8 Surface plasmons and theblinking phenomena
As described in section 3 the photoluminescence emission of quantum wells canstrongly be enhanced through the coupling to surface plasmons in metals near
the quantum well [37]. We investigated the influence of this enhancement on theblinking points by enhancing the photoluminescence through gold nanoparticles
on the sample surface. The proceedings and obtained results will be described inthis chapter.

8.1 Photoluminescence enhancement
To enhance the photoluminescence intensity of the quantum well through resonant
coupling to surface plasmons the right nanoparticles had to be chosen. For oursample we decided to use gold nanoparticles of 50 nm diameter as these particles
have a resonance spectrum that was likely to suit our samples. The resonancemaximum of gold nanoparticles of a diameter of 48 nm is located at about 533
nm. This coincides almost perfectly with the main emission peak of the 540 nm
sample (fig. 3.3) [35]. In addition, gold nanoparticles are very stable comparedto other metal nanoparticles. The nanoparticles were fabricated by Nanocs Inc.
and dissolved in double distilled water with a gold concentration of 0.01% whichcorresponds to a density of 4.5 · 1010 particles/ml.
A drop of this so called colloidal gold was placed on the sample. After the liquid
had evaporated completely and only the gold remained, the sample was observedwith photoluminescent microscopy.
On the 540 nm sample the area covered by nanoparticles showed significantly
enhanced photoluminescence (fig. 8.1) and the drop was easily visible in the photoluminescence picture. The whole area inside the drop appeared much brighter
than the surrounding area. All the strong luminescent centers and blinking points
that could be observed before disappeared in this strong light emitting area (seefig. 8.2).
The quantum well resonantly coupled to surface plasmons in the gold-semiconductor
interface. The gold nanoparticles were separated from the active InGaN layer
by a 5 nm GaN cap layer. The penetration depth of the fringing field of surface
plasmons in a gold-GaN interface into the semiconductor is about 33 nm [37] and55

8 Surface plasmons and the blinking phenomena

Figure 8.1: The left picture shows the photoluminescence of the 540 nm sample with a dropof gold nanoparticles. Inside the drop the photoluminescence is strongly enhanced by quantum
well surface plasmon coupling. The same effect could be observed on the blue 460 nm sample(right). The lower half of the sample is covered with gold, the upper half not. The border of themask can clearly be seen in the photoluminescence picture.

thus larger than the distance of the gold to the InGaN layer. If additionally theresonance condition between emission spectrum and plasmon frequency is fullfilled
the quantum well couples to surface plasmons in the gold-GaN interface. The
emission is enhanced when the surface plasmons subsequently couple to light. The
inherent curvature of nanoparticles or the roughness of a gold film can efficiently
scatter surface plasmons making them loosing momentum and enabling them tocouple to light (cf. section 3).
The same experiment was conducted with the 510 nm and the 460 nm samples
using the same type of nanoparticles. Also these two samples showed significantly
enhanced photoluminescence in the area covered by gold nanoparticles. Fig. 8.1
shows a photoluminescence picture of the blue 460 nm sample, where the lower halfis covered with gold. Compared to the upper half photoluminescence enhancement
is noticeable.
The green 540 nm sample showed the most and the easiest to identify blinking
points. Furthermore, the strongest photoluminescence enhancement was observedon this sample. For these reasons this sample was used for all the following
experiments regarding quantum well surface plasmon coupling.

8.2 Changing emission pattern
To analyze the influence of the nanoparticles on the photoluminescence of thesample on a smaller scale than the size of the drop, the particles had to be moved.
Through moving them their density was reduced locally and effects on small scales
could be observed. For this a aceton soaked cotton bud was rubbed against the56

8.3 SEM images

Figure 8.2: Two photoluminescence pictures at the border of a drop of gold nanoparticles on
the green 540 nm sample. Outside the drop, intense light emitting centers and blinking points
remain, inside the drop they dissapear in a strongly emitting area.

