Home

Search

Collections

Journals

About

Contact us

My IOPscience

Pure, single crystal Ge nanodots formed using a sandwich structure via pulsed UV excimer laser annealing

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 165301 (http://iopscience.iop.org/0957-4484/26/16/165301) View the table of contents for this issue, or go to the journal homepage for more

Download details: IP Address: 149.150.51.237 This content was downloaded on 29/03/2015 at 10:59

Please note that terms and conditions apply.

Nanotechnology Nanotechnology 26 (2015) 165301 (8pp)

doi:10.1088/0957-4484/26/16/165301

Pure, single crystal Ge nanodots formed using a sandwich structure via pulsed UV excimer laser annealing Ting-Wei Liao, Hung-Ming Chen, Kuan-Yuan Shen and Chieh-Hsiung Kuan Graduate Institute of Electronic Engineering and the Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan, People’s Republic of China E-mail: [email protected] Received 31 December 2014, revised 10 February 2015 Accepted for publication 14 February 2015 Published 27 March 2015 Abstract

In this paper, a sandwich structure comprising a SiO2 capping layer, amorphous Germanium (aGe) nanodots (NDs), and a pit-patterned Silicon (Si) substrate is developed, which is then annealed by utilizing a pulsed ultraviolet excimer laser in order to fabricate an array of pure, single crystal Ge NDs at room temperature. A wide bandgap SiO2 capping layer is used as a transparent thermally isolated layer to prevent thermal loss and Si–Ge intermixing. The twodimensional pit-patterned Si substrate is designed to confine the absorbed laser energy, reduce the melting point, and block the surface migration of the Ge. After optimizing the laser radiation parameters such that the laser energy density is 200 mJ cm−2, the laser annealing period is 10 s, and the number of laser shots is 10, pure, single crystal Ge NDs that have both a regular arrangement and a uniform size distribution are obtained in the pits of the Si substrates. The Raman spectrum shows a highly symmetric Ge transversal optical peak with a full width at half maximum of 4.2 cm−1 at 300.7 cm−1, which is close to that of the original Ge wafer. In addition, the high-resolution transmission electron microscopy image for the Ge NDs and the corresponding selected area electron diffraction pattern shows a clear single crystalline structure without any impurities. Keywords: pure, single crystal, germanium, nanodots, excimer laser, sandwich structure, pitpatterned (Some figures may appear in colour only in the online journal) 1. Introduction

layer [15–20]. For LED or PD devices, the quantum confinement effects provided by Ge NDs may relax the selection rule for the conservation of momentum and, hence, enhance radiative recombination [21–24] or increase detection efficiency [25–28]. These studies have identified Ge NDs as a potential candidate for a new generation of semiconductor devices. However, in order to create high performance Ge ND-based devices, the fabrication of Ge NDs with high crystallinity is strongly required. Moreover, precise control of the size uniformity and position of the Ge NDs is also demanded. Several methods have been proposed for fabricating crystalline Ge NDs with high uniformity [1–8, 29–36]. Conventionally, Ge NDs are epitaxially grown on pre-patterned Si substrates, either by using molecular beam epitaxy

Ge nanodots (NDs) or nanocrystals have received considerable attention and have been investigated extensively [1–8] because of their novel electrical [9–11] and optical [12–14] properties, as well as for their compatibility with Si complementary metal–oxide–semiconductor technology. Common applications for Ge NDs include the nonvolatile memory (NVM), light-emitting diode (LED), and photodetector (PD) fields. Ge NDs are embedded inside the oxide layer of NVM devices to serve as separated charge storage sites, providing long retention times and small operating voltages. Fast write/ erase speeds have been achieved thanks to the large band offset at the Ge/oxide interface and the thin tunneling oxide 0957-4484/15/165301+08$33.00

