Dependence of the optoelectronic properties of selenium-hyperdoped silicon on the annealing temperature

Selenium-hyperdoped silicon was prepared by ion implantation at 100 eV to a dose of 6×10¹⁵ Se/cm², followed by furnace annealing at 500–900 °C for 30 min. A phase transition from amorphous to crystalline was observed for the sample annealed at 600 °C. Carrier density in the Se doping layer gradually increases with the annealing temperature and a high carrier/donor ratio of 7.5% was obtained at 900 °C. The effects of annealing temperature on the rectifying behavior and external quantum efficiency of n⁺p junctions formed on Se-hyperdoped silicon were also investigated. We found that 700 °C was the optimal annealing temperature for improving the crystallinity, below-bandgap absorption, junction rectification and external quantum efficiency of Se-doped samples.     

A Monté Carlo simulation study of high-dose and low-energy boron implantation into LPCVD-NiDoS polycrystalline thin films

This work presents a comprehensive study of boron profile implants predicted just after an implantation process and before any thermal anneal treatments generally used for boron redistribution. The study is carried out by using Low-Pressure Chemical Vapor Deposition-Nitrogen-Doped Silicon thin films (LPCVD-NiDoS). The implantation process has been experienced on films having 250 nm of thickness, as-implanted with boron ions at high dose (5×10¹⁵ cm⁻²) and low energy (10 keV). Prior to do these implantations, thin films were first grown under classical LPCVD technique by using two gas sources: Si₂H₆ and NH₃, but the value of the ratio NH₃/Si₂H₆ is deliberately chosen to obtain samples, in-situ nitrogen doped with different nitrogen concentrations. Taking into account the specific structure of these films, an attempt to built a numerically prediction model based on a binary collision concept in a multi-atomic target was performed.As previously found, the presence of nitrogen in the target is known to reduce boron penetration in the polycrystalline silicon films. This effect is significantly increased at high nitrogen concentrations. On the contrary, the characteristic fine-grained structure of these films, which corresponds to the presence of grains (G) and grain boundaries (GB), is known to enhance the opposite phenomenon. This last behavior is taken into consideration by assuming that the grain boundary plays an effective role in the scattering calculation process of the incident ions. In our analysis we postulate that: the incident ion, after interaction with GB, should only produce a change in direction and not a loss of its energy.     

Effects of defect,carrier concentration and annealing process on the photoluminescence of silicon pn diodes

Silicon pn diodes were fabricated by ion implantation of B and P ions with different doses and subsequent annealing processes. Room temperature photoluminescence (PL) were investigated and the factors affecting the PL intensity were analyzed. Results show that both kinds of pn diodes have PL peak centered at about 1140 nm. Dislocation loops resulted from ion implantation and annealing process may enhance the light emission of silicon pn diode due to its band quantum confinement effect to carriers. The luminescence intensity depends on the carrier concentrations in the implantation region. It should be controlled at the range of 1–6×10¹⁶ cm⁻³. Moreover, the PL intensities of pn diodes with furnace annealing (FA) are higher than those with rapid thermal annealing, and the annealing temperature range for FA is 900–1100 °C.     

Surface photovoltage measurements for Cu,Ti and W determination in Si wafers

p-type (1 0 0) Cz silicon wafers were contaminated with Cu, W, and Ti in the dose range of 5×10⁹–1×10¹⁴ cm⁻² by ion implantation. Surface photovoltage measurements were used to detect the metal impurities after annealing. Fast and slow cooling have been used and calibration curves have been obtained in all cases. Higher sensitivity has been determined for slow cooling (Cu and W) or fast cooling (Ti) depending if deep levels are associated with substitutional (Cu and W) or interstitial (Ti) position.     

