Photoluminescence of the VCSEL structure
The photoluminescence (PL) emission was excited by a 325 nm He-Cd laser with a spot
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size of about 2-μm-diameter. By using the microscopy system (WITec, alpha snom), the emission was collected into a spectrometer/CCD (Jobin-Yvon Triax 320 Spectrometer) with a spectral resolution of ~0.15 nm for spectral output measurement. Figure 3.11 shows the PL emission of the as-grown structure and the VCSEL structure, and the reflectivity spectrum of the VCSEL structure. The PL emission peak wavelength and the FWHM of emission spectrum of MOCVD grown structure were 465 nm and 10.5 nm, respectively. After the coverage of top reflector, the PL emission of and the FWHM of emission spectrum of overall VCSEL structure became 464 nm and 0.6 nm, respectively. It is obvious that the PL emission peak wavelength of overall VCSEL structure was modified by cavity mode, which could be seen from the reflectivity spectrum of full structure (the dip in stop-band), and centered at 464 nm. The narrow FWHM of 0.6 nm is an evidence of the strong Febry-Perot cavity effect existing in our VCSEL sample.
Quality factor of the VCSEL structure
A narrow PL emission with full width at half maximum of 0.6 nm corresponds to the cavity resonant mode at 464 nm was observed. It indicates the spontaneous emission generated from MQWs was well-aligned the narrow vertical-cavity mode resulting from the high reflectivity of AlN/GaN DBR and dielectric mirror. The cavity quality factor is a value usually used to evaluate how good a cavity is. Generally, the cavity quality factor is defined as
λ Δ
= λ
Q , where λ is the wavelength emitted form cavity and Δλ is the FWHM of the emission peak. Therefore, we could obtain the Q value of our VCSEL structure to be about 777. This value was further confirmed and roughly consistent with the theoretical estimation using the following equations:
e L is absorption coefficient of GaN, and n is refractive index of GaN. Here we consider the
absorption loss in GaN at 460 nm is 1 - 200 cm-1 at 460 nm [12], and the high reflectivity of top and bottom reflectors are both 99% at 460 nm.
Optical pumping setup
The optical pumping of the sample was performed using a frequency-tripled Nd:YVO4
355-nm pulsed laser with a pulse width of ~ 0.5 ns at a repetition rate of 1 kHz. The pumping laser beam with a spot size of 60 μm was incident normal to the VCSEL sample surface. The light emission from the VCSEL sample was collected using an imaging optic into a spectrometer/CCD (Jobin-Yvon Triax 320 Spectrometer) with a spectral resolution of ~0.15 nm for spectral output measurement. Figure 3.12 shows the schematic diagram of the setup.
3-3.2 Characteristics of Nitride-based VCSELs Threshold characteristics
The light emission intensity from the VCSEL as a function of the pumping energy is shown in figure 3.13. A distinct threshold characteristic was observed at the threshold pumping energy (Eth) of about 180 nJ corresponding to an energy density of 6.4 mJ/cm2 (threshold energy density is 2.56 mJ/cm2 if considering some energy loss due to reflectivity of DBR). Then the laser output increased linearly with the pumping energy beyond the threshold.
The carrier density at the threshold is estimated to be about 2×1019 cm-3, assuming the reflectivity of the top mirror at pumping wavelength of 355 nm was 40%, the absorption coefficient of the GaN was about 105 cm-1 at 355 nm [12] and the quantum efficiency was 10%
[1]. We estimated the threshold gain (gth) of the VCSEL cavity using the equation:
)
where L is effective cavity length (including penetration depth of DBR, which could be c estimated using Eq. (2. 20), αi is absorption coefficient of GaN at lasing wavelength, N is w the number of quantum wells, L is the width of each quantum well and w R1, R2 are the reflectivity of the top and bottom mirrors, respectively. We obtained the required threshold
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gain is about 8.7×103 cm-1. The parameters used in the estimations of carrier density at threshold and threshold gain are listed in the Table 3.1. The threshold gain value at the threshold carrier density is slightly lower than the gain value of Park’s report [13].
