Preparation And Properties Of ZnCdO Thin Films And ZnCdO/ZnO Heterojunctions

Zinc oxide(ZnO)is a direct wide bandgap(?3.37 eV)semiconductor with a large exciton binding energy(?60 meV)at room temperature.Due to the exciton binding energy,which is higher than that in GaAs(?10 meV),ZnO has a great application prospects in photoelectron semiconductor,such as light-emitting diodes(LEDs),laser diodes(LDs)and UV detectorsetc.The bandgap of ZnO can be adjusted by doping Mg,Cd etc,which is important to achieve the luminescence from ultraviolet to green light spectra.Base on the theory of bandgap engineering and the property of ZnO material,heterojunction and superlattice can be fabricatedwhich have a strong carrier confinement.Further,the special band luminescencecan be realized by the ZnO optoelectronic device.At present,the research of ZnCdO/ZnO heterojunction focus on the calculation of band offset with a single-concentration.There have been,however,few researches about the dependence of heterojunction band offset with Cd concentration.As we know,the heterojunction band offset parameter has very important reference value to the design of band structure in bandgap engineering.In order to adjust the band offset of heterojunction and abtain the band offset parameter,the energy band structure must be controlled,which is closely related to the Cd concentration.Therefore,preparing epitaxy Zn1-xCdxO films with different Cd concentration,obtaining the accurate bandgap and researching the independence of heterojunction band offset with Cd concentration provide a important refrence value for the design of Zn1-xCdxO heterojunction.The followingwork has been carried out:(1)The high-quality ceramic ZnCdO targets with different Cd concentration have been baked by solid state reaction method,successfully.A series of Zn1-xCdxO films and Zn1-xCdxO/ZnO heterojunctions have been fabricated on Al2O3 substrate by laser molecule beam epitaxy(L-MBE).The growth rate(?0.05 nm/s)of film has been obtained.We find out that the growth rate will decline when the thickness exceed 100 nm.The molar ratios of Cd and Zn in Zn1-xCdxO films(x=5.0%,6.6%,7.5%,8.2%and 10.0%)have been calculated by X-ray photoelectron spectrum(XPS).(2)The structural property of films have been investigated by X-ray diffraction(XRD),the result indicates that the films have a high single-orientation of c-axis without any impurity phase.With the Cd concentration increasing,the full width at half maximum(FWHM)of ZnCdO(0002)increases from 0.248° to 0.411°,which indicates that the crystal quality of the films decrease with the Cd concentration increasing.At the same time the 20of ZnCdO(0002)characteristic peak decline,which illustrates that the lattice constant increaseswith the Cd concentration increasing.We calculate the absorption coefficient(a)of Zn1-xCdxO films by UV-vis transmittance(T)and reflectance(R),the optical bandgap is obtained by(ahv)Z-hv curve.The result indicates that the band gap of Zn1-xCdxO films can be adjusted by tuning the Cd concentration.(3)In order to measure the band offset of the Zn1-xCdxO/ZnO heterojunctions,XPS has been carried out.The result indicates that both of the conduction band minimum(CBM)and valence band maximum(VBM)are on the ZnCdO side,which illustrates that the heterojunction belongs to type-I alignment.The band offset increases with the Cd concentration increasing.The regulation mechanism of band offset with Cd concentration has been analyzed by first-principles theory.With the Cd concentration increasing,the VBM raise due to the enhancing of p-d repulsion effect of Zn 3d and Cd 4d with O 2p.At the same time,the CBM fall down due to the enhancing of the hybrid effect of Cd 5s and Zn 4s.Therefore,the conduction(valence)band offset will increases with the Cd concentration increasing in the films.At the same time,the experiment result shows that the increasing degreed of conduction band offset is smaller than that of the valence band offset.The carrier concentration>5.7×1017/cm3)has been measured by hall effect testing instrument,the result illustrates that the ZnCdO film belongs to n-type semiconductor.The fermi level is close to conduction band.Therefore,the increasing degreed of conduction band offset is smaller than that of the valence band offset with the Cd concentration increasing in Zn1-xCdxO/ZnO heterojunctions.