Metal-organic chemical vapour deposition (MOCVD) of compound semiconductors. Part 2.–Preparation of ZnSe epitaxial layers on (100) orientated GaAs single-crystalline substrates

Abstract

ZnSe epitaxial layers grown from DEZ and H2Se by metal-organic chemical vapour deposition (MOCVD) contain impurities such as Na and Li which act as electron acceptors. These impurities originate either in the silica reactor walls or in the metal-alkyl precursor and are incorporated into the epitaxial layers during growth. In addition Ga, which is incorporated from the GaAs substrate at the relatively high temperature of growth ( > ca. 773 K), acts as an electron donor. In contrast, ZnSe grown by MOCVD from DMZ and H2Se at lower temperatures (ca. 573 K) contains fewer impurities. The photoluminescence spectra at 10 K are dominated by narrow, bound-exciton emission lines attributed to the presence of paired, neutral donors (denoted Ix), and the blue emission persists with appreciable intensity to room temperature. Virtually no deep-level emission is observed. The material is of low resistivity (ρ298 K≈ 10 Ω cm), has a mobility of µ298 K= 200 cm2 V–1 s–1, a carrier concentration of n298 K= 5 × 1015 cm–3 and has a low compensation ratio NA/ND= 0.25. The electrical properties of ZnSe material grown over the temperature range 523–773 K have been studied as a function of temperature down to ca. 100 K. The highest quality material shows little evidence of Ga incorporation from the GaAs substrates. Satisfactory n-type doping over the carrier concentration range ca. 5 × 1015 to ca. 1 × 1018 cm–3 has been achieved using a novel Ga-containing adduct precursor, trimethylgallium triethylphosphine (TMG-TEP)2, and the electrical properties of the material have been characterised. The incorporation of nitrogen as a p-type dopant, employing gaseous ammonia in high concentration, is sufficient to modify the luminescence properties of the ZnSe but insufficient to convert the undoped, low-resistivity material from n-type to p-type. High-resistivity ZnSe with ρ298 K= 5.1 × 105Ω cm and µ298 K= 300 cm2 V–1 s–1 has been obtained.