sample. The round strongly emitting area of the drop that could be observed
before dissolved and the emission pattern changed. Fig. 8.3 shows different
emission patterns after redistributing the gold with the cotton bud. Each picture
is taken after rubbing with the aceton soaked cotton bud against the sample. The
gold moved and the emission pattern changed strongly each time. Areas covered
with gold can clearly be distinguished from ’clean’ areas by their strong brightness(cf. fig. 8.1). The photoluminescence enhancement appears in form of extended
areas but also in form of small points of high luminescence.
To recognize the same points on the sample, the sample was mapped. At 430 nm
excitation many scratches were clearly visible. These scratches and the edge ofthe sample served as reference lines.
The gold strongly enhanced the photoluminescence of the 540 nm sample thatshows the blinking effect, it can be moved easily and the emission pattern changes
strongly. The enhanced areas can be extended large areas or only small points ofhigh luminescence. All this are the ideal premises to test if blinking points can becreated through surface plasmons.

8.3 SEM images
To confirm that the changing emission patterns were really caused by the nanoparticles scanning electron microscope (SEM) images were taken and compared withthe corresponding photoluminescence pictures and movies.
The first SEM pictures where taken after cleaning the sample in aceton in aultrasonic cleaner for 60 min. The photoluminescence picture showed brightemitting areas which were apparently due to the nanoparticles. The enhanced
emission patters could still be observed. However, analysis with the SEM showed57

8 Surface plasmons and the blinking phenomena

Figure 8.3: After rubbing with a aceton soaked cotton bud the emission pattern of the sample
changes. The six different photoluminescence pictures were taken of the same area on the sample
but the gold distribution and thus the emission pattern changed.

that no nanoparticles had remained on the sample. As a test a new drop ofparticles was disposed on the sample and the nanoparticles in this drop could
easily be observed with the SEM. Fig. 8.4 shows the border of the drop. In thedrop nanoparticles and darker spots can be seen. The darker spots are probably
areas of atomic gold. The nanoparticles left ’footprints’ of atomic gold on thesample (circled spots in the right picture of fig. 8.4) that have the same size as
the nanoparticles.
To investigate on this dark spots, that seemed to play a crucial role in theenhancement process and the changing emission pattern, the sample was cleaned
again in aceton in a ultrasonic cleaner for 60 min. By this all nanoparticles were
removed. With an aceton soaked cotton bud the emission pattern could still bechanged, even though there were no nanoparticles on the sample.
Further SEM analysis revealed that the gold nanoparticles did not only leavesmall footprints of gold on the sample. Films and extended areas of gold could
be found on SEM images. The typical structure of such a gold film can be seenin fig. 8.5. The yellow point in fourth picture of fig. 8.5 has the diameter of the
goldparticles and acts as a reference. The sizes of the dots of atomic gold ranged
from several µm to sizes much smaller than the nanoparticles.
Quantum well surface plasmon coupling is possible in the interface of GaN with
nanoparticles or a metal film. Strong enhancement was observed when the sample
was covered by nanoparticles but also when it was only covered by the gold
film. This shows that the photoluminescence enhancement is caused by both gold
nanoparticles and the film of gold.

58

8.3 SEM images

Figure 8.4: The white points show gold particles, the dark areas a gold film. The gold film is
especially found at the border of the drop. The circles in the right picture show small areas of
atomic gold (gold film) that have about the same area as the nanoparticles. The nanoparticles
seem to leave footprints of atomic gold on the sample.

Figure 8.5: Typical structure of a gold film after redistributing the gold with a aceton soaked
cotton bud. The gold film occurs in the form of dots of various sizes, ranging from several µm
to sizes much smaller than the nanoparticles. The yellow point in the fourth micrograph has adiameter of 50 nm which is the size of the nanoparticles.

598 Surface plasmons and the blinking phenomena

Figure 8.6: The four photoluminescence pictures show a drop of evaporating aceton on thesample and the accompanying changes in the photoluminescence pattern. The left picture wastaken right after dropping the aceton, the right picture after the aceton had evaporated.

