SiGe Surface Research Articles

Study of High-Ge-Content Si0.16Ge0.84 Gate Stack by Low Pressure Oxidation

In this study, we employed low-pressure oxidation (LPO) to achieve a high-quality dielectric gate stack on high-Ge-content (HGC) Si0.16Ge0.84. According to X-ray photoelectron spectroscopy depth-profiling results, lowering the oxidation pressure can drive out-diffusion of Si atoms from the HGC SiGe surface, resulting in the suppression of GeOx formation. A nearly GeOx-free interfacial layer detected at an O2 pressure of 0.01 torr at 600°C was used. Consequently, the formation of stable SiOx resulted in the improvement of Dit and gate leakage current, which in accumulation (VFB−1V) was also considerably reduced with an equivalent oxide thickness of 1.5 nm. Finally, by applying the LPO process and optimized process conditions, the interface trap density (Dit) was reduced to 2 × 1012 eV−1cm−2.

Wet and Siconi® cleaning sequences for SiGe p-type metal oxide semiconductor channels

The low temperature integration of new materials (such as SiGe channels for the holes) is mandatory in advanced metal oxide semiconductor field effect transistors (i.e. in 14nm technology node devices and beyond). In this paper, we have investigated the removal of SiGe oxides prior to Selective epitaxial Growth of Si or SiGe:B in Sources/Drains regions. A very efficient removal of contaminants (C, F, O…) is mandatory if the H2 bake that precedes epitaxy is removed because of thermal budget constraints. As germanium is very reactive in the air, in-situ surface preparation schemes (conducted for instance in a Siconi® chamber) might be useful on SiGe surfaces. This way, the queue-time issues associated with “HF-Last” (HF/HCl follow by deionization water rinse) processes in single wafer wet cleaning tools are avoided. Germanium-rich SiGe layers (Si0.6Ge0.4) were used to characterize the native oxide removal efficiency of “HF-Last” and Siconi® processes. Then, a new surface preparation strategy was developed based on i) a wet chemical oxide formation followed by ii) a standard Siconi® process whose efficiency towards SiO2 has conclusively been demonstrated. Parallel Angle Resolved X-ray Photoelectron Spectroscopy was used to study the chemical composition of the native or chemical oxide and evaluate the efficiency of that treatment on carbon, germanium oxide and silicon oxide.

Stability of SiGe(100) Surfaces after Ammonium Sulfide Passivation

Germanium and silicon germanium are attractive channel materials for transistors. Germanium has a smaller band gap and higher hole mobility than silicon, and varying the Ge concentration is one means of tuning the properties of SiGe alloys. Although these materials have many potential benefits, they are challenging to work with. The native oxide, GeO2, is water soluble and therefore cannot protect the surface during aqueous processes. One option is to deposit a layer that chemically passivates the surface after cleaning to protect it from reoxidation. Sulfur could passivate the surface due to its size and valency. Since the dominant cleaning technology is wet, aqueous ammonium sulfide could be a viable option. While ammonium sulfide processes deposit sub-monolayer coverages of sulfur on germanium, the surface reactions on SiGe surfaces are vastly different. Our study focuses on the treatment of Si-rich SiGe surfaces (Si1-xGex x=0.25) with aqueous (NH4)2S. We varied the concentration of (NH4)2S in solution with and without added acid and characterized the surfaces using X-ray Photoelectron Spectroscopy (XPS). Immersions in (NH4)2S alone oxidized the surface and did not deposit a sulfide layer. Instead the surface oxidized as a function of the (NH4)2S concentration. In order to remove the oxide and slow down or stop the oxidation, small volumes of HF and HCl were added to the mixture. Addition of halogen acids dropped the pH of the solution from 10 to 8 and sulfides were detected with XPS in the S 2p region. Sulfur coverage increased with increasing concentrations of HF and HCl, however oxides increased as well. The simultaneous deposition of sulfur and oxygen suggests that oxidized sulfur, such as sulfates and thiosulfates, deposited on the surface. In addition to chemical characterization of the surface, metal-insulator-semiconductor capacitors (MISCAPs) were fabricated after three different surface treatments: 1) clean and dry (control), 2) cleaning and immersion in (NH4)2S and 3) cleaning and immersion in (NH4)2S with added acid. Capacitance-voltage measurements were obtained for all three surface treatments. The density of interface defects (Dit) was calculated for each process from conductance and capacitance data. Processes that yielded the highest sulfur coverage (ammonium sulfide with added acid) showed a reduction in Dit (1.4 x 1012 cm-2 eV-1) compared to samples that were cleaned and dried only (6.4 x 1012 cm-2 eV-1) and samples treated with aqueous ammonium sulfide (4.4 x 1012 cm-2 eV-1).

