Silicon-based ZnO(MgZnO) Films And Light-emitting Devices

In recent years,enormous research enthusiasm onâ…¡-â…¥group semiconductor ZnO has been greatly spurred due to that it has a direct band gap of 3.37 eV and a large exciton binding energy of 60 meV and,moreover,it has distinct advantages over GaN, such as lower material cost,higher radiation hardness,simpler processing of device owing to amenability to chemical wet etching and ease of film deposition.Currently, great efforts have been expended on development of ZnO-based light emitting devices (LEDs)in order that ZnO can be an alternative to GaN as the semiconductor for short-wavelength optoelectronics.Unfortunately,the strategy of p-type doping for ZnO has not been substantially defined,thus leading to considerable difficulties in achieving ZnO-based light emitting and laser diodes of p-n junction.In order to avoid the difficulties in p-type doping of ZnO,the heterojunctions and metal-insulatorsemiconductor (MIS)devices have been employed in attempt to realize the ultraviolet (UV)electroluminescence(EL)from ZnO.It is well known that silicon is the most important semiconductor as the base material for integrated circuits.However,silicon cannot be applied to LEDs due to its intrinsic indirect bandgap.Evidently,the EL from ZnO and its alloy films on silicon substrate will find applications in silicon-based optoelectronics.In this dissertation,the silicon-based ZnO materials and LEDs have been intensively addressed.Based on the fabrication of ZnO(MgZnO)/Si heterojunctions and silicon-based ZnO(MgZnO)MIS devices,the light emission characteristics and related mechanisms for these two kinds of LEDs have been systematically investigated.In the following,the primary achievements in this work are described.(1)Electroluminescent ZnO/Si and MgZnO/Si heterojunctions were fabricated by deposition ZnO and MgZnO films on silicon substrates of n- and p-type with heavy-and light-doping(i.e.,n⁺,n^-,p⁺ and p^-),respectively.The ZnO(MgZnO)/n⁺-Si heterojunction exhibited UV light characteristics of near-band-edge(NBE)emission from ZnO(MgZnO)and defect-related visible light under the forward bias but only visible light under reverse bias;while the ZnO(MgZnO)/n^--Si heterojunction only emitted relatively weak UV and visible light under the forward bias.On the other hand,the ZnO(MgZnO)/p⁺-Si heterojunction was electroluminescent in the visible region under the forward bias while in both the UV and visible regions under the reverse bias;and the ZnO(MgZnO)/p^--Si heterojunction did not exhibit detectable EL under either forward or reverse bias.Starting from the current-voltage characteristics and the energy-band diagrams under different biases for these heterojunctions,the mechanisms underlying the EL performances as mentioned above have been essentially elucidated.(2)With the silicon-based MIS devices wherein the ZnO or MgZnO film and SiO_x (x≤2)film acted as the semiconductor and insulator respectively,fairly pure UV light emission from ZnO or MgZnO film was achieved and,moreover,the blue-shift of UV light could be realized by adjustment of Mg content in MgZnO. It is definitely pointed out that an appropriately thick SiO_x film is critical for the fairly pure UV light emission,while,the confinement of carriers in the region near the SiO_x/ZnO(MgZnO)interface is the root cause for the enhancement of UV emission and the suppression of visible light.(3)With the silicon-based polycrystalline ZnO film MIS device,the electrically pumped ZnO film random lasing was well demonstrated.Along with the increase of forward bias on the MIS device,the EL of ZnO film transformed from spontaneous emission to random lasing.Furthermore,the random lasing at shorter wavelengths from MgZnO was also realized.The reason for the random lasing is as follows:the in-plane propagation of electroluminescent light within the near-surface region of ZnO film is inevitably subjected to the scattering by the ZnO grain structure,due to the short scattering mean-free path,a part of scattered light will return to the scatterer from which it has been scattered before, thus,forming closed-loop random cavities for the light.With sufficiently high forward bias on the MIS device,in some cavities,the optical gain of stimulated emission will be equal to and even larger than the optical loss,thus leading to random lasing.(4)The luminescence of N₂ microplasma was found in the case of the ZnO,MgZnO and Si based MIS device applied with high forward voltage.Through the in-depth analysis of electron transportation in the MIS device,it is pointed out that the N₂ microplasma was formed by the activation of air around the surface of MIS device by the highly energetic electrons emitted from the MIS device under the high voltage. (5)Taking advantage of the silicon-based ZnO(MgZnO)MIS device applied with different bias,the electric-field-controlled photoluminescence(PL),i.e., electro-photoluminescence of ZnO(MgZnO)was investigated.Compared with the PL of ZnO(MgZnO)film in the case where there was no bias on the MIS structure,the positive bias with negative voltage applied on silicon substrate significantly enhanced the NBE UV emission while suppressing the defect-related visible emissions.This is due to the following reasons:under the positive bias,the photo-generated electrons accumulated in the region near the SiO_x/ZnO(MgZnO)interface,which,on one hand,made the PL of ZnO (MgZnO)proceed in the near-surface region;on the other hand,enhanced the inter-band radiative recombination and significantly reduced the amount of defects involved in PL.In contrast,the negative bias on the MIS device hardly changes the PL of ZnO film.(6)Taking advantage of the silicon-based ZnO(MgZnO)MIS device on the silicon substrate applied with appropriate forward bias,the random lasing from ZnO (MgZnO)film was induced by the electric field under a constant He-Cd laser illumination.It was found that with sufficiently high forward bias on the MIS device,random lasing occurred at certain wavelengths of photoluminescent UV light of ZnO(MgZnO)film.Such a random lasing action is due to that the strong accumulation of photo-generated electrons in the near-surface region of ZnO (MgZnO)film under the high forward bias significantly enhances the PL therein, in this case,a part of sufficiently strong photoluminescent light propagating in the plane of ZnO(MgZnO)film achieves optical gain and oscillates in the closed-loop random cavity,thus leading to random lasing.