Peter J. Hansen

Scanning capacitance microscopy imaging of threading dislocations in GaN films grown on (0001) sapphire by metalorganic chemical vapor deposition

A combination of atomic force microscopy and scanning capacitance microscopy was used to investigate the relationship between the surface morphology and the near-surface electrical properties of GaN films grown on c-axis sapphire substrates by metalorganic chemical vapor deposition. Local regions surrounding the surface termination of threading dislocations displayed a reduced change in capacitance with applied voltage relative to regions that contained no dislocations. Capacitance–voltage characteristics obtained from these regions indicated the presence of negative charge in the vicinity of dislocations.

A new high performance phase shifter using Ba/sub x/Sr/sub 1-x/TiO 3 thin films

In this paper, a new device topology has been proposed to implement parallel plate capacitors using Ba/sub x/Sr/sub 1-x/TiO/sub 3/ (BST) thin films. The device layout utilizes a single parallel capacitor and minimizes conductor losses in the base electrode. The new design simplifies the monolithic process and overcomes the problems associated with electrode patterning. An X-band 180/spl deg/ phase shifter has been implemented using the new device design. The circuit provided 240/spl deg/ phase shift with an insertion loss of only 3 dB at 10 GHz at room temperature. We have shown a figure of merit 93/spl deg//dB at 6.3 GHz and 87/spl deg//dB at 8.5 GHz. To our knowledge, these are the best figure of merit results reported in the literature for distributed phase shifters implemented using BST films at room temperature.

Impact of thermal strain on the dielectric constant of sputtered barium strontium titanate thin films

Barium strontium titanate thin films were deposited by sputtering on Pt/SiO2 structures using five different host substrates: magnesium oxide, strontium titanate, sapphire, silicon, and vycor glass. These substrates were chosen to provide a systematic change in thermal strain while maintaining the same film microstructure. All films have a weakly textured microstructure. Temperature dependent dielectric measurements from 100–500 K determined that decreasing thermal expansion coefficient of the host substrate (i.e., larger tensile thermal strain) reduced the film dielectric permittivity. The experimentally determined Curie–Weiss temperature decreased with increasing tensile thermal strain and the Curie–Weiss constant increased with tensile strain as predicted by Pertsev et al. [J. Appl. Phys. 85, 1698 (1999)].

High tunability barium strontium titanate thin films for rf circuit applications

Large variations in the permittivity of rf magnetron sputtered thin-film barium strontium titanate have been obtained through optimization of growth conditions for maximum dielectric strength and zero-field permittivity in a parallel-plate capacitor structure. Using nominal target compositions of Ba0.5Sr0.5TiO3, and Pt electrodes on c-plane sapphire substrates, adjustment of the O2 partial pressure during deposition was used to vary the excess Ti incorporation into the films, which influenced the low-field permittivity, loss tangent, and dielectric strength. By balancing the benefits of a high permittivity with dielectric strength and loss, we have produced films capable of sustaining short-duration fields greater than 4MV∕cm with over 13:1 (>90%) change in dielectric constant, and greater than 5:1 tunability in bias fields under 1MV∕cm.

ZrO2 gate dielectrics produced by ultraviolet ozone oxidation for GaN and AlGaN∕GaN transistors

We investigated the suitability of ZrO2 as a high-k dielectric for GaN material systems. Thin Zr films (4nm) were deposited by electron-beam evaporation at room temperature on n-type GaN and Al0.22Ga0.78N(29nm)∕GaN high electron mobility transistor (HEMT) structures. The Zr-coated samples were subsequently oxidized at temperatures in the range of 200–400 °C in an ozone environment. Atomic force microscopy studies after oxidation show that the ZrO2 forms a conformal layer on the underlying GaN template. Cross-section transmission electron microscopy studies showed little intermixing of the ZrO2 with the AlGaN∕GaN. The relative dielectric constant of the ZrO2 was determined to be 23. In comparison with HEMTs with bare gates (no dielectric between the gate metal and AlGaN), the HEMTs with ZrO2 showed two to three order of magnitude reduction in gate leakage current. Optimization of the HEMT process on sapphire substrates with ZrO2 under the gates yielded devices with powers of 3.8W∕mm and 58% power-added eff...

