Langmuir 1992,8, 125-129
125
Kinetics of Field Evaporation during Hydride Formation on GaP Surfaces: A FIM and Atom-Probe Study A. Gaussmann,*p+W. Drachsel, and J. H. Block Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6,D-1000 Berlin-33, FRG Received May 14, 1990. I n Final Form: December 10, 1990 Field evaporation of hydrides from different GaP surfaces was studied in the temperature range of
26-340 K. Field evaporation rates spanning up to 3 orders of magnitude were measured as a function of
temperature and at different gas pressures. The temperature dependence of the evaporation rate showed a pronounced minimum at 200 K. Below 200 K the evaporation rate was anisotropic; (111) surface planes showed higher rates than (001) planes. Below 200 K the evaporation rate decreased with increasing temperature, while above 200 K the evaporation rate increased with increasing temperature. Above 200 K an activation energy of 662 meV was found. The occurrence of reversed signs in the temperature coefficientsof field evaporation can be explained by the influence of field-adsorbed molecular hydrogen. Pulsed-laser-stimulatedfield evaporation studies of GaP in the presence of hydrogen showed the formation of singly and doubly charged hydrides of phosphorus while no gallium hydrides are detected. The hydride formation rate and the ion distribution depend strongly on the hydrogen supply to the surface. Under vacuum conditions mostly singly and doubly charged phosphorus dimer and trimer hydrides but no tetramers are found. In hydrogen, the ratio P/Ga increased with higher field strengths. It is likely that the evaporation mechanism consists of a combination of gallium hydride evaporation and phosphorus ion evaporation. Introduction Hydrogen adsorption on semiconductor surfaces is of great fundamental and technological interest. In particular, the binding of hydrogen to unsaturated dangling bonds on silicon surfaces is a property of importance in semiconductor devices, where hydrogen reduces the density of defect statesin the band gap.' Hydrcgen adsorption may affect the surface reactivity, since the number of dangling bonds will be thereby reduced. On the other hand, hydrides are believed to be precursors in the chemical etching of silicon surfaces in high electrostatic fieldsS2.3 These reactions may be rate-limited by the hydrogen supply. FIM studies of silicon and of GaP in hydrogen reported an anomalous reduction of the evaporation field strength due to the presence of h y d r ~ g e n . ~Sakata ? ~ and Block6 concluded that the observed effect on the field evaporation was rate-determined by surface hydride formation. They proposed a mechanism of field-induced dissociation of field-adsorbed molecular hydrogen at the semiconductor surface, followed by surface hydride formation and field evaporation of these hydrides. This mechanism was experimentally confirmed by Kellogg, who used a pulsedlaser-induced atom probe for investigating silicon.' One aim of the present investigation was to characterize the low-temperature behavior of the field evaporation process for GaP. For this purpose, the accessible range of tip temperature was extended down to 26 K. In a former study, GaP showed different evaporation rate regimes in the temperature range of 80-330 K.5 Below room tem+
Present address: EidgenBssische Technische Hochschule,Uni-
versititstrasse 6, CH-8092 Ztirich, Switzerland. (1)Fritche, H.; Tsai, C. C.; Pearson, P. Solid State Technol. 1978,21, 55. (2) Kobayashi, H.; Edamoto, K.; Onchi, M.; Nishijima, M. J. Chem. Phys. 1983, 78, 7429. (3) Melmed, A. J.; Stein, R. J. Surf. Sci. 1975, 49, 645. (4) Sakurai, T.; Culbertson, R. J.; Melmed, A. J. Surf. Sci. 1978, 78, L221. (5) Sakata, T.; Block, J. H.; Naschitzki, M.; Schmidt, W. A. J. Phys. (Paris) 1987, 48, (26-239. ( 6 ) Sakata, T.; Block, J. H. Surf. Sci. 1982, 116, L183. (7) Kellogg, G. L. Phys. Reu. B. 1983, 28, 1957.
