Moira Miller1,2,Marshall Tellekamp2,Anthony Rice2,Jacob Cordell1,2,Xerxes Steirer1,2,Garritt Tucker1,Stephan Lany2,Adele Tamboli2,1
Colorado School of Mines1,National Renewable Energy Laboratory2
Moira Miller1,2,Marshall Tellekamp2,Anthony Rice2,Jacob Cordell1,2,Xerxes Steirer1,2,Garritt Tucker1,Stephan Lany2,Adele Tamboli2,1
Colorado School of Mines1,National Renewable Energy Laboratory2
In past decades traditional III-V materials and devices spurred major advances in optoelectronic technologies, including light-emitting diodes (LEDs), high-efficiency photovoltaics (PV), improved lasers, sensors and displays. However, despite these advances, lattice mismatch between GaN and InGaN in conventional III-V (In<sub>x</sub>Ga<sub>1-x</sub>N) LEDs limits emission efficiency for wavelengths greater than ~500 nm. The strain caused by lattice mismatch leads to piezoelectric polarization creating reduced electron/hole wavefunction overlap and efficiency due to the quantum confined stark effect. The wurtzite II-IV-N<sub>2</sub> material system is currently being investigated as alternative for applications in optoelectronics, especially LEDs. The ternary structure allows for independent tuning of the band gap and lattice constants by cation disorder and alloying, allowing for wide material applications. In this work we explore an alternative material for green LEDs, ZnGeN<sub>2</sub>.<br/>ZnGeN<sub>2</sub> is proposed as a green-to-amber emitter, which, due to its similar structural properties to GaN, can be easily integrated into existing III-V heterostructures for faster industry deployment. The ZnGeN<sub>2</sub>/GaN material system has smaller lattice mismatch and polarization than the traditional III-V system, allowing for increased wavefunction overlap internal quantum overlap. The theoretically predicted type II band alignment between GaN and ZnGeN<sub>2</sub> would improve hole confinement. ZnGeN<sub>2</sub> has been grown by sputtering, HVPE, and MOCVD. Our group has demonstrated high quality MBE ZnGeN<sub>2</sub> growth on GaN and AlN. This talk will focus on ZnGeN<sub>2</sub>/GaN heterostructure device modelling and the experimental results that helped elucidate important input parameters for the simulations.<br/>The modelling program, SiLENSe, is a 1D drift-diffusion model simulation tool used to determine the wavefunction and spectra of an LED from various inputs, including polarization, bias, and device design. We incorporated preliminary band offset measurements of ZnGeN<sub>2</sub> on GaN found in our experimental work, and these preliminary results will also be presented. These band offsets were found by an X-ray photoelectron spectroscopy (XPS) study of ZnGeN<sub>2</sub> band edge positions. We found that the valence band edge of ZnGeN<sub>2</sub> is at least 0.65 eV higher in energy than the valence band edge of GaN, enabling the expected type-II heterojunction. We are performing more experiments to improve the accuracy of these measurements by accounting for band bending and polarization. These band offsets are incorporated in device modelling that helps inform device architecture. Also included as input parameters for the modelling experiments were wurtzite lattice constants found during growth on AlN. GaN is the optimal substrate for ZnGeN<sub>2</sub> in LEDs, but due to their structural and optoelectronic similarity, growths on AlN are required to investigate structural, morphological, and optical properties. Growth on AlN also allowed for high quality lattice constant measurements of relaxed ZnGeN<sub>2</sub>, which is useful for more precise device modelling. We will present how varying heterolayers thicknesses and positions affects wavefunction overlap and emission spectra region. The characterization and modelling presented here represent an important advancement for theorized GaN/ZnGeN<sub>2 </sub>heterostructures in future nitride LEDs.