Zero-phonon lines of nitrogen-cluster states in GaN x As1-x : H identified by time-resolved photoluminescence.

Abstract

The incorporation of only a few percentage of nitrogen atoms into a III–V host material, e.g., GaAs or GaP, leads to dramatic changes in the electronic and optical properties of these compounds [1]. These materials show a very strong band gap bowing with increasing nitrogen concentration [2, 3]. Post-growth annealing of these dilute nitrides induces a noticeable blue-shift of the band-gap [1], while post-growth hydrogenation effectively passivates the incorporated nitrogen atoms due to the formation of various N–H complexes [4, 5]. Both post-growth effects have been studied intensively [1]. Numerous experimental results have shown that the nitrogen-induced disorder leads to the formation of various localized states related to isolated N atoms, NN-pair states, and nitrogen-clusters (NC) even in the very dilute regime (nitrogen concentrations lower than 0.3%) [6, 7]. The modification of the electronic properties, e.g., the electron effective mass and the gyromagnetic factor of electrons [8, 9], by these localized states cannot be explained in the framework of k p models taking into account only the interaction between the host conduction-band (CB) and the localized state of a single nitrogen atom [10, 11]. The theoretical description of the band-formation process in dilute nitrides must be modified to include various N cluster states [12, 13]. In former photoluminescence (PL) experiments the various optical transitions corresponding to different NC states were often attributed to LO-phonon replicas [6, 14]. In this article, we use time-resolved photoluminescence (TRPL) measurements to clarify that actually only zerophonon lines are observed in our PL spectra. Two GaNxAs1-x epitaxial layers of 0.5 lm thickness grown by metal-organic vapor-phase epitaxy are studied. The effective nitrogen concentration xeff is determined by the free exciton recombination energy (see Ref. [8]) to be 0.049% and 0.111%, respectively. The sample containing x = 0.111% of nitrogen is irradiated by a low-energy ion gun with different hydrogen doses (3.5 10–5 10 ions/cm) at 300 C. This reduces the effective nitrogen concentration xeff of the hydrogenated samples to about 0.083% and 0.002%, respectively [15, 16]. The samples are mounted onto a cold finger of a helium cryostat. A 100 fs Ti/Sapphire laser centered at 760 nm with a repetition rate of 80 MHz is used for optical excitation. The emitted light is collected in a backscattering geometry and is dispersed by a spectrometer with a spectral resolution of 0.5 nm. TRPL measurements are performed at low excitation densities (qexc * 8 W/cm ) and low temperatures (T B 70 K) since only at these experimental conditions the emission of various NC states are clearly resolved. The time resolution of the cooled S1 streak camera in these experiments is 15 ps. K. Hantke S. Horst (&) S. Chatterjee P. J. Klar K. Volz W. Stolz W. W. Ruhle Faculty of Physics and Material Sciences Center, Philipps-Universitat Marburg, Renthof 5, 35032 Marburg, Germany e-mail: swantje.horst@physik.uni-marburg.de