nitride lasers technology
rhombus4
cladding layers, GaN waveguide layers and an active
region containing InGaN layers with different indium
content on a c-plane GaN-substrate (Fig. 3). The laser
characteristics include a threshold current of about
125mA, which is only four times the threshold level of
currently available blue InGaN laser diodes.
Several hurdles had to be overcome to extend our lasers
to 500 nm and beyond. Probably the biggest of these
was improving the crystal quality of the high-indium-
content quantum wells needed for green emission. The
quality of this layer can be assessed with micro-
photoluminescence mappings (fig. 4). The black spots in
the left image of fig. 4 are areas of weaker green
spontaneous photoluminescence emission, due to
segregation of indium atoms.
A strong correlation exists between the formation of low Fig.3. Osram’s green-emitting laser is a conventional ridge laser design
spontaneous emission areas and high densities of non- that includes GaN waveguides, an AlGaN electron-blocking layer and
radiative defects. The right image in fig. 4 shows an AlGaN cladding layers
incredibly uniform green photoluminescence from a laser
structure with higher crystal quality. Employing improved
growth parameters and designs on c-plane GaN
substrates formed this structure. No black spots can be
seen, indicating improved crystal quality. This material
produces devices with a higher peak gain for lasing.
The behaviour of our lasers is influenced by the strong
piezoelectric fields within the quantum wells. The strain-
induced piezoelectric fields and tilted energy potentials
reduce the band gap. This is the so-called Stark-Effect.
The lower band gap helps to reach the long emission
wavelength without changing the material composition.
However, lasers operate at current densities that are
typically orders of magnitude higher than LEDs, which Fig. 5 Left: The piezoelectric fields in polar material alter the conduction
partially screen the internal fields, leading to a blue shift of and valence band profiles of the within InGaN-quantum wells (left).
the laser emission wavelength (Fig. 5 right). However, if Carrier separation at the quantized band states is indicated by the
the laser threshold current can be reduced, the Stark- calculated carrier envelopes, which show a reduced electron-hole overlap
Effect in the polar growth direction can be used to shift compared to a rectangular quantum well profile (not shown here).
the laser towards longer wavelengths without increasing Right: Increasing the current density of a polar green InGaN-based LED
the indium-content in the In
X
Ga
1
-
X
N quantum wells. to values needed to drive a laser produces a large blue-shift in the
spontaneous emission wavelength. Nitride lasers with similar indium-
content in the quantum wells that are grown on non-polar substrates
produce a far smaller variation in emission at shorter wavelengths
(dashed line)
Fig.4. Micro-photoluminescence mappings reveal the
quality of the quantum well layers. Left: Lower material
quality, including dark spots due to indium-segregation.
Right: Uniform green photoluminescence emission of
InGaN quantum wells with higher material quality
January/February 2010
www.compoundsemiconductor.net 23
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