Research On The Epitaxial Growth And Characterization Of Semi-Polar AlGaN Materials

With the quick development of the fabrication technology for group ?-nitrides-based light-emitting diodes(LEDs),the Al-rich AlGaN-based ultraviolet LEDs(UV-LEDs)have become the hot spot of recent researches.However,due to the so-called quantum-confined Stark effect(QCSE),conventional polar AlGaN-based UV-LEDs suffer from a quite low internal quantum efficiency.In contrast,with semi-polar AlGaN materials the QCSE caused by spontaneous piezoelectric strain-induced built-in fields could be reduced,and thus the light efficiency of UV-LEDs could be increased.Therefore,it is meaningful to conduct systematic researches on the semi-polar AlGaN materials.In this dissertation,the epitaxial growth of semi-polar(11-22)plane AlGaN materials on m-plane sapphire were investigated experimentally using metalorganic chemical vapor deposition(MOCVD).The Si doping of semi-polar AlGaN epi-layers and the growth of semi-polar AlGaN multiple quantum wells(MQWs)were also performed.The major research contents and the results achieved in this dissertation are listed as follows:1.The growth of semi-polar AlGaN epi-layers was conducted on m-plane sapphire substrates using a low-temperature AIN(LT-AlN)nucleation layer followed by a high-temperature AIN(HT=AIN)buffer.The effects of the growth conditions for nucleation layer and the buffer thickness on the crystalline quality of the semi-polar AlGaN epi-layers were studied in detail.It was found that smooth and crack-free semi-polar(11-22)plane AlGaN epi-layers could be obtained with the help of LT-AlN and HT-AIN layers,and the lowest full width at half maximum(FWHM)of the X-ray curve(XRC)was achieved to be 1,330 arcsec when the growth temperature for nucleation layer was 520 ?,the growth duration was 5.8 min,and the thickness of HT-AlN buffer was 80 nm.2.Three growth techniques were developed to improve the crystalline quality of semi-polar AlGaN epi-layers.1)The pulsed NH3 flow growth technology was adopted to accelerate the surface diffusion of AI atoms and suppress the pre-reaction,leading to the decrease in surface roughness and the enhancement in the crystalline quality for semi-polar AlGaN epi-layers.2)The strain-relaxation interlayers were employed to compensate the in-plane strain induced by lattice mismatch and to block the propagation of the threading dislocations,resulting in the improvement in the crystalline quality for semi-polar AlGaN epi-layers.3)The Indium-surfactant-assisted growth process was developed to reduce the nitrogen vacancies and the native electron concentration,leading to the improvement in electronic properties for the undoped semi-polar AlGaN epi-layers.3.The Si-doped semi-polar(11-22)plane AlGaN epi-layers were successfully deposited on m-plane sapphire substrates,and the effects of Si-doping levels on the structural,electrical and optical characteristics of the epi-layer samples were studied systematically.The characterization results showed that the Si-doping with appropriate silane(SiH4)mole flow rate is beneficial to the interaction and annihilation of the dislocations,but the over-doped samples were suffering from a tensile strain which could be attributed to the limited dislocation movements by the Si "pin" effect,resulting in a deteriorated crystal quality.4.The semi-polar MQWs were successfully fabricated on m-plane sapphire substrates,in which the period was 15 nm and the well width was 7 nm.Furthermore,the impact of growth rate for MQWs on the interface quality and the optical property of MQWs was studied.The SEM characterization results showed that the heterointerfaces in MQWs were smooth and sharp.The photoluminescence(PL)spectra indicated that the emission peak of MQWs was located at approximately 280 nm and the FWHM of the emission peak was relatively small for the sample grown at a low growth rate.