Atomic‐Scale Mechanism of Spontaneous Polarity Inversion in AlN on Nonpolar Sapphire Substrate Grown by MOCVD

Abstract

In the absence of inversion symmetry along the c-direction, group III-nitride materials on sapphire substrates (AlN, GaN, InN) and their alloys can exhibit either an N-polar or a metal-(Al, Ga, In) polar surface. [1] Here, N-polar or metalpolar refers to the situation where the epilayer grows along the c or −c direction. Although having identical bulk properties (e.g., refractive index, bandgap, etc.), the two polarities exhibit noticeable differences in surface energy, [2] growth mode, [3] nonlinear optical property, [4] and susceptibility to chemicals. [5] Manipulating the polarity, therefore, is a fertile playground for discovering novel device concepts in form of heteropolar junctions, such as nonlinear optical devices, [3,4,6] polar discontinuity devices, [7] and high-performance GaN high-electron-mobility transistors (HEMT). [8] Particularly, a number of The performance of nitride devices is strongly affected by their polarity. Understanding the polarity determination and evolution mechanism of polar wurtzite nitrides on nonpolar substrates is therefore critically important. This work confirms that the polarity of AlN on sapphire prepared by metalorganic chemical vapor deposition is not inherited from the nitrides/sapphire interface as widely accepted, instead, experiences a spontaneous polarity inversion during the growth. It is found that at the initial growth stage, the interface favors the nitrogen-polarity, rather than the widely accepted metal-polarity or randomly coexisting. However, the polarity subsequently converts into the metal-polar situation, at first locally then expanding into the whole area, driven by the anisotropy of surface energies, which results in universally existing inherent inverse grain boundaries. Furthermore, vertical two-dimensional electron accumulation originating from the lattice symmetry breaking at the inverse grain boundary is first revealed. This work identifies another cause of high-density defects in nitride epilayers, besides lattice mismatch induced dislocations. These findings also offer new insights into atomic structure and determination mechanism of polarity in nitrides, providing clues for its manipulation toward the novel hetero-polarity devices.

Behaviours of the lattice-polarity inversion in AlN growth on c-Al2O3 (0001) substrates by ammonia-free high temperature metalorganic chemical vapor deposition

“…The Al-polar AlN trapezoid formation may relate to the spontaneous polarity inversion mechanism. 58 Further research work is necessary. Following further growth, the Al-polar AlN trapezoid expands laterally, forming a coalesced Al-polar AlN layer (Fig.…”

“…In fact, lattice-polarity inversion phenomena in III-nitrides grown by various growth techniques have been widely observed. 42,[47][48][49][50][51][52][53][54][55][56][57][58] Mechanisms for lattice-polarity inversion in III-nitrides have been investigated. [48][49][50][51][52][55][56][57][58] Among them, most mechanistic analyses link the origin of the latticepolarity inversion point to the formation of oxygen-related alloy (AlON).…”

“…42,[47][48][49][50][51][52][53][54][55][56][57][58] Mechanisms for lattice-polarity inversion in III-nitrides have been investigated. [48][49][50][51][52][55][56][57][58] Among them, most mechanistic analyses link the origin of the latticepolarity inversion point to the formation of oxygen-related alloy (AlON). [48][49][50][51][52]55,56 However, oxygen (O) accumulation around the IDB region 55 is not detectable in our STEM samples by careful energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) measurements.…”

“…Recently, a spontaneous polarity inversion mechanism has been proposed. 58 It is argued that lattice-polarity inversion is both inherent and spontaneous, and it can happen without the participation of foreign atoms. Although the origin of the lattice-polarity inversion in our AlN growth is not completely clear at the present stage, we believe that the spontaneous polarity inversion mechanism is compatible with our case and it can give us important hints towards a full understanding of lattice-polarity inversion behaviour in our next research work.…”

Annihilation of Nanoscale Inversion Domains in Nitrogen-Polar AlN under High-Temperature Annealing

“…To further verify the atomic structure of the AlN IDs, a HRTEM image along the [11][12][13][14][15][16][17][18][19][20] axis pinpointing a thin stripe is shown in Figure 2c. A sharp interface perpendicular to the sample surface is clearly observed with a small tilt angle around 1.7°.…”