New phenomenon of accumulation layer (AL) creation is achieved due to Cs and Ba adsorption on the n-GaN(0001) surface. Experiments were performed in situ in ultrahigh vacuum. . Experimental and theoretical studies of photoelectron emission of Cs/n-GaN(0001) and Ba/n-GaN(0001) ultrathin interfaces are carried out[1]. It was found that the adsorption drastically change electronic properties of surface and subsurface layer. First, photoemission is observed by light excitation in transparency region of GaN. Data evidence appearance of an AL(2D electron channel) at the surface. Upon Cs and Ba adsorption, minimum value of the thresholds are obtained to equal to ~1.4 eV at Cs coverage of ~0.5 ML and to ~1.9 eV at Ba coverage of ~0.4ML. Note that GaN is among wide-bang-gap semiconductors with a band-gap width of 3.4 eV. Therefore, photoemission under light excitation in the transparency region of GaN cannot be originated from the valence-band states. This is the principal distinction from the photoemission processes for all the known semiconductor photocathodes when the valence electronic states are excited. Thus, the data obtained evidence that the photoemission proceeds from a quasi-metallic bulk states in space-charge region of length L ~ 20-30 nm near the surface, that is, AL is revealed. Electronic and energy parameters of AL is found to change through increasing Cs and Ba submonolayer coverages. Theoretical study of photoemission from AL is carried out in the framework of Urbakh's theory[2]. Matrix elements and energy parameters of AL for Cs/n-GaN and Ba/n-GaN are found. Calculated spectra are in well agreement with the experimental ones. Second, a new phenomenon is revealed, namely, the appearance of oscillation structure in photoemission spectra. To explain origin of oscillation structure, a model taking into account both the electron accumulation layer and multi-beam interference in parallel-sided plate of GaN is suggested.
This work was supported by Grant of RFBR # 07-02 and by Grant of Programm P-28 of Presidium of Russian Academi of Sciences.
[1] G.V. Benemanskaya, V.S. Vikhnin, N.M. Shmidt at. al. Appl. Phys. Lett. 85, 1365(2004)
[2] G.V. Benemanskaya, M.N. Lapushkin, M.I. Urbakh, Sov.Phys. JETP 75, 899(1992). |