High Electron Mobility Transistor Heterostructures Research Articles

Two-dimensional electron gases in AlYN/GaN heterostructures grown by metal–organic chemical vapor deposition

Wurtzite AlN alloyed with group 3 elements Sc and Y boosts the performance of GaN-based high-electron-mobility transistors (HEMTs) significantly as they increase the spontaneous polarization of the barrier layer and, thus, enhance the charge carrier density ns in the two-dimensional electron gas (2DEG) formed at the interface with the GaN channel. The emerging nitride Al1−xYxN additionally features an a lattice parameter matching to that of GaN at x = 0.07–0.11, allowing for the growth of strain-free barriers. Here, we demonstrate the growth of Al1−xYxN/GaN heterostructures for HEMTs by metal–organic chemical vapor deposition for the first time. The effect of the Y concentrations on the 2DEG is investigated in a Y concentration range from 3% to 15%. At 8% Y, a record mobility of 3200 cm2/(Vs) was measured at a low temperature (7 K). Room and low-temperature ns was at 1–2 × 1013 cm−2. Al0.92Y0.08N barriers were coherently strained to the GaN channel for barrier thicknesses from 5 to 15 nm. Finally, the deposition of Al1−xYxN/GaN heterostructures deposited on 4″ 4H–SiC wafers had a room-temperature mobility close to 1400 cm2/(Vs). AlYN/GaN heterostructures may offer advantages over AlScN/GaN heterostructures not only for the lower price and higher abundance of the raw material but also in terms of electrical characteristics and may be more suitable for power amplifying applications due to increased electron mobility.

Surface morphology evolution of N-polar GaN on SiC for HEMT heterostructures grown by plasma-assisted molecular beam epitaxy

The surface morphology evolution of N-polar GaN with growth time was investigated and compared with Ga-polar GaN. N-polar GaN directly grown on SiC substrates was found to have slower 3D-to-2D growth transformation and less coalescence than the Ga-polar counterpart, resulting in rougher surface morphology, whereas the AlN nucleation layer accelerated 3D-to-2D transformation, resulting in smoother surface morphology. N-polar GaN was found to have mound-type surface morphology with clustered atomic steps, unlike the regular screw-type dislocation-mediated step-flow growth observed for Ga-polar GaN. This was explained by the lower diffusion of adatoms on the N-polar surface due to its higher surface energy and higher Ehrlich–Schwoebel barrier. In addition, the increased III/V ratio in N-polar GaN growth was found to reduce the surface roughness from 2.4 nm to 1 nm. Without Si doping, the N-polar GaN high electron mobility transistor (HEMT) heterostructures grown under optimized conditions with smoother surface morphologies exhibited a sheet carrier density of 0.91 × 1013 cm−2 and a mobility of 1220 cm2 (V s)−1. With Si δ-doping, the sheet carrier density was increased to 1.28 × 1013 cm−2 while the mobility was reduced to 1030 cm2 (V s)−1. These results are comparable to the state-of-the-art data of plasma-assisted molecular beam epitaxy-grown N-polar GaN HEMT heterostructures on SiC substrates.

Influence of the temperature on growth by ammonia source molecular beam epitaxy of wurtzite phase ScAlN alloy on GaN

Due to its large piezoelectric and spontaneous polarization coefficients combined with the possibility of being grown lattice-matched with GaN, wide bandgap ScAlN is becoming a promising material in III-nitride semiconductor technology. In this work, and for the first time, ScAlN growth has been performed by molecular beam epitaxy with ammonia source as nitrogen precursor. High electron mobility transistor heterostructures with a 26 nm thick Sc0.15Al0.85N barrier have been grown on GaN-on-sapphire substrates. The effect of growth temperature, ranging between 620 and 800 °C, was carefully investigated. A smooth surface morphology with a mean roughness below 0.5 nm is obtained whatever the temperature while for 670 °C the (0002) and (101̄3) x-ray diffraction rocking curves show minimum full-width at half-maximum of 620 and 720 arc sec, respectively. Furthermore, two-dimensional electron gases with a high density of 3-3.5 × 1013/cm2 were evidenced in the heterostructures grown below 720 °C.

