Chihyu Chen

Transport properties of ZnTe:N thin films

Highly mismatched alloys have been predicted to exhibit enhanced thermoelectric properties. Here we report on transport properties of one such system, nitrogen-doped ZnTe epitaxial layers on GaAs (100). Hall effect, electrical resistivity, and Seebeck coefficient measurements were performed between 5 K and 300 K for samples with a room temperature hole concentration of 0.34–2.16 × 1019 cm−3. Significant phonon-drag thermopower reaching 1.5–2.5 mV K−1 was observed. Fermi-Dirac statistics was used to analyze the transport parameters of ZnTe:N films assuming a single parabolic band. The power factor demonstrates a measurable improvement with increasing nitrogen concentration.

Intermediate band to conduction band optical absorption in ZnTe:O

ZnTe doped with high concentrations of oxygen has been proposed in previous works as intermediate band (IB) material for photovoltaic applications. The existence of extra optical transitions related to the presence of an IB has already been demonstrated in this material and it has been possible to measure the absorption coefficient of the transitions from the valence band (VB) to the IB. In this work we present the first measurement of the absorption coefficient associated to transitions from the IB to the conduction band (CB) in ZnTe:O.

Chemical epitaxy and interfacial reactivity in solution deposited PbS on ZnTe

Despite their promise, the efficiency of quantum dot solar cells is currently low (<10%), where an improved understanding of interfaces and contact layers adjacent to the quantum dots is presently a major limitation. The present work aims at optimizing the conditions for growing PbS thin films on ZnTe in light of the solubility of ZnTe in the alkaline environment required for chemical bath deposition of PbS. The morphology and properties of these films are strongly affected by altering the solution pH, temperature and the reagent concentrations. A detailed structural and chemical analysis reveals the different orientation relationships present between the constituent layers and highlights the spontaneous formation of new, distinct intermediate layers in the unique local environment formed due to the simultaneous dissolution of the ZnTe layers in the presence of precursors for PbS thin film deposition.

Heterojunction Band Offset Limitations on Open-Circuit Voltage in p -Z n T e/n -Z n S e Solar Cells

Limitations on the open-circuit voltage of p-ZnTe/n-ZnSe heterojunction solar cells are studied via current-voltage (I-V) measurements under solar concentration and at variable temperature. The open-circuit voltage reaches a maximum value of 1.95 V at 77 K and 199 suns. The open-circuit voltage shows good agreement with the calculated built-in potential of 2.00 V at 77 K. These results suggest that the open-circuit voltage is limited by heterojunction band offsets associated with the type-II heterojunction band lineup, rather than the bandgap energy of the ZnTe absorber material.

Intermediate band solar energy conversion in ZnTeO

Energy conversion in solar cells incorporating ZnTeO base layers is presented. The ZnTeO base layers incorporate intermediate electronic states located approximately 0.4eV below the conduction band edge as a result of the substitution of O in Te sites in the ZnTe lattice. Cells with ZnTeO base layers demonstrate optical response at energies lower than the ZnTe bandedge, a feature that is absent in reference cells with ZnTe base layers. Quantum efficiency is significantly improved with the incorporation of ZnSe emitter/window layers and transition from growth on GaAs substrates to GaSb substrates with a near lattice match to ZnTe.

Passivation of long-wave infrared InAs/GaSb superlattice detectors with epitaxially grown ZnTe

In past decade, T2SL detectors with promising performance have been reported by various institutions thanks to the extensive modeling efforts, improvement of T2SL material quality, and development of advanced low-dark-current architectures with unipolar barriers (Xbn, CBIRD, pBiBn, M-structure, etc). One of the most demanding challenges of present day T2SL technology is the suppression of surface leakage currents associated with the exposed mesa sidewalls, which appear during the definition of device optical area. Typical FPA pixels have large surface/volume ratio and their performance is strongly dependent on surface effects. In order to overcome the limitation imposed by surface leakage currents, a stable surface passivation layer is needed. In this paper we report on InAs/GaSb T2SL detectors operating in the LWIR spectral region (100% cut-off wavelength of ~10 μm at 77K) passivated with epitaxially grown ZnTe. In order to compensate for the high conductivity of ZnTe passivation it was doped with chlorine to 1 × 1018cm−3 concentration. Dark current measurements reveal the significant reduction of noise current after ZnTe passivation.

Epitaxial growth of ZnTe on GaSb(100) using in situ ZnCl2 surface clean

The epitaxial growth of high-quality ZnTe on GaSb substrates is demonstrated by molecular beam epitaxy without the use of a group-V beam flux or intermediate GaSb buffer layer. A reduced surface cleaning temperature is achieved using a combination of HCl etching prior to loading into the growth chamber and use of a ZnCl2 flux during the thermal clean step. This procedure results in a surface clean temperature of approximately 440 °C, in comparison to 500 °C for an as-received GaSb substrate, providing a means to achieve a clean GaSb surface for ZnTe epitaxy without the requirement for a group-V flux to stabilize the surface to prevent noncongruent sublimation of GaSb. The resulting ZnTe epitaxial layers demonstrate good surface morphology and structural properties based on Nomarski microscope images, transmission electron microscopy images of the ZnTe/GaSb interface, and x-ray diffraction measurements demonstrating a rocking curve with a full width at half maximum of 40 arc sec for the ZnTe (004) reflection.

Evolution of self-assembled Type-II ZnTe/ZnSe nanostructures: Structural and electronic properties

The strain-mediated evolution of epitaxial ZnTe/ZnSe quantum structures is studied at the atomic scale using spherical aberration-corrected scanning transmission electron microscopy, coupled with electronic properties characterized by photoluminescence spectroscopy. The growth development of these buried quantum dots clearly demonstrates a homogeneous profile with similar pyramidal geometry rather than bi-modal distribution; contradicting prior reports on ZnTe/ZnSe quantum dots. The result is consistent with atomistic theoretical calculations on strain distribution and electronic structure of a modeled quantum dot of similar geometry using a valence force field model. It is also found that the transition from 2-D islands to 3-D quantum dots involves thermally activated carrier transfer process and follows up with formation of extended defects at the quantum dot surface, acting as an effective source for remnant misfit strain relaxation. The new physical understanding concerning the growth of self-assemble...