Secondary Ion Mass Spectrometry Profiles Research Articles

Germanium out diffusion in SiGe-based HfO2 gate stacks

The authors report about a detailed study of the chemical composition of advanced HfO2/interfacial layer/SiGe stacks for future p-channel metal-oxide-semiconductor field-effect transistors. Several state-of-the-art characterization techniques are implemented to provide consistent and complementary information about interfacial chemical states and Ge diffusion along the stack. Angle-resolved x-ray photoelectron spectroscopy is performed in both standard and parallel modes. Results highlight the presence of elemental Ge in the HfO2, suggesting some Ge out diffusion from the SiGe substrate. This trend is confirmed by time-of-flight secondary ion mass spectrometry and plasma profiling time-of-flight mass spectrometry, a recently developed technique to monitor thin films compositions during device manufacturing.

Hydrogen passivation of poly-Si/SiOx contacts for Si solar cells using Al2O3 studied with deuterium

The interplay between hydrogenation and passivation of poly-Si/SiOx contacts to n-type Si wafers is studied using atomic layer deposited Al2O3 and anneals in forming gas and nitrogen. The poly-Si/SiOx stacks are prepared by thermal oxidation followed by thermal crystallization of a-Si:H films deposited by plasma-enhanced chemical vapor deposition. Implied open-circuit voltages as high as 710 mV are achieved for p-type poly-Si/SiOx contacts to n-type Si after hydrogenation. Correlating minority carrier lifetime data and secondary ion mass spectrometry profiles reveals that the main benefit of Al2O3 is derived from its role as a hydrogen source for chemically passivating defects at SiOx; Al2O3 layers are found to hydrogenate poly-Si/SiOx much better than a forming gas anneal. By labelling Al2O3 and the subsequent anneal with different hydrogen isotopes, it is found that Al2O3 exchanges most of its hydrogen with the ambient upon annealing at 400 °C for 1 h even though there is no significant net change in its total hydrogen content.

Particle-based chemical oscillation as a function of depth in latex films using gas cluster ion beam secondary ion mass spectrometry profiling

Depth profiles of thin, latex films using gas cluster ion beam (GCIB) secondary ion mass spectrometry (SIMS) show an oscillation of surfactants and polymer signal that is related to the organization of the particles in the film as layers. These results demonstrate the application of GCIB-SIMS to the distribution of water soluble species with molecular sensitivity, which has implications to film performance in areas of adhesion, appearance, and cohesion. Specifically, surfactant species were found at the highest concentrations at the air interface, decreasing through the top few particle layers to a steady state, whereas salt-rich species (sulfates, oligomers) were found at every particle boundary with a high concentration at the substrate interface.

High level active n+ doping of strained germanium through co-implantation and nanosecond pulsed laser melting

Obtaining high level active n+ carrier concentrations in germanium (Ge) has been a significant challenge for further development of Ge devices. By ion implanting phosphorus (P) and fluorine (F) into Ge and restoring crystallinity using Nd:YAG nanosecond pulsed laser melting (PLM), we demonstrate 1020 cm−3 n+ carrier concentration in tensile-strained epitaxial germanium-on-silicon. Scanning electron microscopy shows that after laser treatment, samples implanted with P have an ablated surface, whereas P + F co-implanted samples have good crystallinity and a smooth surface topography. We characterize P and F concentration depth profiles using secondary ion mass spectrometry and spreading resistance profiling. The peak carrier concentration, 1020 cm−3 at 80 nm below the surface, coincides with the peak F concentration, illustrating the key role of F in increasing donor activation. Cross-sectional transmission electron microscopy of the co-implanted sample shows that the Ge epilayer region damaged during implantation is a single crystal after PLM. High-resolution X-ray diffraction and Raman spectroscopy measurements both indicate that the as-grown epitaxial layer strain is preserved after PLM. These results demonstrate that co-implantation and PLM can achieve the combination of n+ carrier concentration and strain in Ge epilayers necessary for next-generation, high-performance Ge-on-Si devices.

