AIN: a solid Al+ ion source

Vacuum/volume36/numbers 11/12/pages 977 to 980/1986

0042-207X/8653.00 + .00

Printed in Great Britain

AIN:

Pergamon Journals Ltd

a solid AI + ion source

J P e l l e t i e r , Physique des Milieux Ionisds, USTMG CNRS UA 844, CIVET BP 98 38243, Meylan Cedex, France

Because of its physical, chemical and electrical properties, AIN is an attractive material for positive surface ionization applications and a likely candidate for AI + thermal emission. The computed composition and pressure of the vapour phase over AIN indicate that sublimation is stoichiometric and that the equilibrium pressure is high, even at 1500 K. Moreover, the bulk electronic properties of AIN are such that work functions as high as 6 eV can be obtained, provided that surface states are not present in the band gap. The evolution of the work function vs temperature was measured on an n-type AIN single crystal, using the Shelton retarding field method. The work function is temperature independent, which infers Fermi level pinning at the surface by an empty intrinsic surface state band. With a value of 5.35 eV for the work function, AIN seems to be a practical alternative to iridium for surface ionization applications. Moreover, direct thermal AI + emission from AIN, expected because of the relatively low ionization energy of Al, has been observed by mass spectrometry. Finally, the mechanisms of thermal positive ion emission from solid sources are discussed in the light of experimental results.

A

Introduction

AB ~

Since Beattie I first pointed out in 1899 the possibility of positive ion emission from heated salts, a great deal of research has been devoted to the study of this phenomenon. Until now, the majority of experimental investigations has been geared towards the production of intense positive ion beams by use of this process. The most famous and efficient ion emitter, the fl-eucryptite (Li20, A1203, 2 SiO2), can deliver Li ÷ current densities as high as several mA cm- 2 (refs 2-4). The advantage and attraction of such a solid emitter over classical surface ionization sources lie in its simplicity, reliability and small size. The pumping requirements are modest and, from pure materials, only a single species is produced. Finally, the source's lifetime, dependent on the intensity of lithium ion emission and the amount of lithium stored inside ~-eucryptite, can reach several hundred hours, i.e. sufficient for most applications. However, although the thermoionic properties of fl-eucryptite have, since its discovery by Blewett and Jones 5, been extensively studied 2'3, the exact mechanism for the emission of positive ions from such a compound has not yet been completely clarified. The purpose therefore of this paper is to propose, with a new example, the thermal AI ÷ emission from A1N, a more complete description of the mechanisms responsible for thermal positive ion emission from solid sources.

These equilibria are governed by the Saha equation:

Theoretical discussion

Consider a plasma in local thermodynamic equilibrium at temperature Tcomposed of molecules AB and its dissociation and ionization products, atoms A and B, electrons e- and positive ions A ÷ (only atoms A can be ionized). The reactions within the plasma are described by the two independent balanced equations: A =

A++e -

(1)

+ B.

(2)

[A +] [e-] [A] Ks(70

(3)

and a similar mass action law for dissociation equilibrium:

[A] IS] [AB]

-

K°(70

(4)

where IX] represents the density of species X. If the terms relative to the excited electronic states can be neglected in the electronic partition functions, the equilibrium constants K,(T) can be expressed as follows:

Ks--geg+gAN © e x p ( - - ~ ) K --~---'ffAffB(-mA~ms~~3/2(2r~kT) -x/2 e x p ( o - 2I

gAB \ m A d- roll/

(5) ~)

(6)

where gx and mx are respectively the statistical weight of the ground state and the mass of species X, N==(2nmekT/h2)3/2 represents the volume in the phase space for electrons, E A is the ionization energy of atom A, E D the dissociation energy of molecule AB, and to and I the pulsation and the inertia momentum of molecule AB. At thermodynamic equilibrium, the electron concentration on a surface exhibiting a work function ~bis given by the RichardsonDushman law: [ e - ] = 9eN= e x p ( - if-T).

(7) 977

J Pelletier: AIN: a solid AI + ion source

Expression (3) is reduced to the Saha-Langmuir equation: [A+]

K+(T) = g+

['¢--EA'~

(8)

If the sum of the concentration of all A- and B-bearing species has a fixed value n, [AB]+ [A] + [A +] = n

(9)

[ B A ] + [B] = n

(10)

the system is therefore completely described by equations (4) and (8)-(10). Then the ion concentration can easily be calculated and its precise expression is:

[A+]=2

Ko [ _ 1 + [ -

4nK+

1 +

7112- ]

Ko(I+K+)J J"

(11)

Now, two practical cases can be considered: (i) K o ~>n (high dissociation constant) K+ [A+] =n - - ; I+K+

(12)

(ii) K D,~ n (low dissociation constant) [ [A+]=

K+

] 1/2

nKOl+K +

~n.