The Impact of Dummy Gate Processing on Si-Cap-Free SiGe Passivation: A Physical Characterization Study on Strained SiGe 25% and 45%

High mobility channel materials like SiGe, Ge and IIIV receive lots of interest in order to enable the continuation of Moore’s path during the upcoming technology nodes. Recently Si-cap-free SiGe passivation with the number of interface states (NIT) down to 2 1011 cm-2 have been demonstrated. [1-3] A clear correlation was established between Ge-content in the SiGe-oxide and NIT. Unfortunately no process information has been provided on how the Ge-oxide-free interlayers were obtained. In a separate study the impact of HF, HCl and SC1 cleaning solution on the SiGe surface and oxide composition was reported. [4] A comparison of SiGe Gate-All-Around (GAA) and FinFET devices using a Si-cap-free SiGe 25% passivation scheme in a replacement metal gate (RMG) integration indicate the device performance is improved by a TMAH (aq) treatment prior to SiGe interlayer formation and high-k deposition. [5] Here the formation of a SiGe wet chemical oxide will be presented. The impact of dummy gate processing on the interlayer composition is reported. Also the impact of a TMAH (aq) treatment prior to chemical oxide formation has been analyzed and will be reported. Blanket strained SiGe 25% and 45% were grown by epitaxy. The SiGe-substrate and -oxide composition were measured by angle-resolved XPS using the Si2p and Ge3d peaks. The Ge3d peak is fitted with three different compounds: elemental Ge, GeO2 and Ge-suboxide. The angle dependence lets us estimate the distribution of the different components with respect to the sample top surface. An concentration increase with increasing angle (relative to the surface normal) indicates the compound is located closer to the sample surface. And reversely a concentration increase with increasing angle indicates the compound is located deeper into the sample. A native oxide was grown on SiGe 25 and 45% by exposing the wafers to the cleanroom air for approx. 1 week. The native oxide was found to be stoichiometric or nearly stoichiometric with a uniform Ge content throughout the oxide for SiGe 25 and 45% respectively. A wet chemical oxide was grown using an ‘imec clean’ [6] having a final O3/HCl (aq) rinse. The wet chemical oxide is Ge-poor compared to the SiGe substrate. No or only a small variation in Ge content of the SiGe-oxide is seen with ARXPS. However the fraction Ge-suboxide decreases with increasing angle, indicating the Ge-suboxide are located deeper within the sample, so closer to the SiGe-oxide/SiGe-substrate interface. The Ge-content of the SiGe-substrate after wet chemical oxidation shows a weak incident-angle-dependence, indicative a limited Ge enrichment of the SiGe substrate towards the SiGe-oxide/-substrate interface. The low Ge content of the SiGe-oxide is attributed to the water solubility of GeO2. The increase in Ge-content towards the SiGe-oxide/-substrate can be (at least partially) attributed to the poor water solubility of Ge-suboxide. The small increase in Ge content of the SiGe substrate can be attributed to the thermodynamically preferred oxidation of Si over Ge. The combined impact of dummy gate deposition, spike anneal and dummy gate removal prior to SiGe interlayer formation was investigated by AR-XPS and SIMS. The temperature used during spike anneal was found to have a significant impact on the Ge-content of the SiGe-oxide formed during an O3-last imec clean. The results will be reported in detail at the conference. The impact of TMAH (aq) on the chemical oxide composition and surface roughness will also be reported. TMAH (aq) was found to improve SiGe device performance. [5] Also the interaction with the subsequent HfO2 deposition will be reported. In summary SiGe is found to behave as a non-linear combination of Si and Ge. In HF-free aqueous solutions, the SiGe oxide composition is a combination of (1) thermodynamically preferred oxidation of Si versus Ge; (2) poor water-solubility of SiO2 and/or its low solubilization rate; and (3) fast dissolution of GeO2 but slow dissolution of Ge-suboxides. [1] CH Lee et al. VLSI 2016. [2] S. Siddiqui et al. presented MRS Spring Meeting 2016. [3] CH Lee et al. IEDM 2016. [4] S.L. Heslop et al. ECS Trans 69 (2015) 287. [5] H. Mertens et al VLSI 2015. [6] M. Meuris et al. Solid State Phenom, July 1995, p. 109. Figure 1

