Precision, all-optical measurement of external quantum efficiency in semiconductors

Abstract

External quantum efficiency of semiconductor photonic devices is directly measured by wavelengthdependent laser-induced temperature change (scanning laser calorimetry) with very high accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with an independent measurement of power-dependent temperature or power-dependent photoluminescence. Time-resolved photoluminescence lifetime and power-dependent photoluminescence measurements are used to evaluate unprocessed heterostructures for critical performance parameters. The crucial importance of parasitic background absorption is discussed.External quantum efficiency (EQE or g ext ) is an important parameter that characterizes many photonic devices. It is widely used to evaluate light emitting diodes, photovoltaics, semiconductor lasers, and emerging technology such as laser-induced refrigeration of solids. 1 The essential idea is to have a single coefficient that accounts for both the efficiency of the photon-electron conversion process (internal quantum efficiency) and the efficiency of moving light into and/or out of the device (coupling efficiency). Internal quantum efficiency is deleteriously affected when electronic excitations lose energy through the production of heat. This nonradiative recombination can be mediated by phonons, interfaces, surfaces, dislocations, defects, and even other charge carriers. The light coupling efficiency is driven by Fresnel reflection and the condition of total internal reflection. This leads to photon recycling and the concomitant production of wasteful heat due to the presence of parasitic background absorption. It is often difficult to model or even anticipate how the various disparate mechanisms degrade performance. Instead, empirical data should guide device development. It is therefore crucial to have in place an experimental scheme for precision measurement of efficiency. This allows such problems to be identified and addressed systematically.There will be different descriptions of EQE and varying design goals depending on the application. A high performance solar cell, for example, must make maximum use of the available broadband spectrum and the resulting photoexcitations to produce an external current. A single color LED design, on the other hand, strives to couple the generated narrow-spectrum of light from the device. In this paper, we present a procedure for precision quantification of photoluminescence external quantum efficiency in bulk GaAs heterostructures. Device characterization is performed in the context of optical refrigeration (laser cooling in solids), where the demands on EQE are extreme. For this applica-tion, EQE is defined as the fraction of photo-excited electron-hole pairs which produce luminescence photons that escape the device into free-space. It has been established that laser-induced cooling of GaAs will occur only when EQE exceeds 99%, i.e., far greater than needed for useful operation of other semiconductor photonic devices. 2 In addition, it becomes p...

Observation of suppressed Auger mechanism in type-I quantum well structures with delocalized electron-hole wavefunctions

“…The radiative recombination coefficient (B) at room temperature was calculated analytically based on van Roosbroeck-Shockley relationship 19,20 and it varies as T −3/2 with the temperature. [20][21][22] The calculation of B is in close agreement with the average experimental values, confirming that the analytical evaluation is robust and valid as reported by Ref. 18.…”

Cryogenic optical refrigeration

“…This effect limits the possibility of laser cooling of rare-earth-doped solids at temperatures below about 50 K. In principle this limit does not exist for laser cooling of semiconductors whose electrons and holes are indistinguishable and which thus obey Fermi-Dirac statistics. The feasibility of laser cooling in semiconductors has been extensively investigated both theoretically [17,[51][52][53][54][55][56][57][58][59][60][61] and experimentally [61][62][63][64][65][66][67][68][69]; however, no net temperature reduction has been observed yet. This failure is due to stringent purity requirements, complications associated with inefficient light extraction from the high-refractive-index substrate (η e < 0.2 for nearly index-matched dome [17,66]), and many-body effects such as a carrier-density-dependent quantum efficiency.…”

Thermal imaging with high spatial and temperature resolution

“…11 Pump luminescence does not contribute significantly to the detected signal due to the counterpropagating geometry, in which case the pump-induced fluorescence is absorbed in the sample, when monitored in transmission. In the current proof-of-principle implementation of the experiment, scan is automated only along one of the directions, while the scan in the orthogonal direction is performed manually by translating a focusing lens by calibrated distances.…”