Although there are numerous optical spectroscopy techniques and methods that have been used to extract the fundamental bandgap of a semiconductor,most of them belong to one of these three approaches:(1)the excitonic a...Although there are numerous optical spectroscopy techniques and methods that have been used to extract the fundamental bandgap of a semiconductor,most of them belong to one of these three approaches:(1)the excitonic absorption,(2)modulation spectroscopy,and(3)the most widely used Tauc-plot.The excitonic absorption is based on a many-particle theory,which is physically the most correct approach,but requires more stringent crystalline quality and appropriate sample preparation and experimental implementation.The Tauc-plot is based on a single-particle theo⁃ry that neglects the many-electron effects.Modulation spectroscopy analyzes the spectroscopy features in the derivative spectrum,typically,of the reflectance and transmission under an external perturbation.Empirically,the bandgap ener⁃gy derived from the three approaches follow the order of E_(ex)>E_(MS)>E_(TP),where three transition energies are from exci⁃tonic absorption,modulation spectroscopy,and Tauc-plot,respectively.In principle,defining E_(g) as the single-elec⁃tron bandgap,we expect E_(g)>E_(ex),thus,E_(g)>E_(TP).In the literature,E_(TP) is often interpreted as E_(g),which is conceptual⁃ly problematic.However,in many cases,because the excitonic peaks are not readily identifiable,the inconsistency be⁃tween E_(g) and E_(TP) becomes invisible.In this brief review,real world examples are used(1)to illustrate how excitonic absorption features depend sensitively on the sample and measurement conditions;(2)to demonstrate the differences between E_(ex),E_(MS),and E_(TP) when they can be extracted simultaneously for one sample;and(3)to show how the popular⁃ly adopted Tauc-plot could lead to misleading results.Finally,it is pointed out that if the excitonic absorption is not ob⁃servable,the modulation spectroscopy can often yield a more useful and reasonable bandgap than Tauc-plot.展开更多
Wurtzite structure gallium nitride GaN,a direct bandgap semiconductor,is an ideal material for fabrication of blue/green light emitting diodes,laser diodes, and high powder Integrated Circuites.Up to now,small single ...Wurtzite structure gallium nitride GaN,a direct bandgap semiconductor,is an ideal material for fabrication of blue/green light emitting diodes,laser diodes, and high powder Integrated Circuites.Up to now,small single crystals,powders and nanomaterials of GaN have successfully synthesized as well as applied films gro wn by MOCVD.In this report,another condensed state of GaN,nanocrystal assembled bulk,was synthesized and its spectra are investigated. Metal gallium or gallium alloys are used as starting materials and haloids used as catalyzer in ammonia.Buff transparent GaN bulks were obtained at 350—500℃.P owder X ray diffraction indicated that the bulks are wurtaite GaN single phase. HRTEM confirmed that the bulks are composed of nanoparticles with average size o f 12mm.The well crystallized particle shows clear diffraction spots.PL spectra of the material are similar to that of GaN singe crystals under the ultraviolet excitation,but blue shift is observed near gap band.Red shift occurs in Raman scattering comparing with single crystals.The materials have the broad potential in the future because it not only possesses of mechanical and optical characteri stics owned by single crystals but also holds the nano properties of the nanoma terials.展开更多
Silicon carbide (SiC) single crystal,which hasn’t melting point at normal pressu r e and sublimates at temperature above 2000℃,is a wide bandgap semiconductor.Si lic on carbide has more than 200 kinds of polytype.Am...Silicon carbide (SiC) single crystal,which hasn’t melting point at normal pressu r e and sublimates at temperature above 2000℃,is a wide bandgap semiconductor.Si lic on carbide has more than 200 kinds of polytype.Among these polytypes,3C SiC、6H SiC and 4H SiC are the most common ones,the band width of them are 2.4eV,3.0eV , an d 3.4eV,restpectively.For its high temperature tolerance and radiation resistanc e,silicon carbide semiconductor can be extensively used to fabricate the power d evi ces and electroluminescence devices operating at high power,high frequency and high radiation environments. The aim of this paper is to introduce our research results of the growth of larg e SiC single crystals by physical vapor transport method.The seed is SiC single crystal wafer with perfect (0001)Si face,which is chosen from the furnace growi ng the green abrasive material of SiC in industry.The source is green powder of SiC .The seed and the source are placed into the graphite crucible of a graphite res i stively heated vacuum furnace.The growth chamber is filled with the atmosphere o f pure araon.When the temperature of source rises to 2300℃,the crystal growth p ro ceeds.The rate of crystal growth is dependent on the growth temperature,the pres sure in furnace and the temperature gradient and distance between the seed and t h e source.Under the controlled growth conditions,the bulk SiC crystal with a diam eter of 40mm and a thickness of 15mm is obtained.The crystal appears to be n type electrical conductivity,the results of X ray Laue photography analysis indicat e that it is 6H SiC polytype.The defects of the crystal are also studied by many kinds of method.展开更多
The wide band gap characteristics of gallium oxide make it very suitable for the preparation of solar-blind ultra-violet photodetectors.The responsivity of ultraviolet photodetectors based on thin films is often low.H...The wide band gap characteristics of gallium oxide make it very suitable for the preparation of solar-blind ultra-violet photodetectors.The responsivity of ultraviolet photodetectors based on thin films is often low.However,nanomaterials have excellent photoelectric properties in device applications due to the high stability brought by high specific surface area and high crystal quality.Here,we successfully depositedβ-Ga_(2)O_(3) nanopores on the double-pass ordered porous AAO tem-plate by PLD.The porous AAO template is used as the growth space of nanomaterials,and the gallium oxide material is filled into the pores of the template to form a nanotube structure.By optimizing the preparation process,the relationship be-tween the performance of gallium oxide nanopores and the growth time was studied in depth.With the increase of growth time,the responsivity of the detector was improved.The rise timeτr=0.7 s,the decay timeτ_(d)=1.3 s,and the responsivity reached 4.63 mA·W^(-1),which was higher than 2.24 mA·W^(-1)of the responsivity of silicon-based gallium oxide nanorods.展开更多
