs homoepitaxy. • Application of RA spectroscopy to surface stoichiometry prior to homoepitaxy of GaAs. Abstract We report on a computer algorithm for the in situ analysis of reflectance anisotropy (RA) spectra in a time frame compatible with the epitaxial growth of cubic semiconductors. This algorithm allows for the in situ acquisition of RA spectra and their decomposition into two components whose amplitude depends on the As coverage of the semiconductor surface. One of such components is associated with the surface orthorhombic strain due to the surface reconstruction and has an amplitude that strongly depends with surface reconstruction and thus As coverage. This fact opens the possibility of using reflectance anisotropy spectroscopy (RAS) as an optical probe to characterize the As surface coverage in real time. To demonstrate the performance of the algorithm we report on RA measurements carried out during the homoepitaxial growth of GaAs (0 0 1). We show that the algorithm is capable of analyzing a set of 500 RA spectra in a time span of about 10 s. This allows for a range of applications for the developed algorithm, including the surface characterization and fine tuning of the substrate stoichiometry just before epitaxial growth, during the growth of the buffer layer. [ABSTRACT FROM AUTHOR]

Highlights • Nanocolumn selective-area growth was achieved by nanosphere lithography and etching. • The shadow effect and Ti growth suppression contribute to selective-area growth. • InGaN sections selective-area grown could be beneficial to uniform In distribution. Abstract In this paper, the selective-area growth (SAG) of GaN-based nanocolumns (NCs) on silicon (Si) has been investigated experimentally. A low-cost and facile method has been proposed and implemented to fabricate the SAG NCs, which contains the processes of nanosphere lithography (NSL) and plasma-assisted molecular beam epitaxy (MBE). SAG NCs grown by this method indicates a higher ability to modulate the NC distributions than those grown by the conventional patterned method. According to photoluminescence results, InGaN/GaN NCs grown on Si nanopillars produced by NSL and etching show the more uniform indium (In) distribution than those grown on the openings of patterned Si substrate. Furthermore, according to the kinetic processes during MBE growth, it is proposed that the SAG of NCs results from the combination of the shadow effect of Si nano-pillars and the growth suppression of titanium (Ti) film. [ABSTRACT FROM AUTHOR]

Highlights • Deep-UV emission at 260 nm has been obtained from AlGaN-based MQW structures. • High AlGaN and AlGaN/AlN MQW structures has been grown by PA-MBE. • Larger Al flux ratio and higher growth temperature contribute to increasing Al content of AlGaN. • The emission wavelength was tuned by controlling the thickness of AlGaN quantum well. Abstract Deep ultraviolet (UV) emission at 260 nm has been obtained from high Al content AlGaN/AlN multi-quantum well (MQW) structures by plasma-assisted molecular beam epitaxy (PA-MBE) on AlN template on sapphire substrates. The effect of III/V ratio and growth temperature on structural and optical properties of AlGaN was studied. It was observed that Al content in AlGaN material increased with increasing Al flux ratio or growth temperature. By altering growth conditions, AlGaN with high Al content of 47–80% was obtained. Further studies indicate that AlGaN layers have excellent surface morphology and optical properties grown in slightly metal-rich condition. As for AlGaN/AlN MQW, the emission wavelength was tuned by controlling the thickness of AlGaN quantum well, and deep ultraviolet emission below up to 260 nm was obtained from AlGaN (2.4 nm)/AlN(6.9 nm) MQW at room temperature. [ABSTRACT FROM AUTHOR]

Highlights • Experimental Estimation of diffusion length of Ga adatoms on GaP (0 0 1) • Correlation between the behavior of diffusion length and surface morphology as a function of the temperature. • Observation of anisotropy along < 1 1 ¯ 0 > and < 1 1 ¯ 0 > directions. Abstract In this work Ga adatom incorporation diffusion length was quantified in two different directions, 〈1 1 0〉 and < 1 1 ¯ 0 > , during MBE growth of a GaP layer. Thickness distribution was measured for the GaP layer grown on striped patterned GaP substrate and the data was analyzed based on a one-dimensional diffusion growth model between two different adjacent facets. It was quantitatively revealed that the diffusion length of the Ga adatoms increased with growth temperature and an anisotropy in the diffusion length was observed along < 110 > and < 1 1 ¯ 0 > directions. The influence in the morphology due to the diffusion length anisotropy was discussed with the RHEED patterns. [ABSTRACT FROM AUTHOR]

Highlights • Lattice deformations in InGaAs/GaAs(1 1 1)A were studied by in situ RSM. • Anomalous lattice shrinkage along the c-axis was found in InGaAs/GaAs(1 1 1)A. • InGaAs/InAs/GaAs(1 1 1)A structure effectively improved crystal quality of InGaAs. Abstract Using in situ synchrotron X-ray reciprocal space mapping, this study investigates the evolution of lattice deformations in the InGaAs/GaAs(1 1 1)A structure with or without a thin InAs layer grown by molecular beam epitaxy. During the initial growth phase, InGaAs directly grown on GaAs(1 1 1)A showed an anomalous lattice shrinkage along the c-axis, with no change in the indium composition. Conversely, the InGaAs grown on InAs/GaAs(1 1 1)A showed no initial lattice distortion, but a variable indium composition. The evolution of the diffraction peak broadening was also monitored. The results confirmed that the thin InAs layer effectively improved the crystal quality during the initial growth of InGaAs. [ABSTRACT FROM AUTHOR]

