spin valve transistor.pdf

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J. Phys. D: Appl. Phys. 33 (2000) 29112920. Printed in the UK PII: S0022-3727(00)13623-X

The spin-valve transistor

P S Anil Kumar and J C Lodder

Information Storage Technology Group, MESA+ Research Institute, University of Twente,PO Box 217, 7500AE, Enschede, The Netherlands

Received 28 April 2000

Abstract. The spin-valve transistor is a magnetoelectronic device that can be used as amagnetic field sensor. It has a ferromagnetsemiconductor hybrid structure. Using a vacuummetal bonding technique, the spin-valve transistor structure Si/Pt/NiFe/Au/Co/Au/Si isobtained. It employs hot electron transport across the spin valve (NiFe/Au/Co). The hotelectrons are injected into the spin valve across the Si/Pt Schottky diode. After traversingacross the spin valve these hot electrons are collected across the AuSi Schottky diode withenergy and momentum selection. The output current is found to be extremely sensitive to thespin-dependent scattering of hot electrons in the spin valve. This gives a magnetocurrentabove 200% in a few oersted of magnetic field at room temperature. The different physicaleffects which govern the output current of the device are examined by studying differenttypes of spin-valve transistors that have Si/Au, Si/Pt and Si/Co collector Schottky diodes andSi(100) and Si(111) orientations. It has been observed that along with the Schottky diodes thevacuum metal bonding also plays an important role in determining the output current. Inaddition, it is realized that collector diodes with extremely low leakage currents, are essentialin order to observe huge magnetotransport properties at room temperature.

1. Introduction

Magnetoelectronic devices are becoming potential candi-dates for technological applications, such as in non-volatile

memory elements, magnetic read and write heads, magnetic

field sensors, etc [1]. In these one exploits the spin of the

electron along with its charge. The discovery of giant mag-

netoresistance in magnetic multilayers has triggered intense

experimental as well as theoretical studies to exploit the po-

tential technological applications and to understand the un-

derlying physicalphenomenathat causethis effect [24]. The

giant magnetoresistance is realized in metallic multilayers

where alternating ferromagnetic and non-magnetic layers are

stacked together. The resistance of such a multilayer stack

depends on the relative alignment of the magnetizations of

the ferromagnetic layers due to spin-dependent scattering atthe interface or the bulk of the ferromagnetic layer. Different

systems such as Fe/Cr, Co/Cu, NiFe/Cu etc, have been exten-

sively studied in order to understand the underlying physical

phenomena [57]. It has been shown that in a Co/Cu mul-

tilayer system, for a certain thickness of the Cu layer, the

adjacent Co layers are antiferromagnetically coupled. In this

antiferromagnetically coupled state, since the magnetization

of the magnetic layers are aligned opposite to each other a

state of high resistance results due to spin-dependent scat-

tering. When a high magnetic field, which can overcome

the antiferromagnetic coupling, is applied, the magnetic lay-

ers are aligned in the same direction and a low-resistancestate results. It has been seen that an approximately 65%

Presented at the TMR Summer School on Mesomagnetism, SpinDynamics and Spin Electronics, Rhodes, September 1999.

magnetoresistance is observed in a Co/Cu system at room

temperature, however one requires a few kilooersted of mag-

netic field to obtain this effect [6]. In the NiFe/Cu system[7] with a relatively low field one can obtain larger effects

(1% Oe1), but these are still much smaller than the sim-

ple anisotropic magnetoresistive NiFe film which exhibits a

magnetoresistance of about 4% Oe1 [8]. For use as a mag-

netic field sensor or a magnetic read head one requires much

higher sensitivity, in accordance with the rapidly expanding

data storage capabilities of the high-density recording me-

dia, so new concepts and technologies are emerging to cope

with these technological demands. In addition to the con-

ventional giant magnetoresistive multilayers, exchange de-

coupled soft spin valves, magnetic tunnel junctions, colossal

magnetoresistive oxides, spin-valve transistors etc are also

being investigated.

An exchange de-coupled soft spin-valve system, where

two ferromagnetic materials whichhave differentcoercivities

are separated by a non-magnetic spacer layer, gives much

higher sensitivity than, for example, the NiFe/Cu/Co system,

giving a magnetoresistance of about 14% with a few

oersted of magnetic field [9, 10]. Here the ferromagnetic

layers can be individually switched by the application of

a suitable magnetic field. The properties of these kinds

of structures are further improved by using pinning layers,

which pin one of the ferromagnetic layers to achieve

well defined magnetic states [11]. It has been observed

that when the conventional current in plane geometry isreplaced by a current perpendicular to the geometry one

obtains enhanced magnetoresistive properties; here the

high sensitivity originates from the fact that the electrons

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P S A Kumar and J C Lodder

pass through all the magnetic layers [12]. However, the

low resistance in the current perpendicular to the plane

geometry gives technological difficulties, due to the high

contact resistance with the electrical probes. Using a

superconducting electrode this problem can be overcome,

but room-temperature operation is not possible. Nowadays,

novel concepts of making nanometre-sized pillars of themultilayers opens the way to have the current perpendicular

to the plane geometry at room temperature [13].