1

© 2015 IOP Publishing Ltd Printed in the UK

Nanotechnology 26 (2015) 165301

T-W Liao et al

[1–4, 29–32] or via chemical vapor deposition (CVD) [6, 33– 35]. The resulting ordered and uniform NDs exhibit superior photoluminescence properties [4, 32, 36]. Recently, approaches that include sputtering [9, 37–39], ion implantation [16, 17, 40], and evaporation [5, 8] accompanied by a high temperature thermal annealing process (>700 °C), have also proved effective in fabricating suitable crystalline Ge NDs. Gao et al created Ge NDs that had an average size of 9.8 nm in a Si and Ge oxide superlattice structure using magnetron sputtering and thermal annealing in an N2 ambient at 750 °C [37]. Volodin et al demonstrated the creation of Ge NDs with a sharp Raman peak at about 300 cm−1 using electron gun (egun) evaporation and post annealing at 800 °C [5]. In [8], Ge NDs with a domain size of over 20 nm were also achieved by evaporating a Ge thin film on a patterned Si substrate and then capping it with a SiO2 layer before annealing at 900 °C. Although each of these approaches have illustrated the capability of fabricating highly crystalline Ge NDs, a severe problem still exists regarding the intermixing of Si and Ge due to the high temperature of the annealing treatment, which may degrade the performance and decrease the efficiency of the Ge ND devices [41–43]. In addition, the uniformity of the Ge NDs needs to be further improved for practical applications. In this paper, a novel method is proposed for fabricating an array of pure, single crystal Ge NDs at room temperature. A sandwich structure is developed, which is then annealed using a pulsed ultraviolet (UV) excimer laser. First, amorphous Ge (a-Ge) NDs with a controlled size, density, and arrangement are selectively deposited on a pit-patterned Si substrate. These NDs are then capped with a SiO2 layer and subsequently irradiated with UV radiation using a pulsed excimer laser. The laser radiation provides the energy required to melt the a-Ge, and the sandwich structure composed of the SiO2 capping layer, the a-Ge NDs, and the pitpatterned Si substrate helps to confine the absorbed energy, induce additional stress, and, furthermore, prevent Si–Ge intermixing. In addition, the NDs exhibit excellent size uniformity and arrangement. Finally, highly crystalline and pure Ge NDs are obtained after optimizing the laser radiation parameters.

substrate (sample-OGP). In order to investigate the effect of the sandwich structure, some a-Ge NDs were capped either with Si (sample-SGP) using e-gun evaporation, or with Si3N4 (sample-NGP) using plasma enhanced CVD rather than SiO2, and some sandwich structures were fabricated directly on a flat Si substrate (sample-OGF) rather than a patterned Si substrate. The samples were subsequently irradiated using a pulsed UV excimer laser with a wavelength of 248 nm in dry N2. The parameters for the excimer laser, including the energy density, E (mJ cm−2), the annealing period, P (s), and the number of shots, N, were further optimized in order to obtain the highly crystalline Ge NDs. The degree of crystallinity of the NDs was determined from their Raman spectra [39, 44] and verified using high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED). The morphology and uniformity of the NDs was inspected using SEM. In our experiments, the Raman spectra of the Ge NDs was measured by a micro-Raman spectrometer (Horiba Jobin Yvon-T64000) at room temperature. An Nd: YAG diode laser with wavelength of 532 nm as the excitation source. The incident light was focused through a 50× objective, and the back-scattered light was coupled into the spectrometer. The power of the laser beam reaching the sample was about 50 mW, and the spot size was about 2 μm. To avoid heating by the laser beam, the power can not be too strong [45]. Therefore, the sample was measured at the same position for three times to further confirm the results.

3. Results and discussions 3.1. Sandwich structure and smoldering process

Figure 1(b) shows a schematic cross section of the sandwich structure (sample-OGP) that was irradiated using a pulsed UV excimer laser for a specific period of time. The capping layer is highly transparent and barely absorbs the laser energy because the bandgap of SiO2 (9 eV) is larger than the photon energy of the UV laser (5 eV). Thus, only the underlying a-Ge NDs and the pit-patterned Si substrate will be substantially annealed. As the laser energy is absorbed, it is transferred to the lattice and instantaneously causes local heating [46]. As the SiO2 capping layer has poor thermal conductivity (0.014 W cm−1 K−1), it is expected to prevent heat dissipation through the air, and thus the heat can only be diffused via thermal conduction through the Si substrate. Sedky et al [47] simulated the heat distribution on the surface of pit-patterned Si substrates irradiated using short excimer laser pulses. They found that the thermal energy can be concentrated by using an array of pits. Moreover, the effect of local heating is most significant if the pitch is approximately twice the diameter of the pits. In this study, the square array of circular pits on the Si substrate is intentionally fabricated to further confine the thermal energy and maintain the temperature within the pattern area for a longer time so as to improve the crystallinity of the Ge NDs. After irradiation using the UV laser, the a-Ge NDs start to melt and crystallize while absorbing the thermal energy that is confined within the sandwich structure. At the

2. Experimental procedure P-type Si (001) substrates were first treated using RCA cleaning procedures and a pattern consisting of a square array of circles was transferred to the substrates via electron beam lithography e-beam and reactive-ion etching. The diameter, pitch, and depth of the resulting circular pits were 100, 200, and 30 nm, respectively. Amorphous Ge NDs with an average thickness of 150 nm were then selectively deposited into the pits using an e-gun evaporation and lift-off process. Figure 1(a) is the scanning electron microscopy (SEM) image of the cone-shaped a-Ge NDs on the pit-patterned Si substrate. These a-Ge NDs were subsequently capped with a 100 nm SiO2 layer to complete the sandwich structure of the SiO2 capping layer, the a-Ge NDs, and the pit-patterned Si 2

Nanotechnology 26 (2015) 165301

T-W Liao et al

Figure 1. (a) The SEM image for the cone-shaped a-Ge NDs on the pit-patterned Si substrate. (b) The schematic cross section of the sandwich structure (sample-OGP) irradiated using a pulsed UV excimer laser, at various values for the laser annealing period, P, laser energy density, E and number of laser shots, N.

