My general interest concentrates on the nanometre-scale engineering of low-dimensional semiconductor structures (such as quantum dots and quantum wires) by using molecular beam epitaxy and the development of novel optoelectronic devices including lasers, detectors, and modulators.

Research
Figure 1: Semiconductor quantum dots are zero-dimensional crystal whose size in the nanometre (1 nanometre=10-9 meter) scale in three spatial directions. Figure (a) shows the AFM image of uncapped InAs/GaAs quantum dots. And Figure (b) shows a typical high-resolution TEM image of single InAs/GaAs quantum dots.

Figure 2: The formation of dislocation during the growth of multilayer quantum-dot structures degrades the performance of quantum-dot lasers. The Dark field (200) cross-sectional TEM image of 5-layer InAs/InGaAs dot-in-a-well sample is shown in Figure (b). With introducing the growth approach, i.e., the high-growth-temperature GaAs spacer layers, the formation of dislocation is suppressed. As shown in Figure (a), there is no any dislocation observed in a number of TEM images with using this growth approach. The performance of quantum-dot laser is dramatically improved by using this growth technique.

Recent Publications

• H. Y. Liu, D. T. Childs, T. J. Badcock, K. M. Groom, I. R. Sellers, M. Hopkinson, R. A. Hogg, D. J. Robbins, D. J. Mowbray, and M. S. Skolnick, High-Performance 3-layer 1.3 µm InAs/GaAs Quantum-Dot Lasers with Very Low Continuous-Wave Room-Temperature Threshold Currents, IEEE Photon. Technol. Lett., 17, 1139 (2005)
• H. Y. Liu, I. R. Sellers, T. J. Badcock, D. J. Mowbray, M. S. Skolnick, K. M. Groom, M. Gutiérrez, M. Hopkinson, J. S. Ng, J. P. R. David, and R. Beanland, Improved performance of 1.3 µm multi-layer InAs quantum dot lasers using a high growth temperature GaAs spacer layer, Appl. Phys. Lett. 85, 704 (2004)

Semiconductor quantum-dot (QD) lasers are theoretically predicted with revolutionary characteristics. These include temperature-independent operation, reduced drive current, reduced sensitivity of the damage, and higher operation speed. But the performance of quantum-dot device was limited by the material quality, in particular for multilayer QD structures. We proposed and demonstrated the high-growth-temperature-spacer-layer growth technique for multilayer QD structures to significantly improve the material quality and device performance. By using this technique, the high-performance of quantum-dot laser is demonstrated with the record-low threshold current density and high outpower for multilayer QD laser under cw operation at room temperature.

• H. Y. Liu, M. J. Steer, T. J. Badcock, D. J. Mowbray, M. S. Skolnick, P. Navaretti, K. M. Groom, M. Hopkinson, and R. A. Hogg, Long-wavelength InAs/GaAs quantum dots and quantum-dot laser with GaAsSb strain-reducing layer, Appl. Phys. Lett. 86, 143105 (2005)
• H. Y. Liu, M. J. Steer, T. J. Badcock, D. J. Mowbray, M. S. Skolnick, and F. Suarez, J. S. Ng, M. Hopkinson, and J. P. R. David, Room-temperature 1.6 µmmlight emission from InAs/GaAs quantum dots with a thin GaAsSb cap layer, J. Appl. Phys. 99, 046104 (2006)

Semiconductor lasers with emission around 1310 nm and 1550 nm are required to take full advantage of the local and global minima in the attenuation of standard optical fiber. It is important to extend the emission of GaAs-based emitter to telecom wavelength around 1550 nm to overcome the limitation of InP-based devices, which is currently used in telecom systems. The emission wavelength of GaAs-based quantum-well emitters is limited by the strain up to 1250 nm. We proposed and demonstrated the room-temperature lasing near 1300 nm and the over 1600nm emission for GaAs-based InAs/GaAsSb quantum-dot structure with engineering the band gap of novel type-II semiconductor.

• T. J. Badcock, H. Y. Liu, K. M. Groom, C. Y. Jin, M. Gutiérrez, M. Hopkinson, D. J. Mowbray, and M. S. Skolnick, 1.3 µm InAs/GaAs quantum-dot laser with a low threshold current density and negative characteristic temperature above room temperature, Electron. Lett. 42, 922 (2006)
• C. Y. Jin, H. Y. Liu, K. M. Groom, Q. Jiang, and M. Hopkinson, T. J. Badcock, R. J. Royce, and D. J. Mowbray, Effects of photon and thermal coupling mechanisms on the characteristics of self-assembled InAs/GaAs quantum dot lasers. Phys. Rev. B, 76, 085315, (2007)

We first demonstrated a negative characteristic temperature over the temperature range from -50 to 40°C by combining high-growth-temperature-spacer-layer growth technique with p-type modulation doping for a 5-layer QD device. Although the temperature-independent operation of quantum-dot laser is theoretically predicted, the negative characteristic temperature is not expected. A theoretical model, which takes into account a photon coupling process between the ground and first excited states of different sized dots, is proposed to fully explain the novel temperature dependence of the threshold current density p-doped lasers.