Investigation of responsivity decreasing with rising bias voltage in a GaN Schottky barrier photodetector

The unexpected decrease in measured responsivity observed in a specific GaN Schottky barrier photodetector (PD) at high reverse bias voltage was investigated and explained. Device equivalent transforms and small signal analysis were performed to analyse the test circuit. On this basis, a model was built which explained the responsivity decrease quantitatively. After being revised by this model, responsivity curves varying with bias voltage turned out to be reasonable. It is proved that the decrease is related to the dynamic parallel resistance of the photodiode. The results indicate that with a GaN Schottky PD, the choice of load resistance is restricted according to the dynamic parallel resistance of the device to avoid responsivity decay at high bias voltage.

Spin dynamics in the second subband of a quasi-two-dimensional system studied in a single-barrier heterostructure by time-resolved Kerr rotation

Spin dynamics in the first and second subbands have been examined simultaneously by time resolved Kerr rotation in a single-barrier heterostructure of a 500 nm thick GaAs absorption layer. By scanning the wavelengths of the probe and pump beams towards the short wavelength in the zero magnetic field, the spin coherent time T-2(1)* in the 1st subband E-1 decreases in accordance with the D'yakonov-Perel' (DP) spin decoherence mechanism. Meanwhile, the spin coherence time T-2(2)* in the 2nd subband E-2 remains very low at wavelengths longer than 810 nm, and then is dramatically enhanced afterwards. At 803 nm, T-2(2)* (450 ps) becomes ten times longer than T-2(1)* (50 ps). A new feature has been discovered at the wavelength of 811nm under the bias of -0.3V (807nm under the bias of -0.6V) that the spin coherence times (T-2(1)* and T-2(2)*) and the effective g* factors (vertical bar g*(E-1)vertical bar and vertical bar g*(E-2)vertical bar) all display a sudden change, presumably due to the "resonant" spin exchange coupling between two spin opposite bands. Copyright (C) EPLA, 2008.

A new method to reduce the dark current of GaN based Schottky barrier ultraviolet photodetector

A new method to reduce the dark current of GaN based Schottky barrier ultraviolet photodetector is proposed. In comparision with conventional i-CaN/n(+)-GaN structure, an additional thin p-GaN cap layer is introduced on the i-GaN(n(-)-GaN) in the new structure. The simulation results showed that the additional layer makes the dark current to decrease in the photodetector due to the increase of the Schottky barrier height. The effects of thickness and carrier concentration of p-GaN layer on the dark current of the photodetector were also studied. It is suggested that the dark current of the new structure device could be better reduced by employing p-GaN with higher carrier concentration as the cap layer.

Reply to

The present paper is the reply to Sandu's Comment on our paper [Phys. Rev. B 72, 153314 (2005)], i.e., the effect of the current operator on the spin-dependent tunneling through a barrier in the presence of the Dresselhaus spin-orbit interaction (DSOI). We demonstrate theoretically and numerically that our previous numerical result is correct when there is no DSOI in the contact region and it remains a good approximation in the presence of the k(3)-DSOI in the contact regions.

Hydrogen sensors based on AlGaN/AIN/GaN Schottky diodes

Pt/AlGaN/AIN/GaN Schottky diodes are fabricated and characterized for hydrogen sensing. The Pt Schottky contact and the Ti/Al/Ni/Au ohmic contact are formed by evaporation. Both the forward and reverse currents of the device increase greatly when exposed to hydrogen gas. A shift of 0.3 V at 300K is obtained at a fixed forward current after switching from N-2 to 10%H-2+N-2. The sensor responses under different concentrations from 50ppm H-2 to 10%H-2+N-2 at 373K are investigated. Time dependences of the device forward current at 0.5 V forward bias in N-2 and air atmosphere at 300 and 373K are compared. Oxygen in air accelerates the desorption of the hydrogen and the recovery of the sensor. Finally, the decrease of the Schottky barrier height and sensitivity of the sensor are calculated.

Effect of silver growth temperature on the contacts between Ag and ZnO thin films

Highly c-axis oriented ZnO thin films were deposited on Si substrates by the pulsed laser deposition (PLD) method. At different growth temperatures, 200 nm silver films as the contact metal were deposited on the ZnO thin films. The growth temperatures have great influence on the crystal quality of Ag films. Current-voltage characteristics were measured at room temperature. The Schottky contacts between Ag and ZnO thin films were successfully obtained when silver electrodes were deposited at 150A degrees C and 200A degrees C. Ohmic contacts were formed while the growth temperatures were lower than 150A degrees C or higher than 200A degrees C. After analysis, the forming of Ag/ZnO Schottky contacts was shown to be dependent on the appearance of the p-type inversion layer at the interface between Ag and ZnO layers.

Electric-tunable magneto resistance effect in a two-dimensional electron gas modulated by hybrid magnetic-electric barrier nanostructure

The electric-tunable spin-independent magneto resistance effect has been theoretically investigated in ballistic regime within a two-dimensional electron gas modulated by magnetic-electric barrier nanostructure. By including the omitted stray field in previous investigations oil analogous structures, it is demonstrated based on this improved approximation that the magnetoresistance ratio for the considered structure can be efficiently enhanced by a proper electric barrier up to the maximum value depending on the specific magnetic suppression. Besides, it is also shown the introduction of positive electrostatic modulation can effectively overcome the degradation of magnetoresistance ratio for asymmetric configuration and enhance the visibility of periodic pattern induced by the size effect, while for an opposite modulation the system magnetoresistance ratio concerned may change its sign. (C) 2009 Elsevier B.V. All rights reserved.

