phys. stat. sol. (c) 3, No. 12, 4184 – 4187 (2006) / DOI 10.1002/pssc.200672882

Resonant tunneling effect and tunneling magnetoresistancein GaMnAs quantum-well double-barrier heterostructures
S. Ohya*, 1, P. N. Hai1, Y. Mizuno1, and M. Tanaka1, 2
1

Department of Electronic Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
2
Japan Science and Technology Corporation, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Received 10 August 2006, accepted 14 October 2006
Published online 23 January 2007
PACS 72.25.Dc, 73.40.Gk, 73.43.Qt, 75.50.Pp, 85.75.Mm
We have investigated the d2I/dV2-V characteristics and tunneling magnetoresistance (TMR) in theGaMnAs quantum-well (QW) double-barier heterostructures, and compared them with those of theGaMnAs-based single-barrier heterostructure. We obtained a relatively high TMR ratio of 126.5% in thesingle-barrier heterostructure at 2.6 K. The value of d2I/dV2 and the TMR ratio of this single-barrier heterostructure monotonically approached zero with increasing the bias voltage. On the other hand, in thedouble-barrier structures, oscillatory behaviors were seen in the negative bias region of d2I/dV2-V curves.
With increasing the QW width, these peaks shifted to a smaller voltage and the period of the oscillation
became short. This means that these oscillatory behaviors are induced by the resonant tunneling effectmediated by holes tunneling through the GaMnAs QW. Also, we observed that TMR is increased at theseresonant peak bias voltages in the double-barrier heterostructures.
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Ferromagnetic-semiconductor heterostructures containing GaMnAs are hopeful candidates for future spintronic devices. Large tunneling magnetoresistance (TMR) of 75% (8.0 K) and 290%(0.39 K) were observed in the GaMnAs-based single-barrier heterostructures [1, 2]. Also, the GaMnAsbased quantum heterostructures are expected to realize new functions by combining TMR and the quantum-size effect (resonant tunneling effect). For example, large enhancements of TMR up to ~800% [3]and to more than ~106% [4] are expected to be obtained in GaMnAs-based resonant tunneling diode(RTD) structures. In magneto-optical measurements of the GaMnAs quantum well (QW) heterostructures, blue shifts of the magneto-optical spectra have been observed, suggesting the existence of thequantum-size effect in GaMnAs [5, 6]. However, there are no reports on the clear observation of theresonant tunneling effect in ferromagnetic-semiconductor quantum heterostructures [4, 7, 8]. We investigated the d2I/dV2-V characteristics and TMR in the GaMnAs-QW double-barier heterostructures withvarious QW thicknesses, and compared them with those of the GaMnAs-based single-barrier heterostructure. We clearly observed the resonant tunneling effect and the TMR increase induced by it in the GaMnAs-QW double-barrier heterostructures. In this paper, we present these experimental results.
2 Growth and measuring methods The GaMnAs-QW double-barrier heterostructurses investigatedhere are composed of Ga0.95Mn0.05As(20 nm)/ GaAs(1 nm)/ Al0.5Ga0.5As(4 nm)/ GaAs(1 nm)/
Ga0.95Mn0.05As QW (d nm)/ GaAs (1 nm)/ AlAs (4 nm)/ GaAs:Be(Be: 1×1018cm–3) with various d from
3.8 to 20 nm. These heterostructures were grown on p-GaAs(001) substrates by molecular-beam epitaxy(MBE). The 1-nm-thick spacer layers were inserted in order to prevent the Mn diffusion into the barrier*

Corresponding author: e-mail: ohya@cryst.t.u-tokyo.ac.jp, Phone: +81 3 5841 6729, Fax: +81 3 5803 3975

