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  • 1
    Digitale Medien
    Digitale Medien
    Diffraction of high-energy synchrotron radiation (invited) (abstract) : Proceedings of the 5th international conference on synchrotron radiation instrumentation (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 1614-1614 
    Details
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: Diffraction of high-energy synchrotron radiation at energies above 100 keV combines advantages of conventional x-ray diffraction and neutron diffraction. For hard x rays absorption in matter is weak with typical mean free paths of several millimeters. Bulk properties are studied on large samples. With a three-crystal diffractometer an excellent k-space resolution of about 10−5 A(ring)−1 transversal and 10−4 A(ring)−1 longitudinal is achieved. In this contribution the particularities of hard x rays, the instrumental setup, and the k-space resolution are discussed and presented. The potential of the new method will be demonstrated on two examples: magnetic diffraction from MnF2 and the structural phase transition of SrTiO3. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1146465
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  • 2
    Digitale Medien
    Digitale Medien
    Ag-induced multistep formation on Si(001) (1995)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 67 (1995), S. 2185-2187 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The step topography of 4°-misoriented Si(001) surfaces was examined by scanning tunneling microscopy and spot profile analyzing–low-energy electron diffraction. The clean Si(001)-(2×1) surface proves to be a single domain substrate, which is characterized by a regular array of double steps. This step structure is changed dramatically upon adsorption of submonolayer quantities of Ag at ∼700 K. In this case, the formation of multisteps with fourfold, sixfold, and eightfold step heights occurs. The single domain character of the substrate is preserved, now showing a Ag-induced (3×2) reconstruction. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.115097
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  • 3
    Digitale Medien
    Digitale Medien
    Energy selective optically pumped stimulated emission from InGaN/GaN multiple quantum wells (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 73 (1998), S. 560-562 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Optically pumped stimulated emission (SE) from InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition has been systematically studied as a function of excitation photon energy (Eexc) to further understand the origin of SE in these structures. Optically pumped SE was observed for excitation photon energies well below that of the absorption edge of the MQWs, indicating the states responsible for the soft absorption edge in these structures can efficiently couple carriers with the gain region. "Mobility edge"-type behavior in the SE peak was observed as Eexc was varied. The effective mobility edge measured in these SE experiments lies ∼110 meV above the main spontaneous emission peak and ∼62 meV above the SE peak. Tuning the excitation energy below the mobility edge was found to be accompanied by a drastic increase in the SE threshold due to a decrease in the effective absorption cross section. The experimental results indicate that the SE peak observed here has the same microscopic origin as the spontaneous emission peak, i.e., radiative recombination of localized states. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.121855
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  • 4
    Digitale Medien
    Digitale Medien
    Stimulated emission characteristics of InGaN/GaN multiple quantum wells: Excitation length and excitation density dependence (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 73 (1998), S. 3689-3691 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Optically pumped stimulated emission (SE) from InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition has been systematically studied as a function of excitation length (Lexc). Two distinct SE peaks were observed from these structures: one that originates at 425 nm at 10 K (430 nm at 300 K) and another that originates at 434 nm at 10 K (438 nm at 300 K). The SE threshold for the high-energy peak was observed to always be lower than that of the low-energy peak, but the difference was found to decrease greatly with increasing Lexc. A detailed study of the emission intensity of these two SE peaks as a function of excitation density shows that the two peaks compete for gain in the MQW active region. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.122864
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  • 5
    Digitale Medien
    Digitale Medien
    Single-electron effects in slim semiconductor superlattices (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 73 (1998), S. 1982-1984 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We fabricated laterally confined GaAs–AlGaAs superlattices with diameters between 500 nm and 2 μm. With decreasing device diameter, a gap evolves in the current–voltage curve around zero bias and steps show up at the onset of the current. This behavior is interpreted in terms of Coulomb blockade, a depletion of the center of the superlattice, and single-electron tunneling through donor levels. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.122342
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  • 6
    Digitale Medien
    Digitale Medien
