Si interfaces and their electrical properties

......... C R Y S T A L QIROWTH

ELSEVIER

Journal of Crystal Growth 157 (1995) 344-348

Formation of buried a-Si/A1/Si, a-Si/Sb/Si and a-Si/B/Si interfaces and their electrical properties A.V. Zotov a.b,., F. Wittmann a, j. Lechner a, S.V. Ryzhkov h, V.G. Lifshits b, I. Eisele

a

Institut fiJr Physik, Universitht der Bundeswehr Mi~nchen, D-85577 Neubiberg, Germany b Institute of Automation and Control Processes, Russian Academy of Sciences, 5 Radio Street, 690041 Vladivostok, Russian Federation

Abstract

Si : A1, Si : Sb and Si : B surface phases capped by amorphous Si layers were grown by MBE. The formation of the buried interfaces was studied by low-energy electron diffraction and Auger electron spectroscopy which revealed that, except for the stable Si(111)~/3 × v~-B, the majority of the surface phases suffer either from in-plane redistribution (i.e. break-up of surface reconstruction) or surface segregation of the dopant atoms. The electrical characterization of the grown samples included conductivity and Hall effect measurements between 20 and 300 K. It was found that, while the buried Si(111)x/3 × vr3-B shows metallic behaviour, the buried Si : A1 and Si : Sb surface phases show negligible activation of dopants.

1. Introduction

Considerable interest exists in the formation of delta-function-like-doped spikes in Si (Si delta doping) not only from scientific viewpoint but also because of the technological applications [ 1-6]. The new promising step in the development of the delta doping technique is connected with the formation of the delta-doped layers with the ordered two-dimensional structures of dopants at the buried interface (ordered delta doping). The most apparent way for achieving the ordered delta doping is the formation of the ordered surface phases of adsorbates on Si surfaces [7] followed by deposition of a Si-capped layer. In this work, we have studied the formation and electrical properties of several surface phases of Al,

* Corresponding author. Fax: e91 bet9@ rz.unibw-muenchen.de.

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Sb and B on Si surfaces capped by an amorphous Si (a-Si) layer. We used a-Si deposited at room temperature (RT) as capping material, since growth at such a low temperature provides the greatest promise for preserving the ordered dopant structure at the buried interface.

2. Sample preparation

The samples with buried surface phases were grown in an MBE chamber under ultra-high vacuum conditions with a growth pressure of ( 4 - 8 ) × 10-10 Torr. The in situ formation procedure includes (a) preparation of atomically clean Si(111) and Si(100) surfaces, (b) formation of the surface phases and (c) subsequent deposition of amorphous Si layers. Atomically clean Si surfaces were prepared by heating Si wafers (B-doped 10 f~. cm) at 1200-1250°C for several minutes. After this treatment, no impurity was detected by Auger electron spectroscopy (AES)

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A.V. Zotov et a l . / Journal of Crystal Growth 157 (1995) 344-348

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A. V. Zotov et al. / Journal of Crystal Growth 157 (1995) 344-348

