MOCVD Growth Of ZnO Films And ZnO/Si Light-Emitting Devices

As an importantâ…¡-â…¥group compound semiconductors,ZnO has attracted much attention for short-wavelength optoelectronic device applications due to its wide band gap(E_g =3.37 eV at room temperature) and relatively large exciton binding energy(60 meV).It has broad prospects in application to solid state lighting,full color display and optical information storage,etc.However,high performance ZnO-based optoelectronic devices have not been achieved yet.One key problem is the lack of device-quality ZnO films,especially the stable and reproducible p-type ZnO films.Even though several groups have successfully developed ZnO-based light emitting diodes(LEDs),all the reported LEDs have low light-emitting efficiency up to now.Therefore,it is necessary to prepare high-quality n-type and reproducible p-type ZnO films simultaneously.In order to solve the above-mentioned problem,our research has been focused on preparing high-quality undoped and doped ZnO films and fabricating ZnO-based heterojunctions by metal-organic chemical vapor deposition(MOCVD) and photo-assisted MOCVD(PA-MOCVD) techniques.The MOCVD growth conditions of ZnO films have been optimized by means of orthogonal design.The experimental results demonstrate that the optimized growth conditions of ZnO films are 630℃in growth temperature,-18℃in Zn source temperature,and 200 sccm in flow rate of oxygen.Employing orthogonal design in the MOCVD growth of ZnO films not only reduce the experimental numbers and costs,but also can obtain the optimized growth conditions quickly and scientifically.This method is a good assistant for the growth.The effects of the growth temperature of ZnO buffer layer on the ZnO epitaxial layers have been studied.Both samples with and without buffer layer have good c-axis preferential orientation.However,with the increasing of ZnO buffer layer growth temperature,the intensity of(002) diffraction peak is obviously strengthened in X-ray diffraction pattern,and the stress caused by the lattice and thermal mismatch between the films and the substrates is almost completely released when the growth temperature is beyond 350℃.Additionally,the samples with ZnO buffer layer have better optical quality according to the intensity ratio of near band emission and deep level emission in PL spectra.It indicates that proper growth temperature of ZnO buffer layer can significantly improve the crystalline and optical quality of ZnO epitaxial layers.The optimal growth temperature of ZnO buffer layer is 450℃.ZnO films are grown by PA-MOCVD using tungsten-halogen lamp as a light source. The effects of light irradiation on the structural,surface morphology and optical properties of the deposited ZnO films have been investigated.Compared with the samples without irradiation,several characteristics of ZnO films with irradiation are improved,including an improvement in the crystallinity of c-axis orientation,an increase in the grain size and an improvement in optical quality of ZnO films.These results indicate that photo irradiation plays an important role in improving the quality of ZnO films prepared by PA-MOCVD. Furthermore,with the increasing of photo irradiated intensity,the resistivity of ZnO films gradually decreases.It is noted that high resistivity and unintentional p-type ZnO films are obtained when the photo irradiated voltage is not higher than 65 V.These results establish the foundation for our further preparation of N-doped p-type ZnO films and indicate that PA-MOCVD is a good method for the growth of high-quality ZnO films.NH₃-doped ZnO films are grown by MOCVD and PA-MOCVD techniques,respectively. N is incorporated into ZnO films during the as-grown process and bonding with the unintentional introduction elements(such as H and C),which passivate or compensate the NO (N occupied O position) acceptors.However,when proper photo irradiation is introduced during the growth,NO acceptors are activated and p-type N-doped ZnO films are obtained. The electrical properties of N-doped p-type ZnO films show hole concentration of 3.61×10¹⁷ cm^-3,mobility of 0.627 cm²/V·s,and resistivity of 27.5Ω·cm.Compared with other methods used to improve the activity of NO acceptors,introducing photo irradiation during the film growth process is simple and it is worth spreading.p-type ZnO films have been prepared by P atoms thermal diffusion from InP substrates. The typical electrical properties of these films show hole concentration of 9.02×10¹⁷ cm^-3, mobility of 1.05 cm²/V·s,and resistivity of 6.6Ω·cm.Temperature-dependent PL spectra and X-ray photoelectron spectra are performed to verify the role of P in these films.The results give direct experimental evidence that P_Zn-2V_Zn shallow acceptor complex most likely contribute to the p-type conductivity,of P-doped ZnO films.The acceptor binding energy is estimated to be～123 meV.This indicates that P is one of the good p-type dopants of ZnO films.In addition,diffusion doping technique is easy and available to prepare P-doped p-type ZnO films.ZnO/Si heterojunction LEDs are fabricated on both high resistivity(p^--) and low resistivity(p⁺-) Si substrates by MOCVD technique.Fairly good rectifications are observed from the current-voltage curves of the both heterojunctions.With increasing temperature,the turn-on voltage and reverse breakdown voltage of n-ZnO/p⁺-Si heterojunction are reduced; the ideal factor n also decreases with increasing temperature,when the voltage increases to a certain degree,the ideal factor essentially unchanged.According to the basic parameters of two semiconductor materials,the ideal energy band diagram of n-ZnO/p⁺-Si heterojunction has been described,and the calculated electronic potential barrier～1.046 eV approaches to the measured heterojunction turn-on voltage～1.25V.Ultraviolet(UV) and blue-white electroluminescence(EL) from ZnO layer are observed only from ZnO/p⁺-Si heterojunction under forward bias at room temperature(RT).Simultaneously,strong infrared(IR) EL emissions are detected from both ZnO/p^--Si and ZnO/p⁺-Si heterojunctions.The analytical results show that the blue-white EL is associated with ZnO deep-level related emissions and the IR EL comes from Si substrates related emissions.Among many ZnO-based heterojunctions,the n-ZnO/p-Si structure is especially attractive due to the well-known advantage of Si substrate(such as low cost,conductive and easy to cleave) and its potential application in Si-based optoelectronic integrated circuits(OEICs).Therefore,the realization of RT EL in UV-visible and IR region on Si substrates will greatly increase the potential application in Si-based OEICs.