Contents lists available at SciVerse ScienceDirect

Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 16 (2013) 89–98

1369-80

http://d

n Corr

E-m

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

Heterojunction performance of dip coated n-Cd0.5Zn0.5S thin filmson different metal sulfide substrates

M. Abdel Rafea a, A.A.M. Farag b,n, Sara Gad a, N. Roushdy a

a Electronic Department, Advanced Technology and New Material institutes, City for Scientific Research and Technology Applications, New Borg El Arab city,

Alexandria, Egyptb Thin Film Laboratory, Physics Department, Faculty of Education, Ain Shams University, Heliopolis, Roxy, Cairo 11757, Egypt

a r t i c l e i n f o

Available online 28 July 2012

Keywords:

Heterojunction

Current–voltage

Capacitance–voltage

Photovoltaic characteristics

01/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.mssp.2012.06.011

esponding author. Tel.: þ20 33518705; fax:

ail address: alaafaragg@yahoo.com (A.A.M. Fa

a b s t r a c t

In this work, n-type cadmium zinc sulfide (n-Cd0.5Zn0.5S) films were grown by a dip

coating technique on different p-type metal sulfide substrates. The morphology, structure,

and composition of the yielded materials have been confirmed by scanning electron

microscopy, X-ray diffraction, energy-dispersive X-ray analysis, respectively. Using the

absorption measurements, the direct allowed band gap energies for Cd0.5Zn0.5S, PbS, CuS,

CuInS2, were found to be 2.93, 1.83, 1.98 and 2.5 eV at room temperature, respectively.

The substrate dependence of the current density–voltage (J–V) characteristics of

n-Cd0.5Zn0.5S/p-PbS, n-Cd0.5Zn0.5S/p-CuS, n-Cd0.5Zn0.5S/p-Cu0.8In1.2S2, n-Cd0.5Zn0.5S/p-

Cu0.9In1.1S2, n-Cd0.5Zn0.5S/p-CuInS2, n-Cd0.5Zn0.5S/p-Cu1.1In0.9S2 and n-Cd0.5Zn0.5S/p-

Cu1.2In0.8S2 heterojunctions were measured at room temperature (�300 K). These

characteristics showed a rectifying behavior consistent with a potential barrier formed at

the interface for all the studied devices. The forward current density–voltage characteristics

under low voltage bias were explained on the basis of thermionic emission mechanism.

Heterojunction parameters such as ideality factor, n, series resistance, RS and barrier height,

Fb were obtained from J–V measurements using Cheung’s method. The heterojunctions show

non-ideal J–V behavior with an ideality factor greater than unity. Analysis of the experimental

data under reverse voltage bias suggests that Schottky effect is the dominant mechanism. The

dark capacitance–voltage characteristics of the heterojunctions were studied at 1 MHz. High

value of built-in potential of 0.58 V was obtained for n-Cd0.5Zn0.5S/p-Cu0.9In1.1S2 heterojunc-

tion as compared to the other studied heterojunctions. The photovoltaic characteristics were

analyzed for the heterojunctions under illumination of 100 mW/cm2.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

It is well known that the II–VI compounds are poten-tially interesting and important because of their majorcontribution to solar cells [1,2] and various opto-electronicdevices [1]. The structures of these materials depend uponstoichiometery of elements and preparation conditions[1,2]. In fact, CdZnS, a ternary chalcogenide semiconductor,

ll rights reserved.

þ20 22581243.

rag).

has a variable band-gap energy of 2.4–3.7 eV, primarilydependent on a relative Cd:Zn ratios [3]. CdZnS has beenwidely used as a wide bandgap window material inheterojunction photovoltaic solar cells, photoconductivedevices [4,5], low-voltage cathode luminescence, high-density optical recording and blue, and ultraviolet laserdiodes [6,7]. Keeping these aspects in view, more attentionis being given in producing good quality CdZnS thin filmsfor comprehensive optical studies and their various appli-cations [8,9].

Most of the studies so far on the above topic wereconcentrated on structures prepared using techniques

Fig. 1. Schematic diagram of Cd0.5Zn0.5S/metal sulfide heterojunction.

M. Abdel Rafea et al. / Materials Science in Semiconductor Processing 16 (2013) 89–9890

such as physical vapor deposition [10], chemical vapordeposition [11] and sputtering [12]. The relatively low costand large area techniques like chemical solution deposi-tion [13], electroless deposition [14], electrodeposition[15] and screen printing [16] are getting acceptance infabrication of solar cells and photodetector structures [17].

Nanostructure films of CdxZn1�xS have been preparedby our research group using low cost dip coating method.However, there are few reports about formation andcharacterization of low cost CdxZn1�xS on different sulfidecompounds substrates [18,19]. These metal sulfide mate-rials were preferred as substrates in the present work dueto improvements in heterojunction performance andconversion efficiency of cells based n-Cd0.5Zn0.5S andeffective lowering of the manufacturing costs comparedto expensive crystalline and polycrystalline silicon tech-nology [20,21]. Moreover, devices with Cd0.5Zn0.5 S/metalsulfide as a heterojunction partner layer have shownbetter blue photon response due to their higher bandgap as compared to devices with CdS [20,21]. Attemptshave been made for searching non-toxic, cost-effectivenew materials devices which are tailor-made for thatpurpose [20,21].

