2DEG System Research Articles

Backgated layers and nanostructures

The characteristics of a novel backgated two-dimensional electron gas (2DEG) system and its application to layers and nanostructures are summarized. In the backgated system, the 2DEG is formed in an undoped heterostructure through a field effect, and we can expect high-mobility and precise controllability. We have successfully fabricated backgated electron bilayer systems, quantum point contacts and lateral periodic structures with a short period. The results clearly show that a backgated system has much potential for application to layers and nanostructures.

Contactless heating oftwo-dimensional electron gas systems by strip-line guidedmicrowaves

A practical contactless method for the determinationof electrical transport properties of heated two-dimensionalelectron gas (2DEG) systems is presented. The transportparameters have been obtained from Shubnikov-de-Haas (SdH)oscillations of the conductivity derived from the absorption ofmicrowaves. The microwave electric field was guided in acoplanar strip-line on top of the sample and was attenuated bythe transport electrons of the 2DEG. Heating of the electronscould be achieved by increasing the microwave power. Theelectron temperature Te was determined from theamplitude of the SdH oscillations. The variation of theabsorbed power Q with the electron temperature Tefollows a dependence Q∝Te5-TL5with TL being the lattice temperature.

Gas source molecular beam epitaxy of InP-based microstructures: material growth, characterization and device applications

InP epitaxial layers and InGaAs/InAlAs/InP modulation doped heterostructures were grown by gas source molecular beam epitaxy, in which arsenic and phosphorus beams were obtained by thermal cracking of AsH 3 and PH 3. High purity InP layers have been obtained by optimizing growth parameters. To identify the species of impurity and defect in high quality InP layers, low temperature photoluminescence (PL) and photothermal ionization spectroscopy measurements were carried out. The residual donors were determined to be silicon and sulfur. To obtain a deep insight to the electronic subband structure of the two-dimensional electron gas (2DEG) system, Shubnikov-de Haas, quantum Hall effect measurements and field-dependent cyclotron resonance measurements were conducted. The electron density on each subband in 2DEG was determined to be 2.84×10 12 and 3.4×10 11 cm −2, respectively. The effective masses of the two subbands are (0.061±0.001) m 0 for the zero subband and (0.049±0.001) m 0 for the first subband. An optoelectronic integrated circuit (OEIC) structure consisting of high electron mobility transistor (HEMT) and metal semiconductor metal photo detector (MSM-PD) was achieved by gas source molecular beam epitaxy with one growth run. The responsibility of MSM-PD is 0.5 A/ W. DC transconductance of an InGaAs/InAlAs HEMT with 1.5-μm gate length is 305 mS mm −1; the maximum saturation current is 350 mA mm −1 and the pinch-off voltage is −1.4 V. The gain of the HEMT amplifier is 14 dB. The integrated photoreceiver could successfully operate at a data rate of 622 Mbit s −1.

Ultra-low-temperature cooling of two-dimensional electron gas

A new design has been used for cooling GaAs/Al x Ga 1− x As sample to ultra-low-temperatures. The sample, with electrical contacts directly soldered to the sintered silver powder heat exchangers, was immersed in liquid 3 He , which was cooled by a PrNI 5 nuclear refrigerator. The data analysis shows that the two-dimensional electron gas (2DEG) was cooled to 4.0 mK at the refrigerator base temperature T b of 2.0 mK . The design with heat exchanger cooling is applicable to any ultra-low-temperature transport measurements of 2DEG system.

High frequency conductivity of the high-mobility two-dimensional electron gas

We measure the real and imaginary conductivity σ(k=0,ω) of a high-mobility two-dimensional electron gas (2DEG) system at frequencies below and above the momentum scattering rate. The imaginary part of the 2DEG impedance is observed to be inductive, consistent with the Drude model. Using this kinetic inductance, we construct a transmission line by capacitively coupling the 2DEG to an Al Schottky barrier gate separated by 5000 Å from the 2DEG. The measured wave velocity and temperature-dependent damping of this transmission line are in good agreement with a simple Drude model. Exciting these modes is equivalent to exciting a 2D plasma mode strongly modified by the interaction between the 2DEG and the gate.