Raman spectra, the two peaks at around 300 and 390 cm−1 correspond to the transversal optical (TO) Raman-active mode of the Ge crystal [39, 49–51] and the Si–Ge intermixing signal [50, 51], respectively. These peaks are further fitted using Lorentz distribution to obtain the full width at half maximum (FWHM) shown in the figure, which represents the crystallinity of the Ge NDs [39, 44, 49, 52]. For sample-OGP, the FWHM of the Raman signal from the Ge NDs (6.2 cm−1) is narrower than that of sample-OGF (7.5 cm−1) and sampleRef (8.3 cm−1), indicating the higher crystallinity of the Ge NDs obtained using this sandwich structure. This result can be attributed to the smoldering process by the SiO2 capping layer and the pit-patterned substrate. As discussed earlier, the poor thermal conductivity of SiO2 restricts the heat dissipation through the air, meaning that the temperature can be effectively maintained within the pit-patterned substrate area [47] for a longer time, as well as providing the compressive stress for the crystallization of the Ge. The sandwich structure using the pit-patterned substrate (sample OGP) is more favorable for inducing the smoldering process than those structures using the flat substrate (sample-OGF) or the sample-Ref structure without a capping layer. Additionally, the material used for the capping layer should be appropriately selected so as to prevent Si–Ge intermixing. As shown in the figure 2, the signal indicating obvious Si–Ge intermixing appears if the SiO2 capping layer is changed to Si3N4 (sample-NGP) or Si (sample-SGP), implying the occurrence of heating in the capping layer. The Si atoms diffuse from the capping layer to the Ge NDs when sufficient thermal energy is received. This result can be explained by the lower bandgap of the Si3N4 (4.7 eV) and the Si (1.12 eV) compared to the photon energy of the UV laser (5 eV), thus the capping layer absorbs a portion of the laser energy. The broadening (15 cm−1) and down-shifting of the Ge TO signal also indicates the structural change (bond lengths and angles) of the Ge NDs due to the Si–Ge intermixing [53]. Therefore, the sandwich structure based on a pitpatterned substrate plays an important role in obtaining highly crystalline Ge NDs. Moreover, in this case, SiO2 is a more suitable choice for the capping layer since it effectively

Figure 2. The Raman spectra obtained after the laser annealing

procedure for various sandwich structures, including pit-patterned (sample OGP) or flat Si (sample-OGF) substrates, Si3N4 (sampleNGP) or Si (sample-SGP) capping layers, and a-Ge NDs on a flat Si substrate without a capping layer (sample-Ref).

same time, the smaller thermal expansion coefficient of the Si (2.5 × 10−6 °C−1) compared to the Ge (6.0 × 10−6 °C−1) can potentially give rise to additional compressive stress on the Ge NDs deposited inside the pits, which, in turn, can lower the melting point of the Ge and contribute to the crystallization process [8, 48]. The crystallization of the Ge NDs due to the combination of the confined thermal energy as well as the compressive stress is thus called the ‘smoldering process’ in the following text. 3.2. Effect of the pit-patterned Si substrate and capping layer

Figure 2 shows the Raman spectra of the Ge NDs obtained after applying the laser annealing procedure. The investigated samples include the sandwich structures with either pit-patterned (sample OGP) or flat Si (sample OGF) substrates, the sandwich structure using either Si3N4 (sample NGP) or Si (sample SGP) capping layers, and a-Ge NDs on a flat Si substrate without a capping layer (sample Ref). All samples were annealed using six shots of laser irradiation with an energy density of 200 mJ cm−2, and the period of each shot is 10 s including an irradiation time of 25 ns. As shown by the 3

Nanotechnology 26 (2015) 165301

T-W Liao et al

Figure 3. (a) The FWHM and peak position for the Ge TO signal at laser energy densities ranging from 50 to 400 mJ cm−2. (b) The Raman

spectra for the sample-OGP annealed at energy densities of 50, 200 and 400 mJ cm−2, respectively. The inset shows the SEM image for the sample OGP annealed at an energy density of 400 mJ cm−2 after the capping layer was removed using BOE. (c) The FWHM of the Ge TO signal for laser annealing periods ranging from 10−5 to 10 s. The inset shows the SEM image for the results at 1e−5 and 10 s, respectively. (d) The FWHM and peak position of the Ge TO signal for a range of laser shots ranging from 1 to 12. The inset shows the Lorentz-fitted Raman spectrum for the crystallized Ge NDs after 10 shots of laser radiation compared with a pure Ge wafer.

prevents Si–Ge intermixing and assists in obtaining pure Ge NDs. In the next section, the effect of further optimizing the laser annealing parameters in order to improve the crystallinity of the Ge NDs in the sandwich structure is investigated.