Films can be formed easily from their precursor solu-tions by common techniques such as dipping, spinning orspraying [22,23]. Of these, dip coating is well known to besimple, cost effective method which can be implementedeither as batch or continious process.Other advantages ofa dip coating process are its ability to coat large surfaceareas,complex shapes,and both sides of aplate at the sametime [22]. Herein, we report a convenient new route tosynthesize n-Cd0.5Zn0.5 S on different p-type substrates ofPbS, CuS, Cu0.8In1.2S2, Cu0.9In1.1S2, CuInS2, Cu1.1In0.9S2

and Cu1.2In0.8S2 heterojunctions using dip coatingmethod. The current density–voltage (J–V) measurementshave been measured at room temperature (300 K). More-over, the effect of substrate on capacitance–voltage char-acteristics and photovoltaic characteristics were alsoconsidered.

2. Experimental

2.1. Thin films and junction formation

High purity cadmium acetate, zinc acetate, thiourea(5 N) were used to prepare the powder and thin films ofCd0.5Zn0.5S. 0.01 M of cadmium acetate and zinc acetatewere dissolved in 100 ml absolute ethanol in 100 mlbeaker. This solution was stirred for 10 min. The molarratios of Cd, Zn acetates and thiourea were taken as 1:1:2respectively in order to obtain Cd0.5Zn0.5S stiochiometriccomposition. To obtain a nano-Cd0.5Zn0.5S, the driedpowder was heated at 200 1C in a muffle for 3 h. Suchpreparation procedure was similar to that published byXu and Zhang [24] for the preparation of metal sulfidenano-powder materials.

Thin films of nanocrystalline Cd0.5Zn0.5S were depositedon different p-type compact disks substrates of PbS, CuS,Cu0.8In1.2S2, Cu0.9In1.1S2, CuInS2, Cu1.1In0.9S2 and Cu1.2In0.8S2

to form different heterojunctions. The thickness of all thesubstrates was nearly the same and found to be 300 mm.

The top and bottom ohmic contacts were deposited usingthermal evaporation technique. The schematic diagram ofthe heterojunction is shown in Fig.1.

2.2. Characterization and measurements

Scanning electron microscopy, SEM of type JEOL-JSM-636 OLA was used to study the surface morphology of thedeposited films. The film thickness was determined by thecross sectional imaging of the edge for the film and thesubstrate interface. The forward and reverse bias I–V

measurements of the heterojunctions under investigationwere carried out by using a highly impedance electrometertype Keithley 617 source meter at room temperature. Thesource of light was a high power halogen lamp containingiodine vapor and tungsten filament. The intensity of lightwas measured with a solar power meter (TM-206, Taiwan).

3. Results and discussion

3.1. X-ray diffraction studies

X-ray diffraction pattern, XRD was used to study thecrystalline structure properties of the synthesizedCd0.5Zn0.5 S, PbS, CuS, Cu0.8In1.2S2, Cu0.9In1.1S2, CuInS2,Cu1.1In0.9S2 and Cu1.2In0.8S2 as shown in Fig. 2(a–d).A polycrystalline nature with single phase are observedfor all the studied materials and the diffraction planesthat characterize the structure are cited on each graph.The interplanar spacing (not shown here) were calculatedfrom the peak positions using Bragg’s equation andcompared with XRD standard cards using the ICDD VIEW2006 data base. The observed d-values were found to bematched well with the cubic systems of Cd0.5Zn0.5 S, CuS,PbS and tetragonal system of CuInS2.

3.2. Morphology and composition analysis

The surface morphology of the Cd0.5Zn0.5 S, PbS, CuSwere studied by SEM as shown in Fig. 3(a)–(c), respec-tively. The surface image revealed that the all preparedmaterials have a fine-grained nanostructure withoutcracks or peels which confirms the high quality of thedeposited film. There are fine particles grown on thesurfaces which are nearly ordered but differ in grain size.There are fine particles which are nearly ordered butdiffer in grain size each of them consists of smallerparticles of size in the range of nanostructural materials.This means that the fine nano-particles agglomerate tobuild a bigger grain in each case especially for CuS.

Cross sectional image of Cd0.5Zn0.5 S was magnified bySEM in order to test the film–substrate interface. Goodadhesion of the film on the substrate was observed and

Fig. 2. X-ray diffraction patterns of (a) Cd0.5Zn0.5S (b) PbS (c) CuS and (d)

CuInS2 of different compositions.

Fig. 3. SEM images of (a) Cd0.5Zn0.5S, (b) PbS and (c) CuS.

M. Abdel Rafea et al. / Materials Science in Semiconductor Processing 16 (2013) 89–98 91

the film vertical boundaries determine the film thicknessas shown in Fig. 4.

The energy dispersive X-ray spectroscopy (EDX)attached to the SEM unit (JEOL-JSM-636 OLA) was used

to investigate the quantitatively chemical composition inweight and atomic percentages of composition ofCd0.5Zn0.5S, Cu0.8In1.2S2, Cu0.9In1.1S2, CuInS2, Cu1.1In0.9S2,Cu1.2In0.8S2, PbS and CuS. Table 1 lists the average calcu-lated composition of the precursor concentration for eachmaterial. The measured values of composition for eachmaterial are close to the calculated values which giveindication for the nearly stoichiometric structure ineach case.

