Thin Solid Films 518 (2009) 1489–1492
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Structural and optical properties of ultrananocrystalline diamond / InGaAs/GaAs quantum dot structures C. Popov ⁎, A. Gushterov, L. Lingys, C. Sippel, J.P. Reithmaier Institute of Nanostructure Technologies and Analytics (INA), University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany
a r t i c l e
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Available online 2 October 2009 Keywords: Quantum dots Ultrananocrystalline diamond ﬁlms Morphology Photoluminescence
a b s t r a c t The combination of the unique properties of ultrananocrystalline diamond (UNCD) ﬁlms and of semiconductor quantum dot (QD) structures could signiﬁcantly improve the performance of different electronic and optoelectronic devices, where e.g. good thermal management and advanced mechanical parameters are required. In the current work quantum dot InGaAs/GaAs heterostructures have been grown by molecular beam epitaxy (MBE) with different densities between 1.6 × 1010 cm− 2 and 1.6 × 1011 cm− 2 controlled by the substrate temperature in the range between 490 and 515 °C. These structures were overgrown with UNCD by microwave plasma chemical vapor deposition (MWCVD) using methane/nitrogen mixtures at 570 °C. Scanning electron microscopy (SEM) reveals that without ultrasonic pretreatment the diamond nucleation density on QD structures is low and only separate islands of UNCD are deposited, while after pretreatment thin closed ﬁlms are formed. From the cross-section SEM images a growth rate of ca. 3 nm/min is estimated which is very close to that on silicon at the same deposition conditions. The UNCD coatings exhibit a morphology consisting of two types of structures as shown by atomic force microscopy (AFM). The ﬁrst one includes nodules with diameters between 180 and 350 nm varying with the density of the underlying QDs; the second is formed by a kind of granular substructure of these nodules with diameters of about 40 nm for all QD densities. The optical properties were investigated by photoluminescence (PL) spectroscopy before and after the deposition of UNCD. The PL signals of QD structures overgrown with UNCD, although with decreased intensity, remain almost unchanged with respect to the peak positions and widths, revealing that the UNCD/QD structures retain the optical properties of uncoated InGaAs/GaAs quantum dots. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Quantum dots (QDs) of III–V semiconductors have been intensively investigated in the last decade because of their unique physical properties. These structures attract great scientiﬁc interest due to their potential applications for fabrication of different electronic and optoelectronic devices, such as superluminescent diodes for optical sensors , quantum dot lasers [2–4], single electron transistors , and single photon emitters . The electronic and optical properties of the QD structures can be tailored by their composition, size and spatial arrangements, controlled by the growth conditions. Another material of interest for preparation of solid-state quantum devices is diamond . In addition to its superior mechanical, tribological and chemical properties, diamond possesses very wide optical bandgap, low electron concentrations and low phonon scattering at room temperature . This allows the fabrication of high quality factor two-dimensional photonic crystal microcavities, e.g. in nanocrystalline diamond (NCD) ﬁlms, enhancing the light–matter interaction for quantum electrodynamic applications
⁎ Corresponding author. E-mail address: [email protected]
(C. Popov). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.097
. The combination of the unique properties of diamond and of semiconductor quantum dot structures could lead to the preparation of novel quantum devices. Another common application of semiconductor QDs and diamond is in high-power devices, e.g. lasers, which require good thermal management in order to overcome the production of heat in very small areas during their operation. A possible solution is the deposition of a thin layer of material with high thermal conductivity between the device and the cooling system. Polycrystalline diamond (PCD), prepared by chemical vapor deposition, possesses thermal conductivity on the order of 2000 W/mK , superior to copper over a wide temperature range and has already found application in the thermal management of ICs  and high-power laser diodes . Nanocrystalline diamond (NCD) ﬁlms deposited at lower temperatures retain to a great extent the superior properties of PCD, being at the same time much smoother. The thermal conductivity of such ﬁlms depends on the thickness and is about half of that of bulk diamond due to scattering by the grain boundaries, but still reaching values up to ca. 1400 W/mK , which makes them suitable for thermal management in high-power devices. In the present work we have studied the possibility for deposition of ultrananocrystalline diamond ﬁlms on InGaAs QDs and investigated the morphology and optical properties of the obtained structures.
