Structure design

3.1.1 GaN-based VCSELs with two mirrors

In this study we propose a VCSEL structure consisting two dielectric DBRs and a GaN-based resonant cavity. An epitaxially grown, thick (~4 μm) GaN-based cavity incorporated with InGaN MQWs was separated from the sapphire substrate by using laser lift-off and then embedded the cavity between two dielectric DBRs. By using different dielectric materials with large difference in refractive index (for example, the difference in SiO2 and TiO2 is ~1.58 at 430 nm), a DBR with a high reflectivity and wide stop band could be achieved with less DBR pairs.

SiO2/TiO2 and SiO2/Ta2O5 DBRs were used in our GaN-based VCSELs. The difference of refractive index between SiO2 and TiO2 is larger than that between SiO2 and Ta2O5, therefore SiO2/TiO2 DBR can achieve a high reflectivity with less pair than SiO2/Ta2O5 DBR. Since the absorption coefficient of Ta2O5 for the pumping laser (Nd:YVO4 laser with laser wavelength of 355 nm) is smaller than SiO2, SiO2/Ta2O5 DBR was used in order to reduce the absorption of pumping laser as the pumping laser passes through the DBR. The thickness of p-GaN is chose to be 1.5λ in order to maximize the overlap between anti-node and MQWs. The structure for simulation is based on the fabricated dielectric DBRs VCSLEs. Figure 3.1 shows the simulated standing wave (square of electric field) patterns calculated by transfer matrix inside the cavity of the VCSEL structure [3.1]. The numerical simulated indicates that the ten pairs MQWs cover

efficiently. The thick n-type GaN layer in the structure can prevent the damage on the InGaN/GaN MQWs since the dislocation or defect might migrate into the MQWs during the laser lift-off process [3.2]. From the numerical simulation of the VCSEL structures with different p- and n-GaN thickness, we also found that the overlap between optical field and MQWs strongly depends on the thickness of p-GaN layer, but not on the thickness of the n-type GaN. In addition, since the MQW region with 1/2λ optical thickness can compensate the possible misalignment between the anti-nodes of the standing wave pattern and the active region position, the effect of the thickness variation of n-GaN that can not be controlled preciously during laser lift-off can be minimized.

3.1.2 Laser lift-off technique

In 1999, Song et al. demonstrated a dielectric DBR VCSEL structure consisting of InGaN MQWs and 10-pair HfO2/SiO2 top and bottom DBR using laser left-off (LLO) technology [3.3]. The reflectivity of top and bottom DBRs were 99.5% and 99.9%, respectively. Now, we also use the same technique to fabricate our sample. Then, the bonding energy of GaN is high as 8.92 eV/molecule, results in the higher melting temperatures and good thermal stability of the GaN compounds compared to other compound semiconductors. The activation energy for GaN decomposition is 3.25 eV/atom. As to the observation of Ga droplets during decomposition in vacuum indicating that GaN decomposes into solid gallium and gaseous nitrogen was reported by Groh et al [3.4]. Sun et al. [3.5] found the thermal decomposition of MOCVD grown GaN on r-plane sapphire occur at a temperature of 1000 ^oC in a hydrogen ambient. Their report indicated decomposition of the GaN→2Ga(l)+N2(g) will occur at a critical temperature of ~1000 ^oC at atmospheric pressure [3.4, 3.6]. In this study, a KrF excimer

laser with a wavelength of 248 nm (5 eV) was used to decompose the GaN grown on c-plane sapphire. The laser illuminated on the surface between GaN and sapphire and decomposed the GaN into Ga and N2, hence, the grown GaN-based LED or micro-cavity structure were transferred from the sapphire substrate to host substrate.

3.1.3 Sample structure

The GaN/InGaN microcavity devices was fabricated by a standard epitaxial growth, followed by dielectric coating, laser lift off, and another dielectric coating to finally form a surface emitting microcavity. The device was grown on a (0001)-oriented sapphire substrate by metalorganic chemical vapor deposition (MOCVD). The layer structures are:

a 30nm nucleation layer, a 4 μm GaN bulk layer, MQWs consisting of 10 periods of 5 nm GaN barriers and 3 nm In0.1Ga0.9N wells, and a 200 nm GaN cap layer. The photoluminescence (PL) emission peak of the fabricated MQW was at 420 nm. Then, a 6 pairs of SiO2/TiO2 dielectric DBR was deposited on the top surface. In order to measure the spectral reflectivity of the deposited DBR, a glass substrate served as a monitor sample was deposited in the same deposition run. The reflectance spectrum of the SiO2/TiO2 DBR is obtained by measuring the monitor sample and the PL spectrum of the as-grown sample as shown by Figure 3.2. Next, a silica substrate was expoxied onto the BDR surface, which is nearly transparent to the wavelengths of the pumping laser and our VCSEL. In order to enhance the adhesion between the epitaxial layers and the silica substrate, an array of disk-like patterns with a diameter of 60 μm was formed by standard photolithography and the SiO2/TiO2 DBR mesas were formed using a buffer oxide etcher.

A pulsed excimer laser was then focused through sapphire substrate onto sapphire GaN interface to remove sapphire substrate by thermal ablation. After the LLO process, the

sample was dipped in HCl solution to remove residual Ga droplets on the exposed GaN buffer layers. In the next step, the sample was lapped and polished using diamond powders to smooth the GaN surface since the LLO process left a roughened surface.

However, to prevent the possible degradation of the quality of MQWs during lapping, the 4.2 μm GaN bulk layer was preserved, followed by a eight pairs of SiO2/ Ta2O5 DBR dielectric coating on the polished GaN surface. The final finished Fabry-Perot cavity formed by these two DBR mirrors has a vavity length of 4 μm. Figure 3.3 shows the complete sample structure and Figure 3.4 shows the fabrication process of the GaN-based dielectric DBRs VCSEL. Figure 3.5 (a) shows the microscopic image of a fabricated 2x2 VCSEL array and the circular areas are the locations of VCSELs with DBRs, also serving the emission apertures. Figure 3.5 (b)、 (c) shows a photograph of the fabricated VCSEL on a silica host substrate and a Si substrate, respectively. In this chapter, we observed the characteristics of VCSEL bonded on the Si substrate.