Functional Materials 12, No.4 (2005)

© 2005 — Institute for Single Crystals

Effects of doping with manganese on the optical properties of magnesium aluminate spinel crystals V.T.Gritsyna, Yu.G.Kazarinov, V.B.Kol'ner , L.A.Lytvynov , K.E.Sickafus V. Karazin Kharkiv National University, 4 Svobody Sq., 61077 Kharkiv, Ukraine Institute for Single Crystals, STC "Institute for Single Crystals" National Academy of Sciences of Ukraine, 60 Lenin Ave., 61001 Kharkiv, Ukraine ' "Los Alamos National Laboratory, Los Alamos, NM 87545, USA The effects of doping with manganese ions on the optical properties of magnesium aluminate spinel crystals grown by Czochralski method were investigated. The optical spectra of the doped crystals have intense absorption in the range of 4 . 5 - 6 . 7 eV which was fit with three absorption bands; the intensity and energy position of these bands vary in dependence on the concentration of doping ions. This effect is explained by formation of .F-type centers near the incorporated Mn ions which leads to changes of electronic proper­ ties of .F + -centers and formation of complex defects including Mn-ions. The neutral atmos­ phere of crystal growth in Czochralski method stimulates the formation of anionic vacancy trapped one electron ( ^ - c e n t e r ) . Radio- and photoluminescence spectra demonstrate the bands related to the recombination luminescence at antisite defects, to emission of impu­ rity ions Mn2+ and Cr 3+ -ions, parameters of which indicate the processes of segregation and coagulation antisite defects during the growth of spinel crystals. Исследовано влияние активации ионами марганца на оптические свойства кристал­ лов магний-алюминиевой шпинели, выращенных методом Чохральского. Оптические спектры активированных кристаллов содержат интенсивное поглощение в области 4 . 5 6.7 эВ, которое представляет суперпозицию трёх полос поглощения, интенсивность и спектральное положение которых изменяются в зависимости от концентрации активаторных ионов. Этот эффект объясняется образованием .F-типа центров, расположенных вблизи ионов марганца, что приводит к изменению электронных свойств ^ - ц е н т р о в и образованию комплексов дефектов, включающих ионы марганца. Нейтральная среда выращивания кристаллов по методу Чохральского обусловливает образование анион­ ных вакансий, захвативших один электрон (^ + -центры). Спектры рентгено- и фотолю­ минесценции содержат полосы, относящиеся к рекомбинационной люминесценции на дефектах антиструктуры, а также к излучению примесных ионов Мп и Сг , парамет­ ры которых свидетельствуют о наличии процессов сегрегации и коагуляции дефектов антиструктуры в процессе роста кристаллов шпинели. Magnesium-aluminate spinel crystals (МдА1 2 0 4 ) a r e p r o s p e c t i v e m a t e r i a l f o r a p ­ p l i c a t i o n i n s c i e n c e a n d t e c h n o l o g y as m a ­ t r i x for fiber-optic t e m p e r a t u r e sensors, t u n a b l e solid s t a t e lasers, s u b s t r a t e for mi­ c r o e l e c t r o n i c s . A l s o t h i s s p i n e l is v e r y spe­ cific i n r e s p e c t t o t h e c r y s t a l l i n e s t r u c t u r e d u e t o t h e e x i s t e n c e of t h e p a r t i a l i n v e r ­ Functional

materials,

12, 4, 2005

s i o n , i.e. t h e c a t i o n i c d i s o r d e r w h i c h l e a d s t o t h e f o r m a t i o n of h i g h - c o n c e n t r a t i o n of 3+ c h a r g e d d e f e c t s (Mg +octa ) and (Al tetra )+, s o - c a l l e d a n t i s i t e d e f e c t s . T h e d e g r e e of in­ v e r s i o n of n a t u r a l s p i n e l c r y s t a l s is low b u t artificially grown crystals have u p to 30 % of d i s o r d e r e d c a t i o n s . B e c a u s e t h e n a t u r a l spinel crystals contain the large concentra719

