Collisional desorption of Cs+ and CsX+ ions from GaAs by Cs+ impact

Nuclear Instruments and Methods in Physics Research B 90 (1994) 482-486 North-Holland

Beam Interactions with Materials & Atoms

Collisional desorption of Cs’ and CsX+ ions from GaAs by Cs’ impact Edward W. Thomas * and David McPhail Department of Materials, Imperial College of Science Technology and Medicine, Prince Consort Road, London SW7 ZBP, UK

During Cs+ bombardment of a GaAs matrix there is a high flux of sputtered Cs+ ions which rises exponentially with fluence; equilibrium is reached within a projectile Cs+ dose comparable with the surface density of a Cs monolayer on GaAs (4X 1014 atoms cm-*). Signals of CsGa+ and C&s’ are respectively two and three orders of magnitude smaller but exhibit similar behavior. The observations are consistent with the incident Cs’ Ions . not occupying sites related to their range distribution but rather precipitating immediately to the surface. On the basis that the subsequent ejection process is collisional desorption the cross sections for Cs desorption from a GaAs surface is evaluated as 1.61 X lo-l5 for 12 keV impact at 45” from the surface normal. Cross sections for desorption of the molecular species are similar. The variations of the cross sections of the three species have been studied for impact energies from 4 to 12 keV and impact angles from 0 to 60”. Implications for understanding Csf profiling in SIMS will be discussed.

1. Int~uction Secondary ion mass spectrometry (SIMS) is often performed with a Cs+ probe in order to enhance the production of secondary negative ions and so obtain optimum detection sensitivity. Cesium induced positive molecular ions, CsX+, are also present and may have advantages for monitoring concentration of an impurity X. It has been suggested that the signals of the CsX’ species are linearly proportional to X concentration even at high densities [1], and variations in sensitivity between species X are small [Z-4]. Moreover, the CsX+ signals are free of unexplained variations at interfaces

El. We have studied ejection of Cs+, CsGa+ and CsAs+ ions ejected by Cs+ impact on GaAs. The signal from a Cs related species will initially, at the start of the ~mbardment, be zero and then increase with implanted dose. By monitoring signals as a function of fluency at low ion beam current densities we demonstrate that the Cs atoms precipitate to the surface and that subsequent removal of the Cs related ions is consistent with a collisional desorption mechanism. In order that signals should be related only to the incorporation of the incoming Cs projectiles it is quite essential to eliminate other changes to the sample which may change signal levels. We therefore perform

* Corresponding author, School of Physics, Georgia Institute of Technology, Atlanta, GA 30332-0430, USA, tel. + 1 404 894 5249, fax + 1 404 853 9958.

a prelimina~

bombardment with Xe’ to both sputter clean the surface and also to create a damaged layer characteristic of the collisional processes that would be observed with Cs+. Only after this preliminary treatment were measurements undertaken.

2. Experiment The studies were performed with a commercial quadrupole based SIMS instrument model ADIDA 6500 manufactured by Atomika. This is provided with both a gas ion source for Xe+ and a cesium ion source which may be used independently to erode the sample. Angle of incidence between the target surface and each incident beam may readily be varied from 0 to 90”. Ejected ions are mass analyzed in a quadruple, placed at approximately 45” to the plane containing the target surface normal. Projectile beams are rastered over a 400 micron square area (measured perpendicular to the beam). To avoid irregular signals from the crater edge, only signals from the central 25% of the area were recorded. When the target is inclined at an angle B to the beam (measured from the surface normal) then the bombarded area is increased by COS-~@ this factor must be accommodated when analyzing the data in terms of incident beam current density. The samples were MBE grown GaAs with a Si doping level of about 1017 atoms cmm3. Nominally the surface was (110) but it is likely that the heavy damage caused by ion bombardment will amorphize the sample. Target

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E. W. Thomas, D. McPhail/ Nucl. Instr. and Meth. in Phys. Res. B 90 (1994) 482-486

