Photoluminescence from anodized and thermally oxidized porous germanium

Thin Solid Films 255 ( 1995) 99~-102

Photoluminescence S. Miyazaki, Department

qf Electrical

from anodized and thermally oxidized porous germanium K. Sakamoto, Engineering,

Hiroshrma

K. Shiba, Unirrrsit~~.

M. Hirose

Higushi-Hirmhimu

724, Japan

Abstract A broad photoluminescence (PL) band at - 1.17 eV is observed for as-anodized porous Ge (PG) at room temperature. Oxidation at 600 “C induces a new intense PL band at - 2.15 eV whose spectral shape remains almost unchanged with progressive oxidation. Considering this result and the observed temperature dependence and decay time of the PL from thermally oxidized PG, it has been suggested that the radiative recombination through localized states is a possible pathway of the emission. Kqw_w~ls: Germanium:

Luminescence;

Nanostructures;

Oxidation

1. Introduction Semiconductor nanostructures have attracted increasing attention in fundamental and applied physics because they have great potential to reveal novel quantum phenomena and to open up new device applications. In particular. the recent discoveries of intense visible light emission from porous Si [ 11, Si [2] and Ge [3, 41 nanocrystallites embedded in SiO, matrix and quantized planar Ge structures [5] have created great interest in the possibility of group IV semiconductors for optoelectronic devices. More recently the photoluminescence (PL) properties of porous Si, _,Ge, (0.04 < x < 0.41) [7], porous Sic [S] and carbon clusters embedded in SiO, [9] have also been reported. Despite many efforts to clarify the origin of the luminescence and its mechanism, these topics are still under discussion. In this paper the luminescence properties of asanodized and thermally oxidized porous Ge (PG) layers have been characterized to gain an understanding of the light emission from nanostructures composed of group IV semiconductors. Based on a structural analysis and the temperature dependence and temporal decay of the emission, a possible luminescence mechanism for thermally oxidized PG is discussed. 2. Sample preparation PG -0.4 pm thick was prepared c-Ge( 100) wafers with a resistivity 0040-6090/95jrS9_5C SSDI

0040-6lNO(

(‘

by anodizing p-type of - 30 0 cm in an

1995 ~~~Elsevier Science S.A. All rights reserved

94)05630-7

HF (50 wt.‘%) solution with a current density of l-5 mA cm-‘. During anodization the Ge wafer was illuminated by a 500 W halogen lamp placed 20 cm from the wafer surface. Subsequent oxidation was performed at a temperature of 600 “C in a furnace with a constant gas flow of 10% 0, diluted with N2 for l-120 min.

3. Results and discussion The Fourier transform IR (FT-IR) absorption spectrum of as-anodized PG (Fig. 1) shows distinct absorption bands at _ 580, _ 830 and -2030 cm -’ due to rocking, bending and stretching vibrational modes of hydrogen bonded to the Ge crystalline surface respectively. No significant absorption originating from germanium oxide is observable just after anodization as in the case of porous Si [lo], though the oxidation of the PG proceeds in part by exposure to air at room temperature. When the as-anodized PG is oxidized at 600 “C in 10% O2 diluted in N, for 5 min, no bonded hydrogen remains in the PG and the pore surface is completely covered with oxygen. Transmission electron microscopy (TEM) observation was carried out to reveal the structure of the thermally oxidized PG. As seen in Fig. 2, the high resolution TEM image of the PG oxidized for I5 min clearly indicatds the existence of Ge nanocrystallites with a grain size of 3- 10 nm.

S. Miyazaki et al. 1 Thin Solid Films 255 (1995) 99-102

100

AS-ANODIZED

-

AFTER

7DAYS

IN AIR

6OO’C, OXIDATION,

GeH,

5mln

1.0

0.6

1.8

1.4 PHOTON

4000

3600

3200

2800

2400

2000 WAVENUMBER

1600

1200

800

400

Fig. 3. Room temperature oxidized PC layers.

ENERGY

PL spectra

2.2

II,

(ev)

for as-anodized

and thermally

(cm-‘)

Fig. 1. IR absorption spectra for an as-anodized PC layer, after exposure to air for 7 days and after 600 “C oxidation for 5 min.

1.21

E t

I

I

1.0

5 rd

(r ”

0.8.

E o.6 (0 5 0.4 5 ii!

0.2

0.8

1.0

1.2 PHOTON

1.4 ENERGY

1.6

1.8

2.0

(e’.‘)

(a) 2.4

I

OXIDIZED AT 6OO’C, 18Omin (N,: O,= 10 : 1) Art 48Bnm

Fig. 2. High resolution TEM image of a thermally The oxidation time is 15 min.

oxidized

PC layer.

The as-anodized PG exhibits room temperature luminescence centred at - 1.17 eV with a full width at half-maximum (FWHM) of -0.61 eV under 488 nm excitation as shown in Fig. 3. This broad PL band at an energy above the bulk Ge band gap suggests that the luminescence might occur through a similar mechanism to the case of porous Si. After 1 min of oxidation at 600 “C a new luminescence band with an FWHM of -0.5 eV appears at -2.15 eV at room temperature, band is almost comwhile the - 1.17 eV luminescence pletely quenched. The intensity of the -2.15 eV luminescence band after 30 min of oxidation is enhanced up to 145 times larger than for the as-anodized PG, though the spectral shape shows no significant change. This implies that the luminescence from the thermally oxidized PG occurs through localized states at or near the Ge-GeO, interface rather than via excitonic recombination related to the quantized states in Ge nanocrystallites. Further oxidation causes a slight reduction of

o.oL --7.2

1.4

1.8

1.6 PHOTON

2.0 ENERGY

2.2

2.4

(eV)

