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Thermal decomposition of arsine on GaAs( 100)
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Surface Science North-Holland
surface science ::...::+... ,,.. _. .......:.~:~.~~~~~:~:~iiiijj ~~~~z~:~:~:~:~:i:8~~.:~:~:~,~,.~,~ ,:.: iiir’.:.: ......~.~...,:;,
275 (1992) 41-51
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Thermal decomposition
of arsine on GaAs( 100)
M. Wolf, X.-Y. Zhu, T. Huett and J.M. White Department of Chemistry and Biochemistry, Center for Materials Chemistry, University of Texas, Austin, TX 78712, USA Received
17 January
1992; accepted
for publication
13 April
1992
The thermal dissociation pathways of ASH, on Ga-rich (4 x 6) and As-rich ~$2 X 8) GaAs(100) surfaces have been studied with high-resolution electron energy loss (HREELS), X-ray photoelectron (XPS) and thermal desorption spectroscopy (TDS). Arsine adsorbs molecularly at 115 K and, based on XPS data, the saturation coverage is 0.16 f 0.04 ML. In TDS, desorption of the parent molecule is accompanied by As-H bond dissociation starting at temperatures as low as 140 K. Of the resulting AsH, (x = 1, 2) and H, part recombines to liberate arsine but approximately 15% of the initial ASH, coverage dissociates irreversibly resulting in As deposition on the GaAs surface. While on the As-rich surface there is no evidence for Ga-H formation, we do observe transfer of hydrogen from As to Ga sites on the Ga-rich surface. Recombinative hydrogen desorption from Ga-H occurs around 500 K on Ga-rich surfaces, whereas H, desorbs at higher temperatures (- 550 K) from As-H on As-rich surfaces. With regard to atomic layer epitaxy (ALE) of GaAs, these results imply that, in the final stages of an As cycle, removal of hydrogen from As sites limits the rate whereas in the initial stages, Ga-H is the reaction intermediate.
1. Introduction
The growth of high-quality, layered structures of compound semiconductors is a key technology for the development of optoelectronic devices, e.g., semiconducting lasers [ll. In order to shrink the dimensions of such devices and to minimize thermal stress and interface reactions, it is desirable to reduce the operating pressures and temperatures in organometallic chemical-vapor deposition (OMCVD) processing [2,31. Moreover, selective growth mechanisms need to be found, especially for three-dimensional structures. Thus, su@zce chemical reactions will play an increasingly important role in materials processing. However, there is often a surprising lack of knowledge about the surface chemistry of precursor molecules used in OMCVD and atomic layer epitaxy (ALE) [4,5]. The molecule used here, arsine, is the most widely used As precursor in OMCVD and ALE of GaAs and other As-based compound semiconductors. Its thermal decomposition is catalyzed by As, Ga or GaAs surfaces [6,7]. This has been 0039-6028/92/$05.00
0 1992 - Elsevier
Science
Publishers
demonstrated by isotope exchange measurements where no HD could be observed during pyrolysis of ASH, in a D, atmosphere above a GaAs substrate [8]. However, the detailed reaction mechanism is not understood and has even been called a mystery [9]. While Nishizawa and Kurabayashi reported that arsine dissociates rapidly at 920-980 K [lo], Luckerath et al. determined, from in situ CARS measurements, that dissociation sets in at 330-450 K [ll]. The surface chemistry of ASH, on GaAs has been studied only very recently. In a very interesting paper, Banse and Creighton report on the TDS of adsorbed ASH, on GaAs(100) enriched to various levels with As [12]. Complementing this important work, we present here a detailed study of the dissociation pathway of ASH, on one Ga-rich and one As-rich GaAs(100) surface using TDS, HREELS and XPS. A brief report has been published recently [ 131. Because it is the most commonly used substrate for OMCVD and ALE growth of device structures, we chose the (100) surface of GaAs. Its surface structure is relatively complex; several
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42
M. Wolf et al. / Thermal decomposition of arsine on GaAs(100)
reconstructions with distinct LEED patterns and different surface stoichiometries are known [14161. One key structure is the (2 x 4)-c(2 X 8) Asrich surface for which there is STM [17] and theoretical work [18]. The unit cell consists of three As dimers (parallel to [Oil]) and one missing dimer exposing the underlying Ga atoms. Thus, the As coverage, for an ideal c(2 x 8) Asrich surface, is 0.75 ML (1 ML = 6.26 x 1014 cm-*, the surface density for a complete As or Ga layer). The structures of the various Ga-rich surfaces are not so well established. A recent STM investigation reveals, for the c(8 X 2) “(4 X 1)” Ga-rich surface, a “ridge and trough” structure [19] which can be modelled in terms of a (4 x 2) unit cell with double rows of Ga dimers and two missing Ga dimers which lead to the exposure of second-layer Ga atoms. The surface Ga concentration in this structure is 0.75 ML and there are a number of chemically different possible adsorption sites within the unit cell. Another reconstruction, denoted “(4 X 6)“, has been discussed as an overlap of “(4 X 1)” and “(2 X 6)” domains [14,20]. We have studied the decomposition of arsine on two structures: the Ga-rich (4 x 6) surface and the As-rich GaAs(lOO)-c(2 X 8) surface. These two cases model the initial and final stages, respectively, of an As cycle in ALE.
2. Experimental The experiments were performed in a previously described [21] ultrahigh-vacuum chamber with a base pressure of 2 X lo- lo Torr. It was equipped with a quadrupole mass spectrometer (QMS) for thermal desorption spectroscopy (TDS) and residual gas analysis, a high-resolution electron energy loss spectrometer for vibrational analysis (HREELS), a hemispherical energy analyzer and an X-ray source for X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED) optics. The sample (1.5 X 10 X 1 mm3) was cut from a semi-insulating GaAs(lOO) wafer (10i5/cm3 Stdoped). To improve resistive heating, a 3000 A thick Ta film was sputtered onto its backside. The sample was held with two Ta clips spot-welded to
MO rods. The latter were connected to a copper sample mounting block which connected to a cooled reservoir. The sample temperature was monitored by a chromel-alumel thermocouple spot-welded to a small Ta clip which was glued, with a high-temperature cement (Aremco-571), to the bottom edge of the sample. The crystal could be cooled to 115 K by contact with liquid nitrogen, and heated to 900 K with a typical rate of 7 K/s. No temperature calibration, i.e., pyrometry, was done; however, we observed the onset of atomic As and Ga desorption at 580°C in agreement with previous studies [20,21]. The sample was cleaned by cycles of: (1) Ar+ bombardment (20 min, 3.5 keV, 2-3 pAA), (2) annealing at 500°C for 10 min, and (3) briefly flashing to 600°C. After this procedure, the surface cleanliness was verified by XPS and LEED. The latter showed a “(4 x 6)” pattern typical for the Ga-rich surface [14,15,20] with very bright and sharp 4-fold spots and much weaker 6-fold spots. As discussed previously [19,20], this Ga-rich surface consists mainly of (4 x l), with some (2 X 6), domains. To ensure reproducibility, the sample was routinely flashed to 600°C before each ASH, dose. Amine (ASH,: Linde 99.9995% purity; AsD,: MSD 98% isotopic purity) was cleaned by freeze-pump-thaw cycles. It was dosed, with the sample at 115 K, through a retractable 2 pm pin-hole doser which terminated approximately 2 mm from the sample. This minimized adsorption on components other than the sample. Reproducible fluxes were achieved by fixing the pressure, measured with a capacitance manometer, behind the pin-hole. Doses were then proportional to the exposure time. The As-rich GaAs(lOO)-c(2 X 8) surface was prepared by dosing N 4500 L ASH, with the sample at 275°C and subsequent annealing to 460°C [22,23]. After annealing, LEED showed a weak c(2 x 8) pattern indicating some disorder between small (2 X 8) domains. HREEL spectra were recorded with a primary electron beam energy of 3 eV and an elastic peak FWHM of 7-8 meV. XP spectra, measured at a detection angle of 75”, with respect to the surface normal, to enhance the surface sensitivity, were excited with AlKa radiation (1486.6 eV) and
M. Wolf et al. / Thermal decomposition of arsine on GaAs(100)
were measured with a hemispherical analyzer operated at 30 eV pass energy. All HREEL and XP spectra were taken at 115 K. TD spectra were recorded with the sample in the line of sight with the QMS ionizer. To reduce electron-stimulated surface processes, the sample was shielded from the ionizer by a grounded shroud with 5 mm diameter mesh-covered entrance aperture.
3. Results 3.1. Thermal desorption
Fig. 1 shows a series of ASH, and H, thermal desorption spectra for different ASH, exposures (given in s for a fixed flux) onto a gallium-rich GaAs(lOO)-(4 X 6) surface. Below 850 K, molecular arsine and hydrogen are the only desorption products for all exposures. ASH, desorbs in two peak - the major peak, which shifts with increasr
I
I
I
I
I
500
1
K
,
/ t 144
K
ASH, /GaAs( 100)
A
T,,,=115K
a”
2’ L
lot 1
I
200
I
300
I
400
I
500
I
600
17
700
Temperature [K] Fig. 1. Thermal desorption spectra (TDS) of ASH, adsorbed on the Ga-rich GaAs(lOO)-(4x6) surface at 115 K. Bottom panel: TDS of molecular arsine (m/e = 78) taken after the indicated exposures. Top panel: H, desorption spectra (m/e = 2). The H, signal below 200 K is attributed to a cracking of AsH, in the QMS ionizer.