Another promising concept is magnetic tunnel junctions,

where a magnetoresistance of about 11% is observed

with a few oersted of magnetic field [14]. Presently a

magnetoresistance of about 40% is observed in this kind

of tunnel junction [15]. Another class of system that is

emerging as a candidate for magnetoelectronic applications

are the half-metallic ferromagnetic oxides. These oxides

show colossal magnetoresistance at low temperatures or

at high magnetic fields [16, 17], and show a high spin

polarization at low temperatures. Efforts are now being

made to make materials with a high ferromagnetic transitiontemperature so that they can have a high degree of spin

polarization at room temperature [18]. These materials are

being made in theform of multilayer stacks(heterostructures)

to exploit the possibilities of using them as room-temperature

magnetoelectronic devices. Other systems of recent interest

are the pure Bi film that shows a magnetoresistance of about

250% at room temperature at an applied magnetic filed of 5 T

[19]. There have been studies on Ni nanocontacts which give

a magnetoresistance of about 280% with a 100 Oe magnetic

field at room temperature [20].

The functional integration of semiconductor with

ferromagnet resulted in a new magnetoelectronic device, thespin-valve transistor [21]. The first spin-valve transistor

showed a huge magnetocurrent at low temperatures.

Subsequently, the room-temperature operation with 15%

magnetocurrent was achieved [22] using a novel vacuum

metal bonding technique [23]. Recently we have been able to

demonstrate a huge magnetocurrent of over 200% in a spin-

valve transistor at room temperature at very low magnetic

fields [24]. However, the output current of the device was

too low to use in technological applications. It is realized

that an understanding of the different physical phenomena,

which play a role, is crucial in order to improve the figure of

merit of the spin-valve transistor.

In this paper we discuss in detail the operation of the

spin-valve transistor and the fabrication steps to make a spin-

valve transistor that operates at room temperature with huge

sensitivity. In addition, we discuss the results of different

types of spin-valve transistor configurations that have been

made in order to understand some of the physical phenomena

that control the spin-valve transistor characteristics.

2. The concept of the spin-valve transistor

The spin-valve transistor is a three-terminal device analogous

to a metal base transistor. Here, a metal base that contains

a spin valve is sandwiched between two n-type Si wafers, asshown in figure 1. It employs hot electron transport across

the spin valve. In order to achieve injection of hot electrons

into the spin valve a metalsemiconductor interface with a

high Schottky barrier is created at the emitter of the spin-

valve transistor. Since SiPt gives a high Schottky barrier,

the SiPt interface is used as the emitter diode. The injected

hot electrons traverse through the spin valve and reach the

collector side of the spin-valve transistor. The collector

Schottky diode is defined in such a way that it has a lower

barrier height compared to the emitter diode. Here a SiAuSchottky diode is used as the collector diode. It has nearly

0.1 eV less barrier height compared to the SiPt Schottky

diode. In figure 2, the schematic energy level diagram of

a representative spin-valve transistor with a SiPt emitter

diode, SiAu collector diode and a NiFe/Au/Co spin valve is

given (NiFe = Ni0.81Fe0.19). A forward bias of the emitter

injects hot electrons into the spin valve (emitter current IE).

The collector Schottky diode is either reverse biased or zero

biased. While the hot electrons traverse the spin valve

they undergo elastic as well as inelastic scattering. Those

electrons which approach the collector barrier with sufficient

energy and suitable momentum surmount the collector barrier

and constitute the collector current (IC). The inelastic

scattering of hot electrons results in the loss of energy for

these electrons and most of these electrons are not collected,

as they have insufficient energy to overcome the collector

barrier. Even the electrons which are elastically scattered are

sometimesnot collectedbecause theirmomentum is changed.

The electrons which are incident on the collector at an angle

normal to the surface or very close to the normal are only

collected due to the need for momentum conservation [25].

