Electroreflectance from Gallium Nitride Using Second-Harmonic Generation
- PDF / 442,046 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 80 Downloads / 201 Views
541 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
EXPERIMENTAL PROCEDURE The GaN sample was grown on sapphire using metal organic chemical vapor deposition [11]. The as-grown single-crystal film was oriented with the (0001) surface parallel to the (0001) surface of the substrate with a 300 azimuthal rotation of the GaN layer with respect to the underlying sapphire substrate as verified by x-ray diffraction. A DC electric field was applied to the GaN sample by placing the semiconductor film in contact with an HCl aqueous solution. The DC potential difference and field strength in the GaN space-charge region were controlled with a standard potentiostat in a three-electrode quartz cell using a Pt counter electrode and a saturated calomel reference electrode (SCE). The nanosecond pulse, tunable dye laser source used for the SH measurements has been previously described in the literature [12]. Linearly s-polarized radiation was incident at an angle of 450 to the GaN/electrolyte interface so as to generate a specularly reflected p-polarized SH signal. Typical incident power densities at the GaN surface were kept below 1 MW/cm 2 so as to minimize sub-bandgap photocarrier generation from possible deep-level traps in the GaN sample. The reflected signal at 20) was detected with a high-gain photomultiplier tube (PMT) and analyzed with a gated integrator. The SH nature of the signal was verified by demonstrating its quadratic dependence to the incident light intensity. Linear ER measurements were performed using s-polarized light from a xenon lamp that was transmitted through a monochromator and focused onto the GaN/electrolyte interface at an angle of 450. The DC electric field in the depletion layer was modulated between the flatband (zero field) and a reverse bias condition by superimposing an AC square-wave voltage (220 Hz) on the DC potential. After detection with a PMT, the AC component of the reflected light was analyzed with a lock-in amplifier and normalized to the zero DC field reflectance. EXPERIMENTAL RESULTS Prior to the optical investigations, the DC field characteristics of the n-type GaN film (Nd - 3x10' 7cmn3 ) were analyzed with a series of AC capacitance measurements. In these electrical F" measurements, the AC current "CZ 6 .............................. !............................... a 3lekHz, ................................ ....................... ......i response or"m .s.th A Cctovoltageap p t10n mV i)(superim posedw a
"-
on the DC applied potential) was w The a twGaN/electrolyte o o-phase lock in amplifier.
5 ............................. ............................... ...............................
interface was modeled as a simple RC series circuit, where the semiconductor space-charge 2 was considered the dominant "__ _ _capacitance relative to the solution phase [13]. The depletion layer capacitance was 0 , 3.5 de-convoluted from the AC 2.375 1.25 0.125 -1.0 current and plotted in a MottApplied Potential (Volts vs. SCE) Schottky format as shown in Figure 1. Mott-Sc
Data Loading...
