Carrier recombination in 1.3 μ m Ga As Sb ∕ Ga As quantum well lasersK. Hild, S. J. Sweeney, S. Wright, D. A. Lock, S. R. Jin, I. P. Marko, S. R. Johnson, S. A. Chaparro, S.-Q. Yu,and Y.-H. Zhang Citation: Applied Physics Letters 89, 173509 (2006); doi: 10.1063/1.2369649 View online: http://dx.doi.org/10.1063/1.2369649 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/89/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Nonradiative recombination in 1.56 μ m GaInNAsSb/GaNAs quantum-well lasers Appl. Phys. Lett. 95, 231104 (2009); 10.1063/1.3271182 Recombination processes in midinfrared InGaAsSb diode lasers emitting at 2.37 μ m Appl. Phys. Lett. 89, 051104 (2006); 10.1063/1.2243973 Theoretical study of Auger recombination in a GaInNAs 1.3μ m quantum well laser structure Appl. Phys. Lett. 84, 1826 (2004); 10.1063/1.1664033 Temperature dependence of continuous wave threshold current for 2.3–2.6 μm InGaAsSb/AlGaAsSb separateconfinement heterostructure quantum well semiconductor diode lasers Appl. Phys. Lett. 74, 2990 (1999); 10.1063/1.123989 Analysis of temperature dependence of the threshold current in 2.3–2.6 μm InGaAsSb/AlGaAsSb quantum-welllasers Appl. Phys. Lett. 74, 2743 (1999); 10.1063/1.124000

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Carrier recombination in 1.3 �m GaAsSb/GaAs quantum well lasersK. Hild, S. J. Sweeney,a� S. Wright, D. A. Lock, S. R. Jin, and I. P. MarkoAdvanced Technology Institute, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom

S. R. Johnson, S. A. Chaparro, S.-Q. Yu, and Y.-H. ZhangMBE Group, Arizona State University, Tempe, Arizona 85287

�Received 14 August 2006; accepted 19 September 2006; published online 26 October 2006�

In this letter the authors present a comprehensive study of the threshold current and its temperaturedependence in GaAsSb-based quantum well edge-emitting lasers for emission at 1.3 �m. It is foundthat at room temperature, the threshold current is dominated by nonradiative recombinationaccounting for more than 90% of the total threshold current density. From high hydrostatic pressuredependence measurements, a strong increase in threshold current with pressure is observed,suggesting that the nonradiative recombination process may be attributed to electron overflow intothe GaAs/GaAsP barrier layers and, to a lesser extent, to Auger recombination. © 2006 AmericanInstitute of Physics. �DOI: 10.1063/1.2369649�

Lasers emitting close to 1.3 �m are of considerable im-portance for the development of metro-area networks.1

Progress in this area has been hindered largely by the need tosignificantly reduce the cost of the laser module itself. Theincumbent InGaAsP quantum well �QW� material systemused to make such lasers suffers from two problems: Firstly,the fact that they are grown on InP makes it difficult toproduce vertical cavity surface emitting lasers �VCSELs�which are vastly more cost effective but are better suited toGaAs substrates. Secondly, the devices are highly susceptibleto temperature variations resulting in the need to incorporatesophisticated temperature control electronics into the pack-age, leading to an order of magnitude increase in cost.Hence, there has been considerable effort devoted to the de-velopment of GaAs-based laser active regions which emit at1.3 �m. InAs quantum dots2 and GaInNAs-based QWs havebeen the subject of extensive research. However, the proper-ties of even the best quantum dot lasers are far from ideal,since their threshold current density increases quickly withtemperature around room temperature due to nonradiativerecombination resulting in a low characteristic temperature,T0�50 K �T0= �d ln Ith /dT�−1�, similar to that of standard1.3 �m QW based lasers.3 P-doped quantum dot lasers canexhibit very high T0 values �even infinite over a narrow tem-perature range�, but this is achieved at the expense of higherthreshold currents compared with undoped devices.4 ForGaInNAs-based QW lasers, it has been shown that even forthe best 1.3 �m devices available, approximately 50% of thethreshold current at room temperature may be attributed todefect-related recombination.5 The implications of this onlong-term device stability have yet to be fully addressed.Another possibility is the use of GaAsSb/GaAs QWs.6 La-sers based upon this material have been produced,7 but little,if any, research has been undertaken to assess the carrierrecombination and temperature dependent processes occur-ring in such devices. The aim of this letter is to consider thecharacteristics of GaAsSb/GaAs-based edge-emitting lasersand to explore the potential of GaAsSb/GaAs active regionsfor use in 1.3 �m VCSELs.

