Although the optimization of the edge emitting laser keeps going, some properties of this kind of laser are unfavorable. One of those properties is its elliptic beam shape. On one hand, the coupling efficiency would be low as the elliptic beam is coupled into optical fiber (typically in the form of circular core). On the other hand, for the application of storage, the elliptic beam not only makes each writing pixel larger but also raises expenses for correcting light shape. Usually, this kind of laser shows slightly large divergence angle to be over ten degree. This also is disadvantageous to the projection. Furthermore, the side emitting laser devices also makes the testing of devices a tough task. The wafer should be cut into several stripes (several laser devices on one strip) before the testing. For a commercial product, the complicated testing would result in a poor producing efficiency and be disadvantageous. Therefore, in order to have a superior laser device, K. Iga demonstrated a new kind of laser diodes, vertical cavity surface emitting laser, in 1977. Vertical cavity surface emitting laser (VCSEL) is a vertical-emitting-type laser.
It is formed by sandwiching a few-lambda cavity in a pair of reflectors, usually in the
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form of distributed Bragg reflector (DBR), with a very high reflectivity (>99.9%) as shown in Fig. 1.2. In contrast to EELs, photons in the cavity of VCSEL are vertically in resonance and emit outside perpendicularly to the surface of the structure. This laser diode can have many advantageous properties than conventional edge emitting laser, such as circular beam shape, lower divergence angle, two-dimensional laser array possible, efficient testing, low threshold, and so on. Owing to these superior performances, VCSELs had become very attractive and started to be applied to the commercial products at long wavelength range. In fact, short-wavelength VCSELs are also very promising for the applications of storage, display, and projection. In particular, the use of two-dimensional arrays of blue VCSELs could further reduce the read-out time in high density optical storage and increase the scan speed in high-resolution laser printing technology. In recent years, several efforts have been devoted to the realization of nitride-based VCSELs. In recent years, several efforts have been devoted to the realization of nitride-based VCSELs [8-17].
Fig. 1.2 The schematic diagram of a vertical-cavity surface emitting laser diode.
1.2.1 Fully Epitaxial Grown VCSELs
In 2005, J. F. Carlin [17] and E. Feltin [18] demonstrated the wholly epitaxial and high quality nitride-based micro-cavity (as shown in Fig. 1.3(a)) using
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metalorganic vapor phase epitaxy (MOVPE or MOCVD). They used the lattice-matched AlInN/GaN as the bottom and top reflectors to avoid cracks happened due to the accumulation of the strain after stacking large pairs of layers.
The reflectivity of AlInN/GaN could be achieved as high as 99.4%. They showed the 3/2-lambda cavity emitted a very narrow emission with a linewidth of 0.52 nm, corresponding to a quality factor of ~800.
1.2.2 VCSELs with Two Dielectric Mirrors
Compared to epitaxial grown reflectors, the fabrication of dielectric mirrors is relatively simple. Furthermore, the large index difference of dielectric mirrors makes them could easily have wide stop band (>50nm) and high reflectivity (>99%) by coating just few stacks of 1/4-lambda-thick layers. Therefore, using dielectric mirrors to accomplish nitride-based VCSELs had begun attractive. Song et al. [9], Tawara et al. [10] and J. T. Chu et. al [12]successively reported the structure (as shown in Fig. 1.3(c)) after 2000. They employed some process techniques such as wafer bonding and laser lift-off to make dielectric mirrors be coated onto both sides of nitride-based cavity. They showed a micro-cavity could have a very high quality factor to be greater than 400 and achieve lasing action using optical pumping. In addition, Takashi Mukai et al. [13] have demonstrated the CW lasing at room temperature in a GaN-based vertical-cavity surface-emitting laser (VCSEL) by current injection in 2008. Its optical cavity consisted of a 7λ-thick GaN semiconductor layer and an indium tin oxide layer for p-contact embedded between two SiO2/Nb2O5 dielectric distributed Bragg reflectors. The threshold current of VCSEL is 13.9kA/cm2 and the lasing wavelength is about 414 nm. However, the fabrication technique of this kind of VCSEL is relatively complicated, and its cavity
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length cannot be efficiently controlled due to polishing problems.
1.2.3 VCSELs with Hybrid Mirrors
The so-called hybrid mirrors are a combination of two different kinds of reflectors, for example, a dielectric mirror and an epitaxial reflector. Typically, the fabrication of this structure is to grow bottom reflector and cavity using MOCVD and then coat dielectric mirror to complete VCSEL structure (as shown in Fig.
1.3(b)). The hybrid-cavity nitride-based VCSEL\formed by the dielectric mirror and the epitaxially grown high-reflectivity GaN/AlxGa1-xN DBR was reported earlier. In 1999, Someya et al. [8] used 43 pairs of Al0.34Ga0.66N/GaN as the bottom DBR and reported the lasing action at ~400nm. Then, Zhouet al. [11]also employed a bottom DBR of 60 pairs Al0.25Ga0.75N/GaN and observed the lasing action at 383.2nm. Both these AlGaN/GaN DBR structures required large numbers of pairs due to the relatively low refractive index contrast between AlxGa1-xN and GaN. Therefore, recently some groups began to study the AlN/GaN for application in nitride VCSEL.
The DBR structure using AlN/GaN has higher refractive index contrast (Δn/n=0.16) [19]that can achieve high reflectivity with relatively less numbers of pairs. It has wide stop band that can easily align with the active layer emission peak to achieve lasing action. However, the AlN/GaN combination also has relatively large lattice mismatch (~2.4%) and the difference in thermal expansion coefficients between GaN (5.59x10-6/K) and AlN (4.2x10-6/K) that tends to cause cracks in the epitaxial film during the growth of the AlN/GaN DBR structure and could result in the reduction of reflectivity and increase in scattering loss. With the mature of epitaxy techniques, the high-reflectivity AlN/GaN DBR structure with relatively smooth surface morphology has become possible with just twenty or thirty pairs [20]. In
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comparison of these three VCSELs, it doesn’t require complicated process such as laser lift-off technique to complete a hybrid VCSEL device. This means the fabrication of such structure is stable and reliable comparing to other structures.
Thus, the hybrid structure is more advantageous in the aspects of fabrication and commercialization In fact, the investigation of the characteristics of the GaN-based VCSELs has gradually attracted more attentions. Kako et al. [21]investigated the coupling efficiency of spontaneous emission (β) and the polarization property of the nitride VCSEL and obtained a high β value of 1.6×10-2 and a strong linear polarization of 98%. Tawara et al. [10]also found a high β value of 10-2 in the nitride VCSEL with two dielectric mirrors. Honda et al. reported the estimation of high characteristics temperature of GaN-based VCSEL [22]. These all mean the development of nitride-based VCSEL and the understanding of the laser performance has become more and more important.
Fig. 1.3 The schematic diagram of three nitride based VCSELs structures