Electrical and photovoltaic properties of p-n heterojunctions obtained using sol gel derived nanostructured ZnO:La films onto p-Si

Highlights

• The p-Si/n-ZnO:La heterojunctions were obtained using different dopant levels (0.2, 0.4, 0.6, 0.8 and 1%) nanostructured ZnO films.

• These structures were shown to have a good diode-like rectifier in the dark.

• The diode parameters were calculated using different methods.

• Also, these diodes exhibit a photovoltaic behavior under illuminations.

Abstract

The p-Si/n-ZnO:La heterojunctions were obtained using different dopant levels (0.2, 0.4, 0.6, 0.8 and 1%) nanostructured ZnO films. To characterize the electrical behavior of these structures, the current-voltage (I-V) analysis were performed. It was observed that all of them showed good rectifying behavior. The diode parameters were determined by using different methods. To characterize the photovoltaic behavior of these diodes, I-V measurements were performed under illuminations (20–100 mW/cm²). The results indicated that obtained photodiode exhibited a good photoconducting behavior.

Introduction

In recent years, deposition and applications of nanostructured zinc oxide (ZnO) films have been intensely studied due to their morphological, electrical, thermal and optoelectronic properties and the wide applicability that has been applied for use in gas sensors, solar cells, photodiodes, transistors and supercapasitors [[1], [2], [3], [4], [5]]. ZnO possesses some outstanding properties such as non-toxic and ample on earth, wide band gap (about 3.37 eV @RT), high transparency and low resistivity, can be easily produced on substrates at low temperature [6]. ZnO is an n-type oxide semiconductor in nature because of deviation from stoichiometry [7]. The free charge carriers primarily cause the shallow donor levels regarding interstitial zinc atoms and oxygen vacancies. Accordingly, by manipulating these naturel defects, the resistivity can be decreased. To further expand the area of the applications, ZnO can be doped with different dopant elements. Doping of ZnO with rare-earth (RE) elements (Lanthanum (La), Erbium (Er), Europium (Eu), Cerium (Ce)…) can change and develop some physical properties of ZnO film [[8], [9], [10], [11], [12]].

RE elements doped ZnO films can be produced successfully with expensive solutions such as chemical vapor deposition (CVD) [13,14] and magnetron sputtering [15,16] as well as inexpensive solution methods such as sol gel [[17], [18], [19]], spray pyrolysis [[20], [21], [22]] and electrochemical deposition [23]. Among them, sol gel method is currently used for basic and applied research and for the deposition of homogeneous, nanostructure, multi-component oxides or organic coatings/films. This method is advantageous because of its simplicity, safety, easy control of the film composition, non-vacuum system for deposition, cheapness of the equipment and raw materials. By spin coating technique, substrate is mounted onto the sample holder, the prepared sol is dropped onto and rotated. In order to obtain high quality film with this technique, it is very important to choose sol and deposition parameters such as the sol concentration, sol aging and mixing time, annealing temperature, number of layers, spin speeds, etc. These parameters generally affect the characteristics of the films.

In the available literature, although there are several characterization studies related to La doped ZnO films obtained by sol gel method [8,9,17,24,25], the p-Si/n-ZnO:La heterojunction properties of these films have been rarely studied [26,27]. There is the role of La concentration in to ZnO lattice. That is, the excess electrons from La³⁺ substituted at the Zn²⁺ sites, could show the better performance in terms of devices. Ilican and Ilgu [27] deposited undoped and 1% La doped ZnO films via sol gel method using dip coating technique onto p-Si substrates to investigate its morphological, structural, optical and electrical properties. Also, they fabricated p-n heterojunction using this film and investigated current-voltage (I–V) characteristics. They calculated ideality factor (n) and barrier height (Ø_b) for p-Si/n-ZnO:La heterojunction, and found to be 2.85 and 0.72 eV, respectively. They measured the I–V characteristic under illumination (100 mWcm⁻²) and reported that this diode had showed a photodiode characteristic. Furthermore, p-ZnO:La/n-Si heterojunction diodes were also fabricated, and their electrical properties were investigated. Habayashi et al. [28] prepared undoped and La doped (1, 2, 4, 5 at.%) ZnO films by spray pyrolysis method onto glass and n-Si substrates. They reported the results about structural, morphological and optical properties of prepared La doped ZnO films. They also fabricated Au/ZnO:La/n-Si/Al structures to analyze electrical properties, and reported that these Schottky diodes had good rectifying properties. They calculated n, Ø_b and series resistance (R_s) values range of 1.47–1.91, 0.28–0.69 eV and 81–307 kΩ, respectively.

In this study, the p-Si/n-ZnO heterojunction structures were obtained using different La dopant level (0.2, 0.4, 0.6, 0.8 and 1.0%) spin-coated ZnO films. Hereafter the names of the obtained diodes are DZLa-02, DZLa-04, DZLa-06, DZLa-08 and DZLa-1 for 0.2%, 0.4%, 0.6%, 0.8% and 1% La doped ZnO based devices, respectively (Table 1). The electrical analysis of the obtained p-n heterojunction diodes was investigated by I–V measurements under dark and illumination conditions. The diode parameters (n, Ø_B and R_s) were calculated by using different methods.