InN Films Research Articles

Study on Preparation and Properties of InN Films on Self-Supporting Diamond Substrates Under Different Nitrogen Flows

Several InN film samples with superb properties were prepared on a self-supporting diamond substrate for different nitrogen flow rates using an electron cyclotron resonance plasma-enhanced metal-organic chemical vapor deposition (ECR-PEMOCVD) system. After the InN film samples were obtained, the samples were characterized via reflected high-energy electron diffraction (RHEED), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscope (AFM), and electron probe micro-analysis (EPMA) to study the effect of the nitrogen flow on the quality of the InN films. The experimental results show that the variation in the nitrogen flow has a great impact on the preferential growth of the (0002) crystal plane of the InN thin-film. By increasing the nitrogen flow moderately, the crystal quality of the film is improved. Under the growth condition of appropriate nitrogen flow, InN thin-films with a preferred orientation along the c-axis can be obtained, and the surface of the resulting InN thin-films is relatively flat. However, a high nitrogen flow does not improve the film crystal quality. The results of the experiment and of the analysis show that the InN films prepared with a nitrogen flow rate of 80 sccm have an excellent preferential orientation. The result of the EPMA test shows that the percentages of the In and N atoms in the prepared film samples are close to a ratio of 1:1, and a small amount of metal In droplets is present. In addition, the InN thin-films prepared in such condition have an excellent surface morphology and composition.

Kinetically stabilized high-temperature InN growth

Abstract We report on indium nitride growth on sapphire substrates by migration-enhanced plasma-assisted metal organic chemical vapor deposition (MEPA-MOCVD). The growth is studied in the temperature range from 700 °C to 957 °C, well above the decomposition temperature of indium nitride in conventional MOCVD. Raman spectroscopy, atomic force microscopy, and X-ray diffraction indicate polycrystalline grainy InN film growth.

Phase transition of InN films via trimethylindium flows

Wurtzite (w) and zincblende (zb) InN films have been grown on (011) SrTiO3 (STO) substrates by metal- organic chemical vapor deposition, the epitaxial relation- ships and optical properties are characterized by X-ray diffraction (XRD), absorption and photoluminescence (PL). Based on XRD θ –2θ and Φ scanning results, the epi- taxial relationships between (w- and zb-) InN films and STO substrates are determined, that is, (0001)[1120]w-InN (011)[100]STO and (100)[011]zb-InN //(011)[100]STO. Compared with the w-InN films, the zb-InN films exhibit a red shift in absorption edge and PL spectra, and a much nar- rower and stronger PL spectrum, implying a better optical quality of zb-InN films. The structure transition is supposed to be due to the difference in atom and bond areal density between the crystal plane of w-InN(0001) and zb-InN(100).

Thermal evolution and migration behavior of ion-implanted nitrogen in ZnO:In-N films

Thermal evolution and migration behavior of nitrogen (N) dopants in indium (In) doped ZnO films implanted with high-dose N ions (ZnO:In-N) were investigated by means of experiment and first-principles calculations. The results demonstrate that N-dopants have poor thermal stability, which has a significant impact on N local chemical states. In particular, two different temperature regions can clearly be distinguished in the annealing process. At low-temperature region, the interaction of substitutional nitrogen (NO) acceptor and interstitial nitrogen (Ni) starts to occur, which leads to a decrease in NO acceptor and the formation of additional molecular nitrogen at oxygen site [(N2)O]. In contrast, at high-temperature region, annealing favors energetically the generation of abundant oxygen vacancies near the surface and simultaneously induces the serious out-diffusion of N-dopants. Combined with the calculated migration barriers, oxygen vacancies are deemed to assist the out-diffusion of N-dopants via a vacancy mechanism. This work provides insights into the formation and evolution of different N-related defects and their interaction with intrinsic defects.

