The structure and magnetic properties of Co-doped ZnO films are discussed in relation to cobalt doping levels and growth conditions. Films were deposited by pulsed-laser deposition (PLD) from ZnO targets containing cobalt concentrations from 0 to 30 at.%. The structure of the films is examined by x-ray diffraction (XRD) and transmission electron microscopy (TEM), and optical absorption is used to infer the substitution of cobalt inside the ZnO lattice. Magnetic properties are characterized by superconducting quantum interference device (SQUID) magnetometry. Films doped with cobalt concentrations of a few per cent appear to be composed of two magnetic components: a paramagnetic component and a low-field ferromagnetic component. Films doped with 30% cobalt show a larger FM signature at room temperature with clear hysteretic shape, but films grown at low pressure are plagued by the precipitation of metallic cobalt nanoparticles within the lattice which can be easily detected by XRD. These particles are well oriented with the ZnO crystal structure. By increasing the base pressure of the vacuum chamber to pressures above 1×10−5 Torr, metallic cobalt precipitates are undetectable in XRD scans, whereas the films still show an FM signature of ∼0.08 μB/Co. Depositions in the presence of oxygen background gas at 0.02 mTorr decreases the magnetization. The decreased magnetization with oxygen suggests that the activation of ferromagnetism depends on defects, such as oxygen vacancies, created during growth. Optical absorption measurements show a sequential increase in the Co+2 absorption peaks in these films, along with an almost linearly increasing bandgap with cobalt concentration suggesting a large solubility of cobalt in ZnO. Bright-field TEM imaging and electron diffraction do not show signs of precipitation; however, dark-field imaging shows circular areas of varying contrast which could be associated with cobalt precipitation. Therefore, the possibility that ferromagnetism results from secondary phases cannot be ruled out.

We have studied the electrical and optical characteristics of isotype p-P organic heterojunctions (HJ) consisting of CuPc (copper phthalocyanine) and PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride). It is found that the charge-transport properties of the heterojunction are limited by thermionic emission of holes over the energy barrier at the CuPc/PTCDA heterojunction at low forward and reverse bias, and by series resistance at high voltage. The heterojunction energy barrier at the CuPc/PTCDA valence-band edge was measured using both current-voltage and capacitance-voltage analysis and was found to be ΔEvC,P=0.48±0.05 eV. Similar measurements made for HJs consisting of CuPc and PTCDA in combination with another perylene-based material, 3, 4, 9, 10-perylenetetracarboxylic-bis-benzimidazole (PTCBI), suggest that the band offsets for these three materials follow a transitive relationship. That is, ΔEvC,P=ΔEvC,B−ΔEvB,P, where subscripts C, P, and B refer to CuPc, PTCDA, and PTCBI, respectively. The results are discussed in terms of energy-band and molecular energy-level models.

We describe investigations of the effects of inserting a thin, low-doped layer into the emitter of an InP/In0.53Ga0.47As heterojunction phototransistor (HPT). This high-low emitter structure has improved sensitivity and bandwidth over conventional structures at low input optical power by decreasing the bulk recombination current at the heterointerface. Experimental data show that the photocurrent gain is independent of the incident optical power at high input powers, corresponding to a heterojunction ideality factor of 1. At low input power, the gain is found to have a small power dependence, with an ideality factor of 1.25. A current gain as high as 260 is obtained at an input power of only 40 nW. These results, which are consistent with numerical simulations of the HPTs, give direct evidence that bulk recombination in the space- charge region at the emitter/base junction is the major source of recombination current for an InP/In0.53Ga0.47As HPT. A second structure is also proposed to improve the sensitivity by inserting a heavily doped layer into the base.

The substrate coupling effects of two adjacent coplanar spiral inductors are characterized and modeled. The noise magnitude between two 45pm-away inductors can be reduced by 6.83 dB by using guard-ring surrounding each inductor, and improved by 10.28 dB further by adding patterned ground polysilicon shield beneath at 3 GHz. The inductor with patterned polysilicon shield beneath shows improved quality factor and noise isolation. Moreover, a macro model is presented for modeling quality factor and inductance of on-chip spiral inductor and associated neighboring inductor's coupling noise effect.

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