This paper reviews semi-polar GaN surfaces, of interest for light emitting devices, from both theoretical and experimental perspectives. Theoretical results on polarization charges at InGaN/GaN heterointerfaces and In incorporation into InGaN films are presented for polar (0001), semi-polar (1122) and nonpolar (1100) surfaces. Specific features of semi-polar InGaN/ GaN structures are emphasized which can be beneficial for improving optical and transport properties of quantum-wellbased light emitting devices. The analysis favours semi-polar surfaces such as the (1122) surface as growth plane for longwavelength light emitters. Therefore, the experimental sections emphasize progress towards long-wavelength LEDs and lasers by growth of InGaN/AlGaN/GaN(1122) heterostructures on large-area GaN(1122)/m-sapphire templates. The current status of such templates as grown by hydride vapour phase epitaxy is presented. The implementation of an epitaxial lateral overgrowth method on such templates to improve device performances is demonstrated.

The boundary conditions for a multicomponent effective wave function are obtained in the eight-band Kane model. The relations are established between boundary-condition matrix elements and the Kane Hamiltonian parameters in constitutive semiconductors. In general, these relations are incompatible with component-bycomponent continuity of the effective wave function, traditionally employed in multiband models. We show that the boundary conditions in the isotropic eight-band Kane model involve three linearly independent phenomenological parameters. Neglecting the spin-orbit interaction at the heterointerface, only two parameters are required to completely describe the matching conditions. These parameters do not depend on the energy of the charge carrier state, hence the nonparabolic regime is described in the most natural way. The boundary condition matrices are derived also for the most important approximate limits: the six-band Kane model, describing the energy spectrum of narrow-gap semiconductors, and the four-band Luttinger model, describing the valence-band top energy region in zinc-blende and diamondlike semiconductors. ͓S0163-1829͑98͒07032-5͔

The authors demonstrate a double quantum well GaSb-based diode laser operating at 2.4μm with a room-temperature cw output power of 1050mW and a maximum power-conversion efficiency of 17.5%. Laser differential gain with respect to current increases by a factor of 2 and laser threshold current is nearly halved when the compressive strain in the quantum wells is increased from 1.2% to 1.6%. This improvement is due to substantially improved hole confinement in the heavily compressively strained active region.

Diode lasers emitting at 3.0 m were designed and fabricated. Device active region contained two compressively strained InGaAsSb quantum wells incorporated in quinternary AlInGaAsSb barriers. Laser output power at room temperature was 130 mW in continuous wave regime and more than 1 W in pulse.

Abstract-InGaAsSb/AlGaAsSb quantum well (QW) diode laser structures with either 1% or 1.5% compressively strained QWs were grown on GaSb substrates by molecular beam epitaxy. Widestripe lasers fabricated from structures of both types have roomtemperature operating wavelengths near 2.3 microns. The roomtemperature threshold current density of 1-mm-long uncoated devices with 1.5% strained QWs was lower than threshold current density of the 1.0% strained QW devices by nearly a factor of two (120 A/cm 2 versus 230 A/cm 2 ). Experiment shows that the reduction in threshold current density with increasing QW strain is related to the increase in differential gain and decrease in transparency current density. Optical gain calculations prove that improvement of the QW hole confinement reduces the threshold carrier concentration in laser structures with heavily strained low arsenic content quantum wells.Index Terms-Mid-infrared, GaSb-based, type-I, high power, heavy compressive strain, differential gain, hole confinement.

Interwell optical-phonon-assisted transitions are studied in an asymmetric double-quantum-well heterostructure comprising one narrow and one wide coupled quantum wells (QWs). We show that the depopulation rate of the lower subband states in the narrow QW can be significantly enhanced thus facilitating the intersubband inverse population, if the depopulated subband is aligned with the second subband of the wider QW, while the energy separation from the first subband is tuned to the highest energy optical-phonon mode.

Electron-phonon scattering rates and intersubband optical gain spectra were calculated, including the optical phonon confinement effect in AlGaAs/GaAs/AlGaAs quantum well heterostructures. Comparison of the calculated gain spectra with those calculated using the bulk phonon approximation shows that details of the phonon spectrum have a strong influence on the intersubband optical gain.

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