(Invited) GaN Wafers with Dramatically Improved Crystalline and Electrical Properties
Authors: Freitas Jaime A. , Culbertson James C , Mahadik Nadeemullah A , Tadjer Marko J. , Sochacki Tomasz , Bockowski Michal 
Published: February 2020

Gallium Nitride (GaN) and its alloys with Indium and Aluminum, part of third semiconductors generation, are revolutionizing both the optoelectronic and electronic device industries. Despite a previous lack of native substrates, a number of devices exceeding the performance of those fabricated from well-developed semiconductors were realized. However, the fabrication of higher performance devices with higher yields requires higher crystalline quality substrates with fully controlled electronic transport properties. Bulk ammonothermal GaN substrates, recently developed and commercialized, are characterized by low dislocation density, a single crystal nature, and high flatness; but they lack full control of the electrical properties. It is expected that films deposited by vapor transport techniques, typically with low impurities background, would allow the growth of materials with high crystalline quality and improved electrical properties. More recently, it was demonstrated that thick GaN films deposited by Hydride Vapor Phase Epitaxy (HVPE, a fast deposition process) on ammonothermal GaN substrates reproduce the high crystalline quality of those substrates, and have free carrier concentrations that are several orders of magnitude lower that of the substrates [1]. We have verified that such HVPE GaN wafers can be used for further homoepitaxial growth. This is extremely important, because it demonstrates the usefulness of this new type of substrate to fabricate highly efficient optoelectronic and electronic devices.
An ~800 mm thick HVPE film deposited on an (0001) epi-ready Ammono substrate (~0.3 degree toward “m”) was removed from the reactor and its surface was chemical-mechanically polished (CMP). This template was put back in the HVPE reactor and an additional ~800 mm was deposited. The final crystal consisted of two sequentially grown HVPE-GaN layers on the Ammono substrate. The top HVPE layer was sliced off and three samples were diced from different regions of this wafer. The surfaces of these samples were mechanically polished and CMP finished. A combination of techniques was employed to investigate the structural, optical, and electrical properties of the crystals.
High resolution XRD was measured on two samples using a Rigaku Smartlab diffractometer with a four bounce Ge (220) monochromator. Lattice parameters were measured using symmetric and asymmetric scans: sample A: c=5.1856Å and a=3.1824Å; sample B were, c=5.1856Å and a=3.1839Å. These values are very close to the bulk lattice constants. The rocking curves for these samples were measured and had full widths at half maximum of ~16 arcsecs, indicating superior crystalline quality.
High lateral and depth resolution micro-Raman scattering (RS) measurements were carried out on the front and back surfaces of two samples. The E2
2 phonon frequency was mapped on both faces of two samples; the distribution of Raman shifts was 567.45±0.05 cm-1, indicating stress free GaN. The A1(LO) phonon frequencies and the linewidths measured on the Ga-polar face were larger than those measured on the N-polar face; this is consistent with a larger incorporation of donor impurities as the growth proceeds. Low temperature, high spectral resolution photoluminescence (PL) measurements carried out on the Ga- and N-polar faces of these samples were also consistent with more doping as the growth proceeds [2]. SIMS depth profile measurements also confirm this observation. Hall measurements provided information about the free-carrier concentration and carrier mobility.
[1] T. Sochacki, et al., J.J. Appl. Phys., 53 (2014)05FA04.
[2] J.A. Freitas, Jr., et al., Cryst. Growth & Design, 15 (2015) 4837.