Hachizo Muto

Effect of ZnO buffer layer on the quality of GaN films deposited by pulsed laser ablation

Abstract ZnO, GaN and ZnO/GaN films have been deposited on c-cut sapphire wafers by Nd:YAG pulsed laser deposition to develop preparation techniques of multi-layer devices in optoelectronics. The quality of the films was investigated as functions of deposition temperature and pressure by means of X-ray diffraction, reflection high energy electron diffraction, scanning electron microscopy and optical transmission spectroscopy. It was found that GaN films could be epitaxially grown on ZnO buffered sapphire substrates even at a low substrate temperature of 300 °C and inactivated N2 flow, confirming the significant role of ZnO as a buffer layer. The crystallinity, surface morphology and optical transmittance of these films could be improved with increasing deposition temperature and were optimized at 800 °C. The optical band gaps of ZnO and GaN were estimated from the optical transmission spectra to be 3.31 and 3.40 eV, respectively.

Control of polytype formation in silicon carbide heteroepitaxial films by pulsed-laser deposition

Control of silicon carbide (SiC) polytype formation has been achieved. 3C, 2H, and 4H SiC heteroepitaxial films are fabricated on a sapphire (0001) substrate at a low temperature of 1100 °C using a pulsed-laser deposition (PLD) method. Images made by transmission electron microscopy clearly show that each film consists of a single polytype. The polytype of the SiC films can be changed by varying just two easily controlled parameters: the laser pulse frequency and the substrate temperature. These results suggest that precise control of growth conditions, which is essential for polymorphic materials, is possible using the PLD method.

Growth of c-axis oriented GaN films on quartz by pulsed laser deposition

Abstract We have successfully fabricated highly oriented GaN films on ZnO-buffered quartz by Nd:YAG pulsed laser deposition to develop large area and low cost optoelectronic devices. The quality of the films was investigated as functions of deposition temperature and ambient pressure by means of X-ray diffraction, scanning electron microscopy, infrared reflectance and optical transmission spectroscopy. It was found that c-axis oriented GaN films could be grown on ZnO-buffered quartz substrates at temperatures from 300 to 800 °C and inactivated N2 flow from 10−3 to 2.0 Torr. The crystallinity, surface morphology and optical transmittance of the films could be improved with increasing deposition temperature and were optimized at 800 °C. A reststrahlen band was observed at 568 cm−1, confirming that GaN was indeed overlaid on ZnO-buffered quartz. The present GaN/ZnO bilayer structure has showed optical transmittance of about 60%.

Pinning effect of a LaFeO3 buffer layer on the magnetization of a La1−xPbxMnO3 layer

Epitaxial bilayered films of La0.55Pb0.45MnO3/LaFeO3 were fabricated on LaAlO3–Sr2AlTaO6 (111) and (001) single-crystal substrates by a pulsed-laser ablation-deposition method to examine the influence of spin orientation in the antiferromagnetic LaFeO3 buffer layer on the magnetization of manganite. The metal–insulator transition temperature (Tρmax) of the bilayered films with (111) orientation increased remarkably compared with that of the monolayered manganese-oxide films, while the Tρmax of the (001) oriented films increased only slightly. Since the spins on the (111) and (001) planes of LaFeO3 are aligned parallel (uncompensated) and antiparallel, respectively, the remarkable increase of Tρmax observed for the former is mainly ascribed to strong pinning of the manganite layer’s magnetization by the uncompensated spins on the (111) plane. This strong pinning was confirmed by ferromagnetic resonance measurements of the bilayered and monolayered films.

Influence of argon gas pressure on the crystallinity of α-SiC epitaxial films fabricated by Nd:YAG pulsed-laser deposition

Abstract Fabrication of SiC crystalline films at low temperatures is examined by pulsed-laser deposition (PLD), since it is important for SiC device technology. The growth temperature of α-SiC hetero-epitaxial films is considerably reduced to 820 °C by fabricating under argon (Ar) atmosphere using a very high fluence of the fourth harmonics of Nd:YAG laser. Influence of the Ar gas pressure on the crystallinity of α-SiC film is studied with X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED). It is found that the optimal pressure is critical and is ∼50 Pa for the following experimental conditions: laser fluence F=20 J/cm2/pulse, distance between a substrate and target dTS=20 mm, and substrate temperature TS=820 °C. Under lower pressure than 40 Pa, the film becomes amorphous. On the other hands, higher pressure than 50 Pa results in poly-crystalline films. These results are plausibly explained by the high kinetic energy of ejected species by laser ablation with high fluence and their collision with the atmospheric Ar gas atoms.

In situ hole doping of wide-gap semiconductors by dual-target simultaneous laser ablation: GaN and SiC epitaxial films

Apparatus for dual-target simultaneous laser ablation deposition and in situ doping techniques have been developed to achieve p-type doping during epitaxial growth of wide-band-gap semiconductors. The apparatus has two target holders with a target-rotation mechanism and a rotation-axis adjusting mechanism to obtain homogeneously doped films. Mg-doped GaN films have been fabricated on 6H–SiC(0001) and Si(111) substrates in NH3 ambient by simultaneous ablation of GaN and Mg-metal targets using two lasers. Junctions of the films with n-type substrates show a diode curve characteristic of p-n junctions, but not for junction with p-Si, indicating hole doping without further procedures. In situ p-type doping to SiC was also achieved by using SiC and Al4C3 targets.

