Vikram L. Dalal

Amorphous and polycrystalline silicon films deposited by electron cyclotron resonance reactive plasma deposition

We describe the preparation and properties of hydrogenated amorphous silicon (a‐Si:H) and polycrystalline Si (poly‐Si) films deposited by electron cyclotron resonance (ECR) reactive plasma deposition. Hydrogen radicals are generated in a physically remote ECR plasma source and are allowed to interact with silane near the growth surface. By controlling the flux of reactive hydrogen species, polycrystalline and amorphous silicon films were systematically grown. Poly‐Si films having large Hall mobilities of 35 cm2/V s were deposited at temperatures as low as 450 °C without subsequent annealing. High‐quality a‐Si:H films were deposited at temperatures as high as 450 °C. Plasma properties near the growth surface were characterized using both optical emission spectroscopy and Langmuir probe techniques.

Epitaxial silicon solar cell

A new design for silicon solar cells employing epitaxial deposition is presented. It is shown that the use of epitaxial solar cells may result in higher efficiencies for converting solar energy into electricity. Preliminary experiments are reported which show the feasibility of making near−ideal junctions with high quantum yields.

Growth and properties of nanocrystalline germanium films

We report on the growth characteristics and structure of nanocrystalline germanium films using low-pressure plasma-assisted chemical vapor deposition process in a remote electron-cyclotron-resonance reactor. The films were grown from mixtures of germane and hydrogen at deposition temperatures varying between 130°C and 310°C. The films were measured for structure using Raman and x-ray spectroscopy. It is shown that the orientation of the film depends strongly upon the deposition conditions. Low-temperature growth leads to both ⟨111⟩ and ⟨220⟩ orientations, whereas at higher temperatures, the ⟨220⟩ grain strongly dominates. The Raman spectrum reveals a sharp crystalline peak at 300cm−1 and a high ratio between crystalline and amorphous peak that is at 285cm−1. The grain size in the films is a strong function of hydrogen dilution, with higher dilutions leading to smaller grain sizes. Growth temperature also has a strong influence on grain size, with higher temperatures yielding larger grain sizes. From these...

Electron and hole drift mobility measurements on methylammonium lead iodide perovskite solar cells

We report nanosecond domain time-of-flight measurements of electron and hole photocarriers in methylammonium lead iodide perovskite solar cells. The mobilities ranged from 0.06 to 1.4 cm2/Vs at room temperature, but there is little systematic difference between the two carriers. We also find that the drift mobilities are dispersive (time-dependent). The dispersion parameters are in the range of 0.4–0.7, and they imply that terahertz domain mobilities will be much larger than nanosecond domain mobilities. The temperature-dependences of the dispersion parameters are consistent with confinement of electron and hole transport to fractal-like spatial networks within nanoseconds of their photogeneration.

Growth chemistry of amorphous silicon and amorphous silicon–germanium alloys

It is generally found that the optimum growth of a-Si:H and a-(Si,Ge):H films and devices depends critically upon the particular growth technique, and the particular growth parameters used to grow the film. Different techniques give very different results, which are sometimes contradictory. The standard model for growth assumes that the Si surface is mostly covered with H bonds, and that growth takes place primarily from silyl radicals. The model assumes that excess surface H is eliminated by a silyl radical, and that the adjacent or wing bonded H atoms are eliminated by a spontaneous bond breaking and H2 formation. In this paper, we show that this model is thermodynamically incorrect, and that it does not explain many of the experimental data, such as why bombardment with inert gases reduces H content, and why material grown at higher growth rates is more unstable. Rather, we suggest that the fundamental limitation to the growth of a-Si:H and a-(Si,Ge):H is the elimination of excess H, both from the surface and from the bulk. The excess H is not eliminated by spontaneous reactions, nor by interactions with the silyl molecule. Rather, it is eliminated by interactions with free H radicals and ions. Inert ions, such as He and Ar, can accelerate the desorption of H from the surface. Atomic and ionic H can diffuse into the material, and also remove subsurface excess H, reforming the Si microstructure. We also show that the influence of ion bombardment is critical for growing high quality a-(Si,Ge):H alloys, and that deposition conditions that lead to low ion bombardment flux can produce poor materials.

