Optical polarisation anisotropy in a -plane GaN/AlGaN multiple quantum well structures (original) (raw)
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Structural, electrical, and optical characterizations of a-plane InGaN/GaN quantum well structures
2009
GaN and related ternary compounds have been widely used for fabrication of light emitting diodes (LEDs) and laser diodes (LDs). Especially, the low-dimensional systems such as quantum wells (QWs), quantum wires, and quantum dots have been investigated as an effective structure for improving the efficiency of light-emitting devices such as light emitting diodes and laser diodes. Generally, the quantum well active regions in I¿-nitride optoelectronic devices grown on conventional templates along the polar orientation have critical problems given by the quantum confined Stark effect (QCSE) due to the effects of strong piezoelectric and spontaneous polarizations. However, the QWs grown on nonpolar templates along aor m-directions are free from the QCSE since the polar-axis lies within the growth plane of the template. In this study, we achieved high quality a-plane GaN films on sapphire substrates and characterized structural, electrical and optical properties in the a-plane InGaN/GaN QW structures. High quality of a-plane GaN templates was confirmed by using selected area diffraction (SAD) patterns and high resolution x-ray diffraction (HRXRD) results. To investigate the electrical properties of aplane GaN QWs structures, the temperature-dependent carrier depth profiles which can determine the carrier confinement with nanoscale spatial resolution were studied. And the redshift of photoluminescence (PL) peaks with increasing temperature will be intensively discussed.
Science and technology of advanced materials, 2016
We report on a comparative study of the low temperature emission and polarisation properties of InGaN/GaN quantum wells grown on nonpolar ([Formula: see text]) a-plane and ([Formula: see text]) m-plane free-standing bulk GaN substrates where the In content varied from 0.14 to 0.28 in the m-plane series and 0.08 to 0.21 for the a-plane series. The low temperature photoluminescence spectra from both sets of samples are broad with full width at half maximum height increasing from 81 to 330 meV as the In fraction increases. Photoluminescence excitation spectroscopy indicates that the recombination mainly involves strongly localised carriers. At 10 K the degree of linear polarisation of the a-plane samples is much smaller than of the m-plane counterparts and also varies across the spectrum. From polarisation-resolved photoluminescence excitation spectroscopy we measured the energy splitting between the lowest valence sub-bands to lie in the range of 23-54 meV for the a- and m-plane sampl...
Applied Physics Letters, 1997
Effects of well thickness and Si doping on the optical properties of GaN/AlGaN ͑MQWs͒ have been investigated by picosecond time-resolved photoluminescence ͑PL͒ measurements. Our results have yielded that ͑i͒ the optical transitions in nominally undoped MQWs with narrow well thicknesses (L w Ͻ40 Å) were blue shifted with respect to the GaN epilayer due to quantum confinement, however, no such blue shift was evident for the MQWs with well thicknesses larger than 40 Å, ͑ii͒ the band-to-impurity transitions were the dominant emission lines in nominally undoped MQWs of large well thicknesses (L w Ͼ40 Å) at low temperatures, and ͑iii͒ Si doping improved significantly the crystalline quality of MQWs of large well thicknesses (L w Ͼ40 Å). The implications of these results on the device applications based on III-nitride MQWs have been discussed.
Applied Physics Letters, 2005
The properties of nonpolar a-plane InGaN/GaN multiple-quantum wells ͑MQWs͒, grown simultaneously on lateral epitaxially overgrown ͑LEO͒ a-plane GaN and planar a-plane GaN, were studied. High-resolution x-ray diffraction analysis revealed that the In mol fraction in the MQWs grown on LEO-GaN was significantly lower than that on planar a-plane GaN. The lower In incorporation was confirmed by microphotoluminescence ͑µ-PL͒ and wide-area photoluminescence measurements, which showed a redshift of the MQW emission from 413 nm for the nearly defect-free laterally overgrown regions to 453 nm for the defective "window" regions of the LEO a-plane GaN, to 478 nm for the high-defect density planar a-plane GaN. µ-PL measurements also demonstrated that the emission from the nearly defect-free wings of the LEO a-plane GaN was more than ten times stronger than the emission from the defective windows.