The devices in this study consist of a tripleGaAs0.9P0.1 /GaAs/GaAs0.7Sb0.3/GaAs/GaAs0.9P0.1 �9 nm/5 nm/7 nm/5 nm/9 nm� strain compensated QW active re-gion grown at 585 °C by molecular beam epitaxy. The nomi-nal 30% Sb concentration is estimated based on photolumi-nescence measurements. The GaAsP layers provide straincompensation to increase the maximum number of highlystrained QWs that can be grown pseudomorphically and toreduce strain driven in-plane Sb segregation that leads toinhomogeneous linewidth broadening. The active region issandwiched between two 20 nm Al0.25Ga0.75As layers, two150 nm graded-index �GRIN� AlGaAs layer with Al concen-tration linearly increased from 25% to 65%, one 2 �mn-type �Si doped, 2�1018 cm−3� Al0.65Ga0.35As claddinglayer followed by 500 nm GaAs buffer layer at bottom, andone 2 �m p-type �Be doped, 2�1018 cm−3� Al0.65Ga0.35Ascladding layer followed by 100 nm GaAs cap layer at top.The doping concentration is decreased from 2�1018 to1�1017 in both GRIN layers from cladding layer to activeregion and is increased to 2�1019 in GaAs cap layer fromcladding layer to surface. The devices are fabricated using atypical broad contact edge-emitting laser process. The deviceridges �50 and 100 �m wide� were defined using photoli-thography and inductively coupled plasma dry etching. Byetching down through the p-GaAs contact layer and stoppingabout 0.1 �m above the active region, these ridges providecurrent confinement as well as waveguiding. Ti/Pt/Aup-contact stripes ranging from 40 to 90 �m were depositedusing a second mask, after which the wafers are lapped downto 100 �m and AuGe/Ni/Au n-metal contacts are depositedon the backside of the substrate; this is followed by rapidthermal annealing for both metal contacts. The devices weremeasured as cleaved. Their room temperature emissionwavelength was measured to be 1.27 �m.

Temperature dependence measurements were performedwith a standard closed cycle cryostat setup over the tempera-ture range of 60–290 K. The lasers were driven under pulsedoperation with 200 ns long pulses at a duty cycle of 10 kHzin order to minimize Joule heating effects. We investigatedthe temperature and current dependencies of the spontaneousemission spectra from which we extracted the radiative cur-rent �since the integrated spontaneous emission is propor-

a�Author to whom correspondence should be addressed; electronic mail:s.sweeney@surrey.ac.uk

APPLIED PHYSICS LETTERS 89, 173509 �2006�

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tional to the radiative current�. To measure the spontaneousemission, we milled a window in the n side of the devicesand aligned an optical fiber to collect the spontaneous emis-sion. This technique is described in detail elsewhere.8 Hydro-static pressure measurements were also performed on the de-vices. The application of high hydrostatic pressure causes anincrease in the direct band gap, mimicking the effect of al-loying, thereby allowing one to investigate wavelength de-pendent properties of semiconductor lasers. It can also beused to vary the alignment between direct ��� and indirect�X, L� bands and to alter the band offsets between layerswhich have different pressure coefficients.9 In this work, hy-drostatic pressure measurements were performed over therange of 0–8 kbar using a gas compressor system.

The integrated spontaneous emission �Lspon� versus cur-rent at two different temperatures, 110 and 260 K, is shownin Fig. 1. In both cases the integrated spontaneous emissionpins at threshold due to the fact that the carrier density isclamped by the lasing process. The two curves have beennormalized to the value of Lspon at threshold �=Lpin�, so thatthe subthreshold shape of both curves can be easily com-pared and contrasted. There is a clear difference in the be-havior of the device at the two temperatures. At 110 K, thesubthreshold Lspon versus current curve is linear, suggestingthat the primary current path flowing through the laser maybe associated with radiative recombination. Furthermore, thefact that the curve remains linear down to low currents sug-gests that defect-related recombination makes a relativelysmall contribution to the threshold current. In stark contrast,the sublinear behavior of the Lspon versus current curve at260 K suggests that a nonradiative process is present andthat this process has a stronger carrier density dependencethan the radiative current.

In Fig. 2 we show the normalized �at 60 K� temperaturedependence of the threshold current density Jth �diamonds�.

We find that the devices are highly temperature sensitivewith a characteristic temperature T0�60–70 K at RT com-parable to values that have been reported elsewhere forGaAsSb/GaAs devices7 and InGaAs�P� / InP devices8 emit-ting around 1.3 �m. From measurements on probed broadarea devices at 300 K, we estimate Jth to be �1.6 kA cm−2

Also shown in Fig. 2 is the temperature dependence of theradiative current density Jrad �squares�, which was foundfrom the pinning level of the integrated spontaneous emis-sion, Lpin, as seen in Fig. 1, since at threshold Lpin�Jrad.From this, we estimate that at 290 K the radiative recombi-nation process accounts for �10% of the total threshold cur-rent. Clearly, nonradiative processes dominate and accountfor �90% of Jth close to RT. From the inset in Fig. 2, itcan also be seen that Jrad has the ideal QW linear tempera-ture dependence,10 leading to the conclusion that the lossmechanism�s� must increase superlinearly with increasingtemperature.

