Intrinsic InN Research Articles

Adsorption characteristics of Ni cluster Modified-InN monolayer for indoor noxious gases (CH2O, C6H6, and NH3)

With the continuous enhancement of people's health awareness and the pursuit of high-quality life, the noxious gases generated after the renovation of new houses are receiving more attention. The gas sensing properties of InN monolayer modified with Ni clusters (1–3 Ni atoms) on CH2O, C6H6, and NH3 were analyzed based on DFT calculations, including the adsorption energy, charge transfer, density of states, bandgap structure, molecular orbitals, electrostatic potential, and desorption time investigation of gas adsorption systems. Moreover, different numbers of Ni atoms were introduced and their adsorption energies were analyzed. The results showed that the intrinsic InN and Ni–InN were suitable materials for gas sensors, while Ni2–InN and Ni3–InN were more suitable as adsorbents for clearing these three gases. The change in conductivity upon gas adsorption provides a theoretical basis for identifying the concentration of mixed gases of CH2O, C6H6, and NH3.

Low-Loss and Tunable Localized Mid-Infrared Plasmons in Nanocrystals of Highly Degenerate InN

Plasmonic response of free charges confined in nanostructures of plasmonic materials is a powerful means for manipulating the light-material interaction at the nanoscale and hence has influence on various relevant technologies. In particular, plasmonic materials responsive in the mid-infrared range are technologically important as the mid-infrared is home to the vibrational resonance of molecules and also thermal radiation of hot objects. However, the development of the field is practically challenged with the lack of low-loss materials supporting high quality plasmons in this range of the spectrum. Here, we demonstrate that degenerately doped InN nanocrystals (NCs) support tunable and low-loss plasmon resonance spanning the entire midwave infrared range. Modulating free-carrier concentration is achieved by engineering nitrogen-vacancy defects (InN1- x, 0.017 < x < 0.085) in highly degenerate NCs using a nonequilibrium gas-phase growth process. Despite the significant reduction in the carrier mobility relative to intrinsic InN, the mobility in degenerate InN NCs (>60 cm2/(V s)) remains considerably higher than the carrier mobility reported for other materials NCs such as doped metal oxides, chalcogenides, and noble metals. These findings demonstrate feasibility of controlled tuning of infrared plasmon resonances in a low-loss material of III-V compounds and open a gateway to further studies of these materials nanostructures for infrared plasmonic applications.

Hexagonal Nanopyramidal Prisms of Nearly Intrinsic InN on Patterned GaN Nanowire Arrays

By using multiple growth steps that separate the nucleation and growth processes, we show that nearly intrinsic InN single nanocrystals of high optical quality can be formed on patterned GaN nanowire arrays by molecular beam epitaxy. The InN nanostructures form into well-defined hexagonal prisms with pyramidal tops. Micro-photoluminescence (μ-PL) is carried out at low temperature (LT: 28.2 K) and room temperature (RT: 285 K) to gauge the relative material quality of the InN nanostructures. Nanopyramidal prisms grown using a three-step growth method are found to show superior quantum efficiency. Excitation and temperature dependent μ-PL demonstrates the very high quality and nearly intrinsic nature of the ordered InN nanostructure arrays.

Convergence of valence bands for high thermoelectric performance for p-type InN

Band engineering to converge the bands to achieve high valley degeneracy is one of effective approaches for designing ideal thermoelectric materials. Convergence of many valleys in the valence band may lead to a high Seebeck coefficient, and induce promising thermoelectric performance of p-type InN. In the current work, we have systematically investigated the electronic structure and thermoelectric performance of wurtzite InN by using the density functional theory combined with semiclassical Boltzmann transport theory. Form the results, it can be found that intrinsic InN has a large Seebeck coefficient (254μV/K) and the largest value of ZeT is 0.77. The transport properties of p-type InN are better than that of n-type one at the optimum carrier concentration, which mainly due to the large Seebeck coefficient for p-type InN, although the electrical conductivity of n-type InN is larger than that of p-type one. We found that the larger Seebeck coefficient for p-type InN may originate from the large valley degeneracy in the valence band. Moreover, the low minimum lattice thermal conductivity for InN is one key factor to become a good thermoelectric material. Therefore, p-type InN could be a potential material for further applications in the thermoelectric area.

