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Suboxide Desorption of Elemental Sources in an Oxygen Background

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Suboxide Desorption of Elemental Sources in an Oxygen Background Drastically enhanced cation incorporation in the epitaxy of oxides due to formation and evaporation of suboxides from elemental sources Georg Hoffmann,∗ Zongzhe Cheng, Oliver Brandt, and Oliver Bierwageny Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V., Hausvogteiplatz 5-7, 10117 Berlin, Germany In the molecular beam epitaxy of oxide films, the cation (Sn, Ga) or dopant (Sn) incorporation does not follow the vapor pressure of the elemental metal sources, but is enhanced by several orders of magnitude for low source temperatures. Using line-of-sight quadrupole mass spectrometry, we identify the dominant contribution to the total flux emanating from Sn and Ga sources at these temperatures to be due to the unintentional formation and evaporation of the respective suboxides SnO and Ga2O. We quantitatively describe this phenomenon by a rate-equation model that takes into account the O background pressure, the resulting formation of the suboxides via oxidation of the metal source, and their subsequent thermally activated evaporation. As a result, the total flux composed of the metal and the suboxide fluxes exhibit an S-shape temperature dependence instead of the expected linear one in an Arrhenius plot, in excellent agreement with the available experimental data. Our model reveals that the thermally activated regimes at low and high temperatures are almost exclusively due to suboxide and metal evaporation, respectively, joined by an intermediate plateau-like regime in which the flux is limited by the available amount of O. An important suboxide contribution is expected for all elemental sources whose suboxide exhibits a higher vapor pressure than the element, such as B, Ga, In, La, Si, Ge, Sn, Sb, Mo, Nb, Ru, Ta, V, and W. This contribution can play a decisive role in the molecular beam epitaxy of oxides, including multicomponent or complex oxides, from elemental sources. Finally, our model predicts suboxide-dominated growth in low-pressure chemical vapor deposition of Ga2O3 and In2O3. I. INTRODUCTION edly high metal incorporation into the films for the used, comparably low metal effusion-cell temperature. Indeed, un- Transparent conducting oxides (TCOs), transparent semi- expectedly high Si concentrations in MBE grown, Si-doped conducting oxides (TSOs), and multicomponent or complex Ga2O3 have recently been reported by Kalarickal et al. [31], oxides gained more and more interest within the last years and attributed to the formation and desorption of volatile because of their great potential for electronic devices, photo- SiO at the Si effusion cell, that exceeds the expected Si flux. voltaics and sensors[1–6]. For the growth of device quality This additional, unintentional mechanism of SiO formation single-crystalline thin films, molecular beam epitaxy (MBE) at comparably low Si temperatures can be understood as a established itself as the major growth technique [7–10], while direct oxidation of the elemental Si charge in the oxygen low pressure chemical vapor deposition (LPCVD) has been background of the growth chamber and plays a key role for demonstrated to be capable of delivering thick films of high the understanding of Si doping concentrations in oxide MBE quality for the case of Ga2O3 [11, 12] and In2O3 [13, 14]. For [31]. the case of MBE, semiconducting oxides (e.g. ZnO [15], SnO [16], SnO2 [17, 18], In2O3 [18, 19], and Ga2O3 [18, 20, 21]) In this article, we show that this mechanism is crucial for as well as complex oxides have been grown by the reaction understanding metal incorporation in the epitaxy of oxides of the vapor from a metal charge placed in a heated effusion in general. In particular, we demonstrate experimentally that cell with reactive oxygen (an oxygen plasma or ozone) on the the metal incorporation is dominated by a contribution due to heated substrate in an ultra-high vacuum chamber [3, 22]. the unintentional formation and evaporation of the suboxide. During LPCVD, metal sources and substrate are essentially Specifically, we investigate the metal sources Sn and Ga placed in a tube furnace held at a temperature ranging from by quadrupole mass spectrometry (QMS) with respect to ◦ 900 to 1050 C, and a mixture of Ar and O2 with total pres- their metal and suboxide fluxes when exposed to an oxygen sure in the mbar-range passes over the liquid Ga or In to background that is typical for oxide MBE. In addition, we transport Ga- or In-containing species to the substrate, on developed a kinetic model that allows us to quantitatively which they form the oxide film with the O2 [11–14, 23]. For describe the total (suboxide and metal) flux from the metal the case of Ga2O3 growth, the Ga-containing species have sources when used in oxygen background. This model allows been described as Ga evaporating