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Physica E 40 (2008) 3004–3008

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Hot-electron transport properties of CoFe/n-Si andCoFe/Cu/n-Si junctions

Xiao-Li Tang�, Huai-Wu Zhang, Hua Su, Zhi-Yong Zhong, Yu-Lan Jing

State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu,

Sichuan 610054, China

Received 26 July 2007; received in revised form 17 November 2007; accepted 25 March 2008

Available online 4 April 2008

Abstract

The hot-electron transport properties of different thickness of CoFe films deposited on n-Si substrate with and without Cu layer were

investigated. Diode characteristics were tested to obtain the heights of Schottky barrier for different samples. The dependences of

Schottky heights on CoFe thickness were studied. The research shows that the height of the Schottky barrier can be adjusted and a good

Schottky diode can be obtained by controlling the thickness of CoFe film accurately. The results are very important for the application of

spintronic devices, such as spin valve transistor (SVT) and magnetic tunnel transistor (MTT).

r 2008 Elsevier B.V. All rights reserved.

PACS: 75.50.�y; 75.70.�i; 75.50.Bb

Keywords: Schottky barrier; Transport properties; Spintronics; Polarizability; Magnetic film

1. Introduction

Recently, spintronics has emerged as an active researchfield. The integration of magnetism and electronics is a maingoal of research for innovative applications [1–3]. The devicesconstructed as a hybrid of magnetic metals and semiconduc-tors have been proposed [4]. Spin valve transistor (SVT) andmagnetic tunnel transistor (MTT) are proposed as devicesusing this new method using. In SVT Schottky barrier is usedas an emitter barrier and a collector barrier [5], and in MTTthe tunnel barrier is used as an emitter barrier and theSchottky barrier is used as a collector barrier [6]. Electronsare only able to enter the collector if they have retainedsufficient energy to overcome the energy barrier at thecollector side, which is chosen to be somewhat lower thanthat of the emitter barrier. Therefore, in these two structures,the height of the Schottky barrier is very important. It toobtain the desired high-quality Schottky barrier with goodrectifying behavior is very important. In research [5], Pt andAu were used to incorporate at the emitter and collector side

e front matter r 2008 Elsevier B.V. All rights reserved.

yse.2008.03.008

ing author. Tel.: +862883201440.

ess: tangtang1227@163.com (X.-L. Tang).

to separate the magnetic layers from direct contact with Si,and rectify the Schottky height. In other researches [7,8], athin insulator such as Al2O3 has been chosen to insert at theinterface to adjust the barrier. These ways work very well.At present, the impact of the thickness of the film on the

Schottky barrier is seldom considered. According to thesemiconductor physics, the Schottky barriers are onlyrelated to the type of the metal. When the normal metal ischanged to magnetic metal, the question posed is ‘‘will themagnetic moment at the ferromagnet/n-Si interface effectthe transport of the hot-electron?’’. The state of themagnetic moment distribution at the interface is relatedto with the film thickness. Therefore, in this paper, wemainly discussed the relationship between the Schottkybarrier and the magnetic film thickness. Otherwise, in orderto find the real reason for the effects of the ferromagnet/n-Si contact, we also fabricated ferromagnet/normal metal/n-Si junctions for comparison.

2. Fabricated procession

Because CoFe has large polarization [9,10], it is oftenselected as a polarization layer in fabricating spintronic

ARTICLE IN PRESSX.-L. Tang et al. / Physica E 40 (2008) 3004–3008 3005

devices [4,11]. So, CoFe was chosen as the magneticlayer, and the typical sample structures in this paper wereCoFe/n-Si and CoFe/Cu/n-Si.

The doped concentration is 1� 1016/cm3 for n-Si. Thesubstrates were prepared for deposition by a cleaningprocess consisting of a dip into a 1% HF solution toremove the native oxide layer, followed by an ultrasonicbath in glassware cleaning solution, and a final rinse in astream of ultrapure distilled water. They were then loadedimmediately into the load chamber. The films werefabricated by LS500 DC and RF sputtering system; thedeposition rate for CoFe and Cu layers was approximately0.1 nm/s and was controlled by a quartz-crystal depositionmonitor. All junctions with size of 200 mm� 400 mm wereformed using shadow masks, which are shown in Fig. 1.

During fabrication, the base vacuum and workingpressure were 8� 10–8 and 7� 10–4mbar, respectively.A permanent magnet which produced a field of 300Oeparallel to the substrate surface along the long axis of thejunction was present during film growth to develop the easyaxis anisotropy. At first, CoFe(3–8 nm) or CoFe(3–8 nm)/Cu(3 nm) films were deposited directly onto the n-Sisubstrate by DC sputtering. Next, a thick Al2O3 (200 nm)

Fig. 1. Fabricated masks of ferromagnet/semiconductor junction.

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40

50

60

703nm4nm5nm6nm7nm8nm

Applied Voltage(V)

Cur

rent

(µA

)

-0.5 0.0 0.5 1.0 1.5

Fig. 2. (a) I–V curves for type-I CoFe (3–8 nm)/n-Si and (b

was grown for insulation between the top electrode and thesubstrate by RF reaction sputtering with a mixed gas ofoxygen and argon in a 1:10 ratio. At last the top electrodewas fabricated for testing.

