Chapter Optimization of GaN-based SESAM Chapter Optimization of GaN-based SESAM In Chapter 4, the non-monolithic GaN-based SESAM with broad stopband and high maximum reflectance was successfully fabricated using dielectric DBR and AR coatings However, severe interference-induced reflectance fluctuations, which might lead to the instability in passive mode-locking, were found within the stopband of the SESAM Optimization in the device structure was therefore necessary to suppress the reflectance fluctuations before the SESAM can be reliably used in passive mode-locking In this chapter, Section 5.1 investigates the origin of the interference fringes within the stopband of the GaN-based SESAM It is found that the thick GaN buffer is the major cause of the reflectance fluctuations By performing simulations on the layer structures, a new SESAM structure with thinner GaN buffer to suppress the reflectance fluctuations was then designed Subsequently, the fabrication of the new SESAM structure is described in Section 5.2, which, according to the simulation, is able to suppress the reflectance fluctuations The fabrication process includes wafer bonding, laser lift-off of the sapphire substrate, inductively 153 Chapter Optimization of GaN-based SESAM coupled plasma (ICP) etching of the GaN buffer and the PECVD deposition of the AR coating Finally, in Section 5.3, the surface morphology, crystal quality of the SESAM structure and the emission property of the QWs will be analyzed to examine the effects of the fabrication process on the characteristics of the SESAM This whole chapter will be focused on the GaN-based SESAM sample fabricated by the InGaN/GaN 8-QW saturable absorber, as discussed in Chapter 5.1 Simulation First of all, the origin of the interference fringes within the stopband of the GaN-based SESAM was investigated As shown in the unmodified GaN-based SESAM structure sketched in Fig 5.1 (same as Fig 4.12 without the AR coating, and the LT and HT GaN buffers were shown together as GaN buffer), there were large variations of the refractive indices at the interfaces of sapphire substrate / air (Δn ≈ 0.7), GaN buffer / sapphire substrate (Δn ≈ 0.8), and GaN cap / first SiO2 layer of DBR (Δn ≈ 1) These three interfaces are referred to as interfaces A, B and C, respectively, as indicated in Fig 5.1 The cavities formed by the above interfaces are the sources of the light interferences Therefore, there are two major sources which can cause the interferences: the cavity formed by interfaces A and B and that formed by interfaces B and C According to the period of the interference fringes (16 nm in average as shown in Fig 4.13) which is related to the thickness and refractive index of the layers between two interfaces, the observed interference fringes within the SESAM stopband were caused by the cavity formed by the interfaces B and C The interference caused by the much thicker sapphire substrate (~ 500 μm) which is the 154 Chapter Optimization of GaN-based SESAM cavity formed by interfaces A and B would give a high-frequency oscillation beyond the wavelength resolution of the spectrophotometer used, so the effect of the sapphire substrate on the reflectance fluctuations was not observed in our experiment Figure 5.1 Schematic structure of the GaN-based SESAM (This is same as Fig 4.11 without the AR coating.) The interference-induced reflectance fluctuations caused by the cavity formed by interfaces B and C can be controlled by thinning the GaN buffer and other epilayers (GaN barriers / cap and QWs) The purpose of thinning these layers is to increase the period of the interference fringes so as to reduce the reflectance fluctuations within a small wavelength span For example, in the oscillation fringes in Fig 4.13, there is a valley at 417 nm and an adjacent peak at 425 nm The oscillation period around this wavelength region is about 16 nm, and the valley-to-peak 155 Chapter Optimization of GaN-based SESAM reflectance difference is about 6.5 %, which is also the oscillation magnitude If the period of the fringes is largely increased, assuming that the oscillation magnitude is unchanged, the reflectance difference between 417 nm and 425 nm will be much less than the oscillation magnitude (6.5 %) Hence, the reflectance fluctuations within this wavelength span (417 – 425 nm) are suppressed For the thinning of the GaN-based layers, obviously the dimension of the QW structure (the barriers, QWs and cap) can not be modified by this process as it provides the desired nonlinear saturable absorption characteristics