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Vacuum 67 (2002) 125–129 Study and characterization of the structures Au/Al 2 O 3 /Si and Au/Al 0 /Al 2 O 3 /Si B. Gruzza a, *, B. Akkal a , L. Bideux a , Z. Benamara b , C. Robert a a Laboratoire des Sciences des Mat ! eriaux pour l’Electronique et d’Automatique, Universit ! e Blaise Pascal de Clermont II, Les C ! ezeaux 63177, Aubi " ere Cedex, France b Laboratoire de Micro- ! electronique Appliqu ! ee, D ! epartement D’ ! electronique, Facult ! e des Sciences de l’Ing ! enieurUniversit ! e Djillali Liab " es de Sidi Bel Abb " es, 22000, Sidi Bel Abb " es, Algeria Abstract The performances of the metal/insulator/semiconductor structures are conditioned by the quality of insulator and the control of the physico-chemical properties of the insulator/semiconductor and metal/insulator interfaces. Within the framework of development of the MIS structures on a silicon substrate, we carried out deposits of gold on two types of surfaces (alumina or aluminum/alumina). The combination of the Auger electrons spectroscopy, elastic peak electron spectroscopy and the electrical measurements shows that the results of electric properties are much better when the deposition of gold is made on surfaces of aluminum/alumina. The interfacial state density N ss in the mid-gap evaluated for the structures Au/Al 2 O 3 /Si and Au/Al 0 /Al 2 O 3 /Si are about 10 12 and 5.2 10 11 eV 1 cm 2 , respectively. From these measurements, we noted that the structures Au/Al 2 O 3 /Si present a flat band voltage V FB of about 8.15 V, which indicates the presence of charges in insulator and in the insulator/semiconductor interface of about 1.92 10 12 charges per cm 2 . r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Metal/insulator/semiconductor structures; Electron spectroscopic methods; Electrical measurements 1. Introduction The comprehension of the structure and the properties of metal/ceramic interface is fundamen- tal for the improvement of the electric character- istics of the electronic structures. The defects of crystal structure influence the properties of surface of ceramic of metallic oxide type. These defects which are often charged modify the electronic structure of the perfect crystal and consequently the conditions of the formation of the interface metal/ceramic. Pedraza et al. [1,2] studied the influence of the irradiation of alumina before the deposit of gold on the force of adhesion between these two elements. To understand the role of oxygen in the adhesion of gold on alumina, the irradiations were carried out in three different atmospheres (air, oxygen, or argon +4% H 2 ). It proved that the force of adhesion increases when the substrate is irradiated in oxygen rich in atmosphere. This improvement suggests that the adherence of the atoms of gold on alumina is stronger when the layer of alumina is saturated with oxygen. It thus *Corresponding author. Fax: +33-4-73-40-73-40. E-mail address: [email protected] (B. Gruzza). 0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0042-207X(02)00197-5

Study and characterization of the structures Au/Al2O3/Si and Au/Al0/Al2O3/Si

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Vacuum 67 (2002) 125–129

Study and characterization of the structures Au/Al2O3/Si andAu/Al0/Al2O3/Si

B. Gruzzaa,*, B. Akkala, L. Bideuxa, Z. Benamarab, C. Roberta

aLaboratoire des Sciences des Mat!eriaux pour l’Electronique et d’Automatique, Universit!e Blaise Pascal de Clermont II,

Les C!ezeaux 63177, Aubi"ere Cedex, FrancebLaboratoire de Micro-!electronique Appliqu!ee, D!epartement D’!electronique, Facult!e des Sciences de l’Ing!enieurUniversit!e Djillali

