The influence of water and ethanol adsorption on the opticalblinking in InGaN quantum wells
Shunra Yoshida1 , Yusuke Fujii1 , Giovanni Alfieri2 , & Ruggero
Micheletto1 *1. Yokohama City University, 22-2 Seto, Kanazawa-ku, 236-0027, Japan
2. Hitachi Energy, Fabrikstrasse 3, Aargau, 5600 Lenzburg,
E-mail: ruggero@yokohama-cu.ac.jp

Abstract. We studied the adsorption of liquids over the surface of InGaN quantum wellbased wide band-gap devices and found that the immersion in certain liquids has noticeable
effects on the optical blinking phenomena. We used two samples with different indium
concentrations, emitting on the green and blue range, and immersed them while under direct
illumination with 365 nm UV light. We found that especially water and ethanol provokedevident optical variations compared to observation in air. While blinking spots can be observed
irrespective of the In concentration, their contrast and luminosity increased for samples withthe emission in the 510 nm range, rather than for those in the 460 nm. Based on these results,
we put forward the hypothesis that the presence of liquids induces the formation of radiative
centers, possibly complexes related to intrinsic defects binding with adsorbed impurities, suchhydrogen or oxygen.

Submitted to: Semicond. Sci. Technol.

The influence of water and ethanol adsorption on the optical blinking in InGaN quantum wells2
1. Introduction
Due to the progress in GaN growth and device manufacture, light emitting diodes (LED)are currently employed for a variety of applications, like visible light communication ortraffic lights. Despite the importance of GaN-based LED, the correct functionality of suchdevices is still undermined by a variety of issues, e.g., high density of defect states or large
strain-induced polarization caused by lattice mismatch. In particular, the optical blinking
phenomenon and the optical memory effects[1, 2] affect the optical quantum efficiency of
InGaN single quantum wells (QW).
The blinking phenomenon (blinking in short) is observed not only in InGaN QW, but also
in many crystals such as CdSe nanocrystals[3] and GaAs quantum dots (QD)[4], where theemission intensity becomes unstable. In InGaN, blinking is induced by illuminating thesurface of the sample with a 365 nm light that excites not only the InGaN layer, but also
the GaN layer. Blinking is known to occur in localized domains where the optical emission
is more intense than the average, called intense luminous centers (ILCs)[1]. While blinking
always occurs in ILCs, not all of them are blinking. As a matter of fact, most ILCs show only
stronger optical intensity, which is caused by accumulation of carriers in localized regionswith high In content[5, 6, 7, 8]. Based on this, a correlation between such spatial nonuniformity of In composition and blinking was also put forward[9]. This effect was confirmed
in several studies, mainly in samples with emission peaks of 510 nm and 540 nm. For samples
that emit blue light such as 460 nm, the same optical blinking can be observed, but it is lessprominent and noticeable[1, 8].
Among the possible causes of blinking in an InGaN QW, the quantum confined Stark
effect is one candidate[1, 10, 11]. However, this cannot explain the experimentally observed
intensity distribution of the blinking[1, 12]. Later, it was proposed that the interference ofthermal vibration of the crystal lattice may be the cause of the unstable blinking[13]. However,this idea considers local phenomena and does not concern the external species present on
the sample surface. In a recent study, Tsutsumi et al. found that the concentration of ILCs
increased when oxygen is adsorbed on the surface of an InGaN QW[14], suggesting thatblinking could be caused by the interaction of surface molecules with impurities present
within the QW.
By considering the above hypotheses, we decided to verify whether or not blinking is causedby the interaction between species adhering to the surface and intrinsic impurities. In orderto do this, we present a detailed study on the effects of liquid adsorption, mainly water andethanol, on the total luminosity and presence of ILCs in two InGaN single QW devices withemission peaks of 460 nm and 510 nm.
2. Experimental Details
The samples used in this experiment are a stack of undoped GaN layer (4 µm ) with a thin
Inx GaN(1−x) active layer (3 nm) capped by an undoped GaN layer (5 nm). The sample isgrown in the (0001) direction by metal-organic vapor phase epitaxy (MOVPE) on sapphire.

