Open Access
Add to BibSonomyAbstract
We demonstrate InGaN/GaN superluminescent diodes with broadened emission spectra fabricated on surface-shaped bulk GaN (0001) substrates. The patterning changes the local vicinal angle linearly along the device waveguide, which results in an indium incorporation profile in InGaN quantum wells. The structure was investigated by microphotoluminescence mapping, showing a shift of central emission wavelength from 413 nm to 430 nm. Spectral full width at half maximum of processed superluminescent diodes is equal to 6.1 nm, while the reference chips show 3.4 nm. This approach may open the path for using nitride devices in applications requiring broad emission spectrum and high beam quality, such as optical coherence tomography.
© 2016 Optical Society of America
1. Introduction
Superluminescent diodes (SLD) are semiconductor light sources combining a high spatial coherence, like that of laser diodes, with a broadband emission spectrum, like that of light emitting diodes (LED). These features are interesting for applications such as fibre optic gyroscopes (FOG), Optical Coherence Tomography (OCT) and micro- or picoprojectors. In case of both OCT and FOG the spectral width of the emitted light is an important parameter determining the usefulness of the light source. For example, the axial resolution of OCT is proportional to the square of central emission wavelength divided by the width of the spectrum (λ2/Δλ). In contrast to other types of broadband devices like supercontinuum lasers and ultrashort pulse lasers, superluminescent diodes have the advantage of small size and low price. That is why arsenide and arsenide-phosphide SLDs are indispensable for many applications.
In the particular case of nitride superluminescent diodes, their shorter emission wavelengths open up new measurement possibilities. For OCT, the short wavelengths strictly lead to better in-plane resolutions and increase the detection signal-to-noise ratio. We believe that the blue or violet light source can be successfully used in material science for inspection of transparent materials (e.g. examining defects in transparent substrates like GaN). On the other hand, the increasing role of plastic fibers in optoelectronics creates a need for blue-green emitters which match their transmission windows. The broadband nitride light sources are good candidates as light sources for plastic-fibre-based devices like FOG. Finally, it is beneficial for projection applications to have a light source based on red, green, and blue monolithic emitters. In such a set, the blue and green components would be fabricated using nitride material system. At the same time low time coherence (wide emission spectrum) is required to reduce the unwanted interference effect - speckles, which influence the quality of the projected image. This problem is especially important for the green emitters which match the maximal sensitivity of the human eye and because of this produce strongly visible speckles.
In case of a standard semiconductor light emitter, the width of the spontaneous emission limits the maximum achievable width of the emitted light spectrum. In the field of infrared SLDs, an extensive work was done to surpass this limitation, for example by: fabricating an active region consisting of quantum wells having different widths [1]; using quantum dots with variable sizes or composition in the active area [2–4]; growing a stack of quantum dot layers in a matrix with varying composition [5]; growing hybrid quantum well/quantum dot active region [6, 7]; bonding several chips fabricated separately [8]. In all cases, utilising a spectral broadening strategy led to a decrease of the λ2/Δλ parameter to around 20 μm or below, even down to 2.5 μm [7]. However, all presented approaches suffer from increased optical losses or light reabsorption; and were not applied for nitride devices.
The history of nitride SLDs is short, but marked by a rapid progress both in terms of general understanding of the device physics and in obtained quality of the devices. The first nitride SLDs were introduced in 2009 by Feltin et al [9]. In the following years a great progress has been achieved thanks to the effort of many groups contributing to this field. A lot of work has been done to examine the different cavity suppression strategies [10–13]. The influence of work temperature has also been investigated [12]. Simultaneously, the devices were optimized in order to reach high output powers [11, 15, 16]. Our recent paper [17] describes in detail the properties of nitride superluminescent diodes with bend-waveguide geometry, emphasizing the influence of the waveguide bend angle on the spectral modulations. As nitride SLDs reach maturity, their properties can be tuned to match the specific requirements of existing and emerging applications.
