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Submitted on 1 Jan 1986
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PULSE LASER ATOM PROBE STUDY OF GaP
T. Adachi, T. Kuroda, S. Nakamura
To cite this version:
T. Adachi, T. Kuroda, S. Nakamura. PULSE LASER ATOM PROBE STUDY OF GaP. Journal de
Physique Colloques, 1986, 47 (C2), pp.C2-293-C2-296. �10.1051/jphyscol:1986244�. �jpa-00225678�
PULSE LASER ATOM PROBE STUDY OFGaP
T. ADACHI, T. KURODA and S. NAKAMURA
Inst. Sci. Ind. Res., Osaka University, Ibaraki, Osaka 567, Japan
Abstract
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Gap tips prepared f r a n whiskers grown by the vapour phase reaction, were analyzed using our ctmbined-type atom-probe FIM by laser pulse operation instead of high voltage pulse operation. It was found that ( 1 ) The m p o s i - tion is Ga : P=l : l at cryogenic temperature 45K in vacuum of betterthan 1 x 10-Ptorr. ( 2 ) Much more cluster ions of phosphorus atan in laser pulse mode are detected in canparison to that in high voltage pulse mode.
(3) Mass resolution for laser pulse operation is high enough to resolved the hydride ions even at linear type mode. We also make clear the influences of laser beam power density irradiated on the tip for field evaporation.
The TOF atcan-probe technique has only been applied to metallic samples due to the difficulty to transmit the nano-second high-voltage pulse through sample tips of low electrical conductivity. Another problem is distortion or reflection of high voltage pulse which result in anenergy spread of the fieXd eeMprated ions.
Kellogg and Tsong proved that laser-assisted field evaporation can be used for TOF atom probe operation /l/. Using laser pulses, as v e d to high-voltage pulses, for TOF atan-probe operation eleiminates the above-mentioned problems and gives applicability to low-conductivity materials. The field evaporation of silicon in ultrahigh vacuum and at cryogenic temperature is qualitatively the same as for metal /2/, /3/. Recently, Cerezo et a1 show a pulse laser atan probe (PLAP) /l / mass spectrum for a GaAs whisker, giving a stoichimtry of &/AS = 0.99 0.03 /4/.
They also give that the value obtained for Inks whisker is In/& = 0.97
+
0.05 /4/.We have also done nearly the same experiment for Gap whisker.
As the sample of the atom probe, GaP whiskers withfhe [TT71 growth orientation were used in the same m e r as the previous experiments /5/. The resistivity of GaP was estimated to be more than 0.004nan. The sharp emitters of GaPwereprepared by dipping the specimens for a few minites in the hot solution of HNOj : HC1 (1 : 3).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986244
JOURNAL DE PHYSIQUE
The laser system employed for this experiment was a nitrogen laser (Molectron UV- 22) having a pulse width of lOns and an energy of-9mJ/pulse in a rectangular beam of-YO.7 X 3cm cross section. The adjustment of forcusing condition was made empirically by observing a field-ion image of the tip and moving the lens until the desired evaporation rate was achieved at various applied d.c. voltage. The results of the measurement are displayed in Fig. 1 which shows the applied d.c.
voltages for pulse laser field evapration of W tip at various distance frcm forcus to the tip. The tip temperature was estimated fran the applied d.c.
voltage for field-evaporation. The specimen temperature could be estimated frcm the temperature dependence of field-evaporation voltage for W tip at constant rate of evaporation /l /. In Fig. 1, specimen tip temperature of TG300K would be correspond to the distance D=7m £ran focus to specimen.
I11
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REmnTS AND D I S C U S S I ~Fig. 2(a) shows a mJlP mass spectrum for a (TOO) plane of GaP whisker, giving a stoichimtry of P/&=? .00. Analysis carried out in MCUUm of about 1 O-'Otorr at 40K which is a specimen temperature before applying the laser beam to the tip.
!the total number of events was 1668 counts per 10000 pulses. The laser beam forcused at D=12cm in front of the tip and applied d.c. voltage held at 10.55kV.
By canparison, Fig. 2(b) is a similar spectrum frcm the same Gap tip under different conditions; laser beam forcused at D=6an, the applied d.c. voltage 5.95kV and at roan temperature before applying the laser beam to the tip. Total number of events was 1886 counts per 10000 pulse. In the later case, the
measured stoichianetry was incorrect, giving P/Ga=O.81. The specimen temperature should be high enough to migrate the surface atcms. A lot of P cluser ions were observed in m s s spectrum. However there were no Ga cluser and GaP ions. In the case of field e m a t i o n at ( i f 7) plane, a lot of P'
,
pZtand P: ion were always observed in mass spectra. e mall amount of and P,? ion were detected f ran the field evaporation at (1 11 ) plane for high field evapration rate and at low p e r density of laser beam applying to the tip. Almost of our results fran( f ii)
,
give incorrect stoichicmetries (p/Ga>l) at low temperature. In all case the measured stoichianetries are sumxized in Fig. 3(a) and (b). Fig. 3 suggests that it is necessary to be found optimum conditions to achieve true canposition by varying the intensity of laser beam applying to the specimen tip and selecting the crystal plane on the tip. The mechanism for the apparent loss of phosphorus or gallium in P Whas not been fully explained at present. To achieve true acmposition, the specimen should be held at law temperature when applying the laser beam to the tip. The field-evaporation rate should also be kept at low one (less than-0.1 events per pulse). However these emration rate are still one order high canpiring with the voltage pulse atan probe at the same probe area on the tip /6/.1.2
1
Dnstance f r o m l o c u s D lcml
Fig.1
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Applied d.c. voltages V.S. distances £ran focus to the W tip. m e field evaporation rate was one monolayer per 150 pulses. The distance frcm focus is a measure of the power density of the laser beam in our system.0
0 20 40 60 80 100 120 MASS-TO-CHARGE R A T I O
Fig. 2
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Pulse laser atom probe mass spectra of ~ a ~ ( i 0 0 ) (a) the tip temperature To -40K before applying the laser beam and LklZan. (b) To-roan temperature before applying the laser beam and D=6cn./l/ G.L.Kellogg and T.T.Tig, J. Appl. Phys.
51
(1980) 1184./2/ G.L.Kellogg, Phys. Rev. (1983) 1957.
/3/ T.T.Tsong, Appl. Phys. Lett. 5 (1984) 1149.
/4/ A.Cerezo, C.R.M.Grovenor and G.D.W.Smith, Appl. Phys. Lett. 5 (1 985) 567.
/5/ Y.Ohno, S.Nakamura, T.Adachi and T.Kuroda, Surface Sci.
69
(1977) 521 , /6/ M.Yamamoto, D.N.Seidman and S.Nakamura, Surface Sci,118
(1982) 555.This work was partly supported by a Grant-in Aid for Scientific Research frcm The Ministry of Education Science and Culture.
JOURNAL DE PHYSIQUE
p i s t a n c e f r o m f o c u s t o s p e c i m e n D ( c m )
Fig. 3(a)
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Variation in measured stoichimetry for analysis of &P using pulse laser for different sample distance £ran forms point.
To -+40' K, P--l0-'"torr .
2.0
1.5
0
ld W
,1.0
W
\ PI
0.5
0
0 (ioo)
*
(111)-
e (Tic)4 (110) e
0
a
3'
@ * a
-
X 84
X
I I I i I I
l 4 6 8 10 1 2
D i s t a n c e f r o m f o c u s t o s p e c i m e n D (cm)
Fig. 3(b)