An additional scattering mechanism which is governed by

the spins of the electrons plays a dominant role is the spin-

dependent scattering when two magnetic layers are present in

the spin valve. The spin-dependent scatteringof hot electronsis determined by the magnetic state of the spin valve. As

the permalloy (NiFe) and cobalt magnetic layers have two

different coercivities and are separated by a non-magnetic

gold spacer layer, they can be aligned, either with their

magnetizations parallel to each other or antiparallel to each

other by applying suitable magnetic fields. The hot electrons

after passing to the permalloy layer are spin polarized as most

of the minority spin carriers have extremely low mean free

paths. These spin-polarized electrons traverse through the

Au layer and reach the Co layer. Now, depending upon the

direction in which the Co layer is magnetized, they either

pass through or are scattered. When the magnetizationsof the permalloy and the Co are parallel to each other the

spin-dependant scattering is less and most of these electrons

are expected to pass through the Co layer, and we obtain a

maximum collector current. When the magnetizations are

aligned opposite to each other, due to high spin-dependent

scattering we obtain a lower collector current. So the

collector current is expected to be extremely sensitive to

the magnetic state of the spin valve. As the magnetic state

of the spin valve can be intelligently controlled by suitable

magnetic fields we get a magnetic field dependence for the

collector current. As the electrons traverse across both

the magnetic layers this configuration resembles the current

perpendicular to the plane geometry of conventional giantmagnetoresistive multilayers; this is expected to increase the

sensitivity of the device. Additionally here the electrons

have an energy that is nearly 0.9 eV above the Fermi level

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Figure 1. Schematic diagram of the cross-section of a spin-valvetransistor showing the emitter, base and collector. The emitter isforward biased and the collector is reverse biased. IE is the emittercurrent andIC is the collector current. The base layer contains aspin valve (NiFe/Au/Co) in addition to a SiPt emitter diode and aSiAu collector diode.

Figure 2. The schematic energy diagram of the spin-valvetransistor showing the SiPt emitter and SiAu collector Schottkybarriers and the spin-valve base. EFis the Fermi level, VBE is thebase-emitter bias,VBC is the base-collector bias. The Schottkybarrier heights are also given.

and the spin-dependent scattering asymmetry is expected

to be higher than that observed for the Fermi electrons

[26].

3. Experimental details

3.1. Si wafer preparation

For the fabrication of the spin-valve transistor it is essential

to grow the metal layers directly on Si. The presence of

native oxide on the surface of a semiconductor suppresses

the tunnelling of metal electrons into the forbidden gap of the

semiconductor and, also, it reduces interdiffusion. However,

it is notdesirable as it produces interfacestates andit altersthe

Schottky barrier height, resulting in uncontrollable Schottky

barrier heights [27]. For well defined Schottky barriers it is

desirable to have an oxide-free Si substrate. Also, smooth

growth of the metal layers is important for the vacuum metal

bonding (discussed in the next section). So the processing

of Si wafer is first optimized to obtain oxide-free substrate

with very low surface roughness. At first, the n-type Si wafer

is oxidized thermally to obtain 30 nm of silicon oxide on

top of it. This wafer is annealed at 1100 C to obtain a

well defined Sisilicon oxide interface [28]. Now, a thickphotoresist layer is spun on top of it and sawn into the

required sizes. These small pieces were further processed to

remove the oxide layer and the sawdust just before loading

into the system. The photoresist layer is stripped off using

concentrated HNO3. Then it was subjected to a 1% HF

treatment to remove the oxide covering the sawdust. Then

5% tetra methyl ammonium hydroxide at 80 C is used to

remove the Si sawdust. Finally, 50% HF is used to remove

the thermally grown silicon oxide and is expected to leave the

surface with hydrogen termination [29]. These pieces wereused for the deposition of the metal layers.

3.2. Optimization of the spin valve

The current in-plane magnetoresistance of the spin valve

has to be optimized to incorporate it into the spin-valve

transistor. Here the metal layer has to be directly grown on

Si without any underlying layers, as is usually done to realize

good magnetoresistive properties. Also, the optimization

should be based on the minimum thickness possible for

the individual metal layers, otherwise in the spin-valve

transistor configuration the hot electrons will lose energy

before reaching the collector as the collector current has anexponential dependence on the thickness [21]. The spin-

valve layer has to be grown on either the AuSi or PtSi

surface for the Schottky barrier requirements. It is found

that the metal layers grown on SiPt have better properties

compared to those grown on SiAu. So the spin valve

(NiFe/Au/Co) is deposited on SiPt. The thickness and the

growth of each of these layers is optimized to obtain well

defined switching of the individual magnetic layers and a

in-plane magnetoresistance of about 1% with a few oersted

of applied magnetic field. In figure 3 the magnetic field

dependence of the magnetization of an optimized spin valve

is given. It is seen that we achieved well defined switching

of the Co and NiFe layers. When a high magnetic field is

applied both the permalloy and Co align in the same direction

and we obtain a high net magnetic moment. Now, when the

applied field is decreased to zero both permalloy and Co are

still magnetized in one direction. When the field is reversed

and increased above the coercive field of the permalloy layer,

the permalloy layer switches to the opposite direction and we

obtain a lownet magnetization. In thisstate the magnetization

of the Co and permalloy layers are aligned opposite to each

other. When we again increase the applied field in the

negative direction and when the applied field is greater than

the coercive field of the Co layer, the Co magnetization

also switches in the applied field direction. Again both thepermalloy and Co magnetizations are aligned parallel to each

other and we obtain a higher net magnetization. In figure 4

the magnetoresistance of this spin valve with a Au cap layer

is given; we observe a magnetoresistance of 1%. In this

figure we can see that the Co layer has a lower coercivity

compared to the previous magnetization measurement. This

is believed to be due to the surface oxidation of the Co in the

previous case, as there is no Au caping layer. The optimized

thicknesses (for the minimum thickness of the metal layer)

are Si/Pt (2 nm)/NiFe (3 nm)/Au (3.5 nm)/Co (3 nm).

