III-nitride growth and characteristics on ferroelectric materials using plasma-assisted molecular beam epitaxy

Abstract

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.

The origin of the residual conductivity of GaN films on ferroelectric materials

“…These samples had a FWHM of 1.38111 and 1.36441 with 880 and 1037Å thickness. Note that these samples grown in the EGR regime have more phase separation (LiNbO 3 +LiNb 3 O 8 ) [7,8], confirmed by XRD measurement, as shown in Fig. 2.…”

“…Surface acoustic wave (SAW) devices were implemented by AlN epitaxial layers on LN, resulting in partially relieving the temperature instability and increasing the SAW velocity [4,6]. Stoichiometric LN (50 mole% of Li 2 O) showed better crystalline stability than Congruent LN (48.39 mole% of Li 2 O), resulting in improved device performance [7]. Furthermore, LT was considered as a substrate for III-nitride growth [8].…”

“…The LN samples were annealed at 500 1C in a dry air environment to enhance the adhesion of GaN films [7][8][9]. GaN films were grown with various gallium fluxes and constant nitrogen flow at the same growth temperatures to verify the effect of V/III ratio on the GaN/LN interface with different growth regimes [10].…”

Lithium niobateX-cut,Y-cut, andZ-cut surfaces fromab initiotheory

“…Periodically poled LN structures 5 and substrate-assisted self-assembly of nanostructures 6 are examples of the emerging applications. Furthermore, LN has attracted a lot of attention as a substrate for the GaN growth, either for the realization of monolithic optoelectronic amplifiers [7][8][9] or as an alternative substrate ͑to ␣-Al 2 O 3 ͒ for the quasilattice matched growth of GaN. [10][11][12][13] The microscopic understanding of LN surfaces is comparatively poor, however.…”

GaN growth on LiNbO3 (0001) – a first‐principles simulation

“…Epitaxially grown GaN is affected by a high density of lattice defects, due to the considerable lattice mismatch (around 16%) between GaN and α-Al 2 O 3 , the most widely used substrate [1]. This might not be relevant for the realization of devices like light emitting diodes [2], but certainly represents a major issue for the realization of devices which require a precise band engineering (e.g.…”