Semiconducting glass-ceramics

JOURNAL o r NON-CRYSTALLINE SOLIDS 2 (1970) 250--277 © North-Holland Publishing Co., A m s t e r d a m

SEMICONDUCTING GLASS-CERAMICS L. L. HENCH Department of Metallurgical and Materials Engineering, University of Florida, Gainesville, Florida, U.S.A. Studies of the microstructure and thermal treatment of V2Os-P205 based glasses have shown an initial ordering of the glass structure followed by detectable crystallization. DC conductivity increases markedly with microstructural changes. Unusual electrical behavior is associated with the partially crystallized glasses including ac absorption, elimination of high frequency conductivity dispersions and inversion of frequency effects at elevated temperatures. The effect of forming variables such as quenching temperatures and rates on electrical properties are documented. Neutron irradiation has also been shown to dramatically change certain characteristics of these materials, whereas other features are irradiation insensitive to flux levels of 1 × 1017nvt. A summary of modified band theory calculations that are consistent with the above results is presented. 1. Introduction The general phenomena of electronic conduction in oxide glasses containing transition metal ions and chalcagonide glasses have been well reviewed by Mackenzie 1) and Pearson 2) among others. Theoretical analysis of the transport mechanism in these glasses, however, is still relatively obscure. Consequently, one of the objectives of the research reported in the present paper is to obtain additional experimental information which may be used as a basis for establishing a more complete theoretical understanding of the electron transport phenomena in semiconducting amorphous solids. A more detailed statement of the theoretical treatment of these results is presented in another paper3). The title "Semiconducting Glass-Ceramics" was used for this communication to emphasize the point that the properties of semiconducting glasses are dependent upon previous thermal history and can be specially modified by using controlled thermal treatments. Glass-ceramics is a term used to describe partially crystalline ceramic materials which have been obtained by thermally treating preformed glassy objects4). The thermal treatment usually involves two steps. The first step is a partial ordering of the glassy structure resulting in the formation of small nuclei of crystalline order. The second step involves the growth of the nuclei into well developed crystalline islands in the glassy matrix. The size of the crystals and the volume fraction 250

SEMICONDUCTING GLASS-CERAMICS

251

of crystal phase is determined by the concentration of nuclei present and the duration of the crystal growth heat treatment. Crystallization of semiconducting glass was shown by Hamblen et al. s) to be an extremely important variable. Certain oxide glass compositions showed an increase in ac electrical conductivity by as much as six to seven orders of magnitude with crystallization, whereas other compositions usually showed a decrease in electrical conductivity with crystallization. These interesting results have prompted the second objective of this investigation, which has been to examine in detail the electrical property changes accompanying thermal treatment of semiconducting glasses and to understand the nature of the structural changes which produce the modification of the electrical behavior. Fabrication of electronic materials, be they thin film, thick film, or bulk materials, often influences the properties obtained. Consequently, in order to characterize the behavior and performance of semiconducting glasses it is important first to evaluate such forming variables as the casting temperature of the glass, the annealing time and temperature of the glass, and the annealing of electrodes. Changes in both ac and dc properties associated with these variables will be discussed. Another question of interest is the relative sensitivity of the electronic behavior of semiconducting glasses to radiation exposure. As reviewed by Kreidl~), the bulk properties of glasses are little affected by fast neutron radiation fluences as large as 1019 nvt or more. Hench and Daughenbaugh 7) have shown that the dc and ac electrical properties of quenched semiconducting oxide glasses are also unaffected by fast neutron dosages of as much as 4 x 1017 nvt and 7-ray dosages in the range of 2 x 108 rad. An additional objective of the present investigation is to establish whether the radiation stability of the electrical properties of a semiconducting glass-ceramic is comparable to that of the quenched glass.

2. Experimental procedures The glass samples investigated in this program were prepared by melting reagent grade chemicals in platinum crucibles in an electrically heated muffle in air and casting into steel molds. The melting-casting temperature for the glass specimens was approximately 860°C unless otherwise noted. Melting times were approximately four to eight hours. Homogeneity of the melts was checked by examining glass fibers pulled from the crucibles. The crucibles were covered during melting and no appreciable weight losses from the initial compositions were noted. An annealing treatment for the glass samples was generally required in

252

L.L.HENCH

order to avoid sample breakage during grinding and polishing procedures. The annealing temperatures were established by studying the effect of annealing time and temperature on electrical properties. Unless noted, the annealing was done at temperatures in the range of 200 to 225 °C for periods of two to three hours. No noticeable changes in electrical properties were observed as a result of the annealing treatments. After casting and annealing, the 1.75 cm OD by 0.5 cm thick samples were ground and polished to parallel surfaces and gold electrodes were vacuum evaporated in a double guard ring configuration. The electrodes were annealed for one hour at 200°C except as noted. Samples were placed in specially designed dielectric sample holders 8) with coaxially guarded leads and evacuated to a 1 ~tm vacuum. AC measurements of conductance and capacitance employed standard bridge techniques. Samples were thermally heat treated in electric muffle furnaces with temperature control of _ 0 . 5 ° C or better. Electrodes were removed from the sample surfaces before each heat treatment in order to prevent long range diffusion of gold into the specimens. Neutron irradiation exposures were performed in the Wright-Patterson Air Force Base test reactor. The WPAFB reactor has a fast neutron flux capability of 1.5 x 1013 n/cm 2 (~b > 0.1 meV). The fast neutron flux was measured using a 58 nickel [n, p] 59 cobalt reaction with a 2.9 meV threshold energy. Fluxes reported are based on activations measured 48 hr after the samples were removed from the reactor. Temperature monitoring of the reactor indicated that the sample temperatures were in the range of 50 °C throughout the exposures. Measurement times after irradiation generally involved a lapse of four weeks or so to reduce activity to a tolerable level.