3.3. Vacuum metal bondingIn order to achieve the structure shown in figure 1 we

developed a vacuum metal bonding technique [23]. As

it is not possible to grow device quality Si layers on

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Figure 3. Magnetization as a function of the magnetic field of thespin valve (NiFe (3 nm)/Au (3.5 nm)/Co (3 nm)) grown on Si/Pt(2 nm). The curve shows well defined switching of the Co and

permalloy layers at the respective coercive fields.

Figure 4. The magnetoresistance against the magnetic field of thespin valve (NiFe (3 nm)/Au (3.5 nm/Co (3 nm)/Au (2 nm)) grownon Si/Pt (2 nm) showing a magnetoresistance of about 1%.

top of metal layers this vacuum metal bonding technique

becomes extremely important in order to realize the spin-

valve transistors. A bonding robot that operates in a

molecular beam epitaxy (MBE) system is used for thevacuum metal bonding. Two cleaned Si pieces were loaded

to the two arms of the bonding robot, a metal layer is then

deposited on both Si pieces and they were brought together

under vacuum to realize the vacuum metal bonding at room

temperature. This technique does not require any further

heat treatment. The Si wafer preparation as described in

the previous section is very crucial in realizing these bonded

structures. If dust particles or Si sawdust are present on

the surface, this can sometimes prevent the bonding as these

micrometre-sized entities prevent the two wafers, to within

a few tens of nanometres of the metal layer, from coming in

contact. So the removal of the Si sawdust is an extremely

important step. In order to prevent the problem arising dueto the dust particles, all of the preparation work is carried out

in a class 100 cleanroom environment. The bonding is tested

for several metal layers, i.e. PtPt, CoCo, AuAu, CuCu,

etc. It was found that the bonding of Au is extremely good

and so Au was chosen as the bonding layer in most of the

prepared spin-valve transistors. In figure 5 the transmission

electron micrograph of an Au bond layer is shown. Here,

5 nm of Au is deposited onto two Si pieces, which were then

bonded under vacuum at room temperature. It is seen from

the micrograph that this technique does not leave any tracesof an interface between the two bonded metal layers. After

testing the reliability of bonding, the required metal layers

for the spin-valve transistor layer (Pt (2 nm)/NiFe (3 nm)/Au

(3 nm)/Co (3 nm)) are now grown on one piece of Si while

a shutter masks the other piece. Then the shutter is opened,

2 nmof Au(or Pt) isdeposited on both pieces and the bonding

is carried out to realize the structure Si/Pt/NiFe/Au/Co/Au

Au/Si.

3.4. Processing of the bonded structure

Since bonding is carried out on large pieces of Si we obtain a

large bonded area (here typically 1 cm2). The leakage currentin the Schottky diodes for such a large area is extremely

high. So the bonded structure is processed into 50 spin-

valve transistors with different sizes, i.e. 350 350 m,

500 500 m, 750 750 m and 1 1 mm emitters.

At first the Si wafer on the emitter side is thinned down

to 30 m (figure 6(a)) using 5% tetra methyl ammonium

hydroxide at 80 C. In this process we have to protect the

collector wafer from etching due to tetra methyl ammonium

hydroxide. A simple solution was to use the anisotropic

etching of Si in the (111) and (100) directions. Si(111) is

etched much less compared to Si(100) in 5% tetra methyl

ammonium hydroxide at 80

C [30]. So Si(111) is used atthe collector side of the spin-valve transistor for this purpose.

When we wanted to use Si(100) as the collector, the back of

this Si wafer had to be protected. For this we have used

a double-side-polished SOI wafer with about 500 m of

handling wafer, 1 m of SiO and 2m of device Si. Here

the 500 m of handling wafer of the SOI wafer is used as the

collector. Now, in tetra methyl ammonium hydroxide, the

2m of Si is first etched away, leaving the SiO layer as the

protection layer from the tetra methyl ammonium hydroxide

etching. After etching down the emitter to the required size

this oxide layer is removed using BHF. From now on the

processing steps for both the Si(100) and Si(111) collectorsare the same. The emitter area is defined using photoresist

