Abstract The structure of a passivating sulfide layer on Ge(001) was studied using X-ray standing waves and X-ray fluorescence. The sulfide layer was formed by reacting clean Ge substrates in (NH 4 ) 2 S solutions of various concentrations at 80°C. For each treatment, a sulfide layer containing approximately two to three monolayers (ML) of S was formed on the surface, and an ordered structure was found at the interface that contained approximately 0.4 ML of S. Our results suggest the rapid formation of a glassy GeS x layer containing 1.5-2.5 ML S residing atop a partially ordered interfacial layer of bridge-bonded S. The passivating reaction appears to be self-limited to 2-3 ML at this reaction temperature

D L Marasco

D T Keane

K D Breneman

M J Bedzyk

O Sakata

P F Lyman

T.-L Lee

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Surface Science 462 (2000) L594–L598
www.elsevier.nl/locate/susc
Surface Science Letters
Structure of a passivated Ge surface prepared
from aqueous solution
P.F. Lyman a,b, O. Sakata a, D.L. Marasco a, T.-L. Lee a, K.D. Breneman a,
D.T. Keane a, M.J. Bedzyk a,c,*
a Department of Materials Science and Engineering and Materials Research Center, Northwestern University, Evanston, IL 60208, USA
b Department of Physics, University of Wisconsin-Milwaukee, Milwaukee, WI 53201, USA
c Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
Received 20 December 1999; accepted for publication 7 April 2000
Abstract
The structure of a passivating sulfide layer on Ge(001) was studied using X-ray standing waves and X-ray
fluorescence. The sulfide layer was formed by reacting clean Ge substrates in (NH4)2S solutions of various
concentrations at 80°C. For each treatment, a sulfide layer containing approximately two to three monolayers (ML)
of S was formed on the surface, and an ordered structure was found at the interface that contained approximately
0.4 ML of S. Our results suggest the rapid formation of a glassy GeS
x
layer containing 1.5–2.5 ML S residing atop a
partially ordered interfacial layer of bridge-bonded S. The passivating reaction appears to be self-limited to 2–3 ML
at this reaction temperature. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Chalcogens; Chemisorption; Compound formation; Crystalline–amorphous interfaces; Germanium; Surface chemical
reaction; Surface structure, morphology, roughness, and topography; X-ray standing waves
The chemical and electronic passivation of the properties of SiO2, whereas the poor electrical
properties [4] of III–V surfaces and their oxidesurface of semiconductors is vital to their utility
interfaces have hampered widespread implementa-for device construction. Truncation of the surface
tion of these semiconductors. An effective passiv-of a crystal induces electronic defect states [1,2]
ation treatment should be chemically stable,that allow carriers to recombine, and ‘fixed’
protect the substrate from oxidation, remove sur-charges residing at a semiconductor/insulator
face-induced [1,2] carrier recombination states,interface can hinder performance of a metal–
and exhibit a low density of fixed charges. Forinsulator–semiconductor capacitor (and hence pre-
ease of processing, it is desirable that such aclude construction of a working device) [3]. The
passivation reaction be carried out chemically atpredominance of Si for semiconductor devices can
atmospheric pressure. Further, it would be conve-largely be attributed to the remarkable passivative
nient if the treatment relied upon a self-limiting
reaction, i.e. a process whose kinetics slow down* Corresponding author. Fax: +1-847-467-2269.
E-mail address: bedzyk@northwestern.edu (M. Bedzyk) considerably after initial reaction. Of course, these
0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0039-6028 ( 00 ) 00508-2
face sulfided in (NH4)2S solution. Although our
high-resolution X-ray standing wave data support
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L595P.F. Lyman et al. / Surface Science 462 (2000) L594–L598
criteria are all satisfied by the oxidation of silicon,
but oxidation of Ge or III–V substrates does not
fulfil these requirements. occupation of a bridge site for some of the reacted
S, we found that 2.4 to 3 ML of S reacted withGreat improvements in the lifetimes of carriers
for AlGaAs/GaAs transistors were realized with a the Ge(001) substrate under a wide range of
