Chapter 4 Results and discussions
4.2 Optical pumping of VCSELs
4.2.3 Polarization
A polarizer was placed in front of the spectrometer in the optical pumping system and the emission spectra of VCSELs were measured at variable angle of the polarizer.
The laser emission intensity as a function of the angle of the polarizer was described as shown in Fig 4.14. The solid line in the figure is the fitting curve which follows the function of cos2θ, where θ is the polarizer rotation angle. The intensity of ideal linear polarized light transmitted through a polarizer is the function of cos2θ. In Fig 4.14, it can be seen the experimental data partially matched the fitting curve.
Besides, the results showed a degree of polarization of about 70% estimated using equation (4.4), where Imax is maximum transmitted output power and Imin is minimum transmitted output power. The value of about 70% and partial a cosine square variation suggest a strong linear polarization property of VCSELs.
min
less sensitive to change in temperature. It is well known that high-temperature sensitivity of the lasing threshold usually limits the performance of a laser under high-temperature operation. On the other hand, weak temperature dependence for the lasing threshold implies that the high-temperature performance limit for laser operation can be significantly expanded.[32]
The variation of the lasing threshold as a function of temperature is plotted in Fig. 4.15. The solid line in Fig. 4.15 represents the best fit of the experimental data to the empirical form (4.5) for the temperature dependence of the lasing threshold.
0 where I0 is a constant, Ith is the threshold optical pumping power, T is the operating temperature and T0 is characteristic temperature.
The characteristic temperature T0 was estimated to be 278 K over the temperature range from 80 to 300 K for our VCSELs. This value of T0 for GaN based VCSELs is slightly lower than the estimated value of ~300K reported by Iga et al.[33]
This is probably because the experimental T0 might be suppressed by leakage currents and non-radial recombination which was not considered in the modal theory occurred in the active layer.
4.2.5 The spontaneous emission factor (β)
The spontaneous emission factor (β) indicates the coupling efficiency of the spontaneous emission to the lasing mode. This factor could be generally obtained from the difference between the heights of the emission intensities on a logarithmic scale before and after lasing. Therefore, Fig. 4.16 replots the emission intensities shown in Fig. 4.8 on a logarithmic scale and the value of the spontaneous emission factor β is about 10-3. The value of β (~10-3) is smaller than the value of about 10-2 mentioned in others’ papers.[34-35] This is partially because the cavity length of our devices are larger than that of their devices and the spontaneous emission factor (β) decreases with a raise in the cavity mode volume.
Figures of chapter 4
Fig.4.1. The PL spectra of as-grown GaN wafers. It showed the emission peak centered at 414nm with FWHM of 18nm,
350 400 450 500
Fig.4.2. The PL spectra of cavity with only one side high reflectivity DBR, DBR
350 375 400 425 450 475 0
500 1000 1500 2000 2500
Intensity(a.u.)
Wavelength(nm)
Fig.4.4. The related PL spectra of structure I
380 400 420 440 460 480 500
0 2 4 6 8 10
1.66um 2.2um 2.8um
4.0um(unpolished)
Intensity(a.u.)
Wavelength(nm)
Fig.4.3. The PL spectra of the complete VCSEL structure, which was named as structure II, and the inset is the layer structure of structure II.
Glass
4µm GaN
DBR
Epoxy DBR
MQW Structure II
350 400 450 500 550 600
Fig.4.5. The related PL spectra of structure I and structure II with a cavity length of about 1.66μm.
350 400 450 500 550 600
0
PL of GaN microcavity
Fig.4.6 The simulated reflectivity and experimental PL spectrum of a
Fig.4.7. This diagram showed the complete structure of the GaN-based VCSELs with two dielectric DBRs and the direction of collected emission light for PL spectra or emission images.
host substrate FWHM of emission wavelength
FWHN(nm)
Fig.4.8. Laser emission intensity and FWHM of emission wavelength as a function of the pumping energy operated at room temperature. The threshold pumping energy is about 270 nJ.
390 400 410 420 430 440
0 500 1000 1500 2000 2500 3000 3500 4000
0.8nm 0.25nm
0.27Pth Pth
Emission Intensity(a.u.)
Wavelength(nm)
1.07Pth
Fig.4.9. Emission spectra from the GaN-VCSEL at various pumping energy.
The lasing emission wavelength is 414 nm with a linewidth of 0.25 nm.
(a)Below threshold (b)Above threshold
Lasing by cracks
Fig.4.10. Emission images of cracks on low quality GaN-based LED without DBR structure (a)below and (b)above the lasing threshold.
