Growth rates and alloy composition of AlGaN grown by MOVPE is often very temperature dependent due to the presence of gas-phase parasitic chemical processes. These processes make wafer temperature measurement highly important, but in fact such measurements are very difficult because of substrate transparency in the near-IR (~900 nm) where conventional pyrometers detect radiation. The transparency problem can be solved by using a mid-IR pyrometer operating at a wavelength (~7500 nm) where sapphire is opaque. We employ a mid-IR pyrometer to measure the sapphire wafer temperature and simultaneously a near-IR pyrometer to measure wafer pocket temperature, while varying reactor pressure in both a N2 and H2 ambient. Near 1300 °C, as the reactor pressure is lowered from 300 Torr to 10 Torr the wafer temperature drops dramatically, and the ∆T between the pocket and wafer increases from ~20 °C to ~250 °C. Without the mid-IR pyrometer the large wafer temperature change with pressure would not have been noted. In order to explain this behavior we have developed a quasi-2D thermal model that includes a proper accounting of the pressure-dependent thermal contact resistance, and also accounts for sapphire optical transmission. The model and experimental results demonstrate that at most growth conditions the majority of the heat is transported from the wafer pocket to the wafer via gas conduction, in the free molecular flow limit. In this limit gas conductivity is independent of gap size but first order in pressure, and can quantitatively explain results from 20 to 300 Torr. Further analysis yields a measure of the thermal accommodation coefficients; α(H2) =0.23, α(N2) =0.50, which are in the range typically measured.
On August 15, 2016, Sandia hosted a visit by Professor Venkatesh Narayanamurti. Prof Narayanamurti (Benjamin Peirce Research Professor of Technology and Public Policy at Harvard, Board Member of the Belfer Center for Science and International Affairs, former Dean of the School of Engineering and Applied Science at Harvard, former Dean of Engineering at UC Santa Barbara, and former Vice President of Division 1000 at Sandia). During the visit, a small, informal, all-day idea exploration session on "Towards an Engineering and Applied Science of Research" was conducted. This document is a brief synopsis or "footprint" of the presentations and discussions at this Idea Exploration Session. The intent of this document is to stimulate further discussion about pathways Sandia can take to improve its Research practices.
An AlN barrier high electron mobility transistor (HEMT) based on the AlN/Al0.85Ga0.15N heterostructure was grown, fabricated, and electrically characterized, thereby extending the range of Al composition and bandgap for AlGaN channel HEMTs. An etch and regrowth procedure was implemented for source and drain contact formation. A breakdown voltage of 810 V was achieved without a gate insulator or field plate. Excellent gate leakage characteristics enabled a high Ion/Ioff current ratio greater than 107 and an excellent subthreshold slope of 75 mV/decade. A large Schottky barrier height of 1.74 eV contributed to these results. In conclusion, the room temperature voltage-dependent 3-terminal off-state drain current was adequately modeled with Frenkel-Poole emission.
Illumination by a narrow-band laser has been shown to enable photoelectrochemical (PEC) etching of InGaN thin films into quantum dots with sizes controlled by the laser wavelength. Here, we investigate and elucidate the influence of solution pH on such quantum-size-controlled PEC etch process. We find that although a pH above 5 is often used for PEC etching of GaN-based materials, oxides (In2O3 and/or Ga2O3) form which interfere with quantum dot formation. At pH below 3, however, oxide-free QDs with self-terminated sizes can be successfully realized.
Three-dimensional phase-field simulations of GaN growth by selective area epitaxy were performed. The model includes a crystallographic-orientation-dependent deposition rate and arbitrarily complex mask geometries. The orientation-dependent deposition rate can be determined from experimental measurements of the relative growth rates of low-index crystallographic facets. Growth on various complex mask geometries was simulated on both c-plane and a-plane template layers. Agreement was observed between simulations and experiment, including complex phenomena occurring at the intersections between facets. The sources of the discrepancies between simulated and experimental morphologies were also investigated. The model provides a route to optimize masks and processing conditions during materials synthesis for solar cells, light-emitting diodes, and other electronic and opto-electronic applications.
We report here the characteristics of photoelectrochemical (PEC) etching of epitaxial InGaN semiconductor thin films using a narrowband laser with a linewidth less than ∼1 nm. In the initial stages of PEC etching, when the thin film is flat, characteristic voltammogram shapes are observed. At low photo-excitation rates, voltammograms are S-shaped, indicating the onset of a voltage-independent rate-limiting process associated with electron-hole-pair creation and/or annihilation. At high photo-excitation rates, voltammograms are superlinear in shape, indicating, for the voltage ranges studied here, a voltage-dependent rate-limiting process associated with surface electrochemical oxidation. As PEC etching proceeds, the thin film becomes rough at the nanoscale, and ultimately the self-limiting etch kinetics lead to an ensemble of nanoparticles. This change in InGaN film volume and morphology leads to a characteristic dependence of PEC etch rate on time: an incubation time, followed by a rise, then a peak, then a slow decay.