Illumination and annealing characteristics of two-dimensional electron gas systems in metal-organic vapor-phase epitaxy grown AlGaN/AlN/GaN heterostructures

We studied the persistent photoconductivity (PPC) effect in AlGaN/AlN/GaN heterostructures with two different Al-compositions (x=0.15 and x=0.25). The two-dimensional electron gas formed at the AlN/GaN heterointerface was characterized by Shubnikov-de Haas and Hall measurements. Using optical illumination, we were able to increase the carrier density of the Al0.15Ga0.85N/AlN/GaN sample from 1.6x10^{12} cm^{-2} to 5.9x1012 cm^{-2}, while the electron mobility was enhanced from 9540 cm2/Vs to 21400 cm2/Vs at T = 1.6 K. The persistent photocurrent in both samples exhibited a strong dependence on illumination wavelength, being highest close to the bandgap and decreasing at longer wavelengths. The PPC effect became fairly weak for illumination wavelengths longer than 530 nm and showed a more complex response with an initial negative photoconductivity in the infrared region of the spectrum (>700 nm). The maximum PPC-efficiency for 390 nm illumination was 0.011% and 0.005% for Al0.25Ga0.75N/AlN/GaN and Al0.15Ga0.85N/AlN/GaN samples, respectively. After illumination, the carrier density could be reduced by annealing the sample. Annealing characteristics of the PPC effect were studied in the 20-280 K temperature range. We found that annealing at 280 K was not sufficient for full recovery of the carrier density. In fact, the PPC effect occurs in these samples even at room temperature. Comparing the measurement results of two samples, the Al0.25Ga0.75N/AlN/GaN sample had a larger response to illumination and displayed a smaller recovery with thermal annealing. This result suggests that the energy scales of the defect configuration-coordinate diagrams for these samples are different, depending on their Al-composition.Comment: 27 pages, 8 figure

Weak antilocalization and zero-field electron spin splitting in AlGaN/AlN/GaN heterostructures with a polarization induced two-dimensional electron gas

Spin-orbit coupling is studied using the quantum interference corrections to conductance in AlGaN/AlN/GaN two-dimensional electron systems where the carrier density is controlled by the persistent photoconductivity effect. All the samples studied exhibit a weak antilocalization feature with a spin-orbit field of around 1.8 mT. The zero-field electron spin splitting energies extracted from the weak antilocalization measurements are found to scale linearly with the Fermi wavevector with an effective linear spin-orbit coupling parameter 5.5x10^{-13} eV m. The spin-orbit times extracted from our measurements varied from 0.74 to 8.24 ps within the carrier density range of this experiment.Comment: 16 pages, 4 figure

Metal-semiconductor-metal ultraviolet photodetectors based on gallium nitride grown by atomic layer deposition at low temperatures

Cataloged from PDF version of article.Proof-of-concept, first metal-semiconductor-metal ultraviolet photodetectors based on nanocrystalline gallium nitride (GaN) layers grown by low-temperature hollow-cathode plasma-assisted atomic layer deposition are demonstrated. Electrical and optical characteristics of the fabricated devices are investigated. Dark current values as low as 14 pA at a 30 V reverse bias are obtained. Fabricated devices exhibit a 15× UV/VIS rejection ratio based on photoresponsivity values at 200 nm (UV) and 390 nm (VIS) wavelengths. These devices can offer a promising alternative for flexible optoelectronics and the complementary metal oxide semiconductor integration of such devices. © 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

InGaAs-based high-performance p-i-n photodiodes

Cataloged from PDF version of article.In this letter, we have designed, fabricated, and characterized high-speed and high-efficiency InGaAs-based p-i-n photodetectors with a resonant cavity enhanced structure. The devices were fabricated by a microwave-compatible process. By using a postprocess recess etch, we tuned the resonance wavelength from 1605 to 1558 nm while keeping the peak efficiencies above 60%. The maximum quantum efficiency was 66% at 1572 nm which was in good agreement with our theoretical calculations. The photodiode had a linear response up to 6-mW optical power, where we obtained 5-mA photocurrent at 3-V reverse bias. The photodetector had a temporal response of 16 ps at 7-V bias. After system response deconvolution, the 3-dB bandwidth of the device was 31 GHz, which corresponds to a bandwidth-efficiency product of 20 GHz

Low-temperature self-limiting atomic layer deposition of wurtzite InN on Si(100)

In this work, we report on self-limiting growth of InN thin films at substrate temperatures as low as 200 °C by hollow-cathode plasma-assisted atomic layer deposition (HCPA-ALD). The precursors used in growth experiments were trimethylindium (TMI) and N2 plasma. Process parameters including TMI pulse time, N2 plasma exposure time, purge time, and deposition temperature have been optimized for self-limiting growth of InN with in ALD window. With the increase in exposure time of N2 plasma from 40 s to 100 s at 200 °C, growth rate showed a significant decrease from 1.60 to 0.64 Å/cycle. At 200 °C, growth rate saturated as 0.64 Å/cycle for TMI dose starting from 0.07 s. Structural, optical, and morphological characterization of InN were carried out in detail. X-ray diffraction measurements revealed the hexagonal wurtzite crystalline structure of the grown InN films. Refractive index of the InN film deposited at 200 °C was found to be 2.66 at 650 nm. 48 nm-thick InN films exhibited relatively smooth surfaces with Rms surface roughness values of 0.98 nm, while the film density was extracted as 6.30 g/cm3. X-ray photoelectron spectroscopy (XPS) measurements depicted the peaks of indium, nitrogen, carbon, and oxygen on the film surface and quantitative information revealed that films are nearly stoichiometric with rather low impurity content. In3d and N1s high-resolution scans confirmed the presence of InN with peaks located at 443.5 and 396.8 eV, respectively. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) further confirmed the polycrystalline structure of InN thin films and elemental mapping revealed uniform distribution of indium and nitrogen along the scanned area of the InN film. Spectral absorption measurements exhibited an optical band edge around 1.9 eV. Our findings demonstrate that HCPA-ALD might be a promising technique to grow crystalline wurtzite InN thin films at low substrate temperatures. © 2016 Author(s)