Besides, only part of the GaN buffer can be removed as one layer of GaN is needed to form the barrier for the InGaN/GaN QW Based on the above, we can reduce the reflectance fluctuations within the stopband by removing the sapphire substrate and thinning the GaN buffer But then the interface between the thinned GaN buffer and air may also cause interference This can be reduced by an AR coating on the top of the thinned GaN buffer For this AR coating, the substrate is now GaN instead of the sapphire substrate for the AR coating used for the unmodified SESAM in Fig 4.12 Hence, the single quarter-wavelength SiO2 AR coating, as illustrated in Table 4.1, should be used Reflectance simulations using Essential Macleod Thin Film Design Program were then carried out on the various structures Figure 5.2 shows the simulated reflectance spectra of the SESAM: (a) for the unmodified structure shown in Fig 5.1; (b) after sapphire substrate removal; (c) after further thinning of the GaN buffer until ~350-nm GaN is left; and (d) eventually with the SiO2 AR coating on the thinned GaN buffer As expected, the high-frequency oscillations, caused by the sapphire substrate, as seen 156 Chapter Optimization of GaN-based SESAM in Fig 5.2 (a), disappeared after the removal of the sapphire substrate as shown in Fig 5.2 (b) Furthermore, after thinning the GaN buffer, the period of the interference-induced oscillation fringes was increased and there was only one interference minimum falling into the DBR stopband as can be seen from Fig 5.2 (c) Due to the large refractive index difference between the air and the thinned GaN buffer, the surface reflectance could be as high as about 30 % The SESAM at this stage acted as resonant type We can reduce the reflectance from the thinned GaN buffer with an AR coating to make this SESAM anti-resonant After adding a SiO2 layer with the optical thickness of 1/4 λ (λ = 425 nm) as an AR coating, the surface reflectance could be reduced to less than 0.7 %, resulting in the reduction of the oscillation amplitude as shown in Fig 5.2 (d) These simulation results show that, by sapphire substrate removal, GaN layer thinning and the additional AR coating, the interference-induced reflectance fluctuations within the stopband of the SESAM can be effectively suppressed 157 Chapter Optimization of GaN-based SESAM Figure 5.2 Simulated reflectance spectra of the SESAM (a) for the unmodified structure shown in Fig 5.1 (taken from the sapphire substrate side), (b) after sapphire substrate removal (taken from the GaN buffer side), (c) after further thinning part of the GaN buffer (taken from the thinned GaN buffer side), and (d) eventually with the SiO2 AR coating on the thinned GaN buffer (taken from the AR coating side) 158 Chapter 5.2 Optimization of GaN-based SESAM Experiments Figure 5.3 Experimental reflectance spectra of the SESAM (a) for the unmodified structure shown in Fig 5.1 (measured from the sapphire substrate side), (b) after laser lift-off of the sapphire substrate (measured from the GaN buffer side), (c) after the ICP etching of the GaN buffer (measured from the etched GaN side), and (d) eventually with the SiO2 AR coating on the etched GaN layer (measured from the AR coating side) 159 Chapter Optimization of GaN-based SESAM According to the above reflectance simulations of various structures, a series of experiments was subsequently conducted A Shimadzu UV-VIS-NIR scanning spectrophotometer was used to perform the reflectance measurements The recorded reflectance spectra of the SESAM after each step are shown in Fig 5.3 Figure 5.3 (a) is the measured reflectance spectrum of the unmodified SESAM structure sketched in Fig 5.1, and it shows severe interference-induced reflectance fluctuations over the DBR stopband Comparing Fig 5.3 (a) with the corresponding simulation result, Fig 5.2 (a), one can see that the high frequency fringes did not appear in Fig 5.3 (a) The high frequency oscillations have a period of less than nm, which is unable to be picked up by the spectrophotometer with a wavelength accuracy of ± 0.5 nm The low frequency oscillation predicted in simulation (Fig 5.2 (a)) has the same period as that observed in the experiment (Fig 5.3 (a)) 5.2.1 Wafer bonding Before the sapphire substrate removal, the SESAM sample as sketched in Fig 5.1 was first bonded to another supporting substrate for easier handling Here a second sapphire substrate was used as the supporting substrate because it is chemically stable at elevated temperatures (ex 280oC) A benzocyclobutene (BCB) 3025 polymer [Niklaus2001] was chosen as the bonding medium because it could sustain the high temperature during the final PECVD deposition of the AR coating To perform the wafer bonding, the SESAM was cut into a mm × mm square piece 160 Chapter Optimization of GaN-based SESAM which was then bonded to an mm × mm supporting sapphire The bonding procedure was as follows: 1) Both the SESAM and the supporting substrate were cleaned with acetone, methanol and rinsed with deionized (DI) water 2) After drying in an oven at 200oC, the supporting substrate was spin-coated with an adhesion promoter, followed by ~ 3-μm BCB 3025 polymer 3) The supporting substrate was placed on a hotplate at 100oC for 10 minutes to evaporate the solvent 4) The SESAM was then attached to the supporting substrate with the DBR side facing the bonding interface, so that the sapphire substrate to be removed was exposed Pressure is applied to remove any trapped air 5) The bonded structure was finally cured at 280oC on a hotplate for 1hr The structure after wafer bonding is illustrated in Fig 5.4, and it was ready for laser lift-off of the sapphire substrate Figure 5.4 Schematic structure of the SESAM sketched in Fig 5.1 after being bonded to the supporting sapphire and ready for laser lift-off 161 Chapter Optimization of GaN-based SESAM 5.2.2 Laser lift-off As the double-side polished sapphire was used as the substrate for the unmodified SESAM, it is transparent to the excimer laser beam Wong et al have demonstrated the damage-free separation of GaN thin films from sapphire substrates by pulsed KrF excimer laser [Wong1998] The transparency of sapphire and the thermal decomposition of GaN at the interface were combined to realize the separation of the sapphire substrate from the rest of the structure In our work, a Novaline 100 KrF excimer laser with a wavelength of 248 nm and a pulse width of 25 ns was used The laser beam spot-size was mm × mm The laser pulse with an energy of 285 mJ was incident through the sapphire substrate and irradiated at the sapphire / GaN interface The pulsed UV irradiation and short GaN optical absorption length resulted in the localized heating of the GaN at the interface [Wong1999] A distinct change in the interfacial region from transparent to a metallic silver color was observed, which indicated that the interfacial GaN had decomposed into metallic gallium and N2 gas Four laser pulses were used to cover each corner of the SESAM sample, respectively The sapphire substrate was then detached from the GaN film leaving a shiny gallium surface The excess gallium on the surface was subsequently removed in diluted HCl (HCl : H2O = 1:1) Figure 5.3 (b) shows the reflectance spectrum of the SESAM after removing the surface gallium Figure 5.3 (b) is essentially similar to Fig 5.3 (a), because the interference caused by the sapphire substrate was not resolvable in Fig 5.3 (a) as explained earlier Comparing Fig 5.3 (b) with Fig 5.2 (b) (the corresponding 162 Chapter Optimization of GaN-based SESAM simulated reflectance spectrum), it was found that the oscillation magnitude obtained from the sample was much smaller than that from the simulation Note that the nature of the surface morphology was not considered in the simulation; however, the sample showed a poor surface morphology after laser lift-off of the sapphire substrate, which will be shown later in Section 5.3 The rough surface of the sample would compromise the cavity effect, therefore resulting in the much smaller oscillation magnitude in the reflectance spectrum 5.2.3 ICP etching Thinning of the GaN buffer was then performed in inductively coupled plasma (ICP) chamber with the substrate holder maintained at the temperature of 6oC and a chamber working pressure of mTorr BCl3 and Cl2 were used as the etchants A similar GaN buffer thinning process after laser lift-off has also been demonstrated by Wong et al using an ion milling technique [Wong19991] The ICP etching process was controlled until there was ~ 350-nm GaN buffer left The remaining GaN buffer was not only used to maintain the integrity of the QW structure, but also to prevent the QW regions from being damaged during the ICP etching The reflectance spectrum after GaN etching is shown in Fig 5.3 (c) It agrees well with the simulated result shown in Fig 5.2 (c) The period of the oscillation fringes was greatly increased 5.2.4 AR coating deposition SiO2 of quarter-wavelength optical thickness was then deposited onto the 163 Chapter Optimization of GaN-based SESAM thinned GaN buffer as an AR coating This layer was