Sheet Carrier Density Research Articles

Effect of III/V ratio on the polarity of AlN and GaN layers grown in the metal rich growth regime on Si(111) by plasma assisted molecular beam epitaxy

Wet chemical etching, reflection high energy electron diffraction, scanning electron microscope and convergent beam electron diffraction have been employed to study the polarities of AlN and the subsequently grown GaN as a function of metal flux in the metal rich growth regime. Both AlN and GaN exhibited metal polarity in the intermediate growth conditions. However, in the droplet growth regime, the polarity of AlN and GaN were N polar and Ga polar, respectively. It was observed that Ga polar GaN could be obtained on both Al and N polar AlN. AlGaN/GaN high electron mobility transistor (HEMT) heterostructure exhibiting hall mobility of 900 cm2 V−1 s−1 and sheet carrier density of 1.2 × 1013 cm−2 was demonstrated using N polar AlN which confirmed Ga polarity of GaN. Al metal flux was likely to play an important role in controlling the polarity of AlN and determining the polarity of the subsequent GaN grown on Si(111) by plasma assisted molecular beam epitaxy (PA-MBE).

Dynamics of photogenerated nonequilibrium electronic states inAr+-ion-irradiatedSrTiO3

A metallic surface is realized on stoichiometric and insulating (100) SrTiO$_3$ by Ar$^+$ - ion irradiation. The sheet carrier density and Hall mobility of the layer are $\sim$$4.0$ $\times$ $10^{14}$ $/cm^2$ and $\sim$$2$ $\times$ $10^3$ $cm^2/Vs$ respectively at 15 K for the irradiation dose of $\sim$4.2 $\times 10^{18}$ $ions/cm^2$. These samples display ultraviolet light sensitive photoconductivity (PC) which is enhanced abruptly below the temperature ($\approx$100 K) where SrTiO$_3$ crystal undergoes an antiferrodistortive cubic-to-tetragonal ($O_h^1$ $\rightarrow$ $D_{4h}^{18}$) structural phase transition. This behaviour of PC maps well with the temperature dependence of dielectric function and electric field induced conductivity. The longevity of the PC-state also shows a distinct change below $\approx$100 K. At $T > 100$ K its decay is thermally activated with energy barrier of $\approx$36 meV, whereas at $T < 100$ K it becomes independent of temperature. We have examined the effect of electrostatic gating on the lifetime of the PC state. One non-trivial result is the ambient temperature quenching of the photo-conducting state by the negative gate field. This observation opens avenues for designing a solid state photo-electric switch. The origin and lifetime of the PC-state are understood in the light of field effect induced band bending, defect dynamics and thermal relaxation processes.

(Invited) Failure Mechanisms in AlGaN/GaN HEMTs Irradiated with 2MeV Protons

Gallium nitride high electron mobility transistors (HEMTs) have shown the potential to be highly resistant to radiation damage, making them ideal for use in microwave power amplifiers and DC/DC converters in space-based applications. To investigate the mechanisms of radiation-induced degradation in AlGaN/GaN HEMTs, 2MeV protons were used to simulate the space radiation environment, and to probe the second order effect of inherent material defects on the response the initial material quality was intentionally varied by studying HEMTs on sapphire, Si, and SiC substrates. Irradiation was performed incrementally up to a fluence of 6x1014 cm-2. The Hall mobility and 2DEG density was measured on Van der Pauw structures before irradiation and at each dose. The decrease in mobility can be attributed to increased carrier scattering in the 2DEG as a result of radiation-induced defects while the decrease in sheet carrier density is attributed to screening of the 2DEG from charged trap formation [1]. The magnitude of change in the 2DEG density is the same for all substrate materials (1x1012 cm-2), though the percent change varies due to different initial values. Note that mobility begins to degrade at lower fluence than sheet carrier density. FET I-V analysis indicated increased ON-resistance and decreased saturation current, consistent with the reduced mobility and sheet carrier density observed in the Hall measurements. The off-state leakage current decreased and threshold voltage was shifted slightly positive, again due to the reduced 2DEG density. The trapping mechanisms were further probed using pulsed I-V measurements at varying quiescent bias conditions. While the dynamic ON-resistance degradation under quasistatic conditions was consistent with DC measurements, the degradation under off-state stress conditions initiated at a lower fluence, and a culminated with a 10X higher degradation at the final fluence. This implies that dynamic ON-resistance, a parameter related to charge trapping, is much more sensitive to radiation than DC characteristics and must be considered when evaluating the radiation hardness of HEMT devices. TEM imaging was employed to directly probe the mechanisms suggested by the electrical measurements. Process-induced defects were visible in both reference and irradiated samples, notably an overflow of Au in the metal stack at the gate edge, possibly related to PECVD deposition of passivation layers. In addition a new, radiation-induced void at both edges of the gate in the Ni region of the Ni/Au gate metallization was revealed [2]. EDS line scans indicate that, after radiation, the 20 nm Ni layer diffused up into the Au, and to a lesser extent, into the AlGaN, leaving voids protruding ~300 nm under the gate edges. The mechanism of the void formation shows all features of Ni/Au inter-diffusion through vacancy exchange, known as the Kirkendall effect. The exact process is still under investigation, but the voids result in reduced gate area, which is responsible for the degraded off-state performance. It is particularly rare that such pronounced diffusion occurs at room temperature. Since the voids only occur at the gate edges, an additional electrochemical or strain-induced driving force is likely present. Therefore the role of the SiNx passivation layer, as well as strain, at the gate edge will be factor in this ongoing research. 1. T.J. Anderson, et. Al. IEEE Electron Dev. Lett. 35 (8), 826-828 (2014) 2. A.D. Koehler, et. Al. IEEE Electron Dev. Lett. 35 (10) (2014)

