Browsing by Subject "Semiconducting gallium compounds"

Now showing 1 - 18 of 18

Open Access
Anharmonicity in GaTe layered crystals
(Wiley-VCH Verlag GmbH & Co. KGaA, 2002) Aydınlı, Atilla; Gasanly, N. M.; Uka, A.; Efeoglu, H.
The temperature dependencies (10-300 K) of seven Raman-active mode frequencies in layered semiconductor gallium telluride have been measured in the frequency range from 25 to 300 cm -1. Softening and broadening of the optical phonon lines are observed with increasing temperature. Comparison between the experimental data and theories of the shift of the phonon lines during heating of the crystal showed that the experimental dependencies can be explained by contributions from thermal expansion and lattice anharmonicity. Lattice anharmonicity is determined to be due to three-phonon processes.
Open Access
Anharmonicity of zone-center optical phonons: Raman scattering spectra of GaSe0.5S0.5 layered crystal
(IOPscience, 2002) Gasanly, N. M.; Aydınlı, Atilla; Kocabas, C.; Özkan H.
The temperature dependencies (10-300 K) of the eight Raman-active mode frequencies and linewidths in GaSe0.5S0.5 layered crystal have been measured in the frequency range from 10 to 320 cm-1. We observed softening and broadening of the optical phonon lines with increasing temperature. Comparison of the experimental data with the theories of the shift and broadening of the interlayer and intralayer phonon lines showed that the temperature dependencies can be explained by the contributions from thermal expansion, lattice anharmonicity and crystal disorder. The purely anharmonic contribution (phonon-phonon coupling) is found to be due to three-phonon processes. It was established that the effect of crystal disorder on the broadening of phonon lines is greater for GaSe0.5S0.5 than for binary compounds GaSe and GaS.
Open Access
Atomic strings of group IV, III-V, and II-VI elements
(American Institute of Physics, 2004) Tongay, S.; Durgun, Engin; Çıracı, Salim
A systematic first-principles study of atomic strings made by group IV, III-V, and II-VI elements has revealed interesting mechanical, electronic, and transport properties. The double bond structure underlies their unusual properties. We found that linear chain of C, Si, Ge, SiGe, GaAs, InSb, and CdTe are stable and good conductor, although their parent diamond (zincblende) crystals are covalent (polar) semiconductors but, compounds SiC, BN, AlP, and ZnSe are semiconductors. First row elements do not form zigzag structures.
Open Access
Dark current reduction in ultraviolet metal-semiconductor-metal photodetectors based on wide band-gap semiconductors
(IEEE, 2009-10) Bütün, Serkan; Gökkavas, Mutlu; Yu, HongBo; Strupinski, Vlodek; Özbay, Ekmel
Photodetectors on semi-insulating GaN templates were demonstrated. They exhibit lower dark current compared to photodetectors fabricated on regular GaN templates. Similar behavior observed in photodetectors fabricated on epitaxially thick SiC templates. © 2009 IEEE.
Open Access
Defect luminescence in undoped p-type GaSe
(Taylor & Francis, 2001) Aydınlı, Atilla; Gasanly, N. M.; Gökşen, K.
Photoluminescence (PL) spectra of undoped single crystals of the layered semiconductor GaSe have been measured in the temperature range from 10 K to room temperature and in the wavelength range from 635 to 750 nm. Two wide bands centred at 644 and 695 nm have been observed at T = 10 K. A detailed analysis of the spectra obtained by varying the excitation intensity and temperature resulted in the identification of the levels involved. A simple model is proposed to account for the observed data.
Open Access
Excitonic improvement of colloidal nanocrystals in salt powder matrix for quality lighting and color enrichment
(OSA - The Optical Society, 2016) Erdem, T.; Soran-Erdem, Z.; Kelestemur, Y.; Gaponik, N.; Demir, Hilmi Volkan
Here we report excitonic improvement in color-converting colloidal nanocrystal powders enabled by co-integrating nonpolar greenand red-emitting nanocrystal energy transfer pairs into a single LiCl salt matrix. This leads to nonradiative energy transfer (NRET) between the cointegrated nanocrystals in the host matrix. Here we systematically studied the resulting NRET process by varying donor and acceptor concentrations in the powders. We observed that NRET is a strong function of both of the nanocrystal concentrations and that NRET efficiency increases with increasing acceptor concentration. Nevertheless, with increasing donor concentration in the powders, NRET efficiency was found to first increase (up to a maximum level of 53.9%) but then to decrease. As a device demonstrator, we employed these NRET-improved nanocrystal powders as color-converters on blue light-emitting diodes (LEDs), with the resulting hybrid LED exhibiting a luminous efficiency >70 lm/Welect . The proposed excitonic nanocrystal powders potentially hold great promise for quality lighting and color enrichment applications.
