Thermal annealing dependence of some optical properties of plasma-modified porous silicon

The photoluminescence and reflectance of porous silicon (PS) with and without hydrocarbon (CH_x) deposition fabricated by plasma enhanced chemical vapour deposition (PECVD) technique have been investigated. The PS samples were then, annealed at temperatures between 200 and 800 °C. The influence of thermal annealing on optical properties of the hydrocarbon layer/porous silicon/silicon structure (CH_x/PS/Si) was studied by means of photoluminescence (PL) measurements, reflectivity and ellipsometry spectroscopy. The composition of the PS surface was monitored by transmission Fourier transform infrared (FTIR) spectroscopy. Photoluminescence and reflectance measurements were carried out before and after annealing on the carbonized samples for wavelengths between 250 and 1200 nm. A reduction of the reflectance in the ultraviolet region of the spectrum was observed for the hydrocarbon deposited polished silicon samples but an opposite behaviour was found in the case of the CH_x/PS ones. From the comparison of the photoluminescence and reflectance spectra, it was found that most of the contribution of the PL in the porous silicon came from its upper interface. The PL and reflectance spectra were found to be opposite to one another. Increasing the annealing temperature reduced the PL intensity and an increase in the ultraviolet reflectance was observed. These observations, consistent with a surface dominated emission process, suggest that the surface state of the PS is the principal determinant of the PL spectrum and the PL efficiency.     

退火对ZnS/PS复合体系光致发光特性的影响

用脉冲激光沉积(PLD)技术以多孔硅(PS)为衬底生长了ZnS薄膜,分别测量了ZnS、PS以及ZnS/PS复合体系在室温下的光致发光(PL)光谱。结果发现,ZnS/PS复合体系的PL光谱中PS的发光峰位相对于新制备的PS有所蓝移。把该ZnS/PS样品分成三块,在真空400℃分别退火10,20,30 min,研究不同退火时间对ZnS/PS复合体系光致发光特性的影响。发现退火后样品的PL光谱中都出现了一个新的绿色发光带,归结为ZnS的缺陷中心发光。随着退火时间的延长,PS的发光强度逐渐降低且峰位红移。把ZnS的蓝、绿光与PS的红光相叠加,整个ZnS/PS复合体系在可见光区450~700 nm形成一个较宽的光致发光谱带,呈现较强的白光发射。     

稀土铒离子注入多孔硅的发光

稀土离子Er注入多孔硅中.在350keV能量,1×1012~1×1015/cm2剂量范围内,注入后的多孔硅仍保持明亮的可见光发射.退火后,在近红外区测到1.54μm附近Er3+的特征发射.其发射强度比硅单晶对照样品明显增强,实验表明这增强作用来源于多孔硅的表面发光层.电化学制备过程中在表面层中带入的O、C、F等多种杂质可能是Er3+发光增强的原因.     

Photoluminescence excitation properties of porous silicon with and without Er-Yb-containing complex

Two bands in the photoluminescence excitation spectra have been studied for the red, blue, and IR emission of oxidized porous silicon (PS) and PS with Er³⁺- and Yb³⁺-containing gadolinium oxychloride complex (PS:Er,Yb), introduced by thermal diffusion. These two spectral bands were shown to reflect contributions of two different mechanisms of excitation-emission processes. The UV band for the IR emission of PS:Er,Yb rose sharply at about 290 nm and was explained by the direct photoemission of carriers from the valence band of Si crystallites into the conduction band of the oxide shell. The second band was found to be common for the red and blue emission and assosiated with the carriers photoexcitation inside the Si crystallites. Lifetimes for both bands were measured and the structure of the blue emission from PS:Er,Yb with peaks at 416, 440, 466, and 500 nm from PS:Er,Yb was observed.     

Effect of preliminary oxidation annealing on properties of porous silicon impregnated with a tungsten-tellurite glass activated by Er and Yb

The effect of preliminary oxidation annealing of porous silicon (PS) on photoluminescence (PL) under laser pumping at wavelengths of 532 and 980 nm, EPR, and transverse current transport in structures based on PS with a fused tungsten-tellurium glass (TTG) doped with Er and Yb has been studied. It has been shown that such annealing and the presence of silicon nanocrystals (nc-Si) in PS promote multiple PL enhancement for both Er ions in TTG and nc-Si in PS at wavelengths of 750 and 1540 nm, respectively. As TTG is fused into PS, P _b -centers of nonradiative recombination are suppressed, while retaining discrete electron tunneling through nc-Si grains in PS.     

