High Two-dimensional Electron Gas Research Articles

A Study of Defects and Surface Properties in AlGaN/GaN High Electron Mobility Transistors (HEMTs) Grown by MOCVD on Semi-Insulating SiC Substrates

ABSTRACTAlGaN/GaN HEMTs (High Electron Mobility Transistors) grown on semi-insulating (SI) SiC substrates are very promising for high power, high speed, and high temperature operation with great potential for both military and commercial applications. These high performance characteristics are possible due to presence of high two-dimensional electron gas (2 DEG) charge sheet density maintaining a high Hall mobility at the AlGaN barrier/GaN buffer hetero-interface. However, reliability of AlGaN HEMTs still remains a major concern because of the large number of defects and traps present both in the bulk as well as at the surface leading to current collapse. We report on the study of defects and surface properties in MOCVD-grown Al0.27Ga0.73N HEMT structures on SI SiC substrates. Our HEMT structures consist of a 25nm thick undoped AlGaN barrier layer and a 3μm thick undoped GaN buffer layer grown on a 100nm thick AlN nucleation layer. Hall measurements showed a charge sheet density of ∼1013/cm2 and a Hall mobility of ∼1500cm2/V·sec. Both cross-sectional and plan view TEMs were employed to study defects in the heterostructures and XPS (X-ray Photoelectron Spectroscopy) and AES (Auger Electron Spectroscopy) employed to study surface properties in both GaN and AlGaN layers. DC characterization results from AlGaN Schottky diodes with Pt/Au Schottky contacts are also reported along with results from AlGaN/GaN HEMT devices.

Properties of III-N MOS structures with low-temperature epitaxially regrown ohmic contacts

A significant limitation in the fabrication of III-N MOSFET relates to the formation of ohmic contacts for enhancement-mode MOSFET structures. Unlike existing III-N HFET devices, which include a high free-carrier density two-dimensional electron gas (2DEG) in the semiconductor substrate, a MOSFET in either accumulation or inversion mode require low free-carrier concentrations for the semiconductor channel to have an off-state. The applied gate bias enhances the free-carrier density in the channel, turning on the FET. Unfortunately, a low free-carrier density substrate is problematic for the formation of ohmic contacts, a problem usually dealt with in silicon MOS through self-aligned ion implantation. The high annealing temperatures associated with activating implanted dopants to substitutional sites limits the use of ion implantation for III-N MOSFET fabrication. To overcome this difficulties, selected area epitaxial re-growth of doped III-N materials was developed to form source-drain contacts on otherwise low-doped III-N epitaxial substrates, yielding the needed N+/n−/N+ or N+/p−/N+ structures. Contact re-growth was performed by MOVPE using a silicon nitride dielectric mask defining plasma-etched recesses in the source-drain region. A significant acceleration in the growth rate and surface roughening was observed following re-growth relative to a non-selective area epitaxial growth due to the reduced fill-factor, motivating a general change in MOVPE-operating conditions during re-growth. As the re-growth was intentionally designed to limit the lateral extent of the source-drain regions, the MOVPE re-growth process was performed under conditions limiting lateral overgrowth. III-N MOSFET structures with epitaxial regrown contacts are projected to provide a pathway for low threshold voltage devices suitable for amplifier or logic applications.

High drain current density and reduced gate leakage current in channel-doped AlGaN∕GaN heterostructure field-effect transistors with Al2O3∕Si3N4 gate insulator

Channel-doped AlGaN∕GaN heterostructure field-effect transistors (HFETs) with metal-insulator-semiconductor (MIS) structures have been fabricated to obtain the high drain current density and reduced gate leakage current. A thin bilayer dielectric of Al2O3(4nm)∕Si3N4(1nm) was used as the gate insulator, to simultaneously take advantage of the high-quality interface between Si3N4 and AlGaN, and high resistivity and a high dielectric constant of Al2O3. A MIS HFET with a gate length of 1.5μm has exhibited a record high drain current density of 1.87A∕mm at a gate voltage (Vg) of +3V, which is ascribed to a high applicable Vg and a very high two-dimensional electron gas (2DEG) density of 2.6×1013cm−2 in the doped channel. The gate leakage current was reduced by two or three orders of magnitude, compared with that in normal HFETs without a gate insulator. The transconductance (gm) was 168mS∕mm, which is high in the category of the MIS structure. Channel-doped MIS HFETs fabricated have thus been proved to exhibit the high current density, reduced gate leakage current, and relatively high transconductance, hence, promising for high-power applications.

