Study On The MOCVD Growth Of The N-polar GaN Films On Sapphire Substrates And The Properties Of Its Related Light Emitting Devices

According to the different stacking sequence of atomic,the Ga N with hexagonal wurtzite structure grown along c axis has two opposite polar states: Ga-polar and N-polar.At present,relatively comprehensive research has been conducted on Ga-polar Ga N materials.Especially Ga-polar Ga N-based light-emitting devices(LEDs)with a high luminous efficiency have been realized and commercialized successfully.In theory,the LEDs with N-polar Ga N material possess higher internal quantum efficiency and carrier injection efficiency than Ga-polar devices.As a result,N-polar Ga N is considered to have great potential in the application of LEDs.However,the research on the growth of N-polar Ga N films is still in the preliminary stage.The N-polar Ga N films grown by metalorganic chemical vapor deposition(MOCVD)or molecular beam epitaxy(MBE)always exhibit poor surface morphology,high dislocation density and uncontrolled background carrier concentration,which hampers the development of N-polar Ga N based LEDs.In this thesis,a series of research works were carried out on the above problems.Finally,blue-violet LEDs based on N-polar Ga N were prepared,and their performance was studied.The specific research contents of this thesis are as follows:(1)The N-polar Ga N films with smooth surface were prepared on sapphire substrate by using the MOCVD method.Moreover,the crystal quality of N-polar Ga N was improved by inserting the in-situ Si Nx mask in the films.The influence of the deposition time and insertion location of the Si Nx mask on the dislocation density and residual stress of the N-polar Ga N films was studied in detail.Under the optimum conditions,the edge dislocation density and residual tensile stress of the film can be reduced by 20 times and 3.7 times,respectively.The electrical and optical properties of the samples with and without Si Nx mask were characterized.It proved that the inserted Si Nx mask could also reduce the unintentional doping concentration and improve the optical quality of the N-polar Ga N film.(2)The N-polar n-type Ga N with controllable electron concentration was successfully achieved by the delta doping method.Firstly,the n-type Ga N films were prepared by the uniform Si doping method.Our measurement results show that many V-shaped pits were presented on the surface of the n-type Ga N films,and the electron concentration of the n-Ga N films couldn't be controlled by the Si concentration.Then,the delta doping method was used.We found that the V-shaped defects on surface of the N-polar n-type Ga N were successfully suppressed by this method.Besides,the electron concentration of the n-type Ga N grown by this method could be controlled by changing the growth condition.By comparison,we could find that the uniform doping method had a higher Si incorporation efficiency,and the delta doping method led to a better optical quality of Ga N film.(3)The N-polar p-type Al Ga N with a high hole concentration was achieved by using polarization-induced doping method.The principle of polarization-induced doping method was interpreted in detail.The advantage of N-polar Al Ga N film on realizing the polarization-induced doping was analyzed.The calculation formula of the theoretical hole concentration in polarization-induced doping layer was inferred.The N-polar Mg-doped Al Ga N films with a linear distribution of Al component were prepared by MOCVD method,and their electrical characteristics were characterized at different temperatures.According to the rule of measured temperature-dependent hole concentration,we found that there were two ionization mechanism for the holes in the graded p-Al Ga N layer,polarization-induced ionization and thermally activated ionization.The hole concentration of p-type Al Ga N was further improved by inserting the in-situ Si Nx mask in the N-polar Ga N template and optimizing the Mg flow rate.Under the optimum conditions,the hole concentration of Al Ga N reached to 9.0×1017cm-3.(4)Based on the optimized film growth and doping conditions mentioned above,the N-polar blue-violet LEDs with and without polarization-induced tunneling junction were prepared.The advantage of N-polar LED under large current was proved.Firstly,by adjusting the structure and growth conditions,the hexagonal structures on the surface of the N-polar multi-quantum wells,which were caused by the Ga N barriers grown at low temperature,were suppressed.Then,a blue-violet N-polar LED wafer with the horizontal structure was prepared based on the obtained growth conditions.The advantages of N-polar LED on turn-on voltage and efficiency droop effect compared to the Ga-polar device were analyzed.Finally,the luminous intensity was improved by adding the polarization-induced tunnel junction in the LED.As a consequence,the external quantum efficiency value was increased by 68% at 40 m A,which was attributed to the better current expansion.