Exoelectronic spectroscopy of intrinsic and extrinsic ... - Science Direct

208

Journal of Non-Crystalline Solids 134 (1991) 208-217 North-Holland

Exoelectronic spectroscopy of intrinsic and extrinsic color centers in surface layers of alkali silicate glasses V.I. A r b u s o v b, A.F. Z a t s e p i n a, V.S. K o r t o v a, M . N . Tolstoi b a n d V.V. T y u k o v a a Urals Politechnical Institute, 620002 Sverdloosk, USSR b State Optical Institute, 199034 Leningrad, USSR Received 14 May 1989 Revised manuscript received 20 March 1991

The possibilities of exoelectronic spectroscopy method for investigation of the nature of color centers (CCs) and electronic processes connected with them in surface layers of alkali silicate glasses of high purity were demonstrated. It was shown that the localized electronic states of matrix together with the extrinsic Fe 2+ ions were the main electron donors. It was determined that the energetic parameters of emission-active centers are closely analogous to those of bulk CCs. The necessary stages of photo- and thermally stimulated exoemission mechanism are the trapping of electron on intrinsic localized states in the vicinity of the bottom of the conduction band, thermo-ionization of the relaxed state of forming centers and electron yield into vacuum. The process of Fe E+ tunnel ionization dominates at q u a n t u m energies of h~0 < 5.0 eV in the formation stage of photoemission from unstable centers.

1. Introduction

Much consideration is given to study of intrinsic and extrinsic color/centers (CCs) in silicate glasses [1,2], since the processes of their formation and decay have impact on practical properties of glasses. Today some progress is achieved in classification of CCs on the basis of their spectroscopic characteristics, obtained from optical and luminescent measurements [1-3]. Mechanisms for CC formation and decay in the bulk of glass are suggested and investigated. However there are no such data for CCs localized in thin surface glass layers. Meanwhile, the specific properties of surface CCs, photo- and thermally stimulated electronic processes near the surface, are important properties in the development of wave guide optics. Exoelectronic spectroscopy is one of the most sensitive methods for detection of charge localization centers in subsurface layers of solids. Important information about energetic parameters, spatial distributions and mechanisms of centers

formation and decay [7-9] can be deduced from such spectra. This paper is addressed to an investigation of the characteristics of exoemission centers and mechanisms of photo- and thermally stimulated electronic processes in surface layers of alkali silicate glasses.

2. Experimental The 2 2 R 2 0 . 3 C a O . 75SIO 2 (R = Li, Na, K) glasses of high purity [10], in which Fe content was no more than 5 x 1 0 -'1 wt%, were investigated. A buffer impurity, CaO, was added to glass. It did not change optical absorption [4,5] and had no impact on photoemission properties of glass surfaces. To eliminate surface leaching, the glasses were subjected to 'back' ion exchange in a liquid of the appropriate nitrates at 400 ° C for 30 rain according to recommendations in ref. [11]. The samples (10 x 10 x 2 mm 3) were put into a vacuum chamber with 1 0 - 4 Pa residual pressure

0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

ILL Arbusou et al. / Intrinsic a n d extrinsic color centers

and annealed at 6 0 0 ° C for 3 min. The exoelectronic emission (EE) current was registered by a secondary electron emission multiplier ( - 1 0 -18 A sensitivity). The error of measurements was not more _+6%. The CCs formed in the surface layers of samples when irradiated by X-rays (Co-anode, D = 2 × 105 R exposure dose), or electron beams (1 keV, current - 1 ~tA, time of irradiation - 5 min) or UV-light with q u a n t u m energy in the range of 3.5-6.2 eV. W h e n measuring thermally stimulated processes, the emission current was registered while samples were heated at a linear rate between 100 and 673 K at a rate of 0.3 K / s . The kinetic parameters of thermally stimulated exoemission (TSEE) were calculated according to the method in ref. [12] with c o m p u t e r resolution of T S E E data into elementary maxima. A calibrated channel of photostimulation which consisted of a deuterium lamp, double m o n o c h r o m a t o r and a system of optical input into the vacuum chamber was used to measure photostimulated exoemission (PSEE) current and to obtain the corrected PSEE spectra, ~-(hc0), over the entire spectral range. The n u m b e r of quanta, falling on the glass surface was constant for these measurements. Spectral resolution of the optical channel was < 3 nm. To determine the role of optical and thermal stages in the mechanism of electronic processes, the PSEE current was measured from the surface of unexcited glasses at linear heating and illuminating with monochromatic UV light.

