Heavily-doped Silicon Research Articles

Intense pulsed light annealing of solution-based indium–gallium–zinc–oxide semiconductors with printed Ag source and drain electrodes for bottom gate thin film transistors

In this study, an intense pulsed light (IPL) annealing process for a printed multi-layered indium–gallium–zinc–oxide (IGZO) and silver (Ag) electrode structure was developed for a high performance all-printed inorganic thin film transistor (TFT). Through a solution process using IGZO precursor and Ag ink, the bottom gate structure TFT was fabricated. The spin coating method was used to form the IGZO semiconductor layer on a heavily-doped silicon wafer covered with thermally grown silicon dioxide. The annealing process of the IGZO layer utilized an optimized IPL irradiation process. The Ag inks were printed on the IGZO layer by screen printing to form the source and drain (S/D) pattern. This S/D pattern was dried by near infrared radiation (NIR) and the dried S/D pattern was sintered with intense pulsed light by varying the irradiation energy. The performances of the all-printed TFT such as the field effect mobility and on–off ratio electrical transfer properties were measured by a parameter analyzer. The interfacial analysis including the contact resistance and cross-sectional microstructure analysis is essential because diffusion phenomenon can occur during the annealing and sintering process. Consequently, this TFT device showed noteworthy performance (field effect mobility: 7.96 cm2/V s, on/off ratio: 107). This is similar performance compared to a conventional TFT, which is expected to open a new path in the printed metal oxide-based TFT field.

TEM-compatible microdevice for the complete thermoelectric characterization of epitaxially integrated Si-based nanowires.

Nanostructured materials present improved thermoelectric properties due to non-trivial effects at the nanoscale. However, the characterization of individual nanostructures, especially from the thermal point of view, is still an unsolved topic. This work presents the complete structural, morphological, and thermoelectrical evaluation of the selfsame individual bottom-up integrated nanowire employing an innovative micro-machined device compatible with transmission electron microscopy whose fabrication is also discussed. Thanks to a design that arranges the nanostructured samples completely suspended, detailed structural analysis using transmission electron microscopy is enabled. In the same device architecture, electrical collectors and isolated heaters are available at both ends of the trenches for thermoelectrical measurements of the nanowire i.e. thermal and electrical properties simultaneously. This allows the direct measurement of the nanowire power factor. Furthermore, micro-Raman thermometry measurements were performed to evaluate the thermal conductivity of the same suspended silicon nanowire. A thermal profile of the self-heating nanowire could be spatially resolved and used to compute the thermal conductivity. In this work, heavily-doped silicon nanowires were grown on this microdevices yielding a thermal conductivity of 30.8 ± 1.7 W Km-1 and a power factor of 2.8 mW mK-2 at an average nanowire temperature of 400 K. Notably, no thermal contact resistance was observed between the nanowire and the bulk, confirming the epitaxial attachment. The device presented here shows remarkable utility in the challenging thermoelectrical characterization of integrated nanostructures and in the development of multiple devices such as thermoelectric generators.

Compact Micro Thermal Sensor Based on Silicon Thermocouple Junction and Suspended Fluidic Channel

This research proposed a micro biochemical thermal sensor with compact integration capability by combining a novel single thermocouple junction and suspended microfluidic channel. While the thermal sensor was fabricated through standard MEMS processes on the silicon-on-insulator (SOI) wafer, a heavily-doped silicon of high Seebeck coefficient was employed as the highly sensitive material for thermocouple junction. To enable a high sensitivity and fast response detection, a unique structure of air-suspended bridge and a built-on SU-8 polymer microfluidic channel was fabricated to reduce heat loss and sample loading dose for trace amount of heat detection. As high as a sensitivity of 0.27 V/W and rapid thermal response less than 200 ms are achieved in this thermal sensor, which allows for the enzymatic bio-reaction heat detection and laid great potential for portable healthcare applications.

Heavily-Doped Bulk Silicon Sidewall Electrodes Embedded between Free-Hanging Microfluidic Channels by Modified Surface Channel Technology.

