Morphology, electrical, and optical properties of heavily doped ZnTe:Cu thin filmsFikry El Akkad and Yaser Abdulraheem

Citation: Journal of Applied Physics 114, 183501 (2013); doi: 10.1063/1.4829453 View online: http://dx.doi.org/10.1063/1.4829453 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/114/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The effects of high temperature processing on the structural and optical properties of oxygenated CdS windowlayers in CdTe solar cells J. Appl. Phys. 116, 044506 (2014); 10.1063/1.4891235 Preliminary study of CdTe and CdTe:Cu thin films nanostructures deposited by using DC magnetron sputtering AIP Conf. Proc. 1555, 48 (2013); 10.1063/1.4820991 Transitions of bandgap and built-in stress for sputtered HfZnO thin films after thermal treatments J. Appl. Phys. 114, 084503 (2013); 10.1063/1.4819232 High-temperature stability of postgrowth-annealed Al-doped MgxZn1-xO films without the phase separationeffect J. Vac. Sci. Technol. B 30, 061201 (2012); 10.1116/1.4754813 Highly conducting zinc oxide thin films achieved without postgrowth annealing Appl. Phys. Lett. 97, 241903 (2010); 10.1063/1.3525575

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Morphology, electrical, and optical properties of heavily doped ZnTe:Cu thinfilms

Fikry El Akkad1,a) and Yaser Abdulraheem2

1Physics Department, College of Science, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait2Department of Electrical Engineering, College of Engineering and Petroleum, Kuwait University,Safat 13060 Kuwait

(Received 29 July 2013; accepted 22 October 2013; published online 8 November 2013)

We report on a study of the physical properties of ZnTe:Cu films with Cu content up to �12 at. %

prepared using rf magnetron sputtering. The composition and lateral homogeneities are studied using

X-ray photoelectron spectroscopy (XPS). Atomic force microscopy measurements on films deposited

at different substrate temperatures (up to 325 �C) yielded activation energy of 12 kJ/mole for the

grains growth. The results of XPS and electrical and optical measurements provide evidence for the

formation of the ternary zinc copper telluride alloy in films containing Cu concentration above �4 at.

%. The XPS results suggest that copper is incorporated in the alloy with oxidation state Cu1þ so that

the alloy formula can be written Zn1�yCuy Te with y¼ 2�x, where x is a parameter measuring the

stoichiometry in the Cu site. The formation of this alloy causes appreciable shift in the binding

energies of the XPS peaks besides an IR shift in the energy band gap. Detailed analysis of the optical

absorption data revealed the presence of two additional transitions, besides the band gap one,

originating from the C8 and C7 (spin-orbit) valence bands to a donor level at �0.34 eV below the C6

conduction band. This interpretation yields a value for the valence band splitting energy D ffi 0.87 eV

independent of copper concentration. On the other hand, the mechanism of formation of the alloy is

tentatively explained in terms of a point defect reaction in which substitutional Cu defect CuZn is also

created. Assuming that substitutional Cu is the dominant acceptor in the Zn rich alloy as in ZnTe,

its formation energy was determined to be 1.7 eV close to the theoretical value (1.41 eV) in

ZnTe. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4829453]

I. INTRODUCTION

The II-VI compound ZnTe is considered as a potential

candidate for applications in the field of optoelectronic devi-

ces particularly green light emitting diodes (LED’s) and solar

cells.1–4 Its incorporation as part of hybrid systems such as,

for example, HgZnTe,5 MgZnTe,6 and CdZnTe7 widens its

range of applications to include infrared detectors, blue

LED’s, and tandem solar cells, respectively. Moreover, the

feasibility of photovoltaic devices using ZnTe homojunctions

has recently been demonstrated.8 The conductivity of the non-

intentionally doped ZnTe is p-type due to self doping by intrinsic

defects (Zn vacancies).9 The conductivity can further be

enhanced through doping with substitutional acceptors of group I

(Cu, Au, Li) or group V (P, N) elements.10 Previous investiga-

tions showed that Cu is a suitable dopant for several applications

because it produces a relatively shallow acceptor defect

(EA¼ 0.12–0.15 eV) that can be introduced either during growth

or by post- preparation diffusion.11–13 Among the important pos-

sible applications of Cu-doped ZnTe is its use as ohmic contact

to CdTe in the CdS/CdTe solar cell, which is a potential candi-

date for wide scale photovoltaic applications.14,15 All these appli-

cations have stimulated a large amount of work on ZnTe:Cu thin

films prepared by various techniques including electro-deposi-

tion,16 metalorgnic vapor phase epitaxy,17 molecular beem epi-

taxy,18 vacuum evaporation,19 and rf sputtering.15,20–22

Despite the numerous publications on the physical prop-

erties of rf sputtered ZnTe:Cu thin films, little is known

about the electrical and optical properties of films containing

Cu concentration beyond �5 at. % (Ref. 13) and even less is

known about the morphology of this type of films. Although

at such high Cu concentration, the possibility of formation of

the ternary ZnCuTe alloy was recognized by previous

authors13–15 yet the signature of the alloy on the electrical

and optical properties has been given relatively little atten-

tion. The identification of the alloy is made difficult by the

close similarity between the lattice constants of ZnTe and

Cu2Te in the cubic phase, which makes difficult the identifi-

cation of the alloy using X-ray diffraction measurements.23

Further difficulty comes about from the scarce information

on the ZnCuTe alloy particularly in the form of thin films.

To our knowledge, the only report on the preparation and

properties of ZnCuTe thin films is the work of Pistone

et al.23 on films prepared by electrodeposition. Some of the

electrical and optical properties of these films have been

reported.

