Electrical characteristics of UV photodetectors based on ZnO/diamond film structure

Ultraviolet photodetectors based on ZnO/diamond film structure were fabricated. The properties of Au/ZnO contacts and effects of grain sizes on the electrical characteristics of photodetectors were discussed. Due to the bombardment with Au atoms and the annealing process, fine ohmic contacts were formed between Au electrodes and ZnO films. Dark currents and photocurrents of the photodetectors were related to sputtering time and the grain size of ZnO films. For the photodetector with a bigger grain size, a lower dark current and a higher photocurrent were obtained under 10 V bias voltage. The time-dependent photocurrent confirmed the carrier trapping effect.     

AlGaN-Based Solar-Blind Schottky Photodetectors Fabricated on AlN/Sapphire Template

We report AlGaN-based back-illuminated solar-blind Schottky-type ultraviolet photodetectors with the cutoff- wavelength from 280nm to 292nm without bias. The devices show low dark current of 2.1× 10^-6A/cm^2 at the reverse bias of 5 V. The specific detectivity D＊ is estimated to be 3.3 × 10^12cmHz^1/2 W^-1 . To guarantee the performance of the photodetectors, the optimization of AlGaN growth and annealing condition for Schottky contacts were performed. The results show that high-temperature annealing method for Ni/Pt Schottky contacts is effective for the reduction of leakage current.     

Spectral sensitivity of IR photodetectors in monolithic bipolar integrated circuits

We examine different designs for photodetectors within bipolar monolithic integrated circuits and study their spectral characteristics. We have analyzed the effect of buried n⁺-type layers in the photosensitive region of the photodiode on its spectral sensitivity. We show that the highest maximum spectral sensitivity in the IR region of the spectrum is exhibited by a tandem photodetector and an n⁺-n-p epitaxial photodiode. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 4, pp. 588–591, July–August, 2008.     

Effects of thermal annealing on the properties of GaN MSM UV photodetectors

Metal-semiconductor-metal-structured GaN ultraviolet photodetectors have been fabricated on sapphire substrates by metalorganic chemical vapor deposition. The properties of GaN photodetectors have been improved through thermal annealing. With a 3 V bias, the very low dark current is about 200 pA, the maximum responsivity of 0.19 A/W is achieved at 362 nm, and the corresponding detectivity is 1.2×10¹¹ cm Hz^1/2/W. The physical mechanism of the effects of thermal annealing also has been studied.     

Exploring spatial resolution in high-sensitivity nanogap quantum dot photodetectors

We propose a new approach to experimentally determine the spatial resolution of nanogap quantum dot (QD) photodetectors consist of solution-processed QDs. Cross talk between a pair of closely positioned QD photodetectors was measured. Devices with 200?nm spacing exhibit low crosstalk of 8.4%. A single QD photodetector also shows high sensitivity, with a lowest detectable optical intensity of 95.3 fW/μm² achieved. The results show the potential of nanogap QD photodetectors for applications in high-density imaging/sensing arrays.     

Evolution of ideas in photon detection

In this paper we consider an evolution of ideas in vacuum photodetector developments. Diverse approaches in developments of vacuum photodetectors (classical photomultipliers and hybrid phototubes) for the last half of century are covered. A particular emphasis is made on large area vacuum photodetectors developments. Some other issues concerning WLS and light guide techniques for increasing photodetectors sensitivity are highlighted as well.     

p型金属氧化物半导体场效应晶体管低剂量率辐射损伤增强效应模型研究

对一种非加固4007电路中p型金属氧化物半导体场效应晶体管(PMOSFET)在不同剂量率条件下的电离辐射损伤效应及高剂量率辐照后的退火效应进行了研究. 通过测量不同剂量率条件下PMOSFET的亚阈I-V特性曲线,得到阈值电压漂移量随累积剂量、退火时间的变化关系. 实验发现,此种型号的PMOSFET具有低剂量率辐射损伤增强效应. 通过描述H⁺在氧化层中的输运过程,解释了界面态的形成原因,初步探讨了非加固4007电路中PMOSFET低剂量率辐射损伤增强效应模型. 关键词： p型金属氧化物半导体场效应晶体管 60Co γ射线')" href="#">⁶⁰Co γ射线 电离辐射损伤 低剂量率辐射损伤增强效应     

