Si heterojunction photodetector

Journal Pre-proof Preparation of visible-enhanced PbI2 /MgO/ Si heterojunction photodetector Raid A. Ismail, Ali M. Mousa, Suaad S. Shaker

PII:

S0030-4026(19)31483-4

DOI:

https://doi.org/10.1016/j.ijleo.2019.163585

Reference:

IJLEO 163585

To appear in:

Optik

Received Date:

21 June 2019

Revised Date:

27 September 2019

Accepted Date:

11 October 2019

Please cite this article as: Ismail RA, Mousa AM, Shaker SS, Preparation of visible-enhanced PbI2 /MgO/ Si heterojunction photodetector, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163585

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Preparation of visible-enhanced PbI2/MgO/ Si heterojunction photodetector Raid A.Ismail, Ali M.Mousa, Suaad S.Shaker

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Department of Applied Science, University of Technology, Baghdad, Iraq

Abstract

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Fabrication and characterization of p-PbI2/MgO/n-Si photodetectors by pulsed laser deposition PLD under different deposition temperatures Ts were demonstrated for the first time. Structural,

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optical and morphological properties of nanostructured MgO and PbI2/MgO films were investigated using x-ray diffraction XRD, UV-Vis absorption and scanning electron microscope

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SEM. The XRD results revealed a single crystalline MgO with cubic structure along the (200) plane, while the PbI2 film deposited on MgO was a single crystalline with hexagonal phase along

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(001) plane. The optical energy gaps of PbI2 films deposited on MgO film were found to be within (2.7-2.5) eV. The current-voltage and capacitance-voltage properties of PbI2/MgO/Si photodetector were investigated. The best responsivity of p-PbI2/MgO/n-Si photodetectors was ~

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0.88A/W at 410nm when the photodetector was prepared at Ts = 45°C. Keywords: PbI2; MgO; PLD; Heterojunction; Silicon; photodetector; Deposition temperature

1. Introduction

Lead iodide has a hexagonal structure with optical band gap around 2.2eV at room temperature

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[1, 2]. Lead iodide has been used in numerous applications, for example, light emitting diodes, perovskite solar cells, photodetectors, X-ray detectors, photoconductors, biological labeling and diagnostics, , active matrix flat panel imagers, and γ-ray detectors [3, 4]. PbI2 films are usually prepared thermal evaporation, chemical methods, pulsed laser deposition, atomic layer deposition, and electron beam evaporation [5-8]. The wider optical energy gap MgO (7.3 eV at room temperature) is a non-toxic, high specific surface reactivity and cubic crystal structure with Fm-3m space group and transparent at the visible light. MgO films have been used in many

applications [9-11] and prepared by many methods [12,13]. PLD technique is one such technique is simple, cost-effectiveness, large deposited area, good control on the film thickness, and high purity film [14-18]. Herein, we report the preparation and characterization of new visibleenhanced PbI2/MgO/Si heterojunction photodetector by PLD technique. The MgO film acts as buffer layer between nanostructured PbI2 and silicon substrate. 2. Experimental work MgO thin film was deposited on stringently cleaned quartz and silicon substrates at 100°C by

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lab-assembled PLD system [14]. N-type silicon substrates of (111) orientation, mirror-like, 1 cm2 area, and (3-5) Ωcm electrical resistivity were used. These silicon substrates were cleaned and

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etched with diluted hydrofluoric acid to remove the surface oxide. The MgO pellet was irradiated with7 ns, 532nm wavelength Nd: YAG laser at fluence level of 4.3 J/cm2. The film thickness was measured using laser interferometer system. In the second step, PbI2 thin film was deposited

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on MgO film by PLD using a laser fluence of 3.9 J/cm2 at temperatures (25, 45, 65, and 100)°C. X-ray diffractometer (XRD-6000, Shimadzu) was used to investigate the film microstructure,

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while its surface morphology was investigated by scanning electron microscope SEM (T-scan Vega III Czech). A double beam UV-Vis spectrophotometer (Metratech SP 8001) was employed

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to measure the optical absorption of MgO film and PbI2 film deposited on the MgO. Raman scattering experiments were performed using system (LabRam HR 800. The ohmic contacts of the fabricated PbI2/MgO/Si photodetectors were established by depositing In on the PbI2 film

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and Al on the back side of silicon substrate as shown in Fig.1. Current-voltage characteristics of

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the photodetector under dark and illumination conditions were measured at room temperature.