基金Supported by Bissell Distinguished Professor Endowment Fund at UNC-Charlotte。
文摘Although there are numerous optical spectroscopy techniques and methods that have been used to extract the fundamental bandgap of a semiconductor,most of them belong to one of these three approaches:(1)the excitonic absorption,(2)modulation spectroscopy,and(3)the most widely used Tauc-plot.The excitonic absorption is based on a many-particle theory,which is physically the most correct approach,but requires more stringent crystalline quality and appropriate sample preparation and experimental implementation.The Tauc-plot is based on a single-particle theo⁃ry that neglects the many-electron effects.Modulation spectroscopy analyzes the spectroscopy features in the derivative spectrum,typically,of the reflectance and transmission under an external perturbation.Empirically,the bandgap ener⁃gy derived from the three approaches follow the order of E_(ex)>E_(MS)>E_(TP),where three transition energies are from exci⁃tonic absorption,modulation spectroscopy,and Tauc-plot,respectively.In principle,defining E_(g) as the single-elec⁃tron bandgap,we expect E_(g)>E_(ex),thus,E_(g)>E_(TP).In the literature,E_(TP) is often interpreted as E_(g),which is conceptual⁃ly problematic.However,in many cases,because the excitonic peaks are not readily identifiable,the inconsistency be⁃tween E_(g) and E_(TP) becomes invisible.In this brief review,real world examples are used(1)to illustrate how excitonic absorption features depend sensitively on the sample and measurement conditions;(2)to demonstrate the differences between E_(ex),E_(MS),and E_(TP) when they can be extracted simultaneously for one sample;and(3)to show how the popular⁃ly adopted Tauc-plot could lead to misleading results.Finally,it is pointed out that if the excitonic absorption is not ob⁃servable,the modulation spectroscopy can often yield a more useful and reasonable bandgap than Tauc-plot.
文摘Wurtzite structure gallium nitride GaN,a direct bandgap semiconductor,is an ideal material for fabrication of blue/green light emitting diodes,laser diodes, and high powder Integrated Circuites.Up to now,small single crystals,powders and nanomaterials of GaN have successfully synthesized as well as applied films gro wn by MOCVD.In this report,another condensed state of GaN,nanocrystal assembled bulk,was synthesized and its spectra are investigated. Metal gallium or gallium alloys are used as starting materials and haloids used as catalyzer in ammonia.Buff transparent GaN bulks were obtained at 350—500℃.P owder X ray diffraction indicated that the bulks are wurtaite GaN single phase. HRTEM confirmed that the bulks are composed of nanoparticles with average size o f 12mm.The well crystallized particle shows clear diffraction spots.PL spectra of the material are similar to that of GaN singe crystals under the ultraviolet excitation,but blue shift is observed near gap band.Red shift occurs in Raman scattering comparing with single crystals.The materials have the broad potential in the future because it not only possesses of mechanical and optical characteri stics owned by single crystals but also holds the nano properties of the nanoma terials.
文摘Silicon carbide (SiC) single crystal,which hasn’t melting point at normal pressu r e and sublimates at temperature above 2000℃,is a wide bandgap semiconductor.Si lic on carbide has more than 200 kinds of polytype.Among these polytypes,3C SiC、6H SiC and 4H SiC are the most common ones,the band width of them are 2.4eV,3.0eV , an d 3.4eV,restpectively.For its high temperature tolerance and radiation resistanc e,silicon carbide semiconductor can be extensively used to fabricate the power d evi ces and electroluminescence devices operating at high power,high frequency and high radiation environments. The aim of this paper is to introduce our research results of the growth of larg e SiC single crystals by physical vapor transport method.The seed is SiC single crystal wafer with perfect (0001)Si face,which is chosen from the furnace growi ng the green abrasive material of SiC in industry.The source is green powder of SiC .The seed and the source are placed into the graphite crucible of a graphite res i stively heated vacuum furnace.The growth chamber is filled with the atmosphere o f pure araon.When the temperature of source rises to 2300℃,the crystal growth p ro ceeds.The rate of crystal growth is dependent on the growth temperature,the pres sure in furnace and the temperature gradient and distance between the seed and t h e source.Under the controlled growth conditions,the bulk SiC crystal with a diam eter of 40mm and a thickness of 15mm is obtained.The crystal appears to be n type electrical conductivity,the results of X ray Laue photography analysis indicat e that it is 6H SiC polytype.The defects of the crystal are also studied by many kinds of method.
文摘The wide band gap characteristics of gallium oxide make it very suitable for the preparation of solar-blind ultra-violet photodetectors.The responsivity of ultraviolet photodetectors based on thin films is often low.However,nanomaterials have excellent photoelectric properties in device applications due to the high stability brought by high specific surface area and high crystal quality.Here,we successfully depositedβ-Ga_(2)O_(3) nanopores on the double-pass ordered porous AAO tem-plate by PLD.The porous AAO template is used as the growth space of nanomaterials,and the gallium oxide material is filled into the pores of the template to form a nanotube structure.By optimizing the preparation process,the relationship be-tween the performance of gallium oxide nanopores and the growth time was studied in depth.With the increase of growth time,the responsivity of the detector was improved.The rise timeτr=0.7 s,the decay timeτ_(d)=1.3 s,and the responsivity reached 4.63 mA·W^(-1),which was higher than 2.24 mA·W^(-1)of the responsivity of silicon-based gallium oxide nanorods.
