JP2890030B2 - Quantum beat device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quantum device having a coupled quantum wire structure for ultra-high speed optical / electronic devices such as a terahertz (THz) electromagnetic wave generating device and an optical-optical switch which can operate in the future femtosecond region. It relates to a beat element.

[0002]

2. Description of the Related Art With the advance of laser technology, it has become possible to generate ultrashort light pulses of several femtoseconds as the limit of ultra-high speed. On the other hand, in the semiconductor optical / electronic device technology, it is only possible to handle a signal in a picosecond region at last, and the femtosecond time region is an unexplored region in current device technology.

[0003] Looking at the flow of research on ultra-high speed semiconductor optical devices, the upper limit of operation is determined by the CR limit in electric field control type optical modulators and optical switches, which are being actively researched. The operating speed is at the upper limit. On the other hand, in the light control type device, the problem of CR limitation can be avoided, but when actual excited carriers are used, there is a problem that the operation speed is limited by the time until the excited carriers are relaxed (relaxation time). In order to solve this problem, a method of reducing the carrier lifetime using non-radiative recombination, a method of using a high-speed relaxation phenomenon, and the like have been studied. However, when the relaxation phenomenon is used, it is difficult to increase the speed over sub-picoseconds, and light and light operating in the femtosecond region are difficult.
In order to realize the electronic fusion device, it is considered that a device that operates according to a new principle without a relaxation time limit is required.

[0004]

A quantum beat, which is one of the coherent phenomena in a semiconductor, is a beat of polarization generated when two excited levels are excited, and is a space in which the wave functions of the two excited levels are different. When having a distribution, there is a spatial vibration of the wave packet. The oscillation period of a quantum beat is determined by the energy difference between two levels, and by increasing this, ultrahigh-speed operation in the femtosecond region is expected. For example, 300
With an energy difference of about meV, an ultrafast vibration of the order of 10 fs can be obtained. In fact, radiation of a terahertz electromagnetic wave accompanying a quantum beat (spatial oscillation of an electron wave packet) in a semiconductor quantum structure has been observed [1]. However, the quantum beats reported in semiconductor quantum thin films (two-dimensional semiconductors) that have been reported so far are multi-level systems, so the coherency of the electron wave is poor, and the low-temperature operation and short connection time of the beats are required. There's a problem.

[0005]

According to the quantum beat device of the present invention, a barrier layer made of a material having a large band gap and two quantum wires or quantum boxes made of a material having a small band gap are formed on a substrate. And the two quantum wires or quantum boxes have a coupled quantum wire structure or quantum box having two or more coupled levels, respectively. .

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to apply the coherent phenomenon to a device, it is essential to increase the phase relaxation time and to reduce the number of excitation levels. Low-dimensional structures are very promising [2] [3]. We have proposed a quantum beat device using a coupled low-dimensional structure (coupled quantum wire / coupled quantum box structure) shown in FIG. 1 in order to improve the coherency of an electron wave. This element controls the spatial oscillation of the electron wave packet, including the phase, by using femtosecond ultrashort light pulses, and is capable of operating in the femtosecond region, such as a terahertz electromagnetic wave generating element or an ultrafast optical-optical switch. It is expected to be applied to electronic fusion devices.

[0007]

EXAMPLES Production of coupled quantum wires As methods for producing a quantum wire, a method of processing a quantum thin film structure by EB lithography and etching, a method of selectively growing on an uneven substrate, a method of using a self-forming method, and the like are considered. Can be

In order to produce a coupled quantum wire structure, it is desirable to use a method capable of producing an extremely fine and strongly coupled structure that exhibits the characteristics of a one-dimensional system. The method using the selective growth [4] enables the layer thickness to be controlled to the order of the atomic layer, and a structure having a very strong bond can be manufactured by arranging quantum wires in the layer thickness direction.