Effects of RTA temperatures on conductivity and micro-structures of boron-doped silicon nanocrystals in Si-rich oxide thin films

In this work, the B-doped Si rich oxide (SRO) thin films were deposited and then annealed using rapid thermal annealing (RTA) to form SiO₂-matrix silicon nanocrystals (Si NCs). The effects of the RTA temperatures on the structural properties, conduction mechanisms and electrical properties of B-doped SRO thin films (BSF) were investigated systematically using Hall measurements, Fourier transform infrared spectroscopy and Raman spectroscopy. Results showed that the crystalline fraction of annealed BSF increased from 41.3% to 62.8%, the conductivity was increased from 4.48×10⁻³ S/cm to 0.16 s/cm, the carrier concentration was increased from 8.74×10¹⁷ cm⁻³ to 4.9×10¹⁸ cm⁻³ and the carrier mobility was increased from 0.032 cm² V⁻¹ s⁻¹ to 0.2 cm² V⁻¹ s⁻¹ when the RTA temperatures increased from 1050 °C to 1150 °C. In addition, the fluctuation induced tunneling (FIT) theory was applicable to the conduction mechanisms of SiO₂-matrix boron-doped Si-NC thin films.     

Photo-patternable polyimide gate insulator with fluorine groups for improving performance of 2,7-didecyl[1]benzothieno[3,2-b][1]benzothiopene (C10-BTBT) thin-film transistors

Surface properties of gate insulators strongly affect the device performance of organic thin-film transistors (OTFTs). To improve the performance of OTFTs, we have developed photo-sensitive polyimide gate insulator with fluorine groups. The polyimide gate insulator film could be easily patterned by selective UV exposure without any photoinitiator. The polyimide gate insulator film, fabricated at 130 °C, has a dielectric constant of 2.8 at 10 kHz, and leakage current density of <1.6 × 10^?10 A/cm² while biased from 0 to 90 V. To investigate the potential of the polyimide with fluorine groups as a gate insulator, we fabricated C₁₀-BTBT TFTs. The field-effect mobility and the on/off current ratio of the TFTs were measured to be 0.76 ± 0.09 cm²/V s and >10⁶, respectively.     

Metallic conduction behavior in selenium-hyperdoped silicon

Hall measurements at 80–300 K are performed on crystalline silicon doped with selenium exceeding the equilibrium solid solubility limit using ion implantation combined with furnace annealing. The temperature dependence of free carrier density and sheet conductivity of the Se doping layer changes with implantation dose. Metallic conduction behavior is well observed in the sample doped with selenium to be 7.4×10²⁰/cm³. The overlapping between Se impurity states below Si conduction band might give a microscopic explanation.     

Prediction of the morphology of the as-implanted damage in silicon using a novel combination of BCA and MD simulations

In order to predict type and amount of defects created by keV ions under realistic implantation conditions, a combination of computer simulations based on the binary collision approximation (BCA) with classical molecular dynamics (MD) calculations is proposed. Time-ordered BCA simulations are applied to ballistic processes with characteristic energies above several 10 eV. Athermal, rapid thermal, and thermally activated processes with lower characteristic energies are treated by MD simulations. They yield the as-implanted defect state formed several 10 ps after ion impact. The MD calculations are performed in cells which are much smaller than the entire volume of the collision cascade of an incident ion but much larger than the distance between nearest-neighbor atoms in the lattice. The as-implanted damage produced by a single ion in a certain cell is found to be completely determined by the nuclear energy deposition of the ion into the cell. Therefore, the MD calculations need to be performed only in one cell for different values of nuclear energy deposition, and statistical considerations based on BCA simulations can be employed to obtain the depth profile and the total number of different defect species (vacancies, interstitials, disordered atoms, etc.) created on average per incident ion. The novel simulation method is applied to investigate the damage morphology produced by 15 keV B⁺, 30 keV P⁺, and 15 keV As⁺ implants.     

