High Performance Wide Spectrum Calibrators Based on Silicon Carbide

Solid-state wide spectrum avalanche (pre-breakdown) Light Emitting Diodes (LEDs) can replace filament and discharge lamps from a great number of commercial applications in the fields of space exploration, military, geophysics, meteorology, and astronomy. The potential applications include universal precise high stability optical sources for laboratory, field, and on-orbit calibration of various sensors and sensing systems, spectrophotometers, solar and sky-luminary radiation simulation, and point source imitators [1-3].

The spectra of injective type LEDs are greatly  dependent on the applied voltage and temperature, and the lifetime of the p-n junctions is relatively short.

Investigation of Silicon Carbide (SiC) Schottky barriers with different metals represents a great number of materials for reproducible device fabrication. The solar spectrum and the spectrum from reverse biased (pre-breakdown or avalanche type) structures are quite similar.

The objective of this project was to develop and fabricate stable, high performance, SiC-based, avalanche wide-spectrum calibrators for laboratory and optical sensing instrumentation systems working in the solar radiation-related spectral range. The main tasks that have been completed in this project are:

  • Light emitting structure and housing design
  • Optimization of single crystalline SiC wafers etching for Schottky barrier formation
  • Development of the process for Ohmic contact formation
  • Optimization of the conditions for aluminum layer deposition in order to form bulk  Schottky  barrier contacts to SiC
  • Optimization of the process for rectifying (light emitting) contact formation in ultra-high vacuum conditions
  • Development of the processes for assembly and protection of the barrier from the media
  • Characterization of the emission area, I-V, and emission power output
  • Characterization of the emission spectrum in the visible region
  • Characterization of the thermal dependence of the emission power output in the range from +60 to -60 o
  • Preliminary lifetime and drift measurements.

A typical light emission spectrum of the developed LED is shown in the picture below:

Light emission

The following achievements have resulted from this project:

  • The dependence of the emission power output in hA of the sensor photocurrent is very close to linear. The absolute emission power achieved from 0.4 diameter emission area at 1 A through the LED structure is about 3 m Higher emission power output amplitudes can be achieved at higher LED’s currents in a pulsed mode.
  • The shape of the emission spectrum slightly changes with the change of applied current only near the maximum due to the thermal dependence of the SiC bandgap.
  • The tilt of the curve in the region from 640 nm to 800 nm shows that extension of the spectrum further into IR region is evident. The small “noise” in the region from 320 to 400 nm (spectrum in the Appendix 1) is an extension of the spectrum into the near UV region absorbed by the wafer on the energy level corresponding to the energy of 6H-SiC bandgap (3.2 eV).
  • The thermal stability measurements show that the emission thermal coefficient of the LED in the temperature range of -60 to +60 oC is less than 0.2 %/deg and can be reduced down to 0.1/deg. in the same temperature range for LEDs working in the pulsed mode at higher current densities.

The temporal instability of the best samples is in the range of 3-5 % for more than 900 hours of continuous operation after at least 300 hours of preliminary electrical annealing.

Integrated Broad Band Optical Calibration Sources Based on Wide Bandgap Semiconductors for Star Simulation

The objective of this project is the development, fabrication, and testing of miniature high-stability integrated super broadband optical emission sources for in-flight calibration of stellar photometers and spectrometers in vacuum and cryogenic environments. The capability of simulating any source planned to be demonstrated by initially building a class C source solar spectrum with the aim of achieving a class A type source, as defined by ASTM standards.

The high stability and high performance of the proposed devices meant to be provided by employment of the avalanche electroluminescence process and implementation of improved materials quality and advanced processing methods [4-9].

The extended emission spectrum ranging from at least 280 nm to beyond 1000 nm, can be achieved by using the broad-band emissions from SiC, Si, and tuned III nitride-based avalanche LED structures and their combinations, integrated on a single chip. Individual addressing of each LED in the structure allows for spectral simulation of a large variation of different class stars.

Schematic of the fabricated Si/3C-SIC based device structure in the first stage of the project is presented in the picture below:

The results of the first stage of the project indicated the following:

  1. Fabrication of integrated 3C-SiC/Si based avalanche light emission diodes is feasible. To the best of our knowledge avalanche emission was observed for the first time from 3C-SiC grown on Si wafers. The optical emissions measured from the fabricated samples cover the entire near-UV, visible, and near-IR parts of the optical spectrum and can be employed for optical calibration in a very broad spectral range.
  2. Thin (~2-3 mm) 3C-SiC layers grown on Si wafers at Cree Research are suitable for “side-by-side” type integration since the 3C-SiC carbide layer can be patterned using an RIE technique. However thicker (~20 mm) 3C-SiC layers grown at Novasic can be used for “vertical alignment”-type integration by wet etching through the Si layer and releasing the optical emission through free-standing 3C-SiC films. We have shown that such approach also allows extension of the emission into the UV by generating and transmitting photons with energies larger than the material bandgap.
  3. Employment of SnO2 layers deposited by spay pyrolysis for avalanche LED fabrication is limited only to p-type semiconductor materials. In addition oxidation of the semiconductor material occurs during the SnO2 deposition. Alternative deposition methods, such as rf sputtering, e-beam deposition, and spin coating should be considered for deposition of SnO2 of both n- and p-type conductivity as well as high-quality Indium Tin Oxide (ITO) layers.

