In this letter we report the synthesis of superconducting
T′-La2-x
Ce
x
CuO4 on SrTiO3(001) by molecular beam
epitaxy. Although T′-La2-x
Ce
x
CuO4 with a very limited range
of x can be prepared by bulk synthesis using rather complicated techniques, this
compound is very easy to obtain by thin film synthesis. Crucial to our success was the
use of SrTiO3 substrates, which seems to stabilize the formation of the
T′ structure by an epitaxial effect. The best T
c was
∼30 K by onset and ∼28 K by zero resistance, which was obtained for
x=0.109. The resistivity showed a metallic temperature dependence, and the film's
overall behavior was similar to that for the best (Nd, Ce)2CuO4 or
(Pr, Ce)2CuO4 films. Moreover, the single phase of the T′
structure formed for a composition range of 0.09<x<0.22, which is much wider than
for previously reported bulk synthesis.
Highly p-doped silicon/silicon–Germanium (SiGe) quantum well structures have been grown by molecular beam epitaxy on 〈100〉 Si substrates as detectors in the midinfrared regime (3–5 μm; 8–12 μm). These detectors—operating from about 3 to 20 μm—are based on heterointernal photoemission of photogenerated holes across the Si/SiGe valence band from a highly p-doped SiGe layer which is grown on an undoped (50 Ω cm), double-sided polished Si substrate. The thickness of the well, its doping level, and its Ge content determines the valence band offset and thus the operating wavelength of the detector. This layer sequence has been repeated up to five times and the structure has been terminated with a p-doped SiGe contact layer on top. The samples grown have been extensively analyzed by secondary ion mass spectroscopy, x-ray diffraction, Rutherford Backscattering, and absorption spectroscopy. Mesa detectors of varying diameters have been fabricated using a standard Si processing technique, and the photocurrent and dark current have been measured at 77 K. A broad quantum efficiency of ηext=0.3% has been obtained (at 4 μm and 77 K) with dark current densities as low as 10−8 A/cm2. The photoresponse spectrum shows a broad maximum between 3 and 5 μm; it is shown that the peak in this spectrum can be tuned over a fairly broad range by the choice of layer composition.
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