Evaluation of the influence of an embedded porous silicon layer on the bulk lifetime of epitaxial layers and the interface recombination at the epitaxial layer/porous silicon interface

Porous silicon plays an important role in the concept of wafer‐equivalent epitaxial thin‐film solar cells. Although porous silicon is beneficial in terms of long‐wavelength optical confinement and gettering of metals, it could adversely affect the quality of the epitaxial silicon layer grown on top of it by introducing additional crystal defects such as stacking faults and dislocations. Furthermore, the epitaxial layer/porous silicon interface is highly recombinative because it has a large internal surface area that is not accessible for passivation. In this work, photoluminescence is used to extract the bulk lifetime of boron‐doped (10¹⁶/cm³) epitaxial layers grown on reorganised porous silicon as well as on pristine mono‐crystalline, Czochralski, p⁺ silicon. Surprisingly, the bulk lifetime of epitaxial layers on top of reorganised porous silicon is found to be higher (~100–115 µs) than that of layers on top of bare p⁺ substrate (32–50 µs). It is believed that proper surface closure prior to epitaxial growth and metal gettering effects of porous silicon play a role in ensuring a higher lifetime. Furthermore, the epitaxial layer/porous silicon interface was found to be ~250 times more recombinative than an epitaxial layer/p⁺ substrate interface (S ≅ 10³ cm/s). However, the inclusion of an epitaxially grown back surface field on top of the porous silicon effectively shields minority carriers from this highly recombinative interface. Copyright © 2013 John Wiley & Sons, Ltd.     

A Bondable Metallization Stack That Prevents Diffusion of Oxygen and Gold into Monolithically Integrated Circuits Operating Above 500°C

We investigate use of tantalum silicide (TaSi₂, 400 nm)/platinum (Pt, 200 nm)/iridium (Ir, 200 nm)/platinum (Pt, 200 nm) as both a bond metal and a diffusion barrier to prevent oxygen (from air) and gold (from the wire bond) from infiltrating silicon carbide (SiC) monolithically integrated circuits operating above 500°C for over 1000 h in air. The TaSi₂/Pt/Ir/Pt metallization is easily bonded for electrical connection to off-chip circuitry and does not require extra anneals or masking steps. It can be used directly on ohmic contact metals, dielectric insulating layers, or interconnect metal, because it adheres to silicon dioxide (SiO₂), silicon nitride (Si₃N₄), and titanium (Ti). In this study, we investigate use of the new metallization of TaSi₂/Pt/Ir/Pt (in deposition order) with TaSi₂ resting on top of a Ti-SiC contact annealed at 600°C for 30 min in nitrogen, which allows the TaSi₂ layer to react with the bottom platinum layer to form the Pt₂Si diffusion barrier at the Pt-Ir interface. Since the iridium layer does not readily form a silicide, it prevents the silicon from migrating into the topmost platinum layer during further annealing or high-temperature integrated circuit operation. This leaves a pure platinum layer at the surface, ideal for gold wire bonding. We discuss the characteristics of the TaSi₂/Pt/Ir/Pt metallization at 500°C after 10 h, 100 h, and 1000 h in air ambient and N₂ ambient. Auger electron spectroscopy (AES) depth profiles of the metallization and field-emission scanning electron microscopy-focused ion beam (FESEM-FIB) cross-sections are also discussed.     

Formation and characterization of porous silicon films obtained by catalyzed vapor-chemical etching

Porous silicon films obtained by the metal-assisted vapor-chemical etching technique have been characterized. For the film formation, epitaxial (100) N/P⁺, 1–5 Ω cm monocrystalline silicon wafers were used. The vapors of an alcoholic solution of H₂O₂/HF were drawn towards the silicon surface, which was previously covered with a thin layer of gold (~8 nm) for the catalytic etching. For the optical and morphological characterization of porous films, Raman spectroscopy, Ellipsometry, FTIR spectroscopy and SEM images were used. The films thickness kept a linear relationship with etching time. A porosity gradient from the surface towards the interface (65% to 12%) was observed in the films. A large amount of Si–H bonds as related to O–Si–O bonds were observed and the pore size depends on the HF concentration. Irregular morphology was found in films formed with 50% HF.     

