A structural analysis of ultrathin barrier (In)AlN/GaN heterostructures for GaN-based high-frequency power electronics

Metal–organic chemical vapor deposition (MOCVD) is one of the best growth methods for GaN-based materials as well-known. GaN-based materials with very quality are grown the MOCVD, so we used this growth technique to grow InAlN/GaN and AlN/GaN heterostructures in this study. The structural and surface properties of ultrathin barrier AlN/GaN and InAlN/GaN heterostructures are studied by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. Screw, edge, and total dislocation densities for the grown samples have been calculated by using XRD results. The lowest dislocation density is found to be 1.69 × 10⁸ cm⁻² for Sample B with a lattice-matched In_0.17Al_0.83N barrier. The crystal quality of the studied samples is determined using (002) symmetric and (102) asymmetric diffractions of the GaN material. In terms of the surface roughness, although reference sample has a lower value as 0.27 nm of root mean square values (RMS), Sample A with 4-nm AlN barrier layer exhibits the highest rough surface as 1.52 nm of RMS. The structural quality of the studied samples is significantly affected by the barrier layer thickness. The obtained structural properties of the samples are very important for potential applications like high-electron mobility transistors (HEMTs).     

Electron Transport in Ga-Rich InxGa1-xN Alloys

Resistivity and Hall effect measurements on n-type undoped Ga-rich InxGa1-xN （0.06 ≤ x ≤ 0.135） alloys grown by metal-organic vapour phase epitaxy （MOVPE） technique are carried out as a function of temperature （15-350 K）. Within the experimental error, the electron concentration in Inx Ga1-x N alloys is independent of temperature while the resistivity decreases as the temperature increases. Therefore, Inx Ga1-xN （0.06 ≤ x ≤0.135） alloys are considered in the metallic phase near the Mort transition. It has been shown that the temperaturedependent metallic conductivity can be well explained by the Mort model that takes into account electron-electron interactions and weak localization effects.     

Overtone spectroscopy of some benzaldehyde derivatives

Overtone spectrum of o, m and p-nitrobenzaldehydes and p-chlorobenzaldehyde has been studied in 2000–12000 cm⁻¹ region. Vibrational frequencies and anharmonicity constants for aryl as well as alkyl CH stretch vibrations have been determined. We have also determined the internuclear distances for the aryl CH bond in the different molecules. The small variation observed in these distances is an indication of the substitution effect. It is observed that in the case of p-disubstituted benzens, the shift in aryl CH bond is proportional to sum of the Hammet σ of the substituents. However in the case of o-disubstituted benzenes it is only 80% of the para-substituted shift.     

The effect of InxGa1−xN back-barriers on the dislocation densities in Al0.31Ga0.69N/AlN/GaN/InxGa1−xN/GaN heterostructures (0.05 ≤ x ≤ 0.14)

Al_0.31Ga_0.69N/AlN/GaN/In_xGa_1?xN/GaN heterostructures grown with the metal-organic chemical vapor deposition (MOCVD) technique with different In_xGa_1?xN back-barriers with In mole fractions of 0.05 ≤ x ≤ 0.14 were investigated by using XRD measurements. Screw, edge, and total dislocations, In mole fraction of back-barriers, Al mole fraction, and the thicknesses of front-barriers and lattice parameters were calculated. Mixed state dislocations with both edge and screw type dislocations were observed. The effects of the In mole fraction difference in the back-barrier and the effect of the thickness of front-barrier on crystal quality are discussed. With the increasing In mole fraction, an increasing dislocation trend is observed that may be due to the growth temperature difference between ultrathin In_xGa_1?xN back-barrier and the surrounding layers.     

DX-center energy calculation with quantitative mobility spectrum analysis in n-AlGaAs/GaAs structures with low Al content

Experimental Hall data that were carried out as a function of temperature (60–350 K) and magnetic field (0–1.4 T) were presented for Si-doped low Al content (x=0.14) n–Al_xGa_1−xAs/GaAs heterostructures that were grown by molecular beam epitaxy (MBE). A 2-dimensional electron gas (2DEG) conduction channel and a bulk conduction channel were founded after implementing quantitative mobility spectrum analysis (QMSA) on the magnetic field dependent Hall data. An important decrease in 2DEG carrier density was observed with increasing temperature. The relationship between the bulk carriers and 2DEG carriers was investigated with 1D self consistent Schrödinger–Poisson simulations. The decrement in the 2DEG carrier density was related to the DX-center carrier trapping. With the simulation data that are not included in the effects of DX-centers, 17 meV of effective barrier height between AlGaAs/GaAs layers was found for high temperatures (T>300 K). With the QMSA extracted values that are influenced by DX-centers, 166 meV of the DX-center activation energy value were founded at the same temperatures.     

