Tobias Nowozin

A write time of 6ns for quantum dot–based memory structures

The concept of a memory device based on self-organized quantum dots (QDs) is presented, enabling extremely fast write times, limited only by the charge carrier relaxation time being in the picosecond range. For a first device structure with embedded InAs∕GaAs QDs, a write time of 6ns is demonstrated. A similar structure containing GaSb∕GaAs QDs shows a write time of 14ns. These write times are independent of the localization energy (e.g., storage time) of the charge carriers and at the moment are limited only by the experimental setup and the parasitic cutoff frequency of the RC low pass of the device.

The QD-Flash: a quantum dot-based memory device

We demonstrate the large potential of III?V compound semiconductors for a novel type of Flash memory. The concept is based on self-organized III?V quantum dots (QDs). Here the advantages of the most important semiconductor memories, the dynamic random access memory and the Flash are merged. A non-volatile memory with fast access times ( 1015 write/erase cycles) as an ultimate solution seems possible. A storage time of 1.6 s at 300 K in InAs/GaAs QDs with an additional Al0.9Ga0.1As barrier was demonstrated and a retention time of 106 years is predicted by us for GaSb QDs in an AlAs matrix. In addition, a minimum write time of 6 ns is obtained for InAs/GaAs QDs. First prototypes with all-electrical data access prove the feasibility of the concept. The stored information is read-out by a two-dimensional hole gas underneath the QD layer. Time-resolved drain-current measurements demonstrate the memory operations.

Hole-based memory operation in an InAs/GaAs quantum dot heterostructure

We present an InAs/GaAs quantum dot (QD) memory structure with all-electrical data access which uses holes as charge carriers. Charging and discharging of the QDs are clearly controlled by a gate voltage. The stored information is read-out by a two-dimensional hole gas underneath the QD-layer. Time resolved drain-current-measurements demonstrate the memory operation. Present write times are 80 ns.

800 meV localization energy in GaSb/GaAs/Al0.3Ga0.7As quantum dots

The localization energies, capture cross sections, and storage times of holes in GaSb quantum dots (QDs) are measured for three GaSb/GaAs QD ensembles with different QD sizes. The structural properties, such as height and diameter, are determined by atomic force microscopy, while the electronic properties are measured using deep-level transient spectroscopy. The various QDs exhibit varying hole localization energies corresponding to their size. The maximum localization energy of 800 (±50) meV is achieved by using additional Al0.3Ga0.7As barriers. Based on an extrapolation, alternative material systems are proposed to further increase the localization energy and carrier storage time of QDs.

Temperature and electric field dependence of the carrier emission processes in a quantum dot-based memory structure

Hole emission processes from self-organized GaAs0.4Sb0.6/GaAs quantum dots embedded in a p-n diode are studied by capacitance-voltage spectroscopy. The method introduced allows the investigation of the temperature and electric field dependence of carrier emission with time constants from below nanoseconds up to thousands of seconds. Different emission processes are clearly distinguished, such as tunneling, phonon-assisted tunneling, and thermal activation, each important for quantum-dot-based memory structures. The erase time was determined to 1.5 ms for an electric field of about 200 kV/cm. At 500 kV/cm, 10 ns are predicted sufficient for fast erasing.

Materials for future quantum dot-based memories

The present paper investigates the current status of the storage times in self-organized QDs, surveying a variety of heterostructures advantageous for strong electron and/or hole confinement. Experimental data for the electronic properties, such as localization energies and capture cross-sections, are listed. Based on the theory of thermal emission of carriers from QDs, we extrapolate the values for materials that would increase the storage time at room temperature to more than millions of years. For electron storage, GaSb/AlSb, GaN/AlN, and InAs/AlSb are proposed. For hole storage, GaSb/Al0.9Ga0.1As, GaSb/GaP, and GaSb/AlP are promising candidates.

230 s room-temperature storage time and 1.14 eV hole localization energy in In0.5Ga0.5As quantum dots on a GaAs interlayer in GaP with an AlP barrier

A GaP n+p-diode containing In0.5Ga0.5As quantum dots (QDs) and an AlP barrier is characterized electrically, together with two reference samples: a simple n+p-diode and an n+p-diode with AlP barrier. Localization energy, capture cross-section, and storage time for holes in the QDs are determined using deep-level transient spectroscopy. The localization energy is 1.14(±0.04) eV, yielding a storage time at room temperature of 230(±60) s, which marks an improvement of 2 orders of magnitude compared to the former record value in QDs. Alternative material systems are proposed for still higher localization energies and longer storage times.

3 ns single-shot read-out in a quantum dot-based memory structure

Fast read-out of two to six charges per dot from the ground and first excited state in a quantum dot (QD)-based memory is demonstrated using a two-dimensional electron gas. Single-shot measurements on modulation-doped field-effect transistor structures with embedded InAs/GaAs QDs show read-out times as short as 3 ns. At low temperature (T = 4.2 K) this read-out time is still limited by the parasitics of the setup and the device structure. Faster read-out times and a larger read-out signal are expected for an improved setup and device structure.

Antimony-based quantum dot memories

As a type-II heterostructure with exclusive hole confinement GaSb/(Al,Ga)As QDs are an ideal candidate for a QD based memory device operating at room temperature. We investigated different Antimony-based QDs in respect of localization energies and storage times with 8-band-k•p calculations as well as time-resolved capacitance spectroscopy. In addition, we present a memory concept based on self-organized quantum dots (QDs) which could fuse the advantages of todays main semiconductor memories DRAM and Flash. First results on the performance of such a memory cell are shown and a closer look at Sb-based QDs as a storage unit is taken.

Self-organized quantum dots for future semiconductor memories

A memory structure based on self-organized quantum dots (QDs) combining the advantages of dynamic random access memory (DRAM) and flash memory which enables extremely fast write times (<1 ns) together with long storage times ( years) at room temperature is presented. A storage time of 1.6 s at 300 K in InAs/GaAs QDs with an additional Al0.9Ga0.1As barrier—100 times longer than in a DRAM—is demonstrated. Much longer retention times of 106 years are predicted for GaSb QDs in an AlAs matrix. A minimum write time of 6 ns is currently obtained for InAs/GaAs QDs, of the order of those for present DRAMs. An even faster write time below 1 ns, only limited by charge carrier capture and relaxation times (in the order of picoseconds), is predicted for a slightly improved device structure.