Ge TO signal, and an a-Ge signal can be observed at 270 cm−1 [39, 52], which means that an energy density of 50 mJ cm−2 is not sufficient for the Ge NDs to crystallize. Moreover, the volatile GeO can be formed and structures may be damaged during annealing [56]. When the energy density reaches 200 mJ cm−2, only a sharp and narrow Ge TO signal is observed, indicating that most of the Ge NDs have crystallized at this energy density. As the energy density increases to 400 mJ cm−2, the Ge TO signal becomes broader and weaker compared to 200 mJ cm−2, and, moreover, a slight down-shift is observed. The inset in figure 3(b) is the SEM image for the sample-OGP annealed at an energy density of 400 mJ cm−2 after the capping layer is removed using a buffered oxide etch (BOE). The SEM image shows that most of the Ge NDs have disappeared, and only small Ge crystals remain in the pits of the Si substrate, indicating that the weaker and down-shifted Raman signal is due to the conspicuous ablation of the Ge NDs through vaporization after the laser irradiation at a high energy density [40, 54]. Therefore, a suitable laser energy density should be selected for the crystallization of Ge NDs in the sandwich structure since the NDs only slightly crystallize at a low energy density and ablate at a high energy density.

3.3. The laser annealing parameters 3.3.1. Laser energy density, E. During laser irradiation, the

grain size and crystallinity of the Ge NDs greatly depends on the laser energy density [46, 54, 55]. In this section, the effect of energy density on the crystallinity of the Ge NDs in the sandwich structure is demonstrated. Figure 3(a) shows the FWHM and peak position of the Ge TO signal at laser energy densities ranging from 50 to 400 mJ cm−2. The sample OGP is annealed using six shots of laser radiation with a period of 10 s. As the energy density increases from 50 to 200 mJ cm−2, the FWHM can be seen to obviously decrease from 13.3 to 6.7 cm−1, and the peak position is up-shifted from 298.9 to 299.5 cm−1. These results indicate that the crystallinity of the Ge NDs is obviously improved because the size of the crystallized region becomes gradually larger [39, 49, 52, 54]. However, when the energy density is set to more than 200 mJ cm−2, the FWHM broadens to 16.1 cm−1 and the peak position is down-shifted to 297.7 cm−1, which indicates that there has been a significant deterioration in the crystallinity of the Ge NDs. Figure 3(b) shows the Raman spectra for the sample-OGP annealed at an energy density of 50, 200 and 400 mJ cm−2, respectively. When the energy density is 50 mJ cm−2, the Raman spectrum shows a broad and weak

3.3.2. Laser annealing period, P. After laser irradiation, the

crystallinity of the Ge NDs can be further improved by using the smoldering process thanks to the thermal energy confined in the sandwich structure. However, the temperature within 4

Nanotechnology 26 (2015) 165301

T-W Liao et al

the pattern area can only be maintained for a short period of time [47]. Thus, the effect of the laser annealing period on the smoldering process must be investigated so as to obtain a high crystallinity for the Ge NDs. Figure 3(c) shows the FWHM of the Ge TO signal for laser annealing periods ranging from 10−5 to 10 s. The sample-OGP is annealed using two shots of laser irradiation with an energy density of 200 mJ cm−2. For this experiment, the laser annealing period is composed of the radiation time and the thermal duration. The radiation time is a transient of about 25 ns for each shot of the pulsed UV excimer laser, and the thermal duration represents the period of the smoldering process for the sandwich structure where the thermal energy is confined and the temperature is maintained. The result of this experiment shows that the FWHM decreases obviously as the period increases from 10−5 to 10 s. The decrease in FWHM means that the crystallization region of the Ge NDs is expanded as the thermal duration period increases. For the shortest period (10−5 s), the maximum FWHM (16.8 cm−1) is obtained, and the size of the Ge NDs is smaller than the as-deposited Ge NDs shown in the left-hand inset of figure 3(c). It may be that the shortest period does not provide a sufficient thermal duration for the crystallization of the Ge NDs, and also raises the energy density, which results in the ablation of the Ge NDs, meaning that their size is therefore decreased. Additionally, as the thermal duration period is increased to more than 1 s, the FWHM values almost the same, which implies that the thermal energy begins to dissipate and, hence, the crystallinity will not be further improved. Thus, we can deduce that the thermal duration of the smoldering process, in which the thermal energy is confined, is approximately 1 s. When the thermal duration period is increased to 10 s, the FWHM is 9.14 cm−1, and the Ge NDs exhibit a rounded top, as shown in the right-hand inset of figure 3(c). These results imply that the Ge NDs are still not fully melted and crystallized. Therefore, to prolong the thermal duration for the complete crystallization of the Ge NDs, the number of laser shots should be increased.

