532 nm laser photodoped semi-insulating Gallium Arsenide (GaAs) transmission and surface reflection RF voltages in the 107 to 160 GHz, and DC transmission and surface reflection voltages

Published: 5 December 2020| Version 1 | DOI: 10.17632/zt2b597w3g.1
Biswadev Roy


We have used a newly developed contactless time-resolved millimeter-wave conductivity (TR-mmWC) system operated in the D-band (107, 110, 120, 130, 140, 150, and 160 GHz) to acquire the photodoped sample radiofrequency(RF) responses once from surface reflection, and another time using through sample transmission. The DC responses are collected only in transmission through sample mode. Millimeter waves obtained from backward wave oscillator (BWO). The GaAs sample is of commercial grade and is rotated at an angle of 65.40 from the probe beam for reflection mode. GaAs sample has high resistivity (10**7 Ohm-cm, bandgap 1.441 eV at 300K, refractive index 3.4 for D band probe signal in 110-160 GHz range, and 4.13 for laser emission wavelength 532 nm). This is a direct bandgap III-V compound semiconductor and has very good application in high radiation environments with good light emission efficiency. Schottky diode (ZBD) with a responsivity ~ 1.6 V/W and 3.2V/mW is used for signal detection. The stimulus of GaAs surface is provided using a 532 nm laser with pulse-width 0.69 ns repeated every millisecond. Laser intensity is fixed at 10.1µJ/cm2 with a spot size ~ 23.75 square mm and probed with a spot size of approximately 6.6 square mm (ratio: ~4:1). In all RF data acquisition, the laser fluence is kept constant and only the probe frequency is swept without altering the characteristic probe beam power of the oscillator at those frequencies. In both RF and DC measurements, the probe beam frequency is swept automatically using LABVIEW, and the sampling period is 500ms. The DC voltage response file (GaAs-Transmission-110-160GHz.csv) were stored in is an ASCII-delimited file, the column 1 of this file is the probe-beam frequency (swept with a resolution of 0.5 GHz), column2 is the free-space transmitted (reference) voltage (average of 30 samples) obtained from direct detection by the ZBD, the third column is the standard deviation of the reference voltages for the 30 samples. The fourth column is the same as column 2 but when the GaAs sample is placed in the path between probe beam and detector, and the fifth column is the same as the third column except, for the through-sample transmitted voltage standard deviations in Volts. The RF (photodoped) output files comprise 8 transmission files and 6 reflection files. These files are also ASCII-delimited comma-separated variable (.csv) files obtained after low noise amplification with gain ~34.5 and averaging about 5000 times. Column 1 represents the delay (seconds) and column 2 is the RF voltage (Volts) after amplification. The filename themselves provide the probe frequency information (GHz). In RF reflection mode we note positive transient. A reversal happens around 133.85 GHz. The reflected probe beam power vanishes exactly at 133.65 GHz. We note positive transients at 109.15 GHz, and no signal is observed at 107 GHz. This does not happen when I collect transients in the transmission mode.


Steps to reproduce

High resistivity SI GaAs wafer to be used with microwave pump-probe system comprising of a tunable 110-170 GHz millimeter wave generator and a 532 nm DPSS 0.69 ns FWHM laser as pump source repeated at 1ms; detection system using antenna required with bias Tee and low noise amplification suitable for 110MHz-6 GHz response with 50-ohm output impedance and bias tee having RF and DC outputs separately. The RF signal is collected by the 20 GSa/s digitizer that has 65500 averaging capability 8-bit resolution. DC data collected from DC output of bias Tee connected to a DMM and probe frequency controlled by using GPIB using LabVIEW.


North Carolina Central University


Physics, Materials Testing, Photovoltaics, Microwave