Figure 3.14 shows the variation of emission spectrum with the increasing pumping energy.
A dominant laser emission line at 456 nm appears above the threshold pumping energy. The laser emission spectral linewidth reduces with the pumping energy above the threshold energy and approaches 0.2 nm above the pumping energy of 1.3Eth.
The emission images under different pumping energy were shown in figure 3.15. The blue spontaneous emission could be seen as the pumping energy was below threshold. With the pumping energy increasing above threshold, a laser spot with relatively strong intensity appears, and the intensity of the spot rapidly increases. This result shows the nitride-based VCSEL is a spot-type laser.
Laser polarization
The contrast of laser emission intensity between two orthogonal polarizations was measured by rotating a polarizer in front of the laser beam. Figure 3.16 shows the laser emission intensity as a function of the angle of the polarizer at the pumping energy of 1.71Eth. The variation of intensity with the angle of the polarizer shows nearly a cosine square variation. The degree of polarization (P) is defined as
min
the maximum and minimum intensity of the nearly cosine square variation, respectively. The result showed the laser beam has a degree of polarization of about 84% suggesting strong polarization property of the laser emission.
Near field pattern (NFP) and far field pattern (FFP)
The near-field pattern (NFP) and far-field pattern (FFP) of the laser were detected by the CCD and were plotted as shown in figure 3.17(A) and (B). Figure 3.17(A) shows the near-field emission intensity as a function of the spatial position. From the figure, we could
obtain the spot size defined by the FWHM and 1/e2 of the intensity profile to be about 1.3 µm and 3 µm, respectively. Figure 3.17(B) shows the far-field emission intensity as a function of the angle between the light direction and the axial perpendicular to the surface. The divergence angle determined by the FWHM could be estimated from the far-field profile to be as small as about 7.6o. Furthermore, both NFP and FFP show that the intensity distributions along two cross axes are almost the same. That is, the laser is actually a vertically emitted single-mode and circular-shape emission beam.
Characteristic Temperature
Figure 3.18 shows the semi natural-logarithm plot of the dependence of the threshold pumping energy (lnEth) on the operation temperature (T). The threshold energy gradually increased as the operation temperature rose from 120 K to 300 K. The relationship between the threshold energy and the operation temperature could be characterized by the equation Eth=E0×eT/T0, where T0 is the characteristic temperature and E0 is a constant. Therefore, we obtain a high characteristic temperature of about 244 K by linear fitting the experiment data.
This high T0 could be understood by some temperature-dependent properties of the components in the nitride structure, active region and DBR. The lasing wavelength shows a slight red shift about 1.6 nm as the temperature rose from 120 K to 300 K as shown in figure 3.19. The PL emission of the MQW was also measured and shows a red shift about 2.9 nm over the same temperature range. It should mean the gain peak moves somewhat faster than the cavity mode about 1.3 nm over the temperature range of 170 K. This shift is so small that the gain peak almost keeps aligning the cavity mode. In fact, the reflectivity of nitride-based DBR is also almost independent with the variation of temperature as shown in the figure 3.20.
That is, the slightly shifted gain peak actually could keep meeting the highest reflectivity although temperature was varied. Therefore, the superior high temperature performance of the GaN-based VCSEL structure could be attributed to the almost invariant reflectivity spectrum of AlN/GaN DBR, and less shift of the gain peak and cavity mode as the temperature rises,
47
and the ten-pair In0.2Ga0.8N/GaN MQW structure which could suppress the carrier leakage from the MQW active layers to the cladding layers and) the thick GaN cavity (880 nm in thickness) providing a good heat dissipation path during the high carrier injection and high temperature conditions [14].
