Carrier separation effects in hydrogenated amorphous silicon photoconductors with multilayer structures

Photogenerated electrons and holes can be separated efficiently by the built-in field present in hydrogenated amorphous silicon (a-Si:H) photoconductors with multilayer (ML) structures. Because of the carrier separation, the recombination rate of photogenerated carrier's decreases and this can lead to high photosensitivity and small photoinduced changes in a-Si:H ML photoconductors. We have observed that such carrier separation effects indeed occur in a-Si:H p-i-p type ML photoconductors. Some comments are made on the origin of the photoinduced changes in a-Si:H from the experimental results obtained for the p-i-p type photoconductors.     

Changes in photovoltaic and dark electrical properties of hydrogenated amorphous silicon diodes induced by forward bias carrier injection

The photovoltaic and dark electrical properties of hydrogenated amorphous silicon diodes were changed by forward bias carrier injection for several hours. These changes were similar to photoinduced (PI) changes previously reported, and this result supports previous explanations for PI changes. The differences between these two types of change are also discussed.     

Direct tunneling at the front contact of amorphous silicon p-i-ndevices

The device physics behind hole direct tunneling currents at the front contact of a-Si:H p-i-n homojunction have been explored. In this paper, the dark I-V, the light I-V, and the QE characteristics of this structure with and without hole direct hole tunneling currents are evaluated and compared. The three differential equation systems of the Poisson's equation, the continuity equation for free electrons, and the continuity equation for free holes have been solved with allowances for direct tunneling currents. Hole direct tunneling currents at the front contact of a-Si:H p-i-n homojunctions give rise to a significant increase in the dark current level at high forward voltages and to an increase in the open-circuit voltage of the light I-V characteristic when the front electron barrier is low. The hole thermionic emission current and the hole direct tunneling current have been carefully compared to the front contact. Hole tunneling currents introduce important modifications to the carrier transport physics not only to the front contact but also in the bulk of the a-Si:H p-i-n homojunction     

An amorphous silicon solar cell having a conversion efficiency of 10.50 percent

We report on a hydrogenated amorphous silicon (a-Si:H) p-i-n solar cell having a conversion efficiency of 10.50 percent (1.05-cm²area). The cell was prepared by a conventional plasma-enhanced chemical vapor deposition process using a structure back electrode n-i-p/SnO2-ITO/glass. This conversion efficiency was obtained from the reduction and control of impurity levels in the i-layer. Photovoltaic characteristics of prepared cells were measured under AMI (100 mW/cm²). The impurity levels of oxygen and boron were measured by SIMS and are discussed.     

Photoinduced conductivity change in erbium-doped amorphous hydrogenated silicon films

Changes in the dark conductivity of erbium-doped amorphous hydrogenated silicon (a-Si:H(Er)) films after their preliminary illumination at room temperature have been studied. The effect of a compensating boron impurity on the photoinduced change in the conductivity of a-Si:H(Er) films is analyzed. It is established that the magnitude and the sign of the change in conductivity depend on the duration of illumination and position of the Fermi level in the mobility gap. Possible mechanisms leading to a photoinduced change in the conductivity of a-Si:H(Er) films are discussed.     

Study of photoconductivity and persistent photoconductivity in sulphur-doped amorphous hydrogenated silicon

We report on the spectral response and intensity dependence of photoconductivity (PC) and persistent photoconductivity (PPC) in plasma-enhanced chemical vapour deposition grown sulphur-doped n-type a-Si:H films. From the intensity dependence of PC it is found that the addition of sulphur changes the recombination mechanism from monomolecular for intrinsic and low-doped films to bimolecular at a high sulphur doping level. The photo-induced metastable increase of dark conductivity in these films is found to be quite similar to that for compensated and doping-modulated a-Si:H films. The PPC effect is detectable up to an illumination temperature of at least 380 K the highest temperature used in this study. At 300 K the conduction persists at a level of one order higher than the equilibrium dark conductivity for over 10³ s after removing the excitation. The PPC in a-Si, S:H is explained in terms of the valence alternation pair model.     

