Oxide planar p–n heterojunction prepared by low temperature solution growth for UV-photodetector applications

Abstract

The paper presents the low temperature growth of a planar p-NiO/n-ZnO/FTO heterostructure for efficient detection of soft UV light. n-ZnO was prepared at 60 °C using an aqueous bath of zinc nitrate precursor. The 2D layer was uniform and well-covered the FTO substrate. NiO was electrodeposited on top of this layer at 90 °C in a dimethyl-sulfoxide (DMSO)-based electrolytic solution. The use of an aprotic solvent is shown to lead to the direct formation of nickel oxide. The p-type conductivity of NiO was demonstrated by the rectifying character of the heterostructure. The p-NiO/n-ZnO planar heterostructured-heterojunction demonstrated UV-photodetection properties with a good sensitivity under forward and reverse bias. A response S_UV ≈ 2.46 at −1 V applied bias and a relatively low turn-on voltage of about 0.76 V were measured. The latter is much lower compared to turn-on voltages for other p-NiO/n-ZnO heterostructures reported in the literature. The elaborated method can serve as a new paradigm in simple and low-temperature deposition of type II heterostructures with large area and high separation efficiency for fabrication of high-performance optical devices, as well as for other types of applications such as gas sensors and catalysis.

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1. Introduction

Low-temperature syntheses of semiconductor transition-metal oxides (TMO) and related heterostructures have attracted tremendous research activity in recent years due to their importance for various advanced opto-electronic applications.^1–3 In this context, ZnO and NiO can be regarded as important visible transparent n-type and p-type semiconductor materials.⁴ Both have a wide bandgap reported at 3.37 eV and 3.6–3.8 eV at room temperature, respectively. Several physical and chemical methods have been described for the synthesis of thin films of these TMOs such as sputtering,⁵ pulsed laser deposition,⁶ chemical precipitation,⁷ hydrothermal processes,^8,9 sol–gel methods^10,11 and electrochemical deposition (ECD).^12–16 The latter technique is of special interest since it is (i) well-suited for scaling-up, (ii) high-quality crystal layers can be prepared even without annealing, (iii) homogeneous deposition can be performed on arbitrary substrate shapes, (iv) the morphology and size can be tuned by manipulating the deposition parameters, (v) the precise control of deposition position by selective patterning of the substrate is possible, (vi) the electrical contact between structures and substrate is excellent and (vii) there is a minimum inter-reaction or inter-diffusion between as-deposit and substrate due to the low deposition temperature. Well-crystallized ZnO layers with controlled morphological, structural, electrical and optical properties have been prepared by ECD. The technique has also provided dense or porous/nanostructured NiO films implemented in various systems such as alkaline batteries,⁹ electrochemical capacitors¹⁰ and electrochromic devices.^11–13 In most of the researches on nickel oxide electrodeposition, aqueous growth solutions have been used, leading to the formation of precursor layers made of nickel hydroxides and/or oxy-hydroxides that required a subsequent heat treatment to produce the NiO semiconductor compound.^18–22 The use of aprotic media has been described by a few authors to avoid hydrogen incorporation and favour the direct oxide ECD.^23–27 Some works focused on the use of ionic liquid based electrolytes whose main drawback is the high cost,^23–26 whereas the implementation of aprotic organic solvents remains almost unexplored. To the best of our knowledge this approach has only been reported by Su et al.²⁷ who used a dimethyl formamide and nickel chloride precursor bath to directly cathodically deposit nickel oxide without a post-deposition heat treatment. It should be noted that other potential interests in using an organic solvent instead of water include (i) the possibility of working at temperatures above 100 °C to get a better crystallized deposited layer, (ii) the avoiding of layer cracking and (iii) the enlargement of the electrochemical potential window for the layer deposition.

p-NiO and n-ZnO can be combined to form a type II p–n junction that extends their real application.^28–43 p-NiO/n-ZnO heterojunctions have attracted great interest due to very efficient charge separation, which is very useful for applications in transparent electronics, visible-blind UV light photodetector,^35–40 UV photovoltaic cells,⁴¹ sensors,⁴² light emitting diodes,^28–30 photocatalysis,³¹ etc. For formation of highly efficient heterojunctions with large contact area, a low cost well-controllable method for deposition of layers is desirable.^34,43 Accordingly, in this work, a formation of p-NiO/n-ZnO heterojunctions by ECD methods with good UV sensing properties, which can be attributed to excellent separation of photogenerated electron–holes pairs, is reported. High crystallinity of the deposited layers was confirmed by XRD, optical measurements and Raman spectroscopy. It revealed excellent ability of ECD method to growth semiconducting oxides with excellent structural properties. A well-defined rectifying current–voltage characteristic was observed for these all-oxide devices.

