The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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A comprehensive review of zno materials and devices

Wurtzitic ZnO is a wide-bandgap (3.437 eV at 2 K) semiconductor which has many applications, such as piezoelectric transducers, varistors, phosphors, and transparent conducting films. Most of these applications require only polycrystalline material; however, recent successes in producing large-area single crystals have opened up the possibility of producing blue and UV light emitters, and high-temperature, high-power transistors. The main advantages of ZnO as a light emitter are its large exciton binding energy (60 meV), and the existence of well-developed bulk and epitaxial growth processes; for electronic applications, its attractiveness lies in having high breakdown strength and high saturation velocity. Optical UV lasing, at both low and high temperatures, has already been demonstrated, although efficient electrical lasing must await the further development of good, p-type material. ZnO is also much more resistant to radiation damage than are other common semiconductor materials, such as Si, GaAs, CdS, and even GaN; thus, it should be useful for space applications.

Recent Advances in ZnO Materials and Devices

A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.

The structural and luminescence properties of porous silicon

no attention has been given to the photoconducting properties of nanowires despite the exciting possibilities for use in optoelectronic circuits. Here, we show the possibility of creating highly sensitive nanowire switches by exploring the photoconducting properties of individual semiconductor nanowires. The conductivity of the ZnO nanowires is extremely sensitive to ultraviolet light exposure. The light-induced conductivity increase allows us to reversibly switch the nanowires between “OFF” and “ON” states, an optical gating phenomenon analogous to the commonly used electrical gating. [2,3,10]

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Nanowire ultraviolet photodetectors and optical switches

羟氨苄青霉素引起大疱性类天疱疮样疹

We report enhanced terahertz emission from N-face InN/InGaN multiple quantum wells relative to that from bulk N-face InN when excited by 800 nm femtosecond optical pulses with low pump intensities.

Terahertz Emission from Indium Nitride Multiple Quantum Wells

The use of low temperatures (∼77K) during Au Schottky contact deposition onto n-GaAs produces an increase in barrier height from 0.73eV for room temperature diodes to 0.82eV. There is no evidence of drift in the forward current in either type of diode and the low temperature deposited samples show smoother Au layers and more abrupt Au∕GaAs interfaces as determined by x-ray reflectivity measurements. Both types of diodes show surface and bulk contributions to the reverse bias current. The diodes with Au deposited at cryogenic temperature did show higher ideality factors, which may result from contaminants gettered to the cold GaAs surface.

Improved Au Schottky contacts on GaAs using cryogenic metal deposition

The development of low cost and compact biological agent identification and detection systems, which can
be employed in place-and-forget applications or on unmanned vehicles, is constrained by the photodetector currently
available. The commonly used photomultiplier tube has significant disadvantages that include high cost, fragility,
high voltage operation and poor quantum efficiency in the deep ultraviolet (240-260nm) necessary for methods such
as fluorescence-free Raman spectroscopy. A III-Nitride/ SiC separate absorption and multiplication avalanche
photodiode (SAM-APD) offers a novel approach for fabricating high gain photodetectors with tunable absorption
over a wide spectrum from the visible to deep ultraviolet. However, unlike conventional heterojunction SAM APDs,
the performance of these devices are affected by the presence of defects and polarization induced charge at the
heterointerface arising from the lattice mismatch and difference in spontaneous polarization between the GaN
absorption and the SiC multiplication regions. In this paper we report on the role of defect density and interface
charge on the performance of GaN/SiC SAM APDs through simulations of the electric field profile within this
device structure and experimental results on fabricated APDs. These devices exhibit a low dark current below 0.1
nA before avalanche breakdown and high avalanche gain in excess of 1000 with active areas 25x larger than that of
state of the art GaN APDs. A responsivity of 4 A/W was measured at 365 nm when biased near avalanche
breakdown.

III-nitride/SiC avalanche photodetectors for enabling compact biological agent identification and detection

We have used femtosecond time-resolved optical techniques to study fundamental materials issues in III-nitride semiconductors relevant to the realization of high quality ultraviolet photodetectors. Intensity dependent pump-probe reflectivity and transmission measurements have been employed in the investigation of carrier dynamics in AlGaN alloys with Al content ranging from ~0.15 to 0.4. For the Al0.15Ga0.85N sample, the intensity dependence of the (Delta) R decay suggests that at high intensity the shallow traps are saturated and ultrafast nonradiative recombination dominates the carrier dynamics. For the Al0.25Ga.75N and Al0.4Ga0.6N samples (Delta) R decays faster with decreasing intensity and changes sign. Moreover, the decays are faster for a given in tensity in the higher Al content sample. This behavior suggests that in these cases the dynamics are governed by trapping at localized states that become deeper and more numerous as the Al content increases. Within this context the sign change in (Delta) R in A;0.4Ga0.6N may be indicative of the onset of photoinduced absorption associated with the excitation of carriers from the localized states to the bands, which has also been observed in time-resolved transmission measurements. This localization may be associated with alloy fluctuations that broaden the absorption edge of the material and degrade the long-wavelength performance of photodetectors. In addition, time-resolved electroabsorption measurements on an AlGaN/GaN heterojunction p-i-n photodiode have been used to study the transient electron velocity overshoot for transport in the c-direction in wurzite GaN. The velocity overshoot is observed at fields well below the field at which the calculated peak steady-state velocity occurs, and it increases with electric field up to ~320 kV/cm, at which field a peak velocity of 7.25x107 cm/s is attained within the first 200 fs after photoexcitation. These results are consistent with theoretical Monte Carlo calculations incorporating a GaN full-zone band structure, which show that because of band nonparabolicity in the (Gamma) valley the majority of electrons do not attain sufficient energy to effect intervalley transfer until they are subjected to higher fields (>325kV/cm). This behavior may have important implications for avalanche photodiodes, for which electrons are promoted to higher lying bands for participating in the avalanche process.

Femtosecond studies of fundamental materials issues in III-nitride ultraviolet photodetectors

The pump-probe technique has been used to perform room temperature studies of the photoinduced changes in the reflectivity {Delta}R associated with exciton and carrier dynamics in GaN prepared by lateral epitaxial overgrowth. For resonant excitation of cold excitons, the {Delta}R decay possesses a 720 ps component attributed to the free exciton lifetime in this high quality material. For electrons with small excess energy ( 100 meV) electron relaxation are attributed to electron-hole scattering in conjunction with the screened electron-LO phonon interaction.

Hongen Shen

Papers

Terahertz Emission from Indium Nitride Multiple Quantum Wells

Improved Au Schottky contacts on GaAs using cryogenic metal deposition

III-nitride/SiC avalanche photodetectors for enabling compact biological agent identification and detection

Femtosecond studies of fundamental materials issues in III-nitride ultraviolet photodetectors

Picosecond photoinduced reflectivity studies of GaN prepared by lateral epitaxial overgrowth