Entanglement in 2DEG systems: towards a detection loophole-free test of Bell's inequality

We make use of the intrinsic noiseless and lossless aspect of an electron source in a 2DEG system to implement a new test of Bell's inequality. The generated entanglement can be tested by two-particle interferometry. Preparation and detection schemes of two complete sets of Bell states are given. A novel type of Bell's inequality is then derived in terms of noise correlation measurements. The characteristics of the electron source are essential to exhibit a violation. This electron system could close the detection efficiency loophole.

Heat instability of quantum Hall conductors

Breakdown of the integer quantum Hall effect (IQHE) is discussed based on the consideration of heat stability of a two-dimensional electron gas (2DEG). First, the 2DEG system in the IQHE state is suggested to become thermally unstable when the Hall electric field ${E}_{y}$ reaches a threshold value ${E}_{b}.$ Above ${E}_{b},$ a small number of excited electrons in the higher Landau level, which are initially present in the conductor as fluctuation, are accelerated by ${E}_{y},$ and the 2DEG thereby undergoes a transition to a warm dissipative state. This abrupt transition is referred to as the bootstrap-type electron heating (BSEH). Second, the critical field, ${E}_{b},$ for BSEH is theoretically derived and compared with the experimental values of the IQHE breakdown reported earlier by different groups. The agreement is satisfactory, suggesting that BSEH is the basic mechanism behind the IQHE breakdown. Third, dynamical aspects of BSEH are discussed. It is argued that the BSEH is a process of the avalanche-type electron-hole pair multiplication, in which a small number of nonequilibrium carriers gains kinetic energy within a Landau level and excites other electron-hole pairs via the inter-Landau-level impact ionization. Electrons travel a macroscopic distance during the process of electron-hole multiplication. This feature is confirmed by experiments. Experiments indicate that ${E}_{y}$ does not locally influence the longitudinal conductivity when the IQHE breaks down, providing definite proof of BSEH. Finally, a variety of characteristics of the IQHE, reported in earlier works, is reviewed and suggested to be consistent with the BSEH model.

Correlation Between Sheet Carrier Density-Mobility Product and Persistent Photoconductivity in AlGaN/GaN Modulation Doped Heterostructures

High quality Al0.25Ga0.75N/GaN modulation-doped heterojunction field-effect transistor (MOD-HFET) structures grown on sapphire substrates with high sheet carrier density and mobility products (nsμ > 1016/Vs at room temperature) have been grown by metal organic chemical vapor deposition (MOCVD). The optimized structures were achieved by varying structural parameters, including the AlGaN spacer layer thickness, the Si-doped AlGaN barrier layer thickness, the Si-doping concentration, and the growth pressure. In these structures, the persistent photoconductivity (PPC) effect associated with the two-dimensional electron gas (2DEG) system was invariantly observed. As a consequence, the characteristic parameters of the 2DEG were sensitive to light and the sensitivity was associated with permanent photoinduced increases in the 2DEG carrier mobility (μ) and sheet carrier density (ns). However, we observed that the magnitude of the PPC and hence the photoinduced instability associated with these heterostructures were a strong function of only one parameter, the product of ns and μ, which is the most important parameter for the HFET device design. For a fixed excitation photon dose, the ratio of the low temperature PPC to the dark conductivity level was observed to decrease from 200% to 3% as the nsμ (300 K) product was increased from 0.048 × 1016/Vs to 1.4 × 1016/Vs. Based on our studies, we suggest that the magnitude of the low temperature PPC can be used as a sensitive probe for monitoring the electronic quality of the AlGaN/GaN HFET structures.

Magnetization reversal and magnetoresistance in a lateral spin-injection device

We have investigated the magnetization reversal and magnetoresistance (MR) behavior of a lateral spin-injection device. The device consists of a two-dimensional electron gas (2DEG) system in an InAs quantum well and two ferromagnetic (Ni80Fe20) contacts: an injector (source) and a detector (drain). Spin-polarized electrons are injected from the first contact and propagating through InAs are collected by the second contact. By engineering the shape of the permalloy film distinct switching fields (Hc) from the injector and the collector have been observed by scanning Kerr microscopy and MR measurements. Magneto-optic Kerr effect (MOKE) hysteresis loops demonstrate that there is a range of magnetic field (20–60 Oe), at room temperature, over which magnetization in one contact is aligned antiparallel to that in the other. The MOKE results are consistent with the variation of the magnetoresistance in the spin-injection device.