Highlights • In 0.74 Ga 0.26 As detector structures with superlattice barrier are grown on InP. • The effects of barrier thickness and period number are investigated. • Dark currents are reduced for the moderate barrier thickness and period number. • Excessive barrier thickness and period number induce material deterioration. Abstract The effects of thickness and period number of In 0.6 Ga 0.4 As/InAs superlattice electron barrier on the performances of InP-based metamorphic In 0.74 Ga 0.26 As detectors have been investigated. The samples were grown by molecular beam epitaxy and characterized by high resolution X-ray diffraction reciprocal space maps, photoluminescence, atomic force microscope, plan view transmission electron microscopy and the device dark current measurements. Results show that the dark currents are reduced by insertion of an electron barrier with moderate thickness and period number in the absorption layer, but a superlattice electron barrier structure with excessive thickness and period number can lead to the deterioration of the material quality and device performances. [ABSTRACT FROM AUTHOR]

Highlights • MBE growth of carbon doped GaAs-(1 1 1)B layers on a miscut substrate. • Carbon doping concentration from 10 16 c m -3 to 10 20 c m -3 • Smooth GaAs(1 1 1)B surfaces (RMS 0.3 nm) up to N C 10 20 c m -3 were achieved. • p-type behavior and electrically active donor incorporation up to N C ≤ 10 19 c m -3. • The carbon ionization energy in GaAs(1 1 1)B was determined to E A = 26.3 ± 0.1 meV. Abstract Here we present a study of the solid source molecular beam epitaxy growth of carbon doped GaAs(1 1 1)B oriented layers in a wide doping concentration range from 10 16 to 10 20 cm - 3 . The layers have been fabricated on GaAs(1 1 1)B substrates with a 1° miscut towards (2 1 1) and carbon was supplied by a heated graphite filament source. Atomic force microscopy revealed smooth surfaces (root mean square roughness 0.3 nm) up to the highest carbon concentration. The carbon concentration was determined by secondary ion mass spectroscopy. All samples exhibit p-type conductivity and temperature-dependent Hall- and resistivity measurements show carrier freeze out for doping concentration in the 10 16 cm−3 and 10 17 cm−3 range, whereas we observe degenerated behavior for carbon concentrations above ca. 1 × 10 19 cm−3. The hole mobility decreases with increasing carbon concentration. The acceptor ionization energy for carbon was determined via low-temperature photoluminescence measurements to 26.3 meV. From the free hole density determined from Hall-measurements at 300 K and the overall carbon concentration, we estimate that approximately 100% of the carbon atoms are electrically active acceptors for carbon concentrations below 1 × 10 18 cm−3. For concentrations above 1 × 10 19 cm−3 compensation sets in and the doping efficiency is well below 100%. For comparison, samples in the standard (1 0 0)-orientation have been produced and carbon incorporation behavior during growth is very similar. [ABSTRACT FROM AUTHOR]

Highlights • Epitaxial growth of Fe delta-doped (In,Fe)Sb thin films. • Intrinsic ferromagnetism confirmed by MCD spectroscopy measurements. • T C lower than the uniformly-doped (ln,Fe)Sb with the same average Fe concentration. • Much higher Hall sensitivity of the δ-doped samples than those of uniformly-doped. • Very promising for highly sensitive anomalous Hall effect sensors. Abstract We have grown Fe δ-doped (In,Fe)Sb thin films by low-temperature molecular beam epitaxy for magnetic-field sensors with ultrahigh Hall sensitivity. Magnetic circular dichroism spectroscopy measurements indicate intrinsic ferromagnetism in the Fe δ-doped (In,Fe)Sb layers. Although the Fe δ-doped (In,Fe)Sb thin films have lower Curie temperature than the Fe uniformly-doped (ln,Fe)Sb with the same average Fe concentration, their anomalous Hall effect is much stronger. The Hall sensitivity of the Fe δ-doped (In 1−〈 x 〉 ,Fe 〈 x 〉)Sb sample with an average Fe concentration 〈 x 〉 = 17% reaches 1522 Ω/T at room temperature, which is much higher than those of Fe uniformly-doped (In,Fe)Sb, (Ga,Fe)Sb, and commercial InSb Hall sensors. [ABSTRACT FROM AUTHOR]

Highlights • Two-dimensional (2D) film formation in group-III nitrides is theoretically studied. • There is faint charge between III-nitrides with 2D structure and Ag(1 1 1) substrate. • The lattice constraint is negligible for III-nitride with hexagonal structure. • The hexagonal structure is stabilized up to 15 monolayers due to interlayer interactions. Abstract The formation of two-dimensional (2D) ultrathin films in III-nitride compound on Ag(1 1 1) substrate is investigated using density functional calculations. The calculations demonstrate that there is faint charge distribution between III-nitrides with 2D hexagonal structure and Ag(1 1 1) substrate while metallic bonds are formed at the interface for III-nitrides with wurtzite structure. This indicates that the lattice constraint of Ag(1 1 1) substrate is negligible for III-nitride compound ultrathin films with hexagonal structure. Furthermore, the hexagonal structure is more stable than wurtzite phase for the films up to 15 monolayers due to interlayer electrostatic interactions. The results suggests that various 2D III-nitride compound ultrathin films can be formed on Ag(1 1 1) substrate. [ABSTRACT FROM AUTHOR]

Highlight • III-V laser monolithically integrated on a CMOS-compatible Si substrate. • MBE-grown InAs/GaAs quantum dot laser on Si with low threshold current density (160 A/cm2) • Single facet output power of 48 mW at injection current density of 500 A/cm2 at room temperature. • Laser operation up to 52 °C under continuous-wave operation mode with characteristic temperature of 60.8 K. Abstract The concept of high-efficiency, high-reliability and low-threshold electrically pumped lasers monolithically grown on silicon has attracted great attention over the past several decades, as a promising on-chip optical source for Si photonics. In this paper, we report an electrically pumped continuous-wave (CW) 1.3 µm InAs/GaAs quantum dot (QD) lasers grown on a complementary metal-oxidesemiconductor (CMOS) compatible Si exact (0 0 1) substrate with reduced GaAs buffer thickness down to ∼2 µm. A threshold current density (J th) as low as ∼160 A/cm2 has been achieved at room temperature. The characteristic temperature (T 0) obtained is ∼60.8 K and laser operation is observed up to 52 °C under CW mode. These results suggest that an O-band InAs/GaAs QD laser could be very promising to develop a monolithically integrated on-chip optical source for Si photonics. [ABSTRACT FROM AUTHOR]

