Liang He, Lei Ma, and Frank Tsui Epitaxial Alloys of CoMnGe: from Highly Spin Polarized Metals to Magnetic Semiconductors Liang He, Lei Ma, and Frank Tsui Department of Physics and Astronomy University of North Carolina at Chapel Hill Yong. S. Chu Advanced Photon Source, Argonne National Laboratory Good morning. I am liang in Dr. Tsui’s group. I am going to talk about the growth properties and some magnetism and transport properties. Of these tow kinds of magnetic material. But mainly focus is growth part. The work is supported in part by NSF DMR-0108605

Motivation and Approach Ternary alloys of CoxMnyGe1-x-y have been predicted to contain many magnetic materials including half-metals that are compatible with semiconductor processing, spintronic and quantum computer applications Approach: Combinatorial synthesis and characterization are necessary In-situ measurements: real-time scanning RHEED, SQUID magnetometry and magnetooptic Kerr effect (MOKE) The ternary alloys of CoMnGe have been predicted to contain many magnetic materials. So, how can we do it. Because of there are 3 materials, so the composition and temperature are relatively complexly. And also the interesting phases as we will see later will only occur in a very narrow range of compostions and processing conditions. So we have to use our combinatorial synthesis to grow the continuous composition sample at a time. For the measurement we have in-situ real-time Rheed image, STM, SEMM. And out side of the chamber, we have squid magento meter and MOKE. Many components, e.g. composition, temperature, Interesting phases occur in narrow range of compositions and processing conditions

Combinatorial MBE Synthesis Motorized Sample Manipulator/ Heater Real time RHEED Imaging Scanning RHEED Gun Quartz Crystal Motorized Masks RGA Flux Monitor AA Monitors This is the image of our real MBE system. In side of the big chamber the sample is facing down. And all the sources are at the bottom of the chamber facing up. We use effusion cells and electron guns to heat all the sources. And there are the real-time scanning RHEED imaging system. Here is the 6-axes precision sample manipulator. With it we can control the sample position very precision. Here is the quartz crystal to read the flux of sources. And also we have tow sets of A-A monitors to monitor the real time flux. And there are STM and sample storage chamber and load lock. Pneumatic Shutters K-Cell E-Gun

MBE Growth and Analysis System Sample manipulator Features: 6-axes precision sample manipulator Sample temperature from -100 to 1500 °C Up to 8 controlled sources In-situ STM, AFM, and SEMM Liquid He magnet cryostat for in-situ MOKE and SEMM SPM Real-time scanning RHEED imaging Sample storage Loadlock Sources

Schematic Diagrams of Combinatorial Sample Growth 2D – Ternary Sample 1D – Binary Sample Flux ~ 2Å Mask Growth Direction Here are the diagrams of the samples shown how we grow . For are the 2D-spread of our CoMnGe ternary sample. We do multi layer wedge growth to realized the combinatory samples. And thins every lay is very thin, about 2A. They 3 material will mix at growth. We call them binary , because across the sample, Ge is constant. Ge substrate (111) or (100) Co Mn Ge

Magnetism and Structure of CoMnGe Grown Epitaxially on Ge (111) Grown at 350°C Annealed at 500°C Ge HCP FCC The X-ray Lattice map shows that the whole sample is grown epitaxialy. But of course the lattice changes with the composition. Here I want to mention that the right green area are FCC structure while the rest of the sample are HCP structure. So alone the black line from right to left ,the reciprocal lattice decreasing. The MOKE saturation Image shows that our sample has many magnetic features. The understand the magnetic properties of deferent part and to know the structure in detail, the following I going to present data alone this black line. The 1-D “binary” sample. HCP Co Mn Room Temperature Saturation MOKE Image X-ray Reciprocal Lattice Map

(CoxMn1-x)0.75 Ge0.25: Magnetic Metals Here is the data. The structure part: 1. The RHEED intensity show a double peak. The d-spacing is increasing from the right to left as I mentioned. And the crystal structure also changes. From FCC to HCP. The interesting thing is there are 3 HCP phase. The are corresponding to the deferent lattice constant. 2. The MOKE intentisy here are show a broad peak here . And the Tc tells us most of the magnetic part has a very high Tc. And the saturation moment is around 100 emu/cc. Composition x 1 Composition x

Structural Evolution of (CoxMn1-x)0.8Ge0.2 Grown on (111) Ge Then we will see how these interesting faseHere is the structural evolution data. First ,we can see that these two order structure correlate to the MOKE intensity precisely. The RHEED intensity initially is very high along the sample, then as we depositing, they go very low even the ordered part. and then they slowly comes back. And they form two order peaks. The d-spacing evolution also verified that the ordered phase are about 1.5-2% bigger than pure Ge. Composition x Composition x

Composition dependent Structure and Magnetism of (Co2/3Mn1/3)xGe1-x (x=0-15%) high low a b RHEED Patent Now I am going to talk about the magnetic semiconductors part. This is the MOKE saturation image of the ternary sample grown on Ge (100). Before is (111) substrate. We are mainly focus the upper small part. And we grow a binary sample along the doted line. From the RHEED scan, across the sample they are all epitaxially grown. But there is a 2D growth to 3d growth transition happens around 8%. And this is the RHEED patents of the 2D and 3D area. The Tc also changes along the conpositions. a: 2D b: 3D

Cross-sectional TEM of CoMn Doped Ge (100) (Co2Mn)0.05Ge0.95 100Å Ge substrate + buffer layer + sample The left one is lower doped Ge. 2D growth, very smooth surface. Right hand one is the higher doped sight, 3D growth, rough surface. This is doping unhomgenoius. But distribute homoegius .stacking falt, interstidal doping. Not diferent Crystal, But defect , or High epitaxial quality.. Atomically smooth surface Large scan, good interface quality.But the doping is inhomegenious.but informally dispersed. FFT FFT film Substrate Ge 200Å 200Å Dr. Chikyow et al., NIMS, Japan

Confirming High Crytalline Quality X-ray Bragg Scan of CoMn Doped Ge (100) Confirming High Crytalline Quality dfilm/dGe=0.99965(5) x = 1% Film under slight compressive strain. log (Int) dfilm/dGe =1.0011 x = 4% The red line is the simulation of the 205 unitcells of thin-film over the unperturbed Ge(001) substrate. 205 unitcell = 5.431*205 =1086 angstrom thick! confirming Film under slight tensile strain. x n q

Semiconducting Transport Properties Semiconducting and can be changed gradually by doping CoMn into Ge Transport & Magnetism please come to talk by Ma et al. on Thursday, Room 16A, session X24.005

Summary A range of ferromagnetic alloys of CoxMnyGe1-x-y with TC > 300 K can be grown coherently on Ge by MBE Magnetic ordering shows strong correlation with structural ordering A new class of magnetic semiconductors with high TC has been discovered, which is compatible with current semiconductor processing technology