Spectroscopic investigations of GaAsSb/GaAs basedstructures for 1.3mm VCSEL applications

G. Blume, T.J.C. Hosea, S.J. Sweeney, S.R. Johnson, J.-B. Wang and Y.-H. Zhang

Abstract: A spectroscopic investigation of GaAsSb/GaAs quantum well (QW) structures grown onGaAs substrates by molecular beam epitaxy is presented. Besides studying their temperature-dependent photoluminescence (PL), the low temperature PL of a series of GaAsSb/GaAs/AlGaAsstructures, with varying GaAs spacer thickness is investigated. This latter study was undertaken toinvestigate the GaAsSb/GaAs band alignment. Blue shifts in the PL peak as a function of excitationlaser intensity at a temperature of 10 K for the range of spacer thickness variation are studied. It isobserved that significant blue shifting occurs only for spacers thicker than ,2 nm. It is tentativelysuggested that this is indicative of a transition from the electrons and holes being predominantlyconfined in the same layer (the QW) to being more strongly confined in adjacent layers, as thespacer thickness increases. The angle-dependent photomodulated reflectivity (PR) of similarsamples is investigated. Here strong low energy interference oscillations (LEIO) are encountered,which tend to obscure any PR signals arising from the QW. The latter are exploited to estimate therefractive index of the layer responsible for the LEIO, and thus identify it. However, a way to avoidthe LEIO is shown, by shortening the laser excitation wavelength, which results in measurable PRsignals from the QW region, yielding several QW transition wavelengths.

1 Introduction

Semiconductor lasers emitting close to 1:3 mm are ofconsiderable interest for local or ‘metro’ area applications.Lasers at this wavelength, optimised for the dispersionminimum of silica fibres, enable fast fibre-to-the-homebased networks. Vertical-cavity surface-emitting lasers(VCSELs) are the ideal source for such applications,which primarily demand low cost and uncooled stableoperation. However, the traditional InGaAsP=InP activeregions used in conventional 1:3 mm edge-emitting lasersare incompatible with the GaAs material system, which hasbeen successfully used to produce VCSELs, for example, at850 nm and 650 nm. This has led to the search for newGaAs-based active regions, which provide photon emissionat 1:3 mm: Materials that have been considered includeInAs=GaAs quantum dots and dilute-nitride quantum wells(QWs). Whilst lasers based upon these materials have beendemonstrated, their performance is non-ideal due largely toinhomogeneities in the growth of quantum dots [1, 2] andhigh defect densities in the dilute-nitride system [3].

GaAsSb=GaAs QWs have been proposed as an alterna-tive material system for use in the active region of 1:3 mmVCSELs and successful VCSEL operation has recently beendemonstrated [4]. There are, however, uncertainties aboutthe nature of the band alignment in GaAsSb for Sb fractions

�30–40%; as required for 1:3 mm emission [5]. Manyreports in the literature claim that the QW is type II in nature[6, 7] whilst others suggest weak type I alignment [8](see Fig. 1). For successful device optimisation it isimportant to have an understanding of the recombinationprocesses in such structures, which will be stronglyinfluenced by the band alignment.

Here, we report on investigations of GaAsSb=GaAs QWsusing conventional photoluminescence (PL) andphotomodulated reflectivity (PR) techniques. We study theblue shift of the PL as a function of temperature and at lowtemperatures as a function of the excitation intensity.The latter study enables us to investigate the changes in thealignment of the conduction band of the GaAsSb=GaAsinterface as a function of the thickness of GaAs spacers in aseries of samples. Furthermore, initial PR measurementsshow strong low energy interference oscillations (LEIOs)when certain excitation lasers are used, which tend toobscure any PR signals from the QW. However, we exploitthese to obtain an average refractive index of the region ofthe device being modulated, which in turn suggests that theinterface of the substrate with the epitaxial layers isresponsible for these oscillations. We show that the LEIOscan be avoided by using a laser with a shorter wavelength,which decreases the absorption depth of the modulation, andthus yields clear PR signals from the QW region. From a fitto the QW PR signals we deduce the energies of the room-temperature ground-state and two higher-order QWtransitions.

