**1. Introduction**

Photonic crystal (PhC) surface emitting lasers (PCSELs) utilizing Bragg diffraction mechanism have considerable amounts of publication during the past few years1,2,3,4. Such PhC lasers have many excellent advantages to attract the attention especially in controlling the specific lasing modes such as longitudinal and transverse modes, lasing phenomenon over the large area, and narrow divergence beam. Therefore, we fabricated the GaN-based PCSELs devices with AlN/GaN distributed Bragg reflectors (DBR) and analyzed the PhC laser characteristics caused by the surrounding PhC nanostructure. However, there were many theoretical methods calculating the photonic band diagrams and the distribution of electric or magnetic field of the PhC nanostructure in the past few years, such as 2-D plane wave expansion method (PWEM)2,5, finite difference time domain (FDTD)6,7, transfer matrix method, and multiple scattering method (MSM), etc. Many different advantages and limitations occur while using these methods. Therefore, in our case, we applied the MSM and PWEM to calculate the PhC threshold gain and photonic band diagram by using our PCSEL device structure.

In this chapter, the fabrication process of PhC lasers will be introduced in section 2. They can be divided into two parts, the epitaxial growth and the device fabrication. Section 3 will show the the foudamental mode characteristics of PhC laser, such as laser threshold pumping power, far-field pattern, MSM theoretical calculation methods, and divergence angles. Section 4, in the Bragg diffraction mechanism, each PhC band-edge mode is calculated and exhibits other type of wave coupling mechanism. Section 5, the photinc band diagrams of foundamental and high order lasing modes can be observed by the angular-resolved μ-PL (AR μ-PL) system. Comparing with the theoretical calculation resulted by PWEM and the experiment results of photonic band diagrams measured by AR μ-PL, they can be well matched and show the novel PhC characteristics. Besides, the fundamental and high order PhC lasing modes would be calculated in this section.

Angular-Resolved Optical Characteristics and Threshold

**(a)**

**AlN/GaN**

**GaN**

**DBR**

**Cavity**

Fig. 2. (a) OM and (b) cross-sectional TEM images of the as-grown micro-cavity sample.

**2.2 The fabrication process of photonic crystal surface emitting lasers (PCSELs)** 

The PhC nanostructure was fabricated on the epitaxial wafers by the following process steps as shown in Fig. 3. In the beginning, the hard mask SiNx 200 nm was deposited on as-grown samples by PECVD. Then, PMMA layer (150 nm) was spun by spinner and exposed by using E-beam writer to form a soft mask. The pattern on the soft mask was transferred to SiNx film to form the hard mask by using ICP-RIE (Oxford Plasmalab system 100), and then, the PMMA layer was removed by dipping ACE. The pattern on hard mask was transferred to GaN by using ICP-RIE (SAMCO RIE-101PH) to form the PhC layer. In order to remove the hard mask, the sample is dipped in BOE. Finally, the PCSEL devices have been fabricated as shown in Fig. 4. Fig. 5. shows the plane-view (a) and the cross section (b) of SEM images of our PCSELs. Although the hole profiles of PhC nanostructure etched through the MQWs region are not perfect due to the lateral plasma etching by ICP-RIE shown in Fig. 5(b), the PhC nanostrustructure near the sample surface which has smooth

Gain Analysis of GaN-Based 2-D Photonics Crystal Surface Emitting Lasers 5

**1**μ**m**

**SLs**

**(b)**

**As-grown 2-inch wafer**