3. E x p e r i m e n t a l r e s u l t s

Figures 1 and 2 show typical T S E E data from the surface of alkali silicate glasses after X-ray and electron irradiation at different temperatures and glass surface preparation. It is seen that the change of alkaline ion type has a small effect on T S E E curves (figs. l(a) and 2(a)). At the same time the T S E E integrated intensity changes monotonically in homological row of glasses, increasing from lithium to potash silicate glasses. X-ray excitation of glasses at 100 K results in emission current maxima between 125 and 145 K, 200 and 210 K,

209

1 .~,~_

E ~.d.~,?. E

Ic::~

";O

I

c~~,rev I

d

I

I

-

I0~

tOO

~0~

T',K

400

'-

500

600

Fig. 1. Characteristic TSEE curves for glasses with different type of alkaline modificator (a) and for potassium sihcate glasses under different conditions of excitation and preparation of surface (b) after X-raying: 1, 2, 3 - Li, Na, K silicate glasses (irradiation temperature =100 K); 4 - irradiation temperature = 120 K; 5 - irradiation temperature = 280 K; 6 - glass without regenerative treatment of surface (irradiation temperature = 100 K).

250 and 260 K, 290 and 330 K, 395 and 400 K, 500 and 525 K. M a x i m u m in the range of 3 9 5 - 4 0 0 K was not simple, since for irradiation of samples at elevated temperatures a splitting into peaks between 350 and 380 K and 400 and 415 K (fig. l(b), curve 5) was observed. In addition the T S E E m a x i m u m between 465 and 470 K and sharp changes in the range of 6 0 0 - 6 1 0 K appear for potash silicate glasses. The electron b o m b a r d m e n t , unlike the X-ray excitation, results in emission m a x i m u m between 500 and 525 K and a new peak between 565 and 585 K (fig. 2(a)) is observed. Evidently the T S E E current between 175 and 275 K is caused by decay of adsorbtive centers, since there was a pressure increase in the v a c u u m c h a m b e r in this temperature range. This assumption is confirmed by experiments in ultrahigh vacuum. A characteristic

210

V.L Arbusov et al. / Intrinsic and extrinsic color centers

feature of samples with leached surface (without treatment by the ' b a c k ' ion exchange method) is a reduced intensity of the 125-145 K and 290-330 K TSEE maxima associated with decay of structural centers (figs. 1 and 2, curves 6). The analysis of the forms of emission current maxima over the entire temperature range shows that monomolecular kinetics are characteristic for thermally stimulated exoemission from the surface of our samples, which is evidence in favor of an ionization mechanism for decay centers and agrees with the data presented in ref. [13]. The parameters for local centers, calculated according to TSEE data, as well as thermally stimulated luminescence (TSL) and optical absorbtion (OA) are given in table 1.

The calculation of energy depth, E, for exoemission centers according to TSEE data takes into account the surface potential barrier value, X, as the energy of thermal activation is d~a = E + XThe resulting error of calculation of the energetic parameters was not more than + 0.05 eV, x-values [19] at 300 K increase monotonically from 0.05 eV to 0.15 eV in transition from potassium to lithium glasses. Based on this regularity, we can explain the TSEE and PSEE intensity changes in the homological row of alkali silicate glasses. The low x-value is due to the fact that electron emission is most probable from surface 'points' with low electron affinity. The value of X decreases at elevated temperatures, which may be a result of diffusion alkali ions from the interior to the surface. The

Table 1 P a r a m e t e r s of subsurface emission-active centers and b u l k color centers in alkali silicate glasses a c c o r d i n g to TSEE, t h e r m a l l y s t i m u l a t e d l u m i n e s c e n c e and optical a b s o r p t i o n CC

Optical

T y p e of

TSEE parameters

type

absorbtion m a x i m u m (eV) ( ETSL, eV)

glass

Tmax (K)

El-

1.3-2.0 [14] (0.15-0.45 [1])

Li Na K

Ef

2.1-2.2 [2] (0.6-0.7 [1,15])

-

Po (l/s)

X (eV)

E (eV)

(E ) (eV)

