A new approach for the fabrication of n-type porous silicon layer is proposed. A hole-rich p-layer is arranged underneath the n-layer, and the np-junction is under forward biased condition in the etching process. Therefore sufficient holes can drift straight-upward and pass across the np-junction from p-region to n-region to participate in electrochemical reaction during the etching process with an unfailing supply. Illumination is an optional hole-supplier in this approach, so the problem of illumination-depth limitation can be overcome. Strong visible photoluminescence emissions are demonstrated on the hole-poor n-type porous layer at about 650 nm.
©2006 Optical Society of America
As we know, porous silicon (PS) structures consisting of many pores and pillars are widely used to yield efficient visible photoluminescence (PL) at room temperature [1,2]. Such light-emission behaviors are primarily attributed to electron confinement in the nano-crystals that constitute the porous structure [3–5].
The techniques of PS formation have been developed by electrochemical anodization [4,6,7], stain etching [8–10], spark-erosion  and vapor etching methods [12, 13]. Among them, electrochemical anodization is the most commonly used. The electrochemical etching is performed in a hydrogen fluoride (HF) solution. As to the etching rates, they are controlled by adjusting the electrolyte compositions and etching current densities. It is well known that the etching current is governed by the hole concentration (accumulation) in the adjacent regions of HF electrolyte and Si atoms to assist anodic oxidation during the electrochemical etching .
For p-type Si, the holes are the majority charge carriers, so the p-type PS (p-PS) layers are fairly easily produced. Conversely, n-type PS (n-PS) is very difficult to form, as a result of the lack of holes. Previously, most of the research done on PS has been made on hole-rich p-type Si. However, the control of n-PS is always required, for example, in light emitting diode (LED) technology and microelectronic technology [15,16]. Up till now, to get an n-PS layer, illumination has still been the popular way (even indispensable way) to generate the holes required in the electrochemical etching process on the hole-poor n-type samples [17, 18].
Yet, the actual photo-energy absorbing by Si atoms is very sensitive to the illumination-source intensity and electrolyte environment, and the conditions are not constant in the process. Since the illumination intensity is related to the distance besides the absorption of the light in the device itself, only top-layer atoms are photo-excited and only the surface layers under illumination generate the electron-hole pairs. The etching rate will gradually decrease with depth from top- to bottom- layers because the illumination is very difficult to reach the deeper layers. Therefore, cone-shape pores in PS structures are often formed by this way . In other words, the only-illumination-assisted etching approach is depth-limited. The illumination related factors and limitation may affect the PS morphology and formation. Besides, it is also known that the n-PS obtained under illumination usually consists of a nano-PS layer and macro-PS layer. . Macro-PS is difficult to show room temperature light emitting in the visible range. 
In this paper, a new approach (named as bottom-hole-assisted approach) is proposed for manufacturing n-PS layers. A mount of holes are supported by a forward biased np-junction in which the p-layer plays the role as a hole-source underneath the n-layer. For the hole-assisting direction, it is bottom-up and straight-upward. Therefore, no depth-limitation should be taken into consideration in this approach. The pillars and pores are narrower, straighter, deeper, and more condensed than those prepared by the conventional only-illumination-assisted approach. The proposed bottom-hole-assisted approach also can be superimposed on the illumination-assisted approach in the electrochemical anodization (named as superimposition-assisted approach).
The schematic diagram of the experimental setup for preparing n-PS layer is given in fig. 1(a). The main body of the HF electrolyte container is made of Teflon materials. And, the apparatus is designed in a vertical arrangement. The sample is face-up so as to easily remove the hydrogen bubbles from the PS surface. In the bottom-hole-assisted approach, the illumination source is optional (not indispensable).
Also, the PS samples are prepared on Si (100). Here a mixture of HF and C2H5OH is utilized as the etching solvent. The illustrative equation of the overall process during PS formation can be expressed as below :
In the equation, the etching rate is determined by the hole (h +) generation.
