# Blue-green and white color tuning of monolithic light emitting diodes

Benjamin Damilano, Pierre Demolon, J. Brault, Thomas Huault, Franck

Natali, Jean Massies

► To cite this version:

Benjamin Damilano, Pierre Demolon, J. Brault, Thomas Huault, Franck Natali, et al., Blue-green and white color tuning of monolithic light emitting diodes. Journal of Applied Physics, 2010, 108 (7), pp.073115. 10.1063/1.3490895. hal-05223848

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Distributed under a Creative Commons Attribution 4.0 International License
Blue-green and white color tuning of monolithic light emitting diodes

B. Damilano,ª P. Demolon,ᵇ J. Brault, T. Huault,ᶜ F. Natali,ᵈ and J. Massies

Centre National de la Recherche Scientifique - Centre de Recherche sur l'Hétéro-Epitaxie et ses Applications, Rue B. Gregory, 06560 Valbonne, France

A blue light emitting diode (LED) is grown on top of a (Ga, In)N/GaN multiple quantum well acting as a light converter from blue to green-yellow wavelength. The blue light is produced by electrical injection, while the green-yellow emitting QWs are optically pumped by the blue photons. It is shown that the final color of the LED is strongly dependent on the blue pumping wavelength, the absorption and the internal quantum efficiency of the light converter. Depending on these parameters blue to green LEDs or even white LEDs can be obtained. In addition the current dependence between 10 to 100 mA of the LED electroluminescence is measured and analyzed. The reduced blue-shift as a function of the current indicates that the carrier density per quantum well in the light converter stays relatively small compared to classical current injected green LEDs.

ªElectronic mail: bd@crhea.cnrs.fr

ᵇCurrent address: Centre de Recherche en Nanofabrication et en Nanocaractérisation CRN²
Université de Sherbrooke 2500 boul. Université Sherbrooke, QC, J1K 2R1, Canada

ᶜCurrent address: RIBER S.A., 31 rue Casimir Périer, BP 70083, 95873 Bezons cedex, France

ᵈCurrent address: MacDiarmid Institute of Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand
# I. INTRODUCTION

White light emitting diodes (LEDs) are the subject of an intense research activity because of their potential advantages for general lighting against standard lamps such as incandescent bulbs or even fluorescent tubes.¹,² The main advantage of white LEDs compared to non-semiconductor based-lamps is their superior luminous efficiency which, in addition, is improving every year. Record luminous efficiencies now exceed 100 lm/W.²,³,⁴ Most generally, these devices are made of a blue (Al, Ga, In)N-based LED chip recovered by a luminophore (typically a YAG: Ce phosphor) which absorbs part of the blue photons and re-emits then yellow light.⁵ Although these devices reach large luminous efficiencies, the addition of the phosphor complicates the LED process which in turn increases the LED cost manufacturing and can reduce the white LED reliability compared to monochromatic (Al, Ga, In)N blue or green LEDs. The use of phosphors can be avoided by the realization of white LEDs by wafer bonding of a blue (Al,Ga,In)N LED and a red AlGaNP LED.⁶ It has been also proposed that the phosphor can be replaced by semiconductor light converters such as CdSe/ZnS quantum dots or AlGaNP layers.⁷,⁸,⁹,¹⁰ But as in the case of phosphors, these approaches need to add foreign materials to the (Al, Ga, In)N blue LED.

This is why some groups are working on the realization of “monolithic” or “phosphor free” white LEDs.¹¹,¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸,¹⁹,²⁰ The basic idea comes from the observation that blue to red emission can be achieved with (Ga, In)N (GaNN) quantum wells (QWs), then it is possible, for example, to mix blue and yellow emitting QWs inside the p-n junction of the LED and get white light emission.¹¹ This approach presents the advantage of avoiding any additional technological steps compared to simple blue or green GaInN LEDs, contrary to phosphor-based white LEDs. However, there are several drawbacks associated with this kind of monolithic white LEDs. The most important one is that the efficiency of monolithic white LEDs is still lower than the one of phosphor-based LEDs due to the internal quantum
efficiency decrease for yellow emitting GaInN quantum wells.¹,¹² There is only one published value of luminous efficiency for monolithic white LEDs,¹² illustrating the difficulty to get high performance devices. Many groups are currently working on the improvement of long-wavelength emitting GaInN heterostructures and it is worth mentioning the recent successful achievements of green GaInN-based laser diodes.²¹,²²,²³ In the peculiar case of monolithic white LEDs, several ways are explored to improve the yellow emission efficiency such as for example pre-strained GaInN quantum wells.¹⁷,²⁰ Another crucial problem is the non-uniform carrier injection among the different GaInN QWs. It has been reported that the color of monolithic white LEDs strongly depends on the injection current due to this phenomenon.¹⁴