structure, and, hence, contributes to the crystallization process of the Ge NDs. Moreover, the provision of thermal energy for a sufficient length of time can be achieved by increasing the number of laser shots, which results in the highly crystalline Ge NDs obtained through the smoldering process using only 10 shots of laser radiation. The inset in figure 3(d) shows the Lorentz-fitted Raman spectrum for the crystallized Ge NDs using 10 shots of laser radiation compared with an original Ge wafer. The crystallized Ge NDs show a highly symmetric Ge TO peak at 300.7 cm−1, which can be well fitted using Lorentz distribution with a FWHM of 4.2 cm−1. These results suggest that the crystallinity of the Ge NDs is close to that of a original Ge wafer (where the FWHM is 4.0 cm−1and the peak position is at 300.7 cm−1). In addition, it is worth noting that no Si–Ge intermixing signal was observed from the Raman spectrum of the crystallized Ge NDs, even if the number of laser shots was increased to more than 10. Therefore, the thermal duration can be effectively prolonged by increasing the number of laser shots such that the crystallinity of the processed Ge NDs is close to that of the original Ge wafer. 3.4. Shape, size distribution and crystal structure

Pure, single crystal Ge NDs have been obtained, and the optimized annealing parameters have found to be a laser energy density of 200 mJ cm−2, a laser annealing period of 10 s, and a maximum of 10 laser shots. In this section, SEM and TEM images are utilized to confirm the shape, size distribution, and structure of the Ge ND crystals. Figure 4(a) shows the SEM image for the Ge NDs obtained in the optimal sample OGP after removing the SiO2 capping layer using BOE. The SEM image shows that the shape of the Ge NDs has been transformed from a cone to a dot, and all of the NDs are regularly arranged in the pits of the Si substrate. These results imply that not only have the Ge NDs completely crystallized, but also that the pit-patterned Si substrate has effectively blocked the surface migration of the Ge NDs during the laser annealing. The inset of figure 4(a) shows the SEM image for the crystallized Ge NDs in the sample OGF after removing the SiO2 capping layer using BOE; this sample was annealed using six shots of laser radiation with an energy density of 200 mJ cm−2 and a period of 10 s. The SEM image shows that some of the Ge NDs have been dislodged from their original position, which suggests that the pit-pattern is necessary in order to achieve the regular arrangement of the Ge NDs. Figure 4(b) shows the size distributions (3 × 3 μm) of 140 Ge NDs, where the mean value is 56.9 nm and standard deviation is 7.29. These results indicate that the size uniformity of the processed Ge NDs is excellent. In this experiment, the uniform Ge NDs can be observed over an area of 600 μm2, and the area density is about 3.9 × 109 cm−2. Figure 5(a) shows the corresponding HR-TEM image for the pure, single crystal Ge NDs obtained in the optimal sample OGP, where their size is estimated to be 60 nm. Figure 5(b) shows the high magnification HR-TEM images for the Ge NDs on the right-hand side of figure 5(a), where the clear lattice structure of the Ge NDs without any defects can be

3.3.3. Number of laser shots, N. Figure 3(d) shows the

FWHM and peak position of the Ge TO signal for laser shot numbers ranging from 1 to 12. The energy density and the laser radiation period is 200 mJ cm−2 and 10 s, respectively. As the number of laser shots increases, the FWHM decreases obviously from 18.8 to 4.2 cm−1, and the peak position shows an up-shift from 297.7 to 300.7 cm−1. When the number of laser shots is increased to more than 10, the narrowest FWHM of 4.2 cm−1 and a peak position of 300.7 cm−1 are obtained. These results indicate that the crystallinity of the Ge NDs has been greatly improved [39, 44] by prolonging the thermal duration as the number of shots is increased. One reason for this could be that the conversion of the Ge NDs from the amorphous phase to a highly ordered crystalline phase not only requires thermal energy, but also requires a sufficient time for the crystals to be formed, rearranged and ordered. After laser radiation, the smoldering process helps to maintain the temperature for a period of time thanks to the sandwich 5

Nanotechnology 26 (2015) 165301

T-W Liao et al

Figure 4. (a) The final shape of the crystallized Ge ND array as observed using SEM, and (b) the size distribution versus the number of 140

Ge NDs.

