Research Focus 1

Comments & Views on Current Projects at NTT Laboratories

Self organization of Strained InGaAs Quantum Disks

A semiconductor quantum box, where electrons are confined in three directions, represents the ultimate low-dimensional quantum structure. The zero dimensional confinement in a quantum box leads to new electronic properties which can be used to improve the performance of optical devices such as semiconductor lasers with ultra low threshold current and non-linear optical switches. However, the precise fabrication of such nanometer-scale quantum boxes is still a major challenge in modern microstructure materials science.

We have found a new self-organizing phenomenon in the metallo-organic vapor phase epitaxy of strained InGaAs/AlGaAs multilayer structures on GaAs (311)B substrates that overcomes the present difficulties associated with the fabrication of quantum-box structures. After the growth of a strained InGaAs layer onto a thick AlGaAs buffer layer, the originally flat InGaAs film organizes spontaneously into homogeneous nanoscale islands embedded in AlGaAs microcrystals (see figure). Since no AlGaAs has been grown after the InGaAs film, this implies that the AlGaAs microcrystals are formed due to lateral mass transport from the buffer layer. Details of the formation mechanism can be found in Nature, vol. 369, p.131, (1994). This unique interaction between the strained InGaAs film and the AlGaAs buffer layer produces well ordered and high density arrays of AlGaAs microcrystals containing disk-shaped InGaAs quantum-box structures. Moreover in our system we can control independently the size and distance of the disks in a wide range by the In content and the InGaAs layer thickness, respectively.

Since no artificial patterning procedure is necessary to define the lateral structure, our quantum disks exhibit excellent crystal quality and homogeneity in size which is most directly evidenced in the high luminescence efficiency at room temperature and the narrow linewidth. In fact, the narrow luminescence linewidth at room temperature (13 meV) reveals reduced thermal broadening in our disks due to efficient lateral confinement of the photogenerated carriers, highlighting the potential of this new self-organizing quantum-box structure for many applications in advanced optoelectronic devices.

Richard Nötzel,
NTT Optoelectronics Laboratories
Present address:
Department of Electrical Engineering
Hokkaido University
e-mail: notzel.ricardel@E5.hines.hokudai.ac.jp

How many NTT postdocs does it take to change a light bulb?
(a) Only one, but he gets three technical papers out of it.
(b) Only one, but it takes two years.
(c) Two: one to handle the bulb, while the other entertains him with this silly joke!