A near-field scanning optical microscopy study of the uniformity of GaAs surface passivation

Abstract

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...

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

“…The lateral extent of the probed volume in the NSOM-based PL experiment is determined by the probe size, surface recombination velocity, and the diffusion of photogenerated carriers in the semiconductor material. 17 This lateral resolution, estimated from the PL images, is better than ϳ200 nm.…”

MORPHOLOGY AND CHEMISTRY OF S-TREATED GaAs(001) SURFACES

“…These effects were shown to be strongly process-dependent. 10,11 Depending on chemical treatment, postetch water rinse, subsequent water rinse after the sulfur treatment, sulfur treatment time and temperature, quite different surface roughness values have been observed. Chemical properties of S-passivated and unpassivated GaAs surfaces were studied by X-ray photoemission 12 and synchrotron radiation photoemission spectroscopy (SRPES).…”