There is a deal number of literature on evaporation processes of GaAs, but much of this is contradictory. At the same time, within the framework of electron theory of semiconductor binary compounds developed at our laboratory [1], an intimate connection between collective processes in the samples, stoichiometry violations at the surface and flows of atoms and molecules, which are desorbed from the surface, become more pronounced.

The evaporation dynamics of non-doped GaAs crystals under their heating (600-1250 K) in vacuum (10^-8 Torr) was studied by an Isotope Induction Mass-spectrometer MI-1201 (USSR). The resolution of the system was more than 5000 carbon units, the sensitivity was greater than 10¹⁰sec^-l.

It has been found that:

1. the spectral composition of GaAs evaporation products consists of As₁, As₂, As₃, As₄ atom flows and atoms of Ga. It is our opinion that the origin of As_l, As₃ molecules is associated with the dissociation under the action of the ionising electron beam in the ion path;
2. during prolonged heating at a fixed temperature of 900 K, there is no stabilization of Ga flows' quantity, non-metal flow (N_x) sum and especially, their ratio NGa/N_x at the level 1 as opposed to crystals with more pronounced ionic bond II-VI. For GaAs this relation is far less than 1, which points to the high depletion of As atoms for the near-surface layer;
3. the quantity NGa/N_x = f(T) is subject to sharp fluctuation in the region of 915 K, which is due to formation of Ga drops on the surface at this temperature. A discontinuity of the function ln NGa = f(l/T) at this temperature, where for the low-temperature region the evaporation activation energy Q Ga is -0.93 eV, and for the high-temperature region it is -1.57 eV also bears witness to this. In so doing Q for As is equal to 2.67 eV all components.

The composition changes of GaAs crystals exposed to thermal treatment as compared to the test crystals were studied by an Auger-spectral analysis method (JAMP- IOSX).

In addition the changes of composition of GaAs crystals and microimpurity distribution with depth were analysed by the method of Time of Flight Mass-spectrometry with laser Excitation ofthe Specimen in the follow regimes: vacuum 10^-9 Torr, mass registration 1-300 amu, resolution 300, sensitivity ~10¹⁶ sm^-3, depth of analysis ~1 µm.

References

1. Georgy Ya. Pikus and others, Ukr. Fiz. Zhurn., vol.37, N 7, pp.1021, (1992)