Wafer-scale high sensitive UV photodetectors based on novel AlGaN/n-GaN/p-GaN heterostructure HEMT

We report a novel high sensitive ultraviolet (UV) photodetector based on AlGaN/n-GaN/p-GaN heterostructure high electron mobility transistor (HEMT) on sapphire substrates and have prepared four types of devices with identical lengths but different widths of the effective region on one wafer. Since the existence of the built-in electric field within p-GaN and n-GaN, the two-dimensional electron gases (2DEGs) at the AlGaN/n-GaN interface is effectively depleted, leading to ultra-low dark current at pA level. Under UV LED irradiation with wavelength of 365 nm, the photodetectors demonstrate extremely high light on/off ratio (Ilight/Idark) of about 106 and high photoresponse speed with the rise time (τrise)/decay time (τdecay) of 1.53 ms/2.21 ms at Vds = -5 V under light power density of 236.97 mW/cm2. Furthermore, the R of the photodetector reaches 9.12 A/W, the EQE and D* are as high as 3100 % and 1.95 × 1013 Jones, respectively, indicating overall high performances of the device in terms of photoresponsivity, EQE, detectivity and robust stability. The proposed photodetectors with merits of p-GaN etchless compared with conventional E-mode p-GaN HEMT and wafer-scale mass production may pave the way for the application of UV communication, Internet of Things and medical treatment.

Enhanced AlScN/GaN Heterostructures Grown with a Novel Precursor by Metal–Organic Chemical Vapor Deposition

Growth of AlScN high‐electron‐mobility transistor (HEMT) structures by metal–organic chemical vapor deposition (MOCVD) is challenging due to the low vapor pressure of the conventionally used precursor tris‐cyclopentadienyl‐scandium (Cp3Sc). It is shown that the electrical and structural characteristics of the AlScN/GaN heterostructure improve significantly by using bis‐methylcyclopentadienyl‐scandiumchloride ((MCp)2ScCl), which has a higher vapor pressure and allows for an increased molar flow and thus higher growth rate (GR). AlScN/GaN HEMT heterostructures with superior electrical characteristics deposited at different barrier growth temperatures are presented. The sheet resistance of 172 Ω sq−1 obtained at 900 °C barrier growth temperature is among the lowest reported so far for AlScN/GaN HEMT structures. The sheet charge carrier density is and the electron mobility μ is 1124 cm2 Vs−1.

Electrical properties of Cu/Pd2Si Schottky contacts to AlGaN/GaN-on-Si HEMT heterostructures

In this study, we analyze gold-free Cu/Pd2Si Schottky contacts to AlGaN/GaN-on-Si HEMT (high electron mobility transistor) heterostructures. The silicide layers are fabricated through magnetron sputtering deposition using a stoichiometric Pd2Si target and are subjected to post-gate annealing at different temperatures. After annealing at 500 °C the leakage currents of the planar diodes are reduced below 3.2 mA/cm2. The microstructures of the diodes are analyzed through transmission electron microscopy (TEM). The post-gate annealing increases lateral dimensions of silicide grains. The electrical properties of the contacts are determined by performing temperature-dependent current-voltage (I-V-T) measurements. The forward current exhibits a specific voltage dependence, which is interpreted based on the two-diode model, reflecting the properties of the Pd2Si/heterostructure interface along with those of the heterostructure itself. The model is fitted to extract the parameters of the Schottky contacts. The barrier height of 0.91 eV and ideality factor n = 1.5 are obtained after post-gate annealing at 500 °C. The effects of post-gate annealing on the forward conduction and leakage mechanisms are then discussed. A simple method is proposed to estimate the polarization charge density, which is required to calculate the electric fields in the heterostructure in the reverse bias. It is observed that post-gate annealing affects the I-V characteristics, likely by reducing the density of the unintentional donors and trap states, leading to an increase in the apparent barrier height along with a reduction in the density of defects contributing to the leakage current.