Surface Channel Technology is known as the fabrication platform to make free-hanging microchannels for various microfluidic sensors and actuators. In this technology, thin film metal electrodes, such as platinum or gold, are often used for electrical sensing and actuation purposes. As a result that they are located at the top surface of the microfluidic channels, only topside sensing and actuation is possible. Moreover, in microreactor applications, high temperature degradation of thin film metal layers limits their performance as robust microheaters. In this paper, we report on an innovative idea to make microfluidic devices with integrated silicon sidewall electrodes, and we demonstrate their use as microheaters. This is achieved by modifying the original Surface Channel Technology with optimized mask designs. The modified technology allows to embed heavily-doped bulk silicon electrodes in between the sidewalls of two adjacent free-hanging microfluidic channels. The bulk silicon electrodes have the same electrical properties as the extrinsic silicon substrate. Their cross-sectional geometry and overall dimensions can be designed by optimizing the mask design, hence the resulting resistance of each silicon electrode can be customized. Furthermore, each silicon electrode can be electrically insulated from the silicon substrate. They can be designed with large cross-sectional areas and allow for high power dissipation when used as microheater. A demonstrator device is presented which reached at a power of , limited by thermal conduction through the surrounding air. Other potential applications are sensors using the silicon sidewall electrodes as resistive or capacitive readout.

Fabrication of silicon electrodes used for micro electrochemical machining

In micro electrochemical machining (ECM), stray corrosion always causes undesired material dissolution and deteriorates the machining localization. Adopting sidewall insulating films on the electrode is proven to be an effective approach for reducing stray corrosion. Sidewall films with characteristics of good insulation, thin thickness, and excellent durability are required. In previous work, we proposed a heavily-doped silicon electrode, on which silicon-based films were prepared and acted as the insulating films. To fabricate silicon electrodes, this research investigates wet etching behavior of heavily doped Si (100) and then presents a fabrication process. Firstly, shapes of micro electrodes are designed on photomasks of lithography. Aiming at patterned profiles, effects of etchants on etch rate, dimension accuracy and surface integrity are investigated. Experimental results indicate that tetramethylammonium hydroxide (TMAH) leads to smoother electrode surfaces and fewer defects compared with potassium hydroxide. Then, insulation and durability performances of silicon-based films are investigated. 500 nm SiO2 and 300 nm Si3N4 composite film achieves more durable insulation and excellent compactness. Consequently, silicon electrodes with the dimension of 86 × 54 μm are fabricated on the Si (100) wafer with 25 wt% TMAH etchant, on which SiO2 and Si3N4 film is deposited by low-pressure chemical vapor deposition. In ECM experiments, stray corrosion of the non-machined surface is significantly reduced, and the sidewall taper of the machined structure is reduced to 5.5°. The validity and durability of silicon electrodes for micro ECM processes are verified.

Contactless, nondestructive determination of dopant profiles of localized boron-diffused regions in silicon wafers at room temperature

We develop a photoluminescence-based technique to determine dopant profiles of localized boron-diffused regions in silicon wafers and solar cell precursors employing two excitation wavelengths. The technique utilizes a strong dependence of room-temperature photoluminescence spectra on dopant profiles of diffused layers, courtesy of bandgap narrowing effects in heavily-doped silicon, and different penetration depths of the two excitation wavelengths in silicon. It is fast, contactless, and nondestructive. The measurements are performed at room temperature with micron-scale spatial resolution. We apply the technique to reconstruct dopant profiles of a large-area (1 cm × 1 cm) boron-diffused sample and heavily-doped regions (30 μm in diameter) of passivated-emitter rear localized-diffused solar cell precursors. The reconstructed profiles are confirmed with the well-established electrochemical capacitance voltage technique. The developed technique could be useful for determining boron dopant profiles in small doped features employed in both photovoltaic and microelectronic applications.