Influence of plasma-activated nitrogen species on PA-MOCVD of InN

We report on the influence of various plasma species on the growth and structural properties of indium nitride in plasma-assisted metalorganic chemical vapor deposition. Atomic emission spectroscopy was used to quantify the molecular, neutral, and ionized nitrogen species concentrations above the growth surface. Reflectance and Raman spectroscopy and X-ray diffraction techniques were used to characterize the grown InN films. It has been found that ionized rather than molecular or neutral nitrogen species is positively correlated with the InN growth rate. We conclude that InN formation in the present case is due to the chemical combination of atomic nitrogen ions with indium.

Correlation of Threading Dislocations with the Electron Concentration and Mobility in InN Heteroepitaxial Layers Grown by MBE

The quantitative interdependencies of growth conditions, crystal defects and electrical/electronic properties of InN thin films, grown by plasma-assisted molecular beam epitaxy on GaN (0001) buffer layers have been investigated. InN epilayers with thickness near 700 nm, grown under different substrate temperature and/or growth rate, have been analyzed. Bulk electron concentration (Nbulk) and mobility values were extracted for each InN film using the inverted version of the multilayer Petritz model, subtracting the conductivity of a corresponding 120 nm InN film. The results indicate a significant reduction of the threading dislocation density by enhancing the diffusion length of indium adatoms during InN growth, through increase of substrate temperature and reduction of growth rate. The electrical characteristics deteriorate with increasing threading dislocation density. Assuming threading dislocations as exclusive sources of donors in InN, their charge state could be between +1 and +2 per c lattice constant length of dislocation line for Nbulk ≈ 4.0–5.7×1017 cm−3, and approximately +2 or larger for Nbulk ≈ 1.5–1.8×1018 cm−3. The scattering effect of threading dislocations is significantly weaker compared to reported theoretical calculations, i.e. it would correspond to an order of magnitude lower threading dislocation density than the experimentally observed density in the range of 1010 cm−2.

(Invited) A Surface Science Toolbox for Understanding Atomic Layer Epitaxy

Atomic Layer Epitaxy (ALEp) is a promising subset of atomic layer processes (ALPs) which has the potential to open a new realm of non-equilibrium semiconductor growth. In ALEp, the objective is to grow crystalline, epitaxial layers on a crystalline substrate for active regions of electronic and optoelectronic devices. This requires expansion of the atomic layer deposition (ALD) processing space to higher temperatures and adds constraints of crystallinity and purity; electronic grade requires impurity concentrations less than ppm. While ALEp has been shown to maintain the self-limiting nature of ALD at temperatures up to 500°C, the additional materials quality criterion requires a more complete understanding of the ALEp process if it is to be fully successful. In this regard, it is essential to develop a set of surface science tools that can be employed either in situ or in vacuo to ensure that atmospheric exposure does not influence or interfere with observed process mechanisms. In this work, we highlight the development and early application of a suite of in situ or in vacuo characterization techniques aimed at providing surface and near-surface structure assessments: low energy electron diffraction (LEED) and grazing incidence small angle x-ray scattering (GISAXS); as well as surface chemistry assessments: (x-ray photoelectron spectroscopy (XPS), resonant ion trap mass spectrometry (RIT-MS) and reflection-absorption infrared spectroscopy (RAIRS). We present select results from these methods during efforts to develop plasma-assisted ALPs for GaN surface preparation for epitaxy and for early plasma-assisted ALEp growth of heteroepitaxial AlN, InN and AlInN films on GaN and Al2O3 substrates. In the former example, in situ GISAXS studies have been used to optimize an emulated gallium flash off (GFO) ALP of as-received GaN substrate surfaces as part of a preparation of such surfaces for epitaxy. A combination of an ex situ UV/O3 oxidation and concentrated HF etch followed by 10 cycles of an emulated GFO ALP result in the smoothest, cleanest surfaces – highly suitable for epitaxy [1,2]. In the latter application, in situ GISAXS studies are used to characterize the nature of the growth mode of AlN and InN binaries and select AlInN ternaries achieved through digital alloying on optimally prepared GaN substrate surfaces. For InN binary growth, which ultimately grows in a 3D mode, the duration of the plasma pulse is shown to influence the growth mode between a bimodal distribution of islands for short pulses to a single mode distribution for intermediate pulses to etching for the longest pulses [3]. Further, we found that the transition from early 2D to ultimate 3D growth and the shape and size of the islands that result are quite sensitive to growth temperature even inside the “ALEp window”[4]. We discovered that although pure InN and AlN grew in 3D and 2D modes, respectively, the InAlN growth mode did not follow a simple trend as the nominal In composition was tuned from InN to AlN. Instead, select compositions (50% and 83% In) showed more 3D growth while others (19% and 64% In) showed 2D growth. Changes in plasma chemistry are also found to affect growth mode and film quality and will be highlighted. These changes in film properties with plasma pulse variations are correlated to independent measurements of plasma properties in an effort to establish plasma process – film property relationships. A combination of RAIRS and RIT-MS will be presented to further illustrate the role of plasma chemistry, while combined in vacuo LEED and XPS will be presented to highlight early film growth evolution. [1] S.G. Rosenberg, D.J. Pennachio, C. Wagenbach, S.D. Johnson, N. Nepal, A.C. Kozen, J.M. Woodward, Z.R. Robinson, H. Joress, K.F. Ludwig, C.J. Palmstrom and C.R. Eddy, Jr., Journal of Vacuum Science & Technology A 37, 020908 (2019). [2] S.G. Rosenberg, C. Wagenbach, V.R. Anderson, S.D. Johnson, N. Nepal, A.C. Kozen, J.M. Woodward, Z.R. Robinson, M. Munger, H. Joress, K.F. Ludwig and C.R. Eddy, Jr., Journal of Vacuum Science & Technology A 37, 020928 (2019). [3] N. Nepal, V. Anderson, S. D. Johnson, B. P. Downey, D. J. Meyer, Z. Robinson, J. M. Woodward, K. F. Ludwig, C. R. Eddy, Jr., Journal of Vacuum Science and Technology A 37, 020910 (2019). [4] J.M. Woodward, S.G. Rosenberg, A.C. Kozen, N. Nepal, S.D. Johnson, C. Wagenbach, Z.R. Robinson, K.F. Ludwig, Jr. and C.R. Eddy, Jr., Journal of Vacuum Science & Technology A 37, 030901 (2019). Figure 1