V.T.Gritsyna et al. / Effects of doping with manganese... was measured in t h e r a n g e 1.2-6.4 eV using either a single or dual beam spectro­ p h o t o m e t e r . I r r a d i a t i o n was performed using a Cu X-ray tube operating at 40 kV and 10 mA. For ultraviolet (UV) irradiation there were used either mercury 600 W lamp or deuterium 400 W lamp, both with quartz tubes. During irradiation the samples were cooled with powerful fan to keep its tem­ p e r a t u r e below 30°C. Radioluminescence (RL) was excited using standard X-ray tube with Cu anode operated at 45 kV and 0.3 mA. Light emission was dispersed with MDR-1 g r a t i n g monochromator and re­ corded in the range 1.55-6.2 eV using FEU106 p h o t o m u l t i p l i e r . Spectral resolution was 1.6 nm in the range 2 0 0 - 4 0 0 nm and 3.2 nm in 4 0 0 - 8 0 0 nm. Photoluminescence (PL) measurements were provided at the room temperature by using two monochromators: one — for excitation (MDR-12), an­ other — for registration of emission (MDR-1). All spectra were corrected on photomulti­ plier spectral sensitivity function. Before each measurement of RL and PL the sam­ ples were annealed at 650°C during 0.5 h. X-ray diffraction analysis of crystal compo­ sition shows only spinel phase of the same lattice parameter independently on the Mnconcentration (Table). Optical absorption spectra of nominally pure crystals contain wide almost continu­ ous absorption from - 4 . 5 to 6.6 eV, the in­ tensity (Fig. 1) with some indication on two bands at - 4 . 8 and -5.4 eV. The experimen­ tal spectra could be fit with three bands of the Gaussian form at 4.8, 5.4, and 6.77 eV. Doping with Mn-ions leads to the changes of the form of absorption spectra, but still all spectra could be fit with three bands the energy positions and intensities of which depend on the concentration of manganese. The first of the indicated bands at 4.8 eV is characteristic for absorption of F + -centers (anionic vacancy captured one electron) in stoichiometric spinel crystals [4], but the position of this band shifts to higher energy

tion of impurity ions such as Cr, Mn, and Fe, this will be one of the reasons for for­ mation of ordered spinel s t r u c t u r e . The presence of transition metal impurities in oxide crystals affects quite markedly the optical, electrical, and mechanical proper­ ties. The effect is strongly dependent on the valence, aggregation state, and concentra­ tion of the impurity. Incorporation of aliovalent impurities in nominally p u r e or doped oxide crystals causes the appearance of charge compensat­ ing "free" and "bond" defects and forma­ tion of the charged or neutral defect clus­ ters. In sapphire crystals doped with mag­ nesium А120з:Мд the FM„~ centers are formed which r e p r e s e n t s the F + -centers near Mg 2+ -ions [1]. In chromium doped MgO:Cr the Cr 3+ centers with Mg vacancies (VMg) on nearest-neighbor and on the nextnearest-neighbor lattice sites have been identified [2]. Moreover, the doping of CaO with Mg 2+ isovalent ions leads to the forma­ tion of F- and F + -centers at regular anion vacancies along with FA- and F^ + -centers on anionic vacancies for which the nearest Ca 2 + is replaced with Mg 2+ . There is some evidence for spatial correlated F- and FAcenters [3]. Therefore by v a r i a t i o n of aliovalent or isovalent i m p u r i t y concentra­ tion we can manipulate the physical proper­ ties which are related to point defects in undoped and impurity-doped crystals. In this paper we investigated the optical prop­ erties of magnesium aluminate spinel crys­ tals grown by Czochralski methods and doped with manganese to different concen­ trations. We investigated stoichiometric spinel crystals MgAI 2 0 4 grown by Czochralski method in argon atmosphere from spinel powder MgAI 2 0 4 as nominally p u r e and doped with Mn to concentration of 0.1 m a s s . % . Samples with dimensions of 10x10 mm 2 and 0.7 mm thickness were cut from single crystals and polished on both sides to an optical finish. Optical absorption

Table. Variation of lattice parameter in spinel crystals grown by Czochralski method and doped with manganese to different concentrations Concentrat. of Mn

Orientation

Angle of deviation

Nominally pure 0.02 mass.% 0.04 mass.% 0.1 mass.%

[310] [310] [310] [100]

