The ZnO-crystal as sensitive and selective detector for atomic hydrogen beams

NUCLEAR

INSTRUMENTS

AND METHODS

57 (t967) 22--28;

© NORTH-HOLLAND

PUBLISHING

CO.

THE ZnO-CRYSTAL AS SENSITIVE A N D SELECTIVE D E T E C T O R F O R A T O M I C H Y D R O G E N BEAMS* K. H A B E R R E C K E R , E. MOLLWO and H. SCHREIBER

lnstitut fiir Angewandte Physik tier Universitiit Erlangen-Niirnberg

and H. HO1NKES, H. NAHR, P. LINDNER and H. WILSCH

Physikalisches lnstitut I der Universitiit Erlangen-Niirnberg Received 21 July 1967 The change in surface conductivity of a ZnO-single-crystal if atomic hydrogen is adsorbed is used to determine the intensity of atomic beams. The lower limit of detection is 2 × 10 n atoms/

cm".sec. The apparatus is described and measurements in a source for polarized protons are discussed.

By the adsorption of atoms and molecules on the surface of semiconductors the electric conductivity may be changed essentially. The adsorbed particles act as surface acceptors or donors. Compared to the density of conduction electrons in the crystal, these acceptors or donors decrease this density in a depletion layer or increase it in an accumulation layer. This may be seen easily with ZnO which is of n-conducting type. The adsorption of oxygen decreases the conductivity, that of hydrogen increases its). Heiland 2) showed that molecular hydrogen does not cause any change in conductivity of ZnO-crystals in the temperature region from 90 to 300°K even in a time of 6 h. Atomic hydrogen, however, produces within seconds a surface layer of good conductivity which in vacuum remains for a long time in the temperature region given above. This surface layer disappears by heating the crystal in vacuum up to 500-600°K or by admission of oxygen. According to Langmuir 3) the change of conductivity in time depending on the H t-partial pressure respectively the number of atoms hitting tile crystal per unit time and area may be described by the following. If a ZnO-crystal is exposed to an atmosphere of atomic hydrogen, the density of physically adsorbed atoms at the surface reaches very soon a density of equilibrium. Be v* the surface density of crystal atoms. Then only a certain surface density of H-atoms v~ which is a small fraction of v* can be chemisorbed by transferring an electron to the crystal. Of the number N of atoms hitting the crystal per unit time and area, only a fraction k N has sufficient energy to be chemisorbed. On the other hand there is a mean lifetime r, after which the atoms leave the crystal surface again. For a certain temperature r is given by the activation energy E, of the chemisorption. So the

real density of chemisorbed atoms depends on the equilibrium of incoming atoms k N and leaving atoms. (The presence of physically adsorbed atoms yields only in a decrease of the activation energy Ea compared to that in vacuum. It will be assumed that E, and therefore ~ do not depend essentially on N under the conditions of this work. Only at N = 0 that means in high vacuum without any physically adsorbed H-atomlayer, z should be very much increased.) With these conceptions the change in density of chemisorbed atoms or electrons transferred to the crystal is given by

dv/dt= {0% -- v)/v*}kN-(v/z).

(1)

After integration one has

v(t) = [v~/ {1 + v* /(kNr)}] x

(2)

x

The change dv/dt at the time t-=O, that means for very small v is then always proportional to the number

of incoming atoms per unit time and area: (dv/dt)o = (v~/v*)kN. The saturation value of chemisorbed atoms is ,,~ = v~ / {l + v*/(kNr)} and is proportional to N, if k N r ~ v*, that means for small concentrations of atomic hydrogen or small intensities of atomic beams. If kNz>>v*, w = v ~ independent ofN. A graphical representation of formula (2) is given in fig. 1. A measurement of the time dependence of v de* Work supported in part by Deutsche Forschungsgemeinschaft and Bundesministerium fiir wissenschaftliche Forschung. 22

DETECTOR

FOR ATOMIC 11"

v(r) voo

z

HYDROGEN

23

BEAMS

I

z- e t ~ -

I+ kNr

v__L" ~,th v'=u7 lo'5[c~_7 and T = 30gs] is kN-c

v(t) Voo

/qo# o.o3

(3

~o

0.I0

Q30

kNrcr~2 3'] 4.10 r5 !3.10 ~5 ,~ .10 I4 13,10

1o

4.1013

3.O

~3. I0 ~3

I0

4 "I012

O5 g (1,

~0

70

#0 o

70

relvtlve durahon of exposure

T

Fig. 1. T h e theoretical density o f c h e m i s o r b e d a t o m s as f u n c t i o n o f the time, if the ZnO-crystal is exposed to a t o m i c h y d r o g e n at t = 0. F o r each curve the p r o d u c t k N (proportional to N ) a n d the experimentally d e t e r m i n e d m e a n life time T ~ 30 s is given.

termines the number k N of atoms per unit time and area which are able to be chemisorbed. If one takes = voJvs

=

for the time constant of the exponential term in eq. (2), one gets k N = (filet)v*.