43
ing coverage from 165 to 144 K, and a weaker peak around 290 K. The H, signal observed below 300 K is proportional to the ASH, signal and is ascribed to cracking of ASH, in the QMS ionizer. At higher temperatures, H, desorbs in a broad range around 500 K, in agreement with studies of atomic hydrogen adsorption, i.e., H atom recombination [24,251. This indicates that some arsine dissociates and, after H, desorption, deposits atomic As. Consistent with the XPS results presented below, we attribute the arsine TDS peak at 144 K to desorption from weakly bound ASH,. Based on HREELS and XPS described below, the ASH, desorption around 300 K is ascribed to recombination of ASH, (x = 1, 2) and H, not to desorption of chemisorbed molecular species. As mentioned earlier, the Ga-rich (4 X 6) surface offers several distinct potential sites for adsorption and dissociation. To address the role of specific sites in the recombinative arsine desorption, we dosed ASH, for 1200 s, flashed off the low-temperature peak at 144 K and redosed for 1200 s. The resulting TD spectra are indistinguishable from the corresponding spectrum for 1200 s in fig. 1. This indicates that the dissociation products [ASH, (x = 1, 2) and/or H] occupy specific sites and, once these are filled, no further dissociation occurs. Fig. 2 shows the variation, with arsine exposure, of the TDS peak areas. It is obvious that the hydrogen desorption at 500 K saturates for doses longer than 300 s while the ASH, desorption peaks continue to increase, at least to 1200 s. The ratio between the two arsine TDS areas is (within *25%) independent of coverage, suggesting, in agreement with the above discussion, that there are at least two distinct adsorption sites within the unit cell - one nonreactive site leading to molecular desorption (144 K) and one reactive site leading to partial dissociation. That the population ratio is coverage independent, shows that there is no exchange between these two sites. Thus, the dissociation process, monitored by the recombination peak (N 290 K) does not compete with molecular desorption in the 144 K state. The absolute coverages indicated on the righthand axis in fig. 2 are derived from XPS data
M. Wolf et al. / Thermal decomposition
44
0.0
i
I
I I
I
I
0.00 0.10 0 $ 2 0.05 E
0.00 ASH, Dose [s]
Fig. 2. TDS peak areas as a function of ASH, exposure, Lower panel: The relative ASH, peak areas of the 144 (open circles) and 290 K (filled circles), normalized by the 200 s value of the total ASH, area. Upper panel: The H, TDS area normalized by its saturation value. The coverages indicated on the right-hand axis are derived by XPS (see section 3.3).
presented in section 3.3. The estimated ASH, flux through our pinhole doser is 2 X 1014 molecules/(s cm*). Using this value, together with the coverages derived from the XPS, the initial sticking coefficient at 115 K is roughly 10p3. In the following HREELS and XPS experiments, if not otherwise stated, the initial arsine coverage equals that of the 1200 s exposure in fig. 1.
of arsine on GaAs(100)
adsorbed on GaAs(100) (2100-2110 cm-‘) [9,25,281. Substituting AsD, (see fig. 41, the As-D stretch appears at 1490 cm-’ leading, as expected, to an isotope effect of 1.41 k 0.03. After annealing the Ga-rich surface to 200 K, there is no shift in the As-H frequency, but there is a significant intensity decrease attributable to desorption of molecular arsine. Upon annealing to 300 K, a new loss at 1870 cm-’ appears; it is assigned to the Ga-H stretch on the basis of losses observed for atomic hydrogen adsorbed on GaAs(100) (1855-1875 cm-‘) [9,25,28]. Thus, arsine dissociates at temperatures below 300 K and part of the atomic hydrogen adsorbs on gallium sites (some attaches to As sites, see discussion, and some may directly desorb). Annealing to 400 K leads to further growth of the Ga-H and further decay of the As-H intensities. Above 450 K, the Ga-H stretch dominates until H, desorption is complete (550 K); at this point the HREEL
As-rich
Ga-rich T annsdng 550 K
Tann.a~~ng x500 : rrcryrw2i 2100
600 K
500 K
450 K
3.2. HREELS
I
I
AsH,/GaAs(lOO)
*Y_j\“Y500K
results
To establish As-H and Ga-H bonds within the adsorbed species, we have recorded, at 115 K, HREEL spectra for ASH,, unannealed and annealed, on both the Ga-rich (4 X 6) and the Asrich c(2 x 8) surfaces. The bottom trace in fig. 3 (left) is a spectrum of unannealed ASH, on the Ga-rich surface. It is dominated by the losses due to the surface optical phonons at 287, 575, 876, and -287 cm-’ corresponding the single, double, and triple phonon loss and the phonon gain peaks, respectively [26,27]. The only loss due to ASH, appears at 2100 cm-’ and is characteristic for the As-H stretch. The frequency lies very close to the gas-phase value (symmetric and asymmetric stretches at 2116 and 2123 cm-‘, respectively) [28] and to that for atomic hydrogen
2000
Electron
Energy
Loss
3000
[cm-‘]
Fig. 3. High-resolution electron energy loss spectra (HREELS) taken at 115 K after adsorption of ASH, at 115 K and annealing to the temperatures indicated. Above 250 K on the Ga-rich GaAs(lOO)-(4 x 6) surface (left panel), H transfer from As (2100 cm-t) to Ga species (1870 cm-‘) is observed. On the As-rich GaAs(lOO)-c(2 X 8) surface only the As-H stretch (2100 cm-‘) is observed. The peaks below 1500 cm-’ are due to the surface optical phonons of GaAs (see text).
M. Wolf et al. / Thermal decomposition of arsine on GaAs(lO0)
spectrum becomes indistinguishable from that of the clean surface. The top two traces in fig. 4 compare spectra for ASH, and AsD, after annealing to 450 K. The Ga-D stretch appears at 1360 cm-’ (isotope effect of 1.38 + 0.03). Within our experimental error, no kinetic isotope effect for the formation of the Ga-H (Ga-D) species was observed. The right-hand panel in fig. 3 shows a similar series of HREEL spectra on the As-rich GaAs(lOOM2 X 8) surface. For all annealing temperatures up to 540 K, the As-H stretch at 2100 cm-’ is present; unlike the Ga-rich surface, there is no evidence for Ga-H species. Even though Ga-H forms easily on the Ga-H surface, it does not accumulate on the As-rich surface, probably because Ga sites that bind H are blocked by As. Fig. 5 summarizes and compares the TD spectra and the HREELS intensities for the Ga-rich and the As-rich surfaces. For the former surface, it is obvious that the sharp drop in the HREELS intensity of the As-H stretch at low temperatures
ArsinelGaAs
0
1000 Electron
2000 Energy
Loss
-
3000 [cm-‘]
Fig. 4. HREEL spectra for ASH, and AsD, adsorbed on the Ga-rich surface. At 115 K, only the As-hydrogen stretch is present - 2100 cm-’ for As-H and 1490 cm-’ for As-D species, respectively. After annealing to 450 K, Ga-hydrogen species are exclusively formed with stretching frequencies of 1875 cm-’ for the Ga-H and of 1360 cm-’ for the Ga-D species, respectively. In both cases an isotope effect of 1.41+ 0.03 is obtained.
45
1.0 0.E
As rich
_
Ga rich
_
0.6 0.4 0.2 0.0 i.a 0.6 0.6 0.4 0.2 0.0
9
6
11IO
300
500
700
Temperature [K] Fig. 5. HREELS intensities of the As-H and Ga-H stretch versus annealing temperature for the As-rich (upper panel) and the Ga-rich (middle panel) surfaces, respectively. The As-H intensities have been normalized to that for 115 K. The bottom panel compares the thermal desorption traces of ASH, and H, from the Ga-rich (solid lines) and As-rich surfaces (dots), respectively.
(middle panel) correlates with the desorption of molecular arsine in the 144 K state (solid line, bottom panel). Above 200 K the As-H loss decreases monotonically while the Ga-H loss grows, reaching its maximum at 400 K. Paralleling the increased Ga-H intensity, desorption of ASH, is observed, mostly due to recombination of dissociation products. Above 400 K, H, desorption sets in and the Ga intensity decreases. Because the Ga-H stretch dominates the HREEL spectra above 450 K, we conclude that on the Ga-rich surface recombinative H, desorption around 500 K originates predominantly from Ga-H species. Turning to the As-rich surface, the dotted data in fig. 5 (bottom panel) show TDS of ASH, and H, after the same exposure as for the Ga-rich surface. At low temperatures, ASH, desorption
peaks at the same position (144 K> as for the Ga-rich surface, but it is broader and tails to higher temperatures. This corresponds to the rather slow monotonic decrease of the As-H HREELS intensity (upper panel). Dihydrogen desorption and the decay to zero of the HREELS peaks make a very interesting point: H remova from the C&-rich surface is completed at about 600 K but extends up to 700 K on the As-rich surface, as reported previously [263. These results accord with recent work [12], except we resolve only one peak, not two, in the region of the recombinative arsine desorption (200-450 K). This difference is likely due to slightly different preparation procedures, particularly of the Asrich surfaces, where we are unable to dose as heavily. Further, although we believe it is negligible, we cannot rule out a small amount of electron-induced damage (from the QMS ionizer) during TPD, even though we have the grounded grid in place.
