Dynamical electron and phonon processes in the excited state of LGO: Cr3+ crystals

LUMINESCENCE

Journal 01 Luminescence ~3 ( 1992) ~4_.~7

JOURNAL OF

Dynamical electron and phonon processes in the excited state of LGO : Cr3~crystals S.A. Basun , S.P. Feofilov and M.Yu. Sharonov ‘~

A.A. Kaplyanskii

‘,

A.B. Bykov ~, BK. Sevastyanov ~

A.F. IOffC P/ivs~co—Technical Ins/itute, Academy oJ S’c,e,icys of Rusoa, 194021 St. l’c,ervlnirg, Russia A. V. S/iubnikoc Institute fdr (‘n’stallograplu, Academy of Sciences of Russia, 11 7333 Moscow, Russia

In pseudo—Stark effect studies on the R. R’ lines of [GO :r~ the ~i point symmetrs 0! both types of fr’ centers iii the low—temperature ferroelectric (C ) phase as well as evidence for local compensation ii ( ‘r ICc ) extra charge in LGO lattices were established. In direct photoelectric studies it was found that ( r photoionization takes place n a two-quantum process via the intermediate 2E excited state. With the help of fluorescent phonon detection the (liflusive regime of THz aCoustiC phonon propagation in [GO at T = 2 K was observed: the ariharmonic phonon decay times were measured for frequencies 5.5 cm I IS!) ns) and 63 cm (75 us).

1. Introduction

used in nonequilibrium phonon studies. Because in the low-temperature C~ phase of [GO two

In Cr-doped LGO (Li 2Ge7O15) crystals 3~ionsthe in “classic” high crystal with field broad spectraabsorption of Cr octahedral positions hands (4A 4T-,, etc.) and sharp R lines (4A~—E)in 4— absorption and fluorescence are observed. At low temperatures two R line pairs can he seen due to two types of Cr3 + centers: R 1 (14360 cm ), R (14423 cm ‘), and R’1 (14375 cm’), R’, (14430 cm ) [1]. At T= 10°Cthe LGO crystal undergoes the ferroelectrie phase transition D~h—C~5 (see ref. [2] and references therein). We present here some results of optical and photoelectric studies of excited state processes in 3 crystals. LGO : 0.05%Cr -

-

2. Nonequilibrium phonon dynamics Fluorescent detection of resonant phonons by observation of Cr3’ ion R~ fluorescence [31was (.oi espondenic to Dr S A B isun A 1 Ioftc Physico t c h nical Institute. Academy of Sciences of Russia. 194)121 s~. Petersburg, Russia. )1i22-23l3/92/$05.O)) s~1992

—

types of Cr~ exist63with J[ possistate splittings U centers 55 and cm different ). it was ble to study the behavior of phonons of these two frequencies. In heat pulse experiments phonons were injected at T 2 K from a constantan thin film =

=

deposited on the crystal surface and heated with short electric current pulses. The detector (d) was a small CW-Ar laser excited crystal volume. With the phonon detector (d) close to the heater (h) the increase of phonon-induced R~fluorescence pulse decay time (from 0.5 to 1.5 ~rs) with of heater energy density (from to It) increase .iJ/mrn~)was observed: the shapes R,(0.7 t) being practically the same for R and R’~pulses. These results give evidence for the important role of phonon—phonon interaction processes in heat pulse phonon kinetics and for the so-called “hotspot [4] creation close to the heater at high injected energies. When the h—d distance is increased up to I mm the R~ pulse leading edge is elongated. thereby giving evidence for the diffusive regime of phonon propagation. The phonon mean free

Elsevier Science Publishers By. All rights reserved

S.A. Basun ci al.

/ Dynamic electron and phonon

3

processes in the excited state of LGO: Cr

+

25

crystals

mode-averaged phonon lifetime may include a contribution of optical phonons, the frequencies twice lower than detected are in purely acoustic

9,

:T;:n::;::::ivity;

I

0

200

spectral ::::n-

400 t, ns

Fig. 1. Phonon-induced R,(t) and R~(t)fluorescence pulses.

