Making Semiconductors Ferromagnetic: Opportunities and Challenges

Transcription

1 Making Semiconductors Ferromagnetic: Opportunities and Challenges J.K. Furdyna University of Notre Dame Collaborators: X. Liu and M. Dobrowolska, University of Notre Dame T. Wojtowicz, Institute of Physics, Polish Academy of Sciences W. Walukiewicz and K.M. Yu, Lawrence Berkeley Nat l Lab University of Virginia, June 19-23, 2006

2 Ferromagnetic (FM) Semiconductors: Outline Why make semiconductors ferromagnetic? Properties of III 1-x Mn x V alloys Mn interstitials: their role and detection Effect of the Fermi energy on the growth of III 1-x Mn x V alloys Future directions FM semiconductor devices

3 Motivation: semiconductor-based spintronics spintronics Today s electronics and data processing uses semiconductor chips that take advantage of the electrical charge of electrons. Recently a new field of semiconductor technology began to emerge, with the hope of using electron spin in addition to its charge in semiconductor devices ( spintronics spintronics ) ). This could combine many functionalities (information storage, logic and data processing, communications) in a single chip. Ultimate goal: quantum computation via spin entanglement

4 Examples of ferromagnetic semiconductors Ga 1-x Mn x As (T c ~170K) In 1-x Mn x As (Tc ~ 60K) Ga 1-x Mn x Sb (Tc ~ 30K) In 1-x Mn x Sb (Tc ~12K) Note: Mn goes into the III-V lattice as divalent Mn ++ substitutionally for the Group-III element. In this situation it is both a magnetic moment and an acceptor.

5 How to make III-Mn Mn-V V ferromagnetic? Ruderman-Kittel Kittel-Kasuya-Yosida (RKKY) interaction carrier mediated can be ferromagnetic Requirements: Large Mn conc. x very high p II-Mn-VIs difficult to dope III-Mn-Vs easier, since Mn also provides holes Ga 1-x Mn x As T C =170K

6 In practice, high Curie temperature T C will be needed for many applications; Mean field theory predicts: C Mn 2 * 1/3 T = CN β m p N Mn : concentration of uncompensated Mn spins; β : coupling constant between localized Mn spins and the free holes (p-d coupling), m* : effective mass of the holes, p : free hole concentration.

7 The x and p dependence of T C in GaMnAs F. Matsukura et al., PRB 57, R2037 (1998) K. C. Ku et al., APL 82, 2302 (2003).

8 H. Ohno et al., Nature 2000 Carrier-controlled ferromagnetism in In 1-x Mn x As

9 Fabrication

10 MBE Facility at Notre Dame Typical growth temperature for Ga 1-x Mn x As: 260 o C

11 Structures used in our studies Growth method: Low Temperature (LT) MBE nm Ga 1-x Mn x As; Ga 1-y Be y As; Ga 1-y-z Be y Al z As or Ga 1-x-y Mn x Be y As or Ga 1-x Mn x As/Ga 1-z Al z As:Be T s ~ ºC ~3nm LT GaAs T s ~ 265 ºC 230 nm In 1-x Mn x Sb T s 160 ºC 100 nm LT-InSb T s 210 ºC 2 nd MBE process ~450nm GaAs buffer T s = 590 ºC (100) GaAs substrate ~4.5 µm CdTe buffer (100) GaAs substrate 1 st MBE process (Poland)

12 Ion Implantation & Pulsed Laser Melting (II-PLM) Ions excimer laser pulse damaged Ga 1-x Mnlayer x As Damaged layer Liquid Regrown Layer substrate Substrate Substrate Substrate Ion Implantation kinetic process exceed equilibrium solubility crystal damaged (annealing necessary) Pulsed Laser Melting ultra-fast solidification non-thermal equilibrium growth can model PLM & Mn incorporation

13 Pulsed Laser Melting (PLM) excimer laser pulse Ga 1-x Mn x As SIMS As implanted 50 kev 2x10 16 /cm 2 Regrown Damaged LiquidLayer layer Liquid Substrate Melt Front Equilibrium Solubility Melt Depth 0.2 J/cm 2 1.1x10 16 /cm 2 retained ultra-fast solidification m/s front velocity solute trapping can model PLM & Mn incorporation PLM: repair lattice & maintain supersaturation

14 Magnetic Anisotropy

15 Magnetic Anisotropy and Strain an Overview Perpendicular Uniaxial Anisotropy H M GaMnAs GaAs H M GaMnAs GaInAs

16 Origin of magnetic anisotropy in III-Mn-V: strain induced uniaxial anisotropy Biaxial compressive tensile strain strain induces uniaxial induces anisotropy: uniaxial anisotropy: Easy axis is Easy typically axis is out typically of plane in plane E F e hh lh ANISOTROPY FIELD [T] GaMnAs / GaAs GaMnAs / GaInAs compressive tensile M s M s 3.5x10 20 cm x10 20 cm BIAXIAL STRAIN ε xx [%] Dietl, Ohno, Matsukura, PRB 2001

