Double-channel Heterostructure Research Articles

Studies on the coupling correlation and strain state of AlGaN/GaN double channel heterostructures grown by metal organic chemical vapor deposition

In this work, high quality AlGaN/GaN double channel heterostructures were grown by metal organic chemical vapor deposition and the coupling correlation between the two channels, as well as the strain state of this multilayer structure are investigated in detail. It was revealed that the stronger coupling correlation affect the 2DEG distribution and leads to the lower channel is similar to the upper channel with the thinner thickness of the upper channel. In addition, the impact of the multilayer strain state on the characteristics of double channel heterostructures was discussed. Moreover, the effects of coupling correlation and strain state on the performance of double channel high electron mobility transistor (HEMT) was shown by 2-D simulation. This work is not only beneficial for the further study of the double (multiple) channel heterostructures and devices, but also instructive to the design and realization of the future atomic hyperfine structures of nitride semiconductors.

Confined and propagating optical phonons in double-channel AlGaN/GaN heterostructures

Based on the dielectric continuous model, the confined and propagating optical phonon modes in AlGaN/GaN double-channel heterostructures are studied by using the transfer matrix method . Their dispersion relations and electrostatic potentials are discussed considering the ternary mixed crystal and size effects. The results show that both confined and propagating phonon modes can be classified into two groups: low- and high-frequency modes. Ternary mixed crystal effect only affects the low-frequency confined phonons , but has little influence on the high-frequency ones. The thicknesses of the AlGaN barriers have a significant impact on the confined phonons oscillating in both barriers and channels, but the thicknesses of the GaN channels only affect the confined phonons oscillating in channels. It is also found that the propagating modes can be ignored in the further researches because of the narrow frequency range allowing them to exist. These findings indicate that the scattering from the optical phonons in double-channel heterostructures can be modulated to improve the performance of related devices by adjusting the Al composition of AlGaN and the size of each layer. • Confined and propagating optical phonon modes were studied in AlGaN/GaN double-channel heterostructures. • The dispersion relation and electrostatic potential of phonon were analyzed in this structure. • The ternary mixed crystal effect and size effect were discussed in detail. • The anisotropy of wurtzite nitrides was included.

Interface optical phonons in double-channel AlGaN/GaN heterostructures: The ternary mixed crystal effect and size effect

Considering the anisotropy of wurtzite semiconductors, the interface optical phonons in double-channel AlGaN/GaN heterostructures are investigated by using a dielectric continuous model and transfer matrix method. Also, the ternary mixed crystal effect and size effect on the dispersion relations and electrostatic potentials of phonons are analyzed in detail. The results show that there are six branches of interface phonon modes in a double-channel heterostructure. For some values of Al composition, however, the phonon mode with the highest frequency may not exist, especially when the thicknesses of materials and the wave vectors of phonons are small. The ternary mixed crystal effect and size effect not only influence the values of phonon frequency and electrostatic potential, but also change the vibration mode of interface phonons. This suggests that the interface phonon vibrations can be controlled to reduce their adverse effects by changing the Al composition of AlGaN and the thickness of each layer in a double-channel heterostructure.

Studies on the InAlN/InGaN/InAlN/InGaN double channel heterostructures with low sheet resistance

High quality InAlN/InGaN/InAlN/InGaN double channel heterostructures were proposed and grown by metal organic chemical vapor deposition. Benefiting from the adoption of the pulsed growth method and Two-Step AlN interlayer, the material quality and interface characteristics of the double channel heterostructures are satisfactory. The results of the temperature-dependent Hall effect measurement indicated that the transport properties of the double channel heterostructures were superior to those of the traditional single channel heterostructures in the whole test temperature range. Meanwhile, the sheet resistance of the double channel heterostructures reached 218.5 Ω/□ at 300 K, which is the record of InGaN-based heterostructures. The good transport properties of the InGaN double channel heterostructures are beneficial to improve the performance of the microwave power devices based on nitride semiconductors.

Gbps terahertz external modulator based on a composite metamaterial with a double-channel heterostructure.

