A study of the distribution of hydrogen and strain in proton-bombarded liquid-encapsulated Czochralski-grown GaAs by double-crystal x-ray diffraction and secondary ion mass spectrometry

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Proton bombardment is an effective method of providing electrical and optical isolation for devices in GaAs integrated circuits. In this study, n-type LEC GaAs has been implanted with protons at doses of 10 15 and 10 16 ions cm −2 at an energy of 190 keV. Double-crystal X-ray diffractometry of the as-implanted material reveals complex rocking curves featuring high intensity damage-related peaks. These have been analysed by comparison with simulated rocking curves which were calculated using the dynamic theory of X-ray diffraction with assumed strain profiles. This simulation-matching technique has enabled the determination of the strain distributions for the two doses studied. For the 10 15 cm −2 dose the peak strain is 3.8 × 10 −4 and for the 10 16 cm −2 dose it is 2.2 × 10 −3 with the peak strain for both samples occurring at a depth of 1.6 μm. Secondary ion mass spectrometry (SIMS) profiles confirm that the peak in the hydrogen concentration occurs close to the peak in strain. A further feature of the strain profiles in the as-implanted sample is that a close match with the experimental data is only obtained when a highly strained layer close to the surface is put into the simulation. Although this was unexpected, it was possible to substantiate its existence by etching studies. Subsequent annealing of the damage over the temperature range 150–500 °C (less than 200 °C in air, and more than 200 °C in arsine) has enabled modifications to the strain profile as a function of annealing temperature to be determined. There is no change at 150 °C but above this temperature the level of strain reduces as the annealing temperature increases and the structural damage recovers between 400 and 500 °C which approximates to the optimum annealing temperature required to achieve good isolation properties. SIMS profiles of the annealed samples show that hydrogen is not mobile at 150 °C but above 150 °C it moves down the strain gradient towards the surface. The data are discussed in the light of the currently perceived model of radiation damage annealing in GaAs, i.e. there are two recovery stages at about 200 °C and 400 °C.