Impact of silicon substrate germanium doping on diode characteristics and on thermal donor formation

doi:10.1016/j.physb.2009.08.150

Physica B: Condensed Matter

Volume 404, Issues 23–24, 15 December 2009, Pages 4723-4726

https://doi.org/10.1016/j.physb.2009.08.150 Get rights and content

Abstract

Ge-doped Si is of potential interest for specific microelectronics applications; in particular, it is known that its incorporation can lead to an improvement of the radiation hardness of the Si. However, the role of the introduced Ge in the electrical stability of the material is also of special concern. In this contribution we investigate the effects of thermal anneals on the electrical characteristics of p-on-n diodes fabricated on Czochralski-grown Ge-doped-Si and on control Si wafers. Diodes fabricated on high resistivity (HR) magnetic CZ, as well as standard HR float zone (FZ) and oxygenated FZ are also added for comparison. The results show little differences between the electrical characteristics of the devices fabricated on the CZ Si and CZ SiGe substrates; however, the differences increase after thermal anneals. Interestingly and important for device applications, less thermal donor generation is found for the case of the Ge-doped material. The obtained results should be taken into account when defining potential applications of Ge-doped materials.

Introduction

Ge-doped Si is of potential interest for application in microelectronics. In particular, it has been reported that Si–Ge solid solutions at low Ge contents, as well as Si₁₋_xGe_x epitaxial layers, exhibit higher radiation resistance than Si [1], [2], [3]. The improved radiation-hardness is attributed to the role of Ge atoms as centers of annihilation of radiation-induced primary defects [1], [3]. Moreover, its higher absorption coefficient than Si makes it suitable for certain radiation detectors and imaging applications [4]. It has also been reported that doping Si with Ge leads to a reduced formation of oxygen-related thermal donors (TDs) [5], [6], [7]. This may be of particular interest for processing of Si radiation detectors, where temperatures in the range of 450 °C may be reached during back-end and post-metallization annealing. It is believed that Ge can pair with oxygen and vacancies to form stable complexes, while reducing the oxygen flux leading to the small oxygen clusters related to TDs [7]. However, the generation of thermal donors may significantly depend on the involved temperatures and thermal history of the material [8]. With this respect, recently it has been observed that the formation of thermal donors in Ge-doped Czochralski Si may be even higher than in control Si by the application of a rapid thermal annealing pre-treatment [9].

In order to shed some further light on the behavior of Ge-doped Czochralski (CZ) Si material, n-type crystals with (CZ SiGe) and without Ge doping (CZ Si) were studied in the present work. As a difference to previous studies, in which in general unprocessed material was used, in the present study p-on-n diodes were fabricated on polished wafers prepared from both crystals following a well-established diode fabrication process [10]. To our best knowledge, this is the first time that a thermal donor generation comparison between standard Si and Ge-doped Si is performed on fabricated diodes (not on raw/as-received material). This means that the studied materials have been subjected to all the thermal steps of a typical diode fabrication process, including high temperature steps like field isolation and impurity activation anneal. Such thermal donor generation study is of particular interest for back-end steps, as the final metal annealing, which is performed at temperatures and ambient in the range of the thermal treatments studied in the present work. Moreover, such a device-level approach is currently especially opportune, since opposite results for thermal donor generation in standard and Ge-doped Czochralski Si have been published recently [8], [9], indicating that the thermal history of the material can have an important impact on the thermal donor generation in Ge-doped Si compared to control standard Si. For comparison, devices processed on other types of Si materials with different oxygen contents, i.e. high resistivity (HR) standard and oxygenated float zone (FZ and OXFZ, respectively), as well as HR magnetic CZ (MCZ) Si, were also included in the study. The effects of thermal anneals in the range of 250–450 °C on the electrical characteristics of the fabricated p-on-n diodes were investigated.

Section snippets

Experimental details

Two n-type CZ crystals were pulled by QL electronics, in collaboration with State Key Laboratory of Silicon Materials (Hangzhou, PR China). One of the crystals was doped with a Ge concentration of about 1×10¹⁹ cm⁻³, whereas the second did not receive any Ge doping. Both crystals were grown under the same pulling conditions, giving rise to similar thermal histories, final resistivities and interstitial oxygen contents ([O_i]) (Table 1, Table 2). The fabricated p-on-n diodes, with an active area of

Results and discussion

Fig. 1 shows typical current–voltage characteristics (J_leak≡I_leak/Area) measured for two sets of 10 CZ Si and CZ SiGe diodes. From the figure, and also from wafer mapping of the I_leak−V measurements, a clear homogeneity in the characteristics was observed. Slightly higher reverse current levels have been observed for the CZ SiGe diodes compared to their CZ Si counterparts (Fig. 1). Although a somewhat higher free carrier concentration was extracted for the CZ SiGe diodes (2.20×10¹⁴ cm⁻³)

