Hole storage in GaSb/GaAs quantum dots for memory devices

Abstract

The hole confinement of self-organized GaSb/GaAs quantum dots embedded in n + p-diodes is investigated experimentally by admittance spectroscopy. The highest thermal activation energy obtained, 400 meV, refers to only weakly charged quantum dots. Detailed bias-dependent investigations allow to study state-filling and Coulomb charging effects. State filling lowers the activation energy down to 150 meV in quantum dots charged with the maximum number of about 15 holes. The observed thermal activation barrier for GaSb/GaAs quantum dots is about twice as high as for structurally comparable InAs/GaAs quantum dots.The ultimate non-volatile electronic memory cell confines a single carrier to represent a bit of information [1]. As conventional material processing technology is not yet able to manufacture suitable semiconductor structures, self-organized quantum dots (QDs) offer an elegant method to create huge ensembles of electronic traps for single or very few carriers [2]. So far, mainly the applicability of InAs/GaAs QDs for memory concepts has been studied, see e.g. Refs. [3,4]. A crucial parameter for memory operation is the thermal activation barrier for the trapped carriers -the larger it is, the longer the storage time becomes. In order to investigate the electronic properties and the carrier dynamics in QD systems, capacitance spectroscopy has been demonstrated to be a feasible tool [5]. Previously, we have employed deep level transient spectroscopy (DLTS) [6] and admittance spectroscopy to study InAs/GaAs [7][8][9] and Ge/Si QDs [10]. QD formation in the latter material system, however, typically leads to rather large islands and low area densities, which renders it considerably less attractive. InAs/GaAs QDs, on the other hand, typically exhibit electron and hole ground state localisation energies in the order of up to about 200 meV. GaSb/GaAs QDs have similar structural properties as typical InAs/GaAs QDs, but a significantly larger hole ground state localization energy is expected due to the type-II band alignment attractive for holes [11]. Here, we study the thermal activation barrier of hole-charged GaSb/GaAs QDs by admittance spectroscopy for varying filling conditions. The layer structure of the investigated GaSb QD sample, grown by metal organic chemical vapor deposition [12,13], is schematically depicted in Fig. 1a. A 500 nm thick highly p-doped contact layer (p = 1 × 10 18 cm -3 ) and a 700 nm thick p-doped GaAs layer (p = 3 × 10 16 cm -3) were deposited on top of a semi-insulating GaAs substrate. Subsequently, a layer of GaSb QDs (2.8 ML nominally) was created in the Stranski-Krastanow growth mode. The QD layer is sandwiched between two 10 nm thick undoped GaAs spacers, and followed by 500 nm p-doped GaAs (p = 3 × 10 16 cm -3 ). Finally, 400 nm of highly ndoped GaAs (n = 7 × 10 17 cm -3 ) was deposited in order to form a n + p diode structure. Employing standard optical lithography and chemical wet-etching, mesa structures with a diameter of 800 µm were created. Standard Ohmic contacts were formed on...

Origin of the blueshift of photoluminescence in a type-II heterostructure

“…The wavefunction of the indirect exciton is significantly extended in space compared with that of a direct exciton in a type-I system where both electrons and holes are confined in the same layer, which allows large controllability of the wavefunction distribution. In addition, the long radiative lifetime originating from spatially indirect recombination is attractive for applications such as optical memories [10,11]. …”

Time-resolved photoluminescence of type-II Ga(As)Sb/GaAs quantum dots embedded in an InGaAs quantum well

“…Ga(As)Sb/GaAs quantum dots (QDs) have recently attracted scientific interest because of their staggered (type-II) band alignment, wide band-gap range, and large valence band offset [1,2], along with the zero-dimensional density of states (DOS) [3]. So far, several groups have reported the optical properties and carrier dynamics in type-II Ga(As)Sb QDs grown by molecular beam epitaxy (MBE) [4][5][6][7][8][9][10][11][12][13] or metalorganic chemical vapor deposition [14,15]. Above all, incorporation of type-II QDs into a type-I quantum well (QW) offers intriguing optical properties due to their possible extended emission wavelength over 1.6 μm because of their large offsets of both conduction-and valence-bands.…”

Lasing characteristics of GaSb∕GaAs self-assembled quantum dots embedded in an InGaAs quantum well

“…4͒ and type-II GaSb/ GaAs QDs can be used for demonstrating similar results in the near-IR. [5][6][7][8][9][10][11][12][13][14][15] In addition, type-II QDs could be also useful for single carrier, even unipolar storage devices including optical memory 15 owing to their longer decay time ͓ϳ10 ns ͑Refs. 7 and 9͔͒.…”

“…Several groups have so far reported the optical properties of type-II GaSb QDs using the Stranski-Krastanov ͑SK͒ growth mode by molecular beam epitaxy [5][6][7][8][9][10][11][12][13] ͑MBE͒ or metal-organic chemical vapor deposition. 14,15 Our group has demonstrated high optical quality in both SK and interfacial misfit 16 ͑IMF͒ growth modes with photoluminescence ͑PL͒ indicating quantized energy levels and blueshift of the PL peak with increased pump power. 12 Moreover, the electroluminescence ͑EL͒ from GaSb IMF QDs at Х1.3 m has been demonstrated, which would be applicable to light sources in fiber-optic communication systems.…”