Compositional contrast in AlxGa1−xN/GaN heterostructures using scanning spreading resistance microscopy

Scanning spreading resistance microscopy has found extensive use as a dopant-profiling technique for silicon-based devices, and to a lesser extent for some III-V materials. Here we demonstrate its efficacy for wide bandgap nitrides and, in particular, show that it may be used to differentiate between layers of different Al-content in an Al_xGa_1−xN/GaN heterostructure. A monotonic increase in resistance signal with increasing Al-content is demonstrated, under optimal imaging conditions. The variation in measured resistance with applied bias is shown to be dependent on the aluminium content, and this is discussed, along with other issues, in the context of potential quantification of unknown samples. The procedure for forming an optimal image is different from that for silicon, in terms of contact forces and applied biases.     

Fröhlich electron-interface and -propagating optical phonon interactions in a wurtzite multi-shell cylindrical heterostructure

Under the dielectric continuum model and Loudon's uniaxial crystal model, the polar optical phonon modes in a wurtzite multi-shell cylindrical heterostructure are analyzed and discussed. The analytical electrostatic potential functions are presented for all the five types of polar optical phonon modes including the interface optical (IO) modes, the propagating (PR) modes, the quasi-confined (QC) modes, the half-space-like (HSL) modes and the exactly confined (EC) modes. By adopting a transfer matrix method, the free IO and PR phonon fields and corresponding Fröhlich electron -IO and -PR interaction Hamiltonians are obtained via the method of electrostatic potential expansion. The analytical formulas are universal and can be applied to single, double and some complex cylindrical wurtzite quantum systems.     

The quasi-confined optical phonons in wurtzite GaN/AlN multiple quantum wells

Within the framework of the dielectric-continuum model and Loudon's uniaxial crystal model, the equation of motion for p-polarization field in arbitrary wurtzite multilayer heterostructures are solved for the quasi-confined phonon (QC) modes. The polarization eigenvector, the dispersion relation, and the electron-QC interaction Fröhlich-like Hamiltonian are derived by using the transfer-matrix method. The dispersion relations and the electron-QC coupling strength are investigated for a wurtzite GaN/AlN single QW. The results show that there are infinite branches of dispersion curve with definite symmetry with respect to the center of the QW structure. The confinement of the quasi-confined phonons in the QW leads to a quantization of q_z,j characterized by an integer m that defines the order of corresponding quasi-confined modes. The QC modes are more dispersive for decreasing m. The QC modes display an interface behavior in the barrier and a confined behavior in the well. The symmetric modes have more contribution to electron-QC interaction than the antisymmetric modes. The strains have more effect on symmetry modes, and can be ignored for symmetry modes.     

Magnetotransport, optical, and electronic subband properties in AlxGa1−xN/AlN/GaN heterostructures

The Shubnikov-de Haas (S-dH) results at 1.5 K for Al_xGa_1−xN/AlN/GaN heterostructures and the fast Fourier transformation data for the S-dH data indicated the occupation by a two-dimensional electron gas (2DEG) of one subband in the GaN active layer. Photoluminescence (PL) spectra showed a broad PL emission about 30 meV below the GaN exciton emission peak at 3.474 eV that could be attributed to recombination between the 2DEG occupying in the AlN/GaN heterointerface and photoexcited holes. A possible subband structure was calculated by a self-consistent method taking into account the spontaneous and piezoelectric polarizations, and one subband was occupied by 2DEG below the Fermi level, which was in reasonable agreement with the S-dH results. These results can help improve understanding of magnetotransport, optical, and electronic subband properties in Al_xGa_1−xAs/AlN/GaN heterostructures.     

Calculated phonon dispersion of infinite-layer compounds and the effects of charge fluctuations

In a first step we use an ab initio rigid-ion model (RIM) to calculate the lattice parameters and the phonon dispersion of the infinite-layer compounds CaCuO₂, SrCuO₂, and BaCuO₂. We find an increase of both the planar and the axial lattice constant when going from CaCuO₂ through SrCuO₂ to BaCuO₂. The rate of increase of the planar lattice constant with respect to the alkaline-earth ionic radius is calculated to be smaller for the replacement of Sr by Ba than for the replacement of Ca by Sr. Both results are in accordance with experimental studies. The phonon dispersion in the RIM exhibits several unstable branches mainly related to axial displacements of the oxygens, indicating the tendency of the crystal to reconstruct in a lower-symmetry structure. The structural stability increases, however, towards BaCuO₂; simultaneously, the maximum phonon frequency decreases. AnA _2u zone-center mode with very large LO-TO-splitting exists in all three compounds (ferroelectric mode). In a second step charge fluctuations (CF) are taken into account at the copper- and oxygen ions, using SrCuO₂ as an example. Due to the vanishing of the ferroelectric split a branch with very steep dispersion forms in the [001] direction in the metallic phase whereas the zone-centerA _2u modes are unchanged in the insulating phase because of the two-dimensional (2D) electronic structure assumed. Characteristic nonlocal electron-phonon-interaction effects are associated with theZ-point Sr-axial-breathing mode: CF of uniform sign within the CuO planes but alternating sign in consecutive planes do occur in the metalic phase. This interplane charge transfer is, on the other hand, suppressed in the insulating phase due to the 2D electronic structure assumed. Instead, large induced site-potential changes emerge in this case.     