In order to propose solutions to these problems we have recently demonstrated a monolithic white LED using a light converter,²⁴ i.e. a LED constituted by a GaInN/GaN multiple (M)QWs emitting blue light under current injection which optically pumps a green-yellow emitting MQW. The purpose of this work is to go further in the comprehension of such devices and in particular to analyze the link between absorption, internal quantum efficiency of the GaInN/GaN MQW light converter and the final color of the LED.

## II. EXPERIMENTAL

The structure of the LED studied in this work is illustrated in Fig. 1. The sample was grown on a sapphire substrate. At first, metal organic chemical vapor deposition (MOCVD) in a (3 x 2 inches) Thomas Swan close-coupled shower head reactor is used for the growth of a 6.6 µm-thick non intentionally doped GaN layer, followed by a 20 periods GaInN (3.1 nm) / GaN (20 nm) MQW which will serve as the green-yellow light converter. The growth temperatures are 700 and 975°C for GaInN and GaN, respectively. The last barrier consists in a 300 nm - thick undoped GaN layer. After MOCVD growth, room temperature photoluminescence (PL) using the 325 nm line of a 15 mW HeCd laser as the excitation source is performed. An In composition gradient exists perpendicularly to the wafer flat which leads to a shift of the
GaInN QW emission wavelength from 500 nm near the flat to 580 nm near the opposite edge. For lines parallel to the flat the wavelength emission is quasi constant. This spatial repartition is schematized in Fig. 2a. The blue LED structure is grown on this template by molecular beam epitaxy (MBE) in a Riber 32P system. At this stage, it is important to note that a full MOCVD grown structure (light converter plus blue LED) resulted in a wafer with large opaque parts and the LEDs processed in these zones were not working properly. This presumably comes from the high temperature steps used to grow the blue LED by MOCVD, inducing a thermal annealing effect in the high In content GaInN/GaN MQW converter which in turn provokes metallic clustering inside the GaInN layers. To avoid this problem, we have grown the blue LED structure by MBE to take advantage of the lower growth temperatures allowed by this technique. Note that the output power of blue LEDs measured at the wafer level is typically 0.5-1 mW whatever the growth technique we use (MBE or MOCVD). For MBE, standard effusion cells are used for Al, Ga, In, Si and Mg and N is provided by in situ ammonia cracking. The sample is heated at 860°C under ammonia flux in order to remove oxygen surface contamination due to the exposition to air. The temperature is set at 820°C for the growth of 2.2 µm-thick GaN: Si (the n-doping level is measured at $3x10^{18}$ cm$^{-3}$ on a separate calibration sample). Then, a 5 period GaInN/GaN MQW is grown. The growth temperatures are 570°C and 800°C for the GaInN QWs and the GaN barrier layers, respectively. There is a thickness gradient with a circular geometry of the GaInN QWs which induces a wavelength emission shift from 405 nm at the center to 450 nm at the edge, as illustrated in Fig. 2b. A 20-nm thick Al$_{0.1}$Ga$_{0.9}$N: Mg blocking layer is deposited on top of the GaInN QWs and the structure is terminated by a GaN: Mg layer grown at 760°C. Photolithography, metal deposition and dry etching steps are used to define the LED pattern. Note that the p-type contact is constituted by a thick Ni/Au (30 nm / 100 nm) layer which is not transparent and actually acts as a low reflectivity mirror. All the electroluminescence (EL)
experiments are performed at room temperature at the wafer level with an injection current between 10 and 100 mA under continuous wave conditions. Light is collected from the backside by an optical fiber and analyzed by a Peltier-cooled BWtek spectrometer.