Reconfigurable Ion Gating of 2H-MoTe2 Field-Effect Transistors Using Poly(ethylene oxide)-CsClO4 Solid Polymer Electrolyte.

Transition metal dichalcogenides are relevant for electronic devices owing to their sizable band gaps and absence of dangling bonds on their surfaces. For device development, a controllable method for doping these materials is essential. In this paper, we demonstrate an electrostatic gating method using a solid polymer electrolyte, poly(ethylene oxide) and CsClO4, on exfoliated, multilayer 2H-MoTe2. The electrolyte enables the device to be efficiently reconfigured between n- and p-channel operation with ON/OFF ratios of approximately 5 decades. Sheet carrier densities as high as 1.6 × 10(13) cm(-2) can be achieved because of a large electric double layer capacitance (measured as 4 μF/cm(2)). Further, we show that an in-plane electric field can be used to establish a cation/anion transition region between source and drain, forming a p-n junction in the 2H-MoTe2 channel. This junction is locked in place by decreasing the temperature of the device below the glass transition temperature of the electrolyte. The ideality factor of the p-n junction is 2.3, suggesting that the junction is recombination dominated.

Unintentional gallium incorporation in AlN and its impact on the electrical properties of GaN/AlN and GaN/AlN/AlGaN heterostructures

Thin AlN interlayers are widely used in (In,Al,Ga)N based high-electron-mobility transistors to improve the mobility of the two-dimensional electron gas forming at the GaN/(In,Al,Ga)N interface. AlN layers grown by metal-organic chemical vapor deposition, however, were recently shown to contain high amounts of gallium caused by carry over reactions, resulting in AlxGa1−xN layers with x ∼ 0.5 under typical deposition conditions. By modifying the AlN growth conditions in this study, layers with an Al mole fraction up to 0.78 were obtained. The unintentional Ga incorporation had a negligible effect on the electronic properties of GaN/AlN/AlGaN structures with nominally 0.7 nm thick AlN interlayer and sheet carrier densities in the order of 1 × 1013 cm−3. It resulted, however, in low electron mobility values for samples with thicker nominal AlN layers and/or higher sheet carrier densities.

(Invited) Neutron Irradiation Effect on GaN-Based Materials

Effects of neutron irradiation on GaN-based materials were investigated. Temperature dependent Hall measurements were used to study the electrical property of two dimensional electron gases (2DEG). The mobility of 2DEG was sensitive to neutron, while sheet carrier density (ns) showed nearly no decrease until a fluence of 3.66×1015cm-2. Raman measurement was used to examine the strain in AlGaN/GaN heterostructure, and no peak shift was observed in the spectrum. In order to investigate the influence of neutron irradiation on the optical properties of GaN, persistent photo conductivity (PPC) and low temperature photoluminescence (PL) were measured. Pronounced PPC was observed, but no yellow luminescence (YL) band appeared in PL spectrum measured at 5K, suggesting that PPC and YL were not related. Thermal stimulated current (TSC) and low temperature photoluminescence (PL) techniques were used to determine the position of deep levels in GaN with high resistance. Four traps are identified in neutron irradiated GaN.

