E. V. Astrova, V. B. Voronkov, I. V. Grekhov, A. V. Nashchekin, and A. G. Tkachencko A. F. Ioffe Physicotechnical Institute, Russian Academy of Sciences, St. Petersburg
共Submitted July 16, 1999兲
Pis’ma Zh. Tekh. Fiz. 25, 72–79共December 12, 1999兲
An investigation is made of the diffusion of boron and phosphorus impurities in macroporous silicon with a regular structure of deep cylindrical pores, for which through doping of the walls was achieved. The⬃150m layers obtained were quasiuniformly doped and had a planar diffusion front, and their electric parameters were very similar to those of the
doped single crystal. It is demonstrated that deep diffusion of phosphorus may be used to fabricate n –n⫹ structures. © 1999 American Institute of Physics.关S1063-7850共99兲01212-4兴
INTRODUCTION
The development of the technology to produce deep cy- lindrical pores 0.6–20m in diameter and up to 500m deep1–4 in n-type silicon opens up extensive prospects for using this material in various fields.5–9 Macroporous aniso- tropic silicon (ma –Si兲 is also a promising material for elec- tronics. Unlike microporous silicon, macroporous silicon has a low porosity (⬍10%) and in many respects retains the properties of the single crystal. Thus, macroporous silicon may be considered as some new material having effective values of macroscopic parameters, such as resistivity, per- mittivity, and so on. The aim of the present paper is to study various possibilities for using macroporous silicon to fabri- cate semiconductor structures.
FABRICATION TECHNOLOGY FOR MACROPOROUS SILICON
The starting material was
具
100典
-oriented non-crucible zone-refined n-type silicon having resistivity ⫽200 ⍀•cm. After thermal oxidation, the 400m thick polished wafers underwent photolithographic treatment which opened up a square network of circular holes in the oxide 10m in di- ameter and a⫽30m apart. Standard alkali etching formed pits in the oxide, in the form of inverted pyramids with a square surface. Deep pores were etched electrochemically using a 2.5% aqueous solution of HF for 4 h with the back of the wafer illuminated by its self-emitted light (⭐800 nm兲 at 4 V and current density j⫽1.66 mA/cm2, which corresponds to a current Ip p⫽15 nA per pore. The shape and size of thepores was monitored using a transverse cleaved section of the sample using a scanning electron microscope共see Figs. 1a and 1b兲. These pores had a depth l⫽120m and diameter d⫽10m, and their density was 1.1⫻105pore/cm. These technological parameters correspond to a stable etching re- gime when the current density at the bottom of the pore is equal to the critical current density for transition to elec- tropolishing jps⫽20 mA/cm2 共Ref. 2兲 and the diameter is
satisfactorily described by3 d⫽a共 j/ jps兲1/2⫽8.6m.
The porosity is easy to estimate using the formula
p⫽ 4
冉
d a冊
2 ,which in our case gives p⫽8.7%.
DOPING OF MACROPOROUS SILICON
Previous attempts have been made to use a porous struc- ture to introduce slowly diffusing impurities into silicon. However, the method has not been widely used because of the radical difference between the properties of ‘‘ordinary’’ microporous silicon and single-crystal Si. Using macroporous silicon can avoid this major disadvantage. There have already been reports of this being used to form deep vertical p-walls preventing the breakdown of high- voltage p –n junctions along their edges.11The regular pore structure used by us can produce well-controlled porous lay- ers having the same wall thickness, which ensures uniform doping of the porous layer when a diffusion source is intro- duced into the inner surface of the pores共Fig. 1c兲. In order to achieve through doping of the walls with boron acceptor impurities, the diffusion zones must overlap in the horizontal plane and the diffusion front must be equalized near the lower part of the pores. If the depth of the p –n junction in the single-crystal silicon is x, the following condition must be satisfied for the boundaries of the p –n junctions to close around the pores
xj min⫽
1
2共
冑
2a⫺d兲,i.e., for our samples xjmin⫽16.2m. Attention should be
drawn to the fact that if the scale of the entire structure is changed, for example, by reducing a and d with the ratio d/a kept constant, the depth xjmay be reduced but the properties
of the material are conserved, since the porosity remains the same. For example, for a⫽3m and d⫽1m we obtain xj
min⫽1.62m and to obtain a p –n junction of this depth the temperature and duration of diffusion may be reduced appre- ciably.
TECHNICAL PHYSICS LETTERS VOLUME 25, NUMBER 12 DECEMBER 1999
958
DIFFUSION OF BORON
In our experiments macroporous silicon was doped with boron from a 1% alcohol solution of H3BO3 at T⫽1250°C for 3 h to the depth xj⫽30m in the single-crystal region,
which is almost twice xjmin. This allowed us to obtain a p –n
junction 150m deep in the macroporous silicon 共see Fig. 2兲. Staining a polished section showed that the boundary
of the p –n junction is horizontal in the region where the pores enter the plane of the cross section and between rows
共see Fig. 3兲.
