Qing Paduano

Inversion domains in AlN grown on (0001) sapphire

Al-polarity inversion domains formed during AlN layer growth on (0001) sapphire were identified using transmission electron microscopy (TEM). They resemble columnar inversion domains reported for GaN films grown on (0001) sapphire. However, for AlN, these columns have a V-like shape with boundaries that deviate by 2 {+-} 0.5{sup o} from the c-axis. TEM identification of these defects agrees with the post-growth surface morphology as well as with the microstructure revealed by etching in hot aqueous KOH.

Correlation between optoelectronic and structural properties and epilayer thickness of AlN

AlN epilayers were grown by metal organic chemical vapor deposition on sapphire substrates. X-ray diffraction measurements revealed that the threading dislocation (TD) density, in particular, the edge TD density, decreases considerably with increasing the epilayer thickness. Photoluminescence results showed that the intensity ratio of the band edge emission to the defect related emission increases linearly with increasing the epilayer thickness. Moreover, the dark current of the fabricated AlN metal-semiconductor-metal deep ultraviolet (DUV) photodetectors decreases drastically with the AlN epilayer thickness. The results suggested that one effective way for attaining DUV optoelectronic devices with improved performance is to increase the thickness of the AlN epilayer template, which results in the reduction of the TD density.

Two-step Process for the Metalorganic Chemical Vapor Deposition Growth of High Quality AlN Films on Sapphire

Epitaxial AlN films were deposited on (0001) sapphire using a new metalorganic chemical vapor deposition process in which optimal substrate nitridation is combined with modulated ammonia flow and a growth pressure reduction from 150 torr to 40 torr after the first stage of growth. A significant improvement in the full width half maximum (FWHM) of the rocking curves was obtained. The best layers had FWHM of 330 arc sec/650 arc sec for the (0002)/(1012) reflections, respectively.

Optimized Coalescence Method for the Metalorganic Chemical Vapor Deposition (MOCVD) Growth of High Quality Al-Polarity AlN Films on Sapphire

Epitaxial Al-polarity AlN films were deposited on (0001) sapphire using a metalorganic chemical vapor deposition process involving a modulated ammonia/trimethyl aluminum flow, a pressure reduction from 150 Torr to 40 Torr after the first stage of growth, and growing the first ~0.1 µm of AlN at a moderate V/III ratio, followed by a switch to low V/III for the remainder of the growth. Smooth, flat epitaxial films with a full width half maximum of the X-ray rocking curves as low as 177 arcseconds/650 arcseconds for the (0002)/(1012) reflections were obtained.

Self-terminating growth in hexagonal boron nitride by metal organic chemical vapor deposition

In this work, we demonstrate the growth of atomically smooth few-layer hexagonal boron nitride (h-BN) on sapphire substrates by metal organic chemical vapor deposition using triethylboron (TEB) and NH3 as precursors. Changing the V/III ratio in a certain temperature and pressure range was found to change the growth mode from random 3D nucleation to self-terminating growth. Infrared reflectance and Raman spectroscopy were used to identify the h-BN phase of these films. Atomic force microscopy measurements confirm that the surfaces are smooth and continuous even over atomic steps on the surface of the substrate. Using X-ray reflectance measurements, the thicknesses of films grown under self-terminating conditions were determined to be ~1.6 nm and independent of the growth time (1 to 60 min) and TEB flux rate. On the basis of the results of this study, a possible mechanism for the observed self-terminating growth behavior is discussed.

Determination of alloy composition and residual stress for AlxGa1-xN/GaN epitaxial films

The alloy composition and the residual stress in Al x Ga 1-x N on GaN epi-layers can be estimated by measuring both the a-axis and c-axis lattice constants using HRXRD. Using linear elastic theory, we derive the alloy composition of a biaxial strained layer from a simple equation relating the measured c-axis and a-axis lattice constants, the lattice constants for unstrained Al x Ga 1-x N (assuming Vegards law), and the ratio of elastic stiffness constants. The method was used to calculate the composition and strain, as well as error estimates, for a set of Al x Ga 1-x N layers grown by MOCVD with 0.23 ≤ x ≤ 0.84.