In the present work, we use Atomic Force Microscopy

(AFM), X-ray photoelectron spectroscopy (XPS), and elec-

trical and optical measurements for the study of some physi-

cal properties of ZnTe:Cu films containing Cu concentration

up to �12 at. %. The formation of the alloy has been con-

firmed through a close investigation of the XPS core levels

spectra and detailed analysis of the optical absorption spec-

tra. The results of electrical measurements are analyzed ona)Author to whom correspondence should be addressed. Email:

f.elakkad@ku.edu.kw.

0021-8979/2013/114(18)/183501/10/$30.00 VC 2013 AIP Publishing LLC114, 183501-1

JOURNAL OF APPLIED PHYSICS 114, 183501 (2013)

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the basis of a proposed point defect model, which leads to

the determination of the formation energy of the substitu-

tional Cu acceptor (CuZn). Also the activation energy of the

grain growth has been estimated from AFM results on films

deposited in the range 60–325 �C.

II. EXPERIMENTAL

ZnTe thin films were prepared in the chamber of

Edwards Auto 360 rf magnetron sputtering unit. The cham-

ber was evacuated to a pressure of about 10�5 Torr (meas-

ured by a Penning gauge) using an oil diffusion pump and a

liquid nitrogen trap. A high purity Argon gas (99.999% pu-

rity) is then admitted with a flow rate adjusted to maintain a

pressure of 7.5 � 10�3 Torr using a pneumatic valve. The

substrates were soda lime glass of dimension 2 cm � 2 cm

placed 5.0 cm above the center of the target. The substrate

was cleaned by soaking in a solution of 5% detergent (Tide)

in de-ionized water which was placed in an ultrasonic

cleaner for 30 min. It was then sequentially rinsed in two

beakers of clean de-ionized water. A hot pressed ZnTe target

(delivered by E-Vac company) of diameter 6.4 cm and purity

99.999% was used. Before each deposition, the target was

pre-sputtered for 10 min while covering the substrate with a

shutter in order to remove any contamination and to elimi-

nate any preferential sputtering effects. All the films were

prepared using an rf power density of 1 W/cm2. The substrate

temperature was measured using a chromel-alumel thermo-

couple and controlled in the range 30�Ts� 325 �C using an

IR heater (quartz halogen lamp). For the preparation of

Cu-doped films, a number of Cu strips (99.999% purity)

each of dimension �2 mm � 4 mm (measured by a traveling

microscope) were uniformly distributed on the ZnTe target

along the circumference of a circle of radius equals to half

the radius (3.2 cm) of the target. The strips had a total surface

area S up to 3% of the total target area (32 cm2). The Cu con-

centration in the 2 � 2 cm2 films was found to be uniform to

613% using XPS measurements (Sec. III B). The films’

thicknesses were measured using a Tencor instrument pro-

filer type Alpha-step 200.

AFM was used to study the samples morphology. The

measurements system (type Agilent 5420 AFM) utilizes the

AC-AFM scanning mode to ensure the highest possible reso-

lution without damaging the samples. Silicon cantilevers for

non-contact mode AFM imaging were used (Nanosensors)

with a typical tip radius <10 nm. The cantilevers were

coated with aluminum on the detector side to increase mea-

surement sensitivity.

XPS measurements were carried out on model Thermo

ESCALAB 250Xi spectrometer using Monochromator with

Al Ka radiation (1486.6 eV) with X-ray spot size 380 lm.

The spectral acquisition and processing were carried out by

means of a vantage V 4.74 data system. The sample was

carefully introduced into the preparation chamber with the

sample holder. It is then degassed until good vacuum was

achieved, then it was transferred into the analysis chamber.

The analyses were carried out with the following parameters:

analysis chamber pressure 10 �9 Torr, step size 0.1 eV, dwell

time 100 ms, and pass energy of 20 eV. All binding energy

values were determined with respect to C1s line (284.6 eV)

originating from adventitious carbon. Etching was performed

using an argon ion gun with voltage of 2 kV, current of 2 lA,

and raster size of 2 mm2.

Hall coefficient and sheet resistance were measured

using a Hall system, type MMR technologies H-50. For this,

four contacts with the Van Der Pauw configuration were

formed by evaporating Au spots (3 mm diameter) under vac-

uum of 10�5 Torr on the sample surface. The leads to the

external circuit were made by soldering copper wires to the

Au contacts using indium. For the study of the conductivity

vs substrate temperature relationship, the lateral resistivity

was measured using gap electrodes by vacuum depositing

two Au films each of dimension 2 mm � 2 mm separated by

2 mm gap on the film surface. Keithley electrometers type

617 were used for voltage and current measurements.

III. RESULTS AND DISCUSSION

Undoped and Cu-doped ZnTe films were prepared using

rf magnetron sputtering at substrate temperature Ts between

60 �C and 325 �C. Preliminary investigation of the structural

characteristics using XRD measurements showed that the

films are polycrystalline with hexagonal structure below

Ts¼ 150 �C and orthorhombic structure above that tempera-

ture. In the following sections, the characterization of these

films using AFM, XPS, and electrical and optical measure-

ments will be presented.

A. Morphology

The surface morphology of the films was studied using

AFM. Fig. 1 shows typical 2D and 3D images for films de-

posited at Ts¼ 100 �C, 200 �C, and 325 �C. The topography

images show that the films surfaces consist of grains of size

increasing from an average value below 5 nm for Ts¼ 60 �Cto about 100 nm for Ts¼ 325 �C.