Electronic and optical properties of nonpolar a-plane GaN quantum wells
Physical Review B, 2010
In this paper we present a detailed study of the electronic band structure of a series of nonpolar a-plane GaN/AlGaN multiple quantum wells ͑QWs͒ of varying well width using complementary results from x-ray diffraction, polarization-dependent photoluminescence excitation spectroscopy, and k • p theory. When excited with unpolarized light, excitonic transitions involving different electron subbands are resolved in the excitation spectra. For linearly polarized ͑E Ќ c , E ʈ c͒ excitation, these are shown to consist of overlapping transitions involving different hole subbands. These results are then analyzed in detail using strain data determined by the x-ray diffraction measurements in combination with the k • p theory to calculate the bulk band structure and the relative oscillator strength of an a-plane GaN film under strain. The results are compared with those of an unstrained c-plane film. This analysis reveals that the experimentally observed polarization anisotropy can be attributed to anisotropic strain in the c plane. Based on the k • p Hamiltonian, we apply an effective mass approximation, taking into account strain and nonparabolicity effects, to calculate the single-particle states and energies for the different quantum wells. The possible influence of the weak spin-orbit coupling on the results is studied in detail. Starting from the single-particle energies and including excitonic binding energies, the band edge optical transitions are calculated and successfully compared to the experimental data. Our analysis gives an estimate for the conduction-to valence-band offset ratio of 45:55 for nonpolar GaN/AlGaN QW structures. Additionally, our study also allows us to investigate the magnitude of the crystal-field splitting and spin-orbit coupling in GaN systems.
AlGaN-GaN Quantum well optical properties
Intersubband transitions in Si-doped molecular beam epitaxy grown GaN/AlGaN multiple quantum wells on c-plane sapphire were investigated using the Fourier-transform infrared optical absorption technique. Several GaN quantum well samples were grown with either AlGaN bulk or GaN/AlGaN short period superlattice barriers. The measurements were made in a waveguide configuration utilizing a facet polished at 45°to the c plane. The integrated area of the intersubband transitions in several waveguides cut from different location of the wafer was measured, from which we estimated the two-dimensional electron gas density ͑͒. The measured values of are about two orders of magnitude larger than the Si doping level of ϳ8ϫ10 17 cm Ϫ3 , which is consistent with the polarization effects, particularly considering the large number of GaN/AlGaN interfaces. The internal quantum efficiency of the intersubband transitions was estimated to be on the order of 40% for samples with superlattice barriers.
Semiconductor Science and Technology, 2005
We have studied the influence of an AlGaN insertion layer in a GaN quantum well on the light emission from a strained GaN/AlGaN multiple-quantum well system. The structural properties of GaN/AlGaN multiquantum wells of the same composition were studied by using the reciprocal space mapping and simulation techniques of high-resolution x-ray diffraction. The lattice constants along the in-plane direction determined from the (1 0 5) reciprocal space maps were a = 3.161Å for normal multiquantum wells and a = 3.152Å for interlayer multiquantum wells. The spatial localization of the quantum well emission was unambiguously determined by monochromatic cathodoluminescence measurements. This observation indicates that the blue emission near 2.96 eV in the interlayer multiquantum wells originates from the MQW region, rather than from a deep level in the GaN buffer layer. These transitions could be a quantum confined Stark effect due to the piezoelectric field caused by the strain between the well and barrier.
Fabrication and optical studies of AlGaN/GaN quantum-well waveguides
Applied Physics Letters, 2001
We report the successful fabrication and optical study of submicron waveguide structures based on AlGaN/GaN multiple-quantum wells (MQWs). The MQW structures were grown by metalorganic chemical vapor deposition on sapphire substrates and the waveguides were fabricated by electron-beam lithography and inductively coupled plasma dry etching. The waveguides were patterned with a fixed width of 0.5 μm but with orientations varying from −30° to 60° relative to the a axis of GaN. Optical emission from these structures was studied by photoluminescence spectroscopy. The peak position and linewidth of the emission peak were found to vary systematically with the orientations of the waveguides and followed the sixfold symmetry of a wurtzite structure. This is most likely related to the anisotropy of the exciton/carrier diffusion coefficient along the different crystal orientations in the quasione-dimensional case. The implication from the results is that in proper designs of photonic and elect...
Journal of Applied Physics, 2014
We have performed a detailed study of the impact of basal plane stacking faults (BSFs) on the optical properties of both a-plane InGaN/GaN quantum wells (QWs) and GaN template samples grown on r-sapphire. In particular we have used polarised photoluminescence excitation spectroscopy (P-PLE) to investigate the nature of the low temperature recombination as well as extracting information on the valence band (VB) polarisation anisotropy. Our low temperature P-PLE results revealed not only excitons associated with intersubband quantum well transitions and the GaN barrier material but also a transition associated with creation of excitons in BSFs. The strength of this BSF transition varied with detection energy across the quantum well emission suggesting that there is a significant contribution to the emission line width from changes in the local electronic environment of the QWs due to interactions with BSFs. Furthermore we observed a corresponding progressive increase in the VB splitting of the QWs as the detection energy was varied across the quantum well emission spectrum.