p-InSe heterojunctions

    Fabrication and characterization of photosensitive n-ZnO/p-InSe heterojunctions Z. Kudrynskyi, V. Khomyak, V. Katerynchuk, M. Kovalyuk, V. Netyaga, B. Kushnir PII: DOI: Reference:

S0040-6090(14)01117-1 doi: 10.1016/j.tsf.2014.11.015 TSF 33885

To appear in:

Thin Solid Films

Please cite this article as: Z. Kudrynskyi, V. Khomyak, V. Katerynchuk, M. Kovalyuk, V. Netyaga, B. Kushnir, Fabrication and characterization of photosensitive n-ZnO/p-InSe heterojunctions, Thin Solid Films (2014), doi: 10.1016/j.tsf.2014.11.015

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ACCEPTED MANUSCRIPT Fabrication and characterization of photosensitive n-ZnO/p-InSe heterojunctions Z. Kudrynskyi 1,*, V. Khomyak 2, V. Katerynchuk 1, M. Kovalyuk 1, V. Netyaga 1, B. Kushnir 1

Frantsevich Institute for Problems of Materials Science, National Academy of Sciences of

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Ukraine, Chernivtsi Branch, str. I. Vilde 5, 58001 Chernivtsi, Ukraine, (e-

Yuriy Fedkovich Chernivtsi National University, str. Kotsubinsky 2, 58012 Chernivtsi, Ukraine

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Corresponding

author:

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mail: [email protected]);

e-mail: [email protected];

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Abstract

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Indium monochalcogenide (InSe) with a band gap of 1.25 eV is a promising material for photovoltaic applications. In this work, photosensitive anisotype n-ZnO/p-InSe heterojunctions were fabricated by means of radio-frequency magnetron sputtering of the zinc oxide onto freshly

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cleaved (0001) van der Waals surface of p-InSe single-crystal. Structural properties of the obtained heterostructures were investigated by means of X-ray diffraction. Surface morphology of the grown ZnO thin films was studied by means of atomic force microscopy. The electrical and photoelectrical properties of the heterojunctions were investigated using the current-voltage characteristics measured at different temperatures, capacitance-voltage characteristics and photoresponse spectra. The dominating current transport mechanisms through the heterojunctions under investigation were determined at forward and reverse bias. It was found that the developed heterojunctions n-ZnO/pInSe show photosensitivity in the photon energy range (1.25 - 3.20) eV at room temperature. In addition, we analyzed the influence of vacuum annealing of the heterojunctions at different temperatures on their photoelectric properties. 1

ACCEPTED MANUSCRIPT Keywords: heterojunction, layered crystals, InSe, ZnO, van der Waals surfaces, magnetron

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sputtering.

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1. Introduction

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Wide-band-gap conductive films of metal oxides are widely used in fabrication of the heterojunctions. They act as a “window” layer in heterojunctions because they are transparent for the optical irradiation of wide spectral range. In addition, the low surface resistance of the films

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provides formation of the depletion region mostly in the semiconductor substrate. The mentioned

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factors promote efficient light-to-electrical energy conversion. Zinc oxide (ZnO) and indium monoselenide (InSe) layered crystals are the semiconductors which are of great interest from the point of view of heterojunction fabrication [1-4]. With a band

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gap of 1.25 eV, InSe is a promising material for the solar cell technology [5-9]. There have been several papers reporting the optimization of parameters of InSe-based heterojunctions for solar cell

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application, e.g. ITO/p-InSe [9], intrinsic oxide/p-InSe [8] etc. The largest solar efficiency ever reported for an InSe-based solar cell (8% for unannealed samples and 10% for annealed samples)