    High-temperature stimulated emission in optically pumped InGaN/GaN multiquantum wells (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 72 (1998), S. 1623-1625 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Stimulated emission (SE) in optically pumped InGaN/GaN multiquantum well (MQW) structures grown by metalorganic chemical vapor deposition was experimentally studied in the temperature range of 175–575 K. The GaN barriers were intentionally doped with a different Si concentration ranging from 1×1017 to 3×1019 cm−3 and the effects of Si doping of GaN barriers on the optical properties of InGaN/GaN MQWs were investigated. The SE threshold was measured as a function of temperature and compared with bulk GaN. We observed that the SE threshold had a low value and a weak temperature dependence: for example, ∼25 kW/cm2 at 175 K, ∼55 kW/cm2 at 300 K, and ∼300 kW/cm2 at 575 K for one of the samples. Low SE thresholds are attributed to the high-quantum efficiency of the MQWs, possibly associated with the large localization of excitons. The characteristic temperature of 162 K was derived from the temperature dependence of the SE threshold. The integrated emission intensity versus pumping density was examined for different temperatures. This study shows that InGaN/GaN MQWs are suitable for development of laser diodes that can operate well above room temperature. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.121133
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  • 7
    Digitale Medien
    Digitale Medien
    Pump-probe spectroscopy of band tail states in metalorganic chemical vapor deposition-grown InGaN (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 73 (1998), S. 1892-1894 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Nanosecond nondegenerate optical pump-probe experiments have been performed on InGaN thin films and InGaN/GaN multiple quantum wells. Bleaching of absorption of the localized band tail states was observed with increasing excitation density (Iexc) of the pump pulse. The dynamics of the bleaching was found to depend on the localization depth of the band tail states and on Iexc. With high Iexc, large blueshifts in the spontaneous emission luminescence peaks were also observed, the magnitude of which was again found to depend on the localization depth of the band tail states. Stimulated emission is observed from the samples with increasing Iexc and correlates with significant changes in the behavior of the absorption bleaching. The observed bleaching dynamics of the band tail states are well explained by considering the effective lifetime of the band tail states as measured by time-resolved photoluminescence experiments. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.122317
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  • 8
    Digitale Medien
    Digitale Medien
    Single- and multi-wall carbon nanotube field-effect transistors (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 73 (1998), S. 2447-2449 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We fabricated field-effect transistors based on individual single- and multi-wall carbon nanotubes and analyzed their performance. Transport through the nanotubes is dominated by holes and, at room temperature, it appears to be diffusive rather than ballistic. By varying the gate voltage, we successfully modulated the conductance of a single-wall device by more than 5 orders of magnitude. Multi-wall nanotubes show typically no gate effect, but structural deformations—in our case a collapsed tube—can make them operate as field-effect transistors. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.122477
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  • 9
    Digitale Medien
    Digitale Medien
    Large optical nonlinearities near the band gap of GaN thin films (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 72 (1998), S. 1504-1506 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The interband optical transitions in single-crystal GaN films grown by metal organic chemical vapor deposition have been studied at 10 K and room temperature using nondegenerate nanosecond optical pump-probe techniques. At low temperatures, strong, well-resolved features are seen in the absorption and reflection spectra corresponding to the 1s A and B exciton transitions. These features broaden and decrease in intensity due to the presence of a high density of photoexcited free carriers and are completely absent in the absorption and reflection spectra as the excitation density, Iexc, approaches 3 MW/cm2, resulting in induced transparency in transmission measurements. The absorption spectra also show induced absorption below the band gap as Iexc is increased. Both the observed induced transparency and induced absorption were found to be extremely large, exceeding 4×104 cm−1 as the pump density approaches 3 MW/cm2 at 10 K. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.121040
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  • 10
    Digitale Medien
    Digitale Medien
    Temperature dependence of interband transitions in GaN grown by metalorganic chemical vapor deposition (1995)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 66 (1995), S. 985-987 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The interband transitions in single-crystal GaN films grown by metalorganic chemical vapor deposition (MOCVD) have been studied as a function of temperature (15≤T≤300 K) by reflectance and photoluminescence measurements. At low temperatures, well-resolved spectral features corresponding to the GaN band structure were observed. The energies of the excitonic interband ΓV9−ΓC7,ΓV7 (upper band)−ΓC7 and ΓV7(lower band)−ΓC7 transitions are found to be 3.485, 3.493, and 3.518 eV at 15 K, respectively, for the MOCVD GaN. The spectral features are broadened and shift to lower energy as temperature increases. At room temperature (300 K), the ΓV9−ΓC7and ΓV7 (upper band) −ΓC7 transition energies of this wide band-gap material are determined to be 3.420 and 3.428 eV, respectively. The temperature dependence of these two transitions have been determined using the Varshni empirical relation. Our results yield E0(T)=3.486–8.32×10−4 T2/(835.6+T) eV for the ΓV9−ΓC7 transition and E0(T)=3.494–10.9×10−4 T2/(1194.6+T) eV for the ΓV7 (upper band) −ΓC7 transition. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.113820
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