MEMORY DEVICE COMPRISING A STRAINED SEMICONDUCTOR DOUBLE - HETEROSTRUCTURE AND QUANTUM DOTS

The present invention relates to memories, and more particu ^¬ larly to semiconductor memories.

Background of the invention

Distinct types of memories will combine the advantages of nonvolatility of the Flash-memory [1] and the performance and endurance of the dynamic random access memory (DRAM) [2] . A large variety of such memory concepts has been proposed using different approaches, like FeRAM, MRAM, PCRAM, etc. [3] . One of the most promising options for chargebased memories is based on self-organized quantum dots (QDs) as memory units. Memory operation for III-V QD structures has been demonstrated, either based on optically [4-6] or electrically con ^¬ trolled charge storage [7-9] .

Objective of the present invention

An objective of the present invention is to provide a memory which provides a long storage time in combination with fast write and erase time and high endurance.

Brief summary of the invention

An embodiment of the invention relates to a memory comprising a strained double-heterostructure having an inner semiconduc ^¬ tor layer which is sandwiched between two outer semiconductor layers. The lattice constant of the inner semiconductor layer differs from the lattice constants of the outer semiconductor layers, the resulting lattice strain in the double-hetero ^¬ structure inducing the formation of at least one quantum dot inside the inner semiconductor layer. Said at least one quan- turn dot is capable of storing charge carriers therein. Due to the lattice strain, the at least one quantum dot has an emis ^¬ sion barrier of 1,15 eV or higher, and provides an energy state density of at least three energy states per 1000 nm ³ , all said at least three energy states being located in an en ^¬ ergy band of 50 meV or less.

Preferably, each of said at least three energy states is ca ^¬ pable of storing two charge carriers.

Each of said at least three energy states preferably stores holes and is capable of storing two holes. Energy states of confined holes are more closely spaced than those of elec ^¬ trons, and thus show a much higher carrier density in terms of their energy distribution than electrons. As such, energy states for holes can store more carriers per unit volume than energy states for electrons. A larger number of stored carriers per binary information unit increase the reliability of the stored information.

According to a preferred embodiment, said at least three en ^¬ ergy states are energetically located above the Fermi-level if the strained double-heterostructure is unbiased.

The memory preferably comprises a 2-dimensional hole gas layer capable of transporting holes for charging or discharging the at least one quantum dot. According to a very preferred embodiment, a semiconductor su- perlattice is arranged between said 2-dimensional hole gas layer and the at least one quantum dot. Said semiconductor superlattice may comprise at least two quantum wells, each of which providing at least one energy state . The hole-energy states of the two quantum wells may be lo ^¬ cated above the Fermi-level if the strained double- heterostructure is unbiased.

The energetic positions of the energy states of the least two quantum wells preferably differ from another if the strained double-heterostructure is unbiased.

The energy states of the at least two quantum wells can pref ^¬ erably be shifted relative to each other and relative to the energy states of the at least one quantum dot by applying an external bias voltage to the strained double-heterostructure.

By applying an erase voltage to the strained double- heterostructure, the energy states of the at least two quan- turn wells may be aligned such that holes can tunnel from the at least one quantum dot through the semiconductor superlattice based on resonance tunneling. In an unbiased state, the energy levels of the at least two quantum wells may mismatch and resonance tunneling of holes may be disabled. With this embodiment just a very small erase voltage is needed to switch from the storage situation to the erase situation. The electric field is only increased marginally leading to a high endurance of the memory cell. The 2 -dimensional hole gas layer may be part of a field ef ^¬ fect transistor of the memory, the gate electrode of said field effect transistor allowing to apply a voltage to the strained double-heterostructure. An intermediate layer may be arranged between the gate elec ^¬ trode of the field effect transistor and the strained double- heterostructure, said intermediate layer having a smaller band gap than the adjacent outer semiconductor layer of the strained double-heterostructure .

A conduction band discontinuity may be positioned at the in ^¬ terface between the intermediate layer and the adjacent outer semiconductor layer. Such a conduction band discontinuity may prevent electrons from reaching the strained double- heterostructure when the strained double-heterostructure is biased . The gate contact of the field effect transistor may be an oh- mic contact, and the intermediate layer may be n-doped.

Alternatively, the gate contact of the field effect transis ^¬ tor may be a Schottky contact.

The inner semiconductor layer and the two outer semiconductor layers may consist of Ga(As,Sb) and (Al,Ga)As, respectively, or of Ga(As,Sb) and GaP, respectively. The quantum dots in the inner semiconductor layer may be arranged in an array.

