Kevin W. Haberern

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.

SCANNING TUNNELING MICROSCOPY COMPARISON OF GAAS(001) VICINAL SURFACES GROWN BY MOLECULAR BEAM EPITAXY

We report the first scanning tunneling microscope observations of molecular beam epitaxy grown GaAs(001) vicinal surfaces cut 2° towards (111)A and 2° towards (111)B. The A‐type step edges are found to be relatively straight, with 16 A kinks occurring typically every 100 A along the step. In contrast, the B‐type step edges are found to be very ragged. On both surfaces, the terrace widths varied considerably. The details of the two step structures are dominated by the structure of the (2×4) unit cell.

MBE growth on vicinal GaAs(001) surfaces studied by scanning tunneling microscopy

We have imaged several different vicinal GaAs(001) surfaces, grown by molecular beam epitaxy (MBE), with the scanning tunneling microscope (STM). All the samples were grown under arsenic-rich conditions and had a (2 × 4)/c(2 × 8) surface reconstruction. The structure of the steps was found to be determined by the structure of the (2 × 4) unit cell. Atomic resolution images of the steps show that there is no step edge reconstruction and that the steps are built up from complete (2 × 4) unit cells. Vicinal GaAs(001) surfaces cut 2° towards (111)A and 2° towards (111)B and grown under step-flow growth conditions were compared. The A-type steps are relatively straight whereas the B-type steps are very ragged. Changing the angle of misorientation of the sample was not found to have a major effect on the step structure. A-type stepped surfaces grown under 2D nucleation growth conditions were found to be less regular than those grown under step-flow growth conditions. The results are discussed in terms of the growth mode of GaAs(001) and the prospects for growing sufficiently regular stepped surfaces for use in device structures such as quantum wires are considered.

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.

Defect characterization of etch pits in ZnSe based epitaxial layers

Three distinct etch pit features in ZnSe based epitaxial layers have been identified. The features were observed with optical dark field microscopy and confirmed to be pits using scanning electron microscopy. Using transmission electron microscopy, we associated different etch pits with characteristic crystallographic defects which are common in epitaxially grown II–VI materials. Frank‐type stacking faults form the largest etch pit followed by a paired configuration of Shockley‐ type stacking faults. The smallest etch pit is due to a single Shockley‐type stacking fault. This study represents one of the first examples of identifying crystallographic defects in II–VI wide bandgap materials using etch pit delineation.

Electrical characterization of p‐type ZnSe:N and Zn1−xMgxSySe1−y:N thin films

Differential van der Pauw–Hall effect and resistivity measurements have been performed to determine the concentration and mobility of free holes in nitrogen doped ZnSe and Zn1−xMgxSySe1−y thin films. Hall data taken between 120 and 300 K, with magnetic fields up to 4 kG yielded an activation energy of the nitrogen acceptors in ZnSe:N of 104 meV. Donor compensation in the ZnSe:N samples was negligible. Compared with ZnSe:N, samples of Zn1−xMgxSySe1−y:N, exhibited a significantly lower value of room‐temperature mobility of holes, and fast‐carrier freeze‐out at relatively high temperature, approximately T=200 K.

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.

Thermal characteristics of blue‐green II‐VI semiconductor lasers

Threshold current densities and wavelengths of gain maximum and longitudinal modes have been determined as a function of temperature for various laser structures. The onset of intrapulse heating has been studied and interpreted. In Zn0.92Mg0.08S0.12Se0.88/ZnS0.06Se0.94 /Zn0.8Cd0.2Se lasers, thermal resistances have been measured, using substrate‐up and substrate‐down mounting. From these, continuous‐wave lasing regimes have been determined.

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.