Doping and residual impurities in GaAs layers grown by close-spaced vapor transport

Abstract

The close-spaced vapor transport (CSVT) technique is used to grow GaAs epitaxial layers from various n- or p-type doped GaAs sources. The transport agent is H2O with PH2O = 4.58 Torr. n-type layers can be grown with Te- or Ge-doped GaAs sources. The transport coefficients of both dopants (ratio of the electrically active dopant concentration in the layer to the electrically active dopant concentration in the source) is 100% for Te or Ge, in the substrate temperature range comprised between 750 and 850 °C. p-type layers are obtained with Zn-doped GaAs sources. The transport coefficient of Zn is about 1% and is also independent of the substrate temperature. The transport coefficients and their independence on temperature are in agreement with a mass-transport controlled model based on the hypothesis that the transport reactions of GaAs and the doping impurities are in equilibrium at the source and substrate temperatures. Si-doped GaAs cannot be used as a source to obtain conductive n-type layers. When undoped semi-insulating (SI)-GaAs wafers are used as sources in CSVT, n-type layers are obtained. They are characterized by ND−NA=9×1015–3×1016 cm−3 and μ300K=3000–4000 cm2 V−1 s−1, independent of the temperature, in the temperature range investigated. Glow discharge mass spectroscopy analyses performed on a source and on a layer indicate that C, O, Si, and S are the major residual impurities in the GaAs layer. All these impurities have their origin in the technique (reactor, transport agent). Ge is also present in the layers, as indicated by photoluminescence. It is a minor impurity. Its origin is probably the SI-GaAs source.