at SciVerse ScienceDirect

Current Applied Physics 13 (2013) 1256e1263

Contents lists available

Current Applied Physics

journal homepage: www.elsevier .com/locate/cap

Electrical characterization and modeling of Au/MEH-PPV/porousnþ-GaAs/nþ-GaAs heterojunction in direct and alternating currentmode

Taoufik Ben Jomaa a,*, Mourad Nouiri b, Lotfi Béji c, Abdelaziz Bouazizi a

a Laboratoire de la Matière Condensée et des Nanosciences (LMCN), Département de Physique, Faculté des Sciences de Monastir, Avenue de l’Environnement5019 Monastir, Tunisiab Laboratoire de Physique des Matériaux et des Nanomatériaux Appliquée à l’Environnement, Département de Physique, Faculté des Sciences de Gabès, CitéErriadh 6072 Zrig, Gabès, Tunisiac Laboratoire d’Énergie et de Matériaux (LABEM), École Supérieure des Sciences et de Technologie de Hammam Sousse, Tunisia

a r t i c l e i n f o

Article history:Received 4 January 2013Received in revised form19 March 2013Accepted 25 March 2013Available online 10 April 2013

Keywords:Porous GaAsOrganic/inorganic heterojunctionCurrent and capacitance vs. voltageHigh frequency capacitance andconductanceSCLC and thermionic conductionDepletion zone width

* Corresponding author. Tel.: þ216 22062004; fax:E-mail addresses: taoufik_benjomaa@yahoo.fr (T.

gmail.com (M. Nouiri).

1567-1739/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.cap.2013.03.024

a b s t r a c t

Assembled heterojunction was fabricated by spin-coating poly[2-methoxy-5-(20-ethyl-hexyloxy)-1,4-phenylene-vinylene)] (MEH-PPV) thin layers on straight and porous nþ-GaAs substrates. The currentevoltage and capacitanceevoltage studies have shown an abrupt junction behavior with current con-duction governed by SCLC and thermionic modes. Andersons’ rules were used to determine depletionwidth and balance bands discontinuities for both heterojunctions. Capacitance and conductance vs.frequency techniques were used to evaluate the density of interface states. Density values obtained fromboth techniques were in a good agreement.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

For the past years, silicon (Si), gallium arsenide (GaAs), silicondioxide insulators, and metals such as aluminum and copper havebeen the backbone of the semiconductor industry. However, therehas been a growing research effort in “organic electronics” toimprove the semiconducting, conducting, and light-emittingproperties of organic solids (molecular organic semiconductorsand polymers) and hybrids (organiceinorganic composites). Per-formance improvements, coupled with the ability to process these“active” materials at low temperatures over large areas on flexiblematerials such as plastic or paper, may provide unique technologiesand generate new applications.

However, little interest is being shown toward amorphous IIIeVsemiconductors. This is presumably due to the fact that the physicsof these materials is not yet completely understood. This deficiencyof information led to an increasing interest in investigating hybrid

þ216 75392421.Ben Jomaa), nouirimourad@

All rights reserved.

organiceinorganic system made of polymer and several semi-conductors [1e9] such as GaAs [7,8], Si [2,3] and porous silicon[1,4e6], because their applicability can be expanded for the pro-duction of new photovoltaic and optoelectronic devices. Similar tothe attempt done using porous silicon to produce hybrid structures,we have deposited p-type MEH-PPV thin film on n-type porousGaAs, which to our knowledge, has not been reported or investi-gated. In this work, we elaborated and investigated a porous n⁺-GaAs/MEH-PPV heterojunction in order to obtain a structure, whichbenefits of both materials properties and can be useful as newphotovoltaic device generation. Because there are no reportsdealing with the electrical characteristics of n-type porous GaAs/MEH-PPV and n-type GaAs/MEH-PPV, this work presents aninitial stage and preliminary interpretation of the electrical char-acteristics in AC and DC modes of such heterostructures.