8.4 Cleaning the sample
As described above the nanoparticles themselves could be removed easily by placing
the sample in aceton and a ultrasonic cleaner. After 60 min no nanoparticles
could be found on the sample anymore. But SEM images showed that a film ofgold remained.
This film couldn’t be removed even after applying the same procedure several
times. The emission pattern could still be easily changed by rubbing with a aceton
soaked cotton bud. Even a evaporating drop of aceton could cause this changes(fig. 8.6). The photoluminescence pictures still showed big areas of enhanced
intensity. All this were signs of remaining gold. A check with the SEM confirmed
this assumption.
Even though cleaning the whole sample didn’t succeed, it was possible to decrease
the amount of gold significantly on small areas. For that the sample was cleanedin aceton and ultrasonic for 60 min. After taking the sample out of the aceton
the remaining aceton was removed with pressurized air. If doing this before theaceton evaporated, a small area got quite clean. This method was used in thefollowing experiments where the photoluminescence of an area with and without
gold was compared.
A promising method to clean the whole sample would be the so called piranha
cleaning or using turpentine.

8.5 Photoluminescence changes
The aim of the experiments described in this chapter was to investigate on theinfluence of the emission enhancement on the blinking points. The best experiment
for this purpose would be comparing an area on a clean sample (without gold)with the same area after putting gold on it.
As described above the sample couldn’t be cleaned perfectly. For this reason theexperiment was conducted with a quite clean area on the sample that was created608.5 Photoluminescence changes
by pressurized air. A movie file of the photoluminescence and the blinking pointsof this area was taken. Then the gold on the sample was moved with the aceton
soaked cotton bud and a new movie file of the same area was taken, now withmore gold on this area. To recognize the same area the edges and scratches of thesample served as reference points.
The comparison between the movie files before and after moving the gold (fig.
8.7, fig. 8.8, fig. 8.9) shows some interesting results.
In general the whole appearance of the emission pattern is stronger in the second
pictures. More gold was moved to the observed area causing more surface plasmon
enhanced emission.
Some of the intense luminescent centers that already existed were further enhanced.
The circles in fig. 8.7 mark such points that appear brighter in the second pictures.
Furthermore new intense emitting centers appeared in areas where they could not
be observed before moving the gold. The comparison in fig. 8.8 shows an example
for a new not blinking intense emitting point. In contrast the circled points in fig.
8.9 show new strong light emitting centers that blinked.
This means new blinking points could be created through the resonant couplingof the quantum wells to surface plasmons in the gold-GaN interface and theaccompanying emission enhancement. This observation confirms the assumptionthat the blinking is caused by trapping of carriers in localized states. The local
electric field of a trapped carrier deforms the quantum well and thus changes
transition probability and emission intensity. New intense emitting centers werecaused by the resonant coupling of the quantum well to surface plasmons in
the GaN-gold interface. The inherent curvature of the nanoparticles and therough structure of the gold film can scatter the surface plasmons reducing their
momentum and enabling them to couple to light. The spontaneous emission
into surface plasmons is much faster than the normal spontaneous emission ofquantum wells into free space. Therefore this process enhances the emission of
the quantum wells (cf. chapter 3.3). If there is a deep trap in the vicinity of sucha enhanced bright point, the trapping and detrapping process of a carrier cancause the blinking.
Another observation that was made in this context is that it seems to depend on
the size of the bright points if they blink or not. As already mentioned above,
intense luminescent centers and blinking points disappear in an extended areaof enhanced emission. Fig. 8.10 illustrates this observation. In the highlighted
area of the left picture most bright points blink. This blinking points disappear
in a extended area of enhanced emission in the right picture. When the blinking
points get to large they stop blinking. In fig. 8.11 most bright points in thehighlighted area blinked. When they got to big (right picture) they didn’t show
blinking behavior anymore.
The same experiments were conducted with the blue 460 nm sample. Also on thissample a enhancement of light emission due to the drop of nanoparticles had been61

8 Surface plasmons and the blinking phenomena

Figure 8.7: Two photoluminescence pictures of the same area on the sample. On the left withless gold than the right one. After rubbing with the cotton bud already existing intense lightemitting centers and blinking points get stronger.

Figure 8.8: Two photoluminescence pictures of the same area on the sample. On the left withless gold than the right one. After rubbing with the cotton bud new intense light emitting
centers appear.

62

8.5 Photoluminescence changes

Figure 8.9: Two photoluminescence pictures of the same area on the sample. On the left withless gold than the right one. After rubbing with the cotton bud new blinking points appear.

