Atomic Modulation of Interdiffusion at Au-GaAs Interfaces

Abstract

Monolayer thicknesses of Al are observed at Au-GaAs(HO) interfaces modulate the relative Ga to As diffusion into Au by over an order of magnitude. This new phenomenon at compound semiconductor/metal interfaces reveals a systematic relationship between the local atomic bonding and the extended chemical structure of the interface.The movement of semiconductor and metal atoms across a metal-semiconductor (M-SC) interface is a widely investigated phenomenon. 1 This interdiffusion is of particular significance for compound semiconductors 1 such as GaAs (Ref. 2) because it correlates with changes in interface electronic properties. The preferential diffusion of one constituent out of the semiconductor lattice can produce electrically active defects with energy levels in the band gap. Along with new states produced by metal diffusion into the semiconductor, these interface features strongly influence the Schottky barrier (SB) formation. Several ultrahigh-vacuum (uhv) studies of M-SC interfaces have recently emphasized the dominant influence of microscopic M-SC bonding in determining SB heights. 3 ' 5 For interfaces exhibiting no strong chemical reaction, 3 significant interdiffusion can take place, the initial stages of which have recently been studied for Au on GaAs(110) by soft-x-ray photoemission spectroscopy (SXPS). 5 » 6 In this Letter, we describe the dramatic effect which reactive monolayers of Al at the Au-GaAs (110) interface have on the macroscopic diffusion of Ga and As from the semiconductor crystal into a metallic Au overlayer. Specifically, the ratio of Ga to As outdiffusion increases systematically over an order of magnitude as the Al interface layer varies from fractions of a monolayer to several monolayers. This novel phenomenon demonstrates that microscopic interfacial bonding plays a central role in forming the macroscopic chemical composition and thereby the electronic structure of M-SC junctions.SXPS experiments were carried out at the University of Wisconsin Synchrotron Radiation Center in a uhv chamber (base pressure p = 5x 10" n Torr). GaAs single crystals (Laser Diode, n = 3.2 x 10 17 Te) were cleaved in uhv to expose (110) surfaces, onto which were evaporated thin Au and Al films. Metal deposition was monitored by a quartz-crystal oscillator positioned next to the cleaved surface. Pressure rise during evaporation was in the 10' 10 -Torr range. Spectra of Ga 3d, As 3d, Al 2p, and Au 4/ core levels were obtained at hv = 130 and 80 eV with a cylindrical mirror analyzer (CMA). The CMA cylindrical axis was at 45° to the cleaved (110) surface normal and at 78° to the incident light beam. Overall energy resolution was 0.3 eV. Al coverages were also checked from the ratio of Al 2p to Ga 3d integrated core-level intensities atfty = 130 eV (Al^iso/Cxa^o) as calibrated previously. 7 The core-level intensities reported here are integrated peak areas normalized to photon beam intensity and with a conventional background subtraction. For a given crystal cleavage, all intensities correspond to the same visual...

The initial growth of vapour deposited gold films

“…The intermixing of gold with that surface starts from the beginning of the deposition as indicated by both the gold and arsenic signals. After the gold atoms have attached to the surface, the intermixing occurs via inward diffusion of gold atoms in agreement with observations on the (110) surface (34,35,36). It is interesting to note that deficiencies produced in this way have also been observed (20) at gold/Si(111) interfaces.…”

“…A similar phenomenon has been observed for the GaAs surface when partially covered by aluminium (34,35,36). Deposition of gold onto aluminium!…”

Abruptness of Semiconductor-Metal Interfaces

“…For Al, both CT and EM act to decrease 7 As , as observed. 3 J As decreases less for 10 A of Al than for 10 A of Ti, scaling with AH R . For In, Zn, and Au, no strong reactions occur and Ax is expected to dominate.…”

“…This attenuation depends strongly upon "chemical trapping" (CT) of the anion by metal atoms near the interface. 3 In general, the larger the heat of interface reaction 4 AH R [^^(semiconductor) -H F (metal-anion complex)], 5 the stronger the anion attenuation rate. Here H F values are a measure of bond strength 6 but do not imply formation of stoichiometric bulk compounds at the interface.…”

Microscopic control of semiconductor surface oxidation

“…Thin metal overlayers deposited on atomically clean semiconductor surfaces can change dramatically the reactivity of the surface for reactions with gas speciesl-4 and metals. 5 - 6 We have recently shown, for example, that thin Cr overlayers on Si(l1l) surfaces can act both as passivating layers and as catalysts for Si(lll)-Au interface reaction, so that one can control and modulate interdiffusion by varying the thickness of the Cr interlayer. 6 Similar effects have been observed by Brillson and co-workers for Al atoms at the GaAs(11O)-Au interface.5 As far as reactions with gaseous species are concerned, a few pioneering studies have addressed the effect of Ag and Au overlayersl,3 on the oxidation of Silicon surfaces, while the presence of an Al overlayer has been shown to induce the formation of a potentially stable new oxide phase on Ge(lll) surfaces.…”