G. C. Hua

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 Ω cm2. 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.

Blue and green diode lasers in ZnSe‐based quantum wells

Laser diode operation has been obtained from (Zn,Cd)Se/ZnSe and (Zn,Cd)Se/Zn(S,Se) quantum well structures in the blue and the green. The devices, prepared on p‐ and n‐type (In,Ga)As or GaAs buffer layers for lattice matching purposes to control the defect density, have been operated at near‐room‐temperature conditions and briefly at room temperature with uncoated end facets. Quasi‐continuous wave operation has been obtained at T=77 K.

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.

Room temperature blue light emitting p-n diodes from Zn(S,Se)-based multiple quantum well structures

Blue (494 nm) light emitting quantum well diodes based on Zn(S,Se) p‐n junctions are demonstrated at room temperature. P‐type Zn(S,Se) is realized by using a nitrogen rf plasma cell. The light emitting diode is formed on homoepitaxial GaAs buffer layers by molecular beam epitaxy. The S fraction of the alloy is selected to provide a lattice match between the II‐VI active region and the GaAs buffer; the result is an active region having a dislocation density below 105/cm−2. The letter discusses emission characteristics as well as the x‐ray rocking curve and transmission electron microscopy characterization of the structures.

Blue/green pn junction electroluminescence from ZnSe‐based multiple quantum‐well structures

The successful p doping of ZnSe by substitutional nitrogen using a plasma cell incorporated into the molecular beam epitaxy chamber has led to the development of electroluminescent devices based on carrier injection at a pn junction. The light emitting diode structures described here are grown on a GaAs substrate using a tetragonally distorted (In,Ga)As buffer layer to provide lattice matching between the substrate and the active II–VI region. The result of the incorporation of the buffer layer is an essentially dislocation‐free active region. The letter discusses optical properties as well as the x‐ray and transmission electron microscopy characterization of the quantum well device structures.

Suppression of twin formation in CdTe(111)B epilayers grown by molecular beam epitaxy on misoriented Si(001)

CdTe(lll)B layers have been grown on misoriented Si(001). Twin formation inside CdTe(lll)B layer is very sensitive to the substrate tilt direction. When Si(001) is tilted toward [110] or [100], a fully twinned layer is obtained. When Si(001) is tilted toward a direction significantly away from [110], a twin-free layer is obtained. Microtwins inside the CdTe(111)B layers are overwhelmingly dominated by the lamellar twins. CdTe(111)B layers always start with heavily lamellar twinning. For twin-free layers, the lamellar twins are gradually suppressed and give way to twin-free CdTe(111)B layer. The major driving forces for suppressing the lamellar twinning are the preferential orientation of CdTe[11-2] along Si[1-10] and lattice relaxation. Such preferential orientation is found to exist for the CdTe(111)B layers grown on Si(001) tilted toward a direction between [110] and [100].

Study of Luminescent Region in Anodized Porous Silicons by Photoluminescence Imaging and Their Microstructures

The luminescent region in anodized porous silicon was examined by means of a photoluminescence imaging technique. It was found for the first time that the luminescence in the visible region originates from the topmost surface layer. This was confirmed not only by the surface photoexcitation but also by the excitation of the cleaved edge. This topmost surface layer does not show any diffraction spots with transmission-electron microscopy and is regarded as an amorphous layer. The microstructure of this layer observed by means of secondary-electron microscope (SEM) consisted of microparticles with the dimensions of 5~30 nm, where the lower dimension is limited by the SEM resolution.

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.