Contact resistance between pentacene and indium–tin oxide (ITO) electrode with surface treatment

Contact resistance between indium–tin oxide (ITO) electrode and pentacene was studied by transmission line method (TLM). Organic solvent cleaned, inorganic alkali cleaned, and self-assembled monolayer (with OTS: octadecyltrichlorosilane) modified ITO electrode structures were compared. Pentacene layer of 300 Å thickness was vacuum deposited on patterned ITO layer at 70 °C with a deposition rate of 0.3 Å/s. Alkali cleaned and SAM modified ITO gave a lower contact resistance of about 6.34 × 10⁴ Ω cm² and 1.88 × 10³ Ω cm², respectively than organic solvent cleaned ITO of about 6.58 × 10⁵ Ω cm². Especially with the SAM treatment, the work function of ITO increased closer to the highest occupied molecular orbital (HOMO) level of pentacene, which lowers the injection barrier between ITO and pentacene. It was also believed that pentacene morphology was improved on SAM modified ITO surface due to the lowering of the surface energy. We could obtain the low contact resistance with SAM treatment which is comparable to the measured value of gold–pentacene contact, 1.86 × 10³ Ω cm². This specific contact resistance is still much higher than that of amorphous silicon thin film transistor (0.1–30 Ω cm²).     

Properties of fluorine-doped SnO2 thin films by a green sol–gel method

Fluorine doped tin oxide (FTO) films were fabricated on a glass substrate by a green sol–gel dip-coating process. Non-toxic SnF₂ was used as fluorine source to replace toxic HF or NH₄F. Effect of SnF₂ content, 0–10 mol%, on structure, electrical resistivity, and optical transmittance of the films were investigated using X-ray diffraction, Hall effect measurements, and UV–vis spectra. Structural analysis revealed that the films are polycrystalline with a tetragonal crystal structure. Grain size varies from 43 to 21 nm with increasing fluorine concentration, which in fact critically impacts resultant electrical and optical properties. The 500 °C-annealed FTO film containing 6 mol% SnF₂ shows the lowest electrical resistivity 7.0×10⁻⁴ Ω cm, carrier concentration 1.1×10²¹ cm⁻³, Hall mobility 8.1 cm²V⁻¹ s⁻¹, optical transmittance 90.1% and optical band-gap 3.91 eV. The 6 mol% SnF₂ added film has the highest figure of merit 2.43×10⁻² Ω⁻¹ which is four times higher than that of un-doped FTO films. Because of the promising electrical and optical properties, F-doped thin films prepared by this green process are well-suited for use in all aspects of transparent conducting oxide.     

Characterization of p-type multicrystalline silicon prepared by cold crucible continuous melting and directional solidification

A solar-grade boron doped silicon ingot with the cross section of 62 mm× 62 mm was cast by cold crucible continuous melting and directional solidification (CCDS). The characterization of this p-type multicrystalline silicon (mc-Si) was measured and evaluated. The results indicate that the ingot mainly consists of uniform columnar grains preferentially aligned parallel to the ingot growth direction. The average density of dislocations (N_dis) in the center area varies from 4 × 10⁴ cm⁻² to 4 × 10⁵ cm⁻², and it is much lower than that in the peripheral area. Comparing with the raw material, the oxygen concentration in the cast ingot is much lower while the carbon holds the same level. The electrical resistivity distributes uniformly and its average value is same as that of the raw material. The minority carrier lifetime (MCLT) is higher than that of the raw material and no region with obvious low MCLT is observed. CCDS has shown to be a potential process to produce mc-Si for solar cells with no crucible contamination and consumption, high production efficiency and uniform quality.     

2 MeV ion irradiation effects on AlGaN/GaN HFET devices

AlGaN/GaN heterostructure field effect transistors (HFETs) were irradiated with 2 MeV protons, carbon, oxygen, iron and krypton ions with fluences ranging from 1 × 10⁹ cm^?2 to 1 × 10¹³ cm^?2. DC, pulsed I–V characteristics, loadpull and S-parameters of the AlGaN HFET devices were measured before and after irradiation. In parallel, a thick GaN reference layer was also irradiated with the same ions and was characterized by X-ray diffraction, photoluminescence, Hall measurements before and after irradiation. Small changes in the device performance were observed after irradiation with carbon and oxygen at a fluence of 5 × 10¹⁰ cm^?2. Remarkable changes in device characteristics were seen at a fluence of 1 × 10¹² cm^?2 for carbon, oxygen, iron and krypton irradiation. Similarly, remarkable changes were also observed in the GaN layer for irradiations with fluence of 1 × 10¹² cm^?2. The results found on devices and on the GaN layer were compared and correlated.     