Fig. 1 Schematic diagram of PbI2/MgO/Si photodetector under reverse bias

The C-V characteristics of the heterojunction were measured using LCR meter at 100 kHz. The responsively of PbI2/MgO/Si photodetector was measured using calibrated Jobin Yvon monochromator. 3. Results and discussion Fig 2 illustrates the XRD patterns of MgO and PbI2 /MgO thin films prepared at different

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deposition temperatures. The reflection peak of MgO thin film was observed at 2θ = 43.74° corresponding to plane (200) plane which belong to crystalline MgO of cubic phase according to

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JCPDS # 78-0430, with lattice constant of 0.418nm. Two reflection peaks of the PbI2 film were observed at 12.7° corresponding to (001) hexagonal phase according to JCPD # 07-0235[19] and a second peak related to MgO thin film [14]. The intensity and crystallinity quality of PbI2 thin

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film were improved at higher substrate’s temperature. The PbI2 lattice constant was 0.45nm and the average crystallite size (calculated for (001) plane) were found to be 43, 48, 50, and 66 nm

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corresponding to substrate temperature of 25, 45, 65 and 100°C, respectively, with the average

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particle size of 16 nm.

450

300 250 200

Ts= 100°C

Ts= 65°C

Ts=45°C

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Intensity (a.u)

350

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400 PbI2(001)

150

Ts= 25°C

100

MgO (200)

50

MgO thin film

0 10

15

20

25

30

35 2θ (degree)

40

45

50

55

60

Fig .2.XRD patterns MgO and PbI2 /MgO thin films

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Fig.3 shows the SEM image of MgO film. The film is compact, smooth, uniform and cracks free

Fig.3. SEM image of nanostructured MgO film

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Some irregular micro-sized particles were noticed on the surface due to laser splashing. Fig.4- (ad) illustrates the SEM images of PbI2 film grown on MgO film at different substrate

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temperatures. The deposited films were dense and have different islands.

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Fig.4. SEM images PbI2 film deposited on MgO film at different substrate temperatures (a) 25°C

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(b)45°C (d) 65°C and (e) 100 °C

The film deposited at 65°C showed formation of some nanorods. At higher temperature than 45°C, the film is converted from 3D to 3D [19]; see Fig 4(c, d). The SEM image of cross-

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sectional view of PbI2/MgO/Si prepared at 65°C is depicted in Fig.5 which shows 2D and 3D grown modes.

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PbI2 film

MgO film Silicon substrate

Fig. 5.SEM image of cross- sectional view of PbI2/MgO/Si deposited at 65°C

The optical absorption spectrum of PbI2/MgO/quartz thin films is shown in Fig.6-a, with maximum absorption observed at 45°C. For all samples, the absorption decreases sharply after 305nm due to the absorption edge of MgO film, then decreases slowly after 520nm due to absorption edge of pbI2 film. The film absorption decreases as substrate temperature increases.

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Fig.6-b shows the variation of (αhν)2 with photon energy hν of PbI2 films deposited on MgO film at different deposition temperatures. The direct transition films have energy gaps decreased from

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2.75 to 2.5eV as the deposition temperature increased from 25 to 100°C due to the increased grain size and the thickness [20,21]. Fig.7 shows linearly increased film thickness with

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deposition temperature as a result of increasing the deposition rate.

Absorbance

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0.6

Mg Qua

0.7

PbI2

0.8

25˚C 45˚C

0.5 0.4 0.3 0.2 0.1

(a)

0 220 320 420 520 620 720 820 920 1020 Wavelength (nm)

25ᴼC 45ᴼC 65ᴼC 100ᴼC

2 1.5 1

(b)

0.5 0

2

2.5

3

3.5

4

4.5

5

5.5

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Photon energy (eV)

6

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1.5

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(α hυ) 2 (eV/cm) 2 ×1011

2.5

Fig.6. (a) Optical absorption of PbI2 deposited on MgO film at various substrate temperatures (b)

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(αhν)2 versus photon energy 320

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310

290 280 270 260 250

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240

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Thickness(nm)

300

0

25

50

75

100

125

Substrate temperature (°C)

Fig.7. Thickness of PbI2 film deposited on MgO film versus substrate temperature

The forward and reverse dark I-V properties of p-PbI2/MgO/n-Si photodetectors prepared at different deposition temperatures are presented in Fig.8. All heterojunctions showed rectification factor depended on the substrate temperature. In these heterojunctions, the forward currents transport mechanism is attributed to the diffusion–tunneling (two regions shown in Fig.8) except the heterojunction prepared at 100°C where the forward current increases exponentially with bias voltage [22,23]. The largest rectification factor (~22.3) was noticed for heterojunction deposited at 45°C, due to the structural defects [24-26]. The saturation current and ideality factor of the

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heterojunctions were found to be depended on the deposition temperature; see Table 1. The optimum ideality factor value (2.2) obtained for heterojunction deposited at 45°C indicates an

100

-6

-4.5

-3

0 -1.5 0 -100 -200

1.5

3

4.5

6

7.5

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-7.5

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200

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Current (µA)

300

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improved PbI2/Si heterojunction properties by the MgO buffer layer [27,28].

