Kuang-Po Hsueh

Low Hysteresis Dispersion La2O3 AlGaN ∕ GaN MOS-HEMTs

AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs) using electron-beam evaporated high-dielectric-constant (high-k) lanthanum oxide layer (La 2 O 3 ) as the gate insulator have been investigated and compared with the traditional GaN HEMTs. The dielectric constant of the La 2 O 3 insulator layer developed in this study was 13.1. In addition, a negligible hysteresis voltage shift in the capacitance-voltage curves can be obtained after high temperature annealing. The compositions and the crystalline structures of La 2 O 3 with different annealing temperatures were observed by X-ray photoelectron spectroscopy and X-ray diffraction, respectively. The La 2 O 3 thin film achieved a good thermal stability after 200, 400, and 600°C postdeposition annealing owing to its high binding energy (835.7 eV) characteristics. Moreover, the gate leakage current of a traditional metal gate GaN HEMT can be suppressed for 1 order of magnitude after inserting a La 2 O 3 insulator between Ni and AlGaN, resulting in a better pulsed-mode operation. The device linearity was also improved due to its flat and wide transconductance (g m ) distribution, which was analyzed by a polynomial curve-fitting technique. Therefore, La 2 O 3 is a potential candidate high-k material for the gate insulator to enhance the GaN-based field effect transistor performance while scaling down the device dimension and device reliability at high power operation.

Temperature-dependent study of n-ZnO∕p-GaN diodes

This work investigates the temperature dependence of the current-voltage (I-V) characteristics of n-ZnO∕p-GaN junction diodes. The n-ZnO films were deposited on top of the p-GaN by dc sputtering with subsequent annealings at 500, 600, 700, and 800°C for 60s. The Hall measurement and the x-ray diffraction pattern are measured to study the n-ZnO films. The temperature sensitivity coefficients of the I-V characterizations are obtained by different substrate temperatures (25, 50, 100, and 150°C) and the extracted values are 2.10, 1.93, 3.22, and 1.36mV∕°C in the forward bias and 8.7, 8.0, 4.6, and 2.3mV∕°C in the reverse bias, respectively. The fabricated n-ZnO∕p-GaN diode with ZnO annealing temperatures at 800°C demonstrates the lowest temperature dependence.

Effect of Cl2∕Ar dry etching on p-GaN with Ni∕Au metallization characterization

This work investigates the effect of Cl2∕Ar dry etching on p-GaN. The root-mean-square (rms) surface roughness is measured, and the depth display (Bearing analysis) is monitored. The current-voltage characteristics of etched p-GaN with Ni(20nm)∕Au(20nm) metallization are studied. Experimental results indicate that the etching rate does not increase significantly with the Cl2 flow rate at a constant power or chamber pressure. The Bearing ratio data exhibit a much stronger variation with etch conditions, the rms displays the same trend but to a lesser extent.

Effect of rapid thermal annealing on MgxZn1-xO films prepared by radio-frequency magnetron sputtering

This study investigates the effects of thermal annealing on the MgxZn1−xO films. MgxZn1−xO films were deposited by a radio-frequency magnetron sputtering system using a 6 in. ZnO/MgO (80/20 wt %) target. The Hall results, x-ray diffraction (XRD), transmittance, and x-ray photoelectron spectroscopy (XPS) were measured. The XRD results indicate that the appearance of only (111) peaks in the as-grown MgxZn1−xO film is a sign of the cubic single phase, whereas the appearance of ZnO (002) peaks in MgxZn1−xO films annealed at 700 and 800 °C confirms the formation of a wurtzite single-phase crystal. The existence of a weak (002)-wurtzite peak besides the (111)-cubic peak indicates the coexistence of two phases. The absorption spectra of MgxZn1−xO annealed at 700 and 800 °C show two stages at wavelengths of 357 and 261 nm. The XPS spectra of MgxZn1−xO films were also demonstrated. The results of this study show that the ZnO films were separated from MgxZn1−xO films after higher thermal annealing.

Temperature-Dependent Characteristics of a GaN/InGaN/ZnO Heterojunction Bipolar Transistor

This work presents dc characteristics (gain and leakage current) of a GaN/InGaN/ZnO npn collector-up heterojunction bipolar transistor (HBT) measured at 300, 200, and 100 K. The GaN-based epilayers of HBT were grown by metallorganic chemical vapor deposition, and the n-ZnO film was then deposited on top of p-InGaN by sputtering. X-ray diffraction analysis demonstrates that annealing at 600°C for 60 s in ambient N 2 improved the quality of the ZnO film. Moreover, this study shows that the current gains (β) from the Gummel plot increased as the temperature decreased because the leakage current of the collector current (I C ) is not a strong function of temperature. However, gains (G CE ) from the measured common-emitter current-voltage characteristics increased slightly as the temperature decreased due to a reduction in the recombination current coupled with lower injection efficiency. These preliminary results show the potential of fabricating GaN/InGaN/ZnO HBTs without dry etching to reach the p-GaN layer, as required in the emitter-up geometry.

