Doping of GaN with transition metals (TMs) introduces deep acceptor levels which allow to manufacture highly resistive layers for high efficiency GaN-based transistors and magnetic semiconductor materials for room temperature spintronics applications. Fe, which appears as Fe3+ in the neutral state and Fe2+ in the ionized one, shows great promise for such applications. However, the mechanism of trapping to the Fe centers in GaN and the relationship between carrier lifetime and trap concentration need to be understood in order to control electrical properties of GaN:Fe.
Electrical and optical properties of GaN:Fe have, to some extent, been studied. However, dynamics of carrier recombination and Fe role in that process has not been explored. To that end, time resolved photoluminescence (TRPL) represents a powerful tool combining temporal and spectral resolution capabilities. It allows to relate the photoluminescence (PL) lifetime to the Fe concentration, establishing TRPL as an effective nondestructive characterization technique for the doping levels.
Room temperature TRPL studies were performed on GaN:Fe doped during growth by metal organic vapour phase epitaxy on sapphire substrates with concentrations from 1 to 10x1018 cm-3. The Fe concentration was measured by secondary ion mass spectrometry; X-ray diffraction showed a uniform crystal quality in the whole range of Fe concentration. PL was excited with femtosecond pulses at 267 nm central wavelength from the third harmonic of a mode-locked Ti:sapphire laser.
A comparison between the PL lifetime in as-grown samples, around hundreds of ps, and doped samples, down to 4 ps, suggests that Fe centers are efficient non radiative recombination channels. A linear relation between inverse of the PL lifetime and the iron concentration has been established, confirming the role of Fe ions as centers of nonradiative recombination. Assuming constant Fe2+ concentration originating from the compensation of the free charges in the as-grown sample, the electron capture cross section was found equal to σe=1.9x10-15 cm2. The hole capture cross section was estimated by using computer simulations of the PL decay. The found upper limit is σe=1x10-15 cm2.
|