Influence of plasma chemistry on the properties of a-(Si, Ge) :H alloys

Abstract We report on the effect of plasma chemistry on the growth and properties of a-(Si,Ge):H films and devices. The films and devices were grown using low pressure electron-cyclotron-resonance (ECR) discharge, with hydrogen or helium as the diluent gas. The plasma chemistry at the surface of the growing film was changed by changing the diluent gas from hydrogen to helium, and by changing the deposition pressure. Optical and electronic properties, including band gap ,defect density, Urbach energy and mobility–lifetime products for both electrons and holes were found to depend upon plasma chemistry. Lower pressure growth produced films with properties approaching device properties. The device properties also depended upon pressure and plasma chemistry. Defect density was measured in nin type devices, and was smaller under conditions of lower pressure and greater ion bombardment.

Novel Hybrid Amorphous/Organic Tandem Junction Solar Cell

We report on a novel hybrid amorphous Si-organic series-connected tandem junction solar cell. The solar cell is fabricated on indium tin oxide (ITO)-coated glass and uses an a-(Si,C):H as the first cell and a P3HT/PCBM organic cell as the second cell. An intermediate ITO layer is used as an ohmic layer which provides an excellent contact to both the first and the second cells. By adjusting the bandgap and thickness of the first a-(Si,C):H cell, we achieve an almost complete matching of currents produced by the first and the second cells. The first cell produces ~0.95-1.0-V open-circuit voltage, and the second cell produces ~0.6-V open-circuit voltage. The combined cell produces 1.5-V open-circuit voltage and had a fill factor of 77%, showing the effectiveness of the intermediate ITO layer to act as an excellent connecting layer between the two cells. When such an ITO layer is not used, the fill factor is very poor. The solar conversion efficiency of the organic cell was 4.3%, whereas the efficiency of the tandem cell was 5.7%. We also measured the stability of the organic cell with and without an inorganic cell acting as a filter in front. It is shown that the degradation of the organic cell is much higher when it is subjected to a full solar spectrum, as compared with when it is subjected to light passing through an inorganic cell first, which filters out ultraviolet (UV) and blue photons. Thus, we show that this new cell combination has the potential to significantly increase the efficiency of organic cells while also decreasing the instability. We also discuss the potential of achieving much higher efficiencies, that is approaching 20%, by using an appropriate combination of amorphous and organic cells. An example is shown next.

AVALANCHE MULTIPLICATION IN BULK n‐Si

The avalanche coefficient of electrons α has been measured in bulk n‐Si for the first time. The measured values of α fit the theoretical curves of Baraff with eg < ei < 1.5 eg and 60 < λ < 80 A, where ei is the ionization energy and λ is the phonon mean‐free path. The values of α agree with the extrapolated values from the p‐n junction data of Lee, et al., but disagree with the results of Ogawa.

Growth of high quality amorphous silicon-germanium films using low pressure remote electron-cyclotron-resonance discharge

Abstract The growth and properties of amorphous silicon-germanium [a-(Si,Ge):H] films using a low pressure remote electron-cyclotron-resonance (ECR) discharge are reported on. It is shown that the use of ion bombardment using He ions at low pressures leads to the growth of material with low H concentration and excellent electronic properties. High photo-sensitivities and low Urbach energies and sub-gap defect densities using He discharges were obtained. It is also shown that the band gap of the film for a given Ge Si ratio is less when He is used as the plasma gas compared with when H is used. The use of He allows a lower gap to be achieved even for a-Si:H, without sacrificing the electronic properties.

Preparation and properties of high quality crystalline silicon films grown by ECR plasma deposition

A process for growing high quality epitaxial silicon on heavily doped silicon (100) wafers at temperatures below 525/spl deg/C has been developed using a high vacuum electron cyclotron resonance (ECR) plasma deposition system. Plasma diagnostic work was done in order to optimize the growth conditions. The crystalline quality of our films has been verified using TEM, Raman and UV reflectance. Spreading resistance profiles (SRP) indicate that our undoped films are n-type with free carrier concentrations between 3/spl times/10/sup 16/ cm/sup -3/ and 3/spl times/10/sup 17/ cm/sup -3/. The junction between the heavily doped wafer and the undoped epi layer is shown to be abrupt. The mobilities of the carriers were measured using Hall measurements, and were found to be as high as in the best crystalline materials. This new technique may have significant applications for low cost Si solar cells.