chamber pressure was 5 x lop9 Torr or lower. To emphasize the transient behavior all studies were performed at low ion beam currents of approximately 2 to 5 nA. The system includes an energy filter in front of the quadrupole. A bias on the target and the setting of the energy filter determine a window of the ejection energies in which ions are detected. Operating conditions were chosen to give maximum ion signals; at this point the signal levels are insensitive to small variations in ejected ion energy distribution. All measurements presented here are relative as a function of dose, angle and energy; we did demonstrate that the observations of relative variation were independent of changes to the energy filter window and to target bias. The mass spectrum of positive ions ejected from GaAs by Cs+ impact shows significant signals of Cs+ from the implanted projectile, and of Ga+ and As+ from the matrix. There are also signals of the diatomic combinations of the matrix atoms with the projectile, CsGa+ and CsAs+, as well as signals of Gal, As; and GaAs+. When sputtering the target with Xe+ the same matrix species are observed and those involving cesium are absent. We discuss here only the data for the Cs related species; a full discussion of the other ions will be presented elsewhere. An important experimental step is a preliminary bombardment of the sample with Xe+ at 12 keV energy. Oxygen signals decreased rapidly with dose indicating cleaning of the surface. The ion signals from matrix species also changed with Xe+ dose indicating a change to ejected ion fractions related possibly to both oxygen contamination and also to radiation damage. The preliminary bombardment was continued until the oxygen signal disappeared and all ion signals became constant. For 12 keV energy at 45” incidence this required a dose of about 1.5 x 1015 ions cm-‘. After a preliminary bombardment with Xe+ we changed to the Cs+ beam with a smaller rastered area and directed into the center of the preliminary crater to avoid edge effects. All data for Cs+ induced signals were obtained in this mode. The projectile beam flux was measured as the current into a Faraday cup in the target plane. The recessed geometry of the cup and the use of retarding potential inhibits loss of secondary charged particles. Comparing ion currents into the Faraday cup (representing the ion beam flux) with apparent currents to the GaAs when a 50 V positive bias was applied to the surface (representing the sum of incident ion flux and ejected positive ion flux) we measure a total yield of ejected positive ions under equilibrium conditions of approximately 0.4 (ions out per ion in) at 45” incidence and 0.7 at 60” both for 12 keV Cs+ impact. From the mass spectra it is clear that the dominant positive ion is Cs+ implying that ejected Cs has almost unity probabil-

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ity of being ionized. An earlier observation of a high ionization efficiency of ejected cesium has been reported by Wittmaack in studies of bombardment of Si [6]. Apparently the ionization probability of Cs ejected from GaAs is approximately unity; an important observation which we shall utilize later.

3. Observations Fig. 1 shows examples of Cs+ signal variation with time during Cs+ bombardment. Clearly the Cs+ signal increases exponentially with fluence towards an equilibrium value. The density of a Cs monolayer adsorbed in GaAs (110) has been measured as 4.0 X lOi atoms cm-‘, or 0.9 atoms per GaAs (110) unit cell [7]. Note that the Cs+ signal has reached approximately half its equilibrium value after a dose of 4 X lOi Cs+ ions cmm2. The density of Cs at the surface will be governed by a competition between an increasing density due to the arrival of Cs in the ion beam and a decreasing density due to sputter erosion of the target. The rate of removal of a species i (such as the cesium) may be expressed in terms of the partial sputtering yield yi and the ion beam flux I, (particles s-l cm-‘) by Ri = yil,.

(1)

The rate of removal Ri (atoms of species i ejected per second from unit area) can also be related to the area1 density Ni (atoms cmP2) of the species and it is more satisfactory [8] to write Ri = NiuDZB.

(2)

The proportionality constant uo will have the dimensions of cross section (cm’) and in cases where Eq. (2) is used to describe the ejection of an adsorbed species, then Ni is the species surface density and I_T,, is the desorption cross section. The value of uo will be independent of surface coverage for conditions where a single binding energy is appropriate. The mass spectra show that the Cs+ signal is two orders of magnitude greater than other species, suggesting that the removal of Cs is principally as the monatomic species. Let us for the moment assume that the signal of Cs+ is proportional to the total rate of Cs ejection and that in turn, through Eq. (2), is proportional to the near surface density of Cs. One possible model to represent the surface Cs density is that the cesium implants to a depth distribution related to range and then the surface erodes into the distribution thereby revealing the Cs. We have simulated the surface density of Cs on this basis by a procedure similar to that shown diagrammatically by Williams and Baker [9] and to the analytical treatment of Schulz and Wittmaack [lo]. The TRIM code (verVI. SPUlTERING/DESORPTION

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E. W Thomas, D. McPhail/Nucl.