and sharp 7 x 7 and 2 X 1 low-energy electron diffraction (LEED) patterns were displayed for S i ( l l l ) and Si(100) samples, respectively. The desired surface phase was formed by deposition of a specific amount of adsorbate onto the Si substrate held at a fixed temperature. The surface phases under consideration were: S i ( l l l ) v ~ " × v ~ - - B (0.33 ML B, 700°C) [8,9], Si(lll)x/'3Vc3--Al (0.33 ML AI, 600°C) [10], Si(111)a7 X 7-A1 (0.25 ML AI, 400°C) and Si(111)a7 x 7-A1 with AI(111) islands (10 ML A1, 400°C) [11], Si(l 11)v~ X 7r3--Sb (1 ML Sb, 600°C) [4], S i ( l l l ) - A 1 " y - p h a s e " (0.6 ML A1, 600°C) [11], Si(100)2X 2-A1 (0.5 ML A1, 20°C) [12], Si(100)c(4× 12)-AI (0.5 ML A1, 600°C) [12,13]. (1 monolayer (ML) equals 7.8 X 1014 a t o m s / c m 2 for Si(111) and 6.8 X 1014 a t o m s / c m 2 for Si(100), the site density for unreconstructed Si surfaces.) The LEED patterns of the surface phases are shown in Fig. 1. Composition of the surface phases was controlled by AES. After the formation of the desired surface phase, the sample was cooled to RT and an a-Si capping layer of 2 0 - 3 0 nm thickness was deposited. For reference, samples which have undergone the same treatments, except for the dopant deposition (i.e. a - S i / S i samples), were also grown. The formation of the a-Si layer was evidenced by attenuation of all LEED spots after deposition of I - 2 ML Si. The monitoring of the attenuation of the LEED spots during the initial stages of a-Si deposition was used to elucidate the stability of the surface reconstructions. The LEED assessment of the structure stability was based on the assumption that, in case of a stable reconstruction, the attenuation of the LEED normal and extra reflections with a-Si deposition should occur simultaneously at approximately the same rate, while, in case of disruption of the original superstructure, the extra reflections should fade more abruptly as compared to the normal spots. It was found that while Si(111)x/3- × v~--B, Si(111)-A1 " y - p h a s e " , S i ( l l l ) c t 7 × 7 - A I and Si(100)c(4 × 12)-A1 show promise for preserving their ordered structures at a buried interface, other above men-

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Si layer thickness, nm Fig. 2. AES signal of (a) Sb and (b) AI as a function of Si capping layer thickness. Solid lines are calculated dependencies for the growth of Si layers in absence of surface segregation of dopant arums. The surfaces before Si deposition had Si(11 l)!/r3 X Vr3-Sb and Si(111)~7 × 7-A1 structures for (a) and (b), respectively. tioned reconstructions are disrupted upon deposition of 0.2-0.4 ML a-Si. [14]. The redistribution of the dopants in the direction normal to the surface plane was controlled by monitoring the AES signal as a function of a-Si layer thickness. Fig. 2 shows the data for the surface phases of AI and Sb (the absence of surface segregation of B at RT has been confirmed elsewhere [8,15,16]). The solid lines correspond to calculated dependencies for layer-by-layer growth of Si in the absence of smearing. One can see

Fig. 1. The LEED patterns of the surface phases under consideration: (a) Si(111)vr3 X Vr3-B, electron energy E is 82 eV; (b) Si(l 1l)v~ x v/3--AI, E = 55 eV; (c) Si(11])t~ 7 X 7-AI, E = 55 eV; (d) Si(11 l)a7 X 7-Al with Al(111) islands, E = 57 eV; (e) Si(111)v/3 x Vr3-Sb, E = 85 eV; (f) Si(111)-Al '"},-phase", E = 55 eV; (g) Si(100)2 X 2-Al, E = 50 eV; (h) Si(100)c(4 x 12)-Al, E = 50eV.

A.V. Zotov et al. /Journal of Crystal Growth 157 (1995) 344-348

no noticeable surface segregation at RT formation of the buried a - S i / S b / S i interface, while a smearing by a few monolayers occurs in case of a-Si/A1/Si. Thus, this set of samples represents the various situations for the formation of the buried surface phases. First, the surface phases before deposition show a variety of structures and compositions. Second, upon RT deposition, the surface phases demonstrate the different extent of stability: Si(lll)v~X v~--B has the highest stability for both in-plane redistribution (i.e. break-up of reconstruction) and surface segregation. S i ( l l l ) v ~ - X f 3 - S b looses its ordered structure but all Sb atoms seem to remain at the buried interface; in case of Si : A1 surface phases, a certain portion of A1 is smeared towards the surface but, nevertheless, S i ( l l l ) - A 1 "y-phase", S i ( l l l ) a 7 x 7-AI and Si(100)c(4 X 12)-A1 reconstructions might be partially preserved at the buried interface.

