Comparison of crystal growth and thermoelectric properties of n-type Bi-Se-Te and p-type Bi-Sb-Te nanocrystalline thin films: Effects of homogeneous irradiation with anelectron beamMasayuki Takashiri, Kazuo Imai, Masato Uyama, Harutoshi Hagino, Saburo Tanaka, Koji Miyazaki, andYoshitake Nishi Citation: Journal of Applied Physics 115, 214311 (2014); doi: 10.1063/1.4881676 View online: http://dx.doi.org/10.1063/1.4881676 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/21?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Significantly enhanced thermoelectric figure of merit through Cu, Sb co-substitutions for Te in Ga2Te3 Appl. Phys. Lett. 101, 081908 (2012); 10.1063/1.4747621 Experimenting with hot isostatically pressed (HIP) nano grained bismuth-telluride-based alloys AIP Conf. Proc. 1449, 544 (2012); 10.1063/1.4731614 Influence of processing parameters on the thermoelectric properties of (Bi0.2Sb0.8)2Te3 sintered by ECAE AIP Conf. Proc. 1449, 111 (2012); 10.1063/1.4731509 Thermoelectric properties of compacted Bi2-xSbxTe3 nanoplatelets with nominal composition of x = 1.5 AIP Conf. Proc. 1449, 103 (2012); 10.1063/1.4731507 Fabrication of Bi 2 Te 3 / Sb 2 Te 3 and Bi 2 Te 3 / Bi 2 Te 2 Se multilayered thin film-based integrated coolingdevices J. Vac. Sci. Technol. A 28, 679 (2010); 10.1116/1.3292600

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Comparison of crystal growth and thermoelectric properties of n-typeBi-Se-Te and p-type Bi-Sb-Te nanocrystalline thin films: Effectsof homogeneous irradiation with an electron beam

Masayuki Takashiri,1,a) Kazuo Imai,1 Masato Uyama,1 Harutoshi Hagino,2 Saburo Tanaka,3

Koji Miyazaki,2 and Yoshitake Nishi11Department of Materials Science, Tokai University, 4-1-1 Kitakaname, Hiratsuka, Kanagawa 259-1292,Japan2Department of Mechanical and Control Engineering, Kyushu Institute of Technology, 1-1 Sensui,Tobata-ku, Kitakyushu 804-8550, Japan3Department of Mechanical Engineering, College of Engineering, Nihon University, Nakagawara,Tokusada, Tamuramachi, Koriyama, Fukushima 963-8642, Japan

(Received 5 March 2014; accepted 23 May 2014; published online 4 June 2014)

The effects of homogenous electron beam (EB) irradiation on the crystal growth and thermoelectric

properties of n-type Bi-Se-Te and p-type Bi-Sb-Te thin films were investigated. Both types of thin

films were prepared by flash evaporation, after which homogeneous EB irradiation was performed at

an acceleration voltage of 0.17 MeV. For the n-type thin films, nanodots with a diameter of less than

10 nm were observed on the surface of rice-like nanostructures, and crystallization and crystal

orientation were improved by EB irradiation. The resulting enhancement of mobility led to increased

electrical conductivity and thermoelectric power factor for the n-type thin films. In contrast, the

crystallization and crystal orientation of the p-type thin films were not influenced by EB irradiation.

The carrier concentration increased and mobility decreased with increased EB irradiation dose,

possibly because of the generation of defects. As a result, the thermoelectric power factor of p-type

thin films was not improved by EB irradiation. The different crystallization behavior of the n-type

and p-type thin films is attributed to atomic rearrangement during EB irradiation. Selenium in the

n-type thin films is more likely to undergo atomic rearrangement than the other atoms present, so

only the crystallinity of the n-type Bi-Se-Te thin films was enhanced. VC 2014 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4881676]

