M-plane Freestanding GaN Substrates Research Articles

Possible origin of double-peak emission in InGaN quantum wells on m-plane free-standing GaN substrates

A new theoretical model has been proposed to explain the origin of the double-peak emission observed characteristically in m-plane InGaN quantum wells (QWs). Although the emission spectrum with a double-peak structure is generally regarded as evidence of In compositional phase separation or extended crystal defects that generate localized energy states, such crystal irregularities cannot be observed by transmission electron microscopy or three-dimensional atom probe in the QWs. It has been clarified, by our model, that only the slowly decaying tailing of the density of states can cause the double-peak structure. This is consistent with experimental results, and furthermore, the measured temperature and In composition dependences of photoluminescence spectra with the double-peak emission can also be successfully reproduced by theoretical calculation based on our model.

Advantages and remaining issues of state-of-the-art m-plane freestanding GaN substrates grown by halide vapor phase epitaxy for m-plane InGaN epitaxial growth

Advantages and remaining issues of state-of-the-art m-plane freestanding GaN (FS-GaN) substrates grown by halide vapor phase epitaxy (HVPE) for m-plane InxGa1 − xN epitaxial film growth by metalorganic vapor phase epitaxy are described. Because of the low threading dislocation and basal-plane stacking fault densities, improved quantum efficiency and short radiative lifetime are achieved for the near-band-edge emission of 200–250 nm thick m-plane pseudomorphic InxGa1 − xN (x ⩽ 0.14) epilayers. Values of full-width at half-maximum for the x-ray ω-rocking curves remain unchanged as the substrate values being 80 and 60 arcsec for the (100) diffraction with 〈0001〉 and azimuths, respectively, and 80 arcsec for the (102) diffraction. As the surface flatness is greatly improved, the In-incorporation efficiency (ηIninc) is lower than the cases for conventional c-plane growth and m-plane growths on defective GaN bases. The former originates from nonidentical surface kinetics, and the latter is due to the reduction in the area of inclined and tilted planes. Sub-micrometer-wide zonary patterns parallel to the c-axis and 2 μm long axis figure-of-eight patterns parallel to the a-axis are clearly visualized in the monochromatic cathodoluminescence (CL) intensity images. Because the spatio-time-resolved CL measurement reveals very little spatial variation of low-temperature radiative lifetime, the slight peak energy variation is interpreted to originate from nonidentical ηIninc for the growing surfaces exhibiting various miscut angles. The figure-of-eight patterns are ascribed to originate from the anisotropic, severe m-plane tilt mosaic along the a-axis of the GaN substrate, and the zonary patterns may originate from the m-plane tilt mosaic along the c-axis. Further reduction in the tilt and twist mosaics is necessary for HVPE of FS-GaN substrates, in order to grow homogeneous InGaN epilayers.

Photoluminescence characteristics of polar and nonpolar AlGaN/GaN superlattices

High quality Al0.2Ga0.8N/GaN superlattices (SLs) with various (GaN) well widths (1.6 to 6.4 nm) have been grown on polar c-plane and nonpolar m-plane freestanding GaN substrates by metal-organic chemical vapor deposition. Atomic force microscopy, high resolution x-ray diffraction, and photoluminescence (PL) studies of SLs have been carried out to determine and correlate effects of well width and polarization field on the room-temperature PL characteristics. A theoretical model was applied to explain PL energy-dependency on well width and crystalline orientation taking into account internal electric field for polar substrate. Absence of induced-internal electric field in nonpolar SLs was confirmed by stable PL peak energy and stronger PL intensity as a function of excitation power density than polar ones.

Impacts of anisotropic lattice relaxation on crystal mosaicity and luminescence spectra of m-plane AlxGa1−xN films grown on m-plane freestanding GaN substrates by NH3 source molecular beam epitaxy

In-plane anisotropic lattice relaxation was correlated with the crystal mosaicity and luminescence spectra for m-plane AlxGa1−xN films grown on a freestanding GaN substrate by NH3-source molecular beam epitaxy. The homoepitaxial GaN film exhibited A- and B-excitonic emissions at 8 K, which obeyed the polarization selection rules. For AlxGa1−xN overlayers, the m-plane tilt mosaic along c-axis was the same as the substrate as far as coherent growth was maintained (x≤0.25). However, it became more severe than along the a-axis for lattice-relaxed films (x≥0.52). The results are explained in terms of anisotropic lattice and thermal mismatches between the film and the substrate. Nonetheless, all the AlxGa1−xN films exhibited a near-band-edge emission peak and considerably weak deep emission at room temperature.

Improved characteristics and issues of m-plane InGaN films grown on low defect density m-plane freestanding GaN substrates by metalorganic vapor phase epitaxy

Improved quantum efficiency and short radiative lifetime were demonstrated for the near-band-edge emission of nearly stacking-fault-free, 200–250-nm-thick, m-plane pseudomorphic InxGa1−xN (0<x≤0.14) films grown by metalorganic vapor phase epitaxy on the low threading dislocation density (<5×106 cm−2) freestanding (FS) GaN substrates. Values of full width at half maximum of x-ray ω-rocking curves of the InxGa1−xN films remain unchanged as the substrate values being 80 and 60 arcsec for the (101¯0) diffraction with ⟨0001⟩ and ⟨112¯0⟩ azimuths, respectively, and 80 arcsec for the (101¯2) diffraction. As the surface flatness was improved, the incorporation efficiency of In was lower than the cases for c-plane growth and m-plane growth on a defective GaN substrate, according to nonidentical surface kinetics and absence of inclined/tilted planes, respectively.

Optical properties of nearly stacking-fault-free m-plane GaN homoepitaxial films grown by metal organic vapor phase epitaxy on low defect density freestanding GaN substrates

Prospective optical properties were demonstrated for nearly stacking fault-free m-plane GaN homoepitaxial films grown by metal organic vapor phase epitaxy on the m-plane freestanding GaN substrates. Values of full width at half maximum of x-ray rocking curves were close to the substrate values being 31arcsec for the (101¯0) diffraction with ⟨0001⟩ azimuth and 48arcsec for the (101¯2) diffraction. Threading dislocation densities were lower than 5×106cm−2. The film surfaces exhibited atomically flat morphology with well-aligned monolayer steps. Low-temperature photoluminescence (PL) spectra exhibited polarization-dependent well-resolved bound and free exciton emission lines, and a characteristic π(k⊥c,E∥c)-polarized PL line was also observed. Room-temperature effective PL lifetime of the free exciton peak increased with increasing supply ratio of ammonia to trimethylgallium, and a record long value for m-plane GaN (268ps) was obtained.