Open Access
Growth of high crystalline quality semi-insulating GaN layers for high electron mobility transistor applications
(2006) Yu H.; Caliskan, D.; Özbay, Ekmel
Semi-insulating character (sheet resistivity of 3.26 × 10 11 Ω/sq) of thick GaN layers was developed for AlGaN/GaN high electron mobility transistor (HEMT) applications on an AlN buffer layer. Electrical and structural properties were characterized by a dark current-voltage transmission line model, x-ray diffraction, and atomic force microscope measurements. The experimental results showed that compared to semi-insulating GaN grown on low temperature GaN nucleation, the crystal quality as well as surface morphology were remarkably improved. It was ascribed to the utilization of a high quality insulating AlN buffer layer and the GaN initial coalescence growth mode. Moreover, the significant increase of electron mobility in a HEMT structure suggests that this is a very promising method to obtain high performance AlGaN/GaN HEMT structures on sapphire substrates. © 2006 American Institute of Physics.
Open Access
High-performance solar-blind photodetectors based on AlxGa 1_xN heterostructures
(IEEE, 2004) Özbay, Ekmel; Bıyıklı, Necmi; Kimukin, I.; Kartaloglu, T.; Tut, T.; Aytür, O.
Design, fabrication, and characterization of high-performance AI xGa1-xN-based photodetectors for solar-blind applications are reported. AlxGa1-xN heterostructures were designed for Schottky. p-i-n, and metal-semicondnctor-metal (MSM) photodiodes. The solar-blind photodiode samples were fabricated using a microwave compatible fabrication process. The resulting devices exhibited extremely low dark currents. Below 3 fA, leakage currents at 6-V reverse bias were measured on p-i-n samples. The excellent current-voltage (I-V) characteristics led to a detectivity performance of 4.9×1014 cmHz1/2W -1. The MSM devices exhibited photoconductive gain, while Schottky and p-i-n samples displayed 0.09 and 0.11 A/W peak responsivity values at 267 and 261 nm, respectively. A visible rejection of 2×104 was achieved with Schottky samples. High-speed measurements at 267 nm resulted in fast pulse responses with greater than gigahertz bandwidths. The fastest devices were MSM photodiodes with a maximum 3-dB bandwidth of 5.4 GHz.
Open Access
Hot electron effects in unipolar n-type submicron structures based on GaN, AlN and their ternary alloys
(The Institution of Engineering and Technology, 2003) Sevik, C.; Bulutay, C.
The authors present an analysis of impact ionisation (II) and related hot electron effects in submicron sized GaN, AlN and their ternary alloys, all of which can support very high field regimes, reaching a few megavolts per centimetre (MV/cm). The proposed high field transport methodology is based on the ensemble Monte Carlo technique, with all major scattering mechanisms incorporated. As a test-bed for understanding II and hot electron effects, an n+-n-n+ channel device is employed having a 0.1 μm thick n-region. The time evolution of the electron density along the device is seen to display oscillations in the unintentionally doped n-region, until steady state is established. The fermionic degeneracy effects are observed to be operational especially at high fields within the anode n+-region. For AlxGa1-xN-based systems, it can be noted that due to alloy scattering, carriers cannot acquire the velocities attained by the GaN and AlN counterparts. Finally, at very high fields II is shown to introduce a substantial energy loss mechanism for the energetic carriers that have just traversed the unintentionally doped n-region.
Open Access
Low-temperature photoluminescence spectra of layered semiconductor TlGaS2
(Pergamon Press, 1998) Gasanly, N. M.; Aydınlı, Atilla; Bek, A.; Yilmaz, I.