and optical lithography (figure 6(b)). Then Cr (10 nm),

followed by Au (100 nm), are deposited for ohmic contacts

(figure 6(c)). A lift-off process using acetone in an ultrasonic

bath defines the Au contact to the emitter (figure 6(d)). The

unwanted Si is etchedawayusing 5% tetra methylammonium

hydroxide at 80 C with Au as the mask, and this gives the

emitter (figure 6(e)). Again optical photolithography is used

to define the base (figure 6(f)). Ion beam etching is then used

to remove the unwanted metal layers from the base in order

to define the base layer (figure 6(g)). We kept the ion beam

energy to 500 eV so as to minimize the damage created by the

ion beam bombardment. However, with these small energiesthe Si below the removed metal layer is also damaged. This

damage may leave the Si with high doping. This can lead

to a high leakage current for the defined collector Schottky

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Figure 5. The cross-sectional transmission electron micrograph of a Si(100)/Au (5 nm)Au (5 nm)/Si(100) bonded structure.

diodes. So the damaged Si outside the base layer is removedusing 5% tetra methyl ammonium hydroxide at 80 C and

the Au back contact is given (figure 6(h)). This processed

structure is fixed to a PCB and Au wire connections are

made using ultrasonic welding. It has to be noted that we

have selected Si wafers that are suitably doped on the back

for ohmic contacts. All the measurements reported in thisarticle aremade on spin-valve transistors with a 350350m

emitter and a 350 700m metal base.

4. Results and discussion

4.1. Schottky barriers

In figure 7, the IVcharacteristics of the SiPt emitter and

SiAu collectordiode aregiven. From thefigureit is seen that

the SiPt emitter diode has an extremely low leakage current(0.1 nA at 1 V reverse bias). The Schottky barrier height

estimated from the above figure is nearly 0.85 eV. Whereas,

the SiAu collector diode has a higher leakage current (1 nA

at 1 V reverse bias) and the Schottky barrier height estimated

is nearly 0.76 eV. The emitter diode is defined by chemical

etching of the Si wafer whereas the collector diode is defined

by the ion beam etching of the base layer. It was observedthat a low leakage current for the collector diode is observed

when the process step in figure 6(h) is performed. This step

is extremely important as it controls the leakage current. The

implication of this leakage current on the magnetotransport

properties will be discussed later. In figure 8, the temperaturedependence of the ideality factors of the emitter and collector

diodes are given in the form ofnkT/q against kT/q plots

from 90 to 300 K. The full line gives the plot ofn = 1.02. It

is seen from the figure that the emitter diode has an ideality

factor of a perfect Schottky diode, because at all temperatures

the curve isclose to the line definingn = 1.02. This shows anideal thermionic emission at all temperatures [31]. Whereas

the collector diode has an ideality factor close to 1.02 atroom temperature, the ideality factor increases drastically as

the diode is cooled. These phenomena show that the ideal

thermionic emission is not present in thecollector diode andit

appears that thermionic field emission may also be dominant

in the collector [31]. This behaviour may arise due to the

damage created by the ion beam etching.

4.2. Electrical characterization of the spin-valve

transistor

4.2.1. Si(100)/Pt/NiFe/Au/Co/Au/Si(111). The fabricated

spin-valve transistors are extensively characterized at room

temperature as well as at low temperatures. For optimized

spin-valve transistors the output has a huge response to the

magnetic field. The output, i.e. the collector current, changes

by a factor of three in a few oersted of magnetic field at room

temperature. In figure 9, the magnetic field dependence of

the collector current for the spin-valve transistor is given at

room temperature as well as at 83 K. Here the emitter diode

is forward biased and an emitter current of 2 mA is injected

into the spin valve. The collector diode is zero biased. When

a magnetic field of 100 Oe is applied both the permalloy and

Co layer in the spin valve are magnetized in the direction of

the magnetic field and we obtain a higher collector current

(11.6 nA). When the field is reversed and is just above the

coercive field of permalloy the magnetizations of the Co

and the permalloy are aligned opposite to each other andwe obtain a lower collector current (3.76 nA). This gives

rise to a huge magnetocurrent of about 208% within a few

oersted of the magnetic field, yielding a relative sensitivity of

approximately 130% Oe1 at room temperature. We define

the magnetocurrent (MC = (IPc IAPc )/I

APc , where I

Pc and

IAPc are the collector currents for parallel and antiparallel

alignment of the magnetization, respectively). At 83 K this

effect is about 402%. This shows that the collector current

(Ic) is very sensitive to the spin-dependent scattering of hot

electrons in the spin valve.