reaction conditions (1 ML=6.25×1014 cm−2). Wepassivation scheme in which the GaAs surfaces
were sulfided using Na2S solution [5]. This proto- explain our findings with a model of the surface
where several layers of amorphous GeS
x
arecol allowed a 60-fold improvement in the current
gain b of the transistor. Studies of the sulfidation formed by the reaction, and a submonolayer
of S occupies ordered bridge sites at theof other III–V surfaces, notably InP(001) [6–8],
followed, using Na2S and (NH4)2S. Sulfiding was GeSx/Ge(001) interface. The results are reminis-
cent of the native oxidation of Si(001).found to remove surface states from the band gap
[6 ] and to slow down deleterious oxidation reac- Experiments were carried out on polished
Ge(001) single crystal substrates. As discussed intions [7,8]. Later, the sulfidation of a Ge(001)
surface using (NH4)2S was studied by Anderson more detail below, a variety of passivation proto-
cols were used, but generally involved successiveet al. [9] using low energy electron diffraction
(LEED) and X-ray photoelectron spectroscopy degreasing, rinsing in deionized water, HF etching,
sulfidation in hot (80°C) ammonium sulfide( XPS). While also motivated by possible passiva-
tive applications, this work focused on the struc- [(NH4)2S] solution, rinsing in either water or meth-
anol, and blow drying.ture of the interface, and found that a (1×1)
LEED pattern was observable upon insertion in a Coverage calibrations were made by compari-
son of the X-ray fluorescence from the samples tovacuum chamber.
Since the (001) face of a diamond-structure a S coverage standard. The coverage standard was
constructed by evaporating ~10 Å of Sb2S3 ontosemiconductor has two dangling bonds per surface
atom, a monolayer (ML) of a Group VI element, a Si substrate, and then capping this layer with
~200 Å of Al. This composition was chosen forwhich will form two covalent bonds, may satisfy
all the dangling bonds of the substrate by occupy- several reasons. The use of Si and Al as substrate
and cap should allow the S content to be measureding a bridge site in a (1×1) geometry. Accordingly,
it was demonstrated by LEED and XPS that 1 ML directly with Rutherford scattering spectrometry
(RBS). Moreover, Sb2S3 evaporates as a moleculeof elemental S deposited on Ge(001) in ultrahigh
vacuum (UHV ) leads to this predicted arrange- [14], ensuring a stoichiometric film. Since the
Rutherford cross-section for Sb is much greaterment [10]. In addition, Te adsorbs on Si(001)
[11,12] and Ge(001) [13] with the same local than for S, the use of Sb2S3 allowed a more
sensitive (if indirect) measurement of the S content.geometry, although stress-relieving defects are pre-
sent to accommodate the size mismatch between RBS spectra were analyzed by RUMP [15], which
found a Sb coverage of 2.1±0.2×1015 cm−2,this larger chalcogen and the substrate. For the
aqueously sulfided Ge(001) surface [9], the implying a S coverage of 3.2±0.3×1015 cm−2 (the
dominant source of the coverage uncertainty is theappearance of a (1×1) LEED pattern and results
from XPS suggested that these surfaces were sim- uncertainty in the ion stopping power tables used
to analyze the RBS spectra [16 ]). Due to the lowilar to the surfaces prepared [10] in UHV.
Accordingly, Anderson et al. concluded that the atomic number of S, the direct measurement by
RBS of the S coverage had large error bars, butstructure of the aqueously sulfided Ge(001) surface
contained 1 ML of bridge-bonded S atoms. The the resultant value and uncertainty encompassed
the indirect coverage value derived from the Sbsulfided surfaces were also stable in air [9], lending
support to the idea that a S-terminated (1×1) measurement.
Both the X-ray fluorescence ( XRF) measure-surface would be (at least chemically) passivated.
The present paper describes a series of measure- ments and structural measurements by X-ray
standing waves ( XSW ) were conducted at thements that probe the structure of a Ge(001) sur-
L596 P.F. Lyman et al. / Surface Science 462 (2000) L594–L598
DuPont–Northwestern–Dow consortium’s undu-
lator beamline 5ID-C of the Advanced Photon
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Source (APS) at Argonne National Laboratory.
A high-heat-load Si(111) monochromator was
used to select a 7 keV beam from the undulator
output, and then the beam was further monochro-
mated and dispersion-matched to the Ge(001)
