Oxidation of AlInAs for current blocking in a photonic crystal laser

laser

Citation for published version (APA):

Zhang, R., Tol, van der, J. J. G. M., Ambrosius, H. P. M. M., Thijs, P. J. A., Smalbrugge, B., Vries, de, T., Roelkens, G., Bordas, F., & Smit, M. K. (2010). Oxidation of AlInAs for current blocking in a photonic crystal laser. In J. Pozo, M. Mortensen, P. Urbach, X. Leijtens, & M. Yousefi (Eds.), Proceedings of the 15th Annual Symposium of the IEEE Photonics Benelux Chapter, 18-19 November 2010, Delft, The Netherlands (pp. 237-240). TNO.

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Oxidation of AlInAs for Current Blocking in a

Photonic Crystal Laser

R. Zhang’, J.J.G.M. van der To11 , H. Ambrosius1, P. Thijs3, B. Smaibrugge,

T. de Vries’, G. Roelkens2, F. Bordas1and M.K. Smit1

1COBRA Research Institute, Technische Universiteit Eindhoven Address: P.O. Box 513,5600 MB Eindhoven, The Netherlands

2Photonics Research Group, Ghent University-IMEC, Ghent, Belgium 3PHILIPS, The Netherlands

To make a direct electrically pumped photonic crystal membrane laser is a challenging task. One of the problems is how to avoid short circuiting between the p- and n-doped parts of the laser diode, when the membrane thickness is limited to 200-300nm.In COBRA, we want to use the oxide of AlInAs to realize the current blocking Junction. In this way, together with a subinicron selective area re-growth technique, we expect to make electrically injected Photonic crystal lasers with higher pumping efficiency and small threshold currents, as well as low power consumption. In this paper results are presented on the development of oxidation of AlInAs. The results show that it is practically feasible to use oxide of AlInAs for current blocking in a photonic crystal

laser in a InP-based membrane system.

1 Introduction

Wet oxidation of Al containing materials has brought clear improvements in Vertical Cavity Surface Emitting Laser (VCSEL) performance ~. Use of this native

oxide as a dielectric aperture for optical and current confinement has enabled ultra-low threshold currents and high-output power VCSELs2. Building on a submicron selective area re-growth technique3, we want to use the oxide of AlInAs for current blocking in an InP-based membrane photonic crystal laser (Fig. 1). Due to the high resistance in the oxide, carriers are forced through the active region in the cavity. Thus the pumping efficiency is increased. Therefore the threshold current and power consumption will be reduced.

2

Design and Fabrication

2.1 Experimentalset-up

tvletal electrodes Active region

nandp

I

Current blocking layer (oxide of AlinAs)

/

The oxidation experiment was carried out in a horizontal quartz tube in a furnace. This

furnace is fed by nitrogen gas passed through a water bubbler maintained at 95 °C.

2.2 Layer stack of the test wafer

Fig. 2 shows the layer stack of the test wafer. On top is a 6Onm InP cap layer. Both

225nm p-InGaAs and p-InGaAsP are for the p side-metal contact. Beneath them are the

35nm InP layer and most importantly lOOnm of AlInAs layer for oxidation. The

substrate is n-type InP and n side metal contact is deposited on the back side.

6OnmInP 225 nm p++ InGaAs 50 nm p-doped InGaAsP 35nn .:

100 nm AilnAs

Fig. 2. Layer stack of thetest wafer made by PHILIPS

2.3 Process exploration

Various kinds of oxidation tests have been done in order to find the right process

window. After rounds of process exploration and optimization, good quality of

oxidization of the AlInAs is obtained as shown in Fig.3. This oxidation test is done at

450°C for about 2hrs. The oxidation depth is around 600nm which is enough for future

applications in photonic crystal lasers, The depth of oxide formation as a function of

oxidation time follows to a square-root functiont. In this test the oxidation rate is

relatively low (0.O6um/min112) because of the relatively low oxidation temperature.