Metastability of phosphorus and boron in hydrogenated amorphous silicon

Thermodynamic equilibration kinetics of phosphorus- and boron-doped hydrogenated amorphous silicon (a-Si:H) are explored using dark conductivity measurements. the equilibration kinetics of phorphorus- and boron-doped a-Si:H are different and there is a strong dependence on the deposition temperature (T_s). Small variations in temperature of the semiconductor cause variations in active dopant concentration, which affects the conductivity. the transformation of phorphorus into the electronically active phase (activation) follows a stretched exponential time dependence with a temperature-independent dispersion parameter, β, of 0.85, whereas the transformation into the electronically passive bonding configuration (passivation) is also a stretched exponential but with β ⩽ 0.8. the kinetics of boron metastability are similarly a stretched exponential, but with equal β values for activation and passivation. the time constant, τ, to achieve equilibrium for both transformations is thermally activated with energies E_act ∽ 1.1 eV for phosphorus and boron. an increase in T_s leads to an increase of E_act. the data are discussed and interpreted based on the hydrogen migration model.     

Sputtered hydrogenated amorphous silicon

Hydrogenated a-Si, whose properties can be modified by impurity doping, can be produced either by decomposition of silane or by reactive sputtering of Si in an argon-hydrogen plasma. This article reviews advances made during the past few years in the preparation and characterization of films produced by the second method. The basic deposition conditions are summarized. The conclusions of analytical studies (photo-emission, infrared spectroscopy, and volumetric measurements of evolved gases), regarding the amount of hydrogen and its bonding configuration in the network, are outlined. The optical, carrier transport, photoconductivity and photoluminescence properties as a function of hydrogen content and doping are described. Electron drift mobilities, deduced from steady state and transient photoconductivity are presented. The transport and recombination properties are discussed with existing models of amorphous semiconductors, and are found to be consistent with atomic relaxations, i.e. polaronic effects.     

Superlattices and multilayers in hydrogenated amorphous-silicondevices

An investigation of the physical properties of ultrathin multilayer structures that consist of hydrogenated amorphous silicon (a-Si:H) and silicon-based compounds such as a-Si_1-xN_x :H and a-Si_1-xC_x:H prepared by either plasma chemical vapor deposition (CVD) or direct photo-CVD is discussed. X-ray interference in the multilayers shows that the heterojunction interface is atomically flat and abrupt. The band offset is successfully determined by X-ray photoelectron spectroscopy. The energy band profile in the superlattices can therefore be designed. The carrier confinement in the ultrathin a-Si:H layers causes a blue shift of the optical bandgap and luminescence spectrum due to the quantization effects. Better understanding of the quantum size effects in the multilayers permits the introduction of superlattice structures to devices such as solar cells, thin-film transistors, and light-emitting diodes. Some of the experimental results are discussed     

Silicon matrix disorder in amorphous hydrogenated silicon alloys

The authors report a detailed investigation of correlations between Urbach energies from photothermal deflection spectroscopy and Raman half-widths of transverse optic (TO)-like Si-Si modes as a measure of silicon matrix disorder in glow-discharge amorphous hydrogenated silicon (a-Si:H) and a-SiGe:H, as well as in glow-discharge and sputtered a-SiC:H and a-SiN:H. A corresponding decrease in TO full width at half-maximum (FWHM) and Urbach energy E₀ for soft deposition techniques yields bond angle distributions as narrow as 8.5° for the best a-Si:H films. Even at the lowest levels of nitrogen incorporation, simultaneous increases in E₀ and TO-like half-widths indicate that lattice distortions occur due to threefold coordination of nitrogen in the a-Si:H matrix. In contrast, no deviation of silicon TO-FWHM could be detected in a-SiC:H of up to 35 at.% of carbon content, whereas Urbach edges broaden in a well-known manner that is interpreted in terms of -CH₃ incorporation into the amorphous network. Diborane doping and sputter deposition, however, give rise to lattice distortions in a-SiC:H, which reflects changes in the carbon coordination     

1/f noise in amorphous silicon and hydrogenated amorphous silicon thin films

Excess noise measurements have been carried out on either sputtered a-Si or sputtered a-Si:H thin films, at 300 K and in the absence of light. The dependence of excess noise amplitude on hydrogen partial pressure in the sputtering chamber during the film growth has been studied. Investigations of 1/f noise in B-doped a-Si:H thin films have also been carried out. The results of this study show that1/f noise can be correlated to both the hydrogen content in a-Si:H films and to the doping of the films. The noise is explained in terms of fluctuations in the number of gap states due to thermally activated configurational changes involving hydrogenated bonds and dangling bonds.     

