C. Boney

Dual-band UV/IR optical sensors for fire and flame detection and target recognition

Several military and industrial applications require simultaneous or at least spatially synchronized detection of optical emissions in different spectral regions. The ability to grow III nitrides on Si wafers is considered to be key to the development of multi-color detectors ranging from the UV to IR wavelengths. GaN/InGaN p-n heterostructures grown on Si wafers indicated sensitivity in a wide spectral range from near UV to near IR. Employment of Schottky barrier photodiode structures based on AlGaN alloys allows extension of the spectral sensitivity further into the UV range beneficial for solar-blind sensing. An alternative way to combine sensitivities in separated IR (provided by silicon) and UV (featured by III nitrides) bands by employment of commercially available silicon-on sapphire (SOS) wafers is discussed.

Molecular beam epitaxy growth of InGaN-GaN superlattices for optoelectronic devicesa)

In the absence of native substrates for InGaN films, the achievement of thick InGaN films of high structural quality remains a challenge. The investigation of InGaN-GaN superlattice (SL) structures is one potential way to increase optical absorption at energies below the GaN bandgap while reducing the formation of detrimental defects. In this article the authors evaluate the structural and optical properties of InGaN-GaN superlattices grown by plasma assisted molecular beam epitaxy with indium compositions of up to 38% and periods from 8 to 20 nm. Of primary concern was the degree of film relaxation as determined by x-ray diffraction (XRD) reciprocal space mapping as a function of indium content and thickness of the InGaN layers. Indium well fractions of up to 0.15 were found to exhibit little or no relaxation for the structures tested by x-ray diffraction. For indium well fractions near ∼0.2, relaxations of the superlattices were in the range of 35% depending on total layer thickness. The samples with in...

Fabrication and characterization of 2.3eV InGaN photovoltaic devices

In this work we present the design, fabrication and characterization of 2.3 eV InGaN-based solar cells confirming the feasibility of high indium content III-Nitride materials for photovoltaics. Growth of single phase InxGa1-xN for x up to 0.4 is achieved using Molecular Beam Epitaxy (MBE) with flux modulation for active species. The material is characterized by x-ray diffraction and photoluminescence to quantify the layer quality and indium content, while the optical properties are characterized by studying spectral response and absorption. The fabricated devices are then studied for photo-response under AM0 and intense UV spectral conditions to evaluate solar cell characteristics. The dark and illuminated I–V results indicate the existence of significant shunt and series resistances. The causes of these behaviors are known and ultimately resolvable.

InGaN devices for high temperature photovoltaic applications

In this work we present the temperature-dependent photovoltaic behavior of InGaN homojunction structures confirming the feasibility of these materials for use in high temperature photovoltaics. Homojunction p-n and p-i-n structures from materials with In content >30% have been processed into PV test devices and, despite being not optimized, without surface passivation or anti-reflection coating (ARC), demonstrated significant spectral response at energies above 2.0 eV. J-V curves under AM0 or concentrated UV illumination were performed at temperatures from 25 °C up to 250 °C. Devices exhibited fractional Jsc reduction of ∼15% of their room temperature value at 200 °C, and ∼20% at 250 °C. Although typical Si-and GaAs-based solar cells tend to increase Jsc slightly when heated, the drop for the InGaN devices is a very encouraging result, since their lack of passivation and other optimizations will have an effect on their temperature dependent behavior.

Self-packaged boron nitride capacitor for high temperature applications

In this work, we investigated applicability of boron nitride (BN) and boron oxynitride (BNO) thin films to fabricate multilayer ceramic capacitors (MLCCs) for high temperature and high frequency applications. Advantages of BN include high temperature and chemical resistance, which should result in more compact and reliable devices. Deposited BN layers by a filamentless ion source assisted physical vapor deposition technique show a high thermal stability up to 1000 °C and a very high breakdown voltage of about 600 V/μm. A 15 mm × 15 mm capacitor geometry was picked to create a simpler packaging scheme. Rectangular electrodes are offset and layered to build up the capacitor and a metallization technique is used to produce high temperature oxidation resistant Au/Ti tab electrodes. We have seen consistent results in terms of: stable capacitance values versus frequency from 10 kHz to 2 MHz; near ideal phase angle (low parasitic inductance); and high quality factors values. Laboratory prototype capacitors with ...

Properties and modelling of InGaN for high temperature photovoltaics

In this work we present the temperature-dependent electrical properties of undoped, n-type, and p-type InGaN layers and p-type superlattices at temperatures up to 200 °C. The analysis takes into account contributions from underlying GaN buffer layers and piezoelectric polarization at the GaN-InGaN interface. P-type InGaN-GaN superlattices show much greater conductivity than p-GaN, but less change relative to room temperature than p-GaN. In light of these results, InGaN photovoltaic performance has been modeled from room temperature to 200 °C for bandgaps between 2.2 and 1.8 eV. For these bandgaps, the efficiency of p-i-n solar cells is projected to drop around 30% at 200 °C relative to the efficiency at 25 °C.

MgF 2 /BN double layer antireflection coating for photovoltaic application

Modeling and fabrication of double layer MgF2/BN antireflection coatings have been undertaken. Minimal reflection losses (≪5%) over a wide range of the solar irradiance (1.1 to 3 eV) are realized for Si and GaAs. Dispersion (n,k) analyses of individual BN films using spectroscopic ellipsometry indicated a nearly constant index of refraction of ∼ 2.8 and a negligible transmission loss over the useful range of the solar spectrum (0.7 to 3.2 eV). This spectral stability in conjunction with its robust ceramic nature and its fairly wide bandgap (6.2 eV) makes BN thin film well adapted for integration in multi-junction and space solar cells.