A team led by Darrell Schlom at Cornell University grew a semiconductor film on sapphire that sets a new bandgap record. By adding silicon to aluminum gallium oxide, the researchers kept the material conducting electricity well. This breakthrough could lead to smaller, more efficient electronics that handle much higher voltages.
Semiconductors with wider bandgaps could pave the way for smaller, more efficient electronics that can handle far higher voltages, but the wider this gap becomes, the harder it is to control how well a material conducts electricity. Now, a team led by Darrell Schlom at Cornell University has shown that adding silicon to thin films of aluminum gallium oxide can keep this control intact at bandgaps wider than any semiconductor before it. Their research has been published in Nature.
Benefits in wider bandgaps
In a semiconductor, electrons occupy two distinct ranges of energy. In the lower "valence band," they are bound tightly to their atoms and can't carry a current. If they gain enough energy, however, they can jump into the higher "conduction band," where they move freely.
A semiconductor's bandgap describes the energy required for an electron to make this leap and allows semiconductors to be switched between insulating and conducting states. Since wider bandgaps let materials withstand stronger electric fields, wide-bandgap semiconductors like gallium nitride and silicon carbide are already used in everything from fast phone chargers to electric vehicles.
The aluminum problem
Aluminum gallium oxide is an especially promising material. By adjusting its ratio of aluminum to gallium, its bandgap can be tuned anywhere from 5.4 to 8.6 electronvolts (eV), the widest range of any known semiconductor. But as the aluminum content rises, it becomes far harder to supply the material with free electrons. Until now, aluminum-rich films have conducted electricity several orders of magnitude more poorly than gallium-rich ones, which has made them impractical for electronics.
Schlom's team tackled this challenge through a careful fabrication approach involving a sapphire substrate. To grow the oxide films, they applied a technique called molecular-beam epitaxy, which builds up a material one atomic layer at a time.
As each film grew on top of the sapphire, they added small amounts of silicon, whose atoms donate spare electrons to the material. Finally, they measured the films' bandgaps and electrical properties and tested how the films behaved inside two key electronic components: a diode and a transistor.
Record-breaking semiconductor
The silicon-doped films conducted electricity at bandgaps of more than 7 eV, beating the previous record held by cubic boron nitride. Above 6 eV, a range the team calls "colossal bandgap," the films conducted electricity at room temperature over 100 million times better than any previous report. Both the diode and the transistor worked successfully with these colossal-bandgap materials.
The approach could also make next-generation semiconductors far more practical to produce. Compared with other ultra-wide-bandgap materials like diamond and cubic boron nitride, sapphire is abundant, cheap and already manufactured at high quality at a massive scale.
Based on their success, Schlom's team now hopes that their films could pave the way for a new generation of power electronics that waste far less energy as heat when converting and controlling electricity.