Study on ingan gan quantum structures and their applications in semiconductor saturable absorber mirror 3

Chapter Saturable Absorbers with GaN-based Quantum Structures Chapter Saturable Absorbers with GaN-based Quantum Structures As mentioned in Section 1.4, for high density high speed data storage applications, high-repetition-rate ultra-short optical pulse sources in the blue/UV wavelength region are required To date, the blue/UV ultra-short optical pulses have mainly been obtained by frequency conversion methods from mode-locked lasers operating in the infrared wavelength region, such as Ti:sapphire lasers and Cr:LiSAF lasers The direct generation of ultra-short blue/UV pulses by passive mode-locking has not yet been demonstrated Although SESAMs have been successfully developed to mode-lock solid state lasers, fiber lasers, and semiconductor lasers in a wide wavelength region to produce ultra-short optical pulses, there is so far no SESAM available in the blue/UV region One of the major problems is the difficulty in monolithically fabricating broadband high-reflective GaN-based DBRs This problem will therefore be discussed and the solution presented in Chapter On the other hand, to fabricate saturable absorber for the blue/UV region, 78 Chapter Saturable Absorbers with GaN-based Quantum Structures GaN-based quantum structures are considered to be suitable candidates As discussed in Chapter 1, owing to their large direct bandgaps, GaN-based materials have great potentials for fabricating short-wavelength optoelectronic devices; and the research in the GaN-based quantum structures has been greatly developed during the past decade Hence, in this thesis, the GaN-based quantum structures will be used to fabricate the saturable absorbers operating in the blue region Also, in view of the large absorption coefficients of the GaN-based materials in the UV region, this study will only be focused on the saturable absorbers and SESAMs operating in the blue region In this chapter, Section 3.1 will be focused on the nonlinear property of the InGaN/GaN quantum well saturable absorber In addition, aiming at the efficient pulse shaping for future passive mode-locking applications, a novel and easy method to reduce the absorption recovery time of the InGaN/GaN quantum well saturable absorber will be presented in Section 3.2 With this method, InGaN/GaN MQW samples with reduced GaN buffer thicknesses were fabricated, and the absorption recovery time was controlled via the crystal quality of the active region Because these InGaN/GaN MQW samples exhibited high-density large V-pits, which could be related with their ultra-short recovery times, Section 3.3 will be dedicated to the study on the effects of V-pits on the morphological and optical properties of InGaN/GaN quantum wells Finally, in Section 3.4, the InGaN/GaN quantum dot saturable absorber will be fabricated and its nonlinear property will be investigated Also, a brief comparison between the InGaN/GaN quantum well and quantum dot saturable absorbers will be presented 79 Chapter 3.1 Saturable Absorbers with GaN-based Quantum Structures InGaN/GaN quantum well saturable absorbers In this work, InGaN/GaN MQW samples with different numbers of quantum wells have been studied for application as saturable absorbers Similar characteristics have been found for these quantum well saturable absorbers As mentioned in Section 1.4.2, the modulation depth of a saturable absorber increases with the increased number of quantum wells once the quantum well composition and width are fixed But when the number of quantum wells further increases, the crystal quality in the active region would be degraded Hence, in this section, we present the results from an InGaN/GaN MQW sample consisting of eight periods of quantum wells (hereafter referred to as InGaN/GaN 8-QW saturable absorber) It has a relatively large number of quantum wells while still showing reasonably good crystal quality The schematic structure of this sample is shown in Fig 3.1 The sample was grown on a double-side polished c-plane (0001) sapphire substrate by MOCVD A 30-nm LT grown GaN buffer layer was first grown to accommodate the lattice mismatch between GaN and sapphire Then a 1.8-µm HT grown GaN buffer layer was grown, followed by eight periods of 3-nm In0.1Ga0.9N (nominal composition) wells and 15-nm GaN barriers On the top of the last quantum well was a 30-nm