Picosecond photocarrier transport in hydrogenated amorphous-siliconp-i-n diodes

Optically detected, picosecond-domain photocarrier transport measurements for hydrogenated amorphous silicon (a-Si:H) p-i-n diodes with uniform and nonuniform electric fields are reported. The transient electroabsorption bleaching mechanism for optical detection is described, and the interpretation of these transients is extended to the case of nonuniform electric fields. A technique for using surface-absorbed light and optical detection for measurement of nonuniform electric fields is presented     

Passivation of p-n junction in crystalline silicon by amorphous silicon

Hydrogenated amorphous silicon, a-Si:H, is shown to be an excellent passivant for crystalline silicon (c-Si) p-n junctions. A two-orders-of-magnitude reduction in reverse leakage current from that of a typical thermal oxide passivated junction is obtained. This is achieved through a lowering of the interface state density by hydrogenation of the c-Si surface. Superior bias-temperature stability of the passivated junctions also is observed. There is evidence that the hydrogen in the bulk of the a-Si:H can act as a hydrogen reservoir for rehydrogenation of the interface between c-Si and a-Si:H. Thermal stability of the a-Si:H is adequate for temperatures up to 500°C for 30 min, which is sufficient for most device-processing requirements. Above 550°C, significant dehydrogenation from both the interface and the bulk a-Si:H regions and an increase in leakage are observed. The passivation properties were assessed through studies of the current-voltage and current-temperature characteristics of the p-n junctions.     

Properties of magnetron sputtered hydrogenated amorphous silicon

Hydrogen concentrations and bonding configurations were studied in hydrogenated amorphous silicon (a-Si:H) films deposited at 50‡C using the magnetron mode of sputtering with partial hydrogen concentrations between 0 and 90 percent in flowing argon. Hydrogen content within the films was determined from nuclear reaction analysis, and the chemical bonding of hydrogen was determined from infrared absorption of as-deposited, thermally annealed, and ion-bombarded films. Hydrogen/silicon ratios in the films increase to a maximum of 0.31 with increasing hydrogen in the deposition system. Ion backscattering shows ∼ 6 at.% argon trapped in the films, but no oxygen was detected by either ion backscattering or by sputter-Auger analysis. The wag and bend modes for Si-H in the films are typical of sputter-deposited a-Si:H; however, the stretch mode region is atypical with absorption near 2000 cm^−l dominating even for H/Si ratio of 0.27. From results of thermal annealing and post-deposition ion bombardment, it is concluded that argon ion bombardment during deposition produces enhanced absorption near 2000 cm^-1 in these a-Si:H films deposited by magnetron sputtering. This work was sponsored in part by the U. S. Department of Energy, under Contract DE-AC04-76-DP00789 and the U. S. Army Research Office, Contract DAA29-79-C-0026. U. S. Department of Energy facility.     

n-i-p型非晶硅太阳电池中p/ITO界面特性研究

采用射频等离子体增强化学气相沉积(RF-PECVD)技术,制备n-i-p型非晶硅(a-Si)太阳电池,采用反应热蒸发法制备ITO薄膜作为太阳电池的前电极。通过改变B2H6的掺杂浓度获得了不同晶化率的p层,详细研究了p层性能对p/ITO界面特性以及电池性能的影响。结果表明,在合适晶化率的p层上沉积ITO薄膜有利于优化p/ITO界面的接触特性,将其应用于n-i-p型a-Si太阳电池,能够显著改善电池的开路电压(Voc)和填充因子(FF),最终,在不锈钢(SS)衬底上获得了转换效率为6.57%的单结a-Si太阳电池。     

Optimization of n/i and i/p buffer layers in n-i-p hydrogenated microcrystalline silicon solar cells

Hydrogenated microcrystalline silicon (μc-Si:H) intrinsic films and solar cells with n-i-p configuration were prepared by plasma enhanced chemical vapor deposition (PECVD). The influence of n/i and i/p buffer layerson the μc-Si:H cell performance was studied in detail. The experimental results demonstrated that the efficiency is much improved when there is a higher crystallinity at n/i interface and an optimized a-Si:H buffer layer at i/p interface. By combining the above methods, the performance ofμc-Si:H single-junction and a-Si:H/μc-Si:H tandemsolar ceils has been significantly improved.     