Enhanced spin–orbit coupling and charge carrier density suppression in LaAl1−xCrxO3/SrTiO3 hetero-interfaces

We report a gradual suppression of the two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 interface on substitution of chromium at the Al sites. The sheet carrier density at the interface (n□) drops monotonically from ∼2.2 × 1014 cm−2 to ∼2.5 × 1013 cm−2 on replacing ≈60% of the Al sites by Cr and the sheet resistance (R□) exceeds the quantum limit for localization (h/2e2) in the concentrating range 40–60% of Cr. The samples with Cr ⩽40% show a distinct minimum (Tm) in metallic R□(T) whose position shifts to higher temperatures on increasing the substitution. Distinct signatures of Rashba spin–orbit interaction (SOI) induced magnetoresistance (MR) are seen in R□ measured in out of plane field (H⊥) geometry at T ⩽ 8 K. Analysis of these data in the framework of Maekawa–Fukuyama theory allows extraction of the SOI critical field (HSO) and time scale (τSO) whose evolution with Cr concentration is similar as with the increasing negative gate voltage in LAO/STO interface. The MR in the temperature range 8 K ⩽ T ⩽ Tm is quadratic in the field with a +ve sign for H⊥ and −ve sign for H∥. The behaviour of H∥ MR is consistent with Kondo theory which in the present case is renormalized by the strong Rashba SOI at T < 8 K.

Quasi-two-dimensional electron gas at the epitaxial alumina/SrTiO3 interface: Control of oxygen vacancies

In this paper, we report on the highly conductive layer formed at the crystalline γ-alumina/SrTiO3 interface, which is attributed to oxygen vacancies. We describe the structure of thin γ-alumina layers deposited by molecular beam epitaxy on SrTiO3 (001) at growth temperatures in the range of 400–800 °C, as determined by reflection-high-energy electron diffraction, x-ray diffraction, and high-resolution electron microscopy. In situ x-ray photoelectron spectroscopy was used to confirm the presence of the oxygen-deficient layer. Electrical characterization indicates sheet carrier densities of ∼1013 cm−2 at room temperature for the sample deposited at 700 °C, with a maximum electron Hall mobility of 3100 cm2V−1s−1 at 3.2 K and room temperature mobility of 22 cm2V−1s−1. Annealing in oxygen is found to reduce the carrier density and turn a conductive sample into an insulator.

Impact of the silicon-nitride passivation film thickness on the characteristics of InAlAs/InGaAs InP-based HEMTs

In this research, the DC and RF characteristics for our own InAlAs/InGaAs InP-based high-electron-mobility transistors (HEMTs) with 0-nm, 50-nm, and 200-nm silicon-nitride (Si3N4) passivation films have been investigated comprehensively and systematically. With increasing Si3N4 passivation film thickness, the device exhibits an apparent improvement in the output conductance and a distinct drift of the pinch-off voltage toward a positive value. Especially, the device passivated by using a 50-nm Si3N4 film demonstrates the highest extrinsic transconductance and channel current. The appeared changes of the DC properties can be interpreted as due to the passivationinduced positively-charged surface state increasing the sheet carrier density with the carrier mobility and parasitic resistance of device being deteriorated by long-time high-stress-intensity ion bombardment. Additionally, the degradations of the current-gain cutoff frequency (f T) and maximum oscillation frequency (f max) can be attributed to the drastically-increased parasitic capacitance and resistance induced by the passivation process. This work will be of great importance in fabricating InP HEMTs with high performances.

Low temperature transport property of the InSb and InAsSb quantum wells with Al0.1In0.9Sb barrier layers grown by MBE

We investigated difference in the transport properties between Al0.1In0.9Sb/InSb and Al0.1In0.9Sb/InAs0.1Sb0.9 quantum wells (QWs). The resistivity of InSb QWs increases exponentially with decreasing temperature, while that of InAs0.1Sb0.9 QWs is much lower than that of InSb QWs, showing low resistivity even at low temperature. Energy band diagram calculations of InAsxSb1−x/Al0.1In0.9Sb quantum wells (QWs) revealed that the bottom of the conduction band of the InAsxSb1−x well moves downward with increasing As content x. The bottom of the InSb QWs lies above the Fermi level (EF), resulting in depletion of the QW, while that in InAs0.1Sb0.9 QWs lies under EF at low temperature. These calculation results support the experimental difference of the resistivity and sheet carrier density between two types of the QWs well.

