High quality GaMnAs films grown with As dimers

We demonstrate that GaMnAs films grown with As2 have excellent structural, electrical and magnetic properties, comparable or better than similar films grown with As4. Using As2, a Curie temperature of 112K has been achieved, which is slightly higher than the best reported to date. More significantly, films showing metallic conduction have been obtained over a much wider range of Mn concentrations (from 1.5% to 8%) than has been reported for films grown with As4. The improved properties of the films grown with As2 are related to the lower concentration of antisite defects at the low growth temperatures employed.Comment: 8 pages, accepted for publication in J. Crystal Growt

Molecular beam epitaxy of free-standing bulk wurtzite AlxGa1-xN layers using a highly efficient RF plasma source

Recent developments with group III nitrides suggest AlxGa1-xN based LEDs can be new alternative commer-cially viable deep ultra-violet light sources. Due to a sig-nificant difference in the lattice parameters of GaN and AlN, AlxGa1-xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE) using a novel RF plasma source. Thick wurtz-ite AlxGa1-xN films were grown by PA-MBE on 2-inch GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1-xN samples. Growth rates of AlxGa1-xN up to 3 μm/h have been demonstrated. Our novel high efficiency RF plasma source allowed us to achieve free-standing bulk AlxGa1-xN layers in a single day’s growth, which makes our MBE bulk growth technique commercially vi-able

Molecular beam epitaxy of highly mismatched N-rich GaNSb and InNAs alloys

GaN materials alloyed with group V anions form the so-called highly mismatched alloys (HMAs). Recently, the authors succeeded in growing N-rich GaNAs and GaNBi alloys over a large composition range by plasma-assisted molecular beam epitaxy (PA-MBE). Here, they present first results on PA-MBE growth and properties of N-rich GaNSb and InNAs alloys and compare these with GaNAs and GaNBi alloys. The enhanced incorporation of As and Sb was achieved by growing the layers at extremely low growth temperatures. Although layers become amorphous for high As, Sb, and Bi content, optical absorption measurements show a progressive shift of the optical absorption edge to lower energy. The large band gap range and controllable conduction and valence band positions of these HMAs make them promising materials for efficient solar energy conversion devices

Molecular beam epitaxy of free-standing bulk wurtzite AlxGa1-xN layers using a highly efficient RF plasma source

Recent developments with group III nitrides suggest AlxGa1-xN based LEDs can be new alternative commer-cially viable deep ultra-violet light sources. Due to a sig-nificant difference in the lattice parameters of GaN and AlN, AlxGa1-xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE) using a novel RF plasma source. Thick wurtz-ite AlxGa1-xN films were grown by PA-MBE on 2-inch GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1-xN samples. Growth rates of AlxGa1-xN up to 3 μm/h have been demonstrated. Our novel high efficiency RF plasma source allowed us to achieve free-standing bulk AlxGa1-xN layers in a single day’s growth, which makes our MBE bulk growth technique commercially vi-able

Unintentional boron incorporation in AlGaN layers grown by plasma-assisted MBE using highly efficient nitrogen RF plasma-sources

Plasma-assisted molecular beam epitaxy (PA-MBE) is now widely used for the growth of group III-nitrides. Many years ago it became clear that during PA-MBE there is unintentional doping of GaN with boron (B) due to decomposition of the pyrolytic boron nitride (PBN) cavity of the RF plasma source. In this paper we discuss the unintentional B incorporation for PA-MBE growth of GaN and AlxGa1−xN using a highly efficient RF plasma source. We have studied a wide range of MBE growth conditions for GaN and AlxGa1−xN with growth rates from 0.2 to 3 µm/h, RF powers from 200 to 500 W, different nitrogen flow rates from 1 to 25 sccm and growth times up to several days. The chemical concentrations of B and matrix elements of Al, Ga, N were studied as a functions of depth using secondary ion mass spectrometry (SIMS). We demonstrate that boron incorporation with this highly efficient RF plasma source is approximately 1×1018 to 3×1018 cm−3 for the AlxGa1−xN growth rates of 2 – 3 µm/h

Thermal and thermo-electric transport properties of quantum point contacts

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Thermo-electric properties of quantum point contacts

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Coulomb-regulated conductance oscillations in a disordered quantum wire

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Terahertz conductivity of the highly mismatched amorphous alloy, GaNBi

We report terahertz optical conductivity measurements of the highly mismatched alloy, GaNBi. We find that in these amorphous GaNBi epilayers grown using plasma assisted molecular beam epitaxy, the optical conductivity is enhanced in the samples grown at higher gallium beam equivalent pressure (BEP). The optical conductivity spectra in these pseudo-amorphous epilayers follow a Drude–Smith behaviour due to charge confinement effects. The direct current conductivity in the epilayers grown at the highest Ga BEP (3.1 × 10−7 Torr) show an increase of three orders of magnitude compared to the one grown at the lowest Ga BEP (2.0 × 10−7 Torr). Our measurements suggests a percolative transition from an insulating nature in the GaNBi epilayers grown at low Ga BEP to a highly conducting phase in the epilayers grown at high Ga BEP

Magnetoconductance of two point contacts in series

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe