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Journal of Magnetism and Magnetic Materials 400 (2016) 290–294
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic-field-induced photocurrent in metal-dielectric-semiconductor heterostructures based on cobalt nanoparticles SiO2(Co)/GaAs V.V. Pavlov a,n, L.V. Lutsev a, P.A. Usachev a, A.A. Astretsov a, A.I. Stognij b, N.N. Novitskii b, R.V. Pisarev a a b
Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia Scientific and Practical Materials Research Centre, National Academy of Sciences of Belarus, 220072 Minsk, Belarus
art ic l e i nf o
a b s t r a c t
Article history: Received 21 June 2015 Received in revised form 14 July 2015 Accepted 20 July 2015 Available online 21 July 2015
Magnetic-field influence on photocurrent in heterostructures of silicon dioxide films with cobalt nanoparticles SiO2(Co) grown on gallium arsenide GaAs substrate has been studied in the avalanche regime at room temperature. High values of magnetic-field-induced photocurrent were found in the vicinity and above the GaAs bandgap of ∼1.4 eV. For photon energies E > 1.4 eV the photocurrent significantly increases, while the avalanche process is suppressed by the magnetic field, and the current flowing through the heterostructure decreases. The photocurrent is enhanced in the SiO2(Co 60 at%)/GaAs heterostructure at the magnetic field H ¼1.65 kOe by factor of about 10 for the photon energy E¼ 1.5 eV. This phenomenon is explained by a model based on electronic transitions in magnetic fields with the spindependent recombination process at deep impurity centers in the SiO2(Co)/GaAs interface region. & 2015 Published by Elsevier B.V.
Keywords: Magnetic-field-induced photocurrent Ferromagnet-semiconductor heterostructures Magnetic nanoparticles
1. Introduction Unique physical properties of ferromagnet-semiconductor heterostructures related to low-dimensional or quantum-dimensional effects are subject of intensive research due to promising applications for spin-electronic devices [1–3]. Potentially, interesting candidates for such kinds of applications are heterostructures composed of ferromagnetic metal nanoparticles in a dielectric matrix. These heterostructures on semiconductor substrates exhibit large magneto-resistance [4–8] and negative photoconductance effects [9]. Magneto-transport properties in these heterostructures can be complementarily observed by exploring optical effects. For example, effective ways to generate and detect spin-polarized carriers are optical pumping and detection [10–12]. Magnetically controlled optoelectronic devices can be considered as new alternative integrated devices for spintronics [13]. Here we report on strong magnetic-field-induced photocurrent in SiO2(Co)/GaAs heterostructures in the avalanche regime. The photocurrent increase is observed for photon energies E greater than the GaAs bandgap Eg. The photocurrent is enhanced by a factor of about 10 for the photon energy E ¼1.5 eV in the magnetic field H ¼1.65 kOe for the SiO2(Co)/GaAs heterostructure with a Co n
Corresponding author. E-mail address: [email protected] (V.V. Pavlov).
http://dx.doi.org/10.1016/j.jmmm.2015.07.063 0304-8853/& 2015 Published by Elsevier B.V.
atomic concentration of 60%.
2. Materials and method The metal-dielectric heterostructures were composed of thin film of amorphous silicon dioxide containing cobalt nanoparticles SiO2(Co) on gallium arsenide n-GaAs(001) substrates. The SiO2(Co) films were prepared at temperature of 200 °C from a composite cobalt-quartz target using the ion-beam deposition technique. GaAs substrates with thickness of 0.4 mm had electrical resistivity of 0.93 × 105 Ω cm . Prior to the deposition process, substrates were polished by a low-energy oxygen ion beam [14]. After this procedure the substrate roughness was less than 0.5 nm. The Co concentration in SiO2 matrix was specified by a ratio of cobalt and quartz surface areas. After preparation the film composition was determined by the nuclear physical methods of element analysis based on the Rutherford scattering spectroscopy. The atomic Co concentration x in SiO2(Co)/GaAs heterostructures was in the range of 45–71 at% for different samples. The SiO2(Co) film thickness was 40 nm. The average size of Co particles was determined by the small-angle X-ray scattering. The size of Co particles increased with the concentration growth x as follows: from 3.0 nm at x¼ 45 at% to 4.0 nm at x¼71 at%. At x ¼60 at% the average size of Co particles was equal to 3.5 nm. We note that according to the study of SiO2(Co)/GaAs heterostructures using the surface
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291
Fig. 1. A scheme of the experimental geometry for the heterostructure SiO2(Co)/GaAs with an applied voltage U, magnetic field H and light illumination with the photon energy E = =ω .
scattering of synchrotron radiation [15], the concentration of Co nanoparticles at the interface is much lower than that in the bulk of the granular film, whereas the distance between particles is significantly larger than that in the bulk of the film. Fig. 1 shows a scheme of the experimental geometry for the heterostructure SiO2(Co)/GaAs in magneto-resistance, magnetooptical and magnetic-field-induced photocurrent experiments. In the magneto-resistance and magnetic-field-induced photocurrent experiments a protective Au layer with the thickness of 3–5 nm sputtered on SiO2(Co) films was used as a contact. The second contact was prepared by a silver paste on the GaAs back surface. To study the magnetic-field-induced photocurrent we used a white light source having the fluence of 2.6 mW/cm2 and a chopper intensity modulation of light at the frequency of 40 Hz. The photocurrent detection was based on a lock-in technique. The light beam has been passing through the Au contact layer, the SiO2(Co) film and the GaAs substrate.
Fig. 2. Injection magneto-resistance ratio IMR as a function of the voltage U in the heterostructure SiO2(Co)/GaAs with Co concentration 60 at% at the magnetic field of 2.1 kOe.
3. Characterization of heterostructures SiO2(Co)/GaAs The metal-dielectric heterostructures SiO2(Co)/GaAs were characterized by measurements of charge current in electric and magnetic fields. Electrical resistivity of SiO2(Co) films was measured by the dc four-probe method at room temperature and decreased from 1.0·102 Ω cm (45 at%) to 4.0 Ω cm (71 at%). The resistivity of GaAs is higher than the resistivity of the film and the applied voltage primarily falls on the GaAs substrate. When positive voltage is applied to the heterostructures SiO2(Co)/GaAs as it is shown in Fig. 1, electrons are injected from the granular film into the substrate. The resistivity of GaAs substrate is higher than the resistivity of the SiO2(Co) films, therefore applied voltage primarily falls in the semiconductor substrate. Fig. 2 illustrates the magnetoresistance effect as a function of the applied voltage. The injection magneto-resistance coefficient is IMR = (R H − R0 ) /R0 [7,8], where R0 and RH are the resistances of the SiO2(Co)/GaAs heterostructure without and in an applied magnetic field H, respectively. For applied voltage U > 52 V , a sharp increase of the electrical current is observed due to the process of impact ionization. Influence of the magnetic field on the current flowing in SiO2(Co)/GaAs heterostructures in the avalanche regime was analyzed in Ref. [8]. The largest magneto-resistance coefficient was observed for the SiO2(Co)/GaAs heterostructure with Co concentration of 60 at%, therefore it was used in experiments on the magnetic-field-induced photocurrent [16]. Fig. 3a shows the current j in the SiO2(Co 60 at%)/GaAs structure as a function of the magnetic field H at room temperature. The applied field suppresses the avalanche process and the current flowing in the SiO2(Co)/GaAs heterostructure decreases [8]. The suppression of the avalanche process causes the sharp growth of the magneto-resistance coefficient. Fig. 3b demonstrates a
Fig. 3. (a) Current j flowing in the SiO2(Co)/GaAs structure with 60 at% Co versus the magnetic field H at room temperature at different values of the applied voltage. (b) Magnetic hysteresis curve obtained by the longitudinal magneto-optical Kerr effect.
magnetic hysteresis curve obtained by longitudinal magneto-optical Kerr effect. The hysteresis curve has a small coercivity due to direct exchange in clusters of Co nanoparticles.
4. Experiments on the magnetic-field-induced photocurrent Spectral dependencies of the photocurrent change Δj caused by the polarized light at different voltages U without magnetic field and in the field H¼ 2.5 kOe are shown in Fig. 4. The photocurrent
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Fig. 4. Spectral dependencies of the photocurrent change Δj in SiO2(Co)/GaAs heterostructure with 60 at% Co caused by the linearly polarized light at different values of the applied voltage U without magnetic field and in the field H¼ 2.5 kOe.
dependencies are different for high and low values of the photon energy E in the magnetic field and without a field. For photon energies E > 1.36 eV at voltages U ≥ 65 V in the magnetic field the photocurrent increases. For the photon energies E < 1.36 eV at high voltages a negative photoconductance is observed [9]. Fig. 5 shows the photocurrent change Δj at photon energies 1.375, 1.4, 1.45 and 1.5 eV versus the magnetic field H for different applied voltages U at room temperature. If the photon energy E is larger than the GaAs bandgap energy E g ≃ 1.4 eV , the light penetration depth in GaAs is low, and the light is absorbed in SiO2(Co)/GaAs heterostructure near the interface. For the case of E ≥ E g , the photocurrent increases and the enhancement of the photocurrent is observed. The photocurrent is increased by a factor of about 10 for the photon energy E¼ 1.5 eV, the applied voltage U¼80 V and the magnetic field H ¼1.65 kOe. The subsequent magnetic field growth leads to the avalanche suppression and to a photocurrent decrease.
5. Theoretical description and discussion The injection magneto-resistance effect and decrease of the current flowing in the SiO2(Co)/GaAs structure are described by the model based on the spin-dependent potential barrier and on
Fig. 5. Photocurrent Δj in SiO2(Co)/GaAs heterostructure with 60 at% Co caused by the linearly polarized light for different photon energies and values of the applied voltage U versus the magnetic field H at room temperature.
the avalanche suppression in the magnetic field [8,17]. According to the model, accumulation electron layer in GaAs is formed in the interface region and the potential barrier is localized at a certain distance from the semiconductor/film interface. Fig. 6 shows a schematic band diagram of the SiO2(Co)/GaAs heterostructure. The accumulation electron layer contains oxygen ions left after the polishing process. According to Refs. [18,19], in addition to the EL2 defect level there are oxygen-ion levels in the GaAs bandgap with energies E1 = 0.48 eV , E2 ¼ 0.74 eV, E3 ¼1.0 eV, and E4 ¼ 1.25 eV counting from the bottom of the conduction band. At room
V.V. Pavlov et al. / Journal of Magnetism and Magnetic Materials 400 (2016) 290–294
-
conduction band
Au
A
B L
293
+
Avalanche process
holes
valence band Ñ
Co
GaAs
SiO2(Co)
Fig. 6. Schematic band diagram of the GaAs in the avalanche regime for the SiO2(Co)/GaAs heterostructure. A is a potential maximum of barrier, B is the start point of the avalanche, C marks holes produced by the avalanche, L marks energy levels of defects inside the GaAs bandgap.
temperature levels with energies E2, E3 and E4 are occupied by electrons. Conductivity in the conduction band is determined by thermally activated electrons from the level E1 ¼0.48 eV [7] which is shown in Fig. 6 by L. Electrons on the oxygen levels are coupled by the exchange interaction with 3d-electrons of Co nanoparticles via electrons in the accumulation layer. Next we consider the photocurrent enhancement for the light with the photon energy E ≥ E g . Light is absorbed in the semiconductor region near the interface and creates electrons in the conduction band and holes in the valence band in the interface region. According to theoretical models [20–27], recombination of charge carriers is related mainly to nonradiative electron transitions through localized electron levels in the forbidden energy band. The rate of electron–hole recombination depends on the spin states of both paramagnetic centers in the bandgap and free carriers. In order to describe the photocurrent enhancement in the magnetic field, we use Lepine's model with small modifications [20,21,23]. The spin-dependent recombination is preceded by the trapping of conduction electrons on localized electron levels (Fig. 7). We suppose that the level E (a), where conduction electron is trapped, is the level of a shallow state and the level E (b) is a partially occupied oxygen level E1 = 0.48 eV . When the triplet electron
states
|T −〉 = | ↓ ↓ 〉 of the
E (a )
|T0 〉 =
E (a )
and
1 2
(| ↑ ↓ 〉 + | ↓ ↑ 〉),
E (b )
|T+〉 = | ↑ ↑ 〉
and
pair appear (Fig. 7a), the transition
E (b )
is forbidden by the spin conservation law. In this case, a → further recombination is highly suppressed. By contrast, for the singlet state |S〉 = 1 (| ↑ ↓ 〉 − | ↓ ↑ 〉) the spin conservation law 2
permits an electron transition from the first level E (a) to the second level E (b) and a further recombination occurs (Fig. 7b). Under the assumption that triplet states cannot recombine and singlet states recombine with a probability rs, the average recombination probability of electrons on levels E (a) and E (b) is written as R = rs/4 . In the presence of the external magnetic field H the levels E (a) and E (b) are split. Taking into account the exchange interaction between electrons on the oxygen levels with 3d-electrons of Co nanoparticles via electrons in the accumulation layer and assuming that the exchange interaction is isotropic, we obtain that the splitting gap is determined by the effective field H + H (exc ) = αH , where H (exc ) is the field caused by the exchange interaction, α⪢1. Neumann's density operator ρ of electrons on splitted levels in a thermal equilibrium is represented in the Boltzmann form [20,21,23]:
Fig. 7. Spin-dependent recombination through a pair of localized electron levels E (a) and E (b) split by the magnetic field. (a) For the triplet spin state of the pair of electrons on levels E (a) and E (b) the spin-dependent recombination is forbidden. (b) For the singlet spin state the process (1) → (2) → (3) of the recombination of charge carriers is permitted.
ρ0 =
∑i4=1 exp ( − εi /kT )|i〉〈i| ∑i4=1 exp ( − εi /kT )
, (1)
where T is the temperature, |i〉 = {|S〉, |T0 〉, |T+〉, |T −〉}, εs = 0 , ε0 = 0 , ε+ = gμ B αH , ε− = − gμ B αH are Zeeman energies of singlet and triplet states, respectively, g is the g-factor of electrons on localized states, μ B is the Bohr magneton. We consider the change of the average recombination probability ΔR of electrons between the thermal equilibrium electron ensemble (1) and the highly saturated non-equilibrium state with the density operator 1 ρsat = 4 ∑i4=1 |i〉〈i|. Then, the change of the average recombination versus the magnetic field H is
⎡ ⎤ ⎛ gμ B αH ⎞ rs ΔR = rs Tr ⎢|S〉〈S|(ρsat − ρ 0 ) ⎥ = − tanh2 ⎜ ⎟. 4 ⎣ ⎦ ⎝ 2kT ⎠
(2)
Taking into account that the photoconductivity sph and the number of photo-induced electrons ne in the conduction band and the number of holes nh in the valence band are inversely proportional to the recombination probability R and ΔR⪡R , in the second order of the Taylor-series expansion of the relation (2) we find 2 Δσ ph Δne Δnh ΔR ⎛ gμ B αH ⎞ = = = − =⎜ ⎟ . σ ph ne nh R ⎝ 2kT ⎠
(3)
In a magnetic field electron–hole recombination is suppressed. Additional holes Δnh move toward the barrier and are accumulated in the barrier region, thus decreasing the barrier height. This leads to the observed increase of the electron current and to the enhancement of the avalanche in the applied magnetic field. Such positive feedback produced by photo-induced holes and additional photo-induced electrons Δne result in strong changes in the electron current Δj . The experimentally observed quadratic-like dependence of the photocurrent Δj from the magnetic field H at
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low field values (Fig. 4, |H| < 1 kOe) corresponds to the relation (3) and confirms the magnetic dependence obtained in theoretical models developed in [20–22]. We note that increase of the applied voltage changes the photocurrent as shown in Fig. 5. At the magnetic field |H| > 1 kOe the photocurrent increases with the applied voltage. This can be explained by growth of number of electrons which reach the potential barrier and form the avalanche process in the GaAs substrate. At high values of the magnetic field H, the avalanche is suppressed and the photocurrent decreases with the applied magnetic field.
6. Conclusions We observed a significant influence of the magnetic field on the photocurrent in SiO2(Co)/GaAs heterostructures in the avalanche regime at room temperature. At photon energies E larger than the GaAs bandgap energy Eg, at small values of magnetic field the photocurrent increases in accordance with a quadratic-like dependence. The enhancement factor is about 10 for the photon energy E ¼1.5 eV and magnetic field H ¼1.65 kOe. The subsequent field growth suppresses the avalanche process leading to a photocurrent decrease. The photocurrent enhancement is explained by the spin-dependent recombination through the localized electron levels in the semiconductor. In a magnetic field electron–hole recombination is suppressed. This leads to additional free electrons in the conduction band and holes in the valence band. As a result, photo-induced holes are accumulated in the barrier, the photocurrent grows and the avalanche feedback can be manipulated by the light. The photocurrent enhancement caused by the applied magnetic field can be used in field-controlled singlephoton avalanche diodes.
Acknowledgments The authors gratefully acknowledge the assistance of V.M. Lebedev for characterization of the film composition. This work was supported by the Russian Government Project no. 14.B25.31.0025,
the Russian Foundation for Basic Research Project nos. 10-0290023, 13-02-00754, 15-52-12015 and 15-02-06208, and the Programs of Presidium of the Russian Academy of Sciences.
References [1] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M. L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488. [2] G. Schmidt, J. Phys. D: Appl. Phys. 38 (2005) R107. [3] I. Žutic, J. Fabian, S. Das Sarma, Rev. Mod. Phys. 76 (2004) 323. [4] H. Akinaga, M. Mizuguchi, K. Ono, M. Oshima, Appl. Phys. Lett. 76 (2000) 357. [5] H. Akinaga, Semicond. Sci. Technol. 17 (2002) 322. [6] M. Yokoyama, T. Ogawa, A.M. Nazmul, M. Tanaka, J. Appl. Phys. 99 (2006) 08D502. [7] L.V. Lutsev, A.I. Stognij, N.N. Novitskii, JETP Lett. 81 (2005) 514. [8] L.V. Lutsev, A.I. Stognij, N.N. Novitskii, Phys. Rev. B 80 (2009) 184423. [9] L.V. Lutsev, V.V. Pavlov, P.A. Usachev, A.A. Astretsov, A.I. Stognij, N.N. Novitskii, Appl. Phys. Lett. 101 (2012) 242104. [10] A.F. Isakovic, D.M. Carr, J. Strand, B.D. Schultz, C.J. Palmstrøm, P.A. Crowell, J. Appl. Phys. 91 (2002) 7261. [11] B. Huang, I. Appelbaum, J. Appl. Phys. 100 (2006) 034501. [12] Y.B. Xu, S.M. Thompson (Eds.), Spintronic Materials and Technology, Taylor & Francis, New York, 2006. [13] V.K. Dugaev, Yu. Vygranenko, M. Vieira, V.I. Litvinov, J. Barnas, Physica E 16 (2003) 558. [14] A.I. Stognij, N.N. Novitskii, O.M. Stukalov, Tech. Phys. Lett. 28 (2002) 17. [15] N.A. Grigor'eva, A.A. Vorob'ev, V.A. Ukleev, E.A. Dyad'kina, L.V. Lutsev, A. I. Stognij, N.N. Novitskii, S.V. Grigor'ev, JETP Lett. 92 (2010) 767. [16] V.V. Pavlov, L.V. Lutsev, P.A. Usachev, A.A. Astretsov, A.I. Stognij, N.N. Novitskii, R.V. Pisarev, Appl. Phys. Lett. 106 (2015) 152404. [17] L.V. Lutsev, J. Phys.: Condens. Matter 18 (2006) 5881. [18] A.L. Lin, E. Omelianovski, R.H. Bube, J. Appl. Phys. 47 (1976) 1852. [19] P.W. Yu, Appl. Phys. Lett. 44 (1984) 330. [20] D.J. Lepine, J.J. Prejean, in: S.P. Keller, J.C. Hensel, J. Stern (Eds.), Proceedings of the 10th International Conference on the Physics of Semiconductor, US Atomic Energy Commission, USA, 1970, p. 805. [21] D.J. Lepine, Phys. Rev. B 6 (1972) 436. [22] R.M. White, J.F. Gouyet, Phys. Rev. B 16 (1977) 3596. [23] J.-M. Spaeth, H. Overhof, Point Defects in Semiconductors and Insulators. Determination of Atomic and Electronic Structure from Paramagnetic Hyperfine Interactions, Springer-Verlag, Berlin, 2003. [24] D. Kaplan, I. Solomon, N.F. Mott, J. Phys. (Paris) - Lettres 39 (1978) L51. [25] A.V. Barabanov, O.V. Tretiak, V.A. Lvov, Phys. Rev. B 54 (1996) 2571. [26] L.S. Vlasenko, Yu.V. Martynov, T. Gregorkiewicz, C.A.J. Ammerlaan, Phys. Rev. B 52 (1995) 1144. [27] E.L. Ivchenko, V.K. Kalevich, A.Yu. Shiryaev, M.M. Afanasiev, Y. Masumoto, J. Phys.: Condens. Matter 22 (2010) 465804.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic-field-induced photocurrent in metal-dielectric-semiconductor heterostructures based on cobalt nanoparticles SiO2(Co)/GaAs V.V. Pavlov a,n, L.V. Lutsev a, P.A. Usachev a, A.A. Astretsov a, A.I. Stognij b, N.N. Novitskii b, R.V. Pisarev a a b
Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia Scientific and Practical Materials Research Centre, National Academy of Sciences of Belarus, 220072 Minsk, Belarus
art ic l e i nf o
a b s t r a c t
Article history: Received 21 June 2015 Received in revised form 14 July 2015 Accepted 20 July 2015 Available online 21 July 2015
Magnetic-field influence on photocurrent in heterostructures of silicon dioxide films with cobalt nanoparticles SiO2(Co) grown on gallium arsenide GaAs substrate has been studied in the avalanche regime at room temperature. High values of magnetic-field-induced photocurrent were found in the vicinity and above the GaAs bandgap of ∼1.4 eV. For photon energies E > 1.4 eV the photocurrent significantly increases, while the avalanche process is suppressed by the magnetic field, and the current flowing through the heterostructure decreases. The photocurrent is enhanced in the SiO2(Co 60 at%)/GaAs heterostructure at the magnetic field H ¼1.65 kOe by factor of about 10 for the photon energy E¼ 1.5 eV. This phenomenon is explained by a model based on electronic transitions in magnetic fields with the spindependent recombination process at deep impurity centers in the SiO2(Co)/GaAs interface region. & 2015 Published by Elsevier B.V.
Keywords: Magnetic-field-induced photocurrent Ferromagnet-semiconductor heterostructures Magnetic nanoparticles
1. Introduction Unique physical properties of ferromagnet-semiconductor heterostructures related to low-dimensional or quantum-dimensional effects are subject of intensive research due to promising applications for spin-electronic devices [1–3]. Potentially, interesting candidates for such kinds of applications are heterostructures composed of ferromagnetic metal nanoparticles in a dielectric matrix. These heterostructures on semiconductor substrates exhibit large magneto-resistance [4–8] and negative photoconductance effects [9]. Magneto-transport properties in these heterostructures can be complementarily observed by exploring optical effects. For example, effective ways to generate and detect spin-polarized carriers are optical pumping and detection [10–12]. Magnetically controlled optoelectronic devices can be considered as new alternative integrated devices for spintronics [13]. Here we report on strong magnetic-field-induced photocurrent in SiO2(Co)/GaAs heterostructures in the avalanche regime. The photocurrent increase is observed for photon energies E greater than the GaAs bandgap Eg. The photocurrent is enhanced by a factor of about 10 for the photon energy E ¼1.5 eV in the magnetic field H ¼1.65 kOe for the SiO2(Co)/GaAs heterostructure with a Co n
Corresponding author. E-mail address: [email protected] (V.V. Pavlov).
http://dx.doi.org/10.1016/j.jmmm.2015.07.063 0304-8853/& 2015 Published by Elsevier B.V.
atomic concentration of 60%.