A Study on the Hopping Nature of an Excess Tunnel Current in Heavily Doped Silicon p − n Junction Diodes at Cryogenic Temperatures

Purpose of the work is to study a nature of the excess tunnel current in heavily doped silicon p − n junction diodes with lengthy compensation region in the p − n junction. In such the diodes, formation of the system of electron and hole “lakes” and hopping conduction via them is possible at low temperatures. And indeed, electric measurements have shown that this excess tunnel current is characterized by Mott’s temperature dependency. It is reasonable to investigate additionally an influence of magnetic field. Previous studies of the magnetic field influence were limited by the field of 9.4 T and alone temperature of 4.2 K. Now the measurements have been carried out at several temperatures close by the liquid helium temperature and at the magnetic fields up to 13.6 T. It was found that in magnetic fields beyond 9.4 T, the diode magnetoresistance demonstrates transition to field dependency predicted theoretically for variable range hopping conduction of Mott’s type via impurity centers. Thus the excess tunnel current in the diode has character of hopping conduction via impurity centers, irrespective of whether the system of electron and hole “lakes” is generated in compensation region of the p − n junction or not.

Diamond film, single-layer carbon nanosheet film and diamond/carbon nanosheet composite film synthesis and field emission performance comparison

A series of diamond films, single-layer carbon nanosheet (SCNS) films and diamond/carbon nanosheet (D/CNS) composite films have been prepared on the heavily-doped n-type silicon substrate by using microwave plasma chemical vapor deposition (MPCVD) techniques and vacuum electron beam evaporation techniques. The films were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction(XRD), Raman spectroscopy. Field emission test results showed that diamond film, SCNS film and D/CNS composite film respective field emission current density is 0.004 mA/cm2 (there's almost no electron emission), 0.45 mA/cm2 and 3.21 mA/cm2 at an electric field of 7.2V/μm. This means that the field emission maximum current density of D/CNS composite films is 7.1 times that of SCNS films at an electric field of 7.2 V/μm. At the same time the D/CNS composite film exhibits the advantage of improved reproducibility and long term stability. Diamond layer can effectively improve the field emission characteristics of CNS film. The reason may be due to the diamond film acts as the electron acceleration layer, meanwhile, the heterogeneity of diamond and CNS material could lead to lowering of the interfacial potential barrier.

0.1-20 THz ultra-broadband perfect absorber via a flat multi-layer structure.

An ultra-broadband perfect absorber based on graded-index mechanism is designed and fabricated. The perfect absorber is comprised of a heavily-doped silicon absorption substrate and a flat six-layer antireflective structure. The refractive index of each layer was widely tuned by hollow polystyrene microsphere and TiO2 nanoparticle dopants, which can offer a gradually changed refractive index profile from 1.3 to 2.9. The experimental results show that 98% absorption can be achieved within the range of 0.1-20 THz. Moreover, the high absorption efficiency as well as the ultra-broad range can maintain for incident angle from 0 to 75° by the theoretical simulation.

On the composition of luminescence spectra from heavily doped p-type silicon under low and high excitation

We present a systematic investigation of the effects of doping on the luminescence spectra from p-type crystalline silicon wafers. First, we explain the difference in the approaches between the line shape analysis and the generalized Planck’s law in modeling the spectral shape, and connect the two methods together. After that, we elucidate the separate impacts of individual phenomena including band gap narrowing, band tails and dopant bands, Fermi level shifting, and hot carriers on the luminescence spectra in heavily-doped silicon. Finally, employing appropriate excitation levels, we can unambiguously re-examine the growing of the band tails and the broadening of the shallow dopant band, as well as the merging of the valence and shallow dopant bands together, on measured luminescence spectra.

Characterizing amorphous silicon, silicon nitride, and diffused layers in crystalline siliconsolarcellsusingmicro-photoluminescence spectroscopy

We report and explain the photoluminescence (PL) spectra from crystalline silicon (c-Si) wafers passivated by hydrogenated amorphous silicon (a-Si:H) films under various measurement conditions, utilizing the different absorption coefficients and radiative recombination mechanisms in c-Si and a-Si:H. By comparison with the luminescence properties of a-Si:H, we also demonstrate that SiNx films deposited under certain silicon-rich conditions yield luminescence spectra similar to those of a-Si:H, indicating the presence of an a-Si:H-like phase in the SiNx films. This causes a reduction in the blue response of the solar cells via parasitic absorption. In addition, with the ability to detect the specific emission from heavily-doped silicon via band-gap narrowing effects, we can unambiguously separate individual spectral PL signatures of three different layers in a single substrate: the SiNx passivation films, the diffused layers, and the underlying c-Si substrate. Finally, we apply this technique to evaluate parasitic absorption in the passivation films, and the doping density of the diffused layers on different finished solar cells, highlighting the value of this nondestructive contactless, micron-scale technique for photovoltaic applications.

