Junction Plane Research Articles

II–VI blue-green light emitters

Mg in ZnCd-chalcogenides opens up a wide range of II–VI materials having both a wide energy gap and a large lattice constant. ZnMgSSe, fully lattice-matched to GaAs (001) substrates, with an energy gap tunable from 2.8 up to ∼ 4.5 eV, has made possible the photopumped operation of MBE or MOCVD-grown Zn(Cd)Se/ZnSSe/ZnMgSSe separate-confinement heterostructure samples and continuous-wave operation of MBE-grown green and blue laser diodes at room temperature. Valence-band engineering by using a superlattice based on the amphoteric native defect model is proposed for removing the doping limit in alloy semiconductors. The doping concentration could be increased by one order of magnitude for (ZnSe)m(ZnMgSSe)n superlattice with m = 4–6 and n = 10–15. The device characteristics of the MBE-grown II–VI wide-gap light emitters are becoming as good as those of the established III–V materials-based LDs except for the device lifetime. We have reported structural studies of degraded ZnMgSSe-based LEDs. Pre-existing stacking faults turn into non-radiative regions with a high density of dislocation dipoles and dislocation loops, spreading out in the 〈100〉 directions during current injection. The dislocation dipoles themselves are aligned along both 〈110〉 directions lying in the {111} plane, with Burgers vectors of the type (a2)〈011〉 inclined at an angle of 45° against the (001) junction plane. Analysis of the degradation process shows that no catastrophically fast degradation occurs when II–VI LDs degrade. We believe that we can establish the reliability of the ZnMgSSe-based LDs through prevention of the start of rapid degradation by eliminating the pre-existing defects, and by slowing down the gradual degradation through reduction of point defects.

Structural study of degraded ZnMgSSe blue light emitters

Using electroluminescence (EL) topography and transmission electron microscopy (TEM), we investigated the nonluminescent regions which form while current is being injected into ZnMgSSe/ZnSSe/ZnCdSe-based blue light emitters. Small dark spots were observed just after turn-on and spread out forming rough nonluminescent triangles in the directions in the EL image of the active region. TEM studies showed that the small dark spots are pre-existing stacking faults originating at the substrate/epitaxial layer interface. The nonluminescent triangles were found to be a dense region of dislocation dipoles and dislocation loops. Each dipole was aligned along two directions in the {111} planes. The Burgers vectors were of the type a/2 inclined at 45° to the (001) junction plane.

Effects of band nonparabolicity on the gain and current density in EuSe-PbSe/sub 0.78/Te/sub 0.22/-EuSe IV-VI semiconductor quantum-well lasers

In this work, a theoretical model that calculates the gain versus current density relationship for IV-VI semiconductor quantum-well lasers was developed. The model, based on Kane's two-band model, solves for the anisotropy in the constant energy surfaces and for the strong nonparabolicity of the bands. The system investigated was the EuSe-PbSe/sub 0.78/Te/sub 0.22/ quantum-well structure at 77 K. The nonparabolicity of the bands in the growth direction was found to shift the energy levels in the PbSe/sub 0.78/Te/sub 0.22/ quantum well to lower energies as compared to a quantum well with parabolic bands. Nonparabolicity of the bands also resulted in an energy dependent density of states in the junction plane of the structure. The effect of nonparabolicity in all directions on the gain versus current density relation is a reduction in the current density needed for any given gain and an increase in the gain saturation level. In addition to the 20% shift in the output lasing energy, nonparabolicity of the bands lowers the values of the confinement factor relative to those for the parabolic bands which in turn lowers the modal gain values.

Characteristics of Gain-Guided Laser Diodes with Cylindrical-Mirror Cavity

We have developed highly reliable etched-mirror laser diodes using a dry etching method. The lasers without facet-coating have been operating stably over 2500 h under automatic-power control (APC) at a power of 3 mW/facet at 50°C. The gain-guided laser diodes with a cylindrical-mirror cavity (CMC) have coaxial mirrors and a fan-shaped stripe structure. By decreasing the curvature radius of the inner facet or increasing the stripe width of the inner facet, the beam waist parallel to the junction plane can be moved outside of the laser diode, while the beam waist perpendicular to the junction plane stops at the mirror facet. A particular CMC laser has a low astigmatism of 4.1 μm and a low relative intensity of noise (RIN) less than –134 dB/Hz at 4 mW under 0–1% optical feedback without high frequency current superposition.

