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276 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 5, MARCH 1, 2007 Cladding Layer Impedance Reduction to Improve Microwave Propagation Properties in p-i-n Waveguides Fang-Zheng Lin, Yi-Jen Chiu, and Tsu-Hsiu Wu Abstract—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. Index Terms—Cladding layer, high speed, microwave propagation loss, optical–electrical (OE) response, p-i-n waveguide, semiconductor, undercut etching. I. INTRODUCTION H IGH-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 Manuscript received September 1, 2006; revised November 28, 2006. This work was supported by grants from NCF (NSC 93-2215-E-110-018 and NSC 93-2215-E-110-013), by the project “Aim for the Top University Plan,” Ministry of Education of Taiwan, and by the Technology Development Program for Academia (92-EC-17-A -07-S1-025) in Taiwan. The authors are with the Institute of Electro-Optical Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, R.O.C. (e-mail: [email protected]). Digital Object Identifier 10.1109/LPT.2006.890044 Fig. 1. Schematic plots for the two cross sections of two p-i-n waveguides, UEARW and RW, and their corresponding equivalent circuits. 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. Fig. 1 shows two schematic cross sections of p-i-n waveguides with the equivalent transmission-line-circuit model, namely the conventional ridge waveguide (RW) and the undercut-etching-active-region waveguide (UEARW). Two governing parameters of microwave properties dominating the high-speed performance are 1) characteristic impedance (related to the electrical reflection) and 2) complex propagation (the real and imaginary parts stand for microwave constant loss and wave number). These two parameters are extracted by the methods cited in [7], [8], [11], and [12], where the waveguide is primarily formed by several elements, namely cladding layer resistance , the intrinsic capacitance of the and metal impedance waveguide, and the inductance of conducting metal. In this experiment, is set as the controlled factor, while the remaining parameters of both waveguides (RW and UEARW) are intentionally kept similar. For example, with the same metalization on the non-Ferroand can magnetic semiconductor material, the same be obtained in two structures. By undercut-etching the active region, UEARW active region can be engineered to get the same width as RW’s. Filled with the low-dielectric constant in UEARW is mainly material into the undercut portion, the ); therefore, attributed to the active region (i.e., the similar can be achieved in both structures. According the circuit model [7], [8], [12], the characteristic impedances and microwave velocities of the waveguide can be approximately and , suggesting that the high-speed determined just by responses for both structures due to such effects can thus be neglected. However, as for electrical propagation loss, it might 1041-1135/$25.00 © 2007 IEEE LIN et al.: CLADDING LAYER IMPEDANCE REDUCTION TO IMPROVE MICROWAVE PROPAGATION be different. Based on the general conditions of , and midfrequency range (10–100 GHz) in p-i-n diode, the complex electrical propagation constants can in the be expressed by first-order approximation [7], [8], [12], where is the radius frequency. On the other hand, in the generation of distributed photocurrent, the high-speed optical–electrical (OE) response can be obtained from the photon-generated current propagating in the p-i-n waveguide as [7], [14] 277 Fig. 2. Schematic diagram of photocurrent experiments. The lines of from A to B and from C to D are CPWs situated on semi-insulating (S.I.) InP layers. The line from B to C is the p-i-n waveguide served as optical, and also electrical, waveguides, where the cross sections of p-i-n waveguides are shown in Fig. 1. The distributive photocurrent (OE conversion) is generated through the input optical power and collected through the load CPW line (C to D , forward term). (1) where is a constant depending on the material, is the microwave reflection coefficient ) and waveguide, at the junction of load (with impedance is the microwave transmission coefficient, is the optical wave propagation constant inside the waveguide, and is the waveguide length. The overall OE response includs the microwave multireflection effects on junction [denoted in term (a)], electrical forward wave [term (b)] with respect to optical wave and electrical backward wave [term (c)]. It could be can lead to the expected that the effect of cladding layers quite different electrical loss and also the device performance. II. FABRICATION, MEASUREMENT, AND RESULTS The p-i-n epi-layer is grown in a metal–organic chemical vapor deposition system. The active region (nondoping) sandwiched by top p-InP and bottom n-InP layers is deposited with InGaAsP–InGaAsP MQWs, serving as optical and electrical waveguides. The i-layer thickness is 185 nm. UEARW and RW of 400- m length are first processed on the same wafer with corresponding cladding layer widths of 2 and 6 m. The active region of UEARW is then further selectively undercut-etched by a H PO H O H O solution from InP until the same width with RWs is obtained. The p- and n-layers contact metalizations are Ti–Pt–Au and Ni–AuGe–Ni–Au. A PMGI (Polymethylglutarimide, MicroChem Inc.) layer was then spun and patterned for passivating the etching surface, planarization, and bridging the high-speed interconnection. Finally, the same metalizations of 2- m-thick coplanar waveguide (CPW), shown in Fig. 2, are