It is known that in a metal base transistor the collector

current Ic ew/, wherew is the thickness of the base layer

and is the electron mean free path [32]. In a spin-valve

transistor, the base layer consists of four different layers and

the derivation of a straightforwardexpression for the collector

current as a function of the thicknesses and electron mean

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Figure 6. The processing steps (a) to (h) after bonding to realizespin-valve transistors of different size.

free paths of individual layers is tedious. However, it can

be assumed that Ic has a strong dependence on. It is also

known that an imbalance of spin populations at the Fermi

level constitutes spin-polarized transport in materials. In

ferromagnetic metals the density of states available to spin-up

and spin-down electrons is almost identical, but these states

are shifted in energy with respect to each other. This results

in an unequal filling of the bands which in turn makes the

spin-up and spin-down carriers at the Fermi level unequal in

number and mobility. This causes a spin-dependent transport

for the Fermi electrons across the spin-valve layer and is

believed to be the origin for the magnetoresistance in the

normal spin-valve configuration. However, in the case of

a spin-valve transistor, the charge carriers are hot electronsand have an energy of about 0.9 eV above the Fermi level.

Calculations have shown that at this electron energy the spin

scattering asymmetry is different than that at the Fermi level

Figure 7. The IVcharacteristics of the SiPt emitter and SiAucollector diodes. The barrier heights estimated from these curvesare also given.

Figure 8. The plots ofnkT/qas a function ofkT/qfor the SiPtemitter and SiAu collector diodes from 90 to 300 K. The linegives the plot of the ideality factorn = 1.02.

[33, 34]. Therefore the spin-dependent scattering is believed

to be different at this energies. Hence in the spin-valve

transistor, the spin-dependent perpendicular transport of hot

electrons, the energy and momentum selection of the hot

electrons at the collector and the exponential dependence

of the collector current on the electron mean free path are

responsible for the huge magnetocurrent observed.In figure 10, the dependence of the collector current

on the magnetic field for the same transistor connected in

the reverse mode is given. Here the SiAu acts as the

emitter and SiPt acts as the collector. Since the SiPt

diode has a barrier height 0.1 eV higher than that of the

SiAu diode one normally expects no collector current.

However here we also observe a magnetocurrent of about

231% at room temperature. The maximum collector current

obtained in this configuration is approximately 0.5 nA.

The origin of this collector current is believed to be from

the inhomogeneities of the Schottky barrier height for the

collector and emitter diodes. It has previously been shown,

using ballistic electron emission microscopy, that a Schottkybarrier height distribution of about 0.1 eV was present for

a SiAu interface [35]. This barrier height inhomogeneity

allows injection of hot electrons across the SiAu interface

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Figure 9. The magnetic field dependence of the collector currentof the spin-valve transistorSi(100)/Pt/NiFe/Au/Co/AuAu/Si(111): (A) at room temperatureshowing a magnetocurrent of 208% and (B) at 83 K showing a

402% magnetocurrent. The emitter current IE = 2 mA and thecollector biasVBC = 0 [24].

with energies above 0.76 eV due to a higher local barrier

height. Some of these electrons can be collected across the

SiPt interfacewhere, locally, a lowerSchottkybarrier height

is present. This could be the reason for a finite, but small,

collector current in this configuration. Here we observe a

larger magnetocurrent compared to the normal configuration.

This is believed to be due to the lower leakage current of the

SiPt diode compared to the SiAu diode. Although there

is no intentional bias of the collector diode (VBC ) due to the

finite resistance of the base layer and the emitter current,

there is a slight negative bias for the collector diode. This

small negative bias increases the leakage current. So even

at zero bias there is finite leakage current. This leakage

current modulatesthe collectorcurrent that in turn reduces the

magnetocurrent. In the case of the SiPt diode the leakage is

extremely low compared to the SiAu diode and we observe

a higher magnetoresistance when we use the SiPt diode as

the collector diode.

4.2.2. Si(100)/Pt/NiFe/Au/Co/Au/Si(100). In order to

understand the role of the orientation of the Si on the

magnetotransport properties, a spin-valve transistor with

Si(100) on both the emitter and collector sides is fabricated.In figure 11 the magnetic field dependence of the collector

current of this spin-valve transistor at room temperature

is given. It has a magnetocurrent of about 240%. The

Figure 10. The magnetic field dependence of the collector currentin the reverse mode of the spin-valve transistorSi(100)/Pt/NiFe/Au/Co/AuAu/Si(111): showing a

magnetocurrent of 231% at room temperature. The emitter currentIE = 2 mA, and the collector bias VBC = 0 [24].

marginal increase in the magnetocurrent is due to the

lower leakage current of the collector diode. The I

V characteristics of the emitter and the collector diodes

show that both have barrier heights that are comparable

to those of the previous spin-valve transistor. The main

point here is the increase in the collector current in this

configuration compared to the previous spin-valve transistor.

The collector current is increased by a factor of 1.7 for

the same emitter current. At this stage it is not possible

to come up with a definite reason for this behaviour: thedifference between the AuSi(111) and AuSi(100) collector

has to be investigated in detail. There has already been some

controversy, based on ballistic electron emission microscopy,

on the transmission probability for the hot electrons in Au

Si(111) and AuSi(100) interfaces [36, 37]. In the case of

spin-valve transistors a number of additional factors such as

the difference in the surface roughness between the two types

of Si wafers, the quality of the AuAu bond in the two cases,

etc have to be investigated in detail. Efforts are being made to

evaluate these properties so that the increase in the collector

current for this spin-valve transistor could be explained.

