Effect of Doping on the Properties of Hydrogenated Amorphous Silicon Irradiated with Femtosecond Laser Pulses

A comparative analysis of the effect of femtosecond laser irradiation on the structure and conductivity of undoped and boron-doped hydrogenated amorphous silicon (a-Si: H) is performed. It is demonstrated that the process of nanocrystal formation in the amorphous matrix under femtosecond laser irradiation is initiated at lower laser energy densities in undoped a-Si: H samples. The differences in conductivity between undoped and doped a-Si: H samples vanish almost completely after irradiation with an energy density of 150–160 mJ/cm².     

Effect of preparation condition on conductivity activation energy and downward conductivity kink of undoped hydrogenated amorphous silicon

Temperature dependence of conductivity has been measured for a series of undoped a-Si:H films prepared by glow discharge decomposition of silane. Conductivity activation energy and the existence of downward conductivity kinks around 430 K were observed to depend on preparation conditions such as substrate temperature, annealing temperature and discharge configuration. The undoped a-Si:H films whose Fermi levels are well-above the midgap, always show downward conductivity kinks. Variation of conductivity activation energies among undoped a-Si:H films can be interpreted by the transfer of electrons between islands and tissue.     

Order parameters in a-Si systems

We have used Raman scattering to characterize the change in structural order for three types of a-Si materials: fluorinated, a-Si:F:H; hydrogenated, a-Si:H; and sputtered, a-Si (sput) under heat-treatment. The narrower TO linewidth, the higher TO-phonon frequency, the lower TA-phonon frequency, and the lower “crystallization temperature”, all indicate an increase in the local order in the amorphous phase for a-SiH, and particularly for a-Si:F:H. This improved structural order is consistent with the reduction of electronic gap-states.     

Luminescence and magnetic resonance in post-hydrogenated microcrystalline silicon

We report photoluminescence, EPR and ODMR studies of plasma hydrogenated LPCVD microcrystalline silicon thin films. Apart from the dangling bond EPR resonance which is similar to that observed in a-Si : H, and possibly the emission at 1.3 eV, we find emission bands and resonance signals quite unlike those observed in normal c-Si and a-Si : H. We conclude that μc-Si : H cannot be regarded purely as crystallites of pure silicon embedded in a matrix of a-Si : H.     

Accumulation and annealing of implantation damage in a-Si:H

Hydrogenated amorphous silicon (a-Si:H) films have been irradiated with H⁺, B⁺, P⁺, and Ar⁺ ion beams. The accumulation and the annealing of irradiation-induced defects has been investigated through a series of electronic transport and PDS measurements. We find that for all projectiles damage accumulation is dominated by atomic displacement collisions with the damage saturating for energy transfers in excess of about 10 eV/target atom. Annealing at elevated temperatures causes the conductivity of doped and irradiated a-Si:H films to increase according to stretched exponential decay curves. All annealing parameters derivable from such fits scale with the energy originally dissipated into atomic displacement collisions. For energy transfers up to 10 eV/target atom the activation energy for annealing increases up to a saturation value and, at the same time, an increasing fraction of the irradiation-induced defects becomes stable against annealing at moderate temperatures (T _a<250° C). We discuss these results with respect to damage accumulation data in crystalline silicon (c-Si) and with regard to the annealing of metastable defects in a-Si:H.     

Observation of field quenching of photo-induced effects in hydrogenated amorphous silicon

Field quenching phenomena were observed in the photo-induced changes in dark current—voltage and dark low frequency capacitance-voltage characteristics of hydrogenated amorphous silicon (a-Si:H) diodes. The photo-induced changes in photoconductivity of undoped a-Si:H measured in coplanar type samples also depended on the externally applied electric field. The mechanisms of the field quenching were discussed referring to trapping and/or recombination of photogenerated carriers in a-Si:H.     

Employment of absorbing a-C:H films in reflective liquid-crystal light modulators

The effectiveness of employing a-C:H films as light-blocking layers in reflective liquid-crystal light modulators with a-Si:H and a-Si:C:H photosemiconductor layers is investigated. The possibility of reducing the light flux penetrating into a photosemiconductor by 100 fold using an a-C:H film with a thickness of 1 μm and an extinction coefficient equal to 5×10⁴ cm⁻¹ at a wavelength of 632.8 nm is demonstrated. Zh. Tekh. Fiz. 69, 80–81 (March 1999)     

