A Study of the Group-IV Diluted Magnetic Semiconductor GeMn. Melissa Commisso Dolph Stu Wolf Group December 4 th, 2008
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1 A Study of the Group-IV Diluted Magnetic Semiconductor GeMn Melissa Commisso Dolph Stu Wolf Group December 4 th, 2008
2 Outline Motivation and Background Our DMS : GeMn Hall Bars Summary / Future Work
3 Motivation Moore s Law reaching its limits Size of circuit elements scaling into a regime where leakage impedes functionality Search for successor to conventional CMOS e- spin as state variable to develop new technologies that can be incorporated with or replace CMOS Benefit of lower power dissipation Our goal is to create a spin based logic memory device using magnetic quantum dots MQDs ( ) We will be studying the diluted magnetic semiconductor GeMn Understand the magnetism Control the magnetism Mn implantation Electrically / Left gate Left QD Right QD Right gate Center gate Tunnel barrier Conducting layer Magnetic Exchange Switch with 2 MnGe QDs
4 Background Concepts for a MQD Device: Quantum-dot Cellular Automata ( QCA) QCA: uniform grid of cells, simplest model has 4 QDs arranged in a square e-s can tunnel into QDs energy states, and arrange themselves into lowest QDs respond to charge states of neighbors writing
5 QCA Shift from 0 1 :, Read-out Quantum Point Contact Single Electron Transistor :, : - QPC changes in conductance SET changes in capacitance due to single e
6 Magnetic QCA ( MQCA) Magnetic QDs ( MQDs): control carrier spin and interactions between carriers Gated control of MQDs Carrier concentration and state Coupling b / w dots Lowest energy, Pauli exclusion
7 MQCA B I I B Read-out : Magnetic Tunnel Juntion Free layer magnetized by external field Applying small bias voltage creates tunneling current thru dielectric, Relative size of current gives state of fixed layer (larger current smaller MR parallel orientations) Switch direction of free layer remeasure current FM (free) layer Tunneling dielec. FM (fixed) layer
8 Magnetic Spin Switch Electrically controlled interactions between MQDs Blue features are QDs + are holes ( ) Arrows are Mn spins Side gates -- carrier concentration and adjacent spin coupling. - Ferromagnetic vs anti ferromagnetic behavior Top gates -- read-out MTJ between top gate and QD yield MR which gives spin orientation
9 What is a Diluted Magnetic Semiconductor? (,, ) (,, ) Portion of semiconductor atoms Si Ge GaAs replaced with magnetic atoms Mn Fe Co Mn atoms introduce Spins Holes ( mediate coupling b/ w spins) Ferromagnetic behavior
10 DMS Magnetic Interactions Ferromagnetic behavior between nearby Mn Long range ferromagnetic ( FM) behavior dominates short range anti- FM behavior Zener double exchange Mn spin couples, to nearby hole which is also coupled to another Mn spin ( not nearest neighbor)
11 The DMS GeMn Hole mediated FM exchange Temp dependence of resistivity has semiconducting characteristics T Curie linearly ( K) with Mn concentration ( %) Success in electrical modification of FM in other materials, but T Curie too low Example: Group III-V GaMnAs T ~ K, Curie Ge shows promise Higher hole mobility than GaAs and Si +/- 0.5 V gates control hole density compatible with CMOS technology Zener model shows high T Curie Computed T Curie for various p-type semiconductors containing 5% Mn and 3.5 x holes/cm 3 Reference: Park et al., Science 295, (2002) Dietl et al., Science 287, 1019 (2000)
12 GeMn Magnetism Forms of magnetism: ferro- para, -, superpara- magnetic Moment vs. Field: Moment vs. Temperature: µb/mn AsImplanted 300C1min 400C1min 500C1min T/K Ferromag signal - Mn concentration - anneal conditions Saturation moment Coercive field T Curie Transition temps Work done by Wenjing Yin Moment/emu Moment/emu 1.2x x x x x x x x x x x10-6 ZFC 5K 10K 15K 25K 50K 150K H/Oe 300C 1min FC 300C 1min ZFC H = 100 Oe Moment T/K Virgin curves (0 1T) show evidence of shift from FM to SPM ZFC H (Oe) (a) Field cooled (FC) and Zero-Field Cooled (ZFC) curves diverge at T Curie (a) Deng-Lu et al, Chinese Physics, 2002
13 GeMn Magnetism Picture courtesy of Wenjing Yin a) b) TEM images: a) Bougeard et. al. PRL, 97, (2006) b) Li He, Robert Hull (UVA / RPI)
14 Outline Motivation and Background Our DMS : GeMn Hall Bars Summary / Future Work
