Ga-polar GaN Research Articles

Step-flow growth mode instability of N-polar GaN under N-excess

GaN layers were grown on N-polar GaN substrates by plasma-assisted molecular beam epitaxy under different III/V ratios. Ga-rich conditions assure step-flow growth with atomically flat surface covered by doubly-bunched steps, as for Ga-polar GaN. Growth under N-excess however leads to an unstable step-flow morphology. Particularly, for substrates slightly miscut towards ⟨101¯0⟩, interlacing fingers are covered by atomic steps pinned on both sides by small hexagonal pits. In contrast, a three-dimensional island morphology is observed on the Ga-polar equivalent sample. We attribute this result to lower diffusion barriers on N-polar compared to Ga-polar GaN under N-rich conditions.

Continuous-Flow MOVPE of Ga-Polar GaN Column Arrays and Core–Shell LED Structures

Arrays of dislocation free uniform Ga-polar GaN columns have been realized on patterned SiOx/GaN/sapphire templates by metal organic vapor phase epitaxy using a continuous growth mode. The key parameters and the physical principles of growth of Ga-polar GaN three-dimensional columns are identified, and their potential for manipulating the growth process is discussed. High aspect ratio columns have been achieved using silane during the growth, leading to n-type columns. The vertical growth rate increases with increasing silane flow. In a core–shell columnar LED structure, the shells of InGaN/GaN multi quantum wells and p-GaN have been realized on a core of n-doped GaN column. Cathodoluminescence gives insight into the inner structure of these core–shell LED structures.

Nucleation and Coalescence of Indium Rich InGaN Layers on Nitridated Sapphire in Metal–Organic Vapor Phase Epitaxy

We report on the evolution of N-polar wurtzite (0001̄) InGaN layers with high indium content grown directly on nitridated (0001) sapphire by metal–organic vapor phase epitaxy. In-situ ellipsometry showed an initial growth delay of 5–8 nm for layers with more than 45% indium content. Atomic force microscopy showed small dense islands which reached their final size after 5–8 nm deposition. The roughness saturated only after 15 nm, and the width of the X-ray diffraction (00.2) reflex showed a similar behavior. Therefore, N-polar InGaN grows via nucleation (d < 10 nm) followed by coalescence (d < 20 nm). With less than 40% indium the coalescence is much slower, very similar to Ga-polar (0001) GaN despite N-polarity of the InN layers. This indicates towards a surface transition between 40 and 50% indium content.

Interface trap characterization of atomic layer deposition Al2O3/GaN metal-insulator-semiconductor capacitors using optically and thermally based deep level spectroscopies

Quantitative measurements of interface state density and energy distribution profiles within Al2O3/GaN interfaces were obtained by constant capacitance deep level transient spectroscopy and deep level optical spectroscopy (CC-DLTS/DLOS). The new application of CC-DLOS to interface state measurement is described, which allows interrogation of very deep interface states. A series of Al2O3/GaN metal-insulator-semiconductor (MIS) devices prepared as a function of Al2O3 thickness via atomic layer deposition, on NH3-MBE-grown n-type Ga-polar GaN layers enabled a systematic study. The overall shape and magnitude of the interface trap distribution, Dit, were determined to be nearly identical, independent of Al2O3 thickness. The Al2O3/GaN Dit spectra had an overall U-shape with Dit ∼1012 cm−2 eV−1 near the conduction band edge, ∼1011 cm−2 eV−1 mid-gap, and ∼1014 cm−2 eV−1 near the valence band edge. However, the interface states near the GaN conduction band showed a slight inverse dependence on Al2O3 thickness, suggestive of annealing effect during deposition. The high near valence band state concentrations are consistent with expectations from residual carbon impurities at the GaN surface. A method for discriminating between bulk and interface states in the CC-DLTS signal is demonstrated, using the results on MIS capacitors in combination with spectroscopy results on a Schottky diode structure.

