Optical and electrical study of deformed hydrogenated bulk Cd0.96 Zn0.04 Te single crystal

Optical and electrical study of deformed hydrogenated bulk single crystal

F. Lmai ,

, N. Brihi ,

, Z. Takkouk ,

, K. Guergouri ,

, F. Bouzerara ,

, and M. Hage-Ali ,

Citation: Journal of Applied Physics 103, 084504 (2008); doi: 10.1063/1.2836483 View online: http://dx.doi.org/10.1063/1.2836483

View Table of Contents: http://aip.scitation.org/toc/jap/103/8

Published by the American Institute of Physics

Optical and electrical study of deformed hydrogenated bulk Cd _0.96 Zn _0.04 Te single crystal

F. Lmai,

N. Brihi,

Z. Takkouk,

K. Guergouri,

F. Bouzerara,

and M. Hage-Ali

1

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

2

Laboratoire d’étude des matériaux, Département de Physique, Faculté des Sciences, Université de Jijel 18000, Algérie

3

Laboratoire de Physique chimie des Semiconducteurs, Université de Constantine, Algérie

共 Received 23 June 2007; accepted 4 December 2007; published online 21 April 2008 兲

The effect of hydrogenation on defects associated with dislocations has been studied in the p-type Cd

Zn

Te 共CZT兲 semiconductor, grown by the horizontal Bridgman method, with the help of current I共V兲, capacity C共V兲 measurements, photoluminescence spectra, and cathodoluminescence imaging. To generate dislocations by plastic deformation we have used a Vickers microhardness instrument on several cadmium 共 Cd 兲共 111 兲 and telluride 共 Te 兲共 1 ¯ ¯ 1 ¯ 1 兲 CZT faces. Hydrogenation was carried out by exposure of the samples to hydrogen plasma at 150 ° C and 3 mbar for 3 h. The analysis of the results obtained confirms both the reduction of the acceptor concentration, that acceptors may be passivated by formation of neutral complexes with hydrogen, and that the majority defect on each face is the tellurium vacancy, V

. A reduction of the broadband at 1.547 eV 共 V

− D 兲 is observed, while the donor bound exciton D

X is increased on the Cd face and finally, it seems that the hydrogen stabilizes dislocations and prevents their removal. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2836483兴

I. INTRODUCTION

The study of p-type Cd

Zn

Te crystals, grown by the horizontal Bridgman process is connected to both the improvement of the crystalline quality of CdTe crystals al- loyed with Zn atoms

and the increase in the resistivity.

This material is principally used as a detector for gamma-ray im- aging applications, which requires high crystalline quality of the basic material. The choice of 4% Zn is essentially to reduce a mismatch between the CdHgTe layers and CdZnTe when the latter is used as a substrate for these layers.

The effect of dislocation on the semiconductors proper- ties has been studied. Guergouri et al. had shown that a plas- tic deformation of CdTe or CdZnTe led to the formation of acceptor states.

Hydrogen is a common compensator in all semiconduc- tors. It interacts with intrinsic defects, impurities such as shallow dopants and deep centers, and the host crystal struc- ture. It is well established that hydrogen passivates the elec- trical defects in semiconductors by forming close pairs. The passivation is most pronounced for acceptors in the case of CdTe.

In this paper we present a study of the dislocations when hydrogenated using photoluminescence excitation I共V兲 and C 共 V 兲 characterization.

II. EXPERIMENTAL

Our study has been carried out on samples of 1 mm thickness and 0.5 cm

surface area. These samples are me- chanically polished using diamond paste with a grain size 0.25 ␮ m and then mechanochemically polished using a bro- mine methanol solution.

The measured net acceptor concen- tration is then 1.14 ⫻ 10

cm

共 before indentation 兲 .

Local plastic deformation has been introduced by means of a Vickers’s microhardness instrument, which consists of applying a pyramidal diamond indenter using a chosen weight of 50 g and an applied time of 30 s. Cathodolumines- cence images obtained after applying a 50 g load reveal well- developed indentation rosettes which show the following features:

共a兲 a strongly perturbed central zone appearing as a very dark area with a diameter ranging from three to four times the diameter of the inner indentation imprint 共Fig. 1兲 and

共b兲 six double arms, each one consisting of a short and a long arm. In the Cd face the long arms of the rosette structure are composed of Cd 共g兲 dislocations, the Te vacancies being the major defect. While the Te face has its longer rosette arms composed of Te 共g兲 dislocations, the Cd vacancies being the major defect in this case.

Hydrogenation was carried out by exposure of the samples to hydrogen plasma at 150 ° C and 3 mbar for 3 h, which was inductively excited by a radio frequency field 共30 kHz兲. To be able to distinguish between the heating and the hydrogenation effect, we have also annealed the sample at 150 ° C for 3 h in closed ampoules.