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was attained by the authors of [9]. Moreover, it has been recently established that the exfoliated InSe nanosheets are optically active at T=300 K, in the technologically important near-infrared spectral range between 1 and 0.8 μm, thus offering unique opportunities for planar device architectures and near-infrared optical sensing [10]. It is well known that even a small lattice mismatch or difference in linear expansion coefficients usually lead to a high concentration of defects at the heterointerface between n- and ptype semiconductors [11]. Such defects make impossible fabrication of high quality heterojunctions. However, the cleaved facets of InSe layered crystals present an atomically smooth surface (rootmean-square roughness is about 0.05 nm) with saturated chemical bonds (van der Waals surface) [12]. Such surface does not need any additional chemical or mechanical treatment and is an ideal 2

ACCEPTED MANUSCRIPT model of a substrate for heterojunction fabrication. Lattice mismatch is not an obstacle for fabrication of high quality diode structures on the basis of the III-VI layered crystals [13]. So the development of heterojunctions using the van der Waals surface of the layered crystals and their

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investigation are of great research interest both from the practical and fundamental point of view

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[14].

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It is well known that the properties of ZnO films significantly depend on substrate onto which they were grown. The ZnO films have never been reported to be grown onto InSe surface. In the present paper we fabricated n-ZnO/p-InSe heterojunctions, studied surface topology

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of the ZnO films and photoelectrical properties of the p-n-junction. It should be noted that the

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optimization of parameters of the n-ZnO/p-InSe heterojunctions for enhancing solar cell efficiency goes beyond the scope of this paper and will be the subject of our future studies.

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2. Experimental details

We used p-InSe single crystals as substrates. The crystals were grown by the Bridgman

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method. The InSe crystals were doped by adding cadmium in order to obtain p-type conductivity. The content of cadmium dopant was 0.01%. The grown single crystals had a well pronounced

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layered structure and mirror-like cleaved surfaces. At room temperature for the p-InSe crystals the concentration of uncompensated acceptors and majority carrier mobility were measured to be p=1014 cm-3 and μp=100 cm2/V·s, respectively. ZnO thin films were deposited onto the van der Waals surfaces (0 0 1) of the freshly cleaved p-InSe single-crystal substrates (with typical dimensions 5 x 5 x 0.4 mm) heated up to T=150 °C. The films were grown by means of radio-frequency (RF) magnetron sputtering using the VUP-5M multipurpose deposition system. A disk of pure zinc (99.999%) of 40 mm in diameter was used as a target. High purity argon and oxygen were used with the Ar : O2 ratio of 4 : 1. RF frequency generation was 13.56 MHz. The grown thin ZnO layers had n-type conductivity. Front electric contact to the ZnO film was formed by thermal deposition of aluminum at 100 °C. High purity indium (In) was used as a back contact material. To eliminate the non-ohmicity of the back contact 3

ACCEPTED MANUSCRIPT p-InSe/In, we used a method proposed in [11]. This method consists of formation of a large number of recombination centres in the region of the semiconductor which is adjacent to the heterointerface “semiconductor/metal” by means of damaging the InSe surface mechanically. In this case the shunt

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currents of the barrier increase significantly and the voltage drop over the contact is much smaller

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than the voltage drop over the heterojunction.

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The schematic diagram of the developed heterojunction is shown in Fig. 1. Vacuum annealing of the obtained n-ZnO/p-InSe heterojunctions was conducted at T = 150°C during 2 hours.

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The crystal structure of the developed heterojunction was investigated by means of an X-ray diffraction (XRD) system (DRON-2.0 diffractometer) using conventional Bragg-Brentano (θ-2θ)

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measurement configuration with a Cu Kα source. LАТТIК–КАRТА software was used to analyze the obtained XRD patterns.

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The current-voltage (I-V) characteristics of the heterojunction were measured in the

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temperature range 254÷332 K using a “Schlumberger SI.1255” setup.

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Photosensitivity spectra of the n-ZnO/p-InSe heterojunctions were measured by means of MDR-3 monochromator (reciprocal linear dispersion 2.6 nm/mm) at room temperature.

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Surface morphology of the CdO thin films was studied by atomic force microscopy (AFM) using Nanoscope IIIa Dimension 3000SPM (Digital Instruments).