Study of SiGe Surface Cleaning

Recently Si can be replaced with SiGe in FinFETs (field-effect transistors) and GAA (gate all around) structures in order to get the higher carrier mobility. However, Ge oxide concentration of SiGe oxide causes an increase in interface trap density (Dit) and becomes a defect of device. Although Si and Ge surface treatments have been studied so far, SiGe surface treatment is still not well understood. Recently cleaning of SiGe surface is required and began to be studied for Ge concentration of SiGe oxide by dry and wet cleaning has been reported [1-2]. For cleaning of SiGe surface, this study focused on wet process solutions of O3 water and acids (HF and HCl) with Si1-xGex (X=25-45%) wafer. The behavior of oxide is shown by using these chemical solutions individually or in combination. XPS measurement showed differences of Ge oxide concentration. As first experiment, native oxide shows Ge concentration of oxide and substrate are same or slightly less than substrate. On the other hand, chemical oxide by using O3 water showed concentration of Ge oxide is 10% less than substrate on Si75Ge25 wafer. This study shows the behavior of SiGe surface can be controlled by changing chemical concentration or chemical combination. Details of results will be reported in presentation. [1]S.L.Heslop,et al.,in:ECS Transactions,69 (8) 287-293 (2015) [2]Sang Wook Park,et al., in:J.Vac.Sci.Technol.A,Vol.33,No.4,Jul/Aug 2015

Stability of SiGe(100) Surfaces after Ammonium Sulfide Passivation

Si1-xGex (x=0.25) substrates were treated with aqueous ammonium sulfide with the goal of passivating the surface both electrically and chemically. X-ray photoelectron spectroscopy (XPS) showed that no sulfur was deposited. Instead the surface was oxidized in solution, and the higher the ammonium sulfide concentration, the more oxide that formed. Spectroscopic ellipsometry showed that samples treated with dilute ammonium sulfide oxidized much more rapidly when exposed to air than those treated with a higher concentration. Addition of halogen acids to the sulfur solution deposited sulfides, but oxides were also present. The density of interface defects (Dit) was extracted from capacitance-voltage measurements of metal-insulator-semiconductor capacitors for the clean acid last surface and after either ammonium sulfide alone or ammonium sulfide plus acid. Samples treated in ammonium sulfide plus acid showed the smallest increase in interface defects compared to the other two surfaces.

