B. Liu

Deep-level traps induced dark currents in extended wavelength InxGa1−xAs/InP photodetector

The dark current mechanism of extended wavelength InxGa1−xAs photo-detectors is still a debated issue. In this paper, the deep-level transient spectroscopy (DLTS) and dark current characteristics of InxGa1−xAs/InP detectors are investigated. Using trap parameters obtained from DLTS measurement, the device simulations of current-voltage characteristics are carried out by Silvaco Altas. The results reveal that the dark current at the low reverse bias voltage is associated with deep level trap induced trap assisted tunneling and Shockley-Read-Hall generation mechanism. The reduction of the deep level trap concentration in InxGa1−xAs absorption layer could dramatically suppress the dark current near zero bias in extended wavelength InxGa1−xAs/InP detectors.

Microstructure and dislocation of epitaxial InN films revealed by high resolution x-ray diffraction

This article reports on the study of microstructure and dislocation of InN films using high resolution x-ray diffraction grown on sapphire (0001) both by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). The mosaic tilt, twist, and correlation lengths of InN films are determined by using symmetrical and asymmetrical reflections as well as reciprocal space mapping. Deducing from these results, MBE-grown InN film exhibits the edge-type dislocations of 4.0×109 cm−2, which is about ten times higher than the density of screw-type dislocations. In MOCVD-grown InN sample, the edge-type dislocations density is as high as 2.1×1010 cm−2, and the screw-type dislocations density is 1.3×109 cm−2. They indicate that edge type is the predominant dislocation type in the InN films. By comparing the reported transmission electron microscopy results, the accuracy of evaluation for the dislocation density using the mosaic model is proved.

Anisotropic crystallographic properties, strain, and their effects on band structure of m-plane GaN on LiAlO2(100)

The m-plane GaN films grown on LiAlO2(100) by metal-organic chemical vapor deposition exhibit anisotropic crystallographic properties. The Williamson–Hall plots point out they are due to the different tilts and lateral correlation lengths of mosaic blocks parallel and perpendicular to GaN[0001] in the growth plane. The symmetric and asymmetric reciprocal space maps reveal the strain of m-plane GaN to be biaxial in-plane compress exx=−0.79% and ezz=−0.14% with an out-of-plane dilatation eyy=0.38%. This anisotropic strain further separates the energy levels of top valence band at Γ point. The energy splitting as 37meV as well as in-plane polarization anisotropy for transitions are found by the polarized photoluminescence spectra at room temperature.

High-quality Schottky contacts to n-InGaN alloys prepared for photovoltaic devices

Large-area Au∕Pt∕n-In0.2Ga0.8N Schottky contacts have been fabricated for photovoltaic devices. The current transport mechanisms of the Schottky contacts to n-In0.2Ga0.8N with different background carrier concentrations are investigated. The thermionic emission is a dominating current transport mechanism at the Pt∕n-InGaN interface in a low background carrier concentration sample, while the defect-assisted tunneling current and trap-related recombination current play important roles in high background carrier concentration samples. The Schottky diode fabricated using the low background carrier concentration sample gives much better Schottky barrier characteristics and exhibits a three to four order of magnitude higher spectral responsivity and a larger rejection ratio in comparison with those fabricated using the high background carrier concentration samples.

Improved Performances of InGaN Schottky Photodetectors by Inducing a Thin Insulator Layer and Mesa Process

The performances of InGaN Schottky photodetectors with varied fabrication processes were investigated. The photoresponse and dark current of InGaN Schottky photodetectors can be obviously improved by inserting a thin Si3N4 passivation layer between the InGaN layer and the Schottky metal. Furthermore, a mesa process gives not only a further increase in the photoresponse but also a pronounced reduction in the reverse leakage current of about two orders of magnitude. A lateral surface leakage current mechanism associated with the 2-D variable-range hopping conduction through high-density surface states in InGaN is proposed to explain the reduction of the reverse leakage current after etching the mesa.