2. Experimental

ZnO layers were prepared directly on cleaned fluorine-doped tin oxide (FTO) coated glass substrate (TEC-10) in a three-electrode cell. It was used as the working electrode and rotated at 145 rpm upon the deposition process. The counter-electrode was a zinc wire and the applied potential was controlled versus a saturated calomel reference electrode (SCE). The deposition bath was a solution of Zn(NO₃)₂ at a concentration of 0.08 M dissolved in MilliQ quality water (18.2 MΩ cm) maintained at 60 °C.⁴⁴ The deposition was performed at a constant applied voltage of −0.9 V/SCE for 500 s. The NiO layers were deposited on the electrodeposited ZnO layers used as electrode. It was positioned vertically in a 50 mL three electrode electrochemical cell. A platinum wire was used as counter electrode. The bath was a DMSO solution containing 0.2 M nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O) (Alfa Aesar, 98%) and maintained at 90 °C. A constant potential of −0.92 V vs. SCE was applied for 1000 s. These films were subsequently dried for 10 minutes at 200 °C to remove the solvent traces and annealed at 450 °C for 2 h.

The sample morphologies were examined with a high resolution Ultra 55 Zeiss FEG scanning electron microscope (FE-SEM) at an acceleration voltage of 10 kV. The structure of oxide films was characterized by a PANalytical X-Pert high-resolution X-ray diffractometer (XRD) operated at 40 kV and 45 mA and using CuKα radiation with λ = 1.5406 Å. The film total transmission and total reflection were measured with a Cary 5000 UV-Vis-NIR spectrophotometer equipped with an integrating sphere. The absorbance spectrum was calculated from these two parameters. The photoluminescence (PL) measurement system combined a YAG:Nd laser and a HR250 monochromator (Jobin-Yvon) coupled to a UV-enhanced intensified charge-coupled device (ICCD; Roper). The excitation wavelength was 320 nm. Raman scattering spectra were measured at room temperature with a Renishaw INVIA apparatus equipped with a microscope and a CCD detector. A 532 nm solid-state green laser was used for off-resonance excitation with 10–50 mW power. The instrument was calibrated using a silicon standard. The heterojunction was point contacted by Au/Ga/In shined by a UV-light emitting diode with λ = 365 nm at a power density of 10 mW cm⁻². The current–voltage I–V curves were measured using an EGG PAR273 potentiostat.

3. Results and discussion

The deposition current recorded upon the oxide deposition is presented in Fig. 1a. For ZnO, the cathodic current first increased due to the progressive coverage of the FTO substrate and reached a deposition current plateau of −1.2 mA cm⁻² after more than 120 s of deposition time.⁴⁵ The FTO/ZnO layer was subsequently used as electrode for the electrodeposition of NiO. The current presented a maximum after 30 s and then slowly decreased to reach a constant deposition current of −2.1 mA cm⁻². The rather high value of the deposition current show that the electron flow is not blocked by the n-type ZnO layer at the applied negative deposition electrochemical potential for which the ZnO layer is in a charge accumulation regime.

Fig. 1 (a) Deposition current curves for ZnO electrodeposition on FTO/glass and of NiO on ZnO/FTO/glass; (b) X-ray diffraction patterns of nickel oxide layers grown by electrodeposition on FTO substrate before and after annealing. (c) Raman spectra of the NiO films obtained in DMSO solution before and after thermal annealing at 450 °C for 2 h. The * marked peaks are assigned to the FTO substrate.