3x103

4x103

PbSCuInS2

CuS

M. Abdel Rafea et al. / Materials Science in Semiconductor Processing 16 (2013) 89–9892

3.3. Absorption characteristics and energy gap

determination

The spectral dependence of the absorption coefficientcharacteristics of PbS, CuS and CuInS2 were determinedfrom the extinction coefficient for each material in thewavelength region 300–1100 nm. The absorption coeffi-cient, a is related to the extinction coefficient k throughthe relation where a¼4pk/l where l is the wavelength[25]. The results for the absorption coefficients are shownin Fig. 5. We notice that the onset of the absorption in thesamples is different and therefore the correspondingband-gap energies of resulting materials can be expected.The energy band gaps of PbS, CuS and CuInS2 werecalculated with the help of the optical absorption spectra.The information about direct or indirect inter-band tran-sitions was determined from the analysis of the absorp-tion spectral dependence near the fundamentalabsorption edges within the framework of one electrontheory. To determine the energy band gap, the relation of(ahn)2 versus photon energy are plotted. The theory ofinterband absorption shows that at the optical absorptionedge the absorption coefficient, a varies with the photonenergy hn according to the following relation [25]:

ðahnÞ2 ¼ Aðhn�EgÞ, ð1Þ

where A is a constant and Eg is the optical band gap. Thus,a plot of (ahn)2 versus (hn) is a straight line whoseintercept on the energy axis gives the energy gap, Eg.These graphs have been given in Fig. 6 and indicates thatthe main transition is a direct allowed in all the studiedmaterials. The band gap energies of PbS, CuS and CuInS2

have been determined by the extrapolation of the linearregions on the energy axis (hn), as shown in Fig. 3 andfound to be 1.83, 1.98 and 2.5 eV. The higher values of the

Fig. 4. Cross sectional image using SEM for Cd0.5Zn0.5S film.

Table 1Experimental composition of the constituent elem

CuInS2, Cu1.1In0.9S2 and Cu1.2In0.8S2 , PbS and CuS sa

Sample Weight% (measured)

Cd0.5Zn0.5S (Cd:Zn:S¼26.50:26.53: 4

CuInS2 (Cu:In:S¼26.31:25.71: 47

Cu1.1In0.9S2 (Cu:In:S¼27.07:22.6:50.3

Cu0.8In1.2S2 (Cu:In:S¼20.51: 30.49: 4

Cu1.2In0.8S2 (Cu:In:S¼30.68: 20.15: 4

Cu0.9In1.1S2 (Cu:In:S¼22.94: 26.61: 5

PbS (Pb:S¼49.33: 50.67 )

CuS (Cu:S¼48.88:51.12)

obtained energy gap of these materials as compared tothat published before can be attributed to the strongconfinement effect when the crystallite size is approachesto the dimension of Bohr exciton [26] as confirmed fromthe SEM images.

According to the absorption coefficient characteristicsof PbS, CuS and CuInS2 materials, the results suggest thatthese materials can be used as absorbing material in itsdevices with n-Cd0.5Zn0.5S which has an energy gap of2.93 eV as shown in Fig. 7.

3.4. Substrate dependence of the forward bias J–V

characteristics

Current–voltage characteristics provide clear illustra-tions about the conduction mechanism inside the inves-tigated devices (n-Cd0.5Zn0.5S on different p-typesubstrates of PbS, CuS, Cu0.8In1.2S2, Cu0.9In1.1S2, CuInS2,Cu1.1In0.9S2 and Cu1.2In0.8S2). Fig. 8 illustrates a set ofsemi-logarithmic plots of forward and reverse bias J–V

characteristics of the heterojunction devices at roomtemperature (300 K). These curves were definitely of thediode type, with the forward direction corresponding tothe negative potential on n-Cd0.5Zn0.5S. This behavior canbe understood by p–n junction, namely the barrier at theinterface limits forward and reverse carrier flow acrossthe junction, where the built-in potential could be devel-oped [27].

ents in Cd0.5Zn0.5S, Cu0.8In1.2S2, Cu0.9In1.1S2,

mples.

Atomic% (calculated)

4.64) (Cd:Zn:S¼25:25:50)

.98) (Cu:In:S¼25:25:50)

3) (Cu:In:S¼27.5: 22.5: 50)

9 ) (Cu:In:S¼20: 30: 50)

9.17) (Cu:In:S¼30: 20:50 )

0.45) (Cu:In:S¼22.5:27.5: 50)

(Pb:S¼50: 50)

(Cu:S¼50: 50)

400 600 800 10000

1x103

2x103

λ (nm)

α (c

m-1

)

Fig. 5. Spectral dependence of absorption coefficient for PbS, CuInS2

and CuS films.

0.0

5.0x105

1.0x106

1.5x106

2.0x106PbSCuS

(αhν

)2 (e

V/c

m)2

hν (eV)1.0 1.5 2.0 2.5 0 1 2 3 4 5

0

1x107

2x107

3x107

4x107

(αhν

)2 (e

V/c

m)2

hν (eV)

CuInS2

Fig. 6. Plot of (ahn)2 versus hn for (a) PbS, CuS and (b) CuInS2 films.

