Growth mechanism of CuZnInSe2 thin films grown by molecular beam epitaxy

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Growth mechanism of CuZnInSe

2 thin ﬁlms grown by molecular

beam epitaxy

Ya Hsin Tseng

a,b

, Chu Shou Yang

a,n

, Chia Hsing Wu

a

, Jai Wei Chiu

a

, Min De Yang

c

,

Chih-Hung Wu

c

a

Graduate Institute of Electro-optical Engineering, Tatung University, Taipei, Taiwan

b

Department of Electro-physics, National Chiao-Tung University, Hsin-Chu, Taiwan

c

Institute of Nuclear Energy Research, P.O. Box 3-11, Lungtan 32500, Taiwan

a r t i c l e

i n f o

Available online 4 January 2013 Keywords:

A1. X-ray diffraction A3. Molecular beam epitaxy B1. CuZnInSe2

B2. Semiconductor materials

a b s t r a c t

CuZnInSe2(CZIS) has potential application in solar cell for absorption layer, and give an advantage to change the band gap from CuInSe2 (1.02 eV) to ZnSe (2.67 eV). Using molecular beam epitaxy technology, the CZIS thin films were grown via CuInSe (CIS) and ZnSe base. In the case of CIS, thin films were grown on Mo-coated soda lime glass with various zinc flux. CIS was transformed into chalcopyrite and sphalerite coexisting CZIS easily but it is difficult to transform into the pure sphalerite CZIS. Zn/(Znþ Inþ Cu) ratio has limited to approximate 36 at% and the excess-Zn played a catalyst role. In the case of ZnSe base, which was grown on GaAs (001), various In and Cu flux defined as the TInseries and TCuseries, respectively. There are four types of compound in the TInseries and TCuseries, including ZnSe, InxSey, ZnIn2Se4(ZIS) and CZIS. In the TInseries under the lowest In and Cu flux, selenium (Se) were randomly combined with cations to form the CZIS. When TInis increased in this moment, the CZIS was transformed into ZIS. In the TCuseries, CZIS demonstrated via In-rich ZIS (Zn(In, Cu)Se) and InxSey base ((Zn, Cu)InSe). It is chalcopyrite and sphalerite coexisting structure in the medium TCuregion. In the high TCuregion, it is transformed into the Zn-poor and Cu-rich CZIS.

1. Introduction

The quaternary compounds, I–II–III–VI2 group, have high

absorption coefﬁcient about 104_–105_cm1_{within widely}

absorp-tion range from visible to near infra-red region, outdoor stability and a signiﬁcant resistance to radiation damage [1]. CuZnInSe (CZIS) is one of the potential materials for absorber layer in solar cell applications. The consisted within I–II–III–VI2group elements

is deﬁned by (CuInSe2)1 x–(2ZnSe)xsolid solution[1–3]. It has

binary structures, chalcopyrite and sphalerite (zinc blende like), which were associated with the Zn content[4]. The chalcopyrite structure resembles CIS, with the Cu and In sites randomly substituted by Zn atoms. The sphalerite structure is like ZnSe, with the position of metal sub-lattice occupied statistically by Cu, In and Zn atoms.

A number of methods for growing CuZnInSe absorber layers have been reported so far. These methods include Bridgman growth[5], chemical vapor transport[5], two-stage technological process: standard thermal evaporation and selenization[6], radio frequency (RF)-sputter[7], pulsed laser deposition (PLD)[7]and

dc sputter [5]. Gremenok et al. 2,4 established the transition region from chalcopyrite structure to the sphalerite structure. The performance of CZIS solar cell linearly depends on composition. The highest solar efficiency was achieved to 7.2%. In order to further improve the conversion efficiency, it is important to understand the detail growth mechanism of CZIS. In this study, CZIS thin films were demonstrated using molecular beam epitaxy (MBE) via the precursor of CuInSe2(CIS) and ZnSe to understand

the growth mechanism. The physical properties of CZIS are characterised by using X-ray diffraction, Raman scattering, and EDX measurements.

2. Experiments

The CZIS thin ﬁlms were demonstrated via the precursor of CuInSe2(CIS) and ZnSe with varied K-cell temperature of Zn, In,

and Cu, which are denoted as TZn, TIn, TCu series. The growth

parameters of CZIS thin ﬁlms are listed inTable 1. In samples-1–5, i.e. TZnseries, the CZIS thin ﬁlms were deposited on Mo-coated

soda lime glass. The growth condition is based on chalcopyrite CIS with supplied zinc source, which varied TZnfrom 300 to 340 1C.