### III. ELECTROLUMINESCENCE CHARACTERISTICS

#### A. Fixed pump wavelength and variable light converter wavelength

We first present the electroluminescence characteristics at a 20 mA injection current regarding the LEDs with a blue wavelength pump fixed near 430 nm and a variable light converter wavelength. In practice, we have investigated LEDs located along a half-circle with a fixed radius. The EL spectrum of a reference blue LED is reported in Fig. 3a, this LED was grown with the same structure and growth conditions than those described in the experimental part excepted that there is no light converter. There is only one peak near 430 nm present on this spectrum and no emission in the green-yellow wavelength range. When we measure the EL of the same blue LED structure grown on the 20 periods light converter, we see that an additional peak appears (Fig. 3b). The position of this peak varies from 510 nm to 573 nm, as illustrated by the magnification on the normalized long wavelength peaks in Fig. 3c. Part of the blue photons emitted through electrical injection are absorbed by the light converter which reemits green to yellow light depending on the In composition at the specific position on the wafer. In order to compare the relative intensity of the green-yellow peaks the spectra of Fig. 3b are normalized to the blue peak. We can observe that there is a decrease of the EL intensity of the green-yellow peak when its wavelength increases. The corresponding EL integrated intensity is reported in Fig. 4 as well as the integrated PL intensity of the 20 periods GaInN/GaN intensity measured before the growth of the blue LED (see experimental part). The EL and PL intensity decrease as a function of the wavelength emission is comparable showing that it is mainly due to the intrinsic quality of the GaInN QWs of the light converter,
i.e. to their internal quantum efficiency (IQE). This decrease of internal quantum efficiency for GaInN QWs when the wavelength increases has been often reported in the literature.¹,¹² In our sample the PL or EL intensity decreases by a factor ~30 between 500 and 570 nm, whereas it has been reported that this factor can be as small as 2.¹² The reason for this behavior is that the sample we investigate presents an emission gradient and is not optimized for yellow emission. Nonetheless, the intensities are sufficient to study the phenomena involved in the original structure presented here. For the LED which presents both blue and yellow peaks we could expect white light emission, but the relative intensity of the yellow peak to the blue one is too weak. We will see in the next sections that it is possible to reinforce the yellow peak to finally get white light emission.

We have evaluated the luminous flux emitted by the LED through the substrate with the pump at 430 nm and the light converter at 510 nm by using a calibrated Si photodiode. For a 50 mA injection current we get 200 lm of mixed blue and green light. This value is rather low, but it has to be remembered that these measurements are made directly on wafer without any technological steps to increase the light extraction efficiency. A decrease of the luminous flux with the increase of light converter wavelength is observed in agreement to the EL measurements.

## B. Variable pump wavelength and fixed light converter wavelength

Now we investigate the LEDs located along a radius of the sample parallel to the flat. In this case, the EL blue peak varies from 405 to 450 nm, as shown in Fig. 5a, while the EL green peak stays constant near 520 nm. As previously, the EL spectra have been normalized to the intensity of the blue peak. We can remark that there is an increase by a factor of more than 10 of the green peak intensity when the blue LED wavelength blue-shifts from 450 to 405 nm.

To explain this phenomenon we use a simple model giving the ratio of the green to blue photon fluxes by:
$$ \frac{\Phi_G}{\Phi_B} = \frac{A\eta}{1-A} \quad (1), $$

where A is the absorption of the blue photons by the light converter and η is the internal quantum efficiency of the light converter. This model implies that if there is initially a photon flux coming from the blue LED φ<sub>B</sub>, Aφ<sub>B</sub> photons per unit time are absorbed by the light converter and ηAφ<sub>B</sub> green-yellow photons are emitted per unit time. In this simple description we do not take into account the difference in the extraction efficiency coefficients between blue and green photons. The absorption A of the light converter depends on the bandgap, thickness and number of GaInN QWs included in the light converter and also on the excitation wavelength. In the case of Fig. 5a, the absorption of the light converter is modified by changing the pump wavelength. The shorter the pump wavelength is, the larger the absorption and therefore the green to blue ratio.