2.00 2.25 2.50 2.75 3.000.0

2.0x1010

4.0x1010

6.0x1010

8.0x1010

1.0x1011

(αhν

)2 (eV

/cm

)2

hν (eV)

Cd0.5Zn0.5S

Fig. 7. Plot of (ahn)2 versus hn for Cd0.5Zn0.5S film.

M. Abdel Rafea et al. / Materials Science in Semiconductor Processing 16 (2013) 89–98 93

The rectifying ratio for the devices was estimated at71 V. The calculated rectification ratio for the investi-gated heterojunctions were collected in Table 2. Thecalculated values of RR showed an enhancement of thedevice rectification ratio in the device of n-Cd0.5Zn0.5S/p-Cu1.2In0.8 S2. Fig. 9(a) shows the plot of rectification ratio,RR versus the type of the used substrate for the device.The proportionate enhancement in the rectification ratioreveals that, the current in the heterojunction is con-trolled by space charge limited theory [28,29]. Accordingto the space charge theory, the unbalanced charge can beeasily produced by the applied voltage and also signifythe trap induced current flow [30].

The current density–voltage characteristics under for-ward bias voltage in the region �0.1 VrVr0.5 V canbe explained using thermionic emission theory and givenby [31]

J¼ J0 expqðV�JRs

nkT

� ��1

� �, ð2Þ

where V is the voltage across the diode, Rs is the seriesresistance, n is the ideality factor and Jo is the saturation

current density given by the following expression [31]:

J0 ¼ AnT2 exp�qFb

kT

� �, ð3Þ

where A* is the effective Richardson constant and Fb is thebarrier height. The saturation current density value foreach heterojunction (listed in Table 2) was determinedfrom the y-axis intercept of the semilogarithmic plot ofJ–V characteristics and shown in Fig. 9(b). As observedfrom this figure that an improvement for the reversesaturation current density for the n-Cd0.5Zn0.5S/p-Cu1.2In0.8S heterojunction as compared to n-Cd0.5Zn0.5S/p-CuS heterojunction. Moreover, the ideality factor isdecreased from 2.2 for n-Cd0.5Zn0.5S/p-CuS heterojunctionto 1.1 for n-Cd0.5Zn0.5S/p-Cu1.2Ga0.8S heterojunction. As aconsequence, the ideality factor derived from the J–V

characteristics deviates far from unity for n-Cd0.5Zn0.5S/p-CuS. At enhanced structure, the ideality factor ofthe device approaches 1, which shows that the width ofthe depletion layer is narrow and becomes comparable tothe diffusion length of the carriers [32]. In addition, it isclear that the value of the ideality factor, n calculated inthe downward curvature region of the Semilog J–V char-acteristics is smaller than those obtained from the linearregion of the same characteristics. This downward curva-ture region may be attributed to the series resistance andthe interface effects [33]. As a result, the barrier heightshowed a voltage biasing dependence due to the potentialchange across the interfacial layer thickness as a result ofthe applied voltage. The series resistance Rs here includesthe contact resistance and the thermionic emission isassumed to be the most predominant mechanism [34].The effect of Rs is significant in the non-linear region ofthe forward voltage biasing and results in reducing thelinear range of the forward J–V curves [35].

Various methods have been suggested to determine theseries resistance which effect on electrical parameters ofdevices. For this, we can determine the series resistancefrom J–V characteristics of the heterojunctions using

10-9

10-8

10-7

10-6

10-5

10-4

10-3

0.0 0.2 0.4 0.6 0.8 1.01.0 0.8 0.6 0.4 0.2 0.0

10-9

10-8

10-7

10-6

V (Volt)

Forward

J (A

/cm

2 )

V (Volt)

Reverse

Fig. 8. Semilogarithmic plot of forward and reverse J–V characteristics for n-Cd0.5Zn0.5S/different substrates.

Table 2Parameters extracted from J–V characteristics for n-Cd0.5Zn0.5S/different-type substrates heterojunction at room temperature.

Heterojunction type RR (at71 V) n J0 (A/cm2) m b (eV cm1/2V�1/2)

n-Cd0.5Zn0.5S/p-CuS 45 2.14 1.19�10�9 2.22 3.84�10�5

n-Cd0.5Zn0.5S/p-PbS 83 1.62 5.26�10�9 1.94 3.18�10�5

n-Cd0.5Zn0.5S/p-Cu0.8In1.2S2 124 1.415 1.25�10�8 2.03 2.96�10�5

n-Cd0.5Zn0.5S/p-Cu0.9In1.1S2 215 1.25 3.30�10�8 2.09 2.85�10�5

n-Cd0.5Zn0.5S/p-CuInS2 325 1.15 8.77�10�8 1.92 2.80�10�5

n-Cd0.5Zn0.5S/p-Cu1.1In0.9S2 692 1.02 2.43�10�7 2.25 2.82�10�5

n-Cd0.5Zn0.5S/p-Cu1.2In0.8S2 978 1.01 6.40�10�6 2.72 2.77�10�5

0

200

400

600

800

1000

0.0

2.0x10-7

4.0x10-7

6.0x10-7C

u 1.2

In0.