4.2.3. Si(100)/Pt/NiFe/Au/Co/Pt/Si(111). In figure 12, the

magnetic field dependence of collector current for a spin-

valve transistor with SiPt as both the emitter and collector

diodes is given. The aim was to have same barrier heights on

both emitter andcollector diodes. A magnetocurrent of 220%

is observedin this system. However, the collector current was

found to be less compared to a transistor which has SiAu as

the collector diode. The IVcharacteristics of the collector

and emitter diodes have shown that the barrier heights are

not equal for the SiPt emitter and SiPt collector. The SiPt

emitter has a Schottky barrier of about 0.85 eV, whereas

the SiPt collector barrier has a Schottky barrier height of

about 0.80 eV. The difference in the barrier height is believedto be due to processing. The chemical etching defines the

emitter diode whereas the collector diode is defined by ion

beam etching. During the ion beam etching the damage

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Figure 11. The magnetic field dependence of the collector currentof the spin-valve transistor Si(100)/Pt/NiFe/Au/Co/AuAu/Si(100)at room temperature showing a magnetocurrent of 240%. The

emitter currentIE = 2 mA and the collector bias VBC = 0.

Figure 12. The magnetic field dependence of collector current ofthe spin-valve transistor Si(100)/Pt/NiFe/Au/Co/PtPt/Si(111) atroom temperature showing a magnetocurrent of 220%. Theemitter currentIE = 2 mA and the collector bias VBC = 0.

created at the edges of the diode may be responsible for

the lowering of the barrier height. Moreover, the collectorhas a higher leakage current compared to the emitter. This

also supports the fact that the differences are due to the

difference in processing. The lower collector current of this

spin-valve transistor compared to a spin-valve transistor that

has a SiPt emitter and a SiAu collector is probably due

to the smaller difference in the barrier height. However,

when the transistor is reverse connected the collector current

is drastically decreased and it has a much smaller collector

current than in figure 10. This shows that other factors are

also important in defining the collector current along with the

differences in the Schottky barrier height. Here the bonding

is carried out in a Pt layer. It was earlier observed that thePtPt bond leaves a bonded interface. So the poor quality

of the Pt bond could also be a reason for the lower collector

current in this transistor.

Figure 13. The magnetic field dependence of collector current ofthe spin-valve transistor Si(100)/Pt/NiFe/Au/CoCo/Si(111) at90 K showing a magnetocurrent of 640%. The emitter current

IE = 2 mA and the collector bias VBC = 0.

4.2.4. Si(100)/Pt/NiFe/Au/Co/Si(111). In order to

understand the roll of emitter and collector Schottky barrier

heights on the magnetotransport properties another system

with a higher difference in the barrier heights is chosen. Here

the emitter is again defined by a SiPt diode and the collector

Schottky diode is defined by a SiCo Schottky diode. Here

the bonding is carried out in the Co layer so the thickness

of Co is nearly 4 nm. The SiCo Schottky diode at room

temperature has a high leakage current (0.1 A). Hence

the magnetocurrent at room temperature was negligible, due

to the large leakage current. So the measurements were

carried out at low temperature. In figure 13, the dependence

of the collector current on the magnetic field at 90 K is

given. A huge magnetocurrent of 640% is observed at this

temperature. The switching of the Co layer seems to be very

sharp and this is believed to be due to the greater thickness

of the Co layer compared with the previous transistors.

The collector current is about 18 nA which is a marginal

increase compared to the previous transistors. Although one

complete layer (Au or Pt at the collector side for the previous

transistors) is absent in this transistor and the difference inthe barrier height is greater compared to the other transistors,

no drastic increase in the collector current is observed. The

TEM micrographs of a CoCo bond layer shows that the

bonded interface is not perfect, as a thin amorphous layer

is observed at the bonded interface. This could be the

reason for the lack of increase in the collector current for this

transistor. This shows that the quality of the bonded interface

is extremely important in obtaining higher collector currents.

The temperature dependence of the magnetocurrent of this

transistor is given in figure 14. It is seen that above 230 K

the magnetocurrent falls drastically and has extremely low

values at room temperature. The temperature dependence ofthe leakage current of the collector diode increases rapidly

above 230 K and is responsible for the lower magnetocurrent

at higher temperatures.

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Figure 14. The temperature dependence of magnetocurrent of thespin-valve transistor Si(100)/Pt/NiFe/Au/CoCo/Si(111). Theemitter currentIE = 2 mA and the collector bias VBC = 0.