Growth and characterization of AlGaN/GaN heterostructure using unintentionally doped AlN/GaN superlattices as barrier layer

AlGaN/GaN heterostructure using unintentionally doped AlN/GaN superlattices (SLs) as barrier layer is grown on C-plane sapphire by metal organic vapor deposition (MOCVD). Compared with the conventional Si-doped structure, electrical property is improved. An average sheet resistance of 287.1 Omega/square and high resistance uniformity of 0.82% are obtained across the 2-inch epilayer wafer with an equivalent Al composition of 38%. Hall measurement shows that the mobility of two-dimensional electron gas (2DEG) is 1852 cm(2)/V s with a sheet carrier density of 1.2 x 10(13) cm(-2) at room temperature. The root mean square roughness (RMS) value is 0.159 nm with 5 x 5 mu m(2) scan area and the monolayer steps are clearly observed. The reason for the property improvement is discussed. (c) 2008 Elsevier Ltd. All rights reserved.

Properties of AlyGa1-yN/AlxGa1-xN/AlN/GaN Double-Barrier High Electron Mobility Transistor Structure

Electrical properties of AlyGa1-yN/AlxGa1-xN/AlN/GaN structure are investigated by solving coupled Schrodinger and Poisson equation self-consistently. Our calculations show that the two-dimensional electron gas (2DEG) density will decrease with the thickness of the second barrier (AlyGa1-yN) once the AlN content of the second barrier is smaller than a critical value y(c), and will increase with the thickness of the second barrier (AlyGa1-yN) when the critical AlN content of the second barrier y(c) is exceeded. Our calculations also show that the critical AlN content of the second barrier y(c) will increase with the AlN content and the thickness of the first barrier layer (AlxGa1-xN).

Effect of metal contact's reflection on the effective coupling coefficient of second-order DFB laser diodes

For a second-order DFB-LD, the presence of a metal contact layer can reduce I-st-order radiation. Part of the reflected power is redistributed into guided modes and results in a variation of the effective coupling coefficient kappa(eff). In this paper, we study the effect of the Au top contact's reflection on the kappa(eff) of 2(nd)-order DFB lasers. (C) 2004 Wiley Periodicals, Inc.

Reflected in-plane conductance near a barrier with Dresselhaus spin-orbit coupling

We have calculated the in-plane conductance of a barrier with the Dresselhaus spin-orbit interaction, which is sandwiched between two spin-polarized materials aligned arbitrarily. Besides a transmitted in-plane current which arises on the drain side as pointed out in Phys. Rev. Lett. 93, 056601 (2004), a reflected in-plane current always appears simultaneously on the source side near the interface of the barrier. The spin polarization of the source affects the transmitted current more than the reflected one, and conversely the spin polarization of the drain affects the reflected current more. The relationship between transmitted current and the reflected one has been studied.

Photoinduced voltage shift in a three-barrier, two-well resonant tunneling structure integrated with a 1.2-mu m-thick n-type GaAs layer

By integrating a three-barrier, two-well resonant tunneling structure with a 1.2-mu m-thick, slightly doped n-GaAs layer, a photoinduced voltage shift on the order of magnitude of 100 mV in resonant current peaks has been verified at an irradiance of low light power density. The 1.2-mu m-thick, slightly doped n-GaAs layer manifests itself of playing an important role in enhancing photoelectric sensitivity. (c) 2005 Elsevier B.V. All rights reserved.

A new type of photoelectric response in a double barrier structure with a wide quantum well

We have calculated the photoelectric response in a specially designed double barrier structure. It has been verilied that a transfer of the internal photovoltaic effect in the quantum well to the tunnelling transport through above-barrier quasibound states of the emitter barrier may give rise to a remarkable photocurrent.

Dynamics of spin-dependent tunneling through a semiconductor double-barrier structure

The dynamics of spin-dependent tunneling through a nonmagnetic semiconductor double-barrier structure is studied including the k(3) Dresselhaus spin orbit coupling is solved by the time-dependent Schrodinger equation with a developed method for the finite-difference relaxation. The resonant peak and quasibound level lifetime are determined by the in-plane wave vector and the applied electric field. The buildup time and decay lifetime of resonant probability amplitude are different for the spin-down and spin-up electrons due to the Dresselhaus spin-orbit coupling. Further investigation shows that the steady spin-polarization in both the well and collector regions has been obtained in the time domain. (C) 2007 American Institute of Physics.

A new Schottky barrier structure of GaN-based ultraviolet photodetector

A new GaN-based ultraviolet photodetector with Schottky barrior structure is proposed. Comparied with the conventional i-GaN/n(+) -GaN structure, there is an additional thin n-AlGaN cap layer on the i-GaN in the new structure. The simulation result demonstrates that the new structure leads to an increased quantum efficiency in GaN photodetection, since the negative effect of surface states on the photodetector is reduced in the new structure. In addition, it is suggested that the performance of device with the new structure could be further improved by employing an even thinner AlGaN cap layer with higher carrier concentration.