| Boron nitride is a poly-phase system of different stacking/layered structure:hexagonal boron nitride (hBN)，cubic boron nitride (cBN)，wurtzitic boron nitride(wBN) and rhombohedral boron nitride (rBN). cBN and hBN attracted researcher’sattention because of their excellent physicochemical properties, semiconductorproperties and wide bandgap. They have broad potential applications in the fields ofmicroelectronic and optoelectronic devices. At present, the major research is focusedon the preparation of high quality BN single crystals and thin films. Less attention ispaid to devices, especially to the BN based deep ultraviolet (UV) photodetectors. Inthis dissertation, primary studies on the surface polarity, doping, metal-semiconductorcontact of cBN, preparation and property of BN thin films, and design and simulationof BN based UV photodetectors, etc. were carried out. The main research results arelisted as follow:(1) The studies of the surface polarity of cBN single crystals. According tothe truth that cBN crystal samples have obvious color zoning, a simple method ofdistinguishing {111}B and {111}N faces by microscopy was found. The defectsand impurities incorporate in {111}N sectors serving as the amber color centers,while very little defects and impurities exist in {111}B sectors, so {111}B sectorsare colorless. Compared with {111}B faces,{111}N faces have faster etch rateand larger surface leakage current, and the disorder-activated Raman scatteringhappens in {111}N faces because of the high density of defects and impurities. Allresults indicate that {111}B faces have better crystalline and chemical stability.(2) The studies of doping cBN crystals. Si was doped into cBN crystals bythermal diffusion at high temperature. The typical doping temperature is850~950℃，and the doping time is about4~5hours. XPS analysis indicated that the content of Siimpurity at the surface layer of cBN samples was more than2.7%. Si mainly occupiedB lattices and became substitutional donor impurities. The measured activation energyof doped cBN samples were larger than0.4eV, so it was difficult for Si impurities tobe ionized at room temperature, and the doped cBN samples still had very high resistivity and the mechanism of space charge limited current. The activation energyand density of Si impurity can be controlled by the doping time and temperature. Thelonger the doping time is, or the higher the doping temperature is, the lower theactivation energy becomes.(3) The studies of metal-semiconductor contact of cBN crystals. Au and Alelectrodes were evaporated on the cBN samples in vacuum. Cr and Ti electrodes weredeposited on the cBN samples by RF magnetron sputtering. The contact properties ofthese metal electrodes, as well as Au pastes and Ag pastes, with cBN samples wereinvestigated. The results revealed that Al, Cr, Ti can formed Schottky contacts withcBN crystals, and Au, Au paste or Ag paste were able to form the ohmic contacts withcBN. After vacuum annealing at450℃and900℃for20minutes respectively, thecontact quality of Cr/Au with cBN was improved.(4) The preparation and characterization of BN films. BN films weredeposited on the substrates of N-type Si and quartz glass by using RF magnetronsputtering, and were characterized by FTIR, UV-Via spectra etc. FTIR resultsrevealed that the fastest growth rate of BN films obtained when Ar:N2was4:1. WhenAr:N2was1:1, the BN films including more than80%hBN phase were attained. Theoptical bandgap of this film was about5.8eV, and the absorption factor was40000cm-1at185nm. However, this BN film was unstable and could become intoboric acid in moist air, which will make the films break and separate from thesubstrate. In order to obtain the stable film with wide bandgap, Fe was added in thegrowth process of BN films, and FeBNO films were achieved. XPS results showedthat the FeBNO films mainly included Fe-N-O, B-N-O, and Fe-O etc. chemical bonds.The FeBNO films were deposited for120minutes with the sputtering power of150W,the substrate temperature of300℃in pure argon atmosphere. The FeBNO films weretransparent for the visible light, and the optical bandgap and the absorption factors ofthe films were5.32eV and100000cm-1at185nm respectively. Moreover, theheterojunction between the FeBNO films and N-type Si substrate had goodrectification characteristics, the rectification ratio reached103. The FeBNO films arevery stable, tightly bond with the substrate, and have very strong absorption in UVregion, therefore, they are suitable to manufacture the UV photodetectors.(5) The design and simulation of cBN UV photodetectors. A parameter libraryof cBN single crystal and a model of a cBN MSM UV photodetector are established with the simulation package Atlas. The simulation results showed that under less than5V voltage, the UV detector with doping concentration of1013cm-3had highresponsibility. When the voltage was greater than5V, the photodetector with dopingconcentration of1014cm-3have higher responsibility. The device with3μm electrodewidth and10μm electrode spacing had the lowest dark current and the highest ratioof photocurrent to dark current. As for the devices with the same electrode width andspacing, the dark current reduced rapidly with the increase of the size of electrodes,but the photocurrent was almost the constant.Two kinds of MSM cBN UV photodetectors with interdigitated finger electrodeswere fabricated by evaporation, sputtered and lithography. One was thephotoconductive device with Au electrodes, and the other was photovoltaic devicewith Cr electrodes. The dark currents and UV photoresponses of the two devices weremeasured. The ratio of photocurrent to dark current the former was only2-3, as forthe latter, the ratio reached56, which was in accordance with the simulation. Becauseof the bad quality of cBN crystals, the performances of cBN UV photodetectors needto be improved further. |
PDF Full Text Request