Charge accumulation layers and surface states in ultrathin Cs,Ba/ n -GaN(0001) interfaces
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Charge Accumulation Layers and Surface States in Ultrathin Cs,Ba/n-GaN(0001) Interfaces G. V. Benemanskayaa, G. E. Frank-Kamentskayab, N. M. Shmidta, and M. S. Dunaevskiœa a b
Ioffe Physicotechnical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia St. Petersburg State Technological Institute, Zagorodnyi pr. 49, St. Petersburg, 198013 Russia e-mail: [email protected] Received March 15, 2006
Abstract—It is found that ultrathin cesium and barium coatings radically change the electronic properties of the surface and the near-surface region of epitaxial n-GaN(0001) layers. A charge accumulation layer serving as a quasi-two-dimensional electronic channel is first formed by adsorption on the surface of a semiconductor. It is revealed that photoemission from the accumulation layer is excited by visible light from the transparency region of GaN and is characterized by a high quantum yield. It is found that the photoemission thresholds hνs and hνp for s- and p-polarized excitation are equal to each other and correspond to the work function. The lowest work function for Cs,Ba/n-GaN interfaces is observed at Cs or Ba coverages close to 0.5 monolayer. Two bands induced by the local interaction of cesium (barium) adatoms with gallium dangling bonds are detected in the electronic spectrum of surface states of Cs,Ba/n-GaN interfaces. An oscillation structure is observed in spectral dependences of the photoyield. This effect is new for photoemission. A model of the effect is proposed. It is found that electronic and photoemission properties of the interfaces correlate with the structural perfectness of the epitaxial n-GaN(0001) layers. PACS numbers: 73.20.-r, 79.60.-i DOI: 10.1134/S1063776106090147
1. INTRODUCTION Studies in recent years have shown that the surface of group III nitrides makes a substantial contribution to many optical and electronic processes. At the same time, works devoted to studying nitride surface properties are few in number. For example, the effect of structural features of group III nitrides on their surface properties has not been understood. It is known that group III nitrides differ substantially from conventional III–V semiconductors in the character of structural organization. A complex character of structural organization is typical for the hexagonal modification of group III nitrides, which serve as a basis for devices emitting in the shortwavelength region of the spectrum. This organization involves not only a high density of dislocations (about 109 cm–2) but also a mosaic structure composed of domains with different tilt and twist angles with respect to each other [1–9]. The relaxation of the system is very sensitive to small changes in technological parameters, which results in a wide variety of group III nitride species from almost nonaggregated domains 100–1000 nm in size to well-aggregated domains with relaxed boundaries. Materials with aggregated domains can also differ in the character of structural organization, depending on
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