Properties Of Two-dimensional Electron Gas Research Articles

Quantum transport properties of two-dimensional electron gases under high magnetic fields

We study quantum transport properties of two-dimensional electron gases under high perpendicular magnetic fields. For this purpose, we reformulate the high-field expansion, usually done in the operatorial language of the guiding-center coordinates, in terms of vortex states within the framework of real-time Green functions. These vortex states arise naturally from the consideration that the Landau levels quantization can follow directly from the existence of a topological winding number. The microscopic computation of the current can then be performed within the Keldysh formalism in a systematic way at finite magnetic fields $B$ (i.e., beyond the semiclassical limit $B=\ensuremath{\infty}$). The formalism allows us to define a general vortex current density as long as the gradient expansion theory is applicable. As a result, the total current is expressed in terms of edge contributions only. We obtain the first and third lowest order contributions to the current due to Landau-levels mixing processes, and derive in a transparent way the quantization of the Hall conductance. Finally, we point out qualitatively the importance of inhomogeneities of the vortex density to capture the dissipative longitudinal transport.

Improved electrical properties in AlGaN∕GaN heterostructures using AlN∕GaN superlattice as a quasi-AlGaN barrier

The authors report the electrical properties of two-dimensional electron gas (2DEG) in AlGaN∕GaN heterostructures using AlN∕GaN superlattices working as a quasi-AlGaN barrier layer. It is found that the electrical properties (2DEG mobility and sheet carrier density) in the quasi-AlGaN∕GaN heterostructure are greatly improved compared to those in the conventional alloy-AlGaN∕GaN one at the high Al composition more than 0.35. The improved 2DEG properties result in the reduction of the sheet resistance to as low as 172Ω∕□, which is extremely important for the high power and high frequency device application. Theoretical calculations clearly indicate that the quasi-AlGaN barrier plays an important role in enhancing the confinement of the carrier at the quasi-AlGaN∕GaN interface.

Orbital magnetism of two-dimensional electron gas in a crossed electromagnetic field: theeffect of spin–orbit interaction, confined geometries and defects

I have studied the magnetic properties of two-dimensional electron gas in the presence of acrossed electromagnetic field under a confining harmonic potential in various regimes oftemperature and magnetic field. It is found that the magnetization shows four types ofoscillation depending on the applied electromagnetic field and temperature: de Haas–vanAlphen (dHvA) oscillation, Aharonov–Bohm oscillation, Landau diamagnetism andmesoscopic fluctuations. I have shown explicitly how the spin–orbit interactionsplits up the dHvA oscillation into ‘spin-up’ and ‘spin-down’ branches. The effectof the restricted geometries has been mimicked in the model by choosing theparameters of the confining potential in such a way that it represents mesoscopicsamples. The dephasing and the reduction of the dHvA oscillations in the presenceof any kind of inhomogeneity have been discussed in this paper in great detail.

The effect of AlN growth time on the electrical properties of Al 0.38Ga 0.62N/AlN/GaN HEMT structures

Al 0.38Ga 0.62N/AlN/GaN HEMT structures have been grown by metal–organic chemical vapor deposition (MOCVD) on 2-inch sapphire substrates. Samples with AlN growth time of 0 s (without AlN interlayer), 12, 15, 18 and 24 s are characterized and compared. The electrical properties of two-dimensional electron gas (2DEG) are improved by introducing AlN interlayers. The AlN growth time in the range of 12–18 s, corresponding to the AlN thickness of 1–1.5 nm, is appropriate for the design of Al 0.38Ga 0.62N/AlN/GaN HEMT structures. The lowest sheet resistance of 277 Ω sq −1 and highest room temperature 2DEG mobility of 1460 cm 2 V −1 s −1 are obtained on structure with AlN growth time of 12 s. The structure with AlN growth time of 15 s exhibits the highest 2DEG concentration of 1.59×10 13 cm −2 and the smallest RMS surface roughness of 0.2 nm.

