Elsevier

Journal of Crystal Growth

Volume 548, 15 October 2020, 125852
Journal of Crystal Growth

Structural evaluation of low-temperature-grown InGaAs crystals on (0 0 1) InP substrates

https://doi.org/10.1016/j.jcrysgro.2020.125852Get rights and content

Highlights

  • Segregation of As precipitates at epi/sub interface in undoped LTG-InGaAs.

  • Absence of precipitates in Be-doped LTG-InGaAs after annealing.

  • Absence of precipitates at dislocation core in LTG-InGaAs after annealing.

  • Detection of surface defects due to evaporation of As atoms during annealing.

  • Physical origin for absence of precipitates in Be-doped InGaAs after annealing.

Abstract

Through transmission electron microscopy and related techniques, we performed structural evaluation of low-temperature grown (LTG) InGaAs crystals on (0 0 1) InP substrates. These crystals were grown at a substrate temperature of 200–250 °C. The results revealed that As precipitates (diameter: 7–15 nm) are generated in undoped InGaAs crystals (mainly near the interface between the epi-layer and the substrate) after annealing at 550 °C for 1 h. However, no precipitates were observed in Be-doped LTG InGaAs crystals before and after annealing. Moreover, in the case of Be-doped LTG-InGaAs crystals highly mismatched with the InP substrate, we observed no preferential generation of precipitates at the core of the misfit dislocations. Based on these findings, the generation mechanism of the precipitates in undoped LTG-InGaAs crystals during the annealing process and physical origin for the absence of precipitates in Be-doped LTG-InGaAs crystals after the annealing are discussed. Apart from the precipitates, we have also found that surface defects elongated in the [1 1 0] direction are formed in the InGaAs crystal, which are presumably generated by the evaporation of As-atoms from the surface during annealing.

Introduction

Photoconductive antennas (PCAs) activated by femtosecond-regime optical pulses have been widely used for terahertz (THz)-wave emission and detection in THz time-domain spectroscopy (THz-TDS) [1], [2], [3]. Low-temperature-grown (LTG) GaAs is characterized by a relatively high carrier mobility (~200 cm2/V·s), an ultrashort carrier lifetime (<1 ps), and a large resistivity (~107 Ω·cm) [4], [5], [6], which represent highly desirable properties for PCAs. For the PCAs fabricated from LTG GaAs, mode-locked Ti:sapphire lasers with wavelengths of a 0.8 μm-region are used as light sources for the emission and detection of THz waves by PCAs. Recently, an attempt has been made to replace these lasers with femtosecond fiber lasers characterized by wavelengths of a 1.5 μm-region. This will allow widespread use of THz-TDS systems because fiber lasers are less expensive and more compact than Ti:sapphire lasers. Therefore, InGaAs is the most widely studied material for PCA applications [7], [8], [9], [10], [11], [12]. Difficulties persist, however, in simultaneously realizing an ultrashort carrier lifetime and a large resistivity in InGaAs. The three aforementioned properties of LTG GaAs originate from the defects caused by As antisites (AsGa) and Ga vacancies incorporated during low-temperature growth and from the high density of As precipitates formed after annealing [5], [6], [13]. Therefore, incorporation of these point defects into the crystals may also play a key role in the case of LTG InGaAs. A previous study confirmed, via transmission electron microscopy (TEM), the formation of As precipitates in undoped LTG InGaAs after annealing [14]. Other previous studies indicated that Be-doping of LTG InGaAs reduces the residual carrier density and lifetime [15], [16], [17], [18], [19]. Nevertheless, using scanning tunneling microscopy and spectroscopy (STM and STS), one of these studies indicated that the Be-doped LTG InGaAs contained significantly fewer AsGa defects than the undoped material [19]. Systematic TEM investigations of As precipitate formation in undoped and Be-doped LTG InGaAs have been reported in only a few studies. Hence, in this work, defects including As precipitates in both undoped and Be-doped InGaAs crystals are evaluated and compared via TEM and related techniques.

Section snippets

Experimental procedure

Using solid sources of In, Ga, and As, the LTG InGaAs samples were grown on semi-insulating (0 0 1)-oriented InP substrates via molecular beam epitaxy (MBE). Details of the growth procedures are reported elsewhere [20], [21]. We also grew an undoped LTG-InxGa1-xAs layer for comparison of defect features with those of the Be-doped layer. The V/III beam equivalent pressure (BEP) ratios were adjusted in the range between 26.7 and 102.4 to control the In content, x, in InxGa1-xAs. After growth, the

Evaluation of undoped-InGaAs crystals: Precipitates

We evaluated the MBE-grown undoped InGaAs crystals using TEM. Fig. 1(a) shows a low-magnification (1 1 0) bright-field cross-sectional TEM (XTEM) image of an undoped InGaAs crystal grown on a (0 0 1) InP substrate. This image was obtained from the 220 reflection with a positive s, which corresponds to a deviation parameter from the perfect Bragg conditions. The crystal was grown at 240 °C and annealed at 550 °C for 1 h (see Sample 1 in Table 1). Two types of defects are observed: i) threading

Conclusions

Undoped and Be-doped LTG-InGaAs crystals have been evaluated via TEM and related techniques. In the undoped InGaAs crystals, As precipitates (diameter: 7–15 nm) were observed after annealing at 550 °C for 1 h. The results revealed that the precipitates are formed mainly near the interface between the epi-layer and the substrate and that the density of these precipitates decreases significantly in the upper region of the epi-layer. However, in Be-doped LTG InGaAs crystals no precipitates were

CRediT authorship contribution statement

Osamu Ueda: Conceptualization, Data curation, Funding acquisition, Investigation, Project administration, Resources, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing. Noriaki Ikenaga: Investigation, Resources, Writing - review & editing. Shingo Hirose: Investigation. Kentaro Hirayama: Investigation. Shunsuke Tsurisaki: Investigation. Yukihiro Horita: Investigation. Yoriko Tominaga: Funding acquisition, Investigation, Resources, Writing - review &

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to express their appreciation to Professor Yutaka Kadoya of Hiroshima University for providing some of the InGaAs samples. We also thank Dr. Makoto Maeda of Natural Science Center for Basic Research and Development, Hiroshima University, for his help with SEM observation. This work was partially supported by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) KAKENHI [grant numbers JP17K05044, JP18K14140, and JP19H04548] Japan, and the

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  • 1

    Present affiliation: Graduate School of Advanced Science and Engineering, Hiroshima University, Japan.

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