Investigation of Thin-Barrier AlGaN/GaN HEMT Heterostructures for Enhanced Gas-Sensing Performance

In this study, a theoretical relationship between two-dimensional electron gas (2DEG) properties of AlGaN/GaN high-electron-mobility transistor (HEMT) hetero- structure and various gas-sensing characteristics using this structure was established. Based on the analytical study, it is proposed that a thinner barrier layer in AlGaN/GaN HEMT heterostructure should result in lower detection limits, higher sensing response, and a faster response time. To prove the analytical study, a thin-barrier (~8 nm) AlGaN/GaN HEMT structure was designed and grown to study its sensing characteristics. The results are compared with the thick-barrier (~18 nm) gas sensor structure. For the thin-barrier NO2 sensor, the sensing response increased by almost ten times when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sf \Delta {I}$ </tex-math></inline-formula> was changed by more than 25%. The sensor also showed a faster response time of 10 s when compared to the thick barrier sensor. Similarly, the thin-barrier NH3 sensor showed a remarkable improvement in sensing response by 17 times and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sf \Delta {I}$ </tex-math></inline-formula> by two times. Based on the response time, the thin-barrier NH3 sensor was also found to be 40% more faster compared to the thick-barrier AlGaN/GaN HEMT sensor.

Strain-engineering in AlGaN/GaN HEMTs: impact of silicon nitride passivation layer on electrical performance

In this work, we present a physics-based analysis of two-dimensional electron gas (2DEG) sheet carrier density and other microwave characteristics such as transconductance and cutoff frequency of AlxGa1−xN/GaN high electron mobility transistors (HEMT). An accurate polarization-dependent charge control-based analysis is performed for microwave performance assessment in terms of current, transconductance, gate capacitances, and cutoff frequency of lattice-mismatched AlGaN/GaN HEMTs. The influence of stress on spontaneous and piezoelectric polarization is included in the simulation of an AlGaN/GaN HEMT. We have shown the change in threshold voltage (Vt) due to tensile and compressive strain with different gate lengths. Also, the influence of stress due to the change in nitride thickness is presented. Our simulation results for drain current, transconductance, and current-gain cutoff frequency for various gate length devices are calibrated and verified with experimental data over a wide range of gate and drain applied voltages, which are expected to be useful for microwave circuit design. The predicted transconductance, drain conductance, and operation frequency are quite close to the experimental data. The AlGaN/GaN heterostructure HEMTs with nitride passivation layers show great promise as a candidate in future high speed and high power applications.

The Kondo effect and mesoscopic quantum coherence in a 2D electron gas of magnetically undoped AlGaN/AlN/GaN heterostructures

We report an observation of the Kondo effect in two-dimensional electron gas (2DEG) of magnetically undoped AlGaN/GaN high electron mobility transistor heterostructures and analyze the 2DEG magnetoresistance oscillations to reveal the small-scale structure of the confining potential. Temperature-dependent zero-field resistivity data exhibit an appreciable upturn below 120 K, followed by the standard low-temperature weak-localization (WL) behaviour below 50 K, crossing over to a pronounced weak antilocalization (WAL) picture at T < 3 K. Magnetotransport characterization was carried out in the temperature range 100 mK ÷ 300 K in the magnetic fields B up to 8 T, applied perpendicular to the 2DEG plane. The Altshuler-Aronov-Spivak oscillations of the magnetoresistance with period h/2e, clearly observed against the background of its smooth behavior, are determined by the inhomogeneity of the 2 DEG confinement and allows to estimate its characteristic spatial scales.

Influence of doping profile of GaN:Fe buffer layer on the properties of AlGaN/AlN/GaN heterostructures for high-electron mobility transistors

The effect of the Fe doping profile of the GaN buffer layer in the heterostructures for high-electron mobility transistors was studied experimentally and by computer simulation. The exponential Fe tail extending to the nominally undoped layers may greatly affect the properties of the structure. Reducing the distance between the channel and the Fe-doped buffer to less than 1 μm results in a decrease in the density and mobility of the two-dimensional electron gas. It also leads to the higher off-state avalanche breakdown voltage and reduced leakage current. A good agreement between simulation and experimental data is obtained when taking into account a Fe segregation effect, while an abrupt doping profile lead to significant discrepancies between them

Optimization of passivation in AlGaN/GaN heterostructure microwave transistor fabrication by ICP CVD

