Microstructures And Properties Of Semiconductor Materials With Pulsed Laser Deposition (PLD)

Gallium nitride (GaN) and zinc oxide (ZnO) are the hot subjects in opto-electronicsemiconductors in recent years. GaN is a nitride semiconductor material with high meltpoint, strong rigidity, large direct energy band gap and chemically stable properties inair. It has a wide applied potential and a good market prospect at light emitting diodes(LEDs), blue light laser and ultraviolet (UV) detector. ZnO is also a wide band-gapsemiconductor material. ZnO thin films have many good properties such as transparentelectrical conductivity, piezoelecteicity, photoelectric, gas sensitivity and pressessensitivity. It can be integrated with many kinds of semiconductor materials.In this research, we get the GaN thin films and ZnO thin films with high crystalquality by the plused laser deposition (PLD) and get the high quality of the GaN thinfilms by further annealed in the high temperature annealing system. We studied theeffect of substrate temperature, annealing temperature, the distance between the lensand the target (Dl-t) and gas pressure on the growth of the thin film, and the results of theexperiment have also been analyzed. The main content is as follow:1. We analyzed the principle of the PLD technique at first and studied the effectsof laser energy density, substrate temperature and gas pressure on the growth of the thinfilms.2. Because GaN has strong rigidity and high melt point. We got the high qualityof the GaN crystal films on Si and sapphire substrates with simple two step growthpattern. The GaN thin films were deposited by PLD at first and then annealed in NH3ambient for 15 min. Because ZnO is oxide compound with a low relative melting pointand easy to crystallize, the high quality of the ZnO thin films can be obtained by PLDdirectly. A Quanta-Ray DCR-3 Nd: YAG laser was focused on a rotating target which isthe same component as the preparing films. The anneal process was carried in an opentube furnace. The main test methods in experimental are X-ray diffraction (XRD),scanning electron microscopy (SEM), atomic force microscopy (AFM), transmissionelectron microscopy (TEM), infrared (IR) spectrum, photoluminescence (PL) spectrumand Raman spectrum.3. The preparation, microstructure and properties of the GaN thin films werestudied. The results of the experiment were analyzed. We studied the influence of thedifferent annealed temperature (900-1050 ℃) and different nitrogen pressure (0.15-15Pa) on the surface morphology, crystallinity and internal stress of GaN thin films. Theresults show that the high quality GaN thin films can not be obtained by PLD. But afterbeing annealed in NH3 ambient for 15 min, the change of the annealed temperature canaffect the migration and the growth orientation of the atoms on the film surface. So theobvious change can be observed with increasing the annealed temperature. When theannealed temperature is fixed, the nitrogen pressure can influence the kinetic energy andthe size of arriving particles. The high quality of the GaN thin films can be obtainedonly when the nitrogen pressure and annealed temperature get the best match value. Itwas summarized that the GaN thin films grown on the Si(111) substrate at 15 Panitrogen pressure and annealed at annealing temperature of 1000 ℃ are thepolycrystalline films with hexagonal wurtizte structure. With the sapphire substrates, theGaN thin films grown at nitrogen pressure of 0.75 Pa and annealed at annealingtemperature of 1000 ℃ have the highest crystal quality, smallest root mean square(RMS) roughness and smallest mechanism.4. We studied the influences of the substrate temperature (100-500 ℃) and thedistance between the lens and the target (52.5-70 cm, the focal length is 70 cm) on thequality of the ZnO thin films deposited on Si substrates. During the deposition, theatomic kinetic energy is mainly determined by substrate temperature when the incidentlaser energy is fixed. The influence of the deposition temperature on the film thickness,crystallinity, surface morphology and optical properties of ZnO thin films were shownin this paper. When the Dl-t is changed, by using the energy density and spot size of theincident laser together, the change of the atomic kinetic energy and deposition rate canbe explained. So the quality of the films becomes better with the decrease of Dl-t. Theresults show that the ZnO thin films deposited at 400 ℃ and at lens-target distance of59.5-70 cm have the highest crystalline quality, best surface morphology and lestdefects.