Photoluminescence (PL) spectra of TlGaS2 layered single crystals were studied in the wavelength region 500-860 nm and in the temperature range 9.5-293 K. We observed a total of three PL bands centered at 568 nm (2.183 eV, A-band), 718 nm (1.727 eV, B-band) and 780 nm (1.590 eV, C-band) at various temperatures. We have also studied the variations of the A- and B-band intensities vs excitation laser density in the range from 7 × 10-2 to 9 W cm-2. The A- and B-bands were found to be due to radiative transitions from the deep donor levels located at 0.362 and 0.738 eV below the bottom of the conduction band to the shallow acceptor levels at 0.005 and 0.085 eV located above the top of the valence band, respectively. The proposed energy-level diagram permits us to interpret the recombination processes in TlGaS2 layered single crystals. © 1997 Elsevier Science Ltd.
Open Access
Low-temperature photoluminescence study of GaS0.5Se0.5 layered crystals
(Elsevier Science, 2001) Aydnl, A.; Gasanly, N. M.; Gökşen, K.
Photoluminescence (PL) spectra of GaS0.5Se0.5 layered single crystals have been studied in the wavelength range of 565-860 nm and in the temperature range of 15-170 K. Two PL bands centered at 585 nm (2.120 eV) and 640 nm (1.938 eV) were observed at T = 15 K. Variations of both bands were studied as a function of excitation laser intensity in the range from 10-3 to 15.9 W cm-2. These bands are attributed to recombination of charge carriers through donor-acceptor pairs located in the band gap. Radiative transitions from shallow donor levels located at 0.029 and 0.040 eV below the bottom of the conduction band to deep acceptor levels located 0.185 and 0.356 eV above the top of the valence band are suggested to be responsible for the observed A- and B-bands in the PL spectra, respectively. A simple energy level diagram explaining the recombination process is proposed.
Open Access
Optical studies of molecular beam epitaxy grown GaAsSbN / GaAs single quantum well structures
(A I P Publishing LLC, 2007) Nunna, K.; Iyer, S.; Wu, L.; Bharatan, S.; Li J.; Bajaj, K. K.; Wei, X.; Senger, R. T.
In this work, the authors present a systematic study on the variation of the structural and the optical properties of GaAsSbNGaAs single quantum wells (SQWs) as a function of nitrogen concentration. These SQW layers were grown by the solid source molecular beam epitaxial technique. A maximum reduction of 328 meV in the photoluminescence (PL) peak energy of GaAsSbN was observed with respect to the reference GaAsSb QW. 8 K and RT PL peak energies of 0.774 eV (FWHM of ∼25 meV) and 0.729 eV (FWHM of ∼67 meV) (FWHM denotes full width at half maximum) corresponding to the emission wavelengths of 1.6 and 1.7 μm, respectively, have been achieved for a GaAsSbN SQW of N∼1.4%. The pronounced S -curve behavior of the PL spectra at low temperatures is a signature of exciton localization, which is found to decrease from 16 to 9 meV with increasing N concentration of 0.9%-2.5%. The diamagnetic shift of 13 meV observed in the magnetophotoluminescence spectra of the nitride sample with N∼1.4% is smaller in comparison to the value of 28 meV in the non-nitride sample, indicative of an enhancement in the electron effective mass in the nitride QWs. Electron effective mass of 0.065 mo has been estimated for a SQW with N∼1.4% using the band anticrossing model.
Open Access
Temperature dependence of the first-order Raman scattering in GaS layered crystals
(Pergamon Press, 2000) Gasanly, N. M.; Aydınlı, A.; Özkan, H.; Kocabaş, C.
The temperature dependence (15-293 K) of the six Raman-active mode frequencies and linewidths in gallium sulfide has been measured in the frequency range from 15 to 380 cm-1. We observed softening and broadening of the optical phonon lines with increasing temperature. Comparison between the experimental data and theories of the shift and broadening of the interlayer and intralayer phonon lines during the heating of the crystal showed that the experimental dependencies can be explained by the contributions from thermal expansion and lattice anharmonicity. The pure-temperature contribution (phonon-phonon coupling) is due to three- and four-phonon processes.
Open Access
Temperature-dependent Raman scattering spectra of ε-GaSe layered crystal
(Elsevier Science, 2002) Gasanly, N. M.; Aydnl, A.; Özkan, H.; Kocabaş, C.
