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Nano memories

Quantum dot memory

Abb. 8: (a) Schnittzeichnung einer Speicher-Struktur mit selbstorganisierten Quantenpunkten als Speicherelemente.
Abb. 8: (b) Schematisches 3D-Bild eines Quantenpunkt-basierten Speicher¬elements mit den Kontakten zur Steuerung des Beladungszustandes und zur Kontaktierung des 2DEG.

The two most important commercially available semiconductor memories are the dynamic random access memory (DRAM) and flash memories. DRAMs are mainly used as the main memory in computer, flash memories are mainly used in mp3-players, memory sticks and digital cameras. A DRAM combines high durability with a fast access but DRAMs are volatile, which means that their information has to be reloaded every ten milliseconds. In contrast to DRAMs flash memories are nonvolatile, the information can be stored up to ten years, but they possess a slower access to the stored information.

The ambition of the investigation in the field of quantum memory devices is to realize a nano-flash combining a long storage time up to ten years with short reading, clearing and writing times and a good durability. Such a memory device should be highly superior to memory devices of today.

The basic concept of quantum dot memory devices is to use quantum dots as storage elements embedded in a p-n diode structure. Providing charge-carrier confinement which allocates electrons or holes in the quantum dots, quantum dots are ideal storage elements.

Pn-diode structures have a space charge area which can changed size by applying an external voltage. Depending on the applied voltage the quantum dots lie within or outside the space charge area. To realize writing, clearing and reading the properties of a pn-structure are used. Reading of stored information, to determine the charge status of the quantum dots, happens without changing the charge status. For detection a two-dimensional electron gas shall be implemented underneath the quantum dot layer. By measuring a current in the two-dimensional electron gas the charge status of the quantum dots will be determined. Figure 8 schematically shows such a structure.


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