substrate using a Si(004) two-bounce channel-cut
crystal. The X-ray flux through a 3 mm wide by
1 mm high slit placed just upstream of the sample
was approximately 1012 photons/s. Using a
vacuum chuck, the samples were gently held on a
four-circle diffractometer inside a thin polyethylene
bag flushed with He (to eliminate Ar fluorescence
and reduce absorption of the 2.3 keV S fluorescent
K X-rays). An energy-dispersive Ge detector opti-
mized for soft X-rays was used to detect the
S XRF.
Fig. 1. XSW scan for the (004) diffraction planes for sulfidedCoverage measurements were conducted by
Ge(001) (sample A). The S coverage is 2.4 ML. The solid linescomparing the S fluorescence from the
are best fits to the reflectivity R and S fluorescence yield usingAl/Sb2S3/Si standard with that from the S/Ge dynamical diffraction theory.
samples under identical conditions. (The absorp-
tion length of the S X-rays in Al is 2.4 mm, so that
absorption in the 200 Å Al capping layer of the samples were all prepared by the following pro-
cedure. Syton polishing was followed by rinsing instandard was negligible.) The XSW measurements
were made by scanning the sample through the water, degreasing in trichloroethane and acetone
with ultrasonic agitation, and rinsing in deionized~50 mrad wide Ge(004) Bragg reflection with a
microstepping motor driving the v rotational stage water. The samples were etched in 29% HF solu-
tion (3:2::HF:H2O, using 48% HF solution), andof a Huber 422 two-circle goniometer. At each
angular step in the scan, an X-ray fluorescence became hydrophobic. Next, they were directly
transferred to a beaker of (NH4)2S solution heldspectrum was collected using the Ge detector
described above and the diffracted X-ray intensity at 80°C. As indicated in Table 1, sample A was
immersed in undiluted, 24 vol% (NH4)2S aqueouswas measured simultaneously using an ion cham-
ber. The XSW technique as applied to surface solution, sample B was immersed in (NH4)2S
solution diluted to 4 vol% with water, sample Cstructural determinations has been reviewed by
Zegenhagen [17]. was immersed in (NH4)2S solution diluted to
0.4 vol% with water, and sample D was immersedA typical XSW scan from a (NH4)2S-reacted
Ge(001) sample is shown in Fig. 1. The S fluo- in (NH4)2S solution diluted to 0.1 vol.% with
methanol. The samples were allowed to react inrescent yield (normalized to unity for off-Bragg
angles) is represented by the open circles, while solution for 20 min, then rinsed several times in
methanol with ultrasonic agitation, and finallythe incident X-ray reflectivity is represented by the
closed circles. Best fits to the reflectivity (using blown dry with difluoroethane. In addition to
those reported in Table 1, the values of P004 anddynamical diffraction theory) and the S fluores-
cence yield are shown by the smooth lines. For F004 reported for sample A were closely reproduced
in several other preparation trials using undilutedthis sample, we found values of P004=0.90±0.01
and F004=0.15±0.02. (NH4)2S solution (24 vol.%) followed by rinsing
in water. (Coverage measurements were not con-The results from a series of XRF and XSW
measurements are summarized in Table 1. These ducted for these samples.)
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L597P.F. Lyman et al. / Surface Science 462 (2000) L594–L598
Table 1
Representative S coverages and XSW results for indicated surface preparations of S/Ge(001)
Sample (NH4)2S solution h (ML) P004 F004 hc (ML)
A 24% in H2O 2.4±0.3 0.90±0.01 0.16±0.01 0.39±0.05
B 4% in H2O 2.3±0.3 0.90±0.01 0.15±0.01 0.35±0.05
C 0.4% in H2O 1.9±0.3 0.90±0.01 0.18±0.01 0.35±0.06
D 0.1% in methanol 2.9±0.3 0.88±0.01 0.13±0.01 0.38±0.05
The last column in Table 1 reports the coherent the reacted layer that is correlated to the Ge
substrate is highly regular and repeatable. Thecoverage hc for each sample. The coherent cover-
age, which is defined as the product of the coherent consistency of this result, despite the variation in
the total S coverage, suggests that the ordered Sfraction and the total coverage, hc=F004h, is a
measure of the total number of S atoms (in ML) population is located at the Ge interface.
The XSW results reported in Table 1 are consis-that are located at a position in registry with the
Ge substrate. tent with a S submonolayer located at a bridge-
like position. The sum of covalent radii [18] for SThe LEED and XPS results of Anderson et al.
