An 8 bit current steering DAC for offset compensation purposes in sensor arrays

Abstract. An 8 bit segmented current steering DAC is presented for the compensation of mismatch of sensors with current output arranged in a large arrays. The DAC is implemented in a 1.8 V supply voltage 180 nm standard CMOS technology. Post layout simulations reveal that the design target concerning a sampling frequency of 2.6 MHz is exceeded, worst-case settling time equals 60.6 ns. The output current range is 0–10 μA, which translates into an LSB of 40 nA. Good linearity is achieved, INL < 0.5 LSB and DNL < 0.4 LSB, respectively. Static power consumption with the outputs operated at a voltage of 0.9 V is approximately 10 μW. Dynamic power, mainly consumed by switching activity of the digital circuit parts, amounts to 100 μW at 2.6 MHz operation frequency. Total area is 38.6 × 2933.0 μm2

Role of the TMG flow rate on the GaN layer properties grown by MOVPE on (hkl) GaAs substrates

International audienceThe role of the TMG flow rate on the properties of the GaN layer grown by MOVPE on (001) and (11n)/n=2,3 GaAs substrates were investigated. The surface morphology, crystalline quality and optical property were found to be strongly dependent on the TMG flow rate. As the latter decreased to 16 μmol/min, in-situ reflectance measurements showed a constant signal. This is attributed to the enhanced coalescence process, which resulted in the improvement of the surface morphology. A high TMG flow rate of 40 μmol/min sccm promoted predominantly vertical growth and resulted in the formation of a three-dimensional island. The lowest YL intensity and FWHM values of near band edge emission were obtained for GaN layers grown on (001) GaAs substrate with a TMG flow rate of 16 μmol/min, indicating an improvement of the optical properties of the GaN layer. This improvement is attributed to the coalescence process at the initial growth stage of GaN and the lateral growth process. All these behaviors were always observable whatever the used substrates. Depth resolved-CL showed that a mechanism of phase transformation in response to changing the substrate orientations

An 8 bit current steering DAC for offset compensation purposes in sensor arrays

An 8 bit segmented current steering DAC is presented for the compensation of mismatch of sensors with current output arranged in a large arrays. The DAC is implemented in a 1.8 V supply voltage 180 nm standard CMOS technology. Post layout simulations reveal that the design target concerning a sampling frequency of 2.6 MHz is exceeded, worst-case settling time equals 60.6 ns. The output current range is 0–10 μA, which translates into an LSB of 40 nA. Good linearity is achieved, INL < 0.5 LSB and DNL < 0.4 LSB, respectively. Static power consumption with the outputs operated at a voltage of 0.9 V is approximately 10 μW. Dynamic power, mainly consumed by switching activity of the digital circuit parts, amounts to 100 μW at 2.6 MHz operation frequency. Total area is 38.6 × 2933.0 μm2

Study of cubic GaN clusters in hexagonal GaN layers and their dependence with the growth temperature

International audienceThe inclusion of cubic phase in MOVPE-grown hexagonal GaN on GaAs substrate and its dependence with the growth temperature are investigated by high-resolution X-ray diffraction (HR-XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and cathodoluminescence (CL). It is observed that the GaN layers surface exhibits 3D-grains structure. The density and shape of these grains are largely dependent on the growth temperature. HR-XRD study reveals the presence of cubic GaN clusters in the hexagonal GaN layer. Using CL we show that the cubic inclusions are not localized at the substrate/epilayer interface but propagate throughout the film

Investigations of in situ reflectance of GaN layers grown by MOVPE on GaAs (001)

International audienceThe growth of low temperature GaN (LT-GaN) layers on GaAs (0 0 1) substrate was performed by metal organic vapor phase epitaxy (MOVPE) at growth temperature range of 500–800 °C. Laser reflectometry (LR) was employed for in situ monitoring of all growth steps. The simulation of experimental time reflectance traces shows that at the first growth stage, the surface roughness increases to reach a limit value depending on growth temperature. Due to surface roughness profile the growth rate time-dependence was found non negligible at the first growth stage. The investigations of in situ reflectance give more precise measurement of growth rates that yields to thermal activation energy close to 0.12 eV. The ex-situ analyses by spectral reflectance (SR) and Atomic Force Microscopy (AFM) showed that the better surface morphology was obtained when the GaN buffer layer is grown at lower temperature, while three dimensional (3D) growth mode was observed at higher temperature. A series of high temperature (800 °C) GaN (HT-GaN) layers were grown on different thicknesses of low temperature (550 °C) GaN buffer layer. The results showed that high density of nucleation sites enhances the initial growth rate and improves the morphological quality of GaN active layer

Effect of GaAs substrate orientation on the growth kinetic of GaN layer grown by MOVPE

International audienceWe have investigated the kinetic growth of low temperature GaN nucleation layers (LT-GaN) grown on GaAs substrates with different crystalline orientations. GaN nucleation layers were grown by metal organic vapor phase epitaxy (MOVPE) in a temperature range of 500-600°C on oriented (001), (113), (112) and (111) GaAs substrates. The growth was in-situ monitored by laser reflectometry (LR). Using an optical model, including time-dependent surface roughness and growth rate profiles, simulations were performed to best approach the experimental reflectivity curves. Results are discussed and correlated with ex-situ analyses, such as atomic force microscopy (AFM) and UV-visible reflectance (SR). We show that the GaN nucleation layers growth results the formation of GaN islands whose density and size vary greatly with both growth temperature and substrate orientation. Arrhenius plots of the growth rate for each substrate give values of activation energy varying from 0.20 eV for the (001) orientation to 0.35 eV for the (113) orientation. Using cathodoluminescence (CL), we also show that high temperature (800-900°C) GaN layers grown on top of the low temperature (550°C) GaN nucleation layers, grown themselves on the GaAs substrates with different orientations, exhibit cubic or hexagonal phase depending on both growth temperature and substrate orientation