Electro Optical Characterisation of Short Wavelength Semiconductor ...

Data Analysis
for the whole devices on a bar.
For the 6µm planars most of the devices needed a large amount of current to start emitting
and degraded very fast. This indicates a bad metal contact with relatively high resistance
and high amount of impurities in the crystal which lead to non-radiative recombinations.
Applying more and more current will consequently lead to heat and degradation of the
device. Fig. 4.16 shows the output light power vs. current density diagram as well as the
spectrum of the only lasing device of 6µm planar . A high threshold current density (28.9
KA/cm 2 ) is observable.
Output Light Power [W]
0,0002
0,00015
0,0001
5e-05
g0975/planar/unmirrored
width: 6um, length: 1035.20 um
0 10000 20000
Current Density[A/cm
30000 40000
2 0
]
Cts/s
3e+05
2,5e+05
2e+05
1,5e+05
1e+05
50000
0
g0975/planar/6um
spectrum
360 380 400 420 440
Wavelength [nm]
Figure 4.16: L/I behaviour as well as spectrum of the only unmirrored lasing device
belonging to 6µm planars
At the first glimpse at the spectrum there is no lasing available. By zooming in, some
details get cleared. Fig. 4.17 shows the zoomed spectrum of the lasing device as well as
the far field photo taken by a regular digital camera. As it can been seen from the spectrum
there is indeed a multimode lasing which happens to occur on top of an LED regime. The
fact that a multimode lasing happens is just normal and because of the many standing
waves which are achieved in different physical places of the active region with possibly
different indium contents which leads to different energy gaps. The strongest modes seem
to be at around 390nm. The LED regime which can be seen as a long bowing blue light
perpendicular to the lasing ray in the far field photo (Fig. 4.17), is actually due to the
photoluminescence of the substrate. As already mentioned the III-V crystals characterised
in this thesis are all homoepitaxially grown, i.e. growth on GaN substrate. Because of
the lower refracting index difference than in sapphire (Al2O3) case, it is possible that the
wave propagates into the GaN and causes luminescence. To stop this, one way would be
increasing the thickness of the Si-doped AlGaN cladding layer. This possibility had been
taken as a basis for growing g0937, g0938 and g0942.
As mentioned on device of the 10µm shallow ridge structure was also able to lase exactly
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