F. Stietz et al.: Interaction of H and Methane with InP and GaAs Surfaces 185 phys. stat. sol. (a) 159, 185 (1997) Subject classification: 68.35.Bs; S7.11; S7.12 Interaction of Hydrogen and Methane with InP(100) and GaAs(100) Surfaces F. Stietz (a), J. Woll (b), V. Persch (c), Th. Allinger (b), W. Erfurth (d), A. Goldmann (b), and J. A. Schaefer (a) (a) Institut fur Physik, TU Ilmenau, D-98684 Ilmenau, Germany (b) Fachbereich Physik, Universitat GH Kassel, D-34132 Kassel, Germany (c) Physikalische-Technische Bundesanstalt, D-14195 Berlin, Germany (d) Max-Planck-Institut fur Mikrostrukturphysik, D-06120 Halle, Germany (Received August 1, 1996; in revised form November 20, 1996) The interaction of thermally activated hydrogen atoms and methane molecules with InP(100) and GaAs(100) surfaces was studied by X-ray and UV-induced photoelectron spectroscopy (XPS, UPS), high resolution electron energy loss spectroscopy (HREELS), low energy electron diffraction (LEED), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and desorption spectroscopy (DS). In most cases the interaction causes a strong decomposition of the surface due to a preferential loss of the group-v and an enrichment of the group-iii elements. Hydrogenation of the clean InP(100) 4 2 surface can be divided into three stages. First, there is a saturation of dangling bonds, where the 4 2 reconstruction is preserved. Then, there is a breaking of the In dimers present at the surface, resulting in a 4 1 LEED structure. Finally, a loss of phosphorus and the build-up of metallic indium droplets follows. Bombardment of InP(100) surfaces with methane ions results in the formation of In±C and P±C species. Hydrogen exposure of GaAs(100) surfaces is more effective, since it changes the surface structures already at the initial stages of interaction. This is corroborated by the HREELS and DS data, which give strong evidence for a preferential loss of arsenic. Desorption spectra were taken during the hydrogen exposure and they show for low hydrogen pressure directly the desorption of AsH 3. For high hydrogen pressure p 1 10 4 Torr) GaH 3 is detected in addition. The intensity ratio of desorbing species (AsH 3 /GaH 3 decreases with increasing hydrogen pressure. After extrapolation a value of one results at a pressure of p ˆ 1 10 3 Torr. Models for the interaction of hydrogen and methane with InP(100) and GaAs(100) are discussed in detail. 1. Introduction The chemical reactions of hydrogen and methane with III±V semiconductors are of practical interest, because they are very important in the production of semiconductor devices. Therefore, a fundamental understanding of a number of different reaction steps is necessary for a successful control of passivation, diffusion, carrier compensation, dry etching processes, minimization of surface damage, etc. From a surface science point of view, we are interested in different bonding configurations during and after the reaction of hydrogen and/or methane with the technically relevant (100) surfaces of GaAs and InP. The change in electrical or optical characteristics is not discussed in this paper. We concentrate on the chemical composition of differently prepared surfaces, as probed by X-ray and UV-induced photoelectron spectroscopy (XPS, UPS), high resolution electron energy loss spectroscopy (HREELS), low energy electron diffraction (LEED), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), electron energy loss spectroscopy (EELS)
186 F. Stietz et al. and desorption spectroscopy (DS). We used the hot filament technique to generate mainly methane and hydrogen radicals, simulating the chemical aspect of a plasma or the bombardment with methane ions in order to simulate reactive ion etching conditions [1]. This enables us to differentiate between chemical and energetic aspects of the reactions. We prepared the indium-rich InP(100) 4 2 surfaces by ion bombardment and annealing. 2 4 surfaces relate to phosphorus-rich surfaces correspondingly [2, 3]. By this way it is assumed that these surfaces consist of three In dimers and one missing dimer [4]. That these surfaces are In-rich was proved by H-titration experiments and HREELS. Galliumrich GaAs(100) c 8 2 surfaces were treated by ion bombardment and annealing, respectively [5]. Arsenic-rich GaAs(100) c 4 4 surfaces were grown in a MBE system and controlled by reflection high energy electron diffraction (RHEED) and then transferred under UHV conditions to perform hydrogenation experiments. 