Comparative Study of the Properties of the Cu-M-O thin films (M=In and Sb)

Comparative Study of the Properties of the Cu-M-O thin films (M=In and Sb)

Nadia Chaglabou

Photovoltaic and Semiconductor Materials, Laboratory, ENIT, Tunisia

Mounir Kanzari

Photovoltaic and Semiconductor Materials Laboratory, ENIT, Tunisia

Abstract

The purpose of this work is to investigate and compare structural and optical properties of the Cu-M-O thin films (M=In and Sb). Samples were prepared via sequential thermal vacuum deposition of Cu and M on glass substrates after what they were heated in vacuum at 200°C for 1 hour followed by an annealing at 400 °C in air atmosphere. XRD of Cu/In system show the presence of Cu9In11 phase after annealing in vacuum and In2O3, CuO and Cu phases after annealing in air atmosphere. For Cu/Sb system, we note the formation of Cu2Sb and Sb after annealing in vacuum, and the presence of -Sb2O4 ,Sb2O3 and CuO phases after annealing in air atmosphere. The optical study indicated that the absorption coefficient of Cu-M-O thin films in all cases is in the range 104-105 cm-1. The electrical measurements show a conversion from a metallic phase to the semiconductor phase after annealing.

1. Introduction

   Recently, a great deal of interest has been focused on transparent conducting oxides (TCOs) due to their wide application in transparent electronic devices such as transparent electrodes for liquid crystal displays (LCDs), organic light emitting diodes (OLEDs) and solar cells [1,2]. Among binary compounds that have attracted much attention as new materials for TCO films we cite ZnO, MgO, In2O3, Ga2O3, and SnO2 [35]. ZnO,

   In2O3, and SnO2 TCO films all have n-type conductivity in nature [610]. Oxides of copper are also attracting renewed interest as promising TCO. Two common forms of copper oxide are Cu2O and CuO. Both the CuO and Cu2O are p-type semiconductors [11]. TCOs based on a Cu+ based system were also examined as CuMO2 (M=Al,Ga,In) [12],[13] and [14], SrCu2O2[15],

   LnCuOS (Ln=lanthanide) [16,17] and systems Ag- Cu-O [18], Cu-In-O [19] and Cu-Sb-O [20] .

   In this paper, we report the preparation and characterization of Cu-M-O system (M=Sb or In).

   The effect of the annealing in air atmosphere of the sample, the order of deposition of materials (Cu and M or M and Cu)) and the number of couples (Cu/M/Cu/M) on structural, optical and electrical properties was studied.

2. Experimental procedures

   The starting intermetallic multilayer systems used in this study, were prepared according the previous paper [20] by vacuum thermal evaporation in a sequential mode of pure (99.999%) Cu and M (M=Sb or In) and a thermal annealing. Thermal evaporation sources were used which can be controlled either by the crucible temperature or by the source power. The distance from crucible to sample holder was 12 cm. The pressure during

   evaporation was maintained between 105 and 106 Torr. The glass substrates were previously cleaned with washing agents commercial (detergent,

   acetone, ethanol and de-ionized water) before being introduced into the vacuum system. The substrate temperature was measured using a chromel-alumel thermocouple during the evaporation process. All obtained samples were prepared by two processes: an annealing at 200 °C for 1h in vacuum (106 Torr) followed by an annealing at 400 °C in air atmosphere at different times. The crystalline

   structure of the synthesized films was investigated by using X-ray diffraction (XRD) with a Philips D8 equipment using a monochromator CuK radiation (=1.54056Ã…) at room temperature.

   Optical transmission and reflection for the samples were measured at normal incidence with an UV visible-NIR Shimadzu spectrophotometer equipped with an integrated sphere in the wavelength range 300-1800 nm. Conductivity types of the samples were measured with the hot probe method.

3. Results and discussion

   1. Structural properties

      The structural properties of the intermetallic multilayer systems (Cu/M)j=1,2,3 and (M/Cu)j=1,2,3

      (M=Sb or In) for both films after annealing in vacuum at 200°C for 1 hour and films after annealing in air atmosphere at 400°C for different times were analyzed by XRD. The XRD spectra of the intermetallic multilayer systems (Cu/M)j=1,2,3 and (M/Cu)j=1,2,3 (M=Sb or In) annealed in vacuum at 200°C for 1 hour are presented in Figure 1 and Figure 2 .