The influence of water and ethanol adsorption on the optical blinking in InGaN quantum wells3
The emission peaks of the samples depend on the In concentration x in the layer, this can range
between 400 nm up to 540 nm or over[5]. For the current study we employed two samples,
one emitting at 460 nm the other at 510 nm.
Prior to optical observation, samples underwent one minute in ultrasound sonic bath(US-2R, AS ONE, Japan) in distilled water. The samples were inserted in a custom madecell and fully immersed in different liquids, then photoluminescence (PL) measurements
were performed by attaching a CCD camera (HDR-SR1, SONY) to a selective excitation
fluorescence microscope (BX51WX, Olympus). The optical emission from the surface wasrecorded at 30 frames per seconds and stored in AVI format. We grouped our recordings in
30 s chunks for analysis (the short initial lamp induced transients were cut away). Ultraviolet
Hg lamps were used as excitation light (365 nm and 405 nm emission lines), the thicknessof the liquid layer was 500 µm, the absorption spectra of water and ethanol in those two UV
ranges is nearly identical[15]. Since the main purpose of the investigation was to detect the
ILCs changes caused by the presence of liquids, all our observations were made using the sole
365 nm excitation light that excites both the InGaN layer and the GaN layer[1].

Figure 1. The photoluminescence of the sample in air and fully immersed in liquid at twoemission wavelength. The photoluminescence maps are in panels (a), (b) and (c) for the
sample emitting at 460 nm, in air, water and ethanol respectively. In panels (e), (f) and (g)again air, water and ethanol for the other sample emitting at 510 nm. It can be seen that thebright spots (ILCs) are increased due to the adhesion of the liquid in spite of the fact thattemperature, optical excitation, location and any other physical parameters are identical. All
the samples are excited by a UV light of 365 nm. All images have the same scale of panel (a).

3. Results and Discussion
Once fully immersed in a liquid, different optical fluorescent patterns and an enhanced
blinking dynamics were detected. To verify this effects, tests were carried out using several
liquids. We found that water and ethanol are the two liquids that visibly alter the optical InGaNThe influence of water and ethanol adsorption on the optical blinking in InGaN quantum wells4
emission pattern and remarkably increase the number of blinking points in a reproduciblemanner, please see the videoclips in the supplementary media (they show 30 seconds ofrecording for the 510 nm samples in air and ethanol).
Fig. 1 is a comparison of the sample surface before and after the immersion in liquid.
Although the area had in air few ILCs (panels a and e), their number increased noticeably inpresence of water (b and f) and ethanol (c and g), while the blinking became distinctly more
intense after ethanol. In addition, while the number of ILC increased, there were regions
where the light emission was weakened by the presence of the liquids and become darker,
improving the overall contrast.
As the figure shows, since the number of ILCs increased in the presence of liquids andsince they occur in regions of high In content[1, 5, 6, 7, 8], our results suggest that the adhesionof the liquid to the sample surface facilitates the collection of carriers in such In-rich regions,where radiative carrier recombination occurs with higher rate. More specifically, the factthat carriers are localized in In-rich regions[6, 16] suggests that the liquids might introduce
impurities that lead to the formation of highly localized radiative centers.
Concerning those domains that look darkened in presence of liquids, we can hypothesize thatthe missing carriers migrated to those ILCs that have become more intense. This is consistent
with previous studies reporting movement of carriers from low optical emission regions tohigher optical emission regions[5] and longer lifetime localized areas[6].
In fig. 2, we map the time-wise standard deviation of the optical emission intensity in thesame regions of fig. 1. This was done in order to quantitatively demonstrate that the ILCs are
actually changing intensity in time (blinking). The standard deviation was calculated pixel-bypixel, analyzing 30 s of data, for the sample in air ((a) and (b)), water ((c) and (d)) and ethanol((e) and (f)). In short, the map represents the optical variability of each pixel. The wider theintensity distribution of the blinking points, the larger the standard deviation. In these maps
absence of blinking, despite the higher intensity, results in a lower standard deviation.
Fig. 3 represents the distribution of the standard deviation’s histogram cumulative frequency
integrated along all the sample surface for each sample in air, water and ethanol, for emission
at 460 nm (fig.3a) and 510 nm (fig.3b). Although the histograms might appear similar for eachcondition, by looking at the steep rise of the cumulative curve, the effect of liquid adhesion
is apparent. In fact, when the samples are fully immersed in water or ethanol, the curve rises
slower. We have to keep in mind that the horizontal axis represents the optical intensity
fluctuations (standard deviation) and, since a higher standard deviation indicates stronger
blinking, a slower growth of the cumulative value signifies a spread of the histogram towardhigher values of standard deviation, that is intense blinking. As it can be understood, thecumulative histogram is a way to make some of the features of the histogram more evident.
In our case, it exposes the higher blinking in a consistent way, better suitable than the barehistogram plot where the three standard deviation histograms (for air, water and ethanol)
overlap and their differences are difficult to grasp.
To have a quantitative insight into this effect, we fitted the cumulative frequency with a

