n-Si半导体作为光电化学分解水阳极的稳定性研究

单晶Si由于它的禁带宽度（1.1ev）与太阳光谱相匹配，作为光活性电极材料被广泛研究。但它作为光阳极不稳定，易腐蚀。所以提高单晶Si作为光阳极时的稳定性就成了关键问题。我们所采用的是在n-Si表面沉积保护膜的方法。首先用电沉积法在n-Si表面沉积一层Pd，光电流得到了很大的提高。从未沉积Pd前的2mA／cm~2增加到29.1mA／cm~2（电极电位在1.5V vs，SCE），并且稳定性增强，能在光电解水体系中稳定工作4小时，但4小时后光电流衰减，所以在n-Si表面沉积Pd有一定的保护作用，但保护作用是有限的。我们在n-Si/Pd电极上用化学沉积法再沉积一层锰的氧化物膜，大大提高了电极的稳定性，在0.5MKOH溶液中，电位控制在0.4V（vs.SCE）时，可以稳定工作110小时。化学沉积锰氧化物膜的烧结温度为250℃（在N_2或Ar保护下），得到的锰氧化物膜经X－射线光电子能谱确定锰的价态为＋3价。RuO_2在n-Si/Pd/Mn_2O_3表面的沉积对氧的析出有催化作用，与未沉积前相比不电流起始电位负移了0.15V，比氧析出可逆电位负80mV [氧析出中逆电位在0.5M KOH溶液中（pH＝13.7）为0.18v（vs.SCE）]，沉积RuO_2的n-Si/Pd/Mn_2O_3/RuO_2电极，也具有同样好的稳定性，连续工作112小时光电流无明显变化。由交流阻抗法求得它的平带电位为-0.5V vs.SCE （0.5M KOH溶液）和0.0V vs.SCE（0.5M K_2SO_4溶液），并由此进一步得出它在0.5M KOH溶液中的能级结构。据此分析了n-Si电极在不加偏压下不能实现水的光电化学分解的原因。由光谱响应实验结果得到。沉积保护膜后的n-Si电极在整个可见光及近红外区内仍然有较好的光吸收。由光电流起始波长1130nm计算出的禁带宽度为1.1eV，与文献值一致。

EFFECTS OF 1-MEV-ELECTRON IRRADIATION ON A-SI SOLAR-CELLS

In this report we present the effects of 1 MeV-electron irradiation on i a-Si:H films and solar cells. It is observed that in the dose range of 1.4-8.4 x 10(15) cm(-2) the defect creation has not reached its saturation level and the metastable defects caused by the irradiation cannot be completely removed by a two hour annealing at 200 degrees C for i a-Si:H films or at 130 degrees C for a-Si:H solar cells. The results may be understood in terms of a model based on two kinds of metastable defects created by 1 MeV-electron irradiation.

ESTIMATION OF THE STRAIN STATE OF GEXSI1-X/SI STRAINED-LAYER SUPERLATTICES BY DOUBLE-CRYSTAL X-RAY-DIFFRACTION

20-period strained-layer superlattices of nominal composition and width Ge0.2Si0.8 (5 nm)/Si(25 nm) and Ge0.5Si0.5 (5 nm)/Si(25 nm) were studied by double-crystal X-ray diffraction. The Ge content x was determined by computer simulation of the diffraction features from the superlattice. This method is shown to be independent of the relaxation of the superlattice. Alternatively, x can be obtained from the measured difference DELTAa/a in lattice spacing perpendicular to the growth plane. It is sensitive to the relaxation. Comparing the results obtained in these two different ways, information about the relaxation of the superlattices can be obtained.

A NEW METHOD FOR DETECTING SUBTLE CHANGES IN SI-H BONDS OF A-SI-H

A new method developed for detecting possible subtle changes in Si-H bonds of a-Si:H after light soaking is described in detail. The method promises a sensitivity orders of magnitude higher than that reached by the usual IR spectrometer. Some preliminary results on phosphorus doped a-Si:H are given.

HYDROGEN DIFFUSION IN A-SI-H/A-SI STRUCTURE UNDER ELECTRICAL BIAS

By using the technique of elastic recoil detection (ERD), we have measured the hydrogen profiles in a-Si:H/a-Si structure samples annealed at various temperatures with and without electrical bias, and investigated the influence of electrical bias on hydrogen diffusion. The results show that hydrogen diffusion in a-Si is significantly enhanced by the action of electrical bias. The existence of the excess carriers, which are introduced by electrical injection, is considered to be responsible for the enhancement of hydrogen diffusion, and the microprocess of hydrogen transport has been exploited.

INTERVALLEY-GAMMA-CHI DEFORMATION POTENTIALS IN III-V ZINCBLENDE AND SI SEMICONDUCTORS BY AB-INITIO PSEUDOPOTENTIAL CALCULATIONS

Intervalley GAMMA - X deformation potential constants (IVDP's) have been calculated by first principle pseudopotential method for the III-V zincblende semiconductors AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs and InSb. As a prototype crystal we have also carried out calculations on Si. When comparing the calculated IVDP's of LA phonon for GaP, InP and InAs and LO phonon for AlAs, AlSb, GaAs, GaSb and InSb with a previous calculation by EPM in rigid approximation, good agreements are found. However, our ab initio pseudopotential results of LA phonon for AlAs, AlSb, GaAs, GaSb and InSb and LO phonon for GaP, InP and InAs are about one order of magnitude smaller than those obtained by EPM calculations, which indicate that the electron redistributions upon the phonon deformations may be important in affecting GAMMA - X intervalley shatterings for these phonon modes when the anions are being displaced. In our calculations the phonon modes of LA and LO at X point have been evaluated in frozen phonon approximation. We have obtained, at the same time, the LAX and LOX phonon frequencies for these materials from total energy calculations. The calculated phonon frequencies agree very well with experimental values for these semiconductors.

