Composition dependent structural properties of Pb12xEuxSe thin ®lms

P.C. Sharma*

Department of Electrical Engineering, University of California, Los Angeles CA 90095-1594, USA

Abstract

We report on the composition dependent structural properties of Pb12xEuxSe thin ®lms deposited by a co-evaporation method. X-ray

diffraction studies have shown that all the ®lms deposited over a composition range of x � 0:1 to 0.45 are polycrystalline in nature with sharp

diffraction peaks independent of compositions and growth conditions. The lattice parameter of the ®lms calculated using the strongest

re¯ection (200) has been found to deviate from Vegard's law at higher compositions. The grain sizes have been found to be in the range of

100±450 AÊ with (200) as the preferred orientation of the grains. The dependence of grain size and orientation on ®lm composition and growth

temperature has been studied in detail to understand the mechanisms governing the polycrystalline grain growth. The incorporation of

europium and surface re-evaporation of selenium have been found to critically affect the polycrystalline grain growth. q 1999 Elsevier

Science S.A. All rights reserved.

Keywords: Lead salt alloys; Co-evaporation; Polycrystalline ®lms

1. Introduction

The ternary narrow gap IV±VI compounds and their

compositional alloys have received a great deal of research

attention due to their suitability for infrared (IR) device

applications [1±3]. While compositional alloys of lead

salts like Pb12xSnx Te and Pb12xSnxSe have found applica-

tions in the wavelength range of 8±12 mm, Pb12xEuxSe has

been found to be extremely useful for 2.5±7 mm infrared

detection [4]. The ability to change the band gap of this

material very easily by adjusting the composition (x) helps

in realizing compositions required to cover a broad range of

IR wavelengths.

It is relatively easy to grow compositional ®lms of

Pb12xEuxSe using a wide range of growth techniques as

both PbSe and EuSe crystallize in the face centered cubic

structure with close values of bulk lattice parameters.

Though MBE grown Pb12xEuxSe ®lms have been studied

for IR device applications [5], there have been no reports

of basic studies on Pb12xEuxSe thin ®lms grown by any other

standard growth technique. Studies on polycrystalline

Pb12xEuxSe ®lms help in determining the suitability of this

material system for thin ®lm IR detector applications. We

report here on the structural properties of Pb12xEuxSe ®lms

deposited by a co-evaporation method. Co-evaporation is a

simple and effective method to deposit compositional ®lms

as it obviates the need to synthesize the compositional mate-

rial besides offering the advantage of independent control

over all the evaporant sources.

2. Experimental

Thin ®lms of Pb12xEuxSe were deposited on ultrasoni-

cally cleaned glass slides using a high vacuum multi-source

co-evaporation system. PbSe, Eu and Se of 5 N purity were

used as source materials. The evaporation sources were

boats made of molybdenum with small ori®ces on top.

The substrate holder, capable of accommodating several

substrates, was heated radiatively in the temperature range

of 30±4008C. The typical boat temperature ranges for PbSe,

Eu, and Se were 600±750, 450±550, and 150±2508C, respec-

tively. The source and substrate temperatures were indepen-

dently controlled to an accuracy of ^ 18C by using PID type

temperature controllers. The details of the system con®g-

uration are discussed elsewhere [6]. The additional Se

source was used to ensure ef®cient incorporation of Eu as

also to check stoichiometric deviations arising out of PbSe

evaporation. The growth rates of individual sources and that

of the compositional ®lms were determined by a set of

independent depositions. The optimum deposition para-

meters to achieve speci®c ®lm composition at a particular

growth temperature were determined through a series of ®lm

depositions and compositional analyses. Films whose

compositions were determined by X-ray photoelectron

spectroscopy (XPS), X-ray ¯uorescence spectroscopy

(XRFS) and electron probe microanalysis (EPMA) were

Thin Solid Films 355±356 (1999) 12±16

0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.

PII: S0040-6090(99)00437-X

www.elsevier.com/locate/tsf

* Tel.: 11-310-206-0925; fax: 11-310-206-8495.

E-mail address: pcsharma@ee.ucla.edu (P.C. Sharma)

used for structural studies. The thickness of the ®lms, as

measured by a -step method using DekTak thickness pro®l-

ometer, was in the range of 1200±1600 AÊ . The structural

characterization of the ®lms was done using X-ray diffrac-

tion methods. A Phillips (1710) X-ray diffractometer and a

Rigaku diffractometer with Cu Ka (l � 1:542 AÊ , as radia-

tion source were used for this purpose. Normal diffraction

patterns were obtained in the range of 5±908 with a scan

speed of 28/min. Slow scans were also done to accurately

determine the angle at which maximum intensities were

observed, and the accuracy in recording the angle was

0.0058. All the diffractograms recorded were compared

with the ASTM data ®le cards [7] to identify different

peaks, phases and orientations of the grains. The interplanar

spacing, d, calculated using the corrected diffraction angle

was used to calculate the lattice constant (ao) for each re¯ec-

tion by using the formula

a0 � dhkl

����������������h2 1 k2 1 l2

pAfter identifying all the diffraction peak positions in the

X-ray diffractograms obtained from normal scans, a slow

scan of 0.258/min was taken within ^ 28 of each and every

peak angle. While the instrument automatically incorporates

the Cu Ka doublet broadening correction in the outputs, the

instrumental broadening correction was applied to the

measured full width at half maximum (FWHM) by using

a graphical method. The corrected FWHM (b ) was then

used to calculate the grain size (D) according to the follow-

ing formula [8]