Growth of GaN Nanorods and Wires and Spectral Tuning of Whispering Gallery Modes in Tapered GaN Wires

This paper reports on the growth of GaN nanorods and wires by metal–organic vapor phase epitaxy. Density, height and diameter are strongly influenced by the growth time. A deposition time of a few minutes leads to the formation of GaN nanorods. Increasing the deposition time up to 1 h yields wires with heights exceeding 47 µm. Transmission electron microscopy and convergent beam electron diffraction measurements are showing the presence of N- and Ga-polar GaN in a single nanorod. Cathodoluminescence measurements are performed showing the appearance of whispering gallery modes. Due to slight tapering of the wires the whispering gallery modes can be spectrally tuned by changing the position of the exposing electron beam at the sidewall facet of the rod.

Electrical characterization of Schottky contacts to N-polar GaN

The Schottky barrier heights of several metals (Cu, Au, Pd, Ni, and Pt) to N-polar GaN were extracted using current–voltage and capacitance–voltage measurements. The dependence of barrier height on metal was found to vary linearly with the electronegativity of the metal as predicted by the metal-induced gap states (MIGS)-and-electronegativity model. However, the magnitude of the barrier heights are lower than those predicted by the MIGS model for GaN and lower than the experimentally measured barrier heights for Ga-polar GaN. It is likely that the polarization-induced charge at the N-polar GaN surface is responsible for the reduced barrier height.

Ga-polar GaN nanocolumn arrays with semipolar faceted tips

Selective area growth of GaN nanocolumns (NCs) by molecular beam epitaxy on laser ablated pre-patterned GaN(0001) templates is shown to provide regular arrays of Ga-polar NCs. The Ga diffusion-assisted growth mechanism is analyzed and the experiments suggest that the effective growth conditions vary with the height of the NCs due to Ga diffusion on the mask and the NC sidewalls, ranging from N-rich up to stoichiometry. The obtained morphology with semipolar facets at the tip is discussed within the framework of equilibrium thermodynamics, which provides a consistent picture also for the growth of N-polar NCs with flat tips. The structural investigation reveals almost defect-free semipolar GaN facets at the top of the NCs, which is known to be a promising way of producing templates for nanoscale semipolar GaN-based heterostructures. Almost no polarization discontinuity is expected for InxGa1−xN/GaN interfaces on such facets.

On the hydrophobicity of modified Ga-polar GaN surfaces

In this communication, we show that chemical modifications of etched GaN (with different morphological features) through treatment with trichlorododecylsilane or sputtering nano-Au films followed by covalent binding of 1-dodecanethiol strongly affect surface water repellency characteristics. Binding of 1-dodecanethiol to Au-coated whiskers results in GaN crystals becoming superhydrophobic with the contact angles approaching 150°. For a single type of surface morphology, a pairwise comparison of contact angle values of trichlorododecylsilane- and Au/1-dodecanethiol-modified surfaces provides an insight into relative abundance of –OH groups on GaN's Ga-polar side, which mediate and control binding of the hydrophobic silane compound, but not of thiols.

Deep level defects in Ga- and N-polarity GaN grown by molecular beam epitaxy on si(111)

Deep level transient spectroscopy has been used to characterize the deep level traps in Ga- and N-polarity GaN films grown by plasma assisted molecular beam epitaxy on Si(111) substrate. The two deep level traps at Ec−Et∼0.21eV (E1) and Ec−Et∼0.48eV (E2) have been detected in Ga-polarity GaN. The E1 level commonly observed deep trap related to the nitrogen vacancy in GaN. It is found that, the E2 level exhibits logarithmic capture kinetic behavior and substantially increases its capture cross section from 10−15 to 10−12cm2 by employing different pulse width ranging from 5ms to 35ms. Such behavior of E2 trap with filling pulse length attributes that, the trap is originated from threading dislocations. In case of N-polarity GaN, we observed two deep level traps with activation energies of Ec−Et∼0.53eV (E3) and Ec−Et∼0.89eV (E4). The estimated capture cross-sections (σS) for these defects were found to be ∼2.51×10−15cm2 and ∼5.21×10−16cm2 respectively. The E3 and E4 are nitrogen antisite point defect and extended defect related to dislocation deep levels, results from growth on N-polarity crystal structure.

