Structure, binding energy and optoelectrical properties of p-type CuI thin films: The effects of thickness

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Please cite this article as: W. Peng, L. Li, S. Yu, H. Zheng, P. Yang, Structure, binding energy and optoelectrical properties of p-type CuI thin films: The effects of thickness, Applied Surface Science (2019), doi: https://doi.org/ 10.1016/j.apsusc.2019.144424

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Structure, binding energy and optoelectrical properties of ptype CuI thin films: The effects of thickness Wei Peng, Lingxia Li*, Shihui Yu*, Haoran Zheng, Pan Yang School of Microelectronics and Key Laboratory for Advanced Ceramics and Machining Technology, Tianjin University, Tianjin, 300072, P. R. China * Corresponding author E-mail: [email protected] (Lingxia Li), [email protected] (Shihui Yu). Abstract: Ultra-high transparent p-type copper iodide (CuI) thin films were fabricated by solid iodization of evaporated Cu precursor layers at room temperature. The effect of the thickness on microstructure, binding energy and optoelectrical properties is systematically studied. X-ray diffraction measurements show the polycrystalline nature of the CuI thin films with zincblende type structure. The X-ray photoelectron spectroscopy (XPS) analysis indicates that the oxidation state of Cu is +1 and the estimated value of [Cu]/[I] at 100 nm is 0.87. Excess iodide ions trap considerable holes, causing CuI thin films to exhibit the p-type conductivity, which is consistent with the results of the Hall effect measurement and the non-linear characteristics of the CuI/ITO structure. Moreover, the CuI thin films with thickness of 100 nm exhibits an ultra-high optical transmittance of 95.5% in the wavelength of 380~780 nm and an excellent conductivity of 34 S/cm. These results prove the great potential of CuI as a promising p-type optoelectronic material. Keywords: X-ray photoelectron spectroscopy; Surface; CuI; Film; Optoelectrical 1. Introduction 1

Recent years have witnessed a growing interest in transparent conductors (TCs), which makes itself an important tool for some fascinating applications in transparent electronics, such as thin-film transistors, light emitting diodes (LED), flat-panel display, gas sensors and solar cells [1-11]. Especially, great advances have been made in the research of n-type TCs, especially indium tin oxide (ITO) [12], fluorine-doped tin oxide (FTO) [13], Al-doped zinc oxide (AZO) [14], and indium-gallium-zinc oxide (IGZO) [15]. Despite these advances, research on p-type counterparts is still rare, and only a few (ZnO [16-17], CuSCN [18], SrCu2O2 [19]) have been reported. The active devices prepared by using existing p-type thin films, like thin-film transistors and LEDs, cannot supply the demand of the practical application. Nevertheless, p-type doping of n-type TCs is extremely difficult, which is primarily related to self-compensation in widebandgap semiconductors [20-21]. Hence, it is of great importance to explore new ptype TCs with superior performance. γ-CuI is a p-type conductive semiconductor material with unique optoelectronic properties, including wide band gap (3.1 eV), high hole mobility (>40 cm2V−1s−1 in bulk), and large room-temperature exciton binding energy (62 meV) [22-23]. Because of these advantages, CuI has become one of the most promising p-type TCs [22]. And it will be hopefully applied in scintillators [24], organic electronics [25], and bipolar diodes [26]. The development of high-quality bipolar diodes opens up the application of CuI in transparent electronics. A variety of techniques have been developed to prepare CuI thin films, including chemical bath deposition [27-28] (CBD), vapor or solid iodization [29-34], successive ionic layer adsorption and reaction (SILAR) [29, 2