3. Results and discussion

Two types of junctions CoFe(3–8 nm)/n-Si (Type-I) andCoFe(3–8 nm)/Cu(3 nm)/n-Si (Type-II) were fabricated forresearch. Their typical current–voltage (I–V) characteristicsobserved were tested by the Keithley 2400 source/measurement unit using the four-terminal method imme-diately after deposition without applying a magnetic field.The positive bias was applied to the magnetic film. Thetransport measurements results are displayed in Fig. 2.It is evident from Fig. 2(a) that the thickness of CoFe

layer affects the I–V characteristics obviously when it isdirectly deposited on the Si substrate with the thicknesschanged from 3 to 8 nm as type-I. However, when thejunction was fabricated as type-II, the thickness of CoFelayer hardly affects the I–V characteristics, which is shownin Fig. 2(b).

(a) Junction, (b) isolator layer (Al2O3) and (c) top electrode.

3nm4nm5nm6nm7nm8nm

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70

80

90

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Cur

rent

(µA

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) I–V curves for type-II CoFe (3–8 nm)/Cu (3 nm)/n-Si.

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Table 1

The Schottky heights and ideality factors for Type-I and Type-II with

changing of CoFe film thickness

Type Thickness (nm) Schottky height (eV) Ideality factor

Type-I 3 0.62 1.5

4 0.62 1.5

5 0.62 1.5

6 0.65 1.72

7 0.66 1.78

8 0.68 1.78

Type-II 3–8 E0.59 1.5

X.-L. Tang et al. / Physica E 40 (2008) 3004–30083006

According to the thermionic-emission theory [12], thecurrent density at a certain temperature T is given by

J ¼ J0 exp½qðV þ DjBiÞ=kBT � (1)

J0 ¼ AnnT2 exp½�qjB=kBT � (2)

where A**, q, jB, DjBi stand for the Richardson constant( ¼ 120A/cm2/K2), the electron charge, the Schottkybarrier height, and the lowering of the barrier due to theimage force. Therefore, the Schottky height for differentthickness of CoFe can be calculated based on the currentoffset (I0) at zero applied bias:

qfB ¼ �kBT lnI0

AreaAnnT2

� �(3)

where kB is the Boltzman constant and Area is the area ofjunction; the ideality factor n can be also calculated basedon the slope of the straight line:

n ¼kT

q

dðln IdiodeÞ

dV

� ��1(4)

The measured I–V characteristic is plotted on a semi-logscale, shown in Fig. 3. Only two different thicknesses of thefilm about type-I are displayed; the other samples can beachieved in the same way.

Therefore, the heights of Schottky barrier and idealityfactor n, which are listed in Table 1, can be achieved.

The typical barrier height for Cu/n-Si Schottky barrier is0.6 eV. Our testing value 0.59 is quite a reasonablevalue compared with it. From Table 1, it is seen that thethickness of CoFe layer hardly has any impact was onthe height of the Schottky barrier when it is fabricated onCu layer. However, when CoFe layer was directlydeposited on Si substrate, its thickness made a greatimpact on the barrier height. It was so excited, since themagnitude of emitter and collector current of SVT andMTT directly depends on the Schottky barrier height.

-0.3

1E-8

1E-7

1E-6

1E-5

1E-4

ForwardReverse

I0Cur

rent

(abs

)[A

]

Applied Voltage(V)

3nm

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Fig. 3. (a) Semi-log plot of the absolute value of the diode current vs. the applie

of the diode current vs. the applied voltage of a CoFe (8 nm)/n-Si.

Therefore, adopting Type-I can rectify the barrier heighteasily and control the current by changing the magneticfilm thickness.According to semiconductor physics, the height of

Schottky barrier is decided by the metal which is directlycontacted with it. In our experiments, this layer is changedfrom normal metal to magnetic metal. Therefore, thechanging of barrier height may be attributed to themagnetic properties of CoFe layer. For a deeper insightinto the mechanisms involved in the effects of the CoFelayer on Schottky barrier, the hysteresis loops of type-I andtype-II were tested. The hysteresis loops were measured bya BHV-525 vibrating sample magnetometer (VSM) and theapplied field was parallel to the long axis of the sample inthe sample plane. Fig. 4 shows the hysteresis loops of CoFefilm fabricated directly on Si substrate and Cu(3 nm) layer.The CoFe films fabricated directly on Si substrate have

high squareness ratios than the films fabricated on Culayer, but the coercivity Hc for all samples decreases withincrease in the thickness of CoFe layer. The decreaseof Hc is possibly attributed to the decrease of surfaceroughness, which reduces the surface macro- and micro-pinning effects [13].

8nm

1E-9

1E-8

1E-7

1E-6

1E-5

ForwardReverse

Cur

rent

(abs

)[A

]

I0

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Applied Voltage (V)

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d voltage of a CoFe (3 nm)/n-Si and (b) semi-log plot of the absolute value

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-75

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H(Oe)

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Fig. 4. Magnetization hysteresis loops of (a) CoFe(3–8 nm)/n-Si and (b)CoFe(3–8nm)/Cu(3 nm)/n-Si juctions.

