M. Leroux

Temperature quenching of photoluminescence intensities in undoped and doped GaN

This work discusses the temperature behavior of the various photoluminescence (PL) transitions observed in undoped, n- and p-doped GaN in the 9–300 K range. Samples grown using different techniques have been assessed. When possible, simple rate equations are used to describe the quenching of the transitions observed, in order to get a better insight on the mechanism involved. In undoped GaN, the temperature dependence of band edge excitonic lines is well described by assuming that the A exciton population is the leading term in the 50–300 K range. The activation energy for free exciton luminescence quenching is of the order of the A rydberg, suggesting that free hole release leads to nonradiative recombination. In slightly p-doped samples, the luminescence is dominated by acceptor related transitions, whose intensity is shown to be governed by free hole release. For high Mg doping, the luminescence at room temperature is dominated by blue PL in the 2.8–2.9 eV range, whose quenching activation energy is in...

From visible to white light emission by GaN quantum dots on Si(111) substrate

GaN quantum dots (QDs) in an AlN matrix have been grown on Si(111) by molecular-beam epitaxy. The growth of GaN deposited at 800 °C on AlN has been investigated in situ by reflection high-energy electron diffraction. It is found that a growth interruption performed at GaN thicknesses larger than three molecular monolayers (8 A) instantaneously leads to the formation of three-dimensional islands. This is used to grow GaN/AlN QDs on Si(111). Depending on their sizes, intense room-temperature photoluminescence is observed from blue to orange. Finally, we demonstrate that stacking of QD planes with properly chosen dot sizes gives rise to white light emission.

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.

Nitridation of sapphire. Effect on the optical properties of GaN epitaxial overlayers

GaN layers were grown by gas‐source molecular beam epitaxy on sapphire substrates using ammonia as a nitrogen source. The nitridation of an Al2O3 surface prior to the GaN growth was followed in situ by reflection high‐energy electron diffraction. A strong variation of the surface lattice parameter was observed corresponding to the formation of an AlN relaxed layer. The nucleation of GaN on such a nitridated surface is facilitated, as checked by atomic force microscopy. Optimization of the nitridation procedure is achieved by investigating the photoluminescence properties of GaN thin films grown for different nitridation times. It is found that the band‐edge and the yellow‐band luminescences are strongly dependent on the nitridated starting surface. Finally, the optimized nitridation process is used to grow high‐quality GaN epitaxial layers.

Luminescence and reflectivity studies of undoped, n- and p-doped GaN on (0001) sapphire

GaN grown by three different methods (MOVPE, GSMBE and HVPE) have been studied using temperature (T) dependent reflectivity and photoluminescence (PL). Both non intentionally doped (MOVPE, GSMBE and HVPE), n- and p-doped samples (MOVPE and GSMBE) have been investigated. Reflectivity is used to obtain intrinsic transition energies. These energies vary with the amount of strain in the crystal. Growth parameters influencing this strain state are discussed. Using MOVPE (T-g approximate to 1050 degrees C) and GSMBE (T-g approximate to 800 degrees C), it is possible to grow samples whose low temperature PL spectra are dominated by free and bound excitons and their phonon replica. Intentional n-type doping up to 10(20) cm(-3) is easily achieved with Si. For n much greater than 10(18) cm(-3), the spectra broaden and exhibit a blue shift, attributed to band filling. p-Type doping has been attempted using Mg, C and Ca. Ca doping led to compensated samples. C doping using CCl4 resulted in n-type samples, due to simultaneous oxygen incorporation in the layers; a strong enhancement of the 3.27 eV donor acceptor pair PL is also observed in this case. p-Type doping up to 10(18) cm(-3) has been achieved with Mg. With increasing densities, a deepening of the donor acceptor pair PL energy is observed. For high Mg doping, the spectra are dominated by a blue band in the 2.8 eV range, involving deep electron states

Molecular-beam epitaxy of gallium nitride on (0001) sapphire substrates using ammonia

Ammonia is used for growing undoped GaN layers by gas source molecular-beam epitaxy on c-plane sapphire substrates. The growth mode is layer by layer as shown by the observation of reflection high-energy electron diffraction intensity oscillations. The structural quality is studied by x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Low-temperature photoluminescence (PL) and reflectivity demonstrate intrinsic excitonic emission. Room-temperature PL exhibits a strong band-edge intensity and a weak deep-level emission, the so-called yellow band. Finally, secondary ion mass spectroscopy is carried out to check the residual impurity levels of Si, C, and O.

Self-limitation of AlGaN/GaN quantum well energy by built-in polarization field

Wurtzite AlGaN/GaN quantum well (QW) structures were grown by molecular beam epitaxy on c-plane sapphire substrates and the QW transition energies were measured by low temperature photoluminescence. Both the well widths and the Al mole fraction in the AlxGa1−xN(0<x<0.3) 100-A-thick barriers have been varied in order to assess the built-in electric field present in the quantum heterostructures. It is found that the electric field increases linearly with the Al composition. The magnitude of this electric field is as high as 1 MV/cm for an Al mole fraction of 0.27. The main consequence is that whatever the investigated Al composition range, the well thickness must be lower than 30 A in order to get a transition energy greater than the band gap of GaN.

Gas source molecular beam epitaxy of wurtzite GaN on sapphire substrates using GaN buffer layers

Wurtzite GaN was grown by gas source molecular beam epitaxy on (0001) sapphire substrates. Taking advantage of the catalytic decomposition of ammonia on the growing surface, high growth rates (>1 μm/h) were achieved for substrate temperatures ranging between 800 and 850 °C. Surface morphology, structural, and optical properties of thick (2–4 μm) GaN films were investigated versus the growth temperature of the GaN buffer layer. It is shown that this parameter has a drastic influence on the GaN properties.

GaN grown on Si(111) substrate: From two-dimensional growth to quantum well assessment

We report on the epitaxial growth of high quality GaN films on Si(111) substrates by molecular beam epitaxy using ammonia. The surface morphology and crystallinity of thick undoped GaN films are characterized by reflection high-energy electron diffraction (RHEED), scanning electron microscopy, and x-ray diffraction. Films having compact morphologies and flat surfaces are obtained and RHEED intensity oscillations are demonstrated for GaN and (Al, Ga)N alloys indicating two-dimensional growth. This has been applied to the growth of AlGaN/GaN quantum well (QW) structures. Low-temperature photoluminescence (PL) spectra of GaN are dominated by a strong and narrow (full width at half maximum=5 meV) band edge luminescence intensity at 3.471 eV assigned to donor bound exciton recombination. PL properties of AlGaN/GaN QW are also very similar to those obtained on equivalent structures grown on sapphire.

Epitaxial relationships between GaN and Al2O3(0001) substrates

GaN thin layers (200 Angstrom) were grown by gas-source molecular beam epitaxy on c-plane Al2O3 substrates, Transmission electron microscopy reveals that two different epitaxial relationships may occur, The well-known GaN orientation with the c axis perpendicular to the Al2O3 surface and [ ](GaN)parallel to[ ](Al2O3) is observed when the substrate is nitridated prior to the growth. On the other hand, GaN layers deposited on bare Al2O3 surfaces exhibit a different crystallographic orientation: [ ](GaN)parallel to[ ](Al2O3) and [ ](GaN)parallel to[ ](Al2O3). This corresponds to a tilt of about 19 degrees of the c axis with respect to the substrate surface