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The researchers worked with molybdenum sulphide (MoS2), an inexpensive semiconductor material with electronic and optical properties similar to materials already used in the semiconductor industry.
However, MoS2 is different from other semiconductor materials because it can be grown in layers only one atom thick without compromising its properties.
In the new technique, researchers place sulphur and molybdenum chloride powders in a furnace and gradually raise the temperature to 8500C, which vaporises the powder. The two substances react at high temperatures to form MoS2. While still under high temperatures, the vapour is then deposited in a thin layer onto the substrate.
“The key to our success is the development of a new growth mechanism, a self-limiting growth,” Cao says.
“Using this technique, we can create wafer-scale MoS2 monolayer thin films, one atom thick, every time,” Cao explains. “We can also produce layers that are two, three or four atoms thick.”
Cao’s team is now trying to find ways to create similar thin films in which each atomic layer is made of a different material. Cao is also working to create field- effect transistors and LEDs using the technique. Cao has filed a patent on the new technique.
Oclaro to showcase expanded laser diode portfolio
The lineup includes an 80 W fibre laser pump, an 150ps DFB seed laser, high performance diode bars at 10xx and 14xx nm and higher power 63x red diode lasers in a smaller package
Oclaro showcased several new, innovative laser diode products for fibre laser, direct diode, medical and consumer applications at the LASER World of PHOTONICS conference in Munich.
MoS2 structure
The researchers can precisely control the thickness of the MoS2 layer by controlling the partial pressure and vapour pressure in the furnace. Partial pressure is the tendency of atoms or molecules suspended in the air to condense into a solid and settle onto the substrate. Vapour pressure is the tendency of solid atoms or molecules on the substrate to vaporise and rise into the air.
To create a single layer of MoS2 on the substrate, the partial pressure must be higher than the vapour pressure. The higher the partial pressure, the more layers of MoS2 will settle to the bottom.
If the partial pressure is higher than the vapour pressure of a single layer of atoms on the substrate, but not higher than the vapour pressure of two layers, the balance between the partial pressure and the vapour pressure can ensure that thin-film growth automatically stops once the monolayer is formed. Cao calls this “self-limiting” growth.
Partial pressure is controlled by adjusting the amount of molybdenum chloride in the furnace - the more molybdenum is in the furnace, the higher the partial pressure.
Leveraging the newest generation 9xx chip design, the BMU80 fibre-coupled, single-emitter module delivers 80 W power in a 105 µm fibre, enabling fibre laser manufacturers to reduce system complexity and achieve higher power with fewer modules.
The 1060 nm DFB seed laser, capable of 800mW peak power at 150 ps pulses and 150 kHz line-width in CW mode, enables short pulse lasers for materials processing applications with improved precision and reduced thermal effects.
Oclaro is also expanding their high power laser diode portfolio to include 10xx and 14xx nm wavelengths. Highly reliable 10xx bars enable multi-Kw direct diode systems through wavelength multiplexing with the 9xx wavelengths. The new family of 14xx nm bar and single emitter products can be used for medical, cosmetics and industrial applications.
The new additions to the visible laser diode family are 100 mW, 633 nm and 150 mW, 638 nm laser diodes.
The 633 nm laser diode offers a compact, high efficiency alternative to the existing bulky He-Ne gas lasers and enables system designers to achieve measurement accuracy, stability and speed which are essential for biomedical and inspection applications.
The 638 nm laser diode delivers 25 percent more output June 2013
www.compoundsemiconductor.net 129
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