It is interesting to note that the chromatic coordinates extracted from the spectra of Fig. 5a change from (0.148, 0.166) to (0.168, 0.615) when the pump wavelength blue-shifts from 450 nm to 405 nm (see chromaticity diagram in Fig. 5b). This means that the color of the LED as perceived by the eye is tuned from quasi pure blue to quasi pure green (see also photos of Fig. 5c) without changing the wavelength of the light converter.

### C. White light emission

At the end of section 3.A we pointed out that the ratio of yellow to blue was too weak to obtain white light. Following equation (1), we see that it is possible to increase this ratio by using a shorter pump wavelength to increase the light converter absorption and also by slightly decreasing the light converter wavelength to increase its internal quantum efficiency (Fig. 4). We can find LEDs corresponding to these criteria on the wafer (the EL spectrum is shown in Fig. 6). The pump wavelength is 406 nm and the light converter peaks at 565 nm. The resulting chromaticity coordinates are (0.328, 0.351) which corresponds to white light
with a correlated color temperature of 5683K and a general color rendering index (Ra) of 41.6 (Fig. 6b and c). Even if we can play with the light absorption to enhance the yellow-to-blue ratio to get white light, the final luminous efficiency will depend on the yellow light converter internal quantum efficiency. Therefore, to obtain a very efficient white light emitting diode, the yellow emission has to be improved. The LED color corresponds to mid-morning or mid-afternoon sunlight. The low color rendering index is not an insurmountable problem as it can be easily improved by increasing the full width at half maximum of the yellow peak. This can be realized by using GaInN QWs emitting at different wavelengths inside the light converter instead of having identical QWs as in the present case.

# IV. VARIATION OF THE LED COLOR WITH ELECTRICAL INJECTION

## A. Electroluminescence as a function of current

We describe and discuss in this section the EL evolution as a function of the electrical injection. The general trend is the same for all the LEDs whatever the pump wavelength and the light converter wavelength. Therefore, we focus the discussion on only one significant LED with a pump wavelength at 408 nm and a light converter wavelength at 529 nm. Figure 7a shows the EL spectra (not normalized, log scale) of this LED for currents ranging from 10 mA to 100 mA. As expected the EL intensity increases when the current increases. The general shape of the EL spectrum is not strongly modified in this current range: the two peaks at 408 nm and 529 nm are well defined whatever the current. The calculation of the chromatic coordinates is reported in Fig. 7b. The chromatic coordinates are (0.218, 0.498) at 10 mA and stay approximately constant between 10 and 60 mA. Above 60 mA, the variation becomes significant and values of (0.201, 0.429) are achieved at 100 mA. This indicates that the blue component of the LED color increases. Figure 8a shows the integrated intensity of each peak of the LED spectrum. The variation as a function of the current is similar for both
contributions, excepted in the 80-100 mA range for which there is a stronger saturation of the green peak. The intensity ratio between green and blue peak is about 1.1 between 10-60 mA and decreases down to 0.9 for 100 mA (inset of Fig. 8a). In our previous work we had seen a larger intensity ratio variation which was related to the large wavelength blueshift of the light converter.²⁴ The situation is different in the present case since the 1 nm blueshift of the light converter between 10 to 100 mA (Fig. 9) seems too small to explain the observed variation. Actually, we interpret this variation as mainly due to thermal heating at large currents. This is confirmed by the wavelength variation as a function of the current of the blue LED pump. The blue LED wavelength is quasi constant at 408 nm between 10-60 mA, whereas a significant shift is observed above 60 mA, the wavelength reaches 409.3 nm at 100 mA. This red-shift is due to the bandgap narrowing due to Joule effect thermal heating of the active zone. As it is explained in section 3.2, the absorption of the light converter decreases when the pump wavelength increases. This in turn causes the decrease of the green to blue intensity ratio when the current increases above 60 mA. According to the data of Fig. 5, the wavelength shift from 408 to 409.3 nm should induce a decrease of the green/blue intensity ratio by 14%. This is somewhat lower than the observed 20% decreases when the current increases from 10 to 100 mA (inset of Fig. 8(a)). The difference can probably be explained by a small decrease of the internal quantum efficiency of the green emitting QWs due to heating of the device.