Ultra-High Purity Hydrazine Delivery for Low Temperature Metal Nitride ALD

Introduction Next generation channel materials (SiGe, Ge and InGaAs) place very difficult thermal constraints (<400C) on Metal Nitride deposition methods. New low temperature methods are potentially needed for: SiN passivation layers on new channel materialsSiN sidewall spacers and contact etch stop layersTiN for metal gate electrodesTiN or WN barrier layers for W contactsTaN barrier layers for Cu Early studies have shown the potential viability of hydrazine and its derivatives as a low temperature Nitrogen source in thermal ALD.1-3 Challenges Though theoretically appealing, an early problem encountered in this approach was the lack of vaporization sources for pure anhydrous H2NNH2gas. There are several challenges associated with the use of anhydrous hydrazine, as it is known to be highly flammable and toxic. In addition, current commercial sources lack the purity needed for semiconductor manufacturing. Membrane Technology Our approach toward increasing safety entails the use of a membrane delivery system where hydrazine is dissolved in an organic solvent (Figure 1). Hydrazine gas permeates the membrane and is delivered to the process chamber, while the solvent does not permeate and remains in the liquid state. Hydrazine vapor pressure is maintained at levels viable for current atomic layer deposition process requirements. The addition of a high boiling point solvent raises the solution flash point, reducing explosive risk. Low Temperature Deposition Initial implementation of our novel hydrazine vaporizer focused on the passivation of SiGe(110) surfaces. Atomic layer deposition of a silicon rich SiNx control layer on Si0.7Ge0.3(001), Si0.5Ge0.5(001), and Si0.5Ge0.5(110) surfaces has been achieved by sequential pulsing of Si2Cl6 and N2H4 precursors at a substrate temperature of 285°C. XPS spectra show a higher binding energy shoulder peak on Si2p indicative of SiOxNyClz bonding while Ge2p and Ge3d peaks show only a small amount of higher binding energy components consistent with only interfacial bonds and indicating the growth of SiOxNy on the SiGe surface with negligible subsurface reactions. Scanning tunneling spectroscopy (STS) measurements confirm the SiNx interfacial control layer forms a passivating interfacial layer on both p-type Si.70Ge.30(001), Si.50Ge.50(110), and Si.50Ge.50(001) surfaces as the surface Fermi level was unpinned and the electronic structure was free of states in the band gap. Significant amounts of oxygen are incorporated into the SiNx layer due to handling procedures as well as the inherent moisture levels in the starting liquid hydrazine material. However in this instance, SiOxNy layer serves to improve the SixGe1-xsurface by preventing formation of Ge-O bonds, therefore it is not a detriment as would be expected in other applications. Hydrazine Measurement and Purification Measurement of water levels inherent to liquid hydrazine has been established to a LDL of 10 parts-per-million using Karl-Fischer and GC-MS methods. Gas phase moisture measurements have been carried out to an LDL of 800 parts-per-billion. Water content in gas phase hydrazine has now reached less than 1.0ppm, which is comparable to Semiconductor grade ammonia. Conclusion There is an emerging need for sub-400C metal nitride deposition. We have developed a novel hydrazine vaporization source and demonstrated that hydrazine is a viable precursor to enable low temperature ALD. Lack of liquid source purity initially posed a problem for films such as TiNx. Our new results indicate that we have overcome the purity issue by reaching hydrazine purity levels comparable to semiconductor grade ammonia. Ultra-high purity hydrazine is now being used for sub 400C deposition of high quality TiNx. XPS, AFM and sheet resistance of this material will be presented. References S. Wolf, M. Edmonds, T. Kent, D. Alvarez, R. Droopad and A. C. Kummel, AVS 2015, San Jose, CA, Paper EM+NS+PS-MoA7, (2015).K. Bernal-Ramos, T. Chen, R. Kanjolia, Y. J. Chabal, “The Role of Hydrazine and its Derivatives in Atomic Layer Deposition of Tungsten Nitride Thin Films”, presented at AVS ALD, Kyoto, Japan, June 2014.B. B. Burton, S. W. Kang, S. W. Rhee and S. M. George, J. Electrochem. Soc., 155(7), D508-D516, (2008). Figure 1

Low temperature thermal ALD of a SiNx interfacial diffusion barrier and interface passivation layer on SixGe1− x(001) and SixGe1− x(110)

Atomic layer deposition of a silicon rich SiNx layer on Si0.7Ge0.3(001), Si0.5Ge0.5(001), and Si0.5Ge0.5(110) surfaces has been achieved by sequential pulsing of Si2Cl6 and N2H4 precursors at a substrate temperature of 285 °C. XPS spectra show a higher binding energy shoulder peak on Si 2p indicative of SiOxNyClz bonding while Ge 2p and Ge 3d peaks show only a small amount of higher binding energy components consistent with only interfacial bonds, indicating the growth of SiOxNy on the SiGe surface with negligible subsurface reactions. Scanning tunneling spectroscopy measurements confirm that the SiNx interfacial layer forms an electrically passive surface on p-type Si0.70Ge0.30(001), Si0.50Ge0.50(110), and Si0.50Ge0.50(001) substrates as the surface Fermi level is unpinned and the electronic structure is free of states in the band gap. DFT calculations show that a Si rich a-SiO0.4N0,4 interlayer can produce lower interfacial defect density than stoichiometric a-SiO0.8N0.8, substoichiometric a-Si3N2, or stoichiometric a-Si3N4 interlayers by minimizing strain and bond breaking in the SiGe by the interlayer. Metal-oxide-semiconductor capacitors devices were fabricated on p-type Si0.7Ge0.3(001) and Si0.5Ge0.5(001) substrates with and without the insertion of an ALD SiOxNy interfacial layer, and the SiOxNy layer resulted in a decrease in interface state density near midgap with a comparable Cmax value.

Electronic structure of O-doped SiGe calculated by DFT + U method**Project supported by the Natural Science Foundation of Yunnan Province, China (Grant No. 2015FB123), the 18th Yunnan Province Young Academic and Technical Leaders Reserve Talent Project, China (Grant No. 2015HB015), and the National Natural Science Foundation of China (Grant No. U1037604).

