Dopant incorporation in epitaxial germanium grown on Ge(100) substrates by MBE

Journal of Crystal Growth 111 (1991) 847—855 North-Holland

847

Dopant incorporation in epitaxial germanium grown on Ge( 100) substrates by MBE V.P. Kesan, S.S. Iyer and J.M. Cotte IBM Research Division, T.J. Watson Research Center, Yorktown Heights, New York 10598, USA

We have conducted the first studies of dopant incorporation (gallium, boron, and antimony) in MBE germanium grown on Ge(100) substrates. Excellent germanium films have been grown on Ge substrates with little demarcation of the substrate—epi interface. A first order kinetic incorporation model has been used to describe the behavior of gallium in germanium. Gallium doping of germanium takes place through an adlayer it the growth front and between growth temperatures of 450 and 550°C successful p-doping of germanium by gallium can be accomplished. Boron is an excellent p-dopant in germanium with good activation at high concentrations and sharp transition profiles. The incorporation of antimony in germanium at growth temperatures 450°Cis poor and needs further investigation.

1. Introduction There is renewed interest in germanium-based devices for FET and bipolar applications. High carrier mobilities at room temperature and 77 K, superior projected high frequency performance, and the possibility of a wide bandgap silicon emitter are some of the advantages of a germanium-based technology, There have been several studies describing growth of Ge on GaAs or vice versa by MBE [1—3].Attempts to grow gallium-doped germanium layers on GaAs have been quite unsuccessful, with poor dopant activation [3]. In this paper we report the first studies of both p-type (gallium- and boron-doped) and n-type (antimony-doped) doping in MBE germanium grown on Ge(100) substrates.

2. Substrate preparation and growth conditions Ge(100) substrates were cleaned using a modified RCA clean suitable for germanium. The first step consisted of a degrease in a 1 : 1 : 4 solution of HC1 : H202: H20 at room temperature for 3 mm followed by a rinse in dc-ionized water. The Ge 0022-0248/91/$03.50 ~ 1991

—

wafers were then etched in H202 for 3 mm while stirring continuously, and then rinsed in dc-ionized water. This was followed by a quick dip in 1 : 10 HF: H20 for 10 s which leaves the wafer surface hydrophobic and then loaded into the MBE systern. The wafers were heated at 650°C for 1—2 mm in the MBE growth system just prior to growth. It is important to keep this step short, since long term heating of Ge wafers at high temperatures before growth results in a rough, undulating substrate (see fig. 1). Fig. 1 shows a cross-sectional TEM of a boron-doped Ge film grown at 450°C on such a Ge substrate and subsequent growth on this rough Ge substrate is seen to cover the rough substrate—epi interface. Also, germanium films grown at high (>650°C) temperatures exhibit poor morphology. Fig. 2 shows a cross-sectional TEM of a boron-doped Ge film grown at 450°C on a Ge substrate cleaned using the procedure described above. The substrate—epi interface is significantly better than in fig. 1. The excellent quality of the epitaxial boron-doped Ge layer is also evident in fig. 2. Germanium films grown at temperatures between 350—550°C on Ge substrates consistently resulted in excellent epitaxial layers. In order to study the incorporation of p and n

Elsevier Science Publishers B.V. (North-Holland)

848

VP. Kesan et aL

/ Dopant

incorporation in epitaxial Ge grown on Ge(100) by MBE

Fig. 1. Cross-sectional TEM micrograph of a boron-doped Ge film grown on a Ge substrate at 450°C. Prior to growth, the Ge substrate was taken to elevated temperatures ( > 650°C),which results in a rough, undulating, substrate—epi interface.

dopants in MBE grown germanium, effusion cell sources were used for antimony and gallium, and boron doping was achieved with a custom c-beam source. The Ge layers were analyzed using SIMS, spreading resistance analysis (SRA), and planar and cross-sectional TEM. The spreading resistance profiles were primarily used to determine dopant activation only, since the transition between doped

and undoped layers in spreading resistance profiles are often inaccurate particularly when the layer thicknesses are less than 0.5 ~tm. Absolute values of carrier concentration in the SRA profiles are within ±20%. A kinetic model for dopant incorporation was used to analyze the observed doping behavior in germanium, and this is described in the next section.

Fig. 2. Cross-sectional TEM micrograph of a boron-doped Ge film grown on a Ge substrate at 450°C. Prior to growth the Ge substrate was cleaned using a modified RCA clean suitable for germanium and the improved substrate—epi interface (compared to fig. 1) is evident.

