Bibliothek

Notizen: The nested carbon nanotubes discovered by Ijima and co-workers as a by-product of fullerene production extended the dimension and geometries of fullerenes into the domain of nanoparticles. These nanotubes are fiberlike structures consisting of concentric graphite sheets nested along the axis extending as long as several micrometers, and diameters of these nanotubes are limited to less than 100 nm. In this paper we report preparation, structural and magnetic properties of cobalt-doped carbon nanotubes. We observed a tubular structure made of coaxial graphite sheets. The inside, of these carbon nanotubes, was filled with cobalt particles. The magnetic properties of the Co-doped nanotubes were measured using a SQUID magnetometer. The undoped (pure) carbon nanotubes exhibit diamagnetic behavior. The Co-doped nanotubes exhibit superparamagnetic behavior with saturation of magnetization at about 0.5 T and coercive field of 750 G. © 1996 American Institute of Physics.

Notizen: We have determined that silicon films have nearly ideal properties for use as a diffusion mask and encapsulation coating for InP and GaAs. The Si films, composed of a single element, are easily and reproducibly deposited by electron beam evaporation at low temperatures. Sharp features can be defined by standard photolithography and freon-plasma etching. The thermal coefficient of expansion of silicon nearly matches that of InP and GaAs so that problems due to film stress are avoided. Additionally, the interaction of Si with InP and GaAs crystals, under severe thermal treatments often used in device fabrication, was found to be negligible. Finally, we found that the Si film acts as a good diffusion mask for Zn which is a common p-type impurity for forming p-n junctions in III-V compounds.

Notizen: A practical dual-wavelength light-emitting double-diode (LEDD) device structure is presented. This device is relatively simple to fabricate and requires a single device-to-fiber alignment for packaging. Additionally, the individual wavelengths can be selected within the 0.8 to 1.6-μm low-loss spectral region of optical fibers. A dual-wavelength LEDD emitting at 1.3 and 1.1 μm is demonstrated. Powers exceeding 10 μW for each wavelength at 50-mA drive current are coupled into a lensed, graded index, 62.5-μm core, 0.29-NA optical fiber.

Notizen: Channeled-substrate buried heterostructure (CSBH) lasers which were purged from populations undergoing high reliability qualification have been studied in detail. Gradual and rapid degradation mechanisms leading to accelerated aging failure modes have been analyzed by transmission electron microscopy, convergent beam electron diffraction, electroluminescence, energy dispersive x-ray analysis, and chemical etching. The gradual degradation mode of CSBH lasers is characterized by (1) a gradual increase in room-temperature threshold current; (2) a decrease in external quantum efficiency, typically a drop in peak value of dL/dI greater than 25%; (3) a drop in forward voltage at low current, indicating a change in junction characteristics; (4) a large peak inI(dV/dI) below threshold (at around 3 mA); and (5) an enhancement in the peak in I2(d2V/dI2) at laser threshold. A defect mechanism associated with the gradual degradation begins with a nucleation of extrinsic dislocation loops along the V-groove {111} p-n–type sidewall interfaces between the Cd-diffused p-InP and liquid-phase-epitaxial-grown n-InP buffer inside the groove. These dislocation loops subsequently grow out of the interfaces into the n-InP buffer region in the direction of minority-carrier injection, indicating a nonradiative recombination-assisted defect growth process. For those loops which enter the quaternary active region near the tip of the active crescent, the growth rate along the (001) and (010) planes is greatly enhanced and the loops eventually cut across the active stripe and become dark-line defects, as confirmed by electroluminescence. Nucleation of dislocation loops is not observed along the {111} p-p–type sidewall interfaces above the active stripe. The fact that the dislocation loops are all extrinsic in nature implies that the {111} sidewall interfaces as well as the quaternary active region contain a high density of interstitials. The possible causes for the generation and growth of the dislocation loops and the high density of point defects are discussed. The rapid degradation mode of the CSBH laser is characterized by a sudden drop in light intensity during the aging process. The associated defect mechanism starts with localized melting at the mirror facet or inside the lasing cavity. A metal-rich droplet subsequently forms which propagates along the center of the active stripe in the direction towards the cavity center via a meltback-regrowth process; i.e., material melts in front of the droplet and regrows after it propagates by. The nonideal condition of regrowth results in the formation of a wormlike defect composed of a cylinder of defective materials bounded by an off-stoichiometric interface. The wormlike defect is dark under electroluminescence. Complicated dislocation structures can also be grown from the wormlike defect under a nonradiative recombination-assisted defect growth process. These phenomena are presented and discussed.

Notizen: A two-step, open-tube diffusion process has been developed to form p+-p-n junctions in GaAs. n-type GaAs substrates were zinc diffused at 550 °C to form a p+ layer, capped with thermally deposited silicon nitride, and annealed in an open-tube furnace. The zinc redistributes during the anneal, forming a p layer adjacent to the p+ layer. The peak carrier concentration of the p+ and p layers and the depth of the p+-p and p-n junctions are controlled by the time and temperature of the anneal. The anneal time was varied from 30 to 180 min and the temperature range was 700–850 °C. Both the p+-p and p-n junction depths show a square root dependence on annealing time. Diffusion coefficients and activation energies for the zinc diffusion which occurs during the anneal are determined.