To obtain a more complete picture of arsine thermal dissociation, we recorded a series of Xray photoelectron spectra for both the Ga- and As-rich surfaces. We chose to follow the As2pJi2 and Ga2p,/, peaks because they have the highest binding energies (BE), i.e., lowest kinetic energies, accessible with AlECar radiation and are, therefore, the most surface sensitive. To further enhance surface sensitivity, the spectra were taken at a detection angle of 75” with respect to the surface normal. Fig. 6 shows the As2p,/, (lefthand panel) and Ga2p,,, (right-hand panel) XP spectra after adsorption and annealing of AsH, on the Ga-rich surface. The background was removed by subtracting a straight line plus a smoothed step function fit to the regions outside the peak. Significantly, a clean surface is recovered after annealing to 870 K (top curve in fig. 6) and, under these conditions, both the As and Ga spectra can be fit by a single Gaussian-Lorentzian function yielding a binding energy of 1322.7 eV for As 2p, 2 (1.85 eV FWHM) and 1117.1 eV for Ga ZP,,, i 1.75 eV FWHM), respectively. After adsorption of molecular arsine at 115 K
r
I
1326
I
1
I
1323
I
I
I
1320
Binding
I
1
1120
Energy
I
1117
I
I
r_
1114
[eV]
Fig. 6. X-ray photoelectron spectra (KPS) of the AsZp,,, (left-hand panel) and Ga2p,,, core levels tright-hand panel) for ASH, adsorbed on the Ga-rich GaAs(lOOM4x 6) surface. The photoelectron detection angle was 7.5”with respect to the surface normal. The solid lines were obtained from fits to linear combinations of Gaussian-Lorentzian functions after bac~ro~~d subtraction. The Ca2pji, core iever can be fit with a single peak with 1117.1 eV binding energy (BE). The As2p,,, peak is deconvoluted into three contributions (dashed curves) which are assigned to substrate As (BE = 1322.7 ev), ASH, (x = 1,2) (BE = 1324.5 eV) and AsH, (BE = 1325.4 eV), respectively.
(bottom curve of fig. 61, there is a well-resolved additional As peak at 1325.4 eV Be - 2.7 eV higher than the substrate As atoms. If we analyze the series of XP spectra assuming only one high BE As peak for all ASH, (x = 1, 2, 3) species, the peak broaderrs and shifts ~u~~~u~~~a~~~ towards lower BE with increasing annealing temperature. We prefer an alternative; assuming two fixed-
47
M. Wolf et al. / Thermal decomposition of amine on GaAs(IO0)
..
;n N
A l.O-
, ,
2
.P z %
2
A&,,
I
I
BE
-
I 1322.7 eV
I
I,
0.2-y j~l:,
i&Q@.... _ P? “Q 0.0 _ d e‘&. -___1:~~5.:-_1~~=~------._,_____*/ I I I I
0.1 -
100
300
Annealing
500
700
Temperature
900
[K]
Fig. 7. Temperature dependence of the ratio of the As2p,,, integrated peak area to that for Ga2p,,,. Fits of the XP spectra in fig. 6 were used. The upper panel shows the ratio for the As substrate peak (BE = 1322.7 eV) and the lower pane1 the corresponding ratios .for ASH, (x = I,21 (BE = 1324.5 eV) and AsH, (BE = 1325.4 eV), respectively. See text for details.
width (1.85 eV) higher BE As peaks, one associated with molecular and the other with dissociated ASH,, the As2p,,, region can be successfully deconvoluted (dashed lines in fig. 6). This deconvolution gives a third peak (1324.5 eV> which intensifies with annealing up to 200 K while the 1325.4 eV peak decays. For 200 K, and higher, the low-temperature arsine desorption is complete and, as anticipated, there is no 1325.4 eV peak. Complementary HREELS data show Ga-H in this temperature regime, i.e., arsine has undergone dissociation leaving ASH, (x = 1, 2) and Ga-H. We, therefore, assign the As2p,/, XPS peak at 1324.5 eV to the dissociation products ASH, (x = 1,2) and the 1325.4 eV peak to molecularly adsorbed arsine (ASH,). From this XPS data, we are not able to resolve adsorbed ASH and ASH,. More quantitatively, the As 2p,,, and Ga 2p,,, ratios are s~marized for the various As peaks in fig. 7. The upper panel describes the substrate As peak, BE = 1322.7 eV. Amine desorption, which increases the relative number of substrate As atoms within the XPS sampling volume, accounts for the slight rise up to 200 K. No~alization by
sensitivity factors for the As2p,,, and Ga 2ps/2 XPS peaks, 1&/i& = 1.57 [29], indicates that there is excess Ga in the surface region, as expected for the GaAs(lOO)-(4 X 6) surface. The lower panel of fig. 7 shows the peak ratios for the molecular (1325.4) eV) and dissociated (1324.5 eV> arsine. The sharp decrease of the molecular ratio below 200 K correlates with the desorption of molecular arsine seen in TDS and HREELS (fig. 5). The variation of the ratio describing the dissociation products makes a key point: this curve (open circles) indicates that arsine starts to dissociate at temperatures as low as 140 K. At 200 K, there is no XPS evidence for molecular arsine, supporting our interpretation that the highertemperature (290 K) ASH, TDS peak involves recombination of ASH, and H. In the following, a simple model is used to derive absolute coverages from our XPS measurements [30], We assume that the GaAs crystal is composed of alternating homogeneous As and Ga layers and that the Ga-rich surface is terminated by Ga, relative concentration xoa. Summing over the contributions of individual layers, the intensities of the As and Ga peaks, IAs and 1o, are: 1:s I,, = 1 - exp( -d/A,, x fc1 -%a)
cos 0)
+Xcia exp( -b/A,,
cos @)I, (1)
CL! 43, =
1 - exp( -d/hGa
cos 0)
X [xGa + (1 --xGa) ew( --b/h,,
~0s @)I, (2)
where d is the Ga-Ga distance, b the As-Ga distance, 0 the detection angle and A,, and A,, are the mean free path lengths of the As2p3,z and Ga2~,,, Photo-electrons, respectively. Evaluation of this model results in a Ga coverage of 0.73 f 0.1 ML for the GaAs(lOO)-(4 x 6) surface (1 ML = 6.26 X lOi cmp2, surface density for a complete Ga layer). This is in agreement with the early work of Drathen et al. 1141 and a more recent STM study of the GaAs(100) surface [19].
M. Wolf& al. / Thermal decomposition of amine on GaAs(100)
48
As a control we measured XPS signals along the surface normal, 0 = 0” (not shown). Consistent with the more accurate results at 0 = 75”, we calculate a surface Ga concentration of 0.65 f 0.2 ML. Using the model of eqs. (1) and (21, we derive (see table 1) an ASH, coverage, on the Ga-rich surface (1200 s dose at 115 K, fig. 2), of 0.16 k 0.04 ML. According to fig. 7, approximately 50% of this undergoes dissociation (partial) upon annealing to 200 K. Our TDS results show that at 350 K, recombinative arsine desorption is almost completed and that the remaining dissociation products decompose irreversibly, leading to As incorporation into the GaAs lattice. From fig. 7 we estimate that 1.5f 5% of the initial ASH, coverage, i.e., 0.025 & 0.008 ML, undergoes irreversible dissociation to add As to the surface. In terms of the bulk As signal, this amount lies within the scatter of the 1322.7 eV BE data (upper panel of fig. 7). We have also recorded a similar series of XP spectra for the As-rich surface (fig. 8). After annealing the As-rich surface to 650 K, two Gaussian-Lorentzian peaks are required to fit the As2p,,* region (bottom spectrum). The spectrum is dominated by an As substrate peak at 1322.7 eV BE (1.85 eV FWHM) with an As: Ga peak ratio of 1.7, i.e., 1.6 x that observed for the Ga-rich surface. The higher BE (1324.5 eV> is assigned to excess As, denoted As..,, not incorporated into the GaAs lattice. One might expect such As atoms to be stabilized by remaining hydrogen atoms, i.e., As-H bonds. However, our HREELS and TDS data, as well as a previous study [26], all indicate that annealing to 650 K completely removes of hydrogen. The relative contributions of the high and low BE As peaks is shown in the inset of fig. 8. Using the model Table 1 Coverages
derived
from XPS analysis
(1 ML = 6.26
x
1
10“’ surface
d t
-
Ga rich “4 x 6”
1.0 -, 0.4 -’
0.2-•
1 I oo.o-.-~-..
b
I I
..._...____ o--~
‘a \.
-1:
100
I
1326
I
1322
1324 Binding
Energy
1320
1318
[eVl
Fig. 8. X-ray photoelectron spectra of the As2p,,, core level for the clean As-rich GaAs(lOO)-c(2 X 8) surface (bottom spectrum) and for the arsine-saturated surface. The inset shows the peak intensities of substrate As (BE = 1322.7 eV), ASH, (X = 1,2) (BE = 1324.5 eV), and ASH, (BE = 1325.4 eV), respectively, each divided by the intensity of the Ga2p,,, peak.
presented above, we calculate (summarized in table l), for the As-rich surface, a total As coverage of 0.76 + 0.1 ML in agreement with previous studies [14,19]. The excess As atoms account for
atoms/cm*)
(ML)
Ga
ASH, (115 K)
ASH,
Ga-rich (4 x 6) As-rich c(2 x 8)
0.73 f 0.1 0.24 f 0.1
0.164 f 0.04 0.154 f 0.04
0.08 + 0.02 0.05 + 0.02 a)
with excess As (XPS BE = 1325.3 eV).
