APPLIED PHYSICS LETTERS 90, 162505 共2007兲

Spin-dependent transport properties in GaMnAs-based spin
hot-carrier transistors
Yosuke Mizuno
Department of Electronic Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan

Shinobu Ohya
Department of Electronic Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan, and PRESTO, JST, 4-1-8 Honcho, Kawaguchi-shi, Saitama 332-0012, Japan

Pham Nam Hai and Masaaki Tanakaa兲
Department of Electronic Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan共Received 11 January 2007; accepted 19 March 2007; published online 18 April 2007兲
The authors have investigated the spin-dependent transport properties of GaMnAs-based“three-terminal” semiconductor spin hot-carrier transistor 共SSHCT兲 structures. The emitter-basebias voltage VEB dependence of the collector current IC, emitter current IE, and base current IB showsthat the current transfer ratio ␣ 共=IC / IE兲 and the current gain ␤ 共=IC / IB兲 are 0.8–0.95 and 1–10,
respectively, which means that GaMnAs-based SSHCTs have current amplification capability. In
addition, the authors observed an oscillatory behavior of the tunneling magnetoresistance ratio with
the increasing bias, which can be explained by the resonant tunneling effect in the GaMnAs
quantum well. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2724771兴
III-V-based ferromagnetic-semiconductor heterostructures containing GaMnAs can be a good model system for
future spintronic devices. Large tunneling magnetoresistance
共TMR兲 of 75% 共8.0 K兲 共Ref. 1兲 and 290% 共0.39 K兲 共Ref. 2兲
were observed in the GaMnAs-based single-barrier magnetic
tunnel junctions 共MTJs兲. Also, the resonant tunneling effectwas observed in the GaMnAs quantum-well heterostructures,
which indicates that highly coherent tunneling occurs in theGaMnAs-based heterostructures.3 However, there are no reports concerning ferromagnetic-semiconductor-based “threeterminal” spin devices with amplification capability. In
three-terminal devices, the spin-dependent transport properties can be changed by bias voltages. If amplifiability is attached to the devices, they can be applied to integrated circuits with many functionalities. As such spin devices,magnetic tunnel transistors 共MTTs兲 have recently captured a
lot of attention.4–6 Metal-based MTTs are composed of ferromagnet 共FM兲/insulator 共I兲 / FM/ semiconductor, whose output currents can be controlled by the bias voltages and magnetization orientation of the FM layers. However, it is very
difficult to fabricate metal-based MTTs with amplifiability
because the transfer ratio ␣ defined by IC / IE 共IC and IE are
the collector and emitter currents, respectively兲 is very low共艋10−2兲 共Refs. 4–6兲 due to the frequent scatterings in themetallic base.
Here, we propose and fabricate a semiconductor spin
hot-carrier transistor 共SSHCT兲. SSHCT is structurally similar
to conventional metal-based MTTs but composed of allepitaxial semiconductor heterostructures, which enables advanced band engineering and has good compatibility withthe existing semiconductor technology. Recently, we havetheoretically shown that, unlike metal-based MTTs, SSHCTs
potentially have amplifiability and that we can control thea兲

Electronic mail: masaaki@ee.t.u-tokyo.ac.jpsign of their magnetocurrent ratio due to the resonant tunneling effect.7 By taking advantage of these features, SSHCTs
are expected to have application possibilities not only fornonvolatile memories and high-accuracy magnetic-field sensors but also for the components of multivalued logic andreconfigurable logic circuits.8 In this letter, we report on thespin-dependent transport properties of GaMnAs-based threeterminal SSHCT structures.
We grew SSHCT structures composed of 共from thetop surface to the substrate兲 Ga0.95Mn0.05As 共30 nm兲 / GaAs
共1 nm兲 / AlAs
共2 nm兲 / GaAs
共1 nm兲 / Ga0.95Mn0.05As
共30 nm兲 / Be-doped GaAs 共30 nm兲 on p-type GaAs共001兲 substrates using low-temperature molecular-beam epitaxy. TheBe concentration of the Be-doped GaAs 共GaAs:Be兲 layer
was 1 ⫻ 1017 cm−3. Figure 1共a兲 shows their layered structure,
the substrate temperature, and the reflection high-energy
electron diffraction 共RHEED兲 pattern during the growth process. The two 1-nm-thick GaAs spacer layers are inserted toprevent the Mn diffusion into the AlAs layer and to obtainatomically flat interfaces. Figure 1共b兲 shows the schematic
valence band diagram of the GaMnAs-based SSHCT structures. In this diagram, spin-polarized hot holes are injectedfrom the GaMnAs emitter into the GaMnAs base by tunneling. During the transmission through the ferromagnetic
GaMnAs base, they lose energy due to spin-dependent scattering. Only those carriers that maintain enough energy to goover the Schottky-like barrier at the base/collector interface
can contribute to the collector current. The SSHCT structures
were fabricated into three-terminal devices with an emitter
共E兲, a base 共B兲, and a collector 共C兲, as schematically shownin Fig. 1共c兲, by standard photolithography and chemical wet
etching techniques. Etching was stopped in the base layer, on
which a metal electrode was formed by evaporating Au. The
active area of the tunnel junction and the total area of thebase layer are 50⫻ 100 and 100⫻ 200 ␮m2, respectively.
The following transport measurements were performed with