      2

      2

      (003)

      (101)

      (003)

      (101)

      (111)

      (111)

      (006)

      (006)

      Cu Sb Sb

      (c)

      (b)

      Intensity a.u.

      Intensity a.u.

      (012)

      (012)

      (202)

      (202)

      (a)

      (003)

      (101)

      (003)

      (101)

      (110)

      (111)

      (110)

      (111)

      (104)

      (112)

      (200)

      (015)

      (006)

      (104)

      (112)

      (200)

      (015)

      (006)

      (c)

      (b)

      (a)

      10 20 30 40 50 60

      Bragg angle 2(°)

      Figure 1. XRD patterns of (Cu/Sb)j=1 (a), (Cu/Sb)j=2 (b), (Cu/Sb)j=3 (c), (Sb/Cu)j=1 (d), (Sb/Cu)j=2 (e)

      and (Sb/Cu)j=3 (f) annealed in vacuum

      at 200°C for 1 hour

      It is clear from Figure 1 that the patterns of (Cu/Sb)j=1,2,3 and (Sb/Cu)j=1,2,3 showed a mixture of Cu2Sb and antimony phases, and the sequences starting with antimony, i.e. (Sb/Cu)j=1 (d), (Sb/Cu)j=2 (e) and (Sb/Cu)j=3 (f) appear most crystallized that the sequences starting with copper

      i.e. (Cu/Sb)j=1 (a), (Cu/Sb)j=2 (b) and (Cu/Sb)j=3 (c).

      Cu In

      11 9

      11 9

      (402)

      (002)

      (402)

      (002)

      (511)

      (313)

      (511)

      (313)

      (f)

      Intensity a.u.

      Intensity a.u.

      (e)

      (d)

      (c)

      (b)

      (a)

      10 20 30 40 50 60

      Bragg angle 2(°)

      Figure 2. XRD patterns of (Cu/In)j=1(a), (In /Cu)j=1 (b), (Cu/ In)j=2(c), (In /Cu)j=2 (d), (Cu/ In)j=3(e)

      and ( In /Cu)j=3 (f) annealed in vacuum at 200°C for 1 hour

      The patterns of (Cu/In)j=1,2,3 and (In/Cu)j=1,2,3 show that all peaks were identified at monoclinic Cu11In9 phase (Figure 2), and the order of deposition of materials (Cu and In or In and Cu) has no effect on the crystallinity of the samples.

      Figure 3 and Figure 4 show the results of our XRD measurements after annealing in air atmosphere at 400°C for different times of the intermetallic multilayer systems (Cu/M)j=1,2,3 and (M/Cu)j=1,2,3 (M=Sb or In). It can be seen that the structural properties of the (Cu/In) annealed system are very similar for all the sequences (Figure 3), whereas the structural properties of annealed intermetallic

      multilayer system Cu/Sb strongly depend on the sequence (Figure 4). For the (Cu/In) system, the patterns of (Cu/In)j=1,2,3 and (In/Cu)j=1,2,3 show In2O3 as main crystalline phase with a preferential orientation (222) in addition to CuO and a small amount of metallic copper for all annealed multilayer systems excepted (Cu/In)j=1 (a) and (Cu/In)j=2 (b) we note non existence of metallic copper. For the (Cu/Sb) system (Figure 4), the patterns of (Cu/Sb)j=1 (a), (Cu/Sb)j=2 (b), (Cu/Sb)j=3

      1. and (Sb/Cu)j=3 (f) annealed in air atmosphere, show two phases, -Sb2O4 and CuO, and it is clear

         that no crystalline phases of copper or antimony metal were detected by X-ray diffraction, indicating a total oxidation of the intermetallic multilayer systems. For the rest of the sequences

         i.e. (Sb/Cu)j=1 (d) and (Sb/Cu)j=2 (e) annealed in air atmosphere, two phases Sb2O3 and CuO were

         detected with a dominant peak corresponding to the Sb2O3 phase with a preferential orientation (222). We can note also the presenc of antimony phase

         only for (Sb/Cu)j=1 (d) after annealing in air atmosphere at 400°C for 3h.

         that optical transmission of the sequences starting with indium, i.e. (In/Cu)j=1 (b), (In/Cu)j=2 and (In/Cu)j=3 (f) in particular the sequence(Cu/In)j=2 (c) when the optical transmission reaches 60% .

