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Physica E 26 (2005) 422–426

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Carrier capture and relaxation of self-assembled ZnTe/ZnSequantum dots prepared under Volmer–Weber and

Stranski–Krastanow growth modes

M.-E. Leea, Y.-C. Yehb, Y.-H. Chungb, C.-L. Wub, C.-S. Yangc, W.-C. Chouc,C.-T. Kuob, D.-J. Jangb,�

aDepartment of Physics, National Kaoshiung Normal University, Kaoshiung, 80264 Taiwan, ROCbDepartment of Physics, National Sun Yat-sen University, 70 Lienhai Road, Kaoshiung, 80441 Taiwan, ROC

cDepartment of Electrophysics, National Chiao-Tung University, Hsinchu, 30056 Taiwan, ROC

Available online 24 November 2004

Abstract

The carrier capture and relaxation of type II ZnTe/ZnSe quantum dots have been investigated with ultrafast time-

resolved photoluminescence upconversion. The carrier capture times were 7 and 38 ps for the Volmer–Weber mode and

Stranski–Krastanow mode, respectively. We found that the carrier relaxation of QDs exhibits faster decay under the

Volmer–Weber growth mode than under the Stranski–Krastanow growth mode. We attribute the difference of carrier

relaxation to the wetting layer formed in the Stranski–Krastanow growth mode.

r 2004 Elsevier B.V. All rights reserved.

PACS: 78.47.+p; 73.63.�b; 78.67.�n

Keywords: Type II quantum dots; Time-resolved photoluminescence; Carrier capture; Carrier relaxation; Auger process

Semiconductor quantum dots (QDs) have beenextensively studied for potential applications inoptoelectronic devices and for their novel electro-nic and optical properties in 3-D confined nanos-tructures [1,2]. Carrier capture and relaxation areamong the important properties that affect thedevice performance such as threshold current and

e front matter r 2004 Elsevier B.V. All rights reserve

yse.2004.08.092

ng author. Fax: 886-75253709.

ss: djjang@mail.nsysu.edu.tw (D.-J. Jang).

temperature stability. Recently, the impact of thewetting layer on the carrier capture and relaxationin QDs has been investigated. Several reportsindicated that the wetting layer is important forcarriers to relax excess energy easily through acontinuum tail of wetting layer defect states [3].The carrier dynamics in the wetting layer inrelation to the capture into the QDs have alsobeen investigated [4]. However, recently, a reportfound that the two-dimensional character of the

d.

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wetting layer is not relevant in determining the QDcapture and relaxation [5]. While the details of themechanism of carrier capture and carrier relaxa-tion remain controversial and most of the studieswere focused on type I QDs of III–V materials, thepresent work demonstrate that the wetting layer issignificant to the carrier dynamics in type II QDsof II–VI materials.In order to identity the impact of wetting layers

on the carrier capture and relaxation, we investi-gate the carrier capture and carrier relaxation ofself-assembled ZnTe quantum dots grown in aZnSe matrix by molecular beam epitaxy with twogrowth modes. One of the growth methods is theStranski–Krastanow (SK) mode that a layer wasgrown first in two dimensions then followed bythree-dimensional islands after a critical thickness.The other growth mode is the Volmer–Weber(VW) mode in which no two-dimensional layerwas formed before three dimension islands weregrown. The type-II QD structure, with electronsand holes confined in different spatial regions, alsoexhibits interesting physical properties, such asslower radiative lifetime, tunability of emissionenergy, and reduction of electron–hole interaction.While holes are confined in the ZnTe layer,electrons are localized in ZnSe barriers. We studythe carrier capture and relaxation with ultrafasttime-resolved photoluminescence (PL) that pro-vides a temporal resolution of better than 300 fs.The high temporal resolution is essential to studythe fast carriers captured by QDs and the PLdecay at the first 20 ps after photoexcitation. Thecarrier capture times were determined from thetime-resolved photoluminescence to be 7 and 38 psfor the VW mode and SK mode, respectively. Wefound that carrier relaxation of QDs in the VWgrowth mode exhibits faster decay than that ofQDs in the SK growth mode due to the wettinglayer in the SK mode providing a pathway forcarriers to diffuse and migrate from large (small)to small (larger) QDs.Self-assembled ZnTe QDs were grown in a ZnSe

matrix on the GaAs substrate by using a Riber 32Pmolecular beam epitaxy system with two growthmodes of VW and SK. The VW mode of QDs with2.9MLs coverage were capped with a ZnSethickness of 5 nm and the SK mode of QDs with