700
As-rich GaAs
Coverage
overlaps
I 500
Annealing Temp. [K]
GaAs(100) surface
a) Estimated:
-----0.-- . . . . . . . . . . . . .. . . . . . o
. ..___-_*--__~---__.._.... I 300
0.0 -,
I I
(x = 1, 2) (200 K)
M. Wolf et al. / Thermal decompositionof arsine on GaAs(lO0)
0.12 + 0.05 ML or _ 16% of the total As coverage. The 1325.3 eV peak, due to adsorbed molecular arsine, is not well resolved in the upper A~2p~,~ spectrum but can be deconvoluted as shown. We did not try to further deconvolute the relative contributions of the excess As atoms and possible dissociation products ASH, (x = 1, 2). However, the temperature dependence of the ratio for the 1324.5 eV peak (open circles in the inset) indicates that ASH, dissociates (we estimate at least 30%) on the As-rich surface. That part of this dissociation is irreversible, is evident from the H, desorption between 400 and 700 R (fig. 5). The coverages derived for the As-rich surface are summarized in table 1.
4. Discussion In the following, we outline a chemical pathway for arsine decomposition on GaAs(100) that is consistent with the literature and with our XPS, TDS and HREELS results. It is evident, pa~icularly from XPS where there is only one chemical state, that ASH, adsorbs molecularly at 115 K on both the Ga- and As-rich surfaces (reaction (3)).
Af%(g)
115
ASH,(a).
(3)
The thermal de~mposition pathway can be described in terms of three temperature regimes: (I) 140-200 K, (II) 200-400 K, and (III) 400-700 K. Region I: Annealing through this region leads to parent desorption (via reaction (4)) and, based on XPS, to dissociation with the As-containing products remaining on the surface.
~%(a)
140-200 ASH,(g),
(4)
On the Ga-rich surface, dissociation accounts for approximately 50% of the initial arsine coverage. On both the As- and Ga-rich surfaces, only the As-H stretch (HREELS) is observable in this temperature region. Thus, dissociation populates surface As, not Ga, sites with H. This is expected in the As-rich case. For the Ga-rich surface, the preference for As-H formation can be rational-
49
ized using the structure model proposed by Biegelsen et al. [19]. According to this model, the Ga-rich ~$8 x 2)-GaAs(lO0) surface consists of ridges and troughs. The Ga dimers forming the rows of ridges would be possible adsorption sites for reversible adsorption and desorption according to reactions (3) and (4). The troughs contain plausible dissociation sites; Ga atoms from the second layer are accessible and they are surrounded by the As atoms of the top layer. If ASH, dissociates preferentially on Ga sites within the troughs, the neighboring structure conveniently places As atoms to receive the abstracted hydrogen (and from which, as discussed below, recombination could easily occur). For energetic reasons, we expect that As-H dissociation proceeds stepwise, rather than in concerted fashion, via reactions (5) and (6) (the subscript “As-sites” denotes arsenic sites): ASH,(a)
‘4o-200 ASH,(a)
AsI&
‘140
+ H(aAs+ites),
AsH( a) + H( aAs_sites).
(5) (6)
At N 200 K the desorption of molecular arsine (via eq. (4)) is completed and ASH, (x = 1, 2) and H are the only products remaining on the surface. Region II: Above 200 K, we observe additional ASH, desorption in a broad range, peaked around 300 K. We attribute this to the recombination of dissociation products, and/or to disproportionation of neighboring ASH,. Keeping in mind that more than 5 of the dissociation appears to be reversible, it is obvious that disproportionation alone is insufficient. Since some As is deposited, the stepwise decomposition of arsine must, with increasing temperature, proceed in competition with the recombination process. Thus, at least two additional reactions must be considered: AsH( a) 2oo-400 As(a) + H(a),
(7)
ASH,(a)
(8)
+ H(a) 200-400 ASH,(g).
On the Ga-rich surface, N 15% of the initial arsine coverage irreversibly dissociates leading to As deposition via eq. (7). The recombination (reaction (8)) requires activation and migration of either ASH,(a) and/or H. H migration, consistent with the observed activated transfer of hydrogen on the Ga-rich surface, forms Ga-H
M. Wolfet al. / Thermal decomposition of arsine on GaAs(100)
50
which, on this surface, is thermally more stable than As-H: H(a.4s-sites
1 2oo-400
H( aoa_sites), (Ga-rich) . (9)
Region ZZZ:After the recombination process, reaction (81, is complete (400 K), hydrogen recombines in a broad temperature range. Based on HREELS, H, is from Ga-H on the Ga-rich surface, and from As-H on the As-rich surface, i.e., H(
aGa-sites
H(a.4s-sites)
1 4oo-600
H,(g),
(Ga-rich),
(lOa)
45o-700
H,(g),
(As-rich).
(lob)
Interestingly the temperature for hydrogen desorption from the As-rich surface is higher than from the Ga-rich surface. Thus, the activation energy for recombinative H,-desorption appears to be higher on the As-rich GaAs(lOO)-c(2 x 8) surface. Considering practical applications, the higher H, desorption temperature on the As-rich surface implies that removal of hydrogen from As sites kinetically limits the final stages of an arsenic ALE cycle. In the initial (Ga-rich) stages of an As ALE deposition cycle, hydrogen will preferentially stay on Ga and the desorption temperature will be lower (short residence time) compared to the final (As-rich) stages. The overall reaction rate will be effectively limited by the low reactive sticking coefficient (1O-6) of arsine 1211. However, the temperature window for ALE growth of GaAs is still mainly determined by the thermal stability of the Ga precursor. For example, using trimethylgallium (TMG) as a precursor, epitaxial growth is achievable at 450-500°C while triethylgallium (TEG) requires only about 400°C [5]. New chemically designed precursor molecules may lead to a further reduction in these temperatures [31]. If this is possible, surface mobility may well control the growth rate, crystal quality and surface stoichiometry. These considerations will also be important in low-pressure OMCVD where surface kinetics are important. As a final note, we observe the onset of ASH, decomposition at a temperature below 140 K much lower than the range of 300-450 K re-
ported under OMCVD conditions [ll]. Considering that conventional OMCVD is rate-limited by mass transport, not surface kinetics, this difference might well be attributed to reactions in the gas-phase diffusion region above the substrate at high pressures. The low reactive sticking coefficient of ASH, limits the extent to which its pressure can be lowered. This restriction might be overcome by laser-assisted deposition techniques which can operate at low temperatures and pressures [32].
5. Summary The results of this paper can be summarized as follows: (1) Arsine adsorbs molecularly on GaAs(100) at 115 K with a saturation coverage of 0.16 + 0.04 ML (1 ML = 6.26 x 1014 cm-*). (2) Around 140 K, the parent molecules desorbs accompanied by the onset of dissociation to form ASH, (X = 1,2) and H. (3) On Ga-rich GaAs(lOO)-(4 X 6), N 50% of the initial coverage dissociates. A significant fraction of this recombines and desorbs as ASH, at 290 K. But approximately 0.025 ML (- 15% of the initial coverage) is finally deposited as As on the surface. In parallel with recombinative ASH, desorption between 200 and 400 K, some H transfers from As to Ga sites. (4) On As-rich GaAs(lOO)-c(2 X 81, H stays exclusively on As sites until recombination and desorption of H, occurs. H, desorption extends to 700 K, N 100 K higher than on the Ga-rich surface.
Acknowledgements
The authors would like to thank B.A. Banse and J.R. Creighton for helpful comments and for providing the AsD, sample. M.W. acknowledges a Feodor Lynen research fellowship of the Alexander von Humboldt Society. This work was supported in part by the Science and Technology Center Program of the National Science Foundation, grant CHE 8920120.
M. Wolf et al. / Thermal decomposition of amine on GaAs(100)
References HI J.L. Jewel], J.P. Harbison,
A. Scherer, Y.H. Lee and L.T. Florez, IEEE Quantum Electron. 27 (1991) 1332. El F.T.J. Smith, Prog. Solid State Chem. 19 (1989) 111. 131 L.M. Miller and J.J. Coleman, CRC Crit. Rev. Solid State Mater. Sci. 15 (1988) 1. and M.V. Pessa, J. Appl. Phys. 60 141 C.H.L. Goodman (1986) R65. Annu. Rev. Mater. Sci. 21 151 A. Usui and H. Watanabe, (1991) 185, and references therein. 161 K. Tamaru, J. Phys. Chem. 59 (1955) 777. 171 M.R. Leys and H. Veenvliet, J. Cryst. Growth 55 (1981) 145. k31C.A. Larsen, NJ. Buchanan and G.B. Stringfellow, Appt. Phys. Lett. 52 (1988) 480. 191 P.E. Gee and R.F. Hicks, Mater. Res. Sot. Symp. Proc. 222 (1991) 43. J. Cryst. Growth 93 ml J. Nishizawa and T. Kurabayashi, (1988) 98. P. Tommak, A. Hertling, H.J. Koss, P. [ill R. Luckerath, Balk, K.F. Jensen and W. Richter, J. Cryst. Growth 93 (1988) 151. WI B.A. Banse and J.R. Creighton, Surf. Sci. Lett., submitted. [131 X.-Y. Zhu, M. Wolf, T. Huett, J. Nail, B.A. Banse, J.R. Creighton and J.M. White, Appl. Phys. Lett. 60 (1992) 977. [141 P. Drathen, W. Ranke and K. Jacobi, Surf. Sci. 77 (1978) L162. Ml R.Z. Bachrach, R.S. Bauer, P. Chiaradia and G.V. Hansson, J. Vat. Sci. Technol. 18 (1981) 797.