GaN cap layer For the nonlinear transmittance study, the light is incident from the sapphire substrate side as indicated in Fig 3.1 So a dielectric AR coating was deposited onto the sapphire substrate by PECVD to avoid the reflection from sapphire and air interface The details of this AR coating will be presented in Section 5.1 80 Chapter Saturable Absorbers with GaN-based Quantum Structures Figure 3.1 Schematic structure of the InGaN/GaN 8-QW saturable absorber The photoluminescence (PL) spectra from the QWs were measured at room temperature using the Accent Rapid Photoluminescence Mapping System (RPM 2045) with He-Cd laser (325nm) as the excitation light source Reflectance from DBR and SESAM were measured using the Shimadzu UV-VIS-NIR Scanning Spectrophotometer The nonlinear transmittance of the InGaN/GaN QW sample was measured based on an experimental configuration of the single-beam Z-scan technique A substantial advantage of the Z-scan technique is that the power of the incident laser beam is kept constant The variation of incident energy fluence is obtained by moving the sample along the beam axis about the focal point The light source used was a Kerr-lens mode-locked Ti:sapphire laser tunable between 720 and 920 nm wavelength with a repetition rate of 90 MHz The output pulses of the 81 Chapter Saturable Absorbers with GaN-based Quantum Structures Ti:sapphire laser were then frequency-doubled using 1-mm-thick BBO crystal A bandpass filter was used to eliminate the fundamental infrared pulses from the second-harmonic generated near-UV pulses The transverse mode of the second-harmonic beam was of near TEM00 mode The wavelength of the near-UV pulses with pulse duration of about 200 fs (FWHM) was tuned between 390 and 410 nm in our measurement A short focal length lens (f = 25 mm) was used to focus the beams on the sample Figure 3.2 shows the linear transmittance spectrum from this InGaN/GaN 8-QW saturable absorber It was recorded at room temperature using the spectrophotometer An enlarged plot near the GaN band edge is shown in the inset The absorption band edge of GaN can be clearly observed at around 365 nm, whereas the absorption by InGaN wells is too small to be observed because of the thin quantum well layers Also, severe interference-induced oscillating fringes were observed on the transmittance spectrum This interference is mainly caused by the thick GaN HT buffer, and it may cause similar interference fringes on the stopband of the corresponding SESAM structure This issue will be further investigated in Chapter 82 Chapter Saturable Absorbers with GaN-based Quantum Structures Figure 3.2 Linear transmittance spectrum from the InGaN/GaN 8-QW saturable absorber The inset shows an enlarged part in the wavelength range of 350 - 500 nm Because the absorption wavelength of the quantum wells cannot be obtained in the linear transmittance spectrum, PL was then measured at the room temperature with the PL mapping system to obtain the emission property of the quantum wells From our experience, the absorption wavelength of a MQW sample is normally about 5-10 nm shorter than the PL emission wavelength Thus, with the PL emission wavelength, we can then estimate the absorption wavelength of the quantum well and tune our laser source to find the suitable incident wavelength for the nonlinear transmittance/absorption measurements Figure 3.3 shows the room temperature PL spectrum from the InGaN/GaN 8-QW saturable absorber As can be observed, the PL emission peaked at around 414 nm Therefore, the wavelength of 408 nm was used as the incident laser wavelength for the nonlinear transmittance/absorption measurements 83 Chapter Saturable Absorbers with GaN-based Quantum Structures 414 nm Figure 3.3 absorber Room-temperature PL spectrum from the InGaN/GaN 8-QW saturable Figure 3.4 shows the transmittance of the 408-nm light through the InGaN/GaN 8-QW saturable absorber at different incident energy fluencies The transmittance curve clearly exhibited a nonlinear behavior Also, the nonlinear transmittance curve has been converted to saturable loss versus incident energy fluence using Eq (1.7), and the results are plotted in the inset of Fig 3.4 As can be observed, the modulation depth of this sample was about 6.5 % Because the sample was grown under normal crystal growth conditions and there was no further post-growth treatment (such as ion bombardment in order