Figure 1(b) shows the case of difficulty in electrochemical etching in n-Si sample without any hole-assistance (illumination-assistance and bottom-hole-assistance). When the assistance of illumination is applied, as shown in Fig. 1(c), the electron-hole pairs are generated and accumulating only on the top surface layers in which some tiny lateral etching accompanies with the vertical etching. On the other hand, in the bottom-hole-assisted approach as Fig. 1(d), an electrical field across the anode and cathode forward biases the np junction, and holes flow straight-upward from p- to n-region. Sufficient holes can reach to the pore tips to participate in chemical reaction during the etching process, even in the dark. The top view and cross section view of PS films are taken using scanning electron microscopy apparatus (SEM, JEOL JSM-6335FNT). In addition, the PL properties are measured on the PL setup (He-Cd laser and a Hamamatsu R928 photomultiplier detector).
3. Results and discussion
PS layers are successfully fabricated by the bottom-hole-assisted approach with or without illumination-assistance. The top view and cross-section SEM images of the PS samples are shown in Figs. 2 and 3, respectively. Two PS samples with n-epilayers on n-substrates (n-on-n) and four PS samples with n-epilayers on p-substrates (n-on-p) are prepared, and named as samples A, B, C, D, E and F, respectively.
In sample A (n-on-n), not any hole-assistance is applied on the sample, and then the electrochemical reaction can not successfully proceed. The PS layer fails to be formed on sample A. For the conventional case based on the only-illumination-assistance, it is applied on sample B (n-on-n). In sample C (n-on-p), which is prepared by bottom-hole-assisted approach, the hole-supplying relies on the p-substrate without any illumination. In samples D, E and F (n-on-p), the illumination-assistance and bottom-hole-assistance are simultaneously superimposed (superimposition-assisted approach). Furthermore, etching solution ratio of C2H5OH: HF=2:1 is used in samples A, B, C and D. In samples E and F, the ratios of 4:1 and 6:1 are used, respectively.
In the samples with illumination-assistance (samples B, D, E and F), the macropores clearly appear on the surface. The surface pores are cone-shape (V-groove), especially even clearer in sample B. In sample B, the electron-hole pairs only rely on the top-surface layers by illumination. After surface pores (of several ten um) are formed, photo-energy is not completely received by the deeper layers. Therefore the deeper layers are illumination-limited and the PS pores are difficult to be formed . A lateral etching keeps on while the vertical etching is proceeding, hence crosswise-branched pores and tissue-like branch structures are formed.
Conversely, in sample C which is based on the bottom-hole-assisted approach, the hole-supplying path is bottom-up and no depth-limitation. The pore shapes are narrower, deeper and straighter. Also, high-aspect-ratio nano-scaled pores (aspect ratio=300) can be obtained, and the pore structures are channel-like instead of tissue-like that always occurs in illumination-assisted approach. As shown in Fig.3, the pore depths of the PS layers can be up to 100 µm and the pore width can less than 300 nm. It is useful to serve as thick-layer light emitting applications. To enhance the etching rate and etching depth, superimposition-assisted approach is applied on samples D, E and F.
An excitation wavelength of 325 nm from a He-Cd laser is used for the PL measurements. Figure 4 shows the PL spectra of PS samples A~F. Clearly, the superimposition-assisted approach can produce a high PL intensity. Also, it means that the bottom-hole-assisted approach can effectively improve the conventional only-illumination-assisted approach.
The measured PL that is usually attributed to quantum size effects is a combination result of light emissions from upper layer (macroporous) and lower layer (nanoporous). Thus no significant size dependence of the PL peak energy can be easily observed and concluded. The experimental results show that the PL intensity increases with the increasing of HF concentration. The naked-eye photos of the PS samples under ultraviolet (UV) light at room temperature are also shown on the Fig.4. Obviously, the PS samples produce a strong emission of orange-red luminance under UV illumination.
In conclusion, a new bottom-hole-assisted approach based on a forward biased np-junction for manufacturing n-PS layer is proposed. Illumination is an optional hole-source in the fabrication of n-type PS. The bottom-hole-assisted approach can overcome the illumination-limitation and depth-limitation problems in conventional only-illumination-assisted approach. With the bottom-hole- assistance, the anodization etching is almost anisotropic. This feature has great potential in fabricating optoelectronic and electronic devices, photonics and chemical sensors.
In the study, clear visible PL emissions at room-temperature are obtained on the hole-poor n-type samples which are fabricated by the new proposed approach even in the dark. Also, the visible luminescence of the n-PS can be drastically enhanced by the superimposition of bottom-hole-assistance and illumination- assistance. This method can be used to improve the performance of photovoltaic devices that convert higher energy into visible wavelength light.
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