Comparative study between different quantum infrared photodetectors

This paper presents a theoretical analysis for the dark current characteristics of different quantum infrared photodetectors. These quantum photodetectors are quantum dot infrared photodetectors (QDIP), quantum wire infrared photodetectors (QRIP), and quantum well infrared photodetectors (QWIP). Mathematical models describing these devices are introduced. The developed models accounts for the self-consistent potential distribution. These models are taking the effect of donor charges on the spatial distribution of the electric potential in the active region. The developed model is used to investigate the behavior of dark current with different values of performance parameters such as applied voltage, number of quantum wire (QR) layers, QD layers, lateral characteristic size, doping quantum wire density and temperature. It explains strong sensitivity of dark current to the density of QDs/QRs and the doping level of the active region. In order to confirm our models and their validity on the practical applications, a comparison between the results obtained by proposed models and that experimentally published are conducted and full agreement is observed. Several performance parameters are tuned to enhance the performance of these quantum photodetectors through the presented modeling. The resultant performance characteristics and comparison among them are presented in this work. From the obtained results we notice that the total dark current in the QRIPs can be significantly lower than that in the QWIPs. Moreover, main features of the QRIPs such as the large gap between the induced photocurrent and dark current open the way for overcoming the problems of quantum dot infrared photodetectors.     

The influence of char surface oxidation on thermal annealing and loss of combustion reactivity

Thermal annealing associated with heat treatment of coal chars affects gasification reactivity and levels of unburned carbon in residual ash from coal-fired furnaces. The present study addresses the effect of char surface oxidation, occurring upon exposure to oxygen, on the course of thermal annealing, and related loss of combustion reactivity. This goal is pursued by comparing the extent of thermal annealing suffered by coal char upon heat treatment in a nitrogen atmosphere with that of chars that underwent oxidation prior to or during heat treatment. Oxidation of char was accomplished by supplying single or multiple pulses of air during the heat treatment, which were sufficient to oxidize the char surface but small enough to limit carbon gasification to less than 5%. The extent of thermal annealing was characterized both in terms of the loss of combustion reactivity and of the development of structural anisotropy of char samples, investigated by HRTEM. Results of the present study confirm that heat treatment reduces oxyreactivity of char samples, the effect being more pronounced at temperatures exceeding 1200 °C. Oxidation of samples mitigates the effects of heat treatment, as demonstrated by the smaller loss of gasification reactivity and by the more limited development of structural anisotropy of oxidized samples. Correspondingly, elemental analysis of samples indicates the formation of stable surface oxides upon oxidation, that are subsequently desorbed upon heat treatment. At temperatures exceeding 1200 °C, the effect of oxidation vanishes. Results are analysed and discussed in the light of the possible hindrance of thermal annealing due to the formation of stable surface oxides and of the parallel modifications occurring to the ash constituents.     

High-efficiency metal-semiconductor-metal photodetectors on heteroepitaxially grown Ge on Si

We demonstrate extremely efficient germanium-on-silicon metal-semiconductor-metal photodetectors with responsivities (R) as high as 0.85 A/W at 1.55 microm and 2V reverse bias. Ge was directly grown on Si by using a novel heteroepitaxial growth technique, which uses multisteps of growth and hydrogen annealing to reduce surface roughness and threading dislocations that form due to the 4.2% lattice mismatch. Photodiodes on such layers exhibit reverse dark currents of 100 mA/cm2 and external quantum efficiency up to 68%. This technology is promising to realize monolithically integrated optoelectronics.     