Brief description of the drawings

In order that the manner in which the above-recited and other advantages of the invention are obtained will be readily un ^¬ derstood, a more particular description of the invention briefly described above will be rendered by reference to spe ^¬ cific embodiments thereof which are illustrated in the ap- pended figures. Understanding that these figures depict only typical embodiments of the invention and are therefore not to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail by the use of the accompanying drawings in which

Figure 1 shows a first embodiment of a memory according to the invention,

Figure 2 shows a schematic illustration of the storage

(a) , write (b) , and erase (c) operations in the embodiment of Figure 1, shows (a) hysteresis at 50 K, and (b) temperature dependence of the hysteresis opening for a sweep time of 1 and 100 ms, respectively,

Figure 4 shows drain current transients at 50 K at a stor ^¬ age voltage of 0 V (a), 0.4 V (b) , and 0.7 V (c) , wherein insets show the valence band profiles at the given voltages,

Figure 5 shows write times (a) and erase times (b) in de ^¬ pendence on the pulse voltage,

Figure 6 shows a second embodiment of a memory according to the invention in an unbiased state,

Figure 7 shows the second embodiment during write opera ^¬ tion,

Figure 8 shows the second embodiment during erase opera ^¬ tion, Figure 9 shows a third embodiment of a memory according to the invention in an unbiased state, Figure 10 shows the third embodiment during erase opera ^¬ tion, and

Figure 11 shows achievable storage times for various mate ^¬ rial systems.

Detailed description of the preferred embodiments

The preferred embodiments of the present invention will be best understood by reference to the drawings, wherein identi- cal or comparable parts are designated by the same reference signs throughout. It will be readily understood that the pre ^¬ sent invention, as generally described herein, could vary in a wide range. Thus, the following more detailed description of the exemplary embodiments of the present invention, is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention.

The embodiments described below may be based on III-V quantum dots (QD) which allow the storage of holes. Hole storage of ^¬ fers significant advantages with respect to scalability and storage time. The energy levels of confined holes in a QD are much more closely spaced than those of electrons due to their larger effective mass. Thus, at least one order of magnitude more holes can be stored in a given volume than electrons. In addition, hole- confining type- I I systems (e.g., GaSb/AlGaAs QDs) provide huge hole localization energies leading to stor- age times of more than ten years at room temperature, a basic prerequisite for a nonvolatile memory.

The localization energies and therefore the emission barriers for confined holes in the ground state of the QD are summa ^¬ rized in Fig. 11 for different III-V QDs [10] . The storage time shows an exponential dependence on the localization en ^¬ ergy as predicted by the common rate equation of thermally activated emission. The storage time increases by one order of magnitude for an increase of the localization energy of about 50 meV. It can be seen that memories based on

Ga (As, Sb) / (Al, Ga) As and Ga (As, Sb) /GaP may reach storage times above 10 years. A first exemplary embodiment of the present invention relates to an InAs/GaAs QD memory which uses holes as charge carriers instead of electrons. For monitoring, the commonly used elec ^¬ tron channel is replaced by a 2D hole gas (2DHG) embedded un ^¬ derneath the QD layer. Static and time-resolved measurements of storage, write, and erase times demonstrate the feasibil ^¬ ity of the hole-based QD-memory concept.

The exemplary embodiment of Figure 1 consists of a quantum- well modulation doped field-effect transistor with a single InAs/GaAs QD layer in close vicinity to the 2DHG. A sche ^¬ matic cross section of the layer structure is shown in Figure 1. A 1 μπι thick undoped GaAs buffer layer 10 was grown first on an undoped GaAs substrate 15. The 2DHG was formed with 40 nm p- doped (p=2*10 ¹⁸ cnf ³ ) GaAs layer 20, a 7 nm thick undoped GaAs spacer layer 25, and a 8 nm thick Ino.25Gao.75As quantum well 30. Subsequently, a 20 nm undoped GaAs layer 35 was de ^¬ posited, followed by a single InAs QD layer 40 (nominally ~1.8 ML) . Finally, the structure was completed by a 180 nm undoped GaAs cap 45.

The heterostructure was processed into Hall bars with an ac- tive area of 310*460 μπι ² using chemical wet etching (see Figure 1 (b) ) .

The source and drain areas 50, 55 were metallized using a Ni/Zn/Au alloy which was annealed at 400 °C for 3 minutes to form ohmic contacts 60 down to the 2DHG. The gate 65 was formed by Ni/Au as a Schottky contact. Hall measurements at gate electrode 70 yielded a charge carrier density and a mo ^¬ bility of the 2DHG at 77 K of 8*10^ cnT ² and 4350 cm ² /Vs, re ^¬ spectively.

