Al heterojunction diode by sol–gel spin technique

Journal of Alloys and Compounds 550 (2013) 129–132

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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Growth and characterization of Ag/n-ZnO/p-Si/Al heterojunction diode by sol–gel spin technique E.F. Keskenler a,⇑, M. Tomakin a, S. Dog˘an b, G. Turgut c, S. Aydın c, S. Duman d, B. Gürbulak d a

Department of Physics, Faculty of Arts and Sciences, Recep Tayyip Erdog˘an University, Rize 53100, Turkey Department of Electrical and Electronics Engineering, Faculty of Engineering and Architecture, Balikesir University, Balikesir 10145, Turkey c Department of Physics Education, Kazim Karabekir Education Faculty, Atatürk University, Erzurum 25240, Turkey d Department of Physics, Faculty of Science, Atatürk University, Erzurum 25240, Turkey b

a r t i c l e

i n f o

Article history: Received 11 August 2012 Received in revised form 24 September 2012 Accepted 26 September 2012 Available online 5 October 2012 Keywords: Ag/n-ZnO/p-Si/Al heterojunction Sol–gel Diode Ideality factor

a b s t r a c t Polycrystalline ZnO thin film was obtained on the p-Si for the heterojunction diode fabrication by sol–gel method. X-ray diffraction study showed that the texture of the film is hexagonal with a strong (0 0 2) preferred direction. Scanning electron microscope image of ZnO showed that the obtained ZnO thin films had more porous character. High purity vacuum evaporated silver (Ag) and aluminum (Al) metals were used to make Ohmic contacts to the n-ZnO/p-Si heterojunction structure. The electrical properties of Ag/n-ZnO/p-Si/Al diode were investigated by using current–voltage measurements. Ag/n-ZnO/p-Si/Al heterojunction diode showed a rectification behavior, and its ideality factor and barrier height values were found to be 2.03 and 0.71 eV by applying a thermionic emission theory, respectively. The values of series resistance from dV/d (ln I) versus I and H(I) versus I curves were found to be 42.1 and 198.3 X, respectively. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In transparent conducting oxide films, Zinc Oxide (ZnO) has gained substantial interest in the research community in part and widely studied in electronic and optical applications due to its excellent opto-electrical properties. ZnO is used in a wide range of optoelectronic applications such as light-emitting diodes (LEDs), flat display panels, solar cells, thin film photovoltaic cells, sensors and detectors [1–4]. It is one of the most promising candidate material for optoelectronic devices such as photodetectors and heterojunction diodes, due to its large exciton binding energy (60 meV) which could lead to lasing action based on exciton recombination even above room temperature [5] and suitable bandgap (3.37 eV) [6,7]. Ohmic and Schottky contacts are essential for ZnO-based diodes to obtain some electrical parameters, almost all of these devices mentioned above require high-quality ohmic and rectifying contacts to get maximum efficiency out of them. Fabricating a good quality rectifying contacts on n-type ZnO is more difficult compared to obtaining Ohmic contacts, due to the high donor existence at the surface region, caused by defects such as zinc interstitials (Zni) and oxygen vacancies (Vo) [8]. The chemical reactions between metal and semiconductor, the defects in the surface

⇑ Corresponding author. Tel.: +90 5376104849. E-mail address: [email protected] (E.F. Keskenler). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.09.131

region and the metal diffusion into the semiconductor are wellknown problems of preventing the formation of Schottky contacts. Özgür et al. had reported that a high work function has to be applied to the surface of a ZnO crystal in order to create Schottky barrier with undoped ZnO [5]. Although Gold (Au) has some serious problems at temperatures higher than 330 K [9,10], it has been widely used to form Schottky barriers to ZnO. Silver (Ag) has been used as a Schottky contact metal by some researchers [11–13] as well. Sheng et al. studied the Ag/ZnO Schottky diode and determined the flatband barrier height to be 0.89 and 0.92 eV by current–voltage and capacitance–voltage measurements, respectively. The ideality factor was also found to be 1.33 [13]. Since the use of heterostructure provides an advantage in the control of the electronic and optoelectronic properties of semiconductor devices [14,15], a number of significant studies, especially on photodiode properties of n-ZnO/p-Si heterojunctions, have been carried out on the fabrication of ZnO-based heterojunctions using materials, such as p-Si and p-AlGaN. ZnO/Si heterojunctions are particularly interesting due to their more cost effectiveness and flexibility for optoelectronic device fabrications [16]. The fabrication and electroluminescence of an n-ZnO nanorod/p-Si heterojunction have been reported by Sun et al. [17] and Chen et al. [18]. Recently, there are reports about high quality ZnO films grown by the well-known techniques such as metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), radio frequency magnetron sputtering, pulsed laser deposition (PLD),

