Monolithic Integration of Membrane-Based Butt-Jointed Built-in DFB Lasers and p-i-n Photodiodes Bonded on Si Substrate

We demonstrate a monolithic integration of lateral-current-injection (LCI)-type membrane-based distributed-feedback (DFB) lasers and p-i-n-photodiodes (PDs) using a butt-jointed built-in (BJB) structure bonded on a Si substrate using benzocyclobutene. The BJB structure to be integrated to the membrane optical devices was prepared by organometallic vapor-phase epitaxy. A threshold current of 280 μA was obtained under a room-temperature continuous-wave condition by adopting a strongly index-coupled DFB laser with a surface grating structure and a λ/4-shift region. A low dark current of 0.8 nA was obtained with the p-i-n-PD at a bias voltage of -1 V, and its photocurrent property coincided with the light output property of the membrane DFB laser.

I. Introduction

Optical interconnects have been adopted in large data centers or super computers [1]. Recently, optical communication has exhibited better performance than electrical interconnection in short range and this trend will continue. Furthermore, we expect the introduction of optical interconnection to on-chip interconnection [2]–[4], which is ultimately a short-reach communication. In the on-chip interconnection technology, the current copper interconnects in the global wire layers limit the performance of large-scale integration (LSI) circuits because of its RC delay and heat generation [5], [6]. Therefore, introduction of optical communication to on-chip interconnection is one of the solutions of such problems. In contrast to conventional optical devices for long-haul transmissions, which are required to emit output power from a few milliwatts to tens of milliwatts, the required output power for on-chip interconnection is from a few tens of microwatts to hundreds of microwatts when the minimum receivable power of a p-i-n-photodiode (PD) at a signal speed of 10 Gb/s is considered. In particular, the energy cost of the transmitter must be significantly less than 100 fJ/bit [7]. For this purpose, lasers with ultra-low power consumption, such as vertical-cavity surface-emitting lasers (VCSELs) [8]–[10] and photonic-crystal (PhC) lasers [11]– [13], have been extensively investigated. VCSELs with 45° mirrors have been reported for in-plane optical interconnection [14]. A PhC laser with an extremely low energy cost of 4.4 fJ/bit was demonstrated at a signal speed of 10 Gb/s using an extremely small volume of active region and a high-sensitivity avalanche PD (APD) for a received optical power of approximately −30 dBm (1 μW) [15]. However, the applied voltage of a typical APD is only a few tens of volts, which is not suitable for integration with LSI chips. Thus, adoption of a typical p-i-n-PD will be more realistic for on-chip optical interconnects, and the required optical power will be one order of magnitude higher than that using an APD [16].