Measurements of surface roughness RRMS showed that it

increases with the substrate temperature Ts and decreases

with the film thickness. The results are summarized in Table

I. Fig. 2 shows an Arrhenius plot for the reduced roughness

(RRMS/d) from which an activation energy ECG¼ 12 kJ/mole

can be estimated (solid line) for the columnar growth of the

grains above �150 �C. This value of ECG is of the same

order of magnitude as the values observed in other II-VI

compounds (for ZnO ECG¼ 22–24 kJ/mole).24

The decrease of RRMS with the increase of d at constant

temperature (325 �C) is displayed in the inset of Fig. 2. This

behavior disagrees with the observation of Guo et al.25 who

reported an increase in surface roughness with thickness on

ZnTe epilayers grown on GaAs substrates. One possible rea-

son for this disagreement may be the dependence of the grain

growth on the nature of the substrate as reported by

Tokumitsu et al.26 who observed that the crystalline quality

of ZnTe films is affected significantly by the substrate na-

ture. Another reason may be the possible island formation

during nucleation and growth processes of thin films. During

the initial stages of thin film growth, a few hemispherical

grains nucleate and grow giving rise to a film roughness typi-

cally of the order of half their diameter. Upon further growth,

183501-2 F. El Akkad and Y. Abdulraheem J. Appl. Phys. 114, 183501 (2013)

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a continuous film starts to form resulting in a decrease in sur-

face roughness.

B. Elemental concentrations and doping effects

The surface and in-depth (Ar-etching for 200 s) Cu con-

centration (up to 12.2%) and stoichiometry (given in terms

of the atomic ratio Zn/Te) are presented in Table II. Little

difference is observed between the surface and the in-depth

values for most of the samples. Concerning stoichiometry,

the majority of the Cu-doped samples studied were found to

exhibit zinc deficiency while undoped samples showed

excess zinc in agreement with literature results on ZnTe and

ZnCdTe films deposited using rf sputtering.14,27,28 The

excess Zn concentration may be due to preferential target

sputtering while the deficiency of Zn in Cu-doped samples

can possibly be due to re-sputtering of Zn from the growing

film by the Cu recoil neutrals from the target as suggested by

Gessert et al.14

As the Cu concentration increases, the possibility of for-

mation of ZnCuTe alloy increases. The features of the XPS

spectra will depend on the possible appearance of the alloy

and also on the chemical activity of excess Te and Zn

besides contaminant elements such as O and C particularly

at the surface (for this reason, we use the in-depth XPS read-

ings for the analysis of the results). All the studied samples

had variable concentrations of O and therefore the possibility

of formation of auxiliary oxides exists.

We first show, in Fig. 3, the Te3d5 and Zn2p3 core

level spectra for a typical undoped sample. The Te3d5

spectrum can be deconvoluted into two Gaussian peaks TA

at 572.63 eV and TB at 576.16 eV. TA is assigned by a

number of authors to Te-Zn bond in ZnTe,29,30 while TB is

believed to be due to Te-O bond in TeO2.31 On the other

hand, The Zn2p3 broad peak can be deconvoluted into two

peaks ZA at 1021.35 eV due to the Zn-Te bond in ZnTe30

and ZB at 1022.93 eV due to Zn-O bond in ZnO.31

Therefore, the XPS spectra of Te3d5 and Zn2p3 in non-

intentionally doped samples consist essentially of peaks

due to the Zn-Te and Te-O bonds in ZnTe and TeO2,

respectively.

FIG. 1. 2D (upper) and 3D (lower) AFM 10 lm � 10 lm images for ZnTe:Cu films: (a) Ts¼ 100 �C; (b) Ts¼ 200 �C; and (c) Ts¼ 325 �C.

FIG. 2. Arrhenius plot for the reduced roughness (RRMS/d). The solid line

represents an activation energy 12 kJ/mole. Inset: A plot showing the tend-

ency of RRMS to decrease with thickness for films deposited at 325 �C.

TABLE I. Thickness d and RMS roughness RRMS for ZnTe:Cu films pre-

pared at different substrate temperature Ts.

Ts ( �C) d (nm) RRMS (nm) (RRMS /d) � 102

60 510 2.8 0.55

100 700 12 1.7

150 486 11 2.3

200 560 17 3.0

300 575 51 8.9

325 700 44 6.3

325 675 46 6.8

325 370 70 19

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Cu is expected to have several roles in this type of films.

It can substitute for Zn forming the CuZn acceptor9 or com-

bine with the excess tellurium forming Cu-Te compounds or

it can be incorporated in the ternary ZnCuTe alloy. We pres-

ent, in Fig. 4, a comparison between the XPS binding energy

(BE) spectra for Cu, Zn, and Te for two samples containing

Cu concentration that differs by 14% but a concentration of

activated Cu that differs by 7600%. Our aim was to see what

effect this large difference in the concentration of activated

Cu will have on the position of the XPS peaks.

The studied samples in Fig. 4 are ZT43, which contains

12.2 at. % Cu of which 94% (i.e., 11.5 at. %) is activated and

ZT53 containing 10.7 at. % Cu of which only 1.4 � 10�2%

of Cu (i.e., 0.15 at. %) is activated (Table III). In the later

sample, two peaks appear in the Cu2p3 spectrum after

deconvolution (Fig. 4(a)) at 932.97 (CA) and 933.94 (CB).

The BE of peak CA is close to that (932.7 eV) attributed by

Carmona et al.32 to the oxidation state Cu1þ as in Cu2�xTe.

Evidence for the presence of Cu-Te phases in our samples is

also obtained from the XPS results on Te as will be shown

later. The peak CB cannot be attributed to a different oxida-

tion state of copper since all the states in metallic Cu, CuTe,

and Cu2Te were reported to have nearly equal Cu2p3 bind-

ing energy within 0.05 eV (Ref. 33) nor can it be attributed

to the existence of CuO since this compound is characterized

by high intensity satellites at higher binding energies than

the 2p3 and 2p1 peaks,34 which are not observed in our XPS

spectra. Hence, we assign this peak to Cu-Te bond in the

ZnCuTe alloy environment (evidence for the presence of

ZnCuTe phase will also be obtained from the next results).

