Plasmas on Quantized Accumulation Layers on ZnO Surfaces

Abstract

Low-energy~electron-loss spectroscopy is used to study collective surface excitations on accumulation layers on ZnO. A prominent loss peak is observed which shifts in energy with electron concentration. The results are well accounted for theoretically and provide direct evidence for the existence of two-dimensional plasmons.PACS numbers: 71.45.Gm, 73, 79.20.Kz The elementary collective excitations of solid surfaces play a fundamental role in a number of recently discovered phenomena. These include some novel device applications like the generation of far-infrared radiation,^ surface photochemistry,^ and surface-enhanced adsorbate spectroscopy.^ The primary tools for probing such excitations have been optical. While optical spectroscopy yields excellent energy resolution, its spatial resolution is limited by its wavelength. In this Letter we report a study of surface excitations by low-energy-electron-loss spectroscopy (LEELS). We present experimental data for LEELS on ZnO surfaces and interpret the data theoretically. The surface properties of ZnO are changed by varying the electron density in the accumulation layer. The elementary excitations consist of coupled two-dimensional (2D) plasm on and phonon modes.The possibility of 2D plasmas in quantized space-charge layers was first pointed out by Stern"* in 1967, but their detection was found to be experimentally quite difficult. To attain the quantized space-charge layers one usually works with the silicon metal-oxide-semiconductor (MOS) structure, where the silicon is not very amenable to direct exterior probes. Thus experimental evidence for the existence of 2D plasmons on MOS structures was found only recently.^'^ LEELS was developed"^ to probe the electronic excitations at the surface and is natural for the study of 2D plasmas, provided that one can produce a quantized space-charge layer on difree surface. We performed our measurements on ZnO surfaces, one of the few semiconductors where quantized accumulation layers on the free surface can be obtained with relative ease.^"^° Our preliminary results^^ contained a coarse background in the energy-loss spectra which was attributed to scattering by 2D plasmons. The present improved data show a pronounced peak due to the plasmons. The energy of the peak increases monotonically with the surface electron concentration N^. Measurements were performed up to 2^3^2x10^^ cm"^, about an order of magnitude higher than on silicon.The theory of 2D plasmas in ZnO-a polar crystal-is somewhat more complicated than in silicon. The plasmon mode couples to the surface and bulk phonon modes and forms three hybrid branches, termed plasmarons .^^ Some theoretical studies of electron scattering from solid surfaces have appeared in the literature.'^ In our previous work^^ we developed a theory for ZnO in which the coupling to the plasmaron modes of the surface was manifest. A number of approximations were made, however, making a detailed comparison between theory and experiment difficult. These limitations have now been removed. Thus...

Growth and structure of Pd films on ZnO(0001)

“…The spectrum is in good agreement with those reported previously for ZnO͑0001͒. 11,[23][24][25] The remaining spectra in Fig. 1 were obtained following deposition of 0.35, 0.75, 1, 1.5, and 3 ML of Pd.…”

Inversion and accumulation layers at InN surfaces

“…The same property is apparent in other semiconductors with large cation-anion size-and electronegativity-mismatches, such as ZnO (which also exhibits surface electron accumulation [11]) and CdO [8,12]. To a lesser extent, the G-point CBM is even quite low in GaN, which explains why p-type doping of GaN is also inefficient.…”

Metallization ofZnO(101¯0)by adsorption of hydrogen, methanol, and water: Angle-resolved photoelectron spectroscopy

“…It has been known that the H atoms adsorbed on and doped in ZnO increase the electric conductivity by forming the charge accumulation layer. 3,4 Thus, the donor character of H on ͑in͒ ZnO has been acknowledged. Nevertheless, the ZnO-H interaction has attracted renewed attention [5][6][7][8][9] since the work by Van de Walle.…”

“…3,4 Therefore, it is important to investigate the adsorption-induced change in the surface electronic structure by direct methods. In the present study, we employed angle-resolved photoelectron spectroscopy ͑ARPES͒ to determine the valence-band structure of ZnO͑1010͒ and its change by adsorption of hydrogen and methanol.…”