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Journal of Theoretical and Applied Physics

, Volume 13, Issue 3, pp 269–275 | Cite as

Effect of doped Zn–PbI2 nanostructures on structural and electrical properties of photodetector applications

  • Safaa Idan MohammedEmail author
  • Haider Mohammed Shanshool
  • Khalil Ibraheem Imhan
Open Access
Research
  • 78 Downloads

Abstract

The present study focuses on the structural and electrical properties of doped zinc–lead iodide (Zn–PbI2) as-deposited film. Lead iodide (PbI2) nanostructure was successfully prepared by thermal evaporation method on a glass substrate at room temperature. The analysis, characterization, and structural properties of PbI2 were achieved using X-ray diffraction (XRD) and scanning electron microscopy. The PbI2 was polycrystalline and had a hexagonal structure as proved using XRD. The measured values are in agreement with other experimental and theoretical data. Furthermore, the present research studied the effect of doping on the physical properties of lead iodide with zinc dopants at different weights (0.02, 0.04, 0.06, and 0.08) mg. The electrical properties of the fabricated metal–semiconductor–metal photodetector based on PbI2 and Pb1−xZnxI2 layers prepared on glass substrates by thermal evaporation method were investigated. The obtained results of Schottky barrier heights for Pb0.98Zn0.02I2 were significant. The current–voltage characteristics of the Pb0.98Zn0.02I2 thin film have acted as a Schottky contact in dark and under white light, 460-nm light. The light responsivity has shown a peak at 460-nm chopped light. At a bias voltage of 1, 3, and 5 V, the photocurrent rise and decay times were investigated. The device has shown faster response times for 460-nm light. This fast response was attributed to the high quality of polycrystalline and showed a high quantum efficiency of 9.19 × 102% when it was illuminated by 460-nm light under the bias of 3 V.

Keywords

Pb1−xZnxI2 nanostructures IV characteristics MSM PD Thermal evaporation method 

Introduction

Lead iodide (PbI2) is a wide bandgap semiconductor (Eg = 2.32 eV) and has been a semiconductor material for use in solid-state X-ray and gamma-ray detectors [1, 2]. The unit cell dimension is a = 4:557 and c = 6:979 Å [3]. Bhavsar et al. [4] studied thin films of Zn-doped and undoped lead iodide gel-grown crystals, by thermal evaporation method, and highly uniform thin films were obtained. Verma et al. [5] in another study synthesized the thin film of pure ZnO, Zn0.97Nd0.03O and Zn0.97Sm0.03O by using metallo-organic decomposition method. The average size of ZnO nanoparticles with Nd and Sm doping is 10 nm, confirmed with atomic force microscopy. The Nd and Sm doping into ZnO thin films significantly enhances absorption in the UV region and suggests its usability for UV detectors. Under UV irradiation (l = 325 nm), the value of photocurrent in Nd and Sm/ZnO thin films is highly enhanced for possible use in UV sensors. Lam et al. [6] in their study fabricated and characterized the ultraviolet (UV) photodetectors with ZnO nanorods (NRs)/Cds thin-film heterostructures on glass substrates. The study last comes to the conclusion that there is significantly high sensitivity in the range of UV light which can be useful for the application of UV detection. Bhardwaj et al. [7] in their study gave an explicit comparison on the basis of structure and electronic properties of ZnO nanostructures which were synthesized by sol–gel and hydrothermal method. Field-emission scanning electron microscopy revealed the formation of ZnO spherical nanoparticle structure for sol–gel method and flower like m-structure for ZnO prepared through hydrothermal route. Among the conclusions, they research into the idea that further extended X-ray absorption fine-structure revealed a similar local atomic structure for both samples despite having different morphologies. Some researchers have investigated the structural properties of PbI2 [8, 9]. Because several methods for depositing films require sublimation or evaporation of the material, it should have a temperature for appreciable evaporation or at least a temperature for appreciable sublimation as films that can be grown at temperatures lower than 250 °C, the maximum temperature for the detector stability during deposition. The material must be thick enough to absorb a high percentage of X-rays, taking into consideration its absorption coefficient. The grain size must be less than 100 μm, and therefore, small-grain-size polycrystalline films or epitaxial films are required [10]. In this work, the deposited film of doped Zn–PbI2 with different weights of zinc dopants on the glass substrate was prepared successfully. The structural and electrical properties of samples were studied. To the best of the author’s knowledge, the preparation of these samples with different weights of Zn dopant and studied their effect on electrical properties are considered to be the first work.