Since 94% of the Cu in sample ZT43 is incorporated in

acceptor sites (i.e., 11.5 at. %) where Cu is in oxidation state

Cu1þ, one expects the single peak in the Cu2p3 spectrum for

this sample to be due to Cu-Te bond in ZnTe or ZnCuTe

alloy. The later possibility is more likely since the BE of the

peak (930.55 eV) is much lower than that of the Cu-Te peak

in ZnTe.35 The shift of the peak to lower BE relative to the

peak CB in sample ZT53 can readily be attributed to a

change in the alloy composition. Therefore, some of the fea-

tures of the Cu spectra are suggestive of the formation of

ZnCuTe alloy.

On the other hand, the Zn2p3 spectrum (Fig. 4(b)) con-

sists of a single Gaussian peak for each sample (at 1019.5 eV

for ZT43 and 1021.73 eV for ZT53) which we attribute to

the Zn-Te bond in the alloy environment with different com-

positions. The higher binding energy for the peak in sample

ZT53 indicates that the composition of the alloy in this sam-

ple is closer to ZnTe.35 However, the concentration of non-

activated Cu (likely to be incorporated in ZnCuTe alloy or

Cu-Te compounds) is much higher in this sample than in

sample ZT43 (10.6 at. % and 0.15 at. %, respectively). This

discrepancy suggests that the position of the BE peaks

depends on the total Cu content (which is lower for ZT53)

rather than on the distribution of Cu among different sites.

The Te3d5 core level for sample ZT53 (Fig. 4(c)) can be

deconvoluted into two main BE peaks, TA at 575.99 eV and

TB at 574.16 eV and a weaker peak TC at 572.77 eV. The

higher BE peak TA whose position varies in the range

575.78 eV–576.86 eV in the studied samples is close to the

chemical state of Te in TeO2 (Ref. 31) similar to undoped

samples but can well be attributed to Cu-Te phase on the ba-

sis of similarity between the origin of the peaks among all

copper chalcogenides.35,36 This however does not discard the

possibility of existence of TeO2 as may be shown in the

spectrum of sample ZT43 where two peaks appear at nearly

the same position which can be assigned to Cu2Te and TeO2

(peaks TA and TC, respectively). The lower BE energy peak

TB whose position falls in the range 572.95 eV–573.58 eV in

the studied samples was identified in the literature as being

due to Te-Zn bond in ZnTe29,30 but the possibility that this

bond be in ZnCuTe cannot be discarded. The peak denoted

TC in sample ZT53 is due to an unknown chemical state for

Te. Therefore, the XPS spectra of Te3d5 show the presence

of ZnTe (or ZnCuTe), Cu2Te, and TeO2 phases.

TABLE II. Summary of the optical data for samples having different Cu content and Zn/Te ratio. E1, E2, E3 are optical transitions energies, g is the Urbach

tail, Eg is the energy band gap, ED is the donor level depth, and D is the valence band splitting energy.

Cu (at. %) Zn/Te E1 E2 E3 g Eg Ed D

Sample t¼ 0 t¼ 200 s t¼ 0 t¼ 200 s eV eV eV eV eV eV eV

ZT3 0.0 0.0 1.7 1.6 1.985 2.248 2.878 0.013 2.261 0.263 0.893

ZT55 4.4 4.3 0.40 0.39 1.473 1.916 2.340 0.193 2.109 0.443 0.867

ZT52 7.1 6.4 0.86 0.92 1.356 1.664 … 0.155 1.819 0.308 …

ZT53 8.2 10.7 0.90 0.90 1.122 1.407 1.986 0.292 1.699 0.285 0.864

ZT43 10.6 12.2 0.44 0.44 1.768 2.194 2.400 0.211 1.355 0.426 0.632

FIG. 3. XPS binding energy spectra forTe3d5 and Zn2p3 core levels in

undoped ZnTe film.

183501-4 F. El Akkad and Y. Abdulraheem J. Appl. Phys. 114, 183501 (2013)

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It may be concluded from the above results that due to

the formation of ZnCuTe alloy, a shift in the XPS peaks

occurs which is pronounced for the peaks CA, ZA, TB and

less pronounced for TA as illustrated in Fig. 5. It is clear

from Fig. 5 that the positions of the peaks depend on the

overall concentration of Cu in the sample rather than on the

distribution of Cu between active and non-active sites. The

shift above about 6 at. % points to the possibility of ZnCuTe

alloy formation. This dependence on the overall Cu concen-

tration indicates that Cu has the same oxidation state inde-

pendent on whether it acts as acceptor in substitutional site

for Zn or incorporated with full co-ordination in the alloy.

Since the Cu acceptor is in oxidation state Cu1þ, so it will be

in the alloy. It follows that the formula for the alloy must be

Zn1�y CuyTe with y¼ 2�x, where x is a measure of the

deviation from stoichiometry in the Cu site (proportional to

the Cu vacancy VCu concentration). The VCu defect is

expected to be the dominant native defect in the Cu-rich side

of the alloy solidus (similar to Cu2�xTe); while in the Zn

side (as in our case), substitutional Cu is expected to be the

dominant defect. Moreover, it is inferred from the above

XPS results that all the studied samples seem to contain

appreciable amount of excess Te in a Cu-Te phase together

with the Zn1�y CuyTe phase.

