Sb Doped GeSn Growth by MOCVD with Newly Employed Source Gases

Abstract

We demonstrated MOCVD (Metal Organic Chemical Vapor Deposition) GeSn growth co-doped with Sb. The Sb doping accelerated the Sn incorporation in the GeSn epitaxial film. The Sn and Sb concentrations were 6.6% and 0.5%, respectively. We consider the surfactant effect by the Sb as well as atmospheric H2 suppress the surface migration and therefore the precipitation of Sn. 1. Introduction GeSn is a promising material for large scale optoelectronic integrated devices on the Si platform [1]. It is reported that GeSn with Sn concentration more than approximately 10% has direct bandgap and emits light efficiently. In addition, GeSn is also expected to improve the performance of MOSFET, when it is used as a material for the channel or source/drain. In order to enjoy the high performance of various GeSn based devices, it is necessary to achieve both high Sn concentration and high crystallinity. However, it has been difficult to increase Sn concentration, because the solid solubility limit of Sn is approximately 1%. Higher Sn concentration than the solubility limit has been demonstrated by MBE at extremely low growth temperature; however, the low temperature growth generally results in the poor crystal quality. Donor or acceptor doping in the GeSn layer has not been reported so much yet, although it should be indispensable for the device application. In this study, we examined MOCVD GeSn growth with Sb doping and found that the high concentration Sn and Sb doping can be achieved simultaneously. 2. Experiments In our experiments, we used tertiarybutylgermane (t-C4H9GeH3) and tetra-ethyl tin (C2H5)4Sn as MOCVD precursors with H2 carrier gas. The Sb is doped from the atmosphere with residual triisopropyl-antimony [(i-C3H7)3Sb]. These newly employed precursors are significantly safer than conventional Ge2H6 and SnH4 because of their non-pyrophoric and non-explosive characteristics. We used Ge(001) as the substrate. The deposition was carried out in 30 Torr atmosphere for 120 min at 320-360°C. The injection rate of the Ge and Sn precursors were controlled between approximately 3.5 × 10−6 and 1.4 × 10−4 mol/min. H2 supply was at 20-40 sccm. The Sn and Sb concentrations were measured by the combination of RBS and SIMS. TEM observation was also carried out for the evaluation of the crystal quality. 3. Results and Discussion Figure 1 shows the depth profiles of Sn and Sb concentration in the epitaxial film. Here, the (Sn+Sb) profile was determined by RBS first. Then, the Sn/Sb ratio was measured by SIMS. The Sn concentration close to the epitaxial interface may be more than 10% and then decreases to the constant of approximately 6.6%, while the Sb concentration is quite uniform along the depth at approximately 0.5%. The surface segregation for Sn and Sb should be the artifacts of the measurement. No obvious defects or precipitations were found by the TEM observation. The Sn concentration is much higher than that obtained without Sb doping; therefore, we believe the Sb doping promoted the Sn incorporation in the film. It is also remarkable that Sb was doped up to such high concentration without precipitation at this relatively high growth temperature. We have verified the existence of H2 in the MOCVD atmosphere promoted the Sn incorporation in the GeSn film, probably due to the surfactant effect. Sb might bring the similar surfactant effect for the GeSn growth. The H2 suppress the Sn surface migration therefore suppress the precipitation [2]. It has been also reported that Sb during the InGaAsN growth lowers surface energy and suppress both surface migration and island formation [3]. The surfactant effects provided by H2 and Sb, in which the surface migration and therefore the precipitation were suppressed similar to the case of the low temperature growth, might promote the Sn and Sb incorporation beyond the solubility limits. 4. Conclusions In this study, we have demonstrated high concentration Sn and Sb incorporation in the epitaxial Ge grown by MOCVD. The Sn and Sb concentrations were 6.6% and 0.5%, respectively. These high concentration may be due to the surfactant effect of both H2 and Sb.