Growth of SiC Hetero-Epitaxial Films by Pulsed-Laser Deposition -Laser Frequency Dependence-

Influence of laser frequency ( f) on the quality of SiC epitaxial films fabricated using pulsedlaser deposition method is investigated by x-ray diffraction, reflection high-energy electron diffraction (RHEED), and x-ray photoelectron spectroscopy. It was found that polyt y e of the prepared SiC films changed from -phase to -phase when f was increased from 1 Hz to 2 Hz. The crystallinity of the α-SiC films is improved with increasing f. However, RHEED images show a pattern of 3×3 surface reconstruction in addition to a strong streak pattern due to epit axial SiC for films fabricated with higher laser frequency than 5 Hz, which suggests that Si l yer precipitates on the SiC films. These results indicate that the laser frequency is one of the e ssential parameters to fabricate high quality SiC films. Introduction Silicon carbide (SiC) is eminently suitable for high temperature , high power, and high frequency devices [1, 2]. Fabrication of the epitaxial film is one of the key technologies for such a device processing. A lot of effort has been devoted to fabricate hetero-epitaxial SiC films, however, most of them were 3C-SiC [3-5]. It is difficult to fabricate the fil ms of -polytype since growth temperature of -SiC is high ( 2000oC) [6]. We have reported formation of -SiC hetero-epitaxial films using pulsed-laser deposition (PLD) method, although the crystallinity was not go d yet [7, 8]. The quality of the film strongly depends on the deposition conditions such a s laser fluence, substrate temperature, deposition atmosphere, and so on [7, 9]. There remain a lot of parameters to be optimized for obtaining the high quality epitaxial films. In this s tudy we present the influence of laser conditions, especially laser frequency on the crystallinity and surfa ce quality of SiC films. Experimental Silicon carbide films were fabricated on sapphire (0001) substrate (Furuuchi Chemical Co.) by the PLD method, using the 4 th harmonic (266nm) of a Nd:YAG pulsed-laser (Spectron Laser Systems ; SL803G). The laser energy used is 50 mJ/pulse, which is focused to a fluence of =1 J/cm on to the target. Distance between the target and the substrate is 30 mm. Laser frequency ( f) was varied from 1 to 10 Hz, while the total laser shot was set to 3600 times for all the samples. Sintered -SiC target (Sumitomo Osaka Cement Co., LTD) with the purity of 99.99% and density 3.20 g/cm was used. Ultrasonic cleaning in acetone solvent was made for the substrat es before use. The substrate was clamped on the substrate holder and the holder was heated to 1220oC with a p yrolytic-graphite heater. The temperature of the substrate surface is estimated to b about 1100oC with a pyrometer, which is lower by 120oC than that of the substrate holder because of i nsufficient thermal contact. Vacuum pressure in the deposition chamber was kept within the order of 10 Torr during the film deposition. Crystallinity and surface precipitates of the prepared f ilms were studied by RHEED, x-ray diffraction (XRD) using -2 and -scan, and x-ray photoelectron spectroscopy (XPS). Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 217-220 doi:10.4028/www.scientific.net/MSF.433-436.217

Preparation of SiC/Si(111) Hetero-Epitaxial Junctions by PLD and Crystallographic and l-V Characterization

Fabrication technique of SiC epitaxial films is studied using pulse d laser ablation-deposition (PLD) method. Hetero-epitaxial films of α-SiC have been successfully prepared on Si(111) substrates at ~1300 as well as sapphire (0001) plane. I-V characteristic of the S iC/p-Si junctions is studied in addition to the crystallographic properties of the films. A nonlinear I-V characteristic with a breakdown voltage of 100-300 V is observed for the SiC/p-Si junctions with epitaxial SiC lattices, which is ascribed to p-n junction. P olycrystalline films give poor I-V characteristic. Introduction Silicon carbide (SiC) is expected to be the next-generation semi conductor, which is applicable for high power, high voltage, high temperature, and high frequency devices [1-4]. Especially, the high-temperature stable types (such as 4H and 6H α-SiC) are strongly expected, since they have a wider band gap (3.0-3.2eV) than the low temperature one (β-SiC; 2.2eV). However, high melting point of SiC (m.p. > 2600) makes it difficult to prepare single crystals. Many trials were made for preparing SiC hetero-e pitaxial films that also can be used as wafer [5-9], however, they have failed except for β-SiC hetero-epitaxial growth by MBE techniques and CVD method [10,11]. Device processing of SiC has other probl ems to be solved. They also arise from the very high m.p. And relate to high te mperature processing such as annealing at around the transition temperature from β-SiC to α-SiC (1600 ) after ion implantation for doping. We study fabrication techniques of SiC films using PLD method, since PLD may produce epitaxial films even at low temperatures via non-equilibrium reac tion mechanism involved. In the previous preliminary report, we could successfully fabricate S iC epitaxial films on Si(111) substrates at 1200~1300 in addition to sapphire (0001) plane [12,13]. These results suggest a possibility of low-temperature preparation of a p-n junc tion by PLD. The present work reports the fabrication of SiC hetero-epitaxial films on p-t ype Si(111) substrates and characterization of their crystallographic and electric prope rties. A non-linear I-V characteristic like p-n junctions or Schottky diode with a breakdow n voltage of 100-300 V was observed for SiC/Si junctions with epitaxial SiC lattices. Experiments PLD experiments were made using the 4 th harmonic (266nm) of nano-second Nd:YAG laser (Spectron LS-803), a laboratory-designed chamber (Sanko-Keisoku Servic e Co.). Sintered targets of 6H α-SiC and Si(111) single crystal substrates were used for PLD. The optimized conditions are as follows. Temperature of the heater on which the substrate is attached is Th=1300 , laser energy E=50 mJ/pulse, fluence (laser energy density deposited on the target) *Corresponding author:Tel:+81-52-736-7317,fax:+81-52-736-7400; E-mail: hachizo.muto@aist.go.jp Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 225-228 doi:10.4028/www.scientific.net/MSF.433-436.225