Effects of optical coupling in III-V multilayer systemsCarsten Baur, Martin Hermle, Frank Dimroth, and Andreas W. Bett Citation: Applied Physics Letters 90, 192109 (2007); doi: 10.1063/1.2737927 View online: http://dx.doi.org/10.1063/1.2737927 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Temperature dependence of defect-related photoluminescence in III-V and II-VI semiconductors J. Appl. Phys. 115, 012010 (2014); 10.1063/1.4838038 Effect of barrier layers on electroluminescence from Si/SiOxNy multilayer structures Appl. Phys. Lett. 102, 081114 (2013); 10.1063/1.4794079 Exact solution of the spectrum and magneto-optics of multilayer hexagonal graphene J. Appl. Phys. 110, 013725 (2011); 10.1063/1.3603040 Probing and modulating surface electron accumulation in InN by the electrolyte gated Hall effect Appl. Phys. Lett. 93, 262105 (2008); 10.1063/1.3062856 Uncooled photodetectors for the 3 – 5 μ m spectral range based on III–V heterojunctions Appl. Phys. Lett. 89, 083512 (2006); 10.1063/1.2337995

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Effects of optical coupling in III-V multilayer systemsCarsten Baur,a� Martin Hermle, Frank Dimroth, and Andreas W. BettFraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany

�Received 28 February 2007; accepted 17 April 2007; published online 8 May 2007�

A method to visualize and investigate radiative recombination processes in compoundsemiconductor materials by utilizing the effect of optical coupling in III-V multilayer systems ispresented. For this purpose, a semiconductor material of interest is grown on an activatedgermanium �Ge� substrate which then serves as a photodiode. By means of spectral responsemeasurements of the Ge photodiode, a response signal from the upper layers can be detected. It isproven both by experiment and by modeling that the signals from these layers can only be explainedby optical transport mechanisms, i.e., radiative recombination. © 2007 American Institute ofPhysics. �DOI: 10.1063/1.2737927�

The influence of radiative recombination processes inIII-V semiconductor materials becomes more and more a de-sign criteria for the optimization of device performance.1

Since most of the commercially used III-V materials are di-rect semiconductors, radiative recombination is the dominat-ing recombination process if the material is of high purityand crystal perfection. With the continuous improvements ofthe manufacturing processes the electrical properties of thematerials are often close to their theoretical limits. Photonrecycling �PR� and optical coupling effects, therefore, playan important role when analyzing the electrical properties ofthe semiconductor materials.2

We make here the distinction between PR processes onthe one hand and optical or radiative coupling on the otherhand. PR is understood as the process in which a photoncreated by radiative recombination of an electron-hole pair isreabsorbed in one and the same layer generating anotherelectron-hole pair. For electrical simulations and minoritycarrier lifetime analysis of double heterostructures this effectis often considered by introducing an average lifetime en-hancement factor �PR.3,4 However, in some cases, this sim-plification is not accurate enough because of the spatial de-pendence of the photon recycling.5 In contrast to that, opticalor radiative coupling describes processes where photons gen-erated by radiative recombination processes are reabsorbedin another layer surrounding the radiating material.

The principles of both processes are the same and wereaddressed by Dumke.6 He already showed that PR results inextended minority carrier lifetimes observed predominantlyin direct semiconductor materials. The positive effects of PRon electrical performance were demonstrated especially fordevices such as double heterostructures3,4 or solar cells 7,8

but also the properties of LEDs or lasers are affected by PR.Although, the process of PR is extensively analyzed

from a theoretical point of view,3,8–10 the impact itself is stillhard to detect and to measure, especially in a quantitativemanner. The same is in principle true for optical couplingeffects. In most of the investigations of PR phenomena pub-lished, contributions from layers other than the active onehave always been neglected since they were small. Only re-cently, it was demonstrated that optical coupling effects can

have a considerable impact on the performance or the behav-ior of certain devices.1,2 It was shown, for example, that thephotocurrent generation within a GaAs single-junction solarcell could only be modeled correctly by taking into accountthe optical coupling between the cell and the underlyingGaAs substrate.2 Nevertheless, this contribution was againquite small.

