R. Pillai

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...

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.

Visible-Blind UV/IR Photodetectors Integrated on Si Substrates

A concept based on structures fabricated using stacked semiconducting layers to obtain a multi spectral photoresponse is investigated. Issues related to III nitride layer growth on thin Si wafers, such as substrate temperature recalibration and mechanical stress due to the lattice mismatch, have been studied. The grown on Si substrate III nitride layers were characterized by using spectroscopic ellipsometry and capacitance measurements. Fabrication of a dual-band UV/IR photodetector with a reasonable responsivity at room temperature has been demonstrated. The integrated device is capable of detecting optical emissions separately in the UV and IR parts of the spectrum. The responsivities of this device are ∼0.01 A/W, at a peak wavelength of 300 nm and ∼0.08 A/W, at a peak wavelength of 1000 nm, respectively. The described dual-band photodetectors can be employed for false alarm-free fire/flame detection and advanced hazardous object or target detection and recognition in several industrial, military, and space applications.

Employment of III-nitride/silicon heterostructures for dual-band UV/IR photodiodes

Employment of layered structures made of semiconductor materials with different optical absorption bands is a new way of realizing either broad band spectrum or selective multiple band photodetectors. A new concept of structures fabricated using stacked semiconducting layers to obtain a multi band spectral response is reported. Based on this approach, fabrication of a Solar-blind dual-band UV/IR photodetectors is demonstrated. Optimization of the device was carried out by modeling of the electric field distribution and developing tunneling barriers. The optimized Solar-blind UV/IR photodiode UV spectral response turns-on approximately around 265 nm (solar-blind) and peaks at 230 nm with a responsivity of approximately 0.0018 A/W. The IR diode response peaks at 1000nm with a responsivity of approximately 0.01 A/W.

InGaN/silicon heterojunction based narrow band near-infrared detector

While InGaN devices have shown great potential as photodetectors and light emitting devices in the UV to visible range, extension of their applications to the near-infrared (NIR) region has been less successful due to challenges faced in developing high Indium content InGaN layers. Here, the authors present our results on the development of an InGaN/Silicon heterojunction structure with a > 70% In content in the InGaN layer, showing a measurable response in the NIR range. The achieved results show that the band offset between InGaN, buffer layer and Silicon interface can be used as a mean for fabrication of photodiode structures with spectral sensitivity varying with indium content. However, the authors observed that the resulting photoresponse is still a combination of photoresponses produced by absorption of photons in both InGaN and silicon materials.

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.

Hardened planar nitride based cold cathode electron emitter

Low threshold electron emission from planar AlN/Silicon heterostructures is reported. The surface emitting ballistic electron structure consisted of an undoped AlN layer grown on Silicon by Molecular Beam Epitaxy, a Ti/Au Ohmic contact, and a thin Pt Schottky contact fabricated by e-beam deposition. Tunnel-transparent Pt Schottky contact was deposited on a 1 μm thick Silicon Dioxide (SiO2) layer and covered a 4 x 4 matrix of 50 μm diameter via produced in the SiO2 layer using photolithography The measurements were performed in vacuum (~10-8 Torr) using a metal grid separated from the structure by a 60 micron thick Kapton® polyimide film having an opening aligned with the via. Bias voltages in the range of 0-130 V were applied across the Schottky diode, while currents were recorded across the structure for grid voltages ranging from 0 to 50 V. The field emission nature of the measured currents was confirmed by plotting the Fowler-Nordheim dependence. Current density of at least 2.5x10-4A/cm2 was achieved for a grid voltage of 50 V and a bias of 130 V. Degradation of the structure performance was observed at bias voltages exceeding 90 V as a result of Schottky barrier modification under the elevated temperature and high electric field operation. The solid-state electron emitting structure indicated a threshold field as low as 0.2 V/μm under applied grid voltage of 12 V.