Characterization of buried SiO2 layers formed by ion implantation of oxygen

Oxygen has been ion implanted (200 keV) into silicon at doses ranging from 2E17/cm² to 2E18/cm². The peak oxygen concentration occurs at a depth of 0.5 μm. These doses produce peak oxygen concentrations which are below and above the concentrations necessary to form stoichiometric SiO₂. If the oxygen concentration exceeds stoichiometry, a buried SiO₂ layer is formed with a thin superficial silicon layer on the surface. This superficial silicon layer has been used as a seed for growing single crystal silicon epi. The resulting Silicon on Insulator (SOI) structure has been characterized by Rutherford backscattering, cross-sectional TEM, AES, optical microscopy, spreading resistance probe, Hall effect and infrared transmission measurements. The effects of dose, substrate temperature during the implant, and subsequent anneal conditions have been examined.     

Deep levels of platinum in silicon

Platinum has received renewed attention of late from device engineers as a means of lifetime-control in silicon. The energy levels assigned to platinum are not well defined, however, and ambiguities exist in the literature. In this work platinum was introduced into n- and p-type silicon and the energy levels and concentrations studied by thermally stimulated current and capacitance techniques. p]Schottky barrier diodes were used to study electron emission from two levels found in the upper half of the band gap. The use of Schottky barriers eliminated the problem of process-induced defect introduced by high concentration p⁺ diffusions. Phosphorus-diffused n⁺ - p diodes were used to study hole emission from three levels found in the lower half of the band gap. Platinum concentration versus distance profiles were obtained from thermally stimulated capacitance measurements. Experimental results indicate that the platinum concentration follows the boron concentration distribution near the junction in p-type silicon.     

Solubility of gold in p-type silicon

The solubility of gold in silicon heavily doped with boron has been investigated using radio-tracer techniques. ¹⁹⁸Au was diffused to saturation into silicon samples at temperatures between 1273 and 1573°K. The gold solubility was found to be enhanced in silicon uniformly doped with a boron concentration of 9 × 10¹⁹ cm^?3 compared with that in “intrinsic” silicon. The experimental results in this work were found to be consistent with calculations based on a Fermi level effect model which assumes that the gold acceptor and donor levels remain fixed with respect to the conduction band edge as the band gap shrinks with increasing temperature. The experimental data reported by other workers for enhancements of gold solubility in both heavily doped n- and p-type silicon were also consistent with the above model.     

Electrical characterization of semiconducting diamond thin films and single crystals

Naturally occurring semiconducting single crystal (type IIb) diamonds and boron doped polycrystalline thin films were characterized by differential capacitance-voltage and Hall effect measurements, as well as secondary ion mass spectroscopy (SIMS). Results for natural diamonds indicated that the average compensation for a type IIb diamond was >17%. Mobilities for the natural crystals varied between 130 and 564 cm²/V·s at room temperature. The uncompensated dopant concentration obtained by C-V measurements (2.8 ± 0.1 × 10¹⁶ cm⁻³) was consistent with the atomic B concentration measured by SIMS performed on similar samples (3.0 ± 1.5 x 10¹⁶ cm⁻³). Measurement of barrier heights for three different metals (platinum, gold, and aluminum) found essentially the same value of 2.3 ± 0.1 eV in each case, indicating that the Fermi level was pinned at the diamond surface. Polycrystalline semiconducting diamond thin films demonstrated a complex carrier concentration behavior as a function of dopant density. This behavior may be understood in terms of a grain boundary model previously developed for polycrystalline silicon, or by considering a combination of compensation and impurity band conduction effects. The highest mobility measured for a polycrystalline sample was 10 cm²/V·s, indicating that electrical transport in the polycrystalline material was significantly degraded relative to the single crystal samples.     

