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.