Z. H. Ma

Temperature dependence of the responsivity of II–VI ultraviolet photodiodes

High-temperature dependence, up to 150 °C, of the photoresponsivity of ZnS, ZnSTe, and ZnSSe photodiodes was investigated in this study. It was found that, in general, the responsivity at higher temperatures will shift to longer wavelengths because of band-gap narrowing. A remarkable observation is that the near-band-edge responsivities of these diodes increase at higher temperature. We believe that this observation is attributed to the change of the density-of-state distribution due to lattice expansion at high temperatures, and a simplified model is used to illustrate this hypothesis.

Photoresponse studies of ZnSSe visible–blind ultraviolet detectors: A comparison to ZnSTe detectors

This work focuses on the investigation of the difference between the photoresponse of ZnS, ZnSSe, and that of ZnSTe Schottky-barrier photodiodes, with a particular aim to reveal the underlying causes of the gradual turn-on characteristic of low-Te-containing ZnSTe Schottky barrier photodiodes. To form the bottom electrode layer for the newly developed ZnSSe diode, n-type doping of ZnSSe by incorporating Al flux during molecular beam epitaxial growth was studied. Excellent-to-good dopant activation is achieved for Se composition up to 50%. The measured photoresponse of the diodes clearly indicates that the Te isoelectronic trapping effect is responsible for the gradual turn-on characteristic of low-Te-containing ZnSTe Schottky-barrier photodiodes. The results also reveal that the ZnSSe diode, having a much better visible rejection power, is a more suitable choice for high-performance visible–blind ultraviolet detection applications.

Aluminum-doped n-type ZnSTe alloy grown by molecular beam epitaxy

Successful n‐type doping of ZnSTe alloy using elemental aluminum source has been carried out by molecular beam epitaxy. Hall effect measurement (300–77 K) was performed on as‐grown ZnS0.977Te0.023 epilayers with various dopant concentrations. Electron carrier concentration as high as 1.3×1019 cm−3 has been achieved. For carrier concentration higher than 5×1018 cm−3, the carrier concentration is independent of temperature, possibly indicating formation of a very shallow donor level. A group of ZnS1−xTex epilayers with different x values was doped using a constant aluminum beam flux for studying the dependence of the dopant activation on Te composition. Good activation of Al dopant was obtained for x value from 0 to a few percent, but it became poor for larger x value and finally Al became inactive for x values higher than 10%. Room temperature photoluminescence measurements on doped and undoped ZnS and ZnS1−xTex layers indicate that Al dopants form deep‐level radiative centers in addition to a shallow dono...

ZnSTe-based Schottky barrier ultraviolet detectors with nanosecond response time

ZnSTe-based Schottky barrier photovoltaic detector arrays were fabricated on GaP(100) using a two-step molecular beam epitaxy growth approach. These detectors exhibit visible blind and ultraviolet (UV) sensitive response with a peak UV responsivity of 0.13 A/W and 1.2×106 V/W at 320 nm. The built-in potential of these detectors was determined to be 1.7 V. The temporal photocurrent response of a 400×400 μm2 detector was measured to be 1.2 ns, limited apparently by the resistance-capacitance (rc) constant of the detector structure.

HIGH PERFORMANCE ZNSTE PHOTOVOLTAIC VISIBLE-BLIND ULTRAVIOLET DETECTORS

ZnS1−xTex-based Schottky photodiodes have been fabricated on various substrates using the molecular beam epitaxy technique. The photovoltage output of these photovoltaic devices is determined using Fourier transform interferometric spectroscopy. The results show that these devices (with Te<10%) are highly sensitive in the ultraviolet but are visible blind. An external quantum efficiency of over 50% has been achieved on a device grown on a GaP substrate and over 40% on a Si substrate.