Figure 8.10: Two photoluminescence pictures of the same area on the sample, before and after
rubbing with a cotton bud. In the highlighted area of the left picture a lot of blinking points
can be observed. In the right picture all the blinking points in the highlighted area disappeared
covered by a uniformly strong light emitting area.

63

8 Surface plasmons and the blinking phenomena

Figure 8.11: Two photoluminescence pictures of the same area on the sample, before andafter rubbing with a cotton bud. In the highlighted area of the left picture a lot of blinking
points can be observed. In the right picture most of the former blinking points got bigger andstopped blinking.

observed (fig. 8.1). Changes in the emission pattern after redistributing the gold
could also be observed on this sample. But, as the blue sample in general shows
very low density of intense light emitting centers and blinking points, no changes
in the blinking behavior could be found.

8.6 Statistics on the changes
As the sample couldn’t be cleaned completely a statistic on the number of blinking
points before and after disposing the drop of nanoparticles couldn’t be conducted
directly.
Instead of that the same area on a sample with different emission patterns wasobserved (fig. 8.12), everytime with a gold film and nanoparticles on it but under
different configurations i.e. different distribution of the film and particles resulting
in different emission patterns. For that each movie was divided in squares of 10µm x 10 µm each. In these squares the number of blinking points was counted andthe density of blinking points was calculated. This was done for three different
gold distributions. Fig. 8.12 shows that the average number of blinking points
per area strongly depends on the emission pattern and thus on the distributionof the gold film and the nanoparticles. This dependence can be caused by two
different effects. First, already existing blinking points can disappear in extended
areas of strong emission. This reduces the density of blinking points. On theother hand nanoparticles or small dots of gold can locally enhance the emission
and thus create new intense emission centers and blinking points. By this thenumber of blinking points is increased.
Another statistic on the number of blinking points was made by comparing movies

648.7 Comparisons between photoluminescence movies and SEM images

Figure 8.12: The three photoluminescence pictures show the same area on the 540nm sample
with different gold distributions resulting in different emission patterns. The corresponding
density of blinking points strongly depends on the gold distribution.

of the sample without gold to movies of the sample with gold. As the sample
couldn’t be cleaned this statistic could not be conducted based on subsequent
videos. For this reason old videos that had been taken before the gold was dropped
on the sample were used. These movies were taken at Yokohama City University
and by Ruggero Micheletto at Kyoto University.
Squares of 40 µm x 40 µm were randomly chosen and the number of blinking
points in these areas were counted. This was done for videos of the sample withand without gold. The histogram of the number of blinking points in the squares
is shown fig. 8.13. The average number of blinking points per square is almost
the same for the situations with and without gold. But in the case with gold the
distribution appears broader. This is consistent with the above made observations,that blinking points can be created by dots of atomic gold or nanoparticles but
can also disappear in an extended enhanced area. Problems in this statistic were
that, in parts, the videos were taken under different conditions such as different
excitation intensities. Furthermore the blinking points had to be counted on quite
large scale which made it difficult to tell between blinking and not blinking points.

8.7 Comparisons between photoluminescencemovies and SEM images
In order to confirm that the new blinking points were really due to the nanoparticles
or dots of the gold film on the sample photoluminescence videos and SEM imagesof corresponding areas were compared. In doing so blinking points that werelocated at the same places as nanoparticles or dots of atomic gold were searched.
The distinctive scratches that had been used as references in the photoluminescence
experiments couldn’t be observed with the SEM. The only noticeable reference
point that could be found both with the SEM and photoluminescent microscopy

65

8 Surface plasmons and the blinking phenomena

Figure 8.13: The histogram shows the number of blinking points in squares of 40 µm x 40 µm
on the sample with and without gold. The average of blinking points in the squares is almost
the same, but in the case with gold the distribution appears broader.