The method presented in this letter is intended to analyzeradiative recombination processes in compound semiconduc-tor layers grown on top of a low band gap photodiode. Itallows for investigating the PR effect and its dependency onparameters such as growth properties, material parameters,doping levels, or bias conditions. This leads directly to im-portant information about the quality of the grown materials.

The redistribution of excited minority carriers due to PRor optical coupling only happens from a material with ahigher band gap to a material with an equal or lower bandgap. Also, the higher band gap material must not be domi-nated by nonradiative recombination processes such asShockley-Read-Hall recombination and must not comprise acurrent sink such as a pn junction. Thus, an ideal structure toinvestigate radiative recombination processes is shown inFig. 1. A pn junction is formed in a Ge substrate by diffusionof As or P leading to an active Ge photodiode. On top of theGe diode, a double heterostructure of Ga0.5In0.5P andGa0.99In0.01As layers is grown lattice matched to the under-lying substrate. All these layers are n doped. The lowerGaInP layer acts as a barrier to prevent diffusive transport ofminority carriers from the GaInAs into the Ge photodiode.This structure was grown using an AIX2600 G3 metal or-ganic vapor phase epitaxy reactor with the typical sources,Ga�CH3�3, Al�CH3�3, In�CH3�3, SiH4, AsH3, and PH3.

a�Present address: European Space Agency �ESTEC�, Keplerlaan 1, 2200AG Noordwijk ZH, The Netherlands.

FIG. 1. Layer structure to investigate radiative coupling of photons ab-sorbed and reemitted in the upper GaInAs and GaInP layers and reabsorbedin the underlying Ge photodiode.

APPLIED PHYSICS LETTERS 90, 192109 �2007�

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The Ge photodiode was characterized using the spectralresponse measurement setup of the Fraunhofer ISE.11 In thissetup, the Ge photo cell is irradiated with a mixture ofchopped monochromatic light generating an ac signal and abroad spectrum from a halogen bias lamp. Electron-holepairs that are excited in the Ge can be separated by the diodeand detected as an external current. Through the use oflock-in techniques the sensitivity of the Ge diode to mono-chromatic light can be separated.

Figure 2 shows the result of the external quantum effi-ciency �EQE� measurement of the structure given in Fig. 1�open symbols�. The measurement clearly reveals a nonzerosignal for photons of wavelengths �890 nm. Due to the highabsorption coefficients in the range of 104–106 cm−1 evenclose to their band edges12 and taking into account theirthicknesses these photons are completely absorbed in the up-per GaInP ���780 nm� and GaInAs ���890 nm� layers.Thus, without the effect of optical coupling these photonswould be lost and would not be contributing to the externalquantum efficiency of the Ge diode. However, since the up-per layers are directly optically coupled to the Ge photodi-ode, a significant amount of the photons can be recycledpredominantly through radiative recombination in GaInAsand reabsorption of the light in the Ge cell. Therefore, theexternal quantum efficiency in the wavelength range between400 and 890 nm is a measure for the material quality of theGaInAs layer.

One sample was irradiated with 1 MeV electrons at afluence of 1�1015 cm−2. This causes the formation of de-fects in the crystal which act as nonradiative recombinationcenters decreasing the Shockley-Read-Hall �SRH� lifetime�SRH. Eventually, �SRH dominates the effective minority car-rier lifetime �eff given by

1

�eff=

1

�rad+

1

�SRH+

1

�Auger, �1�

where �rad is the radiative lifetime and �Auger is the Augerlifetime. �SRH includes both nonradiative recombinations inthe bulk and at the interfaces.

