P. Fini

Polarization effects, surface states, and the source of electrons in AlGaN/GaN heterostructure field effect transistors

The origin of the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructure field effect transistors is examined theoretically and experimentally. Based on an analysis of the electrostatics, surface states are identified as an important source of electrons. The role of the polarization-induced dipole is also clarified. Experimental Hall data for nominally undoped Al0.34Ga0.66N/GaN structures indicate that ∼1.65 eV surface donors are the actual source of the electrons in the 2DEG, which forms only when the barrier thickness exceeds 35 A.

Homoepitaxial growth of GaN under Ga-stable and N-stable conditions by plasma-assisted molecular beam epitaxy

The structure, morphology, and optical properties of homoepitaxial GaN layers grown by molecular beam epitaxy on metalorganic chemical vapor deposition (MOCVD)-grown GaN “template” layers were investigated as a function of the group III/group V flux ratio during growth. GaN layers grown with a low III/V ratio (N-stable growth) displayed a faceted surface morphology and a tilted columnar structure with a high density of stacking faults. In contrast, films grown with a high III/V ratio (Ga-stable growth) displayed comparable structure to the underlying MOCVD-grown template. The transition from N-stable to Ga-stable growth modes was found to occur over a narrow range of Ga fluxes at a growth temperature of 650 °C. Evidence of Ga accumulation and step-flow growth was observed for films grown under Ga-stable conditions, leading to the formation of spiral growth features at the surface termination of mixed edge/screw dislocations. Photoluminescence measurements indicate that the deep-level (∼550 nm) emission is...

POLARIZATION-INDUCED CHARGE AND ELECTRON MOBILITY IN ALGAN/GAN HETEROSTRUCTURES GROWN BY PLASMA-ASSISTED MOLECULAR-BEAM EPITAXY

The formation of the two-dimensional electron gas (2DEG) in unintentionally doped AlxGa1−xN/GaN (x⩽0.31) heterostructures grown by rf plasma-assisted molecular-beam epitaxy is investigated. Low-temperature electrical-transport measurements revealed that the two-dimensional electron gas density strongly depends on both the thickness of the AlGaN layer and alloy composition. The experimental results agree very well with the theoretical estimates of the polarization-induced 2DEG concentrations. Low-temperature electron mobility was found to be much higher in the structures with lower electron sheet densities. Interface roughness scattering or alloy disorder scattering are proposed to be responsible for this trend. A maximum mobility of 51 700 cm2/V s (T=13 K) was obtained in the Al0.09Ga0.91N/GaN structure with a two-dimensional electron gas density of 2.23×1012 cm−2.

Electrical characterization of GaN p-n junctions with and without threading dislocations

The effect of dislocations on the electrical characteristics of GaN p-n junctions has been examined through current–voltage measurements. Lateral epitaxial overgrowth (LEO) was used to produce areas of low dislocation density in close proximity to areas with the high dislocation density typical for growth on sapphire. A comparison of p-n diodes fabricated in each region reveals that reverse-bias leakage current is reduced by three orders of magnitude on LEO GaN. Temperature-dependent measurements on the LEO diodes indicate that the remaining leakage current in these devices is associated with a deep trap level.

Dislocation mediated surface morphology of GaN

The surfaces of GaN films grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were studied using atomic force microscopy (AFM). Due to the high dislocation densities in the films (108 cm−2), the typical surface morphologies of layers grown by both techniques were dominated by three dislocation mediated surface structures—pinned steps, spiral hillocks, and surface depressions. The characteristics of these surface structures were found to depend on growth technique (MOCVD vs MBE) and the group-III to group-V ratio used in the growth of MBE GaN films. Pinned steps, created by the intersections of mixed character dislocations with the free surface, were found on all GaN films. The pinned steps were observed to be predominantly straight on the MOCVD GaN and curved into spiral hillock formations on the MBE GaN. Spiral growth hillocks form when pinned steps grow outward and around the dislocation under step-flow growth conditions. The tightness of the spiral hillocks on MBE G...