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sol–gel method and etc. [6,19–25]. In this study, the sol–gel method was used to synthesize ZnO thin films on p-Si due to some advantages such as low cost, simplicity and versatility of its experimental procedure, large area production and homogeneity. To the best of our knowledge, there are only few reports [2,16] on ZnO heterojunction diodes. In this paper, we report the device performance of the Ag/n-ZnO/p-Si/Al diode with structural and optical properties of ZnO thin film. 2. Experimental ZnO thin films have been grown by sol–gel spin coating method onto p-type Si with resistivity of 0.01–0.03 X-cm and with [1 0 0] orientation and onto glass substrate. Zinc acetate dihydrate [Zn(CH3COO)2H2O] was used as a starting material. 2-Methoxyethanol (C3H8O2) and Monoethanolamine (C2H7NO, MEA) were used as solvent and stabilizer, respectively. The solution was prepared as 0.5 M and the molar ratios of metals to MEA were taken in the ratio of 1:1. The sol was stirred at 60 °C for 2 h to obtain a clear and homogenous solution by magnetic stirrer. Si and glass substrates were cleaned by a solvent clean procedure in acetone and methanol by using an ultrasonic cleaner, rinsed in de-ionized (DI) water and dried by nitrogen gas flow. Then RCA cleaning procedure was applied and rinsed in DI water. Finally it was dipped in HF, rinsed by DI water and dried with nitrogen. The precursor solution was dropped on polished surface of p-Si substrate and spin-coated at a speed of 3000 rpm for 25 s. In order to evaporate the solvent, the as-coated film was sintered at 250 °C for 10 min. This procedure was repeated for 10 times and finally it was annealed in air and nitrogen mixed ambient at 500 °C for 30 min. to ensure the reducing the crystal defects and formation of homogeneity. The thickness of the film was determined using cross-section SEM images and was found to be 417 nm. All vapor deposition processes were carried out in a thermal vacuum coating unit (PVD-HANDY/2S-TE, Vaksis Company) at about 5  105 Torr. Vacuum evaporated Ag and aluminum (Al) metals with thickness of 100 nm were used for ohmic contacts to the n-ZnO layer and p-Si substrates, respectively. The morphology of the ZnO film was obtained from scanning electron microscope (SEM) image and determined with Nova Nanosem 430. X-ray diffraction (XRD) patterns were taken using a Rigaku D/Max-IIIC diffractometer. The diffractometer reflections were investigated at room temperature and the values of 2h were altered between 20o and 80o. The incident wavelength was 1.5406 Å. The current–voltage (I–V), characteristics of the Ag/ZnO/p-Si/Al diode were investigated with 4200 Keithley semiconductor characterization system.

3. Results and discussion The XRD result of the ZnO film on p-type Si substrate is shown in Fig. 1. As can be seen in Fig. 1, there appears ZnO (0 0 2) peak at about 2h = 33.8o indicating that the hexagonal wurtzite structure is predominant in ZnO film. The low intensity peak located at 2h = 61.20° is attributed to (1 0 3) plane of hexagonal ZnO. Periasamy et al. [26] found almost similar result for ZnO thin film prepared on p-type Si substrate by vacuum coating technique. The crystallite

sizes of the ZnO thin film were determined by the means of Xray line-broadening method using the Scherrer equation:

D¼

ð1Þ

where D is grain size, b is broadening of diffraction line measured at half of its maximum intensity in radians, h is Bragg angle and k is wavelength of X-rays. The crystallite size of samples was calculated from the (0 0 2) and (1 0 3) peak. The mean size of the ZnO crystallite is 31.4 nm (±0.1 nm). The lattice parameters a and c of unit cell were calculated according to the following relation, 2

1

4 h þ hk þ k ¼ 2 3 a2 d

2

!

