Neeraj Nepal

Assessment of GaN Surface Pretreatment for Atomic Layer Deposited High-k Dielectrics

We report the effects of GaN surface pretreatments on the material and electrical properties of Al2O3 dielectric deposited by atomic layer deposition (ALD). A layer of Al2O3 was deposited at different temperatures on metal organic chemical vapor deposition grown n-GaN that was treated with either H2O2:H2SO4 (1:5, piranha), HCl:H2O (1:1, HCl), or HF:H2O (1:1, HF) prior to Al2O3 deposition. The Al2O3 layers on piranha- and HF-treated GaN were observed to be uniformly smooth. The piranha pretreatment resulted in the lowest hysteresis. Pretreatment of the GaN surface with piranha removes carbon and hydroxylates the surface, resulting in better quality ALD Al2O3.

Epitaxial graphene surface preparation for atomic layer deposition of Al2O3

Atomic layer deposition was employed to deposit relatively thick (∼30 nm) aluminum oxide (Al2O3) using trimethylaluminum and triply-distilled H2O precursors onto epitaxial graphene grown on the Si-face of silicon carbide. Ex situ surface conditioning by a simple wet chemistry treatment was used to render the otherwise chemically inert graphene surface more amenable to dielectric deposition. The obtained films show excellent morphology and uniformity over large (∼64 mm2) areas (i.e., the entire sample area), as determined by atomic force microscopy and scanning electron microscopy. X-ray photoelectron spectroscopy revealed a nearly stoichiometric film with reduced impurity content. Moreover, from capacitance-voltage measurements a dielectric constant of ∼7.6 was extracted and a positive Dirac voltage shift of ∼1.0 V was observed. The graphene mobility, as determined by van der Pauw Hall measurements, was not affected by the sequence of surface pretreatment and dielectric deposition.

Atomic Layer Epitaxy AlN for Enhanced AlGaN/GaN HEMT Passivation

Enhancements in AlGaN/GaN high-electron-mobility transistor (HEMT) performance have been realized through ultrathin (4 nm) AlN passivation layers, formed by atomic layer epitaxy (ALE). A combination of ex situ and in situ surface cleans prepare the surface for deposition of ALE AlN. HEMTs passivated by high crystallinity AlN, grown at 500 °C, show improvements in 2-D electron gas sheet carrier density, gate leakage current, off-state drain leakage current, subthreshold slope, and breakdown voltage. In addition, degradation of dynamic on resistance during pulsed off-state voltage switching stress is suppressed by ~50% compared with HEMTs passivated by conventional plasma enhanced chemical vapor deposition SiNx.

Epitaxial Growth of III–Nitride/Graphene Heterostructures for Electronic Devices

Epitaxial GaN films were grown by metal organic chemical vapor deposition (MOCVD) on functionalized epitaxial graphene (EG) using a thin (~11 nm) conformal AlN nucleation layer. Raman measurements show a graphene 2D peak at 2719 cm-1 after GaN growth. X-ray diffraction analysis reveals [0001]-oriented hexagonal GaN with (0002) peak rocking curve full width at the half maximum (FWHM) of 544 arcsec. The FWHM values are similar to reported values for GaN grown by MOCVD on sapphire. The GaN layer has a strong room-temperature photoluminescence band edge emission. Successful demonstration of GaN growth on EG opens up the possibility of III–nitride/graphene heterostructure-based electronic devices and promises improved performance.

Epitaxial growth of AlN films via plasma-assisted atomic layer epitaxy

Thin AlN layers were grown at 200–650 °C by plasma assisted atomic layer epitaxy (PA-ALE) simultaneously on Si(111), sapphire (112¯0), and GaN/sapphire substrates. The AlN growth on Si(111) is self-limited for trimethyaluminum (TMA) pulse of length > 0.04 s, using a 10 s purge. However, the AlN nucleation on GaN/sapphire is non-uniform and has a bimodal island size distribution for TMA pulse of ≤0.03 s. The growth rate (GR) remains almost constant for Tg between 300 and 400 °C indicating ALE mode at those temperatures. The GR is increased by 20% at Tg = 500 °C. Spectroscopic ellipsometry (SE) measurement shows that the ALE AlN layers grown at Tg ≤ 400 °C have no clear band edge related features, however, the theoretically estimated band gap of 6.2 eV was measured for AlN grown at Tg ≥ 500 °C. X-ray diffraction measurements on 37 nm thick AlN films grown at optimized growth conditions (Tg = 500 °C, 10 s purge, 0.06 s TMA pulse) reveal that the ALE AlN on GaN/sapphire is (0002) oriented with rocking curve f...

