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Register a new userHighly enhanced Curie temperatures in low temperature annealed (Ga,Mn)As epilayers
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We report Curie temperatures up to 150 K in annealed Ga1-xMnxAs epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) show that the higher ferromagnetic transition temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material - suggesting that the free surface of Ga1-xMnxAs epilayers is important in determining their physical properties.
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We study the effects of growth temperature, Ga:As ratio and post-growth annealing procedure on the Curie temperature, Tc, of (Ga,Mn)As layers grown by molecular beam epitaxy. We achieve the highest Tc values for growth temperatures very close to the 2D-3D phase boundary. The increase in Tc, due to the removal of interstitial Mn by post growth annealing, is counteracted by a second process which reduces Tc and which is more effective at higher annealing temperatures. Our results show that it is necessary to optimize the growth parameters and post growth annealing procedure to obtain the highest Tc.
The effect of microscopic Mn cluster distribution on the Curie temperature (Tc) is studied using density-functional calculations. We find that the calculated Tc depends crucially on the microscopic cluster distribution, which can explain the abnormally large variations in experimental Tc values from a few K to well above room temperature. The partially dimerized Mn_2-Mn_1 distribution is found to give the highest Tc > 500 K, and in general, the presence of the Mn_2 dimer has a tendency to enhance Tc. The lowest Tc values close to zero are obtained for the Mn_4-Mn_1 and Mn_4-Mn_3 distributions.
(Ga,Mn)As and related diluted magnetic semiconductors play a major role in spintronics research because of their potential to combine ferromagnetism and semiconducting properties in one physical system. Ferromagnetism requires ~1-10% of substitutional Mn_Ga. Unintentional defects formed during growth at these high dopings significantly suppress the Curie temperature. We present experiments in which by etching the (Ga,Mn)As surface oxide we achieve a dramatic reduction of annealing times necessary to optimize the ferromagnetic film after growth, and report Curie temperature of 180 K at approximately 8% of Mn_Ga. Our study elucidates the mechanism controlling the removal of the most detrimental, interstitial Mn defect. The limits and utility of electrical gating of the highly-doped (Ga,Mn)As semiconductor are not yet established; so far electric-field effects have been demonstrated on magnetization with tens of Volts applied on a top-gate, field effect transistor structure. In the second part of the paper we present a back-gate, n-GaAs/AlAs/GaMnAs transistor operating at a few Volts. Inspired by the etching study of (Ga,Mn)As films we apply the oxide-etching/re-oxidation procedure to reduce the thickness (arial density of carriers) of the (Ga,Mn)As and observe a large enhancement of the gating efficiency. We report gatable spintronic characteristics on a series of anisotropic magnetoresistance measurements.
We investigate the relationship between the Curie temperature TC and the carrier density p in the ferromagnetic semiconductor (Ga,Mn)As. Carrier densities are extracted from analysis of the Hall resistance at low temperatures and high magnetic fields. Results are found to be consistent with ion channeling measurements when performed on the same samples. We find that both TC and the electrical conductivity increase monotonically with increasing p, and take their largest values when p is comparable to the concentration of substitutional Mn acceptors. This is inconsistent with models in which the Fermi level is located within a narrow isolated impurity band.
We examine the Mn concentration dependence of the electronic and magnetic properties of optimally annealed Ga1-xMnxAs epilayers for 1.35% < x < 8.3%. The Curie temperature (Tc), conductivity, and exchange energy increase with Mn concentration up to x ~ 0.05, but are almost constant for larger x, with Tc ~ 110 K. The ferromagnetic moment per Mn ion decreases monotonically with increasing x, implying that an increasing fraction of the Mn spins do not participate in the ferromagnetism. By contrast, the derived domain wall thickness, an important parameter for device design, remains surprisingly constant.
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