3. Compositional effects Previous results reported by the author 9) showed that a 70 mole% V205-30 mole% P205 glass exhibited a marked divergence between ac and dc conductivity below a critical transition temperature. Extending these measurements to compositions of both lower and higher vanadia contents indicates that the low temperature enhancement of ac conductivity may be a general behavior of amorphous semiconductors. It can be seen in fig. 1 that the ac dispersion for the 60 mole% V z O 5 ~ 0 m o l e % P205 and an 80 mole%VzO5-20 mole% P205 glass is similar. The influence of the larger V205 content is generally one of increasing the de conductivity and decreasing the transition temperature at which the ac enhancement effect is observed. Fig. 1 also shows that there is a definite low temperature departure from linearity in the 1/T plot for the low frequency conductivity when the temperature becomes sufficiently low. It can be seen, for example, in the

SEMICONDUCTING

253

GLASS-CERAMICS

6 0 % VzO5 - 4 0 % P40io

164

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Fig. 1. Temperature dependence of the conductivity of a quenched 80 mole ~ V205-20 mole~o P~O~ glass and a 60 mole~o V205-40 mole~ P205 glass at 10a Hz and 10° Hz.

80/20 glass that the low frequency conductivity temperature dependence tends toward zero when the temperature becomes sufficiently low. The value of the temperature dependence for the low temperature dc conductivity appears to be in the range of 0.04 eV. These observations indicate that at least six significant factors must be included in the description of the total conductivity of a quenched, i.e., non thermally treated, semiconducting glass. These are: 1) the exponential increase of dc conductivity with temperature at high temperatures; 2) the decrease in dc conductivity activation energy with low temperatures; 3) the increase in dc conductivity with increasing V20 5 content; 4) the total conductivity increases as a function of the applied frequency when the frequency exceeds 10 S Hz; 5) the apparent activation energy decreases as a function of frequency below a transition temperature; 6) the ac conductivity transition temperature is a function of the V205 content. It would appear that a reasonable criterion for theoretical analysis of

254

L.L. HENCH

electron transport in quenched semiconducting glasses would be to provide a consistent explanation for each of the above factors. 4. Fabrication variables Any variable that can alter either the order or the homogeneity of a glass can pontentially affect the properties of that glass. Consequently, in order to characterize and understand the behavior of semiconducting glasses it is important to obtain an appreciation of the relative importance of casting temperatures, annealing temperatures, and annealing times on the electrical properties of these glasses. A 33 mole% KPO3-67 mole% V205 glass was chosen as the material for this investigation. This glass was selected in part because of the wide disparity in dc conductivity between the quenched glass and the crystallized glass reported by Hamblen et al.5). A study of the effect of various glass casting temperatures on the dc 10 -~

3 3 Mol % K a P O a - V 2 0

s Glass

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Effect of casting temperatures on the conductivity-frequency dependence of a 33 mole % K2POs-V~O5 semiconducting glass. Measured at 23 °C.

SEMICONDLICTING GLASS-CERAMICS

255

conductivity and the conductivity frequency dependence of the above glass is summarized in fig. 2. These results were obtained by melting and homogenizing the glass at 850 °C. The glass was then equilibrated at a lower furnace temperature from which it was cast into the steel molds. It can be noted that the general effect of decreasing the casting temperature is to increase the dc conductivity of the glass. This effect is presumably the result of an increase in the short range order of the glass as a result of being equilibrated at the lower temperature. A marked structural change, however, can be seen to have occurred between the casting temperature of 700 to 550°C. Other results indicate that the threshold for this change is probably in the range of 600°C. An additional investigation into the subject of heterogeneities in semiconducting glasses 10) has conclusively shown that the glasses cast from 550°C contain crystalline heterogeneities in the size range of 1/~m or less. A volumetric analysis indicated that the volume fraction of the crystals was less than 1~. Consequently, detection of the crystals that produce the large change in dc conductivity shown in fig. 2 could not be detected with the use of the standard X-ray diffractometer. The identification of the presence of these crystals was obtained through the use of an X-ray Guinier-DeWolff diffraction camera 11). The Guinier-DeWolff camera permits very long time exposures of the powder samples, thus enabling very small volume fractions of crystals to be observed. These results thus indicate that there is a gradual increase in order of the glass matrix until a sufficiently low temperature of the melt is reached that the ordered regions form clusters which can effectively serve as nucleation sites for the crystals to grow in the glass during the cooling from the quenched temperature. The results shown in fig. 2 also indicate that the frequency dependence of the conductivity for semiconducting glass is also sensitive to the casting temperature. The highly quenched glass in general tends to show an ~o1 dependence over a long range of frequencies tending toward 092 dependence at frequencies in excess of l 0 6 Hz. As the quenching temperature is lowered, however, a region of frequency independent conductivity appears and seems to gradually increase in importance until the sample quenched from 550°C shows a frequency independent region over nearly four orders of magnitude of frequency. The sample quenched from 550 °C appears to go directly from the frequency independent region into an ~o2 dependent region. These results would seem to indicate that the relative degree of order in the glass matrix influences the electron transport mechanism operating, thereby resulting in a change in the frequency dependence of the electronic processes. Another manifestation of the importance of the casting temperature on the electronic properties of a semiconducting glass is shown in fig. 310).

256

L . L . HENCH

33Mol % K2PO3-VzOs Gloss POURING TEMPERATURES O 850"C IO,O

/

~

B 800"C V 750"C {) 700"C

I.O L~ Z t--

0.1

0.01

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103

i

I

104 105 FREQUENCY (Hz)

i

106

.-.-.I

I0r

Effect of casting temperature on the dielectric loss angle versus frequency spectra of a 33 mole % K2POz-V205 semiconducting glass measured at 23 °C

The tangent of the loss angle delta is shown as a function of the logarithm of the measuring frequency. Tan 6 is plotted as a log scale in order to include the complete range of casting temperatures. The most noticeable effect that one sees from this study is that the glass cast from 550°C exhibits a large pronounced dielectric peak in the range of 105 Hz. This loss peak can be successfully explained in terms of a heterogeneous dielectric loss process associated with the presence of the crystals as heterogeneities in the glass matrix. Since the crystals are of higher conductivity than the glass, there must be an interfacial region separating the two phases. It is within this interfacial region that these proposed charge oscillations in phase with the applied field occur, giving rise to dielectric relaxation losses in the low frequency range. It can also be seen in this figure that there are more subtle variations in the dielectric loss spectrum for all of the sample casting temperatures investigated. The only two consistent trends seem to be a tendency for the samples to develop a small loss peak in the range of 105 to 10 6 Hz as the quenching temperature is lowered. Additional studies in the temperature range between 700°C and 550°C would be useful to ascertain whether the development of the loss process can be followed in detail. There also appears

