Study on ingan gan quantum structures and their applications in semiconductor saturable absorber mirror 4

Chapter Design & Fabrication of GaN-based SESAM Chapter Design & Fabrication of GaN-based SESAM In Chapters 3, the properties of GaN-based quantum wells and quantum dots for the applications as saturable absorbers have been investigated In this chapter and the following chapter, we incorporate the InGaN/GaN quantum well saturable absorber to fabricate a GaN-based SESAM structure To achieve passive mode-locking, a DBR and an AR coating are normally integrated with a saturable absorber to form a SESAM, which can then be directly used for short pulse generation As mentioned in Chapter 1, one of the most challenging problems for fabricating the SESAM operating in the blue region is the difficulty in monolithically fabricating broadband high-reflective GaN-based DBRs In this chapter, this problem will be discussed and the GaN-based SESAM operating in the blue wavelength region will be designed and fabricated As introduced in Chapter 1, there is so far no SESAM available for the blue wavelength region One of the most challenging problems is the difficulty in 131 Chapter Design & Fabrication of GaN-based SESAM monolithically fabricating broadband high-reflective GaN-based DBRs, due to the lack of suitable semiconductor DBR materials lattice matched to GaN Though GaN/AlN and AlN/AlGaN DBRs monolithically grown on GaN have been reported [Ponce2003; Moustakas2001; Waldrip2001; Yao2004; Lin2004], they had very narrow stopbands (less than 40 nm near 415-nm wavelength) due to the small refractive index difference between the high index and low index materials For example, the reflectance spectra of three GaN/AlN DBRs reported by Yao et al are shown in Fig 4.1 To achieve short pulses by passive mode-locking, the fabrication of a broadband SESAM is necessary, which in turn requires a broadband DBR operating in the blue wavelength region In addition, in order to avoid the large surface reflection when light enters the SESAM structure, an AR coating has to be deposited on the incident surface Figure 4.1 Experimental reflectance spectra of three 30-pair AlN/GaN DBR (solid lines) and numerical simulations of their reflectance spectra (dash lines) [Yao2004] 4.1 Material selection for DBR and AR coatings To fabricate the high-quality DBR (reflectance > 99.9%) and AR coatings 132 Chapter Design & Fabrication of GaN-based SESAM for the SESAM operating in the blue region, both high refractive index and low refractive index materials, which are transparent for blue light and have low absorption over a broad wavelength region, are required Materials that can be used in the blue and long violet regions are very limited In view of the performance limitations of the GaN-based monolithic DBRs mentioned earlier, dielectric materials were considered instead Thus, we have more freedom in choosing the materials for DBR and AR coatings, and these dielectric coatings can be fabricated after the MOCVD growth of the GaN-based saturable absorber Some promising dielectric material candidates for the blue region are zirconium oxide (ZrO2, n=2.05), yttrium oxide (Y2O3, n=1.85), hafnium oxide (HfO2, n=2.01), tantalum pentoxide (Ta2O5, n=2.19), silicon nitride (Si3N4, n=2.00) for the high-index layers, and silicon oxide (SiO2, n=1.46) for the low-index layer The refractive indices shown here are the estimated values for the blue light Although a larger index contrast Δn = n − n1 is preferred, in this work, considering the availability of a PECVD system, the Si3N4/SiO2 pair was chosen as the high-index / low-index materials for DBR and AR coatings The index contrast for this Si3N4/SiO2 pair is about 0.55, which is much larger than any GaN-based monolithic DBR pair (ex., Δn=0.2 for the GaN/AlGaN pair) 4.2 Theories of DBR and AR coating design To design suitable DBR and AR coatings, the basic theories on the high-reflective coating and anti-reflective coating were first studied 133 Chapter Design & Fabrication of GaN-based SESAM For high-reflective coatings, the optimal structure varies depending on the relative refractive index of the substrate material compared to the deposited high-index film material The optimal structures are as indicated below Each high-index or low-index layer is of quarter-wavelength thickness Design I: If the refractive index of the high-index (H) layer is higher than that of the substrate (S), maximum reflectance is obtained with an odd number of layers having the high-index layers outermost H L Sub/(HL)pH/Air design H p pairs L nH > nS H S