Journal of Alloys and Compounds 603 (2014) 197–201

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Journal of Alloys and Compounds

journal homepage: www.elsevier .com/locate / ja lcom

Ni/SiC–6H Schottky Barrier Diode interfacial states characterizationrelated to temperature

http://dx.doi.org/10.1016/j.jallcom.2014.02.1770925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +213 552 555 338.E-mail address: anani66@yahoo.fr (M. Anani).

Mekki Benamara a, Macho Anani b,⇑, Boudali Akkal a, Zineb Benamara a

a Applied Microelectronics Laboratory (AMEL), Electronics Department, Faculty of Technology, Djillali Liabes University of Sidi Bel Abbes, BP 89, 22000 Sidi Bel Abbes, Algeriab Laboratoire des Réseaux de Communications, d’Architecture et de Multimédia, Electronics Department, Faculty of Technology, Djillali Liabes University of Sidi Bel Abbes,BP 89, 22000 Sidi Bel Abbes, Algeria

a r t i c l e i n f o

Article history:Received 5 March 2010Received in revised form 27 February 2014Accepted 27 February 2014Available online 6 March 2014

Keywords:Ni/SiC–6H Schottky Barrier DiodeInterfacial statesC(VG) characterizationI(VG) characterization

a b s t r a c t

This study presents a Ni/SiC–6H Schottky Barrier Diode (SBD) characterization at different temperaturesgoing from 77 K to 450 K. The electronic properties of this diode were reported by the analysis of its C(VG)and I(VG) characteristics as a function of temperature.

At low temperature when T < 100 K the high part ideality factors nH were close to 2 showing that theconduction is dominated by the generation–recombination at deep centers. Also, the values of low partideality factor nL varied from 2.69 down to 1.89. These values were also much closer to 2, showing thatthe conduction mechanism was then dominated by a tunneling current assisted by default.

The mean interfacial states density Ds(mean) decreased with increasing temperature from 1.2�1013

eV�1 cm�2 to 6.3 � 1012 eV�1 cm�2. This reducing appeared to be due to the restructuring and rearrange-ment which occurs under molecules thermal activation within the Ni/SiC–6H metal/semiconductorinterface.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Wide bandgaps semiconductors are best suited for high powerand high temperature applications [1–3]. Silicon carbide SiC isone of the most investigated materials and has several characteris-tics that offer a lot of applications inaccessible to silicon. SchottkyBarrier Diode is one of the most fundamental devices in the area ofsemiconductor device technology. In the current research, the for-mation of potential barrier with reasonable inter-face states at me-tal–silicon carbide (M–SiC) contact is of prime interest [4,5].Commercially available SiC wafers contain dislocations, typicallyranging from 103 to 104 cm�2, and most are introduced duringcrystal growth [6]. The use of silicon carbide (SiC) power electron-ics could enable systems that are significantly lighter, smaller, andelectrically more efficient than systems comprised of more con-ventional silicon (Si) based electronics. Although some SiC devices(e.g., Schottky diodes and field-effect transistors (FETs)) have beendeveloped, the performance of most SiC power devices is seriouslydegraded because of a high density of defects in all commercially-available SiC semiconductor wafers [7]. The electrical parametersof bipolar devices can be deteriorated by the presence of point de-fects, which cause carrier trapping, increased leakage current, and

reduction in minority carrier lifetimes [8]. Then, the electronicapplications are limited by several problems, such as imperfectquality control of the material which is closely linked to the qualityof used substrates and the presence of a large amount of defects oralso the nature of the metal/semiconductor interface [9,10]. Inaddition, the presence of traps levels in the bandgap plays a keyrole on the electrical properties of the semiconductor and relatedcomponents [11]. Indeed, charges captured or emitted by deeptraps can offset the concentration of donor or acceptor levelsresulting in increasing the resistivity and reducing the mobilityand then the global performance of the device [12,13].

Electronic properties of a Schottky Barrier Diode are character-ized by its main electrical parameters such as leakage current, bar-rier height (/B) formation at metal/semiconductor interface,ideality factor n, series resistance (Rs) and interface states (Nss)[14]. Shunt resistance (Rsh) of the diode is also an importantparameter which can cause the electrical characteristics of SBDto be non-ideal. For the case of SBD with sufficiently high Rs value(a few X) I–V characteristics of the diode deviate from the linearityespecially at high direct bias region. On the other hand, when diodehas a low Rsh (kX), leakage or reverse bias currents of diode be-comes considerably high [15]. Also, controlling interface states(Nss) and series resistance (Rs) is an important issue for highpower, high speed and high temperature applications [16].