I. INTRODUCTION

Bismuth telluride (Bi2Te3)-based alloys are remarkable

materials with extraordinary thermoelectric properties. They

have been used in thermoelectric devices such as power

generators1–3 and Peltier coolers.4,5 Bulk Bi2Te3-based

alloys possess the highest known thermoelectric figure of

merit at room temperature (RT).6 The thermoelectric figure

of merit is defined as ZT¼ S2rT/j, where S is the Seebeck

coefficient, r is the electrical conductivity, T is absolute tem-

perature, and j is the thermal conductivity, which contains

contributions from electrons and phonons. From the above

definition, the thermoelectric figure of merit is increased by

the thermoelectric power factor S2r and decreased by the

thermal conductivity. Many approaches have been adopted

to enhance the thermoelectric properties of Bi2Te3 and its

alloys, including changing the composition from stoichiome-

try, use of polycrystalline materials with different grain

sizes,7,8 intentional introduction of structural defects,9,10 and

incorporation of dopants such as Sb or Se into the Bi2Te3 lat-

tice.11 In general, bismuth selenium telluride (Bi-Se-Te) acts

as an n-type material, whereas bismuth antimony telluride

(Bi-Sb-Te) acts as a p-type one. This characteristic is favor-

able to fabricate thermoelectric devices because the thermal

expansion rate of the two materials is almost the same, so the

devices are able to tolerate thermal stress.1

To date, thermoelectric devices have mainly been fabri-

cated using bulk materials. However, it is difficult to minia-

turize devices using bulk materials. Thin film process

technology is one approach to downscale devices. Favorable

results were achieved for Bi2Te3/Sb2Te3 superlattices pro-

duced by metal-organic chemical vapor deposition, which

exhibited a ZT of approximately 2.4 at RT.12 In addition,

PbSeTe-based quantum dot superlattice structures grown by

molecular beam epitaxy achieved a ZT of 2.0 at RT.13 The

main way to improve ZT is to control the size of structures

while maintaining high crystallinity. However, although the

above deposition methods are suitable for producing nano-

sized structures with high crystalline quality, the apparatus is

complicated and the deposition rate is low, resulting in high

manufacturing cost. Conversely, conventional deposition

methods such as flash evaporation,14,15 sputtering,16,17 and

electrodeposition18,19 are able to reduce the manufacturing

cost, but it is challenging to produce nanostructures with

high crystallinity. The most common method used to

improve crystallinity is thermal annealing, but the size of

crystal grains increases during this process.

Irradiation with energetic particles such as ions, neu-

trons, and electrons is a possible method to achieve structural

control as an alternative to thermal annealing. In particular,

an electron beam (EB) can modify the atomic structure of a

a)Author to whom correspondence should be addressed. Electronic mail:

takashiri@tokai-u.jp

0021-8979/2014/115(21)/214311/7/$30.00 VC 2014 AIP Publishing LLC115, 214311-1

JOURNAL OF APPLIED PHYSICS 115, 214311 (2014)

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target material without any impurity doping through mecha-

nisms such as elastic interactions (knock-on mechanism) and

excitation-related processes involving breakage or rearrange-

ment of unstable bonds.20–22 Fe-Zr-B alloys with nanocrys-

talline structure have previously been produced by EB

irradiation with controlled irradiation dose.23 Amorphous

Al2O3 has also been crystallized by atomic rearrangement

under EB irradiation.24 Most of the investigations of

EB-irradiated crystal growth have used the focused EB of a

transmission electron microscope. This method is quite use-

ful for clearly revealing the mechanism of crystal growth,

but it is challenging to enlarge the crystallized area. In con-

trast, homogeneous EB irradiation method enables crystalli-

zation of large areas of materials.

In this study, we investigate the effects of homogene-

ous EB irradiation on the crystal growth and thermoelectric

properties of n-type Bi-Se-Te and p-type Bi-Sb-Te thin

films. We fabricate n- and p-type thin films using a flash

evaporation method, and then the resulting thin films are

exposed to homogeneous EB irradiation. The structural

properties of both types of thin films are analyzed by scan-

ning electron microscopy (SEM) and X-ray diffraction

(XRD). The thermoelectric properties of the films, includ-

ing electrical conductivity and Seebeck coefficient, are

examined. Finally, we discuss the origin of the difference

of crystallization observed between the n- and p-type thin

films.

II. EXPERIMENTAL

N-type Bi-Se-Te and p-type Bi-Sb-Te thin films were

fabricated on glass substrates (Corning 7059, size:

50� 25 mm, 1.1 mm thick) by a flash evaporation method.