2 Samples

We investigated a set of samples grown on (100) n-typeGaAs substrates using solid source molecular beam epitaxy[9], which consists of a 400 nm GaAs buffer layer, QWregion and a 30 nm GaAs top layer. The QW regioncomprises a 7 nm GaAs0:64Sb0:36 single QW (SQW) with

q IEE, 2005

IEE Proceedings online no. 20055024

doi: 10.1049/ip-opt:20055024

G. Blume, T.J.C. Hosea and S.J. Sweeney are with the AdvancedTechnology Institute, University of Surrey, GU2 7XH, UK

S.R. Johnson, J.-B. Wang and Y.-H. Zhang are with the MBE Group,Arizona State University, Tempe, Arizona, USA

E-mail: j.hosea@surrey.ac.uk

Paper first received 29th June and in revised form 8th December 2004

IEE Proc.-Optoelectron., Vol. 152, No. 2, April 2005110

GaAs spacers on either side of width varying between 1 and9 nm, placed within 75 nm Al0:25Ga0:75As carrier-confine-ment layers (see Table 1). Further details can be found inref. [8].

We also investigated a related sample with two7 nm GaAs0:64Sb0:36 QWs, 3 nm GaAs spacers, and 50 nmAl0:3Ga0:7As carrier-confinement layers, but with 8 nmGaAsP layers interposed between the GaAs spacers andAlGaAs layers, for the purposes of strain compensation,similar to structures studied by Zheng et al. [10] (althoughthese had no confinement layers). All of the grown epitaxiallayers were nominally undoped.

3 Low temperature photoluminescence

PL is a convenient technique for obtaining structuralinformation from optoelectronic semiconductor samples.The samples are excited by a mechanically-chopped laser,and the resulting spectral emission detected phase sensi-tively with a lock-in amplifier. PL spectra are often ratherbroad and dim at room temperature, so samples aregenerally cooled.

In our PL set-up we used a 2 mW, 633 nm HeNeexcitation laser focussed onto a �1mm2 spot on thesamples. The sample temperature was varied between 10

and 325 K, using a closed cycle liquid helium cryostat.The PL was measured with a 0.3 m monochromator, InGaAsphotodiode, and a Stanford SR830DSP lock-in amplifier,and was subsequently corrected for instrumental response,using the calibrated output spectrum of a Bentham 605 lightsource.

3.1 Temperature dependentphotoluminescence

Figure 2 shows the temperature dependent PL spectra of the3 nm sample, from the spacer variation series (Table 1), at alaser power density of 0:2W=cm2: At room temperature, thesample exhibits a very broad QW emission centred on0.967 eV, with a full width of half maximum (FWHM) of�140meV: When cooled, the PL signal becomes muchmore pronounced, the peak blue shifts to 1.039 eV, and theFWHM drops to 14 meV. This is 75% narrower than the PLlinewidth of 54 meV reported by Zheng et al. [10] forsimilar samples, but without Al0:3Ga0:7As cladding layers,which is consistent with the improvement in carrierconfinement in the present samples. Note the second PL

Fig. 1 Schematic of possible conduction (CB) and valence band (VB) alignments in examples of present structures with GaAsSb QWs,GaAs spacers, and AlGaAs barriers, showing schematic wave functions (grey)

a For type I alignment the electrons and holes are both confined to the QW, allowing direct transitions to occur. If the alignment is type II, the electronconfinement varies with spacer thicknessb Small spacers may increase the electron confinement energy above the band-offset and cause the wave function to remain in the GaAsSb layer, so thatrecombination is direct and the sample behaves as if it were type Ic Wider spacers may favour a probability of the electrons being confined in the GaAs spacers so that recombination would occur by cross-interface transitions

Table 1: Nominal structure of the GaAs spacer series ofsamples, where the samples had spacer thicknesses of 1,2, 3, 6 and 9 nm. Note the 400 nm-thick undoped GaAsbuffer next to the n-type substrate, which is ofimportance in the PR measurements