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Appl. Phys. Lett. 90, 162505 共2007兲

FIG. 3. Collector current IC, emitter current IE, and base current IB as afunction of VEB at 2.6 K at VBC = 0.1 mV. The inset shows the current gain ␤
共=IC / IB兲 as a function of VEB.

FIG. 1. 共Color兲 共a兲 Schematic layered structure of GaMnAs-based semiconductor spin hot-carrier transistors 共SSHCTs兲 consisting of Ga0.95Mn0.05As
共30 nm/ 兲GaAs 共1 nm兲 / AlAs 共2 nm兲 / GaAs 共1 nm兲 / Ga0.95Mn0.05As
共30 nm兲 / GaAs: Be 共30 nm; doping density NA = 1 ⫻ 1017 cm−3兲 grown onp-type GaAs共001兲 substrates, accompanied by substrate temperature Ts andRHEED patterns during the growth process. 共b兲 Schematic valence band
diagram of GaMnAs-based SSHCT structures. Region 1 is the emitter, region 2 is the AlAs tunnel barrier, and region 3 is the base. The collector isthe p-type GaAs共001兲 substrate 共region 4兲. 共c兲 Schematic device structure ofthe GaMnAs-based SSHCTs fabricated in this study.

a conventional direct-current 共dc兲 method in common-baseconfiguration at 2.6 K, which is sufficiently lower than theCurie temperature 共⬃40 K兲 of the GaMnAs layers.
Figure 2 shows the transistor characteristics, i.e., the VBC
dependence of the collector current IC with various VEB ranging from 0 to 1000 mV with 100 mV steps. With increasing

VEB, the energy of the hot holes injected into the base layer
becomes higher, which results in the increase of the numberof those carriers that can go over the collector barrier. Meanwhile, with increasing VBC, the effective thickness of theSchottky-like collector barrier is reduced, which promotes
the tunneling phenomena through the barrier. In this way, ICis well controlled both by VBC and by VEB. From Fig. 2,
using the method proposed by Lacour et al.,9 we also
confirmed that there is no contribution of the leakage current
to IC.
Figure 3 shows the VEB dependence of IC, IE, and IB
when VBC = 0.1 mV in parallel configuration with no magnetic field applied. The current transfer ratio ␣ 共=IC / IE兲 isestimated to be 0.8–0.95, which is much higher than themaximum value 共␣ = 0.03兲 reported in metal-based MTTs.6
The inset of Fig. 3 shows the current gain ␤ 共=IC / IB兲 vs VEB.
The noise at low VEB 共⬍0.04 V兲 is due to the detection limitof our measuring equipment. The obtained ␤ increases linearly with VEB, and is of the order of 10, which means that
GaMnAs-based SSHCTs have current amplifiability. In asimple model, ␣ is given by ␣ = exp 共−t / ␭兲, where t is thebase layer thickness and ␭ is the hole energy attenuationlength. In general, metal-based MTTs are thought to have ␭of less than 3 nm.10,11 However, GaMnAs-based SSHCTs are
considered to have ␭ of several tens of nanometers,3 since
they are composed of fully epitaxial single crystal layers,which leads to the drastic enhancement of ␣ and ␤.
The inset of Fig. 4共a兲 shows the TMR curve when thebias voltage VEC between the emitter and the collector is
350 mV at 2.6 K with a magnetic field B applied along the关1̄10兴 axis in plane. Here, RA in the vertical axis is the resistance area 关共tunnel resistance VEC / IC兲⫻共junction area兲兴.
The base electrode was kept open. Clear TMR 共maximum of
5.3%兲 was observed. In this way, IC is well controlled by themagnetization orientation of the emitter and the base layer.
Figure 4共a兲 shows the VEC dependence of the TMR ratio at
2.6 K when a magnetic field B is applied along the 关1̄10兴
axis in plane. With increasing VEC, the TMR ratio dropped
sharply at around VEC = 77 mV. Then it reached its maximumof 5.3% when VEC is 350 mV, and monotonically decreased
after that. Even when VEC = 2 V, clear TMR 共less than 0.1%兲
still remained. This bias dependence of the TMR ratio had
good reproducibility in different devices. On the grounds that
TMR disappears at some hundreds of millivolts in normal
GaMnAs-based single-barrier MTJs,12,13 the actual voltage

