A-plane InN Research Articles

Effect of indium accumulation on the characteristics of a-plane InN epi-films under different growth conditions

This study investigated the influence of indium accumulation happened on the surface of a-plane InN grown under different growth conditions. Three different growth rates with N/In ratio from stoichiometric to N-rich were used to grow a-plane InN epifilms on GaN-buffered r-plane sapphires by plasma-assisted molecular beam epitaxy. When a-plane InN was grown above 500°C with a high growth rate, abnormally high in-situ reflectivity was found during a-plane InN growth, which was resulted from indium accumulation on surface owing to In-N bonding difficulty on certain crystal faces of a-plane InN surface. Even using excess N-flux, indium accumulation could still be found in initial growth and formed 3-dimension-like patterns on a-plane InN surface which resulted in rough surface morphology. By reducing growth rate, surface roughness was improved because indium atoms could have more time to migrate to suitable position. Nonetheless, basal stacking fault density and crystal anisotropic property were not affected by growth rate.

Angular-dependent Raman study of a- and s-plane InN

Angular-dependent polarized Raman spectroscopy was utilized to study nonpolar a-plane (11¯20) and semipolar s-plane (101¯1) InN epilayers. The intensity dependence of the Raman peaks assigned to the vibrational modes A1(TO), E1(TO), and E2h on the angle ψ that corresponds to rotation around the growth axis, is very well reproduced by using expressions taking into account the corresponding Raman tensors and the experimental geometry, providing thus a reliable technique towards assessing the sample quality. The s- and a-plane InN epilayers grown on nitridated r-plane sapphire (Al2O3) exhibit good crystalline quality as deduced from the excellent fitting of the experimental angle-dependent peak intensities to the theoretical expressions as well as from the small width of the Raman peaks. On the contrary, in the case of the s-plane epilayer grown on non-nitridated r-plane sapphire, fitting of the angular dependence is much worse and can be modeled only by considering the presence of two structural modifications, rotated so as their c-axes are almost perpendicular to each other. Although the presence of the second variant is verified by transmission electron and atomic force microscopies, angular dependent Raman spectroscopy offers a non-destructive and quick way for its quantification. Rapid thermal annealing of this sample did not affect the angular dependence of the peak intensities. The shift of the E1(TO) and E2h Raman peaks was used for the estimation of the strain state of the samples.

Growth of a-Plane InN Film and Its THz Emission

We report the growth of a-plane InN on an r-plane sapphire substrate by plasma-assisted molecular-beam epitaxy. It is found that the a-plane InN is successfully grown by using a GaN buffer layer, which has been confirmed by reflection high-energy electron diffraction, x-ray diffraction and Raman scattering measurements. The Hall effect measurement shows that the electron mobility of the as-grown a-plane InN is about 406 cm2 /V·s with a residual electron concentration of 5.7 × 10 cm−3. THz emission from the a-plane InN film is also studied, where it is found that the emission amplitude is inversely proportional to the conductivity.

Surface states and electronic structure of polar and nonpolar InN – An in situ photoelectron spectroscopy study

Valence band structure and surface states of InN with (0001), (000-1), (1-100), and (11-20) orientation were investigated in situ after growth using photoelectron spectroscopy. Depending on surface orientation, different occupied surface states are identified and differentiated from bulk contributions. For N-polar, m-plane, and a-plane InN, the surface states are located at the valence band maximum, while In-polar InN features surface states close to the Fermi level. The surface band alignment correlates with the position of surface states. For InN(0001), a much larger surface downward band bending is observed compared to N-polar, m-plane, and a-plane InN, where almost flat band conditions occur.

Influence of structural anisotropy to anisotropic electron mobility in a-plane InN

This study reports on the anisotropic electron transport properties and a correlation between the electron mobility (μ) and the stacking faults (SFs) in the a-plane InN film. Electron mobilities measured by terahertz time-domain spectroscopy and Hall effect measurement along the in-plane [1¯100] (c⊥) orientation were much higher than those of the in-plane [0001] (c∥) orientation. This result shows a sharp contrast to higher defect density for the c⊥ orientation as measured by x-ray diffraction. The electrons transporting through the planar SFs aligned along the c⊥ direction are expected to experience more scattering by defects, resulting in lower μ for the c∥ orientation.