8S2

Cu 1

.1In

0.9S

2

CuI

nS2

Cu 0

.9In

1.1S

2

Cu 0

.8In

1.2

S2

PbS

CuS

J0

n

Substrate type

J 0 (A

/cm

2 )

1.2

1.6

2.0

n

Rec

tific

atio

n R

atio

(RR

)

Substrate type

Cu 0

.8In

1.2

S2

Cu 0

.9In

1.1S

2

CuI

nS2

Cu 1

.1In

0.9S

2

Cu 1

.2In

0.8S

2

CuS

PbS

Fig. 9. (a) Rectification ratio, and (b) saturation current density and ideality factor for n-Cd0.5Zn0.5S/different substrates.

M. Abdel Rafea et al. / Materials Science in Semiconductor Processing 16 (2013) 89–9894

Cheung and Cheung method [36]. Furthermore, the idealityfactor, n and the barrier height, Fb were also evaluated.According to Cheung–Cheung method, the forward bias J–V

characteristics due to the thermionic emission having theseries resistance can be expressed as [36]

dV

dðlnJÞ¼ n

kT

qþRsJ ð4Þ

and

HðJÞ ¼ JRsþnFb, ð5Þ

where

HðJÞ ¼ V�nkT

q

� �ln

J

AnT2

� �ð6Þ

0.5 0.75 1

1E-6

1E-5

1E-4

1E-3

0.01

J (A

/cm

2 )

V (Volt)

Fig. 11. Logarithmic plot of J–V characteristics for n-Cd0.5Zn0.5S/different

substrates.

M. Abdel Rafea et al. / Materials Science in Semiconductor Processing 16 (2013) 89–98 95

The dV/d(ln(J)) plot is a straight line region wheredominates the series resistance. According to Eqs. (4) and(5), in the plot of dV/d(ln(J)) versus J and H(J) versus J, itsslope gives the series resistances while its intercept withthe y-axis gives the ideality factors. Fig. 10 represents dV/dln(J) (a) and H(J) (b) vice versa current density, respec-tively. These plots are drawn using the data of the down-ward concave curvature region in the forward bias J–V

characteristics given in Fig. 8. As indicated above, thisregion results from the effect of parameters such as theinterfacial layer thickness between the metal and thesemiconductor. The values of n obtained from the verticalaxis intercepts of H(J)� J and dV/d ln(J)� J curves and thevalues of Rs from their slopes were listed in Table 3. Thus,it can clearly be seen that there is relatively differencebetween the values of n obtained from the downwardcurvature regions of forward bias J–V plots and from thelinear regions of the same characteristics. The reason ofthis difference can be attributed to the existence of effectssuch as the series resistance and the bias dependence ofthe Schottky barrier height according to the voltage dropacross the interfacial layer and change of the interfacestates with bias in this concave region of the J–V plot

0.0 5.0x10-5 1.0x10-4 1.5x10-4

0.08

0.16

0.24

0.32

H (V

)

dV/d

(lnJ)

, (V

)

J (A/cm2)

Fig. 10. (a) Plot of dV/d(ln J) versus J, (b) Plot of H

Table 3Parameters extracted from dV/d(ln J)� J and H(J)-J characteristics for n-Cd0.5Zn

Heterojunction type dV/d(ln J)� J

Rs (kO cm2)

n-Cd0.5Zn0.5S/p-CuS 3.474

n-Cd0.5Zn0.5S/p-PbS 2.475

n-Cd0.5Zn0.5S/p-Cu0.8In1.2S2 1.796

n-Cd0.5Zn0.5S/p-Cu0.9In1.1S2 1.256

n-Cd0.5Zn0.5S/p-CuInS2 1.177

n-Cd0.5Zn0.5S/p-Cu1.1In0.9S2 0.568

n-Cd0.5Zn0.5S/p-Cu1.2In0.8S2 0.538

[36,37]. The barrier height can be also calculated using Eq.(5). A plot of H(J) versus J will be linear and intercept ofthis plot gives the barrier height (Fig. 10b). From the H(J)

8.0x10-6 1.6x10-5 2.4x10-5

1.06

1.08

1.10

1.12

1.14

1.16

J (A/cm2)

versus J of n-Cd0.5Zn0.5S/different substrates.

0.5S/different-type substrates heterojunction at room temperature.

H(J)� J

n Rs (kO cm2) Fb (eV)

4.47 2.630 0.32

3.89 2.515 0.38

3.7 1.905 0.42

3.36 1.440 0.50

2.67 1.290 0.58

2.73 0.940 0.60

2.23 0.843 0.70

M. Abdel Rafea et al. / Materials Science in Semiconductor Processing 16 (2013) 89–9896

versus J plot, the barrier height of the device is calculatedfor each heterojunction and listed in Table 3. Theobserved difference in barrier height calculated fromJ�V characteristics and H(J) versus J plot could be dueto the interface state and series resistance effects [36,37].

