Sachin Bet

Laser doping of chromium in 6H-SiC for white light emitting diodes

Laser doping has been utilized for fabricating white light emitting diodes with 6H-SiC wafers. The emission of different colors to ultimately generate white light is tailored on the basis of donor acceptor pair (DAP) recombination mechanism for luminescence. Chromium (Cr), which is an unconventional dopant that produces multiple acceptor sites per atom, was incorporated into SiC and conventional dopants such as aluminum (Al) and nitrogen (N) were also laser-doped to produce acceptor and donors states, respectively. A p−n junction was fabricated with these dopants and an electroluminescent broad spectrum (400–850 nm) corresponding to white light was observed. This white light is a result of the combination of red, green, and blue lights formed due to DAP recombination between Al–N (blue, 460–498 nm), Cr–N (green, 521–575 nm) and additional other impurity state transitions (red, 698–738 nm).

Laser forming of silicon films using nanoparticle precursor

Polycrystalline silicon films containing cubic silicon crystallites of size 3–4 µm have been formed on nickel substrates by fusing and sintering silicon nanoparticle precursors using a laser. A mechanism for the fusion and sintering of these nanoparticles, resulting in reduced heat input and continuous film formation by surface and grain boundary diffusion, is discussed. Films were characterized by optical microscopy, scanning electron microscopy, energydispersive spectroscopy, and Raman spectroscopy. Films were doped with n- as well as p-type dopants by using a laser doping technique and their current-voltage (I–V) characteristics were measured.

Laser-Patterned Blue-Green SiC LED

Laser direct writing and doping has already been successfully demonstrated for fabrication of PIN diodes, Ohmic and Schottky contacts and other optical structures in silicon carbide (SiC). Although SiC is an indirect bandgap semiconductor a SiC green light emitting diode (LED) has been recently shown to perform better than a GaN green LED. This observation is primarily due to 1) a simple device structure in SiC as opposed to the multilayered structure in GaN and other high bandgap semiconductor compounds and 2) high thermal conductivity. This initial phase focuses on the fabrication of reference SiC LEDs to baseline this novel laser direct writing and doping process. 6H:SiC (n-type) wafer samples were used for fabrication of LEDs. A Nd:YAG laser (1064nm wavelength) was used in continuous wave mode as well as pulsed mode for doping. Doping process iterations were carried out to obtain a uniform doped layer. Two different techniques were employed for patterning: 1) top doping where the incident laser beam assists in doping on the top surface and 2) bottom doping where the bottom surface is doped using the same top incident laser beam. Different structures were synthesized using these two doping techniques. P-type dopants such as boron (in powder form) and aluminum (metalorganic compound tri-methyl aluminum) and n-type dopant (pure nitrogen gas) were used. The doped structures were characterized for I-V characteristics, C-V characteristics and electroluminescence. Electroluminescence (EL) spectrum of the doped samples showed the output in the range of blue to blue green for different samples and the different doped structures. The feasibility of wavelength tuning using nanostructure patterning has also been explored.

Silicon carbide white light LEDs for solid-state lighting

White light emitting diodes (LEDs) have been successfully fabricated for the first time in silicon carbide substrates (4H-SiC) using a novel laser doping technique. The donor-acceptor pair (DAP) recombination mechanism for luminescence has been used to tailor these LEDs. Chromium (Cr), which produces multiple acceptor sites per atom, and selenium which produces multiple donor sites per atom were successfully incorporated into SiC for the first time using laser doping. Aluminum (Al) and nitrogen (N) were also laser-doped into SiC. Green (521-575 nm) and blue (460-498 nm) wavelengths were observed due to radiative recombination transitions between donor-acceptors pairs of N-Cr and N-Al respectively, while a prominent violet (408 nm) wavelength was observed due to transitions from the nitrogen level to the valence band level. The red (698-738 nm) luminescence was mainly due to nitrogen excitons and other defect levels. This RGB combination produced a broadband white light spectrum extending from 380 to 900 nm. The color space tri-stimulus values were X = 0.3322, Y = 0.3320 and Z = 0.3358 as per 1931 CIE (International Commission on Illumination) for 4H-SiC corresponding to a color rendering index of 96.56; the color temperature of 5510 K is very close to average daylight (5500 K).