Alloy effects in Ga1−xInxN/GaN heterostructures

We show that the large band offsets between GaN and InN and the heavy carrier effective masses preclude the use of the virtual crystal approximation to describe the electronic structure of Ga_1−xIn_xN/GaN heterostructures, while this approximation works very well for the Ga_1−xIn_xAs/GaAs heterostructures.     

Growth and properties of magnetron cosputtering grown MnxGe1−x on Si (001)

We have grown Mn_xGe_1−x films (x=0, 0.06, 0.1) on Si (001) substrates by magnetron cosputtering, and have explored the resulting structural, morphological, electrical and magnetic properties. X-ray diffraction results show there is no secondary phase except Ge in the Mn_0.06Ge_0.94 film while new phase appears in the Mn_0.1Ge_0.9 film. Nanocrystals are formed in the Mn_0.06Ge_0.94 film, determined by field-emission scanning electron microscopy. Hall measurement indicates that the Mn_0.06Ge_0.94 film is p-type semiconductor and hole carrier concentration is 6.07×10¹⁹ cm⁻³ while the Mn_xGe_1−x films with x=0 has n-type carriers. The field dependence of magnetization was measured using alternating gradient magnetometer, and it has been indicated that the Mn_0.06Ge_0.94 film is ferromagnetic at room temperature.     

AlN, GaN, AlxGa1 − xN nanotubes and GaN/AlxGa1 − xN nanotube heterojunctions

Semiconductor optoelectronic devices based on GaN and on InGaN or AlGaN alloys and superlattices can operate in a wide range of wavelengths, from far infrared to near ultraviolet region. The efficiency of these devices could be enhanced by shrinking the size and increasing the density of the semiconductor components. Nanostructured materials are natural candidates to fulfill these requirements. Here we use the density functional theory to study the electronic and structural properties of (10,0) GaN, AlN, Al_xGa_1 − xN nanotubes and GaN/Al_xGa_1 − xN heterojunctions, 0<x<1. The Al_xGa_1 − xN nanotubes exhibit direct band gaps for the whole range of Al compositions, with band gaps varying from 3.45 to 4.85 eV, and a negative band gap bowing coefficient of −0.14 eV. The GaN/Al_xGa_1 − xN nanotube heterojunctions show a type-I band alignment, with the valence band offsets showing a non-linear dependence with the Al content in the nanotube alloy. The results show the possibility of engineering the band gaps and band offsets of these III-nitrides nanotubes by alloying on the cation sites.     

Electronic and structural properties of zincblende AlxIn1−xN

Numerical calculations based on first-principles are applied to study the electronic and structural properties of ternary zincblende AlInN alloy. The results indicate the lattice constant has a small deviation from the Vegard’s law. The direct and indirect bowing parameters of 4.731 ± 0.794 eV and 0.462 ± 0.285 eV are obtained, respectively, and there is a direct-indirect crossover near the aluminum composition of 0.817. The bulk modulus is monotonically increased with an increase of the aluminum composition, and the deviation parameter of bulk modulus of 10.34 ± 9.37 GPa is obtained. On the contrary, the pressure derivative of bulk modulus is monotonically decreased with an increase of the aluminum composition.     

Structural and optical properties of an InxGa1−xN/GaN nanostructure

The structural and optical properties of an In_xGa_1−xN/GaN multi-quantum well (MQW) were investigated by using X-ray diffraction (XRD), atomic force microscopy (AFM), spectroscopic ellipsometry (SE) and photoluminescence (PL). The MQW structure was grown on c-plane (0 0 0 1)-faced sapphire substrates in a low pressure metalorganic chemical vapor deposition (MOCVD) reactor. The room temperature photoluminescence spectrum exhibited a blue emission at 2.84 eV and a much weaker and broader yellow emission band with a maximum at about 2.30 eV. In addition, the optical gaps and the In concentration of the structure were estimated by direct interpretation of the pseudo-dielectric function spectrum. It was found that the crystal quality of the InGaN epilayer is strongly related with the Si doped GaN layer grown at a high temperature of 1090 °C. The experimental results show that the growth MQW on the high-temperature (HT) GaN buffer layer on the GaN nucleation layer (NL) can be designated as a method that provides a high performance InGaN blue light-emitting diode (LED) structure.     