Porous silicon layers prepared by electrochemical etching for application in silicon thin film solar cells

In this paper, multilayer structures of porous silicon were fabricated by using electrochemical etching and characterized for its optical properties and surface morphology. Samples of monolayer of porous silicon were grown to study the characteristics of porous layer formation with respect to applied current density, etching time and hydrofluoric acid concentrations. Photoluminescence peaks of red emission at wavelength 695 and 650 nm were observed from multilayer porous silicon structures. By atomic force microscopy measurement, hillocks like surface were clearly observed within the host material, which confirmed the formation of pores.     

Efficient antireflective downconversion Er3+ doped ZnO/Si thin film

This study is an investigation of the potential of Er doped ZnO thin films for downconversion photons and an antireflective layer when placed in front of the silicon solar cells. We optimized the properties of the film with appropriate deposition conditions on Si (111) substrate by aerosol assisted chemical vapor deposition (AACVD) process. An enhancement of both crystallinity and optical response was achieved in the case of film doped with 2.504 at.% Er³⁺. A low reflectance and high refractive index of the film were obtained at around 632 nm. Downconversion process was also reached for this film under visible excitation to near-infrared (NIR) 980 nm photons useful for Si solar cell.     

热退火对SiO2：Er薄膜发光特性的影响

利用离子注入法制备SiO₂:Er样品,并在不同温度下进行退火处理。通过微区拉曼光谱、吸收光谱、X射线光电子能谱等手段对其进行结构表征,并进行了室温和变温的光致发光特性研究,得到了可见区和红外区的光致发光。其中,⁴S_3/2→⁴I_15/2的发光强度随温度的升高,先增强后减弱,呈现出反常的温度淬灭效应,此现象是由Er³⁺与SiO₂的缺陷之间的能量传递造成的。     

Magneto-optical study of Er-related center in selectively doped Si:Er

Photoluminescence at λ=1.54 μm from an Er³⁺-related center dominant in a sublimation MBE-grown multi-layer Si/Si:Er structure is investigated in magnetic fields up to 6 T. The magnetic-field-induced splitting is observed for all the main lines of the Er-related photoluminescence spectrum. For the most intense emission line, angular dependence of the splitting is measured in the (011) crystallographic plane of the sample. The effective g-tensor, corresponding to the difference between individual g-tensors of the lowest multiplets of the ground and the first excited states, is experimentally determined. In this way the symmetry of the Er-related optically active center dominant in the structure is found to be orthorhombic I (C_2v). From temperature dependence of the intensity of the magnetic field split components, individual g-tensors of the ground and the excited states are separated. No influence of the growth direction on the symmetry of Er-related center was found.     

Improvement of the luminescence in p-type as-prepared or dye impregnated porous silicon microcavities

We have realized distributed Bragg reflectors and microcavities with a remarkable optical quality (R_max.=99.5% at 850 nm, FWHM=5 nm at 772 nm) with low doped p-type silicon. This is due to a strong decrease of the porous Si/bulk Si interface roughness that was obtained by low-temperature anodization. The properties of porous silicon microcavities are investigated by photoluminescence and reflection measurements. We also have filled porous silicon with Rhodamine 800 dye. The spontaneous emission spectrum of the optically excited Rhodamine 800 is drastically modified by microcavity effect: the peak emission intensity is increased, the line width is narrowed. The results demonstrate that using all porous silicon or dye-filled microcavities provides new possibilities to improve the properties of photonic devices.     