Highlights • Methodology to determine flux and temperature distributions for stationary MBE. • GaAsBi growth regimes determined in high resolution with respect to the As/Ga ratio. • Strong variation of Bi incorporation and structural properties. • Material optimization by fine-tuning the As/Ga ratio. Abstract The control of Bi incorporation and material properties in III-V-Bi alloys has proved challenging due to their high sensitivity to the epitaxial growth parameters. Here, we present a methodology for determining the variation in the Ga, As, and Bi fluxes and the temperature across a stationary substrate in molecular beam epitaxy. By correlating the flux distributions with material properties, we identify distinct regimes for epitaxy of GaAsBi. In particular, we devise a detailed image of the interplay between Bi incorporation and structural properties of a bulk GaAs 0.96 Bi 0.04 layer grown on GaAs(1 0 0) with respect to the As/Ga ratio. The influence of As/Ga is analyzed with high resolution over the important stoichiometric range (i.e. As/Ga = 0.6–1.6). Growth outside the near-stoichiometric As/Ga regime leads to decreased Bi incorporation, decreased structural quality and the formation of Ga, Ga/Bi or Bi droplets. On the other hand, growth at As/Ga = 1.00–1.17 leads to maximized material quality. For this regime, the surface roughness is further optimized by fine-tuning the As/Ga ratio to suppress surface mounding to a value of 0.5 nm. The results reveal the extreme sensitivity of GaAsBi growth to small variations in the As/Ga ratio, and demonstrate the applicability of stationary growth in studying these effects. [ABSTRACT FROM AUTHOR]

Graphical abstract Growth of self-assembled and position-controlled InN nanowires on Si (1 1 1) substrates by plasma-assisted molecular beam epitaxy is demonstrated. By variation of the growth parameters, fundamental knowledge of the nanowire formation process is obtained. Structural analysis of a self-assembled InN nanowire reveals that the nanowire has a wurtzite crystal structure covered by an indium oxide shell. In addition, it is shown that self-assembled InN nanowires can exhibit both lattice polarities. Highlights • Decomposition and redeposition effects dominate at high growth temperatures. • Self-assembled InN NW exhibits wurtzite crystal structure covered by an oxide shell. • Self-assembled InN NWs show both lattice polarities. • Growth of position-controlled InN NWs on different substrates possible. Abstract This work presents a systematic growth study of self-assembled and position-controlled InN nanowires (NWs) on Si (1 1 1) substrates by plasma-assisted molecular beam epitaxy without using a buffer layer. By variation of the substrate temperature, the III/V flux ratio, and the growth time, optimal parameters for self-assembled and position-controlled NW growth are found and compared. This leads to a fundamental understanding of the underlying growth mechanism for InN NW formation. Scanning transmission electron microscopy measurements of a self-assembled InN NW show a wurtzite crystal structure with a hexagonal cross-section and flat lateral facets covered by an approximately 5 nm thick indium oxide shell. In addition, Kelvin probe force microscopy measurements indicate the existence of both lattice polarities in case of self-assembled InN NWs. [ABSTRACT FROM AUTHOR]

Highlights • Efficient Fe doping into GaN buffer layer achieved using solid Fe source in NH 3 MBE growth. • Semi-insulating (S.I.) GaN buffer is realized by adding Fe impurities. • The increased sites occupied by too many Fe atoms will damage the lattice order. • Unnecessary Fe dopants over 1019 cm−3 tends to increase the residual stress. Abstract The electrical properties of AlGaN/GaN high electron mobility transistor structures with composite Fe-doped GaN/undoped GaN buffers grown on (1 1 0) Si substrates by ammonia molecular beam epitaxy were reported. Fe concentration in the range of 1017 cm−3 is sufficient to compensate the residual donors (<1016 cm−3) of undoped GaN and demonstrated a highly resistive GaN on (1 1 0) Si substrates. For buffers with Fe concentration of 1 × 1017 ∼ 1 × 1018 cm−3, the buffer was semi-insulating, with the Fermi level pinned near 0.5–0.7 eV which is determined by temperature-dependent I-T measurements. From the PICTS measurements, the Fe-related peaks of 0.69 eV (B) and 0.86 eV (C) are observed. The peak B is most likely due to the center pinning the Fermi level which is close to the activation energy observed in the temperature-dependent current measurement of Fe-doped sample, tentatively attributed to complexes between substitutional Fe ions and native defects such as nitrogen vacancies. The peak C is similar to the hole trap of 0.85 eV previously reported. In spite of similar transfer device characteristics, the Fe-doped buffer shows a noticeable difference in the off-state leakage behaviors. The sample grown with the optimized Fe concentration of 1 × 1017 cm−3 had a buffer resistivity of approximately 3.84 × 1011 Ω cm, increased by almost four orders of magnitude with respect to that (4.5 × 107 Ω cm) of the reference sample with undoped GaN buffer. [ABSTRACT FROM AUTHOR]

Highlights • Epitaxial regrowth accomplished on a membrane that was removed from its native substrate. • Power-dependent photoluminescence shows comparable recombination rates versus baseline. • Cathodoluminescence map indicates uniform bulk crystal quality on membrane epitaxy. Abstract Micro-transfer printing has demonstrated value for device- and array-level heterogeneous integration in a variety of applications. Here, it is used instead to assemble unique substrates with the potential to enable III–V growth templates that are both low-cost and specified at any desired lattice parameter within the range of III–V materials. Critical to this, epitaxial growth crystal quality must be established as comparable to that produced on conventional epi-ready substrates. We demonstrate epitaxial regrowth on gallium arsenide membranes that were transfer printed from their source substrate and onto a target handle. Both morphology and luminescence properties were characterized. Comparing the transfer printed membrane epitaxy with epitaxy on a conventional substrate, we found recombination rates and spatial uniformity from power-dependent photoluminescence and cathodoluminescence, respectively, to yield similar results, suggesting material quality was not adversely limited by the engineered substrate. [ABSTRACT FROM AUTHOR]

