Formation of Buried Porous Silicon Structure by Hydrogen Plasma Immersion Ion Implantation
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ABSTRACT Plasma Immersion Ion Implantation (PIll) excels in several areas over conventional ion implantation, for example, higher dose, shorter implantation time, and lower overall cost. The technique can be used to fabricate buried porous silicon. In our experiment, hydrogen is implanted into Si by PHI at 5-30kV to form underlying porous silicon (PS) which emits light at an energy higher than the Si bandgap. The optical properties of the PS samples as measured by photoluminescence are quite good. The PIII technique therefore offers an alternative means to fabricate buried porous silicon structures which can potentially be used to fabricate optoelectronic devices in silicon.
INTRODUCTION In the last few years, strong room-temperature light emission from electrochemically etched porous silicon (PS) [1] has attracted a lot of attention. The technology has the potential to integrate optoelectronic and microelectronic devices on a single Si wafer. Unfortunately, the chemically etched PS process is difficult and not compatible with traditional Si fabrication technology because the HF solution which is the anodic electrolyte [1] causes a rough surface and leaves surface impurities after the etching process [2]. Furthermore, the mechanism of light emission from PS is still unclear. In this paper, we introduce an alternative way to prepare "clean" PS. Our method avoids contamination from acid and has higher compatibility with Si-base IC fabrication [3]. Hydrogen is implanted into a crystalline Si wafer by plasma immersion ion implantation (PII) to generate innumerable nano-sized bubbles under the surface [4]. When the silicon walls between these bubbles are thin enough to allow quantum effects to take place, silicon will be transformed from an indirect bandgap material to a direct bandgap one and give out light at an energy higher than the silicon bandgap. We call it buried porous silicon (BPS) since its structure is similar to
ordinary porous silicon. PIll is a novel ion implantation technology that was first introduced by Conrad and coworkers at University of Wisconsin [5]. It has many advantages when compared to a
conventional implanter, especially high throughput and low cost. In Pill, the whole wafer is immersed in a plasma and pulsed or DC biased. Positive ions are implanted normally into the wafer, and the implantation time is consequently independent of the wafer size. It is thus a 427
Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
promising method to produce low-cost large-area Si-based display divices and silicon-on-insulator
materials[6,7]. EXPERIMENT P-type Czochralski grown (100) Si wafers with resistivity from 1 to 50 f-cm were used in our experiments. The PHI rowave
-V
chamber was pumped down to 2xl0-6r
before starting the implantation. A negative dc bias, from -5kV to -30kV, was applied to
the Si wafer that was immersed in a hydrogen plasma created by an ECR Fig.1 Schematic of hydrogen plasma (Electron Cyclotron Resonance) source. Positive ions were accelerated cross the Imm
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