Figure 2 schematically shows the valence band of the struc ^¬ ture for the three memory operations: storage, writing, and erasing. At the storage position (Fig. 2(a)), the binding potential of holes in the QDs represents the emission barrier, needed to store a logical "1". To store a logical "0" (de ^¬ fined as empty QDs) a capture barrier is necessary, which is formed by the band-bending of the Schottky contact.

The storage time for both logic states is limited by the emission and capture processes of the QDs. In the structure, thermally assisted tunneling across the emission and capture barriers initiates the discharging and charging processes. The emission and capture rates depend on the barrier height (i.e., localization energy and capture barrier height), the temperature, and the electric field. To write a logical "1" (Fig. 2(b)), a negative bias is applied to the gate. This completely eliminates the capture barrier formed by the band- bending and fast write times down to nanoseconds can be real- ized. Thus, the QD-memory concept solves the drawbacks of Flash' s Si0 ₂ barriers by using a large barrier height which can, however, be decreased to almost zero during write opera ^¬ tion. Write times similar to those of DRAMs or even shorter are possible.

To erase the information (Fig. 2(c)) the electric field at the position of the QDs is increased by applying a positive bias such that tunnel emission occurs. The read-out of the stored information is done via the 2DHG below the QD layer. Carriers stored in the QDs reduce the charge density and the mobility in the 2DHG resulting in a lower conductance of the 2DHG when the QDs are occupied. To investigate the influence of holes stored in the QDs on the conductance of the 2DHG, the drain current I _D versus gate voltage V _G was measured in the dark with a fixed drain-source voltage of 100 mV.

Figure 3 (a) shows the measured hysteresis at a temperature of 50 K. The measurement cycle starts with a 10 ms long charging pulse (V _G =-1 V) , which shifts the QD states below the Fermi level, charging them with holes from the 2DHG (see Fig.

2 (b) ) .

When the gate voltage is now swept to 1.5 V the drain current decreases until the 2DHG is pinched off at about 1.1 V. Dur ^¬ ing the down sweep the QDs remain occupied if the sweep time is shorter than the hole storage time in the QDs.

At VG =1.5 V the QD states are far above the Fermi level (see Fig. 2(c)) and tunnel emission discharges the QDs. When the gate voltage is swept back to -1 V a larger current is ob ^¬ served leading to a distinct hysteresis opening. The hystere- sis originates from the influence of holes stored in the QDs on the conductance of the 2DHG during the down sweep.

The charged QDs act as Coulomb scattering centers, reducing the mobility of the 2DHG. In addition, using Gauss' law it was predicted that the transfer of holes in QDs lead to a re ^¬ duction in the carrier density in the 2DHG. Both the lowered charge carrier density and the decreased mobility reduce the conductance during the down sweep, resulting in a lower cur- rent trace compared to the up sweep. The maximum hysteresis opening with respect to the up sweep is shown in Fig. 3(b) as a function of temperature for two different sweep times. Us ^¬ ing a sweep time of 1 ms, the hysteresis opening drops from 32% at 20 K to almost zero at 85 K. The descent has its ori- gin in the reduced charge carrier storage time of QDs with increasing temperature, i.e., at higher temperatures more holes are emitted during the down sweep.

In [7] a high temperature memory effect due to deep levels is reported for a different InAs QD-structure . Here, the absence of such high temperature memory effects proves that not deep levels but in fact the QDs act as memory units. This conclu ^¬ sion is confirmed by previous investigations of hole emission from similar InAs/GaAs QDs by deep level transient spectros- copy, which resulted in a thermal emission time constant of 5 ms at 90 K for the QD hole ground state, in agreement with the disappearance of the hysteresis at 85 K for a sweep time of 1 ms . A sweep time of 100 ms further reduces the hystere ^¬ sis opening as compared to 1 ms, since more holes, stored in the QDs, are emitted during the slower down sweep.

An increased maximum hysteresis opening is expected using larger QD densities and/or multiple QD layers. The memory op- eration of the QD-memory prototype is studied by time- resolved measurements of the drain current at different stor ^¬ age voltages Vst , with either initially occupied or empty QDs . The QDs are charged or discharged by applying a gate voltage of -0.8 or 2 V, respectively.

After this initialization of the logical "1" or "0", the gate voltage was abruptly changed to the storage voltage and the drain current was measured as a function of time.

