Annealing behavior of ZnTe investigated with 111mCd-emission channeling

Beam interactions with Meter&Is & Atoms

ELSEVIER

Nuclear

Instruments

and Methods

in Physics

Research

B 136.-138

( 1998) 751-755

Annealing behavior of ZnTe investigated with

l1

‘“Cd-emission

Abstract The Emission Channeling technique has been used to study the annealing behavior of ZnTe implanted with different doses of Cd. “‘mCd ions of 60 keV energy were implanted at 100 K into a single crystal sample at the on-line isotope separator, ISOLDE, at CERN. Emission channeling measurements were performed along the (1 0 0) and (1 1 0) axial directions on the conversion electrons emitted in the “‘“Cd decay. The temperature at which practically complete lattice recovery is achieved in the neighbourhood of the implanted probes, with the probes on substitutional sites, was found to depend strongly on implantation dose. For an implantation dose of 1.3 x 10” Cd/cm? essentially full recovery of the lattice occurred at 500 K, while for a dose of 1.8 x 10’” Cd/cm’ lattice recovery was achieved at 550 K. At a dose of 2.3 x 10” Cd/cm’ no recovery was observed up to a temperature of 550 K. 0 1998 Published by Elsevier Science B.V. Ke~c~rls:

Ion implantation:

Lattice site; Radiation

damage;

1. Introduction

The large band gap II-VI semiconductors have attracted considerable attention for applications in optoelectronic devices. Amongst these ZnTe with its band gap of 2.26 eV at 300 K and the relative ease of high p-type doping is a promising material for the fdbricdtiOn of blue-green light-emitting diodes (LEDs) and laser diodes. Two essential requi-

* Corresponding author. Tel.: +27 3 I 204 4663; fax: +27 3 I 204 4795: e-mail: [email protected]. ’ The ISOLDE Collaboration. CERN, CH-1211 Geneva 23. Switzerland. 0168-583X/98/$19.00 0 1998 Published P~~SO168-583X(97)00864-1

by

Annealing

sites for such applications are high quality crystalline material and the production of doped layers with negligibly small strain in the neighbourhood of the dopants. Two recent attempts at producing high quality ZnTe layers have been reported [1,2], using molecular beam epitaxy (MBE) and metallorganic vapour phase epitaxy (MOVPE), respectively. Although LED’s employing ZnTe-based materials have been produced recently [3], a difficulty with producing ZnTe based p-n junctions is the incorporation of n-type dopants [a]. MOVPE produced samples showed the presence of unintentional C doping [2], which may be a general problem with thermal processes as a means of incorporating controlled concentra-

Elsevier Science B.V. All rights reserved

tions of desired dopants in the ZnTe layers. Ion implantation is thus likely to be the most successful means of incorporating dopant atoms in the IIVI compounds. In addition to offering control of dopant species, and accurate control of dopant concentration and profile, the process also offers the possibility to control the local stoichometry of a layer. However, a major difficulty with ion implantation is the lattice damage accompanying the implantation process, which is compounded in the II-VI semiconductors by the large variety of defects which can be formed [5]. Early ion implantation studies in ZnTe are those of Pautrat et al. [6,7], who implanted N, 0. AI. Ne, Zn and Tl ions into samples of ZnTe. Their results showed that the generation of defects with ion implantation is a very general phenomenon in ZnTe, and is largely independent of the nature and energy of the impinging ions. These authors also found that below a critical implantation dose of 5 x 10” cm ~’ the lattice damage anneals at about 573-673 K. These results were in agreement with earlier channeling and cathodoluminescence measurements [8,9], unpublished but quoted in Ref. [6], but in contrast with transmission electron microscopy (TEM) studies on Ar irradiated ZnTe samples [IO]. The latter study showed that up to temperatures as high as 573 K thermal treatment alone could not fully conduct the lattice recovery, and electron irradiation enhanced the annealing considerably. The present measurements were therefore undertaken to contribute to the search for effective dopant implantation conditions and annealing procedures in ZnTe. A sensitive technique for such investigations is Emission Channeling [I 1,121. which is especially suited for the study of annealing behavior of implantation defects and for the determination of impurity lattice sites directly after implantation and after subsequent annealing procedures. In the compound II-VI (and 111-V) semiconductors ion implantation produces a variety of different defects and defect complexes. Isoelectronic impurities, however, are expected to occupy substitutional sites after annealing, the sub-lattice depending on whether the impurity is a group II or VI element. In the present investigations we have used the isomerit radioactive nuclide “‘“Cd as probe. Our