C. Popov et al. / Thin Solid Films 518 (2009) 1489–1492
Table 1 QD densities and heights as well as PL signal parameters before and after the overgrowth with UNCD. Sample
LD-QD UNCD/LD-QD HD-QD UNCD/HD-QD
Height of QD
PL signal Position
1.6 × 1010
3.5 ÷ 5.5
1.6 × 1011
3.0 ÷ 7.7
1.320 1.316 1.211 1.220
53 50 71 67
2. Experimental Quantum dot In0.6 Ga0.4As heterostructures have been grown on (100) GaAs substrates using GEN II molecular beam epitaxy (MBE) system. All structures included 200 nm GaAs buffer layer deposited at a substrate temperature of 590 °C, which was lowered to the desired one for the subsequent formation of the QDs without interruption of the growth process. For the quantum dot formation a thin In0.6 Ga0.4As layer with a nominal thickness of 1.4 nm was deposited on the buffer layer. The as-grown QDs were covered with a 50 nm GaAs barrier layer and a second QD uncapped layer for morphology investigations, both grown at the same substrate temperature like the ﬁrst QD layer. The density of the dots was controlled by variation of the substrate temperature  as shown in Table 1. The QD structures were coated with ultrananocrystalline diamond (UNCD) ﬁlm by microwave plasma chemical vapor deposition (MWCVD) using 17% CH4/N2 gas mixtures in the set-up described elsewhere . The substrate temperature was 570 °C, the working pressure 2.3 kPa, the forwarded MW power 800 W, the deposition time 30 min. Under similar growth conditions ﬁlms composed of diamond nanocrystallites (3–5 nm in diameter) embedded in an amorphous carbon matrix have been deposited on different substrates, like silicon, polycrystalline diamond, c-BN, TiN, etc [16,17]. In the current work bare GaAs substrates were also coated with UNCD ﬁlm for comparison. In order to enhance the diamond nucleation, part of the samples were pretreated ultrasonically for 60 min in a suspension of 80 mg ultra-
disperse diamond powder (3–5 nm grain size) and 50 mg diamond powder (250 nm grain size) in n-pentane prior the deposition. The topography of the QD structures before and after coating with UNCD ﬁlm was investigated by atomic force microscopy (AFM, NanoScope III) in a tapping mode and the analysis of the images was performed with Gwyddion software. The morphology and the thickness of the top UNCD layer were studied by scanning electron microscopy (SEM, Hitachi 3000). The optical properties of the uncoated and coated samples were measured with a standard photoluminescence (PL) set-up. A DPSSlaser with a wavelength of 532 nm and a spot of ca. 400 µm with Gaussian intensity distribution was used for the optical excitation, and the emitted PL light in reﬂection was focused into a 1/4 m monochromator. The resulting signals were detected by a thermoelectrically cooled silicon photodiode in combination with a Lock-In ampliﬁer.
3. Results and discussion The density and height of the QDs before the overgrowth with UNCD were determined from the AFM images, as presented in Fig. 1, and the results are summarized in Table 1. Varying only the substrate temperature QDs with different densities were prepared: 1.6×1011 cm− 2 at 490 °C (HD-QD in the following) and 1.6×1010 cm− 2 at 515 °C (LD-QD). With increasing the substrate temperature the density of the dots decreases, while their height increases, due to the increased surface migration length of In atoms at higher temperatures . The topography of the prepared UNCD/QD structures was also studied by AFM. From the images the existence of two types of structures in the UNCD morphology is observed. The ﬁrst one consists of nodules with diameters of 180–350 nm, decreasing in the order bare GaAs substrate — LD-QD–HD-QD, and accompanied by a decrease of the height difference (Fig. 1 (d)–(f)). The second type of structures is represented by a kind of granular substructure of these nodules with diameters of about 40 nm irrespective of the size of the main structures. It can be supposed that the size and density of the main structures are determined by the initial nucleation density on the substrates overgrown with UNCD, where the density of the QDs plays an additional role to the nucleation process. This is the reason for the smaller size of the main UNCD structures grown on the QDs with higher density. The size of the substructures is determined to a
Fig. 1. AFM images of (a) bare GaAs substrate, (b) QDs with low density (LD-QD), (c) QDs with high density (HD-QD), (d) GaAs substrate coated with UNCD layer, (e) LD-QD coated with UNCD layer, (f) HD-QD coated with UNCD layer.
