Silicon Monohydride Research Articles

A TPD, HREELS, and XPS study of electron-induced deposition of germanium on Si(100)

The decomposition of digermane and deposition of germanium via electron irradiation of digermane overlayers on Si(100) at 110 K was studied using temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and X-ray photoelectron spectroscopy (XPS). At high digermane exposures, hydrogen TPD traces exhibit three peaks at 140, 575, and 740 K, corresponding to desorption of molecularly adsorbed digermane, hydrogen desorption from adsorbed germanium, and hydrogen desorption from the silicon monohydride state, respectively. HREELS spectra of Si(100) following high exposures to digermane, exhibit losses at 820 and 2050 cm −1, and a shoulder at 2150 cm −1, corresponding to the GeH x scissor, Ge–H stretching, and Si–H stretching vibrational modes, respectively. Electron irradiation ( E=150 eV) of digermane overlayers on Si(100) causes a decrease in the intensity of the 140-K TPD peak and an increase in the intensity of the 575-K TPD peak, and a decrease in the intensity of the HREELS losses. The relative concentration of germanium on Si(100) (as determined by XPS) after annealing the surface to 200 K is at least 2 times higher on electron-irradiated surfaces than on surfaces that were not exposed to the electron beam.

High-resolution electron energy loss spectroscopy evidence for electron beam-induced decomposition of trimethylsilane adsorbed on Si(100)

The effects of incident electrons on trimethylsilane ((CH 3) 3SiH) adsorbed onto Si(100) at 110 K are examined using high-resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). TPD from trimethylsilane-covered Si(100) show hydrogen desorption from silicon monohydride states at 780 K ( β 1); at high trimethylsilane exposures, two low-temperature (130 K and 155 K) molecular TPD states appear. The absence of silicon dihydride species on the trimethylsilane covered Si(100) surface was demonstrated by comparing the data obtained from this surface with that obtained from disilane-covered Si(100). HREELS and TPD data indicate that carbon deposition on Si(100) is enhanced by irradiating the trimethylsilane covered surface only when the molecular states are present.

Stable Ge–H phase at 620 K on Si 1− xGe x/Si(001) surface: a high-resolution electron energy loss spectroscopy study

The products of decomposition by the hot wire technique of disilane, germane and mixtures of these gases on a Si(001) surface held at 620 K are analysed using high-resolution electron energy loss spectroscopy. Silicon monohydride (Si–H) is detected for decomposed Si 2H 6 on Si(001), and both Si–H and Ge–H are present on Si 1− x Ge x /Si(001) ( x<0.5). In contrast, no hydride is detected on a pure Ge surface prepared in the same experimental conditions. From that, it is deduced that a Ge–H phase is stabilized at 620 K due to the presence of the neighboring silicon atoms in the alloys. Further room-temperature exposure to atomic hydrogen reveals additional hydrogen adsorption on Ge atoms. Short- versus long-range interaction mechanisms are discussed to explain the present observations.

Transmission-ion-channeling studies of the silicon (111) monohydride surface

Transmission ion channeling was used to examine the atomic position of deuterium on the silicon (111) monohydride surface. A 2-MeV {sup 4}He{sup +}-ion beam was used to elastically recoil deuterium from the beam-exit surface of a thin silicon crystal. The yield of recoiled deuterium was measured versus angle between the analysis beam and the channeling axis near the {l_angle}111{r_angle}, {l_angle}110{r_angle}, and {l_angle}100{r_angle} axes. The location of the surface deuterium relative to the silicon lattice was examined by comparing the measured yields with computer simulations of ion channeling. Channeling measurements were consistent with approximately half of the deuterium occupying a site close to the site predicted by {ital ab initio} calculations of the structure for the Si(111) 1{times}1 monohydride surface, with the remaining deuterium being disordered or displaced from the predicted site. {copyright} {ital 1997} {ital The American Physical Society}

Electron irradiation effects on digermane adsorbed on Si(100) surfaces

The effects of electron-beam irradiation on digermane adsorbed on Si(100) at 120 K have been investigated by temperature programmed desorption (TPD). Digermane adsorbed on clean Si(100) at low exposures exhibits a single H 2 TPD peak at 800 K, which originates from the silicon monohydride state on the Si(100) surface. At higher digermane exposures, a new peak appears between 550 and 570 K, and the 800 K peak shifts to lower temperatures. The 570 K peak is believed to originate from surface germanium hydride species. At the highest digermane exposures, a molecular desorption peak appears at approximately 140 K. Electron exposure of Si(100) dosed heavily with digermane causes the molecular TPD peak at 140 K to decrease in intensity and broaden, with a concomitant increase in TPD peak intensity at 570 K, indicating that the electron beam dissociates physisorbed digermane species via deposition of GeH x species onto the Si(100) surface.

