Optimizing the Stoichiometry of Ga₂O₃ Grown by RF‐Magnetron Sputter Deposition by Correlating Optical Properties and Growth Parameters

Abstract

β‐Ga2O3 thin films are deposited by radiofrequency (RF)‐magnetron sputtering on quartz and c‐sapphire substrates using a ceramic stoichiometric Ga2O3 target and a constant flux of argon process gas. Oxygen flux, heater power, and sputtering power are varied in the synthesis of the layers. The resulting Ga2O3 layers are analyzed in terms of their structural and optical properties. Based on this analysis, the process parameters leading to the formation of an optimized β‐Ga2O3 layer are identified. The main challenge in obtaining the stoichiometric β‐Ga2O3 thin films by sputter deposition is to overcome the influence of a strong preferential sputtering of Ga from the ceramic target. This can be achieved by adding a suitable fraction of oxygen to the argon process gas used in the deposition process. Furthermore, it is demonstrated that the refractive index dispersion of β‐Ga2O3 depends strongly on its composition. Thus, a combined analysis of refractive index dispersion and optical bandgap position may serve as a valuable preliminary probe of the thin film's composition.

Progress in Sputter Growth of β‐Ga2O3 by Applying Pulsed‐Mode Operation

“…The layer thicknesses obtained translate to growth rates of around 50–120 nm min −1 plasma on‐time. Compared with the growth rate of approximately 4–5 nm min −1 found for conventionally sputtered layers, this is a significant improvement.…”

“…First, we will recap tendencies observed in the conventional RF sputter deposition of Ga 2 O 3 . The optical bandgap strongly depends on the deposition parameters oxygen flux, heating power, and RF power.…”

“…Thus, series I resembled conventional RF sputter deposition at fixed PRF and PDC values, whereas series II illustrated the impact of using the pulsed mode in the sputter deposition of β ‐Ga 2 O 3 . The results of the series were compared with measurements conducted on samples which were conventionally sputtered using the same setup and reported previously . All samples were unintentionally doped and, thus, did not show significant electrical conductivity, which was most likely driven by the pronounced grain structure intrinsic for sputtered thin films.…”

“…The temperature range was chosen in accordance with the findings of Akazawa and Zhang et al, who observed the threshold temperature of crystalline Ga 2 O 3 at 500 and 300 °C, respectively . Details on the distinct temperature characteristics of the setup can be found elsewhere …”

“…Therefore, an increase in the growth rate is often required to ensure use on an industrial scale. In those cases, sputtering techniques seem to be viable candidates for achieving this goal . Atoms ejected from the target's surface by energetic ion impingement are neutral unless they are ionized, inter alia, by application of very high power to the discharge .…”