On the nature of exoemission of ZnO powder

Pkysica 49 (1970) 157-164

ON THE

NATURE

0 North-Holland Publishing Co.

OF EXOEMISSION

OF ZnO POWDER

D. DE MUER and W. MAENHOUT-VAN

DER VORST

Laboratorium voor Kristallografie en S&die van de Vaste Stof, Rijksuniversiteit, B-gooo Gent, Belgil Received 23 February 1970

Synopsis The exoemission glow curve of ZnO powder was recorded in the temperature range between - 180 and + 170°C. Six glow peaks are present and analysis indicates that the exoemission of ZnO follows predominantly second-order kinetics, and that it can not originate from trapping centers. A comparison of the results obtained with two different types of counting tubes also enables to conclude that the emitted particles are negative ions.

1. Introd&iorz. The so called exoelectron emission (or exoemission) is a rather new method to study surface properties of solids. In nearly all the papers on this topic it is implicitly assumed that the emitted particles are electrons; evidence that this is so is, however, mostly lacking. Up to now, the exoemission from semiconductor surfaces has practically not been studied and nearly nothing is known about the nature of the responsible emission centers. In the case of Ge, Seegerl) supposed that the trapping centers at the surface cannot act as emission centers, because the emission at low temperatures is inconsistent with the high electron affinity of Ge (about 4 eV). Seidl2) and Menold3) came to the same conclusion resp. for CuzO and ZnO. The parameters of the emission centers of semiconductors were never evaluated. In the present work we calculate the parameters of the exoemission centers of ZnO for two peaks of the glow curve. The experiments with two different counters, i.e. an electrostatic and a magnetic one, enabled to prove that the emitted particles in the case of ZnO are not electrons but negative ions. 2. Experimental techniques. The samples were pure ZnO powder deposited on a gold base by electrostatic dispersion. This plate is placed on a special sample holder mounted in a “Quick Fit” vacuum vessel. The specimen holder is movable so that it can be easily and rapidly moved from the excitation source to the detector. The sample block can be cooled to liquidair temperature and linearly warmed up till 175°C. 157

158

I>. DE MUEK

AND

W. MAENHOUT-VAN

L>EK VORST

It was possible to excite the specimen either by X rays or UV light. After excitation at - 185°C during one hour the specimen is brought into the vicinity of the counter dynode and slowly warmed up at a constant rate of 4”C/min. As detector we used two kinds of counters, an open electrostatic

electron

plier (Bendix

multiplier

(EM1 type 9603)

and a magnetic

electron

multi-

type M 306).

3. Experimental results. 3.1. Detection with an electrostatic electron multiplier. After irradiation with UV light, the ZnO powder did not exhibit any exoemission at all, even when the sample was stimulated with visible light after excitation. On excitation of the sample with X rays, an intense exoemission was observable. The glow curve of this emission is presented in fig. 1. Between - 180 and + 17O”C, six glow peaks were found respectively at -160, -130, -60, -25, $90 and + 135°C. At + 170°C the shape of the curve indicates that a 7th peak might be present at higher temperatures. This could not be verified as it was impossible to warm the specimen up to higher temperatures. When the specimens were examined several months after their formation and had been exposed to air during that period it appeared that the shape of the glow curve between 20” and 170°C was different. Indeed, such a specimen shows besides the already mentioned low-temperature peaks only one peak at + 1 lo’%, those previously present at +90 and + 135°C are no longer observed (see fig. 2). Up to now, we are unable to explain the origin of this phenomenon. These results, however, throw some light on the fact that some authors found only one peak 4,5), and others more peaksa) in the temperature interval 20-200°C.

Fig. 1. Glow curve of the exoemission of ZnO powder. In the temperature I, II and III, the proportionality factor cxis resp. = 1, 10 and 100.

regions

ON THE

NATURE

OF EXOEMISSION

OF ZnO POWDER

159

15-

;;

5

5-

+80

+90

+lOO

Fig. 2. Exoemission

+llO

4120

peak of ZnO powder

+130

+1

at 110°C.