The Cr31’ center photoionization processes in LGO were studied by photoconductivity measurements. The electric field E iO~V/cm was ap-

path estimated from the delay of R 2(t) pulse maximum is 1 0.1 mm. Such a small 1 value in LGO may be connected with intermode scattering between acoustic and optical vibrations (frequencies 55 and 63 cm are in resonance with low-lying optical vibrations of LGO lattice [51), such as scattering on lattice imperfections and/or ferroelectric domainwith walls.optical generation and In experiments detection of phonons in LGO : Cr31’ the Cu-laser pulses were used (~t = 20 ns, A = 510, 578 nm, frep = 10 kHz). The resonant 55 and 63 cmt phonons were generated in the excited region of the crystal due to nonradiative relaxation of excited Cr31’ ions and were detected by observation of R 2(t) pulses [61.At sufficiently low generated phonon occupation numbers, when the phonon— phonon interaction effects are negligible, and if the time Td of diffusive phonon escape from excited volume (dimension d >> 1) is large, the R2 fluorescence decay time i-, ~ = + r~ will reflect the resonant phonon anharmonic decay time T~ (see ref. [31). Indeed, the R2 (63 cm_t), and R’2 (55 cmt) luminescence decay time for exciting beam diameter d = 0.5—1 mm and laser power density 2 was powerpulse independent and 1.5—20 kW/mm was 75 ns for R 2(t) and 150 ns for R~(t)(fig. I). It is natural to consider these decay times as anharmonic decay times of 63 and 55 cm phonons, respectively, their ratio fitting well to v’ dependence, characteristic decay into two acoustic phonons. Though for the the frequencies 55 and 63 cm are in resonance with ‘~

low-lying optical vibrations and the measured

plied at T= 2 and 77 K to thin (—~ 0.2 mm) LGO : 0.05 %Cr platelets with transparent electrodes deposited on their surfaces. Spectral dependence of photocurrent under excitation with different Ar laser lines correlates with R-luminescence excitation spectrum in broadband absorption region, which directly that the conduc3~ionshows photoionization. The tivity is because of Cr dependence of the observed photocurrent (I) on exciting light intensity (P) for excitation with Ar and He—Ne laser lines is quadratic, I P2. This dependence gives evidence for the two-quantum mechanism of Cr3’1’ photoionization. The measured photocurrent I—V characteristic is I E3~2 what is also observed in some other insulator ‘~

crystals (e.g. YAG : Ti) in which the nonlinear photoionization takes place.

,

P1 ~

‘ 810

~‘

~ ~

‘

/

-‘

P.

EHc H200 kV/cm P°50mW

2 t

11+480

16440 16600 16360 o,cm Fig. 2. Luminescence and photocurrent spectra at T = 2 K (solid lines). R 2, R’, fluorescence lines are shown with dashed line for T = 77 K.

21+

5.4. lha,sun c al.

/

Dynamic electron and phiotion t,rocesses

The photoconductivity spectral dependence in R-line region (690—700 nm) measured by scanning the dye-laser frequency has many peaks in contrast to simple optical absorption and fluores— cence spectra in which R1, R and R’1 and R’~ 3 centers dominate (fig. 2). lines of main Cr Some weak ‘‘satellite” lines in luminescence spectrum with intensities by a factor of 1000 less than that of main R lines correspond to photocurrent spectrum maxima. Thus the peaks connected with R line satellites belonging to special Cr3 1’ centers dominate in resonant region of nonlinear two-quantum photoconductivity spectrum. The following explanations may he suggested. I) Two-quantum photocurrent excitation is connected with summation of 2[ energies of two excited Cr3 + ions, resulting in the transition of 4A one ion to the ground state ( 2) whereas the other ion is excited to the upper lying state overlapping with conduction hand (CB).3~This process [7] up-conis similar YAG :coupled Cr version in to theobserved system ofinweakly Cr31’ ion as a result of such process of CrBe2A one ) state. ions was excited to upper-lying ( cause the photoionization up-conversion process 9 1’ is for relatively close interacting Cr at ioneffective pairs photocurrent spectrum exhibits peaks shifted R line frequencies corresponding to 2 F pairs

—

in

thu e.rc,ted state of 1.0 C:

long (millisecond) photocurrent decay was ohserved. 2) In two-quantum (two-step) photocurrent cxcitation the second step is the photoionization of 3 ion from 2 F state by the optical quantum Cr from R line spectral 3region. Strong shift to of CTB 2 F + centers relative levels ofmay distorted Cr a higher efficiency of their bottom result in photoionization by R light in comparison with main Cr31’ centers. It may appear that for main Cr31’ centers the distance from 2E to the CB bottom is somewhat higher than the R line energy and thus the discussed photoionization tran-

‘

sri vials

sition from F is el’t’ectivc only under shoi’tcrwavelength excitation (Ar and He—Ne laser lines). Different distances front 2 F to CB for main and distorted centers may play also a role in alternative mechanism dtscussed above. Iof nonlinear photoconductivity Observed spectral position of long-wavelength edges of strong absorption from Cr3 excited F—state and froni Cr3 ground 4A state at IS 000 and at 281300 cm . respectively, shows that indeed the energy distance from 2 F state of main Cr9 centers to CB bottom is very close to R-line frequency. ‘