The experimental setup has been described in detail previ-

ously.25 The distance between the tungsten boat and sub-

strate was set to 200 mm. The starting materials were n-type

Bi2.0Te2.7Se0.3 and p-type Bi0.4Te3.0Sb1.6 spherical powders

prepared by a reported centrifugal atomization method.26

We confirmed that the average size of both types of powder

was approximately 200 lm. Thin films were prepared using

the same deposition conditions with the exception of the

elemental composition of starting powders. In both cases,

the resulting films had a uniform thickness across the sub-

strate of 0.49 lm for the n-type Bi-Se-Te thin film and

0.73 lm for the p-type Bi-Sb-Te thin film. To deposit the

thin films, we first loaded the starting powder into the pow-

der vessel and placed a glass substrate on the holder. The

chamber was evacuated to 1.4� 10�3 Pa and then a current

of 80 A was applied to the tungsten boat until the tempera-

ture of the substrate reached 200 �C. Finally, by gradually

tilting the powder vessel, the powder was fed into the tung-

sten boat, which caused the powder to evaporate onto the

glass substrate. We confirmed that the atomic compositions

of both types of thin films were almost the same as those of

the starting powders.27,28

Next, the n-type Bi-Se-Te and p-type Bi-Sb-Te thin

films were homogenously irradiated with an electron-

curtain processor at RT (CB175/15/180L, Energy Science,

Inc., Iwasaki Electric Group Co., Ltd.).29–31 A tungsten

filament under vacuum was used to generate an EB with a

voltage of 0.17 MeV and irradiating current of 2.0 mA. The

thin films were irradiated with the EB through a titanium

window in the vacuum chamber. To prevent oxidation, the

samples were kept in a nitrogen atmosphere at a pressure of

0.1 MPa with a residual oxygen concentration of less than

0.04%. The flow rate of nitrogen gas was 1.5 l/s. The EB

irradiation depth (Dth:m) can be expressed according to the

density (q: kg/m3) and irradiation voltage (V: kV) by the

following equation:32

Dth ¼ 66:7 V5=3=q: (1)

Table I presents the estimated EB irradiation depth for n-

and p-type samples, including the thin films and substrate.

The calculated EB irradiation depths of n-type Bi-Se-Te and

p-type Bi-Sb-Te were 46.0 lm and 52.2 lm, respectively.

These depths are larger than the film thickness of both types

of thin films. In contrast, the EB irradiation depth of the glass

substrate was 126.1 lm, which is smaller than the thickness

of the substrate. Therefore, in both types of thin films, the

EB penetrated through the thin films, and then stopped in the

substrate.

The surface morphology of the thin films was investi-

gated by SEM (S-4800, Hitachi) using an electron accelerat-

ing voltage of 10 keV. This voltage is sufficiently lower

than the electron irradiation voltage (0.17 MeV), so the con-

dition of the thin films was not changed during SEM obser-

vation. The crystallographic properties of the thin films

were evaluated by XRD (Mini Flex II, Rigaku) using Cu-Ka

radiation (k¼ 0.154 nm). The average crystal grain size was

estimated from the full-width at half maximum of the XRD

peaks using the Scherrer equation. The carrier concentration

and mobility of the thin films were measured at RT using a

Hall effect measurement system (HMS-3000, Ecopia). The

in-plane electrical conductivity of the thin films was meas-

ured at RT by the four-point probe method (RT-70 V,

Napson). The in-plane Seebeck coefficient of the films was

also measured at RT. One end of each thin film was con-

nected to a heat sink and the other end to a heater. The

Seebeck coefficient was determined as the ratio of the

potential difference along the film to the temperature differ-

ence, which was measured using two K-type thermocouples

with a diameter of 0.1 mm pressed onto the thin film. The

distance between the thermocouples was 13 mm. The

in-plane thermoelectric power factor, S2r, of each film was

calculated from its measured electrical conductivity and

Seebeck coefficient.

TABLE I. Calculated EB irradiation depths of n-type Bi-Se-Te, p-type Bi-

Sb-Te, and a glass substrate.