257

SEMICONDUCTING GLASS-CERAMICS

to be a consistent development of a dielectric absorption in the range of 3 x 106 Hz to 5 × 106 n z for all of the glasses quenched from high temperatures. This absorption peak may in some way be associated with the general ac enhancement process of the highly quenched glasses, although this interpretation has little foundation at the present time. The results shown in fig. 3 seem to be consistent with the interpretation offered above that the matrix of the glass increases in order as the quenching temperature is decreased until eventually critical size nuclei form which result in stable crystals during the cooling of the cast sample. However, interpretation of the dielectric loss results is complicated by the fact that such a se-

1044

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16 5

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o

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Fig 4. Influence of a 15 min annealing at various temperatures on the electrical conduc tivity of 33 mole % KPO3-VzO6 glasses measured at 23°C.

258

L.L.HENCH

quence of structural changes means that both the conductivity of the matrix is changing along with the formation of the second phase particles. Consequently, one might expect that the nature of the interface between the two phases will vary as the quenching temperature is altered. Thoeretical analysis of the variations in such volumetric interfaces is complicated indeed. Specific interpretation of the influence of the interfaces on the total electronic behavior of the materials is even more difficult. Because it is necessary to handle glass specimens, and often essential to cut, grind or polish the material, an important part of the glass fabrication process is to relieve the stresses in the cast objects by an annealing step. The importance of carefully selecting the annealing time and temperature of semiconducting glasses is emphasized in fig. 4. Variation in the low frequency and high frequency conductivity of a 33 mole% KPO3-V205 glass as a function of annealing temperature for a 15 minute annealing is shown. When the annealing temperature exceeds 250°C, a sharp increase in the dc conductivity by four order of magnitude occurs. The ac conductivity measured at 2 x 10 6 Hz increases by two orders of magnitude. Similar to the results discussed above, this effect of annealing temperature can also be attributed to structural changes taking place in the glass. The nature of the structural changes will be discussed in greater detail in the following section on thermal treatment. The point to be emphasized in the present case is that selection of annealing temperatures for semiconducting glasses must include a check as to whether the electrical behavior of the glasses is altered as a result of the annealing treatment. Selection of an annealing treatment based entirely upon strain relief may result in samples with properties that are not characterized as to either quenching conditions or thermal history. Another caution to be exercised in the preparation of semiconducting glass specimens is the selection of an electrode annealing schedule. Vapor deposition of electrodes often does not result in a completely intimate, ohmic contact with the sample. An annealing treatment of the electrodes which serves to relieve stresses in the gold film and at the same time promote chemical bonding between the electrode area and the sample is often required. Again, care must be exercised that the electrode annealing temperature and time do not exceed the conditions required to produce structural changes in the glass. 5. Thermal treatments

The use of thermal treatments ot modify and control electrical properties of insulating glass-ceramics (ionic conductors) has been well reviewed by McMillan 12). The purpose of the present study is to examine the dependence of the electrical properties of a semiconducting glass on the thermal treat-

259

SEMICONDUCTINGGLASS-CERAMICS

ments of the glass. Fig. 5 illustrates the large magnitude of changes that result from relatively minor thermal treatments of a semiconducting glassceramic. The log conductivity versus lIT curve for the highly quenched glass-ceramic exhibits a nearly linear temperature dependence of the dc conductivity with an activation energy of 0.6 eV. A large ac conductivity I©%

,o-:

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0

NCHED

°Dc

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o

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I0~0(*K-) Fig. 5. Temperature dependence of the electrical conductivity of a quenched 33 mole ~o KPO3-V~O5 glass compared with the conductivity after a 30 rain and a 30.8 hr heat

treatment at 288 °C.

enhancement effect is also seen to be present below a transition temperature, similar to results previously reported 9) and as shown in fig. 1 for the binary vanadia-phosphate glass. This result indicates that the addition of ternary components to the semiconducting glass in general does not change the transport behavior. Heat treating the glass for 15 min at 288 °C produces a pronounced effect on both the dc and ac electrical properties. It can be seen in fig. 5 that the

260

L.L. HENCH

dc conductivity at room temperature has increased by as much as four orders of magnitude. The activation energy for the dc conductivity has decreased to 0.2 eV. Further increases in thermal treatment result in still higher values for the dc conductivity, but the activation energy remains relatively constant at 0.2 eV. Fig. 5 shows that the ac conductivity enhancement effect is further increased by heat treatments. However, the relative ratio of the ac to dc effect is decreased. Consequently, the spread in values between the dc and the four MHz values becomes less as the heat treatment becomes greater, but the magnitude of the ac effect is larger. The temperature dependence of the ac conductivity at four megacycles appears to remain constant at approximately 0.04 eV. As will be discussed later in the section on theoretical interpretation, the changes in the activation energies resulting from thermal treatments appear to be clues as to the netura of the transport process occurring in the semiconducting glass-ceramics. Another interesting feature that appears in the conductivity-temperature behavior of the heat treated glass-ceramics is an inversion of the frequency effect above a critical temperature. The inversion results in the total conduc-

213

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Type I] / TC = 120o C ..-/

IJ~

' 41{IOeHz

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i

i

i

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......

. . . .

. . . .

I000 (OK',)

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Fig. 6. Comparison of the transition temperatures of the ac conductivity frequency inversion for two semiconducting glass-ceramics. Both are of 33 m o l e ~ KPO3-V~O5 content but type I has been quenched from 850 °C and heat treated for 4¼ hr at 288 °C. Type II was quenched from 550°C.

