Bao-Ping Zhang

Fabrication and characterization of InGaN p-i-n homojunction solar cell

In x Ga 1 -x N p-i-n homojunction solar cells with different In content are studied. The measured open circuit voltages ( V oc ) are 2.24, 1.34, and 0.96 V, for x = 0.02 , 0.12, and 0.15, respectively. By comparing the x-ray rocking curves, the I-V characteristics and the external quantum efficiencies, it’s demonstrated that the deterioration of InGaN crystal quality for larger In contents causes the decrease of V oc . The result demonstrates that reduction of defect is a key factor in the fabrication of nitride solar cell.

Strain relaxation and its effect in exciton resonance energies of epitaxial ZnO layers grown on 6H-SiC substrates

Thickness-dependent strain relaxation and its role on exciton resonance energies of epitaxial ZnO layers grown on 6H-SiC substrates have been studied. The magnitudes of strain were determined experimentally by x-ray diffraction measurements. The strain ratios under biaxial stresses (Δc/c0)/(Δa/a0) of epitaxial ZnO layers grown on SiC and Al2O3 substrates were estimated to be 0.38 and 0.50, respectively. The strain-induced band shift δEA/δezz for ZnO/SiC and ZnO/Al2O3 heterostructures was analyzed by photoluminescence with the values of 13.1 and 14.6 eV, respectively. These comparative strain-induced band shifts, as well as Poisson ratios, evidenced the role of lattice deformation kinetics induced by different lattice mismatches in the ZnO/SiC and ZnO/Al2O3 heterostructures.

Efficient hole transport in asymmetric coupled InGaN multiple quantum wells

InGaN based light emitting devices (LEDs) with asymmetric coupled quantum wells (AS-QWs) and conventional symmetric coupled quantum wells (CS-QWs) active structures were grown by metal-organic chemical vapor deposition technique. The LEDs with AS-QWs active region show improved light emission intensity and reduced forward voltage compared with LEDs with CS-QWs active region. Based on the electroluminescence measurements and the devices structure analysis, it can be concluded that these improvements are mainly attributed to the efficient hole tunneling through barriers and consequently the uniform distribution of carriers in the AS-QWs

Temperature-dependent exciton dynamics in a ZnO thin film

We measured the temperature-dependent photoluminescence (PL) decay time of the mixed system of free exciton (FX) and donor-bound exciton (DX0) in a ZnO thin film. From the measured cw PL spectra, we also calibrated the individual spectral widths and integrated PL intensities of FX and DX0 by fitting the spectra with four Lorentzian functions. The radiative lifetime of the mixed system was then calibrated based on the thermal quenching rate of the total integrated PL intensity of the system. With the system radiative lifetime data, FX radiative lifetimes were estimated by using a theoretical relation between the lifetime and the spectral width. From the results of FX radiative lifetime, we calibrated the DX0 radiative lifetimes by considering the emission intensity ratio between FX and DX0. The results support our model that the DX0 radiative behavior is similar to that of FX when the thermal energy is smaller than the donor binding energy.

Photoluminescence and built-in electric field in ZnO/Mg0.1Zn0.9O quantum wells

Photoluminescence study of ZnO∕Mg0.1Zn0.9O quantum wells with graded well width (Lw) was carried out at 4.2K. The emission evolution from quantum confinement regime to quantum-confined Stark regime was observed clearly. For large Lw, the emission splits into two peaks which are attributed to the emissions of ZnO band edge and separately localized carriers, respectively. The internal electric field in the well layer was estimated to be ∼0.3MV∕cm, being similar to previous reports. The results are useful in designing ZnO QW based optoelectronic devices.

