GaN Heterojunction Research Articles

Interface states at the SiN/AlGaN interface on GaN heterojunctions for Ga and N-polar material

Dielectric passivation is important to improve the stability and reliability of gallium nitride based semiconductor devices. We need to characterize various dielectrics and their interfaces to nitrides accurately to be able to exploit the benefits efficiently. Earlier, B. L. Swenson and U. K. Mishra [J. Appl. Phys. 106, 064902 (2009)] have detailed a photo-assisted high frequency CV characterization technique for the Ga-polar SiN/GaN interface that gives an accurate value of interface state density (Dit) across the bandgap of the dielectric. In this work, we extend the technique to study the interface states at the SiN/AlGaN interface on GaN for Ga and N polar material. This simulates the AlGaN/GaN HEMT structure. A MIS-type structure comprised of a metal on SiN on an AlGaN/GaN heterojunction was used for the study. For a structure with 1 nm AlGaN interlayer, a peak interface state density of 2.8 × 1012 cm−2 eV−1 was measured. For Ga polar devices, the measured Dit decreases with increasing AlGaN thickness. In the N-polar case, the measured Dit increases with increasing AlGaN thickness. The variations of measured Dit with AlGaN thickness, in both cases, can be explained by screening from the accumulation charge at the AlGaN/GaN interface.

Interfacial misfit dislocation scattering effect in two-dimensional electron gas channel of GaN heterojunction

Interfacial misfit dislocation in AlxGa1−xN/GaN heterojunction was studied as a function of barrier layer thickness and lattice mismatch degree. The sheet charge density induced by interfacial dislocation was obtained. Based on the interfacial dislocation model, the mobility was calculated in two-dimensional electron gas channel. It was found that the mobility was dominated by the interfacial dislocation scattering at low temperature, and depended on the barrier layer thickness and lattice mismatch degree.

Local Electronic Structure and UV Electroluminescence of n-ZnO:N/p-GaN Heterojunction LEDs Grown by Remote Plasma Atomic Layer Deposition

ABSTRACTNitrogen-doped ZnO (ZnO:N) films have been prepared by remote plasma atomic layer deposition (RP-ALD) and treated by rapid thermal annealing (RTA) in oxygen atmosphere. The local electronic structures of the (ZnO:N) films were investigated by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES) at the O K-edge. The XPS reveals the presence of the Zn-N bond in the ZnO:N films, indicating that partial amounts of oxygen sites are occupied by nitrogen species. This is correspondent with the decrease of electron concentration in ZnO:N films with the nitrogen doping concentration, as indicated by the Hall effect measurement. The RP-ALD technique was applied to fabricate the n-type ZnO:N/p-type GaN heterojunction LEDs. Dominant ultraviolet electroluminescence at 371 nm from the ZnO:N layer was observed at room temperature.

Envelope tracking power amplifier with saturated driver amplifier optimized for overall efficiency

AbstractThis letter presents a new technique to enhance the overall efficiency of a two‐stage envelope tracking (ET) power amplifier (PA). Because of heavy gain compression, ET amplifiers can suffer from inefficient driver stages. By reducing the supply voltage of the driver amplifier, its efficiency can be improved without degrading the linearity performance of the overall PA system because the final‐stage ET PA allows the decrested RF signal as an input. This technique is demonstrated for a single carrier WCDMA signal using a RF PA based on a GaAs HVHBT with a dynamic supply modulator and a driver stage based on a GaN heterojunction field‐effect transistor (HFET). The measurement shows that the drain efficiency of the driver amplifier is improved from 25.6 to 41.5% and the overall efficiency of the ET PA system is improved from 46.2 to 48.3% while the efficiency of the final‐stage ET PA is maintained at higher than 55%. The average output power is greater than 44.6 dBm, the overall gain is greater than 20 dB, and the adjacent channel leakage ratio is less than −46 and −53 dBc at 5‐ and 10‐MHz frequency offsets, respectively. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett, 54:505–507, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26527

White-Light Electroluminescence From n-ZnO/p-GaN Heterojunction Light-Emitting Diodes at Reverse Breakdown Bias

White-light electroluminescence (EL) from n-type ZnO (n-ZnO)/p-type GaN (p-GaN) heterojunction light-emitting diodes operated at reverse breakdown bias was reported. The n-ZnO epilayers were grown by atomic layer deposition on p-GaN. The electron tunneling from the deep-level states near the ZnO/GaN interface to the conduction band in n-ZnO is responsible for the reverse breakdown. The EL spectrum was composed of the blue light at 450 nm and the broadband around 550 nm, which originated from the Mg acceptor levels in p-GaN and the deep-level states near the ZnO/GaN interface, respectively. The chromaticity coordinate of the EL spectrum was (0.31, 0.36), which is very close to (0.33, 0.33) of the standard white light.

