Research On GaAs/AlGaAs Quantum Well Material Micro-structures And Device Characteristics

The new-type GaAs/AlGaAs quantum well infrared photodetectors are combinedby the advanced film growth technology and micro-electronics. Because it has suchadvantages as even thickness, mature preparation technology, low cost, goodanti-irradiation properties, easy to implement multi-color detection and so on. Recentlyit has been main stream technology of infrared acquisition aid due to its wide use in thefollowing fields: biological and medical imaging, space resources determination, hightechnology and modern armaments in military arena, mines detection, infraredguidance-system, battlefield surveillance, thermal transmission microscopy ofanti-tank guided missiles and so on. However, its bigger dark current, lower quantumefficiency and narrower bandwidth have inhibited fast development.Optimized material growth and device microstructures are clearly needed to fullyexploit the potential of GaAs/AlGaAs QWIP based on single photon detectors. MetalOrganic Chemical Vapor Deposition (MOCVD) is used to grow the material, andautomatic testing system is designed for performance parameters of QWIP. HighResolution Transmission Electron Microscopy (HRTEM) is adopted to analyze samplemicrostructure, while Room Temperature Photoluminescence (RT-PL) is used to gainGaAs well and AlGaAs barrier PLs to study micrographic. Thus, the photocurrent canbe improved while dark current can be lower, and the relationship is establishedbetween microstructure and macro photoelectric performance, which will lay solidfoundations for energy structural design and optimized material growth of QWIP. Themain contents, conclusions and results in the dissertation are as follows:1. The30~50periods GaAs/Al0.3Ga0.7As QWIP samples with different pressurewelding area size and position, whose photosensitive surface is300μm×300μm andpeak wavelength is about8.5μm, are grown by MOCVD. Their photoelectriccharacteristics are tested and analyzed from experimental and theoretical research.The results indicated that dark current, noise, response, voltage-current characteristicand detection are asymmetry under forward and opposite voltage. Combined withmicrostructure of samples by HRTEM, the findings suggest: the thread dislocation nearAlGaAs is widened, which leads to Al atom detached from AlGaAs and causes phaseseparated. These are the fundamental reason that leads to photoelectric characteristicsworse. The asymmetry of interface between GaAs and AlGaAs caused by materialgrowth technology or doping elemental diffusion, moreover, massive defects in GaAsincluding point defects and buffer layer defects caused by crystal lattices mismatch in hetero epitaxial layers, which is fundamental reason that leads to asymmetry of devicemacro photoelectric characteristic curves. The electrode in photosensitive surface willincrease the dark current in junction, and so do surface and electrode leakage.2. The GaAs/AlGaAs QWIP desired potential well model is established accordingto structure parameters. The QWIP samples are cleavage and the peak wavelengths ofGaAs well and AlGaAs barrier PLs are gained. Thus, the structural parameters can beobtained by desired potential well model and the band theory, which includes Al content,well width, transition-energy between subbands, peak wavelength and so on. The resultsshow: the actual well width deviated from the designed one near to1~2atoms while theactual Al content deviated from the designed one near to1%~2%. The actual one hassome deviation from the designed one. But the work will help to keep chips withdeviation from preparing devices and improve MOCVD.3. The band model for peak wavelength of QWIP is built. The relationship betweenpeak wavelength of QWIP and Al content in AlGaAs and the one betweenmicrostructure and macro photoelectric characteristics are determined. Sample-deviceswith Al content0.23and0.32are grown by MOCVD. Results of spectrum test show:Peak wavelengths from test of3#and4#are8.36μm and7.58μm while the ones fromschroedinger equation are9.672μm and7.928μm, and the error between them is15.6%and4.6%respectively. Results indicates: All these display that diffusion of Al atom ininterface between GaAs and AlxGa1-xAs causes quantum-well (QW) changing fromnormal square model to the one determined by complementary error, which results inQW reduce, GaAs width increase, sub-band energy change, and peak wavelength blueshift, which illustrates optical excitation carriers are changing from bound-to-continuumto bound-to-quasi-bound model. It is found the reason leads to3#error much larger thatis crystal lattices mismatch caused by dislocation and worse control precision for thematerial by use of HRTEM. If Al content is reduced, the sub-band distance will shrinkand peak wavelength will be red shift. Results of RT-PL are compatible with theoreticalcalculations. Obviously, Al content can modify optoelectronic characteristics of QWIP.4. The relationship between peak wavelength of QWIP and well width of GaAs isestablished. GaAs/Al0.3Ga0.7As QWIPs with well width of4.5nm and5.0nm are grownby MOCVD based on two-dimensional perfect normal well model. The photoresponespectra of samples are tested by Fourier spectrometer in liquid nitrogen temperature(77K) while GaAs well and AlGaAs barrier PL ones are obtained bu RT-PL. Results ofspectral response indicate peak wavelengths from experiment of5#and6#are8.39μmand7.69μm while the ones from two-dimensional perfect normal well model are 8.924μm and8.051μm, and the error between them is6.36%and4.7%respectively. Atthe same time, full width at half maximum (FWHM) rises from27.3%to44.2%. Butresults of PL show: PL agrees with two-dimensional perfect normal well model andschroedinger equation. And micro-structure of samples is studied in detail byHRTEM. The above shows that if GaAs well is widened, peak spectral response willshift towards high energy and FWHM will go up. These make it clear that E1ofground state is falling relative to bottom of well, which leads to sub-band distance larger,peak wavelength blue shift in which FWHM is increasing. And E2of excited state ismoving from well inside to well outside, that’s optical excitation carriers are changingfrom bound-to-continuum to bound-to-quasi-bound model.5. After micro-structure of samples is studied in detail by HRTEM. The conclusioncan be obtained: Al atom detached from AlGaAs, which will lead to photoelectron andthermal excited electron in well speed and direction changed and alter macrographyphotoelectric characteristics of device. The thread dislocation along (100) can’t smooththe stress-strain caused by crystal lattice mismatch between GaAs and AlGaAs forintroducing into GaAs. And it will bend a little near the area of QW. Thus, this willaffect output response.