Process flowchart

The AlN- VCSEL was fabricated by six process steps. In the beginning, mesa was etched by ICP dry etching, as shown in Fig. 5.7. Then 200nm SiNx layer was deposited by PECVD and patterned to defined current injection aperture as shown in Fig. 5.8. The 30nm ITO were deposited by sputter and annealed at oven 2hr and nitrogen ambient. The inner diameter is 25um and the outer diameter is 80um as shown in Fug. 5.9 .Through the ring shape ITO the VCSEL cavity will become an ideally cavity without ITO absorption. At current injection, the current will laterally spreading into aperture (mark in Fig.5.9.2) by high sheet resistance of AlN current blocking layer and the turn on image is shown in Fig. 5.9.1. The Ti/Al/Ni/Au and Ni/Au contacts were deposited to serve as n-type and p-type electrode respectively, as shown in Fig. 5.10. The VCSEL was completed by capping the structure with 10 periods of SiO2/Ta2O5 DBR stack (R~99%), as shown in Fig. 5.11

Fig. 5.7 Mesa etching step and the device turn on image

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Fig. 5.8 Deposited SiNx as the passivation layer

Fig.5.9.1 The ring shape ITO

Fig. 5.9.2 The ring shape ITO and its CCD image.

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Fig. 5.10 deposited n-contact and p-contact

Fig. 5.11 Deposited 10-pair Ta2O5/SiO2 DBR

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5.3Characteristics of electrically pumped GaN-based VCSELs with AlN current blocking layer(CBL)

5.3.1 The emission images of AlN VCSEL

Figure 5.12 shows the light emission photograph of AlN-VCSEL operated at 0.03mA.

The bright apertures show AlN layer can effective block the current. The sizes of aperture were defined 3um, 5um, 7um, 10um.. We can clearly find the light emission is concentrate in the current aperture defined by AlN.

In order to observe more detail in the current aperture, we also observed the optical intensity distribution by means of CCD and Beam-view program. The color on Fig 5.12(c) represents the relative optical intensity emitted from observed device, and this figure shows the current injection uniformly into the aperture. We can observe the light beam showing a Guass distribution in Fig. 5.12(d).Fig. 5.8 shows the VCSEL before contact process image, the ring shape ITO with 25um and 80 um, inner and outer diameter respectively. Fig. 5.9 shows the turn on image of VCSEL without ITO layer, which injection current laterally flow through p-GaN layer. The concentrated emission light not only confirms that current is uniformity injection into the current aperture but also shows that the AlN plays a role of current blocking layer successfully.

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Fig. 5.12 Emission images of AlN VCSEL

Fig. 5.13 (a)The emission image of aperture size of 3μm under high magnification CCD image (b)(c)(d)Beam-view on 3um aperture emission

Fig. 5.8 The ring shape ITO and CCD image.

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Fig. 5.9 Current lateral spreading by AlN current blocking, the white mark point out the 10um current aperture.

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5.3.2The L- I-V curves of AlN blocking layer and conventional VCSEL

The current-voltage (I-V) characteristics of AlN and conventional device are both shown in Figure 5.14. The turn on voltage and resistance of the conventional VCSEL which is 10um current aperture define by SiNx passivation was about 7.14V and 385Ω respectively. The 10um current aperture of VCSEL with AlN CBL shows 7.3V and 389Ω respectively. The slightly higher voltage and resistance compared with the conventional device because the current path could be longer than the conventional VCSEL, such results are realizable and acceptable.

Fig.5.14 I-V curve

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5.3.3The optical characteristics of AlN VCSEL

Fig.5.16 shows the AlN VCSEL device structure and turn on image. Fig. 5.17(

a) shows the EL spectrum of AlN VCSEL. The equation, which associates the quality factor and total loss, is as follow:

Δλ

λδ

By the equation, the quality factor estimated about 1100, which is at the same order we have measured with the bottom DBR reflection 99.5%.

Fig.5.15AlN VCSEL structure and CCD image of 10um aperture.

The EL spectrum and divice structure of AlN and conventional device are both shown in Figure 5.17. In the EL spectrum, take ring shape ITO layer as contact layer shows a high Q (1100) than circular ITO shape (650). This information would cause by the cavity without ITO layer become a more ideally resonant cavity, which is effective reduce the optical loss.

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Fig. 5.16EL spectrum and structure of conventional and AlN CBL device.

5.4 Summary

In summary, GaN-based vertical cavity surface emitting laser using AlN re-growth on MQWs for current blocking layer is fabricated and measured. The re-growth temperature and pressure are 1020^oC and 100torr respectively. At the same DBR reflectance, the cavity without ITO layer shows a high Q (1100) than conventional device (650). The turn on voltage and series resistance were 7.3V and 389 Ω respectively, which close the conventional device with the same current aperture.

The current blocking layer not only confines the injection current but also can makes cavity become a more ideally resonant cavity by taking away the ITO layer. These characteristic suggests that AlN current blocking layer during the process of GaN-based VCSEL should be an effective and feasible way for current confinement and reduce the optical loss.

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Chapter 6 Conclusions

In conclusion, a 7λ GaN-based VCSEL using AlN as current blocking layer was fabricated and measured. The AlN layer re-growth condition is 1020oC and 100torr is determined. The AlN layer R.M.S and sheet resistance are 2.93nm and 8KΩ respectively. The device structure is composed of high-reflectivity AlN/GaN bottom DBRs (97%) and SiO2/Ta2O5 top DBRs (99%). The turns on voltage and series resistance were about 7.35V and 389Ω respectively. The emission peak wavelength of the AlN VCSEL was located at 447nm with a narrow line-width of 0.4nm. Compared to the emission spectrum of the bottom DBR reflection 99.5% device, the device with bottom DBR reflection 97% shows the same order Q factor of 1100, which without ITO layer. Moreover, the degrees of polarization of about 88.7% were measured. The high degrees of polarization might elucidate optical confined in the VCSEL cavity.

While the reason for high degrees of polarization might attribute to the low refractive index of AlN .However, we have not measured the threshold current, which may be attributed to lack of reflection from bottom DBR. But the improvement of Q factor proves that taking AlN as current blocking layer indeed plays an important role to fabricate this kind of high Q device like VCSEL. In order to achieve a certain both upper and bottom DBR reflectivity in requirements and solve the heat dissipation problem due to worse thermal conductivity of sapphire substrate, we may also choose laser liftoff process in our VCSEL structure. We expect combine the AlN structure and laser liftoff technique and finally reach the optimum structure in GaN-based VCSEL.

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