A 2 in. diam single crystal of lithium tetraborate (Li2B4O7) was successfully grown by the Czochralski method. The crystal was free from macrodefects and had a dislocation density as low as 100/cm2. It had an excellent homogeneity of the refractive index and a wide transparency down to 170 nm. The optical damage threshold was 40 GW/cm2. Second-harmonic generation and sum frequency generation were investigated in association with the generation of the fourth and fifth harmonics of a Q-switched Nd:YAG laser. The conversion efficiency of the second-harmonic generation from the green (532 nm) light was 20%.

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We report characteristic behaviors of medium-deep electron traps (EL5, EL6, etc.) in n-type bulk GaAs crystals observed by isothermal constant-capacitance–voltage transient spectroscopy (CCVTS). Each CCVTS spectrum of EL5 and EL6 was broader than a theoretical one expected for a single level, and was found to consist of more than two trap components. Anomalous filling time dependence of CCVTS peak heights for these traps was observed when the filling pulse width was varied in a wide range, i.e., one component of EL6 decreased to about 50% of its maximum, while one component of EL5 increased and saturated. The decrement of the constituent of EL6 was nearly equal to the increment of that of EL5. These variations could be reversed by controlling electron occupation fractions of these traps by means of changing the interval time between two adjacent filling pulses in application of the isothermal “double-pulse CCVTS” method. Such interaction was commonly observed in n-type bulk GaAs irrespective of carrier concentration and growth method. These behaviors of medium-deep electron traps are interpreted well by a model of an electronically controlled bistable reaction.

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