{1

Experimentally the density v of chemisorbed atoms, respectively their change in time d v / d t is determined by measuring the surface conductivity g of the ZnO-

and fl = ( k N / v * ) { 1 + v* /(kNO}, zl g(t) ,dg=

N~

~o

14

-2-I

N:6.10[cm

-~ o.~

s]

N= 3 .lOr4/cm~~S' J

/

tj

~

"G 070, ~"

I0 o

8. I0'3[c

J

10 7

T~

10 2

t IS; "

du~oi'on o r exposure

Fig. 2. T h e experimentally m e a s u r e d surface conductivity o f a ZnO-crystal as function o f the time o f exposure to an a t o m i c hydrogen beam. F o r each curve the b e a m intensity m e a s u r e d by a c o m p r e s s i o n tube is given. T h e value for Jg(t)/dgoo with N--->0o is m e a s u r e d u n d e r the influence o f various very high b e a m itensities.

24

K. H A B E R R E C K E R

F

?L! ¸ iiiiiiiiii

iiii .......J ~tJL

y-il .....

•

Fig. 3. ZnO-crystal in holder. In the bore-hole the crystal needle m a y be seen.

crystal, respectively their change in time dg/dt. Fig. 2 shows some measured curves, using an atomic beam of hydrogen of known intensity. This procedure will be described in detail late,'. By comparison of these curves to the theoretical ones in fig. 1 one finds k ~ 0 . 1 5 (this is important for an absolute determination of N). For a quick and relative determination of N one measures only the increase of surface conductivity which is proportional to N:

dg/dt = el~(dv/dt) = elL(V,/v*)kN, where e = e l e m e n t a r y c h a r g e = 1.6 x 10-z9 C; l~=mobility of electrons in Z n O ~ 10z cm2/Vs, Vx/V* ~ 10 - 2

The crystals used in our investigations (0.2-0.4 mm dia., 6-12 mm length) are grown in the gas phase and doped with Cu. Their bulk conductivity is 10 -~ 10-a/ohm-cm. In order to construct a simple and sensitive detector for atomic hydrogen it is necessary to transforln this increase of surface conductivity (causing an increase in current) into a voltage signal, which is proportional to the concentration of atomic hydrogen (or the intensity in an atomic beam). After such a measurement the crystal is regenerated by heating off the atomic layer. Fig. 3 shows a ZnO-crystal suitable for this purpose mounted in a polystyrol holder. Care has to be taken that the contacts of the crystal are absolutely ohmic. The crystal is therefore treated in an atomic hydrogen atmosphere of 10 -z Torr, betk3re gold contacts are deposited under vacuum. A first detector setup operating with a ZnO-crystal was developed by Haberrecke,-4). Such a detector for atomic hydrogen based on this principle is interesting in some aspects: For the construction of sources for polarized protons

e l a/.

and deuterons following the atomic beam method, detailed information concerning the atomic beam is necessary. Furthermore, the transition probability for the Abragam-Winter adiabatic passage method 5) which is used to increase the polarisation, can be measured without ionizing and accelerating the beam, if a suitable detector and polarisation analyser ~'7) is available. The detector is sensitive enough to measure the intensity of hydrogen atoms, after they have been polarized and adsorbed on surfaces and evaporated again. Compared with a compression tube (as described later) for measuring atomic beam intensities, the ZnOcrystal offers mainly 3 advantages: 1. a much better sensitivity; 2. a much smaller sensitive area and so a better spatial resolution; 3. specific sensitivity only for atomic hydrogen or deuterium (for example the compression tube measures undissociated H2-molecules in a beam also). Another detector for atomic hydrogen beams based on the recombination heat is described in'°). A complete electronic circuit (block diagram, fig. 4) for the application of the crystal to experiments with atomic hydrogen beams was built up. The circuit consists of a timing unit, an analysing part with amplification and registration of the crystal signal and a part which restores the surface of the crystal by heating. The time for one cycle of measurement is essentially determined by features of the ZnO-crystal and amounts to 18 sec only. The cycle (fig. 5a) consists of: 1. putting the crystal into the measuring circuit; 2. switching on the atomic beam by moving a diaphragm ("shutter") (fig. 5b); 3. measurement of the "beam signal"; 4. switching off the atomic beam; 5. switching the crystal out of the measuring circuit into the heating circuit ; 6. heating voltage on; 7. heating voltage off. A three step oscillator triggers at times t 1, t2 and t 5 three unsymmetrical multivibrators. The sweep back into the initial state defines the times t3, t4, t6 respectively. The three multivibrators are joined by emitter followers and relais. As the ZnO-crystal shows a decrease in its resistance R~ when it is exposed to atomic hydrogen, the time derivative of Rc is to be measured immediately after t2. R, is measured in a tunable bridge (fig. 4) which