Carbon doping in MgB2: role of boron and carbon px(y) bands

We have studied the changes in the electronic structure and the superconducting transition temperature T_c of Mg(B_1−xC_x)₂ alloys as a function of x with 0≤x≤0.3. Our density-functional-based approach uses the coherent-potential approximation to describe the effects of disorder, the Gaspari-Gyorffy formalism to estimate the electron-phonon matrix elements and the Allen-Dynes equation to calculate T_c in these alloys. We find that the changes in the electronic structure of Mg(B_1−xC_x)₂ alloys, especially near the Fermi energy E_F, come mainly from the outward movement of E_F with increasing x, and the effects of disorder in the B plane are small. In particular, our results show a sharp decline in both B and C p_x(y) states for 0.2≤x≤0.3. Our calculated variation in T_c of Mg(B_1−xC_x)₂ alloys is in qualitative agreement with the experiments.     

Electron-phonon interaction in NbB2: a comparison with MgB2

We present a comparison of electron-phonon interaction in NbB₂ and MgB₂, calculated using full-potential, density-functional-based methods in P6/mmm crystal structure. Our results, described in terms of (i) electronic structure, (ii) phonon density of states F(ω), (iii) Eliashberg function α²F(ω), and (iv) the solutions of the isotropic Eliashberg gap equation, clearly show significant differences in the electron-phonon interaction in NbB₂ and MgB₂. We find that the average electron-phonon coupling constant λ is equal to 0.59 for MgB₂ and 0.43 for NbB₂, leading to superconducting transition temperatures T_c at around 22 K for MgB₂ and 3 K for NbB₂.     

Effect of pressure on the energy band gaps of wurtzite GaN and AlN and electronic properties of their ternary alloys AlxGa1−xN

Using the Empirical Pseudopotential Method (EPM) combined with an improved Virtual Crystal Approximation (VCA), where the effect of compositional disorder is included as an effective periodic potential, we have calculated the electronic band structure of GaN and AlN under hydrostatic pressure up to 200 kbar and 160 kbar, respectively. The behavior of electronic properties of their alloys Al_xGa_1−xN in the wurtzite structure have been predicted at zero pressure, for the entire range of alloy concentrations.     

Composition modulation analysis of InxGa1−xP layers grown on (0 0 1) germanium substrates

The development of new photovoltaic approach to improve costs and efficiencies is focused on the new materials and new technologies. InGaP is, in this sense, a key material for solar conversion. In particular, in the solar concentration approach, this material is part of multiple junction solar cells. Its low lattice mismatch with germanium and its adequate bandgap make it very promising. This paper shows how compositional modulation can affect the InGaP emitter and the AlGaAs tunnel junctions. The influence of the growth conditions, on the compositional modulation and misfit and threading dislocations, in In_0.49Ga_0.51P layers is demonstrated by TEM on purposely grown single InGaP layers. High resolution electron microscopy (HREM) intensity profiles showed no elastic lattice related modulation.     

Effects of hydrostatic pressure on ionized donor bound exciton states in strained wurtzite GaN/AlxGa1−xN cylindrical quantum dots

Based on the effective-mass approximation and variational procedure, ionized donor bound exciton (D⁺, X) states confined in strained wurtzite (WZ) GaN/Al_xGa_1-xN cylindrical (disk-like) quantum dots (QDs) with finite-height potential barriers are investigated, with considering the influences of the built-in electric field (BEF), the biaxial strain dependence of material parameters and the applied hydrostatic pressure. The Schrödinger equation via the proper choice of the donor bound exciton trial wave function is solved. The behaviors of the binding energy of (D⁺, X) and the optical transition associated with (D⁺, X) are examined at different pressures for different QD sizes and donor positions. In our calculations, the effective masses of electron and hole, dielectric constants, phonon frequencies, energy gaps, and piezoelectric polarizations are taken into account as functions of biaxial strain and hydrostatic pressure. Our results show that the hydrostatic pressure, the QD size and the donor position have a remarkable influence on (D⁺, X) states. The hydrostatic pressure generally increases the binding energy of (D⁺, X). However, the binding energy tends to decrease for the QDs with large height and lower Al composition (x<0.3) if the donor is located at z₀≤0. The optical transition energy has a blue-shift (red-shift) if the hydrostatic pressure (QD height) increases. For the QDs with small height and low Al composition, the hydrostatic pressure dependence of the optical transition energy is more obvious. Furthermore, the relationship between the radiative decay time and hydrostatic pressure (QD height) is also investigated. It is found that the radiative decay time increases with pressure and the increment tendency is more prominent for the QDs with large height. The radiative decay time increases exponentially reaching microsecond order with increasing QD height. The physical reason has been analyzed in depth.     