It is also important to note, that the emission spectrum of nitride superluminescent diodes shows a significantly smaller bandwidth than the spontaneous emission. This results from a narrow gain spectrum [18] which is a consequence of the large effective mass of electrons in InGaN quantum wells [19]. For example, our earlier work [13, 16, 17] reports emission spectrum width of 3-4 nm under standard operation conditions (miliwatt range of optical power) for devices with no spectral-broadening strategy applied, in contrast to 8 nm or more for low current range when spontaneous electroluminescence dominates.
The aim of this work is to broaden the emission spectra of nitride superluminescent diodes. By means of proper substrate patterning, we introduce a linear change of the indium content in quantum wells along the device waveguide, as shown in Fig. 1, which leads to a shift of central emission wavelength.
Fig. 1 Scheme of the proposed method of spectral broadening: (a) top view of the superluminescent diode chip, (b) spatial change in indium content, (c) bandgap for three selected positions in the chip.
The resulting spectrum of the device is a sum of all the guided components emitted at different positions and centred at different wavelengths. Such a spectrum is significantly broader than that of standard superluminescent diodes having constant indium composition in the quantum wells, which were presented in the previous reports [13, 16, 17]. Apart from the difference in the active region (indium content profile) we use a standard fabrication technology and bend-waveguide geometry [16, 17].
In order to minimize the light reabsorption, the indium content decreases towards the front facet region. We applied a continuous change of indium content as opposed to a stepwise design to achieve a better overlap of gain spectra of neighboring areas of the device and thus to obtain smoother emission spectrum. In our design the band gap of quantum wells increases in the direction of the chip’s front facet, preventing light from reabsorption.
2. Substrate patterning
The method of achieving spectral broadening proposed in this work is based on the experiments presented by Sarzynski et al [20, 21]. These papers demonstrate the existence of a strong dependence of the indium incorporation as a function of substrate vicinal angle (off-cut; the angle between the crystallographic plane and substrate surface). For example, for layers grown at the same growth conditions (gas flows and temperature 820°C), but on substrates having vicinal angles of 0.2° and 2.5°, the indium content changed from about 14% to 7%, respectively. Moreover, this work presents a technology of substrate patterning, which allows to fabricate a substrate with different local off-cut; Fig. 2. The method requires a multilevel photolithography (contrary to the well-known binary pattern photolithography). A substrate is spin-coated with a photoresist layer and exposed to light while its doses are spatially varied in a continuous way. Therefore, after the photoresist development, we obtain a resist film of a spatially variable thickness. Next, the sample is dry-etched by Inductively Coupled Plasma Reactive-Ion Etching (ICP RIE). As the etching rate of GaN almost equals the etching rate of the developed photoresist, we are able to transfer the pattern onto the GaN substrate.
Fig. 2 Subsequent fabrication steps of substrate pattern: (a) deposition of photoresist, (b) exposure to light with spatially varying doses, (c) photoresist development, (d) transfer of the pattern onto the substrate by dry etching. (e) Scheme of the proposed shape of the substrate pattern, characterized by smooth change of vicinal angle across a single chip area. It provides a spatial difference of indium incorporation during the epitaxy of InGaN quantum well layers according to the design shown in Fig. 1.
In order to achieve the indium content profile, as described in the previous section, we need to tune the vicinal angle of the substrate from a low value in the area of the rear facet to a high value at the front facet. Additionally, the change should be smooth and continuous. A design, which matches all this requirements, is an inclined plane similar to the shape of an airplane propeller; Fig. 2(e).