To more in depth understand the doping effects of oxygen on SiGe alloys, both the micro-structure and properties of O-doped SiGe (including: bulk, (001) surface, and (110) surface) are calculated by DFT + U method in the present work. The calculated results are as follows. (i) The (110) surface is the main exposing surface of SiGe, in which O impurity prefers to occupy the surface vacancy sites. (ii) For O interstitial doping on SiGe (110) surface, the existences of energy states caused by O doping in the band gap not only enhance the infrared light absorption, but also improve the behaviors of photo-generated carriers. (iii) The finding about decreased surface work function of O-doped SiGe (110) surface can confirm previous experimental observations. (iv) In all cases, O doing mainly induces the electronic structures near the band gap to vary, but is not directly involved in these variations. Therefore, these findings in the present work not only can provide further explanation and analysis for the corresponding underlying mechanism for some of the experimental findings reported in the literature, but also conduce to the development of μc-SiGe-based solar cells in the future.

Abrupt SiGe and Si Profile Fabrication by Introducing C Delta Layer

Epitaxial SiGe is widely used for many applications e.g. stressor for C-MOS channel (1), NPN HBT base (2) and SiGe superlattice bolometer (3). However, in order to fabricate shallow and abrupt SiGe profiles, diffusion control at Si and SiGe interface is important, especially at high Ge content. To prevent the interdiffusion, a possible solution could be to lower the temperature during and after the SiGe epitaxy, however it is not preferred because flexibility of process integration is restricted. To reduce the interdiffusion, introduction of C delta layer at the interface is reported (4). Here we investigated the influence of the C delta layer on the interdiffusion and crystal quality. Epitaxial Si/SiGe growths are carried out using a single wafer reduced pressure CVD system. After native oxide removal of a Si(001) wafer by 1000oC prebake in H2, ~10 nm thick Si0.5Ge0.5 is deposited using H2-SiH4-GeH4 at 500oC. Then a Si cap layer is deposited at 500oC to 575oC using H2-Si2H6. To discuss the influence of a C delta layer on the interdiffusion and crystallinity, CH3SiH3 exposure with H2 carrier gas is performed at 500oC before/after the SiGe growth. Profiles of Ge content and crystal perfection are simulated by fitting of XRD rocking curves of Si(004) diffraction. Crystal perfection ranges from 1 (ideal crystal) to 0 (amorphous/polycrystalline). Crystal perfection of SiGe with C delta layers at different positions without/with postannealing at 575oC is shown in Fig. 1. In the case of samples without postannealing, the influence of C delta layers on the crystal perfection is minor. On the other hand, in the case of samples after the postannealing at 575oC, suppression of crystallinity degradation is observed only for the samples with C delta layer at the SiGe surface. C delta layer at the interface between Si and SiGe is less impacting on the suppression of crystallinity degradation. The degradation of SiGe crystallinity is also detected by spectroscopic ellipsometry analysis. The C delta layer at the surface seems to be suppressing defect formation of SiGe at 575oC. Further investigation is required for clarifying the mechanism behind. Figure 2 shows crystal perfection of a Si cap layer deposited on SiGe without/with C delta layers at different temperatures. Note that no defects are observed for all Si growth temperatures used if the Si layer is deposited on Si surface. That means the Si2H6 based Si epitaxy for its own is working perfectly. In the case of the sample without C spike at the interfaces, crystal perfection is ~0.8 from 500oC to 550oC, indicating high crystal quality. But at 575oC, crystal perfection becomes zero because Si is grown on defective SiGe. On the other hand, in the case of the sample with C delta layers at the interface, low crystal perfection of Si cap layer is observed at 500oC. With increasing Si cap growth temperature the crystal perfection of the Si cap becomes higher. At above 550oC, crystal perfection reached ~0.9. The high crystallinity of Si cap at 575oC is caused by maintained crystallinity of the SiGe by the C delta layer. A possible interpretation of low crystallinity Si growth at 500oC and improved Si crystal quality at higher temperature is; Adsorbed CH3 species on SiGe surface at 500oC causes probability of disordered Si2H6 adsorption.At higher temperature, hydrogen desorption from the SiGe surface and reallocation are pronounced. The CH3species may react with dangling bond at substitutional site. (C desorption is not evidenced by SIMS measurement). SiGe/Si cap profiles simulated by XRD without and with C delta layers at interface are shown in Fig. 3 (a). By depositing Si cap at 550oC, smearing of SiGe profile is observed for sample without C delta layers. On the other hand, in the case of sample with C delta layer at interface, steep SiGe profile is observed. This diffusion suppression by C delta layer is also detected by SIMS (Fig. 3 (b)). The SiGe interdiffusion process is reported as vacancy mechanism (5). Possible assumption of the diffusion suppression effect is vacancy trap near the interface by C. These results shown here offer the opportunity of interdiffusion control of SiGe/Si interface using C delta doping while maintaining high crystal perfection. References (1) J. Kasim et al. Solid-State Electron. 110 (2015) 19 (2) A. Fox et al. IEEE Electron Device Lett. 36 (2015) 642 (3) L. Zhang et al. DOI: 10.1109/RSETE.2012.6260364 (4) T. Hirano et al. Thin Solid Films 518 6 1 (2011) S222 (5) N. Ozguven et al. J. Appl. Phys 90 (2007) 082109 Figure 1

