Chuanxin Lian

Polarization-Induced Hole Doping in Wide–Band-Gap Uniaxial Semiconductor Heterostructures

Activating Stubborn Dopants Many applications of semiconductor light-emitting diodes and lasers, such as reading optical disks, benefit from shorter wavelengths, but this requires materials with larger energy gaps between their valance and conduction bands. The electronic conductivity of these materials often has to be increased by doping with impurity atoms. However, in nitride materials, such as GaN and AlGaN, hole doping with acceptor atoms such as Mg is ineffective at room temperature. Simon et al. (p. 60) grew a gradient of AlGaN on the surface of GaN and found that the polarization of the layer could field-ionize the acceptor dopants efficiently at room temperature. The heterostructure was used successfully in a light-emitting diode that emits in the ultraviolet. A compositional gradient of two semiconductors creates an electronic polarization that ionizes and activates dopant atoms. Impurity-based p-type doping in wide–band-gap semiconductors is inefficient at room temperature for applications such as lasers because the positive-charge carriers (holes) have a large thermal activation energy. We demonstrate high-efficiency p-type doping by ionizing acceptor dopants using the built-in electronic polarization in bulk uniaxial semiconductor crystals. Because the mobile hole gases are field-ionized, they are robust to thermal freezeout effects and lead to major improvements in p-type electrical conductivity. The new doping technique results in improved optical emission efficiency in prototype ultraviolet light-emitting–diode structures. Polarization-induced doping provides an attractive solution to both p- and n-type doping problems in wide–band-gap semiconductors and offers an unconventional path for the development of solid-state deep-ultraviolet optoelectronic devices and wide–band-gap bipolar electronic devices of the future.

Gate-Recessed Enhancement-Mode InAlN/AlN/GaN HEMTs With 1.9-A/mm Drain Current Density and 800-mS/mm Transconductance

Having a drain current density of 1.9 A/mm, a peak extrinsic transconductance of 800 mS/mm (the highest reported in III-nitride transistors), ft/fmax of 70/105 GHz, and Vbr of 29 V, 150-nm-gate enhancement-mode InAlN/AlN/GaN high-electron-mobility transistors are demonstrated on SiC substrates using plasma-based gate-recess etch. The possible plasma-induced damage in the gate region was investigated using interface-trap states extracted from temperature-dependent subthreshold slopes.

Polarization‐engineering in group III‐nitride heterostructures: New opportunities for device design

The role of spontaneous and piezoelectric polarization in III-V nitride heterostructure devices is discussed. Problems as well as opportunities in incorporating polarization in abrupt and graded heterojunctions composed of binary, ternary, and quaternary nitrides are outlined.

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.

Quantum transport in graphene nanoribbons patterned by metal masks

Graphene nanoribbons (GNRs) were fabricated by metal mask lithography and plasma etching. GNRs with width ∼20 nm show field-effect conductance modulation of ∼12 at room temperature and >106 at 4.2 K. Conductance quantization due to quantum confinement in low field transport was observed. Landauer formula was utilized to fit the experimental data and excellent agreement was obtained. The extracted subband energy separation was found to deviate from the predicted values of perfect armchair GNRs. Transmission probability is much smaller than unity due to scattering by GNR edge/bulk disorder and impurities, indicating a mean free path ∼40 nm. High field family I-Vs exhibited current saturation tendency and current density as high as 2 A/mm has been measured at low temperature.

Threshold Voltage Control in

The first demonstration of high-Al-composition (> 70%) AlGaN high electron mobility transistors (HEMTs) is reported. High electron mobility (~1300 cm2/Vs at room temperature) was achieved in novel high-Al-composition AlGaN 2-D electron gas structures. The threshold voltages (Vth) of Al0.72Ga0.28N/AlN/GaN HEMTs were shifted from -1.0 to -0.13 V by employing different gate metal stacks, Al/Au and Ni/Au, respectively. With a 4-nm Al2O3 gate dielectric on top of the nitride heterostructures, the ~0.9-eV work-function difference between Al and Ni induced ~0.9-V Vth shift in the pairs of the Al/Au and Ni/Au gate HEMTs, which indicates that the Fermi level is unpinned at the ALD Al2O3/AlGaN interface. The results were reproducible for HEMTs of various gate lengths. The results suggest that it is possible to obtain enhancement- and depletion-mode AlGaN HEMTs using work-function engineering which can enable integrated monolithic digital circuits without postgrowth recess etching or ion implantation.

\hbox{Al}_{0.72} \hbox{Ga}_{0.28}\hbox{N/AlN/GaN}

We have fabricated AlGaAs/GaAs/GaN heterojunction bipolar transistors (HBTs) formed by direct wafer fusion with different fusion temperatures. By employing a low wafer fusion temperature of 550 degC, current gains as high as ~9 and output currents as high as ~65 mA (emitter size of 100times120 mum2) were obtained. The effective minority carrier lifetime in the base was estimated to have decreased ~20 times due to the fusion process. In comparison, HBTs produced with higher wafer fusion temperatures (600 degC and 650 degC) exhibit lower current gains (~2-3) and higher base-collector leakage currents

HEMTs by Work-Function Engineering

GaAs/GaN pn heterojunction diodes have been fabricated by direct wafer fusion and characterized by capacitance-voltage (C-V) measurements and temperature dependent current-voltage (I-V) measurements. The wafer-fused pn diode showed a good rectifying behavior, but a small turn-on voltage was observed, which was attributed to defect-assisted tunneling-recombination. The flat-band voltage extracted from C-V is around 0.46 V, much smaller than the built-in voltage calculated for an ideal GaAs/GaN pn heterojunction. A band diagram including interface charge effects together with a possible energy barrier, stemming from a layer of disordered material at the fused GaAs/GaN interface, has been proposed to explain the experimental observations.

DC Characteristics of AlGaAs/GaAs/GaN HBTs Formed by Direct Wafer Fusion

The surface potential distribution across lateral Ni–(Al)GaN Schottky junctions was measured by scanning Kelvin probe microscopy. The bare surface barrier heights of unintentionally doped Al0.22Ga0.78N and n‐GaN in air were estimated to be ∼1.15 and 0.7 eV, respectively. Upon 364.5 nm (band edge for GaN) illumination, the surface barriers of both n‐GaN and AlGaN∕GaN were observed to decrease. The minority carrier diffusion length in n‐GaN(Si∼3.5×1017cm−3) was extracted from the surface photovoltage profile near the Schottky junction, ∼1.8±0.4μm. The scanning Kelvin probe surface photovoltage technique for measuring minority carrier diffusion length, while similar to the electron beam induced current technique, offers greater accuracy and higher spatial resolution due to separation of the minority carrier excitation source (relatively large area, above band gap light beam) from the nanometer-size probe (scanning force microscope tip).

Electrical transport properties of wafer-fused p-GaAs/n-GaN heterojunctions

The authors have compared AlGaAs∕GaAs∕GaN heterojunction bipolar transistors (HBTs) formed by wafer fusion with AlGaAs∕GaAs∕GaAs as-grown HBTs subject to high temperature annealing conditions similar to those used in the wafer fusion process. The high temperature annealing alone is found to cause gain degradation by a factor of 2–6, a result of reduction in minority carrier lifetime in the base. Detailed analysis indicates that the fused HBTs also suffer from higher recombination in the emitter-base junction, exacerbated base degradation as well as effective potential barriers formed at the GaAs base/GaN collector junction.