Hall Effect Studies of AlGaAs Grown by Liquid-Phase Epitaxy for Tandem Solar Cell Applications

We report results from Hall effect studies on Al (x) Ga1-x As (x = 0.23-0.24) with bandgap energies of 1.76 +/- A 0.01 eV grown by liquid-phase epitaxy (LPE). Room-temperature Hall measurements on unintentionally doped AlGaAs revealed p-type background doping for concentrations in the range 3.7-5.2 x 10(16) cm(-3). Sn, Te, Ge, and Zn-doped AlGaAs were also characterized to study the relationship between doping concentrations and the atomic fractions of the dopants in the melt. Temperature-dependent Hall measurements were performed to determine the activation energies of the four dopants. Deep donor levels (DX centers) were dominant for Sn-doped Al0.24Ga0.76As, but not for Te-doped Al0.24Ga0.76As. Comparison of the temperature-dependent Hall effect results for unintentionally and intentionally doped Al0.24Ga0.76As indicated that the impurity contributing to the p-type background doping had the same activation energy as Mg. We thus suggest a Te-doped emitter and an undoped or Ge-doped base to maximize the efficiency of Al (x) Ga1-x As (x similar to 0.23) solar cells grown by LPE.