Magnetoelectric Transportation Measurement Systems And Transparent Conductive Oxide Semiconductor Properties

With the continuous reduction of the chip feature size, the traditional semiconductor technology is approaching the physical limit. People are trying to find new solutions for replacing the current silicon technology. Growing up techniques like wide band gap semi-conductors and spintronics have opened a new era for electronic devices. Wide band gap semiconductors are known as the third generation semiconductors, with huge market po-tential in the field of integrated circuits, electronic communication and photoelectronics. Compared to previous semiconductors, favorable characteristics such as wider band gap, higher breakdown voltage and larger saturated electron drift velocity make them more suitable for constructing productions like power devices, high frequency and high speed devices, microwave and millimeter wave devices, light emitting devices and high density integrated electronic devices. On the other hand, quantum effects can appear when the chip feature size approaches the de Broglie wavelength of electron. These effects may be harmful if haven’t been taken into considerations when designing electronic circuits. Conversely, new type of quantum and spintronic devices which can work with lower pow-er consumption and higher integration density may be built by taking advantage of these effects. It will greatly improve the performance of integrated circuit devices. Transparent conductive oxide (TCO) semiconductor is a typical branch of wide band gap semicon-ductors. It plays a significant role in the area of field effect transistor (FET), invisible electronic devices and flat panel displays, etc. However, a higher resistivity and lower transparency make p-type TCOs more elusive in contrast with traditional n-type TCOs. It impedes the development of TCO devices to a certain extent. Recently, delafossite-structured ABO2 oxides become a promising candidate for p-type TCOs due to the pi-oneer work on transparent CuAlO2 conductive film. Among them, CuCr1-xMgxO2 (C-CMO) has the highest conductivity. At the same time, the unique magnetic properties dominated by Cr spin characteristics brings a bright application prospect in the field of spintronic devices for CCMO. In this work, various physical properties have been char-acterized for CCMO samples. Several models have been established to explain the ex-perimental results. Based on these analyses, a new type of TCO heterojunction device has also been constructed with good performance, which is expected to accelerate the application of room temperature electronic devices for CCMO.In addition, with the development of semiconductor devices towards the direction of high frequency and low power consumption, the measurement of the characteristic sig-nals is becoming more challenging. Traditional technology has been difficult to meet the requirements of speed and precision for measurements. In particular, during the develop-ment process of the new type semiconductor devices, extremely environment with high temperature, high magnetic field or high frequency are frequently accompanied. How to obtain accurate parameters for devices under complex environments has become a chal-lenging key for the industry. In view of the technical challenges mentioned above, a trans-portation data acquisition and analysis system with accuracy, speed and stability has been developed, which plays an important role in promoting the research and development for new type of semiconductor devices.The main works and innovations of this dissertation are listed as follows:1. A transportation data acquisition and analysis system based on digital lock-in am-plification technology has been developed. An automatic measurement systems for multiple electrical properties of semiconductors under complex environment has also been developed.The main innovative points of the digital lock-in system are as follows:(a) The utilization of digital lock-in technique with independent program controllable signal to obtain the AC signals, and the introduction of phase markers to provide the reference signal for lock in amplifiers. The whole system is able to achieve good noise suppres-sion, and can be used to study the characteristics of the AC signal transportation with both accuracy and speed. (b) The measurement is carried out automatically by com-puter. The speed has been increased by several tens of times compared to traditional methods. Signals can be measured in real-time, making the results more intensive and accurate, (c) The system has favorable software structure, design patterns like state machine and producer-consumer are used for main programming body, making the structure more stable and scalable.The main innovative points of the automatic electrical properties measurement systems are as follows:(a) The system can measure the electrical parameters of vari-ous semiconductors under complex environment like deep low temperature or strong magnetic field. Final results can be automatically saved as