GaN Single Crystals Research Articles

Influence of Morphology on Current–Voltage Behavior of GaN Nanowires

We demonstrate the effect that the different morphologies of molecular beam epitaxy-grown GaN nanowires (NWs) can have upon current-voltage (I-V) behavior. Two aspects of NW morphology were investigated. The first aspect was the NW diameter, d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">NW</sub> . For single-crystal Si-doped GaN NW devices with d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">NW</sub> <; 120 nm, I-V curves were nonlinear. In contrast, single-crystal Si-doped NWs from the same growth run with d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">NW</sub> > 120 nm consistently showed ohmic I-V behavior. This discrepancy is likely the result of the comparatively larger surface depletion in thin NWs, which contributes to 1) an increased contact barrier, and 2) a barrier resulting from an axial band offset between the portion of the NW directly beneath the contact and the portion extending from the contact. The second aspect of NW morphology that we investigated was NW coalescence, which occurs when neighboring NWs fuse together during growth. I-V measurements of undoped coalesced NWs showed that these structures can have a free carrier concentration that is significantly higher than the background carrier concentration that is present in single-crystal (noncoalesced), undoped NWs.

Synthesis and Growth Mechanism: A Novel Fishing Rod-Shaped GaN Nanorods

A novel fishing rod-shaped GaN nanorod is successfully fabricated through a new method by using the two-step growth technology. This growth method is applicable to continuous synthesis and is able to produce a large number of single-crystalline GaN nanorods with a relatively high purity and at a low cost. X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy are used to characterize the as-synthesized nanorods. The results show that most of the nanorods consist of a main rod and a top curved thread. It is single-crystal GaN with hexagonal wurtzite structure. The representative photoluminescence spectrum at room temperature exhibits a strong UV light emission band centered at 370.8 nm. Furthermore, a possible two-stage growth mechanism of the fishing rod-shaped GaN nanorod is also briefly discussed.

Nonlinear optical effect upon the irradiation of GaN with cluster ions

The effect of the irradiation of single-crystal GaN films with accelerated atomic (P) and molecular (PF4) ions with an energy of 0.6 keV amu−1 on the crystal structure and optical properties of the films is considered. It is shown that, compared to irradiation with atomic ions, irradiation with small clusters induces (i) more rapid accumulation of structural defects, (ii) more efficient suppression of the violet luminescence band, and (iii) shorter decay times of the intensity of this band.

Electrochemical deposition of copper on single-crystal gallium nitride(0001) electrode: nucleation and growth mechanism

The electrodeposition of Cu on n-type single-crystal GaN(0001) electrode from sulfate solution was investigated by electrochemical techniques and scanning electron microscopy. It was found that Cu deposition on GaN(0001) commenced at a large negative potential of −840mV vs. Pt/Pt2+ and was quasi-reversible and mass transfer limited. On the basis of Tafel plot, a low exchange current density of ∼ 2.3×10−6mAcm−2 was calculated. This was mainly due to the limited free electrons in the conduction band of GaN. In addition, the current transient measurements revealed that the deposition process followed the progressive nucleation in 0.5M H2SO4+5mM CuSO4. The instantaneous nucleation was observed only at the large applied potential of −1.1V.

Analysis of reciprocal space maps of GaN(0001) films grown by molecular beam epitaxy

The reciprocal space map of a heteroepitaxial single-crystal GaN(0001) film on 6H-SiC(0001) is analyzed. The film contains a high density of threading dislocations which intersect the film parallel to the surface normal. The strain field associated with these dislocations is expected to broaden all reciprocal lattice points to discs perpendicular to the dislocation lines,i.e.parallel to the surface. Experimentally, however, the reflection is observed to be broadened also perpendicular to the surface and is rotated towards the surface normal. Using Monte Carlo simulations, it is shown that both of these features are a natural consequence of the presence of misfit dislocations at the film/substrate interface.

Enhancement of photoluminescence efficiency from GaN(0001) by surface treatments

We investigated the photoluminescence (PL) efficiency of GaN(0001) single crystals with clean and well-defined surfaces using the PL technique in ultrahigh vacuum in situ. We found typical degradation factors: native oxides at the top surface, damaged layers in the subsurface, and hydrogenated non-radiative states inside bulk GaN. By eliminating the degradation factors, a band-to-band PL intensity of approximately 120 times higher than that of the as-received samples was achieved. The PL efficiency enhancement mechanism is discussed, and the role of hydrogen in GaN crystals is proposed.

Damage-free and Atomically-flat Finishing of Single Crystal SiC by Combination of Oxidation and Soft Abrasive Polishing

Plasma-assisted polishing (PAP), which combined the irradiation of atmospheric-pressurewater vapor plasma and ceria abrasive polishing, was proposed for the finishing of difficult-to-machine materials, such as single crystal SiC, GaN, sapphire and diamond. In the case of PAP was applied to a 4H-SiC-Si face, an oxide layer (SiO2) was generated after plasma irradiation and it was removed by polishing using soft abrasives. In this way, a damage-free and atomically-flat 4H-SiC-Si face could be obtained. In this study, soft abrasive polishing (CeO2)of a plasma-oxidized SiC-Si face and a thermally oxidized SiC-C face was respectively conducted. The generation of a well-ordered one-bilayer step-terrace structure on both of the polished Si face and C face was confirmed by atomic force microscopy (AFM). However, due to the high temperature of thermal-oxidation, many pits were generated on SiC-C face after polishing.