3. Results and discussion 3.1. Structural properties The XRD result justified the formation of ZnO film onto the InSe substrate. It was found that the grown ZnO film was polycrystalline with a hexagonal structure (P6_3mc space group) and had the following lattice parameters: a = 3,242 Å, c = 5,188 Å. The InSe substrate was singlecrystal with a rhombohedral structure and had the following lattice parameters: a = 4,002 Å, c = 24,9678 Å.

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ACCEPTED MANUSCRIPT The obtained results allow us to determine the lattice mismatch between the InSe substrate and grown ZnO film: ε = [aInSe- aZnO]/aInSe= 19 %.

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3.2. Surface topology

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Fig. 2 shows the surface topology of the grown ZnO film investigated using AFM.

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It is clearly seen that the film has a grain-like structure. The grains are randomly orientated. Such structure of the film indicated that it is polycrystalline. Each grain presents an ordered crystalline ZnO structure. If this is so then an abrupt edge of photosensitivity should be observed in

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the high-energy region (corresponding to light absorption in ZnO) of the photocurrent spectra which

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reflect light absorption both in the substrate and the oxide film. An abrupt edge of photocurrent or absorption is characteristic of single-crystal semiconductors. It is seen in Fig. 2 that there is light-coloured (oxidized) and dark-coloured (underoxidized)

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regions in this image. The nucleus oxide molecules are gathered into particle associations with a dome-like shape. The density of such agglomerates is quite high and their sizes are in the range of

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5-15 nm. Such nanostructures indicate weak wetting of one material (ZnO) by another one (InSe substrate). Multiple channels caused by the nonuniformity of oxide growth emerge in the film.

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Because the oxide film is formed in such a way, it is interesting to study the p-n-junction from the point of view of the influence of the shunt currents. Moreover, it is interesting to establish which factors, e.g. thermal annealing in vacuum, can result in improvement of properties of the film and heterojunction.

3.3. Electrical properties The capacitance-voltage (C-V) characteristics of the n-ZnO/p-InSe heterojunctions plotted in 1/C2 vs. V coordinates are linear indicating that the p-n-junction is abrupt. The I-V characteristics of the n-ZnO/p-InSe heterojunctions are non-linear, which demonstrates that the obtained heterojunctions exhibit diode behaviour (Fig. 3).

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ACCEPTED MANUSCRIPT The shape of the I-V characteristics at forward bias is similar to an exponential dependence. At back bias the saturation current begins changing only at higher temperatures and higher voltage. The I-V characteristics of the n-ZnO/p-InSe heterojunction at are shown in Fig. 4. The

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forward branches of the I-V characteristics (before vacuum annealing) are described by an

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exponential function, so they are plotted in semilogarithmic scale (Fig. 4,a).

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The experimental results can be described by the following equation [15]:

 qV  J  exp    nkT 

(1)

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where J – the current density, n – is the ideality factor, q is the electron charge, V is the applied voltage, k is the Boltzmann constant, T is the temperature. If n=1 then the diffusion current

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is dominant; if n=2 then the recombination current is dominant. If both currents are of comparable values then the value of n is between 1 and 2.

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In our case, at small bias the slope of the I-V characteristics is approximately 3.7 and almost

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does not depend on the temperature. The fact that n is >2 and is temperature independent indicate

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that here we can have be both tunnel and tunnel-recombination current transport mechanisms. In both cases, the slope of the I-V characteristic does not depend on temperature as well as in our case [11]. But quantum-mechanical tunneling is possible only when the thickness of the potential

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barriers is smaller then the de Broglie wavelength for an electron (< 100 Ǻ). Such situation occurs in highly doped and degenerated semiconductors. Such current transport mechanisms are not possible in the n-ZnO/p-InSe heterojunction, because the concentration of charge carriers in InSe is far from degeneration. Another alternative current transport mechanism through the heterojunction can be shunt currents, which are added to other currents and distort our understanding of the electronic processes. Usually these currents are dominant at low applied voltage. When the voltage is increasing, the diffusion current predominates over the shunt current because of their different exponential dependences. At the same time, the value of the shunt currents depends on the quality of the heterojunction. Such shunt currents are caused by the defects and mechanical strain which occur at 6

