N. Dharmarasu

Demonstration of AlGaN/GaN high-electron-mobility transistors on 100 mm diameter Si(111) by plasma-assisted molecular beam epitaxy

AlGaN/GaN high-electron-mobility transistor structures grown on 100 mm high-resistivity Si(111) substrates using plasma-assisted molecular beam epitaxy are reported. The two-dimensional electron gas (2DEG) formation in the heterostructures was realized by the growth optimization of two-step low temperature and high temperature AlN layers and GaN buffer layer. High-electron mobility of 1100 cm2/V s with a sheet carrier density of 9×1012 cm−2 was achieved. The presence of 2DEG in the AlGaN/GaN interface was confirmed by temperature dependent Hall measurements and capacitance-voltage carrier profiling. The fabricated 1.5 μm gate length high electron mobility transistor exhibited a maximum drain current density of 530 mA/mm and a peak extrinsic transconductance of 156 mS/mm.

Demonstration of AlGaN/GaN High-Electron-Mobility Transistors on 100-mm-Diameter Si(111) by Ammonia Molecular Beam Epitaxy

We demonstrate, for the first time, crack-free AlGaN/GaN high-electron-mobility transistors (HEMT) on 100 mm Si(111) by ammonia molecular beam epitaxy. High growth rate accelerates rapid transition from three-dimensional (3D) to two-dimensional (2D) growth and reduces the defect density. With increasing GaN buffer thickness, FWHM of GaN(002) XRD peak and dislocation density decrease. Highest electron mobilities of 1350 and 4290 cm2/Vs were measured at RT and 90 K, respectively. Submicron gate devices exhibited good pinch-off characteristics with a maximum drain current (IDmax) of 768 mA/mm at Vg= +1 V and a maximum extrinsic transconductance (gmmax) of 190 mS/mm at VD= 6 V.

Growth and characterization of AlGaN/GaN/AlGaN double-heterojunction high-electron-mobility transistors on 100-mm Si(111) using ammonia-molecular beam epitaxy

To improve the confinement of two-dimensional electron gas (2DEG) in AlGaN/GaN high electron mobility transistor (HEMT) heterostructures, AlGaN/GaN/AlGaN double heterojunction HEMT (DH-HEMT) heterostructures were grown using ammonia-MBE on 100-mm Si substrate. Prior to the growth, single heterojunction HEMT (SH-HEMT) and DH-HEMT heterostructures were simulated using Poisson-Schrodinger equations. From simulations, an AlGaN buffer with “Al” mole fraction of 10% in the DH-HEMT was identified to result in both higher 2DEG concentration (∼1013 cm−2) and improved 2DEG confinement in the channel. Hence, this composition was considered for the growth of the buffer in the DH-HEMT heterostructure. Hall measurements showed a room temperature 2DEG mobility of 1510 cm2/V.s and a sheet carrier concentration (ns) of 0.97 × 1013 cm−2 for the DH-HEMT structure, while they are 1310 cm2/V.s and 1.09 × 1013 cm−2, respectively, for the SH-HEMT. Capacitance-voltage measurements confirmed the improvement in the confinement of ...

GaN Schottky Metal–Semiconductor–Metal UV Photodetectors on Si(111) Grown by Ammonia-MBE

For the development of GaN-based ultraviolet (UV) photodetectors, a simple epilayer structure consisting of GaN (600 nm)/AlN (200 nm) was grown on 100-mm Si substrate using ammonia-molecular beam epitaxy growth technique. The epilayers were crack-free and showed good surface and optical quality. Metal-semiconductor-metal (MSM) interdigitated Schottky-based contacts, fabricated using Ni/Au metallic layers, showed a low dark current of 0.43 nA at 15 V. The analysis of dark current as a function of applied bias revealed that the major current conduction mechanism was through thermionic emission over a Schottky barrier of 0.902 eV. Moreover, the Schottky barrier was found to reduce with the bias, which has been attributed to the image force reduction in the devices. The MSM devices exhibited a peak responsivity of 0.183 A/W at an incident wavelength of 362 nm with a UV/visible rejection ratio of 170. The peak responsivity corresponds to external quantum efficiency of ~70%. The devices also showed good linearity and almost flat responsivity with input power for the applied bias beyond 7 V.

