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Title Page
Abstract
Contents
Abbreviations 20
Chapter 1. Background on long-wavelength light sources 23
1.1. Introduction 23
1.2. Limitation on InP-based long-wavelength light sources 27
1.3. Potential active material: Germanium (Ge) 31
1.3.1. Ge properties and its applications 33
1.3.2. Single-crystal Ge film using the different Ge precursors 39
1.4. III-V/Ge/III-V double-heterostructures (DHs) for GaAs-based long-wave length light sources 41
1.4.1. Polar (III-V)-on-non-polar (Ge) epitaxy 41
1.4.2. Cross-doping 43
1.5. High-quality polar-on-non-polar epitaxy approaches 45
1.5.1. Prelayers such as As or Ga 45
1.5.2. Low-temperature GaAs buffer layer 47
1.5.3. AlxGa₁-xAs buffer layer[이미지참조] 47
1.6. Research objective and outline 49
Chapter 2. Experimental methods 52
2.1. Ge/III-V heterostructures using a LP-MOCVD system 52
2.2. Epitaxial Growth 54
2.3. Material analysis and tools 55
2.3.1. Long-wavelength photoluminescence (PL) system 55
2.3.2. Material Characterization 57
2.3.3. Light-current-voltage (L-I-V) set-up 58
Chapter 3. GaAs/Ge/GaAs double-heterostructures (DHs) 60
3.1. Single-crystalline Ge films using an IBuGe precursor 60
3.2. Polar (GaAs)-on-non-polar (Ge) epitaxy 70
3.3. Characterization of GaAs/Ge/GaAs DHs 79
3.3.1. Secondary ion mass spectrometry (SIMS) 79
3.3.2. High-resolution transmission electron microscopy (HRTEM) 80
3.3.3. STEM-energy dispersive X-ray spectroscopy (EDS) 81
3.4. Strong room-temperature (RT) photoluminescence of ultra-thin Ge film 85
3.5. Conclusion 99
Chapter 4. GaAs-based long-wavelength light sources 100
4.1. Introduction 100
4.2. Top-emitting Ge light emitting diodes (LEDs) 100
4.2.1. Fabrication procedures 103
4.2.2. Optical and electrical characteristics 105
4.2.3. Optical and electrical characteristics with the varied ambient temperatures 107
4.3. Edge-emitting Ge laser diodes (LDs) 110
4.3.1. Fabrication procedures 110
4.3.2. Fabry-Perot mirror 112
4.3.3. Optical and electrical characteristics 113
4.4. Conclusion 115
Chapter 5. Conclusion and future work 116
5.1. Conclusion 116
5.2. Future work 118
5.2.1. Long-wavelength VCSELs onto a GaAs substrate 118
5.2.2. Thin-film Ge LEDs and LDs 119
References 120
Research achievements 140
Figure 1.1. Emission wavelength range of near-infrared light sources. 23
Figure 1.2. The 1310, 1550 nm optical communication windows with the lowered optical loss. 24
Figure 1.3. Lidar technology with long-wavelength light sources. 25
Figure 1.4. Absorption of fatty tissue in accordance with the light sources emitting at the different wavelength. 26
Figure 1.5. Bandgap energy according to the lattice constant. 27
Figure 1.6. Widespread adoptions of the InP wafers. 28
Figure 1.7. Limitation on InP-based long-wavelength VCSELs. 30
Figure 1.8. Potential active material: Germanium (Ge). 31
Figure 1.9. Various applications using the Ge materials. 33
Figure 1.10. Lattice constants of the Si₁-xGex alloys with Ge contents.[이미지참조] 34
Figure 1.11. InGaP/(In)GaAs/Ge tandem solar cell applications. 35
Figure 1.12. Lattice constants of GaAs, AlAs and Ge materials. 36
Figure 1.13. Bowed/warped epi wafers 37
Figure 1.14. VCSEL reflectivity Maps: Ge & GaAs wafer 37
Figure 1.15. E-K diagram of the indirect-bandgap Ge materials. 38
Figure 1.16. Product data sheet of IBuGe (Dow Electronic Materials). 40
Figure 1.17. APD formation during polar-on-non-polar epitaxy. 42
Figure 1.18. Cross-doping of the GaAs/Ge heterostructures. 44
Figure 1.19. AlxGa₁-xAs role of the GaAs/Ge heterostructures.[이미지참조] 44
Figure 1.20. APD formation during polar-on-non-polar epitaxy. 46
Figure 1.21. Polar-on-non-polar epitaxy using Ga or As prelayers. 46
Figure 1.22. AFM images of the GaAs/AlxGa₁-xAs/Ge structures with (a) x＝0 (b) x＝0.3, (c)x＝0.6, and (d) x＝1.0. The rms of the structures determined from...[이미지참조] 48
Figure 1.23. Motivation of GaAs-based long-wavelength light sources using Ge QWs. 50
Figure 2.1. Photographs of the LP-MOCVD using an IBuGe source 53
Figure 2.2. Long-wavelength photoluminescence system. 56
Figure 2.3. L-I-V set up for GaAs-based long-wavelength light sources. 59
Figure 3.1. The 1-μm thick Ge film is epitaxially grown on n-GaAs (001) substrates with a miscut of 6° towards [111] via a LP-MOCVD system using a... 60
