Microscopic Mechanisms of Self-Compensation in Si $\delta$-Doped GaAs

We combined systematic cross-sectional scanning tunneling microscopy and spectroscopy investigations with Hall measurements on single Si $\delta$-doped layers, as well as Si $\delta$-doped superlattices in GaAs. We found that Si self-compensation involves nucleation and growth of electrically neutral Si precipitates at the expense of the conventional donor Si phase.

Figure 1

Hall-derived free-electron density per period in Si $\delta$-doped GaAs samples. We compare results for single $\delta$-doped layers as grown (open squares), after a 2 h postgrowth anneal (open circles), and for ten period Si $\delta$-doping superlattices (open diamonds). The solid line denotes the Si surface density per period $n_s$ (from Si flux calibrations). Solid symbols show the number of ${\mathrm{S}\mathrm{i}}_{\mathrm{G}\mathrm{a}}$ donors (solid squares) and Si atoms within neutral Si precipitates (solid circles) as derived from XSTM studies of the topmost (last-deposited) superlattice period.

Figure 2

(a)–(e) XSTM topographic images of the intersections of Si layers in GaAs with the (110) cleavage surface for $n_s$ ranging from 0.02 to 1 ML. Substitutional Si donors ${\mathrm{S}\mathrm{i}}_{\mathrm{G}\mathrm{a}}$, surface gallium vacancies $V_{\mathrm{G}\mathrm{a}}$, and Si precipitates C are visible. (f) and (g) images of C defects at 0.2 ML. (h) Image of a minority unidentified defect $X$. The tunneling current was 0.2 nA , $V_b$ was 1.8 V for (a) and (b) and between $-1.5$ and $-1.8\mathrm{V}$ for (c)–(h).

Figure 3

Distribution of the density of the ${\mathrm{S}\mathrm{i}}_{\mathrm{G}\mathrm{a}}$ donors (open bars) and of the Si atoms in the C regions (solid bars) along the [001] growth direction. The origin is the center of the Si distribution within the C regions.

Figure 4

(a) Tunneling spectra measured close to and in the C region shown in (b). The peak at 1.5 V in spectrum 1 is caused by tunneling of the free carriers in the conduction band CB within the $n$-doped region. (c) potential measured along the [001] direction across a 1 ML-thick Si region. The maximum is in the region of the ${\mathrm{S}\mathrm{i}}_{\mathrm{G}\mathrm{a}}$ donors; the sharp minimum is caused by the Si-rich layer C; the broad minima correspond to the intrinsic regions on either side of the Si layer.