Phosphor-converted light-emitting diode based on ZnO-based heterojunction

A phosphor-converted light-emitting diode (LED) was realized by coating BaMg 2 Al 16 O 27 :Eu 2+ ·Mn 2+ and (SrCaPO 4 )·B 2 O 3 :Eu 2+ ·Na + phosphors onto an n - ZnO/i - MgO/p - GaN heterojunction diode. Two emission bands at around 450 and 520 nm were observed in the phosphor-converted LED under the injection of continuous current. By analyzing the optical properties of the heterojunction diode and phosphors, it is concluded that the emission at 450 nm comes from (SrCaPO 4 )·B 2 O 3 :Eu 2+ ·Na + phosphor, while the one at 520 nm comes from BaMg 2 Al 16 O 27 :Eu 2+ ·Mn 2+ phosphor under the excitation of the light emitted from the n - ZnO/i - MgO/p - GaN heterojunction diode. The results reported in this paper may provide a route to ZnO-based phosphor-converted LEDs for future lighting or displaying purpose.

GaN Power Transistors on Si Substrates for Switching Applications

In this paper, GaN power transistors on Si substrates for power switching application are reported. GaN heterojunction field-effect transistor (HFET) structure on Si is an important configuration in order to realize a low loss and high power devices as well as one of the cost-effective solutions. Current collapse phenomena are discussed for GaN-HFETs on Si substrate, resulting in suppression of the current collapse due to using the conducting Si substrate. Furthermore, attempts for normally off GaN-FETs were examined. A hybrid metal-oxide-semiconductor HFET structure is a promising candidate for obtaining devices with a lower on-resistance (Ron) and a high breakdown voltage (Vb).

Polarization-Induced Zener Tunnel Junctions in Wide-Band-Gap Heterostructures

The large electronic polarization in III-V nitrides allows for novel physics not possible in other semiconductor families. In this work, interband Zener tunneling in wide-band-gap GaN heterojunctions is demonstrated by using polarization-induced electric fields. The resulting tunnel diodes are more conductive under reverse bias, which has applications for zero-bias rectification and mm-wave imaging. Since interband tunneling is traditionally prohibitive in wide-band-gap semiconductors, these polarization-induced structures and their variants can enable a number of devices such as multijunction solar cells that can operate under elevated temperatures and high fields.

HIGH CURRENT DENSITY/HIGH VOLTAGE AlGaN/GaN HFETs ON SAPPHIRE

AlGaN / GaN heterojunction field effect transistors (HFETs) on sapphire substrates for power-switching applications have been fabricated. Design parameters such as source-gate spacing (Lsg), gate length (Lg), and gate width (Wg) have been varied to check their effects on the device performances. For a gate-drain spacing (Lgd) of 10µm, a specific on-resistance (ARon) of 1.35mΩ-cm2 and off-state breakdown voltage (BV) of 770V was achieved, which is close to the 4- H SiC theoretical limit.

4-NM AlN BARRIER ALL BINARY HFET WITH SiNx GATE DIELECTRIC

Undoped AlN / GaN heterostructures, grown on sapphire by molecular beam epitaxy, exhibit very low sheet resistances, ~ 150 Ohm/sq, resulting from the 2-dimensional electron gas situated underneath a 4 nm thin AlN barrier. This extraordinarily low sheet resistance is a result of high carrier mobility and concentration (~ 1200 cm2/Vs and ~ 3.5×1013 cm-2 at room temperature), which is ~ 3 × smaller than that of the conventional AlGaN / GaN heterojunction field effect transistor (HFET) structures. Using a 5 nm SiN x deposited by plasma enhanced chemical vapor deposition as gate-dielectric, HFETs were fabricated using these all binary AlN / GaN heterostructures and the gate tunneling current was found to be efficiently suppressed. Output current densities of 1.7 A/mm and 2.1 A/mm, intrinsic transconductance of 455 mS/mm and 785 mS/mm, were achieved for 2 µm and 250 nm gate-length devices, respectively. Current gain cut-off frequency fT of 3.5 GHz and 60 GHz were measured on 2 µm and 250 nm gate-length devices, limited by the high ohmic contact resistance as well as the relatively long gate length in comparison to the electron mean free path under high electric fields.

