Unlike silicon and traditional III-V semiconductors, the III-nitrides exhibit high spontaneous and piezoelectric polarization charges at epitaxial polar heterojunctions. In the process of investigating scaling properties of gate-stacks consisting atomic-layer deposited Al 2 O 3 /III-Nitride heterojunctions, we find interface charges that appear closely linked to the polarization charges of the underlying nitride substrate. Through capacitance-voltage measurement on a series of samples of varying dielectric thicknesses, we find the presence and propose an origin of benign donor-type interface charges (Q it $6 Â 10 13 cm À2 ) at the AlN/Al 2 O 3 junction. This interface charge is almost equal to the net polarization charge in AlN. The polarization-related dielectric/AlN interface charge and the role of oxygen in the dielectric as a possible modulation dopant potentially offer opportunities for various device applications.GaN high electron mobility transistors (HEMTs) outperform Si devices for high voltage switching by virtue of their large bandgap and additionally possess the potential for very high speed switching. This requires highly scaled low sheetresistance HEMT structures with very thin barriers. However, ultrathin epitaxial barriers (such as AlN or InAlN) result in substantial leakage currents preventing the capability to block high drain voltages, and dielectrics can substantially mitigate this problem. Thus, dielectrics such as SiO (Ref. 4) are being investigated intensively both for composite gate stacks as well as for the suppression of current collapse 5 by passivating surface states in these devices. Atomic layer deposited (ALD) Al 2 O 3 has drawn the attention of the community due to its large bandgap and outstanding dielectric 6 and passivation 7 properties. The superior quality (in terms of uniformity) of ALD over sputtering and electron-beam deposition, coupled with high band gap ($6.5 eV), 8 high dielectric constant ($9.1), high break down field ($10MV/cm), high thermal (amorphous $1000 C), and chemical stability of ALD-grown Al 2 O 3 makes it a natural choice as a gate insulator for AlN/ GaN HEMTs (Ref. 9) and its variants. The study of the ALD Al 2 O 3 /III-nitride interface is of prime importance for device characteristics of AlN/GaN HEMTs. In this work, we present a comprehensive characterization of AlN/GaN MOS-HEMT gate stacks with ALD Al 2 O 3 of various thicknesses. Through capacitance-voltage (C-V) measurement, we find the presence and propose an origin of benign donor-type positive interface charge (Q it ) at the AlN/Al 2 O 3 junction and relate its presence to the polarization charges in AlN. The presence of Q it explains the trend of pinch-off voltage and twodimensional electron gas (2DEG) density with ALD thicknesses both qualitatively and quantitatively. Recent report 10 (appeared after this submission) on ALD/GaN structure also invokes positive interface charge to explain the trend of pinch-off voltage with ALD thicknesses. AlN/GaN HEMT structures were grown in a Veeco Gen 930 molec...
Electrically injected deep ultra-violet emission is obtained using monolayer thin GaN/AlN quantum structures as active regions. The emission wavelength is tuned by controlling the thickness of ultrathin GaN layers with monolayer precision using plasma assisted molecular beam epitaxy. Single peaked emission spectra are achieved with narrow full width at half maximum for three different light emitting diodes operating at 232 nm, 246 nm, and 270 nm. 232 nm (5.34 eV) is the shortest electroluminescence (EL) emission wavelength reported so far using GaN as the light emitting material and employing polarization-induced doping.
The ruggedness, portability, high-efficiency, and microfabrication benefits of solid-state semiconductor light sources over conventional lamps became clear in the last decade for visible wavelengths in the solid-state lighting revolution, and gave birth to several new applications. A similar revolution is expected in the deep-UV spectrum. Semiconductor light sources such as Light-Emitting Diodes (LEDs) and Lasers in the deep ultraviolet (UV) spectrum have versatile applications in water and air purification, in healthcare applications of biophotonic diagnostics and sterilization, in food preservation, in security and environmental monitoring and in industrial curing. The semiconductor material substrate of choice for deep-UV photonic devices is direct-bandgap AlN with an energy bandgap of ~6.2 eV (200 nm), and the active regions where photons are produced are various ternary compositional alloys of AlN with GaN of bandgap ~3.4 eV (365 nm).For deep-UV LEDs, quantum well active regions composed of AlGaN have been used to push the interband optical transition to high energies [1][2][3][4][5]. The internal quantum efficiency in high Al containing AlGaN Quantum Wells (QWs)/barrier structures is limited by the quantum confined stark effect (QCSE) [6][7][8], edge emission due to valence band structure re-ordering [9][10][11], combined with material defect (e.g. dislocation) induced non-radiative recombination. Compositional fluctuations of Al and Ga concentrations in ternary AlGaN alloy layers degrade efficient optical emission in the deep-UV range [12], and together with the other effects degrade the LED efficiency.Distinct from the alloy AlGaN layers, deep-UV emission down to 224 nm has been achieved in binary GaN/AlN heterostructures [13][14][15][16][17]. As a significant advantage, the polarization of the emitted photons in ultrathin GaN QWs and quantum dots/disks (QDs) is perpendicular to the c axis, making them propagate parallel to the c-axis [9,11]; this surface emission property is highly favorable for light extraction.We recently demonstrated deep UV LEDs [18-20] emitting as short as 232 nm by incorporating 2 monolayer (ML) thick GaN QDs in AlN barriers. As the height of the QD reduces and the oscillator strength increases [21], the radiative lifetime decreases significantly, increasing the internal quantum efficiency. Shortening the emission wavelength even deeper below 230 nm by utilizing GaN QDs embedded in AlN barriers will further enable applications in sensing and toxic gas detection applications. Tunable sub-230 nm deep-UV emission was demonstrated by Molecular Beam Epitaxy (MBE) growth of 2 ML GaN QDs using a modified Stranski-Krastanov (m-SK) growth method [22]. The m-SK technique uses thermal annealing of the 2 ML GaN quantum well structure sandwiched between AlN barriers. In this letter, we present an alternative approach to realize tunable sub-230 nm emission with higher internal quantum efficiency using SK growth of 2 ML GaN QD structures by MBE. Unlike the earlier work based on m-SK method, cont...
We overview recent progress in growth aspects of group III-nitride heterostructures for deep ultraviolet (DUV) light-emitting diodes (LEDs), with particular emphasis on the growth approaches for attaining high-quality AlN and high Al-molar fraction AlGaN. The discussion commences with the introduction of the current status of group III-nitride DUV LEDs and the remaining challenges. This segues into discussion of LED designs enabling high device performance followed by the review of advances in the methods for the growth of bulk single crystal AlN intended as a native substrate together with a discussion of its UV transparency. It should be stated, however, that due to the high-cost of bulk AlN substrates at the time of this writing, the growth of DUV LEDs on foreign substrates such as sapphire still dominates the field. On the deposition front, the heteroepitaxial growth approaches incorporate high-temperature metal organic chemical vapor deposition (MOCVD) and pulsed-flow growth, a variant of MOCVD, with the overarching goal of enhancing adatom surface mobility, and thus epitaxial lateral overgrowth which culminates in minimization the effect of lattice-and thermal-mismatches. This is followed by addressing the benefits of pseudomorphic growth of strained high Al-molar fraction AlGaN on AlN. Finally, methods utilized to enhance both p-and n-type conductivity of high Al-molar fraction AlGaN are reviewed.
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