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

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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.