Growth of GaNxAs12x atomic monolayers and theirinsertion in the vicinity of GaInAs quantum wells

M. Le Du, J.-C. Harmand, K. Meunier, G. Patriarche and J.-L. Oudar

Abstract: The deposition of N-rich GaNAs atomic monolayers was investigated. Such layers weresuccessfully grown while exposing a GaAs surface to a nitrogen plasma source during a growthinterruption at 400 8C in a molecular beam epitaxy reactor. N accumulation was confirmed andevaluated by secondary ion mass spectroscopy. This process is compatible with regrowth, as in situmonitored by reflection high-energy electron diffraction. The crystal shows good structural quality,as displayed by transmission electron microscopy, that reveals that the accumulation occurredwithin 1 nm. In a series of samples, two of these ultrathin GaNAs layers were inserted in GaAsbarriers, on each side of a GaInAs quantum well (QW). A drastic effect of the N-rich layers on theQW photoluminescence (PL) intensity was observed, as well as on the carrier recombinationdynamics, with a strong influence of the spacer thickness between the QW and the N-rich layers.A time-resolved PL analysis of these samples evidenced nonradiative relaxation times in the rangeof a few ps. This very short carrier lifetime is attributed to the presence of nonradiative centresrelated to the N-rich layers close to the QW, and can be used to design ultrafast optical devices.

1 Introduction

Fast recovery time is a key parameter for the design of newhigh transmission rate optical devices, like ultrafast opticalswitches for telecommunication applications. Opticalswitches can consist of semiconductor saturable absorbersthat regenerate the optical signal by excitonic absorption ina QW-based structure. To reach high transmission capacity,the essential parameter for saturable absorbers is therecovery time after absorption, which is governed by thecarrier recombination rate and is typically around a few nsfor semiconductors. To prepare the next generation ofsystems, possibly operating at 40Gbit=s; it is necessary tobring this recombination rate down to 5 ps. To shorten theresponse of a device, several methods were proposed. Fastrecovery is usually achieved through rapid nonradiativecarrier recombination on localised mid-gap states created bycrystal defects. Those point defects are usually generatedby various means, like low-temperature epitaxial growthð200 �CÞ [1, 2] or heavy ion irradiation [3]. In theseapproaches, defect density and spatial localisation cannot beaccurately controlled, especially in ion irradiation, whichcreates defects throughout the whole structure. It has beenreported that the addition of N shortens the carrier lifetimesof III – V alloys, due to the presence of traps andnonradiative centres related to nitrogen [4–6]. This shortrecombination rate could be attractive to design structuresfor high-speed optical devices. Here we propose the designof an active optical layer using III–V–N alloys to monitorthe recombination rate of photocarriers generated in a QW.As compared to the previous approaches, we expect several

advantages: (i) ease of production, the defects being createdduring the epitaxial growth (no extra technological stepneeded); (ii) excellent control of defect localisation in thegrowth direction (resolution in the nm range) for maximumefficiency; and (iii) easy adjustement of defect density bycontrolling the N concentration.

In this paper, we present a study of a structure whereGaNAs ultrathin layers (GaNAs UTLs) are used in order toshorten the recovery time of optical absorption in a QW.For an N composition of 2% and no rapid thermal annealing,we measured relaxation times of 50 ps in a GaInNAs QW.To bring this relaxation time down to 5 ps, morenonradiative centres need to be created. However, theintroduction of N in a QW also results in a drasticbroadening of its absorption edge, due to a complex bandtailing. Moreover, the bandgap of the QW material is indeedvery dependent on the N composition [7]. For these reasons,it is desirable to design a structure where the QW absorptioncharacteristics and the carrier recombination dynamics canbe controlled independently. Hence, we studied thedeposition of N-rich GaNAs UTLs close to a GaInAs QW.

2 Experiments and results

The samples under investigation were grown on semi-insulating GaAs (100) substrates in a solid-source molecularbeam epitaxy reactor. The growth chamber was equippedwith a radiofrequency (RF) plasma cell, used to generateactive N species from pure (7N) injected N2 gas. Constantplasma conditions were used in the following experience(0.25 sccm N2 flow, 600 W RF power). However, the activeN flux could be adjusted with a valve between both plasmaand growth chambers. Ga and In fluxes were supplied fromconventional effusion cells and As, in the form of As2; wassupplied from a cracker source. The GaAs growth rate was0:25 nm=s: GaNAs UTLs were grown at low temperature,typically 400 �C;which is usual for dilute nitrides in order toavoid GaN-phase segregation, as follows: the Ga and Asshutters were closed and the GaAs surface was exposed toactive N species for 10 s.