from the liquid Ga source to explain literature data on Sn doping [27–30] and Ga2O3 [23]. growth rate [24–26] in oxide MBE, and predicts the suboxide Regarding MBE grown samples, a critical inspection of flux from the metal source to be orders of magnitude higher arXiv:2109.01195v1 [cond-mat.mtrl-sci] 2 Sep 2021 reported Ga2O3 growth rates [24–26] and Sn-concentrations than the pure metal flux during reported growth of Ga2O3 [11, in doped Ga2O3 [27–29] and In2O3 [30] testifies unexpect- 12, 23] and In2O3 [13, 14] by LPCVD. Finally, we predict by the comparison of the vapor pressure curves of selected pure elements to those of their suboxide, that dominant suboxide desorption from the elemental source can be relevant for ∗ Electronic mail: Hoff[email protected] oxide MBE using B, Ga, In, Sb, La, Ge, Si, Sn, Mo, Nb, y [email protected] Ru, Ta, V, and W sources, whereas we can exclude this 2 mechanism for Ba, Al, Ti, and Pb sources. ing levels, cell design, or position of the cell with respect to the substrate. Nevertheless, data for lower )Ga, available from other MBE systems ([24, 25, 39] and [26]) again show II. REPORTED FLUXES FROM THE OXIDE-MBE that the incorporated Ga-flux is significantly higher than ex- LITERATURE pected from the vapor pressure curve as well as measured BEP. This deviation increases with decreasing cell tempera- During MBE-growth the metal flux from an effusion cell ture: the BEP decreases according to the Ga-vapor pressure is essentially proportional to the metal vapor pressure at the curve, whereas the incorporated flux shows a much weaker given effusion-cell temperature, and it is often measured in dependence on )Ga, almost forming a plateau [see inset of the absence of an oxygen background as beam-equivalent Fig.1(b)]. pressure (BEP) by a nude ion gauge at the substrate posi- Likewise, significantly higher incorporated fluxes than ex- tion. Consequently, the dependence of metal flux and BEP pected from the vapor pressure curve with an almost plateau- on effusion-cell temperature is well described by the vapor- like cell-temperature dependence have recently been demon- pressure curve of the metal. Figure1 summarizes published strated for Si during the MBE growth of Si-doped Ga2O3, data on incorporated Sn and Ga fluxes during oxide MBE and explained by the formation and desorption of the volatile together with the corresponding measured or extrapolated suboxide SiO instead of Si from the Si source [31]. In the BEPs for a wide range of metal-effusion-cell temperature in following we will show theoretically and experimentally, that an Arrhenius diagram with a fixed ratio U between flux- and the very same mechanism is responsible for the unexpectedly BEP-scale, and includes dashed lines representing the vapor high Ga- and Sn-incorporation during oxide MBE growth at pressure of the respective metal. Specifically, the incorpo- low cell temperatures. The apparent discrepancy between rated metal fluxes were calculated by multiplying growth BEP and incorporated flux can be immediately understood rate with doping concentration (of Sn-doped Ga2O3 [27–29] by the absence and presence of an oxygen background pres- −6 and In2O3 [30] films) or cation density (of SnO2 [32–34] or sure (typically in the 10 mbar-range) in the growth chamber Ga2O3 [24–26, 36–38] films) for the case of a dopant flux during BEP measurement and film growth, respectively. or cation flux, respectively. We assume that the incorpo- rated metal flux reflect the source-metal flux at the substrate since the chosen data largely corresponds to non-metal-rich III. EXPERIMENTAL growth conditions, under which full metal incorporation can be expected [18]. As shown in Fig.1(a), at high Sn-cell temperatures Two different methods were used for the experimental in- ◦ vestigation. The first method is the measurement of the ()Sn ≥ 1000 C) the incorporated Sn-fluxes and Sn-BEPs (from three different MBE systems) show the expected behav- direct beam flux using a QMS. The QMS experiments were ior of approximately following the vapor pressure behavior of performed in a test chamber as shown in the inset of Fig.2. Sn, denoted by a dashed line. The ratio of Sn-BEP to Sn-flux Further, Fig.2 shows a typical QMS spectrum of the direct Sn is almost equal for the MBE systems used in Refs. [30, 32] flux 9(= and SnO suboxide flux 9(=$ from a single-filament and [33] (BEP and related flux datapoints overlap using the effusion cell revealing the presence of SnO suboxide that same U) suggesting that the BEP can be used to estimate the forms in a controlled O-background pressure ( 9$ ). Due to metal flux independently of the used MBE system. Moving the isotopic distribution, Sn and SnO can easily be identi- ◦ fied. For the QMS experiments, the device ionizer was run to significantly lower Sn-cell temperature ()Sn ≤ 800 C), however, the incorporated metal flux becomes two to four or- at an electron energy of 50 eV to maximize its sensitivity.
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