The resistivity was measured using a four-probe method in the porous and single-crystal regions immediately after diffusion 共Figs. 2a and 2b兲 and after grinding the p-layer in the single-crystal region共Fig. 2c兲. These values are given in
FIG. 1. Macropores in Si: a and b — SEM images, c — diagram showing introduction of dopant.
FIG. 2. Schematic of sample structure: a兲 top view; deep pores etched in central region; b兲 cross section; c兲 cross section after grinding a 35m thick layer from above; d兲 structure to measure the parameters of a p –n junction on single-crystal and macroporous Si.
FIG. 3. Optical-microscope image of cross section through boron-doped macroporous silicon sample after exposing p –n junction共the p region is a light color兲. On the left-hand side of the photograph the pores do not reach the plane of the polished section although the formation of a deep p –n junction can be seen.
959
Table I where the average resistivity of the layer ¯
⫽RS•xj, and the corresponding average boron concentration N¯B are taken from published data for uniformly doped
single-crystal silicon.12In regions with and without pores we fabricated p –n junctions with vertical walls, obtained by mechanical polishing an annular groove, and Ni contacts
共Fig. 2d兲. These structures were used to measure the hole
lifetimes p by the Lax method. Table I gives values ofp
measured by switching from the forward current Ij⫽100 mA
to the reverse Ir⫽50 mA in the single-crystal and
macroporous Si regions, respectively. It can be seen that¯ for doped macroporous silicon is slightly higher than that for single-crystal silicon although this has a sufficiently low value for many applications andp is slightly lower.
DIFFUSION OF PHOSPHORUS
Diffusion of phosphorus into macroporous silicon with a pore depth l⫽165m was carried out using a 10% alcohol solution of H3PO4at 1240°C for 5 h (xj⫽25m兲. Figure 4a
shows the depth distribution of the surface resistance Rs ob-
tained as a result of layer-by-layer grinding of the sample on the pore side. The measurements were made in regions 1, 2, and 3 shown on the inset. Note that the shunting influence of the lower n-Si layer can be neglected when analyzing the results of the measurements in regions 1 and 2 since its re- sistance is several orders of magnitude higher than the resis- tance of the diffusion layer. It can be seen that in the single- crystal region 2, the n⫹layer ceases to have an influence at x⬇50m whereas for the macroporous silicon region 1 this is ⬃200m. The resistivity of phosphorus-doped macroporous silicon estimated using the formula ¯⫽Rs
⫻(l⫹xj⫺x)⫽Rs(190⫺x)⫻10⫺4⍀•cm is plotted in Fig.
4b. It can be seen that¯ remains low as far as values of x corresponding to the pore depth.
The standard substrate for the formation of bipolar de- vices is a highly doped thick n⫹ layer. Its role is to reduce the thickness of the base and thus the resistance connected in series with the p –n junction. This n –n⫹structure is usually fabricated by epitaxial growth of thin n-Si on a highly doped substrate or a thick n⫹ layer on single-crystal Si whose thickness is then reduced 共‘‘inverted’’ epitaxial structures兲. By using deep doping of macroporous silicon with donor impurities, it is possible to replace the existing epitaxial tech- nology for fabricating n –n⫹ structures with diffusion tech- nology. Measurements of Rs in region 3 共Fig. 4a兲 demon- strate an effective reduction in the resistance achieved using this type of layer. Because of the low porosity of macroporous silicon this substrate should possess good me- chanical strength and the problem of compatibility between this technology and other operations involved in fabricating devices may be resolved by masking the inner surface of the pores with thermal oxide. There is reason to assume that replacing the complex and ecologically harmful epitaxy pro- cesses with relatively simple etching and diffusion processes is economically viable.
CONCLUSIONS
We have therefore confirmed that replacing single- crystal silicon with macroporous silicon can appreciably in- crease the thickness of the diffusion layers while negligibly changing the properties of the material and can be used to fabricate device structures, in particular n –n⫹ structures to produce a thick n⫹layer.
FIG. 4. Resistivity distribution in various regions of the sample after phos- phorus diffusion: a — surface resistivity, b — volume resistivity in region containing pores.
TABLE I. Parameters of layers obtained by boron diffusion.
Rs,⍀/䊐 ,⍀•cm NB, cm⫺3 p,s
After diffusion Single-crystal Si 0.261 7.5⫻10⫺4 1.8⫻1020 8
After diffusion Macroporous Si 0.168 2⫻10⫺3 6⫻1019 4
After removing 35m Macroporous Si 0.470 4⫻10⫺3 3⫻1019
This work was financed by the RFBR Program of Sup- port for Scientific Schools and the Ministry of Science Pro- gram ‘‘Physics of Solid-State Nanostructures.’’
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