Shihab Al-Daffaie

1-D and 2-D Nanocontacts for Reliable and Efficient Terahertz Photomixers

New types of continuous-wave (CW) terahertz (THz) photomixers were fabricated using 1-D and 2-D nanocontacts on low-temperature-grown (LTG) GaAs as well as nitrogen-ion-implanted (N+i) GaAs. The 1-D and 2-D nanocontacts were formed by silver nanowires and graphene sheets, respectively. A silver nanowire (Ag-NW) with a physical diameter of \varnothing 60 nm and \varnothing 120 nm can handle currents of and , respectively, without electromigration, thus enabling reliable high photocurrents. The small diameter of such a nanowire electrode in 1-D photomixers reduces the device capacitance by a factor of more than ten compared with photomixers with conventional interdigital fingers. Thus, these nanowire properties can be used for the fabrication of photomixers with cut-off frequencies and with extremely reliable operation. A multilayer graphene (MLG) of 6-8 layers shows unique properties with high short-circuit current capabilities of and reliable high photocurrents. Furthermore, the MLG absorbs only 2.3% of white light and is ideal as a transparent nanocontact. The use of these graphene electrodes in 2-D photomixers allows an efficient optical power illumination for high photocurrent generation and shows the potential for reliable CW THz emission. Initial THz measurements confirmed the high possibilities of these new nanocontact photomixers.

Efficiency enhancement of InGaN/GaN light-emitting diodes with pin-doped GaN quantum barrier

Blue InGaN/GaN light-emitting diodes with undoped, heavily Si-doped, Si delta-doped, heavily Mg-doped, Mg delta-doped, and Mg–Si pin-doped GaN barrier are investigated numerically. The simulation results demonstrate that the Mg–Si pin-doping in the GaN barrier effectively reduces the polarization-induced electric field between the InGaN well and the GaN barrier in the multiple quantum well, suppresses the quantum-confined Stark effect, and enhances the hole injection and electron confinement in the active region. For this light-emitting diode (LED) device structure, we found that the turn-on voltage is 2.8 V, peak light emission is at 415.3 nm, and internal quantum efficiency is 85.9% at 100 A cm−2. It is established that the LED device with Mg–Si pin-doping in the GaN barrier has significantly improved efficiency and optical output power performance, and lower efficiency droop up to 400 A cm−2 compared with LED device structures with undoped or Si(Mg)-doped GaN barrier.

Photocathodes based on graphene nanoplatelet emitters on semi-insulating GaAs photoswitch

A simple photocathode based on graphene nanoplatelets glued on semi-insulating (s.i.) GaAs was fabricated and used for field electron emission in a diode configuration. The graphene nanoplatelets act as field emitter array with low turn-on field. The photomodulation was achieved with a GaAs photoswitch in series to the bottom of the graphene nanoplatelet emitters. The position and power of the laser illumination (800 nm) was not critical for photomodulation.

CNT photocathodes based on GaAs high-frequency photoswitches

A high-frequency photocathode based on carbon nanotube (CNT) blocks on semiisolating GaAs or low-temperature grown GaAs was fabricated and used for electron emission in a diode configuration. The CNT blocks can achieve high currents and current densities of I=0.5 mA and J=28 mA/cm2 at E=2.1 V/μm, respectively. The photomodulation was achieved with a GaAs photoswitch electrically connected to the bottom of the CNT block emitter. This photocathode can find applications in high frequency tubes and optically driven X-ray sources.

Resonant tunneling transport in Zn x Be 1-x Se/ZnSe/Zn y Be 1-y Se asymmetric quantum structures

II-VI compounds are promising materials for the fabrication of room-temperature terahertz devices due to their beneficial properties like as type-I conduction band alignment, high breakdown field strength (~331 kV/cm for ZnSe vs. ~80 kV/cm for GaAs), and higher values of the conduction band offset (1.5 eV for BeSe/ZnSe vs. 0.7 eV for AlAs/GaAs). In this paper we report on numerical study of the resonant tunneling transport in ZnBeSe/ZnSe/ZnBeSe symmetric and asymmetric resonant tunneling diodes (RTDs). The negative differential resistance feature is observed in the current-voltage characteristics of the ZnSe-based RTDs. It is found that the maximum of peak-to-valley ratio (PVR) of the current density is equal to 6.025 and 7.144 at 150 K, and to 1.120 and 1.105 at 300 K for the symmetric and asymmetric RTDs, respectively. The effect of barrier heights on the frequency and output power performance of RTD devices are studied and discussed.

Ultrafast photocathodes based on n-doped graphene emitters on compound semiconductor photoswitches

A photocathode based on n-doped graphene emitters in series to a GaAs photoswitch was fabricated and used for pulsed field electron emission. The n-doped graphene array emits at low turn-on fields (<; 3 V/μm). A fast photomodulation of the emission current with high on/off ratio ~200 was achieved with a semi-insulating (s.i.) GaAs photoswitch in a diode configuration. Different laser sources were used for these investigations.