2

þ

l c2

ð2Þ

where d is interplanar spacing of atomic planes and (hkl) is Miller indices. The lattice constants determined from the 2h values of (0 0 2) and (1 0 3) diffraction peaks were c = 5.306 Å and a = 3.353 Å (±0.005 Å). The grown ZnO thin film has larger a and c lattice constants than those of standard data of Joint Committee on Powder Diffraction Standards (c = 5.20661 Å and a = 3.24982 Å) [27]. SEM micrograph of ZnO thin film was also shown in the inset to Fig. 1. SEM images of ZnO showed that the grown films had more porous character which may affect the electronic characteristic of devices fabricated on the surface. Ismail et al. [28] reported that the granular and polycrystalline structure of ZnO had played an important role in the electrical properties. The optical transmittance spectra of ZnO film grown onto the glass substrate as a function of wavelength are shown in Fig. 2. The grown ZnO thin film had the maximum transmittance value of 70%. The band gap energy, Eg, was determined by using the dependence of absorption coefficient (a) on the photon energy,

a ¼ Aðhm  Eg Þ1=2

ð3Þ

where A is a constant. Eg is obtained by extrapolating the linear part of (ahm)2 versus (hm). The variation of (ahm)2 as a function of hm is shown in inset to Fig. 2. The band gap of the sample has been estimated to be 3.27 eV. This value is found to be in good agreement with those reported by Li et al. [29]. They found that the optical band gap of ZnO:Cd film prepared with sol–gel spin coating method is gradually decreased from 3.27 to 3.16 eV with Cd doping concentration increases. In addition, Li et al. [30] studied the effects of rapid thermal annealing in different ambient on the structural, electrical and optical properties of the sol–gel derived ZnO thin

84

8000

Grain size D = 31.35 nm Lattice parameter c = 5.306 Å a = 3.353 Å

10

2000

14

3000

-1 2

(103)

4000

8 6

2

5000

E g = 3.27 eV

60

4

(αhν) (eVm ) ×10

6000

72

Transmittance (%)

(002)

7000

Intensity (arb.unit)

0:94k b cos h

48 36 24 12

1000 0 25

30

35

40

45

50

55

60

65

2θ (degree) Fig. 1. X-ray diffraction spectra of the ZnO film on p-Si substrate and inset figure shows the SEM image of the porous ZnO film.

0 35 0

2 0 3.0 3.1 3.2 3.3 3.4 h ν (eV)

400

45 0

500

55 0

60 0

65 0

70 0

75 0

80 0

Wavelength (nm) Fig. 2. Optical transmittance spectra of ZnO and the inset graph is the plots of (aht)2 versus photon energy (ht) for the ZnO film.

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  I ¼ I0 eqV=nkT  1

1.3

1.0

0.09

0.9

0.06

dV/d(ln I)

0.12

1.1

0.8

ð4Þ

0.7

where I0 is the saturation current, q is the electronic charge, V is the applied voltage, n is the ideality factor, k is Boltzmann’s constant and T is the temperature in K. The saturation current I0 is defined by,

0.6

I0 ¼ A T 2 eq/b0 =kT

0.15

H(I), RS = 198.3 Ω; φb0= 0.81 eV

1.2

H(I)

films and they found that the sharp absorption edges occurred at the wavelength of about 379 nm, which corresponding to a band gap of about 3.27 eV. Also, in the previous reports, the value of 3.27 eV for the band gap was also determined to be the same as in this study by other researchers [31,32]. Fig. 3 shows the semi–logarithmic forward bias I–V characteristic of Ag/ZnO/p-Si/Al structure. It was observed from Fig. 3 that the I–V characteristic of the sample shows rectifying behavior. The thermionic emission current–voltage relation of a p-n heterojunction is usually written as a function of the applied voltage (V) as [33],

0.03 dV/(dlnI), RS = 42.1 Ω; n = 1 0.00 0

3

6

9

12 15 -4 I (A)×10

18

21

24

ð5Þ

Fig. 4. The values of series resistance and ideality factor from dV/d (ln I) versus I curve and the barrier height and series resistance from H(I) versus I plot.

where A is the theoretical Richardson constant (32 A/cm K for ZnO), A is the diode area and Ubo is the zero bias barrier height. The ideality factor, n, can be calculated from the slope of the straight line region of the forward bias ln (I)–V plot and can be written as,

(Ubo = 0.79 eV) [16]. One of reasons of the deterioration in the ideal diode rectifying behavior is the series resistance effect. The diode current–voltage relation with series resistance (RS) is given by [33],

2

⁄

2

q dV n¼ kT dðln IÞ

ð6Þ

I0 can be determined by an extrapolation of the forward bias ln(I)–V curve to V = 0. If n varies between 1 and 2, the tunneling current mechanism is dominant. If n = 2, the generationrecombination current mechanism is the dominant. If n > 2, it means the leakage current mechanism is dominant [34]. Ubo is calculated by the following formula,