Figure 3.2. The 10 x 10-um² AFM images of the 175-μm thick p-Ge substrate (Ge ref) and 1-μm thick Ge film (As-grown Ge) on n-GaAs substrates. 61
Figure 3.3. The HAADF-STEM image with the defect-free epitaxial Ge film onto the n-GaAs substrates. 62
Figure 3.4. The rocking curves from the (004) Bragg line of Ge ref and as grown Ge are investigated with HR-XRD measurements. 63
Figure 3.5. Raman shifts of Ge ref and as-grown Ge are observed at 300.9 and 301.6 cm¯¹, respectively. 64
Figure 3.6. The optical properties of Ge ref and as-grown Ge are investigated using room-temperature PL measurements. 66
Figure 3.7. Normalized room-temperature PL behaviors of p-Ge substrate (Ge ref). 67
Figure 3.8. Normalized room-temperature PL behaviors of the epitaxial Ge film on the tilted n-GaAs substrate (As-grown Ge). 67
Figure 3.9. Ultra-thin flexible Ge solar cells for lattice-matched thin-film InGaP/(In)GaAs/Ge tandem solar cells 69
Figure 3.10. MOCVD-grown GaAs/Ge/GaAs DHs 70
Figure 3.11. The 10 × 10-μm² AFM image of the GaAs/Ge/GaAs DHs 71
Figure 3.12. TEM images of the un-doped GaAs/Ge/GaAs DHs. 72
Figure 3.13. The diffusion depth of Ga, As, and Ge atoms in GaAs/10-nm thick Ge film/GaAs DHs. 73
Figure 3.14. Un-doped GaAs/Ge/GaAs DHs with a two-step (2T) GaAs growth technique. 74
Figure 3.15. The 10×10μm² AFM images of the GaAs/Ge/GaAs DHs in accordance with GaAs buffer layer growth time from 1 to 10 mins. 75
Figure 3.16. The 10×10μm² AFM images of the GaAs/Ge/GaAs DHs in accordance with/ without the prelayers such as Ga or As. 76
Figure 3.17. GaAs/Ge/GaAs DHs with the differently off-oriented GaAs substrates. 77
Figure 3.18. PL behaviors of GaAs/Ge/GaAs DHs on the differently tilted n-GaAs substrates. 78
Figure 3.19. The diffusion depth of Ga, As, and Ge atoms in the well-defined interfaces of the GaAs/10-nm thick Ge/GaAs DHs with (w/) 2T GaAs growth technique. 79
Figure 3.20. The HRTEM images with average background subtraction filter (ABSF) 80
Figure 3.21. The STEM image corresponding to the line profiles. 82
Figure 3.22. The line profiles of the GaAs/Ge/GaAs DHs. 83
Figure 3.23. The mapping images examined for the quantized high-resolution analysis of crystalline GaAs and Ge materials in the GaAs/Ge/GaAs DHs. 84
Figure 3.24. GaAs/Ge/GaAs DHs with the varied Ge film thickness from 140 to 2 nm. 85
Figure 3.25. Room-temperature PL behaviors of GaAs/Ge/GaAs DHs with the varied Ge film thickness from 140 to 2 nm. 86
Figure 3.26. The PL intensity in accordance with the bulk-type Ge film thickness. 87
Figure 3.27. The PL intensity in accordance with the Ge QW thickness. 89
Figure 3.28. The normalized PL intensity with the varied Ge thickness from 140 to 35 nm. 91
Figure 3.29. The normalized PL intensity with the 10-nm thick Ge film. 92
Figure 3.30. The change of the energy bandgap at Γ and L valley of Ge films in the GaAs/Ge/GaAs DHs as a function of Ge thickness from 10 to 140 nm. 93
Figure 3.31. The normalized Raman spectra stemming from the varied Ge thickness from 140 to 2 nm in the GaAs/Ge/GaAs DHs. 95
Figure 3.32. The room-temperature strong PL intensity induced by the ultra-thin Ge (3.5 nm) QW compared to as-grown Ge (1-μm-thick Ge film). 97
Figure 3.33. A strong PL intensity of the 3.5-nm thick Ge film 98
Figure 4.1. Top-emitting Ge LED structures 100
Figure 4.2. Cross-sectional TEM image of the Ge LED structures 101
Figure 4.3. EDS mapping image of the top-emitting Ge LED structures. 102
Figure 4.4. Top-emitting Ge LED fabrication procedures 104
Figure 4.5. The light-current-voltage (L-I-V) system. 105
Figure 4.6. The L-I-V curves of the top-emitting Ge LEDs 106
Figure 4.7. The L-I-V curves of the top-emitting Ge LEDs with the ambient temperatures from 5 to 55 °C. 108
Figure 4.8. Edge-emitting Ge LD fabrication procedures 111
Figure 4.9. Fabrication procedures of the edge mirror of the Ge LDs. 112
Figure 4.10. The schematic and L-I-V curves of the edge-emitting Ge LD. 113
Figure 4.11. The schematic and L-I-V curves of the edge-emitting Ge LD. 114
Figure 5.1. Long-wavelength VCSELs onto a GaAs substrate. 118
Figure 5.2. Thin-film Ge LEDs and LDs. 119
초록보기
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 ℃) followed by high-temperature (HT) GaAs growth (650 ℃) 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.MARC 보기
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