Influences of the dislocation density on the electric behavior of n-CdTe

Materials Science and Engineering B 137 (2007) 49–52

Influences of the dislocation density on the electric behavior of n-CdTe

N. Brihi

, F. Lmai

, Z. Takkouk

, F. Ayad

, M. Ayoub

, M. Hage-Ali

aLaboratoire d’´etude des Mat´eriaux, Facult´e des Sciences, Universit´e de Jijel, Algeria

bInstitut d’Electronique du Solide et des Syst`emes, 23 rue du loess, BP 20 CR, F-67073 Strasbourg Cedex 2, France Received 3 March 2006; received in revised form 10 October 2006; accepted 11 October 2006

Abstract

It is well known that the CdTe material suffers from the presence of native defects, due to the introduction of dislocations by pressure owing to its mechanical brittleness. Within this framework, we present a study of the dislocations effect on the electric properties of the CdTe material, using measurements byC(V) andI(V) and IR absorption. The adopted method for the plastic deformation is the Vickers microhardness at room temperature and under various weights (10, 25, 50 and 80 g). The following results have been obtained: a reduction in the donors concentration, an increase in the leakage current as well as a reduction of the band gap and finally a drastic variation of the Schottky barrier where we could observe two distinct behaviours depending on the used side (Cd or Te) for the contact deposition.

© 2006 Elsevier B.V. All rights reserved.

Keywords: Cadmium telluride; Defects; Microindentation; Electrical measurements

1. Introduction

The study of the electric properties of the II–VI semicon- ductors was, for many years the object of many publications, mainly devoted to the study of crystals[1]and the correlation between crystalline quality and electronic properties were rela- tively more recent[2–4]. In this II–VI semiconductors family, CdTe is still the most important one, considering its increased interest in the X- and␥-ray detection[5,6], and in the medical imagery[7]. CdTe is known also to be the best substrate in the CdHgTe layers epitaxy[8]for the IR detection. We can mention also the photorefractivity[9]and solar cells[10]applications.

The use of this material requires then a good comprehension, as well as a good control of the physical phenomena like the struc- tural defects and imperfect ions position like the dislocations [11]. This work is especially devoted to the study of the disloca- tions behavior, which is often considered harmful for the correct operation of the electronic devices with good electric properties.

We have introduced punctual dislocations in the material, with different density, by the plastic deformation using the Vickers

∗Corresponding author. Tel.: +33 3 88 10 65 42; fax: +33 3 88 10 62 30.

E-mail address:lmai@iness.c-strasbourg.fr(F. Lmai).

microhardness method. In order to determine the electric proper- ties of these defects and their influence on the material behavior.

We have used few methods such asI(V) andC(V) and IR absorp- tion methods.

2. Experiment

The preparation of the samples obtained from horizontal Bridgman method ingots [12], consists on a mechanical pol- ishing with diamond of the wafers (grain of 0.25␮m), fol- lowed by a mechano-chemical etching (by brome-methanol 0.5% [12]). Dislocations are obtained by pyramidal micro Vickers hardness instrument deformation, with various weights (10, 25, 50 and 80 g) at room temperature and normal atmo- sphere, on the two sides A (Cd) and B (Te) of our n-type undoped CdTe (n= 10⁺¹⁴cm⁻³). The investigations were car- ried out using electrical I(V),C(V) and optical measurements near-visible IR absorption lights. Schottky and ohmic contacts were obtained, respectively, with gold and indium evaporation [13].

In our experiment we carried out six indentations prints uni- formly distributed on a 2 mm diameter circle hexagonal. The Schottky contact covered the indented surface. Thus, we took care to measure only the capacity of the deformed surface

0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.mseb.2006.10.003

50 N. Brihi et al. / Materials Science and Engineering B 137 (2007) 49–52

Fig. 1. Cathodoluminescent image of the indentation rosette SA, short arms;

LA, long arms.

and tried to avoid any contribution from a undeformed areas.

The morphology of the print is characterized by a geometrical rosette figure with six couple of arms: a long arm (LA) and a short one (SA), this figure is called indentation rosette (Fig. 1) [14].