[9] suggested that only a single monolayer of S and Ge would place a bridge-site S atom at an
adsorption height of 1.25 Å above the last Geexists at the chemically reacted surface, and that
this monolayer passivates the Ge(001) surface layer. That position is consistent with our XSW
results, which show that a S population of approxi-from further chemical activity. The S monolayer
was thought [9] to be similar in structure to one mately 0.4 ML is located at an adsorption height
of 1.27±0.02 Å. In this model, the remaining 1.5prepared by deposition of elemental S on clean
Ge(001) in UHV. In that structure, the S atoms to 2.5 ML of S occupy positions uncorrelated with
the Ge lattice, consistent with bonding in a glassyoccupy bridge positions in a (1×1) arrangement,
leading to only one surface-related Ge oxidation network of GeS
x
.
Our data are also consistent with a closelystate [10]. Our data are not consistent with this
model for the (NH4)2S-passivated surface. Instead, related model of the surface layer. The ordered S
component could represent the transition from awe propose that a disordered (glass-like) GeS
x
layer resides atop a partially ordered interfacial locally ordered, crystalline phase of GeS
x
residing
at the interface and an amorphous GeS2 layerlayer.
Our XRF results indicate that, under a wide (that would be expected for thick layers). An
appealing picture is that this transition results fromrange of concentrations of (NH4)2S solution, 2–
3 ML of S are reacted with the Ge substrate. Thus, the creation of a partially ordered (perhaps epitax-
ial ) GeS
x
compound layer whose registry with thethere does appear to be a self-limiting reaction of
Ge sulfidation in (NH4)2S solution, but the reacted substrate is lost within a few angstroms, presuma-
bly due to different lattice constants and the pro-layer is more extensive than the 1 ML suggested
by the results of Anderson et al. [9]. The sulfidation clivity [19] of Ge and S to form glassy structures.
This model would be reminiscent of an often-of Ge(001) is instead reminiscent of the oxida-
tion of Si(001), which forms a thin native oxide proposed picture of the intensely studied oxidation
of Si: there have been recurring reports (see e.g.limited to ~15 Å under a wide range of oxidation
conditions at room temperature. Ref. [20]) of a partially ordered interfacial SiO
x
compound whose order completely breaks downPerhaps the most striking result reported in
Table 1 is the consistency of the coherent coverage for SiO2 layers more than several angstroms thick.
While we have found significant differenceshc under a wide variety of preparation conditions.
Even though the concentration of the (NH4)2S between the structure of a (NH4)2S-passivated
Ge(001) surface and that reported by Andersonsolution was varied by a factor of 240, the resul-
tant hc varied by only 10%. Thus, the structure of et al., several of our findings are consistent with
L598 P.F. Lyman et al. / Surface Science 462 (2000) L594–L598
their results. In both studies, a self-limiting, passiv- Contracts No. DMR-9632472 to the MRC at
Northwestern University, DMR-9632593 andating reaction was found, rendering the surface
S
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stable in air. Both studies lend support for at least DMR-9973436 to MJB, and DMR-9304725 to
DND, and by the State of Illinois under Contracta partial layer of bridge-bonded S at the Ge
surface. However, we find that 2 to 3 ML of S No. IBHE HECA NWU 96 to DND.
reacts with the Ge substrate over a wide range of
(NH4)2S concentrations and two different solvents.
Our results suggest that the sulfidation of Ge more References
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[2] W. Shockley, Phys. Rev. 56 (1939) 317.We cannot, however, specify with the present
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Report RC9250, 1982 (unpublished).P. Baldo in acquiring the RBS spectra. This work
[17] J. Zegenhagen, Surf. Sci. Rep. 18 (1993) 199.was supported by the US Department of Energy
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Rev. Lett. 59 (1987) 213.and by the National Science Foundation under


Structure of a passivated Ge surface prepared from aqueous solution,

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Structure of a passivated Ge surface prepared from aqueous solution,

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