Absorption tail and fundamental absorption edge in vanadium phosphate glassesGordon Wood Anderson Citation: Journal of Applied Physics 44, 406 (1973); doi: 10.1063/1.1661895 View online: http://dx.doi.org/10.1063/1.1661895 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/44/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Structural investigation of phosphate - bismuth glasses with vanadium AIP Conf. Proc. 1565, 122 (2013); 10.1063/1.4833710 Absorption spectra of plutonium in phosphate and borosilicate glasses AIP Conf. Proc. 532, 161 (2000); 10.1063/1.1292242 Ultraviolet optical absorptions of semiconducting copper phosphate glasses J. Appl. Phys. 73, 7760 (1993); 10.1063/1.353975 Raman spectra of vanadium phosphate glasses J. Appl. Phys. 50, 2765 (1979); 10.1063/1.326239 Polaronic Hopping Conduction in Vanadium Phosphate Glasses J. Appl. Phys. 42, 2857 (1971); 10.1063/1.1660640

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Absorption tail and fundamental absorption edge in vanadium phosphate glasses

Gordon Wood Anderson Naval Research Laboratory, Washington, O. C. 20390 (Received 21 March 1972; in final form 4 August 1972)

Optical measurements of the fundamental absorption edge and broad absorption tail of 50 mole% V,O, vanadium phosphate glass films were made in the range 3850--30 ()()() cm-' (2.6--0.33 IL) before and after reducing and oxidizing anneals. The absorption in the tail region of the fundamental absorption increased during reducing anneals and decreased during oxidizing anneals. A band at 19400 cm-' (2.41 eV) was introduced by the reducing anneals. The band gap Eg was determined to be 2.71, 2.92, and 2.82 eV after successive anneals in Qxygen, hydrogen, and oxygen, respectively. Evidence for direct forbidden transitions was observed from fundamental absorption edge measurements similar to that for crystalline V,O,. The concentration of reduced V4+ ions is concluded to be largely responsible for the variation of Eg and of the absorption tail.

INTRODUCTION

Vanadium phosphate glasses based on the system VP5-PP5 have received considerable attention recent­ly.I-18 They are extrinsic, n-type semiconductors, and many authors believe electronic conduction occurs by polaron transfer between transition metal ions. 1,2,5_7,10_13,15_17 Quantitative structural studies have not been done, though it is known by means of electron microscopy that thin, blown films contain no crystals larger than 20-40 A. 3,8,9 It has been suggested that phase separation occurs in all bulk samples, 9 and preliminary investigations are in concurrence with this speculation. 18

Optical properties have received little attention with most reports emphasizing the infrared. 2,3,14,15 One study on blown films was reported covering the far infrared through the near ultraviolet (500.0-0.35 Jl) presenting in detail the vibrational absorption bands and the fundamental absorption edge. 3 The fundamental absorption edge of the glasses and crystalline VP5 near 2.5 eV 3,19,20 fits the criterion for direct forbidden transitions,

K-(hv -Eg )3 /2/hv,

much better than that for direct allowed transitions,

K- (hv - Eg)1/2,

(1)

(2)

where K is the absorption coefficient and Eg is the band gap. 21 A broad absorption tail extends from the absorp­tion edge to the lattice vibrational absorption bands at about 7 Jl. in the glasses but not in crystalline VP5 and brings about their black color. The fundamental absorp­tion edge shifts to higher energies with increasing PP5 content and occurs at slightly higher energies than that of crystalline VP5'

It has been suggested3 that the broad absorption tail and the optical band gap energy, E" may be related to the high concentration of V4+ ions1,2,4,5,18 in this glass system. Thus electronic excitations of the d-d type may contribute to this absorption tail. In addition, a decrease in concentration of nonbridging oxygen ions with increas­ing P 20 S content, resulting from the increase in V4+ ion concentration with increasing P20 5 content, 2,4,5,18 may occur. Because charged nonbridging oxygen ions as­sociated with the VP5-like network would introduce 2p-like energy levels higher than those of bridging oxygen ions, an increase in Eg with increasing V4+ ion content is reasonable. The purpose of this paper is to place these speculations on a quantitative basis by

406 J. Appl. Phys .• Vol. 44, No.1, January 1973

investigating the changes in these absorptions in given samples after reducing and oxidizing annealing treatments.