51
[16] J. Massies, P. Etienne, F. Dezaly and N.T. Linth, Surf. Sci. 99 (1980) 121. [17] M.D. Pashley, K.W. Haberern, W. Friday, J.M. Woodall and P.D. Kirchner, Phys. Rev. Lett. 60 (1988) 2176. [18] D.J. Chadi, J. Vat. Sci. Technol. A 5 (1987) 834. [19] D.K. Biegelsen, R.D. Bringans, J. Northrup and L.-E. Swartz, Phys. Rev. B 41 (1990) 5701. [20] J.R. Creighton, Surf. Sci. 234 (1990) 287. 1211 J.R. Creighton and B.A. Banse, Mater. Res. Sot. Symp. Proc. 222 (1991) 15. [22] M.L. Yu, U. Memmert and T.F. Kuech, Appl. Phys. Lett. 55 (1989) 1011. 1231 B.A. Banse and J.R. Creighton, Appl. Phys. Lett., submitted. [24] D.J. Frankel, C. Yu, J.P. Harbison and H.H. Farrell, J. Vat. Sci. Technol. B 5 (1987) 1113. [25] J.R. Creighton, J. Vat. Sci. Technol. A 8 (1990) 3984. 1261 H. Liith and R. Matz, Phys. Rev. Lett. 46 (1981) 1652. 1271 L.H. Dubois and G.P. Schwartz, Phys. Rev. B 26 (1982) 794. 1281 V.M. McConagie and H.H. Nielsen, Phys. Rev. 75 (1949) 633. 1291 C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy (Perkin-Elmer, Eden Prairie, MN, 1979). [30] W. Ranke and K. Jacobi, Surf. Sci. 63 (1977) 33. [31] A.H. Cowley, R.A. Jones, P.R. Harris, D.A. Atwood, L. Contreras and C.J. Burek, Angew. Chem., in press; J.E. Miller and J.G. Ekerdt, Chem. Mater., submitted. 1321 X.-Y. Zhu, M. Wolf and J.M. White, J. Chem. Phys., in press.
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Surface Science North-Holland
surface science ::...::+... ,,.. _. .......:.~:~.~~~~~:~:~iiiijj ~~~~z~:~:~:~:~:i:8~~.:~:~:~,~,.~,~ ,:.: iiir’.:.: ......~.~...,:;,
275 (1992) 41-51
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Thermal decomposition
of arsine on GaAs( 100)
M. Wolf, X.-Y. Zhu, T. Huett and J.M. White Department of Chemistry and Biochemistry, Center for Materials Chemistry, University of Texas, Austin, TX 78712, USA Received
17 January
1992; accepted
for publication
13 April
1992
The thermal dissociation pathways of ASH, on Ga-rich (4 x 6) and As-rich ~$2 X 8) GaAs(100) surfaces have been studied with high-resolution electron energy loss (HREELS), X-ray photoelectron (XPS) and thermal desorption spectroscopy (TDS). Arsine adsorbs molecularly at 115 K and, based on XPS data, the saturation coverage is 0.16 f 0.04 ML. In TDS, desorption of the parent molecule is accompanied by As-H bond dissociation starting at temperatures as low as 140 K. Of the resulting AsH, (x = 1, 2) and H, part recombines to liberate arsine but approximately 15% of the initial ASH, coverage dissociates irreversibly resulting in As deposition on the GaAs surface. While on the As-rich surface there is no evidence for Ga-H formation, we do observe transfer of hydrogen from As to Ga sites on the Ga-rich surface. Recombinative hydrogen desorption from Ga-H occurs around 500 K on Ga-rich surfaces, whereas H, desorbs at higher temperatures (- 550 K) from As-H on As-rich surfaces. With regard to atomic layer epitaxy (ALE) of GaAs, these results imply that, in the final stages of an As cycle, removal of hydrogen from As sites limits the rate whereas in the initial stages, Ga-H is the reaction intermediate.
1. Introduction
The growth of high-quality, layered structures of compound semiconductors is a key technology for the development of optoelectronic devices, e.g., semiconducting lasers [ll. In order to shrink the dimensions of such devices and to minimize thermal stress and interface reactions, it is desirable to reduce the operating pressures and temperatures in organometallic chemical-vapor deposition (OMCVD) processing [2,31. Moreover, selective growth mechanisms need to be found, especially for three-dimensional structures. Thus, su@zce chemical reactions will play an increasingly important role in materials processing. However, there is often a surprising lack of knowledge about the surface chemistry of precursor molecules used in OMCVD and atomic layer epitaxy (ALE) [4,5]. The molecule used here, arsine, is the most widely used As precursor in OMCVD and ALE of GaAs and other As-based compound semiconductors. Its thermal decomposition is catalyzed by As, Ga or GaAs surfaces [6,7]. This has been 0039-6028/92/$05.00
0 1992 - Elsevier
Science
Publishers
demonstrated by isotope exchange measurements where no HD could be observed during pyrolysis of ASH, in a D, atmosphere above a GaAs substrate [8]. However, the detailed reaction mechanism is not understood and has even been called a mystery [9]. While Nishizawa and Kurabayashi reported that arsine dissociates rapidly at 920-980 K [lo], Luckerath et al. determined, from in situ CARS measurements, that dissociation sets in at 330-450 K [ll]. The surface chemistry of ASH, on GaAs has been studied only very recently. In a very interesting paper, Banse and Creighton report on the TDS of adsorbed ASH, on GaAs(100) enriched to various levels with As [12]. Complementing this important work, we present here a detailed study of the dissociation pathway of ASH, on one Ga-rich and one As-rich GaAs(100) surface using TDS, HREELS and XPS. A brief report has been published recently [ 131. Because it is the most commonly used substrate for OMCVD and ALE growth of device structures, we chose the (100) surface of GaAs. Its surface structure is relatively complex; several
B.V. All rights
reserved
42
M. Wolf et al. / Thermal decomposition of arsine on GaAs(100)
reconstructions with distinct LEED patterns and different surface stoichiometries are known [14161. One key structure is the (2 x 4)-c(2 X 8) Asrich surface for which there is STM [17] and theoretical work [18]. The unit cell consists of three As dimers (parallel to [Oil]) and one missing dimer exposing the underlying Ga atoms. Thus, the As coverage, for an ideal c(2 x 8) Asrich surface, is 0.75 ML (1 ML = 6.26 x 1014 cm-*, the surface density for a complete As or Ga layer). The structures of the various Ga-rich surfaces are not so well established. A recent STM investigation reveals, for the c(8 X 2) “(4 X 1)” Ga-rich surface, a “ridge and trough” structure [19] which can be modelled in terms of a (4 x 2) unit cell with double rows of Ga dimers and two missing Ga dimers which lead to the exposure of second-layer Ga atoms. The surface Ga concentration in this structure is 0.75 ML and there are a number of chemically different possible adsorption sites within the unit cell. Another reconstruction, denoted “(4 X 6)“, has been discussed as an overlap of “(4 X 1)” and “(2 X 6)” domains [14,20]. We have studied the decomposition of arsine on two structures: the Ga-rich (4 x 6) surface and the As-rich GaAs(lOO)-c(2 X 8) surface. These two cases model the initial and final stages, respectively, of an As cycle in ALE.
2. Experimental The experiments were performed in a previously described [21] ultrahigh-vacuum chamber with a base pressure of 2 X lo- lo Torr. It was equipped with a quadrupole mass spectrometer (QMS) for thermal desorption spectroscopy (TDS) and residual gas analysis, a high-resolution electron energy loss spectrometer for vibrational analysis (HREELS), a hemispherical energy analyzer and an X-ray source for X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED) optics. The sample (1.5 X 10 X 1 mm3) was cut from a semi-insulating GaAs(lOO) wafer (10i5/cm3 Stdoped). To improve resistive heating, a 3000 A thick Ta film was sputtered onto its backside. The sample was held with two Ta clips spot-welded to
MO rods. The latter were connected to a copper sample mounting block which connected to a cooled reservoir. The sample temperature was monitored by a chromel-alumel thermocouple spot-welded to a small Ta clip which was glued, with a high-temperature cement (Aremco-571), to the bottom edge of the sample. The crystal could be cooled to 115 K by contact with liquid nitrogen, and heated to 900 K with a typical rate of 7 K/s. No temperature calibration, i.e., pyrometry, was done; however, we observed the onset of atomic As and Ga desorption at 580°C in agreement with previous studies [20,21]. The sample was cleaned by cycles of: (1) Ar+ bombardment (20 min, 3.5 keV, 2-3 pAA), (2) annealing at 500°C for 10 min, and (3) briefly flashing to 600°C. After this procedure, the surface cleanliness was verified by XPS and LEED. The latter showed a “(4 x 6)” pattern typical for the Ga-rich surface [14,15,20] with very bright and sharp 4-fold spots and much weaker 6-fold spots. As discussed previously [19,20], this Ga-rich surface consists mainly of (4 x l), with some (2 X 6), domains. To ensure reproducibility, the sample was routinely flashed to 600°C before each ASH, dose. Amine (ASH,: Linde 99.9995% purity; AsD,: MSD 98% isotopic purity) was cleaned by freeze-pump-thaw cycles. It was dosed, with the sample at 115 K, through a retractable 2 pm pin-hole doser which terminated approximately 2 mm from the sample. This minimized adsorption on components other than the sample. Reproducible fluxes were achieved by fixing the pressure, measured with a capacitance manometer, behind the pin-hole. Doses were then proportional to the exposure time. The As-rich GaAs(lOO)-c(2 X 8) surface was prepared by dosing N 4500 L ASH, with the sample at 275°C and subsequent annealing to 460°C [22,23]. After annealing, LEED showed a weak c(2 x 8) pattern indicating some disorder between small (2 X 8) domains. HREEL spectra were recorded with a primary electron beam energy of 3 eV and an elastic peak FWHM of 7-8 meV. XP spectra, measured at a detection angle of 75”, with respect to the surface normal, to enhance the surface sensitivity, were excited with AlKa radiation (1486.6 eV) and
M. Wolf et al. / Thermal decomposition of arsine on GaAs(100)
were measured with a hemispherical analyzer operated at 30 eV pass energy. All HREEL and XP spectra were taken at 115 K. TD spectra were recorded with the sample in the line of sight with the QMS ionizer. To reduce electron-stimulated surface processes, the sample was shielded from the ionizer by a grounded shroud with 5 mm diameter mesh-covered entrance aperture.