At relatively high forward voltages (0.5rVr2 V), adifferent mechanism is operating. As observed in Fig. 11,the current shows a power-law exponent of the formJaVm. The m values were calculated from the slopes of thelog J–log V characteristics and listed in Table 2. The domi-nant charge transport mechanism for the device wasdetermined by obtaining m value from the slope of linearregion in Fig. 8 after re-plotted as log J–log V [38]. This curveshows quadratic voltage dependence at this voltage region[38]. The space charge limited current (SCLC) according totraps free Child’s law can be expressed as follows [39]:

J¼9

8ee0m

V2

d3, ð7Þ

where d is the thickness of n-Cd0.5Zn0.5S, m is the carriermobility, V is applied voltage and e0 and e are the vacuumand relative permittivities, respectively.

0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

10-8

10-7

10-6

10-5

J R (A

/cm

2 )

V1/2 (Volt )1/2

Fig. 12. Semilogarithmic plot of reverse current density versusV1/2 for

n-Cd0.5Zn0.5S/different substrates.

10

20

30

40

-2.0 -1.6 -1.2 -0.8 -0.4 0.0 -2.0 -1.6 -1.2 -00.0

2.0x10-3

4.0x10-3

6.0x10-3

8.0x10-3

1/C

2 (pF-2

)

V

C (p

F)

V (Volt)

Fig. 13. (a) Capacitance–voltage characteristics b) 1/C2 versus voltage

3.5. Substrate dependence of the reverse bias J–V

characteristics

In order to determine conduction mechanism of n-Cd0.5Zn0.5S on different p-type substrates of PbS, CuS,Cu0.8In1.2S2, Cu0.9In1.1S2, CuInS2, Cu1.1In0.9S2 and Cu1.2In0.8S2

heterojunction under the reverse bias voltage, the differentconduction mechanisms were tested for these devices. Thedominant mechanism was found to be Schottky effect,which is known as the modification of the potential barrierof the metal semiconductor interface [40]. Emission of thecarriers is analogous to thermoionic emission, the appliedfield contributes to decrease extraction work of metal–semiconductor. This can explained in terms of the chargetransport mechanism operating in the device in the differentvoltage ranges [40]. The current for this conductionmechanism is given as [40]

JR ¼ Js expbs

kT

� �V

d

� �1=2" #

, ð8Þ

where Js is a constant, V is the applied voltage and d isthickness of the material and bs is the Schottky constant,which is determined as[40]:

bs ¼e3

4pee0

� �1=2

, ð9Þ

where e is carrier charge, e0 is the dielectric constant ofvacuum and e is the dielectric constant of the material. Theplot of semilogarithmic JR versus V1/2 for the studied devicesis shown in Fig. 12. It is observed that the obtained averageexperimental bs is closer to the theoretical value of bs forthe all studied devices as listed in Table 2. This suggests thatthe charge transport mechanism under reverse bias isthrough the Schottky mechanism.

3.6. Substrate dependence of capacitance–voltage

characteristics

Capacitance–voltage measurement is one of the mostimportant nondestructive methods for obtaining information

0.1

0.2

0.3

0.4

0.5

0.6

Vb

(V)

Substrate type

CuS

PbS

Cu 0

.8In

1.2S

2

Cu 0

.9In

1.1S

2

CuI

nS2

Cu 1

.1C

u 0.9

S2

Cu 1

.2 In

0.8S

2

.8 -0.4 0.0 0.4 0.8

(Volt)

and (c) built-in potential of n-Cd0.5Zn0.5S/different substrates.

0.0 0.1 0.2 0.3 0.4 0.50.0

1.0x10-4

2.0x10-4

3.0x10-4

J (A

/cm

2 )

V (Volt)

Fig. 14. J–V characteristics of n-Cd0.5Zn0.5S/different substrates under

illumination of 100 mW/cm2.

M. Abdel Rafea et al. / Materials Science in Semiconductor Processing 16 (2013) 89–98 97

on rectifying contacts interfaces [41]. Capacitance–voltagecharacteristics in this work (Fig. 13(a)) were performed at asufficiently high frequency of 1 MHz for n-Cd0.5Zn0.5S ondifferent p-type substrates of PbS, CuS, Cu0.8In1.2S2,Cu0.9In1.1S2, CuInS2, Cu1.1In0.9S2 and Cu1.2In0.8S2 heterojunc-tions. This high frequency made the interface state chargesdo not contribute to the diode capacitance [41–43]. This willoccur when the time constant is too long to permit thecharge to move in and out of the interface states in responseto an applied signal [42]. The junction capacitance can beexpressed as [43]

C�2¼

2ðVbþVÞ

qee0A2N, ð10Þ

where N is the doping carrier concentration, Vb is the built-in potential at zero bias, which determined from theextrapolation of the linear C�2–V plot to the V-axis asshown in Fig. 13(b).

Fig. 13(c) shows plots of the built-in potential versus thesubstrate type of the heterojunction obtained from the slopeof the linear plot of C�2–V curves. As observed Vb increasesfrom 0.11 V for n-Cd0.5Zn0.5S/p-PbS heterojunction to 0.58for n-Cd0.5Zn0.5S/p-Cu1.2In0.8S2 heterojunction. This behaviorcan be explained by whether the interface state charges thatcontribute to the diode capacitance or the charge at theinterface states that can follow an ac signal [44,45].

