Ultrafast carrier dynamics in InN epilayers

doi:10.1016/j.jcrysgro.2004.05.028

Journal of Crystal Growth

Volume 269, Issue 1, 15 August 2004, Pages 10-14

https://doi.org/10.1016/j.jcrysgro.2004.05.028 Get rights and content

Abstract

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.

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InGaN is one of the important ternary alloys which enables the nitrides to have bandgap energy in a wide range of 0.77–3.44 eV corresponding to the photon wavelength of 360–1610 nm by simply adjusting In content. However, the research on the InGaN has primarily focused on low-dimensional structures though knowledge of the physical properties of bulk InGaN thin-film is important in designing optoelectronic applications that utilize thick InGaN layers. In this study, we revisited partially strain relaxed bulk InGaN thin-films of different In compositions. We found that fast carrier decay was caused by hot carrier cooling due to the presence of oxygen impurity, and the slow carrier decay was governed by the carrier localization initiated by V-shape defects which concentrated at the bottom of the InGaN layer starting at InGaN/GaN interface. We also observed that the V-shape defects became severe, and the slow decay carrier lifetime decreased as In composition of the InGaN layer increased. Our observation gives guidance in designing optoelectronic applications using partially relaxed thick InGaN layers, such as buffer layers for long-wavelength InGaN light emitting diodes as well as solar absorber layer for InGaN photovoltaic cells.
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We have demonstrated that the thickness of the catalyst layer plays a significant role in the morphology and the material quality of the InN nanocolumns grown on Si(111) substrate by plasma-assisted molecular beam expitaxy using vapor-liquid-solid method. A systematic investigation of In catalyst films was undertaken, revealing that high density uniform InN can not be obtained when the In catalyst is either too thin or too thick and the appropriate thickness of the deposited In was 1 nm. The influence of V/III radio on the growth procedure is also discussed, and as proved, a higher V/III radio is necessary to be used to get high density nanocolumns and improved optical property.
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III-N semiconductors are useful for solid state lighting [1], high power electronics [2], ultraviolet emitters [3,4], solar energy conversion [5,6], and sensors [7]. Within the III-N family, InN stands out due to its narrow bandgap (0.65 eV) [8,9], very high electron mobility [10–16] and large saturation velocity, making it a potential candidate for high-speed and high frequency electronic [17,18], optoelectronic [19], and energy harvesting applications [20,21]. The synthesis of planar InN films is hampered by the lack of a lattice-matched substrate and the thermal instability of InN [22], leading to highly defective films.
We report on a systematic growth study of the nucleation process of InN nanowires on Si(1 1 1) substrates using plasma assisted molecular beam epitaxy (PAMBE). Samples are grown with various substrate temperatures and III/V ratios. Scanning electron microscopy, X-ray diffraction spectroscopy, energy dispersive X-ray spectroscopy, and photoluminescence are carried out to map out the variation in structural and optical properties versus growth conditions. Statistical averages of areal density, height, and radius are mapped as a function of substrate temperature and III/V ratio. Three different morphological phases are identified on the growth surface: InN, α-In and β-In. Based on SEM image analysis of samples grown at different conditions, the formation mechanism of these phases is proposed. Finally, the growth phase diagram of PAMBE grown InN on Si under N-rich condition is presented, and tapered versus non-tapered growth conditions are identified. It is found that high growth temperature and low III/V ratio plays a critical role in the growth of non-tapered InN nanowires.
Optical and structural studies of InN/GaN dots with varying GaN cap thickness
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The PL spectrum of the un-capped InN dots shows peak energy at 0.807 eV, with a FWHM of 104 meV. The emission energy of the un-capped InN dots is higher than the reported bandgap energy of 0.69 eV [13], and indicates a strong Burstein–Moss (BM) effect due to the presence of a high electron concentration in the InN dots. Cimalla et al. [14] reported that the high surface electron concentration generated by the high surface density of states was due to the high ratio of surface area to the volume of dots.
This paper reported a structural and optical study of single InN/GaN dots with varying GaN cap thickness. The surface morphology of the 10-nm thick GaN capping layer shows a truncated pyramidal shape and changed to a dome shape when the thickness of the capping layer increases to 20-nm. The increase in compressive strain of InN dots with the increase in the cap thickness was analyzed by high resolution x-ray diffraction (HRXRD). A redshift was observed in the photoluminescence (PL) peak energy of the InN dots with increasing GaN capping thickness. The redshift of the PL peak energy of the 20-nm thick GaN capping layer sample was believed to be due to the GaN capping avoids InN decomposition and decreases the surface defect density. In addition, the maximum PL intensity of the 20-nm thick GaN capping layer sample was ∼2.5 times higher than that of the uncapped sample.
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High density InN/GaN nanodots were grown by pulsed mode (PM) metal–organic chemical vapor deposition (MOCVD). InN nanodots density of up to ∼5×10¹⁰ cm⁻² at a growth temperature of 550 °C was achieved. The high diffusion activation energy of 2.65 eV due to high NH₃ flow rate generated more reactive nitrogen adatoms on the growth surface, and is believed to be the main reason for the growth of high density InN nanodots. In addition, an anomalous temperature dependence of the PL peak energy was observed for high density InN nanodots. The high carrier concentration, due to high In vacancy (V_In) in the InN nanodots, thermally agitated to the conduction band. As the measurement temperature increased, the increase of Fermi energy resulted in blue-shifted PL peak energy. From the Arrhenius plot of integrated PL intensity, the thermal activation energy for the PM grown InN nanodots was estimated to be E_a∼51 meV, indicating strong localization of carriers in the high density InN nanodots.
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Future bandwidth demand in optical communications requires all-optical devices based on optical non-linear behavior of materials. InN, with a room temperature direct bandgap well below 0.82 eV (1.5 μm) is very attractive for these applications. In this work, we characterize the non-linear optical response and recombination lifetime of the interband transition of InN layers grown on GaN template and Si(1 1 1) by molecular beam epitaxy. Non-linear characterization shows a decrease of the third-order susceptibility, χ⁽³⁾, and an increase of recombination lifetime when decreasing the energy difference between the excitation and the apparent optical band-gap energy of the analysed samples. Taking into account the non-linear characterization, an optically controlled reduction of the speed of light by a factor S=4.2 is obtained for bulk InN at 1.5 μm. The S factor of InN (5 nm)/In_0.7Ga_0.3N (8 nm) multiple quantum well heterostructures at the same operation wavelength is analysed, predicting an increase of this factor of three orders of magnitude. This result would open the possibility of using InN-based heterostructures for all-optical devices applications.

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Ultrafast carrier dynamics in InN epilayers

Abstract

J. Crystal Growth

Phys. Stat. Sol. (a)

Appl. Phys. Lett.

Jpn. J. Appl. Phys. Part 2

Appl. Phys. Lett.

Appl. Phys. Lett.

Phys. Stat. Sol. B

Phys. Rev. B