On the Cd side the (LA) is a Cd dislocation where the majority carrier defect is Te vacancies, while on the Te side the (LA) is a Te (g) dislocation where the majority carrier defects is the acceptor Cd vacancies [15]. An estimate of the density of dislocations for each weight 0, 10, 25, 50 and 80 g is given, respectively, by 0.6×10⁵, 7×10⁵, 21×10⁵, 34×10⁵and 45×10⁵cm⁻². 3. Results

For the current studies Au was evaporated on the indented face. Fig. 2shows the current variation as a function of the a variation of the applied voltage on the two sides Cd and Te for load 80 g, for others loads (10, 20 and 50 g) the investigated effect of indentation on the leakage current and on the capacity is very small. So we prefer to not display the associated figures for avoiding useless obstruction.Fig. 3shows the capacity mea-

Fig. 2. CharacteristicsI(V) for the two sides (Te and Cd) of the CdTe indented by the 80 g load.

Fig. 3. Characteristics 1/C²(V) for CdTe indented by 80 g load for Te and Cd side.

Table 1

Electrical measurements before and after indentation under different loads

0 g 10 g 25 g 50 g 80 g

Side Cd Nd×10¹⁴(cm⁻³) 1.05 0.79 0.53 0.29 0.07 φb(eV) 0.72 0.91 0.90 0.91 0.5 Side Te Nd×10¹⁴(cm⁻³) 1.00 0.82 0.54 0.30 0.073

φb(eV) 0.92 0.70 0.71 0.71 0.50

surements as function of the applied voltage which allows us to draw the curves 1/C²(V). The analysis of these curves allow then the calculation of the donors concentration using Eq.(1), as well as the Schottky barrier height (φb) obtained by the intersection between the 1/C²(V) line and the voltage axis:

ND−NA = 2

A²qε0εr(d(1/C²)/dV) (1) whereND,NAare, respectively, the concentration of the donors and the acceptors. Theε0,εr permittivity vacuum and CdTe.

Table 1summarizes the results.

We can notice that for the Cd face the value of φb shifts from∼0.72 eV for the undeformed sample to∼0.91 eV after indentation and it shifts from∼0.92 to∼0.71 eV for the Te face under the loads of 10, 25 and 50 g. Special case for the 80 g load, whereφbvalue of 0.5 eV for both sides has been obtained.

Concerning donors concentration, we notice that it decreases in the same way and order of magnitude for the two sides (Table 1).

The results drawn inFig. 4andTable 2show that the edge of light

Table 2

Absorption results before and after different loads indentation

Applied weights (g) Absorption edgeEg(eV)

0 1.50

10 1.495

25 1.49

50 1.484

80 1.479

N. Brihi et al. / Materials Science and Engineering B 137 (2007) 49–52 51

Fig. 4. UV–vis spectra for CdTe indented by various loads.

absorption is shifted proportionally to the applied weight loads towards the lower light energies (larger wave length), which is a clear indication of a reduction of the band gap.

4. Discussion

The light absorption results for both Cd and Te sides show that the increase of the weight load and consequently the increase dislocation density lead to the absorption edge displacement towards the low energies, by narrowing of band gap in the field of elastic tension of dislocations. Since the donors concentration decreases drastically in the both case too, leading on the contrary an extension of the band gap. We must suppose then that we have a drastic increase of acceptor level concentration near the valence band which can then explain this band gap reduction (VCd−are often present in CdTe especially in deformed material).

In addition, the electrical measurements have shown a low increase of the negative leakage current due to the accep- tors minority carriers, which is lower on the Cd side, than on the Te side. That can be explained by the φb higher, but mainly, to the fact that dislocations in the Cd side create acceptors, which influence the leakage current and then reduce the donors concentration. We are certainly in presence of a compensation phenomena of donors (InCd+, Br⁺, . . .) by the indentation structural defects (VCd2−). The complex accep- tor VCd2−+ InCd+→(VCd2−, InCd+)⁻ was observed by Brihi and co-workers[15], by photoluminescence spectra studies of indented samples. That means that dislocation density increasing induces a rise in the concentration of the Cd vacancies, leading to an additional reduction ofNd.

