Carrier excitation dynamics in black silicon nano pillar arrays

Black silicon has attracted a great deal of interest for its promising photonic applications and exciting physical properties. Several approaches have been used to demonstrate the possibility of producing black silicon with CW light emission, but the investigation of a detailed radiative dynamical properties of the recombination process is still lacking. Here, we present ultrafast radiative recombination phenomena from black silicon consisting of quantum pillars produced by plasma ion etching. An ultrafast blue luminescence component competing with non-radiative recombination at surface defects was identified as no-phonon recombination process. This component involves two decay processes with a peak energy at around 480 nm, which have the fast component of about 10 ps followed by a component of around 50 ps decay time constant. The emission exhibits slow component in red spectral region with time constant ranging from 1.5 to 2.5 ns. When the surface of nano pillars is smoothed, the slow component at around 600 nm is enhanced to the detriment of blue-green emission, increasing the lifetime of carriers within the Si core of the quantum pillars. This process results in a slower sates assuming a 3-component exponential decay as measured by Streak camera. The ultrafast PL decay leads to a transfer of carriers to long-lived defect states as evidenced by a red emission at around 2 eV. The results are interpreted through the presence of quantum confinement at the tip regions of the pillars and surface defects originating from the oxide environment surrounding the nanometer size pillars. long lived Time-correlated single counting nanosecond these considering

electrical properties offering a great potential for promising applications for advanced scientific research subjects 8- 10 . For solar energy harvesting applications through photovoltaic effect, black silicon surface is a perfectly compatible material with silicon solar cells providing a natural advantage with its ease of fabrication 11 . Solar cells fabricated using b-Si exhibited conversion efficiencies ranging from 10% up to 19% depending on the fabrication method [12][13][14][15][16][17][18][19] . Not only capturing the solar spectrum but also turning these pillars into field effect devices can offer a great advantage for sensor applications 20 . Light trapping can enable the realization of the photothermic conversion devices 21 . Isolating particles of interest within the forest of quantum nanosized pillars or using surface functionalizing techniques enables us to enhance the sensitivity while providing an advantage of filtering the particles to be detected 8, 20 . Black silicon may find applications in imaging and micro-electro-mechanical systems (MEMS) as active and passive micro and nanostructured semiconductor quantum structures 22,23 .
The surface formed on silicon wafers by above mentioned techniques consists of tapered pillars presenting quantum size structures down to a nanometer size tips 4 . It is interesting to see that the surface so obtained soaks all the visible light making the surface appear as a deep black color. Nevertheless, all those pillars were found to be crystalline as we have already demonstrated through transmission electron microscope (TEM) analysis 4 . However, the surfaces of these pillars are not atomically flat, representing some surface defects 3 like structure, missing atoms and oxidation induced capping regardless of wafer orientation. The presence of oxide cap layer or the oxidation of the surface of the pillars was also demonstrated by Fourier Transformed Infrared (FTIR) measurements of vibrational spectra. The presence of stretching modes of Si-O-Si vibrations at around 1090 cm -1 provides a clear evidence of the oxide formation at the surfaces of the pillars.
Owing to its quantum size surface structure, we have already reported an efficient continuous wave (CW) photoluminescence (PL) activity in a broad spectral range going from visible to near infrared region 4 . CW PL exhibits wafer (p-type or n-type), crystal structure and temperature dependent features both in the visible and in the near infrared region. In litterature, one can find a number of studies on time-resolved photoluminescence of silicon microstructures and nanostructures 24,25 . Despite several studies on CW photoluminescence and time-resolved PL on micro and nanostructures, there is no study on the radiative recombination dynamics of the quantum pillars in black Si. This work aims at filling this gap of exciting scientific and technological interest in such a surface.
Transformation of silicon surface to black silicon nanometer size pillars. The b-Si quantum pillars or whiskers were fabricated by a reactive ion etching (RIE) of thermally oxidized and photoresist (named as PMMA) coated 3 inch size p-type Si wafers having <100> and <111> crystal orientations as reported earlier 4 . Wafer resistivities used in the fabrication of b-Si in our work were around 10 Ω-cm. For the RIE of these wafers, we used chlorine plasma, which has led to the formation of quantum sized pillars on the surface of the Si wafer. Following this process, the wafer surface becomes black as appearance, since quantum pillars absorb all the white light turning the surface of the wafer to a deep black color.