Conclusions

The effects of thermal anneals were investigated in the range between 250 and 450 °C on the electrical characteristics of p-on-n diodes fabricated on Czochralski-grown Ge-doped-Si (CZ SiGe) (with [Ge]=1×10¹⁹ cm⁻³) and control CZ Si devices. Diodes fabricated on high resistivity (HR) magnetic CZ, as well as standard HR float zone (FZ) and oxygenated FZ were also included for comparison. The results show little differences between the electrical characteristics of the devices fabricated on the CZ

Acknowledgements

J. Chen and J. Vanhellemont acknowledge the National Natural Science Foundation of China (NSFC, Grant nos. 50832006 and 50911130014) and the Research Foundation-Flanders (FWO) for financial support.

References (14)

C. Cui et al.
Mater. Sci. Semicond. Process.
(2006)
L.I. Khirunenko et al.
Sov. Phys. Semicond.
(1987)
H. Ohyama et al.
IEEE Trans. Nucl. Sci.
(1994)
M.S. Saidov et al.
Phys. Solid State
(2007)
A. Ruzin et al.
J. Appl. Phys.
(2004)
Y.M. Babitskii et al.
Sov. Phys. Semicond.
(1988)
E. Hild et al.
Appl. Phys. Lett.
(1998)

There are more references available in the full text version of this article.

Cited by (8)

Germanium doping for improved silicon substrates and devices
2011, Journal of Crystal Growth
Citation Excerpt :
Relation (4) is only valid for oxygen concentrations well above 4×1017 cm−3 [33] which explains the less good fit closer to the surface especially for the GCZ diodes. Besides the slower TDD formation in Ge doped substrates, there is a slower thermal donor formation during thermal anneals in a N2 ambient compared to a N2/H2 ambient [20]. This is due to a hydrogen mediated increase of interstitial oxygen diffusivity resulting in a higher TDD formation rate for anneals in a hydrogen containing atmosphere [30].
During the last decade, the 300 mm silicon wafer has been optimized and one is studying the move to 450 mm crystals and wafers. The ever increasing silicon crystal diameter leads to two important trends with respect to substrate characteristics: the interstitial oxygen concentration decreases while the size of grown in voids and crystal originated particles (COPs) in vacancy-rich crystals is increasing.
The first effect is due to the large melt in which movements have to be controlled and partly suppressed by the use of magnetic fields. This magnetic confinement leads to a more uniform dopant incorporation but at the same time to a more limited transport of oxygen from the quartz crucible to the melt and the growing crystal. The reduced interstitial oxygen concentration and the lower thermal budget of modern device processing leads to strongly reduced oxygen precipitation and thus internal gettering capacity.
The increasing COP size (accompanied by a decreasing density) is caused by the decreasing pulling rate and thermal gradient that have to be used in order to avoid dislocation formation. The slower cooling of the crystal leads to a decreased void nucleation rate and at the same time to an increased thermal budget for void growth as well as a larger number of vacancies available per void.
In the present paper the effect of germanium doping in the range between 10¹⁶ and 10¹⁹ cm⁻³ on COP formation and oxygen precipitation is discussed and illustrated. Also the beneficial effect of germanium doping with respect to wafer breakage during processing, with respect to the suppression of thermal donor formation and with respect to improving device radiation hardness is addressed.
Investigating impurities and surface properties in germanium co-doped multi-crystalline silicon: a combined computational and experimental investigation
2024, Journal of Materials Science: Materials in Electronics
Diode characteristics and thermal donor formation in germanium-doped silicon substrates
2013, ECS Transactions
The effect of impurity-induced lattice strain and Fermi level position on low temperature oxygen diffusion in silicon
2011, Journal of Applied Physics
Electron irradiation induced defects in germanium-doped Czochralski silicon substrates and diodes
2011, Physica Status Solidi (C) Current Topics in Solid State Physics
Carrier lifetime studies in diode structures on Si substrates with and without Ge doping
2011, Solid State Phenomena

View all citing articles on Scopus

View full text

Impact of silicon substrate germanium doping on diode characteristics and on thermal donor formation

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusions

Acknowledgements

Mater. Sci. Semicond. Process.

Sov. Phys. Semicond.

IEEE Trans. Nucl. Sci.

Phys. Solid State

J. Appl. Phys.

Sov. Phys. Semicond.

Appl. Phys. Lett.