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Correlation between quantum well morphology, carrier localization and the optoelectronic properties of GaInNAs/GaAs light emitting diodes
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2006 Semicond. Sci. Technol. 21 1047
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I NSTITUTE OF P HYSICS P UBLISHING S EMICONDUCTOR S CIENCE AND T ECHNOLOGY Semicond.Sci.Technol.21(2006)1047–1052doi:10.1088/0268-1242/21/8/011
Correlation between quantum well morphology,carrier localization
and the optoelectronic properties of GaInNAs/GaAs light emitting diodes
J M Ulloa1,4,A Hierro1,J Miguel-S′a nchez1,A Guzm′a n1,
J M Chauveau2,A Trampert2,E Tourni′e3and E Calleja1
1ISOM—Universidad Polit′e cnica de Madrid,Ciudad Universitaria s/n,E-28040Madrid,
Spain
2Paul-Drude-Institut f¨u r Festkoerperelektronik,Hausvogteiplatz5-7,D-10117Berlin,
Germany
3Universit′e Montpellier II,CEM2,UMR CNRS5507,F-34095Montpellier Cedex5,France
E-mail:ahierro@die.upm.es
Received21February2006,in?nal form8May2006
Published28June2006
Online at https://www.360docs.net/doc/d716545528.html,/SST/21/1047
Abstract
The impact of carrier localization on the optoelectronic properties of
GaInNAs/GaAs quantum well(QW)light emitting diodes(LED)grown by
molecular beam epitaxy with different QW morphologies is studied.The
QW morphology is determined by transmission electron microscopy,and is
directly related to carrier localization,which is quanti?ed with two different
approaches:analysing the broadening of the low energy side of the
photocurrent(PC)spectrum,and deriving a Stokes shift from
electroluminescence(EL)and PC spectroscopic measurements.Carrier
localization is found to be much stronger when the transition from a two-to
a three-dimensional(2D,3D,respectively)growth mode has started,
indicating that once the3D growth appears,carrier localization is dominated
by the resulting upper interface modulation or island formation,regardless
of the In and N contents.Nevertheless,within a given growth mode,2D or
3D,carrier localization increases with the alloy effective band gap
wavelength,i.e.,with the In and N contents.This effect is also related to the
QW morphology,because high In and N contents induce compositional
?uctuations,which are responsible for generating localization states in2D
QWs,and accelerate the transition to the3D growth mode.In3D
GaInNAs/GaAs QW LEDs,the EL emission is affected by carrier
localization even at room temperature,while in2D LEDs localization is only
relevant at very low temperatures.Indeed,LEDs with3D GaInNAs/GaAs
QWs have external quantum ef?ciencies which are determined by the
presence of localized carriers in states below the conduction band
edge.
4Present address:COBRA Inter-University Research Institute,Eindhoven
University of Technology,PO Box513,5600MB Eindhoven,The
Netherlands.
0268-1242/06/081047+06$30.00?2006IOP Publishing Ltd Printed in the UK1047
J M Ulloa et al
1.Introduction
Laser diodes based on GaInNAs quantum wells(QW) emitting around1.3μm have been quickly and successfully developed in the past few years[1–3],and the theoretically predicted advantages over the commonly used InGaAsP devices regarding high temperature performance have been demonstrated[4,5].In spite of the fast progress,very little is unambiguously known about the in?uence of QW morphology and carrier localization on device performance. The introduction of N has been shown to induce compositional ?uctuations in the QW that can provoke interface roughening and accelerate the transition from the two-to the three-dimensional(2D and3D)growth mode[6,7].All these features strongly affect the optical properties of the QW, and may be the origin of the commonly observed carrier localization effect[8,9].This last phenomenon has been extensively characterized by photoluminescence(PL) measurements in simple QWs,but it has not been de?nitely linked to the QW morphology.Moreover,investigations of its effect on device performance have started only very recently [10],and it has not been quanti?ed as a function of emission wavelength.Finally,if carrier localization is strong enough,it could affect device performance even at room temperature,as it does in other material systems,such as InGaN[11].
In this work,we compare the optoelectronic properties of light emitting diodes(LED)based on GaInNAs/GaAs QWs grown by molecular beam epitaxy(MBE)with different morphologies.Transmission electron microscopy(TEM) measurements show that the different QW structures can be classi?ed into three types:homogeneous2D QWs with no apparent alloy?uctuations,2D QWs with compositional ?uctuations and QWs with quasi-3D island formation.The impact of carrier localization on device performance is studied by measuring the electroluminescence(EL)and photocurrent (PC)spectra of the LEDs as a function of temperature,which allows the quanti?cation of the effect of carrier localization through the Stokes shift and the broadening of the PC spectra. The relative in?uence on carrier localization of the QW morphology on one side,and the In/N content on the other, is analysed in LEDs emitting between1.2and1.6μm.The different degrees of the impact of carrier localization on the external ef?ciency of the LEDs are studied as a function of the injected current,paying special attention to the effect of post-growth annealing.