Structural and optical properties of SiC-SiO2 nanocomposite thin films

This study deals with the deposition of thin films of a SiC-SiO2nanocomposite deposited on silicon substrates. The deposition is carried out by a co-sputtering RF magnetron 13.56 MHz, using two targets a polycristallin 6H-SiC and sprigs of SiO2. In order to study the influence of the deposition time on the morphology, the structural and optical properties of the thin films produced, two series of samples were prepared, namely a series A with a 30 min deposition time and a series B of one hour duration. The samples were investigated using different characterization techniques such as Scanning Electron Microscope (SEM), X-ray Diffraction (DRX), Fourier Transform Infrared Spectroscopy (FTIR), Secondary Ion Mass Spectrometry (SIMS) and photoluminescence. The results obtained, reveal an optical gap varies between 1.4 and 2.4 eV depending on the thickness of the film; thus depending on the deposition time. The SIMS profile recorded the presence of oxygen (16O) on the surface, which the signal beneath the silicon signal (28Si) and carbon (12C) signals, which confirms that the oxide (SiO2) is the first material deposited at the interface film - substrate with an a-OSiC structure. The photoluminescence (PL) measurement exhibits two peaks, centred at 390 nm due to the oxide and at 416 nm due probably to the nanocrystals of SiC crystals, note that when the deposition time increases, the intensity of the PL drops drastically, result in agreement with dense and smooth film.

Enhanced blue responses in nanostructured Si solar cells by shallow doping

Optimally designed Si nanostructures are very effective for light trapping in crystalline silicon (c-Si) solar cells. However, when the lateral feature size of Si nanostructures is comparable to the junction depth of the emitter, dopant diffusion in the lateral direction leads to excessive doping in the nanostructured emitter whereby poor blue responses arise in the external quantum efficiency (EQE). The primary goal of this study is to find the correlation of emitter junction depth and carrier collection efficiency in nanostructured c-Si solar cells in order to enhance the blue responses. We prepared Si nanostructures of nanocone shape by colloidal lithography, with silica beads of 520 nm in diameter, followed by a reactive ion etching process. c-Si solar cells with a standard cell architecture of an Al back surface field were fabricated varying the emitter junction depth. We varied the emitter junction depth by adjusting the doping level from heavy doping to moderate doping to light doping and achieved greatly enhanced blue responses in EQE from 47%–92% at a wavelength of 400 nm. The junction depth analysis by secondary ion mass-spectroscopy profiling and the scanning electron microscopy measurements provided us with the design guide of the doping level depending on the nanostructure feature size for high efficiency nanostructured c-Si solar cells. Optical simulations showed us that Si nanostructures can serve as an optical resonator to amplify the incident light field, which needs to be considered in the design of nanostructured c-Si solar cells.

Effects of SF6 plasma treatment on the properties of InGaZnO thin films

The effects of sulfur hexafluoride (SF6) plasma on the properties of amorphous InGaZnO (a-IGZO) thin films were examined. The properties of the a-IGZO thin films were characterized by Hall effect measurement, dynamic secondary ion mass spectroscopy (SIMS), and X-ray photoelectron spectroscopy (XPS). The IGZO thin films treated with SF6 plasma before annealing had a very high resistance mainly owing to the inclusion of S into the film surface, as evidenced by SIMS profiles. On the other hand, the samples treated with SF6 plasma after annealing showed better electrical properties with a Hall mobility of 10 cm2/(V·s) than the untreated samples or the samples SF6 plasma-treated before annealing. This was attributed to the increase in the number of oxygen vacancy defects in the a-IGZO thin films owing to the enhanced out-diffusion of O to the ambient and the increase in the number of F-related donor defects originating from the incorporation of a much larger amount of F than of S into the film surface, which were confirmed by XPS and SIMS.

Doping profile measurement on textured silicon surface

In crystalline silicon solar cells, the front surface is textured in order to lower the reflection of the incident light and increase the efficiency of the cell. This texturing whose dimensions are a few micrometers wide and high, often makes it difficult to determine the doping profile measurement. We have measured by secondary ion mass spectrometry (SIMS) and electrochemical capacitance voltage profiling the doping profile of implanted phosphorus in alkaline textured and in polished monocrystalline silicon wafers. The paper shows that SIMS gives accurate results provided the primary ion impact angle is small enough. Moreover, the comparison between these two techniques gives an estimation of the concentration of electrically inactive phosphorus atoms.

Careful stoichiometry monitoring and doping control during the tunneling interface growth of an n + InAs(Si)/p + GaSb(Si) Esaki diode

In this work, we report on the growth of pseudomorphic and highly doped InAs(Si)/GaSb(Si) heterostructures on p-type (0 0 1)-oriented GaSb substrate and the fabrication and characterization of n+/p+ Esaki tunneling diodes. We particularly study the influence of the Molecular Beam Epitaxy shutter sequences on the structural and electrical characteristics of InAs(Si)/GaSb(Si) Esaki diodes structures. We use real time Reflection High Electron Diffraction analysis to monitor different interface stoichiometry at the tunneling interface. With Atomic Force Microscopy, X-ray diffraction and Transmission Electron Microscopy analyses, we demonstrate that an “InSb-like” interface leads to a sharp and defect-free interface exhibiting high quality InAs(Si) crystal growth contrary to the “GaAs-like” one. We then prove by means of Secondary Ion Mass Spectroscopy profiles that Si-diffusion at the interface allows the growth of highly Si-doped InAs/GaSb diodes without any III-V material deterioration. Finally, simulations are conducted to explain our electrical results where a high Band to Band Tunneling (BTBT) peak current density of Jp = 8 mA/μm2 is achieved.