Figure 4 shows the transients at 50 K for three different storage voltages (0, 0.4, and 0.7 V) . The upper transients represent hole capture into initially empty QDs, leading to a decrease in the conductance of the 2DHG and, hence, to a de- crease in the drain current. The lower transients represent hole emission out of fully occupied QDs and, thus, the drain current increases to the equilibrium state. A change of the storage voltage from 0 to 0.4 V and further to 0.7 V causes multiple effects (Figs. 4(a)-4(c)); the time constants of both transients increase, the amplitude of the capture tran ^¬ sient is reduced, and the amplitude of the emission transient is increased. These effects can be explained by the changes of the capture and emission processes when applying a posi ^¬ tive storage voltage to the structure.

On the right hand side of Figs. 4 (a) -4(c) the valence band profiles for the three storage voltages are shown. The ampli ^¬ tudes of the transients represent the amount of transferred holes and are correlated with the number of levels which are below the Fermi level (for capture) and above the Fermi level (for emission) . A larger positive storage voltage shifts the Fermi level toward the QD ground state and, hence, more holes are emitted, less holes are captured, and thus the amplitudes vary .

The prolongation of the time constants is also related to the Fermi level shift as it leads to an increased capture and emission barrier height (Ebar in Fig. 4) .

The write and erase times of the memory structure were meas ^¬ ured. To determine write and erase times, a method was used which allowed to study emission from or capture into QDs across an enlarged span of time constants. The hysteresis opening at a storage position of 0.4 V was measured after ap ^¬ plying write/erase pulses with successively reduced pulse widths down to 10 ns . When the pulse width was too short for any charging/discharging of the QDs, the hysteresis opening vanished. The write/erase times were defined as the pulse width, at which the hysteresis opening drops to 50% of the maximum value. Figure 5(a) shows the write time in dependence on the write pulse voltage at 20 and 50 K. A more negative write pulse leads to a reduction in the capture barrier during writing and, hence, the write time decreases exponentially. For write pulses larger than |0.5| V the write time starts to saturate and reaches a minimum at 80 ns for a write pulse of -1.75 V.

This saturation has presently its origin in a parasitic cut ^¬ off frequency of about 2 MHz of the RC low pass of our pre ^¬ sent devices. Much faster write times are expected for smaller devices having larger parasitic cut-off frequencies.

The erase times are shown in Fig. 5(b) . A minimum erase time of 350 ns at 50 K was obtained for an erase pulse of 2.5 V. The temperature dependence of the write and erase times re ^¬ flects again the increased thermal capture and emission rates at higher temperatures. Summarizing, the first exemplary embodiment relates to a hole based memory device using InAs/GaAs QDs for charge carrier storage. Charging and discharging of the QDs are clearly controlled by a gate voltage. The read-out of the stored infor ^¬ mation uses a 2DHG, with a relative hysteresis opening up to 32%. Write times down to 80 ns - only a factor of 8 larger than for a typical DRAM - and erase times of 350 ns - four orders of magnitude faster than for a typical Flash memories - were demonstrated. The results support the assumption that the QDs act as memory.

A second exemplary embodiment of the present invention will be explained in further detail with reference to Figures 6 - 8. Figure 6 shows a memory 105 comprising a strained double- heterostructure 110 having an inner semiconductor layer 115 which may consist of Ga(As,Sb) and which is sandwiched be ^¬ tween two outer semiconductor layers 120 and 125. The outer semiconductor layers 120 and 125 may consist of (Al,Ga)As.

The lattice constant of the inner semiconductor layer 115 differs from the lattice constants of the outer semiconductor layers 120 and 125, such that lattice strain is generated. The resulting lattice strain in the double-heterostructure 110 induces the formation of at least one quantum dot inside the inner semiconductor layer, said at least one quantum dot providing at least three energy states 186 capable of storing charge carriers therein. Due to the lattice strain, the at least one quantum dot has an emission barrier Eb of 1,15 eV or higher, and provides an energy state density of at least three energy states per 1000

3

nm . All of the at least three energy states are located m an energy band AWb of 50 meV or less. The energy band AWb is provided such that each of said at least three energy states is capable of storing two holes. Figure 6 shows the memory 105 in an unbiased state. It can be seen that the at least three energy levels of holes are ener ^¬ getically above than the Fermi-level Ef.

Moreover, Figure 6 shows a 2-dimensional hole gas layer 130 which is capable of transporting holes for charging or discharging the at least one quantum dot. The 2-dimensional hole gas layer 130 is part of a field effect transistor of the memory. The gate electrode 135 of the field effect transistor allows applying a voltage to the strained double-hetero- structure 110.