DC characteristics of AZO/GaN heterojunction bipolar transistors

A new npn Al-doped zinc oxide (AZO)/GaN heterojunction bipolar transistor (HBT) is presented. The fabricated HBT with an emitter size of 120times120 mum 2 exhibits a current gain of 5.8 at V BE =3.0=V. The anomalous high current gain is observed at low current level due to the leakage current. The common-emitter output characteristics demonstrate DC current gain of 1.2. In addition, improved device characteristics are observed after the P 2 S 5 /(NH 4 ) 2 S treatment.

High-quality InGaN∕GaN heterojunctions and their photovoltaic effects

High-quality p-GaN∕i-In0.1Ga0.9N∕n-GaN heterojunctional epilayers are grown on (0001)-oriented sapphire substrates by metal organic chemical vapor deposition. The Pendellösung fringes around the InGaN peak in high-resolution x-ray diffraction (HRXRD) confirm a sharp interface between InGaN and GaN films. The corresponding HRXRD and photoluminescence measurements demonstrate that there is no observable phase separation. The improvement in crystal quality yields high-performance photovoltaic cells with open-circuit voltage of around 2.1eV and fill factor up to 81% under standard AM 1.5 condition. The dark current-voltage measurements show very large shunt resistance, implying an insignificant leakage current in the devices and therefore achieving the high fill factor in the illuminated case.

Electron irradiation of AlGaN∕GaN and AlN∕GaN heterojunctions

The effects of 10MeV electron irradiation on AlGaN∕GaN and AlN∕GaN heterojunctions grown by molecular beam epitaxy are reported. The irradiation increases the resistivity of the GaN buffer due to compensation by radiation defects with levels near Ec−1eV and decreases the mobility of the two-dimensional electron gas (2DEG) near the AlGaN∕GaN (or AlN∕GaN) interface. The bulk carrier removal rate in the GaN buffer is the same for both types of structures and similar to carrier removal rates for undoped n-GaN films. In structures with a density of residual donors of ∼1015cm−3, irradiation with electron doses of ∼5×1015cm−2 renders the buffer semi-insulating. The 50% degradation of the 2DEG conductivity happens at several times higher doses (close to 3×1016cm−2 versus 6.5×1015cm−2) for AlN∕GaN than for AlGaN∕GaN structures, most likely because of the lower thickness of the AlN barrier.

Cross-sectional scanning photoelectron microscopy and spectroscopy of wurtzite InN∕GaN heterojunction: Measurement of “intrinsic” band lineup

A method for studying heterojunction band lineups on the submicrometer scale is demonstrated by using synchrotron-radiation photoelectron microscopy and spectroscopy. In particular, an in situ sample cleavage technique is adopted here to reveal the cross-sectional, nonpolar a-plane face of InN∕GaN heterojunction grown on Si(111) along the polar −c axis with fully relaxed lattice structure, eliminating the polarization effects associated with the interface charge/dipole normal to the cleaved surface. The “intrinsic” valence band offset at the cleaved InN∕GaN heterojunction has been determined to be 0.78eV. Additionally, using known material parameters, the values of InN∕GaN conduction band offset and InN electron affinity are also estimated.

Very low sheet resistance and Shubnikov–de-Haas oscillations in two-dimensional electron gases at ultrathin binary AlN∕GaN heterojunctions

Very low sheet resistance two-dimensional electron gases have been obtained at ultrathin AlN∕GaN heterojunctions. By investigating the molecular beam epitaxy growth conditions, the interface roughness at the heterojunction has been reduced, leading to high room-temperature electron mobilities (μ∼1400–1600cm2∕Vs) and high electron sheet densities due to spontaneous and piezoelectric polarization (ns∼1–3×1013cm−2) depending on the AlN thickness. This combination led to a large reduction of the room-temperature sheet resistance of 148Ω∕◻, a new record. The improved transport properties have made it possible to observe Shubnikov–de-Haas oscillations of the magnetoresistance of the two-dimensional electron gas at ultrathin binary AlN∕GaN heterojunctions for the first time. The results are indicative of the vast potential of high-quality AlN∕GaN structures for a variety of device applications.