perature the evaporation rate was found to decrease with increasing temperature, while at higher temperatures the evaporation rate increased with increasing temperature. In a recent study the influence of the hydrogen supply on the field evaporation of GaP was characterized in detail? Hydrogen may be supplied directly from the gas phase, or via diffusion from original adsorption sites at the shank, or from central areas of crystal planes to kink sites, where field evaporation occures. With this model, the experimentally observed dependence of the field evaporation rate of GaP on the hydrogen partial pressure could be explained. In order to complete this study, the hydride formation on GaP surfaces was analyzed by the atom-probe method. Information about the evaporation mechanism could be obtained. It is likely that, for this system, the evaporation mechanism consists of a combination of gallium hydride evaporation and phosphorus ion evaporation, which is strongly dependent on the chosen surface plane. Experimental Section This investigation was performed in two different experimental devices. The first one is a conventionalultrahigh vacuum (UHV) field ion microscope,which is equipped with a closed cycle helium refrigerator. The tip temperature can be adjusted from 26 to 500 K. Measurements of evaporation rates could be performed in this apparatus. The second device was a pulsed-laser atom probe, described in detail e l s e ~ h e r e .In ~ this apparatus 5-11s laser pulses from a combination of a nitrogen laser and a dye laser (main wavelength 600 nm) were focusedontothe tip. Energy densities of up to J/pulse/cm-*were transferred to the tip surface. According to the high band gap of GaP, the light frequency of the laser had to be doubled in order to reach the absorption edge and to produce enough thermal energy for stimulated field evaporation.'O The specimen support consisted of a four-lead holder, which allows measurements of the sample temperature. The GaP used of sulfur;the specific was n-doped with approximately,1017/cm-3 (8) Gaussmann, A.; Drachsel, W.; Block, J. H. J. Phys. (Paris) 1989, 50. ~.C8-141. --
.
(9) Drachsel, W.; Nishigaki, S.; Block, J. H. Int. J.Mass Spectrom. Ion Phys. 1980,32, 333. (10) Landolt-Bornstein 1982, III, 17a, 2.9, 496.
0743-7463/92~2408-0125$03.00/0 0 1992 American Chemical Society
Gaussmann et al.
126 L
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Figure 1. Calibration curve for the evaporation rate k of the gallium phosphide (111)plane: temperature T = 28 K, gas Pa, voltages U = 8.03 kV and U = 8.25 kV. pressure P = 4 X To maintain a constant evaporation rate k, one must increase the voltage U for each evaporated surface layer by 6.5 V. resistivity was of the order of low2Q-cm. Specimenswere prepared by cutting a GaP wafer in thin (110)oriented rods. These rods were fixed by a conductive epoxy glue to a platinum support loop which was used for sample heating. Chemical etching was performed in a freshly prepared solution of 1:3 HNOdHCl at about 350 K. Needle-shaped specimens with a tip radius of 100 nm were obtained. Once the etching procedure was completed, the sample was rinsed in methanol. The surface of the freshly etched tip was observed by optical microscopy: Only very smooth samples were chosen for further experiments. Once the GaP sample was introduced into the chamber and UHV conditionswere established, a cleaning procedure, consisting of neon ion bombardment," field evaporationin neon, and fieldinduced chemical etching by hydrogen gas: was applied. No thermal annealing was performed, because the epoxy glue used for sample attachment decomposes at temperatures above 500 K. Segregation within the compound semiconductor specimen might also occur at higher temperatures. In the atom-probe apparatus no voltages higher than 10 kV could be applied to the specimen. This instrumental limitation restricted an extended application of the field evaporation method necessary for developing surface planes. Nevertheless, clean surfaces and stable structures could be obtained and imaged. Due to the missing resolution of surface planes, in this part of the study it is only possible to distinguish between dark and bright areas on the tip, which were discerned using hydrogen as the image gas. The field evaporation rate k is defined by the number of surface planes of a particular surface orientation which is evaporated per unit of time (layers/& The other symbols are the vibrational frequency y, a factor a taking care for energy transfer, the activation energy Qn(F), which is a function of the field strength F, the Boltzmann constant k B , and the absolute temperature T. The field evaporation rate k was measured by observing the
shrinking and final removal of individual planes, which evaporated via kink sites. In (111)planes a layer consists of an equal number of gallium and phosphorus atoms with gallium_atomson top in (111)planes and phosphorusatoms on top in (111)planes. The measurement of evaporation rates has been described in more detail elsewhere.6J1 In order to maintain a constant local field strength, the applied voltage had to be corrected, because of the increase of the tip radius during continuous field evaporation. This correction also takes care of the increase of the area of layers with increasing radius (see Figure 1).
