Cladding Layer Impedance Reduction to Improve Microwave Propagation Properties in p-i-n Waveguides

The cladding layer effect on microwave propagation properties of semiconductor p-i-n waveguides is investigated in this letter. Through the optical excitation in quantum wells of p-i-n waveguides, high-speed photocurrent is used to examine the microwave propagation. Two devices of p-i-n waveguides with different cladding layers are fabricated and measured, showing that a higher speed is found in the waveguide of wider cladding width. Verified by the microwave propagation properties, the higher speed is mainly attributed to lower microwave propagation loss due to the lower impedance in the wider cladding layer, suggesting this kind of structure can be applied to high-speed waveguide-based devices

I. Introduction

High-Speed high-efficiency semiconductor devices based on p-i-n waveguides have been widely used in the optoelectronics application. Due to the highly confined electric field in i-layers of p-i-n waveguides, the devices can be operated in a high-electric-field-driven regime with low driving voltage, allowing wide applications in optoelectronic fields, such as high-speed electroabsorption modulators, electrooptical phase modulators, and waveguide photodetectors [1]–[6]. Among these devices, the traveling-wave structure has been widely used to overcome the resistance capacitance element in order to obtain high-speed and also high-efficiency in long waveguides [2]–[9]. However, due to the finite resistance of cladding layers (p-and n-layers), the highly loaded capacitances resulted from the highly confined electric fields in the small volume of i-layers will cause the so-called “slow-wave” problems, i.e., the electrical wave propagation properties of 1) high propagation loss, 2) slower velocity than optical wave, and 3) low impedance. These factors mainly determine the overall performance of the devices and limit the whole design tolerance. Therefore, it is quite important to investigate the cladding-layer effect in p-i-n structures on the microwave propagation properties of waveguides. Based on the p-i-n heterostructure, p-i-n waveguides can be served as optical and also microwave waveguides. In this letter, based on the device structures of [2], the high-speed photocurrent from different cladding layers of waveguides is generated to test the device performance and microwave propagation through electrical-field-driven optical absorption in multiple quantum-wells (MQWs), namely quantum-confine-stark effect [10]. It is found that lowering the impedance of cladding layers can improve the device bandwidth by lowering propagation loss, further verified by measuring the basic electrical wave properties.