Физика твердого тела, 2008, том 50, вып. 9 Ground state measurement of Pr 3+ in Y 2 O 3 by photoconductivity © Dongdong Jia ∗ , Xiao-jun Wang ∗∗,∗∗∗ , W.M. Yen ∗∗∗∗ * Department of Geology and Physics, Lock Haven University, PA 17745 Lock Haven, USA ** Department of Physics, Georgia Southern University, GA 30460 Statesboro, USA *** Key Laboratory of Excited State Processes, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 130033 Changchun, China **** Department of Physics and Astronomy, University of Georgia, GA 30602 Athens, USA E-mail: xwang@georgiasouthern.edu 1 at.% Pr 3+ doped Y 2 O 3 single crystal fiber is prepared unsing a laser beated pedestal growth method. Emission and excitation spectra of the fiber have been measured. The emissions of 1 D 2 to 3 H 4 and 1 D 2 to 3 H5 4 f -4 f transitions are found at 620 and 720 nm, respectively. The 3 P 2 , 3 P 1 , 1 I 6 and 3 P 0 ,4 f -4 f absorptions are observed at 456, 472, 482 and 492 nm respectively. The 4 f -5d absorption band is detected at 288 nm. Photoconductivity result shows that the 4 f -5d transition of Pr 3+ produces a direct photocurrent around 285 nm. Taking the onset photocurrent at 320 nm, the ground state of Pr 3+ is determined at 1.7 eV above the valense band of the host. One of the authors (X.J.W.) is grateful for the support by the MOST of China (2006CB601104, 2006AA03A138) and the National Natural Science Foundation of China (10574128). PACS: 72.40.+w, 71.55.Ht 1. Introduction Trivalent rare earth (RE 3+ ) ions are important for various applications, such as lamp phosphors and information display materials [1–3]. Recently, some research work on RE 3+ doped luminescence materials has been to study the delocatization os excited state electrons [4]. Excitation state electron delocalization is one of the major problems that may cause quenching of luminescence and laser action [5,6]. In general, such a delocatization happens to the d electrons rather that f electrons because the d state of the dopants usually overlap the conduction band of the hosts, which creates a physical path for delocatlization [7]. Therefore, the excited state structure of the RE 3+ ions relative to the host band gap becomes the key factor that determines the delocalization of electrons. Theoretical predictions of the band structure relative the host band gap have been carried out in many hosts for RE 3+ and RE 2+ ions [8]. But these predictions yield also some incorrect information [9–11]. Thus experimental determinations of the band structures are needed. The first set of band structure of RE 3+ in yttrium aluminum garnet (YAG) host was determined and extimated using a photoemission method [12,13]. The results revealed that the 5d states of the ions are close to host conduction band and the 4 f ground states are close to the host valence band. In this work, the band structure of the Pr 3+ ions relative to Y 2 O 3 host is determined using a photoconductivity measurement. It is a following effort in determining the band structures of the RE 3+ in Y 2 O 3 . Using the photoconductivity method, the ground states of Ce 3+ , Tb 3+ and Er 3+ ions have been found to be 2.8, 1.3 and -1. 0 eV respect to the host valence band [14–16]. Pr 3+ is the ion next to Ce 3+ with two valence electrons. Both 4 f -4 f and 4 f -5d transitions exist in Pr 3+ doped materials. The competition of 5d state and 1 S 0 state of Pr 3+ in strontium aluminates is of interest in quantum cutting phosphors [17,18]. Pr 3+ has also important ap- plications in long persistent phosphors and light emitting diode phosphors (LED). In these cases, photoionization of excited state electrons are important to the performance of these phosphors, so that the band structure will provide information to the applications [19,20]. 2. Experimental The Y 2 O 3 : Pr 3+ single crystal fibers have been pre- pared using a laser heated pedestal growth (LHPG) method [15,21].Y 2 O 3 and Pr(NO 3 ) 3 powder mixtures were made with a proper mole ration (1% doping concentration) and were heat-treated at 900 ◦ C for 2h. The treated raw materials were re-mixed, pressed into pellets and sintered at 1200 ◦ C in air in a Linderburg blue tube furnace for 2 h. The sintered pellets were cut and polished into 1 × 1 mm square rods for laser pulling. The single crystal fibers were reduced at 1350 ◦ C in a 5%H 2 + 95% N 2 gas flow in order to obtain Pr 3+ . The fiber samples were polished into 300 μm thin slabs along their fiber axis with two parallel side surfaces. Photoconductivity spectra of the samples were measured at room temperature. Ni meshes were used to serve as the electrodes. The light source for excitation was an Oriel 200W xenon lamp filtered through an ISA Jobin Yvon Spex 1610