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-300 Bais voltage (V)

Fig.8. Dark I-V characteristics of PbI2/MgO /Si heterojunction photodiode under forward and reverse bias prepared at different substrate temperatures

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Table 1: Saturation currents and ideality factor of heterojunction as function of substrate temperature Substrate temperature (°C ) 25

Saturation current (μA)

Ideality factor

0.2

3

45

0.15

2.2

65

0.4

3.8

100

0.67

4

The variation of C-2 with reverse bias voltage is given in Fig.9. The linear relationship indicates abrupt heterojunctions [29, 30]. Extrapolating the linear part of C-2 gives the value of Vbi which was found in the range of (0.7 -1.5) V depending on the deposition temperature. Fig.10. reveals the illuminated I-V curves of PbI2/MgO/Si photodetector at white light intensity of (28 mW/cm2). After illumination of the photodetector, the photocurrent increased due to the e-h pairs

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generation in the heterojunction depletion region.

Fig.9. C-2 versus reveres volatge plot of p-PbI2/MgO/n-Si heterojunction prepared at different substrate temperatures

At substrate temperature of 45°C, the photocurrent rises due to the large depletion width, high absorption coefficient, good film stoichiometric, and low density of structural defects [31]. Further increase in the deposition temperature decreases the photocurrent due to the

recombination and trapping centers [32]. The on/off ratio of PbI2/MgO/Si photodetector at reverse bias voltage of 7.5V was calculated. The maximum value was obtained for the photodetector prepared at 45°C. Enhancing the morphology and increasing the absorption coefficient could have been the main reasons behind the increase of the on/off ratio. Fig.11 shows a plot of responsivity of PbI2/MgO/Si heterojunction photodetectors prepared at different deposition temperatures.

Reverse voltage (V) -4

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0 -1 0 -1000

-2

-2000 -3000 25°C -4000 45°C -5000

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65°C

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-5

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-6

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-7

Phototcurrent (μA)

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-6000

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Fig.10. Illuminated I-V characteristic of PbI2/MgO/Si photodiode deposited at different substrate temperatures

1.2

25°C 45°C

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Responsivity (A/W)

1

65°C 100°C

0.8 0.6 0.4 0.2

0 375

475

575

675

Wavelength (nm)

775

875

Fig.11. Responsivity of PbI2/MgO /Si heterojunction photodetector

The photodetector deposited at 25°C exhibited three distinct response peaks located at 410, 660 and 860nm with responsivity values of 0.76, 0.48 and 0.59A/W, respectively. The peak at 410nm is related to the PbI2 film absorption edge and the peak at 660nm is ascribed to the deep level defect, while the third peak is related to the bulk silicon absorption edge. We have observed a

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small shift in response peak of the photodetector prepared at larger than 45°C was noticed arising from the decrease in the energy gap and increase in the particle size of PbI2 film. The maximum

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visible responsivity obtained was 0.88A/W at 410nm at preparation temperature of Ts = 45°C. The high responsivity at visible region can be attributed to the presence of MgO buffer layer

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which reduces the effect of lattice misfit.

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3. Conclusion

We have successfully prepared high visible responsivity nanostructured PbI2/MgO/Si

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heterojunction photodetector by PLD method without using post annealing. The effect of substrate temperature on the structural, optical and electrical properties of PbI2/MgO was demonstrated. The grown PbI2 film was single crystalline hexagonal phase (2H-polytype) while

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the MgO was single crystalline cubic structure. The intensity and crystallinity quality of PbI2 thin film were enhanced after increasing the substrate temperature. SEM investigation showed a temperature dependent morphology and size of PbI2 film deposited on the MgO film. Maximum responsivity of 0.88A/W at 410nm was obtained at preparation temperature of 45°C. The presence of MgO buffer layer between PbI2 film and silicon substrate improved remarkably the

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junction characteristics. Based on the obtained results, PbI2/MgO/Si photodetector can be used efficiently for detecting visible weak signals.

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