XPS measurements were also used to study the lateral

homogeneity of the films. Fig. 6 shows the in-depth compo-

sition (200 s of Ar sputtering) as a function of the lateral dis-

tance for a representative sample. The average concentration

over 1.5 cm is found 44.0 61.0 at. %, 52.16 0.56 at. %, and

2.37 6 0.31 at. % for Te, Zn, and Cu, respectively. The

standard deviations in the concentrations values are of the

same order of magnitude as those obtained using conven-

tional Cu-doped targets.2,13,14

C. Electrical properties

The Hall coefficient RH and sheet resistance Rsh were

measured on a number of Cu-doped samples prepared at dif-

ferent substrate temperatures. The obtained values of the

hole concentration p (calculated using p¼ 1/q RH) and the

sheet resistance Rsh are listed in Table III. Table III displays

also the percentage of activated copper F¼ p/NCu; where

FIG. 4. XPS Core level spectra: Cu2p3

(a), Zn 2p3 (b), and Te3d5 ((c) and

(d)) for two samples ZT43 and ZT53

of Cu content in the “bulk”12.2 at. %

and 10.7 at. %, respectively.

TABLE III. Summary of electrical results for films prepared with

Ts¼ 60–325 �C and S¼ 2%–2.9%.

Ts p Rsh F¼ p/Ncu Er

Sample�C cm�3 W /sq % meV

ZT3 325 5.22 � 1015 2.31 � 105 … 121

ZT52 100 3.66 � 1016 7.84 � 106 2.9 � 10�3 115

ZT53 60 2.88 � 1017 3.06 � 105 1.4 � 10�2 161

ZT55 150 2.28 � 1018 4.37 � 103 0.27 65

ZT43 325 2.28 � 1021 2.07 � 102 94 164FIG. 5. Dependence of binding-energy peaks positions on copper concentration.

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NCu¼Q Cu/100; Cu being the Cu concentration (in at. %)

and Q¼ 1.98 � 1022 cm�3 mole�1 for ZnTe. It can be seen

that the percentage of activated copper increases by several

orders of magnitude as the substrate temperature increases

from the 60–100 �C range to 325 �C. This reflects the tend-

ency towards increasing solubility of Cu in acceptor sites

with increasing temperature. It is noticed that F reaches 94%

for the sample containing the highest free hole concentration

of 2.28 � 1021 cm�3 prepared at the highest temperature of

325 �C attained in the present study (S¼ 2.45% was used for

the preparation of this sample). It is one of the samples stud-

ied more closely in Sec. III B using XPS measurements.

The room temperature conductivity increases exponen-

tially with the substrate temperature above about 150 �C (Fig.

7). This change in r with Ts can be taken as a measure of

change in p since p /1/Rsh (Table III), which implies p / r(neglecting to the first approximation the change of carrier

mobility with p). It is to be noted that some samples in Fig. 7

had a resistivity too high to allow measurements using our

MMR technologies setup. Hence, in order to conduct the con-

ductivity measurements under the same experimental condi-

tions, we have used gap electrodes for all the samples in this

set. The straight line of the Arrhenius plot in Fig. 7 at high

temperatures yields activation energy ECu¼ 0.85 eV.

The activation energy of the electrical conductivity Er

has been determined using measurements of the dark current

at constant applied voltage as a function of temperature in

the range 77–300 K. Both heating and cooling cycles were

carried out with negligible difference in the recorded curves.

Typical Arrhenius plot for the current is shown in Fig. 8. The

obtained activation energy values determined from the

slopes of similar plots are given in Table III. It is noticed

that the first three samples in Table III have Er in the range

115–161 meV, close to the literature value for the ionization

energy for the CuZn acceptor center and the second ioniza-

tion state of the native vacancy of zinc defect

(120–140 meV).37 This suggests that the variation in carrier

mobility does not significantly affect the activation energy of

the conductivity in these samples and that the Cu acceptor

center is the dominant defect despite the possible formation

of the alloy. However, the mobility effect may be significant

for the rest of the samples particularly the degenerate one

ZT43 for which no change in the carrier concentration with

temperature is expected.

D. Optical transitions

In this section, we study the optical absorption spectra

in Cu doped samples with the aim of identifying the optical

transitions and investigating the dependence of their energies

on the Cu concentration. For this, the transmission T and

reflectivity R in the wavelength range 300 nm–3000 nm have

been measured on films with different Cu content. Typical

results are shown in Fig. 9 for an undoped sample (Tables II

and III). The absorption coefficient a was calculated using38

a¼ � 1=df ln ½T=ð1� RÞ2�g; (1)

where d is the sample thickness. The analysis of the absorp-

tion spectra is based on the equation relating the absorption

coefficient with the photon energy;38

ai h�¼A ½ h� � Ei� m; (2)

FIG. 6. In depth concentrations (200 s below the surface) of Te, Zn, and Cu

as a function of lateral distance in ZnTe:Cu film.

FIG. 7. Dependence of the room temperature conductivity r on substrate

temperature Ts plotted on a semi-logarithmic scale (Arrhenius plot).

FIG. 8. Typical Arrhenius plot for the dark current in ZnTe: film containing

12.2 at. % Cu.

183501-6 F. El Akkad and Y. Abdulraheem J. Appl. Phys. 114, 183501 (2013)

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where ai is the absorption coefficient at photon energy

h� associated with the transition of energy Ei (where i¼ 1,

2….), A is a constant which depends on the nature of the

transition, and m is a number that can take the values 1=2, 3/2,

2, or more depending on whether the transition is direct

allowed, direct forbidden, indirect allowed, or indirect for-

bidden, respectively. Our analysis of the absorption spectra

revealed the presence of three direct transitions of energies

denoted E1, E2, and E3. The analysis is based on the fact that

the measured absorption is the sum of contributions from all

optical transitions. The absorption coefficient can therefore

by written as39

a ¼ a1 þ a2 þ a3;

where a1is the absorption coefficient due to the transition of

energy E1and so forth. Since the transitions are usually well

separated in energy, each transition will dominate in a lim-

ited energy range, which can be used to determine its energy.

