ELECTRON BEAM SOURCE MOLECULAR BEAM EPITAXY OF AlxGa1-xAs GRADED BAND GAP DEVICE STRUCTURES

Abstract

ABSTRACT A new method has been developed for the growth of graded AlxGai_xAs alloys by molecular beam epitaxy which is based upon electron beam evaporation of the Group III elements. The metal evaporation rates are measured real-time and feedback controlled using beam flux sensors. The system is computer controlled which allows precise programming of the Ga and Al evaporation rates. The large dynamic response of the metal sources enables for the first time the synthesis of variable Alx Ga!_xAs with arbitrary composition profiles. This new technique has been demonstrated in the growth of unipolar hot electron transistors, graded base bipolar transistors, and M- shaped barrier superlattices.The optical and electronic properties of Alx Gai_xAs/GaAs heterojunctions and quantum wells have been extensively studied in materials grown by molecular beam epitaxy (MBE). There are, however, a number of interesting structures which have been proposed which require precise alloy grading over atomic dimensions. The ability to tailor band structure to obtain novel electrical and optical properties is termed band-gap engineering11 .'1 ' Previous attempts to obtain alloy grading by varying the temperature of MBE effusion cells have been only marginally successful and have serious limitationsJ2~ 41 This is related to the large thermal inertia and restrictions in heating and cooling rates of the MBE effusion cells which leads to slow modulation of the beam flux along with a time lag in response. In addition, variations in the growth rates determined by the Group III molecular beam fluxes seriously complicates the problem of obtaining arbitrary alloy grading. Thus Group III sources with large dynamic response are required for band gap engineering of A^Ga^xAs. Several different approaches could be used which include electron beam heated sources which are typically used to evaporate Si and refractory metals,I5' gas sources,'6' and ion beam sources.'7'