a set of database after processed by background algorithm. (b) The measurement can be carried out auto-matically by computer, with good human-computer interaction interface and real-time display of the characteristic curves. Dynamic call technique has also been involved in the event structure to achieve good decoupling characteristics, which greatly enhance the stability of the system. (c) Shielding protections for the whole system are quite good. The system can provide accurate current as low as pA level and can measure the voltage as low as nV level. The transportation properties for samples with extremely low or high resistance can also be measured accurately. The program controlled pin switch technique during the Van der Pauw measurements enhance the stability and speed of the system. (d) The system can help to study transport characteristics for samples under complex environment with illumination. And the sample is complete-ly isolated from the light supply, so heating on samples from the light source can be neglected.2. The carrier dynamics for CCMO samples have been studied from the results of Terahertz spectrum and Hall measurement.The main innovative points of this research section are as follows:(a) The Tera-hertz reflection spectrum of the CCMO sample exhibits a characteristic peak at about 15 THz, and the center of this peak shows a red-shift trend with increasing tempera-ture. In the frequency region below 6 THz, the free carrier absorption effect becomes more prominent. (b) Hall coefficient shows a turning-point at about 220,206, and 194 K for the composition of x=0.02,0.06 and 0.10, respectively. The phenomena can be attributed to the transition of carrier transport mechanism from a thermal activation behavior to a variable range-hopping one. (c) We obtained various of electrical param-eters from the fitting and calculation of the Terahertz spectrum, which were quite in consistent with those from electrical characterization methods. It provides another ac-cess path to the electrical parameters in addition to traditional electrical measurement methods.3. By analyzing the optical, electrical and magnetic properties, the energy band structure, transition and vibration models for CCMO have been discussed in de-tail.The main innovative points of this research section are as follows:(a) It has been proved that the upper part of the valence band for CCMO has primarily the Cr 3d character with a minor contribution from Cu 3d. The spin-orbit interactions of Cr3+ ions in octahedral environment make the 3d states more disperse, which can contribute to raising the conductivity. (b) Strong exciton excitation around 1.8 eV has been ob-served due to the naturally low-dimensional structure of the delafossite, which can be modulated by temperature and hole concentration. (c) An abnormal dependence of optical band gap as well as the E9 mode center with the temperature has been found for lightly doped samples(x=0.02 and 0.04). At low temperature region, the gap en- ergy shows a red-shift trend with decreasing the temperature. It is due to the strong Cr-O-Cu interaction in the upper part of the valence band, which can be weakened by heavily Mg doping. Over-doping Mg not only disturb the local spin fluctuation at Cr sites, but also increase the Cu2+ defect states.(d) By studying the Terahertz spectrum and Magnetoresistance effect at extremely low temperatures, an energy level spacing diagram taking spin flip into account for CCMO has been established.(e) Two suc-cessive magnetic transitions have been observed at corresponding Neel temperature TN1 around 24.7 K and TN2 around 23.0 K, as manifested by magnetoresistance mea-surements. The increasing spin-charge coupling caused by out-of-plan structure below TN2 not only results in the enhancement of negative MR at strong magnetic field, but also brings back the abnormal red-shift below TN2 for heavily doped samples.4. CuCrO2-based heterojunctions with favorable electrical characteristics have been fabricated by combining p-type Mg-doped CuCrO2 and n-type Al-doped ZnO. By studying the Ⅰ-Ⅴ characteristic curves and electrical properties under magnetic field, the intrinsic transport mechanism and the prospects for room temperature application of the device have been discussed.The main innovative points of this research section are as follows:(a) CuCrO2-based heterojunctions with favorable electrical characteristics have been fabricated by combining p-type Mg-doped CuCrO2 and n-type Al-doped ZnO. (b) Positive magne-toresistance (MR) effect for the heterojunction can be observed at room temperature due to the tunneling-induced antiparallel spin polarization near the heterostructure in-terface. (c) It was found that the current for the heterojunction in low bias voltage re-gion is dominated by the trap-assisted tunneling mechanism. The MR effect becomes enhanced with the magnetic field, and shows the maximum at a bias voltage around 0.5 V. The phenomena indicate that the CuCrO2-based heterojunction is a promising candidate for low-power semiconductor spintronic devices.