Heterojunction (original) (raw)

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Interface between two layers or regions of dissimilar semiconductors

A heterojunction is an interface between two layers or regions of dissimilar semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction. It is often advantageous to engineer the electronic energy bands in many solid-state device applications, including semiconductor lasers, solar cells and transistors. The combination of multiple heterojunctions together in a device is called a heterostructure, although the two terms are commonly used interchangeably. The requirement that each material be a semiconductor with unequal band gaps is somewhat loose, especially on small length scales, where electronic properties depend on spatial properties. A more modern definition of heterojunction is the interface between any two solid-state materials, including crystalline and amorphous structures of metallic, insulating, fast ion conductor and semiconducting materials.

Manufacture and applications

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Heterojunction manufacturing generally requires the use of molecular beam epitaxy (MBE)[1] or chemical vapor deposition (CVD) technologies in order to precisely control the deposition thickness and create a cleanly lattice-matched abrupt interface. A recent alternative under research is the mechanical stacking of layered materials into van der Waals heterostructures.[2]

Despite their expense, heterojunctions have found use in a variety of specialized applications where their unique characteristics are critical:

Energy band alignment

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The three types of semiconductor heterojunctions organized by band alignment.

Band diagram for stradding gap, _n_-n semiconductor heterojunction at equilibrium.

The behaviour of a semiconductor junction depends crucially on the alignment of the energy bands at the interface. Semiconductor interfaces can be organized into three types of heterojunctions: straddling gap (type I), staggered gap (type II) or broken gap (type III) as seen in the figure.[9] Away from the junction, the band bending can be computed based on the usual procedure of solving Poisson's equation.

Various models exist to predict the band alignment.

The typical method for measuring band offsets is by calculating them from measuring exciton energies in the luminescence spectra.[13]

Effective mass mismatch

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When a heterojunction is formed by two different semiconductors, a quantum well can be fabricated due to difference in band structure. In order to calculate the static energy levels within the achieved quantum well, understanding variation or mismatch of the effective mass across the heterojunction becomes substantial. The quantum well defined in the heterojunction can be treated as a finite well potential with width of l w {\displaystyle l_{w}} {\displaystyle l_{w}}. In addition, in 1966, Conley et al.[14] and BenDaniel and Duke[15] reported a boundary condition for the envelope function in a quantum well, known as BenDaniel–Duke boundary condition. According to them, the envelope function in a fabricated quantum well must satisfy a boundary condition which states that ψ ( z ) {\displaystyle \psi (z)} {\displaystyle \psi (z)} and 1 m ∗ ∂ ∂ z ψ ( z ) {\displaystyle {\frac {1}{m^{*}}}{\partial \over {\partial z}}\psi (z)\,} {\displaystyle {\frac {1}{m^{*}}}{\partial \over {\partial z}}\psi (z)\,} are both continuous in interface regions.

Nanoscale heterojunctions

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Image of a nanoscale heterojunction between iron oxide (Fe3O4 — sphere) and cadmium sulfide (CdS — rod) taken with a TEM. This staggered gap (type II) offset junction was synthesized by Hunter McDaniel and Dr. Moonsub Shim at the University of Illinois in Urbana-Champaign in 2007.

In quantum dots the band energies are dependent on crystal size due to the quantum size effects. This enables band offset engineering in nanoscale heterostructures. It is possible[17] to use the same materials but change the type of junction, say from straddling (type I) to staggered (type II), by changing the size or thickness of the crystals involved. The most common nanoscale heterostructure system is ZnS on CdSe (CdSe@ZnS) which has a straddling gap (type I) offset. In this system the much larger band gap ZnS passivates the surface of the fluorescent CdSe core thereby increasing the quantum efficiency of the luminescence. There is an added bonus of increased thermal stability due to the stronger bonds in the ZnS shell as suggested by its larger band gap. Since CdSe and ZnS both grow in the zincblende crystal phase and are closely lattice matched, core shell growth is preferred. In other systems or under different growth conditions it may be possible to grow anisotropic structures such as the one seen in the image on the right.

The driving force for charge transfer between conduction bands in these structures is the conduction band offset.[18] By decreasing the size of CdSe nanocrystals grown on TiO2, Robel et al.[18] found that electrons transferred faster from the higher CdSe conduction band into TiO2. In CdSe the quantum size effect is much more pronounced in the conduction band due to the smaller effective mass than in the valence band, and this is the case with most semiconductors. Consequently, engineering the conduction band offset is typically much easier with nanoscale heterojunctions. For staggered (type II) offset nanoscale heterojunctions, photoinduced charge separation can occur since there the lowest energy state for holes may be on one side of the junction whereas the lowest energy for electrons is on the opposite side. It has been suggested[18] that anisotropic staggered gap (type II) nanoscale heterojunctions may be used for photocatalysis, specifically for water splitting with solar energy.

