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Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 3 — Feb. 11, 2013
  • pp: 3133–3137

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A 340-nm-band ultraviolet laser diode composed of GaN well layers

Yoji Yamashita, Masakazu Kuwabara, Kousuke Torii, and Harumasa Yoshida  »View Author Affiliations


Optics Express, Vol. 21, Issue 3, pp. 3133-3137 (2013)
http://dx.doi.org/10.1364/OE.21.003133


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Abstract

We have demonstrated the laser operation of a short-wavelength ultraviolet laser diode with multiple-quantum-wells composed of GaN well layers. The laser action has been achieved in 340-nm-band far from the wavelength corresponding to GaN band gap under the pulsed current mode at room temperature. The device has been realized on the Al0.2Ga0.8N underlying layer. The AlN mole fraction of the underlying layer is 0.1 lower than that of the underlying layer which was used for the previously reported 342 nm laser diode. These results provide a chance to the next step for a shorter-wavelength ultraviolet laser diode.

© 2013 OSA

1. Introduction

Nitride-based ultraviolet laser diodes have considerably interest for a number of applications including chemical analysis, medical devices, bio-agent detection and material processing. The laser diodes promise higher power, high efficiency, smaller size, and less cost than currently used bulky ultraviolet lasers. In nitride-based light emitting diodes (LEDs), the emission wavelengths were already expanded down to 210 nm, and high external quantum efficiency of over 10% was achieved even at a short-wavelength of 278 nm [1

Y. Taniyasu, M. Kasu, and T. Makimoto, “An aluminium nitride light-emitting diode with a wavelength of 210 nanometres,” Nature 441(7091), 325–328 (2006). [CrossRef] [PubMed]

,2

M. Shatalov, W. Sun, A. Lunev, X. Hu, A. Dobrinsky, Y. Bilenko, J. Yang, M. Shur, R. Gaska, C. Moe, G. Garrett, and M. Wraback, “AlGaN deep-ultraviolet light-emitting diodes with external quantum efficiency above 10%,” Appl. Phys. Express 5(8), 082101 (2012). [CrossRef]

]. By optical pumping, laser actions of nitride materials were also demonstrated in 200-nm-band [3

H. Kawanishi, M. Senuma, and T. Nukui, “Anisotropic polarization characteristics of lasing and spontaneous surface and edge emissions from deep-ultraviolet (λ ≈ 240 nm) AlGaN multiple-quantum-well lasers,” Appl. Phys. Lett. 89(4), 041126 (2006). [CrossRef]

–5

T. Wunderer, C. L. Chua, Z. Yang, J. E. Northrup, N. M. Johnson, G. A. Garrett, H. Shen, and M. Wraback, “Pseudomorphically grown ultraviolet c photopumped lasers on bulk AlN substrates,” Appl. Phys. Express 4(9), 092101 (2011). [CrossRef]

]. In contrast, progress in developing short-wavelength ultraviolet laser diodes has been still limited in 330-nm-band, because laser diodes require a more complex structure, thicker layers and higher crystalline quality than LEDs [6

S. Masui, Y. Matsuyama, T. Yanamoto, T. Kozaki, S. Nagahama, and T. Mukai, “365 nm ultraviolet laser diodes composed of quaternary AlInGaN alloy,” Jpn. J. Appl. Phys. 42(Part 2, No. 11A), L1318– L1320 (2003). [CrossRef]

–18

H. Yoshida, M. Kuwabara, Y. Yamashita, K. Uchiyama, and H. Kan, “The current status of ultraviolet laser diodes,” Phys. Status Solidi A 208(7), 1586–1589 (2011). [CrossRef]

]. In order to shift in the lasing wavelength toward shorter ultraviolet wavelength, more effective carrier recombination in active region and the laser-diode structure composed of relatively low AlN mole fractions in AlGaN layers are indispensable.