-10

1.5x10-10

2.0x10-10

2.5x10-10

3.0x10-10

3.5x10-10

4.0x10-10

4.5x10-10

C (F

)

77K 100K 150K 200K 250K 300K 350K 400K 450K

198 M. Benamara et al. / Journal of Alloys and Compounds 603 (2014) 197–201

Some researchers have investigated the electrical properties ofthe Au/n–GaAs SBD to explore the possibility of its use in photovol-taic applications as a solar cell. Others have investigated the illumi-nation effect on main electrical parameters such as n, /B0, WD andND in Al–TiW–Pd2Si/n–Si SBDs under dark and in a wide range ofillumination levels at room temperature. They showed that theelectrical characteristics of SBDs are considerably affected underillumination [17].

In this work, we have studied anomalies observed on the SBDoutput characteristics SBD based on a Ni/SiC–6H and contributedto a better understanding of the origin of these deficiencies by ana-lyzing the role of traps on the performance of these components inthe metal/semiconductor interface.

5.0x10-11

1.0x10

T=(77/100/150)°K

VG (V)-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0

Fig. 1. C(VG) temperature related characteristic.

2. Experimental details

First things first, the plates were chemically cleaned in order to clean them fromany organic or mineral contamination. Then, the surface of the SiC device was ther-mally oxidized under continuous H2O flow, afterward the oxide was removed athigh frequency. Next, it has been carried out the standard chemical cleaning‘‘RCA’’ which generated, in a reproducible way, a well suitable oxide for the SiC sur-face. Schottky metal was then rapidly deposited till the layer formed a square con-tact of variable side. The size of the pattern was defined by the mask used, ingeneral META1. The active layer was n-doped with ND = 5 � 1015 cm�3 intendedfor the study of the Schottky contact.

The rear ohmic contact was obtained by Ti(1000 Å)/Pt(1000 Å) depositing onthe back face, after an RIE engraving in order to remove the oxide coating, this cre-ated backside in the thermal oxidation [18,19].

Electrical current were measured using an HP4145B ‘‘Semiconductor ParameterAnalyser’’. Each SMU can be programmed to develop an electrical potential goingfrom 0 V to ±100 V. Temperature dependent I(V) measurements were producedeither by heating support, or by cooling with liquid nitrogen. This system allowedthe temperature to be varied from 77 K up to 600 K.

As a result, the 6H–SiC Schottky diode formed on n-type epitaxial layers grownby CREE USA research, are realized by depositing an activated layer doped at5 � 1015 cm�3 with 7 lm thickness [18,19].

The metallic layer Ni produced a square Schottky contact of (1.6 � 1.6) mm2

area.Capacitance was measured using a HP 4275A ‘‘Multifrequency LCR Meter’’,

which makes possible the measurement of the diode’s capacitance in a frequencyrange going from 10 KHz to 10 MHz, for voltage in the range ±100 V.

1015

1016

1017

T=300°KT=77°K

F=1 MHz

Nd

(cm

-3)

W ( µm)0.2 0.4 0.6 0.8 1.0 1.2 1.4

Fig. 2. Doping profile at two different temperatures.

3. Results and discussion

The nature and characteristics of the Ni contact on SiC is deter-mined by the thermal treatment of the deposited film. Saxena et al.have stated that as-deposited Ni film forms a Schottky contactwhich remains stable at temperature up to 973 K. Therefore, after1273 K annealing was believed to be the reason for ohmic contactformation [11].

Other authors declared that after annealing at 1223 K, close tothe preceding value also, an ohmic contact was obtained, andthere, the contact resistance was equal to 4.8 � 10�5 X cm2 [12].

Fig. 1 shows the C(VG) characteristic as a function of tempera-ture for our Ni/SiC–6H Schottky diode. Let’s remark that the illus-trated C(VG) curve have shown the same appearance that of anideal Schottky contact. The evolution of this curve related to tem-perature is remarkable and this effect is due to thermal activationof carriers acting in the depletion zone.

According to the relation of Mott [20], the variation of thedepletion zone width W is defined as:

W ¼ Ses

CðVGÞð1Þ

C(VG) is the measured capacitance, S is the diode section and es is thepermittivity of the semiconductor.

