Sheet Charge Density Research Articles

Unpassivated AlGaN-GaN HEMTs with minimal RF dispersion grown by plasma-assisted MBE on semi-insulating 6H-SiC substrates

High electron mobility transistors (HEMTs) are fabricated from AlGaN-GaN heterostructures grown by plasma-assisted molecular beam epitaxy (MBE) on semi-insulating 6H-SiC substrates. At a sheet charge density of 1.3 × 10/sup 13/ cm/sup -2/, we have repeatedly obtained electron mobilities in excess of 1350 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs. HEMT devices with a gate length of 1μm, a gate width of 200 μm, and a source-drain spacing of 5 μm show a maximum drain current of 1.1 A/mm and a peak transconductance of 125 mS/mm. For unpassivated HEMTs, we measured a saturated power output of 8.2-W/mm continuous wave (cw) at 2 GHz with an associated gain of 11.2 dB and a power-added efficiency of 41%. The achievement of high-power operation without a surface passivation layer suggests that free surface may not be the dominant source of radio-frequency (RF) dispersion in these MBE-grown structures. This data may help discriminate between possible physical mechanisms of RF dispersion in AlGaN-GaN HEMTs grown by different techniques.

Comprehensive analysis of small-signal parameters of fully strained and partially relaxed high Al-content lattice mismatched Al/sub m/Ga/sub 1-m/N/GaN HEMTs

Proposes an accurate model to investigate the small-signal microwave parameters of fully strained (FS) and partially relaxed (PR) Al/sub m/Ga/sub 1-m/N/GaN high electron-mobility transistors (HEMTs). It is observed that elastic strain relaxation of the Al/sub m/Ga/sub 1-m/N layer imposes an upper limit on the maximum two-dimensional electron-gas sheet charge density and is, thus, extremely critical in determining the microwave performance of high Al-content Al/sub m/Ga/sub 1-m/N/GaN HEMTs. The model incorporates the effects of strain relaxation of the barrier layer, field-dependent mobility, parasitic source/drain resistance, and velocity saturation to evaluate drain current, transconductance, drain conductance, cutoff frequency, and transit time of FS and PR Al/sub m/Ga/sub 1-m/N/GaN HEMTs with different Al mole fractions. The proposed model predicts a high drain current of 5.94 A/mm for a PR 0.3-/spl mu/m Al/sub 0.4/Ga/sub 0.6/N/GaN HEMT, which is in close proximity with previously published simulated results. A peak transconductance of 154 mS/mm is also estimated for a 1-/spl mu/m gate-length device with aluminum concentration of 15% (FS), which is in close agreement with previously published measured data. A high cutoff frequency of 21.09 GHz was predicted for a 0.6-/spl mu/m device with an Al mole fraction of 0.5 (PR), thus showing the potential of AlGaN/GaN HEMTs for microwave applications.

A SPICE-compatible model for nanoscale MOSFET capacitor simulation under the inversion condition

A SPICE-compatible charge model for nanoscale MOSFET is proposed. Based on the solution of Schrodinger-Poisson (S-P) equations, the developed compact charge model is optimized with respect to: 1) the position of the charge concentration peak; 2) the maximum of the charge concentration; 3) the total inversion charge sheet density; and 4) the average inversion charge depth, respectively. This model can predict inversion layer electron density for various oxide thicknesses and applied voltages. Compared to the S-P results, our model prediction is within 5% of accuracy. Application of this charge quantization model to the C-V measurement produces an excellent agreement. This compact model has continuous derivatives and is therefore amenable to a device simulator. It can also be easily incorporated into circuit simulator for modeling ultrathin oxide MOSFET C-V characteristics.

