ELECTRONIC STATES AND TRANSPORT PROPERTIES OF AN n-TYPE δ-FUNCTION DOPING LAYER IN p-TYPE Si

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ELECTRONIC STATES AND TRANSPORT PROPERTIES OF AN n-TYPE δ-FUNCTION

DOPING LAYER IN p-TYPE Si

G. Tempel, F. Koch, H. Zeindl, I. Eisele

To cite this version:

G. Tempel, F. Koch, H. Zeindl, I. Eisele. ELECTRONIC STATES AND TRANSPORT PROPERTIES

OF AN n-TYPE

δ-FUNCTION DOPING LAYER IN p-TYPE Si. Journal de Physique Colloques,

1987, 48 (C5), pp.C5-259-C5-262. �10.1051/jphyscol:1987555�. �jpa-00226759�

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JOURNAL DE PHYSIQUE

Colloque C5, suppl6ment au noll, Tome 48, novembre 1987

ELECTRONIC STATES AND TRANSPORT PROPERTIES OF AN n-TYPE &-FUNCTION DOPING LAYER IN p-TYPE Si

G. TEMPEL, F. KOCH, H.P. ZEINDL* and I. EISELE*

Physik-Department E 16, Technische Universitdt ~iinchen, D-8046 Garching, F.R.G.

'~achbereich Elektrotechnik, Universitdt der Bundeswehr, D-8014 Neubiberg, F.R.G.

Des couches bien definies e t extrbement minces d'atomes de Sb ont Bt'e introduites dans l e Si de type p pendant l a deposition par MBE.

Nous avons mesure l e transport parallel e t perpendiculaire a' l a couche dopee.

Ainsi nous avons determine 1 es niveaux el ectroni ques des sous-bandes 2-dimensionel 1 es.

Abstract

An extremely sharp and well-defined sheet of Sb-dopant atoms can be embedded in Si with high p-type background during MBE growth.

W have measured the transport parallel and perpendicular to the doping layer. The e electronic 1 eve1 s of 2-dimensional subbands have been detected.

Introduction

For various electronic applications, such as the camelback diode and transistor devices, i t i s required to have extremely sharp and abrupt high density doping profiles. In Ref. /1/ we have shown how an n-type Sb-layer in Si can be incorporated in an epitaxial layer during MBE growth. The method involves a stop and grow pro- cedure whereby Sb ions are deposited a f t e r the growth has been interrupted and the layer cooled t o room temperature. The Sb i s subsequently covered with ^%30

0

of amor- phous Si and f i n a l l y recrystallized a t the growth temperature of 7000 C when growth i s continued. In t h i s wnner a doping layer of Sb atoms has been achieved that extends only a few l a t t i c e planes in the growth direction.

In e a r l i e r work the so-called 6-doping layer has been embedded in weakly p-type ( 2 x 1016 cm-3) material. Quantized electronic subbands have been observed that con- firm the sharpness of the layer. In addition the sharp Sb doping layer has been observed directly in a TEM bright f i e l d image.

Present work has aimed t o produce the n-type Sb layer next to strongly p-type material in order t o simulate the conditions necessary in devices where on the scale of a few 100

6I

the potential i s to r i s e rapidly. The unoccupied electronic subbands above EF a r e t o be confined by the p-type depletion barrier potential and not i n a near accumulation situation such as i n the previous work. Our concern in t h i s work has been t o study the transport in the layer and via tunneling currents normal to the layers.

Experimenta I Notes

A large number of &-layer samples with different Sb density has been grown in succession in order to establish the technique of solid phase epitaxy. Doping levels of 1015 to l o t 3 cm-2 have been given for the conductivity studies. The number of Sb atoms actually incorporated d u r i n g the growth i s not known precisely.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987555

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JOURNAL DE PHYSIQUE

PI&

^{r i g}t f i e l d TEM image of an Sb 6-doping layer with density 5 x 1013 cm-3 and 200

1

distance from the Si surface

Fig. 2: Resistance per square vs. NS from-Hall measurements f o r many different

6-layer samples. The transition t o an insulatin s t a t e occurs j u s t below

3

% 1 x 1 0 1 cm-2.

The sharpness of the layer t h a t has been achieved is i l l u s t r a t e d most convin- cingly by the bright f i e l d TEM images. Fig. 1 i s an example with an Sb areal density of 5 x 1013 cK2. The dark band shows i t t o be confined t o ^%2 nm. The estimated resolution of the microscope i s i t s e l f limited t o scattering of the order of 1 nm.

A controlled thickness of surface layer (b 20 nm) i s subsequently added a f t e r the solid phase epitaxy t o achieve the right magnitude of tunneling resistance t o the surface Schottky barrier contact. The peaked doping profile seen in the figure i s certainly much sharper than could be measured by SIMS. The l a t t e r technique has been used to provide a count of the Sb atoms f o r comparison Mith the electrically

measured density of active dopant.

In order t o achieve an abrupt and high density p-n junction, we proceed as fol- lows. MBE growth i s carried out in the presence of a Ga flux from an effusion c e l l . The partial pressure i s such a s t o produce ^%f018 cm-3 p-type material in calibra- tion experiments. I t i s well known that because of segregation the growing surface i s covered under these conditions with a very high concentration of Ga atoms. When interrupting the growth we f i r s t heat the sample t o 900° C from i t s growth tem- perature (% 7000 C ) t o drive off excess Ga. This step i s followed by the room temperature deposition of the Sb 6-layer.