2. Materials and method The metal-dielectric heterostructures were composed of thin film of amorphous silicon dioxide containing cobalt nanoparticles SiO2(Co) on gallium arsenide n-GaAs(001) substrates. The SiO2(Co) films were prepared at temperature of 200 °C from a composite cobalt-quartz target using the ion-beam deposition technique. GaAs substrates with thickness of 0.4 mm had electrical resistivity of 0.93 × 105 Ω cm . Prior to the deposition process, substrates were polished by a low-energy oxygen ion beam [14]. After this procedure the substrate roughness was less than 0.5 nm. The Co concentration in SiO2 matrix was specified by a ratio of cobalt and quartz surface areas. After preparation the film composition was determined by the nuclear physical methods of element analysis based on the Rutherford scattering spectroscopy. The atomic Co concentration x in SiO2(Co)/GaAs heterostructures was in the range of 45–71 at% for different samples. The SiO2(Co) film thickness was 40 nm. The average size of Co particles was determined by the small-angle X-ray scattering. The size of Co particles increased with the concentration growth x as follows: from 3.0 nm at x¼ 45 at% to 4.0 nm at x¼71 at%. At x ¼60 at% the average size of Co particles was equal to 3.5 nm. We note that according to the study of SiO2(Co)/GaAs heterostructures using the surface
V.V. Pavlov et al. / Journal of Magnetism and Magnetic Materials 400 (2016) 290–294
291
Fig. 1. A scheme of the experimental geometry for the heterostructure SiO2(Co)/GaAs with an applied voltage U, magnetic field H and light illumination with the photon energy E = =ω .
scattering of synchrotron radiation [15], the concentration of Co nanoparticles at the interface is much lower than that in the bulk of the granular film, whereas the distance between particles is significantly larger than that in the bulk of the film. Fig. 1 shows a scheme of the experimental geometry for the heterostructure SiO2(Co)/GaAs in magneto-resistance, magnetooptical and magnetic-field-induced photocurrent experiments. In the magneto-resistance and magnetic-field-induced photocurrent experiments a protective Au layer with the thickness of 3–5 nm sputtered on SiO2(Co) films was used as a contact. The second contact was prepared by a silver paste on the GaAs back surface. To study the magnetic-field-induced photocurrent we used a white light source having the fluence of 2.6 mW/cm2 and a chopper intensity modulation of light at the frequency of 40 Hz. The photocurrent detection was based on a lock-in technique. The light beam has been passing through the Au contact layer, the SiO2(Co) film and the GaAs substrate.
Fig. 2. Injection magneto-resistance ratio IMR as a function of the voltage U in the heterostructure SiO2(Co)/GaAs with Co concentration 60 at% at the magnetic field of 2.1 kOe.
3. Characterization of heterostructures SiO2(Co)/GaAs The metal-dielectric heterostructures SiO2(Co)/GaAs were characterized by measurements of charge current in electric and magnetic fields. Electrical resistivity of SiO2(Co) films was measured by the dc four-probe method at room temperature and decreased from 1.0·102 Ω cm (45 at%) to 4.0 Ω cm (71 at%). The resistivity of GaAs is higher than the resistivity of the film and the applied voltage primarily falls on the GaAs substrate. When positive voltage is applied to the heterostructures SiO2(Co)/GaAs as it is shown in Fig. 1, electrons are injected from the granular film into the substrate. The resistivity of GaAs substrate is higher than the resistivity of the SiO2(Co) films, therefore applied voltage primarily falls in the semiconductor substrate. Fig. 2 illustrates the magnetoresistance effect as a function of the applied voltage. The injection magneto-resistance coefficient is IMR = (R H − R0 ) /R0 [7,8], where R0 and RH are the resistances of the SiO2(Co)/GaAs heterostructure without and in an applied magnetic field H, respectively. For applied voltage U > 52 V , a sharp increase of the electrical current is observed due to the process of impact ionization. Influence of the magnetic field on the current flowing in SiO2(Co)/GaAs heterostructures in the avalanche regime was analyzed in Ref. [8]. The largest magneto-resistance coefficient was observed for the SiO2(Co)/GaAs heterostructure with Co concentration of 60 at%, therefore it was used in experiments on the magnetic-field-induced photocurrent [16]. Fig. 3a shows the current j in the SiO2(Co 60 at%)/GaAs structure as a function of the magnetic field H at room temperature. The applied field suppresses the avalanche process and the current flowing in the SiO2(Co)/GaAs heterostructure decreases [8]. The suppression of the avalanche process causes the sharp growth of the magneto-resistance coefficient. Fig. 3b demonstrates a
Fig. 3. (a) Current j flowing in the SiO2(Co)/GaAs structure with 60 at% Co versus the magnetic field H at room temperature at different values of the applied voltage. (b) Magnetic hysteresis curve obtained by the longitudinal magneto-optical Kerr effect.
magnetic hysteresis curve obtained by longitudinal magneto-optical Kerr effect. The hysteresis curve has a small coercivity due to direct exchange in clusters of Co nanoparticles.
