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
Description
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 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 0.4 to 0.7 meV. GaN refractive index for 532 nm laser pulse is 2.33 with large penetration depth compared to its thickness. Zero-bias Schottky diode (ZBD) with a responsivity ~ 3.6V/mW is used for signal detection. Stimulus of 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 function of laser intensity, and whether the differences in 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 seen either in reflection or transmission mode) however, changes in d.c. voltages are exhibited when GaN surface is illuminated with laser pulse in the intensity range 10.1µJ/cm2 to 5.3nJ/cm2 . The differences between the probe reflection voltages while laser is ON (illuminated by a spot diameter ~10mm) and OFF (dark) when plotted as function of laser intensity, a rapid change from slightly negative to a steep positive transition occur 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 file. Probe beam frequency is swept automatically using LABVIEW and sampling period is 500ms. The column 1 of each file is probe beam frequency, column2 is the reflected voltage (average of 30 sample) from ZBD, an average of 30 samples collected for each probe frequency bin. Third column is the standard deviation of the laser ON voltage sample set. Fourth column is same as column 2 but when laser is switched off (dark) and fifth column is same as third column except for the reflected probe beam voltage standard deviation under dark condition.
Files
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)