Distribution Of Two-dimensional Electron Gas Research Articles

Electron Population Dynamics in Optically Pumped Asymmetric Coupled Ge/SiGe Quantum Wells: Experiment and Models

n-type doped Ge quantum wells with SiGe barriers represent a promising heterostructure system for the development of radiation emitters in the terahertz range such as electrically pumped quantum cascade lasers and optically pumped quantum fountain lasers. The nonpolar lattice of Ge and SiGe provides electron–phonon scattering rates that are one order of magnitude lower than polar GaAs. We have developed a self-consistent numerical energy-balance model based on a rate equation approach which includes inelastic and elastic inter- and intra-subband scattering events and takes into account a realistic two-dimensional electron gas distribution in all the subband states of the Ge/SiGe quantum wells by considering subband-dependent electronic temperatures and chemical potentials. This full-subband model is compared here to the standard discrete-energy-level model, in which the material parameters are limited to few input values (scattering rates and radiative cross sections). To provide an experimental case study, we have epitaxially grown samples consisting of two asymmetric coupled quantum wells forming a three-level system, which we optically pump with a free electron laser. The benchmark quantity selected for model testing purposes is the saturation intensity at the 1→3 intersubband transition. The numerical quantum model prediction is in reasonable agreement with the experiments and therefore outperforms the discrete-energy-level analytical model, of which the prediction of the saturation intensity is off by a factor 3.

Electron Distribution in AlGaN/GaN Modulation-doped Heterostructures

We present a calculation of the distribution of two-dimensional electron gas (2DEG)along the quantization direction in an AlGaN/GaN modulation-doped heterostructure (MDHS).The main confinement sources from ionized donors, 2DEG and polarization charges are prop-erly taken into account. We show that the 2DEG distribution near the MDHS interface dependsstrongly on the model of potential barrier in use. Within the ideal model of infinite potential bar-rier, the 2DEG distribution near the interface is increased with a rise of the sheet densities of2DEG and polarization charges. On the contrary, this distribution is decreased within the realisticmodel of finite potential barrier. Since the key mechanisms limiting the 2DEG mobility in MDHSare alloy disorder and surface roughness scatterings that are very sensitive to the near-interface2DEG distribution, with a rise of the sheet densities of 2DEG and polarization charges the 2DEGmobility is decreased within the infinite-barrier model, while increased within the finite one.

Spatial distribution of two-dimensional electron gas in a ZnO/Mg0.2Zn0.8O heterostructure probed with a conducting polymer Schottky contact

A ZnO/Mg0.2Zn0.8O heterostructure was characterized at T=2 K through capacitance measurements with using a conducting polymer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), as a Schottky contact. The Nyquist diagram, which is the trajectory curve of the complex impedance vector, appeared to be an excellent semicircular shape, implying that the PEDOT:PSS/ZnO/Mg0.2Zn0.8O junction can be described with an equivalent single RC parallel circuit. Capacitance-voltage characteristics elucidate the existence of a two-dimensional electron gas, where 1019 cm−3 electrons are confined within 5 nm at the ZnO/Mg0.2Zn0.8O heterointerface.

DC Variable Harmonic Pass Band Operation of AlGaN/GaN Surface Acoustic Wave Devices

We have experimentally studied the DC bias dependent frequency characteristics of surface acoustic wave (SAW) and Sezawa modes and their higher frequency modes up to 3.0 GHz, on AlGaN/GaN film SAW devices with different line widths and spacing ratio. By applying a DC bias signal to input and output interdigital transducer (IDT), the distribution of two-dimensional electron gas (2DEG) near the IDT electrodes is modulated and many pass bands can be observed in a wideband frequency range. These pass bands show interesting DC bias dependent frequency characteristics. In particular, pass bands at 1.92 and 2.1 GHz show remarkable ON/OFF switch operation in a DC bias voltage range from 0 to -20 V. Compared with AlGaN/GaN films SAW devices with the same line width to spacing ratio, ITD electrodes with different line width to spacing ratios are very effective for obtaining a good higher-mode-frequency response.

Function of quantum-confinement effect in the AlGaN∕AlN∕GaN heterostructure with an AlN interfacial layer

This is a theoretical study of the distribution of two-dimensional electron gas (2DEG) in an AlGaN∕AlN∕GaN heterostructure. It is shown that the addition of an AlN interfacial layer leads to a slight increase of 2DEG density. The role of quantum-confinement effect in the change of 2DEG distribution with an inserted AlN interfacial layer is revealed by comparing calculated 2DEG distributions in an AlGaN∕(AlN)∕GaN heterostructure with and without considering the quantum-confinement effect. In the classic model, insertion of an AlN interfacial layer has a negligible effect on the 2DEG distribution. While taking the quantum effect into account, we can clearly see that incorporation of an AlN interfacial layer results in a substantial influence of 2DEG distribution, with the electron spillover into barrier layer reduced from 1.35×1012to0.23×1012cm−2, which are 9.8% and 1.5%, respectively, of the total 2DEG sheet density.

Impact of dopants in GaN on the formation of two-dimensional electron gas in AlGaN/GaN heterostructure field-effect transistors

The two-dimensional electron gas distribution in AlGaN/GaN high electron mobility transistors is determined from the solution of the coupled Schrodinger’s and Poisson’s equations. Considering the piezoelectric effect, the two-dimensional electron gas concentration is calculated to be as high as 7.7×1019 cm-3. In order to obtain an understanding of how the two-dimensional electron gas distribution is influenced by dopants in GaN, we observed the two-dimensional electron gas concentration and occupation of sub-bands versus dopant concentration in the GaN layer of an AlGaN/GaN heterostructure. Our results show that the two-dimensional electron gas concentration is slightly increased at higher doping levels in GaN, while the quantum confinement in the AlGaN/GaN heterostructure is weakened with the increase of donor concentration in the GaN layer.

Influence of doping on the two-dimensional electron gas distribution in AlGaN/GaN heterostructure transistors

A study of two-dimensional electron gas distribution in AlGaN/GaN heterostructure field effect transistors is performed by solving the coupled Schrödinger’s and Poisson’s equation self-consistently. Taking the piezoelectric effect into account, the two-dimensional electron gas concentration is calculated to be as high as 1019 cm−3. To gain an understanding on how the two-dimensional electron gas distribution is influenced by dopant concentration in material, we observed the two-dimensional electron gas concentration and occupation of subbands versus doping level in GaN and in AlGaN layer. The results show that the two-dimensional electron gas concentration depends much more strongly on the doping level in AlGaN than in GaN. And besides, the heavier doping in GaN should weaken the quantum confinement in the AlGaN/GaN heterointerface.

An envelope function description of the quantum well formed in strained layer SiGe/Si modulation doped field effect transistors

Schrödinger and Poisson equations are solved self-consistently to calculate the quantum mechanical properties of the quantum well (QW) formed in SiGe/Si/SiGe strained layer modulation doped field effect transistors. The strain produces an electron quantum well in Si. The two-dimensional electron gas distribution is calculated to show excellent confinement at low temperature. The confinement is poor at very low electron concentration at room temperature. However, it improves with increasing number of channel electrons. Moreover, a large electron concentration in a wide QW may result in the loss of confinement.