V.P. Kesan et aL

/ Dopant

incorporation in epitaxial Ge grown on Ge(100) by MBE

3. Model for dopant incorporation The kinetics of spontaneous dopant incorporation during epitaxial germanium growth by MBE can be described by a simple adsorption—incorpo-

849

Let us now consider a clean host surface and apply a step flux, FD; i.e., at t < 0, FD =0 and at t 0, FD FD. We then have =

dNDs/dt

(K1 + KD)NDS, FD ‘1 —(K!+KD)t K1 + KD ~ e

=

FD

—

(8a)

ration—desorption model [4]. In the most general form this is

N

dN0~/dt FD

The time constant of importance here, by

=

—

~KDP(NDS)~

—

K1N05,

(1)

~

—

Dskt)

—

where N~5is the surface concentration of the adsorbed dopant species at the growth front, FD is the dopant flux, KD is the desorption coefficient, p is the order of desorption, and K1 is the incorporation coefficient. The incorporation and desorption processes are activated processes with characteristic energies associated with them. Eq. (1) can be solved once a value for p is chosen. The case p 1 is the most relevant and has been used to describe dopant incorporation in silicon [4]. Eq. (1) then reduces to

T

dNDS/dt= FD

NDR(t)

=

—

KDNDS

—

KINDs.

(2)

Analyzing eq. (2) in the steady state, i.e., with dNDS/dt 0, we get =

ND5

=

FD/( KD + K1).

(3)

=

—

)

(K + K I

—

exp[—(K1

+

KD)tI). (10)

If the growth rate is constant the temporal vanation is easily transformed to a spatial variation by

V=Foe/NO.~

=

(11)

Vt, .

where x is the coordinate of film thickness and v the velocity of growth. This velocity is given by .

.

(12)

Hence, NDB(x)

=

FD ( s~—N0~1 exp —

/ K1 + KD \ — ~

~

(5) D

(13) In the above transient analysis we have assumed

The resultant steady state bulk doping concentration NDB in the epitaxial Ge film is given by

that K1 and K0 are constants, which means that the temperature is constant and there are no

—

S~

(6)

saturation effects. The quantity [(K1 + KD)/v A is the spatial counterpart of r. It represents the thickness of film over which variations in

where N0 is the number of germanium atoms per 3. Simplifying eq. (6), we get cm NDB S(FD/FGe)NO. (7)

doping occur response changes density in doping flux. inFurther, bothtor sudden and A become smaller as the temperature of growth is increased.

NOB

dN

—

K1 K1+K

(9)

—

=s~-Not1

dN1~~/dt=K1 FD/(KD+Kl).

dNjnc/dt FD

is given

and is the characteristic dopant incorporation time. When the Ge—dopant system is subjected to a sudden change of flux, the system approaches its new steady state in an exponential manner in time. The initial rate of approach is given by 1/’r. Since the incorporation rate follows the surface concentration behavior, the bulk doping concentration as a function of time is F

x

The rate of the number of atoms that incorporate to those that are incident, on a per unit time basis in steady state, is the sticking coefficient, s. Thus

T,

D

Once steady state has been reached, the number of incident atoms per second is equal to the sum of those that desorb and incorporate per second. Hence, the rate of incorporation is given by (4)

8b

.

=

~

‘d ~ N0, Ge

=

=

850

VP. Kesan et aL

/ Dopant

incorporation in epitaxial Ge grown on Ge(100) by MBE

The dopant and germanium fluxes are determined from growth rates and effusion cell calibration. The characteristic dopant incorporation time, T, can be determined from the slope of the trailing/leading edge of the dopant profile in the epitaxial Ge layer and from eqs. (7) and (13). Using eqs. (3) and (9), we can determine the surface concentration of the adsorbed dopant species, NOB. The incorporation coefficient, K1,

not readily incorporate in germanium and instead doping takes place through the formation of an adlayer at the growth front. Fig. 3a shows the SIMS profile of a 4000 A Ga doped Ge layer grown on a 1000 A undoped Ge buffer with a 1000 A undoped Ge cap at 550°C.The transition between the doped and undoped layers on the leading and trailing edges of the Ga doped film is gradual clearly indicating incorporation from an adlayer at the growing surface. Fig. 3b, which shows a spreading resistance profile of the same layers seen in fig. 3a, indicates about 22% gallium activation in the doped germanium layer. Also seen is some gallium segregation at the Ge substrate—epi interface and fig. 3b shows this gallium at the interface to be active. In addition to thick Ga doped Ge films, 1000 A Ga doped Ge films separated by 1000 A undoped layers were grown at different growth temperatures between 350°C and SIMS 550°Cprofile at different Ga afluxes. the of such film Fig. with4aa shows total thickness of 1.1 ~tm and grown at 550°C. A

can then be determined from eqs. (3), (7), and (9). Since r is known, we can now compute the desorption coefficient, K0, and the sticking coefficient, s. This model is now applied to study galhum incorporation in germanium.