Study of silicon doped with zinc ions and annealed in oxygen

The results of studies of the surface layer of silicon and the formation of precipitates in Czochralski n-Si (100) samples implanted with ⁶⁴Zn⁺ ions with an energy of 50 keV and a dose of 5 × 10¹⁶ cm^–2 at room temperature and then oxidized at temperatures from 400 to 900°C are reported. The surface is visualized using an electron microscope, while visualization of the surface layer is conducted via profiling in depth by elemental mapping using Auger electron spectroscopy. The distribution of impurity ions in silicon is analyzed using a time-of-flight secondary-ion mass spectrometer. Using X-ray photoelectron spectroscopy, the chemical state of atoms of the silicon matrix and zinc and oxygen impurity atoms is studied, and the phase composition of the implanted and annealed samples is refined. After the implantation of zinc, two maxima of the zinc concentration, one at the wafer surface and the other at a depth of 70 nm, are observed. In this case, nanoparticles of the Zn metal phase and ZnO phase, about 10 nm in dimensions, are formed at the surface and in the surface layer. After annealing in oxygen, the ZnO · Zn₂SiO₄ and Zn · ZnO phases are detected near the surface and at a depth of 50 nm, respectively.     

Isolation of silicon film grown on porous silicon layer

We have investigated a new technology for dielectric isolation of a Si film grown epitaxially on a porous silicon layer. After oxidation of the porous silicon layer, a Si on Ohcidized Porous Silicon(SOPS) structure can be obtained. It is proposed that micropores pinch off quickly in the interfacial region between the porous silicon layer and the epitaxial film. A minimum yield calculated from Rutherford backscattering spectra of the epitaxial silicon film is 5.3%, and an electron Hall mobility of 600cm²/V.s is obtained in the film with a carrier concentration of 1 x 10¹⁷ /cm³. MOSFETs were fabricated on the SOPS structure.     

Surface texture of single-crystal silicon oxidized under a thin V<Subscript>2</Subscript>O<Subscript>5</Subscript> layer

The process of surface texturing of single-crystal silicon oxidized under a V₂O₅ layer is studied. Intense silicon oxidation at the Si–V₂O₅ interface begins at a temperature of 903 K which is 200 K below than upon silicon thermal oxidation in an oxygen atmosphere. A silicon dioxide layer 30–50 nm thick with SiO₂ inclusions in silicon depth up to 400 nm is formed at the V₂O₅–Si interface. The diffusion coefficient of atomic oxygen through the silicon-dioxide layer at 903 K is determined (D ≥ 2 × 10^–15 cm² s^–1). A model of low-temperature silicon oxidation, based on atomic oxygen diffusion from V₂O₅ through the SiO₂ layer to silicon, and SiO _x precipitate formation in silicon is proposed. After removing the V₂O₅ and silicon-dioxide layers, texture is formed on the silicon surface, which intensely scatters light in the wavelength range of 300–550 nm and is important in the texturing of the front and rear surfaces of solar cells.     

Formation of thermal donors in SIMOX material

Thermal oxygen donor generation in SIMOX material formed in Czochralski (CZ) and oxygen free float zone (FZ) silicon was investigated by Hall and photoluminescence techniques. It was determined that residual interstitial oxygen was introduced to silicon by the SIMOX buried oxide formation process thus increasing the possibility of thermal donor creation. Significantly, thermal donor generation was identified and localized to the top silicon region in FZ material. The detected concentration of residual oxygen was on the order of 5 × 10¹³ cm^-3 and is negligible when compared to the intrinsic oxygen concentration of the starting CZ bulk material.     

Investigation of Multicarrier Transport in LPE-Grown Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te Layers

A detailed study is presented of multicarrier transport properties in liquid-phase epitaxy (LPE)-grown n-type HgCdTe films using advanced mobility spectrum analysis techniques over the temperature range from 95 K to 300 K. Three separate electron species were identified that contribute to the total conduction, and the temperature-dependent characteristics of carrier concentration and mobility were extracted for each individual carrier species. Detailed analysis allows the three observed contributions to be assigned to carriers located in the bulk long-wave infrared (LWIR) absorbing layer, the wider-gap substrate/HgCdTe transition layer, and a surface accumulation layer. The activation energy of the dominant high-mobility LWIR bulk carrier concentration in the high temperature range gives a very good fit to the Hansen and Schmit expression for intrinsic carrier concentration in HgCdTe with a bandgap of 172 meV. The mobility of these bulk electrons follows the classic μ ~ T ^−3/2 dependence for the phonon scattering regime. The much lower sheet densities found for the other two, lower-mobility electron species show activation energies of the order of ~20 meV, and mobilities that are only weakly dependent on temperature and consistent with expected values for the wider-bandgap transition layer and a surface accumulation layer.     

