Jae Bin Lee

Deposition of ZnO thin films by magnetron sputtering for a film bulk acoustic resonator

Abstract To fabricate lateral-field excitation (LFE)-mode solid mounted resonator (SMR)-type film bulk acoustic resonators (FBARs), piezoelectric ZnO layers were deposited in an RF magnetron sputtering system. Control of the crystallinity, microstructure and electric properties of the piezoelectric layers was essential for fabricating high-quality LFE-mode SMR-type FBARs. In the appropriate deposition condition for FBAR devices, ZnO thin films with highly c-axis-preferred orientation (XRD rocking curve, σ=2.17°), high resistivity of 106 Ω cm and surface roughness of 10.6 A were deposited. Optimal substrate rotation was especially important for improvement of the c-axis-preferred orientation of ZnO films. Plasma properties such as the electron temperature, plasma density and saturated ion current were also analyzed for optimal ZnO deposition conditions using a Langmuir double-probe system. The resonator, for which the active piezoelectric area was 200×200 μm2, consisted of 1.25-μm-thick ZnO film and a 110-nm Au electrode. Its series and parallel resonance frequencies appeared at 1.68 and 1.71 GHz, respectively, and the quality factor was 201.4±7.4.

Microstructural evolution and preferred orientation change of radio‐frequency‐magnetron sputtered ZnO thin films

Crystallographic orientation and microstructural changes of ZnO thin films deposited by rf‐magnetron sputtering were investigated with changing deposition parameters. Positional dependence of texture orientation and microstructural features, which might be attributed primarily to bombardment of high energetic oxygen species on the film in the region of the target erosion area, were observed. The films deposited at low temperature and/or using pure argon had (002) preferred orientation and a smooth columnar structure. At increasing substrate temperature and/or oxygen partial pressure, c‐axis orientation of the film was severely degraded and abnormal elongated crystallites were also developed. As the films became thicker, c‐axis orientation changed from normal to the substrate to parallel to the substrate. In this case, the microstructure was altered from a smooth fine columnar structure to abnormally grown elongated crystallites that were developed to a faceted texture structure.

Electrical Properties of the La2O3/4H-SiC Interface Prepared by Atomic Layer Deposition Using La(iPrCp)3 and H2O

The La2O3 and Al2O3/La2O3 layers were grown on 4H-SiC by atomic layer deposition (ALD) method. The electrical properties of La2O3 on 4H-SiC were examined using metal-insulator-semiconductor (MIS) structures of Pt/La2O3(18nm)/4H-SiC and Pt/Al2O3(10nm)/La2O3(5nm)/4H-SiC. For the Pt/La2O3(18nm)/4H-SiC structure, even though the leakage current density was slightly reduced after the rapid thermal annealing at 500 oC, accumulation capacitance was gradually increased with increasing bias voltage due to a high leakage current. On the other hand, since the leakage current in the accumulation regime was decreased for the Pt/Al2O3/La2O3/4H-SiC MIS structure owing to the capped Al2O3 layer, the capacitance was saturated. But the saturation capacitance was strongly dependent on frequency, indicating a leaky interfacial layer formed between the La2O3 and SiC during the fabrication process of Pt/Al2O3(10nm)/ La2O3(5nm)/ 4H-SiC structure.

4H-SiC Planar MESFETs on High-Purity Semi-Insulating Substrates

Planar MESFETs were fabricated on high-purity semi-insulating (HPSI) 4H-SiC substrates. The saturation drain current of the fabricated MESFETs with a gate length of 0.5 μm and a gate width of 100 μm was 430 mA/mm, and the transconductance was 25 mS/mm. The maximum oscillation frequency and cut-off frequency were 26.4 GHz and 7.2 GHz, respectively. The power gain was 8.4 dB and the maximum output power density was 2.8 W/mm for operation of class A at CW 2 GHz. MESFETs on HPSI substrates showed no current instability and much higher output power density in comparison to MESFETs on vanadium-doped SI substrates.