## B. Comparison of standard current injected green LEDs with optically pump green light emitting devices

We investigate in this section the current dependence of the wavelength and the intensity of the green peak. It is very interesting to note that there is a blue shift of only 1 nm between 10 mA and 100 mA (Fig. 9). This contrasts to the large shifts (about 10 nm) observed for commercially available green emitting LEDs.²⁵,²⁶ In order to compare our results in the same experiment setup, we have grown in our MOCVD reactor a green LED. The active zone is
constituted of a 5 period green emitting GaInN/GaN QWs. For this LED the wavelength blue-shift between 10 mA and 50 mA is 12.7 nm (Fig. 9), which is even larger than observed in Ref. 25 and 26. This blue-shift is generally attributed to the progressive screening of the internal electrical field by the injected carriers when the current increases. This effect is particularly marked due to the fact that the carrier distribution is not homogeneous in all the GaInN wells: the carrier density is much larger in the first QW next to the p-GaN.²⁷,²⁸ We do not observe this large wavelength shift in the case of the monolithic LED with light converter because the GaInN green emitting QWs are excited optically and not electrically. In a rough description, the carrier density in the n<sub>th</sub> QW of the light converter (counting from the surface to the substrate of the sample) should be proportional to:

$$ \alpha L_w e^{(-\alpha n L_w)} (2), $$

where α is the absorption coefficient of GaInN at an energy corresponding to the pump wavelength, and L<sub>w</sub> the QW thickness. Of course, this relation indicates that the photo-generated carrier density decreases as a function of the QW number n. More interesting is the fact that as the QW thickness is only a few nanometers the absorption of each QW will be relatively small and, therefore, so does the carrier density in each QW. Indeed, as shown by the experimental results, the carrier density in each QW stays much lower than for an electrically injected green LED. It should be pointed out that if the green QWs were electrically injected, the blue-shift as a function of current would be also large and very similar to the one observed in ref. 25 and 26 for example. Actually the interest of using optical pumping against electrical pumping is to better distribute the carriers in the 20 green emitting QWs, resulting in a lower carrier density in each QW.

Another problem of green LEDs is their decreased efficiency at large current which is often named as efficiency droop. Best efficiencies are then obtained at low current density. Once again, in the case of optical injection of green QWs, the carrier density is relatively low and
therefore, the efficiency remains close to the maximum value at large injection currents. Note
that by using this approach large green output power can be achieved provided all the blue
photons are absorbed and converted in green.

V. CONCLUSION

We have investigated the working of a monolithic LED with a MQW GaInN/GaN light converter. We have shown that the color of the LED depends both on the green-yellow light converter wavelength and on the intensity ratio between the blue and green-yellow peak. This ratio is critically affected by the internal quantum efficiency of the green-yellow emitter and its absorption. When this ratio is properly adjusted, white light can be obtained. The recent realization of high efficiency yellow emitting GaInN QWs on non-polar or semi-polar GaN orientation,²⁹ paves the way for the fabrication of efficient white LEDs following the approach described here. Also, owing to the small wavelength shift of the green-yellow peak with increasing current density, we can imagine the realization of pure green or yellow LEDs provided that complete absorption of the blue light is realized. It would be of interest to grow the entire structure using a MOCVD process, as the current LED production is based on this technique. To this end, growth strategies have to be developed to avoid the degradation of the light converter during the MOCVD growth of the blue LED, such as the development of a lower temperature growth procedure for GaN.

ACKNOWLEDGMENTS

The authors would like to thank S. Chenot for the process of the samples, D. Lefebvre for his
invaluable assistance, B. Poulet for taking the pictures, M. Leroux and P. De Mierry for
critical reading of the manuscript. This work is supported by ANR-PNANO 06 Project
“DEMONI”.
¹M.R. Krames, O.B. Schekin, R. Mueller-Mach, G.O. Mueller, L. Zhou, G. Harbers, and M.G. Craford, *IEEE J. Disp. Technol.* **3**, 160 (2007).

