A Study Of The Two-color AlGaN/GaN Superlattice Infrared/Ultraviolet Photodetector

The forbidden gap of the AlxGa1-xN material can be adjusted continuously from3.4eV (for GaN) to6.2eV (for AlN). It is a good alternative for optical detection inthe Ultraviolet Solar Blind Area (200-280nm). The conduction band discontinuity ofthe AlN and GaN can reach2eV which endows this system a huge competitiveadvantage in the infrared photodetection. The problem is the intensity of the ultravioletlight decreases quickly in the near-surface atmosphere, and the effective detectionrange is not longer than500m, while the infrared light which can be used to realize theremote probing has a considerable background radiation. If we can use the infrareddetection to realize the remote probing and switch to the ultraviolet mode when thedistance from the target is not longer than500m, the efficiency of the target trackingand identification can be enhanced and the influence of the background radiation canbe weakened. Further more, if the detection of the two different wavelengths can berealized with a single device and just one cooling system, the cost and the volume ofthe detector will be decreased and the application areas will be expanded. The purposeof this subject is exactly what we have discussed above, the detection of the ultravioletlight is achieved by the interband transition of the AlGaN/GaN superlattice while thedetection of the infrared light is realized by intersubband transition of the superlattice.We focus our attention on the infrared detection, because the selective absorption ofthe superlattice structure to the infrared light must consider all kinds of parameters ofthe superlattice, and this process is more complicated when the polarization effect istaken into account.This dissertation refers to the entire process of the design of the AlGaN/GaNsuperlattice photodetection, including the simulation of the superlattice structure, theepitaxial growth of the superlattice with MOCVD, the measurement of the optical andelectric characteristics of the superlattice. The major achievements are listed below:1. With the use of the software SILVACO and NEXTNANO3, the structures ofthe AlGaN/GaN are defined, and the influence of each parameter on the position of thesubband is analyzed. The analyzed structure parameters include the doping in the wells,the thicknesses of the barriers and wells, the Al component of the barriers, thetemperature, and the Al component in the bottom buffer layers et al. Due to thepolarization effect in GaN-based material, the influence of these parameters on the energy band of the superlattice is different from that of the ordinary quadrate potentialwells. For example, the structure must be high concentration doped, or the electrons inthe wells will be depleted and cannot be used to detect the photons. Also, the barrierthickness will influence the position of the subband in the well which is also beyondimagination in nonpolarized materials.2. According to the simulation results, the AlGaN/GaN superlattice is epitaxialgrown on the twin polished sapphire substrates by MOCVD. Then, the epitaxialstructures are characterized by means of AFM, HRXRD and Raman et al. The resultshows the high quality superlattice can be realized in our lab. Through analyze to theHRXRD results, the thicknesses of the barriers and the wells can be determined, thethickness of a single layer can reduce to1.7nm, and can be adjusted in1nm range.3. The optical characteristics of the superlattices are measured. To prepare thesamples met the requirements, a polishing platform is self-made. The result indicatesthat the infrared detection wavelength of the superlattice cannot be longer than6.15μm, because of the restriction of the sapphire. The transmission spectrum of thesuperlattice is obtained by the subtract of the transmission spectrums of the sampleswith and without superlattice, the result shows that AlGaN/GaN superlattice whoseinfrared absorption wavelength locates between2.92and4.65μm can be realized inour lab. The photoluminescence spectrum shows that the optical detection in theUltraviolet Solar Blind Area (200-280nm) can also be realized.4. For the subsequent examination of the electric characteristics of thesuperlattice, a testing method is proposed in our lab to qualitatively and quantitativelydetect the traps at the interfaces of the AlGaN and GaN.