Fabrication and Characterization of 4H-SiC Planar MESFET Using Ion- Implantation

4H-SiC planar MESFETs having submicron-gate length were fabricated using ion-implantation and their DC and RF performances were characterized. The ion-implantation process which is essential to fabricate a planar device was investigated. Activation annealing after ion-implantation was performed in induction heating system under Ar atmosphere and the annealing condition was optimized. The fabricated MESFET showed good contact properties and pinch-off characteristics. The possibility of application of planar MESFETs in high voltage operation was suggested. Introduction SiC is expected to be an attractive candidate for the application of high-frequency and high-power devices due to its superior electrical, chemical and thermal properties. 4H-SiC MESFET, especially, has many advantages benefiting from the merits of 4H-SiC [1,2]. However, there is much to be desired in applying typical recess-etched gate structure to SiC because recess dry etching of the gate region degrades the Schottky characteristics [3]. To improve the device performances, planar MESFET having good ohmic and Schottky contact properties was fabricated without recess gate etching [4]. Ion-implantation was used as an essential process to fabricate a planar device [5]. In this work, 4H-SiC planar MESFETs having submicron-gate length were fabricated and the DC and RF performances were characterized. The surface morphology and electrical properties of ion-implanted region were investigated by AFM and Hall measurements. Experimental Fig. 1 shows the schematic cross-sectional structure and microscope image of a fabricated MESFET. The gate-to-source spacing, gate length and gate-to-drain spacing were 1.5 μm, 0.5 μm and 2.0 μm, respectively. The substrate was an n-type 4H-SiC purchased from Cree, Inc., with a lightly doped, 5 μm-thick p-type buffer layer and a 0.2 μm-thick n-type channel layer doped at 1.9 × 10 cm. The fabrication process included mesa isolation, ion-implantation and activation anneal, field oxide formation, ohmic contact formation, gate definition, and pad metallization. The cleaning of substrate was performed very carefully before each procedure. Mesa isolation was performed by reactive ion etching (RIE) using a SiO2 mask. To form highly doped source and drain regions, high-temperature and multiple-energy ion-implantation with phosphorous was carried out. Induction heating system was used to activate the implanted ions. All metals were deposited by e-beam evaporation. Evaporated Ni obtained using post-deposition annealing (PDA) at 1000oC for 2 min in Ar atmosphere was used for ohmic contacts. Gate contact was formed using Ni followed by the deposition of Pt Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 1181-1184 doi:10.4028/www.scientific.net/MSF.457-460.1181

Improvement of the reverse characteristics of Ti/4H-SiC schottky barrier diodes by thermal treatments

Ti/4H-SiC Schottky barrier diodes were fabricated under 500, 750, 1000 °C thermal treatment conditions. After the heat treatment at 750 °C, formation of TiC(111) and Ti5Si3(210) phases was confirmed by XRD analysis. Formation of Ti carbide and silicide phase increased breakdown voltage VB from 545 V to 830 V. An improvement of breakdown voltage (VB) was observed in case of the thermal treatment in nitrogen ambient at 750 °C for 2 min. Ideality factor (n), specific on resistance (Ron), and Schottky barrier height (Φb) were 1.04, 2.7 m-cm2, 1.33 eV respectively.

The effects of capping barrier layers on the compositional and structural variations of integrated Pb(Zr, Ti)O 3 ferroelectric capacitor having the dimension 3 × 3 μm 2

Structure and composition of the ferroelectric Pb(Zr, Ti)O 3 layers in a capacitor of the ferroelectric random-access memory (FeRAM) device having a density of 64 k were investigated by transmission electron microscopy (TEM) together with the energy-dispersive spectroscopy (EDS) technique. The 250 nm thick PZT layer derived by the sol-gel route showed a 2–3% Pb-deficient, 3–4% Ti-deficient, and 5–7% Zr-excess composition at the top electrode interface compared to the bulk composition when they were as-fabricated. The local compositional nonuniformity became more critical as the integration process proceeded, which seriously degraded the ferroelectric hysteresis and the device yield. The major cause of the compositional variation was the outward diffusion of Pb through the capping barrier TiO 2 layer during annealing at 650 °C. The AlN capping barrier layer was also not effective in suppressing the diffusion of Pb. However, the Al 2 O 3 /TiO 2 double capping layer was very effective in suppressing the outward diffusion of Pb, and excellent ferroelectric characteristic was expected.