The past few decades have witnessed a substantial increase in terahertz (THz) research. Utilizing THz waves to transmit communication and imaging data has created a high demand for phase and amplitude modulation. However, current active THz devices, including modulators and switches, still cannot meet THz system demands. Double-channel heterostructures, an alternative semiconductor system, can support nanoscale two-dimensional electron gases (2DEGs) with high carrier concentration and mobility and provide a new way to develop active THz devices. In this Letter, we present a composite metamaterial structure that combines an equivalent collective dipolar array with a double-channel heterostructure to obtain an effective, ultrafast, and all-electronic grid-controlled THz modulator. Electrical control allows for resonant mode conversion between two different dipolar resonances in the active device, which significantly improves the modulation speed and depth. This THz modulator is the first to achieve a 1 GHz modulation speed and 85% modulation depth during real-time dynamic tests. Moreover, a 1.19 rad phase shift was realized. A wireless free-space-modulation THz communication system based on this external THz modulator was tested using 0.2 Gbps eye patterns. Therefore, this active composite metamaterial modulator provides a basis for the development of effective and ultrafast dynamic devices for THz wireless communication and imaging systems.

The effect of GaN thickness inserted between two AlN layers on the transport properties of a lattice matched AlInN/AlN/GaN/AlN/GaN double channel heterostructure

One AlInN/AlN/GaN single channel heterostructure sample and four AlInN/AlN/GaN/AlN/GaN double channel heterostructure samples with different values of the second GaN layer were studied. The interface profiles, crystalline qualities, surface morphologies, and dislocation densities of the samples were investigated using high resolution transmission electron microscopy, atomic force microscopy, and high-resolution X-ray diffraction. Some of the data provided by these measurements were used as input parameters in the calculation of the scattering mechanisms that govern the transport properties of the studied samples. Experimental transport data were obtained using temperature dependent Hall effect measurements (10–300K) at low (0.5T) and high (8T) magnetic fields to exclude the bulk transport from the two-dimensional one. The effect of the thickness of the second GaN layer inserted between two AlN barrier layers on mobility and carrier concentrations was analyzed and the dominant scattering mechanisms in the low and high temperature regimes were determined. It was found that Hall mobility increases as the thickness of GaN increases until 5nm at a low temperature where interface roughness scattering is observed as one of the dominant scattering mechanisms. When GaN thicknesses exceed 5nm, Hall mobility tends to decrease again due to the population of the second channel in which the interface becomes worse compared to the other one. From these analyses, 5nm GaN layer thicknesses were found to be the optimum thicknesses required for high electron mobility.

Current-Transport Mechanisms in the AlInN/AlN/GaN single-channel and AlInN/AlN/GaN/AlN/GaN double-channel heterostructures

Current-transport mechanisms were investigated in Schottky contacts on AlInN/AlN/GaN single channel (SC) and AlInN/AlN/GaN/AlN/GaN double channel (DC) heterostructures. A simple model was adapted to the current-transport mechanisms in DC heterostructure. In this model, two Schottky diodes are in series: one is a metal–semiconductor barrier layer (AIInN) Schottky diode and the other is an equivalent Schottky diode, which is due to the heterojunction between the AlN and GaN layer. Capacitance–voltage studies show the formation of a two-dimensional electron gas at the AlN/GaN interface in the SC and the first AlN/GaN interface from the substrate direction in the DC. In order to determine the current mechanisms for SC and DC heterostructures, we fit the analytical expressions given for the tunneling current to the experimental current–voltage data over a wide range of applied biases as well as at different temperatures. We observed a weak temperature dependence of the saturation current and a fairly small dependence on the temperature of the tunneling parameters in this temperature range. At both a low and medium forward-bias voltage values for Schottky contacts on AlInN/AlN/GaN/AlN/GaN DC and AlInN/AlN/GaN SC heterostructures, the data are consistent with electron tunneling to deep levels in the vicinity of mixed/screw dislocations in the temperature range of 80–420K.