135-145 130-140 120-130

-

0.20 0.15 0.10

0.07-0.13 0.10-0.16 0.14-0.20

0.10 0.13 0.17

Li Na K

290-300 290-300 300-330

- 1011 1011 - 1011

0.15 0.10 ~)

0.45-0.75 0.40--0.70 0.35-0.65

0.60 0.55 0.50

3.7 [16] (0.8 [1,17])

Li Na K

345-375 370-385 380-385

- 101° _ 101o - 101o

0.10 a) a)

0.60-0.80 0.60-0.90 0.60--1.00

0.70 0.75 0.80

E4

5.3-5.4 [2] (0.8-1.0 [1,15])

Li Na K

400-405 410-415 410-415

- 1011 - 1011 - 1011

0.10 a) a)

0.70-0.90 0.70-0.90 0.70-0.90

0.80 0.80 0.80

-

(1.0 [15])

K

465

~ 1011

a)

1.00

1.00

Li Na K

500 500 520-525

- 106 - 106 - 106

a) a) ~)

0.95--1.15 0.95-1.15 0.95-1.15

1.05 1.05 1.05

Li Na K

570-575 545-565 580-585

- 101o - 10 l° _ 101o

a) ~) a)

1.10-1.20 1.10-1.20 1.10-1.20

1.15 1.15 1.15

K

600-610

-

~)

1.20-1.30

1.25

-

E'

-

5.8 [18] (1.2 [15])

~) W i t h i n the limits of m e t e r i n g error (0.05 eV).

- -

V.I. Arbusov et al. / Intrinsic and extrinsic color centers

data for energetic depth of centers and the process frequency factor, P0, presented in table 1, were calculated from several TSEE curves for various kinds and conditions of excitation. A temperature decrease of sample resulted in an activation energy decrease, being the most distinctive in the low temperature part of the TSEE curve. Electron bombardment, as compared with X-ray irradiation, gave rise to centers at greater depth and to an increase in frequency factor for TSEE processes. This fact indicates that the energetic depth for most exoemission centers in silicate glasses is dispersed about an average ( E } value. The abnormally low value of frequency factor was found for the center, which decays between 500 and 525 K. The spectral dependences of PSEE current for sodium silicate glass sample are given in fig. 3. Their characteristic feature is an exponential de-

~9

,,°'j U

,,

&._ =.a"

t0

x' o

00

200

500

A0U g00 T,K -

BOO

Fig. 2. Characteristic TSEE curves for glasses with different type of alkaline modificator (a) and for potassium silicate glasses under different conditions of excitation and preparation of surface (b) after electron bombardment: 1, 2, 3 Li, Na, K silicate glasses, respectively, (irradiation temperature = 180 K); 4 - irradiation temperature = 300 K; 5 - irradiation temperature = 390 K; 6 - glass without regenerative treatment of surface (irradiation temperature = 180 K).

211

\

t0~_

\\\

-, \

"~x.~ q

~0 ° I

I

~0

[

I

I

I

I

~0.~ +~,eV

[

x

~ \

[

["

I

,4,0

Fig. 3. PSEE spectra for the initial (1, 3) and X-rayed (2) sodium silicate glasses at 300 K (1, 2) and 220 K (3).

pendence in the 5.4-6.0 eV interval and deviations from an exponential dependence in the low energy part of spectrum. A temperature decreases from 300 K to 220 K does not change the form of ~'(hw) function, but decreases considerably the absolute PSEE yield in the whole spectral range, and increases the deviations from the exponential dependence in the low energy part of spectrum. X-ray irradiation of glasses also results in a decrease of the value of ~-(h0~) in the range 4.6-5.3 eV, widening of the exponential region, and a sharp drop of PSEE intensity at ho~ < 4.6 eV. The unirradiated alkali silicate glasses samples, heated while illuminated with monochromatic light with energy between 3.5 and 6.2 eV, resulted in PTSE at temperatures corresponding to individual TSEE maxima of these glasses (fig. 4). PTSE maxima between 270 and 318 K and 485 and 530 K, which are also present in TSEE spectra (figs. 1 and 2), indicate that the emission centers are formed in glass surface layers during a photostimulated process. These centers are similar to those formed under X-ray irradiation and electron bombardment. A rough estimation of PTSE centers energies by emission peak temperature gives values which agree with the TSEE data (see table 1). PTSE current kinetics become bimolecular. It should be noted that the splitting of PTSE maximum was observed in the range 270-318 K.