66

8.8 Changes in vacuum
was the edge of the sample. For this reason all the comparisons were made atareas near characteristic structures at the edge of the sample. But even with thesestructures as references, the comparison between photoluminescence movies and
SEM images remained difficult. Areas in photoluminescence videos could easily
be recognized in the SEM images too, but a exact points could only roughly belocated because of the fundamentally different imaging mechanisms.
As seen from fig. 8.4 gold nanoparticles and clusters of gold nanoparticles appear
as white points in SEM images, whereas the film of atomic gold and dots of atomic
gold appear dark. To find blinking points that are caused by either nanoparticles
or dots of gold, an area of low nanoparticle/goldfilm density had to be created.
Areas with high densities of nanoparticles and areas covered with an extended
film of gold show overall bright enhanced emission but no intense light emitting
centers and thus no blinking points. Due to the complicated surface plasmon
resonance conditions and the imperfection of the quantum well not every single
nanoparticle or dot of atomic gold can cause a blinking point.
However, both small dots of atomic gold and a cluster of nanoparticles were foundat places were a blinking point appeared in the photoluminescence video. Fig.
8.14 shows two blinking points at the same place where two dots of atomic gold
could be found in the SEM picture (dark points). Fig. 8.15 and fig. 8.16 show thephotoluminescence and SEM pictures of the same area on the sample. A blinking
point appears in the photoluminescence video at the same point where a clusterof nanoparticles can be found in the SEM image.
These observations support the assumption that the new blinking points are causedby plasmon resonance in the gold on the sample and that both gold nanoparticles
and dots of atomic gold can create them.

8.8 Changes in vacuum
For the measurements on the temperature dependence of the blinking in section
7 the sample was put in the vacuum of a cryostat. When venting the sample
chamber and removing the vacuum significant changes in the photoluminescence
could be observed.
Fig. 8.17 shows the same area on the sample of 540 nm central emission with (left)
and without vacuum (right). Two areas with different emitting characteristics
could be observed. The uniformly emitting area that doesn’t show intense lightemitting centers is typical for emission enhancement by an extended film of gold.
In this area no blinking points could be found. After venting the chamber thephotoluminescence intensity of this area increased. The other area showed many
intense light emitting centers many of which blinked. When removing the vacuum
the whole pattern of bright points changed. Many of the points disappeared
while other new ones appeared. The pressure was about 4.0 · 10−5 mbar when the67

8 Surface plasmons and the blinking phenomena

Figure 8.14: The circled points in the photoluminescent microscopy image (top) show blinking
points. The same area observed by SEM shows dark spots at the same points. The dark spots
indicate dots of atomic gold.

688.8 Changes in vacuum

Figure 8.15: The circle in the photoluminescence microscopy image shows a blinking point at
the same place were a cluster of nanoparticles can be found in the SEM image (fig.8.16)

69

8 Surface plasmons and the blinking phenomena

Figure 8.16: The circle in the SEM image shows a cluster of nanoparticles at the same place
were a blinking point can be found in the photoluminescence image (fig.8.15)

70

8.8 Changes in vacuum

Figure 8.17: The images show the photoluminescence of a area on the sample with 540 nm
central emission in vacuum (left) and without vacuum (right). The photoluminescence patternchanges. Blinking points appear and disappear.

chamber was evacuated and 1.0 · 103 mbar after venting.
These observations might be explained by the highly sensitive resonance conditions
for quantum well surface plasmon coupling. The resonance depends on manyfactors such as the size and shape of the nanoparticles, the structure and thicknessof the goldfilm, the distance of the metal to the quantum well and the surrounding
material (section 3). The conditions can be changed when venting the vacuum
chamber and could cause the observed changes.

71

72

9 Memory effect
The overall photoluminescence of the 510 nm sample changed strongly rightafter starting to illuminate the sample. Fig. 9.1 shows the difference between
the photoluminescence when starting the excitation and after one minute ofcontinuous illumination with the 365 nm line of a mercury lamp. The emission
appears much brighter and the contrast and granularity appear more dominant
in the second case. Motivated by this observation the temporal behavior after
starting to illuminate the sample was studied. The measurements described inthis section were conducted by Masayoshi Abiko at Kyoto University.

Figure 9.1: The photoluminescence of the 510 nm sample right after starting to illuminate thesample (left) and after one minute of continuous illumination with the 365 nm line of a mercury
lamp (right). The overall emission appears brighter and the contrast and granularity appear
more dominant in the later case.