3. Results 3.1. Thermal desorption
Fig. 1 shows a series of ASH, and H, thermal desorption spectra for different ASH, exposures (given in s for a fixed flux) onto a gallium-rich GaAs(lOO)-(4 X 6) surface. Below 850 K, molecular arsine and hydrogen are the only desorption products for all exposures. ASH, desorbs in two peak - the major peak, which shifts with increasr
I
I
I
I
I
500
1
K
,
/ t 144
K
ASH, /GaAs( 100)
A
T,,,=115K
a”
2’ L
lot 1
I
200
I
300
I
400
I
500
I
600
17
700
Temperature [K] Fig. 1. Thermal desorption spectra (TDS) of ASH, adsorbed on the Ga-rich GaAs(lOO)-(4x6) surface at 115 K. Bottom panel: TDS of molecular arsine (m/e = 78) taken after the indicated exposures. Top panel: H, desorption spectra (m/e = 2). The H, signal below 200 K is attributed to a cracking of AsH, in the QMS ionizer.
43
ing coverage from 165 to 144 K, and a weaker peak around 290 K. The H, signal observed below 300 K is proportional to the ASH, signal and is ascribed to cracking of ASH, in the QMS ionizer. At higher temperatures, H, desorbs in a broad range around 500 K, in agreement with studies of atomic hydrogen adsorption, i.e., H atom recombination [24,251. This indicates that some arsine dissociates and, after H, desorption, deposits atomic As. Consistent with the XPS results presented below, we attribute the arsine TDS peak at 144 K to desorption from weakly bound ASH,. Based on HREELS and XPS described below, the ASH, desorption around 300 K is ascribed to recombination of ASH, (x = 1, 2) and H, not to desorption of chemisorbed molecular species. As mentioned earlier, the Ga-rich (4 X 6) surface offers several distinct potential sites for adsorption and dissociation. To address the role of specific sites in the recombinative arsine desorption, we dosed ASH, for 1200 s, flashed off the low-temperature peak at 144 K and redosed for 1200 s. The resulting TD spectra are indistinguishable from the corresponding spectrum for 1200 s in fig. 1. This indicates that the dissociation products [ASH, (x = 1, 2) and/or H] occupy specific sites and, once these are filled, no further dissociation occurs. Fig. 2 shows the variation, with arsine exposure, of the TDS peak areas. It is obvious that the hydrogen desorption at 500 K saturates for doses longer than 300 s while the ASH, desorption peaks continue to increase, at least to 1200 s. The ratio between the two arsine TDS areas is (within *25%) independent of coverage, suggesting, in agreement with the above discussion, that there are at least two distinct adsorption sites within the unit cell - one nonreactive site leading to molecular desorption (144 K) and one reactive site leading to partial dissociation. That the population ratio is coverage independent, shows that there is no exchange between these two sites. Thus, the dissociation process, monitored by the recombination peak (N 290 K) does not compete with molecular desorption in the 144 K state. The absolute coverages indicated on the righthand axis in fig. 2 are derived from XPS data
M. Wolf et al. / Thermal decomposition
44
0.0
i
I
I I
I
I
0.00 0.10 0 $ 2 0.05 E
0.00 ASH, Dose [s]
Fig. 2. TDS peak areas as a function of ASH, exposure, Lower panel: The relative ASH, peak areas of the 144 (open circles) and 290 K (filled circles), normalized by the 200 s value of the total ASH, area. Upper panel: The H, TDS area normalized by its saturation value. The coverages indicated on the right-hand axis are derived by XPS (see section 3.3).
presented in section 3.3. The estimated ASH, flux through our pinhole doser is 2 X 1014 molecules/(s cm*). Using this value, together with the coverages derived from the XPS, the initial sticking coefficient at 115 K is roughly 10p3. In the following HREELS and XPS experiments, if not otherwise stated, the initial arsine coverage equals that of the 1200 s exposure in fig. 1.
of arsine on GaAs(100)
adsorbed on GaAs(100) (2100-2110 cm-‘) [9,25,281. Substituting AsD, (see fig. 41, the As-D stretch appears at 1490 cm-’ leading, as expected, to an isotope effect of 1.41 k 0.03. After annealing the Ga-rich surface to 200 K, there is no shift in the As-H frequency, but there is a significant intensity decrease attributable to desorption of molecular arsine. Upon annealing to 300 K, a new loss at 1870 cm-’ appears; it is assigned to the Ga-H stretch on the basis of losses observed for atomic hydrogen adsorbed on GaAs(100) (1855-1875 cm-‘) [9,25,28]. Thus, arsine dissociates at temperatures below 300 K and part of the atomic hydrogen adsorbs on gallium sites (some attaches to As sites, see discussion, and some may directly desorb). Annealing to 400 K leads to further growth of the Ga-H and further decay of the As-H intensities. Above 450 K, the Ga-H stretch dominates until H, desorption is complete (550 K); at this point the HREEL
As-rich
Ga-rich T annsdng 550 K
Tann.a~~ng x500 : rrcryrw2i 2100
600 K
500 K
450 K
3.2. HREELS
I
I
AsH,/GaAs(lOO)
*Y_j\“Y500K
results
To establish As-H and Ga-H bonds within the adsorbed species, we have recorded, at 115 K, HREEL spectra for ASH,, unannealed and annealed, on both the Ga-rich (4 X 6) and the Asrich c(2 x 8) surfaces. The bottom trace in fig. 3 (left) is a spectrum of unannealed ASH, on the Ga-rich surface. It is dominated by the losses due to the surface optical phonons at 287, 575, 876, and -287 cm-’ corresponding the single, double, and triple phonon loss and the phonon gain peaks, respectively [26,27]. The only loss due to ASH, appears at 2100 cm-’ and is characteristic for the As-H stretch. The frequency lies very close to the gas-phase value (symmetric and asymmetric stretches at 2116 and 2123 cm-‘, respectively) [28] and to that for atomic hydrogen
2000
Electron
Energy
Loss
3000
[cm-‘]
Fig. 3. High-resolution electron energy loss spectra (HREELS) taken at 115 K after adsorption of ASH, at 115 K and annealing to the temperatures indicated. Above 250 K on the Ga-rich GaAs(lOO)-(4 x 6) surface (left panel), H transfer from As (2100 cm-t) to Ga species (1870 cm-‘) is observed. On the As-rich GaAs(lOO)-c(2 X 8) surface only the As-H stretch (2100 cm-‘) is observed. The peaks below 1500 cm-’ are due to the surface optical phonons of GaAs (see text).
M. Wolf et al. / Thermal decomposition of arsine on GaAs(lO0)
spectrum becomes indistinguishable from that of the clean surface. The top two traces in fig. 4 compare spectra for ASH, and AsD, after annealing to 450 K. The Ga-D stretch appears at 1360 cm-’ (isotope effect of 1.38 + 0.03). Within our experimental error, no kinetic isotope effect for the formation of the Ga-H (Ga-D) species was observed. The right-hand panel in fig. 3 shows a similar series of HREEL spectra on the As-rich GaAs(lOOM2 X 8) surface. For all annealing temperatures up to 540 K, the As-H stretch at 2100 cm-’ is present; unlike the Ga-rich surface, there is no evidence for Ga-H species. Even though Ga-H forms easily on the Ga-H surface, it does not accumulate on the As-rich surface, probably because Ga sites that bind H are blocked by As. Fig. 5 summarizes and compares the TD spectra and the HREELS intensities for the Ga-rich and the As-rich surfaces. For the former surface, it is obvious that the sharp drop in the HREELS intensity of the As-H stretch at low temperatures
ArsinelGaAs
0
1000 Electron
2000 Energy
Loss
-
3000 [cm-‘]
Fig. 4. HREEL spectra for ASH, and AsD, adsorbed on the Ga-rich surface. At 115 K, only the As-hydrogen stretch is present - 2100 cm-’ for As-H and 1490 cm-’ for As-D species, respectively. After annealing to 450 K, Ga-hydrogen species are exclusively formed with stretching frequencies of 1875 cm-’ for the Ga-H and of 1360 cm-’ for the Ga-D species, respectively. In both cases an isotope effect of 1.41+ 0.03 is obtained.
45
1.0 0.E
As rich
_
Ga rich
_
0.6 0.4 0.2 0.0 i.a 0.6 0.6 0.4 0.2 0.0
9
6
11IO
300
500
700
Temperature [K] Fig. 5. HREELS intensities of the As-H and Ga-H stretch versus annealing temperature for the As-rich (upper panel) and the Ga-rich (middle panel) surfaces, respectively. The As-H intensities have been normalized to that for 115 K. The bottom panel compares the thermal desorption traces of ASH, and H, from the Ga-rich (solid lines) and As-rich surfaces (dots), respectively.
