We report the structural, electrical and optical properties of bulk InAsN alloy with various nitrogen contents deposited on (100) InP substrates using plasma-assisted gas-source molecular beam epitaxy. From absorption measurements, it is found that the fundamental absorption energy of InAsN is higher than that of InAs due to the Burstein-Moss effect resulting from the high residual carrier concentration in InAsN. To deduce the 'real' band-gap energy of InAsN samples, the energy shift due to the Burstein-Moss effect and the band-gap narrowing effect are calculated by using a self-consistent approach based on the band-anticrossing (BAC) model [Shan et al.: Phys. Rev. Lett. 82 (1999) 1221. After correction, the 'real' band-gap energy of InAsN samples decreases as N increases. The electron effective mass of InAsN is also investigated by plasma-edge measurement. We found a sizeable increase of the electron effective mass in these InAsN alloys, which is consistent with the theoretical predictions based on the BAC model.

We report magnetotransport measurements and scaling analysis on a series of center-doped GaAs quantum wells. Sharp phase transitions were observed in the magnetic field sweep and it was found that, depending on the doping concentration in the quantum wells, the insulating phase can make transitions to quantum Hall phase with Landau-level filling factors ͑͒ of 2, 6, and 8. The critical exponents vary from sample to sample and are mobility dependent. The longitudinal resistivities of these samples at the phase transition points decrease with increasing , and for a sample with higher mobility, its value is close to h/e 2 .

Highly strained InGaAs/GaAs quantum wells grown at very low temperature (380 °C) have been studied. The critical thickness of the In 0.38 Ga 0.62 As quantum well is 8.8 nm and the photoluminescence peak is at 1.24 m. An edge-emitting In 0.388 Ga 0.612 As/GaAs quantum-well laser demonstrates an emission wavelength of 1.244 m at 18 °C. The threshold current density is 405 A/cm 2 for an as-cleaved diode laser with 873-m cavity length. The internal quantum efficiency and laser cavity loss are 93% and 6.4 cm Ϫ1 , respectively.

Magnetic-field-induced phase transitions were studied with a two-dimensional electron Al-GaAs/GaAs system. The temperature-driven flow diagram shows the features of the Γ(2) modular symmetry, which includes distorted flowlines and shiftted critical point. The deviation of the critical conductivities is attributed to a small but resolved spin splitting, which reduces the symmetry in Landau quantization. [B. P. Dolan, Phys. Rev. B 62, 10278.] Universal scaling is found under the reduction of the modular symmetry. It is also shown that the Hall conductivity could still be governed by the scaling law when the semicircle law and the scaling on the longitudinal conductivity are invalid. *corresponding author:yhchang@phys.ntu.edu.tw PACS numbers: 1 Magnetic-field-induced phase transitions in two-dimensional electron systems (2DESs) have been an active research topic since the discovery of the quantum Hall effect. [1-18] The law of corresponding states proposed by Kivelson, Lee, and Zhang (KLZ) [2], which was based on the effective field Maxwell-Chern-Simon theory, provides a powerful method for classifying quantum Hall states and the transitions between them. According to the law of corresponding states, all the magnetic-field-induced phase transitions are of an equivalent class. In the integer quantum Hall effect (IQHE), the equivalence is established by the Landau-level addition transformation [1,2]. Magnetic-field-induced phase transitions are believed to be good examples of quantum phase transitions. [1,19] Universal properties suchas the reflection symmetry [3], universality of critical conductivities [1,4], and the universal scaling with same critical exponent [5,6] are expected and in addition, it is also expected that the temperature-driven flow lines [1,7,8] are governed by the semicircle law.Because of the existence of the law of corresponding states, the phase diagram of the QHE has a symmetry equivalent to the Γ 0 (2) symmetry group, which is a subgroup of the modular group. [8][9][10][20][21][22][23][24] The universal properties mentioned above can be taken as the manifestations of the Γ 0 (2) modular symmetry. [8-10] However, this symmetry relies on the assumption that all the Landau bands are equally spaced in energy, a condition satisfied