25 r

T(T)

Fig. 3. Exoemission

glow curve of CaS04 after netic electron

X-ray

multiplier.

excitation,

detected

by a mag-

D. DE MLJER AND

160

W. MEANHOUT-VAN

3.2. Detection with a magnetic electron netic electron multiplier was especially adapted electrons (see ref. 6). In addition, an electrostatic

DEK

VORST

multiplier. The magto detect low-energetic lens was mounted near

to the entrance grid, and the sample holder was isolated, so that a variable accelerating field (up to 700 V) could be applied between the sample and the entrance grid. In spite of these different precautions, we were unable to detect any exoemission of ZnO with the magnetic electron multiplier. In order to check that this was not due to the experimental setup the ZnO sample was replaced by a metal or an ionic crystal. A very intense exoemission was recorded with abraded aluminium and with an irradiated CaS04 crystal (see fig. 3). This allows to conclude that no electrons are emitted in the case of ZnO but rather negative ions and more specifically oxygen ions. 4. Calculation of the parameters of the emission centers. 4.1. Basic e quat ions. The basic equations used in the theory of thermoluminescence arc also valid for exoelectron emission. So the intensity of the exoemission can be defined as follows:

I=-$, with 0 < r/( 1 and where n denotes the density of filled emission centers. The probability l/r for thermal emptying of the filled emission centers is given by the equation 1 -=_ 7

1

exp

70

E ~ kT > ’

(

where l/r0 is a frequency factor, and E is the thermal activation energy of the emission centers. When I is proportional to n (resp. ns), the exoemission follows first- (resp. second-) order kinetics. 4.2. Calculation from the shape of the glow peaks. We applied the universal method for analysis, which we developed in another paper recently publishedlo). Suppose that the heating starts at the temperature T1 and at the moment tl; the density of filled emission centers is then n(tl). The two curves ln

1

Wdt

_

[

vs.

1

~

kT

and ln_,fi:l

dnldt] ?I?

1 vs.

--

kT

(4)

ON THE

-91

NATURE

I

OF EXOEMlSSION

I

I

29

I

30

of the functions

1:

2 = -

dn/dt -; n

curves2:

Z=

I

1

(3) and (4) for the peak

and the peak at + 110°C (lower Curves

I 31

l/kT[(e"?

Fig. 4. Calculation

161

OF ZnO POWLIEK

-

n(tl) dn/dt 922

at -25°C

(upper

part)

part).

; curves 3 : common tangent.

162

D. DE

MUEIi

AND

W. MAENHOUT-VAN

DEK

VOKST

have a common tangent at the temperature TI. The slope of this line determines the activation energy. It was shown that the equation of this straight line can be written as ln i

Z 111-l7

-

k”l:,

70

if the excitation of excitation.

source is sufficiently

intense

to reach the saturation

value

We have made these calculations only for the glow peaks which are sufficiently intense and which were completely isolated in the glow curve, i.e. the peaks at -25 and + 110°C. The curves (3) and (4) were calculated for these two peaks (fig. 4). It appears that for the peak at -25”C, the curves 2 and 3 coincide. For the peak at + 1 10°C the divergence of curve 2 with respect to curve 3 is much less than the divergence of curve 1. From these results we may draw the following conclusion (see ref. 10). The exoemission at -25°C follows pure second-order kinetics; at + 110°C there is also a first-order component, which is, however, much less important than the second-order component. The intensity of the X-ray source was sufficiently large, so that the curves 3 in fig. 4 are described by eq. (5). H ence the intersection of this straight line with the ordinate axis gives the frequency factor ~/TO.The values of E and ln( 1/TO)that were calculated from fig. 4 are represented in table I. The errors which are mentioned, are the calculated dispersions. TABLE

Parameters

of two

1

emission

centers,

calculated

from the shape of the corresponding glow peaks, by means of the universal method Peak

E (ev) 0.01

79 *

6

2.33 & 0.01

66 f

8

1.77 5

-25T +lloOc

W/To)

4.3. Calculation from two points of the glow peaks. We make use of the method of Kelly and Laubitz7). The activation energy can be calculated by the following formulas: E = Cl

E = C2

kT,T’ T,

-

T’ ’

kTmT” T” -

T,

’

(6) (7)