Figure 5. (a) A TEM cross section image for the sandwich structure, and (b) a magnified view of the right-hand side of the crystallized Ge ND illustrated in figure 5 (a), with the corresponding SAED depicted in the inset.

observed. The inset in figure 5(b) shows the corresponding SAED pattern for figure 5(b). The SAED pattern clearly represents the structure of a single crystal and no crystal structure for any other material can be observed. Therefore, these results reconfirm that the processed Ge ND is a single crystal without any impurities.

Ge NDs. The Raman spectrum shows that the crystallinity of the Ge NDs is close to that of the original Ge wafer. In addition, these NDs exhibit excellent size uniformity, arrangement, and a clear crystal structure without any impurities.

Acknowledgments This research is supported by the National Science Council of the Republic of China (Taiwan) under contract no. NSC 1022221-E-002-151-MY3 and NSC 102-2120-M-009-002.

4. Conclusion Pure, single crystal Ge NDs have been obtained using a sandwich structure composed of a SiO2 capping layer, a-Ge NDs, and a pit-patterned Si substrate using pulsed UV excimer laser annealing. The sandwich structure induces a smoldering process that plays an important role in confining the thermal energy and prolonging the thermal duration for the crystallization of the Ge NDs. Moreover, the SiO2 capping layer effectively prevents Si–Ge intermixing. The laser annealing parameters have been further optimized to obtain the optimal energy density and thermal duration for the complete crystallization of the

References [1] Huangfu Y, Zhan W, Hong X, Fang X, Ding G and Ye H 2013 Heteroepitaxy of Ge on Si(001) with pits and windows transferred from free-standing porous alumina mask Nanotechnology 24 185302 [2] Grydlik M, Langer G, Fromherz T, Schaffler F and Brehm M 2013 Recipes for the fabrication of strictly ordered Ge

6

Nanotechnology 26 (2015) 165301

[3]

[4] [5] [6]

[7]

[8]

[9]

[10]

[11] [12] [13] [14] [15]

[16]

[17]

[18] [19]

[20]

[21]

T-W Liao et al

islands on pit-patterned Si(001) substrates Nanotechnology 24 105601 Ma Y J, Zhong Z, Yang X J, Fan Y L and Jiang Z M 2013 Factors influencing epitaxial growth of three-dimensional Ge quantum dot crystals on pit-patterned Si substrate Nanotechnology 24 015304 Huangfu Y, Zhan W, Hong X, Fang X, Ding G and Ye H 2013 Optimal growth of Ge-rich dots on Si(001) substrates with hexagonal packed pit patterns Nanotechnology 24 035302 Volodin V A, Marin D V, Rinnert H and Vergnat M 2013 Formation of Ge and GeSi nanocrystals in GeOx /SiO2 multilayers J. Phys. D: Appl. Phys. 46 275305 Chen H-M, Kuan C-H, Suen Y-W, Luo G-L, Lai Y-P, Wang F-M and Chen S-T 2012 Thermally induced morphology evolution of pit-patterned Si substrate and its effect on nucleation properties of Ge dots Nanotechnology 23 015303 Perez del Pino A, Gyorgy E, Marcus I C, Roqueta J and Alonso M I 2011 Effects of pulsed laser radiation on epitaxial self-assembled Ge quantum dots grown on Si substrates Nanotechnology 22 295304 Chiu C W, Liao T W, Tsai K Y, Wang F M, Suen Y W and Kuan C H 2011 Fabrication method of high-quality Ge nanocrystals on patterned Si substrates by local melting point control Nanotechnology 22 275604 Zhang B, Yao Y, Patterson R, Shrestha S, Green M A and Conibeer G 2011 Electrical properties of conductive Ge nanocrystal thin films fabricated by low temperature in situ growth Nanotechnology 22 125204 Katsaros G, Spathis P, Stoffel M, Fournel F, Mongillo M, Bouchiat V, Lefloch F, Rastelli A, Schmidt O G and De Franceschi S 2010 Hybrid superconductor– semiconductor devices made from self-assembled SiGe nanocrystals on silicon Nat. Nanotechnology 5 458 Fujii M, Mamezaki O, Hayashi S and Yamamoto K 1998 Current transport properties of SiO2 films containing Ge nanocrystals J. Appl. Phys. 83 1507–12 Chang J E et al 2012 Matrix and quantum confinement effects on optical and thermal properties of Ge quantum dots J. Phys. D: Appl. Phys. 45 105303 Garoufalis C S 2009 Optical gap and excitation energies of small Ge nanocrystals J. Math. Chem. 46 934 Palummo M, Onida G and Sole R D 1999 Optical properties of germanium nanocrystals Phys. Status Solidi A 175 23 de Sousa J S, Peibst R, Erenburg M, Bugiel E, Farias G A, Leburton J-P and Hofmann K R 2011 Single-electron charging and discharging analyses in Ge-nanocrystal memories IEEE Trans. Electron Devices 58 2 Ang R, Chen T P, Yang M, Wong J I and Yi M D 2010 The charge trapping and memory effect in SiO2 thin films containing Ge nanocrystals J. Phys. D: Appl. Phys. 43 015102 Yang M, Chen T P, Liu Z, Wong J I, Zhang W L, Zhang S and Liu Y 2009 Effect of annealing on charge transfer in Ge nanocrystal based nonvolatile memory structure J. Appl. Phys. 106 103701 Yuan C L and Lee P S 2008 Enhanced charge storage capability of Ge/GeO2 core/shell nanostructure Nanotechnology 19 355206 Chiang K H, Lu S W, Peng Y H, Kuan C H and Tsai C S 2008 Characterization and modeling of fast traps in thermal agglomerating germanium nanocrystal metal–oxide– semiconductor capacitor J. Appl. Phys. 104 014506 Lee P F, Lu X B, Dai J Y, Chan H L W, Jelenkovic E and Tong K Y 2006 Memory effect and retention property of Ge nanocrystal embedded Hf-aluminate high-k gate dielectric Nanotechnology 17 1202 Peng Y H, Hsu C H, Kuan C H, Liu C W, Chen P S, Tsai M J and Suen Y W 2004 The evolution of