The first transition energy E1 is determined by plotting b1 vs

hv where b1¼ (a1 h�)2 in the photon energy range where it

dominates (in the low energy side of the absorption edge)

and then finding the intercept of the linear part of the plot

with the h� axis (Eq. (2)). Once E1 is determined, the line b1

vs h� is extrapolated to higher photon energies and the

absorption coefficient a2 associated with the second transi-

tion is calculated at a given photon energy using the equa-

tion, a2¼ a – a1¼ 1h� b1=2 � b1=2

1

h i. This procedure leads to

finding the absorption curve for the second transition from

which E2 is determined using the same procedure as for E1.

Finally, the absorption curve for the third transition is calcu-

lated by extrapolating b2 vs h� to higher photon energies and

repeating the above procedure.

Fig. 10 shows typical results for the three transitions in

the undoped sample (Tables II and III). Table II collects the

transition energies determined for the studied samples. The

transition energy E2 at low Cu concentration is close to the

energy band gap of ZnTe (2.26 eV). We therefore assign

this energy to an optical transition between the C8 valence

band (VB) and the C6 conduction band (CB). However, in

order to determine an accurate value for the energy gap Eg,

one must correct this optical band gap Ego (C8 - C6 transi-

tion) for shifts due to the Moss-Burstien (MB) effect,40

resulting from the penetration of the Fermi level into the

VB at high free carrier concentration, and the Urbach tail41

due to the perturbation in the local potential associated with

the random distribution of impurities. The equation for Eg

is then written

Eg¼Ego � vþg; (3)

where v and g are the magnitudes of MB and Urbach

shifts, respectively. The MB shift in all samples except ZT43

is inexistent since the free hole concentration is below

the effective density of states in the valence band Nv¼ 1.16

� 1019 cm� 3 (using effective mass of holes of 0.60 mo),

indicating that the Fermi level is above the VB edge (non-de-

generate case). For the degenerate sample ZT43, the Femi

level is deep inside the VB at a distance 1.06 eV from the

edge (calculated using p¼Nv F1/2 (/) where F1/2 (/) is the

Fermi integral and /¼Ev�Ef/kT; Ef being the Fermi

energy, and Ev is the energy of the VB edge).42 Concerning

the magnitude of the Urbach tail g, it has been determined

experimentally from the slope of the linear plot (log a vs hv)

in the region of photon energy just below the absorption

edge using the equation a¼ ao exp (hv/g).41 The obtained

values of g are tabulated in Table II and an example of the fit

is displayed in Fig. 11. The corrected values of the energy

gap Eg calculated using Eq. (3) are also shown in Table II

and plotted as a function of Cu concentration in Fig. 12. It is

observed that Eg decreases with the increase of Cu concen-

tration, which provides further evidence for the formation

of the ZnCuTe alloy. The decrease of Eg with Cu concentra-

tion is consistent with the expected gradual shift towards

the much lower value of energy gap for Cu2�xTe.

Domashevskaya et al.43 reported an energy gap for Cu2�xTe

of 1 eV for the stoichiometric compound (x¼ 0). The magni-

tude of the change in Fig. 12 suggests a relatively large bow-

ing parameter in this alloy system.

The transition energy E1 lies below E2 by 0.34 6 0.08.

We therefore assign E1 to a transition from the C8 VB to a

donor level at about 0.34 eV below the CB edge. The donor

level depth calculated from ED¼E2 – E1 is given in Table II

for the studied samples. It falls in the range 0.26–0.44 eV.

FIG. 9. Transmission T and reflectivity R spectra for undoped ZnTe film of

thickness 480 nm.

FIG. 10. (ah�)2 against photon energy plot showing the three optical transi-

tions identified in the undoped film of Fig. 9.

183501-7 F. El Akkad and Y. Abdulraheem J. Appl. Phys. 114, 183501 (2013)

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The much higher transition energy E3 for the third tran-

sition suggests that it is due to an electronic transition

involving the spin orbit band. A transition between the

spin-orbit VB C7 and the donor level would imply a splitting

energy of about 0.87 eV for Cu concentration up to 10.7 at.

% in good agreement with the reported value for

ZnTe (0.90–0.91 eV).44,45 The values of D calculated using

D¼E3–E1 are given in Table II. The smaller value of D for

the degenerate sample ZT43 is not surprising since the first

transition E1 originates from the Fermi level in this sample

rather than from the top of the C8 VB so that the separation

energy between the initial states is narrower. Moreover, there

is some inaccuracy in the calculation of the Fermi level

depth in this sample due to uncertainty in the effective mass

value used for the calculation of the effective density of

states Nv. At such high level of degeneracy, the effective

mass at the Fermi level must be used, which is expected to

be significantly different from the density of states effective

mass (0.6) used in the calculation of Nv. Fig. 13 shows

schematically the transitions assigned to E1, E2, and E3

besides the values of the three main energy intervals

(ED, Eg, and D) according to the present work.

E. Defect model

Our results of XPS and electrical and optical measure-

ments provide evidence that Cu is distributed between

ZnCuTe and Cu-Te phases besides acting as acceptor center

in substitutional site for zinc as commonly believed.9,37

Bearing in mind these results, one may speculate about the

possible mechanism of creating the alloy system. Because

most of the Cu doped samples contain excess Te and because

of evidences obtained from XPS measurements here and in

the literature32,35,36 that excess Te in ZnTe:Cu films is

mainly involved in Cu-Te compounds, it seems plausible to

consider a role for these compounds in the creation of the

alloy during high temperature deposition. A possible reaction

involving Cu2Te would be

2Cu2TeþZnZn¼Cu�Znþ hþZnCu2TeþCuTe:

This reaction creates the ternary alloy with the correct oxida-

tion state for copper (Cu1þ as evidenced in the present work)

and creates a copper acceptor defect, thus explaining the

increase in the electrical conductivity as the substrate tem-

perature increases (Fig. 7).

