Controlling the growth morphology and phase segregation of Mn-doped Ga2Se3on Si(001)

Abstract

The growth and phase segregation properties of the potential dilute magnetic semiconductor alloy (MnSe)${}_{x}$(Ga${}_{2/3}$Se)${}_{1\ensuremath{-}x}$ are studied as a function of thickness, Mn concentration, postgrowth annealing, and the presence or absence of undoped Ga${}_{2}$Se${}_{3}$ buffer and capping layers. This system is an unusual case in heteroepitaxy where two-phase $\mathrm{MnSe}+{\mathrm{Ga}}_{2}{\mathrm{Se}}_{3}$ has better lattice matching than the (MnSe)${}_{x}$(Ga${}_{2/3}$Se)${}_{1\ensuremath{-}x}$ alloy. Despite this peculiarity, this system shows a modified form of Stranski-Krastonow growth: laminar films are observed up to a certain $x$-dependent critical thickness, above which islands are observed by scanning tunneling microscopy. The island morphology depends on the presence or absence of an undoped Ga${}_{2}$Se${}_{3}$ buffer layer and postgrowth annealing. A kinetically stabilized platelet morphology is observed at the crossover point between laminar and islanded films. Based on Mn and Se $K$-edge extended x-ray absorption fine structure and x-ray absorption near-edge structure spectroscopy, there are two types of Mn in islanded films: Mn that remains doped in the Ga${}_{2}$Se${}_{3}$ but oxidizes upon exposure to air, and Mn that participates in the islands, which are precipitates of the MnSe phase. Consistent with MnO or MnSe, $L$-edge x-ray absorption on air-exposed films suggests the Mn is in the formal $+$2 oxidation state. No $L$-edge x-ray magnetic circular dichroism signal is observed at 20 K, which may be due to surface effects or to a lack of magnetic order.