Interface Engineering for Atomic Layer Deposited Alumina Gate Dielectric on SiGe Substrates.

Optimization of the interface between high-k dielectrics and SiGe substrates is a challenging topic due to the complexity arising from the coexistence of Si and Ge interfacial oxides. Defective high-k/SiGe interfaces limit future applications of SiGe as a channel material for electronic devices. In this paper, we identify the surface layer structure of as-received SiGe and Al2O3/SiGe structures based on soft and hard X-ray photoelectron spectroscopy. As-received SiGe substrates have native SiOx/GeOx surface layers, where the GeOx-rich layer is beneath a SiOx-rich surface. Silicon oxide regrows on the SiGe surface during Al2O3 atomic layer deposition, and both SiOx and GeOx regrow during forming gas anneal in the presence of a Pt gate metal. The resulting mixed SiOx-GeOx interface layer causes large interface trap densities (Dit) due to distorted Ge-O bonds across the interface. In contrast, we observe that oxygen-scavenging Al top gates decompose the underlying SiOx/GeOx, in a selective fashion, leaving an ultrathin SiOx interfacial layer that exhibits dramatically reduced Dit.

(Invited) Rapid In-Situ Carbon and Oxygen Cleaning of In0.53Ga0.47As(001) and Si0.5Ge0.5(110) Surfaces via a H2 RF Downstream Plasma

Channel passivation of SiGe and InGaAs requires removal of dangling bonds and formation of a thermally stable interface. On InGaAs, one solution is the deposition by self-limiting CVD or ALD of a few monolayer of silicon, silicon oxide, or silicon nitride. To form an Si-OH terminated In0.53Ga0.47As(001) surface, two monolayers of Si were deposited at 350°C with self-limiting CVD using Si2Cl6 dosing following by dosing anhydrous HOOH(g). Scanning tunneling microcopy (STM) shows and ordered surface is form while scanning tunneling spectroscopy (STS) shows the surface Fermi level position moves towards midgap due to a surface dipole formation from –OH groups and oxygen bonding to the surface. TMA was dosed at 250°C onto the Si-OH terminated InGaAs(001), and XPS shows the emergence of the Al 2p and C 1s peaks indicative of TMA surface nucleation. The TMA shifts the InGaAs Fermi level back towards the conduction band, consistent with unpinning. MOSCAPs were fabricated to further demonstrate Fermi level unpinning using the Si-OH termination of InGaAs. While this same Si deposition process could be employed on SiGe, a simple annealing process can be employed to form a silicon terminated SiGe surface. A saturation dose of H2O2(g) at 25oC chemisorbs to on Si0.5Ge0.5(110) and Si0.47Ge0.53(001) surfaces leaving the Fermi level on the surface unpinned, and the surface is functionalized with mainly Si-OH, Ge-OH and Si-O-Ge bond. After a subsequent TMA dose at 25oC and annealing at 300oC, XPS and STM verify that a thermally stable and well-ordered monolayer of Al2O3 is formed on SiGe(110) and (001) surfaces with only Al-O-Si bonds and no detectable Ge-O-Si bonds. The H2O2(g) functionalization provides 3 times higher O sites on the surface and 3 times higher TMA nucleation density than H2O(g) at 25°C and 120°C. A variety of strongly bound passivating atoms can induce formation of a nearly pure Si interfacial layer between high-k dielectrics and SiGe. This annealing procedure was employed to form MOSCAP on SiGe(001) using Al2O3 and HfO2 gate oxides after either (NH4)2S or NH3 plasma passivation. For NH3 plasma passivation, angle-resolved XPS showed that the annealing allows formation of nearly Ge-free SiON layer. For higher Ge concentrations, the annealing process is unlikely to form a nearly Ge-free SiON layer so a novel plasma-free chemistry has been develop to deposit SiON using ALD with N2H4(g) as a precursor. Figure 1

