Μm Stripe Width Research Articles

Maskless Lateral Epitaxial Overgrowth of GaN on Sapphire

ABSTRACTWe demonstrate a technique of lateral epitaxial overgrowth (LEO) of GaN, termed 'maskless' LEO, in which no mask is deposited prior to LEO regrowth. Instead, a bulk (> 2 μm) GaN layer on sapphire is selectively dry etched, leaving ∼5 μm-wide stripe mesas oriented in the <1010>GaN direction, with a 20 μm period. These stripes serve as seeds for LEO GaN growth, which proceeds from the tops of the stripes and expands laterally, resulting in a ‘T’, or overhang, morphology. As for LEO over an SiO2 mask, significant defect reduction (from ∼109 cm−2 to ∼106 cm−2 ) is observed in cross-sectional transmission electron microscopy (TEM). Atomic force microscopy of the top surface of the LEO GaN reveals that no threading dislocations with screw component terminate at the surfaces of laterally overgrown regions. X-ray diffraction measurements reveal that the wings exhibit a crystallographic tilt away from the seed regions in an azimuth perpendicular to the stripe direction; the tilt angle (∼0.4 – 0.5°) is relatively independent of growth temperature and wing aspect ratio.

Magnetic domains and magnetization reversal in epitaxial Fe layers patterned by the atomic saw method

We present a study of the magnetic domains structure and the magnetization reversal in systems of stripes and dots patterned on Fe films by the atomic saw method. Continuous epitaxial 50 and 20 Å Fe films have been grown by molecular beam epitaxy deposition method, then by applying strain on MgO monocrystalline substrate and controlling the dislocation slipping, micronic iron stripes and dots have been obtained. For the system of 1.4 μm wide stripes characterized by a strong uniaxial magnetic anisotropy (Ha=1500 Oe) resulting from a uniaxial relaxation of Fe lattice parameter, large magnetic domains have been observed by Kerr microscopy imaging. This structure is not directly correlated to the geometry of ribbons. It reveals the strong influence of the mutual dipolar coupling leading to a quasi-collective magnetization reversal. In contrast, Kerr microscopy observations done on samples structured into 1–3 μm dots show that the domain wall propagation is hindered by the net of orthogonal steps generated by the process. The domain architecture is then directly twinned to the geometry of dots. Both dot mean size and effective size dispersion which is imposed by the dipolar field seem to be the two relevant parameters that govern the two steps magnetization reversal. Calculations of coupling dipolar field are done showing its strong influence upon the magnetization reversal in the stripes case.

CW operation of 3.4 µm optically-pumped type-IIW laser to 220 K

An optically-pumped type-II W laser emitting at 3.4 µm operated CW up to 220 K, with a characteristic temperature of 51 K for the range 78–220 K. At 78 K, the output power was 90 mW from a 12 µm wide stripe, and the differential power conversion efficiency was 2.9% per facet (uncoated).

Room-temperature continuous-wave operation of GaInN/GaNmultiquantum well laser diode

Continuous-wave operation at room temperature was demonstrated in a GaInN/GaN multiquantum well (MQW) laser grown by metal organic chemical vapour deposition (MOCVD) using a horizontal reactor. The laser structure was grown on a (0001) c-plane sapphire substrate. A 1 mm long cavity with a 4 µm wide ridge stripe was formed by cleaving along the (11-20) plane of the GaInN/GaN epitaxial layers. Stimulated emission was observed at a wavelength of 411 nm with a threshold current density of 11.7 kA/cm2.

Magnetic properties of atomically-thin epitaxial dots and stripes with micrometer lateral size

Molecular beam epitaxy grown micrometer-sized ultrathin dots and stripes of Fe on W(110) are investigated in situ by scanning Kerr microscopy (SKEM). All structures are found to be in a single domain state. A 5- μm wide Fe stripe having a thickness of 12 atomic layers (AL) and an aspect ratio of 500 is magnetized along the W[11̄0] direction. This is found to be independent of the stripe's orientation.

8.8 W CW power from broad-waveguide Al-free active-region(λ = 805 nm) diode lasers

Al-free active-region (λ = 805 nm) diode lasers with 1 µm thick InGaP waveguide layers provide continuous wave powers as high as 8.8 W from 1.25 mm long devices with 4%/95% facet-coating reflectivities and 100 µm wide stripes. The transverse beam pattern is Gaussian-like with a 36° beamwidth and the series resistance is only 48 mΩ.