Instr. and Meth. in Phys. Res. B 90 (1994) 482-486

700

Combining the predicted surface density Ni with Eq. (21, and assuming as before that the Cs+ ion signal is proportional to the surface concentration, then the variation of the signal with time is given by

600 500

S = Sin[l - exp( -Zea,t)],

I

0

20

40

60

60

100

120

I40

STEP

Fig. 1. Cs+ signal as a function of the experimental time step for 12 keV bombardment of GaAs at 45”, 30” and 15” (in descending order) from the surface normal. The projectile Cs+ beam current (2.75 nA) provides the following fluxes of incident particles for each experimental time step: 6.1~10’~ at 45”, 7.4~ 1013 at 30” and 8.3 X 1013 at 15”, each in particles incident per cm2 per time step. Data points are the experimental signals. The lines are the fittings of Eq. (5) with (TV = 1.61 X 10-15, 9.2~ lo-l6 and 8.0X lo-l6 cm’, respectively. The isolated data points are the predictions of the implant erode model for the 45” case alone.

sion 91.14) is used to calculate both range and sputtering information. The resulting density of Cs is assumed, through Eq. (2), to be proportional to the Cs+ signal and for one case is shown normalized to the Cs+ signal on Fig. 1. There is no correspondence between the behavior of the Csc signal and the predicted Cs density. The Cs+ signal attains its equilibrium value more rapidly than is predicted on the basis that the surface must erode to the implanted distribution. Our alternative mode1 is derived from the observation that the Cs+ signal very rapidly reaches saturation at a cumulative dose comparable with the surface density of a Cs monolayer on GaAs. We propose that the incoming Cs+ does not remain at the depth to which it is implanted, but rather precipitates immediately to the surface. The rate of arrival at the surface would then be equal to a fraction F of the incident ion beam flux density I,. The fraction F would include a correction for those incident Cs+ ions which are kinematically reflected; this is expected to be of the order of one percent or less [ll]. The rate of removal is given by Eq. (2), leading to a time dependence of the Cs surface density

dN. 2 =Z,F-JBNiu,, dt which, with the boundary condition that the surface density at the start of bombardment is zero, integrates to give N,=&[l-exp-Z.u,t)].

(4)

(5)

Where Si, represents the signal at saturation. Eq. (5) exactly represents the form of the observed signals and by fitting to observations one may arrive at the cross section u,,. Such a fit to the data is shown in Fig. 1 and the agreement is excellent. The behavior of the CsGa+ and CsAs+ signals are very similar to those of the Cs+ ions; the signal rises to reach an equilibrium within a fluence comparable with the density of an absorbed monolayer of Cs and can be accurately fitted with Eq. (5). The main difference with the Cs+ is that the signals are respectively two and three orders of magnitude smaller. Space does not permit display of the experimental data but in Table 1 we have given selected values of the apparent desorption cross section uo. Based on the reproducibility of the cross section evaluations we estimate the uncertainty in un to be f 10% for Cs+, f 20% for CsGa+ and &30% for CsAs+. The lower reliability for the molecular species relates to the lower signal levels and therefore decreased statistical reliability. As a subsidiary experiment we also performed direct measurements of sputtering coefficients by measuring the volumes of the sputtered craters after extended bombardment using a Tenco Alpha Step-200 profileometer. Crater depths were typically 1500 A and corresponded to a fluence almost two orders of magnitude greater than that at which the Cs+ signal saturated; thus the sputtering coefficients are for GaAs with an equilibrium density of Cs. Sputter coefficients as a function of angle are shown in Fig. 2; we estimate the reliability as &lo%, based on the repeatability of the measurements. The measured yields are approximately 30% lower than calculations using the TRIM code (version 91.14). Sputtering yields were also measured for Xe+ ions under similar conditions and were

Table 1 Apparent desorption cross section (ho for desorption of Cs, CsGa and CsAs, obtained by fitting Eq. (5) to the data Energy

12 12 12 12 12 6 4

[keV]

Angle

Cross section

[degl

Cs

uo [IO-l6

cm’]

CsGa

CsAs

60 45 20 15 0 45 45

23 16 9.2 8.0 6.2 31 41

13 8.8 7.4 5.9 5.2 23 28

29 20 13 10 8.5 51 57

E. W. Thomas, D. McPhail /Nucl. Znstr. and Meth. in Phys. Res. B 90 (1994) 482-486

Fig. 2. Variation with angle of the sputtering yield and oo cross sections for 12 keV Csf impact on GaAs. The sputtering yield is in ions/atom (bold line). Cross sections (ho for removal of Cs, CsGa and CsAs are shown in units of lo-l6 cm*. The lines connecting the data points are for guidance only.

essentially the same values as Cs+; since the two species have similar masses and should exhibit similar physical sputtering, this observation suggests that chemical to erosputtering by Cs+ is not a major contribution sion rate. As expected the sputtering coefficients are very high, 15 atoms per ion for 12 keV Cs+ at 45” incidence angle.