Thickness

(lm)

Density

(g/cm3)

EB irradiation

depth (lm)

n-type Bi-Se-Te 0.49 7.57 46.0

p-type Bi-Sb-Te 0.73 6.67 52.2

Glass substrate

Corning 7059

1100 2.76 126.1

214311-2 Takashiri et al. J. Appl. Phys. 115, 214311 (2014)

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III. RESULTS AND DISCUSSION

A. Structural properties of n-type Bi-Se-Te and p-typeBi-Sb-Te thin films

Fig. 1 shows the surface morphology of n-type Bi-Se-Te

and p-type Bi-Sb-Te thin films observed by SEM. The SEM

image of the untreated n-type thin film shows that it was

composed of numerous rice-like nanostructures with diame-

ters of 10–20 nm and lengths of 30–100 nm that were bonded

with each other (Fig. 1(a)). The surface morphology of the

n-type thin film treated with an EB irradiation dose of 0.22

MGy was similar to that of the untreated n-type thin film

(Fig. 1(b)). In contrast, when the EB irradiation dose was

increased up to 0.43 MGy, the surface morphology changed

markedly. A number of nanodots with diameters of less than

10 nm were observed on the surface of rice-like nanostruc-

tures (Fig. 1(c)). When the EB irradiation dose was increased

further (up to 1.07 MGy, see Fig. 1(d)), the surface morphol-

ogy was similar to that of the thin film obtained at a dose of

0.43 MGy, but the boundaries of the nanodots became more

defined. In contrast, for the p-type Bi-Sb-Te thin films, the

SEM image of the untreated n-type thin film shows that it

was composed of numerous spherical nanostructures with

diameters of 20–30 nm (Fig. 1(e)). The surface morphologies

of the p-type thin films treated with an EB irradiation dose

ranging from 0.22 to 1.07 MGy were similar to that of the

untreated p-type thin film (Figs. 1(f)–1(h)). Therefore, the

morphology of the p-type thin films was not strongly influ-

enced by EB irradiation.

XRD patterns of the n-type Bi-Se-Te and p-type Bi-Sb-

Te thin films obtained after EB irradiation with various doses

are presented in Fig. 2. All of the peaks in the patterns of the

n-type and p-type thin films correspond to the reflections of

the rhombohedral phase of Bi2Te3 (JCPDS 15-0863). For the

n-type thin films, the XRD peaks consistent with (0 1 5) and

(1 1 10) planes were mainly observed for the untreated thin

film but their intensities were low, which revealed that the

untreated thin film contained some amorphous phase. Only

the intensity of the (0 1 5) peak was enhanced with increas-

ing EB irradiation dose. This was particularly obvious when

the EB irradiation dose was 0.43 MGy, while other peaks

weakened or even disappeared. This trend corresponds to the

change of surface morphology at this EB irradiation dose

observed by SEM. Therefore, we think that the nanodots on

the rice-like structures were crystalline, and the further gen-

eration of nanodots led to higher crystallinity. In contrast, for

p-type thin films, the XRD peaks attributed to the (0 1 5) and

(0 0 1) planes consistent with c-axis orientation were

observed for the untreated thin film. Even when the EB irra-

diation dose was increased to 1.07 MGy, the XRD peaks did

not change much, and their intensities remained low, so the

p-type thin films contained amorphous phase following EB

irradiation.

Fig. 3 illustrates the relationship between the EB irradia-

tion dose and crystallographic properties (average grain size

and crystal orientation) of the n-type Bi-Se-Te and p-type

Bi-Sb-Te thin films. In Fig. 3(a), the estimated crystal grain

sizes of the untreated n-type and p-type thin films were 13

and 24 nm, respectively. The crystal grain size of n-type thin

films increased slightly with EB irradiation dose, whereas

that of the p-type thin films remained almost constant. The

increase of crystal grain size of both types of thin films with

FIG. 1. Surface morphologies of n-type Bi-Se-Te and p-type Bi-Sb-Te thin

films on glass substrates imaged using SEM. (a)–(d) are images of n-type

thin films that were irradiated at doses of 0, 0.22, 0.43, and 1.07 MGy,

respectively. (e)–(h) are images of p-type thin films irradiated at doses of 0,

0.22, 0.43, and 1.07 MGy, respectively.