Result and Oi scussion

For high density of Sb atoms in the 6-layer the electron system i s degenerate.

The electronic levels form a s e t of 2-dimensional subbands in a V-shaped potential we1 1.

We have studied both the Hall effect and surface r e s i s t i v i t y of many &-layer samples in order t o characterize t h e i r electrical properties and t o learn how the nominal Sb concentration introduced during growth relates t o the measured c a r r i e r density Ns. Fig. 2 shows the results of p, vs. the measured Hall density N,. Samples are such that the Sb-layer i s embedded in nominally undoped background material, sufficiently f a r from the surface in order t o avoid significant c a r r i e r transfer t o the surface. W note the sharp r i s e of the resistance a t low e NS indicating the

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Fig. 3: Tunneling current derivative

-%

Potential distribution and m a s observed f o r the &-layer energy evels f o r the &-layer sample sample. The subband energies a r e with p-background doping of

marked according t o the calculation. 1.0 x 1018 cm-3.

t r a n s i t i n to an insulating s t a t e . The average separation f o r the c r i t i c a l density i s % 30

!.

The Hall densities Ns do not necessarily measure correctly the electron concentration i n the &-layer because two or more subbands a r e occupied. The subband electrons have different transport masses and mobilities. Measurements that compare the total SIMS Sb concentration with the Hall density NS are in progress.

In order t o characterize the sharp p-n junction we have chosen to examine the tunneling current flowing from the &-layer to a surface Schottky contact (see i n s e r t in Fig. 3). Such measurements have- previously been carried out f o r &-layers in GaAs /2/ and i n our previous work / I / . The steepness of the potential well i s characterizd best by examining the unoccupied electronic levels above EF. These a r e seen f o r electron currents tunneling into the doping layer fram the metal contact f o r negative values of the Schottky gate potential. Fig. 3 shows an example of such a tunneling spectrum. The sample has a &-layer with design doping of ^Q,2 x 1013 cm-2 Sb on a p-type background of 1.0 x 1018 Ga cm-3. The Schottky barrier contact i s made by evaporating Ni-Cr and Ag. The contact has a typical barrier height of 0.65 eV.

The dI/dVg structures in Fig. 3 are particularly clearly resolved f o r negative Vg as expected. On1 y weak tunneling conductivity structures identify the occupied levels f o r positive Vg. The situation i s similar to that found by Kunze /3/ f o r tunneling into the Si-MOS structures in the presence of a strong substrate bias potential.

W proceed w i t h the analysis of the data in Fig. 3 by calculating the potential e profile and subband levels f o r the known p-type doping and the expected surface barrier potential ^Q,0.65 eV). The number of 6-layer electrons corresponding t o the density of 2 x 1013 Sb atoms i s adjusted t o produce the f i t of levels to the dI/dV structures as indicated by arrows i n Fig. 3. The marks a r e corrected with regar! t o the change of the energy levels when a gate voltage i s applied.

In Fig. 4 the calculation of the potential nd subband structure with a gate voltage of 200 mV and Ns value f 1.3 x 1013 i s shown. Levels 0 and 0' are found to be occupied. 0.7 x 1013 cmdq electrons are l o s t ' t o the depletion charge and the Schottky gate. The levels a r e labelled in accordance w i t h the nomenclature estab- 1 ished f o r MOS inversion layer subbands on (100) Si. Their degeneracies a r e respec- tively gv = 2 and 4 f o r the unprimed and primed numbers. The subband energies of 0 and 0' are very sensitive to the spread of the &-doping. Spreading fluctuations in

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C5-262 JOURNAL DE PHYSIQUE

t h i s case may account f o r the missing of these levels which were seen clearly in previous tunneling spectra 111. Insofar as the calculation f o r Fig. 4 reproduces the dI/dV structures seen in the experiments i t provides a measure of the potential profiye in the surface p-n junction region.

When a structure such a s employed here i s illuminated with band-gap l i g h t one expects the dI/dVg peaks to s h i f t , because the p-n junction potential i s reduced under nonequilibr~um conditions. For the present example the Tight e f f e c t i s quite small. This can be understood by considering the valence band and acceptor s t a t e s also shown in the diagram. For the high doping levels the junction width i s so small that extra electrons due t o the illumination in the higher subbands are rapidly l o s t by tunneling t o neutral acceptor s t a t e s under the nonequilibrium conditions. I t follows t h a t i t i s more d i f f i c u l t t o perturb the electronic s t a t e s in such a sharp doping prof i 1 e.

This work was supported by Siemens AG Miinchen, SFE "Physik und Technologie fur mikrostrukturierte Bauelemente". The authors a r e grateful t o T. Wegehaupt f o r his SIMS measurements and H. Oppolzer f o r his TEM pictures.

References :

/1/ H.P. Zeindl, T. Wegehaupt, I. 'Eisele, H. Oppolzer, H. Reisinger, G. Tempel, and F. Koch, Appl. Phys. Lett. 50 (171, p. 1164

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1166, f987

/2/ ^M. Zachau, F. Koch, K. PlooK P. Roentgen, H. Beneking, Solid State Commun.

59 (81, p. 591

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594, 1986

/3/ Kunze, G. Lautz, Solid State Commun.

42

( I ) , p. 27

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^{31, 1982}