Synthesis of 2D yttrium zinc oxide nanosheets via simple chemical route toward high performance Schottky diode based on large charge carriers density and photoresponsivity

Yttrium zinc oxide (Zn0.85Y0.15O) nanostructures were stoichiometrically prepared by co-precipitation method. XRD, EDX, XPS, SEM, and TEM spectroscopy were examined to investigate structure, composition, and morphological characteristics. The synthesized nanocomposite exhibited polycrystalline structure with small crystallite size ∼ 27 nm in which the particles appeared in sheets like shape with high atomic density on surface. The optical parameters including energy gap (Eg) and refractive index (n) were investigated from (T%) and (R%) measurements through wavelength range from 300–900 nm. Al/Y:ZnO/p-Si/Ag Schottky diode was fabricated using thermal evaporating technique and its current–voltage (I–V) was analyzed using different models. The photodiode showed non-ideal behavior with ideality factor greater than unity and small potential barrier. Under various illuminations, the photodiode has revealed high photosensitivity attributed to trapped charge carriers at the interface. The charge carrier density Nd and built-in voltage Vbi were estimated from Mott Schottky (M–S) function suggesting high Schottky diode efficiency.


Introduction
Industry relies heavily on petroleum, coal, and natural gas as main conventional energy resources.However, according to recent investigations these resources are projected to be depleted within the next few decades due to rapid industrialization, modern civilization, and global increasing energy demand.Additionally, the massive carbon-dioxide emission by fossil fuels combustion is one of the critical issues causing environmental pollution and greenhouse gases.Therefore, seeking reliable resources is so important to overcome shortage of energy and maintain a clean environment.The solar power has emerged as a promising technology owing to its unlimited availability and environmental friendliness [1,2].Solar cells are special types of optoelectronics that generate electricity by converting incident optical energy from the sun into electrical signals.When photons incident on an optical device, it generates free charge carriers electrons and holes (e-h) that significantly contribute to electrical conductivity.Currently, researchers are working on innovative strategies for manufacturing photovoltaic devices-based nanotechnology [3,4].Nanoscale metal oxides including stannous oxide (SnO 2 ), zinc oxide (ZnO), gallium doped zirconium oxide (Ga/ZrO 2 ), lanthanum doped copper oxide (La/CuO), and palladium doped manganese oxide (Pd/MnO) have been widely utilized in electronics and energy storage thanks to their semiconducting nature, large surface area, and tunable topological architecture.
In particular, transparent semiconductor zinc oxide has attracted notable attention due to its optical transparency, high chemical reactivity, and thermal stability.Zinc oxide nanoparticles (ZnO NPs) possess favorable energy band structure, high bulk electron mobility of 200 cm 2 V −1 s −1 , and excellent structuralmorphological features [5,6].However, nanoparticles agglomeration, large exciton binding energy of 60 meV, and charge carriers recombination impede its electrical conductivity and limit the optoelectronic performance [6,7].Several studies have reported that, integration of ZnO with different impurity atoms will modify the band gap by generating additional energy levels, enhancing light absorption, and reducing photocarriers recombination [7,8].Further, combining ZnO with different additives facilitates exciton separation and increases free charge carriers mobility [8].Nobel metals like gold (Au), silver (Ag), ruthenium (Ru), palladium (Pd), indium (In), yttrium (Y), and lanthanum (La) have been used as superior dopants for providing new class of nanocomposite materials [9,10].Compared with various elements, yttrium (Y) has strong corrosion resistance and single filled 4d-orbital which produces electron configurations for photoluminescence advantages and electronic conductivity.Adding yttrium ions (Y 3+ ) inside ZnO will improve its photoactivity and UV emission.When trivalent Y 3+ ion as donor type dopant substitutes divalent Zn 2+ into the host ZnO matrix, the charge concentration increases by donating one more electron makes it a better photoactive nanomaterial [11].Saha et al have synthesized n-type Y-doped ZnO nanowires by chemical bath deposition (CBD) method achieving low power UV response based on the huge UV/blue luminescence and oxygen vacancy providing an insight for fabricating highly efficient heterojunction UV detectors [12].In general, integration of ZnO framework with rare earth elements increases crystal defects, oxygen vacancies, and tunable its energy gap [13].Arafat Toghan et al have reported the possibility of using Y/ZnO NPs in electronic industry by controlling dopant contents and morphological shape [14].Engineering the surface morphology in one (1D) or two (2D) dimensional nanomaterials comprising nanorod, nanoneedle, nanosheet, nanocomb, or nanotube has been considered for enhancement of nanoparticles activity by supporting plenty active sites [15].Recently, ZnO nanorods were utilized for optimizing electromagnetic spectrum of ultraviolet radiation in metalsemiconductor-metal (MSM) photodetector applications.Arif Kösemen has fabricated Y/ZnO nanorods as electron transport layer for developing organic solar cells efficiency from 2.35% to 3.87% [16].Also, Shaan Bibi Jaffri et al have prepared semiconducting Y 2 O 3 /ZnO stacked nano fibers via microwave approach for improving the power conversion efficiency (PCE) of photovoltaics and electrical charge storage specific capacitance in energy storage devices [17].In the present study, Y/ZnO nanosheets were prepared by cost-effective chemical co-precipitation approach and their microstructure and optical characteristics were investigated.The electronic parameters obtained from I-V and C/G-V analysis emphasized the high Al/Y:ZnO/p-Si/Ag photodiode performance.In this chemical reaction, 7.12 g zinc chloride dihydrate was dissolved in 60 ml deionized water and 2.25 g yttrium (III) chloride hexahydrate in 30 ml using magnetic stirrer.Yttrium chloride solution was added to zinc chloride with continuous stirring to avoid nanoparticles agglomeration.Thereafter, a solution of precipitating agent, 3.76 g NaOH was dissolved in 70 ml deionized water then slowly added drop by drop to the mixture of aqueous chloride solution.At pH reaction 10, ~a homogeneous white precipitate was obtained then separated using filter paper.The resulting precipitate powder washed many times by deionized water to remove any residuals or impurities.To obtain nanometal oxide Zn 0.85 Y 0.15 O, the resulting product was dried in a furnace at 75 °C overnight and eventually calcined at 400 °C in the air for 3 h.

