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Jo: Surface Reactions of Atomic Hydrogen with Ge(100) in Comparison with Si(100)

Abstract

The reactions of thermal hydrogen atoms H(g) with the Ge(100) surface were examined with temperature-programmed desorption (TPD) mass spectrometry. Concomitant H2 and CH4 TPD spectra taken from the H(g)-irradiated Ge(100) surface were distinctly different for low and high H(g) doses/substrate temperatures. Reactions suggested by our data are: (1) adsorbed mono(β1)-/di-hydride(β2)-H(a) formation; (2) H(a)-by-H(g) abstraction; (3) GeH3(a)-by-H(g) abstraction (Ge etching); and (4) hydrogenated amorphous germanium a-Ge:H formation. While all these reactions occur, albeit at higher temperatures, also on Si(100), H(g) absorption by Ge(100) was not detected. This is in contrast to Si(100) which absorbed H(g) readily once the surface roughened on the atomic scale. While this result is rather against expectation from its weaker and longer Ge-Ge bond as well as a larger lattice constant, we attribute the absence of direct H(g) absorption to insufficient atomic-scale surface roughening and to highly efficient subsurface hydrogenation at moderate (>300 K) and low (≤300 K) temperatures, respectively.

I. Introduction

The chemistry of gaseous hydrogen atoms, H(g), on and in crystalline group-IV semiconductors has been an active research area for decades [17]. Reactions that occur, when H(g) is exposed to Si(100), have thus been well documented: surface-adsorbed mono- and di-hydride formation [67]; H(a)-by-H(g) abstraction [8]; surface etching as a result of the SiH3(a)-by-H(g) abstraction [11]; amorphous hydride (a-Si:H) formation from subsurface hydrogenation [12]; and direct absorption of H(g) by the crystalline bulk [8,13]. The relative importance of theses reactions is determined mostly by the substrate temperature. While the surface etching of Si(100) by H(g) is slow and anistropic at moderately elevated temperatures, it becomes faster and rather isotropic at room temperature [8,9]. At even lower temperatures, subsurface Si(100) hydrogenation/amorphization is a dominant reaction channel [12]. The Si(100) surface becomes rougher on the atomic sale at elevated temperatures as a result of anisotropic etching by H(g) [9]. It was found that the surface roughened by H(g) etching provides a low-barrier channel for direct H(g) absorption by Si(100) [8]. H atoms incorporated into crystalline silicon (c-Si) deactivates dopants, other impurities, and lattice vacancy defects, profoundly affecting fabricated Si devices [15,14].
On the other hand, much less research attention has been paid to the surface chemistry of H(g) on Ge(100) compared with Si(100) [1519]. While germanium attracts renewed attention in view of the next-generation semiconductor technology due to its high carrier mobility [2030], we decided to perform a comparative study on the H(g)/Ge(100) system. Compared with c-Si, c-Ge has a larger lattice constant and a weaker Ge-Ge bond on average.
We hereby report the effects of substrate temperature and exposure on the surface reactions of H(g) on Ge(100) in comparison with the previous results for Si(100). Similarly, H(g) adsorption to form surface mono- and di-hydrides, H(a)-by-H(g) abstraction, GeH3(a)-by-H(g) etching, and sub-surface hydrogenation all occurred, albeit at lower temperatures. In contrast to Si(100), however, no absorption of H(g) has been detected even with extremely large H(g) exposures on Ge(100) despite its larger lattice constant (more spacious interstices) and its weaker bond strengths (facile hydrogenation and/or etching). We discuss this difference and propose a plausible explanation for the lack of expected H(g) absorption.