Highlights • Demonstration of dislocation-free AlGaN nanowire on Al coated Si. • Demonstration of AlGaN nanowire wavelength tuning on Al coated Si. • Demonstration of deep ultraviolet nanowire LEDs on Al coated Si. Abstract We have demonstrated the epitaxial growth of AlGaN nanowires on Al coated Si (0 0 1) substrate. The as-grown nanowires feature diameters of >200 nm and relatively uniform height distribution. AlGaN nanowires with emission wavelengths from 340 nm to 288 nm have been successfully achieved by varying Al/Ga beam equivalent pressure ratio and growth temperature. Detailed structural characterization suggests that AlGaN nanowires grown on Al template are free of dislocations. We have further demonstrated functional AlGaN nanowire deep ultraviolet (DUV) light emitting diodes, which exhibit a turn-on voltage of 7 V and a single peak EL emission at 288 nm. The realization of high quality AlGaN nanostructures on reflective Al template provides a promising approach for achieving high efficiency DUV light emitters. [ABSTRACT FROM AUTHOR]

Highlights • Defect creation in GaP-Si(0 0 1) structures was investigated by XRD and TEM. • Anti-phase boundaries (APBs) were observed in GaP-Si(0 0 1) structures. • APBs increased local elastic stress and stimulated creation of SFs and MTs. • MBE growth conditions postpone onset of relaxation of GaP up to >250 nm thick. Abstract This work investigates defect formation and evolution associated with the deposition of GaP layers on precisely oriented Si(0 0 1) substrates. The GaP layers were grown with thicknesses ranging from ∼37 nm to ∼2 µm at a growth rate of 0.52 μm/hr using molecular beam epitaxy (MBE). The crystallinity of thin (37-nm) MBE-grown GaP layers was also compared with thin GaP layers grown by migration-enhanced epitaxy (MEE). The MBE growth procedure was shown to postpone relaxation of the epitaxial GaP layers up to a thickness of ∼250 nm. Detailed analysis of high-resolution X-ray diffraction patterns and comparison with cross-sectional transmission electron micrographs clarified the defect formation mechanism. Thin GaP layers showed very low defect densities except for anti-phase boundaries, whereas substantial threading defects predominated in the thicker, noticeably relaxed structures. [ABSTRACT FROM AUTHOR]

Highlights • Atomistic mechanisms for observations of As-for-Sb exchange and Sb segregation. • Ab initio calculations of pure and Sb-exposed InAs(0 0 1) surface reconstructions. • Experimental and calculated phase diagrams of Sb-terminated InAs surfaces. • Varied group V-rich and stoichiometric surfaces to control crystal properties. Abstract Interfacial broadening that occurs due to intermixing of Sb and As at the interfaces is a challenge for growing high quality mixed anion heterostructures. In this work, we examine the atomic scale interaction between As and Sb. Experimental observations show that the surface consists primarily of (4 × 3) and c(4 × 4) reconstructions as a function of As and Sb overpressure. Density functional theory calculations confirm the experimental phase diagram and show that many of the reconstructions contain both Sb dimers and sub-surface Sb. This indicates concurrent mechanisms for interfacial broadening: some Sb atoms are weakly bound in dimer sites and segregate during growth, while some Sb atoms are driven to infiltrate the crystal and intermix with As. The calculations also predict a stoichiometric Sb-terminated α2(2 × 4) to be stable, suggesting that abrupt III-As/III-Sb heterostructures could be obtained by growing on this surface. While not trivial to achieve, this surface can be experimentally obtained by depositing Sb directly on an In-terminated surface. [ABSTRACT FROM AUTHOR]

Highlights • A series of lattice-matched InAlN films was grown over a wide range of growth conditions. • Growth conditions include those reported in the literature as conducive to homogeneous InAlN. • Modification of growth parameters does influence the process of indium segregation. • Formation of the honeycomb structure is attributed to a delayed formation of an indium adlayer. Abstract The impact of growth conditions on indium segregation in metal-polar In 0.17 Al 0.83 N lattice-matched to GaN grown by plasma assisted molecular beam epitaxy is reported. A series of lattice-matched InAlN films was systematically grown over a wide range of temperatures and active nitrogen fluxes. The growth conditions include those reported in the literature as conducive to homogeneous In 0.17 Al 0.83 N (Kaun et al., 2014). However, honeycomb indium composition modulation was found in all InAlN films. For samples grown at the highest temperature and nitrogen flux, InAlN layers exhibit larger and more irregular cells with emergence of open cell boundaries. We conclude that the studied growth conditions alone are not sufficient to eliminate the ‘honeycomb’ structure in metal-polar InAlN lattice-matched to GaN, and that the physical process necessary for reproducible growth of homogeneous In 0.17 Al 0.83 N requires further clarification. [ABSTRACT FROM AUTHOR]