Due to the strong internal electric field in AlGaN heterostructure, the spatial separation between electron and hole wavefunctions leads to the decrease of electron-hole recombination probability. Narrowing the well width in quantum well is expected to be an effective way to increase the wavefunctions overlap. In addition, that leads to the shift of the emission wavelength toward shorter wavelength region as a secondary effect. Hirayama et al. reported the effects of ultraviolet LEDs [19

H. Hirayama, N. Noguchi, T. Yatabe, and N. Kamata, “227 nm AlGaN light-emitting diode with 0.15 mW output power realized using a thin quantum well and AlN buffer with reduced threading dislocation density,” Appl. Phys. Express 1, 051101 (2008). [CrossRef]

].

AlGaN layers with higher AlN mole fractions are favorable for the layer structure design of short-wavelength ultraviolet laser diodes. The AlGaN layers with relatively high AlN mole fraction were used for the underlying and the cladding layers of the previously reported ultraviolet laser-diodes [9

H. Yoshida, Y. Takagi, M. Kuwabara, H. Amano, and H. Kan, “Entirely crack-free ultraviolet GaN/AlGaN laser diodes grown on 2-in. sapphire substrate,” Jpn. J. Appl. Phys. 46(9A), 5782–5784 (2007). [CrossRef]

, 12

H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, “A 342-nm ultraviolet AlGaN multiple-quantum-well laser diode,” Nat. Photonics 2(9), 551–554 (2008). [CrossRef]

, 13

H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, “Demonstration of an ultraviolet 336 nm AlGaN multiple-quantum-well laser diode,” Appl. Phys. Lett. 93(24), 241106 (2008). [CrossRef]

]. Generally, the output power of ultraviolet laser diodes and LEDs decrease with the increase of AlN mole fractions in the AlGaN layers. Because the AlGaN layers have serious problems due to lack of suitably lattice-matched substrates. The growth of AlGaN layers with higher AlN mole fractions on lattice-mismatched substrates causes dislocations and eventually crack-generation because of tensile strain and thermal mismatch. Moreover the increase of the AlN mole fractions in p-type AlGaN layers leads to the decrease of hole concentration, due to the poor crystalline quality and the high activation energy of Mg-acceptor.

From these points of view, it is important to demonstrate the laser operation of ultraviolet laser diodes in which the quantum well consists of quite narrow well width and/or AlN mole fractions of AlGaN layers are relatively low. In this paper, we report on the laser operation of a short-wavelength ultraviolet laser diode with multiple-quantum-wells (MQWs) composed of GaN well layers. The laser operation has been achieved in 340-nm-band far from the wavelength corresponding to GaN band gap under the pulsed current mode at room temperature. This record is the shortest lasing wavelength ever reported for a semiconductor laser composed of binary compound well layer. Moreover, the device has been realized on the Al0.2Ga0.8N underlying layer. The AlN mole fraction of the underlying layer is 0.1 lower than that of the underlying layer of the previously reported 342 nm laser diode [12

H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, “A 342-nm ultraviolet AlGaN multiple-quantum-well laser diode,” Nat. Photonics 2(9), 551–554 (2008). [CrossRef]

]. We believe that these results provide a potential breakthrough to next stage for a shorter-wavelength ultraviolet laser diode.

2. Experimental

Figure 1 shows a schematic illustration of the layer structure of the ultraviolet laser diode. Material growth was carried out by metalorganic vapor phase epitaxy. The laser structure is similar to the previously reported ones [12

H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, “A 342-nm ultraviolet AlGaN multiple-quantum-well laser diode,” Nat. Photonics 2(9), 551–554 (2008). [CrossRef]

, 13

H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, “Demonstration of an ultraviolet 336 nm AlGaN multiple-quantum-well laser diode,” Appl. Phys. Lett. 93(24), 241106 (2008). [CrossRef]

, 16

H. Yoshida, M. Kuwabara, Y. Yamashita, K. Uchiyama, and H. Kan, “Radiative and nonradiative recombination in an ultraviolet GaN/AlGaN multiple-quantum-well laser diode,” Appl. Phys. Lett. 96(21), 211122 (2010). [CrossRef]

