E. Monroy

Wide-bandgap semiconductor ultraviolet photodetectors

Industries such as the automotive, aerospace or military, as well as environmental and biological research have promoted the development of ultraviolet (UV) photodetectors capable of operating at high temperatures and in hostile environments. UV-enhanced Si photodiodes are hence giving way to a new generation of UV detectors fabricated from wide-bandgap semiconductors, such as SiC, diamond, III-nitrides, ZnS, ZnO, or ZnSe. This paper provides a general review of latest progresses in wide-bandgap semiconductor photodetectors.

The effect of the III/V ratio and substrate temperature on the morphology and properties of GaN- and AlN-layers grown by molecular beam epitaxy on Si(1 1 1)

Abstract We have studied the effect of the III/V ratio and substrate temperature on the growth of GaN and A1N films on Si(1 1 1) substrates by molecular beam epitaxy, where active nitrogen was generated by a radio frequency plasma source. In the case of GaN, two distinct regimes of growth (Ga-rich and N-rich conditions) lead to different crystal morphologies and luminescence properties. Scanning electron micrographs of the cleaved edges of films grown under highly N-rich conditions reveal columnar features, while growth under Ga-excess results in compact layers. The lowtemperature photoluminescence associated with the N-rich films is dominated by intense and narrow exciton lines, with peaks having full-width at half-maximum of less than 2 meV, whereas the Ga-rich films exhibit weaker and broader emissions. For increasing substrate temperatures above 700°C, stoichiometry is reached at higher Ga/N ratios, pointing to an enhancement of Ga desorption characterized by an activation energy of 2.5 eV. A similar study of A1N films shows that the desorption of A1 in terms of growth rate is not relevant for the substrate temperature range studied (850–920°C). III/V ratios close to the stoichiometric value and substrate temperatures above 900°C lead to high-quality A1N layers on Si(l 1 1) substrates. Complete relaxation is reached, for both GaN and A1N, in films with thicknesses well below 1 μm.

III nitrides and UV detection

III nitrides have become the most exciting challenge in optoelectronic materials in the last decade. Their intrinsic properties and an intense technological effort have made possible the fabrication of reliable and versatile detectors for short wavelengths. In this work, materials and devices issues are considered to provide a full picture of the advances in nitride UV photodetection. First, basic structures like photoconductors, Schottky, p-i-n and metal-semiconductor-metal photodiodes and phototransistors are compared, with emphasis on their specific properties and performance limitations. The efforts in the design and fabrication of more advanced detectors, in the search for higher quantum efficiency, contrast, signal-to-noise or speed operation, are reviewed afterwards. Metal-insulator-semiconductor diodes, avalanche photodetectors and GaN array detectors for UV imaging are also described. Further device optimization is linked with present materials issues, mainly due to the nitride quality, which is a direct result of the substrate used. The influence of substrates and dislocations on detector behaviour is discussed in detail. As an example of AlGaN photodetector applications, monitoring of the solar UV-B radiation to prevent erythema and skin cancer is presented.

High-performance GaN p-n junction photodetectors for solar ultraviolet applications

The fabrication and characterization of ultraviolet photodetectors based on GaN p-n junctions is reported. The devices are grown by metalorganic vapour phase epitaxy on basal-plane sapphire substrates. These detectors are visible-blind with a sharp wavelength cut-off at 360 nm. The photocurrent is linear with incident power from up to , with a responsivity of at 360 nm. The device time response is dominated by the effective resistance-capacitance time constant, and a 105 ns response is estimated for very low load resistances. A comparison with the response of GaN photoconductor detectors is also presented. The application of these high-performance photodetectors for solar ultraviolet monitoring is described.