Spontaneous Nucleation and Growth of GaN Nanowires: The Fundamental Role of Crystal Polarity

We experimentally investigate whether crystal polarity affects the growth of GaN nanowires in plasma-assisted molecular beam epitaxy and whether their formation has to be induced by defects. For this purpose, we prepare smooth and coherently strained AlN layers on 6H-SiC(0001) and SiC(0001̅) substrates to ensure a well-defined polarity and an absence of structural and morphological defects. On N-polar AlN, a homogeneous and dense N-polar GaN nanowire array forms, evidencing that GaN nanowires form spontaneously in the absence of defects. On Al-polar AlN, we do not observe the formation of Ga-polar GaN NWs. Instead, sparse N-polar GaN nanowires grow embedded in a Ga-polar GaN layer. These N-polar GaN nanowires are shown to be accidental in that the necessary polarity inversion is induced by the formation of Si(x)N. The present findings thus demonstrate that spontaneously formed GaN nanowires are irrevocably N-polar. Due to the strong impact of the polarity on the properties of GaN-based devices, these results are not only essential to understand the spontaneous formation of GaN nanowires but also of high technological relevance.

Asymmetric interfacial abruptness in N-polar and Ga-polar GaN/AlN/GaN heterostructures

In this letter, we report on the interfacial abruptness of GaN/AlN/GaN heterostructures with pulsed laser atom probe tomography (APT). The samples were grown by plasma-assisted molecular beam epitaxy (MBE) under both metal-rich and N-rich conditions on both Ga-polar (0001) GaN templates and N-polar (0001¯) GaN substrates. An NH3 assisted MBE technique was involved to grow similar Ga-polar and N-polar structures on GaN:Fe substrates and GaN/Al2O3 templates, respectively, for a comparison study. We find in all cases the interface with net positive polarization charge was chemically abrupt, whereas the interface with net negative polarization charge was diffuse. We discuss the implications on device design and performance. These data validate the efficiency of APT in studying interfaces for better performance in devices.

Selective-area growth of thin GaN nanowires by MOCVD

We report the selective-area growth (SAG) of Ga-polar thin GaN nanowires on patterned GaN/sapphire (0001) substrates using metalorganic chemical vapor deposition (MOCVD) with a continuous gas supply. It has been found that the anisotropy in the growth rates of the (0001) and the {1–100} facets of GaN reaches a maximum at low precursor flow rates for both Ga source and NH3. It has also been revealed that the SAG efficiency which is dependent on pattern fill-factor should be properly taken into account in order to grow thin GaN nanowires. Based on these findings, we demonstrate the growth of GaN nanowires with a diameter of 50nm, which is the smallest reported so far by selective-area MOCVD. Optical properties of a single GaN nanowire have been investigated by low temperature micro-photoluminescence. It has been shown that an appropriate shell layer is effective to considerably improve the properties, suggesting the importance of controlling surface states.

Homoepitaxial N-polar GaN layers and HEMT structures grown by rf-plasma assisted molecular beam epitaxy

The authors have investigated the growth and structural and electrical properties of homoepitaxial GaN layers and GaN/AlGaN heterostructures grown on free-standing, hydride vapor phase epitaxy grown, N-polar GaN:Fe substrates by rf-plasma molecular beam epitaxy. Secondary-ion mass spectroscopic analysis of unintentionally doped and Be-doped N-polar GaN layers indicate that oxygen is the dominant impurity in all layers and is largely insensitive to growth temperature in the range investigated (675 °C < TS < 760 °C). Transmission electron microscopy (TEM) indicates that threading dislocations are generated at the regrowth interface in these samples; in contrast to homoepitaxial growth on Ga-polar GaN, and that the density of threading dislocations diminishes as the growth temperature increases. However, examination by TEM indicates that threading dislocations are not generated at the regrowth interface of samples subjected to pregrowth substrate surface cleaning by gallium deposition and desorption and subsequent growth of ultrathin (15 Å) initial AlN layers. N-polar GaN/AlGaN heterostructures grown on Be-doped homoepitaxial N-polar GaN buffers exhibit low buffer leakage and Hall mobilities up to 1680 cm2/Vs at sheet densities of 1.3 × 1013 cm−2. High electron mobility transistors have been fabricated on these structures; drain current densities over 700 mA/mm and breakdown voltages as high as 70 V have been measured.