Secondary defect profile related to low energy implanted boron measured up to 3.5 µm depth into Si-substrates

Low energy implantation is one of the most promising options for ultra shallow junction formation in the next generation of silicon BiCMOS technology. Among the dopants that have to be implanted, boron is the most problematic because of its low stopping power (large penetration depth) and its tendency to undergo transient enhanced diffusion and clustering during thermal activation. This paper reports an experimental study of secondary defect profiles of low energy B implants in crystalline silicon. Shallow p⁺n junctions were formed by low energy B implantation—10¹⁵ cm⁻² at 3 keV—into a reference n-type crystalline silicon or pre-amorphized n-Si with germanium −10¹⁵ cm⁻² at 30 keV, 60 keV, and 150 keV. Rapid Thermal Annealing (RTA) for 15 s at 950°C was then performed. Secondary defect profiles induced by this process are measured with isothermal transient capacitance in association with Deep Level Transient Spectroscopy (DLTS). Relatively high concentrations of electrically active defects have been obtained up to 3.5 μm into the crystalline silicon bulk. The relation of these defects with boron is discussed. The results of this study are in agreement with boron transient enhanced diffusion in Si-substrate as has been reported by Collart using Secondary Ion Mass Spectrometry (SIMS) measurements.     

Influence of working pressure on the structural,optical and electrical properties of sputter deposited AZO thin films

Highly oriented crystalline aluminum doped zinc oxide (AZO) films were sputter deposited on glass substrates and a systematic investigation on the as deposited and etched films was reported for its further application in silicon thin film solar cell. Influence of the deposition pressure (from 2 to 8 mTorr) and post-annealing temperature (at 400 °C for 5 min) on the structural, optical and electrical properties of the as-deposited and etched samples were analyzed. The optimum condition for its reproducibility and large area deposition is determined and found that the depositions made at 8 mTorr at 200 W having the distance from source to substrate of 9 cm. All the AZO films exhibited a c-axis preferred orientation perpendicular to the substrate and their crystallinity was improved after annealing. From the XRD pattern the grain size, stress and strain of the films were evaluated and there is no drastic variation. Optical transmittance, resistivity, Hall mobility and carrier concentration for the as deposited and etched-annealed films were found to improve from 79 to 82%; 2.97 to 3.14×10⁻⁴ Ω cm; 25 to 38 cm²/V s; 8.39 to 5.96×10²⁰/cm³ respectively. Based on the triangle diagram between figure of merit and Hall mobility, we obtained a balance of point between the electrical and optical properties to select the deposition condition of film for device application.     

Low temperature Solution-Processed ZnO film on flexible substrate

Transparent conductive ZnO films were directly deposited on unseeded polyethersulfone (PES) substrates with a spin-spray method using aqueous solution at a low substrate temperature of 85 °C. All ZnO films were crystalline with wurtzite hexagonal structure and impurity phases were not detected. ZnO films deposited without citrate ions in the reaction solution had a rod array structure. In contrast, ZnO films deposited with citrate ions in the reaction solution had a continuous, dense structure. The transmittance of the ZnO films was improved from 11.9% to 85.3% as their structure changed from rod-like to continuous. After UV irradiation, the ZnO films with a continuous, dense structure had a low resistivity of 9.1×10⁻³ Ω cm, high carrier concentration of 2.7×10²⁰ cm⁻³ and mobility of 2.5 cm² V⁻¹ s⁻¹.     

