Small size GaN nanorods (GaN NRs) show new features of the photoluminescence (PL) associated with the enhanced surface effects and the strong electron-phonon coupling. A dominant emission is observed at 3.43 eV; a lower energy with respect to the commonly reported (DXA)-X-0 peak for thicker NRs. The phonon replicas of the silent B1l acoustic phonon mode are well represented in the PL spectra at temperatures up to 150 K inferring the enhanced Frohlich electron-phonon coupling. The B1l phonon mode was previously detected for the grown GaN NRs by resonant Raman scattering under resonance excitation. The enhanced electron-phonon coupling through Frohlich interaction is proven by the calculated Huang-Rhys factor. The low-energy dominant peak intensity is around six times the (DXA)-X-0 peak intensity indicating that the majority of excitons occupy the surface shell of GaN NRs. This study provides new insights on small-size GaN NRs that greatly influence their physical properties for applications in optoelectronics, UV and blue lasers, and high-temperature/high-power electronic devices. (C) 2021 Elsevier B.V. All rights reserved.

Molecular Beam Epitaxy: Materials and Applications for Electronics and Optoelectronics is presented in five parts: Fundamentals of MBE; MBE technology for electronic devices application; MBE for optoelectronic devices; Magnetic semiconductors and spintronics devices; and Challenge of MBE to new materials and new researches. The book offers chapters covering the history of MBE; principles of MBE and fundamental mechanism of MBE growth; migration enhanced epitaxy and its application; quantum dot formation and selective area growth by MBE; MBE of III-nitride semiconductors for electronic devices; MBE for Tunnel-FETs; applications of III-V semiconductor quantum dots in optoelectronic devices; MBE of III-V and III-nitride heterostructures for optoelectronic devices with emission wavelengths from THz to ultraviolet; MBE of III-V semiconductors for mid-infrared photodetectors and solar cells; dilute magnetic semiconductor materials and ferromagnet/semiconductor heterostructures and their application to spintronic devices; applications of bismuth-containing III–V semiconductors in devices; MBE growth and device applications of Ga2O3; Heterovalent semiconductor structures and their device applications; and more.

Wurtzite semiconductor compounds have two silent modes, B1 l and B1 h. A silent mode is a vibrational mode that carries neither a dipole moment nor Raman polarizability. Thus, they are forbidden in both infrared reflectivity and Raman spectroscopy. Astonishingly, we detected the B1 l mode in high-quality, ultra-narrow GaN nanowires using resonant Raman scattering, although the B1 h was not observed, and there is no immediate explanation for this asymmetric finding. The Raman experiments were performed using several laser lines from 647 to 325 nm; the latter is a wavelength in which Raman becomes resonant. Actually, we observed the B1 l mode only in resonance, indicating that the appearance of this mode is related to Fröhlich electron-phonon interactions; i.e., a dipole moment emerging in the B1 l silent mode may not be present in the B1 h mode. To shed light onto the physical origin of these observations, we performed density functional theory calculations of the lattice dynamics in GaN. We performed a careful analysis of the different physical mechanisms that allow the forbidden mode to appear to explain the physics underlying the nonzero dipole moment in the B1 l mode, and the reason why this dipole moment is not present in the B1 h mode.

Thallium-containing III-V (Tl-III-V) and bismuth-containing III-V (III-V-Bi) alloy semiconductors were first proposed as novel functional semiconductors. They are alloys consisting of semiconductors (III-V) and semimetals (Tl-V, III-Bi), and are important materials for the fabrication of temperature-insensitive lasing wavelength laser diodes as well as long wavelength infrared (LWIR) optical devices. However, the growth conditions for alloys containing Tl and Bi are very strict and the growth windows are narrow. In this chapter, the expected properties of these semiconductors and the experimental results for the growth, characterization, and device applications are described.

Local structures around gadolinium atoms in rare-earth (RE)-doped InGaGdN thin films were studied by means of fluorescence extended X-ray absorption fine structure (EXAFS) measured at the Gd LIII-edges. The samples were doped with Gd in-situ during growth by plasma-assisted molecular beam epitaxy (PAMBE). Gd LIII-edge EXAFS signal from the GaGdN, GdN and Gd foil were also measured as reference. The X-ray absorption near edge structure (XANES) spectra around Gd LIII absorption edge of InGaGdN samples observed at room temperature indicated the enhancement of intensities with the increase of Gd composition. Further EXAFS analysis inferred that the Gd atoms in InGaN were surrounded by similar atomic shells as in the case of GaGdN with the evidence indicating majority of Gd atoms substituted into Ga sites of InGaGdN. A slight elongation of bond length for the 2nd nearest-neighbor (Gd–Ga) of sample with higher Gd concentration was also observed.

