Takao Miyajima

Characterization of threading dislocations in GaN epitaxial layers

We investigated Si-doped GaN epitaxial layers on a (0001)-sapphire substrate using a HCl vapor-phase etching technique, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. Three kinds of distinctive etch pits correspond to three different types of threading dislocations, edge, mixed, and screw types. Photoluminescence intensity increases with the decrease in the number of etch pits corresponding to mixed and screw dislocations. The number of etch pits corresponding to edge dislocations, however, did not change. We concluded, therefore, that threading dislocations having a screw-component burgers vector act as strong nonradiative centers in GaN epitaxial layers, whereas edge dislocations, which are the majority, do not act as nonradiative centers.

Epitaxial Growth of ZnMgSSe on GaAs Substrate by Molecular Beam Epitaxy

We propose a new material, ZnMgSSe, as the cladding layer of a blue-light laser diode. Band-gap energy can be varied from 2.8 to near 4 eV, maintaining lattice-matching to a (100)GaAs substrate. The band-gap energies of MgS and MgSe (zincblende structure) are estimated to be about 4.5 eV and 3.6 eV, and the lattice constants are 5.62 ? and 5.89 ?, respectively. The refractive index of ZnMgSSe lattice-matched to GaAs is smaller than that of ZnSSe lattice-matched to GaAs. ZnMgSSe meets the requirements of the cladding layer of ZnSSe for fabricating the blue-light laser diode.

Epitaxial growth of ZnMgSSe on GaAs substrate by molecular beam epitaxy

Abstract We propose a new material, ZnMgSSe, as the possible cladding layer of blue laser diodes. The band-gap energy can be varied from 2.8 to near 4 eV, maintaining lattice-matching to a (100) GaAs substrate. From the quarternary data, the band-gap energies of MgS and MgSe (zincblende structure) are estimated to be about 4.5 and 3.6 eV, and the lattice constants are 5.62 and 5.89 A, respectively. The refractive index of ZnMgSSe lattice-matched to GaAs is smaller than that of ZnSSe lattice-matched to GaAs. ZnMgSSe therefore meets the requirements of the cladding layer of ZnSSe for fabricating blue laser diodes.

EPITAXIAL GROWTH OF P-TYPE ZNMGSSE

N‐doped p‐type ZnMgSSe was grown by molecular beam epitaxy. Nitrogen was induced by electron cyclotron resonance plasma. The maximum net acceptor concentration (NA−ND) and the activation energy of the nitrogen acceptor (EN) depend on the band‐gap energy of ZnMgSSe. With increasing band‐gap energy, the maximum NA‐ND is decreased and EN is increased. The maximum NA‐ND and the EN of ZnMgSSe with a band‐gap energy of 3.05 eV at 77 K are 2.5×1016 cm−3 and 140 meV, respectively.

Electroluminescence in an Oxygen-Doped ZnSe p-n Junction Grown by Molecular Beam Epitaxy

Blue electroluminescence has been obtained from ZnSe p-n junctions. ZnSe films were grown on n-type GaAs(100) substrates by molecular beam epitaxy. The dopants used for n-and p-type ZnSe were Ga and O, respectively. The electron-beam-induced current strongly suggests the formation of a p-n junction. The built-in Potential of the p-n junction and carrier concentration of p-type ZnSe layer estimated from the capacitance-voltage relation were about 2.3 V and 1.2×1016cm-3, respectively. The electroluminescence spectra from the p-n junction were dominated by band-edge emissions of 466 nm at room temperature and 446 nm at 77 K.

12W peak-power 10ps duration optical pulse generation by gain switching of a single-transverse-mode GaInN blue laser diode

We have generated optical pulses at 405nm from a single-transverse-mode GaInN blue laser diode under intensive gain-switching operation with a minimum pulse duration of less than 8ps. The maximum optical peak power was as high as 12W with a pulse width of 10ps. The peak power obtained is the highest value for optical pulses ever generated from a single transverse-mode GaInN laser diode.

