Studies Of The Growth Of InN Materials By RF-MBE And The Fabrication Of InN-based Light Emitting Devices

Indium nitride, as the least understood group-III nitride compound, has attracted an extensiveinterest over the past decade due to its possible narrow band gap. The early synthesized InNshowed absorption data for~1.9eV, and this value was generally accepted to be the energy gapof intrinsic InN at the early stage. With improvements in growth techniques, the appearance of0.7-0.9eV photoluminescence (PL) and absorption data indicates that InN is probably a narrowband gap semiconductor, although some counter-arguments remain. Consequently, furtherstudy of the optical properties of InN is necessary to evaluate the material properties of InN. Inlight of the newly revealed narrow band gap of InN, this nitride alloy system could cover abroader wavelength range from ultraviolet to near-infrared (NIR), which makes it particularlysuitable for semiconductor optoelectronic and microwave device applications. The narrowdirect gap InN is also applicable to the fabrication of non-toxic near-infrared light emittingdiodes (LEDs) and high speed laser diodes (LDs) for optical communication systemapplications which can replace the conventional semiconductor technology. Consequently, therealization of electroluminescence (EL) is of considerable importance to the material physicsaspects of InN research and for the fabrication of InN-based NIR devices. This dissertationmainly focused on the growth, characterization and NIR devices fabrication of InN material byRF molecular beam epitaxy (RF-MBE).InN films have been prepared by RF-MBE on Si and GaN/Al2O3substrates. The effects oftemparure of In source and the flow regulating of N2on the properties of InN film are discussedon Si substrates. The experimental results demonstrate that the samples with stoichiometric proportion would be obtained when the temperature of In source is650℃and the flow rate ofN2is3sccm. The nucleation layer is employed to improve the crystalline of InN material, andthe optimal growth temperature of InN nucleation layer is435℃. The effect of the temperaturefor main layer has been investigated. It indicates that the stress caused by the lattice betweenthe films and the substrate can almost be completely released by the nucleation layer. However,the morphology is highly sensitive to the growth temperature of the main layer.The effects of the growth temperature of InN main layer on the GaN substrates have alsobeen discussed. It is found that higher temperature favors the lateral growth of the InN mainlayer, and the growth of490℃is in a two-dimensional growing model. In addition, the PLspectrum shows an obvious redshift and a narrow PL peak with the increasing temperature. Theeffects of different temperature nucleation layer on the surface morphology, crystal quality andoptical properties of InN main layer are analysed and the optimized growth conditions areobtained. The results show that the InN main layers has similar XRD dates and PL spectrawhen the temperature of nucleation layer is385℃and415℃, respectively, whereas the InNmain layer with narrowest FWHM of the XRD2-and scans has a PL FWHM asnarrow as154nm. The above results indicate that the optical quality of InN materials could beapproximately determined by the evaluation of the relative crystal quality.We investigate the characteristics of InN materials grown on sapphire substrate by usingtwo-step method and three-step method. The RHEED investigation results of latticedeformations illustrate that the InN sample grown by two-step method has poor crystallinequality. However, the RHEED images imply that well-controlled nitridation of sapphiresubstrate, which results in the formation of AlN nucleation layer, can play an important role toreduce lattice mismatch, and then InN materials with good crystalline quality could be obtained.In addition, the InN sample grown by two-step method has a broad PL spectrum and a largerabsorption edge relative the one grown by three-step method. And the XRD measurementsindicate that the InN sample with a broad PL spectrum has poor crystalline quality whichprobably leads to a strong Burstein-Moss effect, consequently, the above InN sample has abroad PL spectrum and a larger absorption edge.In-situ X-ray photoelectron spectroscopy measurements were performed on the InN samplesgrown on different substrates. Detailed analysis of In3d5/2and In M4N45N45gives the exact modified Auguer parameters of In3d in InN is852.324eV, and we mark it in Wagner plotwhich is used to identify the change of chemical states. In addition, the InN sample with abroad PL spectrum has a high proportion of energy-loss peak which indicates its strongersurface electron accumulation. Consequently, this method is probably helpful in evaluating thecharacteristics of InN materials.We report the fabrication of n-InN/p-GaN heterojunction LED devices by directly growingn-type InN epitaxial layer on p-type GaN film with a sapphire substrate. XRD and PLmeasurements indicate that the InN film has better crystalline and optical quality. The lightemitting diode shows typical rectification characteristics with a turn-on voltage of around0.8V.A dominant narrow NIR emission peak centered at1573nm was observed from the InN sideunder applied forward bias. By comparing with the photoluminescence spectrum, the ELemission peak at1573nm was attributed to the band-edge emission of the InN film.The n-InN nanodots/p-Si(111) heterojunction diode was fabricated by plasma-assistedmolecular beam epitaxy. The device shows a clear rectifying behavior with a turn-on voltage ofapproximately1.2V at room temperature. The NIR electroluminescence can be observed underforward bias, which covers a wide wavelength range(1100nm-1700nm). By comparing with theInN LED film devices, there is an obvious enhancement of the less dominant transitions on theshort wavelength side of the EL spectra, which may arises from the recombination of theinjected holes with the extreme high-density surface electrons of InN nanodots.