We report on a detailed investigation of molecular beam epitaxy and characterization of GaN epilayers on Si (0 0 1) substrate through controlled nanowire coalescence. By varying the substrate temperature, the axial/lateral growth ratio of GaN nanowires on Si (0 0 1) can be modulated by nearly two orders of magnitude. As such, the transition from GaN nanowire arrays to coalesced GaN epilayers can be obtained through a relatively thin (∼150 nm) intermediate layer, which leads to the subsequent formation of crack-free GaN epilayers on Si (0 0 1) substrate. Detailed scanning transmission electron microscopy (STEM) studies suggest that the resulting GaN epilayers are nearly free of dislocations and stacking faults. Controlled p-type conduction is further achieved for Mg-doped GaN epilayers. Hole concentrations ∼5.6 × 10 17  cm −3 were measured at room temperature, with mobility values ∼3 cm 2 /V·s. Moreover, we have demonstrated functional InGaN/GaN LEDs on Si (0 0 1) substrate, wherein the active region and p-contact layer consist of InGaN/GaN disks-in-nanowires and Mg-doped GaN epilayers, respectively. The devices exhibited a turn on voltage of 2.7 V and strong emission at 525 nm. [ABSTRACT FROM AUTHOR]

We demonstrate a catalyst-free method to grow lateral InAs nanowires on GaAs (1 0 0) substrate by means of molecular beam epitaxy. By applying pre-surface treatment under oxygen plasma, lateral InAs nanowires with lengths of 1–2 μm and width of approximately 30–80 nm are epitaxial grown along [ 1 1 ¯ 0 ] direction. Stacking faults are not observed in the epitaxial process, which is usually an issue for InAs nanowires grown vertically on (1 1 1) substrates. Photo-luminescent measurements were performed for both single and multiple layers of InAs nanowires. A spectrum peak at the wavelength of 1625 nm is observed for a single wire at 5 K and room temperature emission is obtained for three layers of InAs nanowires. In addition, InAsSb nanowires are achieved along [1 1 0] direction, with a length of 0.4–0.8 μm and a width of 60–80 nm. [ABSTRACT FROM AUTHOR]

Bismuth containing semiconductor alloys are of increasing interest for their potential applications as infrared devices, though material quality in these materials has often proven problematic. This work shows a series of GaAsBi films grown by molecular beam epitaxy over a range of temperatures, 340 °C–315 °C, with all other parameters equal. Atomic force microscopy shows that there is an increase in the surface roughness of the films from 0.8 nm to 2.7 nm RMS. Transmission electron microscopy and atom probe tomography reveals the onset of spontaneous nanostructure formation in the films including nanopores, lateral composition modulation, and Bi rich GaAsBi clusters. The phenomena are shown to correlate and linked to reaching a growth instability due to the differences in surface diffusivity of Bi and As adatoms. [ABSTRACT FROM AUTHOR]

In this article, we report the formation of InAs quantum ring nanostructures (QRNs) on GaSb (0 0 1) surface by droplet epitaxy (DE) mode using molecular beam epitaxy. We examined the impact of various growth conditions, including substrate temperature (T s ), As 2 beam equivalent pressure (BEP) and surface stoichiometry, on the shape, density and size of the InAs QRNs. We confirmed that the InAs QRNs have better rotational symmetry at relatively high T s and low As 2 BEP. The symmetry of the QRN is due to the isotropic indium (In) out-migration along [1 1 0] and [1 −1 0], controlled via change in stoichiometry (surface As coverage) with temperature and the As 2 BEP. These results indicate that we can realize InAs QRN on GaSb surface by DE process. [ABSTRACT FROM AUTHOR]

A 99.6% relaxed InSb layer is grown on a 6° offcut (1 0 0) Si substrate via an AlSb/GaSb buffer using molecular beam epitaxy (MBE). A 200 nm GaSb buffer is first grown on Si and the lattice mismatch between them is accommodated by an interfacial misfit (IMF) array consisting of uniformly distributed 90° misfit dislocations. Si delta doping is introduced during the growth of GaSb to reduce the density of threading dislocation. Subsequently, a 50 nm AlSb buffer is grown followed by a 0.8 µm InSb layer. The InSb layer exhibits a 300 K electron mobility of 22,300 cm 2 /Vs. An InSb photoconductor on Si is demonstrated with a photoconductive gain from 77 K to 200 K under a 700 °C maintained blackbody. [ABSTRACT FROM AUTHOR]

The evolution of lateral InSb nanowires (NW) grown on CdTe (0 0 1) substrate by increasing of the InSb thickness is reported. At 10-nm growth of InSb, NW with narrow lateral width of ∼ 70 nm and high NW density (∼18 μm−1) are obtained. The evolutional description of InSb NW growth based on the atomic force microscopic characterization is presented. The intrinsic strain at the interface and top InSb layer is revealed by x-ray diffraction analysis. The strong polarization-dependent photoluminescence (PL) of NW is observed. The polarization degree of NW is ∼ 80% and 56% when the excited laser and the emission PL are polarized, respectively. The latter can be attributed to the shape anisotropy. [ABSTRACT FROM AUTHOR]

In-situ UV illumination influences the incorporation dynamics of bismuth adatom in GaAs. Here we use the inherent variation of the fluence across the sample to explore the role of the incident irradiation. With illumination it is found that steady state growth processes are achieved more quickly resulting in more abrupt interfaces, as well as uniform GaAs 1 - x Bi x epi-layers. Comparisons of low temperature photoluminescence spectra show an increasing density of clusters of incorporated bismuth atoms with decreasing incident fluence. [ABSTRACT FROM AUTHOR]

The growth of undoped GaSb epilayers on GaAs (0 0 1) substrates with 2° offcut towards 〈1 1 0〉, by molecular beam epitaxy system (MBE) at low growth temperature is reported. The strain due to the lattice mismatch of 7.78% is relieved spontaneously at the interface by using interfacial misfit array (IMF) growth mode. Three approaches of this technique are investigated. The difference consists in the steps after the growth of GaAs buffer layer. These steps are the desorption of arsenic from the GaAs surface, and the cooling down to the growth temperature, under or without antimony flux. The X-ray analysis and the transmission electron microscopy point out that desorption of arsenic followed by the substrate temperature decreasing under no group V flux leads to the best structural and crystallographic properties in the GaSb layer. It is found that the 2 µm-thick GaSb is 99.8% relaxed, and that the strain is relieved by the formation of a periodic array of 90° pure-edge dislocations along the [1 1 0] direction with a periodicity of 5.6 nm. [ABSTRACT FROM AUTHOR]