[0009] The MOCVD method and M
In the method of manufacturing a quantum wire on the V-groove using the BE method, the width of the quantum wire in the lateral direction is as large as about 30 nm to 50 nm.
A quantum wire structure in which the characteristics of the one-dimensional system completely appear has not been manufactured. On the other hand, the flow rate modulation MOCVD method can improve the selective growth ratio of the quantum wire to the bottom of the V-groove by alternately supplying the growth material. Fine wires can be produced. [5] FIG. 2 shows a flow rate modulation MOC.
FIG. 2 shows a schematic view of a coupled quantum wire structure created by the VD method. First, a V-groove 2 is formed on a GaAs substrate 1 in the (100) direction.
GaAs buffer layer 3 and AlGaAs
An s buffer layer 4, a GaAs quantum wire 5, an AlGaAs barrier layer 6, a GaAs quantum wire 7, and an AlGaAs protective layer 8 were grown to form a coupled quantum wire structure. In order to obtain an extremely fine quantum wire structure, a flow rate modulation MOCVD method is used only when growing a GaAs quantum wire, and the other layers are made of a normal MOCV.
Growth was performed using D. Next, in order to obtain light emission from the quantum wires, the (100) quantum well layer above the V mesa and the quantum well layer on the (111) V mesa side wall around the V mesa were removed by lithography and etching techniques. FIG. 3A shows an example of a cross-sectional TEM photograph at this time, and FIG.
(B). 5 nm thick and 30 nm wide at the bottom of the V-groove
(Effective width 15 nm) Ultrafine two-layer quantum wires and thickness 2
It is shown that a coupling structure composed of a nm AlGaAs barrier layer is formed. In addition, since the selective growth ratio between quantum wires and quantum thin films was improved using the flow rate modulation method,
The thickness of the quantum thin film layer on the (111) V mesa sidewall is suppressed to about 1 nm.

[0010] The coupled quantum coupling effects 4 and the light emitting characteristics of the thin lines, conceptual view of the energy level of binding quantum wires when bound two independent and thin barrier layer of the energy level and between the quantum wire Is shown. In a symmetric coupling structure where two energy levels are the same, each level splits after coupling into energy levels having symmetric and antisymmetric wave functions.
The split energy difference (split energy) is enlarged by reducing the thickness of the barrier layer and increasing the coupling.
As is clear from the theoretical analysis of the coupled quantum well,
Even in an asymmetric bond structure having two different energy levels, if the bond splitting energy is larger than the energy difference between two quantum wires, the energy difference between the low energy level and the high energy level is enlarged by the bond. here,
The level on the low energy side split by bonding is referred to as a bonding ground level, and the level on the high energy side is referred to as a bonding higher level. On the other hand, regarding the hole level, the ratio of the width and the thickness of the cross section of the manufactured quantum wire structure is three times or more, and it is considered that the quantum wire structure is close to the heavy hole and the light hole of the quantum well. This is called a quasi heavy hole or quasi light hole level. From the above, the optical transitions allowed in this coupled quantum wire are the same for the electron-pseudo heavy hole transition (Echh1), the electron-pseudo light hole transition (Eclh1), and the higher-order coupling level for the ground coupling level. The transition between an electron and a suspicious heavy hole (Echh
2), transition between electron and pseudo write hole (Eclh2) is allowed, and a total of four transitions are possible [6].

FIG. 5 shows the temperature characteristics of the photoluminescence (PL) emission spectrum of a single quantum wire having almost the same layer thickness for comparison with the fabricated coupled quantum wire having a layer thickness of 5 nm and a barrier layer thickness of 3 nm. . At the bottom of each figure, a PL emission spectrum upon strong excitation at low temperature is shown.

In the case of low-temperature weak excitation, both are single peaks, whereas in the case of room-temperature weak excitation or low-temperature strong excitation, a large subpeak is observed in the case of a coupled quantum wire. On the other hand, in the case of a single quantum wire, a large subpeak is not seen. This is because the bond level split by the bond (Echh
1, Eclh1).