ATOMIC FORCE MICROSCOPY STUDIES OF ZNSE SELF-ORGANIZED DOTS FABRICATED ON ZNS/GAP

ZnSe self-organized dot structures on ZnS thin films were fabricated by the molecular beam epitaxy technique. In situ reflection high-energy electron diffraction studies reveal that growth interruption is required for the formation of the dot structure. Atomic force microscopy (AFM) images of the dots taken within the same day of growth reveal that the dot density increases with increasing ZnSe coverage. A density of 18 μm−2 was achieved with a coverage of 8.0 ZnSe monolayers. AFM images taken at later times (up to six months later) show ripening effects. The average dot size measured at various times after growth is consistent with the prediction of the Ostwald ripening model with a growth time constant of 4±1 days for the structure with a coverage of 8.0 ZnSe monolayers. The dot size and density in the fully ripened state are essentially independent of the initial ZnSe coverage.

Strong surface segregation of Sb atoms at low temperatures during Si molecular beam epitaxy

Surface segregation of Sb atoms at low temperatures below 400°C during Si molecular beam epitaxy (MBE) growth is studied by ex situ X-ray reflectivity measurements and secondary ion mass spectroscopy (SIMS). One monolayer of Sb atoms was first deposited at the temperature of 300°C, followed by a 23-nm thick Si overlayer grown at different temperatures of 250, 300, 350, and 400°C. The decay lengths of dopant Sb distribution profiles are obtained to be 0.45, 0.95, 3.5, and >20 nm by simulations of their X-ray reflectivity curves, respectively. A strong surface segregation of Sb atoms is observed at temperatures of 350 and 400°C, which is also confirmed by the SIMS profiles. Surface structure change caused by a high coverage of Sb is suggested to explain such a strong segregation.

Temperature dependence of the responsivity of ZnS-based UV detectors

This work focused on the studies of the high-temperature dependence of the responsivity of ZnS, ZnSTe and ZnSSe photodiodes. It was found that in general the responsivity at higher temperatures will shift to longer wavelengths because of band gap narrowing. A remarkable observation is that the near-band-edge responsivities of these diodes increase at higher temperature. We believe that this observation is attributed to the change of the density of state distribution due to lattice expansion at high temperature and a simplified model is used to illustrate this hypothesis.

Secondary ion mass spectroscopy study of Al diffusion in sandwich-ZnSTe structures grown by molecular beam epitaxy

Sandwiched structures that consist of ZnSTe/ZnSTe:Al/ZnSTe were fabricated by the molecular beam epitaxy technique on three GaAs substrates oriented along (100), (511), and (711), respectively, to study the thermal diffusion of Al dopant in ZnS0.986Te0.014 matrix by secondary ion mass spectroscopy depth profiling. The relative sensitivity factor of Al respect to Zn was determined to be 4.5±0.5×1019 cm−3. The Al diffusion coefficients at annealing temperature of 450 and 550 °C were found to be dependent on Al concentration. The upper and lower limits of the diffusion coefficients were obtained through a data fitting program based on Fick’s second law, the results suggest that the diffusion is anisotropic possibly due to channeling effect. The results also reveal that Al in the matrix is thermally stable at temperature as high as 300 °C and thus prove that Al is a good candidate for the n-type doping of ZnSTe alloy system in terms of thermal stability.

XPS studies of Auger parameter shift of ZnS1–xTex alloys

Abstract X-ray photoelectron spectroscopy (XPS) was used to study the photoelectron emission of a relatively unexplored semi-insulator ZnS 1– x Te x alloy system. The investigation was focused on the Auger parameter shifts of Zn and Te as a function of composition. Taking into account the alloy disordering and the semi-insulating characteristics of the ZnS 1– x Te x alloy system, a model based on the concept of relaxation is used to explain the polarization change of this alloy system due to the existence of the two core holes left in the Auger process. To complement this theoretical model, the dielectric constants of ZnS 1– x Te x alloys as a function of composition were measured using a structure similar to a parallel plate capacitor. The Auger parameter shifts calculated from this model are in good agreement with that obtained from the XPS measurement.