Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2001"

The Oxygen Vacancy as the Origin of a Green Emission in Undoped ZnO

Abstract: We have investigated undoped ZnO single crystals, which are commercially available from Eagle-Picher, by photoluminescence (PL) and optically detected magnetic resonance (ODMR) spectroscopy. The electrical properties of this material are very similar to the samples investigated. The total residual shallow donor concentration is about 1 x 10{sup 17} cm{sup -3}. The low temperature emission is dominated by the donor bound exciton (D{sup 0}X) at 3.366 eV. At 2.45 eV the broad, unstructured ''green'' emission is located, its full width at half maximum is 320 meV. The temperature dependence of the PL reveals that this green band maintains its peak energy up to 450 K, which is a feature typical of a recombination within a localised defect, while the D{sup 0}X emission follows the shrinkage of the bandgap with increasing temperature. (orig.)

Epitaxial Lateral Overgrowth of GaN

Abstract: Since there is no GaN bulk single crystal available, the whole technological development of GaN based devices relies on heteroepitaxy. Numerous defects are generated in the heteroepitaxy of GaN on sapphire or 6H-SiC, mainly threading dislocations (TDs). Three types of TDs are currently observed, a type (with Burgers vector 1/3〈〉); c type (with 〈0001〉) and mixed a+c (1/3〈〉). The Epitaxial Lateral Overgrowth (ELO) technology produces high quality GaN with TD densities in the mid 106 cm—2, linewidth of the low-temperature photoluminescence (PL) near-bandgap recombination peaks <1 meV and deep electron traps reduced below 1014 cm—3 (compared to mid 1015 cm—3 in standard GaN). Numerous modifications of the ELO process have been proposed in order either to avoid technological steps (mask-less ELO) or to improve it (pendeo-epitaxy). Basically developed on either sapphire or 6H-SiC, the ELO technology is also achievable on (111)Si or (111)3C-SiC/Si provided that an appropriate buffer layer is grown to avoid cracks. More sophisticated technologies have been implemented to further increase the useable part of the ELO GaN surface (two technological steps, three-step ELO). Unfortunately, in-depth understanding of the basic ELO process is still missing, i.e. of the growth anisotropy and bending of dislocations.

Playing with Polarity

Abstract: We review the influence of GaN crystal polarity on various properties of epitaxial films and electronic devices. GaN films grown on sapphire by MOCVD or HVPE usually exhibit Ga-face polarity. N-face polarity is obtained either on the backside of such layers after removal from the substrate, or by turning the crystal polarity in MBE growth via a thin AlN buffer layer. In addition to rather obvious differences in their structural and morphological features, Ga- and N-face samples differ also in their electronic properties. Thus, different Schottky barrier heights are observed for both polarities, the position and detailed properties of spontaneously formed two-dimensional electron gases vary with polarity, and the adsorption of gases and ions also show an influence of the two different surfaces. A particular interesting possibility is the growth of lateral polarity heterostructures with predetermined macroscopic domains of different polarity separated by inversion domain boundaries. These structures make use of the crystal polarity as a new degree of freedom for the investigation of electronic properties of III-nitrides and for novel devices.

Band Anticrossing in III-N-V Alloys

Abstract: Recent high hydrostatic pressure experiments have shown that incorporation of small amounts of nitrogen into conventional III–V compounds to form III–N–V alloys leads to splitting of the conduction band into two subbands. The downward shift of the lower subband edge is responsible for the observed, large reduction of the fundamental band gaps in III–N–V alloys. The observed effects were explained by an anticrossing interaction between the conduction band states close to the center of the Brillouin zone and localized nitrogen states. The interaction leads to a change in the nature of the fundamental from the indirect gap in GaP to a direct gap in GaNP. The predictions of the band anticrossing model of enlarged electron effective mass and enhanced donor activation efficiency were confirmed by experiments in GaInNAs alloys.