Is the Fermi‐level pinned on InN grown surfaces?

AbstractIn this paper, we discuss the Fermi‐level pinning issue on InN grown surfaces, in particular the nonpolar m ‐planes. Detailed angle‐resolved X‐ray photoelectron spectroscopy studies show that for intrinsic InN nanowires the Fermi‐level on the nonpolar grown surfaces is located 0.1 to 0.2 eV below the conduction band edge, i.e., not pinned in the conduction band. This indicates that the commonly measured Fermi‐level pinning in the conduction band on nonpolar grown surfaces is not an intrinsic property of InN. Furthermore, our studies also suggest that surface states are positioned near the band edge and therefore do not contribute to the Fermi‐level pinning. This is consistent with recent theoretical and experimental studies. In the end, with the absence of the Fermi‐level pinning in the conduction band and surface electron accumulation, the unique electrical and optical properties of intrinsic InN nanowires are discussed, and compared with previously reported InN epilayers and n ‐type InN nanowires. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Occupation statistics of the 5/7-atom dislocation core structure within n-type indium nitride

Recent density functional calculations by Kalesaki et al. [Appl. Phys. Lett. 98, 072103 (2011)] and by Takei and Nakayama [J. Cryst. Growth 311, 2767 (2009)] have shown that the 5/7-atom dislocation core structure possesses a donor character within intrinsic wurtzite InN. In the present work, we relax the assumption that wurtzite InN is intrinsic and obtain the occupation statistics of the different possible ionization states of the 5/7-atom dislocation defect sites as a function of the bulk doping concentration. An underlying Gibbs factor formalism, similar to that applied earlier to the problem of obtaining the occupation statistics of the dislocation defect sites within n-type gallium nitride, has been employed for the purposes of the present analysis. The occupation statistics results of this analysis suggest that dislocation line densities below 1010 cm−2 are necessary in order to achieve bulk free electron concentrations lower than 1017 cm−3 within wurtzite InN. Our occupation statistics results are shown to be consistent with existing electron mobility measurements from the literature.

Probing the electrical transport properties of intrinsic InN nanowires

We have studied the electrical transport properties of intrinsic InN nanowires using an electrical nanoprobing technique in a scanning electron microscope environment. It is found that such intrinsic InN nanowires exhibit an ohmic conduction at low bias and a space charge limited conduction at high bias. It is further derived that such InN nanowires can exhibit a free carrier concentration as low as ∼1013 cm−3 and possess a very large electron mobility in the range of 8000–12 000 cm2/V s, approaching the theoretically predicted maximum electron mobility at room temperature. In addition, charge traps are found to distribute exponentially just below the conduction band edge, with a characteristic energy ∼65 meV.

Observation of phonon sideband emission in intrinsic InN nanowires: a photoluminescence and micro-Raman scattering study

In this work, photoluminescence and micro-Raman scattering experiments were performed on undoped InN nanowires. It was found that, besides the main photoluminescence peak, a clear phonon sideband emission peak, with an extremely narrow linewidth ∼9 meV, was measured. The phonon spectrum revealed by micro-Raman scattering indicates only uncoupled LO phonons are involved in such phonon sideband emission. The clearly resolved phonon sideband emission peak with a narrow linewidth, together with the uncoupled LO phonon modes, suggests the superior quality of the presented InN nanowires, i.e., extremely low residual electron density and the absence of surface electron accumulation, which is consistent with the physical properties of intrinsic InN nanowires as in the previous studies. The detailed phonon sideband properties are also discussed in the text.

High-pressure optical absorption in InN: Electron density dependence in the wurtzite phase and reevaluation of the indirect band gap of rocksalt InN

We report on high-pressure optical absorption measurements on InN epilayers with a range of free-electron concentrations ($5\ifmmode\times\else\texttimes\fi{}{10}^{17}$--$1.6\ifmmode\times\else\texttimes\fi{}{10}^{19}$ cm${}^{\ensuremath{-}3}$) to investigate the effect of free carriers on the pressure coefficient of the optical band gap of wurtzite InN. With increasing carrier concentration, we observe a decrease of the absolute value of the optical band gap pressure coefficient of wurtzite InN. An analysis of our data based on the k\ifmmode\cdot\else\textperiodcentered\fi{}p model allows us to obtain a pressure coefficient of 32 meV/GPa for the fundamental band gap of intrinsic wurtzite InN. Optical absorption measurements on a 5.7-\ensuremath{\mu}m-thick InN epilayer at pressures above the wurtzite-to-rocksalt transition have allowed us to obtain an accurate determination of the indirect band gap energy of rocksalt InN as a function of pressure. Around the phase transition (\ensuremath{\sim}15 GPa), a band gap value of 0.7 eV and a pressure coefficient of \ensuremath{\sim}23 meV/GPa are obtained.