9.1 Method
To have a closer look at the temporal behavior of the photoluminescence theexperimental setup shown in fig. 9.2 was used. It consisted of a photoluminescent
microscope coupled with a monochromator to observe the time-intensity profilesof the photoluminescence at single wavelengths. A CCD camera recorded theintensity changes. The samples were excited with an ultraviolet Hg lamp (365 nm

73

9 Memory effectand 405 nm emission lines). As the band gap of InGaN is about 3.4 eV (365 nm)
and the one of InGaN is under 2.95 eV (over 420 nm), the 365 nm line excited
both the GaN barrier and the InGaN active layer whereas the 405 nm line only
excited the InGaN active layer. The illumination intervals could be controlled bya shutter at the mercury lamp.

Figure 9.2: The experimental setup used to analyze the photoluminescence changes. A
photoluminescent microscope is coupled with a monochromator and a CCD camera.

9.2 Photoluminescence changes
Fig. 9.3 shows the spectrum of the 460 nm centred sample. With the monochromator the temporal behavior of both the main emission peak and the yellow band
was recorded. This measurement was done right after starting to illuminate thesample with the 365 nm line. Fig. 9.4 shows that the yellow band reaches amaximal value immediately, whereas the intensity of the main emission peak rises
exponentially.
Exciting the sample with the 365 nm and the 405 nm line of the Hg lamp revealed that the behavior of the main emission peak depends on the excitation
wavelength. When the sample was excited with 405 nm light a different behavior
could be observed (fig. 9.5). The intensity reaches a maximal value and then falls
exponentially.
The graphs for 365 nm excitation were fitted with
I = Y0 − A1 exp(−t/τ1 ) − A2 exp(−t/τ2 )

74

(9.1)

9.2 Photoluminescence changes

Figure 9.3: The spectrum of the InGaN/GaN sample at room temperature shows the main
emission peak at about 460 nm and the characteristic yellow band.

Figure 9.4: After starting to illuminate the sample with 365 nm UV light the intensity of themain peak and the yellow band showed different behavior. The yellow band reaches a maximal
intensity immediately, whereas the intensity of the main emission peak rises slowly.

75

9 Memory effectFigure 9.5: The temporal behavior of the main emission peak intensity after starting theillumination depends on the exciting wavelength. Excited with 405 nm light the intensity reaches
a maximal value and then falls exponentially, excited with 365 nm light the intensity doesn’treach its maximal value immediately but rises exponentiallyand the 405 nm excited ones with
I = Y0 + A1 exp(−t/τ1 )(9.2)

9.3 Systematic illumination interval sequence
To examine if the timeconstants depend on previous illumination and dark times
a systematic illumination interval sequence was applied. Measurements of 40 s
illumination periods were taken after different darktimes. A initializing darktimeof 1 hour was followed by darktimes increasing in 5 min steps starting from 5 min
up to 60 min. Fig. 9.6 illustrates this darktime illumination sequence. The 40 stime intensity profiles were recorded subsequent to each darktime and fitted withequation 9.2. The resulting time constants τ1 and τ2 were plotted in dependenceof the previous darktime. Another series of measurements was done in reversetime order, starting with a darktime of 60 min down to 5 min in 5 min steps.
Fig. 9.7 reveals that the time constants of the photoluminescence rise depend on
the previous darktimes. Moreover a comparison to the time inverted illuminationinterval sequence demonstrated that the time constants not only depend on theprevious darktime but on the whole previous illumination history. The time
constants in the second graph are almost double than in the first graph after
corresponding darktimes. This shows a clear memory of the previous illumination
history.
We think that the behavior observed in these experiments is due to carriers trappedin the quantum well that act like Coulomb barriers and inhibit recombination.

769.3 Systematic illumination interval sequence

Figure 9.6: The scheme shows the systematic illumination interval sequence. After a darktimeof one hour the sample was illuminated for 40 s and the temporal behavior of the intensity
recorded. After that the darktime was varied from 5 min up to 60 min (5 min steps), eachdarktime followed by a 40 seconds measurement when the sample was illuminated. Anothermeasurement was done in reverse time order.

Figure 9.7: The time constants of the main emission peak excited with 365 nm light. The
illumination interval sequence was conducted for darktimes from 60 min down to 5 min (left)
and form 5 min up to 60 min (right). The time constants depend on the previous dark time
and the whole previous illumination history.