Few-layer black phosporous PMOSFETs with BN/AI 2 O 3 bilayer gate dielectric: Achieving I on =850μA/μm, g m =340μS/μm, and R c =0.58kΩ·μm

In this paper, high-performance few-layer black phosphorus (BP) PMOSFETs have been demonstrated by using MOCVD BN and ALD Al<inf>2</inf>O<inf>3</inf> as the top-gate dielectric as well as the passivation layer. Highest Ion of 850μA/μm (V<inf>ds</inf> = −1.8 V) and g<inf>m</inf> of 340μS/μm (Vds = −0.8 V) have been achieved with the 200nm chancel length (L<inf>ch</inf>) devices. Record low contact resistance (R<inf>c</inf>) of 0.58kΩ·μm has been obtained on BP transistors by contact engineering. The gate leakage of the BN/Al<inf>2</inf>O<inf>3</inf> bilayer gate dielectric is less than 10⁻¹²A/μm² (V<inf>g</inf> = −1V) with an EOT of 3nm. SS and hysteresis voltage as low as 70mV/dec and 0.1V have been achieved, indicating a high quality interface between BP and BN.

Flexible Gallium Nitride for High‐Performance, Strainable Radio‐Frequency Devices

Flexible gallium nitride (GaN) thin films can enable future strainable and conformal devices for transmission of radio-frequency (RF) signals over large distances for more efficient wireless communication. For the first time, strainable high-frequency RF GaN devices are demonstrated, whose exceptional performance is enabled by epitaxial growth on 2D boron nitride for chemical-free transfer to a soft, flexible substrate. The AlGaN/GaN heterostructures transferred to flexible substrates are uniaxially strained up to 0.85% and reveal near state-of-the-art values for electrical performance, with electron mobility exceeding 2000 cm2 V-1 s-1 and sheet carrier density above 1.07 × 1013 cm-2 . The influence of strain on the RF performance of flexible GaN high-electron-mobility transistor (HEMT) devices is evaluated, demonstrating cutoff frequencies and maximum oscillation frequencies greater than 42 and 74 GHz, respectively, at up to 0.43% strain, representing a significant advancement toward conformal, highly integrated electronic materials for RF applications.

How Important Is the Metal–Semiconductor Contact for Schottky Barrier Transistors: A Case Study on Few-Layer Black Phosphorus?

Black phosphorus (BP) is a recently rediscovered layered two-dimensional (2D) semiconductor with a direct band gap (0.35–2 eV), high hole mobility (300–5000 cm2/Vs), and transport anisotropy. In this paper, we systematically investigated the effects of metal–semiconductor interface/contacts on the performance of BP Schottky barrier transistors. First, a “clean” metal–BP contact is formed with boron nitride (BN) passivation. It is found that the contact resistance of the clean metal–BP contact is seven times less than the previously reported values. As a result, high-performance top-gate BP transistors show a record high ON-state drain current (Ion) of 940 μA/μm. Second, BN tunneling barriers are formed at the source/drain contacts to help understand the abnormally high OFF-state drain current (Ioff) in devices with clean metal–BP contacts. This high Ioff is attributed to the electron tunneling current from the drain to the channel. Finally, the Ion/Ioff of BP field-effect transistors can be significantly improved by using an asymmetric contact structure. By inserting a thin BN tunneling barrier at the drain side, Ioff is reduced by a factor of ∼120 with a cost of 20% reduction in Ion. This case study of contacts on BP reveals the importance of understanding the metal–semiconductor contacts for 2D Schottky barrier transistors in general.

Asymmetric S/D contacts with BN tunneling barrier on black phosphorous FETs

This paper reveals the significance of contact tunneling barrier for narrow bandgap black phosphorus (BP) PMOSFETs. Without the tunneling barrier, record high I<inf>on</inf> of 940µA/μm is achieved however I<inf>off</inf> is also increased due to the reverse electron tunneling current. With an asymmetric BN tunneling barrier at source side, I<inf>off</inf> is reduced by a factor of 5×10³ at a cost of 20% I<inf>on</inf> reduction.