Recently, Prof. Xinyu Liu’s team from IMECAS, and their collaborators (Xianping Wang from ISSPCAS, and the advanced process center of IMECAS) have made the innovative progress in the origin of GaN interface state.
The GaN interface state is the core problem that has been faced since the research of the III-N material system, which restricts the scale and practicality of the device. Most researchers usually focus on the source of deep interface traps because of its obvious negative effects on the performance. The density (Dit) of the deep states on the (Al)GaN surface can be effectively suppressed to the order of 1011-1012 cm-2 eV-1, by a passivation layer such as SiO2, SiNx, and AlN, etc. However, the states located close to the conduction band (EC) of (Al)GaN are still relatively high and is commonly higher than 1013 cm-2 eV-1. Due to few report about the experimental value of the capture cross section (σn), σn is typically inadequately assumed to be constant across the whole interface at a value of 10-14-10-16 cm-2. Such an assumption would enormously underestimate the emission time constant (τe), which could give rise to a current collapse at low frequencies (e.g., <1 MHz), if the actual σn is lower than 10-16 cm-2. Furthermore, the origin of the near-conduction band (NCB) level is also unclear, and it lacks self-contained experimental and theory investigations; hence, it is difficult to present a comprehensive proposal to fabricate the robust interface between the dielectric and the (Al)GaN layer. Nitridation, oxidation, crystallization and other solutions were once adopted to treat the interface to improve performance, but the underlying logic is not always consistent. Hence, it is necessary to have more insight into the interface states and its potential origin whether shallow or deep.
In response to the above key scientific and technical issues, the constant-capacitance deep-level transient Fourier spectroscopy is utilized by Xinyu Liu’s team to characterize the interface between a GaN epitaxial layer and a SiNx passivation layer grown by low-pressure chemical vapor deposition (LPCVD). A near-conduction band (NCB) state ELP (EC-ET = 60 meV) featuring a very small capture cross section of 1.5×10-20 cm-2 was detected at 70 K at the LPCVD-SiNx/GaN interface. A partially crystallized Si2N2O thin layer was detected at the interface by high-resolution transmission electron microscopy. Based on first-principles calculations of crystallized Si2N2O/GaN slabs, it was confirmed that the NCB state ELP mainly originates from the strong interactions between the dangling bonds of gallium and its vicinal atoms near the interface. The partially crystallized Si2N2O interfacial layer might also give rise to the very small capture cross section of the ELP owing to the smaller lattice mismatch between the Si2N2O and GaN epitaxial layer and a larger mean free path of the electron in the crystallized portion compared with an amorphous interfacial layer. This discovery reveals the theoretical origin of the near-conduction band state from a new perspective, and provides a profound theoretical and practical basis for solving the interface state problem. On the other hand, this work also released a crystal medium Si2N2O, which is highly matched with GaN<11-20> and <1-100> directions, and is expected to give birth to new research hotspots in the field of material growth.
This work was published in the journal "ACS Appl. Mater. Interfaces" under the title "Insight into the near-conduction band states at the crystallized interface between GaN and SiNx grown by low-pressure chemical vapor deposition"(DOI: 10.1021/acsami.8b04694). Related patents have been applied.
The research was supported by the National Natural Science Foundation's major instrument project/key project/general project, the Chinese Academy of Sciences' key frontier project/STS project, and National Key R&D Program of China.
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Link: https://pubs.acs.org/doi/10.1021/acsami.8b04694
