Observation of linear and quadratic magnetic field-dependence of magneto-photocurrents in InAs/GaSb superlattice
© Li et al.; licensee Springer. 2014
Received: 24 March 2014
Accepted: 5 May 2014
Published: 31 May 2014
We experimentally studied the magneto-photocurrents generated by direct interband transition in InAs/GaSb type II superlattice. By varying the magnetic field direction, we observed that an in-plane magnetic field induces a photocurrent linearly proportional to the magnetic field; however, a magnetic field tilted to the sample plane induces a photocurrent presenting quadratic magnetic field dependence. The magneto-photocurrents in both conditions are insensitive to the polarization state of the incident light. Theoretical models involving excitation, relaxation and Hall effect are utilized to explain the experimental results.
MPE has been observed in InGaAs/InAlAs two-dimensional electron gas, GaAs/AlGaAs quantum well, graphene and so on [5–7]. By comparison, the InAs/GaSb type II supperlattice has some advantages in investigating spin transport and fabricating spintronic devices for its properties of large SOI in InAs and GaSb, relatively high carrier mobility in InAs and peculiar energy band structure [8, 9]. Previously, the InAs/GaSb type II superlattice has been extensively researched as an infrared detector. The studies have been mainly focused on carrier recombination, interface properties, tailoring of energy bands and so on [10–17]. The zero-field spin splitting has also been observed in InAs/GaSb quantum wells by Shubnikov-de-Haas oscillation , while the investigations on the magneto-photo effect is seldom concerned. In the present paper, we investigate the MPE in the InAs/GaSb type II supperlattice. Unlike the previous researches of the magnetic field strength dependence of the photocurrents, we mainly focus on the magnetic field direction dependence of the photocurrents in this structure. By varying magnetic field direction in or out of the sample plane, we observed linear and quadratic magnetic field dependence of the photocurrents, respectively. More information about excitation and relaxation of electrons in this structure were obtained from the experiments.
The InAs/GaSb superlattice was fabricated by molecular beam epitaxy technique on semi-insulating (001)-oriented GaAs substrate. The 500-nm GaAs and 1,000-nm GaSb buffers were deposited on the substrate to relieve the lattice mismatch. Then an InAs/GaSb superlattice of 155 periods was deposited. The monolayer thicknesses of InAs and GaSb are 3.85 and 2.60 nm, respectively. The sample was not intentionally doped. The energy gap of this structure calculated by the k ·p theory is 129.5 meV. The standard Hall measurement demonstrates that the sample is n-type at room temperature, i.e. electrons are the main carriers contributing to transport. Since in the n-type superlattice spin relaxation time and lifetime of holes are much shorter than those of electrons, we neglect the contribution of holes to the magneto-photocurrents. Four pairs of ohmic contact electrodes which are parallel to , ,  and  crystallographic directions were equidistantly made on the edges.
The experimental setup is shown in Figure 1b. A linearly polarized 1,064-nm laser normally irradiated on the center of the sample to excite direct interband transition of electrons. Hence, the circular photogalvanic effect and linear photogalvanic effect  are forbidden in this C 2v symmetry structure for the normal incidence case. A permanent magnet was used to generate magnetic field which can be along arbitrary direction in the sample plane. The investigation of photogalvanic effect was carried out at room temperature by rotating the magnetic field. The data were collected by a standard lock-in amplification technique. Specifically, the laser power was about 63 mW, the light spot diameter was 1.2 mm and the permanent magnet strength was 0.1 T. Besides, we choose x, y and z to be along ,  and  crystallographic directions, respectively.
Results and discussion
In-plane magnetic field-dependent MPE
By extracting the peak-to-peak values of the currents (Jpp) in four crystallographic directions, we observed that Jpp in the  and  crystallographic directions are larger than that in the  and  directions. Merely considering the SOI-induced anisotropic splitting of the energy bands (see ) seems unable to explain this experimental result. Actually, the total photocurrents(described by Jpp) are decided by both SOI and Zeeman splitting. The SOI generates the spin-dependent asymmetric transition matrix elements and scattering matrix elements in excitation and relaxation processes, respectively, which lead to the asymmetric distribution of electrons in each spin-splitting subband. The Zeeman splitting transforms the net spin currents to charge currents. Hence, the photocurrents are proportional to the Zeeman split energy and then the electron effective g-factor g ∗. In view of this, there are no common anion and cation in the InAs/GaSb superlattice interface; this structure belongs to the C 2v symmetry. Hence, g ∗ presents in-plane anisotropy when the magnetic field is in different crystallographic directions . We speculated that the co-effect of the anisotropic SOI and g ∗ make Jpp in the  and  crystallographic directions larger.
Similar to the parameters in Equations 1 and 2, S1± denote radiation polarization unrelated currents. Linearly and circularly polarized light related currents are described by S2±, S3± and S4±, respectively. C1′ and C2′ are background currents.
Fitting results of the parameters
S 1 ′
S 2 ′
S 3 ′
S 1 +
S 1 −
S 2 +
S 2 −
S 3 +
S 3 −
ϕ is the angle between the wave vector and the x direction. α is the angle between the plane of linear polarization and the x direction. Considering the contribution of asymmetric relaxation of electrons to the current, we should add an additional term to the . Then the in Equation 6 includes contributions of both excitation and relaxation. Owing to the magneto-photocurrent in this superlattice is independent of the radiation polarization, it can be deduced that is much larger than and . This conclusion is similar to that in  which that reported always overwhelms and theoretically.
The radiation polarization independent of MPE generated by direct interband transition had also been observed in the BiTeI film . However, in (110)-grown GaAs/Al x Ga1−x As quantum wells, MPE generated by indirect intrasubband transition shows clear relations to the radiation linear polarization state . The reason may be that in the intrasubband transition process, spin-dependent asymmetric electron-phonon interaction which contributes to the magneto-photocurrent is sensitive to the radiation polarization state. It leads to the relative magnitudes of and in Equation 6 increase. More practically, the phonon effect may be taken into account when designing optically manipulated spintronics devices in the future.
Tilted magnetic field-dependent MPE
ε x i ′ and ε y i ′ are also mixing parameters due to the Hall effect. C x ′ and C y ′ are background photocurrents.
In summary, we have researched magneto-photocurrents in the InAs/GaSb superlattice when an in-plane and tilted magnetic field were applied respectively. The magneto-photocurrents in both conditions are insensitive to the polarization state of the incident light. A theoretical model involving anisotropic photo-excited carriers density is utilized to explain the in-plane magnetic field-induced MPE. Compared to the direct electron-photon interaction, the asymmetric electron-phonon interaction which contributes to the magneto-photocurrent may be more sensitive to the radiation polarization state. The quadratic magnetic field dependence of the magneto-photocurrents can be well illustrated by an additional Hall effect model.
The work was supported by the 973 Program (2012CB921304 and 2013CB632805) and the National Natural Science Foundation of China (Nos. 60990313, 61176014, 61307116 and 61290303).
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