Study on GaN buffer leakage current in AlGaN/GaN high electron mobility transistor structures grown by ammonia-molecular beam epitaxy on 100-mm Si(111)

The effect of carbon doping on the structural and electrical properties of GaN buffer layer of AlGaN/GaN high electron mobility transistor (HEMT) structures has been studied. In the undoped HEMT structures, oxygen was identified as the dominant impurity using secondary ion mass spectroscopy and photoluminescence (PL) measurements. In addition, a notable parallel conduction channel was identified in the GaN buffer at the interface. The AlGaN/GaN HEMT structures with carbon doped GaN buffer using a CBr4 beam equivalent pressure of 1.86 × 10−7 mTorr showed a reduction in the buffer leakage current by two orders of magnitude. Carbon doped GaN buffers also exhibited a slight increase in the crystalline tilt with some pits on the growth surface. PL and Raman measurements indicated only a partial compensation of donor states with carbon acceptors. However, AlGaN/GaN HEMT structures with carbon doped GaN buffer with 200 nm thick undoped GaN near the channel exhibited good 2DEG characteristics.

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).

Effect of Stress Mitigating Layers on the Structural Properties of GaN Grown by Ammonia Molecular Beam Epitaxy on 100 mm Si(111)

The effect of AlGaN and AlN/GaN stress mitigating layers (SMLs) on the structural and morphological properties of GaN grown by ammonia molecular beam epitaxy (MBE) on 100 mm Si(111) has been studied. GaN grown on both AlGaN and AlN/GaN SMLs showed two-dimensional (2D) growth mode whereas the growth mode was three-dimensional (3D) for the GaN grown without the use of any SML. GaN on AlN/GaN SML showed lesser pit density on the surface, higher residual compressive strain and lower dislocation density compared to the GaN grown on AlGaN SML. Further enhancement in surface morphology with pit-free surface and reduced surface roughness was obtained by increasing the GaN growth rate from 0.22 to 0.70 µm/h in AlN/GaN SML structure.

Stress evolution of GaN/AlN heterostructure grown on 6H-SiC substrate by plasma assisted molecular beam epitaxy

The stress evolution of GaN/AlN heterostructure grown on 6H-SiC substrate by plasma assisted molecular beam epitaxy (PA-MBE) has been studied. AlN nucleation layer and GaN layer were grown as a function of III/V ratio. GaN/AlN structure is found to form buried cracks when AlN is grown in the intermediate growth regime(III/V∼1)and GaN is grown under N-rich growth regime (III/V<1). The III/V ratio determines the growth mode of the layers that influences the lattice mismatch at the GaN/AlN interface. The lattice mismatch induced interfacial stress at the GaN/AlN interface relaxes by the formation of buried cracks in the structure. Additionally, the stress also relaxes by misorienting the AlN resulting in two misorientations with different tilts. Crack-free layers were obtained when AlN and GaN were grown in the N-rich growth regime (III/V<1) and metal rich growth regime (III/V≥1), respectively. AlGaN/GaN high electron mobility transistor (HEMT) heterostructure was demonstrated on 2-inch SiC that showed good ...

Characterization of highly-doped GaN as a new material for plasmonic applications

We report on the optical properties of highly doped Gallium Nitride (GaN:Si) layers in regard to plasmonic applications. Samples with different carrier concentrations were grown by PA-MBE and characterized by spectroscopic ellipsometry. Functions for the isotropic and anisotropic complex refractive indices are presented and discussed in the context of the possible excitation of plasmonic states.