Experimental process

Lead iodide (PbI2) powder preparation

Lead iodide (PbI2) was prepared in the laboratory by the reaction of potassium iodide KI with lead nitrates Pb(NO3)2. In accordance with the following procedures, 1 mol/L of potassium iodide KI was dissolved in DI water at boiling temperature and vigorously stirred for 10 min. Same procedure was followed with 2 mol/L of lead nitrate Pb(NO3)2. After that, the two solutions were mixed in one beaker and placed on a hot plate at a temperature of boiling and stirred quickly for 1 h. (K) and (Pb) are the positive ions, while (I) and (NO3) are the negative ions. The (NO3) group is radical. We can observe from the sudden appearance of yellow precipitation that a chemical change has occurred. This proves the formation of PbI2 crystalline in this solution. The positive charged (Pb) has combined with the negative charged I. Similarly, the K and (NO3) group are combined. The equation describing the reaction may be written as follows:
$$2{\text{KI}} + {\text{Pb}}\left( {{\text{NO}}_{3} } \right) \to {\text{PbI}}_{2} + 2{\text{KNO}}_{3}$$

Soon, a yellow deposit appeared at the bottom of the beaker. This deposit is the lead iodide (PbI2), which is not soluble in water; while the potassium nitrates (KNO3) are solved in water. Then, the water is removed from the beaker using a scaled syringe to avoid any disturbance may occur. After the deposited material is dried, it was removed from the bottom of the beaker and kept in desiccators.

Preparation of the thin-film samples

PbI2 thin films were deposited on glass substrates at room temperature using electron beam evaporation (Auto 306 Vacuum Coater). The principal reason for using this technique is to allow the deposition of a large area in a cost-effective manner [11]. Glass substrates were provided of thickness 1 mm and dimensions 15 × 20 mm prior to the thermal evaporation of PbI2. Glass slides are often cleaned by a solvent clean, followed by a deionized water (DI) rinse, a mild acid clean, DI rinse, and blow-dry. It involves the following steps: Solvents can clean oils and organic residues which appear on glass surfaces by the following steps:
  1. (a)

    Pour acetone into a glass container.

     
  2. (b)

    Pour methanol in a separate container.

     
  3. (c)

    Place acetone on a hot plate to warm up (do not exceed 55 °C).

     
  4. (d)

    Place glass wafer in a warm acetone bath for 10 min.

     
  5. (e)

    Remove and place in methanol for 2–5 min. Remove and rinse in DI water.

     
  6. (f)

    Blow-dry with nitrogen.

     

The thermal evaporator deposits the metal onto the substrate. Under vacuum, the current passes through a filament which heats up the metal present in the boat. When the current reaches the evaporation value, the melted metal evaporates onto the substrate. This is done under high vacuum to allow the vapor to reach the substrate without reacting with or scattering against other gas-phase atoms in the chamber. Pb1−xZnxI2 nanostructure was deposited on a glass substrate using thermal evaporation under 5 × 10−5 torr vacuum. The contact as silver (Ag) mask was deposited via thermal evaporation on Pb1−xZnxI2 surface to create metallization grid patterns. First, the Pb1−xZnxI2 photodetector was annealed at 200 °C for 1/2 h to enhance the metal–semiconductor contact.

Characterization techniques

The weight method was adopted to measure the thickness. Sensitive electrical balance Metler AE-160 was used, with precision reaching 10−4 g. The structural properties of the deposited PbI2 thin films were investigated by using X-ray diffraction to determine whether the sample is crystalline or not, also the diffraction is used to determine spacing, preferred orientation, and the average grain size. In our measurements, we used X-ray diffraction system (Philips PW 1710 X-ray diffractometer) with the following characteristics: source radiation of CuKα with 1.54 Ǻ wavelength, incidence angle: 10°–60°, and scanning speed: (5°/min). The scanning electron microscope (SEM) images of these films were obtained by (SEM) model (JEOL-JSM-6460 LV). The energy bandgap from optical absorption is measured from Zn–PbI2 thin films deposited on glass substrates using a UV–Vis spectroscopy. IV characteristics are measured by (Keithley, 2400 system) high-voltage source. Two needle-like probes are positioned on the contacts under voltage − 5.0 to + 5.0 V. It results in a current flow between the contacts which is measured via ammeter. These results are displayed on a screen or a monitor. The photo- and dark currents were taken into consideration. In addition, ideality factors and SBH depend on IV measurements.

