This paper reports that cathodoluminescence(CL) measurements have been done to study the alloy fluctuation of the Al_(0.3)Ga_(0.7)N layer in Al_(0.3)Ga_(0.7)N/GaN heterostructures.The CL images and linescanning results demonstrate the existence of compositional fluctuation of Al in the Al_(0.3)Ga_(0.7)N barrier.A model using aδ-shape perturbation Hamilton function has been proposed to simulate the scattering probability of the two dimensional electron gases (2DEG) induced by Al composition fluctuation.Two factors,including conduction band fluctuation and polarization electric field variation,induced by the Al composition fluctuation have been taken into account.The scattering relaxation time induced by both factors has been estimated to be 0.31 ns and 0.0078 ns,respectively,indicating that the variation of the piezoelectric field is dominant in the scattering of the 2DEG induced by Al fluctuation.
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.
Ridge InGaN multi-quantum-well-structure (MQW) edge-emitting laser diodes (LDs) were grown on (0001) sapphire substrates by low-pressure metal-organic chemical vapour deposition (MOCVD).The dielectric TiO2/SiO2 front and back facet coatings as cavity mirror facets of the LDs have been deposited with electron-beam evaporation method.The reflectivity of the designed front coating is about 50% and that of the back high reflective coating is as high as 99.9%.Under pulsed current injection at room temperature,the influences of the dielectric facets were discussed.The threshold current of the ridge GaN-based LDs was decreased after the deposition of the back high reflective dielectric mirrors and decreased again after the front facets were deposited.Above the threshold,the slope efficiency of the LDs with both reflective facets was larger than those with only back facets and without any reflective facets.It is important to design the reflectivity of the front facets for improving the performance of GaN-based LDs.
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.
We present the theoretical results of the electronic band structure of wurtzite GaN films under biaxial strains in the (11■2)-plane.The calculations are performed by the k · p perturbation theory approach through using the effective-mass Hamiltonian for an arbitrary direction.The results show that the transition energies decrease with the biaxial strains changing from 0.5% to 0.5%.For films of (11■2)-plane,the strains are expected to be anisotropic in the growth plane.Such anisotropic strains give rise to valence band mixing which results in dramatic change in optical polarisation property.The strain can also result in optical polarisation switching phenomena.Finally,we discuss the applications of these properties to the (11■2) plane GaN-based light-emitting diode and lase diode.
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.
