Sunghun Jung

Determination of band gap energy (Eg) of Cu2ZnSnSe4 thin films: On the discrepancies of reported band gap values

We demonstrate experimental data to elucidate the reason for the discrepancies of reported band gap energy (Eg) of Cu2ZnSnSe4 (CZTSe) thin films, i.e., 1.0 or 1.5 eV. Eg of the coevaporated CZTSe film synthesized at substrate temperature (Tsub) of 370 °C, which was apparently phase pure CZTSe confirmed by x-ray diffraction (XRD) and Raman spectroscopy, is found to be around 1 eV regardless of the measurement techniques. However, depth profile of the same sample reveals the formation of ZnSe at CZTSe/Mo interface. On the other hand, Eg of the coevaporated films increases with Tsub due to the ZnSe formation, from which we suggest that the existence of ZnSe, which is hardly distinguishable from CZTSe by XRD, is the possible reason for the overestimation of overall Eg.

Enhanced exciton separation through negative energy band bending at grain boundaries of Cu2ZnSnSe4 thin-films

Local surface potential of Cu2ZnSnSe4 thin-films was investigated by Kelvin probe force microscopy. The surface potential profile across grain boundaries (GBs) shows a rise of 200–600 meV at GBs in a Cu-poor and Zn-poor film with 3.8% efficiency, which means positively charged GBs. In contrast, the GBs in a Cu-poor and Zn-rich film with 2% efficiency exhibit lowering of surface potential by 40 meV. The results indicate that GBs of Cu2ZnSnSe4 films play a role for exciton separation and governing defects for high efficiency could be not only CuZn but also VCu as explained theoretical predictions.

Direct electronic transport through an ensemble of InAs self-assembled quantum dots

Electronic transport properties through an ensemble of InAs self-assembled quantum dots are reported. A metal–semiconductor–metal diode with self-assembled quantum dots has been fabricated. Clear staircases are observed in the current–voltage characteristics measured from the diode, and several peak structures are identified in the differential conductance. These conductance peaks are interpreted as due to resonant tunneling through the energy states of the self-assembled quantum dots.

Fabrication and electrical characterization of planar resonant tunneling devices incorporating InAs self-assembled quantum dots

Planar-type quantum-dot devices have been fabricated and characterized. Aluminum metal electrodes with interelectrode spacing of 30 nm have been deposited on an InAs self-assembled quantum-dot wafer to form the quantum-dot devices. The current–voltage characteristics measured from the devices, in which a single quantum dot is placed in between the electrodes, exhibit negative differential resistance effects at the temperature above 77 K. They are interpreted as due to three-dimensional–zero-dimensional resonant tunneling through the InAs self-assembled quantum dot.

Band gap determination of Cu 2 ZnSnSe 4 thin films

We demonstrate experimental data to elucidate the reason for the discrepancies of reported band gap energy (E<inf>g</inf>) of Cu<inf>2</inf>ZnSnSe<inf>4</inf> (CZTSe) thin films, i.e., 1.0 or 1.5 eV. E<inf>g</inf> of the co-evaporated CZTSe film synthesized at substrate temperature (T<inf>sub</inf>) of 370 °C, which was apparently phase pure CZTSe confirmed by X-ray diffraction (XRD) and Raman spectroscopy, is found to be around 1 eV regardless of the measurement techniques. However, depth profile of the same sample reveals the formation of ZnSe at CZTSe/Mo interface. On the other hand, E<inf>g</inf> of the co-evaporated films increases with T<inf>sub</inf> due to the ZnSe formation, from which we suggest that the existence of ZnSe, which is hardly distinguishable from CZTSe by XRD, is the possible reason for the over-estimation of overall E<inf>g</inf>.