DETECTOR

FOR

ATOMIC

HYDROGEN

BEAMS

25

S TAB

200

rUN,IBLE

V

H, c -

SLaB

L i

(

i

Q"

6 v

ST,~B

_ _ ]

I L - - T-

SWITCM

ME4SURING BRIDGE

faRE -

SECOND -

PLOTTER

A MPLIFIER

Fig. 4. Block-diagram of electronic circuit. gives the signal voltage Ud. The change of Ud is proportional to that of Rc in all interesting cases. A direct voltage preamplifier follows. The amplifier input is short cut from t4 to tl. The output is connected with a two way differentiating element R d , C d and R 'd ~ dt '. , After a time characterised by the time constant Td gdC d (330 ms) the differentiated voltage signal is proportional to d R j d t . A second direct voltage amplification yields a voltage, which is registrated by a Moseley plotter 680 M. The plotter input is also cut short fi'om t4 to tt. The voltage amplification over all is ==

R E L A I S I CRYSTAL I N M E A S U R I N G CIRCUIT

[ i

9,

I

I RELAIS

lI.

BEAM

ON

55

i'

] I

]

I

I

1

I

1

I'-

I

t

I I

I

I

I

I

'1

I

I

1

I RELAZS [ , I

[[[:HEATZNG

I I

VOLTAGE

ON

I

I

0

3

8 9 I0 tt

tSs

t~

t2

t3

tT=t ~

t,;

]tsl

i

[

j

t5 t 6

Fig. 5a. Time diagram of one cycle. SHUTTER

"~> t-

]

2. -.......... f/- .......

Aro~4zc BEASf

.-,>

-'~ Cm'SrAL

Fig. 5b. Experimental arrangement (schematically).

V~=2.6 x 103 V/(V/sec). The bridge is tuned in such a way that Ua is slightly negative in the regeneration phase while Ud becomes positive if hydrogen atoms hit the crystal. So the disturbing pulse from opening the amplifier input becomes negative and is cut off by a clipper diode in the second amplifier. Between amplifier output and plotter input there is an integrating element in order to depress the thermal noise of the crystal. The rate of change in resistance d R J d t is constant during the "'beam-on"-time t 2 to t 3 only for intensities from 1012 a t o m s / c m / - s to 10 ~3 atoms/cm2.s of the beam. With greater intensities the crystal reaches the saturation region (fig. 1) already within this time, and d R j d t is a proportional measure for the intensity only immediately after t 2 and decreases towards t 3. Because of the time constants of plotter, differentiating and integrating element r 0, r 0 and r~ the final signal is delayed and reaches its maximum when d R c / d t is already decreasing again. So the final signal is smaller for greater time constants. In order to restore a clean surface after exposure to atomic hydrogen, a current pulse of 20 ms duration is used to produce a temperature, at which the hydrogen layer is reevaporated. Because of the good conductivity of the surface the current pulse heats essentially this one, and the resistance drops further. The volume of the crystal remains cool, so that the temperature equilibrium is reached after 8 sec and a new measurement may be carried out. Tile current pulse is produced by discharging a

26

K.

;): C ); /¸4¸¸¸¸¸¸ : ._.q ;> O un

~E

g+

g ~P

.l::=

~E o

I

H g~ e-

'-.-,

;2

g~ 0/1

,~

.-= o ._= =o

4 aj

el a/.

HABERRECKER

With the arrangement described the profile of the intensity ! in an atomic hydrogen beam with rotational symmetry around the beam axis was measured. The ZnO-crystal was moved in a plane perpendicular to the beam axis in the two coordinate directions .v and y (fig. 6). A calculation of the absolute change in conductivity following the theory given above is not possible, as some parameters are not known. Furthermore the time constants of the electronic measuring device change the result. So the sensitivity curve of the crystal was determined experimentally. Starting with the highest intensity / o the mean intensity is reduced to c~=~ by interrupting the beam periodically. This new value Ii = a / o is reproduced by reducing the atomic beam and then 12=~21o is taken, etc. So the intensity is decreased step by step, until the lower limit of sensitivity is reached. The interruptions are made by a quickly rotating disk with cogs. Though the experiment shows that the final signal is not proportional to the beam intensity, this procedure is allowed, as the interruption period (1 ms) is small compared to the time constant of chemisorption. Furthermore the maxwellian velocity distribution smears over the single portions of the beam within the time of flight between disk and crystal. It was experimentally ensured that there is no influence of' the unsteady beam. The best crystal used showed a lower limit of sensitivity o f / r = 4 . 2 × 10 _4 / 0 which is determined by thermal noise. The absolute value of I 0 was measured with a compression tube of entrance area A. The atoms of the beam enter a measuring volume through a tube of vacuum conductance /. without striking the wall. They produce an increase of pressure zip in the volume of a commercial ion gauge tube until equilibrium of incoming particles q~i and outgoing particles 4 ) o = l . A p is established. So the intensity is given by

e,

I = q,~/A = L z l p / A .