SEMICONDUCTING GLASS-CERAMICS

26 !

tivity at four megacycles being less than that measured at dc or low frequencies when the critical temperature is exceeded. The critical temperature at which this effect occurs appears to be independent of the thermal treatment of the semiconducting glass-ceramic. However, a study of this effect resulting from heterogeneities grown into the glass from quenching reported by the author 10) showed that the temperature of inversion is different from the sample reported herein. It is believed that the difference encountered is related to the size of heterogeneity and the nature of the interface between the heterogeneity and the glass matrix. A more detailed investigation of the ac conductivity inversion for a 33 mole~ KPOa-V205 glass-ceramic heated for four and three-quarter hours at 288 °C is shown in fig. 6. This glass is denoted as type I in the figure. Before heat treating the sample was produced by quenching from 850°C. It can be seen from the figure that as a critical temperature is approached the ac conductivity enhancement effect diminshies and the parameter plotted (aac-adc)/tr, c tends toward zero. When the critical temperature of 56°C is exceeded, the ac conductivity becomes less than that of the dc conductivity, and the magnitude of the ac inversion increases as the frequency of the ac field increases. A comparison of the type I semiconducting glass-ceramic which is heterogeneous as a result of a controlled thermal treatment is made with a semiconducting glass-ceramic which has crystals grown in it by quenching from a low temperature, 550°C. The results show that the frequency inversion effect exists for the type II semiconducting glass-ceramic but the critical temperature is increased to a value of 120°C. As discussed in the theoretical section these results seem to indicate that there is some type of inductive effect occurring in the heterogeneous semiconducting glass. Since the inversion effect has not been detected for the highly quenched semiconducting glasses, it is likely that the inductive behavior is related to the interfacial barriers existing in the heterogeneous glasses. As shown previously in fig. 3 the type II glass-ceramic, quenched from 550°C, exhibits a dielectric relaxation loss maxima. It has also been shown 10) that a type I semiconducting glass-ceramic heat treated for 4¼ hr at 288°C, exhibits dielectric relaxation loss peaks. When the measuring temperature is increased, the location of the dielectric loss peak shifts to higher frequencies for both the type I and the type II glass-ceramics. A comparison of the temperature dependence of the loss peak shifts for both type I and type II glass-ceramics is shown in fig. 7. The data are reported as the logarithm of the location of the loss peak maxima as a function of reciprocal temperature in °K-1. The activation energy for the loss process can be calculated from the slope of the curves in fig. 7. The results show that

262

L.L, HENCH

the type I semiconducting glass-ceramic has an activation energy for the loss process of 0.20 eV. The activation energy for the type II semiconducting glass-ceramic is 0.27 eV. In both cases the activation energy is comparable to that calculated for the dc conductivity in these materials. Evidence previously presented 10) demonstrates that the loss process occurs when the samples have had sufficient thermal treatment that the crystals begin to be detectable in X-ray diffraction experiments employing the

x

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to ~ - - ~ - - -

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C o m p a r i s o n of the temperature dependence of the dielectric loss process for a type I and type I I semiconducting glass-ceramic.

Guinier-DeWolff X-ray camera. The volume fraction of the crystals, however, for a heat treatment of 15 min at 288°C is in the range of 0.1~ or less. Heat treating the semiconducting glass to as much as 100 hr at 288 °C still does not produce sufficient volume fraction of crystallization to be observable using a normal X-ray diffractometer. As shown in fig. 5, longer thermal treatments produce large increases in the dc conductivity of the material, and it is found that when the dc conductivity becomes sufficiently large the capacitance of the sample decreases to 0.01 pF, or less, throughout the frequency ranges measured. Consequently, the state of high conductivity in

SEMICONDUCTING GLASS-CERAMICS

263

the samples of exceptionally long thermal treatment destroys the dielectric loss process. Additional information as to the type of microstructural changes resulting from thermal treatments of semiconducting glasses have recently been obtained through the use of X-ray small angle scattering measurements 13). An incident X-ray beam in the range of 10-3 to 1°20 will be scattered when passed through a sample that contains heterogeneities of different electron density from that of the sample matrix. Heterogeneities from l0 A to 1000 A in size will give rise to a small angle X-ray scattering. Such a technique was used to examine the structure of an as-cast type I semiconducting glassceramic which had no subsequent thermal treatment. The sample exhibited a conductivity behavior as shown in fig. 6. A small amount of X-ray small angle scattering was observed in the quenched material. Analysis of the X-ray results indicates that both the Guinier and the Porod radii are equal to 92 A. Analysis of the small angle scattering of an sample heat treated at 288 °C, electrical properties shown in fig. 5, show that the particle size of the scattering regions has increased to a range of 115 to 120 A. The similarity of the Guinier radius and the Porod radius for both the as-cast and the heat treated sample indicates that there is essentially no size distribution of the scattering regions. Analysis of the total integrated intensity of both the quenched and heat treated samples show that the product of the volume fraction and the electron density difference squared increases by a factor of three as a result of the thermal treatments. Since the particle size of the scattering regions was shown to be relatively unaffected by the thermal treatment, it can be concluded that the large increase in the scattering intensity is the result of the nucleation and growth of new scattering regions. It cannot be ascertained from these results whether or not the scattering regions are a glass and glass separation which continues to develop with thermal treatment or whether the scattering centers are the result of nucleation and growth of extremely small crystals. Since it is known from the X-ray Guinier-DeWolff diffraction results that diffraction lines associated with the crystalline phase do develop within a four and three-quarter hour thermal treatment, it is concluded at the time of this writing that the heterogeneities giving rise to the small angle scattering results are highly disordered crystalline regions in the size range of 90 to 120 A. It is suggested that the degree of disorder in the micro heterogeneities is sufficiently great in the as-cast samples that the regions are hardly distinguishable by any experimental techniques from a glass. The major difference lies in the compositional concentration of the V4+ ions in the scattering regions, thereby producing a region of higher conductivity

264

L.L.HENCH

in a matrix of low conductivity. The theoretical section of this paper will attempt to show that such a conclusion provides a consistent explanation of all of the microstructure and electrical behavior results.