Improving the electron mobility of polycrystalline InN grown on glass substrates using AlN crystalline orientation layers

Herein, polycrystalline InN films with flat surfaces and improved electron mobility were grown atop AlN orientation layers on glass substrates by pulsed sputtering deposition. X-ray diffraction and transmission electron microscopy revealed that the InN films comprise dense c-axis-oriented grains. The electron mobilities of these c-axis-oriented InN films were higher than those of the InN directly grown on the glass substrate, reaching as high as 427 cm2 V−1 s−1. To demonstrate a practical application of the developed InN film, a thin-film transistor was fabricated on a 5-nm-thick c-axis-oriented InN film on an AlN orientation layer and operated successfully with a field-effect mobility of 60 cm2 V−1 s−1.

Improvement of polycrystalline InN thin films properties by localized ion source under low RF plasma ambient

Polycrystalline InN/Si(111) were prepared using plasma-assisted reactive evaporation technique under localized ion source (LIS). It was shown that in presence of LIS the structural, compositional, morphological and optical properties of the films can be engineered. XRD and Raman results revealed that shorter LIS-to-substrate distance ( $$d_{LIS - s} )$$ results in higher crystallinity. From XPS and EDX mapping it was concluded that as $$d_{LIS - s}$$ decreases, the incorporation of N into the InN films is enhanced. More uniform, compact and smooth films are obtained as the LIS is placed more adjacent to the substrate. From the optical results it was found that by using LIS, the band gap energy ( $$E_{g}$$ ) of the samples decreases from 1.97 to 1.23–1.37 eV which could be applicable in solar cell devices.