Large-area wafer processing for 0.78-/spl mu/m AlGaAs laser diodes

Large-area wafer processing for buried-ridge loss-guided inner-stripe AlGaAs laser diodes (LD's) has been developed for the first time. High uniformity of ridge waveguides on a 3-in wafer is realized by introducing selective wet etching. The standard deviations of the ridge width and the remaining p-cladding layer thickness on either side of the ridge, which have a great influence on the device characteristics, are 0.07 μm and 0.01 μm, respectively. As a result, good uniformity of laser characteristics has been obtained, for example, the distribution of full width at half maximum angles of far-field pattern in the direction parallel to the junction plane (/spl theta//sub ///) is only 0.22/spl deg/ (1/spl sigma/) for 1000 chips across a 3-in wafer.

Hot electron light-emitting semiconductor heterostructure device--type 2

A novel hot electron light-emitting device is proposed which operates by the application of longitudinal electric field, i.e. in the plane of the GaAs quantum wells, which are placed next to the junction plane of an n-Ga1-xAlxAs--p-GaAs heterostructure. Application of high electric fields results in the transfer of hot electrons via tunnelling and thermionic emission, from the quantum well in the depletion region, into the GaAs inversion layer. The hot holes in the p-GaAs, initially away from the junction, then diffuse towards the junction plane to recombine with the excess hot electrons, giving rise to electroluminescence (EL) which is representative of the GaAs band-to-band emission. As the applied field is increased, a high-energy tail in the EL spectrum develops, and, photons with energies greater than the el-hhl transition energy in the quantum well are absorbed and re-emitted by the quantum well. Thus a second peak develops in the EL spectra which becomes stronger with increasing applied electric field. The device has been theoretically modelled, by solving Schrodinger and Poisson's equations self-consistently, to understand the processes leading to EL emission in the various channels.

How dislocations affect transport

Dislocations crossing a junction in HgCdTe have little effect on detector responsivity, but are known to reduce the zero bias impedance RoA. and increase the leakage current, especially at low temperatures where RoA is dominated by tunneling and generation/recombination processes. We have calculated the Coulomb and piezoelectric fields associated with dislocations in an attempt to interpret their effect on the junction’s transport properties. Dislocation electric fields can affect transport since they are superimposed on the built-in and applied junction fields which control the currents. The screening of the fields in the neutral region is consistent with the dislocations’ small effect on responsivity. Their impact in the space charge region is found to be significant and consistent with the nonlinear dependence of performance on dislocation density. The piezoelectric potential of the typical 60° dislocation in a sphalerite crystal, and the Coulomb potential of a dislocation crossing the junction plane other than normally, are angularly varying in the junction plane. Angular variation of the potentials can be qualitatively interpreted as an angular modulation of the potential barrier. Because of the nonlinear dependence of junction currents on the barrier (or the junction potential), the angular variation of the currents does not vanish upon averaging. We find that the range of the Coulomb potential is too small to account for a major portion of the experimentally reported performance degradation but may be responsible for the reduction of RoA at cryogenic temperatures and low dislocation density, and that the longer range piezoelectric potential may be important. We also find that superposing the potentials of neighboring dislocations, because of the nonlinear dependence of junction leakage currents on junction potentials may account for the observed nonlinearity of performance degradation with dislocation density as measured by etch pit density.

Output beam characteristics of 1.3 µm GaInAsP/InPSL-QW lasers with narrow and circular output beam

It is verified that the output beam for lasers with a mode-field converter is quite stable against output power up to 30 mW and operating temperature up to 70°C. The threshold current and differential quantum efficiency of an HR-coated device are 14 mA and 45%, respectively. The FWHMs of the FFPs parallel and perpendicular to the junction plane are as small as 10 and 11°, respectively.

Structural study of defects induced during current injection to II–VI blue light emitter

We have carried out structural studies of nonluminescent areas developed by current injection in ZnMgSSe alloy-based II–VI blue light emitting diodes by electroluminescence topography and transmission electron microscopy. The nonradiative regions, which spread out in the 〈100〉 direction during current injection, consist of a high density of dislocation dipoles and dislocation loops. The source of these defects is the preexisting stacking faults originating at the substrate/epilayer interface. The dipoles themselves are aligned along both of the 〈110〉 directions lying in the {111} plane. Their Burgers vectors were of the type (a/2)〈011〉 inclined at 45° to the (001) junction plane.

Steady expiratory flow in a model symmetric bifurcation.