Preparation of GaN nanowires by nonammonia method and their photoelectronic properties

Single-crystal hexagonal wurtzite GaN nanowires were successfully synthesized by using plasma-enhanced chemical vapor deposition (PECVD) via vapor-liquid-solid (V-L-S) mechanism, under the condition of non-ammonia at 1050 ℃. Raman spectra show that the as-synthesized nanowires have large disorder surface, in which there is a significantly small size effect. Furthermore, it is also observed that the prepared nanowires have typical photoluminescence characteristics and good field emission properties.

Investigation of Elastic Energy on Single Crystal GaN Nanobeams with Different Span

This research presents a novel technique which can more efficiently fabricate different spans of nanobeams on the same substrate. It requires less time to prepare specimen and further shortens the process of aligning, clamping, and testing. Also, we probe into the elastic deformation properties of clamped freestanding GaN nanobeams with different spans. In the bending process, displacement,D, corresponding to load,Pis strongly dependent on the span of nanobeam at the same penetration depth and a distinct linearity is observed. Young’s moduliEof the GaN in this study are calculated as 171.3 GPa ±5.4% and 264.2 GPa ±4.7% by strain energy methods, respectively, for the longer and shorter spans of nanobeams, serving as a simple supporting beam of elastic material under small deformation. The result shows that, even under small deformation, the rigidity enhancement helps the shorter nanobeam store more elastic energy.

Structural characterizations of GaN nanowires grown on Si (111) substrates by thermal evaporation

In this work wurtzite GaN nanowires (NWs) on silicon (111) substrates were grown, using commercial GaN powder by thermal evaporation in an atmosphere of argon (Ar) gas. Structural and optical characterizations were performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution X-ray diffraction (HR-XRD) and photoluminescence (PL) spectroscopy. The obtained results of GaN NWs indicated that the NWs on Si (111) substrates are of good quality single-crystal hexagonal GaN due to the small lattice mismatch between the NWs and substrates compared to the growth of GaN NWs on other substrates.

&gt;1200 V GaN‐on‐silicon Schottky diode

AbstractM/A‐COM Technology Solutions has continuing joint development efforts sponsored by the Department of Energy with MIT main campus and MIT Lincoln Laboratory to develop GaN on silicon two and three terminal high voltage/high current switching devices. The initial developmental goals were for a Schottky diode that has a reverse breakdown blocking voltage of &gt;600 V and is capable of handling 10 A of forward current. A comparison of the M/A‐COM Technology Solutions lateral GaN Schottky diode on‐resistance as a function of reverse breakdown voltage for a number of both lateral and vertical GaN Schottky diode geometries taken from the literature is presented. The substrates employed for all of these data points are either sapphire, SiC, silicon, and even one study which utilized single crystal GaN. Also included in this plot are theoretical limits for the basic materials typically used in GaN Schottky diode construction. It can be seen that the reverse breakdown results of approximately 1500 V for M/A‐COM Technology Solutions lateral anode connected field GaN Schottky diodes on silicon substrates compare extremely favorably with the reported performance of the state‐of‐the‐art devices, regardless of substrate material or design geometry. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Hydrophobic Growth of GaN Material: Current Status and Future Potential

Due in part to the high breakdown voltage, gallium nitride is among a select few semiconductors investigated for next generation high power electronic devices. However, GaN based devices are currently performing far below their theoretical potential. Manufacturers of GaN HEMTs and FETs must de-rate the voltage of the device to maintain a high reliability. Conventional epitaxial growth of GaN thin films by HVPE, MOCVD, and MBE have been performed on nonnative substrates of Si, SiC, or sapphire, which results in defect formation and ultimately lowers operating voltages. To reduce the defects, several growth techniques have been utilized by researchers to produce bulk GaN single crystals. However, the fabrication of bulk GaN crystals of substantial size and suitable quality has proved quite difficult. This paper will review the current state of the art for the Ammonothermal, Flux growth, and HVPE techniques currently being pursued to produce bulk GaN.

Scallion-Root-Shaped GaN Nanorods Grown by Two-Step Method and Study on their Properties

Two-step growth technology to successfully synthesize scallion-root-shaped GaN nanorods was presented in this paper. This growth method is applicable to continuous synthesis a large number of single-crystalline GaN nanorods with a high purity at a low cost. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) are employed to characterize the structure, composition and morphology of as-grown GaN nanorods. The results show that the obtained nanorods are single-crystal GaN with hexagonal wurtzite structure and have a relatively high purity. The diameter of the nanorods is about 500nm with length up to several tens of micrometers. The representative photoluminescence spectra (PL) measured at room temperature exhibited a strong and broad emission peak at 388nm corresponding to the strong-band-emission in wurtzite GaN, indicating that the nanorods have a good emission property. The growth mechanism is also briefly discussed.