ACCEPTED MANUSCRIPT the heterointerface as a result of temperature changing during the fabrication of a heterojunction and ohmic contacts. The defects are resulted by the differences between two contacting materials. Thermal annealing of the samples in vacuum is an effective method to decrease the number of

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defects and therefore decrease the shunt currents. If the shunt currents are decreased after annealing

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then the ideality factor n is increased.

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The back branches of the I-V characteristics (before vacuum annealing) are shown in Fig. 4, b. Extension of the p-n-junction leads to the suppression of injection currents and emergence of the currents limited by the space charge. These currents are resulted by the current through an insulator

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[16]. According to [16] at not high electric field strength:

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J  

V2 L3

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where  is the relative permittivity;  is the electron mobility; L is the thickness of the p-n-

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junction. This equation corresponds to the trapless quadratic law. The back branches of the I-V characteristics of the n-ZnO/p-InSe heterojunction are plotted in log-log scale because for the

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currents limited by the space charge the I-V characteristics correspond to the power law. As it is seen from Fig. 4,b the I-V characteristics are described by the quadratic law in the investigated

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temperature range. It is worth noting that at higher temperatures (335 K) the ohmic dependence is dominating in the quadratic law. Hence, a depletion region is formed as a result of fabrication of the n-ZnO/p-InSe heterojunction. The barrier region leads to the injection of electrons into the semiconductor at forward bias and to the currents limited by space charge at back bias. At forward bias the forward currents are added with the shunt currents, which leads to overestimated values of the ideality factor n. Vacuum annealing of the samples is an approach to improve the ideality factor n. The n-ZnO/p-InSe heterojunction was annealed in vacuum at T = 150°C during 2 hours. In Fig. 5 one can see the I-V characteristics of the heterojunction measured after the vacuum annealing. They indicate that the low-temperature annealing does not change significantly the diode properties of the heterojunction compared to the unannealed samples. But in order to establish the 7

ACCEPTED MANUSCRIPT changes in the functional dependence of current on voltage we plotted the I-V characteristics in semilogarithmic and log-log scales (Fig. 5). It is seen from Fig. 5 that at forward bias all the curves are shifted parallel. It means that

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after the annealing the slope of the curves does not change. However, the ideality factor n changed

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from 3.7 to 2.7 in comparison with unannealed samples. So the shunts currents were reduced and

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the diode properties of the heterojunction were improved after the annealing. This indicates that the thermal annealing of the samples in vacuum has a positive influence on the properties of the heterojunction, i.e. the shunt currents are decreased without changing the injection properties of the

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p-n-junction. Such changes can be resulted by an increase in mechanical strain at the

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heterointerface, elimination of short-circuit channels and other defects. It is worth noting that the I-V characteristics at back bias also did not change significantly (Fig. 5, b). It is clearly seen that at all temperatures all the branches are characterized by two slopes

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with the exponent 1 at low bias voltage and the exponent 2 at higher bias voltage. According to [16] such situation takes place in the model of depletion region, which presents a trapless insulator with

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equilibrium free carriers. If we assume that there is certain concentration of thermally activated electrons then an ohmic law of I-V should be observed. Shallow donors with relatively low binding

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energy can be a source of such electrons [17]. There are no deviations from the Ohm law until the average concentration of injected non-equilibrium free carriers is comparable with the concentration of thermally excited carriers. In this case, at higher back bias voltage a crossover to the trapless quadratic law, i.e. currents limited by the space charge, occurs (Fig. 5, a).