expected to reduce the surface reflectance to less than 0.7% The AR coating deposition was also conducted in the Nextral (NE) D200 Unaxis PECVD system described in Section 2.2 at the chamber temperature of 280oC The backside coating process on the exposed GaN buffer after laser lift-off was also reported by Song et al in the fabrication of a vertical injection blue light emitting diode [Song1999] The final reflectance spectrum after AR coating is shown in Fig 5.3 (d) It is clear that the interference-induced reflectance fluctuation is effectively suppressed The experimental results agree very well with the simulation We also noted that the oscillation fringes outside the high-reflectance stopband showed some deviation from the simulated spectra This is because the layer thicknesses in the real sample are not exactly the required thicknesses (λ/4n) These deviations were un-avoidable due to the limitation in the very precise thickness control during the crystal growth and etching A tiny variation in the thickness of one of the layers would cause the interference fringes to change outside the DBR stopband Fortunately, this deviation does not play any role in the operation of the SESAM because it is outside the working wavelength range of the SESAM In fact, the great agreement between the experimental and the simulation results within the DBR stopband wavelength region indicates that our thickness controls during the growth and etching processes are rather good 5.3 Characterizations In this section, the surface morphology, crystal quality and emission 164 Chapter Optimization of GaN-based SESAM property of the SESAM sample during and after the optimization of the structure were analyzed by the characterization techniques of AFM, XRD, SEM and PL to examine the effects of the fabrication processes on the characteristics of the SESAM 5.3.1 Surface morphology The surface morphology of the SESAM sample after each optimization step was characterized using a Digital Instruments Nano-scope III AFM under tapping mode Figure 5.5 AFM images (5 μm × μm) of the surface profiles of (a) the eight-period quantum well (8-QW) saturable absorber (taken from the GaN cap surface), (b) the SESAM after laser lift-off of the sapphire substrate (taken from the exposed GaN buffer surface), (c) the SESAM after the ICP etching of the GaN buffer (taken from the etched GaN surface), and (d) the SESAM eventually with the SiO2 AR coating on the etched GaN layer (taken from the AR coating surface) The lighter colors represent higher features, while the darker colors represent lower features 165 Chapter Optimization of GaN-based SESAM Figure 5.5 illustrates the AFM images of the surface profiles of (a) the 8-QW saturable absorber, (b) the SESAM after laser lift-off of the sapphire substrate, (c) the SESAM after the ICP etching of part of the GaN buffer, and (d) the SESAM eventually with the SiO2 AR coating on the etched GaN The roughness root-mean-square (RMS) value of the GaN buffer surface after the laser lift-off of the sapphire substrate was 9.2 nm The high-density tiny bumps on the surface after the laser lift-off could be related the high-density dislocations in the GaN at the sapphire/GaN interface After ICP etching of the GaN, it was observed that the surface morphology was significantly improved, with an RMS value of 6.1nm Some pinholes were observed on the surface after ICP etching A series of experiments confirmed that these pinholes are broadened etch-pits resulting from the termination of the dislocations When the SiO2 AR coating was finally deposited, the surface became smoother, indicated by an RMS value of 5.3 nm Moreover, the pinholes introduced by the dry etching were filled up The AR-coated surface morphology of the SESAM after structure modifications is comparable with that of the original MOCVD grown 8-QW saturable absorber, which had an RMS value of 5.4 nm This indicates that the above structure modifications can effectively suppress the interference-induced reflectance fluctuations without introducing much adverse effect on the surface morphology 5.3.2 Crystal quality The crystal quality of the SESAM sample after each optimization step was 166 Chapter Optimization of GaN-based SESAM analyzed by XRD using the double-crystal high-resolution X-ray (Cu Kα1) diffractometer (X’Pert-MRD, Philips) Figure 5.6 shows the φ-circle scan of the reflections from GaN (10 12) atom planes for the SESAM after the laser lift-off It shows the six-fold symmetry of the GaN on the supporting sapphire, indicating that the GaN retains its