Results and discussion

XRD of PbI2

Figure 1 shows the XRD pattern of the grown polycrystalline PbI2. XRD pattern was obtained with scanning angles of 2θ = 10°−60° for a high-purity PbI2 crystal, containing the complete set of reflections. The sharp diffraction peak originates at 2θ = 25.8° which corresponds to (011) reflections of the wurtzite (hexagonal) phase of PbI2 according to the standard database ASTM (ICSD, 01-079-0803). Other lines accompanying the reflections belong to (001), (102), (003), (110), and (004) which correspond with 2θ = 12.6°, 34.1°, 38.4°, 39.5°, 52.4°, respectively [12]. PbI2 is a hexagonal semiconductor material with lattice constants of a = 4.562 Å and c = 6.985 Å [13].
Fig. 1

XRD patterns of purified PbI2 materials

SEM of Pb1−xZnxI2

The SEM of Pb1−xZnxI2 nanostructures revealed the formation of hexagonal platelets and rods grown on the substrate as well as grains and voids. The morphology of the grain is mostly of closely irregular platelets and rods. Figure 2 shows that the grain size of Pb1−xZnxI2 is not affected by increasing Zn doping which is consistent with the XRD results.
Fig. 2

SEM image spectra of Pb1−xZnxI2 nanostructures with various Zn moles fractions xax = 0.02, bx = 0.04, cx = 0.06, and dx = 0.08

Metal–semiconductor–metal photodetector-based on PbI2

The following subsections present the characterizations of the fabricated device.

Electrical measurements of Ag-based Schottky contact

Figure 3 shows the plot of ln I as a function of voltage to calculate the barrier height for Ag contact on PbI2 and Pb0.92Zn0.08I2.
Fig. 3

Plot of voltage as a function of ln I to calculate the barrier height for Ag contact on undoped and doped under a illumination and b dark conditions

Figure 4 shows the IV characteristics from − 6 to 6 V of the PbI2 and Pb1−xZnxI2 measured in the dark and white light (tungsten lamp) and under 460 nm. Figure 4 shows the current–voltage (IV) characteristics of Ag-based Schottky contact for (a) PbI2 and (b, c, d, and e) doped Pb1−xZnxI2, respectively. The Schottky barrier heights (SBH) and ideality factor of these contacts were calculated through the IV measurements using equations
$$I = I_{^\circ } \exp \left( {\frac{qV}{{nK_{B} T}}} \right)\;{\text{and}}\;I_{^\circ } = A^{ * } AT^{2} \exp \left( {\frac{{ - q\phi_{B} }}{{K_{B} T}}} \right)$$
where \(I_{^\circ }\) is the saturation current, n is the ideality factor, KB is the Boltzmann’s constant, T is the absolute temperature, \(A^{*}\) is the effective Richardson coefficient, A is the area of the Schottky contact, and ΦB is the barrier height. The leakage current value was determined at (2 V). It can be seen from Fig. 4b that the curves show a rectifying behavior and this improved with doped, indicating that the addition of 0.02 Zn to PbI2 produces a strong chemical reaction and better contact. Ag metal contact has a high work function (4.70 eV) [14]. As a result, (b) shows the highest Schottky barrier value compared with the rates of other zinc. This indicates that it has good surface morphology. The mechanism to change the value of current at higher electric fields depends on changing the energy gap. The Schottky barrier height SBH and ideality factor are summarized in Table 1.
Fig. 4

IV characteristics of Ag-Schottky contact with a PbI2, (be) Pb1−xZnxI2 nanostructures

Table 1

Structural and electrical properties of PbI2 and Pb1−xZnxI2 nanostructure

Sample

Ideality factor (n)

Barrier height (eV)