Results of the FIM Investigation The crystallographicplanes were identified on the basis of the symmetry properties and the angular relations known from stereographic projections of the lattice. One (11)Kellogg, G.L. J. VUC.Sci. Technol., A 1984,2,1597. (12)Sakata, T.;Block, J. H.Surf. Sci. 1983,130,313.
Figure 2. FIM pattern of a clean field-evaporated (011)oriented gallium phosphorus emitter: tip voltage U = 12.50kV, tip temPa. perature T = 50 K,hydrogen pressure P = 4.0 X
10.51
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, 40
.
, 60
.
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.
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Temperature I K
.
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.
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Figure 3. Temperature dependence of the evaporation voltage at constant k of the gallium phosphide (111)surface in the temperature range of 28-150 K: hydrogen pressure P = 4 X Pa, evaporation rate k = 0.1 layers/s.
aim of this study was to compare evaporation rates from different surface planes, in particular from the gallium (111) and, simultaneously, from the phosphorus (111) planes. For this reason (011) oriented emitters were chosen, because both planes of interest are placed symmetrically around the (011) plane, with an angle of 35O26' to the pole axis, respectively. Furthermore, two (001) planes are lying orthogonally to the [lll]zone but with a greater angle of 4 5 O to the pole axis, respectively (which results in a longer projection distance in the FIM pattern). A typical FIM pattern of a clean field evaporated GaP tip is shown in Figure 2. The metallic (gallium) (111)plane was easy to index, because this plane appeared first and was brighter and better resolved than the nonmetallic (phosphorus) (111)~ 1 a n e . l ~ In hydrogen, the reduction of the field evaporation voltage Uat constant evaporationrate k could be observed down to 26 K. The measurement was performed in such a way that the voltage U was adjusted to a value where the evaporation rate precisely reached the value of k = 0.1 layers/s. Below 80 K the field evaporationdecreased more sharply. As shown in Figure 3, a change in temperature of 60 K from 86 to 26 K corresponds to a reduction in the field evaporation voltage of 2.0 kV or 15% The evaporation rate k at constant field strength F in the temperature range of 30-100 K was determined for
.
(13)Arthur, J. R. J . Appl. Phys. 1966,37,3057.
Langmuir, Vol. 8, No. 1, 1992 127
Kinetics of Field Evaporation 600. 550.
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Figure 4. Temperature dependence of the evaporation rate k for different gallium phosphite surface planes in the temperature range from 30 to 100 K. The evaporation voltage was kept constant a t U = 14.00 kV. 71
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Figure 6. Gallium phosphide under UHV conditions: temperature 92 K, applied voltage 6.5 kV, residual gas pressure 2.5 X lo-* Pa, 4251 laser pulses, random rate 10 counts/s.
to temperatures of 200 K a slight decrease of the evaporation rate k was found, while at higher temperatures a strong increase of k was observed. The welldeveloped minimum of hydride evaporation at constant field strength was determined to be 200 f 30 K for different tip radii. For the regime above 200 K the observed increase of the evaporation rate with temperature could be assigned to a thermally activated evaporation process with an activation energy of 662 f 240 meV. A discussion of the evaporation process will follow later on. Below 200 K, precursor states in the adsorption of hydrogen cause an unusual temperature dependence as to be discussed below.