Bioconjugation of Highly Luminescent Colloidal CdSe–ZnS Quantum Dots with an Engineered Two-Domain Recombinant Protein

Abstract: We present a novel approach, based on molecular self-assembly driven by electrostatic attractions, for conjugating inorganic colloidal semiconductor nanocrystals (quantum dots: QDs) having negatively charged surfaces with a two-domain recombinant protein bearing a positively charged C-terminal leucine zipper domain. Aggregation-free QD/protein conjugate dispersions were prepared. Conjugates retain both properties of the starting materials, i.e., biological activity of the protein and spectroscopic characteristics of the QDs. Such hybrid bio-inorganic conjugates represent a powerful fluorescent tracking tool, because they combine advantages of CdSe–ZnS quantum dots, such as chemical stability and a wide range of size-dependent luminescence emission properties, with a straightforward electrostatic conjugation approach. We describe the design and preparation of a model QD/protein conjugate and present functional characterization of the conjugate using luminescence and bioassays.

Extracting quantitative information from high resolution electron microscopy

Abstract: Despite the development of high-resolution electron microscopy (HREM) that allows imaging of most materials, the extraction of quantitative information at atomic scale still requires considerable additional efforts. This review presents the recent developments on techniques that can be used to determine the local strain, chemical composition or atomic structure retrieval in HREM. The source of noise in images and effective methods for improving the signal-to-noise ratio in direct or Fourier space are discussed. The artefacts of filtering are commented. In all the methods, the sample thickness has to be determined. A detailed analysis of the thin foil relaxation effect on the measured distortion fields is presented as well as the possibilities of using finite element calculations for its modelling. The local composition measurement based on the chemically sensitive reflections; pattern recognition and the measurement of lattice parameters are described. Examples from semiconductor heterostructures are discussed. The current state of the strategies used for the retrieval of the atomic configuration of defects from HRTEM images is briefly presented. The limits of detection and the accuracy of the methods are summarised; and it is pointed out that electron holography, focal series reconstruction and the coming Cs corrected microscopes will help to obtain this information with better accuracy.

Colloidally Prepared CdHgTe and HgTe Quantum Dots with Strong Near‐Infrared Luminescence

Abstract: Several series of CdHgTe composite nanocrystals were prepared using thiol-capped CdTe nanocrystal precursors to which subsequent layers of HgTe and CdTe were added. The position of the ‘excitonic’ photoluminescence peak measured at room temperature was red-shifted to the near infrared to give emission wavelengths ranging from 800 to 1100 nm depending on the quantum dot composition, with quantum efficiency (QE) significantly increased over the pure CdTe material (QE up to around 40%). Thiol-capped HgTe nanocrystals synthesized in aqueous solution show a broad photoluminescence with a QE of ∼ 50%. It has been shown using D2O as a solvent that by varying the synthesis conditions it is possible to tune the luminescence of HgTe quantum dots to the desired wavelength in the range of 900–2000 nm. HgTe nanocrystals passivated at the surface with a thick CdS layer have been shown to be much more robust towards heating and “aging” of the optical properties.

Investigating Alternative Gate Dielectrics: A Theoretical Approach

Abstract: We describe several applications of first-principles computational methods based on density functional theory (DFT) to the study of potential gate dielectric materials. First we investigate the stability of binary alkaline earth oxides in contact with Si and SiO 2 . In particular, we consider the case of SrO, which is important for the epitaxial growth of the SrTiO 3 perovskite structure on the Si (001) surface. Then we discuss the energetics of the SrTiO 3 (001) surface. We conclude with a brief discussion of the structure and electronic properties of crystalline ZrO 2 and HfO 2 .

Theory of Trion Spectra in Semiconductor Nanostructures

Abstract: A consistent density-matrix approach for the absorption spectra of semiconductor nanostructures at finite carrier densities is presented that describes both bound and scattering three-particle states (trions). It automatically includes the two-particle bound states (excitons). In optical transitions, the initial electron or hole momentum is transferred to the final trion state, which - due to recoil energy - gives rise to low-energy tails at the trion and exciton line. A high-energy tail at the exciton is due to the exciton-electron scattering states (trion continuum). The incomplete transfer of exciton oscillator strength to the main trion line and the trion continuum is exemplified by a quantum wire calculation.