Thermal annealing effects on the dynamic photoresponse properties of Al-doped ZnO nanowires network

In this study, UV photodetectors based on a network of aluminium-doped zinc oxide (AZO) nanowires were manufactured at a low cost; for this purpose, a fast and simple fabrication process that involved dropping nanowires dispersion solution was employed instead of the conventional e-beam lithography process that is used to manufacture single nanowire–based UV photodetectors. It was demonstrated that nanowire network–based UV photodetectors provide a much faster UV photoresponse than conventional single nanowire–based UV photodetectors. The fast UV photoresponse of the fabricated UV photodetector can be attributed to the fact that the potential barriers formed in the nanowire network junctions effectively block the flow of electrons during the process of photocurrent decay. Furthermore, the UV photoresponse under illumination by a 254 nm UV source was studied as a function of the annealing temperature of the AZO nanowires network at a bias of 5 V. The fabricated UV photodetector showed the fastest response of 2 s to UV illumination in air when the sample was annealed in air for 1 h at 300 °C.     

Analysis of the effect of an electric-field profile on the gain bandwidth product of avalanche photodetectors

We investigate the effect of the electric-field profile on the gain-bandwidth product of avalanche photodetectors with separate absorption and multiplication. We show that for a given multiplication layer thickness the electric-field profile plays an important role in determining the gain-bandwidth product. The calculation results show that an increasing triangular electric-field profile yields a larger gain-bandwidth product than most other profiles for Si/InGaAs avalanche photodetectors.     

Barrier Enhancement Effect of Postannealing in Oxygen Ambient on Ni/AIGaN Schottky Contacts

Al0.2 Ga0.8N/GaN samples are grown by metalorganic chemical vapour deposition （MOCVD） method on （0001） sapphire substrates. A 10nm-thick Ni layer is deposited on AlGaN as the transparent Schottky contact. The effect of postannealing in oxygen ambient on the electrical properties of Ni/AlGaN is studied by current-voltage- temperature （I-V-T） measurement. The annealing at a relatively low temperature of 300℃ for 90 s results in a decrease of the ideality factor from 2.03 to 1.30 and an increase of the Schottky barrier height from 0.77eV to 0.954 e V. The I-V-T analysis confirms the improvement originated from the formation of NiO, a layer with higher resistance, which could passivate the surface states of AlGaN and suppress the tunnelling current. Furthermore, the annealing also leads to an increase of the transmittance of the contacts from 57.5% to 78.2%, which would be favourable for A1GaN-based photodetectors.     

Si-based near infrared photodetectors operating at 10 Gbit/s

We report on fast p-i-n near infrared photodetectors in pure germanium on silicon. The diodes were fabricated by chemical vapor deposition at 600 °C followed by thermal annealing at 900 °C. We also verified that bypassing the thermal treatment did not have significant effects on the resulting crystal quality, allowing to considerably reducing the thermal budget hence simplify the integration with silicon. We demonstrate the operation of photodiodes with responsivities of 0.4 and 0.2 A/W at 1.3 and 1.55 μm, respectively, as well as open-eye diagrams at 10 Gbit/s.     

基于PBDT-TT-F∶PCBM体异质结红光探测器的光电特性

采用旋涂工艺与蒸镀工艺结合的方法制备了PBDT-TT-F∶PCBM体异质结红光探测器,研究了活性层的混合比例和厚度、退火温度等因素对器件光电特性的影响。实验结果表明:活性层PBDT-TT-F∶PCBM的混合质量比为1∶1.5、厚度为150 nm、退火温度为100℃、时间为15 min时制备的器件性能最佳,在波长为650nm、功率为6.6 m W/cm~2的光照下,探测器光电流密度可达到0.85 m A/cm~2,光响应度达到128 m A/W。     

Infrared photodetectors based on graphene van der Waals heterostructures

We propose and evaluate the graphene layer (GL) infrared photodetectors (GLIPs) based on the van der Waals (vdW) heterostructures with the radiation absorbing GLs. The operation of the GLIPs is associated with the electron photoexcitation from the GL valence band to the continuum states above the inter-GL barriers (either via tunneling or direct transitions to the continuum states). Using the developed device model, we calculate the photodetector characteristics as functions of the GL-vdW heterostructure parameters. We show that due to a relatively large efficiency of the electron photoexcitation and low capture efficiency of the electrons propagating over the barriers in the inter-GL layers, GLIPs should exhibit the elevated photoelectric gain and detector responsivity as well as relatively high detectivity. The possibility of high-speed operation, high conductivity, transparency of the GLIP contact layers, and the sensitivity to normally incident IR radiation provides additional potential advantages in comparison with other IR photodetectors. In particular, the proposed GLIPs can compete with unitravelling-carrier photodetectors.     

