Lian Gui-Jun

GaN-Based Thin Film Vertical Structure Light Emitting Diodes Fabricated by a Modified Laser Lift-off Process and Transferred to Cu

GaN-based thin film vertical structure light-emitting diodes (VS-LEDs) were fabricated by a modified YAG laser lift-off (LLO) process and transferred to Cu substrates. With a comparison of the electrical and optical properties of conventional LEDs on sapphire substrates and of lateral structure thin film LEDs by a KrF LLO process, the vertical structure of LLO LEDs shows obvious superiority. LLO VSLEDs made by modified YAG LLO process show less increase of leakage current than the devices made by conventional KrF LLO process. Furthermore, owing to the well current spreading and less current path, the ideality factors and series resistance of vertical structure LEDs reduce greatly and the efficiency increases more obviously than the lateral structure LEDs, which is also reflected on the relative L-I curves. The output power of vertical structure LEDs is over 3 times greater than that of the lateral structure LLO LEDs within 300mA.

Difference of Oxide Hetero-structure Junctions with Semiconductor Electronic Devices

Charge carrier injection performed in Pr0.7Ca0.3MnO3 (PCMO) hetero-structure junctions exhibits stable without electric fields and dramatic changes in both resistances and interface barriers, which are entirely different from behaviors of semiconductor devices. Disappearance and reversion of interface barriers suggest that the adjustable resistance switching of such hetero-structure oxide devices should associate with motion of charge carriers across interfaces. The results suggested that injected carriers should be still staying in devices and resulted in changes in properties, which guided to a carrier self-trapping and releasing picture in strongly correlated electronic framework. Observations in PCMO and oxygen deficient CeO2 devices show that oxides as functional materials could be used in microelectronics with some novel properties, in which interface is very important.Charge carrier injection is performed in Pr0.7Ca0.3MnO3 (PCMO) hetero-structure junctions, exhibiting the stability without electric fields and dramatic changes in both resistance and interface barriers, which are entirely different from behaviour of semiconductor devices. The disappearance and reversion of interface barriers suggest that the adjustable resistance switching of such hetero-structure oxide devices should associate with motion of charge carriers across interfaces. The results suggest that injected carriers should be still staying in devices and result in changes of properties, which lead to a carrier self-trapping and releasing picture in a strongly correlated electronic framework. Observations in PCMO and oxygen deficient CeO2-δ devices show that oxides as functional materials could be used in microelectronics with some novel properties, in which the interface is very important.

Relationship of Polaron Exchange with Ferromagnetic and Insulator--Metal Transitions in Doped Manganites

We report the experiment results and data analyses based on a polaron exchange model for La0.7 Ca0.3 MnO3 and Pr0.7 (Sr1-x Cax)0.3MnO3 epitaxial thin films. In the polaron exchange model with an energy balance condition, critical temperature of TC for stable ferromagnetic (FM) ordering should depend on δE as kBTC = E0 exp(- δE/kbTc), where δE denotes the potential barrier for the exchange polarons to overcome. Using the small polaron hopping model, the resistivity peak temperature TP is a function of the hopping energy Ehop. The dependence of TP on Ehop is similar to the dependence of TC on δE, which reveals that the polaron exchange relates to FM and insulator–metal transitions. The result indicates that the polaron exchange model is a simple way for describing the FM ordering, and is very helpful for understanding of complex doped manganites.

Fantastic Behaviour of High-TC Superconductor Junctions: Tunable Superconductivity

Carrier injection performed in oxygen-deficient YBa2Cu3O7(YBCO) hetero-structure junctions exhibited tunable resistance that was entirely different with behaviors of semiconductor devices. Tunable superconductivity in YBCO junctions, increasing over 20 K in transition temperature, has achieved by using electric processes. To our knowledge, this is the first observation that intrinsic property of high TC superconductors superconductivity can be adjusted as tunable functional parameters of devices. The fantastic phenomenon caused by carrier injection was discussed based on a proposed charge carrier self-trapping model and BCS theory.

