1.
Introduction

The ZnO film with wide band gap has the characteristics of low cost, high transparency, low resistivity, and large exciton binding energy (60 meV). It is a good candidate semiconductor material for light emitting diodes^[1–5]. The ZnO/Si heterojunction diode can be integrated with the Si process, and can also be used in many fields such as photo-detector and emission devices^[6–9]. However, due to the lattice mismatch between ZnO and Si, there are a large number of interface states while directly depositing ZnO on Si, which seriously affect the photoelectric performance of the ZnO/p-Si heterojunction.



CuI is a high-conductivity p-type semiconductor material with a large energy gap of 3.1 eV. It is a molecular crystal that forms few stress and interface states and it has been widely used in solar cells^[10–14]. In this work, n-type ZnO thin film was deposited on a p-Si substrate to form an n-ZnO/p-Si heterojunction substrate. To passivate the ZnO/Si interface, a thin CuI film interface passivation layer was inserted at the ZnO/p-Si heterojunction interface.

2.
Experiment

A p-type Si (100) wafer with a resistivity of 50 Ω·m was used as a substrate for ZnO thin film deposition. The Si wafer was boiled for 8 min in a mixed solution of ammonia water, hydrogen peroxide and deionized water (1 : 2 : 5 by volume) and then boiled for 8 min in a mixed solution of hydrochloric acid, hydrogen peroxide and deionized water (1 : 2 : 8 by volume). The Si wafer was then cleaned ultrasonically in ethanol for 15 min, and finally was washed in deionized water and blown dry with argon before use. Sputtering of ZnO was carried out in an argon atmosphere at atmospheric pressure of 0.6 Pa, and the power of DC sputtering was 300 W. The sputtering lasted for 15 min. Al₂O₃-doped ZnO ceramics (1 : 100 by molar ratio) were used as sputtering targets. An Al back surface field was deposited on the reverse side of the ZnO/p-Si heterojunction by sputtering the high-purity Al target, and then annealed at 400 °C for 4 h in the argon atmosphere to form better ohmic contact.



The CuI film interface passivation layer was prepared on a p-type Si wafer by the successive ionic layer adsorption and reaction (SILAR) method^{[13, 15–17]} due to its low cost.



A mixed solution of CuSO₄·5H₂O (0.1 mol/dm³) and Na₂S₂O₃ (0.1 mol/dm³) was used as a cationic precursor. The volume ratio of CuSO₄·5H₂O to Na₂S₂O₃ is 5 : 2. KI at a concentration of 0.025 mol/dm³ was used as an anion precursor for growing CuI. If the substrate for growing CuI film is immersed in the cationic precursor for 5 s, Cu⁺ is adsorbed on the surface of the substrate. The less adsorbed Cu⁺ was washed away in the process of rinsing the cation precursor for 5 s with deionized water. The substrate was then immersed in an anionic precursor for 20 s so that the Cu⁺ reacted with the I^- to formation of CuI. Finally, the substrate was rinsed with deionized water for 5 s to remove the less adsorbed ions in the powdery structure. This process was repeated for 6 cycles. The process for preparation of CuI films by SILAR is shown in Table 1.

Chemical component	CuSO4·5H2O + Na2S2O3	Deionized water (H2O)	KI	Deionized water (H2O)
Concentration	5 × 0.1 mol/dm3 + 2 × 0.1 mol/dm3	With electrical resistivity of 18 MΩ·cm	0.025 mol/dm3	With electrical resistivity of 18 MΩ·cm
PH	～5	7	～6	7
Specific reaction	${left[ {{ m{Cu}}left( {{{ m{S}}_{ m{2}}}{{ m{O}}_3}} ight)} ight]^ - } leftrightarrow { m{C}}{{ m{u}}^ + } + {{ m{S}}_{ m{2}}}{ m{O}}_3^{2 - }$	Rinsing off the less absorbed Cu+	${ m{C}}{{ m{u}}^ + };{{ + }};{{{ m I}}^ - } to { m{CuI}}$	Rinsing off the less absorbed Cu+
Soaking time	5	5	20	5

Table1.
Process for preparation of CuI films by SILAR.