212

V . L A r b u s o v et a L /

Intrinsic a n d extrinsic color c e n t e r s

20~ %

t ~Lu o..

~0

~00

200

400

I 600

500

The component a t T m a x = 273 K does not change its position under different light quantum energies whereas the positions of component between 293 and 318 K and PTSE m a x i m u m at 500 K depend on the energy of light quantum (fig. 5(a)). Figure 5(b) shows spectral, ~-(h¢0, Tmax), dependences for PTSE maxima intensity, when the number of quanta falling on the sample is constant in the whole spectral range. The similarity in spectra for PSEE (fig. 3) and PTSE, is noteworthy. Moreover E 0 calculated from the exponential part of spectral dependence for the 273 K and 500 K PTSE maxima from z( ho~, Tmax) - exp( hco/E ) coincides with the exponential part of r(h~0) PSEE and equals 0.22 eV for sodium silicate glass sampies. In the range 293-318 K, the parameter, E 0, is greater for the high temperature component of PTSE m a x i m u m ( E 0 = 0.23-0.24 eV).

Fig. 4. Characteristic PTSE curves for sodium silicate glass at various light quanta energies: 1, 3.6 eV; 2, 3.8 eV; 3, 4.5 eV; 4, 5.0 eV; 5, 6.0 eV.

4.

I

"T :I

I

I

i

1

I

I "~

I

~

|

I

I

I

I

I

~

o4

/ :

I

I

I

I

I

I

I

I

l

I iOs --

N,.

7

"-".q\.

XX.N

\-".b\. \N

~£ t

I

I

I

I

I

60

I

I

I

~0 _,,

I

J

I

b-

P

4.0 "kw, ~'Y

Fig. 5. Temperature position of PTSE at 290-318 K (1, 2) and 460-530 K (3, 4) for lithium silicate (1, 3) and sodium silicate (2, 4) glasses (a) and the intensity of PTSE maxima at 290-318 K (5), 273 K (6), 460-530 K (7) for sodium silicate glass (b) as a function of light quanta energy.

Discussion

4.1. The nature and properties of exoemission centers The TSEE maxima for our alkali silicate samples at 125-145 K, 290-330 K, and 400-415 K correspond to the decay temperatures of the known bulk CCs of E;-, E 2 and E 4 type [1-3,15,17,20] in high purity sodium silicate glasses according to data on thermal annealing of induced optical absorption bands and TSL. The observed decrease of TSEE maxima intensity from the leached surface of glasses (without ' b a c k ' ion exchange treatment) is indicative of electronic CCs of Et~2 type near the surface being connected with alkaline ions which have trapped electrons and are in different structural positions, as was supposed for bulk Ei -centers [1,21]. The frequency factor value, P0, for these TSEE centers is near the intrinsic phonon frequency of glass network but is less than p0-values for TSL-centers [6,17]. This fact may indicate a sufficiently strong interaction of the subsurface CCs with matrix and specific character of electronic processes near the surface. Taking into account the presence of band state 'tails', a characteristic feature for the band structure of

17.I. Arbusov et al. / Intrinsic and extrinsic color centers

glassy solids [22,23] and the conductance band edge in silicate glasses formed by alkaline ion states [24], it may be supposed that localized matrix states are the precursors of emission-active E,--type CCs. In this case the ET-type CCs will be formed as a result of electron autolocalization on states of alkaline atoms (polaron state) [1,25]. The nature of the center, responsible for the TSEE maximum between 360 and 380 K and TSL in the same temperature range [1,15,17], is not definite. The energetic depth of this center is close to that of E£-center. Investigation of isochronous annealing of induced absorption [16] and ESR signal [17] shows that the intrinsic electron center with 3.7 eV maximum of optical absorption and g = 1.99 decays in this temperature range. According to ref. [15], the electronic CC with 5.8 eV optical absorption decays at about 550 K [18]. This CC is close to the known E'-center in quartz glass [26,27] and has similar optical and paramagnetic properties. The known data on the microheterogeneous structure of alkali glass [28] confirm the validity of these assumptions, as the E'-center is a typical defect of silicon oxigen glass network in the glasses low in alkaline ions. Hence TSEE maximum between 545 and 585 K may be associated with the decay of E'-centers in such microheterogeneous quartz-like regions of the glass surface layer. The TSEE maximum with the greatest width in the range of 500 K is characterized by an unusually low frequency factor. We suggest that an important role for ion processes in this temperature range is indicated. The aforesaid is verified by the fact that the ion conductance of glassy sodium trisilicate attains a value sufficient for ions to participate in recombination processes at 450 K [17]. The annealing activation energy (--- 1 eV) of centers is comparable with that for thermal dissociation or ionization of small clusters of alkaline metals. The observed discrepancy between the electronic and hole trap centers in irradiated silicate glasses according to ESR-data [1,26] points to the existence of alkaline ion clusters which trap electrons and which lack paramagnetic properties. So, it may be supposed that TSEE at 500 K is associated with ionization or decay of alkali metal clusters which are the diamagnetic electron traps.

