Physics of the Solid State, Vol. 46, No. 6, 2004, pp. 1001–1007. Translated from Fizika Tverdogo Tela, Vol. 46, No. 6, 2004, pp. 972–978. Original Russian Text Copyright © 2004 by Andreev. SEMICONDUCTORS AND DIELECTRICS Stark Structure of the Spectrum and Decay Kinetics of the Er3+ Photoluminescence in Pseudoamorphous a-nc-GaN Films A. A. Andreev Ioffe Physicotechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia Received April 29, 2003 Abstract—Amorphous films containing nanocrystalline inclusions, a-nc-GaN, were codoped with Er in the course of magnetron sputtering. Intense intracenter emission of Er3+ ions in the wavelength region λ = 1510– 1550 nm was obtained only after multistage annealing at temperatures of 650–770°C. The intracenter emission was excited indirectly through electron–hole pair generation in the a-nc-GaN matrix by a pulsed nitrogen laser. Subsequent recombination via a number of localized states in the band gap transferred the excitation energy to the Er3+ ions. Measurements of photoluminescence (PL) spectra carried out with time resolution in various annealing stages at different temperatures in the range 77–500 K and in the course of decay permitted us to establish the Stark nature of the PL spectrum and to reveal the dynamic and nonequilibrium character of redis- tribution of the intracenter emission energy among the Stark modes in the course of the excitation pulse relax- ation. The information thus obtained was used to interpret the dominant hot-mode contribution and the complex decay kinetics. © 2004 MAIK “Nauka/Interperiodica”. 1. INTRODUCTION energy of the electron–hole pairs created in the matrix with Eg = 3.4 eV. In the course of interband recombina- The interest in studies of the luminescence of triva- tion involving a broad spectrum of localized states, the lent erbium ions in various semiconductor matrices energy of the pairs is transferred to the erbium ions non- stems primarily from the possibility of developing nar- resonantly via the GaN lattice–Er3+ bonds; this row band, thermally stable emitters for the wavelength (1535 nm) corresponding to minimum losses in quartz accounts for the multitude of intracenter transitions, of fiber waveguides [1]. This subject is also of fundamen- which we are interested only in the transition connect- tal significance, because many attendant issues still ing the first excited state of the ion to the ground state, 4 4 λ ≅ remain unsolved. In particular, the specific features of i.e., the I13/2 I15/2 transition at the wavelength erbium photoluminescence (PL) at high concentrations 1535 nm. The nonresonant mechanism of energy trans- of optically active ions (≥1020 cm–3), the mechanism of fer to the ion is very poorly understood, but it undoubt- energy transfer from the pumped matrix to erbium ions, edly has specific features and should affect the Er3+ PL and the PL dynamics at high pulsed pump power levels pattern. Furthermore, the high excitation density 2 4 on the order of 100 kW/cm , i.e., in the conditions close reached in a pulse makes the population of the I13/2 to the superluminescence threshold [2], are very poorly Stark states a nonequilibrium process, a factor that sub- understood. stantially modifies the PL decay kinetics. While the present study deals to a certain extent with these issues, it does not claim to have solved them and should be considered rather as a step in this direction. 2. EXPERIMENTAL This study includes two new elements, namely, (1) the choice of an amorphous a-nc-GaN matrix for Er3+ ions, Films of amorphous a-GaN measuring 4–7 nm with which allows erbium doping to high concentrations (up a fairly uniform distribution of nc-GaN nanocrystallites to 5 × 1020 cm–3) [3], and (2) the use of a pulsed nitro- of hexagonal symmetry were prepared by magnetron gen laser with a pulse power of 1.6 kW to excite the sputtering of metallic Ga in a reactive gas mixture Ar + matrix–erbium ions system. The laser radiation does N2. The critical parameter for the process is the sub- not coincide in energy (បν = 3.68 eV) with any of the strate temperature Ts, which was found to be optimal 4f 11 erbium ion shell levels and, therefore, cannot be around 300–350°C. It was established that, at higher ° transferred to the erbium ion through direct resonance values of Ts approaching 500 C, films tend toward absorption of a photon. Thus, the erbium ions can be uncontrollable crystallization under subsequent anneal- excited only indirectly, through dissipation of the ing. The erbium ion PL in such films is unstable. 