Fabrication and Luminescence of Etched Quantum Rings and Vertically Coupled Dots
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Mat. Res. Soc. Symp. Proc. Vol. 406 © 1996 Materials Research Society
quantum well of 4 nm, 12 nm of AlGaAs, 35 nm of AlGaAs and a 10 nm GaAs cap layer. Unless indicated the AlGaAs layer had 33% Al. The layers were nominally undoped.
7II1 r--7:-(d)
:: (f)
(e)
(g)
m
(c)
(b)
(a)
(i)
(h)
Top metallic layer
High molecular weight PMMA (ELV)
E ]Low
molecular weight PMMA (BDH)
Polyimide
M
Photoresist
Bottom metallic layer
S.
Top contact (NiCr)
Figure 1. Fabrication steps for quantum dots and rings (a) to (e) and for vertically coupled quantum dots (a) to (i) The fabrication sequence consisted of several steps shown in figure 1. Dot and ring patterns were defined by electron beam lithography using a standard bi-layer PMMA resist system (Fig. Ia) which allows lift off of metal layers up to 60 nm. For the 500 nm dots the dose / spot size used were 650 Ccm- 2 / 112 nm, for the 250 nm dots 400 Ccm- 2 / 40 nm, respectively. For rings with 400 nm diameter (350 nm inner diameter) these values were 600 Ccm- 2 / 15 nm. After exposure the pattern was developped in MiBK:IPA 1:3 solution at 23.1*C (Fig. lb). After deoxidation a metal mask for dry etching was deposited on the substrate (Fig. Ic). For rings the mask was 30 nm thick and for dots it was 20 nm NiCr, 18 nm Au and 20 nm NiCr. The metal mask for dots also served as a metallization of the top of these pillars thus permitting a small fraction of the emitted light to be transmitted. This is different from the standard p-contact for GaAs (15 nm Cr, 20 nm Au, 40 nm Zn, 300 nm Au) which is totally opaque at the required wavelength. After evaporation lift-off was performed in hot acetone (Fig. ld), followed by Reactive Ion Etching
(RIE - Fig. le and Fig. 3 left). For the coupled quantum dot samples the 500 nm etch depth was
achieved with a flow of 8 sccm of SiCl4, 30 W rf power and -70 V bias for 5.5 minutes. Ring samples were etched in a flow of 6 sccm of SiC14, 100 W rf power and a bias of -380/-340 V for 1 minute to obtain a depth of 200 nm (Fig. 2). The following steps deal with contacting the pillars. After metallization of the downside of the wafer with the standard n-contact (Fig. If) both contacts were annealed using rapid thermal annealing in Argon for 20s at 340°C to provide ohmic contacts. To contact the top of the pillars the 308
structure needed to be planarised with an insulating material. Two layers of polyimide (XU 218, Ciba-Geigy) were coated on the sample (Fig. 1g). It was found that better planarisation results if two layers of polyimide are spun onto the surface, baking the sample at 180C for 2hrs after spinning the first layer. The optimum planarisation is obtained by spinning an additional 350 nm thick layer of S1400-17 (Shipley) photoresist above the polyimide, and then baking it at 120C for 2 hrs. At this temperature, the photoresist flows into the valleys between the pillars leading to a flat surface. The sample was then etched in oxygen to expose the top metallic contacts (Fig. lh). Since S1400-17 photoresist and XU-218 polyimide h
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