Understanding the role of Si doping on surface charge and optical properties: Photoluminescence study of intrinsic and Si-doped InN nanowires

In the present work, the photoluminescence (PL) characteristics of intrinsic and Si-doped InN nanowires are studied in detail. For intrinsic InN nanowires, the emission is due to band-to-band carrier recombination with the peak energy at $\ensuremath{\sim}$0.64 eV (at 300 K) and may involve free-exciton emission at low temperatures. The PL spectra exhibit a strong dependence on optical excitation power and temperature, which can be well characterized by the presence of very low residual electron density and the absence or a negligible level of surface electron accumulation. In comparison, the emission of Si-doped InN nanowires is characterized by the presence of two distinct peaks located at $\ensuremath{\sim}$0.65 and $\ensuremath{\sim}$0.73--0.75 eV (at 300 K). Detailed studies further suggest that these low-energy and high-energy peaks can be ascribed to band-to-band carrier recombination in the relatively low-doped nanowire bulk region and Mahan exciton emission in the high-doped nanowire near-surface region, respectively; this is a natural consequence of dopant surface segregation. The resulting surface electron accumulation and Fermi-level pinning, due to the enhanced surface doping, are confirmed by angle-resolved x-ray photoelectron spectroscopy measurements on Si-doped InN nanowires, which is in direct contrast to the absence or a negligible level of surface electron accumulation in intrinsic InN nanowires. This work elucidates the role of charge-carrier concentration and distribution on the optical properties of InN nanowires.

Tuning the Surface Charge Properties of Epitaxial InN Nanowires

We have investigated the correlated surface electronic and optical properties of [0001]-oriented epitaxial InN nanowires grown directly on silicon. By dramatically improving the epitaxial growth process, we have achieved, for the first time, intrinsic InN both within the bulk and at nonpolar InN surfaces. The near-surface Fermi-level was measured to be ∼0.55 eV above the valence band maximum for undoped InN nanowires, suggesting the absence of surface electron accumulation and Fermi-level pinning. This result is in direct contrast to the problematic degenerate two-dimensional electron gas universally observed on grown surfaces of n-type degenerate InN. We have further demonstrated that the surface charge properties of InN nanowires, including the formation of two-dimensional electron gas and the optical emission characteristics can be precisely tuned through controlled n-type doping. At relatively high doping levels in this study, the near-surface Fermi-level was found to be pinned at ∼0.95-1.3 eV above the valence band maximum. Through these trends, well captured by the effective mass and ab initio materials modeling, we have unambiguously identified the definitive role of surface doping in tuning the surface charge properties of InN.

Molecular beam epitaxial growth and characterization of catalyst-free InN/InxGa1−xN core/shell nanowire heterostructures on Si(111) substrates

We report on the achievement of, for the first time, InN/InGaN core/shell nanowire heterostructures, which are grown directly on Si(111) substrates by plasma-assisted molecular beam epitaxy. The crystalline quality of the heterostructures is confirmed by transmission electron microscopy, and the elemental mapping through energy dispersive x-ray spectrometry further reveals the presence of an InGaN shell covering the sidewall and top regions of the InN core. The optical characterizations reveal two emission peaks centered at ∼1685 nm and 1845 nm at 5 K, which are related to the emission from the InGaN shell and InN core, respectively. The InN/InGaN core/shell nanoscale heterostructures exhibit a very high internal quantum efficiency of ∼62% at room temperature, which is attributed to the strong carrier confinement provided by the InGaN shell as well as the nearly intrinsic InN core.