Theφb variation is explained by the pinning model of the Fermi level which can be summarised as following[16,17]: the Schottky barrier height is affected by the majority carrier defects existing at the studied surface. The long arms of the indentation rosette at the Cd side and Te side is consisting in a gliding (g) dislocations of Cd (g) and Te (g)[14]. This implies at the Cd side the existence of Te vacancies (VTe+) in great concentration inducing the pinning of the Fermi level according to their level position in the band gap (Ev(Te) near 0.9 eV)[17], this value is

in good agreement with that obtained for (φb= 0.9 eV) for the loads (10, 25 and 50 g). At the Te side, the majority carriers is still the cadmium vacancies which lead to the pinning of the Fermi level position in the band gap according to their level position in the band gap (Ev(Cd)= 0.7 eV). This leads to a perfect agreement with the obtained value height of the barrier height (φb= 0.7 eV).

Concerning the barrier height of 0.5 eV induced by the 80 g load, we are working with low resistivity material, the depleted zone is then really thin in the same order of magnitude of the indented layer. The electrical measurements are only deal- ings with depleted layer (∼80 tonnes/cm²) and at 80 g load dislocations are so high in this region. It can be in this case, that the material is nearly amorphous or at highly polycrys- talline which contain many type of defects leading to high carrier charge concentration and lowering the height Schottky barrier.

5. Conclusion

The work undertaken in this paper is related to the study of the dislocations effect density on the electric properties of not inten- tionally doped n-CdTe. The increase of dislocations leads to:

• A reduction in the concentration of existing donors due to the creation of new concentration of Cd vacancies (VCd2−), which leads to the passivation of donors.

• An increase of the leakage current, which is due to the rise of the concentration of surface states.

• The creation of the shallow defect levels close to the conduc- tion band leads to a reduction of the band gapEg.

• The load of 80 g gives aφbof 0.5 eV for both Cd and Te sides.

For the others loads (10, 25 and 50 g) we observed two values ofφb:φb∼0.91 eV for Cd side andφb∼0.71 eV for Te side.

This study can be of great interest in understanding the material behavior in presence of choc impact leading to high dislocation density and to highlight the importance of the direc- tion crystal and of the stress matrix effect in II–VI material.

References

[1] K. Guergouri, R. Triboulet, J. Cryst. Growth 132 (1993) 20.

[2] A. Gukasyan, A. Kvit, Y. Klevkov, Solid State Commun. 97 (10) (1996) 897.

[3] A. Castaldini, A. Cavallini, B. Fraboni, L. Polenta, P. Fernandez, J. Piqueras Mater. Sci. Eng. B42 (1996) 302.

[4] A. Zumbiehl, S. Mergui, M. Ayoub, M. Hage-Ali, A. Zerrai, K. Cherkaoui, G. Marrakchi, Y. Darici, Mater. Sci. Eng. B71 (2000) 297.

[5] F. Glasser, V. Gerbe, P. Ouvrier-Buffet, M. Accensi, J.L. Girard, M. Renaud, J.L. Gersten-Mayer, Nucl. Instrum. Methods Phys. Res. A458 (2001) 544.

[6] R. Sudharsanan, K.B. Parnham, N.H. Karam, Lazer Focus World 32 (1996) 199.

[7] K. Kinkade, Lazer Focus World 28 (1997) 120.

[8] K. Guergouri, M.S. Ferah, R. Triboulet, Y. Marfaing, J. Cryst. Growth 139 (1994) 6.

[9] J.P. Zielinger, et al., Mater. Sci. Eng. B16 (1993) 273.

[10] G. Khrypunov, A. Romeo, F. Kurdesau, D.L. B¨atzner, H. Zogg, A.N. Tiwari, Solar Energy Mater. Solar Cells 22 (2005).

[11] N. Brihi, K. Guergouri, Sci. Technol. 13 (2000) 25.

52 N. Brihi et al. / Materials Science and Engineering B 137 (2007) 49–52 [12] R. Triboulet, Y. Marfaing, J. Electrochem. Soc. 120 (1973) 1260.

[13] A. Zerrai, K. Cherkaoui, G. Marrakchi, G. Bremond, J. Cryst. Growth 197 (1999) 646.

[14] A. Rivi`ere, B. Sieber, J.P. Rivi`ere, Microx. Microanal. Microstruct. 2 (1991) 257.

[15] K. Guergouri, N. Brihi, R. Triboulet, J. Cryst. Growth 209 (2000) 709.

[16] I.M. Dharmadasa, J.M. Thornton, R.H. Williams, Appl. Phys. Lett. 54 (1988) 137.

[17] Z. Sobiesierski, I.M. Dharmadasa, R.H. Williams, Appl. Phys. Lett. 53 (1988) 2623.