Antisite effect on hole-mediated ferromagnetism in (Ga,Mn)As

We study the Curie temperature and hole density of (Ga,Mn)As while systematically varying the As-antisite density. Hole compensation by As-antisites limits the Curie temperature and can completely quench long-range ferromagnetic order in the low doping regime of 1%–2% Mn. Samples are grown by molecular beam epitaxy without substrate rotation in order to smoothly vary the As to Ga flux ratio across a single wafer. This technique allows for a systematic study of the effect of As stoichiometry on the structural, electronic, and magnetic properties of (Ga,Mn)As. For concentrations less than 1.5% Mn, a strong deviation from $T_C\propto p^{0.33}$ is observed. Our results emphasize that proper control of As-antisite compensation is critical for controlling the Curie temperatures in (Ga,Mn)As at the low doping limit.

Figure 1

Quenched ferromagnetism by As compensation. Data from (Ga,Mn)As films grown at $250°\mathrm{C}$ using substrate rotation. (a) Samples were grown using a constant growth rate and Mn concentration but varying As:Ga. Room temperature hole density $\left(p\right)$ is plotted on the left-hand vertical axis as a function of As:Ga. The corresponding Curie temperature $\left(T_C\right)$ is plotted on the right-hand vertical axis. (b) Similar data plotted for samples grown with a constant As:Ga, but varying Mn concentration. (c) Data in (a) and (b) are replotted with $T_C$ as a function of $p$. A fit to $T_C\propto p^{0.33}$ is shown for the ferromagnetic samples. (d) Magnetization versus temperature scans (field warmed with $B=5\mathrm{mT}$) used to extract $T_C$ shown in (b).

Figure 2

Geometric flux gradients in the MBE system. As:Ga is continuously varied and optimized in a single nonrotated growth. (a) Deposition geometry (not to scale) in the MBE system. The relative positions of the As, Ga, and Mn Knudsen cells with respect to the substrate are shown from side and front views. (b) The flux gradient of Ga and As molecular beams along the $y$ axis of the wafer normalized to the center flux value. Measured values (data points) and fits to second order polynomials (lines) are shown. (c) and (d) The Mn:Ga flux variation along the $x$ axis (c) and $y$ axis (d) of the wafer measured from hole density $\left(p\right)$ variation in nonrotated samples grown at $400°\mathrm{C}$, where As flux does not affect carrier density. The line in (d) is the inverse of the Ga-flux variation, shown as a line in (b), which accounts for the variation in the Mn:Ga ratio along the $y$ axis.

Figure 3

Morphology of GaAs and (Ga,Mn)As films as a function of As:Ga (nonrotated growths). AFM micrographs ($1\times 1\mu {\mathrm{m}}^2$ scans) are plotted for a GaAs growth, but are qualitatively identical to (Ga,Mn)As films in the same wafer position. The RMS roughness measured from such AFM scans is plotted as a function of As:Ga (along the $y$ axis of the wafer) for both a GaAs and (Ga,Mn)As sample. The value of As:Ga at $y=0$ for both films is 8.3. The stoichiometric region is shaded grey, where excess As is minimized, two-dimensional growth (2D) is maintained, however, some granular structure is observed.

Figure 4

Film strain due to As nonstoichiometry and Mn incorporation in GaAs and (Ga,Mn)As nonrotated films measured by $\omega \text{−}2\theta$ HRXRD scans near the (004) peak. (a) and (b) Data from a GaAs film at the As-rich wafer position (a) and stoichiometric position (b). Data (points) and dynamical simulations (lines) are plotted for various ${\mathrm{As}}_{\mathrm{Ga}}$ densities. (c) and (d) Data from a (Ga,Mn)As film at the As-rich wafer position (c) and stoichiometric position (d). Data (points) and dynamical simulations (lines) are plotted using different Mn densities.

Figure 5

The As-compensation gradient in nonrotated (Ga,Mn)As films. The room temperature hole density $\left(p\right)$ variation along films grown with different Mn concentrations, (a) as a function of $y$ position, and (b) as a function of As:Ga. (c) and (d) Similar data for samples with the same Mn concentrations but different As:Ga at $y=0$. Lines in each plot connect data points from the same wafer.

Figure 6

The Curie temperature gradient in nonrotated (Ga,Mn)As films due to As compensation. The hole density $\left(p\right)$ and $T_C$ are plotted as a function of As:Ga for two wafers with different Mn concentrations, (a) 1.25%, and (b) 1.50%. Lines guide the eye. The stoichiometric region is shaded grey, where excess As is minimized.

Figure 7

Curie temperature $\left(T_C\right)$ versus hole density $\left(p\right)$ in several nonrotated (Ga,Mn)As films showing deviation from $p^{0.33}$. (a) Data are plotted from the As-rich region of the wafer $(\mathrm{As}:\mathrm{Ga}⩾9)$, and (b) from the Ga-rich region $(\mathrm{As}:\mathrm{Ga}⩽9)$. The shaded region marks $p$ values at which ferromagnetism is observed in As-rich material (a) and not in Ga-rich material (b). The line fit of data in (a) to $T_C\propto p^{0.09}$ is replotted in (b).