Characterization of Structural Defects in (Cd,Zn)Te Crystals Grown by the Travelling Heater Method

Structural defects and compositional uniformity remain the major problems affecting the performance of (Cd, Zn)Te (CZT) based detector devices. Understanding the mechanism of growth and defect formation is therefore fundamental to improving the crystal quality. In this frame, space experiments for the growth of CZT by the Travelling Heater Method (THM) under microgravity are scheduled. A detailed ground-based program was performed to determine experimental parameters and three CZT crystals were grown by the THM. The structural defects, compositional homogeneity and resistivity of these ground-based crystals were investigated. A ZnTe content variation was observed at the growth interface and a high degree of stress associated with extensive dislocation networks was induced, which propagated into the grown crystal region according to the birefringence and X-ray White Beam Topography (XWBT) results. By adjusting the growth parameters, the ZnTe variations and the resulting stress were efficiently reduced. In addition, it was revealed that large inclusions and grain boundaries can generate a high degree of stress, leading to the formation of dislocation slip bands and subgrain boundaries. The dominant defects, including grain boundaries, dislocation networks and cracks in the interior of crystals, led to the resistivity variation in the crystals. The bulk resistivity of the as-grown crystals ranged from 109 Ωcm to 1010 Ωcm.

Thermal Analysis of AlGaN/GaN HEMTs Considering Anisotropic And Inhomogeneous Thermal Conductivity

To examine the differences of thermal characteristics introduced by material thermal conductivity, anisotropic polycrystalline diamond (PCD) and GaN are analyzed based on the accurate model of grain sizes in the directions of parallel and vertical to the interface and an approximate solution of the phonon Boltzmann transport equation. Due to the space-variant grain structures of PCD, the inhomogeneous-anisotropic local thermal conductivity, homogeneous-anisotropic thermal conductivity averaged over the whole layer and the typical values of inhomogeneous-isotropic thermal conductivity are compared with/without anisotropic GaN thermal conductivity. The results show that the considerations of inhomogeneous-anisotropic PCD thermal conductivity and anisotropic GaN thermal conductivity are necessary for the accurate prediction of temperature rise in the GaN HEMT devices, and when ignoring both, the maximum temperature rise is undervalued by over 16 K for thermal boundary resistance (TBR) of 6.5 to 60 m2K/GW at power dissipation of 10 W/mm. Then the dependences of channel temperature on several parameters are discussed and the relations of thermal resistance with power dissipation are extracted at different base temperature. Compared with GaN, SiC and Si substrates, PCD is the most effective heat spreading layer though limited by the grain size at initial growth interface.

Three Dimensional Effect of Axial Magnetic Field to Suppress Convection in the Solvent of Ge0.98Si0.02 Grown By the Traveling Solvent Method

A three dimensional numerical simulation of the effect of an axial magnetic field on the fluid flow, heat and mass transfer within the solvent of GE0.98Si0.02 grown by the travelling solvent method is presented. The full steady state Navier-Stokes equations, as well as the energy, continuity and the mass transport equations, were solved numerically using the finite element technique. It is found that a strong convective flow exists in the solvent, which is known to be undesirable to achieve a uniform crystal. An external axial magnetic field is applied to suppress this convection. By increasing the magnetic induction, it is observed that the intensity of the flow at the centre of the crucible reduces at a faster rate than near the wall. This phenomenon creates a stable and flat growth interface and the silicon distribution in the horizontal plane becomes relatively homocentric. The maximum velocity is found to obey a power law with respect to the Hartmann number Umax Ha⁻⁷/⁴

Three Dimensional Effect of Axial Magnetic Field to Suppress Convection in the Solvent of Ge0.98Si0.02 Grown By the Traveling Solvent Method

A three dimensional numerical simulation of the effect of an axial magnetic field on the fluid flow, heat and mass transfer within the solvent of GE0.98Si0.02 grown by the travelling solvent method is presented. The full steady state Navier-Stokes equations, as well as the energy, continuity and the mass transport equations, were solved numerically using the finite element technique. It is found that a strong convective flow exists in the solvent, which is known to be undesirable to achieve a uniform crystal. An external axial magnetic field is applied to suppress this convection. By increasing the magnetic induction, it is observed that the intensity of the flow at the centre of the crucible reduces at a faster rate than near the wall. This phenomenon creates a stable and flat growth interface and the silicon distribution in the horizontal plane becomes relatively homocentric. The maximum velocity is found to obey a power law with respect to the Hartmann number Umax Ha⁻⁷/⁴