The analytical expression for determining the doping in thedepletion zone, neglecting the effect of the voltage drop due tointerfacial states Ds, is given by [3]:

NDðWÞ ¼2

ðqesS2Þ dC�2ðVGÞ

dVG

ð2Þ

ND(W) is the concentration of doping impurities.Fig. 2 shows the doping profile of the Ni/SiC–6H diode esti-

mated in Eqs. (1) and (2). This gives a doping value in good agree-ment with those given by the manufacturer, while doping profileincreases slightly at 300 K as it depleted the active layer. This is ex-plained by a not entirely homogeneous doping scheme.

Fig. 3 displays the forward I(VG, T) characteristics of the Ni/SiC–6H SBD knowing that a current reading is performed every 50 K.

To interpret the anomalous behavior of such SBD one must splitthe I(VG) characteristic in two segments. The total current is thesum of the thermionic currents from each barrier (a high barrierof IH

S ;/HBn;n

H , and SH parameters and a low barrier of ILS;/

LBn;n

L,and SL factors). SH and SL areas are related to every barrier [10].

The models of I–V calculations could be divided into three lev-els. The first is based on the simple engineering model using the

10-15

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

T=77°KT=100°KT=150°K T=200°KT=250°KT=300°KT=350°K T=400°K T=450°K

I (A

)

VG (V)0.0 0.5 1.0 1.5 2.0

Fig. 3. I(VG, T) Ni/SiC diode characteristic.

0.0 0.5 1.0 1.5 2.0-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

(2)(1)

Ln I'

= L

n [I/

(1-e

-qVG

/kT)]

IsH

IsB

VG (V)

Fig. 4. Ln½I=ð1� e�qVG=kT Þ� ¼ f ðVGÞ curve.

M. Benamara et al. / Journal of Alloys and Compounds 603 (2014) 197–201 199

thermionic emission theory, the second level consists in includinganalytical solution of the transport equations in the base of a diodeand the third and last level use a set of Poisson’s equation and apair of continuity equations completed by a pair of drift diffusionequations [21].

For Ocaya, also, thermionic emission theory is the simplisticmethod model to extract parameters instead of physical analyticalmodels to solve transport equations and numerical models basedon Poisson’s equation together with drift–diffusion and continuityequations [22].

For n-type Schottky contact semiconductors under forward biasfour basic transport processes can take place [23]. The four pro-cesses are:

1. Emission of electrons from the semiconductor over the top ofthe barrier into the metal.

2. Quantum mechanical tunneling through the barrier.3. Recombination in the space-charge region.4. Recombination in the neutral region (called hole injection).

For the lowly doped semiconductor the current flows as a resultof thermionic emission with electrons thermally excited over thebarrier.

In the intermediate doping range, thermionic field emission(TFE) dominates.

For high doping, the barrier is sufficiently narrow at or near thebottom of the conduction band for the electrons to tunnel directly,known as field emission (FE) [23].

The three regimes can be distinguished by considering the dis-tinctive energy E00 which is the characteristic tunneling energythat is related to the tunnel effect transmission probability [23].

E00 ¼h

4p

ffiffiffiffiffiffiffiffiffiffiND

m�ees

s¼ 1:183� 10�11

ffiffiffiffiffiffiND

pð3Þ

where ND is the doping concentration, m�e (�0.25 m0 for 6H–SiC) isthe effective tunneling mass, and es (�10 e0) is the dielectric con-stant for 6H–SiC.

A comparison of E00 to the thermal energy kT shows that whenthermionic emission dominates kT� E00, for thermionic-fieldemission kT � E00, and for field emission kT� E00. For simplicity,

the range of each regime can be chosen by E00 6 0.2kT for TE,0.2kT < E00 < 5kT for TFE, and E00 P 5kT.

For 6H–SiC with a tunneling effective mass of 0.25 m0, thiscorresponds approximately to TE for ND 6 1.86 � 1017 cm�3, TFEfor 1.86 � 1017 cm�3 < ND < 1.15 � 1020 cm�3, and FE forND P 1.15 � 1020 cm�3.

In our case, in the whole doping level range of ND =[2.19–5] � 1015 cm�3, we remain in the low level doping so inthe pure thermionic emission (TE) zone [23].

Then, for the real Schottky diode, the thermionic emission pro-cess is the dominant current transport mechanism and it is namelyused by a majority of authors [25–36].

Such an expression of the thermionic emission current is givenby [10].

