Electrical and Photoresponse Properties of p ...

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Journal of Nanoelectronics and Optoelectronics Vol. 9, 584–589, 2014

Electrical and Photoresponse Properties of p-CuGaO2 -on-p-Si/Al Photodiode I. A. Elsayed1 2 , T. Fahmy1 3 , Farid El-Tantawy4 , W. A. Farooq5 , and F. Yakuphanoglu6 ∗

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Physics Department, College of Science and Humanitarian Studies, Salman bin Abdulaziz University, 11942, Saudi Arabia 2 Physics Department, Faculty of Science, Damietta University, 34517, Egypt 3 Polymer Research Group, Physics Department, Faculty of Science, Mansoura University, 34517, Egypt 4 Department of Physics, Faculty of Science, Suez Canal University, Ismailia, 41522, Egypt 5 Department of Physics and Astronomy, College of Science, King Saud University, Riyadh 11451, Saudi Arabia 6 Physics Department, Faculty of Science, Firat University, Elazig, 23169, Turkey We synthesized CuGaO2 film by sol gel method to prepare a photodiode of p-Si/CuGaO2 . The structural properties of the CuGaO2 film were investigated using atomic force microscope (AFM). The optical band gap of the CuGaO2 film was found to be 3.64 eV. The photoresponse characteristics indicate that the diode exhibits a photoconducting behavior. The photosensitivity value under 100 mW/cm2 was found to be of 6.80 × 102. The ideality factor and barrier height of the diode were obtained to be 2.02 ± 0.06 and 0.60 ± 0.4 eV, respectively. The interface states of the diode were determined by conductance technique and was found to be ∼1015 eV−1 cm−1 . It is evaluated that p-CuGaO2 -on-p-Si/Al photodiode can be used for optoelectronics applications.

Keywords: CuGaO2 Film, Photodiode, Sol–Gel. 1. INTRODUCTION Since the discovery of p-type transparent conducting oxides (TCOs) in 1997, the counterpart of n-type TCOs, it becomes an essential materials in a variety of usages and applications. A numerous efforts from the researchers all over the world have been done since then to understand and utilize it especially in solar energy and optoelectronics.1 The delafossites metal oxides are TCOs if the monovalent cations is Cu or Ag atom. The high transparency and electrical conductivity of delafossite metal oxide semiconductor make it a good candidate in the field of optoelectronics.2 Copper oxide based delafossite is an example where it has a wide optical band gap of about 3.1 eV.3 Being a transparent p-type semiconductor, all transparent p–n junctions are recently fabricated using these delafossite compounds.4 Delafossite CuGaO2 one of these promising materials and because of its large band gap (3.6 eV) it appears very attractive as a transparent conductive oxide. One of the superior advantage of these delafossite compounds is the capability to fabricate the junction using the low temperature, low cost, mass production easy technology sol–gel method.5 In present paper, we used current–voltage and impedance spectroscopy to present the optical and ∗

Author to whom correspondence should be addressed.

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J. Nanoelectron. Optoelectron. 2014, Vol. 9, No. 5

electrical properties of the CuGaO2 film and Al/pSi/CuGaO2 /Al Schottky diode under various illumination conditions.

2. EXPERIMENTAL PROCEDURE We synthesized the CuGaO2 film using sol–gel spin coating method. For this, the used precursors are the copper acetate monohydrate, gallium nitrate, ethanol amine and ethanol. The nominal moles of copper acetate monohydrate and gallium nitrate were dissolved in ethanol for 10 min and after 2 h, the stirring procedure was completed at room temperature. The gel of solutions was obtained to prepare the solid films. To prepare the diode, firstly, p-type silicon wafer (600 m in thickness and resistivity (5–10 -cm), with 111 orientation) was etched by HF for 1 min and was rinsed in distilled water using an ultrasonic bath for 10–15 min. The ohmic contact was prepared by evaporating of Al metal by thermal evaporation system. After evaporation of Al metal onto silicon, Al/p-Si was annealed at 570 C for 5 in N2 atmosphere. The film of CuGaO2 was spin coated onto the silicon wafer with a rotating speed of 2000 rpm for 20 s. The obtained film was dried on a hot plate at 150 C for 10 min and then, it was annealed at 500 C for 1 h. The top contact was prepared onto CuGaO2 film using Au metal. The diode contact area was determined to be 3.14 × 10−2 cm2 . The optical spectra 1555-130X/2014/9/584/006

doi:10.1166/jno.2014.1637

Electrical and Photoresponse Properties of p-CuGaO2 -on-p-Si/Al Photodiode

Elsayed et al.

of the films were measured using a Shimadzu 3600 UVVIS-NIR spectrophotometer. The electrical characterizations of the diode were performed using a Keithley 4200 semiconductor system and solar simulator.

