Improved field-emission characteristics
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Improvedfield-emission characteristics of GaN by BN coating Chiharu Kimura,Tomohide Yamamoto,and Takashi Sugino a)Department of Electrical Engineering,Osaka University,2-1Yamadaoka,Suita,Osaka565-0871,Japan͑Received3April2002;accepted12August2002;published4February2003͒Using coating with a boron nitride͑BN͒film,we attempted to improvefield-emission characteristics of gallium nitride͑GaN͒cold cathodes.First,we measured thefield-emission characteristics of BN/n-Si samples to investigate the electron-emission mechanism of the BNfilm.We discuss the electron-emission process of the BNfilm in terms of the surface roughness dependence of the field-emission characteristics.We suggest that the coating with a BNfilm thinner than10nm is effective in reducing the turn-on voltage of the electron emission.Second,field-emission characteristics are examined for the hexagonal n-type GaN layers roughened with H2plasma treatment.Moreover,nanocoating with a BNfilm is carried out on the surface of the GaN sample for the BN/GaN sample.We achieved a turn-on electricfield as low as4.6V/m.©2003 American Vacuum Society.͓DOI:10.1116/1.1516185͔I.INTRODUCTIONDevelopment of cold cathodes with high performancesuch as a high-emission current operation at a low voltage isstrongly desired for application to vacuum microelectronicsandfield-emission displays.III-nitride materials such as alu-minum nitride͑AlN͒,boron nitride͑BN͒,and gallium nitride ͑GaN͒are believed to be some of the most promising mate-rials forfield emitters.This is because,in addition to prop-erties such as chemical inertness and mechanical hardness,the negative electron affinity appears on the surface of AlN ͑Ref.1͒and BN͑Refs.2and3͒,and the pyramid shape of GaN can be formed using the selective growth of hexagonal GaN by metalorganic chemical-vapor deposition ͑MOCVD͒.4–7Concerningfield emission from BNfilms, there have been several reports.8–14A low turn-on electric field has been reported for the BNfilm synthesized by laser ablation.8A significant reduction in the turn-on electricfield has been demonstrated for the Si tipfield-emitter array coated with the BNfilm by plasma-assisted chemical-vapor deposition͑PACVD͒.15The coating technique with BNfilms is effective in improving thefield-emission characteristics of cold cathodes.For a practical application,it is important to understand thefield-emission mechanism of the BNfilm.This article describes the electron-emission process ofBN/n-Si samples.We suggest that coating with nanosizedthin BNfilm is effective in reducing the turn-on voltage ofthe electron emission.Finally,the nanocoating with BNfilmis carried out on the GaN surface treated with hydrogen(H2)plasma.II.EXPERIMENTBNfilms were deposited on Si and GaN substrates byPACVD using N2and BCl3as source materials.The growthapparatus has been reported previously.16The growth tem-perature was650°C.N2plasma was produced by supplyingrf power of40W to a turn coil installed around the reactor.dc negative bias of200V was applied to the substrate.Hexagonal n-type GaN layers grown on sapphire sub-strates by MOCVD were used in the present experiment. AlN layers were used as a buffer layer.GaN layers were grown at1100°C.The n-type GaN layer was doped with Si using SiH4gas.The GaN thickness was about2.0m.The electron density of the n-type GaN layer was estimated to be 2ϫ1017cmϪ3at room temperature by the Hall measure-ment.The GaN surface was treated with H2plasma.The H2gas flow rate,the gas pressure,and the microwave power were regulated at50sccm,40Torr,and300W,respectively.The treatment time wasfixed at5min.The surface morphology was observed for the BN and GaN samples by atomic-force microscopy͑AFM͒.Field-emission characteristics were measured in the chamber evacuated to3–7ϫ10Ϫ7Torr.The spacing be-tween the sample surface and the anode electrode wasfixed at125m.A Si wafer with3mmϫ3mm was used as an anode electrode.A series resistance of10M⍀was connected to the sample to protect the measurement