Effect of plastic deformation on the optical and electrical properties in Cd0.96Zn0.04Te single crystals

Author’s Accepted Manuscript

Effect of plastic deformation on the optical and eletrical properties in Cd _0.96 Zn _0.04 Te single crystals

F. Lmai, R. Moubah, A.El. Amiri, A. Boudali, E.K. Hlil, H. Lassri

PII: S0022-3697(16)30272-4

DOI: http://dx.doi.org/10.1016/j.jpcs.2016.07.009 Reference: PCS7811

To appear in: Journal of Physical and Chemistry of Solids Received date: 10 April 2016

Revised date: 10 June 2016 Accepted date: 6 July 2016

Cite this article as: F. Lmai, R. Moubah, A.El. Amiri, A. Boudali, E.K. Hlil and H. Lassri, Effect of plastic deformation on the optical and eletrical properties in Cd _0.96 Zn _0.04 Te single crystals, Journal of Physical and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2016.07.009

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1

in ptical and eletrical properties ation on the o

lastic deform Effect of p

crystals single

0.04

Te

0.96

Zn Cd

F. Lmai

^a,b,

*, R. Moubah

^c,

*, A. El. Amiri

^c

, A. Boudali

^d

, E. K. Hlil

^e

, H. Lassri

^c

a

LPTA, Faculté des Sciences Aïn Chock, Université Hassan II de Casablanca, Morocco

b

Institut d’Electronique du Solide et des Systèmes, 23 rue du loess, BP 20 CR, F-67073, Strasbourg Cedex 2, France

c

LPMMAT, Faculté des Sciences Aïn Chock, Université Hassan II de Casablanca, Morocco

d

Laboratory of Physico-chemical Studies, University of Saida, Saida, Algeria

e

Institut Néel, CNRS et Université Joseph Fourier, BP 166, F-38042 Grenoble cedex 9, France

Abstract

Using UV-visible, photoluminescence, electrical measurements and ab-initio calculations, we study the effect of introduced dislocations on the optical and electrical properties in Cd

_0.96

Zn

_0.04

Te crystals. To generate dislocations, a plastic deformation on the Cd(111) and Te

( 1 1 1 faces was induced. It is shown that the plastic deformation results in: i) a decrease in ) Zn concentration in the deformed regions, which is higher on the Cd face, ii) decrease in the

band gap energy, iii) an increase of acceptor concentration, and iv) the leakage current is higher on the Te face. Calculation of barrier height has led to identify the dominant defect, which is the complex Cd vacancies, acceptor center [V

_Cd

, A

_Cd

] on the Cd face and V

_Te

on the Te side, respectively. Electronic structure calculations based on full potential linearized augmented plane waves (FPLAPW) method were performed as well and have shown that the optical band gap energy decrease upon deformation can be understood by the decrease in Zn content in the deformed regions.

Keywords: Impurity and defect levels; II-VI Semiconductors; Optical properties;

Photoluminescence; Structure electronic calculation

*Corresponding authors: reda.moubah@hotmail.fr; lmai.fatima@gmail.com

1. Introduction CdTe is a well-known semiconductor and

has been extensively studied in recent years due to its high potential applications

in x- and -ray detectors [1,2]. Generally, the use of CdTe in spectroscopic detectors requires a high crystalline quality of the material [3,4]. Doping CdTe crystals with Zn atoms appears to be a good route for improving the crystallinity [5]. Furthermore, it permits to increase the electrical resistivity [6,7].

On the other hand, it is known that the structural defects can affect significantly the physical properties of materials [8]. In case of CdTe, most of these defects are situated in the dislocation network, which can be due to growth process, e.g., local nonstoichiometry and dislocation movements. Other defects may also appear close to the walls of the ampoule at the stage of the solidification process of crystals. In this work, we investigate the influence of an induced plastic deformation on the optical and electrical properties of Cd_0.96Zn_0.04Te crystals using photoluminescence, UV-visible, electrical measurements and ab-initio calculations. The mechanical deformation was induced by means of Vickers’s microhardness method. The effect of a bad crystalline quality (deformed crystals) on the properties of devices is reported. In particular, we focuse on the Cd(111) and Te

( 1 1 1 )

faces because there is a polarity effect in the <111> direction. Furthermore, electronic structure calculations based on full potential linearized augmented plane waves method were performed as well and computed band gap energies were compared to our experimental data.