q IEE, 2004

IEE Proceedings online no. 20040889

doi: 10.1049/ip-opt:20040889

The authors are with CNRS/Laboratoire de Photonique et deNanostructures, Route de Nozay, 91460, Marcoussis, France

Paper first received 3rd May and in revised form 7th June 2004

IEE Proc.-Optoelectron., Vol. 151, No. 5, October 2004254

In order to give a qualitative hint on the structural qualityof the GaAs barrier grown on top of the GaNAs UTLs,growth was in situ monitored by reflection high-energyelectron diffraction (RHEED). Figures 1a and 1b show theRHEED patterns obtained after a 10 s exposure to N, andduring regrowth over such a layer. No change in surfacereconstruction was evidenced compared to a N-free GaAsgrowth interruption=regrowth at 400 �C: The streakypatterns obtained in Figs. 1a and 1b indicate that the growthfront does not roughen during N exposure nor at regrowth.However, an exposure lasting more than 2 min resulted in aspotty RHEED pattern. In that case, such a result probablymeans that the GaAs surface has been saturated withnitrogen, making proper regrowth of GaAs over such a GaNlayer impossible. These RHEED observations allowed us toestablish boundary conditions, such as maximum durationof the N exposure and=or maximum valve aperture of theplasma cell, before a spotty RHEED pattern is obtained.

N accumulation was confirmed and estimated by asecondary ion mass spectroscopy (SIMS) analysis oncalibrating samples. Results of the SIMS depth profileare shown on Fig. 2, using different deposition conditions.As Fig. 2 shows, the total amount of nitrogen deposited onGaAs depends on the duration of the exposure and on thevalve aperture of the plasma cell. By integrating the SIMSdata giving volumic nitrogen concentration as a function ofdepth, we calculated the N sheet concentration for eachdeposition condition. For a 30 s exposure and a largevalve aperture, the N concentration is determined to be2:5� 1014 cm�2: For the same valve aperture but with ashorter 10 s exposure, one finds 6:3� 1013 cm�2; and fora 10 s exposure but a smaller valve aperture, the sheet

concentration is then reduced to 2:8� 1013 cm�2: TheseSIMS results show that N incorporation is found to beroughly proportional to the duration of the growthinterruption. It is worth noting that these sheet concen-trations correspond to less than a monolayer of GaNwhich corresponds to a sheet concentration of about6:2� 1014 cm�2: A structural study was also carried outby transmission electron microscopy (TEM). TEM imagesof the previous sample show thin and dark layers embeddedin GaAs, with an accentuated contrast for longer exposuretimes or larger valve aperture. Layer thicknesses were foundto be in the range 0.6–1.2 nm. Before N deposition, thesubstrate temperature was lowered to 400 �C during theGaAs growth. At this temperature, the GaAs surface is notideally flat. A surface roughness of a few monolayers caneasily develop due to the short Ga diffusion length [8]. It istherefore not surprising that the GaNAs UTLs do not appearas ideal planar monolayers. Nevertheless, it can be seen thatthe GaNAs UTLs are well defined and relatively uniform.There is no sign of phase separation ðGaAsþ GaNÞ; norevidence of surface segregation during the GaAs regrowth.If we assume that the layers are ideally uniform with N onsubstitutional sites, then the N concentrations are found to be1–1:4% for the reduced N flux and a 10 s exposure, 3– 4:7%for the maximum N flux and during 10 s and 10–14% for themaximum N flux and the longest 30 s exposure. There isindeed a possibility that some N has been incorporated oninterstitial sites, antisites or has formed N complexes.However, we believe that most of the N is on substitutionnalsites since TEM did not reveal a significant deviation tocrystallinity.

In order to investigate the effect of such GaAsN UTLson the relaxation time of photoexcited carriers, a seriesof samples were grown consisting of a single 8 nm-thickGa1�yInyAs QW with yIn ¼ 0:30; sandwiched between two50 nm-thick GaAs barriers. On each side of the GaAsbarriers, two 20 nm-thick GaAlAs layers were also includedfor carrier confinement. The QW structure was then cappedwith 5 nm of GaAs. Apart from the reference sample,GaNAs UTLs were added in the GaAs barriers, on each sideof the GaInAs QW. Samples A, B and C had GaNAs UTLs

Fig. 1 RHEED patterns

a GaAs (011) surface after a 10 s exposure to the N fluxb Same GaAs (011) surface after N exposure and at the beginning of GaAsregrowth