]. An Al0.2Ga0.8N underlying layer was grown on a c-sapphire substrate by a hetero-facet-controlled epitaxial lateral overgrowth technique [20

K. Hiramatsu, K. Nishiyama, M. Onishi, H. Mizutani, M. Narukawa, A. Motogaito, H. Miyake, Y. Iyechika, and T. Maeda, “Fabrication and characterization of low defect density GaN using facet-controlled epitaxial lateral overgrowth (FACELO),” J. Cryst. Growth 221(1-4), 316–326 (2000). [CrossRef]

, 21

S. Kamiyama, M. Iwaya, S. Takanami, S. Terao, A. Miyazaki, H. Amano, and I. Akasaki, “UV light-emitting diode fabricated on hetero-ELO-grown Al0.22Ga0.78N with low dislocation density,” Phys. Status Solidi A 192(2), 296–300 (2002). [CrossRef]

]. The laser structure consists of a 2.8-μm-thick n-Al0.2Ga0.8N n-contacting layer, a 0.6-μm-thick n-Al0.2Ga0.8N cladding layer, a 120-nm-thick Al0.1Ga0.9N guiding layer, GaN/Al0.1Ga0.9N MQWs, a 120-nm-thick Al0.1Ga0.9N guiding layer, an AlGaN electron-blocking layer, a 0.5-μm-thick p-Al0.2Ga0.8N cladding layer, and a 25-nm-thick p-GaN contacting layer.

Fig. 1 Schematic illustration of the layer structure of ultraviolet laser diode.

The laser stripes with a width of 3 μm were defined by lithography. Both of the formation of ridge structure and the exposure of n-contacting layer were carried out by dry etching. Au and Al based contacting pads were deposited onto the p- and n-contacting layers, respectively. Cavity facets were also formed by dry etching for the laser diodes with a cavity length of 500 μm. No reflective coating was employed for the cavity facets. The GaN well width was estimated to be around 1 nm from the growth rate, the photoluminescence measurement, and the theoretical calculation of a potential well model. On the negative side, the optical confinement of the laser diodes are relatively lower than that of the previously reported ones, because of the narrower well width despite the same number of quantum wells in the MQWs.

3. Results and discussion

Figure 2 shows a series of lasing spectra of the device. These spectra were measured under the pulsed current mode with a pulse duration of 10 ns and a repetition frequency of 5 kHz at room temperature. The device starts lasing at a threshold current of approximately 700 mA and exhibits sharp lasing emissions around a wavelength of 345 nm above the threshold.We have realized laser operation of the laser diode composed of GaN well layers in 340-nm-band, the shortest wavelength ever reported for a semiconductor laser diode with binary compound well layer. We already reported the GaN/AlGaN MQW laser diodes with the similar layer structure, but with a wider GaN well width of 3 nm [9

H. Yoshida, Y. Takagi, M. Kuwabara, H. Amano, and H. Kan, “Entirely crack-free ultraviolet GaN/AlGaN laser diodes grown on 2-in. sapphire substrate,” Jpn. J. Appl. Phys. 46(9A), 5782–5784 (2007). [CrossRef]

]. Those previous laser diodes lased in the peak wavelength range from 355.4 to 361.6 nm under a 100-ns-pulse condition at room temperature. There is a clear correlation between the emission wavelength and the well width. The relation of the wavelength and the well width approximately agrees with the theoretical calculation of the simple potential well model. At a current of 914 mA, the device exhibits the sharp spectrum with a full-width at half-maximum of 0.5 nm, which is slightly larger than the resolution limit of the spectrometer. The emission spectrum is thought to be a multimode one. The peak wavelength shift of 0.7 nm above the threshold is comparable to that of the previously reported 342 nm laser diode [12

H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, “A 342-nm ultraviolet AlGaN multiple-quantum-well laser diode,” Nat. Photonics 2(9), 551–554 (2008). [CrossRef]

