II-VI and Nitride Wide Bandgap Semiconductors

In association with faculty members: Professor John Hegarty, Professor James Lunney and Dr. Eithne McCabe

Other group members: Mr. Gary Laird

Background

 There has been an enormous increase in interest in wide bandgap semiconductor materials over the last 5 years. The recent introduction of the nitride based laser diode by Nichia has given a further boost to the research effort. We have been working principally on II-VI laser materials over the same period but have recently extended our interest into nitride materials. We have had a close association with the Sony Research Center in Japan who are the principle industrial developer of II-VI materials. Our studies have looked at recombination processes in these II-VI materials at room temperature. We have found that although II-VI materials degrade quickly at high carrier concentration, these materials can also exhibit point defect annealing which may be useful for prolonging device lifetime. We have filed a joint patent with Sony on this interesting new results.

Results

We have studied recombination processes in II-VI  quantum well materials at room temperature. We find that electron-hole recombination dominates at high carrier density although we cannot rule out that Coulomb enhancement effects influence the actual lifetime value. We determine a value of the radiative recombination coefficient B of 5 x 10^-10 cm³s^-1 and a value of 1 x 10⁸ s^-1 for the nonradiative coefficient A. While the B value is somewhat larger than GaAs based III-V materials, the A coefficient is much greater than high quality III-V's indicating that further improvement in growth quality is possible and of course desirable for device applications.

To date, the longest lived II-VI laser diode operated for about 500 hrs. This much be improved by an order of magnitude for commerical development. Using a confocal optical microscope, we have imaged the degradation process in II-VI laser strructrues. Dr. Eithne McCabe is the expert on the use of the confocal microscope. The results are given in the figures below. We observe the <100> oriented dark line defects as dark areas where non-radiative recombination dominates. We also observe a general brightening of the area illuminated in the spot. This uniform brightening is an athermal annealing effect which we believe to be a recombination enhanced defect reaction involving point defect in the active layer of the structure. This important result shows that there is considerable potential to improve the light emitting characteristics of II-VI layers.

Within the Department, Professor James Lunney produces GaN materials using pulsed laser deposition. We have begun a study of these materials to look at recombination and degradation effects in these layers.

There is already strong interaction with Optronics Ireland which is an Irish government sponsored initiative whose aim is to bring suitable optoelectronic research ideas to prototype stage and beyond. Professor John Hegarty is the technical director of Optronics Ireland.
 
 
 
 
 

 
 
 (a)                                                                        (b)

 (c)                                                                         (d)
 

PL confocal micrographs showing the progressive stages of degradation and annealing in a ZnCdSe/ZnSSe/ZnMgSSe separate confinement heterostructure: (a) t=0, (b) t=15 sec., (c) t=2 min., (d) t= 40 min.
 
 

For further information contact:

Dr. John Donegan
Department of Physics,
Trinity College,
Dublin 2,
Ireland.

jdonegan@tcd.ie

•  Japanese Patent H9-103153, Filed April 21, 1997. [J F Donegan, C Jordan, D T Fewer, E M McCabe, A Ishibashi, A Toda, M Shiraishi, Y Sanaka and N Noguchi.]

‘Defect Annealing in a II-VI laser diode structure under intense optical excitation’ Applied Physics Letters 72 (1998) 194-196 [ C. Jordan, D.T. Fewer, J.F. Donegan, E.M. McCabe, A. Huynh, F.P. Logue, S. Taniguchi, T. Hino, K. Nakano, and A. Ishibashi]

 ‘Optical gain in (Zn,Cd)Se-Zn(S,Se) quantum wells’, Journal of the Optical Society of America B, 15 (1998) 1295-1304 [F.P. Logue, P.Rees, J.F. Heffernan, C. Jordan, J.F. Donegan, J. Hegarty, F. Hiei, S. Taniguchi, T. Hino, K. Nakano and A. Ishibashi]

 ‘Optical gain in (Zn,Cd)Se-Zn(S,Se) quantum wells as a function of temperature’ Journal of Crystal Growth 184/185 (1998) 623-626 [ F.P. Logue, P. Rees, C. Jordan, J.F. Donegan, J. Hegarty, F. Hiei, S. Taniguchi, T. Hino, K. Nakano and A. Ishibashi]

 ‘Photoluminescence of localized excitons in pulsed-laser-deposition GaN’ Applied Physics Letters 73 (1998) 3390-3392 [ M. Cazzanelli, D. Cole, J.F. Donegan, J.G. Lunney, P.G. Middleton, K.P. O’Donnell, C. Vinegoni and L. Pavesi]

 ‘Pulsed laser deposition of GaN thin films’ Material Science and Engineering (in the press) [M. Cazzanelli, D. Cole, J. Versluijs, J.F. Donegan, and J.G. Lunney]

‘Optical gain and linewidth enhancement factor in bulk GaN’, Semiconductor Science and Technology 14 (1999) 517-520 [F. Renzoni, J.F. Donegan and C.H. Patterson]

 ‘Carrier density dependence of the photoluminescence lifetimes in ZnCdSe/ZnSSe quantum wells at room temperature’ Applied Physics Letters 74 (1999) 3359-3361      [ C. Jordan, J.F. Donegan, J. Hegarty, B. Roycroft, S. Taniguchi, T. Hino, E. Kato,  N. Noguchi, and A. Ishibashi]

 ‘Recombination lifetimes in undoped and doped ZnCdSe laser structures’ Society of Photo-Optical Instrumentation Engineers Proceedings 3625 ( invited paper, to be published, Apr. 1999) [ J.F. Donegan, C. Jordan, G. Laird, S. Taniguchi, T. Hino, E. Kato, N. Noguchi, and A. Ishibashi]

 ‘ Confocal photoluminescence microscopy in II-VI materials: annealing and degradation dynamics’ Society of Photo-Optical Instrumentation Engineers Proceedings Proceedings 3605 (1999) 65-72 [ E.M. McCabe, C. Jordan, D.T. Fewer, J.F. Donegan, S. Taniguchi, T. Hino, A. Ishibashi, P. Uusimaa, and M. Pessa]