FIG. 2. XRD patterns of (a) n-type Bi-Se-Te thin films and (b) p-type Bi-Sb-

Te thin films.

214311-3 Takashiri et al. J. Appl. Phys. 115, 214311 (2014)

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EB irradiation dose was negligible compared with that

reported for thin films treated by thermal annealing.7,28,33 In

Fig. 3(b), the crystal orientation is defined as the ratio of the

intensity of the (0 1 5) XRD peak of to the sum of the inten-

sities of all XRD peaks. The crystal orientations of the

untreated n-type and p-type thin films were 0.52 and 0.22,

respectively. The crystal orientation of n-type thin films

increased linearly with EB irradiation dose up to 0.43 MGy,

reaching 0.91 at this dose. This indicates that preferential ori-

entation along the (0 1 5) plane of the n-type Bi-Se-Te thin

films was adopted. The crystal orientation was saturated at

approximately 0.9 even when the EB irradiation dose was

further increased. Therefore, the homogeneous EB irradia-

tion treatment formed crystals in the n-type thin films with

preferential orientation while maintaining the relatively

small size of the crystal grains. In contrast, although the crys-

tal orientation of p-type Bi-Sb-Te thin films increased

slightly with EB irradiation dose from 0.43 MGy ([0 1

5]/R[h k l]: 0.19) to 1.07 MGy ([0 1 5]/R[h k l]: 0.31), the

crystal orientation of p-type thin films was much lower than

that of the n-type thin films.

B. Thermoelectric properties of n-type Bi-Se-Teand p-type Bi-Sb-Te thin films

Fig. 4 shows the relationship between the EB irradiation

dose and thermoelectric properties of the n-type Bi-Se-Te

and p-type Bi-Sb-Te thin films, including the carrier concen-

tration and mobility estimated from Hall effect measure-

ments. Fig. 4(a) reveals that the carrier concentration of all

of the n-type thin films was approximately 5.0� 1019 cm�3,

which was the similar value of the established n-type bulk

alloy with same atomic composition.6 Therefore, we think

that homogeneous EB irradiation treatment did not affect the

carrier concentration of n-type thin films much. In contrast,

the carrier concentration of the p-type thin films increased

with EB irradiation dose from 1.3� 1020 cm�3, which was

also the similar value of the established p-type bulk alloy

with same atomic composition,6 for the untreated thin film to

5.3� 1020 cm�3 for an EB irradiation dose of 1.07 MGy.

This increase of carrier concentration in the p-type thin films

may be caused by defects generated by EB irradiation.34,35

The carrier mobility of the n-type thin films increased

with EB irradiation dose from 7.3 cm2/V/s for the untreated

thin film to 23.3 cm2/V/s for an EB irradiation dose of 1.07

MGy (Fig. 4(b)). We expect that this enhancement of the

mobility of n-type thin films induced by homogeneous EB

irradiation was caused by the improvement of crystallinity

and crystal orientation. Nonetheless, the mobility of estab-

lished n-type bulk alloys was more than 100 cm2/V/s,6 so

there is still room to improve the mobility of such films by

optimizing the EB irradiation conditions. In contrast, the car-

rier mobility of the p-type thin films decreased as the EB

irradiation dose was increased, from 9.9 cm2/V/s for the

untreated thin film to 3.4 cm2/V/s for an EB irradiation dose

of 1.07 MGy. The decreased mobility of p-type thin films

with increasing EB irradiation dose is probably because elec-

trons were trapped in the defects generated by EB

irradiation.

FIG. 3. (a) Average crystal grain size and (b) crystal orientation of n-type

Bi-Se-Te and p-type Bi-Sb-Te thin films as a function of EB irradiation dose

determined from XRD peaks.

FIG. 4. (a) Carrier concentration and (b) mobility of n-type Bi-Se-Te and

p-type Bi-Sb-Te thin films as a function of EB irradiation dose determined

by Hall effect measurements.

