2DEG Sheet Carrier Density Research Articles

Method for noncontact measurement of 2DEG mobility and carrier density in AlGaN/GaN on semi-insulating wafers

We present a charge-assisted sheet resistance technique for noncontact wafer level determination of two-dimensional electron gas (2DEG) mobility versus sheet carrier density without any test structures or gates. Instead, the electrical biasing of the 2DEG is provided by surface charge deposition, using a corona charging method. Analysis of the sheet resistance versus deposited charge identifies the 2DEG full depletion condition and enables calculation of the 2DEG sheet carrier density required for the mobility. Results for AlGaN/GaN heterostructures on semi-insulating SiC and sapphire substrates show good agreement with Hall results at a zero-bias condition.

Enhancement of 2D Electron Gas Mobility in an AlN/GaN/AlN Double‐Heterojunction High‐Electron‐Mobility Transistor by Epilayer Stress Engineering

Herein, 2D electron gas (2DEG) enhancement in an AlN/GaN/AlN double‐heterojunction high‐electron‐mobility transistor (DH‐HEMT) is achieved by epilayer stress engineering. The epistructures are grown on a SiC substrate using plasma‐assisted molecular beam epitaxy (PA‐MBE). The stress in the AlN buffer is systematically studied as a function of the III/V ratio. An optimized AlN buffer layer with the relaxation of 66% and a smooth surface morphology with a root mean square (RMS) roughness of 0.5 nm is grown using the two‐step growth method with recovery via metal consumption. It is observed that the stress in the GaN channel is influenced by the stress in the AlN buffer nonlinearly. Furthermore, the carrier mobility increases as the stress in the GaN channel is increased. However, the 2DEG density remains unchanged. An optimized AlN/GaN/AlN DH‐HEMT epistructure with very high 2DEG sheet carrier density of 4.1 × 1013 cm−2 and carrier mobility of 613 cm2 V−1 s−1 is achieved, which leads to higher device output power density.

Non-Reciprocal Acoustic Transmission in a GaN Delay Line Using the Acoustoelectric Effect

This letter reports on first-time demonstration of non-reciprocal acoustic transmission in a gallium nitride (GaN) delay line structure. The split of forward (S21) and backward (S12) transmissions is observed by applying a dc electric field through the active area with a two-dimensional electron gas (2DEG) sheet. The non-reciprocity (~20.7 dB/mm max. in this letter) varies by tuning the gate voltage to deplete the 2DEG sheet carrier density, which agrees with model prediction for the interaction between 2DEG and surface acoustic waves. Our preliminary results prove the feasibility of implementing chip-scale non-reciprocal acoustic devices in a GaN platform through acoustoelectric amplification.

Study of Two-Dimensional Electron Gases for GaN-Based Heterostructure

Two dimensional electron gas (2DEG) sheet carrier density is investigated for AlGaN/GaN, AlN/GaN and InGaN/GaN heterostructures using a self-consistent Schrodinger-Poisson equation simulator. The simulation also takes into account the underlying physics behind the characteristics of the 2DEG density nature. The dependence of 2DEG density on alloy composition, thickness of the channel layer and the change of gate voltage are investigated in the process. This simulation displays the comparison of 2DEG sheet carrier density among different heterostructures. The maximum 2DEG sheet carrier density obtained for AlGaN/GaN is 1.76×1013 cm-2, for AlN/GaN is 2.16×1013 cm-2 and for InGaN/GaN is 1.82×1013 cm-2 without any bias voltage.

Degradation mechanisms of 2 MeV proton irradiated AlGaN/GaN HEMTs

Proton-induced damage in AlGaN/GaN HEMTs was investigated using energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM), and simulated using a Monte Carlo technique. The results were correlated to electrical degradation using Hall measurements. It was determined by EDS that the interface between GaN and AlGaN in the irradiated HEMT was broadened by 2.2 nm, as estimated by the width of the Al EDS signal compared to the as-grown interface. The simulation results show a similar Al broadening effect. The extent of interfacial roughening was examined using high resolution TEM. At a 2 MeV proton fluence of 6 × 1014 H+/cm2, the electrical effects associated with the Al broadening and surface roughening include a degradation of the ON-resistance and a decrease in the electron mobility and 2DEG sheet carrier density by 28.9% and 12.1%, respectively.