3.4. Photoelectrical properties The photosensitivity spectra of the n-ZnO/p-InSe heterojunction are shown in Fig. 6. It is well seen that the photosensitivity spectrum is limited from both sides and corresponds to light absorption in narrow band gap InSe (~ 1.25 eV) and wide band gap ZnO (~ 3.2 eV) which is characteristic of the spectrum shape for heterojunctions. The abrupt edge of the photosensitivity spectrum in the high energy region indicates that the grown ZnO film is of a good quality and has 8

ACCEPTED MANUSCRIPT semiconductor properties. The feature of this heterojunction is that due to the low-resistivity of the ZnO film the whole depletion region is located in InSe. This results in overlapping of the light absorption region and the p-n-junction, where generated photocarriers are separated by the electric

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field. The photoelectrical measurements showed good photoresponse, suitable for photosensor

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applications.

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It is also worth noting that the n-ZnO/p-InSe heterojunction fabricated on the basis of the InSe layered crystals were of high quality even despite a significant lattice mismatch between ZnO

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and InSe.

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4. Conclusions

The n-ZnO/p-InSe heterojunctions were fabricated and investigated. ZnO thin films were grown onto freshly cleaved van der Waals surfaces (0001) of p-InSe single-crystal substrates by

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means of RF magnetron sputtering.

It is shown that, despite the large lattice mismatch of 19% between InSe and ZnO, a high

semiconductors.

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quality heterojunction with a well pronounced diode-like behaviour can be fabricated using these

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The formation of ZnO film was justified by XRD results. The AFM results revealed that the oxide layer has a grain structure. Multiple channels were detected in the film indicating the complexity of substrate wetting by ZnO. The current transport mechanisms were analyzed by investigating temperature dependences of the I-V characteristics. It was found that the true picture of the current transport mechanisms was distorted by the shunt currents. The shunt currents are resulted by the presence of mechanical strain and various defects at the heterointerface. At forward bias the ideality factor n is equal to 3.7 indicating that it is hard to define the real current transport mechanism. At back bias the I-V characteristics are described by the currents limited by the space charge by the exponential dependence where index of power is equal to 1 and 2.

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ACCEPTED MANUSCRIPT The vacuum annealing of the n-ZnO/p-InSe heterojunctions allowed us to improve the ideality factor from 3.7 to 2.7 by decreasing the influence of the shunt currents. The obtained result proofs the significant influence of the shunt currents on the I-V characteristics. This approach opens

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up possibilities of improving electrical properties of the n-ZnO/p-InSe heterojunctions and makes

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promising further studies of the influence of vacuum annealing on their parameters.

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In addition, we established that the developed heterojunctions are photosensitive in the photon energy range from 1.12 to 3.75 eV, which makes them promising for photosensor

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application.

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Fig. 1. Schematic diagram of the n-ZnO/p-InSe heterojunction.

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Fig. 2. 2D AFM image of the surface of ZnO film.

Fig. 3. I-V characteristics of the n-ZnO/p-InSe heterojunction measure at different temperatures

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before and after vacuum annealing.

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Fig. 4. The I-V characteristics of the n-ZnO/p-InSe heterojunctions measured at different temperatures T under forward bias in the semilogarithmic scale (a) and under back bias in log-log

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scale (b).

Fig. 5. The I-V characteristics of the n-ZnO/p-InSe heterojunctions measured (after vacuum

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annealing at T=150 °C during 2 hours) at different temperatures T under forward bias in the

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semilogarithmic scale (a) and under back bias in log-log scale (b).

Fig. 6. The photosensitivity spectra of the n-ZnO/p-InSe heterojunction measured at room temperature. The inset shows the low energy edge of the photosensitivity spectra which corresponds to the light absorption in InSe.

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ACCEPTED MANUSCRIPT Highlights

1. Thin ZnO films were grown onto van der Waals surface of InSe substrate.

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2. n-ZnO/p-InSe heterojunctions were fabricated.

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3. The heterojunctions are photosensitive in photon energy range from 1.12 to 3.75 eV.

4. Despite the lattice mismatch of 19% the heterojunctions exhibit diode-like

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behavior.

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5. Vacuum annealing improves electrical properties of the heterojunctions.

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