2. Experiment and electronic structure calculations

Cd0.96Zn0.04Te single crystals were prepared by horizontal Bridgman method ingots. In order to get different faces (Cd and Te), the as-grown crystals were mechanically polished using diamond paste with a 0.25 µm grain size, then they were mechano-chemically polished using bromine methanol solution [9].The structural and

chemical characterizations of the resulting crystals were previously reported using x-ray diffraction (XRD) and electron probe microanalysis (EPMA) [10,11]. The XRD data attest to the high structural quality of the Cd_0.96Zn_0.04Te crystals, with no mosaic structure [10]. The EPMA measurements show that all samples are uniform, and chemically homogeneous, with the expected stoichiometry in a good agreement with the XRD data [11]. Surface imaging was carried out using cathodoluminescence (CL) imaging spectroscopy. Local plastic deformation has been operated by means of Vickers’s microhardness method. This technique consists in applying a pyramidal diamond indenter into surface of material under controlled magnitude and rate of loading [Figure 1(a)]. The force is usually obtained with different weights. In this work, we have applied weights of 10, 25 and 50 g during 30 seconds at room temperature and normal atmosphere, on the two sides Cd and Te of our p-

type Cd_0.96Zn_0.04Te crystals ( n = 1.1x 10¹³ cm^-3). Figure 1(b) shows an image of an indented crystal using a weight of 50 g. We can clearly see well developed rosettes with a dark zone located at the center induced by indentation with a diameter ranging from 3 to 4 times the diameter of the inner indentation imprint. Six double arms appear and each one consists of short and long arms. Face metallization was performed by an Edward's 306 evaporator to improve Schottky contacts, and by electroless H₂AuCl₃ to obtain ohmic contact [12]. Electrical characterizations were carried out using a Keithley 617 digital electrometer for I(V) measurements, and a Keithley 590 meter for C(V) measurements. The frequency used for C(V) measurements was 1 MHz. UV-visible measurements were performed using a U-3000 Hitachi spectrophotometer [13,14]. Photoluminescence experiments [15] were carried out at 4 K using a (FTIR) BOMEM DA8 spectrometer used in emission mode.

Excitation light was provided by a 488 nm wavelength line of an Ar+ laser. The exit density power and beam spot diameter were 10 W.cm^-2 and 200 µm, respectively.

Electronic structure calculations of Cd

_0.96

Zn

_0.04

Te and CdTe were performed using FPLAPW method within the density functional theory framework as implemented in the WIEN2K package [16,17]. For exchange-correlation functional parameterization, we have used Tan-

Blaha Modified Beck and Johson (TB-mbj) approximations [18]. The radius of muffin-tin atomic spheres (RMT) was set to 2.5 u.a for all atoms. Inside the muffin-tin spheres the maximum value of quantum angular momentum l

max

=10 was taken for the wave function expansion. The maximum value of RMT×K

_max

was 7. The self-consistent criterion of convergence total energy was set to 10

^-4

Ry. The maximum number of K-points in the whole Briliouin zone (BZ) was 250 points. Pure CdTe crystallizes in zinc blend structure with lattice parameter a = 6.48 Å [19]. To simulate the Zn insertion in the CdTe matrix, we have considered supercells 3×2×2 and 3×2×1. The former contains 48 atoms in which one Cd atom is substituted by Zn one, thus the supercell is Cd

23

ZnTe

24

the concentration of Zn is therefore 4.16 %.

3. Results and discussion The optical absorption spectra as a function of the photon energy (hυ) for undeformed and deformed Cd_0.96Zn_0.04Te crystals were displayed in figure 2. The absorption edge for the undeformed crystal is located at around 1.54 eV, which corresponds to the band gap energy (E_g). After indentation and for both the Te and Cd faces, the absorption edges were shifted to lower energies highlighting a reduction in band gap energy, which can be attributed to the defects induced by indentation. These defects are excpetecd to create additional band tail states within the band gap which leads to a shrinkage in band gap. It should be noted that the band gap decrease is more pronounced in the case of Cd face than the Te one. In order to have more information on the change in band gap for the Cd and Te faces, we use the following formula which links the band gap energy of Cd_{1_x}Zn_xTe and the Zn concentration (x) at room temperature [20]:

E_g (300 K) = 1.5045 + 0.631x + 0.128x² (1) Where 1.5045 eV is the band gap energy value of pure CdTe.

