Maarten Buijs

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.

Carrier leakage in blue‐green II–VI semiconductor lasers

Carrier confinement in blue‐green II–VI semiconductor lasers was investigated. For devices longer than 300 μm an energy barrier of 260–280 meV was found to confine the electrons, the carrier being mainly responsible for leakage, within the active region. Shorter devices show more leakage due to an increased importance of mirror losses which require higher threshold gain. Due to the low conductivity of the p‐type cladding layer there is a sizable contribution of drift to the total leakage current.

Thermal index guiding in gain‐guided blue‐green II–VI semiconductor lasers

We have studied the lateral waveguiding properties of gain‐guided and index‐guided II–VI lasers under pulsed conditions and their influence on threshold current density Jth and differential quantum efficiency η. Thermal index guiding was found to reduce the astigmatism of the gain‐guided devices. The thermally induced lateral field confinement leads to a lowering of Jth and an increase of η with pulse width with a maximum after a few μs because of maximum overlap of the near field with the gain profile. For the index‐guided devices the lateral waveguiding is fully determined by the built‐in refractive index profile and no dependencies on pulse width are observed.

II-VI index-guided lasers for optical recording

We have demonstrated the feasibility of reading a high density Digital Versatile Disc using a green ZnSe based laser as the light source. In order to achieve this, high quality optical and electrical properties are required from the laser. We fabricated index-guided lasers to produce a single mode gausian optical beam having 7 micrometers of astigmatism. Operating electrical and optical parameters were measured and compare well with currently available semiconductor lasers for optical storage systems. The read- out experiment shows the favorable noise characteristics of ZnSe based lasers.

Dependence of threshold current density on the energy‐band offsets in Zn1−uCduSe/ZnSzSe1−z/Zn1−xMgxSySe1−y separate‐confinement heterostructures lasers

We use a theoretical model of semiconductor lasers to calculate threshold current density (Jth), optical gain (g), and spontaneous emission rate (rsp) as a function of the two heterojunction band offsets between Zn1−uCduSe(well)/ZnSzSe1−z(guiding) layers and ZnSzSe1−z (guiding)/ Zn1−xMgxSySe1−y(cladding) layers. We find that for the conduction‐band offset between the well and the guiding layers ranging from ∼0.55ΔEg to ∼0.75ΔEg (ΔEg being the energy band‐gap difference), there is very little change in Jth, g, rsp. Furthermore, these three quantities are very weakly dependent on the conduction‐band offset between the guiding and the cladding layers ranging from 0.2ΔEg to 0.5ΔEg. However, if the offset between the well and the guiding layers is increased from ∼0.75ΔEgto ∼0.8ΔEg, the calculations indicate a sharp drop in Jth, g, and rsp. We explain this behavior as due to the unbinding of the ground‐state light‐hole subband as the valence band offset is reduced.

Lateral waveguiding in blue-green II–VI semiconductor lasers

Abstract We have studied the lateral waveguiding properties of gain-guided and index-guided II–VI lasers under pulsed conditions and their influence on threshold current density J th and differential quantum efficiency η. Thermal index-guiding was found to reduce the astigmatism of the gain-guided devices. The thermally induced lateral field confinement leads to a lowering of J th and an increase of η with pulse width with a maximum after a few microseconds because of maximum overlap of the near field with the gain profile. For the index-guided devices the lateral waveguiding is fully determined by the built-in refractive index profile and no dependencies on pulse width are observed.