1. Introduction

Visible photoluminescence property from germanium (Ge) nanoparticle at room temperature has been reported over years even though bulk Ge is an indirect band gap semiconducting materials [1–6]. Quantum confinement effect plays an important role in optical absorption and luminescence in nano-scale particles and structures of semiconductors [6]. Such a nanostructure is conventionally fabricated by dry fabrication method: electric spark processing [6], gas evaporation [7], rf magnetron cosputturing [1, 2], cluster-beam evaporation [8], and so on. In recent years, laser ablation is used for many application system and production of local nanostructures [9] on specific area with a high spatial resolution. These applications include micro-structuring of three dimensional features as a component for photonics and preparation of nano size structures from metallic silver [10]. The preparation of nanostructure surface of germanium by femtosecond laser irradiation of a single crystal was already reported [11–13]. Bulk Ge exhibits two different ablation thresholds as resulting in amorphous layer on the surface of exposed area. At the second threshold fluence, especially nanoparticle dangled on the irregular microstructures of Ge was massively formed upon photoexciation with single fslaser pulse [11]. The processed surface was also found to be photoluminescent at room temperature. Although the mobility of holes in germanium is larger than for any other common semiconductors, it has still a limit to use because of difficult processing. Furthermore in optoelectronics for advanced display systems, microelectronics, and micro-and nanobiosensors, local creation of nanostructure is critical to use, which is an advantage of femtosecond laser irradiation on materials.

In this work, we investigate the effects of the processing conditions on the formation and its optical properties of the Ge nanostructures. We observe that the environment conditions during the processing with various ambient gases play an important role in encapsulation of Ge nanostructures with an oxidized layer as well as decision of their size.

2. Experiments

Photoluminescent Ge nanoparticles are prepared by femtosecond laser ablation of undoped (001) germanium single crystal with diamond cubic structure. The sample preparation method has been already described in detail elsewhere [11]. A 150 fs laser pulse (Quantronix, USA) at the wavelength of 800 nm is irradiated on germanium substrate. We make various ambient environments reducing pressure to about 10 mtorr using chamber, and pure nitrogen gas conditions are with different pressures of 0.7 and 4.6 atm. To prevent an effect resulted from following successive pulse, single shot configuration is adapted in the current work by using a fast mechanical shutter. We characterize the surface of processed Ge with atomic force microscopy (AFM, PSIA, Korea) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM 3010) with a cross-sectional technique. The size of processed Ge nanostructures is estimated from the AFM images with image processor (Image J, Wrightm). In addition, Raman scattering signals are collected with a backscattering geometry at the excitation wavelength of 488 nm and dispersed by a double monochromator (Jobin-Yvon U-1000) at room temperature. We remove the excitation laser light efficiently by inserting a holographic optical filter (Kaiser, USA). The laser power and the spectral line width are kept less than 4 mW and 2 cm^-1, respectively [14]. The time-resolved photoluminescence measurement is performed using a time-correlated single photon counting (TCSPC) system consisted of a pulsed picosecond dye laser and a micro-channel plate photomultiplier (MCPPMT). Frequency-doubled laser output at 290 nm is implemented to the photoexcitation of the processed samples [15].

3. Results

An AFM image of Ge surface processed in air is shown in Fig. 1(a). There are many nanostructures with the size of several tens of nanometers on the microstructures formed by femtosecond laser irradiation. Beyond the region shown in Fig. 1(a), whole surface of processed Ge reflects the spatial profile of the laser pulse directly as volcanic shape and numerous hemispheres are distributed on that. Figure 1(b) shows cross-sectional TEM image of processed Ge nanoparticle, which apparently shows the crystal structure of germanium and amorphous layer with several nano meter thick.

 

Fig. 1. (a) AFM image of surface of ablated Ge which contains numerous nanoparticles. The nanoparticles are distributed within 10–30 nm size. The TEM image (b) reveals existence of several nano meter oxide layer on the surface of the formed nanoparticle. (c) EDS (energy dispersive spectrometry) spectrum of Ge surface after processing shows oxide component besides Ge. (d) The measured photoluminescent spectrum from Ge nanoparticles, which are processed with different laser fluence, shift toward longer wavelength as fluence increase.

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Since the energy dispersive measurement in Fig. 1(c) reveals the appearance of the GeO_x under only fs-laser irradiation, it is reasonable that the chemical composition of the observed layer from HRTEM is mainly due to the formation of thin GeOx films on Ge nanostructures. We study in detail the dependence of photoluminescent peaks on the laser fluence from 4.9 J/cm 2, to 24.5 J/cm ² in air of which value is above 0.492 J/cm ² [16] for undoped germanium threshold of ablation. The photoluminescence at room-temperature is obtained with processed Ge upon photoexcitation at 325 nm. It is interesting to note that the spectral feature of the photoluminescence exhibits apparent peak shift to the longer wavelength.

To confirm that the appearance of GeO_x layers formed by surface oxidation reaction, which should be responsible for the room-temperature photoluminescence, we have exposed the germanium surface in vacuum less than 1 mtorr to the femtosecond laser pulse with a fluence of 9.8 J/cm ². Figure 2 shows photoluminescent spectral features observed just after processing within 10 minutes. Two sharp peaks observed in the spectrum Fig. 2(a) are due to not photoluminescence from the sample but a background light during PL measurement.