Liab"es de Sidi Bel Abb"es, 22000, Sidi Bel Abb"es, Algeria

Abstract

The performances of the metal/insulator/semiconductor structures are conditioned by the quality of insulator and the

control of the physico-chemical properties of the insulator/semiconductor and metal/insulator interfaces. Within the

framework of development of the MIS structures on a silicon substrate, we carried out deposits of gold on two types of

surfaces (alumina or aluminum/alumina). The combination of the Auger electrons spectroscopy, elastic peak electron

spectroscopy and the electrical measurements shows that the results of electric properties are much better when the

deposition of gold is made on surfaces of aluminum/alumina. The interfacial state density Nss in the mid-gap evaluated

for the structures Au/Al2O3/Si and Au/Al0/Al2O3/Si are about 10

12 and 5.2� 1011 eV�1 cm�2, respectively. From these

measurements, we noted that the structures Au/Al2O3/Si present a flat band voltage VFB of about 8.15V, which

indicates the presence of charges in insulator and in the insulator/semiconductor interface of about 1.92� 1012 chargesper cm2. r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Metal/insulator/semiconductor structures; Electron spectroscopic methods; Electrical measurements

1. Introduction

The comprehension of the structure and theproperties of metal/ceramic interface is fundamen-tal for the improvement of the electric character-istics of the electronic structures. The defects ofcrystal structure influence the properties of surfaceof ceramic of metallic oxide type. These defectswhich are often charged modify the electronicstructure of the perfect crystal and consequently

the conditions of the formation of the interfacemetal/ceramic.Pedraza et al. [1,2] studied the influence of the

irradiation of alumina before the deposit of goldon the force of adhesion between these twoelements. To understand the role of oxygen inthe adhesion of gold on alumina, the irradiationswere carried out in three different atmospheres(air, oxygen, or argon +4% H2). It proved thatthe force of adhesion increases when the substrateis irradiated in oxygen rich in atmosphere. Thisimprovement suggests that the adherence of theatoms of gold on alumina is stronger when thelayer of alumina is saturated with oxygen. It thus

*Corresponding author. Fax: +33-4-73-40-73-40.

E-mail address: [email protected]

(B. Gruzza).

0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 1 9 7 - 5

appears that gold can have reactions with oxygenbut gold can also react with aluminum [3,4].To understand the role of interfaces in the

metal/ceramic contact, we carried out two kinds ofdeposition of gold: on alumina and on aluminum-rich surface.We also controlled the quality of insulating the

alumina obtained while ensuring ourselves of theabsence of any impurity in the volume of ourlayers. For that, we combined the Auger electronsspectroscopy (AES), the spectroscopy of the elasticpeak as well as electric characterization of thestructures using capacitance voltage (C2V ) mea-surement.

2. Experimental procedure

The samples of silicon (1 0 0) are p-type wafersat different doping levels (1015–1017 atoms cm�3).They were chemically cleaned according to amethod based on successive baths of H2SO4solution, methanol solution 3% bromine, anddeionized water [5]. The deposition of aluminawas performed in an ultra-high-vacuum (UHV)chamber at a pressure of 10�7–10�8 Pa.The sample surface was controlled in situ using

AES with a retarding field analyzer (RFA). A lowrate of carbon and oxygen contamination atomswas detected. These impurities were removed insitu by cleaning with low-energy Ar+ ionsbombardment (ions energy=300 eV, ionic curren-t=2 mAcm�2) [6].To realize a metallic gate, we have used a

molybdenum mask. This mask allows to achieveelectrical measurements with a gold gate of 1mmdiameter and layer thickness of about 1000 (A. Thecapacitance C as a function of bias voltage VGplotted at the frequency of 1MHz, was measuredwith a PAR 129 A two-phase lock-in amplifier.

3. Results and discussion

Figs. 1 and 2 show CðV Þ experimental charac-teristics measured at a high frequency (1MHz) ofthe structures Au/Al2O3/Si and Au/Al

0/Al2O3/Si.These curves have the typical form of MIS

structures; for the Au/Al2O3/Si samples, theexperimental characteristic is strongly shiftedtowards the negative tensions. To evaluate the

Fig. 1. Experimental and theoretical CðV Þ curves of Au/Al2O3/Si structures.

Fig. 2. Experimental and theoretical CðV Þ curves of Au/Al0/Al2O3/Si structures.