         2 3

         2 3

         (222)

         (222)

         In O

         (-111)

         (111)

         (332)

         (111)

         (-111)

         (111)

         (332)

         (111)

         CuO Cu

         2

         2

         (112)

         ( 004 )

         (112)

         ( 004 )

         (-111)

         (-111)

         (111)

         (111)

         Sb Sb O4 Sb O

         (211)

         (211)

         2 3

         (440)

         (440)

         (622)

         (622)

         CuO (f)

         (111)

         (111)

         (112)

         (112)

         (222)

         (222)

         (f)

         (003)

         (003)

         (-111)

         (-111)

         (111)

         (111)

         (e)

         (332)

         (332)

         (111)

         (111)

         (111)

         (111)

         (-202)

         (-202)

         (e)

         Intensity a.u.

         Intensity a.u.

         (222)

         (222)

         Intensity a.u.

         Intensity a.u.

         (112)

         (112)

         (400)

         (400)

         (444)

         (444)

      2. (d)

         (211)

         (211)

         (-111)

         (-111)

         (111)

         (111)

         (111)

         (111)

         (440)

         (440)

         (622)

         (622)

         ( 011)

         ( 011)

         (111)

         (111)

         ( 004 )

         (113)

         (-111)

         ( 020 )

         (111)

         ( 004 )

         (113)

         (-111)

         ( 020 )

         (111)

         ( 024 )

         ( 221 )

         ( 222 )

         ( 310 )

         ( 024 )

         ( 221 )

         ( 222 )

         ( 310 )

         (c)

         (b)

         (a)

         10 20 30 40 50 60 70

         Bragg angle 2(°)

         Figure 3. XRD patterns of (Cu/In)j=1 (a), (Cu/ In)j=2 (b), (Cu/ In)j=3 (c), (In /Cu)j=1 (d), (In /Cu)j=2 (e)

         and (In /Cu)j=3 (f) annealed in air atmosphere at 400°C

   2. Optical properties

Figure 5-8 show the spectral distribution both of the transmission T and the reflection R at normal incidence for the annealed intermetallic multilayer systems (Cu/M)j=1,2,3 and (M/Cu)j=1,2,3 (M=Sb or In) in air atmosphere at 400°C.

Optical transmission of Cu/Sb system was about 60% except the samples (Sb/Cu)j=1 (b) and (Sb/Cu)j=2 (d) annealed in air atmosphere, when antimony films are deposited firstly (Figure 5). It is clear from Figure 7, that after annealing in air atmosphere at 400°C the optical transmission of the sequences starting with copper, i.e. (Cu/In)j=1 (a), (Cu/In)j=2 (c) and (Cu/In)j=3 (e) is most important

(c)

(b)

(a)

10 20 30 40 50 60

Bragg angle 2(°)

Figure 4. XRD patterns of (Cu/Sb)j=1 (a), (Cu/ Sb)j=2 (b), (Cu/ Sb)j=3 (c), (Sb /Cu)j=1 (d), (Sb /Cu)j=2 (e)

and (Sb /Cu)j=3 (f) annealed in air atmosphere at 400°C

The optical absorption coefficients were evaluated from the transmission and reflection data taken at 300K using the formula [21];

ln

ln

1 1 R 2

d T

Where is the absorption coefficient in cm-1, d is the thickness of the film, T and R are the transmission and reflectance, respectively. Fig 9 and Fig 10 show the absorption coefficients as a function of the photon energy. It can be seen that all the films have relatively high absorption

coefficient (104-105 cm-1) in the visible range and near-IR spectral range.

