Electrical transport in AlGaN/GaN heterostructures
Date Issued
2005
Date
2005
Author(s)
Wu, Kun-Ta
DOI
en-US
Abstract
In this thesis, I will report on two measurements on AlGaN/GaN high electron mobility transistors (HEMTs). This thesis consists of the following two parts.
1.Transport in AlxGa1-xN/GaN HEMTs with different Al compositions
We performed measurements on three AlxGa1-xN/GaN HEMTs with different Al contents (11%, 15%, and 25% respectively). All three samples are grown on sapphire substrates, and the mobility measurements indicate that AlxGa1-xN/GaN HEMTs with 15% Al content have the highest mobility (6600 cm2/Vs at 10K). If the Al content of AlxGa1-xN/GaN HEMTs exceeds 15%, the mobility will drop drastically.
In addition, we found that the mobilities of the three samples are almost identical at room temperature. This is because electron-phonon scattering dominates the electrical scattering at room temperature, and the electron-imperfection scattering due to impurities in the sample is small compared with electron-phonon scattering.
Furthermore, we found that the 2DEG concentration increases with increasing Al content in AlxGa1-xN. Quantitative calculation of the 2DEG concentration was performed and the same trend was obtained. The reason for this is that when the Al fraction increases, the polarization in AlxGa1-xN increases and induces more sheet charges at the interface of AlxGa1-xN and GaN. Therefore, for an AlxGa1-xN/GaN heterostructure with a higher Al composition, more electrons would be attracted to compensate for the sheet charges near the AlxGa1-xN/GaN interface. Hence the 2DEG concentration would be higher for an AlxGa1-xN/GaN sample with a higher Al composition.
We also measured the activation energy of AlxGa1-xN/GaN HEMTs, and found that the activation energy increases with increasing Al content. As the Al content increases, the lattice mismatch between AlxGa1-xN and GaN becomes more prominent. This increased lattice mismatch could induce a larger defect density. Therefore a higher activation energy is required to thermally activate the carriers into the 2DEG region.
Finally, the fact that AlxGa1-xN/GaN HEMTs with 15% Al content has the highest mobility suggests that we can grow other AlGaN/GaN HEMTs with 15% Al content for the greatest mobility.
2.Measurements of AlGaN/GaN HEMTs grown on p-type silicon substrates
The substrate used in the previous section is sapphire. However, sapphire is not as popular as Si in the industry. We therefore attempt to grow our samples on Si substrates. However, growing high-quality GaN on silicon substrates proves difficult because of the large lattice mismatch (about 17%) and large thermal mismatch (about 54%) between Si and GaN.
In this section, we inserted a thin Si5N4 film with deposition time of 5 seconds and 10 seconds to improve the GaN quality, and observed how such a thin film could affect the 2DEG mobility at the interface of AlGaN and GaN in comparison with the case without a Si5N4 thin film. At 10 K, the sample without the Si5N4 thin film had mobility 744 cm2/Vs, whereas for the samples with 5 seconds and 10 seconds deposition time of Si5N4 it was 2323 cm2/Vs and 2387 cm2/Vs respectively. The result showed that the mobility was greatly enhanced by as much as three times after the Si5N4 thin film was inserted.
Upon comparing the mobility with the results of the previous chapter, we see that Al0.15Ga0.85N/GaN HEMTs on sapphire substrates had a much higher mobility of 6600 cm2/Vs. We conclude that it is difficult to grow AlGaN/GaN HEMTs on silicon substrates with mobilities as high as those grown on sapphire substrates. Nevertheless, this great enhancement of the mobility demonstrates the usefulness of our technique. If we perform further investigations on the optimization of our growth temperature, HEMT structure, and most importantly, Si5N4 treatment technique, it is expected that the quality of HEMTs grown on Si substrates may well be as high as those grown on conventional sapphire substrates.
Subjects
霍爾效應
氮化鋁鎵
氮化鎵
藍寶石基板
矽基板
Hall effect
AlGaN
GaN
sapphire substrate
silicon substrate
Type
thesis
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