Focused ion molecular-beam epitaxy - a novel approach to 3D device fabrication using simultaneous p- and n-type doping

The combination of molecular-beam epitaxial (MBE) growth and in situ focused ion beam (FIB) technology, has attracted considerable research interest in recent years since early studies in this area by Hirayama and Okamoto and Miyauchi et al. on III-V semiconductor material systems. In this paper, we describe a novel fabrication technique that directly integrates a FIB column into an MBE growth chamber. This `FIMBE' system allows the generation of novel III-V device structures that would otherwise be impossible to fabricate via conventional lithographic techniques. The use of a Au70Si15Be15 liquid metal ion source (LMIS) in our FIMBE system offers for the first time both n and p-type selective-area doping during MBE growth. A mass filtered 2 kV FIB column was used to selectively dope regions of a single GaAs epilayer using both Be and Si, with landing energies of 30 and 25 eV, respectively. High-quality bulk GaAs was grown with 77 K mobilities of 0.3 m2 V-1 s-1 at a carrier concentration of 5×1023 m-3 using the Si2+ species, and 0.065 m2 V-1 s-1 at a carrier concentration of 1×1023 m-3 for Be2+. The epilayer quality is thus comparable to that obtained from conventional thermally doped material. Low-energy ion-doped 2D hole gases (2DHGs) over selective areas have also been fabricated for the first time using 30 eV Be2+ ions as the dopant species in a GaAs/AlxGa1-xAs heterostructure. This resulted in a 2DHG with an unilluminated 1.7 K mobility of 8.8 m2 V-1 s-1 at a carrier concentration of 2.6×1015 m-2. A Si2+ ion doped HEMT was also grown, attaining a 1.7 K 2DEG mobility of 60.7 m2 V-1 s-1 at a carrier concentration of 1.9×1015 m-2 after illumination. This compares favourably with a thermally doped control structure grown immediately afterwards that attained a 1.7 K 2DEG mobility of 80.7 m2 V-1 s-1 at a similar carrier concentration after illumination.