Undoped semi-insulating (SI) InP wafers were obtained by high-temperature annealing under high phosphorus over pressure. These wafers show resistivities higher than 107 Ω cm, with mobilities greater than 4000 cm2 V−1 s−1. The SI properties could be held even after cap annealing with SiNx films at 700 °C for 15 min. The activation energy of deep levels causing the semi-insulation was estimated as 0.64 eV. Photoluminescence measurements made on undoped SI InP show hitherto unknown peaks in the long wavelength region between 1000 and 1400 nm.

InP polycrystals grown by the HB technique and InP single crystals grown by the liquid-encapsulated Czochralski technique have been evaluated by photoluminescence. It was found that as the carrier concentration is decreased, the photoluminescence spectrum shows finer structures. When the carrier concentration is less than 9×1014 cm−3, a strong free-exciton peak could be observed.

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Recently, it was found that semi-insulating behavior of undoped InP can be realized by high pressure annealing of undoped high purity InP. In the present work, studies related with the achievement of the semi-insulating state are reviewed. Purification of raw materials, effect of native defects, effect of high pressure annealing, contamination of Fe are discussed. The semi-insulation mechanism is explained by the Shockley diagram. The semi-insulating state is supposed to be achieved by the annihilation of shallow donors (presumably phosphorus vacancies) and the compensation of the residual donors with a small amount of Fe deep acceptor.

The effect of thermal annealing of semi-insulating InP wafers in the 660–820 °C temperature range under SiNx capping condition is studied by electron paramagnetic resonance (EPR) spectroscopy. The annealing leads to the formation of electrically active, deep thermal donors with total defect concentrations up to 1016 cm−3. The thermal donors are of intrinsic origin. By transient EPR spectroscopy the activation energies for electron emission of the dominant thermal donors were determined to be 0.40 and 0.14 eV, respectively.