(a) 0.3

h 1/n = Ah − Eg

0.1

0.0 400

600

700

800

900

1000

1200

1400

Wavelength (nm) (b) 100 80

60

40

20

0 200

(1)

where A is constant, Eg is the band gap and n is an exponent to determine type of optical transitions. The optical band gap was determined from Figure 3(c) and the optical band gap of CuGaO2 was found to be 3.64 ± 0.02 eV.

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3.1. Surface Morphology and Optical Properties of the CuGaO2 Film The AFM image (1 m × 1 m) of the CuGaO2 film is shown in Figure 1. As seen in Figure 1, the film is composed of nanoparticles with size range 30–80 nm. The film thickness was determined to be 100 nm. The optical band gap of the film was studied by measuring the optical absorbance and transmittance around the visible region. Figure 2 shows the optical absorption behavior of the film. As seen in Figure 2(a), the absorption is less than 0.2% in the wavelength range of 400–1200 nm. For transmittance, the percentage has a sharp increase from a few tenth at 300 nm to more than 50% and continues to increase to 84% at 800 nm, as shown in Figure 2(b). As clearly seen in Figure 2(b), the absorption drop about order of magnitude between the wavelengths range of 280–334 concurrently with the onset increase of the transmittance to more than 50% (Fig. 2(b)). This suggests that the optical band gap is found to be about 3.71 eV.6 To determine the type of optical band gap, we used the following relation,7

T%

3. RESULTS AND DISCUSSION

Abs. (a.u)

0.2

400

600

800

1000

Wavelength (nm) (c) 4

(αhν)2 (a.u)

3

2

1

0 2.0

2.5

3.0

3.5

4.0

hν (eV) Fig. 2. (a) Optical absorbance (b) optical transmittance (c) plot of ( h 2 versus h for the CuGaO2 film.

Fig. 1.

AFM image of CuGaO2 film.

J. Nanoelectron. Optoelectron. 9, 584–589, 2014

The obtained value of optical band gap is in agreement with those reported by Ueda,8 Mine9 and Alias.10 The preparation of the Schottky diode Al/ p-Si/CuGaO2 /Al procedure follows Gupta recipe.5 A p-CuGaO2 is coated onto p-Si/Al substrate so that, 585


The ideality factor of the diode was determined using the linear part of the I –V curve at forward bias region under dark and before the RS contribution. The ideality factor was found to be 2.02 ± 0.06. The ideality factor higher than unity and the non-ideal behavior can be owed to the inhomogeneity of Schottky barrier heights, the presence of interface states, and oxide layer on silicon wafer and series resistance.14 Because the series resistance effect cannot be ignored, the barrier height b can be calculated using the modified Norde equations15–17 I V q V ln F V = − (4) KT AA∗ T 2

10–3

10–4

10–5

I (A)

10–6

Dark 10 mW 30 mW 60 mW 80 mW 100 mW

10–8

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10–9

and

10–10 –6

–4

–2

0

2

4

6

V (V) Fig. 3. I –V characteristics of the diode under dark and illumination conditions.

a CuGaO2 /p-Si junction can be formed and Al can be deposited on top of the CuGaO2 film. Because of the work function m of aluminum, 4.08 eV, is lower than that of the p-type silicon, s > 415 eV, a barrier is formed in the semiconductor near the contact with the metal. 3.2. Current–Voltage Characteristics of the Al/p-CuGaO2/p-Si/Al Diode The voltage–current characteristics of the prepared Al/pCuGaO2 /p-Si/Al diode were investigated under different illumination conditions. Figure 3 shows I –V characteristics of the diode under dark and various lights intensities. As seen in Figure 3, the current of the diode increases exponentially at lower bias voltages. This suggests that the charge transport mechanism of the diode is controlled by thermionic emission mechanism since the data agree with the thermionic model.11 The I –V characteristics of the diode can be analyzed by the following relations qV − I RS (2) I = Io exp nKT where n is the ideality factor, q is the electronic charge, k is the Boltzmann’s constant, T is the absolute temperature, V is the applied voltage, Rs is the series resistance, and Io is the reverse saturation current. The Io value can be expressed by the following relation, q Io = AA∗ T 2 exp − b (3) KT where b is the barrier height, A∗ is the effective Richardson constant (32 A/cm2 K2 for p-type silicon) and A is the effective device area.12 13 586