The influence of water and ethanol adsorption on the optical blinking in InGaN quantum wells5

Figure 2. Standard deviation map of samples in (a) and (b) air, (c) and (d) water, (e) and (f)ethanol, for 460 nm ((a), (c), (e)) and 510 nm ((b), (d), (f)) emissions. The blinking points
appear to increase in number and variability due to the adhesion of the liquid (note the color
scale differences). Two ILC, labeled A and B, will be discussed later in the text. All images
have the same scale of panel (b).

Boltzmann sigmoid
f0 − f∞(1)
1 + e(x−x0)/τ
where x0 is the center of the sigmoid, f0 and f∞ are the initial and final value of it and τ
represents its slope inclination. Plotting τ for the three conditions studied (air, water andethanol), the effect of the liquids is clearly noticeable as shown in fig. 4.
Since the blinking points are located at ILCs in In-rich areas[1, 5, 6, 7, 8], theirdistribution, in the standard deviation map of fig. 2, reflects the spatial distribution of the
In composition in the sample. Even if the presence of the liquids enhances the standard
deviation in these local domains, it is difficult to detect blinking averaging the optical standard
deviation on all the surface of the sample, because this means averaging the local indium
fluctuations. In fact, the average standard deviation calculated on all the surface, for eachsample, is approximately 2 (a.u.), whereas in confined regions of the maps of fig. 2, blinking
points shows standard deviations of 10 or more, that is 5 times or higher. This averaging
explains the reason why previous large scale studies on InGaN did not show fluctuations or
blinking[1].
With our methodology we could compare individual ILCs optical emission profile forf = f∞ +The influence of water and ethanol adsorption on the optical blinking in InGaN quantum wells6

Figure 3. Histogram of the standard deviation maps in figure 2. Panel (a) represents the
460 nm sample and (b) the 510 nm one. The horizontal axis is the standard deviation. The
histogram is clipped up to the range where their cumulative relative frequencies approach
approximately to 1. The cumulative line plots indicate that the standard deviation, that
represents the optical blinking, is increasing due to the presence of water and ethanol, withethanol showing consistently a stronger effects on both samples.

each condition. Figure 5 shows the change in the intensity of the emission of two ILC (thoseindicated by the arrow in fig. 2). The three plots show the photoluminescence in the same
local area for each of them. The blue-emitting 460 nm (ILC A) in ethanol shows less prominent variations when immersed in different liquids compared to the 510 nm ILC B (notice thescale that is higher for the greener sample). In contrast, when immersed in water or ethanol,
ILC B shows an intensity much higher than prior to the immersion. Since the 460nm sample
has less Indium than 510nm, these results further suggest that In concentration favors opticalemission following adsorption of liquids compared to PL in air and, at the same time, it promotes blinking in same areas. Moreover, although blinking was previously thought to havean adverse effect on light emission, this experiment shows that liquid adsorption has a strong
effect in the confined region of ILC, but improves or does not alter much the total PL emission
averaged on all the surface of the sample. In addition, since it is considered that the ILC emit
light by concentrating a large number of carriers in a confined region, our results suggest thatthe blinking in the InGaN QW depends not only on the presence of In-rich regions, but might

The influence of water and ethanol adsorption on the optical blinking in InGaN quantum wells7

Figure 4. The slope τ of the cumulative frequencies of the distributions in figure 3 when fittedwith equation 1. The increased τ resulting when immersed is noticeable on both samples. All
the τ shown and error bars are the result of a fit with R2 better than 0.99.