渐变带隙氢化非晶硅锗薄膜太阳能电池的优化设计

利用一维微电子-光电子结构分析软件(AMPS-1D)在AM1.5G(100 mW/cm2)、室温条件下模拟和比较了有、无渐变带隙氢化非晶硅锗(a-SiGe:H)薄膜太阳能电池的各项性能.计算结果表明:渐变带隙结构电池具有较高的开路电压(V oc)和较好的填充因子(FF),转换效率(E ff)比非渐变带隙电池提高了0.477%.研究了氢化非晶硅(a-Si:H)、氢化非晶碳化硅(a-SiC:H)和氢化纳米晶硅(nc-Si:H)三种不同材料的窗口层对a-SiGe:H薄膜太阳能电池性能的影响.结果显示:在以nc-Si:H为窗口层的电池能带中,费米能级E F已经进入价带,使得窗口层电导率及电池开路电压有所提高,又由于ITO与p-nc-Si:H的接触势垒较低,使得接触处的电场降低,更有利于载流子的收集.另一方面,窗口层与a-SiGe:H薄膜之间存在较大的带隙差,在p/i界面由于能带补偿作用形成了价带势垒(带阶)?E v,阻碍了空穴的迁移,因此我们在p/i界面引入缓冲层,使得能带补偿作用得到释放,更有利于空穴的迁移和收集,得到优化后单结渐变带隙a-SiGe:H薄膜结构太阳能电池的转换效率达到了9.104%.     

Substitutional doping of amorphous silicon

It is shown that the electrical conductivity of a tetrahedral amorphous semiconductor can be controlled over many orders of magnitude by doping with substitutional impurities. Experiments were carried out on a-Si specimens prepared by the glow discharge technique, with phosphorus and boron impurity levels between 5 × 10^?6 and 10^?2 ppv.     

Estimation of localized state distribution profiles in undoped and doped a-Si:H by measuring space-charge-limited current

The characteristic temperature which characterizes the localized states of amorphous silicon-hydrogen (a-Si:H) has been obtained by measuring space-charge-limited (SCL) currents in planar structure gap cells for undoped and nitrogen doped films. The SCL current is confirmed by measuring the critical voltage as a function of electrode gap spacing. The characteristic temperature obtained is 1060 ± 150 K for undoped a-Si:H films. The value of nitrogen doped film prepared under the condition of 0.4% N₂ gas partial pressure becomes less than 400 K.     

Simulation of daytime variations in the characteristics of a-Si:H solar cells

The time dependences of the key characteristic of a-Si:H solar cells over daylight hours are theoretically simulated. The model is used to calculate the time dependences for an arbitrary geographic latitude in the interval 30°–60° and arbitrary day of the year. The calculated results are illustrated for a geographic latitude of 45° and equinox. The relative variations in the characteristics of the a-Si:H solar cells are valid with a relatively high accuracy for the solar cells based on alternative semiconductors provided that their efficiency ranges form 7 to 20%.     

On the role of the dangling bond as a radiative centre in a-Si:H

A comparison of recent ODMR results a-Si : H with previously reported ESR and LESR measurements suggests that a principal radiative electron centre has g=2.0055 and may therefore be identified as the dangling board. Our results suggest that the dangling bond (DB) can participate in two distinct PL transitions, centred near 1.25 eV and 0.9 eV, which are present to a measurable extent in all samples examined, in the medium-to-high quantum efficiency range (η_PL ～ 0.1?1.0). This conclusion is in contrast with the generally accepted role of the dangling bond in a-Si : H as primarily a non-radiative recombination centre, but is consistent with a variety of data from other experimental techniques.     

A study of built-in potential ina-Si solar cells by means of back-surface reflected electroabsorption

The electroreflectance (ER) signal has been studied for the purpose of identifying the built-in field in practical amorphous silicon (a-Si∶H) solar cells. Through both theoretical and experimental considerations, it has been confirmed that the ER signal essentially comes from the light which is reflected at the back surface and hence experiences the internal electric field within thea-Si∶H layer. By analyzing the ER signal, which is really the back-surface reflected electroabsorption signal, the built-in potentialV _bcan be evaluated. This method has been applied to various types ofp-i-n junctiona-Si solar cells.V _bof a usual homojunction solar cell was about 0.85 V. Increases ofV _bby 50≈130mV have been found in heterojunction solar cells constructed withp-type amorphous silicon carbide (a-SiC∶H) and/orn-type microcrystalline silicon (μc-Si) as compared with homojunctionp-i-n solar cells. Moreover, a clear dependence ofV _bon the substrate materials has been observed. These experimental results are described in connection with cell performances.     