measurements were therefore expected to monitor the annealing of implantation defects from probe atoms on substitutional Zn sites. From measurements of the channeling effects of the conversion electrons emitted in the “‘“‘Cd decay (r,;? =48.6 min), we have studied the implantation sites of Cd atoms and the annealing behavior of lattice as a function of implantation dose.

2. Experimental

details

In Emission Channeling measurements radioactive probe nuclei are implanted into single crystal samples, and the yield of emitted charged particles (a, fl’, b- or conversion electrons) is measured outside the sample as a function of angle relative to the different crystallographic axes and planes. For an electron-emitting probe atom located at a substitutional site the channeled electrons are bound to the positively charged atomic rows. and consequently. result in enhanced emission yields along all crystallographic axes. If the probe atom is located away from the atomic rows the channeling effects are reduced. In particular, in the (1 1 0) axial direction the electrons emitted from interstitial sites produce a nearly isotropic distribution. A comparison of measured yields with simulated channeling patterns for different lattice sites (substitutional, interstitial) thus allows a direct determination the lattice sites of the implanted atoms. Comparisons between the observed and simulated channeling spectra also permit one to determine the fractional occupancy of lattice sites. and to study annealing characteristics. The single crystal ZnTe sample was supplied by CRYSTAL GmbH. It was grown from the gas phase, and had a specific resistance of > 10’ R cm. The crystal had a mechanically polished upper surface which from Laue X-rays photographs was determined to have a (3 3 1) orientation. Before implantation the ZnTe crystal was cleaned in a 0.5% bromine+methanol solution. It was mounted in the vacuum chamber of a three-axis channeling goniometer. At the on-line isotope separator “‘“Cd ions were implantISOLDE at CERN [13] ed with an energy of 60 keV, into the ZnTe crystal through a 3 mm diameter aperture. The implanta-

B 136~138 (1998)

tions were done with the beam incident normal to the surface of the sample (because of its (3 3 1) orientation). The mean implantation range was est_imated from TRIM calculations [14] to be 262 A, with a straggle of 130 A. In an initial study a low dose of 0.5 x lOI crnm2 Cd probe atoms was implanted into the sample at room temperature, in order to provide a comparison for the low temperature implantations and annealing studies. Subsequent implantations, of doses of 1.3, 1.8 and 2.3 x 1O’j Cd cm-‘, were made at a sample temperature of 100 K. K- and L-conversion electrons from the “‘%d, 396 + 245 keV transition were detected in a Si surface barrier detector with an angle resolution of 0.2”. The conversion electron emission yields were measured over an angular range of *3” about the (1 1 0) or (1 1 1) crystallographic directions. A careful annealing study was done in the case of the 1.3 x lOI cm-’ implantation dose, with the sample annealed for 1 min at several temperatures in the temperature range lOOG550K. In the case of the 1.8 x 10” cm ’ dose annealing was performed between 300 and 550 K. The measurements at the highest implantation dose focused on annealing temperatures in the region where maximum lattice recovery had been observed, i.e. from 400 to 550 K.

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2.4 2.2

a) l%e: 1.3x10” cd/an2 cl10

2.0 I 1.8 1.61.4 -

YIU

an



11 T,=5OOK T,,=550K

l

1.8

1,6 c) Dose: 2.3 x lO”cn~~~ Cl10

I.