4. Pseudo-Stark splitting of R lines (‘r

‘

ion fluorescence spectra (R

1 and R’1 lines) were studied at 7’ = 2 K in external electric fields up to 350 kV/crn. applied along a, 6 and c’ orthorhombic crystal axes. linear in field symmetric pseudo-Stark [9] The doublet splitting of R 1 and R’1 lines was observed in field L H a. The splitting ‘/( V/cm) line and was 0.38 0.7 ~ 10/ 10 cni cm ‘/(V/cm) for for R’ R 1. In

the field inE lb splitting was axis)lO notimes less. whereas E H the c (ferroelectric splitting was observed at all. From an analysis of these results it follows that in C 25 [GOin crystal phase both 3 centers are situated field types of Cr with triclinie C site symmetry. This conclusion is in full agreement with the results of ESR studies of [GO : Cr31’, where two types of [SR spectra of Cr3 centers both belonging to C local symmetry were observed in ferroelectric C 25 phase and the genesis of these centers from one C type 3 center in paraelectric D Cr 11 phase was shown. 3 It is natural to assume that pair of optical Cr centers (C 1 ) corresponds to pair of [SR centers (C’) in [GO (C 2~). In both dipole types of centers are the directed effective nearly R-line transition momenta parallel to a-axis. This may he explained by the existence of local charge compensation for Cr3 ions [101, replacing Ge41’ in octahedral positions the charge compensating defects in Cr1~ vicinity according to [10] are just in a direction relative to Cr31’ position Some asymmetry of R~atid ‘=

I

levels of pair” ions single rather ions thannot at participating main R line frequencies of Cr9 in excitations summation (see also ref. [8]). This mechanism is in agreement with the results of preliminary photocurrent kinetics measurements under chopped optical excitation in which the

(‘r

I

—

S.A. Basun

Ct

R~pseudo-Stark doublets components intensities was observed, especially pronounced if during the sample cooling from 300 to 2 K the electric field was applied along a-axis. This asymmetry (which can be switched by cooling in opposite electric field) evidences for the possibility of effective electric dipole momenta reorientation for both types of Cr31’ centers, due to reorientation of charge compensating defect position relative to Cr31’. Taking into account the found C 1 siteinsymme31’ centers ferrotry of both types electric phase (C of optical Cr 25) and in accordance with ESR data [101it can be predicted that these two types would become physically identical (C1 symmetry) after the LGO transition to parae[ectric phase D2h at T= 10°C and that R and R’ lines in spectra would merge at T> 10°C.The 31’common centers “structural” in LGO C genesis of two optical Cr 25 phase is confirmed also by similarity of R and R~transition properties (close spectral positions, similar electric dipole momenta directions, the possibility of their reorientation in external field).

References [1] R.C. Powell, Phys. Rev. 173 (1968) 358; J. AppI. Phys. 39

(1968) 4517.

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crystals

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[2] Y. Iwata, I. Shihuya, M. Wada. A. Sawada and Y. Ishibashi, J. Phys. Soc. Jpn. 56 (1987) 2420. [3] K.F. Renk, in: Nonequilibrium Phonons in Nonmetallic Crystals. Modern Problems in Condensed Matter Sciences, vol. 16, eds. W. Eisenmenger and A.A. Kaplyanskii (North-Holland. Amsterdam. 1986) pp. 277 and 317. l-Iensel and R.C. Dynes. Phys. Rev. Lett. 39 (1977) 969 M. Greenstein. MA. Tamor and J.P. Wolfe. Phys. Rev. B 26 (1982) 5604 [5] A.A. Volkov. G.V. Kozlov, Y.G. Goncharov, M. Wada. A. Sawada and Y. Ishibashi. J. Phys. Soc. Jpn. 54 (1985)

[41J.C.

818. Dijkhuis, A. van der Pol and H.W. de Wijn, Phys. [6] J.1. Rev. Left. 37 (1976) 1554;

R.S. Meltzer and J.E. Rives, Phys Rev. Lett. 38 (1977) 421 [7] R. Wannemacher and J. Heber. J. Lumin. 39 (1987) 49. [8] R. Buisson and J.-C. Vial, J. de Phys. Lett. 42 (1981) [115. [9] W. Kaiser, S. Sugano and DL. Wood, Phys. Rev. Left. 6

(1961)Galeev, 605 [10] A.A. N.M. Hassanova, A.B. Bykov. V.M. Vinokurov. N.M. Nizamutdinov and G.R. Bulka, in: Spectroscopy, Chemistry and Real Structure of Minerals and Their Analogs (Kazan University Publishing, Kazan, 1990) ~. 77 (in Russian).