$p$-type codoping effect in (Ga,Mn)As: Mn lattice location versus magnetic properties

In the present work, we perform a systematic investigation on $p$-type codoping in (Ga,Mn)As. Through gradually increasing Zn doping concentration, the hole concentration increases, which should theoretically lead to an increase of the Curie temperature ($T_{\mathrm{C}}$) according to the p-d Zener model. Unexpectedly, although the film keeps its epitaxial structure, both $T_{\mathrm{C}}$ and the magnetization decrease. The samples present a phase transition from ferromagnetism to paramagnetism upon increasing hole concentration. In the intermediate regime, we observe a signature of antiferromagnetism. By using channeling Rutherford backscattering spectrometry and particle-induced x-ray emission, the substitutional Mn atoms are observed to shift to interstitial sites, while more Zn atoms occupy Ga sites, which explains the observed behavior. This is also consistent with first-principles calculations, showing that the complex of substitutional Zn and interstitial Mn has the lowest formation energy.

Figure 1

Cross-sectional high-resolution TEM micrographs of samples (a) Zn-0, (b) Zn-2, and (d) Zn-8. The sample surface is exemplarily marked by a dashed line in (a).

Figure 2

(a) Channeling [011] Rutherford backscattering spectra of samples Zn-0 (dot) and Zn-8 (dash). A pure GaAs wafer (dash dot) is placed as a reference for comparison as well as the random spectra (solid line). The inset shows the sign of interstitial Mn atoms in sample Zn-8. (b) Calculated minimum backscattering yield ${\chi }_{\min }$ [32] for all samples together with the pure GaAs for the reference.

Figure 3

(a) Temperature-dependent sheet resistance at zero field and (b) magnetoresistance under a 5-T field for (Ga,Mn)As and (Ga,Mn)As:Zn samples. (c) The magnetic-field-dependent Hall resistance at room temperature of all samples.

Figure 4

(a) Magnetization as a function of magnetic field at 5 K when magnetic field is applied along the $\left[1\overline{1}0\right]$ direction. (b) Temperature-dependent remanent magnetization under zero field after cooling down process under a field of 2 kOe.

Figure 5

Temperature-dependent magnetization after zero-field-cooling (closed symbol) and field-cooling (open symbol) procedure of samples (a) Zn-0, (b) Zn-1, (c) Zn-2, (d) Zn-4, and (e) Zn-8 under fields of 50 Oe (circles), 100 Oe (up triangles), 200 Oe (down triangles), and 500 Oe (diamonds). A peak around 10 K can be observed in (d) as the sign of the possible antiferromagnetic coupling in sample Zn-4.

Figure 6

Full particle-induced x-ray emission (PIXE) spectra of all samples if the He beam is aligned along the (a) [011] and (b) [001] directions. Enlarged Mn $K\alpha$ peak of the PIXE spectra (open scatters: random spectrum; solid scatters: channeling spectrum) for all samples if the He beam is aligned along the (c)–(h) [011] and (i)–(n) [001] directions. The difference between the yield of these two directions shows that the interstitial Mn can be only observed along the [011] direction.

Figure 7

Zn fluence dependence of (a) hole concentration, (b) Curie temperature $T_{\mathrm{C}}$ and saturation magnetization $M_{\mathrm{S}}$, and (c) substitutional fraction ${\chi }_{\mathrm{sub}}$ and the estimated total Mn concentration.

Figure 8

(a) The structure of Zn- and Mn-codoped GaAs, with (b),(d),(f),(h) substitutional Mn atoms and interstitial Zn atoms and with (c),(e),(g),(i) substitutional Zn atoms and interstitial Mn atoms. The red, violet, green, and gray balls refer to Ga, Mn, As, and Zn atoms, respectively. The substitutional atoms in (b),(c),(f),(g) are far away from the interstitial ones, so they are not in sight. The interstitial atoms occupy two different sites which are in a tetrahedral position (b)–(e) with Ga atoms as the nearest neighbors [solid symbols in (j)] or (f)–(i) with As atoms as nearest neighbors [open symbols in (j)]. (j) Upon decreasing the distance between the doped interstitial and substitutional atoms, the distance dependence of cell energy relative to the lowest-energy cases, until the generation of dimers between substitutional and interstitial atoms occurs. The case of substitutional Mn and Zn atoms is added for comparison.