]. The wavelength shift is caused principally by the self-heating, the band-gap renormalization, the band filling (Burstein-Moss shift), and the screening of quantum-confined Stark effect (QCSE). The remarkable blue-shift tendencies were observed in the polar (c-plane) nitride-based laser diodes compared to the non-polar (m-plane) ones [22

M. Kubota, K. Okamoto, T. Tanaka, and H. Ohta, “Continuous-wave operation of blue laser diodes based on nonpolar m-plane gallium nitride,” Appl. Phys. Express 1(1), 011102 (2008). [CrossRef]

, 23

K. Okamoto, T. Tanaka, and M. Kubota, “High-efficiency continuous-wave operation of blue-green laser diodes based on nonpolar m-plane gallium nitride,” Appl. Phys. Express 1, 072201 (2008). [CrossRef]

]. In the case of polar nitride-based laser diodes, due to spontaneous and piezoelectric polarizations, the QCSE is thought to be the dominant effect on the shift to shorter wavelength. Similarly in this device, a relatively strong screening of the internal electric field by free carriers can be anticipated around the threshold even in the narrower well width.

Fig. 2 Series of lasing spectra of the device operating above threshold.

The corresponding threshold current density is estimated to be 47 kA/cm2, which is still higher than 8.7 kA/cm2 of the previously reported 342 nm laser diode. The threshold current density is sensitive to multiple factors including, but not limited to, the carrier recombination rate, the optical confinement in the active region, the reflectivity and/or the surface profile of cavity mirrors, and the ridge-waveguide geometry. It would be considered that the high threshold current density is mainly due to decreased optical and carrier confinements in the narrow wells.

Figure 3 shows transverse electric (TE) and transverse magnetic (TM) spectra operating above the threshold. Excellent TE/TM ratio has been also confirmed due to the dominance of TE optical gain in the GaN/AlGaN MQWs as well as the high reflectivity for the TE mode by the cavity facets [3

H. Kawanishi, M. Senuma, and T. Nukui, “Anisotropic polarization characteristics of lasing and spontaneous surface and edge emissions from deep-ultraviolet (λ ≈ 240 nm) AlGaN multiple-quantum-well lasers,” Appl. Phys. Lett. 89(4), 041126 (2006). [CrossRef]

, 12

H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, “A 342-nm ultraviolet AlGaN multiple-quantum-well laser diode,” Nat. Photonics 2(9), 551–554 (2008). [CrossRef]

, 13

H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, “Demonstration of an ultraviolet 336 nm AlGaN multiple-quantum-well laser diode,” Appl. Phys. Lett. 93(24), 241106 (2008). [CrossRef]

]. The light output power and the other detail characteristics will be described later [24

M. Kuwabara, Hamamatsu Photonics K.K., 5000 Hirakuchi, Hamakita-ku, Hamamatsu 434–8601, Japan, Y. Yamashita, K. Torii, and H. Yoshida are preparing a manuscript to be called “Laser operation of nitride laser diodes with GaN well layer in 340 nm band.”

].

Fig. 3 Transverse electric (TE) and transverse magnetic (TM) spectra operating above threshold.

By increasing the AlN mole fraction in each layer of the laser diodes, it would be possible to shift the lasing wavelength into shorter wavelength region. Based on the results that have been demonstrated in this work, we expect further shift in the lasing wavelength of the previous reported 336 nm laser diode [13

H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, “Demonstration of an ultraviolet 336 nm AlGaN multiple-quantum-well laser diode,” Appl. Phys. Lett. 93(24), 241106 (2008). [CrossRef]

] into 320-nm-band or shorter wavelength band by applying both the narrower well width and the AlN mole fraction margin.