(13)

Equation (13) points out that, if the dissociation rate is not close to unity, the ionization rate cannot reach unity, either. O n the other hand, when total dissociation occurs, the ionization rate reaches unity only if the condition: K+ >> 1 i.e. ¢ > E a

AH~ is the activation enthalpy of vaporization which can be determined from the temperature dependence of the pressure. Finally, the temperature dependence of the positive ion current density is, also, accurately determined from equations (11) and (15)-(17).

Properties of AIN It clearly appears from the above that the two conditions for an efficient source delivering monoatomic ions are: dissociative sublimation and work function in the order of magnitude of ionization energy. With these considerations in mind, a methodology for a search for solid ion sources may be worked out as follows: firstly, at least one of the elements of the compound must exhibit a low ionization energy; secondly, the element to be ionized must be present in a dissociated form in the vapour phase above the solid; thirdly, the electronic properties of the compound must be such that high work functions may be expected. Given its physical, chemical and bulk electronic properties, AIN, a III-V semiconducting compound, appears to be a likely candidate for use as a solid ion emitter: the ionization energy of A1 is fairly low, i.e. 5.98 eV -,~hereas the energies for nitrogen are 14.53 eV for atoms and 15.58 eV for molecules. The composition and pressure of the vapour phase at thermodynamic equilibrium above AIN 6 are indicated in Table 1. It appears that evaporation is stoichiometric and that A1N dissociation is complete. The evolution of A1 pressure with temperature (cf. Figure 1) exhibits an activation enthalpy of vaporization AH~ =4.4 eV. I0J

(14)

is fulfilled (equations (8) and (12)). The maximum ion current density which can be extracted is equal to the random thermal flow of ions A + leaving the surface:

40' t i

where [A ÷] is a function ofn and n a function of the gas pressure at the surface. In the case of solid ion sources, the flow of A-bearing species at the surface is equal to the vacuum evaporation rate. So, the concentration of all A-bearing species at the surface may be expressed as:

n=eP/kT

•~

Hv =

5

7<

(16)

where e is the evaporation coefficient (e < 1) and P the equilibrium vapour pressure above the solid: P=k~(T)exp

4.4eV

( 'JAM:

i 5.5

6.0 1 0 4

Reciprocal temperature

- ~-~.

(17)

TCK)

Figure l. A] partial pressure over A I N as a function of reciprocal temperature.

Table 1. Composition and vapour pressure (in atmospheres) above AIN vs temperature Vapour composition

978

i 6.5

T(K)

N

N 2

AI

AIN

2250 2000 1750 1500

1.96 × 10 - 9 2.31×10 -11 ---

7.76 x 10-3 4.69×10 '* 1.25 x 10 -5 9.76x 10 -8

1.55 × 10 2 9.38×10 -4 2.50x 10 -5 1.95×10 -7

1.11 x 10 s 8.01 x 10-11

Total pressure 2.33 x 10 2 1.41 × 10 -3 3.76× 10 5 2.93 × 10- 7

J Pelletier: A I N : a solid AI + ion source

The work function of a semiconductor is defined as the energy difference between the Fermi level and the vacuum level at the surface. Consequently its work function is linked, not only to the Fermi level in the bulk, but also to the position of the intrinsic surface state bands with respect to the bulk energy levels: a filled surface band leads to Fermi level stabilization in p-type materials, whereas an empty surface band in the forbidden zone causes level stabilization in n-type materials 7. As the sum of the energy band gap (6.2 eV) plus the electron affinity (estimated to be 2.2 eV 7) is quite high for AIN, work functions as high as 6eV can be obtained even at high temperatures, provided that p-type doping is available and that surface states are not present in the band gap. The evolution with temperature of the measured work function of an n-type AIN crystal 7 has shown that its value, 5.33 eV, is temperature independent, indicating Fermi level pinning at the surface by an empty intrinsic surface state band 7. With p-type AIN, a higher work function value could be expected. Unfortunately, for want of p-type AIN, it was not possible to verify such a possibility. Nevertheless, in the light of the work function values already obtained, AIN appears likely to produce AI ÷ ions.