We fabricated such inclined planes on a c-plane bulk GaN substrate grown by the ammonothermal method [22] with the initial vicinal angle of 0.3° to the m-axis and 0° to the a-axis. Further in this paper we will refer to the vicinal angle only along a-axis, because this is the direction in which the change of vicinal angle is applied. After spin-coating a 2μm layer of a photoresist, the sample was exposed in a laser writer system (Microtech Laserwriter LW405B). The multilevel photolithography pattern was used to form two regions: one flat, preserving the original vicinal angle of the substrate made as reference and the second one containing the inclined planes. Between the incline planes, flat areas were left for the purpose of fabricating reference devices near the spectrally-broadened SLDs with as similar growth conditions as possible. As a next step the photoresist was developed and the sample was dry-etched by ICP RIE to transfer the pattern onto the bulk GaN crystal. The etching parameters were chosen so that the etching rate of GaN and the developed photoresist are very similar. By this means we fabricated a substrate which allows a simultaneous fabrication of several spectrally-broadened and standard SLDs. The profilometer scans measured along one of the inclined planes are shown in Fig. 3(a) and the calculated values of surface off-cut are presented on Fig. 3(b). We were able to change the vicinal angle from 0.9° to below 0.2°. The surface profiles turned out to be practically identical across the processed wafer, which proves the effectiveness of our patterning technology. The highest vicinal angle (0.9°) reported in this work is not a limit of this approach. Greater angles can be reached by using a thicker layer of photoresist or by decreasing the inclined plane width. In this work we chose to limit the vicinal angle to about 0.9°, as the growth at too high vicinal angle results in poorer morphology. This patterning method can be also applied to other types of substrates, provided that the photoresist properties and the etching conditions are chosen so that the etching rate of photoresist and the substrate material are similar.
Fig. 3 Characterization of the patterned substrate shape: (a) profilometer scans parallel to the m-plane and (b) corresponding change in the vicinal angle.
3. Device fabrication and microphotoluminescence mapping
On the substrate, prepared by the described method, we grew an epitaxial structure by metalorganic vapour phase epitaxy (MOVPE) with a graded-index separate-confinement heterostructure (GRINSCH) design [23]. The structure consists of a thick bottom cladding, graded bottom waveguide layer, active layer with three quantum wells and quantum barriers overgrown by thin cap layer, electron blocking layer, graded upper waveguide and cladding layer. The structure was grown at temperatures in the range of 760°C to 1050°C. The level of doping was 1019 cm−3 for Mg and 5∙1018 cm−3 for Si. The scheme of the structure is shown in Fig. 4(a).
The quality of the fabricated structure was examined by transmission electron microscopy and x-ray diffraction. Figure 4(b) presents a TEM image of the active region of the structure of the patterned area of the crystal. The bright line visible below the three quantum wells corresponds to a high indium content layer, formed here for technical purposes. Figure 5 shows High Resolution X-Ray Diffraction (HR-XRD) scans of the discussed structure, measured in the flat and patterned region. Simulation was shown on the same picture and was done using EPITAXY software provided by Panalytical. Comparison of measurements and simulations indicates a very good crystal quality of the structure. The increased smoothness of the curve measured on patterned part of the substrate results from the varying indium content in this region, which is averaged by the XRD measurement. The results also suggest that the quantum wells grown in the patterned area are around 0.2 nm wider when compared to these from the flat part.
In this work we did not study in detail how the morphology of the substrate changes for different vicinal angles, but such a comparison was done in the earlier work [21]. The optimal morphology, smooth and single atomic steps, can be achieved for vicinal angle of around 0.6° - 0.8°, depending on the growth conditions. But satisfactory structural quality can be also obtained when the steps start to be wavy and some step-bunching appears. Thanks to this, the usable window of the vicinal angle values increases to around 0.3° to 1.2°. This window can be shifted by changing the growth conditions of the epitaxial layers. In this work, the situation is complicated, as the prepared substrate has 3D shapes on the surface. However, the satisfactory structural quality shown in Fig. 5 proves, that the substrate patterning did not cause any growth instabilities. It is important to note, that the substrate was initially misoriented with a vicinal angle of 0.3° in m-axis. So, even in the flat area of the substrate, with no vicinal angle produced by patterning, the growth is stable. Additionally, Fig. 6(a) shows an optical microscope image with Nomarski interference contrast of the substrate with epitaxial structure, confirming that the designed pattern is preserved during growth process.
Fig. 6 Characterization of the epitaxial structure fabricated on the patterned substrate. Panel (a) shows an optical microscope image with Nomarski interference contrast, where the designed inclined plane is marked with the white dashed line. Panel (b) presents a microphotoluminescence map of this area, proving that the indium content decreases along the waveguide (schematically marked by the white lines).