2. Experimental

nþ-type porous GaAs sample was fabricated by electrochemicalanodization of (100) oriented nþ-type GaAs straight substrate doped

T. Ben Jomaa et al. / Current Applied Physics 13 (2013) 1256e1263 1257

with silicon (Si) (n ¼ 1.3 � 1018 cm�3) using HF-Et-OH (1: 2) as elec-trolyte under 24 mA cm�2 current density for 12 s. Prior to polymerdeposition, porous and straight GaAs substrates were cleaned in ul-trasonic bath of acetone and dried. MEH-PPV the p-type semi-conductor, was dissolved in tetrahydrofuran (THF) at about 3%/volume. MEH-PPV thin films were obtained from the prepared solu-tion by spin coating at rate of 2000 rd/mn onto nominal and porousGaAs. Polymer layer thickness is about 1mm,deducedby interpolationfrom the thickness vs. MEH-PPV concentration curves, calibrated onstarting nþ-type GaAs. On the top of the polymer layer a 2 mmdiameter, gold contacts were deposed by high vacuum evaporationtechnique at room temperature and at pressure of about1.33�10�3 Pa. I (V)measurementswere performed using a KEITHLEY236, as a DC source measure unit. To perform capacitanceevoltageanddielectric investigations, a 4192AHEWLETT-PACKARD impedanceanalyzerwas used. All experimentswere performed in thedark underambient conditions. A schematic of the investigated structures isillustrated in Fig. 1.

3. Result and discussion

3.1. Samples SEM morphology

Fig. 2 illustrates surface SEM images of the used porous GaAssubstrate. The brightness in Fig. 2 a shows macropores; themselvesare constituted of elongated pores and coarse underlying structuresupporting the fine structure. Magnifications shown in Fig. 2b and chighlight that anodization of nþ-GaAs produced a bimodal effect inthe pore size distribution. We find pores with diameter greater than500 nm spaced by large areas constituted with smaller pores. Fig. 2cexhibits the porous nature of the layer.Wenote 50e300nmdiameterpores with irregular shapes, spaced by 50e200 nm. More details ofstructural and morphological studies were reported in a previouswork [10].

Fig. 3 illustrates, through SEM images, a comparison betweenMEH-PPV/nþ-GaAs (sample A) and MEH-PPV/Porous nþ-GaAscontacts (sample B). In fact, with sample B the polymer seeps intopores, forming thin polymer nanosized filaments (Fig. 3b). In thecaseofMEH-PPVdepositedon the startingnþ-GaAs (Fig. 3a), contactis taking place only at the interface between the two materials. Wenote that, in the porous nþ-GaAs structure, there is larger specificarea, thus we expect strong interaction between the two materials.

3.2. Currentevoltage characteristics

Investigated structures consist on MEH-PPV/GaAs based hybridheterojunction. First structure is assembled as follows: Au/MEH-PPV/nþ-GaAs, labeled “sample A”. The second one was prepared

Fig. 1. Schematic representation of th

using porous nþ-GaAs, (Au/MEH-PPV/Porous nþ-GaAs/nþ-GaAs),labeled as “sample B” (Table 1). MEH-PPV is a p-type organicsemiconductor acts as electron acceptor and hole transport mate-rial [11].

IeV characteristics are showed in Fig. 4. A rectifying behaviorwas found with sample A and B. This indicates the establishment ofthe expected depletion zone. Since Au work function is closer to theHOMO value of MEH-PPV, no diode behavior will be expected inAu/MEH-PPV contact. Therefore, we can assume that, the existingdepletion region is taking place in MEH-PPV/nþ-GaAs and MEH-PPV/Porous nþ-GaAs junction, respectively for sample A and B.Sample A shows better rectifying ratio (RR) compared to sample Bwhich is found about 1.4 and 2 Respectively with sample A and B,measured atH4 V. It is noteworthy sample A shows higher currentvalue for all bias range. This result was correlated to the existence ofa porous layer in sample B that provides large number of recom-bination centers.

IeV curves are fitted using the following relation [12]:

I ¼ Is

�exp

�qV

nKBT

�� exp

��aqVKBT

��(1)

where:

Is ¼ A*T2exp��qfB

KBT

�(2)

Is is the saturation current, n the ideality factor, q is the elementaryelectron charge, fB is the potential barrier height and a is a physics’parameter related to both semiconductors dielectric constants andcarrier densities, through[12]:

a ¼ 32Na

32Na þ 31Nd(3)

31 and 32 are the dielectric constants and Nd and Na are donor andacceptor charge carrier densities respectively. Subscript numbers 1and 2 refer to nþ-GaAs (or porous nþ-GaAs) and MEH-PPVrespectively.

