Thomas M. Marshall

Blue‐green injection lasers containing pseudomorphic Zn1−xMgxSySe1−y cladding layers and operating up to 394 K

We describe the performance of blue‐green injection lasers containing Zn1−xMgxSySe1−y cladding layers. The devices have yielded the lowest reported threshold current densities (500 A/cm2) and the highest reported pulsed output powers (500 mW) at room temperature. Lasing has been observed at temperatures as high as 394 K. The room temperature and 85 K lasing wavelengths are 516 and 496 nm, respectively. The use of Zn1−xMgxSySe1−y, instead of ZnSzSe1−z, cladding layers provides a clear improvement in optical confinement, demonstrated by the widening of the far‐field pattern in the direction perpendicular to the layers. The lasers are separate‐confinement heterostructures with a ZnS0.06Se0.94 waveguiding region and a single Cd0.2Zn0.8Se strained quantum well. The entire structure is pseudomorphic with the GaAs substrate.

Spectral shifts associated with dark line defects in degraded II-VI laser diodes

Spectral shifts associated with 〈100〉 dark line defects of degraded II-VI laser diodes based on ZnCdSe/ZnSSe/MgZnSSe separate confinement heterostructures have been studied by spatially resolved cathodoluminescence at room temperature. Dark line defects were induced by electron-beam bombardment. Peak shifts as large as 2 nm were observed towards the blue or the red depending on the local circumstances. Peak widths usually became narrower after degradation. Redshifts and blueshifts are explained in terms of strain relaxation and Cd out-diffusion associated locally with degradation, as well as the kinetic energy dependence of the degradation-related carrier capture cross section.

Characterization of low defect density blue-green lasers

Abstract We have used a combination of techniques to characterize low defect density (≤ 10 5 cm −2 ) blue-green separate confinement heterostructure lasers. The limits of lattice mismatch between the substrate and quaternary cladding layers that result in a pseudomorphic laser structure were determined by X-ray diffraction and transmission electron microscopy to be ≤ 0.0015. The determination of defect density (stacking faults, threading dislocations, etc.) in the active layer was performed by optical imaging. Photoluminescence imaging is nicely suited for defect observation, because of the nonradiative transitions associated with the defects and can easily be performed over large areas. Propagation of defects during device operation (device degradation) was monitored in real time with optical imaging. The degradation was observed to start at grown-in defect sites (in the active layer) that emanate from 〈100〉 dark line defects. From the direct observation of device degradation, a mechanism involving the creation of new defects from nonradiative recombination at existing defects sites is proposed.

DIFFUSION LENGTHS OF EXCITED CARRIERS IN CDXZN1 -XSE QUANTUM WELLS

Diffusion lengths of excited carriers in a CdxZn1−xSe multiple quantum well structure were determined for temperatures between room temperature and 8 K from cathodoluminescence measurements. The diffusion length was found to depend upon temperature and Cd concentration of the quantum well. For the highest Cd concentration (x=0.43), the diffusion length increased with temperature up to 225 K and then dropped at higher temperatures. Diffusion lengths were 0.21 μm at 8 K, 0.38 μm at 225 K, and 0.24 μm at room temperature. For the well with least Cd concentration (x=0.24), longer diffusion lengths were obtained. The nature of the diffusing carriers is also discussed.Diffusion lengths of excited carriers in a CdxZn1−xSe multiple quantum well structure were determined for temperatures between room temperature and 8 K from cathodoluminescence measurements. The diffusion length was found to depend upon temperature and Cd concentration of the quantum well. For the highest Cd concentration (x=0.43), the diffusion length increased with temperature up to 225 K and then dropped at higher temperatures. Diffusion lengths were 0.21 μm at 8 K, 0.38 μm at 225 K, and 0.24 μm at room temperature. For the well with least Cd concentration (x=0.24), longer diffusion lengths were obtained. The nature of the diffusing carriers is also discussed.