Spectroscopic imaging study on CIGS thin film solar cells

Cu(In1−xGax)Se (CIGS) based thin film solar cells are usually built by depositing CdS as a buffer layer and ZnO as a window layer on top of the CIGS absorber layer. In order to optimize their performances, it is essential to understand the interactions between the layers. In this study, we have investigated the interactions between the layers by examining the optical properties of the CIGS solar cell structure at each step of the fabrication process―CIGS, CIGS/CdS, and CIGS/CdS/ZnO― using photoluminescence and Raman spectroscopic imaging techniques. The images of the intensity of the Raman peak at 175 cm−1 due to the A1 vibration mode show that the homogeneity improves after CdS deposition. Micro-PL intensity imaging also confirmed this observation. Furthermore, the PL peak intensity and the energy position vary after each layer deposition.

Optical Characterization of Cu2ZnSnSe4grown by thermal co‐evaporation

Raman scattering and photoluminescence on Cu 2 ZnSnSe 4 thin films grown by thermal co‐evaporation were performed. The photoluminescence spectrum shows a peak below 1.0 eV. The Raman spectra of Cu 2 ZnSnSe 4 show two main peaks at 170, 192–195 cm −1 and additional Raman mode at 260 cm −1 , which is ascribed to the Cu 2− x Se phase. The lateral distribution of the Cu 2− x Se phase in Cu 2 ZnSnSe 4 thin films is investigated by scanning micro‐Raman scattering. In Cu‐rich samples, the distribution of the Cu 2− x Se phase is found to be uneven.

Optical Characterization of Cu[sub 2]ZnSnSe[sub 4] grown by thermal co-evaporation

Raman scattering and photoluminescence on Cu 2 ZnSnSe 4 thin films grown by thermal co‐evaporation were performed. The photoluminescence spectrum shows a peak below 1.0 eV. The Raman spectra of Cu 2 ZnSnSe 4 show two main peaks at 170, 192–195 cm −1 and additional Raman mode at 260 cm −1 , which is ascribed to the Cu 2− x Se phase. The lateral distribution of the Cu 2− x Se phase in Cu 2 ZnSnSe 4 thin films is investigated by scanning micro‐Raman scattering. In Cu‐rich samples, the distribution of the Cu 2− x Se phase is found to be uneven.

Electrical Properties Of E-Beam Exposed Silicon Dioxides And Their Application To Nano-Devices

It has been well known that carbon contamination layers build up on thin films when they are exposed by high energy electron beams [l]. Such contamination layers were commonly used as etch-masks in earlier e-beam lithography and pattern transfer processes [2]. However, the electrical properties of the e-beam induced contamination layers and their application to nano-device fabrication have not been focused so far. We would like to present the electrical properties of e-beam induced carbon contamination layers on SiO,. It has been found that the aluminum can make electrical contacts to the contamination layers, and the current-voltage (I-V) characteristics of various Al/contamination layer/Al structures have been measured. Figure 1 (a) and (b) show in-situ scanning Auger electron spectroscopic data for the bare and the e-beam exposed SiO, films, respectively. The carbon layer with the thickness of 3 nm can be clearly identified in the case of the e-beam exposed S O z film. Figure 2 shows the SEM photo of a typical sample structure for the electrical measurements. The 40 pm long, 1 pm wide line patterns have been exposed between two aluminum pads. Figure 3 (a) and (b) show the current-voltage (I-V) and the differential conductance-voltage (dI/dV-V) characteristics of 3 samples with different ebeam doses. While the current measured between A1 pads on bare SiO, is negligible, the I-V from all 3 exposed samples exhibit conducting properties. Furthermore, both the I and the dI/dV systematically increase with the amount of the e-beam dose, as is summarized in Fig. 4 plotting dI/dV at 1 V as a function of the dose. In conclusion, the electrical measurement of carbon contamination layers on SiO, has been systematically performed. The conducting properties of thin carbon layer can be utilized in the one step fabrication of nano-meter sized wire structures and barrier structures. Characterization results of such e-beam induced nano-structures will also be presented at the conference.