[22] [23] [24]

[25] [26]

[27]

[28] [29]

[30] [31] [32] [33]

[34]

[35]

[36]

[37]

[38]

[39] [40]

7

electroluminescence in Ge quantum-dot diodes with the fold number Appl. Phys. Lett. 85 6107–9 Chang W-H et al 2003 Room-temperature electroluminescence at 1.3 and 1.5 um from Ge/Si self-assembled quantum dots Appl. Phys. Lett. 83 2958–60 Stoffel M, Denker U and Schmidt O G 2003 Electroluminescence of self-assembled Ge hut clusters Appl. Phys. Lett. 82 3236–8 Klein M V, Sturge M D and Cohen E 1982 Exponential distribution of the radiative decay rates induced by alloy scattering in an indirect-gap semiconductor Phys. Rev. B 25 4331–3 Cosentino S et al 2011 High-efficiency silicon-compatible photodetectors based on Ge quantum dots Appl. Phys. Lett. 98 221107 Singha R K, Manna S, Das S, Dhar A and Ray S K 2010 Room temperature infrared photoresponse of self assembled Ge/Si (001) quantum dots grown by molecular beam epitaxy Appl. Phys. Lett. 96 233113 Tzeng S S and Li P W 2008 Enhanced 400–600 nm photoresponsivity of metal–oxide–semiconductor diodes with multi-stack germanium quantum dots Nanotechnology 19 235203 Tong S, Liu F, Khitun A and Wang K L 2004 Tunable normal incidence Ge quantum dot midinfrared detectors J. Appl. Phys. 96 773–6 Hrauda N, Zhang J J, Groiss H, Etzelstorfer T, Holy V, Bauer G, Deiter C, Seeck O H and Stangl J 2013 Strain relief and shape oscillations in site-controlled coherent SiGe islands Nanotechnology 24 335707 Rinke G, Mussler G, Gerharz J, Moers J and Gr¨utzmacher D 2009 Growth of Ge dots on templated Si substrates with diffusion-altered holes Europhys. Lett. 85 58002 Dais C, Solak H H, Müller E and Grützmacher D 2008 Impact of template variations on shape and arrangement of Si/Ge quantum dot arrays Appl. Phys. Lett. 92 143102 Grützmacher D et al 2007 Three-dimensional Si/Ge quantum dot crystals Nano Lett. 7 3150 Chen Y R, Kuan C H, Suen Y W, Peng Y H, Chen P S, Chao C H, Liang E Z, Lin C F and Lo H C 2008 Highdensity one-dimensional well-aligned germanium quantum dots Appl. Phys. Lett. 93 083101 Stoica T, Shushunova V, Dais C, Solak H and Grutzmacher D 2007 Two-dimensional arrays of self-organized Ge islands obtained by chemical vapor deposition on pre-patterned silicon substrates Nanotechnology 18 455307 Olzierski A, Nassiopoulou A G, Raptis I and Stoica T 2004 Two-dimensional arrays of nanometer scale holes and nanoV-grooves in oxidized Si wafers for the selective growth of Ge dots or Ge/Si hetero-nanocrystals Nanotechnology 15 1695–700 Chen Y, Pan B, Nie T, Chen P, Lu F, Jiang Z and Zhong Z 2010 Enhanced photoluminescence due to lateral ordering of GeSi quantum dots on patterned Si(001) substrates Nanotechnology 21 175701 Gao F, Green M A, Conibeer G, Cho E-C, Huang Y, Pere-Wurfl I and FlynnK C 2008 Fabrication of multilayered Ge nanocrystals by magnetron sputtering and annealing Nanotechnology 19 455611 Das K, Goswami M L N, Dhar A, Mathur B K and Ray S K 2007 Growth of Ge islands and nanocrystals using RF magnetron sputtering and their characterization Nanotechnology 18 175301 Fujii M, Hayashi S and Yamamoto K 1991 Growth of Ge microcrystals in SiO2 thin film matrices: a Raman and electron microscopic study Japan. J. Appl. Phys. 30 687–94 Kanemitsu Y, Masuda K, Yamamoto M, Kajiyama K and Kushida T 2000 Near-infrared photoluminescence from Ge nanocrystals in SiO2 matrices J. Lumin. 87 457–9