Rutile films grown by molecular beam epitaxy on GaN and AlGaN∕GaN

Titanium dioxide (TiO2, with the rutile structure) was grown on (0001) oriented GaN and (0001) Al0.33Ga0.67N∕GaN heterostructure field effect transistor (HFET) structures by molecular beam epitaxy. X-ray diffraction showed (100)TiO2‖(0001)GaN(AlGaN) and [001]TiO2‖⟨112¯0⟩GaN(AlGaN) with three rotational variants of the TiO2. Transmission electron microscopy of 50nm thick TiO2 films on GaN and AlGaN∕GaN showed sharp interfaces with no intermixing or reaction between the oxide and semiconductor. The TiO2 exhibited a columnar film microstructure with a lateral domain size of a few nanometers parallel to (101)TiO2 and a few tens of nanometers parallel to (101¯)TiO2. Metal–oxide HFETs with 50nm thick TiO2 dielectric layers under the gate were processed and compared to HFETs without the TiO2 dielectric layer. The transconductance of the HFETs with TiO2 was 140mS∕mm, approximately 20% less than HFETs with no dielecric, and the pinchoff voltages of the two stuctures were comparable. The dielectric constant of the ...

Effect of thermal strain on the ferroelectric phase transition in polycrystalline Ba0.5Sr0.5TiO3 thin films studied by Raman spectroscopy

We have applied Raman spectroscopy to study the influence of thermal strain on the vibrational properties of polycrystalline Ba0.5Sr0.5TiO3 films. The films were grown by rf magnetron sputtering on Pt∕SiO2 surface using different host substrates: strontium titanate, sapphire, silicon, and vycor glass. These substrates provide a systematic change in the thermal strain while maintaining the same film microstructure. From the temperature dependence of the ferroelectric A1 soft phonon intensity, the ferroelectric phase transition temperature, TC, was determined. We found that TC decreases with increasing tensile stress in the films. This dependence is different from the theoretical predictions for epitaxial ferroelectric films. The reduction of the ferroelectric transition temperature with increasing biaxial tensile strain is attributed to the suppression of in-plane polarization due to the small lateral grain size in the films.

Influence of stoichiometry on the dielectric properties of sputtered strontium titanate thin films

The dielectric permittivity, dielectric quality factor (inverse dielectric loss), and lattice parameter of 140 nm sputtered SrTiO3 films were dependent on the oxygen partial pressure and total chamber pressure (O2+Ar) during film growth. Films were grown at 25 and 75 mTorr (mT) in an oxygen rich and oxygen deficient sputtering gas environment concurrently on (100) SrTiO3 and (111) Pt/(0001) Al2O3 substrates. Films were deposited on platinized sapphire for electrical characterization and the homoepitaxial films were used as a structural and chemical standard. High resolution triple axis x-ray diffraction results showed an increase in mismatch between the film and substrate (200) peak in homoepitaxial SrTiO3 films with higher total growth and lower oxygen pressures. Dielectric quality factors of the SrTiO3 films on platinized sapphire at 1 MHz for the 25 mT (50 sccm Ar/50 sccm O2), 25 mT (90 sccm Ar/10 sccm O2), 75 mT (50 sccm Ar/50 sccm O2), and 75 mT (90 sccm Ar/10 sccm O2) film growths were 320, 251, 209...

LiNbO3 thin film growth on (0001)-GaN

LiNbO3 thin films were grown by rf magnetron sputtering on (0001)-GaN templates and AlGaN∕GaN structures. The films were characterized by four-circle x-ray diffraction, atomic force microscopy (AFM), and transmission electron microscopy (TEM). No second phases, such as a Li-excess or Li-deficient phase, were detected by θ-2θ scans and the films were highly (0001) textured. LiNbO3{202¯4} φ-scans and the electron diffraction pattern show that the films were epitaxially grown on GaN with crystallographic registry. The LiNbO3 c-plane was parallel to the c-plane of the GaN, but there was a 30° in-plane rotation between the LiNbO3 and GaN so that [11¯00]LiNbO3‖[112¯0]GaN(AlGaN) and the LiNbO3 films had two variants of grains rotated 60° in-plane to each other. It was confirmed by high resolution TEM that there was a transition layer between LiNbO3 and GaN. The films were annealed to improve the crystallinity and following annealing investigated using convergent beam electron diffraction (CBED) to determine the ...

Phase shifters using (Ba,Sr)TiO/sub 3/ thin films on sapphire and glass substrates

In this paper, we present results from distributed phase shifter circuits that were fabricated on sapphire and glass substrates. The circuits employ voltage tunable (Ba,Sr)TiO/sub 3/ (BST) thin films deposited by rf magnetron sputtering. Both parallel-plate and interdigital capacitor structures are investigated. K/Ka-band phase shifters demonstrated a phase shift of 265/spl deg/ with an insertion loss of 5.8 dB at 20 GHz and 180/spl deg/ phase shift with an insertion loss of 4 dB at 30 GHz. Both circuits demonstrated a promising figure of merit /spl sim/60/spl deg//dB at 10 GHz. A C/X-band phase shifter demonstrated a phase shift of >460 degrees with an insertion loss of 8.8 dB at 8 GHz.