Abstract
In this paper we report on the optical properties of a series of InGaN polar quantum well structures where the number of wells was 1, 3, 5, 7, 10 and 15 and which were grown with the inclusion of an InGaN Si-doped underlayer. When the number of quantum wells is low then the room temperature internal quantum efficiency can be dominated by thermionic emission from the wells. This can occur because the radiative recombination rate in InGaN polar quantum wells can be low due to the built-in electric field across the quantum well which allows the thermionic emission process to compete effectively at room temperature limiting the internal quantum efficiency. In the structures that we discuss here, the radiative recombination rate is increased due to the effects of the Si-doped underlayer which reduces the electric field across the quantum wells. This results in the effect of thermionic emission being largely eliminated to such an extent that the internal quantum efficiency at room temperature is independent of the number of quantum wells.
Highlights
There have been several recent investigations [1,2,3,4,5,6,7,8,9,10,11,12,13,14] into the effects of the inclusion of layers grown prior to the first quantum well (QW) in InGaN/GaN light-emitting diodes (LEDs) and in structures for photoluminescence (PL) experiments
It has been shown that the Fermi level pinning caused by n-type doping can be used to manipulate the strength of this field close to the surface of a GaN structure [12,13,14,16,17], and that this affects the emission properties of QWs grown in this region through modifications to the quantum confined Stark effect (QCSE) [12,13,14,17]
It has been suggested that the efficiency improvements reported for structures containing ULs are due to the increased electron-hole wave function overlap leading to an increased radiative recombination rate following a decrease in the electric field across the QWs [11,12,13]
Summary
There have been several recent investigations [1,2,3,4,5,6,7,8,9,10,11,12,13,14] into the effects of the inclusion of layers grown prior to the first quantum well (QW) in InGaN/GaN light-emitting diodes (LEDs) and in structures for photoluminescence (PL) experiments. It has been suggested that the efficiency improvements reported for structures containing ULs are due to the increased electron-hole wave function overlap leading to an increased radiative recombination rate following a decrease in the electric field across the QWs [11,12,13]. This effect can be overcome in multiple quantum well (MQW) structures due to the recapture of the photo-excited carriers by other QWs in the stack [20] It is the purpose of this paper to determine whether the effects of thermionic emission can be overcome by the inclusion of ULs which lead to a reduced radiative recombination rate to such an extent that the thermally driven escape of carriers can be negated
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