E. Muñoz

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 responsivity and internal gain mechanisms in Au-ZnMgO Schottky photodiodes

Schottky photodiodes based on Au-ZnMgO/sapphire are demonstrated covering the spectral region from 3.35 to 3.48 eV, with UV/VIS rejection ratios up to ∼105 and responsivities as high as 185 A/W. Both the rejection ratio and the responsivity are shown to be largely enhanced by the presence of an internal gain mechanism, by which the compensated films become highly conductive as a result of illumination. This causes a large increase in the tunnel current through the Schottky barrier, yielding internal gains that are a function of the incident photon flux.

Bowing of the band gap pressure coefficient in InxGa1-xN alloys

The hydrostatic pressure dependence of photoluminescence, dEPL/dp, of InxGa1−xN epilayers has been measured in the full composition range 0 0.4 and a relatively steep dependence for x<0.4. On the basis of the agreement of the observed PL pressure coefficient with our calculations, we confirm that band-to-band recombination processes are responsible for PL emission and that no localized states are involved. Moreover, the good agreement between the experimentally determined dEPL/dp and the theoretical curve of dEG/dp indicates that the hydrostatic pressure dependence of PL measurements can be used to quantify changes of the band gap of the InGaN ternary alloy under pressure, demonstrating that the disorder-re...

AlGaN ultraviolet photodetectors grown by molecular beam epitaxy on Si(111) substrates

Abstract The performance of AlGaN metal–semiconductor–metal (MSM) photodetectors grown on Si(111) is presented in this article. It is shown that the growth of an adequate AlN buffer layer is critical to achieve visible-blind devices, and that its role as an effective electrical insulator of the conductive substrate was found to be more efficient for N-excess AlN growth. The increase of Al content produced a transition from photoconductor to MSM photodiode behaviour, as determined from the detector responsivity, temporal response, and UV/visible contrast. The effect of the contact metal on photoconductive gain and UV/visible contrast was also studied.

Dilute nitride based double-barrier quantum-well infrared photodetector operating in the near infrared

Near-infrared detection is reported for a double-barrier quantum-well infrared photodetector based on a 30-A GaAs1−yNy (y≈0.01) quantum well. The growth procedure using plasma-assisted molecular-beam epitaxy is described. The as-grown sample exhibits a detection wavelength of 1.64 μm at 25 K. The detection peak strengthens and redshifts to 1.67 μm following rapid thermal annealing at 850 °C for 30 s. The detection peak position is consistent with the calculated band structure based on the band-anticrossing model for nitrogen incorporation into GaAs.

Properties of a hole trap in n-type hexagonal GaN

Minority carrier transient spectroscopy is performed in Schottky diodes fabricated on hexagonal n-type GaN grown by metalorganic chemical vapor deposition, either doped with two concentrations of Si or unintentionally doped. Capacitance transients are measured after a light pulse sent through the semitransparent contact which generates electron–hole pairs in the depletion zone. They display the characteristic sign of hole emission. The same deep level is detected in all the samples, independent of the doping level and doping species, with a concentration of some 1015 cm−3, even in the sample prepared by epitaxial lateral overgrowth. The ionization energy and capture cross section deduced from Fourier Transform transient spectroscopy are respectively 0.81±0.03 eV and 2×10−14 cm2. Such a capture cross section for holes indicates an attractive potential and hence a negatively charged center before the hole capture. Hole emission is suppressed by electron–hole recombination when a sufficiently long majority c...

Carrier compensation by deep levels in Zn1−xMgxO/sapphire

A systematic analysis of the deep level spectrum in the lower half of the bandgap of Au–Zn1−xMgxO (0.056<x<0.18) Schottky diodes is presented. Two deep levels are observed at Ev+580 and Ev+280 meV regardless of the bandgap energy with trap concentrations linearly increasing with the Mg content. The Ev+280 meV trap concentration becomes as high as 1.01×1018 cm−3 at 18% Mg, partially compensating the films and causing a decrease from 8.02×1016 to 1.27×1016 cm−3 in the net electron concentration and an increase by three orders of magnitude in the diode series resistance due to electron trapping.

Application and Performance of GaN Based UV Detectors

In this work, we discuss the state-of-the-art of different types of AlGaN-based ultraviolet photodetectors, including metal-semiconductor-metal photodetectors, Schottky barrier photodiodes and p-i-n structures. New metal-insulator-semiconductor photodiodes will be proposed as low-noise UV photodetectors, improving the detectivity by more than one order of magnitude in comparison with standard Schottky photodiodes, and with a higher fabrication yield than metal-semiconductor-metal photodiodes.

Effect of nitrogen on the optical properties of InGaAsN p-i-n structures grown on misoriented (111)B GaAs substrates

In this work, we demonstrate the growth, characterization, and processing of InGaAsN single quantum well p-i-n structures by solid-source molecular-beam epitaxy on misoriented GaAs (111)B substrates. Two different misorientations were studied simultaneously, 1° toward [−211] and 2° toward [2-1-1], the latter showing the highest optical quality. The roles of the arsenic flux, substrate misorientation, and amount of active nitrogen on the optical properties and crystal quality are discussed. We demonstrate photoluminescence emission at wavelength as long as 1.42 μm at 16 K on (111)B GaAs. Postgrowth rapid thermal annealing improves crystal quality and typical blueshifts of the peak emission are observed, like the case of (100).

Depth-resolved analysis of spontaneous phase separation in the growth of lattice-matched AlInN

We report the detection of phase separation of an Al1?xInxN/GaN heterojunction grown close to lattice-matched conditions (x ~ 0.18) by means of Rutherford backscattering spectrometry in channelling geometry and high-resolution x-ray diffraction. An initial pseudomorphic growth of the film was found, with good single crystalline quality, the nominal composition and very low strain state. After ~50?nm, a critical thickness is reached at which the InN molar fraction of the films drops to ~15% and at the same time the single crystalline quality of the films degrade drastically. This spontaneous effect cannot be ascribed to strain relaxation mechanisms since both techniques show a good single crystalline growth of the ternary under lattice matched conditions.