1 For exposure to methane and/or hydrogen the samples were positioned 5 cm in front of a hot tungsten filament 2100 100 K) and the gases were introduced through separate leak valves. The values for exposure were determined as the product of the uncorrected ion gauge reading and the time span the filament or the acceleration voltage was switched on. The composition of gas mixtures was also determined by an ion gauge. For methane ion bombardment we used a standard ion gun with an accelerating voltage of 50 V and an uncorrected ion gauge reading of 10 3 Torr. In situ mass spectra were taken during the exposure of GaAs(100) surfaces to atomic hydrogen with a differentially pumped quadrupole mass spectrometer (QMS), which was positioned 1 cm in front of the sample surface. Therefore, measurements could be taken up to a pressure of 7 10 4 mbar in the UHV system. The ionization energy of the QMS was set to 90 ev. 2. Results 2.1 Indium phosphide Fig. 1 shows the intensity ratio of the P2p and the In3d lines as a function of hydrogen exposure for an initially clean InP(100) 4 2 surface (left) and a contaminated InP(100) Fig. 1. P2p/In3d 3=2 intensity ratio as a function of the hydrogen exposure (L). Left: for a clean InP(100) 4 2 surface, right: for a contaminated InP(100) surface 1 These measurements were performed at Montana State University in the group of G. J. Lapeyre.
Interaction of Hydrogen and Methane with InP(100) and GaAs(100) Surfaces 187 Fig. 2. Left panel: EELS spectra (2nd derivative of the energy distribution N E of InP(100) after a hydrogen exposure of 10 7 L for three different primary beam energies: 150 (a), 750 (b), 1550 ev (c). Right panels: surface and bulk plasmon as a function of the hydrogen exposure surface before ion bombardment and annealing (right). For the clean surface the intensity ratio is almost constant up to an exposure of 10 4 L, whereas for the contaminated sample a change occurs after 10 6 L. At these very high hydrogen exposures the sample temperature was set to 500 K in order to increase the reaction rate. Consequently the intensity ratio has dropped by about a factor of three, similar to the situation for the initially clean surface (left panel). For this latter situation, Fig. 2 represents EELS data for three different primary beam energies: 150, 750, and 1550 ev. These spectra are Fig. 3. HREELS spectra for differently prepared InP(100) surfaces after a hydrogen exposure of 100 L. (a) Ion bombarded surface, (b) after annealing of an ion bombarded surface to 500 K, which exhibits a 1 1 LEED pattern, (c) 4 2 surface
188 F. Stietz et al. dominated by surface- and bulk-plasmon loss structures of metallic indium at 8.5 and 11.5 ev, respectively. In addition, their multiples and combination losses are observed, as indicated. Their relative intensities increase drastically for exposures higher than 10 4 L, as evidenced in the right part of Fig. 2. There, the relative intensity of both signals is plotted as a function of hydrogen exposure. For low hydrogen exposures 10 4 L) there appears a feature around 8.5 ev in the EELS spectra, but this structure is related to a transition from the valence band to the conduction band (loss energy 8.8 ev). The structure at 18.1 ev for high hydrogen exposures is explained by a transition from the In4d core level to the conduction band [6]. Finally, it should be mentioned that the spectrum obtained after an exposure of 10 7 L of hydrogen shows strong similarities to spectra taken from a clean In metal. HREELS spectra shown in Fig. 3 are able to give detailed information about the chemical situation of differently prepared surfaces. In each case a hydrogen exposure of 100 L was used to probe an ion bombarded surface (curve a), a slightly annealed surface up to 500 K after ion bombardment (curve b) and a well ordered 4 2 surface after ion bombardment and annealing up to 650 K (curve c). For the ion bombarded surface an energy loss at 285 mev (P±H stretching vibration) dominates the spectrum besides the contamination peak at about 360 mev, which is due to carbon±hydrogen species. At the 1 1 and the 4 2 surfaces the strongest feature appears at 212 mev, which is related to the excitation of In±H stretching vibrations. Fig. 4. XPS spectra of InP after different treatments: (1) as built in, (2) after CH 4 ion bombardment (50 V) corresponding to a total exposure of 10 7 L, (3) after subsequent annealing to 470 K, (4) final annealing to 870 K
