0.94 µm diode lasers based on Stranski-Krastanow and sub-monolayer quantum dots

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Semicond. Sci. Technol. 15 (2000) 1061–1064. Printed in the UK PII: S0268-1242(00)11516-0

0.94 µm diode lasers based onStranski–Krastanow andsub-monolayer quantum dots

S S Mikhrin†, A E Zhukov†, A R Kovsh†, N A Maleev†,V M Ustinov†, Yu M Shernyakov†, I P Soshnikov†, D A Livshits†,I S Tarasov†, D A Bedarev†, B V Volovik†, M V Maximov†,A F Tsatsul’nikov†, N N Ledentsov†, P S Kop’ev†, D Bimberg‡and Zh I Alferov†

†Ioffe Physico-Technical Institute, Russian Academy of Sciences 194021 St Petersburg,Politekhnicheskaya 26‡Institut fur Festkorperphysik, Technische Universitat Berlin, D-10623 Berlin, Germany

E-mail: mikhrin@pop.ioffe.rssi.ru

Received 2 February 2000, accepted for publication 5 October 2000

Abstract. A comparative analysis is made of laser diodes based on Stranski–Krastanow (SK)and sub-monolayer (SML) InAs/GaAs quantum dots, emitting at about 940 nm. Owing to thebetter uniformity of sub-monolayer quantum dots, the SML QD laser surpasses the SK QDone in power characteristics. A maximum output power of 3.9 W and a peak powerconversion efficiency of 59% have been achieved for SML QD 100 µm wide lasers at 10 ◦C.

1. Introduction

Ytterbium-doped yttrium-aluminium garnet (Yb:YAG) crys-tal has emerged recently as a promising candidate for a solid-state laser host. As compared to Nd:YAG, the much broaderabsorption band of the material leads to its weaker sensitivityto diode wavelength specifications, while the longer pumpwavelength (941 nm) makes the material suitable for pump-ing with InGaAs diodes [1]. To the best of our knowledge,the highest reported continuous-wave (CW) output power(∼10 W per 100 µm) was achieved on strained quantum-well diode lasers emitting in the 915–970 nm wavelengthrange [2–4]. The theoretically predicted advantages of thequantum dot (QD) laser and recent results in the field of self-organized QDs [5] show promise of further improvementsin power characteristics. However, the relatively low sur-face density and poor uniformity of QDs fabricated in theStranski–Krastanow (SK) growth mode result in a steep sat-uration of the optical gain. In turn, this restricts the flexibilityof the diode design and makes it impossible to achieve lowthreshold current density and high slope efficiency simulta-neously. This problem can be solved by using several sheetsof QDs [6].

We have recently reported creation of InGaAs/GaAsquantum dots of non-Stranski–Krastanow class by depositionof alternate InAs and GaAs layers with effective InAscoverage less than one monolayer [7]. These QDs, alsocalled sub-monolayer QDs (SMLQD), embedded in a GaAsmatrix, show a luminescence peak in the practically important0.92–1 µm spectral range. By simply changing the InAs

coverage, GaAs spacers and number of cycles, a desirablewavelength can be achieved. Moreover, as shown in thiswork, the sub-monolayer QDs are characterized by muchbetter uniformity and, when used as an active medium ofa diode laser, make it possible to achieve narrower gainspectrum, higher differential gain and lower threshold currentdensity.

In the present work, we studied characteristics of 0.94µm diode lasers based on self-organized quantum dots of twodifferent types, conventional SK and SML QDs. SML QD-based lasers are superior in CW power and efficiencyperformance. A high output power of 3.9 W from a 100 µmwide uncoated aperture and high power conversion efficiencyof 59% are reported.

2. Experiment

The laser structures under investigation were grown by solid-source molecular beam epitaxy on n+-GaAs(100) substratesusing the AlGaAs/GaAs graded-index separate-confinementheterostructure design. The first structure denoted as SK QDlaser contained 10 sheets of In0.5Ga0.5As QDs separatedby 5 nm Al0.15Ga0.85As spacers. The active region of thesecond (SML QD) laser represented an array of InGaAsQDs formed by alternate deposition of InAs (0.5 ML) andGaAs (2.5 ML), repeated 10 times. The details of MBEfabrication of these two types of structure were described in[8, 7], respectively. To compare both these QD lasers withthe conventional quantum-well (QW) laser we fabricated it in

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S S Mikhrin et al

the same geometry. The active region of this QW laser serveda 8.5 nm thick InxGa1−xAs QW with In content (x) of 16%.The epitaxial wafers were then processed into 100 µm widebroad-area stripe diodes by chemical etching through a p+-GaAs contact layer. No facet coating was used. Devices ofvaried length L were tested under pulsed-current conditions(5 kHz / 1 µs). For CW measurements, the devices weremounted p-side down on a copper submount and tested at10 ◦C.