Modeling of 2DEG sheet carrier density and DC characteristics in spacer based AlGaN/AlN/GaN HEMT devices

In this paper, 2-D physics based model for two-dimensional electron gas (2DEG) sheet carrier density ns and DC characteristic of the proposed spacer layer based AlxGa1−xN/AlN/GaN High Electron Mobility Transistors (HEMTs) is modeled by considering the triangular quantum well. To obtain charge density ns, the variation of Fermi level with supply voltage and the formation of Energy sub-bands E0, E1 is considered. The obtained results are simple and easy to analyze the sheet charge density, DC characteristics model for spacer layer based AlxGa1−xN/AlN/GaN HEMT power devices. Due to large two-dimensional electron gas (2DEG) sheet carrier density and high velocity, the maximal drain current density achieved is very high. An expression for the two-dimensional electron gas (2-DEG) charge density ns, valid in all regions of device operation is studied and applied to derive drain current along with channel length modulation and velocity saturation effects. The spacer layer based AlGaN/AlN/GaN heterostructure HEMTs shows excellent promise as one of the candidates to substitute present AlGaN/GaN HEMTs for future high speed and high power applications. The proposed model is in a good agreement with experimental data for analyzing both sheet carrier density and drain current over a range of applied voltages and different gate geometries.

Gate metal induced reduction of surface donor states of AlGaN/GaN heterostructure on Si-substrate investigated by electroreflectance spectroscopy

The surface donor state of AlGaN/GaN heterostructure on Si-substrate was investigated at the presence of Schottky gate metal (Au,Ni) on the surface by using electroreflectance spectroscopy. The sheet carrier density of two-dimensional electron gas (2DEG) decreased significantly from 1.03×1013cm-2 to 5.74×1012cm-2 for Au and from 1.03×1013cm-2 to 3.68×1012cm-2 for Ni upon the introduction of Schottky gate metal. Through the analysis of the depletion of 2DEG, which is attributed to both Schottky junction and the lowered energy of Schottky barrier height, it was concluded that the diminished sheet carrier density of 2DEG originated from the reduction of surface donor state induced by gate metal.

Mobility spectrum analysis of anisotropic electron transport in N-polar GaN/AlGaN heterostructures on vicinal sapphire substrates

In this work, we present results of a study of anisotropic two-dimensional electron gas (2DEG) transport in N-polar GaN/AlGaN heterostructures grown on slightly mis-oriented sapphire substrates. High-resolution mobility spectrum analysis of magnetic-field dependent Hall-effect and resistivity indicate an isotropic 2DEG sheet carrier density, yet significant anisotropy was observed in carrier mobility. A single electron species with a narrow mobility distribution was found to be responsible for conduction parallel to the multi-atomic steps resulting from growth on the vicinal substrates; whilst in the perpendicular direction two distinct electrons peaks are evident at T ⩽ 150 K, which merge near room temperature. The linewidth of the mobility distributions for transport in the perpendicular direction was found to be significantly broader than that of the single electron in the parallel direction. The broader mobility distribution and the lower average mobility for the 2DEG in the perpendicular direction are attributable to interface roughness scattering associated with the GaN/AlGaN interfacial steps.

Extraction of AlGaN/GaN HEMT Gauge Factor in the Presence of Traps

Repeatable gauge factors (GFs) of an AlGaN/GaN high-electron mobility transistor (HEMT) device were obtained after eliminating parasitic charge-trapping effects. Many GFs for AlGaN/GaN HEMTs are reported in the literature, and charge traps could be responsible for the four orders of magnitude variation in reported GFs. By employing continuous subbandgap optical excitation, the effect of nonrepeatable charge-trapping transients was effectively minimized, allowing the GF to be accurately measured. The measured GF (-2.8 0.4) is reasonably close to the simulated GF (-7.9 5.2) calculated from stress-induced changes in the 2-DEG sheet carrier density and mobility.