3.8°

720

4°

1.3° 4°

a, c

Center 8.0845 8.0845 8.0848 8.0838

Edge 8.0848 8.0842 8.0842 8.0858

Functional materials, 12, 4, 2005

V.T.Gritsyna et al. / Effects of doping with manganese... I.a.u. E.ev

Fig. 1. Absorption spectra of MgAI204:Mn spinel crystals grown by Czochralski method. The inset plot shows the dependences of en­ ergy position of absorption bands on the con­ centration of Mn-ions in crystals. 1(U) — nom pure; 2(P), 3(A), 4(V) — 0.02, 0.04, 0.1 % Mn. at the increasing of manganese concentra­ tion. The definite i n t e r p r e t a t i o n of this band is difficult because at this energy c o u l d be s i t u a t e d a l s o t h e F + - b a n d in a-AI 2 0 3 a n d / o r absorption band related to charge transfer transition in Fe 3 + ions in oxide crystals. Absorption related to charge transfer transitions in Fe 3 + ions in spinel consist of two bands at 4.75 and 6.4 eV which allow us to conclude on the improbablity of this identification, because the second band is absent. The F + -centers in a-AI 2 0 3 show also two absorption bands at 4.83 and 5.45 eV [5] which is very close to experimental bands. Nevertheless, the be­ havior of these two bands with manganese concentration excludes also this identifica­ tion because the position and intensity of 5.4 eV band do not depend on the Mn-ion content, but the intensity of band at 4.8 eV grows linearly with Mn concentration and its position shifts to higher energy up to 5.2 eV. The energy position of the last absorp­ tion band changes with Mn-concentration in opposite direction from 6.77 eV for nomi­ nally pure crystals to 6.4 eV in spinel crys­ tals containing 0.1 mass.% of manganese, the intensity of this band increases also but it tends to saturation at high concentration. In the crystals doped with the highest Mn concentration the experimental spectra could be fit only with two bands at energies of 5.4 and 6.2 eV. Functional materials, 12, 4, 2005

I,a.LI.

r

Fig. 2. Difference absorption spectra of MgAI204 spinel crystals doped with manga­ nese to different concentration and irradiated with UV-light: ЦU) — nom pure; 2(P), 3(A), 4(V) — 0.02, 0.04, 0.1 % Mn. Therefore, we have concluded t h a t maxi­ mum absorption at 4.8 eV is superposition of two bands one of which is caused by F + -centers in the presence of some amount of a-AI 2 0 3 phase and another one is related to F + - c e n t e r s s i t u a t e d near Mn-ions in spinel lattice. The formation of F-type cen­ ters related to anionic vacancies in the Czo­ chralski grown crystals is consequence of reducing growth atmosphere. The energy position of the t h i r d band and its depend­ ence on manganese concentration allow to ascribe it t e n t a t i v e l y to complex defects which include lattice defects and impurity ions. Usually isolated anionic and cationic va­ cancies in as-grown crystals are optically inactive, b u t irradiation with UV-light or X-rays leads to charge exchange between de­ fects and impurities and formation of hole centers at cationic vacancies and F-type cen­ ters at anionic vacancies. The difference spectra of i r r a d i a t e d and non-irradiated spinel samples doped with manganese to dif­ ferent concentrations are shown in Fig. 2. Irradiation of nominally pure spinel crystals with UV-light demonstrate the prominent radiation-induced absorption bands at 3 . 1 , 4.75, and 5.35 eV which were identified with isolated hole and electron centers. Also there is strong absorption in the vicinity of ~4 eV, and the deconvolution of absorption spectra in Gaussians bands gives additional bands at 3.78 and 4.15 eV, which were identified with hole and electron centers at antisite defects [4]: 721

V.T.Gritsyna et al. / Effects of doping with manganese... I

I. а.и.; 100.

C.J

16

12

10 г

a u

?-E*430nm ' 2- Ex-450 nm 3- Em-520 nm 2

i • Excitation spectra

•э

Emission spectra 0.D0

0.04

0.08 C.maes* i\

•;

V

\

i *2 1 S

8

:\

4 0.1

0 200

300

400

500

600

700

X, nm

350

400

450

500

550

X, nm

Fig. 3. Radioluminescence spectra of MgAI204:Mn spinel crystals doped with man­ ganese to different concentrations: 1(U) — nom pure; 2(P), 3(A), 4(V) — 0.02, 0.04, 0.1 % Mn.