Dislocation and morphology control during molecular-beam epitaxy of AlGaN/GaN heterostructures directly on sapphire substrates

We report on the growth and transport characteristics of high-density (∼1013 cm−2) two-dimensional electron gases confined at the AlGaN/GaN interface grown by plasma-assisted molecular-beam epitaxy on sapphire substrates. For structures consisting of a 25 nm Al0.30Ga0.70N barrier deposited on a 2 μm insulating GaN buffer, room-temperature mobilities averaging 1400 cm2/V s at a sheet charge density of 1.0×1013 cm−2 are consistently achieved. Central to our approach is a sequence of two Ga/N ratios during the growth of the insulating GaN buffer layer. The two-step buffer layer allows us to simultaneously optimize the reduction of threading dislocations and surface morphology. Our measured sheet resistivities as low as 350Ω/□ compare favorably with those achieved on sapphire or SiC by any growth method. Representative current–voltage characteristics of high-electron-mobility transistors fabricated from this material are presented.

Study of charge distribution across interfaces in GaN/InGaN/GaN single quantum wells using electron holography

Strong internal electric fields (of the order of MV/cm) influence the electronic properties and light emission process of InGaN quantum wells. We have used electron holography to directly profile the potential and charge distribution across a GaN/In0.18Ga0.82N/GaN quantum well structure, and present here an analysis of the fine-scale potential variations at the quantum well. The potential profiles show a drop of 0.6±0.2 V across, and an internal electric field of −2.2±0.6 MV/cm in the quantum well. An analysis based on Poisson’s equation suggests that the field is caused by electronic charges with a peak density of 8×1020 cm−3, corresponding to a sheet charge density of 0.027 C/m2. Free electron and hole densities of the order of 1020 cm−3 are confined separately in the quantum well. These free carriers screen only part of the electric field due to the polarization effect.

Properties of Delta Doped Al0.25Ga0.75N and GaN Epitaxial Layers

ABSTRACTDelta doping (paused growth doping) was investigated as an alternative to uniformly distributing the dopant in the nitride semiconductor layer. In this work, delta doped layers were produced in MOVPE-grown AlGaN and GaN layers at a susceptor temperature of 1220°C by turning off the group III precursors (TMG and TMA) and introducing into the reactor a silicon precursor Si2H6 (disilane) for a fixed period (pause time) before growth was restarted. The compositional and electrical properties as a function of aluminum content and dopant flux were investigated for nitride layers on 2 inch c-plane sapphire substrates. Secondary ion mass spectroscopy (SIMS) measurements revealed a sharp silicon peak with a FWHM of 5.7 ± 0.6 nm for an Al0.25Ga0.75N sample and 10.0 ± 0.6 nm for a GaN sample with sheet charges of 7.9×1012 cm−2 and 9.9×1012 cm−2,respectively. Room temperature Hall mobility as high as 265 cm2 V−1s−1 for a sheet charge 7.9×1012 cm−2 was demonstrated for delta doped Al0.25Ga0.75N layers, but the mobility enhancement saturated and then decreased with increasing sheet charge. Room temperature sheet charge increased with increasing dopant flux for delta-doped AlGaN and GaN layers. Sheet charge density as high as 2.2×1013 cm−2 and 1.3×1013 cm−2 was measured at room temperature for Al0.25Ga0.75N and GaN delta doped layers, respectively. Under identical doping conditions, the Hall sheet charge of the delta doped Al0.25Ga0.75N layer was approximately half as large as GaN layers. The impurity and electrical characteristics of the delta doped layers are further discussed.

An accurate charge control model for spontaneous and piezoelectric polarization dependent two-dimensional electron gas sheet charge density of lattice-mismatched AlGaN/GaN HEMTs

The present paper proposes an improved charge control model of lattice-mismatched AlGaN/GaN HEMTs, valid over the entire operating region. The model for estimation of two-dimensional electron gas (2-DEG) sheet carrier concentration accounts for the strongly dominant spontaneous and piezoelectric polarization at the AlGaN/GaN heterointerface. The dependence of 2-DEG sheet carrier concentration on the aluminum composition and AlGaN layer thickness has been investigated in detail. Current–voltage characteristics developed from the 2-DEG model include the effect of field dependent mobility, velocity saturation and parasitic source/drain resistances. Close proximity with experimental data confirms the validity of the proposed model.

