Determination of carrier diffusion length in MOCVD‐grown GaN epilayers on sapphire by optical techniques

Two optical techniques for the determination of a bipolar diffusion length LD of optically excited carriers in GaN epitaxial layers, namely a time-resolved picosecond four-wave mixing (FWM) on free carrier grating and time-integrated photoluminescence (PL) are presented and examined. The PL technique is based on time-integrated photoluminescence (PL) spectra measurements from the front and back sides of the sample under cw and nanosecond pulsed laser excitation. The another method utilizes time-resolved picosecond four-wave mixing (FWM) at various light-induced grating periods to extract diffusion coefficient and carrier recombination lifetime. The value of the diffusion length derived by means of FWM decreases with GaN layer thickness from LD = 260 nm (for 1.7 μm-thick layer) to LD = 100 nm (for 0.3 μm-thick layer). The integral PL measurements give the value of LD = 120-130 nm for the 620 nm layer under pulsed excitation intensities up to 200 kW/cm2. It increases to 150-170 nm at the excitation intensity enhancement to 1 MW/cm2. These values are close to the value of the diffusion length equal to 160 nm obtained using FWM for this layer thickness evidencing the compatibility of both methods. The changes in the value of LD are discussed in terms of the defect distribution in the epitaxial GaN layer. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)