ALBERT

Notes: 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.

Notes: 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.

Notes: 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.

Notes: The diffusivity of the zinc Ga1−xAlxAs at 650 °C was studied using sealed-ampoule diffusion. Whereas Ageno, Roedel, Mellen, and Escher [Appl. Phys. Lett. 47, 1193 (1985)] found dramatic decreases in the zinc diffusivity at xAl≈0.05 and xAl≈0.20 using open-tube diffusion, the results of the present study show that the sealed-ampoule technique results in a more uniform dependence of the zinc diffusivity on aluminum concentration.

Notes: With x-ray diffraction techniques, it is possible to routinely measure lattice parameters to several parts in 104 for thin-film samples. However, measurements of lattice parameter changes for quaternary device structures several microns in width are not usually feasible with x-ray diffraction techniques. For this reason, transmission electron microscopy has been used to determine the position of higher-order Laue zone lines within convergent-beam electron diffraction patterns from thin foil cross sections of planar quaternary layers grown on InP substrates. A calibration curve has been generated which describes the position of higher-order Laue zone lines as a function of the lattice parameter determined from x-ray diffraction measurements. For the active quaternary region of an elecro-optical device structure, it is shown that ths calibration procedure may be sensitive to a relative change in lattice parameter as small as ±2 parts in 104.

Notes: 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.

Notes: 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.

Notes: 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.

Notes: 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.

Notes: High-power operation of AlGaAs multi-quantum-well laser diodes emitting near 730 nm is reported. 1000 h, reliable operation at a power of 1.0 W for 100 μm emission aperture and 500 μm cavity length devices has been demonstrated at room temperature. These devices have threshold current densities of 770 A/cm2 with the characteristic temperature coefficients of threshold current, T0, and external differential quantum efficiency, T1, of 152 and 167 K, respectively. © 1999 American Institute of Physics.