Formation of a functional homo-junction interface through ZnO atomic layer passivation: Enhancement of carrier mobility and threshold voltage in a ZnO nanocrystal field effect transistor

Highlights

• We formed a ZnO-NC/ZnO-ALD planar-homojunction interface, passivating the surface.

• Carrier mobility in the accumulation layer was enhanced by surface passivation.

• Suppression of surface depletion by ALD are crucial in assembling devices.

Abstract

We report enhancement of mobility and increase in mobile carrier concentration in zinc oxide (ZnO) nanocrystal (NC) field effect transistors (FETs) through the formation of a homo-junction interface using atomic layer deposition (ALD) passivation. An ultrathin ALD-ZnO passivation film deposited on a ZnO NC film not only increased the FET mobility from 4.6ⅹ10⁻⁶ to 1.4ⅹ10⁻⁴ cm²/V but also caused earlier turn-on of the ZnO NC FETs, shifting the threshold voltage from 18.9 to −4.6 V. The enhanced FET mobility and earlier turn-on in the FET are attributed to reduced localized state density on the ZnO NC surface through ALD-ZnO passivation. Passivation of the surface states mitigates carrier depletion in the ZnO NC film through oxygen adsorption on the ZnO surface. We also observed that the presence of saturation of the drain in a high drain-source voltage region depends on the ALD-ZnO passivation and its origin is discussed.

Introduction

Zinc oxide (ZnO) materials have been widely used in energy and display devices [[1], [2], [3], [4]]. Mainly, ZnO-based emissive layer-transport layer heterostructures in light emitting diodes (LEDs) have been extensively researched because of its functional tunability through electronic and structural modification [[5], [6], [7], [8], [9]]. In photovoltaics (PVs), it has been demonstrated that the combination of light sensitizer and electron accepting layer (ZnO) is crucial in improving the power conversion efficiency through adjusting energy level band offset and the density of localized states. Importantly, tuning the interfacial electronic structre through structural modification of the electron transport layer has been proved to be effective for device optimization, allowing for efficient charge transport and transfer [[7], [8], [9]]. Recently, nanostructured ZnO materials including nanocrystals (NCs) and nanowires (NWs) have been coupled to organic semiconducting materials and organometal halide perovskite materials to fabricate highly efficient PVs and photodetectors [[10], [11], [12]].

To adjust the electronic properties of ZnO for highly efficient charge transfer and transport, researchers have modified electronic and structural properties of ZnO through a variety of strategies including incorporation of the self-assembled monolayer at the surface, layering a functional thin film, formation of solid solution using chemical doping, creation of core/shell heterostructure and bulk defect engineering [[13], [14], [15], [16], [17], [18], [19], [20], [21], [22]]. Most of all, device optimization through passivation of ZnO surface defects has been considered to be most effective. Yang et al. demonstrated that the intrinsic bulk defects have little effect on charge transfer/transport by fabricating PVs with ZnO films possessing different form of defects [23]. Instead, rinsing the ZnO surface after synthesis which removes byproduct of ZnO synthesis enhanced the PV efficiency regardless of the type of defects. This result indicates that altering the interfacial properties through surface chemistry-induced immobilization of functional molecules can change the electrical properties of the whole layers governing the device efficiency in nano-sized devices. Spalenka et al. reported, indeed, on the passivation of the surface defect using carboxylic acids leading to a significant mobility improvement, demonstrating that surface treatment can change the electrical properties of the ZnO film, altering carrier concentration and mobility [24]. The origin of the enhanced electrical properties through surface treatment has been attributed to surface defect passivation, modifying interfacial electrostatic conditions. X-ray photoelectron spectroscopy study to probe the effect of surface passivation of ZnO through electrochemical grafting revealed that covalently-anchored organic layers to the ZnO surface could alter surface band bending which can adjust charge transfer and transport [25]. In the framework, improvement in inverted organic PVs through silane-capping of ZnO NCs is attributed to passivation of surface-adsorbed oxygen defects, altering energy band bending [26].