original wurtzite crystal structure after the lift-off and substrate transfer The cross-sectional SEM image, taken by a JEOL 6700F field emission microscope operating at kV, at the bonding interface after the substrate transfer is shown in the inset of Fig 5.6 As can be observed, the transferred film was flat Figure 5.6 XRD φ-circle scan of the GaN (10 12) reflections for the SESAM after laser lift-off (measured from the GaN buffer side) The inset shows the cross-sectional SEM image of the bonding interface after substrate transfer Figure 5.7 shows the XRD spectra in ω–2θ geometry from the 8-QW saturable absorber (Fig 5.1 without the DBR coatings, measured from a bigger-sized sample before being cut into the 3mm × 3mm piece), the SESAM on the supporting 167 Chapter Optimization of GaN-based SESAM sapphire after laser lift-off, and the SESAM after ICP etching The reflections from GaN (0002) and Al2O3 (0006) atom planes were clearly observed for all these samples The Al2O3 (0006) reflection peaks at near 21o were aligned for the three curves so that the reflections from GaN (0002) atom planes can be directly compared Figure 5.7 XRD ω–2θ scans from the 8-QW saturable absorber (measured from the GaN cap side from a bigger-sized sample before being cut into the mm × mm piece), the SESAM on the supporting sapphire after laser lift-off (measured from the GaN buffer side), and the SESAM after ICP etching (measured from the etched GaN side) As shown in Fig 5.7, after laser lift-off, the reflection from GaN (0002) shifted towards the higher angles by ~ 0.066o, while the intensity of the reflection reduced to about 1/6 of the intensity from the 8-QW saturable absorber The shift of the GaN (0002) reflection towards the higher angle corresponds to the increase of the GaN lattice constant by ~ 0.37%, which indicates the occurrence of relaxation of the original compressive strain, brought about by the laser lift-off and substrate transfer The intensity reduction after laser lift-off is mainly due to two factors: 1) the smaller 168 Chapter Optimization of GaN-based SESAM size of the sample (3 mm × mm) compared with the X-ray beam size, 2) the degradation of the surface morphology as indicated by the AFM images in Fig 5.5 (b) Furthermore, after the ICP etching of the GaN layers, the reflection from GaN (0002) was not shifted further, but the intensity was further reduced by about 83% This indicates that there was no further stress developed during the etching process; and the reduction in intensity is due to the much smaller thickness of GaN after thinning (The GaN layer was thinned down from about μm to ~ 350 nm.) Figure 5.8 are the XRD rocking curves of the GaN (0002) ω-scan reflections from (a) the 8-QW saturable absorber (before being cut into the mm × mm piece), (b) the SESAM on the supporting sapphire after laser lift-off, and (c) the SESAM after ICP etching It shows that the full width at half maximum (FWHM) of the GaN (0002) reflection was only slightly increased by 0.002o after the laser lift-off and substrate transfer Therefore, no evident thermal and mechanical damage has been introduced to the films during the laser lift-off and substrate transfer After the ICP etching of the GaN buffer, a larger FWHM broadening of 0.012o was observed Although the removal of the more defective part of the GaN buffer might result in a slight narrowing of the FWHM, the observed FWHM broadening is mainly due to the damage by the ion bombardment during the ICP etching It should be noted that, although the damage encountered during the laser lift-off and GaN etching resulted in a small broadening at the FWHM of the GaN (0002) reflection, the final FWHM after ICP etching was only 0.100o, which still indicates the good crystal quality 169 Chapter Optimization of GaN-based SESAM Figure 5.8 XRD rocking curves of the GaN (0002) reflections from (a) the 8-QW saturable absorber (measured from the GaN cap side from a bigger-sized sample before being cut into the mm × mm piece), (b) the SESAM on the supporting sapphire after laser lift-off (measured from the GaN buffer side), and (c) the SESAM after ICP etching (measured from the etched GaN side) 170 Chapter Optimization of GaN-based SESAM 5.3.3 Emission property The influence of the structure modifications on the emission from the InGaN/GaN quantum wells was studied by a micro-PL system at room temperature, and the samples were excited by a 325-nm He-Cd laser Figure 5.9 shows the room temperature PL spectra of the 8-QW saturable absorber and the SESAM after ICP etching to thin down