Ultrahigh power-bandwidth-product performance of low-temperature-grown-GaAs based metal-semiconductor-metal traveling-wave photodetectors

Abstract

High-output-power and high-bandwidth performances are usually two tradeoff parameters in the design of high-speed photodetectors. In this letter, we report high peak-output-voltage ͑ϳ20 V͒ and peak-output-current ͑ϳ400 mA, 50 ⍀ load͒ together with ultrahigh-speed performances ͑1.5 ps, 220 GHz͒, observed in low-temperature-grown-GaAs ͑LTG-GaAs͒ based metal-semiconductor-metal ͑MSM͒ traveling-wave photodetectors ͑TWPDs͒ at a wavelength of 800 nm. Ultrahigh-peak-output-power and ultrahigh-electrical-bandwidth performances were achieved due to the superior MSM microwave guiding structure and a short carrier trapping time in the LTG-GaAs layer, which reduced the space-charge screening effect and increased the photoabsorption volume without sacrificing electrical bandwidth significantly. We also observed different bias-dependent nonlinear behaviors in MSM TWPDs under high and low illuminated optical power excitations, which are possibly dominated by the space-charge screening and the lifetime increasing effects, respectively.Ultrahigh speed photodetectors ͑PDs͒ attract much attention due to their applications in fiber communication and optical measurement systems. 1 Recently, the development of optical amplifier has created a new demand of high inputoptical power durability in these high-speed PDs. Some receiver circuits use fiber amplifiers as preamplifiers, which relieve or even eliminate the necessity of electrical amplifiers. 1 By utilizing photomixing techniques, microwave photonic systems are expected to generate tunable and highpower microwaves or submillimeter waves under intense optical excitation. 2 However, ultrahigh bandwidth PDs are known to suffer bandwidth degradation problems under intense optical illumination. 3,4 Maximum output current/power and electrical bandwidth performances are usually two tradeoff parameters in the design of high-speed PDs. 1 By properly scaling down the size and photoabsorption volumes of PDs, ultrahigh speed performances can be achieved due to the reduction of parasitic capacitance and resistance in PDs. 5 However, the small photoabsorption volume ͑ϳ1 m 3 ͒ would cause high densities of photogenerated free carriers inducing strong space-charge fields that screen the external applied bias field. The device electrical bandwidth would thus degrade seriously due to the reduction in drift velocities of photogenerated carriers. 3,4 On the other hand, by reducing the optical modal absorption constant and increasing the photoabsorption volume, the output power can be increased significantly at the expense of the electrical bandwidth limited by R-C time constant and high microwave loss in the large-area and long-absorption-length devices. 6,7 There are two major ways to increase the output-saturation-current͑power͒-electrical-bandwidth-product performances. One is to distribute the photocurrents along edge-coupled PDs, such as the velocity match distributed photodetectors ͑VMDPs͒, 8 the other is to increase the carrier velocity using unitraveling carrier PDs ͑UTC-PDs͒. 1 In this letter...

Controlling electronic properties of epitaxial nanocomposites of dissimilar materials

“…The high resistivity and short carrier lifetimes of some metal/semiconductor composites such as LTG GaAs have enabled fabrication of high-speed photodiodes and detectors [36][37][38][39]. The short carrier lifetimes allow the detectors to resolve very short picosecond pulses while the high resistivities permit high biases and large changes in photocurrent.…”

Traveling-Wave Photodetectors With High Power–Bandwidth and Gain–Bandwidth Product Performance

“…Fig. 5(a) and (b) shows the measured impulse full-width at half-maximum (FWHM) under short and long wavelength excitations versus dc bias voltage at different optical illumination powers, respectively [30], [45]. As shown in Fig.…”

“…The dc bias voltage (also 30 V) thus limited the maximum output peak voltage due to external circuit saturation effects. The peak voltage ( 30 V) and electrical bandwidth (190 GHz) product (5.7 THz-V) is the highest among all the reported ultrahigh-speed PDs, including LTG-GaAs-based p-i-n TWPDs ( 1400 fC, 6 ps) [44], MBE annealed MSM-TWPD ( 1600 fC, 1.5 ps, 220 GHz, 4.4 THz-V) [45], InGaAsbased vertical p-i-n PD ( 7.2 ps, 68 fC) [46], GaAs-based p-i-n TWPD ( 5.5 ps, 59 fC) [47], unitraveling carrier PD (UTC-PD, 3.1 ps, 115 GHz, : 1.92 V) [20], and VMDP (40-50 GHz, : 2.5 V) [48]. This excellent power-bandwidth product of MSM-TWPD is due to the MSM microwave guiding structure but also due to the short carrier trapping time and the high-voltage capability of LTG-GaAs.…”

“…5(a), we can clearly see that in most optical pumping energies, as shown in traces of , an optimal bias point, which will increase with the optical excitation energy, for the fastest device response exists. We attributed this nonlinear behavior to the combination of different physical processes including carrier lifetime increasing [52], defect saturation, and space-charge screening effects [44], [45]. In the low optical excitation regime the dominant bandwidth limiting factor is carrier lifetime increasing and the FWHM increases with the bias voltage due to carrier heating and intervalley scattering [30], [31] instead of the reported coulomb-barrier lowering effect [52], because the electric field in the optical guiding mode center of our device is not high enough to induce this effect significantly [28], [30], [31].…”

Optimization of photomixers and antennas for continuous-wave terahertz emission

“…The accuracy of such measurements is dependent upon the wavelength and power of the exciting laser, as well as the correct interpretation of various artifacts that can arise. In addition, photoreflectance curves take no account of the possible effect of the electric field on the lifetime [35], [36]. However, the technique provides valuable and quantitative estimates for the lifetime, which are sufficient for comparative measurements.…”