GAAS EPITAXIAL LAYERS OBTAINED BY CLOSE-SPACED VAPOR TRANSPORT IN H2-FINE CONTROL OF THE GROWTH-RATE AND ITS EFFECT ON THE ELECTRICAL-PROPERTIES OF THE LAYERS(H2O AND H2+CO2 AMBIENTS )

In view of developing the close-spaced vapor transport technique (CSVT ) to obtain III/V homojunction solar cells, it is necessary to finely control the growth rate of GaAs epitaxial layers. This has been perfor med either by controlling the water vapor pressure, P(H2O), injected i n the reactor along with H-2, in H-2 + H2O ambient, or by controlling the water vapor pressure generated in situ by the reaction of H-2 + CO 2 in the reactor. For H-2 + CO2 ambient, P(CO2), controls P(H2O) accor ding to the following reaction: 2 H-2(g) + CO2(g) reversible 2 H2O(g) + C(s). The growth rates calculated with a diffusion controlled model are in agreement with the experimental values for both ambients, inclu ding the observation of a maximum in the evolution of the growth rate with P(CO2). Controlling the growth rate of GaAs by changing P(H2O) af fects the carrier density (N(A) - N(D)) of P-type layers grown from Zn -doped GaAs, sources. In both ambients (N(A) - N(D)) is a function of P(H2O)1/2. Such a behavior is also obtained for the calculated carrier densities. It is the result of the transport of Zn as ZnO in CSVT. In H-2 + CO2 ambient, where H2O and C are generated in situ, carbon is n ot incorporated as a major p-type doping impurity, contrarily to expec tations. n-type GaAs layers were also obtained from Te-doped GaAs sour ces. In that case, the measured N(A) - N(D) values are not affected by changes in P(H2O) because water is not involved in the transport of T e in CSVT.