._~

capacitor (32 I~F) of 40-200 V depending on the special crystal used. This voltage has to be chosen carefully. It determines the sensitivity of the crystal and the reproducibility of measurements. New crystals also need some hours of "training time" (running repeatedly through the above cycle) until stable surface conditions are produced.

The vacuum conductance for molecular hydrogen H 2 has to be used in this formula because the incoming atoms all recombine in the tube. L was determined experimentally. 10 was found to be (3.5 _0.5)x 10 ~5 atoms/cm 2.s. The lower limit for reproducible measurements is therefore 1.5 x 1012 atoms/cm2.s, the lower limit for detection only is about 2 x 10 ~l atoms/ cm

2 - s.

Fig. 7 shows the calibration curve for a special ZnOcrystal. The deviation from linearity at high intensities

DETECTOR

arbitrary

FOR

ATOMIC

HYDROGEN

27

BEAMS

units

75oo arbitrary

units

5o~

u]

(3

lo(x

INTENSITY

crn = s

INTENSITY

Fig. 7. Calibration curve of = complete ZnO-crysta] detector device.

is produced by the time constants of the electronic circuit whereas that at low intensities is characteristic for the crystal itself. As an example for application in the field of atomic beams the measurement of the velocity distribution of atoms produced by a high frequency discharge is describedS). A beam of atomic hydrogen with slit profile (0.1 x 9 mm 2) passes through an inhomogeneous magnetic field with constant field gradient. The deviation of the atoms from the axis to the side depends essentially on the velocity (s,-~ l/v2). As the atoms of the beam have different velocities and there are two possibilities of orientation for their magnetic moment in the field given, the beam is spatially split up into

*

i

,

*

*

e ~ p e r l m e n t a l points

\ --

theory

.x\ \

POSITION OF CRYSTAL

Fig. 8. Spatial distribution of intensity in the atomic beam due to the velocity distribution after passing through a magnetic field with constant field gradient.

two broad components. The intensity distribution in these components represents the velocity distribution of the atoms in the beam. It is compared to a maxwellian distribution (fig. 8) of the form

dl = ( 21/v~)v3 e x p ( -

vZ/v2)dv,

where dl is the intensity of atoms in the velocity interval (v, v + d v ) and Vh=(2kT/mtt) ~ the most probable velocity. So one finds 265°C for the effective temperature of the gas in this discharge tube and Vh=3.64X l03 m/s for the most probable velocity of the atoms, if the discharge tube is operated under the usual optimal conditions. The deviation between theory and experiment at high velocities may be explained by the following argument: the maxwellian distribution is valid only for particles in thermal equilibrium with environment which is not given in a high frequency discharge tube. There is a radial temperature distribution with high temperature at the axis and low temperature at the (cooled) wall of the tube. This yields an overlap of several maxwellian distributions. Beyond that the pressure in the discharge tube and the form of the exit nozzle show a small influence on the distribution, especially at low pressures the conformity between theory and experiment is better. The ZnO-detector has been used as a reliable instrument in our developmental work on polarized ion sources')). The authors wish to thank Dipl.-Phys. J. Witte for some contributions in the early stage of this work.

28

K. HABERRECKER el aL

References a) G. Heiland, E. Mollwo, F. St6ckmann, Electronic processes in zincoxide in Solid State Physics (ed. Seitz-Turnbull, Academic Press, New York, 1959). z) G. Heiland, Z. Physik 148 (1957) 15. a) I. Langmuir, J. Amer. chem. Soc. 40 (1918) 1369. 4) K. Haberrecker, Diplomarbeit (Erlangen, 1965) unpublished.

'~) A. Abragam and G. M. Winter, Phys. Rev. Letters 1 (1958) 374. 6) H. Hoinkes, Diplomarbeit (Erlangen, 1966) unpublished. 7) H. Nahr, Diplomarbeit (Erlangen, 1966) unpublished. 8) p. Lindner, Diplomarbeit (Erlangen, 1966) unpublished. 9) H. Wilsch and H. Nahr, Verhandl. DPG (VI) 2 (1967) 366. 10) H. Hirsch, Diplomarbeit (Freiburg, 1966) unpublished.