Influence of growth conditions on undoped and sulfur-doped InP grown by molecular-beam epitaxy

Abstract

Undoped and sulfur-doped InP have been grown by molecular-beam epitaxy. Undoped InP is n type and contains residual sulfur incorporated from the phosphorus source material. Increasing substrate temperatures during growth cause a decrease in the residual doping level to 2–3×1015 cm−3 but are associated with an increase in the compensation ratio and an increase in the concentration of epilayer defects, up to 105 cm−2. Doping with sulfur produced from an electrochemical sulfur cell has been studied in the range 5×1016–6×1019 cm−3. Highly doped layers show greatly improved surface morphology compared to low and undoped InP layers. Sulfur is shown not to diffuse to any measurable extent during MBE growth; however, at high growth temperatures (530 °C) there is loss of sulfur as a volatile indium sulfide. The removal of the surface oxide from the InP substrate before growth has been studied as a function of substrate temperature and phosphorus overpressure. The oxide layer can be removed at a low temperature (∼480 °C) by using a low pressure of P4 rather than P2 as is used during growth. The low-temperature oxide removal leads to improved epilayer morphology for undoped layers. The removal of the surface oxide and the loss of sulfur during growth as a volatile sulfide are discussed in terms of the free energy of formation of the various possible products.