Band Gap Of InN Research Articles

On the crystalline structure, stoichiometry and band gap of InN thin films

Detailed transmission electron microscopy, x-ray diffraction (XRD), and optical characterization of a variety of InN thin films grown by molecular-beam epitaxy under both optimized and nonoptimized conditions is reported. Optical characterization by absorption and photoluminescence confirms that the bandgap of single-crystalline and polycrystalline wurtzite InN is 0.70±0.05eV. Films grown under optimized conditions with an AlN nucleation layer and a GaN buffer layer are stoichiometric, single-crystalline wurtzite structure with dislocation densities not exceeding mid-1010cm−2. Nonoptimal films can be polycrystalline and display an XRD diffraction feature at 2θ≈33°; this feature has been attributed by others to the presence of metallic In clusters. Careful indexing of wide-angle XRD scans and selected area diffraction patterns shows that this peak is in fact due to the presence of polycrystalline InN grains; no evidence of metallic In clusters was found in any of the studied samples.

Theoretical study of the band-gap anomaly of InN

Using a band-structure method that includes the correction to the band-gap error in the local-density approximation (LDA), we find that the band gap for InN is 0.8±0.1eV, in good agreement with recent experimental data, but is much smaller than previous experimental value of ∼1.9eV. The unusually small band gap for InN is explained in terms of the high electronegativity of nitrogen and, consequently, the small band-gap deformation potential of InN. The possible origin of the measured large band gaps is discussed in terms of the nonparabolicity of the bands, the Moss–Burstein shift, and the effect of oxygen. Based on the error analysis of our LDA-corrected calculations we have compiled the band-structure parameters for wurtzite AlN, GaN, and InN.

Comment on “Mie Resonances, Infrared Emission, and the Band Gap of InN”

A Comment on the Letter by T. V. Shubina et al., Phys. Rev. Lett. 92, 117407 (2004). The authors of the Letter offer a Reply.

Anisotropy of the dielectric function for wurtzite InN

A ( 11 2 ̄ 0 ) a -plane InN film grown by molecular beam epitaxy on ( 1 1 ̄ 02 ) r -plane sapphire substrate with an AlN nucleation layer and a GaN buffer was studied by spectroscopic ellipsometry. The data analysis yields both the ordinary and the extraordinary dielectric tensor components perpendicular and parallel to the optical axis, respectively. Strong optical anisotropy is demonstrated over the whole energy range from 0.72 up to 9.5 eV. The line shapes of the tensor components and the polarisation behaviour are in very good agreement with the results of recently published band structure and dielectric function calculations. Above the band gap, five van Hove singularities are evidenced from the ordinary component, while three are resolved from the extraordinary part. The polarisation dependence below 1 eV can be interpreted in terms of optical selection rules for three energetically split valence bands around the Γ -point of the Brillouin zone, similar to the well known behaviour of wurtzite GaN. This emphasises a band gap of hexagonal InN of about 0.7 eV.

Effect of N isotopic mass on the photoluminescence and cathodoluminescence spectra of gallium nitride

GaN is a wurtzite-type semiconductor at ambient conditions whose natural composition consists of almost pure 14N (99.63% 14N and 0.37% 15N) and a mixture of 60.1% 69Ga, and 39.9% 71Ga. We report a low-temperature photoluminescence and cathodoluminescence study of GaN thin films made from natural Ga and N, and from natural Ga and isotopically pure 15N. The contribution of the nitrogen vibrations to the bandgap renormalization by electron-phonon interaction has been estimated from the nitrogen isotopic mass coefficient of the bound exciton energy. The temperature dependence of the bandgap of GaN can be explained with the measured isotopic mass coefficients of Ga and N. We have estimated the aluminum and indium contribution to the bandgap renormalization in AlN and InN from the temperature dependence of the AlN and InN bandgap up to 300 K, assuming that the N contribution is similar to that found in GaN. The similar bandgap isotopic mass coefficients of C, N, and O, of Al, Si and P, of Zn, Ga and Ge, and of Cd and In suggests that elements of the same row of the periodic table have similar bandgap isotopic mass coefficients.