Next, we processed the structure to fabricate 3 μm wide ridge waveguides in a bend shape as described by Kafar et al. in [16] and deposited nickel-gold contacts on top of the ridges. At the end, we cleaved the wafer to form 1000 μm long devices. A certain area of the wafer was left unprocessed enabling microphotoluminescence mapping.
As the next step we performed microphotoluminescence mapping (μPl) of the contact-free chips from the patterned substrate. Figure 6(b) shows a map of central emission wavelengths. The vertical area from 0.05 mm to 0.17 mm in the x direction corresponds to the fabricated inclined plane. We can clearly see that the applied method leads to a significant change in the central emission wavelength of the active region in a single-chip area – from about 430 nm to 413 nm. On such a sample, the device’s ridge waveguide (marked schematically by a white line) spans over the entire range of wavelengths and combines all the local spectra. By these means, we can significantly broaden the emission spectrum.
4. Superluminescent diode characterization
The fabricated devices were characterized in a standard test setup for light-current-voltage measurements. The emission spectra were acquired by a Horiba Jobin-Yvon FHR 1000 spectrometer with a diffraction grating having 3600 grooves per mm and with a high-resolution Synapse 2048x512 CCD camera. We show a comparison of parameters measured for the devices fabricated on an inclined plane and a flat area of the substrate.
Figure 7 presents a comparison of the emission spectra measured under operating current of 100 mA and 400 mA for a SLD fabricated on the patterned and flat areas. The reference SLD was fabricated on the same substrate during the same process, but on a neighbouring region with constant vicinal angle (uniform thickness of resist during fabrication), so with uniform indium content along the waveguide.
Fig. 7 Comparison of emission spectra of a superluminescent diode fabricated on a patterned substrate and a reference diode fabricated on flat area of the same substrate. The spectra were taken under current operation of 100 mA (a) and 400 mA (b).
Additionally, in Fig. 8(a) we present a comparison of spectral full width at half maximum and modulation depth acquired for both devices at different operating currents. Clearly, the emission spectrum of the superluminescent diode fabricated on the patterned substrate is broader, but the difference decreases with increasing current. At the operating current of 400 mA, which corresponds to useful optical powers, the FWHM of spectrally-broadened SLD equals to 6.1 nm, while the reference device shows FWHM of 3.4 nm. Both SLDs have the same bend waveguide shape and any difference between them should result only from the shapes of the substrate surface. The increase of FWHM value corresponds to a decrease of the λ2/Δλ parameter from about 54 μm (reference) to about 29 μm, while for commercial SLDs this parameter ranges from 60 to 10 μm.
Fig. 8 Comparison of FWHM (a) and modulation depth (b) dependences on current, measured for devices presented in Fig. 5. For 400 mA full width at half maximum was increased from 3.4 nm to 6.1 nm.
There is also a clear difference in the modulation depth of the spectra presented in Fig. 8(b), which is a result of two mechanisms. Firstly, in the SLD with broadened spectra the light propagating towards the rear facet is strongly absorbed, as it travels in the direction of a higher indium content and smaller bandgap. This supresses light oscillation in the waveguide and decreases the modulation depth. This reasoning is confirmed by the observation of a very weak and red-shifted emission from the back-side of the chip. Secondly, the difference in central emission wavelengths between the two devices corresponds to a different effective reflectivity of the front-facet, which also influences the modulation depth. The observed shift in the central emission wavelengths (between devices fabricated on patterned and flat substrate) is a result of the fabrication strategy. The low off-cut of the reference diode substrate results in a high amount of indium in the quantum wells.