Intersubband transition in symmetric AlxGa1 − xN/GaN double quantum wells with applied electric field

Influence of the applied electric field (AEF) on the intersubband transitions (ISBTs) in symmetric Al_xGa_1 − xN/GaN double quantum wells (DQWs) is investigated by self-consistent calculation. It is found that three- and four-energy-level DQWs can be realized when suitable electric field is applied. When the AEF is 0.93 MV/cm, the 1_odd-2_even ISBT has a comparable absorption coefficient with the 1_even-2_odd ISBT, and the four-energy-level DQWs are realized. The wavelengths of the 1_odd-2_even and 1_even-2_odd ISBTs are located at 1.31 and 1.62 μm, respectively. When the AEF is 1.10 MV/cm, the 1_odd-2_even and 1_odd-2_odd ISBTs have comparable absorption coefficients, and the three-energy-level DQWs are realized. The wavelengths of the 1_odd-2_even and 1_odd-2_odd ISBTs are located at 1.30 and 1.55 μm, respectively. The results suggest promising application to two-color optoelectronic devices operating within optical communication wavelength band, and this study also provides a method for realizing the two-color optoelectronic devices by using the AEF.     

Electronic structures of zigzag AlN, GaN nanoribbons and AlxGa1−xN nanoribbon heterojunctions: First-principles study

The electronic and structural properties of zigzag aluminum nitride (AlN), gallium nitride (GaN) nanoribbons and Al_xGa_1−xN nanoribbon heterojunctions are investigated using the first-principles calculations. Both AlN and GaN ribbons are found to be semiconductor with an indirect band gap, which decreases monotonically with the increased ribbon width, and approaching to the gaps of their infinite two dimensional graphitic-like monolayer structures, respectively. Furthermore, the band gap of Al_xGa_1−xN nanoribbon heterojunctions is closely related to Al (and/or Ga) concentrations. The Al_xGa_1−xN nanoribbon of width n=8 shows a continuously band gap varying from about 2.2 eV-3.1 eV as x increases from 0 to 1. The large ranged tunable band gaps in such a quasi one dimension structure may open up new opportunities for these AlN/GaN based materials in future optoelectronic devices.     

Numerical optimization of carrier confinement characteristics in (AlxGa1−xN/AlN)SLs/GaN heterostructures

We present numerical optimization of carrier confinement characteristics in (Al_xGa_1−xN/AlN)SLs/GaN heterostructures in the presence of spontaneous and piezoelectrically induced polarization effects. The calculations were made using a self-consistent solution of the Schrödinger, Poisson, potential and charge balance equations. It is found that the sheet carrier density in GaN channel increases nearly linearly with the thickness of AlN although the whole thickness and equivalent Al composition of Al_xGa_1−xN/AlN superlattices (SLs) barrier are kept constant. This result leads to the carrier confinement capability approaches saturation with thicknesses of AlN greater than 0.6 nm. Furthermore, the influence of carrier concentration distribution on carrier mobility was discussed. Theoretical calculations indicate that the achievement of high sheet carrier density is a trade-off with mobility.     

Surface and interface optical phonon modes and electron-phonon interaction in a multi-shell spherical system

Within the framework of the dielectric continuum model, interface optical(IO) and surface optical(SO) phonon modes and the Fr?hlich electron-IO (SO) phonon interaction Hamiltonian in a multi-shell spherical system were derived and studied. Numerical calculation on CdS/HgS/H₂O and CdS/HgS/CdS/H₂O spherical systems have been performed. Results reveal that there are two IO modes and one SO mode for the CdS/HgS/H₂O system, one SO mode and four IO modes whose frequencies approach the IO phonon frequencies of the single CdS/HgS heterostructure with the increasing of the quantum number l for CdS/HgS/CdS/H₂O. It also showed that smaller l and SO phonon compared with IO phonon, have more significant contribution to the electron-IO (SO) phonon interaction. Received 16 October 2001 and Received in final form 23 January 2002 Published online 25 June 2002