I. S. Tarasov

Ultralow internal optical loss in separate-confinement quantum-well laser heterostructures

Internal optical loss in separate-confinement laser heterostructures with an ultrawide (>1 smm) waveguide has been studied theoretically and experimentally. It is found that an asymmetric position of the active region in an ultrawide waveguide reduces the optical confinement factor for higher-order modes and raises the threshold electron density for these modes by 10–20%. It is shown that broadening the waveguide to above 1 µm results in a reduction of the internal optical loss only in asymmetric separate-confinement laser heterostructures. The calculated internal optical loss reaches ∼0.2 cm−1 (for λ≈1.08 µm) in an asymmetric waveguide 4 µm thick. The minimum internal optical loss has a fundamental limitation, which is determined by the loss from scattering on free carriers at the transparency carrier density in the active region. An internal optical loss of 0.34 cm−1 was attained in asymmetric separate-confinement laser heterostructures with an ultrawide (1.7 µm) waveguide, produced by MOCVD. Lasing in the fundamental transverse mode has been obtained owing to the significant difference in the threshold densities for the fundamental mode and higher-order modes. The record-breaking CW output optical power of 16 W and wallplug efficiency of 72% is obtained in 100-µm aperture lasers with a Fabry-Perot cavity length of ∼3 mm on the basis of the heterostructures produced.

Finite time of carrier energy relaxation as a cause of optical-power limitation in semiconductor lasers

It is shown that the reason why the maximum attainable optical power in semiconductor lasers is limited is the finite time of carrier energy relaxation via scattering by nonequilibrium optical phonons in the quantum-well active region. The power and spectral characteristics of semiconductor lasers are studied experimentally at high excitation levels (up to 100 kA/cm2) in pulsed lasing mode (100 ns, 10 kHz). As the drive current increases, the maximum intensity of stimulated emission tends to a constant value (“saturates”), and the emitted power increases owing to extension of the spectrum to shorter wavelengths. The intensity saturation is due to limitation of the rate of stimulated recombination, caused by a finite time of the electron energy relaxation via scattering by polar optical phonons. It is found that the broadening of the stimulated emission spectrum is related to an increase in carrier concentration in the active region, which enhances the escape of electrons into the waveguide layers. As the drive current increases, the carrier concentration in the waveguide reaches its threshold value and there appears an effective channel of current leakage from the active region. The experiment shows that the appearance of a band of waveguide lasing correlates with a sharp drop in the differential quantum efficiency of a semiconductor laser.

High-Power Laser Diodes Based on Asymmetric Separate-Confinement Heterostructures

Asymmetric separate-confinement laser heterostructures with ultrawide waveguides based on AlGaAs/GaAs/InGaAs solid solutions, with an emission wavelength of ∼1080 nm, are grown by MOCVD. The optical and electrical properties of mesa-stripe lasers with a stripe width of ∼100 μm are studied. Lasers based on asymmetric heterostructures with ultrawide (>1 μm) waveguides demonstrate lasing in the fundamental transverse mode with an internal optical loss of as low as 0.34 cm−1. In laser diodes with a cavity length of more than 3 mm, the thermal resistance is reduced to 2°C/W, and the characteristic temperature T0= 10°C is obtained in the range 0–100°C. A record-breaking wallplug efficiency of 74% and an output optical power of 16 W are reached in CW mode. Mean-time-between-failures testing for 1000 h at 65°C with an operation power of 3–4 W results in the power decreasing by 3–7%.

Spinodal Decomposition of GaxIn1−xAsyP1−yQuaternary Alloys

Using X-ray structural analysis, scanning electron microscopy, atomic force microscopy, and photoluminescent spectroscopy, it is shown that it is possible to obtain a small-scale domain structure on the surface of liquid-phase epitaxial heterostructures. The domain structure emerges as a result of spinodal decomposition of the GaxIn1 − xAsyP1 − y quaternary alloy due to immiscibility of its components and relaxation of its lattice parameter to surrounding layers.

Spinodal Decomposition of Ga{sub x}In{sub 1-x}As{sub y}P{sub 1-y}Quaternary Alloys

Using X-ray structural analysis, scanning electron microscopy, atomic force microscopy, and photoluminescent spectroscopy, it is shown that it is possible to obtain a small-scale domain structure on the surface of liquid-phase epitaxial heterostructures. The domain structure emerges as a result of spinodal decomposition of the GaxIn1 − xAsyP1 − y quaternary alloy due to immiscibility of its components and relaxation of its lattice parameter to surrounding layers.

The substructure and luminescence of low-temperature AlGaAs/GaAs(100) heterostructures

The substructure and luminescence of low-temperature epitaxial AlGaAs alloys are studied by Raman spectroscopy and photoluminescence spectroscopy. It is shown that the experimental data obtained in the study are consistent with the results of the previous structural and optical study. The assumption that, at high concentrations of carbon acceptors, the acceptor atoms concentrate at lattice defects of the AlGaAs crystal alloys to form carbon nanocrystals is confirmed.

Effect of silicon on relaxation of the crystal lattice in MOCVD–hydride AlxGa1 − xAs:Si/GaAs(100) heterostructures

The X-ray diffraction and infrared spectroscopy data for MOCVD-hydride AlxGa1 − xAs:Si/GaAs(100) heterostructures and homoepitaxial GaAs:Si/GaAs(100) structures doped with Si to a content of up to ∼1 at % are reported. It is shown that, in the homoepitaxial heterostructures, the formation of alloys with Si yields a decrease in the crystal lattice parameters of the epitaxial layer and a negative lattice mismatch with the single-crystal substrate (Δa < 0). At the same time, the formation of quaternary alloys in the AlxGa1 − xAs:Si/GaAs(100) heterostructures is not accompanied by any pronounced strains in the crystal lattice. By introducing Si into the epitaxial layers of these heterostructures, it is possible to attain complete matching of crystal lattice parameters of the film and substrate in the appropriately chosen technological conditions of growth of the epitaxial layers.

X-ray diffraction studies of heterostructures based on solid solutions AlxGa1 − xAsyP1 − y: Si

The growth of MOCVD-hydride epitaxial heterostructures based on ternary solid solutions AlxGa1−xAs heavily doped with phosphorus and silicon has been studied using high-resolution X-ray diffraction and X-ray microanalysis. The prepared epitaxial films are five-component solid solutions (AsxGa1−xAsyP1 − y)1 − zSiz.

Structural and spectral features of MOCVD AlxGayIn1 − x − yAszP1 − z/GaAs (100) alloys

The study is concerned with MOCVD epitaxial heterostructures grown on the basis of AlxGayIn1 − x − yAszP1 − z quinary alloys in the region of alloy compositions isoperiodic to GaAs. By the X-ray diffraction technique and atomic force microscopy, it is shown that, on the surface of the heterostructures, there are nanometric objects capable of lining up along a certain direction. From calculations of the crystal lattice parameters with consideration for internal strains, it can be inferred that the new compound is a phase based on the AlxGayIn1 − x − yAszP1 − z alloy.

Study of non-diffracting light beams from broad-stripe edge-emitting semiconductor lasers

Broad-stripe edge-emitting semiconductor lasers have been used to obtain propagation-invariant (nondiffracting) light beams with powers and diameters of the central ray acceptable for optical manipulation and tweezing. The results of investigations of the propagation of Bessel beams generated from broad-stripe lasers with spectrally selective resonator show that the spatial homogeneity of emission plays a much greater role than the temporal coherence in the formation of Bessel beams. The main factors limiting the length of non-diffracting beam propagation (without distortion of the central ray) are the astigmatism and multimode character of laser radiation.