3. Results and discussion

3.1. Formation of sub-monolayer QDs

The sequence of SML quantum dots formation isschematically shown in figures 1(a)–(d). It was foundpreviously [9] that, when the InAs coverage is less then1 ML and optimized MBE growth conditions are used, thinInAs film transforms into an array of 1 ML high islandspartly covering the surface. The islands of the second InAslayer separated from the first layer by a thin GaAs spacerspatially correlate with those of the preceding layer. As aresult of multiple sub-monolayer deposition, In-rich QD-like clusters consisting of several 1 ML high islands areformed, located one above another. A cross-sectional TEMimage of the sub-monolayer InAs (0.5 ML)/GaAs (2.5 ML)superlattice is shown in figure 1(e). The QDs are visible asdark contrasting regions. The vertical correlation of QDsseparated by sufficiently thin spacers has been previouslyobserved in Stranski–Krastanow systems and explained bythe effect of non-uniform strain fields [10]. In the case of thesub-monolayer growth mode, the essential point is that allthe constituent islands are characterized by the same heightof 1 ML. Also, we can assume that the pyramidal SK QDshape with usually large base-to-height aspect ratio resultsin a strong effect of small deviations of the QD height andside facet angle on the quantization energy. Contrastingly,SML QDs are characterized by better stability with respect tosmall deviations in size and shape, owing to the symmetricalshape and aspect ratio close to unity (see figure 1). Thus,we expect much better uniformity of the SML QD arrayas compared to the QDs formed in the three-dimensionalStranski–Krastanow mode.

Figure 2 shows photoluminescence (PL) spectra for bothstructures with chemically etched-off contact p+-GaAs layer.It can be seen that the full-width-at-half-maximum of theSML QD PL line is about 19 nm, which is 3.5 times narrowerthan that of the SK QD, estimated to be 67 nm. We believe thisto be due to the optimized growth mode used to form thesesub-monolayer QDs. This narrowing of the PL spectrumshows that SML QDs have much better uniformity, whichapparently could give a saturated gain sufficiently high toenable lasing on a single row of SML QDs.

3.2. Threshold current density and differential gain

Figure 3 plots the threshold current density (Jth) versusthe reciprocal cavity length (1/L) for both the lasers underinvestigation. The dependence of the lasing wavelength on1/L is also shown. For 2 mm long cavities, both the lasershave similar threshold current densities and wavelengths of

(a)

(b)

(c)

(d)

(e)

5 nm

Figure 1. Scheme of SML quantum dot formation: (a) depositionon InAs (<1 ML) on GaAs to give 1 ML high islands, (b) flatsurface after deposition of several ML thick GaAs cap, (c) verticalcorrelation of InAs islands, (d) InGaAs QDs formed by SMLdeposition and (e) cross-sectional TEM image of an SML QDstructure.

750 800 850 900 950 1000 1050

RT 100 W cm−2

67 nm

19 nmSK QD

SML QD

Wavelength (nm)

PL in

tens

ity (

arb.

uni

ts)

Figure 2. Room temperature PL spectra taken from SK QD (•)and SML QD (◦) structures with chemically etched-off contactp+-GaAs layer.

