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GaN-based VCSELs
GaN-Based wide band-gap materials have been widely used in the fabrication of light-emitting devices such as light-emitting diodes (LEDs), and laser diodes (LDs). The edge-emitting type GaN-based LDs were reported under continuous-wave (CW) operation at room-temperature (RT) by Nakamura et al. in 1996. Since then, the edge-emitting type GaN-based LDs have become prevalent as dominant light sources in the high-density optical storage system. On the other hand, the GaN-based vertical-cavity surface-emitting lasers (VCSELs) have attracted much attention because of their superior characteristics such as low threshold current, single longitudinal mode operation, symmetric and low divergence angle, and two dimensional array capability. These superior characteristics inherited in GaN-based VCSELs have many applications such as high density optical storage system, laser printing, laser mouse and micro- or pico-projectors. However, there are still many challenges to realize a GaN-based VCSEL.
The key issues limiting the development of GaN-based VCSELs are the lattice mismatch between GaN and sapphire substrates, the difficulty in growing high-quality and high-reflectivity GaN-based distributed Bragg reflectors (DBRs) and low optical gain exhibited in InGaN multiple quantum wells (MQWs) due to the complicated issues such as built-in polarization field in the quantum well, seriously broadening linewidth due to the phase separation with In-rich clusters and the carrier leakage out of the active regions.
There have been many research groups focused on the growth and fabrication of GaN-based VCSEL. In 1996, Redwing et al reported the optical pumping results of GaN-based VCSEL structure at room temperature for the first time. Their VCSEL structure consisted of all epitaxially grown layers, including a 10 μm GaN active region and 30-period Al0.4Ga0.6N/Al0.12Ga0.88N-based top and bottom mirror. The threshold pumping energy was as high as 2.0 MW/cm2 due to the low mirror reflectivity of about 84%~93% and the thick GaN active layer. Subsequently, the In0.1Ga0.9N VCSEL was fabricated and observed lasing action at 77K by Arakawa et al. in 1998. The structure was constructed by a hybrid type cavity consisting of an In0.1Ga0.9N active layer grown on a 35-pair Al0.34Ga0.66N/GaN-based bottom DBR with the reflectivity of 97%, and a 6-pair TiO2/SiO2 dielectric top DBR with the reflectivity of 98%. In addition, Song et al. using laser lift-off technology demonstrated the dielectric type VCSEL structure consisting of 10-pair SiO2/HfO2 top and bottom dielectric DBRs and InGaN MQWs. The cavity quality factor was as large as 600 due to the high reflectivity of the top and bottom DBRs exceeding 99.5% and 99.9%, respectively. Moreover, the room temperature (RT) lasing at blue wavelength in the hybrid type GaN-based VCSEL under optical pumping has been reported by Someya et al. in 1999. The structure was formed by an InGaN MQW sandwiched between nitride-based and oxide-based DBRs. Thereafter, Carlin et al. reported the crack-free fully epitaxial nitride microcavity in 2005. The cavity was formed by lattice-matched AlInN/GaN-based DBR with reflectivity up to 99%. We also reported optically pumped GaN-based VCSEL structures in dielectric type cavity and hybrid type cavity structures.
The electrically-pumped GaN-based VCSEL has not been reported until 2008 by our group. We first demonstrated electrically-pumped GaN-based VCSEL with hybrid DBRs at 77K. The GaN-based VCSEL structure consisted of a 10-pair InGaNGaN MQW active layer embedded in a GaN hybrid microcavity of 5λ optical thickness and sandwiched between an epitaxial AlNGaN DBR and a Ta2O5SiO2 dielectric DBR with reflectivity 99.4% and 99%, respectively. A 240 nm Indium-tin-oxide (ITO) was deposited on top of the aperture to serve as the transparent contact layer. CW laser action was achieved at a threshold injection current of 1.4 mA in a 10-m aperture at 77 K. The laser emitted a blue wavelength at 462 nm with a narrow linewidth of about 0.15 nm. In the same year, Higuchi et al. demonstrated CW lasing GaN-based VCSEL at room temperature. The structure of the VCSEL consisted of top and bottom dielectric SiO2/Nb2O5 DBRs and a two-pair InGaN/GaN quantum well active layer. The optical cavity consisted of a 7λ-thick GaN semiconductor layer. A 50-nm ITO layer was deposited as a p-type ohmic contact and current spreading layer. The laser lift-off (LLO) technique was used to remove the sapphire substrate, and GaN-based VCSEL was mounted on a Si substrate by wafer bonding. The threshold current was 7.0 mA for an 8 μm aperture device and the emission wavelength was approximately 414 nm. In 2009, the same group further improved the lasing characteristics of GaN-based VCSEL fabricated by using GaN substrates. The results showed a higher maximum output power and a longer lifetime than that fabricated using a sapphire substrate due to the reduction of dislocation density. In 2010, we demonstrated the GaN-based VCSEL with a hybrid cavity under current injection operating at room temperature with the lasing wavelength of 412 nm.
However, there are still many issues of VCSEL structure to be further improved. The crystal quality of the active layer has to be refined to reduce the nonradiative recombination channels. By using free standing GaN substrate or applying superlattice layers before the growth of InGaN MQWs could effectively reduce the defect density to enhance the GaN VCSEL performance. On the other hand, the effective current injection is required for the low threshold current operation. Since the current leakage paths can be vertically passing through the InGaN MQW and/or horizontally extending out of the current aperture. Although we have applied AlGaN electron blocking layer in the GaN-based VCSEL structure, the strain-induced polarization field could adversely lower the barrier height, which the leakage current could still occur in the vertical current path. As a result, introduction of polarization-matched InGaN/AlInGaN MQWs could independently control the interface polarization charges and bandgap, which can help to design a preferable electron blocking layer. As for the effective current injection in the horizontal direction, the current aperture should get close to the InGaN MQW as much as better. In addition, the present GaN-based VCSEL structure requires transverse optical confinement to further reduce the threshold current and properly control the laser beam quality. This could be rather challenging because oxidation of the GaN-based material remains difficult. Nonetheless, a properly designed transverse optical confinement layer can combine the function of the current aperture to realize a low threshold high efficiency GaN-based VCSEL.