Correlation between Sheet Carrier Density-Mobility Product and Persistent Photoconductivity in ALGAN/GAN Modulation Doped Heterostructures

AbstractHigh quality Al0.25Ga0.75N/GaN modulation-doped heterojunction field-effect transistor (MOD-HFET) structures grown on sapphire substrates with high sheet carrier density and mobility products (nsμ > 1016/Vs at room temperature) have been grown by metal organic chemical vapor deposition (MOCVD). The optimized structures were achieved by varying structural parameters, including the AlGaN spacer layer thickness, the Si-doped AlGaN barrier layer thickness, the Si-doping concentration, and the growth pressure. In these structures, the persistent photoconductivity (PPC) effect associated with the two-dimensional electron gas (2DEG) system was invariantly observed. As a consequence, the characteristic parameters of the 2DEG were sensitive to light and the sensitivity was associated with permanent photoinduced increases in the 2DEG carrier mobility (μ) and sheet carrier density (ns). However, we observed that the magnitude of the PPC and hence the photoinduced instability associated with these heterostructures were a strong function of only one parameter, the product of ns and μ, which is the most important parameter for the HFET device design. For a fixed excitation photon dose, the ratio of the low temperature PPC to the dark conductivity level was observed to decrease from 200% to 3% as the nsμ (300 K) product was increased from 0.048 × 1016/Vs to 1.4 ‘times; 1016/Vs. Based on our studies, we suggest that the magnitude of the low temperature PPC can be used as a sensitive probe for monitoring the electronic quality of the AlGaN/GaN HFET structures.

High-quality two-dimensional electron gas at an inverted undoped heterointerface

A back-gated undoped heterostructure, in which a two-dimensional electron gas (2DEG) is formed at the inverted undoped heterointerface through the back-side field effect, offers the possibility of high mobility and the feasibility of fabricating several kinds of back-gated structures. We used such a 2DEG system to fabricate a Corbino-disk structure. The results for the back-gated Corbino-disk structure show that the density of the 2DEG is well controlled by the back-gate bias and the fine structures corresponding to the integer and fractional quantized Hall effects are clearly observed, reflecting the high quality of the 2DEG formed in the undoped heterostructure. The characteristics in a low magnetic field region confirm the homogeneous back-gate control of the 2DEG down to a density of less than 1010cm−2.

Coupled cyclotron resonance transitions of bilayer 2DEG systems in GaAs

Electron cyclotron resonances in bilayer quasi two-dimensional electron systems are examined in the extreme magnetic quantum limit. The variation of the resonance frequencies with electron density strongly depends on the thickness of the barrier between the two layers. In samples with sufficiently thin barriers there is strong evidence for the presence of interlayer Coulomb coupling and influences of wavefunction delocalization effects.

Phase coherence of edge states over macroscopic length scales

Phase coherence of edge states in the integer quantum-Hall states is studied through the observation of interference effects occurring between two macroscopically separated gated regions of a two-dimensional electron gas (2DEG) system. When a 2DEG density in the two-gated regions varies simultaneously while the 2DEG regions without the gates are maintained in an integer quantum-Hall state, the transition width between successive integer quantum-Hall states significantly narrows as compared to the case when only one of the two gates is biased. This suggests coherent interference between the electron states in the separated two gated regions. By externally introducing partial phase-breaking into the electron system by weakly coupling the system to the Ohmic contact, the size of the phase coherence length of edge states is estimated to be 5.4 mm at a temperature of 20 mK and a magnetic field of 3.3 T.