A model symmetric bifurcation was employed to simulate steady expiratory flow in the upper part of the human central airways. A two color, two component laser Doppler anemometer was used to measure both the axial flow and the secondary flow at three different Reynolds numbers of 518, 1036, and 2089, corresponding to Dean numbers of 98, 196, and 395. The test section is a symmetric bifurcation of constant cross-sectional area with a branching angle of 70 degrees. The flow rate into the two daughter branches was about the same. Results show that in the junction plane, velocity profiles in the daughter branches are skewed towards the inner walls. In the parent tube, just downstream of the flow divider, the velocity profile is biconcave with a dip at the center but this is rapidly transformed into a velocity peak. In a plane transverse to the bifurcation plane, parabolic velocity distribution was conserved through the daughter branches. In the parent tube, the transverse profiles became flat downstream of the flow divider and developed a defect at the center further downstream towards the end of the parent tube part of the bifurcation. The velocity defect was confined to a small region in the vicinity of the centerline. Helical motion typified by symmetric vortices was observed in the daughter branches. In the parent tube, a set of four vortices induced by the turning of the flow was observed.

Resistive measurement of the temperature dependence of the penetration depth of Nb in Nb/AlOx/Nb Josephson junctions

The temperature dependence of the penetration depth of Nb films was determined from resistive transitions of Nb/AlOx/Nb Josephson junctions in a constant magnetic field applied parallel to the junction planes. Distinct resistance peaks were observed as temperature decreases and those peaks were found to appear when the total flux threading the junction equals an integral multiple of the flux quantum. From this condition, the penetration depth at those peak positions has been determined. The temperature dependence was well described by either the dirty local limit or the two-fluid model. This method can be useful for a highly fluctuating system such as high-temperature superconductors.

Defect-related generation-recombination region in heterojunction diodes

A numerical analysis is presented of semiconductor heterojunctions containing deep defect levels, in order to study the mechanism of generation or recombination of electron-hole pairs induced by thermal or optical transitions involving deep defect energy levels. The final aim is to determine the width of the effective generation or recombination region Wgr and compare to the width of the space-charge region W. Numerical solution of Poisson’s equation and the current-transport equations yield the electrical potential and the quasi-Fermi levels for electrons, holes, and occupied deep defects. The resulting transition rates yield the net generation rate of electron-hole pairs as function of position x. An application is made to an InGaAs/InP heterojunction with a midgap level on the InGaAs side. Results for a Si homojunction with a deep acceptor level at Ec−0.54 eV are given for comparison. The applications show that in all cases of reverse bias Wgr is smaller than W, reaching for some parameters describing the heterojunction system a value of 1/3 for the ratio Wgr/W. Usually the two widths are assumed to be equal. The physical origin of this reduction is the existence of competing recombination processes due to free-carrier tails extending from the neutral regions into the space-charge region. In heterojunction structures, this effect is even enhanced in case of accumulation of free carriers due to conduction- or valence-band discontinuities at the junction plane. It is shown that if one considers defects states only in a finite region close to the junction, this may yield marked differences in the value of the resulting current as compared to the homojunction case.

Flux pinning in a slab model of grain and twin boundaries

The flux pinning potential and critical current of a grain and a twin boundary are evaluated for a S-S’-S junction model and a configuration of parallel magnetic field and junction plane. In the grain boundary model, the S’ domain is assumed to be disordered, to mimic oxygen disorder. Large critical currents ≈O(106 amp/cm2) are estimated for both models. The grain boundary model exhibits a “nonweak” link behavior and moderate critical current dependence on the grain misalignment angle. We briefly dicuss an approach to enhance critical currents based on our grain boundary model.

Large-scale metalorganic chemical vapor deposition growth of highly reliable 780 nm AlGaAs multiple quantum well high-power lasers

Successful large-scale (twelve 2 inch diamter wafers/run) metalorganic chemical vapor deposition (MOCVD) growth of AlGaAs multiple quantum well (MQW) structure for 780 nm high-power lasers using a barrel-shaped reactor is demonstrated. Excellent uniformity of growth rate (∓3.5%) and Al content (∓0.3%) for an Al 0.48Ga 0.52As layer within a 2 inch diameter wafer has been obtained. Very accurate layer thickness control down to 1 nm has also been realized for AlGaAs well layer. Application of this technique to 780 nm AlGaAs MQW high power laser diodes realized excellent uniformity of the laser characteristics. The uniformity of the beam divergency to the junction plane (θ ⊥), lasing wavelength and threshold current were 27∓1°, 785∓3 nm and 48∓7 mA, respectively. In addition, it is shown that the reduction of the oxygen concentration in the laser crystal below 1x10 17 cm -3 is essential to obtain low threshold current with good reproducibility. Stable operation of the laser diodes for more than 1000 h has been also confirmed even at accelarating condition of 60°C, 50 mW.