Ar等离子体处理PET在非晶硅太阳电池中的应用

在射频等离子体增强化学气相沉积(RF-PECVD)系统的腔室内,对聚对苯二甲酸乙二醇酯(PET)塑料薄膜进行Ar等离子体预处理,并采用光发射谱(OES)对不同处理参数下的氩气辉光状态进行在线监测。对处理前后PET及PET/ITO进行透过谱、原子力显微镜(AFM)以及扫描电镜(SEM)的测试结果表明,Ar等离子体处理改善了PET塑料薄膜的表面形貌,使之更适合ITO薄膜的生长。以Ar等离子体处理的PET/ITO为衬底,在沉积温度为125℃条件下,制备出效率为5.4%的p-i-n型非晶硅(a-Si)柔性太阳电池。     

The effects of hydrogen on aluminum-induced crystallization of sputtered hydrogenated amorphous silicon

The effects of hydrogen on aluminum-induced crystallization (AIC) of sputtered hydrogenated amorphous silicon (a-Si:H) were investigated by controlling the hydrogen content of a-SiH films. Nonhydrogenated (a-Si) and hydrogenated (a-Si:H) samples were deposited by sputtering and plasma-enhanced chemical vapor deposition (PECVD). All aluminum films were deposited by sputtering. Hydrogen was introduced into the sputter-deposited a-Si films during the deposition. After deposition, the samples were annealed at temperatures from 200°C to 400°C for different periods of time. X-ray diffraction (XRD) patterns were used to confirm the presence and degree of crystallization in the a-Si:H films. For nonhydrogenated films, crystallization initiates at a temperature of 350°C. The crystallization of sputter-deposited a-Si:H initiates at 225°C when 14% hydrogen is present in the film. As the hydrogen content is decreased, the crystallization temperature increases. On the other hand, the crystallization initiation temperature for PECVD a-Si:H containing 11at.%H is 200°C. Further study revealed that the crystallization initiation temperature is a function, not only of the total atomic percent hydrogen in the film, but also a function of the way in which the hydrogen is bonded in the film. Models are developed for crystallization initiation temperature dependence on hydrogen concentration in a-Si:H thin films.     

Hydrogenated amorphous silicon thin film transistor fabricated onplasma treated silicon nitride

We have demonstrated that the performance of the inverted staggered, hydrogenated amorphous silicon thin film transistor (a-Si:H TFT) is improved by a He, H₂, NH₃ or N₂ plasma treatment for a short time on the surface of silicon nitride (SiN _x) before a-Si:H deposition. With increasing plasma exposure time, the field-effect mobility increase at first and then decrease, but the threshold voltage changes little. The a-Si:H TFT with a 6-min N₂ plasma treatment on SiN_x exhibited a field effect mobility of 1.37 cm²/Vs, a threshold voltage of 4.2 V and a subthreshold slope of 0.34 V/dec. It is found that surface roughness of SiN_x is decreased and N concentration in the SiN _x at the surface region decreases using the plasma treatment     

酞菁锌/氢化非晶硅复合薄膜光电性质的研究

Composites consisting of hydrogenated amorphous silicon （a-Si： H, inorganic） and zinc phthalocyanine （ZnPc, organic） were prepared by vacuum evaporation of ZnPc and sequential deposition amorphous silicon via plasma enhanced chemical vapor deposition （PECVD）. The optical and electrical properties of the composite film have been investigated. The results demonstrate that ZnPc can endure the temperature and bombardment of the PECVD plasma and photoconductivity of the composite film was improved by 89.9% compared to pure a-Si： H film. Electron mobility-lifetime products/lr of the composite film were increased by nearly one order of magnitude from 6.96 × 10^-7 to 5.08 × 10^-6 cm2/V. Combined with photoconductivity spectra of the composites and pure a-Si： H, we tentatively elucidate the improvement in photoconductivity of the composite film.