Fig. 2 SIMS depth profile of three GaNAs UTLs deposited duringgrowth interruption, showing incorporation of nitrogen at differentgrowth conditions

a Large valve aperture of the plasma cell and 30 s exposure resultingin 2:5� 1014Natoms=cm2

b Same large valve aperture and 10 s exposure resulting in 6:3�1013Natoms=cm2

c Smaller valve aperture and 10 s exposure resulting in 2:8� 1013 Natoms=cm2

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deposited, respectively, 4 nm, 10 nm and 2.5 nm away fromthe QW. This variable spacer thickness was introduced inorder to study and identify the mutual influence of theGaNAs UTLs and the QW on their optical properties.The schematic structure of such a sample is shown in Fig. 3.A 10 s exposure with a large plasma cell valve aperture waschosen to elaborate the GaNAs UTLs. These conditions arecompatible with regrowth as shown by the preliminaryRHEED and TEM study. It must be added that at no timeduring the growth of the quantum well, nor of the secondGaNAs UTL, did the RHEED turn spotty. Figure 4 displaysa TEM image of sample C. It shows good structural quality,and a steep barrier–QW interface, indicating that thedeposition of GaNAs UTLs did not affect the growth ofthe QW. However, the QW interfaces appear darker than thecore of the well itself, and extend on about 1.2 nm. Thisbehaviour has already been reported and discussed in otherstudies [9–11], and is attributed to a weaker indiumcomposition at the interfaces than in the core of the QW.The QW had to be grown at low temperature, as the distancebetween the GaNAs UTLs and the QW was rather short,from 2.5–10 nm, there was not enough time to raise thetemperature for the QW growth.

The effect of the GaNAs UTLs on the optical propertiesof the GaInAs SQW was first investigated by low-temperature PL. The PL characteristics of the sampleswere measured at 11 K, using a 532 nm laser and detected bya cooled Ge detector. Figure 5 shows the 11 K spectra ofsamples A and B, as well as two reference samples, oneconsisting of a single GaInAs QW (SQW) without anyGaNAs UTLs, and the other consisting of only two GaNAsUTLs, without the QW. The SQW reference has a peakemission wavelength of 1010 nm (1227 meV) and a full

width at half maximum (FWHM) of 26 meV. This slightlybroad FWHM of the reference SQW sample is attributed tothe low temperature growth. The GaNAs UTL referencesample exhibits broad emissions below the GaAs band gap,showing that a wide distribution of deep energy levels iscreated by the GaNAs UTLs. The PL spectra of the sampleshaving GaNAs UTLs embedded next to the QW mainlyconsist of two convoluted peaks: a high-energy peak H witha peak emission wavelength of 1020 nm (1215 meV), and abroad low-energy peak L. When comparing to the referencesamples, one can associate the low-energy emission L to thedeep energy levels created by the GaNAs layers, and thehigh-energy peak H to the QW luminescence. The smalldifference between peak H energy and the SQW referencepeak might be due to fluctuation of In concentration. All thesamples have a nearly constant peak H linewidth, but peakH intensity decreases as the thickness of the spacerbetween the GaInAs QW and the GaNAs UTLs is reduced.In addition, the intensity ratio of peak H over peak L alsodecreases when this spacer thickness is reduced. Theseresults show the drastic effect of the GaNAs UTLs on theQW luminescence intensity, with a strong influence ofthe spacer thickness.

To clearly identify the emission mechanism and isolatethe origin of each peak, further measurements were done.The PL dependence on temperature and excitation powerwere studied. This analysis was done on sample A. Resultsare, respectively, reported in Figs. 6a and 6b. In Fig. 6a, theintensities of peaks H and L have been plotted as a functionof temperature, 10–150K: A remarkable difference isobserved between the PL intensity variations of peak Hand peak L, suggesting that the recombination process ofcarriers leading to peak L differ significantly from thecarriers emitting at higher energies. An S-shaped tempera-ture dependence of the lower energy peak L is observed.This typical dependence has been reported by severalgroups studying III–V-nitride alloys and is attributed toexciton localisation at low temperature due to N-relatedalloy fluctuations [4–6]. The broadband low-energy emis-sion is likely to come from the GaNAs UTLs, acting like adefect layer and trapping excitons on localised states createdby potential fluctuations. Furthermore, it is found that peak

Fig. 3 Structure of the Ga0:7In0:3As single quantum well samples,with GaNAs UTLs grown in the GaAs barriers at a distanced ¼ 2:5 nm; 4 nm and 10 nm from each side of the QW