II. Experimental

Experiments were carried out in a turbo-molecular pumped ultrahigh vacuum (UHV) system with a base pressure of 4 × 10−11 Torr. The main 500 L/s turbo pump was backed by a 500 L/s oil diffusion pump and in turn by a mechanical pump. The UHV system was equipped with a single-pass cylindrical mirror analyzer (PHI 3017 model) for the Auger electron spectroscopic (AES) analysis of surface elemental compositions, a differentially pumped triple-filter quadrupole mass spectrometer (QMS, and Thermo-Fisher Scientific Smart IQ + 300D) for residual gas analysis (RGA) and for temperature-programmed desorption (TPD) measurements of desorbing species from the sample surface. The experimental system was also equipped with a high-purity gas delivery system with a tubular gas doser connected to a precision leak valve.
The Ge(100) sample used here was p-type (B-doped; ρ = 0.01~0.39 ohm-cm) and cut into a 10 × 18 mm2 piece. It was then mounted on an XYZ-rotational manipulator for liquid nitrogen sample cooling, electrical feedthroughs for dc resistive sample heating for TPD measurements, and a type-K thermocouple inserted and glued (Aremco 516 high-temperature ceramic adhesive) into a small hole drilled on the sample edge for accurate Ts measurements. The sample temperature (Ts) was regulated precisely by a dc power supply, which is in turn controlled for a linear temperature ramping by an electronic temperature programmer (Eurotherm 2404) interfaced to PC control software.
The sample, introduced into our UHV chamber without pretreatment, was cleaned in situ with several cycles of a 30-min 1-keV Ar+-ion sputtering at 700 K and a subsequent 10-min annealing at 900 K. H2 gas (99.999 purity) was introduced into the chamber through a tubular doser controlled by a pinhole disc. Thermal-energy hydrogen atoms H(g) were produced by a hot (~1800 K) spiral W filament positioned ~6 mm in front of an H2 gas dosing tube facing the sample surface from a distance of 3 cm. H(g) exposures are reported here in monolayers (ML), calibrated against the exposure of 155-L H2 to give a saturation monohydride coverage, Ge(100)-2 × 1:H. Following each H(g) dose, the sample was rotated 90° away from the tubular gas doser and positioned in front of a 3 mm diameter apertured cone to face the differentially pumped QMS from a line-of-sight distance of 1 mm. TPD measurements were made with a 2.5 K/s ramp rate. Ultrahigh-purity (99.999%) H2 gas was further purified with a liquid nitrogen trap in a turbo-pumped (<2 × 10−7 Torr) gas manifold and checked for impurity gases with RGA.

