GaAs Field Emitter Arravs

1

J

R . Z. Bakhtizin, S. S . Ghots. and E. K . Ratnikova

I Abstract-The paper reports on technological peculiarities of

fabrication of large-area gallium arsenide multipoint cathodes; the results of emission characteristics investigation are de- scribed. Expressions for the noise power spectral density func- tion and the time constant of degradation for such cathode ar- rays have been obtained.

I . INTRODUCTION ALLIUM ARSENIDE (GaAs) is believed to be a G promising material for development of large-area ef-

ficient sources of electrons and a new type of photocath- odes owing to high mobility of electrons. short lifetime of the minor current carriers. and broader forbidden zone as compared with that of Si and Ge. The above features present an opportunity to fabricate, for instance, photo- detectors which are capable of operating in a wide spec- tral range at relatively high temperatures and are remark- able for a low level of dark current.

However, there are several obstacles to the realization of the above-mentioned features. As opposed to Si, gal- lium arsenide does not produce a stable, high-quality . and easily formable natural oxide. Moreover, GaAs is a two- component compound; the GaAs surface is known to be more sensitive to the influence of various chemical sub- stances. Thus we were forced to develop a new approach to fabrication of the multipoint structures.

11. FABRICATION It is a common practice to fabricate multipoint cathodes

by means of a photolithographic method with subsequent chemical etching [ 11, [2]. By optimizing the above method by making use of the dielectric Ta205 sublayer, we are able to produce the multipoint cathodes on various semi- conductors, including GaAs [ 31. Having introduced a new procedure in the traditional photolithographic method, i.e., the formation of columns, we succeeded in a much more dense arrangement of the tips on the multipoint cathode. Fig. l(a) shows a scheme of the cathode fabri- cation.

As a result, we fabricatet tip matrices with the follow- ing parameters: r < 1000 A , heights from 5 to 30 pm at densities up to 1.0 X 10h/cm2, and the total area is 1 to 4 cm2. Fig. l(b) demonstrates a typical tip matrix ob- tained on the basis of GaAs according to the modified method.

Manuscript received Jul ) 2 5 . 1990. The authors are with the Department o f Physical Electronic.\. Bashkir

State Univcrsity. 450074 Uta, USSR. IEEE Log Number 9101922.

2

Fig. I . ( a ) Main \rage\ ol lahricarion ot GaA\ l icld eiiiitter arra)a. l - Cleaning o l iiionocr)\taIIinc platc I I ) : a plate I\ c(rated mith a tantalum mask ( 2 ) by mean\ 0 1 i nagwt ron \puttc.ring. 2-Oxidation 0 1 Ta to oxide and production ol protcctcd Tn,O, mash ( 3 1 . 3-b'oriiiation ( i t ' the poin t structure 14) b) i(in-plnsm;i etching 4 --Rcmo\al o l thc pi-otecti\e mask h) a high-frequency discharge i n the .irgon mediuiii. ( h ) A dctail of the tip matrix

0018-938319111000-2398501 00 c 1991 l t t t

BAKHTIZIN cf al.: GaAs FIELD EMITTER ARKAYS 2399

Initially, liquid-chemical etching in a standard etchant was used for fabrication of the tip structures which was later replaced by ion-plasma etching. Due to the high vol- atility and fluidity of arsenic we had to reduce: i) the du- ration of various operations; ii) the temperature of the plate treatment; and to completely automate the techno- logical process. Auger spectroscopy and secondary ion mass spectroscopy were used to check the treatment of GaAs surface, since it had been experimentally found that depending upon the discharge power there can arise con- siderable changes in the structure of the surface layers during the ion-plasma etching of two-component mate- rials; the above changes being caused by preferential sputtering of one of the components and differential intru- sion of the surface atoms in the crystal lattice.

A subsequent comparative analysis of emission char- acteristics of GaAs cathodes showed that the ion-plasma etching was likely under certain conditions to reduce the level of the dark current as compared to that of the sam- ples obtained by chemical etching. One possible reason for that is a decrease of the surface generation velocity for the current carriers due to the surface disorder resultant from such a treatment. Also, concentration of the deep levels in the subsurface layer may vary during the ion- plasma treatment, which is dependent on the concentra- tion ratio, of Ga and As atoms, i.e., it depends on the amount of deviation from the stoichiometry. Fig. 2 shows typical current-voltage characteristics of p-GaAs multi- point cathode taken at different modes of operation.

111. NOISE CHARACTERISTICS AND UNIFORMITY OF EMISSION

It is known that stability and high uniformity of emis- sion over the whole emitting surface are crucial factors determining the range of applicability of the GaAs mul- tipoint cathodes. To solve the problem it is desirable to investigate the fluctuation characteristics of the GaAs multipoint cathodes and to appropriately correct the pro- cess of their fabrication.

