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Single Dopants in Semiconductors Paul M REVIEW ARTICLE PUBLISHED ONLINE: 24 JANUARY 2011!|!DOI: 10.1038/NMAT2940 Single dopants in semiconductors Paul M. Koenraad1 and Michael E. Flatté2 The sensitive dependence of a semiconductor’s electronic, optical and magnetic properties on dopants has provided an extensive range of tunable phenomena to explore and apply to devices. Recently it has become possible to move past the tunable properties of an ensemble of dopants to identify the e!ects of a solitary dopant on commercial device performance as well as locally on the fundamental properties of a semiconductor. New applications that require the discrete character of a single dopant, such as single-spin devices in the area of quantum information or single-dopant transistors, demand a further focus on the properties of a specific dopant. This article describes the huge advances in the past decade towards observing, controllably creating and manipulating single dopants, as well as their application in novel devices which allow opening the new field of solotronics (solitary dopant optoelectronics). arly on in semiconductor research, the ubiquity of unintentional behaviour. About 10 years ago it was realized that single impurities dopants meant that reproducible results were rare; as late as 1931, could be used to physically realize the ‘qubits’ of quantum physicist Wolfgang Pauli opined in a letter1 to Rudolph Peierls computation, with a single phosphorus dopant’s nuclear spin7, or a E 8 9 that “one shouldn’t work on semiconductors, that is a !lthy mess; who single dopant’s bound electron spin or charge . Single dopants are knows whether any semiconductors exist.” Eight decades later, the suggested not only as qubits for quantum computing but also as non- purity of germanium is better than 1 part in 1011, permitting almost classical light sources in quantum information science for quantum nine orders of magnitude variability in doping concentration. Silicon key distribution systems, quantum repeaters, quantum lithography, purity is similar. Yet even with this current exquisite control, the multivalent logic and local sensing. Single impurities have already dopants usually still play a supporting role in current devices, by mod- shown their importance for non-classical light sources through the ifying the chemical potential of a material. At these levels of purity, demonstration of single-photon emission from nitrogen-vacancy each unintentional dopant is on average more than a micrometre (NV) centres in diamond10, isoelectronic tellurium impurity centres away from any other unintentional dopant. "us devices on the nano- in ZnSe (ref. 11) and N pairs in GaP (ref. 12) as well as triggered scale can be expected to have absolutely no unintentional dopants, single-photon emission in the case of N-acceptors in ZnSe (ref. 13) and if the doping is carefully controlled a device can be constructed and NV centres in diamond14. with one and only one dopant — a solitary dopant opto electronic, "e above examples may be viewed as the initial demonstrations or solotronic, device. And, as atoms are the building blocks of mat- and model device designs on the path to a fully $edged solotronic ter, a solotronic device is where the miniaturization of semiconductor technology, which requires considerable additional fundamen- devices reaches the limits set by the discrete nature of matter. tal study and device application. "e desirable features of solitary Although the properties of individual dopants determine some dopants, such as the reproducible quantized properties of a speci!c aspects of a doped device (such the dependence of carrier freeze- dopant, make them ideal objects for further scienti!c study and out on the dopant binding energy), the !rst e#ect of individual robust applications. However, a reliance on the properties of a soli- dopant dynamics on devices was through electronic noise. About tary dopant generates new challenges: device behaviour depends 30 years ago one of the main sources of Lorentz noise in metal– on the local environment (strain, electric, magnetic and optical oxide–semiconductor !eld-e#ect transistor (MOSFET) devices was !elds) and the dopant position within the device. Scienti!c study traced to the trapping and detrapping of charge carriers at impu- of single-atom behaviour is most advanced in single-atom and rity centres close to the conduction channel. Owing to the high single-ion electromagnetic traps in vacuum; study of the solitary purity of the semiconductor materials and the increasingly small dopant in a solid also takes advantage of a natural form of trap- device sizes, discrete impurities started to show up in most device ping, as the solitary impurity is held in place by the electromagnetic transport properties. Random telegraph noise from an individual !elds of the host solid around it, but on a length scale orders of impurity was !rst observed at low temperatures in MOSFETs2 and magnitude smaller. quantum point contact structures3 and later