Vo KT − (5) q Where f Vo is the minimum value of F V , Vo = 011 V is the corresponding voltage, = 3 is the integer greater than n, I V is the current obtained from I –V curves, A is the diode contact area and A∗ is the Richardson constant. The barrier height of the diode was found to be 0.60 ± 0.4 eV. As clearly seen in Figure 3, the forward bias current is nearly independent of the illumination conditions, whereas, the reverse current of the diode is increased with solar light illumination. The curve of photoresponse (Iphoto /Idark versus power was plotted and it is shown in Figure 4. It exhibits a linear relation of the relative current Iph /ID with the illumination power, Figure 4. This entitled the diode to be used as a device in optoelectronic applications. The transient photocurrent time response of the diode is shown in Figure 5 at turning on and off at 100 mW/cm2 illumination. The figure indicates charge carriers generation and trapping processes in the deep levels.18

b = F Vb +

3.3. Capacitance and Conductance–Voltage Characteristics The interruption of lattice periodicity at the interface region beside many other factor such as impurities and 800 700 600 500

Iph/Idark

10–7

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400 300 200 100 0 0

20

40

60

80

100

Power (mW/cm2) Fig. 4. Iph /Idark versus power plot for the Al/p-CuGaO2 /p-Si/Al diode.



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5 × 10–5

100 mW/cm2

Current (A)

4 × 10–5

3 × 10–5

2 × 10–5

1 × 10–5

0 0

5

10

15

20

25

30

35

40

45

Time (s)

formation of an interfacial layer lead to existence of localized electronic stated associated with the surface region.19 20 The admittance spectroscopy is used to probe the electrical properties of the diode where the capacitance and conductance as a function of the applied bias voltage in the frequency range of 10–1000 KHz of the Al/pCuGaO2 /p-Si/Al diode was studied, as shown in Figure 6. In the forward bias voltage region, the capacitance is clearly independent of the frequency or bias voltage while it shows a noticeable variation at the reverse bias voltage. Because of the ability of the charge carriers to follow the electric field at low frequency while it fail at higher frequency, there is a dispersion in the capacitance.21 22 At a low frequency the charges at the interface states can follow the field and this leads to a depletion capacitance that dominate the overall capacitance. At a higher frequency the charges fail to do so and their contribution to the capacitance can be neglected. At a high frequency, the capacitance is affected more by the series resistance rather than the depletion capacitance.20 23–25

CADJ =

G2m + 2 Cm2 Cm a2 + 2 Cm2

(6)

G2m + 2 Cm2 a a2 + 2 Cm2

(7)

GADJ =

Where CADJ and GADJ are series resistance compensated capacitance and conductance respectively. a = Gm − G2m + G2m Rs

(8)

The compensated conductance dependence on the applied voltage and frequency is shown in Figure 8. As seen in Figure 8, the curves peak in reverse bias voltage region and the intensity of the peaks increase with the frequency. The peak intensity–frequency relation suggests that the interface states can follow the ac signal.5 According to Hill and Coleman,13 the interface states density (Dit can be calculated by the following relation, Gmax / 2 (9) Dit = qA Gmax /Cox 2 + 1 − Cm /Cox 2 1.6×10–2

50 KHz 100 KHz 200 KHz 300 KHz 400 KHz 500 KHz 600 KHz 700 KHz 800 KHz 900 KHz 1 MHz

2

1

1.2×10–2

G (S)

C (nF)

3

50 KHz 100 KHz 200 KHz 300 KHz 400 KHz 500 KHz 600 KHz 700 KHz 800 KHz 900 KHz 1 MHz

8.0×10–3

4.0×10–3

0.0 0 –6

–4

–2

0

2

4

6

V (V) Fig. 6. C–V characteristics of Al/p-CuGaO2 /p-Si/Al diode at various frequencies.


–6

–4

–2

0

2

4

6

V (V) Fig. 7. G–V characteristics of the Al/p-CuGaO2 /p-Si/Al diode at various frequencies.

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Fig. 5. The transient photocurrent time response plot of the diode at 100 mW.

Although dispersion of the capacitance can give us information about the energy distribution and density of interface states, the interface state capacitance is buried in many other capacitance. For this reason, the parallel conductance was measured which depends solely on the steady-state capture and emission of the carriers by the interface states.26 Figure 7 shows the conductance–voltage relations at different frequencies. In forward bias region, the conductance is nearly independent of the applied bias voltage and frequency, while it shows strongly dependence on them in the reverse bias voltage region. When the Schottky diode is reversely biased and probed with a small ac signal a strong accumulation region is established at high frequency. In this case the series resistance Rs is the main reason for the energy loss. In order to correct for the series resistance for both C–V and G–V relations, we used the following two equations26 27


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4.0 50 KHz 100 KHz 200 KHz 300 KHz 400 KHz 500 KHz 600 KHz 700 KHz 800 KHz 900 KHz

