Molecular beam epitaxy of multilayer structures with GaAs and AlxGal-xAs

As indicated in the opening article of this issue [11 thin epitaxial films of some III-Y semiconductors have properties which are important in high-frequency and optoelectronic applications. Such films can be prepared by conventional growth techniques such as liquid-phase epitaxy (LPE) or vapour-phase epitaxy (YPE). For particular applications it may be advantageous to use metal-organic vapour-phase epitaxy (MO-YPE), discussed in the previous articles [21[31,or molecular beam epitaxy (MBE) [41. MBE is a refined form of ultra-high vacuum evaporation. In this technique thermally generated collision-free molecular (or atomic) beams of the constituent elements, formed in Knudsen sources, are deposited on a heated substrate where they react to form an epitaxially related crystalline film. Growth tem·peratures are usually somewhat lower than those used in the more conventional growth techniques, while growth rates are in the range 0.01 to 1 nm/s. Because the molecular beam Duxes can be started or stopped rapidly using a simple mechanical shutter, atomically abrupt interfaces can be obtained. Using in situ analysis it is possible to measure directly the intensity of the various molecular beams and as a result films of high crystalline quality with precise control of the thickness, composition and doping level can be obtained. Changes in composition and doping level on an atomic scale facilitate the formation of well-defined multilayer structures with special properties [51. The growth of III-Y semiconductor films by MBE has been studied extensively at Philips Research Laboratories in Redhill, England. This study included the preparation of various types of III-Y compounds and layered structures, investigation of the mechanisms controlling the growth and dopant incorporation, characterization of surfaces and interfaces,