Laser fabrication of silicon carbide light emitting diodes

With the shrinkage in the size of the semiconductor devices, greater shift has been observed towards use of lasers, electron and x-ray beams and advanced optics for small area device fabrication. A novel laser direct-write doping and metallization technique provides a single step approach for processing wide bandgap materials for electronic and optoelectronic device applications, which are difficult to dope using conventional techniques. Laser doping opens the opportunity to use unconventional dopants for customizing emission wavelength. It effectively reduces the number of fabrication steps and allows for selective area doping and direct metallization without metal deposition. To demonstrate this technology a pulsed Nd:YAG laser (1064nm wavelength) was used to fabricate blue light emitting laser diodes in a silicon carbide (SiC) 6H:SiC (n-type) wafer substrates. A p-n junction was created by laser doping aluminum (p-type) and nitrogen (n-type) on clean SiC wafer. Pure indium wire was used to obtain good ohmic contacts. These devices were characterized by capacitance-voltage (C-V), current-voltage (I-V), and electroluminescence (EL) measurements. A narrow electroluminescence (EL) peak at 481.82 nm and broad EL peak around 498.8 nm wavelengths were observed with two different detectors, characterizing the p-n junction as a blue light emitter.With the shrinkage in the size of the semiconductor devices, greater shift has been observed towards use of lasers, electron and x-ray beams and advanced optics for small area device fabrication. A novel laser direct-write doping and metallization technique provides a single step approach for processing wide bandgap materials for electronic and optoelectronic device applications, which are difficult to dope using conventional techniques. Laser doping opens the opportunity to use unconventional dopants for customizing emission wavelength. It effectively reduces the number of fabrication steps and allows for selective area doping and direct metallization without metal deposition. To demonstrate this technology a pulsed Nd:YAG laser (1064nm wavelength) was used to fabricate blue light emitting laser diodes in a silicon carbide (SiC) 6H:SiC (n-type) wafer substrates. A p-n junction was created by laser doping aluminum (p-type) and nitrogen (n-type) on clean SiC wafer. Pure indium wire was used to obtain good ...

CO2 Laser Drilling of Microvias Using Diffractive Optics Techniques: I — Mathematical Modeling

This paper is the first of three parts summarizing the research to use diffractive optics at the CO2 laser wavelength to drill microvias in the 40–50 μm range for organic packaging applications. This first part mainly focuses on mathematical modeling of the drilling process, which is used to define the characteristics of the laser beam necessary to achieve the required geometry of the microvias. These laser characteristics and the properties of the incoming laser beam of the CO2 laser system are then used to provide a deterministic approach for obtaining performance data for the diffractive optics design. The targeted optics are designed based on the modeling result and are then integrated into a prototype system to execute the drilling operation. The model is based on the conversion of optical energy into thermal energy due to laser-substrate interaction, propagation of thermal energy in the substrate, thermal as well as radiation damage threshold of the substrate and other important laser drilling parameters (e.g., fluence, temporal and spatial characteristics of the beam, residue at the via bottom).Copyright

