Green and blue emission from stain-etched porous silicon

Visible light emission is reported from boron doped p+ type porous silicon films. These films were prepared by stain etched method. In this method KOH pallets were used in de-ionized water at 75°C for etching silicon samples. Blue and green light area observed from these porous silicon samples. The visible photoluminescence originates from direct transitions between energy levels in the quantum wells.


INTRODUCTION
The surface morphology of photoluminescent porous silicon has turned the much attention to its electronic and optical properties [1][2] . The discovery of photoluminescence in porous silicon 3 at room temperature generated its use in optoelectronic industries. The increasing use of silicon in integrated circuits and their techniques have been developed. The restriction on silicion over GaAs in making optoelectronic devices is due to indirect band gap of silicon. So efficient light emitting devices have not been achieved in silicon technology. Therefore an extensive research on formation of porous silicon and observation of efficient visible light at room temperature have been going on since last decade. Rather photoluminescence, electroluminescence 4 is achieved at room temperature from porous silicon based optoelectronic devices. The formation of porous silicon in hydrofluoric acid electrolyte under anodic bias was reported first by Turner 5 and Uhlir 6 . The anodic oxidation is widely used technique for preparation of porous silicon [7][8][9] . However electrodeless (stain-etching) techniques [11][12] can also be used to grow porous silicon layers. Both thickness and structure of pore size can be controlled with anodic oxidation technique. This controls the electrical and optical behaviour 10 of porous silicon layers. In this letter, we present a novel nano engineering technique of porous silicon preparation and optical properties of the formed porous silicon films. This new technique enable us to grow PS from vapors etchant instead of anodic oxidation. This technique can be used for etching the selective area of surface of semiconductor. It is an electrodeless technique for making less thicker layer compared to anodic oxidation. This technique presents a new nano structure for optical and electrical properties of interest of porous silicon. The capability of fabricating electroluminescence and photoluminescence devices, the pattern of nano-structured silicon is very important for application in flat panel display devices and monolithic integration. The electroluminesence is reported from stain-etched (HF:HNO 3 :H 2 O mixture) optical devices by Xu 11 . In this paper we present green and blue emission under direct UV light illumination.

Fabrication
Porous silicon samples were prepared by gas enchant method. In this method silicon wafers with boron doped P + type (100) of resistivity 10-15 -cm were used in 80% KOH solution (100 ml demonized water + 80 gm KOH pallets). The whole system is kept in temperature controlling bath at 75°C. A telfon cell used in order to expose silicon wafer surface to vapors evaporating from KOH solution as shown in fig. 1. expected that the origin of these photoluminescence is from 'S' shaped quantum wires spread over the exposed surface. These 'S' shaped structure, called quantum wires, are observed to run perpendicular to surface and a morphology of interconnected pores and quantum wires is found. The transition energy (E) for green colour is ~2.27 eV with 70% measured porosity while transition energy (E) for the colour with 80% porosity is ~2.8 eV. It is also expected that the photoluminescence originates from transition in degenerate 13 energy levels in the quantum wires. The direct transition dominated mechanism is belived. It is known that the different wavelengths are emitted from samples of different porosities and size of quantum wires in porous silicon. Table -1 shows the combination of photoluminescence and quantum wells with porosity. The dimensions of the observed surfaces are summarized in the table 1. The dimension of sample-1 and sample-2 are compared, the quantum wells of small width are observed form sample-2 and it is noted that the quantum wells of 150 nm width are observed for sample-1, and quantum wells of 50 nm width are for sample-2. However the lateral extension of the surface remain almost constant. So after being exposed to silicon surface, vapors are constantly exhausted from the surface. The growth rate is about 50nm per minute. A UV lamp (365 nm) is kept at 30cm distant. During of porous silicon, the UV light exposure makes PL intensity stable. On decreasing the percentage of KOH solution, a monhomogeneous porous film is obtained. So to require a homogeneous layer of porous silicon the percentage of KOH solution can be increased. In this method the etching rate is less but a smooth and homogeneous PS layer is achieved. The samples were then rinsed in de-ionized water and dry with nitrogen. All measurements were carried out at room temperature under-ambient atmosphere.

RESULTS AND DISCUSSION
A weak photoluminescence of green (546nm) and blue (450nm) from porous silicon wafers were observed under excitation of UV lamp at the end of porous silicon formation. The formation mechanism of both samples was identical. The peak maxima of both samples are found at 546nm and at 450nm, which presented in fig. 5 and fig. 6. It is A blue of shift is observed as porosity is decreased. The photoluminescence spectrum taken from porous silicon films under UV (365) excitation has emission bands peaked at ~546 nm and at ~450nm. The recombination mechanism of luminescence is proposed by almost everyone. On the basis of experimental studies of photoconductive 14 properties of porous silicon as anodized PS excited with UV light emit efficient visible photoluminescence and emission of electroluminescence at room temperature is Tyagi & Pathak, Mat. Sci. Res. India., Vol. 4(2), 501-504 (2007)

Fig. 1: Schematic diagram of a gas etchant cell. In this silicon surface is ethed by fumes
observed by applying forward bias voltage, so there exist a correlation between the porosity 15 of porous silicon and photoluminescence. The results indicates that some carries are influenced by UV illumination and also, it enhances the PL intensity of emitted light to some extent. This silicon nano structure is obtained by gas etchant, core radius ionic cell is ~5 nm and therefore possibility of defect generation is negligible. This indicates that both green and blue emission might come from quantum wires via transition in conduction band.
However photoluminescence intensity could be enhanced. Although radiative recombination centers can be generated during illumination but these have no effect on photoluminescence intensity. It remains almost constant during experiment as shown in fig.1. The intense blue emission from carbon-plasma implanted porous silicon reported silicon reported by Liu 16 et al., but a sustainable and stable intense photoluminescence is questionable. However the photoluminescence are confirmed by quantum wire model. To the best of our knowledge direct monochromatic visible light silicon sources are not present. A need of novel quantum wire engineering is to be developed to achieve low visible wavelength.

CONCLUSION
We reported porous silicon films fabricated with KOH fumes as etchant by exposing silicon wafer surfaces. The prepared films exhibit weak intense green and blue emission. This emission of visible light is due to transition mechanism among the energy levels in quantum wires. The high porous surfaces shows the blue shift. If emission of stable and intense visible light would be achieved from porous silicon, the crystalline silicon become the heart of optoelectronic industries. For this a quantum mechanical study is under way and would be presented in future.