214311-4 Takashiri et al. J. Appl. Phys. 115, 214311 (2014)

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Fig. 5 presents the relationship between the EB irradia-

tion dose and in-plane thermoelectric properties of the

n-type Bi-Se-Te and p-type Bi-Sb-Te thin films, including

electrical conductivity, absolute Seebeck coefficient, and

thermoelectric power factor. Fig. 5(a) indicates that the in-

plane electrical conductivity of an untreated n-type thin film

was 64.3 S/cm, and it increased markedly with EB irradiation

dose. The electrical conductivity reached 205.6 S/cm at an

EB irradiation dose of 1.07 MGy. We attribute this improve-

ment of electrical conductivity to increased mobility,

because the carrier concentration was almost constant

throughout our experiment. The in-plane electrical conduc-

tivity of an untreated p-type thin film was 203.1 S/cm, much

higher than that of an untreated n-type thin film. The electri-

cal conductivity increased with homogeneous EB irradiation

dose until it became saturated at approximately 300 S/cm.

This saturation occurred because the carrier concentration

increased and mobility simultaneously decreased as the EB

irradiation dose increased.

Fig. 5(b) shows the dependence of the absolute in-

plane Seebeck coefficient of n-type Bi-Se-Te and p-type Bi-

Sb-Te thin films on EB irradiation dose. It is noted that the

original Seebeck coefficient of the n-type Bi-Se-Te thin

film was negative, and that of the p-type Bi-Sb-Te thin film

was positive. We used absolute Seebeck coefficients for

convenience of visualization. The absolute Seebeck coeffi-

cient of the n-type thin films increased slightly with EB

irradiation dose from 46.2 lV/K for the untreated thin film

to a maximum of 68.5 lV/K for an EB irradiation dose of

1.07 MGy. In contrast, the absolute Seebeck coefficient of

the p-type thin films decreased markedly with increasing

EB irradiation dose initially, from 95.4 lV/K for the

untreated thin film to a minimum of 40.3 lV/K for an EB

irradiation dose of 0.43 MGy. The Seebeck coefficient did

not change much with further increases of EB irradiation

dose. The decrease of Seebeck coefficient of the p-type thin

films with increasing irradiation dose was possibly related

to the increased carrier concentration caused by the genera-

tion of defects.

Fig. 5(c) reveals the dependence of the in-plane thermo-

electric power factor of the n-type Bi-Se-Te and p-type Bi-

Sb-Te thin films on EB irradiation dose. The thermoelectric

power factor of the n-type thin films increased linearly with

EB irradiation dose. The maximum thermoelectric power

factor of the n-type thin films was 0.96 lW/cm/K2 at an EB

irradiation dose of 1.07 MGy, which was approximately

seven times higher than that of the untreated n-type thin film.

However, this magnitude of thermoelectric power factor is

still low compared with those reported for established n-type

bulk alloys.6,27 This is because the EB-irradiated thin films

had the lower electrical conductivity and Seebeck coefficient

that those of bulk Bi2Te3 alloys. Therefore, there is still

room to enhance the thermoelectric properties of such films

by optimizing the EB irradiation conditions. In contrast, the

thermoelectric power factor of the p-type thin films

decreased substantially with increasing EB irradiation dose

initially, from 1.85 lW/cm/K2 for the untreated p-type thin

film was to a minimum of 0.46 lW/cm/K2 at an EB irradia-

tion dose of 0.43 MGy. Then, the thermoelectric power fac-

tor remained approximately constant as the EB irradiation

dose increased further.

C. Crystal growth mechanism of n-type Bi-Se-Te andp-type Bi-Sb-Te thin films

As mentioned above, the crystal growth and thermoelec-

tric properties of the n-type Bi-Se-Te thin films were quite

different from those of the p-type Bi-Sb-Te thin films even

though the both types were treated with EB irradiation under

exactly the same conditions. Homogeneous EB irradiation of

the n-type thin films enhanced their crystallization and crys-

tal orientation, and mobility was improved. As a result, the

electrical conductivity and thermoelectric power factor of

the n-type thin films were increased by EB irradiation. In

contrast, the crystallinity and crystal orientation of p-type

Bi-Sb-Te thin films were not influenced by homogeneous EB

FIG. 5. (a) Electrical conductivity, (b) Seebeck coefficient, and (c) thermo-

electric power factor of n-type Bi-Se-Te and p-type Bi-Sb-Te thin films as a

function of EB irradiation dose.