equipment.The cathode electrode was attached to the Si rear face and on the GaN samples with Ag epoxy.III.RESULTS AND DISCUSSIONPrior to characterizing GaNfield emitters coated with BN films,we investigated thefield emission for BNfilms depos-ited on Si substrates.Figure1showsfield-emission charac-teristics measured for BNfilms with variousfilm thick-nesses.The measurement limit of the emission current was about1ϫ10Ϫ12A in the present experiment.The anode volt-age at the emission current of1ϫ10Ϫ11A is designated as a turn-on voltage.The turn-on voltages are estimated to be 610,970,1900,and1040V for the BN samples,Nos.1–4 with thicknesses of70,20,70,and10nm,respectively.No significant thickness dependence of the turn-on voltage is found for the BNfilms.The Fowler–Nordheim͑FN͒plots of thefield-emission characteristics are also indicated in thea͒Electronic mail:sugino@pwr.eng.osaka-u.ac.jp544544 J.Vac.Sci.Technol.B21…1…,JanÕFeb20031071-1023Õ2003Õ21…1…Õ544Õ4Õ$19.00©2003American Vacuum Societyinset of Fig.1.The FN plots fell on a straight line,suggest-ing the presence of the FN tunneling in the electron-emission process.Figures 2͑a ͒and 2͑b ͒depict the surface morphologies and roughness profiles of the two BN samples,Nos.2and 4,the turn-on voltages of which are almost the same.We found a significant change in the surface roughness between the two samples.It should be noted here that the roughness nonuni-formity was observed on the surface.The surface roughness was estimated as follows.17,18Three parts of unevenness la-beled as r 1,r 2,and r 3are chosen from the largest in the roughness profile.After the estimation just mentioned is car-ried out for the roughness profiles of the three different parts in the AFM image of 1m ϫ1m,the roughness is defined here as the average of nine values.The range of nine values is indicated with an error bar.The turn-on voltage is plotted as a function of the surface roughness in Fig.3.We consider that the turn-on voltage also depends on the emission site density.However,even though the emission site density due to a change of the roughness profiles is taken into account,we find that for the sample Nos.1–3,the turn-on voltage depends on the surface rough-ness.However,in spite of the smooth surface as shown in Fig.2͑b ͒,the turn-on voltage as low as 1040V is obtained for the sample No.4.Sample No.4does not belong to the group of sample Nos.1–3.The surface roughness depen-dence of the turn-on voltage is shown for the sample Nos.1–3to be thicker than 20nm.However,it may be suggested that the BN film which is as thin as 10nm makes it possible to lower the effective potential barrier height through which FN tunneling occurs.Various characteristics of the BN film are needed to dis-cuss the field-emission mechanism of the BN/n -Si sample.Properties of the BN films are summarized in Table I.IntheF IG .1.Field-emission characteristics of BN/n -Si samples.The inset shows FNplots.F IG .2.Surface morphologies and roughness profiles of ͑a ͒BN/n -Si sample No.2and ͑b ͒BN/n -Si sample No.4.F IG .3.Surface roughness dependence in relation to turn-on voltage.T ABLE I.Properties of the BN films.Characteristic Measurement Reference No.Crystal structure Hexagonal polycrystalline16Band gap6.0eV 16Electron affinityNegative 3Fermi level (E F ϪE V ) 2.8eV 3Resistivity106⍀cm 19Conduction typep -type19JVST B -Microelectronics and Nanometer Structurescase of the thick BN film,the energy-band diagram of the BN/Si sample is illustrated in Fig.4͑a ͒.In order to discuss the electron-emission mechanism,three processes are con-sidered,as shown in Fig.4͑a ͒.Because the field-emission characteristics are dominated by the FN tunneling process,process I is ruled out.In the case of process II where electrons are emitted due to FN tunneling through the potential barrier higher than 3eV ,it is necessary to enhance the electric-field strength at the BN/Si interface.However,there exists no large field-enhancement factor at the smooth Si surface.Moreover,because the BN film has p -type conduction,the band bending near