High-Performance Metamorphic InP/GaAsSb/InP “Type-II” DHBTs Grown on GaAs Substrates

1)Y. Zeng, 1)H.G. Liu, 1)N.G. Tao, and 1)C.R. Bolognesi,

2)W. Tang, 2)W. Zhou, and 2)K.M. Lau

1)Compound Semiconductor Device Laboratory (CSDL), Simon Fraser University, 8888 University Drive Burnaby BC, Canada, V5A 1S6 (E-mail : colombo@ieee.org / tel: 604-291-5964)

2)Dept. of Electrical & Electronic Engineering, HKUST

Clear Water Bay, Hong-Kong (E-mail: eekmlau@ust.hk / fax: 852-2358-1485)

Keywords: HBTs, Metamorphic Growth on GaAs Substrates, Type-II Band Alignment Abstract We report the fabrication of high-performance metamorphic (MM) InP/GaAsSb/InP DHBTs grown on GaAs substrates which show performances comparable or, in certain respects, superior to similar structures grown on InP substrates. The emitter base junction non-ideality factor is found to be significantly higher in the MM devices when compared to the lattice-matched layers: this is believed related to a significantly rougher emitter base junction compared to layers grown lattice-matched on InP. Current gain cutoff frequencies fT as high as 110 GHz have been achieved with a 30 nm base and a 200 nm collector. INTRODUCTION

The staggered (“type-II”) band lineup InP/GaAsSb heterojunction system has proven extremely advantageous in the development of manufacturable high-performance double heterojunction bipolar transistor (DHBTs) by eliminating the requirement for compositional grading at the base/collector heterojunction. The staggered band lineup at the base/collector hetero-interface allows the fabrication of high-performance transistors with uniform layers, and without the growth and processing complications associated with the use of compositional grading schemes. This favorable combination of properties has enabled InP/GaAsSb/InP DHBTs with cutoff frequencies exceeding 300 GHz and breakdown voltages BVCEO > 6 V with a 2000 Å InP collector [1], and we have recently demonstrated a record fT × BVCEO product of > 2300 GHz-V in the same system [2]. Interest in the growth of InP-based device structures on cheaper and sturdier GaAs substrates has existed for some time now because of the cost and potential yield challenges associated with manufacturing on InP. In the present work, we report the fabrication of high-performance metamorphic (MM) InP/GaAsSb/InP DHBTs

with performances comparable, and in certain respects even superior to similar structures grown on InP substrates.

EXPERIMENTAL WORK

The samples used here were grown by MOCVD on GaAs substrates at the Hong Kong University of Science and Technology (HKUST). From the substrate up the layers consist of a 700 nm InP buffer, a 300 nm GaInAs sub-collector layer doped with Si at 1019 /cm3, a 200 nm InP collector with a Si doping ranging from 5- to 2 × 1016 /cm3, a 50 (30) nm GaAs0.51Sb0.49

base layer doped with C at 3 (5) × 1019 /cm3 providing a sheet resistance of ~1,400 (~1,200) Ω/sq., a 50 nm InP emitter layer doped with Si at 3 × 1017 /cm3, and a 100 nm GaInAs emitter contact layer doped with Si at 1019 /cm3. Self-aligned transistors were fabricated by optical contact lithography at the SFU CSDL using a process similar to that reported in [1].

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B

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Fig. 1) B/C diode characteristics for MM InP/GaAsSb DHBT grown on GaAs substrate: The ideality and breakdown characteristics correspond to those observed on InP.

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Fig. 1 shows the base-collector diode I-V characteristics for the metamorphic DHBT: the collector diode ideality factor is nC ~ 1.3, which is exactly the same value found in structures grown on InP substrates. On the other hand, the emitter-base diode ideality factor in the MM structures is nE = 1.6-1.7 which is significantly higher than the expected value of 1.05, and suggestive of grading and/or roughness at the interface between GaAsSb and the InP emitter. It is not yet clear if the discrepancy is related to the metamorphic growth or to the protocol for the E/B interface. Atomic force microscopy (AFM) characterization on the layers reveals a much rougher surface for the MM layer stack when compared to similar structures grown on matched InP substrates. Fig. 2 compares AFM measurements for layers grown on a) InP and b) GaAs substrates: whereas the layers in a) provide a smooth featureless surface (RMS roughness = 0.36 nm; average roughness 0.27 nm), the MM structure is characterized by an RMS roughness of 6.2 nm and an average roughness 4.9 nm. This may well account for the high non-ideality factor of MM E/B junctions.

a)

b)

Fig. 2) AFM scans on InP/GaAsSb DHBT structures grown on a) lattice-matched InP substrates, and b) GaAs substrates.