Improved electrical properties of the two-dimensional electron gas in AlGaN/GaN heterostructures using high temperature AlN interlayers

The electrical properties of two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures using high temperature (HT) AlN interlayers (ITs) grown on c-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD) have been investigated. It is found that the electrical properties (electron mobility and sheet carrier density) are improved compared with those in the conventional AlGaN/GaN heterostructures without HT AlN ITs, and the improved 2DEG properties result in the reduction of the sheet resistance. The results from high resolution X-ray diffraction (HRXRD) and Raman spectroscopy measurements show that HT AlN ITs increase the in-plane compressive strain in the upper GaN layer, which enhances the piezoelectric polarization in it and consequently causes increasing of 2DEG density at the AlGaN/GaN interface. Meanwhile, the compressive strain induced by HT AlN ITs leads to a less tensile strain in AlGaN barrier layer and causes positive and negative effects on the sheet carrier density of 2DEG, which counteract each other. The HT AlN ITs reduce the lattice mismatch between the GaN and AlGaN layers and smooth the interface between them, thus increasing the electric mobility of 2DEG by weakening the alloy-related interface roughness and scattering. In addition, the surface morphology of AlGaN/GaN heterostructures is improved by the insertion of HT AlN ITs. The reason for the improved properties is discussed in this paper.

An analytical model for current–voltage characteristics of AlGaN/GaN HEMTs in presence of self-heating effect

A 2-D analytical thermal model for the I– V characteristics of AlGaN/GaN is presented. The effect of self-heating is studied by investigating the temperature effects on various parameters: the 2DEG sheet carrier density, the Fermi level, the electron mobility, the saturation velocity and the critical field. After incorporating self-heating effect in calculations of current–voltage characteristics, our results agreed well with published experimental data.

Electrical behavior of AlGaN/GaN heterostuctures upon high-temperature selective oxidation

In this paper, the influence of a high-temperature (900 °C) selective oxidation process on the electrical properties of AlGaN/GaN heterostructures was investigated. In particular, electrical measurements performed on appropriate devices and test patterns demonstrated that the current flow through the two-dimensional electron gas (2DEG) was suppressed, even if the thickness of the local oxide did not reach the AlGaN/GaN interface. The combination of macroscopic current-voltage and capacitance-voltage measurements with depth-resolved scanning capacitance microscopy elucidated the doping dependence and the compositional stability of the material during high-temperature oxidation. The reduction in the 2DEG sheet carrier density and the variation of the threshold voltage of simple high electron mobility transistor structures upon high-temperature annealing were also discussed.

Magnetotransport studies of AlGaN/GaN heterostructures grown on sapphire substrates: Effective mass and scattering time

Temperature-dependent magnetotransport measurements have been carried out on high-quality two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures with different Al content grown on sapphire substrates. The GaN electron effective mass and the quantum scattering time are determined by well-resolved Shunbikov–de Hass oscillations. The electron effective mass is determined to be 19m0. The ratio of the classic scattering time to the quantum scattering time increases with increasing 2DEG sheet carrier density, which agrees very well with the previous calculation based on an ideal 2DEG in conventional semiconductor systems. Our result indicates that a low density of deep centers results in the much higher mobility of our structure compared with other reports, which is of critical importance in fabricating a high-quality 2DEG structure in AlGaN/GaN systems.

Classical Edge Magnetoplasmon in a GaAs/AlGaAs Two-Dimensional Electron System

We have studied the edge magnetoplasmon (EMP) in a GaAs/AlGaAs two-dimensional electron gas (2DEG) system under conditions where the quantum Hall effect (QHE) is not observed; we call this a classical EMP. The classical EMP's are excited in a 2DEG disk by radio frequency electric field excitation (≤1 GHz) in a high magnetic field B (≤5 T) with the 2DEG sheet carrier density n s between 2×10 15 m -2 and 1×10 16 m -2 . The resonant frequency is found to be almost proportional to n s and inversely proportional to B , which agrees quantitatively with the theory proposed by Volkov and Mikhailov, Sov. Phys.-JETP 67 (1988) 1639 by assuming the effective dielectric constant \(\hat{\varepsilon }=(1+\varepsilon _{\rm GaAs})/2\) where e GaAs is appropriate to the GaAs. A comparison of the experimental results with the calculations revealed that the electron mobility plays an important role in the definition of the EMP frequency.