[I(]
Figure 5. Temperature dependence of the evaporation rate k of the gallium phosphide (111)and (001) surfaces in the temperature range of 32-336 K. k is plotted relative to the constant Pa. The field strength was kept hydrogen pressure P = 2 X constant (relative to U = 10.57 kV by adding a correction of 6.5 V for each evaporated layer).
several surface planes using two different hydrogen pressures. Figure 4 shows the results. No significant difference in k could be determined for the close-packed gallium (111)and the phosphorus (111) planes. However, the less densely packed (001) plane, with a concentration of 50% gallium atoms and 50% phosphorus atoms showed an evaporation rgte lowered by afactor of 0.7, as compared to (111)and (111)planes. This result is surprising, since the number of surface sites per unit area in the (001) plane is comparably smaller (70% of (111)).Accordingly, we have to conclude that there is a high anisotropy in field evaporation. The reason for this may be found in differences of the adsorption behavior on (111)and (001) planes, which will be shown later in the atom-probe investigations. The importance of hydrogen adsorption for field evaporation of all surfaces follows from the proportionality between the field evaporation rate k and the hydrogen pressure P, which is valid for each of the planes (see Figure 4). However, the interaction of hydrogen with phosphorus surface atoms is much more pronounced than with gallium. The temperature dependence of the evaporation rate k in the whole experimentally accessible range from 30 to 336 K is shown in Figure 5. The measurements were performed for the (111)and the (001) planes. The field strength F was kept constant. In the temperature ranges from 30 to 120 K and from 270 to 336 K, the hydrogen gas Pa, while in the range from 120 pressure was P = 5 X to 270 K, the measurements were performed at a 4 times Pa. This higher hydrogen gas pressure P = 2 X procedure was found necessary because of the dim images at these temperatures. By using the proportionality between the evaporation rate k and the hydrogen gas pressure P, the experimental data could be correlated. Up
Results of the Atom-Probe Investigation Under UHV conditions the GaP field evaporates in the form of singly and doubly charged phosphorus cluster ions) whereas gallium evaporates only as singly charged ions according to its natural abundance of isotopes. Phosphorus is detected mainly as singly charged P2+ and P3+ ions, the amounts of PI+(or Pz2+)and P4+are smaller by a factor of 10 and 5, respectively. No GaP molecular ions were found. The stoichiometric analysis, i.e., the total amount of phosphorus atoms compared to the total amount of gallium atoms, gives a P/Garatio of 1.2. Figure 6 shows the results. Our findings are in good agreement with former investigations on this subject.l4-l6 In the presence of hydrogen, phosphorus atoms evaporate preferentially as hydrides while no gallium hydrides are detected. No Gal' hydrides were found either. The surface composition of the GaP is strongly inhomogenous and depends on the location of the probe hole. One can distinguish between so-called dark and bright areas during imaging of the tip. Figure 7 shows a mass spectrum of a dark area and Figure 8 a mass spectrum of a bright area. On dark areas gallium ions dominate largely, no hydrogen at all is detected, and the phosphorus cluster distribution coincides with the vacuum mass spectra shown in Figure 6. Only the minor part of the phosphorus clusters formed monohydrides. On bright areas mostly phosphorus hydrides and hydrogen ions are detected. The gallium ion production rate in bright areas was found to be smaller by a factor of 10 than on dark areas. This finding may be due to an experimental artifact. From the high concentration of hydrogen on bright areas, one can deduce that, in this region of the tip, the local field strength F might be higher. Because of the lower ionization potential of gallium compared to phosphorus (Ita = 6.06 eV and I p = 10.57 (14) Ohno, Y.; Kuroda, T.; Nakamura, S. Surf. Sci. 1978, 75, 689. (15) Nishikawa, 0.;Nomura, E.; Yanagisawa, M.; Nagai, M. J. Phys. (Paris) 1986, 47, C2-303. (16)Tomita, M.; Kuroda, T. Surf. Sci. 1985,201, 385.
