We report scanning tunneling microscopy ͑STM͒ images of surfaces of GaN films and the observation of luminescence from those films induced by highly spatially localized injection of electrons or holes using STM. This combination of scanning tunneling luminescence with STM for GaN surfaces and the ability to observe both morphology and luminescence in GaN is the first step to investigate possible correlations between surface morphology and optical properties.
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 ...
We have achieved spatially resolved photoluminescence ͑PL͒ from metalorganic vapor phase epitaxy ͑MOVPE͒ grown GaAs surfaces by near-field scanning optical microscopy ͑NSOM͒. We have performed the topography, reflection, and PL measurements by NSOM combined with the topography measurements by atomic force microscopy ͑AFM͒ on the as-grown and (NH 4 ͒ 2 S-passivated GaAs samples. The uniformity of GaAs with a thin Al 0.65 Ga 0.35 As cap layer has also been studied and compared with the ͑NH 4 ͒ 2 S treated samples. We found the submicron scale variations in PL intensity which were not correlated to the topographic features. The PL intensity variation was related to the changes in the surface state density. ), particularly after exposure to air.1 These surface states are nonradiative recombination centers which impact the performance of many minority carrier devices, such as light emitting diodes, solar cells, heterojunction bipolar transistors, and lasers. This high density of surface states leads to Fermi level pinning near-midgap, resulting in difficulties in the Ohmic contact formation and in the fabrication of metal-insulatorsemiconductor field-effect transistors ͑MISFET͒.2 In recent years, many studies have focused on the effective passivation of the compound semiconductor surfaces.3 ͑NH 4 ͒ 2 S passivation of GaAs surfaces has been proven to be very successful and easy to implement. 4 Several characterization techniques have been used to probe the properties of the passivated surfaces. 5,6 The uniformity of passivation at submicron scale, however, has rarely been studied. As the device dimensions decrease to the nanoscale, the surface properties and uniformity of these properties at the submicron scale become increasingly important.7 Near-field scanning optical microscopy ͑NSOM͒ has been demonstrated to be a powerful tool for performing optical measurements at a much higher spatial resolution than conventional far-field optics. 8 In this work, the spatially resolved PL measurements on the passivated GaAs surfaces, using NSOM, are presented. These quantitative measurements provide information on the surface state density variation as well as the independently determined surface morphologies of the passivated surface. Measurements of the optical uniformity of ͑NH 4 ͒ 2 S-passivated surfaces are compared to control samples consisting of a thin Al 0.65 Ga 0.35 As cap layer on an epitaxial GaAs layer. This structure serves as an alternative GaAs surface passivation. The epitaxially grown GaAs-Al x Ga 1Ϫx As heterostructure has a low interface state density and interface recombination velocity 9,10 and should have more uniform properties than the free GaAs surface.The NSOM is a home-built system with a commercial scanning control unit.11 All the experiments are done in illumination mode where the NSOM tip serves as the excitation source. The optical collector is a parabolic reflector with the tip at its focal point, maximizing the collection efficiency. A low level He-Ne laser light is used for the standard shear f...
We have achieved spatially resolved room temperature photoluminescence (PL) from MOVPE GaAs (001) surfaces by Near-Field Scanning Optical Microscopy (NSOM). The PL intensity variation was related to the change of surface state density. Using this technique, the uniformity of surface passivation after (NH 4)2S treatment has been studied. We have performed the topography, reflection and PL measurements by NSOM as well as the topography measurements by Atomic Force Microscopy (AFM) on the as grown, etched and sulfur passivated GaAs samples. The uniformity of GaAs with a thin Al0 65Ga0.35As cap layer has also been studied and compared with the (NH 4)2S treatment. We found the sub-micron scale variations in PL intensity which were not correlated to the topographic features. Theoretical modeling has been used to obtain semi-quantitative analysis of the experimental results.
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