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 I_x), and the blue emission persists with appreciable intensity to room temperature. Virtually no deep-level emission is observed. The material is of low resistivity (ρ_{298 K}≈ 10 Ω cm), has a mobility of µ_{298 K}= 200 cm² V^–1 s^–1, a carrier concentration of n_{298 K}= 5 × 10¹⁵ cm^–3 and has a low compensation ratio N_A/N_D= 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 ρ_{298 K}= 5.1 × 10⁵Ω cm and µ_{298 K}= 300 cm² V^–1 s^–1 has been obtained.