The fitting values of IeV plots deduced from eq. (1) wereinjected in eq. (2) to deduce fB values. For sample A, from the a-value we deduce a carrier density of about Na ¼ 2 � 1017 cm�3 forMEH-PPV layer, since all other parameters are well known. Na isthen used in eq. (3) to evaluate the carrier density in the porous-GaAs layer. Electrical permittivity of the porous-GaAs layeris determined from its relationship with corresponding porosity,eq. (4) [13]:

e assembled A and B structures.

Fig. 2. SEM images of the used porous nþ-GaAs substrate.

Fig. 3. SEM images of MEH-PPV deposited on nþ-GaAs (a) and MEH-PPV deposited on porous nþ-GaAs (b), red circle delimits the area where the polymer is removed to illustratepolymer infiltration into porous nþ-GaAs layer. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

T. Ben Jomaa et al. / Current Applied Physics 13 (2013) 1256e12631258

P ¼ 1�ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

3porous

3

r(4)

non�porous

Porosity is reported, in our previous work, to be 60%, deducedusing mass-method [14]. All values are listed in Table 2.

The perfect match between theoretical and experimental data(Fig. 4) confirms that the current transport is governed essentiallyby thermionic emission through potential barrier for both

structures. However, different mechanisms can be involved inelectrical transport. The logelog forward current IeV plots ensuredetermination of these mechanisms. Such manner was reportedby several organic related works, such as [15]. Related plots areshown in Fig. 5. As seen the voltageecurrent dependence can bedescribed as a power law (I f Vn). For sample A, we distinguishthree distinct regions, denoted region I, II and III. For sample B,only about two regions were distinguished. For low voltages

Table 1Investigated structures.

Samples details Samples symbols

Au/MEH-PPV/nþ-GaAs AAu/MEH-PPV/Porous nþ-GaAs/nþ-GaAs B

T. Ben Jomaa et al. / Current Applied Physics 13 (2013) 1256e1263 1259

(V < 0.1 V), sample A shows an ohmic conduction behavior,indicating the existence of a small amount of interface barrier.Therefore, the external applied-field is small; this barrier hindersthe charge injection. In region II, n value is found to be about 2.This indicates a space charge limited current mechanism (SCLC).We record a rapid increase of the current by increasing bias, sincethe applied voltage is passed through the threshold of existingblocking. For organic materials, the SCLC model has been widelyused to describe the behavior of organic diodes [16,17]. Third re-gion is characterized by n factor superior to 2. Hence, it isdescribed as a trap-filling region. We record a decrease of thecurrent variation rate with the applied voltage value. In thismechanism, the current is limited by trapped charges. For sampleB, unlike sample A, the ohmic attitude, for small voltages, is notfound. Even under small-applied field, current is described as SCLCmechanism. The second region is trap filling one. The absence ofthe ohmic region is attributed to the incorporation of polymer intoGaAs pores, which eliminates blocking barrier. This explains theweak current values compared to sample A. This situation(absence of an ohmic behavior at low bias) was also reported withMEH-PPV/n-GaN structure [18].

3.3. Capacitanceevoltage characteristics

The CeV characteristics of both heterojunctions were measuredat a frequency of 200 kHz. The capacitance of a heterojunction isdescribed by the Anderson’smodel, which can be expressed as [19]:

C�2 ¼ 2ð 31N1 þ 32N2ÞðVbi � V � KBT=qÞq 31 32N1N2A2 (5)

where q is the electron elementary charge, Vbi is the diffusion po-tential (the voltage drop through the pen junction in thermalequilibrium), V is the applied voltage, and A is the cross sectional

-4 -2 0 2 4

-20

-10

0

10

20

30

40

Experimental Curve

Theoretical Curve

Cu

rre

nt (m

A)

Applied bias v

Au/MEH-PPV/n+

-GaAs

Fig. 4. Theoretical (lines) and experimental (symbols) curre

area which is, in this case, equal to the effective contact area. Fig. 6shows the C�2eV measured curves, where a linear behavior isobserved in the 0.2e0.6 V and 0.5e1.5 V ranges for samples A and Brespectively. This fact is a clear indication that the fabricated de-vices are abrupt junctions [20].