The temperature dependencies (15-300 K) of seven Raman-active mode frequencies and linewidths in layered gallium selenide have been measured in the frequency range from 10 to 320 cm-1. We observed softening and broadening of the optical phonon lines with increasing temperature. Comparison between the experimental data and theories of the shift and broadening of the intralayer phonon lines during heating of the crystal showed that the experimental dependencies can be explained by the contributions from thermal expansion, lattice anharmonicity and crystal disorder. The pure-temperature contribution (phonon-phonon coupling) is due to three-phonon processes. Moreover, it was established that the effect of crystal disorder on the linewidth broadening of TO mode is stronger than that of LO mode.
Open Access
Thermally stimulated current observation of trapping centers in undoped GaSe layered single crystals
(Wiley, 2001) Gasanly, N. M.; Aydınlı, A.; Salihoglu, Ö.
Undoped p-GaSe layered single crystals were grown using Bridgman technique. Thermally stimulated current measurements in the temperature range of 10-300 K were performed at a heating rate of 0.18 K/s. The analysis of the data revealed three trap levels at 0.02, 0.10 and 0.26 eV. The calculation for these traps yielded 8.8 × 10-27, 1.9 × 10-25, and 3.2 × 10-21 cm2 for capture cross sections and 3.2 × 1014, 1.1 × 1016, and 1.2 × 1016 cm-3 for the concentrations, respectively.
Open Access
Thermally stimulated currents in layered Ga4SeS3 semiconductor
(2004) Aytekin, S.; Yuksek, N.S.; Goktepe, M.; Gasanly, N.M.; Aydınlı, Atilla
Thermally stimulated current (TSC) measurements are carried out on nominally undoped Ga4SeS3 layered semiconductor samples with the current flowing along the c-axis in the temperature range of 10 to 150 K. The results are analyzed according to various methods, such as curve fitting, initial rise and Chcn's methods, which seem to be in good agreement with each other. Experimental evidence is found for the presence of three trapping centers in Ga4SeS3 with activation energies of 70, 210 and 357 meV. The calculation yielded 7.9 × 10-21,7.0 × 10 -19 and 1.5 × 10-13 cm2 for the capture cross section, and 1.6 × 1010, 6.5 × 1010 and 1.2 × 1011 cm-3 for the concentration of the traps studied. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinlteim.
Open Access
Trap levels in layered semiconductor Ga2SeS
(Elsevier, 2004) Aydınlı, Atilla; Gasanly, N. M.; Aytekin, S.
Trap levels in nominally undoped Ga2SeS layered crystals have been characterized by thermally stimulated current (TSC) measurements. During the measurements, current was allowed to flow along the c-axis of the crystals in the temperature range of 10-300 K. Two distinct TSC peaks were observed in the spectra, deconvolution of which yielded three peaks. The results are analyzed by curve fitting, peak shape and initial rise methods. They all seem to be in good agreement with each other. The activation energies of three trapping centers in Ga2SeS are found to be 72, 100 and 150 meV. The capture cross section of these traps are 6.7×10-23, 1.8×10-23 and 2.8×10-22cm2 with concentrations of 1.3×1012, 5.4×1012 and 4.2×1012cm-3, respectively.
Open Access
Trapping centers in undoped GaS layered single crystals
(Springer, 2003) Gasanly, N. M.; Aydınlı, Atilla; Yüksek, N. S.; Salihoglu, Ö.
Nominally undoped p-GaS layered single crystals were grown using the Bridgman technique. Thermally stimulated current measurements in the temperature range 10-300 K were performed at a heating rate of 0.10 K/s. The analysis of the data revealed six trap levels at 0.05, 0.06, 0.12, 0.63, 0.71, and 0.75 eV. The calculations for these traps yielded 1.2 × 10-21, 2.9 × 10-23, 2.4 × 10-21, 8.0 × 10-9, 1.9 × 10-9 and 4.3 × 10-10 cm2 for the capture cross sections and 1.6 × 1013, 5.0 × 1012, 7.3 × 1012, 1.2 × 1014, 8.9 × 1013 and 2.6 × 1013 cm-3 for the concentrations, respectively.