Normalized optical emission spectra from: 3C-SiC-based sample (blue line); Si-based sample (red line); integrated device (green line, modeled) are shown in the picture below:

In the course of the next project stage the feasibility of fabricating integrated structures based on III-Nitrides with broad band optical emission spectra matching a class B solar simulator with possibilities of attaining a class A type solar simulator have been demonstrated. The results also have shown that the SiC based integrated structures fell short of the required emissions in the 500 nm to 600 nm wavelength range to properly simulate a class C solar simulator. It was experimentally demonstrated that optical emission from reverse biased avalanche LED devices is much more stable than that exhibited from conventionally forward biased injective type LEDs.

The integrated III-Nitride-based optical emitter sample was grown on a 5 µm thick n-GaN template on sapphire. The MBE-grown structure began with 500 nm of n-GaN, followed by 5 In0.10Ga0.90N quantum wells. The wells were 4.5 nm thick and were separated by 15.5 nm GaN barrier layers. On top of the well structure, 120 nm of p-GaN was grown.

P-mesas were defined by using a mesa photolithography mask. The patterned sample was etched in a Reactive Ion Etching system and etched in a chlorine gas atmosphere. The samples were then re-patterned using negative resist for both p-contacts (Ni/Au) and n-contacts (Ti/Al/Ti/Au).

The figure below shows the photograph of the processed sample, with the device structures of varying optical emmision areas:

Precessed Sample

The results of this project are summarized below:

  • The forward biased injective LED thermal coefficient of optical power shows approximately a 0.5 to 0.6 % change with temperature under cryogenic and heated environments respectively. Whereas, in the case of reverse biased avalanche LEDs, the variations were 0.04% under similar cryogenic temperatures and 0.32 % under similar elevated temperature conditions.
  • Feasibility of fabricating III-Nitride LEDs with power densities of up to 0.43 µW/cm2 at 80 mA of current at forward bias has been demonstrated. For this LED, the forward bias (injective) and reverse bias (avalanche) optical emission spectra peaks are at 444 and 395 nm, respectively.
  • Compared to several tested commercial LED structures, the LEDs fabricated in this project show a much better Gaussian distribution in the optical emission spectrum measured in avalanche mode at reverse bias conditions. The fabrication of improved InGaN LED structures optimized for performance under reverse bias conditions can enable the realization of Class A solar simulators.

The figure below shows room temperature reverse bias optical emission spectra for 460 nm, 505 nm, and 525 nm of the fabricated nitride-based (GaN/InGaN) LED structures:

Optical Calibration Sources Based on Micro-Column Arrays

Formation of Micro Column Arrays (MCA) by laser ablation of a Si target irradiated with an excimer laser has been confirmed by using a large variety of pulsed laser sources. The general feature of these structures is their protrusion above the initial surface of the sample by 10–20 µm. Such MCA produced by laser ablation of solids represent a new type of solid surface processing which results in materials with very interesting characteristics, such as large specific area, low-threshold electron field emission, and unique optical properties [10-12].

The main objective of this project was to demonstrate the feasibility of fabricating Three-Dimensional (3D) MCA for calibration sources having high optical output from the same orifice in both Visible (VIS) and Infrared (IR) parts of the spectrum. Pulsed laser ablation of heat-resistant stainless steel and refractory metals foils have been used for fabrication of such arrays. Due to unique micro-structural properties, the ablated foil surface behaves as a black body emitter with a high output in both VIS and IR parts of the spectrum at temperatures close to 2500 °F (1360 °C) achieved by resistive heating of the foil strip.

The emissive properties of MCA depend on the material properties (such as: composition, melting temperature, corrosion resistance), as well as on the laser ablation conditions (such as: laser power, wavelength, pulse duration, and environment). The appropriate selection of the materials and investigation of the process parameters in the project was the main key to achieving desirable device parameters.

A typical SEM image of a stainless steel sample with MCA is shown in the picture below:

The characterization results indicated that samples based on Mo, Ti, and Alloy 321 exhibit emittances in the range ~90-95% at angles in the range 15-60deg. and 75-85% at the angle of 75deg. Sample based on W exhibits emittance in the range of 75-90% at angles in the range 15-60deg. and 60-75% at the angle of 75deg. Sample based on Ta exhibits emittance in the range of 80-85% in the whole range of angles: 15-75deg.

The project results show that MCA can be fabricated on various materials including stainless steels and refractory metals. Samples, based on MCA fabricated on these materials exhibit optical properties close to those of blackbody emitters. From the measured samples best results in stability and reproducibility of the optical characteristics were achieved on alloy 321 and tantalum-based samples. III nitride-based coatings for improvement of the MCA samples stability have a high application potential.



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