ILðVGÞ ¼ ILs 1� e�qVG=kT� �

eqVG=nLkT ð4Þ

IHðVGÞ ¼ IHs 1� e�qVG=kT� �

eqVG=nHkT ð5Þ

ITðVGÞ ¼ bILðVGÞ þ ð1� bÞIHðVGÞ ð6Þ

b represents the surface ratio between the majority region SH andthe minority area SL.

Here, VG is the drop of the applied voltage across the semicon-ductor surface depletion layer, q is the electron charge, K is Boltz-mann constant, n is the ideality factor, T is the temperature inKelvin and Is the reverse saturation current expressed also by [10]:

Is ¼ SA��T2 exp � qkT

/Bn

� �ð7Þ

/Bn is the barrier height, A�� is the effective Richardson constant,

A�� ¼ 120m�em0½A=cm2 K2� ð8Þ

The theoretical value of A�� is about 146 A/cm2 K2 for 6H–SiC[9].

Hence, characteristics parameters were determined by I–V anal-ysis following the thermionic-emission theory. The current–voltagecharacteristics constructed in semi logarithmic coordinates of theNi/SiC–6H sample is shown in Fig. 4. The intersection between

12

1.2x1013

1.5x1013

1.8x1013

V-1

cm-2

)

200 M. Benamara et al. / Journal of Alloys and Compounds 603 (2014) 197–201

the straight line interpolations of the first linear segment of thecurve with the currents axis can determine the saturation current IL

S.

LnI0 ¼ Ln1

1� e�qVG

kT

" #¼ f ðVGÞ ð9Þ

Özavcı et al. explained the fact that the I–V plot for every tem-perature has two distinct linear regions with different slopes [24].In our case the frontier occurs near the value of 1 V, less corre-sponding to the low region and more corresponding to the high re-gion. This result is the same that Vassilevski et al. [37] andSochacki et al. reached even if the metal was not exactly the same[38].

The ideality factor nL is extracted from the slope of the first lin-ear segment interpolation. It seems to be estimated at more than2.69 at 77 K and less than 1.89 at 450 K. It decreases, then, withincreasing temperature, showing that the SBD is more idealizedat high temperatures.

The height of the low barrier /LBn at the origin (zero polarization)

is:

/LBn ¼ �

kTq

LnIL

s

SA�T2

!ð10Þ

Substituting the values of saturation current of the low barrierILS in Eq. (7), the evaluated low barrier height /L

Bn passes from0.30 to 1.26 V when the temperature increases from 77 K to450 K, augmenting when temperature is rising.

By the same way, the saturation current IHs , the ideality factor nH

and the high barrier height /LBn are estimated from the intersection

of the straight line interpolation of the second linear segment ofthe relation (9) also. The same trend is remarked and the SBD ismore close to the ideal one at high temperatures.

These results were corroborated by those obtained from Özavcıet al. where the ideality factor (n) decreases while the effective bar-rier height (/B0) increases with increasing temperature (80–340 K)[24].

Note that for low currents, the bias voltage VG can be consideredequal to the voltage V applied across the surface layer depletion ofthe SiC–6H semiconductor. Thus, in this case, the solely parasiticsingle phenomenon which affects the I(VG) characteristic is thedensity of interfacial states Ds [10]. Therefore, when the bias flowbecomes relatively high, this version is no longer valid and onemust take into account the voltage across the series resistance Rs.

The electrical parameters dropped from the I(VG) characteristicfor the Ni/SiC–6H sample are set in Table 1. These parameters areclose to those obtained by Perez et al. [39].

To estimate the average distribution of interfacial states in theNi/SiC–6H contact, we calculated Ds(VG) using the followingexpression given by [12]:

nL ¼ 1þ dei

es

Wþ qDs

h ið11Þ

ei is the interfacial layer permittivity and d is the thin insulatinglayer between the metal and the semiconductor.

Table 1Electrical parameters evaluated from I(VG) characteristic.

T (K) 77 100 150 200 250 300 350 400 450nH 2.23 1.76 1.59 1.47 1.37 1.22 1.22 1.1 1.12nL 2.69 2.55 2.42 2.25 2.13 2.07 2.02 1.96 1.89

/HBn (eV) 0.79 0.98 1.19 1.32 1.41 1.51 1.55 1.68 1.7

/LBn (eV) 0.30 0.56 0.48 0.61 0.74 0.87 1 1.13 1.26

RHs (X) 2 2 3 4 4 4 7 10 13

RLs (KX) 136 135 126 117 103 67 33 19 10

By replacing the values of nL the ideality factor evaluated fromthe I(VG) characteristic for each temperature in expression (11),and the value of the experimental area of the depletion zone W(when VG = 0), and taking ei = 3.9, and d0 = 20 Å, we have evaluatedthe change in the density of interfacial states as a function oftemperature.