Using equation (1), we found that the Zn concentrations for the Cd and Te faces are 0.019 and 0.029, respectively. As a consequence, the larger decrease in E_g upon indentation for the Cd face, can be understood by its smaller Zn concentration in the deformed region with respect to the Te side.

Figure 3 displays the photoluminescence spectra recorded upon deformation for the Cd and Te sides measured at 4 K. As can be observed, the spectra present different peaks. These peaks are characteristic of Cd_0.96Zn_0.04Te compound and can be attributed to:

X: peak due to recombination of free excitons

D°X: peak due to recombination of excitons bound to neutral donors

A°X: peak due to recombination of excitons bound to neutral acceptors

A°X- LO: phonon replicas of A°X

DA: donor-acceptor band

3

We note that the principle peak located at around 1.61 eV is shifted to lower energies for the Cd face. The band gap energy values deduced from photoluminescence measurements at 4 K were found to be 1.609 and 1.612 eV for deformed Cd and Te faces, respectively. These results are similar to those deduced from the UV-visible measurements at room temperature showing that the bang gap reduction is more pronounced in case of the Cd side with respect the Te face.

Figure 4 displays the I versus V curves for undeformed (0 g) In/Cd

_0.96

Zn

_0.04

Te/Au heterostructure for both Cd and Te faces. As can be observed for both faces, the I (V) curves are highly asymmetric for negative and positive biases. We note that I increases significantly at high positive bias. This diode like behavior is typical for semiconductors with Schottky contacts [21]. At low voltage, electrons have not enough energy to overcome the barrier height of the Schottky barrier, as a consequence the current is low. By increasing the bias voltage, electrons will have more energy to jump the Schottky barrier, and thus the current increases significantly. The asymmetry between negative and positive biases in the I(V) curve is due to the different electrodes used for electrical contacts (In and Au) which have different barrier heights (different work functions), and thus the asymmetry is induced. One can notice that the leakage current is higher on the Te face compared to the Cd side which can be explained by its higher acceptor concentrations with respect Cd side. Figure 5 shows the change of 1/C

²

as a function of V for undeformed (0 g) In/Cd

0.96

Zn

0.04

Te/Au sample for the Cd and Te faces recorded at room temperature. As expected, 1/C

²

increases linearly with increasing V. The relationship linking 1/C

²

and V can be expressed as:

(2) Where A is the effective diode area, ε is the dielectric constant, N

a

and N

_d

are the acceptor and

donor concentrations, respectively. The acceptor concentration and barrier height 

B

have been determined for undeformed and deformed crystals and are displayed in table 2. For the undeformed Cd face (0 g), the barrier height value was found to be 0.62 eV, which corresponds to a relative pinning energy of 0.62 eV. According to Molva et al. [22], this pinning energy is mainly associated with the complex defect (V

Cd

A

Cd

) (V

Cd

=Cd vacancy), (A

Cd

=acceptor center). We suggest that this type of defect is dominant in our samples, because V

_Cd

and A

_Cd

defects contribute together to the A°X-LO recombination, as the photoluminescence measurements have shown. After indentation, the pinning energy increases to around 0.7 V, which can be attributed to Te vacancies [23]. The same approach

was applied for the Te face. Before indentation, the majority of defects are due to Te vacancies, which becomes the complex (V

_Cd

, A

_Cd

) after indentation. An estimate of dislocation densities for different applied weights (different pressures) 0, 10, 25 and 50 g gives rise values of 110

⁴

, 14.510

⁴

, 4810

⁴

and 14710

⁴

cm

^-2

,respectively. The disclocation density increases with increasing the appplied weight. We note that these densities values are lower than those found in pure CdTe [24], which can be understood by the insertion of Zinc element within CdTe matix, which improves the cristalline structure (less dislocations) [25].