Molecular Beam Epitaxial Growth of Nonpolar a-plane InN/ GaN Heterostructures

ABSTRACTNonpolar a-plane InN/GaN heterostructures were grown by plasma assisted molecular beam epitaxy. The growth of nonpolar a- plane InN / GaN heterostructures were confirmed by high resolution x-ray diffraction study. Reflection high energy electron diffraction patterns show the reasonably smooth surface of a-plane GaN and island-like growth for nonpolar a-plane InN film, which is further confirmed by scanning electron micrographs. An absorption edge in the optical spectra has the energy of 0.74 eV, showing blueshifts from the fundamental band gap of 0.7 eV. The rectifying behavior of the I-V curve indicates the existence of Schottky barrier at the InN and GaN interface. The Schottky barrier height (φb) and the ideality factor (η) for the InN/GaN heterostructures found to be 0.58 eV and 2.05 respectively.

Influence of biaxial strain on near-band-edge optical properties of c- and a-plane wurtzite-InN films

Optical properties of c- and a-plane wurtzite-InN films with different in-plane strain are investigated by using 8 × 8 k·p method. Taking into account the influence of carrier density, the imaginary parts of ordinary and extraordinary dielectric function of c- and a-plane InN near bandedge are presented. For c-plane InN, the shift of ordinary absorption edge in the case of isotropic in-plane strain in comparison to the unstrained one is very small. For a-plane InN, the splitting between the ordinary and extraordinary components below 1 eV can be modulated with different anisotropic in-plane strain. Furthermore, the difference of splitting between the cases of in-plane strain and strain-free slightly decreases with increasing carrier concentration, which implies the influence of in-plane strain becomes weaker in higher carrier concentration. The spontaneous emission spectrum shows the ratio of peak intensity of y- and z-polarizations increases with enhancement of the splitting. It is considered that the distinct polarization dependent optical anisotropy is expected to be observed in non-polar InN film by appropriately modulating the in-plane strain.

Epitaxial Growth of High Quality Nonpolar InN Films on LiGaO2 Substrates

A new system for obtaining high quality nonpolar InN films has been demonstrated. It is found that a-plane InN epitaxially grows on LiGaO2 (001) with high phase purity and smooth surface. The in-plane epitaxial relationships are InN [0001]// LiGaO2 [100], and InN [11̅00]// LiGaO2 [010]. The interface between the substrate and a-plane InN is atomically abrupt. The threading dislocation density drops dramatically from 2 × 1010 cm−2 of its initial growth stage to 3 × 109 cm−2 when the InN film thickness reaches 30 nm, which we believe is driven by dislocation merging or annihilation near the nucleation layer. The band edge emission from its photoluminescence spectra of as-grown a-plane InN is located at 0.72 eV. This novel high quality nonpolar a-plane InN on a LiGaO2 (001) system opens up a new possibility for high-efficiency nonpolar InN devices.

MOCVD growth of a-plane InN films on r-Al 2O 3 with different buffer layers

InN films have been grown on (1 –1 0 2) r-plane Al 2O 3 substrates by metalorganic chemical vapor deposition with GaN buffer, with GaN and low-temperature (LT) InN buffers, and without any buffer layer. It is found that all of the samples are in the non-polar (1 1 –2 0) a-plane and an increase in the growth temperature of GaN buffer to about 800 °C can improve the crystal quality of the InN films as compared to the film grown on GaN buffer at 550 °C, while 550 °C is the usual GaN buffer growth temperature in c-plane InN growth. Surface morphologies of the films show a stripe-like pattern. Introducing GaN buffer can reduce the anisotropy of the full-width at half-maximum of X-ray (1 1 –2 0) rocking curves along different directions in plane. The added LT-InN layer cannot improve the morphology and the crystallography of a-plane InN, but it can increase the intensity of photoluminescence (PL) spectrum remarkably and make about 10 meV blueshift as to the sample with only GaN buffer. The PL measurements reveal that the InN films have a fundamental band gap of about 0.75 eV.

Structural anisotropy of nonpolar and semipolar InN epitaxial layers

We present a detailed study of the structural characteristics of molecular beam epitaxy grown nonpolar InN films with a- and m-plane surface orientations on r-plane sapphire and (100) γ-LiAlO2, respectively, and semipolar (101¯1) InN grown on r-plane sapphire. The on-axis rocking curve (RC) widths were found to exhibit anisotropic dependence on the azimuth angle with minima at InN [0001] for the a-plane films, and maxima at InN [0001] for the m-plane and semipolar films. The different contributions to the RC broadening are analyzed and discussed. The finite size of the crystallites and extended defects are suggested to be the dominant factors determining the RC anisotropy in a-plane InN, while surface roughness and curvature could not play a major role. Furthermore, strategy to reduce the anisotropy and magnitude of the tilt and minimize defect densities in a-plane InN films is suggested. In contrast to the nonpolar films, the semipolar InN was found to contain two domains nucleating on zinc-blende InN(111)A and InN(111)B faces. These two wurtzite domains develop with different growth rates, which was suggested to be a consequence of their different polarity. Both, a- and m-plane InN films have basal stacking fault densities similar or even lower compared to nonpolar InN grown on free-standing GaN substrates, indicating good prospects of heteroepitaxy on foreign substrates for the growth of InN-based devices.