Temperature dependent energy relaxation time in AlGaN/AlN/GaN heterostructures

The two-dimensional (2D) electron energy relaxation in Al_0.25Ga_0.75N/AlN/GaN heterostructures was investigated experimentally by using two experimental techniques; Shubnikov-de Haas (SdH) effect and classical Hall Effect. The electron temperature (T_e) of hot electrons was obtained from the lattice temperature (T_L) and the applied electric field dependencies of the amplitude of SdH oscillations and Hall mobility. The experimental results for the electron temperature dependence of power loss are also compared with the current theoretical models for power loss in 2D semiconductors. The power loss that was determined from the SdH measurements indicates that the energy relaxation of electrons is due to acoustic phonon emission via unscreened piezoelectric interaction. In addition, the power loss from the electrons obtained from Hall mobility for electron temperatures in the range T_e > 100 K is associated with optical phonon emission. The temperature dependent energy relaxation time in Al_0.25Ga_0.75N/AlN/GaN heterostructures that was determined from the power loss data indicates that hot electrons relax spontaneously with MHz to THz emission with increasing temperatures.     

Well parameters of two‐dimensional electron gas in Al0.88In0.12N/AlN/GaN/AlN heterostructures grown by MOCVD

Resistivity and Hall effect measurements were carried out as a function of magnetic field (0‐1.5 T) and temperature (30‐300 K) for Al_0.88In_0.12N/AlN/GaN/AlN heterostructures grown by Metal Organic Chemical Vapor Deposition (MOCVD). Magnetic field dependent Hall data were analyzed by using the quantitative mobility spectrum analysis (QMSA). A two‐dimensional electron gas (2DEG) channel located at the Al_0.88In_0.12N/GaN interface with an AlN interlayer and a two‐dimensional hole gas (2DHG) channel located at the GaN/AlN interface were determined for Al_0.88In_0.12N/AlN/GaN/AlN heterostructures. The interface parameters, such as quantum well width, the deformation potential constant and correlation length as well as the dominant scattering mechanisms for the Al_0.88In_0.12N/GaN interface with an AlN interlayer were determined from scattering analyses based on the exact 2DEG carrier density and mobility obtained with QMSA. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Extremes of nuclear structure

With the advent of medium and large gamma detector arrays, it is now possible to look at nuclear structure at high rotational forces. The role of pairing correlations and their eventual breakdown, along with the shell effects have showed us the interesting physics for nuclei at high spins — superdeformation, shape co-existence, yrast traps, alignments and their dramatic effects on nuclear structure and so on. Nuclear structure studies have recently become even more exciting, due to efforts and possibilities to reach nuclei far off from the stability valley. Coupling of gamma ray arrays with ‘filters’, like neutron wall, charged particle detector array, gamma ray total energy and multiplicity castles, conversion electron spectrometers etc gives a great handle to study nuclei produced online with ‘low’ cross-sections. Recently we studied, nuclei in mass region 80 using an array of 8 germanium detectors in conjunction with the recoil mass analyser, HIRA at the Nuclear Science Centre and, most unexpectedly came across the phenomenon of identical bands, with two quasi-particle difference. The discovery of magnetic rotation is another highlight. Our study of light In nucleus, 107In brought us face to face with the ‘dipole’ bands. I plan to discuss some of these aspects. There is also an immensely important development — that of the ‘radioactive ion beams’. The availability of RIB, will probably very dramatically influence our ‘conventional’ concept of nuclear structure. The exotic shapes of these exotic nuclei and some of their expected properties will also be touched upon.     

Preparation, structural analysis and dielectric properties of Bi x La1−x FeO3 perovskite

Rare earth element substituted bismuth ferrites (BiFeO₃) are of enormous importance as magnetoelectric materials. The polycrystalline samples of Bi _x La_1−x FeO₃ (x=0, 0.2, 0.4, 0.6, 0.8) were prepared by solid-state reaction using standard ceramic method. The single-phase formation of these compounds was confirmed by X-ray diffraction (XRD) studies. The samples with x=0, 0.2, 0.4, 0.6 are found to be orthorhombic while the sample with x=0.8 is triclinic. The dielectric constant (ε′) and dissipation factor (tan δ) were measured in the frequency range 100 Hz to 1 MHz at room temperature and as a function of temperature at certain fixed frequencies (1 kHz, 10 kHz, 100 kHz, 1 MHz). All the samples showed dielectric dispersion. The dielectric constant with temperature shows a broad peak; the peak temperature shifts with frequency which reflects the relaxor-type behavior. The peak above 600 K in the measured temperature range corresponds to antiferromagnetic ordering temperature (Néel temperature). The broadness of the peak changes with composition. The ac conductivity as well as ε′ are found to be maximum for the sample x=0.2 at room temperature.