Terahertz waves sit between microwaves and infrared light. They pass through paper, plastic, and fabric, but not through metal or water. That makes them handy for inspecting packages without opening them or scanning luggage at the airport. But receivers for these waves have so far been made from pricey materials like gallium arsenide. Silicon, the backbone of all electronics, was not up to the job: it 'remembers' light for too long.
This new work changes the picture. Instead of crystalline germanium, which is usually grown on silicon, the team used amorphous germanium — no strict atomic order. In such a disordered lattice, electrons knocked out by light lose energy almost instantly. The charge carrier lifetime is just 1.1–1.4 picoseconds. That is a thousand times faster than in regular germanium.
The film was deposited by two methods: plasma-enhanced chemical vapor deposition and magnetron sputtering. The first produced a smoother, more uniform surface. That detector performed best: a signal-to-noise ratio of 32 decibels and a bandwidth of about 2 terahertz. That is a 2.5 times stronger signal than the device made from the sputtered film.
The same material also works as an emitter. An antenna made of amorphous germanium on silicon produces terahertz pulses with a signal-to-noise ratio of 40 decibels and a bandwidth of up to 3 terahertz. That means you can put both a transmitter and a receiver on the same chip.
Now terahertz systems can be built on silicon fabrication lines, just like ordinary integrated circuits. That will make scanners cheaper for food quality control, medical devices for skin inspection, and security systems. Amorphous germanium is becoming a bridge between the world of electronics and the world of invisible light.
Terahertz radiation is sometimes called the 'ray of the future': it sees through packaging but, unlike X-rays, does not harm cells.