We report on the growth technique and electrical properties of epitaxial Si films grown at low temperatures using an electron-cyclotron-resonance plasma deposition technique. We have used standard high vacuum apparatus to grow high quality films at 450-525°C. A critical step in achieving high quality films is an in situ hydrogen plasma cleaning of the wafer before growth. We have systematically studied the influence of ion bombardment during growth by biasing the substrate, and find that the films are crystalline for substrate bias voltages less negative than about Ϫ15 V, but become polycrystalline as the magnitude of the negative bias is increased. The crystallinity of the film was measured using Raman spectroscopy. The undoped films are n type with carrier concentrations in the 10 16 -10 17 cm Ϫ3 range. The Hall mobilities measured for the films are comparable to values obtained in bulk Si crystals. We can achieve abrupt profiles in carrier concentrations between the heavy doped substrate and the epilayer, with no evidence of diffusion. © 1995 American Institute of Physics.Low temperature epitaxial growth of silicon is of significant current interest due to its potential for making devices with small feature sizes.1 A low growth temperature ͑400-700°C͒ prevents the deleterious effects of autodoping from the substrate and lateral diffusion from contacts. To achieve such a low temperature growth, various techniques, such as ultrahigh vacuum chemical vapor deposition ͑UHV-CVD͒, 1,2 very low pressure CVD, 3 plasma enhanced CVD, 4 and low pressure UHV electron-cyclotron-resonance ͑ECR͒ CVD [5][6][7][8] have been used. The advantages of ECR-CVD are the low particulate production in the reactor, enhanced growth rate at low temperatures, and the ability to control plasma potentials, and hence ion bombardment of the substrate, during growth. In this paper, we report on a systematic study of conditions leading to the growth of very high quality Si films using a controlled, low pressure ECR plasma of silane and hydrogen at low temperatures ͑450-525°C͒.The growth apparatus is similar to the one used in previous work by Mui et al. 8 and Tae et al. 6 However, unlike these previous authors, we did not use a UHV system, but merely a standard high vacuum system equipped with O-rings. The system has previously been described in detail. 9The base pressure in the system is in the range of 5 -9 ϫ10 Ϫ8 Torr. These vacuum conditions require the growth rate to be maximized in order to reduce the incorporation of impurities as previously described by Comfort and Reif. 3 Most of the current techniques for growing epitaxial silicon films below 600°C require UHV conditions due to their extremely low growth rates ͑Ͻ1 Å/s͒. 2,[5][6][7][8] In contrast, the enhanced surface mobility of radicals due to ion bombardment in the ECR plasma allows typical growth rates of more than 3.5 Å/s to be achieved. The growth rate in this type of system is very sensitive to the process conditions, especially the pressure, and silane to hydrogen ratio. W...

The development of a process for the low temperature (< 600°C) growth of epitaxial silicon is an important technological issue. Conventional growth processes involve temperatures in excess of 1000°C. At these temperatures autodoping and impurity redistribution limit the feature size achievable in VLSI fabrication. As the typical feature sizes move into the submicron region, new processes for epitaxial silicon deposition will be needed. Another application for a low temperature growth process is the fabrication of solar cells on inexpensive metallurgical grade silicon wafers. Impurity diffusion from the wafer during conventional epitaxial silicon growth limits the quality of the solar cells if expensive high purity wafers are not used. We have used electron cyclotron resonance (ECR) plasma deposition to grow high quality epitaxial silicon films on silicon wafers. This growth technique relies on the deposition of silicon from a highly energetic hydrogen and silane plasma. The presence of the hydrogen in the plasma provides reactive etching of the silicon surface during growth. This reduces the oxygen and carbon contamination in the film as well as increasing the number of available growth sites on the surface by displacing the adsorbed hydrogen. By optimizing the growth pressure, substrate temperature, microwave power, substrate bias and silane to hydrogen ratio we have developed a process which provides enhanced growth rates and good uniformity at temperatures (425-575°C) significantly below those used in conventional processes. The structural and electrical properties of the films have been characterized using SEM, TEM, Raman spectroscopy, UV reflectance, spreading resistance profiles. Hall mobility measurements, and both four-point probe and van der Pauw resistivity measurements.

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Chapter I. Research on a-Si Materials and Devices I.1 Introduction I.2 Considerations about growth chemistry I.3 Use of ECR reactor as a controlled plasma tool I.4 Single junction device growth and results I.5 Tandem junction device growth and results I.6 Conclusions from a-Si:H research Chapter II. Research on a-(Si,Ge) materials and devices photovoltaics; a-Si materials and devices; ECR plasma deposition; single-junction solar cells; tandem-junction cells; graded-gap cells; plasma chemistry; electron cyclotron resonance 16. PRICE CODE 17. SECURITY CLASSIFICATION OF REPORT Unclassified

A process for growing high quality epitaxial silicon on heavily doped silicon (100) wafers at temperatures below 525°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 3~1 0 '~ cm-3 and 3~1 0 '~ ~m -~. 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.