Surface, bulk, and interface electronic properties of nonpolar InN

The electronic properties of a-plane and m-plane InN have been investigated by x-ray photoemission spectroscopy, infrared reflectivity, and surface space-charge calculations. Electron accumulation has been observed at the surface of nonpolar InN and the surface Fermi level has been found to be lower than previously observed on InN samples. A high electron density in the InN close to the interface with GaN was found in each nonpolar InN sample.

Influence of buffer layers on the microstructure of MOVPE grown a-plane InN

We report a comparative study of the microstructure of a-plane ( 1 1 2 ¯ 0 ) InN epilayers grown on different buffer layers via metalorganic vapour phase epitaxy. Under optimized growth conditions, the crystalline quality of the InN epilayer is found to be best on an AlN buffer layer compared to that on GaN buffer layers or InN nucleation layers. All a-plane InN epilayers show an anisotropy in the in-plane mosaicity with the in-plane tilt value being different along the c- and m-directions. We also observe a Nagai-like tilt in all the epilayers. The magnitude and nature of this anisotropy and the Nagai-like tilt is also strongly influenced by the buffer layer. Our investigations show that InN grown on an AlN buffer has smaller in-plane tilt, basal plane stacking fault density, and background carrier concentration resulting in a higher mobility and a low band gap energy of ∼ 0.70 eV .

Optimization of a-plane [formula omitted] InN grown via MOVPE on a-plane GaN buffer layers on r-plane [formula omitted] sapphire

We have performed a comprehensive investigation of the growth parameter space for the MOVPE of a-plane ( 1 1 2 ¯ 0 ) InN on a-plane GaN buffer layers deposited on r-plane ( 1 1 ¯ 0 2 ) sapphire substrates. About 0.2 μ m thick a-plane InN epilayers were grown on 1 μ m thick a-plane GaN buffer layers in a close-coupled showerhead reactor. The growth parameters—substrate temperature, reactor pressure, V/III ratio—were systematically varied and their effect on structural, electrical, optical and morphological properties of a-plane InN films were studied. All a-plane InN epilayers show an anisotropy in the in-plane mosaicity. The ( 1 1 2 ¯ 0 ) ω -fwhm varies depending on the scattering vector being parallel to the c-direction or the m-direction. The magnitude and nature of this anisotropy is strongly influenced by the growth parameters. In general compared to c-plane InN, we observed a higher growth rate and a slightly higher optimum growth temperature for the a-plane InN epilayers. The optimum growth conditions are found at T=550 °C, P=500 Torr, V/III = 11,000, where the ω -fwhm for symmetric ( 1 1 2 ¯ 0 ) reflections are 0.83° and 1.04° along [0 0 0 1] and [ 1 1 ¯ 0 0 ] directions, respectively, and for the skew-symmetric ( 1 0 1 ¯ 1 ) plane is 1.47°. The optimized a-plane InN has a photoluminescence peak emission at 1750 nm (0.71 eV) at low temperature (11 K) and a mobility of 234 cm 2/V s, carrier concentration 1.4×10 19 cm −3 at room temperature.

Epitaxial growth, electrical and optical properties of a-plane InN on r-plane sapphire

The heteroepitaxy of a-plane (112¯0) InN films on r-plane (11¯02) sapphire substrates, by nitrogen radio frequency plasma-assisted molecular beam epitaxy, has been investigated and compared to that of c-plane (0001) InN. The epitaxial growth of a-plane InN proceeded through the nucleation, growth, and coalescence of three-dimensional islands, resulting in surface roughness that increased monotonically with epilayer thickness. The full width at half maximum of (112¯0) x-ray diffraction rocking curves decreased significantly with increasing InN thickness, characteristic of structural improvement, and it reached the value of 24 arcmin for a 1 μm thick film. Hall-effect measurements exhibited a similar dependence of electron concentration and mobility on thickness for both the a- and c-plane InN films. The analysis of the Hall-effect measurements, by considering the contribution of two conducting layers, indicates a similar accumulation of low mobility electrons with Ns>1014 cm−2 at the films’ surface/interfacial region for both the a- and c-plane InN films. From optical transmittance measurements, the absorption edge of 0.768 eV was determined for the 1 μm a-plane film, consistent with the expected Burstein–Moss effect. Photoluminescence spectra exhibited a lower energy peak at 0.631 eV, suggesting defect-related transitions.