–4

4×10

2×10–4

50 KHz 100 KHz 200 KHz 300KHz 400 KHz 500 KHz 600 KHz 700 KHz 800 KHz 900 KHz 1 MHz

3.5 3.0

Rs (kΩ)

Gadj (S)

6×10–4

2.5 2.0 1.5 1.0 0.5

0 –2.0

–1.5

–1.0

–0.5

0.0

0.5

1.0

1.5

2.0

V (V)

–6

Where Gmax the maximum peak value of Gm and Cox the capacitance of the oxidation layer. The calculated value of Dit is shown in Figure 9. The values of Dit are decreased with increasing frequency. 2 Gmax / Dit = (10) qA 0402 The behavior of the Rs with bias voltage at different frequencies is shown in Figure 10. The plot shows some peaks that their magnitude is decreased with increasing frequency and shift their positions. The Rs causes a dissipation in the diode. At a low frequency, the charge carriers can follow the ac field allowing for capture and emission of majority carrier by interface states. This process absorbs energy from the ac field. This dissipation in energy is manifested as increase in Rs . At the contrary, at a higher frequency, the charge carriers cannot follow the ac field signal which leads to decrease in dissipation. Therefore, the series resistance demolished at the strong accumulation region. Both the estimated value of the density of interface states, Dit and the calculated value of the series resistance, Rs confirm the presence of interface states. 5×1011

4×1011

Dit (eV–1cm–2)

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Fig. 8. Gadj –V characteristics of the Al/p-CuGaO2 /p-Si/Al diode at different frequencies.

3×1011

2×1011

1×1011

200 k

400 k

600 k

800 k

f (Hz) Fig. 9. Plot of Dit versus f of Al/p-CuGaO2 /p-Si/Al diode.

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0.0

1M

–4

–2

0

2

4

6

V (V) Fig. 10. Rs –V characteristics of the Al/p-CuGaO2 /p-Si/Al diode at different frequencies.

4. CONCLUSIONS We have synthesized and investigated the optical and electrical properties of a CuGaO2 film and a Al/p-CuGaO2 / p-Si/Al diode. The film has a nano-size structure and optical band gap of 3.6 eV. The diode has exhibited a highly photo-response in reverse bias voltage region. The conductance and capacitance measurements have confirmed the existence of interface states which dictate the diode behavior. Acknowledgment: The authors would like to thank Salaman Bin Abdulaziz University KSA and Deanship of Scientific Research for their supporting. The work is Supported by the Deanship of Scientific Research in Salman Bin Abdul-Aziz University, Saudi Arabia under Grant No. 973/01/2014.

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14. F. Yakuphanoglu, Y. Caglar, M. Caglar, and S. Ilican, Mater. Sci. Semicond. Process 13, 137 (2010). 15. H. J. Norde, Appl. Phys. 50, 5052 (1979). 16. F. J. Yakuphanoglu, Alloys Compd. 494, 451 (2010). 17. O. Madelung, M. Schulz, and H. Weiss, Semimagnetic Semiconductors, Landolt-Bornstein, New, Series, Group III, Vol. 17, Part B, Springer, Berlin (1982). 18. F. Yakuphanuglo, Solar Energy 85, 2518 (2011). 19. D. Michel, V. Roland, L. Willy, and C. Felix, Solid State Electronics 37, 433 (1994). 20. E. H. Nicollian and J. R. Brews, Metal Oxide Semiconductor (MOS) Physics and Technology, Wiley, New York (1982). 21. F. Yakuphanoglu and S. Okur, Microelectron. Eng. 87, 30 (2010).

22. D. T. Phan, R. K. Gupta, G. S. Chung, A. A. Al-Ghamdi, O. A. Al-Hartomy, F. El-Tantawy, and F. Yakuphanoglu, Sol. Energy 86, 2961 (2012). 23. S. M. Sze, Physics of Semiconductor Device, 2nd edn., John Wiley & Sons, New York (1981). 24. H. Tecimer, H. Uslu, Z. A. Alahmed, F. Yakuphano˘glu, and S. ¸ Altındal, Composites: Part B 57, 25 (2014). 25. S. ¸ Karata¸s and F. Yakuphano˘glu, Mater. Chem. Phys. 138, 72 (2013). 26. E. H. Nicollian and A. Goetzberger, Bell Syst. Technol. J. 46, 1055 (1967). 27. ˙I. Dökme, S. ¸ Altındal, T. Tunç, and ˙I. Uslu, Microelectron. Reliab. 50, 39 (2010).

Received: 5 October 2014. Accepted: 5 November 2014.

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