Sampling of RF signals with LTG-GaAs based MSM structures

Abstract

In this work, we study photoconductive sampling of RF signals using metal semiconductor metal structures with low-temperature grown GaAs (LTG-GaAs) as active media. The sampler is a photoswitch (PS) embedded in a microwave coplanar waveguide (CPW) and it is triggered by ultra short optical pulses. Both the design of the electrodes and the semiconductor (SC) characteristics are optimized to ensure high RF isolation in the dark state [1], large electrical bandwidth and a short response-time to optical exciting pulses. We present here a complete set of results concerning device characterization and demonstrate sampling operation. DC electrical characterization, optoelectronical impulse response and sampling of synchronized RF signals are obtained. From these data, we derive important SC material parameters such as electron mobility and lifetime. Finally we also investigate and explain through a phenomenological model the nonlinearities observed in the optoelectronical response of the sampler.Among the SC material properties, dark conductivity, carrier lifetime and mobility play a major role in optical sampling of mm-wave. For such a purpose it has been demonstrated that LTG-GaAs grown at 250°C with 1 ps response time can be used but at the price of rather low mobility [2]. For our dedicated RF application, i.e. sampling of signal up to 40 GHz, we have chosen higher growth temperatures (Tg 320 or 430°C) that lead to electron lifetime of 7 or 60 ps as determined by optical pump -THz probe experiments. In order to decrease the equivalent serial capacitor of the PS [1] the GaAs wafer is chemically etched at the PS location before growing the LTG-GaAs layer. Finally, using standard photolithographic techniques, Au electrodes are deposited onto the layer to form the CPW with the 50x70 pim2 PS.DC electrical characterization of the devices shows nonlinearity of the I-V curves that is mainly attributed to space-charge limited current (SLC). Due to this effect, the total current density results from both carrier photo-injection and injection at the electrodes, whose behavior is depicted by Poisson's law and photoconduction equation. Comparing experimental data with a phenomenological model of SLC for different CW optical powers (Fig. 1) yields to an electron mobility of 2400cm2/V/s (Tg=320°C) and a defect concentration of Nd=2x1016 cm-3. These values are well consistent with the ones that can be found in the literature for LTG-GaAs grown at such average temperatures. 100 3 D2 < 1 Zy 0 D LU Measurement (Popt=3mW) BIAS VOLTAGE (V) TIME (ps) Fig. 1: I-V curves for different optical powers. Fig. 2: Sampling of 10 and 20 GHz signals Using 100 fs laser pulses, we perform dynamical characterization of the fabricated structures that show transient electrical responses of 33 ps under a 20 V biased voltage. Microwave characterization of the device in the dark shows that only -35 dB of the incident 20 GHz signal is detected at the output of the CPW, confirming the enhanced RF isolation and the small equivalent capacitor of the PS (1m fF...

Colloidal upconversion nanocrystals enable low-temperature-grown GaAs photoconductive switch operating at λ = 1.55 μm

“…Besides the short carrier lifetime which is necessary to reduce the recovery time of the switch, LT-GaAs also exhibits high carrier mobility and high dark resistivity, all of which are factors contributing to the intensive research so far performed on photoconductive switches based on this material. [17][18][19][20] Nevertheless, the operation of LT-GaAs-based photoconductive switches is limited to an optical excitation source with a wavelength () shorter than 800 nm due to the direct bandgap of GaAs (1.42 eV 21 at room temperature), while common optical communications are typically operated at wavelengths of 1.3 3 µm or 1.55µm. In fact, photoconductive switches based on most of the above-mentioned material systems cannot achieve functionality at these short-wave infrared wavelengths.…”

“…demanded for a wide range of applications, such as electrical signal switching [1][2][3][4][5][6][7], signal sampling [8][9][10], voltage pulse [11,12], waveform generation [13][14][15], and terahertz emitters [16]. So far, popular choices of semiconductors to achieve microwave photoconductive switches are chromium-doped gallium arsenide [6], low-temperature-grown gallium arsenide (LT-GaAs) [17][18][19][20], indium phosphide (InP) [1,3], silicon (Si) and its derivatives [4,5,8,10,11,13] (e.g. amorphous Si and Si on sapphire).…”

“…Among them, LT-GaAs was developed with a growth temperature less than 200 °C in order to have abundant arsenic antisite-related deep states which lead to subpicosecond carrier lifetime in the samples. Besides the short carrier lifetime which is necessary to reduce the recovery time of the switch, LT-GaAs also exhibits high carrier mobility and high dark resistivity, all of which are factors contributing to the intensive research so far performed on photoconductive switches based on this material [17][18][19][20].…”