4. Experiments on the magnetic-field-induced photocurrent Spectral dependencies of the photocurrent change Δj caused by the polarized light at different voltages U without magnetic field and in the field H¼ 2.5 kOe are shown in Fig. 4. The photocurrent
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Fig. 4. Spectral dependencies of the photocurrent change Δj in SiO2(Co)/GaAs heterostructure with 60 at% Co caused by the linearly polarized light at different values of the applied voltage U without magnetic field and in the field H¼ 2.5 kOe.
dependencies are different for high and low values of the photon energy E in the magnetic field and without a field. For photon energies E > 1.36 eV at voltages U ≥ 65 V in the magnetic field the photocurrent increases. For the photon energies E < 1.36 eV at high voltages a negative photoconductance is observed [9]. Fig. 5 shows the photocurrent change Δj at photon energies 1.375, 1.4, 1.45 and 1.5 eV versus the magnetic field H for different applied voltages U at room temperature. If the photon energy E is larger than the GaAs bandgap energy E g ≃ 1.4 eV , the light penetration depth in GaAs is low, and the light is absorbed in SiO2(Co)/GaAs heterostructure near the interface. For the case of E ≥ E g , the photocurrent increases and the enhancement of the photocurrent is observed. The photocurrent is increased by a factor of about 10 for the photon energy E¼ 1.5 eV, the applied voltage U¼80 V and the magnetic field H ¼1.65 kOe. The subsequent magnetic field growth leads to the avalanche suppression and to a photocurrent decrease.
5. Theoretical description and discussion The injection magneto-resistance effect and decrease of the current flowing in the SiO2(Co)/GaAs structure are described by the model based on the spin-dependent potential barrier and on
Fig. 5. Photocurrent Δj in SiO2(Co)/GaAs heterostructure with 60 at% Co caused by the linearly polarized light for different photon energies and values of the applied voltage U versus the magnetic field H at room temperature.
the avalanche suppression in the magnetic field [8,17]. According to the model, accumulation electron layer in GaAs is formed in the interface region and the potential barrier is localized at a certain distance from the semiconductor/film interface. Fig. 6 shows a schematic band diagram of the SiO2(Co)/GaAs heterostructure. The accumulation electron layer contains oxygen ions left after the polishing process. According to Refs. [18,19], in addition to the EL2 defect level there are oxygen-ion levels in the GaAs bandgap with energies E1 = 0.48 eV , E2 ¼ 0.74 eV, E3 ¼1.0 eV, and E4 ¼ 1.25 eV counting from the bottom of the conduction band. At room
V.V. Pavlov et al. / Journal of Magnetism and Magnetic Materials 400 (2016) 290–294
-
conduction band
Au
A
B L
293
+
Avalanche process
holes
valence band Ñ
Co
GaAs
SiO2(Co)
Fig. 6. Schematic band diagram of the GaAs in the avalanche regime for the SiO2(Co)/GaAs heterostructure. A is a potential maximum of barrier, B is the start point of the avalanche, C marks holes produced by the avalanche, L marks energy levels of defects inside the GaAs bandgap.
temperature levels with energies E2, E3 and E4 are occupied by electrons. Conductivity in the conduction band is determined by thermally activated electrons from the level E1 ¼0.48 eV [7] which is shown in Fig. 6 by L. Electrons on the oxygen levels are coupled by the exchange interaction with 3d-electrons of Co nanoparticles via electrons in the accumulation layer. Next we consider the photocurrent enhancement for the light with the photon energy E ≥ E g . Light is absorbed in the semiconductor region near the interface and creates electrons in the conduction band and holes in the valence band in the interface region. According to theoretical models [20–27], recombination of charge carriers is related mainly to nonradiative electron transitions through localized electron levels in the forbidden energy band. The rate of electron–hole recombination depends on the spin states of both paramagnetic centers in the bandgap and free carriers. In order to describe the photocurrent enhancement in the magnetic field, we use Lepine's model with small modifications [20,21,23]. The spin-dependent recombination is preceded by the trapping of conduction electrons on localized electron levels (Fig. 7). We suppose that the level E (a), where conduction electron is trapped, is the level of a shallow state and the level E (b) is a partially occupied oxygen level E1 = 0.48 eV . When the triplet electron
states
|T −〉 = | ↓ ↓ 〉 of the
E (a )
|T0 〉 =
E (a )
and
1 2
(| ↑ ↓ 〉 + | ↓ ↑ 〉),
E (b )
|T+〉 = | ↑ ↑ 〉
and
pair appear (Fig. 7a), the transition
E (b )
is forbidden by the spin conservation law. In this case, a → further recombination is highly suppressed. By contrast, for the singlet state |S〉 = 1 (| ↑ ↓ 〉 − | ↓ ↑ 〉) the spin conservation law 2
permits an electron transition from the first level E (a) to the second level E (b) and a further recombination occurs (Fig. 7b). Under the assumption that triplet states cannot recombine and singlet states recombine with a probability rs, the average recombination probability of electrons on levels E (a) and E (b) is written as R = rs/4 . In the presence of the external magnetic field H the levels E (a) and E (b) are split. Taking into account the exchange interaction between electrons on the oxygen levels with 3d-electrons of Co nanoparticles via electrons in the accumulation layer and assuming that the exchange interaction is isotropic, we obtain that the splitting gap is determined by the effective field H + H (exc ) = αH , where H (exc ) is the field caused by the exchange interaction, α⪢1. Neumann's density operator ρ of electrons on splitted levels in a thermal equilibrium is represented in the Boltzmann form [20,21,23]:
Fig. 7. Spin-dependent recombination through a pair of localized electron levels E (a) and E (b) split by the magnetic field. (a) For the triplet spin state of the pair of electrons on levels E (a) and E (b) the spin-dependent recombination is forbidden. (b) For the singlet spin state the process (1) → (2) → (3) of the recombination of charge carriers is permitted.