The photoluminescence of Gd-doped GaN multi-quantum wells (MQWs) is presented and discussed considering the formation of a Gd:Nitrogen-vacancy (N-vacancy) complex. A lower energy photoluminescence peak was observed for the Gd-doped GaN MQW sample with respect to the main peak assigned to a neutral donor bound exciton (DX) of the undoped GaN MQW sample. The X-ray absorption near edge structure spectrum observed at Gd LIII-edge indicates a nitrogen vacancy adjacent to the Gd substituting the Ga ion in Gd-doped GaN MQW sample. Local stresses around the Gd dopants in Gd-doped GaN matrix generated due to the larger diameter of the Gd ion with respect to the Ga ion can be relieved by the creation of vacancies. The lower formation energy of N-vacancies in GaN matrix introduce them as a preferred candidate to relieve the generated stresses. A Gd:N-vacancy complex consisting of a Gd ion and the created nitrogen vacancy adjacent to the Gd dopant is likely to form in GaN:Gd matrix. The lower photoluminescence peak energy observed in the Gd-doped GaN MQW sample is assigned to the recombination of an exciton captured at the Gd:N-vacancy complex forming a small polaron-like state. A model is presented considering the small exciton-polaron population in defect sites captured around the Gd ions in the Gd-doped GaN. 2014 Elsevier B.V. All rights reserved.

The photoluminescence of Gd-doped GaN multi-quantum wells (MQWs) is presented and discussed considering the formation of a Gd3+:Nitrogen-vacancy (N-vacancy) complex. A lower energy photoluminescence peak was observed for the Gd-doped GaN MQW sample with respect to the main peak assigned to a neutral donor bound exciton ((DX)-X-0) of the undoped GaN MQW sample. The X-ray absorption near edge structure spectrum observed at Gd L-III-edge indicates a nitrogen vacancy adjacent to the Gd substituting the Ga ion in Gd-doped GaN MQW sample. Local stresses around the Gd dopants in Gd-doped GaN matrix generated due to the larger diameter of the Gd3+ ion with respect to the Ga3+ ion can be relieved by the creation of vacancies. The lower formation energy of N-vacancies in GaN matrix introduce them as a preferred candidate to relieve the generated stresses. A Gd3+:N-vacancy complex consisting of a Gd3+ ion and the created nitrogen vacancy adjacent to the Gd3+ dopant is likely to form in GaN:Gd matrix. The lower photoluminescence peak energy observed in the Gd-doped GaN MQW sample is assigned to the recombination of an exciton captured at the Gd3+:N-vacancy complex forming a small polaron-like state. A model is presented considering the small exciton-polaron population in defect sites captured around the Gd3+ ions in the Gd-doped GaN. (C) 2014 Elsevier B.V. All rights reserved.

Mn-doped ZnSnAs2 (ZnSnAs2:Mn) thin films with 5.0 and 6.5% Mn composition were epitaxially grown by molecular beam epitaxy on InP (001) substrates. These films had a Curie temperature of 334 K, corresponding to room-temperature ferromagnetism. The local structures around Mn atoms in ZnSnAs2:Mn were studied by analysis of the X-ray absorption fine-structure spectra. It was found that the Mn atoms substitute into Zn or Sn cation sites, and the Mn‒As bond length is 2.50 Å, which is slightly smaller than the value of 2.53 Å in a sphalerite (zinc-blende) ZnSnAs2 bulk crystal. The Mn‒As bond length in ZnSnAs2:Mn is consistent with the value obtained from GaMnAs, and has a smaller value than that obtained from the zinc-blend MnAs thin films grown on a InP substrate.

Two nano-scale disks with the different structures, a single AlGaN/GaN(2~3 nm)/AlGaN and a multi-AlN(1nm)/GaN(9nm)/AlN(1nm), in columnar forms are optically examined. The multiple resonant peaks concerning phonon from the GaN disks are observed in the distinct spectrum form between both GaN disks at low temperature. In the sample with the multi-disk structure, of which diameters are 40 ~ 60 nm, a peak line with the energy separation of ~91 meV is observed and attributed to resonant scattering by an A1-LO phonon. The single disk structure also presents a multi-peak line spectrum but gives the unique temperature dependence in the spectrum structure: the energy interval between the adjacent peaks spreads with increasing temperature.

The effect of growth temperature on the nitrogen concentration and optical properties of the InGaP(N) epilayer was investigated. The nitrogen concentration of InGaPN is dependent on the growth temperature (450–490°C), whereas the indium concentration is independent of the growth temperature. Nitrogen concentration decreased the photoluminescence (PL) intensity and increased the full width at half maximum of the PL peak. Raman analysis revealed that the preferred atomic configuration of InGaPN consisted of Ga–N and In–P bonds, which induced higher local strains during growth. The degradation of PL efficiency was related to this bonding configuration. Nitrogen concentration reduced the coupling constant of electron–phonon interaction, thus, reducing the temperature dependence of the PL peak shift. The reduced PL quenching around room temperature was attributed to the decrease of band discontinuity at the InGaPN/GaAs heterointerface due to the increase in N concentration.

It has been reported that composition modulation is naturally formed in some of the epitaxially-grown thin films. Structural characterization of these materials is necessary for controlling their nanostructures precisely. Here, we prepared epitaxially-grown III–V semiconductor alloys and characterized their atomistic structures by means of diffraction crystallography and electron microscopy techniques. As a consequence, we found that the following quantum well structures are spontaneously formed: (1) ultrashort period lateral composition modulation (LCM) with a modulation period of ∼1 nm; (2) complex vertical composition modulated (VCM) structures consisting of two modulated structures with a different period (∼4 and ∼25 nm). The former LCM structure is created via nanoscale phase separation at the growth surface, while the shorter-period modulation in the later VCM structure is induced by rotating a substrate through an inhomogeneous distribution of the anion flux within a chamber.

