26 Oct 2001 - Cambridge, UK
Breakthrough may herald new concepts in communications technology
Scientists have found that cloud-like particles in semiconductors called trions are able to conduct electricity. The discovery, which will be reported this week in the research journal Science, resulted from collaboration between Toshiba Research Europe Limited (Cambridge, UK), the University of Cambridge and the University of Nijmegen (the Netherlands).
The conduction of electricity in cables or household appliances is due to the movement of electrons in response to an applied voltage; indeed electricity is the movement of electrons. Electric current due to the movement of positively charged holes, which correspond to the absence of an electron, is also possible and is utilised in some types of transistor.
Now a new type of current has been found due to a composite particle, comprising two electrons and a hole, which can be moved around the semiconductor using applied voltages in a similar fashion to individual electrons and holes. The two electrons attach to the hole in the semiconductor in much the same way as electrons to the nucleus of an ion, or, to draw another analogy, the planets to the sun of a small solar system. When a voltage is applied all three particles are found to move through the semiconductor together.
The Joint Managing Director of Toshiba Research Europe, Professor Michael Pepper, comments “It actually moves surprisingly quickly for such a large particle, being only three times slower than an individual electron.”
This composite particle, called a trion or charged exciton, is more than 500 Angstroms in diameter, compared to a typical distance between atoms in a semiconductor of 3 Angstroms and the diameter of an electron of less than 1 million millionth of an Angstrom (an Angstrom is one millionth of the diameter of a human hair). Previously it was thought that such a large particle was fixed by imperfections in the semiconductor, but the new findings have shown that this is not the case.
A key feature of this development is the ability of physicists to produce new types of semiconductor with a high degree of crystalline perfection. This has been a goal of numerous laboratories world-wide, as it is the presence of impurities and defects in the semiconductor which degrades many of their desirable properties. The ability to produce extremely high quality crystals of the semiconductor Gallium Arsenide resulted in the trion being free to move. Previously it was thought that the irregularities in the crystal, such as impurities, would always prevent the trion from being free to move around when an electric field was applied.
Electrons and holes play an important role in determining the optical properties of semiconductors, for instance they are responsible for light generation in light emitting diodes. Since the neutral exciton of one electron and one hole has no net charge, it cannot be moved by applied voltage. However, the researchers found that by attaching an extra electron to the exciton, thereby forming the trion with a negative charge, they were able to control its position by applied voltage.
According to Dr. Andrew Shields of Toshiba Research Europe Ltd: “These results show that trions move around in an electric field in a similar way to electrons. It suggests we may be able to control light emission in semiconductors in previously unimagined ways. For instance, in the future it may be possible to use this phenomenon to make light sources where we can modulate the output intensity or wavelength by moving the trions around in the chip. Such devices could allow more data to be sent on optical communications systems, while also reducing their cost.”
Technical Background
This new phenomenon was demonstrated using the transistor structure shown in Fig.1, which allows control of the concentration of excess electrons in a thin semiconductor layer under the gate region by applying a voltage between the gate and drain contacts.
An incident laser excites electron-hole pairs within the semiconductor under the gate region. An electron and hole bind to form a complex called an exciton, which has energy states analogous to those of the hydrogen atom. Since the exciton formed between the negatively charged electron and positively charged hole has no net charge, it is insensitive to an electric field created by a voltage applied between the source and drain of the transistor. Thus light emission originates from the same area of the device as that excited by the laser.
However, strikingly different behaviour is observed after introducing excess electrons to the quantum well. In this case the photo-excited electron-hole pairs bind to an excess electron, to form a three-particle exciton, called a trion or negatively charged exciton. The exciton in a semiconductor can be regarded as analogous to a hydrogen atom and the trion can be regarded as the analogue of the negative hydrogen ion, H-. Now the scientists have discovered, that due to their net negative charge, trions are able to drift in an applied electric field, travelling over distances as long as several micrometers. Thus upon applying a voltage between the source and drain of the transistor, the light emissive area, as seen through a special optical microscope, is skewed away from the incident laser spot (Fig.2). The trions drift in the opposite sense to the field because of their negative charge. Reversing the polarity of the applied field causes the trions to drift in the opposite direction. This is the first time that trions have been manipulated in this way by applied voltages.
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