Fabrication of InN on epitaxial graphene using RF-MBE

This paper reports the fabrication of InN layers on the epitaxial graphene (EG) using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). Prior to the fabrication of InN, single crystalline EG with step and terrace structure was formed on 6H-SiC (0001) substrate in an Ar ambient by the Si sublimation method. Single crystalline epitaxial layers of InN with smooth surfaces are successfully fabricated on the EG using RF-MBE. InN layers with terrace and step structure are grown on the graphene surface up to 2MLs, and InN are grown in a layer by layer 2D growth mode. If the number of layers is increased above 3 MLs, the terrace and steps disappear, and the growth mode changes to 3D mode. The Raman spectroscopy analysis shows that the interfacial stress is reduced for the InN layer grown on the EG surface. The quality of the grown InN layer on the EG surface achieved at present is comparable to the InN film grown on sapphire. This work opens the possibility of growing high-quality InN layers on the EG surface in the near future.

Observation and mitigation of RF-plasma-induced damage to III-nitrides grown by molecular beam epitaxy

In this work, radio-frequency (RF) plasma-induced damage to III-nitride surfaces and bulk defects is observed and mitigated. It is shown that for InN films, the surface is more sensitive to plasma-induced damage than GaN films, as observed via atomic force microscopy and reflection high energy electron diffraction. In order to isolate any possible plasma-induced damage, a growth window for InN is established, and temperature ranges are determined for other damaging effects which include roughening due to low adatom mobility, InN decomposition, and indium desorption. In situ plasma monitoring and optimization are accomplished with a combination of optical emission spectroscopy as well as a remote Langmuir probe. It is shown that by increasing the plasma nitrogen flow, the positive ion content increases; however, the ion acceleration potential reduces. Additionally, a reduced RF plasma power results in a reduction of atomic nitrogen species. These plasma species and energetic variations result in variations in the bulk unintentional background electron concentrations observed by room temperature Hall effect measurements of ∼1 μm thick InN films. By increasing the nitrogen flow from 2.5 to 7.5 sccm for a constant RF power of 350 W, the background electron concentration decreases by 74% from 1.36 × 1019 cm−3 to 3.54 × 1018 cm−3, while maintaining a smooth surface morphology. Additionally, photoluminescence spectra indicate optical emission energies shift from ∼0.81 to 0.71 eV (closer to the fundamental bandgap of InN) by limiting the damaging plasma species. Finally, conditions are presented to further minimize plasma-induced damage in III-nitride devices.

Growth of narrow substrate temperature window on the crystalline quality of InN epilayers on AlN/Si(1 1 1) substrates using RF-MOMBE

Epitaxial InN films were grown on AlN/TMAl seed layer/Si(1 1 1) substrates in the range of temperature between 500 and 540 °C using pulsed-mode RF-MOMBE. The structural properties of the InN films were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy, and transmission electron microscopy (TEM). The electrical properties were characterized by Hall effect measurements. The results indicated that small changes in temperature significantly affect the quality of InN films. The XRD and TEM results indicated that the InN films grew oriented along the c-axis. Phi-scan XRD patterns indicated that the InN films grew epitaxially on the Si(1 1 1) substrates with the orientation relationship InN [1 1 −2 0]InN//[11 0]Si. When the substrate temperature at 530 °C, the TEM images indicated three-dimensional InN/AlN growth and growth rate was approximately 0.2 μm/h. The cross-sectional TEM images and SAD pattern showed a sharp InN/AlN interface and clear epitaxial orientation relationships of InN (−2 1 1 0)InN//(−2 1 1 0)AlN//(2 –2 0)Si, [0 0 0 1]InN//[0 0 0 1]AlN//[1 1 1]Si. The electron concentrations of the InN films were measured from 5.5 × 1019 to 1.3 × 1020 cm−3, and the mobility was measured from 116 to 360 cm2/V·s. The results indicate that controlling the substrate temperature is essential for engineering the growth of InN films on silicon wafers.

Characterization of the electrical properties of an InN epilayer using terahertz time-domain spectroscopic ellipsometry

We demonstrate the measurement of electrical properties of InN layers grown by RF-MBE using terahertz time-domain spectroscopic ellipsometry (THz-TDSE). Dependences of carrier density and mobility on InN film thickness are characterized using THz-TDSE. It is found that these electrical properties of the InN films are improved with the increase in the film thickness. Although the results obtained by Hall effect measurement also show the same tendency, the electrical properties of InN obtained by THz-TDSE is superior to those by Hall effect measurement. In addition, we also apply a two-layer model for THz-TDSE measurement to analyze the electrical properties of the bulk InN region and the surface electron accumulation layer independently. The mobility ranging from 50 to 140 cm2 V−1 s−1 are obtained for the surface electron accumulation layer of InN.