The prepared layers were characterized by X-ray diffraction. Patterns of the as-grown/dried and of the annealed NiO films are presented in Fig. 1b as curves 2 and 3, respectively, and they are compared to the FTO substrate pattern (curve 1). For the layer dried after deposition, no NiO characteristic peak was detected due to the poor crystallinity of the layer. After annealing at 450 °C, the several characteristic peaks of cubic nickel oxide marked with “+” correspond to the (111), (200) and (220) planes (space group Fm3̄m) at 2θ = 37.1°, 43.2° and 62.6°, respectively. The lattice parameters are a = b = 4.1944 Å and α = β = γ = 90°. The XRD peaks of NiO(111) and (220) planes are present only as shoulders on left and right sides of tin oxide (200) and (310) planes, respectively. The best defined XRD peak centered at 43.2° corresponds to NiO(200). The peak broadness gives insights into nano-crystallinity of the ECD samples. A XRD diffraction peak centered at about 44.6° and present both before and after annealing could be assigned to cubic metallic nickel (space group Fm3m) Ni(111) plane (pdf card #04-0850) or to other nickel oxide phase (pdf card #14-0841 and 14-0481). It suggests that after thermal annealing, the electrodeposited layer is a mixture of mainly cubic-NiO and other nickel oxide phases. According to our experimental observations an increase in the temperature of thermal annealing in air is necessary to improve crystallinity of the layers prepared in DMSO solution of nickel salt.

The NiO layers have been further investigated by micro-Raman spectroscopy before and after thermal annealing (Fig. 1c). Before annealing, the broad Raman peak centered at 509 cm⁻¹, has been assigned to the first order longitudinal optical phonon (1LO) mode of NiO and the band centered at 1030 cm⁻¹ to the 2LO mode of NiO.^46,47 Globally, no Raman signal due to α-Ni(OH)₂ was found for the dry samples showing the direct deposition of NiO.

After annealing at 450 °C (curve (2)), the spectrum was characterized by two broad Raman emissions centered at 523 cm⁻¹ and 1075 cm⁻¹. The former was assigned to the 1LO mode and the latter to the 2LO mode of NiO. The NiO 1TO mode is present as a shoulder. In the case of NiO/ZnO annealed heterostructure the (1LO) and (2LO) modes were also found at 524 and 1091 cm⁻¹ respectively. In addition the E2-high ZnO mode was present at 438 cm⁻¹ (curve (3)).

The heterostructure absorbance spectrum presented in Fig. 2a is dominated by the band-to-band transition of ZnO below 380 nm. NiO absorption is not clearly observed due to the high bandgap energy of NiO (reported above 3.6 eV) which is masked by F:SnO₂. Fig. 2a also shows the low absorbance of the NiO/ZnO layers in the visible-near infrared wavelength. The optical absorption coefficient α was computed using classical Tauc analysis as reported in previous works^3,48 and displayed in Fig. 2b. Coefficient α depends on the photon energy near the ultraviolet edge of the ECD NiO/ZnO heterostructure. A direct energy bandgap is found from Fig. 2b by extrapolating the linear region of the curve until the interception with the hν axis. The estimated value is ≈3.1 eV, which can be assigned to the energy band gap of the ZnO layer.^3,48 PL measurements showed no special features in the case of NiO layers deposited on FTO either before or after annealing (not shown). In Fig. 2c, the ZnO spectrum is characterized by a strong UV emission centered at 383 nm with FWHM ≈ 25 nm, attributed to near band edge emission (NBE) (curve 1). The high optical quality of the ZnO layer is shown by the absence of significant visible emission due to intrinsic defects. In the case of PL data for NiO/ZnO layer (Fig. 2c, curve 2), no significant differences could be observed in comparison with ZnO layer, i.e. apparition of new peaks with significant intensity. The main reason can be attributed to d–d transitions in the NiO layer, which are dipole-forbidden by the Laporte selection rule.⁴⁹ In results, the recombination processes occur mainly in the ZnO layer.^50,51 Also no shift in position of peak attributed to NBE of the ZnO layer can be observed, which was also observed for other NiO/ZnO heterostructures,^50,51 and is an indicator that recombination of electrons and holes occurs in the ZnO layer.^49–52

Fig. 2 (a) Plot of absorbance versus wavelength for NiO/ZnO heterostructure annealed at 450 °C for 2 h. (b) Tauc plot of (ανh)² vs. photon energy for NiO/ZnO layers annealed at 450 °C for 2 h. (c) Room-temperature photoluminescence spectra of (1) ZnO and (2) NiO/ZnO heterostructure annealed at 450 °C for 2 h.

