Surface morphology of $\text{Cr}:{\text{Ga}}_2{\text{Se}}_3$ heteroepitaxy on Si(001)

Addition of the transition-metal dopant Cr to ${\text{Ga}}_2{\text{Se}}_3$ during heteroepitaxial growth on Si(001), a system of interest as a prototype silicon-compatible, dilute magnetic semiconductor, has been studied with scanning-tunneling microscopy and scanning Auger microscopy as a function of Cr concentration and the presence or absence of an undoped buffer or capping layer. Chromium incorporates into laminar ${\text{Ga}}_2{\text{Se}}_3$ films up to a solubility limit of several atomic percent, after which Cr-rich islands nucleate. At low Cr concentrations, the vacancy-ordered nanoridge structure characteristic of pure ${\text{Ga}}_2{\text{Se}}_3$ remains but nanoridge aspect ratios decrease with Cr concentration; this is likely associated with Cr removing intrinsic vacancies. At higher Cr concentrations, faceted, Cr-rich islands nucleate, often surrounded by trenches, and the terrace morphology no longer resembles pure ${\text{Ga}}_2{\text{Se}}_3$. Growth of Cr-doped ${\text{Ga}}_2{\text{Se}}_3$ directly on Si(001):As is qualitatively similar to growth on a pure ${\text{Ga}}_2{\text{Se}}_3$ buffer layer; however, the island structure changes dramatically upon coverage of a highly doped layer with undoped ${\text{Ga}}_2{\text{Se}}_3$. Addition of Cr stabilizes cubic overlayer growth under Se-poor growth conditions beyond that of pure ${\text{Ga}}_2{\text{Se}}_3$; no growth of the hexagonal layered structure characteristic of bulk GaSe was observed.

Figure 1

(Color online) Surface topography of $\text{Cr}:{\text{Ga}}_2{\text{Se}}_3$ with STM (empty state images). Growth parameters of Cr concentration are: doped only for (a)–(c): (a) 6%, inset 3%; (b) 9%; (c) 17%; doped/pure for (d)–(f): (d) $6/0\%$; (e) $9/0\%$; (f) $16/0\%$; pure/doped for (g)–(i): (g) $0/6\%$; (h) $0/9\%$; (i) $0/17\%$. The right column shows line profiles along (green) lines from the above images: (j) doped only; (k) doped/pure; (l) pure/doped. STM images are $100\times 100{\text{nm}}^2$. Vertical scale color range = 0.6 nm except (i), where it is 1.2 nm. Additional information is in Table .

Figure 2

(Color online) Expanded STM view of regions between islands for (a) pure ${\text{Ga}}_2{\text{Se}}_3$ (1.5 bilayers) [$-4\text{V}$, 0.09 nA]; (b) pure ${\text{Ga}}_2{\text{Se}}_3$ (3.2 bilayers) [$-5.4\text{V}$, 0.09 nA]; (c) 3% film in Fig. 1a inset; (d) 9% film in Fig. 1b; and (e) 17% film in Fig. 1c. ($25\times 42{\text{nm}}^2$, color $z$ scale range 0.6 nm), images (a) and (b) adapted from Ref. 25.

Figure 3

(Color online) Local structure of $\text{Cr}:{\text{Ga}}_2{\text{Se}}_3$ island regions with STM. (a) Three-dimensional (3D) profile expanded region of 16% $\left(2.7\text{nm}\right)+0\%$ (2.3 nm) sample shown in Fig. 1f (5V tip, 0.2 nA, and $42\times 42{\text{nm}}^2$). Island top 0.4 nm above the surrounding surface while sitting in a 0.4-nm-deep depression; (b) 3D profile of expanded region of 0% (1.3 nm) $+9\%$ (4.7 nm) sample (3.34 V tip, 0.2 nA, and $42\times 42{\text{nm}}^2$). Island top is 2-nm higher than the surrounding surface, in a 0.8-nm-deep depression; (c) expansion of flat area to the lower left of the island for 0/9% film in (b); and the white spots are about 2-nm apart (3.34V, 0.2nA, and $20\times 20{\text{nm}}^2$). The bottom row shows line profiles from the above images and the arrows indicate the line along which the 3D profile was taken while the dotted lines on the profiles show the facet angle.

Figure 4

(Color online) (a) SEM image of the surface of 16/0% sample in Figs. 1f, 3a (Field of view: $0.3\mu \text{m}$; 20.0 keV; and scale bar is $0.02\mu \text{m}$) and (b) spot Auger profiles from points marked in (a), revealing differential local composition between terrace and island regions. Approximate electron-beam size $\sim 10\text{nm}$.