4. Discussion Our fitting procedure has related a model for Cs surface density to the measured Cs+ ion flux and assumes that the fraction of ejected Cs emerging in the ion state is independent of the Cs density. Let us examine that assumption. Traditionally the probability of ionization is considered to be related to the difference between the ionization potential of the ejected species (here Cs) and the work function of the target through the electron tunneling model (see, e.g., ref. [12]). If one assumes that the implanted Cs causes the work function to change, then the ionization probabilities should be altered in a predictable manner. Using measurements by Heskett et al. [7] of how the work function relates to the density of a Cs overlayer on GaAs, one may use the electron tunneling model to predict that the ionization probabilities of Ga and As should decrease by two orders of magnitude during bombardment to saturation. While the signals of Ga+ do decrease by that order of magnitude, the signals of As+ are essentially invariant, contrary to the tunneling model. The model would also predict that the ionization probability of Cs should remain unchanged at unity until a Cs density of 1 x 1014 Cs cm-’ is reached because at lower densities the work function of the surface will be greater than the ionization potential of

485

Cs. For higher coverage the ionization probability should decrease by about two orders of magnitude towards saturation in a manner similar to that indicated by Yu [12] in a study of adsorbed Cs sputtering from Si. This is not consistent with our observations of ion ejection yield discussed in section 2, which show ionization efficiency to be approximately unity at saturation. Moreover, the data of Fig. 1, and the many other data sets not reproduced here, show that the model with a constant ionization probability reliably predicts signal changes at low dose (where the ionization probability is expected to be unity and constant) just as well as at high dose (where the electron tunneling model suggests a probability of the order lo-‘). We would conclude that the electron tunneling model related to the work function does not predict the observed behavior and that for the Cs+ signals the assumption of a constant ionization probability is valid. This does not necessarily imply a disagreement with the work of Yu [12] on the variation of Cs ionization probability with the work function for adsorbed layers of Cs on Si. In that case the projectile (Ne+), energy (500 eV> and fluence were chosen to produce negligible surface damage. In the present case with sputtering coefficients of 15 or more, as shown on Fig. 2, the ionization probability is likely to be governed more by the local properties of the collision cascade rather than an average property such as the work function. In Fig. 2. we show the angular variation of (ho, compared with the sputtering coefficient to which they are normalized. Note from Eqs. (1) and (2) that the sputtering yield can be written in terms of a cross section for ejection. The similarity of the four curves in Fig. 2 suggests that total sputtering and the ejection of the three specific ions arises from the same type of collisional event. There are no previous studies of this type which provide data with which the observed cross sections may be compared. We do note, however, that they are of the same magnitude as true desorption cross sections for removal of heavy adsorbates by ions of comparable mass [8]. Moreover, the increase of a factor of three from 0 to 60” incidence angle is also comparable with the behavior of desorption. Wittmaack [13] has presented measurements of saturation signals of CsGa+ and CsAs+ for 15 keV Cs+ impact on GaAs using an instrument similar to the present work. The observed change to saturation signal levels between 15 and 45” are consistent with the observations shown in Fig. 2.

5. Conclusion The most important observation is that the saturation signals related to Cesium approach their equilibrium in a rather small fluence, corresponding to injecVI. SPUTTERING/DESORPTION

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E. W; i%omas, D. McPhail /Nucl.

Ins&. and Meth. in Phys. Res. 3 90 (1994) 482-486

tion into the sample of approximately one monolayer. The fluence dependence is consistent with a desorption model for which the cross section is similar to those for heavy particle desorption processes. The rapid rise to equilibrium may explain why, in SIMS, the Cs+ profiling through interfaces provides a good measure of material concentration while in the profiling with 0; there are significant irregularities which render accurate measures of density impossible [5]. In the case of oxygen the equilibrium composition of the surface is only established after erosion of a

layer comparable with the oxygen range and during this period changes to surface composition give rise to changes in signal levels which are not representative of sample composition. By contrast, when profiling with Csc in GaAs, the equilibrium surface layer is established in a very low dose equal to deposition of one monolayer of cesium. We suggest that an advantage of profiling a layered structure with Cs+ is that unexplained irregularities at interfaces are likely to be minimized due to the rapid approach to equilibrium surface conditions.

Acknowledgement E.W.T. expresses gratitude to the Materials Science Department of the Imperial College London for their

hospitality during the period when this work was performed.