Rapid thermal annealing was used to produce n⁺-surface layers in tin- and selenium-implanted GaAs. Hall effect and differential Van der Pauw measurements were performed and peak electron carrier concentrations of about 9 x 10¹⁸ cm⁻³ with corresponding sheet resistivities as low as 28 Ω/□, were achieved for high-dose, room-temperature Sn and Se implants. To realise this, development of new encapsulants and improvements to existing annealing methods were made. A novel method of depositing evaporated AIN for protecting the GaAs surface has been developed. This consists of the evaporation of Al in the presence of pure ammonia gas or an aqueous NH3/N5 gas mixture. The evaporated AlN layers were characterised using RBS and RHEED techniques and found to consist mainly of Al, N and 0 (as contaminant). These layers were used to successfully encapsulate GaAs at temperatures of up to 1100°C which is well above 950°C, the characteristic maximum useful temperature achieved using the existing CVD Si₃N₄. A double-layer encapsulant, consisting of ≈ 300 Å CVD Si₃N₄ plus 600 Å evaporated AlN was also developed and found to withstand even higher temperatures (≈1150°C) and to provide more reliable and reproducible results than either AlN or Si₃N₄ when used singly.