Mg acceptor energy levels in AlxInyGa1-x-yN quaternary alloys: An approach to overcome the p-type doping bottleneck in nitrides

The Mg-Ga acceptor energy levels in GaN and random Al8In4Ga20N32 quaternary alloys are calculated using the first-principles band-structure method. We show that due to wave function localization, the MgGa acceptor energy level in the alloy is significantly lower than that of GaN, although the two materials have nearly identical band gaps. Our study demonstrates that forming AlxInyGa1-x-yN quaternary alloys can be a useful approach to lower acceptor ionization energy in the nitrides and thus provides an approach to overcome the p-type doping difficulty in the nitride system.

Enhancement of electroluminescence in p-i-n structures with nano-crystalline Si/SiO2 multilayers

Nano-crystalline Si/SiO2 multilayers were prepared by alternately changing the ultra-thin amorphous Si film deposition and the in situ plasma oxidation process followed by the post-annealing treatments. Well-defined periodic structures can be achieved with 2.5 nm thick SiO2 sublayers. It is shown that the size of formed nano-crystalline Si is about 3 nm. Room temperature electroluminescence can be observed and the spectrum contains two luminescence bands located at 650 nm and 520 nm. In order to improve the hole injection probability, p-i-n structures containing a nanocrystalline Si/SiO2 luminescent layer were designed and fabricated on different p-type substrates. It is found that the turn-on voltage of p-i-n structures is obviously reduced and the luminescence intensity increases by 50 times. It is demonstrated that the use of a heavy-doped p-type substrate can increase the luminescence intensity more efficiently compared with the light-doped p-type substrate due to the enhanced hole injection.

Photovoltaic effects in InGaN structures with p-n junctions

InGaN photovoltaic structures with p-n junctions have been fabricated by metal organic chemical vapour deposition. Using double-crystal X-ray diffraction measurements, it was found that the room temperature band gaps of p-InGaN and n-InGaN films were 2.7 and 2.8 eV, respectively. Values of 3.4 x 10(-2) mA cm(-2) short-circuit current, 0.43 V open-circuit voltage and 0.57 fill factor have been achieved under ultraviolet illumination (360 nm), which were related to p-n junction connected back-to-back with a Schottky barrier and many defects of the p-InGaN film. 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

High-Temperature Continuous-Wave Single-Mode Operation of 1.3 mu m p-Doped InAs-GaAs Quantum-Dot VCSELs

In this letter, we have demonstrated continuous-wave single-mode operation of 1.3-mu m InAs-GaAs quantum-dot (QD) vertical-cavity surface-emitting lasers (VCSELs) with p-type modulation-doped QD active region from 20 degrees C to 60 degrees C. The highest output power of 0.435mW and lowest threshold current of 1.2 mA under single-mode operation are achieved. The temperature-dependent output characteristics of QD-VCSELs are investigated. Single-mode operation with a sidemode suppression ratio of 34 dB is observed at room temperature. The critical size of oxide aperture for single-mode operation is discussed.

Design and Fabrication of AlGaN-Based Resonant-Cavity-Enhanced p-i-n UV PDs

AlGaN-based resonant-cavity-enhanced (RCE) p-i-n photodetectors (PDs) for operating at the wavelength of 330 nm were designed and fabricated. A 20.5-pair AlN/Al0.3Ga0.7N distributed Bragg reflector (DBR) was used as the back mirror and a 3-pair AlN/Al0.3Ga0.7N DBR as the front one. In the cavity is a p-GaN/i-GaN/n-Al0.3Ga0.7N structure. The optical absorption of the RCE PD structure is at most 59.8% deduced from reflectance measurement. Selectively enhanced by the cavity effect, a response peak of 0.128 A/W at 330 nm with a half-peak breadth of 5.5 nm was obtained under zero bias. The peak wavelength shifted 15 nm with the incident angle of light increasing from 0 degrees to 60 degrees.