Bandgap, electrical and structural properties of thick InN (0001) films grown under optimal conditions

The improvement potential for the structural, electrical and opto-electronic properties of heteroepitaxial InN-on-GaN (0001) films by using optimal conditions (substrate temperature, In and N fluxes) of plasma-assisted molecular beam epitaxy and increasing the epilayer thickness to few micrometres has been investigated. The increase of InN thickness to 3.7 μm resulted to a-type component threading dislocation density of 6x109 cm−2 and directly measured electron mobility of 2330 cm2/Vs and concentration of 4.5x1017 cm−3. The optical bandgap of this film at 300K was 0.637 eV. However, a degradation in the integrity of the interfacial InN/GaN region was observed in films thicker than 1 μm, with the formation of voids and the nucleation of microcracks, which may be related to strain relaxation or thermal decomposition.

Improvement of luminescence properties of InN by optimization of multi-step deposition on sapphire

Thin films of InN were grown on sapphire by metalorganic chemical vapor deposition using multi-step growth technique (thermal cleaning and nitridation of sapphire substrate, deposition and annealing of nucleation layer, and growth of the active InN layer). The effects of different growth steps on the structural quality and luminescence properties of the samples grown for the study were analyzed by X-ray structural analysis (ω-2Θ scans, measurement of rocking curves) and photoluminescence spectroscopy with sub-micrometer spatial resolution enabled by applying confocal microscopy. The influence of nitridation duration, variation of thickness and annealing conditions of nucleation layer, and the temperature of deposition of the active InN layer were studied and discussed.

Investigation on the formation mechanism of p-type ZnO:In-N thin films: experiment and theory

Effects of post-annealing on electrical properties of N+ ion-implanted into indium doped ZnO (ZnO:In-N) films are investigated. p-type conduction of ZnO:In-N films with the appropriate annealing have been confirmed by Hall-effect measurements and electrical rectification behavior of homojunctions. The transmitted spectra and temperature-dependent photoluminescence (PL) spectroscopy results show that the band gap of p-type ZnO:In-N is narrow as compared to ZnO:In and the ionization acceptor energy is estimated to be 128 meV, which agrees well with our calculated transition levels $$\varepsilon \left( {0/ - 1} \right)$$ of InZn-2NO. X-ray photoelectron spectroscopy (XPS) spectra confirm the dominant existence of N-related acceptor defect complexes in p-type samples. Combining with transition state calculations, we find that the incorporation of In can facilitate p-type effective doping by pinning a passive impurity band above the valence band maximum (VBM) and decreasing thermal activation energy of N interstitial (Ni) in ZnO.

Features of the Formation of Ohmic Contacts to n+-InN

We report about a study of the formation and current transport mechanism of ohmic contacts to n+-InN with electron concentrations of 2×1018, 8×1018, and 4×1019 cm−3. Pd/Ti/Au ohmic contacts are formed by the proposed approach of simultaneous magnetron metal deposition and in-situ temperature annealing, which allows obtaining a low contact resistivity (4.20±2.67)×10−6 Ohm· cm2. The additional rapid thermal annealing in the temperature interval 350–400 ∘C is used to improve further contact characteristics. Optimal parameters of the temperature treatment are determined by statistic methods. As for the current transport mechanism, the unusual growing temperature behavior of contact resistivity is observed in the wide temperature range 4.2–380K for each doping level of InN films. The mechanism of thermionic current flow explains the current transport through metal shunts, which is associated with the conducting dislocations. The extracted density of conducting metal shunts has a good agreement with experimental values of the screw and edge dislocation densities experimentally obtained by high-resolution X-ray diffraction. Additionally, from the obtained contact resistivity temperature dependences, we can argue about the metal, which penetrates dislocations and forms shunts.