The light-current curves of the described devices are shown in Fig. 9(a). This measurement was done under CW operation with temperature stabilised at room temperature. Clearly, the device with broadened spectrum emits much smaller optical power. This is in agreement with the basic equation describing the optical power emitted from a single-pass superluminescent diode [24]:
where Psp is the spontaneously emitted power, g is the effective gain coefficient, α are the waveguide internal optical losses and L is the waveguide length. In the case of a device having a varying indium content along the waveguide, the gain spectrum shifts following the indium content changes. Because of this, a particular wavelength can be strongly amplified only near the position where it matches the gain maximum. As light travels towards the front facet the gain of this wavelength decreases and, eventually, becomes negative (losses). Therefore all wavelengths are amplified on distances much smaller than the chip length. As Popt depends on an exponent of the amplification length, the optical power decreases significantly with respect to the reference device. However, if the substrate patterning is used with a high-power SLD design, it is possible to preserve moderate optical powers. It is important to note, that for applications such as OCT, the width of emission spectra is much more important than high optical power (the required amount is in the range of 5 to 10 mW). In this case it is favourable to increase the emission bandwidth even with the cost of optical power reduction.Fig. 9 Comparison of light-current (a) and current-voltage (b) dependences of described devices shown in Fig. 7. The superluminescent diode with spectral broadening is characterised by a significantly lower optical power than the reference device, but also by significantly lower operating voltage.
Furthermore, optical losses also strongly influence the achievable optical power, by decreasing the intensity of light propagating through the waveguide. It is especially important in our approach, where the emission spectrum of the device’s rear region is red-shifted with respect to the front-region gain spectrum. We calculated the internal optical losses for the examined structure by measuring gain spectra of a laser diode fabricated on the same substrate on a flat area by Hakki-Paoli method [25]. The obtained value equals 25 cm−1. In future work, the output power of broad-emission nitride SLDs can be definitely increased by further decreasing the internal optical losses through optimizing the epitaxial structure and processing.
A significant decrease of operating voltage for spectrally-broadened SLD is shown in Fig. 9(b). We attribute it to the difference in vicinal angles of the patterned GaN substrate and the reference device. As reported by Perlin et al. in [26], epitaxial layers grown on substrates with higher vicinal angle show higher hole concentrations in Hall effect measurements. This agrees with our observation, as the higher hole concentrations lead to decrease of the voltage drop on p-type layers. This effect is statistically valid and was observed on many other devices fabricated on patterned and flat area of the substrate.
In the end, we would also like to focus the reader’s attention, on the difference in the way the FWHM changes for both devices, which is shown in Fig. 8(a). In case of the reference SLD, the FWHM drops quickly and after 200 mA is almost stable. Such a behaviour is presented in more details in our earlier work [16]. On the other hand, the spectrally-broadened SLD shows a slower and rather linear decrease of FWHM. The fact that the emission spectrum combines the different wavelength components allows to challenge the material restrictions on the FWHM. Additionally, this method is quite flexible and can be further developed. One of the possible strategies is to create nonlinear indium content profile along the waveguide, which could help to increase the width of the emission even more.
5. Conclusions
We presented a method of broadening the emission spectra of nitride superluminescent diodes. The approach is based on the observed dependence of indium incorporation in the InGaN layers grown on the substrate with different vicinal angles. We used a substrate patterning method in order to fabricate an active layer with spatially changing indium content. The purpose of the design is to achieve decreasing indium content along the waveguide, from the back facet towards the front facet. Microphotoluminescence maps of the chip area confirm the presence of indium content distribution according to our design, showing that the central emission wavelengths vary from 430 nm to 413 nm. We characterized devices fabricated on a patterned substrate and achieved an increase of spectral width from 3.4 nm to 6.1 nm. This increase corresponds to a decrease of the λ2/Δλ parameter from 54 to 29 μm.
Acknowledgments
We wish to thank E. Grzanka for performing the XRD characterisation and J. Smalc-Koziorowska for TEM imaging. This work was supported within the research projects PRELUDIUM no. 2013/11/N/ST7/02714 and OPUS no. 2014/15/B/ST3/04252 founded by the Polish National Science Centre and grant no. POIG.01.04.00-14-007/12 founded by Polish National Center for Research and Development.
