Electro Optical Characterisation of Short Wavelength Semiconductor ...

GaN Devices
treated. Generally the achieved waves in each of the three wells of a GaN laser could be
energetically different, if the thickness and/or indium content of the wells are not the same.
It would be a relief finding a theoretical method to predict the exact problem of a grown
emitting crystal only by seeing the spectra. This could keep back the arduous preparation
of the specimens for TEM iv to determine the thickness of the active region. Fig. 4.21
sketches some attempts being done for this sake.
Cts/s
spectrum fitted with y=a0*exp(-((x-a1)^2)/a2) +a3*exp(-((x-a4)^2)/a5)+ a6*exp(-((x-a7)^2)/a8) +a9
10000
8000
6000
4000
2000
0
spectrum
non-linear fitted curve
y=a0*exp(-((x-a1)^2)/a2)
y=a3*exp(-((x-a4)^2)/a5)
y=a6*exp(-((x-a7)^2)/a8)
g0942/e08
2,8 3 3,2 3,4 3,6
Energy [eV]
Cts/s
spectrum fitted by y=a0*exp(-((x-a1)^2)/a2) +a3*exp(-((x-a4)^2)/a5)+ a6*exp(-((x-a7)^2)/a8) +a9
20000
15000
10000
5000
0
spectrum
non-linear fitted curve
y=a0*exp(-((x-a1)^2)/a2)
y=a3*exp(-((x-a4)^2)/a5)
y=a6*exp(-((x-a7)^2)/a8)
g0942/e14
2,8 3 3,2 3,4 3,6
Energy [eV]
Figure 4.21: Fitted spectra of two devices from g0942 series. The wavelength is converted
to the corresponding energy.
The measured spectra were fitted with Gaussian functions, as a spectrum is expected to
be Gaussian. The best fit achieved was with three different Gaussian functions as the
Fig. 4.21 shows. This could mean three differently emitting sources, but not necessarily.
The procedures inside the active region could be much more complicated. Processes like
constructive or destructive interference etc. could always take place and this doesn’t allow
too easy interpretations.
Whatever have been done, there was always a variable too much, so that this problem
cannot be solved without a TEM picture with statements about the thickness of each of
the wells or an HRXRD with the exact indium contents.
Another point observed was a systematic threshold voltage shifting by cw measurements
as seen in Fig. 4.22. Lasing or not the devices here are all diodes. Thus, a diode like
behaviour is expected and is indeed the case. The threshold voltage shifting due to the
differences in AlGaN cladding layer seemed to be unusual. The length a layer shouldn’t
have anything to do directly with the energy barriers (see Fig. 1.9). But a higher threshold
voltage means a higher needed energy to overcome the barrier. Therefore, it would be useful
to solve the Schrödinger-Poisson equation for each structure. The Poisson equation yields
the distribution of the charge carriers and the structure of the conducting and the valence
band. This band structure is then the basis of the Schrödinger equation to find the energy
iv Transmission Electron Microscopy
49