Pulsed metal organic chemical vapor deposition of nearly latticed-matched InAlN/GaN/InAlN/GaN double-channel high electron mobility transistors

High quality, nearly lattice-matched InAlN/GaN/InAlN/GaN double-channel heterostructures were grown on sapphire by pulsed-metal-organic-chemical-vapor-deposition (PMOCVD). High electron mobility of 1414 cm2/Vs was achieved along with a two-dimensional-electron-gas density of 2.55 × 1013 cm−2. We attribute it to the high quality PMOCVD-grown InAlN barriers and, additionally, to the novel GaN layer growth between two InAlN barriers, which consists of a thin GaN spacer to prevent indium-redistribution and indium-cluster formation during the subsequent growth and a relatively thick GaN channel to enhance electron mobility. High-electron-mobility-transistors fabricated on these heterostructures with 0.8-μm-length gate exhibit a maximum drain current of 906 mA/mm and a transconductance of 186 mS/mm.

Highly uniform sheet resistance of the double-channel AlInN/GaN heterostructure

A high uniformity of sheet resistance was achieved in the double-channel (DC) Al 0.82In 0.18N/GaN heterostructure by lowering the interface roughness scattering effect. The variation of the AlInN/GaN interface roughness as a key factor influenced the uniformity of the sheet resistance. In the DC heterostructure, the distribution of the two dimension electron gas (2DEG) was modified to reduce interface roughness scattering effect. As a result, the uniformity of the sheet resistance was enhanced, and the nonuniformity of the sheet resistance in the DC Al 0.82In 0.18N/GaN could be reduced to 0.7% after structure optimization.

High electron mobility and low sheet resistance in lattice-matched AlInN/AlN/GaN/AlN/GaN double-channel heterostructure

High electron mobility and low sheet resistance were achieved in lattice-matched AlInN/AlN/GaN/AlN/GaN double-channel (DC) heterostructure. Two-dimensional electron gas (2DEG) of the DC heterostructure was divided into the double channels and the room-temperature mobility was increased to 1430 cm2/V s by reducing the 2DEG density in each channel, compared with low electron mobility (1090 cm2/V s) for lattice-matched AlInN/AlN/GaN single-channel heterostructure. It was found that the 2DEG mobility was limited by thickness of the AlN interlayer between the double channels. After the structure optimization, the room temperature electron mobility of the DC heterostructure reached 1570 cm2/V s with sheet resistance of 222 Ω/◻.

Use of double-channel heterostructures to improve the access resistance and linearity in GaN-based HEMTs

Double-channel structures have been used in AlGaN/GaN high electron mobility transistors to reduce the access resistance. Carrier densities as high as 2.9/spl times/10/sup 13/ cm/sup -2/ and mobilities in the 1300 cm/sup 2//V/spl middot/s range have been obtained in the access region. Also, the correct design of the potential barrier between the different channels allowed tailoring the differential access resistance to enhance the linearity of the transistors. This increase in linearity has been measured as a flatter profile of the transconductance and cutoff frequency versus current and as an improvement of more than 2 dB in large-signal two-tone linearity measurements.

Enhanced Current-Voltage Characteristics of Al0.25Ga0.75As/In0.25Ga0.75As/GaAs P-HEMTs Using an Inverted Double Channel Structure

An inverted double channel Al0.25Ga0.75As/In0.25Ga0.75As/GaAs pseudomorphic high electron mobility transistor (P-HEMT) grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) has been demonstrated for the first time. The inverted double channel heterostructure shows a high two-dimensional electron gas (2-DEG) concentration of 4.53×1012 cm-2 along with a large mobility of 5010 cm2/V·s at 300 K, respectively. The fabricated P-HEMT device with a gate dimension of 1.8×200 µm2 shows a maximum drain current of as high as 820 mA/mm and a maximum extrinsic transconductance of 320 mS/mm at 300 K. Also, extrinsic transconductance is sustained over a wide range of gate voltages from -2.0 V to 1.8 V. In addition, a high two-terminal gate-drain reverse breakdown voltage of -17 V is obtained. The results obtained show a great potential of the inverted double channel P-HEMT for power applications.