Quantum corrections to the conductivity and Hall coefficient of a two-dimensional electron gas in a dirtyAlxGa1−xAs∕GaAs∕AlxGa1−xAsquantum well: From the diffusive to the ballistic regime

We report an experimental study of the quantum corrections to the longitudinal conductivity and the Hall coefficient of a low mobility, high density two-dimensional two-dimensional electron gas in a AlGaAs/GaAs/AlGaAs quantum well in a wide temperature range (1.5 K - 110 K). This temperature range covers both the diffusive and the ballistic interaction regimes for our samples. It was therefore possible to study the crossover region for the longitudinal conductivity and the Hall effect.

Investigation into the Energy Band Diagram and ChargeDistribution inAlGaN/GaN Double Heterostructures by Self-ConsistentPoisson–Schrödinger Calculations

The energy band diagram and charge distribution of the unintentionaldoped AlGaN/GaN/AlGaN/GaN double heterostructure wereobtained by self-consistent Poisson–Schrödinger calculations. Thesevere band tilting and high two-dimensional electron gas (2DEG)density mainly attribute to the large internal polarization intensity,which is close to a linear function of Al composition. The influence ofAl composition is investigated. The results show that band tiltingenlarges and 2DEG gains with Al composition, and two-dimensional holegas occurs when Al composition reaches a certain extent. The influenceof Al composition and two-dimensional hole gas (2DHG) on devices isdiscussed.

Doping design of GaN-based heterostructure field-effect transistors with high electron density for high-power applications

For fully exploiting high potentiality of AlGaN/GaN heterostructure field-effect transistors (HFETs) for high-power applications, back-doping (BD) and channel doping (CD) designs have been proposed that make it possible to obtain high two-dimensional electron gas (2DEG) densities even for the devices with thin AlGaN barrier layers. Furthermore, a novel metal–insulator–semiconductor (MIS) HFET using a bi-layer of Al2O3/Si3N4 thin film has been proposed to efficiently suppress the gate leakage current in the devices with thin barrier layers, by taking advantage of highly-resistive Al2O3 layers and excellent quality of Si3N4/AlGaN interface. In BD and CD designs where an asymmetric double-heterostructure and a single-heterostructure are employed, respectively, donor atoms are additionally doped in the backside region beneath the 2DEG position and electrons are supplied also from these doped regions. By using BD and CD designs, very high 2DEG densities around 3 × 1013 cm−2 have been achieved in the Al0.3Ga0.7N/GaN HFETs whose barrier layer (Al0.3Ga0.7N) is designed to be as thin as 120 Å. A BD-HFET with the gate-length (Lg) of 1.5 μm has exhibited a high current density (Id) of 1.2 A/mm and a high transconductance (gm) of 200 mS/mm, which is ascribed to high 2DEG densities and thin barrier layers in these devices. A novel MIS structure mentioned above has been applied to CD-HFET, and a fabricated MIS CD-HFET with Lg = 1.5 μm has exhibited a state-of-the-art current density of 1.6 A/mm and gm = 145 mS/mm, along with reduced gate leakage current. Thus, BD- and CD-HFETs, combined with Al2O3/Si3N4 MIS structure, are promising for high-power applications and should be inevitably required in the future for further improving the device performance. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AlGaN/GaN HFETs fabricated on 100-mm GaN on silicon ( [formula omitted]) substrates

