E. M. Arakcheeva

Fabrication of semiconductor-and polymer-based photonic crystals using nanoimprint lithography

The technology of fabricating photonic crystals with the use of nanoimprint lithography is described. One-and two-dimensional photonic crystals are produced by direct extrusion of polymethyl methacrylate by Si moulds obtained via interference lithography and reactive ion etching. The period of 2D photonic crystals, which present a square array of holes, ranges from 270 to 700 nm; the aperture diameter amounts to the half-period of the structure. The holes are round-shaped with even edges. One-dimensional GaAs-based photonic crystals are fabricated by reactive ion etching of GaAs to a depth of 1 μm through a mask formed using nanoimprint lithography. The resulting crystals have a period of 800 nm, a ridge width of 200 nm, and smooth nearly vertical side walls.

Reflecting p-contact based on thin ITO films for AlGaInN flip-chip LEDs

A reflecting contact to a p-GaN layer, used in fabrication of blue flip-chip light-emitting diodes, has been produced by deposition of thin indium tin oxide (ITO) films by electron-beam evaporation. The high reflectance of the contact, which exceeds that of a Ni/Ag contact, provides a 15–20% increase in the external quantum efficiency of light-emitting crystals. The forward voltage drops for crystals with an ITO(5 nm)/Ag(220 nm) contact are comparable with the corresponding values for crystals with a Ni(1.5 nm)/Ag(220 nm) contact. The specific resistance of the contact with an ITO layer is 3.7 × 10−3 Ω cm2. It is shown that, for ITO films produced by the given method, the optimal thicknesses providing the best electrical and optical characteristics of the crystals are in the range 2.5–5.0 nm.

Room-temperature lasing in microring cavities with an InAs/InGaAs quantum-dot active region

Microring cavities (diameter D = 2.7–7 μm) with an active region based on InAs/InGaAs quantum dots are fabricated and their characteristics are studied by the microphotoluminescence method and near-field optical microscopy. A value of 22 000 is obtained for the Q factor of a microring cavity with the diameter D = 6 μm. Lasing up to room temperature is obtained in an optically pumped ring microlaser with a diameter of D = 2.7 μm.

Effect of an excited-state optical transition on the linewidth enhancement factor of quantum dot lasers

An analytical expression is derived for the linewidth enhancement factor of a quantum-dot laser, which makes it possible to describe its dependence on optical loss and photon density in an explicit form. The model accounts for refractive index variations at the ground-state optical transition due to gain/absorption variations upon the first excited-state transition in quantum dots. It is shown that a decrease in optical loss, an increase in saturated gain, and an increase in the energy separation between the excited-state and ground-state transitions results in a decrease in the α factor both at and above the lasing threshold.

Raising the quantum efficiency of AlGaInN flip-chip LEDs by Reactive ion etching of the outer side of SiC substrates

A method for creating a light-scattering microprofile on the outer side of SiC substrates in order to diminish the light extraction loss caused by the effect of total internal reflection in light-emitting AlGaIn/GaN structures has been further developed. It is suggested to use thin photoresist layers as random masks for reactive ion etching of silicon carbide substrates. A microprofile with the required parameters has been obtained on the substrate surface by optimizing the etching modes, with the result that the external quantum efficiency of light-emitting crystals increased by more than 25%.

Blue flip-chip AlGaInN LEDs with removed sapphire substrate

Characteristics of AlGaInN LEDs with removed sapphire substrate are studied. To remove the substrate from a finished LED crystal mounted by the flip-chip method onto a silicon wafer, the laser lift-off technique was used. To raise the light output efficiency, a scattering profile was formed on the n-GaN surface by ion etching in a Cl2: Ar gas mixture. This resulted in the 25–30% increase in the external quantum efficiency of LEDs. The LEDs fabricated in this way demonstrate stable operation at drive currents of up to 300 mA with an optical power as high as 110 mW.

Effect of the nonlinear saturation of the gain on the peak modulation frequency in lasers based on self-assembled quantum dots

Peak modulation frequency of lasers based on self-organized quantum dots is calculated taking into account the effect of nonlinear gain saturation. Because of a large nonlinear gain coefficient and a reduction in the differential gain with increasing optical losses, the peak modulation frequency is attained for an optimum loss level that is significantly lower than the saturated optical gain in the active region. For lasers based on multiply stacked arrays of quantum dots, the peak modulation frequency first increases with increasing number of quantum-dot layers before leveling off, with the limiting value being inversely proportional to the nonlinear gain coefficient.

A temperature-stable semiconductor laser based on coupled waveguides

The design of a stripe semiconductor laser with a composite waveguide formed of two tunneling-coupled waveguides, one of which is narrow and contains the active medium and the other of which is wide and forms the optical mode is considered. It is shown that temperature stabilization of the lasing wavelength can be attained with such a semiconductor laser design. Stabilization of the wavelength can be brought about by lasing in antiresonance conditions of coupling of two waveguides; these conditions arise in a narrow range of wavelengths that depends on temperature only slightly. Calculations performed at the parameters characteristic of the InAlGaAsP laser system show that it is possible to reduce the temperature dependence of the lasing wavelength by a factor of 3 in the temperature range with the width of 60 K.

Optical studies of a two-dimensional photonic crystal with the InAs/InGaAs quantum-dot structure as an active region

A two-dimensional semiconductor photonic crystal with a hexagonal lattice of submicrometer holes is produced by etching the GaAs/AlGaAs planar structure containing the InAs/InGaAs quantum dots in the waveguide layer. By analyzing the reflectance spectra at variable angles of incidence and polarizations of light, the photonic band structure is determined. The Fano resonance peaks revealed in the reflectance spectra at the TM (TE) polarization along the Γ-K (Γ-M) symmetry direction are due to the resonance interaction of optically active photonic bands with the incident light. The band structure of the radiation leakage modes is investigated by studying the angular dependence of the photoluminescence intensity. A threefold increase in the photoluminescence intensity revealed at the resonance frequency of the photonic crystal is attributed to the Purcell effect.

Process of fabricating semiconductor microcavities and photon crystals

A process of fabricating microcavities and photon crystals in GaAs structures by means of electron lithography and reactive ion etching is described. Two types of structures, with micropillars and with photon crystals, are considered. The latter structures have the form of a square or hexagonal array of holes in a planar waveguiding structure. The minimal diameter of the micropillars is 100 nm, and their height is 700 nm. The size of the holes in the photon crystals and the photon crystal period are controllably varied from 140 to 500 nm and from 400 to 1000 nm, respectively. The etch depth of the crystals is more than 350 nm.