Investigation of Cr- and Al-based metals for the reflector and Ohmic contact on n-GaN in GaN flip-chip light-emitting diodes

This letter investigates three composite metals used as a reflector and Ohmic contact on n-GaN to simplify the process in a flip-chip light-emitting diode (FCLED). The investigated composite metals were Ti∕Al∕Ti∕Au, Cr∕Al∕Cr∕Au, and Cr∕Ti∕Au. The specific contact resistivities of the Ti∕Al∕Ti∕Au, Cr∕Al∕Cr∕Au, and Cr∕Ti∕Au Ohmic contacts on n-GaN were changed from 1.4×10−4, 1.7×10−4, and 1.9×10−4Ωcm2 to 1.3×10−4, 1.1×10−4, and 3.3×10−5Ωcm2, respectively, after 500h of thermal stress. The corresponding operating voltages of FCLEDs with different composite metals were changed by less than 1%. After 96h of thermal stress, the luminous intensities of the three structures decreased by 6.2%, 11.1%, and 1.4%, respectively. The GaN FCLED that was fabricated with Cr∕Ti∕Au as a reflector and an Ohmic contact on n-GaN exhibits good thermal stability and luminous intensity.

High-temperature stability of postgrowth-annealed Al-doped MgxZn1-xO films without the phase separation effect

This study investigates the effects of thermal annealing on Al-doped MgxZn1−xO (AMZO) films. AMZO films were deposited by a radio-frequency magnetron sputtering system using a 4 in. ZnO/MgO/Al2O3 (76/19/5 wt. %) target. This study measures and reports the Hall results, x-ray diffraction (XRD), transmittance, and x-ray photoelectron spectroscopy (XPS) data. XRD results show that the ZnO (002) and MgO2 (002) wurtzite peaks in addition to the (111)-cubic peak disappeared after 1000 °C annealing. This indicates the coexistence of two phases in the as-grown AMZO films rebuilt after higher thermal treatment. The absorption edges of these as-grown AMZO films shifted toward the short wavelength of 323 nm under 80% transmittance, implying that band gaps can be tuned by changing the Mg content of the AMZO layer. The XPS spectra of AMZO films were also used to analyze the composition of the as-grown and annealed AMZO films.

GaN lattice matched ZnO/Pr 2 O 3 film as gate dielectric oxide layer for AlGaN/GaN HEMT

In this work, we perform AlGaN/GaN MOS-HEMT by using ZnO/Pr<inf>2</inf>O<inf>3</inf> as gate dielectric. After 600°C annealing, the XRD analysis shows ZnO thin films with highly crystalline characteristics, which exhibit a lattice constant (a=3.2498, c=5.2066) matched to GaN (a=3.1890, c=5.1855). The gate leakage current can be improved significantly by inserting ZnO/Pr<inf>2</inf>O<inf>3</inf> dielectric layer; meanwhile, ZnO/Pr<inf>2</inf>O<inf>3</inf> MOS-HEMT shows superior breakdown voltage performance toward ZnO MOS-HEMT and conventional Ni/Au HEMT. From these results, ZnO/Pr<inf>2</inf>O<inf>3</inf> dielectric is promising for low leakage current of AlGaN/GaN based MOS- HEMT.

Low Schottky barrier to etched p‐GaN using regrown AlInGaN and InGaN contact layer

The p-type AlInGaN and InGaN contact layers were regrown on the etched p‐GaN to study the Ni∕Au contact current-voltage (I‐V) characteristics. The thickness of the contact layer was 100nm and regrown by metalorganic chemical vapor deposition. By using the regrown contact layer on etched p‐GaN, Schottky barrier height (SBH) from the I‐V characterization was reduced. The SBH of 0.65eV from the contact to the etched p‐GaN was reduced to 0.56eV and 0.58eV, respectively, after the AlInGaN and InGaN contact layers were formed. In addition to the I‐V characterization of Ni∕Au contacts, surface morphology and x-ray analysis were studied.

Physical properties of Al-doped MgZnO film grown by RF magnetron sputtering using ZnO/MgO/Al2O3 target

This study investigates the physical properties of Al-doped MgxZn1-xO (AMZO) films. Al-doped MgxZn1-xO films were deposited by radio-frequency (RF) magnetron sputtering system using a 4 inch ZnO/MgO/Al2O3 (76/19/5 wt%) target. This study determined the resulting x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), Hall measurement, and transparent performance of the films. XRD results indicate that the diffraction angles of the annealed AMZO film shifted toward the high-angle side, indicating that thermal annealing could relax the compressive strain components in the as-deposited samples. XPS results reveal a high carbon content on the surface of MgxZn1-xO. This may be due to contamination. The average Mg content of the as-grown AMZO is about 19.23 at. % at a depth of 40 nm. The Al-doped MgxZn1-xO film in this study shows high transparency with transmittances over 95 % in the visible region (400 ~ 800nm), and a sharp absorption edge is visible in the UV region due to the Mg content. The Hall measurement of Al-doped MgxZn1-xO films deposited at lower RF power show higher doping concentrations, lower resistivity and higher mobility as a function of the annealing temperature. Experimental results indicate that Al-doped MgxZn1-xO film with 1000 °C annealing contains more oxygen vacancies, which play the role of donor. Oxygen vacancies generate states in the band gap and increase conductivity.