III. Results and Discussion

It has been well established that both Si(100) and Ge(100) surfaces reconstruct to make (2Í1) surfaces, on which each surface atom has only one dangling bond. Such dangling bonds are readily passivated by H(g) with a nearly unity sticking probability, leading to well-ordered monohydride-saturated surfaces, Si(100)-2 × 1:H [812] and Ge(100)-2 × 1:H [1619,2728]. Schematic views of the unit cell of a diamond crystal lattice and the (100)-2 × 1:H surface are drawn in Figure 1. Surface dimer bonds on these (100)-2×1:H (1 ML θH) surfaces are strained (weak) and are vulnerable to the attack of H(g) to make a (100)-3×1:H (1.33ML θH) phase, which is composed of alternating monohydride dimer and dihydride rows. Both the interatomic distances on the surface (labelled A and B in the Figure 1) and the lattice constant are larger by ~4% for Ge(100). On average, Ge-Ge and Ge-H bonds are weaker and longer than the corresponding Si-Si and Si-H bonds. With these structural and chemical similarities and differences in mind, we analyze and compare our results for Ge(100) obtained in this work with the literature results in terms of various reaction channels as functions of the substrate temperature Ts and the H(g) dose.
Displayed in Figure 2 is a series of low-dose H2 TPD spectra taken from the Ge(100)-2 × 1 surface pre-exposed to indicated H(g) exposures (ML) at 300 K. The red-colored desorption curves are those with an additional shoulder peak (labelled β2 at 520 K) indicative of dihydrides, GeH2(a), which grows beyond the primary peak (β1 at 570 K) for monohydrides, GeH(a). Similar double-peaked desorption behaviors have been reported for both Si(100) [812] and Ge(100) surfaces [812,1619,2728]. However, there is one distinct aspect observed here: the β2-peak grew, albeit relatively slowly compared with Si(100), well beyond the 1.33-ML H(a) coverage corresponding to a (3 × 1):H phase on Ge(100) (see Figure 3).
A strong dependence on the surface temperature (Ts) was observed as summarized in Figure 3. The β2-H2 desorption peak, which is H2 desorption from surface dihydride GeH2(a) species, increases sharply with decreasing Ts for a fixed dose of 13-ML H(g). Together with the 500-K GeH4 desorption (See the right panel of Figure 3), the strong β2-H2 peak intensities obtained with Ts ≤ 300 K indicate the formation of surface multi-hydrides GeHx(a) (x = 2 and 3) and surface etching. It was previously shown that Si etching via SiH3(a)-by-H(g) abstraction during H(g) dose also occurs at adsorption Ts that leads to strong β2-H2 and SiH4 desorption in TPD [810,31]. The detection of GeH4 TPD peak at 490 K, resulting from the recombination of two surface adsorbates, SiH3(a) and H(a), suggests that SiH3(a) species are formed and abstracted by H(g) during the H(g) dose. We thus conclude that H(g) atoms, when exposed to Ge(100) at Ts below 300 K, not only form surface GeHx(a) (x > 1) species but also abstract GeH3(a) to produce the gaseous etch product GeH4 from Ge(100) during H(g) dose. A very similar behavior was observed on Si(100), although the threshold Ts is about 200 K lower on Ge(100) [811,1618,31]. It should be noted that the β2-H2 peak grows at the sacrifice of the β1-H2 peak with decreasing Ts: The β1-peak intensity decreased by ~33% when Tads decreased from 450 K to 120 K. This suggests that the increase of the total H2 desorption with decreasing Tads was due to the increased concentration of increased di-and tri-hydrides on the surface, rather than to etching-induced roughening and a subsequent increase in the area of the surface.
The evolution of H2 desorption peaks, as a function of H(g) exposure in the high-dose regime at low surface temperatures (Tads = 120, 200, and 300 K), is shown in Figure 4. Noteworthy features are: (1) the β2-to-β1 peak intensity ratio for a given dose of 13-ML H(g) increases with decreasing Ts; (2) the amount of overall H2 desorption decreases with increasing H(g) dose; and (3) the H2 desorption decreases more quickly at a lower Tads with increasing H(g) dose. Previously, very similar results were obtained from Si(100) at Ts between 115 and 300 K [12]. Low-energy electron diffraction (LEED) and TPD analyses revealed that hydrogenation proceeds beyond the topmost surface atoms, forming amorphous hydrogenated Si layers, a-Si:H, in the subsurface of Si(100). We thus conclude that a large H(g) dose at a low Ts ≤ 300 K leads to subsurface hydrogenation of Ge(100). The nearly complete suppression of both β1- and β2-H2 desorption (see the top curve on the left panel), together with increased desorption of GeHx (x < 4) species in TPD (not shown), is ascribed to a-Ge:H formation on Ge(100) [12]. The quicker decrease in H2 desorption at a lower Ts suggests that the subsurface hydrogenation occurs more rapidly at lower Ts. From comparison with Si(100) [12], it is also concluded that Ge(100) is hydrogenated more readily and to a greater extent than Si(100). The difference might be reconciled with the weaker and longer Ge-Ge and Ge-H bonds and with the larger lattice constant of Ge(100).
Figure 5 shows H2 TPD spectra taken from the Ge(100) surface pre-exposed to very large H(g) doses at 400 K. The β1-H2 peak intensity maximizes at a 1-ML H(g) and then decrease slowly with increasing H(g) exposure up to 20,000 ML. The decreasing β1-H2 intensity parallels the increasing β2-H2 desorption. This trend was common for Tads down to 120 K, as can be seen in Figure 4. Moreover, the β21 peak intensity ratio is smaller than those of low-Tads H(g) doses, suggesting that etching would occur mostly a step edge sites, leading to slow but anisotropic surface etching. However, the total (β1- and β2-) H2 desorption increased only by ~15% from 1-ML to 20,000-ML H(g). This very small increase suggests that GeH3(a)-by-H(g) etching occurs too slowly to result in a significant increase in the surface area on Ge(100). This is in sharp contrast with Si(100), where anisotropic etching dominates at moderate temperatures, leading to a roughened surface with a more than doubly increased area, estimated by the total H2 desorption of surface-bound H(a) [8]. Once roughened significantly by etching by H(g), the Si(100) surface directly absorbed H(g) unlimitedly within a 400–600 K Ts window. H(g) atoms absorbed by the crystalline bulk of Si(100) evolved out as strong H2 peak at 860–900 K, well-resolved from the 780-K β1- and 650-K β2-H2 desorption peaks for surface-bound hydrogen atoms [8,10].
From the absence of no additional H2 TPD peak even with an extremely large H(g) dose of 20,000 ML, except the well-known β1- and β2-H2 peaks for surface-adsorbed H(a), we conclude that the crystalline bulk of Ge(100) does not absorb H(g). This suggests that the Ge(100) surface also needs to be pre-roughened significantly on the atomic scale for direct H(g) absorption into the Ge(100) lattice bulk to occur. This result is rather surprising when the larger lattice constant of Ge(100), together with longer and weaker Ge-Ge and Ge-H bonds on average, is considered. Weber et al. [20] also suggested an effective diffusion barrier for atomic H to explain the low H concentration incorporated in the crystalline bulk of germanium from a hydrogen plasma. Both atomic and molecular H2 are important in preparation and chemical processing of semiconductor surfaces [19,2428]. Isolated H atoms play an important role of passivating imperfections in the crystalline lattice of semiconductors in the manufacture of electronic devices [3233]. While the insufficient surface roughening and/or anisotropic etching with the much greater H(g) is concluded to be the lack for the bulk H(g) absorption by Ge(100) in this work, further studies with a pre-roughened, e.g., HF-etched, Ge(100) surface would confirm our conclusion [10].