Kinetics of hydrogen desorption from germanium-covered Si(100)

Two models from the recent literature, proposed to describe the apparent effect of Ge coverage on silicon monohydride desorption kinetics, are evaluated based on their ability to fit temperature-programmed desorption data, an evaluation of the physical consistency of the estimated kinetic constants, and a comparison with the effects of other atomic impurities (B, P, C) on hydrogen desorption from Si(100). The first model attributes the decrease in the peak temperature for silicon monohydride ( β 1) desorption with increasing Ge coverage to a long-range electronic effect that reduces the activation barrier uniformly over the entire surface. It is shown that this model fails to fit the high Ge coverage data unless the preexponential factor also decreases by approximately nine orders of magnitude, which is physically implausible. The second model considers the possibility of an alternate pathway to depopulate the silicon monohydride phase, in which hydrogen diffuses from the silicon to the germanium surface phase, and desorbs rapidly from a short-lived GeH intermediate. The estimated activation barrier for the surface migration step of 25 ± 1 kcal mol −1 is thermodynamically consistent with the energetics of hydrogen desorption from Si and Ge, and its magnitude is intermediate between the estimated activation barriers for hydrogen surface diffusion on Si and Ge. Although Ge may modify the energetics of the interaction of hydrogen and silicon, it is concluded that such an effect alone is insufficient to describe the shift and broadening of the β 1 feature, while the GeH intermediate model succeeds, even in the absence of any such effect.

Ab initio calculation of hydrogen abstraction energetics from silicon hydrides

In this article, we present calculated energies for the abstraction of hydrogen from silicon monohydride and silicon dihydride surface bonding units by atomic hydrogen obtained using ab initio configuration interaction theory. Three and four silicon atom clusters are used to model the dihydride and monohydride units, respectively. Heats of reaction and activation energy barriers are calculated, including the vibrational energies of the initial, final, and transition states. Hydrogen abstraction from a Si–H unit (H+Si4H10→Si4H9+H2) is found to be exothermic by 9.4 kcal/mol with a transition state energy barrier of 5.5 kcal/mol when H approaches along the surface normal. The dihydride abstraction reaction, H+Si3H8→Si3H7+H2, is exothermic by 7.7 kcal/mol and has an energy barrier of 7.3 kcal/mol when H is approaching along Si–H axis. The barrier is larger for hydrogen atom approaching along the surface normal. The larger barrier for abstraction from a dihydride unit is consistent with our experimental observation of a preferential reduction in monohydride bond concentrations when hydrogenated silicon films are exposed to atomic hydrogen during plasma deposition.

An infrared study of SiH cluster formation in a-Si: H thin films

Hydrogenated amorphous silicon (a-Si: H) thin films have been extensively investigated as a low-cost material for solar cell production. Photovoltaic properties and degradation characteristics are strongly dependent on the various possible hydrogen-silicon bonding configurations, and these can be determined by infrared spectroscopy very rapidly and non-destructively. In this paper we report, for sputter-deposited a-Si: H films which were further thermal annealed at temperatures up to 600°C, infrared spectroscopy (FTIR) results that indicate the occurrence of silicon monohydride clustering. Films were deposited under different substrate temperatures and in-chamber annealing conditions, and cluster formation appears to have taken place regardless of the initial bonding configuration distribution. For a-Si: H films with a predominantly monohydride distribution, annealing temperatures as low as 200°C resulted in extensive changes in HSi bonding configurations, hydrogen motion and cluster formation.

Nearest-neighbor repulsion-perturbed silicon monohydride bond-stretching vibrations in hydrogenated and deuterated silicon nitrogen alloy films

It is shown that increases in the frequency of the silicon monohydride (SiH) bond-stretching vibration in hydrogenated silicon nitrides, over and above what can be attributed to chemical induction effects, are generated by a repulsive interaction between the H-atom of the SiH group, and a near-neighbor SiNH group. This interaction increases the effective bond-stretching force constant of the H-atom of the SiH group. This effect has been verified by ab initio quantum chemistry calculations that have been used to calculate the energy of the H-atom of the SiH group as a function of the relative separation between near-neighbor SiH and SiNH groups.