Magnetotransport measurements on modulation Si δ-doped pseudomorphic In 0.2Ga 0.8As/GaAs quantum wells

The effect of doping concentration on electrical properties of Si δ-doped pseudomorphic In 0.2Ga 0.8As/GaAs quantum wells (QWs) was examined in this work. Magnetotransport measurements were carried out in the magnetic fields up to 12 T in the dark at the temperature of T=1.7 K. The results were analysed using the fast Fourier transform (FFT). It was found that one electron subband was occupied in the QW with the doping concentrations of 2.3×10 12 or 6.0×10 12 cm −2. The parallel conduction was present in both samples, which increased with an increase of Si δ-doping concentration. The well-developed plateaus arising from the quantised Hall effect were observed in the Si δ-doped In 0.2Ga 0.8As/GaAs QW with reduced parallel conduction. The two subbands remain occupied within the V-shaped potential well formed at the Si doping plane when the Si δ-doping concentration was high. The results of measurements in tilted magnetic fields, confirming the attribution of FFT peaks to the particular 2DEG systems, are also presented.

Two-dimensional electron gas in AlGaN/GaN heterostructures

The formation of a two-dimensional electron gas (2DEG) system by an AlGaN/GaN heterostructure has been further confirmed by measuring its electrical properties. The effect of persistent photoconductivity (PPC) has been observed and its unique features have been utilized to study the properties of 2DEG formed by the AlGaN/GaN heterointerface. Sharp electronic transitions from the first to the second subbands in the 2DEG channel have been observed by monitoring the 2DEG carrier mobility as a function of carrier concentration through the use of PPC. These results are expected to have significant implications on field-effect transistor and high electron mobility transistor applications based on the GaN system.

Determination of significant transport parameters of two-dimensional electron gas systems by microwave methods

The main transport parameters of a two-dimensional electron gas (2DEG) system, namely, the effective mass m*, electron sheet concentration ns, transport lifetime τc, and quantum lifetime τq, were determined exclusively by contactless microwave transmission methods in the frequency range 26.5–40 GHz. Shubnikov–de Haas oscillations were detected by measuring σxx with parallel waveguides while crossed waveguides were used for Hall measurements. The microwave results are in good agreement with values obtained from conventional direct current investigations. The 2DEG is realized in a modulation doped InGaAs/InP double heterostructure.

Atomic Force Microscope Nanofabrication of InAs/AlGaSb Heterostructures

We employ an atomic force microscope (AFM) for the fabrication of InAs/Al(Ga)Sb nanostructures. In order to make nanoscale modifications in the two-dimensional electron gas (2DEG) system, we apply an AFM oxidation process for GaSb, for the first time. We found that the oxidation of GaSb occurs for tip voltages over 7–8 V and that the oxidized GaSb region can be selectively removed by immersion in water for 10 min. We successfully fabricate a lateral periodic structure on the GaSb cap layer using AFM oxidation with a 0.3 µ m spacing. The induced increase in the 2DEG concentration is as high as 80% due to the surface modification. The magnetotransport measurement also implies structural modification of the 2DEG system.

Coulomb blockade as a non-invasive probe in double layer 2DEG systems

We have used a double two-dimensional electron gas (double 2DEG) sample to measure the density of states in one of the 2DEGs over a submicron area. The lower 2DEG screens the electric field between the plates of a mesoscopic capacitor made up of a quantum dot in the upper 2DEG and a backgate behind both 2DEGs. The capacitance is measured by observing the Coulomb blockade period of the dot as a function of backgate voltage. The density of states is inferred from the screening, the technique is sensitive to both localised and extended electronic states.

Transition from Semi-Classical to Quantum Transport in Quasi-Ballistic Wires

We have found evidence of a transition from the semi-classical to quantum transport in quasi-ballistic narrow wires constructed in 2DEG system of a GaAs/AlGaAs heterojunction. By increasing the ratio of the mean free path to the wire length while keeping the wire width constant, we find a clear offset between the semi-classical and the quantum behaviors in the two-terminal measurement of the wire resisitance. We can explain such an offset by taking into account the transmission probability of electron waves propagating in the wire.

Persistent Photoconductivity in p-Type GaN Epilayers and n-Type AlGaN/GaN Heterostructures

ABSTRACTPersistent photoconductivity (PPC) effect has been observed in p-type GaN epilayers grown both by metal-organic chemical vapor deposition (MOCVD) and reactive molecular beam epitaxy (MBE) as well as in a two-dimensional electron gas (2DEG) system formed by an AlGaN/GaN heterostructure grown by MOCVD. Its properties have been investigated at different conditions.