We have studied the effect of silicon nitride (SiN) dielectric passivating film deposition by inductively coupled plasma chemical vapor deposition (ICP CVD) on the parameters of AlGaN/GaN heterostructure high electron mobility transistors (HEMT). Study of the parameters of the dielectric layers has allowed us to determine the effect of RF and ICP power and working gas flow ratio on film growth rate and structural perfection, and on the current vs voltage curves of the passivated HEMT. The deposition rate changes but slightly with an increase in RF power but increases with an increase in ICP power. Transistor slope declines considerably with an increase in RF power: it is the greatest at minimum power RF = 1 W. In the beginning of growth even at a low RF power (3 W) the transistor structure becomes completely inoperable. Dielectric deposition for HEMT passivation should be started at minimum RF power. We have developed an AlGaN/GaN microwave HEMT passivation process providing for conformal films and low closed transistor drain–source currents without compromise in open state transistor performance: within 15 and 100 mA, respectively, for a 1.25 and 5 mm common T-gate (Ug = –8 V and Ud-s = 50 V).

Impact of n-GaN cap layer doping on the gate leakage behavior in AlGaN/GaN HEMTs grown on Si and GaN substrates

The gate leakage characteristics of n-GaN- and i-GaN-capped AlGaN/GaN high-electron-mobility transistor (HEMT) heterostructures grown on various substrates of Si, SiC, and GaN were investigated. HEMT heterostructures were grown by metal-organic vapor phase epitaxy, and the effect of n-GaN cap layer doping on the gate leakage characteristics was investigated depending on the dislocation densities. For i-GaN capped HEMT heterostructures grown on GaN substrates, the current–voltage characteristics were well explained by the theoretical calculation based on thermionic emission, thermionic-field emission, and field emission. Alternatively, for the AlGaN/GaN HEMT heterostructures grown on Si substrates that contain a high threading dislocation density of 8.2 × 109 cm−2, a drastic increase in the reverse leakage current of approximately five orders of magnitude was observed compared to the reverse leakage current observed for the i-GaN-capped HEMT heterostructures. Conductive atomic force microscope analysis revealed that the dislocation-induced surface pits acted as leakage paths only for the n-GaN-capped HEMT heterostructures. Furthermore, the temperature dependence of the leakage current through the surface pit was investigated using a conductive AFM system, and the temperature-dependent behavior associated with one-dimensional variable-range-hopping (1D-VRH) was confirmed. These results indicate that the surface pits have a large impact on the leakage characteristics of the n-GaN capped structures with high dislocation density, and the electron transport through dislocations based on 1D-VRH plays an important role. Furthermore, we demonstrated a significant reduction in the reverse leakage current for n-GaN-capped AlGaN/GaN HEMT heterostructures using low-dislocation-density GaN substrates, i.e., by reducing dislocation-induced leakage paths. Therefore, we believe that the HEMT heterostructures grown on GaN substrates have an advantage in suppressing frequency dispersion and current collapse with minimized impact on the gate leakage behavior of the Schottky-gated HEMTs with an n-GaN cap layer.

The Origin and Influence of Compensatory Current in AlGaN/GaN Type High Electron Mobility Transistor Heterostructures with Two Conducting Channels on the Hall Measurements

The second conducting channel is created in AlGaN/GaN type high electron mobility transistor (HEMT) heterostructures deposited by the metal organic vapor phase epitaxy (MOVPE) technique, in which the pressure changes during the growth of the buffer GaN layer to ensure its high resistivity. It is stated that a second, parasitic, conducting channel is induced as a result of nonintentional doping that occurs at the GaN–GaN interface. The Hall measurements, in the wide range of temperatures, from 77 to 420 K, are used to obtain sheet resistivity, sheet carrier concentration, and electron mobility of the heterostructures. The theoretical model of the multilayer transport in AlGaN/GaN type HEMT heterostructures, based on an equivalent circuit, allows for estimation of compensatory current. Based on the theoretical model, the correction map for the Hall measurement of the samples with two conducting channels is evaluated. The measured electron mobility μmeas obtained from Hall measurement is applied for the determination of the 2D electron gas (2DEG) mobility μ1 of the samples with two conducing channels using the equation μ1 = αμmeas. It is observed that the appropriate correction coefficient α depends on second channel parameters, i.e. the sheet resistance and mobility of the second conducting channel.