Leakage current

Undoped PbI2

 Light

1.7

0.64

− 7.967657E−05

 Dark

2.6

0.67

− 1.369305E−05

Pb0.98Zn0.02I2

 Light

1.6

0.84

− 5.202599E−08

 Dark

1.08

0.83

− 5.380798E−09

Pb0.96Zn0.04I2

 Light

2.1

0.76

− 2.046658E−04

 Dark

6

0.79

− 3.346693E−06

Pb0.94Zn0.06I2

 Light

3.6

0.40

− 1.053405E−05

 Dark

1.2

0.64

− 7.373940E−06

Pb0.92Zn0.08I2

 Light

2.6

0.64

− 4.969795E−04

 Dark

3.9

0.68

− 2.677603E−06

Photocurrent response of Pb0.98Zn0.02I2 device

The light responsivity (R) of Pb0.98Zn0.02I2 was calculated using equation
$$R = \frac{{I_{\text{Ph}} }}{{AP_{\text{opt}} }}$$
where Iph is the photocurrent, A is the illuminated area, and popt is the power of the incident light.
Figure 5 shows a plot of the responsivity against the wavelength (under bias voltage of 3 V). As shown in Fig. 5, the highest R-value was observed at 460 nm. The photocurrent response of the Pb0.98Zn0.02I2 device was studied at various bias voltages under illumination with 460-nm chopped light. Figure 6 shows the photocurrent response curves at applied bias voltages of 1, 3, and 5. The current was increased to saturation when it was illuminated and decreased again when the light was switched off. When the device is illuminated with sufficient energy, electron–hole pairs are formed by optical absorption. These pairs are separated under the applied electrical field at the Zn–PbI2 interface. The rise time of the device exposed to 460-nm light ranged from 53 to 61 ms and the fall time from 57 to 63 ms depending on the bias voltage. Table 1 lists the rise time and decay time in Fig. 7. The sensitivity of the fabricated device was determined at a different applied bias voltage using equation \(S = \frac{{I_{\text{Light}} - I_{\text{Dark}} }}{{I_{\text{Dark}} }} \times 100\%\), and the device was more sensitive to 460 nm light than to illumination at other wavelengths. At b bias voltage of 3 V, the sensitivity of the device under 460-nm light was 5472% as shown in Table 2. Quantum efficiency (ƞ) is an important parameter to evaluate the performance of the photosensitive device. It is related to the number of electron–hole pairs excited by absorbed photons as given by equation
$$\eta = \frac{hc}{e\lambda }R_{\lambda }$$
where h is Planck’s constant, c is the speed of light, e is the electron charge, λ is the wavelength, and \(R_{\lambda }\) is the spectral responsivity. The efficiency (ƞ) values of the fabricated Pb0.98Zn0.02I2 detector were found to be dependent on the bias voltage and the wavelength of the incident light as shown in Table 2. The high quantum efficiency of Pb0.98Zn0.02I2 photodetector is related to the large value of the carrier mobility lifetime.
Fig. 5

Light responsivity versus wavelengths of light fabricated polycrystalline PbI2 photodetector

Fig. 6

Photoresponse of Pb0.98Zn0.02I2 device illuminated by chopped 460-nm light at various bias voltages a 1 V, b 3 V, and c 5 V

Fig. 7

Scheme to clarify the calculation of rise time and decay time

Table 2

Photoelectrical parameters of Pb0.98Zn0.02I2 photodetector device

Sample

Wavelength (nm)

Bias voltage (V)

Rise time (ms)

Decay time (ms)

Sensitivity (%)

Quantum efficiency (ƞ) (%) (× 102)

Pb0.98Zn0.02I2

460

1

61

63

2890.656

2.95

3

53

57

5472.738

9.19

5

57

59

583.9345

6.80

Conclusions

Thin-film samples of doped Zn–PbI2 with different weights of zinc dopants on glass substrate were prepared successfully. The Ag-based Schottky contacts based on PbI2 and Pb1−xZnxI2 nanostructures have been investigated. All the samples revealed a variable behavior, and they become more rectifying when adding Zn impurity with good electrical properties of Pb0.98Zn0.02I2. Good results for SBH have been obtained at Pb0.98Zn0.02I2 due to good structural and electrical properties for this sample. In addition, the surface morphology plays an important role in the interface between the metal and Zn–PbI2 surface, which in turn reduces tunneling effect and increases the thermionic emission which, eventually, leads to improving the Schottky barrier height.

Notes

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Authors and Affiliations

  1. 1.Ministry of Science and TechnologyBaghdadIraq

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