Dislocations in GaN‐Based Laser Diodes on Epitaxial Lateral Overgrown GaN Layers

We have investigated dislocations in GaN-based laser diodes (LDs) on epitaxial lateral overgrown (ELO) GaN layers using transmission electron microscopy and cathodoluminescence microscopy and found a correlation between dislocations and device reliability. Dislocation density in the seed regions of ELO GaN layers is of the order of 10 8 cm -2 , while that in the wing regions is less than mid-10 6 cm -2 . The origin of dislocations in the wing regions is the extension of defects in highly defective regions near the GaN layer/substrate interface in the seed regions. The lifetime of LDs has a strong correlation with consumption power. However, some LDs have a shorter lifetime although their consumption power is almost the same. In the LDs with short lifetimes, dislocations lying in the c-plane were formed below the active regions, bent towards the c-axis and threaded upwards to active regions. These newly created dislocations can become detrimental to the device lifetime.

GaN-based blue laser diodes

We report our recent progress on GaN-based high-power laser diodes (LDs), which will be applied as a light source in high-density optical storage systems. We have developed raised-pressure metal-organic chemical vapour deposition (RP-MOCVD), which can reduce the threading-dislocation density in the GaN layer to several times 108 cm-2, and demonstrated continuous-wave (cw) operation of GaN-based LD grown by RP-MOCVD. Furthermore, we found that the epitaxial lateral overgrowth (ELO) technique is useful for further reducing threading-dislocation density to 106 cm-2 and reducing the roughness of the cleaved facet. By using this growth technique and optimizing device parameters, the lifetime of LDs was improved to more than 1000 hours under 30 mW cw operation at 60 °C. Our results proved that reducing both threading-dislocation density and consumption power is a valid approach to realizing a practical GaN-based LD. On the other hand, the practical GaN-based LD was obtained when threading-dislocation density in ELO-GaN was only reduced to 106 cm-2, which is a relatively small reduction as compared with threading-dislocation density in GaAs- and InP-based LDs. We believe that the multiplication of non-radiative centres is very slow in GaN-based LDs, possibly due to the innate character of the GaN-based semiconductor itself.

Electroluminescence from a ZnSe p-n Junction Fabricated by Nitrogen-Ion Implantation

Electroluminescence has been obtained from ZnSe p-n junctions. ZnSe films were grown on n-type GaAs substrates by molecular beam epitaxy. The dopant used for n-type ZnSe was Ga, and p-type ZnSe was formed by nitrogen ion implantation into undoped ZnSe which was grown on the Ga-doped ZnSe layer. Rapid thermal annealing was performed using an infrared lamp in N2 ambient. The formation of a p-n junction was confirmed by measuring electron beam-induced current. The electroluminescence spectra were dominated by two peaks of band-edge emission at 446 and 459 nm at 77 K.

Picosecond optical pulse generation from self-pulsating bisectional GaN-based blue-violet laser diodes

Self-pulsations of 407 nm emitting GaN-based blue-violet laser diodes with bisectional (BS) electrodes were demonstrated by applying a reverse bias (VSA) to the subelectrode of the saturable absorber (SA) section. By increasing the injected current to the main electrode of the gain section with a reverse bias of VSA=−12 V, the optical pulses shortened to 30 ps. The optical peak output power was as high as 2.4 W with a pulse width of 30 ps and a repetition frequency of 0.9 GHz. This is so far the shortest pulse width from a self-pulsating BS GaN-based laser diode achieved by applying a reverse bias to the SA section.Self-pulsations of 407 nm emitting GaN-based blue-violet laser diodes with bisectional (BS) electrodes were demonstrated by applying a reverse bias (VSA) to the subelectrode of the saturable absorber (SA) section. By increasing the injected current to the main electrode of the gain section with a reverse bias of VSA=−12 V, the optical pulses shortened to 30 ps. The optical peak output power was as high as 2.4 W with a pulse width of 30 ps and a repetition frequency of 0.9 GHz. This is so far the shortest pulse width from a self-pulsating BS GaN-based laser diode achieved by applying a reverse bias to the SA section.