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

Figure 4.4 Simple schematic structure of a GaN-based saturable absorber Sapphire substrate QW or QD GaN cap Case I Case II Figure 4.5 Schematic structures of two possible designs for

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

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

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=n2−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 materialsfor DBR and AR coatings The index contrast for this Si3N4/SiO2pair 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

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

In design I, the total reflectance

2 2 2

2 2

))/()/(1

)/()/(1(

s H

p L H

s H

p L H

n n n n

n n n n R

+

−

where p is the number of high-index/low-index pairs, n S is the refractive index of the

substrate material, and n H, nL are the refractive indices for the high-index and low-index materials, respectively

In design II, the total reflectance

2

))/(1

)/(1(

s

p L H

s

p L H

n n n

n n n R

+

−

where p is the number of high-index/low-index pairs, n S is the refractive index of the

substrate material, and n H, 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

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

λ0 Δλ

2 2

0 1 2

0 1

−

=+

s s

As indicated in Fig 4.3 (a), n 1 is the refractive index of the single quarter-layer

coating material, and n 0 and n S are the refractive indices of the incident medium (air in most cases) and the substrate material, respectively The minimum reflectance can be achieved if is satisfied The reflectance of the two-layer quarter/quarter

AR coating is

2

1 0 S

n = ⋅ n n

As indicated in Fig 4.3 (b), n 1 and n 2 are the refractive indices of the first and second

quarter-layer coating materials, respectively, and n 0 and n S are the refractive indices of the incident medium (air in most cases) and the substrate material, respectively The minimum reflectance can be achieved if n / n22 12 =n / n0 S is satisfied

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

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

Figure 4.4 Simple schematic structure of a GaN-based saturable absorber

Sapphire substrate

QW or QD GaN cap

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

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

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

Table 4.1 Simulated reflectance spectra of four possible coating designs on GaN using Si3N4 and/or SiO2

Schematic structure Simulated reflectance spectrum

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

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

Table 4.2 Simulated reflectance spectra of four possible coating designs on sapphire using Si3N4 and/or SiO2

Schematic structure Simulated reflectance spectrum

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

Maximum reflectance: 99.9836% Minimum reflectance: 0.2502%

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

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,

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