Semi-insulating Gallium Nitride on sapphire (GaN) surface reflection voltages in 107.35 to 165 GHz when stimulated using 0.69ns pulsed laser with low power densities
We have used a newly developed quasi-optical free-space time-resolved millimeter-wave conductivity (TR-mmWC) system operated in the D-band (107.35 GHz to 165 GHz) with 0.5GHz resolution to acquire surface reflected probe beam voltages from high resistivity (105 Ohm-cm) 434 µm thick semi-insulating n-type gallium nitride (GaN) wafer with thickness 5 µm on sapphire. The source for millimeter waves is the backward wave oscillator (BWO) with a spot diameter ~3mm, and the GaN sample is of commercial grade and is rotated at an angle of 65.40 from the probe beam direction. Probe beam photon energies are in the range of 0.4 to 0.7 meV. GaN refractive index for 532 nm laser pulse is 2.33 with a large penetration depth compared to its thickness. Zero-bias Schottky diode (ZBD) with a responsivity ~ 3.6V/mW is used for signal detection. The stimulus of the GaN surface is provided using a 532 nm DPSS laser with pulse-width 0.69 ns repeated every millisecond. The idea of performing this experiment was to note changes in photo-emission induced reflection voltages (dc) off of GaN surface as a function of laser intensity, and whether the differences in the illuminated and dark state reflected voltages bear any relationship with the laser fluence. GaN has a bandgap ~3.4 eV we use the 532nm pulse with energy hence no radio-frequency signal due to excess charge carrier kinetics is observed (no transients are seen either in reflection or transmission mode) however, changes in d.c. voltages are exhibited when GaN surface is illuminated with a laser pulse in the intensity range 10.1µJ/cm2 to 5.3nJ/cm2 . The differences between the probe reflection voltages while the laser is ON (illuminated by a spot diameter ~10mm) and OFF (dark) when plotted as a function of laser intensity, a rapid change from slightly negative to a steep positive transition occurs when the laser intensity is around 0.65µJ cm-2. Four sets of data are uploaded for interested users. The laser pump intensity in micro-Joules per sq. cm appears in the filename itself for each ASCII delimited numeric data saved as a comma-separated variable (.csv) file. The probe beam frequency is swept automatically using LABVIEW and the sampling period is 500ms. Column 1 of each file is probe beam frequency, column2 is the reflected voltage (average of 30 samples) from the zero-bias diode detector (ZBD), an average of 30 samples collected for each probe frequency bin. The third column is the standard deviation of the laser ON voltage sample set. The fourth column is the same as column 2 but when the laser is switched off (dark) and the fifth column is the same as the third column except for the reflected probe beam voltage standard deviation under dark condition.
Steps to reproduce
Apparatus defined in RSI paper "A time-resolved millimeter wave conductivity (TR-mmWC) apparatus for charge dynamical properties of semiconductor, Review of Scientific Instruments, Vol. 89, 104704 (2018)