"Totally new physics" yields first germanium laser

Laura S. Marshall, laura.marshall@photonics.com

It’s the very first germanium laser capable of emitting wavelengths useful for optical communications. It’s also the first operable at room temperature. And this new laser not only holds promise for optical computing but also proves that indirect-bandgap semiconductors can yield practical lasers.

“The laser is just totally new physics,” said Lionel C. Kimerling of MIT, whose Electronic Materials Research Group developed the germanium laser. Kimerling is the Thomas Lord professor of materials science and engineering. The research team published its results online in Optics Letters in January.

A germanium laser chip and a cross-sectional scanning electron microscope picture of the Ge waveguide are shown here. The patterns that appear on the laser chip are the result of the scattering of light by the Ge waveguides. Courtesy of Jifeng Liu, MIT.

Previously, it was believed that indirect-bandgap semiconductors could not be used for practical lasers. Within semiconductor crystals, an excited electron will free itself and enter the conduction band so it can move freely around the crystal. But that excited electron will be in one of two states: In the first, it releases extra energy as a photon; in the second, the extra energy is released another way – heat, for example.

In direct-bandgap materials, the first, photon-emitting state is a state of lower energy than the second state; in indirect-bandgap materials, the reverse is true. And because an excited electron will occupy the lowest-energy state available, they tend to go into the photon-emitting state in direct-bandgap materials such as gallium arsenide but not in indirect-bandgap materials such as germanium.

“In indirect gap semiconductors, there is a mismatch in momentum between the electrons in the indirect conduction valleys and the holes in the valence band,” said lead author Jifeng Liu, a postdoctoral associate who co-authored the article with Kimerling, Jurgen Michel, the group’s principal research investigator, and graduate students Xiaochen Sun and Rodolfo Camacho-Aguilera.

“Since any transition needs to conserve the momentum, light emission cannot happen in indirect-bandgap semiconductors unless the electrons happen to obtain an adequate amount of momentum from the waves of atomic vibrations in the material, known as ‘phonons,’ to compensate this mismatch,” Liu said.

“It is similar to the situation of penguins waiting to catch the right sea wave in order to hop onto an iceberg. Therefore, the light emission in indirect-gap semiconductors is very inefficient, so these materials are considered unsuitable for practical lasers. Historically, scientists simply avoid using indirect-gap materials for light-emitting devices,” he said.

For the new laser, Liu and colleagues forced excited germanium electrons into the photon-emitting state – the higher-energy state – using two strategies common to chip manufacturing.

In the first approach, the group doped the germanium with phosphorous, which has five outer electrons; germanium has only four. That extra electron filled the lower-energy state in the conduction band, causing excited electrons to “spill over” into the higher-energy state and emit photons.

In the second strategy, the team “strained” the germanium, prying its atoms slightly farther apart than they naturally would be. To do this, they grew the germanium directly atop a layer of silicon, which lowered the energy difference between the two states, enabling excited electrons to spill over into the photon-emitting state instead of releasing their extra energy in another way.

Thus, they lured the electrons into the photon-emitting state and produced a practical laser with indirect-bandgap semiconductors.

For optical computing, it is essential to develop cheap, practical ways to integrate optical and electronic components onto silicon chips. Lasers used today for communications systems must be built separately from expensive materials such as gallium arsenide and then grafted onto silicon chips, a process that takes more time and is more costly than if they were constructed directly on the silicon itself.

Germanium, it should be noted – unlike typical laser materials – is easy to use in existing silicon-chip manufacturing processes. Liu said that germanium and silicon are in the same group in the periodic table and have the same crystal structure and number of valence electrons. “Therefore, introducing germanium directly onto silicon chips does not induce any dopant contamination to existing silicon transistor devices as typical laser materials such as GaAs do.”

“There are two major steps in further development of this technology,” he said. “First, we will develop germanium laser diodes that are directly powered by electrical current. In fact, we demonstrated the first germanium light-emitting diode on silicon last year, so we believe that an electrically pumped laser diode can be achieved with improved device design. Second, we will further increase the doping level in germanium to enhance its efficiency. We have found some good approaches to achieve this goal, but I cannot disclose it yet due to proprietary issues.”