Surface defect passivation has been carried out with a number of methods. Through insertion of a functional layer, device optimizations in PVs and LEDs were proven to be successful. Cao et al. realized hysteresis-free and stable PVs by depositing a thin MgO layer on a ZnO electron transport layer in perovskite PVs, reducing electron recombination at the interface [27]. Al₂O₃ passivation of ZnO NCs using atomic layer deposition (ALD) was also successful in achieving a higher power conversion efficiency with a higher short circuit current resulting from suppression of surface defects [28]. Kim et al. also reported surface passivation through ALD in which surface defects of the ripple-structured ZnO thin films were passivated by ALD-ZnO reducing the quenching sites by surface traps [29]. Wang et al. investigated the effect of ZnO-indium (In) nano-junction on charge transfer, finding that lower work function In layer positions the Fermi energy level closer to the conduction band edge of ZnO compared to that of ZnO without In layer [19]. The formation of ZnO–In nano-junction fills up the potential trapping centers through electron transfer to ZnO. The resulting change in the energy band bneding led to more efficient charge transfer for electron extraction from phenyl-C61-butyric acid methyl ester to ZnO. These results clearly demonstrate that surface defect passivation alters charge transfer by altering energy band bending induced by carrier trap filling.

On the other hand, device optimization through ZnO chemical doping has been carried out [30,31]. Oxygen vacancies (V_O) and Zn interstitials (Zn_i) have been accepted as the origin of n-type charge transport in native ZnO films [4]. The electrical conductivity of ZnO film was enhanced by incorporating Group-III elements (Al, Ga and In) into ZnO as substitutional elements for Zn with an increased electron concentration [1]. Electrical optimization through chemical doping, however, introduces structural defects which serve as carrier trapping centers, increasing carrier recombination. The resulting carrier loss and energetic disorder at the interface where ZnO films are in contact with other materials deteriorate device performance [32,33]. Recent results show, however, that formation of solid solution using Mg doping is effective in tuning interfacial energy band offset enhancing device efficiency in PVs and LEDs. Mg composition-dependent open circuit voltage and short circuit current indicate that interfacial energy band offsets can be modified in PVs [34]. In LEDs, charge balance was achieved by Mg doping into the ZnO layer, lowering charge injection rate due to a higher charge injection barrier [35].

Facile tuning of electrical and optical properties through size and composition control during solution processing has, importantly, aroused compelling attention accelerating fundamental understanding of electronic properties of ZnO NCs [14,32,33]. Energy band-offset tuning through compositional engineering has increased compatibility with a class of materials enabling the formation of various functional hetero-junction interfaces. However, a high surface to volume ratio of nanostructured ZnO materials including NCs, nanorods and NWs increased necessity of surface passivation, requiring compatibility with other component layers for various device applications. Very recently, Ozu et al. reported ZnO NW passivation using an ultrathin SnO₂ layer, reducing the density of the deep traps by the absorbed oxygen and hydroxyl groups on the ZnO NW surface [20]. SnO₂ passivation not only reduced the surface recombination but also led to an upward shift of the Fermi energy, causing more efficient electron transfer to ZnO.

Effect of surface passivation on nanostructured ZnO electrical properties was more fundamentally investigated using field effect transistors (FETs), emphasizing that surface defects are potential sites for oxygen adsorption which deplete mobile carriers, altering energy band bending. ZnO NCs exhibited, indeed, low carrier mobility and relatively high threshold voltage in FETs because of its large surface to volume, featuring a high density of localized states [32,33,[36], [37], [38]]. It has also been reported that carrier depletion induced by surface states suppresses efficient mobile carrier transport [39,40].

As mentioned until now, most of surface passivation was carried out through formation of hetero-junction interface which is unavoidable for device operation. For various device applications, however, surface passivation by formation of homo-junction is attractive in that it not only reduces lattice mismatch at the interface involving surface passivation layer but also causes a homogeneous change in the electrical properties, enhancing compatibility with other layers. In the perspective, we created a planar homo-junction interface by passivating the surface of a ZnO NC layer with an ultrathin conformal ALD-ZnO layer, resulting in a reduction in the density of surface trapping centers on which oxygen molecules are captured immobilizing free electrons. Through formation of the ZnO homo-junction with a reduced number of localized states, change in the original energy band offset can be minimized even after ZnO surface passivation using ALD-ZnO layer. To probe the effect of the planar homo-junction interface on charge transport properties, we adopted field effect devices and analyzed the device parameters, including the mobility and the threshold voltage.