Diffusion lengths of carriers in n- and p-type ZnMgSSe cladding layers of green laser diodes

We have used cross sectional cathodoluminescence microscopy as a fast and nondestructive tool to characterize II–VI based green laser diodes. We find evidence for carrier mediated excitation of semiconductor layers that are not directly irradiated by the focused electron beam, from which diffusion lengths of lower mobility carriers (presumably holes) can be estimated. We find that N-doped (p-type) ZnMgSSe exhibits a very low (near) band edge luminescence efficiency. The diffusion length of minority carriers in p-type ZnMgSSe:N [(1–2)×1017 cm−3 net acceptor concentration] was found to be lower than for n-type ZnMgSSe:Cl with roughly equal dopant concentration. The diffusion length of minority carriers in n-type ZnMgSSe:Cl decreases from 0.21 μm for a doping level of (1–2)×1017 cm−3 to <0.05 μm when the n-type doping is increased to (2–4)×1018 cm−3. This decrease in diffusion length is accompanied by an increase of a broad luminescence band around 550 nm, which is attributed to Cl-related defects in the gap...

ACTIVATION ENERGY OF NONRADIATIVE PROCESSES IN DEGRADED II-VI LASER DIODES

A spatially resolved cathodoluminescence study of 〈100〉 dark line defects (DLDs) of degraded II–VI laser diodes based on a ZnCdSe/ZnMgSSe separate confinement heterostructure has been carried out at temperatures between room temperature and 8 K. Cathodoluminescence line scans were used to measure the change of contrasts between the DLDs and the adjacent material. The contrast decreased with decreasing temperature, which suggests that the nonradiative recombination processes associated with DLDs are thermally activated. Activation energies were found to be about 16 and 6 meV for temperatures above and below 200 K, respectively, which may reflect a transition between free carriers and bound excitons at this temperature.

Electrical transport in n-type ZnMgSSe grown by molecular beam epitaxy on GaAs

Significant progress in improving the performance of blue-green II-VI semiconductor injection lasers has come about from advances in the epitaxial growth and doping of ZnMgSSe on GaAs substrates. This paper investigates electrical transport and its relation to structural quality in n-type Zn1-yMgy SxSe1-x epilayers doped with Cl, grown by molecular beam epitaxy. The composition parameters x and y vary from about 0.12-0.18 and 0.08-0.15, respectively. The quaternary epilayers studied are lattice-matched (or nearly so) to the GaAs substrate. Temperature-dependent Hall-effect measurements are performed on seven n-type ZnMgSSe:Cl epilayers, and a technique is presented whereby the resulting mobility-vs-temperature data is compared with data for ZnSe to obtain a structural figure of merit that is useful in characterizing the quaternary epilayer.

Electrical characterization of p-type Zn(Se,Te):N semiconductor layers

We have grown p-type ZnSe1-xTex:N (x equals 0.08 - 1.0) epilayers by molecular beam epitaxy on GaAs substrates, and characterized their electrical behavior. The Te fraction x was determined by energy-dispersive x-ray spectroscopy and by high-resolution x-ray diffraction. The free-hole concentrations and mobilities were determined by Hall-effect measurements, and the contact resistances of evaporated PdAu metal to the epilayers were measured using standard transmission-line techniques. The contact resistance decreases sharply with increasing Te content, falling from 0.6 (Omega) cm2 for a film with 8% Te to 3.5 multiplied by 10-7 (Omega) cm2 for a pure ZnTe film. Under the growth and doping conditions used, the hole mobility shows a minimum of about 1 cm2/Vs at about 25% Te. It is expected that by optimizing these single-layer properties, the building blocks of an improved electrical contact to ZnSe can be obtained.

Advances in blue-green laser diodes

Great strides have been made in the area of Il-VI based short wavelength laser diodes since the first demonstration of pulsed blue-green lasers by 3M Company in 1991. These advances have been primarily due to a greater understanding of the material growth, including substrate preparation, the use of the MgZnSSe quaternary system, defect reduction, and p-type ohmic contacts.

Growth and characterization of low threshold-current-density II-VI blue-green laser diodes grown by molecular beam epitaxy

We describe the growth and characterization of Zn/sub 1-x/Mg/sub x/S/sub y/Se/sub 1-y//Zn/sub 1-u/Cd/sub u/Se-based laser diodes using ZnSeTe graded-gap contacts, and related materials. Contact performance, degradation and defect generation, thermal transport, and life testing are discussed.