Fabrication of strained InAs island ensembles on nonplanar patterned GaAs(001) substrates

We report on the behavior of InAs deposited on nonplanar GaAs(001) substrates patterned with ≲0.5 μm wide stripe mesas oriented along the [11̄0] and 〈100〉 directions and with square mesas with a lateral size of ≳0.5 μm oriented along the 〈100〉 directions. Interfacet migration of In from the sidewalls to the mesa top leads to an enhanced InAs island density on the stripe as well as square mesa tops compared to that on the planar unpatterned region. Using such interfacet migration and InAs deposition amount less than needed for island formation on planar GaAs(001), we demonstrate complete selectivity in the positioning of InAs islands on the [11̄0] oriented stripe mesas of widths ≲100 nm, with islands forming exclusively on the mesa tops. These islands arrange in mesa-width-dependent parallel chains. They show photoluminescence (PL) comparable to that from the islands on the planar substrates. The polarization dependence of the PL suggests the presence of anisotropy in strain fields and potential elongation of islands in the [11̄0] direction. The significance of the stripe mesa edge orientation to the island formation is revealed by vastly different island densities and InAs morphology on the [11̄0] oriented versus 〈100〉 oriented stripe mesas. These differences reflect difference in migration from the different sidefacets that surround the (001) mesa top.

V-groove side-pumped 1.5 µm fibre amplifier

A side-pumping technique, using an imbedded v-groove is used to efficiently pump a double-clad Er/Yb doped fibre amplifier operating at 1.5 µm and pumped at 0.98 µm. A diode-to-fibre coupling efficiency of 76% was measured for a 100 µm wide broad stripe laser diode and a fibre with a 50 × 120 µm first cladding. A maximum output power of 185 mW, and separately, a small signal gain of 35 dB, are demonstrated.

An ultrahigh vacuum scanning Kerr microscope

A new ultrahigh vacuum instrument allowing in situ Kerr microscopy and scanning tunneling microscopy is described. The Kerr microscope has a spatial resolution of about 1 μm. First experimental results are reported on the magnetism of a 5 μm wide stripe consisting of six atomic layers of Fe grown in situ by molecular beam epitaxy on a W(110) surface.

Replicated Mach-Zehnder interferometers with focusing grating couplers for sensing applications

A novel type of Mach-Zehnder structure which is suitable for the low-cost fabrication of miniature sensor modules is described. It consists of a nano- and micro-structured polymer substrate which is coated over its whole area with a high-index inorganic dielectric film. Ridge structures of about 3 μm height, together with focusing input and output gratings of about 20 nm depth are replicated in a single embossing step into a polymer foil, followed by waveguide deposition. The focusing grating couplers enable coupling of collimated free-space beams into and out of the stripe waveguide structures without the need for external optical elements. This leads to relaxed alignment tolerances and eases the miniaturization of the complete sensor system. In addition, fabrication based on replicated plastic substrates offers the opportunity of low-cost mass production. Waveguide and interferometer structures of this type are characterized by exciting modes in stripe waveguides via the focusing grating couplers. Successful light input and output from a 4 μm wide stripe waveguide as well as light transmission through a replicated Mach-Zehnder structure formed of 3 μm wide waveguide stripes demonstrate the feasibility of this approach. The use of high-index waveguiding films on low-index substrates makes the structure well suited for sensing applications.

Theoretical estimation of threshold current of cubic GaInN/GaN/AlGaN quantum well lasers

Cubic GaInN/GaN/AlGaN quantum well (QW) lasers are theoretically analyzed. For the gain calculation, the dipole moment of cubic GaN which is extrapolated from those of other several III–V compounds is corrected by fitting the theoretical light absorption coefficients to the experimental values. Using the corrected dipole moment, the threshold current density value of 1.0 kA/cm 2 is theoretically calculated in the case of the single QW laser with the lasing wavelength of 415 nm. For the 10 μm wide stripe single QW lasers, the threshold currents ( I th) are calculated as a function of the cavity length. The I th drastically decreases with decreasing the cavity length because the corrected dipole moment gives the higher differential gain. The I th values below 20 mA are theoretically given for a short cavity length of 100 μm.

Q-switching in single-heterojunction lasers and generation of ultrahigh-power picosecond optical pulses

Q-switching has been obtained for single-heterojunction lasers. The ultrafast saturable absorber in the laser cavity is created by implantation of high-energy heavy ions. The peak power generated by the lasers in a 150 μm stripe width is 380 W with a pulse length of 40 ps.

Magnetotransport in microstripes patterned in ultrathin cobalt films

The magnetotransport properties are an attractive alternative to standard observation techniques for the characterization of the magnetization reversal in structures with reduced lateral dimensions (stripes). In particular, magnetic films exhibit the so-called anomalous Hall effect which is very sensitive to the local magnetic domain structures. We began to work with the model system Co/Au(111) grown on silicon dioxide. We present here a new method for microstructuring magnetic multilayers, that avoid the use of technological processes on the multilayers. Preliminary magnetotransport results in 50 μm wide stripes are presented and discussed.

MOVPE-grown high CW power InGaAs/InGaAsP/InGaP diode lasers

Al-free InGaAs/InGaAsP/InGaP laser structures grown by MOVPE have yielded new records in performance. CW front-facet-emitted powers of 5.8 W are achieved from optimized double-quantum-well lasers with 100 μm wide stripes and 1 mm long cavity lengths. Devices with a 0.5 mm cavity length exhibit total power conversion efficiencies as high as 59%. A novel facet passivation technique, consisting of laser-assisted deposition of ZnSe, is shown to increase the COD level by 50%. Single-mode, CW output powers of 400 mW are obtained from triple-core ARROW lasers fabricated by a two-step MOVPE growth process.