Notizen: The electrical and optical properties of AlGaAs grown by metalorganic molecular-beam epitaxy using triethylaluminum, tri-isobutylaluminum, and trimethylamine-alane are compared. It is found that tri-isobutylaluminum yields the lowest residual carbon incorporation in the layers (Na − Nd = 4 × 1015 cm−3) and the highest electron and hole mobilities. Photoluminescence spectra for the higher-quality AlGaAs, grown using TiBAl, show excitonic luminescence. However, this luminescence appears to be defect related.

Notizen: Recently, a number of studies have reported a correlation between variations in threshold voltage of field effect transistors and the nonuniformity of the luminescence efficiency of semi-insulating GaAs crystals grown by the liquid-encapsulated Czochralski technique. The changes in luminescence efficiency and subsequently the variations in threshold voltages were dramatically reduced by postgrowth annealing of the GaAs crystals under a variety of conditions. In this study, we employ the technique of spatially resolved cathodoluminescence (CL) to carefully examine the changes in luminescence efficiency due to postgrowth annealing. In agreement with previous work, we find that the CL variations are greatly reduced from a factor of ∼2 to ∼5% by thermal annealing at 800 °C for 30 h or at 1200 °C for 6 h followed by slow cooling. The latter thermal treatment is the same as that experienced by crystals during growth by the horizontal gradient freeze (HGF) technique. The extremely uniform luminescence efficiency of HGF crystals is thus believed to be a result of the thermal treatment during growth. Using evacuated sealed ampoule annealing in the temperature range of 550–650 °C, we show for the first time that improvements in CL uniformity are a result of a diffusion process which involves an As vacancy. Due to the dependence on As loss, values of the diffusivity (D) depend on the surface conditions. Values of D=9×106 exp(−2.6 eV/kT) cm2/s and D=1×107 exp(−2.5 eV/kT) cm2/s are obtained for polished and as-cut surfaces, respectively.

Notizen: Zinc was diffused into GaAs, Al0.38Ga0.62As, and GaAs0.6P0.4 using a 2000–3000-A(ring)-thick yttria-stabilized cubic-zirconia (YSZ) protection layer to produce planar p-n junctions. The YSZ layer greatly reduced thermal decomposition of the semiconductor while allowing zinc to diffuse into the III-V semiconductors. The diffusion depth showed a square-root-of-time dependence for all samples protected with YSZ. Characterization of the GaAs diffused with the YSZ protection process indicates that the YSZ layer has virtually no effect on either the carrier concentration profile or activation energy. No thermal decomposition was observed by visual inspection following a diffusion using only elemental Zn on the GaAs, Al0.38Ga0.62As, or GaAs0.6P0.4 samples protected with a YSZ layer. Photoluminescence analysis shows that the YSZ-protected GaAs samples have higher luminescence efficiency relative to unprotected samples. Device fabrication has been demonstrated by combining the YSZ passivation process with selective area diffusion. Our results indicate that the YSZ diffusion procedure has greater tolerance to process variations than previously reported diffusion procedures for zinc in III-V semiconductors.

Notizen: The reverse-biased performance of a molecular-beam-epitaxy-grown high-power optothyristor has been systematically characterized for pulsed power-switching applications. The device has a P+N-SI-PN+ thyristor-like structure with the bipolar junctions formed by AlGaAs. The semi-insulating (SI) GaAs used is liquid-encapsulated-Czochralski grown, undoped, and 650 μm in thickness. It is found that the reverse-biased optothyristor can be triggered by a light-emitting diode operated at 10−5 W, and miniature semiconductor lasers can trigger the switch with 132 A current using only a 1-mm-diam optical aperture. The reverse switching di/dt and the maximum peak current are reported as a function of blocking voltage. The effects of bipolar junctions on both sides of the SI-GaAs are also reported by comparing the bulk photoconductive current with the optothyristor switched current. It is shown that a laser beam of 0.05 μJ can be used to trigger on and switch about the same current as a 0.3 μJ laser beam, suggesting the possibility of integrating miniature semiconductor lasers and the optothyristors on the same chip to form a portable, compact, high-power solid-state pulser.

Notizen: In this paper we describe the properties of high-quality, semiconductor laser facet coatings based on yttria-stabilizied cubic zirconia (90-m% ZrO2/10-m% Y2O3). We have found that cubic zirconia films can be reproducibly deposited by electron-beam evaporation with an index of refraction of 1.98 at 6328 A(ring), almost ideal for use as a single-layer antireflection coating for GaAs/GaAlAs-based lasers. ZrO2 has a monoclinic crystal structure at room temperature, but changes to tetragonal, hexagonal, and cubic phases upon heating to higher temperatures. However, the addition of the Y2O3 stabilizes ZrO2 in the cubic form, thus allowing electron-beam deposition of thin films of this material to be more controllable and reproducible without the usual addition of oxygen into the vacuum chamber during deposition. Preliminary aging tests of high-power GaAs/GaAlAs lasers show that cubic zirconia films suppress the photo-enhanced oxidation of laser facets that degrades device performance.