Modeling of Intrinsic Electron and Hole Trapping in Crystalline and Amorphous ZnO

Abstract

Furthermore, even thermodynamically stable phases, for example, zincblende or rocksalt, are only grown under very specific conditions of substrates and/or at high pressure (see, e.g., refs. [8-12]). Although successful growth of thin a-ZnO films with thicknesses ranging from 5 to 100 nm using different techniques and on different substrates has been reported, [13-18] the morphology of these films remains controversial due to the lack of high intensity X-ray diffraction or grazing incidence X-ray and neutron diffraction characterization. Many of such films are reported to contain nanocrystalline inclusions. [13,16,18-20] Amid largely unsuccessful attempts to grow good quality a-ZnO films, theoretical predictions of a-ZnO have been more optimistic. Stable a-ZnO structures have been produced using mainly ab initio molecular dynamics (MD) melt and quench (MQ) methods. [21-26] These computationally demanding calculations use small periodic cells, very high cooling rates, and provide limited statistics of structural characteristics. The sampling of the a-ZnO configurational space has been, therefore, poor and a distribution of electronic properties has not been provided yet. Recently, we investigated the ability of bulk ZnO to form glass structures using interatomic potentials (IPs) and an MQ procedure within isothermal-isobaric (NPT) ensemble. [27] This allowed us to use large (up to 768 000 atoms) periodic cells and improve the statistics of the distribution of a-ZnO structural characteristics. These calculations have demonstrated that cooling rates in an MQ procedure around 100 K ps −1 lead to the formation of stable and uniform amorphous structures. ZnO samples show, however, different degrees of crystallinity at lower cooling rates. The average density of a-ZnO samples produced using IPs is about 5.04 g cm −3 and the coordination numbers of Zn and O atoms are around 3.9, reflecting the strong propensity to crystallization. Still, the stability tests carried out using the activation-relaxation technique (ART) [28-32] and simulated annealing demonstrated that the obtained amorphous structures are stable. [27] Advances in ultrafast liquid quenching and deposition of thin films on cold substrates [33-36] make achieving growth of amorphous ZnO films a tangible prospect. Their potential use in (photo)electronic devices will expose these films to electrons and holes. Therefore, questions arise whether a-ZnO films will retain good electron mobility and whether there is any possibility for electron or hole localization due to disorder. Such Recent advances in ultrafast liquid quenching and deposition of thin films on cold substrates make growing amorphous (a)-ZnO films increasingly feasible. The electronic structure and electron and hole trapping properties of amorphous ZnO are predicted using density functional theory (DFT) simulations with a hybrid density functional (h-DFT). An ensemble of fifty 324-atom structures is employed to obtain the distribution of structural and electronic properties of a-ZnO. The results demon...

Disorder-induced electron and hole trapping in amorphous TiO2

“…We note that distribution of calculated E T is affected by several factors compared to similar calculations in the crystal phase, as discussed in detail in Ref. [56]. One of the major factors is the partially localized character of the initial state, which is discussed below along with the other aspects.…”

“…Following the success in experimental preparation of metastable metal alloys [67], theoretical models of oxide glasses are also usually obtained using a melt-quench procedure and molecular dynamics (MD) [68]. This technique has been used to model structures of amorphous a-HfO 2 , a-SiO 2 , a-Al 2 O 3 , a-ZnO, and a-Sm 2 O 3 [47,50,56,[69][70][71] as well as other non-glass-forming oxides [72]. Similarly, classical force-fields, [39,73-80] density-functional-based tight-binding [81] and DFT [41][42][43]64] simulations have been used to create models of a-TiO 2 structures.…”

“…The degree of localization of these states was further analyzed by calculating the IPR spectrum, which has been used to characterise the localization of electronic states in amorphous materials including TiO 2 [41,42] and other (more complex) amorphous structures, see, e.g., Refs. [56,69,110,111]. IPR is calculated for each energy eigenstate of the system and the formulation used here has been reported previously [56].…”

“…[56,69,110,111]. IPR is calculated for each energy eigenstate of the system and the formulation used here has been reported previously [56]. The average a-TiO 2 IPR spectrum shown in Fig.…”

“…Holes have been shown to trap at low-coordinated oxygen sites in most amorphous oxides amorphous oxides, including ZnO, Al 2 O 3 , HfO 2 and SiO 2 . [46,56].…”