Role of the dopant aluminum for the growth of sputtered ZnO:Al investigated by means of a seed layer concept

Abstract

This work elucidates the effect of the dopant aluminum on the growth of magnetron-sputtered aluminum-doped zinc oxide (ZnO:Al) films by means of a seed layer concept. Thin (<100 nm), highly doped seed layers and subsequently grown thick (∼800 nm), lowly doped bulk films were deposited using a ZnO:Al2O3 target with 2 wt. % and 1 wt. % Al2O3, respectively. We investigated the effect of bulk and seed layer deposition temperature as well as seed layer thickness on electrical, optical, and structural properties of ZnO:Al films. A reduction of deposition temperature by 100 °C was achieved without deteriorating conductivity, transparency, and etching morphology which renders these low-temperature films applicable as light-scattering front contact for thin-film silicon solar cells. Lowly doped bulk layers on highly doped seed layers showed smaller grains and lower surface roughness than their counterpart without seed layer. We attributed this observation to the beneficial role of the dopant aluminum that induces an enhanced surface diffusion length via a surfactant effect. The enhanced surface diffusion length promotes 2D-growth of the highly doped seed layer, which is then adopted by the subsequently grown and lowly doped bulk layer. Furthermore, we explained the seed layer induced increase of tensile stress on the basis of the grain boundary relaxation model. The model relates the grain size reduction to the tensile stress increase within the ZnO:Al films. Finally, temperature-dependent conductivity measurements, optical fits, and etching characteristics revealed that seed layers reduced grain boundary scattering. Thus, seed layers induced optimized grain boundary morphology with the result of a higher charge carrier mobility and more suitable etching characteristics. It is particularly compelling that we observed smaller grains to correlate with an enhanced charge carrier mobility. A seed layer thickness of 5 nm was sufficient to induce the beneficial effects.

Field Emission at Grain Boundaries: Modeling the Conductivity in Highly Doped Polycrystalline Semiconductors

“…Anticipating results of μ-T fits, one estimates reasonable values for Θ and λ tr to be 1000 K and 0.3, respectively, resulting in an electron-phonon scattering mobility of 200 cm 2 =V s. Note that this value is similar to 210 cm 2 =V s derived by Ellmer [21]. Figure 4 shows the mobility data of ZnO∶Al films obtained by Berginski et al [56] and Sommer et al [57]. By varying the target doping concentration (TDC) and deposition temperature, samples with various carrier concentrations and mobilities are obtained [56].…”

“…Hence, the seed-layer approach [56]. Furthermore, the data of a seed-layer approach are added [57]. Dashed lines represent the results of the conductivity model assuming the trap density to be independent of the carrier concentration.…”

“…III A. Details about the approach can be found in Sommer et al [57]. Here, it is important only that the application of a 40-nm-thin seed layer improves the mobility of the 800-nm-thick total layer significantly.…”

“…Deposition conditions may alter the concentration of dopant atoms. For instance, the increase of deposition temperature leads to an increase of aluminum concentration in ZnO∶Al due to a favored reevaporation and resputtering of zinc in comparison to aluminum [57]. The increasing dopant concentration thus induces a higher density of trap states and a higher carrier concentration.…”

High-Hall-Mobility Al-Doped ZnO Films Having Textured Polycrystalline Structure with a Well-Defined (0001) Orientation

“…In this work, on the basis of many reports on controlling the crystallinity and/or surface morphology of AZO films in the early growth stages [11][12][13][14][15][16][17][18], we propose a resolution to the issue; a critical layer (CL) made from 10-nm-thick Ga-doped ZnO (GZO) films with a preferential c-axis orientation normal to the substrate as an interface layer between thicker AZO films by DC-MS and substrates. The CL comprises GZO films deposited by ion plating (IP) with dc arc discharge.…”