Material Thickness (nm)

GaAs 30

Al0:25Ga0:75As 75

GaAs 1–9

GaAs0:64Sb0:36 7

GaAs 1–9

Al0:25Ga0:75As 75

GaAs 400

n-type GaAs substrate

Fig. 2 Temperature-dependent photoluminescence of GaAsSb=GaAs=AlGaAs SQW sample with 3 nm GaAs spacers

The PL exhibits a broad (34 meV) QW feature peaked at 0.969 eV at 300 K,narrowing to 12 meV FWHM, and moving to 1.036 eV, when cooled to10 K. A possible impurity transition (IMPT) can be seen near 1.18 eV in thelow-temperature PL

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peak at about 1.49 eV below 50 K, which can be attributed toa direct transition of the GaAs spacers. Between the QW andGaAs peaks there appears a very broad feature centred near1.18 eV below 150 K. This is at too high an energy(�140meV) above the main PL peak to be due to ahigher-order QW transition, since the manifold of QWtransition energies was identified subsequently using roomtemperature PR and was found to occur over a muchnarrower energy-range above the QW ground-state tran-sition energy (see Section 4.2). Rather, we believe this peakis most likely due to deep-level defects and impurities in theAl0:3Ga0:7As confinement layers, such as Ga vacancies asdescribed in [11].

3.2 Intensity dependent photoluminescence

In these experiments we studied the peak PL wavelength asa function of the power of the excitation laser. The intensityof most HeNe lasers is fixed, so in order to vary this weinterposed a series of different neutral density filters into thebeam, giving laser power-density at the sample between 2and 200mW=cm2: Since we could clearly only reduce thelaser intensity by this method, we were forced to performthis study at a low temperature of 10 K, where sufficientinitial PL signal was present.

Considering first the results for the wider spacer samples,Fig. 3a shows an example of intensity-dependent QWemission peaks from the GaAs spacer variation series(Table 1), for a 6 nm spacer sample. Here, compared to thePL peak position at the lowest available laser intensity, weobserve a total blue shift of �12meV; as the excitationpower is increased by two orders of magnitude, whichsaturates at higher power. This behaviour is quantified inFigs. 4 and 5, as discussed later. Comparable behaviourhas also been observed in GaAsSb=GaAs QW samples byChiu et al. [12] and Dinu et al. [6], who attributed it to theexistence of a type II band alignment, which results inelectrons and holes being confined in adjacent layers.The Coulomb attraction between the optically-excitedelectrons and holes on either side of the GaAsSb=GaAsinterface creates an electric field across the interface, whichleads to significant band bending. Chow et al. [13] also

Fig. 3 Photoluminescence at 10 K obtained for the AlGaAs=GaAs=GaAsSb SQW sample with 6 and 1 nm GaAs spacersrespectively, against power density of HeNe laser excitation source(indicated next to each spectrum)

a The spectra of the 6 nm spacer sample show a significant total blue shift(amounting to 12 meV) when the excitation intensity is increased by twoorders of magnitude from app. 2mW=cm2 to 200mW=cm2

b In contrast, the spectra of the 1 nm spacer sample show no measurableshift

Fig. 4 10 K PL peak positions of the 6 nm GaAs spacer sample(open circles) in Fig. 3a against low laser excitation intensityð� 20 mW=cm2ÞThe curve is an empirical fit using (1)Insets: Schematics of the band structure of the QW region with the ground-state electron and hole wave functions (grey) for wide GaAs spacersassuming a type II GaAsSb=GaAs band offset (see Fig. 1). The large blueshift with increased laser excitation can be explained by charge separationof the excited carriers and subsequent band bending, as discussed in themain text

Fig. 5 Maximum relative blue shift DEmax of PL peaks observedfor full range of investigated GaAs spacer thickness variation(open circles)