Nitrogen and hydrogen content, morphology and phase composition of hot filament chemical vapor deposited diamond films from NH3/CH4/H2 gas mixtures

In this paper, we present our studies related to phase composition, nitrogen and hydrogen content and morphology of diamond films deposited by hot filament chemical vapor deposition from NH3/CH4/H2 gas mixtures. Raman spectroscopy is used to study the phase composition and chemical nature of the diamond films. The appearance of a nitrogen associated 1190cm−1 peak and carbon‑nitrogen bonding configuration are discussed in light of NH3/CH4 ratios. Scanning electron microscopy reveals nano- and microcrystalline structure of diamond films, depending on the ratio of NH3/CH4 in the gas mixture. Secondary ion mass spectroscopy (SIMS) is used to study the extent and uniformity of nitrogen and hydrogen content within the films. An increase in nitrogen concentration in the films is primarily attributed to the increased NH3/CH4 ratio and also to the microstructure of the deposited film. SIMS profiles of hydrogen reveal a correlation between hydrogen and nitrogen in the diamond films, from which nitrogen bonding configuration can be studied.

Gallium In-Depth Profile in Bromine- Etched Copper-Indium-Galium-(Di)selenide (CIGS) Thin Films Inspected Using Raman Spectroscopy.

In the thin film solar cells domain, copper indium galium (di)selenide (CIGS) is a material with well-established photovoltaic purpose. Here the presence of a suitable [Ga]/([Ga]+[In]) (GGI) in-depth profile has proved to play a key role in the performance of cells. The implementation of a routine method based on reliable but easily available experimental techniques is mandatory to obtain information on the GGI profile of any CIGS layer, in order to achieve high efficiency chalcogenide layers. In this vein, we here propose and systematically test a simple method for the GGI profile determination based on repeated bromine etching of CIGS thin films followed by Raman analysis of the A1 peak position. The reliability of the proposed approach is verified using a methodical comparison with energy-dispersive X-ray spectroscopy (EDS) analysis and secondary ion mass spectroscopy (SIMS) profiles, showing a good agreement with the GGI in-depth profiles determined using Raman analysis on bromine etched samples.

SIMS of Delta Layers in Organic Materials: Amount of Substance, Secondary Ion Species, Matrix Effects, and Anomalous Structures in Argon Gas Cluster Depth Profiles

A study is reported of the quantification of the amount of matter by secondary ion mass spectrometry (SIMS) when depth profiling a nominally 3.1 nm thick delta layer of FMOC-l-pentafluorophenylalanine in Irganox 1010. The depth profiles are made using 5 keV Ar2300+ cluster ions with analysis by 25 keV Bi3+ ions. Data for 89 negative secondary ions shows profiles whose integrated intensities as a function of depth, even when normalized to the intensity for the pure material, still vary over a factor of 12. This variation mainly arises from matrix effects that are measured here using separate samples with mixed layers of three intermediate compositions of the two materials. Matrix enhancements or suppressions vary widely from secondary ion to secondary ion and are not related to the energy of the analyzing Bi3+ ion. Strong effects can cause the delta layer signal to show structure that may be misinterpreted. The compositional profile is established by using trial Dowsett profiles, representing the composition, which are then enhanced or reduced according to the measured matrix effect with the result then fitted to the normalized intensity data. By fitting each secondary ion profile separately, the computed amount of matter still varies weakly with the enhancement. This arises as a result of a longer wavelength roughening, equivalent to a root-mean-square value of about 2.5 nm, which causes, for example, the measured maximum intensity to be lower than the actual maximum intensity appropriate for the delta layer. The effective matrix enhancement is thus reduced by 20%. When this is included, it is found that the variation with enhancement disappears and the amount of matter is found to be equivalent to 3.22 ± 0.07 nm although the scatter from individual ions has a standard deviation of 0.45 nm. It is concluded that the matrix terms used are a good description of the phenomenon and that SIMS profiles may be made quantitative if suitable secondary ions are available and the matrix terms are measured. In the absence of measured matrix terms, the measured quantities are prone to large errors.