On mobility of electrons in a shallow Fermi sea over a rough seafloor

Several doped semiconductors, in contrast to heavily-doped silicon and germanium, host extremely mobile carriers, which give rise to quantum oscillations detectable in relatively low magnetic fields. The small Fermi energy in these dilute metals quantifies the depth of the Fermi sea. When the carrier density exceeds a threshold, accessible thanks to the long Bohr radius of the parent insulator, the local seafloor is carved by distant dopants. In such conditions, with a random distribution of dopants, the probability of finding an island or a trench depends on on the effective Bohr radius, and the carrier density, n. This picture yields an expression for electron mobility with a random distribution of dopants: , in reasonable agreement with the magnitude and concentration dependence of the low-temperature mobility in three dilute metals whose insulating parents are a wide-gap (SrTiO3), a narrow-gap (PbTe) and a ‘zero’-gap (TlBiSSe) semiconductor.

Solution-processed hafnium oxide dielectric thin films for thin-film transistors applications

Hafnium oxide (HfOx) dielectric thin films were fabricated on heavily-doped silicon (100) substrates by a solution process. The precursor solution was prepared by dissolving HfCl4 in ethanol. The annealing effects on the microstructural and electrical properties of HfOx thin films were studied. The HfOx thin film annealed at 500°C exhibited the best insulating performance with a current density of 1×10−9A/cm2 at an electric field of 4.5MV/cm. In order to demonstrate the possible application of HfOx thin films in thin-film transistors (TFTs), the indium-zinc oxide (IZO) channel layer was fabricated by magnetron sputtering at room temperature. The IZO TFT based on 500°C-annealed HfOx can be operated under an operation voltage of 5V, with a high field-effect mobility of 36.9cm2/Vs, a threshold voltage of 1.8V, a subthreshold swing of 0.38V/dec, and an on/off current ratio of 109. The results demonstrate that HfOx thin film prepared by the solution process is a promising gate dielectric for high-performance oxide electronic devices.

A Processing Window for Fabricating Heavily Doped Silicon Nanowires by Metal-Assisted Chemical Etching

Heavily doped silicon nanowires (SiNWs) with controllable doping concentrations can be used in many fields because of their unique characteristics. However, it is difficult to fabricate long heavil...

Temperature-dependent 29Si NMR resonance shifts in lightly- and heavily-doped Si:P

Careful NMR measurements on a very lightly-doped reference silicon sample provide a convenient highly precise and accurate secondary chemical shift reference standard for 29Si MAS-NMR applicable over a wide temperature range. The linear temperature-dependence of the 29Si chemical shift measured in this sample is used to refine an earlier presentation of the paramagnetic (high-frequency) 29Si resonance shifts in heavily-doped n-type silicon samples near the metal–nonmetal transition. The data show systematic decreases of the local magnetic fields with increasing temperature in the range 100–470K for all samples in the carrier concentration range from 2×1018cm−3 to 8×1019cm−3. This trend is qualitatively similar to that previously observed for the two-orders of magnitude larger 31P impurity NMR resonance shifts in the same temperature and concentration ranges. The 29Si and 31P resonance shifts are not related by a simple scaling factor, however, indicating that impurity and host nuclei are affected by different subsets of partially-localized extrinsic electrons at all temperatures.

Strong absorption and selective emission from engineered metals with dielectric coatings

We demonstrate strong-to-perfect absorption across a wide range of mid-infrared wavelengths (5-12µm) using a two-layer system consisting of heavily-doped silicon and a thin high-index germanium dielectric layer. We demonstrate spectral control of the absorption resonance by varying the thickness of the dielectric layer. The absorption resonance is shown to be largely polarization-independent and angle-invariant. Upon heating, we observe selective thermal emission from our materials. Experimental data is compared to an analytical model of our structures with strong agreement.

