The effect of stress stimulation on gold (Au) induced lateral crystallization (GILC) of amorphous Ge on insulating substrates is investigated. As a result, the GILC is significantly enhanced by using compressive residual stresses of up to 200 MPa in TEOS-SiO2. In addition, it is found that the annealing temperature necessary to cause the GILC for a short annealing time (60 min) can be decreased to 130 °C. We have demonstrated that the GILC enhancement was caused by the stress stimulation contributed to bond rearrangement and Au easily diffused into Ge. This study proposes a unique low temperature crystallization technique that introduces a residual film stress on metal induced lateral crystallization, paving the way for the low-cost fabrication of flexible electronic devices on low-softening temperature plastic substrates.

Low temperature formation of a crystalline semiconductor on insulating substrates plays an important role in the fabrication of flexible electronic devices, including various functional devices (such as logic circuits, solar cells, and memories) built on a flexible plastic substrate.1,2 The temperature of the fabrication process is required to be lower than 150 °C that is the softening temperature of low-cost plastic (e.g., polypropylene) substrates.3 In line with this, Ge is not only superior in electrical properties, such as high carrier mobility, but also has low temperature solid phase crystallization (SPC) compared to Si, making it an attractive material for flexible devices. Nevertheless, the SPC of amorphous Ge on insulating substrates requires annealing temperatures higher than 400 °C,4–8 which is a huge barrier to the realization of advanced flexible devices.

Recently, metal induced crystallization (MIC) attracts attention as a low temperature crystallization method of amorphous Ge thin films.5,9–17 In particular, gold induced crystallization has an outstanding advantage in the aspect of low SPC temperature (∼200 °C).18–27 Moreover, we have significantly enhanced gold induced lateral crystallization (GILC) of amorphous Ge by introducing point defects by high energy electron irradiation,24 demonstrating that the superposition of different types of energy is effective in promoting GILC. However, it is still difficult to form crystalline Ge films below the softening temperature of low-cost plastic substrates (150 °C). Therefore, a new approach to reduce the crystallization temperature is strongly required.

Hekmatshoar et al. have investigated the low temperature crystallization at 130 °C of Ge thin films by Cu induced crystallization with an externally applied stress,13,14 and they suggested the advantage of stress assistance on MIC. However, because the strain is mechanically applied and causes inhomogeneity in the strain distribution, the formed poly Ge layer has cracks, resulting in the degradation of electrical properties of Ge films. Thus, a technique to apply strain uniformly in the plane of amorphous Ge is rather desirable for practical applications. In our present work, we have developed a low temperature crystallization technique for amorphous Ge by the combination of gold induced lateral crystallization and stress stimulation, which is applied by utilizing the residual stress of a SiO2 layer grown by chemical vapor deposition (CVD) on amorphous Ge.28 

The experimental procedure is schematically shown in Fig. 1. Cz n-type Si (100) substrates are covered with thermally oxidized SiO2 films (thickness: 250 nm) after RCA cleaning. Amorphous Ge films (thickness: 100 nm) were deposited on the SiO2/Si substrate using a DC magnetron sputtering system with Ar plasma (Ar gas purity: > 99.9995 vol. %, Ar gas flow rate: 10 sccm, pressure during deposition: 2.0 Pa, DC power: 50 W, and deposition rate: 15 nm/min) at room temperature. Subsequently, Au films (thickness: 200 nm and diameter: 3 mm) were patterned using a metal mask during the evaporation on amorphous Ge films in a vacuum chamber (base pressure: 1.5 × 10−2 Pa). Finally, tetraethyl-orthosilicate SiO2 (TEOS-SiO2) films (thickness: 500 nm) were deposited on patterned-Au/amorphous Ge/SiO2/Si substrates using a PD-100ST plasma chemical vapor deposition system (Samco, Inc.) at 130 °C for 5 min (deposition rate: 100 nm/min). Here, TEOS-SiO2 films have a residual stress whose amount and direction (tensile or compressive) can be easily adjusted by the CVD process conditions (e.g., RF power, process pressure, gas flow rate, and flow ratio of TEOS/O2). The residual stress of TEOS-SiO2 thin films was estimated using the Stoney equation29 from the measurement results of FLX-2320 (KLA Tencor). We expected that weakening the Ge–Ge bond strength decreases the activation energy and results in crystal nucleation at a very low temperature. These samples were annealed below the eutectic temperature of the Ge–Au system (∼361 °C)30 by furnace annealing in a N2 ambient atmosphere (N2 gas purity: 99.9995 vol. % and N2 gas flow rate: 200 sccm).

FIG. 1.

Experimental procedure.

FIG. 1.

Experimental procedure.