The samples with the wider spacer thicknesses exhibit a clear blue shift,while those with smaller spacers show no measurable shiftInset: Energy shift DE (see (1)) over the whole range of excitation intensityfor both samples shown in Fig. 3: 6 nm spacers (open circles) and 1 nmspacers (full circles)The curves in the Figure and the Inset are guides to the eye

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observed a large blue shift, but in GaAsSb=InGaAs QWswhere the band alignment is type II in nature. Theyinvestigated this theoretically using microscopic modellingand found several opposing contributions to a net blue shift:they argue that, for the most part, this is due to the effects ofband filling and the charge separation itself, but that bandbending and many-body effects partly screen these effects.However, our experiment cannot distinguish the interplaybetween these simultaneously occurring contributions andwe follow the argument of [6] and [12], as follows. Whenthe excitation is increased, additional carriers are created,which enhances the electric field and thus the band bending.This in turn distorts the QW from a simple rectangularprofile (see Fig. 1c) to a roughly triangular form, asexplained schematically in the insets in Fig. 4. This raisesthe confinement energies, which will thus initially blue shiftas laser power increases.

The saturation behaviour of the blue shift at high laserpower (see inset of Fig. 5 for the 6 nm spacer sample) is alsoconsistent with this picture: since the electron wave-function now extends significantly into the GaAsSb layer,direct recombination with the holes becomes much morelikely, thus effectively causing it to behave partly like atype I interface (see right inset in Fig. 4).

Figure 4 shows the PL peak positions of Fig. 3a, E, forthe lower range of laser excitation powers, I. We may fit thisbehaviour with an empirical description given by:

EðIÞ ¼ E0 þ DEðIÞ ¼ E0 þ að1 � e�bIÞ ð1Þ

where a and b are fitting parameters and E0 the unperturbedPL peak position at zero laser power, I ¼ 0; which in thecase of the 6 nm spacer sample is estimated to be 1:007 eV�1meV: This allows us to depict the relative energy shiftDE as a function of excitation power, as indicated in theinset of Fig. 5, which illustrates the different behaviour ofsamples shown in Fig. 3.

Alternatively, when the GaAs spacers are narrow(as illustrated in Fig. 1b) we observe no significant shift inthe PL peak position for any laser excitation power (seeFig. 3b and inset of Fig. 5 for the 1 nm GaAs spacer sample).Here, the initial peak position E0 at 10 K (see (1)) is at ahigher energy than that of the wide spacer samples (see e.g.Fig. 3) due to stronger quantum-confinement, as discussedextensively by Johnson et al. [8]. Our observed decrease ofE0 with increasing spacer thickness is similar to that of [8]and so we do not show these data, but our results may bedescribed empirically by:

E0ðxÞ ¼ 1:011 þ 0:047 expð�0:586xÞ ð2Þ

where x is spacer thickness in nm and E0 is in eV. In thenarrow spacer case, because of the additional AlGaAsconfinement, the transition is spatially direct owing to thefact that both carrier types are predominantly localised inthe centre of the QW region (see Fig. 1b). Thus, it behavesin a similar manner to a type I band alignment, and thetransition energy is not influenced significantly by increas-ing the laser power.

Figure 5 summarises the results for the total observed PLblue shift for laser intensities between zero and 200mW=cm2; for the range of GaAs spacer thicknesses in thesamples. One can see that for the narrow spacers (1–2 nm)there is no measurable blue shift, while for wider spacers(3, 6 and 9 nm) a blue shift clearly occurs. This is consistentwith the picture of an evolution from a situation where theelectrons and holes are confined in the same QW layer, tobeing confined in adjacent layers, as the spacer thicknessincreases from 2 to 3 nm, as depicted schematically in

Fig. 1b and c. We tentatively suggest that this is due to areduction in the ground-state confinement energy when theconfinement region gets wider with increasing spacerthicknesses. This is supported by the aforementionedreduction in E0 with increasing spacer thickness, (2). In thewide spacer case the ground-state electron wave-functionextends further out into the GaAs spacers, reducing theoverlap with the ground-state hole wave-function, thusgiving a spatially-staggered arrangement normallyassociated with type II behaviour (see Fig. 1c).

