The characteristics of III-nitrides grown on zinc- and oxygen-face ZnO by plasma-assisted molecular beam epitaxy were investigated. The reflection high-energy electron diffraction pattern indicates formation of a cubic phase at the interface between III-nitride and both Zn- and O-face ZnO. The polarity indicates that Zn-face ZnO leads to a single polarity, while O-face ZnO forms mixed polarity of III-nitrides. Furthermore, by using a vicinal ZnO substrate, the terrace-step growth of GaN was realized with a reduction by two orders of magnitude in the dislocation-related etch pit density to ∼108cm−2, while a dislocation density of ∼1010cm−2 was obtained on the on-axis ZnO substrates.
Demonstration of AlGaN/GaN high-electron-mobility transistors on 100 mm diameter Si(111) by plasma-assisted molecular beam epitaxy Appl. Phys. Lett. 97, 232107 (2010); 10.1063/1.3518717 Effect of the growth temperature and the AlN mole fraction on In incorporation and properties of quaternary IIInitride layers grown by molecular beam epitaxy J. Appl. Phys. 104, 083510 (2008); 10.1063/1.2999564III-nitride growth and characteristics on ferroelectric materials using plasma-assisted molecular beam epitaxy Two-dimensional electron gases induced by polarization charges in AlN/GaN heterostructure grown by plasmaassisted molecular-beam epitaxy
Recently published methods that answer the previously unresolved critical issue of in situ growth regime determination during molecular beam epitaxy of AlN are used to address issues of material quality and intergrowth nonuniformity for improved repeatability using a modulated flux technique. A shutter modulation growth technique, defined as metal modulation epitaxy (MME), using the previously published reflection high-energy electron diffraction (RHEED) signatures was developed with the goal of obtaining materials with the properties of droplet regime materials, without the adverse effect of droplets. The films grown using MME were compared to films grown with no shutter modulation, and the surface roughness determined by atomic force microscopy was improved. For an unmodulated sample without droplets, the rms surface roughness was 6.9nm, while a sample with droplets had a rms surface roughness of 1.2nm. For the same Al flux that resulted in droplets with the unmodulated sample, the MME sample had no droplets and had a surface roughness of 3.3nm. Furthermore, while a nearly 20% increase in the Al flux still did not result in droplets for MME, a nearly 50% increase in the al flux did finally result in droplets. Therefore, by using MME, a wider range of Al flux is allowed for Al-rich growths without droplets. The results from the recently published RHEED transient characterization provide a powerful new tool that can be used as repeatable growth indicators that can possibly be used to standardize growths with techniques such as MME presented herein.
The characteristics and growth of III-nitride materials on ferroelectric lithium niobate with varying Li mole composition have been investigated to achieve a better device performance of AlGaN∕GaN heterojunction structures. III-nitride growth on lithium niobate has been performed after high temperature (1000°C) furnace thermal treatments in dry air environment resulting in atomically flat surfaces on lithium niobate (LN). However, while this furnace thermal treatment results in improved surface smoothness and III-nitride adhesion, it also causes repolarization, ferroelectric domain reversal from a +z spontaneous polarization to a −z spontaneous polarization in the surface of congruent LN (48.39mole% of Li2O). On the other hand, near-stoichiometric LN (49.9mole% of Li2O) did not develop repolarization during the identical thermal treatment. Furthermore, as determined in situ by spectroscopic ellipsometry, congruent LN (CLN) shows a bigger variation of the pseudorefractive index and pseudoextinction coefficient during annealing at various temperatures in the range of 100–800°C, indicating surface modification at those temperatures. On the other hand, near-stoichiometric LN (SLN) shows little change in these indexes implying that SLN provides better surface stability at elevated temperatures in the vacuum environment. This improved vacuum tolerance of SLN has allowed Al0.20Ga0.80N∕GaN heterostructures to be grown on CLN and SLN substrates, with two times higher channel mobility resulting on SLN substrates. Thus, SLN appears to be a viable substrate for molecular beam epitaxial growth of III-nitrides in various applications.
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