The group III-nitride material system has been demonstrated by many groups to produce high performance, heterostructure field effect transistors (HFETs). AlGaN/GaN heterostructures yield high two-dimensional electron gas densities with high carrier mobilities and simultaneous high breakdown field. Devices based on this structure perform well at high power and at high frequency operating conditions. Most AlGaN/GaN HFETs to date have been produced on sapphire or silicon carbide substrates due to the limited availability of bulk GaN substrates. There are limitations in using these substrate materials in either thermal conductivity, cost or wafer diameter. The use of silicon substrates can overcome the issues of sapphire and SiC that limit manufacturability. In this work, results from HFETs fabricated on 100-mm silicon substrates using a proprietary MOCVD reactor design will be presented. The quality and uniformity of the GaN epitaxy, the microwave characterization of these devices at 2 GHz, and the thermal performance of large periphery devices on this material will be detailed. The electrical performance of these devices is found to be comparable to that of early devices on sapphire and SiC. The results will illustrate the viability of silicon as a low cost, manufacturable platform for AlGaN based devices from the standpoint of epitaxy, device performance, and thermal power handling.

AlGaN/GaN Heterostructure Field-Effect Transistors with Back-Doping Design for High-Power Applications: High Current Density with High Transconductance Characteristics

ABSTRACTElectron transport properties and DC device characteristics have been examined in the AlGaN/GaN heterostructure field-effect transistors (HFETs) with back-doping design that makes it possible to obtain high two-dimensional electron gas (2DEG) densities even for the devices with thin AlGaN barrier layers. In the back-doping design, an asymmetric double-heterostructure is employed, and donor atoms are doped not only in the surface-side AlGaN layer but also in the underlying AlGaN layer. In this structure, electrons are efficiently supplied also from the back-doped AlGaN barrier layer to the GaN channel and merged into a single 2DEG layer, with the help of the negative polarization charges at the heterointerface between the GaN channel and the underlying AlGaN barrier layer. By using back-doping design, very high 2DEG densities around 3×1013 cm−2 has been achieved in the Al0.3Ga0.7N/GaN HFET whose barrier layer (Al0.3Ga0.7N) is designed to be as thin as 120 Å. An HFET with the gate-length of 1.5 μm has exhibited a high current density of 1.2 A/mm and a high transconductance of 200 mS/mm, which is ascribed to high 2DEG densities and thin barrier layers in these devices. HFETs with the back-doping design are thus promising for high-power applications.

Back-Doping Design in AlGaN/GaN Heterostructure Field-Effect Transistors for High-Power Applications

AbstractA novel doping design has been proposed that yields high two-dimensional electron gas (2DEG) densities in the AlGaN/GaN heterostructure field-effect transistors (HFETs) even when the AlGaN barrier layers are designed to be very thin. In the novel doping design, an asymmetric double-heterostructure is employed, and donor atoms are doped not only in the surface-side AlGaN layer but also in the underlying AlGaN layer. In this structure, electrons are efficiently supplied also from the back-doped AlGaN barrier layer to the GaN channel, with the help of the negative polarization charges at the heterointerface between the GaN channel and the underlying AlGaN barrier layer. High 2DEG densities can thus be obtained. Moreover, relatively high 2DEG mobilities can be obtained for high 2DEG densities, because back-doped donor atoms are sufficiently remote from the position of the 2DEG so that the 2DEG is less subjected to the ionized impurity scattering due to the relevant donor atoms. By using this back-doping design, a very high 2DEG density of 2.8x1013 cm-2 (2DEG mobility is 850 cm2/Vs) has been obtained at 300 K in the Al0.3Ga0.7N/GaN HFET whose barrier layer (Al0.3Ga0.7N) is as thin as 120 Å. Thus, the back-doping design is effective to obtain high 2DEG densities in the HFETs with thin barrier layers, and promising for high-power applications.

