Nanofabrication

Finding the right balance for SnO growth enables the realization of all-oxide SnO/Ga2O3 vertical pn heterojunction diodes

Oxide electronics is a rapidly developing field of research, yielding opportunities for transparent devices, solar-blind UV sensors, or energy-efficient power electronics. Currently, Ga2O3 is considered a champion semiconducting material for high-voltage power electronics that is predicted to outperform even GaN and SiC but can only be doped n-type. This doping asymmetry precludes the implementation of p-type functionalities based on Ga2O3, in particular technologically important pn-junctions which are building blocks for many types of devices. To make up for this shortcoming, we have prepared a pn-junction by combining the naturally p-type semiconducting oxide SnO with Ga2O3. The high hole mobility of SnO is beneficial for this and other applications but its growth is severely challenged by metastability with respect to the metallic Sn and the n-type semiconducting SnO2.

Figure 1: (a) Finding the right growth conditions for SnO based on in-situ measurements of the growth rate of SnO2. (b) Schematics of the processed, vertical SnO/Ga2O3 pn-diode. (c) Current-voltage characteristics of the diode including fit (orange, broken line) to the diode equation. The inset indicates the low turn-on voltage.

Therefore, the growth of SnO by molecular beam epitaxy (MBE) requires a finely tuned balance of provided oxygen and Sn metal fluxes as well as a suitable growth temperature to prevent the formation of unwanted Sn or SnO2. We solved this metastability problem 2 by applying a lesson previously learned from the MBE growth of SnO2: The growth of SnO2 proceeds in two reaction steps. In the first step SnO is formed from the Sn vapor and the oxygen. In the second step any oxygen that is left over can oxidize SnO further into SnO2. This behavior can be followed in Fig. 1 (a) that shows the SnO2 growth rate measured during growth with different amounts of supplied oxygen. At the oxygen flux of 0.15 sccm, marked with the green circle, growth of SnO2 ceases and just enough oxygen is provided for the first reaction step that forms the SnO. In this fashion the sweet spot for SnO growth was rapidly found in a single experiment and used for subsequent growth of a p-type SnO layer on an n-type semiconducting Ga2O3 substrate at a lower growth temperature Tg of 400°C.

 

After growth, the SnO/Ga2O3 sample was processed by our technologists into a vertical diode,1 whose structure is schematically shown in Fig. 1 (b). Our partners from the University of Leipzig, with whom we collaborate in the framework of GraFOx (http://grafox.pdi-berlin.de/), took current-voltage measurements between the top and bottom Ti/Au-contact of this diode. The results shown in Fig. 1 (c) reveal a diode-like characteristics with high rectification of 2 x 108 at +/-1 V and an ideality factor of 1.16, indicating a high-quality pn-junction. The related pn-junction isolation even prevented parallel conduction in the highly conductive Ga2O3 substrate during measurements of the electrical properties of the SnO layer on top, highlighting the potential for decoupling the p-type functionality in lateral transport devices made of SnO from that of the underlying n-type Ga2O3 substrate. In addition, the pn junctions may contribute to field management required to reach higher voltage capabilities in Ga2O3 devices.

2 Author M. Budde , P. Mazzolini , J. Feldl , C. Golz , T. Nagata , S. Ueda , G. Hoffmann , F. Hatami , W. T. Masselink , M. Ramsteiner , O. Bierwagen
Title

Plasma-assisted molecular beam epitaxy of SnO(001) films: Metastability, hole transport properties, Seebeck coefficient, and effective hole mass

Source Phys. Rev. Mater. , 4 , 124602 ( 2020 )
DOI : 10.1103/physrevmaterials.4.124602 | Download arXiv: 2007.13448v2 | 3234 Cite : Bibtex RIS
M. Budde, P. Mazzolini, J. Feldl, C. Golz, T. Nagata, S. Ueda, G. Hoffmann, F. Hatami, W. T. Masselink, M. Ramsteiner, and O. Bierwagen

1 Author M. Budde , D. Splith , P. Mazzolini , A. Tahraoui , J. Feldl , M. Ramsteiner , H. von Wenckstern , M. Grundmann , O. Bierwagen
Title

SnO/beta-Ga2O3 vertical pn heterojunction diodes

Source Appl. Phys. Lett. , 117 , 252106 ( 2020 )
DOI : 10.1063/5.0031442 | Download: PDF | 3220 Cite : Bibtex RIS
M. Budde, D. Splith, P. Mazzolini, A. Tahraoui, J. Feldl, M. Ramsteiner, H. von Wenckstern, M. Grundmann, and O. Bierwagen