@I

Fig. 4. Temperature dependence of luminescence spectra for (a) as-anodized and (b) thermally oxidized PG layers. The oxidation time is 180 min.

the PL intensity, which is presumably due to the decrease in the total volume of the luminescent region. Fig. 4 presents the temperature dependence of the PL spectra for the as-anodized and thermally oxidized PG layers. The luminescence peak intensity for the asanodized PG is gradually increased by a factor of 20 as the temperature decreases from 292 to 18 K, while the peak energy exhibits a blue shift of - 0.13 eV, which is about twice as large as the increase in the bulk Ge band gap, as shown in Fig. 4(a). For the thermally oxidized

S. Miyazaki

0 PHOTON

ENERGY

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et al. 1 Thitz Solid Films 255 (1995) 99- 102

5

10

15

20

25

30

35

40

TIME (nsec)

(eV)

Fig. 5. Comparison between room temperature luminescence spectra for the same sample as shown is Fig 3 measured using the 488 nm line from an Ar’ laser or a 337.1 nm light pulse from an N2 laser.

Fig. 6. PL decay curves measured at various emission wavelengths for the same sample as shown in Figs. 2 and 5. The temporal response in the measuring system is also presented as a reference.

PG the temperature dependence of the luminescence becomes weak. As shown in Fig. 4(b), the PG oxidized at 600 “C for 180 min shows a blue shift of -0.06 eV in the PL peak energy and a monotonic increase by a factor of 1.8 in the PL peak intensity with decreasing temperature down to 18 K. The weak temperature dependence of the luminescence for the thermally oxidized PG might be attributable to the improvement in carrier confinement by the formation of GeOz. When the thermally oxidized PG is excited with a 337.1 nm light pulse with a width of 4-8 ns from an Nz laser, a luminescence centred at -2.65 eV with an FWHM of - 1.45 eV is obtained at room temperature as shown in Fig. 5. The marked change in the PL spectrum suggests that the radiative recombination occurs through localized states. If the luminescence mechanism is dominated by the excitonic recombination related to the quantized states in Ge nanocrystallites, the Stokes shift under 488 nm excitation should be close to the binding energy of excitons. However, the observed Stokes shift of -0.4 eV seems to be too large for the binding energy of exictons even in the Ge nanocrystallites. In order to gain further insight into the luminescence mechanism of the thermally oxidized PG, timeresolved luminescence measurements were carried out in the temperature range from 18 to 293 K. As seen in Fig. 6, the luminescence decay for an emission in the energy range from 1.9 to 2.8 eV takes place in the nanosecond region, but not in the time range from microseconds to milliseconds as typically observed for the red or yellow emission from porous Si [ 111, and exhibits such a weak temperature dependence that the lower energy emission has a slightly slower decay at a lower temperature. Since the luminescence efficiency is still low compared with the case of most luminescent porous Si, it is likely that non-radiative recombinations mainly contribute to this fast decay. It is interesting to note that the spectral shape, temperature

dependence and decay time of the emission band from the thermally oxidized PG are quite similar to those from Ge nanocrystallites embedded in SiOz prepared by an r.f. sputtering technique [2, 31. However, taking into account the fact that the consecutive blue shift in the luminescence cannot be observed with progressive oxidation, the luminescence from the thermally oxidized PG might be associated with the radiative recombination through localized states at or near the Ge nanocrystallite-GeO> interface, whose structure is not yet identified.

4. Conclusions The oxidation of as-anodized PG at 600 ‘C causes complete desorption of the surface hydrogen bonds, resulting in the generation of Ge nanocrystallites covered with GeO,. The broad luminescence band at - 1.17 eV which is observed for as-anodized PG at room temperature is completely quenched by the thermal oxidation and a new broad PL band at -2.15 eV is developed with no significant change in the spectral shape. These results suggest that the emission process of the thermally oxidized PG is governed by the carrier recombination through localized states. This plausible argument can also explain the observed weak temperature dependence and fairly short decay time of the luminescence.

[I] L. T. Chanham, Appl. Phys. Left., 57 ( 1990) 1046. [2] H. Takagi, H. Ogawa, Y. Yamazaki, A. Ishizaki and T. Nakagiri, Appl. Phys. Left., 56 (1991) 2379. [3] Y. Maeda. N. Tsukamoto. Y. Yazawa, Y. Kanemitsu and Y. Masumoto. Appl. Phys. Lett., 59 (1991) 3168.

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[4] Y. Kanemitu, H. Uto, Y. Masumoto and Y. Maeda, A@. Phys. Lett., 61 (1992) 2187. [5] T. Kawaguchi and S. Miyazima, Jpn. J. Appl. Phys., 32 (1993) L215. [6] R. Venkatasubramanian, D. P. Malta, M. L. Timmons and J. A. Hutchby, Appl. Phys. Lett., 59 (1991) 1603. [7] A. Ksendzov, R. W. Fathauer, T. George, W. T. Pike, R. P. Vasquez and A. P. Taylor, Appl. Phys. Lett., 63 (1993) 200.

[8] T. Matsumoto, J. Takahashi, T. Tamaki, T. Futagi, H. Mimura and Y. Kanemitsu, Appl. Phys. Lett., 64 (1994) 226. [9] S. Hayashi, M. Katoaka and K. Yamamoto, Jpn. J. Appl. Phys., 32 ( 1993) L274. [IO] S. Miyazaki, K. Shiba, K. Sakamoto and M. Hirose, Optoelectronics, 7 (1992) 9. [ 111 S. Miyazaki, K. Shiba, K. Sakamoto and M. Hirose, MRS Symp. Proc., 283 (1993) 269.