New self assembly monolayer onto SiGe as a selective biosensor for single-strand DNA

An important step of fabrication of selective DNA probe/receptor is the functionalization of semiconducting surfaces with a self-assembled monolayer (SAM) with an appropriate surface termination to interact with DNA. In this work, we studied an immobilization of single-strand DNA (ssDNA) onto self-assembly monolayer of both the heavily exploited aminopropyltriethoxy silane (APTES) and n-(2-aminoethyl)-11-aminoundecyltrimethoxysilane (NAATS), a bi-functional amino silane onto the surface of SiGe substrate. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) techniques were employed to characterize the silanization of SAMs on SiGe surface. The immobilization process was confirmed using fluorescence intensity measurements. The coverage values of APTES and NAATS SAMs were estimated to be 71.3 and 83 XPS% at 78°, respectively from the C/Ge ratio. Comparing the performance towards both specific and non-specific ssDNA complements for both silanes, it was observed that a higher selectivity and sensitivity was obtained by using NAATS SAM. This work highlighted the importance of SAM chain length by comparing one short alkyl chains, APTES with that long-chain counterpart, NAATS.

Low thermal budget for Si and SiGe surface preparation for FD-SOI technology

Ultra thin Silicon films of Silicon-on-Insulator technology are metastable and thus cannot be submitted to high temperature treatments that may roughen or disrupt the film during the set of technological steps required for device fabrication. This paper concerns the development of an efficient low temperature cleaning process of Si and SiGe surfaces that enables a subsequent good-quality epitaxy of raised source and drain. For this purpose wet-clean, plasma-clean and several combinations of both are used. We thus propose two effective surface cleaning processes with low thermal budget optimized for FD-SOI technology.

Direct MOCVD epitaxy of GaAsP on SiGe virtual substrate without growth of SiGe

GaAsP has important applications for solar cells and light-emitting diodes on silicon substrates. Here we demonstrate that GaAsP can be directly grown by metal-organic chemical vapor deposition on previously-prepared lattice-matched SiGe virtual substrates, without prior in-situ growth of SiGe. By optimizing the growth pressure and AsH3 pre-exposure time, the surface morphology of GaAsP is improved and the pinhole densities are reduced by two orders of magnitude from 2.4×107/cm2 to 1.2×105/cm2. The physics is suggested to be an optimization between complete As-termination and AsH3 etching effect of SiGe surface.

Chemically selective formation of Si–O–Al on SiGe(110) and (001) for ALD nucleation using H2O2(g)

Passivation and functionalization via atomic hydrogen, hydrogen peroxide (H2O2(g)), and trimethylaluminum (TMA) on clean silicon–germanium (Si0.5Ge0.5(110) and Si0.47Ge0.53(001)) surfaces were studied and compared at the atomic level using ultra-high vacuum (UHV) scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and X-ray photoelectron spectroscopy (XPS) to understand the topological, electronic, and chemical structures of the surfaces. STM and XPS indicate that a sputter-cleaned SiGe(110) surface is terminated with adatoms, while a SiGe(001) surface is terminated with germanium dimers. STS demonstrates that the Fermi level on a clean SiGe(110) surface is pinned near mid-gap due to surface dangling bonds, while the Fermi level on a clean SiGe(001) surface is consistent with unpinning. A saturation dose of H2O2(g) at 25°C chemisorbs to SiGe surfaces, leaving the Fermi level at the surface consistent with unpinning, and the surface is functionalized mainly with Si–OH, Ge–OH, and Si–O–Ge bonds on both SiGe(110) and (001). After a subsequent TMA dose at 25°C, XPS and STM verify that a thermally stable and well-ordered monolayer of Al2O3 is formed on SiGe(110) and (001) surfaces, resulting in the formation of Al–O–Si bonds. The H2O2(g) functionalization provides three times more oxygen sites on the surface and three times as great a TMA nucleation density than does H2O(g) at both 25°C and 120°C. STS demonstrates that H2O2(g)- and TMA-dosed SiGe surfaces show a Fermi level consistent with unpinning and a local density of states (DOS) without any states between the conduction and valence band edge, indicating an ideal template for further atomic layer deposition (ALD) nucleation of high-k materials on SiGe(110) and (001) surfaces.