On the contrary, the TInseries were based on ZnSe with turning

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the TInfrom 575 to 645 1C. In TCuseries, these sample parameters

are similar to sample-10. The variable factor is the TCu. The

thicknesses of samples-1–4 are around 300 nm, which were

determined by cross-sectional scanning electronic microscopy images. The growth rate of samples-1–4 is 1.25 nm/min, which is independent to the varying degree of zinc ﬂux until the TZn

reached 340 1C. In sample-5, the growth rate is dropped to 0.88 nm/min. The reason of decreasing growth rate is discussed latter. For TInand TCu series, the growth rate is around 0.9 nm/

min. The cation atomic ratio is calculated by the element atomic percentage, which is determined by energy dispersive spectrum. Raman scattering spectroscopy was excited by 514.5 nm-Ar ionic

Table 1

Growth parameters of CuZnInSe2compounds in TZn, TIn, and TCuseries.

Sample number Sample Substrate (o C) Selenium (o C) Zinc (o C) Indium (o C) Copper (o C) Zn/(Zn þCu þIn) (at%) Cu/(ZnþCu þIn) (at%)

In/(Znþ Cuþ In) (at%) CuInSe 500 203 655 1150 1 TZnseries 550 203 300 655 1150 20.4 53.3 26.2 2 310 28.8 56.7 14.4 3 320 35.6 48.4 15.8 4 330 36.1 48.6 15.3 5 340 29.2 50.3 20.4 ZnSe 300 198 315 6 TInseries 500 198 315 575 1000 52.4 18.9 28.7 7 585 36.9 38.7 24.4 8 600 61.5 21.9 16.6 9 615 38.3 16.5 45.2 10 630 8.7 12.7 78.6 11 TCuseries 500 198 315 630 1012 33.9 24.7 41.4 12 1025 38.8 22.9 38.3 13 1037 39.6 26.6 33.8 14 1050 32.2 27.4 40.4 15 1062 42.6 34.2 23.2 16 1075 9.8 59.6 30.6 17 1087 10.7 61.7 27.6 18 1100 8.5 62.3 29.2 150 160 170 180 190 200

Sample-5

Raman shift (cm-1₎

CuInSe

Sample

-

1 *

Intensity (a.u.)

Fig. 1. Raman scattering spectra of CuInSe2and CuZnInSe2of samples-1 and 5.

27.0 27.3 27.6 27.9 40 45 50 55 60

*

Sample-4 Sample-5 Sample-3 Sample-2 2 Theta (degree) (220/204) (116/312) Mo CuInSe₂ Sample-1 (112) plane of CuInSe (112) plane of CuZnInSe

*

Intensity (a.u.)

Fig. 2. XRD patterns of CuZnInSe2 in TZnseries.

Fig. 3. Top view and cross-sectional SEM images of CuZnInSe2 thin ﬁlm with

TZn¼(a) 330 1C (sample-4) and (b) 340 1C (sample-5). The inset is a bright ﬁeld

high-resolution transmission electron microscopy image of CZIS wire, taken from sample-5.

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laser and analyzed by a JY-550 spectrometer with the spectrum resolution of about 0.05 nm. The slit width was set at 100

mm, and

grating with 2400 grooves/mm was used. The structure analysis was performed using a D2 PHASER X-ray diffract meter employ-ing CuKaradiation with a wavelength of 1.5419 ˚A.

3. Results and discussions

3.1. TZnseries: CuInSe2incorporated with varied content of Zn Fig. 1 shows the Raman spectra of CuInSe2 and CuZnInSe,

which is grown at TZn¼300 1C (sample-1) and 340 1C (sample-5).

The CIS A1 phonon mode is slightly shifted to high frequency

when zinc atom incorporated into CIS. It refers to the heavy atoms of indium (M ¼114.82) that have replaced by slight atoms of zinc (M¼65.38). Additionally, when Cu–Se bonding was replaced by Zn–Se, the phonon frequency shifts toward high frequency has been mentioned [6]. It implies that the Zn–Se bonding was produced in the CuInSe2crystal. XRD spectra of CZIS samples-1–

5 and a reference sample of CIS are shown inFig. 2. The XRD peaks

are associated to CZIS and CIS (112), (220/204), and (116/312), respectively. CIS and CZIS compounds are co-existed in sample-1 (TZn¼300 1C). Further increasing TZn, the crystal structure

trans-forms to CZIS completely and the diffraction angle shifts to larger degree. According to the Raman and XRD spectra, the crystal structure of CZIS in TZnseries is assumed to be chalcopyrite and

sphalerite mixed. When TZn¼340 1C (sample-5), the XRD peak

shifts to inverse direction. This behavior is understood by the varied cation ratio. The cation ratio of Zn/(Zn þCuþIn) is increased with TZnraised until TZn¼330 1C, as shown inTable 1.

100 200 300 400 500 ZnSe(LO) Sample-6 Sample-7 Sample-8 Sample-9 ZnIn₂Se₄ (B₂)

_*

Sample-10 x1/3 InSe Ramanshift (cm-1₎ A'₁(1) E"E"(TO) A'1(2) E"(LO) 2E'(TO) 2E'(LO)

Fig. 4. Raman spectra of CZIS thin ﬁlms in TInseries. Raman spectra of ZnSe and

InSe are used for reference.

10

20

30

40

50 Sample-6

Intensity(a.u.)