1063-7834/04/4606-1001 $26.00 © 2004 MAIK “Nauka/Interperiodica” 1002 ANDREEV Codoping with erbium was achieved in the course of 3. RESULTS AND DISCUSSION film growth by placing small Er-metal platelets of cali- 3.1. Changes in the Er3+ Spectra in the Course brated area into the magnetron discharge. The concen- of Optical Activation tration thus introduced was varied in the range (1–10) × 1020 cm–3. The maximum intensity was achieved at an It is known [1, 3, 6] that the mechanism of optical Er concentration of 5 × 1020 cm–3. A further increase in activation consists in the formation of a local atomic the concentration resulted in a strong decrease in the PL environment around the erbium ion, generating a crys- lifetime. tal field of electrostatic nature without inversion sym- metry. Such a crystal field results in Stark splitting of Structural characterization of the films was done by the erbium 4f 11 multiplets. In addition, because there is means of x-ray diffraction, Raman spectral measure- no inversion site symmetry, the allowed f–d transitions ments, and direct HRTEM observation of the micro- become admixed to the f–f transitions forbidden by structure. The HRTEM method permitted us to detect selection rules, thus making the f–f intracenter transi- inclusions of nanocrystals and their belonging to the c- tions possible, although with a low probability, and this GaN hexagonal phase and determine their dimensions manifests itself, as will be seen from what follows, in (5–7 nm), the fractional volume of the phase (≈10%), the low PL decay rate. and its variation under annealing, namely, the increase The most efficient crystal field is generated by in the volume fraction of the nanocrystals without an atoms of highly electronegative elements, such as nitro- increase in their size [4, 5]. The HRTEM data were gen and oxygen. The absence of any experimentally kindly provided by Prof. H.P. Strunk (Erlangen, Ger- observable optical activity of the erbium ion in as-pre- many). While the vibrational mode structure in the pared films implies that nitrogen does not create the Raman spectra obtained by V.Yu. Davydov (Ioffe Insti- 3+ tute, Russian Academy of Sciences, St. Petersburg) is required configuration in the Er ion environment. fairly diffuse, the maxima of the A (TO) + E + E and Such a configuration does not form even though there 1 1 2 is an excess of chemically active nitrogen in the form of A1(LO) + E1(LO) modes nevertheless coincide in posi- plasma ions during the film growth. Our experiment tion with those for the c-GaN. The Raman spectrum of shows that optical activity appears only at temperatures the films sharpens with annealing, which is evidence favoring the generation of nitrogen vacancies in the there is a growing contribution from the nanocrystalline GaN structural network [7]. This takes place in the tem- fraction, and is in agreement with the HRTEM data. perature interval 650–770°C. This allows two alterna- Despite this trend, the a-nc-GaN films retain their two- ° tives, namely, either the released nitrogen becomes phase (a + nc) microstructure up to at least 775 C, dem- chemically bonded to Er3+ or the process of nitrogen onstrating also high adhesion to such substrates as c-Si generation is conducive to the diffusion of residual oxy- and fused quartz. gen (which is always present in the matrix in dissolved Directly after deposition, Er-doped films do not form) to the erbium ions. This residual oxygen is get- exhibit the characteristic erbium PL at λ ≅ 1535 nm. To tered efficiently by the erbium to form the ErOx com- obtain this emission, one has to optically activate the plex, whose erbium ion is acted upon by the field of erbium ions, a process consisting in a series of high- negative oxygen charges. Irrespective of the way in ° temperature anneals with Tann in the range 650–770 C. which this process evolves, one can be confident that it To achieve the maximum Er3+ PL intensity, several does not occur quickly and is extended both in time and 3+ cycles with Tann increased judiciously in each subse- temperature. By measuring the Er PL spectra in dif- quent cycle will suffice. Annealing at a temperature in ferent stages of optical activation, one can follow the ° mechanism of crystal field formation, as well as draw excess of the maximum Tann, 775 C, initiates a clearly pronounced macrocrystallization, which brings about some conclusions concerning the field symmetry and, film turbidity and a complete or partial disappearance hence, understand the nature of the Er3+ PL spectra. ° of the erbium PL. The first anneal (Tann = 650 C) gives rise to a very 3+ PL measurements were carried out with an optical weak Er PL signal with a fine structure consisting of resolution of 5 nm. The InGaAs-based photodiode with a set of narrow, well-resolved peaks. Curve 1 in Fig. 1 a sensitivity of 25 µV/mW provided by L.B. Karlina displays this spectrum, obtained by S.B.