However, it should be mentioned that, at this stage, wecannot discount the possibility that the excitation-dependentblue shift may be in part due to inhomogeneous materialcaused by strain-driven segregation, that depends on growthtemperature, Sb mole fraction, growth rate, and possibly theexistence of unintentional surface segregating impuritiesas seen by Khreis et al. [14] and Kaspi et al. [15].Nevertheless, the fact that we observe a clear differencebetween the wide and narrow spacer layers supports theinterpretation given above, in terms of the changes inelectron confinement. Clearly, however, a full theoreticalmodelling of the band alignment in these structures isneeded before firm conclusions can be drawn. This will bean aim of future, more detailed, investigations.

4 Room temperature photomodulatedreflectance

PR is a non-destructive, all-optical technique and ofcomparable simplicity in use to room temperature PL.However, it is far more versatile, e.g. allowing investi-gations of indirect band-gap materials [16] or even complexmultilayer structures such as vertical cavity structures [17].It generally produces much sharper features from criticalpoints at room temperature than PL and cooling is thereforenot usually required. Furthermore, while PL generally onlygives the ground-state transition in QW samples, PR oftenreveals this, together with higher-order transitions from theQWs and other layers in the structures. This has provedextremely useful for extracting structural details such aslayer thickness, strain, and composition [18, 19]. Since thepresent PL measurements gave only a single peak from theQWs, it was of interest, therefore, to measure the samesamples with PR to see if more information about the QWscould be gleaned.

The mechanism behind PR may be summarised briefly asfollows. If during a normal DC measurement of thereflectivity (R) spectrum, the semiconductor sample issimultaneously modulated with a chopped laser, thischanges the carrier density, thus the dielectric functionand hence produces small additional reflectance pertur-bations, DR: These AC components of R are readilydetected phase sensitively with a simple conventionaldetector=lock-in amplifier arrangement. The resulting PRspectra generally have sharp differential-like lineshapescentred on optical transitions in the material. The PR signalis defined as the relative change in reflectance, DR=R; whicheffectively normalises out any spectral variations owing tothe measurement equipment itself.

For our experiment we used the same equipment (laser,chopper, spectrometer, detector and lock-in amplifier) as forthe PL, except that the set-up was changed to theconventional PR arrangement [20], as follows. A 100 Wtungsten filament lamp was used as input to the spec-trometer to provide the monochromatic incident light on thesample, which was then reflected (at a variety of angles, seelater) into the InGaAs detector. The same spot on the sample

IEE Proc.-Optoelectron., Vol. 152, No. 2, April 2005 113

was simultaneously modulated with the chopped (330 Hz)HeNe laser. The DC (R) and AC ðDRÞ components of thesignal were recorded simultaneously as a function ofwavelength.

4.1 Angular dependent room-temperaturephotomodulated reflectance

Although, as mentioned earlier, PR often yields clearfeatures from QWs and other layers in semiconductorsamples, the spectra may be occasionally confused byobtrusive unwanted extra features arising from opticalinterference effects. These may be manifested as widely-spaced, decaying fringes, at energies below the band-gap ofthe substrate: so-called low energy interference oscillations(LEIO) [21]. Unfortunately, we find that LEIO do indeedoccur in the PR of the present samples (see later).The occurrence of LEIO in PR has been studied earlier[22, 23] and attributed to interference between light raysreflected from adjacent interfaces within the structuresstudied. In those studies, positive identification of the originof the LEIO was achieved by varying the thickness of thelayers responsible, by destructive etching of the samples.However, an easier, non-destructive, parameter to changeduring the PR measurements is the angle of incidence. Thischanges the optical thickness of any layers responsible forLEIO, and thus their position and spacing in the PRspectrum, while the electronic transition features cannotdepend on incidence angle, and are therefore static. Thisallows one to identify quickly which PR features are due toelectronic transitions and which are due to interference [24].It is well known that, e.g. destructive interference betweentwo rays reflected from the top and bottom of a givenuniform layer within a structure, of effective refractiveindex n, occurs at a wavelength given by:

lðyÞ ¼ lð0Þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 � sin2 y

n2

sð3Þ

where y is the angle of incidence and lð0Þ the wavelengthat normal incidence [25]. In LEIO we expect that the

wavelengths of all the features will vary with angleaccording to (3).