Applying the law of mass action to the above reaction,

½Cu�Zn�p½ZnCuTe �½CuTe�½Cu2Te�2

¼ K (4)

with

K ¼ Ko exp ð�DH=kTÞ;

where DH is the enthalpy of the above reaction and Ko is a

constant. DH is also the formation energy of the CuZn

defect according to the above reaction. This energy can be

determined by inserting the neutrality condition [Cu�Zn]¼ p

FIG. 11. Semilogarithmic plot of the absorption coefficient vs photon energy

in the sub band edge region for undoped film. The Urbach tail is calculated

from the slope SL of the line using SL¼ (2.3g).

FIG. 12. Energy band gap Eg as a function of photon energy for ZnTe:Cu

films.

FIG. 13. Schematic diagram showing the initial and final states assigned to

the three identified transitions E1, E2, and E3 in Cu-doped films. ED, Eg, and

D are the donor level depth, the band gap energy, and the spin-orbit splitting

energy, respectively. The subscripts v and c denote the valence and conduc-

tion bands, respectively.

183501-8 F. El Akkad and Y. Abdulraheem J. Appl. Phys. 114, 183501 (2013)

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into Eq. (3), which yields p¼Ko0 exp (�DH/2kT). Given

the arguments mentioned in Sec. III C and assuming that

the formation of the alloy does not invalidate the condition

r/ p, it follows that DH¼ 2ECu where ECu is the activa-

tion energy of the electrical conductivity in Fig. 7, which

amounts to 0.85 eV. Therefore, DH¼ 1.70 eV. It is interest-

ing to note that this energy is close to the theoretically cal-

culated value (1.41 eV) for the formation energy of the

CuZn defect in ZnTe with the release of a Zn atom to the

vapor phase.46

IV. CONCLUSION

Study of the surface morphology using AFM showed

that rf sputtered ZnTe:Cu films consist of grains of colum-

nar structure whose growth is thermally activated with

energy of 12 kJ/mole. Using XPS measurements, evidence

is obtained for the formation of alloy with formula

Zn1�yCuyTe where copper is incorporated in the oxidation

state Cu1þ (y¼ 2�x where x is a measure of the stoichiom-

etry in the Cu site). The shift of the energy band gap Eg to

lower values as the copper concentration increases provides

further evidence for the creation of the alloy. The analysis

of the optical absorption results revealed the presence of

two additional optical transitions, besides the C8 VB-C6 CB

transition (Eg), originating from the C8 and the split off C7

valence bands to a donor level at Ec - 0.34 eV. The valence

band splitting energy is determined to be 0.86 -0.89 eV,

close to the reported value for ZnTe (0.90–0.91 eV).

The overall results are used to speculate about the mecha-

nism of formation of the alloy and the substitutional Cu

acceptor CuZn. It is suggested that the creation of these spe-

cies takes place via a reaction involving Cu2Te second

phase. Based on this reaction, the formation energy of the

substitutional Cu acceptor is determined to be 1.7 eV close

to the theoretical value 1.41 eV for the creation of this

defect in ZnTe.

ACKNOWLEDGMENTS

The authors would like to thank both the Kuwait

Foundation for the Advancement of Science (KFAS) for

funding the present work and the Research Administration of

Kuwait University for continuous support (Project #

2011/1413/01). Their thanks are also due to Mr. Mathew

Joseph and Mr. Shaji Michael for their help in electrical

measurements and to Mrs. Terresia Joseph and Sudeep

Joseph for their help in XPS measurements. Thanks also to

the general facility of the Faculty of Science (Projects GS

02/08 and GS03/01) and of the Faculty of Engineering

(Projects GE0107, GE01/08, and GE02/08) for their valuable

technical support.

1T. Tanaka, K. Saito, M. Nishio, Q. Guo, and H. Ogawa, Appl. Phys.

Express 2, 122101 (2009).2T. A. Gessert, W. K. Metzger, P. Dippo, S. E. Asher, R. G. Dhere, and M.

R. Young, Thin Solid Films 517, 2370 (2009).3T. Tanaka, CLEO: Applications and Technology (California United States,

San Jose, 2012).

4K. Sato, T. Asahi, M. Hanafusa, A. Noda, A. Arakawa, M. Uchida, O.

Oda, Y. Yamada, and T. Taguchi, Phys. Status Solidi A 180, 267 (2000).5F. El Akkad and A. Farhan, J. Phys. D: Appl. Phys. 28, 1958 (1995).6F. El Akkad, M. Abdel Naby, and M. Ali Omar, J. Vac. Sci. Technol. A

13, 2797 (1995).7Y. G. Xiao, Z. Q. Li, M. Lestrade, and Z. M. S. Li, Proc. SPIE 7771,

77710K (2010).8T. Tanaka, K. M. Yu, P. R. Stone, J. W. Beeman, O. D. Dubon, L. A.

Reichertz, V. M. Kao, M. Nishio, and W. Walukiewicz, J. Appl. Phys.

108, 024502 (2010).9B. K. Meyer and W. Stadler, J. Cryst. Growth 161, 119 (1996).

10M. Nishio, K. Kai, K. Saito, T. Tanaka, and Q. Guo, Thin Solid Films 520,

743 (2011).11J. Tang, D. Mao, T. R. Ohno, V. Kaydanov, and J. U. Trefny, in

Conference Record of the Twenty-Sixth IEEE Photovoltaic SpecialistsConference (1997), p. 439.

12B. Monemar, P. O. Holtz, H. P. Gislason, and N. Magnea, J. Lumin. 34,

245 (1986).13T. A. Gessert, A. R. Mason, R. C. Reedy, R. Matson, T. J. Coutts, and P.