to decrease the recovery time), the sample is expected to have good crystal quality with relatively long carrier lifetime Thus, it should work as a slow absorber Simulation based on the experimental data and slow absorber theory verifies that this absorber is indeed a slow absorber In the inset of Fig 3.4, the solid squares are the experimental data, and the 84 Chapter Saturable Absorbers with GaN-based Quantum Structures solid curve is the simulation curve fitted by Eq (1.5) derived from the slow absorber theory Our experimental data matches well with the simulation curve for the following values: Fsat = 5.5 μJ/cm2 and q0 = 0.080 Figure 3.4 Transmittance versus the incident energy fluence at the incident wavelength of 408 nm for the InGaN/GaN 8-QW saturable absorber, measured at room temperature The inset shows the saturable loss versus incident energy fluence (solid squares) and the simulation curve based on the slow absorber theory (solid curve) As shown in Fig 3.4, clear nonlinear transmittance property has been observed in InGaN/GaN quantum well saturable absorbers However, it is so far difficult to determine the appropriate modulation depth value for GaN-based saturable absorbers The answer could only be clear after the actual mode-locking experiment is conducted using GaN-based saturable absorbers The modulation depth can then be adjusted by adjusting the number of quantum wells 85 Chapter 3.2 Absorption Saturable Absorbers with GaN-based Quantum Structures recovery time reduction in InGaN/GaN quantum well saturable absorbers In Section 3.1, the nonlinear transmittance properties of InGaN/GaN quantum wells for application as saturable absorbers have been investigated For passive mode-locking by SESAMs, the absorption of a saturable absorber should recover to its initial state in a short time (a few picoseconds to a few tens of picoseconds), in order to achieve efficient pulse shaping But the absorption recovery times of the epitaxially grown compound semiconductors normally fall in the nanosecond range, as will be shown later in Fig 3.9 Therefore, many methods have been developed to purposely reduce the recovery times of the saturable absorbers The most common methods are LT growth [Gupta1992], ion implantation [Delpon1998], and proton bombardment [Gopinath2001] More recently, recovery time reduction by controlling the InP buffer thickness has been demonstrated in the metamorphically grown GaAs-based SESAMs [Suomalainen2005] The purpose of these methods is to introduce deep levels in the bandgap of the active material so as to reduce the carrier lifetimes through the enhanced non-radiative recombination In this section, we present a novel and easy method to reduce the absorption recovery time of GaN-based saturable absorbers The absorption recovery time is controlled via controlling the crystal quality at the active region through engineering the GaN buffer thickness The influence of GaN buffers on the crystal quality and absorption recovery time of the InGaN/GaN quantum well saturable absorber is investigated 86 Chapter Saturable Absorbers with GaN-based Quantum Structures Three InGaN/GaN MQW samples, marked as A, B and C, were grown by MOCVD on double-sided polished c-plane sapphire substrates Before the growth of MQWs, the LT GaN buffer was first deposited at 520oC, followed by a HT GaN buffer grown at 1020oC The InGaN/GaN MQWs consisted of five periods of 3-nm In0.18Ga0.82N (nominal composition) wells and 15-nm GaN barriers Finally, a 30-nm GaN cap was deposited The schematic structure of these InGaN/GaN MQW samples is shown in Fig 3.5 The LT and HT GaN buffer thicknesses of each sample are listed in Table 3.1 These InGaN/GaN MQW samples act as saturable absorbers Figure 3.5 Table 3.1 Schematic diagram of the InGaN/GaN MQW structures LT GaN buffer and HT GaN buffer thicknesses of samples A to C The absorption recovery times of the saturable absorbers studied in this section were obtained by a pump-probe setup The light sources used in this 87 Chapter Saturable Absorbers with GaN-based Quantum Structures field and narrower well-widths would have a much higher internal quantum efficiency than that of the c-plane MQWs [Nishizuka2004] Therefore, it is possible for PL band C3 to have a stronger intensity than band C1, although only a small number of {11 2m } faceted MQWs exist Recent studies on the semi-polar GaN films [Baker2006] and