S. Dalmasso

Built-in electric-field effects in wurtzite AlGaN/GaN quantum wells

AlGaN/GaN quantum well (QW) structures are grown on c-plane sapphire substrates by molecular beam epitaxy. Control at the monolayer scale of the well thickness is achieved and sharp QW interfaces are demonstrated by the low photoluminescence linewidth. The QW transition energy as a function of the well width evidences a quantum-confined Stark effect due to the presence of a strong built-in electric field. Its origin is discussed in terms of piezoelectricity and spontaneous polarization. Its magnitude versus the Al mole fraction is determined. The role of the sample structure geometry on the electric field is exemplified by changing the thickness of the AlGaN barriers in multiple-QW structures. Straightforward electrostatic arguments well account for the overall trends of the electric-field variations.

Room-temperature blue-green emission from InGaN/GaN quantum dots made by strain-induced islanding growth

InGaN/GaN self-assembled quantum dots (QDs) were obtained by molecular beam epitaxy making use of the Stranski–Krastanov growth mode. Room-temperature photoluminescence (PL) energy of QDs was observed from 2.6 to 3.1 eV depending on the dot size. PL linewidths as low as 40–70 meV at 10 K and 90–110 meV at 300 K indicate low dot size dispersion. The comparison of PL intensity versus temperature of an InGaN epilayer and InGaN/GaN QDs demonstrates the higher radiative efficiency of the latter.

Pyramidal defects in metalorganic vapor phase epitaxial Mg doped GaN

A transmission electron microscopy study of structural defects induced by the introduction of Mg during the growth of metalorganic vapor phase epitaxy GaN is presented. These defects are assumed to be pyramidal inversion domains with an hexagonal base and {1123} inclined facets. The tip of the pyramids is always pointing toward the [0001] direction, i.e., in a Ga-terminated film, toward the substrate and in a N-terminated film, toward the surface. A chemical quantitative analysis shows that these pyramidal defects are Mg rich. They are present in all the studied films, independent of the doping level.

Injection dependence of the electroluminescence spectra of phosphor free GaN-based white light emitting diodes

Multicolor, multi-quantum well light emitting diodes have been fabricated by molecular beam epitaxy by inserting quantum wells of various widths in the active region. The In content of the wells is 15%-20% and the color is governed by carrier confinement and the Stark effect. Combining a proper number of blue and yellow quantum wells allows to obtain monolithic white LEDs. The electroluminescence spectra of the diodes have been studied. At low injection, the luminescence intensity varies quadratically with the injection current, showing that the electroluminescence originates from the depleted region of the diode, and that non-radiative recombination paths exist. However, for higher injection currents, the luminescence efficiency of the wells situated near the n-side of the junction starts to vary linearly with the current, and this is accompanied by the appearance of GaN electroluminescence. We show that this is due to the entering of these wells into the neutral region of the diode, explaining the injection dependence of the color of these multicolor LEDs.

Green InGaN Light-Emitting Diodes Grown on Silicon (111) by Metalorganic Vapor Phase Epitaxy

We report on the growth and characterization of green InGaN light-emitting diodes (LEDs) grown on Si (111) substrates using metalorganic vapor phase epitaxy. A single InGaN quantum well active layer has been used. The optical qualities of InGaN on Si(111) and the p–doping efficiency of GaN are discussed. The turn-on voltage of the LED is 6.8 V and the operating voltage is 10.7 V at 20 mA. Electroluminescence of the LEDs starts at a forward bias of 3.5 V. The electroluminescence peaks at 508 nm, with a full-width at half maximum of 52 nm. An optical output power of 6 µW (in ~ 8π/5 sr) was achieved for an applied current of 20 mA.

GaInN GaN multiple-quantum-well light-emitting diodes grown by molecular beam epitaxy

GaInN and GaN were grown by molecular beam epitaxy on c-plane sapphire using NH3. 9 K photoluminescence performed on both GaInN thin layers and GaInN/GaN multiple-quantum wells (MQWs) exhibits narrow emission (∼50 meV linewidths). Transmission electron microscopy images show sharp GaInN/GaN interfaces and homogeneous GaInN layers. Strong indium surface segregation is also evidenced. Light-emitting diodes were fabricated from 5×GaInN (25 A)/GaN (35 A) MQW heterostructures. The 300 K electroluminescence yields blue light at 440 nm.

Optical characterization of AlxGa1-xN alloys (x < 0.7) grown on sapphire or silicon

The optical properties of Al x Ga 1-x N samples (x < 0.7) have been studied by photoluminescence (PL) and reflectivity in the 10-300 K temperature range. Various physical properties have been studied as a function of composition, such as Stokes shift, alloy broadening, exciton localization, and Huang-Rhys factor. Up to x 0.3, a band gap bowing factor of ∼0.9 eV accounts for the variation of PL and reflectivity energies. At higher compositions, luminescence energies deepen with regard to this behaviour. This is interpreted as a consequence of the Γ 9 -Γ 7 crossover of the valence band maxima. This is confirmed by the linear polarization of the luminescence studied under oblique observation.

Influence of high Mg doping on the microstructural and optoelectronic properties of GaN

A transmission electron microscopy study of a wide range of p-type GaN samples reveals that high Mg doping has a strong influence on the polarity of GaN. The main characteristic of Mg-doped metal organic vapour phase epitaxy (MOVPE) and bulk GaN is the presence of pyramidal inversion domains (PIDs). It is shown that the appearance of PIDs is correlated with a decrease of the free hole concentration and with the appearance of the blue photoluminescence band which is characteristic of MOVPE-grown Mg-doped GaN. A tentative model based on electrostatic considerations is proposed for this blue luminescence band.

Structural defects and relation with optoelectronic properties in highly Mg-doped GaN

A transmission electron microscopy of pyramidal inversion domains induced by Mg doping in MOVPE and bulk GaN is presented. Based on high resolution observations and EDX analysis, two atomic models are proposed for the Mg-rich (0001) inversion domain boundaries. These structural defects appearing for Mg concentrations in the 10(19) cm(-3) range are shown to be possible origins for the auto-compensation and the blue luminescence.

Optoelectronic characterization of blue InGaN/GaN LEDS grown by MBE

Light emitting diodes were grown by molecular beam epitaxy using NH, as nitrogen precursor. The active layer is composed by a single plane of undoped InGaN layer with about 15% of In. The structure was buried by 2700 Angstrom of Mg-doped GaN (p = 1 x 10(17) cm(-3)). The turn on voltage is at 4.5 V and the operating voltage is 6.1 V at 20 mA. Temperature dependent I(V) characteristics reveal the predominance of tunneling injection current. We measure room temperature electroluminescence in the blue from 440 to 490 nm with a narrow full width at half maximum