Photoemission study of the Na/ZnSe„100… interface Zhonghui Chen* and D. Eich Experimentelle Physik II, Universita ¨t Wu ¨rzburg, Am Hubland, D-97074 Wu ¨rzburg, Germany G. Reuscher and A. Waag Experimentelle Physik III, Universita ¨t Wu ¨rzburg, Am Hubland, D-97074 Wu ¨rzburg, Germany R. Fink and E. Umbach Experimentelle Physik II, Universita ¨t Wu ¨rzburg, Am Hubland, D-97074 Wu ¨rzburg, Germany Received 26 February 1999 We report on a comprehensive study of the ZnSe(100)- c (2 2)-Na interface using x-ray and UV photo- emission, and x-ray-induced Auger spectroscopy. Spectra were taken after stepwise Na deposition onto a clean c (2 2)-reconstructed ZnSe100 surface at room temperature up to a saturation coverage of about 1 ML of Na and after annealing. Based on the analysis of Auger parameters and of the relative intensity evolution of various Na, Zn, and Se species, we present the following model for the ZnSe(100)- c (2 2)-Na interface: below a coverage of 0.5 ML, Na is adsorbed on Zn vacancy sites; above 0.5 ML, a cation exchange reaction occurs between Na and Zn atoms; Zn atoms segregate on top of the Na overlayer forming metallic Zn. In addition, band-bending, surface dipole, valence-band and surface states will be discussed. S0163-18299902336-X I. INTRODUCTION ZnSe is one of the important II-VI semiconductors due to its application in optoelectronic devices. The metal-ZnSe in- terface is an important aspect for such devices. Photoemis- sion studies of metal-ZnSe interfaces provide microscopic insight into the chemical and electronic properties of the interface. 1–3 Moreover, the deposition of alkali metals on solid surfaces is extensively employed in technological pro- cesses. For example, alkali metals adsorbed on semiconduc- tor surfaces were shown to be efficient catalytic promoters in the oxidation and nitridation of semiconductors. 4,5 In addi- tion, alkali metals can also be used to control band offsets at heterojunctions, as was experimentally shown for the SiO 2 /Si interface doped with a Cs interlayer. 6 Alkali-metal/ZnSe interfaces are of particular interest for several reasons. First, substitutional alkali atoms in ZnSe serve as acceptors. Up to now, it has been a puzzle that the compensated acceptor concentration of alkali-doped ZnSe can hardly exceed 10 17 cm -3 . 7 The electronic and chemical information on alkali/ZnSe interfaces, as extracted from pho- toemission spectra, is expected to be very useful for an un- derstanding of this puzzle. Second, the Zn-Se bond has a relatively large ionic character. The heat of formation of ZnSe -163 kJ/mol is nearly twice as large as that for the covalent semiconductors GaAs and InP -88 and -88.7 kJ/ mol, respectively. 8 Comparison with these materials may then provide insight into reaction pathways and interdiffu- sion. For example, the Au/GaAs interface is reactive, leading to some exchange of atoms, 9 while the Au/ZnSe interface remains abrupt mainly due to its different cohesive energy. 1 Third, alkali-metal overlayers have been regarded as model systems for metal-semiconductor interfaces due to their simple electronic structure and their chemically active na- ture. The most extensive studies on alkali adsorbates on semiconductor surfaces using photoemission have concen- trated on group-IV and -III-V semiconductors, particularly Si and GaAs. 10 To our knowledge, there are no photoemission studies on alkali/ZnSe interfaces yet. Moreover, previous studies on interfaces of alkali/Si, alkali/GaAs, etc. 10 have shown that only approximately 1 ML of alkali atoms can be grown on semiconductor surfaces at room temperature. Fi- nally, it appears feasible to identify semiconductor surface states from valence-band spectra by Na exposure. In this paper we report on an x-ray photoemission XPS, UV photoemission UPS, and x-ray-induced Auger spec- troscopy XAES study of the room-temperature evolution of the ZnSe(100)- c (2 2)-Na interface. We begin with a dis- cussion of the chemistry of Na/ZnSe interfaces at room tem- perature. Based on the Auger parameters and the relative intensity evolution of various Zn, Se, and Na species, we conclude that the Na/ZnSe(100)- c (2 2) interface is reac- tive with the appearance of a ‘‘metallic’’ Zn species at Na saturation coverage, while the Na species remain nonmetallic even at saturation coverage. The valence bands of the Na/ ZnSe100 interface exhibit a metal-like Fermi cutoff due to the formation of Zn clusters. Quantitative XPS analyses in- dicate that Na can be deposited on the ZnSe(100)- c (2 2) surface at room temperature up to saturation coverage ( 1 ML of Na, and that the growth of Na on ZnSe(100)- c (2 2) consists of two stages which are separated by a critical Na coverage of 0.5 ML. Below 0.5 ML of sodium is prob- ably adsorbed on Zn sites, while above 0.5 ML a reaction occurs, for which an interface model is suggested. Band bending, surface dipoles, and Fermi-level pinning will be discussed. Finally, we shall identify a peak in the ZnSe va- lence band as a surface state. II. EXPERIMENTAL DETAILS The experiments were performed on undoped ZnSe layers approximately 1 mm thick grown by molecular-beam epi- PHYSICAL REVIEW B 15 SEPTEMBER 1999-II VOLUME 60, NUMBER 12 PRB 60 0163-1829/99/6012/89159/$15.00 8915 ©1999 The American Physical Society