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Fig. 5. I–V curves for CoFe (3–8 nm)/n-Si under an external field of 50Oe.

X.-L. Tang et al. / Physica E 40 (2008) 3004–3008 3007

In order to analyze the reason for changing the heightsof Schottky barrier, we considered that the distributions ofmagnetic moments at CoFe/n-Si interface may be theprimary reason for the impact on the hot-electrontransport properties. According to Fig. 4, 3–8 nm CoFefilms have different Hc and magnetization hysteresisloops. It means that the switching procedure for CoFemagnetic moments is different. Therefore, the distributionsand magnitude of the moments at the interface are differentfor the samples after deposition. When the electronstransport from semiconductor to magnetic film, theelectrons with spin aligns with the moments (spin-up) atthe interface are scattered less than the electrons (spin-down) with spin opposites to it, and the scatteringintensities increase with the magnitude of the momentsalong the easy axis. In ferromagnet/n-Si junctions, theeffects of magnetic moments scattering are just likeadding an additional barrier at the interface and thisbarrier height is different for the spin-up and spin-downelectrons. Since the Hc decreased with increase in thethickness of the CoFe layer, the magnetic moments maytend to be the direction of easy axis in the thick CoFe layer.Considering the discussion above, with the increase ofCoFe layer, more amounts of spin-down electrons may getstronger scattering due to the larger amount of spin alongthe easy axis, and not retain sufficient energy to overcomethe Schottky barrier; so the current decreases as shown inFig. 2(a).

For type-II, varieties of CoFe film thickness impacted onthe current slightly and the Schottky barrier heights werealmost the same. It is based on the normal metal Cu, whichcontacts with n-Si, deciding the barrier height. The slightchanging of the current might be due to the bulk scatteringof the magnetic film. It also confirms that when CoFe filmis directly in contact with n-Si (Type-I), the spin-dependentinterface scattering is the crucial factor that affects theSchottky height.

In addition, an external magnetic field of 50Oe was alsoapplied during the measurements of I–V curves for CoFe(3–8 nm)/n-Si junctions, which are shown in Fig. 5. It isobvious from Fig. 5 that the thickness of CoFe layer affectsthe I–V characteristics minor. The external field 50Oe cansaturate 3–8 nm CoFe films, and cause the magneticmoments along the direction of the external magnetic field.Therefore, the additional barrier produced by the spin stateat the interface may be the same for CoFe (3–8 nm)/n-Sijunctions. So, the difference between I–V curves becameminor. The results further confirm that the distribution ofmoments at the interface is the crucial reason for theimpact on the hot-electron transport properties.

4. Conclusion

The impact of magnetic film thickness on the Schottkybarrier was demonstrated. For CoFe/n-Si junction, controlling

ARTICLE IN PRESSX.-L. Tang et al. / Physica E 40 (2008) 3004–30083008

the thickness of CoFe film accurately, the height of theSchottky barrier can be adjusted. This characteristic isimportant for spintronic devices, such as SVT and MTT.For CoFe/Cu/n-Si junction, the variety of CoFe film thicknesshas little impact on the Schottky barrier height. This wasattributed to the spin-dependent interface scattering, whichaffects the Schottky height.

Acknowledgments

The authors are grateful for the support of the NaturalScience Foundation of China (Nos. 90306015, 60721001,and 60771047).

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] F.J. Jedema, A.T. Filip, B.J. van Wees, Nature (London) 410 (2001)

345.

[3] F.J. Jedema, H.B. Heersche, A.T. Filip, J.J.A. Baselmans,

B.J. vanWees, Nature (London) 416 (2002) 713.

[4] S.H. Jang, T. Kang, J.H. Lee, K.Y. Kim, J. Magn. Magn. Mater.

272–276 (2004) 1930.

[5] R. Jansen, J. Phys. D.: Appl. Phys. 36 (2003) R289.

[6] Sebatiaan van Dijken, Xin Jiang, Stuart S.P. Parkin, Appl. Phys.

Lett. 82 (2003) 775.

[7] O.M.J. van’t Erve, G. Kioseoglou, A.T. Hanbicki, C.H. Li,

B.T. Jonker, Appl. Phys. Lett. 84 (2004) 4334.

[8] J.H. Lee, S.Y. Park, Kyung-In Jun, K.-H. Shin, Jinki Hong, K. Rhie,

Phys. Stat. Sol. (B) 241 (2004) 1506.

[9] Evgeny Y. Tsymbal, Oleg N. Mryasov, Patrick R. LeClair, J. Phys.:

Condens. Matter 15 (2003) R109.

[10] Jagadeesh S. Moodera, George Mathon, J. Magn. Magn. Mater. 200

(1999) 248.

[11] Sebastiaan van Dijken, Xin Jiang, Stuart S.P. Parkin, Appl. Phys.

Lett. 80 (2002) 3364.

[12] S.M. Sze, Physics of Semiconductor Device, Wiley, New York, 1981,

p. 245.

[13] M. Li, G.-C. Wang, J. Magn. Magn. Mater. 217 (2000) 199.