Ref HAL: hal-00750406_v1
DOI: 10.1063/1.4749789
WoS: 000309072200043
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22 Citations
Résumé:

We compute using envelope function calculations the energy and the oscillator strength of excitons in zinc-blende/wurtzite quantum wells (QWs), such as those that appear in many examples of semiconductor nanowires, and in basal plane stacking faults (BSFs). We address specifically the model-case of GaN. In addition to the electron-hole Coulomb interaction, we account for the quantum-confined Stark effect. We demonstrate that despite the type-II band alignment at the zincblende/ wurtzite interfaces, a significant binding and a rather strong oscillator strength are preserved by excitonic effects. When adjacent crystal phase QWs are coupled together, we compute increased as well as decreased exciton oscillator strength with respect to the single QW case, depending on the QW-QW coupling scheme. Comparing the results of our calculations with available data, we finally conclude in favor of the absence of built-in electric fields perpendicular to the BSF planes.

Résumé:

I present an overview of the specific properties of high-Al content AlGaN-based materials and nanostructures. More specifically, valence-band state symmetries, biexcitonic effects, internal polarizations are discussed. Experimental and instrumental issues regarding the short wavelengths are also introduced, in the context of the characterization of specific defects by time-resolved spectroscopy.

Ref HAL: hal-00726087_v1
DOI: 10.1016/j.mee.2012.07.024
WoS: 000309497200083
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5 Citations
Résumé:

E-beam lithography was used to pattern a titanium mask on a GaN substrate with ordered arrays of nanoholes. This patterned mask served as a template for the subsequent ordered growth of GaN/InGaN nanorods by plasma-assisted molecular beam epitaxy. The mask patterning process was optimized for several holes configurations. The smallest holes were 30 nm in diameter with a pitch (center-to-center distance) of 100 nm only. High quality masks of several geometries were obtained that could be used to grow ordered GaN/InGaN nanorods with full selectivity (growth localized inside the nanoholes only) over areas of hundreds of microns. Although some parasitic InGaN growth occurred between the nanorods during the In incorporation, transmission electron microscopy and photoluminescence measurements demonstrated that these ordered nanorods exhibit high crystal quality and reproducible optical properties.

Ref HAL: hal-00708483_v1
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Self-assembled InxGa1-xN nanocolumns (NCs) were fabricated by plasma-assisted MBE on Si(111) substrates. The samples were analyzed by continuous-wave and time-resolved photoluminescence (PL) in order to gain control about the emission characteristics by changing the growth parameters. NCs were grown on Si (111) in a two-step procedure: (i) growth of GaN NCs at 800 ºC and (ii) growth of InGaN NCs on top of the GaN NCs at different sample temperatures (from 650 ºC to 750ºC) with the same nominal flux of In and Ga. This growth approach was chosen due to a drastic suppression of non-radiative recombination centers inside the NCs, a significant enhancement of the internal quantum efficiency and a superior light extraction efficiency. Within the growth temperature range of 650 to 750 °C, an increase in In incorporation for decreasing temperature is observed. This effect allows tailoring the InGaN nanocolumns emission line shape by using temperature gradients during growth. Depending on the gradient rate, span, and sign, broad emission line shapes are obtained, covering the yellow to green range, even yielding white emission.

Commentaires: Co-auteurs de cette communication: S. Albert, A. Bengoechea-Encabo, M. A. Sanchez-Garcia, E. Calleja.

Ref HAL: hal-00708726_v1
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Since the demonstration of nanowires utilization in optoelectronics [1], nano-objects have attracted a growing interest from the scientific community. In particular, GaN nanowires are strain- and defect-free UV light emitters that can be grown on Si substrates. However, as proposed recently in [2], the low-temperature donor-bound exciton emission from ensemble of narrow GaN nanowires is broad, due to the distribution of the distance between the donor nuclei and the surface. In this work, we study the influence of the surface and of the dielectric mismatch on the binding energy of donor atoms and, therefore, on the emission energy of donor bound excitons in GaN nanostructures. We compute by envelope function calculations, performed in the effective potential formalism, the binding energy of donor atoms in thin air/semiconductor/air slabs. We not only account for the Coulomb interaction between the electron and the donor nucleus, but also for the attraction between the electron and the image charges of the nucleus and for the repulsion between the electron and its images (i.e. the electron self-energy). We first consider the case of a donor nucleus located at the center of a nanostructure. Comparing our data with tight-binding calculations performed in Ref. [3], we show that the planar symmetry used here reproduces well the results obtained for cylindrical wires. Strikingly, the combination of the confinement and the dielectric mismatch in the narrowest nanostructures leads to donor binding energies exceeding four times the bulk Rydberg [4]. When the donor nucleus is located at the air/semiconductor interface, its binding energy is as expected reduced compared to the bulk case [5]. However, we obtain that this reduction is only by a factor of two, while a factor of four is computed when the dielectric mismatch is neglected [5]. Finally, we discuss the relative influence of the dielectric mismatch, surface and confinement on the donor binding energy with respect to the thickness of the nanostructure. [1] for a review, see C. M. Lieber et al., MRS Bulletin 32, 99 (2007). [2] P. Corfdir et al., JAP 105, 013113 (2009), O. Brandt et al. PRB 81, 045302 (2010). [3] M. Diarra et al., Phys. Rev. B 75, 045301 (2007). [4] G. Bastard, Phys. Rev. B 24, 4714 (1981). [5] S. Satpathy, Phys. Rev. B 28, 4585 (1983).

Ref HAL: hal-00708711_v1
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Contrary to GaN-based structures grown along [0001], nonpolar heterostructures allow for the growth of thick QWs, while keeping an optimal overlap between electron and hole wave functions. However, the use of lattice-mismatched substrates induces high densities of stacking faults and dislocations. Then, even at low temperature (T), the exciton decay is dominated by the capture on defects [1]. We investigate here the dynamics of excitons in AlGaN/GaN QWs deposited on the a-facet of GaN crystals. We first determine by cathodoluminescence QW dislocation density and exciton diffusion length at 300 K of 2.105 cm-2 and 100 nm, respectively, demonstrating that dislocations do not affect QW excitons at room-temperature [2]. We then study the time-resolved photoluminescence (PL) of excitons in the 10-320 K range for QW samples with various widths and barrier Al-contents. For all samples, the effective lifetime of QW excitons increases with T, indicating the absence of nonradiative phenomena in the low-T range. We observe purely radiative recombination up to 240 K for a 7 nm thick Al0.06Ga0.94N/GaN QW, i.e. a QW with small exciton localization energy (2 meV) [3]. This observation therefore demonstrates the possibility of achieving UV emitters with a rather narrow emission line and a good radiative efficiency at 300 K. In the high-T range, a drop in the QW PL lifetime is accompanied by an increase in the barrier PL lifetime, until both emissions follow the same dynamics. Supported by a model accounting for the equilibrium between excitons and free carriers in the QWs and the barriers, we show that the nonradiative recombination of charge carriers in the AlGaN barriers is the mechanism limiting the PL lifetime of excitons in the QWs [3]. The nonradiative recombinations at 300 K are suppressed by reducing the thermal escape of carriers from the QW. This is achieved by the growth of thick QWs rather than increasing the barrier Al-content, which is important from the defect/strain generation point of view. [1] P. Corfdir et al., J. Appl. Phys. 107, 043524 (2010). [2] P. Corfdir et al., Phys. Rev. B 83, 245326 (2011). [3] P. Corfdir et al., J. Appl. Phys. 111, 033517 (2012).