4. Gallium doping of gennanium The gallium K-cell was calibrated by growing amorphous Si films at 250°Cand a wide2.range of s have gallium flux from 10b0_1013 atoms/cm been examined in these experiments. Gallium does

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VP. Kesan et a!.

/ Dopant

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incorporation in epitaxial Ge grown on Ge(IOO) by MBE

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Fig. 4. (a) SIMS profile of 1000 A Ga-doped Ge films separated by 1000 A undoped layers grown at 550°C at different Ga fluxes. The total film thickness is 1.1 jim. (b) Spreading resistance profile of the same film as seen in (a).

spreading resistance profile of the same film is seen in fig. 4b. Ga doped Ge layers similar to the ones shown in figs. 3 and 4 were grown at different temperatures and their SIMS and SRA profiles were analyzed using the model described in the previous section. The characteristic dopant incorporation time, T, for a growth temperature of 450°Cwas 150—250 s and at 550°C, T was 50—100 s. Fig. 5 shows the incorporation coefficient, K1, for two different growth temperatures as a function of surface gallium concentration, NOB. The incorporation coef ficient is constant with increasing surface coverage indicating that an increase in surface gallium concentration translates directly into higher incorporation in the bulk layer (see eqs. (5) and (7)). In addition, fig. 5 shows that no saturation effects (as indicated by a decreasing K1) are observed for surface coverages up to 0.5—LO ml. The incorpora tion coefficientbetween also increases almost order of magmtude growth by at 450 andan 550°C and hence the resulting galhum concentration in films grown at 550°Cis almost an order of mag-

nitude larger than in films grown at 450°C. The activation energy for incorporation obtained from a plot of K 1 versus 1/T was found to be apI0’

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2) . . (atoms/cm Fig. 5. Incorporation coefficient, K 1, for two different growth temperatures as a function of surface gallium concentration, N09.

852

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CONCENTRATION (cm SRA data for Fig. 6. Activation ATOMIC ratio obtained from SIMS and two different growth temperatures as a function of the SIMS atomic doping concentration,

proximately 1.3 eV. This value is very similar to the activation energy for incorporation of gallium in silicon [5]. This suggests that gallium incorporation in germanium, like in silicon, involves the clustering of gallium atoms on the surface, with subsequent incorporation proceeding by gallium I

I

~I2Ol3OI4OI5L DEPTH (~sm) Fig. 7. (a) SIMS profile of a 4000

A

I

breaking away from the periphery of these

the activation energy for incorporation obtained in our experiments should match data on the surface diffusion activation energy for gallium on germanium. The desorption coefficient, K0, was

-

U

atoms

clusters and surface diffusing to an available nearby kink site. If this mechanism were true, then

-

0

in epitaxial Ge grown on Ge(JOO) by MBE

less than 5 x i0—~and the sticking coefficient, s, for gallium on germanium was between 0.25 and 1.0 for the entire range of growth conditions cxamined. Fig. 6 shows a plot of the activation ratio obtained from SIMS and SRA data for two different growth temperatures as a function of theactivaSIMS atomic doping concentration. The dopant tion of gallium increases with increasing atomic concentration and reaches unity at doping levels greater than 1019 cm The unity activation at high doping maybe due to band-tailing effects. The poor activation at higher growth temperatures, for a particular value of atomic concentration (see fig. 6), may be attributed to the fact that there is increased clustering of gallium atoms at the surface at higher temperatures. Here, it is important to note that no gallium precipitates ~.

a

DEPTH (sm)

boron-doped Ge film grown on a 500 A undoped buffer at 450°C. (b) The corresponding spreading resistance profile of the same sample.

VP. Kesan et at

/ Dopant incorporation in epitaxial

Ge grown on Ge(100) by MBE

1019,

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Boron doping of germanium was accomplished using an c-beam source for boron. Fig. 7a shows the SIMS profile of a 4000 A boron-doped Ge film grown on a 500 A undoped buffer at 450°C and fig. 7b shows the corresponding spreading resistance profile. Fig. 8a shows the SIMS profile of a sample with 500 A boron-doped germanium pulses separated by 500 A undoped Ge layers and fig. 8b shows the SRA profile of the same sample. The limited dynamic doping range seen in this film is due to our custom c-beam source. The interfacial boron spike [6] can be seen in both samples shown in figs. 7 and 8. By comparing the

I

-

I0~~oI2

DEPTH (sm)

5. Boron doping of gennanium

I

~ 1018

03

014

05

DEPTH (~a.m)

Fig. 8. (a) SIMS profile of 500 A boron-doped germanium pulses separated by 500 profile of the same sample.

were seen in cross-sectional TEM even at the highest doping levels obtained of 1020 cm3. While growth at high temperatures around 550°C may result in increased dopant incorporation, the fraction that is active is actually reduced compared to growth at 450°C. Thus the window for optimal growth of gallium doped Ge films is quite small, between 450 and 550°C. Through the use of pre-buildup and flash-off techniques [4], it may be possible to obtain gallium doped germanium with sharp transition profiles.