High Resolution Transmission Electrom Microscopy (HRTEM) images. TEM studies on these wafers
have shown that wafer surface consists of Si quantum pillars or whiskers type of extensions, which are aligned vertically to the wafer plane. As evidenced from the atom interference fringes observed in TEM as shown in Fig. 1a, these pillars are crystalline and their dimensions can be down to few atoms thick tips. On 4 their surface, some missing atom positions resulting in a certain irregular surface profile is visible. As shown in Fig. 1b their length can be up to 200 nm long with some core-shell like structure having a crystalline Si core, which is encapsulated by a surface oxide 4 . Within the Si core, regular atomic planes with inter atomic distances of 0.34 nm can be measured and the surface irregularities due to missing atoms are also visible. FTIR measurements in Fig. 1d indicate there is a native oxide on these pillars as evidenced from a relatively strong Si-O-Si absorption band related silicon-oxygen-silicon stretching vibrations at 1085 cm -1 . In order to determine the role of the oxide on the quantum pillars we have exposed the pillars to the vapor of HF:HNO3 acid mixture 30 . The results are summarized in Fig. 2b, which shows the streak camera image for the treated b-Si sample. As shown in the image, the radiative recombination line peaks at around 100 ps and decays very fast right away from the ultra-fast emission line. The slower decay time in this surface smoothining process is probably indicative of the removal of defects on the surface of the pillars as evidenced from the slower decays in ultrafast radiative recombination regime.
Recombination dynamics. In Fig. 3, we observe the temporal and spectral evolution of the radiative recombination at 300K. The Fig.3a shows the amount of photon counting as a function of the decay time in picosecond at different wavelengths as deduced from the Fig.2. TRPL spectra were simulated by a curve (a) (b) 10 fitting program resulting in three components decay times as typical dynamical behavior of b-Si samples.
For fitting the following expresssion was found to be well describing the luminescence decay  Fig. 4b as an insert.

Smoothing of nano pillar surfaces.
It has already been shown that the exposure of silicon rods to the vapor of an acid mixture containing HF and HNO3 has the effect of smoothing the surface of the rods 30 . Similar effects are expected for the b-Si quantum pillars. Therefore, the PL decay measurements have beeen performed also on b-Si quantum pillar samples, which were subjected to the surface smoothing procedure as described earlier 30 . This procedure removes some of the oxide from the shell region on the pillar surfaces. 12 The results obtained from these investigations on such samples are displayed on Fig.4  Time-correlated single photon counting. Figure 2 shows the time-resolved PL dynamics spectrally integrated from 450 nm to 600 nm for a black-Si produced on Si(111) as a function of decay time. The solid lines indicates a global fitting of the PL dynamics to a triple exponential decay functions. As indicated in The left hand side is the silicon core where the blue-green emission at around 2.5 eV occur due to the confinement effect; the middle part indicates traps involved recombination due to localized states between the core and the surface oxide; the right hand side is the red recombination originating from the oxide around the pillars.
Non-radiative recombination at traps was shown to be playing a significant role in photoluminescence decay in semiconductors 4 . Threrefore, the intermediate decay component of around 50 ps could be attributed to the presence of traps at the surfaces of quantum pillars. Radiative or non-radiative recombination at these 16 traps are expected to play a significant role in determining the magnitudes of the decay time components in b-Si. While the non-radiative recombination reduces the emission intensity, the radiative one contributes to the spectral region between the blue and the red region.
From these results, we observe that the intensity of light emission at around 480 nm resulting from the ultrafast decay is rather weak. The reason of this can be the following: 1) quantum size concerns only the tip regions of relatively small volume as compared to the whole pillar volume, 2) electron-hole pair generated at the tips of the quantum pillars diffuses fast away from the tip regions to trap states or larger dimensional parts of the silicon core thus reducing the rate of the quantum confined radiative recombination of carriers. These processes are likely at the origin of the weak emission at around 480 nm. The most of the excitation light is absorbed in the remaining parts of the pillars thus contributing light emission originating from the slow states at around 600nm. A typical shape of a pillar can be assumed to be a cone with a height of about 250 nm and with a cone diameter of around 100 nm as shown at the insert in Fig. 1d. The surface area of such a pillar can be estimated from = , where r and x are the radius and the length of the lateral surface, respectively.
Using these parameters, the surface area of the pillar can be estimated to be ≈ From these studies, we have shown that silicon nano pillars in black silicon exhibit exciting optical properties, which offer promising application possibilities in ultra large scale integration of sensors, memory, switching devices, imaging, energy harvesting, quantum hardware fabrication for quantum technologies and optical information processing platform integrating near infrared optoelectronic components.