128 Langmuir, Vol. 8, No. 1, 1992
Gaussmann et al.
69Ga+ 1200,
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Figure 7. Dark area of the gallium phosphite surface: temperature 92 K, applied voltage 7.1 kV,pressure 2 X Pa of hydrogen, 3127 laser pulses, random rate 114 countsis.
3501 ,
,--H3+
assigned as a precursor of the field-induced formation of surface hydrides. The field evaporation rate of silicon hydrides was enhanced due to two reasons. When dangling bonds are removed during the formation of Si-H surface bonds (i) the field penetration into the solid enlarges and (ii) the back-bonding of surface Si atoms to the lattice weakens. Therefore,also with GaP the interaction of the hydrogen can be understood by formation of surface hydrides. Below 200 K the influence of the hydrogen pressure P and the difference in hydride formation on the (111)and (001) surface planes can be interpreted on the basis of a field adsorption/desorption equilibrium of molecular hydrogen on the emitter. If one assumes that the weak bond of field-adsorbed molecular hydrogen can be described by a vibrational mode of T O = s, then it is possible to derive from the Frenkel equation
300
T
= T,,exp-
H
Discussion The temperature dependence of the field evaporation rate k of GaP in the presence of hydrogen shows two different regimes with opposite temperature gradients of field evaporation rates. The interaction of hydrogen may be expected to be similar to findings with silicon.6 At low temperatures field-adsorbed molecular hydrogen was
kBT a value for the mean residence time at T = 30 K of T = s. Therefore, for hydrogen pressures in the 10-3-Pa range, the surface coverage 0 would be rather small and far below the monolayer limit. The proportionality between the evaporation rate k and the hydrogen pressure Pare expected,because the impingement rate j of hydrogen is directly proportional to the gas pressure P. The higher the gas supply, the more hydrogen can adsorb and the faster is the rate k. The observed leveling off of the evaporation rate k below T = 50 K in Figures 4 and 5 can be understood as the supply limit of molecular hydrogen. At T = 50 K , molecular hydrogen loses the ability for surface diffusion. Hydrogen molecules which are impinging on terrace sites cannot reach kink sites which are ready to field evaporate. At temperatures above 200 K the evaporation rate k increases drastically. The Arrhenius plots in the temperature range of 200-360 K yield an activation energy of Q = 662 meV. This activation energy may be explained by a reduced influence of hydrogen on the process, because for T > 200 K the surface concentration of field-adsorbed hydrogen is very small. In this temperature range the GaP field evaporates as atoms or clusters, rather than as hydrides. On the other hand, Yamamoto et al. detected phosphorus hydrides also at room temperature and observed a layer by layer evaporation of gallium and phosphorus from the (111)planes.'* In particular, gallium field evaporated much faster than phosphorus, which can be understood by the difference in their ionization potentials. On the basis of these findings one may interpret the energy Q = 662 meV as the activation energy necessary for field evaporation of phosphorus. This value will 'strongly depend on the field strength F and the hydrogen gas pressure P. The atom-probe analysis confirms the presumed hydride formation and indicates that the evaporation rate is enhanced by the hydrogen supply. The more hydrogen is available the more phosphorus hydride ions are observed at the expense of pure phosphorus species. It is known that, at low temperatures and under vacuum conditions, the normal field evaporation is somewhat sporadic and characterized by cluster f0rmati0n.l~ Therefore, at low coverages of hydrogen, phosphorus hydride cluster evaporation and phosphorus cluster evaporation take place simultaneously. However, no phosphorus trimer and tet-
(17) Wagman,D. D.; et al. The NBS tables of chemical thermodynamic properties. Selected values for inorganic and C1and C2 organicsubstances in SI units. J.Phys. Chem. Ref. Data, Suppl. 1982, 11, 2.
