Progress on 850 nm Flip Chip Bondable VCSEL for Optical Interconnects
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Abstract We report here efforts to advance 850 nm VCSELs designed specifically for integration to SiCMOS. Our VCSEL design is based on an oxidized current aperture and oxidized top mirror with the intent that the device operate as a bottom emitter after integration. Our design also employs intercavity contacts and the first non-alloyed buried contact have been realized. The devices produce resonantly enhanced light emission with record output of 70 ýlW/st.rad. I.
Introduction.
Recently, optical interconnects have been widely pursued as a viable means to overcome the well known bottlenecks [1] in information flow on and off Si electronics. A key milestone in this technology consists in integrating 850 nm MQW modulators to VLSI Si electronics by flip chip bonding [2]. System demonstrations involving this technology have been accomplished for photonic switching application [31 photonic ram [4], optical backplanes [5]- where up to 4000 MQW elements have been utilized along with about 50k transistors on roughly a one cm size chip. In these applications the modulators serve as both the receiver for optical input and the transmitter for optical output. Future applications include WDM wavelength monitoring, optical cross connects, speech processing and switched networks. Even though modulators offer distinct advantages for some cases their contrast ratio is often less than 3:1 and this can be a serious limitation in general optical interconnect applications. It would therefore be advantageous to integrate lasers to achieve larger contrast ratios. In order to combine integration with electronic functionality these lasers should have a low threshold current z lmA and preferably be in a surface normal mounting configuration. VCSELs appear to be an optimum device. One promising structure to investigate is that based on current confinement by oxidation of an AlAs layer as a constriction aperture. Such a layer also gain guides the light output. However, to accomplish this objective for 850nm operation requires that several issues be advanced to further VCSEL technology. They are as follows: i; arrays of VCSELs with uniformity in threshold current and light output, ii; current aperture for low threshold current and low aspect topology in top mirror, iii; coplanar contacts for added electronic functionality, iv; no degradation in VCSEL performance under flip chip bonding. Even though VCSELs has undergone rapid performance improvements in recent years their status for the above applications remains questionable. For instance, the sub-mamp current threshold devices have just now been achieved at 850nm using an oxidized current aperture but with a conventional semiconductor top mirror [6]. This advancement could improve the electronic compatibility with Si but the large topographical features (top mirror) still remain problematic for the ensuing integration steps. On the other hand, low topology VCSELs based on a current confinement aperture and an oxidized top mirror has only been reported at 980 num but not at 850 nim [7]. Furthe
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