Fig. 3a and b shows SEM images of ZnO under-layer deposited on FTO/glass at low and high magnifications, respectively. As can be seen, ZnO films consisted of densely packed crystallites with rounded shape that completely covered the FTO layer. The size of the ZnO crystals forming the layer ranged between 150 and 500 nm. The electrodeposited ZnO layer was uniform with a mean thickness of 400 nm (see Fig. 3c). Fig. 3d, e and S1† show SEM top-view images of NiO deposited on ZnO/FTO/glass. They show that NiO grew in form of large grains with a size in the 300 nm to 3 μm range and made of very densely packed nanoparticles (Fig. 3e). A cross-sectional view of the NiO/ZnO/FTO/glass structure is shown in Fig. 3f. NiO forms a homogeneous 2D layer on top of the ZnO film with a measured thickness in the range of 400–420 nm range.

Fig. 3 SEM images of (a and b) ZnO underlayer deposited on FTO/glass; (c) cross-sectional view for ZnO underlayer deposited on FTO/glass substrate; (d) NiO top-layer deposited on ZnO/FTO/glass; (e) NiO top-layer deposited on ZnO/FTO/glass magnified; (f) cross-sectional view for NiO/ZnO layers deposited on FTO/glass substrate.

Fig. 4a shows the general structure of the device elaborated in this work and Fig. S2† is the energy band diagram of the p-NiO/n-ZnO heterostructure at equilibrium. The planar heterostructure was contacted on the FTO layer (−contact) and on the NiO layer (+contact). In Fig. 4b the I–V curves recorded in the dark and under UV illumination (λ = 365 nm, 10 mW cm⁻²) from the NiO layer side is shown. The device demonstrated a rectifying I–V curve typical of a heterojunctions diode, whereas a linear curve was found when the contacts were taken on the ZnO and FTO. This establishes that the electrodeposited NiO was actually p-type.³⁴ The observed I–V characteristics in the dark and under UV illumination can be explained aided by the energy band diagram of the p-NiO/n-ZnO heterojunction under reverse bias conditions. In the dark, the p-type NiO blocks the electron injection from the Au/Ga/In contact and the n-type ZnO layer blocks hole injection from the FTO cathode. Under illumination at 365 nm, UV-light is absorbed in the ZnO layer and hole–electron pairs are generated (hv → e⁻ + h⁺), which are separated by the generated built-in electric field (Fig. 4c). Thus, from p-NiO to n-ZnO region the electron flow takes place, while from n-ZnO to p-NiO region the hole flow takes place, leading to enhanced photocurrent.¹⁷

Fig. 4 (a) Illustration of device structure (p-NiO/n-ZnO/FTO/glass) and respective connection for electrical measurements. (b) Typical current–voltage characteristic of the p-NiO/n-ZnO heterostructure based device in the dark and under UV illumination (λ = 365 nm), (c) illustration of energy band diagram of reverse biased p-NiO/n-ZnO heterostructure under illumination with UV light.

In dark conditions, a dark leakage current of 1.15 mA (at reverse bias of −2 V) was detected. Moreover, the turn-on voltage of ≈0.76 V of our structures is much lower compared to the values of other p-NiO/n-ZnO heterostructures reported in literature (see Fig. 4c).^40,50,51 Thus, taking into account the relatively high forward current (in range of mA) it can be concluded that contacts on p-NiO and n-ZnO layers are of low resistance, and the observed turn-on voltage results from rectifying properties of the p-NiO/n-ZnO heterocontact.³⁴ Microcracks in NiO may alter the electrical behaviour and lead to direct contact to the ZnO layer. The estimated series resistance (R_s) from I–V characteristics in Fig. 4a amounts to 260 Ω, which also demonstrates low resistance of the formed contacts.²⁶

Under illumination with λ = 365 nm, ratio of the photo- to dark current at reverse and forward bias is 1.9 (for instance both at −2 V and +1.6 V), demonstrating good sensing properties of the soft UV light. Illumination with deeper UV light at λ = 254 nm, gives rise to a lower photocurrent signal under reverse bias (Fig. S3a, ESI†). In this case light is absorbed in the NiO layer and the low current suggests the presence of defects in NiO which induces charge recombination. Consequently, a lower ratio of 1.2 for forward current was observed. This ratio is even lower (1.07) at reverse current (Fig. S3a†), demonstrating very low sensitivity to UV illumination with λ = 254 nm.

