SIMS profile analyses for Ge-, Co- and Fe-doped InP substrates used in epitaxial growth

Abstract

Cockayne and co-workers [1, 2] have shown recently that germanium (Ge) or cobalt (Co) doping of liquid-encapsulation Czochralski (LEC) grown indium phosphide (InP) single crystals produces potentially useful properties in the application of this I I I -V compound as a substrate material for the manufacture of a number of electronic devices. At high concentrations (>1019 atoms cm-3), germanimn hardens the lattice sufficiently to inhibit dislocation generation during LEC growth and allows single crystals with dislocation densities close to zero to be obtained [1]. Cobalt, on the other hand, can be used at low doping levels (~3 x 101Satomscm-3) to induce resistivities in the region of 107ohmcm [2]. An important factor in determining applications for these materials is a knowledge of the extent to which cobalt and germanium migrate into epitaxial layers grown on to substrates doped with these elements. Here, we report the results of secondaryion mass spectrometric (SIMS) analysis of undoped layers grown on to such substrates by either vapour phase epitaxy (VPE) or molecular beam epitaxy (MBE). Comparison is made with layers grown similarly on to Fe-doped substrates. The single crystals used as substrates were all grown by the standard LEC technique using r.f. heating [3] from polycrystalline material supplied by MCP (Electronic Materials) Ltd. This yields undoped crystals with a residual donor concentration within the range 2 to 5 x l0 is cm -a at RT (room temperature) and with a corresponding mobility range of 4000 to 5000cm 2 V -1 sec -1. The highpurity dopants, germanium, cobalt or iron were added to the starting charge prior to melting and growth. The dopant levels, quoted subsequently, were determined either chemically by spark-source mass spectrometry or from electrical data obtained by the Van der Pauw technique. Epitaxial layers were grown on to (1 0 0) substrates from the crystals using either VPE or MBE techniques. Prior to growth polished substrates were degreased in propan-2-ol, dried, dipped in 0.3% bromine in methanol solution for 60 sec, rinsed in deionized water and then blow dried. The VPE process employed was the well established technique [4] based on the reaction between indium and PC13 in a stream of H2. In this process, the substrate is heated in H~ to the growth temperature of 650 ° C over a period of 30 min followed by growth for a further 40 rain to produce layers 2 to 3 #m thick. The MBE process used indium and P2 beams in ultra-high vacuum [5], wherein the substrates are heated for 5 min at 505 ° C in a P2 flux to facilitate cleaning without surface degradation followed by growth at 420 ° C for 3 h to produce layers 0.5 to 1 #m thick. SIMS analysis was performed with a Cameca 1MS3F instrument. All samples were analysed under standard conditions using an O~ primary beam to erode through the epitaxial layer into the substrate. The isotopes counted were S6Fe+, S9Co + and 72Ge+, all for a period of 30 sec at each sampling point. Each sample was analysed by rastering a beam of either 0.3/2A or 1 gA over an area of 500#m square. An aperture diaphragm was inserted into the secondary ion beam to restrict ions counted to a central area of 60gin diameter, to avoid crater edge effects. The energy discriminating slits between the electrostatic and magnetic analysers were opened to allow a 10 eV bandpass centred on the maximum energy of an aluminium standard specimen, no offset being used. All other apertures and slits were set for maximum trans-