17 Magnetic Domain Structure

18 Method for mapping domains in III-Mn Mn-V V alloys * * Collaboration with Ulrich Welp and V. K. Vlasko-Vlasov at Argonne National Laboratory

19 Magneto-optical optical images of domain boundaries in Ga 1-x Mn x As U. Welp et al. PRL (2003). H [100], T=15 K (left) and 35 K (right); each square is 1 mm 1 mm

20 Planar Hall Effect (PHE) in GaMnAsSb alloy (a) H 1 H 2 (b) [010] [-110] [-100] [110] 5 4 M H [100] ϕ=88 o [1-10] [0-10] I koe [-1-10] (c)

21 Attempts to increase T C by doping to increase the hole concentration and the role of Mn ions in Interstitial lattice positions

22 Temperature dependence of resistivity of Ga 1-x-y Mn x Be y As with x=0.055 T C =61K T C =42K T Be =0C 11011cte 11010ete 11011bte 11011ate 11010ate 11010dte Log 10 (ρ(0) [Ωcm]) 10-2 T C =30K T C =24K T C =19K T Be =1010C T Be =1017C T Be =1025C Does T C decrease with Be co-doping? T C <11K T Be =1038C T Be =1045C Temperature [K]

23 Role of lattice location of Mn in III 1-x Mn x V alloys Substitutional Mn Interstitial Mn Random clusters not commensurate Mn III Mn Ga Mn III acceptor Mn Ga with E act =110 mev III (Ga, In) V (As, Sb) Mn Mn I double donor (passivates 2 Mn III acceptors) Electrically inactive Mn-related small clusters (and possible MnV precipitates) No direct measurement of Mn location in III 1-x Mn x V

24 Lattice location determined by angular scan of c-rbs/pixec <111> <111> 2 MeV 4 He + <110> RBS <110> Combined channeling Rutherford backscattering (RBS) and particle induced x ray emission (PIXE) PIXE

25 Ga As (R BS) Mn (P IXE) y~0. 01<110> T =30K C PIXE/RBS angular scans for Ga 1-x-y Mn x Be y As y=0 <110> T C =61K y~0.01 <110> T C =30K y~0.05 <110> T C <5K y~0.11 <110> 0.8 normalized yield, χ <111> GaAs (RBS) Mn (PIXE) <111> <111> <111> Huge increase of Mn I with Be content tilt angle (degree)

26 Correlation of Mn location, p and T C in Ga 1-x-y Mn x Be y As [Mn I ] and random Mn-related clusters increase with Be content. Mn I are unstable and form random clusters after LT-annealing Hole concentration relatively constant at ~5x10 20 cm -3

27 Angular scans for Ga 1-x Mn x As Ga 1-x Mn x As x=0.02 <110> Ga 1-x Mn x As x=0.08 <110> GaAs (RBS) Mn (PIXE) 1.0 <111> <111> tilt angle ( o ) tilt angle (degree) Random fraction Mn Ran (clusters) typically 0.05 [Mn I ] up to ~ for high x (>0.05) Attempts to correlate T C with x not meaningful XRD cannot accurately determine x

28 Low Temperature annealing

29 LT-annealing: resistivity and magnetization T C increases for optimal annealing Magnetization (emu/cm 3 ) Magnetization (emu/cm 3 ) T=5 K Magnetic field (Gs) 5 as-grown in plane field annealed H=10 Gs Temperature (K) resistivity decreases saturation magnetization M S increases when the samples are annealed at around 280 C (indicates that the concentration of magnetically active active Mn ions increases).

30 LT-annealing: Mn location, p and T C LT-annealing (280ºC) breaks antiferromagnetically coupled Mn I - Mn Ga pairs Mn clusters, increases p = 1x10 21 cm -3 ([Mn Ga ]-2[Mn I ]) [Mn Ga ] active spins higher T C =110K 350ºC annealing drives the system towards equilibrium random Mn precipitates and/or clusters from Mn Ga Changes observed are unlikely due to As Ga defects (stable up to 450ºC)

31 Correlation of Mn location, p and T C in Ga 1-x-y Mn x Be y As [Mn I ] and random Mn-related clusters increase with Be content. Mn I are unstable and form random clusters after LT-annealing Hole concentration relatively constant at ~5x10 20 cm -3

32 Role of interstitials in limiting (reducing) T C Electrical compensation by interstitial Mn I leads to a reduction of the hole concentration (and T C ~ p 1/3 ) Mn I donors tend to drift toward Mn III to form Mn III -Mn I pairs in III 1-x Mn x V alloys Such Mn III -Mn I pairs then couple antiferromagnetically