Fabrication of 4H-SiC Planar MESFETs on High-Purity Semi-insulating Substrates

4H-SiC planar MESFETs were fabricated using ion-implantation on high-purity semi-insulating substrate, and their DC and RF performances were characterized. A modified RCA method was used to clean the substrate before each procedure. Sacrificial oxide was grown after channel layer etching to eliminate plasma damage to the gate region. A thin, thermal oxide layer was grown to passivate the surface and then a thick field oxide was deposited by CVD. The maximum oscillation frequency of 26.4 GHz and the cut-off frequency of 7.2 GHz were obtained. The power gain was 8.4 dB and the output power was 2.8 W/mm at 2 GHz.

Improvements in the Reverse Characteristics of 4H-SiC Schottky Barrier Diodes by Hydrogen Treatments

We fabricated 4H-SiC Schottky barrier diodes with various metals such as titanium, nickel and platinum. Density of interface states Dit and neutral level φo (which is the position that the Fermi level must assume if the surface is electrically neutral) were calculated 1.8×10 12 cm -2 eV -1 and 1.76 eV, respectively. In order to reduce reverse leakage current, hydrogen annealing and hydrogen plasma treatment were performed after Schottky contact metallization. The reverse leakage current measured at -100 V was reduced by one or two order of magnitude. An improvement of reverse leakage current was observed in case of both hydrogen annealing and hydrogen plasma treatment.

Effect of Crucible Design on the Shape and the Quality in 6H-SiC Crystals Grown by Physical Vapor Transport

In this study, we newly designed the inner shape of crucible to improve the growth morphology and the quality of crystal grown by physical vapor transport. The effect of crucible design on the shape and the quality of grown crystal was investigated. Modified crucible contained a truncated-cone shaped pedestal at the bottom. Recrystallization severely occurred on the surface of source in case of a crucible without truncated cone. But for the crucible with it, sublimation of source was relatively uniformly achieved and sublimation yield was also increased. Crystal grown in a crucible with a truncated cone showed better morphology, smaller value of FWHM and less dislocation density than crystal without it. And a further enlargement was achieved in the crystal. The geometry of truncated cone was optimized in order to grow a crystal with best growth morphology and the highest quality. The experimental results were also discussed with the result of temperature distribution calculation. Introduction Silicon carbide (SiC) is expected as a new semiconductor to realize high power, high temperature and high frequency electronic devices. The quality of the grown crystals, however, still needs improvement, because the crystals still contain a lot of micropipes and dislocations. And these improvement of the quality and the enlargement of SiC substrate are main problems to be solved for practical use of the SiC power devices and their applications. Large-sized SiC crystals are generally grown by the physical vapor transport [1]. Enlargement of SiC crystal has been investigated, using the modified platform on which the seed crystal was attached [2] or the lateral growth by newly designed crucible [3]. And a crucible with an inner guide-tube to improve the crystal quality was tried to grow single crystal separated from polycrystal [4]. In this paper, we newly designed the inner structure of the crucible in order to improve the growth morphology and the crystal quality. The effect of crucible design on the shape and the quality of grown crystal was investigated. Experimental SiC single crystals were grown by physical vapor transport using a quartz tube reactor with a water-cooled jacket. Modified crucible contained a truncated-cone shaped pedestal at the bottom of the crucible. It was installed in order to improve the sublimation yield of the source material and the growth morphology of crystal. The geometrical parameter of the modified crucible was shown in Fig.1. The seed crystal was fixed to the graphite lid. Abrasive commercial SiC powder for a source material was charged in the graphite crucible. And then the graphite crucible was heated to 2200 o C by RF induction heating with a frequency of 20 kHz. The top of the crucible was kept at the temperature of 2200 o C and the temperature between the top and the bottom of the crucible was about 120 o C. Crystals were grown in argon atmosphere and process pressure was kept at 10 torr during the growth. Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 103-106 doi:10.4028/www.scientific.net/MSF.457-460.103