High performance pnp AlGaN∕GaN heterojunction bipolar transistors on GaN substrates

We fabricated pnp AlGaN∕GaN heterojunction bipolar transistors (HBTs) with various emitter areas on GaN substrates and investigated their common-emitter current-voltage and high-power characteristics at room temperature. The HBTs with an emitter area of 30×50μm2 exhibited a high performance: a maximum current gain of 85 at a collector current of 30mA and a maximum collector current density of 7.3kA∕cm2 at a collector-emitter voltage of 30V, which corresponds to the maximum power dissipation density of 219kW∕cm2. The current gain and the collector current density increased compared to those on sapphire substrates. For the HBT with the emitter area of 270×450μm2, the current gain was still as high as 47 and the maximum collector current reached as high as 1A, and this single HBT showed a high-power dissipation of 30W. This high performance of the HBTs is ascribed to the low dislocation density and relatively high thermal conductivity of the GaN substrate. Although the emitter crowding and self-heating effects remain, the obtained values are highest among pnp nitride-based HBTs.

Carrier transport mechanisms of Pnp AlGaN∕GaN heterojunction bipolar transistors

We fabricated Pnp AlGaN∕GaN heterojunction bipolar transistors (HBTs) with various base widths WB and investigated their common-emitter current-voltage characteristics at room temperature to clarify their carrier transport mechanisms. The current gain β increased as WB decreased. The maximum current gain βmax was 40 in a HBT with a WB of 30nm. HBTs with different base widths exhibited almost the same tendency for β to increase with increasing the collector current IC, indicating that the carrier transport mechanism is the same in all the n-GaN base layers. With a low IC, recombination in the emitter-base depletion region is the dominant carrier transport mechanism. β was less affected when IC was high, and the carrier transport was dominated by the minority hole diffusion in the neutral base layer. The minority hole diffusion length obtained from the HBT characteristics agrees well with previous results obtained with electron beam induced current measurements, also indicating that βmax was determined by the minority hole diffusion length in the n-GaN base layer.

Ultraviolet bandpass Al0.17Ga0.83N∕GaN heterojunction phototransitors with high optical gain and high rejection ratio

Ultraviolet Al0.17Ga0.83N∕GaN-based heterojunction phototransistors (HPTs) grown by metal-organic vapor-phase epitaxy were demonstrated. The HPTs showed a bandpass spectral responsivity ranging from 280to390nm. With a bias voltage of 6V, the responsivity at an incident of 340nm was as high as 1500A∕W, corresponding to a quantum gain of 5.47×103. In contrast to GaN-based photoconductors, the HPTs also featured high contrast in spectral response. With a bias voltage of 3V, the spectral response showed high rejection ratios of approximately 4×105 and 1×104 for the long-wavelength side (340∕400nm) and the short-wavelength side, respectively. The high contrast of spectral response for the long-wavelength side could be due to the long trapping time of holes blocked by the base-emitter heterojunction and the low defect-to-band response that is caused by defect-related gap states.

Cascade single-chip phosphor-free white light-emitting diodes

In order to resolve the problems existing in the conventional phosphor-converted light-emitting-diodes (LEDs) and red-green-blue LEDs, the cascade single-chip phosphor-free white LED was proposed with GaAs∕GaN heterojunction direct wafer bonding. Corresponding to the color-matching calculation, the white LED demonstrated the CIE chromaticity coordinates of about (0.3, 0.3) at 20mA, which was very close to the ideal white light position (1∕3, 1∕3) on the chromaticity diagram. The fabrication and the electrical and optical performances of such white LEDs were described.

Conduction band offset at the InN∕GaN heterojunction

The conduction-band offset between GaN and InN is experimentally determined. InN∕n-type GaN isotype heterojunctions grown by molecular beam epitaxy are observed to exhibit Schottky-junction like behavior based on rectifying vertical current flow. From capacitance-voltage measurements on the heterojunction, the Schottky barrier height is found to be ∼0.94eV. The photocurrent spectroscopy measurement by backside illumination reveals an energy barrier height of 0.95eV across the heterojunction, consistent with the capacitance measurement. By combining electrical transport, capacitance-voltage, and photocurrent spectroscopy measurement results, the conduction band offset between InN and GaN is estimated to be ΔEC=1.68±0.1eV.