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doi:10.1016/j.jc

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Journal of Crystal Growth 301–302 (2007) 805–808

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Optical properties of p-type modulation-doped InAs quantum dotstructures grown by molecular beam epitaxy

N. Kumagai�, K. Watanabe, Y. Nakata, Y. Arakawa

Nanoelectronics Collaborative Research Center, Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, Japan

Available online 24 January 2007

Abstract

We have performed temperature-dependent studies of photoluminescence (PL) properties of p-type modulation-doped InAs/GaAs

quantum dots (QDs) structures grown by molecular beam epitaxy. In temperature dependence of integrated PL intensity, full width at

half maximum (FWHM) of PL spectra and its peak energy, differences between undoped and p-doped have depended on a modulation-

doping level. We have observed the improvement of temperature stability of PL intensity by p-type modulation doping to InAs QDs in

comparison with that of undoped. FWHM of PL spectrum of p-InAs QDs increases as modulation-doping level increases. Excessive

modulation doping has caused the degrading of PL properties due to Beryllium (Be) diffusion from p-GaAs modulation-doping layer to

InAs layer via undoped GaAs spacer layer. The Be diffusion into InAs region was confirmed by secondary ion mass spectroscopy.

r 2006 Elsevier B.V. All rights reserved.

PACS: 78.66.Fd; 78.67.Hc

Keywords: A1. Modulation doping; A1. Photoluminescence; A1. Quantum dots; A3. MBE; B1. InAs

1. Introduction

Since the proposal of a quantum dot (QD) laser byArakawa and Sakaki [1], self-organized InAs coherentislands on GaAs using Stranski–Krastanov growth modehave been widely investigated because of not only one ofpromising material systems to realize a QD laser but alsophysical interests of zero-dimensional systems. QD laserhas advantages arising from atomic-like density state, andbeen expected as next-generation light sources for opticalcommunication bands. On the other hand, p-type modula-tion doping to QD was proposed to improve a differentialgain in QD lasers by Takahashi and Arakawa [2]. Deppe etal. calculated the modulation characteristics of p-InAs QDlaser by a quasi-equilibriums model and suggested thebandwidth larger than 30GHz [3]. Recently p-InAs QDslasers have been realized and reported positive andnegative effect by p-type modulation doping [4–6].Significant improvements were temperature stabilities ofboth threshold current and differential quantum efficiency.

e front matter r 2006 Elsevier B.V. All rights reserved.

rysgro.2006.11.124

ing author.

ess: nkumagai@iis.u-tokyo.ac.jp (N. Kumagai).

On the other hand, negative effects were decrease indifferential quantum efficiency and increase in thresholdcurrent density as modulation-doping concentration in-creases [4,5]. As pointed out by Novikov et al., it isimportant for p-QD laser to optimize modulation-dopingstructure and the other parameters such as QD density,number of stacking of QD layer and design of claddinglayers. And for the improvement of performance of QDlaser by p-type modulation doping, high doping as possibleis desired theoretically [3,7].Previously, we observed dependence of photolumines-

cence (PL) intensity of p-InAs QDs on modulation-dopinglevel and PL intensity was enhanced to about 10 times atmaximum at room temperature [8,9]. Over the optimizeddoping level of p-GaAs as holes supplier, PL intensityturned to degrade as doping level increased. At a viewpointof improvement of laser performance, degrading of opticalproperty by high doping is undesirable. Therefore, wethought that detailed studies of effect of p-type modulationdoping on optical property were necessary.In this study, we have performed temperature depen-

dence of PL properties of p-InAs QDs with variousmodulation-doping levels. Additionally, in the case of high

ARTICLE IN PRESS

1E-3

0.01

0.1

1

undoped

1e16 cm-3

2e17 cm-3

8e18 cm-3

undoped

1e16 cm-3

2e17 cm-3

8e18 cm-3

undoped

1e16 cm-3

2e17 cm-3

8e18 cm-3

Inte

gra

ted P

L inte

nsity [a.u

.]

0 100 150 200 250 300

0.95

0.96

0.97

0.98

0.99

1.00

1.01

1.02

1.03

1.04

PL p

eak e

nerg

y [e

V]

15

20

25

30

35

FW

HM

[m

eV

]

50

N. Kumagai et al. / Journal of Crystal Growth 301–302 (2007) 805–808806

modulation doping to InAs QDs that the deteriorating ofPL intensity was caused at room temperature, weinvestigate the effect of such a high doping to InAs QDson temperature characteristic. Taking account of ourprevious results about direct doping of Be to InAs QDs[10], we discuss Be diffusion to InAs region as a reason ofthe deteriorating of optical properties.