Surface structure and chemical states of a-plane and c-plane InN films

The surface electron accumulation layer, surface structure, and surface chemical states of a-plane (nonpolar) and c-plane (polar) InN epitaxial films were investigated. Electrical measurements indicated electron accumulation layers on the surface of both the InN films. Angle-resolved x-ray photoelectron spectroscopy (XPS) measurements indicated a strong band bending at both surfaces, thus confirming the surface electron accumulation. Further XPS analysis of the near-surface chemical states indicated an In adlayer at the surface of c-plane InN and an oxygen adsorbed layer on the a-plane InN. These results suggest different ad-layers to cause the surface electron accumulation on c-plane and a-plane InN.

In adlayer mediated molecular beam epitaxial growth and properties of a-plane InN on freestanding GaN

The role of the In adlayer on the morphological and structural properties of nonpolar a-plane InN films was elucidated during the plasma-assisted molecular beam epitaxy on freestanding GaN. Reflection high energy electron diffraction during In adsorption experiments on a-plane InN surfaces revealed a stable In adlayer coverage of ∼2 ML. This In adlayer-mediated growth was responsible for achieving atomically smooth surfaces (rms roughness of <1 nm), phase-pure material with lower x-ray rocking curve widths (Δω<0.5°), lower crystal mosaic tilt/twist, and decreased stacking fault densities, compared to N-rich conditions. The photoluminescence peak emission and band gap energy of the a-plane InN films were ∼0.63 and ∼0.7 eV, respectively.

Absence of Fermi-Level Pinning at Cleaved Nonpolar InN Surfaces

Prior experimental work had found that the Fermi level at InN growth surfaces is pinned well above the conduction band edge, leading to strong surface band bending and electron accumulation. Using cross-sectional scanning photoelectron microscopy and spectroscopy, we show definitive evidence of unpinned Fermi level for in situ cleaved a-plane InN surfaces. To confirm the presence or absence of band bending, the surface Fermi level relative to the valence band edge was precisely measured by using both the Fermi edge of Au reference sample and the core level of ultrathin Au overlayer. It is confirmed that flat surface bands only occur at cleaved nonpolar surfaces, consistent with the recent theoretical predictions.

Ultrafast E1(LO) phonon and plasma dynamics in a-plane wurtzite InN

Coherent phonon spectroscopy of an a-plane wurtzite InN epitaxial thin film is demonstrated with time-resolved second-harmonic generation technique. Coherent E1(LO) phonons are launched via a transient electric field screening process in the near surface region of the a-plane wurtzite InN. Because of the macroscopic electric dipole interaction between coherent E1(LO) phonons and photoexcited plasmons, two hybridized vibration modes, phononlike (L−) and plasmonlike (L+) E1(LO) phonon-plasmon coupling modes, are identified. The investigation of these coupling modes allows us to understand the ultrafast carrier and phonon dynamics in a-plane wurtzite InN.

Characterization of a-plane InN film grown on r-plane sapphire by MOCVD

A-plane InN film has been grown on r-plane sapphire substrate by MOCVD without any buffer. X-ray ω/2 θ scan confirms that only wurtzite-type InN was grown. The in-plane orientation relationship between a-plane InN and r-plane sapphire is 〈0 0 0 1〉 InN//[1¯ 1 0 1] sapphire and [1¯ 1 0 0] InN//[1 1 2¯ 0] sapphire deduced from selected area diffraction. Atomic force microscopy reveals that no stripe features appear on surface of the a-plane InN film, which is different from that of a-plane GaN grown on r-plane sapphire where stripes along [0 0 0 1] direction appear on the surface. The difference originates from the different growth mode of InN and GaN. In addition, photoluminescence and absorption spectra show that our a-plane InN has a narrow band gap of 0.7 eV, which is similar to that of c-plane InN film grown by the same system.

In-adlayers on non-polar and polar InN surfaces: Ion scattering and photoemission studies

The surface structure of In-polarity c-plane InN has been investigated by low energy ion scattering spectroscopy. Comparison of ion scattering spectra recorded along the [1 0 0 0] azimuth with model calculations indicates that the clean In-polarity c-plane InN surface is terminated by In-adlayers with a laterally contracted topmost In layer. This is consistent with previous X-ray photoemission and electron diffraction results. Additionally, the surface properties of a-plane InN have been investigated using core-level and valence band X-ray photoemission spectroscopy (XPS). From the ratio of the In and N core-level XPS signal intensities, the clean a-plane InN surface has also been found to be terminated by In-adlayers. Photoemission measurements of the valence band maximum to surface Fermi level separation for a-plane InN indicate the existence of an electron accumulation layer at the surface. This observation of electron accumulation at non-polar InN surfaces in the presence of In-adlayers is in agreement with the predictions of previous first-principles calculations.