Abstract
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 using a rate-equation model that takes into account the O2 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 exhibits an S-shaped temperature dependence instead of the expected linear one in an Arrhenius plot, which is 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 O2. An important suboxide contribution is expected for all elemental sources whose suboxide exhibits a higher vapor pressure than that of 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.
Highlights
For the growth of device quality single-crystalline thin films, molecular beam epitaxy (MBE) established itself as the major growth technique,7–10 while low-pressure chemical vapor deposition (LPCVD) has been demonstrated to be capable of delivering thick films of high quality for the case of Ga2O311,12 and In2O3.13,14 For the case of MBE, semiconducting oxides (e.g., ZnO,15 SnO,16 SnO2.17,18 In2O3,18,19 and Ga2O318,20,21) and complex oxides have been grown by the reaction of the vapor from a metal charge placed in a heated effusion cell with reactive oxygen on the heated substrate in an ultra-high vacuum chamber
We investigate the metal sources Sn and Ga by quadrupole mass spectrometry (QMS) with respect to their metal and suboxide fluxes when exposed to an oxygen background that is typical for oxide MBE
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 oxide MBE using B, Ga, In, Sb, La, Ge, Si, Sn, Mo, Nb, Ru, Ta, V, and W sources, whereas this mechanism is almost negligible for Ba, Al, Ti, and Pb sources
Summary
Transparent conducting oxides (TCOs), transparent semiconducting oxides (TSOs), and multicomponent or complex oxides gained more and more interest within the last few years because of their great potential for electronic devices, photovoltaics, and sensors. For the growth of device quality single-crystalline thin films, molecular beam epitaxy (MBE) established itself as the major growth technique, while low-pressure chemical vapor deposition (LPCVD) has been demonstrated to be capable of delivering thick films of high quality for the case of Ga2O311,12 and In2O3.13,14 For the case of MBE, semiconducting oxides (e.g., ZnO, SnO, SnO2.17,18 In2O3,18,19 and Ga2O318,20,21) and complex oxides have been grown by the reaction of the vapor from a metal charge placed in a heated effusion cell with reactive oxygen (an oxygen plasma or ozone) on the heated substrate in an ultra-high vacuum chamber. During. Regarding MBE-grown samples, a critical inspection of reported Ga2O3 growth rates and Sn-concentrations in doped Ga2O327–29 and In2O330 testifies that there is unexpectedly high metal incorporation into the films for the used, comparably low metal-effusion-cell temperature. We experimentally demonstrate that the metal incorporation is dominated by a contribution due to the unintentional formation and evaporation of the suboxide at sufficiently low effusion-cell temperature or sufficiently high oxygen background pressure. We developed a kinetic model that allows us to quantitatively describe the total (suboxide and metal) flux from the metal sources when used in the oxygen background This model allows us to explain literature data on Sn doping and the Ga2O3 growth rate in oxide MBE and predicts the suboxide flux from the metal source to be orders of magnitude higher than the pure metal flux during the reported growth of Ga2O311,12,23 and In2O313,14 by LPCVD. 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 oxide MBE using B, Ga, In, Sb, La, Ge, Si, Sn, Mo, Nb, Ru, Ta, V, and W sources, whereas this mechanism is almost negligible for Ba, Al, Ti, and Pb sources
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