Segregation effects and bandgap engineering in InGaN quantum-well heterostructures
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Segregation effects and bandgap engineering in InGaN quantum-well heterostructures Kirill A. Bulashevich, Sergey Yu. Karpov, Roman A. Talalaev, Igor Yu. Evstratov, Yuri N. Makarov 1 Soft-Impact Ltd, P.O.Box 33, 194156 St. Petersburg, Russia 1 STR Inc, P.O.Box 70604, Richmond, VA 23255, USA ABSTRACT The analysis of In surface segregation and its impact on the composition profile and light emission spectra of the InGaN single quantum well heterostructures grown by Metalorganic Vapor Phase Epitaxy (MOVPE) is carried out by coupled solution of the Poisson and Schrödinger equations. Effective methods of controlling the composition profile, indium predeposition and temperature ramping during the cap layer growth are considered in terms of surface segregation model. General trends in spectra transformation upon the forward bias variation and their correlations with the quantum well electronic structure are discussed. INTRODUCTION Recently, In surface segregation has been recognized as a factor critical to control of the composition profiles in III-nitride light emitting diode (LED) quantum well (QW) heterostructures. Much effort was made to find correlations between the growth recipes, segregation effects, and emission characteristics of the grown diodes. Along with variation of growth parameters (temperature, pressure, precursor flow rates, etc.), specific procedures – growth interruption at the QW interfaces [1], indium predeposition [2,3], and temperature ramping during barrier or cap layer growth [4] – were suggested to improve the composition profiles in InGaN QWs, normally serving as active regions in blue and green LEDs. Recent theoretical studies [5,6] considered basic mechanisms of In surface segregation in the multi-layer structures grown by MOVPE and some approaches to control of the composition distributions in the InGaN QW. However, the interrelation between the segregation effects and the characteristics of light emitted from the LED structures still remains poorly understood. In this paper, we report on the quantum-mechanical study of segregation effects on the composition profiles and emission spectra of MOVPE-grown InGaN single-quantum-well (SQW) structures. Special attention is given to the most effective ways of controlling the front and back QW interfaces – indium predeposition and temperature ramping during the cap layer growth. CONTROL OF COMPOSITION PROFILE IN InGaN-SQW HETEROSTRUCTURES Consider a simple heterostructure that consists of an undoped InGaN SQW between n- and pdoped thick GaN layers, grown by MOVPE at 730°C in a vertical rotating-disk reactor under the conditions reported in [1]: pressure of 200 Torr, flow rates of TMGa, NH3 , and N2 (carrier gas) of 10.8 µmole/min, 406 mmole/min, and 196 mmole/min, respectively. TMIn flow rate was adjusted
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to obtain steady-state indium composition of 0.27. We will distinguish between (i) the ideal structure, which can be projected from the steady-state calibration of the precursor flow rates, (ii) the real structure with profile affect
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