2.Experimental procedure
The samples were grown by MBE on(001)GaAs substrates using Ga,As and In solid sources,and a radio-frequency plasma source for N.The active zone of all the samples consisted of a GaInNAs single QW with150–250nm GaAs barriers.The well width ranged from40to90?A for all the samples,and the In and N contents were nominally between 25%to35%and1%to3%,respectively.The QW was embedded in the intrinsic region of a p–i–n junction,except in sample2D-D(see section3.1),which did not have a p-doped layer.The Be and Si concentrations were2×1018cm?3, for the p+and n+regions,respectively.The QWs were grown at temperatures ranging from400to430?C,approximately,Table1.Well width,In and N contents and growth mode of all the analysed QWs.
Sample L w(?A)In(%)N(%)Growth mode
2D-A4031 2.12D
2D-B7030 1.72D
2D-C7531 2.22D
2D-D903532D
3D-A7025 1.13D
3D-B7030 1.33D
and the growth runs were monitored by in situ re?ection high-energy electron diffraction(RHEED).All samples were processed into circular200μm diameter mesa LEDs with Au/AuZn/Au(p-type)and Au/AuGe(n-type)ohmic contacts.
The EL spectra were measured under continuous-wave conditions at low currents,and under pulsed conditions(1μs pulse width and1%duty cycle)at high currents( 5mA) to avoid heating effects.The luminescence was dispersed with a1m monochromator and detected with a cooled Ge photodetector using a lock-in ampli?er.Light from a quart-halogen lamp dispersed through a1/4m monochromator was used as the excitation source for the PC spectroscopic measurements.TEM analysis was carried out with a JEOL 3010microscope operating at300kV.Finally,rapid thermal annealing(RTA)was performed using an AET furnace with the samples capped with GaAs and exposed to a N2atmosphere.
3.Results and discussion
3.1.Growth and structural analysis
The growth temperature is a crucial parameter that determines the morphology of the QW[6].A change in the growth mode as a function of the growth temperature has been previously reported in GaInNAs and GaAsN:for a given growth rate,low temperatures lead to smooth2D growth that turn into3D at higher growth temperatures[12].This2D–3D transition is preceded by an increase in interface roughness,likely induced by strong compositional?uctuations that may be originated by phase separation[7].This tendency for alloy decomposition is also enhanced for a given growth temperature by increasing the In and N contents[13].
The QWs analysed in this work were grown at different temperatures.Samples2D-A,2D-B and2D-C were grown at400?C and2D-D at410?C,while the growth temperature was increased in the other two(3D-A and3D-B)up to around 430?C,calibrated against the(2×4)/c(4×4)transition [14].This increase in the growth temperature was enough to induce the transition to the3D growth mode,as evidenced by the observed change from a streaky RHEED pattern in the low-temperature samples to a modulated or spotty one in the high-temperature ones.The growth mode of all the analysed samples is indicated in table1,in which3D means a rough top interface with quasi-island formation,but not a conventional quantum dot structure(see?gure1(c)).The well width measured by TEM and the In and N contents are also indicated in table1.The In content is the nominal one and the N content was derived by?tting the effective band gap
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Correlation between QW morphology,carrier localization and the optoelectronic properties
Figure 1
002
GaInNAs GaInNAs GaInNAs
g 002
10 n
10 nm GaInNAs
G (c)G a A s G a A s 5 nm a A
s G
a A s G a I n N A s 5 nm
10 nm g 002
(a )
(c )
Figure 1.Dark-?eld TEM images taken under chemical sensitive
g 002conditions of three analysed QWs showing the three different morphologies found.(a )Sample 2D-A:2D QW with weak
compositional ?uctuations.(b )Sample 2D-D:2D QW with strong compositional ?uctuations.(c )Sample 3D-A:3D QW.The growth temperatures were 400,410and 430?C for samples 2D-A,2D-D and 3D-A,respectively.
energy calculated with a two-band anti-crossing model to the one obtained from the PC spectra [15].The well width of the quasi-3D QWs was considered to be the average width.This
can introduce a higher error on the N content determination using the model,but,regardless of the accuracy of the deduced values,it is evident from the effective band gap wavelength that the N content is much smaller in the two quasi-3D QWs than in the 2D ones (see ?gures 3and 4).This conclusion is supported by the fact that the PL
peak energy is higher in the 3D samples at every temperature.