213

The role of extrinsic Fe CCs in high temperature TSL is described in ref. [15]. The TSEE current may be associated with the decay of [Fe3÷]--type CCs, formed due to electron trapping by Fe 3+ ion. However, it is shown in refs. [17,20] that these CCs are stable at 500 K, and the maximum rate of their decay is observed between 600 and 610 K. Moreover, such decay in terbium coactivated glass occurs due to thermostimulated tunneling electron transfer between the main states of [Fe 3÷] and Tb 3+ centers [20]. It may be supposed that, at this temperature, the common thermoionization decay of intrinsic [Fe 3÷] centers with electron yield into conductance band occurs in non-activated glasses, and the high temperature TSEE maximum at 610 K is associated with annealing of extrinsic CCs. The results obtained lead to a conclusion that the bulk intrinsic electronic CCs, formed due to charge exchange and subsequent structural relaxation of matrix electronic states and point defects of structure, take part in surface TSEE processes for alkali glasses. The energetic depth of emission active centers depends slightly on the kind of alkali ion and its value is less compared with that of similar bulk CCs (table 1). A marked dispersion of emission parameters of Ei--centers is a property of disordered systems [13,14,17,22,23]. A detailed investigation of such distributions is possible by the methods of modulation thermoactivated spectroscopy [13,17]. As is noted in refs. [29,30], the localized valency states, taking part in formation of PSEE spectrum exponential region (fig. 3) on one hand, and the ground states of extrinsic Fe 2÷ ions, photoionization of which results in additional $(h~0) growth, on the other hand, may be the electron donors. The possibility of Fe 2÷ ionization by light quanta in a wide energy range is shown for sodium silicate glass in refs. [2,31]. The results of PTSE investigations point to the formation of CCs ( E 2 in particular) as a result of interaction of UV irradiation with the glass and participation of these CCs in thermally stimulated exoemission process. The splitting of PTSE maximum between 273 and 318 K, associated with decay of E2-centers (fig. 4) requires the identification of both components. Some assumptions on their nature

214


may be expressed on the basis of comparison of PSEE spectra (fig. 3) and T(h0~, Tmax) relationships of emission current (fig. 5(b)). The exponential slope for the low temperature T(ht~, 273 K) component for PTSE coincides with that of the ~'(h~) relationship for PSEE but is less than the exponential slope of the high temperature T(ht~, 290-318 K) component for PTSE. In ref. [32], the degree of slope for PTSE exponential edge in optical absorption spectra as a function of structural disorder of the glass matrix is shown. The increase of slope for the high temperature PTSE component may be associated, by analogy, with increased disorder in the region of the center. It may be supposed that the high temperature PTSE component is connected with decay of E 2centers being near the glass surface, whereas the low temperature component corresponds to the similar centers at greater depth.

4.2. Mechanisms of formation and decay of electronic color centers in glass surface layers The formation of electronic CCs as a result of irradiation of dielectrics takes place due to occupation of electronic traps of different kind followed by structural relaxation. In glasses, such processes are efficient under the action of fast electrons and X-ray. Heating at a linear rate of irradiated glasses results in step-by-step annealing of the CCs and appearance of TSEE maxima. Decay of electronic CCs, according to TSEE data (figs. 1 and 2, table 1), is characterized by first order kinetics, which indicates a thermoionization mechanism of their decay. Unlike X-ray excitation and electron bombardment, UV-irradiation of the glass may result in CC formation both above-the-barrier and by tunneling. As is known [2,31,33] the photoionization of extrinsic Fe 2÷ ions, being donors for exoelectrons, proceeds in sodium silicate glass at ho~ > 5 eV according to the above-the-barrier mechanism, with all possible electronic CCs being formed, and by tunneling at low energies as a result of electron phototransfer from the excited level (Fe 2÷)* only onto the levels of precursors of intrinsic electronic CCs. The results of PTSE investigation are indicative of effective generation of