4. Summary

We report on the laser operation of a short-wavelength ultraviolet laser diode with MQWs composed of GaN well layers. The laser operation has been achieved in 340-nm-band far from the wavelength corresponding to GaN band gap under the pulsed current mode at room temperature. This record is the shortest lasing wavelength ever reported for a semiconductor laser composed of not only the GaN well layer but also the binary compound well layer. Moreover, the device has been realized on the Al0.2Ga0.8N underlying layer. The AlN mole fraction of the underlying layer is 0.1 lower than that of the underlying layer of previously reported 342 nm laser-diode. The device configuration provides another approach toward shorter ultraviolet wavelength of nitride-based laser diodes with the difficulty of higher AlN mole fractions.

References and links

1.

Y. Taniyasu, M. Kasu, and T. Makimoto, “An aluminium nitride light-emitting diode with a wavelength of 210 nanometres,” Nature 441(7091), 325–328 (2006). [CrossRef] [PubMed]

2.

M. Shatalov, W. Sun, A. Lunev, X. Hu, A. Dobrinsky, Y. Bilenko, J. Yang, M. Shur, R. Gaska, C. Moe, G. Garrett, and M. Wraback, “AlGaN deep-ultraviolet light-emitting diodes with external quantum efficiency above 10%,” Appl. Phys. Express 5(8), 082101 (2012). [CrossRef]

3.

H. Kawanishi, M. Senuma, and T. Nukui, “Anisotropic polarization characteristics of lasing and spontaneous surface and edge emissions from deep-ultraviolet (λ ≈ 240 nm) AlGaN multiple-quantum-well lasers,” Appl. Phys. Lett. 89(4), 041126 (2006). [CrossRef]

4.

M. Shatalov, M. Gaevski, V. Adivarahan, and A. Khan, “Room-temperature stimulated emission from AlN at 214 nm,” Jpn. J. Appl. Phys. 45(49), L1286– L1288 (2006). [CrossRef]

5.

T. Wunderer, C. L. Chua, Z. Yang, J. E. Northrup, N. M. Johnson, G. A. Garrett, H. Shen, and M. Wraback, “Pseudomorphically grown ultraviolet c photopumped lasers on bulk AlN substrates,” Appl. Phys. Express 4(9), 092101 (2011). [CrossRef]

6.

S. Masui, Y. Matsuyama, T. Yanamoto, T. Kozaki, S. Nagahama, and T. Mukai, “365 nm ultraviolet laser diodes composed of quaternary AlInGaN alloy,” Jpn. J. Appl. Phys. 42(Part 2, No. 11A), L1318– L1320 (2003). [CrossRef]

7.

K. Iida, T. Kawashima, A. Miyazaki, H. Kasugai, S. Mishima, A. Honshio, Y. Miyake, M. Iwaya, S. Kamiyama, H. Amano, and I. Akasaki, “350.9 nm UV laser diode grown on low-dislocation-density AlGaN,” Jpn. J. Appl. Phys. 43(No. 4A), L499– L500 (2004). [CrossRef]

8.

J. Edmond, A. Abare, M. Bergman, J. Bharathan, K. L. Bunker, D. Emerson, K. Haberern, J. Ibbetson, M. Leung, P. Russel, and D. Slater, “High efficiency GaN-based LEDs and lasers on SiC,” J. Cryst. Growth 272(1-4), 242–250 (2004). [CrossRef]

9.

H. Yoshida, Y. Takagi, M. Kuwabara, H. Amano, and H. Kan, “Entirely crack-free ultraviolet GaN/AlGaN laser diodes grown on 2-in. sapphire substrate,” Jpn. J. Appl. Phys. 46(9A), 5782–5784 (2007). [CrossRef]

10.

M. Kneissl, Z. Yang, M. Teepe, C. Knollenberg, O. Schmidt, P. Kiesel, N. M. Johnson, S. Schujman, and L. J. Schowalter, “Ultraviolet semiconductor laser diodes on bulk AlN,” J. Appl. Phys. 101(12), 123103 (2007). [CrossRef]

11.

K. Kojima, U. T. Schwarz, M. Funato, Y. Kawakami, S. Nagahama, and T. Mukai, “Optical gain spectra for near UV to aquamarine (Al,In)GaN laser diodes,” Opt. Express 15(12), 7730–7736 (2007). [CrossRef] [PubMed]

12.