(middle panel) correlates with the desorption of molecular arsine in the 144 K state (solid line, bottom panel). Above 200 K the As-H loss decreases monotonically while the Ga-H loss grows, reaching its maximum at 400 K. Paralleling the increased Ga-H intensity, desorption of ASH, is observed, mostly due to recombination of dissociation products. Above 400 K, H, desorption sets in and the Ga intensity decreases. Because the Ga-H stretch dominates the HREEL spectra above 450 K, we conclude that on the Ga-rich surface recombinative H, desorption around 500 K originates predominantly from Ga-H species. Turning to the As-rich surface, the dotted data in fig. 5 (bottom panel) show TDS of ASH, and H, after the same exposure as for the Ga-rich surface. At low temperatures, ASH, desorption
peaks at the same position (144 K> as for the Ga-rich surface, but it is broader and tails to higher temperatures. This corresponds to the rather slow monotonic decrease of the As-H HREELS intensity (upper panel). Dihydrogen desorption and the decay to zero of the HREELS peaks make a very interesting point: H remova from the C&-rich surface is completed at about 600 K but extends up to 700 K on the As-rich surface, as reported previously [263. These results accord with recent work [12], except we resolve only one peak, not two, in the region of the recombinative arsine desorption (200-450 K). This difference is likely due to slightly different preparation procedures, particularly of the Asrich surfaces, where we are unable to dose as heavily. Further, although we believe it is negligible, we cannot rule out a small amount of electron-induced damage (from the QMS ionizer) during TPD, even though we have the grounded grid in place.
To obtain a more complete picture of arsine thermal dissociation, we recorded a series of Xray photoelectron spectra for both the Ga- and As-rich surfaces. We chose to follow the As2pJi2 and Ga2p,/, peaks because they have the highest binding energies (BE), i.e., lowest kinetic energies, accessible with AlECar radiation and are, therefore, the most surface sensitive. To further enhance surface sensitivity, the spectra were taken at a detection angle of 75” with respect to the surface normal. Fig. 6 shows the As2p,/, (lefthand panel) and Ga2p,,, (right-hand panel) XP spectra after adsorption and annealing of AsH, on the Ga-rich surface. The background was removed by subtracting a straight line plus a smoothed step function fit to the regions outside the peak. Significantly, a clean surface is recovered after annealing to 870 K (top curve in fig. 6) and, under these conditions, both the As and Ga spectra can be fit by a single Gaussian-Lorentzian function yielding a binding energy of 1322.7 eV for As 2p, 2 (1.85 eV FWHM) and 1117.1 eV for Ga ZP,,, i 1.75 eV FWHM), respectively. After adsorption of molecular arsine at 115 K
r
I
1326
I
1
I
1323
I
I
I
1320
Binding
I
1
1120
Energy
I
1117
I
I
r_
1114
[eV]
Fig. 6. X-ray photoelectron spectra (KPS) of the AsZp,,, (left-hand panel) and Ga2p,,, core levels tright-hand panel) for ASH, adsorbed on the Ga-rich GaAs(lOOM4x 6) surface. The photoelectron detection angle was 7.5”with respect to the surface normal. The solid lines were obtained from fits to linear combinations of Gaussian-Lorentzian functions after bac~ro~~d subtraction. The Ca2pji, core iever can be fit with a single peak with 1117.1 eV binding energy (BE). The As2p,,, peak is deconvoluted into three contributions (dashed curves) which are assigned to substrate As (BE = 1322.7 ev), ASH, (x = 1,2) (BE = 1324.5 eV) and AsH, (BE = 1325.4 eV), respectively.
(bottom curve of fig. 61, there is a well-resolved additional As peak at 1325.4 eV Be - 2.7 eV higher than the substrate As atoms. If we analyze the series of XP spectra assuming only one high BE As peak for all ASH, (x = 1, 2, 3) species, the peak broaderrs and shifts ~u~~~u~~~a~~~ towards lower BE with increasing annealing temperature. We prefer an alternative; assuming two fixed-
47
M. Wolf et al. / Thermal decomposition of amine on GaAs(IO0)
..
;n N
A l.O-
, ,
2
.P z %
2
A&,,
I
I
BE
-
I 1322.7 eV
I
I,
0.2-y j~l:,
i&Q@.... _ P? “Q 0.0 _ d e‘&. -___1:~~5.:-_1~~=~------._,_____*/ I I I I
0.1 -
100
300
Annealing
500
700
Temperature
900
[K]
Fig. 7. Temperature dependence of the ratio of the As2p,,, integrated peak area to that for Ga2p,,,. Fits of the XP spectra in fig. 6 were used. The upper panel shows the ratio for the As substrate peak (BE = 1322.7 eV) and the lower pane1 the corresponding ratios .for ASH, (x = I,21 (BE = 1324.5 eV) and AsH, (BE = 1325.4 eV), respectively. See text for details.
width (1.85 eV) higher BE As peaks, one associated with molecular and the other with dissociated ASH,, the As2p,,, region can be successfully deconvoluted (dashed lines in fig. 6). This deconvolution gives a third peak (1324.5 eV> which intensifies with annealing up to 200 K while the 1325.4 eV peak decays. For 200 K, and higher, the low-temperature arsine desorption is complete and, as anticipated, there is no 1325.4 eV peak. Complementary HREELS data show Ga-H in this temperature regime, i.e., arsine has undergone dissociation leaving ASH, (x = 1, 2) and Ga-H. We, therefore, assign the As2p,/, XPS peak at 1324.5 eV to the dissociation products ASH, (x = 1,2) and the 1325.4 eV peak to molecularly adsorbed arsine (ASH,). From this XPS data, we are not able to resolve adsorbed ASH and ASH,. More quantitatively, the As 2p,,, and Ga 2p,,, ratios are s~marized for the various As peaks in fig. 7. The upper panel describes the substrate As peak, BE = 1322.7 eV. Amine desorption, which increases the relative number of substrate As atoms within the XPS sampling volume, accounts for the slight rise up to 200 K. No~alization by
sensitivity factors for the As2p,,, and Ga 2ps/2 XPS peaks, 1&/i& = 1.57 [29], indicates that there is excess Ga in the surface region, as expected for the GaAs(lOO)-(4 X 6) surface. The lower panel of fig. 7 shows the peak ratios for the molecular (1325.4) eV) and dissociated (1324.5 eV> arsine. The sharp decrease of the molecular ratio below 200 K correlates with the desorption of molecular arsine seen in TDS and HREELS (fig. 5). The variation of the ratio describing the dissociation products makes a key point: this curve (open circles) indicates that arsine starts to dissociate at temperatures as low as 140 K. At 200 K, there is no XPS evidence for molecular arsine, supporting our interpretation that the highertemperature (290 K) ASH, TDS peak involves recombination of ASH, and H. In the following, a simple model is used to derive absolute coverages from our XPS measurements [30], We assume that the GaAs crystal is composed of alternating homogeneous As and Ga layers and that the Ga-rich surface is terminated by Ga, relative concentration xoa. Summing over the contributions of individual layers, the intensities of the As and Ga peaks, IAs and 1o, are: 1:s I,, = 1 - exp( -d/A,, x fc1 -%a)
cos 0)
+Xcia exp( -b/A,,
cos @)I, (1)
CL! 43, =
1 - exp( -d/hGa
cos 0)
X [xGa + (1 --xGa) ew( --b/h,,
~0s @)I, (2)
where d is the Ga-Ga distance, b the As-Ga distance, 0 the detection angle and A,, and A,, are the mean free path lengths of the As2p3,z and Ga2~,,, Photo-electrons, respectively. Evaluation of this model results in a Ga coverage of 0.73 f 0.1 ML for the GaAs(lOO)-(4 x 6) surface (1 ML = 6.26 X lOi cmp2, surface density for a complete Ga layer). This is in agreement with the early work of Drathen et al. 1141 and a more recent STM study of the GaAs(100) surface [19].
M. Wolf& al. / Thermal decomposition of amine on GaAs(100)
48
As a control we measured XPS signals along the surface normal, 0 = 0” (not shown). Consistent with the more accurate results at 0 = 75”, we calculate a surface Ga concentration of 0.65 f 0.2 ML. Using the model of eqs. (1) and (21, we derive (see table 1) an ASH, coverage, on the Ga-rich surface (1200 s dose at 115 K, fig. 2), of 0.16 k 0.04 ML. According to fig. 7, approximately 50% of this undergoes dissociation (partial) upon annealing to 200 K. Our TDS results show that at 350 K, recombinative arsine desorption is almost completed and that the remaining dissociation products decompose irreversibly, leading to As incorporation into the GaAs lattice. From fig. 7 we estimate that 1.5f 5% of the initial ASH, coverage, i.e., 0.025 & 0.008 ML, undergoes irreversible dissociation to add As to the surface. In terms of the bulk As signal, this amount lies within the scatter of the 1322.7 eV BE data (upper panel of fig. 7). We have also recorded a similar series of XP spectra for the As-rich surface (fig. 8). After annealing the As-rich surface to 650 K, two Gaussian-Lorentzian peaks are required to fit the As2p,,* region (bottom spectrum). The spectrum is dominated by an As substrate peak at 1322.7 eV BE (1.85 eV FWHM) with an As: Ga peak ratio of 1.7, i.e., 1.6 x that observed for the Ga-rich surface. The higher BE (1324.5 eV> is assigned to excess As, denoted As..,, not incorporated into the GaAs lattice. One might expect such As atoms to be stabilized by remaining hydrogen atoms, i.e., As-H bonds. However, our HREELS and TDS data, as well as a previous study [26], all indicate that annealing to 650 K completely removes of hydrogen. The relative contributions of the high and low BE As peaks is shown in the inset of fig. 8. Using the model Table 1 Coverages
derived
from XPS analysis
(1 ML = 6.26
x
1
10“’ surface
d t
-
Ga rich “4 x 6”
1.0 -, 0.4 -’
0.2-•
1 I oo.o-.-~-..
b
I I
..._...____ o--~
‘a \.