In design I, the total reflectance − (n H / n L ) p (n H / n s ) R=( + (n H / n L ) (n H / n s ) 2p )2 (4.1) where p is the number of high-index/low-index pairs, nS is the refractive index of the substrate material, and nH, nL are the refractive indices for the high-index and low-index materials, respectively 134 Chapter Design & Fabrication of GaN-based SESAM Design II: If the refractive index of the high-index (H) layer is lower than that of the substrate (S), maximum reflectance is obtained with an even number of layers having the high-index layers on the top H Sub/(LH)p/Air design L p pairs H nH < nS L S In design II, the total reflectance − (n H / n L ) p ns R=( ) + (n H / n L ) p n s (4.2) where p is the number of high-index/low-index pairs, nS is the refractive index of the substrate material, and nH, nL are the refractive indices for the high-index and low-index materials, respectively As indicated in Fig 4.2, the width of the stopband (the full width at the half maximum (FWHM) of the ideal non-transmission band) is Δλ = 4λ0 π sin −1 ( nH − nL ) nH + nL (4.3) where λ0 is the center wavelength 135 Chapter Design & Fabrication of GaN-based SESAM Δλ λ0 Figure 4.2 Typical reflectance spectrum of a DBR For AR coatings, they might be single quarter-layer, two-layer quarter/quarter, and multi-layer coatings Although multi-layer coatings may provide broader stopbands and lower minimum reflectance, their complicated design and fabrication process may also compromise these advantages Thus, in this work, only single quarter-layer coating or two-layer quarter/quarter coating was used The reflectance of the single quarter-layer AR coating is R =( n0 − n12 / ns ) n0 + n12 / ns (4.4) As indicated in Fig 4.3 (a), n1 is the refractive index of the single quarter-layer coating material, and n0 and nS are the refractive indices of the incident medium (air in most cases) and the substrate material, respectively The minimum reflectance can be achieved if n12 = n0 ⋅ nS is satisfied The reflectance of the two-layer quarter/quarter AR coating is 136 Chapter Design & Fabrication of GaN-based SESAM R =( n0 − ( n2 / n12 )nS ) n0 + ( n2 / n12 )nS (4.5) As indicated in Fig 4.3 (b), n1 and n2 are the refractive indices of the first and second quarter-layer coating materials, respectively, and n0 and nS are the refractive indices of the incident medium (air in most cases) and the substrate material, respectively The minimum reflectance can be achieved if n2 / n12 = n0 / nS is satisfied Incident medium (n0) Incident medium (n0) 2nd quarter-layer coating (n2) Single quarter-layer coating (n1) 1st quarter-layer coating (n1) Substrate (nS) Substrate (nS) (a) (b) Figure 4.3 Schematic structures of (a) single quarter-layer AR coating and (b) two-layer quarter/quarter AR coating 4.3 Design and simulation of DBR and AR coatings In Chapters 3, the GaN-based saturable absorbers have been demonstrated to be the good candidates for the applications in the blue region, and such saturable absorbers are always grown by MOCVD on the sapphire substrate and with a GaN capping layer on the top of the structure (Fig 4.4) To fabricate a SESAM structure, the DBR and AR coatings are deposited on the opposite sides of a saturable absorber We simulated different designs of DBR and AR coatings based on the saturable absorber structure shown in Fig 4.4, and there are two possible structures for making a GaN-based SESAM, as illustrated in Fig 4.5 The arrows indicate the direction of the incident light As discussed in Section 4.1.1, the Si3N4/SiO2 pair was used for the 137 Chapter Design & Fabrication of GaN-based SESAM DBR coatings, while for the AR coatings, they were designed with one or both of these two materials A center wavelength of 425 nm was chosen for the following designs GaN cap QW or QD Sapphire substrate Figure 4.4 Simple schematic structure of a GaN-based saturable absorber Case I Case II Figure 4.5 Schematic structures of two possible designs for GaN-based SESAM (For the dielectric DBR structures, the black parts indicate the high-index Si3N4 layers and the white parts indicate the low-index SiO2 layers.) Based on the above two possible designs, a series of simulations were conducted The important results are summarized below Case I: 1) DBR coating In case I, the substrate for the DBR coating is sapphire (n=1.67), which refractive index is lower than that of the high-index material Si3N4 (n=1.97 in this work) in DBR Therefore, a DBR structure of the Sub/(HL)pH/Air design, as discussed in the design I of Section 4.1.2, was used for simulation 138 Chapter Design & Fabrication of GaN-based SESAM Figure 4.6 Maximum reflectance values and the stopband widths at 95% of maximum reflectance from the design I DBRs on sapphire with different numbers of Si3N4/SiO2 pairs Figure 4.7 Simulated reflectance spectrum of the design I DBR on sapphire with 15 pairs of Si3N4/SiO2 Figure 4.6 shows the maximum reflectance values of the design I DBRs with different numbers of Si3N4/SiO2 pairs Because a good DBR requires not only the high maximum reflectance but also a broad and flat stopband, the full widths of the stopband at the 95% of the maximum reflectance are also plotted with respect to the 139 Chapter Design & Fabrication of GaN-based SESAM number of pairs As can be seen from Fig 4.6, the DBR consisting of 15 pairs of quarter-wavelength Si3N4/SiO2 layers gives sufficiently high reflectance Also, it is highly reflective across a broad range of wavelengths Although the DBR consisting of 20 pairs of Si3N4/SiO2 may give a slightly higher reflectance and a broader width at 95% of maximum reflectance, but this improvement is insignificant, especially when the increased fabrication complication is considered Therefore, for the design I DBR, a 15-pair Si3N4/SiO2 structure is preferred The simulated reflectance spectrum of this 15-pair DBR is shown in Fig 4.7 For the reference wavelength of 425 nm, the quarter-wavelength thicknesses are ~ 54 nm for Si3N4 and ~ 73 nm for SiO2 The maximum theoretical reflectance calculated from Eqn 4.1 for such a DBR structure centered at 425nm was 99.9740%, and the stopband was 82.6-nm wide (FWHM) as calculated from Eqn 4.3 2) AR coating For case I, the substrate for the AR coating is the GaN capping layer (n=2.48) We simulated two possible single quarter-layer coating designs and two possible two-layer quarter/quarter coating designs on GaN with Si3N4 and SiO2 The simulated reflectance spectra are summarized in Table 4.1 As can be observed, the third and the fourth designs are not acting as AR coatings It turned out that the first design - the single layer of SiO2 with a quarter-wavelength optical thickness (~ 73 nm for a reference wavelength of 425nm) - gives the lowest reflectance as an AR coating The theoretical minimum reflectance calculated from Eq (4.4) was 0.6786% at 425nm 140 Chapter Design & Fabrication of GaN-based SESAM Table 4.1 Simulated reflectance spectra of four possible coating designs on GaN using Si3N4 and/or SiO2 Schematic structure Simulated reflectance spectrum Lowest reflectance Quarter-layer SiO2 GaN Quarter-layer Si3N4 GaN Quarter-layer SiO2 Quarter-layer Si3N4 GaN Quarter-layer Si3N4 Quarter-layer SiO2 GaN 141 Chapter Design & Fabrication of GaN-based SESAM Case II: 1) DBR coating In case II, the substrate for the DBR coating is GaN (n=2.48), which refractive index is higher than that of the high-index material Si3N4 (n=1.97 in this work) in DBR Therefore, a DBR structure of the Sub/(LH)p/Air design, as discussed in the design II of Section 4.1.2, was used for simulation Figure 4.8 Maximum reflectance values and the stopband widths at 95% of maximum reflectance from the design II DBRs on GaN with different numbers of Si3N4/SiO2 pairs Figure 4.9 Simulated reflectance spectrum of the design II DBR on GaN with 15 pairs of Si3N4/SiO2 142 Chapter Design & Fabrication of GaN-based SESAM Figure 4.8 shows the maximum reflectance values of the design II DBRs with different numbers of Si3N4/SiO2 pairs As can be seen, the DBR consisting of 15 pairs of quarter-wavelength Si3N4/SiO2 layers gives sufficiently high reflectance and is highly reflective across a broad range of wavelengths Similar to the case in the design I DBRs, considering the complication in fabricating a 20-pair DBR, the 15-pair Si3N4/SiO2 DBR is preferred for the design II structure The simulated reflectance spectrum of this 15-pair DBR is shown in Fig 4.9 For the reference wavelength of 425 nm, the quarter-wavelength thicknesses are ~ 54 nm for Si3N4 and ~ 73 nm for SiO2 The maximum theoretical reflectance calculated from Eq (4.2) for such a DBR structure centered at 425nm was 99.9836%, and the stopband was 82.6-nm wide (FWHM) as calculated from Eqn 4.3 2) AR coating For case II, the substrate for the AR coating is the sapphire substrate (n=1.67) We also simulated two possible single quarter-layer coating designs and two possible two-layer quarter/quarter coating designs on sapphire with Si3N4 and SiO2 The simulated reflectance spectra are summarized in Table 4.2 As can be observed, the second and the fourth designs are not acting as AR coatings It turned out that the third design - two