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Chemical component	CuSO4·5H2O + Na2S2O3	Deionized water (H2O)	KI	Deionized water (H2O)
Concentration	5 × 0.1 mol/dm3 + 2 × 0.1 mol/dm3	With electrical resistivity of 18 MΩ·cm	0.025 mol/dm3	With electrical resistivity of 18 MΩ·cm
PH	～5	7	～6	7
Specific reaction	${left[ {{ m{Cu}}left( {{{ m{S}}_{ m{2}}}{{ m{O}}_3}} ight)} ight]^ - } leftrightarrow { m{C}}{{ m{u}}^ + } + {{ m{S}}_{ m{2}}}{ m{O}}_3^{2 - }$	Rinsing off the less absorbed Cu+	${ m{C}}{{ m{u}}^ + };{{ + }};{{{ m I}}^ - } to { m{CuI}}$	Rinsing off the less absorbed Cu+
Soaking time	5	5	20	5

The crystal structure of the ZnO thin film deposited on p-Si was measured by X-ray powder diffraction (XRD RINT-2100V, Rigaku, Cu Kα). The current–voltage properties of the ZnO/p-Si heterojunction were measured with an Agilent sourcemeter (model 4165 C). The capacitance-voltage properties were measured with an Agilent LCR meter (model 4824 A). The resistivity of the ZnO film was measured with the four-probe test resistance tester manufactured by Four Probes Technology Co., Ltd. Thickness of thin films was measured by Step Height Measurement (NanoMap-PS).

3.
Experimental result

3.1
The resistivity and thickness of ZnO films and CuI films

According to the measurement results of ZnO film resistivity and thickness, it is found that the presence or absence of the CuI interface layer has little effect on the resistance and thickness of ZnO film. The resistance values of the ZnO thin films are 73 and 72 Ω/□ respectively. The resistance values of the CuI films are 43 Ω/□. The thicknesses of the ZnO films on the Si and the CuI are 135 and 137 nm respectively.

3.2
XRD characterization of ZnO/CuI/p-Si heterojunction

Fig. 1 shows the X-ray diffraction (XRD) pattern of the ZnO/p-Si heterojunction with and without the CuI interfacial layer. From the XRD pattern, it can be found that the insertion of the thin CuI layer has not significantly affected the growth of hexagonal polycrystalline ZnO film. Comparatively, after inserting a layer of CuI film, the diffraction peak of the (200) plane of ZnO is slightly weakened, which could be due to the decrease of ZnO grains grown on the (200) edge of the CuI layer.






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Figure1.
(Color online) X-ray diffraction pattern of ZnO/p-Si and ZnO/CuI/p-Si heterojunctions.

3.3
I–V characteristics of ZnO/CuI/p-Si heterojunctions

Fig. 2 shows the comparison of I–V characteristics of ZnO/p-Si heterojunctions and ZnO/CuI/p-Si heterojunctions. According to the figure, when a CuI film is inserted into the ZnO/p-Si heterojunction interface, the forward current increases ($V > 0.3;{
m{ V}}$), the reverse current is significantly reduced and the rectification ratio increases.






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Figure2.
(Color online) I–V curve for the ZnO/p-Si and ZnO/CuI/p-Si heterojunction.

3.4
C–V characteristics of ZnO/CuI/p-Si heterojunctions

Fig. 3 shows the comparison of the capacitance–voltage ((C_LF–C_HF)/C_HF–V) characteristic curves of ZnO/p-Si and ZnO/CuI/p-Si heterojunctions. The low frequency and high frequency capacitances (C_LF and C_HF) are measured at voltages of 0.5 and 500 kHz, respectively. The measured values of ZnO/CuI/p-Si heterojunctions are smaller than those of ZnO/p-Si heterojunctions, which indicates that the interface of ZnO/CuI/p-Si heterojunction has fewer defects.






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Figure3.
(Color online) Capacitance–voltage characteristic curves (C_LF–C_HF)/C_HF–V of ZnO/p-Si and ZnO/CuI/p-Si heterojunctions.




Fig. 4 is the comparison of the 1/C²–V characteristic curves between ZnO/p-Si heterojunction and a ZnO/CuI/p-Si heterojunction. According to Fig. 4, the fitted straight-line extension of the 1/C²–V characteristic curve of the ZnO/CuI/p-Si heterojunction has a much larger intercept with the abscissa (V axis) than that of the ZnO/p-Si heterojunction, indicating that the ZnO/CuI/p-Si heterojunction has a large internal barrier.






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Figure4.
(Color online) 1/C²–V characteristic curves of the ZnO/p-Si and ZnO/CuI/p-Si heterojunctions.

