Study of CoGa deposition from the single source precursor (CO) 4CoGaCl 2 (THF)

Abstract

Depositions of thin films of CoGa from the single-source precursor (CO) 4CoGaCl 2(THF) ( 1) have been carried out in an epitaxial organometallic chemical vapor deposition reactor. Prior to deposition, mass spectrometric (MS) and thermogravimetric (TGA) analyses of 1 were undertaken. Electron impact MS reveals a tendency for 1 partially to disproportionate (after loss of THF) into ClGa[(CoCO) 4] 2 and GaCl 3 at probe temperature (120°C). The parent molecular ion for 1 appears with only low relative intensity (1%) indicating easy loss of THF as the first step. The principal features of the spectrum can be assigned to step-wise CO-loss series of peaks derived from three parent ions [(CO) 4CoGaCl 2] + (a), [(CO) 4CoGaCl] + (b), and [(CO) 4Co] + (c). The relatively high intensity of the peaks derived from parent ions (a) and (b) indicate a relatively good stability of the CoGa bond through to the species [CoGa] + (rel. int. 15%). TGA of 1 under Ar shows decomposition in a single step in the range 100 to 230°C; residual weight of 44% is higher than expected for CoGa alone. Under H 2 however, a multistep decomposition process is observed culminating at 400–450°C; residual weight is 33%, close to that expected for CoGA. In the (cold wall) CVD reactor, optimum deposition occurs under reduced pressure (0.9–6 torr) and laminar flow conditions using H 2 as carrier gas for (100)GaAs, (100)Si, and Al 2O 3 as substrates; growth rates are about 1.8 μm/h. Polycrystalline films are obtained under all conditions; annealing under H 2 for 2 h at 500°C gives relatively sharp XRD patterns for β-CoGa. For depositions in the range 250 to 350°C the Co/Ga ratio in the films is close to 1/1 showing control of stoichiometry by the precursor. These films however still contain some Cl. In the range 350 to 400°C the Cl content is lowered, however, the films become Co rich indicating loss of Cl is accompanied by loss of Ga. The stoichiometric control achieved in the present work indicates that single source precursors of ratio CoGa 2 could give Ga rich (64 at.% Ga) β-cubic CoGa for lattice matching to (100)GaAs.