Figure 3. 6 The coupling efficiency of spontaneous emission (β) 3-2 Characteristics of electrically pumped GaN-based VCSELs
The electroluminescence (EL) characteristics of the fabricated VCSELs were measured by the probe station system and injected current by the Keithley 238 CW current source as shown in
Electrical pumping setup
Figure 3. 7. Figure 3. 8 is the images of the low temperature EL measurement system. The light output power can be measured by Si-based optical power meter through an integrated sphere. The relative electrical characteristics, such as current-light output intensity(L-I) and current-voltage (I-V) properties, were performed by using the probe station, Keithley 238 CW Current Source, UV power detector, and Newport 1835-C optical power meter. The emission
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light was then collected by a 25 µm-diameter multimode fiber using a microscope with a 40X objective and fed into the spectrometer/CCD (Jobin-Yvon Triax 320 Spectrometer) with a spectral resolution of ~0.15 nm for spectral output measurement.
All the data could be directly feed backed to the computer from these facilities including the optical meter spectrometer and the Keithley 238 current source by the GPIB connector.
Figure 3. 7 The probe station measurement setup.
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Figure 3. 8 The images of low temperature EL measurement system
The light emission intensity from the VCSEL as a function of the injection current is shown in
Threshold characteristics
Figure 3. 9 at 200 K, 240 K, 270 K, and 300 K. All of them has a distinct threshold characteristic was observed at the threshold injection current (Ith) of about 7.5 mA (200 K), 8.2 mA (240 K), 9.2 mA (270 K), and 9.7 mA (300 K). At room temperature(300 K), the threshold current density is 11.4 kA/cm2 similar to the result of the electrical pumped GaN VCSEL with double dielectric mirrors fabricated by Nichia company[5,6].
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0 5 10 15 20
0 1 2 3 4 5
200K 240K 270K 300K
Int e ns it y (a rb. uni t)
Current(mA)
Figure 3. 9 The lasing intensity as a function of injection current under different measurement temperature from 200 K (solid line), 240 K (dash line), 270 K (dot line), and 300 K (dot dash line).
Figure 3. 10 shows the laser device voltage as a function of the injection current at 200 K, 240 K, 270 K, and 300 K. When we increased the measurement temperature from 200 K to 300 K, the series resistance and turn-on voltage of the laser device decreased from 220 Ω to 180 Ω and from 4.55 V to 4.3 V, respectively. In general, this might be caused by the worse hole mobility in p-GaN material in lower temperature (200 K) compared with the room temperature (300 K). In our experiment, the whole mobility in p-GaN material increased when the environment temperature rose to 300 K and further accelerated the recombination of the holes and the electrons. Therefore, the series decreases in higher measurement temperature.
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0 5 10 15 20
0 2 4 6 8 10
Vo lt a g e (V)
Current(mA)
200K 240K 270K 300K
Figure 3. 10 The voltage is a function of injection current at 200 K, 240 K, 270 K, and 300 K.
Figure 3. 11 shows L-I-V curves at 300 K. The dash line is the linear fitting curve of the laser intensity versus injection current. A clear lasing transition from spontaneous emission to stimulated emission can be observed at room temperature.
From the linear fitting curve, the laser threshold current is around 9.7 mA corresponding to the current density of about 12.4 kA/cm2. The relative low threshold at room temperature operation could be due in part to the successful prevention of carrier overflow by using the electron blocking layer on top of the MQWs and the lower internal absorption loss of the thinner ITO layer. The turn-on voltage is about 4.3 V indicating the good electrical contact of the 30 nm ITO transparent layer and the 2 nm-thick InGaN layers. The output laser intensity from the sample increased linearly with current injection beyond the threshold current. However, the laser intensity started to roll over at higher injection current beyond 15 mA due to the thermal effect.
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Figure 3. 11 The lasing intensity and the voltage as a function of injection current at 300 K. The threshold current and turn-on voltage are about 9.7 mA and 4.3 V.
Figure 3. 12 shows the variation of emission spectrum with the increasing pumping energy. From the figure, we can observe the transition behavior from spontaneous emission to stimulated emission. Above the threshold current, one dominant laser emission wavelength at 412 nm appears with a linewidth of about 0.5 nm.
360 380 400 420 440 460
Intensity(arb. unit)
Wavelength(nm)
1.2Ith Ith 0.6Ith
Figure 3. 12 The emission spectra were recorded at injection current of 0.6 Ith, 1 Ith, and 1.2 Ith.
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There are two CCD images below and above threshold current as shown in Figure 3. 13, respectively. The blue spontaneous emission could be seen both in these two images. When we injected the current above the threshold current, a laser spot of about 2 μm in diameter with relatively strong intensity appears and shows the inhomogeneous phenomenon of InGaN material.
Figure 3. 13 The image shows the lasing spot with the diameter of about 2 µm below and above threshold current.
Characteristic Temperature
Figure 3. 14 shows the semi natural-logarithm plot of the dependence of the threshold pumping energy (lnEth) on the operation temperature (T). The threshold current gradually increased as the operation temperature rose from 200 K to 300 K. In general, we will use the relation between the threshold current and the operation temperature could be characterized by the equation Eth=E0×eT T/0, where T0 is the characteristic temperature and E0 is a constant, showing the linear characteristic between measurement temperature and laser threshold power. However, the relationship between the threshold current and the operation temperature in VCSEL devices appear the U-shape property not linear relation. Therefore, we cannot estimate the characteristic temperature in the laser devices.
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 nm as the temperature rose from 200 K to 300 K as shown in Figure 3. 15. This lasing wavelength shift per Kevin degree is so small of about 10-2 nm/K 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 previous report. That is, the slightly shifted gain
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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, 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 (1.1 μm in thickness) providing a good heat dissipation path during the high carrier injection and high temperature conditions [7].
200 220 240 260 280 300
2.0 2.1 2.2 2.3
ln (I
th)
Temperature(K)
Figure 3. 14 Semi natural-logarithm plot of the dependence of the threshold pumping energy (ln(Ith)) on the operation temperature.
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200 220 240 260 280 300
411.0 411.5 412.0 412.5 413.0
W a ve le ngt h( nm )
Degree(K)
9mA 15mA
Linear Fit of 9mA Linear Fit of 15mA
Figure 3. 15 The lasing wavelength of GaN-based VCSEL as a function of temperature at 9 mA and 15 mA.
In order to understand the β of this cavity, we normalized the scales of The coupling efficiency of spontaneous emission (β)
Figure 3.
11 and re-plotted it in a logarithm scale as shown in Figure 3. 16. Besides, we used the Eq. (2. 2 7) to fit our data as shown in chapter 2 and the fitting result shows the β value of the laser is about 5×10-3.
As we mentioned before, this β value of the VCSEL is two order of magnitude higher than that of the typical edge emitting semiconductor lasers (normally about 10-5) [2, 4] indicating the enhancement of the spontaneous emission into a lasing mode by the high quality factor microcavity effect in the VCSEL structure.
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0.1 1
10-2 10-1 100 101 102 103 104 105 106
Fitting curve
Photon number n