D � kl

bcosu

where l is the X-ray radiation wavelength, u is the Bragg

angle and the coef®cient k is taken to be unity. To obtain the

preferred orientation of grains in the ®lms, a quantitative

method was used in which the diffraction peaks were

scanned at a slow speed and the area under each peak

above the background was measured to obtain the integrated

intensity. This integrated intensity was normalized by using

the following formula

NIIkhkll � Area under a particular peak

Total Area under all peaks I=I0

ÿ �where I/I0 is the relative intensity of the corresponding peak

in the ASTM standard ®le card.

3. Results and discussion

The structural properties of compositional ®lms usually

exhibit a strong dependence on composition and substrate

temperature. Thus Pb12xEuxSe ®lm properties like crystal-

linity, lattice constants, grain sizes, preferred orientation of

grains have been investigated in terms of their dependence

on these two parameters. Fig. 1 shows the X-ray diffraction

pattern of a typical Pb12xEuxSe ®lm sample deposited at

3008C for x � 0:4 with all the prominent peaks and their

orientations. All ®lms have been found to be polycrystalline

in nature with sharp diffraction peaks independent of growth

conditions. Comparison of peaks in diffractograms with the

ASTM ®le cards revealed that all ®lms showed only PbSe

phase and no other elemental peaks like Pb, Eu and Se were

present. However, as the d-spacings of EuSe are very close

to that of PbSe, the presence of EuSe phase is very likely.

The ®lms can therefore be treated as compositional alloys of

PbSe and EuSe. The observed reduction in intensity of

peaks as compared to ASTM standards is due to the ®nite

thickness of the samples. The diffraction data of these ®lms

is presented in Table 1. It can be seen from this table that

there is a gradual increase in the d-spacing with increasing

europium content in the ®lms. The lattice constants have

been calculated for the most intense re¯ection correspond-

ing to (200) orientation. Fig. 2 shows lattice constant as a

function of composition for Pb12xEuxSe ®lms deposited at

2508C. As can be seen from this ®gure, the lattice constant

of these ®lms increases with increasing europium content

(x) but not in accordance with Vegard's law which predicts a

linear change in lattice constant with composition in alloys

and solid solutions. Similar deviations have been observed

in studies on thin ®lms of homologous material systems like

Pb12xEuxTe [9], Pb12xYbxTe [10], and Pb12xEuxSeyTe12y

[11]. Both PbSe and EuSe crystallize in the face centered

cubic structure with close lattice constants (PbSe � 6:12 AÊ

and EuSe � 6:19 AÊ for (200)). This helps in forming a

mixed crystal system without drastically changing the

resulting Pb12xEuxSe system. Typically, europium is

expected to take Pb sites in PbSe and form a substitutional

alloy wherein a few selenium atoms are bonded to euro-

pium. This kind of substitutional bonding leads only to a

small increase in lattice constant appreciably smaller than

the lattice constant of EuSe thereby exhibiting a deviation

from Vegard's law. Fig. 3 shows the variation of lattice

constant with substrate temperature for three different ®lm

P.C. Sharma / Thin Solid Films 355±356 (1999) 12±16 13

Fig. 1. X-ray diffractogram of Pb12xEuxSe ®lm showing prominent re¯ec-

tions. Ts � 3008C and x � 0:4. All the prominent re¯ections of PbSe and

EuSe as observed in ASTM ®le cards appear in this diffractogram with

reduced intensities. The reduction in intensity is due to the ®nite thickness

of ®lm samples.

compositions. As is evident from this ®gure, the lattice

constant shows a rapid increase with growth temperature

for low europium compositions while the increase is gradual

for higher compositions. Also, a saturation in the increase of

lattice constant beyond 2508 for higher compositions is

evident from this ®gure. This behavior is possibly due to

the fact that elevated temperatures improve the incorpora-

tion of europium into PbSe which typically occupies Pb

sites or defect vacancies leading to an increase in lattice

constant. However, still higher substrate temperatures ( ,3008C) increase selenium re-evaporation from the growth

surface which checks the increase in lattice constants. Thus

the simultaneous incorporation of europium and loss of sele-

nium saturate the increase in lattice constant of Pb12xEuxSe

®lms.

