A comparison of MBE and MOMBE/CBE growth mechanisms using modulated beam mass spectrometry and RHEED

Abstract

MOMBE and CBE growth has until recently been based on largely empirical studies of the epitaxial process. We have used modulated beam mass spectrometry (MBMS) and reflection high energy electron diffraction (RHEED), previously applied to the study of MBE, to investigate the interaction of metalorganics with As 2 and As 4 with a view to understanding more completely the MOMBE/CBE process. The decomposition of metalorganic (MO) sources under quasi-equilibrium conditions has been studied using time-of-flight (TOF) techniques. The cracking pattern of trimethylgallium (TMGa) has been established using this method. The interaction of TMGa with As 2 and As 4 has been studied in detail on both singular and vicinal plane samples. The growth rate is a complex function of both substrate temperature and group III incident flux and is also influenced by the nature of the group V species and the group V/III ratio. Studies of the species desorbing from the surface during growth by MOMBE using TMGa have shown that in addition to the metalorganics, only methyl radicals are observed, the methane and ethane seen in mass spectrometer studies which have not used modulation techniques are formed in the system. Modulating the absorbed beam (TMGa) has shown that, in the temperature range where significant growth is achieved, no TMGa is desorbed from the surface. The presence of CH 3 (no CH 4 or C 2H 6 is detected) desorbing from the surface suggests the pyrolysis of TMGa via sequential release of methyl radicals. The reaction mechanism for the decomposition of TMGa under MOMBE conditions is complicated with at least one slow step limiting the growth rate of GaAs.