B. Gruzza et al. / Vacuum 67 (2002) 125–129126

alumina thickness of the test structures, we haveused two methods.The first one consists of the estimation of the

thickness of various alumina deposits on siliconusing ellipsometry before the deposition of gold oraluminum; the results obtained with the twosamples Au/Al2O3/Si and Au/Al0/Al2O3/Si is120 nm. In the second method, we used CðV Þcharacteristics, where the capacity in the accumu-lation region is maximum and equal to that ofinsulator. Knowing the dielectric constant ofalumina (ei ¼ 5 for Al2O3 [7]) and gate area S;the thickness of insulator expression is given by [8]

di ¼eoeiCox

S: ð1Þ

The thickness of insulators of the samples Au/Al2O3/Si and Au/Al

0/Al2O3/Si is evaluated to be118 and 143.5 nm, respectively.The insulator thickness values obtained from

CðV Þ characteristics for the sample Au/Al2O3/Si isof the same order of magnitude as those measuredusing ellipsometry [9].To understand the increase in the thickness of

alumina and the influence of aluminum on theelectric characteristics of the structures Au/Al2O3/Si and Au/Al0/Al2O3/Si, we characterized variousaluminas by combining the AES and elastic peakelectron spectroscopy (EPES).Fig. 3 shows the variation of the Auger inten-

sities of alumina during gold deposition for Au/Al2O3/Si and Au/Al

0/Al2O3/Si structures. In thefirst stage of the gold deposition, the Auger signalof alumina increases on substrate Al0/Al2O3/Si,whereas it decreases on substrate Al2O3/Si. Thisdifference can be explained by the transformationof metal aluminum into alumina during thecondensation of the atoms of gold. When wecontinue to deposit gold, the Auger signals ofalumina for the two substrates stagnate, decreas-ing very slowly, indicating the presence of aluminamolecules on the surface.The analysis of the characteristic curves of each

sample before deposit of gold by spectroscopy ofthe elastic peak illustrated in Fig. 4, shows thatAl2O3/Si structure is made up only of alumina,while structure Al0/Al2O3/Si presents a surfacerich in aluminum. During the first 5min of

deposit, we noticed that the elastic coefficient ofreflection for Au/Al0/Al2O3/Si increases whileapproaching that of alumina. This behavior relates

Fig. 3. Variation of the Auger intensities of alumina during

gold deposition for Au/Al2O3/Si and Au/Al0/Al2O3/Si struc-

tures.

Fig. 4. Elastic coefficient of reflection for Au/Al2O3/Si and

Au/Al0/Al2O3/Si during Au deposition.

B. Gruzza et al. / Vacuum 67 (2002) 125–129 127

to the oxidation of aluminum, at the same time,alumina is less and less present at the surface of theAu/Al2O3/Si structure. Thus, we can conclude thatthe Au/Al2O3/Si structure is made up of mainlygold at surfaces with a very weak alumina share,while each volume constitutes pure gold. One canthus consider that at the time of the deposit of goldon alumina, a fine layer of alumina remains on thesurface.For Au/Al0/Al2O3/Si sample, it very clearly

appears that the surface is made up of an Au/alumina mixture, one can thus assume thataluminum diffuses on the surface during thedeposition of gold [10]. After the solution ofPoisson’s equation we can obtain theoretical CðV Þcurve; we take into account that the capacity of theMIS structure is equivalent to two capacities Cscand Cox in serial. The differential capacity Csc dueto the variation of the zone of space charge of thesemiconductor and the Cox capacity characterizesthe insulating layer.Figs. 1 and 2 represent theoretical and experi-

mental CðV Þ curves for the samples Au/Al2O3/Si,Au/Al0/Al2O3/Si. We notice that CðV Þ curvemeasured for the structure Au/Al2O3/Si stronglyshifts towards the negative tensions comparedto theoretical CðV Þ curves. This shift in tensionof experimental CðV Þ curve indicates the pre-sence of charges at the insulator/semiconductorinterface. But in the case of Au/Al0/Al2O3/Sistructures, we can note a strong similarity betweenthe theory and the experiment (nearly identicalslopes) and a weak shift between the theoreticaland experimental curves plotted at 1MHz, thesedata confirm the improvement of the metal/insulator interface.The flat band voltage VFB is estimated starting

from the comparison between the theoretical andexperimental curves, it is about �8.15V for Au/Al2O3/Si and �2.15V for structure Au/Al0/Al2O3/Si (Table 1).