Further evidence for the importance of nonradiative re-combination may be found from high pressure measure-ments. Figure 3 shows the measured room temperature pres-sure dependence of Jth for the GaAsSb devices �circles�. Alsoshown is the ideal expected variation of Jrad�Eg

2 �Ref. 11��dot-dot-dashed line�, where Eg is the band gap �taken fromEg=hc /�lasing, where �lasing is the measured lasing wave-length�. Note that in each case, Jth and Jrad have been nor-malized to their respective values at atmospheric pressure.Clearly, it can be seen that Jth increases more rapidly withpressure than the ideal Jrad. Preliminary pressure measure-ments suggest that the pressure dependence of Jrad in ourdevices is very close to ideal.12 From the earlier temperaturedependence measurements, we found that a nonradiative pro-cess dominated at room temperature forming �90% Jth. Wetherefore conclude that the observed strong increase of Jthwith pressure is due to nonradiative recombination. The con-duction band offset of the GaAsSb/GaAs interface �for 30%Sb� is still a matter of debate with reports in the literature ofboth weak type I �Ref. 13� and type II alignments.14 Thisindicates that the conduction band offset is close to zero. Dueto the slightly larger pressure coefficient of the GaAsSb �Ref.15� QW � minimum compared to the GaAs barrier �minimum16 �difference= +1.2 meV/kbar�, occupation of

FIG. 1. Integrated spontaneous emission as a function of current at 110 K�solid diamonds� and 260 K �open squares�. At 110 K Lspon increases lin-early with current, indicating that it is dominated by radiative recombina-tion. The contrasting sublinear behavior at higher temperatures indicates thepresence of a nonradiative process. Note that at each temperature, Lspon hasbeen normalized to its value at threshold �Lpin�. The lines are guides tothe eye.

FIG. 2. Jth and Jrad as a function of temperature �normalized at 60 K�. Theinset shows the temperature dependence of Jrad on an expanded scale fromwhich it can be seen to exhibit an ideal QW-like linear temperature depen-dence �indicated by the dashed line�.

173509-2 Hild et al. Appl. Phys. Lett. 89, 173509 �2006�

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electrons in the GaAs barrier states will increase with in-creasing pressure, as shown by the calculated dotted line inFig. 3. Furthermore, due to the small or possibly negative�type II� GaAsSb/GaAs band offset, the GaAsP layers act asadditional barriers. With increasing pressure, the X minimain the GaAsP layers move to lower energy and may providean additional leakage path, as shown by the dot-dashed linein Fig. 3. Thus, for a fixed quasi-Fermi-level splitting, theoccupation of the barrier layers will increase exponentiallywith increasing pressure leading to an increased leakage cur-rent. While carrier leakage into the GaAs and/or GaAsP bar-rier layers offers a possible explanation for the increase of Jthwith pressure, it is clear that this in itself is insufficient. Thelower rate of increase of Jth with pressure dependence com-pared to the leakage paths may be explained if carrier leak-age and a nonradiative process that decreases with pressureare both present. In Fig. 3 for comparison we plot the pres-sure dependence of Jth �open squares� and the Auger current�solid line� for a typical 1.3 �m InGaAsP device �taken fromRef. 17�. For the InGaAsP devices, Auger recombination isthe dominant nonradiative process8,10,17,18 causing Jth to re-duce with pressure due to the decrease in the Auger currentwith pressure. We therefore propose that the nonradiativerecombination current which dominates Jth for the GaAsSbdevices at room temperature is due to a combination of car-

rier leakage and Auger recombination. However, based onour data, it is not yet possible to quantify the role of each ofthese nonradiative current paths.

In summary, from our first studies of 1.3 �m GaAsSblasers, we find that at room temperature the threshold currentis dominated by nonradiative recombination. The nonradia-tive process is also responsible for the poor temperature sen-sitivity of the devices resulting in low T0 values around roomtemperature. From our pressure dependence measurements,we observe that the threshold current increases with pressure,consistent with the transfer of carriers into the GaAs/GaAsPbarrier layers due to the decrease of the band offset withpressure. This suggests that carrier overflow into the GaAsand/or GaAsP barrier layers together with Auger recombina-tion forms a large nonradiative current path in these devicesat normal operating temperatures.

The authors thank E. P. O’Reilly and S. B. Healy �Tyn-dall Institute, Ireland� for useful discussions. They also grate-fully acknowledge EPSRC �UK� for supporting this projectunder Grant No. GR/T21516/01.

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FIG. 3. Measured pressure dependence of Jth for the GaAsSb devices�closed circles� and ideal Jrad �Eg

2 dependence� �dot-dot-dashed line�, nor-malized at atmospheric pressure. The dotted line indicates the calculatedpressure dependence of carrier leakage into the GaAs � minimum, while thedot-dashed line indicates the pressure variation of carrier leakage into the Xminima of the GaAsP layers. For comparison, the pressure dependence of Jth

for a 1.3 �m InGaAsP device is shown �open squares� together with thecalculated variation in the Auger current with pressure for the InGaAsPdevice �solid line� �derived from Ref. 17�.

173509-3 Hild et al. Appl. Phys. Lett. 89, 173509 �2006�

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