6. Thermal treatment of miscellaneous compositions An effort has been made to follow the thermal treatment effects of several other compositions in addition to the 33 mole~o K P O 3 - V 2 0 s composition discussed above. Fig. 8 shows the influence of the thermal treatment at 288 °C on the conductivity of an 80~ V 2 0 5 - 2 0 ~ P205 semiconducting glass. The data show only a small tendency for the conductivity to increase with thermal treatment of as much as 94 hr at 288 °C. Small fluctuations in the conductivity at several time increments appear in the data although it appears reasonable that these are a result of small variations in electrode area associated with putting new electrodes on the samples after every heat treatment. The ac enhancement effect appears to increase somewhat with the thermal treatment, although the change is not large and these results are plotted on linear scales. The important conclusion which can be drawn from this study is that as

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Fig. 8. Variationin low frequencyand high frequencyconductivityof an 80 ~ V~O5-20 P205 glass with duration of thermal treatment at 288 °C. Measurements made at 23 °C.

SEMICONDUCTING

265

GLASS-CERAMICS

long as the thermal exposure of the semiconducting glass is kept below a temperature which permits the formation of micro-heterogeneous crystals, the conductivity of the glass, both dc and ac, is time independent. For the glass shown in fig. 8 the temperature at which this time independence still occurs can be as large as 288 °C. Two additional MO-V2Os-P20 5 ternary glasses of the same composition as the potassia-vanadia-phosphate glass discussed previously were examined. The results obtained are shown in fig. 9. Both glasses exhibit the frequency t6 4

Q

QUENCHED

Irl

15 MIN AT 2880C

v

6 0 M I N A T 288"C

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QUENCHED 15 MIN. AT 2 8 8 " C

3 3 M o l % BaPO3-V2OsGIoss ~ . 33Mol % No POa-VzO5 Gloss

/

1(55

10-6

115 102

i 103

i 104

i 105

i IO s

I I07

FREQUENCY {Hz)

Fig. 9.

Frequency dependence of the electrical conductivity of 33 m o l e ~ NaPO~-V~O5

and 33 mole~oNaPOa-V~O5 glasses as a function of thermal treatment. Measured at 23°C.

independent conductivity behavior up to 105 Hz followed by a frequency dependent regime at the higher frequencies similar to that reported for the binary vanadia-phosphate glasses and the other ternary glass. The variation of the conductivity with thermal treatment for these two glasses differs significantly from that experienced in the investigation of the K20-containing glass. A 15 min heat treatment at 288°C of the Na20containing glass shows that only a small change in conductivity in the low frequencies is observed while, however, there is a tendency for the frequency dependence of the conductivity to change somewhat at higher frequencies. More extensive data are available for the BaO-containing glass and the results are most interesting. It can be seen from the data that the thermal treatment results in a progressive decrease in conductivity for the samples.

266

L.L.HENCH

The conductivity is seen to drop by nearly one-half order of magnitude with a 60 rain heat treatment. This behavior is in sharp contrast to the four orders of magnitude increase in conductivity produced by a 15 min heat treatment at 288 °C for the potassia-containing ternary glass. The above measurements were all made at 23 °C. It is well known 12) from extensive studies of alkali-silicate and alkalisilicate-earth glasses that the variations in the type of glass modifying cation can drastically influence the type and magnitude of phase separation that occurs in the glasses. The type of modifier ion is also well known to strongly influence the crystallization kinetics of the glasses. Consequently, it appears reasonable from the results discussed above that the variation of the alkali and alkali-earth contents in the semiconducting glasses changes the tendencies towards phase separation and crystallization and thus produces marked variations in the material's sensitivity to thermal treatments. Consequently, it would appear that there is considerable latitude in modifying the temperature sensitivity and the microstructural dependent properties by careful selection of ternary components to semiconducting glasses. 7. Irradiation effects

Previous results reported from this laboratory have shown that the dc and ac conductivity and temperature dependence of the conductivity of binary VEOs-P205 quenched semiconducting glasses are independent of neutron radiation dosages in the range of 5 × 10~v nvt. The present studies have shown that the ternary 33 mole% KPO3-VEOs semiconducting glass electrical conductivity is also independent of fast neutron irradiation to as much as 3 × l017 nvt, which is the limit of radiation presently investigated. The insensitivity of the electrical properties of the quenched ternary glass is shown graphically in fig. 10, which is a plot of the dielectric loss angle as a function of measuring frequency for a quenched sample exposed to a cumulative series of fast neutron irradiation. It can be seen that there is no appreciable change in the nature of the loss process. There is a slight indication that dielectric losses increase, reflecting the small increase in dc conductivity after neutron irradiation. The tendency for the irradiation to increase the conductivity of quenched semiconducting glasses is previously shown 7) to be related to the gamma radiation flux accompanying the fast neutrons in the reactor environment. The radiation behavior of a semiconducting glass-ceramic which contains structural heterogeneities, however, is quite different from that of the quenched samples. Fig. 11 is a plot of the electrical conductivity frequency dependence of a 33 mole% KPO3-V205 glass which has been heat treated for

267

SEMICONDUCTING GLASS-CERAMICS

&

PR E-RA.DIATION I? 1.0 X I0 NVT

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,

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53 Mol % K2PO3-VzO5

3

I I0

10 2

t

=

IO s

10 4 FREQUENCY

IOs

l0s

(Hz)

Fig. 10. Effect of fast neutron irradiation on the dielectric loss angle of an unbeat treated 33 mole ~ KPO3-V~O5 semiconducting glass. Measured at 23 °C.