214311-5 Takashiri et al. J. Appl. Phys. 115, 214311 (2014)

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irradiation. The carrier concentration increased but mobility

decreased with EB irradiation dose, possibly because of the

generation of defects. As a result, the thermoelectric power

factor of p-type thin films was not able to be improved.

Next, we discuss the origin of this difference of crystalliza-

tion in the n-type and p-type thin films.

We considered two mechanisms of crystallization:

crystallization attributed to the temperature rise during ho-

mogeneous EB irradiation,36 and crystallization caused by

atomic rearrangement during homogeneous EB irradia-

tion.22,24 We first examined how much the temperature

would increase during homogeneous EB irradiation. The

temperature rise, DT, of a sample, including a thin film and

glass substrate can be estimated using the following

equation:

DT ¼ DEB=mC; (2)

where DEB is the EB irradiation dose (J), m is the mass of the

sample (g), and C is the specific heat (J/g/K). Here, the elec-

tron beam penetrated through the thin film and stopped in the

glass substrate, so we estimated the mass of each sample

including the thin film and glass substrate to the depth of EB

penetration. The specific heat of each sample was estimated

from the combined values of the thin film and glass substrate

according to their mass ratio. The temperature rises of the

n-type and p-type samples during EB irradiation are pre-

sented in Table II. The temperature rises of both types of

samples were similar, and the maximum temperature rise

was approximately 26 K at an EB irradiation dose of 1.07

MGy, indicating that the temperature of each sample

increases by at most 50 �C. In our previous study, the crys-

tallization of thin films required thermal annealing above a

temperature of 150 �C.7 Therefore, we conclude that the

effect of heating induced by EB irradiation should be

excluded as a factor contributing to crystallization.

Second, we examined atomic rearrangement induced

by homogeneous EB irradiation. To evaluate the crystalli-

zation process, we needed to know the maximum trans-

ferred energy and displacement threshold energy of each

element in the n-type Bi-Se-Te and the p-type Bi-Sb-Te

thin films. The maximum transferred energy EM from an

incident electron mass me and kinetic energy E to a constit-

uent atom of mass ma can be calculated using the following

equation:37,38

EM ¼ 2maEðEþ 2mec2Þ=½ðma þ meÞ2c2 þ 2maE�; (3)

where c is the speed of light. The equation is derived

from the conservation law of momentum for relativistic

electrons. The calculated EM of Bi, Te, Se, and Sb under

an EB with a voltage of 0.17 MeV are presented in

Table III. The displacement threshold energy, EDT, of

Bi, Te, Se, and Sb is also shown,39–42 but unfortunately

literature data for EDT in Bi-Se-Te and Bi-Sb-Te alloys

are unavailable. We found that Se exhibited the highest

EM (5.5 eV) and Bi the lowest (2.1 eV) of the four ele-

ments contained in the thin films. The displacement

threshold energy of Te and Se were lower (Te:

7.8–8.0 eV, Se: 8.2 eV) than that of Bi (13 eV). The elas-

tic displacement mechanism of crystallization is likely

to be dominant when EM is higher than EDT. In our case,

EM of each element was lower than the corresponding

EDT, indicating that EB irradiation at a voltage of

0.17 MeV did not satisfy the conditions required for

crystallization. However, it should be noted that the

knock-on energy of surface atoms is lower than that of

bulk ones.24 Therefore, it is possible that crystallization

is enhanced on the surface of the thin films under such

irradiation conditions.