Fig. 3 shows typical frequency dependencies of the power spectral density function S( f ) of the field emission current noise for the p-type GaAs multipoint cathode; the measurements were taken in the Fowler-Nordheim region of the current-voltage characteristics. Numerous mea- surements have shown that dependencies obtained for this frequency region are well approximated by the following expression:

-7 i -8 -

-9 -

-10 -

0.2 0.4 1.0

Fig. 2 . Current-voltage characteristics of 4 6 0 4 . cm p-GaAs emitter ar- ray (formed on A( 100) surface) at T = 90 K . I-in darkness; 2-with light excitation.

0 0 I o O

10"

,c IO" v

m

> rn

% w t:

a

p1

0 2

' 1 O

0

0

0

* 0

0 . 0

0

IO0 10 102 lo3 [Hzl FREQUENCY {

Fig. 3 . Spectral density function of the field emission current noise for 3 0 4 . cm GaAs multipoint cathode at field emission current I = 1.9 X

A. I--T = 90 K: 2--T = 300 K .

ters I 2

D(Z) = - 2N

where &(I ) is the value dependent on the field emission current and y is the spectral density function index. This enables one to describe the current fluctuations in terms of the previously proposed statistical model of the quasi- stationary noise [4]. The above model is capable of relat- ing the power (dispersion) of the field emission current fluctuations with the average number N of emission cen-

and it enables one to determine the value N on the basis of measured field emission current I and dispersion D.

As it turned out, the number of emission centers for cathodes with a similar area and number of tips is strongly dependent on the method of surface treatment. During ac- tivation, an increase in the average field emission current was observed to correspond to an increase in the number

IEEE TRANSACTIONS ON ELECTPrY DEVICES. VOL 38. NO IO. OCTOBER 1991

Fig. 4. Distribution of the field emission over the cathode surface when focused on a closely located phosphor screen.

of emission centers. Preliminary ionic treatment of the cathode surface is favorable for a more uniform start of the separate tips. Fig. 4 shows the field emission distri- bution pattem at accelerating voltage of 4.5 kV (the pat- tern is focused on a closely located phosphor screen an- ode).

In view of a strong dependence of the flicker noise upon technology of the multipoint cathode fabrication and tak- ing into account the fact that even similarly fabricated cathodes with very close electrophysical properties show different l/f noise intensities it may be stated that the flicker noise is related to various processes of creation and migration of defects in this material.

IV. LIFETIME A N D DEGRADATION OF SEMICONDUCTOR MULTIPOINT CATHODES

When carrying out noise investigations it is interesting to detect the fluctuation spectrum components with period comparable with the constant of degradation time T,/ of the cathode. In most cases, direct registration of such components is impossible because the measurement time t,, is usually far shorter than T( / , and to obtain the required statistical accuracy it is necessary that the period of the lower limiting frequency T, should be, at least, a fraction oft,,. It has been found, however, that for a case of flicker fluctuations it is quite possible to detect the influence of the slow emitter degradation upon relatively high fre- quency components of the noise current.

For the stationary noise dispersion it may be written

D ( I ) = i: SCf, I ) df (3)

where fu andfi are the upper and lower limiting frequen- cies of the flicker noise, respectively, and A, >> f,. Sub- stituting (1) and ( 2 ) into (3) gives

From a practical point of view, the most important is case (6), where y > 1, because it is always realizable for the field emission. Moreover, (6 ) enables one to estimate ex- perimentally the lower limiting frequency of the processes in the cathode, and thereby provides an estimate of its degradation constant time

1 7,/ = 1.. (7)

J /

For the purpose of using the estimate (6) for determina- tion of fi and 7,/ it is necessary to identify the number N of emission centers independently from the noise mea- surements. For instance, it is possible to find the value of N proceeding from the condition of screening of the ex- temal electric field. It has been shown in [8] that, at least for frequencies fo close to 1 Hz, it is possible to rewrite (6) in the form

“Y - I

where fo = 1 Hz and (Y = 2 X I O p 9 is a constant which was found on the basis of (6) and estimates of N from the condition of screening of the external field. It should be emphasized that (8) is close to the well-known Hooge re- lationship for y = 1. Comparing (6) and (8), we find

(9)

This equation may be immediately used to determine the multipoint cathode lifetime (the degradation constant).

The measurements of fluctuation spectra of the new prepared and placed into the vacuum specimens gave the following estimates for the spectral density function in- dices: y I = 1.33 at T, = 90 K ; y2 = 2.01 at T2 = 300 K (see Fig. 3). Equation (9) yielded, respectively, T, /~ = 5 . 2 x lo6 s = 60.7 days and T,/? = 475 s = 8 min, which is quite realistic (at field emission current I = 1.9 x lo-‘ A).

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