Si-DOPING OF MOCVD GaAs: CLOSER ANALYSIS OF THE INCORPORATION PROCESS

Doping studies have been carried out as a function of silane input fraction over the entire dopant range of silicon in GaAs. In the lower dopant region where the incorporation varies linearly with the silane input mole fraction, the compensation ratio N~/N~ appears to have a constant value of 0.3. In this region the incorporation also is orientation dependent. For SiH 4 input mole fractions higher than ix 10—6 the electron concentration does not change noticeably, however the compensation ratio rises from 0.3 to 0.7. Simultaneously precipitates are observed in the grown layer. From gas phase equilibrium calculations such silicon precipitation is predicted to take place at these higher input pressures of SiH4. As regards the incorporation rate of Si itself, analysis of the doping results reveals that, when corrections are made for differences in GaAs growth rate and small differences in growth temperature, a constant deposition of silicon along the MOCVD reactor is obtained. The absence of depletion effects and the observed temperature dependence imply that the limiting step for the Si incorporation is the decomposition of the SiH4 which happens very close to the hot surface of the GaAs substrate. 1. Introduction used as the doping source. One of the dominant problems inherent with this dopant is a strong Metalorganic chemical vapour deposition temperature dependence which makes uniform (MOCVD) now has become a standard and doping difficult for reactors where the tempera-versatile technique with a proved capability to ture differs over the susceptor. When silane is grow many sophisticated structures such as super-replaced by disilane, the incorporation process lattices, lasers and tandem solar cells. For most appeared to be independent of growth tempera-device applications, a good control of the doping ture [1,21. However, recent analysis has revealed process is necessary, including control of doping that actually disilane should make no difference level, uniformity and doping profile. Up to now, when compared with silane, except when lower no single dopant seems to be able to fulfill all the pressures are used or when the flow and tempera-requirements such as wide doping range, easy ture profiles of the incoming gas are undeveloped control of uniformity, easy to handle, negligible [3,4]. Another well recognized effect is the corn-memory effect, low diffusion coefficient, and so pensation which can occur with silicon. In litera-on. As an n-type dopant, silicon is widely used for ture, for low and moderate Si concentrations, this GaAs. For …