  1. ^ a b Smith, C.G (1996). "Low-dimensional quantum devices". Rep. Prog. Phys. 59 (1996) 235282, pg 244.
  2. ^ Geim, A. K.; Grigorieva, I. V. (2013). "Van der Waals heterostructures". Nature. 499 (7459): 419–425. arXiv:1307.6718. doi:10.1038/nature12385. ISSN 0028-0836. PMID 23887427. S2CID 205234832.
  3. ^ Leu, Sylvère; Sontag, Detlef (2020), Shah, Arvind (ed.), "Crystalline Silicon Solar Cells: Heterojunction Cells", Solar Cells and Modules, vol. 301, Cham: Springer International Publishing, pp. 163–195, doi:10.1007/978-3-030-46487-5_7, ISBN 978-3-030-46485-1, retrieved 2023-04-18
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  5. ^ Yamamoto, Kenji; Yoshikawa, Kunta; Uzu, Hisashi; Adachi, Daisuke (2018). "High-efficiency heterojunction crystalline Si solar cells". Japanese Journal of Applied Physics. 57 (8S3): 08RB20. Bibcode:2018JaJAP..57hRB20Y. doi:10.7567/JJAP.57.08RB20. S2CID 125265042.
  6. ^ "HJT - Heterojunction Solar Cells". Solar Power Panels. Retrieved 2022-03-25.
  7. ^ Kroemer, H. (1963). "A proposed class of hetero-junction injection lasers". Proceedings of the IEEE. 51 (12): 1782–1783. doi:10.1109/PROC.1963.2706.
  8. ^ Ortiz-Quiñonez, Jose-Luis; Pal, Umapada (October 2024). "Interface engineered metal oxide heterojunction nanostructures in photocatalytic CO2 reduction: Progress and prospects". Coordination Chemistry Reviews. 516: 215967. doi:10.1016/j.ccr.2024.215967.
  9. ^ Ihn, Thomas (2010). "ch. 5.1 Band engineering". Semiconductor Nanostructures Quantum States and Electronic Transport. United States of America: Oxford University Press. pp. 66. ISBN 9780199534432.
  10. ^ J. Tersoff (1984). "Theory of semiconductor heterojunctions: The role of quantum dipoles". Physical Review B. 30 (8): 4874–4877. Bibcode:1984PhRvB..30.4874T. doi:10.1103/PhysRevB.30.4874.
  11. ^ Pallab, Bhattacharya (1997), Semiconductor Optoelectronic Devices, Prentice Hall, ISBN 0-13-495656-7
  12. ^ Adachi, Sadao (1993-01-01). Properties of Aluminium Gallium Arsenide. ISBN 9780852965580.
  13. ^ a b Debbar, N.; Biswas, Dipankar; Bhattacharya, Pallab (1989). "Conduction-band offsets in pseudomorphic InxGa1-xAs/Al0.2Ga0.8As quantum wells (0.07≤x≤0.18) measured by deep-level transient spectroscopy". Physical Review B. 40 (2): 1058–1063. Bibcode:1989PhRvB..40.1058D. doi:10.1103/PhysRevB.40.1058. PMID 9991928.
  14. ^ Conley, J.; Duke, C.; Mahan, G.; Tiemann, J. (1966). "Electron Tunneling in Metal–Semiconductor Barriers". Physical Review. 150 (2): 466. Bibcode:1966PhRv..150..466C. doi:10.1103/PhysRev.150.466.
  15. ^ Bendaniel, D.; Duke, C. (1966). "Space-Charge Effects on Electron Tunneling". Physical Review. 152 (2): 683. Bibcode:1966PhRv..152..683B. doi:10.1103/PhysRev.152.683.
  16. ^ Griffiths, David J. (2004). Introduction to Quantum Mechanics (2nd ed.). Prentice Hall. ISBN 0-13-111892-7
  17. ^ Ivanov, Sergei A.; Piryatinski, Andrei; Nanda, Jagjit; Tretiak, Sergei; Zavadil, Kevin R.; Wallace, William O.; Werder, Don; Klimov, Victor I. (2007). "Type-II Core/Shell CdS/ZnSe Nanocrystals: Synthesis, Electronic Structures, and Spectroscopic Properties". Journal of the American Chemical Society. 129 (38): 11708–19. doi:10.1021/ja068351m. PMID 17727285.
  18. ^ a b c Robel, István; Kuno, Masaru; Kamat, Prashant V. (2007). "Size-Dependent Electron Injection from Excited CdSe Quantum Dots into TiO2Nanoparticles". Journal of the American Chemical Society. 129 (14): 4136–7. doi:10.1021/ja070099a. PMID 17373799.