Sample-7

Sample-8

ZIS(002)

Sample-9

Sample-10

InSe(002) InSe(004) (006)

63.5 64.0 64.5 65.0 65.5 66.0

Sample-6

2 theta

CZIS(400)

*

Sample-7 Sample-8 Sample-9

_ZIS(008)

Sample-10

Fig. 5. XRD patterns of CuZnInSe in TInseries.

100 200 300 400 500 600 ZnSe ZnSe (LO)

Sample-10

In-rich ZIS x3 x2 x4 x2 x1/3

Sample-11

Sample-13

Sample-14

(Zn, Cu)InSe In₂Se₃:(Cu, Zn)

Sample-15

Sample-16

Sample-17

Sample-18

Intensity (a.u.) Ramanshift (cm-1) Zn-poor/Cu-rich CZIS Zn(In, Cu)Se

Fig. 6. Raman spectra of CZIS thin ﬁlms in TCuseries and a reference sample

of ZnSe.

Y.H. Tseng et al. / Journal of Crystal Growth 378 (2013) 158–161 160

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Further raising the TZnto 340 1C, Zn/(Zn þCuþIn) is declined. It

implies that the excess-Zn played another role instead of trans-forming into purity sphalerite structure.Fig. 3(a) and (b) displays the top view SEM images of samples-4 and 5, respectively. Sample-5 has been observed as a dense nanowire on the surface. We assumed that the Zn plays a catalyst role in the excess-Zn condition. A bright ﬁle transmission electron microscopy image of a wire, which took from sample-5, is shown in the inset ofFig. 3(b). The lattice constant is determined around 0.408 nm, which is similar to the XRD result (0.409 nm). It implies that wire is CZIS crystal, which is an identical case with thin ﬁlm.

3.2. TInand TCuseries: ZnSe incorporated with varied content of Cu

and In

Fig. 4shows the Raman scattering spectra of TInseries samples,

ZnSe, In2Se3, and InSe. These spectra are strong correlates to the

material content ratio. In the poor indium and copper region, i.e. samples-6–8, the Raman scattering patterns are similar to ZnSe. The longitudinal optical phonon (LO) of ZnSe (251 cm1_{) reveals}

red-shift when the indium and copper incorporated. It is con-sisted with the results of TZnseries. When indium content ratio

dominated in total cation atoms, like sample-9, the phonon energy at around 242 cm1 _{is associated to ZnIn}

2Se4 B2 mode [7]. When TInwas increased to 630 1C (sample-10), the vibration

mode of InSe bonding was presented due to the indium cation content ratio increasing to 78.6%. This phase transformation process is also observed in the XRD patterns.Fig. 5exhibits the XRD spectra of samples-6–10. XRD pattern of CZIS (400), which is marked by star symbol, is observed in samples-6 and 7. The crystalline transfers to ZIS as indium content reached 45.6%. Additionally, the InSe (002), (004) and (006) planes were observed in sample-10. In TIn series, the crystalline of CZIS is

attributed to sphalerite. When the indium cation content ratio higher than 45%, the sphalerite CZIS transformed to tetragonal ZIS and even hexagonal InxSey.

Because the excess-In limited the ratio of x in the (CIS)1 x

-(2ZnSe)xsolid solution, the TInwas ﬁxed at 630 1C in TCu series.

The copper content ratio is increased with the increase in TCu. The

Raman spectra of TCuseries samples are shown inFig. 6.

Accord-ing toFig. 6, the TCuseries samples were separated into four parts:

(1) In sample-10, this Raman spectrum includes the phonon modes of InxSey: (Cu, Zn) and In-rich ZIS:Cu, which are symbolized by

open circle and down-toward-triangle, respectively.

(2) At TCu¼1012–1050 1C, it includes Zn(In, Cu)Se, which means

the In sites of In-rich ZIS was replaced by Cu, and (Zn, Cu)InSe, which means the Zn sites of InxSey: (Zn, Cu) was replaced by Cu.

(3) When TCu¼1062 1C, the phonon mode of InxSey:(Cu, Zn)

disappeared.

(4) When TCu exceeded 1075 1C, Zn-poor/Cu-rich CZIS was

formed.

4. Conclusions

The crystal structure of CuZnInSe2is controlled by turning the

atomic content ratio. CZIS products have no purity sphalerite structure unless the excess Se-rich condition. In the CIS case, the Zn cation content ratio was limited at 37 at% due to the limited of cation/anion ((CuþIn þZn)/Se). Further increasing the TZn, the

Zinc atom plays a role of catalyst to grow nanowire instead of inserting into the crystal structure. In the ZnSe case of TInseries,

purity sphalerite CZIS structure was product in excess-Se-rich condition, and the TInincreasing leads to the transformation from

sphalerite CZIS to tetragonal ZIS structure and even if the InxSey.

Furthermore, a pure chalcopyrite structure is exhibited in the copper-rich condition.

Acknowledgment

This work was supported by the Nation Science Council under Grant number NSC-97–2112-M-036-002-MY3 and Tatung Uni-versity under Grant number B100-O04-044.

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