AlGaN/GaN heterostructures on insulating AlGaN nucleation layers

A single temperature process using AlGaN nucleation layers has been developed that produces device-quality, GaN-based materials with bilayer step surfaces. The AlGaN nucleation layer is deposited by flow modulation organometallic vapor phase epitaxy at temperatures in excess of 1000 °C, where GaN and AlGaN films can be subsequently grown. We have optimized this process on both sapphire and SiC substrates, where the conditions for nucleation are found to be quite different. For growth on SiC, aluminum mole fractions ranging from 6% to 35% result in featureless surfaces. Optimizing the alloy composition and thickness of the nucleation layer on SiC allows the deposition of GaN buffer layers exceeding 5 μm without the formation of cracks. A minimum of 15% aluminum in the nucleation layer is required for smooth growth on sapphire substrates. High room temperature two-dimensional electron gas mobilities of 1575 and 1505 cm2/Vs with sheet charge densities of 1.0×1013 and 1.4×1013 cm−2 are observed in undoped AlGaN/GaN structures placed on insulating AlGaN nucleation layers on sapphire and SiC, respectively.

Characteristics of electron traps in Si-doped Ga 0.51In 0.49P and electrical properties of modulation doped GaInP/InGaAs/GaAs heterostructures

In order to investigate the feasibility of Si-doped Ga 0.51In 0.49P for modulation-doped field effect transistor applications, single Ga 0.51In 0.49P layers and Ga 0.51In 0.49P/In x Ga 1− x As/GaAs ( x=0,0.15 and 0.25) modulation doped heterostructures grown by gas source molecular beam epitaxy were characterized through deep level transient spectroscopy and Hall-effect measurements. Electrical characterization of the undoped and moderately Si-doped ( N D =3×10 17 cm −3 ) GaInP layers yielded an electron trap with an activation energy of 0.75 eV and a temperature dependent capture cross section with a capture barrier of 0.593 eV. The density of this trap increased, and an anomalous decrease in the free carrier concentration of GaInP was observed after the samples were annealed at temperatures typically used in device processing. While, this trap showed characteristics similar to DX centers, it was not detected in highly Si doped ( N D ≈4×10 18 cm −3 ) as grown layers suggesting that the trap is a defect complex including a residual impurity. While very high two-dimensional electron gas density ( 2.6×10 12 cm −2 at 30 K) was achieved in the lattice matched ( x=0) structures, the strained structures were found to be very sensitive to heat treatment, although the InGaAs layers thicknesses were below the theoretical critical thickness. Persistent photoconductivity and a significant reduction in the interface sheet electron density were observed after annealing. The anomalous behavior can be attributed to the decrease in the carrier concentration of the doped GaInP barrier layer and to the strain relaxation at the hetero-interface after annealing. While other explanations may be possible, the decrease in the GaInP electron concentration can be attributed to Si atoms moving from donor to acceptor sites.

In/sub 0.5/(Al/sub x/Ga/sub 1-x/)/sub 0.5/ HEMTs for high-efficiency low-voltage power amplifiers: design, fabrication, and device results

In/sub 0.5/(Al/sub x/Ga/sub 1-x/)/sub 0.5/ high electron-mobility transistors (HEMTs) are expected to have higher two-dimensional electron gas density and larger current drive capability than both Al/sub 0.23/Ga/sub 0.77/As and In/sub 0.5/Ga/sub 0.5/P HEMTs due to the improved conduction-band offsets. In this paper, we performed a systematic investigation of the electrical properties of In/sub 0.5/(Al/sub x/Ga/sub 1-x/)/sub 0.5/P (0/spl les/x/spl les/1) material system lattice matched to GaAs. By considering the conduction-band offset, direct-to-indirect-band electron transfer, donor-related deep levels, and Schottky barrier height, a relatively narrow range of the Al content 0.2/spl les/x/spl les/0.3 was found to be the optimum for the design of In/sub 0.5/(Al/sub x/Ga/sub 1-x/)/sub 0.5/ HEMTs. Under 1.2-V operation, power transistors with the optimum aluminum composition show high drain current density, high transconductance, and excellent power-added efficiency (65.2% at 850 MHz). These results demonstrate that InAlGaP HEMTs are promising candidates for high-efficiency low-voltage power applications.