Understanding the effect of nitrogen plasma exposure on plasma assisted atomic layer epitaxy of InN monitored by real time grazing incidence small angle x-ray scattering

The authors present an in situ study of the effect of nitrogen plasma pulse time on the temporal evolution of the surface morphology of InN growth on a-plane sapphire at 250 °C by plasma assisted atomic layer epitaxy (ALEp). The growth surface evolution was monitored in real-time using grazing incidence small angle x-ray scattering (GISAXS) measurements at an x-ray incidence angle of 0.8°. Nitrogen plasma pulse time (tp) was varied between 15 and 30 s in 5-s steps, and for all tp, the near specular scattering broadens and correlated peaks develop and evolve along the Yoneda Wing (YW). For tp ≥ 20 s, a YW with one correlated length scale evolves. At the end of the growth, the longest correlated length scale is 16.54 nm for tp = 25 s. Porod analysis of GISAXS intensity at high qy for tp = 25 s shows the formation of mounded shapes at the early stage of nucleation that transitioned to cylinders after about 3 unit cells of InN growth. Additionally, at tp = 25 s, the growth rate is highest with root mean square surface roughness and carbon impurity levels at or below atomic force microscopy and x-ray photoelectron spectroscopy sensitivity limits, respectively. At tp < 25 s, the growth surface may be undersaturated and at tp > 30 s, it appears that trimethylindium precursor molecules start to decompose, resulting in higher carbon content in the film. Thus, the nature of GISAXS correlated length scale directly correlates with the material quality. Additional ex situ characterizations reveal an electron mobility of 6–31 cm2/V s for a 3–5 nm thick InN film on a-plane sapphire, which is similar to the reported value of 30 cm2/V s for a 1300 nm thick InN film grown by molecular beam epitaxy directly on sapphire. Thus, the combination of in situ synchrotron x-ray analysis and ex situ characterization is a powerful approach to develop understanding of the growth mechanisms of ALEp of III-N materials in order to improve the quality by reducing impurities and broaden material applications.

Growth of nonpolar InN nanocrystal films by RF plasma-assisted evaporation technique

Nonpolar InN nanocrystal films were grown by RF plasma-assisted evaporation technique on bare Si and quartz substrates, simultaneously. The as-grown InN films were characterized by X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. The synergistic cooperation among the evaporation temperature, chamber pressure and substrate temperature is critical for the formation of nonpolar InN films. The absence of the (0 0 2) peak in the X-ray diffraction patterns indicates that as-grown InN films grew preferentially along the nonpolar plane. In addition, the InN films grown on Si and quartz substrates exhibit different crystallinity, morphology and stress. The In-rich growth condition can be responsible for the formation of nonpolar InN films.

Structural, electrical, and optical properties of annealed InN films grown on sapphire and silicon substrates

N-face {0001} InN films, grown by plasma-assisted molecular beam epitaxy on sapphire (0001) and Si (111) substrates were annealed in a nitrogen atmosphere, with the aim to determine the limits of their thermal stability and look into the degradation of their optoelectronic properties. Indium clusters, indicating InN decomposition, were observed after annealing at 500 °C on the film grown on sapphire, while they were present already after annealing at 450 °C on the film grown on Si. The disintegration of InN films occurred after annealing at 500 °C by through-holes formation, and it was enhanced by cracking and delamination on the Si substrate. A significant deterioration of electron Hall mobility was observed after annealing at 500 °C and it correlated with the extent of disintegration of each film. Both as-grown and annealed films exhibited broad PL spectra, typical for heavily doped semiconductors, with films on the Si substrate exhibiting non-uniformities of PL spectra throughout the surface. The evolution of the PL spectra correlated with the signatures of annealing-caused crystal deterioration observed by other characterisation techniques, and led to similar PL spectra after annealing at 550 °C. The results suggest that InN decomposition is avoided in the films grown on the sapphire substrate if they are annealed at temperatures up to 450 °C. InN grown on the silicon substrate exhibits a slightly faster destruction. The findings indicate correlation between the thermal stability and the defect content of the layers and heteroepitaxial interfaces.