3.7. Substrate dependence of photovoltaic characteristics.

The photovoltaic properties of n-Cd0.5Zn0.5S on differentp-type substrates of PbS, CuS, Cu0.8In1.2S2, Cu0.9In1.1S2,CuInS2, Cu1.1In0.9S2 and Cu1.2In0.8S2 heterojunctions weredetermined by measuring the J–V characteristics using aload resistance under illumination of 100 mW/cm2. So, ithas been suggested that the light is absorbed at p-types ofPbS, CuS, Cu0.8In1.2S2, Cu0.9In1.1S2, CuInS2, Cu1.1In0.9S2 andCu1.2In0.8S2 for each heterojunction. The generation ofphotoelectrons is followed by electron transfer fromp-type semiconductor into n-Cd0.5Zn0.5S through the poten-tial barrier at the interface. This is a result of a difference inelectron affinities between the two semiconductors [46]. Itis note worthy that the photocurrent in the reverse directionis strongly enhanced by photo-illumination. This behavioryields useful information on the electron–hole pairs, whichwere effectively generated in the junction by incidentphotons. Under the influence of the electric field at thejunction, electrons were accelerated towards the n-Cd0.5Zn0.5S, while the generated holes were swept towardsthe other semiconductor along the potential barrier at theinterface [47,48]. It can be seen from this figure that thecurrent value at a given voltage the heterojunctions underillumination is higher than that in the dark. This indicatesthat the light generates carrier-contributing photocurrentdue to the production of electron–hole pairs as a result ofthe light absorption [47,48].

To estimate the photovoltaic parameters of the hetero-junctions, the illuminated devices were connected to avariable (from zero to infinity) load resistance. The currentthrough the load resistor and the voltage across the junctionwere measured and plotted as shown in Fig. 14. It can beseen from this figure that the heterojunctions show

photovoltaic characteristics with Jsc (short-circuit currentdensity) of 0.24 mA/cm2 for n-Cd0.5Zn0.5S/p-CuS increases to0.308 mA/cm2 for n-Cd0.5Zn0.5S/p-Cu1.2In0.8S2 heterojunction.The Voc (open-circuit voltage) of 0.49 V for most of thestudied heterojunctions which give evidence for the littleeffect of the substrate on the value of open circuit voltage.

4. Conclusions

The dark current density–voltage (J–V) and thecapacitance–voltage characteristics for n-Cd0.5Zn0.5S on dif-ferent p-type of PbS, CuS, Cu0.8In1.2S2, Cu0.9In1.1S2, CuInS2,Cu1.1In0.9S2 and Cu1.2In0.8S2 heterojunctions prepared by dipcoating technique illustrated the effect of the substrate on theheterojunction parameters. High rectification characteristicswere obtained for n-Cd0.5Zn0.5S/p-Cu1.2In0.8S2 heterojunctionas compared to the other studied heterojunctions. Twodifferent conduction mechanisms were identified for theinvestigated diode depending on the forward bias range.Under reverse bias, the predominant current is influenced bySchottky mechanism. The built-in potential was estimatedfrom the capacitance–voltage measurements at high fre-quency (1 MHz). Under illumination of �100 mW/cm2, het-erojunctions of n-Cd0.5Zn0.5S on different p-type of PbS, CuS,Cu0.8In1.2S2, Cu0.9In1.1S2, CuInS2, Cu1.1In0.9S2 and Cu1.2In0.8S2

heterojunctions showed photovoltaic characteristics.

Acknowledgments

This work was financially supported by the Science andTechnology Development Fund (STDF), Egypt, Grant no. ID1349.

References

[1] S.D. Chavhan, S.V. Bagul, R.R. Ahire, N.G. Deshpande, A.A. Sagade,Y.G. Gudage, Ramphal Sharma, Journal of Alloys and Compounds436 (2007) 400–407.

[2] A.A. Bassam, A.W. Brinkman, G.J. Russell, J. Woods, Journal ofCrystal Growth 86 (1988) 667–671.

[3] A.M. Salem, Applied Physics A 74 (2002) 205–211.[4] A. Jager-Waldau, Solar Energy Materials and Solar Cells 95 (2011)

1509–1517.

M. Abdel Rafea et al. / Materials Science in Semiconductor Processing 16 (2013) 89–9898

[5] V.B. Sanap, B.H. Pawar, Journal of Optoelectronic and BiomedicalMaterials 3 (2011) 39–43.

[6] H. Liu, Y. Zhu, Materials Letters 62 (2008) 255–257.[7] B. Fisher, J. Biddle, Solar Energy Materials and Solar Cells 95 (2011)

1741–1755.[8] B. Kumar, P. Vasekar, S.A. Pethe, N.G. Dhere, G.T. Koishiyev, Thin

Solid Films 517 (2009) 2295–2299.[9] W. Xia, J.A. Welt, H. Lin, H.N. Wu, M.H. Ho, C.W. Tang, Solar Energy

Materials and Solar Cells 94 (2010) 2113–2118.[10] H. Hoche, J. Schmidt, S. Groß, T. Troßmann, C. Berger, Surface and

Coatings Technology 205 (2011) S145–S150.[11] S. Canovic, B. Ljungberg, M. Halvarsson, Micron 42 (2011) 808–818.[12] Z. Shi, Y. Wang, C. Du, N. Huang, L. Wang, C. Ning, Applied Surface