This doesn’t act within one single illumination event. It cumulates over multiple
on-off sequences and thus remembers previous illumination events.

77

78

SummaryIn the work presented for this thesis, we investigated on a blinking phenomenon
observed in InGaN/GaN single quantum wells. The photoluminescence of quantum
wells with high indium content showed strong luminescent centers. Some of thesebright points were optically not stable, they blinked.
To investigate on this phenomenon we characterized the blinking behavior ofdifferent points. A small amount of points showed random telegraphic behavior
switching between clear on and off states. Statistics on the time intervals spent in
the on or off state revealed two different probability distributions. One distribution
was exponential which could be explained by a simple quantum jump model ina three-level system, the other distribution obeyed a power law which indicateda more complex underlying mechanism. Most points however did not show
telegraphic behavior but a more chaotic one without observable discreate intensity
levels. There is evidence to suggest that the intensity changes are caused by the
local electric field of a carrier in a deep trap in vicinity to a bright point, deforming
the quantum well and thus reducing or increasing the radiative recombination
rate when charged or discharged. Such an interaction of a bright spot witha single switching center results in exponentially distributed times in on andoff states. The other kinds of blinking points can be caused by more complex
situations like the coupling of a bright spot to several deep traps. Simulating sucha situation resulted in blinking behavior without obvious intensity levels similar
to the measurements.
Experiments on the temperature dependence of the blinking points showed thatwith increasing temperatures the switching gets faster, the off times shorter and,
applying the model of the charged traps, the delocalization probability higher.
At low temperatures all blinking points disappeared. The average intensity ofblinking points switching between more than two levels decreased with lower
temperatures, the switching occurred between less distant intensity levels.
In another experiment gold nanoparticles were put on the quantum well. The
motivation for this experiment were former observations of a strong emission
enhancement caused by the resonant coupling of quantum wells to surface plasmons
in metal-semiconductor interfaces. Strong emission enhancement was observed andmoving the gold on the sample resulted in significantly changed emission patterns.
This way new intense emitting centers and blinking points could be created bymeans of gold nanoparticles or small dots of atomic gold on the quantum well. The
quantum well resonantly couples to surface plasmons in the gold-GaN interface,79Summary
the inherent curvature of the nanoparticles and the roughness of the gold film canscatter the surface plasmons making them loose momentum and enabling themto couple to light.
In the last part of the thesis the temporal behavior of the overall photoluminescenceof the main emission peak of a InGaN/GaN single quantum well was studied.
The photoluminescence intensity turned out to rise or fall after starting theillumination depending on the exciting wavelength. A systematic illuminationinterval sequence revealed an optical memory effect. The photoluminescence
temporal behavior turned out to be dependent on previous dark times and thewhole previous illumination history.

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Acknowledgements
There are many people I want to thank for their help, support and encouragement
during my diploma thesis.
Prof. Dr. Ulrich T. Schwarz for introducing me to a very interesting topic, theguidance and encouragement he provided during my stay in Yokohama, forfruitfull discussions and for providing the opportunity to write my thesis incollaboration with Yokohama City University.
Prof. Dr. Ruggero Micheletto for inviting me to his laboratory at Yokohama
City University, for his hospitality, his guidance and continuous support in all
matters.
Dr. Koichi Okamoto and the group of Prof. Dr. Yoichi Kawakami at Kyoto
University for letting me conduct parts of the experiments in their laboratories
and for helpfull information on surface plasmon resonance.
Prof. Dr. Hideho Uchiyama for letting me use the equipment of his laboratory
while we were waiting for our own to arrive.
Masayoshi Abiko from Kyoto University for his measurements on the memory
effect that I used for this thesis.
My laboratory colleges Kotaro Oikawa and Fernanda Maria Avila Ortega for
their help with scientific and language problems and for the great time we spent
together.
My parents who supported me throughout my studies.

Erklärung
Hiermit erkläre ich, dass ich die Diplomarbeit selbstständig angefertigt und keine Hilfsmittel außer den inder Arbeit angegebenen benutzt habe.

Regensburg, den 20. November 2009