Effect of oxygen annealing on the photoresponse of PbSe thin films fabricated by the pulsed laser deposition method

In this work, we have fabricated lead selenide (PbSe) thin films by the pulsed laser deposition method on Si/SiO₂ substrates and investigated the effect of oxygen annealing (sensitization) in these films. The oxygen-sensitized films show high responsivity in the visible (VIS) and the near-infrared (NIR) region at room temperature without cooling. We also demonstrate the effective surface oxidation of PbSe thin films during the oxygen annealing process without treated with commonly used halogens that leads to a better photoresponse in these PbSe films.     

Reactive Solid State Dewetting: Interfacial Cavitation in the System Ag-Ni-O

Micron thick silver films, vapour deposited onto high purity polycrystalline nickel substrates, dewet the substrate after high temperature annealing in oxygen rich atmospheres, while the films remain stable after annealing at the same temperature in a nitrogen atmosphere. Dewetting occurs when a nickel oxide layer is formed at the silver-nickel interface as a consequence of oxygen diffusion through the silver film.The sensitivity of the dewetting process on various parameters such as the annealing: temperature, time and oxygen partial pressure has been determined.Scanning Electron Microscopy (SEM) of cross-sections reveal that the main mechanism of dewetting at short annealing time is the nucleation of cavities at the Ag-NiO interface which grow towards the free surface of the Ag film. They are formed not only at Ag grain boundaries and triple junctions but also in the core of Ag grains. Such cavities do not occur when the Ag film is deposited onto a NiO single crystal. We propose a simple model for the cavitation: a vacancy supersaturation is sustained in Ag, at the Ag-NiO interface, as a result of oxygen consumption by the oxidation reaction. In regions of fast oxidation, the vacancy supersaturation is large enough to promote the nucleation and growth of interfacial cavities. The model qualitatively accounts for all the observed trends; quantitatively, on top of the vacancy supersaturation, extra-contributions to the driving force for cavitation must be invoked.     

ZnO nanorod-based UV photodetection and the role of persistent photoconductivity

We report on the substantial persistent photoconductivity (PPC) response exhibited by the zinc oxide (ZnO) nanorod-based ultraviolet (UV) photodetection system. An increase in photocurrent and, hence, rise in PPC was observed for larger UV exposure times at regular intervals. Triggered by quantum efficiency, the increment in sustained conduction band electrons is proposed as the main reason behind the increased photocurrent response. In contrast, the trap centers located below the conduction band are expected to slow down the recombination rate, which accounts for the rise in PPC. The lowering of PPC upon annealing suggests the surface dependent nature of the PPC. The growth and decay mechanism of PPC has a direct relevance while assessing figure of merit of prototype nanostructure-based optical sensor and UV photodetectors.     

XPS study of the surface chemistry of Ag-covered L-CVD SnO2 thin films

In this paper, we present the results of X-ray photoelectron spectroscopy characterization of SnO₂ thin films prepared by laser chemical vapour deposition (L-CVD) and subsequently covered by Ag atoms just after deposition and after long-term exposed to dry air, subsequent annealing in ultra high vacuum at 400 °C and dry air oxidation at 400 °C. Using the standard analytical procedure based on atomic sensitivity factors, the variation of surface chemistry defined in terms of the relative concentration of the main components of the films after the above-mentioned procedures has been determined. It was confirmed that after dry air exposure as well as dry air oxidation, the layers undergo an oxidation reaching almost SnO₂ stoichiometry. Besides, during ultra high vacuum annealing, the films undergo reduction to almost SnO stoichiometry. At the same time, Ag atoms deposited at the top of layers diffuse into the subsurface layers. This was confirmed by X-ray photoelectron spectroscopy depth profiling analysis.