 

Fig. 2. (a) PL of Ge nanoparticle formed by laser ablation with various ambient surroundings: nitrogen, vacuum and air condition. (b) The PL of Ge nanoparticle processed in nitrogen condition is again measured after 24 hours. After one day oxidized Ge nanoparticle, which is processed in nitrogen condition, reveals PL in contrast with an immediate measurement as shown in (a).

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Under pure nitrogen gas environment with varying the pressure from 0.7 to 4.6 atmospheres, there is no apparent photoluminescence at room temperature. All samples prepared from the above experiments show an apparent photoluminescence at room temperature, however, if the samples have remained 24 hours under ambient conditions. Meanwhile, no other apparent changes in their surface morphology are observed by AFM works. Supposing that the oxidation of single-crystalline germanium should be hardly occurred under ambient conditions, these observations unequivocally led us to conclude that the GeO_x layer, resulted from the surface oxidation during formation of nanostructures, plays a crucial role in the room-temperature photoluminescence.

PL spectral features shown in Fig. 1 and Fig. 2 reveal that more than two different emissive states should be involved in the radiative relaxation processes of the prepared Ge nanostructures upon photoexcitation. To understand deeper on this observation, it is valuable to have further information on the radiative relaxation dynamics for PL. Figure 3(a) and (b) shows typical temporal decaying profiles of PL measured at two different probe wavelengths of 3.1 eV and 1.8 eV, respectively, and the dependence of the observed decay time constants on the probe wavelength in Fig. 3(c). The deconvolution with a measured instrumental response function of the current system reveals that the temporal profiles could be successfully simulated with a single exponential decay component with a time constant about 0.7 ns, of which value is almost invariant over all the emission range Fig. 3(c) [17]. Even though no conclusive evidence is available from the current work, it is quite reasonable to suppose that the observed PL from the prepared Ge nanoparticles in visible region should be originated from either same excited species or thermally equilibrated two different ones. Meanwhile, with changing the preparation conditions, the relative intensity for two PL peaks apparently altered. Especially, the peak in the lower energy gains its intensity with increasing the laser fluence as shown in Fig. 1(d), which has two shoulders in the spectral features.

 

Fig. 3. Time correlated single photon counting spectra at 3.1 eV (a) and 1.8 eV (b), respectively. The decay time constants are invariant at various probe wavelengths (c).

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In order to get further information on the origin of the observed room-temperature photoluminescence from the processed Ge surface, we measure the Raman spectra of the above samples and show the results in Fig. 4. The previous theoretical works [18, 19] reported that the indirect band-gap energy of Ge nanoparticle is very sensitive to the size, and further the spectral feature of Raman band is asymmetrically broadened accompanied with slight lower shifts in its peak position due to the phonon confinement effects [20–25] with decrease in its diameter. According to the reported model, the Raman spectral features with mean crystallite size d may be described by

where ω(q) denotes the phonon dispersive relation and Γ is the natural line width. It is assumed that the Brillouin zone is spherical and the phonon dispersion relations are isotropic. To estimate the mean size d of Ge nanoparticle, the first-order Raman spectrum I(ω) is only considered. The measured Raman peaks are consistent with calculated spectra and estimated mean particle size can be adapted the assumption, which is mentioned above. It should be noted that the samples prepared under vacuum result in the largest size of nanoparticle in diameter, while that formed under 4.6 atm nitrogen environments gives the smallest one.

 

Fig. 4. Raman shift from the Ge nanoparticles formed by laser irradiation in different ambient system. The peak position and broadening of Raman spectrum estimate particle size of nanoparticles. There is good agreement between the measured Raman spectra (black line) and calculated spectra (gray line) from the phonon dispersion relation.

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With careful checking the simulation results, it is reasonable to conclude that with increasing the surrounding pressures of the environment the size of resultant nanoparticle decreased. Supposing that the energy dissipation rate per every collision of the gas molecules to the excited Ge surface be quite fast and effective to be fully relaxed, the formation as well as growth time of the nanoparticles from the molten Ge layer, which is resulted from the laser irradiation, should be inversely correlated with the back-pressure. In practices, high pressure of nitrogen gas of 4.6 atm governs smallest particle formation without any oxide layer, which results in the largest shift in Raman peak as well as broadening compared to the other cases and also is consistent with observations of PL spectrum analysis.

The current observations suppose that nanoparticle size increases as laser fluence increases. Unless the size difference is significant, the quantum size effect on PL as well as Raman spectral feature occurred consistently. It is clear from the PL result of the formed Ge nanoparticles that luminescence at 2.3–2.7 eV arises from Ge being introduced into the GeO_x matrix. The TEM image in Fig. 1(b) also directly shows Ge sample contained a significant amount of GeO_x and oxides, which means that the amorphous GeO_x is to be responsible for the PL peaks.

4. Conclusions

We observe numerous Ge nanostructures on the surface of Ge single crystal by femtosecond laser ablation. The nanostructures give an apparent PL in visible region at room temperature. The measurement of PL and Raman spectrum of the samples formed under various environments and different laser fluence lead us to suppose that that the oxidation reaction plays an important role in formation of photoluminescent Ge nanostructures and its size determination.