35], vacuum thermal evaporation [36-37], pulsed laser deposition (PLD) [38], and sputtering [39-40]. Among these techniques, the solid iodization method is more suitable for mass production because of a simple process and low cost. However, there have been few studies about the effect of the process parameters on the properties of CuI thin films prepared by solid-state iodization. For CuI thin films, the thickness has a great influence on grain growth, optical bandgap and binding energy [41-42]. Therefore, appropriate thickness can effectively improve the optoelectrical properties of CuI thin films. In this paper, the CuI thin films with different thicknesses were prepared successfully by solid iodization of copper evaporated Cu precursor layers. The asprepared CuI thin films have ultra-high transmittance and excellent electrical conductivity. The thickness dependence of microstructure, binding energy and optoelectrical properties is systematically studied. This paper will be an important supplement on knowledge of the p-type TCs. 2. Experimental techniques Cu precursor layers were prepared by vacuum thermal evaporation in a dynamically pumped chamber with a base pressure of 9 ×10-4 Pa on glass substrates. The glass substrates were washed with acetone and alcohol in an ultrasonic bath for 15 minutes respectively, and subsequently dried before the deposition. The substrate temperature for the vacuum evaporation was maintained at room temperature. The vacuum state was maintained during the preparation of CuI thin films. The Cu film thickness was estimated based on the evaporation current and the mass of Cu wires. 3

Some bare high purity (99.9%) Cu wires were used as the evaporation source. For the iodization of Cu precursor layers, the Cu layers were directly placed on the iodine powder with Cu-side facing down at room temperature for 30 minutes to make it fully iodinated. About 4g I2 particles were ground into powder using a mortar to overspread the bottom of a glass bottle. The thicknesses of the as-prepared CuI thin films were between 50 and 430 nm. Moreover, CuI/ITO diodes were prepared on glass substrates. ITO layers were fabricated by RF magnetron sputtering on glass substrates, and then CuI layers were fabricated by the process mentioned. Square gold electrodes were prepared on top of the CuI and ITO layers. For preparing the electrodes, the vacuum thermal evaporation was employed to prevent the excessive energy of the magnetron sputtering from affecting the performance of the film. The crystalline phase of CuI thin films was measured using X-ray diffraction (XRD, D/MAX 2500, Rigaku, Japan) with the grazing incidence and Cu Kα radiation (λ = 0.1542 nm), by acceleration current of 200 mA and electric field of 40 kV. All diffraction patterns were tested from 20° to 55° with θ-2θ mode (Bragg-Brentano mode) and scanning speed of 5.000° min−1. The thickness of CuI films was measured from the Alpha-step surface profiler (D-100, KLA Tencor). The surface morphology and roughness were measured at room temperature by atomic force microscopy (AFM, Solver NEXT, NT-MDT, Russia). X-ray photoelectron spectroscopy was measured at room temperature using monochromatic Al Kα (hν=1486.6 eV) on a Kratos AXIS Ultra DLD to examine the state information of Cu and I of CuI thin films. A four-probe tester (ST-2258A, Jingke Electronic Co. Ltd, Suzhou, China) was employed to measure the 4

sheet resistance of CuI thin films. The carrier concentration and mobility of CuI thin films were investigated using the van der Pauw method at room temperature. An ultraviolet/visible (UV/VIS) spectrophotometer (Model 723PC, Jinghua Instruments, Shanghai, China) was used to obtain the optical absorption and optical transmission spectra in the wavelength range from 350 to 800 nm. A semiconductor parameter analyzer (Keithley 4200A) was employed for current–voltage (I–V) measurements for the CuI/ITO diode structure. 3. Results and Discussion Fig. 1 shows the XRD patterns for CuI thin films prepared on glass substrates with different thicknesses. It is found that all the CuI thin films are well-crystallized with a preferential orientation along the (1 1 1) direction, which indicates the formation of zincblende -CuI. No diffraction peaks of Cu, CuO and Cu2O, are observed in Fig. 1, indicating that Cu is completely transformed into γ-CuI [43-44]. As shown in Fig. 1, for the CuI thin films with thickness of 50 nm, the intensity of the (1 1 1) diffraction peak is weak. As the thickness increases to 430 nm, the diffraction intensity increases gradually. These results mean that the increase in thickness is advantageous to the improvement of crystallinity for CuI thin films. Fig. 2 shows the full width at half-maximum (FWHM) and grain size of the (1 1 1) diffraction peak of the CuI thin films as a function of the thickness. The FWHM values reduce from 0.6 to 0.37 with the thickness increasing from 50 to 430 nm. The crystallite size is calculated according to the Scherrer’s formula [45-46]: D=