EXPERIMENTAL

The samples studied were films about 2 Jl. thick blown from melts containing nominally 50 mole% VP5 upon batching. This composition was chosen because of the rapid change in V4+ ion concentration with initial VP5 concentration at this composition. 2,4 Fifty-gram batches were prepared by mixing reagent grades of V20 5 and phosphoric acid with distilled water added to make a slurry in high purity Thermal Syndicate Ltd. fused quartz crucibles. The mixtures were held 15 h at 115°C to drive off the water. The batches were melted in air at 920°C for over 3 h in the crucibles. Films were blown from a small amount of semimolten material gathered on the end of a refractory tube.

The optical measurements were made in the range 3850-30000 cm-1 (2.6-0.33 Jl.) using a Cary 14 spec­trometer. Annealing of the films was done at about 200°C and 2-atm pressure in sealed glass tubes con­taining hydrogen or oxygen as the reducing or oxidizing atmosphere, respectively. The low annealing tempera­ture was selected because of the low transition tempera­tures of these glasses. Annealing times were typically 1 to 2 weeks.

RESULTS

The optical absorption spectra of a 50 mole% V 20 5 film are shown in Fig. 1 after successive anneals in oxygen, hydrogen, and oxygen and are labeled O2(1), H 2 , and 02(2), respectively. Through most of the visible range the absorption increased and decreased, respectively, after the successive anneals in hydrogen and oxygen. These data are shown in a limited range in Fig. 2 clearly demonstrating a band at about 19400 cm-1

which, respectively, increased and decreased upon reduction and oxidization of the glass. In the infrared these three spectra were indistinguishable within experimental error, and the oscillations in the infrared are interference fringes (Fig. 1). Probably due to ther­mal relaxation there was a small increase in the absorp­tion tail in the visible following the first oxygen anneal with respect to the results before annealing.

In the region of the fundamental absorption edge, the changes in absorption with reducing and oxidizing anneals were reversed, the absorption at a given energy greater than about 21500 cm-1 (2.67 eV) decreasing and

Copyright © 1973 American Institute of Physics 406

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407 Gordon Wood Anderson: Absorption tail in vanadium phosphate glasses 407

>­f-

5.0

4.0

~ 3.0 w o .J « u

~ 2.0 o

1.0

WAVELENGTH (m icrons) 2.50 1.50 1.00 0.80 0.60 0.50

---02 (1)

---- H2

--------- O2 (2)

0.40 0.35

, , I

; / : I : I : I : I

" I / I : I

" / " I : /

: I / / " / !I

II /f

i/ /f

/f 1/

,/1 </

0.04000 8000 12,000 16,000 20,000 24,000 28,000 WAVENUMBER (cm-l)

FIG. 1. Room-temperature optical absorption spectra of a 50 mole% V20 S glass film after successive anneals in oxygen, hydrogen, and oxygen labeled 02(1), H2, and 02(2), respectively.

increasing, respectively. Eg thus increased and de­creased, respectively, with these anneals. There was essentially negligible change in the absorption edge position as a result of the first oxygen annealing treatment.

DISCUSSION

The suggestion that the V4+ ions contribute to the broad absorption tail was based on the observed absorption of V4+ ions in other glass systems in the visible and near infrared,22-28 on the occurrence of the fundamental absorption edge at about 0.4 eV in V02 for temperatures less than the metal-semiconductor transition tempera­ture,29 and on the known occurrence of V4+ ions in significant quantity and the lack of observatiort of more reduced vanadium ions in vanadate glasses. 2,4,S,18 The increase and decrease in the absorption tail with re­duction and oxidization, respectively, are consistent with this interpretation, and it is thus suggested that d-d type electronic transitions at the V4+ ion sites are in large part responsible for this broad absorption tail. The occurrence of a band at about 19400 cm-1 in the annealed vanadate glasses (Fig. 2), moreover, is typical of silicate and borate glasses containing reduced va­nadium which exhibit several absorption peaks in the visible and near infrared. 22_28 Other weaker, broadened d-d transitions also may contribute to the absorption tail of the vanadium phosphate glasses. Positional disorder in the system probably broadens these 3d absorption bands causing the broad absorption throughout the tail region. As in other oxide glasses, though, the disorder probably does not significantly alter Eg itself due to the nearly identical local order in the crystalline and glass states4 and to the relatively large separation between the 2p and 3d bands. 3,19,20,29 The 2p band should