the InGaN/GaN MQWs grown on the semi-polar GaN templates [Sharma2005; Haffouz2006] have similarly demonstrated that the {11 2m } faceted MQWs have higher emission efficiency than c-plane MQWs In the {10 1} faceted MQWs, the piezoelectric field is as weak as that in the {11 2} faceted MQWs However, because of the large bandgaps of these {10 1} faceted wells, both the conduction band and the valence band discontinuities in such MQWs are minimal Moreover, considering that these samples were deliberately designed to give short recovery time with a large number of dislocations and defects, it is easy for the carriers in these {10 1} faceted MQWs to escape from the wells to the barriers, using the defect states as stepping stones Consequently, few carriers can recombine radiatively in the wells This is in close agreement with the phenomenon that, in both samples, bands B2 and C2 are the weakest in PL intensity In summary, in the InGaN/GaN MQW samples B and C with reduced GaN buffer thicknesses, three types of MQWs with different facet morphologies were studied: 1) regular c-plane MQWs; 2) MQWs grown on the {10 1} faceted sidewalls, contributing to a PL band with much higher emission energies than the c-plane MQW emission band; and 3) MQWs grown on the {11 2m } ( m ≥ ) faceted sidewalls, which gave a PL band with emission energies in between those of the first two 116 Chapter Saturable Absorbers with GaN-based Quantum Structures categories The existence of high-density large V-pits and the MQWs with different facet morphologies in samples B and C could be related with their ultra-short absorption recovery times Further investigations are needed to explore this relationship 3.4 InGaN/GaN quantum dot saturable absorber As mentioned in Section 1.3, due to their 3D carrier confinement effect, quantum dot structures are gaining increasing interests for application in the optoelectronic devices In the earlier part of this chapter, the saturable absorbers fabricated with InGaN/GaN quantum wells were studied In the future mode-locking operation, if the bandwidth of a SESAM can be increased, shorter pulses can then be generated For a SESAM, the DBR high-reflectance stopband should be broader than the absorption spectrum of the saturable absorber The effective bandwidth of the SESAM is then mainly related the absorption spectrum of the saturable absorber Compared to the quantum well saturable absorbers, quantum dot saturable absorbers have a broad-band absorption spectrum due to the inhomogeneous broadening associated with a variation of dot sizes, and hence have potential with respect to the generation of much shorter pulses The PbS quantum dots in glass saturable absorbers have been demonstrated for mode-locking the Nd3+:KGd(WO3)2 laser and the Nd3+:Y3Al5O12 laser The absorption recovery time could be as short as 23 ps [Malyarevich2002] Also, because of the ultra-short recovery time of the InAs/InGaAs quantum dot saturable absorber, the pulses as short as 158 fs have been generated from a passively mode-locked Cr4+:forsterite laser [McWilliam2006] Therefore, the 117 Chapter Saturable Absorbers with GaN-based Quantum Structures use of quantum dot saturable absorbers is a promising approach to achieve fast recovery of the absorption without additional post-growth processing or buffer thickness engineering as described in Section 3.2 In this section, in order to obtain ultra-short absorption recovery times in the blue wavelength region, an InGaN/GaN quantum dot saturable absorber operating in the blue region was fabricated, and its basic properties as a saturable absorber were investigated GaN-based quantum dots fabricated using different methods have been reviewed in Section 1.3 The quantum dots can be fabricated by the SK growth mode through the self-assembly method without any surfactant But in some cases, the lattice mismatch induced strain may not be sufficient to result in the SK growth mode in the nitride system Therefore, Si as an anti-surfactant was used to modify the GaN or AlGaN surface states to realize nitride quantum dots In this work, the InGaN/GaN quantum dot structure was fabricated using the method developed by Chen et al [Chen2006] in our group A 30-nm LT GaN buffer layer was first grown by MOCVD on a double-side polished c-plane sapphire substrate A 2-µm HT grown GaN buffer was then deposited as a template for the following quantum dot growth