Chapter 4 Characteristics of electrical pumped GaN-based VCSELs
4.1.2 CW lasing of current injected GaN-based VCSEL at 77k…
With the achievement of optically pumped GaN-based VCSEL, the realization of electrically-injected GaN-based VCSEL has become promising. Fig. 4.5 is the overall current injected VCSEL structure. The resonant cavity structure has an 0ne optical wavelength thickness ITO layer deposited on the p-type GaN layer compared with optical pumped cavity. The ITO layer can be used as a transparent conduct layer(TCL) and improve the current spreading problem resulting from low conductivity of p-GaN.
In addition ,One optical wavelength thickness can match the resonance phase condition of microcavity and reach high transmittance(~98%) for ITO layer.
However , we can find that the quality factor of electrically pumped cavity is about 900 from PL spectrum , as shown in Fig. 4.6. The value of quality factor is about half of the optical pumped result due to additional ITO absorption. We consider the loss of ITO maybe one of the main challenges for us to reach CW lasing in current injected VCSELs at room temperature.
In order to observe the lasing behavior in current injected VCSEL, we packaged our devices into TO can. The packaged VCSEL device was mounted inside a cryogenic chamber for testing under cw current injection condition using a cw current source at 77 K. Fig. 4.7 shows the light output power versus cw injection current and current-voltage characteristics of the VCSEL sample at 77 K. The laser light
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output power showed a distinct threshold characteristic at the threshold current (Ith) of about 1.4 mA and then was linearly increased with the injection current beyond the threshold. The threshold current density is estimated to be about 1.8 kA/cm2 for a current injection aperture of 10 um in diameter, assuming the current is uniformly injected within the aperture. The lasing wavelength is 462.8nm with 0.15nm line width shown in Fig. 4.8. The inset of Fig3.8 is the CCD image of the spatial l emission pattern slightly below threshold. We believe the nonuniformity in the emission intensity across the aperture could be due to the In nonuniformity that creates a nonuniform spatial gain distribution in the emitting aperture as reported earlier[36]
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Fig. 4.5 The schematic diagram of the current injected VCSEL structure
Fig. 4.6 Emission spectrum of the current injected VCSEL structure.
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Fig. 4.7 The light output intensity vs injection current and current–voltage characteristics of GaN VCSEL
Fig. 4.8 The laser emission spectrum at different injection current levels measured at 77 K.
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4.2 The design of electrical pumped VCSEL
4.2.1The reflectance and quality factor simulation with different ITO thickness
So far, our group have fabricated and demonstrated the CW operation of an electrically pumped GaN-based VCSEL at 77 K. Next step, we should work toward the cw lasing of current injected devices at room temperature. In order to reach the goal, we try to reduce the loss of ITO so that the threshold current can also be lower at room temperature. In general, the thinner layer has the smaller absorption for the same material, so we would like to replace previous one optical wavelength thickness ITO with thinner ITO layer.
Figure show the simulation cavity structure with different ITO layer thickness from 0 nm, 30 nm, 120 nm, 210 nm, and 225 nm. Here, the 225nm-thick ITO layer stands for one optical wavelength thickness at 440nm. Owing to DBR reflectivity symmetry, the we chosen 18-pair AlN/GaN DBR. Figure is the simulated reflectance spectra under different ITO thickness. The dip positions in the reflectance spectra represent the cavity modes with different ITO thickness and the quality factor can be estimated from the linewidth of the dip. In
Figure, the cavity mode wavelength is the function of different ITO thickness. The cavity mode wavelength shifts to longer wavelength because of the longer cavity length, but the cavity mode wavelength would turn back to shorter wavelength when ITO thickness is thicker than 120nm due to the exceeding of the stop band region of the lower DBR. In this case, the cavity mode would jump to the (m+1)th mode from the mth mode. Furthermore, the cavity mode also changes to multimode owing to longer cavity length and smaller mode spacing when ITO thickness is larger than 30nm. In Figure , the quality factor is about 700 using the cavity with a 225nm-thick
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ITO layer, but this value is much lower than that without ITO layer of about 3300. If we consider the qualify factor of the cavity with a 30nm-thick ITO layer, the value of about 3100 is a little smaller than that with a 0nm-thick ITO layer but the structure with a ITO layer can be efficiently injected current in our electrical pumped VCSEL devices. Base on the simulation results and reality device requests, we can expect the 30nm ITO layer can efficiently reduce the loss and threshold current density of our VCSEL devices.
Figure 4.9 The simulation cavity structure
Figure 4.10The simulated reflectance spectra with different thickness of ITO
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Figure4.11the simulated cavity mode with different thickness of ITO
Figure 4.12 The simulated quality factor with different thickness of ITO
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