InN p-i-n Nanowire Solar Cells on Si

In this paper, we report the first experimental demonstration of InN nanowire solar cells. By employing an in situ deposited In seeding layer, we have achieved electronically pure, nearly intrinsic InN nanowires directly on Si(1 1 1) substrates by molecular beam epitaxy. The growth and characterization of Si- and Mg-doped InN nanowires is also investigated, which can exhibit superior structural and optical properties. We have further studied the epitaxial growth, fabrication, and characterization of InN:Si/i-InN and InN:Mg/i-InN/InN:Si axial nanowire structures on p-type and n-type Si(1 1 1) substrates, respectively. With the use of a CdS surface passivation, InN:Mg/i-InN/InN:Si nanowire homojunction solar cells exhibit a promising short-circuit current density of ~14.4 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and power-conversion efficiency of ~0.68% under simulated one-sun (AM 1.5G) illumination. This work suggests the first successful demonstration of p-type doping in InN nanowires and also constitutes important progress for the development of InGaN-based, full-solar-spectrum photovoltaics.

Photoluminescence Properties of a Nearly Intrinsic Single InN Nanowire

AbstractIn this paper, the achievement of nearly intrinsic InN nanowire is reported. With the use of an in situ deposited In seeding layer, nearly defect‐free, non‐tapered InN nanowires are grown directly on Si(111) substrates by molecular beam epitaxy. The photoluminescence emission of a single InN nanowire is analyzed, which exhibits, for the first time, a very narrow (∼13 meV) spectral linewidth, a clear band filling effect with the increase of excitation power, and a significant red shift of the peak energy with increasing temperature. Detailed analysis confirms the InN nanowire has a very low residual doping of ∼1 × 1016 cm−3, or less. It is further suggested that there is a small, or negligible level of electron accumulation at the lateral nonpolar surfaces of nearly intrinsic InN nanowires, which is in direct contrast to the commonly observed surface electron accumulation of n‐type degenerate InN.

Photoluminescence Properties of a Nearly Intrinsic Single InN Nanowire

AbstractIn this paper, the achievement of nearly intrinsic InN nanowire is reported. With the use of an in situ deposited In seeding layer, nearly defect‐free, non‐tapered InN nanowires are grown directly on Si(111) substrates by molecular beam epitaxy. The photoluminescence emission of a single InN nanowire is analyzed, which exhibits, for the first time, a very narrow (∼13 meV) spectral linewidth, a clear band filling effect with the increase of excitation power, and a significant red shift of the peak energy with increasing temperature. Detailed analysis confirms the InN nanowire has a very low residual doping of ∼1 × 1016 cm−3, or less. It is further suggested that there is a small, or negligible level of electron accumulation at the lateral nonpolar surfaces of nearly intrinsic InN nanowires, which is in direct contrast to the commonly observed surface electron accumulation of n‐type degenerate InN.

Carrier-induced refractive index change and optical absorption in wurtzite InN and GaN: Full-band approach

Based on the full band electronic structure calculations, first we consider the effect of n-type doping on the optical absorption and the refractive index in wurtzite InN and GaN. We identify quite different dielectric response in either case; while InN shows a significant shift in the absorption edge due to n-type doping, this is masked for GaN due to efficient cancellation of the Burstein-Moss effect by the band gap renormalization. For high doping levels the intraband absorption becomes significant in InN. Furthermore, we observe that the free-carrier plasma contribution to refractive index change becomes more important than both band filling and the band gap renormalization for electron densities above 10$^{19}$~cm$^{-3}$ in GaN, and 10$^{20}$~cm$^{-3}$ in InN. As a result of the two different characteristics mentioned above, the overall change in the refractive index due to n-type doping is much higher in InN compared to GaN, which in the former exceeds 4\% for a doping of 10$^{19}$~cm$^{-3}$ at 1.55~$\mu$m wavelength. Finally, we consider intrinsic InN under strong photoexcitation which introduces equal density of electron and holes thermalized to their respective band edges. The change in the refractive index at 1.55~$\mu$m is observed to be similar to the n-doped case up to a carrier density of 10$^{20}$~cm$^{-3}$. However, in the photoexcited case this is now accompanied by a strong absorption in this wavelength region due to $\Gamma^v_5 \to \Gamma^v_6$ intravalence band transition. Our findings suggest that the alloy composition of In$_x$Ga$_{1-x}$N can be optimized in the indium-rich region so as to benefit from high carrier-induced refractive index change while operating in the transparency region to minimize the losses. These can have direct implications for InN-containing optical phase modulators and lasers.