The homoepitaxial growth of N-polar GaN was investigated by ammonia molecular beam epitaxy. Systematic growth studies varying the V/III flux ratio and the growth temperature indicated that the strongest factor in realizing morphologically smooth films was the growth temperature; N-face films needed to be grown approximately 100 °C or greater than Ga-face films provided the same metal flux. Smooth N-face films could also be grown at temperatures only 50 °C greater than Ga-face films, albeit under reduced metal flux. Too high a growth temperature and too low a metal flux resulted in dislocation mediated pitting of the surface. The unintentional impurity incorporation of such films was also studied by secondary mass ion spectroscopy and most importantly revealed an oxygen content in the mid 10 17 to the mid 10 18  cm −3 range. Hall measurements confirmed that this oxygen impurity resulted in n-type films, with carrier concentrations and mobilities comparable to those of intentionally silicon doped GaN. [ABSTRACT FROM AUTHOR]

We demonstrate GaN nanocolumn growth on fused silica glass by plasma-assisted molecular beam epitaxy. The effect of the substrate temperature, Ga flux and N 2 flow rate on the structural and optical properties are studied. At optimum growth conditions, GaN nanocolumns are vertically aligned and well separated with an average diameter, height and density of 72 nm, 1.2 μm and 1.6 × 10 9 cm −2 , respectively. The nanocolumns exhibit wurtzite crystal structure with no threading dislocations, stacking faults or twinning and grow in the [0 0 0 1] direction. At the interface adjacent to the glass, there is a few atom layers thick intermediate phase with ABC stacking order (zinc blende). Photoluminescence measurements evidence intense and narrow excitonic emissions, along with the absence of any defect-related zinc blende and yellow luminescence emission. [ABSTRACT FROM AUTHOR]

We have demonstrated that in situ annealing effect has to be taken into account in order to realize the 1.31 μm InAs quantum dot (QD) lasers with the dot-in-well (DWELL) structure. The photoluminescence (PL) properties have been investigated for the InAs DWELL samples annealed at different temperatures in situ , simulating the annealing process during the growth of the top cladding AlGaAs layer in the laser structure. The QDs with large size in the DWELL structure are vulnerable to the annealing process at temperatures above 550 °C, revealed by the drastic change in the PL spectra. However, the DWELL structure is stable during the annealing process at 540 °C for three hours. The thermal stability of the QDs in the DWELL structure has to be considered in the growth of QD lasers for long wavelength operation. [ABSTRACT FROM AUTHOR]

Quantum dashes were synthesized in the molecular beam epitaxial growth of InAs on GaAs(2 2 1). By changing the arsenic pressure it was possible to obtain highly ordered one-dimensional InAs arrays as demonstrated by autocorrelation function analysis. Polarized Raman spectroscopy was utilized in order to characterize the samples and to estimate the stress at the InAs/GaAs interface as well as the surface anisotropy imposed by the quasi one-dimensional character of the quantum dashes. The most ordered surface, showed the lowest correlation length, and for this sample the Raman spectra exhibits small shift of the GaAs resonance modes indicating likewise small GaAs tensile strain. [ABSTRACT FROM AUTHOR]

Up-converted photoluminescence (UPL) in InAs/GaAs heterostructures has been investigated. Relaxation process imposes a great challenge for efficient UPL. It is found that efficient UPL can be detected by the luminescence from InAs/GaAs multi quantum well (MQW), and that the intensity could be enhanced by further improving crystalline quality of GaAs barrier. In addition, choosing proper energy states as intermediate states is another important issue to enhance UPL. We describe how the overall UPL efficiency can be controlled by the epitaxial growth and selection of intermediate states. [ABSTRACT FROM AUTHOR]

This work provides a comprehensive study of the incorporation behavior of B in growing GaAs under molecular beam epitaxy conditions. Structural characterization of superlattices revealed a strong dependence of the BAs growth rate on the GaAs growth rate used. In general, higher GaAs growth rates lead to a higher apparent BAs growth rate, although lower B cell temperatures showed saturation behavior. Each B cell temperature requires a minimum GaAs growth rate for producing smooth films. The B incorporation into single thick layers was found to be reduced to 75–80% compared to superlattice structures. The p-type carrier densities in 1000 nm thick layers were found to be indirectly proportional to the B content. Furthermore, 500 nm thick B x Ga 1−x As layers showed significantly lower carrier concentrations, indicating B segregation on the surface during growth of thicker layers. [ABSTRACT FROM AUTHOR]

Self-assembled In x Ga 1−x As quantum dots (QDs) are promising hosts for spin qubits with excellent coupling to photons. Nuclear spin and charge fluctuations lead to dephasing and limit the applicability of QDs as qubits. We show that charge noise can be minimized by high quality MBE growth of well-designed heterostructures yielding natural optical linewidths down to 1.15 µeV. To minimize the nuclear spin noise, one direction would be to reduce the wave function overlap with the nuclei. We show that this is indeed the case for a single hole spin in a QD that we embedded in the intrinsic region of an n-i-p-diode. For random nuclei, the heavy-hole limit is achieved down to neV energies, equivalent to dephasing times of microseconds. [ABSTRACT FROM AUTHOR]

Interlayer exchange coupling (IEC) of GaMnAs-based multilayers has been investigated by varying the structural parameters, such as spacer thickness, carrier doping in the nonmagnetic layers, and repetition number of GaMnAs/GaAs bilayers. The type of IEC in the structure (i.e., either ferromagnetic or antiferromagnetic IEC) was identified from the magnetotransport measurements, which show either anisotropic magnetoresistance or giant magnetoresistance-like behavior. The investigation revealed that the parameters of the ferromagnetic GaMnAs layer (i.e., Mn composition and thickness) are less important than those of the non-magnetic GaAs spacer layer in the structure. In particular, it was found that the presence of carriers in the spacer layer plays a crucial role in determining the type and strength of the IEC in GaMnAs-based multilayer systems. [ABSTRACT FROM AUTHOR]

