Pham Tien Lam

Cooperative transition of electronic states of antisite As defects in Be-doped low-temperature-grown GaAs layers

Magnetic properties resulting from localized spins associated with antisite arsenic ions AsGa+ in Be-doped low-temperature-grown GaAs (LT-GaAs) layers were studied by measuring the magnetization of lift-off samples. With fast cooling, the magnetization of samples at 1.8 K becomes significantly lower than that expected from Curie-type paramagnetism in the range of the applied field to 7 T, and a transition from low magnetization to the magnetization of paramagnetism occurs upon the heating of samples to 4.5 K. With slow cooling, on the other hand, samples have a paramagnetic temperature dependence throughout the measurement-temperature range. The magnetization was found to decrease monotonically when a sample was kept at a fixed low temperature. These observations are explained by the cooperative transition of electron states of AsGa defects, which is closely related to the normal-metastable state transition of EL2 defects in semi-insulating GaAs. The results of the magnetization measurements in the presen...

Non-equilibrium critical point in Be-doped low-temperature-grown GaAs

We studied the transition process of antisite arsenic defects in Be-doped low-temperature-grown GaAs layers by measuring the magnetization. This material exhibits bistability at non-equilibrium; at a fixed temperature in a fixed magnetic field a sample relaxes towards two different states, depending on the preceding cooling process. We observed anomalously large magnetization fluctuations in macroscopic samples during the transition from bistability to monostability with gradual change of the temperature. Slowing down of the relaxation of the magnetization is observed as a sample approaches the transition into monostability. Large fluctuations observed from a two-piece sample exhibit intermittent bursts by high-pass filtering and follow a generalized Gumbel probability density distribution. These observations suggest a possibility of the occurrence of a non-equilibrium critical point in this material. Microscopic processes underlying the observed phenomena are discussed with results of first-principles ca...

First-principles study of hydrogen-enhanced phosphorus diffusion in silicon

We present a first-principles study on the interstitial-mediated diffusion process of neutral phosphorus (P) atoms in a silicon crystal with the presence of mono-atomic hydrogen (H). By relaxing initial Si structures containing a P atom and an H atom, we derived four low-energy P-H-Si defect complexes whose formation energies are significantly lower than those of P-Si defect complexes. These four defect complexes are classified into two groups. In group A, an H atom is located near a Si atom, whereas in group B, an H atom is close to a P atom. We found that the H atom pairs with P or Si atom and changes the nature bonding between P and Si atoms from out-of-phase conjugation to in-phase conjugation. This fact results in the lower formation energies compare to the cases without H atom. For the migration of defect complexes, we have found that P-H-Si defect complexes can migrate with low barrier energies if an H atom sticks to either P or Si atom. Group B complexes can migrate from one lattice site to anothe...

Mechanism of Large Thermal Fluctuations and Slow Relaxation of Elastically Interacting Point-Defect System

The magnetization of a Be-doped low-temperature-grown GaAs layer exhibits large fluctuations at low temperatures, while it slowly relaxes towards a stable value. These changes in the magnetization result from transitions of antisite As defects between substitutional and interstitial sites. We have analyzed magnetization fluctuations by repeating measurements under identical conditions on both small and large numbers of pieces of samples. The analysis shows that observed large fluctuations are made of a number of coherent fluctuations, each of which corresponds to collective transitions of defects in one region of a sample. A number of coherent fluctuations increases with the temperature by following a relation of the canonical distribution, while the relaxation time increases with the temperature. An implication of the results is discussed by focusing on the long-range elastic interactions of defects and its effect on the mixing process of a phase-space trajectory of the system. We suggest that the mixing...

Transition of electron transport process in Be-doped low-temperature-grown GaAs layer

Effects of the structure change of antisite-As (AsGa) defects on the electron transport property in Be-doped low-temperature-grown GaAs layers were investigated. In this material, AsGa atoms cooperatively change their positions between substitutional and interstitial sites. We found an abrupt change in the resistance of a sample at a temperature around 3 K, where a discontinuous decrease of the magnetization was also observed. The mechanism of the transition of the electron transport property is explained by first-principles calculations of the electron state of an AsGa atom accompanied with a shallow acceptor Be atom. At the transition, AsGa+ ions are cooperatively displaced to interstitial sites and become neutral atoms, which result in generation of holes in the valence band. The mechanism of the discontinuous change of the electron transport process in this material is discussed in connection with existing mechanisms such as those of metal-insulator transitions.

Magnetic and electronic properties of be-doped low-temperature grown GaAs layers

Magnetic and electron transport properties of Be-doped low-temperature-grown GaAs layers at low temperatures were studied. A nearly abrupt decrease of magnetization is observed at a temperature around 3.2 K where a discontinuous decrease in resistance is also observed. These observations are explained by cooperative transition of the electron states of ASGa defects. First-principal calculations of the electron state of an ASGa atom with a shallow acceptor Be atom show that at the transition, an ASGa+ion is displaced to the interstitial site and becomes a neutral atom, leading to a magnetization annihilation. In addition, the displacement results in the generation of a hole in the valence band and enhance the electrical conduction.