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layers and to smooth the surface. The GaAs:Be, AlAs, and the lowest GaAs spacer layers were grown athigh temperatures of 600, 550, and 600 ºC, respectively. The GaMnAs layers were grown at low temperature of 225 ºC, and the GaAs/ AlGaAs/ GaAs barrier layers were grown at 205 ºC. In these structures, TMR occurs by tunneling of holes between the ferromagnetic GaMnAs top electrode and the ferromagnetic GaMnAs QW. Also, we grew a single-barrier heterostructure composed of Ga0.95Mn0.05As(20nm)/ GaAs(1 nm)/ AlAs(2 nm)/ GaAs(1 nm)/ Ga0.95Mn0.05As (20 nm)/ GaAs:Be (100 nm)/ p-GaAs(001)as a reference sample. The growth conditions used for the growth of this single-barrier structure were thesame as those used for the double-barrier structure. After the growth, circular mesa diodes with 200 µmin diameter were fabricated by chemical etching. We spin-coated an insulating negative resist on thesample, opened a contact hole with 180 µm in diameter on the top of the mesa, and fabricated a metalelectrode by evaporating Au on this surface. In the following measurements, the bias polarity is definedby the voltage of the top GaMnAs electrode with respect to the substrate. The schematic band diagrams
when positive and negative biases are applied to the double-barrier structure are shown in Figs. 1(a) and(b), respectively. The following tunneling transport measurements were carried out in a cryostat cooled
at 2.6 K with a conventional two-terminal direct-current (DC) method. dI/dV-V and d2I/dV2-V characteristics were derived mathematically from the data of the I-V characteristics measured at every 5 mV. The
results of the bias dependence of TMR were obtained mathematically from the data of I-V characteristics
measured in parallel and antiparallel magnetizations. I-V characteristics in both parallel and antiparallelmagnetizations were obtained at zero-magnetic field.

Fig. 1 Schematic band diagrams of the GaMnAs-QW
double-barrier heterostructure when the bias polarity is(a) positive and (b) negative. Here, the 1-nm-thick GaAs
spacer layers are omitted for simplicity.

3 Results Figures 2 shows (a) dI/dV-V, (b) d2I/dV2-V characteristics and (c) bias dependence of TMR
observed in the GaMnAs/ AlAs/ GaMnAs single-barrier heterostructure (reference sample) at 2.6 K. In(a), solid and dotted curves correspond to the data in parallel and antiparallel magnetizations, respectively. The values of dI/dV and d2I/dV2 monotonically change with increasing the bias voltage |V| except
for those near zero bias. The sharp features near zero bias correspond to the zero-bias anomaly which isusually observed in GaMnAs-based heterostructures [9]. These results of the monotonical changes indI/dV and d2I/dV2 are different from those observed in metal-based magnetic tunnel junctions, whereinelastic scattering peaks of magnons and phonons have been usually observed [10, 11]. The inset of (c)shows the magnetic field dependence of the tunnel resistance at 2.6 K observed in this sample with amagnetic field applied in plane along the [100] direction and with a bias voltage of 1 mV. We can see atypical TMR curve with a relatevely high TMR ratio of 126.5%. Here, TMR is defined as (RAP-RP)/RP,
where RP and RAP are the tunnel resistances (=V/I) for parallel and antiparallel magnetization, respectively. We can see that the TMR value rapidly decreases with increasing the bias voltage. The bias voltage Vhalf, at which the TMR ratio is reduced by half, was 14 mV in this sample.

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S. Ohya et al.: Resonant tunneling effect and magnetoresistance in GaMnAs heterostructures

Fig. 2 (a) dI/dV-V and (b) d2I/dV2-V characteristics ofthe GaMnAs/ AlAs/ GaMnAs single-barrier heterostructure (reference sample) at 2.6 K. In (a), thesolid and dotted curves correspond to the parallel andantiparallel magnetizations, respectively. (c) Bias
dependence of TMR of this sample at 2.6 K, when themagnetic field was applied in plane along the [100]axis. The inset shows the magnetic field dependenceof the resistance-area product (RA) observed in theGaMnAs single-barrier heterostructure at 2.6 K witha bias voltage of 1 mV. Typical TMR curve with arelatively high TMR ratio of 126.5% was obtained.