Voltage-tunable SiO2-isolated a-SiC:H and/or a-SiN:H p–i–n thin-film LEDs fabricated on c-Si

A voltage-tunable amorphous p–i–n thin-film light emitting diodes (TFLEDs) with SiO₂-isolation on n⁺-type crystalline silicon (c-Si) has been proposed and fabricated successfully. The structure of the device with i-a-SiC:H and i-a-SiN:H luminescent layers is indium–tin–oxide (ITO)/p⁺-a-Si:H/p⁺-a-SiC:H/i-a-SiC:H/i-a-SiN:H/n⁺-a-SiCGe: H/n⁺-a-SiC:H/n⁺-c-Si/Al. This device revealed a brightness of 695 cd/m² at an injection current density of 300 mA/cm². Its EL (electroluminescence) peak wavelength exhibited blue-shift from 655 to 565 nm with applied forward-bias (V) increasing from 15 to 19 V, but the EL peak wavelength was red-shifted from 565 to 670 nm with further increase of V from 19 to 23 V. By comparing with the EL spectra from p–i–n TFLEDs with i-a-SiC:H or i-a-SiN:H luminescent layer only, the EL spectrum of this TFLED could consist of three bands of radiations from the tail-to-tail-state recombinations in (1) i-a-SiC:H layer, (2) i-a-SiN:H layer, and (3) i-a-SiC:H/p⁺-a-SiC:H junction.     

Contact resistivity of amorphous and crystalline GeCu2Te3 to W electrode for phase change random access memory

We have investigated the contact resistivity of GeCu₂Te₃ (GCT) phase change material to a W electrode using the circular transfer length method (CTLM). The contact resistivity ρ_c of as-deposited amorphous GCT to W was 3.9×10⁻² Ω cm². The value of ρ_c drastically decreased upon crystallization and crystalline GCT that annealed at 300 °C showed a ρ_c of 4.8×10⁻⁶ Ω cm². The ρ_c contrast between amorphous (as-deposited) and crystalline (annealed at 300 °C) states was larger in GCT than in conventional Ge₂Sb₂Te₅ (GST). Consequently, it was suggested from a calculation based on a simple vertical structure memory cell model that a GCT memory cell shows a four times larger resistance contrast than a GST memory cell.     

Region-dependent behavior of I–V characteristics in n-ZnO:Al/p-Si contacts

The discrepancy of rectifying characteristics in n-ZnO:Al/p-Si heterojunctions from diode to diode was demonstrated by region dependent dark I–V characteristics, where the junction is laterally cut to sequentially decrease the area. Further investigation shows that the junction (2.1×2.1 cm²) with the barrier height Φ=0.693 eV consists of one part (2.1×1.4 cm²) with Φ=0.695 eV and the other part (2.1×0.7 cm²) with Φ=0.686 eV. It is found that reverse currents saturate with different values of 3.6×10^?3, 2.5×10^?3 and 1.58×10^?3 A for the light I–V curves of the three junctions with the same areas. To explain this peculiarity, the probable reason is discussed in terms of carrier transportation through the spatially fluctuating barrier.     

Interaction of ion-implantation-induced interstitials in B-doped SiGe

B-doped Si_0.77Ge_0.23 of various surface-doping levels was used to investigate the evolution of implant damage and the corresponding transient enhanced diffusion of boron as a function of boron concentration. These layers were implanted with a non-amorphizing 60 keV, 1×10¹⁴ cm⁻² Si, and annealed at 750 °C. Plan-view transmission electron microscopy (PTEM) confirmed the formation and dissolution of dislocation loops. Transient enhanced diffusion (TED) is evident in the surface doped SiGe, but the low diffusivity of interstitials in Si_0.77Ge_0.23 and the presence of interstitial traps inhibited TED at the deeper B marker layer.     

Structural,optical and dielectric properties of HfSiO films prepared by co-evaporation method

HfSiO dielectric films were prepared on Si substrate by the co-evaporation method. The chemical composition, crystalline temperature, optical and electrical properties of the compound film were investigated. X-ray photoelectron spectroscopy analysis illustrated that the atom ratio of Hf to Si was about 4:1 and Hf–Si–O bonds appeared in the film. The X-ray diffraction analysis revealed that the crystalline temperature of the film was higher than 850 °C. Optical measurements showed that the refractive index was 1.82 at 550 nm wavelengths and the optical band gap was about 5.88 eV. Electrical measurements demonstrated that the dielectric constant and a fixed charge density were 18.1 and 1.95×10¹² cm⁻² respectively. In addition, an improved leakage current of 7.81 μA/cm² at the gate bias of −3 V was achieved for the annealed HfSiO film.