118, 555. (19) Tsong, T. T. Surf. Sci. 1979, 81, 28.
Figure 8. Bright area of the gallium phosphite surface: temperature 92 K, applied voltage 7.2 kV, pressure 2 X Pa of hydrogen, 1748 laser pulses, random rate 5892 counts/s.
eV),17charged gallium could field evaporate randomly in the electrical field and escape detection by the laser pulses. This hypothesis would explain the high random rate in Figure 8. All phosphorus clusters in the bright area appear as hydrides, viz., P2H+,P2H3+,P2H5+,P2H2+,P2H22+,and P3H2+. No phosphorus tetramer hydrides are formed; Le., the cluster distribution differs from the mass spectrum without hydrogen. From this we deduce that the doubly charged dimers PZ2+may also be field evaporated under UHV conditions. A change in the applied voltage U has a large effect on the total amount of detected hydrogen, gallium, and phosphorus atoms. On dark areas, where gallium dominates, the ratio P/Ga strongly depends on the voltage U. By raising the voltage from 6.3 to 7.2 kV, the ratio P/Ga changes from 0.5 to 2.0. To a lesser extent, this behavior is also observed on bright areas dominated by phosphorus. The ratio P/Ga = 5.0 measured at 6.3 kV increases slightly with the applied voltage U. However, an increase of the total amount of hydrogen relative to gallium and phosphorus is also noticed. Raising the tip voltage U from 6.3 to 7.2 kV doubles the amount of hydrogen. On bright regions, mass spectra confirm the existence of small amounts of contaminants; they are tentatively NO2+,NO+, and COH+. They do assigned to be H30+, not appear under UHV conditions and on dark areas.
(18)Yamamoto, M.; Seidman, D. N.; Nakamura, S. Surf. Sci. 1982,
Langmuir, Vol. 8, No. I, 1992 129
Kinetics of Field Evaporation
ramer evaporation is observed on hydrogen saturated surfaces; exclusively dimer and trimer hydrides are formed. The formation of these hydrides has the effect of providing a smooth field evaporation, layer by layer. This effect, which was already observedin early FIM studies on silicon? is called the anomalous field evaporation. The detection of phosphorus hydrides and the tital absence of gallium hydrides in hydrogen suggest that, under the chosen experimental conditions (at temperatures below 200 K and at hydrogen pressures in the 10-3-Pa range),hydrogen interacts preferentially with phosphorus. This may be expected, because of the large single bond energy in PH of 3.3 eV.20 Furthermore, it is as yet not clear whether the formation of covalent GaH really occurs, because this compound is expected to be energetically not very stable.21 The fact that no charged GaP molecules are detected can be explained by the low binding energy of GaP which is only 910 meV.17 It seems plausible that the large binding energy of P H leads to a preferential evaporation of this charged species, because once hydrogen associates with a phosphorus atom, PH does not dissociate easily. The quantity of hydrogen on the tip varies from one region to another. On dark areas no hydrogen can be detected, while on bright areas hydrogen represents the main component in the mass spectra. This finding is remarkable, since the bright region in a FIM pattern is generally rich in gallium (Le., the (111)pole), while the dark region is rich in phosphorus (respectively the (111) However, the mass spectra indicate the contrary; i.e., on bright areas phosphorus ions predominate, while on dark areas, mainly gallium ions and phosphorus and phosphorus hydride ions are detected. A closer look at the mass spectra shows that on bright areas the random rate (the rate of ions which is not correlated with laser pulses) of the time-of-flight spectrum is very high. This demonstrates a rather high nonstimulated field evaporation. The elevated random rate can be attributed mainly to the desorption of hydrogen (which predominates in this region) and partly to the field evaporation of gallium ions (because of the lower ionization potential of the latter with respect to phosphorus ions17). This depletion of gallium on gallium planes would therefore result in the observed mass spectra with the high amount of phosphorus hydride ions. The hydrogen on the gallium surface can be explained by the fact that gallium does not interact with hydrogen. On dark areas, the low ionization potential of gallium is also an argument for the observed increase of the ratio P/Ga with increasing voltage U. At low field strengths gallium does not field evaporateinstantaneously. As shown in Table I, gallium dominates the mass spectra at low field strengths. However, at high field strength gallium field evaporates (probablynot time correlated and therefore hidden in the random rate) and phosphorus ions and phosphorus hydride ions are mainly detected, so that the ratio P/Ga is exactly inverted. The observation that no hydrogen is found on the phosphorus surface can be explained by the phosphorus hydride formation. This (20)Wolf,H. F.Semiconductors;Wiley-Interscience: New York, 1971; p 21.