Impact of strain relaxation of AlmGa1−mN layer on 2-DEG sheet charge density and current voltage characteristics of lattice mismatched AlmGa1−mN/GaN HEMTs

An accurate charge control model to investigate the effect of aluminum composition, strain relaxation, thickness and doping of the AlmGa1−mN barrier layer on the piezoelectric and spontaneous polarization induced 2-DEG sheet charge density, threshold voltage and output characteristics of partially relaxed AlmGa1−mN/GaN HEMTs is proposed. The strain relaxation of the barrier imposes an upper limit on the maximum 2-DEG density achievable in high Al content structures and is critical in determining the performance of lattice mismatched AlmGa1−mN/GaN HEMTs. The model incorporates the effects of field dependent mobility, parasitic source/drain resistance and velocity saturation to evaluate the output characteristics of AlmGa1−mN/GaN HEMTs. Close proximity with published results confirms the validity of the proposed model.

Bias Stress Measurements on High Performance AlGaN/GaN HFET Devices

High performance AlGaN/GaN heterojunction field effect transistor (HFET) devices have been fabricated, using material grown by MBE on sapphire substrates. The Hall effect 2DEG sheet charge densities and mobilities were as high as 1.68 × 10 13 cm -2 and 1000 cm 2 /Vs respectively. Peak dc currents in excess of 1 A/mm, with transconductances in excess of 220 mS/mm were common. f T and f max scaled linearly with the inverse of the gate length, with maximum values of 39 and 80 GHz recorded for T-gates of length 0.3 μm. The devices with optically defined gates were stressed at V DS = 10 V and V G = 1 V. for times up to 5000 s. The stress decreased both f T and f max , but this degradation mostly recovered after a few days. In contrast, the gate leakage current improved (decreased) with stressing, but did not return to the initial undesirable levels. Thus, we found that electrical bias stressing can have a positive effect on some of the characteristics of our devices.

Bias Stress Measurements on High Performance AlGaN/GaN HFET Devices

High performance AlGaN/GaN heterojunction field effect transistor (HFET) devices have been fabricated, using material grown by MBE on sapphire substrates. The Hall effect 2DEG sheet charge densities and mobilities were as high as 1.68 × 10 13 cm -2 and 1000 cm 2 /Vs respectively. Peak dc currents in excess of 1 A/mm, with transconductances in excess of 220 mS/mm were common. f T and f max scaled linearly with the inverse of the gate length, with maximum values of 39 and 80 GHz recorded for T-gates of length 0.3 μm. The devices with optically defined gates were stressed at V DS = 10 V and V G = 1 V. for times up to 5000 s. The stress decreased both f T and f max , but this degradation mostly recovered after a few days. In contrast, the gate leakage current improved (decreased) with stressing, but did not return to the initial undesirable levels. Thus, we found that electrical bias stressing can have a positive effect on some of the characteristics of our devices.

AlGaN/AlN/GaN high-power microwave HEMT

In this letter, a novel heterojunction AlGaN/AlN/GaN high-electron mobility transistor (HEMT) is discussed. Contrary to normal HEMTs, the insertion of the very thin AlN interfacial layer (/spl sim/1 nm) maintains high mobility at high sheet charge densities by increasing the effective /spl Delta/E/sub C/ and decreasing alloy scattering. Devices based on this structure exhibited good DC and RF performance. A high peak current 1 A/mm at V/sub GS/=2 V was obtained and an output power density of 8.4 W/mm with a power added efficiency of 28% at 8 GHz was achieved.

EXTRACTION OF POLARIZATION-INDUCED CHARGE DENSITY INMODULATION-DOPED AlxGa1-xN/GaN HETEROSTRUCTURETHROUGH THE SIMULATION OF THE SCHOTTKY CAPACITANCE-VOLTAGE CHARACTERISTICS

Polarization-induced charge density in modulation-doped AlxGa1-xN/GaN heterostructures is extracted through the simulation of the Schottky Capacitance-Voltage (C-V) characteristics. C-V measurements were made on Pt Schottky contacts on modulation-doped Al0.22Ga0.78N/GaN heterostructures. The C-V characteristics were simulated numerically using the three-dimensional Fermi model.Influence of sample parameters on C-V characteristics is analized by the simulation.Polarization-induced charge density,n-AlGaN doping level and Schottky barrier height have different influences on the C-V characteristics,and thus the polarization-induced charge density can be extracted accurately.The polarization-induced sheet charge density at the heterointerface is extracted to be 6.78×1012cm-2 in the Al0.22Ga0.78N/GaN heterostructure with the Al0.22Ga0.78 N thickness of 45nm.This work provides a method for quantitative analysis of the polarization-indeced charge in modulation-doped AlxGa1-xN/GaN heterostructures.