The EQE of the Ge diode after electron irradiation isalso given in Fig. 1 �closed symbols�. Obviously, there is noPR signal remaining in the short wavelength region below890 nm. Radiative recombination processes are completelymasked by nonradiative processes. However, the Ge photo-diode itself shows almost no degradation at all. This is con-

sistent with observations made by other groups, that Ge isvery robust to high energy particle irradiation.13

To verify these conclusions and to prove that minoritycharge carriers are effectively blocked by the lower GaInPbarrier layer, theoretical simulations were performed. There-fore, the structure given in Fig. 1 was modeled using a simu-lation tool for III-V compound semiconductors developed atFraunhofer ISE.2 It is based on a synthesis of works pub-lished by Miller,9 Parks et al.10 and Balenzategui et al.8

Figure 3 shows the energy band diagram and the quasi-Fermi potentials of the structure as a function of the devicedepth under short circuit conditions. For the modeling GaAswas used instead of Ga0.99In0.01As as the material parametersare more precisely known and the error seems to be negli-gible due to the small amount of In incorporated in theGa0.99In0.01As layer of our sample �see Fig. 1�. Also the layerthicknesses were changed for simplification without affectingthe main result of this analysis, which is the shape of theband structure in the vicinity of the second GaInP barrierlayer at a depth of about 3 �m. The barrier height for holesshould be sufficient to prevent the diffusion of minoritycharge carriers from the GaAs layer into the Ge. The samestructure was then used also to model the internal quantumefficiencies �IQE� neglecting reflection losses at the devicesurface.

The simulation results for the IQE of the investigatedstructure is shown in Fig. 4 for different SRH lifetimes �SRHin the GaAs layer. As expected from the experimental results,

FIG. 2. EQE of the Ge photodiode before and after irradiation with 1 MeVelectrons at a fluence of 1�1015 cm−2.

FIG. 3. Energy band diagram as a function of the device depth.

FIG. 4. IQE of the device structure in Fig. 3 as a function of the SRHlifetimes in the GaAs layer. Also shown are calculations ignoring opticalcoupling labeled as “w/o PR.”

192109-2 Baur et al. Appl. Phys. Lett. 90, 192109 �2007�

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the magnitude of the radiative recombination signal between400 and 890 nm is a function of the nonradiative SRH life-time. In the worst case, assuming a SRH lifetime of only1 ns, no radiative recombination signal can be found any-more. Figure 4 also shows two calculations where the opticalcoupling has been ignored. In this case the simulation revealsnearly no response in the wavelength region between 400and 890 nm, independent of the assumed SRH lifetime. Thisclearly proves that the non-zero IQE and EQE signals in thiswavelength range between 400 and 890 nm are assigned tooptical coupling between the upper layers and the Ge diode.

With the signals in the wavelength range between 400and 890 nm being then a function of the nonradiative minor-ity carrier lifetime in the double heterostructure layers, con-clusions about the material quality of these layers can bedrawn. Even a quantitative analysis of the material qualityshould be feasible through a comparison with theoreticalsimulations. Therefore, also the influence of parameters suchas the surface recombination velocity, layer thickness, biaslight intensity, bias voltage, or doping concentration whichare known to have an impact on either the radiative recom-bination or the contribution of nonradiative processes has tobe investigated.4,5,14

In summary, a method was presented that allows for de-termining radiative recombination processes utilizing opticalcoupling between different semiconductor layers. Thereby, adouble heterostructure is grown on a Ge photodiode that isused as a detector for the emitted photons from the doubleheterostructure. From spectral response measurements the ef-fectiveness of radiative recombination processes as a func-tion of incident wavelength can be identified. This method

can also be used for a quality assessment of semiconductormaterials.

This work has been supported by the European Commis-sion through the funding of the project FULLSPECTRUM�Reference No. SES6-CT-2003-502620�.

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192109-3 Baur et al. Appl. Phys. Lett. 90, 192109 �2007�

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