Plasma-assisted molecular beam epitaxy (PA-MBE) is now widely used for the growth of group III-nitrides. Many years ago it became clear that during PA-MBE there is unintentional doping of GaN with boron (B) due to decomposition of the pyrolytic boron nitride (PBN) cavity of the RF plasma source. In this paper we discuss the unintentional B incorporation for PA-MBE growth of GaN and Al x Ga 1−x N using a highly efficient RF plasma source. We have studied a wide range of MBE growth conditions for GaN and Al x Ga 1−x N with growth rates from 0.2 to 3 µm/h, RF powers from 200 to 500 W, different nitrogen flow rates from 1 to 25 sccm and growth times up to several days. The chemical concentrations of B and matrix elements of Al, Ga, N were studied as a functions of depth using secondary ion mass spectrometry (SIMS). We demonstrate that boron incorporation with this highly efficient RF plasma source is approximately 1×10 18 to 3×10 18 cm −3 for the Al x Ga 1−x N growth rates of 2 – 3 µm/h. [ABSTRACT FROM AUTHOR]

Two vertically stacked quantum dots that are electronically coupled, so called quantum dot molecules, are of great interest for the realization of solid state building blocks for quantum communication networks. We present a modified gradient approach to realize InAs quantum dot molecules with a low areal density so that single quantum dot molecules can be optically addressed. The individual quantum dot layers were prepared by solid source molecular beam epitaxy depositing InAs on GaAs(100). The bottom quantum dot layer has been grown without substrate rotation resulting in an In-gradient across the surface, which translated into a density gradient with low quantum dot density in a certain region of the wafer. For the top quantum dot layer, separated from the bottom quantum dot layer by a 6 nm thick GaAs barrier, various InAs amounts were deposited without an In-gradient. In spite of the absence of an In-gradient, a pronounced density gradient is observed for the top quantum dots. Even for an In-amount slightly below the critical thickness for a single dot layer, a density gradient in the top quantum dot layer, which seems to reproduce the density gradient in the bottom layer, is observed. For more or less In, respectively, deviations from this behavior occur. We suggest that the obvious influence of the bottom quantum dot layer on the growth of the top quantum dots is due to the strain field induced by the buried dots. [ABSTRACT FROM AUTHOR]

In this work the effects of filling the surface energy states in AlGaAs/GaAs heterostructures on the depletion layer are reported. The depletion layer width was varied from 49 to 10 nm as determined by Raman spectroscopy, allowing to discern their effect on the generation of Franz-Keldysh oscillations observed by photoreflectance spectroscopy. It is found that the photoreflectance modulation process of built-in electric fields at surface is negligible when the surface-levels are filled. This work demonstrates the relationship between the surface-states density, the surface states capability of capture carriers and the layer sequence of the heterostructure. These parameters need to be considered in order to get an adequate analysis of the photoreflectance spectrum of heterostructures. It is shown that if a nearly full-filled condition in the available surface energy levels is established, the Franz-Keldysh oscillations produced by the modulation of the built-in electric field intensity disappears as a result of the reduction in the photogenerated carrier density in the photoreflectance measurement. [ABSTRACT FROM AUTHOR]

Hybrid nanostructures are known to elicit an enhanced optical response. We study the directed alignment of ErAs metal nanoparticle (NP) and InAs quantum dot (QD) using molecular beam eptaxy (MBE) in a GaAs matrix. Due to high surface free energy caused by the crystal structure difference, overgrowth of an ErAs NP with GaAs forms a depression that condenses subsequent InAs adatoms to form an inverted QD self-aligned to the underlying ErAs NP. The ErAs NP growth, GaAs overgrowth, and InAs QD deposition were carefully controlled and studied with transmission electron microscopy (TEM) and atomic force microscopy (AFM) to investigate their effects on the QD-NP alignment. [ABSTRACT FROM AUTHOR]

We report on the growth by solid-source MBE of random-alloy Al x In 1–x As y Sb 1–y layers lattice-matched to (0 0 1)–GaSb substrates, with x Al ∈ [0.25; 0.75]. The samples quality and morphology were characterized by X-ray diffraction, Nomarski microscopy and atomic force microscopy. Layers grown at 400 °C demonstrated smooth surfaces and no sign of phase decomposition. Samples with x Al ≤ 0.60 demonstrated photoluminescence (PL) at 300 K whereas samples with higher Al content only demonstrated PL at low temperature. Samples grown at 430 °C, in contrast, exhibited PL at low temperature only, whatever their composition. Inferred bandgap energies corroborate the estimation of a non-null quaternary bowing parameter made by Donati, Kaspi and Malloy in Journal of Applied Physics 94 (2003) 5814. Upon annealing, the PL peak energies increased, getting even closer to the theoretical values. These results are in agreement with recently published results on digital AlInAsSb alloys. Our work, which reports the first evidence for PL emission from random-alloy AlInAsSb layers lattice-matched to GaSb, opens the way to their use in optoelectronic devices. [ABSTRACT FROM AUTHOR]

The effects of well shapes and waveguide materials on InP-based InAs quantum well lasers have been investigated. The laser structures were grown on metamorphic In 0.65 Al 0.35 As buffers. A novel trapezoidal quantum well composed of In y Ga 1−y As grading and InAs layer was used to improve the quality of quantum well. Quaternary In 0.65 Al 0.2 Ga 0.15 As waveguide was applied instead of ternary In 0.65 Ga 0.35 As to enhance the carrier injection. The material qualities have been characterized by X-ray diffraction, transmission electron microscopy and photoluminescence measurements, while the device properties of the lasers with various structures were investigated at different temperatures. Results show that the laser performances have been improved by the use of trapezoidal quantum wells and InAlGaAs waveguides. [ABSTRACT FROM AUTHOR]