²M.H. Crawford, *IEEE J. Selected Topics Quantum Electron.* **15**, 1028 (2009).

³Y. Narukawa, M. Sano, M. Ichikawa, S. Minato, T. Sakamoto, T. Yamada, and T. Mukai, *Jpn. J. Appl. Phys.* **46**, L963 (2007).

⁴A. Laubsch, M. Sabathil, J. Baur, M. Peter, and B. Hahn *IEEE Transactions on Electron Devices* **57**, (2010).

⁵S. Nakamura and G. Fasol: *The Blue Laser Diode* (Springer, Berlin, 1997).

⁶X. Guo, G. D. Shen, B. L. Guan, X. L. Gu, D. Wu, and Y. B. Li, *Appl. Phys. Lett.* **92**, 013507 (2008).

⁷H. S. Chen, D. M. Yeh, C. F. Lu, C. F. Huang, W. Y. Shiao, J. J. Huang, C. C. Yang, I. S. Liu, and W. F. Su, *IEEE Photon. Technol. Lett.* **18**, 1430 (2006).

⁸S. Nizamoglu, T. Ozel, E. Sari, and H. V. Demir, *Nanotechnology*, **18**, 065709 (2007).

⁹H.S. Jang, B.-H. Kwon, H. Yang, and D.Y. Jeon, *Appl. Phys. Lett.* **95**, 161901 (2009).

¹⁰X.Y. Guo, J.W. Graff, E.F. Schubert, R.F. Karlicek, *Proc. SPIE* **3938**, 60 (2000).

¹¹B. Damilano, N. Grandjean, C. Pernot, and J. Massies, *Jpn. J. Appl. Phys., Part 2* **40**, L918 (2001).

¹²V.V. Nikolaev, M.E. Portnoi, and I. Eliashevich, *Phys. Stat. Sol. (a)* **183**, 177 (2001).

¹³M. Yamada, Y. Narukawa, and T. Mukai, *Jpn. J. Appl. Phys., Part 2* **41**, L246 (2002).

¹⁴S. Dalmasso, B. Damilano, C. Pernot, A. Dussaigne, D. Byrne, N. Grandjean, M. Leroux, and J. Massies, *Phys. Status Solidi* (a) **192**, 139 (2002).

¹⁵S. J. Chang, L. W. Wu, Y. K. Su, C. H. Kuo, W. C. Lai, Y. P. Hsu, J. K. Sheu, J. F. Chen, and J. M. Tsai *IEEE Transaction on Electron. Devices* **50**, 519 (2003).

¹⁶A. Dussaigne, J. Brault, B. Damilano, and J. Massies, *Phys. Status Solidi* (c) **4**, 57 (2007).
¹⁷C.F. Huang, C.-F. Lu, T.-Y. Yang, J.-J. Huang, and C.C. Yang, Appl. Phys. Lett. **90**, 151122 (2007).

¹⁸M. Funato, T. Kondou, K. Hayashi, S. Nishiura, M. Ueda, Y. Kawakami, Y. Narukawa, and T. Mukai, Appl. Phys. Express **1**, 011106 (2008).

¹⁹S.-N. Lee, H. S. Paek, H. Kim, T. Jang, and Y. Park, Appl. Phys. Lett. **92**, 081107 (2008).

²⁰C.-F. Lu, C.-F. Huang, Y.-S. Chen, W.-Y. Shiao, C.-Y. Chen, Y.-C. Lu, and C. C. Yang, IEEE J. Selected Topics Quantum Electron. **15**, 1210 (2009).

²¹T. Miyoshi, S. Masui, T. Okada, T. Yanamoto, T. Kozaki, S. Nagahama, and T. Mukai, Appl. Phys. Express **2**, 062201 (2009).

