Experimental processes

The epitaxial structure of the GaN-based VCSEL was first grown on a (0001)-oriented sapphire substrate by metal organic chemical vapor deposition system.

The structure consists of a 30-nm nucleation layer, 4-µm undoped GaN, a multiple quantum-well (MQW) composed of 10 periods of 5-nm GaN barrier and 3-nm In0.1Ga0.9N well, and 280 nm undoped GaN as shown Fig.3.1. The original epitaxial wafer was cleaved to a size of 1.5×1.5 cm². The backside of the sapphire substrate was polished using diamond slurries in order to reduce scattering of KrF excimer laser during the laser lift-off process. Then a dielectric DBR consisting of 6 pairs of SiO2/TiO2 was evaporated on the top of the grown structure to form a SiO2/TiO2

DBR/InGaN MQW/GaN/sapphire structure. The structure has a reflectivity of 98.3%

at 414 nm measured by a n&k analyzer, as shown in Fig. 3.2. Then, an array of disk-like SiO2/TiO2 DBR mesas with 60 µm in diameter was formed by standard photolithography process and buffer oxide etcher (BOE). The patterned SiO2/TiO2

DBR/InGaN MQW/GaN/sapphire structure was then mounted onto a host fused silica substrate by epoxy bonding processes. The mounted sample was then subjected to a laser lift-off (LLO) process similar to the process we reported earlier [15-16]. A KrF excimer laser at 248 nm was incident on the sapphire substrate to cause the deposition of GaN into gaseous nitrogen and gallium droplets. The average energy density of KrF excimer laser was approximately 600 mJ/cm². Then, the sapphire was separated from the epitaxilly grown structure to form a GaN/InGaN MQW/SiO2/TiO2

DBR/silica substrate configuration. The transferred sample was dipped into HCl solution to remove the residual Ga on the n-GaN. After the residual Ga were removed, the mean surface roughness of the GaN surface measured by atomic force microscopy (AFM) was about 15 nm over a scanned area of 5×5 µm², as shown in Fig. 3.3.

Scanning electron microscope images of the GaN surface before and after HCl dip were shown in Fig. 3.4(a) (b). The GaN surface of the lifted-off structure was then lapped and polished by diamond slurries to assure smooth surface for deposition of the second dielectric DBR. After the polish process, a smoother GaN surface with a mean surface roughness about 1nm over a scanned area of 10×10 µm² was obtained,

than 3nm.

Finally, the second DBR consisting of 8 pairs of SiO2/Ta2O5 was deposited on the top of the polished GaN surface. The reflectivity of the SiO2/Ta2O5 DBR at 414 nm is 97.2% measured by a n&k analyzer, as shown in Fig. 3.6. The complete structure of the GaN VCSEL with two dielectric DBRs is shown in Fig. 3.7(a). Fig.

3.7(b) shows the microscopic top view image of the VCSEL array, the circular disk areas are the location of VCSELs with DBR cavity. Fig. 3.8(a)-(f) show the fabrication steps of the optically pumped GaN- based blue-violet vertical cavity surface emitting laser using wafer bonding and LLO techniques.

3.1.2 Issues of processes for reduction of cavity lengths

The numbers of cavity mode and threshold gain are related to cavity length.

Therefore, the cavity length is demanded to be controlled. ICP etching is a good method to control cavity lengths more exactly than lapping with diamond slurries.

The interface between the GaN layer and sapphire substrate is named as LLO surface when the sapphire is removed from bulk GaN layer by a laser lift-off technique. Fig. 3.9(a) shows the LLO GaN surface after ICP etching without pre-polished by diamond slurries and the etching was conducted under a gas mixture condition of Cl2/Ar = 50/30 standard cubic centimeter min (sccm), the 400W of ICP source power, 40W of bias power and 0.66Pa of chamber pressure for a 1 min etching time. In Fig. 3.9(b), the LLO GaN surface has been lapped by diamond slurries before ICP etching with the same ICP etching recipe. The right part of Fig. 3.9(b) is the region after ICP etching and the left part is the region before ICP etching.

Inspection of Fig. 3.9, the smoother surface (RMS~1nm) of LLO GaN surface with pre-polished was obtained after ICP etching. This is partly because the mean surface roughness (RMS~15nm) of a LLO GaN surface after wet etching is rougher than that (RMS<1nm) of an epitaxial GaN surface. In addition, the LLO GaN surface is a highly defective region due to a GaN buffer layer grown on a sapphire substrate at the low temperature.[17] The existence of defect would cause the higher etching rate of the region near defects and result a rough GaN surface after ICP etching.[18] In order to obtain a smooth GaN surface, it is necessary to remove the GaN buffer layer and to smooth the LLO GaN surface by pre-polished before ICP etching.

In conclusion, after ICP etching, the surface morphology of the sample with

pre-polished is better than that of the sample without pre-polished.

3.2 Laser lift-off technique 3.2.1 GaN Decomposition

In a report by Sun et al. [19] the thermal decomposition of MOCVD grown GaN on r-plane sapphire was found to occur at a temperature of 1000°C in a hydrogen ambient. They reported that the surface of the GaN thin film was totally decomposed leaving only a residual Ga droplet surface, following the equation:

)

Fig. 3.10 shows the equilibrium pressure temperature (P-T) curve for GaN under N2 ambient, determined experimentally by Karpinski et. al.[20] In the recent report [21-23], the critical temperature of GaN decomposition was estimated to be about 1000℃. The recently report [24-25] also shows the calculated P-T curve for GaN as show in Fig. 3.11. The decomposition of GaN→Ga(l)+N2(g) was occurred at a critical temperature of ~1000℃ at 1 atm. The GaN sample after laser irradiation tends to show some material residues such as Ga, and Ga oxide. These residues were then clean up by dilute acid solution such as HCl or H2SO4/H2O2. Besides, structural damage and chemical intermixing resulting from laser processing was minimal and that was confined to approximately the first ≅ 50nm of the resulting material.[26]

3.2.2 KrF excimer laser setup

Fig. 3.12 shows the schematic diagram of the setup for conducting the LLO experiment. A KrF excimer laser (Lambda Physick LPX210) at wavelength of λ=248 nm with pulse width of 25 ns was used for LLO technique. The maximum laser output energy was about 700 mJ. The frequency of laser can be varied from 1 Hz to 100Hz.

The LLO processing beam passed through a optical projection system, and then focuses onto the sample with a square spot size of 1.2×1.2 mm². The samples were placed on the top of working station which can be moved by hand. The decomposition of GaN→Ga(l)+N (g) was occurred at the interface between a GaN layer and a

substrate was easily remove from the LEDs structure by heating the irradiated sample at a Ga melting point of about 30℃.

3.3 Optical measurement instruments