870 IEEE ELECTRON DEVICE LETTERS, VOL. 29, NO. 8, AUGUST 2008 Behaviors of Emission Wavelength Shift in AlInGaN-Based Green Laser Diodes Sung-Nam Lee, H. Y. Ryu, H. S. Paek, J. K. Son, Y. J. Sung, K. S. Kim, H. K. Kim, H. Kim, T. Jang, K. H. Ha, O. H. Nam, and Y. Park Abstract—InGaN quantum-well (QW) green laser diodes (LDs) with an emission wavelength of 483.7 nm were characterized by controlling the injection pulsewidth. The emission wavelength of LDs showed a large blueshift (> 20 nm) of spontaneous emission peak with increasing injection current below the threshold cur- rent. The huge blueshift was ascribed to the deep In localization states and the strong piezoelectric field in the green InGaN QW structure with higher In contents than conventional violet/blue InGaN QWs. However, the lasing wavelength of LDs was slightly redshifted by increasing the injection pulsewidth due to the ther- mal heating effects. Index Terms—Gallium compounds, GaN, laser diode (LD), light-emitting diodes (LEDs), quantum well (QW), semiconductor lasers. B LUE-VIOLET laser diodes (LDs) with InGaN multiple- quantum-well (MQW) structure have been commercial- ized as a light source for next-generation digital versatile disk systems [1]–[3]. Additionally, blue InGaN LDs with an emis- sion wavelength of near 450 nm have been developed for the blue light sources of laser projection display systems and high- resolution printers [3], [4]. However, there are serious problems in achieving full-color laser display systems of smaller size and lower power consumption due to the absence of green LDs that are as small as blue/red LDs. Therefore, GaN-based green LDs should be developed to replace bulk-state lasers, such as Ar-gas and frequency-doubled green lasers. However, in spite of the significant improvement in LD structures with InGaN ac- tive material quality by the optimization of growth conditions, there are still important issues in achieving a lasing wavelength that is longer than 500 nm due to In phase separation, strain- induced piezoelectric field, etc. [5]–[13]. Because of the solid-phase immiscibility in the InGaN–GaN material system, the phase separation was expected to be a Manuscript received April 29, 2008; revised May 23, 2008. The review of this letter was arranged by Editor C. Jagadish. S.-N. Lee, H. S. Paek, J. K. Son, Y. J. Sung, K. S. Kim, H. K. Kim, H. Kim, T. Jang, and Y. Park are with the OS Laboratory, Corporate R&D Institute, Samsung Electro-Mechanics Co., Ltd., Suwon 443-743, Korea (e-mail: snlee@samsung.com; Paek@sm.not; jkson@sm.not; sung@sm.not; kskim@sm.not; hkkim@sm.not; hs20.kim@samsung.com; tjang@sm.not; ypark@sm.not). H. Y. Ryu is with the Department of Physics, Inha University, Incheon 402-751, Korea (e-mail: hanryu@inha.ac.kr). K. H. Ha is with the Semiconductor Device Laboratory, Samsung Advanced Institute of Technology, Suwon 440-600, Korea (e-mail: khha@sm.not). O. H. Nam is with the Department of Nano-Optical Engineering, Korea Polytechnic University, Siheung 429-793, Korea (e-mail: ohnam@kpu.ac.kr). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2008.2001081 significant problem in the growth of a typical InGaN epitaxial layer according to theoretical studies by Ho and Stringfellow [11]. To achieve the green emission, InGaN MQWs should be obtained by higher In content than 20%. Accordingly, In phase separation is easily generated in an InGaN green active layer, resulting in the degradation of emission efficiency and the large blueshift. These are caused by the inhomogeneous emission and the band-filling effect, respectively [12], [13]. The built- in polarization-induced electric field limits the performance of optical devices due to the strong polarization properties of III-nitrides [8]–[10]. Specifically, the spatial separation of the electron and hole wave functions caused by the quantum- confined Stark effect reduces the oscillator strength of transition and recombination efficiency in the InGaN QW structure [9]. In this letter, we demonstrated the AlInGaN-based green LDs with the InGaN single-QW structure by optimizing growth parameters and structure optimizations. LD structures including clad, waveguide, QW, and electron blocking layer were grown on freestanding GaN substrates by metal–organic chemical vapor deposition. The dislocation density of the freestanding GaN substrates was as low as 5 × 10 6 /cm 2 . The active layer consisted of a 20-Å In x Ga 1−x N QW separated by 150-Å In y Ga 1−y N barriers, where the x and y were chosen to be 25% and 2%, respectively, for an emission wavelength of ∼500 nm. To increase the optical confinement factor, an 80-nm-thick In 0.01 Ga 0.99 N double optical confine- ment layer (OCL) structure was introduced around the InGaN single-QW structure. As an electron blocking layer between the upper InGaN OCL and the p-type GaN, AlGaN/GaN mul- tiquantum barriers were grown to efficiently suppress electron overflow to p-type doped layers [3]. LD devices were fabricated as follows: A ridge waveguide LD was fabricated with a width of 2.5 μm and a cavity length of 650 μm. Pd/Ti/Pt/Au and Pd/Ti/Al/Ti ohmic contacts were formed on p-type and n-type GaN, respectively. Mirror facets were prepared by cleaving process, and highly reflective multilayer films were coated on the front facet of 95% and the rear facet of 99%. A schematic structure of the green LD grown on GaN substrate is shown in Fig. 1. The LDs have been characterized under pulse operation modes at room temperature. Fig. 2 showed that the light output power and the operation voltage were plotted as a function of the injection current. The peak wavelength of the lasing emis- sion was observed at 483.7 nm, which was one of the longest lasing wavelength spectra with the single-mode operation of AlInGaN-based LDs, to our knowledge. The threshold current and the threshold voltage were 95 mA and 6.17 V, respectively, 0741-3106/$25.00 © 2008 IEEE Authorized licensed use limited to: Inha University. Downloaded on July 08,2010 at 02:34:20 UTC from IEEE Xplore. Restrictions apply.