²²Y. Enya, Y. Yoshizumi, T. Kyono, K. Akita, M. Ueno, M. Adachi, T. Sumitomo, S. Tokuyama, T. Ikegami, K. Katayama, and T. Nakamura, Appl. Phys. Express **2**, 082101 (2009).

²³A. Avramescu, T. Lermer, J. Müller, S. Tautz, D. Queren, S. Lutgen, and U. Strauss, Appl. Phys. Lett. **95**, 071103 (2009).

²⁴B. Damilano, A. Dussaigne, J. Brault, T. Huault, F. Natali, P. Demolon, P. De Mierry, S. Chenot, and J. Massies Appl. Phys. Lett. **93**, 101117 (2008).

²⁵http://www.nichia.com, datasheet for « truegreen »LEDs

²⁶http://www.osram.com, datasheet for « truegreen »LEDs

²⁷A. David, M. J. Grundmann, J.F. Kaeding, N.F. Gardner, T.G. Mihopoulos, and M.R. Krames, Appl. Phys. Lett. **92**, 053502 (2008).

²⁸M. Peter, A. Laubsch, P. Strauss, A. Walter, J. Baur, and N. Hahn, Phys. Stat. Sol. (c), **6**, 2050 (2008).

²⁹H. Sato, R.B. Chung, H. Hirasawa, N. Fellows, H. Masui, F. Wu, M. Saito, K. Fujito, J.S. Speck, S.P. DenBaars, and S. Nakamura, Appl. Phys. Lett. **92**, 221110 (2008).
FIG. 1. Schematic structure of the device, consisting of a blue LED grown on top of a 20 periods green-yellow emitting GaInN/GaN multiple quantum wells.

FIG. 2. Schematics of the spatial wavelength emission variation on the wafer of the light converter (a) and the blue light emitting diode (b).

FIG. 3. Room temperature electroluminescence spectra for a 20 mA current of: (a) a reference blue light emitting diode, (b) a blue light emitting diode grown on top of the light converter, in which the different spectra are recorded on light emitting diodes distributed on the epitaxial wafer along a circle with a fixed radius giving a constant wavelength for the blue peak and a variable green-yellow emission for the converter. The spectra are normalized to the blue peak intensity. (c) Magnification of the long wavelength peak (normalized) of the spectra in (b).

FIG. 4. Room temperature integrated photoluminescence intensity (continuous line) of the 20 periods GaInN/GaN multiple quantum wells as function of the wavelength recorded before the growth of the blue light emitting diode and integrated electroluminescence intensity (diamonds) of the green-yellow peak extracted from Fig. 3b.

FIG. 5. (a) Room temperature electroluminescence spectra at 20 mA for light emitting diodes located along a radius of the sample parallel to the flat. The pump wavelength varies from 405 nm to 450 nm while the light converter wavelength stays at 520 nm. (b) Chromatic coordinates corresponding to the spectra in (a), the pump wavelength is indicated beside each data point. (c) The 4 photos correspond to the light emitting diode spectra reported in (a) and
are sorted from left to right with an increasing pump wavelength. The symbols displayed in
the left bottom corner correspond to those used to identify the EL spectra in (a).

FIG. 6. (a) Electroluminescence spectrum at room temperature at a 20 mA current of a white monolithic light emitting diode with a light converter at 565 nm, corresponding chromaticity coordinates (b) and picture (c).

FIG. 7. (a) Room temperature electroluminescence spectra as a function of the current of a light emitting diode with a pump at 408 nm and the light converter at 529nm. (b) Corresponding chromaticity coordinates (triangles).

FIG. 8. (a) Integrated electroluminescence intensity of blue (triangles) and green peaks (diamonds) extracted from Figure 7 as a function of the current. The inset reports the integrated intensity ratio of green to blue peaks. (b) Electroluminescence peak wavelength of the blue pump as a function of the current.

FIG. 9. Variation of the electroluminescence peak wavelength as a function of the current for standard green light emitting diodes (triangles, diamonds, circles) and for the green peak of a light emitting diode with a light converter (squares). For this last device, the current corresponds to the injection current in the blue light emitting diode which serves as pump.
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JAP, Damilano et al.
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