Comparative study of the oxidation of thin porous silicon layers studied by reflectometry, spectroscopic ellipsometry and secondary ion mass spectroscopy

We present a systematic study on ultrathin porous silicon (PS) layers (40–120 nm) of different porosities, formed by electrochemical etching and followed by thermal oxidation treatment (300°C and 600°C) and by electrochemical oxidation. The oxidised and non-oxidised PS layers have been analysed by spectroscopic reflectometry (SR), spectroscopic ellipsometry (SE) and secondary ion mass spectroscopy (SIMS). The SR and SE spectra were fitted by a multiparameter fit program and the composition and the thickness of the PS layers were evaluated by different optical models. PS layers, formed electrochemically in the outermost layer of a p/n⁺ monocrystalline silicon junction were successfully evaluated using a gradient porosity optical model. The non-oxidised PS, formed in p-type silicon, can be well described by a simple optical model (one-layer of two-components, silicon and voids). The spectra of the oxidised PS layers can be fitted better using an optical model with three interdependent components (crystalline-silicon, silicon-dioxide, voids). The SIMS results give a strong support for the optical model used for SR and SE.     

Donor-state-enabling Er-related luminescence in silicon: direct identification and resonant excitation

We conclusively establish a direct link between formation of an Er-related donor gap state and the 1.5 microm emission of Er in Si. The experiment is performed on Si/Si:Er nanolayers where a single type of Er optical center dominates. We show that the Er emission can be resonantly induced by direct pumping into the bound exciton state of the identified donor. Using two-color spectroscopy with a free-electron laser we determine the ionization energy of the donor-state-enabling Er excitation as E(D) approximately 218 meV. We demonstrate quenching of the Er-related emission upon ionization of the donor.     

Optical amplification in Er/Yb silicate slot waveguide

Active slot waveguides were fabricated by embedding low-index Er/Yb silicate material in high-index silicon. A 1.7 dB signal enhancement at 1.53 μm in a 6 mm-long slot waveguide was observed through 1476 nm pumping. The peak Er emission cross-section is determined as 7.53×10(-21) cm2 and the excited Er ion fraction is 0.17. Our experiment shows that the defects in upper c-Si of Si-on-insulator (SOI) and deposited α-Si distorts photoluminescence spectrum and prevents further optical amplification. This negative effect can be partly corrected through annealing treatment, which allows better propagation of the pump light, therefore, stronger excitation in the sandwiched Er/Yb silicate. The defects also affect the 1.53 μm decay curve and are the dominant lifetime reduction mechanism in the active slot waveguide.     

INFRARED EMISSION FROM Si IMPLANTED WITH HIGH Er CONCENTRATION

Erbium-doped silicon has been fabricated by ion implantation performed on a metal vapour vacuum arc ion source. After rapid thermal annealing (RTA), 1.54μm photoluminescence was observed at 77K. Rutherford backscattering spectrum indicated that Er ions are mainly distributed near the surface of the samples, and Er concentration exceeded 10²¹cm^-3. Needle nanometre crystalline silicon (nc-Si) was formed on the substrate surface. Band edge emission spectrum at 10K verified that the minority carrier lifetime increased upon RTA. The photocarrier mediated processes enabled energy transferring from nc-Si (or c-Si) to the Er³⁺ ions and resulted in light emission of 1.54μm.     

Effects of boundary reflection on photoacoustic spectrum of porous silicon

Photoacoustic (PA) amplitude and phase spectra are studied on porous silicon (PS) samples. For the sample with a thinner PS layer and a rough interface observed by field-emission scanning electron microscope (FE-SEM), PA amplitude decays rapidly at short wavelengths but stays at a higher level above 650 nm compared with a sample with a thicker PS layer and a smooth interface. In this latter long-wavelength region, phase delay for the former sample is smaller. A model calculation for the two-layer model taking account of scattering of light in the porous media and interface reflection of light gives at least a qualitative explanation of these differences. Received: 30 June 1999 / Accepted: 11 October 1999 / Published online: 23 February 2000     

Er Doping conditions of planar porous silicon waveguides

EDX and infrared photoluminescence (IR PL) analyses performed on erbium-doped porous silicon waveguides (PSWG) were studied using different doping conditions. Both parameters of the cathodisation electrochemical method used for Er incorporation and parameters of thermal treatments required for Er optical activation were taken into consideration. Firstly, by varying the current density and the time of cathodisation, we have shown that a current density of 0.1 mA/cm² for 10 min allows homogeneous Er doping to be achieved throughout the depth of the guiding layer. Then, the PL intensity at 1.53 μm was studied as a function of the oxidation time at 900 °C and Er diffusion temperature for 60 min. Increasing the oxidation time up to 1 h allows PL to be enhanced due to active Si-O-Er complex formation whereas an oxidation time of 2 h induces a decrease in PL because of Er segregation. Moreover, an increase in the diffusion temperature induces an optimal distribution of optically active Si-Er-O complexes inside the crystallites. When the temperature is too high, a PSWG densification and Er segregation occurs inducing a decrease in PL due to energy transfer phenomena.     

