G. P. Yablonskii

Low-threshold 303 nm lasing in AlGaN-based multiple-quantum well structures with an asymmetric waveguide grown by plasma-assisted molecular beam epitaxy on c-sapphire

We report on AlGaN multiple-quantum-well separate confinement laser heterostructures grown by plasma-assisted molecular-beam epitaxy directly on c-sapphire at low temperatures (<800 °C). Threading dislocation density was reduced down to 109–1010 cm−2 owing to both intentionally introduced strained AlGaN/AlN superlattices and self-organized blocking structures in the AlGaN step-graded buffer layers. The quantum wells were fabricated by a submonolayer digital alloying technique. Calculations of the optical gain and confinement in the optically pumped laser structures yielded its optimum design comprising an asymmetric waveguide. Lasing at 303 nm with the relatively low threshold excitation density of 0.8 MW/cm2 at 295K has been achieved.

Crystal growth and properties of LiAlO2 and nonpolar GaN on LiAlO2 substrate

In this study, the growth and properties of LiAlO2 material and a nonpolar GaN-based light-emitting-diode (LED) structure on LiAlO2 have been investigated. The LiAlO2 material is grown by the Czochralski pulling technique and is used as a substrate for nonpolar nitride growth. An improved surface roughness can be obtained by a four-step polishing process. With subsequent nitridation treatment, a pure M-plane (1010) GaN can be obtained. An electron microscope shows an abundance of cracks that are oriented parallel to the (001) and (100) planes of the LiAlO2 substrate on the rear surface of GaN. The absence of the polarization-induced electric field of a GaN-based LED structure on LiAlO2 was shown by using photoluminescence measurements. Therefore, this approach is promising to further increase the luminescence performance of GaN-based LEDs.

Lasing in Cd(Zn)Se/ZnMgSSe heterostructures pumped by nitrogen and InGaN/GaN lasers

Photoluminescence and lasing at a wavelength of λ=510–530 nm (green spectral region) in Cd(Zn)Se/ZnMgSSe structures with a different design of the active region are studied in a wide range of temperatures and nitrogen laser pump intensities. A minimal lasing threshold of 10 kW/cm2, a maximal external quantum efficiency of 12%, and a maximal output power of 20 W were obtained for the structure with the active region composed of three ZnSe quantum wells with fractional-monolayer CdSe inserts. The lasers exhibited a high temperature stability of the lasing threshold (characteristic temperature T0=330 K up to 100°C). For the first time, an integrated converter composed of a green Cd(Zn)Se/ZnMgSSe laser optically pumped by a blue InGaN/GaN laser that is grown on a Si (111) substrate and incorporates multiple quantum wells is suggested and studied.

Blue InGaN/GaN multiple-quantum-well optically pumped lasers with emission wavelength in the spectral range of 450–470 nm

Optically pumped lasing in the wavelength range of 450–470 nm in InGaN/GaN multiple-quantum-well heterostructures grown by metalorganic vapor phase epitaxy was achieved and investigated. The energy and power per pulse of the laser were 80 nJ and 10 W correspondingly for one facet at room temperature. The far-field patterns of the laser emission consisted of three light spots near the angles of +30°, −15°, and −45°. The highest operating temperature was 450 K. The photoluminescence and photoluminescence excitation spectrum structures suggest that the quantum dots inside the quantum wells are involved in the recombination mechanism.

Recombination dynamics and lasing in ZnO/ZnMgO single quantum well structures

We report a time-resolved study of the recombination dynamics in molecular beam epitaxy grown ZnO∕ZnMgO single quantum wells (SQWs) of 1.0–4.5nm width. The SQWs exhibit different emission properties, depending on both the well width and defect density. Stimulated emission has been achieved at room temperature in a separate confinement double heterostructure having a 3nm wide SQW as an active region. It has been found that a critical parameter for the lasing is the inhomogeneous broadening of both QW and barrier emission bands.