Subsequently, a 100-nm SiO2 layer was deposited onto the GaN template by PECVD at 280oC Before the epitaxial growth of InGaN, the sample was dipped in the diluted hydrofluoric acid to remove the SiO2 layer and the native oxide on GaN The purpose of SiO2 deposition and removal was to modify the GaN surface states so as to facilitate the formation of quantum dots, which will be discussed later Immediately after the cleaning process, three pairs of InGaN/GaN 118 Chapter Saturable Absorbers with GaN-based Quantum Structures MQWs were grown at 740oC with the nominal In composition of 17% on the pretreated GaN template The thicknesses of the layers are indicated in Fig 3.18 Finally, a 30-nm GaN capping layer was deposited Figure 3.18 Schematic diagram of the 3-layer InGaN quantum dot sample The formation mechanism of quantum dots fabricated by the above method has been reported by Chen et al [Chen2006] During the deposition of the SiO2, a few Si atoms occupy the surface sites of GaN, and these Si atoms remain on the surface even after the SiO2 removal Therefore, the surface states are modified and result in the subsequent growth of InGaN quantum dots in the MOCVD reactor It is similar to a traditional fabrication method where the Si anti-surfactant was used to modify the surface states of GaN or AlGaN This is the formation mechanism for the first InGaN quantum dot layer in our sample For the subsequent two layers of InGaN, quantum dots can only be formed if the GaN barriers are thin enough to allow the strain created by the quantum dots formed in the first layer to be transmitted to the growing surface 119 Chapter Saturable Absorbers with GaN-based Quantum Structures Therefore, it also requires that the first layer quantum dots were made thick enough As shown in Fig 3.18, the InGaN layer thicknesses decreased from the first layer to the last layer so as to ensure the growth of quantum dots in the subsequent InGaN layers This method of fabricating multi-layer quantum dots is convenient But it would be difficult to grow quantum dots of more than three layers This issue will be further discussed in the later part of this section Because this sample consisted of three layers of InGaN and the surface was capped with a thick GaN layer, the morphology of the InGaN layers could not be directly investigated by AFM However, the surface morphologies of the single-layer InGaN quantum dot samples grown under the same mechanism without the GaN capping layer have been studied [Chen2006] Figure 3.19 shows the AFM images of the uncapped single-layer InGaN quantum dots grown at 740 °C on the SiO2 pretreated GaN surface with the growth duration of 60 s and 120 s The growth rate was about 0.08 nm/s As can be seen, when the InGaN growth lasted for 60 s, the lateral sizes of the quantum dots were 37±11 nm and the average height was about 3.6 nm The quantum dots were distributed with a density of 9×1010 cm-2 For the sample with the InGaN growth duration of 120 s, the lateral sizes of the quantum dots increased to 48±14 nm and the average height increased to about 11 nm; while the density of the quantum dots decreased to 5×1010 cm-2 For our 3-layer quantum dot sample, the thickness of the first InGaN layer was controlled to be about nm, which is in between the InGaN thicknesses of the above two samples (3.6 nm and 11 nm) Therefore, for our 3-layer quantum dot sample, considering that it was fabricated 120 Chapter Saturable Absorbers with GaN-based Quantum Structures under the same growth conditions, it is expected that the lateral sizes and the dot density of the first InGaN quantum dot layer were between those of the two samples shown in Fig 3.19 Figure 3.19 AFM images of uncapped single layer InGaN quantum dot samples grown on the SiO2 pretreated GaN surface at 740oC for (a) 60 s and (b) 120 s [Chen2006] The cross-sectional TEM characterization was also conducted on this 3-layer InGaN quantum dot sample, as shown in Fig 3.20 Because the first InGaN layer was 121 Chapter Saturable Absorbers with GaN-based Quantum Structures the thickest among the three layers and was grown directly on the SiO2 pretreated GaN surface, it clearly showed the formation of quantum dots exhibiting nano-islands in the cross-sectional image The quantum dots observed in the figure has a height of ~ nm and a lateral size of ~ 20 nm In the second InGaN layer, quantum dots with larger lateral sizes were