Ge/III-V heterostructures for GaAs-based long-wavelength light sources
- 주제(키워드) Germanium , Isobutlygermane , metalorganic chemical vapor deposition (MOCVD) , epitaxial growth , single crystal , heterostructure , long-wavelength light source , light-emitting diode (LED) , edge-emitting laser diode (LD) , vertical-cavity surface emitting laser (VCSEL)
- 발행기관 아주대학교
- 지도교수 이재진
- 발행년도 2023
- 학위수여년월 2023. 8
- 학위명 박사
- 학과 및 전공 일반대학원 전자공학과
- 실제URI http://www.dcollection.net/handler/ajou/000000033028
- 본문언어 영어
- 저작권 아주대학교 논문은 저작권에 의해 보호받습니다.
초록/요약
Doubles have been raised about a light source using a germanium (Ge) material due to its indirect bandgap nature in which the minimum value (L valley) of conduction band relies on the different point with the maximum value of valence band in brillouin zone, not being potentially suitable for the light sources due to a phonon-assisted process. Fortunately, a pseudodirect bandgap of Ge, a small energy difference (0.136 eV) between the direct Γ and indirect L valley, has created great impetus for direct bandgap transition of Ge with respect to tensile strain with n-type doping, and quantum confinement effects (QCEs). Recently, the enhanced luminescence spectra from Ge nanomembranes (NMs) were presented in accordance with different levels of biaxial tensile strain, enabling efficient light emission and population inversion. Single-crystalline MOCVD-grown III-V/Ge/III-V double-heterostructures (DHs) are envisioned to open up a GaAs-based long-wavelength light source owing to their closely matched thermal expansion and negligible lattice mismatch, enabling the high-quality III-V/Ge heterostructures with lower dislocation density and larger critical thickness. Especially, the epitaxial ultra-thin Ge films inserted between the wide-bandgap III-V materials can be exploited in quantum well (QW) configuration, enabling the carrier accumulation in the optical and electrical confinement of III-V/Ge/III-V DHs. In fact, the GaAs/Ge heterojunction was firstly created by Alferov for the hope of “ideal couple” that satisfies the thermal and electrical, and crystallochemical properties. A Ge direct bandgap (0.8 eV) transition, corresponding to 1550 nm, can be favorably utilized for fiber-optic communication, providing the efficient coupling to other optical systems with a low-loss optical fiber. Moreover, the radiative transition rate induced by the direct Γ valley promises a significant contribution to the enhanced light emission owing to its several orders of magnitude faster than that of the indirect L valley. The subband position of the Ge indirect L valley in the narrow Ge QWs could be tuned toward bottom of the conduction band in the surround barriers, being potentially higher than the subband position of Ge direct Γ valley, which in turn makes the Ge direct optical transition much more efficient in bulk-type Ge film with large active volume. It is worth noting that the adoption of the appropriate wide bandgap material for the optical confinement of the ultra-thin Ge film could be crucial for the direct Ge band-to-band transition, allowing a strong quantum confinement effect in the L valley, but weak confinement effect of the Γ valley. But, there has so far been little solid investigation of III-V/Ge/III-V DHs for practical and efficient GaAs-based long-wavelength light sources onto the GaAs substrates. The most commercially used method to get the long-wavelength light sources, especially, operating at 1550 nm, includes InGaAsP based systems where InP substrates have been mainly exploited for the lattice-matched high-quality InGaAsP/InP heteorstructures. However, InP substrates are more fragile and considerably expensive than GaAs substrates, which could further be aggravated with the use of larger diameter wafers. Furthermore, there exists a fundamental limitation on the lattice-matched distributed Bragg reflector (DBR) materials with high refractive index contrast in InP substrates, impeding the progress of long-wavelength vertical cavity surface emitting lasers (VCSELs). There exists a significant technical issue related to III-V epitaxy onto Ge materials such as polar (III-V)-on-non-polar (Ge) epitaxy: (a) formation of antiphase boundaries (APBs) at the interfaces of Ge/III-V heterostructures; and (b) the problem of cross-doping; this, in turn