Photoluminescence studies of porous silicon microcavities

Narrow photoluminescence peaks with a full-width at half-maximum of 14–20 nm are obtained from porous silicon microcavities (PSM) fabricated by the electrochemical etching of a Si multilayer grown by molecular beam epitaxy. The microcavity structure contains an active porous silicon layer sandwiched between two distributed porous silicon Bragg reflectors; the latter were fabricated by etching a Si multilayer doped alternatively with high and low boron concentrations. The structural and optical properties of the PSMs are characterised by scanning electron microscopy and photoluminescence (PL). The wavelength of the narrow PL peaks could be tuned in the range of 700–810 nm by altering the optical constants.     

Improved performance of a crystalline silicon solar cell based on ZnO/PS anti-reflection coating layers

A porous silicon (PS) layer was prepared by photoelectrochemical etching (PECE), and a zinc oxide (ZnO) film was deposited on a PS layer using a radio frequency (RF) sputtering system. The surface morphology of the PS and ZnO/PS layers was characterised using scanning electron microscopy (SEM). Nano-pores were produced in the PS layer with an average diameter of 5.7 nm, which increased the porosity to 91%. X-ray diffraction (XRD) of the ZnO/PS layers shows that the ZnO film is highly oriented along the c-axis perpendicular to the PS layer. The average crystallite size of the PS and ZnO/PS layers are 17.06 and 17.94 nm, respectively. The photoluminescence (PL) emission spectra of the ZnO/PS layers present three emission peaks, two peaks located at 387.5 and 605 nm due to the ZnO nanocrystalline film and a third located at 637.5 nm due to nanocrystalline PS. Raman measurements of the ZnO/PS layers were performed at room temperature (RT) and indicate that a high-quality ZnO nanocrystalline film was formed. Optical reflectance for all the layers was obtained using an optical reflectometer. The lowest effective reflectance was obtained for the ZnO/PS layers. The fabrication of crystalline silicon (c-Si) solar cells based on the ZnO/PS anti-reflection coating (ARC) layers was performed. The I–V characteristics of the solar cells were studied under 100 mW/cm² illumination conditions. The ZnO/PS layers were found to be an excellent ARC and to exhibit exceptional light-trapping at wavelengths ranging from 400 to 1000 nm, which led to a high efficiency of the c-Si solar cell of 18.15%. The ZnO/PS ARC layers enhance and increase the efficiency of the c-Si solar cell. In this paper, the fabrication processes of the c-Si solar cell with ZnO/PS ARC layers are an attractive and promising technique to produce high-efficiency and low-cost of c-Si solar cells.     

Optoelectronic characterization of ZnS/PS systems

ZnS thin films are deposited on porous silicon （PS） substrates with different porosities by pulsed laser deposition （PLD）. The photoluminescence （PL） spectra of the samples are measured at room temperature. The results show that the PL intensity of PS after deposition of ZnS increases and is associated with a blue shift. With the increase of PS porosity, a green emission at about 550 nm is observed in the PL spectra of ZnS/PS systems, which may be ascribed to the defect-center luminescence of ZnS films. Junction current- voltage （I-V） characteristics were studied. The rectifying behavior of I-V characteristics indicates the formation of ZnS/PS heterojunctions, and the forward current is seen to increase when the PS porosity is increased.     

Annealing and amorphous silicon passivation of porous silicon with blue light emission

The photoluminescence (PL) of the annealed and amorphous silicon passivated porous silicon with blue emission has been investigated. The N-type and P-type porous silicon fabricated by electrochemical etching was annealed in the temperature range of 700-900 °C, and was coated with amorphous silicon formed in a plasma-enhanced chemical vapor deposition (PECVD) process. After annealing, the variation of PL intensity of N-type porous silicon was different from that of P-type porous silicon, depending on their structure. It was also found that during annealing at 900 °C, the coated amorphous silicon crystallized into polycrystalline silicon, which passivated the irradiative centers on the surface of porous silicon so as to increase the intensity of the blue emission.