Growth Rate and Temperature Dependence of Oxygen Incorporation in Si1-XGex Thin Films

A series of Si1-xGex heterostructures were grown at different substrate temperatures using molecular beam epitaxy (MBE) on Si (100) substrates, where the composition x was varied in a “staircase” pattern by stepping down the germanium flux between constant composition layers while maintaining a constant silicon flux, and vice versa. The silicon flux was supplied with an e-beam evaporator, while the germanium flux was provided with an effusion cell. The germanium composition and oxygen concentration of these staircase structures were then measured using secondary ion mass spectroscopy (SIMS). From the SIMS profiles, it is evident that the oxygen concentration is inversely proportional to and linear with growth rate, regardless of how that rate is changed (either fixing silicon and varying germanium or vice versa). This implies that the dominant source of oxygen contamination in a clean growth system does not originate from either the silicon or germanium, in which case the concentration would be exponential with individual source rate or source flux, but instead comes from the residual background in the MBE chamber. Further, the oxygen concentration retains its linearity in growth rate when changing substrate growth temperature, and shows an exponential dependence in substrate growth temperature for a given rate, where the oxygen concentration increases at lower growth temperature.

Growth Rate and Temperature Dependence of Oxygen Incorporation in Si1-XGex Thin Films

A series of Si1-x Ge x heterostructures was grown at different substrate temperatures using molecular beam epitaxy (MBE) on Si <100> substrates, where the composition x was varied in a “staircase” pattern by stepping down the Ge flux between constant composition layers while maintaining a constant Si flux, and vice versa. The Ge composition and oxygen concentration of these staircase structures were then measured using secondary ion mass spectroscopy (SIMS). From the SIMS profiles, it is evident that the oxygen concentration is inversely proportional to and linear with growth rate, regardless of how that rate is changed. This observation implies that the dominant source of oxygen is the residual background in the MBE chamber. Further, the oxygen concentration retains its linearity in growth rate when changing substrate growth temperature. The oxygen concentration increases at lower growth temperature, and shows an exponential dependence in substrate growth temperature for a given rate.

Analysis of Al diffusion processes in TiN barrier layers for the application in silicon solar cell metallization

An evaporated Al layer is known as an excellent rear metallization for highly efficient solar cells, but suffers from incompatibility with a common solder process. To enable solar cell-interconnection and module integration, in this work the Al layer is complemented with a solder stack of TiN/Ti/Ag or TiN/NiV/Ag, in which the TiN layer acts as an Al diffusion barrier. X-ray photoelectron spectroscopy measurements prove that diffusion of Al through the stack and the formation of an Al2O3 layer on the stack's surface are responsible for a loss of solderability after a strong post-metallization anneal, which is often mandatory to improve contact resistance and passivation quality. An optimization of the reactive TiN sputter process results in a densification of the TiN layer, which improves its barrier quality against Al diffusion. However, measurements with X-ray diffraction and scanning electron microscopy show that small grains with vertical grain boundaries persist, which still offer fast diffusion paths. Therefore, the concept of stuffing is introduced. By incorporating oxygen into the grain boundaries of the sputtered TiN layer, Al diffusion is strongly reduced as confirmed by secondary ion mass spectroscopy profiles. A quantitative analysis reveals a one order of magnitude lower Al diffusion coefficient for stuffed TiN layers. This metallization system maintains its solderability even after strong post-metallization annealing at 425 °C for 15 min. This paper thus presents an industrially feasible, conventionally solderable, and long-term stable metallization scheme for highly efficient silicon solar cells.

Wafer level package of Au-Ge system using a Ge chemical vapor deposition (CVD) thin film

A Ge thin film deposited by chemical vapor deposition (CVD) was used to obtain a uniform bonding between Au and Ge films for applications of wafer level packages (WLPs). This Ge CVD thin film showed selective growth on Au and Cu metals when the substrate has both metal and oxide. A one-step and two-step Ge deposition followed by eutectic bonding method was employed to bond the wafers. The samples were characterized by X-ray diffraction, field emission scanning electron microscopy equipped with an energy dispersive spectroscopy (FESEM-EDS), atomic force microscopy, high resolution Field emission transmission electron microscopy, IR inspection tool and secondary ion mass spectroscopy (SIMS). According to the IR inspection results, the two-step Ge deposited sample showed more uniform film compared to one-step deposition after eutectic bonding. Moreover, an improved bonding quality was obtained from the two-step process. Based on FESEM observations, a uniform and crater-free interface was detected between the bonded 4-inch wafers, in which the presence of Ge beside Au and Si was confirmed by EDS. SIMS profiles proved the formation of a thin Au-Ge interlayer at the bonded interface, which enhanced the bonding conditions.