Laser Doping of Chromium and Selenium in p-Type 4H-SiC

Chromium (Cr) and selenium (Se) are laser doped in silicon carbide (4H-SiC p-type aluminium) to fabricate an embedded light emitting device and to tune the light emission. A near infrared Nd:YAG (1064 nm wavelength) laser source and an organometallic Cr compound (bis (ethyl benzene)-chromium) and organometallic Se compound (dimethyl selenide) were used to laser dope SiC. A p-n junction device structure was created using these dopants. The dopant profiles have been characterized using secondary ion mass spectrometry. Electrical properties were measured using Hall effect measurement. Enhanced diffusivity and solubility with complete activation of dopants was observed for laser doped Cr and Se. Cr and Se are unconventional dopants, which serve as a double donor and a double acceptor respectively, while aluminium (Al) behaves as single acceptor and nitrogen (N) as a single donor in SiC. The defect levels (donor and acceptor) created within the forbidden band gap of SiC due to Se, Cr and Al onsets the donor acceptor pair (DAP) recombination mechanism for luminescence observed in SiC. Electroluminescence studies showed an orange (677 nm) corresponding to Cr-Al and, red (698 nm) and white (380-900 nm) for Se-Al and pure white for Cr-Se-Al. The Cr-Se-Al white light exhibited a correlated color temperature of 4935 K, which compares well to average daylight (5500 K).

Laser doping of germanium

A direct-write pulsed Nd:yttrium-aluminum-garnet laser treatment in an aluminum-containing gas was applied to the polished surface of an undoped Ge wafer. After KOH etching to remove metallic aluminum deposited on the surface, secondary ion mass spectroscopy (SIMS) revealed ~60-200 nm penetration for Al at a concentration of ~1017 cm-3. Atomic force microscopy showed that surface roughness is much less than the measured penetration depth. Laser doping of Ge is a potential low cost, selective-area, and compact method, compared with ion-implantation, for production of high current ohmic contacts in Ge and SiGe opto-electronic devices.

Laser thin film deposition on plastic substrates using silicon nanoparticles for flexible electronics

The melting temperature of silicon nanoparticles decreases significantly compared to the melting temperature of bulk silicon when the particle size is less than 5 nm. This concept is utilized for reducing the processing temperatures to deposit thin silicon films on plastic substrates. An aqueous dispersion of 5 nm silicon nanoparticles was used as precursor to produce polycrystalline silicon (c-Si) thin films on plastic substrates. A Nd:YAG (1064 nm wavelength) laser in continuous mode (CW) was used for deposition, annealing and recrystallization. Experiments were carried out in air as well as in argon atmosphere for obtaining a continuous recrystallized c-Si thin film on nickel substrates. Optimized parameters were utilized for obtaining c-Si films on plastic substrates. The substrate was not preheated and the entire film deposition process was conducted maintaining the substrate at room temperature while the film was heated with the laser beam. The process involved two steps: (i) a film forming step in which the aqueous medium was evaporated and the silicon particles were fused simultaneously to obtain a continuous film, and (ii) a recrystallization step in which the films were annealed and recrystallized by laser heating. The films were characterized by optical microscopy, Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy. Doping of these films with nitrogen and boron was also carried out successfully which was confirmed by the current voltage (I-V) measurements. The oxygen content of the laser air-treated film decreased with increasing both the laser power and the irradiation time. Laser argon-treated films showed comparatively lower amounts of oxygen. Raman spectroscopy showed a shift from amorphous to more crystalline phase with increasing the laser power and irradiation time. The increasing number of silicon crystallites observed using SEM confirmed this transformation.The melting temperature of silicon nanoparticles decreases significantly compared to the melting temperature of bulk silicon when the particle size is less than 5 nm. This concept is utilized for reducing the processing temperatures to deposit thin silicon films on plastic substrates. An aqueous dispersion of 5 nm silicon nanoparticles was used as precursor to produce polycrystalline silicon (c-Si) thin films on plastic substrates. A Nd:YAG (1064 nm wavelength) laser in continuous mode (CW) was used for deposition, annealing and recrystallization. Experiments were carried out in air as well as in argon atmosphere for obtaining a continuous recrystallized c-Si thin film on nickel substrates. Optimized parameters were utilized for obtaining c-Si films on plastic substrates. The substrate was not preheated and the entire film deposition process was conducted maintaining the substrate at room temperature while the film was heated with the laser beam. The process involved two steps: (i) a film forming step in ...