50µm
(a) (b)
(d)
0.2Eth 0.9Eth
1.3Eth 1.2Eth
(c)
360 380 400 420 440
0 1000 2000 3000 4000 5000 6000 7000
Below threshold Above threshold
Intensity(a.u.)
Wavelength(nm)
Fig.4.11. The PL spectra of cracks on low quality GaN-based LED without DBR structure below the lasing threshold and above the lasing threshold
Fig.4.12. Near field emission images of a single GaN-based VCSEL at various pumping energy of 0.2Eth, 0.9Eth, 1.2 Eth, and 1.3 Eth for Fig. 4.12(a), 4.12(b), 4.12(c), and 4.12(d), respectively. The light emission from the circular disk area depict the laser emission pattern.
Fig.4.13. Laser emission patterns from a single GaN-based VCSEL at pumping energy of 1.3 Eth. (a) 2-D emission intensity profiles with a near-Gaussian distribution. (b) 3-D distribution emission distribution of laser emission.
(a) (b)
Fig.4.14. Transmitted output power of VCSEL versus Polarizer rotation
0 50 100 150 200 250 300 350
20 40 60 80 100 120 140 160 180
200 Experimental data Fitting curve
Degree (o)
Intensity (a.u.)
Fig.4.15 The variation of the lasing threshold as a function of temperatures is plotted and the solid line represents the best fit of the experimental data.
50 100 150 200 250 300
6.0 6.2 6.4 6.6 6.8 7.0 7.2
7.4 Experimental data
Linear fit of experimental data
ln(E th/E 0)(a.u.)
Temperature(K)
Fig.4.16 Input-output characteristic on logarithmic scales. Dash line are the theoretical fitting.
50 100 150 200 250 300 350
1E-4 1E-3 0.01 0.1 1 10 100
Intensity(a.u.)
Excitation Energy(nJ/pulse)
sample 1 Sample 2 PL
Calculatedthickness ~1.9um Calculated thickness ~1um SEM
Table 4.1. Structure of samples consist of 4μm GaN layer, a multiple quantum-well (MQW) composed of 10 periods of 5-nm GaN barrier and 3-nm In0.1Ga0.9N well, and 280nm GaN layer fabricated on GaAs substrate by laser lift-off technique and bonding technique. The PL spectra, calculated thickness and SEM pictures of samples were arranged in above table.
350 400 450 500 550 600
1000
350 400 450 500 550 600
1000
Chapter 5 Conclusions and future work
5.1 Conclusions
We have fabricated an optical pumped GaN-based vertical cavity surface emitting laser which was formed by two dielectric DBRs of SiO2/TiO2 and SiO2/Ta2O5. In order to fabricate resonant cavity with high Q factor, we also successfully developed a laser lift-off technique to transfer GaN-based film. Besides, we also optimized the recipe of polish process to get optical flatness for deposition of dielectric DBRs. By utilizing above techniques, we fabricated resonant cavity with high Q factor of 518.
The lasing action was obtained from a GaN-based VCSEL with In0.1Ga0.9N/GaN MQWs and two dielectric DBRs of SiO2/TiO2 and SiO2/Ta2O5
fabricated by a laser lift-off technique. The laser emits blue-violet wavelength at 414 nm under optical pumping at room temperature with threshold energy of 270 nJ. The laser emission has a narrow linewidth of 0.25nm and a degree of polarization of 70%.
The characteristic temperature of our GaN-based VCSEL is about 278K. The laser emission images, including near-field patterns and far-field patterns clearly indicate the vertical lasing emission of the VCSEL.
5.2 Future work
The optical characteristics of GaN-based VCSELs will be investigated in detail, such as influence of temperature for wavelength and so on. Besides, electrical pumped GaN-based VCSELs would be fabricated in the future. The electrical pumped devices are more useful than optical pumped devices depending on the viewpoint of application. Therefore, we will try to realize electrical pumped VCSEL by developing some extra techniques such as deposition of metal, dry etching and ion implant.
However, some difficulties which are not met in optical pumped devices could be anticipated for electrical pumped devices. For example, leakage current and poor electrical properties of contact may be generated during a bonding process. The devices may be damaged by heat before laser threshold. Therefore, we will develop some important techniques as follow:
(1) Metal bonding technique
(2) Ion-implant technique for current confinement (3) Intra-cavity contacts
(4) Thermal annealing technique for ohmic contact and so on to accomplish electrical pumped GaN-based VCSELs.
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