Electrodeposition of Cu-doped ZnO nanowire arrays and heterojunction formation with p-GaN for color tunable light emitting diode applications

Abstract

Copper-doped zinc oxide (ZnO:Cu) nanowires (NWs) were electrochemically deposited at low temperature on fluor-doped tin oxide (FTO) substrates. The electrochemical behavior of the Cu–Zn system for Cu-doped ZnO electrodeposition was studied and the electrochemical reaction mechanism is discussed. The synthesized ZnO arrayed layers were investigated by using SEM, XRD, EDX, photoluminescence and Raman techniques. X-ray diffraction analysis demonstrates a decrease in the lattice parameters of Cu-doped ZnO NWs. Structural analyses show that the nanomaterial is of hexagonal structure with the Cu incorporated in ZnO NWs probably by substituting zinc in the host lattice. Photoluminescence studies on pure and Cu-doped ZnO NWs shows that the near band edge emission is red-shifted by about 5 or 12 nm depending on Cu(II) concentration in the electrolytic bath solution (3 or 6 μmol l⁻¹). Cu-doped ZnO NWs have been also epitaxially grown on Mg doped p-GaN single-crystalline layers and the (ZnO:Cu NWs)/(p-GaN:Mg) heterojunction has been used to fabricate a light-emitting diode (LED) structure. The emission was red-shifted to the visible violet spectral region compared to pure ZnO. The present work demonstrates the ability of electrodeposition to produce high quality ZnO nanowires with tailored optical properties by doping. The obtained results are of great importance for further studies on bandgap engineering of ZnO, for color-tunable LED applications and for quantum well preparation.

Introduction

Zinc oxide (ZnO) is a wide and direct bandgap (3.37 eV at room temperature) semiconductor with a strong cohesive energy of 1.89 eV [1]. Recently, its nanostructures, such as nanowires (NWs) and nanorods (NRs) have attracted great research interest due to superior properties compared to bulk material, such as high crystalline quality, large aspect ratio, and quantum confinement effects [2], [3], [4], [5]. This is most relevant, especially, when ZnO is considered as strong competitor to GaN, which possess the same wurtzite structure [6]. ZnO and GaN nanostructures present enormous interest for low-dimensional light emitters [7], [8] or photodetectors. However, GaN nanowire arrays are more difficult to synthesize [9], [10] in comparison to ZnO NWs/NRs [4]. In comparison to GaN, zinc oxide has a stronger exciton binding energy (60 meV) than GaN (25 meV) that should favor light emission at room temperature [11]. At the same time, small-diameter ZnO nanowires are expected to lower the lasing threshold because quantum effects result in enhancement of density of states near the band edges and radiative recombination due to carrier confinement [12]. Also, high radiation tolerance of ZnO material and improved radiation hardness of ZnO NWs [13], [14], [15], [16] make it a strong candidate for high temperature electronic devices that can reliably be operated in space and other harsh environments [17]. In this way ZnO NWs are very attractive due to their characteristics providing an important application potential [18], [19], [20], [21], [22], [23], e.g. in short-wavelength low-dimensional optoelectronic devices [22], nano-sensors [18], low-threshold excitonic lasers, light-emitting diodes (LEDs) [7], [8], [20], [22] and photodetectors.

Different techniques such as vapor deposition, pulsed laser deposition, molecular beam epitaxy, metal organic chemical vapor deposition (MOCVD), sputtering, electron beam evaporation, spray pyrolysis, sol–gel processing, chemical, and electrochemical deposition (ECD) have been employed to fabricate ZnO NWs/NRs [3], [4], [5], [6], [7], [8], [18], [23], [24], [25]. Among numerous deposition techniques, electrochemical deposition has become an important technique for fabricating ZnO NWs/NRs owing to its simplicity, cost-efficiency, large-area deposition, good-quality NWs/NRs and low synthesis temperatures in comparison to other techniques [24], [25]. We have recently successfully fabricated low voltage UV-light-emitting diodes with an emission layer made of electrodeposited ZnO-NW arrays [8], [22].

However, several issues have to be clarified, such as the possibility of doping NWs through electrochemical process and tuning of chemical and physical properties of nanomaterial by incorporation of dopant in lattices of ZnO. The addition of dopant can permit bandgap engineering in semiconductors and to create barrier layers which will facilitate radiative recombination by carrier confinement [26].

A very important issue in ECD of ZnO NWs/NRs is to study the presence and the effect of copper ions (Cu²⁺) on the structural and optical properties of ZnO nanomaterial grown in similar electrochemical bath, which could be useful for device applications, especially for tuning the emission wavelength of ZnO-based LED and prepare quantum wells. The high ionization energy of copper and the low formation energy of substitutional group-IB elements indicate that high concentration of Cu could be incorporated into ZnO, which could be used for bandgap tuning/engineering [27], [28]. According to previous theoretical reports [28] a defect band can form inside the ZnO bandgap, narrowing the bandgap of ZnO by Cu-doping. The group-IB d state between the group-IB d state and the oxygen p state also suggests that the effective mass of the defect band will be small, which will favor transport properties [28], [29]. In order to explore possible band tuning of ZnO NWs, we used Cu as dopand during of the electrochemical growth of ZnO NWs. In this work, nanomaterials properties of electrodeposited pure and Cu-doped ZnO NWs and device applications of heterojunction-based LED consisting of ZnO:Cu NWs are presented. A Cu-doped ZnO NWs growth mechanism is suggested based on experimental observations. The structural and optical properties of ZnO:Cu NWs are then described. Finally, LED devices containing a n-ZnO:Cu NWs/p-GaN epitaxial heterojunction are presented with an electroluminescence peak centered in the violet spectral region and shifted compared to pure ZnO NWs. The charge carriers transport mechanism and origin of EL peak shift compared to pure ZnO are discussed.