Ferroelectric gate for control of transport properties of two-dimensional electron gas at AlGaN∕GaN heterostructures

The concept of a field-effect transistor with ferroelectric gate has been implemented using the GaN∕AlGaN heterostructure combined with Pb(Zr,Ti)O3 ferroelectric layer. The processing conditions were optimized in a way to obtain textured Pb(Zr,Ti)O3 films without destroying the two-dimensional electron gas (2D gas) situated at 20nm below the interface. Poling the ferroelectric layer through the conductive cantilever of the atomic force microscope results in a stable change of resistance of the 2D gas. Concentration and mobility of electrons in the 2D gas were monitored using Hall effect and four-probe conduction measurements in a wide temperature range from 12to300K. The gate polarization oriented in the direction from bottom to top provokes a partial depletion in the channel, resulting in a conductivity decrease by factor of 2.5–3. The depletion effect is found to be reversible so that the initial conductivity in the 2D gas can be restored by depoling the ferroelectric gate. These results indicate that the ferroelectric gate integrated onto the GaN-based system with 2D gas is potentially interesting for nonvolatile memories. Additionally, direct domain writing on a ferroelectric gate represents a flexible and nondestructive way of making rewritable nanopatterns projected onto low-dimensional semiconductor structures.

Magnetotransport in two-dimensional electron gas: The interplay between spin-orbit interaction and Zeeman splitting

We investigate theoretically the interplay between Zeeman splitting, Rashba spin-orbit interaction (RSOI), and Dresselhaus spin-orbit interaction (DSOI) and its influence on the magnetotransport property of two-dimensional electron gas (2DEG) at low temperature. Our theoretical results show that the nodes of the beating patterns of the magnetoresistivity ${\ensuremath{\rho}}_{\mathrm{xx}}$ for 2DEG with RSOI or DSOI alone depend sensitively on the total spin splitting induced by these three spin splitting mechanisms. It is interesting to find that the eigenstates in the presence of RSOI alone are connected with those in the presence of DSOI alone but with opposite Zeeman splitting by a time-reversal transformation. Consequently, the magnetoresistivities exhibit exactly the same oscillation patterns for these two cases. For strong RSOI or DSOI alone, the magneto-oscillation of ${\ensuremath{\rho}}_{\mathrm{xx}}$ shows two distinct periods. For 2DEG with both RSOI and DSOI, the beating patterns vanish for equal RSOI and DSOI strengths and vanishing Zeeman splitting. They will appear again, however, when Zeeman splitting or the difference between RSOI and DSOI strengths increases.

Theoretical analysis about the influence of channel layer thickness on the 2D electron gas and its distribution in InP-based high-electron-mobility transistors

The principle of high-electron-mobility transistor (HEMT) and the property of two-dimensional electron gas (2DEG) have been analyzed theoretically. The concentration and distribution of 2DEG in various channel layers are calculated by numerical method. Variation of 2DEG concentration in different subband of the quantum well is discussed in detail. Calculated results show that sheet electron concentration of 2DEG in the channel is affected slightly by the thickness of the channel. But the proportion of electrons inhabited in different subbands can be affected by the thickness of the channel. When the size of channel lies between 20—25 nm, the number of electrons occupying the second subband reaches the maximum. This result can be used in parameter design of materials and devices.

Magneto-optics of two-dimensional electron gases modified by strong Coulomb interactions in ZnSe quantum wells

The optical properties of two-dimensional electron gases in ZnSe/(Zn,Be)Se and ZnSe/(Zn,Be,Mg)Se modulation-doped quantum wells with electron densities up to 1.4x10^{12} cm^{-2} were studied by photoluminescence, photoluminescence excitation and reflectivity in a temperature range between 1.6 and 70 K and in external magnetic fields up to 48 T. In these structures, the Fermi energy of the two-dimensional electron gas falls in the range between the trion binding energy and the exciton binding energy. Optical spectra in this regime are shown to be strongly influenced by the Coulomb interaction between electrons and photoexcited holes. In high magnetic fields, when the filling factor of the two-dimensional electron gas becomes smaller than two, a change from Landau-level-like spectra to exciton-like spectra occurs. We attempt to provide a phenomenological description of the evolution of optical spectra for quantum wells with strong Coulomb interactions.

High-quality two-dimensional electron gas at large scale GaN/AlGaN wafer interface prepared by mass production MOCVD systems

Basic electronic properties of two-dimensional electron gas (2DEG) formed at GaN/AlGaN hetero-interface in large-scale (100 mm) wafer made by metal organic chemical vapour deposition (MOCVD) have been reported and discussed. From conventional Hall measurements, highest electron mobility was found to be μ e∼1680 and 9000 cm 2/V s at room temperature and at ∼5 K, respectively, for sheet electron density of n s∼8×10 12 cm −2. In magneto-resistance (MR) measurements carried out at 1.5 K in Hall bar sample defined by photolithography and ion implantation, very clear Schubnikov de-Haas oscillations and integer quantum Hall effect were observed in diagonal ( R xx ) and off-diagonal ( R xy ) resistances, respectively. In addition, a good insulating nature of GaN layer is confirmed by capacitance–voltage ( C– V) measurement. These results suggest the high-qualitiness of our 100 mm GaN/AlGaN high electron mobility transistor (HEMT) wafers comparable to those so far reported.