30 min at 288 °C. As was shown in a previous section, this thermal treatment is sufficient to cause large increases in the magnitude of the dc conductivity and to decrease markedly the relative importance of the ac enhancement effect. Consequently, the conductivity frequency dependence of the preirradiation data for the sample is nearly frequency independent to frequencies as high as 4 x 106 Hz. The results also show that after the sample has been exposed to an integrated neutron dosage of 1 x 1017 nvt, the total conductivity is increased in magnitude and the ac enhancement effect appears again. The ac enhancement effect exhibited by the irradiated sample is akin to that exhibited by the quenched glass prior to thermal treatment. However, the total conductivity of the quenched glass was as much as four orders of magnitude lower than that shown here. A longer radiation exposure resulting in an integrated flux of 2.7 × 1017 nvt can be seen to begin to decrease the total conductivity of the sample with the ac enhancement effect remaining the same as in the samples exposed to the lower cumulative dosage. It was shown above that the dielectric relaxation loss peak was characteristic of heterogeneities in the thermally treated semiconducting glass-cera-

268

L.L.HENCH

3 3 Mol % KPO 3 - V2OsGlass 5 0 rnin at 2 8 8 " C

O PRE- RADIATION E] I X I O I r N V T A 2.7 X IOIrNvT

5~lb 4

(J

--

I0 z

-n

I~1

"~

G

I 103

E! 0

I 104

I 10 5

I I0 s

I

I0 r

F R E Q U E N C Y (Hz)

Fig. 11. Neutron radiation induced changes in electrical conductivity magnitude and frequency dependence of a 33 mole ~ KPOs-VzO~ glass heat treated for 30 min at 288 °C. 23 °C measuring temperature.

mics. It is helpful in attempting to understand the nature of the structure and property changes resulting from neutron irradiation to examine the effects of fast neutrons on the relaxation loss peak. Measurements of this type appear to be particularly important in investigating the structural changes accompanying neutron irradiation based upon the recent results reported for neutron effects in ionically conducting glass-ceramics14). Those results showed that the diffusivity and dissolution rate of metastable crystals in the size range of 100 to 200 A in the glass are greatly enhanced by radiation dosages in the range of 1 × 1017 nvt. More recent unpublished results of the same authors have shown that integrated fluxes of 4 x 1017 nvt can completely destroy the appearance of dielectric loss peaks in the insulating glassceramics. Since the size of the crystals in the semiconducting glass-ceramic is similar to that investigated in the insulating glass-ceramics, the similar radiation behavior of the two systems as shown in fig. 12 is extremely interesting. The data of fig. 12 demonstrates that the exposure of the semiconducting glassceramic to successively higher neutron dosages effectively destroys the di-

269

SEMICONDUCTING GLASS-CERAMICS 70

33 Mol % KzPO ~- V20s Gloss 50 rain ot 288"C

60

(9 PRE- RADIATION El AFTER IX 1017 NVT

50

&. AFTER 2 7 X I0 r~ NVT

40

x30 ,',3 t-2O

I0

0 I0

I0 z

i03

104

105

="1, iOs

FREQUENCY (Hz)

Fig. 12. Variation in the dielectric relaxation of a heat treated semiconducting glass as a result of fast neutron irradiation.

electric loss peak in the material. As was previously discussed in the section on thermal treatment, the appearance of dielectric peaks coincides with the reduction in relative importance of the ac enhancement effect in the heat treated glasses. Present results indicate that a reversal of the process occurs upon exposure to a neutron flux. The ac enhancement effect returns and the dielectric loss peak disappears. An important difference, however, is that the magnitude of total conductivity for the neutron irradiated samples is much higher than that of the quenched specimens which show a large ac enhancement effect. This would seem to indicate that the interfacial barriers giving rise to the dielectric relaxation process are degraded by the radiation exposure. The redevelopment of the ac enhancement is proposed to be a result of ionization of charge carriers in the heterogeneous regions increasing the conductivity of the regions, thus bringing back the relative importance of the frequency dependent heterogeneous contribution to the total conductivity. 8. Theory A quantitative theoretical treatment of all of the results presented above

270

L.L.HENCH

is a formidable task indeed. However, the challenge offered by the appearance of so many different phenomena can be a theoretical asset in that it is likely that only a single model can consistently explain all of the results. A quantitative model consistent with several of the most important observations is reported in an accompanying paperS). A qualitative presentation of the arguments contained in that paper and the explanations proposed for the other phenomena observed follows. The discussion will be presented through the media of a series of schematic representations of the electronic structure of the semiconducting glass-ceramics. Fig. 13 is a schematic representation of the electronic structure that can be expected for a semiconducting glass of varying degrees of disorder. The right-hand side of the figure, section A, is a material of very little structural distortion. In this case the material is crystalline and exhibits an electronic band structure characteristic of the transition metal oxide serving as a major component of the material. The conduction band in the case of a V205 base

:~" , , .

I (C)

(B)

(A)

I• v

2POLARONIC

BAND

~_Ebp

~

L

I

. . . . . . . . . .

F

~

+ E b ~1-,4ev ~

--

V4

~---:~ Eg ~2ev

"~

lC)

z

~

~

~ALtZED P O L A - - ~ ~

Y

~.Olev

"~ ~

z

~' ~

"--, ...,. ---~

(B)

,(A)

I

-~

_ _ _ .

--

Ev

VALENCE BAND

~I0-50%

0% PERCENT DISTORTION OF VO60CTAHEBRA ....-

Fig. 13. Schematicrepresentation of the changes in the electronic structure of a transition metal oxide based material as the atomic structura! distortion increases.

SEMICONDUCTING GLASS-CERAMICS

271

material consists of the electrons in the V 5 + ions. The width of the conduction band is proportional to 2J where J is the overlap integral [see the accompanying theoretical paper3)]. The magnitude of the conduction band can be expected to be in the range of 1 to 0.1 eV. Electrons associated with the V 4+ ions are considered to be polarons and therefore are lowered in energy by an amount proportional to the polaron binding energy. The location of these states can be expected to be approximately 0.1 to 0.4 eV below the conduction band. The energy gap separating the polaronic states and the valence band for the undistorted material is approximately in the range of 2 eV. At the extreme left of the figure is shown a type of electronic structure that can be expected when the distortion of the atomic arrangement in the material is sufficient to satisfy Anderson's criteria, thus resulting in complete localization of the energy levels. The energy levels will be localized polaron states. Electrical conduction in this case can be considered to be exclusively hopping between the polaron states. This is in contrast to the behavior of the material at the extreme right of the figure where both hopping between the polaronic V 4+ states and excitation of the electrons from these states to the conduction band are possible. There will be a continuum of electronic structure between the completely ordered and the completely disordered cases shown at the extremes of the figure. At section B, for example, it can be seen from the figure that one might expect a localization of states at the extremes of the conduction band due to the atomic structural distortion in the material. The localized states will be polaron states and hopping among these states will be the dominant conduction mechanism. At the extreme top of the polaronic states it is possible that a polaronic band will exist if the overlap integral between the states is sufficiently large. The order of magnitude of the width of the polaronic band can be expected to be in the range of 0.01 eV with the band displaced from the conduction band in the range of 0.1 to 0.4 eV. The electronic states associated with the classical valence band should also be expected to become localized at the extremities of the band. At the extreme left of the figure, corresponding to complete disorder, it is possible that the valence band can overlap with the conduction band, producing a continuum of localized states. The density of states in the conduction band that might be expected for the various degrees of disorder in the material is shown in fig. 14. Three possible states of the structure corresponding to the sections shown in fig. 13 are represented. In the highly ordered case (A) a sharp, well defined conduction band and polaronic impurity states associated with the V 4+ ions are shown. Complete disorder (C) in the structure results in complete localization of the

