Nitrides below the tip of the iceberg

Discoveries of new nitride materials are expanding the semiconductor landscape beyond the GaN family.

BY ANDRIY ZAKUTAYEV FROM THE NATIONAL LABORATORY OF THE ROCKIES

By now, you have likely heard of AlScN. If not, it is worth paying attention: it is a ferroelectric nitride that has become a prominent topic in compound semiconductor research. Yet, if one asks the community to provide an overview of nitride semiconductors, most would immediately point to GaN, the material that revolutionised LEDs and is now transforming telecom and defence applications.

However, whether the focus remains on GaN or expands to include AlScN, the discussion may still be confined to only the tip of the nitride iceberg. That is my perspective – and the one that I hope may resonate with other researchers as community interest in emerging nitride materials grows. What follows is an overview of the broader and still-developing landscape of novel nitride semiconductors.

For many decades, GaN and its semiconductor alloys have dominated nitride device research. In 2014, InGaN was recognised with the Nobel Prize in Physics for its role in LED development, helping to drive an energy-efficient lighting revolution. Since then, AlGaN HEMTs have played a crucial role in defence and telecom industries and are now expanding into other applications. However, despite continued advances in these GaN-based materials, there is growing recognition that they represent only a fraction of the broader nitride semiconductor materials space.

More recently, AlScN has risen to prominence in semiconductor research. Ferroelectric switching in this polar nitride was reported a few years ago in Germany and has since attracted significant attention around the world. Device engineers are now exploring its potential as a dielectric barrier layer in HEMTs to increase channel charge density, as well as in switchable ferroelectric field-effect transistors for non-volatile memory applications. Notably, although AlScN is considered a relatively new ferroelectric nitride for semiconductor devices, it has been used for more than a decade as a piezoelectric material in tunable RF filter applications in the telecom industry. Despite this long-term piezoelectric history and the recent ferroelectric breakthrough, it is unlikely that AlScN represents the final stage of nitride semiconductor materials development.

Less visible, but equally important, are advances in fundamental materials chemistry that are laying the groundwork for a wide range of new nitride discoveries. Researchers in this area have predicted and synthesized numerous novel nitride compositions and structures, some exhibiting highly tunable electronic, and even quantum, properties. This raises an important question: what lies beneath the GaN tip of the nitride iceberg?

The research on nitrides already extends beyond diodes and transistors to include their piezoelectric, magnetic and photonic applications. The number next to each icon corresponds to the number of research articles with the word ‘nitride’ and the corresponding application keywords, in the title/abstract/keywords of the papers. For example, 276,000 (276K) is the number of papers mentioning the word ‘nitride’, the middle number of 91K is the number of papers talking about nitride semiconductors, the number of papers about nitride diodes or LEDs is 26K, on so on.

Below the tip of the iceberg
Contrary to common perception, nitride semiconductors are not limited to III-N compositions such as GaN. For example, ZnGeN₂, part of the II-IV-N₂ family, offers the potential for bandgap tunability through cation ordering at a fixed lattice constant. This approach may provide a pathway to addressing the miscibility gap challenges encountered in InGaN for green and amber LEDs.

The II-IV-N₂ family extends beyond ZnGeN₂ to include compounds such as MgSiN₂, ZnTiN₂, Mg₂SbN₃, Zn₂NbN₃ and Zn₃MoN₄, all of which show potential for tunable optical performance.ce without the same degree of lattice-matching constraints.

While nitride semiconductors are often associated with wurtzite structures, this is not their only crystallographic form. Recently, rocksalt nitrides such as MgZrN₂ and Mg₂NbN₃ have emerged as wide-bandgap semiconductors. Despite possessing indirect bandgaps and non-polar structures, unlike the wurtzite nitrides, these materials may find roles as barrier layers in quantum devices, including Josephson junction qubits. Although rocksalt nitrides have historically been associated with superconductivity, such as in NbN, new semiconducting variants are broadening the scope for future device design and fabrication.

Bridging the structural space between wurtzite and rocksalt are layered nitrides. These include materials such as ZnZrN₂, sometimes termed ‘wurtsalt’, which consist of alternating wurtzite and rocksalt layers, and are predicted to exhibit very low hole effective masses.

Beyond the well-known wurtzite-derived structures of AlGaN or InGaN, nitride semiconductors feature other crystal structures under research, including rocksalt-derived, layered (such as wurtsalt, nickeline), and most recently perovskite. Each of these structures contains two distinct metal atoms with different charge marked by orange or red, and a nitrogen atom marked blue.

Although wurtsalt structures remain challenging to synthesize, the related layered ‘rockseline’ structure (combining rocksalt and nickeline motifs), exemplified by MgMoN₂, has recently been realised in thin-film form. There is cautious optimism that the epitaxial growth of rockseline materials, together with their predicted topological properties, could contribute to the development of next-generation nitride-based quantum electronics.

Another emerging area involves nitride semiconductors with non-stoichiometric metal-to-nitrogen ratios, such as nitride perovskites with the general formula ABN₃. Following theoretical predictions a decade ago, recent synthesis advances have enabled the development of oxygen-free nitride perovskites. Examples include LaWN₃ exhibiting high piezoelectric coefficients, and CeTaN₃, demonstrating switchable ferroelectricity. Admittedly, compared with halide perovskites, progress in nitride variants has been more gradual. Nevertheless, these materials may significantly influence the future direction of electronic device research.

Taken together, these research developments suggest that the nitride semiconductor field extends well beyond GaN and its close relatives. Advances in materials chemistry are revealing a diverse family of nitride material systems, ranging from tunable-bandgap wurtzites and emerging rocksalt semiconductors, to layered nitrides and nitride perovskites. It is conceivable that one of these compositions could follow a trajectory similar to AlScN, rapidly gaining prominence due to distinctive functional properties.

From the perspective of a materials scientist, it is my opinion that continued evolution in electronic devices presents an opportunity.

Broadening the materials palette to include the wider family of nitrides may enable new functionalities and device concepts drawn from what lies beneath the tip of the GaN iceberg.