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29 May 2009


Nichia pushes InGaN lasers nearer to green via 515nm emission

Japan’s Nichia Corp has managed to coax continuous 515nm ‘green’ laser light out of an indium gallium nitride (InGaN) structure [Takashi Miyoshi et al, Applied Physics Express, vol.2, p062201, 2009]. The firm’s researchers see possibilities for application in compact next-generation projectors, particularly in a mobile phone format. Existing projectors use direct red and blue lasers, but the green component is achieved using frequency doubling in second harmonic generation (SHG) lasers. Creating a direct green laser diode would enable smaller projectors to be produced, hopefully at lower cost.

Filling the green light gap has been hard. Although lasers based on II-VI compound semiconductor materials were demonstrated in the 1990s, these devices are unreliable under the high currents that are needed to produce lasing.

With the development of blue-violet-ultraviolet LEDs and laser diodes based on the III-nitride system, starting also in the 1990s until the present, it is natural to look to the possibility of extending the wavelength (reducing the frequency) into the green gap. This involves narrowing the band gap from gallium nitride’s energy band gap of ~3.4eV (corresponding to the ultraviolet wavelength 365nm) by introducing indium (to produce InxGa_1-xN). However, to get into the green wavelength range (520-570nm) requires significant amounts of indium, and growing such layers with the required high quality is extremely difficult. Thermal stability and parasitic chemical reactions have been highlighted as particular problems in growing such layers.

The Nichia researchers used a conventional free-standing c-plane GaN substrate on which to grow their layers. Films consisting of various combinations of aluminum, indium, gallium and nitrogen (AlInGaN) were grown using metal-organic chemical vapor deposition (MOCVD). The active layer consists of InGaN multi-quantum wells (MQWs). As for most commercial laser diodes, separate confinement heterostructures (SCHs) with a lower refractive index are used to confine the emitted light within the lasing cavity.

The dimensions of the resulting laser diode ridge were 2 microns x 600 microns. Continuous wave (cw) characterization of the devices was carried out at 25ºC. Below an emission wavelength of 500nm, the devices had a threshold current density of 1-2kA/cm2; this begins to increase as the amount of indium increases to the level needed to achieve a wavelength of 515nm (4.4kA/cm2, corresponding to 53mA for the particular device). It is desirable for the turn-on of lasing (threshold) to be achieved at as low current as possible. The 515nm device had an output power of 5mW at an operating current of 88mA (and an operating voltage of 5.5V).

The paper also shows improved uniformity of photoluminescence compared with previous laser diodes produced by the team that operated at shorter wavelengths (>470nm). The Nichia group explains this as being due to their improved growth of the layers, particularly of the active MQW structure. The previous structures suffered from non-radiating regions with poor crystal quality. These non-radiating regions are not found in the new structures.

The temperature variation and lifetime characteristics were also determined. Devices rated at 510-513nm wavelength were operated cw at 25ºC with automatic power control for 500 hours. This gives an estimate for the lifetime (the point when the operating current has risen by 30% over the initial current) of more than 5000 hours. Also, the increase in emission wavelength with temperature was just 0.022nm/K

Nichia describes its latest laser diodes as ‘green’, presumably on the basis that, in terms of the spectral colors (violet, blue, green, yellow, orange red), ‘green’ is allotted the 495-570nm space. However, green is better perceived when in the range 520-570nm, while blue comes from 440-490nm. The range 490-520nm is better described as ‘blue-green’.

Rival laser manufacturers Rohm of Japan and Osram Opto Semiconductors of Regensburg, Germany have also recently produced InGaN-based LDs with wavelengths approaching 500nm.

See related items:

Osram Opto claims first 500nm InGaN laser

Non-polar InGaN lasers near green light

Search: Nichia Green laser InGaN




The author Mike Cooke is a freelance technology journalist who has worked in the semiconductor and advanced technology sectors since 1997.