Research On The GaN Flip-chip Homogeneous P-N Junction Pair Chip Fabrication And Applications

GaN is an ideal material for short-wavelength light-emitting devices.In recent years,GaN and its related III-V semiconductor materials and devices have been developed rapidly.Among them,P-N junction devices based on In GaN/GaN multiple quantum wells can not only emit light at a specific frequency under current driving,but also generate the corresponding photocurrent at a specific frequency of light.Based on this principle,both light-emitting diodes（LED）and photodetector（PD）can be integrated on the same substrate to form a homogeneous LED-PD micro-unit.In this thesis,we have successfully fabricated a LED-PD device based on a pair of homogeneous P-N junction pair on a sapphire substrate using semiconductor processes and completed the overall package of the device in the form of flip-chip bonding.The P-N junction device unit has the functions of both luminescence and photoelectric detection,which realizes the coupling and matching of luminescence and absorption spectra of a single device.It overcomes the problems of the existing discrete LED light source and the PD light detector being independent of each other,such as poor compatibility and inability to achieve photon integration.On this basis,different application scenarios have been studied.The main research contents are as follows:（1）The idea of GaN-based flip-chip homogenous P-N junctions is proposed for the first time in this thesis.The two types of light-emitting and photosensitive photonic units were fabricated on a GaN wafer grown on the same sapphire substrate containing In GaN/GaN multiple quantum wells（MQW）and integrated as a pair of homogeneous P-N junction pair,which can simultaneously realize the two functions of light emission and detection.（2）The working mechanism and basic parameters of GaN-based homogenous P-N junction devices are studied.The principle of realizing luminescence and light reception in flip-chip GaN homogeneous epitaxy is analyzed.The existence of the Stokes shift reduces the spectral overlap between the emission and absorption spectra of MQW;The doping content x of In constituent of In_xGa_1-xN designed to set as 0.158、0.215 and 0.274 in the fabrication process.Accordingly the chip wavelength of LED chips are 440nm、470nm and513nm.at the same driving current of 10 m A the induced photo-current are 10.604μA、5.165μA、2.509μA.the stocks shift increased and the photo-current decreased accordingly.when the emission wavelength of LED increased even at the maximum Stokes shift,Therefore,in a homogeneous PN junction LED-PD device,due to the same MQW structure,the absorption spectrum of a homogeneous PN junction pair will still overlap with the emission spectrum.the PD in the homogeneous PN junction pair can be used as a detector to detect the light emitted by the LED.（3）Designed a flip-chip fabrication plan,studied the large scale fabrication process of flip-chip P-N junction chips,including GaN epitaxial growth technology,introduced silver mirrors and distributed Bragg reflection layers in the flip-chip fabrication process to increase the chip’s light extraction efficiency.Symmetrical homogenous P-N junction blue chip,green chip and asymmetric homogenous P-N junction chip are designed and prepared.Introduced flip-chip packaging solutions such as silicon substrates and aluminum substrates into the packaging of homogenous P-N junction chips.（4）The homogeneous P-N junction micro-device is packaged on a wearable transparent flexible substrate so that the chip is tightly attached to the skin of the wrist.In the absence of any signal modulation,the pulse signal on the wrist is monitored in real-time through the homogeneous P-N junction micro-device to obtain a clear signal of the heart rate pulse.The real-time heart pulse monitoring chip with monolithic design has broad application prospects for biological and microstructural sensors.In the light emission current range of 0-60 m A,the peak-to-peak amplitude（Ip-p）of the PPG signal changes only0.141-0.155μA,and the shape retention of the heart rate pulse current profile is intact.（5）A high-precision and rapid method for measuring the refractive index of solution based on a homogenous P-N junction chip based on the flip-chip structure is proposed.Compared with the traditional refractive index test method,the exposed transparent sapphire in direct contact with the solution can greatly enhance the Fresnel reflection and total reflection at the sapphire/solution interface,which strengthens the photoelectric coupling process,and provides high sensitivity and consistency.The chip can accurately sense a solution with a refractive index range of 1.3325 RIU to 1.525RIU at room temperature and shows a sensitivity of 7.77μA/RIU.The detection resolution is 6.4×10^-6RIU.（6）The chip structure is further optimized.By optimizing the area and position of the PD and selecting the characteristic blue wavelength of the chip,a wider overlap of the absorption spectrum and the emission spectrum is realized.During the measurement of the water content in high-purity alcohol,the light emitted by the luminous P-N junction is reflected at the interface between the sapphire and the alcohol and then received by the P-N junction.Due to the high hydrophobicity of the sapphire surface,the measurement of water content in alcohol has high sensitivity and transient response.This feature can also be applied to the precise test scenarios of water content in a variety of complex liquid systems.The Experimental results show that the detection sensitivity of the micro-device for water content in ethanol is 7.12n A/（water content in alcohol%）,and the detection resolution is0.007%.