The memory of Figure 6 further comprises an intermediate layer 140 which is arranged between the gate electrode 135 of the field effect transistor and the strained double- heterostructure 110. The intermediate layer 140 has a smaller band gap than the adjacent outer semiconductor layer 120 of the strained double-heterostructure 110. The smaller band gap provides a conduction band discontinuity AE _L at the interface between the intermediate layer 140 and the adjacent outer semiconductor layer 120. The conduction band discontinuity

AE _L prevents electrons originating from gate 135 to get into the outer semiconductor layer 120 if a positive bias voltage is applied. Thus, the conduction band discontinuity AE _L avoids recombination of electrons and holes in the inner semiconductor layer 115.

The gate contact of the field effect transistor is preferably an ohmic contact, and the intermediate layer 140 is prefera ^¬ bly n-doped.

Figure 7 shows the memory 105 of Figure 6 after applying a write-voltage to the strained double-heterostructure . It can be seen that holes 185 may leave the 2-dimensional hole gas layer 130 and drop into the energy states 186 of the quantum dot located in the inner semiconductor layer 115.

Figure 8 shows the memory 105 after applying an erase-voltage to the strained double-heterostructure. It can be seen that the holes 185 stored in the energy states of the quantum dot may tunnel through the energy barrier 190 and reach the 2- dimensional hole gas layer 130. A third exemplary embodiment of the present invention will be explained in further detail with reference to Figures 9-10.

Figure 9 shows a memory 105 which comprises a strained dou ^¬ ble-heterostructure 110 having an inner semiconductor layer 115 which is sandwiched between two outer semiconductor layers 120 and 125. An intermediate layer 140 is arranged be ^¬ tween a gate electrode 135 and the outer semiconductor layer 120. Furthermore, the memory 105 comprises a 2-dimensional hole gas layer 130 which is capable of transporting holes for charging or discharging at least one quantum dot positioned in the inner semiconductor layer 115. Insofar, the third exemplary embodiment corresponds to the second exemplary em ^¬ bodiment described with respect to Figures 6-8. In addition to the second embodiment, the third embodiment comprises a semiconductor superlattice 200 which is arranged between the 2-dimensional hole gas layer 130 and the outer semiconductor layer 125. The semiconductor superlattice 200 comprises two quantum wells 210 and 220. Each quantum well 210 and 220 provides at least one energy state 230.

In Figure 9, the strained double-heterostructure 110 is unbi- ased. It can be seen that the energetic positions of the en ^¬ ergy states 230 of the quantum wells 210 and 220 differ from another. Due to the mismatch of the energy states 230, the holes trapped in the energy states in the quantum dot cannot tunnel through the semiconductor superlattice 200 and thus not reach the 2-dimensional hole gas layer 130.

Figure 10 shows the third embodiment after applying an exter ^¬ nal erase bias voltage to the strained double-hetero ^¬ structure. Due to the bias voltage, the energy states 230 of the quantum wells 210 and 220 are shifted relative to each other and relative to the energy states 186 of the at least one quantum dot. This alignment enables the holes 185 to tun ^¬ nel from the quantum dot through the semiconductor superlattice 200 based on resonant tunneling. The holes can reach the 2-dimensional hole gas layer 130 and the energy states of the at least one quantum dot are emptied.

In the first and second embodiment, the erase process is based on hole tunneling through the entire emission barrier. Hence, the erase time depends on the height of the emission barrier. Regarding an emission barrier of 1.15 eV or higher, high electric fields in the range of MV/cm are needed to reach fast erase times [11] . This high electric fields lead to a high power consumption of the memory cell and to a low endurance, as defects in the lattice could be generated.

The third embodiment solves these problems. There, the erase process is done by resonant tunneling through the superlat- tice. Only a small voltage is needed to align the energy states of the quantum well to allow resonant hole tunneling. As a consequence the electric field is increased just margin ^¬ ally. Therewith, both problems - the high power consumtion and the low endurance - are solved. A memory with long stor ^¬ age time in combination with a fast erase time and high en ^¬ durance is feasible.

Figure 11 shows achievable storage times for various material systems. It can be seen that memories based on

Ga (As, Sb) / (Al, Ga) As and Ga (As, Sb) /GaP may reach storage times above 10 years.

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Reference Signs

10 buffer layer

15 sub strate

20 layer

25 spacer layer

30 quantum well

35 layer

40 QD layer

45 cap

50 source

55 drain

60 ohmic contact

65 gate

105 memory

110 strained double-heterostructure

115 inner semiconductor layer

120 outer semiconductor layer

125 outer semiconductor layer

130 2-dimensional hole gas layer

135 gate electrode

140 intermediate layer

185 hole

186 energy state

190 energy barrier

200 semiconductor superlattice

210 quantum well

220 quantum well

230 energy state

AE _l conduction band discontinuity E _F Fermi energy

E _v valence band conduction band energy band