2. Experiment

P-type modulation-doped InAs QDs samples weregrown on semi-insulating GaAs (0 0 1) substrate bymolecular beam epitaxy (MBE). We used VG V80 mk.III solid source MBE equipment. After growth of 300 nmp-type GaAs buffer layer as a hole supplier at 600 1C byuniform doping of Beryllium (Be), followed by undopedGaAs spacer layer of 30 nm, and InAs QDs were grown ona spacer layer at 500 1C at a low growth rate of 0.009ML/s.The InAs layer has the nominal thickness of 2.4ML. AndInAs QDs layer was capped by undoped GaAs of 100 nmat 480 1C. During a growth from buffer to capping layer,As4 pressure was maintained at 6.2� 10�6 Torr. In the caseof undoped sample for a reference, undoped GaAs bufferwas merely grown. For three p-InAs QDs samples weprepared in this study, modulation-doping concentrationof p-GaAs was 1� 1016, 2� 1017 and 8� 1018 cm�3,respectively. The schematic sample structure is shown inFig. 1. The QD density, PL peak energy and full-width athalf maximum (FWHM) of undoped InAs QDs referencesample at room temperature was �1� 1010 cm�2, �1.3 mmand �21meV, respectively. The QD density of p-dopedInAs QDs did not change in comparison with that ofundoped. PL measurements were performed in variabletemperature closed-cycle helium cryostat under the excita-tion of 633 nm line of He–Ne laser. The PL measurementrange was varied from 10 to 300K.

3. Results and discussion

Fig. 2 shows temperature dependence of (a) integratedPL intensity, (b) FWHM and (c) PL peak energy ofundoped and p-InAs QD samples with various dopinglevel. Integrated PL intensity is normalized by the eachintensity at 10K. As doping level increase to 2� 1017 cm�3,

Undoped GaAs spacer, 30nm

InAs 2.4 ML

GaAs cap,100nm

Be doped p-GaAs, 330nm

S.I. GaAs sub.

Fig. 1. Schematic structure of p-type modulation-doped InAs QDs

sample.

Temperature [K]

Fig. 2. Temperature dependence of PL properties of undoped and p-

doped InAs QDs with various modulation-doping levels. (a)–(c) are the

integrated PL intensity, full width at half maximum (FWHM) and PL

peak energy, respectively. Circle, square, triangle, and inversed triangle

indicate undoped InAs QDs, p-InAs QDs with p-GaAs doping level of

1� 1016, 4� 1016, 2� 1017 and 3� 1018 cm�3, respectively. Each intensity

is normalized.

the quench tendency of integrated intensity of p-dopedInAs QDs have been almost same with that of undoped toaround 225K. From around 225K, falloffs of intensity of

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Table 1

Fitting parameters of Ci and Ei

Sample C1 E1 (meV) C2 E2 (meV)

Undoped 90.8 46.1 1.2� 1011 490.0

1� 1016 cm�3 52.0 38.5 5.8� 109 457.2

2� 1017 cm�3 56.5 36.4 2.0� 109 450.8

8� 1018 cm�3 182.8 49.8 2.0� 108 364.1

N. Kumagai et al. / Journal of Crystal Growth 301–302 (2007) 805–808 807

p-doped InAs QDs have improved in comparison with thatof undoped. In the case of the doping level of8� 1018 cm�3, for a temperature rise, the quench in theintegrated intensity becomes earlier than those of undopedand lower doped samples. And the total quench from 10 to300K in integrated intensity has been more degraded thanthose of lower doped samples. These results indicate thatmodulation-doping level also should be optimized in termsof temperature dependence. About the reason of theimprovement of temperature stability of PL intensity byp-type modulation doping, it is considered that holes atground states of InAs QDs are compensated by modula-tion doping, in spite of thermal escape of holes fromground state with increasing of temperature.