Characterization of nanomaterial
The crystal structure of yttrium zinc oxide was identified by x-ray diffraction (XRD; Bruker D8 Advance Eco) using a copper target, Kα radiation (λ = 1.541Å), worked at voltage of 40 kV and scanning rate of 0.05°/s with 2θ angle varied from 20°-80°.X-ray photoelectron spectroscopy (XPS, Escalab 250 Xi) analysis was performed to identify the electronic structure.The surface morphology and elemental composition were recorded using scanning electron micrograph (SEM; Helios Nanolab.400) attached with Energy dispersive x-ray analysis (EDX).Transmission electron microscopy (TEM; JEOL JEM-2100) was employed to visualize shape and average particle size.A spin coater EMS 150T ES was utilized to fabricate Y/ZnO thin films.The spectrophotometer Schimadzu UV3600 was employed for optical transmittance (T%) and reflectance (R%) measurements.´cm 2 .The electrical measurements were carried out in dark conditions using FYTRONIX 9500 electronic device characterization system (ECS).The photocurrent sensitivity was examined using a solar simulator (TM-206).In the frequency range from 10 khz 1 Mhz, -a computerized FYTRONIX impedance analyzer was executed for capacitancevoltage C V ( -)and series resistance-voltage R V s ( -)analysis.dissimilar electronegativity between Zn (1.65) and Y (1.22) which result in crystal deterioration, lattice expansion, a slide shift to higher 2θ angle position and high internal strain.No additional peaks related to hydroxyl group or unreacted ion salts were detected in the pattern [18].Scherer equation:

Results and discussion
was applied to determine the crystallite size D ( ) from the most prominent peaks (101, 002, 100).The lattice strain , e ( ) dislocation density , d ( ) and crystallinity X c ( ) were evaluated using the formulas: ,

Morphological analysis
The surface morphological nature and particles shape were visualized by SEM micrographs.Figures 3(a), (b) show the particles of Zn 0.85 Y 0.15 O in thin sheets aggregated together with high density on the surface due to Van der Walls intermolecular attraction force between dopant and host molecules.However, the surface was found to be compact, irregular, and rough [23,24].The mean particles size was determined from TEM micrographs.As depicted in figures 3(c), (d), most of the particles possess nanosheet structure linked with little nanospheres.The mean particle size of the nanosheets was found to be 14.78 nm which means that the electrons can move in two dimensions, hence one dimension is quantized [25,26].The morphological analysis emphasized that, the two dimension yttrium zinc oxide (2D Y/ZnO) with quantum size effect and large surface area could play an important role in reinforcement of charge carriers density and development of the photodiode efficacy [27,28].

