# Presentation: 843950

## Subsurfactant Epitaxy:a new route toward high temperature ferromagnetism in semiconductors

- Based on first-principles total energy calculations within density functional theory, a conceptually new approach, Subsurfactant Epitaxy, has been proposed to dope Mn into Ge, taking advantage of the energetic and kinetic characteristics of Mn at the growth front of Ge(100). Experimental confirmation of the prediction has resulted in of Ge(1-x)Mn(x) dilute magnetic semiconductors with magnetic ordering temperatures well over room temperature, at much lower Mn concentration.

*Prediction: W. G. Zhu**1,2**, H. H. Weitering**3,1**, E. G. Wang**2**, E. Kaxiras**4**, & Z. Y. Zhang**1,3**, PRL. ****93****, 126102 (2004).**Confirmation: C. G. Zeng**3**, Z. Y. Zhang**1,3**, K. v. Benthem**1**, M. F. Chisholm**1**, & H. H. Weitering**3,1**, PRL ****100****, 066101 (2008)*
- 1Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
- 2International Center for Quantum Structures and Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People's Republic of China
- 3Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996, USA
- 4Department of Physics and Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA* *

## Subsurfactant Epitaxy:a new route toward high temperature ferromagnetism in semiconductors

**Subsurfactant Epitaxy:*****a new route toward high temperature ferromagnetism in semiconductors***
*Prediction: W. G. Zhu, H. H. Weitering, E. G. Wang, E. Kaxiras, & Z. Y. Zhang, PRL. ****93****, 126102 (2004).*
*Confirmation: C. G. Zeng, Z. Y. Zhang, K. v. Benthem, M. F. Chisholm, & H. H. Weitering, PRL ****100****, 066101 (2008)*

**Subsurfactant epitaxy (theory)**

**0.2%**

**Magnetic measurements as a function of temperature (left) and magnetic field (right)**

- Subsurfactant epitaxy  a new avenue toward superior dopant control in dilute magnetic semiconductors.
- Achieving Tc > RT  alleviating one major technical bottle-neck in spintronic applications
- Subsurfactant epitaxy  stimulus to reassess existing theoretical models on dilute magnetic semiconductors

**Notes:**
Background: Dilute magnetic semiconductors (DMS) with high ferromagnetic ordering temperatures (TC) have vast potential for advancing spin-based electronics or “spintronics”. To date, achieving high-TC DMS typically required doping levels of order 5%. Such high doping levels inevitably compromise the structural homogeneity and carrier mobility of the DMS. 
Aim of the Project: Achieve optimal control of the magnetic dopant distribution in the DMS so as to create a better device material for spintronic applications.
Results: Here, we establish “subsurfactant epitaxy” as a novel kinetic pathway for synthesizing Mn-doped germanium with TC much higher than room temperature, at dramatically reduced doping levels. This is accomplished by optimal control of the diffusion kinetics of the dopant atoms near the growth front in two separate deposition steps. These steps have been visualized in an animation movie. The first step involves a submonolayer dose of Mn on Ge(100) at low temperature; this step populates subsurface interstitial sites with Mn while suppressing lateral Mn diffusion and clustering. The second step involves epitaxial growth of Ge at elevated temperature, taking advantage of the strong floating ability of the interstitial Mn dopants towards the newly defined subsurface sites at the growth front. Most remarkably, the Mn dopants trapped inside the film are uniformly distributed at substitutional sites, and the resulting film exhibits ferromagnetism above 400 K at the nominal doping level of only 0.2%. Subsurfactant epitaxy thus emerges as a new avenue toward superior dopant control in dilute magnetic semiconductors and potentially other semiconductor applications that require doping levels far above the thermodynamic solubility limits. 
Significance of this work: This discovery is expected to have due impacts in two forefront areas of the chemical and physical sciences. First, our discovery of “subsurfactant epitaxy” as a novel kinetic pathway toward the synthesis of non-equilibrium structures and materials is of broad fundamental- and paramount practical interest, particularly within the context of doping functional materials. Secondly, we have successfully applied this conceptual advance to alleviate a major bottleneck problem in spintronics, namely how to fabricate dilute magnetic semiconductors with ferromagnetic ordering temperatures that exceed room temperature by a comfortable margin while minimizing the structural disorder and inhomogeneities that are intrinsic to high doping levels. Here, we demonstrate that controlled doping via subsurfactant epitaxy produces ferromagnetic ordering temperatures well above room temperature at doping levels that are at least 20 times lower than those typically invoked in other fabrication techniques. This furthermore leads to superior structure-property relationships, and confirmed by a variety of physical property measurements. Subsurfactant epitaxy, as established here, thus stands as a new and powerful avenue toward superior dopant control in dilute magnetic semiconductors and potentially other semiconductor applications that require doping levels far above the thermodynamic solubility limit. Finally, this work also represents a major stimulus to reassess most of the existing theoretical models on dilute magnetic semiconductors: how could it be that such dilute doping levels produce such high Tc’s?

## Animation of Subsurfactant Epitaxy