Since characteristics around room temperature areimportant for the application of optical devices anddifferent tendencies of PL intensity with temperatureamong our samples appeared around 150K, we focusedand fitted in high temperature region from 150 to 300K.Fitting results of PL intensity are shown in Fig. 3. Theseexperimental results can be theoretically fitted by thefollowing equation:

IPL ¼ 1

,1þ

Xi¼1

Ci expð�Ei=kTÞ

" #, (1)

where Ci is a constant and Ei is a thermal activation energy.Fitting parameters of Ci and Ei are shown in Table 1. E1

features the slow slope of quench of intensity in lowtemperature region less than around 225K, and E2 featuresthe remarkable falloff in high temperature region morethan 230K. As modulation-doping level increases fromundoped to 2� 1017 cm�3, both E1 and E2 decrease.Therefore, it seems that the effect of p-type modulationdoping to InAs QDs appears as the reduction of activation

150 175 200 225 250 275 300 325

1E-3

0.01

0.1

1

Inte

gra

ted P

L inte

nsity [a.u

.]

Undoped

1e16 cm-3

2e17 cm-3

8e18 cm-3

fitting to undoped

fitting to 1e16 cm-3

fitting to 2e17 cm-3

fitting to 8e18 cm-3

Temperature [K]

Fig. 3. Integrated PL intensity and fitting result from 150 to 300K. Closed

circle, open square, open triangle, and open inversed triangle indicate

undoped InAs QDs, p-InAs QDs with p-GaAs doping level of 1� 1016,

2� 1017 and 3� 1018 cm�3, respectively. Solid line, dashed line, dotted line

and dot-dashed line is fitting result to undoped, 1� 1016, 2� 1017 and

3� 1018 cm�3, respectively.

energy quantitatively. However, in the case of the highestdoped sample, E1 increase while E2 remains decreasingwith increasing of modulation-doping level. Additionally,the C1 also has become larger than that of the others. Theinfluence of the E1 slope to total quench of intensitybecomes large, accordingly the total quench of intensitybecomes larger than that of the other samples in spite ofthe smallest E2. The reduction of emission efficiency andthe small falloff around room temperature by highmodulation doping are thought of as one of the reasonsfor the increase of threshold current and temperaturestability of threshold current around room temperature inp-InAs QD laser.FWHM of p-doped InAs QDs samples show conspic-

uous deviations from that of undoped as temperatureincreases. And the degree of deviation depends onmodulation-doping level. At low temperature region lessthan 50K, since thermal activation of dopant in p-GaAs issuppressed, p-type doping effect is weakened. For thereason of broadening of FWHM as modulation-dopinglevel increases, state filling effect by modulation doping isconsidered. Because the energy distance of discrete levels ofholes are so close that state of holes becomes broad in hightemperature region. The energy distance between groundstate and second state was reported �10meV by Dikshitand Pikal [11].PL peak energy of p-doped InAs QDs samples has been

larger than that of undoped in the range of measurement.However, unlike in integrated PL intensity and FWHM,shifts of PL peak for energy of p-doped InAs QDs have notshown a remarkable modulation-doping dependence.Although the blue shift of emission wavelength by p-typemodulation doping was reported by Shchekin et al., thereason is has not been cleared yet [12]. They mentioned thatchanges of emission wavelength cannot be explained bycarrier filling effect or Coulomb interaction. Indeed, at theviewpoint of carrier filling effect, the larger deviation of PLpeak energy at low temperature than that at highertemperature cannot be explained. P-type modulation-doping effect should be appeared more significant in hightemperature region than low temperature.Here, we discuss about degrading of PL intensity by high

modulation doping. Previously we performed PL measure-ment for InAs QDs sample with direct doping of Be. Theresult suggested that incorporation of Be into InAs QDsinduces degrading of PL properties (intensity and FWHM)[10]. Therefore, we considered the influence of diffusion ofBe into InAs QDs and/or wetting layer, and performed

ARTICLE IN PRESS

BeIn

ten

sity o

f 2

nd

. io

n [

co

un

ts/s

ec]

Be

co

nce

ntr

atio

n [

cm

-3]

As

In

0 20 40 60 80 100 120 140 160 180

0 20 40 60 80 100 120 140 160 180

1014

1015

1016

1017

1018

1019

1014

1015

1016

1017

1018

1019

102

103

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102

103

104

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106

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Be

Inte

nsity o

f 2

nd

. io

n [

co

un

ts/s

ec]

Be

co

nce

ntr

atio

n [

cm

-3]

Depth [nm]

Depth [nm]

As

In

Fig. 4. SIMS profiles of Be in p-type modulation-doped InAs QDs

samples (i-GaAs 100nm/ InAs 2.4ML/ i-GaAs spacer 30 nm/ p-GaAs

400 nm/ SI-GaAs substrate). Secondary ion intensity of Be are converted

to concentration per volume. (a) Doping level of 2� 1017 cm�3 in p-GaAs.