Enhanced Current-Voltage Characteristics of Al0.25Ga0.75As/In0.25Ga0.75As/GaAs P-HEMTs Using an Inverted Double Channel Structure

An inverted double channel Al0.25Ga0.75As/In0.25Ga0.75As/GaAs pseudomorphic high electron mobility transistor (P-HEMT) grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) has been demonstrated for the first time. The inverted double channel heterostructure shows a high two-dimensional electron gas (2-DEG) concentration of 4.53×1012 cm-2 along with a large mobility of 5010 cm2/V·s at 300 K, respectively. The fabricated P-HEMT device with a gate dimension of 1.8×200 µm2 shows a maximum drain current of as high as 820 mA/mm and a maximum extrinsic transconductance of 320 mS/mm at 300 K. Also, extrinsic transconductance is sustained over a wide range of gate voltages from -2.0 V to 1.8 V. In addition, a high two-terminal gate-drain reverse breakdown voltage of -17 V is obtained. The results obtained show a great potential of the inverted double channel P-HEMT for power applications.

Ga0.25In0.75As/InP quantum wells with extremely high and anisotropic two-dimensional electron gas mobilities

Measurements of modulation-doped Ga0.25In0.75As/InP quantum wells show, in the 〈−110〉 direction, a record electron mobility of 520 000 cm2/V s at 300 mK. A mobility difference of 15% between the 〈110〉 direction and the 〈−110〉 direction is observed. This anisotropy is tentatively attributed to an ordering effect. The mobilities at room temperature and at 77 K were 16 100 and 170 000 cm2/V s, respectively. By separating out the ionized impurity scattering from other scattering processes in the quantum well, we conclude that at low electron concentrations ionized impurity scattering is limiting the mobility, while alloy scattering has a strong influence on the mobility at high electron concentrations. From this result we determine the first experimental value of the alloy-scattering potential as ΔV=0.3 eV.

Electron heating effect on transport properties in [formula omitted] modulation doped structures grown by gas source molecular beam epitaxy

We have grown Si SiGe modulation doped structures by gas source molecular beam epitaxy. Magnetotransport measurements were carried out on Hall bar structures at 0.4 K. Pronounced Shubnikov-de Haas oscillations were observed in the longitudinal magnetoresistance, indicative of a high quality two-dimensional electron gas in the Si channel. Electron mobilities up to 95 000 cm 2/V · s with sheet densities of about 8 × 10 11 cm −2 were obtained at low temperature. The oscillations in the magnetoresistance showed obvious damping as the electric field applied to the Hall bar was increased, indicating an effect of electron heating. Electron temperatures were derived from the oscillation ampitude damping and examined as a function of energy loss rate, which can be determined from the input power per electron in the steady state.

MOVPE Growth of High Electron Mobility AlGaN/GaN Heterostructures

ABSTRACTWe have fabricated AlxGa1−xN/GaN heterostructures with high two-dimensional electron gas (2DEG) mobilities and high sheet carrier densities by metalorganic vapor phase epitaxy (MOVPE). The 2DEG sheet density and mobility exhibit a compositional dependence on the Al fraction of the electron donor layer. The highest mobility (5750 cm2/Vs at 16K) was measured in a sample with x=0.15 that had a sheet carrier density of 8.5×1012 cm−2. The undoped AlxGa1−xN layers have low background carrier concentrations and can be intentionally doped n-type using SiH4. The effect of intentional n-type doping of the AlxGa1−xN donor layer on the electrical properties of the 2DEG was studied in structures that included an undoped AlxGa1−xN spacer layer of varying thickness. Higher 2DEG mobilities were obtained when a 100Å thick undoped layer was included in the structure due to spatial separation of the 2DEG from ionized impurities in the doped AlxGa1−xN. These initial results demonstrate that the electrical properties of AlxGa1−xN/GaN heterostructures can be controlled by intentional doping and appropriate layer design.