Electronic and vibrational states in InN and InxGa1−x N solid solutions

The review presents the results of optical studies of the fundamental physical characteristics of InN, the material which remains the least studied among nitrides of Group-III elements. The results of early optical studies of InN are analyzed and compared with recent data. New experimental facts reported in the review refer to hexagonal single-crystal epitaxial InN layers with an electron concentration of (1−2)×1018 to 6×1020 cm−3, which are grown by molecular beam epitaxy (MBE) and metal-organic vapor-phase epitaxy (MOVPE) on Al2O3 substrates. The aim of this review is to make a joint analysis of optical spectra (absorption, photoluminescence (PL), PL excitation, and photomodulated reflection) near the fundamental band gap. Furthermore, basic structural and electrical characteristics that have been obtained by a whole range of techniques are given for epitaxial layers of hexagonal InN. The principal result of recent studies is that the hexagonal InN crystal is a narrow-gap semiconductor with a band gap of 0.65–0.7 eV. Previously, the band gap of this material was considered to be 1.89 eV. It is shown that the Burstein-Moss effect accounts for the strong difference between the band gap and the optical absorption threshold in InN samples with a high concentration of electrons. The small value of the band gap of hexagonal InN is confirmed by optical studies of InxGa1−xN solid solutions at high concentrations of In. Theoretical calculations of the band structure of hexagonal InN crystals are briefly reviewed.

Optical properties and electronic structure of InN and In-rich group III-nitride alloys

The optical properties and electronic structure of molecular-beam epitaxy grown InN and In-rich group III-nitride alloy films are studied. The band gap of InN is determined to be 0.7 eV by optical absorption, photoluminescence, and photo-modulated reflectance. The band gap exhibits weaker temperature and pressure dependencies than those of GaN and AlN. The narrow band gap leads to a strong k·p interaction, resulting in a non-parabolic conduction band, which is studied by the free electron concentration dependence of the electron effective mass. Highly n-type InN exhibits a large Burstein-Moss shift in the optical absorption edge; this effect may be responsible for the 1.9 eV band gap reported previously for some degenerately doped InN films. The band gap bowing parameters of the InGaN and InAlN alloy systems are determined. The band offset of InN with other group III-nitrides is presented and its effect on p-type doping is discussed.

Absorption and photoluminescence features caused by defects in InN

Linear combination of atomic orbitals electron band structure calculations are used to examine the influence of common defect structures that may arise as artifacts during the growth of InN. For 1.9 eV band gap InN, the formation of indium rich In x Al 1− x N or In x Ga 1− x N interfacial layers results in lower band-gap material. Exciton emissions at energies as low as 0.765–0.778 eV for In x Al 1− x N, and as low as 0.50–0.82 eV for In x Ga 1− x N are calculated, which are consistent with the recent observations of luminescence in InN. Optical absorption features are shown to also occur at energies that would interfere with band-gap measurements. The role of oxygen alloying was also examined, and the ternary semiconductor InO y N 1− y with y∼0.1 was identified. It was also found that the presence of this concentration of O atoms in InN decreases the band gap energy. Optical absorption as low as 1.19 eV can be evident, while exciton emissions were found to vary in energy over the range 0.84–1.01 eV. This work suggests that oxygen alloys play no role in raising any supposedly smaller band gap of InN to the observed 1.9 eV.

Ultrafast carrier dynamics in InN epilayers

Ultrafast differential transmission measurements on a series of InN epilayers, grown by molecular beam epitaxy, have been employed to determine the carrier lifetimes and to probe the carrier recombination dynamics at room temperature. Differential transmission spectra reveal a peak energy of ∼0.7 eV on these samples, supporting the existence of the narrow band gap of InN. In addition, we observed a fast initial hot carrier cooling followed by a slow recombination process after pulse excitation. Carrier lifetimes ranging from 48 ps to 1.3 ns have been measured in these samples. An inverse proportionality between the carrier lifetime and the free-electron concentration was observed, suggesting that the donor-like defects or impurities may stimulate a formation of non-radiative recombination centers, reducing the carrier lifetime.