Analytical modeling of stress-induced leakage currents in 5.1–9.6-nm-thick silicon-dioxide films based on two-step inelastic trap-assisted tunneling

The thickness dependence of stress-induced leakage currents (SILCs) has been investigated for silicon-dioxide films with thicknesses between 5.1 and 9.6 nm. Assuming a two-step trap-assisted tunneling process accompanied by an energy relaxation process of trapped electrons, a set of analytical equations is given, which describes quantitatively the SILC dependence on oxide electric field with trap site location, trapped sheet charge density, and trap state energy as characteristic trap site parameters. Applying this model to the SILC data of 5.1–9.6-nm-thick silicon-dioxide films, the best agreement between experimental and calculated I–V data is achieved by a constant trap state energy of 1.93 eV relative to the silicon-dioxide conduction-band edge. Trap sites are located at 4.24 nm from the gate interface for 6.8–9.6-nm-thick films, while the 5.1 nm film exhibits a slightly different trap site location of 4.08 nm. The trapped sheet charge density Qtrap increases linearly with oxide thickness from −0.34×10−6 to −1.29×10−6 C/cm2. As a result, the thickness dependence of Qtrap suppresses the local tunneling current between the gate injection interface and trap sites by a reduction of the local oxide electric field. This fact explains the decrease of SILC with an increase in oxide thickness.

Polarization effects and transport in AlGaN/GaN system

The AlGaN/GaN heterostructure is important for both electronic and optoelectronic devices. In addition to having a large band gap, the heterostructure has a strong piezoelectric effect and a large spontaneous polarization. This allows one to incorporate a large electric field (&amp;gt;106 V/cm) and high sheet charge (&amp;gt;1013 cm−2) without doping. Theoretical studies are done to examine how polarization effects controls the sheet charge density. The studies also focus on how interface roughness, aluminum mole fraction in the barrier, impurity scattering, sheet charge density, and phonon scattering influence mobility. Results are compared with experimental studies on samples grown by both molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). We find that interface roughness is a dominant source of scattering in the samples reported. Due to the variation in growth techniques we find that the MBE samples have a smoother interface quality compared to the MOCVD samples. By carefully fitting the experimental data we present results on interface roughness parameters for MBE and MOCVD samples.

Design, processing and characterization of delta-doped channelAlGaAs/InGaAs/GaAs HFETs

We report on design, processing and characterization ofAlGaAs/InGaAs/GaAs HFETs which feature the power capabilities of doubledelta-doped devices and the low noise performance of the standard HFETs. Tostudy the influence of delta doping position on the subband energies and sheetcharge density, numerical simulations based on a self-consistent solution ofPoisson's and Schrödinger's equations have been used for different channelthicknesses. Three different devices with a single δ-doped layerlocated at three different positions inside the channel have been fabricatedand tested. On-wafer noise measurements have revealed that the position of thedelta-doped layer can be optimized to minimize the noise figure without anyrelevant degradation of the maximum transconductance. The device with thedelta-doped layer closer to the bottom heterointerface was found to exhibitthe best performance in terms of the maximum cutoff and maximum oscillationfrequencies and the minimum noise figure.

Charge control and mobility in AlGaN/GaN transistors: Experimental and theoretical studies

In this article we report on two dimensional sheet charge and mobility in GaN/AlGaN heterostructure field effect transistors. Both experimental and theoretical results are presented. Experimental results are reported on samples grown by metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Theoretical studies are done to examine how spontaneous polarization and piezoelectric effect control the sheet charge density. The studies also focus on how interface roughness, aluminum mole fraction in the barrier and phonon scattering influence mobility. We find that interface roughness is a dominant source of scattering in the samples reported. Due to the variation in growth techniques we find that the MBE samples have a smoother interface compared to the MOCVD samples. By carefully fitting the experimental data we present results on interface roughness parameters for MBE and MOCVD samples.