Fig. 4. Excitation/emission spectra of Mill­ ions in Mn-doped spinel crystals: 1 — nom pure; 2, 3, 4 — 0.02, 0.04, 0.1 % Mn. The inset plot shows the dependencies of intensity of excitation and luminescence bands on the concentration of Mn-ions in MgAl204:Mn crystals.

-> (Mg2+i()0 (band at 3.78 eV),

and impurity ions at the much lower con­ centrations. The RL spectra of nominally pure spinel crystals disclose the prominent emission bands at 4.8 (258 nm), 2.38 (520 nm), and 1.8 eV (688.5 nm) which are related to recombination luminescence at antisite defects, to radiative transitions in Mn 2+ - and Cr 3+ -ions in spinel crystals, re­ spectively (Fig. 3). For the first time the Mn-doped to 0.01 mol% spinel crystals was studied in cathode-luminescence method [7]. The dependence of luminescence properties of Mn 2+ -ions on the ordering of spinel lat­ tice was shown in [8]. Our experiment dem­ onstrates t h a t doping with Mn-ions leads to the vanishing of the 4.8 eV band and the decreasing of Cr 3+ -ions luminescence at the growth of Mn 2+ -ion band. Because of the UV-emission band is identified with the re­ combination luminescence this means t h a t some type of antisite defects disappear in Mn-doped spinel crystals. Magnesium aluminate spinel lattice con­ tains two types of polyhedron formed by oxygen ions: t e t r a h e d r a l and octahedral sites. Because the energy of preference of Mn 2+ -ion for octahedral position is equal to zero, they can occupy both positions. The observed luminescence spectra of manganese doped spinel crystals contain the band at 520 nm, which is usually assigned to tran­ sitions between the spin-orbit components of the 4 T j excited state and the 6A± ground state of Mn 2+ in tetrahedral position. The measured excitation spectra of the Mn 2+ emission are shown in Fig. 4. According to the Tanabe-Sugano scheme the bands at

(Alf+J+ + e- -> (A|3+ ra )0 ;( b

a n d a t 4 Л 5

eV).

Doping with manganese to concentration of 0.02 % leads to disappearance of hole centers at isolated cationic vacancies (the 3.1 eV band), growth of bands at 3.78 and 4.15 eV, at the constant intensity of bands related to F-type centers. In crystals doped with manganese to concentration of 0.04 % and higher there were observed only the bands at 3.78 and 4.15 eV. The previous investigations of kinetics of accumulation and decay of these bands allow us to iden­ tify the spatially correlated antisite defects which are responsible for high resistance of this material to displacive irradiation [5]. At the doping with manganese there appear also band at 1.5 eV the origin of which is need to be studied. Therefore, the doping with manganese leads to formation of an­ tisite defects, which are optically active after UV-irradiation and to disappearance of isolated cationic and anionic vacancies. The existence of defects and impurities could be studied by radioluminescence — the emission which arises under X-ray or gamma-irradiation. At such types of irra­ diation we generate the free electrons and holes which could be captured by defects and impurity ions with subsequent emission of characteristic photons allowing to iden­ tify the n a t u r e of emitting species and de­ fects. Because of the detection level of lu­ minescence methods is several orders of magnitude higher to compare with t h a t of absorption it allows to study the defects 722

Functional materials, 12, 4, 2005

V.T.Gritsyna et al. / Effects of doping with manganese...