FIG. 2. Transistor characteristics, i.e., the collector current IC as a functionof VBC at 2.6 K with various VEB ranging from 0 to 1000 mV 共each step is
100 mV兲.
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Appl. Phys. Lett. 90, 162505 共2007兲

Mizuno et al.

different VEC. The local minima are considered to result fromthe resonant tunneling effect. The quantum level 共77/ 10= 7.7 meV兲 is close to the calculation result of 10 meV of thefirst quantum level of heavy holes.7 We can estimate thespin-splitting energy ⌬EHH of the first quantum level ofheavy holes by the difference of the voltages VEC, where RA
becomes minimal in parallel and antiparallel configurations.
Since the difference of VEC is 1 mV 共=77− 76兲, ⌬EHH iscalculated to be 0.1 meV 共=1 / 10兲, which is consistent withthe calculation result that ⌬EHH is negligibly small in GaMnAs layers when magnetization is in plane.15
In summary, we have measured the spin-dependenttransport properties of GaMnAs-based three-terminal
SSHCT structures. High transfer ratio ␣ and current gain ␤
were obtained, indicating that GaMnAs-based SSHCTs havecurrent amplifiability. The oscillatory behavior of the bias
dependence of the TMR ratio was explained by the resonant
tunneling effect in the GaMnAs quantum well.
This work was partly supported by PRESTO/SORST ofJST, Grant-in-Aids for Scientific Research, IT Program ofRR2002 of MEXT, and Kurata-Memorial Hitachi Scienceand Technology Foundation.
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FIG. 4. 共Color兲 共a兲 TMR ratio as a function of VEC at 2.6 K when a magnetic field B is applied along the 关1̄10兴 axis in plane. The inset shows theTMR curve; the resistance area RA = 共tunnel resistance VEC / IC兲⫻共junctionarea兲 as a function of magnetic field B at 2.6 K when VEC = 350 mV. 共b兲 RAas a function of magnetic field B when the field is applied along the 关1̄10兴axis in plane at 2.6 K. The inset shows its magnified view at the kink around
VEC = 77 mV.

applied between the emitter and the base is assumed to beapproximately one-tenth of VEC because a large part of VEC
contributes only to the deformation of the Schottky-like collector barrier. This assumption is appropriate considering thatVhalf, at which TMR is reduced by half, is about 450 mV
关=800− 350, as shown in Fig. 4共a兲兴, which is ten times larger
than those of GaMnAs-based single-barrier MTJs.2,12,13 The
peak of the TMR ratio at VEC = 350 mV thus corresponds to
the TMR maximum occasionally observed at some tens ofmillivolts in GaMnAs-based MTJs,3 which can be reproduced by a theoretical calculation based on the transfermatrix method and the Esaki-Tsu formula.14
Figure 4共b兲 shows the VEC dependence of RA between
the emitter and the collector in parallel and antiparallel configurations. In Fig. 4共b兲, the two curves have a kink at around
VEC = 77 mV, where the TMR ratio sharply drops. The magnified view at the kink is shown in the inset of Fig. 4共b兲.
Both curves have local minimal values of RA, but at slightly

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