Spin injection and dynamics in p-doped Germanium
F. Rortais1, P. Laczkowski2, S. Oyarzun1, J.-C. Rojas-Sanchez2, N. Reyren2, C. Vergnaud1,
A. Marty1, C. Beigné1, J.-P. Attané1, L. Vila1, G. Desfonds1, S. Gambarelli1, H. Jaffrès2, J.M. George2, M. Jamet1
1 Institute
of NAnoscience and Cryogeny, CEA and Université Joseph-Fourier, Grenoble, France
2Unité Mixte CNRS/Thalès and Université Paris-Sud 11, Palaiseau, France
Presenting author's e-mail address: [email protected]
Spin injection has already been demonstrated in many different semiconducting materials (Si,
GaAs ...) [1]. Among them, germanium appears as a promising material for spintronics since it
makes a good compromise between a long spin diffusion length (due to its crystal symmetry) and
a rather large spin-orbit coupling [2,3]. Spin-orbit coupling is a key parameter to manipulate the
spin in semiconductors. In this presentation we focus on the spin properties of holes in the Ge
valence band where the spin-orbit effects are very large due to the p-type character of the orbitals.
Nevertheless unexpected long hole spin lifetimes have been recently reported in p-type Ge [4].
Here we have used two different techniques to determine experimentally the spin diffusion length
of holes: the weak anti-localisation (WAL) and the electrical spin injection/detection using a threeterminal device. We also demonstrated the spin lifetime modulation by applying an electric field
through a back gate voltage. Finally, using the same device, we could measure the spin
accumulation in Ge by the application of a temperature gradient between the ferromagnetic
electrode and the germanium channel.
In this work we have used germanium on insulator (GOI) wafers made of an 89 nm-thick active
layer highly boron doped up to 7.0.1018 cm-3. Two different germanium devices were prepared: a
double Hall cross of 110 µm long and 20 µm large for weak anti-localisation measurements and
three-terminal devices with a 400x150 µm2 ferromagnetic electrode.
For WAL measurements, a DC current of 10 µA was applied along the Hall bar and we recorded
simultaneously the magnetoresistance and Hall signals. The magnetic field was applied
perpendicular to the film. In figure 1, the magnetoconductance at 2 K clearly exhibits a peak at
zero field which is the signature of WAL. We further extracted the mobility (≈190 cm²/V.s) and
carrier density (≈ 7.1018 cm3) from Hall measurements. We studied the temperature (figure 1a) and
electric field dependences. The magnetoconductance peak fully disappears at 40 K and the
mobility starts decreasing at high gate voltages.
We used the Hikami-Larkin-Nagaoka (HLN) model [5] to fit the magnetoconductance signal:
∆ =−
+
+
+
+
+ l
+
−l
− ln
(1)
where Bφ and Bso are respectively magnetic field characterised by phase coherence length and
spin orbit length.
The experimental data show a very good agreement with preliminary result of this model and
in the field range of the model validity, we obtain reasonable values for the phase coherence length
(70 nm) and the spin-orbit length (12 nm). The experimental temperature dependence of the phase
coherence length is also in agreement with preliminary result of the HLN model (Figure 1b).
Moreover by applying a back gate voltage we could modulate the spin-orbit length and thus the
spin lifetime.
1.31
2K
4K
6K
8K
10K
12K
14K
20K
30K
40K
1.3
1.29
1.27
1.26
Lm
Lp
Ls
70
60
50
Lm (m)
G (e²/h)
1.28
80
40
30
1.25
20
1.24
10
0
1.23
-6
-4
-2
0
Field (T)
a)
2
4
6
10
20
30
40
T (K)
b)
Figure 1 : a) Magnetoconductance of p-type germanium recorded at different temperatures under a perpendicular
applied field. We observe a complete disappearance of the WAL at 40K. b) Characteristics lengths extracted with
HLN model of WAL measurement. Lφ, Lso and lm are respectively phase coherence length, spin orbit length and
mean free path.
We used a 3-terminal device to inject and detect spin currents in p-type Ge. The device is made
of a Ge mesa with two ohmic contacts (Au/Ti) and a magnetic tunnel junction (Ta/CoFeB/MgO)
in between in order to inject and detect the spin polarized current [6]. The interface between the
tunnel junction and p-type Ge shows a very low resistance as compared to n-type Ge. We detected
a spin accumulation in the germanium valence band using the Hanle effect and inverse Hanle
effect. We have performed a complete study of the spin signal as a function of temperature, bias
and electric field. We were also able to detect a tunneling spin Seebeck signal by Joule heating the
Ge channel which sets a temperature gradient between the ferromagnet and the Ge channel [6].
[1] S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. von Molnar, M. L. Roukes, A. Y. Chtchelkanova
and D. M. Treger, “Spintronics: A spin-based electronics vision for the future,” Science 294, pp 1488-1495 (2001).
[2] A. Jain, L. Louahadj, J. Peiro, J. C. Le Breton, C. Vergnaud, A. Barski, C. Beigné, L. Notin, A. Marty, V. Baltz, S.
Auffret, E. Augendre, H. Jaffrès, J. M. George, and M. Jamet, “Electrical spin injection and detection at Al2O3/ntype germanium interface using three terminal geometry,” Appl. Phys. Lett. 99, 162102 (2011).
[3] Y. Zhou, W. Han, L.-T. Chang, F. Xiu, M. Wang, M. Oehme, I. a. Fischer, J. Schulze, R. K. Kawakami, and K. L.
Wang, “Electrical spin injection and transport in germanium,” Phys. Rev. B 84., 125323 (2011).
[4] S. Iba, H. Saito, A. Spiesser, S. Watanabe, R. Jansen, S. Yuasa, and K. Ando, “Spin Accumulation and Spin
Lifetime in p-Type Germanium at Room Temperature,” Appl. Phys. Express 5, 053004 (2012).
[5] E. Akkermans, G. MontambauxY., “Mesoscopic physics of electrons and photons,” Cambridge. (2007).
[6] A. Jain, C. Vergnaud, J. Peiro, J. C. Le Breton, E. Prestat, L. Louahadj, C. Portemont, C. Ducruet, V. Baltz, A.Marty,
A. Barski, P. Bayle-Guillemaud, L. Vila, J.-P. Attané, E. Augendre, H. Jaffrès, J.-M. George, M. Jamet, “Electrical
and thermal spin accumulation in germanium,” Appl. Phys. Lett. 101, 022402 (2012).