quarter-wavelength Si3N4/SiO2 layers with the SiO2 layer outermost - gives the lowest reflectance as an AR coating The theoretical minimum reflectance calculated from Eq (4.5) is 0.2502% at the center wavelength of 425nm 143 Chapter Design & Fabrication of GaN-based SESAM Table 4.2 Simulated reflectance spectra of four possible coating designs on sapphire using Si3N4 and/or SiO2 Schematic structure Simulated reflectance spectrum Quarter-layer SiO2 Sapphire Quarter-layer Si3N4 Sapphire Lowest reflectance Quarter-layer SiO2 Quarter-layer Si3N4 Sapphire Quarter-layer Si3N4 Quarter-layer SiO2 Sapphire 144 Chapter Design & Fabrication of GaN-based SESAM Compared to the case I SESAM, the case II SESAM has a DBR with higher reflectance and an AR coating with lower reflectance The results are summarized in Table 4.3 Hence, the case II SESAM is a better structure for the SESAM fabrication In the following Section 4.1.4, the DBR and AR coatings of the case II structure simulated in this section will be fabricated using PECVD Table 4.3 Summary of the DBR and AR coating properties for the SESAM structures I and II SESAM 15 pairs of DBR Best AR coating Maximum reflectance: 99.9740% Minimum reflectance: 0.6786% Maximum reflectance: 99.9836% Minimum reflectance: 0.2502% Case I Case II 4.4 Deposition of DBR and AR coatings According to the simulated case II SESAM structure in Section 4.1.3, the DBR and AR coatings were first studied and calibrated separately before being 145 Chapter Design & Fabrication of GaN-based SESAM incorporated with the saturable absorber Both the SiO2 and Si3N4 layers were deposited using the Nextral (NE) D200 Unaxis PECVD system described in Section 2.2 at the chamber temperature of 280oC For the DBR studies, a GaN template grown by MOCVD was used as the substrate This GaN layer is similar to the GaN capping layer to be used in the final SESAM structure A DBR consisting of 15 pairs of quarter-wavelength SiO2/Si3N4 layers, with the Sub/(LH)p/Air design, were then deposited on the GaN template For our PECVD system, the calibrated refractive indices for SiO2 and Si3N4 were approximately 1.45 and 1.97, respectively, as discussed in Section 2.2 The quarter-wavelength thicknesses were ~ 54 nm for Si3N4 and ~ 73 nm for SiO2, for the nominal center wavelength of 425 nm Figure 4.10 Experimental reflectance spectrum of the optimized DBR with 15 pairs of SiO2/Si3N4 Through a series of thickness optimization, which is described in Section 2.2, 146 Chapter Design & Fabrication of GaN-based SESAM the maximum reflectance achieved at the top of the stopband is more than 99%, measured by the spectrophotometer The stopband width is as broad as ~ 90 nm at the center wavelength of 425 nm The experimental reflectance spectrum of the optimized DBR is shown in Fig 4.10 The slightly broader stopband of the experimental DBR (90 nm) than that of the simulated DBR (82.6 nm) is mainly due to the deviation of the actual refractive indices of the deposited materials (according to the actual stoicheometry of each layer) from the calibrated values (n=1.97 for Si3N4 and n=1.45 for SiO2) used for simulation Nevertheless, the broad stopband and the high maximum reflectance of the optimized DBR indicate the good process control during the PECVD deposition, and this dielectric DBR has superior properties over the GaN-based monolithic DBRs as shown in Fig 4.1 [Ponce2003; Moustakas2001; Waldrip2001; Yao2004; Lin2004] In addition, in Fig 4.10, the oscillation fringes outside the high-reflective stopband showed some deviation from the simulated spectrum in Fig 4.9 This is un-avoidable due to the limitation in the very precise thickness control during the PECVD deposition A tiny variation in the thickness would cause the changes in the interference fringes outside the DBR stopband Fortunately, this deviation does not play any role in the SESAM operation For the AR coating deposition, a sapphire substrate is used This is exactly the same substrate as that to be used in the actual SESAM The AR coating consisting of a pair of quarter-wavelength Si3N4/SiO2, with the SiO2 layer outermost, was deposited For the nominal center wavelength of 425 nm, the quarter-wavelength 147 Chapter Design & Fabrication of GaN-based SESAM thicknesses were ~ 54 nm for Si3N4 and ~ 73 nm for SiO2 Through the thickness optimization, which is described in Section 2.2, the lowest reflectance of 1.5% was achieved at ~ 425nm The experimental reflectance spectrum is shown in Fig 4.11 This higher minimum reflectance as compared to the simulated value of 0.2502 % in Section 4.1.3 might be due to