E 2-centers in surface layers of the glass under UV irradiation over the entire spectral range. Bimolecular kinetics of PTSE are explained by the fact that photostimulated reoccupation of E~--centers levels, being already thermoionized, takes place under conditions of UV irradiation. The formation of E~--type shallow CCs by trapping electrons on localized states of conductance band at the intermediate stage of emission process explains the dependence of PSEE on temperature, observed experimentally (fig. 3). As the E2--centers in the silicate glass are thermally unstable at 300 K (figs. 1 and 2, table 1), their decay gives rise to PSEE current from the glass surface at the temperature given (fig. 3, curve 1). In fact, the decrease of glass temperature to 220 K, at which annealing of E 2-centers does not yet occur, results in decrease of PSEE intensity over the entire spectral range (fig. 3, curve 3). In this case, the PSEE current is connected mainly with the decay of E~--centers at the temperature above 50 K [6,14]. The agreement revealed between the TSEE current at T = 300 K (fig. 3, curve 1) and intensity of PTSE maximum is the result of decay of E 2centers (fig. 5(b)) over the entire spectral range; also, the existence of a temperature-PSEE relationship points to the fact that direct photoionization of extrinsic and matrix localized states of alkali silicate glass followed by electron emission into vacuum is hardly probable. The PSEE and PTSE processes consist of two stages: the photostimulated formation of unstable CCs and their thermal ionization. A large probability of trapping the electron by the precursors of these CCs is connected with a high concentration of localized electronic states near the bottom of the conduction band. The order of formation and decay for E 7-type subsurface CCs in photo- and thermally stimulated electronic process with Fe 2+ ions as the donor of exoelectrons may be presented as FEZ++ E ° h~)(Fe2+)* + E ° above-the-barrier

tunneling

r~

>I r e

2+1 +

J + E/-

kr)e- T (PSEE, PTSE) + E ° + [Fe 2+ ] +,

(1)

V.I. A rbusov et a L / Intrinsic and extrinsic color centers

where E ° are localized states of the conduction band 'tail' (precursors of E,--centers). We suggest that the similarity of PSEE spectra (fig. 3) and spectral dependences of PTSE maxima (fig. 5(b)) is evidence that the extrinsic Fe z+ ions as well as the localized states of the glass near the top of the valency band may be the donors of electrons in the formation of unstable CCs. Here the following reaction takes place: E, o + H, o ho~, above-the-barriertunneling)E-i + Hi+ kT

>e q' (PSEE,

PTSE)+

E,° + H,+ ,

(2)

where H/° is the localized valence state (precursors of H,+-centers). As follows from eqs. (1) and (2), the observed deviations of PSEE (fig. 3) and PTSE (fig. 5(b)) spectra from an exponential in the low energetic region (hw < 5 eV) are caused by high efficiency of tunneling formation processes of intrinsic electronic CCs, being emission-active at the given temperatures. At hw > 5 eV, some of the electrons in the conduction band may be trapped by the extrinsic Fe 3+ ions, forming deep [Fe3+]--centers [20,34] that decrease the concentration of emission-active E, -type CCs. So, the observed decrease of PSEE intensity after X-raying the glass (fig. 3, curve 2) is explained by decrease of exoelectron donors due to radiational recharge of Fe 2+ ions and ionization of valency matrix states. An appreciable decrease of "r(hw, T) at hw < 4.6 eV, caused by formation of hole exoemission-inactive H2+.4-type CCs, with the optical absorption bands in this spectral range, also points to matrix ionization [1,2]. Proceeding from the formation and decay features of exoemission active CCs, a generalized scheme of photo and thermally stimulated electronic processes in subsurface layers of alkaline silicate glasses is proposed (fig. 6). As mentioned above, the extrinsic Fe 2+ ions and localized matrix states near the top of the valency band are the electron suppliers for formation of intermediate exoemission-active centers. The ground state of Fe z+ ions is approximately 5 eV lower than the boundary of electron mobility [2,31,33]. By virtue of glass structure disorder, the energy of Fe 2+

9+1" e J ~

t

?

215 Vacuum

t, bevee'

5

/

d .5._) ×.