The face metallization has been performed by evapora- tion of In metal to perform the Schottky contacts and the

a兲Electronic mail: [email protected].

b兲Electronic mail: [email protected].

c兲Electronic mail: [email protected].

d兲Electronic mail: [email protected].

e兲Electronic mail: [email protected].

f兲Electronic mail: [email protected].

0021-8979/2008/103共8兲/084504/5/$23.00 103, 084504-1 © 2008 American Institute of Physics

electroless H

AuCl

to obtain the ohmic contacts. The elec- trical characterization was performed using a Keithley 617 digital electrometer for the I共V兲 measurements and a Kei- thley 590 C 共 V 兲 meter for the C 共 V 兲 measurements. The fre- quency used for the C 共 V 兲 measurements was 1 MHz.

Photoluminescence experiments were carried out at liq- uid helium temperature using a Fourier-transform infrared spectrometer BOMEM DA8 used in the emission mode. Ex- citation light was provided by the 488 nm line of an Ar

laser cooled by air. The provided density power is about 10 W cm

and the beam was focused to a spot size of 200 ␮ ^m.

III. RESULTS AND DISCUSSION A. Luminescence properties

The effect of the annealing shown in 共Fig. 2兲 is to re- move the indentation rosette, in contrast to the hydrogena-

tion, in which no change can be observed on the indentation rosette 共 Fig. 1 兲 .

In Figs. 3共a兲, 3共b兲, 4共a兲, and 4共b兲 we show photolumi- nescence spectra after deformation of the Cd face and the Te face, respectively; 共a兲 indicates the case before hydrogena- tion and 共b兲 the one after hydrogenation.

The gap energy 共E

兲 is given by the energy of the free exciton recombination peak 共X兲 and is related to the Zn con- centration x by the empirical formula 共1兲,

E

= 1.604 + 0.42x + 0.33x

. 共1兲

The results obtained are summarized in Table I.

These spectra show an increase in the energy gap after hydrogenation, which would be attributed to a double effect:

That of the temperature and that of the hydrogen-tellurium interaction. The temperature causes the Zn diffusion from the bulk to the surface, containing Cd vacancies, with a strong concentration on the Te face compared with the Cd face. On both faces the hydrogen interacts, at the same time, with Te atoms and Cd vacancies, but the hydrogen has a strong af- finity for Te atoms; this leads to the volatile entity TeH

, illustrated by the following reaction:

2H + Te → TeH

. 共2兲

On the Te face the dominant interaction is made between hydrogen and tellurium, while that of the Cd face is made

FIG. 1.共Color online兲Cathodoluminescence image of indented surface zone of a CdZnTe sample共not hydrogenated and hydrogenated兲. L.A: Long arm, S.A: Short arm.

FIG. 2.共Color online兲Cathodoluminescence image of CdZnTe indented and annealed sample.

FIG. 3. Low-temperature photoluminescence spectra ofp-CdZnTe, face Cd.

共a兲indented and共b兲indented-hydrogenated.

084504-2 Lmaiet al. J. Appl. Phys.103, 084504共2008兲

between hydrogen and Cd vacancies. With the concentration of Cd vacancies being higher on the Te face, it can be noted that the rise in the gap energy on this face is higher than that of the Cd face.

The A

X line decreases on both faces, but more strongly on the Cd face, while the D

X line is relatively enhanced by hydrogenation on the Cd face 共Fig. 3兲. These observations are interpreted in terms of a direct hydrogen-acceptor inter- action, leading to acceptor neutralization.

The higher re- duction of the A

X line on the Te face is explained by the fact that the hydrogen interacts more strongly with vacancies of the cadmium 共dominant element on the Te face兲, but less with the acceptor on the Cd face.

The D − A band at 1.5470 eV, due to the recombination between a shallow donor and a complex acceptor is probably of the 关 V

− D 兴 共 V

= Cd vacancy 兲 .

The reduction of the

FIG. 5.I共V兲curves of indented-hydrogenated In-pCdZnTe-Au structure for both Cd and Te faces.

FIG. 6.C共V兲curves of indented-hydrogenated In-pCdZnTe-Au structure for both Cd and Te faces.

FIG. 7. 共1/C²兲共V兲curves of indented-hydrogenated In-pCdZnTe-Au struc- ture for both Cd and Te faces.

FIG. 4. Low-temperature photoluminescence spectra ofp-CdZnTe, face Te.

共a兲indented and共b兲indented-hydrogenated.

TABLE I. Quantities determined from photoluminescence measurements at low-temperature of indented and indented-hydrogenatedp-CdZnTe for Cd and Te faces:Eg, band gap energy;x: concentration of Zn.