Optical analysis
The optical transmittance (T%) and reflectance (R%) of yttrium zinc oxide thin film are measured through wavelength range from 300-900 nm.As observed in (figure 4(a)), the fabricated film shows good transparency changed between 75%-90% with an intense absorption edge at l = 390 nm close to visible spectrum.This edge owing to electron transition from the valence band V.B ( ) to conduction band C.B .( ) A wide reflectivity band centered at 440 nm attributed to photon-electron interaction on the film surface (figure 4(b)) [29].The optical absorption coefficient a ( ) was given by the equation: Figure 5(a) illustrates the absorption coefficient as a function of photon energy h .n ( ) A strong absorption band was detected at h n = 3.50 eV related to electron transitions from V.B to C.B result in generating free charge carriers [29,30].The energy gap E g is an important optical parameter can be estimated from the absorption coefficient using Tauc relation: A represents an independent constant, h is the Plank's constant, n is the photonic frequency, n is an index takes the values (2, 3, 1/2, 1/3) according to indirect allowed, indirect forbidden, direct allowed, and direct forbidden transitions, respectively.Y/ZnO film has revealed direct allowed band gap of 3.24 eV was determined by extrapolating the linear portion of hnh 2 a n ( ) to zero Y-axis, h 0,   The complex refractive index n* composed of real part (refractive index n) and imaginary part (extinction coefficient k) was identified using the following relation: The thin film exhibited normal dispersion in the visible spectrum changed to anomalous behavior near the interband absorption region due to the scattering energy, (figure 6(a)).The extinction coefficient shows a sharp peak at 390 nm owing to strong optical absorption, (figure 6(b)) [31,32].

I-V analysis under dark
The current-voltage I V ( -)characteristics of Al/Y:ZnO/p-Si/Ag Schottky diode was measured under dark conditions at room temperature.As depicted in (figure 7(a)), the photodiode exhibits high rectification behavior of ratio I I f r / equals 13 10 3 ´at 3 V; I f is the forward current and I r is the reverse current suggesting good switching performance.The presence of Y 3+ work on charge recombination reduction providing more charge carriers transportation.In Schottky barrier diode, the charge carriers transportation between metal contacts and semiconductor (M-S-M) can be analyzed by thermionic emission (TE) theory expressed as: In this equation, q denotes the carrier charge is constant value equals 1.6 10 C, 19 ´k is the Boltzmann constant given by the value 1.38 10 kg m s K , ´--and T is the absolute temperature was 300 Kelvin.The electronic parameters including potential barrier , b f reverse saturation current I , s and ideality factor n have been identified in the forward biased at low voltage 0 V 0.4.> > I s was calculated from the intercept of straight- line portion of semi-log I V -plot at zero potential using the relation: The obtained I s value was used to determine the potential barrier b f deduced by the formula: A signifies the effective metal contact area 0.5 10 cm 2 2 ´--( ) and A* is the effective Richardson's constant (32 Acm k 2 2 -for p-type Si).The fabricated Schottky diode demonstrated small reverse saturation current of about 0.08 nA and low potential barrier 0.647 eV ~indicating low leakage current, small minority charge carriers diffusion, and easily charge passing to metal contacts [33,34].The ideality factor n is an important electronic parameter that describes the photodiode quality can be determined from the slope of ln I V -graph given by the equation: It is important to note that, Schottky diode of ideal behavior has ideality factor of unity which is difficult to be experimentally achieve.In Y:ZnO/p-Si diode, the ideality factor was calculated greater than one which may be ascribed to the presence of series resistance, interface states, native oxides, crystal defects or inhomogeneities between metal and semiconductor [35,36].The series resistance R s has a significant impact on electrical conductivity of photodiode.High photodiode performance requires low series resistance R s and large shunt resistance R sh which are given by junction resistance formula, R .j V I

= ¶ ¶
From R j versus V plot in (figure 7(b)), R s and R sh of Y:ZnO/p-Si diode were calculated to be 2829 W and 1.41 10 8  ´W at 2.5 V  respectively.Moreover, S K Cheung and N W Cheung functions were applied to define R , n, s b f using the equation: R s was determined from the slope of straight line illustrated in (figure 8(a)), whereas n was calculated from the intercept on the I-axis equals nkT q. / A second Cheung function was applied to provide R s and n by the equation: R s was determined again from the slope of H I - ). Norde function is an important model used for evaluating the electronic parameters of an ideal photodiode using the formulas [34,36]: γ is the first integer > n, and I o is the minimum current obtained from I V -corresponding to V o and F V o ( ) was defined from the projection of the minimum point of F (V)-V plot demonstrated in (figure 8(c)) [36, 37].The electronic parameters estimated from TE theory, Cheung, and Norde functions were summarized in table 2. The small potential barrier and relatively low series resistance of the fabricated Schottky diode arise from the presence of 2D Y/ZnO nanosheets as an active layer.Luo et al have studied the significant impact of Y 2 O 3 on electrical resistors reduction of ZnO and improvement of electrical conductivity [38].