In order to determine accurately the QW morphology,all the samples were analysed by cross-sectional dark-?eld
N o r m a l i z e d i n t e n s i t y (a .u .)
Energy (eV)N o r m a l i z e d i n t e n s i t y (a .u .)
Energy (eV)
Figure https://www.360docs.net/doc/d716545528.html,parison of the room temperature PC and EL spectra
from the same LED,for samples 2D-A (homogeneous 2D QW)(a )and 3D-A (3D QW)(b ).The dashed lines on the top of the PC spectra are the resulting ?tted curves using the sigmoidal absorption formula.The vertical dashed lines represent the centre of the EL emissions,and the arrows represent the effective band gap energy obtained from the ?t to the PC spectra.
(This ?gure is in colour only in the electronic version)
TEM measurements with a g =002diffraction vector,sensitive to chemical composition.This sensitivity to chemical composition is expressed as a variation of the image contrast that is dependent on In–N concentrations.Figure 1shows the TEM images of three samples representing the three different morphologies found.A transition from a rather homogeneous 2D QW (?gure 1(a ))to a 2D QW with compositional ?uctuations (?gure 1(b )),and ?nally to a QW with quasi-3D island formation (?gure 1(c ))is observed.The two samples grown at low temperature maintained ?at interfaces.Although alloy homogeneity is not perfect over a large scale in sample 2D-A,it is much better than in 2D-D,which clearly shows a strong periodic contrast variation in the GaInNAs QW that corresponds to compositional ?uctuations.This difference is likely due to the slightly lower growth temperature and the smaller In and N contents of sample 2D-A.In contrast,the sample grown at high temperature (3D-A)shows a strong modulation of the top GaInNAs /GaAs interface,where island formation can be observed,indicating a 3D growth mode.The quasi-3D islands were more marked in sample 3D-B,grown at high temperature with higher In content.Samples 2D-B and 2D-C,grown at low temperature,presented an intermediate situation between 2D-A (weak compositional inhomogeneities on a large scale)and 2D-D (strong compositional ?uctuations),but always with ?at QW interfaces.
3.2.Broadening of the PC spectrum and carrier localization In order to analyse how carrier localization is linked to the QW morphology and how it in?uences device performance,room temperature PC and EL spectra were measured from the processed LEDs.Figure 2shows the pulsed mode EL at 16A cm –2and the PC of the same diode for the devices processed from samples 2D-A and 3D-A (see ?gures 1(a )and (c )).Both the EL and PC spectra are normalized.The full width at half maximum (FWHM)of the EL is 28and 86meV for the 2D and 3D LEDs,respectively,i.e.,the EL from QWs grown under 2D conditions is much narrower.This
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J M Ulloa et al
1100
120013001400
1500
1600
481216202428
?E (m e V )
PC effective band gap (nm)
Figure 3.Broadening parameter of the PC spectra at room
temperature as a function of the PC effective band gap for all the analysed LEDs.
dependence of the EL emission on the QW growth mode agrees well with that observed in PL,where the narrower spectra are obtained from 2D samples [16].
The analysis of the PC spectrum can provide further evidence of the large differences between 2D and 3D LEDs,and allows the study of the carrier localization effect through the broadening of the low energy side of the spectra.The absorption edge can be analysed including the effects of broadening by ?tting the sigmoidal absorption formula [17]to the measured PC spectrum.This formula is given by
α(E)=α0
1+exp E g ?E
E ,(1)which is characterized by an energy E g identi?ed as the ‘effective band gap’of the alloy,and a broadening parameter E .As shown in ?gure 2,this expression provides a good ?t to the experimental data (note that only the ?rst absorption edge is ?tted in the case where there are more than one).The effective band gaps of samples 2D-A and 3D-A derived from the ?ts are 986meV (1258nm)and 1057meV (1173nm),respectively,and are indicated with an arrow in ?gure 2.The values obtained for the absorption edge broadening E are 9and 23meV in 2D-A and 3D-A,respectively.This difference can be explained to result from the greater tailoring of the density of states for the 3D QW caused by the presence of a higher density of localized states below the alloy conduction band edge,likely due to the quasi-3D island formation.