Interaction of Hydrogen and Methane with InP(100) and GaAs(100) Surfaces 189 Fig. 5. P2p/In3d 3=2 intensity ratio as a function of methane exposure during bombardment with an ion gun (50 ev) Bombarding a contaminated InP(100) surface with methane ions (50 ev), the surface composition changes dramatically. After an exposure of 10 7 L, the In3d and P2p signals are drastically reduced, as shown in Fig. 4. Simultaneously, the C1s signal (not shown here) increased substantially. Only after annealing the sample up to 870 K (line 4), the P2p and the In3d signals increase slightly again, after having been almost suppressed after annealing the sample at 470 K (line 3). The change of the intensity ratio of the P2p and the In3d 3=2 lines (Fig. 5) as a function of methane exposure shows first a slight decrease and after high exposure >10 6 L) an increase, which is opposite to the behaviour shown for the hydrogen exposure (Fig. 3). 2.2 Gallium arsenide In Fig. 6 HREELS results are plotted for differently prepared surfaces. The left panel exhibits spectra after a hydrogen exposure of 20 L of an ion bombarded surface (curve a) and of an ion bombarded and annealed surface (curve b). In both cases, the Ga±H stretching vibrations dominate the spectra, whereas the As±H intensities are rather weak. The opposite is true for the spectra shown in the right panel of Fig. 6. In this case, we prepared an arsenic-rich surface by rapid processing, i.e. by fast annealing and cooling procedures, a parameter not taken into account before. 2 The intensity ratio for the two stretching vibrations As±H and Ga±H increases from 1.5 to 2.2 by increasing the hydrogen exposure from 80 (curve a) to 200 L (curve b). In addition, the linewidths of both lines change, too. This indicates different adsorption sites for the two different hydrogen exposures. In Fig. 7 (left) the partial pressure as measured by the quadrupole mass spectrometer of GaH u ˆ 71 and AsH u ˆ 76 as a function of exposure time for a hydrogen pressure of 1:3 10 5 mbar is shown. The GaH signal increases continuously with time till the filament is turned off whereas the AsH signal jumps to a maximum and then slowly decreases. In the right panel of Fig. 7 the mass spectra of GaH and AsH for a hydrogen pressure of 6:7 10 4 mbar are shown. Here the GaH and AsH signals behave in a similar way. Both desorption spectra show a small jump, when the filament is turned on and after that a further increase for both signals occurs. In Fig. 8 (left panel) the partial pressure of AsH 3, AsH 2 and AsH and GaH 3, GaH 2 and GaH as a 2 These measurements were performed at the Forschungszentrum Julich in the group of H. Ibach. 13 physica (a) 159/1
190 F. Stietz et al. Fig. 6. HREELS spectra of differently prepared GaAs(100) surfaces. Left panel: after ion bombardment (a) and after ion bombardment and annealing (b) and hydrogen exposure of 20 L; right panel: after a rapid thermal annealing process and successive hydrogen exposure of 80 L (a) and 200 L (b) Fig. 7. Partial pressure of GaH u ˆ 71 (a) and AsH u ˆ 76 (b) as a function of exposure time for different hydrogen pressures p ˆ 1:3 10 5 mbar (left panel) and p ˆ 6:7 10 4 mbar (right panel). The point of time the filament was turned on and off are indicated by ªonº and ªoffº