Studies on the butt-coupling of InGaAsP-waveguides realized with selective area metalorganic vapour phase epitaxy

A method for the characterisation of the process-induced optical loss in butt-coupled passive waveguides will be described. In principle the results are valid for any active-passive waveguide butt-coupling as well (e.g. laser-waveguide). The method was applied to butt-coupled waveguides, which were grown by selective area metal organic vapour phase epitaxy (MOVPE). Different etching procedures for the preparation of the masked mesa stripes were examined, which considerably influenced the regrowth behaviour close to the mesa edge. The optical loss at the interface was found to depend primarily on the thickness difference of the waveguides. The excess layer thickness due the regrowth near the mesa is proportional to the width of the masked stripe. It reaches a factor of 1.7 at 50 μm wide stripes and decreases exponentially with distance from the stripe ( 1 e length = 26 μ m ). In general the measured loss values agree with the numerical simulation at least for stripe widths ≥30 μm. For stripe widths of ≤30 μm loss values ≤1 dB have been obtained in all cases, but deviations of the calculated loss values occurred, which can be attributed to the regrowth geometry at the interface.

Metal-clad ridge waveguide structure InGaP/InGaAIP visible laser diodes

InGaP/InGaAIP visible-light laser diodes with a metal-clad ridge waveguide structure have been fabricated. Compressively strained SCH-MQW structures were grown by low-pressure MOCVD and the lasing wavelength was 690 nm. The threshold current was 28 mA at 25°C for a 400 μm cavity and 5 μm stripe width. CW operation was obtained as the cleaved device by employing thick gold layer as a heat spreader. The light-output power versus CW current was linear up to a maximum light output power of 38 mW. Stable operation of the fundamental transverse optical mode was maintained up to 30 mW. These results show that this metal-clad ridge waveguide structure provides sufficient current confinement even under high light output power operation.

Continuous-wave operation of ZnMgSSe based blue-greenlaser diodesunder high temperature and high output power

Continuous-wave (CW) operation up to 323K at 1 mW and up to 30 mW at room temperature has been demonstrated for 507nm ZnCdSe/ZnSSe/ZnMgSSe separate-confinement heterostructure (SCH) gain-guided laser diodes. The room temperature CW threshold current and voltage were 27 mA (Jth = 460 A/cm2) and 7.9 V, respectively, with 10 µm-wide stripe and 70/95% reflectivity facet.

Buried heterostructure laser diodes fabricated using in situ processing

An in situ process which includes electron cyclotron resonance plasma etching and molecular beam epitaxial regrowth is applied to the fabrication of buried heterostructure vertical cavity surface-emitting laser (SEL) diodes and edge-emitting laser (EEL) diodes. The buried SEL with a 7.5 μm diameter has a pulsed laser threshold current of 1 mA, and a threshold voltage of 4 V with a peak power of 0.9 mW. The buried EEL with 2.5 μm stripe width and 800 μm cavity length has a threshold current density of 500 A/cm2.

High-power low-threshold Ga 0.88 In 0.12 As 0.10 Sb 0.90 /Al 0.47 Ga 0.53 As 0.04 Sb 0.96 double heterostructure lasers grownby liquid phase epitaxy

Ga0.88In0.12As0.10Sb0.90/Al0.47Ga0.53As0.04Sb0.96 double heterostructure lasers emitting at 2.05 µm have been grown by liquid phase epitaxy on GaSb substrates. For pulsed operation of 300 µm-wide broad stripe lasers, output power as high as 80 mW per facet and threshold current density as low as 1.7 kA/cm2 have been obtained for cavity lengths of 300 and 900 µm, respectively. Mesa-stripe lasers 200 µm long showed room temperature threshold current of 200 mA with a characteristic temperature T0 ≃ 115 K.

Early Electromigration Failure in Submicron width, Multilayer Al Alloy Conductors: Sensitivity to Stripe Length

ABSTRACTThe microstructure of Al alloy integrated circuit conductors strongly affects electromigration failure of the metallization. For submicron linewidths, the microstructure is usually near - bamboo, consisting of alternate polycrystalline and bamboo segments. These polycrystalline segments appear to be much more susceptible to electromigration - induced voiding than the bamboo regions. As a result, microstructural related early failures are a significant concern. In this study we systematically vary line - length to investigate the impact of random microstructural variations on the failure distribution parameters of narrow, multilayer (Ti/TiN/Al/TiN) Al alloy conductors. We show that early failures can occur in 0.6 μm wide stripes. The impact of early void formation on conductor failure, however, is dependent on the resistance failure criterion. By increasing the value, early failures may be eliminated in long line - lengths, suggesting that the resistance failure criterion may provide a means to control the impact of early voiding on the electromigration failure.