IEEE ELECTRON DEVICE LETTERS, VOL. 12, NO. 7, JULY 1991 357

Heterojunction Bipolar Transistors with SiGe Base Grown by Molecular Beam Epitaxy A. Pruijmboom, Jan W. Slotboom, Member, IEEE, D. J . Gravesteijn, C. W. Fredriksz, A. A. van Gorkum, Member, IEEE, R. A. van de Heuvel, J. M. L. van Rooij-Mulder,

G. Streutker, and G. F. A. van de Walle

Abstract-High-quality SiGe heterojunction bipolar transis- tors (HBT’s) have been fabricated using material grown by molecular beam epitaxy (MBE). The height of parasitic barriers in the conduction band varied over the wafer, and the influence of these barriers on collector current, Early voltage, and cutoff frequency was studied by experiments and simulations. Temper- ature-dependent measurements were performed to study the influence of the barriers on the effective bandgap narrowing in the base and to obtain an expression for the collector-current enhancement.

I. INTRODUCTION iGe-BASE heterojunction bipolar transistors (HBT’s) S offer the possibility to exploit the advantages of HBT’s,

using standard silicon processing. Much of the recent effort on SiGe HBT’s is focused on structures grown by chemical vapor deposition (CVD) techniques [ I]-[4]. Structures fabri- cated so far, using molecular beam epitaxy (MBE), generally display poor base-current characteristics [ 5 ] , [6]. The electri- cal characteristics of the Si and SiGe n-p-n transistors, pre- sented in this letter, indicate that high-quality HBT’s can also be realized by MBE. We describe experiments in which the location of the junctions varied over the wafer. The influence of the base-collector (BC) voltage V,, on parasitic conduc- tion-band barriers is studied by simulations and experiments. We will show that these barriers are detrimental for the current gain, the Early voltage, and the cutoff frequency of the transistors. From temperature-dependent measurements, we will demonstrate that the collector-current enhancement of our HBT’s can be described by a single exponential function with a temperature-independent prefactor.

11. DEVICE FABRICATION AND ELECTRICAL RESULTS

Highly uniform Sio,80Geo,20 HBT structures, as well as identical Si base structures, were grown in a previously described MBE system (71, on Sb-doped substrates (10-2Q . cm) of 100 mm diameter. The emitter and collector layers consist of lowly doped (2 x l O I 5 cmp3), respectively, 250- and 300-nm-thick n-type Si. The 50-nm-thick base layer was doped p-type (3 x IO’* ~ m - ~ ) by evaporation of boron-doped Si. Mesa etching and passivation by TEOS were used to

Manuscript received December 10, 1990; revised April 8, 1991. This work was supported in part by ESPRIT project 2016 TIPBASE.

The authors are with Philips Research Laboratories, 5600JA Eindhoven, The Netherlands.

IEEE Log Number 9101230.

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Depth [nm] - Fig. 1. SIMS depth profile of As, P, B, and Ge, as measured in the emitter of a Sio.80Geo.20 HBT. Of the boxlike Ge profile, only the electrically relevant part (above 2%) is shown. Note the difference between the P profile on the center and edge of the wafer, caused by stronger channeling on the center of the wafer.

obtain lateral isolation. The actual emitter was formed using a double implant (Pt: 2 x l O I 3 cm-2, 90 keV; AS+: 2 x IOI5 cmp2, 50 keV). A furnace anneal of 10 min at 850°C was used to activate the emitter- and base-contact implants. Due to channeling, P+ is implanted much deeper than pre- dicted by, e.g., SUPREM3 simulations. This is seen in the SIMS results of Fig. 1. The implantation damage has given rise to enhanced diffusion of the as-grown boxlike B profile. This is confirmed by the fact that the boron profile remained boxlike on those sites on the wafer that did not receive an emitter implant. As a result, the emitter-base (EB) and BC junctions do not coincide anymore with the edges of the Ge profile. Since channeling strongly depends on the implanta- tion angle, there is a considerable difference between the P profiles on the center and edge of the wafer (see Fig. I).

Gummel plots of Si- and SiGe-base transistors on the center of the wafer, with an emitter area of 12 x 62 pm2, are displayed in Fig. 2 . Both the base and the collector current of the Si-base transistor are ideal over several decades, showing, for the first time, high-quality transistors on MBE material. The SiGe HBT shows an ideal collector current and a small nonideality in the base current. Since the characteris- tics improve with increasing annealing time, we speculate that the latter is due to some residual implantation damage. The collector-current enhancement of an n-p-n SiGe HBT, with a constrant bandgap narrowing AE, in the base, rela-

0741-3106/91/0700-0357$01,00 0 1991 IEEE

358

, 10-1 , 1

VCb [vl - I I 1 I I I I I 500 600 700 800 900

v,,Wl - Fig. 2. Gummel plot of a Si (dashed line) and a SiGe (solid line) transistor, with an emitter area of 12 x 62 pm2, on the center of the wafers. The inset displays the effect of V,, on Z, for a Si and SiGe transistor at a fixed V,, of 600 mV.

t c n - centre , . . . . . . . . .