15 GeMn - Doping Si, Ge atoms have 4 valence e- s Nearest neighbors form covalent pairs Extra e- donated to conduction band Accepts e- from valence band Doped n-type ( Phosphorus) and p-type Boron ( ) Boron 4x10 18 /cm 3
16 GeMn Mn Implantation Ion implantation is the method used to introduce Mn into the Ge lattice Old Profile Multiple implantations yield more uniform distribution Implantations done at low temp (75 C) to avoid formation of secondary phases New Profile
17 GeMn Rapid Thermal Anneal Mn implantation damages crystallinity of Ge film Mn tends to cluster Rapid Thermal Anneal ( RTA ) Recovers implantation damage Redistributes Mn Annealing temp affects saturation moment and T Curie Plot courtesy of Wenjing Yin
18 GeMn Pulsed Laser Melting Film becomes damaged after implantation, (~100 ) Laser melts film which quickly ns solidifies as crystalline material Helps avoid clustering and secondary phases Shows better recrystalization than RTA Picture from Prof. Michael Aziz s website XeCl (308 nm) 30 ns Prof. Aziz s lab, Harvard
19 Pulsed Laser Melting Energy (reflected) melt reached SiO2 Energy (reflected) melt ends in Ge XeCl pulse Melt needs to end in crystalline Ge region so that there is a seed layer for Ge to recrystallize Melt depth determined by beam fluence ( energy/ spot size) A-Ge C-Ge SiO Time (ns) Si
20 PLM : X-Ray Diffraction n λ=d sinθ Si (400) Ge (400) # 4 # 5 # 1 # 3 # 2 MnGe (AI) GOI GOI: unimplanted, no melt MnGe: As implanted, no melt #1: UHC 475, melt stops furthest from SiO2 interface #2: UHC 440 #3: UHC 405 #4: UHC 390 #5: UHC 375, melt stops closest to interface
21 PLM : Comparison of Results GOI As Implanted RTA PLM Virgin GOI 1.1%Mn, As Implanted 1.1%Mn, RTA 400C 1 min 0.67%Mn, Laser anneal Intensity (a.u.) 400 Ge Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) 400 Ge theta theta theta theta % Mn, As Implanted % Mn, 300C 1min % Mn, Laser anneal µ B /Mn µ B /Mn µ B / Mn H (Oe) H (Oe) H (Oe) (M vs. H data at 5K)
22 Outline Motivation and Background Our DMS : GeMn Hall Bars Summary / Future Work
23 Hall Effect Charge build up on 1 side creates potential difference Comprised of ordinary and anomalous (, ) Hall effects OHE AHE V Hall αb + βm (α, β OHE AHE are constants dependant on film dimensions, ( / ), sheet resistance resistivity thickness and applied current) AHE can be extrapolated Proportional to magnetization Hall potential gives additional info: Sign of Hall potential n- or p- type semiconductor R Hall = V Hall A / IlB = 1 / nq n = carrier concentration ( cm-3 )
24 Hall Bar Fabrication Start with GeMn thin film Spin on photoresist Chemical, reacts with UV light to become soluble insoluble / Expose using mask Develop Etch exposed Ge Anneal Pattern contacts Evaporate metals ; Remove PR Typical dimensions: Length Width ( left to right ) ~1200 µm ( top to bottom ): 50, 100, 500 µm separation: 50, 100, 150 µm Contact separation
25 Measurement AC Transport 5 point contact, ( Hall ), / + + If voltage leads are not perfectly to bias field measured voltage has additional components besides V Without B field potentiometer b w Va and Vb nulls offset voltage due to sample resistance OHE dominates at high field: Can be subtracted out 0.67% Mn, annealed at 300 C for 1 min Raw V Hall Normalized resistivity 1.0 Magnetic moment Anomalous Hall Resistivity 5K µb/mn V_Hall Raw (V) H (Oe) H (Oe)
26 Electrical Control of FM How do electric fields control the FM? Voltage applied across MOS structure modifies charge distribution in semiconductor P- type semiconductor: V > 0 creates depletion layer by forcing (+) holes away from insulator/ semiconductor interface creating carrier-free region V < 0 attracts holes to interface, Carrier mediated FM so strength of FM dependant upon carrier concentration V Gate = 0 V Gate > 0 V Gate < 0
27 Electrical Control of FM : Gated Hall Bars Work done at UVA, CNS: PECVD : deposit 300nm Si 3 N 4 Lithography/RIE : Pattern/etch holes to Hall bar contacts Lithography : Pattern top contacts and V gate e- beam evaporator : Deposit Ti/Au Gating QDs: Electron Beam Lithography
28 Outline Motivation and Background Our DMS : GeMn Hall Bars Summary / Future Work