IV. Summary

Reactions of atomic hydrogen H(g) with the Ge(100) surface were investigated as a function of surface temperature and H(g) dose. While all surface reactions, i.e., adsorption, abstractive surface etching, and subsurface hydrogenation and amorphization, that occur on Si(100) were confirmed to take place also on Ge(100), direct H(g) absorption by Ge(100) was not detected for wide ranges of substrate temperatures and H(g) doses. The larger lattice constant and weaker Ge-Ge bond were not enough to provide a low-barrier surface channel for direct H(g) absorption. We attribute the absence of H(g) absorption to an insufficient surface roughening of the Ge(100) surface at Ts > 300 K and to predominantly occurring subsurface hydrogenation, forming amorphous germanium hydride a-Ge:H layers, at Ts ≤ 300 K.

Acknowledgement

SKJO thanks Dr. Minbok Jung for his help in carrying out early experiments.

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Figure 1
Model views for the diamond crystal lattice unit cell and the (2 × 1)-reconstructed (100) surface with all dangling bonds passivated each with an H atom. The intra- (A) and inter-dimer (B) Si-Si (Ge-Ge) distances are 2.3 Å (2.4 Å) and 5.38 Å (5.58 Å), respectively, on their (100)-2Í1 surfaces. The lattice constants are 5.431 Å and 5.646 Å for Si and Ge, respectively.
asct-26-174f1.tif
Figure 2
H2 TPD Spectra from the Ge(100) surface pre-exposed to atomic H at 300 K in the low exposure regime. The 1-ML atomic H dose, calibrated against the saturation β2-peak intensity, corresponds to a 155-L H2 dose in front of a hot (~1800 K) spiral tungsten filament.
asct-26-174f2.tif
Figure 3
H2 (Left Panel) and GeH4 (Right Panel) TPD spectra simultaneously taken from the Ge(100) surface pre-exposed to a fixed 13-ML atomic H(g) at the indicated surface temperatures.
asct-26-174f3.tif
Figure 4
H2 TPD spectra from the Ge(100) surface pre-exposed to the indicated amounts of H(g) atoms (in ML) at three different (Tads= 120, 200, and 300 K) surface temperatures. Note that the β2-to-β1 H2 peak intensity ratio increases with the decreasing surface dosing temperature and that both β1- and β2-H2 peak intensities diminish with the increasing H(g) dose more quickly at lower substrate temperatures during dose.
asct-26-174f4.tif
Figure 5
H2 TPD spectra taken from the Ge(100) surface pre-exposed to H atoms at a fixed surface temperature of 400 K. Note that the trend of the decreasing H2 desorption, observed at lower dosing temperatures, does not take place and that the β2-desorption peak rather increases, albeit slowly, steadily to reach ~1.5ML of a total H uptake.
asct-26-174f5.tif
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