We have investigated the spatial evolution of ( n × 3 ) surface reconstructed areas during the molecular beam epitaxial growth of InAs–GaAs(001) by using ab initio -based calculation and in situ observation, in order to understand the mechanism of consequent QD nucleation. Statistical analysis of ( n × 3 ) -reconstructed morphology reveals that the fraction of ( 8 × 3 ) -reconstructed areas, as well as those of ( 4 × 3 ) and ( 6 × 3 ) , appears and decreases in the later stage of the growth. This behaviour is consistent with the result of our ab initio -based calculation incorporating chemical potentials of source materials in the gas phase. In contrast, fragmented ( 2 × 3 ) areas remain throughout the InAs growth as potential QD nucleation sites. [ABSTRACT FROM AUTHOR]

Laterally biased quantum-well infrared photodetectors (LBQWIPs) are expected to exhibit a photoresponse at room temperature. In these devices, the photocurrent is collected by means of two lateral Ohmic contacts on each side of an undoped quantum well (QW), which is coupled by tunneling to another n -doped QW. Photoexcited electrons from the n -doped QW tunnel through to the undoped QW and are swept out via a lateral bias voltage. Up to now, the practical development of these structures has not been yet achieved due to the difficulty of contacting single QWs separated by a few nanometers. In this paper, we report on a viable technology to fabricate LBQWIPs. We present two procedures to contact individual QWs, which are sufficiently close to be coupled by tunneling. The final devices exhibit very low dark-current values and clear infrared absorption peaks at 300 K, in good agreement with the results of numerical simulations. This work demonstrates the practical functionality of the laterally biased structure and paves the way for future developments of room temperature QWIPs. [ABSTRACT FROM AUTHOR]

The surface morphology of GaSb was investigated by changing growth conditions such as thermal oxide desorption temperature, growth temperature, and growth step by solid source molecular beam epitaxy. At high temperature growth, the pits caused by the thermal oxide desorption remained in the GaSb buffer layer surface, while the surface was sufficiently flattened. At low temperature growth, the pits disappeared, while the surface was not enough flattened even in the case of step-flow mode growth. Since the pits disappeared at lower growth temperature regardless of the growth mode, this behavior might be explained by the Ga migration length depending on the growth temperature. By applying two-step high/low temperature growth, where both growth steps proceed in step-flow mode, flat, a pit-free GaSb buffer surface could be obtained. [ABSTRACT FROM AUTHOR]

The generation of rotational twins in the growth of GaAs and GaSb on Ge(111) substrates by molecular beam epitaxy has been characterized by X-ray diffraction. Rotational twins are shown to be less generated in GaSb than in GaAs. It has been demonstrated that the generation of rotational twins in GaAs can be suppressed by inserting GaSb as a buffer layer. It has also been demonstrated that the use of the As 2 beam and vicinal surfaces is effective for suppressing twin generation in GaAs. [ABSTRACT FROM AUTHOR]

We present an initial study of the influence of the growth parameters on the surface morphology and on the interface quality of homoepitaxial GaP(1 1 1) and heteroepitaxial GaP/AlGaP(1 1 1) grown on GaP(1 1 1)B substrates using Gas-Source Molecular Beam Epitaxy (GSMBE). Three different surface reconstructions are identified in the RHEED patterns during the growth runs. The Root Mean Square (RMS) surface roughness measured post-growth by AFM ranges from 3 to 10 nm over 10 × 10 µm 2 areas, for a film thickness of 100–600 nm. The results of 2θ-ω XRD scans on (1 1 1) and (3 1 1) planes reveal a stacking disorder in the AlGaP layer and further XRD phi-scan measurements on GaP (3 1 1) show strong peaks with 3-fold rotational symmetry and additional of 3-fold weak peaks indicating only a negligible fraction of the twinned crystal orientation in the substrate. TEM images of these samples show a smooth interface between the AlGaP layer and GaP substrate, and reveal the presence of a high density of extended defects such as stacking faults, twinning and dislocations lines in AlGaP layer whereas the GaP layer appears as pure Zinc-Blende. Further TEM analysis reveals composition and local strain variations for GaP/AlGaP samples associated with an undulated surface. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate a self-assisted VLS growth of GaAs nanowires directly on GaAs (111)B substrates without assistance of SiO 2 layer. We believe that with this technique, we can overcome some inherent problems that usually occur in conventional self-assisted VLS growth and simplify the process. Moreover, to study the effect of substrate temperature, each nanowire sample was fabricated at the different temperature from 400 °C to 600 °C using Molecular Beam Epitaxy (MBE) technique. Surface morphology, elemental composition, and crystal structure of nanowire samples were characterized by Scanning Electron Microscope (SEM), Energy-dispersive X-ray spectroscopy (EDX), and X-ray Diffraction Analysis (XRD) respectively. [ABSTRACT FROM AUTHOR]

We fabricate self-aligned vertically stacked GaAs quantum dot molecules (QDMs) by filling of self-assembled nanoholes in AlGaAs. The tunable nanoholes are created using local droplet etching (LDE) combining conventional molecular beam epitaxy with self-assembled, lithography-free patterning. The optical emission from single, strain-free QDMs shows clear excitonic features with linewidths below 150 µeV after optimizations of the fabrication process. This allows investigations of the coupling among the individual dots forming a QDM. In electric fields oriented along the axis of the QDM, luminescence emission from direct and indirect transitions can be clearly distinguished. Furthermore, an anti-crossing behaviour demonstrates inter-dot coupling in the QDM. [ABSTRACT FROM AUTHOR]