High-performance (Al,Ga)N-based solar-blind ultraviolet p–i–n detectors on laterally epitaxially overgrown GaN

Solar-blind ultraviolet photodiodes with a band-edge wavelength of 285 nm were fabricated on laterally epitaxially overgrown GaN grown by metalorganic chemical vapor deposition. Current–voltage measurements of the diodes exhibited dark current densities as low as 10 nA/cm2 at −5 V. Spectral response measurements revealed peak responsivities of up to 0.05 A/W. Response times for these diodes were measured to be as low as 4.5 ns for 90%-to-10% fall time. For comparison, diodes were fabricated using the same p–i–n structure deposited on dislocated GaN. These diodes had dark current densities many orders of magnitude higher, as well as a less sharp cutoff, and a significant slow tail under impulse excitation.

Microstructure of GaN laterally overgrown by metalorganic chemical vapor deposition

Extended defect reduction in GaN grown by lateral epitaxial overgrowth (LEO) on large-area SiO2/GaN/Al2O3 wafers by low pressure metalorganic chemical vapor deposition is characterized using transmission electron microscopy and atomic force microscopy. The laterally overgrown GaN (LEO GaN) has a rectangular cross section with smooth (0001) and {1120} facets. The density of mixed-character and pure edge threading dislocations in the LEO GaN (<5×106 cm−2) is reduced by at least 3–4 orders of magnitude from that of bulk GaN (∼1010 cm−2). A small number of edge dislocations with line directions parallel to the basal plane are generated between the bulk-like overgrown GaN and the LEO GaN regions as well as at the intersection of adjacent merging LEO GaN stripes. The edge dislocations are most likely generated to accommodate the small misorientation between bulk-like GaN and LEO GaN regions as well as between adjacent single-crystal LEO GaN stripes.

Defect reduction in (11¯00) m-plane gallium nitride via lateral epitaxial overgrowth by hydride vapor phase epitaxy

This letter reports on extended defect density reduction in m-plane (11¯00) GaN films achieved via lateral epitaxial overgrowth (LEO) by hydride vapor phase epitaxy. Several dielectric mask patterns were used to produce 10 to 100 μm-thick, partially and fully coalesced nonpolar GaN films. X-ray rocking curves indicated the films were free of wing tilt. Transmission electron microscopy showed that basal plane stacking fault (SF) and threading dislocation (TD) densities decreased from 105cm−1 and 109cm−2, respectively, less than 3×103cm−1 and ∼5×106cm−2, respectively, in the Ga-face (0001) wing of the LEO films. SFs persisted in ⟨0001⟩-oriented stripe LEO films, though TD reduction was observed in the windows and wings. Band-edge cathodoluminescence intensity increased 2 to 5 times in the wings compared to the windows depending on the stripe orientation. SFs in the low TD density wings of ⟨0001⟩-stripe films did not appear to act as nonradiative recombination centers.

Characterization of Planar Semipolar Gallium Nitride Films on Spinel Substrates

Specular, planar semipolar gallium nitride films were grown by hydride vapor phase epitaxy. Planar films of (1013) and (1122) GaN have been grown on (1010) m-plane sapphire. The in-plane epitaxial relationship for (1013) GaN was [3032]GaN ∥[1210]sapphire and [1210]GaN ∥[0001]sapphire. The in-plane epitaxial relationship for (1122) GaN was [1121]GaN ∥[0001]sapphire and [1100]GaN ∥[1210]sapphire. The (1013) films were determined to have N-face sense polarity and a threading dislocation density of 9×108 cm-2. The (1122) films have Ga-face sense polarity and have a threading dislocation density of 2×1010 cm-2. The basal plane stacking fault density was 2 ×105 cm-1 for both orientations. The RMS roughness of the films was under 10 nm for a 5 ×5 µm2 area.

Optimization of the surface morphologies and electron mobilities in GaN grown by plasma-assisted molecular beam epitaxy

The morphology and electrical properties of homoepitaxial GaN layers grown by molecular beam epitaxy at 720 °C were investigated as a function of Ga/N ratio. GaN films grown with low Ga/N ratios (N-stable regime) are semi-insulating and have heavily pitted morphologies. GaN films grown with higher Ga/N ratios (intermediate regime) have fewer pits with areas of atomically flat surface. The room-temperature electron mobilities in samples grown in the intermediate regime are greater than 800 cm2/V s and increase with Ga/N ratio. At the highest Ga/N ratios (Ga-droplet regime), Ga droplets formed on the surface during growth. Although the surface morphology is free of pits and atomically flat for films grown within the Ga-droplet regime, the mobility decreases significantly compared to films grown in the intermediate regime. Room-temperature electron mobilities as high as 1191 cm2/V s were measured in a GaN film grown with the highest Ga/N ratio within the intermediate regime.