GAAS QUANTUM STRUCTURES - COMPARISON BETWEEN DIRECT PSEUDOPOTENTIAL AND SINGLE-BAND TRUNCATED-CRYSTAL CALCULATIONS

single-band approach for semiconductor clusters which accounts for the nonparabolicity of the energy bands was recently used by Rama Krishna and Friesner [M.V. Rama Krishna and R.A. Friesner, Phys. Rev. Lett. 6 7, 629 (1991)]. We compare the results of this method (denoted here as single-band truncated-crystal, or SBTC, approximation) with a direct pseudopotential band-structure calculation for free-standing hydrogen- passivated GaAs quantum films, wires, and dots. The direct pseudopoten tial calculation, which includes coupling between all bands, shows tha t isolated GaAs quantum films, wires, and dots have an indirect band g ap for thicknesses below 16, 28, and at least 30 Angstrom (8, 14, and at least 15 ML), respectively; beyond these critical dimensions the tr ansition becomes direct. A comparison of the SBTC approximation with t he direct pseudopotential calculation shows that (i) the confinement e nergy of the valence-band maximum is overestimated by the SBTC method, because the zero-confinement character of this state is neglected; (i i) the confinement energy of the Gamma-derived conduction state (direc t band gap) is slightly overestimated by the SBTC approximation, mainl y because of the assumption of infinite potential barriers at the boun daries; (iii) the confinement energy of the X-derived conduction state (indirect band gap) is severely underestimated by the SBTC method; (i v) while the SBTC approximation predicts ''quantum deconfinement'' (i. e., reduction of gap as size is reduced) for the direct gap of thin Ga As quantum wires, such effect is not present in the direct pseudopoten tial calculation. (C) 1996 American Institute of Physics.