References and links
1. C. F. Lin and B. L. Lee, “Extremely broadband AlGaAs/GaAs superluminescent diodes,” Appl. Phys. Lett. 71(12), 1598–1600 (1997). [CrossRef]
2. Z. Z. Sun, D. Ding, Q. Gong, W. Zhou, B. Xu, and Z. G. Wang, “Quantum-dot superluminescent diode: A proposal for an ultra-wide output spectrum,” Opt. Quantum Electron. 31(12), 1235–1246 (1999). [CrossRef]
3. N. Liu, P. Jin, and Z. G. Wang, “InAs/GaAs quantum-dot superluminescent diodes with 110nm bandwidth,” Electron. Lett. 41(25), 1400–1402 (2005). [CrossRef]
4. Y. C. Yoo, I. K. Han, and J. I. Lee, “High power broadband superluminescent diodes with chirped multiple quantum dots,” Electron. Lett. 43(19), 1045–1047 (2007). [CrossRef]
5. L. H. Li, M. Rossetti, A. Fiore, L. Occhi, and C. Velez, “Wide emission spectrum from superluminescent diodes with chirped quantum dot multilayers,” Electron. Lett. 41(1), 41–43 (2005). [CrossRef]
6. S. Chen, K. Zhou, Z. Zhang, J. R. Orchard, D. T. D. Childs, M. Hugues, O. Wada, and R. A. Hogg, “Hybrid quantum well/quantum dot structure for broad spectral bandwidth emitters,” IEEE J. Sel. Top. Quantum Electron. 19(4), 1900209 (2013). [CrossRef]
7. S. Chen, W. Li, Z. Zhang, D. Childs, K. Zhou, J. Orchard, K. Kennedy, M. Hugues, E. Clarke, I. Ross, O. Wada, and R. Hogg, “GaAs-based superluminescent light-emitting diodes with 290-nm emission bandwidth by using hybrid quantum well/quantum dot structures,” Nanoscale Res. Lett. 10(1), 1049 (2015). [CrossRef] [PubMed]
8. V. Jayaraman, E. Hall, and D. Leonard, “Direct multi-lateral wafer bonding for new functionality in photonic integrated circuits,” Conference on Optoelectronic and Microelectronic Materials and Devices 2006, 180–183 (2006). [CrossRef]
9. E. Feltin, A. Castiglia, G. Cosendey, L. Sulmoni, J.-F. Carlin, N. Grandjean, M. Rossetti, J. Dorsaz, V. Laino, M. Duelk, and C. Velez, “Broadband blue superluminescent light-emitting diodes based on GaN,” Appl. Phys. Lett. 95(8), 081107 (2009). [CrossRef]
10. M. T. Hardy, K. M. Kelchner, Y. D. Lin, P. S. Hsu, K. Fujito, H. Ohta, J. S. Speck, S. Nakamura, and S. P. DenBaars, “m-Plane GaN-based blue superluminescent diodes fabricated using selective chemical wet etching,” Appl. Phys. Express 2(12), 121004 (2009). [CrossRef]
11. H. Ohno, K. Orita, M. Kawaguchi, K. Yamanaka, and S. Takigawa, “200mW GaN-based superluminescent diode with a novel waveguide structure,” Photonics Conference (PHO) 2011 IEEE, 505–506 (2011). [CrossRef]
12. F. Kopp, T. Lermer, C. Eichler, and U. Strauss, “Cyan superluminescent light-emitting diode based on InGaN quantum wells,” Appl. Phys. Express 5(8), 082105 (2012). [CrossRef]
13. A. Kafar, S. Stanczyk, S. Grzanka, R. Czernecki, M. Leszczynski, T. Suski, and P. Perlin, “Cavity suppression in nitride based superluminescent diodes,” J. Appl. Phys. 111(8), 083106 (2012). [CrossRef]
14. K. Holc, L. Marona, R. Czernecki, M. Bockowski, T. Suski, S. Najda, and P. Perlin, “Temperature dependence of superluminescence in InGaN-based superluminescent light emitting diode structures,” J. Appl. Phys. 108(1), 013110 (2010). [CrossRef]