PHOTOVOLTAIC EFFECT AND ITS POLARITY IN SI DOPING SUPERLATTICES

We have studied the photovoltaic effects in Si doping superlattices (nipi) under different excitation conditions with and without additional cw optical biasing using a He-Ne laser. On the basis of the photovoltaic theory of carrier spatial separation in superlattices, we propose the concept of spatial fixity of the photovoltage polarity in type-II superlattices and examine the experimental results. The photovoltaic effect in Si nipi is found mainly from the direct transitions related with shallow impurities in real space, not the electron-hole band-to-band process as in GaAs nipi.

THEORY OF THE ELECTRONIC-STRUCTURE OF POROUS SI

A theoretical model for the electronic structure of porous Si is presented. Three geometries of porous Si (wire with square cross section, pore with square cross section, and pore with circular cross section) along both the [001] and [110] directions are considered. It is found that the confinement geometry affects decisively the ordering of conduction-band states. Due to the quantum confinement effect, there is a mixing between the bulk X and GAMMA states, resulting in finite optical transition matrix elements, but smaller than the usual direct transition matrix elements by a factor of 10(-3). We found that the strengths of optical transitions are sensitive to the geometry of the structure. For (001) porous Si the structure with circular pores has much stronger optical transitions compared to the other two structures and it may play an important role in the observed luminescence. For this structure the energy difference between the direct and the indirect conduction-band minima is very small. Thus it is possible to observe photoluminescence from the indirect minimum at room temperature. For (110) porous Si of similar size of cross section the energy gap is smaller than that of (001) porous Si. The optical transitions for all three structures of (110) porous Si tend to be much stronger along the axis than perpendicular to the axis.

INVESTIGATION OF BURIED ALN LAYERS FORMED BY NITROGEN IMPLANTATION INTO AL

The composition and microstructure of buried layers of AlN formed by high energy N+ ion implantation into polycrystalline Al have been determined. Both bulk and evaporated thin films of Al have been implanted with 100 and 200 keV N+ ions to doses of up to 1.8 x 10(18)/cm2. The layers have been characterised using SIMS, XTEM, X-ray diffraction, FTIR, RBS and in terms of their microhardness. It is found that, for doses greater than the critical dose, buried, polycrystalline AlN layers are formed with preferred (100) or (002) orientations, which are sample specific. With increasing dose the nitrogen concentration saturates at the value for stoichiometric AlN although the synthesised compound is found to be rich in oxygen.

EFFECTS OF DEEP CENTERS AT PT SI INTERFACES AND HYDROGENATION

Electrical measurements were combined with surface techniques to study the Pt/Si interfaces at various silicide formation temperatures. Effects of deep centers on the Schottky barrier heights were studied. Hydrogen plasma treatment was used to passivate the impurity/defect centers at the interfaces, and the effects of hydrogenation on the Schottky barrier heights were also examined. Combining our previous study on the Pt/Si interfacial reaction, factors influencing the PtSi/Si Schottky barrier diode are discussed.

STRUCTURAL DEFECTS AND THEIR ELECTRICAL-ACTIVITY IN GERMANIUM IMPLANTED SILICON

The electrical and structural characteristics of secondary defects in regrown amorphous layers formed in n-type Si(100) with a resistivity of 2 OMEGA cm and 6 OMEGA cm using Ge+ ions, has been studied. The amorphous layers with a thickness of 460 nm are formed by implantation of 1 x 10(15) Ge+ cm-2 at an energy of 400 keV. Both conventional furnace and rapid thermal annealing were used to regrow the amorphous layer and the residual defects have been characterised in terms of their concentration depth distribution and activation energies using C-V and DLTS. Structural information has been obtained from RBS and XTEM. By choosing suitable anneal conditions it is possible to eliminate extended defects, apart from a low concentration of end of range dislocation loops. However, a substantial population of electrically active point defects remain after simple low thermal budget anneals. In a sample implanted with 1 x 10(15) Ge+ cm-2 at 400 keV a region of deep donors approximately 460 nm from the surface is always present When the samples are annealed at higher temperatures (> 850-degrees the total deep donor concentration is reduced by one order of magnitude. Other electrically active defects not observable in the low (750-degrees-C) temperature annealed layers become apparent during anneals at intermediate temperatures.

KINEMATICAL STUDY OF GEXSI1-X/SI STRAINED-LAYER SUPERLATTICE BY A DOUBLE CRYSTAL X-RAY-DIFFRACTION METHOD

Two samples of nominal 20-period Ge0.20Si0.80(5 nm)/Si(25 nm) and Ge0.5Si0.5(5 nm)/Si(25 nm) strained-layer superlattices (SLSs) were studied by the double-crystal X-ray diffraction method. It is convenient to define the perpendicular strains relative to the average crystal. Computer simulations of the rocking curves were performed using a kinematical step model. An excellent agreement between the measured and simulated satellite patterns is achieved. The dependence of the sensitivity of the rocking curves to the structural parameters of the SLS, such as the alloying concentration x and the layer thicknesses and the L component of the reflection g = (HKL), are clearly demonstrated.