Perspectives on future directions in III-N semiconductor research

The family of III-V nitride semiconductors has garnered significant research attention over the last 20–25 years, and these efforts have led to many highly successful technologies, especially in the area of light emitting devices such as light emitting diodes for solid state white lighting and lasers for high density optical read/write memories. These applications have taken advantage of a key material property of the III-N materials, namely a direct, tunable (0.7–6.2 eV, λ ∼ 200 nm to 1.7 μm) bandgap and have been accomplished despite a relatively poor level of material quality. But a direct, tunable bandgap is only one of many interesting properties of III-N materials of interest to potential future technologies. A considerable list of first and second order properties make this family of semiconductors even more attractive—namely, electric polarization, piezoelectricity, high breakdown field, pyroelectricity, electro-optic and photo-elastic effects, etc. The first few of these have found much utility i...

Epitaxial metallic β-Nb2N films grown by MBE on hexagonal SiC substrates

RF-plasma MBE was used to epitaxially grow 4- to 100-nm-thick metallic β-Nb2N thin films on hexagonal SiC substrates. When the N/Nb flux ratios are greater than one, the most critical parameter for high-quality β-Nb2N is the substrate temperature. The X-ray characterization of films grown between 775 and 850 °C demonstrates β-Nb2N phase formation. The (0002) and X-ray diffraction measurements of a β-Nb2N film grown at 850 °C reveal a 0.68% lattice mismatch to the 6H-SiC substrate. This suggests that β-Nb2N can be used for high-quality metal/semiconductor heterostructures that cannot be fabricated at present.

Effect of GaN surface treatment on Al2O3/n-GaN MOS capacitors

The surface preparation for depositing Al2O3 for fabricating Au/Ni/Al2O3/n-GaN (0001) metal oxide semiconductor (MOS) capacitors was optimized as a step toward realization of high performance GaN MOSFETs. The GaN surface treatments studied included cleaning with piranha (H2O2:H2SO4 = 1:5), (NH4)2S, and 30% HF etches. By several metrics, the MOS capacitor with the piranha-etched GaN had the best characteristics. It had the lowest capacitance–voltage hysteresis, the smoothest Al2O3 surface as determined by atomic force microscopy (0.2 nm surface roughness), the lowest carbon concentration (∼0.78%) at the Al2O3/n-GaN interface (from x-ray photoelectron spectroscopy), and the lowest oxide-trap charge (QT = 1.6 × 1011 cm−2eV−1). Its interface trap density (Dit = 3.7 × 1012 cm−2eV−1), as measured with photon-assisted capacitance– voltage method, was the lowest from conduction band-edge to midgap.

Epitaxial Lift-Off and Transfer of III-N Materials and Devices from SiC Substrates

In this paper, electrical characterization results of N-polar GaN high-electron-mobility transistors that have been released from a 6H-SiC wafer and manually transferred to a Si wafer using a novel epitaxial lift-off (ELO) technique are presented. This recently developed ELO method uses a thin sacrificial layer of Nb2N, a hexagonal epitaxial conductor with less than 1% lattice mismatch to 4H- and 6H-SiC, to serve as the template for III-N device heterostructure growth. Measured results of transferred devices indicate that electron transport properties and low power density electrical performance are nominally unchanged relative to values measured before release. This technique has several advantages over competing ELO techniques, such as the well-known smart cut method, including bonding-ready released material with atomically-smooth backsides (≤ 0.5 nm rms), easy substrate reclaim with indefinite recycling potential, and a transfer process that can be performed after full front-side device processing and yield screening has been completed.

Impact of surface treatments on high-κ dielectric integration with Ga-polar and N-polar GaN

Gallium- and nitrogen-polar GaN surfaces are subjected to a variety of pretreatments, including oxidation, before the application of high-κ dielectrics by atomic layer deposition (ALD) in order to assess the “best” preparation of smooth, clean, and electrically high-performing dielectric semiconductor interfaces. In terms of topographical and chemical cleanliness, a pretreatment with a wet chemical piranha etch (H2SO4:H2O2) was found to be optimum for both surfaces, and additionally, (NH4)2S is effective for N-polar surfaces. Both thermal and plasma oxidations were employed for controlled growth of native oxides. For Ga-polar surfaces, all native oxides were as smooth as pretreated surfaces, while for N-polar surfaces, all native oxides are much rougher except for very short, high temperature oxidations. ALD Al2O3 films on Ga-polar surfaces are smoother for pretreated surfaces than for as-received surfaces, whereas for N-polar surfaces the opposite is true. In general, ALD HfO2 films on Ga-polar surfaces ...