at room temperature Why is our focus on the solitary dopant in a semiconductor, in bipolar transistors4. Such developments imply that individual rather than a metal or insulator? To reduce the electronic interac- impurities might ultimately dominate the device characteristics of tion of the single dopant with the host, and preserve some of the future MOSFET transistors5. "e potential $uctuations from indi- discrete character of the dopant, the host should not be a metal; the vidual impurities in a MOSFET transistor are shown in Fig. 1a. overlap of a host metal’s electronic states and impurity states is too Controlled positioning of these impurities can improve threshold large for a single impurity to dominate the properties. However, a voltage reproducibility for MOSFETs, as shown in Fig. 1b–e (ref. 6). host insulator is also less than optimal, for to enable the applica- "e International Technology Roadmap for Semiconductors (ITRS) tion of external electric !elds and couple the single dopant to trans- speci!cally discusses the need for accurate three-dimensional (3D) port, the host should be a semiconductor. "us the exploration and dopant incorporation, pro!ling and modelling. manipulation of a single dopant in a semiconducting host should "e detection of the discrete properties of solitary dopants in be the focus of solotronic research, and holds promise for device individual devices motivated new device designs that required this applications as diverse as scalable sources for quantized emission, 1COBRA Inter-University Research Institute, Department of Applied Physics, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands. 2Department of Physics and Astronomy, University of Iowa, 203 VAN, Iowa City, Iowa 52242, USA. *e-mail: [email protected]; michael_fl[email protected] NATURE MATERIALS | VOL 10 | FEBRUARY 2011 | www.nature.com/naturematerials 91 © 2011 Macmillan Publishers Limited. All rights reserved nmat_2940_FEB11.indd 91 11/1/11 11:56:33 REVIEW ARTICLE NATURE MATERIALS DOI: 10.1038/NMAT2940 a to obtain detailed information about the responsible impurity itself. One exception is the use of noise in a conduction channel to measure the spin state of a single impurity8. A spin-to-charge transformation can occur when the impurity, by virtue of the Pauli principle, can only bind a second electron with opposite spin. Recently this tactic was applied to monitor the spin resonance of a single paramagnetic 15 impurity at the Si/SiO2 interface . Single impurities also manifested their presence in tunnel- ling. In a double-barrier resonant-tunnelling (DBRT) diode with a micrometre-sized diameter, features in the I–V curves could be related to the presence of a single Si donor in the DBRT well16. Although DBRT structures enabled the determination of the spin- splitting of an individual Si doping atom located in a GaAs quantum bc8 8 well17 as well as located in an AlAs barrier18, and an estimate of the Ave. Vth = –0.4V 19 7 7 extension of the wavefunction of the impurity state , their use is o%en hampered by the uncontrolled incorporation and unknown 6 6 chemical nature of the impurity state. Other tunnelling approaches 5 5 have used a single impurity to scan the density-of-states (DOS) 20 4 4 spectrum of a two-dimensional electron gas or shown the presence 21,22 equency equency Ave. Vth = –0.2V of a single impurity in industrial-type nano-MOSFETs . Fr Fr 3 3 Optical identi!cations of an individual semiconductor impurity 2 2 followed a%er tunnelling identi!cation16, by analysing the electro- 1 1 luminescence of a doped large-area DBRT structure and relating sharp lines to the presence of independent acceptors in the DBRT’s 0 0 23 –2 –1 0 12 –2 –1 0 12 well . "e application of standard high-resolution optical spectros- Vth [V] Vth [V] copy on a GaAs sample with a low density of N-pairs allowed for the !rst luminescence study of individual isoelectronic N-pairs24. d Drain e Drain An improved optical assessment of individual impurities, by opti- cal excitation and detection, could be obtained by confocal micro- scopy in combination with solid immersion lenses that reduce the m spot size to about 250 nm in the visible range of the spectrum. 2 1. "is approach allowed the observation of single NV centres in dia- mond25. A length scale of 250 nm is still large compared with the typical distance between impurities, but new approaches, such as stimulated emission depletion microscopy, show a superior reso- lution, better than 10 nm for an impurity in diamond26. For more Sour ce Sour common optical analysis techniques, the impurities have to be spec- ce 100 nm 300 nm troscopically singled out from the background, and their concentra- tion must be very low. For NV centres in diamond, which can have 9 −3 Figure 1 | Impurities in a MOSFET device. a, Simulation of the potential concentrations of the order of 10 cm or even lower depending on distribution in a 50-nm MOSFET device showing the potential fluctuations the growth conditions, this methodology works very well.
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