300

400

500

600

700 JL, nm

Fig. 5. Excitation/emission spectra of Cr3+ions in Mn-doped spinel crystals: 1(U) — nom pure; 2(P), 3(A), 4(—) — 0.02, 0.04, 0.1 % Mn. The inset plot shows the dependencies of ratio of intensity of excitation to luminescence bands on the concentration of Mn-ions in MgAI204:Mn crystals ( 1 — Xex = 560 nm. 2 — Xex = 430 nm). 460, 430, 390 and 360 nm could be identified with transitions from the ground state 4 ^A^S) to the 4T2(4G), A X ( 4 G) + ( 4 £ ) , 4 T 2 ( 4 -D), and 4E(4D) of Mn 2+ -ions in tetrahedral position, respectively. Therefore, both the luminescence and excitation spectra confirmed the tetrahedral coordination of M i l l ions in spinel lattice, and the large widths of these bands indicate the existence of disordering of occupied sites. Taking into account optical absorption spectra, one can propose t h a t Mn 2+ ions are surrounded by anionic vacancies and antisite defects in the second and higher coordination spheres. The concentration dependences of intensity of excitation and emission bands are shown in the insert of Fig. 4. In general, the behavior of these bands is correlated to concentration dependence of absorption band at 6.4 eV. Despite of the doping spinel crystals with manganese, the luminescence of uncontrolled impurities of Cr 3+ ions could be observed also. The experimental spectra of emission and excitation of Cr 3+ ions in spinel crystals is shown in Fig. 5. Zero-phonon emission line at wavelength of 688.5 eV originated from the spin-forbidden transition 2Eg —> 4A2g. in octahedrally coordinated Cr 3+ (so-called Л-line). Excitation spectra consist of two bands at 560 and 430 nm related to transitions from ground state to ^Tog, and 4Tlg, respectively. The position of Cl"3 -emission line certainly indicates the oc­ tahedral coordination of this ion in spinel lattice, but doublet character of excitation Functional materials, 12, 4, 2005

bands related to field-dependent transitions could be explained by localization of defects (including a n t i s i t e defects) near incorpo­ rated Cr 3 + -ions. In opposite to Mn 2+ -ions the efficiency of excitation in both bands is the same, but increasing of manganese concen­ tration to 0.1 mass.% leads to decrease of emission intensity of Cr 3 + -ions, maybe, be­ cause of excitation energy transfer from Cr 3+ to Mn 2+ -ions. It is usually recognized t h a t incorpora­ tion in ionic crystals of aliovalent ion as impurity leads to creation of a t t e n d a n t de­ fects for charge compensation. Also incorpo­ ration even isovalent ions could create com­ plex defects consisted of impurity and lat­ tice defects, like Ш2+-Уд in MgO and CaO crystals [9]. Because Mn 2+ can be placed in tetrahedral and octahedral positions we can expects formation of such complex defects in spinel lattice. Moreover, the inconsis­ tency of size, charge, and electronic struc­ t u r e of impurity ion to the replaced one leads to distortion of regular lattice, which act as attractive or repulsive center for dif­ ferent types of defects including charged antisite defects [10]. Cationic disorder in spinel by itself leads to creation perturbed of Cr 3+ and Mn 2+ ions [8]. Irradiation of nominally p u r e spinel crystals with UVlight or X-rays leads to formation of Vand F-types centers along with hole and electron centers at antisite defects [4]. Dop­ ing with manganese ions causes damping the formation of V- and F-types centers and enhances the optical centers formation at antisite defects (Fig. 2). There is a correla­ tion between efficiency of RL of Mn-ions and irradiation induced optical center for­ mation at antisite defects in dependence on manganese concentration. This means t h a t incorporated manganese ions serve as cen­ ters of segregation and coagulation of an­ tisite defects, some of them is forming ab­ sorption centers, others — centers of lumi­ nescence. The absence of recombination luminescence in UV range for doped crys­ tals related to antisite defects indicates the vanishing of isolated antisite defects. In conclusions, the experimental study of the optical properties of manganese doped spinel crystals shows the existence of ab­ sorption bands which were identified with F + -centers distorted by impurity ions. The n e u t r a l a t m o s p h e r e of crystal growth in Czochralski method stimulates the forma­ tion of anionic vacancy trapped one electron (F + -center). The variation of transition en­ ergy in these centers from 4.8 eV in nomi723