Gas Phase Phosphorus Heavily-Doped FZ Silicon Thermal Field Design and Growth Method

This paper introduces the fabrication method of a kind of gas phase phosphorus heavily-doped float zone (FZ) silicon, including thermal field design (electromagnetic copper coil with double water cooling system). This method solves the problems during the pulling process of heavily-doped FZ silicon crystal of phosphorus doped. The gas phase phosphorus heavily-doped FZ silicon crystal using this methods with low oxygen content (less than 0.2ppma),low radial resistivity variation (less than 10%), low resistivity (the minimum of 0.002 ohm.cm), and is good to meet the transient voltage suppressor (TVS) for silicon substrate material requirements.

Correlating Microscale Thermal Conductivity of Heavily-Doped Silicon With Simultaneous Measurements of Stress

The functioning and performance of today’s integrated circuits and sensors are highly affected by the thermal properties of microscale silicon structures. Due to the well known size effect, the thermal properties of microscale silicon structures are not the same as those of the bulk silicon. Furthermore, stress/strain inside microscale silicon structures can significantly affect their thermal properties. This article presents the first thermal conductivity measurements of a microscale silicon structure under applied compressive stress at 350 K. Atomic force microscope (AFM) cantilevers made of doped single-crystal silicon were used as samples. A resistance temperature detector (RTD) heater attached to another RTD sensor was used as the heating module, which was mounted onto a nanoindentation test platform. This integrated system applied compressive load to the cantilever in the longitudinal direction while supplying heat. The thermal conductivity of the cantilevers was calculated using steady state heat conduction equation. The result shows that the measured thermal conductivity varies from 110 W/(m·K) to 140 W/(m·K) as compressive strain varies from 0.1% to 0.3%. Thermal conductivity is shown to increase with increase in compressive strain. These results match with the published simulation values. The measured thermal conductivity and stress values vary in the similar manner as a function of applied strain.

Infrared plasmons on heavily-doped silicon

We examine the long-wave infrared (LWIR) optical characteristics of heavily-doped silicon and explore engineering of surface plasmons polaritons (SPP) in this spectral region. Both phosphorus (n-type Si) and boron (p-type Si) implants are evaluated and various cap layers and thermal annealing steps are examined. The optical properties are measured using ellipsometry and fit to a Drude model for the infrared (IR) permittivity. The predicted metallic behavior for Si in the thermal IR and its impact on the spatial confinement and dispersion for surface plasmons is studied. We find that the transverse spatial confinement for a surface plasmon on highly doped Si is strongly sub-wavelength near the plasma edge, and the confinement to the surface is enhanced to greater than 10× that of the metal confined SPP over the entire LWIR spectrum.

Analysis for piezoresistive property of heavily-doped polysilicon with upper and lower bounds

This paper proposed an analysis for the piezoresistive property of heavily-doped polycrystalline silicon (polysilicon) with upper and lower bounds derived using the fundamental piezoresistive coefficients of single-crystalline silicon (SCS). The analysis is applicable to the polysilicon that the piezoresistive effect in grain boundaries is negligible and the orientations of the SCS grains are completely random. For deriving the upper and lower bounds, the analysis employed two approximation models on the basis of two extreme assumptions of uniform stress and fractional change in resistivity in each SCS grain of polysilicon. The fundamental piezoresistive coefficients of polysilicon derived from these models were successfully defined as the upper and lower bounds, based on the elastic theories in the micromechanics filed. The applicability of the analysis was discussed by comparing the derived upper and lower bounds with the piezoresistive property of heavily-doped polysilicon measured in some previous works. The measured piezoresistive property of both p-type and n-type polysilicon lay within the theoretical possible range between the upper and lower bounds at the impurity concentration of 1019 cm−3 levels, which suggested the applicability of the proposed analysis for the piezoresistive property of heavily-doped polysilicon.