The Investigation Grain Boundaries Behavior And Carriers Transport Mechanism In ZnO/Si Heterostructure

The use of thin-film semiconducters deposited on a low cost substrate has been regarded as a promising approach toward the mass production of low-cost photovoltaic devices. Zinc oxide (ZnO) is wide band gap semiconductor (-3.37 eV at room temperature) with various applications. Recently, it was considered to be a new photo-electric material in shortwave length as GaN. In1996, stimulated UV emission of ZnO at room temperature was reported, and ZnO as a UV laser material attracted people's attention immediately.The native ZnO is an insulator if it is satisfied stoichiometry. However, the undoped ZnO shows typical n-type conductivity, which is attributed to the structural defects as oxygen vacancy. Recently, pointed this may be related with Hi. The n-type ZnO is easily available while it has been proved that it is very difficult to obtain p-type ZnO, so practical device applications such as LDs and LEDs using ZnO are still experiencing inherent problems. The hetero-epitaxy of ZnO film is seemed to be another way to further realize the ZnO-based devices, and soon quickly become research hotspots.ZnO can be deposition on different substrates, such as GaN, SiC, GaAs and so on. Among these different substrates, Si is nice substrates because of its low cost, easily fabricated electrodes and hold promise to be integrated with ZnO-based devices.ZnO films depositing on a silicon substrate usually form polycrystalline structure due to a large lattice mismatch, so grain boundary has a great influence on carrier transport. When ZnO grains bond with Si layer, the crystal boundary between them can be regarded as intergranular boundary, then a complex sandwich structure in the form of Si-intergranular layer-ZnO grains are formed. The crystallization of ZnO films will be improved after annealing, but the dependence of electrical properties upon grain boundaries in ZnO/Si heterostructure annealing has not yet been well established. Understanding the electrical properties of such structures is clearly a very important ingredient in developing higher efficiency heterojunctions photoelectric devicesIn chapter one, firstly, a comprehensive physical and chemical properties review of ZnO was given; secondly, the methods of preparation ZnO films were briefly introdued, incluing sputtering, pulser laser deposition, chemical vapor deposition, et al. Finally, a present research progress of ZnO/Si heterojunction characteristics is illuminated, indicting the core of the thesis.In chapter two, a detail of samples preparations conditions were given, and then the methods of morphology characterization of ZnO polycrystalline films were briefly introduced, such as atomic force microscope (AFM), X-Ray Diffraction and ellipsometry measurement. Finally, the principles of electrical characterization of ZnO/Si heterojuction were described and deduce some useful expressions.In chapter three, the principle of the deep level transient spectroscopy (DLTS) and thermally stimulated current (TSC) was introduced, and some basic formula derivation that related with our research work were given.In chapter four, ZnO films are deposited on p-type Si substrates by MOCVD with different total gas velocity. The current versus voltage and temperature (I-V-T) characteristics, the DLTS and the photoluminescence (PL) spectra of the samples are measured. DLTS show us two deep level centers of E1 (EC-0.13±0.02 eV) and E2 (EC-0.43±0.05 eV) in sample 1202a. The energy gap?Eb (0.13±0.01eV) between the donor energy levels and conductance band is obtained from the I-T characteristics as well, and it is considered to be the same one as E1, which is related to the band edge emission at about 382nm obtained by PL spectra of sample 1202a. The energy location and the relative of trap densities of E1 will be different while the total gas velocity is changed. The deep level behavior of both E1 and E2 level is investigated.The origination of E1 has a relationship with the interstitial zinc (Zni) in ZnO, we consider that E1 is a luminescence centre of D0h from localization state energy level to valence band. The relative trap density of E1 increasing resulted in mutual renormalization of the band result energy position of E1 has a slightly offset. It is reveal that deep level of E2 has characteristic of recombination center. The E2 level also has an influence on the free exciton emission intensity.In chapter five, ZnO films are deposited on p-type Si substrates by Sol-Gel, and the annealing at O2-600?and O2-900?for 1 hour, respectively.The grain boundary layer behavior in ZnO/Si heterostucture is investigated. The I-V, DLTS and capacitance-voltage (C-V) curves are measured. The interesting phenomenon that the crossing of In I-V curves of ZnO/Si heteroj unction at the various measurement temperatures and the height of its effective barriers decreasing with the decrement of temperature are in contradiction with the ideal heteroj unction thermal emission model is observed.The crossing of In I-V curves and negative temperature characteristics of forward transport currents after O2-600?annealing can be explained using the behaviors of grain boundary layer. The investigation shows that the n type carriers in ZnO film are origination from Zni**.After high temperature annealing at O2 ambience, the microstructures of ZnO/Si heterostructure are decorated and stoichiometric deviation improved, the influence of interfacial states on carriers transport process of ZnO/Si heterostructure is weakened. So the transport currents of ZnO/Si heterojunction are dominated by grain boundary layer as high densities of interfacial states existed.In chapter, six, heteroepitaxial undoped ZnO films were grown on Si (100) substrates by radio-frequency (RF) reactive sputtering, and then some of samples were annealed at N2-800?(Sample 1, S1) and O2-800?(Sample2, S2) for 1 hour, respectively. The characterization of carriers transport in ZnO/p-Si heterostructure and its temperature coefficients of grain boundary resistances are investigated using I-V and DLTS measurements. Two phenomenon were observed. First, the temperature coefficients of grain boundary resistances of S1 were positive (positive temperature coefficients, PTC) while that of both the as-grown sample and S2 were negative (negative temperature coefficients, NTC). Second, the I-V properties of S2 were similar to those common p-n junctions while that of both the as-grown sample and S1 had double schottky barrier behaviors which were in contradiction with the ideal p-n heterojunction model. Combined with DLTS results, the variations of temperature coefficients of grain boundaries resistances can be explained using the change of grain boundaries behavior and the densities of interfacial states. It revealed that the concentrations of intrinsic defects in ZnO grains and the densities of interfacial states in ZnO/p-Si heterojunction varied with the different annealing ambiences, which caused the grain boundary barriers in ZnO/p-Si heterojunction varied. After O2-800?annealing, the lattice mismatch and stoichiometric deviation of ZnO grain are well improved which results the double-Schottky behavior vanished and NTC resistances appears. This resulted adjustment electrical properties of ZnO/p-Si heterojunction that may be suitable in various applications.In chapter seven, ZnO/n-Si isotype heteroj unctions were fabricated using pulse laser deposition (PLD) technique. ZnO films deposited on Si substrates are usually formed polycrystalline structure, and then barrier structure formed at the ZnO polycrystalline grain boundaries. The grains boundary has a great influence on photoelectricity of Zinc Oxide film. For research the influence of grain boundaries, the films annealed in O2 at 500?,600?,700?and 800?, respectively. The optic and electricity characteristics of ZnO films were studied by I-T, C-V, X-ray diffraction (XRD) and ellipsometry.The experiment results show the electron traps in ZnO grain boundaries are filled with electrons which provided by high concentration deep level impurities (Zni), and deep level center located in ZnO forbidden gap ET=EC -0.247±0.08eV When annealed temperature increased to 700?, got neutral donor deep level center (D0) in sample, located at ET=EC -0.134±0.03eV. It can be speculated that D0 is complex defect formed by high temperature oxygen treatment, and had a great influence on optic and electricity characteristics of films. The oxygen was the main reason improved the films microstructure, and resulted photoelectricity of Zinc Oxide film changed.