ATOMIC LAYER ETCHING CHEMISTRY OF CL2 ON GAAS(100)

The reaction of Cl2 on GaAs(100) was studied under UHV conditions using metastable quenching electron spectroscopy (MQS), AES, LEED and TPD. Chlorination of the Ga-rich c(8 x 2)Ga surface- at 85 K by Cl2 forms a GaCl layer, where each surface Ga has one Cl bond and two backbonds to As. The reaction saturates with an exposure of about 1 L Cl2. Formation of the GaCl layer increases the work function by 1.8 +/- 0.2 eV. The GaCl desorbs in a doublet at 620 and 650 K, uncovering the underlying As layer, which desorbs in three peaks at 670, 730, and 810 K. The arsenic 670 K peak is due to desorption of AS2 and AS4, and the 730 and 810 K peaks are due to desorption of AS2. Annealing at 820 K returns the surface to the original c(8 x 2)Ga condition. Flashing the chlorinated c(8 x 2)Ga surface to 650 K removes all the GaCl, leaving a clean As-rich surface. Chlorination of this surface at 85 K followed by TPD gives desorption of AsCl3 at 170 K, GaCl2 at 240 K, and GaCl at 620 and 770 K. The data indicates that the sequence of chlorinating the c(8 x 2)Ga surface at 85 K, flashing it to 650 K to remove the GaCl, chlorinating the resulting As-rich surface at 85 K, and heating to 820 K removes 2 ML of Ga and 2 ML of As, and returns the surface to c(8 x 2)Ga. The surface GaCl and GaCl2 give separate bands in the MQ spectrum, with binding energies of about 12.0 and 6.2 eV, respectively, below the GaAs valence band maximum. Repetitive cycles of exposure of the chlorinated c(8 x 2)Ga surface at 85 K to Cl2 followed by warming to 130 K or above to desorb the excess Cl2 gradually converts GaCl to GaCl2. The data suggests that partially chlorinated arsenic formed at 85 K is converted by heating to GaCl and GaCl2. The present results are consistent with studies of the desorption products from steady-state continuous etching of GaAs(100) by beams of Cl2.