Novel Analytical model for current-voltage characteristics and transconductance of undoped AlGaN/GaN MISHFETs and performance comparison with different high-k dielectrics

In this paper an improved charge controlled model of undoped, lattice mismatched AlGaN / GaN MISHFET is presented .This model gives an accurate estimation of the 2-DEG sheet carrier concentration at the interface contributed entirely by spontaneous and piezoelectric polarization .The dependence of this sheet carrier density on Al composition and the insulator thickness is investigated in detail. Based on these developments an analytical current – voltage model incorporating Source/Drain parasitic resistance is presented. Also, the non-linear behavior of saturated Drain current is properly formulated and computation of transconductance is carried out for the same. Even without any intentional doping the proposed model exhibits fairly encouraging values of output current and conductance.

Growth-induced electron mobility enhancement at the LaAlO3/SrTiO3 interface

We have studied the electronic properties of the 2D electron liquid present at the LaAlO3/SrTiO3 interface in series of samples prepared at different growth temperatures. We observe that interfaces fabricated at 650 °C exhibit the highest low temperature mobility (≈10 000 cm2 V−1 s−1) and the lowest sheet carrier density (≈5×1012 cm−2). These samples show metallic behavior and Shubnikov-de Haas oscillations in their magnetoresistance. Samples grown at higher temperatures (800–900 °C) display carrier densities in the range of ≈2−5×1013 cm−2 and mobilities of ≈1000 cm2 V−1 s−1 at 4 K. Reducing their carrier density by field effect to 8×1012 cm−2 lowers their mobilities to ≈50 cm2 V−1 s−1 bringing the conductance to the weak-localization regime.

Study on temperature-dependent carrier transport for bilayer graphene

In order to investigate the temperature-dependent carrier transport property of the bilayer graphene, graphene films were synthesized on Cu foils by a home-built chemical vapor deposition (CVD) with C2H2. Samples regularity, transmittance (T) and layer number were analyzed by transmission electron microscope (TEM) images, transmittance spectra and Raman spectra. Van Der Pauw method was used for resistivity measurements and Hall measurements at different temperatures. The results indicated that the sheet resistance (Rs), carrier density (n), and mobility (μ) were 1096.20Ω/sq, 0.75×1012cm−2, and 7579.66cm2V−1s−1 at room temperature, respectively. When the temperature increased from 0°C to 240°C, carrier density (n) increased from 0.66×1012cm−2 to 1.55×1012cm−2, sheet resistance (Rs) decreased from 1215.55Ω/sq to 560.77Ω/sq, and mobility (μ) oscillated around a constant value 7773.99cm2V−1s−1. The decrease of the sheet resistance (Rs) indicated that the conductive capability of the bilayer graphene film increased with the temperature. The significant cause of the increase of carrier density (n) was the thermal activation of carriers from defects and unconscious doping states. Because the main influence on the carrier mobility (μ) was the lattice defect scattering and a small amount of impurity scattering, the carrier mobility (μ) was temperature-independent for the bilayer graphene.

Diamond Based Field‐Effect Transistors of Zr Gate with SiNx Dielectric Layers

Investigation of Zr‐gate diamond field‐effect transistor with SiNx dielectric layers (SD‐FET) has been carried out. SD‐FET works in normally on depletion mode with p‐type channel, whose sheet carrier density and hole mobility are evaluated to be 2.17 × 1013 cm−2 and 24.4 cm2·V−1·s−1, respectively. The output and transfer properties indicate the preservation of conduction channel because of the SiNx dielectric layer, which may be explained by the interface bond of C‐N. High voltage up to −200 V is applied to the device, and no breakdown is observed. For comparison, another traditional surface channel FET (SC‐FET) is also fabricated.

Enhancement of Two‐Dimensional Electron‐Gas Properties by Zn Polar ZnMgO/MgO/ZnO Structure Grown by Radical‐Source Laser Molecular Beam Epitaxy

A Zn polar ZnMgO/MgO/ZnO structure with low Mg composition Zn1−xMgxO layer (x = 0.05) grown on a‐plane (11–20) sapphire by radical‐source laser molecular beam epitaxy was reported. The insertion of a thin (1 nm) MgO layer between ZnMgO and ZnO layers in the ZnMgO/ZnO 2DEG structures results in an increase of 2DEG sheet density and affects electron mobility slightly. The carrier concentration reached a value as high as 1.1 × 1013 cm−2, which was confirmed by C‐V measurements. A high Hall mobility of 3090 cm2/Vs at 10 K and 332 cm2/Vs at RT was observed from Zn0.95Mg0.05O/MgO/ZnO heterostructure. The choice of the thickness of MgO was discussed. The dependence of carrier sheet density of 2DEG on ZnMgO layer thickness was calculated in theory and the theoretical prediction and experimental results agreed well.