Edge electroluminescence of silicon: An amorphous-silicon-crystalline-silicon heterostructure

Silicon edge electroluminescence (EL) was observed on an amorphous-silicon-crystalline-silicon heterostructure (a-Si: H(n)/c-Si(p)) in the temperature range from 77 to 300 K. The room-temperature EL internal quantum efficiency of the heterostructure under study was found to be about 0.1%. A theoretical analysis of the emissive properties of the a-Si: H(n)/c-Si(p) heterostructure was made in terms of the model of an abrupt planar p-n junction and showed that, for optimal doping, the internal quantum efficiency of the EL may be as high as a few percent at a modulation frequency of about 50 kHz.     

Paramagnetic states in doped amorphous silicon and germanium

ESR-results for doped hydrogenated amorphous silicon and germanium are discussed. g-values and linewidths of the resonances as well as effective correlation energies of the corresponding energy states are compared for both materials. It is shown that the existing large differences of these quantities between a-Si : H and a-Ge : H can be explained by the differences of spin-orbit coupling and degree of localization.     

“In situ” measurements of the transient photoconductivity in a-Si:H

During glow discharge deposition of a-Si:H the transient photoconductivity is measured with the time-resolved microwave conductivity method. The change of the signal with deposition time is discussed. Evidence is given that the defect density is larger in the surface layer. Analysis of the maximum signal height as a function of the film thickness gives the absorption coefficient at the excitation wavelength. Influence of the deposition temperature on the properties of the film is studied. It is found that lowering the substrate temperature yields films with a larger charge carrier decay rate.     

Transport mechanism and its relationship with heterogeneity in a-Si: H

Hydrogen evolution and dc electrical conductivity measurement have been carried out for a-Si: H prepared by d.c. and r.f. glow discharge decomposition of silane. We show that the kink in log σ vs 1/T plot can be explained in terms of the film heterogeneity. Kinks can appear or disappear after successive annealing depending on the specimens, and the upward shift in kink temperature by annealing is observed for the specimens showing kinks. These results are interpreted to be due to the structural and compositional change in heterogeneity caused by the diffusion and rearrangement of hydrogen.     

Spectrum of photoinduced optical absorption in a-Si: H

Photoinduced absorption below the band gap in a-Si: H is interpreted in terms of transitions between holes trapped in states near the trap quasi-Fermi level and the valence band.     

高绒度掺硼氧化锌透明导电薄膜用作非晶硅太阳电池前电极的研究

将自行研制的具有优异陷光能力的掺硼氧化锌用作p-i-n型非晶硅太阳电池的前电极,并且将传统商业用U型掺氟二氧化锡作为对比电极.相比表面较为平滑的掺氟二氧化锡,掺硼氧化锌表面大类金字塔的绒面结构会在本征层生长过程中触发阴影效应,形成大量的高缺陷材料区和漏电沟道,进而恶化电池的开路电压和填充因子.在不修饰掺硼氧化锌表面形貌的情况下,通过调节非晶硅本征层的沉积温度来消弱高绒度表面形貌引起的这种不利影响,对应的电池开路电压和填充因子均出现提升.在仅有铝背电极的情况下,在本征层厚度为200 nm的情况下,以掺硼氧化锌为前电极的非晶硅太阳电池转换效率达7.34%(开路电压为0.9 V,填充因子为70.1%,短路电流密度11.7 mA/cm2).     

Nanoroughness control of Al-Doped ZnO for high efficiency Si thin-film solar cells

The Al-doped ZnO (ZnO:Al) front transparent conducting oxide (TCO) for high efficiency Si thin-film solar cell has been developed using RF magnetron sputtering deposition and chemical wet etching. Microscopic surface roughness of the as-deposited ZnO:Al film estimated by spectroscopic ellipsometry is closely related to the compactness of the TCO film, and shown to be a straightforward and powerful tool to optimize the deposition conditions for the proper post-etched surface morphology. Wet-etching time is adjusted to form the U-shaped craters on the surface of the ZnO:Al film without sharp etch pits that can cause the crack-like defects in the overgrown microcrystalline Si-absorbing layers, and deteriorate the V_oc and FF of the Si thin-film solar cells. That is to say, the nanoroughness control of the as-deposited TCO film with proper chemical etching is the key optimization factor for the efficiency of the solar cell. The a-Si:H/a-SiGe:H/μc-Si:H triple junction Si thin-film solar cells grown on the optimized ZnO:Al front TCO with anti-reflection coatings show higher than 14% conversion efficiency.