The growth parameter dependence of structural, electrical and magnetic properties in Gd-doped GaN layers grown by plasma-assisted molecular beam epitaxy has been investigated. The structural and magnetic properties of Gd-doped GaN films grown on GaN templates strongly depend on the MBE growth condition. While Gd-doped GaN grown under relatively high Ga fluxes consist of wurtzite GaGdN layers without Gd-related precipitates, Gd-incorporated GaN films grown under low Ga fluxes contain a lot of nanoparticles ranging from several nm to several tens nm in size. The samples with Gd-related nanoparticles exhibit hysteresis in the magnetization–magnetic field curves at 10K. The separation between the field-cooled and zero-field-cooled magnetization–temperature curves is observed at around 30K. This behavior is understood in terms of super-paramagnetism originating from the ferromagnetic nanoparticles observed in the cross-sectional transmission electron microscopy images.

GaDyN/GaN double-barrier magnetic tunneling junctions (DB-MTJs) structures are grown on GaN templates by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). X-ray diffraction (XRD) θ/2θ scan curves exhibited a clear peak of GaDyN at low angle side of the GaN (0002) diffraction peak. From analyzing extended x-ray absorption fine structure (EXAFS), majority of Dy atoms are found to be incorporated into substituting the Ga sites in GaN with wurtzite structure, and GaDyN has longer bond length than that of GaN. From XRD and EXAFS results, the lattice constant of the GaDyN is larger than that of GaN due to large radius of Dy ion. Photoluminescence from GaDyN DB-MTJs structure was found, which is related to the Dy ions. Ferromagnetism is confirmed in the magnetization versus magnetic field curves at room temperature for the all samples. It is found that the magnetization per unit volume and the coercivity show change with increasing the thickness of the middle GaDyN layer. This implies that the interlayer interaction exists between the GaDyN layers.

We have investigated optical property of GaGdN grown on GaN template by radio-frequency plasma-assisted molecular beam epitaxy by varying the concentration of Gd. All the samples exhibit photoluminescence emission at the near band edge of 356nm, a blue luminescence (BL) band around 420nm and a green luminescence (GL) band around 500nm, instead of yellow luminescence (YL) band around 550nm frequently observed in n-GaN layer. The band edge emission is independent of the Gd concentration. This shows that the values of band gap for the GaGdN layers are the same as that of GaN. The temperature dependencies of the GL band intensity and peak wavelength are very similar to those of the YL band observed in the GaN epitaxial layers on the GaN templates. On the other hand, with increasing the Gd concentration, the BL bands observed in the GaGdN layers are red-shifted and their intensities decrease. The BL band intensity remains unchanged in the temperature range of 20–60K. It rapidly decreases at temperature above 100K, and disappears at 200K. The BL band is red-shifted in the temperature range of 4–200K and its value is about twice larger than that of the GL band.

X-ray photoemission spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED) have been used to investigate the initial stages of InP growth on H-terminated Si and native Si oxide surfaces. By annealing at 800°C, the H-terminated Si surface exhibits a (2×1) RHEED pattern while the native oxide surface just shows a halo RHEED pattern. After exposing them to P2 flux, the Si(2×1) clean surface exhibits a (2×1) RHEED pattern while the native Si oxide layer always shows the halo RHEED pattern. XPS measurement reveals that while P atoms adsorb on the Si(2×1) surface, the adsorption of P atoms does not take place on the native Si oxide surfaces. In consequence of the difference in the adsorption of P atoms between the clean Si and Si oxide surfaces, InP does not grow on the Si oxides at temperatures above 450°C while InP grows on the clean Si substrates. Based on the findings, we will discuss the mechanism of the selective area growth of InP on patterned Si substrates with Si oxide mask in terms of desorption of In and P.

The formation of nitrogen-induced defects and the reduction of their concentration after rapid thermal annealing in TlInGaAsN alloy system are studied in detail, employing the photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) studies. XPS analysis revealed that the N-induced defect configurations are most likely attributed to the N–As spilt interstitials and (NAs–AsGa) nearest neighbor pairs. Rapid thermal annealing (RTA) treatment reduces the defect concentration and induces a significant improvement in the PL efficiency of the TlInGaAsN material system.

GaN/AlGaN single quantum disks on GaN nanorods were grown on Si (001) substrate with native SiO2 layer by a plasma-assisted molecular-beam epitaxy under nitrogen-rich conditions. The transmission electron microscopy observations show single GaN nanorods images with an average thickness of 4 nm for the GaN single quantum disk and nanorod diameter of 15 nm. The observed photoluminescence spectra at 8 K show a peak at 3.475 eV, attributed to an exciton recombination in GaN. A strong peak was observed at 3.542 eV. This peak is attributed to the quantum con nement of excitons in the GaN quantum disks.