the GaN Note that the PL from the 8-QW saturable absorber was measured from the GaN cap side, while the PL from the SESAM after sapphire substrate removal and ICP etching of GaN buffer was measured from the thinned GaN buffer side Figure 5.9 Room temperature PL spectra of the 8-QW saturable absorber (measured from the GaN cap side) and the SESAM after ICP etching (measured from the etched GaN side) It was found that the PL peak was red-shifted from ~ 421 nm before any structure modification to ~ 433 nm after the ICP etching Besides, after ICP etching, 171 Chapter Optimization of GaN-based SESAM the PL peak intensity was reduced by about 40 %, and the long-wavelength shoulders disappeared The redshift in PL emissions agrees well with the above XRD result which shows the compressive strain relaxation after the structure modifications (Fig 5.7) For the compressively strained QWs, the heavy-hole / light-hole degeneration at the Γ point of the Brillouin zone happened, pulling down the valance bands and giving PL emissions at the shorter wavelength Therefore, with the compressive strain relaxation, the emission wavelength was redshifted The reduction of PL intensity after ICP etching is mainly due to the increased absorption of the excitation light (325 nm) by the much thicker GaN layer above the QWs in the PL measurements The GaN cap in the 8-QW saturable absorber was only 30 nm, while the remaining GaN buffer above the QWs was ~ 350 nm after ICP etching Furthermore, it should be noted that the PL emission band of the 8-QW saturable absorber is modulated by the interference-induced fringes For example, the long-wavelength shoulders at around 432 nm, and 444 nm etc are mainly due to the interference, which is in a good agreement with the reflectance spectrum in Fig 5.3 (a); and such interference-induced shoulders can be observed more clearly in the logarithmic scaled PL spectrum, as indicated by the arrows in Fig 5.10 After ICP etching, such an interference effect was suppressed and therefore the interference-induced shoulders in the PL spectrum disappeared 172 Chapter Optimization of GaN-based SESAM Figure 5.10 Logarithmic scaled PL spectrum of the 8-QW saturable absorber at the long-wavelength side The arrows indicate the interference-induced shoulders According to the above PL emission results, the InGaN/GaN quantum wells in the SESAM after structure modifications maintain very good emission properties after structure modifications 5.4 Chapter summary In this chapter, structure modifications on the GaN-based SESAM working in the blue wavelength region were reported The interference issue within the stopband of the SESAM was studied A series of fabrication processes was simulated to reduce the interference-induced reflectance fluctuations Laser lift-off of the sapphire substrate, ICP etching of the GaN layers, and PECVD deposition of the SiO2 AR coating were conducted according to the simulation The interference-induced reflectance fluctuations were successfully suppressed in the SESAM after structure modifications Characterizations by AFM, XRD, SEM and PL further indicated that 173 Chapter Optimization of GaN-based SESAM the SESAM after modifications exhibited good surface morphology, crystal quality and emission property After structure modifications developed in this chapter, the GaN-based SESAM with a broad high-reflective stopband in the blue region can be used to passively mode-lock blue lasers such as GaN-based semiconductor lasers to produce ultra-short optical pulses In addition, by adjusting the composition and thickness of the QWs, the absorption wavelength of the saturable absorber can be easily adjusted Also, the high-reflective stopband center of the SESAM can be controlled by adjusting the thicknesses of the DBR quarter-wavelength layers Thus the GaN-based SESAM structure after structure modifications can be simply modified to operate at various wavelengths in the blue region 174 ... fabricated by the InGaN/ GaN 8-QW saturable absorber, as discussed in Chapter 5. 1 Simulation First of all, the origin of the interference fringes within the stopband of the GaN- based SESAM was investigated... Furthermore, after thinning the GaN buffer, the period of the interference-induced oscillation fringes was increased and there was only one interference minimum falling into the DBR stopband as can be... sketched in Fig 5. 1, and it shows severe interference-induced reflectance fluctuations over the DBR stopband Comparing Fig 5. 3 (a) with the corresponding simulation result, Fig 5. 2 (a), one can