3.0MLs coverage were capped with a ZnSethickness of 50 nm. Atomic force microscopy(AFM) showed that two families with differentsizes of diameters, large and small, of QDs weregrown for both samples. The AFM also indicatedthat mainly small size QDs were grown for smallZnTe deposition (thickness of 1.3MLs). Thedependence of the sizes of QDs on ZnTe deposi-tion thickness were also reported for InAs/GaAsQDs [6]. Large QDs with dot densities of 3.6� 108

and 1.5� 109 cm�2 for the VW mode and SKmode, respectively, were estimated by atomic forcemicroscopy as well as the small QDs with dotdensities of 4.0� 109 and 1.0� 1010 cm�2 for theVW mode and SK mode, respectively. The detailsand growth parameters of these two modes aregiven elsewhere [7,8]. To study the carrier captureand relaxation of two different growth mode ofQDs, the all optical PL up-conversion spectro-scopy [9,10] was used to measure the time-resolvedphotoluminescence of these samples. This up-conversion technique, in contrast to a pumpprobe, covers a very wide spectral range of PLwithout the need of a tunable laser source. A beamwith a 150 fs pulsewidth from a Kerr-lens mode-locked Ti:sapphire laser at a repetition rate of76MHz was split into two beams. One of the twobeams was frequency-doubled with a 4 beta-barium-borate (BBO) nonlinear crystal and wasused to illuminate the samples. For the presentwork, 100mW of average power was focused on a50 mm spot on the surface of the sample mountedin a low-vibration closed cycle cryostat kept attemperature of 35K for the present study. The PLwas collected and focused by a pair of off-axialparabolic mirrors onto another BBO nonlinearcrystal, where the PL was mixed with the funda-mental beam that was sent through an opticaldelay. The signal of the sum frequency wasgenerated by angle-tuning the BBO crystal to thespectrum of interest according to the phase-matching condition. The up-converted signal wasfiltered with a band-pass filter to remove anycontribution that might come from either thefundamental or frequency-doubled beams. Thesignal was then dispersed by a monochromator of30 cm focal length to enhance the spectral resolu-tion before collection by a thermal-electrically

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cooled GaAs photomultiplier. Eventually, we useda standard photon-counting instrument to analyzethe signal.The time-integrated PL of the QDs photoexcited

nonresonantly with the frequency-doubled lightfrom the fundamental of the Ti:sapphire lasershows that the peaks of the PL for the VW modeand SK mode QDs are 2.33 and 2.22 eV, respec-tively. The difference of the peak energies of thesetwo similar height QDs is the result of the wettinglayer and larger QDs formed, whose energies arelower, in the SK mode. The width of 40 nm for thetime-integrated PL of the VW mode and 30 nm forthe SK mode are due to the size distribution ofQDs. The time-resolved PL of both samples forenergies at the barriers and the peaks of the time-integrated PL are shown in Fig. 1. The PLintensity of the barrier for the VW mode QDs(Fig. 1(a)) increases rapidly with a time scale ofabout 0.5 ps and is followed by a decay with a timescale of 7 ps. The PL of the barrier for the SKmode QDs reveals a slower rise time of 3 ps and adecay time of 38 ps. The differences of the PL risetimes for these two different growth modes may bedue to the different thicknesses of the cappinglayers. The ratio of the rise time to the decay timeis consistent for both samples, which may indicate

Fig. 1. The time-resolved PL of ZnTe/ZnSe QDs grown by VW

mode ( triangle down) and SK mode (circle) at energy of barrier

(a) and at the peak of the time-integrated PL (b). The inset

shows the time-resolved PL in the first 100 ps. The sample’s

temperature is 35K. The lines are for visual guidance and the

vertical scale is linear.