29 Magnetic Quantum Dots in GeMn Grow Ge QDs ( Jerry Floro group) Use FIB to implant QDs with Mn Gate QDs Use electric fields to modify magnetism through control of carrier concentration in the QD Add side gates control interactions between neighboring QDs Left gate Left QD Right QD Right gate Center gate Tunnel barrier Conducting layer Magnetic Exchange Switch with 2 MnGe QDs
30 Summary We hope to use the DMS GeMn to create a device which can be used in future / logic memory applications Understanding the magnetism in GeMn Effects of Mn concentration AHE data mimics M vs PLM:., annealing on magnetism H data Suspect Mn diffusion to surface, leaving behind some substitutional Mn, ( ) More crystalline less Mn clusters clusters may give strongest FM signal Electrical modification of the magnetism in GeMn Preliminary results do show effect on magnetism due to Vgate Future Work Gated Hall bars PLM with different Mn concentrations, post-anneal conditions Hall effect in MBE grown samples
31 THANKS!
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34 QCA Logic Majority Gate AND : 1 input fixed as 0 Both remaining inputs must be 1 in order to get output of 1 OR : 1 input fixed as 1 Either of the other inputs must be a 1 in order to get output of 1 2 Inverter Only needs 1 input
35 Doping a Semiconductor Si atom has 4 valence e- s Nearest neighbors help form covalent pairs 5 - s P atom has valence e Extra e- donated band n-type to conduction Extra e- donated to conduction band 3 - s B atom has valence e Can accept an e- from valence band, leaving behind a hole p-type Accepts e- from valence band
36 Physics of Magnetism 101 Three types of magnetic interactions Direct exchange Superexchange Indirect exchange Wavefunctions overlap Wavefunctions interact via Wavefunctions interact via directly nonmagnetic neighbors conduction electrons Electrons on different atoms Antiparallel alignment favored (e.g. H 2 molecule) Electrons on different atoms Antiparallel alignment favored (e.g. MnO, MnGe) Electrons on different atoms Parallel or antiparallel alignment favored (RKKY) Electrons on same atom Parallel alignment favored (Hund's First Rule) Ferromagnetic semiconductors Wavefunctions interact via valence holes Parallel alignment favored
37 MQCA B I I B B I Read-out : Magnetic Tunnel Juntion Free layer magnetized by external field Applying small bias voltage creates tunneling current thru dielectric, Relative size of current gives state of fixed layer (larger current parallel orientations) Switch direction of free layer remeasure current FM (free) layer Tunneling dielec. FM (fixed) layer
38 Hall measurement data analysis procedure Resistance(Ohms) K 30K 50K 100K 150K 200K 250K 300K Field(T) ρ ( H ) = R ( H ) µ H + R ( H µ M Hall 0 0 s ) 0
39 DMS format below into bullets, add to previous slide These materials exhibit a spin-dependent coupling between localized states and those in the valence and conduction bands. It is due to this coupling that the holes in dilute magnetic semiconductors mediate a ferromagnetic interaction between the localized spins. This property means that it is possible to control magnetization by an electric field, which affects the carrier density in semiconductors. ( ) These mixed crystals semiconductor alloys may be considered as containing two interacting subsystems. The first of these is the system of delocalised conduction and valence band electrons. The second is the random, diluted system of localised magnetic moments associated with the magnetic atoms. The fact that both the structure and the electronic properties of the host crystals are well known means that they are perfect for studying the basic mechanisms of the magnetic ineractions coupling the spins of the band cariers and the localised spins of magnetic ions The. coupling between the localised moments results in the existence of different magnetic phases ( such as paramagnets, spin glasses and antiferromagnets).
40 Questions (11) How does a spin couple to a hole? - hole has opposite spin as e- (11) RKKY model? not needed (13)? (14) 3? Coercive field Magnetic field in first pics no (25)? AMR - remove (27) $ (3 ) explanation of AMR rd bullet remove
41 Possible scenarios: More crystalline, more Mn is diffused out thru surface More crystalline, less Mn clusters ( clusters may give strongest FM signal) Continue experiments with bigger pieces more Mn implanted, different post-anneal conditions,
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