Sulfur passivation for the formation of Si-terminated Al2O3/SiGe(0 0 1) interfaces

Sulfur passivation is used to electrically and chemically passivate the silicon–germanium (SiGe) surfaces before and during the atomic layer deposition (ALD) of aluminum oxide (Al2O3). The electrical properties of the interfaces were examined by variable frequency capacitance–voltage (C–V) spectroscopy. Interface compositions were determined by angle-resolved X-ray photoelectron spectroscopy (AR-XPS). The sulfur adsorbs to a large fraction of surface sites on the SiGe(001) surface, protecting the surface from deleterious surface reactions during processing. Sulfur passivation (a) improved the air stability of the cleaned surfaces prior to ALD, (b) increased the stability of the surface during high-temperature deposition, and (c) increased the Al2O3 ALD nucleation density on SiGe, thereby lowering the leakage current. S passivation suppressed formation of GeO bonds at the interface, leaving the majority of the Al2O3–SiGe interface terminated with direct SiOAl bonding.

Density-Functional Theory Molecular Dynamics Simulations and Experimental Characterization of a-Al₂O₃/SiGe Interfaces.

Density-functional theory molecular dynamics simulations were employed to investigate direct interfaces between a-Al2O3 and Si0.50Ge0.50 with Si- and Ge-terminations. The simulated stacks revealed mixed interfacial bonding. While Si-O and Ge-O bonds are unlikely to be problematic, bonding between Al and Si or Ge could result in metallic bond formation; however, the internal bonds of a-Al2O3 are sufficiently strong to allow just weak Al bonding to the SiGe surface thereby preventing formation of metallic-like states but leave dangling bonds. The oxide/SiGe band gaps were unpinned and close to the SiGe bulk band gap. The interfaces had SiGe dangling bonds, but they were sufficiently filled that they did not produce midgap states. Capacitance-voltage (C-V) spectroscopy and angle-resolved X-ray photoelectron spectroscopy experimentally confirmed formation of interfaces with low interface trap density via direct bonding between a-Al2O3 and SiGe.

Surface Cleaning of SiGe(100) and Passivation of Ge(100) with Aqeuous Ammonium Sulfide

The effect of standard aqueous cleans including SC-1, HCl, HF, and HCl/HF mixtures on SiGe(100) surfaces with 50, 75 and 85% Ge molar ratios was characterized with x-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry. HF was most effective at removing both Ge and Si oxides and reduced the carbon contamination. SC-1 selectively depleted Ge from the surface. Passivation of Ge(100) surfaces with aqueous (NH4)2S was characterized with XPS as a function of the solution concentration and with the addition of H2O2. The passivation process was independent of the (NH4)2S concentration and approximately 3 Å of S was deposited or about one layer based on XPS. Addition of H2O2 to very dilute (NH4)2S oxidized the surface while addition to higher concentrations showed similar effects as a sequential HCl/H2O2 treatment.

Development of a ReaxFF reactive force field for Si/Ge/H systems and application to atomic hydrogen bombardment of Si, Ge, and SiGe (100) surfaces

A new reactive force field was developed for use in molecular dynamics simulations of chemical systems composed of silicon (Si), germanium (Ge), and hydrogen (H) with the ReaxFF code. The development incorporated Ge into the ReaxFF family of reactive potentials by fitting against a diverse training set of DFT data that pertain to Si/Ge/H bonding environments. The predictive capacity of the force field was manifested in molecular dynamics simulations of the H atom bombardment of the (100) surface of c-Si, c-Ge, and c-SiGe crystalline solid slabs in order to simulate the effects of the H-plasma semiconductor cleaning process in the near-surface region. Phenomena related to surface and subsurface H adsorption, H2 generation, and surface etching were described and compared in relation to material composition and the kinetic energy of the impinging atoms.