One can observe, then in Fig. 5 that Ds decreases when temper-ature increases. This improvement is due to the restructuring andthe reordering of the Ni/SiC–6H metal/semiconductor interface un-der temperature effect, leading to a decrease of the interfacialstates density.

The values of the calculated Ds average density of interfacialstates vary from 1.2 � 1013 eV�1 cm�2 down to 6.3 � 1012 eV�1

cm�2 when the temperature varies between 77 K up to 450 K.Taking into account the voltage drop due to the interfacial statesDs, the expression of the characteristic C�2(VG) shown in Fig. 6 isreplaced after correction by [14]:

C�2ðVGÞ ¼2

esqNdS2

1ð1þ aÞ ðVd � VGÞ ð12Þ

where the parameter a is defined as:

a ¼qDsðmeanÞd

eið13Þ

Substituting the values of the density of interfacial states Ds

estimated from the characteristics I(VG) and C(VG) at the 300 �Ktemperature in Eq. (13), the parameter a is estimated to be almostequal to 1.05.

Knowing the slope of the C�2(VG) characteristic interpolation,we estimated from Eq. (12), the ND concentration of the substrate.Then, the value of ND doping was reported in the order of2.19 � 1015 cm�3. The intersection of the straight line interpolationof the C�2(VG) curve with the voltage axis enabled us to read thevalue of V0 and hence to calculate the diffusion potential Vd.

V0 ¼ ð1þ aÞ Vd �kTq

� �ð14Þ

In our case, the diffusion potential Vd was estimated to be equalto 0.66 V at ambient temperature so near about 300 K. Therefore,after calculating this diffusion potential one could estimate thebarrier height /Bn through:

/Bn ¼ Vd þkTq

LnNc

Nd

� �ð15Þ

3.0x1012

6.0x1012

9.0x10

T (°K)

Ds

(e

100 200 300 400 500

Fig. 5. Interfacial state density variation related to temperature.

-5 -4 -3 -2 -1 0 1 20.0

5.0x1018

1.0x1019

1.5x1019

2.0x1019

2.5x1019

3.0x1019

3.5x1019

C-2

(F-2)

T=300°KF=1 MHz

VG (V)

Fig. 6. C�2(VG) characteristic of the Ni/SiC Schottky contact.

M. Benamara et al. / Journal of Alloys and Compounds 603 (2014) 197–201 201

where Nc is the effective states density in the conduction band.We finally found that the low barrier height deduced from the

I(VG) characteristics at the temperature 300 K was no more than0.87 eV comparable to the value determined from measurementsof the C(VG) where /Bn = 0.89 eV.

4. Conclusions

This work has conducted a study on a Ni/SiC–6H n-type Scho-ttky Barrier Diode intended mainly as power rectifier devices.The electronic properties of this Ni/SiC–6H SBD was tested basedon analysis of C(VG) and I(VG) characteristics related totemperature.

The I(VG) analysis revealed a double inhomogeneity of the bar-rier height. This inhomogeneity is characterized by two parallelindependent diodes one high and the second low. At low temper-ature when T < 100 K, nH idealities factors values are close to 2illustrating the fact that the conduction phenomenon seems tobe dominated by the recombination–generation process on deepcenters. These points are located around the defective areas caus-ing a doubling of the barrier height observed on the direct profile.

Then, values of nL idealities factors dropped, also, from 2.69 to1.89. Hence, these values show that the conduction mechanismwas there dominated by a tunneling current assisted by default.

Finally, the average density of interfacial states Ds(mean) wascalculated by analyzing the C(VG) and I(VG) characteristics as afunction of temperature, also. Ds, then, was found reversely propor-tional to temperature and passes from the order of 1.2 � 1013

eV�1 cm�2 down to 6.3 � 1012 eV�1 cm�2 when the temperaturevaries between 77 K and 450 K. The decrease of the interfacialstates density can be explained by the restructuring and rearrange-ments of the Ni/SiC–6H interface under the effect of temperaturewhich acts as ‘‘healing’’ the metal/semiconductor interfacialdefects.

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