I

853

A undoped Ge layers grown at 450°C. (b)

SR.A

SRA and SIMS profiles in figs. 7 and 8, it is can be seen that 85—100% of the boron in germanium is active. Fig. 2, which shows a TEM cross-section of a boron-doped Ge film, also points to the excellent quality of these layers. Hence, boron is clearly the preferred p-dopant in germanium and uniform doping profiles at high concentrations with sharp transitions can be achieved.

6. Antimony doping of gennanium The antimony K-cell was calibrated using amorphous Si films grown at 250°C and a wide range of antimony flux from 1011 to 5 x 1013 atoms/cm2. s were examined. Fig. 9 shows the SIMS profile of a 0.9 ~&mGe film intended to produce 1000 A thick antimony-doped Ge pulses separated by 1000 A undoped Ge layers and grown at 450°C. The leading edge transition of the first antimony-doped layer is sharp, but over a wide range of surface coverage, the bulk antimony doping concentration is relatively constant and does not turn off even when the surface atomic concentration of antimony is not replenished during growth of the undoped Ge layers. Th same film as shown in fig. 9, when grown at 550°C, results in lower bulk antimony concentration. SR.A profiles

854

VP. Kesan et aL I

• I

I

I

I

I

/ Dopant

incorporation in epitaxial Ge grown on Ge(100) by MBE

fims grown at such low temperatures is correspondingly higher. Clearly, antimony doping of germanium does not proceed along similar lines as in silicon and will be studied in greater detail in the future.

I

• N)

E ~ lOIS e~J\, E ~

2

.

7. Conclusion

~

•~

ANTIMONY K-CELL

•

SHUTTER OPEN ANTIMONY K-CELL SHUTTER CLOSED

~ • W

-

425 375

-

325

-

275

~

Sb

~ ~

-

l0~

I -

• I 1016 0.0

~

0.2

~~\\

0.4 0.6 0.8 DEPTH (sm)

1~

1.0

Fig. 9. SIMS profile of a 0.9 ~smGe films intended to produce 1000 A thick antimony-doped Ge pulses separated by 1000 A undoped Ge layers and grown at 450°C. The antimony K-cell temperature during growth and the K-cell shutter status is shown in the inset.

on these samples showed 20—30% activation of the antimony in the germanium layers. Fig. 10 shows a TEM cross-section of the same sample described in fig. 9. A number of “hair-pin” defects of stacking faults can be seen at the top of the film. It may be possible to achieve spontaneous antimony incorporation by reducing the growth temperature to around 300°C, but the defect density in Ge

In summary, we have studied dopant incorporation (gallium, boron, and antimony) in MBE germanium grown on Ge(100) substrates. Excellent germanium films have been grown on Ge substrates with little demarcation of the substrate—epi interface. Gallium doping of germanium takes place through an adlayer at the growth front, and between growth temperatures of 450°C and 550°C, successful p-doping of germanium by galhum can be accomr~lished Boron is an excellent p-dopant m germamum with good activation at high concentrations and sharp transition profiles. The incorporation of antimony m germamum at growth temperatures 450°C is poor. Further reduction in the growth temperature to around 300 °Cmay increase antimony incorporation, with a penalty of increasing defect density in these films. .

~‘.

Acknowledgements We would like to acknowledge the excellent technical assistance of Bruce Ek and F.K. LeGoues for the invaluable TEM work.

1.

Fig. 10. TEM cross-section of the same sample described in fig. Q. A number of hair-pin~defects of slacking faults can be seen at the top of the film.

VP. Kesan eta!.

/ Dopant

incorporation in epiraxia! Ge grown on Ge(100) by MBE

855

References

[4] S.S. Iyer, in: Epitaxial Silicon Technology, Ed. B.J. Baliga (Academic Press, Orlando FL, 1986).

[1] R.W. Grant, J.R. Waidrop, S.O. Kowalczyk and E.A. 1.raut, J. Vacuum Sci. Technol. B3 (1885) 1295. [2] N. Chand, J. Kiem and H. Morkoç, AppI. Phys. Letters 48 (1986) 484. [3] S. Strite, M.S. Unlu, K. Adomi, G.-B. Bao and H. Morkoç, IEEE Electron Device Letters EDL-11 (1990) 233.

[5] S.S. Iyer, PhD Dissertation, Univ. of California, Los Angeles (1981). [6] S.S. Iyer, S.L. Delage and G.J. Scilla, Appi. Phys. Letters 52 (1988) 486.