272

L. L. HENCH

N(E)

(c)

r

,I

(A/ I /

(A)

/'

/

,'

(811 /

,," /

/

I
I I I $ I

\ \

\ \

\ \

f E~

Fig. 14. Schematic density of states curves for a semi-conducting glass-ceramic of 3 varying degrees of disorder. The letters represent cross sections of the electronic structure

in fig. 13. energy levels with the maximum density of states being located in the range of the polaronic impurity levels. Intermediate structural case (B) will consist of a smearing out of the impurity states into a broader distribution of polaronic states as well as the localization of the energy levels from the bottom of the conduction band. Since it has been established above that an important feature of the semiconducting glasses was the existence of heterogeneities, even in the highly quenched glasses, it is important to consider the possible effects that heterogeneities have on the electronic structure of the material. A possible representation of such a structure is shown in fig. 15. Fig. 15a consists of a heterogeneity of greater order, represented by region A, in a matrix of lesser degree of order, represented by region B. At 0°K electronic population of region A will be in the impurity states. Population of region B will be in the bottom portion of the localized polaronic levels. The occupied states are shown as cross-hatched regions in the bottom of the figure, b. The polaronic band states in region B and the conduction band states in both regions A and B are expected to be unoccupied at 0 °K. Fig. 15b represents the change in the electronic structure of the heterogeneous material that might be expected when the temperature is increased to 300°K. Partial occupation of the conduction band in region A should occur. Occupation of higher lying polaronic levels in region B and perhaps even some of the lower level polaronic band states may occur. In order for the Fermi levels of the two phases to equilibrate, it might be expected that electrons transfer from the higher conductivity region A to matrix B, giving rise to occupation of higher levels at the interface. The net effect of the charge

273

SEMICONDUCTING GLASS-CERAMICS

transfer across the phase boundary should be to produce a space charge, which increases the height of the bands on the A side of the boundary and lowers the bands on the B or matrix side of the boundary. The perturbation of the energy levels in the region of the boundary according to the scheme proposed is shown in the figure. A consequence of the presence of the (B)

(B

.......................... ;;

;-

;~

9"!--

;~

; ;s

; - ; 2",-'~ i

(B)

(8)

+

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+l-

-I+ I

(C)

. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .

~---:..:_:

(c.)

Fig. 15. Representation of the interfacial Shottky barriers occurring at the boundary of heterogeneities in the semiconducting glass. Letters A, B, and C correspond to regions of disorder as shown in figs. 13 and 14. (a) Electron population at 0°K. (b) Change in electron population at 300 °K. (c) Electron population at 300°K when the matrix consists only of localized states.

274

L.L.HENCH

boundary is to produce an ionically charged region around the periphery of the heterogeneity. It is the coupling and oscillation of this region that is proposed to be responsible for the dielectric loss peaks in the heterogeneous semiconducting glass-ceramics. The activation energy associated with the oscillation of the ions in this region and any electrons localized with these ions can be expected to be in the range of 0.2 eV, in agreement with that measured for the temperature dependence of the relaxation process. Another consequence of the existence of the interfacial barrier is the isolation of the charge carriers in the higher conductivity region. Although the electrons in this region are more mobile than those in the matrix, the presence of the interfacial barrier prevents them from contributing to the dc conduction unless tunnelling through the barriers occurs. It is proposed that the ac enhancement effect observed in the quenched and heat treated samples is a result of the high frequency field coupling with the oscillation of the charge carriers in the heterogeneous region, thus giving rise to a large ac contribution to the total conductivity when the frequency exceeds that required for coupling with the charge carriers in the isolated region. It is proposed that the relative importance of the ac enhancement effect is a consequence of the degree of disorder in the glassy matrix. As shown in fig. 15c if the matrix is completely disordered, all of the energy states will be localized and the occupation of the states may be as shown in the shaded region of the figure. The dc conductivity in the matrix of such a material can be expected to be low in magnitude, as is observed for the highly quenched glasses. The charge carriers in the isolated phase, will still have appreciable mobility and give rise to an ac enhancement of the total conductivity when sufficiently high frequencies are applied. After the glass has been exposed to a thermal treatment, the matrix order increases and the magnitude of the dc conductivity increases markedly. The change that might be expected is for the electronic structure to go from that shown in fig. 15c to that shown in fig. 15b. Because of the additional order in the matrix, the charge carriers have higher mobility and the dc conductivity increases many orders of magnitude as is experimentally observed. The additional ordering and increase in size of the isolated phase also increases the conductivity of this phase, but the charge carriers are still isolated by the interfacial charge barriers so the ac enhancement effect is still observed. Because the dc conductivity has increased appreciably, the relative importance of the ac enhancement effect becomes less and the ac-dc dispersion observed in 1/Tplots is less marked. The change in local order of the matrix of the glass results in the decrease in the activation energy for dc conductivity for the heat treated glass from a magnitude of approximately 0.6 eV to a value of 0.2 eV. The mechanism of the ac enhancement is supposed to be unaltered by the