Spectroscopic Evidence for Near-Neighbor Bonded H(D) in A-Si:N:H(D) Thin Films

AbstractIncreases in the frequency of the silicon monohydride (Si-H) bond-stretching vibration in hydrogenated silicon nitrides, over and above what can be attributed to chemical induction effects, are shown to be generated by a repulsive interaction between the H-atom of the Si-H group, and a near-neighbor SiN-H group. This interaction increases the effective bond-stretching force constant of the H-atom of the Si-H group. This effect has been verified by ab-initio quantum chemistry calculations that have been used to calculate the energy of the H-atom of the Si-H group as a function of the relative separation between near-neighbor Si-H and SiN-H groups. Decreases in the bond-stretching frequencies of SiN-H groups in heavily hydrogenated nitrides are attributed to H-bonding effects associated with near-neighbor pairs of SiN-H groups.

Initial stage of native oxide growth on hydrogen terminated silicon (111) surfaces

We investigated the initial stage of native oxide growth on an atomically flat hydrogen terminated silicon (Si) (111) surface by immersion in pure water using Fourier transformed infrared attenuated total reflection spectroscopy and reflection absorption spectroscopy. There is a sharp absorption peak at 2083 cm−1 arising from silicon monohydrides at surface terraces, this peak decreases and broadens with immersion time and seems to separate into two components. This broadening is considered to reflect the progress of very early native oxide growth. The experiment on a vicinal Si(111) surface showed that the absorption peaks arising from monohydrides at the terraces and at the steps on the surface decrease at approximately the same rate with immersion time. This indicates that the native oxide growth arises in spite of terraces and steps on the silicon surface. Furthermore, the large amount of silicon-hydrogen (Si–H) bonds not related with oxygen atoms suggests that the native oxide growth mechanism is not layer-by-layer oxidation.

Infrared Study of the Reactions of Atomic Deuterium with Amorphous Silicon Monohydride

Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) has been applied to characterize amorphous silicon monohydride films before and after reaction with deuterium atoms. The hydride films were grown by chemical vapor deposition on oxide-covered silicon substrates, and the data suggest that the film is terminated by a homogeneous monolayer of primarily dimerized silicon monohydride. Exposure of this film to atomic deuterium causes the replacement of silicon hydride with adsorbed deuterium. Only the monodeuteride is formed by reaction at 200 °C. Reaction at −110 °C produces mono-, di-, and trideuteride, demonstrating that the isolation of insertion products is temperature-dependent.

Electroluminescence in porous silicon

Electroluminescence (EL) in gold-coated porous silicon has been studied under pulse excitation. The room temperature EL is independent of ambient, unlike photoluminescence (PL). It decreases after 30 min annealing treatments, disappearing after about 480 °C, whereas the PL from either a similar or the same sample, observed through the gold contact, is strongly reduced after the same treatment at about 350 °C. The EL peak wavelength, unlike PL, shows no spectral shift on annealing. It is deduced that the origin of the EL is different from that of PL, and is consistent with silicon monohydride.

Atomic Step Structure on Vicinal H/Si(111) Surface Formed by Hot Water Immersion

We investigated the hydride structures at the step edges on Si(111) surfaces after removing surface oxide with HF solution followed by immersion in boiling water, using polarized infrared attenuated total-reflection spectroscopy. Vicinal (111) surfaces misoriented toward the [112̄] direction at angles of 0°, 2°, and 4° were used to elucidate the hydride structures at the steps. We assigned absorption peaks to monohydride chains at the step edges along the [1̄10] direction, vertical dihydride (whose three atoms form a surface vertical to [111]) at the kinks, and monohydride on the terraces. We determined that the silicon monohydride chain forms an atomically straight step edge along the [1̄10] direction with a small amount of vertical dihydride kinks after 5-min immersion in boiling water. Based on these assignments, we reexamined the water temperature and time dependence of these hydride structures and discussed the step formation mechanism.

Effect of deposition parameters on the properties of amorphous silicon-germanium alloy films grown by the photo-CVD method

A significant improvement in structural, optical and electrical properties of a-SiGe:H films grown by the photo-chemical vapour deposition method has been achieved through a selection of chamber pressure, substrate temperature and hydrogen dilution. By varying these process parameters, a-SiGe:H films having dominant silicon and germanium monohydride bonds were produced (from IR analysis) which have less microstructure related disorder, sharp valence band tail width ( E 0) and low dangling-bond density ( N s). As a consequence the magnitude of the photogenerated carrier (generation efficiency-mobility-lifetime product, ημτ) improved by two orders of magnitude. This resulted in a photoconductivity (σ Ph) of 10 −5 S cm −1 along with a dark conductivity (σ D) of 10 −10 S cm −1 for a-SiG:H film having a band gap of 1.5 eV.

