Thermodynamic Modeling of As and P Incorporation in Ga_xIn_(1-x)P_yAs_(1-y)Epitaxial Layers Grown by Organometallic Vapor Phase Epitaxy

摘要

The quaternary epitaxial film Ga_xIn_(1-x)P_yAs_(1-y) (Q) lattice-matched to InP is the active layer in lasers emitting between 1.1 and 1.55 μm. In this paper, we present a thermodynamic analysis of the group V incorporation in Q layers prepared by organometallic vapor phase epitaxy. We have recently given an equilibrium description of the combined pyrolysis of AsH_3 and PH_3 for any input flow rate and H_2 dilution as a function of growth temperature and total pressure, p_(tot). To extend the treatment to gas-solid equilibrium, the Q solid is considered to be a quaternary regular solution subject to the constraint of mixing on both sublattices, for which activities were previously published. Knowing the free-energy of formation of the four bounding binary compounds, a root for y_P at a given temperature, input flow rates and p_(tot) is obtained by iteration that stops at values of p_(As) and p_P_4 simultaneously satisfying the free-energies of formation and the gas flow material balance as well as the site conservation constraint. At a constant p_(tot) in complete thermodynamic equilibrium, y_P slowly increases with temperature (800-1200K). Taking into account the incomplete decomposition of PH_3 and considering the undecomposed fraction of PH_3 as an "inert" gas, the analysis shows a rapid rise in y_P with temperature in the deposition zone. Clearly, to attain the desirable thermodynamic regime at, say, ～650℃, the use of an alternative source, such as tertiarybutylphosphine, is desirable. We present the solid composition, y_P, as a function of temperature and PH_3 flow rate for realistic parameters for Q materials emitting between 1.1-1.55 μm. We also show the predicted lattice mismatch and emission wavelength associated with y_P. A preliminary comparison with experimental data obtained in our laboratory is in reasonable accord with the calculated results.