Far-field solution of the Helmholtz equation for double heterostructure diode lasers

On the basis of the Helmholtz equation the far-field distribution is derived for double heterostructure lasers. The results show that the far-field distribution in the direction normal to the junction plane approaches a Lorentzian function, but parallel to the junction it may be approximated by a Gaussian function. The far-field intensity patterns have “analogous elliptic” form. It is also shown, for the first time, that the separability condition is not strictly valid for the far-field of a laser diode. Only in the vicinity of the optical axis the field can be expressed as a product of two separate functions, each of which depends only on one of the two transverse coordinates parallel and perpendicular to the diode junction.

1.48 mu m high-power GaInAsP-InP graded-index separate-confinement-heterostructure multiple-quantum-well laser diodes

High-power 1.48- mu m graded-index separate-confinement-heterostructure multiple-quantum-well laser diodes (GRIN-SCH MQW LDs) have been investigated in terms of the beam divergence, threshold current, and differential quantum efficiency. A calculation predicts that narrow beam divergence perpendicular to the junction plane could be obtained by the use of small number of step quaternary layers with wide bandgap. Experimentally, the threshold current increased due to the small optical confinement, although the beam divergence became small. The full width at half maximum (FWHM) of the far-field pattern in the horizontal direction was 20 degrees and that in the vertical direction was 25 degrees . Output power as high as 208 mW was achieved at a driving current of 1000 mA for a 1-mm-long device. Stable transverse mode operation was confirmed up to the maximum output power. Coupling efficiency of 89% and output power from a single-mode fiber of 185 mW were obtained at 25 degrees C. >

Critical layer thickness in [formula omitted] heterostructures determined by X-ray diffraction

The critical thickness of strained Al xGa 1−xAs ySb 1−y layers (0.35 ⩽ x ⩽ 0.58) grown by liquid phase epitaxy (LPE) on GaSb (001) oriented substrates was determined by X-ray double-crystal diffractometry on (115) reflection planes. This method allows one to determine the relative lattice-mismatch perpendicular to the growth direction ( Δa a ) τ and in the junction plane ( Δa a ) ∥ . The state of the epitaxial layer (strained or relaxed) was deduced for mismatches ( Δa a ) 0 = (a AlGaAsSb − a GaSb) a GaSb varying from −1 × 10 −3 to 4 × 10 −3. The evaluated critical thickness of layers under tension was: h c (μm) ≈ 0.4 × 10 −3 ( Δa a ) 0 much lower than that of layer under compression: h c (μm) ≈ 2.2 × 10 −3 ( Δa a ) 0 .

Coupling to Lower Hybrid Waves with the Multijunction Grill

Coupling characteristics of the multijunction grill have been studied in detail by experiments on the basis of a simple theory using a multireflection method with the aid of scattering matrices not only at the junction plane but also at the grill mouth. Theoretical values of the global reflection coefficients in the primary waveguide and secondary waveguides are estimated from the vector sum of reflecting components caused by each reflection between two scattering planes. The experimental results of scattering matrices and global coupling agree well with theoretically calculated ones. It is shown that the multiple reflection in secondary waveguides of the four-junction grill for the JIPPT-IIU tokamak is finished up to three or four reflections. The length of the secondary waveguide is confirmed to be one of the parameters determining the grill efficiency.

High power, singlemode InGaAs-GaAs-AlGaAs strained quantum well lasers with new current blocking scheme using GaAs layers grown by MBE at low substrate temperatures

Device results from laser diodes with a new current blocking scheme using low temperature (LT) GaAs grown by MBE at 200°C are reported. The laser structure is grown by metal organic vapour phase epitaxy on channelled MBE-grown LT-GaAs. The LT-GaAs is shown to be effective in preserving the current blocking properties for InGaAs-GaAs-AlGaAs strained quantum well lasers. The CW laser threshold is 50 mA at RT (with a lasing wavelength of 980 nm) and far field patterns show fundamental modes (both lateral and transverse modes) even at an output power of 55 mW (per facet, uncoated). FWHMs of far field patterns parallel and perpendicular to the junction plane are 16 an d 39° at an output power of 55 mW, respectively.

Influence of trapped Abrikosov vortices on the critical current of the Josephson tunnel junction

Values for the maximum Josephson critical current (Icmax) and distortion of the dependence of the critical current (Ic) on magnetic field applied parallel to the plane of the tunnel junction (H/sub ///) are calculated for different configurations of Abrikosov vortices in a wide range of their concentration. An experimental procedure is developed to trap both misaligned dipole vortices and monopole vortices in the Nb/AlOx/Nb Josephson tunnel junction for study of their influence on the Ic(H/sub ///) curves and the value of Icmax. The results of the theoretical calculations agree well with the experimental data.