| AVS 66th International Symposium & Exhibition | |
| Thin Films Division | Wednesday Sessions |
| Session TF+EM-WeA |
| Session: | Emerging Thin Film Materials: Ultra-wide Bandgap and Phase Change Materials |
| Presenter: | Benthara Hewage Dinushi Jayatunga, Case Western Reserve University |
| Authors: | B.H.D. Jayatunga, Case Western Reserve University K. Kash, Case Western Reserve University M.D. Reza, The Ohio State University H. Zhao, The Ohio State University O. Ohanaka, Case Western Reserve University R. Lalk, Case Western Reserve University M. Zhu, The Ohio State University J. Hwang, The Ohio State University |
| Correspondent: | Click to Email |
ZnGeN2 and GaN are almost lattice matched and both have band gaps of approximately 3.4 eV. A large conduction band offset of ~ 1.4 eV results in a type II band alignment that has great potential for novel device structures. [1,2] For the 50-50 alloy, a slightly positive mixing energy, indicating a tendency toward phase separation, has been predicted. [3] For this mixture the lowest energy configuration is predicted to be an octet-rule-preserving orthorhombic Pmn21 phase. Other compositions may in principle be made in octet-rule-preserving (and thus lower energy) phases, compared to those that break the octet rule, by random stacking of ZnGeN2 and GaN layers along the orthorhombic b axis. [3] Whether random stacking, phase separation, or octet rule violations occur will determine whether, and by how much, the band gap may be tuned with composition, and whether the transport properties are isotropic or anisotropic. The only other work on this alloy reported to date employed a gas reduction nitridation method for synthesis of powders of different compositions, from pure ZnGeN2 to a 50-50 mixture, for photocatalytic applications [4].
Here we report the results of MOCVD growth of this alloy on c-, r-, and a-plane sapphire and c-GaN/sapphire substrates, at temperatures varying from 550 0C to 700 0C. Films at the 50-50 composition exhibit better surface morphologies when grown on r-sapphire substrates. Zn incorporation increases with the increase of Ga. The highest growth rate, 3.46 µm/hr, was obtained for a film grown on r-sapphire at 670 0C and 550 torr, for which a 2θ-ω XRD measurement yielded a wurtzite (110) diffraction peak at 2Ɵ = 57.700 with FWHM of 0.760 and an RMS surface roughness of ~ 10 nm by AFM. The Hall mobility is 8.19 cm2/v-s with an n-type carrier concentration of 8.5 x 1018 cm-3. Atomic-resolution HAADF-STEM revealed the atomic arrangement of the film near the substrate interface. Introduction of a low-temperature-grown ZnGeN2 buffer layer (480 0C at 600 torr with low injection of precursors compared to the film growth conditions) led to improved surface morphology and crystal quality, and yielded a room temperature photoluminescence spectrum indicating a band edge at approximately 3.5 eV, close to that predicted for the Pmn21 phase. [3]
The authors acknowledge support from the National Science Foundation DMREF: SusChEM: grant 1533957.
[1] L. Han, K. Kash, H. Zhao, J Appl Phys 120, 103102 (2016)
[2] M. R. Karim, H. Zhao, J Appl Phys 124, 034303 (2018)
[3] B.H.D. Jayatunga, S. Lyu, S. Kumar, K. Kash, W. R. L. Lambrecht, Phys Rev Mat 2 (2018)
[4] T. Suehiro, M. Tansho, T. Shimizu, J Phys Chem C 121, 27590 (2017)