Chapter 1 Introduction
5.5 Effect of Indium inhomogeneity on laser characteristic
The laser emission patterns from the aperture show single-spot and multiple-spots emission patterns under different pumping conditions as shown in Fig.
5.13(a) and 5.13(b) at pumping energy of 1.15Eth and 1.2Eth. The lasing wavelength from the each emission spot also differed in few nanometers. At low pumping energy, the emission pattern showed a single spot with a spot size of about 3 μm. As the pumping energy increased, a second spot appeared showing a double spot emission pattern with a spatial separation of about 22 μm apart. The corresponding emission spectra of these two spots are shown in Fig. 5.13(c). The wavelength of the single emission spot is about 415.9 nm. For the double emission pattern, there is second emission wavelength at about 414.9 nm in addition to the 415.9 nm emission line.
Since the separation between these two spots is large compared to the axial mode spacing, the difference of the emission wavelengths could be caused by either the spatial non-uniformity in InGaN MQWs or the non-uniformity in the DBR cavity.
The band gap of InGaN is decided by the Indium composition in the alloy. The composition dependence of the room-temperature band gap of In1-xGaxN alloys can be
expressed by the following equation [5.21]
( )
x x(
x)
x(
x)
Eg =3.24 +0.771− −1.43 1− .
According to this equation, we can estimate the wavelength variation induced by
Indium composition change. To simply the calculation, the quantum effect is not considered in this estimation since we just discussed the composition dependence of emission wavelength. As shown by the lasing spectra, the difference of lasing wavelength is 1 nm, which can correspond to 0.002 (0.2%) of difference in Indium composition. This amount of variation in In composition is possible and reasonable for InGaN alloy grown by a MOCVD system.
In order to clarify the origin of these emission wavelengths variation through the aperture, we conducted the micro-PL intensity mapping of the VCSEL using a scanning optical microscopy. Figure 5.14(a) shows the intensity mapping of the entire aperture of the VCSEL. With a fine scan inside the square area in Fig. 5.14(a), Fig.
5.14(b) shows the non-uniform PL emission intensity across the aperture has patches
of bright areas with about 2~4 μm in size. The bright areas have about 1.8-times higher intensity than the dark areas. Fig. 5.14(c) shows the PL spectra of bright (marked as A) and dark (marked as B) areas. Nevertheless, spatial inhomogeneity in cavity loss due to potential micrometer-scale imperfection of the DBRs could also cause different threshold gains in spatial distribution. However, micro-PL measurement results in Fig. 5.14(c) show similar linewidth of the spontaneous emission for bright and dark areas, indicating no significant spatially non-uniformity in the DBRs across the circular mesa. On the other hand, the different micro-PL
intensities across the VCSEL aperture imply a non-uniform material gain distribution existed in InGaN/GaN MQW layers. In fact, the indium fluctuation was commonly observed for the expiaxially grown InGaN MQWs and also the subsequent carrier localization effect had been reported [5.22]. Therefore, we believe the indium inhomogeneity in the VCSEL MQWs could be responsible for the appearance of spatially separated lasing spots within the mesa aperture and the difference in the emission wavelength could be due to the slight variation in the indium content of the MQW.
For further investigating the indium inhomogeneity in our InGaN MQWs, the as-grown cavity with InGaN MQWs used in the VCSEL structure was examined.
Figure 5.15 shows a monochromatic CL map of the as-grown InGaN MQWs sample
at 410 nm. The CL image shows that the intensity of 410 nm emission has a non-uniform distribution implying that the In inhonogeneity in the InGaN/GaN MQWs. We believe the spatially non-uniform emission intensity results from the indium inhomogeneity in the MQWs, and consequently result in multiple-spots laser emission pattern. This result also consists with the micro-PL measurement. Figure 5.16 shows a TEM image of the MQWs in the scope of about 4 pairs of well and barrier. In the InGaN layers, some brighter areas (indicated by arrows) which could be due to the indium inhomogeneous distribution were observed. The indium separation
in the InGaN wells is believed contributing to the results we observed in the micro-PL and CL measurement. Conclude from the results of micro-PL, CL, and TEM, the indium inhomogeneity in the InGaN/GaN MQWs have a remarkable influence on the emission characteristics of GaN-basedVCSELs.