Sheldon, J. Electron. Mater. 24, 1443 (1995).14T. A. Gessert, A. R. Mason, P. Sheldon, A. B. Swartzlander, D. Niles, and

T. J. Coutts, J. Vac. Sci. Technol. A 14, 806 (1996).15S. H. Kim, J. H. Ahn, H. S. Kim, H. M. Lee, and D. H. Kim, Curr. Appl.

Phys. 10, S484 (2010).16T. Ishizaki, T. Ohtomo, Y. Sakamoto, and A. Fuwa, Materials

Transactions 45, 277 (2004).17B. Bozzini, M. A. Baker, P. L. Cavallotti, E. Cerri, and C. Lenardi, Thin

Solid Films 361–362, 388 (2000).18T. Tanaka, K. M. Yu, A. X. Levander, O. D. Dubon, L. A. Reichertz, N.

Lopez, M. Nishio, and W. Walukiewicz, Jpn. J. Appl. Phys., Part 1 50,

082304 (2011).19W. Wang, A. Lin, and J. D. Phillips, J. Electron. Mater. 37, 1044

(2008).20G. K. Rao, K. V. Bangera, and G. K. Shivakumar, Mater. Res. Bull. 45,

1357 (2010).21W. G. Wang, K. J. Han, K. J. Yee, C. Ni, Q. Wen, H. W. Zhang, Y.

Zhang, L. Shah, and J. Q. Xiao, Appl. Phys. Lett. 92, 102507 (2008).22F. El Akkad and M. Thomas, Phys. Status Solidi C 2, 1172 (2005).23A. Pistone, A. S. Arico, P. L. Antonucci, D. Sivestro, and V. Antonucci,

Sol. Energy Mater. Sol. Cells 53, 255 (1998).24N. Bouhssira, M. S. Aida, A. Mosbah, and J. Cellier, J. Cryst. Growth 312,

3282 (2010).25Q. Guo, Y. Sueyasu, Y. Ding, T. Tanaka, and M. Nishio, J. Cryst. Growth

311, 970 (2009).26Y. Tokumitsu, A. Kawabuchi, H. Kitayama, T. Imura, Y. Osaka, and F.

Nishiyama, Jpn. J Appl. Phys., Part 1 29, 1039 (1990).27S. Ringel, R. Sudharsanan, A. Rohatgi, M. S. Ownes, and H. P. Gillis,

J. Vac. Sci. Technol. A 8, 2012 (1990).28B. M. Basol and V. K. Kapur, Sol. Cells 30, 143 (1991).29D. Soundarajan, D. Mangalaraj, D. Nataraj, D. Dorosinski, and J. Santoyo-

Salazar, in International Conference on Superconductivity and Magnetism(ICSM2008), Journal of Physics: Conference Series Vol. 153 (2009), p.

012048.30W. Wang, G. Xia, J. Zheng, L. Feng, and R. Hao, J. Mater. Sci.: Mater.

Electron. 18, 427 (2007).31Lasurface and NIST XPS-databases.32J. Carmona-Rodriguez, R. Lozada-Morales, P. del Angel-Vicente, O.

Jimenez-Sandoval, G. Lopex-Calzada, D. Dahlberg, and S. Jimenez-

Sandoval, J. Mater. Chem. 21, 13001 (2011).33G. Teeter, J. Appl. Phys 102, 034504 (2007).34S. Poulston, P. M. Parlett, P. Stone, and M. Bowker, Surf. Interface Anal.

24, 811 (1996).35S. Zalenkiene, “Formation and study of mixed copper sulfide-copper tellu-

ride layers on the surface of polymide 6,” Ph.D. dissertation (Kaunas

University of Technology, Kaunas, 2009), pp. 13–15.36G. She, X. Zhang, W. Shi, Y. Cai, N. Wang, P. Liu, and D. Chen, Cryst.

Growth Des. 8, 1789 (2008).37N. Hammond, A. Kohn, J. L. Debrun, and H. Rodot, J. Phys. Chem. Solids

34, 1069 (1973).38J. I. Pankove, Optical Processes in Semiconductors (Dover Publications

Inc., 1975).39A. E. Rakhshani, J. Appl. Phys. 81, 7988 (1997).40E. Burstein, Phys. Rev. 93, 632 (1954); T. S. Moss, Proc. Phys. Soc.

London, Ser. B 67, 775 (1954).

183501-9 F. El Akkad and Y. Abdulraheem J. Appl. Phys. 114, 183501 (2013)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

128.123.35.41 On: Thu, 04 Sep 2014 09:30:09

41S. John, C. Soukoulis, M. H. Cohen, and E. N. Economou, Phys. Rev.

Lett. 57, 1777 (1986).42J. S. Blakmore, Semiconductor Statistics (Dover Publication, Inc., New

York, 1987).43E. P. Domashevskaya, V. V. Gorbachev, V. A. Terekhov, V. M.

Kashkarov, E. V. Panfilova, and A. V. Shchukarev, J. Electron. Spectrosc.

Relat. Phenom. 114–116, 901 (2001).

44D. Long and J. L. Schmit, Semiconductors and Semimetals, edited by R.

K. Willardson and C. Albert (Academic Press, Inc., New York, 1970),

Vol. 5, p. 195.45J. Schrier, D. O. Demchenko, L. Wang, and A. Paul Alivisatos, Nano Lett.

7, 2377 (2007).46M. A. Berding, M. van Schilfgaarde, A. T. Paxton, and A. Sher, J. Vac.

Sci. Technol. A 8(2), 1103 (1990).

183501-10 F. El Akkad and Y. Abdulraheem J. Appl. Phys. 114, 183501 (2013)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

128.123.35.41 On: Thu, 04 Sep 2014 09:30:09