33 Direct evidence of Fermi-energy energy- dependent formation of Mn interstitials in Ga 1-x Mn x As: Studies of modulation doped Ga 1-x Mn x As/Ga 1-y Al y As:Be heterostructures

34 GaMnAs Quantum Well Geometry 50 ml AlGaAs:Be 20 ml (Ga,Mn)As 100-nm GaAs Buffer (001) GaAs Substrate

35 Log 10 (ρ(0) (Ωcm)) Temperature dependence of resistivity of MD Ga 1-x Mn x As/Ga 1-y Al y As:Be heterostructures vs. thickness of doped region (d( Be ) vs. Be doping level (T( Be ) 1.3x x x x10-3 8x10-3 7x10-3 T ρ =77 K d Be =0 nm T ρ =97 K d Be =5.3 nm x=0.062 d GaMnAs =5.6 nm T Be =1040 o C d Be T ρ =99 K d Be =5.3 nm T ρ =110 K d Be =13.2 nm 6x Temperature (K) Log 10 (ρ(0) (Ωcm)) x10-3 8x10-3 7x10-3 6x10-3 5x10-3 4x10-3 T ρ =72 K T Be =0 o C T ρ =95 K T Be =1020 o C T ρ =73 K T Be =0 o C T ρ =110 K T Be =1050 o C T Be x=0.066 d GaMnAs =5.6 nm d Be =13.3 nm Temperature (K) layer

36 Temperature dependence of remanent magnetization of MD Ga 1-x Mn x As/Ga 1-y Al y As:Be heterostructures growth direction GaMnAs increase of T C in qualitative agreement with self-consistent calculations (Vurgaftman and Meyer) v.b. GaAlAs Magnetization (emu/cm 3 ) In-plane field B = 100 Gs T C =98 K T C =80 K T C =62 K Resistivity (10 3 Ωcm) T ρ =68 K T ρ =87 K undoped first barrier T ρ =110 K 10 second barrier Temperature (K) second barrier doped undoped first barrier doped Temperature (K) v.b. growth direction GaAlAs GaMnAs decrease of T C consistent with strong dependence of Mn I formation on Fermi energy during growth

37 Conclusions Directly identified Mn I in Ga 1-x Mn x As and In 1-x Mn x Sb thin films Mn I are responsible for creation of AF pairs and for the limiting the Curie temperature in ferromagnetic Ga 1-x Mn x As and In 1-x Mn x Sb LT- annealing drives Mn I to random clusters increase of p, of active spins, and of T C Incorporation of nonmagnetic acceptors increases concentration of Mn I and decreases T C. Counter doping with donors as a method to enhance T C. Modulation doping first experimental results that confirms our model of Fermi-energy dependent creation of Mn I

38 Bottom line Incorporation of Mn Ga (and Mn In ) and creation of Mn I results from the value of E F during the growth! This is what limits T C

39 Devices

40 Giant domain wall magnetoresistance in planar devices with nano-constrictions C. Rüester et al., Phys. Rev. Lett. 91, (2003).

41 Giant Planar Hall Effect in Epitaxial (Ga,Mn)As Devices H.X. Tang et al., PRL 90, (2003)

42 Electrical Manipulation of Magnetization Reversal in a Ferromagnetic Semiconductor D. Chiba, M. Yamanouchi, F. Matsukura, H. Ohno, Science 301, 943 (2003)

43 Current-driven magnetization reversal in a ferromagnetic semiconductor based (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction D. Chiba, Y. Sato, T. Kita, F. Matsukura, and H. Ohno, PRL 93, (2004)

44 Current-induced domain-wall switching in a ferromagnetic semiconductor structure M. Yamanouch, D. Chiba, F. Matsukura and H. Ohno, Nature 428, 539 (2004),

45 Magneto-transport devices on vicinal GaMnAs (a) z [001] [110] H θ α x I [110] 1.0 T c ~ 60K (b) [001] α [110] [110] I r xx (T)/r xx (T c ) o 2 o 4 o 5 o GaMnAs Tem perature (K) GaAs

46

47 P b a M close to +H 8 a [010] [110] R xy (Ω) 0 e d α = 5 M o [010] +I [100] -50 b -100 c H (kg) Q H = 0 +I [010] e +I _ [010] M close to -H d M o [010] -H +I [100] c -H +I M o [100]

48 Ω dr /DR AHE (a) (b) 5 o 4 o 2 o sin(a)/ 2 6 dr /DR PHE o 4 o 5 o 4 2 DR AHE /DR PHE a (deg) 0

49 Ω Ω

50 Major challenge: Need for strategies to increase T C Counter doping with donors as a method to enhance T C. Modulation doping first experimental results that confirms our model of Fermi-energy dependent creation of Mn I