NONSTOICHIOMETRY AND CARRIER CONCENTRATION CONTROL IN MBE OF COMPOUND SEMICONDUCTORS

All compound semiconductors remain single-phase over some small range of deviation from stoichiometric proportions. Outside this range precipitates occur. Within it, native point defects including vacancies, interstitials and antisites are generated; these act as shallow or deep acceptors and donors and thus profoundly affect the electronic properties of the crystal. The equilibrium partial pressure of N2 over the compound MN typically increases orders of magnitude as stoichiometry deviation varies from the M-rich to the N-rich phase boundary. Although MBE is not an equilibrium process, film stoichiometry deviation is widely adjustable with the N2/M impingement rate ratio during growth. Closer control of this ratio is possible for the II-VI and IV-VI compounds by using the binary compounds as evaporation sources rather than the constituent elements, since the II-VI compounds evaporate dissociatively and congruently (the elements being more volatile than the compounds) and the IV-VI compounds evaporate mostly as MN. The III-V compounds evaporate dissociatively and noncongruently, since the III elements are less volatile. At growth temperatures just below the re-evaporation temperatures of the compounds, where highest crystal quality is generally obtained, the composition of II-VI compound films is self-regulating within the single-phase field because of the high volatility of the elements. A small fraction of excess impinging Te2 is incorporated into ZnTe, however, probably in the form of Zn vacancies, since these acceptor defects are thought to be responsible for the high p-conductivity of pure ZnTe. GaAs grown with high As2/Ga impingement flux ratio contains a photoluminescent deep defect level which is absent with lower As2/Ga. In addition, the amphoteric impurity dopant Ge may be made to move from Ga (donor) sites to As (acceptor) sites by decreasing As2/Ga, but usually with the appearance of a Ga precipitate. In the IV-VI compounds, stoichiometry deviation produces only shallow vacancy and interstitial defect levels, whixh act as dopants. Defect doping is typically adjustable from 1019 p cm-3 using excess VI flux to 1017 n cm-3 using excess IV flux, but only 0.1-1% of the excess elements are incorporated into the growing film. Most excess Se and Te re-evaporate and most excess Pb precipitates at the growth surface. The M-substitutional impurity dopants T1 (p-type) and Bi (n-type) are more suitable for IV-VI MBE, since nearly all impinging flux is actively incorporated and since higher doping levels (1019-1020 cm-3) are achievable. At the higher doping levels, compensation occurs in some cases as dopant begins to be incorporated on VI sites also, but this effect can be reduced by the use of excess impinging VI. Alternatively, compound dopants such as Bi2Te3, which evaporates molecularly, can be employed. © 1981.