Study of schottky diode behavior under illuminations
The photocurrent I ph ( ) sensitivity and conduction mechanism of Y:ZnO/p-Si Schottky diode were investigated under various illuminations.As illustrated in the logarithmic scale (figure 9(a)), the photocurrent was increased from 18 nA in dark to 25 μ A under100 mW cm −2 .When the photodiode is subjected to light, the trapped charge carriers at heterojunction layers absorb incident energy and jump into conduction band result in increasing photoconductivity.The current transport mechanism was identified according to the power law given by the relation: P represents the power intensity, m is an exponent determines the dominant mechanism, and A is constant.The power m = 0.40 was calculated from the slope, (figure 9(b)) confirming the continuous distribution of trapped centers at the interface states.The transient photocurrent and photocapacitance were investigated under the impact of light intensity.When the device is illuminated, the current fast rises to a specific level reaching a saturation limit until light is switching off, (figure 10(a)).During light on, the trapped carriers in deep levels absorb incident photons and become free [39].
During light off, the charge carriers recombine again and trap in deep levels.Thus, the current gradually decreases returning to its initial value [40].The capacitance sensitivity was studied in (figure 10(b)), when the light on, the capacitance rises to a certain value and takes almost stable value.When the light is off, the capacitance suddenly decreases to its initial value attributed to charge carriers recombination.To better understand the photodetector conduct, the photosensitivity Ps, photoresponsivity PR, quantum efficiency QE, and detectivity D* were studied using the following formulas: P s ,      range from −5 V to + 5 V.As can be seen in (figure 11(a)), the measured capacitance C m ( )peak was gradually decreased from 1.6 nF to 0.75 nF shifted to higher voltage in the reverse bias region [42, 43].The capacitance affected by series resistance was determined by the relation: is the angular frequency and C adj is the adjusted capacitance.As observed in (figure 11(b)), C adj was regularly decrease at low frequencies range of 10 kHz-300 kHz thanks to the presence of charge carrier at the interface states that follow ac signals at low frequencies.
The irregular decrease in the capacitance peak at frequencies greater than 300 kHz is attributed to the impact of series resistance in which the interface states cannot follow ac signals at higher frequencies [43,44].The Schottky diode conductance G m and corrected conductance G adj were measured as a function of frequency using the formula: G m peak was increased in the reverse biased with applied frequency (figure 12(a)).Whereas G adj was increased until 900 kHz (figure 12(b)) [45,46].The anomalous behavior with sharp drop at 1 MHz owing to the high impact of series resistance at interfacial layers in which the charge carriers cannot follow ac signals at frequencies greater than 900 kHz, (figure 12(b)).The R s behavior was analyzed from the capacitance-voltage measurements  As exhibited in (figure 13), R s magnitude was regularly decreased at frequencies greater than 200 kHz dependence of the response of interface states at Schottky diode layers to applied alternating current signals.
Furthermore, Mott Schottky (M-S) formula was utilized to identify the charge carrier density and the built-in voltage V bi at the interface states using the equation: ´-/ is the permittivity of vacuum.V bi was evaluated by extrapolation the linear portion of (M-S) graph to V-axis and N d was obtained from the slope, (figure 14) [46].The   the heterogeneous diode, 2D Y/ZnO nanosheets have revealed high electron-transfer based on high charge carriers concentration and mobility.In addition to appropriate energy level alignment and chemical stability [45][46][47].

Conclusions
Yttrium zinc oxide nanosheets were successfully prepared by a facile chemical approach and their physicochemical aspects were characterized using various techniques.The nanosheets appeared in a large surface area with quantum size effect.The energy gap of the prepared thin film was estimated in the range of 3.24 eV revealing strong optical absorption inside the UV region.Schottky diode performance was analyzed from I-V analysis in dark and under illumination conditions indicating high rectification ratio RR = 13 10 3 ´at 3 V, and a relatively small series resistance R s ( ) in the range of 3.00 kW in addition to high photoconductivity and responsivity.The C/G-V analysis confirmed the high charge carriers response at the interface states to applied ac signals result in enhancement of photodiode performance.In conclusion, the presence of 2D Y/ZnO nanosheets plays an important role in increasing free charge carriers mobility and photodiode efficiency.