Se present in n-type Bi-Se-Te thin film exhibited the

lowest energy gap between EM and EDT, so the n-type Bi-

Se-Te thin films were expected to show enhanced crystalli-

zation through rearrangement of Se atoms as a base point in

the crystals. Conversely, the p-type Bi-Sb-Te thin films did

not contain Se atoms, so atomic rearrangement was less

favored. As a result, the crystallization of p-type Bi-Sb-Te

thin films was not enhanced under the experimental condi-

tions. According to Qui et al.,22 when the amorphous phase

transitions to the crystalline phase as the EB irradiation

energy is increased, the excited amorphous state exists

between them. This excited amorphous state contains stored

energy in the form of disordered atomic configuration, so it

possesses a large quantity of defects resulting from dangling

bonds. We expect that the p-type Bi-Sb-Te thin films formed

a quasi-excited amorphous state under the investigated irra-

diation conditions. As mentioned above, the carrier concen-

tration of p-type Bi-Sb-Te thin films increased with EB

irradiation dose because defects were generated by EB irra-

diation. The formation of a quasi-excited amorphous state of

the p-type Bi-Sb-Te thin films corresponds to the observed

trends of the electrical properties of these films. Finally, we

may conclude that EB irradiation enhanced crystallization

when the energy gap between EM and EDT of each element

was sufficiently small.

TABLE II. Temperature rise of n-type Bi-Se-Te and p-type Bi-Sb-Te thin

films on glass substrates during EB irradiation.

EB irradiation dose

(MGy)

Temperature rise

(K)

n-type Bi-Se-Te/glass 0.22 5.2

0.43 10.2

1.08 25.7

p-type Bi-Sb-Te/glass 0.22 5.3

0.43 10.3

1.08 26.0

TABLE III. EM, EDT, and EDT-EM of elements in the n-type Bi-Se-Te and

p-type Bi-Sb-Te thin films.

Element

Atomic

weight

(g/mol)

Maximum

transferred

energy, EM (eV)

Displacement

threshold

energy, EDT (eV)

EDT-EM

(eV)

Bi 209.0 2.1 13 (Ref. 39) 10.9

Te 127.6 3.4 7.8–8.0 (Ref. 40) 4.4–4.6

Se 79.0 5.5 8.2 (Ref. 41) 2.7

Sb 121.8 3.6 8.5–9.9 (Ref. 42) 4.9–6.3

214311-6 Takashiri et al. J. Appl. Phys. 115, 214311 (2014)

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IV. CONCLUSION

We investigated the effects of homogeneous EB irradia-

tion treatment on the crystal growth and thermoelectric prop-

erties of n-type Bi-Se-Te and p-type Bi-Sb-Te thin films.

Both types of thin films were prepared using a flash evapora-

tion method, after which EB irradiation was performed at an

acceleration voltage of 0.17 MeV under N2 at RT. For the

n-type thin films, numerous nanodots with a diameter of less

than 10 nm were observed on the surface of rice-like nano-

structures for EB irradiation doses of 0.43 and 1.07 MGy.

The crystallization and crystal orientation of the n-type films

were improved by the homogeneous EB irradiation treat-

ment. The resulting enhancement of mobility led to increases

of electrical conductivity and thermoelectric power factor. In

contrast, the crystallization and crystal orientation of p-type

Bi-Sb-Te thin films were not influenced by homogeneous EB

irradiation under the conditions investigated. The carrier

concentration increased and mobility decreased with increas-

ing irradiation dosage, possibly because of the generation of

defects. As a result, the thermoelectric power factor of

p-type thin films was not improved.

We investigated the origin of the difference of crystalli-

zation between the n-type and p-type thin films under EB

irradiation. We conclude that crystallization was caused by

atomic rearrangement during the homogeneous EB irradia-

tion. The crystallization was enhanced when the elements in

the thin films possessed a higher maximum transferred energy

as well as a lower displacement threshold energy. Se was the

element that best satisfied the above conditions of the four

elements in the thin films. Therefore, n-type Bi-Se-Te thin

films showed enhanced crystallization through rearrangement

of Se atoms as a base point in the crystals. The p-type Bi-Sb-

Te thin films did not contain Se atoms, so crystallization was

not improved. Instead, the irradiation energy was stored in

the form of disordered atomic configuration (quasi-excited

amorphous state), so a large amount of defects resulting from

dangling bonds were introduced into the p-type films.

ACKNOWLEDGMENTS

The authors thank Mr. Miyamoto, Mr. Kiyuna, Mr.

Kusagaya, and Mr. Takaya at Tokai University for providing

experimental support.

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