(21)Cotton, F.A.;Wilkinson, G. Anorganische Chemie;Verlag Chemie: Weinheim, Germany, 1980; p 183.
Table I. Percentages of the Ion Yields from a Dark Gallium Phosphide Surface Relative to the Applied Voltage applied phosphorus and voltage/kV
phosphorus hydrides/ %
gallium/ %
hydrogen/ %
5.8 6.2 6.7 7.1
35 50 57 68
65 50 43 32
0 0 0 0
Table 11. Percentages of the Ion Yields from a Bright Gallium Phosphide Surface Relative to the Applied Voltage applied phosphorus and voltage/kV
phosphorus hydrides/ %
gallium/ %
hydrogen/ %
6.3 6.7 7.2
65 60 51
14 8 8
21 32 42
reaction is limited by the hydrogen supply. Phosphorus ions and phosphorus hydride ions appear in Figure 7, but no pure phosphorus ions are detected on the bright (hydrogen rich) area of Figure 8, where an excess of hydrogen is available for the formation of phosphorus hydrides. The supply of hydrogen increases with the field strength. Thus, the amount of surface hydrogen in the bright area increases with the applied voltage (see Table 11). The existence of H3+ signals on hydrogen-saturated surfaces can be explained by the mechanism of H3+ formation for metals as proposed by Ernst.22 H3+ is formed by a fieldinduced reaction at the surface between field-adsorbed diatomic hydrogen and chemisorbed hydrogen atoms. We observed that the H3+ formation is correlated with the surface contaminants H20, CO, and NO, as found by Kellogg at silicon surface^.^ H&+, COH+, and NO+ are detected in our mass spectra. The fact that no CO+ is found can be explained by the higher field strength necessary for desorption of this species.23The observation of contaminants on hydrogen-saturated surfaces may be an indication of the promoted diffusion of hydrogen. Contaminants were not detected either for clean surfaces under vacuum conditions or at low coverage of hydrogen. It seems possible that these species are deposits from the etching procedure and diffuse from the shank of the tip to the apex. This process is possibly promoted by the presence of hydrogen and by the electrical field gradient. This study revealed the existence of different evaporation processes, which are dependent on the field strength. The inhomogeneous surface distribution of gallium and phosphorus in hydrogen results from the presence of domains of gallium and phosphorus (i.e., (111)and (111) planes). The different reaction mechanisms can be described by a model, which is coherent with our results and which, therefore, should stimulate further investigations on similar systems.
Acknowledgment. We acknowledge the helpful discussion with Dr. N. Ernst and Dr.W. A. Schmidt. This work was supported by the Deutsche Forschungsgemeinschaft Bad Godesberg (SFB 6/81). Registry No. Gap, 12063-98-8;gallium hydride, 131641-96-8. (22)Ernst, N.; Bozdech, G.; Allam, S. H.; Block, J. H. In Proceedings of the 27th International Field Emission Symposium Tokyo; Yashiro, Y., Igata, N.Eds.; University of Tokyo: Tokyo, 1980;p 164. (23)Kruse, N. Surf. Sci. 1986,178, 820.