Pseudopotential theory of semiconductor quantum dots

Abstract: This paper reviews our pseudopotential approach to study the electronic structure of semiconductor quantum dots, emphasizing methodology ideas and a survey of recent applications to both free-standing and semiconductor embedded quantum-dot systems.

High Pressure Studies of the Raman‐Active Phonons in Carbon Nanotubes

Abstract: We report high pressure Raman studies on as-prepared, purified, and solubilized single-walled and aligned multi-walled carbon nanotubes. The pressure dependence of radial (R) and tangential (T) vibrational bands in these samples is measured and compared with the results from other studies. In single-walled nanotubes, an abrupt drop in the intensity of these bands is seen near 2 GPa, indicative of a phase transition. Experiments on single (unbundled) tubes reveal a ≈10 cm—1 upshift of the R band relative to its frequency in bundled tubes. This is opposite to the predictions of calculations that include the intertube van der Waals interaction only and is explained by the changes in the electronic band dispersion driven by tube–tube interactions. Surprisingly, the pressure dependence of the R and T bands in unbundled tubes is very similar to that seen in bundled tubes, which indicates that the changes in the electronic band structure might significantly influence the pressure dependence.

Passivation and doping due to hydrogen in III-nitrides

Abstract: We have systematically studied the electronic structure and stability of hydrogen in AlN, GaN, and InN, based on first-principles calculations. In GaN and AIN, H is amphoteric and always compensates the prevailing conductivity: in GaN, H + is stable for Fermi levels below 2.2 eV, and in AlN, H + is stable for E F below 2.5 eV. In InN, we find that H + is stable for all Fermi level positions; i.e., H behaves exclusively as a donor. Consequences for controlling the conductivity of InN are discussed.

Excitons and Trions Modified by Interaction with a Two-Dimensional Electron Gas

Abstract: A concept of mixed exciton-trion states is formulated theoretically and proved experimentally for II-VI semiconductor quantum wells with a two-dimensional electron gas. The concept considers the resonances of neutral excitons and charged excitons (trions) as mixed (with each other) via their interaction with free electrons. Reflectivity spectra of modulation-doped ZnSe/(Zn,Mg)(S,Se) and CdTe/(Cd,Mg)Te quantum wells are analyzed. A good qualitative agreement of the experimental results with model calculations is achieved.

Pressure‐Controlled Solution Growth of Bulk GaN Crystals under High Pressure

Abstract: We have developed the pressure-controlled solution growth (PC-SG) method and indicated the validity of this method for the growth of GaN single crystals. We have investigated the effect of the supersaturation of nitrogen atoms on the size of a GaN single crystal and its morphology. GaN single crystals with a surface area of about 120 mm2 and/or with good morphology were obtained at a rate of increase of nitrogen pressure less than 49 MPa/h. Structural and photoluminescence properties of GaN single crystals were examined and determined that these crystals with good morphology had good crystallinity. Furthermore, we have introduced a new high-pressure furnace and developed the multi-crucible system. The GaN single crystal with a surface area of 300 mm2 was obtained by natural nucleation. The GaN crystal with 47 mm diameter, whose normal axis was the (0001) plane, could be grown on a sapphire substrate.

Capture Kinetics of Electron Traps in MBE-Grown n-GaN

Abstract: The carrier capture kinetics of the E c -0.59 eV and E c -0.91 eV electron traps found in molecular beam epitaxy (MBE)-grown n-GaN have been determined by means of deep level transient spectroscopy (DLTS). The 0.59 eV trap does not show the behaviour of either ideal point defects or line defects. In contrast, the 0.91 eV trap displays the kinetics of linearly arranged interacting point defects, which generate a time-dependent local Coulombic potential with a characteristic time constant of 8.6 μs.

