[Show abstract][Hide abstract] ABSTRACT: Laser irradiation damage in ZnTe epilayers was analyzed in situ by power-density-dependent and time-resolved micro-Raman spectroscopy. Damage by ablation or compound decomposition on the sample surface was revealed by the decrease of the ZnTe–nLO mode intensity with the increase of laser power density. The appearance of the peaks associated with the stronger crystalline-tellurium modes, tellurium aggregates and second-order Raman scattering at room temperature μ-Raman spectra was observed for higher power densities than 4.4 × 105 W cm−2. The Raman signal time transients of ZnTe–nLO and crystalline-tellurium modes reveal an exponential evolution of the laser irradiation damage and a fast formation of crystalline tellurium aggregates on the layer surface.
Semiconductor Science and Technology 09/2011; 26(10):105023. · 1.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The presently unknown band offset in nonpolar cubic GaN/AlN superlattices is investigated by inter-sub-band and interband spectroscopies as well as ab initio calculations. On one hand, the conduction-band offset (CBO) has been determined from the comparison of the measured transition energies with model calculations within the effective mass approximation. On the other hand, the valence-band offset (VBO) and the CBO are accurately simulated by calculating many-body corrections within the GW approximation on top of hybrid-functional density functional theory calculations. Thus, a CBO of (1.4±0.1) eV and a VBO of (0.5±0.1) eV is obtained as a result of both approaches.
Physical Review B 05/2011; 83(19). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Vapor–liquid–solid condensation was utilized to fabricate zinc-blende GaN quantum dots on 3C-AlN (0 0 1) in a molecular beam epitaxy system. By adjustment of deposition parameters and nitridation procedure the density of the quantum dots was controllable in the range of 5e8–5e12 cm−2. The quantum dots in the range of 8e10–5e12 cm−2 have shown strong optical activity in photoluminescence spectroscopy. Furthermore we have demonstrated that vapor–liquid–solid condensation is suitable to tune the emission energy of zinc-blende GaN quantum dots in the range of 3.55–3.81 eV.
Journal of Crystal Growth 05/2011; 323(1):286. · 1.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Non-polar relaxed cubic GaN was grown by molecular beam epitaxy (MBE) on nano-patterned 3C-SiC/Si (0 0 1) substrates with negligible hexagonal content and less defect density than in planar cubic GaN layers. Nano-patterning of 3C-SiC/Si (0 0 1) is achieved by self-ordered colloidal masks for the first time. The method presented here offers the possibility to create nano-patterned cubic GaN without the need for a GaN etching process and thus is a potential alternative to the conventional top–down fabrication techniques.
Journal of Crystal Growth 05/2011; 323(1):84. · 1.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Exciton states have been studied experimentally in strained ZnSe/(Zn,Mg)Se quantum well structures with a Mg content of only 3.6% by means of magnetoreflectivity and ultrafast piezospectroscopy. The small intrinsic band gap difference and the built-in strain in barriers and quantum wells lead to a shallow confinement potential for heavy holes which is smaller than the Coulomb electron-hole interaction. An exciton state formed by a confined electron and heavy-hole continuum states is identified. The experimental findings are supported by numerical model calculations.
[Show abstract][Hide abstract] ABSTRACT: Cubic InGaN/GaN double heterostructures and multi-quantum-wells have been grown by Molecular Beam Epitaxy on cubic 3C-SiC. We find that the room temperature photoluminescence spectra of our samples has two emission peaks at 2.4 eV and 2.6 eV, respectively. The intensity of the 2.6 eV decreases and that of the 2.4 eV peak increases when the In mol ratio is varied between x = 0.04 and 0.16. However, for all samples the peak energy is far below the bandgap energy measured by photoluminescence excitation spectra, revealing a large Stokes-like shift of the InGaN emission. The temperature variation of the photoluminescence intensity yields an activation energy of 21 meV of the 2.6 eV emission and 67 meV of the 2.4 eV emission, respectively. The room temperature photoluminescence of fully strained multi quantum wells (x = 0.16) is a single line with a peak wavelength at about 510 nm.