the cath-ode substrate due to the positive space charge is not ex-pected.The electron emission is probably caused by process III consisting of the hopping conduction in the BN film and FN tunneling from the defect levels to the vacuum.It is reason-able to consider the existence of defects in nanocrystalline BN films.Defect states densely distributed near the Fermi level in the BN film make it possible to transport electrons in the conduction band of the Si to the BN surface due to the hopping conduction.When the defect level is located below the vacuum-energy level of the BN film,electrons in the defect level are emitted due to FN tunneling through the surface-potential barrier.An increase of the field-enhancement factor due to the surface roughness leads to a reduction in the turn-on voltage.Therefore,process III is supported by the experimental data shown in Fig.3.When the BN film is thinner than 10nm so that electrons in the conduction band of the n -Si tunnel through the BN film,the electrons at the BN surface are emitted due to FN tunneling through the surface-potential barrier,as illustrated in Fig.4͑b ͒.An application of the positive bias to the anode electrode makes it possible to lower the effective surface-potential barrier.Therefore,in comparison with the field-emission characteristics due to process III,a significant re-duction in the turn-on voltage is expected.When an increase of the field-enhancement factor at the BN surface or at the BN/Si interface is added,further reduction in the turn-on voltage may be achieved.In order to elucidate the electron-emission mechanism for the BN film as thin as 10nm,itmayF IG .4.Energy-band diagram of BN/n -Si structures with ͑a ͒a thick BN film and ͑b ͒a thin BNfilm.F IG .5.Field-emission characteristics of as-grown,roughened GaN,and BN/GaNsamples.F IG .6.Surface roughness profiles of ͑a ͒as-grown GaN,͑b ͒roughened GaN,and ͑c ͒BN/GaN samples.J.Vac.Sci.Technol.B,Vol.21,No.1,Jan ÕFeb 2003be necessary to discuss the possible ballistic transport in the BNfilm.However,field-emission characteristics obtained here are dominated by FN tunneling.The electrical transport of the thin BNfilm will be investigated in future work.The coating with a nanosized thin BNfilm was applied to improve thefield-emission characteristics of GaN.Sample A was an as-grown GaN layer on the sapphire substrate. Sample B was a GaN layer roughened due to H2plasma treatment.17,18Sample C was a GaN layer coated with BN film as thin as10nm.Figure5showsfield-emission charac-teristics of the three samples.No significant electron emis-sion was detected at an anode voltage as high as5000V for the as-grown sample A with a smooth surface.For sample B prepared with the H2plasma treatment,the turn-on electric field is estimated to be12.4V/m.In contrast to these two samples A and B,a turn-on electricfield as low as4.6V/m is achieved for the BN/GaN sample C.Surface roughness profiles of samples A,B,and C are indicated in Fig.6.The surface roughness was estimated to be5.3–10.6nm and0.5–0.6nm from the roughness profiles of the samples B and C.Though the roughness of sample C was smaller than sample B,a significant effect of the nano-coating with BNfilm was demonstrated.IV.CONCLUSIONSField-emission characteristics were investigated for the BNfilms deposited on n-Si substrates.In the case of the BN film thicker than20nm,we suggest that the electron-emission process consists of the hopping conduction in the BNfilm and FN tunneling at the BN surface.However,when the BNfilm as thin as10nm is deposited,electrons tunnel through the BNfilm,and the effective potential-barrier height is lowered at the BN surface.Therefore,further reduc-tion in the turn-on voltage is expected.Field-emission char-acteristics of the GaN sample nanocoated with a BNfilm are compared with those of the GaN sample roughened due to H2plasma treatment.We estimate a turn-on electricfield as low as4.6V/m for the BN/GaN sample.ACKNOWLEDGMENTSThe authors are indebted to M.Yuri,M.Ishida,and M. 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