Progress in Growth and Physics of Nitride-Based Quantum Dots

Abstract: GaN-based semiconductors have received great attention because of various potential device applications including short wavelength laser diodes (LDs), light emitting diodes (LEDs), photodetectors, and electron devices [1]. Short wavelength LED are used for full-color flat display panes and illumination systems. The blue/violet LDs will be for ultra-high density optical data storage. In addition, GaN electron devices for application to high frequency and high power oscillators are now intensively investigated. GaN and related materials (i.e., InAlGaN alloy) have various unique characteristics, such as wide bandgap energy (1.8–6.2eV), possible large band discontinuity in hete-rostructures, strong polarization effects (≈ 2MV cm−1) due to piezoelectric field effects and spontaneous polarization effects, large binding energy of excitons (> 50meV), high saturation velocity (≈ 2.7 × 10 cm4s−1), and high breakdown field (≈ 2 × 106 V cm−1).

Pressure‐Induced Valence Transitions in Rare Earth Chalcogenides and Pnictides

Abstract: The electronic structure of rare earth chalcogenides and pnictides is calculated with the ab-initio self-interaction corrected local-spin-density approximation (SIC-LSD). This approach allows both an atomic-like description of the rare earth f-electrons and an itinerant description of other electronic degrees of freedom. Specifically, different formal valencies of the rare earth atom, corresponding to different f-shell occupancies, can be studied and their energies compared, leading to a first-principles theory for. pressure-induced valence transitions. SIC-LSD calculations for cerium monopnictides and monochalcogenides. Yb monochalcogenides, and EuS are presented. The observed equilibrium lattice constants are well reproduced assuming a trivalent Cc configuration and divalent Eu and Yb configurations. The trends in the high pressure behavior of these systems are discussed. With applied pressure, isostructural phase transitions are found to occur in CeP and CeS, caused by the delocalization of the Ce f-electron, while in the heavier Cc compounds, the structural B1 --> B2 transition happens before f-electron delocalization occurs. Similarly, both Eu and Yb chalcogenides transfer to trivalent configurations with pressure, in accordance with observation.

Wetting Behaviour of GaN Surfaces with Ga‐ or N‐Face Polarity

Abstract: The wetting behaviour of GaN surfaces with N-face and Ga-face polarity and the influence of different surface treatments is studied by measuring the wetting angle of highly purified water by microscopic imaging. We found that wet thermal oxidation of the surface leads to a decreased wetting angle indicating an improved wetting behaviour. The presence of Al in AIN or AlGaN leads to a further reduction of the wetting angle, which is attributed to the presence of Al 2 O 3 on the surface. In addition the comparison of Ga- and N-face material revealed a lower wetting angle for all N-face samples. XPS analysis showed the enhanced formation of native oxide on the surface with N-face polarity.

Axial Pressure Influence on Dielectric and Ferroelectric Properties of Na0.5Bi0.5TiO3 Ceramic

Abstract: It was shown that axial pressure strongly changes the dielectric and ferroelectric properties of Na 0.5 Bi 0.5 TiO 3 ceramic. This includes: (i) the shift of rhombohedral-tetragonal phase transition to higher temperature, suppressing the dielectric dispersion, and reduction of thermal hysteresis of the electric permittivity which can be evidence of transition nature changes to second order one, and (ii) the decrease of polarization in the long-range ferroelectric phase and the shift of phase decay temperature to higher temperature. Also the thermodynamic parameters of the rhombohedral-tetragonal phase transition were estimated.

Ab initio analysis of surface structure and adatom kinetics of group-III nitrides

Abstract: Recent results based on density-functional theory calculations concerning the structure and stability of group-III nitride surfaces are discussed. An analysis of the thermodynamically stable surface structures for this materials system reveals that the driving mechanisms behind surface reconstructions are fundamentally different to those in ‘traditional” (i.e. arsenic or phosphorus based) III–V semiconductors. Specifically, surfaces are always metal-rich and nitrogen atoms on and in the surface layer are thermodynamically unstable. This feature will be shown to have important consequences on surface morphology, adatom kinetics, growth, reactivity, and alloy formation.

