Haimei Gong

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.

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.

Effects of an anodic oxide passivation layer on mesa-type InGaAs (PIN) photodetectors

Because of the high density of surface states and large surface recombination velocity found in III–V materials, surface passivation remains a crucial processing step for the fabrication of mesa-type InGaAs photodetectors. It is required to find an easy and economical method to form an effective passivation layer on the surface of the detector. In this work, an oxide passivation layer formed by electrochemical anodization was proposed and a 256-element In0.78Ga0.22As photodetector array was fabricated. The dark current performance achieved is as low as 2 × 10−7 A (bias voltage of −0.1 V) at 300 K. The reduction of dark current indicates that the anodic oxide passivation layer is effective in reducing recombination at the exposed mesa surface.

Effects of tensile stress induced by SiO2 passivation layer on the properties of AlGaN∕GaN heterostructure photodiode

The effects of tensile stress induced by SiO2 passivation layer on AlxGa1−xN∕GaN heterostructure photodiode are investigated. The photodiode, with SiO2 layer annealed at 650°C for 30min in O2, shows that reverse current has decreased to 6.16nA∕cm−2 under −10V, two orders lower than that of the device without annealing technique. The responsivity also increases to 0.212A∕W at zero bias. The high-resolution x-ray diffraction, Hall measurements are taken to investigate the surface strain and electrical properties of p-AlGaN surface. These observations indicate that tensile stress induced by SiO2 annealing technique can improve performances of this photodiode greatly.

Damage threshold of HgCdTe induced by continuous-wave CO2 laser

We studied the interaction of p-type HgCdTe with a continuous-wave (cw) carbon dioxide (CO2) laser experimentally and theoretically and obtained an apparent damage threshold temperature, 670–680 K, which corresponds to the temperature of the solid–solid phase transition of HgCdTe. We proposed a two-dimensional thermal conducting model dealing with the thermal diffusion of cw laser processing of materials in this letter. The temperature distribution during the interaction between HgCdTe and the cw CO2 laser is also presented and discussed. All the theoretical calculations are in good agreement with the experimental results.

Reduction of ohmic contact resistivity on p-GaN using N2 plasma surface treatment at room temperature

A reduction of Ni/Au ohmic contact resistivity on p-type GaN was obtained by surface treatment using N2 plasma at room temperature. For p-type GaN with a hole concentration of about 1 × 1017 cm−3, the contact resistivity decreased from 5.0 × 10−2 Ω cm2 to 3.1 × 10−4 Ω cm2 by the N2 plasma treatment, compared to that of the HCl only-treated sample. The O 1s core-level peak in the x-ray photoemission spectra shows that the N2 plasma treatment is effective in the removal of the surface oxide layer. Compared to the HCl only-treated surface, the surface Fermi level on the N2 plasma-treated surface lies about 0.58 eV closer to the valence band edge, resulting in a much smaller surface barrier height to p-GaN than the HCl only-treated surface. The smaller surface barrier height of p-GaN treated with N2 plasma can lead to a lower contact resistivity and can play an important role in lowering the metal contact resistivity to p-GaN.

Inductively coupled plasma etching of AlGaN using Cl2/Ar/BCl3 gases

AlGaN is am important ultraviolet optoelectronic material and inductively coupled plasma (ICP) etching plays an important role in fabrication of mesa structures of AlGaN-based photodiodes. In this work, we investigate ICP etching processes of Al0.32Ga0.68N and Al0.47Ga0.53N. The Al0.32Ga0.68N and Al0.47Ga0.53N materials were firstly tested by transmission spectra and it indicates that they are different materials with different epitaxial quality. Cl2/Ar/BCl3 were used as the ICP gases, and Cl2/Ar mixing ratio was fixed at 4:1. Etching behaviors were characterized by varying the ICP power, the dc bias, Cl2/Ar/BCl3 mixing ratio. ICP power influences etching rates. Dc bias heavily influences the etching rates, and the etching rates increase monotonously with dc bias, which suggests that the ion-bombardment effect is an important factor of these etching processes. BCl3 is the effective removal of oxygen during the etching, and also influences etching rates. The surface rms roughness was measured by an at omic force microscope. The ICP etching surface morphologies were studied by Scanning Electron Microscope (SEM). The results show dc bias and BCl3 are important to electrical characteristics of epitaxial materials. At a relative high dc bias and more BCl3, the etching rate is low, but the damage is low. These results have direct application to the fabrication of AlGaN-based ultraviolet optoelectronic devices.

Electric-field effects on persistent photoconductivity in undoped n-type epitaxial GaN

Investigations of electric-field effects on persistent photoconductivity (PPC) in undoped n-type epitaxial GaN are presented. We have observed that, by applying a high-voltage pulse, the course of PPC was effectively accelerated. Moreover, the current relative responsivity changed with biases across the GaN region, which was not seen in normal photoconductivity phenomena. Experiments were also conducted to indicate notable differences between heating effects and electric-field effects on PPC decay course. Our results show that field-associated changes in trapping parameters are responsible for the above abnormal phenomena.

Study on Au diffusion in 0001 undoped wurtzite GaN

Gold diffusion at 300 °C and 400 °C in 2 µm thick undoped wurtzite GaN grown by the metal–organic chemical vapour deposition technique has been studied by using secondary ion mass spectroscopy (SIMS) depth profiles. The diffusion coefficients D(300 °C) = 1.03 × 10−15 cm2 s−1, D(400 °C) = 1.69 × 10−14 cm2 s−1 and the activation energy Q = 0.93 eV are obtained. The dominant mechanism of Au diffusion in GaN is probably the diffusion of an impurity by the interstitial mode. Comparison with Au diffusion in other semiconductors is given and discussed.

Life signal detection using an on-chip split-ring based solid state microwave sensor

A technique for measuring the amplitude and frequency of breathing and heartbeat has been developed using an on-chip solid state sensor integrating a semiconductor microwave sensor and a split ring operating at a resonance frequency of 4.2 GHz. This technique allows the lock-in amplifier to make real-time measurements, analogous to measurements taken by a vector network analyser through an antenna, but with the advantage of being portable and having a user friendly DC output. The effectiveness of this approach is shown by performing several experiments to determine the breathing and heartbeat frequency with and without the presence of an obstacle between the test subject and the microwave sensor and transducer. The experimental results demonstrate the high sensitivity and large dynamic range over which the proposed system can be used for practical applications.