[Show abstract][Hide abstract] ABSTRACT: Cubic Al y Ga 1-y N/GaN heterostructures on GaAs(001) substrates were grown by radio-frequency plasma-assisted molecular beam epitaxy. High resolution X-ray diffraction, spectroscopic ellipsometry and cathodoluminescence were used to characterize the structural and optical properties of the alloy epilayers. X-ray diffraction reciprocal space maps demonstrate the good crystal quality of the cubic Al y Ga 1-y N films. Both SE as well as room temperature CL of the Al y Ga 1-y N epilayer show a linear increase of the band gap with increasing Al-content. A pseudomorphically strained cubic 10 x (2.4 nm GaN/ 4.8 nm Al 0.12 Ga 0.88 N) multi-quantum well (MQW) structure has been realized. Cathodoluminescence clearly demonstrates strong radiative recombination due to quantized states in the GaN well layer at a photon energy of 3.323 eV.
[Show abstract][Hide abstract] ABSTRACT: We report on carbon doping of cubic GaN by CBr4 using plasma-assisted molecular beam epitaxy. Cubic GaN:C samples were doped at different CBr4 beam equivalent pressures between 2×10−9 and 6×10−6 mbar. The incorporated carbon concentration of up to 1×1020 cm−3 was achieved in c-GaN:C as measured by secondary ion mass spectroscopy. The net donor/acceptor concentration was obtained by evaluation of capacitance-voltage data. Capacitance-voltage measurements on nominally undoped cubic GaN showed n-type conductivity. With an increase in CBr4 flux the conductivity type changed to p-type and for highest CBr4 flux an acceptor surplus of 1×1019 cm−3 was obtained. The electrical properties of the c-GaN:C layers were investigated by current–voltage measurements and a decrease in the serial conductance by two orders of magnitude was demonstrated in c-GaN:C. A blue shift of the 2 K donor–acceptor pair luminescence with an increase in carbon concentration was obtained.
Journal of Crystal Growth 01/2011; 323(1):88-90. · 1.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: By the analysis of intra- and intersubband transitions in GaN∕AlN superlattices the band offset is determined experimentally. Superlattice structures with different period lengths were fabricated by plasma-assisted molecular beam epitaxy 3C-SiC substrates. The structural properties were studied by high resolution X-ray diffraction, revealing a high structural perfection of the superlattice region with several peaks in the X-ray spectra. Infrared absorbance spectroscopy revealed clear intrasubband transitions in the spectral region of 1.55 μm measured at room temperature. Clear intersubband transitions were observed by photoluminescence at room temperature. These transition energies were compared to calculated energies using a 1D Poisson Schro¨dinger solver. For the calculations standard parameters for cubic GaN and AlN were used, while the band offset between GaN and AlN was varied. Optimal agreement between experimental and theoretical data was obtained for a band offset ΔEC:Δ EV of 55:45.
[Show abstract][Hide abstract] ABSTRACT: We report on carbon doping of cubic GaN by CBr4 using plasma-assisted molecular beam epitaxy on 3C-SiC (001) substrates. The samples consist of a 70 nm thick GaN buffer followed by a 550 nm thick GaN:C layer. Carbon doping is realized with a home-made carbon tetrabromide sublimation source. The CBr4 beam equivalent pressure was established by a needle valve and was varied between 2×10-9 mbar and 6×10-6 mbar. The growth was controlled by in-situ reflection high energy electron diffraction. The incorporated carbon concentration is obtained from secondary ion mass spectroscopy. Capacitance voltage characteristics were measured using metal-insulator-semiconductor structures. Capacitance voltage measurements on nominally undoped cubic GaN showed n-type conductivity with ND-NA = 1×1017 cm-3. With increasing CBr4 flux the conductivity type changes to p-type and for the highest CBr4 flux NA-ND = 4.5×1018 cm-3 was obtained.
[Show abstract][Hide abstract] ABSTRACT: Zinc-blende GaN quantum dots were grown on 3C-AlN(0 0 1) by a vapor–liquid–solid process in a molecular beam epitaxy system. We were able to control the density of the quantum dots in a range of 5e8–5e12 cm−2. Photoluminescence spectroscopy confirmed the optical activity of the GaN quantum dots in a range of 1e11–5e12 cm−2. The data obtained give an insight to the condensation mechanism of the vapor–liquid–solid process in general, because the GaN quantum dots condense in metastable zinc-blende crystal structure supplied by the substrate, and not in the wurtzite crystal structure expected from free condensation in the droplet.
Journal of Crystal Growth 10/2010; 312(9):3235. · 1.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Zinc‐blende GaN quantum dots were grown on 3C‐AlN(001) by two different methods in a molecular beam epitaxy system. The quantum dots in method A were fabricated by the Stranski‐Krastanov growth process. The quantum dots in method B were fabricated by droplet epitaxy, a vapor‐liquid‐solid process. The density of the quantum dots was controllable in a range of 1e8 cm−2 to 1e12 cm−2. Reflection high energy electron diffraction analysis confirmed the zinc‐blende crystal structure of the QDs. Photoluminescence spectroscopy revealed the optical activity of the QDs, the emission energy was in agreement with the exciton ground state transition energy of theoretical calculations.
