Usage of lithium niobate in nonlinear optics is seriously hampered by optical damage, in particular, where high intensity is needed. Doping with magnesium can improve its resistance against optical damage. However, since a rather large dopant concentration is required (more than 4.6mol% MgO) and since the distribution coefficient is unfavorable, it is difficult to grow crystals of high optical quality. The authors show that by doping with zirconium, one can obtain at the same time a higher resistance against optical damage, a lower doping threshold (only 2.0mol% ZrO2), a distribution coefficient near 1.0, and a low coercive field that is only one-third of that of congruent LiNbO3. These properties suggest that zirconium-doped lithium niobate is an excellent choice for nonlinear optical applications.

Positron-lifetime and infrared-absorption spectroscopies have been used to investigate the compensation defects that render undoped n-type liquid encapsulated Czochralski-grown InP semi-insulating under hightemperature annealing. The positron measurements, carried out over the temperature range of 25-300 K, reveal in the as-grown material a positron lifetime of 282Ϯ5 ps which we associate with either the isolated indium vacancy V In 3Ϫ or related hydrogen complexes. The shallow donor complex V In H 4 , responsible for much of the n-type conductivity and the strong infrared absorption signal at 4320 nm, is ruled out as a significant trapping site on the grounds that its neutral state is present at too low a concentration. After annealing at 950°C, in conjunction with the disappearance of the V In H 4 infrared-absorption signal, trapping into V In-related centers is observed to increase slightly, and an additional positron trapping defect having a lifetime of 330 ps appears at a concentration of ϳ10 16 cm Ϫ3 , indicating divacancy trapping. These results support the recent suggestion that the V In H 4 complex present in as-grown InP dissociates during annealing, forming V In H n (3Ϫn)Ϫ (0рnр3) complexes and that the recombination of V In with a phosphorus atom results in the formation of EL2-like deep donor P In antisite defect, which compensates the material. It is suggested that the divacancy formed on annealing is V In V P , and that this defect is probably a by-product of the P In antisite formation.

We have investigated the photoluminescence mapping characteristics of semi-insulating (SI) InP wafers obtained by annealing in iron phosphide ambience (FeP 2 -annealed). Compared with as-grown Fe-doped and undoped SI InP wafers prepared by annealing in pure phosphorus vapour (P-annealed), the FeP 2 -annealed SI InP wafer has been found to exhibit a better photoluminescence uniformity. Radial Hall measurements also show that there is a better resistivity uniformity on the FeP 2 -annealed SI InP wafer. When comparing the distribution of deep levels between the annealed wafers measured by optical transient current spectroscopy, we find that the incorporation of iron atoms into the SI InP suppresses the formation of a few defects. The correlation observed in this study implies that annealing in iron phosphorus ambience makes Fe atoms diffuse uniformly and occupy the indium site in the SI InP lattice. As it stands, we believe that annealing undoped conductive InP in iron phosphide vapour is an effective means to obtain semi-insulating InP wafers with superior uniformity.

Fe-doped liquid encapsulated Czochralski InP has been annealed between 500 and 900°C for different durations. The electrical property of annealed InP has been studied by temperature-dependent Hall measurement. Defects in annealed Fe-doped InP have been detected by room-temperature transient photocurrent spectroscopy. Upon annealing, the change of electrical property in this material is indicative of the formation of a high concentration of defects. The formation process of these thermally induced defects is discussed.