2.6 μm MBE grown InGaAs detectors with dark current of SRH and TAT

We fabricate 2.6 μm InGaAs photodetectors by MBE technology and study its dark current mechanisms. Deep-level transient spectroscopy (DLTS) demonstrates a deep-level trap located at E c - 0.25 eV in the absorption layer. Using the trap parameters, a dark current model is constructed and the device simulation generates the dark current characteristic which agrees well with the experimental data. The model suggests that the dark current at low reverse voltage is dominated by the Shockley-Read-Hall (SRH) and trap-assisted tunneling (TAT). Furthermore, it predicts some basic rules for suppressing the dark current in 2.6 μm InGaAs detectors.

Biaxial and uniaxial strain effects on the ultraviolet emission efficiencies of AlxGa1−xN films with different Al concentrations

The influences of biaxial and uniaxial strain on the ultraviolet emission efficiencies of both c- and m-plane AlxGa1−xN films with different Al concentrations are investigated under the framework of k⋅p perturbation theory. The optimal high efficiency windows, for ultraviolet light emissions are quantitatively estimated. c-plane AlxGa1−xN modified by uniaxial strain, shows more advantages over biaxial-strained AlxGa1−xN. This is due to the relatively more flexible tuning range and the advantage of obtaining pure linear polarization, which can be utilized to design polarized emission devices. For m-plane AlxGa1−xN, there are always in-plane polarized emissions under both biaxial and uniaxial strain conditions, thus, it is more likely to obtain high surface emission efficiency.

Strain-modulated valence band engineering for enhancement of surface emission in polar and nonpolar plane AlN films

The k⋅p perturbation theory is adopted to calculate the strain-modulated excitonic transition energies and their polarization properties in c- and m-plane AlN. The two topmost valence subbands exchange their band characteristics at the degenerate point where ezz=0.98% and exx=eyy=−1.70%. The surface emission efficiency of c-plane AlN films can be dramatically enhanced with ezz>0.98% (exx=eyy<−1.70%), where the lowest excitonic transition is predominantly z-polarized. Besides, nonpolar plane (m- or a-plane) AlN experiencing anisotropic in-plane strain can be chosen as a candidate for enhancing the surface emission efficiency by proper strain manipulation.

Observation of hole accumulation at the interface of an undoped InGaN/GaN heterostructure

A pronounced hump structure at about −5 V in the high-frequency capacitance-voltage (C-V) curve of an undoped InGaN/GaN heterostructure is observed and this hump weakens gradually with decreasing measurement frequency, indicating the occurrence of an inversion behavior in the InGaN/GaN heterostructure. The inversion behavior in the C-V curve is attributed to hole accumulation at the heterointerface where a hole well is formed due to the strong piezoelectric polarization effect in the InGaN/GaN heterostructure. The acceptor traps related to Ga vacancies in the InGaN layer are thought to be the source of the minority carriers. The theoretical calculation of band diagram of the InGaN/GaN heterostructure confirms the formation of the hole well at the heterointerface and supports the behavior of hole accumulation under negative bias voltage.

The effect of long-duration high-temperature annealing in an air ambient on the properties of AlGaN∕GaN heterostructures

The effects of a long-duration high-temperature annealing in an air ambient on the strain of the AlGaN barrier layer and high-temperature transport properties of the two-dimensional electron gas (2DEG) in AlGaN∕GaN heterostructures were investigated. The results show that the annealing induces a nonreversible lattice relaxation in the AlGaN layer and increases remarkably the 2DEG density due to the incorporation of oxygen atoms into the AlGaN surface and decreases the 2DEG mobility in the AlGaN∕GaN heterostructure. However, the conductivity of the 2DEG has no obvious change in our samples within the measured temperature range before and after the annealing, indicating that AlGaN∕GaN heterostructures are possibly promising for electron devices operated at high temperatures based on atmospheric exposure.