Substrate impact on the low-temperature growth of GaN thin films by plasma-assisted atomic layer deposition

Gallium nitride (GaN) thin films were grown on Si (100), Si (111), and c-plane sapphire substrates at 200 �C via hollow-cathode plasma-assisted atomic layer deposition (HCPA-ALD) using GaEt3 and N2/H2 plasma as group-III and V precursors, respectively. The main aim of the study was to investigate the impact of substrate on the material properties of low-temperature ALD-grown GaN layers. Structural, chemical, and optical characterizations were carried out in order to evaluate and compare film quality of GaN on different substrates. X-ray reflectivity measurements showed film density values of 5.70, 5.74, and 5.54 g/cm3 for GaN grown on Si (100), Si (111), and sapphire, respectively. Grazing incidence x-ray diffraction measurements exhibited hexagonal wurtzite structure in all HCPA-ALD grown GaN samples. However, dominant diffraction peak for GaN films grown on Si and sapphire substrates were detected differently as (002) and (103), respectively. X-ray diffraction gonio scans measured from GaN grown on c-plane sapphire primarily showed (002) orientation. All samples exhibited similar refractive index values (∼2.17 at 632 nm) with 2-3 at. % of oxygen impurity existing within the bulk of the films. The grain size was calculated as ∼9-10 nm for GaN grown on Si (100) and Si (111) samples while it was ∼5 nm for GaN/sapphire sample. Root-mean-square surface roughness values found as 0.68, 0.76, and 1.83 nm for GaN deposited on Si (100), Si (111), and sapphire, respectively. Another significant difference observed between the samples was the film growth per cycle: GaN/sapphire sample showed a considerable higher thickness value when compared with GaN/Si samples, which might be attributed to a possibly more-efficient nitridation and faster nucleation of sapphire surface. � 2016 American Vacuum Society

High-speed GaAs-based resonant-cavity-enhanced 1.3 μm photodetector

Cataloged from PDF version of article.We report GaAs-based high-speed, resonant-cavity-enhanced, Schottky barrier internal photoemissionphotodiodes operating at 1.3 μm. The devices were fabricated by using a microwave-compatible fabrication process. Resonance of the cavity was tuned to 1.3 μm and a nine-fold enhancement was achieved in quantum efficiency. The photodiode had an experimental setup limited temporal response of 16 ps, corresponding to a 3 dB bandwidth of 20 GHz. © 2000 American Institute of Physic

High bandwidth-efficiency solar-blind AlGaN Schottky photodiodes with low dark current

Cataloged from PDF version of article.Al0.38Ga0.62N/GaN heterojunction solar-blind Schottky photodetectors with low dark current, high responsivity, and fast pulse response were demonstrated. A five-step microwave compatible fabrication process was utilized to fabricate the devices. The solarblind detectors displayed extremely low dark current values: 30lm diameter devices exhibited leakage current below 3 fA under reverse bias up to 12V. True solar-blind operation was ensured with a sharp cut-off around 266 nm. Peak responsivity of 147mA/W was measured at 256 nm under 20 V reverse bias. A visible rejection more than 4 orders of magnitude was achieved. The thermally-limited detectivity of the devices was calculated as 1.8 · 1013 cmHz1/2W 1 . Temporal pulse response measurements of the solar-blind detectors resulted in fast pulses with high 3-dB bandwidths. The best devices had 53 ps pulse-width and 4.1GHz bandwidth. A bandwidth-efficiency product of 2.9GHz was achieved with the AlGaN Schottky photodiodes. (C) 2004 Elsevier Ltd. All rights reserve

A Near-Infrared Range Photodetector Based on Indium Nitride Nanocrystals Obtained Through Laser Ablation

Cataloged from PDF version of article.We present a proof-of-concept photodetector that is sensitive in the near-infrared (NIR) range based on InN nanocrystals. Indium nitride nanocrystals (InN-NCs) are obtained through laser ablation of a high pressure chemical vapor deposition grown indium nitride thin film and are used as optically active absorption region. InN-NCs are sandwiched between thin insulating films to reduce the electrical leakage current. Under -1 V applied bias, the recorded photoresponsivity values within 600-1100-nm wavelength range are as high as 3.05 x 10(-2) mA/W. An ultrathin layer of nanocrystalline InN thin film is, therefore, a promising candidate for NIR detection in large area schemes. © 2014 IEEE

Resonant excitation of surface plasmons in one-dimensional metallic grating structures at microwave frequencies

Grating-coupling phenomena between surface plasmons and electromagnetic waves were studied in the microwave spectrum using metallic gratings. Transmission measurements were carried out to observe the transmitted radiation around the surface plasmon resonance frequencies. Grating structures with subwavelength apertures were designed for transmission experiments. Measurements were made in the microwave spectrum of 10-37.5 GHz, corresponding to a wavelength region of 8-30 mm. The A1 samples had a grating periodicity of 16 mm. A 2 mm wide subwavelength slit was opened for transmission samples. Samples with one/double-sided gratings displayed remarkably enhanced transmission and directivity with respect to the reference sample without gratings. The experimental results agreed well with theoretical simulations. ∼50% transmission at 20.7 mm, ∼25-fold enhancement, and ±4° angular divergence were achieved with a ∼λ/10 aperture. © 2005 IOP Publishing Ltd