ON THE

NATURE

OF EXOEMISSION

OF ZnO

163

POWDER

where T, is the absolute temperature at maximum emission, T’ resp. T” are the absolute temperatures where the emission reaches half of its maximum value in the ascending resp. descending part of the peak, Cl and CZ are parameters that vary with the kinetics of the emission process and with the value of T, and E. The values of Cl and Cs were tabulated by Flemings). The values of the activation energy which were found in this way, are given in table II. TABLE II Activation

energies

(in eV),

assumption

calculated

from

asc. part

-25°C

1.34 + 0.20

+lloOc

1.87 & 0.20

desc. part 0.95 * 1.38 f

of the glow peak,

on the

kinetics Second-order

First order process Peak

two points

of first- and second-order

asc. part

process desc. part

0.20

1.63 f

0.30

1.68 f

0.20

2.25 4

0.30

2.45 j, 0.30

0.30

The errors were calculated assuming that the error for T, - T’ and T” - T, amounts to 2°C. From table II it appears that for both glow peaks, the two energy values which were calculated, assuming a second-order process, agree very well, and also do agree with the energy value in table I. On the assumption of first-order kinetics, however, the two energy values for both peaks diverge. From this, it follows again that the two glow peaks of exoemission which could be analysed, follow predominantly second-order kinetics. 5. Disczlssion of the eqberimental results. 5.1. The centers which are responsible for the exoemission of ZnO, are of another nature than the trapping centers of thermoluminescence. This can be deduced from the following experimental facts : By excitation with UV light, one can detect thermoluminescence, but no exoemission. The glow curve of these two phenomena is completely different. All the peaks of thermoluminescence follow predominantly first-order kinetics 9710). Th e t wo exoemission glow peaks we analysed, follow predominantly second-order kinetics. 5.2. The exoemission is not due to trapping centers in the band gap. This follows from the fact that the trapping centers corresponding with the activation energies given in table I, should be situated under the Fermi level (as was already noticed by Menolds)).

164

ON THE

NATURE

OF EXOEMISSION

OF ZnO

POWDER

The values of ln( 1/TO) also are inconsistent with the theory of trapping centers. Indeed, from table I, we find for the two peaks that we analysed, the following frequency factors: 1/TO= 1034 s-1 resp. 1028 s-i. These values are much larger than the maximal vibrational frequency of the components of the crystal lattice. 5.3. As the measurements with a magnetic electron multiplier, failed for ZnO we may conclude that the emitted particles at ZnO are of another nature than at ionic crystals and metals. Still, they have a negative charge, as could be deduced from the accelerating field within the electrostatic electron multiplier. The magnetic electron multiplier that we used has a tungsten cathode and it is known that the yield of secondary electron emission at tungsten is smaller for impact of ions than for impact of electrons. From the foregoing discussion, we are disposed to accept that the emitted particles during exoemission at ZnO are negative ions. Moreover, we assume that it concerns oxygen ions. In the case of ZnO the name “exoelectrons” has no sense. More evidence will be necessary to confirm the exoemission of ions. 5.4. For the exoemission of ZnO we propose the following mechanism. During excitation with X rays, the remaining oxygen molecules of the ambient gas are ionized. The oxygen ions which are formed in this way and which touch the surface of the sample, are adsorbed at low temperature. During the subsequent heating of the sample, there is a desorption, that gives rise to the exoemission. The emission peaks at different temperatures should correspond to different adsorption states at the surface. We wish to thank Professor W’. Dekeyser for his Acknowledgements. continuous interest and helpful discussions. Oneiofius (D.D M) expresses also his gratitude to the “Institut des Recherches Scientifiques dans 1’Industrie et 1’Agriculture” for a research fellowship.

KEFERENCES

1) 4

Seeger, K., Z. Phys. 149 (1957) 453. Seidl, R., Naturwissenschaften 46 (1959)

3) 4)

Menold, R., 2. Phys. 157 (1959) 499. Hieslmair, H. and Milller, M., 2. Phys. 152 (1958) 642. Sjasjkov, A. S., Morzjoezina, W. M., Krylova, I. W. and Kobozjev,

5)

573.

Khim.

40 (1966)

6) 7)

Leffel, Kelly,

C. S., Rev. sci. Instrum. 35 (1964) 1614. P. J. and Laubitz, M. J., Canad. J. Phys.

8)

Fleming, R. J., Canad. J. Phys. De Muer, D. and Maenhout-Van

46 (1968) 1569. der Vorst, W.,

De Muer,

1.

9) 10)

N. I., Zh. Fiz.

250.

D., Physica

48 (1970)

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