-1:
100
I
1326
I
1322
1324 Binding
Energy
1320
1318
[eVl
Fig. 8. X-ray photoelectron spectra of the As2p,,, core level for the clean As-rich GaAs(lOO)-c(2 X 8) surface (bottom spectrum) and for the arsine-saturated surface. The inset shows the peak intensities of substrate As (BE = 1322.7 eV), ASH, (X = 1,2) (BE = 1324.5 eV), and ASH, (BE = 1325.4 eV), respectively, each divided by the intensity of the Ga2p,,, peak.
presented above, we calculate (summarized in table l), for the As-rich surface, a total As coverage of 0.76 + 0.1 ML in agreement with previous studies [14,19]. The excess As atoms account for
atoms/cm*)
(ML)
Ga
ASH, (115 K)
ASH,
Ga-rich (4 x 6) As-rich c(2 x 8)
0.73 f 0.1 0.24 f 0.1
0.164 f 0.04 0.154 f 0.04
0.08 + 0.02 0.05 + 0.02 a)
with excess As (XPS BE = 1325.3 eV).
700
As-rich GaAs
Coverage
overlaps
I 500
Annealing Temp. [K]
GaAs(100) surface
a) Estimated:
-----0.-- . . . . . . . . . . . . .. . . . . . o
. ..___-_*--__~---__.._.... I 300
0.0 -,
I I
(x = 1, 2) (200 K)
M. Wolf et al. / Thermal decompositionof arsine on GaAs(lO0)
0.12 + 0.05 ML or _ 16% of the total As coverage. The 1325.3 eV peak, due to adsorbed molecular arsine, is not well resolved in the upper A~2p~,~ spectrum but can be deconvoluted as shown. We did not try to further deconvolute the relative contributions of the excess As atoms and possible dissociation products ASH, (x = 1, 2). However, the temperature dependence of the ratio for the 1324.5 eV peak (open circles in the inset) indicates that ASH, dissociates (we estimate at least 30%) on the As-rich surface. That part of this dissociation is irreversible, is evident from the H, desorption between 400 and 700 R (fig. 5). The coverages derived for the As-rich surface are summarized in table 1.
4. Discussion In the following, we outline a chemical pathway for arsine decomposition on GaAs(100) that is consistent with the literature and with our XPS, TDS and HREELS results. It is evident, pa~icularly from XPS where there is only one chemical state, that ASH, adsorbs molecularly at 115 K on both the Ga- and As-rich surfaces (reaction (3)).
Af%(g)
115
ASH,(a).
(3)
The thermal de~mposition pathway can be described in terms of three temperature regimes: (I) 140-200 K, (II) 200-400 K, and (III) 400-700 K. Region I: Annealing through this region leads to parent desorption (via reaction (4)) and, based on XPS, to dissociation with the As-containing products remaining on the surface.
~%(a)
140-200 ASH,(g),
(4)
On the Ga-rich surface, dissociation accounts for approximately 50% of the initial arsine coverage. On both the As- and Ga-rich surfaces, only the As-H stretch (HREELS) is observable in this temperature region. Thus, dissociation populates surface As, not Ga, sites with H. This is expected in the As-rich case. For the Ga-rich surface, the preference for As-H formation can be rational-
49
ized using the structure model proposed by Biegelsen et al. [19]. According to this model, the Ga-rich ~$8 x 2)-GaAs(lO0) surface consists of ridges and troughs. The Ga dimers forming the rows of ridges would be possible adsorption sites for reversible adsorption and desorption according to reactions (3) and (4). The troughs contain plausible dissociation sites; Ga atoms from the second layer are accessible and they are surrounded by the As atoms of the top layer. If ASH, dissociates preferentially on Ga sites within the troughs, the neighboring structure conveniently places As atoms to receive the abstracted hydrogen (and from which, as discussed below, recombination could easily occur). For energetic reasons, we expect that As-H dissociation proceeds stepwise, rather than in concerted fashion, via reactions (5) and (6) (the subscript “As-sites” denotes arsenic sites): ASH,(a)
‘4o-200 ASH,(a)
AsI&
‘140
+ H(aAs+ites),
AsH( a) + H( aAs_sites).
(5) (6)
At N 200 K the desorption of molecular arsine (via eq. (4)) is completed and ASH, (x = 1, 2) and H are the only products remaining on the surface. Region II: Above 200 K, we observe additional ASH, desorption in a broad range, peaked around 300 K. We attribute this to the recombination of dissociation products, and/or to disproportionation of neighboring ASH,. Keeping in mind that more than 5 of the dissociation appears to be reversible, it is obvious that disproportionation alone is insufficient. Since some As is deposited, the stepwise decomposition of arsine must, with increasing temperature, proceed in competition with the recombination process. Thus, at least two additional reactions must be considered: AsH( a) 2oo-400 As(a) + H(a),
(7)
ASH,(a)
(8)
+ H(a) 200-400 ASH,(g).
On the Ga-rich surface, N 15% of the initial arsine coverage irreversibly dissociates leading to As deposition via eq. (7). The recombination (reaction (8)) requires activation and migration of either ASH,(a) and/or H. H migration, consistent with the observed activated transfer of hydrogen on the Ga-rich surface, forms Ga-H
M. Wolfet al. / Thermal decomposition of arsine on GaAs(100)
50
which, on this surface, is thermally more stable than As-H: H(a.4s-sites
1 2oo-400
H( aoa_sites), (Ga-rich) . (9)
Region ZZZ:After the recombination process, reaction (81, is complete (400 K), hydrogen recombines in a broad temperature range. Based on HREELS, H, is from Ga-H on the Ga-rich surface, and from As-H on the As-rich surface, i.e., H(
aGa-sites
H(a.4s-sites)
1 4oo-600
H,(g),
(Ga-rich),
(lOa)
45o-700
H,(g),
(As-rich).
(lob)
Interestingly the temperature for hydrogen desorption from the As-rich surface is higher than from the Ga-rich surface. Thus, the activation energy for recombinative H,-desorption appears to be higher on the As-rich GaAs(lOO)-c(2 x 8) surface. Considering practical applications, the higher H, desorption temperature on the As-rich surface implies that removal of hydrogen from As sites kinetically limits the final stages of an arsenic ALE cycle. In the initial (Ga-rich) stages of an As ALE deposition cycle, hydrogen will preferentially stay on Ga and the desorption temperature will be lower (short residence time) compared to the final (As-rich) stages. The overall reaction rate will be effectively limited by the low reactive sticking coefficient (1O-6) of arsine 1211. However, the temperature window for ALE growth of GaAs is still mainly determined by the thermal stability of the Ga precursor. For example, using trimethylgallium (TMG) as a precursor, epitaxial growth is achievable at 450-500°C while triethylgallium (TEG) requires only about 400°C [5]. New chemically designed precursor molecules may lead to a further reduction in these temperatures [31]. If this is possible, surface mobility may well control the growth rate, crystal quality and surface stoichiometry. These considerations will also be important in low-pressure OMCVD where surface kinetics are important. As a final note, we observe the onset of ASH, decomposition at a temperature below 140 K much lower than the range of 300-450 K re-
ported under OMCVD conditions [ll]. Considering that conventional OMCVD is rate-limited by mass transport, not surface kinetics, this difference might well be attributed to reactions in the gas-phase diffusion region above the substrate at high pressures. The low reactive sticking coefficient of ASH, limits the extent to which its pressure can be lowered. This restriction might be overcome by laser-assisted deposition techniques which can operate at low temperatures and pressures [32].
5. Summary The results of this paper can be summarized as follows: (1) Arsine adsorbs molecularly on GaAs(100) at 115 K with a saturation coverage of 0.16 + 0.04 ML (1 ML = 6.26 x 1014 cm-*). (2) Around 140 K, the parent molecules desorbs accompanied by the onset of dissociation to form ASH, (X = 1,2) and H. (3) On Ga-rich GaAs(lOO)-(4 X 6), N 50% of the initial coverage dissociates. A significant fraction of this recombines and desorbs as ASH, at 290 K. But approximately 0.025 ML (- 15% of the initial coverage) is finally deposited as As on the surface. In parallel with recombinative ASH, desorption between 200 and 400 K, some H transfers from As to Ga sites. (4) On As-rich GaAs(lOO)-c(2 X 81, H stays exclusively on As sites until recombination and desorption of H, occurs. H, desorption extends to 700 K, N 100 K higher than on the Ga-rich surface.
Acknowledgements
The authors would like to thank B.A. Banse and J.R. Creighton for helpful comments and for providing the AsD, sample. M.W. acknowledges a Feodor Lynen research fellowship of the Alexander von Humboldt Society. This work was supported in part by the Science and Technology Center Program of the National Science Foundation, grant CHE 8920120.
M. Wolf et al. / Thermal decomposition of amine on GaAs(100)
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