A Raman spectroscopy study of InN

We report on the Raman analysis of InN films grown by molecular beam epitaxy (MBE), RF sputtering and remote-plasma enhanced chemical vapor deposition (RPE-CVD). Varying the excitation wavelength, information on the homogeneity of the InN films and the free carrier concentration is extracted. The use of UV Raman spectroscopy to probe excess nitrogen in the InN films via a Raman band located around 2300 cm −1 is illustrated. RF sputtered InN was found to contain significantly more excess nitrogen than MBE and RPE-CVD grown InN. The use of resonant Raman scattering spectroscopy to contribute information to the current controversy of the InN band gap is proposed and first results are presented. Resonant Raman scattering under 1.49 eV excitation suggests the presence of a critical point in the InN band structure within 200–300 meV of this laser excitation energy. Further investigations will be needed to clarify whether this is the band gap or a higher critical point in the band structure.

Energy band gaps of InN containing oxygen and of the InxAl1−xN interface layer formed during InN film growth

The effect of known growth artifacts on the absorption and photoluminescence properties of InN films is determined using linear combination of atomic orbitals electron band structure calculations. InxAl1−xN interfacial layers are examined for various atomic fractions of Al, since these layers are observed to be relatively thick (up to 100 nm) for thin films of InN deposited on AlN or sapphire. It is found that for penetration of Al atoms in InN, forming In-rich InxAl1−xN, a decrease of the energy band gap of InN occurs, despite AlN having a much larger band gap than InN. Γc13↔Γν154 exciton emissions for InxAl1−xN are found to have an energy of 0.765–0.778 eV and may explain recent photoluminescence data for InN. Optical absorption for this alloy is dominated by a 1.58–1.62 eV transition. The second artifact investigated here is high concentration oxygen impurity atoms in wurtzite InN. Segregated oxygen species are not considered, only alloyed species with oxygen substituting on the nitrogen site. For this arrangement a new ternary semiconductor InOyN1−y with y∼0.1 is identified. A model of the tetrahedral cell In–O is made and the energy band gap of InOyN1−y is calculated. It is found that the presence of O atoms in InN can decrease the energy band gap. Optical absorption as low as 1.19 eV can be evident. The exciton emissions Γc12↔Γν151 in InOyN1−y were found to vary in energy over the range 0.84–1.01 eV.

Observation of nonequilibrium longitudinal optical phonons in InN and its implications

Nonequilibrium longitudinal optical phonons in a high quality, single crystal wurtzite structure InN sample have been studied by picosecond Raman spectroscopy. Our experimental results demonstrate that the band gap of InN cannot be around 1.89 eV; but are consistent with a band gap of about 0.8 eV. In addition, they disprove the idea that 0.8 eV luminescence observed recently in InN is due to deep level radiative emission in InN.

Band dispersion relations of zinc-blende and wurtzite InN

The fundamental band-gap energy of the III-V compound semiconductor InN has been corrected due to recent experiments on high-quality InN samples. The advances in the InN growth technique also open new possibilities to realize high-frequency field effect transistors at elevated temperatures due to the small effective mass in the InN conduction-band minimum and high optical phonon frequencies. For device simulations the correct band dispersion relations around the InN band gap are of great interest and motivate the reexamination of previous theoretical works on InN. We present the band structure on InN in the zinc-blende and wurtzite-type configuration calculated within the empirical pseudopotential method approach by exploiting the transferability of ionic model potential parameters. The modified potential parameters of In support the achieved reliability of the presented InN zinc-blende and wurtzite band structure and related properties, e.g., Luttinger and Luttinger-like $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ parameters.

Effects of electron concentration on the optical absorption edge of InN

InN films with free electron concentrations ranging from mid-1017 to mid-1020 cm−3 have been studied using optical absorption, Hall effect, and secondary ion mass spectrometry. The optical absorption edge covers a wide energy range from the intrinsic band gap of InN of about 0.7 to about 1.7 eV which is close to the previously accepted band gap of InN. The electron concentration dependence of the optical absorption edge energy is fully accounted for by the Burstein–Moss shift. Results of secondary ion mass spectrometry measurements indicate that O and H impurities cannot fully account for the free electron concentration in the films.