8 MeV Proton Irradiation Damage and Its Recovery by Annealing on Single-Crystalline Zinc Oxide Crystals

ABSTRACTII-VI compound semiconductor ZnO has a potential for high radiation hardness since large threshold displacement energy of constituent atoms can be expected due to the small unit-cell volume and large bandgap energy of 3.37 eV. In order to study the radiation hardness, singlecrystalline c-axis-oriented O-polar ZnO films with and without two-dimensional electron gas, a Zn-polar ZnO bulk crystal, and a Ga-polar GaN bulk crystal for comparison, were irradiated by an 8 MeV proton beam using a tandem-type accelerator. The radiation damage increased the electrical resistance and decreased the photoluminescence (PL) intensity of these samples with the increase of proton fluence over specific threshold values. In agreement with the expectation, ZnO samples were revealed to have superior radiation hardness; the threshold fluences for the deterioration of PL intensity were 3×1013 p/cm2 for the GaN bulk crystal, 2×1014 p/cm2 for the ZnO bulk crystal, and 5×1014 p/cm2 for the two ZnO films, in accordance with the order of the threshold fluences for the electrical resistance increase. The effect of post-irradiation annealing was also studied for these damaged bulk crystals; both electrical and optical properties of the ZnO bulk crystal were almost recovered to the pre-irradiation values, however, only the electrical properties of the GaN bulk crystal were recovered, by the annealing up to 700°C. Such a rapid recovery of the ZnO bulk crystal indicates the easy annihilation of Zn vacancy complexes acting as non-radiative centers by the recombination with interstitial Zn atoms. Since the migration barrier height energy of interstitial Zn atoms is known to be so small that it might occur even at room temperature, we ascribed the superior radiation hardness of ZnO crystals to the restoration of damage-induced defects by a self-annealing effect during irradiation.

Temperature dependent photoluminescence of lateral polarity junctions of metal organic chemical vapor deposition grown GaN

We report on fundamental structural and optical properties of lateral polarity junctions in GaN. GaN with Ga- to N-polar junctions was grown on sapphire using an AlN buffer layer. Results from scanning electron microscopy and Raman spectroscopy measurements indicate a superior quality of the Ga-polar GaN. An extremely strong luminescence signal is observed at the inversion domain boundary (IDB). Temperature dependent micro photoluminescence measurements are used to reveal the recombination processes underlying this strong emission. At 5 K the emission mainly arises from a stripe along the inversion domain boundary with a thickness of 4-5 μm. An increase of the temperature initially leads to a narrowing to below 2 μm emission area width followed by a broadening at temperatures above 70 K. The relatively broad emission area at low temperatures is explained by a diagonal IDB. It is shown that all further changes in the emission area width are related to thermalization effects of carriers and defects attracted to the IDB. The results are successfully used to confirm a theoretical model for GaN based lateral polarity junctions. Due to the strong and pronounced emission of IDBs even at elevated temperatures, it is demonstrated that lateral polarity junctions exhibit a strong potential for future high efficiency devices.

Analysis of leakage current mechanisms in Pt/Au Schottky contact on Ga-polarity GaN by Frenkel-Poole emission and deep level studies

We report the Frenkel-Poole emission in Pt/Au Schottky contact on Ga-polarity GaN grown by molecular beam epitaxy using current-voltage-temperature (I-V-T) characteristics in the temperature ranging from 200 K to 375 K. Using thermionic emission model, the estimated Schottky barrier height is 0.49 eV at 200 K and 0.83 eV at 375 K, respectively, and it is observed that the barrier height increases with increase in temperature. The extracted emission barrier height (ϕt) for Ga-polarity GaN Schottky diode by Frenkel-Poole theory is about 0.15 eV. Deep level transient spectroscopy study shows a deep level with activation energy of 0.44 eV, having capture cross-section 6.09 × 10−14 cm2, which is located between the metal and semiconductor interface, and trap nature is most probably associated with dislocations in Ga-polarity GaN. The analysis of I-V-T characteristics represents that the leakage current is due to effects of electrical field and temperature on the emission of electron from a trap state near the metal-semiconductor interface into continuum states associated with conductive dislocations in Ga-polarity GaN Schottky diode.