Magnetotransport properties of two-dimensional electron gas in AlSb∕InAs quantum well structures designed for device applications

The mobility and the sheet electron density of two-dimensional electron gas in AlSb∕InAs quantum well structures optimized for device applications were measured in the temperature range 4.2K<T<90K. A maximum electron mobility μ=3.24×105 was found at 4.2K at a sheet electron density n2D=1.1×1012cm−2. Measurements of the integral quantum Hall and Shubnikov-de Haas oscillations in the temperature range 0.07–9K were also carried out to obtain additional information on the characteristics of the two-dimensional electron gas. The electron effective mass m* and the effective electron g-factor g* were determined from these measurements and found to be, respectively, 0.032m0 and 14.6. The latter is in good agreement with the recent experimental data obtained from cyclotron resonance and titled magnetic-field experiments.

Magnetotransport properties of two-dimensional electron gas on cylindrical surface

Cylindrical GaAs/AlGaAs quantum wells containing two-dimensional electron gas (2DEG) have been fabricated. To prepare the quantum wells and contact pads for them, the directional rolling of strained heterofilms into tubes was used. For the first time, magnetotransport properties of 2DEG on a cylindrical surface with a radius of curvature of 4 μm have been experimentally studied. The difference between the magnetoresistances of planar and rolled quantum wells have been examined and explained.

Magnetotransport properties of two-dimensional electron gas on cylindrical surface

Cylindrical GaAs/AlGaAs quantum wells containing two-dimensional electron gas (2DEG) have been fabricated. To prepare the quantum wells and contact pads for them, the directional rolling of strained heterofilms into tubes was used. For the first time, magnetotransport properties of 2DEG on a cylindrical surface with a radius of curvature of 4 μm have been experimentally studied. The difference between the magnetoresistances of planar and rolled quantum wells have been examined and explained.

Study of Spin-Polarized Transport Properties for Spin-FET Design Optimization

A Monte Carlo method developed previously for spin dynamics is applied to study spin-polarized transport properties of two-dimensional electron gas in semiconductor spin-FET structure. The specific symmetry of spin-orbit terms (Rashba and Dresselhaus) leads to strong anisotropy of spin dynamics in the low field regime. Coherent spin evolution and spin dephasing are investigated for different orientations of the device channel related to the crystallographic axes. Efforts have been made to suppress spin dephasing while conserving coherent oscillation of spin polarization required for spin-FET design. Results derived from this study provide useful information to assist in optimization of the spin-FET performance.

MBE growth and studies of hybrid heterostructures with II–VI/InAs heterovalent interfaces in the active region

InAs quantum wells (QWs) with AlAsSb and CdMgSe asymmetric barriers have been fabricated by a two-step molecular beam epitaxy on InAs(100) substrates. A 5-monolayer-thick ZnTe layer has been introduced between the InAs and CdMgSe layers as a buffer of the II–VI growth initiation. Ex-situ S-passivation technique applied to the uncapped InAs surface provides formation of defect free III–V/II–VI heterovalent interface. Photoluminescence study proves a strong type I band line-up at the ZnTe-mediated InAs/CdMgSe interface with the conduction and valence band offsets of about 0.6 eV and 1.0 eV, respectively. Transport properties of two-dimensional electron gas confined in a 16-nm-thick AlAsSb/InAs/ZnTe/CdMgSe QWs grown on GaAs (100) are compared with those of pure III–V 2DEG structures. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Large effective mass enhancement of the InAs 1− xN x alloys in the dilute limit probed by Shubnikov-de Haas oscillations

Transport properties of two-dimensional electron gases formed in low-nitrogen-content InAs 1− x N x /InGaAs single quantum wells have been investigated using Shubnikov-de Haas (SdH) oscillations. We determine the nitrogen-content-dependent two-dimensional carrier concentration and electron effective mass by analyzing the SdH oscillation function. The carrier mobility decreases with the increase of nitrogen composition, suggesting some deterioration of crystal quality. Our result shows that even in the dilute alloy limit, the electron effective mass increases considerably, and such an enhancement cannot be explained by the present simple band anticrossing model.