observed, and the height was decreased to ~ nm In the third InGaN layer, there was no clear observation of quantum dots but only small thickness variations across the layer can be seen Therefore, the last InGaN layer can also be simply considered as a quantum well It also contributes to the nonlinear absorption Figure 3.20 X-TEM image of the InGaN/GaN 3-layer quantum dot sample As mentioned above, the quantum dot growth mode can be exhibited in the first InGaN layer because of the modified GaN surface states, and in the subsequent InGaN layers, the quantum dot growth mode would be continued if the first InGaN 122 Chapter Saturable Absorbers with GaN-based Quantum Structures layer is thick enough and the GaN barriers are thin enough to ensure that the strain exists in the growing surface In our sample, as shown in Fig 3.20, the last InGaN layer did not show the clear formation of quantum dots This is mainly because the GaN barriers in this sample are thick (15 nm) If the barrier thickness could be reduced to about 10 nm, it is expected that, the quantum dot formation could still be clearly observed in the third InGaN layer Nevertheless, we believe that, even with thinner GaN barriers, it might still be difficult to maintain the clear quantum dot growth mode for more than three InGaN layers In this work, because all the InGaN/GaN quantum well samples studied earlier are with the GaN barrier thickness of 15 nm, we would like to keep the same barrier thickness for this quantum dot sample Also, the total thickness of the InGaN layers were 16 nm in this work (8 + + 3= 16 nm), which is almost the same as that of the 5-period quantum well samples A, B and C studied in Section 3.2 (3 nm × = 15 nm) Hence, the nonlinear transmittance/absorption results obtained from this quantum dot sample could be compared with the results from quantum wells presented in Section 3.2 The optical properties of this quantum dot sample were then investigated The room temperature PL emission spectrum is shown in Fig 3.21 A strong PL emission band with a peak at 438 nm was observed The bandwidth (FWHM) of this emission band was as large as 200 meV, mainly due to the formation of quantum dots with different sizes and compositions 123 Chapter Saturable Absorbers with GaN-based Quantum Structures 438 nm Figure 3.21 Room temperature PL spectrum of the InGaN/GaN 3-layer quantum dot sample The nonlinear transmittance property of this quantum dot saturable absorber was then characterized by the pump-probe system sketched in Fig 3.6 The incident laser wavelength was 410 nm The repetition rate of the laser source was kHz, which produced an incident energy fluence tunable from hundreds of μJ/cm2 up to 100 mJ/cm2 The maximum incident energy fluence used for this measurement was about 85 mJ/cm2, which was 10 – 20 % below the photo-damage threshold of GaN This measurement condition is the same as that used for the nonlinear transmittance study in quantum well saturable absorbers A, B and C in Section 3.2, so that the transmittance results could be compared with those obtained from the quantum well saturable absorbers (Fig 3.11) Figure 3.22 shows the transmittance of the 410-nm light through the quantum dot sample at different incident energy fluences Also, the nonlinear 124 Chapter Saturable Absorbers with GaN-based Quantum Structures transmittance curve has been converted to saturable loss versus incident energy fluence using Eq (1.7), and the results are plotted in the inset of Fig 3.22 The solid squares in the inset are the experimental data, and the solid curve is the simulation curve fitted by Eq (1.5) derived from the slow absorber theory It can be observed that, at the maximum incident energy fluence of ~ 85 mJ/cm2, the absorption saturation has not been achieved This is similar to the results shown in Fig 3.11 for quantum well saturable absorbers A, B and C In the insets of Fig 3.22, simulation using the slow absorber theory estimated the modulation depth and the saturation fluence of this quantum dot sample to be about 20 % and 10 mJ/cm2, respectively Figure 3.22 Transmittance versus the incident energy fluence at the incident wavelength of 410 nm for the 3-layer InGaN quantum dot sample The inset shows the saturable loss versus incident energy fluence (solid squares) and the simulation curve based on the slow absorber theory (solid curve) As observed in Fig 3.22, the transmittance