Porous Semiconductors: A Tutorial Review
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ABSTRACT Porous semiconductors constitute a class of material that exhibit surprising properties, are quite easy to fabricate, but are however fragile, complex, and difficult to characterise. This tutorial review extracts specific topics from the large knowledge base now available on porous Si that are deemed relevant to other porous semiconductors beginning to receive study. It also highlights topics where controversy is now resolved, where problems remain, and where further effort could be focused. I INTRODUCTION Although porous semiconductors are strictly speaking, not a new class of material, it is only in the 1990's that a significant level of study has occurred. Over the last six years there have been > 1500 papers published with the focus primarily on visibly luminescent high porosity silicon (1). Given such a large body of data, and that work is now starting to appear on a range of other luminescent porous semiconductors (see table 1), it would seem timely to highlight some of the lessons learnt from porous Si work that are directly applicable to porous semiconductors in general. TABLE 1 - INTEREST IN POROUS SEMICONDUCTORS SEMICONDUCTOR
No. of papers
RECENT REFERENCE
RENDERED POROUS Si
> 1500
This MRS proceedings
SiC
> 13
GaP
>8
Si.Ge1 .x
>7
Ge
>4
GaAs
>4
InP
>3
CuInSe 2
> 1
MacMillan et at. J Appl Phys 80, 2412 (1996). Meijerink et al. J Appl Phys 79, 9301 (1996). Schoisswohl et al. J Appl Phys 79, 9301 (1996). Miyazaki et al. Thin Sol Films 255, 99 (1995). Schmuki et al. Appl Phys Lett 69, 1620 (1996). Kikuno et al. Jpn J Appl Phys 34, 177 (1995). Lebedev et al. Tech Phys Lett 22, 467 (1996).
29 Mat. Res. Soc. Symp. Proc. Vol. 452 ©1997Materials Research Society
The focus of this short, highly selective literature review is thus to summarise some of the significant advances made with porous Si over the last few years, but in addition the major issues and problems that are to some degree unique to porous semiconductors. Its aim is hence to attempt to guide new researchers in the field towards making better quality material, avoid some of the reproducibility problems that have arisen in the past, and target some of the key problems in the future. Table 2 illustrates the material complexity involved since both skeleton and pore microstructure influence properties, and can occur over widely varying length scales. TABLE 2
DESCRIPTIONS OF A POROUS SEMICONDUCTOR
MAJOR STRUCTURAL FEATURES
EXAMPLES AND LABELS
EXAMPLES OF MEASUREMENT TECHNIQUES*
SKELETON: Crystallinity Size distribution Morphology Defects Strain
eg poly, amorphous eg gaussian, bimodal eg columnar, fractal eg extended, point eg symmetry and sign
TEM, XRD SAXS, RSS TEM, SPM HRTEM, EPR XRD
SKELETON SURFACE: Chemical composition Surface area Surface bonding Defects
eg eg eg eg
IBA, SIMS GADA, SAXS FTIR, XPS EPR, PAS
POROSITY: Void content
Size distribution
Morphology *
hydride, oxide BET value dihydride, hydroxyl dangling bond
0-30% 30-70% 70-100% < 2 nm 2-50 nm > 50 nm eg columnar,
low porosity medium porosity hi
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