Relax.

Ev

Fig. 6. The scheme of photo- and thermally stimulated emission processes in surface layers of silicate glasses: 1-3, optical transitions of electron from the ground onto the excited states of Fe 2+ ions in the mobility gap (1) and conductance band (2), and from the localized matrix states near the top of valency band onto the localized states near the bottom of conductance band (3) as well; 4-6, above-the-barrier (4, 5) and tunnel (6) electron transition from (Fe 2÷ )* states onto the localized states of matrix (4, 6) and (Fe 3+ )* levels (5) as well; 7-9, structural relaxation of recharged electronic states for E , (7), [Fe 3+] (8), H, + (9).

ground state in the ensemble of extrinsic centers is characterized by a dispersion. Fe z+ ions appear in the excited states, lying in the conduction band (transition 2) or in the forbidden band (transition 1) depending on the energy of absorbed light quantum. In so doing, the above-the-barrier (transition 4, 5) or tunnel (transition 6) electron transfer onto the matrix localized states, respectively, as well as the transfer on the excited levels of extrinsic ions (Fe 3+) (transition 5) occurs. As a result of structural relaxation (transition 7, 8), the E , and [Fe2+] - type exoemission-active centers are formed. The centers of Ei--type may appear also due to the direct ionization of valence band 'tails' (transitions 3 and 7) with simultaneous formation of hole //,.+-centers (transition 9). The electronic centers are destroyed under photo- and thermal stimulation, supplying electrons into the conduction band followed by emission into vacuum. The production of ET-centers during irradiation may be the result of localization of

216


electrons on the alkaline ions being in differing sites. Besides, the ion processes may also play an important role in the origin of the family of E,-centers due to radiation-induced diffusion of alkaline ions by X-rays, electron beams or other radiation. Such processes may take place both at the stage of excitation of glass matrix and in the process of relaxation of recharged electronic states. The role of ion processes in the exoemission requires additional investigation. The spectral dependence of PTSE maxima in the range of 500 K (fig. 5(a), curves 3, 4) clearly demonstrates the existence a distribution of energy parameters for emission active CCs in the glass, presented in fig. 6. A gradual increase of energy to 5 eV results in excitation of Fe 2+ states, being near the boundary of mobility, and further tunneling occupation of more and more shallow energy levels of CCs precursors. This process is displayed in the shift of PTSE maximum, associated with the decay of these relaxed CCs, in the low temperature range. Above-the-barrier photoionization of Fe 2+ is possible at energies > 5 eV. As a result free electrons are trapped mainly on the deepest levels of CCs precursors that leads to a sharp change of Tma× ( h ~ ) curve. Such a feature is indicative of substantial overlapping of excited (Fe2+) * states and the CC precursors states. By contrast, the position of PTSE maxima, connected with E 2-center decay, is either constant or changes slightly mainly in the range > 5 eV (fig. 5(a), curves 1, 2) due to the properties of E l - c e n t e r precursors and their relaxation mechanism. So, the exoemission properties of silicate glasses are explained consistently in terms of mechanism of PSEE and PTSE photostimulated CCs formation followed by their ionization.

5. Conclusion

On the basis of this investigation we conclude the following. (1) Exoemission activity of alkaline silicate glass surface under photo- and thermal stimulation results from thermal ionization of ensembles of bulk electronic CCs formed due to the charge change both of localized states of the conduction

band and point defects in glass network ( E 7centers, E'-centers, alkaline ions clusters and CCs with 3.7 eV absorption). (2) Changing the alkaline cation from Li to Na to K slightly influences the parameters of local TSEE centers, but increases the emission current intensity owing to reduction of surface electron affinity and a narrowing of the mobility gap. (3) Photostimulated exoemission in glasses is a two-stage process. At the first stage the intermediate unstable CCs of El- and E 2 type are formed and at the second one thermoionization of these centers takes place followed by emission of electrons into vacuum. Depending on the light quanta energies, the intermediate unstable CCs are formed either by above-the-barrier or by tunneling phototransport of electrons from Fe 2÷ ions and the intrinsic states of the valence band edge onto the vacant levels of conduction band 'tail'. (4) The tunneling photostimulated formation of CCs in the glass for a wide range of light quanta energy is connected with the extended energetic distributions of exoemission pre-centers and 'tails' of energy bands for electronic states.

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