the following reasons: 1) the non-ideal thickness control of each layer, 2) the deviation of the actual refractive indices (according to the actual stoicheometry of each layer) from the theoretical values (n=1.97 for Si3N4 and n=1.45 for SiO2) applied in simulation, and 3) the non-ideal surface morphology for the actual sample Figure 4.11 Experimental reflectance spectrum of the optimized two quarter-layer AR coating After the separate study and calibration, the DBR and AR coatings were finally incorporated with the InGaN/GaN 8-QW saturable absorber presented in 148 Chapter Design & Fabrication of GaN-based SESAM Section 3.1 for SESAM fabrication The 15 pairs of SiO2/Si3N4 dielectric DBR and the one pair Si3N4/SiO2 AR coating were deposited on the opposite sides of the saturable absorber by PECVD The schematic structure of this GaN-based SESAM structure is shown in Fig 4.12 Figure 4.12 Schematic structure of the GaN-based SESAM The arrow indicates the direction of light incidence Subsequently, the reflectance spectrum of the SESAM was recorded by the spectrophotometer The light was incident from the AR coating side of the SESAM, as 149 Chapter Design & Fabrication of GaN-based SESAM indicated by the arrow in Fig 4.12 As shown in Fig 4.13, the SESAM had a broad stopband of about 90 nm centered at ~ 425 nm, and the maximum reflectance was over 99% It is also noted that, severe oscillation fringes, induced from interference, were observed on the top of the SESAM stopband, and this will be discussed in detail in Chapter The decrease in reflectance at the shorter wavelength side of the spectrum is mainly due to the increasing absorption of GaN-based materials at those wavelengths, which is in agreement with the results of linear transmission experiment for the 8-QW saturable absorber shown earlier in Fig 3.2 In addition, at λ < 365 nm, most of the light was absorbed by the SESAM due to the strong band-to-band absorption in GaN The bandedge of GaN is located at ~ 363 nm Figure 4.13 Reflectance spectrum from the GaN-based SESAM sketched in Fig 4.12 4.5 Drawbacks of the GaN-based SESAM To overcome the limitation of the monolithic GaN-based DBR, we have 150 Chapter Design & Fabrication of GaN-based SESAM fabricated a broadband GaN-based SESAM using the SiO2 / Si3N4 dielectric DBR, as described in Section 4.1 As a result, a broad stopband with high reflectance was achieved However, as shown in Fig 4.13, this non-monolithic SESAM suffered from severe interference-induced reflectance fluctuations within the stopband For example, in a small wavelength span (417 nm – 425 nm), the magnitude of the interference-induced reflectance fluctuations could be as large as ~ 6.5 % (see Fig 4.13), which was comparable to the modulation depth of this 8-QW saturable absorber (~ 6.5 % as shown in Fig 3.4) If used for mode-locking, this SESAM would cause severe instability in the mode-locking Therefore, further modifications in the SESAM structure were necessary to suppress the reflectance fluctuations before the SESAM can be reliably used for passive mode-locking and short pulse generation 4.6 Chapter summary In this chapter, a GaN-based SESAM was fabricated using an InGaN/GaN quantum well saturable absorber The DBR and AR coatings were deposited by PECVD after the MOCVD growth of the saturable absorber A dielectric SiO2/Si3N4 DBR was applied instead of the traditional monolithic DBR This GaN-based SESAM exhibited a broad stopband (~ 90 nm wide) and high maximum reflectance (> 99%) However, this non-monolithic SESAM suffered from severe interference-induced reflectance fluctuations within the stopband, which might result in instability for passive mode-locking application Hence, in the following chapter 5, the reflectance fluctuations in the stopband of the GaN-based SESAM will be investigated and the 151 Chapter Design & Fabrication of GaN-based SESAM further modifications in the SESAM structure will be presented to suppress the interference 152 ... coating After the separate study and calibration, the DBR and AR coatings were finally incorporated with the InGaN/ GaN 8-QW saturable absorber presented in 148 Chapter Design & Fabrication of GaN- based... the GaN- based monolithic DBRs as shown in Fig 4. 1 [Ponce2003; Moustakas2001; Waldrip2001; Yao20 04; Lin20 04] In addition, in Fig 4. 10, the oscillation fringes outside the high-reflective stopband... Schematic structures of (a) single quarter-layer AR coating and (b) two-layer quarter/quarter AR coating 4. 3 Design and simulation of DBR and AR coatings In Chapters 3, the GaN- based saturable absorbers