0.9 

 cos  5

(2)

where D is the average crystallite size, λ is the wavelength of Cu Kα radiation (1.5418Å), β is the full-width-at-half-maximum of the (1 1 1) diffraction peak, and θ is the Bragg diffraction angle of the (1 1 1) peak. As shown in Fig. 2, the crystallite size increases rapidly with the thickness increases from 50 nm to 100 nm, and then the crystallite size increases slowly as the thickness further increases to 430 nm. The crystallite size reaches the maximum value (~23 nm) at thickness of 430 nm. This phenomenon may be attributed to the lattice mismatch and the surface energy. For thinner films, the large specific surface area and the lattice mismatch between CuI thin film and glass substrate introduce several defects [44, 47], resulting in the poor crystallinity. With the thickness increasing, the effect of the lattice mismatch on crystallinity becomes weak. The crystallite size is primarily affected by the surface energy of CuI thin films [48], therefore, the crystallite size increases. In order to explore the exact chemical states, XPS spectra of Cu and I were measured as shown in Fig. 3. Compared with the reported XPS spectra of CuI [49], no other peaks are found in the measured spectra, indicating that the CuI thin films are pure-phase. Cu 2p and I 3d orbitals split into two peaks due to the spin-orbit interaction. As shown in Fig. 3(a), the peaks located at 951.9 and 932.1 eV are compatible with Cu 2p1/2 and Cu 2p3/2, respectively. The non-existent shake-up satellite peaks demonstrate that no Cu2+ is formed [50]. Combining with the results of XRD, it can be concluded that the oxidation state of Cu is +1 [36, 51]. As shown in Fig. 3(b), the peaks located at 631.6 eV and 619.5 eV are consistent with I 3d3/2 and I 3d5/2 for the CuI thin films with thickness of 100 nm. When the thickness is 200 nm, the core-level spectra of I- are 6

composed of two peaks at 631.4 eV and 619.3 eV, assigned to I 3d3/2 and I 3d5/2, respectively. The spectra of I 3d show that the binding energies of CuI thin films with various thicknesses are different, and the binding energy reduces slightly with the increase of thickness. Moreover, the atomic ratio of copper and iodine, [Cu]/[I], is calculated by the relative sensitivity factor method (RSFM) [52] and the intensities of Cu 2p3/2 and I 3d5/2 peaks. The formula as follow:

nCu ICu SCu  nI II SI

(3)

where ICu and II are the peak strength of Cu 2p3/2 and I 3d5/2, SCu and SI are the atomic sensitivity factor of Cu and I, the values are 5.321 and 10.343, respectively. The estimated [Cu]/[I] values are 0.87 and 0.89 for the CuI thin films with 100 nm and 200 nm thickness, respectively, indicating that an excessive amount of I- is incorporated into CuI thin films [53]. Excess iodide ions will trap a large number of holes [54], which indicates that the p-type conductivity of γ-CuI originates from stoichiometrically excess iodine. The morphologies of CuI thin films were characterized by atomic force microscopy (AFM). Fig. 4 (a-e) show the 3D micrographs of CuI thin films with various thicknesses. The AFM images indicate the grain size increases with the thickness increasing, which is consistent with the FWHM results. When the thickness is 50 nm, the surface morphology of CuI thin films is nonuniform and incongruent, indicating that the crystallinity is poor. With the thickness increasing to 200 nm, the surface morphology becomes smooth and well-knit, as shown in Fig. 4(c). As the thickness further increases to 430 nm, the surface becomes more homogeneous and the grain size increases, 7

indicating that the crystallization quality improves. These results suggest that the thickness has a great influence on the surface morphology and crystallization. As shown in Fig. 4(f), The average roughness gradually increases from 1.5 nm to 12 nm with the thickness increasing from 50 nm to 430 nm. In the application of optoelectronic devices, the transmittance is an important criterion for evaluating optical properties in the visible region (380~780 nm). Figure 5(a) shows the transmittance spectra of CuI thin films with different thicknesses in the wavelength range of 350~800 nm. The transmission spectra show fluctuations in the wavelength range of 410~800 nm due to the interference of light on the film/air and film/substrate interfaces [55]. The information of interference stripes obtained with different thickness indicates that the film surface is smooth and homogeneous. In addition, the average optical transmittance Tav is calculated by the following relationship:

