A new interface for efficient electronics

By uniting different forms of the same semiconductor material, KAUST researchers have created a self-powered device that detects ultraviolet light^[1]. The key part of the device is known as a phase heterojunction (PHJ), an arrangement that could open up a host of new electronics applications.

The atoms within a crystalline semiconductor can be arranged in various patterns known as polymorphic phases. Each phase offers distinct properties, such as the light wavelength it absorbs. The new device uses two phases of gallium oxide (Ga₂O₃), a stable and relatively inexpensive material that absorbs deep in the ultraviolet part of the spectrum.

Electronic devices often contain adjacent layers of two semiconductor materials. But marrying two phases of the same semiconductor to create a PHJ offers several advantages over the traditional approach, explains team member Yi Lu.

For instance, PHJs can avoid absorbing unwanted wavelengths of light, and they tend to create a strong electric field at the interface between the phases, which could significantly enhance the performance of devices including solar cells, transistors and photodetectors. “Growing and integrating polymorphic phases of the same material is also more straightforward and economical than combining dissimilar materials, but it is difficult to maintain a high-quality interface” says Lu.

The researchers used a method called epitaxy to prepare their PHJ. This involves firing a laser at a target to generate a stream of atoms that subsequently assemble on a substrate.

First, they grew gallium oxide’s orthorhombic phase (κ-Ga₂O₃) on a sapphire substrate. They used high vacuum conditions, and included some tin in the target, which helped to generate the desired phase. On top of that, they formed a layer of the monoclinic phase (β-Ga₂O₃), using oxygen-rich conditions to ensure the correct crystal structure.

“The main contribution of this work is the first demonstration of a phase heterojunction with a clear, atomically-sharp, and well-defined interface,” says Xiaohang Li, who led the team.

When ultraviolet light hits this PHJ, it excites electrons into a higher energy band, leaving positively-charge ‘holes’ behind in a lower-energy band. Crucially, each of these energy bands differs slightly between the two phases, which creates an electric field at the interface between the layers. This helps to quickly and efficiently separate the electrons and holes, generating a current without having to apply an external voltage — meaning the device is self-powered.

“Researchers have previously tried to demonstrate this PHJ,” says Ph.D. student and team member Patsy A. Miranda Cortez.“However, they just formed randomly distributed mixed phases, which may not be suitable for semiconductor device mass production.”

The PHJ created a current roughly 1000 times greater than similar devices that contained only a single phase of gallium oxide, and it did so much more quickly. Consequently, it could produce a much stronger detection signal in response to very weak deep ultraviolet light.

The team now plans to combine other phases of gallium oxide, and apply their PHJs to areas such as advanced imaging, energy-efficient photonics, and power electronics.