of the quantum dot saturable absorber at the lowest incident energy fluence was about 50% This value is almost 125 Chapter Saturable Absorbers with GaN-based Quantum Structures the same as those of the quantum well saturable absorbers A, B and C in Section 3.2 As mentioned earlier, because there is no AR coating deposited on the incident surface, which is the GaN capping layer, the surface reflection could be as large as about 30% Also, it is observed that the saturation fluence Fsat of this quantum dot saturable absorber was 10 mJ/cm2, which is smaller than those of the quantum well saturable absorbers studied in Section 3.2 (14.7 mJ/cm2, 16.7 mJ/cm2 and 21.5 mJ/cm2 for samples A, B and C, respectively) Therefore, for this quantum dot saturable absorber, lower energy fluence is required for the onset of the nonlinear absorption behavior Moreover, the q0 value (almost identical to the modulation depth) of this 3-layer quantum dot saturable absorber was about 20%, which is much larger than those of the 5-QW saturable absorbers (11.5%, 6.6% and 5.6% for samples A, B and C, respectively, in Section 3.2) The much larger modulation depth in our quantum dot sample is mainly attributed to the enhanced nonlinearity of the zero-dimensional quantum dot structure and also the availability of quantum dots with different sizes But as mention earlier, it is so far difficult to determine the best modulation depth value If a smaller modulation depth is needed, a single-layer quantum dot saturable absorber can then be used Subsequently, the absorption recovery time of this quantum dot saturable absorber was investigated also by the pump-probe technique using the system shown in Fig 3.6 The repetition rate of the laser was 80 MHz and the pump energy fluence was 51 μJ/cm2 These conditions are the same as those used for the measurement of recovery times in quantum well saturable absorber samples A, B and C in Section 3.2 126 Chapter Saturable Absorbers with GaN-based Quantum Structures The time-resolved transmittance response obtained from this quantum dot saturable absorber is shown in Fig 3.23 It is known that a single quantum dot shows a single-exponential decay behavior; while the quantum dot ensemble usually shows a non-exponential PL decay behavior, which is the summation of mono-exponential decays originating from each individual quantum dot in the ensemble [Dworzak2004] Hence, the non-exponential decay behavior of the quantum dot ensemble could be fitted by the following stretched exponential model [Scher1991]: ( y ( t ) = y0 exp ⎡ − t / τ * ⎢ ⎣ ) β ⎤ ⎥ ⎦ (3.1) where τ* is the time constant and β is the stretching parameter As shown in Fig 3.23, the recovery time behavior of this quantum dot saturable absorber was fitted using the above stretched exponential model with a time constant τ* of 60 ps and a stretching parameter β of 0.39 The deviation of β from unity is a measure of the degree of disorder defined by the distribution of quantum dot size and composition A small value of the stretching parameter β indicates the severe size and composition variation of the quantum dots in this sample Dworzak et al have also reported different InGaN quantum dot samples with the stretching parameter β varied from 0.35 to 0.80 By studying the spectral dependence of time constants for single InGaN quantum dots, they showed that a quantum dot with higher transition energy generally has a shorter time constant, because of the increasing importance of escape processes [Dworzak2004] In fact, the dynamics of luminescence are governed by two competing processes: the transition probability which primarily depends on electron-hole wave-function overlap and the probability of nonradiative 127 Chapter Saturable Absorbers with GaN-based Quantum Structures recombination For quantum well saturable absorber samples B and C in Section 3.2, the recovery times were reduced by introducing increased number of dislocations and defects so as to enhance the nonradiative recombination In this section, for our quantum dot sample, the ultra-short recovery time (60 ps) is mainly attributed to the increased transition probability, because of the increased electron-hole wave-function overlap in the quantum dots with three-dimensional confinement This quantum dot saturable absorber with the recovery time of 60 ps could be directly used as a saturable absorber for passive mode-locking and ultra-short