C. Popov et al. / Thin Solid Films 518 (2009) 1489–1492
Fig. 2. SEM micrograph of UNCD / InGaAs/GaAs quantum dot structures.
great extent by the secondary nucleation, inﬂuenced only by the deposition conditions (e.g. gas phase composition, substrate temperature, etc.) which were kept constant during the UNCD deposition. As a result, the variation in the diameter of these substructures is rather low, remaining ca. 40 nm in all cases, including bare GaAs substrates. SEM reveals that without pretreatment the UNCD nucleation density is very low on GaAs without or with QDs and only islands of UNCD are deposited, while the pretreated samples are coated with uniform coatings (Fig. 2). The micrographs show the typical UNCD morphology observed for ﬁlms deposited on other substrates (Si, c-BN, TiN, etc.) [16,18]. From the cross-section images a thickness of about 80–90 nm is determined, corresponding to a growth rate of ca. 3 nm/min. This growth rate is very similar to that for UNCD on Si substrates at the same deposition conditions [17,19]. The PL measurements were performed with the QD samples before and after overgrowth with UNCD. Since the intensities of the PL signals taken from different positions on the samples varied, reﬂecting the nonuniform distribution of the initially grown QDs, signals with averaged intensity between the maximum and the minimum have been considered. Such signals, which in all cases possessed a Gaussian form, are presented in Fig. 3 and their characteristics are given in Table 1. The used laser power was kept as low as possible not to saturate the ground state and to get transitions from excited states so that the form of each PL
spectrum resembles in a ﬁrst approximation the size dispersion of the QDs ensemble. At low dot densities, less dots contribute to the emission and the emission intensity saturates, while more carriers are excited to higher order states, resulting in a shift of the PL spectra of LD-QD to higher energy in comparison with HD-QD. Only marginal changes in the peak positions and widths are observed after the coating of QDs with UNCD (Table 1). The slight decrease of the FWHM is most probably a result from the annealing at 570 °C during the UNCD growth which in general has effect on the QD size distribution and optical properties . Irrespective of the density of the underlying QD ensembles the intensity of the PL signal is lower after the overgrowth with UNCD, which to a great extent could be explained with the optical parameters of the latter. The investigation of the optical properties of the UNCD layers have shown that the extinction coefﬁcient k is about 0.052 at the excitation laser wavelength of 532 nm and between 0.035 and 0.030 in the range of the PL signals . This would lead to a decrease of up to 15%–25% of the intensity of the light passing two times (exciting laser light forward and PL signal backward) through the UNCD layer with thickness of ca. 90 nm, which can not completely explain the overall decrease of the PL signal intensity. The reﬂectivity of the UCND is about 0.12–0.14 in the range of 500–1000 nm , which leads to additional losses of the signal intensity. Finally, some scattering from the surface of the UNCD ﬁlm and its interface with the barrier GaAs layer should also not be excluded. In spite of the drop in the signal intensity, the results from the PL measurements demonstrate that the QD ensembles are not destroyed after the coating with UNCD and remain optically active.
4. Conclusions Ultrananocrystalline diamond ﬁlms have been deposited by MWCVD on the top of InGaAs quantum dot ensembles with different densities grown by molecular beam epitaxy. The investigations of the topography of the obtained UNCD/QD structures by AFM revealed that the density of the quantum dots plays a role in the primary diamond nucleation. This results in a decrease of the size of the main structures composing the UNCD ﬁlms with increasing the density of the underlying QDs. The latter, however, has no inﬂuence on the secondary nucleation rate of UNCD, determined only by the deposition conditions, and reﬂected in the same size of the substructures building up the diamond layer. The PL measurements of the QD ensembles before and after the overgrowth with UNCD showed that although the intensity of the PL signal decreases, its position and FWHM remain unaffected, demonstrating the possibility for the preparation of optically active UNCD/QD structures.
Fig. 3. PL signals of QDs with low and high densities before and after the overgrowth with UNCD layer.
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