Science 258 (2011) 1328–1335.[13] H. He, J. Huang, L. Cao, J. Wu, Sensors & Actuators, B: Chemical 132

(2008) 5–8.[14] M. Alishahi, S.M. Monirvaghefi, A. Saatchi, S.M. Hosseini, Applied

Surface Science 258 (2012) 2439–2446.[15] M. Ebadi, W.J. Basirun, Y. Alias, M.R. Mahmoudian, S.Y. Leng,

Materials Characters 66 (2012) 46–55.[16] A. Li, C. Hui, Y. Li, Y. Wang, Journal of Alloys and Compounds 509

(2011) 203–207.[17] R. Po, C. Carbonera, A. Bernardi, F. Tinti, N. Camaioni, Solar Energy

Materials and Solar Cells 100 (2012) 97–114.[18] H.H. Gurel, O. Akinci, H. Unlu, Thin Solid Films 516 (2008)

7098–7104.[19] Y. Zhang, X. Kong, Y. Qu, P. Jing, Q. Zeng, Y. Sun, A.Y. Wang, J. Zhao,

H. Zhang, Journal of Luminescence 129 (2009) 1410–1414.[20] S. Gimenez, I. Concina, A. Vomiero, I. Mora-Sero, Journal of Physical

Chemistry Letters 2 (2011) 454–460.[21] G. Jia, N. Wang, L. Gong, X. Fei, Chalcogenide Letters 7 (2010)

349–355.[22] K.K. Li, D.H. Heartling, Integrated Ferromagnetic 3 (1993) 81–87.[23] J.M. Dona, J. Herrero, Thin Solid Films 268 (1995) 5–12.[24] Z.Y. Xu, Y.C. Zhang, Materials Chemistry and Physics 112 (2008)

333–336.[25] M.A. Yildirim, A. Ates, A. Astam, Physica E 41 (2009) 1365–1372.[26] C.C. Uhuegbu,Canadian, J. Scie, Industrial Research 2 (2011)

230–241.

[27] A. Ashery, A.A.M. Farag, R. Mahani, Microelectronic Engineering 87(2010) 2218–2224.

[28] S. Darwish, Physica B 349 (2004) 296–303.[29] R.A. Joshi, A. Ghosh, V.S. Taur, S.U. Shaikh, F.Y. Siddiqui,

R.B. Birajadar, A.V. Ghule, R. Sharma, Materials Chemistry andPhysics 127 (2011) 191–196.

[30] H.O. Mohammad, R. Jayaram, Vacuum 42 (2006) 117–120.[31] M.C. Arenas, N. Mendoza, H. Cortina, M.E. Nicho, H. Hu, Solar

Energy Materials and Solar Cells 94 (2010) 29–33.[32] A.A.M. Farag, I.S. Yahia, T. Wojtowicz, G. Karczewski, Journal of

Physics D: Applied Physics 43 (2010) 215102–215108.[33] A.A.M. Farag, I.S. Yahia, M. Fadel, J. Inter., Hydrogen Energy 34

(2009) 4906–4913.[34] C.K. Suman, J. Yang, C. Lee, Materials Science and Engineering B 166

(2010) 147–151.[35] A. Tataroglu, I. Yucedag, S. Altindal, Microelectronic Engineering 85

(2008) 1518–1523.[36] S.K. Cheung, N.W. Cheung, Applied Physics Letters 49 (1986) 85–87.[37] S. Karatasa, A. Turut, Physica B 381 (2006) 199–203.[38] R.K. Gupta, F. Yakuphanoglu, Solar Energy 86 (2012) 1539–1545.[39] H.B. Ouerghemmi, F. Kouki, P. Lang, H. Ben Ouada, H. Bouchriha,

Synthetic Metals 159 (2009) 551–555.[40] R. Sahingoz, C. Soykan, F. Yakuphanoglu, M. Voigt, H. Cetin, Optical

Materials 28 (2006) 962–965.[41] S. Aydogan, M. Saglam, A. Turut, Polymer 46 (2005) 6148–6153.[42] A.A.M. Farag, A. Ashery, M. Yahia Abed, M.A. Shenashen,

T.A. Hindia, International Journal of Modern Physics B 21 (2007)5099–5111.

[43] Y.S. Ocak, M. Kulakci, T. Kılıc-oglu, R. Turan, K. Akkılıc, SyntheticMetals 159 (2009) 1603–1607.

[44] A. Tataroglu, S. Altindal, Microelectronic Engineering 83 (2006)582–588.

[45] A. Tataroglu, S. Altindal, Microelectronic Engineering 85 (2008)2256–2260.

[46] H.S. Soliman, A.A.M. Farag, N.M. Khosifan, M.M. El-Nahass, ThinSolid Films 516 (2008) 8678–8683.

[47] M.M. El-Nahass, K.F. Abd-El-Rahman, A.A.M. Farag, A.A.A. Darwish,Organic Electronics 6 (2005) 129–136.

[48] R.A. Messenger, G.G. Ventre, Photovoltaic Systems Engineering, 3rdEd. CRC Press, 2010.