Structural evaluation of low-temperature-grown InGaAs crystals on (0 0 1) InP substrates
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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Present affiliation: Graduate School of Advanced Science and Engineering, Hiroshima University, Japan.