Investigation of Nucleation and Intermixing at Hetero-Interface in III-Nitride-4H-SiC Structures

III-Nitride/SiC heterostructure devices are a promising approach for improving the performance of SiC devices such as bipolar transistors and avalanche photodiodes. However, the performance of these devices should critically depend on the properties of the hetero-interface that will likely lie within an active region of the device and impact carrier transport. Importantly, intermixing at the hetero-interface can effect doping profiles in these structures as constituent atoms of each semiconductor act as a dopant in the other. In this paper we explore the impact of in situ substrate preparation and migration enhanced epitaxy (MEE) on the nucleation and impurity concentration of thin AlN films grown by plasma-assisted molecular beam epitaxy on 4H-SiC. The surface morphology of the samples were examined by atomic force microscopy and the composition of the films were studies by secondary ion mass spectroscopy (SIMS) and depth profiling x-ray photoemission spectroscopy (XPS). The MEE approach promotes the nucleation of AlN growth on SiC in a 2D mode while suppressing the migration of Al into SiC. Active N that leaks around the closed shutter during in-situ preparation prevents the nucleation of AlN in a 2D growth mode at lower substrate temperatures that is attributed to GaN island formation at the hetero-interface.

Optimized Temperature in Phosphorous Diffusion Gettering Setup of Chromium Transition Metal in Solar Grade Multicrystalline p-Type Silicon Wafer

We have investigated in this work the effect of the temperature profile during homogeneous phosphorous diffusion gettering (PDG) on multicrystalline (mc-Si) silicon p-type wafers destined for photovoltaic solar cells. Temperatures were varied from 800 ◦C to 950 ◦C with time cycle of 90 minutes. Phosphorous profile of np junction was measured by secondary ion mass spectroscopy (SIMS) from 0.45 μm to 2.4 μm. Chromium concentration profile measured on the same samples by SIMS shows a high accumulated concentration of Cr atoms in the gettering layer at 900 ◦C and 950 ◦C, compared to samples obtained at 800 ◦C and 850 ◦C. The effective lifetime (τeff) of minority charge carriers characterized by quasi-steady state photoconductance (QSSPC) is in correlation with these results. From the QSSPC measurements we have observed an amelioration of τeff from 7 μs before PDG to 26 μs in the samples after PDG, processed at 900 ◦C. This indicates the extraction of a non-negligible concentration (5× 10 cm−3 to 5× 10 cm−3) of Cr from the bulk to the surface gettering layer, as observed in the chromium SIMS profiles. A light degradation of τeff (18 μs) is observed in the samples treated at 950 ◦C due probably to a partial dissolution of the metallic precipitates, especially at the grain boundaries and in the dislocations vicinity. The related τCr-Impurity lifetime value of about 8.5 μs is extracted, which is the result of interstitial Cri or CriBs pairs, proving their strongest recombination activity in silicon.

CMOS-compatible method for doping of buried vertical polysilicon structures by solid phase diffusion

Polysilicon receives attention nowadays as a means to incorporate 3D-structured photonic devices into silicon processes. However, doping of buried layers of a typical 3D structure has been a challenge. We present a method for doping of buried polysilicon layers by solid phase diffusion. Using an underlying silicon oxide layer as a dopant source facilitates diffusion of dopants into the bottom side of the polysilicon layer. The polysilicon is grown on top of the oxide layer, after the latter has been doped by ion implantation. Post-growth heat treatment drives in the dopant from the oxide into the polysilicon. To model the process, we studied the diffusion of the two most common silicon dopants, boron (B) and phosphorus (P), using secondary ion mass spectroscopy profiles. Our results show that shallow concentration profiles can be achieved in a buried polysilicon layer using the proposed technique. We present a quantitative 3D model for the diffusion of B and P in polysilicon, which turns the proposed method into an engineerable technique.

SIMS depth profiling and topography studies of repetitive III–V trenches under low energy oxygen ion beam sputtering

Chemical depth profiling of III–V trenches containing InGaAs quantum wells with AlAs barriers selectively grown inside SiO2 cavities was studied using magnetic secondary ion mass spectrometry (SIMS). The authors show that the depth resolution of SIMS profiles of III-As layers degrades under extremely low energy oxygen beam sputtering (&amp;lt;500 eV) due to ripple formation and an increase in surface roughness. Improved SIMS depth resolution was observed by increasing the incident angle (∼50°–65°). Finally, the authors report the effect of sample rotation and orientation of the ion beam with respect to the trenches on depth profiling of III-As layers using time-of-flight SIMS.