also be affected by the presence of impurities (like H, C, O), due to adsorption of liquids,
very near the quantum well (the capping layer of the samples is few nm, so the liquids are inproximity to the QW).
To this end, it can be speculated that the detection of these ILCs, in such In-rich regions, might
be linked to the formation of radiative centers involving intrinsic defects. As a matter of fact,
In-rich regions are typically associated to low crystalline quality[17], due to the presence ofV-shaped defects formed by dislocations gliding to the surface[18]. The interaction between
impurities and dislocations was studied theoretically by Jones et al.[19] and it was concluded
that the formation of complex defects is possible. For this reason, since a high density of dislocations means that the concentration of gallium vacancy (VGa ) and nitrogen vacancy (VN )is high, we can put forward the hypothesis that, in such In-rich regions, complex defects witheither oxygen or with the addition of hydrogen can form. As a matter of fact, a rather highdensity of VGa can be expected in GaN grown on sapphire[20], like the samples analyzed
in this study. In addition, H and O are rather common impurities found in metal-organic
chemical vapor deposition (MOCVD) grown materials and they are energetically favorable atempty N-sites[21]. In particular, O can be adsorbed at GaN surfaces[22, 23, 24]. By considering these factors combined, it can be speculated that the adsorbed O might diffuse along thedislocation[25], where it can give rise to VGa related complexes[26], e.g. VGa -ON is known
to be a source of green luminescence[27]) and is thermally stable up to 1500 K[28].
It can be argued that O diffusion might be difficult to occur at the temperatures employed inthis study (RT). However, Pearton et al. have shown that, after growing SiO2 on GaN at 50 o C,a significant concentration of O could be detected in GaN up to a depth of 40 nm. Since dislocations provide an energetically favorable path for its diffusion[25], with an activation energythat can be half of that required in the bulk[25], it is not unreasonable to hypothesize the diffu-

The influence of water and ethanol adsorption on the optical blinking in InGaN quantum wells8
sion of O occurring even at RT. We point out that, according to theoretical calculations, H canalso be adsorbed on GaN surfaces[29], so the involvement of H in such radiative centers might
not be ruled out. It is also worthwhile noting that radiative complexes might form throughout the InGaN QW, but the lower the In concentration, the lower their density. In addition,a lower In concentration results in deeper electrically active levels in the band gap for thesame defect[30], thus altering the optical emission properties of such radiative complex. As apossible follow-up to the present study, positron annihilation spectroscopy (PAS) or electron
paramagnetic resonance (EPR) might be carried out, in order to verify the presence of these
VGa -related complexes in our samples.

Figure 5. The time profile of the photoluminescence emission of the ILC A (panel (a)) and B(panel (b)) indicated in figure 2 in the three conditions air, water and ethanol. In both regions,but especially for the ILC B that is located on the 510nm emitting sample, the intensity is
increased locally and its variability, the blinking phenomenon, is also evidently enhanced by
the liquids.

Table 1 shows the average value of the overall intensity of the samples, in air or immersedin the liquid. As it can be seen, the overall strength was basically smaller in liquid. However,when water was introduced on the 510 nm sample surface, the overall emission intensity

The influence of water and ethanol adsorption on the optical blinking in InGaN quantum wells9

460nm
510nm

Air
111
68.0

Water
98.8
81.5

Ethanol101
68.0

Table 1. Comparison of overall average strength of each sample in presence of liquids. Clearly
their overall effect on the average intensity is much smaller than the confined effect on local
emitting domains.

increased. Since this is a change in intensity of about 20% compared to the sample in air andis locally dis-homogeneous, this variation is considered not useful for industrial applications.
However, since there is the possibility that the covering model discussed by Tsutsumi et al.[14]can be applied to the current experiment to model the relationship between the surface state
due to liquid adhesion and the emission intensity changes, we plan to investigate the overall
emission variations occurring during the evaporation of the liquid.
4. Conclusions
We exposed InGaN single quantum well samples, with emission wavelengths centered on 460
nm and 510 nm, to water and ethanol, at room temperature and performed photoluminescencemeasurements with excitation light of 365 nm. As a result, we confirmed that the brightspots and blinking domains normally present on these samples in air, increased in both cases.
The facts that the emission intensity can be linked to the presence of In-rich regions andthat blinking requires also a large number of carriers, suggests that such intense luminous
centers might be due to the presence of VGa -related defects, possibly bound to oxygen
donors. We believe that the method of creating an optical variability standard deviation
map and examining its spatial structure reveals novel information about the sample internal
carrier dynamics. Moreover, the histogram analysis has been proven to be useful for theunderstanding of the effect of liquid absorption on the local optical dynamics.
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