that only the thickness of the capping layer affectsthe increase in rise and decay times in the SKmode. Fig. 1(b) shows the time-resolved PL atpeak energies of the time-integrated PL. For theVW QDs, we observe that the PL intensityincreases with a risetime of 2 ps and the decay ofthe PL can be characterized bi-exponentially withtwo time constants: a fast decay time of 20 ps and aslower decay time of 1.6 ns. The figure also shows amuch slower PL risetime of 15 ps and a PL lifetimeof more than 4 ns for the SK growth mode. Thetimes of carrier capture by QDs, determined fromthe lifetimes of the barriers, are 7 and 38 ps for theVW mode and SK mode, respectively. Weattribute the fast PL decay of the VW modewithin the first 20 ps after photoexcitation to theAuger processes due to the higher carrier densitygenerated inside the dots [11]. As the carrierdensity decreases with delay time, the Augerprocess is less effective. Because of the nature ofthe type II QDs structure of ZnTe/ZnSe, holeswere confined in the ZnTe dots and electrons arelocalized in the ZnSe layer around the dots. Thespatial separation of the wave functions of holesand electrons accounts for the slow PL decay timeof 1.6 ns for the VW mode. In comparison, wefound that for the SK mode the PL decay time ismore than 4 ns, which is about 3 times slower thanthat for the VW mode. We attribute this slowdecay, in addition to the spatial separation of wavefunction of holes and electrons, to the wettinglayer formed in this structure. The carriersgenerated from the capping layer by photoexcita-tion are funneled into the wetting layer before theyare captured by QDs. Once the carriers reach thewetting layer, either from barriers or QDs, theywill not escape easily to the barrier as a result ofthe confinement by the barrier, whose band gapenergy is larger than that of the wetting layers.This could also partially explain the slow PL risetime of the SK mode compared to that of the VWmode in Fig. 1(b). In contrast, the carriers inthe QDs of the VW mode are not confined beforethey are captured by the QDs and, thus, theprobability of escaping from the QDs increases.Therefore, the wetting layer in the SK mode playsan important role in carrier capture and relaxa-tion. The slow decay lifetime for both samples are

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slower/comparable to that of type I CdSe/ZnSeQDs [12,13].In order to understand the role of wetting layer

in carrier capture and relaxation, we measured thedecay of PL as a function of energy for bothsamples, as shown in Fig. 2. The PL decay of theSK mode at energies above the peak of the time-integrated PL exhibits bi-exponential decay. Forcomparison, we determined the PL decay at allenergies with two time constants, one refers to theslow decay and the other refers to the fast decay ofthe PL although the PL shown in Fig. 1(b) caneasily be explained by a single exponential. Asshown in Fig. 2, the slow decay time of the SKmode decreases with energy. We attribute thedecrease of slow decay times also to the wettinglayer because the carriers in the small QDs, whoseenergy states are larger, can be excited to thewetting layer either by the Auger process or bythermal emission and will later relax to the largeQDs. Carriers in the confined wetting layermigrate from small QDs to the large QDs bydiffusion. On the contrary, the slow decay times ofthe VW mode level around 1.6 ns indicates that theprobabilities of carrier capture into the QDs areequal and thus the carriers excited by the Augerprocess and thermal emission will relax to large

Fig. 2. The fast and slow PL decay times of ZnTe/ZnSe QDs.

Open (solid) down triangle are the fast (slow) PL decay times

for SK mode. Open (solid) circles are the fast (slow) PL decay

times for VW mode. The scales for data points to the left and

right of the break are on the left and right vertical axes,

respectively. The lines between data points are for visual

guidance. The dashed and solid lines are the normalized time-

integrated PL of VW mode and SK mode, respectively.

and small QDs with the same probabilities sinceno path is provided for carriers to diffuse andmigrate from large (small) to small (larger) QDs.The same probabilities of carrier capture in theVW mode is confirmed by studying the PL risetimes of the QDs and we found that they were allabout 2 ps, regardless the detected energy. Theabove argument could also explain why the slowPL decay times for the SK mode are smaller thanthe VW mode at energies above 2.3 eV. Thecarriers escape away from small QDs in the SKmode and diffuse to the large QDs through thewetting layer. Therefore, fewer carriers are relaxedto the bandgap of the small QDs than to the largeQDs and the slow decay times are thus smallerthan those of the VW mode.In conclusion, we studied the carrier capture

and relaxation of ZnTe/ZnSe QDs grown in theVW and SK modes with ultrafast PL upconver-sion. We found that the carrier capture times were7 and 38 ps for the VW mode and SK mode,respectively. We attribute the fast PL decay of theVW mode within the first 20 ps after photoexcita-tion to the Auger processes. We attribute the slowdecay of the PL for the SK mode, compared tothat of the VW mode, in addition to the spatialseparation of wave function of holes and electrons,to the wetting layer formed in this structure. Wehave demonstrated the wetting layer plays animportant role in carrier capture and relaxation ofType II ZnTe/ZnSe QDs.

This work was supported in part by NationalScience Council, ROC under Grant No. NSC 92-2112-M-110-015.

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