Extraction of admittance parameters of symmetrically doped AlGaN/GaN/AlGaN DH-HEMT for microwave frequency applications

An analytical model for determining intrinsic short-circuit admittance (Y) parameters of AlGaN/GaN/AlGaN Double Heterostructure (DH) High Electron Mobility Transistor (HEMT) is presented. These Y parameters obtained in terms of the various small signal equivalent circuit parameters are in turn used to evaluate the enhanced microwave performance of the DH-HEMT in terms of various parameters including Unilateral Power Gain and maximum oscillation frequency. The cut-off frequency has been evaluated from short circuit current gain. The cut-off frequency (fT) and maximum oscillation frequency (fmax) exhibited for DH-HEMT is 125 GHz and 215 GHz which is comparatively improved as obtained in Single Heterostructure (SH) HEMT (fT = 105 GHz and fmax = 165 GHz). The analytical results obtained for DH and SH-HEMT; have been compared and found to match well with the ATLAS 2D device simulation results thus proving the validity of the model.

Metalorganic Chemical Vapor Phase Epitaxy Growth of Buffer Layers on 3C‐SiC/Si(111) Templates for AlGaN/GaN High Electron Mobility Transistors with Low RF Losses

Herein, the interest of cubic silicon carbide as a template for the growth of AlGaN/GaN high electron mobility transistor (HEMT) heterostructures on silicon substrates for high‐frequency operation is shown. On the one hand, 0.6–0.8 μm‐thick 3C‐SiC grown by chemical vapor deposition on intrinsic silicon substrate having initial resistivity superior to 5 kΩ cm enables the metalorganic vapor phase epitaxy of GaN buffer layers with propagation losses below 0.4 dB mm−1 at 40 GHz and 0.5 dB mm−1 at 67 GHz. On the other hand, an HEMT heterostructure is grown on 1.5 μm‐thick 3C‐SiC on 4° off‐axis silicon substrate having an initial resistivity superior to 200 Ω cm that allows to keep a sufficiently resistive epilayer stack limiting the loss up to 0.78 dB mm−1 at 40 GHz. Device process developed on a piece of the 100 mm diameter wafer leads to the demonstration of DC transistor operation with low leakage currents. Compared with direct growth on silicon, these templates enable reduced radio frequency (RF) propagation losses that are very interesting for high‐frequency transistors and circuits operation.

Analysis of Al0.15Ga0.85N/GaN/Al0.15Ga0.85N DH-HEMT for RF and Microwave Frequency Applications

A charge control based analytical model is followed to study the impact of donor-layer doping and gate-length on microwave frequency performance of AlGaN/GaN/AlGaN double heterostructure high electron mobility transistor (DH-HEMT). DH-HEMT is observed to be more sensitive to gate-length and doping variation as compared to single heterostructure high electron mobility transistor (SH-HEMT). The effect of gate-length and doping on various performance parameters, i.e., transconductance, drain conductance, cut-off frequency and maximum oscillation frequency has been analysed. The results so obtained are compared with simulation results and are found to be in good agreement.

AlGaN/GaN high electron mobility transistor heterostructures grown by ammonia and combined plasma-assisted ammonia molecular beam epitaxy

The structural properties and surface morphology of AlN epitaxial layers grown by ammonia (NH3) and plasma-assisted (PA) molecular beam epitaxy (MBE) at different growth conditions on (0001) sapphire were investigated. The lowest RMS roughness of ∼0.7 nm was achieved for the sample grown by NH3 MBE at a substrate temperature of 1085 °C and NH3 flow of 100 standard cm3 min−1. Atomic force microscopy measurements demonstrated a terrace-monolayer step-like surface morphology. Furthermore, the optimal substrate temperature for growth of GaN and AlGaN layers was determined from analysis of the GaN thermal decomposition rate. Using the optimized growth conditions, high electron mobility transistor heterostructures were grown by NH3 MBE on different types of AlN nucleation layer deposited by NH3 MBE or PA MBE. The grown heterostructures demonstrated comparable two-dimensional electron gas (2DEG) properties. The maximum 2DEG mobility of ∼2000 cm2 V–1 s–1) at a 2DEG density of ∼1.17 × 1013 cm−2 was achieved for the heterostructure with a PA MBE-grown AlN nucleation layer. The obtained results demonstrate the possibility of successful combination of different epitaxial approaches within a single growth process, which will contribute to the development of a new type of hybrid epitaxy that exploits the advantages of several technologies.