Close modal

After annealing, the surface morphology of lateral crystallization regions was evaluated by optical microscopy and scanning electron microscopy using LV100ND (Nikon Inc.) and JSM-7001F (JEOL Ltd.), respectively. The crystallinity of lateral crystallized Ge regions was measured by Raman spectroscopy with an excitation laser wavelength of 457 nm using LabRAM HR Evolution (HORIBA Inc.). The atomic concentration profiles were evaluated by energy dispersive x-ray spectroscopy (SEM-EDX) with an excitation electron beam of 3.5 kV and a cumulative time of 15 min using Aztec energy standard X-act (Oxford Instruments). All measurements were performed at room temperature.

Figures 2(a)–2(e) show Nomarski optical photographs and Raman spectra at several regions of the samples without and with the compressive stressed TEOS-SiO2 cap layer after annealing at 150 °C for 60 min. Au circular pattern regions are located on the left side of the optical photographs defined as “A region,” which act as a seeding area for the GILC of amorphous Ge. Furthermore, amorphous Ge regions are located on the right side of the optical photographs defined as “B region.” In the case of the samples without and with a stress-free (almost 0 MPa) TEOS-SiO2 cap layer corresponding to Figs. 2(b) and 2(c), it is found that the Nomarski optical photographs did not change compared with the before annealing sample [Fig. 2(a)]. In addition, the broad peak corresponding to Ge–Ge bonds in amorphous Ge around 270 cm−1 in “B region,” shown in Figs. 2(a)–2(c), is observed. These results indicate that the phase change from amorphous to crystal does not occur because Au atoms necessary for crystal nucleation cannot be supplied by low temperature annealing.

FIG. 2.

Nomarski optical photographs and Raman spectra at several regions of the samples covered without the TEOS-SiO2 cap layer (a) before and (b) after annealing, and covered with a TEOS-SiO2 cap layer having compressive stresses of (c) almost 0 MPa, (d) 100 MPa, and (e) 200 MPa after annealing at 150 °C for 60 min.

FIG. 2.

Nomarski optical photographs and Raman spectra at several regions of the samples covered without the TEOS-SiO2 cap layer (a) before and (b) after annealing, and covered with a TEOS-SiO2 cap layer having compressive stresses of (c) almost 0 MPa, (d) 100 MPa, and (e) 200 MPa after annealing at 150 °C for 60 min.

Close modal

On the other hand, in the case of the samples covered with TEOS-SiO2 cap layers having compressive stresses of 100 MPa and 200 MPa, another region defined as “C region” formed between the “A region” and “B region” is clearly observed, as shown in Figs. 2(d) and 2(e), respectively Furthermore, the sharp Raman peaks originating from TO phonon mode of Ge–Ge bonds in crystalline Ge are clearly observed around 300 cm−1 in “C region.” These results demonstrate that amorphous Ge was crystallized around the Au circular pattern at a very low temperature (∼150 °C) after short time (∼60 min) annealing by utilizing the compressive stress and the catalytic effect of Au.

The crystallinity of “C region” was evaluated by microprobe Raman mapping measurement (spot size: 1 µm and effective resolution: 0.1 cm−1). The Raman mapping images of the Raman peak positions for the samples without and with the 200 MPa compressive stressed TEOS-SiO2 cap layer after annealing at 200 °C for 60 min are shown in Fig. 3(a). The peak position of Ge–Ge bonds for crystalline Ge tends to move toward the low wave number side with an increase in the distance from the Au pattern edge. Thus, these results indicate that the GILC of amorphous Ge progress concentrically regardless of the TEOS-SiO2 covering. In particular, the Raman peak position at the GILC region of 299.1 cm−1 in the vicinity of the Au pattern edge is nearly the same as the peak position for a single crystal Ge (100) substrate (299.4 cm−1). This result indicates that the GILC region has high crystallinity.

FIG. 3.

(a) Raman mapping images of the Raman peak positions for the samples without and with compressive stress after annealing at 200 °C for 60 min, (b) isochronal annealing characteristics, (c) the critical annealing temperature, (d) isothermal annealing characteristics, and (e) the incubation time of GILC for amorphous Ge without and with compressive stressed TEOS-SiO2 cap layer.

FIG. 3.

(a) Raman mapping images of the Raman peak positions for the samples without and with compressive stress after annealing at 200 °C for 60 min, (b) isochronal annealing characteristics, (c) the critical annealing temperature, (d) isothermal annealing characteristics, and (e) the incubation time of GILC for amorphous Ge without and with compressive stressed TEOS-SiO2 cap layer.