Room temperature continuous wave lasing of electrically injected GaN-based vertical cavity surface emitting lasers

Continuous wave (CW) lasing of electrically injected GaN-based vertical cavity surface emitting lasers (VCSELs) was achieved at room temperature. First, a high quality factor (Q) VCSEL-structured device with very narrow linewidth of 0.12 nm, corresponding to a Q-value of 3570 was obtained through two-step substrate transfer technique. However, poor heat dissipation ability prevented the device from lasing. Based on the high-Q resonant cavity design, we further fabricated vertical-structured VCSELs through metal bonding technique on Si substrate. CW lasing from vertical-structured VCSELs was observed with threshold current of density of 1.2 kA/cm2 and lasing linewidth of about 0.20 nm.

High-Quality ZnO Layers Grown on 6H-SiC Substrates by Metalorganic Chemical Vapor Deposition

In this letter, the 6H-SiC substrate has been studied for the growth of the ZnO layer. The X-ray diffraction measurement clearly showed the c-axis oriented growth of ZnO layers on SiC(0001) substrates. The X-ray rocking curve measurement diffracted a smaller full-width at half maximum of the ZnO layers grown on SiC than that of the same layers grown on the conventional Al2O3 substrate with metalorganic chemical vapor deposition (MOCVD). A distinct free-exciton emission was dominantly observed even at room temperature (RT) while the donor-bound-exciton peaks were disappeared at around ~120 K. In addition, no deep-level emission was observed even at RT in the ZnO/SiC samples. These optical and crystalline properties have hardly been observed in the ZnO/Al2O3 samples grown by MOCVD. Therefore, the higher quality of the ZnO layers grown on SiC might be attributed to the smaller lattice mismatch of ~5% as well as the +c surface orientation in ZnO/SiC sample geometry.

Stimulated Raman scattering from individual single-wall carbon nanotubes

Individual single-wall carbon nanotubes(SWNTs) exhibited continuous-wave stimulated Raman scattering (SRS). The Raman gain is a few orders higher, and the threshold power is a few orders lower, than values ever reported for other bulk materials and is explained as the result of both the large nonlinear property and efficient electron-phonon interaction in the SWNT. The laser-induced variation of the peak position of the SRS line was likely to depend on the linewidth or tube quality. The results demonstrate the high potential of SWNTs in applications of nanoscale nonlinear optical devices such as wide-range tuneable Raman lasers.

Demonstration and Study of Photovoltaic Performances of InGaN p-i-n Homojunction Solar Cells

We reported on the InGaN p-i-n homojunction solar cells with In contents of 0.02 and 0.12. Under the same illumination of Xe lamp, In_0.02 Ga_0.98N cell exhibited a fill factor (FF) of 67.8%, a higher open-circuit voltage (V_oc) of 2.15 V while In_0.12Ga_0.88N cell showed a FF of 64.8% and a lower V_oc of 1.35 V. The measurements of atomic force microscopy (AFM) images and leakage currents revealed that V-shaped defects, which are known to cause the increase in reverse current, were the main factor dominating the V_oc of In_0.12Ga_0.88N cells. Short-circuit current versus open-circuit voltage (J_sc - V_oc) curves were consistent with the characteristics expected from the current-voltage equation. Relative external quantum efficiencies (EQEs) were also measured and showed strong dependence on V-shaped defects. These results indicate that defects in bulk InGaN has a negative impact on the photovoltaic performances of solar cells based on III-nitrides.

Substantial photo-response of InGaN p-i-n homojunction solar cells

InGaN p-i-n homojunction structures were grown by metal-organic chemical vapor deposition, and solar cells with different p-contact schemes were fabricated. X-ray diffraction measurements demonstrated that the epitaxial layers have a high crystalline quality. Solar cells with semitransparent p-contact exhibited a fill factor (FF) of 69.4%, an open-circuit voltage (V-oc) of 2.24 V and an external quantum efficiency (EQE) of 41.0%. On the other hand, devices with grid p-contact showed the corresponding values of 57.6%, 2.36 V, 47.9% and a higher power density. These results indicate that significant photo-responses can be achieved in InGaN p-i-n solar cells.