Kan Mi

Solar-blind AlGaN photodiodes with very low cutoff wavelength

We report the fabrication and characterization of AlxGa1−xN photodiodes (x∼0.70) grown on sapphire by low-pressure metalorganic chemical vapor deposition. The peak responsivity for −5 V bias is 0.11 A/W at 232 nm, corresponding to an internal quantum efficiency greater than 90%. The device response drops four orders of magnitude by 275 nm and remains at low response for the entire near-ultraviolet and visible spectrum. Improvements were made to the device design including a semitransparent Ni/Au contact layer and a GaN:Mg cap layer, which dramatically increased device response by enhancing the carrier collection efficiency.

High-temperature, high-power, continuous-wave operation of buried heterostructure quantum-cascade lasers

We report cw operation of buried heterostructure quantum-cascade lasers (λ=6 μm) using a thick electroplated Au top contact layer and epilayer-up bonding on a copper heat sink up to a temperature of 333 K (60 °C). The high cw optical output powers of 446 mW at 293 K, 372 mW at 298 K, and 30 mW at 333 K are achieved with threshold current densities of 2.19, 2.35, and 4.29 kA/cm2 respectively, for a high-reflectivity-coated, 9-μm-wide and 3-mm-long laser.

Demonstration of a 256×256 middle-wavelength infrared focal plane array based on InGaAs/InGaP quantum dot infrared photodetectors

We report a demonstration of an infrared focal plane array based on InGaAs/InGaP quantum dot infrared photodetectors. The middle-wavelength infrared quantum-dot infrared photodetector (QDIP) structure was grown via low-pressure metal organic chemical vapor deposition. A detectivity of 3.6×1010 cm Hz1/2/W was achieved at T=95 K and a bias of −1.4 V. The background limited temperature of our QDIP was 140 K with a 45° field of view. A 256×256 detector array was fabricated with dry etching, and hybridized to a Litton readout chip by indium bumps. Thermal imaging was achieved at temperatures up to 120 K. At T=77 K, the noise equivalent temperature difference was measured as 0.509 K with a 300 K background and f/2.3 optics.

High detectivity InGaAs/InGaP quantum-dot infrared photodetectors grown by low pressure metalorganic chemical vapor deposition

We report a high detectivity middle-wavelength infrared quantum dot infrared photodetector (QDIP). The InGaAs quantum dots were grown by self-assembly on an InGaP matrix via low pressure metalorganic chemical vapor deposition. Photoresponse was observed at temperatures above 200 K with a peak wavelength of 4.7 μm and cutoff wavelength of 5.2 μm. The background limited performance temperature was 140 K, and this was attributed to the super low dark current observed in this QDIP. A detectivity of 3.6×1010 cm Hz1/2/W, which is comparable to the state-of-the-art quantum well infrared photodetectors in a similar wavelength range, was obtained for this InGaAs/InGaP QDIP at both T=77 K and T=95 K at biases of −1.6 and −1.4 V, respectively.

Future of AlxGa1-xN materials and device technology for ultraviolet photodetectors

Thanks to advances in the quality of wide bandgap AlxGa1-xN semiconductors, these materials have emerged as the most promising approach for the realization of photon detectors operating in the near ultraviolet from 200 to 365 nm. This has in turn spurred the need for such devices in an increasing number of applications ranging from water purification to early missile threat warning systems. Nevertheless, the control of the material quality and doping, and the device technology remain tremendous challenges in the quest for the realization of high performance photodetectors. Design of the photodetector structure is one of the key issues in obtaining high performance devices; especially the thickness of the intrinsic region for p-i-n photodiodes is a crucial value and needs to be optimized. We compare the performance of the p-i-n photodiodes with different widths for the depletion region, which shows a trade-off between speed and responsivity of the devices. Furthermore, another challenge at present is the realization of low resistivity wide bandgap p-type AlxGa1-xN semiconductors. We present here recent advances and propose future research efforts in the enhancement of the AlxGa1-xN p-type conductivity through the use of polarization fields in AlxGa1-xN/GaN superlattice structures.

Transport and photodetection in self-assembled semiconductor quantum dots

A great step forward in science and technology was made when it was discovered that lattice mismatch can be used to grow highly ordered, artificial atom-like structures called self-assembled quantum dots. Several groups have in the meantime successfully demonstrated useful infrared photodetection devices which are based on this technology. The new physics is fascinating, and there is no doubt that many new applications will be found when we have developed a better understanding of the underlying physical processes, and in particular when we have learned how to integrate the exciting new developments made in nanoscopic addressing and molecular self-assembly methods with semiconducting dots. In this paper we examine the scientific and technical questions encountered in current state of the art infrared detector technology and suggest ways of overcoming these difficulties. Promoting simple physical pictures, we focus in particular on the problem of high temperature detector operation and discuss the origin of dark current, noise, and photoresponse.

Demonstration of 256 x 256 focal plane array based on Al-free GaInAs-InP QWIP

We report the first demonstration of an infrared focal plane array based on aluminum-free GaInAs-InP quantum-well infrared photodetectors (QWIPs). The long wavelength QWIP structure was grown via a low-pressure metal-organic chemical vapor deposition. Corrugated light coupling structure of QWIP was fabricated with a dry etching process. A 256/spl times/256 detector array was also fabricated with dry etching and hybridized to a Litton readout integrated circuit via indium bumps. A unique positive lithography method was developed to perform indium-bump liftoff. The noise equivalent differential temperature (NE/spl Delta/T) of 29 mK was achieved at 70 K with f/2 optics.

Characteristics of high-quality p-type AlxGa1−xN/GaN superlattices

Very-high-quality p-type AlxGa1−xN/GaN superlattices have been grown by low-pressure metalorganic vapor-phase epitaxy through optimization of Mg flow and the period of the superlattice. For the superlattice with x=26%, the hole concentration reaches a high value of 4.2×1018 cm−3 with a resistivity as low as 0.19 Ω cm by Hall measurement. Admittance spectroscopy was performed in order to investigate the electrical properties of the superlattices. These measurements confirm that superlattices with a larger period and higher Al composition have higher hole concentration and lower resistivity, as predicted by theory.

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.