Observation of symmetrically decay of A1(longitudinal optical) mode in free-standing GaN bulk single crystal from Li3N flux method

We report Raman analysis of A1(LO) (longitudinal optical) and E2(high) phonon lifetimes in a bulk GaN single crystal and their temperature dependence from 77 K to 770 K. Both the A1(LO) and E2(high) phonons in GaN were observed decaying primarily into two phonons of equal energy [Klemens model, P. G. Klemens, Phys. Rev. 148, 845 (1966)]. This is a rare example of a high-quality free-standing GaN bulk single crystal displaying abnormal A1(LO) phonon decay. These results will have significant impact on designing and understanding of GaN-based high-speed, high-power electric devices.

Resonant Raman scattering in GaN single crystals and GaN-based heterostructures: feasibility for laser cooling

The recent progress on Raman scattering in GaN single crystals and GaN/AlN heterostructures is reviewed. Anti-Stokes Raman scattering is used to determine electron-phonon scattering time and decay time constant for longitudinal-optical phonons. In a typical high electron mobility transistor based on GaN/AlN heterostructures, strong resonances are reached for the first-order and second-order Raman scattering processes. Therefore, both Stokes and anti-Stokes Raman intensities are dramatically enhanced. The feasibility for laser cooling of a nitride structure is studied. A further optimization will enable us to reach the threshold for laser cooling. Raman scattering have potential applications in up-conversion lasers and laser cooling of nitride ultrafast electronic and optoelectronic devices.

Local current conduction due to edge dislocations in deformed GaN studied by scanning spreading resistance microscopy

Local electrical conductivities were measured for plastically deformed n -GaN single crystals by scanning spreading resistance microscopy (SSRM). In the SSRM images, many spots with high conductivity were observed, which can be attributed to introduced edge dislocations whose line direction is along [0 0 0 1] and Burgers vector is b = ( a/ 3)[1 2 ¯ $\bar{2}$ 1 0]. This result is in contrast to the previous studies which showed that grown-in edge dislocations of the same type in GaN films exhibit virtually no conduction. This suggests that the dislocation conduction depends sensitively on the dislocation core structure. Current-voltage spectra indicated a Frenkel-Poole mechanism for the conduction.

Observations of Macroporous Gallium Nitride Electrochemically Etched from High Doped Single Crystal Wafers in HF Based Electrolytes

In this paper, we present a process to produce porous structures by electrochemical etching of highly doped n-type GaN single crystals in aqueous HF solutions. At first, we show that we are able to perform well oriented macropores from 30% HF based solutions. To our knowledge, it's the first time that this morphology is observed in porous GaN. These macropores grow from a 5 μm thick mesoporous layer observable at the sample surface. No significant difference in term of morphology is visible for etching on Ga or N face. Moreover, the influence of the HF concentration on the morphology is also discussed.

Cracks and dislocations at Vickers, Berkovich and cube corner indentations in (0001) GaN single crystals

Abstract(0001) GaN single crystals having a thickness of 3.4 mm and a density of in‐grown dislocations of 3.5 x 106 cm‐2 have been deformed at room temperature using a cube corner, a Berkovich and a Vickers indenter, respectively. Cube corner indentations were performed in a scanning electron microscope in order to observe the deformation in situ. Subsequently, cracks and dislocations at the indentations were investigated by means of optical microscopy, scanning electron microscopy in secondary electron contrast and cathodoluminescence imaging. The arrangement of dislocations is determined by the symmetry of the indented surface. In contrast, the formation of cracks depends on both the shape and the orientation of the indenter. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Structure and luminescence properties of thermally nitrided Ga2O3 nanowires

The structure and luminescence properties of thermally nitrided Ga2O3 nanowires were examined. Transmission electron microscopy and X-ray diffraction confirmed the formation of a uniform GaN shell layer on the surface of the nanowires by thermal nitridation. The core and shell of the nitrided nanowires were monoclinic-structured single crystal Ga2O3 and wurtzite-type hexagonal close-packed-structured single crystal GaN, respectively. The as-synthesized Ga2O3 nanowires exhibited a broad emission band at approximately 570nm in the yellow region. In contrast, the nitrided Ga2O3 nanowires exhibited a much stronger emission band at approximately 455nm in the blue region, which must originate from the newly formed GaN shell layer.

Preparation and characterization of one-dimensional GaN nanorods with Tb intermediate layer

GaN nanorods have been successfully prepared on Si(111) substrates by magnetron sputtering through ammoniating Ga2O3/Tb thin films. X-ray diffraction (XRD), X-ray photoelectron spectroscope (XPS), FT-IR spectrophotometer, scanning electron microscope (SEM), high-resolution transmission electron microscope (HRTEM), and photoluminescence (PL) spectroscopy were used to characterize the microstructures, morphologies compositions and optical properties of the GaN samples. The results demonstrate that the nanorods are single crystal GaN with hexagonal wurtzite structure and high-quality crystalline after ammoniating at 950°C for 15min, which have the size of 100–150nm in diameter. Ammoniating temperatures and times affect the growth of GaN nanorods significantly. The growth procedure mainly follows the Tb catalyst-assisted VLS mechanism.