Elsevier

Journal of Crystal Growth

Volume 395, 1 June 2014, Pages 31-37
Journal of Crystal Growth

Impact of strained GaAs spacer between InP emitter and GaAs1−ySby base on structural properties and electrical characteristics of MOCVD-grown InP/GaAs1−ySby/InP DHBTs

https://doi.org/10.1016/j.jcrysgro.2014.03.010Get rights and content

Highlights

  • We demonstrated novel InP/GaAsSb HBTs with a GaAs spacer between the InP emitter and GaAsSb base.

  • We obtained an abrupt E–B interface by inserting the GaAs spacer.

  • DC current gain increased with increasing thickness of the GaAs spacer when the spacer was 2-nm-thick or less.

  • The 0.25-μm-emitter HBTs with the 2-nm GaAs spacer exhibited β, peak ft, and peak fmax at JC=10 mA/μm2 of over 90, 388 GHz, and 290 GHz, respectively.

Abstract

Novel InP/GaAs1−ySby/InP double-heterojunction bipolar transistors (HBTs) with a GaAs spacer between the InP emitter and GaAs1−ySby base layer were grown by the metalorganic chemical vapor deposition method in order to simplify the switching sequence for forming a high-quality InP-emitter/GaAs1−ySby-base interface. After removal of the InP emitter, the top surface of the GaAs spacer exhibits smooth step-flow-like morphology with root-mean-square roughness of 0.17–0.36 nm, whereas the morphology of the GaAs1−ySby base of the sample without the GaAs spacer is bumpy. Secondary ion mass spectroscopy reveals that the spacer suppresses the incorporation of excess Sb into the InP emitter around the emitter–base junction. The dependence of the current gain on the thickness of the GaAs spacer is investigated and, when the GaAs spacer is 2 nm, the highest current gain is obtained. Therefore, we employ the spacer to scaled-down HBTs with a 0.25-μm-wide emitter. The scaled-down HBTs show high current gain of over 90 at collector current density JC of 10 mA/μm2 even though the space between emitter and base electrodes is just 0.15 μm. We obtain peak current-gain cut-off frequency of 388 GHz and peak maximum oscillation frequency of 290 GHz at JC=10 mA/μm2. This result suggests that the presence of the GaAs spacer does not impose any penalty on the characteristics at high JC. The insertion of the GaAs spacer is a good way to obtain a high-quality E–B interface with a simple precursor-supply sequence and thereby HBTs with both high-current gain and reasonably high RF performance.

Introduction

InP/ GaAs1−ySby /InP double heterojunction bipolar transistors (HBTs) are very attractive for use in terahertz devices and ICs for high-bit-rate optical telecommunications systems because they can provide sufficiently high current-gain cut-off frequency (ft) and breakdown voltage (BVCEO) at the same time. Recently, InxGa1−xAs1−ySby-base sub-terahertz HBTs with a reasonably high breakdown voltage have been reported [1], [2]. However, compared to HBTs with an InGaAs base, obtaining a high current gain is still challenging because of difficulty in forming a high-quality InP/GaAs1−ySby interface.

When the InP-emitter/GaAs1−ySby-base interface is being formed during epitaxial growth, such as by the metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) method, the supply of at least seven species (Ga, In, P, As, Sb, n-, and p-type doping) of precursors should be switched on and off. It is well known that Sb tends to be segregated on top of an epitaxial surface. Therefore, one way to obtain a high-quality InP/GaAs1−ySby interface is to suppress the incorporation of Sb into the InP emitter. For this purpose, exposing the GaAs1−ySby surface to an As-rich atmosphere during the growth interruption, which is expected to remove the excess Sb on the GaAs1−ySby surface, has been proposed by several research groups in both MOCVD [3] and MBE [4]. However, the appropriate growth interruption time and precursor-supply sequence greatly depend on the growth apparatus. The process for optimization of the growth condition thus becomes important and tends to be complicated.

Furthermore, the type-II heterojunction at the emitter–base (E–B) interface enhances the recombination current because electrons injected from the emitter are blocked at the junction. One way to resolve the problem is to use a compositionally graded wide-band-gap In(Al, Ga)P emitter in HBTs [5], [6]. By using high-In-content InAlP or InGaP as the emitter, the conduction band offset at the junction is reduced and the current gain therefore improves. Insertion of InAlAs as the spacer between the InP emitter and GaAs1−ySby base also improves the current gain [7]. In this structure, although the InAlAs spacer is not compositionally graded, the current gain is improved by separating the electron-blocking region from the high-hole-concentration base region. However, in both approaches, the switching sequence of the precursor supply tends to be more complicated because ternary or quaternary alloy is used as the emitter or the spacer.

In this paper, in view of this background, we propose a novel HBT structure that has a thin GaAs spacer inserted between the InP emitter and GaAs1−ySby base in order to obtain a high-quality E–B interface with a very simple precursor-supply sequence. We first explain the advantage of HBTs with the GaAs spacer. Then, the structural properties and DC characteristics of large-area HBTs with the GaAs spacer are shown in order to clarify the influence of the insertion of the spacer and optimize the thickness. Finally, the properties of scaled-down 0.25-μm-emitter HBTs at high-current injection are shown.

Section snippets

Experiments

The HBT layers were grown on 3-in. semi-insulating (001) InP substrates in the vertical reactor of an MOCVD apparatus. We used triethylgallium (TEGa) and trimethylindium (TMIn) for group-III precursors, arsine (AsH3), phosphine (PH3), and trimethylantimony (TMSb) for group-V precursors, and disilane (Si2H6) and carbon tetrabromide (CBr4) for doping. H2 was the carrier gas. The growth temperature was over 500 °C for the base layer.

Fig. 1(a) and (b) shows the proposed HBT structure with the GaAs

Structural properties and surface morphology

In this part, we discuss the structural properties and the crystal quality at the E–B interface of HBTs with the GaAs spacer. Fig. 2 shows (004) XRD patterns of the HBT wafers for LADs with a GaAs spacer of 0–5 nm. The Sb content of the GaAs1−ySby base of the samples is 0.50 (near lattice match). Upper and lower profiles are the measured and simulated profiles, respectively. Simulation was performed under the pseudomorphic condition (no lattice relaxation). The broad diffraction peak at about

Conclusion

We demonstrated novel MOCVD-grown InP/GaAs1−ySby HBTs with a thin GaAs spacer between the InP emitter and GaAs1−ySby base and investigated their structural properties and electric characteristics. The precursor-supply sequence for forming the E–B interface was successfully simplified by inserting the GaAs spacer. The surface of the GaAs spacer after removal of the InP emitter exhibited smooth step-flow-like morphology with RMS roughness of 0.17–0.36 nm, whereas that of the GaAs0.50Sb0.50 base of

Acknowledgments

The authors thank M. Mitsuhara for fruitful discussions, and H. Oohashi, T. Akeyoshi and K. Murata for their encouragement throughout this work.

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