Selective-Area Growth of Crystal Phase Transition Wurtzite InP Film
Authors: Azuma Yuki , Zheng Ziye , Motohisa Junichi , Tomioka Katsuhiro 
Published: December 2024

Nano light-emitting diodes (LEDs) for next generation display devices are required for high brightness, high luminous efficiency, and high color rendering property in visible color region. Especially, yellow color wavelength still lacks moderate-candidate materials. In case of conventional AlGaInP, the bandgap becomes indirect with increasing Al composition, results in low luminous efficiency. While in case of nitride semiconductors, crystal growth of narrow gap nitride is difficult and also decrease luminous efficiency. With this regards, wurtzite (WZ) phosphide (P)-related semiconductors by crystal phase transition are expected to overcome this issue. The crystal phase transition occur that the crystal structure of III-V compound semiconductor stabilized metastable structure and changes the bandgap to direct transition [A. De et al., Phys Rev. B, 81, 155210 (2010)]. Among the WZ III-Vs, WZ-AlInP has a bandgap of the yellow color wavelength. Therefore, exploring the growth technique for pure WZ-AlInP large-area film is important for the application of nano LEDs. Disc-shape WZ-InP was reported [S. Mauthe et al., IEEE J. Sel. Top. Quantum Electron., 25, 8300507 (2019), P. Staudinger et al., Nanotechnology, 32, 075605 (2021)]. However, there was no report on growth technique for large-area WZ III-Vs thin films. Here, we investigated growth in selective-area growth of WZ-InP film toward large-area growth.
In experiment, 20 nm-thick SiO2 sputtered InP(111)A substrates were used for the selective-area growth. For mask openings, periodical openings were formed using lithography and wet etching. Two kinds of shape were designed as mask openings. One was hexagonal openings surrounded with four short {-211} and two long {-211} sides. Another design was octagonal openings surrounded with six short {-211} and two long {-110} sides. Then, InP was grown by low-pressure horizontal MOVPE with H2 carrier gas. The source materials were trimethylindium (TMIn) and tertiarybutylphosphine (TBP). The partial pressure ratio of TMIn and TBP was 24. The growth temperature was 660 ºC. Growth time for the InP was varied from 10 - 45 min. Next, two step growth was introduced for the InP growth to investigate the growth morphologies toward epitaxial lateral-overgrowth (ELO). After the growth of InP fins in 20 min, InP was grown continuously for 15 min at 600 ºC with same V/III partial pressure ratio.
The InP hexagonal- and octagonal-shaped fins as following the designed shape of the mask openings were grown on the substrates. Transmission electron microscopy (TEM) images and selective-area electron diffraction (SAED) patterns indicated that the InP fins had WZ structure in both mask openings. And photoluminescence (PL) spectra showed two peaks at 1.46 eV and 1.42 eV originated from the WZ-InP fins and the interface between WZ-InP fins and zinc-blende (ZB)-InP substrate, respectively. The interesting point that the average fin height of both InP fins had saturated value due to the surface diffusion length of In adatoms. After the fin height reached to the saturation value, the average width of the InP fins was increased. Eventually, the InP fins exhibited random coalescence. The PL spectra of the coalesced fins showed the peak at 1.37 eV and 1.45 eV meaning that the coalesced InP fins had WZ structure including the ZB structure. TEM image and SAED pattern showed the crystal structure of the InP fin inside the coalesced InP part. The bottom of the InP fin had WZ and ZB mixed structure, while the top of the InP fin exhibited ZB structure. As for coalescence part, the crystal structure of the bottom of the coalesced InP part had WZ and ZB mixed structure, and the top of the coalesced InP part had WZ structure. These results mean that the ELO growth process continuously formed from the sidewalls of the WZ-InP fins, while top of the WZ-InP fin and ELO layer, the InP layer formed ZB phase. This was because that the diffused In adatoms on sidewalls were not reached to top planes of the coalesced layer and the effective V/III ratio was decreased.
To suppress the random coalescence growth and unintentional formation of the ZB-InP, two-step selective-area growth combining the fin growth and ELO were introduced. The two-step growth successfully formed large area films. The PL spectra of the InP large-area film had the peaks at 1.34 eV, 1.39 eV, and 1.42 eV. The peak at 1.34 eV means ZB-InP bandgap. The peaks at 1.39 eV and 1.42 eV mean ZB and WZ mixed structure. These PL results assumed that the ZB-InP layer was grown on large-area WZ-InP film. Further investigation of the crystal structure of the large-area InP films by TEM will be discussed on the day.