Interaction of Hydrogen and Methane with InP(100) and GaAs(100) Surfaces 191 Fig. 8. Left panel: partial pressure of AsH u ˆ 76 (a), AsH 2 u ˆ 77 (b) and AsH 3 u ˆ 78 (c) as a function of exposure time; right panel: partial pressure of GaH u ˆ 71 (a), GaH 2 u ˆ 72 (b) and GaH 3 u ˆ 73) (c) as a function of exposure time; hydrogen pressure p ˆ 6:7 10 4 mbar function of exposure time is shown. Besides AsH, AsH 3 and AsH 2 can be detected as well. The spectra show an identical behaviour as a function of exposure time, as compared to the AsH signal. The intensity ratio I(AsH 3 =I(AsH) of the desorption signals yields a value of 0.7. This value agrees very well with the cracking pattern if AsH 3 is ionized by electrons of 90 ev [7]. In Fig. 8 (right panel) the partial pressure of GaH, GaH 2 and GaH 3 is shown as a function of exposure time. Only GaH could be detected in the mass spectrometer. To decide which species was desorbed from the surface the knowledge of the cracking pattern of GaH 3 is necessary, but no exact values are given yet. It is known that GaH 3 is very unstable in the gas phase [7]. 3. Discussion For studying the intensity behaviour of In4d lines for different hydrogen exposures, we used fitting routines given elsewhere [4, 8]. Visual inspection gives direct evidence that at least two doublets have to be used, In in InP and In in a dimer configuration or In as a metal (clusters). The latter two components are degenerate in energy, but their full width at half maximum (FWHM) is rather different. With increasing hydrogen exposure the latter doublet increases in intensity, and its FWHM is decreased from 0.76 to 0.43 ev. From the energetic position of the In bulk component of InP the band bending can be evaluated. The band bending is reduced by roughly 0.2 ev if the core level spectra for the clean sample and the hydrogenated sample (not shown here) are compared. The bulk contribution is shifted from a binding energy of 17.45 to 17.67 ev. In addition, 13*
192 F. Stietz et al. with hydrogenation a strong phosphorus depletion results, as concluded from our XPS results (Fig. 1). This behaviour is true for both kinds of surfaces, the InP(100) 4 2 and the InP(100) contaminated with a thin oxide and some methyl groups. The main difference in the two sets of experiments is, that for P depletion higher hydrogen exposures are needed for a contaminated surface. In line with a phosphorus depletion is the observation of surface and bulk plasmons of metallic indium and its intensity increase (Fig. 2). Fig. 2 shows both features after high hydrogen exposures (10 7 L), combination bands and multiples in the EELS spectra in the second derivative mode of the energy distribution. Fig. 3 demonstrates that HREELS is very sensitive to the chemical bonding situation of differently prepared surfaces. Cleaning a contaminated InP(100) surface prior to annealing by ion bombardment, strong P±H stretching vibrations (Fig. 3, curve a) indicate that phosphorus dangling bonds have been saturated after an exposure of 100 L, whereas at an ion bombarded surface, which was annealed to 500 K (Fig. 3, curve b), In dangling bonds have been saturated, and there seem to be no P dangling bonds available. Again, the situation after further annealing to 650 K is different. There, with LEED we observe a 4 2 structure and with HREELS a structure at about 285 mev (Fig. 3, curve c), which is assigned to the P±H stretching vibration. At this stage, the value of 0.2 for the intensity ratio P±H/In±H corroborates a model for the InP(100) 4 2 surface with three In dimers and one missing dimer. After this passivation process, where still a 4 2 LEED pattern is preserved, further hydrogenation results in bond breaking, giving a 4 1 structure and then a 1 1 structure. After additional hydrogen exposure no LEED pattern is observed, and we end up with indium droplets which can have a size up to 0.2 mm [8]. A way to avoid this droplet formation is the combined use of methane and hydrogen in order to remove indium by the use of methyl groups and phosphorus by taking atomic hydrogen. In order to simulate reactive ion etching (RIE) conditions we used 50 ev methane ions. Fig. 4 shows that the ioninduced process increases the carbon content dramatically at higher exposures. Furthermore, the ions strongly react with indium phosphide, and a rather thick polymeric film results by which the P2p and In3d signals are almost suppressed after an exposure of 10 7 L (Fig. 4). Additional annealing up to 870 K merely changes this situation, as evidenced by the weak P2p and In3d lines (see Fig. 4) and the strong C1s line, not shown here [9]. Very interesting is the fact that the intensity ratio P2p/In3d 3=2 (see Fig. 5) increases with higher exposures, which is opposite to our hydrogenation studies, which were discussed before. Therefore, there is some hope to