The same C�2eV curves were used to evaluate the carrier con-centration of the highly doped side and the diffusion potential (Vbi)values. Thus from the slope of curves, we estimated an averagedoping concentration of around 1.2 � 1018 cm�3 for the porous nþ-GaAs, which agrees with the average free carrier concentrationdeduced previously by IeV investigations and a diffusion potentialvalues deduced from the intercept of potential axis reported inTable 3. The higher built-in potential is due to the higher barrierwidth in such type of heterostructures. In addition, we note that,the non-linear regions of the C�2eV curves may be related to a non-uniform distribution of concentration in the polymer layer due toan interface state and irregular surface profile [15,21].

In case of heterojunctions, depletion width in the contact zoneof both semiconductors, at zero volts is given by Ref. [22]:

w21 ¼ 2 31 32N2ðVbi � KBT=qÞ

qN1ð 31N1 þ 32N2Þ(6)

w22 ¼ 2 31 32N1ðVbi � KBT=qÞ

qN2ð 31N1 þ 32N2Þ(7)

where 1 and 2 refer to nþ-GaAs (or porous nþ-GaAs) and MEH-PPVrespectively. Hence, total heterojunction depletionwidth values aregiven by:

wT ¼ w1 þw2 (8)

As we can see in Table 3, the total depletion width wT is smallerin the case of sample A than that of sample B. This difference can bedue to the minimization of the whole heterojunctions charge car-rier density induced by the fact that porous structure can bedescribed as an addition of vacuum and material. Hence, with thisfact, we assume that the charge carrier recombination zone ispractically deviated in the depletion zone of the inorganic semi-conductor side. We gathered related results in Table 3.

oltage (V)

-4 -2 0 2 4

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Cu

rre

nt (m

A)

Experimental Curve

Theoretical Curve

Au/MEH-PPV/Porous n+

-GaAs/n+

-GaAs

ntevoltage characteristics of heterostructures A and B.

Table 2Fitted physics’ parameters values of A and B samples.

Sample n fB (V) Is (A) Nd (cm�3) Na (cm�3) a 3/ 30

A 35.34 0.463 4.2 � 10�4 1.3 � 1018 1.17 � 1017 0.02598 3MEH�PPV ¼ 353GaAs ¼ 13.16

B 34.63 0.542 2 � 10�5 7 � 1017 1.17 � 1017 0.02614 3MEH�PPV ¼ 3.53Porous-GaAs ¼ 2

10 10 10

10

10

10

10

10

10

10

Au/MEH-PPV/n+

-GaAs

Au/MEH-PPV/Porous n+

-GaAs/n+

-GaAs

I

II

Slope = 1.96

Slope = 3.84

Slope = 3.46

Slope = 1.94

III

Slope = 1.13

Cu

rre

nt (A

)

Applied bias voltage (V)

I

II

Fig. 5. Ln(V) vs. Ln(I) characteristics of heterostructures A and B.

Table 3Capacitance vs. applied bias voltage deduced parameters.

Sample Vbi (V) WMEH�PPV(mm) Wnþ�GaAs(mm) DEc (eV) DEv (eV)

A 0.21 8.577 2.528 0.697 0.343B 0.24 4.01 11.42 1.17 0.59

T. Ben Jomaa et al. / Current Applied Physics 13 (2013) 1256e12631260

3.4. Energy-band diagram

For the electrical characterization of any kind of hetero-junctions, one of the main issues is the evaluation of the con-duction and valence band discontinuities, DEc and DEv. A simplesolution was proposed by Anderson based on previous work ofShockley [23]. The Anderson’s equation states the following:

DEc ¼ dMEH�PPV þ dnþ�ðPorousÞGaAs þ Vbi (9)

And the valence band value is deduced from the well-knownformula:

-0.8 -0.4 0.0 0.4

0

1

2

3

4

5

6

7

8

C-2

(1

01

6

F-2

)