-3

-2

I

I.,

-1

0

T,=550K

:

1

2

3

Tilt Angle (deg.) 3. Results and discussion Sample channeling spectra are displayed in Fig. 1. Quantitative estimates of the fractions of implanted ions at substitutional sites were obtained from channeling patterns calculated within the many-beam formalism based on the dynamical theory of electron diffraction [I 11,with the continuum axial and planar potentials derived from the analytical Doyle-Turner [15] atomic potentials. The effects of dechanneling and thermal vibrations of the probe nuclei were taken into account as discussed in Refs. [11,12]. In the calculations Debye temperatures of Oo = 270 K for the Zn sub-lattice. and On = 213 K for Cd on a Zn-site, were used. The channeling spectrum observed with the low dose, room temperature implantation corresponded to a fractionJ1= 56(5)% occupation of substitu-

Fig. I. Emission channeling spectra of conversion electrons from “‘“Cd implanted in ZnTe at different implantation doses and annealing temperatures TA. The solid lines represent calculated yields based on the dynamical theory of electron diffraction.

tional sites, reflecting the ability of the ZnTe sample to anneal a large fraction of the implantation damage already during implantation. In the low temperature implantations the channeling effects were considerably reduced or not observable directly after implantation, indicating strong distortion of the local lattice around the probe atom. In the annealing study the normalized maximum channeling yields along the (1 1 0) and (1 1 1) axial directions were used as a measure of the ‘goodness’ of the lattice site of the implanted ions, and hence of the structural recovery of the lattice from

K. Bharlrth-Ram et cd. I Nucl. Instr. and Meih in Phjx. Res. B 136138

154

A

Dc6e=1.8xIo’3an-z411> Dose = 1.8 x 10’3ar-2 4 I@

Fig. 2. The maximum normalized channeling the different implantation doses as a function perature.

yield observed for of annealing tem-

implantation defects. This is displayed in Fig. 2(a) and (b), plotted against the annealing temperature TA.

The channeling spectra observed along the (1 1 0) axial direction for the implantation dose of 1.3 x lOI cm :, after annealing the sample at 450 and 500 K, is displayed in Fig. l(a). The solid line represents the channeling pattern calculated assuming a 100% occupancy of substitutional sites by the implanted Cd probe atoms. These results and the annealing measurement displayed in Fig. 2(a) show that after annealing at 500 K practically 100% Cd atoms are at substitutional Zn sites. Annealing at 550 K produced no increase in the channeling yield. It is evident that structural recovery of the ZnTe lattice in the neighbourhood of the implanted Cd ions is complete at TA = 500 K. In Fig. l(b) we present channeling spectra, observed along the (1 1 1) axial direction, for an implantation dose of 1.8 x 10’” cm-“. The solid line

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751-755

again represents calculated spectra for 100% substitutional Cd atoms. At TA = 500 K the channeling yield corresponds to a substitutional fraction of < 60% showing that there is still considerable lattice defect around the implanted probes. Complete lattice recovery sets in at the higher temperature of 550 K. Finally, in Fig. l(c) we present the channeling spectrum observed at 550 K for an implantation dose of 2.3 x lOI cm-‘. At this higher dose there was little change in the channeling spectra over the temperature interval 400--550 K, with the fraction of the Cd ions at substitutional sites being < 30%. suggesting that this dose is perhaps close to the critical dose above which implantation damage of the lattice does not anneal. Our results have shown that in the case of low implantation doses structural recovery of the ZnTe lattice from the implantation induced damage is achieved at annealing temperatures of 500-550 K, with practically 100% of the implanted dopants on or close to substitutional sites. While emission channeling effects are sensitive probes of the overall recovery of the crystallinity of the lattice, they are not sensitive to the microscopic structure of the immediate neighbourhood of the probe atom. Information on this may be obtained from measurements with other techniques such as perturbed angular correlations (PAC), where the electric quadrupole moment of the nuclear probe is very sensitive to interactions with defects in its nearest neighbourhood. In PAC studies on ZnTe (and other II-VI semiconductor) with “‘In probes Wichert et al. [16] observed the formation of InM-VM pairs in which the metal vacancy is trapped at the “‘In probe atom which occupies the Zn sub-lattice. Indeed “‘“‘Cd, the probe used in the present study, because of its decay cascade is also an ideal probe for such study, and when used together with the Emission Channeling measurements should give both direct identification of the lattice sites of the probes and information on probe-defect interactions.