Now, we will describe the electronic structure calculations to see how the Zn concentration

does influence the band gap energy. In figure 6, we have plotted the total density of states

(TDOS) for Cd

23

ZnTe

24

(Cd

0.96

Zn

0.04

Te). Analysis of these curves shows that the core levels

are mainly formed by d bands of Cd and Zn atoms. The valence states are originated from p

bands of Cd and Te atoms while the s bands of Cd and Zn atoms contribute largely to band

conduction. Taken as reference Fermi level position, the band gap induces a zero DOS which

is a signature of a semi conducting-like state. The deduced gap energies calculated at 0 K

were found to be 1.63 and 1.75 eV, for pure CdTe and Cd

_0.96

Zn

_0.04

Te, respectively. This is in

accordance with experimental data showing that the reduction of Zn concentration leads to the

decrease in band gap energy.

4

4. Conclusion To summarize, we have investigated the electrical and optical properties of deformed and undeformed p-type

Cd_0.96Zn_0.04Te crystals on both Cd (111) and Te

( 1 1 1 )

faces. It is shown that the mechanical deformation causes a shrinkage in band-gap energy, which is explained by the reduction of the Zn concentration on both faces. In addition, the plastic deformation creates acceptor centers, which was deduced from the increase of Cd vacancies concentration, and an increase of leakage current. This study also allowed us to specify the type of majority of defects which are: (V_Cd, A_Cd) on the Cd face and V_Te on the Te face, before indentation. After indentation these defects are inverted, and it seems that Zinc stabilizes dislocations. Electronic structure calculations were performed and have supported that the band gap reduction upon indentation can be attributed to the decrease of Zn content in the deformed regions.

Acknowledgement:

The authors would like to thank R. Triboulet (LPSC, CNRS Meudon, France) for providing the crystals used in this study and A. Lusson for the photoluminescence measurements.

Side Eg (eV) Eg

(eV) x

Cd 1.530 0.013 0.019

Te 1.535 0.010 0.029

Table 1 Parameters determined from UV-visible measurements at room temperature: Eg: bandgap energy; Eg:

value of the Eg shift after indentation; x: Zn concentration.

0 g 10 g 25 g 50 g Cd

side

N

_A

x10

¹³

(cm

^-3

) 1.10 1.62 2.13 2.61 Φ

b

(eV) 0.62 0.72 0.71 0.70

Te side

N

_A

x10

¹³

(cm

^-3

) 1.13 1.64 2.10 2.70 Φ

b

(eV) 0.71 0.61 0.60 0.62

Table 2 Electrical parameters of the p-CdZnTe on Cd and Te sides for undeformed (0 g) and deformed crystals: N

_A

: acceptor concentration; 

_B

: barrier height. The deformations were induced by means of Vickers’s microhardness method by applying a pyramidal diamond indenter using different weights: 10, 25 and 50 g.

(b)

(a) 136 Diamond pyramid

indentor

5

Fig. 1. (a) Schematic illustration of the Vickers’s microhardness principle used for indentation. (b) Surface cathodoluminescence image of indented zone of a Cd

0.96

Zn

0.04

Te crystal. L.A: Long arm, S.A: Short arm.

Fig.

2.

Abso rptio n spect

ra of non- inden ted

and indented Te and Cd sides for Cd_0.96Zn_0.04Te crystals measured at room temperature.

1.46 1.48 1.50 1.52 1.54 1.56 1.58 1.60 0.0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Abso rp tio n (a. u.)

h  (eV)

Non-indented

Indented Te side

Indented Cd side

3. Fig.

Low-

tem pera ture

(4

K) photoluminescence spectra of indented Cd

0.96

Zn

0.04

Te, for both Cd and Te faces Fig. 4. I(V) curves of the In- Cd

0.96

Zn

0.04

Te -Au structure for both Cd and Te faces.

1.52 1.54 1.56 1.58 1.60 1.62

0 1 2 3 4 5 6 7 8 9

D°X A°X

A°X-LO

PL intensity (a.u.)

h (eV) Cd side

Te side

DA

-1.5 -1.0 -0.5 0.0 0.5 1.0

-0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30

Cd side Te side

I ( m A )

V ( V )

7 Fig. 5.

(1/C²)(V) curves of the In- Cd_0.96Zn₀

.04Te -Au structure for both

Cd and Te sides.

6. Fig.

Total DOS in Cd

0.96

Zn

0.04

Te compound from FPLAPW calculations.

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0 1 2 3 4 5

0.8 1.2 1.6 2.0 2.4

Cd face Te face

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