Applied bias v

Au/MEH-PPV/n+

-GaAs

-2

19

-2

Fig. 6. C�2 vs. applied bias voltage cu

DEc � Ev ¼ EgMEH�PPV � Egnþ�ðPorousÞGaAs (10)

where dMEH�PPV ¼ EF�HOMO is the height of the Fermi level abovethe HOMO band, dnþ�(Porous)GaAs ¼ KBTln(Nc/Nd) is the position of theFermi level in the nþ-GaAs (or the porous nþ-GaAs) from the bot-tom of the conduction band, EF is the Fermi level, and Eg is the bandgap energy of the respective materials. In the offsets calculation, weused the following parameter value for the MEH-PPV:LUMO ¼ 2.8 eV, HOMO ¼ 5.3 eV and EF ¼ 4.8 eV, for the nþ-GaAs: Ec ¼ 4.14 eV, Ev ¼ 5.6 eV and EF ¼ 4.1 eV and for the porousnþ-GaAs: Ec ¼ 3.68 eV, Ev ¼ 5.6 eV and EF ¼ 4.1 eV. Both valence andconduction bands offsets values indicate that the offset disconti-nuity occurs in the conduction band.

Based on DEc and DEv (Table 3) and the above given energiesvalues we attempted to build a schematic band model of the MEH-PPV/porous nþ-GaAs/nþ-GaAs and MEH-PPV/nþ-GaAs hetero-junction (Fig. 7).

3.5. Dielectric characterizations

The frequency dependent capacitance and conductance ofsemiconductor devices can be used to calculate interface proper-ties. Various techniques have been used to achieve this. One of thetechniques is conductance technique, which determines the point-

oltage (V)

-1.6 -0.8 0.0 0.8 1.6

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

C(10

F)

Au/MEH-PPV/Porous n+

-GaAs/n+

-GaAs

rves of heterostructures A and B.

Vacuum Level

LUMO = 2.8eV

n-GaAs

gap = 1.46eV E = 5.6eV

E = 4.14eV

0

δ = 0.053eV

gap = 2.5eV

HOMO = 5.3eV

X

ΔEV = 0.3eV

δMEH-PPV

= 0.5eV

EF

ΔEC = 0.69eV

MEH-PPV

LUMO = 2.8eV

Porous n-GaAs

gap = 1.92eVE = 5.6eV

E = 3.68eV

0

gap = 2.5eV

HOMO = 5.3eV

X

ΔEV = 0.59eV

δMEH-PPV

= 0.5eV

EF

ΔEC = 1.17eV

MEH-PPV

Vacuum Level

δ = 0.006eV

ω ω ω ω

Fig. 7. Energy band diagram in the interface region of heterostructures A and B at equilibrium.

10-4

10-3

Au/MEH-PPV/Porous n+

-GaAs/n+

-GaAs

Co

nd

uc

ta

nc

e (S

)

Frequency (kHz)

0V

3V

-3V

10 10 10

10

10

Au/MEH-PPV/n+

-GaAs

Co

nd

uc

ta

nc

e (S

)

Frequency (kHz)

0V

3V

-3V

Fig. 8. Conductance vs. Frequency plots of heterostructures A and B for different applied bias.

10 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5Au/MEH-PPV/Porous n

+

-GaAs/n+

-GaAs

Ca

pa

cita

nc

e (n

F)

Frequency (kHz)

0V

3V

-3V

Fig. 9. Capacitance vs. Frequency plots of heterostructures A and B for different applied bias.

Table 4Conductance vs. Frequency deduced trap density and time constant values.

Sample Nss (cm�2 eV�1) Time constant s (s)

3 V 0 V �3 V 3 V 0 V �3 V

A 2.38 � 1015 1.84 � 1015 8.62 � 1014 1.33 � 10�7 1.18 � 10�7 1.82 � 10�7

B 4.70 � 1014 1.65 � 1014 3.22 � 1014 3.33 � 10�7 2.86 � 10�7 2.86 � 10�7

T. Ben Jomaa et al. / Current Applied Physics 13 (2013) 1256e1263 1261

0001008006004002

(F

)

Frequency (kHz)

0V

3V

-3V

Linear Fit Css

= AqNss

arctan (2πfτ)/(2πfτ)

000100101

10

10

10

10 Au/MEH-PPV/Porous n -GaAs/ n -GaAs

Css

(F

)

Frequency (kHz)

0V

3V

-3V

Linear Fit C = AqN arctan (2 f )/(2 f )

Fig. 10. Css vs. Frequency (symbols) with fitted plots (lines) of heterostructures A and B for different applied bias.