4. Conclusions Our measurements ZnTe >/ 50% Cd atoms

have shown when implanted

that in with low

K. Bharuth-Ram

et al. I Nucl. Itutr.. and Meth. in Ph.vs. Rex B 136-138 (1998)

dose (0.5 x lot3 cm-‘) and at room temperature, take up substitutional sites on implantation. At implantation temperatures of 100 K the fraction of implanted ions at high symmetry sites is considerably reduced due to the extended defects accompanying the implantation process. The annealing temperature at which structural recovery of the lattice occurs. with all the implanted ions on substitutioanl sites, depends strongly on the implantation dose. Provided the implantation dose < 2 x 10” cm--’ the ZnTe lattice recrystallizes at TA = 500-550 K. To obtain unambiguous information on microscopic structure of the lattice in the immediate vicinity of the implanted Cd probes the present measurements must be supplemented with perturbed y-y angular correlation measurements on the gamma cascade emitted in the “‘“Cd decay.

References [l]

K. Watanabe, G. Landwehr.

M.Th. Litz. M. Korn. W. Ossau. A. Waag, U. Schiissler. J. Appl. Phys. 81 (1997) 451.

[?I

751-755

155

N. Lovegine, M. Longo. P. Prete. C. Gerardi. L. Calcagnile. R. Congolani. A.M. Mancini, J. Appl. Phys. 81 (1997) 685. 131 M.C. Phillips. M.W. Wang, J.F. Swenberg. J.O. McCaldin. T.C. McGill, Appl. Phys. Lett. 61 (1991) 1962. ST. [41 D.B. Laks. C.G. van de Walle. G.F. Neumark. Pantelides, Phys. Rev. Lett. 66 (1991) 648. 151 T.W. Sigmon. Nucl. tnstr. and Meth. B 718 (1985) 402. WI J.L. Pautrat. D. Bensahel. B. Katircioglu. J.C. Pfister. L. Revoil. Rad. Eff. 30 (1976) 107. [71 J.L. Pautrat. E. Molva. N. Magnea. J.C. Pfister. Inst. Phys. Conf. Ser. 59 (1980) 359. 181 A. Bontemps. E. ligeon, J. Marine, J.C. Pfister. Cent. Conf. Sot. Franc. de Physique, Vittel. 1973. [91 J. Marine, M. Quillec. J.C. Pfister. ESSDERC Conference. Munich. Germany, 1973 [lOI G. Lu. F. Niu. R. Wang. Rad. Eff. 116 (1991) 81. [111 H. Hofs%s, Hyp. Int. 97/98 (1996) 247. [121 H. Hofsiiss. G. Lindner. Phys. Reports 101 (1991) 131. [I31 E. Kugler. D. Fiander. B. Jonson. H. Haas. A. Przewloka. H.L. Ravn, D.J. Simon. K. Zimmer. Nucl. Instr. and Meth. B 70 (1992) 41. iI41 J.F. Ziegler, J.P. Biersack. U. Littmark, The Stopping and Range of Ions in Solids. Pergamon Press. New York. 1985. u51 P.A. Doyle. P.S. Turner, Acta Cryst. A 24 (1968) 390. [I61 Th. Wichert. Th. Krings. H. Wolf, Physica B 185 (1993) ‘97.