pulse generation without further treatment or structure modification to purposely reduce the recovery time Figure 3.23 Time-resolved transmittance response of the InGaN/GaN 3-layer quantum dot saturable absorber at the pump energy fluence of 51 μJ/cm2 The solid curve shows the stretched exponential curve-fitting result according to the experimental data points ΔT/T0 is the change in transmittance, and ΔT/T0 = (T-T0)/T0 (T, T0: transmittance with and without pump beam, respectively) The results presented in this Chapter shows that using quantum dots as the active region of the saturable absorber is an easier method to achieve ultra-short 128 Chapter Saturable Absorbers with GaN-based Quantum Structures recovery time without sacrificing the crystal quality For quantum well saturable absorbers, in order to achieve ultra-short recovery time, dislocations and defects have to be purposely introduced to the active region so as to enhance the nonradiative recombination The degradation in the crystal quality would cause the reduction of the modulation depth In comparison, for the quantum dot saturable absorber, the ultra-short recovery time can be easily achieved through the increased electron-hole wave-function overlap without compromising the crystal quality as well as the modulation depth 3.5 Chapter summary In this Chapter, the nonlinear properties of saturable absorbers fabricated with GaN-based quantum wells operating in the blue wavelength region were first investigated Because quantum well saturable absorbers grown under normal growth conditions exhibit long absorption recovery times (hundreds of picoseconds to more than μs), a novel and convenient method to reduce the recovery time of InGaN/GaN quantum well saturable absorbers was developed The recovery time was effectively reduced by introducing dislocations and defects into the active region through engineering the buffer thickness An absorption recovery time as short as 34 ps was achieved by an InGaN/GaN quantum well saturable absorber with a 15-nm LT GaN buffer and a 500-nm HT GaN buffer The modulation depth was also found to be reduced due to the degraded crystal quality Subsequently, it was found that the quantum well samples with reduced buffer thicknesses exhibited high density of large V-pits, which could be related with their ultra-short absorption recovery time So the 129 Chapter Saturable Absorbers with GaN-based Quantum Structures experimental evidence from SEM, TEM and PL were investigated In addition to the regular c-plane MQWs, the MQWs grown on the {10 1} faceted sidewalls of the V-pits were also observed, which gave much higher emission energies than those of the c-plane MQWs When the low-temperature GaN buffer was very thin, the {11 2m } ( m ≥ ) faceted sidewalls of the V-pits were observed The MQWs grown on such sidewalls had emission energies between those of the c-plane MQWs and those of the {10 1} faceted sidewall MQWs Towards the end of this Chapter, a 3-layer InGaN quantum dot sample was fabricated and its nonlinear property as a saturable absorber was investigated Because of the increased electron-hole wave-function overlap in the quantum dots with three-dimensional confinements, an absorption recovery time of 60 ps was demonstrated The modulation depth of this quantum dot saturable absorber was as large as 20 %, mainly due to the large size and composition variations of the quantum dots Therefore, using quantum dots as the active region of the saturable absorber is an easier way to achieve the ultra-short absorption recovery time without sacrificing the crystal quality as well as the nonlinear property More work is required to further explore the fabrication technique of GaN-based quantum dots and the nonlinear properties of quantum dot saturable absorbers 130 ... Section 3. 3 will be dedicated to the study on the effects of V-pits on the morphological and optical properties of InGaN/ GaN quantum wells Finally, in Section 3. 4, the InGaN/ GaN quantum dot saturable. .. this study will only be focused on the saturable absorbers and SESAMs operating in the blue region In this chapter, Section 3. 1 will be focused on the nonlinear property of the InGaN/ GaN quantum. .. presented 79 Chapter 3. 1 Saturable Absorbers with GaN- based Quantum Structures InGaN/ GaN quantum well saturable absorbers In this work, InGaN/ GaN MQW samples with different numbers of quantum wells