2. 4 .
Fabrication and characterization of Y:ZnO/p-Si Schottky diode Zn 0.85 Y 0.15 O nanosheets were deposited onto p-type silicon via a spin coater technique.Silicon wafers were etched by dilute HF then cleaned with a mixture of deionized water and acetone bath of ratios 3:1 respectively and eventually washed several times using distilled water.Silver (Ag) and aluminum (Al) metal contacts were evaporated via thermal evaporation technique giving diode surface area of 0.5 10 2

24 =
b respectively[19,20].Where, β signifies the full width at half maximum (FWHM) of the corresponding peak measures in radians, k represents the proportionality constant known as shape factor equals 0.94, θ is the Bragg's scattering angle, and λ denotes the wavelength of x-ray (λ = 1.541Å for CuK α radiation).The crystallographic parameters D, , , X c e b ( )were presented in table1.The large strain and dislocation density related to lattice defects and crystal deterioration by Y 3+ support.Whereas the small crystallite size 27 ~nm associated with the high density of the nanoparticles[20,21].The chemical composition and elements concentration were recorded by EDX spectrum.As demonstrated in (figure1(b)), the nanocomposite constitutes of the main elements zinc (Zn), yttrium (Y), and oxygen (O) suggesting the successful integration of Y ions inside the host ZnO[18,22].The peak located at 0.277 keV arises from carbon coated grid during processing.To define the chemical and electronic state of yttrium zinc oxide nanoparticles, XPS was carried out using a standard carbon reference C 1s detected at 285.54 eV.The survey spectrum revealed the presence of Zn, O, and Y with no other elemental detection, (figure2(a)).The XPS core level of Zn 2p, O 1s and Y 3d are shown in figures 2(b)-(d).The high-resolution XPS spectrum of Zn 2p (figure 2(b)) displays two peaks at 1022.09 eV attributed to the 2p 3/2 core levels of Zn and the second peak at 1045.19 eV ascribed to 2p 1/2 core levels of Zn with spin-orbit difference (23.10 eV) [22].The shift in binding energy compared to the standard pure ZnO of 2.85 eV
is attributed to the existence of Y 3+ dopant in the host ZnO matrix which clearly approves the production of Zn 0.85 Y 0.15 O nanocomposite.The XPS spectrum of O 1s (figure 2(c)) shows three characteristic peaks at binding energies 529.98, 531.88, and 533.12 eV.The two high-resolution spectra are associated with the stoichiometric oxygen ions of ZnO matrix and oxygen vacancies, respectively and the weak peak located at 533.12 eV originated from Y-O bonding.Y 3d spectrum has two spin-orbit splitting peaks at 157.98 eV and 160.16 eV assigned to Y 3d 5/2 and Y 3d 3/2 core level of Y-O and Zn-Y-O bonds, respectively while the third peak at 158.63 eV related to Y-OH, (figure 2(d)) [23].

2 a n = ( ) (figure 5
(b)).In fact, wurtzite ZnO NPs have a standard energy gap of 3.37 eV.The synergism between yttrium ions and ZnO was successfully modified the optical band gap by narrowing the band structure[30,31].

Figure 5 .
Figure 5. (a) Optical absorption coefficient a ( ) and (b) Energy gap E g ( ) plot of Y/ZnO thin film.

3. 6 .
Capacitance/conductancevoltage (C/G -V) analysis To study the junction behavior of Al/Y:ZnO/p-Si/Ag Schottky diode, the impact of voltage and frequency on the capacitance was measured.Figures 11(a), (b) shows the measured capacitance-voltage C heterojunction diode at different frequencies within 10 kHz-1 MHz

Figure 9 .
Figure 9. (a) I-V and (b) power law plot under various illuminations of Y:ZnO/p-Si diode.
e denotes the static dielectric constant of semiconducting Y/ZnO, C signifies the depletion layer capacitance, N d provides the concentration of ionized donor (carrier concentration), and 8

= 2 . 8 ×
N d value was utilized to calculate the barrier height b f by the relation: 10 19 cm −3 is the density of states in the C.B. V , bi N , d and b f values were calculated and tabulated in table4.The fabricated Schottky diode shows large charge carrier concentrations and small built-in voltage.In

Figure 13 .
Figure 13.R s -V characteristics of the fabricated diode as a function of various frequencies.

Table 1 .
Crystallographic parameters of yttrium zinc oxide nanoparticles.

Table 2 .
Electronic parameters of I-V characteristics under dark conditions.

Table 3 .
Photoelectrical parameters of the fabricated Schottky diode under illuminations.

Table 4 .
Electronic parameters obtained from C-V analysis.