4.3. The leakage current

To achieve room-temperature operation and huge magne-

tocurrent it is essential to have a collector barrier which has

extremely low leakage. One of the main factors that intro-

duces leakage into the collector diode is the ion beam etching

processing step (figure 6(g)) used to define the base [38]. Al-

though the ion beam etching removes the unwanted metal

layer and defines the base, during the removal of the metal

layers the ion beam damages the Si beneath the metal layer.

This damaged Si (heavily doped) introduces leakage into the

diodes. Therefore, the removal of the damaged Si is impor-tant (figure 6(h)). This aspect is illustrated in the following

figure (figure 15). In figure 15(a), the temperature depen-

dences of the magnetocurrent for different samples are given.

In sample 1 the damaged Si is only partially removed. Hence

at room temperature the leakage dominates and the magne-

tocurrent is very low. When more damaged Si is removed

from sample 1 we obtain less collector leakage, and better

performance at room temperature is realized (sample 1a).

So for room-temperature operation process step (h) (in fig-

ure 6) is very important. Samples 2 and 3 give very large

magnetocurrents at room temperature as the leakage current

is very low after subjecting them to process step h. How-ever, the magnetocurrent has a trend to fall above 300 K.

Therefore, the magnetocurrent above 300 K has to be mea-

sured and the collector leakage has to be further improved

for practical applications as the operating temperature can

often increase above room temperature. The importance of

having extremely low leakage currents is further substanti-

ated in figure 15(b). The magnetocurrent as a function of

the collector bias is given in this figure. It is seen that the

magnetocurrent falls as the bias is increased. This can be

understood in terms of the leakage current: as the bias in-

creases the leakage current increases and the magnetocurrent

decreases. Moreover sample 5 has lowest leakage currentand the magnetocurrent does not fall rapidly with bias com-

pared to the magnetocurrent of sample 2, which has a higher

leakage current.

Figure 15. (A) The temperature dependence of themagnetocurrent of three samples with different collector leakagecurrents. The emitter currentIE = 2 mA and collector biasVBC = 0. (B) The collector bias dependence of the magnetocurrentof two samples with different collector leakage currents. Theemitter currentIE = 2 mA and the collector bias VBC = 0.

4.4. The collector current

The main drawback of the spin-valve transistor at present isthe low collector current. Although we inject 2 mA of cur-

rent into the structure we only obtain a collector current ofaround 10 nA. This gives an extremely low transfer ratio (ofthe order of 106). So the main effort at this stage is the im-

provement of the collector current. Additionally, the collec-tor leakage has to be further improved so that this device can

be operated above room temperature. Therefore, the leakagecharacteristics above room temperature have to be evaluated

and, if necessary, different types of Schottky diodes (differ-ent metalsemiconductorcombinations) have to be fabricated

and tested. In this respect spin-valve transistors with twoother collector Schottky diodes are fabricated and studied:

Si/Pt/NiFe/Au/Co/PtPt/Si and Si/Pt/NiFe/Au/CoCo/Si. Inthe case of the spin-valve transistor with CoSi as the collec-

tor diode the leakage effect at room temperature dominates sothe magnetocurrent at room temperature is negligible. How-

ever, at low temperature the magnetocurrent was comparable

to that of the sample with the AuSi collector diode. Also, inthe sample with the CoSi collector, the collector current didnot increase appreciably although the barrier height differ-

ence is high and there are fewer interfaces. This is believed

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to be due to the poor quality of the CoCo bond. In the caseof

the spin-valve transistor with PtSi as the collector diode the

magnetocurrent at room temperature was same as that of the

spin-valve transistor with AuSi as the collector. However,

the collector current was lower than that of the spin-valve

transistor with the AuSi collector. This is believed to be due

to smaller differences in the Schottky barrier heights betweenthe emitter and collector and the poor quality of the PtPt

bond compared to the AuAu bond. Therefore, in order to

improve thecollector current, thebonding hasto be perfected.

In addition, the scattering centres for the electrons have to be

reduced. This can be achieved by a more controlled growth

of the metal layer, to give very few defects, low diffusion, etc.

5. Conclusion

We have developed a spin-valve transistor with huge mag-

netotransport properties that operates at room temperature.

Perpendicular hot electron transport across the spin valve,

and energy and momentum selection by the collector diode

are believed to be the origin of the huge magnetocurrent.

The principle of operation and the detailed fabrication steps

are also discussed. The spin-valve characteristics were ex-

amined for different configurations of spin-valve transistor.

In all the spin-valve transistors we obtain a magnetocurrent

above 200%. It has been observed that the collector leakage

plays an important role in determining the room-temperature

operation. It is also realized that, along with the Schottky

barriers, the bonded interface is also important in obtaining

high output currents for the device.

Acknowledgments

The authors are thankful to Dr R Keim for the TEM images.

The authors are also thankful to Ir Ing, O M J vant Erve,

Ir R Vlutters and Dr R Jansen for experimental help. The

financial assistance from the Dutch Technology Foundation

(STW) is also duly acknowledged.

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