Studies of the Vaporization Mechanism of Gallium Arsenide Single Crystals

Abstract

The kinetics of vaporization of gallium arsenide single crystals into vacuum have been investigated using microbalance and mass spectrometric techniques in the temperature range 700–900° C. Although gallium arsenide vaporized incongruently to yield liquid gallium and arsenic vapor molecules, initial steady state evaporation rates can be obtained in the temperature range of study. The total evaporation rates and the activation energies of vaporization were found to be the same for both (111) and $(\overline{1}\overline{1}\overline{1})$ faces. The initial vacuum vaporization rates of gallium arsenide single crystals are lower than the maximum rates calculated from equilibrium vapor pressures by about a factor of six and the activation energy of vaporization is ${\mathrm{\Delta \; H}}^{*}\text{=90±3}\mathrm{kcal}/\mathrm{mole}$ . When excess gallium liquid was placed on top of the vaporizing surface, the rate increased by a factor of 2 while the activation energy remained virtually unchanged. Both Te‐doped and Zn‐doped GaAs samples have lower evaporation rates than the pure gallium arsenide crystals. The activation energy for Te‐doped samples is 90 kcal/mole, the same as that found for pure samples. However the activation energy of vaporization is lower for the Zn‐doped samples (76 kcal/mole). When excess gallium liquid was placed on top of the surface of these doped samples, the vaporization rate was found again to have increased with the activation energies remaining the same as those without excess liquid gallium. The vapor compositions for the (111) and $(\overline{1}\overline{1}\overline{1})$ faces were found to be different by mass‐spectrometric studies. Vaporization of the (111) face yielded only tetramers (As₄). However, during the vaporization of the $(\overline{1}\overline{1}\overline{1})$ crystal face both the tetramers (As₄) and the dimers (As₂) were detected. Excess gallium liquid on top of the crystal surfaces does not seem to affect the vapor compositions significantly. Based on these experimental results at least two reaction steps in the sequence of reactions leading to vaporization of gallium arsenide single crystals can be distinguished. (1) The availability of arsenic vacancies V_As or divancies V_AsV_Ga at the vaporizing surface appears to control the rate of sublimation. (2) Once these defects are created the vaporizing arsenic molecules break away from the lattice in a subsequent rapid reaction step which does not control the rate but establishes the vapor compositions over the two different crystal surfaces.