The coupling efficiency of spontaneous emission (β)
In order to understand the β of this cavity, we normalized the scales of figure 3. 13 and re-plotted it in a logarithm scale as shown in figure 3.21. Furthermore, we used the Eq. (2. 35) to fit our data, and the fitting result shows the β value of our VCSEL is about 2×10-2. we also estimated the β value based on the approximation equation (2. 24):
where Fp is the Purcell factor, Q is the cavity quality factor, λ is the laser wavelength, Vc is the optical volume of laser emission, and n is the refractive index. As we have mentioned earlier, the photoluminescence spectrum of our GaN-based VCSEL showed a narrow emission peak with full width at half maximum of 0.6 nm, which corresponds to a cavity quality factor of 777. The refractive index is 2.45 for the GaN cavity. For the estimation of the optical volume, we used the laser spot size (at 1/e2 of near-field intensity profile) shown in figure 3. 17(a) which is about 3 μm and the cavity length of about 7λ with considering the penetration depth of the DBRs to estimate the Vc to be about 4×10-11 cm3. By using these parameters, the Purcell factor of about 2.8×10-2 was obtained, and we estimated the β value to be about 2.2×10-2, which is consistant with the above β value estimated from figure 3.21. This β value of the VCSEL is three order of magnitude higher than that of the typical edge emitting semiconductor lasers (normally about 10-5) [1, 15] indicating the enhancement of the spontaneous emission into a lasing mode by the high quality factor microcavity effect in the VCSEL structure.
References
1.T. Someya, R.Werner, A. Forchel, M. Catalano, R. Cingolani and Y.Arakawa, Science, 285, 1905 (1999)
2. H. Zhou, M. Diagne, E. Makarona, A. V. Nurmikko, J. Han, K. E. Waldrip and J. J. Figiel, Electron. Lett., 36,1777 (2000)
3. T. Ive, O. Brandt, H. Kostial, T. Hesjedal, M. Ramsteiner, and K. H. Ploog, Appl. Phys.
Lett., 85, 1970 (2004)
4. P. Mackowwiak and W. Nakwaski, J. Physics D: Applied Physics 33, 642 (2000) 5. P. Mackowwiak and W. Nakwaski, J. Physics D: Applied Physics 34, 954 (2001) 6. P. Mackowwiak, T. Czyszanowski, R. P. Sarzala, M. Wasiak, and W. Nakwaski,
Opto-Electron. Rev., 11, 119 (2003)
7. G. W. Wood, U. Ozgur, H. O. Everitt, F. Yun, and H. Morkoc, phys. stat. sol. (a), 188, 793 (2001)
8. 李正中, 薄膜光學與鍍膜技術, (1999)
9. E. D. Palik, Handbook of Optical Constants of Solids, Academic Press Inc (1985)
10. G. S. Huang, T. C. Lu, H. H. Yao, H. C. Kuo, S. C. Wang, C. W. Lin and L. Chang, Appl.
Phys. Lett., 88, 061904 (2006)
11. H. K. Cho, J. Y. Lee, and G. M. Yang, Appl. Phys. Lett., 80, 1370 _2002_.
12.G. Yu, G. Wang, H. Ishikawa, and M. Umeno, T. Soga, T. Egawa, J. Watanabe, and T.
Jimbo,Appl. Phys. Lett., 70, 3209 (1997)
13. S. H. PARK, Jpn. J. Appl. Phys., 42, L 170 (2003)
14. M. Ortsiefer, R. Shau, G. Bohm, F. Kohler, J. Rosskopf, and M. C. Amann, Phys. Stat.
Sol. (a)188, No. 3, 913-919 (2001)
15.T. Tawara, H. Gotoh, T. Akasaka, N. Kobayashi, and T. Saitoh, Appl. Phys. Lett., 83, 830 (2003)
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Figure 3.1 Simulated reflectivity spectra of 5, 10, 15, and 25 pairs of AlN/GaN DBR.
400 425 450 475 500
0
400 425 450 475 500
0
Figure 3.2 Simulated reflectivity spectra of three different nitride-based
DBRs with high reflectivity.
300 350 400 450 500 550 600
Figure 3.3 Simulated reflectivity spectra of 3, 5, and 8 pairs of Ta
2O
5/SiO
2DBR.
Figure 3.4 Electric field intensity and refractive index as a function of the
distance from top layer.
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