I

T 6:: z o m

E -0.5

-1 .o

0 0.2 0.4 0.6 0.8

Distance [pm]

Fig. 3. Approximated doping profiles for transistors on the center (solid line) and the edge (dotted line) of the wafer, used as input for 1D-device simulations and the resulting band structures at V,, = 500 mV and V,, = 2.5 V.

tive to one with a Si base is given by

-- (Ic)SiGe (NcNuDn)SiGe

(‘,)si (NcNuDnIsi -

where D, is the electron diffusion coefficient in the base, and N, and Nu are the densities of states. The collector-current enhancement of the fabricated HBT’s appears to be strongly dependent on the location on the wafer. This behavior can be understood in terms of the above-mentioned variations in the location of the junctions. If the EB and/or BC junctions are located in Si instead of in SiGe, parasitic barriers develop in the conduction band [8]. These barriers lead to a much smaller enhancement of the collector current. In order to study these barriers, the SIMS profiles were modeled by Gaussians (see Fig. 3). Using a modified version of the 1D-device simulator TRAP [9], the band structures of Fig. 3

IEEE ELECTRON DEVICE LETTERS, VOL. 12, NO. I, JULY 1991

C- Temp. [“C] 200170140 I I I I 110 80 I 1 50 20

l$ -

2 v

o v } “edge

1 oo L--+--+ 1000[K~l] __c

T

Fig. 4. Zc(SiGe)/Zc(Si) as a function of T - ’ at V,, = 0 and 2 V, for a Sio,80 Ge,,,, HBT and an identical Si-base transistor on the center and edge of the water. The much lower E,’s on the edge of the wafer reflect the presence of the large barrier at the BC junction, which can only be slightly reduced by increasing Vcb.

were obtained. As shown in the inset of Fig. 2, I, of the SiGe HBT on the center of the wafer increases much more strongly with increasing V,, than that of the Si transistor. The strongest increase occurs between 0 and 2 V. This apparent lower Early voltage of the HBT is due to lowering of the barrier at the BC junction for increasing Vc,. It is seen in Fig. 3 that at V,, = 2.5 V no barriers are present for the transistor on the center of the wafer. The transistor on the edge, however, still has considerable barriers. As a result, I, on the center of the wafer is a factor of 13 higher than on the edge of the wafer. Our simulations show that charge storage in the base increases strongly, and hence, fT decreases when barriers occur. On the center of the wafer, lowering the BC barrier, by increasing V,, from 0 to 2 V, leads to a pro- nounced increase in fT from 6 to 12 GHz. On the edge of the wafer, the barrier cannot be removed and, therefore, fT is much lower (1.2 GHz) and almost independent of V,, . An extensive study of these effects is given in [lo]. Similar results have been reported for p-n-p transistors [ l l ] . The effect of the barriers also appears in the temperature depen- dence of I,(SiGe)/Ic(Si). In the case of barriers, (1) should be replaced by the more general expression

For a constant bandgap narrowing in the base and assuming that heavy doping effects are the same for Si and SiGe, EA = AE,(SiGe) and C = ( D n ~ , N ” ) s i G e / ( ~ n N , N u ) s i . When barriers are present, however, EA < A E,(SiGe). The effect of barriers is nicely demonstrated in Fig. 4, where I,(SiGe)/ I,(Si) is plotted as a function of T- ’ . On the center of the wafer, EA = 98 meV is found at V,, = 0 V. At Vcb = 2 V, the BC barrier has been removed completely and EA = 153 meV is obtained. This value is in good agreement with the bandgap narrowing found by Prim et al. [8].

PRUUMBOOM et al. : HETEROJUNCTION BIPOLAR TRANSISTORS WITH SiGe BASE 359

Extrapolating the measurements to T - = 0 yields C = 0.4. This value is equal to the reduction of N,N, calculated for Sio.80Geo,2,, [8], indicating that the minority mobility for electrons in Si and SiGe is about the same.

111. CONCLUSIONS

We have fabricated homojunction and heterojunction tran- sistors, which to our knowledge are the best produced so far using MBE material. We have produced HBT’s with varia- tions over the wafer of the location of the junctions and, hence, of the height of parasitic barriers in the conduction band. We have shown, both from simulations and measure- ments, that these barriers have a detrimental effect on the current gain, Early voltage, and cutoff frequency. From temperature-dependent measurements we have shown that the collector-current enhancement of SiGe HBT’s, with and without barriers, can be described by an exponential function with a temperature-independent prefactor.

ACKNOWLEDGMENT

The authors like to acknowledge J . Blake and J . Slatter for the modification of TRAP for heterostructures and R. Vriezema and P. Zalm for the SIMS measurements.

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