Domain bistability in photoexcited GaAs multiple quantum wells

We report optical studies of stable electric-field domains in an undoped GaAs multiple-quantum-well p-i-n diode. Discontinuities and bistability due to jumps in the domain-wall position were observed in the photocurrent current-voltage characteristic. Simultaneous photoluminescence (PL) measurements confirm the presence of stable domains, and show that the bistability relates to the path history of the domain-wall location. From the PL measurements we have been able to calibrate the field strengths within the domains and determine the sheet charge density in the domain wall.

Growth of high-performance GaN modulation-doped field-effect transistors by ammonia-molecular-beam epitaxy

The growth of AlGaN/GaN modulation-doped field-effect transistors (MODFETs) by ammonia-molecular-beam epitaxy on sapphire substrates is reported. C-doped GaN (2 μm thick) was used as the insulating buffer layer in the device structures. The MODFET structure was completed by the subsequent growth of 2000 Å of undoped GaN as the channel layer and 130 Å of AlxGa1−xN(0.1⩽x⩽0.3) as the donor barrier layer. Sheet carrier densities of up to 2×1013 cm−2 with mobility of ∼1000 cm2/V s have been achieved even without doping the AlxGa1−xN barrier, indicating a large piezoelectric effect and excellent interface quality. The MODFET layers grown exhibited a unique surface morphology showing very flat plateaus with rms roughness of 0.8 nm on the plateaus and rms roughness of 8 nm over a larger area. A 100-μm-wide device with a 1 μm gate length exhibited a maximum dc current drive of 0.9 A/mm, a peak transconductance of 160 mS/mm, a current gain cutoff frequency of 15.6 GHz, and a maximum oscillation frequency of 49.4 GHz. The high dc and rf performance is attributed to the high two-dimensional electron mobility, high sheet charge density, and insulating property of the C-doped GaN buffer.

Characterization of AlGaN/GaN Heterostructures Grown by Metalorganic Chemical Vapor Deposition

We report the growth and characterization of high-quality AlGaN/GaN heterojunctions by low-pressure metalorganic chemical vapor deposition. We have measured the mobility, μn, and sheet charge density, ns of the interfacial two-dimensional electron gas (2DEG) as a function of Al composition, substrate orientation, and modulation doping. We have also attempted to correlate these results with (102) X-ray diffraction linewidths and surface roughness measured by atomic force microscopy. Interestingly, we do not find any correlation between the roughness of the wafer surface and the 2DEG concentration or mobility. Inserting a short NH3/H2 purge immediately before initiating the AlGaN growth improves the 2DEG mobility in these AlGaN/GaN structures. By optimizing the growth conditions, we have achieved Al0.30Ga0.70N/GaN heterostructures on (0001) sapphire with 2DEG mobilities in excess of μn = 1300 cm2/Vs at 300 K and 4500 cm2/Vs at 100 K with ns ∼ 9.4 × 1012 cm—2.

A quantitative analysis of stress-induced leakage currents and extraction of trap properties in 6.8 nm ultrathin silicon dioxide films

An analytical model for the quantitative analysis of stress-induced leakage currents (SILC) in ultrathin silicon dioxide films is described, which enables the extraction of trap parameters, e.g., trap site location. Assuming a two-step trap-assisted inelastic tunneling mechanism, the conduction of electrons through the silicon dioxide film proceeds as follows: First, electrons tunnel from the cathode into neutral trap sites followed by an energy relaxation into the lowest available energy state of these trap sites. Finally, the electrons reach the anode by a direct tunneling process. We applied this model to the SILC characteristics of a stressed 6.8 nm ultrathin silicon dioxide film. The following parameters could be deduced: The trap sites are located at 4.47 nm relative to the cathode interface with a trapped sheet charge density of |6.54×10−8| C/cm2, and a trap state energy of 2.3 eV relative to the conduction band edge of the silicon dioxide.