Nature of crystal Face Eg⫾510⁻⁴ 共eV兲 x⫾10⁻³共concentration兲

Indented Cd 1.6093 0.01

Te 1.6116 0.02

Indented hydrogenated

Cd 1.6118 0.02

Te 1.6168 0.03

D − A band is due to the hydrogen passivation of V

, the hydrogen passivation of shallow donors being not very effective.

Finally, the effect of the annealing in the pres- ence of hydrogen 共plasma兲 illustrated in Fig. 1, no change of the configuration of the indentation rosette is observed, that is explained by the fact that hydrogen stabilizes the disloca- tion, and prevents their removal, in contrast to the standard annealing 共 no hydrogen 兲 , in which the indentation rosette disappears 共 dislocation arms 兲 Fig. 2.

B. Electrics measurements

Using I 共 V 兲 curves 共 Fig. 5 兲 we have estimated the leakage current, which is higher on the Te face compared to that of the Cd face. The surface state density estimated, qualita- tively, from the deviation of C 共 V 兲 curves 共 Fig. 6 兲 with re- spect to the capacitance axis is higher on the Te face than on the Cd face. The acceptor concentration has been estimated and the barrier height ⌽

has been measured from Eq. 共 3 兲 and from curves 共 1 / C

兲共 V 兲 共 Fig. 7 兲 ,

N

− N

= 2 A

q␧

␧

d 共 1 / C

兲 dV

. 共3兲

All these results are plotted in Table II, and indicate a de- crease in the acceptor concentration N

after hydrogenation.

The barrier height value equal to 0.72 eV on Cd face and 0.62 eV on the Te face before hydrogenation, taken as the same value 共0.71 eV兲 on both faces Cd and Te after hydro- genation.

The explanation of the variation of the barrier height is based on the Fermi level—pinning model,

which stipu- lates that the majority defect state causes the pinning of the Fermi level, and the relative pinning energy is characteristic of the nature of the surface state involved. Our results ⌽

= 0.62 eV, corresponding to the pinning energy 0.62 eV in respect to the valence band E

, show that the relative pinning energy on the Te face in mainly due to the complex defect 共V

, A兲. We have suggested this defect as a majority defect because of the formation of the V

on the long arms of the indentation rosette 共the long arms of the indentation rosette are composed of Te 共g兲 dislocations, the Cd vacancies being the major defect兲 and A

defects contribute together to the A

X recombination.

After hydrogenated we find ⌽

= 0.71 eV, corresponding to the pinning energy 0.7 eV / E

, which identified as a Te vacancy, which is explained by an interaction of hydrogen with the Cd vacancies, and by a pref- erential interaction of hydrogen with tellurium, forming the volatile entity TeH

, which leads to the creation of the Te vacancies at the crystal surface. The same approach is made on the Cd face. We have found that the majority defect,

before hydrogenation, is attributed to the Te vacancy 共the long arms of the indentation rosette are composed of Cd 共g兲 dislocation, the Te vacancies being the major defect兲, which creates, within the band gap, an energy level of 0.7eV 共nearly equal to ⌽

兲. After hydrogenation, we have found the same value of the barrier height on the Te face after hydrogenation 共⌽

= 0.71 eV 兲 is attributed to Te vacancies.

The decrease in the acceptor concentration is explained by the passivation of the acceptor by the hydrogen and the formation of a neutral complex 共 H− A 兲 .

IV. CONCLUSION

We mentioned that a plastic deformation of p-type Cd

Zn

Te leads to the formation of acceptors states 共an increase in the acceptor concentration after deformation兲.

We have shown that the incorporation of hydrogen in Cd

Zn

Te semiconductors leads to the neutralization of acceptor defects 共 V

兲 and a strong affinity of Te for H

.

A strong reduction of the acceptor related luminescence lines A

X, and the same behavior for broadband at 1.547 eV 共 V

− D 兲 , while the donor bound excitons D

X is enhanced.

Schottky contacts on indented-hydrogenated crystals are bet- ter on the Cd faces, which present a weak surface state den- sity compared with the Te faces.

A decrease in the acceptor concentration is explained by the passivation of the defects by the hydrogen. Increases of the concentration of Te vacancies become the major defects on both Cd and Te faces and finally we confirm that the hydrogen stabilizes the dislocations up under the effect of the temperature.

ACKNOWLEDGMENTS

The authors would like to thank R. Triboulet 共 LPSC, CNRS Meudon, France 兲 for supplying the crystal used in this study.

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TABLE II. Electrical data of indented and indented-hydrogenatedp-CdZnTe for Cd and Te faces.

Nature of crystal Indented Indented hydrogenated

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Face Cd 2.41 0.72 −2.2 0.20 0.71 −3

Face Te 2.60 0.63 −3.1 0.15 0.71 −4.3

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