Close modal

In order to investigate the effect of the stress stimulation in detail, we defined the GILC length as the perpendicular distance of the “C region,” which is between the Au circular pattern edge and the lateral crystallization front, using Raman mapping results [see Fig. 3(a)]. Figure 3(b) shows the isochronal annealing characteristics for 60 min of the GILC length for the samples without and with the TEOS-SiO2 cap layer at various compressive stresses. The GILC length increases with an increase in the annealing temperature in all samples. It is noted that the same isochronal characteristics were obtained for the samples without and with the stress-free TEOS-SiO2 cap layer. This result indicates that only the deposition of TEOS-SiO2 cap layer is not enough to achieve low temperature GILC of amorphous Ge.

Particularly, it was clearly observed that the GILC of amorphous Ge on insulating substrates was significantly enhanced by utilizing the compressive stress. The critical annealing temperatures to cause the GILC of amorphous Ge are estimated as the temperatures where GILC initiated, and they are summarized as a function of the residual compressive stress of TEOS-SiO2 in Fig. 3(c). This clearly indicates that the critical annealing temperature is significantly decreased with an increase in the compressive stress. As a result, low temperature crystallization below 130 °C becomes possible by utilizing a compressive stress of 200 MPa. These results suggest that applying an external stress is necessary for the decrease in crystallization temperatures of amorphous semiconductors on insulating substrates.

The annealing time dependence of the GILC length for the samples covered with the compressive stressed TEOS-SiO2 cap layer with 200 MPa is shown in Fig. 3(d). It is clearly found that the GILC length increased with an increase in the annealing time. As a result, a wide GILC length (∼20 µm) was obtained after short time annealing (60 min) by using compressive stress stimulated low temperature (∼130 °C) GILC.

The incubation times, at which the GILC length starts to increase, are summarized as a function of the annealing temperature, as shown in Fig. 3(e). The incubation times for the samples covered with the 200 MPa compressive stressed TEOS-SiO2 cap layer are 10 times as fast as those for the samples without the TEOS-SiO2 cap layer. Furthermore, the estimated activation energy of GILC initiation for amorphous Ge on insulating substrates was calculated by an Arrhenius plot of the reciprocal incubation time. The annealing temperature range was 175 °C–250 °C. The activation energy was calculated as 0.33 eV and 0.22 eV for the samples without and with the 200 MPa compressive stressed TEOS-SiO2 cap layer, respectively. The reduction in the activation energy required for crystal nucleation is attributed to stress stimulation by the TEOS-SiO2 cap layer.

GILC propagates from the crystal nuclei around the Au pattern edge to the amorphous Ge region. Therefore, we performed EDX measurements to evaluate the GILC enhancement by the stress stimulation. Figures 4(a) and 4(b) show the EDX line scan profiles of Au atoms of the samples without and with the 200 MPa compressive stressed TEOS-SiO2 cap layer after annealing at 200 °C for 60 min, respectively. The Au pattern edges are defined as zero of the x-axis. Au atomic concentration gradually decreases from the Au pattern edge toward the Ge region. This result indicates that Au atoms diffused into the Ge region in both samples without and with the compressive stressed TEOS-SiO2 cap layer. In particular, when compressive stress was utilized, it was found that the Ge region contains more Au atoms, as compared to the sample without the TEOS-SiO2 cap layer. It suggests that the critical temperature of GILC can be decreased due to the weakening of the Ge–Ge bond strength by compressive stress stimulation.

FIG. 4.

EDX line scan profiles of the samples (a) without and (b) with the compressive stressed TEOS-SiO2 layer and (c) diffusion coefficients after annealing at 60 min.

FIG. 4.

EDX line scan profiles of the samples (a) without and (b) with the compressive stressed TEOS-SiO2 layer and (c) diffusion coefficients after annealing at 60 min.

Close modal

Moreover, it is known that the diffusion species concentration depends on the error function, the solution to Fick’s second law. We have examined the numerical calculation for the EDX line scan profiles using the error function. The estimated diffusion coefficients after annealing for 60 min are summarized as a function of the annealing temperature, as shown in Fig. 4(c). It is found that the diffusion coefficient is increased by using the compressive stressed (200 MPa) TEOS-SiO2 cap layer. Thus, GILC lengths are expanded because the Au atoms easily diffused far away from the Au pattern edge. Therefore, the Au induced lateral crystallization of amorphous Ge on insulating substrates was significantly enhanced by compressive stress.

We have investigated the stress stimulation effect on GILC of amorphous Ge on insulating substrates. The results demonstrated that the GILC of amorphous Ge on insulating substrates was significantly enhanced by using the residual compressive stress of TEOS-SiO2 films. Furthermore, the critical annealing temperature to cause GILC was decreased to 130 °C. This enhancement was attributed to the weakening of the Ge–Ge bond strength and the ease of Au atom diffusion into the Ge region. These results suggest that the low temperature formation of crystalline Ge becomes possible below the softening temperature of low-cost insulating flexible substrates.

Part of this work was supported by JSPS KAKENHI, Grant No. JP 17K06363.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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