Pressure Dependence of Raman Linewidth in Semiconductors

Abstract: (a) Institut de Physique et Chimie desMate´riaux de Strasbourg (IPCMS), UMR 7504duCNRS,Universite´LouisPasteur,23rueduLoess,F-67037StrasbourgCedex,France(b)Max-Planck-Institutfu¨rFestko¨rperforschung,Heisenbergstr.1,D-70569Stuttgart,Germany(ReceivedAugust28,2000)Thepressuredependencesofthephononlifetimesofzone-centeropticalphononsofvariousgroupIVand III–V semiconductors are calculated from first principles using third-order density-func-tionalperturbationtheory.Themicroscopicmechanismresponsible,i.e.theanharmonicdecayofaphonon into phonons of different energy, is revealed and the sensitive dependence on pressure isstudiedindetail.Theresults arecomparedwithrecentmeasurements of thepressure dependenceofthelinewidthsofthefirstorderRamanlinesofSi,Ge,andSiC.

On the theory of thin ferroelectric films

Abstract: A Green's function formalism is used to calculate self-consistently the temperature dependence of the layer polarization and the thickness dependence of the Curie temperature of thin ferroelectric transverse Ising films in the ferroelectric phase, T < T c .

Theoretical Study of the Elastic Properties of III–P Compounds

Abstract: We present the results of an ab initio theoretical study within the local density approximation to determine the pressure dependence of the second-order elastic constants C ij of BP, AlP, GaP and InP in order to clarify the mechanism of the pressure-induced phase transition in these compounds. Our results do not support the idea of a soft acoustic branch as responsible for the phase transition.

Mechanical Properties and Elastic Constants of Zinc‐Blende Ga1—xInxN Alloys

Abstract: We have investigated the mechanical properties and elastic constants of zinc-blende Ga 1-x In x N alloys, using the pseudopotential method within the virtual crystal approximation combined with the Harrison bond-orbital model. The agreement between our results and the available experimental and other theoretical data was generally satisfactory. Special attention has been given to the effect of alloy disorder on the studied quantities. For this purpose, the compositional disorder is added to the virtual crystal approximation as an effective potential. It is found that except for the anisotropy factor, the compositional disorder should not be ignored in the calculations of the remaining studied quantities of alloys of interest.

Mosaicity of GaN Epitaxial Layers: Simulation and Experiment

Abstract: High-resolution X-ray diffraction has been used to analyze GaN epilayers with varying coalescence thickness which were grown by MOVPE on (0001) oriented sapphire. The decrease of the density of edge type threading dislocations with increasing coalescence thickness causes a marked difference in the mosaicity of the samples. As the defects form along the grain boundaries, this corresponds to an increase in lateral coherence length with increasing coalescence thickness. The lateral coherence length has been obtained from simulations of reciprocal lattice points of off-axis Bragg reflections, measured in asymmetric diffraction geometry.

Optical Properties of Dense and Diluted Ensembles of Semiconductor Quantum Dots

Abstract: We examine collective effects in an ensemble of close-packed ultrasmall CdSe quantum dots by comparison of optical properties of dense and diluted thin film composites. Studies of absorption spectra, photoluminescence excitation spectra and electroabsorption of diluted and condensed structures along with numerical simulation confirmed that in a dense ensemble extended electron states occur by analogy with Anderson transitions in disordered solids. Therefore evolution from isolated quantum dots to quantum dot solids in case of strong confinement results in a drastic modification of optical and electronic properties.