Fig. 3 (a) d2I/dV2-V characteristics of GaMnAs-QW
double-barrier heterostructures with various QW thicknesses d in parallel magnetization at 2.6 K. Numbers in
the parentheses express the magnification ratio for thevertical axis. (b) Bias dependence of TMR of thesejunctions at 2.6 K, where the TMR ratios are normalized
by the maximum value of TMR in each curve. The inset
is the magnetic-field dependence of the tunnel resistanceobtained in the junction with d = 12 nm when a magnetic
field was applied in plane along the [100] direction at 2.6
K with the bias voltages of +10 mV (red curve) and -104
mV (blue curve).

Figure 3(a) shows the d2I/dV2-V characteristics of the GaMnAs-QW double-barrier heterostructureswith various d at 2.6 K in parallel magnetization. As well as the single-barrier structure, the zero-bias
anomaly is seen near zero bias in all the curves. The most different features are the oscillatory behaviors
observed in the negative bias region of all the curves. With increasing d, these peaks shift to the smallervoltage and the period of the oscillation becomes short. This means that these oscillatory behaviors are
induced by the resonant tunneling effect mediated by holes tunneling through the GaMnAs QW. Figure© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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3(b) shows the bias dependence of TMR with a magnetic field applied in plane along the [100] direction.
The inset shows the TMR curves observed at 2.6 K in the double-barrier heterostructure with d = 12 nm
when the bias voltage is +10 mV(red curve) and -104 mV(blue curve), where the TMR ratios are 18.9%
and 14.1%, respectively. In contrast to the single-barrier heterostructure, even when the voltage of -104
mV is applied, more than a half of the TMR value obtained at 10 mV is maintained. In the main graph ofFig. 3(b), TMR oscillations can be seen in the negative bias region of all the curves. With increasing d,the TMR peaks shift to a smaller voltage as is the case of the d2I/dV2-V characteristics shown in Fig. 3(a),indicating that these TMR increases are induced by the resonant tunneling effect. Also, Fig. 3(b) indicates that Vhalf can be increased by the resonant tunneling effect. In the negative-bias region of d = 12nm, |Vhalf| was increased to 124 mV due to the TMR increase at -104 mV, whereas Vhalf in the positive
bias was 76 mV. This value is much higher than that obtained in the single-barrier structure mentioned
above, and also higher than those reported in GaMnAs-based single-barrier heterostructures which areusually around 50 mV [2, 12, 13]. This result indicates the possibility of controlling the TMR ratio andVhalf by using the resonant tunneling effect in ferromagnetic-semiconductor quantum heterostructures.
4 Summary We investigated the d2I/dV2-V characteristics and the TMR behaviors in the GaMnAs-QW
double-barier heterostructures, and compared them with those of the GaMnAs-based single-barrier heterostructure. We obtained a relatively high TMR ratio of 126.5% in the GaMnAs single-barrier heterostructure at 2.6 K. The value of the d2I/dV2 and the TMR ratio monotonically approached zero withincreasing the bias voltage in this single-barier structure. On the other hand, oscillatory behaviors wereobserved in the negative bias region of the d2I/dV2-V curves in the double-barrier heterostructures. With
increasing d, these peaks shift to smaller voltages and the period of the oscillation becomes short. Thismeans that these oscillatory behaviors are induced by the resonant tunneling effect mediated by holes
tunneling through the GaMnAs QW. Also, the TMR increases were observed at these resonant peak biasvoltages in the double-barrier heterostructures. These TMR increases induced the enhancement of Vhalf,indicating the possiblity that we can control the TMR ratio and Vhalf by using the resonant tunneling
effect in ferromagnetic-semiconductor quantum heterostructures.
Acknowledgments This work was partly supported by SORST of JST, Grant-in-Aids for Scientific Research, IT
Program of RR2002 of MEXT, and Kurata-Memorial Hitachi Science & Technology Foundation.

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