Resistance Switching Characteristic and Charge Carrier Self-Trapping in Epitaxial Pr0.7(Ca1–xSrx)0.3MnO3 Thin Films

Carrier injection performed in Pr0.7Ca0.3MnO3 junctions demonstrate resistance switching (RS) characteristic with dramatic changes in both resistances and interface barriers, which suggests a charge carrier self-trapping model in strongly correlated electronic framework. Un-stable RS behaviour without electric fields in epitaxial Pr0.7(Ca1–xSrx)0.3MnO3 (PCSMO) films shows dependences on insulator–metal transition temperature, which indicates that RS process is really related to the intrinsic property of carriers. The switched resistance of epitaxial PCSMO films also depends on the amount of current pulses, which should be another evidence of the carrier self-trapping model, similarly to the dependence on the amount of self-trapped charge carriers.

Growth of n-type ZnO thin films by using mixture gas of hydrogen and argon

High-quality oxide semiconductor ZnO thin films were prepared on single-crystal sapphire and LaAlO3 substrates by pulsed laser deposition (PLD) in the mixture gas of hydrogen and argon. Low resistivity n-type ZnO thin films with smoother surface were achieved by deposition at 600°C in 1Pa of the mixture gas. In addition, ferromagnetism was observed in Co-doped ZnO thin films and rectification I−V curves were found in p-GaN/n-ZnO and p-CdTe/n-ZnO heterostructure junctions. The results indicated that using mixture gas of hydrogen and argon in PLD technique was a flexible method for depositing high-quality n-type oxide semiconductor films, especially for the multilayer thin film devices.

Different interdiffusion characteristics between Ag/YBa2Cu3O7−x and Al/YBa2Cu3O7−x contact interfaces

Abstract The differences between the interdiffusion characteristics of Ag YBa 2 Cu 3 O 7−x and Al YBa 2 Cu 3 O 7−x contact interfaces have been revealed by secondary ion mass spectrometry (SIMS). The different electrical properties of Ag YBa 2 Cu 3 O 7−x and YBa2Cu3O7−x films after high temperature treatment are well understood by the SIMS results.

Non-ferromagnetic Insulating Interface Region in La0.14Pr0.56Sr0.3MnO3/SrTiO3 and La0.7Ca0.3MnO3/Gd0.7Ca0.3MnO3 Multilayers

La0.14Pr0.56Sr0.3MnO3/SrTiO3 and La0.7Ca0.3MnO3/Gd0.7Ca0.3MnO3 multilayers have been fabricated by using pulsed laser deposition. The insulator–metal transition temperatures of the multilayers decrease with the decreasing thickness of the ferromagnetic layers. It is confirmed from the experiment and analysis that a non-ferromagnetic insulating region exists near the interface, which could not be attributed to the roughness of the growth surfaces and disorder of the ferromagnetic layers.

Magnetoresistance, special heat and ac magnetic susceptibility in La1-xCaxMnO3 system

We have measured the resistivity, special heat and AC magnetic suceptibility versus temperature in La1-xCaxMnO3(LCMO) bulk samples with x=0.3, 0.4, 0.5, 0.6. In the LCMO (x=0.3 and 0.4) samples a transition from dρ/dT0 to dρ/dT0 was observed, accompanied by a peak in specific heat. In AC magnetic suceptibility measurement, a ferromagnetic and an antiferromagnetic transitions were observed in LCMO (x=0.5) sample.

YBaCuO BICRYSTAL JUNCTIONS AND DC SUPERCONDUCTING QUANTUM INTERFERENCE DEVICES

We have prepared yttria-stabilized-zirconia bicrystal substrates using a simple hot-pressing method. The grain-boundary junctions have been fabricated with YBa2Cu3O7 thin films grown epitaxially on the bicrystals. The patterns are defined by conventional photolithography. The dc and microwave characteristics of the junctions and the dc superconducting quantum interference devices (SQUIDs) have been intensively studied. The current-voltage curves are bridge-typed with noise rounding near the critical current. Resistive tail has been observed in the resistance versus temperature curves. The results are compared with the theoretical prediction for classical Josephson junctions. It is found that the behavior of bicrystal junctions can be described in the frame of classical theory. The deviations are attributed to the nonuniformity of the junctions. The small loop dc SQUIDs demonstrate diffraction and interference effects with regard to the applied magnetic field. A large square-washer with a new configuration has been designed to enhance the effective area of dc SQUID as a magnetometer. It has achieved a magnetic field resolution down to 1 pT/Hz(at 10 Hz) at 77 K.