hinders the implementation of high-quality GaAs onto Ge materials. In order to handle these issues, much effort has eagerly been devoted to high-quality GaAs epitaxy on the misoriented substrates using a two-step GaAs growth technique, AlGaAs intermediate layers, and pre-layers such as As, and Ga. But, to our best knowledge, there is no reports of the GaAs-based long-wavelength light sources using the Ge QW inserted between the wide-bandgap III-V barriers. Here, the high-quality single-crystalline Ge film is epitaxially grown via low-pressure metalorganic chemical vapor deposition (LP-MOCVD) using a novel isobutylgermane (IBuGe) metalorganic source. The high-quality MOCVD-grown III-V/Ge/III-V DHs are investigated by using atomic force microscope (AFM), room-temperature photoluminescence (PL), Raman spectroscopy, secondary ion mass spectrometry (SIMS) measurements, high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDS) analysis. This dissertation focuses on the single-crystal Ge epitaxy onto GaAs substrates and the high-quality polar-on-non-polar epitaxy that plays a crucial role of the GaAs-based long-wavelength light sources using the Ge QWs. Especially, the sharp interfaces at the GaAs/Ge/GaAs DHs are obtained with two-step (2T) GaAs growth technique that utilized low-temperature (LT) GaAs growth (450 °C) followed by high-temperature (HT) GaAs growth (650 °C) above GaAs/Ge heterostructures. A considerably strong room-temperature PL intensity is obtained from the ultra-thin Ge QWs in which the L valley intensity is overlapped with the Γ valley according to the decreased Ge film width. The strong room-temperature PL stemming from the ultra-thin Ge QWs enables the top-emitting light emitting diodes (LEDs), and edge-emitting laser diodes (LDs) operation. The electrical and optical characteristics of the top-emitting Ge LEDs and edge-emitting Ge LDs are investigated with a light-current-voltage (L-I-V) system. To my knowledge, it is the first time to present the light sources using the Ge QWs inserted between the GaAs materials. Furthermore, an adoption of ultra-thin Ge QWs could pave a way toward high-performance long-wavelength VCSELs, possibly using the commonly used AlGaAs/GaAs DBRs in order to achieve the necessary high reflectivity.
more목차
Chapter 1. Background on long-wavelength light sources 1
1.1. Introduction 1
1.2. Limitation on InP-based long-wavelength light sources 5
1.3. Potential active material: Germanium (Ge) 9
1.3.1. Ge properties and its application 11
1.3.2. Single-crystal Ge film using the different Ge precursors 17
1.4. III-V/Ge/III-V double-heterostructures (DH) 19
1.4.1. Polar (III-V)-on-non-polar (Ge) epitaxy 19
1.4.2. Cross-doping 21
1.5. High-quality polar-on-non-polar epitaxy approaches 23
1.5.1 Prelayers such as As or Ga 23
1.5.2 Low-temperature GaAs buffer layer 25
1.5.3 AlxGa1-xAs buffer layer 25
1.6. Research objective and outline 27
Chapter 2. Experimental methods 30
2.1. Ge/III-V heterostructures using a LP-MOCVD system 30
2.2. Epitaxial growth 32
2.3. Material analysis and tools 33
2.3.1. Long-wavelength photoluminescence (PL) system 33
2.3.2. Material characterization 35
2.3.3. Light-current-voltage (L-I-V) set-up 36
Chapter 3. GaAs/Ge/GaAs double-heterostructures (DHs) 38
3.1. Single-crystalline Ge films using an IBuGe precursor 38
3.2. Polar (GaAs)-on-non-polar (Ge) epitaxy 48
3.3. Characterization of GaAs/Ge/GaAs DHs 57
3.3.1. Secondary ion mass spectrometry (SIMS) 57
3.3.2. High-resolution transmission electron microscopy (HRTEM) 58
3.3.3. STEM-energy dispersive X-ray spectroscopy (EDS) 59
3.4. Strong room-temperature photoluminescence 63
3.5. Conclusion 77
Chapter 4. GaAs-based long-wavelength light sources 78
4.1. Introduction 78
4.2. Top-emitting Ge light emitting diodes (Ge LEDs) 78
4.2.1. Fabrication procedures 81
4.2.2. Optical and electrical characteristics 83
4.2.3. Optical and electrical characteristics with the varied ambient temperatures 85
4.3. Edge-emitting Ge laser diodes (LDs) 88
4.3.1. Fabrication procedures 88
4.3.2. Fabry-Perot mirror 90
4.3.3. Optical and electrical properties 91
4.4. Conclusion 93
Chapter 5. Conclusion and Future work 94
5.1. Conclusion 94
5.2. Future work 96
5.2.1. Long-wavelength VCSELs onto a GaAs substrate 96
5.2.2. Thin-film Ge LEDs and LDs 97
References 98
Research achievements 118