200 A cm−2 and 947 nm (SML) and 300 A cm−2 and 942 nm(SK). With decreasing diode length, the lasing wavelengthis blue-shifted and the threshold current density increasesto match the cavity loss. However, it is clearly seen thatthe SML QD laser demonstrates a weaker threshold currentdensity rise and a better wavelength stability with increasingmirror loss. In a 0.5 mm long diode, the wavelength of theSK QD laser reaches 920 nm with Jth of 800–900 A cm−2,while the SML QD laser still operates at a longer wavelengthof 940 nm with a relatively low threshold current density of

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0.94 µm diode lasers based on quantum dots

λ

Jth

0 5 10 15 20 25 300

500

1000

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870

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Reciprocal cavity length 1/ (cm )L −1

Thr

esho

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ensi

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Wav

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gth

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)λ

�� �

�

� � �

�

Figure 3. Threshold current density (Jth) and lasing wavelength(λ) versus reciprocal cavity length (1/L) for 100 µm wideSML QD (•), SK QD (◦) and QW (�) laser diodes tested inpulsed mode 5 kHz / 1 µs.

about 450 A cm−2. The slope of the Jth(1/L) dependence isinversely proportional to the differential gain, which is, thus,about two times higher in the SML QD laser as compared withthe SK QD device. We believe that both these facts are directconsequences of the weaker inhomogeneous broadening ofthe gain spectrum, associated with the better uniformity of theSML QD array. It should be noted here that the densities ofthe SK and SML QD arrays are nearly the same, of the orderof 1011 cm2. To compare these results with those obtainedfor InGaAs QW, we also plotted Jth(1/L) for our QW laserstructure. The QW laser exhibits a much slower increase inJth due to a much higher gain of QW as compared with QDsof both types.

The reciprocal differential efficiency is plotted againstthe cavity length in figure 4. It can be seen that, overthe whole range of cavity lengths studied, the differentialefficiency (external) of the SML QD laser is higher thanthat of SK QD. From this plot, the internal loss (αi) andthe internal quantum efficiency (ηi) were evaluated. Theinternal quantum efficiencies are comparable, being 100%and 96% for the SML QD and SK QD lasers, respectively.This fact reflects the high material quality and similar devicedesigns. However, the internal losses in these cases are quitedifferent. While the SML QD laser shows a reasonably lowαi of 2.3 cm−1, the internal loss in the SK QD laser is muchhigher, 10 cm−1. It has been shown that αi of a self-organizedQD laser is mostly governed by free carrier absorption in thewaveguide layer, the effective volume of the QD region beingnegligibly small. The extremely low internal loss of 1.3–1.5 cm−1 has been reported for 1.24 µm self-organized QDlasers [5, 11]. In these long-wavelength QD lasers, the pile-up of carriers in the waveguide layer is effectively suppressedthrough deep localization in the quantum dots. However, inthe present case of a 0.94 µm QD laser, one could expect astronger effect of free carrier absorption. The lower opticalgain of the SK QD laser forces the quasi-Fermi levels to gethigher, thereby increasing the concentration of free carriers inthe waveguide layer. Thus, the better uniformity of the QDarray, which can be achieved by using the sub-monolayer

0 500 1000 1500 2000 2500

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ipro

cal d

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ienc

y 1/

η D ηα

i

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i

i

= 0.94= 8 cm−1

ηα

i

i

= 1.0= 2.3 cm−1

Cavity length (µm)L

�

�

�

�

Figure 4. Reciprocal differential efficiency 1/ηD versus cavitylength L for the SML QD (•), SK QD (◦) and QW (�) lasers.

growth mode, also results in a lower internal loss. In the caseof QW, internal losses are as high as 8 cm−1. We believe thatthis is associated with the self-absorption in the QW stateslying higher than the Fermi energy.

3.3. Power and efficiency characteristics

It is well known that just after the threshold the outputpower is linearly proportional to the drive current. Thenthe slope efficiency decreases and the output power saturatesor even decreases with increasing current. The reason forthe thermal rollover is that only a part of the input electricpower is converted into light. Another part heats the diodeoperating in the CW mode. For the sake of better heatdissipation, relatively long (1−2 mm) diodes are commonlyused for high-power CW operation, although the slopeefficiency decreases with increasing cavity length. Owingto the combination of a high internal quantum efficiency andrelatively low internal loss, the SML QD laser has noticeablyhigher differential efficiency in longer diodes as compared tothat of the SK QD laser, e.g., 82% against 46% in 1 mm longdiodes. Taking into account the lower threshold currents ofthe SML QD lasers (see figure 3), we can expect superiorpower characteristics in this case.