The system dynamic characteristics were investigated by measuring the transient response at different applied voltage bias. Fig. 5 shows the transient response at +1 V (a), −1 V (b) and −1.5 V (c). Because response and recovery curves are consisting of two components, namely a fast and a slow one,⁵³ the UV response and time components were calculated by bi-exponential fitting^54,55 and gathered in Table 1. The fast component is attributed to fast change in the charge carrier concentration under exposure to UV light, while the slow component is a result of slow oxygen photodesorption/adsorption from the surface of NiO top layer.^53,56,57

Fig. 5 Transient response to UV illumination (λ = 365 nm) under (a) +1 V, (b) −1 V and (c) −1.5 V applied voltage bias.

Table 1 UV-response parameters of the fabricated devices at different applied voltages

Applied voltage	SUV = IUV/Idark	τr1 (s)	τr2 (s)	τd1 (s)	τd2 (s)
+1 V	1.76	11.4	142	27.8	—
+1 V (low UV)	1.70	5.6	158.5	13.8	—
−1 V	2.46	4.6	62.3	22.7	293.3
−1 V (low UV)	2.34	7.2	99.8	23.3	293.6
−1.5 V	3.15	6.5	112.7	26.2	316.3

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The highest UV response is observed at −1.5 V (S_UV ≈ 3.15), while the lowest UV response is at +1 V (S_UV ≈ 1.76). This can be a result of the higher potential barrier created at reverse bias,^34,50,51 which is more efficient for separation of photoexcited holes and electrons. However, the fastest response time is recorded for −1 V, and it is about two times faster than for −1.5 V, while recovery times are comparable. Also, it is very important to mention that an incomplete recovery is found for positive applied voltage (1 V), demonstrating a persistent photoconduction. Thus, the most favourable applied voltage to obtain the fastest UV photodetectors is −1 V. Under UV illumination with lower intensity at applied bias voltage of +1 V and −1 V (Fig. S3b and c, ESI†), the UV response was decreased but no essential differences in time constants were observed (Table 1). Such slow response and recovery of the devices can be attributed to a slow photodesorption process of oxygen molecules from the surface of the NiO layer.^40,50,51

4. Conclusions

We have presented the solution growth by electrodeposition of planar p-NiO/n-ZnO/FTO heterostructures and introduced a new technique for NiO thin film preparation using an aprotic DMSO electrolytic bath. The layers were rapidly prepared at low temperature (below 90 °C). Good crystallinity of the as-deposited layers and improvement by thermal annealing were demonstrated by XRD, optical and micro-Raman measurements. The elaborated prototype devices demonstrated quite good UV-sensing properties at −1 V applied bias with S_UV ≈ 2.46. The relatively low turn-on voltage of ≈0.76 V is much lower when compared to values of other p-NiO/n-ZnO heterostructures reported in the literature and is more favourable for practical applications. Such high UV sensing performances can be a result of high separation efficiency of formed p-NiO/n-ZnO heterostructures with high contact area. The transient UV-response was measured at different applied voltage bias (+1 V, −1 V, −1.5 V) and demonstrated excellent recovery and higher speed at −1 V applied bias (τ_r1 ≈ 4.6 s, τ_r2 ≈ 62.3 s, τ_d1 ≈ 22.7 s, τ_d2 ≈ 293.3 s). Future works will be devoted to the increase of the electrodeposited NiO layer purity, density and coverage and to the improvement of the interface between the two semiconductor layers. Also the improvement of the device design and contact features should permit better UV photodetection performances.

Acknowledgements

Dr Lupan gratefully acknowledges CNRS Council for support as expert scientist at IRCP Chimie ParisTech, Paris. This work was supported by the ANR POSITIF project, grant no. ANR-12-PRGE-0016. This work was partially financially supported by the STCU under the Grant no. 5989. O. Majerus (IRCP, Paris, France) is acknowledged for her help in Raman spectra measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13763f

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