Novel GaAs surface phases via direct control of chemical potential

Using in situ surface electron microscopy, we show that the surface chemical potential of GaAs (001), and hence the surface phase, can be systematically controlled by varying temperature with liquid Ga droplets present as Ga reservoirs. With decreasing temperature, the surface approaches equilibrium with liquid Ga. This provides access to a regime where we find phases ultrarich in Ga, extending the range of surface phases available in this technologically important system. The same behavior is expected to occur for similar binary or multicomponent semiconductors such as InGaAs.

Figure 1

(a) MEM image of a set of planar (001) trail regions produced by the motion of Ga droplets. The primary electron energy was 20 keV and this was offset with respect to surface potential by −0.3 eV. (b) LEEM image of the planar (001) region used for the phase transformation studies. The dashed circle indicates the position of the illumination aperture used to collect the μLEED diffraction data in Fig. 2. The electron energy is 2.4 eV. The scale markers in (a) and (b) are 5 and 2 μm, respectively.

Figure 2

LEEM images and accompanying μLEED diffraction patterns obtained from a GaAs(001) planar region with liquid Ga droplets present on the surface. The dashed circle in (a) indicates the position of the illumination aperture used to collect the μLEED diffraction data. The scale marker in (c) is 2 μm across. Schematic diffraction patterns are also shown, where large circles indicate the positions of $(1\times 1)$ spots. In (a) the pattern indicates a pure $c(8\times 2)$ phase. (b) $c(8\times 2)$ coexists with $(6\times 6)D$ (superimposed red spots and lines). $(6\times 6)D$ is disordered along $\left[\overline{1}10\right]$ and differs from the $(6\times 6)$ phase observed without droplets (see text). (c) Phase coexistence between $(4\times 6)$ (black) and a high-periodicity $c(2\times 12)$ phase (green). Each phase distribution was carefully stabilized by annealing for 60 min. The electron energy was 2.4 eV for LEEM imaging and 10.3 eV for μLEED diffraction.

Figure 3

Phase diagram with and without droplets deduced from LEEM imaging and μLEED as a function of temperature (see Fig. 2). Phase transformations with and without droplets occur at boundaries ${\mathrm{\Gamma }}_1,{\mathrm{\Gamma }}_2,{\mathrm{\Gamma }}_3$ and ${\mathrm{\Delta }}_1,{\mathrm{\Delta }}_2$, respectively.

Figure 4

Model for the variation of Ga chemical potential with temperature. ${\mu }_{\mathrm{Ga}}^d$ and ${\mu }_{\mathrm{Ga}}^b$ are the respective Ga surface chemical potentials with and without droplets present. The droplet chemical potential ${\mu }_{\mathrm{Ga}}^l$ is well approximated by that of liquid Ga, evaluated from the thermodynamic data contained in Ref. [28]. ${\mu }_{\mathrm{Ga}}^b$ and ${\mu }_{\mathrm{Ga}}^d$ (solid line) are calculated from Eqs. (3) and (5), respectively, using illustrative values $c_k=-37\mathrm{eV}{\mathrm{K}}^{-1},r_{\mathrm{Ga}}/r_d=30$, and $(E_d-E_{\mathrm{Ga}})=-0.35\mathrm{eV}$. The temperatures of the actual phase transformations in Fig. 3 are indicated on the ${\mu }_{\mathrm{Ga}}^b$ and ${\mu }_{\mathrm{Ga}}^d$ curves. The dashed curve represents ${\mu }_{\mathrm{Ga}}^d$ for a factor-of-4 decrease in droplet perimeter per unit area. A value of $4kT$ has been added to the vertical chemical potential axis to make the plots more readable.