migrations of excitons and holes

J O U R N A L OF L U M I N E S C E N C E 5 (1972) 117-131 ~g) North-Holland Publishing Co.

MIGRATIONS OF EXCITONS AND HOLES IN LUMINESCENT CRYSTALS OF CsBr E. VASIL'CHENKO, N. L U S H C H I K and CH. L U S H C H I K

Institute of Physics and Astronomy, Toomem~igi, Tartu, Estonian S.S.R., U.S.S.R. Received 14 November 1971 For CsBr, CsBr:TI, CsBr:In and C s B r : N a luminescence has been studied for the uv excitation within the CsBr fundamental absorption. Optically created excitons in CsBr at 80°K give unpolarized emission of 3.49 eV with the decay time of emission z = 10.5 ~s. It was concluded that emission arises after the excitoas have migrated distances of several lattice constants and thin have been self-trapped. Optical creation of excitons in CsBr:TI and CsBr:In induces the luminescence not on!v of self-trapped excitons but also of In + and T1+ centres. The intensity of activator emission does not depend on temperature in the interval from 80 to 140°K and increases with temperature if T > 140°K. At 80°K the decay time of exciton emission in CsBr equals that of CsBr:In. The emission of In + and TI + centres at 80°K is excited by not~relaxed excitons. At high temperatures, T > 150°K, axially relaxed excitons diffuse to impurity centres and excite them. The temperature dependences of the ionization of In + centres by holes have been investigated. At T < 130°K nonrelaxed holes ionize In + centres and the ionization efficiency does not depend on temperature. At T > 130°K the Vk centres are mobile and also ionize In + centres. Various properties of relaxed and nonrelaxed excitons and holes are discussed.

1. Introduction

The luminescence of impurity centres following the creation of electronic excitations of the host is used in many devices. It is very important to clarify the micromechanisms of energy transport through the lattice to impurity centres. This problem has been investigated for alkali h~iides in a number of papers (e.g., see refs. 1-5). In the present commanication migration of energy in the pure and doped CsBr crystals has ~,een examined (see also ref. 6). We have paid special attention to the r~.le of axially relaxed and •~ v ; a l l , , nr~nrahavod h,qla~ and excitcm.g An analouous r~roblem has been investigated for NaC1 7, s) and KI 9, 10). 2. Experimental

Single crystals of CsBr, CsBr" Na, CsBr:In and CsBr'Ti were grown by 117

118

E. VASIL'CHENKO~ N. LUSHCHIK AND CH. LUSHCH|K

the Stockbarger procedure using purified salts. Bromides, e.g., NaBr, InBr and TIBr, were added as impurities. Oriented and nonoriented polished single crystal plates 12 x 8 x 1 mm 3 were examined. Use of oriented samples will be mentioned below. The orientation of single crystal planes was carried out by means of the identification o f the needle prick tracks on the single crystals inserted between two crossed polaroid. The plates with the { 100} faces were prepared. The concentration of indium in CsBr was estimated by means of the Smakula formula. It was supposed that the oscillator strength for the band at 5.35 eV, f~ = 1. Experiments give for KCI: In, f~ = 0.57. For CsBr: Na and CsBr" TI activator concentrations before melting are given. The principal experiments were performed by means of the experimental set up consisting of the vacuum grating monochromator VMR 2, a powerft:i lamp with a hydrogen or carbon dioxide discharge in the capillary and a glass cryostat with LiF windows and an charcoal adsorption pump suitable for the temperature interval from 80 to 400°K. The excitation spectra have been measured by the irradiation of the crystal with equal quantum intensity for all frequencies. The equality of quantum intensities was guaranteed by means of sodium salicylate screen varying the width of monochromator slits. Intensities of the fast luminescence (not stationary luminescence) with the time rise ~ ~ 1 s were registered. The emergent luminescence was registered at the angle of 90 ° to the exciting beam through a monochromator ZMR 3 (or through glass filters) by means of a photomuitiplier FEU 39 with a quartz window and an electronic potentiometer EPP 09. Emission spectra were corrected with respect to photocathode sensitivity and monochromator dispersion. The quantum intensities of emission are given in the figures. A pile-of-plates polarizer has been constructed from six polished LiF plates. The beara fell at the angle of 55030 ' on the surface of LiF plates. The polarizer gave a degree of polarization up to ~;0%. The exciting light fell perpendicular to the surface of the crystal {100), and the electric vector E was oriented along the axis C4 parallel to the monochromator slits. A Franck-Ritter prism was used as an analyzer. The degree of polarization was determined by p = (I, -

I1)/(I,

+ zi),

where It; and I~ are intensities of light, polarized in parallel and perpendicular directions to the polarization of the exciting beam. For investigating fast processes a discharge, operated from tiratron and giving impulses of 10-7 s, was used. Impulses were registered by an oscillograph CI 8A 12). X-ray irradiation at 80°K was carried out by a Roentgen tube (W anode, 60 kV, 14 mA).

MIGRATIONS OF EXCITONS AND HOLES

I 19

3. Electronic excitations of CsBr

One-photon CsBr fu,'.damental absorption has been studied at 295 °K 13, 14), at 80°K 14), and at 10°K is). In fig. 1 (and figs. 2 to 5~ the CsBr fundamental absorption is reproduced on the basis of the data reported by Eby, Teegarden and Dutton14). In the low energy region of fundamental absorption at 80°K four narrow

I

i - C~B,

11 p ~

_,

..... 180./i,ze,~