Tian-Yu Zhou

Frequency Up-Conversion Photon-Type Terahertz Imager

Terahertz imaging has many important potential applications. Due to the failure of Si readout integrated circuits (ROICs) and the thermal mismatch between the photo-detector arrays and the ROICs at temperatures below 40 K, there are big technical challenges to construct terahertz photo-type focal plane arrays. In this work, we report pixel-less photo-type terahertz imagers based on the frequency up-conversion technique. The devices are composed of terahertz quantum-well photo-detectors (QWPs) and near-infrared (NIR) light emitting diodes (LEDs) which are grown in sequence on the same substrates using molecular beam epitaxy. In such an integrated QWP-LED device, photocurrent in the QWP drives the LED to emit NIR light. By optimizing the structural parameters of the QWP-LED, the QWP part and the LED part both work well. The maximum values of the internal and external energy up-conversion efficiencies are around 20% and 0.5%. A laser spot of a homemade terahertz quantum cascade laser is imaged by the QWP-LED together with a commercial Si camera. The pixel-less imaging results show that the image blurring induced by the transverse spreading of photocurrent is negligible. The demonstrated pixel-less imaging opens a new way to realize high performance terahertz imaging devices.

Homogeneous spectral spanning of terahertz semiconductor lasers with radio frequency modulation

Homogeneous broadband and electrically pumped semiconductor radiation sources emitting in the terahertz regime are highly desirable for various applications, including spectroscopy, chemical sensing, and gas identification. In the frequency range between 1 and 5 THz, unipolar quantum cascade lasers employing electron inter-subband transitions in multiple-quantum-well structures are the most powerful semiconductor light sources. However, these devices are normally characterized by either a narrow emission spectrum due to the narrow gain bandwidth of the inter-subband optical transitions or an inhomogeneous broad terahertz spectrum from lasers with heterogeneous stacks of active regions. Here, we report the demonstration of homogeneous spectral spanning of long-cavity terahertz semiconductor quantum cascade lasers based on a bound-to-continuum and resonant phonon design under radio frequency modulation. At a single drive current, the terahertz spectrum under radio frequency modulation continuously spans 330 GHz (~8% of the central frequency), which is the record for single plasmon waveguide terahertz lasers with a bound-to-continuum design. The homogeneous broadband terahertz sources can be used for spectroscopic applications, i.e., GaAs etalon transmission measurement and ammonia gas identification.

Terahertz Quantum Well Photodetectors With Metal-Grating Couplers

In this paper, terahertz (THz) quantum well photodetectors (QWPs) with metal-grating couplers are presented for normal-incident operation. Both one-dimensional and two-dimensional grating samples were fabricated. The photocurrent spectra and the peak responsivity were characterized. The effects of the grating period, fill factor, and shape of the grating patch were investigated. It is found that metal gratings are simple but efficient couplers for THz QWPs, and compared with the 45°-incident facet sample, the peak responsivity of the THz QWP with the optimum grating coupler is improved by 146%.

Effects of annealing temperature on the electrical property and microstructure of aluminum contact on n-type 3C–SiC

The 3C—SiC thin films used herein are grown on Si substrates by chemical vapor deposition. Al contacts with different thickness values are deposited on the 3C—SiC/Si (100) structure by the magnetron sputtering method and are annealed at different temperatures. We focus on the effects of the annealing temperature on the ohmic contact properties and microstructure of Al/3C—SiC structure. The electrical properties of Al contacts to n-type 3C—SiC are characterized by the transmission line method. The crystal structures and chemical phases of Al contacts are examined by X-ray diffraction, Raman spectra, and transmission electron microscopy, respectively. It is found that the Al contacts exhibit ohmic contact behaviors when the annealing temperature is below 550°C, and they become Schottky contacts when the annealing temperature is above 650°C. A minimum specific contact resistance of 1.8 × 10−4 Ωcm2 is obtained when the Al contact is annealed at 250°C.

Direct Observation of Nanoscale Native Oxide on 6H-SiC Surface and Its Effect on the Surface Band Bending

The RCA-cleaned 6H-SiC surface has a 1 nm native oxide layer, which was directly observed by high-resolution transmission electron microscopy and confirmed by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The surface band bending caused by the native oxide layer was studied by synchrotron radiation photoelectron spectroscopy. The binding energy of Si 2p core level for the Ni/oxygen-free SiC interface showed almost zero shift (<0.07 eV). However, it red-shifted about 0.34 eV for the Ni/native-oxide/SiC interface, it indicated that negative charged interface states induced in the Ni/native-oxide interface resulted in the upward bending of the interface energy band.

Effect of annealing temperature on the contact properties of Ni/V/4H-SiC structure

A sandwich structure of Ni/V/4H-SiC was prepared and annealed at different temperatures from 650 °C to 1050 °C. The electrical properties and microstructures were characterized by transmission line method, X-ray diffraction, Raman spectroscopy and transmission electron microscopy. A low specific contact resistance of 3.3 × 10-5 Ω·cm2 was obtained when the Ni/V contact was annealed at 1050 °C for 2 min. It was found that the silicide changed from Ni3Si to Ni2Si with increasing annealing temperature, while the vanadium compounds appeared at 950 °C and their concentration increased at higher annealing temperature. A schematic diagram was proposed to explain the ohmic contact mechanism of Ni/V/4H-SiC structure.

Terahertz imaging using photomixers based on quantum well photodetectors

Due to the fast intersubband transitions, the terahertz (THz) quantum well photodetector (QWP) is supposed to work fast. Recently it has been demonstrated that the THz QWP can detect the THz light modulated at 6.2 GHz and therefore it can be used as a photomixer [H. Li et al., Sci. Rep. 7, 3452 (2017)]. In this work, the authors report a novel active THz imaging using THz QWP photomixers. The THz radiation source used for this imaging application is a multi-mode THz quantum cascade laser (QCL) operating in continuous wave mode. When the fast THz QWP is illuminated by the multi-mode THz radiation, the intermediate frequency signal that is resulted from the frequency beating between the neighbouring THz modes of the QCL can be extracted from the QWP mesa for imaging applications. Employing the technique, the frequency can be down-converted from the THz range to the microwave regime. And therefore, the signal can then be amplified, filtered, and detected using the mature microwave technology.