H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, “A 342-nm ultraviolet AlGaN multiple-quantum-well laser diode,” Nat. Photonics 2(9), 551–554 (2008). [CrossRef]

13.

H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, “Demonstration of an ultraviolet 336 nm AlGaN multiple-quantum-well laser diode,” Appl. Phys. Lett. 93(24), 241106 (2008). [CrossRef]

14.

H. Tsuzuki, F. Mori, K. Takeda, M. Iwaya, S. Kamiyama, H. Amano, I. Akasaki, H. Yoshida, M. Kuwabara, Y. Yamashita, and H. Kan, “Novel UV devices on high-quality AlGaN using grooved underlying layer,” J. Cryst. Growth 311(10), 2860–2863 (2009). [CrossRef]

15.

H. Yoshida, M. Kuwabara, Y. Yamashita, Y. Takagi, K. Uchiyama, and H. Kan, “AlGaN-based laser diodes for the short-wavelength ultraviolet region,” New J. Phys. 11(12), 125013 (2009). [CrossRef]

16.

H. Yoshida, M. Kuwabara, Y. Yamashita, K. Uchiyama, and H. Kan, “Radiative and nonradiative recombination in an ultraviolet GaN/AlGaN multiple-quantum-well laser diode,” Appl. Phys. Lett. 96(21), 211122 (2010). [CrossRef]

17.

K. Nagata, K. Takeda, K. Nonaka, T. Ichikawa, M. Iwaya, T. Takeuchi, S. Kamiyama, I. Akasaki, H. Amano, H. Yoshida, M. Kuwabara, Y. Yamashita, and H. Kan, “Reduction in threshold current density of 355 nm UV laser diodes,” Phys. Status Solidi 8(5), 1564–1568 (2011). [CrossRef]

18.

H. Yoshida, M. Kuwabara, Y. Yamashita, K. Uchiyama, and H. Kan, “The current status of ultraviolet laser diodes,” Phys. Status Solidi A 208(7), 1586–1589 (2011). [CrossRef]

19.

H. Hirayama, N. Noguchi, T. Yatabe, and N. Kamata, “227 nm AlGaN light-emitting diode with 0.15 mW output power realized using a thin quantum well and AlN buffer with reduced threading dislocation density,” Appl. Phys. Express 1, 051101 (2008). [CrossRef]

20.

K. Hiramatsu, K. Nishiyama, M. Onishi, H. Mizutani, M. Narukawa, A. Motogaito, H. Miyake, Y. Iyechika, and T. Maeda, “Fabrication and characterization of low defect density GaN using facet-controlled epitaxial lateral overgrowth (FACELO),” J. Cryst. Growth 221(1-4), 316–326 (2000). [CrossRef]

21.

S. Kamiyama, M. Iwaya, S. Takanami, S. Terao, A. Miyazaki, H. Amano, and I. Akasaki, “UV light-emitting diode fabricated on hetero-ELO-grown Al0.22Ga0.78N with low dislocation density,” Phys. Status Solidi A 192(2), 296–300 (2002). [CrossRef]

22.

M. Kubota, K. Okamoto, T. Tanaka, and H. Ohta, “Continuous-wave operation of blue laser diodes based on nonpolar m-plane gallium nitride,” Appl. Phys. Express 1(1), 011102 (2008). [CrossRef]

23.

K. Okamoto, T. Tanaka, and M. Kubota, “High-efficiency continuous-wave operation of blue-green laser diodes based on nonpolar m-plane gallium nitride,” Appl. Phys. Express 1, 072201 (2008). [CrossRef]

24.

M. Kuwabara, Hamamatsu Photonics K.K., 5000 Hirakuchi, Hamakita-ku, Hamamatsu 434–8601, Japan, Y. Yamashita, K. Torii, and H. Yoshida are preparing a manuscript to be called “Laser operation of nitride laser diodes with GaN well layer in 340 nm band.”