Substantial photo-response of InGaN p-i-n homojunction solar cells

InGaN p-i-n homojunction structures were grown by metal-organic chemical vapor deposition, and solar cells with different p-contact schemes were fabricated. X-ray diffraction measurements demonstrated that the epitaxial layers have a high crystalline quality. Solar cells with semitransparent p-contact exhibited a fill factor (FF) of 69.4%, an open-circuit voltage (V-oc) of 2.24 V and an external quantum efficiency (EQE) of 41.0%. On the other hand, devices with grid p-contact showed the corresponding values of 57.6%, 2.36 V, 47.9% and a higher power density. These results indicate that significant photo-responses can be achieved in InGaN p-i-n solar cells.

Fabrication and modulation characteristics of 1.3 mu m p-doped InAs quantum dot vertical cavity surface emitting lasers

We present the fabrication of 1.3 mu m waveband p-doped InAs quantum dot (QD) vertical cavity surface emitting lasers (VCSELs) with an extremely simple process. The continuous-wave saturated output power of 1.1 mW with a lasing wavelength of 1280 nm is obtained at room temperature. The high-speed modulation characteristics of p-doped QD VCSELs of two different oxide aperture sizes are investigated and compared. The maximum 3 dB modulation bandwidth of 2.5 GHz can be achieved at a bias current of 7 mA for a p-doped QD VCSEL with an oxide aperture size of 10 mu m in the small signal frequency response measurements. The crucial factors for the 3 dB bandwidth limitation are discussed according to the parameters' extraction from frequency response.

Characteristic study of maximum modal gain of p-doped 1.3 mu m InAs/GaAs quantum dot lasers

We report an experimental and theoretical study of maximum modal gain of p-doped 1.3 mu m InAs/GaAs quantum dot (QD) lasers. The maximum modal gain of the QD laser with five stacks of QDs is as high as 17.5 cm(-1) which is the same as that of the undoped laser with identical structures. The expression of the maximum modal gain is derived and it is indicated that p-doping has no effect to the maximum modal gain. We theoretically calculated the maximum modal gain of the QD lasers and the result is in a good agreement with the experimental data. Furthermore, QDs with lower height or smaller aspect ratio are beneficial to achieving a greater maximum modal gain that leads to lower threshold current density and higher differential modal gain, which is good for the application of p-doped 1.3 mu m InAs/GaAs QD lasers in optical communications systems.

Dynamic P-I and P-V Curves for Semiconductor Lasers and Modulators

In this paper, we propose the dynamic P-V curve for modulator and P-I curve for laser diode, and present a simple approach to deriving the curves from the small-signal frequency responses measured using a microwave network analyzer. The linear response range, modulation efficiency, optimal driving conditions at different frequency can, therefore, be determined. It is demonstrated that the large-signal performance of electro-absorption (EA) modulator and the directly modulated semiconductor lasers can be predicted from the dynamic curved surface. Experiments show a good agreement between the evaluated characteristics and the measured large-signal performance.

Band-gap bowing and p-type doping of (Zn, Mg, Be)O wide-gap semiconductor alloys: a first-principles study

Using a first-principles band-structure method and a special quasirandom structure (SQS) approach, we systematically calculate the band gap bowing parameters and p-type doping properties of (Zn, Mg, Be)O related random ternary and quaternary alloys. We show that the bowing parameters for ZnBeO and MgBeO alloys are large and dependent on composition. This is due to the size difference and chemical mismatch between Be and Zn(Mg) atoms. We also demonstrate that adding a small amount of Be into MgO reduces the band gap indicating that the bowing parameter is larger than the band-gap difference. We select an ideal N atom with lower p atomic energy level as dopant to perform p-type doping of ZnBeO and ZnMgBeO alloys. For N doped in ZnBeO alloy, we show that the acceptor transition energies become shallower as the number of the nearest neighbor Be atoms increases. This is thought to be because of the reduction of p-d repulsion. The N-O acceptor transition energies are deep in the ZnMgBeO quaternary alloy lattice-matched to GaN substrate due to the lower valence band maximum. These decrease slightly as there are more nearest neighbor Mg atoms surrounding the N dopant. The important natural valence band alignment between ZnO, MgO, BeO, ZnBeO, and ZnMgBeO quaternary alloy is also investigated.