Nd:YAG laser Triax 320
CCD
Objective lens Sample
Dichroic mirror Flip mirror
Fiber
Figure 5.1 Schematic diagram of measurement setup for the characteristics of the GaN-based two dielectric DBRs VCSEL.
cryostat chamber
100 150 200 250 300 350 0
10 20 30 40 50
E m is si on i n te ns it y (arb. uni t)
Excitation energy (nJ/pulse)
Figure 5.2 Laser emission intensity obtained from the emission spectra as a function of pumping energy at room temperature
390 395 400 405 410 415 420 425 430 435 440
Emission intensity (arb. unit)
Wavelength (nm) 0.25Eth
Eth
1.13Eth
FWHM=2.5Å
Figure 5.3 Spectral evolution of the VCSEL emission different pumping levels.
390 395 400 405 410 415 420 425 430 435 440 0
1 2 3 4 5
E=0.25E
th
In te n s ity (a rb . u n it)
Wavelength (nm)
Figure 5.4 Spontaneous emission spectrum below threshold condition shows multiple cavity modes.
Figure 5.5 Laser emission intensity as a function of normalized pumping intensity in logarithmic scale.
0.1 1
Emission intensity (arb. unit)
Normalized pumping energy (E/E
th)
β ~ 1x10
-3Figure 5.6 2-D contour plot (a) and 3-D isometric plot of the spatial intensity distribution of the laser beam from the VCSEL.
Normalized intensity (arb. Unit)
0 0.5
1
(a)
(b)
0 50 100 150 200 250 300 350 0
40 80 120 160 200 240
Laser intensity (arb. unit)
Polarizer rotation angle (degree)
experimental data fitting curve (cos2θ
)
Figure 5.7 The angle dependent laser intensity.
0 50 100 150 200 250 300 350 400 5.0
5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0
Ln(E o/E th)
Temperature (
oK)
Experimetal data Linear fit
Figure 5.8 Temperature dependence of the lasing threshold of the VCSEL.
Figure 5.9 Normalized photoluminescence spectra of the GaN-based VCSEL under different pumping power levels at 300K.
405 410 415 420 425 430 435 440
0
400 405 410 415 420 425 430 435 440 445 450 455 -3x10
3-2x10
3-1x10
30 1x10
32x10
33x10
3Los s ( c m
-1)G a in ( c m
-1)
Wavelength (nm)
μW 950 850 750 650 550 450 350
Figure 5.10 Gain spectra of the VCSEL under different pumping power levels at 300K.
0 1 2 3 4 5 6 7 1.0x10
31.5x10
32.0x10
32.5x10
33.0x10
3P e ak gain (cm
-1)
Carrier density (x10
19cm
-3) 80K 150K 220K 300K
Figure 5.11 Pumping carrier density dependence of the peak gain of the lasing mode (at ~420 nm) for different measurement temperature.
50 100 150 200 250 300 6.0x10
28.0x10
21.0x10
31.2x10
31.4x10
31.6x10
3g
0(c m
-1)
Temperature (
oK)
Figure 5.12 The g0 factor as a function of temperature.
400 405 410 415 420 425 430 435 0.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Lasing mode
Line w idth e n ha nce m ent fac tor ( α )
Wavelength (nm) 300K
220K 150K 80K
Figure 5.13 Calculated α-factors as a function of wavelength for different measurement temperature.
1st spot
2nd spot 1st spot
50 μm
410 411 412 413 414 415 416 417 418 419 420 0
Figure 5.14 Emission pattern of the VCSEL at pumping energies of (a) 1.15Eth with single laser emission spot and (b) 1.2Eth with two laser emission spots. The arrows indicate the position of the first and second emission spots. (c) Spectra of the first emission spot and second emission spot at pumping energies of 1.15Eth and 1.2Eth, respectively.
(a) (b)
30 55
B
5 μm
A
Figure 5.15 (a) Micro-PL intensity mapping image of the VCSEL aperture. (b) Fine micro-PL scan inside the square area in (a).
(a)
(b)
400 405 410 415 420 425 430 435 440 445 450 0
50 100 150 200 250 300 350 400
PL intensity (arb. unit)
Wavelength (nm)
Bright area (A) Dark area (B)
Figure 5.16 PL spectra of bright point (A) and dark point (B) areas.
X3,000
1 μm
Figure 5.17 Monochromatic CL map of the as-grown InGaN MQWs sample at 410 nm.
InGaN
GaN
5 nm
InGaN
GaN
InGaN
Figure 5.18 TEM image of the as-grown MQWs used in the VCSELs cavity.
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