Amorphous Superlattices
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AMORPHOUS SUPERLATTICES L. LEY Max-Planck-Institut fUr Festk6rperforschung, Heisenbergstrasse 1, 7000 Stuttgart 80, Federal Republic of Germany ABSTRACT In this contribution I review some of the salient results obtained on periodic multilayer structures made from amorphous semiconductors by the method of plasma assisted chemical vapour deposition. Emphasis is placed on evidence for quantum size effects in heterostructures, on the carrier recombination kinetics in amorphous doping superlattices, and on phonon diffraction phenomena. INTRODUCTION Periodic multilayer structures consisting of amorphous semiconductors are commonly albeit not quite correctly referred to as amorphous superlattices. They are the amorphous analogs of their crystalline counterparts. With one exception [1] the materials used are derived from the group IV elements C, Si, and Ge and the Si alloys SiN N (OkxsI.33) and SiOx (O x52). The individual layers are deposited from the gas phase by the plasma assisted chemical vapour deposition of appropriate gases:[2,3] CH4 or C2 H6 , SiH 4 , and GeH 4 for the elemental semiconductors, and gas mixtures such as SiH 4 + NH or SiH 4 + N2 0 for the large band gap Si oxides and nitrides. 3 he films deposited at substrate temperatures between 200 and 350 0 C contain approximately 10 to 20 at. % hydrogen and the material is therefore referred to as a-Si:H, for example. The hydrogen is an essential ingridient since
it
reduces
the
concentration
of deep
defects
(mainly
dang-
ling, i.e. unsatisfied bonds) through passivation to a level low enough (e 101'- cm- 3 eV-' in a-Si:H) to allow substitutional doping of the group IV amorphous semiconductors through the introduction of group III or group VI elements [3]. A low defect density is also essential in order to achieve reasonable photoconductivity and photoluminescence yield because dangling bonds act as efficient non-radiative recombination centers [4]. Two kinds of amorphous superlattices have been prepared and investigated: Heterostructures or quantum well structures in which a low band gap material is sandwiched between layers with a larger band gap [5,6], and doping superlattices [7,8] in which the doping of a-Si:H varies periodically between n-type and p-type with possibly an intermediate layer of intrinsic material (Inipi' structures). Multilayers of either kind are easily grown by switching periodically between two kinds of reactant gas mixtures. Gas exchange times comparable with or smaller than the time needed to grow a monolayer (typical growth rates are _..2 A/sec) should, in principle, give atomically abrupt interfaces. X-ray diffraction [9] and transmission electron-micrographs [10,11] do indeed confirm the growth of highly regular superlattices with an interface roughness no larger than - 6 A. Raman scattering investigations [12,13] of the density of Si-Ge bonds in a-Si/a-Ge multilayers have demonstrated that the interfaces are nearly ideally abrupt. The density of interface Si-Ge bonds is no more than a factor of two larger than the bond denM
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