Carrier localization in gallium nitride

In wide bandgap GaN, a large number of interesting and important scientific questions remain to be answered. For example, the large free electron concentration reaching 10{sup 19} to 10{sup 20} cm{sup - 3} in nominally undoped material are ascribed to intrinsic defects because no chemical impurity has been found at such high concentrations. According to theoretical models, a nitrogen vacancy acts as a donor but its formation energy is very large in n-type materials, making this suggestion controversial. We have investigated the nature of this yet unidentified donor at large hydrostatic pressure. Results from infrared reflection and Raman scattering indicate strong evidence for localization of free carriers by large pressures. The carrier density is drastically decreased by two orders of magnitude between 20 and 30 GPa. Several techniques provide independent evidence for results in earlier reports and present the first quantitative analysis. A possible interpretation of this effect in terms of the resonant donor level is presented

Pressure dependence of Se absorption lines in AlSb

Using far infrared absorption spectroscopy, the authors have investigated electronic transition spectra of Se donors in AlSb as a function of hydrostatic pressure. At least two distinct ground to bound excited state transition lines, which depend quadratically on the pressure, can be seen. At pressures between 30 and 50 kbar, evidence of an anti-crossing between one of the electronic transitions and a peak which they attribute to the 2 zone center LO phonon mode can be seen

Gallium self-diffusion in gallium arsenide: A study using isotope heterostructures

Ga self-diffusion was studied with secondary-ion mass spectroscopy in {sup 69}GaAs/{sup 71}GaAs isotope heterostructures grown by molecular beam epitaxy on GaAs substrates. Results show that the Ga self- diffusion coefficient in intrinsic GaAs can be described accurately with D = (43{+-}25 cm{sup 2}s{sup -1})exp(-4.24{+-}0.06 eV/k{sub B}T) over 6 orders of magnitude between 800 and 1225 C under As-rich condition. Experimental results combined with theoretical calculations strongly suggest Ga vacancy being the dominant native defect controlling the diffusion. No significant doping effects were observed in samples where the substrates were doped with Te up to 4x10{sup 17}cm{sup -3} or Zn up to 1x10{sup 19}cm{sup -3}

Compositional Modulation in InxGa1-xN

Transmission Electron Microscopy and x-ray diffraction were used to study compositional modulation in In{sub x}Ga{sub 1-x} N layers grown with compositions close to the miscibility gap. The samples (0.34 < x < 0.8) were deposited by molecular beam epitaxy using either a 200-nm-thick AlN or GaN buffer layer grown on a sapphire substrate. In the TEM imaging mode this modulation is seen as black/white fringes which can be considered as self-assembled thin quantum wells. Periodic compositional modulation leads to extra electron diffraction spots and satellite reflections in x-ray diffraction in the {theta}-2{theta} coupled geometry. The modulation period was determined using both methods. Larger modulation periods were observed for layers with higher In content and for those having larger mismatch with the underlying AlN buffer layer. Compositional modulation was not observed for a sample with x = 0.34 grown on a GaN buffer layer. Modulated films tend to have large 'Stokes shifts' between their absorption edge and photoluminescence peak

Compositional tuning of ferromagnetism in Ga1-xMnxP

We report the magnetic and transport properties of Ga1-xMnxP synthesized via ion implantation followed by pulsed laser melting over a range of x, namely 0.018 to 0.042. Like Ga1-xMnxAs, Ga1-xMnxP displays a monotonic increase of the ferromagnetic Curie temperature with x associated with the hole-mediated ferromagnetic phase while thermal annealing above 300 C leads to a quenching of ferromagnetism that is accompanied by a reduction of the substitutional fraction of Mn. However, contrary to observations in Ga1-xMnxAs, Ga1-xMnxP is non-metallic over the entire composition range. At the lower temperatures over which the films are ferromagnetic, hole transport occurs via hopping conduction in a Mn-derived band; at higher temperatures it arises from holes in the valence band which are thermally excited across an energy gap that shrinks with x.Comment: To be published in Solid State Communication

CUORE: A Cryogenic Underground Observatory for Rare Events

CUORE is a proposed tightly packed array of 1000 TeO2 bolometers, each being a cube 5 cm on a side with a mass of 760 g. The array consists of 25 vertical towers, arranged in a square of 5 towers by 5 towers, each containing 10 layers of 4 crystals. The design of the detector is optimized for ultralow-background searches: for neutrinoless double beta decay of 130Te (33.8% abundance), cold dark matter, solar axions, and rare nuclear decays. A preliminary experiment involving 20 crystals 3x3x6 cm3 of 340 g has been completed, and a single CUORE tower is being constructed as a smaller scale experiment called CUORICINO. The expected performance and sensitivity, based on Monte Carlo simulations and extrapolations of present results, are reported.Comment: 39 pages, 12 figures, submitted to NI

Star Formation and Dynamics in the Galactic Centre

The centre of our Galaxy is one of the most studied and yet enigmatic places in the Universe. At a distance of about 8 kpc from our Sun, the Galactic centre (GC) is the ideal environment to study the extreme processes that take place in the vicinity of a supermassive black hole (SMBH). Despite the hostile environment, several tens of early-type stars populate the central parsec of our Galaxy. A fraction of them lie in a thin ring with mild eccentricity and inner radius ~0.04 pc, while the S-stars, i.e. the ~30 stars closest to the SMBH (<0.04 pc), have randomly oriented and highly eccentric orbits. The formation of such early-type stars has been a puzzle for a long time: molecular clouds should be tidally disrupted by the SMBH before they can fragment into stars. We review the main scenarios proposed to explain the formation and the dynamical evolution of the early-type stars in the GC. In particular, we discuss the most popular in situ scenarios (accretion disc fragmentation and molecular cloud disruption) and migration scenarios (star cluster inspiral and Hills mechanism). We focus on the most pressing challenges that must be faced to shed light on the process of star formation in the vicinity of a SMBH.Comment: 68 pages, 35 figures; invited review chapter, to be published in expanded form in Haardt, F., Gorini, V., Moschella, U. and Treves, A., 'Astrophysical Black Holes'. Lecture Notes in Physics. Springer 201