Figure 3shows the value of E as a function of E g for all the analysed samples.It is clear that E is much higher in the two samples grown at high temperature,i.e.,in the 3D LEDs.Moreover,this happens independently of the alloy effective band gap,and thus independently of the In and N contents,as evidenced by the fact that the two 3D samples have shorter effective band gap wavelength than the 2D ones.An increase in the Stokes shift (indicating an increase of the magnitude of carrier localization)with the N content has been reported [18].Nevertheless,a QW with low In and N contents can present a much stronger carrier localization than another with much higher In and N contents grown at lower temperature if the former has been grown under a 3D regime.This implies that,1160
1200
1240
1280
1320
13601400
-20
-100102030
40
S t o k e s s h i f t (m e V )
PC effective band gap (nm)
Figure 4.Stokes shift at room temperature as a function of the PC effective band gap for all the analysed LEDs.Negative values
indicate the absence of Stokes shift and the presence of band ?lling.
once the 3D growth appears,carrier localization is strongly increased,regardless of the In and N contents.
From ?gure 3it also seems that,for a given growth mode (2D or 3D),carrier localization increases with In and N contents,as indicated by the increase in E with the alloy effective band gap wavelength.This tendency within a given growth regime can also be related to the QW morphology,because high In and N contents enhance phase separation and facilitate the formation of compositional ?uctuations [13]. E doubles its value from the homogeneous 2D QW 2D-A (?gure 1(a ))to the 2D QW with strong compositional ?uctuations,2D-D (?gure 1(b )).This result directly links the magnitude of carrier localization In GaInNAs QW LEDs to compositional inhomogeneities.Further evidence of the relation between QW morphology and the in?uence of carrier localization on device performance can be obtained from the comparison of the EL and PC spectra,which we analyse next.3.3.Stokes shift and carrier localization
An appropriate parameter to quantify carrier localization in an LED is the Stokes shift [19],de?ned as the energy difference between the effective band gap obtained from the PC spectrum and the EL peak energy.In this analysis,the EL must be measured under low injection currents,in order to not saturate the localized states [10].In our case,no room temperature EL could be detected below 16A cm –2in the less ef?cient sample,so that value was used in all of them for proper comparison.If lower current densities were used,the value of the obtained Stokes shift could be slightly higher,but it does not change the qualitative results obtained in this work.
Figure 4shows the Stokes shift at room temperature as a function of the PC effective band gap for all the samples except 2D-D,for which EL cannot be measured because it is not a p–i–n structure.In this graph,the negative values indicate that there is no Stokes shift.Again,a clear difference is obtained between the samples depending on the QW morphology.In all the 2D samples,the EL peak energy is above the PC gap energy,as is the case in a QW LED when carrier localization is not present.This is due to the fact that in
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Correlation between QW morphology,carrier localization and the optoelectronic properties the PC measurements electron–hole pairs are created in the
minima of the unpopulated conduction and valence bands, while in the EL measurement,due to the injected current, there is a certain band?lling level in both bands which shifts the peak of the spectrum to higher energies.In contrast, in the3D samples a Stokes shift is obtained,which means that the effect of carrier localization can be observed in GaInNAs devices working at room temperature.The values of the Stokes shift are small compared,for example,with those measured in InGaN LEDs[20],but demonstrate that carrier localization can in?uence device performance of a GaInNAs LED even at room temperature.This seems to be possible only in devices based on a3D QW.As was the case for E,the Stokes shift seems to increase with the emission wavelength,or with the In and N content,although more samples must be analysed to con?rm this tendency.The results obtained from the analysis of the Stokes shift agree well with those previously deduced from E,con?rming that carrier localization is much higher in samples with a strong modulation at the upper interface than in those with?at interfaces,even when the latter have higher In and N contents and show longer emission wavelengths.
Although carrier localization is not observed in2D LEDs working at room temperature,it could be detected at low temperature,when carriers have less thermal energy and cannot escape from the potential minima.Figure5(a)shows the Stokes shift as a function of temperature for samples2D-A and3D-B(the latter after an RTA treatment at850?C for 15s),both with a similar band gap energy(～1eV).In the2D case a Stokes shift appears below93K,indicating that the EL emission is dominated by localized states only in that range of low temperatures.In addition,the Stokes shift at17K is only12meV,which means that in this sample the localized states are close in energy to the conduction band edge and/or the density of localized states is small and consequently most of them are?lled at16A cm–2[10].In the3D case,there is a Stokes shift at all temperatures,with values as high as80meV at low temperatures,indicating a much stronger carrier localization effect than in the2D case.There is an initial small increase of the Stokes shift with temperatures up to 100K,due to thermal activation of carriers that can reach the deepest potential minima(in connection with the minimum of the‘S’-shaped curve of the luminescence peak energy versus temperature)[16].For higher temperatures,there is a decrease of the Stokes shift due to progressive carrier population of higher energy localized states.