15. F. Kopp, C. Eichler, A. Lell, S. Tautz, J. Ristic, B. Stojetz, C. Hoss, T. Weig, U. T. Schwarz, and U. Strauss, “Blue superluminescent light-emitting diodes with output power above 100 mW for picoprojection,” Jpn. J. Appl. Phys. 52(8S), 08JH07 (2013). [CrossRef]
16. A. Kafar, S. Stanczyk, G. Targowski, T. Oto, I. Makarowa, P. Wisniewski, T. Suski, and P. Perlin, “High-optical-power InGaN superluminescent diodes with “j-shape” waveguide,” Appl. Phys. Express 6(9), 092102 (2013). [CrossRef]
17. A. Kafar, S. Stanczyk, P. Wisniewski, T. Oto, I. Makarowa, G. Targowski, T. Suski, and P. Perlin, “Design and optimization of InGaN superluminescent diodes,” Phys. Status Solidi., A Appl. Mater. Sci. 212(5), 997–1004 (2015). [CrossRef]
18. U. T. Schwarz, H. Braun, K. Kojima, M. Funato, Y. Kawakami, S. Nagahama, and T. Mukai, “Investigation and comparison of optical gain spectra of (Al,In)GaN laser diodes emitting in the 375 nm to 470 nm spectral range,” Proc. SPIE 6485, 648506 (2007). [CrossRef]
19. M. Goano, E. Bellotti, E. Ghillino, C. Garetto, G. Ghione, and K. F. Brennan, “Band structure nonlocal pseudopotential calculation of the III-nitride wurtzite phase materials system. Part II. Ternary alloys AlxGa1−xN, InxGa1−xN, and InxAl1−xN,” J. Appl. Phys. 88(11), 6476–6482 (2000). [CrossRef]
20. M. Sarzynski, T. Suski, G. Staszczak, A. Khachapuridze, J. Z. Domagala, R. Czernecki, J. Plesiewicz, J. Pawlowska, S. P. Najda, M. Bockowski, P. Perlin, and M. Leszczynski, “Lateral control of indium content and wavelength of III–nitride diode lasers by means of GaN substrate patterning,” Appl. Phys. Express 5(2), 021001 (2012). [CrossRef]
21. M. Sarzynski, M. Leszczynski, M. Krysko, J. Z. Domagala, R. Czernecki, and T. Suski, “Influence of GaN substrate off-cut on properties of InGaN and AlGaN layers,” Cryst. Res. Technol. 47(3), 321–328 (2012). [CrossRef]
22. R. Dwilinski, R. Doradzinski, J. Garczynski, L. P. Sierzputowski, A. Puchalski, Y. Kanbara, K. Yagi, H. Minakuchi, and H. Hayashi, “Excellent crystallinity of truly bulk ammonothermal GaN,” J. Cryst. Growth 310(17), 3911–3916 (2008). [CrossRef]
23. S. Staczyk, T. Czyszanowski, A. Kafar, J. Goss, S. Grzanka, E. Grzanka, R. Czernecki, A. Bojarska, G. Targowski, M. Leszczyski, T. Suski, R. Kucharski, and P. Perlin, “Graded-index separate confinement heterostructure InGaN laser diodes,” Appl. Phys. Lett. 103(26), 261107 (2013). [CrossRef]
24. G. A. Alphonse, “Design of high-power superluminescent diodes with low spectral modulation,” Proc. SPIE 4648, 125–138 (2002). [CrossRef]
25. B. W. Hakki and T. L. Paoli, “Gain spectra in GaAs double-heterostructure injection lasers,” J. Appl. Phys. 46(3), 1299–1306 (1975). [CrossRef]
26. P. Perlin, G. Franssen, J. Szeszko, R. Czernecki, G. Targowski, M. Krysko, S. Grzanka, G. Nowak, E. Litwin-Staszewska, R. Piotrzkowski, M. Leszczynski, B. Łucznik, I. Grzegory, R. Jakieła, M. Albrecht, and T. Suski, “Nitride-based quantum structures and devices on modified GaN substrates,” Phys. Status Solidi., A Appl. Mater. Sci. 206(6), 1130–1134 (2009). [CrossRef]