The charge density in the insulator is given bythe relation [11]

QT ¼Cox

qSðVFB � fmsÞ; ð2Þ

where fms is the difference between the workfunctions of the metal and of the semiconductor.The charge density in the insulator Qot for the Au/Al2O3/Si and Au/Al0/Al2O3/Si samples is esti-mated at 1.92� 1012 charges/cm2 and 4.47� 1011

charges/cm2, respectively.The value of Qot obtained from the analysis of

CðV Þ data is in good agreement with that obtainedby Hollinger et al. [17].We notice that the difference between measured

CðV Þ curve and theoretical CðV Þ curve is notconstant, but varies with the biasing VG: There isnot only the effect of charges Qot and thedifference between the work function of metaland of semiconductor fms; but also that of thedensity of states at the interface Nss which producea distortion of curves CðV Þ:The interfacial density of states Nss is given by

the formula [12,15]

Nss ¼Cox

qS

dðDV Þdcs

; ð3Þ

where q is the electron charge and cS is thepotential surface.To trace Nss versus (Ess � Evs) we established

the correspondence between the potential of sur-face cS and the energy Ess � Evs by means of therelation [13]

Ess � Evs ¼Eg

2þ qðcs � cBÞ; ð4Þ

where Eg; and Evs are, respectively, the gap of thesemiconductor, the energy level of the valenceband at the surface and cB is the potential in thesemiconductor bulk.Fig. 5 shows the variation of NssðEss � EvsÞ for

the samples Au/Al2O3/Si and Au/Al0/Al2O3/Si.

Table 1

Different parameters obtained for Au/Al2O3/Si and Au/Al0/Al2O3/Si structures

Samples di (nm) VFB (V) Qot (charges cm�2) Nss (eV

�1 cm�2)

Au/Al2O3/Si 118.1 �8.15 1.92� 1012 1012

Au/Al0/Al2O3/Si 143.5 �2.15 4.47� 1011 5.2� 1011

B. Gruzza et al. / Vacuum 67 (2002) 125–129128

These curves have even pace than those describedby Sze [8] and other authors [14,16] and theminimum of the state density is roughly in the mid-gap.The values of Nss in the mid-gap calculated for

the samples Au/Al2O3/Si and Au/Al0/Al2O3/Si are

about 1012 and 5.2� 1011 eV�1 cm�2, respectively.For the Au/Al2O3/Si sample the experimental

slope is different from the theoretical slope, whichis confirmed by the presence of a density of defectin Al2O3/Si interface.

4. Conclusion

Within the framework of design of MISstructure, we have carried out deposits of goldon two types of surfaces (alumina or aluminum/alumina) to account for the role played byinsulating surface on the deposit of gold.The study of these surfaces made it possible to

deduce a strong diffusion of the aluminum atomson the surface in the case of an Au/Al0/Al2O3/Si

structure contrary to a structure of the type Au/Al2O3/Si or the alumina diffusion which is muchless significant.It appears from the electric measurements that

the surface has a better quality if it is rich inaluminum. The interfacial state density in the mid-gap evaluated for the Au/Al2O3/Si and Au/Al

0/Al2O3/Si structures is about 1012 and5.2� 1011 eV�1 cm�2, respectively.In the case of Au/Al0/Al2O3/Si structures, we

have noted a strong similarity between the theoryand the experiment (nearly identical slope) and aweak shift corresponding to a flat band voltageVFB ¼ �2:5V, these data confirm the improve-ment of the metal/insulator interface.

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Fig. 5. Distribution of densities of states in the band gap of the

Au/Al2O3/Si and Au/Al0/Al2O3/Si structures.

B. Gruzza et al. / Vacuum 67 (2002) 125–129 129