Novel cascade structure aims for shorter wavelengths

02 Apr 2008

A quantum cascade laser operating in the near infrared could be on the cards, thanks to research that has shown electroluminescence from an unusual combination of materials.

US researchers have fabricated a quantum cascade structure that they believe has the potential to lase at wavelengths as short as 1.5 µm, in comparison with commercial quantum cascade lasers that typically operate beyond 4 µm. Claire Gmachl and co-workers at Princeton University and the City College of New York (CCNY) produced the emitters from II-IV structures deposited on an indium phosphide substrate.

Although quantum cascade lasers (QCLs) can now be purchased commercially, it is very difficult to make these structures emit short wavelengths – the current best is 2.75 µm with antimonide-based structures. If Gmachl's team can produce a QCL that operates in the near-infrared region, it ought to greatly increase the number of applications open to such devices.

However, the team readily admits that their latest results represent only an "initial step towards a II-VI quantum cascade laser". The emitters reported in a recent issue of Applied Physics Letters produce electroluminescence at 4.8 µm when driven electrically – which is very close to the "design" wavelength of 4.4 µm. Encouragingly, for such early-stage work, this emission could be measured from 78 K all the way up to room temperature.

What's unusual about these emitters is that they are fabricated from ZnCdSe/ZnCdMgSe structures deposited on a InP substrate. "A II-VI quantum cascade laser would be a technological accomplishment on its own, as it would be the first such non-III-V device," said Kale Franz, one of the Princeton researchers. "It would make concrete the idea that the quantum cascade is a (synthetic) material class in, and of, itself, with properties completely decoupled from the constituent materials used to fabricate the structure."

To fabricate the structures, the Princeton team enlisted the help of Maria Tamargo's group at CCNY, which specializes in II-VI epitaxy. Their approach was to first deposit a 0.25 µm thick buffer layer on a low-doped InP substrate, and then use a zinc flux treatment to prepare for II-VI epitaxy.

"Both the ZnCdSe and ZnCdMgSe compositions used in this work have the same lattice constant as InP, so in that respect the buffer layer isn't as critical as growing on silicon," explained Franz. "However, one key to starting II-VI growth on InP is the zinc flux treatment. This is a critical step."

After the zinc flux step, the team grew ten periods of active regions, including ZnCdSe wells and ZnCdMgSe barrier layers, using molecular beam epitaxy. Lithography and a wet chemical etch were used to define the electroluminescent structures, which were then formed semi-circular emitters.

The end result was a conventional two-well active region quantum cascade structure. Future development towards a laser is already in progress, with the team currently working on its second-generation designs. And, although much more work will be required before the team can build a useful laser, the Princeton team has shown that making such a device should be possible.