[AI + ] =

nK+

(18)

and the collected ion current density [equations (8) and (15)-(18)]: j= K(T)exp(

A H : + kTEA--¢)

(19)

where K(T) is a pre-exponential factor of which the temperature dependence over the small range studied, is much smaller than that of the exponential term. It clearly follows that, if the A1÷ peak intensity accurately reflects the ion current density emitted by the solid ion source, activation energy for ion emission is (for EA> q~) greater than the activation enthalpy of vaporization. The gap observed is in agreement, within the limits of experimental accuracy, with the difference between the A! ionization energy and the measured n-type A1N work function 7. With p-type A1N, higher work functions and ionization efficiencies close to unity could be obtained. Unfortunately, it has not been possible to verify this point. Nevertheless, the results already obtained from n-type A1N have indicated some of the mechanisms responsible for thermal positive ion emission from solid sources.

The fl-eucryptite

Experimental results The n-type A1N crystal is placed on a molybdenum susceptor heated under vacuum (10- 5 Pa) up to 1850 K by a rfheater and its temperature measured by an optical pyrometer. The 200 V potential difference applied between the source and the mass spectrometer is enough to obtain the saturation current density (~- 1 pA cm -2 at 1850 K). While the neutral spectrum is rich in peaks, A1÷ appears as the only peak present in the ion spectrum. The evolution of the AI + peak intensity with reciprocal temperature is shown in Figure 2. The activation energy given by the slope, i.e. 5.2 eV, is slightly higher than the activation enthalpy of vaporization. Indeed, in the case of A1N, the dissociation constant is high (Ko>>n) and for the n-type, the ionization constant is quite low (K÷ ,~ 1). The A1÷ density becomes (equation 12):

l o3

o

.~" 2 O~

-

:

eV

10

The mechanism of Li + emission from fl-eucryptite (Li20 , A1203, 2 SiO2) or similar compounds can now be quite well explained from the above mentioned analyses. As thermodynamical data on fl-eucryptite are unknown, an accurate study of its vapour in equilibrium with the solid phase is not possible. One approach is to consider ~-eucryptite as the juxtaposition of three solid phases, Li20, SiO 2 and AI203 in equilibrium with the vapour phase. Assuming this is the case, it can be seen that, at intermediate temperatures, lithium oxide, Li20, alone is strongly dissociated6: Li20(s)--+2 L i ( g ) + 1/2 O2(g )

(20)

to give lithium in the vapour phase. If this result can be transposed to fl-eucryptite, the vaporization of lithium is accompanied by an increase in the oxygen pressure above the solid phase. Studies of the electronic properties of nonstoichiometric oxides including work functions and conductivity measurements 8-1° have shown that the thermo-electronic emission of an oxide at high temperature largely depends on the donors and acceptors present in the volume of the oxide. In particular, an increase in the oxygen pressure results in an oxide richer in oxygen. The oxide becomes more p-type and its work function is much higher. In the case of ~-eucryptite, the lithium vaporization gives rise to oxygen pressure and consequently to enrichment of the solid phase, so that its work function increases and becomes close to lithium ionization energy (5.39 eV). High ionization rates can then be observed. On the other hand, if, for one reason or another, the oxygen content in the solid phase decreases, the same occurs for the work function and the ionization rate as well. Such behaviour has been observed when fl-eucryptite is deposited on a metallic substrate acting as a reducer 4.

Conclusion I

i 5.5 Reciprocal

Figure 2.

i 6.0 temperature

1 04

i 6.5

TCK~

k l + peak intensity as a function of reciprocal temperature.

Thermal positive ion emission from solid sources such as A1N and fl-eucryptite has been discussed in the light of experimental results. A better understanding of the mechanisms involved would enable the range of ions, which can be produced by such a remarkably simple process, to be extended. 979

J Pelletier: A I N : a solid AI + ion source

Acknowledgements The author thanks C P o m o t for his constant interest in this work and J Cocagne for his assistance.

References 1 j C Beattie, Philos Mag, 48, 97 (1899). 2 G Couchet, Ann Phys, 9, 731 (1954).

980

3 F M Johnson, RCA Rev, 23, 427 (1962). 4 A Septier and H Leal, Nucl Instrum Meth, 29, 257 (1964). 5 j p Blewett and E J Jones, Phys Rev, 50, 464 (1936). 6 Thermodata, Thermodynamical data bank, BP 66, 38402 St Martin D'Heres Cedex. 7 j Pelletier, D Gervais and C Pomot, J appl Phys, 55, 994 (1984) and references therein. 8 j p Loup and P Odier, Rev Int Hautes Temp Refract, 7, 378 (1970). 9 p Odier, J P Loup and A M Anthony, Rev Int Hautes Temp Refract, 8, 243 (1971). lo j Nowotny, J Chim Phys, 75, 689 (1978).