High‐quality GaN film and AlGaN/GaN HEMT grown on 4‐inch Si(110) substrates by MOCVD using an ultra‐thin AlN/GaN superlattice interlayer

We report the characterization results of GaN films and AlGaN/GaN heterostructure grown on 4‐inch Si(110) substrates by MOCVD using an ultra‐thin AlN/GaN superlattice interlayer (SL IL) structure. The structure optimization of the SL IL, characterizations of the GaN film quality and the strain state are carried out. The SL IL is found to play an important role in controlling the strain states in GaN films. A crack‐free GaN film up to 2 μm thick is achieved by inserting one SL IL. High‐quality GaN is obtained with the FWHM values of (002) and (102) diffraction peaks from a 2 μm‐thick GaN film being as narrow as 427 arcsec and 665 arcsec, respectively. A flat surface morphology of GaN with monolayer steps is obtained. Based on the SL IL technique, AlGaN/GaN heterostructure is grown with 2DEG mobility and sheet carrier density being ∼1500 cm2/Vs and 1.2 × 1013 cm−2. A high‐electron‐mobility transistor (HEMT) is fabricated and good DC characteristics are demonstrated.

Unique model of polarization engineered AlGaN/GaN based HEMTs for high power applications

In this paper, a novel source/drain Schottky contact technology AlGaN/GaN HEMT device is designed and modelled. Based on this concept, effect of mole fraction in three different HEMT devices is studied using the unique mathematical model. An optimized Al0.2Ga0.8N/GaN HEMT shows an excellent improvement in sheet carrier density and breakdown voltage. Break down voltage (BV) is one of the major concerns in the present day High Electron Mobility Transistor device (HEMT). The proposed device with Lgd of 5μm shows a breakdown voltage of 320V, which is in good agreement with experimental data. In addition to this, drain current, transconductance and frequency is also modelled and simulated using TCAD. Hence Schottky based AlGaN/GaN HEMT is one of the promising candidates for high power applications.

Intrinsic carrier multiplication efficiency in bulk Si crystals evaluated by optical-pump/terahertz-probe spectroscopy

We estimated the carrier multiplication efficiency in the most common solar-cell material, Si, by using optical-pump/terahertz-probe spectroscopy. Through close analysis of time-resolved data, we extracted the exact number of photoexcited carriers from the sheet carrier density 10 ps after photoexcitation, excluding the influences of spatial diffusion and surface recombination in the time domain. For incident photon energies greater than 4.0 eV, we observed enhanced internal quantum efficiency due to carrier multiplication. The evaluated value of internal quantum efficiency agrees well with the results of photocurrent measurements. This optical method allows us to estimate the carrier multiplication and surface recombination of carriers quantitatively, which are crucial for the design of the solar cells.

Selective area growth of Bernal bilayer epitaxial graphene on 4H-SiC (0001) substrate by electron-beam irradiation

We report selective area growth of large area homogeneous Bernal stacked bilayer epitaxial graphene (BLEG) on 4H-SiC (0001) substrate by electron-beam irradiation. Sublimation of Si occurs by energetic electron irradiations on SiC surface via breaking of Si–C bonds in the localized region, which allows the selective growth of graphene. Raman measurements ensure the formation of homogeneous BLEG with weak compressive strain of −0.08%. The carrier mobility of large area BLEG is ∼5100 cm2 V−1 s−1 with a sheet carrier density of 2.2 × 1013 cm−2. Current-voltage measurements reveal that BLEG on 4H-SiC forms a Schottky junction with an operation at mA level. Our study reveals that the barrier height at the Schottky junction is low (∼0.58 eV) due to the Fermi-level pinning above the Dirac point.

A high density two-dimensional electron gas in an oxide heterostructure on Si (001)

We present the growth and characterization of layered heterostructures comprised of LaTiO3 and SrTiO3 epitaxially grown on Si (001). Magnetotransport measurements show that the sheet carrier densities of the heterostructures scale with the number of LaTiO3/SrTiO3 interfaces, consistent with the presence of an interfacial 2-dimensional electron gas (2DEG) at each interface. Sheet carrier densities of 8.9 × 1014 cm−2 per interface are observed. Integration of such high density oxide 2DEGs on silicon provides a bridge between the exceptional properties and functionalities of oxide 2DEGs and microelectronic technologies.