Studies of buried interfaces by optical second-harmonic generation

Abstract

Abstract. Optical Second-Harmonic Generation (SHG) has been used to study the technologically very important buried semiconductor-metal and magnetic multilayer interfaces. For the case of GaAs-Au, the SHG intensity is shown to depend on the applied bias and Schottky barrier height, and is strongly affected by the sweepout of the carriers generated by the fs excitation pulses. For the M/Co/M multilayers, with M = Cu or Au, the SHG signals appear to depend strongly on the magnetization and can be shown to be interface specific. PACS: 42.65, 73.30, 75.70 Nonlinear optical techniques like Second-Harmonic Generation (SHG) and Sum-Frequency Generation (SFG) have been proven to be versatile and sensitive surface and interface probes, that derive their surface specificity from the symmetry breaking at interfaces [1,2]. In particular, they are unique for studying buried interfaces, that are practically inaccessible by other than optical techniques. Such interfaces, like semiconductormetal, metal-metal, metal-electrolyte and various other interfaces, are highly relevant from a practical point of view, but also pose many fundamental questions about the interfacial electronic structure and dynamics. In this paper two of such interface problems will be adressed: the electric-field dynamics at a Schottky barrier and the magnetic structure of interfaces between thin films of magnetic and nonmagnetic materials. Though both studies rely on field-induced effects (electrical and magnetic, respectively), the observed signals appear to be quite large and to contain some interesting new results.Paper presented at the 129th WE-Heraeus-Seminar on "Surface Studies by Nonlinear Laser Spectroscopies", Kassel, Germany, May 30 to June 1, 1994 SHG at a metabsemiconductor interfaceThe problem of Schottky barrier formation at a metalsemiconductor interface still attracts a lot of attention, as new developments in material fabrication and structure determination have shown that the precise interface structure plays a crucial role in determining the Schottky barrier height. For electro-optic applications, not only the static electronic structure but even more importantly, the dynamics near these interfaces in extremely relevant, as this will affect the speed of such devices. Recent photoluminescence studies of a Au/GaAs structure have shown unexpected field and laser intensity dependences at quite moderate laser powers (10 mW) that are commonly met in optical semiconducting devices [3,4]. Theoretical models suggest a very rapid drop of the effective barrier height as a result of the spatial separation of the carriers excited by the incident laser puls (carrier sweepout). However, the photoluminescence is only indirectly affected by this interface field, as the signal is coming from a large bulk region. Very recently, Qi et al. [5] showed the power of SHG to probe the band bending region in GaAs. A direct way to probe the (time evolution of the) effective Schottky barrier height would be to measure the field induced SHG with ultra-short excitat...

Depletion-electric-field-induced second-harmonic generation near oxidized GaAs(001) surfaces

“…This is consistent with a recent SHG study of Au/GaAs Schottky barriers as a function of junction bias and SHG probe intensity. 15,16 Additionally, our results support the proposal of Yamada, Kimura, and Fuqua that a correlation observed between ''surface-sensitive'' SHG and simultaneously obtained photoluminescence during photowashing of GaAs wafers was due to unpinning of the Fermi level combined with an EFISH contribution to the SHG. 51 It should be noted that the SHG probe in Ref.…”

“…4,[6][7][8][9][10][11] Several recent reports have demonstrated convincingly that depletion-electric-fieldinduced second-harmonic generation ͑depletion-EFISH͒ can dominate surface contributions in a number of semiconducting samples. [12][13][14][15][16] In the present paper, we demonstrate the importance of the depletion electric field in modifying the SHG observed from oxidized GaAs͑001͒, and conclude that nearly all of the signal attributed to the reduction of symmetry at the surface of highly doped samples results from the near-surface depletion electric field.…”

Nonlinear magneto-optical Kerr spectra of thin ferromagnetic iron films calculated with ab initio bandstructure theory

“…The nonlinear response senses only the electronic structure of the first two top film layers, whereas in the case of linear response the electronic structure of the whole film is involved. This is found to be in agreement with experiments [10][11][12][13].…”

“…Our theory suggests that the experimental enhancement of the NOLIMOKE signal for 3 and 4 layer films at a frequency of 1.55 eV (corresponding to a wavelength of 800 nm, see Fig. 5 in [12]) results from the electronic structure and in particular from the enhancement of the surface magnetization.…”

Does femtosecond time-resolved second-harmonic generation probe electron temperatures at surfaces?

“…PACS: 42.65. Ky, 78.47.+p, 63.20.Kr Nonlinear optical techniques like second-harmonic and sum-frequency generation gain increasing importance for the investigation of surfaces [1], interfaces [2], thin films [3], and multilayers [4]. This trend is intensified by the possibility to investigate the electron dynamics with femtosecond time resolution.…”