Atomic Structure of Extended Defects in Wurtzite GaN Epitaxial Layers

Abstract: Using high resolution electron microscopy, atomistic modelling and image simulations, typical contrast was identified for most of the extended defects which form in epitaxial GaN layers. There are three types of defects which propagate into the active layers: the threading dislocations, prismatic stacking faults and inversion domains. The atomic structure of the a pure edge threading dislocations was shown to exhibit 5/7, or 8 atom cycles. The two configurations were observed at a similar frequency for isolated dislocations and low angle grain boundaries. Coincidence grain boundaries were studied and they were made of pure edge a dislocations with the above identified atomic structures. A topological analysis of high angle grain boundaries was carried out in order to determine the defect content at the interfaces. The reconstruction of some boundaries was only possible by taking into account the occurrence of structural units which exhibit 4 atom ring cycles for the dislocation cores. The {1210} stacking fault has two atomic configurations in wurtzite (Ga, Al, In)N with 1/2 and 1/6 displacement vectors. It originates from steps at the SiC surface and it can form on a flat (0001) sapphire surface. The two atomic configurations have comparable energy in AlN, whereas the 1/2 {1210} atomic configuration should be more stable in GaN and InN. Observations carried out in plan-view show the 1/2 {1210} atomic configuration in GaN layers. The 1/6 configuration was found inside the AlN buffer layer in cross section observations. It folds rapidly to the basal plane, and when hack into the prismatic plane, it bears the 1/2 {1210} atomic configuration. The {1010} inversion domains in GaN layers grown on sapphire substrate were identified in multi-beam dark field images and by convergent beam electron diffraction. In the investigated samples, Holt and V models are shown to form in samples depending on the growth conditions. The samples containing Holt inversion domains exhibited a flat surface morphology, whereas the V IDBs were observed in the centre of small pyramids (100 nm high) protruding at the sample surface. The Holt inversion domains were always smaller (<20 nm), at quite high densities (2.5 x 10 10 cm -2 ), whereas the V ones could reach 50 nm and one order of magnitude lower density. The inversion domains were found to be generated mostly at sapphire surface steps where they minimized the large misfit along the c axis (20%).

Electron Transport in III–V Nitride Two‐Dimensional Electron Gases

Abstract: We present a study of electron scattering processes in AlGaN/GaN two-dimensional electron gases. A theoretical study of the effect of deformation potential scattering from strain fields surrounding dislocations is presented. The most important scattering mechanisms limiting electron transport are identified. We find that for AlGaN/GaN 2DEGs, mobility is limited by alloy scattering at high 2DEG densities. For AlN/GaN 2DEGs, interface roughness scattering limits mobility at high densities; there is a large improvement by the removal of the alloy barrier. At low 2DEG densities, dislocation scattering from charged cores and strain fields are the dominant scattering mechanisms.

Growth Temperature Dependences of MOVPE InN on Sapphire Substrates

Abstract: This paper reports growth temperature dependences of MOVPE InN on sapphire substrates Surface morphology of grown InN is markedly dependent on growth temperature An InN film with a fibrous columnar structure with small grains is grown at temperatures lower than 550 °C At 630-650 °C, a continuous film with enhanced two-dimensional growth is obtained In the temperature range 500-600 °C, the growth rate of InN is found to be increased with increasing growth temperature and shows a saturation against the increase in TMI supply At 630-650 °C, such a saturation is not found and a growth rate as high as 08 μm/h is obtained by increasing TMI supply At a growth temperature lower than 600 °C, carrier concentration shows a marked NH 3 /TMI molar ratio dependence, while at 630-650 °C it is independent on NH 3 /TMI molar ratio In order to obtain InN with a carrier concentration of (3-5) x 10 19 cm -3 , an NH 3 /TMI molar ratio as low as 9 x 10 3 is adequate at around 650 °C, while that of 18 x 10 5 is needed at 500 °C This is due to the higher decomposition rate of NH 3 at 650 °C Atmosphric pressure growth gives better electrical properties than reduced pressure (76 Torr) growth, suggesting that effective NH 3 decompostion rate at 760 Torr is higher than that at 76 Torr The difference in carrier concentration between reduced-pressure grown and atmospheric-pressure grown InN films becomes small with increasing growth temperature