X-ray-emission study of the structure of Si:H layers formed by low-energy hydrogen-ion implantation

Amorphous silicon layers formed by implantation of 24-keV hydrogen ions into SiO₂/Si and Si with doses of 2.7×10¹⁷ and 2.1×10¹⁷ cm^?2, respectively, were studied using ultrasoft X-ray emission spectroscopy with variations in the energy of excitation electrons. It is ascertained that the surface silicon layer with a thickness as large as 150–200 nm is amorphized as a result of implantation. Implantation of hydrogen ions into silicon coated with an oxide layer brings about the formation of a hydrogenated silicon layer, which is highly stable thermally.     

Nanoheterostructures with improved parameters for high-speed and efficient plasmon-polariton light emitting Schottky diodes

As a result of theoretical and experimental analyses, the parameters of heterostructures with InAs quantum dots in a GaAs matrix are determined, which provide the development of high-speed and efficient plasmon-polariton near-infrared light-emitting Schottky diodes based on such structures. The quantum dots should be arranged on a heavily doped (to a dopant concentration of 10¹⁹ cm^–3) GaAs buffer layer and be separated from the metal by a thin (10–30 nm thick) undoped GaAs cap layer. The interface between the metal (e.g., gold) and GaAs provides the efficient scattering of surface plasmon-polaritons to ordinary photons if it contains inhomogeneities shaped as metal-filled cavities with a characteristic size of ~30 nm and a surface concentration above 10¹⁰ cm^–2.     

Formation of the nickel-platinum alloy silicide Schottky barriers

We propose a new technique for the Schottky barrier formation that involves magnetron deposition of a thin film from a multicomponent target consisting of vanadium, platinum, and nickel onto silicon and the subsequent stage thermal treatment. Using the developed technique, we fabricated device structures with the 0.69–0.71-V-high Schottky barriers. It is established that the barrier layer comprises the Ni_{1 ? x} Pt _x Si silicide phase and about 2 at % of platinum in the contact region. We show that the amount of platinum at the interface with silicon determines the barrier’s height. The highest platinum content at the interface is ensured at the two-stage thermal treatment at a first stage temperature of 240–300°C. The use of the two-stage thermal treatment in the silicide formation in the system’s silicon-composite Ni-Pt-V alloy allows obtaining a silicide layer with higher structural quality and a better silicon/silicide interface than the one-stage treatment can yield.     

A 3‐in‐1 doping process for interdigitated back contact solar cells exploiting the understanding of co‐diffused dopant profiles by use of PECVD borosilicate glass in a phosphorus diffusion

Boron and phosphorus doping of crystalline silicon using a borosilicate glass (BSG) layer from plasma‐enhanced chemical vapor deposition (PECVD) and phosphorus oxychloride diffusion, respectively, is investigated. More specifically, the simultaneous and interacting diffusion of both elements through the BSG layer into the silicon substrate is characterized in depth. We show that an overlying BSG layer does not prevent the formation of a phosphorus emitter in silicon substrates during phosphorus diffusion. In fact, a BSG layer can even enhance the uptake of phosphorus into a silicon substrate compared with a bare substrate. From the understanding of the joint diffusion of boron and phosphorus through a BSG layer into a silicon substrate, a model is developed to illustrate the correlation of the concentration‐dependent diffusivities and the emerging diffusion profiles of boron and phosphorus. Here, the in‐diffusion of the dopants during diverse doping processes is reproduced by the use of known concentration dependences of the diffusivities in an integrated model. The simulated processes include a BSG drive‐in step in an inert and in a phosphorus‐containing atmosphere. Based on these findings, a PECVD BSG/capping layer structure is developed, which forms three different n⁺⁺−, n⁺− and p⁺−doped regions during one single high temperature process. Such engineered structure can be used to produce back contact solar cells. Copyright © 2016 John Wiley & Sons, Ltd.     

