Martin Kocan

Ultrafast metal-semiconductor-metal photodetectors on low-temperature-grown GaN

We have fabricated and characterized ultrafast metal-semiconductor-metal photodetectors based on low-temperature-grown (LT) GaN. The photodetector devices exhibit up to 200kV∕cm electric breakdown fields and subpicosecond carrier lifetime. We recorded as short as 1.4-ps-wide electrical transients using 360-nm-wavelength and 100-fs-duration laser pulses, that is corresponding to the carrier lifetime of 720fs in our LT GaN material.

Ion versus pH sensitivity of ungated AlGaN/GaN heterostructure-based devices

We have investigated the pH and ion sensitivity of AlGaN/GaN heterostructure devices; these devices are sensitive to the ion concentration rather than to the pH of the solution. Sheet resistance as a function of pH for calibrated pH solutions and dilute NaOH, HCl, KOH, and NaCl showed an increase as a function of ionic concentration, regardless of whether the pH was acidic, basic, or neutral. An increase in resistance corresponds to accumulation of negative ions at the AlGaN surface, indicating device selectivity toward the negative ions. We attribute this to the formation of a double layer at the liquid/semiconductor interface.

Growth and properties of GaN and AlN layers on silver substrates

We report on the preparation and properties of GaN and AlN layers grown by molecular-beam epitaxy on silver metal substrates. X-ray diffraction rocking curves show polycrystalline character of GaN with high preferential GaN(11-22) orientation. An intermetallic phase of Ga3Ag is found at the GaN∕Ag interface. On the other hand, AlN layers exhibit a monocrystalline structure with a growth direction of (0002). Schottky diodes prepared on GaN layers show good rectifying behavior and relatively low leakage current (∼10−3A∕cm2). These results indicate that the III-nitride growth on metallic substrates might be used for low-cost and large-area electronic and photonic devices.

Monolithic Integration of Ultrafast Photodetector and MESFET in the GaN Material System

We have fabricated and characterized ultrafast metal-semiconductor-metal (MSM) photodetectors integrated with metal-semiconductor-field-effect-transistors (MESFETs) integrated in coplanar strip lines in the GaN/AlN/SiC material system. We recorded electrical transients of the single photodetector as short as 0.9 ps wide by optoelectric pump-probe measurements using 360-nm-wavelength and 100-fs-duration laser pulses. Electric photoresponse transients of the photodetector with 6-mV peak amplitude were amplified by the MESFET, resulting in 4-ps-wide and 35-mV peak amplitude signals. This monolithically integrated optoelectronic circuit is presented as a potential candidate for high-speed ultraviolet optoelectronics.

Femtosecond and highly sensitive GaAs metal–semiconductor–metal photodetectors grown on aluminum mirrors/pseudo-substrates,

In this study, ultrafast GaAs metal–semiconductor–metal (MSM) photodetectors grown on aluminum mirrors/pseudo-substrates were fabricated and tested. Surface characterization measurements revealed the good quality of the surface morphology, while x-ray diffraction measurements showed several crystallographic orientations of the GaAs layer. The material exhibited a 50 fs carrier lifetime due to growth-induced defects. The response of the photodetectors showed a full width at half maximum of 300 fs. These results demonstrate that the growth of GaAs layers on lattice-mismatched metallic substrates with high thermal conductivity is a promising approach for low-cost and large-area fabrication of electronic and ultrafast photonic devices that require a highly effective thermal drain.

Experimental and numerical investigation of the electrical characteristics of vertical n-p junction diodes created by Si implantation into p-GaN

Vertical n-p junction diodes were fabricated by Si+ ion implantation into Mg doped p/p+ GaN, followed by rapid thermal annealing at 1260degC in NH3/N2 for 30 s. Implantations were performed at 40, 60 and 80 keV and circular contacts on the n-region were fabricated with various diameters between 100 and 600 mum. Light emission from the periphery of the contact under forward bias conditions confirmed the existence of an n-p junction. Current-voltage characteristics revealed rectifying behaviour associated with n-p junction. 2-Dimensional Sentaurus Devicetrade was used to simulate the device. Current density distribution indicates high current near the peripheral region and it appears that the performance of the device is not determined by the implantation conditions. Further optimisation of the vertical structure is required to improve the device performance.

Mn incorporation in GaN thin layers grown by molecular-beam epitaxy

GaMnN layers have been grown by MBE on Si(111) in a wide range of growth conditions. For different substrate temperatures the structural and composition dependence of the grown layers has been studied as a function of the Mn supply. We find that at regular substrate temperature the incorporation of Mn into the layer is low and difficult to control. Only at a reduced substrate temperature of 650 °C and under N-rich growth conditions it is possible to grow homogeneous GaMnN layers if the Mn supply is below 15% of the total metal flux. The incorporation efficiency in this range is about 30%, which corresponds to a maximum Mn content in the diluted layers of about 5%. Above a critical Mn supply, GaMn3N precipitates are formed, which often extend out of the surface with a typical pyramidal structure.