J. Appl. Phys., Vol. 44, No.1, January 1973

tail somewhat into the gap due to disorder, but the 3d states are relatively localized and thus less affected by disorder. 1_2, S_7, 10_13, lS_17 It should be noted that the absorption is very high and that the glasses probably absorb in large part by mechanisms involving d-like electrons in this region as in the case of crystalline VP4' It is not conclusive that the absorption tail in the vanadium phosphate glass system is brought about entirely by V4+ ions rather than by more reduced vana­dium ions as well, though this speculation is reasonable because of the highly oxidizing thermal history of these glasses and the lack of observation of more reduced vanadium ions. 2,4, S, 18

It has been suggested that the oxygen 2p bands are about 2.5 eV below the 3d bands in crystalline VP4' 29 and experimental evidence indicates that about the same energy gap occurs in crystalline VPS. 19,20 Thus the oxygen 2p bands probably are about 2.5 eV below the 3d bands in the glasses as well and should not contribute to the absorption tail. Moreover, because the local order at the vanadium ion sites in these glasses is nearly the same as that in crystalline VPs, 4 the nOn­bridging oxygen ions, as in crystalline VPS' 19,20 probably do not contribute significantly to optical ab­sorption in the tail region. A crude picture of the electronic energy levels is thus suggested for the glasses as shown in Fig. 3 in which the upper level of the valence band increases with increasing nonbridging oxygen ion concentration.

Analysis of the fundamental absorption yields values for Eg and allows comparison of the data with the criteria for the types of electronic transitions [Eqs.

WAVELENGTH (microns) 0.20.65 0.60 0.55 0.50 0.45 0.40

0.0i---"'=-----------=:~-----__l

w ~ -0.2 « r u >-t: -0.4 (f) z w o oJ (} -0.6

f= c... o

-0.8

-1.0

15,000

\ \ \ \ \

\\ \

-- H2 - O2 (1)

------- O2 (2) -02 (1)

17,500 20,000 22,500 WAVENUMBER (cm-I )

\ \ \

\

\ \ \ \ \ \ \ \

25,000

FIG. 2. Changes in the room-temperature optical absorption spectra of a 50 mole% V20 5 glass film after successive anneals in hydrogen and oxygen with respect to initial oxygen anneal labeled ~ - ~ (1) and 02(2) - ~ (1), respectively.

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408 Gordon Wood Anderson: Absorption tail in vanadium phosphate glasses 408

FERMI LEVEL~~~~~-L-L~~~~~~~~

VANADIUM 3d BANDS

OXYGEN 2p BANDS

FIG. 3. Possible energy-band representation for vanadate glasses. Nonbridging oxygen ions are suggested to contribute to the electronic levels in the upper part of the valence band as roughly indicated.

(1) and (2)]. To correct for the effect of the absorption tail and reflection losses, a straight line was drawn through the mean points of the optical density absorption maxima and minima in the tail, extended into the region of the fundamental absorption edge, and subtracted from the measured optical density to obtain a corrected optical density fo r the unannealed films (70. 0 and 87. 5 mole% VP5)' For the 50 mole% VP5 annealed films

"' '-C\J

rOOI~

!::: 20001 (/)

z w o ..J « 1500 U i= 0.. 0 0 w f- 1000 u w 0:: 0:: 0 u

0:: W CD :::E ::J Z w > ~ ~

875 700

15 ..J «

Hz u i= . . 0.. 0 . 0 . 10 w . f-• U W 0::

\5

0:: 0 U

L-J

_~o 30,000

FIG. 4. Analysis of fundamental absorption edge of vanadium phosphate glass films at room temperature. Straight lines drawn through points fitting criterion for direct forbidden transitions. 0: unannealed 87.5 mole% V20 5 film; ~: unannealed 70.0 mole% V20 5 film; _, ., .: 50 mole% V20 5 film after suc­cessive anneals in oxygen 02(1), hydrogen H2, and oxygen 02 (2), respectively. Rightmost curve attempts to fit data after hydrogen anneal, H2 • to criterion for direct allowed transitions on right-hand vertical axis.