/bo ¼

kT AA T 2 ln q I0

! ð7Þ

The experimental values of n, I0 and Ubo were given in the inset of Fig. 3. The ideality factor was found to be 2.03. Ideality factor being greater than unity indicates that the diode exhibits a nonideal behavior. The obtained ideality factor value in this study is lower than those reported by Mansour and Yakuphanoglu [16] and Majumdar et al. [35] for ZnO/Si heterojunctions. The barrier height being calculated as 0.71 eV is quite close to the difference between the work functions of Si (4.97 eV) and ZnO (4.25 eV) [36] and lower than that of calculated by Mansour and Yakuphanoglu

  I ¼ I0 eqðVIRS Þ=nkT  1

ð8Þ

Ubo and other main electrical parameters, such as n and Rs can also be obtained using a method developed by Cheung and Cheung [33]. Cheung and Cheung’s functions can be used in order to analyze the series resistance effect. Eq. (8) can be differentiated as follows, dV nkT ¼ IRS þ dðln IÞ q

ð9Þ

The series resistance and ideality factor are determined by using the slope and by extrapolating the linear part of dV/d (ln I) versus I curve, respectively. If n = 1, the Eq. (8) can be written as,

HðIÞ ¼ IRS þ

/b0 q

ð10Þ

where,

HðIÞ ¼ V 

  kT I ln q AA T 2

ð11Þ

The series resistance Rs can be calculated from the slope of the straight line region of H(I)–I curve which provides a second determination of Rs. We can also find out the barrier height Ubo from the -2

10

10 10

-2

-3

n = 2.03 Φb0 = 0.71 eV

-4

I0 = 3.62 μA

Region III -3

10 Current (A)

10

10 10 10 10 10

-5

2.0 Current (mA)

Current (A)

2.4

10

-6

-7

-8

1.6 1.2

Region I -5

10

0.8 0.4 0.0

-9

-1.0 -0.5 0.0 0.5 1.0 Voltage (V)

-10

-1.2

Region II

-4

10

-0.9

-0.6

-0.3 0.0 Voltage (V)

0.3

0.6

0.9

1.2

Fig. 3. Current–voltage characteristics of Ag/n-ZnO/p-Si/Al heterojunction diode.

-6

10

0.01

0.1 Voltage (V)

1

Fig. 5. The I–V characteristic in a double logarithmic plot for the Ag/ZnO/p-Si/Al structure.

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intercept of the H(I) axis. Fig. 4 shows dV/d (ln I) versus I plots and H(I) versus I for the Ag/ZnO/p-Si/Al structure. The series resistance and ideality factor from dV/d (ln I) versus I curve were found to be 42.1 X and 1.0, respectively. The barrier height and series resistance for H(I) versus I plot were obtained to be 0.81 eV and 198.3 X, respectively. The I–V characteristic in a double logarithmic plot for the Ag/ZnO/p-Si/Al structure has been given in Fig. 5. As can be seen, the forward bias characteristic of the Ag/ZnO/ p-Si/Al structure has three distinct linear regions with different slopes. These results indicate that there are different conduction mechanisms for the Ag/ZnO/p-Si/Al structure. The region I shows an ohmic behavior with slopes about unity (1.0). In region II, the slope of I–V characteristic in the double logarithmic scale is increased. It has the value larger than two (2.8) which suggests the possibility of the space charge limited current mechanism (SCLC) [37,38]. The slope of the region III is found to be about 1.6. At high voltages the slope tends to decrease because the device approaches the ‘trap-filled’ limit when the injection level is high whose dependence is the same as in the trap-free space-charge-limited current [39,40].

4. Conclusion We report fabrication of an Ag/n-ZnO/p-Si/Al heterojunction diode by sol–gel spin coating technique. The film exhibits hexagonal wurtzite structure with a strong (0 0 2) preferred direction perpendicular to the substrate. In addition, the low intensity (1 0 3) peak located at 2h = 61.20° was observed in the XRD pattern. The crystallite size of ZnO is found 31.4 nm. The c and a lattice constants of the sample are 5.306 and 3.353 Å, respectively. The band gap of the ZnO film on glass substrate was found to be 3.27 eV. The barrier height and ideality factor values for the diode were found to be 0.71 and 2.03 eV, respectively. The values of series resistance from dV/d (ln I) versus I and H(I) versus I curves were found to be 42.1 and 198.3 X, respectively. Ideality factor of Ag/n-ZnO/p-Si/Al heterojunction diode is greater than unity, which indicates that thermionic emission is not the only conduction mechanism for the current flow.

Acknowledgements This work was supported by the Atatürk University Research Fund, Project no 2011/98. One of the authors (E.F. Keskenler) would like to thank to Mustafa Furkan Keskenler for technical support.

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