Semi-insulating iron-doped InP buffer layers for Al-free GaInP/GaInAs pHEMTs

InP buffer layers for Al-free HEMT applications were deposited by metal-organic vapor-phase epitaxy (MOVPE) on semi-insulating InP substrates. The challenge posed by the production of insulating buffers on InP substrates is a well-known problem. Different growth conditions were chosen to achieve highly insulating layers. Our experiments show that regardless of growth conditions, impurities arising from reactor parts and from the starting substrate lead to an n-type background doping decreasing exponentially from 1×10¹⁷ at the substrate to 4×10¹⁴ cm^-3 for sufficiently thick layers. Highly resistive buffer layers could only be obtained doping InP with iron at a concentration of 6×10¹⁶ cm^-3. Consequently, the sheet resistance of InP could be increased from R_S = 3000 Ω/□ for undoped layers to R_S = 9.4×10⁷ Ω/□, resulting in InP buffer layers that are suitable for high-speed HEMTs. Non-optimized Al-free GaInP/GaInAs pHEMTs with a T-gate footprint of 100 nm achieved a cutoff frequency of f_T = fmax ~ 250 GHz, with a channel mobility and electron density of 10,000 cm²/Vs and 10¹² cm^-2, respectively.