Heterogeneity and photoconductivity kinetics in amorphous hydrogenated silicon

A model of the decay kinetics of photoconductivity in amorphous hydrogenated silicon, in which recombination of excess carriers is assumed to occur via tunnelling, is proposed. It is shown that study of the decay shape after flash illumination can be a very effective way to detect structural inhomogeneities in amorphous or disordered semiconductors. Fiz. Tekh. Poluprovodn. 32, 1266–1268 (October 1998)     

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.     

Recombination in hydrogenated amorphous silicon

Investigations of transient photoconductivity in a-Si: H lead to the conclusion that tunnel recombination of localized excess carriers may predominate in the temperature range from liquid-helium temperatures up to temperatures at which the material was synthesized. Fiz. Tekh. Poluprovodn. 32, 923–930 (August 1998)     

Laser annealing of hydrogenated amorphous silicon

Pulsed laser annealing of hydrogenated amorphous silicon, glow-discharge-deposited on single crystal silicon, shows partial crystallization without significant loss of hydrogen. After laser annealing up to 60 MW/cm², the photoluminescence spectrum exhibits only a very slight shift to lower energy. This contrasts with thermally induced dehydrogenation which causes a large spectral shift. Also thermally induced crystallization occurs above 700?C where dehydrogenation is complete.     

Compensated back-channel TFTs in hydrogenated amorphous silicon

Compensated back-channel inverted staggered TFTs have been made in a-Si:H. Donor impurities were implanted to form a good source and drain ohmic contact followed by an acceptor implant to compensate the channel region. TFTs have been made with no degradation of channel mobility due to the implants and a leakage current between source and drain comparable with the best TFTs made using conventional etched back-channel technology.     

Metastability and relaxation processes in hydrogenated amorphous silicon

The kinetics of structural relaxation in hydrogenated amorphous silicon (a-Si:H) deposited by various methods is investigated by differential scanning calorimetry. The experimental results are used to analyze the nature of the metastable states in a-Si:H and to investigate the relationship between structural relaxation and light-induced metastability (the Staebler-Wronski effect). Fiz. Tekh. Poluprovodn. 31, 1449–1454 (December 1997)     

Trions in silicon nanocrystals in an amorphous hydrogenated silicon matrix

A simple theoretical model of the radiative recombination in amorphous hydrogenated silicon films containing silicon nanocrystals is suggested. This material is used for the fabrication of modern thin-film solar cells. The calculation results are in good agreement with experimental data on the photoluminescence spectra.     

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.     

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.     

Photoluminescence of erbium in amorphous hydrogenated phosphorus-doped silicon

The photoluminescence of erbium ions in phosphorus-doped a-Si:H films has been investigated. The observed increase in the Er photoluminescence with increasing defect density in the samples and the correlation of the temperature variation of the Er photoluminescence and defect-associated photoluminescence intensities are explained on the basis of a model of excitation of Er ions as a result of Auger recombination with defect participation. Fiz. Tekh. Poluprovodn. 31, 869–871 (July 1997)     

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.     

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     

Erbium electroluminescence in p-i-n amorphous hydrogenated silicon structures

A comparative analysis of λ=1.54 μm electroluminescent structures based on amorphous hydrogenated silicon is made. The possibility of obtaining room-temperature electroluminescence from forward-biased conventional p-i-n structures based on this material is demonstrated for the first time, which is of interest for the development of effective emitting structures with current pumping.     

Amorphous chromium silicide formation in hydrogenated amorphous silicon

The interaction between thin films of hydrogenated amorphous silicon and sputter-deposited chromium has been studied. Following deposition of the chromium films at room temperature, the films were annealed over a range of times and temperatures below 350°C. It was found that an amorphous silicide was formed only a few nanometers thick with the square of thickness proportional to the annealing time. The activation energy for the process was 0.55±0.05 eV. The formation process of the silicide was very reproducible with the value of density derived from the thickness and Cr surface density being close to the value for crystalline CrSi₂ for all films formed at temperatures ≤300°C. The specific resistivity of the amorphous CrSi₂ was ≈600 μΩ·cm and independent of annealing temperature.     

Energy distribution of localized states in amorphous hydrogenated silicon

A new method for determining the spectral dependence of the optical-absorption coefficient in amorphous hydrogenated silicon is suggested. The method is based on the analysis of spectral and temperature dependences of transient photoconductivity in this material. Energy distribution of localized states involved in recombination of nonequilibrium holes was calculated.     

Diffusion of chromium in thin hydrogenated amorphous silicon films

The diffusion of a chromium bottom contact has been studied through thin 10-nm amorphous silicon film. The concentration of the diffused impurity has been analyzed by an X-ray photon spectroscopy technique and the diffusion coefficient was estimated. Diffusion annealing was carried out in vacuum (10^?6 mTorr), the temperature was kept at 400°C, and the annealing time was varied from 0 to 300 min. The authors propose that diffusion of chromium in thin hydrogenated amorphous film is limited by silicide formation at the metal-silicon interface.     

Conductivity and structure of Er-doped amorphous hydrogenated silicon films

A columnar structure of a-Si:H(Er) film serving as a working layer in electroluminescent structures has been demonstrated. The diameter of columns is in the range of 60–100 Å. In a structure of this kind, the conductivity depends on the direction of current. In the planar configuration, room-temperature transport occurs through hopping via localized states near the conduction band edge, within the band tail. In the sandwich configuration, the conduction occurs along the column boundaries, where the conductivity is higher, via hopping conduction at the Fermi level.     

隔热结构中非晶硅的释H_2机理与工艺研究

对采用等离子增强化学气相淀积法(PECVD)制备的氢化非晶硅(a-Si:H)薄膜进行了退火释H2实验,并对三文治结构膜层生长工艺作了改进。红外透射光谱表明:提高退火温度及增加退火时间会造成Si-H键断裂释放H2影响器件结构完整;不断改进设计,最终采用精简有效热敏面积及将退火工艺提前以扩张释H2渠道的方案,获得600℃退火后仍保持完整三文治结构的优化设计流程。     

Gamma-irradiation-induced metastable states of undoped amorphous hydrogenated silicon

The conductivity of intrinsic amorphous hydrogenated silicon (a-Si:H) becomes higher upon gamma irradiation. This effect is due to an increase in the number of metastable D ⁺ states in the mobility gap. At the same time, the conductivity of extrinsic (undoped) irradiated a-Si:H decreases. Most likely, gamma irradiation creates hydrogen-containing complexes in this material. The results obtained are discussed in comparison with the known data for B-or P-doped a-Si:H.