Table 5Capacitance vs. Frequency deduced trap density and time constant values.

Sample Nss (cm�2 eV�1) Time constant s (s)

3 V 0 V �3 V 3 V 0 V �3 V

A 1.98 � 1016 1.89 � 1016 3.08 � 1014 2.60 � 10�7 2.20 � 10�7 2.12 � 10�7

B 3.43 � 1015 6.6 � 1015 3.92 � 1014 4.20 � 10�7 2.01 � 10�7 2.99 � 10�7

T. Ben Jomaa et al. / Current Applied Physics 13 (2013) 1256e12631262

to-point density of states throughout the depletion region of suchdevices [24]. The conductance technique determines the surfaceparameters with more accuracy than capacitance technique [25]because the conductance comes only from the interface states[24]. The conductance losses are the base of conductance techniqueresulting from the exchange of majority carriers between theinterface states, when a small AC signal is applied to the devices[26]. In depletion region, theDC biased Fermi level starts to oscillateabout the mean position by applying AC signal [27]. Therefore, it isassumed that there may be a capacitance due to interface states inexcess to depletion layer capacitance, which depends upon therelaxation time of the interface states and the frequency of the ACsignal.

Figs. 8 and 9 show, respectively, the measured conductance (G)and measured capacitance (C) of A and B heterostructure indepletion region as a function of the frequency at different biasvoltages. At higher frequencies, capacitance remains almost con-stant, while at the lower frequencies, capacitance and conductancechange. This means that at higher frequencies, the interface statescannot follow the AC signal [28] and consequently cannotcontribute to the capacitance. It indicates that the interface statesare responsible for the observed frequency dispersion in C(f) andG(f) curves.

In such heterostructures, we can adopt the AC conductanceformula [29,30]:

G ¼ GDC þ AuS (11)

where GDC is the DC conductance, A is a constant and s is the fre-quency exponent factor, which is generally inferior to the unit(s < 1), characterizing a low power response attributed in the mostcases to the hopping conductivity. In summary, this transportprocess is evoked when the carriers localized at Fermi level tend tofollow the applied electric field by jumping into empty statescrossing an electric barrier resulting from material particles fluc-tuation or chemical environments.

According to Nicollian and Goetzberger [31], the interface statesconductance can be quantitatively formulated by:

Gss ¼ AqNss

sln�1þ u2s2

�(12)

2

And they estimate that Gss(umax) z G(umax), which allows asimple deduction of the trap density (Nss) and time constant (s)reported in Table 4.

The interface capacitance Css is related to the measured capac-itance C and the higher frequency capacitance CHF by the followingequation [32]:

Cssðf Þ ¼ Cðf Þ � CHF (13)

According to the same authors [31], this expression is modeledby:

Css ¼ AqNssarctgðusÞ

us(14)

Our best fit (solid line in Fig. 10) showed a good agreement withthe trap density (Nss) and time constant (s) previously found inconductance-frequency investigations. The obtained fitting resultsof Nss and s are listed in Table 5.

4. Conclusion

In this paper,we attempted to evaluatephysical parametersofAu/MEH-PPV/Porous nþ-GaAs/nþ-GaAs and Au/MEH-PPV/nþ-GaAsheterojunctions. This investigation allowed us to determine theconduction process through heterojunction interface. This processwas found to be dominated by both SCLC and thermionic emission ofcharge carriers through the potential barrier. Using CeV data, wecalculated the depletion zonewidth in heterojunction both sides andeventually evaluated a schematic band diagram of each hetero-structure. In addition, trap density was investigated by capacitanceand conductance vs. frequency spectroscopy at relatively high fre-quency, such results showed a good agreement between bothinvestigation techniques. We assume that our results are in the basedata for further investigations of hybrid organic/porous inorganicsemiconductor devices and their applications in photovoltaic, opto-electronic and industry domains.

T. Ben Jomaa et al. / Current Applied Physics 13 (2013) 1256e1263 1263

References

[1] D.P. Halliday, E.R. Holland, J.M. Eggleston, P.N. Adams, S.E. Cox, A.P. Monkman,Thin Solid Films 276 (1996) 299e302.

[2] P. Stallinga, H.L. Gomes, H. Rost, A.B. Holmes, M.G. Harrison, R.H. Friend,Synthetic Metals 111 (2000) 535e537.