Electron mobility in an AlGaN/GaN two-dimensional electron gas I-carrier concentration dependent mobility

The transport properties of two-dimensional electron gas (2-DEG) at the AlGaN/GaN interface were studied by characterizing the 2-DEG mobility dependence on carrier concentration, n/sub s/, and temperature. High-quality AlGaN/GaN heterostructures were grown, and heterostructure field effect transistors (HFETs) using a Fat FET geometry were fabricated. Measurements of 2-DEG mobility were performed by magnetoresistance and capacitance-conductance. In order to understand the dominant transport factors, the mobility was modeled using different scattering mechanisms and compared to our results. It is found that mobility dependence on n/sub s/ shows a bell-shape behavior over the whole temperature range. For low n/sub s/ the mobility is dominated by Coulomb interaction from interface charge, and at high n/sub s/ the mobility is dominated by interface roughness. Using previously reported experimental values of interface charge and interface roughness in our modeling, we show good agreement with mobility measurement results. Scattering from interface states in AlGaN/GaN heterostructures, seems to be related to the high polarization field in the heterointerface. At temperatures higher than 200K polar optical phonon scattering dominates the transport, yet both interface charge and roughness affect the mobility at the low and high n/sub s/, respectively.

Two-dimensional electron gases induced by polarization charges in AlN/GaN heterostructure grown by plasma-assisted molecular-beam epitaxy

We discuss the growth and transport properties of two-dimensional electron gas confined at the AlN/GaN heterointerface grown by plasma-assisted molecular-beam epitaxy. The sheet carrier density was found to be highly dependent on the barrier thickness of AlN grown on a doped or undoped GaN layer. The carrier sheet density monotonously increased from 0.8×1012 to 1.1×1013 cm−2 as the AlN barrier thickness on a semi-insulating GaN layer increased from 15 to 25 Å due to spontaneous and piezoelectric polarization. An AlN barrier of 35 Å in thickness grown on n-GaN gave the highest sheet carrier density, which was 4.3×1013 cm−2. In thin AlN barrier layers, the sheet carrier density was low due to surface depletion.

Designing two-dimensional electron gas in AlGaN/InGaN/GaN heterostructures through the incorporated InGaN layer

In this paper, we took a brief investigation on the properties of two-dimensional electron gas (2DEG) in AlGaN/InGaN/GaN heterostructures. Band profiles and the 2DEG distribution were calculated by self-consistently solving the Schrödinger and Poisson equations. Our calculated results indicate that the 2DEG concentration can be considerably increased with the incorporation of an InGaN layer. Quantum confinement, and thereby the 2DEG distribution are very sensitive to the strain of the incorporated InGaN layer. Hence one can design the 2DEG by incorporating proper InGaN layer into the conventional AlGaN/GaN heterostructures. These novel features are attributed to the strong polarization effect in AlGaN/InGaN/GaN heterostructures.

Transport properties of two-dimensional electron gas in different subbands in triangular quantum wells at AlxGa1−xN/GaN heterointerfaces

Magnetotransport properties of modulation-doped Al0.22Ga0.78N/GaN heterostructures were investigated by means of magnetoresistance measurements at low temperatures and high magnetic fields. Strong Shubnikov–de Haas oscillations with the double periodicity are observed. The mobility spectrum is obtained, which demonstrates that the mobilities of the two-dimensional electron gas (2DEG) in the two subbands in the triangular quantum well at heterointerface. It is found that the mobility of the 2DEG in the second subband is much higher than that in the first one. This is explained that interface roughness scattering and alloy disorder scattering have much stronger influence on transport properties of the 2DEG in the first subband than that in the second subband in AlxGa1−xN/GaN heterostructures.

Strain relaxation correlated with the transport properties of AlN/GaN heterostructure grown by plasma-assisted molecular-beam epitaxy

We report on the transport properties of two-dimensional electron gases confined at the AlN/GaN heterointerface grown by plasma-assisted molecular-beam epitaxy on (0001) sapphire substrates. Two-dimensional (2D) sheet carrier density has been found to vary with the AlN barrier layer thickness due to the existence of strain-induced piezoelectric polarization field at the heterointerface. The highest 2D sheet carrier density achieved was 2.3×1013 cm−2 for a 35-Å-thick AlN barrier layer. Further, with the increase of the AlN barrier width about 50∼60 Å, a drop in 2D sheet carrier density was noticed due to the annihilation of piezoelectric polarization, caused by the partial tensile-strain relaxation of the AlN barrier layer. Inconsistently, the 2D electron mobility was high: about 853 cm2/V s. The interface roughness of the heterostructure was estimated to be 6 Å rms for a 50-Å-thick AlN barrier layer, using grazing incidence x-ray reflectivity studies.