The value of the direct bandgap of InN: a re‐examination

We report the observation of a broad absorption in the 1.25 eV region that is typical of thin InN films. Such a feature we attribute to light absorption at the energy of the fundamental direct band gap of InN, while we attribute the low energy 650–750 meV photoluminescence to an extrinsic recombination process analogous to the process that give the blue band in AlN and the yellow band in GaN. The substantial broadening is attributed to both the residual doping, and strain. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mie resonances, infrared emission, and the band gap of InN.

Mie resonances due to scattering or absorption of light in InN-containing clusters of metallic In may have been erroneously interpreted as the infrared band gap absorption in tens of papers. Here we show by direct thermally detected optical absorption measurements that the true band gap of InN is markedly wider than the currently accepted 0.7 eV. Microcathodoluminescence studies complemented by the imaging of metallic In have shown that bright infrared emission at 0.7-0.8 eV arises in a close vicinity of In inclusions and is likely associated with surface states at the metal/InN interfaces.

Hydrostatic pressure dependence of the fundamental bandgap of InN and In-rich group III nitride alloys

We report studies of the hydrostatic pressure dependence of the fundamental bandgap of InN, In-rich In1−xGaxN (0<x<0.5) and In1−xAlxN (x=0.25) alloys. The bandgap shift with pressure was measured by optical absorption experiments with samples mounted in diamond anvil cells. The pressure coefficient is found to be 3.0±0.1 meV/kbar for InN. A comparison between our results and previously reported theoretical calculations is presented and discussed. Together with previous experimental results, our data suggest that the pressure coefficients of group III nitride alloys have only a weak dependence on the alloy composition. The photoluminescence signals appear to yield significantly smaller pressure coefficients than the bandgap from absorption measurements. This is due to emission associated with highly localized states. Based on these results, the absolute deformation potentials of the conduction and valence band edges are estimated.

Bandgaps of Ga 1− x In x N by all‐electron GWA calculation

The electronic band-structures of wurtzite-type InN and Ga1−xInxN are calculated by using an all-electron full-potential linearized augmented-plane-wave method in the GW approximation (GWA), which is recognized as the most reliable approach for band-structure calculations of semiconductors. Our calculations show that the bandgap of InN is smaller than 1 eV, and strongly support the recently observed smaller bandgaps. The calculated bandgaps of Ga1−xInxN ternary alloys generally agree with the experimental values. Moreover, we also consider bandgaps of other semiconductors in the GWA and the obtained agreement corroborates the accuracy of the calculations. The present work thus provides a strong theoretical framework to consider bandgaps of semiconductor nitrides and further supports a possibility of the bandgap control in all the range of visible light using nitrides alone. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Erratum: Breakdown of the band-gap-common-cation rule: The origin of the small bandgap of InN [Phys. Rev. B67, 165209 (2003)

Erratum: Breakdown of the band-gap-common-cation rule: The origin of the small bandgap of InN [Phys. Rev. B<b>67</b>, 165209 (2003)

Narrow bandgap group III‐nitride alloys

AbstractHigh‐quality wurtzite In‐rich In1−xGaxN (0 ≤ x ≤ 0.5) and In1−yAlyN films (0 ≤ y ≤ 0.25) were grown on sapphire substrates by molecular‐beam epitaxy. Optical absorption, photoluminescence and photomodulated reflectance measurements demonstrate that the fundamental bandgap for InN is only about 0.7 eV. The free electron effective mass is found to vary with free electron concentration, the consequence of a strongly non‐parabolic conduction band caused by the k · p interaction with the valence bands across the narrow bandgap. The bandgap gradually increases with increasing Ga or Al content in In1−xGaxN or In1−yAlyN alloys. The composition dependencies of the bandgaps are well described by bowing parameters of 1.4 eV for In1−xGaxN and 3.0 eV for In1−yAlyN. The direct gaps of the group III‐nitride alloy system cover a very broad spectral range from the near‐infrared in InN to deep‐ultraviolet in AlN. This offers unique opportunities for the use of these alloys in a wide range of optoelectronic and photovoltaic devices. (© 2003 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)