Chemically Homogeneous Complex Oxide Thin Films Via Improved Substrate Metallization

A long‐standing challenge to the widespread application of complex oxide thin films is the stable and robust integration of noble metal electrodes, such as platinum, which remains the optimal choice for numerous applications. By considering both work of adhesion and stability against chemical diffusion, it is demonstrated that the use of an improved adhesion layer (namely, ZnO) between the silicon substrate and platinum bottom electrode enables dramatic improvements in the properties of the overlying functional oxide films. Using BaTiO₃ and Pb(Zr,Ti)O₃ films as test cases, it is shown that the use of ZnO as the adhesion layer leads directly to increased process temperature capabilities and dramatic improvements in chemical homogeneity of the films. These result in significant property enhancements (e.g., 300% improvement to bulk‐like permittivity for the BaTiO₃ films) of oxide films prepared on Pt/ZnO as compared to the conventional Pt/Ti and Pt/TiO_x stacks. A comparison of electrical, structural, and chemical properties that demonstrate the impact of adhesion layer chemistry on the chemical homogeneity of the overlying complex oxide is presented. Collectively, this analysis shows that in addition to the simple need for adhesion, metal‐oxide layers between noble metals and silicon can have tremendous chemical impact on the terminal complex oxide layers.     

Variable area MWIR diodes on HgCdTe/Si grown by molecular beam epitaxy

Molecular beam epitaxy technique has been used to grow double layer heterostructure mercury cadmium telluride materials on silicon substrates for infrared detection in the mid-wavelength infrared transmission band. Test structures containing square diodes with variable areas from 5.76 × 10⁻⁶ cm² to 2.5×10⁻³ cm² are fabricated on them. The p on n planar architecture is achieved by selective arsenic ion implantation. The absorber layer characteristics for the samples studied here include a full width at half maximum of 100–120 arcsec from x-ray rocking curve, the electron concentration of 1−2 × 10¹⁵ cm⁻³ and mobility 3−5 × 10⁴ cm²/V-s, respectively at 80 K from Hall measurements. The minority carrier lifetime measured by photoconductive decay measurements at 80 K varied from 1 to 1.2 μsec. A modified general model for the variable area I–V analysis is presented. The dark current-voltage measurements were carried out at 80 K and an analysis of the dependence of zero-bias impedance on the perimeter/area ratio based on bulk, surface generation-recombination, and lateral currents are presented. The results indicate state-of-the art performance of the diodes in the midwavelength infrared region.     

Variable-Field Hall Measurement and Transport in LW Single-Layer n-Type MBE Hg1−x Cd x Te

Molecular beam epitaxy n-type long-wavelength infrared (LWIR) Hg_1?x Cd _x Te (MCT) has been investigated using variable-field Hall measurement in the temperature range from 50 K to 293 K. A quantitative mobility spectrum analysis technique has been used to determine the role of multicarrier transport properties with respect to epilayer growth on lattice-matched cadmium zinc telluride, as well as lattice-mismatched silicon (Si) and gallium arsenide (GaAs) buffered substrates. Overall, after postgrowth annealing, all layers were found to possess three distinct electron species, which were postulated to originate from the bulk, transitional (or higher-x-value) regions, and an interfacial/surface layer carrier. Further, the mobility and concentration with respect to temperature were analyzed for all carriers, showing the expected mobility temperature dependence and intrinsic behavior of the bulk electron. Electrons from transitional regions were seen to match expected values based on the carrier concentration of the resolved peak. At high temperature, the lowest-mobility carrier was consistent with the properties of a surface carrier, while below 125 K it was postulated that interfacial-region electrons may influence peak values. After corrections for x-value and doping density at 77 K, bulk electron mobility in excess of 10⁵ cm² V^?1 s^?1 was observed in all epilayers, in line with expected values for lightly doped n-type LWIR material. Results indicate that fundamental conduction properties of electrons in MCT layers are unchanged by choice of substrate.