Fabrication of Luminescent Porous Silicon Layers
Using Extremely Dilute HF Solutions

182, 591 (2000).
N. Noguchi and I. Suemune, Appl. Phys. Lett. 62, 1429
(1993).

3

Hideki Koyama and Keiichi Takemura

Figure 1 shows the photoluminescence (PL) intensities
of PSi samples prepared at different HF concentrations
and etching current densities. It is shown that efficient PL
can be obtained at HF concentrations down to 0.1%. In
the case of 0.1 % HF, the most efficiently luminescentsample can be obtained at a very low etching current
density of ~ 0.1 mA/cm2. This is due to reduced criticalcurrent densities as reported by Zhang et al.1 It should be
noted that such low current densities can never be used toobtain luminescent PSi layers at high HF concentrations.
The use of dilute HF solutions, therefore, is advantageous
to obtain a thin layer of luminescent PSi by a reduced
etching current density.

1010.1p-Si (100) 0.01-0.02 Ωcm
Etched in the dark0.1110100

HF concentration (%)

Fig. 1. PL intensities of PSi samples prepared withdifferent HF concentrations and etching current
densities. The PL intensities are classified into fivegrades and shown with different symbols. Crosses
indicate samples that have suffered from partial or total
electropolishing.
35
Etched in the darkwith 0.1% HF for 30min

3025

n-Si (100) 0.05-0.5 Ωcm
0.07 mA/cm

2015

2p-Si (100)0.01-0.02 Ωcm
20.1 mA/cm

10501.4

1.6

1.8

22.2

Photon energy (eV)Fig. 2. PL spectra of two PSi samples prepared with
0.1% HF.
102Illumination intensity (mW/cm )This work was supported in part by a research grantfrom Hyogo Science and Technology Association and by
a Grant-in-Aid for Scientific Research from the Japan
Society for the Promotion of Science.

100

0.01

Figure 2 shows PL spectra of two different PSi
samples prepared with 0.1% HF. In addition to p-type
wafers, n-type wafers can be etched into luminescent PSi
layers without illumination.
The fact that luminescent PSi layers can be formed atvery low etching current densities can also be utilized toobtain luminescent PSi layers without external bias. We
have formed PSi samples on n-type wafers under shortcircuit conditions. The etching current was provided by
the short-circuit photocurrent resulting from illumination
performed during the etching procedures. As shown in
Fig. 3, luminescent PSi layers can be obtained at
illumination intensities as low as ~ 0.02 mW/cm2. These
intensities are remarkably smaller than those required toform luminescent PSi layers by open-circuit
photochemical etching.3

Totally or partiallyelectropolished100-1000
10-1001-100.1-1
0-0.12Etching current density (mA/cm )Light emitting porous silicon (PSi) layers can beformed by anodically etching Si wafers in HF solutions.
In most cases concentrated (10-50 %) HF solutions areused. Zhang et al.1 have measured the critical current
densities for the formation of PSi layers over a wide rangeof HF concentration. Their results show that the criticalcurrent densities do exist at HF concentrations as low as
0.1%. This implies that PSi layers can be formed withthese extremely dilute HF solutions. In their paper,
however, no information is given on the luminescence
properties of their PSi samples. Luminescent PSi layers
prepared with dilute HF solutions are especially beneficialfor applications in the field of education.2 In this study,
we have confirmed that efficient luminescence can beobtained for HF concentrations down to 0.1%. We havealso found some other advantages of preparing
luminescent PSi layers in such dilute HF solutions.

1000

PL intensity (arb. units)Technology Education Group
Department of Practical Life Studies
Hyogo University of Teacher Education
Yashiro, Hyogo 673-1494, Japan

10.1
Inhomogeneous
100-1000
10-1001-100-10.01
n-Si (100) 0.05-0.5 Ωcm0.001
0.1110100

HF concentration (%)
1X.G. Zhang, S.D. Collins, and R.L. Smith, J.
Electrochem. Soc. 136, 1561 (1989).
2V.P. Parkhutik and L.T. Canham, Phys. Stat. Sol. (a)Fig. 3. Results of the short-circuit photoelectrochemicaletching. Measured PL intensities are graded and plottedwith different symbols.