This paper calculates the wavelengths of the interband transitions as a function of the Al mole fraction of Al x Ga 1 x N bulk material.It is finds that when the Al mole fraction is between 0.456 and 0.639,the wavelengths correspond to the solar-blind (250 nm to 280 nm).The influence of the structure parameters of Al y Ga 1 y N/GaN quantum wells on the wavelength and absorption coefficient of intersubband transitions has been investigated by solving the Schro¨dinger and Poisson equations self-consistently.The Al mole fraction of the Al y Ga 1 y N barrier changes from 0.30 to 0.46,meanwhile the width of the well changes from 2.9 nm to 2.2 nm,for maximal intersubband absorption in the window of the air (3 μm < λ < 5 μm).The absorption coefficient of the intersubband transition between the ground state and the first excited state decreases with the increase of the wavelength.The results are finally used to discuss the prospects of GaN-based bulk material and quantum wells for a solar-blind and middle infrared two-colour photodetector.
The influence of applied electric fields on the absorption coefficient and subband distances in asymmetrical AlN/GaN coupled double quantum wells(CDQWs) has been investigated by solving Schrdinger and Poisson equations self-consistently.It is found that the absorption coefficient of the intersubband transition(ISBT) between the ground state and the third excited state(1odd 2even) can be equal to zero when the electric fields are applied in asymmetrical AlN/GaN CDQWs,which is related to applied electric fields induced symmetry recovery of these states.Meanwhile,the energy distances between 1odd 2even and 1even 2even subbands have different relationships from each other with the increase of applied electric fields due to the different polarization-induced potential drops between the left and the right wells.The results indicate that an electrical-optical modulator operated within the opto-communication wavelength range can be realized in spite of the strong polarization-induced electric fields in asymmetrical AlN/GaN CDQWs.
By using temperature-dependent Hall,variable-frequency capacitance-voltage and cathodoluminescence (CL) measurements,the identification of inductively coupled plasma (ICP)-induced defect states around the Al x Ga 1-x N/GaN heterointerface and their elimination by subsequent annealing in Al x Ga 1-x N/GaN heterostructures are systematically investigated.The energy levels of interface states with activation energies in a range from 0.211 to 0.253 eV below the conduction band of GaN are observed.The interface state density after the ICP-etching process is as high as 2.75×10 12 cm 2 ·eV 1.The ICP-induced interface states could be reduced by two orders of magnitude by subsequent annealing in N 2 ambient.The CL studies indicate that the ICP-induced defects should be Ga-vacancy related.
This paper investigates the temperature dependence of the specific resistance in annealed V/Al/V/Au (15 nm/85 nm/20 nm/95 nm) contacts on n-Al0.4Ga0.6 N. Contacts annealed at 700C and higher temperatures show Ohmic behaviour. Annealing at 800 C produces the lowest contact resistance. Samples annealed at 800C have been analysed by using cross-sectional transmission electron microscopy and an energy dispersive x-ray spectrum. Limited reaction depths are observed between V-based contacts and n-AlGaN. The VN grains are found to form in the contact layer of the annealed samples,which can be considered as the key to the successful formation of Ohmic contact. The contact layer adjacent to AlGaN material consists of V-Al-Au-N,AlN and AlAu alloys.
The temperature dependence of carrier transport properties of AlxGa1-xN/InyGa1-yN/GaN and AlxGa1-xN/GaN heterostructures has been investigated.It is shown that the Hall mobility in Al0.25Ga0.75N/In0.03Ga0.97N/GaN heterostructures is higher than that in Al0.25Ga0.75N/GaN heterostructures at temperatures above 500 K,even the mobility in the former is much lower than that in the latter at 300 K.More importantly,the electron sheet density in Al0.25Ga0.75N/In0.03Ga0.97N/GaN heterostructures decreases slightly,whereas the electron sheet density in Al0.25Ga0.75N/GaN heterostructures gradually increases with increasing temperature above 500 K.It is believed that an electron depletion layer is formed due to the negative polarization charges at the InyGa1-yN/GaN heterointerface induced by the compressive strain in the InyGa1-yN channel,which e-ectively suppresses the parallel conductivity originating from the thermal excitation in the underlying GaN layer at high temperatures.