OCIS Codes
(140.3610) Lasers and laser optics : Lasers, ultraviolet
(140.5960) Lasers and laser optics : Semiconductor lasers
(230.5590) Optical devices : Quantum-well, -wire and -dot devices

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: September 27, 2012
Revised Manuscript: December 14, 2012
Manuscript Accepted: December 19, 2012
Published: February 1, 2013

Citation
Yoji Yamashita, Masakazu Kuwabara, Kousuke Torii, and Harumasa Yoshida, "A 340-nm-band ultraviolet laser diode composed of GaN well layers," Opt. Express 21, 3133-3137 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-3-3133


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References

  1. Y. Taniyasu, M. Kasu, and T. Makimoto, “An aluminium nitride light-emitting diode with a wavelength of 210 nanometres,” Nature441(7091), 325–328 (2006). [CrossRef] [PubMed]
  2. M. Shatalov, W. Sun, A. Lunev, X. Hu, A. Dobrinsky, Y. Bilenko, J. Yang, M. Shur, R. Gaska, C. Moe, G. Garrett, and M. Wraback, “AlGaN deep-ultraviolet light-emitting diodes with external quantum efficiency above 10%,” Appl. Phys. Express5(8), 082101 (2012). [CrossRef]
  3. H. Kawanishi, M. Senuma, and T. Nukui, “Anisotropic polarization characteristics of lasing and spontaneous surface and edge emissions from deep-ultraviolet (λ ≈ 240 nm) AlGaN multiple-quantum-well lasers,” Appl. Phys. Lett.89(4), 041126 (2006). [CrossRef]
  4. M. Shatalov, M. Gaevski, V. Adivarahan, and A. Khan, “Room-temperature stimulated emission from AlN at 214 nm,” Jpn. J. Appl. Phys.45(49), L1286– L1288 (2006). [CrossRef]
  5. T. Wunderer, C. L. Chua, Z. Yang, J. E. Northrup, N. M. Johnson, G. A. Garrett, H. Shen, and M. Wraback, “Pseudomorphically grown ultraviolet c photopumped lasers on bulk AlN substrates,” Appl. Phys. Express4(9), 092101 (2011). [CrossRef]
  6. S. Masui, Y. Matsuyama, T. Yanamoto, T. Kozaki, S. Nagahama, and T. Mukai, “365 nm ultraviolet laser diodes composed of quaternary AlInGaN alloy,” Jpn. J. Appl. Phys.42(Part 2, No. 11A), L1318– L1320 (2003). [CrossRef]
  7. K. Iida, T. Kawashima, A. Miyazaki, H. Kasugai, S. Mishima, A. Honshio, Y. Miyake, M. Iwaya, S. Kamiyama, H. Amano, and I. Akasaki, “350.9 nm UV laser diode grown on low-dislocation-density AlGaN,” Jpn. J. Appl. Phys.43(No. 4A), L499– L500 (2004). [CrossRef]
  8. J. Edmond, A. Abare, M. Bergman, J. Bharathan, K. L. Bunker, D. Emerson, K. Haberern, J. Ibbetson, M. Leung, P. Russel, and D. Slater, “High efficiency GaN-based LEDs and lasers on SiC,” J. Cryst. Growth272(1-4), 242–250 (2004). [CrossRef]
  9. H. Yoshida, Y. Takagi, M. Kuwabara, H. Amano, and H. Kan, “Entirely crack-free ultraviolet GaN/AlGaN laser diodes grown on 2-in. sapphire substrate,” Jpn. J. Appl. Phys.46(9A), 5782–5784 (2007). [CrossRef]
  10. M. Kneissl, Z. Yang, M. Teepe, C. Knollenberg, O. Schmidt, P. Kiesel, N. M. Johnson, S. Schujman, and L. J. Schowalter, “Ultraviolet semiconductor laser diodes on bulk AlN,” J. Appl. Phys.101(12), 123103 (2007). [CrossRef]