(a)

(b)

(c)

(d)

(e)

(f)

(a)

(b)

(c)

(d)

(e)

(f)

100

Transmission (%)

Transmission (%)

80

60

40

20

Where A is a constant, h is the Plank constant and n is an exponent that depends on the type of transition. For direct allowed transition n=1/2 and for indirect allowed transition n=2.

The value of the direct and indirect energy gap for films are obtained by extrapolating the linear regions of the (h)2 and (h)1/2 versus h curve to the horizontal photon energy axis.

(a)

(b)

(c)

(d)

(e)

(f)

(a)

(b)

(c)

(d)

(e)

(f)

100

80

Transmission (%)

Transmission (%)

60

0 40

300 600 900 1200 1500 1800

Wavelength (nm)

20

Figure 5. Optical transmission of (Cu/Sb)j=1 (a), (Sb/Cu)j=1 (b), (Cu/Sb) j=2 (c), (Sb/Cu)j=2 (d), (Cu/Sb)j=3 (e), and (Sb/Cu)j=3 (f) annealed in air

atmosphere at 400°C

On other hand, it is clear that the absorption coefficient, , decreases as the number of couples Cu/Sb (or Sb/Cu) increases for Cu/Sb system. The absorption coefficient is related to the energy gap Eg according to the equation [22]:

h Ah Eg n

0

300 600 900 1200 1500 1800

Wavelength (nm)

Figure 7. Optical transmission of (Cu/In)j=1 (a), (In/Cu)j=1 (b), (Cu/In)j=2 (c), (In/Cu)j=2 (d),(Cu/In)j=3 (e), and (In/Cu)j=3 (f) annealed in air

atmosphere at 400°C

For the (Cu/Sb) system annealed in air atmosphere, two direct allowed transitions, Eg1 and Eg2, were

(a)

(b)

(c)

(d)

(e)

(f)

(a)

(b)

(c)

(d)

(e)

(f)

100

80

founded for each sample which corresponds,

Reflexion (%)

Reflexion (%)

respectively, to the band gap of copper oxide and antimony oxide phases. Indeed it has been shown [23-26] that CuO present a band gap of 1.3-2.1eV and the band gap of -Sb2O4 is in general higher than the band gap of Sb2O3 [27]. We note also, for the (Cu/In) system annealed in air atmosphere , the

60 presence of two direct band gaps, Eg1

and Eg2, for

40

20

0

300 600 900 1200 1500 1800

Wavelength (nm)

Figure 6. Optical reflection of (Cu/Sb)j=1 (a), (Sb/Cu)j=1 (b), (Cu/Sb)j=2 (c), (Sb/Cu)j=2 (d),(Cu/Sb)j=3 (e),

and (Sb/Cu)j=3 (f) annealedin air

atmosphere at 400°C

each sample, associated respectively, to the band gap of CuO [23-26] and In2O3 [28] phases.

Optical parameters determined for CuSbO and CuInO systems were summarized in the table 1 and table 2.

For the (Cu/Sb) system annealed in air atmosphere, table 1 show a decrease of the values of the direct energy with the increase of the number of couples. Its similar for (Cu/In) system annealed in air atmosphere (table 2) ; the increase of the number of couples has contributed in the decrease of the values of the direct energy.

100

80

Reflexion (%)

Reflexion (%)

60

(a)

(b)

(c)

(d)

106

(a)

(b)

(c)

(d)

(e)

(f)

(a)

(b)

(c)

(d)

(e)

(f)

-1

-1

Absorption coefficient (cm )

Absorption coefficient (cm )

105

40

104

20

0

300 600 900 1200 1500 1800

Wavelength (nm)

103

1,2 1,4 1,6 1,8 2,0 2,2 2,4

Photon energy h (eV)

Figure 8. Optical reflection of (Cu/In)j=1 (a),(In/Cu)j=1 (b), (Cu/In)j=2 (c), (In/Cu)j=2 (d), (Cu/In)j=3 (e),

and (In/Cu)j=3 (f) annealed in air

atmosphere at 400°C

Table 1. Optical parameters of (Cu/Sb)j=1(a), (Sb/Cu)j=1 (b), (Cu/Sb)j=2(c), (Sb/Cu)j=2 (d), (Cu/Sb)j=3 (e), and (Sb/Cu) j=3 (f) annealed in air atmosphere at 400°C.