Si 2H 6 adsorption and dissociation pathways on Ge(001)2 ×1: mechanisms for heterogeneous atomic layer epitaxy

Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), and reflection high-energy electron diffraction (RHEED) were used to investigate the adsorption, surface dissociation, and thermal desorption of Si 2H 6 on Ge(001)2 × 1. At room temperature, EELS results show that Si 2H 6 is dissociatively adsorbed onto Ge dangling bonds and further decomposes to yield a disordered overlayer consisting of SiH 2, GeH, and undissociated SiH 3. A saturated coverage of approximately 0.5 monolayers is obtained with Si 2H 6 exposures greater than or equal to 1 × 10 15 cm -2. RHEED results indicate that, in contrast to Si 2H 6-saturated Si(001), the saturated Ge(001) surface still exhibits a significant fraction of dimerized bonds. H 2 desorption from the silicon dihydride phase occurs during 1 min anneals at T a > 150°C while hydrogen desorption from the silicon monohydride phase and Ge surface segregation both occur at significant rates at T a ⩾ 350 °C. All hydrogen is desorbed by 450°C, compared with ⋍ 550°C for the saturated Si(001) surface.

Fluorine Adsorption and Etching on Si(111):SiH Surface during Immersion in HF Solution

We examined the infrared spectrum of Si(111) after terminating its surface with silicon monohydride and immersing it in various concentrations of HF solution to induce fluorine adsorption. After immersion in HF solution, a part of the silicon monohydride surface was etched and (100), (010) and (001) steps composed of strained dihydrides appeared. The silicon-hydrogen stretching frequency for the monohydride was shifted by the fluorine adsorption. Angle-resolved X-ray photoelectron spectroscopy revealed that fluorine penetrates the silicon lattice as well as exists close to the surface. We conclude that fluorine does not terminate the surface silicon dangling bond but inhabits interstitial sites just below the silicon surface.

Correlation between silicon hydride species and the photoluminescence intensity of porous silicon

The role of silicon hydride species in the photoluminescence intensity behavior of porous Si has been studied. The surfaces of luminescent porous Si samples were converted to a predominate SiH termination using a remote H plasma. The as-passivated samples were then immersed in various concentrations of hydrofluouric solutions to regulate the recovery of SiH2 termination on the surface. Photoluminescence measurements and transmission Fourier-transform infrared spectroscopy have shown that predominant silicon monohydride (SiH) termination results in weak photoluminescence. In contrast, it has been observed that the appearance of silicon dihydride (SiH2) coincides with an increase in the photoluminescence intensity.

Low-temperature adsorption and decomposition of borazine on the Si(100)-(2 × 1) surface

Borazine adsorption at a surface temperature ( T s) of − 120°C and the thermal decomposition of borazine have been investigated on the Si(100)-(2 × 1) surface using temperature-programmed desorption (TPD) and static secondary ion mass spectrometry (SSIMS). The initial reactive sticking coefficient ( S 0) at this temperature was measured as 0.08. Borazine dissociates by breaking BH and/or NH bonds to form silicon monohydride species, SiH, even at −120°C because of the highly reactive silicon dangling bonds. The monohydride then decomposes at 550°C to desorb H 2. After the initial decomposition stage, subsequent borazine molecules (α 1-borazine) can coadsorb intact with the partially decomposed borazine (β-borazine). α 1-borazine eventually desorbs molecularly with a peak temperature of 110°C. Taking into account the coexistence of both α 1- and β-borazine, the saturation coverage of borazine (θ BZ) was estimated as 1.2 molecules per surface Si atom. After borazine adsorption and subsequent high-temperature annealing (900°C), the surface contains boron and nitrogen originating from decomposed borazine. This surface is less reactive (“poisoned”) towards borazine adsorption. Above 900°C, the nitrogen- and boron-containing Si surface further decomposes to evolve SiN or Si 2N.

Silicon-Monohydride Termination of Silicon-111 Surface Formed by Boiling Water

Aqueous HF etching of the silicon surface removes surface oxide, leaving a silicon surface terminated inhomogeneously by silicon mono-, di-, and trihydrides. We studied the effect of the immersion in water following the HF etching, on the surface hydride structure on Si(111), by measuring Si-H stretching vibration using infrared absorption spectroscopy. Immersion in boiling water (100°C) for 600 s produces a surface homogeneously terminated with silicon monohydride normal to the surface and free of oxidation. We concluded that water can remove silicon dihydride and trihydride and leave a silicon surface terminated with monohydride. The homogeneous surface has a low defect density of less than 0.5%.