Nanotechnology 26 (2015) 165301

T-W Liao et al

[41] Jung J, Yu S F, Olubuyide O O, Hoyt J L, Antoniadis D A, Lee M L and Fitzgerald E A 2004 Effect of thermal processing on mobility in strained Si/strained Si1−yGey on relaxed Si1−yGey (x < y) virtual substrates Appl. Phys. Lett. 84 3319 [42] Dehlinger G, Koester S J, Schaub J D, Chu J O, Ouyang Q C and Grill A 2004 High-speed germanium-onSOI lateral PIN photodiodes IEEE Photonics Technol. Lett. 16 2547 [43] Liu J 2010 private communication [44] Parker J H, Feldman D W Jr and Ashkin M 1967 Raman scattering by silicon and germanium Phys. Rev. 155 712–4 [45] Volodin V A, Marin D V, Sachkov V A, Gorokhov E B, Rinnert H and Vergnat M 2014 Applying of improved phonon confinement model for analysis of Raman spectra of germanium nanocrystals J. Exp. Theor. Phys. 118 65–71 [46] Nickel N H 2003 Laser crystallization of silicon Semicond. Semimetals 75 11–40 [47] Sedky S, Tawfik H, Ashour M, Graham A B, Provine J, Wang Q, Zhang X X and Howe R T 2012 Microencapsulation of silicon cavities using a pulsed excimer laser J. Micromech. Microeng. 22 075012 [48] Rosei F and Raiteri P 2002 Stress induced surface melting during the growth of the Ge wetting layer on Si(0 0 1) and Si (1 1 1) Appl. Surf. Sci. 195 16–9 [49] Vega F, Serna R, Afonso C N, Bermejo D and Tejeda G 1994 Relaxation and crystallization kinetics of amorphous

[50] [51]

[52] [53] [54]

[55] [56]

8

germanium films by nanosecond laser pulses J. Appl. Phys. 75 7287–91 Kolobov A V 2000 Raman scattering from Ge nanostructures grown on Si substrates: power and limitations J. Appl. Phys. 87 2926–30 Tripathi S, Brajpuriya R, Sharma A, Shripathi T and Chaudhari S M 2006 Structural characterization of annealed Si/Ge nanostructures using Raman spectroscopy, XRR and AFM J. Phys. D: Appl. Phys. 39 4848–54 Mestanza S N M, Rodriguez E and Frateschi N C 2006 The effect of Ge implantation dose on theoptical properties of Ge nanocrystals in SiO2 Nanotechnology 17 4548–53 Wenli Z and Tinghui L 2012 Compositional dependence of Raman frequencies in SixGe1−x alloys J. Semicond. 33 112001–5 Park J-H, Han S-M, Park S-G, Han M-K and Shin M-Y 2006 Excimer laser recrystallization of nanocrystalline-Si films deposited by inductively coupled plasma chemical vapour deposition at 150 °C Phys. Scr. T126 85–8 Bidin N and Razak S N A 2012 ArF excimer laser annealing of polycrystalline silicon thin film Crystallization—Sci. Technol. 18 481–506 Marin D V, Volodin V A, Gorokhov E B, Shcheglov D V, Latyshev A V, Vergnat M, Koch J and Chichkov B N 2010 Modification of germanium nanoclusters in GeOx films during isochronous furnace and pulse laser annealing Tech. Phys. Lett. 36 439–42

Pure, single crystal Ge nanodots formed using a sandwich structure via pulsed UV excimer laser annealing.

In this paper, a sandwich structure comprising a SiO2 capping layer, amorphous Germanium (a-Ge) nanodots (NDs), and a pit-patterned Silicon (Si) subst...
1MB Sizes 0 Downloads 10 Views