ERRONEOUS COMPOSITION OF GaAs MASS-ANALYZED BY ATOM-PROBES

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ERRONEOUS COMPOSITION OF GaAs MASS-ANALYZED BY ATOM-PROBES

O. Nishikawa, H. Kawada, Y. Nagai, E. Nomura

To cite this version:

O. Nishikawa, H. Kawada, Y. Nagai, E. Nomura. ERRONEOUS COMPOSITION OF GaAs MASS- ANALYZED BY ATOM-PROBES. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-465-C9-470.

�10.1051/jphyscol:1984977�. �jpa-00224466�

JOURNAL DE PHYSIQUE

Colloque C9, supplément au n012, Tome 45, décembre 1984 page C9-465

ERRONEOUS COMPOSITION OF GaAs MASS-ANALYZED BY ATOM-PROBES O. Nishikawa, H. Kawada, Y. Nagai and E. Nomura

Department of MateriaZs Science and Engineering, The Graduate SchooZ a t Nagatsuta, Tokyo I n s t i t u t e of TeehnoZogg, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan

Résumé - L'analyse en masse par sonde

atome de GaAs donne souvent une composition trop riche en As. La raison du déficit en atomes de Ga a été attribuée

l'évaporation préférentielle de Ga au faible champ exercé par la tension continue pendant les intervalles entre deux impulsions de tension. Dans le but d'examiner cette hypothèse, la relation entre le nombre d'ato- mes Ga manquants et celui des atomes évaporés de champ en ten- sion continue a été cherchée en employant une sonde

atomes avec un temps de vol rectiligne. Le nombre d'atomes évaporés à tension continue diminue presque jusqu'à zéro lorsque la température de la pointe est abaissée et que la fraction de tension d'impulsion est augmentée. Cependant, le nombre d'ato- mes As détectés reste supérieur

celui des atonies de Ga, même

à

110 K. Une étude plus poussée est nécessaire pour expliquer cette richesse en As de la composition.

Abstract - The atom-probe mass analysis of GaAs often gives an As rich composition. The cause of the deficiency of a large number of Ga atoms detected was attributed to the preferential field evaporation of Ga at low field exerted by the dc voltage during the intervals between superposing voltage pulses. In order to examine this presumption, the relation between the number of deficient Ga atoms and that of field evaporated atoms at dc voltages was investigated using an atom-probe with a straight flight path. The number of evaporated atoms at dc voltages decreased to nearly zero as the tip temperature was lowered and the fraction of pulse voltage was increased. How- ever, the number of the detected As atoms remained more than that of Ga even at 110 K. Further study is required to explain the observed As rich composition.

1 - INTRODUCTION

Since the introduction of the atom-probe, it has been expected that its unique capability, atom-by-atom mass analysis, would lead to a new approach to the atomically detailed study of metal-metal and metal- semiconductor interfaces. This application to the study of the initial stage of silicide formation[l-31 and of the Al-Ga exchange reaction at the Al-GaAs interface[4] was successfully demonstrated.

However, it has been also noticed that the atom-probe tends to fail t0 give the stoichiometric composition of compound semiconductors[5,61.

In the case of GaAs the number of As ions detected is usually much

larger than that of Ga ions. It was assumed that the preferential

field evaporation of Ga under the field exerted by the dc voltage

applied to the specimen tip during the intervals between successive

voltagepulsescauses the deficiency of Ga ions. Accordingly, the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984977

C9-466 JOURNAL DE PHYSIQUE

condition which suppresses the preferential field evaporation of Ga and gives the stoichiometric composition is searched.

II - EXPERIMENTAL AND RESULTS

The atom-probe with a 75 cm straight flight path was used. The repetition rate of voltage pulse superposed on the dc voltage was fixed to 15 per sec. Each pulse triggers the 8-channel timer with the time window of 8 msec. GaAs of three different resistivities was used, Table 1. Figure 1 is the histogram obtained by analyzing the l a c m Table 1 - Physical values of GaAs specimen.

Orientation Dopant Concentration Resistivity [O011 Si 3.6 x 10 18 cm-3

1.1 x 1 0 - ~ Q cm [0111 Zn 2.0 x 1019 cm-3 3.5 1 0 - ~ I 0111 si 2.0 x 1016 cm-3 1.0 n cm specimen at room temperature. The smallest mass-to-charge ratio Ms is about 81 for the pulse voltage V =0.75 kV, Fig. 1 (a), and shifts to the larger ratio side for V p = 1 . : kV, Fig. 1 (b) , and then returns to the smaller side again for V p = 1.5 and 2.0 kV, Fig. 1 (c) and (d), respectively. The variation of Ms with temperatures and Vp is shown in Fig. 2. For V p < 1 . 5 kV, Ms drastically decreased from -85 to -75 when the temperature was lowered below 200 K and then gradually

shifted to the larger ratio side with the increase of Vp. For the low resistivity specimen Ms is close to 69 for low Vp, indicating the detection of 6 9 ~ a + , Fig. 3. However, the variation of Ms with Vp is similar to that for the I R c m specimen at low temperatures and no temperature effect is seen. The gradua1 shift of Ms to the larger ratio side with V is due to the voltage drop of Vp while the pulse is travelling througg the semiconductor needle. Since Vp drops more than 50"sor the l n c m specimen, the shift of mass of detected ions to the larger side is significant. Thus the ions giving Ms more than 80 in Fig. 2 are As ions and the ions giving M s - 7 5 below 200 K do not correspond to As but to Ga ions. This indicates that no Ga atoms exist on the surface at the moment of the application of pulse and remaining As atoms only are evaporated. Thus the appearance of Ms- 75 at low temperatures suggests that the reduction of the specimen temperature suppresses the Ga evaporation.