4.
Analysis and discussion

The interface state density of the Schottky junction formed by metal semiconductor contact can be measured by the high and low frequency capacitance method^[18–20]. The relationship between interface state density and high and low frequency capacitance is shown in Eq. (1):

${D_{{ m{it}}}} = {left( {{{q{varepsilon _{ m{s}}}{varepsilon _0}{N_{ m{a}}}}/{2{V_{ m{a}}}}}} ight)^{{1/2}}}{{left( {{C_{{ m{LF}}}} - {C_{{ m{HF}}}}} ight)}/{q{C_{{ m{HF}}}}}},$

(1)

where $q$ is the electron charge, ${varepsilon _{
m{s}}}$ and ${varepsilon _0}$ are the semiconductor dielectric constant and the vacuum dielectric constant, respectively, ${N_{
m{a}}}$ and ${V_{
m{a}}}$ are the doping amount and the band bending amount of the semiconductor, respectively, and ${C_{{
m{LF}}}}$ and ${C_{{
m{HF}}}}$ are the low frequency and high frequency capacitance, respectively. As the impurity concentration of ZnO is much larger than the doping concentration of p-Si, the ZnO/p-Si heterojunction can be approximated by a metal semiconductor contact, and the interface state density can also be expressed by Eq. (1).



According to Eq. (1), the interface state density of the ZnO/CuI/p-Si heterojunction is smaller than that of the ZnO/p-Si heterojunction. The main reason CuI can reduce the interface state is that CuI is a molecular crystal. As CuI is relatively soft, although the thermal expansion coefficients of Si and CuI are different, no dislocations and defects are formed at the interface when the heterojunction is formed^[21]. Therefore, CuI can effectively reduce the interface state of the ZnO/Si interface.



At high frequencies, considering the influence of the interface state of the heterojunction, the capacitance of the n-ZnO/p-Si heterojunction cell can be expressed as^[22] :

${C^{ - 2}} = frac{{2({N_{ m A}}{varepsilon _1} + {N_{ m D}}{varepsilon _2})}}{{q{N_{ m A}}{N_{ m D}}{varepsilon _1}{varepsilon _2}}}frac{1}{{left( {{V_{ m D}} - V} ight)}}({V_{ m D}} - frac{{{Q_{ m IS}}^2}}{{2q({N_{ m A}}{varepsilon _1} + {N_{ m D}}{varepsilon _2})}} - V).$

(2)

At high frequencies, the curve 1/C²–V is still a straight line, and the apparent internal potential V_int can be obtained from the cross-section of the line.

${V_{operatorname{int} }} = {V_{ m D}} - frac{{{Q_{ m IS}}^2}}{{2q({N_{ m A}}{varepsilon _1} + {N_{ m D}}{varepsilon _2})}}.$

(3)

The change in interface state density at the ZnO/p-Si interface can be analyzed according to Eq. (3). According to Fig. 4, the built-in potential of the ZnO/CuI/p-Si heterojunction is about 0.8 V, which is quite different from the value of 0.3 V for the prepared ZnO/p-Si heterojunction. According to analysis by Eq. (3), this is mainly due to the existence of a large number of interface states at the ZnO/p-Si heterojunction interface. After inserting a layer of CuI film, the interface state is reduced, causing the increase in the built-in potential.



Under forward bias, there are three main types of carrier motion in the n-ZnO/p-Si heterojunction, as shown in Fig. 5: (I) Electron motion in the bottom of the p-Si conduction band to the bottom of the conduction band of n-ZnO; (II) Hole movement at the top of the valence band of n-ZnO to the top of the valence band of p-Si; (III) The electrons at the bottom of the n-ZnO conduction band conform to the holes at the top of the p-Si valence band through the interface state. The Ⅰ and Ⅱ processes are beneficial for forward current, and the III process is detrimental to forward current. The greater the interface state, the easier it is for the III process to occur. After inserting the CuI film at the n-ZnO/p-Si heterojunction interface, the interface state is reduced, which weakens the process of the forward voltage download stream motion III, resulting in an increase in the forward current.






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Figure5.
Energy band structure diagram for n-ZnO/p-Si heterojunction with forward applied voltage and the carriers transport.




In the region of $0 leqslant V < 0.3;{
m{ V}}$, the forward current is relatively reduced, which is because hot carrier tunneling is the main conductive mechanism in the forward low voltage region, and the reduced interface state is not beneficial. In the reverse voltage region, the III process is beneficial to the reverse current, so the reverse current of the n-ZnO/p-Si heterojunction is reduced after the CuI film is inserted. Meanwhile, as the insertion of the CuI film reduces the interface state, it increases the effective built-in potential, and increases the rectification ratio of the n-ZnO/p-Si heterojunction.

5.
Conclusion

In this study, a thin CuI layer was deposited at the ZnO/Si interface. It was found that the CuI did not affect the crystallinity of ZnO, while it significantly improved the heterojunction properties, such as reducing the reverse current and increasing the rectification ratio. The possible mechanism has been proposed, and these results showed that the passivation of interface is critical for ZnO/Si heterojunctions.