Polarity dependent structure and optical properties of freestanding GaN layers grown by hydride vapor phase epitaxy

GaN freestanding substrate was obtained by hydride vapor phase epitaxy directly on sapphire with porous network interlayer. The morphologies of Ga-face and N-face of freestanding GaN substrate were analyzed by a variety of characterization techniques before and after etching in boiled KOH for 1min. The obtained characteristics of unetched GaN are strongly dependent on the growth polarity. The N-polar GaN layer has high free electron, impurity and point defect concentrations. In the layers grown on the (0001) Ga-polar side, these concentrations are very low. After etching, the Ga-polar GaN has identical properties to those of the unetched Ga-polar GaN layer. But the etched N-polar GaN has significant difference with unetched N-polar GaN layer in structure and optical properties. The etched N-polar GaN has a smaller (0002) DCXRD width (646″) than the unetched N-polar GaN (1351″). The optical quality of etched N-polar GaN is comparable with that of Ga-polar GaN, and the FWHMs of the D0X line of Ga-face and etched N-face are 9.3 and 12.8meV, respectively. The LPP mode in the Raman spectra and FERB peak in PL spectra were used to analyze the free carrier concentration of two sides of etched and unetched freestanding GaN substrate.

Polarity and Its Influence on Growth Mechanism during MOVPE Growth of GaN Sub-micrometer Rods

The influence of polarity during the MOVPE growth of GaN based sub-micrometer (sub-μm) rods has mostly been neglected up to now. In this paper we demonstrate that the surface polarity plays a crucial role for the morphology of the GaN sub-μm rods. Based on the differences between N-polar and Ga-polar surfaces, a model is suggested to explain the influence of various parameters on the morphology of GaN sub-μm rods for the first time. For Ga-polar GaN, the {10−11} r-planes, similar to N-polar (000−1) c-planes, are terminated by nitrogen atoms. These N-terminated surfaces can be passivated by hydrogen, which leads to a stable surface with low growth rates and therefore tends to keep a pyramidal shape with stable r-planes. For N-polar GaN, {10−1−1} r-planes can be modified by H2 etching, leading to the formation of more stable {1−100} m-planes, hence supporting the formation of sub-μm rods with vertical sidewalls.

Spectroscopic measurements of the surface stoichiometry of chemical vapor deposited GaN

We report on the surface stoichiometry of Ga-polar GaN films grown by metalorganic chemical vapor deposition as studied by x-ray photoelectron spectroscopy. GaN film surfaces are found to be Ga-rich, with Ga:N ratios ranging from 1.3:1 to 3.2:1. In vacuo ion-beam sputter/annealing studies show that these treatments drive the apparent Ga:N surface composition farther from unity, either through a decrease in surface contamination, oxidation of the surface, or both. Simple annealing experiments decrease the Ga:N ratio. The measured GaN ratio is correlated with the GaN growth time, suggesting that residual Ga precursor after growth interacts with the GaN surface as it cools.

“Cube-on-hexagon” orientation relationship for Fe onGaN(0001¯): The missing link in bcc/hcp epitaxy

We investigate, experimentally and theoretically, the epitaxy of body-centered-cubic Fe on hexagonal GaN. For growth on the Ga-polar GaN(0001) surface we find the well-known Pitsch-Schrader orientation relationship between Fe and GaN. On the N-polar $\text{GaN}(000\overline{1})$ surface we observe coexistence between the familiar Burgers orientation and a new orientation in which the Fe(001) plane is parallel to $\text{GaN}(000\overline{1})$. This ``cube-on-hexagon'' orientation constitutes the high-symmetry link required for constructing a symmetry diagram for bcc/hcp systems in which all orientation relationships are connected by simple rotations.