Since the moment of evaporation is not regulated for the ions evapo- rated at Vdc, the flight tirnes of these ions can not be rneasured and these are detected as randomly incoming unidentifiable ions. Flight times of the ions with the mass-to-charge ratio m/n less than 400 are only a few microseconds and are a minute fraction of the time window of the timer, 8 msec. Thus, the ions with longer flight times were counted as the ions evaporated at Vdc.

The number of ions evaporated at dc voltages Ndc is relatively small for the low resistivity specimen and it reduces to nearly zero as the pulse fraction Vp/(Vp+Vdc) increases and/or temperature decreases, Fig. 4. For the 1 R c m specimen Ndc is very large at room temperature and for the pulse fraction below 15%,Fig. 5. As the pulse fraction increases, Ndc sharply decreases corresponding to the decrease of Msr that is, the appearance of Ga ions shown in Fig. 4. At lower temper- atures the variation of Ndc in Fig. 5 is quite silimar to that

observed for the low resistivity specimens shown in Fig. 4.

Mass to charge ratio m/n

Fig. 1 - H i s t o g r a n i s o f G a A s w i t h t h e r e s i s t i v i t y of 1 . 0 n c m m a s s -

a n a l y z e d a t room t e m p e r a t u r e . ( a ) V d c = 7 . 0 1 k V a n d V p = 0 . 7 5 kV. ( b )

V d c = 7 . 0 6 kV a n d V = 1 . 0 kV. ( c ) Vdc

6 . 6 1 k V a n d V p = 1 . 5 kV. ( d ) V d c =

6 . 5 1 k V a n d V p = 2 . 9 k v .

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701 1

0.5 I 1.5 2 2.5

Pulse voltage Vp

k V )

A

300 K

a 2 3 0 K

0

2 0 0 K

o 1 4 0 K x I I O K

L

0.5 1.0 1.5 2 .O

Pulse voltage Vp ( k V ) I I I - DISCUSSIONS

2 - Variation o f the smallest number of

l ^Fig.

mass-to-charge ratio Ms with the pulse voltage Vp a t ver ious tempera- tures. The resistivity of the specimen: 1 cm.

Fig. 3 - Variation o f

with Vp at various temperatures.

Resistivity

3.5 x 10-3 n cm.

The observed results indicate that the preferential evaporation of Ga

can be effectively suppressed by lowering the specimen temperature

and/or increasing the pulse fraction. Under this condition the ratio

of the number of As ions detected NAS to that of Ga ions NG, should

be 1 : 1. Figure 6 shows the variation of N A ~ / N ~ ~ with Vp for the

Fig. 4 - Variation of Ms with pulse fraction Vp/(Vp+Vdc) at room temperature and llOK. Resistivity

1.1 x 10- 3 R cm.

1

160 -

Roorn ternp.

:

230 K a

A

0 . 2 0 0 K

:

1 4 0 K x : I I 0 K

h

.-

^a

.t; E ⁸⁰

C

O

. - _a

V

O

z"

1 A-

10 2 O 30 40

V p / ( V p + V d c ) (%)

Fig. 5 - Variation of Ms with pulse fractions at various temperatures.

Resistivity

1.0 Rcm.

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1 1 1 1

X

O

A

O

( 0 ) : l . 1 ~ 1 0 ~ ~ Qcm (Si doped)

-

x

3.5x10-~ ncm (Zn doped) -

i X ) : l . O n c m ( S i d o p e d ) 110 K

-

Fig. 6 - Variation of the ratio of the number of As ions detected to that of Ga ions with pulse fraction at 110 K.

three specimens. NA^ is always larger than NG,, even if temperature is as low as 110 K and the pulse fraction is more than 30

This suggests that the Ga evaporation is not the fundamental*factor which causes the erroneous composition. The other plausible explanation is the diffusion of Ga £rom the tip apex to the tip shank. However, the Ga diffusion at 110 K and below is very unlikely. Another possibility

is the evaporation of Ga as GaH+ and the dissociation into G ~ + H + immediately after the field evaporation as discussed by Yamamoto et al.

to explain the Ga rich composition obtained by mass analyzing GaP[6].

The neutral Ga atoms would not be detected effectively because of their dispersed flight path and cause the deficiency of detected Ga atoms. However, the single bond energy of GaH(-3eV) is too large to be dissociated and the number of H+ detected in the present study is negligibly small to compensate for the missing Ga atoms.

Accordingly no satisfactory explanation is available for the erroneous composition of GaAs and further study is required to elucidate the observed As rich composition.

REFERENCES

[l] Nishikawa, O., Tsunashima, Y., Nomura, E., Horie, S., Wada, M., Shibata, M., Yoshimura, T. and Uemori, R., J. Vac. Sci. Technol.

B1 (1983) 6.

121 Z s h i k a w a , O., Nomura,E., Wada!M., Tsunashima,Y., Shibata,M., Yoshimura,T. and Uemori, R., J.Vac. Sci. Technol. a (1983)lO.

[3] Nishikawa,O., Shibata,M.,Yoshimura,T. and Nomura,E.,J.Vac. Sci.

Technol. B2 (1984) 21.

[4] ~ishikawa,~., KanedaIo., Shibata,M.and Nomura,E., Proc. 31st Intern. Field Emission Symposium (Paris, 1984).

151 Tsong, T. T., Ng, Y. S., Surf. Sci. 77 (1978) L187.

[6] Yamamoto,M., Seidman, D.N. and ~ a k G u r a , S . , Surf. Sci. 118 (1982)

5 5 5 .