Quantum semiconductor memory device including quantum dots

Details Quantum semiconductor memory device including quantum dots Quantum semiconductor memory device including quantum dots

1999-03-22

2001-08-28

Crane, Sara (Department: 2811)

Active solid-state devices (e.g., transistors, solid-state diode

Thin active physical layer which is

Heterojunction

C257S018000, C257S021000, C257S184000, C257S460000, C257S615000

Reexamination Certificate

active

06281519

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention generally relates to quantum semiconductor devices and more particularly to a quantum semiconductor memory device that uses a quantum dot structure for storage of information and a fabrication process thereof.
In a so-called bulk semiconductor crystal in which there is no carrier confinement, the state density of carriers increases continuously along a parabolic curve with energy. In a so-called quantum well structure in which the carriers are confined one-dimensionally in a two-dimensional plane, on the other hand, there appears a stepwise change in the state density with energy due to the existence of quantum states. In such a system that has a stepwise change in the state density, the distribution of carriers is restricted substantially as compared with the case of a bulk crystal, and a sharp optical spectrum is obtained when the quantum well structure is used for an optical semiconductor device such as a laser diode. By using a quantum well structure, the efficiency of optical emission is improved in such optical semiconductor devices. Further, quantum well structures are used for an energy filter of carriers in electron devices that have a resonant-tunneling barrier such as an RHET (resonant-tunneling heterostructure transistor).
In a quantum wire structure in which the degree of carrier confinement is increased further, the state density of the carriers is modified such that there appears a maximum state density at the bottom edge of each step. Thereby, the sharpness of the energy spectrum increases further.
In an ultimate quantum dot structure in which the degree of carrier confinement is increased further, the state density becomes discrete as a result of the three-dimensional carrier confinement, and the energy spectrum of the carriers becomes also discrete in correspondence to the discrete quantum levels. In the system that has such a discrete energy spectrum, the transition of carriers occurs discontinuously between the quantum levels even at a room temperature in which there exists a substantial thermal excitation. Thus, an optical semiconductor device that uses a quantum dot structure can provide a very sharp optical spectrum even in a room temperature operation.
Further, the energy filter having a quantum dot structure can provide the desired very sharp energy spectrum not only at a very low temperature but also at a room temperature.
Meanwhile, there is a proposal to construct a quantum semiconductor memory device that uses such a quantum dot structure for an optical storage of information. For example, Muto, et al. (Muto, S. Jpn. J. Appl. Phys. vol.34, 1995, pp.L210-212, Part2, No.2B, February 1995) describes a quantum semiconductor memory device that uses a quantum dot structure formed on a stepped semiconductor surface by a lateral epitaxial growth of semiconductor layers. In the proposed structure, the electrons excited as a result of an optical excitation are caused to tunnel to an adjacent semiconductor layer and held therein.
In the foregoing quantum structure, the electron excited in the quantum dot is held stably in the semiconductor layer adjacent to the quantum dot in a spatially separated state from the hole that is created as a result of the optical excitation of the electron and remaining in the quantum dot. By forming the quantum dot from a semiconductor material of a direct-transition type and by forming the adjacent semiconductor layer from a semiconductor material of an indirect-transition type, the optical excitation of carriers in the adjacent semiconductor layer is effectively avoided.
FIG. 1
is a band diagram showing the principle of the conventional quantum semiconductor memory device of the foregoing prior art.
Referring to
FIG. 1
, the quantum semiconductor memory device includes a quantum dot M
1
of GaAs surrounded by a storage layer M
3
of AlAs, with a thin barrier layer M
2
of AlGaAs intervening between the quantum dot M
1
and the storage layer M
3
. It should be noted that GaAs forming the quantum dot M
1
is a typical direct-transition type semiconductor material and causes an excitation of an electron represented by a solid circle to a quantum level L
e
and a hole represented by an open circle to a quantum level L
h
in response to an irradiation of an optical radiation having a wavelength &ngr;.
Thereby, it should be noted that the quantum dot M
1
has a size set such that the quantum level L
e
is located higher than a bottom edge of the conduction band of the adjacent AlAs layer M
3
, so that the electron thus excited in the quantum dot M
1
can fall to the conduction band of the AlAs layer M
3
, after passing through the barrier layer M
2
by tunneling. On the other hand, the hole that is created in the quantum dot M
1
as a result of the optical excitation of the electron remains in the quantum dot M
1
because of the larger effective mass. Thereby, the electron and hole thus excited optically are held stably at respective locations separated from each other spatially.
In the band structure of
FIG. 1
, it should be noted that the optical excitation in the AlAs layer M
3
does not occur substantially. In the AlAs layer M
3
, which is an indirect-transition type semiconductor, the optical excitation of carriers from the valence band to the X-valley of the conduction band occurs only in the presence of the other elementary excitation such as a phonon that satisfies the conservation of momentum. Further, the excitation to the &Ggr;-valley, which does not require such an interaction with other elementary excitations, does not occur because of the very large transition energy necessary for causing the optical excitation. Thus, there occurs no substantial optical excitation in the AlAs layer M
3
.
In order to fabricate the quantum semiconductor memory device having such a quantum dot structure, it is necessary to establish a technology to form a high-quality quantum dot with clearly defined quantum levels so that the desired optical transition of carriers occurs between these quantum levels. In addition, it is necessary that the size of the quantum dot is controlled in the quantum semiconductor memory device of
FIG. 1
such that the quantum level L
e
in the quantum dot M
1
is located at an energetically higher level than the X-valley of the conduction band of the AlAs layer M
3
.
Conventionally, the so-called quantum well structure that confines the carries in a substantially two-dimensional surface has been formed successfully and with reliability by using an MBE (molecular beam epitaxy) process or an MOVPE (metal-organic vapor phase epitaxy) process, such that a very thin quantum layer is sandwiched by a pair of barrier layers. Further, a quantum wire structure, in which the carriers are confined substantially along a one-dimensional wire, can be formed by using a so-called inclined semiconductor substrate having a stepped structure on a principal surface thereof. It is known that a quantum wire can be formed by causing a lateral epitaxial growth of a narrow quantum semiconductor layer having a small thickness and a limited width from each lateral edge of the stepped structure along the stepped surface. Alternatively, a quantum wire may be formed by applying an electron beam lithography.
Thus, it has been thought that a quantum dot structure may also be formed by using a stepped surface of an inclined semiconductor substrate or kink similarly to the case of forming a quantum wire. It turned out, however, that it is difficult to control the stepped surface of the inclined semiconductor substrate such that the desired formation of the isolated quantum dot is formed according to such a lateral epitaxial process. Further, the tendency that a mixing of element occurs at the heteroepitaxial interface of the quantum dot thus formed according to such a lateral epitaxial process makes it difficult to form a clearly defined quantum dot having a boundary where there is a sharp change of composition.
Further, the process of forming a quantum dot by using a photolithographic process

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