High-speed, low-noise metal–semiconductor–metal ultraviolet photodetectors based on GaN

We present the fabrication and characterization of nonintentionally doped GaN and GaN:Mg Schottky metal–semiconductor–metal (MSM) photodetectors, grown on sapphire by metalorganic chemical vapor deposition. Low-leakage, Schottky contacts were made with Pt/Au. The devices are visible blind, with an ultraviolet/green contrast of about five orders of magnitude. The response times of the MSM devices were <10 ns and about 200 ns for GaN and GaN:Mg, respectively. The noise power spectral density remains below the background level of the system (10−24 A2/Hz) up to 5 V, for the undoped GaN MSM detector.

GaN/AlN short-period superlattices for intersubband optoelectronics: A systematic study of their epitaxial growth, design, and performance

We have studied the effect of growth and design parameters on the performance of Si-doped GaN/AlN multiquantum-well (MQW) structures for intersubband optoelectronics in the near infrared. The samples under study display infrared absorption in the 1.3–1.9 μm wavelength range, originating from the photoexcitation of electrons from the first to the second electronic level in the QWs. A commonly observed feature is the presence of multiple peaks in both intersubband absorption and interband emission spectra, which are attributed to monolayer thickness fluctuations in the quantum wells. These thickness fluctuations are induced by dislocations and eventually by cracks or metal accumulation during growth. The best optical performance is attained in samples synthesized with a moderate Ga excess during the growth of both the GaN QWs and the AlN barriers without growth interruptions. The optical properties are degraded at high growth temperatures (>720 °C) due to the thermal activation of the AlN etching of GaN. Fr...

HIGH-QUALITY VISIBLE-BLIND ALGAN P-I-N PHOTODIODES

We report the fabrication and characterization of AlxGa1−xN p-i-n photodiodes (0⩽x⩽0.15) grown on sapphire by low-pressure metalorganic chemical vapor deposition. The devices present a visible rejection of six orders of magnitude with a cutoff wavelength that shifts from 365 to 338 nm. Photocurrent decays are exponential for high load resistances, with a time constant that corresponds to the RC product of the system. For low load resistances, the transient response becomes non-exponential, with a decay time longer than the RC constant. This behavior is justified by the strong frequency dependence of the device capacitance. By an admittance analysis, we conclude that speed is not limited by deep levels, but by substitutional Mg capture and emission time.

Photoconductor gain mechanisms in GaN ultraviolet detectors

GaN photoconductive detectors have been fabricated on sapphire substrates by metal organic vapor phase epitaxy and gas-source molecular beam epitaxy on Si (111) substrates. The photodetectors showed high photoconductor gains, a very nonlinear response with illuminating power, and an intrinsic nonexponential photoconductance recovery process. A novel photoconductor gain mechanism is proposed to explain such results, based on a modulation of the conductive volume of the layer.

Photoconductive gain modelling of GaN photodetectors

A model to explain the behaviour of GaN photoconductive detectors is proposed, and it is based on the idea of a volume modulation rather than a carrier density modulation. Space charge regions inside the semiconductor produce a variation of the conductive volume when carriers are photogenerated. The strong non-exponential photocurrent decays result from carrier capture processes over the barriers associated with space charge regions. By means of computer simulation, this model explains quite well the behaviour of current GaN photoconductor devices and predicts their time response, temperature dependence and responsivity properties.

AlGaN metal–semiconductor–metal photodiodes

We report on the fabrication and characterization of AlGaN metal–semiconductor–metal photodiodes with sharp cutoff wavelengths from 365 to 310 nm. The detectors are visible blind, with an ultraviolet/visible contrast of about 4 orders of magnitude. The photocurrent scales linearly with optical power for photon energies both over and below the band gap, supporting the absence of photoconductive gain related to space-charge regions. No persistent photoconductivity effects have been detected. Time response is limited by the RC product of the measurement system, the transit time of the device being far below 10 ns. The normalized noise equivalent power at 28 V bias is lower than 17 pW/Hz1/2 in GaN detectors, and about 24 pW/Hz1/2 in Al0.25Ga0.75N photodiodes.