3. Electrical characterization

The electrical characterization of the buried surface phases includes conductivity and Hall effect measurements in the temperature range from RT down to 20 K. A magnetic field of 0.5 T was employed. In order to avoid any heat treatment during sample preparation, the contact pads for van der Pauw geometry were formed by sputter etching of the capped layer followed by a deposition of AI. We found that the buried S i ( l l l ) v ~ - × f 3 - B surface phase manifests itself as a degenerate doping layer. A hole concentration of (1.9 + 0 . 2 ) X 1014 cm -z and a mobility of (32 + 3) cmZ/V s were determined at T = 23 K assuming the Hall factor to be 0.75 [17]. Beating in mind that nominal B coverage in the Si(lll)v/3 X v/-3--B phase is 0.33 ML (2.6 x 1014 cm -2) one can suggest that about 75% of boron atoms are electrically active. The present results are in agreement with the data of Headrick et al. [18] who reported for the a - S i / S i ( l l l ) v ~ x v ~ - B interface a carrier density of (2.0 _ 0.3) X l014 c m - 2 and a mobility of (31 _+ 5) cmZ/V s. In case of the buried Si:A1 and Si:Sb surface phases, the freeze-out of carriers at low temperatures was found. In addition, the temperature dependencies of the Hall coefficient and resistance for samples

347

with buried surface phases and for pure a-Si/Si samples coincide within the accuracy of the measurements. Thus, it might be concluded that these buried surface phases do not contribute to the sample conductance, i.e. the electrical activation of the dopants in the phases is negligible. The annealing of the samples at temperatures of epitaxial crystallization converts them to the conventional highly doped delta-type structures. For example, before annealing the sample with Si(lll)x/-3X v/3--Sb (1 ML Sb) surface phase capped by a-Si layer has a carrier density at 23 K of only 3 X 10 9 cm -z, while after annealing at 600°C for 10 min the carrier density becomes 1.1 X 1014 c m -2 with mobility of 52 cm2/V s. (Both latter values are typical for Si : Sb delta-doped layers with high Sb concentration [1,2,5].) The extremely low activation of dopants in the buried Si:Sb and Si:AI surface phases is apparently connected to the amorphous Si cap. We consider difference in electrical conduction of Si(111)f3- X v/3--B and other surface phases capped by a-Si to be connected with the difference in their atomic structure. Though all the Si:A1 and Si:Sb surface phases have different particular atomic arrangements of adsorbates (e.g. Si(1 l l)x/3- X x/3--A1 contains AI adatoms [10], Si(lll)v/3 - X f3--Sb is formed by Sb trimers [19], Si(100)2 X 2-A1 consists of AI dimers [12], the building blocks of Si(100)c(4 X 12)-AI are A1 six-atom clusters [13], etc.), they have one feature in common: adsorbate atoms occupy positions on top of the Si atoms of the substrate. Upon deposition of amorphous silicon they keep their electrically inactive sites within amorphous Si and do not contribute to the sample conduction. In case of Si(111)v~- X v~--B surface phase, B atoms are known to occupy the substitutional sites in the second layer of the Si(111) surface double layer [18], i.e. they already are in the electrically active positions. RT deposition of a-Si does not disturb the structure of the Si:B surface phase and the buried surface phase manifests itself as a metallic layer due to the high concentration of dopants.

4. Conclusion

In conclusion, the formation of the buried surface phases of AI, Sb and B on Si substrates capped by

348

A.V. Zotov et al./ Journal of Crystal Growth 157 (1995) 344-348

amorphous Si were studied by means of low-energy electron diffraction and Auger electron spectroscopy. The Si(111)v~- × V~--B surface phase was found to be the most stable one from the viewpoint of both in-plane redistribution and surface segregation of dopants. Conductivity and Hall effect measurements revealed that only the a - S i / S i ( l l l ) v ~ - × v~--B buried surface phase shows metallic behaviour, while all the Si : AI and Si: Sb surface phases under consideration show extremely small (if any) activation of dopants. The difference in electrical properties can be explained by the difference in the atomic structure of the surface phases: while A1 or Sb atoms occupy the positions on top of the Si substrates, B atoms in the Si(111)¢3 × V~--B phase occupy the substitutional sites in the second layer of the Si(111) surface double layer.

Acknowledgements One of the authors (A.V.Z.) acknowledges the Alexander von Humboldt Foundation for a research fellowship.

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