Tav 

 V ( )T ( )d   V ( ) d 

(4)

where T(λ) is the light transmittance and V(λ) is the photopic luminous efficiency function defining the standard observer for photometry [56]. Fig. 5(b) shows the variation of average transmittance in the visible range (380~780 nm) as a function of the thickness for CuI thin films. The average optical transmittance (Tav) values of 50 nm,100 nm, 200 nm, 330 nm and 430 nm films, according to the formula, are 86.5%, 95.5%, 87.2%, 84% and 69.5%, respectively. When the thickness of CuI thin films is 50 nm, the transmittance is low, which can be illustrated by the fact that the lattice mismatch and poorer crystallinity lead to a large number of the optical scattering and 8

poor light transmittance. As the thickness increases to 100 nm, the lattice mismatch decreases and crystallinity increases, leading to a reduction in optical scattering and an increase in transparency. With the thickness further increasing to 430 nm, the crystallite size increases slightly. The surface roughness deteriorates and absorption raises [57-59], resulting in increasing the surface scattering and decreasing the light transmittance. The optical absorption coefficient can be estimated by the formula [60]:

T  e  t

(5)

where α and t represent the optical absorption coefficient and the thickness, and T is the transmittance of thin films. On the basis of the relationship between the absorption coefficient and the photonic band gap near the absorption edge, the Tauc relationship [61] is accorded with:

（ h）n  A(h  Eg )

(6)

where Eg is the optical band gap, ν is the transition frequency, A is defined as a constant, and the exponent n characterizes the nature of band transition, respectively. n=1/2 and 1/3 correspond to indirect allowed and indirect forbidden transitions and n=2 and 2/3 corresponds to direct allowed and direct forbidden transitions, respectively [57]. In this work, because the CuI thin films belongs to direct allowed transition material, the value of n is 2. Fig. 5(c) shows the curves of α2 versus photon energy for CuI thin films with various thicknesses. The Eg is determined by extrapolating the straight regions of the plots of α2 vs. hv to α2=0 (i.e., αhv=0) [62]. Fig. 5(d) shows the variation of optical band gap as a function of the thickness for CuI thin films. The optical band gap decreases from 3.032 eV to 3.012 eV with the thickness increasing from 50 to 430 nm, which is 9

consistent with the observed red shift in the transmission spectra. The values are consistent with those of bulk CuI crystals [30]. The observed decrease in band gap values is attributed to the enhancement in the crystallinity and the increase of grain size [63]. Fig. 6 shows the square resistance and conductivity of CuI thin films with various thicknesses. It is observed that the square resistance of CuI thin films gradually decreases as the thickness increases from 50 nm to 430 nm. The conductivity initially increases with the thickness, reaches a maximum at thicknesses of 100 nm(34 S/cm), and decreases with further increasing thickness up to 430 nm. This phenomenon is directly related to the changes of carrier concentration and mobility. The conductivity () is determined by the following equations [33]:

  ne

(7)

where n is the carrier concentration, e is the charge of the carrier, and μ is the mobility. Fig. 7 depicts the carrier concentration and mobility as a function of various thicknesses. The Hall effect measurement data indicates, for all the CuI thin films, the sign of the Hall coefficient is positive, which means all the samples are p-type conductive. The mobility improves with the thickness increasing, and carrier concentration exhibits the same trend as the conductivity. The observed less carrier concentration and mobility in 50 nm due to the poor crystallinity and the small grain size. With the thickness increasing to 100 nm, the reduction of grain boundary density due to improvement of the crystallinity and increase of the grain size results in a decline in grain boundary scattering. Hence, both the carrier concentration and mobility increase. As the thickness 10