  11. K. Kojima, U. T. Schwarz, M. Funato, Y. Kawakami, S. Nagahama, and T. Mukai, “Optical gain spectra for near UV to aquamarine (Al,In)GaN laser diodes,” Opt. Express15(12), 7730–7736 (2007). [CrossRef] [PubMed]
  12. H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, “A 342-nm ultraviolet AlGaN multiple-quantum-well laser diode,” Nat. Photonics2(9), 551–554 (2008). [CrossRef]
  13. H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, “Demonstration of an ultraviolet 336 nm AlGaN multiple-quantum-well laser diode,” Appl. Phys. Lett.93(24), 241106 (2008). [CrossRef]
  14. H. Tsuzuki, F. Mori, K. Takeda, M. Iwaya, S. Kamiyama, H. Amano, I. Akasaki, H. Yoshida, M. Kuwabara, Y. Yamashita, and H. Kan, “Novel UV devices on high-quality AlGaN using grooved underlying layer,” J. Cryst. Growth311(10), 2860–2863 (2009). [CrossRef]
  15. H. Yoshida, M. Kuwabara, Y. Yamashita, Y. Takagi, K. Uchiyama, and H. Kan, “AlGaN-based laser diodes for the short-wavelength ultraviolet region,” New J. Phys.11(12), 125013 (2009). [CrossRef]
  16. H. Yoshida, M. Kuwabara, Y. Yamashita, K. Uchiyama, and H. Kan, “Radiative and nonradiative recombination in an ultraviolet GaN/AlGaN multiple-quantum-well laser diode,” Appl. Phys. Lett.96(21), 211122 (2010). [CrossRef]
  17. K. Nagata, K. Takeda, K. Nonaka, T. Ichikawa, M. Iwaya, T. Takeuchi, S. Kamiyama, I. Akasaki, H. Amano, H. Yoshida, M. Kuwabara, Y. Yamashita, and H. Kan, “Reduction in threshold current density of 355 nm UV laser diodes,” Phys. Status Solidi8(5), 1564–1568 (2011). [CrossRef]
  18. H. Yoshida, M. Kuwabara, Y. Yamashita, K. Uchiyama, and H. Kan, “The current status of ultraviolet laser diodes,” Phys. Status Solidi A208(7), 1586–1589 (2011). [CrossRef]
  19. H. Hirayama, N. Noguchi, T. Yatabe, and N. Kamata, “227 nm AlGaN light-emitting diode with 0.15 mW output power realized using a thin quantum well and AlN buffer with reduced threading dislocation density,” Appl. Phys. Express1, 051101 (2008). [CrossRef]
  20. K. Hiramatsu, K. Nishiyama, M. Onishi, H. Mizutani, M. Narukawa, A. Motogaito, H. Miyake, Y. Iyechika, and T. Maeda, “Fabrication and characterization of low defect density GaN using facet-controlled epitaxial lateral overgrowth (FACELO),” J. Cryst. Growth221(1-4), 316–326 (2000). [CrossRef]
  21. S. Kamiyama, M. Iwaya, S. Takanami, S. Terao, A. Miyazaki, H. Amano, and I. Akasaki, “UV light-emitting diode fabricated on hetero-ELO-grown Al0.22Ga0.78N with low dislocation density,” Phys. Status Solidi A192(2), 296–300 (2002). [CrossRef]
  22. M. Kubota, K. Okamoto, T. Tanaka, and H. Ohta, “Continuous-wave operation of blue laser diodes based on nonpolar m-plane gallium nitride,” Appl. Phys. Express1(1), 011102 (2008). [CrossRef]
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  24. M. Kuwabara, Hamamatsu Photonics K.K., 5000 Hirakuchi, Hamakita-ku, Hamamatsu 434–8601, Japan, Y. Yamashita, K. Torii, and H. Yoshida are preparing a manuscript to be called “Laser operation of nitride laser diodes with GaN well layer in 340 nm band.”

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