(b) Doping level of 8� 1018 cm�3 in p-GaAs. The protuberance around

80nm is background noises.

N. Kumagai et al. / Journal of Crystal Growth 301–302 (2007) 805–808808

secondary ion mass spectroscopy (SIMS) to obtain Beprofile in p-InAs QDs samples. Fig. 4 shows Be profiles oftwo p-InAs QDs samples with modulation-doping level of(a) 2� 1017 and (b) 8� 1018 cm�3. In the case of (a), thediffusion of Be was not confirmed around InAs layer.However, for (b) case, the significant existence of Be inInAs layer was confirmed. This result indicates Be diffusioninto InAs layer. Mosca et al. studied Be diffusion in GaAsgrown by MBE and shown the experimental result ofdiffusion from p-GaAs (1� 1019 cm�3) grown at 570 1C top-GaAs (4� 1016 cm�3) grown at 600 1C via undopedGaAs spacer of 30 nm thick [13]. Therefore, it seems thatour result is proper. Our results suggest that the non-radiative center as shown in E2 fitting result is depend onBe diffusion to InAs layer with a increasing of modulation-doping level (Fig. 4).

So far, the reason of an increase of threshold current byp-type modulation doping in InAs QD laser has beenexplained by Auger scattering effect as a doping levelincreases [4,5]. Since our results indicate that the opticalproperties is degraded by Be diffusion into InAs layer, thediffusion is thought of as one of the reason for increase ofthreshold current in p-InAs QD lasers.

4. Summary

In conclusion we have investigated temperature depen-dence of PL properties of p-type modulation-doped InAsQDs, and shown that PL intensity and FWHM arestrongly depended on modulation-doping level. Quenchof PL intensity with increasing of temperature hasimproved by p-type modulation doping. In the case ofexcessive modulation doping, the diffusion of Be causesdegrading of both PL intensity and its temperaturedependence. To supply many holes to QDs and avoiddegrading of optical property due to diffusions of Be toInAs QD regions, our results indicate the importance tooptimize modulation-doping structure.

Acknowledgement

This work was supported by the Focused Research andDevelopment Project for the realization of the world’s mostadvanced IT nation, MEXT. The authors thank M. Ishida,S. Nagahara, and M. Nomura for useful discussions.

References

[1] Y. Arakawa, H. Sakaki, Appl. Phys. Lett. 40 (1982) 939.

[2] T. Takahashi, Y. Arakawa, Optoelectronics 3 (2) (1988) 155.

[3] D.G. Deppe, H. Huang, O.B. Shchekin, IEEE J. Quantum Electron.

38 (2002) 1587.

[4] O.B. Shchekin, D.G. Deppe, Appl. Phys. Lett. 80 (2002) 3277.

[5] I.I. Novikov, N.Yu. Gordeev, L.Ya. Karachinskii, M.V. Makismov,

Yu.M. Shernyakov, A.R. Kovsh, I.L. Krestnikov, A.V. Kozhukhov,

S.S. Mikhrin, N.N. Ledenstov, Semiconductors 39 (2005) 477.

[6] K. Otsubo, N. Hatori, M. Ishida, S. Okumura, T. Akiyama, Y.

Nakata, H. Ebe, M. Sugawara, Y. Arakawa, Jpn. J. Appl. Phys. 43

(B8) (2002) L1124.

[7] O.B. Shchekin, D.G. Deppe, Appl. Phys. Lett. 80 (2002) 2758.

[8] N. Kumagai, K. Watanabe, S. Iwamoto, S. Tsukamoto, Y. Arakawa,

17th IPRM, WP35, 2005.

[9] N. Kumagai et al., to be submitted.

[10] N. Kumagai, K. Watanabe, S. Tsukamoto, Y. Arakawa, The 67th

Autumn Meeting, The Japan Society of Applied Physics, 13.3,1p-P2-

16, 2006.

[11] A.A. Dikshit, J.M. Pikal, IEEE J. Quantum Electron. 40 (2004) 105.

[12] O.B. Shchekin, D.G. Deppe, D. Lu, Appl. Phys. Lett. 78 (2001) 3115.

[13] R. Mosca, P. Bussei, S. Franchi, P. Frigeri, E. Gombia, A. Carnera,

M. Peroni, J. Appl. Phys. 93 (2003) 9709.