Annealing Ag on GaAs: Interplay between cluster formation and Fermi level unpinning

Abstract

It has been shown that upon annealing a 2-monolayer coverage of Ag on UHV-cleaved GaAs at 500 °C, the Ag clusters into islands and the surface Fermi level moves back to within 0.2 eV of the bulk position. In this study, this Fermi level unpinning behavior has been investigated further by using various substrate doping levels and varying the anneal temperature. The Ag clustering process was observed using both ultraviolet photoelectron spectroscopy (UPS) and scanning electron microscopy. Through UPS, the surface Fermi level (Efs) movement was monitored simultaneously with the clustering process. No distinct temperature for the onset of clustering is observed. Rather, the clustering process occurs continuously over the range of temperatures studied (room temperature to 500 °C), with the Ag clusters growing and increasing in separation as anneals are performed at successively higher temperatures. A 10 min annealing time was sufficient to achieve a stable, equilibrium configuration at each annealing temperature. Movement of the Efs back to near the bulk position occurred between 375–450 °C for high doped n-GaAs (6×1018/cm3); whereas no movement of the Efs was observed for low doped n-GaAs (4×1016/cm3) up to 475 °C. Similar results were obtained when the experiment was repeated for high doped p-GaAs (1.4×1019/cm3) and low doped p-GaAs (5×1016/cm3). The absence of movement of Efs in the low doped GaAs is attributed to the longer substrate depletion length, implying that only the areas beneath and within the depletion length of the Ag clusters are pinned.