Ga0:8Al0:2As cladding layers were added for optical confinement

Fig. 4 TEM dark-field cross-section micrograph of as-grownsample, showing the GaInAs QW, interfacial layers and N-richGaNAs UTLs placed 2.5 nm away from the QW

Fig. 5 11 K PL spectra of samples A and B (hollow symbols)

Reference samples (solid symbols) were also included, one consisting of asingle GaInAs QW, and the other consisting of two GaNAs UTLs withoutthe QW

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H follows a rather more conventional temperature depen-dence, with no sign of exciton localisation. Figure 6b showsthe excitation-power-dependent PL spectra of sample A at11 K in the dependence range 2–200 mW. PL spectra arenormalised to peak H. The high-energy peak H has itsintensity increasing with higher excitation power, which istypical of delocalised states. On the contrary, low-energypeak L saturates with excitation. This fading is knownbehaviour for localised states which saturate quickly andhave a very small density of states. This supports thesuggestion that peak L can be assigned to the N-rich layers,with recombination dominated by localised exciton emis-sion. For this series of samples, we assume that a fixednumber of excited carriers are created or transferred into thelayers between the two GaAlAs barriers. Therefore, thedecrease of the intensity ratio of peak H over peak Loccurring when the spacer thickness is reduced illustratesthat more carriers are trapped by the GaNAs UTLs beforethey can recombine radiatively into the QW.

To further our investigation on the effect on carrierrelaxation dynamics of these GaNAs UTLs embedded nextto a GaInAs QW, time-resolved photoluminescence (TRPL)measurements were carried out at room temperature. Theexcitation signal was generated by a tunable Ti:sapphiremode-locked laser having a pulse duration of 5 ps. PL decaytime was analysed using a two-dimensional synchroscanstreak camera. The measurements were performed at theenergy of peak H, related to the QW emission. Figure 7

shows the decay curves for samples B and C. The inset ofFig. 7 is a plot of the nonradiative lifetime of the samplesagainst the spacer thickness s. The nonradiative recombina-tion rate is proportional to the number of excited carriers n,whereas bimolecular recombination varies as n2: Thus, ourTRPL measurements were done at low excitation power inorder to have the PL decay dominated by nonradiativerelaxation. Our results show PL decays that can be wellfitted by double-exponential function, with an early stageexhibiting a very fast decay time. We will focus on this firststage, being the fastest and thus being the most representa-tive of nonradiative recombination. This fast decay timedecreases from 13 ps to 6 ps as the GaNAs UTLs aredeposited closer to the QW. As a reference, 400 �C grownGaAs exhibits a decay time of approximately 40–50 ps [2].Since those TRPL measurements were performed on theQW related peak, the short decay time indicates that thecarriers trapped into the QW have two possible recombi-nation paths. Carriers can recombine radiatively from theirQW levels, or they can transfer to nonradiative centres. Thistransfer could be possible by tunnelling of carriers from theQW energy levels to the deep levels of the GaNAS UTLs.The tunnelling probability would indeed increase as the QWand the GaNAs UTLs become closer to each other.

3 Conclusions

In summary, the deposition of N-rich GaNAs UTLs wasinvestigated. Growth of such layers was successfullyachieved by performing a GaAs growth interruption underan active N species flux. N concentration up to 10–14% wasmeasured in a layer extending over about 1 nm. As in situmonitored by RHEED, this process was compatible withregrowth and enabled us to insert such layers in the vicinityof a GaInAs QW. It is found that the influence of the GaNAsUTLs is strengthened as the distance between the GaNAsUTLs and the QW decreases. The QW PL intensitydecreases at the benefit of broad emissions, which areattributed to the GaNAs UTLs. The identification of thedifferent PL emissions was confirmed by temperature-dependent and excitation-dependent measurements. Time-resolved analysis indicated that the recombination lifetimeof carriers trapped in the QW is shortened to less than 10 pswhen the GaNAs UTLs are placed in the proximity of the

Fig. 6 PL properties of sample A

a Temperature dependence, focusing on the PL shift of peak H and peak Lb Excitation-powerDependence at ILK. The GaInAs QW reference sample (ref QW) was alsoincluded

Fig. 7 TRPL decay for sample B (GaNAs UTLs 10 nm away fromthe QW) and sample C (GaNAs UTLs 2.5 nm away from the QW)

Curves have been horizontally shifted for clarityInset: nonradiative decay time of all the samples against the distance of theGaNAs UTLs from the GaInAs QW

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QW. By varying N content in the GaNAs UTLs and byoptimising the distance to the QW, this kind of structurecould be an easy and flexible means to control the carrierlifetime in a QW for high-speed optical devices.

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