Fatemeh Shahedipour

Photoluminescence band near 2.9 eV in undoped GaN epitaxial layers

The broad photoluminescence band with a maximum at about 2.9 eV widely observed in undoped epitaxial GaN is studied as a function of temperature and excitation intensity. We attribute the band to transitions from a shallow donor to a deep localized acceptor. The zero-phonon transition for this band is at 3.098 eV as determined from the fine structure at low temperatures. A local vibrational mode in the ground state with an energy of 36 meV is found.

Progress in the fabrication of GaN photocathodes

Currently, photo-cathodes hold the highest promise in the near term (next few years) of being able to detect low light level UV signals at high QE while being nearly blind to visible wavelengths. We briefly discuss the requirements for UV detection for astronomical applications, and then we describe our work on producing GaN based photo-cathodes. The p-type GaN films were grown on sapphire at Northwestern University. The films were then converted into opaque photo-cathodes inside photo-tubes at Hamamatsu. Hamamatsu tested detective quantum efficiencies (DQE) of these detectors to be as high as 30% at 200 nm. The ratio of peak DQE at 200 nm to the minimum DQE at 500 nm was measured to be about 6 X 103. We found a dramatic increase in the DQE at 200 nm versus the conductivity, with the break point being near 0.13 1/(Ohm-cm). Based on this dramatic increase, we believe that further improvement in photo-cathode quantum efficiencies can be achieved by increasing the conductivity. We have recently achieved more than an order of magnitude increase in conductivity by co-doping techniques. Improvements in the solar blindness of the devices depend both on characteristics of the film and its surface properties. A detailed discussion of decreasing the visible response and producing a sharper wave-length cutoff is beyond the scope of this work, but we briefly discuss the attributes that most likely affect the wavelength dependence of the photo-cathode response.

Comparative optical studies of p-type and unintentionally doped GaN: The influence of annealing

We present Raman studies of p-type and unintentionally doped GaN epitaxial layers grown by metalorganic vapor phase epitaxy onto c-plane sapphire substrates. The E2 (high) Raman mode from a series of thermally annealed p-type samples shows that a compressive lattice distortion is induced with increasing annealing temperature. This is further corroborated by our photoluminescence measurements which show that the blue luminescence at 2.8 eV undergoes a redshift upon increasing the annealing temperatures beyond 650 °C. In comparing the Raman and photoluminescence spectra from the various samples we discuss the importance of two possible mechanisms: local distortion and longitudinal optical phonon–plasmon coupling.

Lateral epitaxial overgrowth of GaN on sapphire and silicon substrates for ultraviolet photodetector applications

Abstract Lateral epitaxial overgrowth of GaN thin films was conducted by low-pressure metalorganic chemical vapor deposition on basal plane sapphire and (111) silicon substrates. The films were characterized through X-ray diffraction, photoluminescence, scanning electron microscopy, atomic force microscopy and deep level transient spectroscopy. Schottky metal–semiconductor–metal ultraviolet photodetectors were fabricated on LEO grown GaN films for the first time. The spectral responsivity, its dependence on optical excitation power and bias voltage, and the device time decay properties were characterized. The orientation of the interdigitated fingers with respect to the LEO stripes was investigated.

Solar-blind AlxGa1-xN p-i-n photodetectors grown on LEO and non-LEO GaN

There is currently a strong interest in developing solid- state, UV photodetectors for a variety of applications. Some of these are early missile threat warning, covet space to space communications, flame monitoring, UV radiation monitoring and chemical/biological reagent detection. The III-Nitride material system is an excellent candidate for such applications due to its wide, reagent detection. The III-Nitride material system is an excellent candidate for such applications due to its wide, direct bandgaps and robust material nature. However, despite many inherent material advantages, the III-Nitride material system typically suffers from a large number of extended defects which degrade material quality and device performance. One technique aimed at reducing defect densities in these materials is lateral epitaxial overgrowth (LEO). In this work, we present a preliminary comparison between AlGaN UV, solar-blind p-i-n photodiodes fabricated form LEO GaN and non-LEO GaN. Improvements in both responsivity and rejection ratio are observed, however, further device improvements are necessary. For these, we focus on the optimization of the p- i-n structure and a reduction in contact resistivity to p- GaN and p-AlGaN layers. By improving the structure of the device, GaN p-i-n photodiodes were fabricated and demonstrate 86 percent internal quantum efficiency at 362 nm and a peak to visible rejection ratio of 105. Contact treatments have reduced the contact resistivity to p-GaN and p-AlGaN by over one order of magnitude form our previous results.

AlxGa1-xN Materials and Device Technology for Solar Blind Ultraviolet Photodetector Applications

There has been a growing interest for the development of solar blind ultraviolet (UV) photodetectors for use in a variety of applications, including early missile threat warning, flame monitoring, UV radiation monitoring and chemical/biological reagent detection. The AlxGa1-xN material system has emerged as the most promising approach for such devices. However, the control of the material quality and the device technology are still rather immature. We report here the metalorganic chemical vapor deposition, the n-type and the p-type doping of high quality AlxGa1-xN thin films on sapphire substrates over a wide range of Al concentration. The quality of this AlxGa1-xN material was verified through the demonstration of high performance visible and solar blind ultraviolet p-i-n photodiodes with a cut-off wavelength continuously tunable from 227 to 365 nm, internal quantum efficiencies up to 86% when operated in photovoltaic mode, and a ultraviolet-to-visible rejection ratio as high as six orders of magnitude. Both front and back side illuminated p-i-n photodiodes were realized. Photodetector devices were also demonstrated on GaN material obtained using lateral epitaxial overgrowth. The technology for such AlxGa1-xN based devices was improved in an effort to enhance their performance, including the development of ohmic metal contacts to both n-type and p-type AlxGa1-xN films with an Al concentration up to 40%.