deposited as the connecting feed and load lines, forming a traveling-wave structure [2], [4]. Fig. 2 plots the schematic setup for measuring high-speed photocurrent and also electrical scattering matrix ( -parameters) of the devices. As a high-speed modulated optical signal travels through the waveguide (from point “ ” to point “ ”), the forward distributive photocurrent can thus be created and then propagate along the waveguide, forming the “edge-coupled photodetector” structure [5], [6]. Through the collected high-speed photocurrent, the electrical propagation on waveguides can thus be examined. A CW laser diode with power level of 1 mW at a wavelength of 1550 nm Fig. 3. Normalized OE-response with frequency. The experiment data are shown in close-circle points for UEARW and open-circle points for RW. The lines are the calculated results for both structures. modulated by a high-speed LiNbO optical modulator is used as a high-speed modulated optical source. A high-speed microwave vector network analyzer (40 GHz, VNA, Anritsu) is then employed to characterize the high-speed OE response (photocurrent) by feeding and collecting the microwave power through two high-speed microwave probes. The forward photocurrent (point “ ” is collecting point) is the signal to be characterized. Both feed-line and loaded-line CPWs are terminated by a 50- resistor or 50- instrument. In all the measurements, the optical power level is kept low enough in order to avoid the nonlinear optical-power dependent behavior. Fig. 3 shows the normalized OE responses for UEARW and RW structures from dc to 30 GHz. The bias is set as reverse bias of 1 V. It can be clearly seen that the UEARW speed is faster than RWs, where the corresponding 3-dB bandwidth is 12 GHz for UEARW and 8 GHz for RW. The difference in response is found to be about 2 dB at a frequency of 30 GHz. It also should be noted that the difference of responses between both structures increases monotonically as the modulated frequency increases, indicating that the better high-speed performance can be achieved through the wider cladding layers structure. In order to further analyze the high-speed performance of the device, the electrical propagation properties of waveguides should be considered. Through direct measurement on the straight waveguides (without CPW feed and load lines), we obtain the two-port microwave scattering matrix ( -parameters) from VNA, by which the microwave propagation properties, and , can be extracted by the method described by i.e., are about (21 ) for UEARW and [12]. The extracted (22 ) for RW. Fig. 4(a) plots the obtained microwave propagation loss coefficient in term of dB/100 m (real part of ) and 278 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 5, MARCH 1, 2007 (squares), and electrical loss with impedance mismatch (lines). All the factors illustrate similar responses in UEARW and RW except the electrical loss mechanism. And also, as seen in the simulated results [line curves of Figs. 3 and 4(a)], the calculated OE response and microwave propagation properties are quite in agreement with the experimental results, suggesting that the lower impedance in the cladding layer is the main factor enhancing the performance of UEARW. Fig. 4. (a) Extracted microwave propagation constants (including microwave loss and velocity) of UEARW and RW structures. (b) Calculated OE response of UEARW and RW in the conditions of velocity mismatch (triangles), impedance mismatch (circles), and electrical loss (squares) and loss with impedance mismatch (line). The main difference of both structures is mainly attributed by the microwave propagation loss. TABLE I PARAMETERS OF UEARW AND RW FOR THE OE RESPONSE CALCULATIONS III. CONCLUSION Two structures of p-i-n waveguides with different cladding layers are tested for investigating the microwave propagation properties of semiconductor p-i-n waveguides. Through the optical-to-electrical conversion in quantum wells of p-i-n diodes, the higher speed is found in the waveguide of wider cladding width. By extracting the microwave propagation properties from electrical transmission on waveguides, we demonstrate that the higher speed is mainly attributed to the lower microwave propagation loss from the lower impedance in wider cladding layer. REFERENCES phase velocity (from the imaginary part of ). It exhibits that and phase velocity both structures have similar values of within 10% difference, suggesting that the capacitance and inductance in both waveguides are similar[7], [8]. On the other hand, we observed a significant change in the propagation loss term. The higher the frequency, the larger is the difference of loss coefficient. We use the transmission line model (TLM) pattern method and dc forward current of p-i-n of cladding layers [13], diode to extract the resistance m for UEARW and obtaining the values of m for RW. The metal impedance is about m at frequency of 40 GHz, which is much (110 000 m). smaller than the inductance impedance, is low enough to ensure in UEARW strucAs ) dominates the wave ture and also the inductance ( impedance, the loss coefficient reveals a quadratic relation with frequencies of higher than 10 GHz, confirming the relation . The measured loss coefficient in RW is about two times of UEARW for frequency values of higher than 10 GHz, consistent with the measured and further supporting that the lower-resistance cladding layers will lead to the lower microwave propagation loss in such frequency range. 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