Figs. 3-4 compare the x-ray diffraction data for MM and LM DHBT growths. Whereas fine structure to the right of the InP substrate peak is visible in Fig. 3, no such structure can be detected in the MM samples.

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Fig. 4) X-ray diffraction data for MM InP/GaAsSb/InP DHBT structure. No clear fine structure can be discerned around the MM-buffer peak.

Common-emitter current gain values as high as β = 20 were measured at VBC = 0 V. Fig. 5 shows the common-emitter IC-VCE characteristics for a MM-DHBT with a 1 × 24 μm2 emitter contact: the characteristics are well-behaved and show a low collector leakage current ICEO < 60 μA at VCE = 1 V (i.e. open circuited base contact) and a very low collector offset voltage ΔVCE = 25 mV. The transistor features a common-emitter open-base breakdown voltage BVCEO = 8.75 V which is somewhat higher than the ~6V we usually observe with material grown on InP. The observation of higher breakdown voltages in MM structures with presumably higher defect densities than lattice-matched structures is interesting and merits further investigation.

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[V]

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Fig. 5) Common-emitter I-V characteristics for a typical MM InP/GaAsSb DHBT with a 1 × 24 μm2 emitter. The collector offset voltage is 25 mV.

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Fig. 6) Extracted microwave frequency response of 1 × 24 μm2 MM DHBT with a 50 nm base and a 200 nm collector. The base sheet resistance was ~1100 Ohms/sq as determined by TLM measurements.

Fig. 6 shows the measured RF characteristics for a 1 × 24 μm2 transistor at VCE = 1.5 V and JC = 1.1 mA/μm2: extrapolation of the current gain h21 and of Mason’s unilateral power gain U with a –20 dB/dec roll-off yields fT = 90 GHz and fMAX = 100 GHz. We note that the SFU group reported fT = 106 GHz in [3] for a 1 × 24 μm2 transistor built with a 40 nm base and a 200 nm InP collector (BVCEO = 8 V). The present MM-DHBT RF performance with a 50 nm base certainly appears to be comparable to that achieved on similar structures grown on InP substrates.

Fig. 7 shows a similar device built with a 30 nm base layer and using a slightly higher base doping level of 5 × 1019 /cm3: in this case fT = 111 GHz, but fMAX = 60 GHz only at VCE = 1.5V due to a poor base Ohmic contact in this particular process run which showed values of ~2 × 10–6 Ohm-cm2, or about 100× higher than achieved in the 50 nm base sample (this was probably due to an equipment malfunction during the metallization process, because the collector contact was also found to be abnormally high [4×] in this run). It seems likely that the present results

underestimate the potential performance of the 30 nm base sample.

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FT = 111 GHz / F

MAX = 60 GHz

U H21

Fig. 7) Extracted microwave frequency response of 1 × 24 μm2 MM DHBT with a 30 nm base and a 200 nm collector. The base sheet resistance was ~960 Ohms/sq as determined by TLM measurements. CONCLUSIONS This work demonstrates the potential of metamorphic growth for the realization of high-performance InP/GaAsSb/InP DHBTs. Surprisingly, the B/C diode behaves very well despite its proximity to the GaAs substrate. On the other hand, more work is needed to clarify the role of MM growth on the E/B junctions which show abnormally high junction non-ideality factors. ACKNOWLEDGEMENTS The SFU work was supported by NSERC Strategic Partnership and Discovery grants. The HKUST activities were supported by Research Grant Council of Hong Kong (HKUST6249/02E). The authors acknowledge Prof. S.P. Watkins for access to the x-ray diffraction and AFM equipments at SFU. REFERENCES [1] M.W. Dvorak, C.R. Bolognesi, O.J. Pitts, and S.P. Watkins, 300 GHz

InP/GaAsSb/InP double HBTs with high current capability and BVCEO>6 V, IEEE Electron Dev. Lett. 22 (8) pp. 361-363 (2001).

[2] H.G. Liu, C.R. Bolognesi, and S.P. Watkins, 15-nm Base Type-II

InP/GaAsSb/InP DHBTs with fT = 384 GHz and a 6V BVCEO, IEEE Trans. Electron Dev., accepted for publication (2006).

[3] M.W. Dvorak, N. Matine, C.R. Bolognesi, X.G. Xu, and S.P. Watkins,

Design and performance of InP/GaAsSb/InP double heterojunction bipolar transistors, J. Vac. Sci. Tech., 18 (2) pp.761-764 (2000).

ACRONYMS

DHBT: Double Heterojunction Bipolar Transistor MOCVD: Metal Organic Chemical Vapor Deposition AFM: Atomic Force Microscopy MM: Metamorphic TLM: Transmission Line Measurement

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160 CS MANTECH Conference, April 24-27, 2006, Vancouver, British Columbia, Canada