V.T.Gritsyna et al. / Effects n a l l y p u r e c r y s t a l s t o 5.2 eV i n c r y s t a l s d o p e d w i t h m a n g a n e s e t o c o n c e n t r a t i o n of 0 . 1 m a s s . % c a n b e e x p l a i n e d b y t h e defec­ t i v e n a t u r e of t h e f i r s t a n d s e c o n d c o o r d i n a ­ t i o n s p h e r e s a t l o c a l i z a t i o n of M n - i o n s n e a r anionic vacancy c a p t u r e d one electron form­ i n g F + - c e n t e r . T h e l u m i n e s c e n c e of t h e s e Mn2+-ions gives t h e m a i n c o n t r i b u t i o n in RL s p e c t r a of s p i n e l c r y s t a l s . T h e d i s a p p e a r ­ a n c e of t h e U V - b a n d r e l a t e d r e c o m b i n a t i o n l u m i n e s c e n c e in m a n g a n e s e doped spinel c r y s t a l s i n d i c a t e s t h a t Mn-ions also s e r v e s a s c e n t e r s of a t t r a c t i o n of a n t i s i t e d e f e c t s responsible for this emission. Acknowledgements. This research was m a d e p o s s i b l e i n p a r t b y G r a n t # 2 0 5 8 of t h e Science and Technology Center in U k r a i n e (STCU) a n d G r a n t # 0 7 - 1 3 - 0 3 of t h e M i n i s t r y of E d u c a t i o n a n d S c i e n c e of Ukraine.

of doping with

manganese...

References 1. R.Vila, M.Jimenez de Castro, Phys.Rev.B, 59, 7480 (1999). 2. J . E . W e r t z , P.V.Auzins, J. Phys.Chem. Solids, 28, 1557 (1967). 3. F.J.Ahlers, F.Lohse, J.M.Spaeth, Solid State Commun., 4 3 , 321 (1982). 4. V.T.Gritsyna, I.V.Afanasyev-Charkin, Yu.G.Kazarinov, K.E.Sickafus, Nucl. Instr. Meth. Phys.Res., В 218, 264 (2004). 5. A.Morono, E.R.Hodgson, J. Nucl. Mater., 250, 156 (1997). 6. V.T.Gritsyna, Yu.G.Kazarinov, V.A.Kobyakov, K.E.Sickafus, Rad.Eff.Def. Solids, 157, 659 (2002). 7. F.A.Hammel, J.F.Sarver, J. Elecrochem. Soc, 111, 252 (1964). 8. U.R.Rodriguez-Mendoza, V.D.Rodriguez, A.Ibar­ ra, Rad.Eff.Def. Solids, 136, 29 (1995). 9. E.Siegel, K.A.Muller, Phys.Rev.B, 19, 109 (1979). 10. V.T.Gritsyna, Yu.G.Kazarinov, V.A.Kobyakov, K.E.Sickafus, MRS Proc, 792, 117 (2004).

Вплив активацп марганцем на оптичш властивостл кристал1в магнш-алюмш1ево1 шпшел1 В.Т.Грицина, Ю.Г.Казартов, В.Б.Кольнер, Л.А.Литвинов, К.Е.СЬкафус Д о с л у ж е н о вплив активацп юнами марганцю на оптичш властивоси. кристал1в магнш-алюм1н1ево1 шшнел1, вирощених методом Чохральського. Оптичш спектри активованих кристал1в м1стять штенсивне поглинання в област1 4 . 5 - 6 . 7 еВ, яке е суперпозипдею трьох смуг поглинання, штенсившсть та спектральне положения я к и х змгнюються в залежност1 в1д концентрацп активаторних ioHie. Цей ефект пояснюеться створенням .F-типу центр1в, розташованих поблизу ioHie марганцю, що призводить до змгяи електронних властивостей ^ + -центр1в та створення комплекс1в дефекпв до я к и х входять юни марганцю. Нейтральне середовище вирощування кристал1в за методом Чох­ ральського зумовлюе створення ашонних вакансш, я ш захоплюють один електрон (^ + -центри). Спектри рентгено- та фотолюмгнесценцп м1стять смуги, яга вщносяться до рекомбшацшно1 люмшесценци на дефектах антиструктури, випромшювання домпнкових ioHiB Mn та Сг , параметри я к и х вказують на наявшсть процеыв сегрегацп та коагуляцп дефект1в антиструктури у процеы. росту кристал1в шпшел1.

724

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12, 4, 2005