Study On The Growth Of AlGaN/GaN HEMT Structure By MOCVD For HEMT Devices

In recent years, AlGaN/GaN HEMT devices have received intensive attention because of its unique advantage in high frequency, high voltage and high power devices. The level of research on AlGaN/GaN HEMT devices is strongly affected by the quality of the corresponding material. Therefore, This paper mainly studies the growth of AlGaN/GaN HEMT structure by MOCVD for HEMT devices. First of all, Fe-doped high-resistivity GaN films and AlGaN/GaN HEMT structure were grown on sapphire substrates, and the influence of the high-resistivity GaN on HEMT devices was studied. Then, In view of the many advantages of Si substrate, we investigated the growth of GaN on Si substrates. Finally, AlGaN/GaN HEMT structure was grown on Si substrates, which be processed into devices. The major work are listed as below.1. Fe-doped high-resistivity GaN with different Cp2 Fe flow rate was studied. It is found that, high-resistivity can be obtained by Fe impurity introduce Fe3+/2+ deep acceptor level in GaN, which compensate the background carrier concentration. Meanwhile, Fe impurity can introduce edge dislocations acting as acceptor, which also compensate the background carrier concentration to some extent. In a certain range, with the increasement of Cp2 Fe flow rate, the sheet resistance of Ga N material increases. The sheet resistance of GaN as high as 1×1010 ?/? while the Cp2 Fe flow rate is 75 sccm. When Cp2 Fe flow rate continue to increase to 100 sccm, the increase of the sheet resistance of GaN material is not obvious. In addition, the increasement of Cp2 Fe flow rate will lead to a deterioration in quality and surface roughness.2. The influence of the thickness of Fe-doped high-resistivity GaN films on HEMT devices was studied. The Cp2 Fe flow rate of 75 sccm was used to grow AlGaN/GaN HEMT structures with various Fe-doped layer thickness, which be processed into devices. All the HEMT devices possess satisfactory turn-off and gate-controlled characteristics. Besides, the increasement of Fe-doped layer thickness can improve the breakdown voltage of the HEMT device by 39.3 per cent, without degradation of the transfer characteristic.3. The influence of the growth pressure of the high temperature AlN buffer on the properties of GaN grown on Si(111) substrate was studied. It is found that, the surface morphology and structural properties of the GaN epilayer strongly depends on the growth pressure. With the optimized growth pressure, we were able to obtain a 1-?m-thick crack-free GaN layer. The(0002) and(10-12) XRD FWHM of the GaN film are 735 and 778 arcsec respectively, the tensile stress calculated from the Raman spectra is 0.437 GPa.4. The influence of the TMAl source flow rate of the high temperature AlN buffer on the properties of GaN grown on Si(111) substrate was studied. It is found that, the surface morphology and structural and optical properties of the GaN epilayer strongly depends on the TMAl source flow rate. With the optimized TMAl source flow rate, we were able to obtain a 1-?m-thick crack-free GaN layer. The(0002) and(10-12) XRD FWHM of the GaN film are 547 and 563 arcsec respectively, the tensile stress calculated from the Raman spectra is 0.4 GPa, and RMS roughness of AFM 5 ?m×5 ?m scan is 0.539 nm.5. The AlGaN/GaN HEMT structures were grown on Si substrates, which be processed into devices. The experimental results show HEMT structure has high crystal quality, smooth surface and low tensile stress, its two-dimensional electron gas density is 7.38×1012 cm-2, and the mobility is 1560 cm2/V·s. HEMT devices possess satisfactory gate-controlled characteristics, its threshold voltage is-4.4 V, and the breakdown voltage is 575 V.