The power of light emitted from two uncoated facets(Pout) is shown in figure 5 as a function of the drive current(I ) for 1 mm SML QD and SK QD diodes operating inCW mode at 10 ◦C heat-sink temperature. The maximumoutput power of the SK QD laser is 0.9–1 W. The rolloveris caused by the inefficient power conversion resulting fromthe low slope efficiency. The SML QD laser demonstratesmuch higher output power. The maximum output powerachieved for a 1.04 mm long stripe is 3.2 W. This valueis limited by the catastrophic optical mirror damage. Theoutput characteristics of a longer (1.76 mm) SML QD diodeare also presented in figure 5. In this case, the maximumoutput power is even higher, 3.85–3.9 W, owing to the betterheat dissipation.

The total power conversion efficiency (ηC) has beencalculated from an experimental Pout(I ) dependence anda measured diode voltage. Figure 6 shows ηC as a

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0 1 2 3 4 50.0

0.5

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L = 1760 µm

L = 1040 µm

SK QDSML QD

CW 10 °C W = 100 µmO

utpu

t pow

er×

2 fa

cets

(W)

P

Drive current (A)I

Figure 5. Power of light emitted from two uncoated facets (Pout)versus drive current (I ) for 100 µm wide SML QD (full curve fordifferent stripe lengths) and SK QD (broken curve) laser structuresoperating in CW mode at 10 ◦C.

0 1 2 30

20

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Output power (W)Pout

ηC

= 59%peak

ηC

= 21%peak

Con

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effi

cien

cy(%

)η C

Figure 6. Total power conversion efficiency (ηC) versus drivecurrent (I ) for SML QD (•) and SK QD (◦) lasers.

function of drive current for 1 mm long SML QD andSK QD laser diodes. It can be seen that the SK QDdiode has relatively low power conversion efficiency witha maximum value of 21% at 0.7 W output power. Onthe contrary, the SML QD laser exhibits much betterefficiency. The device has a maximum total power conversion

efficiency of 59% at a total power output of 1.5 W. To the bestof our knowledge, this is the highest efficiency ever reportedfor any diodes based on self-organized QDs. The figurealso shows that the SML QD device has a power conversionefficiency exceeding 50% over a wide range of Pout, from0.55 to 3 W. This result unambiguously demonstrates thepotential of QD lasers based on uniform QD arrays for high-power applications.

In conclusion, we compared two types of diode laserswith InGaAs/GaAs QDs as active medium. The activeQD region was formed either by the conventional Stranski–Krastanow technique or by sub-monolayer growth. Owingto the better uniformity of sub-monolayer QDs, the SML QDlaser shows two times higher differential gain and much lowerinternal loss, as compared to the SK QD laser. This results insuperior power characteristics, which compare well with thebest achievements of QW lasers. In particular, a maximumoutput power of 3.9 W and a peak power conversion efficiencyof 59% were achieved in the SML QD laser.

Acknowledgments

This work was supported by the Bundesministerium furBildung und Forschung and Russian Foundation for BasicResearch.

References

[1] Bruesselbach H W, Sumida D S, Reeder R A and Byren R W1997 IEEE J. Quantum Electron. 3 105–16

[2] O’Brien S, Zhao H, Schoenfelder A and Logan R J 1997Electron. Lett. 33 1869–71

[3] Al-Muhanna A, Mawst L J, Botez D, Garbuzov D Z,Martinelli R U and Connoly J C 1998 Appl. Phys. Lett. 731182–4

[4] He X, Srinivasan S, Wilson S, Mitchell C and Patel R 1999Electron. Lett. 34 2126–7

[5] Zhukov A E et al 1999 IEEE Photon. Technol. Lett. 111345–7

[6] Alferov Zh I et al 1996 Semiconductors 30 194[7] Zhukov A E et al 1999 Electron. Lett. 35 1845–7[8] Maximov M V et al 1998 J. Appl. Phys. 83 5561–3[9] Ledentsov N N, Wang P D, Sotomayor-Torres C M,

Egorov A Yu, Maksimov M V, Ustinov V M, Zhukov A Eand Kop’ev P S 1994 Phys. Rev. B 50 12171–4

[10] Ledentsov N N et al 1996 Phys. Rev. B 54 8743–50[11] Lester L F, Stinz A, Li H, Newell T C, Pease E A, Fuchs B A

and Malloy K J 1999 IEEE Photon. Technol. Lett. 11931–3

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