Single crystal n-GaAs substrates have been implanted at room temperature with 120Sn ions at an energy of 70 MeV. The optical microscope and scanning electron microscope studies of the cross section of the implanted samples have shown the formation of two sharp layers at a depth of 8.7 and 11 μm from the surface of the substrate. The electrical characteristics of the Schottky diodes fabricated on the implanted substrates at room temperature indicate the presence of high series resistance due to radiation defects. The electrical properties of the implanted samples were investigated after implantation and annealing to 850 °C. Low temperature resistance measurements of these samples indicate that the samples annealed to 450 °C are dominated by a variable range hopping conduction mechanism, whereas for the samples annealed at 550 and 650 °C the electrical conduction is due to hopping between the neighboring defect sites. At annealing temperatures higher than 650 °C, the electrical transport below room temperature seems to be dominated by carriers in the extended states, which are also responsible for the electrical conduction at room temperature and above, for the samples annealed at temperatures higher than 450 °C.
We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La 0.7 Ca 0.3 MnO 3 (LCMO) and YBa 2 Cu 3 O 7−δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 100 LaAlO 3 (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 µm width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, I c , was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a J c of ∼ 2 × 10 6 A/cm 2 , the LCMO side showed about half the value for the same thickness (1800Å). The difference in J c indicates that a certain thickness of YBCO has become 'effectively' normal due to self-injection. From the measurement of J c at two different thicknesses (1800Å and 1500Å) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection length (at 77 K) was estimated to be ∼ 900Å. To the authors' best knowledge, this is the first time that self-injection length has been quantified. A control experiment carried out with LaNiO 3 deposited by PLD on YBCO did not show any evidence of self-injection.
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