Temperature Sensitivity Dependence on Cavity Length in p-Type Doped and Undoped 1.3-mu m InAs-GaAs Quantum-Dot Lasers

We have fabricated 1.3-mu m InAs-GaAs quantum-dot (QD) lasers with and without p-type modulation doping and their characteristics have been investigated. We find that introducing p-type doping in active regions can improve the temperature stability of 1.3-mu m InAs-GaAs QD lasers, but it does not, increase the saturation modal gain of the QD lasers. The saturation modal gain obtained from the two types of lasers is identical (17.5 cm(-1)). Moreover, the characteristic temperature increases as cavity length increases for the two types of lasers, and it improves more significantly for the lasers with p-type doping due to their higher gain.

p-type doping of GaInNAs quaternary alloys

Using the first-principles band-structure method, we investigate the p-type doping properties and band structural parameters of the random Ga1-xInxN1-yAsy quaternary alloys. We show that the Mg-Ga substitution is a better choice than ZnGa to realize the p-type doping because of the lower transition energy level and lower formation energy. The natural valence band alignment of GaAs and GaInNAs alloys is also calculated, and we find that the valence band maximum becomes higher with the increasing in composition. Therefore, we can tailor the band offset as desired which is helpful to confine the electrons effectively in optoelectronic devices. (C) 2008 Published by Elsevier B.V.

Stable multiplication gain in GaN p-i-n avalanche photodiodes with large device area

Visible-blind p-i-n avalanche photodiodes (APDs) were fabricated with high-quality GaN epilayers deposited on c-plane sapphire substrates by metal-organic chemical vapour deposition. Due to low dislocation density and a sophisticated device fabrication process, the dark current was as small as similar to 0.05 nA under reverse bias up to 20V for devices with a large diameter of 200 mu m, which was among the largest device area for GaN-based p-i-n APDs yet reported. When the reverse bias exceeded 38V the dark current increased sharply, exhibiting a bulk avalanche field-dominated stable breakdown without microplasma formation or sidewall breakdown. With ultraviolet illumination (360 nm) an avalanche multiplication gain of 57 was achieved.

Influence of penetrating V-pits on leakage current of GaN based p-i-n UV detector

The leakage mechanism of GaN-based p-i-n (p-AlGaN/i-GaN/n-GaN) UV detector has been investigated. With the same dislocation density, devices made from material with higher density of V-pits on surface produce larger leakage current. SEM images show that some V-pits penetrate into i-GaN layer, sometimes even the n-GaN layer. If p-ohmic contact metal (Ni/Au) deposits in the V-pits, Schottky contact would be formed at the interface of metal and i-GaN, or form ohmic contact at the interface of metal and n-GaN. The existence of parallel Schottky junction and ohmic contact resistance enhances the leakage current greatly.

A new p-n structure ultraviolet photodetector with p(-)-GaN active region

A new ultraviolet photodetector of employing p menus type GaN (p(-)-GaN) as the active layer is proposed. It is easy to obtain the p(-)-GaN layer with low carrier concentration. As a result, the depletion region can be increased and the quantum efficiency can be improved. The influence of some structure parameters on the performance of the new device is investigated. Through the simulation calculation, it is found that the quantum efficiency increases with the decrease of the barrier height between the metal electrode and the p(-)-GaN layer, and it is also found that the quantum efficiency can be improved by reducing the thickness of the p(-)-GaN layer. To fabricate the new photodetector with high performance, we should employ thin p(-)-GaN layer as the active layer and reduce the Schottky barrier height.