From the diffraction data the (200) re¯ection has been

found to be the re¯ection with maximum intensity in all

the Pb12xEuxSe ®lm samples. Fig. 4 shows the normalized

intensities of peaks drawn against different orientations for

different compositions. It can be seen from this ®gure that

(220) orientation also becomes more and more prominent

with increasing europium content in the ®lms. The (200)

peak is the most prominent re¯ection (I=I0 � 100 for

EuSe) in EuSe XRD data. Based on the relative intensities

of (200) and (220) orientations, it may be concluded that

though the most preferred orientation of crystallites is (200),

(220) orientation also becomes signi®cant as the ®lm

composition moves from PbSe (x � 0) to EuSe (x � 1).

Data shown in Table 2 illustrates this observation in terms

P.C. Sharma / Thin Solid Films 355±356 (1999) 12±1614

Fig. 3. Lattice constant of Pb12xEuxSe ®lms as a function of substrate

temperature for different compositions. The saturation of lattice constant

at higher growth temperatures and compositions is due to the re-evapora-

tion of Se from the growth surface.

Fig. 4. Normalized intensities of prominent peaks from diffraction patterns

drawn for different orientations and different compositions (x) of

Pb12xEuxSe ®lms. The peaks corresponding to orientations (200), (111),

(220), and (420) are present in all the ®lms independent of growth condi-

tions and compositions.

Table 1

XRD data of Pb12xEuxSe ®lms for different values of x for (200) orientation

Composition, x 2u d (AÊ ) ao (AÊ )

0.0 29.160 3.0600 6.120

0.10 29.125 3.0636 6.127

0.20 29.080 3.0682 6.136

0.25 29.065 3.0698 6.140

0.30 29.030 3.0734 6.146

0.40 29.020 3.0745 6.149

0.45 29.010 3.055 6.151

Fig. 2. Lattice constant of Pb12xEuxSe ®lms as a function of composition.

The bulk lattice constants for PbSe � 6:12 AÊ and EuSe � 6:19 AÊ . The

lattice constants in this ®gure are calculated using the d-spacing of (200)

re¯ection. T s � 2508C.

of compared intensities for our ®lms and ASTM data for

PbSe and Euse.

The grain sizes of Pb12xEuxSe ®lms have been found to be

in the range of 100±450 AÊ . Figs. 5 and 6 show the variation

of grain size with ®lm composition and substrate tempera-

ture, respectively. As can be seen from Fig. 5, the incorpora-

tion of europium into PbSe enhances the grain growth by

reducing the growth induced defects. For the grain growth to

progress, it is very essential to have reduced strain and

lattice mismatch. Since our ®lms are grown on glass slides,

substrate induced strain is usually very high. This strain is

accommodated by the formation of small crystallites. This

may be the reason for the small grain sizes observed in our

case. Further, grain growth is enhanced by the ability of

europium to bond with selenium. This process is inhibited

by the re-evaporation of selenium from the surface of the

substrate at higher temperatures. Fig. 6 shows this saturation

in the grain growth at higher substrate temperatures. Thus

effective incorporation of europium and reduced selenium

re-evaporation from the substrate are critical to the poly-

crystalline grain growth in Pb12xEuxSe ®lms.

4. Conclusions

The compositional dependence of structural properties in

Pb12xEuxSe ®lms has been investigated for the ®rst time

using X-ray diffraction methods. The increase in lattice

constant of Pb12xEuxSe shows a signi®cant deviation from

Vegard's law for higher compositions. Polycrystalline grain

growth shows a saturation at higher substrate temperatures

and low europium contents suggesting a strong dependence

on the mechanism of europium incorporation and bonding

with excess selenium. While the ®lms exhibit a preferred

orientation of (200), (220) also becomes prominent with

increasing europium content. The present work demon-

strates the possibility of obtaining thin ®lms of Pb12xEuxSe

over a wide range of compositions. We have shown that by

optimizing the ®lm composition and growth temperatures,

®lms with large grain sizes can be grown reproducibly.

These compositional ®lms may ®nd applications as thin

®lm based IR detectors operating in photovoltaic and photo-

conducting modes.

P.C. Sharma / Thin Solid Films 355±356 (1999) 12±16 15

Table 2

XRD data of Pb12xEuxSe ®lms compared to the ASTM data of PbSe and EuSe

PbSe ASTM Data Pb12xEuxSe Data EuSe ASTM Data

d (AÊ ) I/I0 (hkl) d (AÊ ) (Range) I/I0 (Range) (hkl) d (AÊ ) I/I0 (hkl)

3.54 30 (111) 3.54±3.57 10±30 (111) 3.56 30 (111)

3.06 100 (200) 3.06±3.08 100 (200) 3.087 100 (200)

2.165 70 (220) 2.165±2.176 40±70 (220) 2.184 100 (220)

1.369 25 (420) 1.369±1.371 5±10 (420) 1.261 70 (420)

Fig. 5. Variation of grain size with Pb12xEuxSe ®lm composition. The grain

sizes are estimated by using a graphical method.

Fig. 6. Variation of grain size of Pb12xEuxSe ®lms with substrate tempera-

ture. The saturation seen in the grain sizes is due to the reduced ef®ciency of

europium bonding with Se caused by Se re-evaporation from hot substrate.

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