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Positron lifetime experiments have been performed to study the recovery of point defects in electron-irradiated n-type GaAs, which has been completely compensated in the irradiation. Irradiation temperature was 20 K and isochronal annealings were performed from 77 to 650 K. After all annealings below 450 K positrons detect irradiation-induced Ga vacancies and Ga antisite defects in a negative charge state. The main recovery stage of Ga is at 200--300 K. The Ga antisites anneal out at 500 K. Since the n-type conductivity reappears also after annealing at 500 K, we conclude that the negative Ga antisites have an important role in the compensation of n-type GaAs in the electron irradiation. Removal of compensation at 500 K reveals irradiation-induced As vacancies, which are converted from positive charge state to neutral or negative. The As vacancies introduced in the irradiation have similar ionization levels to those found in as-grown GaAs. The As vacancies recover at 550--650 K and their introduction rate is about 0.5 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ after annealing at 550 K. The low value of the introduction rate may indicate that the recovery of the As vacancies starts already before 550 K.

Positron-lifetime experiments have been performed to investigate the metastability of the point defects produced in the electron irradiation of semi-insulating GaAs. The measurements in darkness indicate the presence of Ga vacancies and Ga antisite defects in a negative charge state. Illumination at 25 K reveals another type of a defect, which has a vacancy in its metastable state. The metastable vacancies can be observed most effectively after illumination with 1.1-eV photons and they are persistent up to the annealing temperature of 80--100 K. The introduction rate of the metastable defects is about 0.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which is close to the values reported earlier for the As antisite. The metastable properties of the defects resemble those of the well-known EL2 center in as-grown GaAs. We associate these defects to As antisites, which exhibit the metastability predicted by the theory: in the metastable configuration the As antisite atom relaxes away from the lattice position, leaving a Ga site vacant.

The electronic properties of native monovacancy defects were investigated using techniques based on positron annihilation in n-type Si-doped GaAs. The positron lifetime measurements as a function of temperature have shown the thermal ionization of native As vacancies from negative to neutral charge state: ${\mathit{V}}_{\mathrm{As}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}${\mathit{V}}_{\mathrm{As}}^{0}$. By combining the results of positron lifetime and two-dimensional angular-correlation-of-annihilation-radiation (2D-ACAR) experiments, the 2D-ACAR distributions of neutral and negative arsenic vacancies were separated from the partial bulk contribution. We found that 2D-ACAR distributions of the As vacancies are different from those of the bulk, and we also observed differences between the distributions of ${\mathit{V}}_{\mathrm{As}}^{0}$ and ${\mathit{V}}_{\mathrm{As}}^{\mathrm{\ensuremath{-}}}$. When compared to the negative charge state ${\mathit{V}}_{\mathrm{As}}^{\mathrm{\ensuremath{-}}}$, the neutral charge state ${\mathit{V}}_{\mathrm{As}}^{0}$ is characterized by a narrower momentum distribution from valence electron annihilations and a lower high-momentum tail from core electrons. These findings lead us to conclude that the electronic and ionic structure of the As vacancies depend strongly on their ionization. We also infer that ${\mathit{V}}_{\mathrm{As}}^{\mathrm{\ensuremath{-}}}$ has a smaller open volume than ${\mathit{V}}_{\mathrm{As}}^{0}$. This conclusion is in agreement with previous positron lifetime results and molecular-dynamics calculations.

Positron-lifetime measurements show that acceptors are produced in semi-insulating GaAs by 1.5-MeV electron irradiation at 20 K. Two types of acceptors can be separated. The first ones are negative vacancy-type defects which anneal out over a very broad range of temperature between 77 and 500 K. The second ones are negative ion-type defects which are stable still at 450 K. The data show that these two types of defects are independent and do not form close pairs. We attribute both to gallium-related defects. We identify the ion-type acceptors as isolated gallium antisites. The vacancy-type acceptors are identified as gallium vacancies which are isolated or involved in negatively charged complexes. The introduction rate of the gallium antisite is estimated to be 1.8\ifmmode\pm\else\textpm\fi{}0.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the fluence range ${10}^{17}$--${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ for 1.5-MeV electron irradiation at 20 K.

A beam of low-energy positrons together with ion-beam techniques have been used to profile defect and hydrogen distributions and their annealing behavior after the room-temperature implantation of 3\ifmmode\times\else\texttimes\fi{}${10}^{15}$ to 1\ifmmode\times\else\texttimes\fi{}${10}^{17}$ 60-keV ${\mathrm{H}}^{+}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ in semi-insulating GaAs. A monovacancy overlayer is observed close to the surface at 0\char21{}330 nm after 1\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{H}}^{+}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ implantation. The vacancies in the stopping range of the implanted H at 330\char21{}720 nm are seen only after the annealing at 200 \ifmmode^\circ\else\textdegree\fi{}C, probably due to a release of hydrogen from the vacancies. The total vacancy depth of 720 nm correlates well with the distribution of displaced atoms and with the energy deposited in the H implantation. Above 350 \ifmmode^\circ\else\textdegree\fi{}C, the monovacancies in the region of 0\char21{}350 nm start to disappear, whereas at the depth of 350\char21{}750 nm stable vacancy-hydrogen agglomerates are formed at 400 \ifmmode^\circ\else\textdegree\fi{}C. The redistribution of the implanted hydrogen at 0\char21{}350 nm was observed to be connected with the migration of complexes containing implantation-induced defects.

Defects induced by electron irradiation in semi-insulating and {ital n}-type GaAs crystals have been characterized by positron-lifetime measurements. We conclude that electron irradiation with energies of 1.5--3 MeV produces negative monovacancies and negative ions at low and room temperature. The results also show that the native monovacancy defects in lightly {ital n}-type GaAs change their properties under irradiation. We relate this change to the existence of an ionization level {minus}{r arrow}0 or 0{r arrow}+ of the native monovacancy defects in the upper half of the band gap. We propose that irradiation produces negative Ga{sub As} antisites and negative {ital V}{sub Ga} vacancies. In {ital n}-type GaAs the behavior of the native defects under irradiation is in agreement with their earlier assignment to {ital V}{sub As}.

Experimental results on positron trapping at vacancies in electron-irradiated silicon are presented. The positron lifetimes 273\ifmmode\pm\else\textpm\fi{}3 and 248\ifmmode\pm\else\textpm\fi{}2 ps in pure Si and heavily-phosphorus-doped Si ([P]=${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) are assigned to a negative monovacancy ${V}^{\mathrm{\ensuremath{-}}}$ and a negative vacancy-phosphorus pair (V-P${)}^{\mathrm{\ensuremath{-}}}$, respectively. In pure Si, positron trapping displays a strong negative temperature dependence, and the specific trapping rate reaches very large values ${(10}^{17--}$${10}^{18}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$) at low temperatures. In Si:P the trapping rate is independent of temperature. These different temperature behaviors are attributed to different positron-trapping mechanisms, a cascade of one-phonon transitions in pure Si, and an Auger process in Si:P.