Self-supporting m-plane GaN substrate improves the performance of UV avalanche photodiode
CompoundSemi website report: Researchers at the Center for Quantum Devices Research at Northwestern University reported on UV photodetectors based on m-plane GaN materials (Z. Vashaei et al., Appl. Phys. Lett. p96, p201908, 2010).
A new type of avalanche photodiode (APD) based on a self-supporting m-plane GaN substrate has lower dark current and higher gain than other GaN-based APDs. There have been commercial suppliers capable of providing the above-described substrate, which was originally intended to improve the performance of a semiconductor light emitting device such as a green semiconductor laser or the like. The m-plane GaN substrate is also attractive to APD devices because devices grown epitaxially on the m-plane have higher ionization coefficients than conventional c-plane GaN substrates.
Researchers are working to replace bulky and fragile glass photomultiplier tubes (PMTs) with UV-APD based on tri-five nitride materials. The operating voltage of the PMT typically requires 1 kV and the internal gain is 106, so its dark current (noise) is very low compared to conventional Si or SiC based semiconductor devices. In addition, semiconductor devices based on Si and SiC also need to filter incident light to remove light in the visible/solar spectral range.
The AlGaN-based semiconductor APD has a band gap that can be adjusted in the range of 3.4 eV (GaN) and 6.2 eV (AlN) depending on the composition, and the corresponding wavelength range is 365 nm to 200 nm. Thus the above devices are naturally unaffected by the visible spectrum/sunlight wavelength (wavelengths greater than 380 nm).
Previous researchers have tried using sapphire or SiC substrates to grow nitride APD. However, the lattice mismatch that exists in the above material system results in a higher dislocation density (about 10 9 /cm 2 ), which increases the leakage current and limits the avalanche gain of the device. Avalanche gain occurs when carriers get enough energy under reverse bias to produce more electron-hole pairs and constant cascading occurs.
Metal Organic Chemical Vapor Deposition (MOCVD) uses a source gas of trimethylgallium and ammonia to epitaxially grow the layers of the device. Silane (SiH 4 ) and magnesium ferrocene (DCpMg) were used as n-type and p-type dopant sources. At first, it was not very successful to attempt to epitaxially grow a GaN layer on m-plane GaN. Later, it was found that it is necessary to add a desorption release step at a high temperature to remove various impurities such as oxygen or silicon on the substrate surface. The next work will be mainly to optimize the MOCVD growth conditions so that the crystal quality of both the undoped and doped layers is optimized.
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