ρ0 =
∑i4=1 exp ( − εi /kT )|i〉〈i| ∑i4=1 exp ( − εi /kT )
, (1)
where T is the temperature, |i〉 = {|S〉, |T0 〉, |T+〉, |T −〉}, εs = 0 , ε0 = 0 , ε+ = gμ B αH , ε− = − gμ B αH are Zeeman energies of singlet and triplet states, respectively, g is the g-factor of electrons on localized states, μ B is the Bohr magneton. We consider the change of the average recombination probability ΔR of electrons between the thermal equilibrium electron ensemble (1) and the highly saturated non-equilibrium state with the density operator 1 ρsat = 4 ∑i4=1 |i〉〈i|. Then, the change of the average recombination versus the magnetic field H is
⎡ ⎤ ⎛ gμ B αH ⎞ rs ΔR = rs Tr ⎢|S〉〈S|(ρsat − ρ 0 ) ⎥ = − tanh2 ⎜ ⎟. 4 ⎣ ⎦ ⎝ 2kT ⎠
(2)
Taking into account that the photoconductivity sph and the number of photo-induced electrons ne in the conduction band and the number of holes nh in the valence band are inversely proportional to the recombination probability R and ΔR⪡R , in the second order of the Taylor-series expansion of the relation (2) we find 2 Δσ ph Δne Δnh ΔR ⎛ gμ B αH ⎞ = = = − =⎜ ⎟ . σ ph ne nh R ⎝ 2kT ⎠
(3)
In a magnetic field electron–hole recombination is suppressed. Additional holes Δnh move toward the barrier and are accumulated in the barrier region, thus decreasing the barrier height. This leads to the observed increase of the electron current and to the enhancement of the avalanche in the applied magnetic field. Such positive feedback produced by photo-induced holes and additional photo-induced electrons Δne result in strong changes in the electron current Δj . The experimentally observed quadratic-like dependence of the photocurrent Δj from the magnetic field H at
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low field values (Fig. 4, |H| < 1 kOe) corresponds to the relation (3) and confirms the magnetic dependence obtained in theoretical models developed in [20–22]. We note that increase of the applied voltage changes the photocurrent as shown in Fig. 5. At the magnetic field |H| > 1 kOe the photocurrent increases with the applied voltage. This can be explained by growth of number of electrons which reach the potential barrier and form the avalanche process in the GaAs substrate. At high values of the magnetic field H, the avalanche is suppressed and the photocurrent decreases with the applied magnetic field.
6. Conclusions We observed a significant influence of the magnetic field on the photocurrent in SiO2(Co)/GaAs heterostructures in the avalanche regime at room temperature. At photon energies E larger than the GaAs bandgap energy Eg, at small values of magnetic field the photocurrent increases in accordance with a quadratic-like dependence. The enhancement factor is about 10 for the photon energy E ¼1.5 eV and magnetic field H ¼1.65 kOe. The subsequent field growth suppresses the avalanche process leading to a photocurrent decrease. The photocurrent enhancement is explained by the spin-dependent recombination through the localized electron levels in the semiconductor. In a magnetic field electron–hole recombination is suppressed. This leads to additional free electrons in the conduction band and holes in the valence band. As a result, photo-induced holes are accumulated in the barrier, the photocurrent grows and the avalanche feedback can be manipulated by the light. The photocurrent enhancement caused by the applied magnetic field can be used in field-controlled singlephoton avalanche diodes.
Acknowledgments The authors gratefully acknowledge the assistance of V.M. Lebedev for characterization of the film composition. This work was supported by the Russian Government Project no. 14.B25.31.0025,
the Russian Foundation for Basic Research Project nos. 10-0290023, 13-02-00754, 15-52-12015 and 15-02-06208, and the Programs of Presidium of the Russian Academy of Sciences.
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