1.5‐µm emission of naturally‐oxidized InN crystals grown by MOVPE

AbstractPhotoluminescence (PL) spectrum of a naturally‐oxidized InN sample has been investigated. The PL‐peak wavelength locates at around 1.53 µm, which is by 250‐300 nm shorter than that of an intrinsic InN. In addition to this PL peak shift of the InN crystals (corresponding bandgap variation) their quality seems to be improved from the analysis of PL spectra. The naturally‐oxidized layer is found to have the original lattice structure of InN from X‐ray diffraction measurements. The 8‐20% of oxygen atoms has been evaluated to be incorporated in the InN layer by the energy dispersive X‐ray spectroscopy. The PL spectra have been affected by temperature. While a PL‐peak intensity increases with temperature decrease, a PL‐peak shifts toward the longer wavelength. Such red‐shift tendency is opposite to the case of other III‐V compounds. This anomalous temperature effect has been proved from variation of PL spectra with time after excitation. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Phonon polariton of InN observed by infrared synchrotron radiation

Temperature dependence of the reflectivity of InN with a carrier concentration of (3.5–4.7)×1017cm3 is investigated from 50to750cm−1 using infrared synchrotron radiation. E1 phonon is separately observed from plasma oscillation, and in the energy range below E1(TO), phonon polariton is observed up to 104cm−1. The lifetime of the E1(TO) phonon is directly determined by the reflectivity measurements. From the temperature dependence of the lifetime, the E1(TO) phonon primarily decays into phonons with a renormalized frequency of 177cm−1. From the plasma edge position the electron effective mass is estimated to be me⊥=0.076m0 for the intrinsic InN.

Photoluminescent properties of InN epifilms

We report a detailed investigation of the photoluminescent properties of InN epifilms with free-electron concentrations ranging from 3.5 × 1017 cm−3 to 5 × 1019 cm−3. It is found that the photoluminescence (PL) peak energy strongly depends on the electron concentration. We show that the broadening of the PL spectra with increasing free-electron concentration arises from the breaking of the k = 0 selection rule. The large asymmetric line shape of the photoluminescence spectra can be well described by the free-electron recombination band model. We establish an empirical relation between the full-width at half-maximum (FWHM) value of the PL spectra and the free-electron concentration, which provides a convenient formula to determine the free-electron concentration in InN epifilms by PL measurement. We point out that the peak energy of the PL spectra does not reflect the real band gap of InN epifilms. Calculations based on the effects of Burstein–Moss absorption, band tail and band renormalization were used to analyse the PL spectra, and the fundamental band gap of the intrinsic InN film was obtained. The corresponding expression for the band gap narrowing effect of the InN film is found to be ΔEBGN = 1 × 10−8n1/3 + 3.6 × 10−7n1/4 + 2.3 × 10−11n1/2 eV. The temperature-dependent band gap of the intrinsic InN was fitted by the Pässler equation. The Pässler parameters of the intrinsic InN are α = 0.55 meV K−1, Θ = 576 K and p = 2.2. It is found that the band gap energies at T = 0 K and room temperature are close to 0.68 eV and 0.62 eV, respectively. In addition, we show that the band gap obtained from the PL spectra is in excellent agreement with that obtained from infrared absorption.

Temperature dependence of Raman scattering in hexagonal indium nitride films

We report on Raman spectroscopy study of hexagonal InN thin films grown by metal-organic vapor-phase epitaxy, with the emphasis on frequencies and linewidths of E2(high) and A1(LO) modes in the temperature range from 93to443K. The present InN exhibits a fundamental band gap of ∼1.2eV from photoluminescence and optical transmission spectra, which is in good agreement with the recent suggested parameter for intrinsic InN. The temperature dependence of the E2(high) and A1(LO) phonons can be described well by a model which has taken into account the contributions of the thermal expansion of the crystal lattice, the strain between InN thin films and sapphire substrates, as well as three- and four-phonon coupling. Micro-Raman mapping results also demonstrate the high uniformity of the studied InN.