further increases to 430 nm, the carrier concentration decreases and the mobility slightly increases. This may be attributed to the decline of relative excessive iodide and the deterioration of surface morphology. The estimated [Cu]/[I] values indicate the relative number of excessive iodine at 100 nm is more than at 200 nm and the relative excessive iodide declines with the thickness increases from 100 nm to 200 nm. The decline of relative excess iodide reduces the carrier concentration [54], and the deterioration of surface morphology causes the increase of electron scattering on the film surface. In order to further determine conductivity type, a CuI/ITO diode structure was prepared on glass substrates. The current-voltage characteristics were measured by semiconductor parameter tester, and drawn in Fig. 8. The diode structure diagram is shown in the bottom right illustration of Fig. 8. The top left inset of Fig. 8 reveals the I-V curves for the Au contacts on the CuI and ITO layers. The I-V curves are all straight lines, indicating that the Au contacts on CuI and ITO are ohmic contact, the same phenomena have been reported [39, 64, 65]. Fig. 8 shows that the I-V characteristics of CuI/ITO diodes is non-linear, which further confirms that the CuI thin films is p-type conductive. In addition, Fig. 8 exhibits the maximum current is ~190 mA when the voltage is 4 V, meanwhile, the forward turn-on voltage (VF) is ~1.8 V here. This indicates the CuI/ ITO diode has rectification characteristics, which will accelerate the development of transparent electronics. 4. Conclusion Ultra-high transparent, p-type CuI thin films were fabricated by solid iodization of evaporated Cu precursor layers at room temperature. The microstructure, binding 11

energy, optoelectrical properties of CuI thin films were observed by means of AFM, Xray diffraction, X-ray photoelectron spectroscopy, the Hall effect measurement, fourpoint probe meter and ultraviolet-visible absorption spectra. Meanwhile, the effect of the thickness on structural, optical and electrical properties for CuI thin film is systematically investigated. X-ray diffraction measurements show the polycrystalline nature of the CuI thin films with zincblende type structure. The analysis of X-ray photoelectron spectroscopy (XPS) data indicates that the oxidation state of Cu is +1 and the estimated value of [Cu]/[I] at 100 nm is 0.87. Excess iodide ions trap considerable holes, causing CuI thin films to exhibit the p-type conductivity. At the thickness of 100 nm, we obtain the highest conductivity of 34 S/cm at the average optical transmittance of 95.5% in the visible region. The p-type conductivity is confirmed by the Hall effect measurement and the non-linear characteristics of the CuI/ITO diode structure. The CuI thin films prepared by solid iodization of evaporated Cu precursor layers could be attractive as proper hole transport materials in optoelectronic and electronic devices. Besides, these also will guide the film preparation by the reaction precursor film such as, CuI by iodization, FeS2 by sulfuration [44]. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grant No. 61671326, 61701338), the National Key Research and Development Program of China (Grant No. 2017YFB0406300). References [1] J. Gao, K. Kempa, M. Giersig, E.M. Akinoglu, B. Han, R. Li, Physics of transparent 12

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Fig. 1. XRD spectra of the CuI thin films as a function of the thickness.

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Fig. 2 FWHM value and grain size of (1 1 1) peaks of CuI thin films as a function of the thickness.

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Fig. 3 XPS analysis of CuI film at 100 nm and 200 nm thickness: (a) Cu 2p, (b) I 3d.

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Fig. 4 Micrographs of atomic force microscopy (AFM) of the CuI thin films prepared with various thicknesses: (a) 50 nm, (b) 100 nm, (c) 200 nm, (d) 330 nm, and (e) 430 nm; (f) surface average roughness of the CuI thin films as a function of the thickness.

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Fig. 5 (a) Optical transmittance as a function of wavelength for CuI thin films with various thicknesses; (b) Average optical transmittance as function of film thickness for CuI thin films; (c) Curves of α2 versus photon energy for the CuI thin films prepared with various thicknesses; (d) optical band gap Eg of CuI thin films as a function of the thickness.

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Fig. 6 Square resistance and conductivity of CuI thin films as a function of the thickness.

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Fig. 7. Carrier concentration and mobility as function of the thickness for CuI thin films.

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Fig. 8 The I–V characteristics of CuI/ ITO heterojunction. The top-left inset: the schematic diagram of CuI/ ITO heterojunction, and the botton-right inset: the I–V curves of gold-contacted CuI and ITO.

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Highlights 1. P-type CuI film is prepared by solid iodization evaporated copper. 2. The exact chemical states of Cu and I is analyzed by XPS spectra. 3. XPS measurements show the films are pure phase and oxidation state of Cu is +1. 4. CuI films exhibit ultra-high transparency (95.5%) and good conductivity (34 S/cm). 5. A CuI/ITO diode structure prepared has good rectification characteristics.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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