New growth directions for nitride based blue lasers and LEDs

Blue and green LEDs and laser diodes are realized using InGaN quantum wells (QWs) on GaN substrates. A major problem for nitride based light emitters is the piezoelectric polarization field which is oriented along the wurtzite c-axis. It is stronger for QWs with high In mole fraction (e.g. for green emission) due to increasing lattice mismatch to the substrate. Since only c-oriented substrates are available in sufficient size, device structures and QWs are grown along the c-axis. This leads to an electric field of several MV/cm across the QWs, causing a separation of the electron and hole wave functions. Therefore common light emitters feature very thin QWs (<3.5 nm) to maximize the wave function overlap. This problem can be solved by changing the growth direction to nonpolar and semipolar orientations that have the polar c-axis inclined or parallel to the growth surface. The change of surface orientation reduces not only the radiative life time but also affects the incorporation of indium into QWs. Such QWs exhibit smaller microscopic wavelength and intensity variations of the luminescence, which is expected to improve the laser performance. When changing the orientation from c-plane to a semipolar orientation the isotropic in-plane symmetry is reduced to a onefold symmetry. This causes polarized emission of LEDs and anisotropic gain and waveguiding properties in edge emitting lasers as shown theoretically and experimentally. The absence of strong polarization fields allows new active region designs with thicker InGaN QWs. Using an active region with a 6.7 nm thick single QW we realized optically pumped laser structures emitting at 400 nm and 450 nm as well as current injection LEDs with violet, blue and green emission.