Mn-doped InAs self-organized diluted magnetic quantum-dot layers with Curie temperatures above 300K
Applied Physics Letters
The magnetic and structural properties of InAs:Mn self-organized diluted magnetic quantum dots grown by low-temperature ͑ϳ270°C͒, solid-source molecular-beam epitaxy using a very low InAs growth rate ͑Ͻ0.1 ML/ s͒ are investigated. A Curie temperature ͑T C ͒ of ϳ350 K is measured in a sample grown with a Mn/ In flux ratio of 0.15. Electron energy-loss spectroscopy confirms that most of the Mn remains within the InAs quantum dots. We propose as a possible explanation for this high T C the effects
... of magnetic and structural disorder introduced by a random incorporation and inhomogeneous distribution of Mn atoms amongst the InAs quantum dots. Among III-V diluted magnetic semiconductors (DMSs), (Ga,Mn)As and its heterostructures have elicited the greatest amount of interest for their potential application in semiconductor spintronic devices. Recent investigations have shown that the ferromagnetic transition, or Curie, temperature ͑T C ͒ may be raised to 150 K in thin ͑15 nm͒ (Ga,Mn)As epilayers, 1 160 K in (Ga,Mn)As trilayer structures, 2 and 172 K in Mn ␦-doped GaAs/ p-AlGaAs triangular quantum wells 3 after low-temperature annealing. For electronic and optoelectronic spin injection devices, it is clearly desirable to achieve spin injection and device operation at room temperature, rendering (Ga,Mn)As unsuitable for such applications at the present time. In this letter, we report the growth of Mn-doped InAs self-organized quantum dots, or "diluted magnetic quantum dots" (DMQDs). The magnetic properties of the quantum-dot layers have been investigated by superconducting quantum interference device (SQUID) magnetometry. Atomic force microscopy (AFM), x-ray energy dispersive spectroscopy (XEDS), electron energy-loss spectroscopy (EELS), and transmission electron microscopy (TEM) were used to examine the microstructure of these materials. A possible explanation for the high T C observed is offered based on recent theoretical studies 4-7 of hole-mediated ferromagnetism, accounting for the effect of disorder. Several InAs:Mn quantum-dot multilayers of varying Mn concentration were grown by low-temperature molecular-beam epitaxy (LT-MBE) on semi-insulating (100)oriented GaAs substrates in a Varian GEN-II chamber. 8 All samples were grown using a relatively low As 4 : Ga beam equivalent pressure (BEP) ratio ͑ϳ16: 1͒, a very low InAs growth rate ͑ϳ0.07 ML/ s͒, and a growth temperature in the range of 250-280°C. A radiatively coupled thermocouple is used to monitor the temperature of the substrate, which is In mounted to a molybdenum block during growth. After depositing a 500-nm GaAs buffer layer at 610°C, the substrate was cooled to a temperature in the range of 250-280°C for the growth of a 100-nm LT-GaAs buffer layer followed by the InAs:Mn QD multilayers. After deposition of a 2.0-ML InAs wetting layer, the Mn effusion cell shutter was opened for the growth of 2.6 ML of InAs:Mn QDs, upon which the RHEED pattern become dim and then spotty. A 45-s growth interruption was allowed for dot formation before capping with 50-56 nm of undoped GaAs. Finally, a 30-nm GaAs capping layer was deposited following the growth of ten InAs:Mn QD layers. The heterostructure for a representative sample is shown in Fig. 1 . Initial optimization of the dot growth conditions was accomplished via AFM. The three-dimensional (3D) AFM image for 4.7 ML of InAs:Mn deposited on LT-GaAs using a very low InAs growth rate ͑0.05 ML/ s͒, a substrate temperature of 280°C, and a Mn/ In flux ratio of 0.05, indicates that the dots are well formed. The base length and height of the dots are 35-37 and 13-15 nm, respectively; the dot density a) Electronic mail: firstname.lastname@example.org FIG. 1. Schematic heterostructure of a typical InAs:Mn quantum-dot multilayer sample grown by LT-MBE.