Three-dimensional numerical simulation of crystal growth using TSM under g-jitter conditions

The goal of this thesis is to study the effect of residual gravity on crystal growth of Silicon Germanium GE0.98 Si0.02 using the Traveling Heater Method (THM). This method has proven to be one of the most efficient techniques to grow high-quality crystals because it can be grown at relatively low temperatures compared to existing crystal growth techniques. Yet, because of natural convection due to earth's gravity, imperfection in terms of silicon distribution along the growth interface occurs. By growing crystals in a space environment, residual gravity represented by a static microgravity component and a sinusoidal component would decrease the intensity of the convective flow, which in return would lead to a more uniform silicon distribution. However, g-jitter fluctuation has proven to have a noticeable effect on the silicon distribution. Therefore, as an initial step to understand the behavior of crystal growth in space, each component of the g-jitter force will be studied thoroughly. The momentum, mass and energy equations, representing the 3D TSM model, were solved using finite element means. The preliminary results indicate that the complexity and the intensity of the silicon distribution along the growth interface are proportional to the convective flow, that partially controls the migration of silicon. Therefore, the quality of the crystal growth is assessed based on the behavior of the flow along the solvent regime. Based on the imposed static gravity in the range of 10-6 go to 10-3 go, the flow was determined to be in a diffusion mode with a velocity ranging from 10-6 cm/sec to 10-3 cm/sec. As a matter of fact, the flow intensity was noted to be positively proportional to the dominant component of both the static and the amplitude of the imposed g-jitter and negatively proportional to the frequency of the sinusoidal g-jitter. Consequently, realistic space growth conditions have proven to be an effective way of producing a homogeneous crystal since a flawless crystal silicon distribution is obtained at the growth interface.

3-D modeling and simulation of crystal growth of GE₀.₉₈ Si₀.₀₂ under the influence of various gravity levels, G-jitter and rotating magnetic field using traveling solvent method

A three-dimensional numerical simulation was conducted to study the effect of a rotating magnetic (RMF) field on the fluid flow, heat transfer and mass transfer in the presence of various gravity levels by utilizing the traveling solvent method (TSM). The presence of the RMF suppressed the buoyancy convection in the GE₀.₉₈ Si₀.₀₂ solution zone in order to get homogeneity with a flat growth interface. It was found that the intensity of the flow at the centre of the crucible decreased at a faster rate compared to the flow near the walls when increasing magnetic field intensity is combined with a certain rotational speed. This behavior created a stable and uniform silicon distribution in the horizontal plane near the growth interface in the terrestrial condition. Different magnetic field intensities for different rotational speeds were examined in both terrestrial and micro-gravity conditions. The effects of residual acceleration, known as G-jitter, on board the International Space Station and European Space Orbiter were also investigated.

Three-dimensional numerical simulation of crystal growth using TSM under g-jitter conditions

The goal of this thesis is to study the effect of residual gravity on crystal growth of Silicon Germanium GE0.98 Si0.02 using the Traveling Heater Method (THM). This method has proven to be one of the most efficient techniques to grow high-quality crystals because it can be grown at relatively low temperatures compared to existing crystal growth techniques. Yet, because of natural convection due to earth's gravity, imperfection in terms of silicon distribution along the growth interface occurs. By growing crystals in a space environment, residual gravity represented by a static microgravity component and a sinusoidal component would decrease the intensity of the convective flow, which in return would lead to a more uniform silicon distribution. However, g-jitter fluctuation has proven to have a noticeable effect on the silicon distribution. Therefore, as an initial step to understand the behavior of crystal growth in space, each component of the g-jitter force will be studied thoroughly. The momentum, mass and energy equations, representing the 3D TSM model, were solved using finite element means. The preliminary results indicate that the complexity and the intensity of the silicon distribution along the growth interface are proportional to the convective flow, that partially controls the migration of silicon. Therefore, the quality of the crystal growth is assessed based on the behavior of the flow along the solvent regime. Based on the imposed static gravity in the range of 10-6 go to 10-3 go, the flow was determined to be in a diffusion mode with a velocity ranging from 10-6 cm/sec to 10-3 cm/sec. As a matter of fact, the flow intensity was noted to be positively proportional to the dominant component of both the static and the amplitude of the imposed g-jitter and negatively proportional to the frequency of the sinusoidal g-jitter. Consequently, realistic space growth conditions have proven to be an effective way of producing a homogeneous crystal since a flawless crystal silicon distribution is obtained at the growth interface.