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Fig.1 A variety of photodiodes are possible using GaN and related materials, including photoconductors, diodes, and a heterostructure field-effect transistor (HFET; upper right). (adapted from M. S. Shur and M. A. Khan, "GaN/AlGaN Heterostructure Devices: Photodetectors and Field Effect Transistors," MRS Bulletin 22(2), 44 (Feb.1997))

The resistivity of semiconducting materials may be changed by many orders of magnitude when they are exposed to radiation, which makes them a natural choice for use in photodetectors. These materials are particularly sensitive to electromagnetic radiation with photon energies that exceed the energy bandgaps of the semiconductor, which produces electron-hole pairs. Recent interest in detectors of ultraviolet (UV) radiation stimulated research in wide-bandgap semiconductors that are sensitive to UV light but not to visible or infrared (IR) light.

Even silicon, with an energy bandgap of 1.12 eV, can be used to detect UV radiation, which corresponds to photon energies higher than 3.18 eV. Silicon, however, is not the best choice for a UV photodiode. While it detects UV radiation, it also responds to the many photons at visible or IR wavelengths that may strike it under typical conditions. Ideally a UV detector would be insensitive to visible and IR radiation, a so-called solar-blind or visible-blind detector. This characteristic provides a unique niche for wide-bandgap semiconductors, which are mostly transparent to these wavelengths. Gallium nitride (GaN) and other nitride-containing semiconductors are prime candidates for visible-blind detectors.

Gallium nitride is a direct-bandgap material with excellent transport and optical properties that has been grown on sapphire, spinel, silicon, and silicon carbide (SiC) substrates. Other GaN-based materials include aluminum nitride (AlN), aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), and recently developed quaternary AlGaInN compounds. Silicon carbide, which also has been used for UV photodetectors, is a better material for this purpose than silicon but still inferior to GaN because it is an indirect-gap semiconductor with a smaller bandgap (depending on a crystal modification of SiC, its polytype).

Producing signals

Photons absorbed in a semiconductor may cause different types of transitions. For UV detectors, three types of transitions are the most relevant: intrinsic transitions, in which an electron or hole moves from the valence to the conductance band; transitions between the valence band and the exciton level; and transitions from the deep-acceptor to the conductance band. An exciton can be visualized as an electron-hole pair formation similar to a hydrogen atom. The excitons can be dissociated through thermal excitation, or electron-hole pairs forming the excitons can be pulled apart in a high electric field.

The light-generated carriers-the holes and electrons-can produce an electric signal in two ways. In one case they can increase the sample conductivity. If the sample is connected to a voltage source, this will cause an additional photoconductive current. If the sample is connected to a current source (which is a voltage source in series with a resistance much greater than the device resistance), the light-generated carriers will cause the voltage across the sample to decrease. Such photodiodes are called photoconductors (see figure).

The other way to produce a signal is to use a semiconductor device that has an internal electric field. It may be a p-n junction diode, a p-i-n diode, or a Schottky barrier diode. In any of these cases, electrons and holes can be separated by the built-in electric field or, more typically, by the electric field that is a combination of the external and internal electric fields. This charge separation leads to an additional light-generated current. The principle of operation of such a detector is very similar to a solar cell, even though the bias conditions and the device designs are quite different. Such a photodiode is called a photovoltaic detector.

The GaN or AlGaN materials used for UV photodetectors are primarily n-type, because the electron mobility in these compounds is much higher than the hole mobility. This makes the fate of light-generated electrons and holes quite different. In each of the three transitions, electrons wind up as free carriers in the conduction band. In an n-type material, we expect that electrons supplied by donors fill the deep-acceptor traps that are always present in this partially compensated material. As a consequence, these negatively charged deep-acceptor-type traps quickly trap the minority carriers (holes). The extra majority carriers can be trapped by neutral-acceptor traps, which have already trapped holes. Because the capture cross section for a neutral trap should be much smaller than that for a charged trap, the electron lifetime can be quite long.

Photoconductive detectors

The transit time of the fastest carriers (usually electrons) determines the detection time of a photodiode. However, the device remains conductive until all light-generated carriers recombine. Hence, the response time of a photoconductor is proportional to the lifetime of the light-generated carriers, which typically varies between 10^-3 and 10^-10 s. Because the gain is proportional to the carrier lifetime, there is a clear trade-off between the gain and the speed of a photoconductor.

The energy gap of nitride-based materials ranges from 1.89 eV for InN to 3.5 eV for GaN and 6.2 eV for AlN, and a wide range of GaN-AlN and InN-GaN solid-state solutions has been demonstrated. Hence, in principle, photodiodes based on these material systems can span the wavelength range from 656 to 200 nm. This includes practically the entire visible range (400 to 770 nm) as well as much of the ultraviolet range (200 to 400 nm).

For example, one of the first GaN photo conductive detectors consisted of an 0.8-µm insulating GaN layer grown over a 0.1-?m AlN buffer.1 Both the optical absorption and the photoluminescence signal peaked at around 365 nm. Peak detector responsivity was more than 1000 A/W. The detector had a very large dynamic range-more than five orders of magnitude for the excitation wavelength of 254 nm. Characteristic response time was on the order of 1 ms.

Photovoltaic detectors

One of the first photovoltaic detectors realized in GaN was the Schottky barrier photodetector on p-type GaN.² The p-type doping level was about 5 x 10¹⁷ cm^-3. The Schottky contact had a turn-on voltage of approximately 1.5 V and a reverse breakdown voltage of 3 V. The detector was illuminated from the bottom (that is, from the sapphire side). The measured responsivity was approximately 0.13 A/W at 320 nm and rapidly decreased with an increase in the wavelength beyond 365 nm. The detector response time was around 1 ?s and was limited by the resistance-capacitance (RC) constant of the measurement setup.

More recently, transparent Schottky barrier detectors using n-type GaN have been reported.³ A transparent Schottky contact allowed for front illumination. The vertical geometry (as opposed to the mesa structures) leads to a dramatic reduction in the series resistance, reducing the RC-limited response time down to 118 ns for a 50-W load.

The responsivity is quite high (up to 0.18 A/W) and, at least at short wavelengths, seems to be entirely limited by the Schottky contact transparency.

In addition to speed and detectivity, noise is an important characteristic of a photodiode. Preliminary studies in GaN photoconductors have shown that 1/f noise, which increases as frequency decreases, is the dominant source of noise in these devices, often exceeding all other contributions at frequencies below about 1 kHz. This will be a major challenge in developing GaN-based photodetectors, a challenge that may be addressed by improving the quality of the material.

Gallium nitride-based materials have demonstrated excellent performance potential for applications in visible-blind detectors, which can operate in harsh conditions and/or at elevated temperatures. The research and development of both types of GaN-based photodiodes is still in initial stages. Improvements in technology and design, in material quality, contacts, and surface passivation, for example, should lead to the development of much better devices and image-processing arrays. Further developments may also result from optoelectronic integration of these photodetectors with GaN-based electronic devices. The full potential of GaN-based photodiodes is still to be discovered.

ACKNOWLEDGMENTS

The authors wish to acknowledge the US Office of Naval Research (Arlington, VA), which partially funded this work (Project Monitors Colin Wood and John Zolper).

REFERENCES

1. M. Asif Khan et al., Appl. Phys. Lett. 60(23), 2917 (1992).
2. M. Asif Khan et al., Appl. Phys. Lett. 63(18), 2455 (1993).
3. Q. Chen et al., Appl. Phys. Lett. 70(17), 2277 (1997).

Tue Jun 01 00:00:00 CDT 1999

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