[3] M. Cakar, A. Türüt, Y. Oganer, Journal of Solid State Chemistry 168 (2002)169e174.

[4] T.P. Nguyen, P. Le Rendu, K.W. Cheah, Physica E 17 (2003) 664e665.[5] H. Amrollahi Bioki, M. Borhani Zarandi, Indian Journal of Physics 86 (6) (2012)

439e441.[6] T.P. Nguyen, P. Le Rendu, M. Lakéhal, M. de Kok, D. Vanderzande, A. Bulou,

J.P. Bardeau, P. Joubert, Physica Status Solidi A 197 (2003) 232e235.[7] D.P. Halliday, J.W. Gray, P.N. Adams, A.P. Monkman, Synthetic Metals 102

(1999) 877e878.[8] J. Yang, I. Shalish, Y. Shapira, Physical Review B 64 (2001) 35325.[9] T. Chassé, C.I. Wu, I.G. Hill, A. Kahn, Journal of Applied Physics 85 (1999) 6589.

[10] L. Beji, T. Ben Jomaa, A. Ltaeif, A. Bouazizi, Physica Status Solidi A 202 (2005)1763e1767.

[11] F. Yakuphanoglu, W.A. Farooq, Acta Physica Polonica A 119 (2011) 890e894.[12] H.A. Kaci, D. Boukredimi, M. Mebarki, Physica Status Solidi A 183 (2001) 345e

351.[13] P.A. Ivanov, M.G. Mynbaeva, S.E. Saddow, Semiconductor Science and Tech-

nology 19 (2004) 319.[14] L. Beji, T. Ben Jomaa, Z. Harrabi, A. Laribi, A. Missaoui, A. Bouazizi, Vacuum 80

(2006) 480e487.[15] Mo Zhu, Tianhong Cui, Kody Varahramyan, Microelectronic Engineering 75

(2004) 269e274.

[16] P.W.M. Blom, M.J.M. de Jong, M.G. van Munster, Physical Review B 55 (1997)R656.

[17] A.J. Campbell, D.D.C. Bradley, D.G. Lidzey, Journal of Applied Physics 82 (1997)6326.

[18] M. Soylu, Optical Materials 34 (2012) 878e883.[19] L. Magafas, N. Georgoulas, A. Thanailakis, Semiconductor Science and Tech-

nology 7 (1992) 1363.[20] P. Rosales-Quintero, A. Torres-Jacome, F.J. De la Hidalga-Wade, C. Zúñiga-Islas,

W. Calleja-Arriaga, C. Reyes-Betanzo, Superficies y Vacío 21 (2008) 1e8.[21] G. Liang, T. Cui, K. Varahramyan, Solid State Electronics 47 (2003) 691.[22] Weiwei Gao, Xuan Sun, Jing Wang, Journal of Applied Physics 109 (2011)

23909.[23] R.L. Anderson, Solid-State Electronics 5 (1962) 341e351.[24] S. Logothetidis, E. Evangelou, N. Konofaos, Journal of Applied Physics 82

(1997) 5017e5020.[25] E.H. Nicollian, A. Goetzberger, Applied Physics Letters 7 (1965) 216e219.[26] E.H. Nicollian, J.R. Brews, MOS (Metal Oxide Semiconductor) Physics and

Technology, Wiley, New York, 1982.[27] S. Karadeniz, A. Birkan Selçuk, N. Tu�gluo�glu, S. Bilge Ocak, Nuclear Instruments

and Methods in Physics Research Section B Beam Interactions with Materialsand Atoms 259 (2007) 889e894.

[28] J. Fernández, P. Godignon, S. Berberich, J. Rebollo, G. Brezeanu, J. Millán, Solid-State Electronics 39 (1996) 1359e1364.

[29] M. Pollak, T.H. Geballe, Physical Review 122 (1961) 1742e1753.[30] N.F. Mott, E.A. Davis, Electronic Process in Noncrystalline Materials, Oxford

University Press, New York, 1979.[31] E.H.Nicollian, A.Goetzberger, Bell SystemTechnical Journal 46 (1967) 1055e1133.[32] A. Singh, Solid-State Electronics 28 (1985) 223e232.