SEMICONDUCTINGGLASS-CERAMICS

275

thermal treatment, so the activation energy remains constant at 0.04 eV, as shown. A summary of the type of changes in the electronic structure with successive thermal treatments consistent with the above results and interpretation is shown in fig. 16. The isolated phase is seen to increase in size and order with a progressive thermal treatment. The matrix phase sharply increases in degree of order in the early stages of the thermal treatment, giving rise to the large increase in the dc conductivity and the decrease in the activation energy of conduction. (c)

(B)

(c)

(B)

(B)

(B)

(B)

(c)

(B)

t1

(B) ~,

(A)

~I

(B)

Fig. 16. Changes in the electronic structure of a semiconducting glass occurring as a result of thermal treatments. A heterogeneous highly conducting phase increases in size,

number and degree of order. The matrix increases in degree of order concurrently as the thermal treatment time proceeds from to to t~. The experimental phenomenon observed that still remains to be interpreted is that of the frequency inversion at temperatures above a critical temperature for heterogeneous semiconducting glass-ceramics. Fig. 17 represents the type of change in the electronic structure that might be expected when a field is applied to a heterogeneous semiconducting glass. The effect of the field is to modify the height of the interfacial space charge barriers surrounding the heterogeneities. It is proposed that when the dc conductivity

276

L.L. HENCH

of the matrix becomes sufficiently high, which will occur at high temperatures, tunnelling across the space charge barrier can occur. Effect of the tunnelling is to inject charge carriers that otherwise would have contributed to the dc conduction into the isolated phase which is giving rise to an ac absorption. When the applied ac frequency becomes sufficiently high that the field effectively couples with the charge barriers in the isolated phase,

- +

+l-

Ev

Fig. 17. Shift in the electronic population and internal interfacial barriers due to applying a dc field. Tunneling through the barrier into the high conductivity isolated phase is now possible. the injected charge carriers in effect become trapped in the isolated phase. The result of this entrapment is to remove charge carriers from the long range conduction process, thus decreasing the dc conductivity contribution to the total conductivity. Since the coupling is frequency dependent, the total conductivity decreases as a function of frequency and decreases as the temperature increases because the probability of the tunnelling becomes greater. The electronic behavior that results is akin to that of a solid state inductive process where it is the heterogeneities that are causing the current to lag the voltage. 9. Conclusions

In addition to the conclusions presented above, it is important to reiterate that the results obtained in this investigation indicate that a successful theoretical analysis of the electronic behavior of amorphous semiconductors requires thorough investigation and understanding of the detailed microstructures in the as-prepared materials. Results also prove that it is important to understand the effect of thermal history on the electronic properties,

SEMICONDUCTINGGLASS-CERAMICS

277

w h e t h e r the t h e r m a l history is a result o f c o n t r o l l e d exposures to a furnace e n v i r o n m e n t o r to resistance heating o f a thin film, o r a p p l i c a t i o n o f an electron b e a m . O n l y with an u n d e r s t a n d i n g o f m i c r o s t r u c t u r e a n d the effects o f t h e r m a l h i s t o r y on the m i c r o s t r u c t u r e can it be expected that a theoretical u n d e r s t a n d i n g a n d k n o w l e d g e a b l e a p p l i c a t i o n o f a m o r p h o u s semiconductors be m a d e .

Acknowledgements The a u t h o r w o u l d like to give special a c k n o w l e d g e m e n t to the e x p e r i m e n t a l assistance o f D. L. K i n s e r a n d A. E. C l a r k t h r o u g h o u t the investigations discussed. T h e s u p p o r t o f the W r i g h t - P a t t e r s o n A i r F o r c e Base r e a c t o r g r o u p in the r a d i a t i o n studies is gratefully a c k n o w l e d g e d , p a r t i c u l a r l y the c o n t i n u i n g assistance o f Lt. Bruce Wilson, M a j o r F r e d Buoni, a n d Mr. A r t h u r Bauer. F i n a n c i a l s u p p o r t o f the A d v a n c e d Research Projects A g e n c y o f the U.S. D e p a r t m e n t o f Defense, m o n i t o r e d by the A F C R L u n d e r C o n t r a c t F-1962868-C-0058 also is m o s t heartily a c k n o w l e d g e d .

References 1) J. D. Mackenzie, in Modern Aspects of the Vitreous State, Vol. 3, Ed. J. D. Mackenzie (Butterworths, Washington, 1964) pp. 126-147. 2) A. D. Pearson, in: Modern Aspects of the Vitreous State, Vol. 3, Ed. J. D. Mackenzie (Butterworths, Washington, 1964) pp. 29-58. 3) H. F. Schaake and L. L. Hench, J. Non-Crystalline Solids 2 (1970) 292. 4) L. L. Hench, Structure and Properties of Glass-Ceramics, in: Proc. 4th Space Congress, pp. 19-1 to 19-26, April 1967. Reprinted as Tech. Paper 377 in Engineering Progress at the Univ. of Florida, June 1967. 5) D. P. Hamblen, R. A. Weidel and G. E. Blair, J. Am. Ceram. Soc. 48 (1965) 311. 6) N. J. Kreidl, in: Modern Aspects of the Vitreous State, Vol. 5, Ed. J. J. Mackenzie (Butterworths, Washington, 1900). 7) L. L. Hench and G. A. Daughenbaugh, J. Nucl. Mater. 25 (1968) 58. 8) L. L. Hench, Dielectric Relaxation in Materials Analysis, Soc. of Aerospace Material Process Engineers, 14th Annual Symp. and Exhibition, Nov. 1968. 9) L. L. Hench and D. A. Jenkins, Phys. Status Solidi 20 (1967) 327. 10) L. L. Hench, A. E. Clark and D. L. Kinser, J. Electrochem. Soc., submitted. 11) D. L. Kinser and L. L. Hench, J. Am. Ceram. Soc. 51 (1968) 445. 12) P. W. McMillan, Glass Ceramics (Academic Press, New York). 13) S. A. Bates and L. L. Hench, X-Ray Small Angle Scattering in a Semiconducting Glass, to be submitted. 14) W. D. Tuohig and L. L. Hench, J. Nucl. Mater. (1969).