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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 Å and 5.89 Å, 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.

Effects of nitrogen ion irradiation on photoluminescence (PL) properties at 77 K of undoped GaN grown by plasma enhanced molecular beam epitaxy were investigated. It was found that for epitaxial layers grown under ion irradiation conditions, the PL spectra were dominated by near band-edge emission at 357 nm (3.47 eV). However, deep-level emission at around 567 nm (2.2 eV) was also observed and the intensity of which was considerably strong. On the other hand, for GaN films grown with no ion irradiation, the intensity of the deep-level emission was two orders of magnitude less than that of the near band-edge emission. This result suggests that the nitrogen ions degrade the PL properties, i.e., create defect levels relating to emission at around 567 nm (2.2 eV).

Nitrogen-doped p-type ZnSe films have been grown by molecular beam epitaxy using electron cyclotron resonance (ECR) plasma of N2. The nitrogen concentration in the film from 1×1017to 6×1018cm-3has been controlled by adjusting both the aperture area of the ECR cell and the input microwave power. A net acceptor concentration as high as 4.5×1017cm-3was obtained by C-V measurements, and up to this level, the ratio of the net acceptor concentration to the incorporated nitrogen impurity concentration was almost unity. Donor-to-acceptor (DA) emission dominated the photoluminescence spectra at 4.2 K, and the emission intensity ratio of deep to DA emissions was less than 1/1000 for the ZnSe films with nitrogen concentrations less than 2×1018cm-3.

We report the first operation of ZnSe-ZnMgSSe double-heterostructure photopumped blue lasers with a threshold excitation intensity at 150 kW/cm2at room temperature. The lasing was confirmed by measuring the polarization. This result indicates that ZnMgSSe is a promising material for the cladding layer of a blue laser diode. Until now, no material had been lattice-matched to a GaAs substrate and had sufficient band-gap energy. If p-type and n-type doping is possible, this ZnSe-ZnMgSSe double heterostructure can be applied to a blue laser diode.

We report results on the 77 K photoluminescence (PL) of undoped GaN epilayers grown on c-plane sapphire substrate by electron cyclotron resonance molecular beam epitaxy (ECR-MBE). It was found that the PL properties of undoped GaN epilayers are strongly affected by the growth conditions such as nitrogen pressure, growth temperature and ECR plasma power. The intensities of the donor-to-acceptor (D-A) pair emission peak at about 3.2 eV and of the deep-level emission peak at around 2.3 eV change with the variation of these three growth conditions. Assuming that the D-A peak and deep-level emission peak are related to native defects and carbon impurities, respectively, the systematic change of the PL spectra with growth conditions can be reasonably interpreted. At the optimum growth conditions, the PL spectra at 77 K are dominated by excitonic peak at 3.47 eV and the intensity of deep-level emission peak is one order of magnitude less than that of the excitonic peak. However, since the region regarded as optimum is relatively narrow, it is important to control the growth conditions precisely.

We observed the lasing operation of ZnSSe-ZnMgSSe double heterostructure (DH) photopumped blue lasers up to 500 K. The wavelength at room temperature (RT) and threshold intensity at 300 K were 464.5 nm and 105 kW/cm2, respectively. Characteristic temperature (T0) was estimated to be 170 K. These results indicate that this structure has good thermal stability. Both n-type and p-type doping in ZnMgSSe were successfully performed. Both n-type and p-type ZnMgSSe show good properties for fabricating laser diodes.

We report that a Ti/Pt/Au multilayered metal structure can make a nonalloy ohmic contact to n-type ZnSe with an electron concentration greater than 1019cm-3. A specific contact resistance as low as 3.4×10-4?-cm2was achieved for Cl-doped ZnSe with an electron concentration of 2×1019cm-3. We think that the lower contact resistance can be ascribed to the higher reactivity and adhesion of Ti metal with the surface of ZnSe at low temperature, and to the lower work function of Ti metal. The dominant current flow may be due to the quantum mechanical tunneling of electrons through the potential barrier at the ZnSe surface, because the specific contact resistance decreases with an increase in the electron concentration. A small improvement in the specific contact resistance was made by thermal annealing at a temperature of 250°C for 5 min, which yielded the lowest value of 1.1×10-4?-cm2. With annealing at 300°C and below, the contact resistance was not drastically increased. The electrical properties of the contact are therefore thermally stable up to 300°C.