Study Of Epitaxial Growth And Optoelectronic Properties Of AlN Thin Films

High-efficiency ultraviolet LEDs and laser diodes are of great interest for a variety of applicationssuch as energy-saving solid state lighting, microelectronic photolithography technology, high density datastorage, biomedical analysis, and environmental sciences. III-V nitride semiconductors including GaN-andAlGaN-based alloys are the most widely used materials for developing UV and deep UV LEDs. Recently, aAlN-based p-i-n LED was demonstrated with the light-emitting wavelength as short as210nm, attributedto the large band gap （6.2eV） of AlN. The AlN-based LED emitting at210nm is the shortest wavelengthreported to date among all kind of LEDs, which has aroused extensive interests in the AlN-based deep UVlight-emitting devices.Epitaxial AlN films with c-axis orientation were grown on both （111） MgO and c-sapphire substratesby laser molecular beam epitaxy （laser-MBE）. The in-plane epitaxial relationships were determined to be[112|0]AlN//[01|1]MgOand [11|00]AlN//[112|0]sapphire, and the lattice mismatch are4.2%and13.2%for AlNfilms on MgO and sapphire, respectively. The AlN films show Al-and N-polar on MgO and sapphire,respectively. The former is supposed to be caused by the center of inversion symmetry of （111） MgOsubstrate, while the latter is due to O-polar of sapphire. The FWHM of ω-scanning spectrum for AlN filmon （111） MgO substrate is smaller than that on c-sapphire substrate. The optical band-gap energies for AlNfilms grown on MgO and sapphire were found to be5.93eV and5.84eV, close to the standard band gap of6.2eV, and the calculated Urbach energies were0.27eV and0.53eV, respectively. These results indicate alower amorphous content and/or less defects/impurities in Al-polar than N-polar AlN. PL spectra show thatboth two samples only one peak emission around378nm, a band centered around3.28eV of deep leveldefects.AlN thin films with a single orientation were on Si substrates under the optimal conditions.Au/（0002）AlN/（111）Si/Al heterojunction was prepared for exploring the electrical properties of thestructure. I-V characteristic curve shows that AlN/Si heterojunction has typical rectifying characteristics.The log-log plot of the I-V data at RT shows that current transport mechanism exhibited in three differentregions. In region I, current follows a linear dependence, that is, I^CV. In this region the current transport is dominated by tunneling at low voltages. For region II, the current exponentially increases following theequation I^exp（αV）, which is usually observed in the wide band gap p-n diodes, due torecombination-tunneling mechanism. For region III, the I-V characteristic follows a power law I^VC, wherethe current conduction is attributed to the space charge limited current （SCLC） for a single carrier holeinjection since concentration of the holes are larger than the electrons in the present case. This SCLCmechanism is a normal phenomenon in the wide band gap semiconductors. Through impedancecharacteristic curve （Cp-V, GP-V） of AlN/Si, we analyzed state density distribution in the interface ofAlN/Si heterojunction. The results show that interface states are mainly shallow level defects andimpurities distributed in near⁰.25eV from the top of valence band in Si substrate. The state density ofheterojunction interface is¹.1×10^-12cm^-2, this result was contained in the range of interface state densityabout Si substrates, between the result of Si substrate with a natural oxide layer, and hydrogen passivationreported by V.V. Afanas’ev. Au/（0002）AlN/（111）Si/Al heterojunction electroluminescent （EL） spectra showthat, a green light centered at518nm （2.4eV） emission can be clearly seen by naked eyes in a dark room atroom temperature under forward bias. We think that was contribution by AlN films deep level impurities ordefects recombination. On the other hand, when the Au/AlN/p-Si heterostructure under reverse biased, nolight emission can be detected.The crystal quality of MgO films with different laser energy and substrate temperature under oxygenatmosphere1.4×10^-2Pa was explored by PLD. High crystal quality single orientation （100） MgO filmswere prepared on Si substrates. Polycrystalline MgO films were prepared under no oxygen atmosphere usePLD technology. The orientation of polycrystalline MgO contains （111）,（100）,（110）. Single oriented（0002） AlN films were grown under optimal conditions use laser-MBE in our laboratory on polycrystallineMgO film that have prepared. We successfully prepared Au/AlN/MgO/Si/Al p-i-n structure on the basis ofabove. Fitting the log-log plot for the I-V data of AlN/MgO/Si structure we found current transmission inaccordance with space-charge-limited current transport mechanism. We explored the interface feature ofAlN/Si with different thickness MgO layer. The interface state density and the stability of the interface ofAlN/Si heterojunction were decreases by inserting MgO layer in AlN/Si. Interface states density ofAlN/Si with²0nm MgO layer was minimum, which is7.8×10¹¹¹.3×10¹²cm^-2. Electroluminescentresults show that the adding of the MgO layer increase the series resistance of the structure and the center of green emission from⁵10nm shifted to534nm. That indicate the interface state transfer to thedeep-level, and the position of EL were redshift.