Figure5(a)also shows the Stokes shift at room temperature for sample3D-B as-grown.The magnitude of the Stokes shift decreased as a result of annealing.Indeed, at room temperature there is almost no Stokes shift in the annealed sample.If this difference is extrapolated to the low-temperature region,values of the Stokes shift as high as 110meV are obtained for the as-grown LED.The same result was obtained for sample3D-A,for which the room temperature Stokes shift was reduced down to2meV after annealing. This clearly indicates that carrier localization is reduced after annealing,in agreement with what has been reported before from PL measurements[16].Although the reason for this is still not completely clear,it could be due to the homogenization of the In and N contents after annealing,an effect which has been demonstrated in2D QWs[7].The driving force for this
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T (K)
~
I
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η
t
x
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Figure5.(a)Stokes shift as a function of temperature for samples 2D-A and3D-B.(b)Low temperature L/I ratio(proportional toηext) as a function of I for samples2D-A and3D-B as-grown.
redistribution is related to the reduction of the local strain, which is generated by the large differences in the atomic radii between Ga and In,and As and N,respectively.
The Stokes shift at50K is eight times larger in the 3D sample than in the2D one.This strong difference in the magnitude of carrier localization at low temperatures is expected to have some effect on the ef?ciency of the device. This becomes clear from the analysis of the integrated EL (L)as a function of the injected current(I).The ratio L/I is proportional to the external quantum ef?ciency(ηext) and its dependence on I is modi?ed if carrier localization is present[10].Figure5(b)shows the L/I ratio versus I at17K,in a logarithmic plot,for the homogeneous2D sample2D-A and the3D sample3D-B.The shape of the 2D curve is close to the typical one of a QW LED,with the initial increase ofηext due to the gradual increase in the radiative to non-radiative recombination rate,and the tendency to saturate at higher I.This initial increase is more pronounced in the3D QW,as a result of the larger non-radiative recombination[21].However,at high currents,the variation of the ef?ciency with I is remarkably different in the two cases.While the ef?ciency of the2D LED decreases slightly with I,this decrease is much more pronounced
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in the3D LED,as a result of the much stronger carrier localization in the3D sample.This effect can be explained as follows.As the localized states are more ef?cient in competing with non-radiative recombination centres,the transition from luminescence dominated by localized states to that dominated by the less ef?cient band edge states implies a reduction in the ef?ciency of the device.As the density of both localized states and non-radiative recombination centres is smaller in the 2D sample,this effect is small in the2D LED.Nevertheless, the carrier delocalization effect strongly affects the ef?ciency of the3D LEDs at low temperatures,making it strongly dependent on the injected current.It should be noted here that in spite of the strong carrier localization in the3D LEDs, the2D LEDs are more radiatively ef?cient,as shown in ?gure5(b),so2D QWs are preferred for device applications.
4.Conclusions
The impact of carrier localization on the optoelectronic properties of GaInNAs/GaAs QW LEDs has been shown to be highly dependent on the morphology of the QW observed by TEM.Particularly,the growth mode seems to be a major factor.Carrier localization is much stronger in devices based on a quasi-3D QW than those in which the QW maintains ?at interfaces,even if the latter have higher In and/or N contents and thus longer emission wavelength.This implies that,once the3D growth appears,carrier localization is dominated by the resulting interface roughness or island formation.On the other hand,if the In/N contents are increased within a given growth mode,2D or3D,carrier localization increases.This is the result of the enhanced alloy decomposition for higher In and N contents in the2D case,which in turn facilitates the transition to the3D growth mode.
In3D GaInNAs/GaAs QW LEDs,the EL emission is affected by carrier localization even at room temperature,as evidenced by the presence of a Stokes shift.In contrast,in2D LEDs localization is only relevant at very low temperatures, the effect still small compared to the3D case,in which the external ef?ciency of the LED is strongly changed by the presence of the localized states.A comparative analysis of as-grown and annealed3D QWs indicates that RTA reduces the Stokes shift and can thus be used to reduce carrier localization in GaInNAs/GaAs QW devices.Acknowledgments
The authors would like to thank Maite Perez and David Lopez for the processing of the samples.This work has been supported by the European Union,project no.IST-2000-26478-GINA1.5,by Comunidad de Madrid and by the Spanish Ministerio de Ciencia y Tecnolog′?a,project TIC2001-3849. References
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