[Show abstract][Hide abstract] ABSTRACT: We report on the optical investigation of single electron spins bound to
fluorine donor impurities in ZnSe. Measurements of photon antibunching
establish the presence of single, isolated optical emitters, and
magneto-optical studies are consistent with the presence of an exciton bound to
the spin-impurity complex. The isolation of this single donor-bound exciton
complex and its potential homogeneity offer promising prospects for a scalable
semiconductor qubit with an optical interface.
[Show abstract][Hide abstract] ABSTRACT: We report on very long electron spin relaxation times in highly n-doped bulk zincblende GaN exceeding 500 ps up to room-temperature. Time-resolved Kerr-rotation measurements show an almost temperature independent spin relaxation time between 80 and 295 K confirming an early prediction of Dyakonov and Perel for a degenerate electron gas.
[Show abstract][Hide abstract] ABSTRACT: Damage caused by laser irradiation on the surface of ZnTe epilayers was studied by micro-Raman and atomic force microscopy (AFM). ZnTe LO-phonon overtones up to four order and TO + (n − 1)LO zone-center phonons were observed in the resonant micro-Raman spectra at room temperature. Discrepancies in the literature regarding the origin of two features observed at low frequencies around 120 and 140 cm−1 in the Raman spectrum of ZnTe are discussed and resolved. These Raman peaks were not detected by using a low excitation laser power density on a Zn-terminated ZnTe surface; however, with the increase of the laser power density they were found to arise irreversibly. The correspondence of these peaks in a wave number with the strongest Raman peaks of the crystalline tellurium phase and the intensity enhancement behavior with the laser power in a similar way as for CdTe strongly suggests the formation of crystalline tellurium aggregates on the layer surface due to laser irradiation damage. AFM data reveal the occurrence of laser ablation on the ZnTe surface even though the surface temperature of the sample is below the melting point.
Semiconductor Science and Technology 06/2010; 25(7):075003. · 1.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: ZnSe <sub>(1-x)</sub> Te <sub>x</sub> (x∼0.06) epilayers were grown on GaAs(001) substrates at 350 ° C by isothermal closed space sublimation (ICSS) technique. The epitaxial growth was performed in low-pressure helium atmosphere (∼0.1 mbar ) by sequential exposures of the substrate to vapors of a solid solution of selenium-tellurium and elemental zinc. The use of a mixed source is proposed in order to regulate the partial vapor pressure of the constituents by composition. Strain and composition of the ZnSe <sub>(1-x)</sub> Te <sub>x</sub> epilayers were extracted from high resolution x-ray reciprocal space mapping. Structural investigations show a reasonably good crystalline quality of the epilayers. Good reproducibility of composition and control of thickness were obtained although atomic layer epitaxy regimen was not achieved. A growth rate of 1.3 monolayers/cycle was ascribed to multilayer adsorption and the existence of an efficient transport of SeTe in graphite under thermodynamic conditions of ICSS. Both Raman and photoluminescence characterizations suggest the existence of random alloy epilayers with larger composition disorder in the mesoscopic scale than those obtained by molecular beam epitaxy.
Journal of Applied Physics 06/2010; · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present our recent results on the growth of cubic AlN (001) layers by plasma assisted molecular beam epitaxy (PAMBE) using freestanding 3C-SiC (001) substrate. For high-quality c-AlN layers reflection high-electron energy diffraction (RHEED) patterns in all azimuths show RHEED patterns of the cubic lattice, hexagonal reflections are absent. High-resolution X-ray diffraction (HRXRD) measurements confirm the cubic structure of the c-AlN layers with a lattice parameter of 4.373 Å. Atomic force microscopy (AFM) scans show an atomically smooth surface with a roughness of 0.2 nm RMS. Ellipsometry studies yield the dielectric function (DF) of c-AlN from 1 to 10 eV. The direct gap is determined with 5.93 eV at room temperature, while the indirect one is below 5.3 eV (onset of adsorption). The high-energy part of the DF is dominated by two transitions at 7.20 and 7.95 eV attributed to critical points of the band structure.
Physica Status Solidi (A) Applications and Materials 05/2010; 207(6):1365 - 1368. · 1.53 Impact Factor