J. Han

Graded band gap ohmic contact to p-ZnSe

We describe a low‐resistance quasi‐ohmic contact to p‐ZnSe which involves the injection of holes from heavily doped ZnTe into ZnSe via a Zn(Se,Te) pseudograded band gap region. The specific contact resistance is measured to be in the range of 2–8×10−3 Ωu2009cm2. The graded heterostructure scheme is incorporated as an efficient injector of holes for laser diode and light emitting diode devices, demonstrating the usefulness of this new contact scheme at actual device current densities.

Microstructure study of a degraded pseudomorphic separate confinement heterostructure blue‐green laser diode

The microstructure of a degraded II‐VI blue‐green laser diode based on the ZnCdSe/ZnSSe/ ZnMgSSe pseudomorphic separate confinement heterostructure has been examined by transmission electron microscopy. Triangular nonluminescent dark defects observed in the laser stripe region by electroluminescence microscopy have been identified to be dislocation networks developed at the quantum‐well region. The dislocation networks have been observed to be nucleated at threading dislocations originating from pairs of V‐shaped stacking faults which are nucleated at or near the II‐VI/GaAs interface and extending into the n‐ZnMgSSe lower cladding layer.

Zinc-blende MnTe: epilayers and quantum well structures

Epilayers of the previously hypothetical zinc‐blende MnTe have been grown by molecular beam epitaxy. Epitaxial layers (0.5 μm thick) of MnTe were characterized using x‐ray diffraction and transmission electron microscopy; optical reflectance measurements indicate a band gap of ∼3.2 eV. A series of strained single quantum well structures was fabricated with zinc‐blende MnTe forming the barrier to CdTe quantum well regions; photoluminescence spectra indicate optical transitions corresponding to strong electron and hole confinement.

Heavy p‐doping of ZnTe by molecular beam epitaxy using a nitrogen plasma source

The successful p‐doping of ZnSe and Zn(S,Se) using a nitrogen plasma source during growth by molecular beam epitaxy has been an important factor leading to the recent realization of blue‐green diode lasers and light‐emitting diodes. This letter reports the results of the nitrogen doping of ZnTe using similar techniques. Doping levels approaching the 1019 cm−3 range are reported along with electrical, optical, and microstructural characterization. Highly doped ZnTe provides an opportunity for forming an ohmic contact to p‐ZnSe.

Blue‐green laser emission from ZnSe quantum well microresonators

Microresonator structures have been fabricated from (Zn,Cd)Se/Zn(S,Se) quantum well material. The resonators, which are designed in the ‘‘whispering gallery’’ geometry, have been optically pumped at room temperature with threshold excitation levels of 100 kW/cm2.

Reduction of structural defects in II–VI blue green laser diodes

Early blue/green laser diodes based on ZnSe exhibited room temperature, continuous wave (cw) lifetimes of the order of a minute. Similar to the history of (Al,Ga)As lasers, the source of the degradation was the presence of extended crystalline defects. The dominant extended defects in the early room temperature cw lasers originated as stacking faults generated at the ZnSe/GaAs heterovalent nucleation event, and exhibited densities of the order of 106 cm−2. In this letter, a procedure is described which will ensure a consistent run to run reduction of the density of such extended defects to the mid to low 103 cm−2 over a 3 in. wafer.

On degradation of ZnSe‐based blue‐green diode lasers

The degradation mechanism of ZnMgSSe/ZnSSe/ZnCdSe separate confinement heterostructure laser diodes is studied under continuous wave and pulsed operation at room temperature. The degradation of the active quantum well is caused by formation of optically inactive areas which nucleate at paired stacking faults. These ‘‘dark defects’’ are identified as areas of dislocation network and point defect segregation.

Pseudomorphic separate confinement heterostructure blue-green diode lasers

The growth and performance of pseudomorphic separate confinement heterostructure blue‐green laser diodes are described. The devices incorporate the (Zn,Mg)(S,Se) quaternary as cladding layers surrounding a Zn(S,Se) waveguiding layer, and having single or multiple quantum wells of (Zn,Cd)Se. Devices have been operated at room temperature under pulsed conditions (∼1 μs, 10−3 duty cycle) for periods up to 1 h. X‐ray rocking curve full width at half‐maxima as low as 44 arcsec were obtained for a laser structure employing quaternary cladding layers (Mg=9%, S=12%), consistent with transmission electron microscope observations showing no dislocations or stacking faults. The Zn(Se,Te) graded contact was adapted to form an ohmic contact to the top p‐type quaternary layer.

Transport study of ZnSe:N employing Zn(Se,Te) graded contacts

Hall effect measurements for temperatures ranging from 77 to 320 K on a series of p‐ZnSe films grown on GaAs substrates are reported. The ZnSe epilayers were doped using a nitrogen plasma source during growth by molecular beam epitaxy. A Zn(Se,Te) graded band‐gap layer was used to provide ohmic contacts over the temperature range of the measurements. The activation energy of nitrogen in ZnSe at the infinite dilution limit was found to be 114 meV, and compensation ratios ranged from 6% to 11%.

Ohmic contacts and transport properties in ZnSe-based heterostructures

In this paper both horizontal and vertical transport properties of ZnSe based heterostructures were studied. Temperature-dependent Hall effect measurements were performed on nitrogen-doped ZnSe, ZnTe, Zn(S,Se) and (Zn,Mg)(S,Se) epilayers; accepter concentration N a , compensation donor concentration N d and the activation energy E a were derived by curve-fitting to the freeze-out behavior of the hole concentrations. Vertical transport study, through the use of an analytical computer simulation, suggested that the electron transport across the n-ZnSe/n-GaAs heterointerfacc is often hindered by the presence of a high density of interface states; both the employment of heavy doping near the interface and the modification of GaAs surface stoichiometry before the nucleation of ZnSe were found effective in reducing the device impedance