Fig. 3. SEM picture of AITnAs oxide from Focused Ion Beam etching

3 Electrical measurement:

3.1 Idea and design

~, HFW mag ~It H~T1 cwr ‘~- 500 nm 1.71 ijm~15OcOO~ 52~ 5.0 mm 2.00 kV~0.21 nA Llgent

One of the key features of the InP-based membrane platform is its ultra-small thickness

(200-300nm)5. Therefore the thickness of the oxide layer should be limited to lOOnm.

Whether this small thickness of oxide can give high enough resistance is what needs to

be determined. Fig.4 shows the schematic view of the p-i-n diode used to measure the

resistance of the AlInAs oxide. Stripes of different widths (from 2um to lOum) have

been formed

3.2-Process optimization

Three optical lithography steps are used. The first step is to defines the stripes. After

that, Bromine wet etching is used to form the mesa. The oxidation is done afterwards at

500°C

for 1 hour. This relatively

high oxidation temperature is

used to obtain a oxidation rate of

0.4um/min”2. This is beneficial

to have stripes with a completely

oxidized

layer.

The

quality

observed on cleaved samples is

similar as the oxidation test done

at

The

second

lithography step is used to open

the top contact for the final

metallization. A final optical

lithography step is performed to

isolate each device from each

other as can be seen in Fig.4.

Finally, metal contact (Pt, Ti and

Au) are deposited on both sides

and the whole process is finished

with an annealing for the metal

contact.

3.3 Resistance measurement

Fig.4. Schematic view of the device for electrical measurement

RW1(5umi

0.0 0.2 0.4 0.6 0.8 1.0

Fig.5 shows the I-V (current-voltage) curve for the un-oxidized sample. The “turn-on” effect of the p-n junction is clearly visible, it is around 0.7 V. Above turn-on voltage, the resistance is around 25≤2.

Fig. 6 shows the I-V curve of the sample (3.2um in width, 8mm in length) whose AlInAs layer (lOOnm in thickness) is completely oxidized. The resistance of AlInAs oxide is at least l00k~2 until 3V which means that wet oxidation of AlInAs brought an increase of the electrical resistance for more than 3 orders. This is sufficiently high for use in photonic crystal laser in InP-based membrane.

1.OxlO’ ~ bd1~dsaI+pie~ 8.0x10’

60x10’

-:~

V(V)

Fig. 6. I-V measurement of a complete oxidized sample.

4 Conclusion

The result shows that it is feasible to use AlInAs oxide in an InP membrane based photonic crystal laser for the current blocking function. This technique, combined with submicron selective area re-growth, will enable us to make direct electrically pumped InP membrane based lasers with low threshold currents and low power consumption, as well as high output power.

Acknowledgements

The authors thank EU Project HISTORIC for its financial support.

References

[1] H. Gebretsadik, K. Kamath, W-D. Zhou, and P. Bhattacharya, “Lateral oxidation of InAlAs in InP based heterostructures for long wavelength vertical cavity surface emitting laser applications,” App!. Phys. Lett. 72 (2), 12 January 1998

[2] D. L. Huffaker,O. Baklenov, L. A. Graham, B. G. Streetman, and D. G. Deppe ,“ Quantum dot

vertical-cavity surface-emitting laser with a dielectric aperture,” App!. Phys. Lett. 70 (18), 5 May

[3] R. Zhang, F. Bordas, J.J.G.M. van der Tol, H. Ambrosius, G. Roelkens, M.A.Dundar and M.K. Smit, “Submicron Active-Passive Integration for InP-based Membranes on Silicon , Proc. ECIO 2010,

Cambridge (UK), paper ThHI.

[4] J,J.G,M, van der To!, R. Zhang, J.Pello, F. Bordas, G. Roelkens. H. Ambrosiusa, P. Th~js, F. Karouta and M. K. Smit, “Photonic Integration in Indium-Phosphide Membranes on Silicon,” JET Optoelectronics, in press