FIG. 6 shows PL emission characteristics of three types of coupled quantum wires having different barrier layer thicknesses. On board surface (100)
From the direction, the measurement was performed separately for the polarization parallel to and perpendicular to the quantum wire. From the surface of the substrate, in the case of a quantum thin film, light emission characteristics without polarization dependence were obtained, whereas from the fabricated device, light emission characteristics with polarization characteristics unique to quantum wires were obtained. From the polarization information, it is considered that the main peak and the sub-peak are due to the transitions Ech1 and Ech2 relating to the pseudo heavy hole. This energy difference (ΔE = Ec
hh2-Ech1) is the splitting energy due to the bond. From the figure, it can be seen that the splitting energy increases as the barrier layer thickness is reduced and the bond is strengthened. This was first observed as an effect of quantum wire coupling.

It should be noted that the splitting of the main peak and the sub-peak of the coupled quantum wire is clearly seen in comparison with the previously reported coupled quantum thin film. Further, when each peak is separated, the full width at half maximum of the main peak is about 19 meV near room temperature, which is sufficiently narrower than that of the quantum thin film (FWHM to 25 meV). These characterize the bonded one-dimensional system and are the first to be clearly seen from the characteristics at room temperature.

[0015]

As described above, the MOC on the V-groove is
Ultra fine (5 nm thick, equivalent width 1) using VD selective growth method
5 nm) of a coupled quantum wire structure. For the first time, a clear coupling effect and the characteristics of a one-dimensional system were observed from the fabricated coupled quantum wires.

The coupled quantum wire structure manufactured this time can generate coherent quantum oscillation (quantum beat) by controlling coherent electrons in the quantum wire using femtosecond light. By utilizing the optical nonlinearity associated with this quantum oscillation, application to ultrafast optical / electronic fusion devices that can operate in the femtosecond range, such as terahertz electromagnetic wave generation devices and ultrafast optical-optical switches, is expected.

Reference [1] G. FIG. Roskos, M .; C. Nuss, J .;
Shah, K .; Leo, D .; A. B. Miller,
A. M. Fox, S.M. S. Rink, and K.K. Ko
hler, Phys. Rev .. Lett. , 68,22
16 (1992) [2] Y. Arakawa and H.C. Sakak
i, Appl. Phys. Lett. , 40,939
(1982) [3] M.P. Asada, Y .; Miyamoto, and
Y. See Suematsu, Jpn. J. Appl. Ph
ys 24, L95 (1985) [4] E.I. Kapon, D .; M. Hwang, and
R. Bhat, Phys. Rev .. Lett. , 63,
430 (1989) [5] X.R. L. Wang, M .; Ogura, Appl.
Phys. Lett. , 66, 1506 (1995) [6] Kawai, J. et al. Kaneko, and
N. Watanabe, J .; Appl, Phys. 5
8, 1263 (1985)

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a quantum beat device according to the present invention.

FIG. 2 is a schematic view showing a structure of a coupled quantum wire.

FIG. 3 is a transmission electron micrograph showing a cross section of a coupled quantum wire (a).
FIG.

FIG. 4 is a diagram illustrating level separation of a coupled quantum wire.

FIG. 5 is a diagram showing PL emission characteristics of a quantum wire and a coupled quantum wire.

FIG. 6 is a diagram showing the emission characteristics of a coupled quantum wire.

[Explanation of symbols]

 Reference Signs List 1 GaAs substrate 2 V groove 3 GaAs buffer layer 4 AlGaAs barrier layer 5 GaAs quantum wire 6 AlGaAs barrier layer 7 GaAs quantum wire 8 AlGaAs protective layer

Claims (1)

(57) [Claims] 1. A barrier layer made of a material having a large bandgap and two quantum wires or quantum boxes made of a material having a small bandgap are formed close to each other on a substrate and quantum-mechanically coupled. A quantum beat device, wherein each of the two quantum wires or quantum boxes has a coupled quantum wire structure or a coupled quantum box structure having two or more coupled levels.