J. Appl. Phys., Vol. 44, No.1, January 1973

this straight line was drawn tangent to the 19400 cm-1

band with a slope slightly less than that of the tail at lower energies. This process which assumes little change in reflectivity at the absorption edge is reason­able because the measured reflectivity of bulk samples changed only slightly in this region. The correction made no qualitative difference in the results of the analysis but did permit quantitative determination of Eg with more confidence.

Figure 4 shows the results of the data analysis. The straight lines drawn through the five sets of data indi­cate the good fit to the criterion for direct forbidden transitions [Eq. (1) J. In each case the edge fits this criterion much better than that for direct allowed transitions [Eq. (2)], as does the edge of crystalline VP5. 19

,20 This comparison is shown for the worst case, the 50 mole% VP5 sample after the hydrogen anneal. The points on the far right in Fig. 4 illustrate an attempt to fit the data for this case to the criterion for direct allowed transitions [Eq. (2) J on the right-hand vertical axis; in this case the material is least like crystalline V 2°5' The slopes of the lines vary only slightly with composition or annealing. Thus, the fundamental ab­sorption edge of these glasses in general is the same as that of crystalline V P5' It may be similar to charge transfer absorption which occurs at about 2.5 eVor higher energies in oxide glasses containing vanadium impurities 23 ,26 and is most likely due to excitation of oxygen 2p-like electrons into the vanadium 3d bands (Fig. 3).

The intersections of the straight lines in Fig. 4 with the horizontal axis determine Eg • These values are 2.38 and 2.47 eV for unannealed samples of slightly over 1 f1 thickness and of 87.5 and 70.0 mole% V20 5 bat ching composition, respectively, and 2.71,2.92, and 2.82 eV for the 50 mole% VP5 film after annealing in oxygen, hydrogen, and oxygen again, respectively.

The pattern of increaSing Eg with increasing PP5 content,3 and thus with increasing V4+ content, 2,4,5,18 for samples of different composition continued as given samples of originally 50 mole% V20 5 content were annealed in reducing and oxidizing atmospheres (Fig. 4). Moreover, the variation of Eg with composition or V4+

content qualitatively should occur in this manner as discussed above. That is, the nonbridging oxygen ion content, which should increase with decreasing V4+ ion content, should introduce higher 2p-like energy levels than those of bridging oxygen ions thus bringing about a decrease in Eg with decreaSing V4+ ion content. The observed variation of Eg with annealing treatment for the 50 mole% VP5 sample is thus consistent with the argument that the absorption tail is enhanced by the presence of reduced vanadium. In addition, because it is often more difficult to oxidize than reduce oxide glasses with an oxidizing thermal history, it is not surprising that the second oxygen anneal does not com­pletely reverse the hydrogen anneal.

CONCLUSIONS

It has been shown that for vanadium phosphate glass films of 50 mole% VP5 bat ching composition both the

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409 Gordon Wood Anderson: Absorption tail in vanadium phosphate glasses 409

absorption tail in the 15000-21500 cm-1 range and the optical band gap energy, E g' reversibly increase and decrease upon reducing and oxidizing anneals, respec­tively, and that an absorption band is induced at about 19400 cm-1 upon reducing anneals. From these results it was suggested that nonbrldging oxygen ions, dependent on V4+ ion content, introduce higher energy levels of the oxygen 2p-like electrons than the levels of the bridging oxygen ions thus decreasing E g • Evidence for a direct forbidden transition mechanism as in crystalline VP5 was presented. It was further suggested that the broad absorption tail in the visible and near infrared in large part is due to d-d type electronic transitions at V4+ ions.

ACKNOWLEDGMENTS

The author wishes to thank D. F. Barbe, J. E. Davey, R.J. Ginther, M.N. Kabler, M.H. Reilly, M. Rubinstein, and G. H. Sigel for helpful discussions.

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