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Sharp enters the non-polar GaN fray


The leading diode laser and consumer electronics manufacturer is hoping to push GaN lasers to longer wavelengths for color display applications.

The field of non-polar GaN lasers, currently dominated by Rohm Corporation and the University of California, Santa Barbara, now has another big player in the shape of Sharp Corporation.

This firm's researchers, like the other groups, have turned to this material to avoid built-in polarization fields found in conventional c-plane GaN. These fields reduce laser output efficiency and shorten emission wavelengths, in a blue-shift effect that worsens at higher injection currents.

Sharp's 400nm blue laser
Sharp's 400nm blue laser

Non-polar m-plane GaN doesn't have these fields, so substrates made of this material are promising candidates for fabricating blue lasers with longer wavelengths, nearer the green part of the spectrum (see related stories).

Therefore, rather than the 400 nm laser diodes it makes using c-plane GaN for its Blu-ray players, the Japanese conglomerate used m-plane GaN substrates to make diodes that emit light at 463 nm.

Sharp says that it needs this longer-wavelength light for diode-laser based displays, as well as 520 nm green lasers that would complete the red-green-blue diode trio.

The crack-free lasers were made at Sharp's Advanced Technology Research labs using MOCVD, according to its Applied Physics Express paper, published online on January 11. This represents a move away from the MBE growth that the company typically uses in its red laser manufacturing.

The research team characterized the blue lasers under pulsed operation, finding the devices' threshold current to be 69 mA at a threshold current density of 7.8 kA/cm2. At 80 mA operating current the laser produced 10 mW output power.

The epitaxial growth started with a 1.8 µm-thick n-type Al0.035Ga0.965N cladding layer deposited on top of the m-plane GaN substrate, followed by a 0.2 µm n-type GaN optical guiding layer.

After this Sharp's team grew a sandwich arrangement, consisting of the 3 nm-thick InGaN double-quantum-well layer surrounded above and below by 70 nm-thick undoped InGaN optical guiding layers.

The deposition was completed with another 5 nm-thick undoped GaN optical guide layer, a p-type Al0.3Ga0.7N evaporation-preventing layer, a 0.55 nm p-type Al0.035Ga0.965N cladding layer and a 0.1 µm-thick p-type GaN contact layer.

• In another paper published in Applied Physics Express on the same day, Rohm emphasised its greater experience in this area with a continuous wave operation m-plane laser diode emitting at 459 nm. Although it has a similar epitaxial structure, Rohm’s laser has a 40 mA threshold current and threshold current density of 5.0 kA/cm2.

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