Low-threshold green laser heterostructures with Zn(Mg)SSe/ZnSe graded-index superlattice waveguide: Structural and optical properties

We report on structural and optical properties of green (λ∼520 nm) ZnCdSe/ZnMgSSe optically pumped laser heterostructures with a Zn(Mg)SSe/ZnSe graded-index superlattice (SL) waveguide, grown by molecular beam epitaxy. The pseudomorphic 400 nm thick waveguide comprising a set of strained ZnMgSSe/ZnSe and ZnSSe/ZnSe SLs of different periods and barrier-to-well thickness ratios at each side of a ZnCdSe quantum well (QW) active region provides efficient transport of nonequilibrium carriers to the QW. This results in reduction in laser threshold down to the extremely low value of 1.5 kW/cm2 at 300 K and increasing the external quantum efficiency above 44%.

Luminescence and lasing in InGaN∕GaN multiple quantum well heterostructures grown at different temperatures

It was found that the decrease of the InGaN∕GaN multiple quantum well (MQW) growth temperature from 865 to 810 °C leads to a MQW emission wavelength shift from the violet to the green spectral region. The lowering of the growth temperature also promotes a decrease of the MQW photoluminescence (PL) intensity at high excitation and a disappearance of the excitonic features from the low-temperature reflection and PL spectra of GaN barriers and claddings. The laser threshold dependence on Tg is not monotonic, with the lowest value of 270kW∕cm2 at Tg=830°C. High-temperature annealing (900 °C, 30 min) leads to a twofold increase of the PL efficiency only from the InGaN QWs grown at the lowest temperature. The results allow one to explain the laser threshold behavior in terms of the heterostructure quality, the defect concentration, In clusterization, and the piezoelectric field dependence on the MQW growth temperature.

Effects of electron–phonon interaction and chemical shift on near-band-edge recombination in GaN

A coherent analysis of the near-band-edge luminescence spectra of undoped and Si-doped GaN layers grown by metalorganic vapor phase epitaxy on a sapphire substrate has been performed with a model including the effect of the charge carrier–longitudinal optical (LO) phonon interaction, based on Frohlich’s polaron theory. This model allows one to correlate the relative intensities of the phonon sidebands to the position of the zero-phonon line from which ionization energies are obtained. Moreover, central-cell corrections are taken into account using two different models: the quantum defect model and the Lucovsky model [Solid State Comm. 3, 299 (1965)]. The effect of the electron-LO phonon interaction has been included in these models. Comparison between theory and experiment through the Huang-Rhys factor [Proc. Roy Soc. A204, 406 (1950)] allows a precise determination of the impurity binding energies and effective radii, as well as a reliable characterization of the charge carrier–LO phonon interaction.

Multiple quantum well InGaN/GaN blue optically pumped lasers operating in the spectral range of 450-470 nm

Lasing under optical pumping by N 2 -laser radiation in InGaN/GaN multiple quantum well heterostructures grown in AIXTRON MOVPE reactors was achieved and investigated in the wavelength range of 450-470 nm. The laser operation wavelength depends most strongly on V/III ratio during quantum well barrier growth. The total energy and power per pulse of the laser were 300 nJ and 40 W, respectively, with differential quantum efficiency of 3% at room temperature. The laser threshold increases exponentially with increasing operation wavelength which is mainly due to the decreasing efficiency of the spontaneous emission and due to an increase of its spectral width.

Influence of UV Light-Assisted Annealing on Optical Properties of InGaN/GaN Heterostructures Grown by MOVPE

The influence of ultraviolet light-assisted annealing on the optical properties of InGaN/GaN single quantum wells (SQW), and thick single and double heterostructures (SH, DH) was investigated. It was shown that annealing promotes an increase of the photoluminescence (PL) intensity from the active layers of the SQW and double heterostructures by as much as one order of magnitude. This is effected mainly by a diminishing of the defect concentration and a smoothing of the potential fluctuations in the upper cladding layer. The operating temperature of the DH lasers was lifted from 220 up to 300 K after annealing. Using nitrogen laser irradiation during annealing led both to a PL efficiency increase and to a phase separation in the SH with a thick active layer.