High-Performance In0.3Ga0.7As/In0.29Al0.71As/GaAs Metamorphic High-Electron-Mobility Transistor

A new high-electron-mobility transistor (HEMT) using InAlAs/InGaAs grown by molecular beam epitaxy on GaAs has been successfully realized. This device, having an In content close to 30%, presents several advantages over conventional pseudomorphic HEMTs on GaAs as well as over lattice matched HEMTs on InP. High electron mobility with high two-dimensional electron gas density ( 25000 cm2/V·s with 3×1012 cm-2 at 77 K) as well as high Schottky barrier quality (V b=0.68 V with η= 1.1) has been obtained for this material system. A 0.4-µm-gate-length device showed an intrinsic transconductance as high as 700 mS/mm with f t=45 GHz and fmax =115 GHz while a minimum noise figure of NFmin =1.1 dB at 18 GHz has been obtained with a 0.2 µm device. This performance is, to the authors' knowledge, the first reported for submicrometer-gate metamorphic InAlAs/InGaAs/GaAs HEMTs.

AlGaAs/InGaAs/GaAs single electron transistors fabricated by Ga focused ion beam implantation

Single electron transistors are formed in an AlGaAs/InGaAs/GaAs modulation-doped heterostructure by Ga focused ion beam implantation. The AlGaAs/InGaAs/GaAs system has a high two-dimensional electron gas density and facilitates a lateral constriction whose depletion length is much smaller than that in a conventional AlGaAs/GaAs system. A dot structure confined by a small depletion spreading of less than 0.15 μm is formed by the ion implantation. This ion implantation is also employed to form in-plane gates for controlling the tunneling junctions between the dot and reservoirs, and the number of electrons in the dot. Coulomb oscillations and a Coulomb staircase have been clearly observed by controlling three in-plane gates.

Very high two-dimensional electron gas concentrations with enhanced mobilities in selectively double-δ-doped GaAs/InGaAs pseudomorphic single quantum well heterostructures

In this letter a new type of GaAs/In0.25Ga0.75As/GaAs pseudomorphic single quantum well heterostructure by selectively double-δ-doping GaAs layers on both sides of the InGaAs channel grown by low-pressure metalorganic chemical vapor deposition is demonstrated. An extremely high two-dimensional electron gas concentration more than 1×1013 cm−2 with enhanced mobility of 2900 cm2/V s is achieved. This type of structure can easily break through the doping limitation in conventional heterostructures while keeping a high mobility.

OMVPE-grown (Al xGa 1− x) 0.5In 0.5P/InGaAs MODFET structures: growth procedure and Hall properties

Modulation doped (Al x Ga 1- x ) 0.5In 0.5P/In y Ga 1- y As/GaAs single quantum well structures have been grown by low pre ssure metal organic vapour phase epitaxy and characterized by Hall measurements and cathodoluminescence. The high conduction band offset in this structure is favourable for the fabrication of MODFET structures with high two-dimensional electron gas densities in the In y Ga 1- y As channel. Electron sheet densities up to 2.75 x 10 12 cm -2 have been obtained for x = 0.4 and y = 0.3 with appropriate doping. The structures have excellent Hall properties at room temperature and at 77 K. The critical interface between the InGaAs channel layer and the AlGaInP barrier layer has been improved by placing a thin AlGaAs interlayer (1.8 nm thick) between these two layers. This arrangement provides an abrupt interface between quantum well and interlayer and suppresses the interaction of the two-dimensional electron gas in the quantum well with the non-ideal interface between the interlayer and the AlGaInP barrier layer. Cathodoluminiscence studies revealed that the mechanism of strain relaxation in these structures is distinctly different from the mechanism observed in AlGaAs/InGaAs/GaAs heterostructures grown by molecular beam epitaxy. The results of this first study on AlGaInP/InGaAs/GaAs heterostructures lend strong support to the hypothesis that structures of this type are capable to displace AlGaAs/InGaAs/GaAs heterostructures for the fabrication of high speed field effect transistors on GaAs substrates.