find the right parameters to etch InP layer by layer. Further experiments in that direction are needed to solve this delicate problem [12]. For GaAs the changes in the core level spectra are not as drastical as for InP which means that the formation of a metallic component is almost suppressed. Detailed results on this issue are given elsewhere [10, 11]. Further information is obtained by HREELS data taken at differently prepared GaAs(100) surfaces by using hydrogen as a probe and as a reactant. After ion bombardment of GaAs we always find a Ga-rich surface, as evidenced by the dominating line of Ga±H stretching vibrations. A similar situation is found after ion bombardment and annealing (see Fig. 6). The reduced linewidth of the Ga±H mode intensity at the ion bombarded surface indicates only one kind of hydrogen bonding, whereas different sites seem to be occupied at an ion bombarded and annealed surface. More experiments are needed to clarify this issue. Most interesting are results obtained after having shortened the time for annealing and cooling the sample. After
Interaction of Hydrogen and Methane with InP(100) and GaAs(100) Surfaces 193 Fig. 9. Intensity ratio of AsH 3 /GaH 3 as a function of hydrogen pressure. Extrapolating the measurements up to a pressure of 1 10 3 mbar, indicated by the dotted line, yields a ratio of one rapid thermal annealing and cooling (RTA) we obtain a rather different situation probed by atomic hydrogen. By this process we are able to produce an arsenic-rich surface. The As±H/Ga±H intensity ratio is 1.5 after an exposure of 80 L, and it increases further to 2.2. An increase in full width at half maximum (FWHM) indicates that there are different adsorption sites available like Ga±As±H, As±As±H or AsH 2 with slightly different vibrational energies. The reduction of the FWHM of the Ga±H line observed simultaneously (right panel of Fig. 6, curves a, b) indicates in addition that there are different adsorption sites available for gallium. The reduction of the linewidth is in line with the hydrogen induced desorption of gallium hydrides observed by mass spectrometry, leaving behind only one kind of bonding. A hydrogen stretching vibration at metallic gallium 1549 cm 1 [13] was not observed. The high arsenic concentration at the surface can have its origin in the high annealing temperature of up to 900 K and the buildup of Ga droplets. Another possibility is, that the high temperature allowed a diffusion of arsenic to the surface and a freeze-in of that situation due to rapid cooling. In principle it is possible to etch As-rich samples with atomic hydrogen as well as Garich samples, as we can demonstrate with our in situ studies with quadrupole mass spectrometry (QMS), shown in Fig. 7 to 9. AsH 3 desorbs from the surface, is cracked due to the ionizing process in the quadrupole mass spectrometer and AsH and AsH 2 are formed. Therefore we tentatively interpret the data in the following way. In agreement with AsH 3, GaH 3 desorbs from the surface and GaH is built up in the mass spectrometer or on the way between the sample and the spectrometer. In Fig. 9 the rate of desorbing AsH 3 and GaH 3 molecules as a function of hydrogen pressure is shown. In all measurements the ratio was higher than one, indicating a gallium enrichment at the surface in very good agreement with earlier results, obtained by Auger electron spectroscopy and high resolution electron energy loss spectroscopy [5, 13, 14]. With increasing pressure the ratio decreases. If one extrapolates our results, an intensity ratio of one would be achieved for a pressure of 1 10 3 mbar, meaning a desorption of equal quantities of GaH 3 and AsH 3 from the surface at this hydrogen pressure. In all cases, the driving force for etching is the breaking of surface bonds by atomic hydrogen and the build-up of hydrides, which desorb into vacuum. In the future, the combination of high resolution electron energy loss spectroscopy (HREELS) with scanning tunneling microscopy (STM), and in situ methods like desorption spectroscopy (DS) will be of high relevance for more detailed information about the different interaction mechanismus involved in fundamental etching processes on an atomic scale.
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