MBE AlGaN/GaN HEMT Heterostructures with Optimized AlN Buffer on Al2O3

The effect of molecular-beam epitaxy (MBE) growth conditions on properties of AlN epitaxial layers was investigated resulting in determination of optimal substrate temperature and ammonia flow. Optimal substrate temperature for growth of GaN and AlGaN layers was determined analyzing thermal decomposition rate of GaN. Based on the information, high electron mobility transistor heterostructures were grown on sapphire substrates using both ammonia and combined plasma-assisted/ammonia MBE modes. The highest achieved 2DEG mobility was 1992 cm2/(V s) (at 2DEG density of 1.17 × 1013 cm–2) which is the current state-of-the-art level.

Effect of surface passivation process for AlGaN/GaN HEMT heterostructures using phenol functionalized-porphyrin based organic molecules

In this work, we investigate an unexplored possibility of passivating the charged surface states on AlGaN/GaN high electron mobility transistor (HEMT) heterostructures by using organic molecules. This has further led to remarkable enhancement in the electrical properties of rectifying metal-semiconductor contacts on AlGaN/GaN. Phenol functionalized Zinc metallated-Tetra Phenyl Porphyrin (Zn-TPPOH) organic molecules were adsorbed on AlGaN/GaN via the solution phase to form a molecular layer (MoL). The presence of the MoL was confirmed using X-ray Photoelectron Spectroscopy (XPS). The thickness of the MoL was assessed as ∼1 nm, using Spectroscopic Ellipsometry and cross-sectional Transmission Electron Microscopy. XPS peak-shift analyses together with Kelvin Probe Force Microscopy revealed that the molecular surface modification reduced the surface potential of AlGaN by approximately 250 meV. Consequently, the Barrier height (ideality factor) of Ni Schottky diodes on AlGaN/GaN was increased (reduced) significantly from 0.91 ± 0.05 eV (2.5 ± 0.31) for Ni/AlGaN/GaN to 1.37 ± 0.03 eV (1.4 ± 0.29) for Ni/Zn-TPPOH/AlGaN/GaN. In addition, a noteworthy decrement in the reverse current from 2.6 ± 1.93 μA to 0.31 ± 0.19 nA at −5 V (∼10 000 times) was observed from Current-Voltage (I-V) measurements. This surface-modification process can be fruitful for improving the performance of AlGaN/GaN HEMTs, mitigating the adverse effects of surface states and polarization in these materials.

Polarization dependent charge control model for microwave performance assessment of AlGaN/GaN/AlGaN double heterostructure HEMTs

An accurate polarization dependent charge control-based analytical model is proposed for microwave performance assessment of $$\hbox {Al}_{0.15}\hbox {Ga}_{0.85}\hbox {N/GaN/Al}_{0.15}\hbox {Ga}_{0.85}\hbox {N}$$ double heterostructure high electron mobility transistors (DH-HEMTs) in terms of current, transconductance, gate capacitances and cutoff frequency. An analytical expression correlating the sheet carrier concentration in the two 2DEGs formed at the upper and lower heterointerfaces of a DH-HEMT is obtained. AlGaN/GaN/AlGaN DH-HEMTs are found to exhibit superior RF performance as compared to its single heterostructure counterpart in terms of higher drain current, improved transconductance, higher gate capacitance and higher unity-gain cutoff frequency. This improvement in the DH-HEMT is mainly attributed to the formation of two 2DEGs (at top and the bottom heterointerface) as compared to the single 2DEG in a SH-HEMT. The variation of drain current with drain voltage and with gate voltage of AlGaN/GaN SH-HEMTs and AlGaN/GaN/AlGaN DH-HEMTs is obtained analytically and found to agree reasonably well with that obtained using ATLAS 2D device simulation, thereby validating the proposed model.