Figure 9. Absorption coefficient spectra of (Cu/Sb)j=1 (a), (Sb/Cu)j=1 (b), (Cu/Sb)j=2 (c), (Sb/Cu)j=2 (d), (Cu/Sb)j=3 (e), and (Sb/Cu)j=3 (f) annealed

in air atmosphere at 400°C

(a)

(b)

(c)

(d)

(e)

(f)

(a)

(b)

(c)

(d)

(e)

(f)

106

Samples Thickness

nc Eg (eV)

Absorption coefficient (cm-1)

Absorption coefficient (cm-1)

105

(f) 701 2.11 1.49 2.20

104

Table 2. Optical parameters of (Cu/In)j=1 (a), (In/Cu)j=1 (b), (Cu/In)j=2 (c), (In/Cu)j=2 (d), (Cu/In)j=3 (e), and (In/Cu)j=3 (f) annealed in air atmosphere at 400°C.

103

1,2 1,4 1,6 1,8 2,0 2,2 2,4

Photon energy h (eV)

Samples Thickness nc (nm)

Eg (eV) Eg1 Eg2

Figure 10. Absorption coefficient spectra (Cu/In)j=1 (a),

(In/Cu)j=1 (b), (Cu/In)j=2 (c), (In/Cu)j=2 (d),

(Cu/In)j=3 (e), and (In/Cu)j=3 (f) annealed in air atmosphere at 400°

(a) 188

(b) 162

1.91

2.13

1.50 3.72

1.50 3.71

3.3 Electrical properties

For the configuration of the electrical

(f) 670 1.90 1.50 2.30

measurements, two gold electrodes were subsequently deposited as shown in figure 11.The (Cu/Sb) system annealed in air atmosphere at 400

°C are highly compensated except the samples (Cu/Sb)j=3 and (Sb/Cu)j=3 which exhibit p-type conductivity. For The (Cu/In) system annealed in air atmosphere at 400 °C, all the sequences starting

with indium, i.e. (In/Cu)j=1, (In /Cu)j=2 and (In/Cu)j=3 show p-type conductivity, and the sequences starting with copper show n-type conductivity with the exception of (Cu/In)j=3 which confirmed also a p-type conductivity. So, it is clear that only the films deprived of copper phase after annealing in air atmosphere (figure 3) show n-type conductivity.

of the films. So the Cu-M-O thin films (M=In or Sb) are interest candidates for technological applications such photovoltaic since antimony, indium and copper are more abundant in nature and their prices are lower than other metallic materials.

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   4. Conclusion

   CuSbO and CuInO systems have been obtained in two stages: sequential evaporation of metal precursors (Cu and Sb or In), and annealing in a vacuum (106 Torr) followed by an annealing in air atmosphere. The most important conclusion pointed out after this study is that the nature of the metal M (antimony or indium), the order of deposition of materials and the number of couples affect structural, optical and electrical properties.

   X-ray diffraction analysis confirmed that antimony improve the crystallinity of thin films compared in indium, and for Cu-Sb-O system, the best crystallinity is obtained in the case when copper films are deposited onto antimony films. For Cu- In-O system, the order of deposition of materials (Cu and In or In and Cu) has no effect on the crystallinity of the samples. The optical parameters (refractive index, n, absorption coefficient and optical band gap, Eg) of the films were determined by simple calculations using the transmission and reflection spectra. The Cu-M-O thin films have two direct band gap energies: between 1.481.50 eV and 2.20-2.95 eV for Cu-Sb-O system and in the ranges of 1.481.51 eV and 1.87-3.72 eV for Cu- In-O system. After annealing in air atmosphere, Cu-Sb-O system is highly compensated except the samples (Cu/Sb)j=3 and (Sb/Cu)j=3 which exhibit p- type conductivity and for Cu-In-O system some

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