A near-field scanning optical microscopy study of the photoluminescence from GaN films

Abstract

We have achieved spatially resolved photoluminescence from GaN films using a near-field scanning optical microscope ͑NSOM͒. GaN films grown by hydride vapor phase epitaxy ͑HVPE͒ and metal-organic vapor phase epitaxy ͑MOVPE͒ on sapphire substrates have been studied. We have performed spatial scans of topography, band edge, and yellow luminescence signals. Atomic force microscopy measurements were also made and compared with the NSOM topography. We have found spatial variations in photoluminescence characteristics at the submicron scale for both HVPE and MOVPE GaN. The observed enhancement of yellow luminescence at multiatomic step edges on the HVPE GaN surface suggests that the yellow luminescence is associated with chemical impurities incorporated during the growth of GaN films. © 1996 American Institute of Physics. ͓S0003-6951͑96͒00549-9͔The successful development of short wavelength light emitting diodes and the most recent realization of nitride semiconductor lasers have stimulated great interests in the applications of this material for blue and ultraviolet optoelectronic devices.2 Epitaxial films grown by chemical vapor deposition or molecular beam epitaxy on conventional substrates, such as Al 2 O 3 or SiC, contain a high density of dislocations, on the order of 10 8 -10 10 cm Ϫ2 . 3 The extended and point defects in GaN films may greatly impact the performance of many devices. For example, the microscopic inhomogeneity could limit the achievable gain of laser structures. Recently, there have been several reports on the studies of microscopic properties of the GaN materials at different spatial scales. [4][5][6] In this letter, we report the high spatial resolution photoluminescence ͑PL͒ measurements of GaN films by near-field scanning optical microscopy ͑NSOM͒. 7We have performed spatially resolved studies of band edge PL ͑BL͒ as well as yellow luminescence ͑YL͒. Yellow luminescence is frequency found in GaN materials and has been attributed to both chemical and physical defects. [8][9][10][11] This optical information is correlated with the detailed morphological features obtained simultaneously by NSOM, as well as atomic force microscopy ͑AFM͒ measurements. Secondary ion mass spectroscopy ͑SIMS͒ measurements were also performed to determine the concentrations of chemical impurities in the GaN films.The NSOM is a home built system with a commercial electronic control unit. 12 All the experiments were performed in illumination mode, where the probe served as an excitation source. The He-Ne laser light, used for the standard shear force scanning distance regulation. 13 A parabolic reflector is used which has been specifically designed to collect the optical signals with optimum efficiency. The NSOM probes used in this study were pulled from UV grade single mode optical fibers using a modified commercial fiber puller 14 and the tips were coated with aluminum. The tip radius and aperture size were determined by low voltage scanning electron microscopy on probes fabricated under similar processing conditions ...

Polarized microphotoluminescence and reflectance spectroscopy of GaN with k perpendicular to c: Strongly π-polarized line near theAexciton

“…It is worth noting that different spatially resolved optical techniques ͑-PL, cathodoluminescence, near-field scanning optical microscopy͒ were previously used to study various properties such as the strain distribution across the layers and the PL efficiency in a quantum well or in the vicinity of defects. 4,7,[10][11][12][13] Recently, we have demonstrated the possibility to measure -PL and R in different linear polarizations from a facet of thick GaN ͑0001͒ epilayers grown by hydride vapor phase epitaxy ͑HVPE͒. 14 The unexpected dominance of a -polarized line ͓marked below as X()͔ in the vicinity of the -polarized A exciton ͓FX A ()͔ was observed inter alia in low-temperature spectra, which obviously contradicts the selection rules for optical transitions in the wurtzite crystal.…”

Spatial distributions of near-band-gap uv and yellow emission on MOCVD grown GaN epifilms

“…In the studies of photoluminescence ͑PL͒ on GaN films, there is a broad emission band near 2.3 eV ͑yellow emission͒, in addition to the large, sharp, near-bandgap PL emission peak at 3.4 eV ͑UV emission͒. 17,18 While the search for the true origin of the yellow band emission is still a very active research area, most agree that its existence is due to defect/impurity assisted transitions and is an indicator of the quality of the films. 19 Reports on the studies of the near-band-gap UV and the yellow emission are plenty, including, for example, UV-to-yellow emission ratio ͑UV/ yellow͒ analysis with bimolecular model.…”

Spatial variation of photoluminescence and related defects in InGaN/GaN quantum wells

“…6,7 To understand the microscopic optical properties associated with the defect distribution of InGaN alloys, near-field scanning optical microscopy ͑NSOM͒ is a very well-suited optical characterization technique. [8][9][10][11] In this letter, we report NSOM results on MOCVD-grown InGaN/GaN quantum wells ͑QWs͒ focusing on the spatial distribution of luminescence near V defects.…”