Figure 6 shows the room temperature PR spectra,obtained with the HeNe laser, as a function of varyingincidence angle for a strain-compensated sample. Mostprominent is a very sharp feature from the GaAs at 1.42 eV.On the high energy side of this are smaller features between1.6 and 1.8 eV; we believe due to a combination of Franz-Keldysh oscillations [26] from the GaAs and the GaAsPstrain-compensation layers. The highest-energy feature at1:83 � 0:01 eV is due to the AlxGa1�xAs barriers, and wasfurther confirmed by a measurement using a 514 nm Arþ

laser (see later). Its position reveals that the aluminiumcontent is actually x ¼ 0:33 � 0:01; slightly higher than thenominal 0.30. All these features are static with angle, asexpected. However, as mentioned earlier, the expected(weak) static PR signals from the QW below the GaAsfeature are completely obscured by strong LEIO. Theidentification of the latter as non-electronic interference-features is evident from their motion with angle.

However, we can exploit these obtrusive features toestimate the refractive index of the layer responsible for theLEIO, by using (3). This was done as follows. The squaresof the wavelength positions of the peaks and troughs in theLEIO in Fig. 6 (indicated by the sloping vertical lines) wereplotted against the square of the sine of the incidence angle(see inset in Fig. 7). According to (3), this yields in a straightline of slope equal to �n�2 and a y-axis intercept equal tol2ð0Þ: The resulting values for n for each of the verticalsloping lines in Fig. 6 are plotted in Fig. 7 against energy,(corresponding to lð0Þ), together with literature values forthe binary compounds of GaAs, GaP and GaSb [27]. Thisstrongly suggests that the LEIO features are mainlyattributable to GaAs.

It may be noted that the two highest-energy values of n inFig. 7 have a somewhat larger uncertainty owing to the factthat the associated interference features are very close to thedecaying low energy tail of the strong GaAs PR signal at1.42 eV. This makes it harder, therefore, to estimate theexact position of the peaks and troughs (see Fig. 6). Wemight also note that the experimental results for the

Fig. 6 Room temperature PR spectra of sample with straincompensated GaAsSb=GaAs=GaAsP=AlGaAs QW, obtained with633 nm HeNe modulating laser, and against incidence angle(indicated to the right)

The PR spectra are offset vertically, for clarity. The low energy regime isdominated by low energy interference oscillations (LEIO) whose peaks andtroughs move with angle (indicated by vertical sloping lines). These LEIOare so strong as to obscure any PR signal from the QW, expected near1.0 eV. The only evident static electronic features are due to the GaAs, theGaAsP and the Al0:3Ga0:7As layers, as indicated. Also shown, at the lowerright, is the clearer PR signal obtained from the AlGaAs layer using a514 nm Arþ laser

Fig. 7 Open circles show resulting values of refractive index nfor five marked LEIO features in Fig. 6, at corresponding photonenergies

The solid and dashed curves show the accepted energy-variations of therefractive indices of GaAs, GaSb and GaP. The experimental results liebetween the region bounded by these three curves, but are closest tothe GaAs behaviour, indicating that the LEIO arise from a GaAs layer in thestructureInset: Example plot of the square of the interference wavelength of thecentral of the five LEIO features in Fig. 6, as a function of the square ofthe sine of the angle of incidence (filled circles). The line is a linearregressive fit of (3) whose slope gives the refractive index, n

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refractive index are slightly below the literature values forundoped GaAs in Fig. 7. This is consistent with a doping-induced Moss-Burstein shift [28] in the GaAs materialresponsible for the LEIO. The only intentionally-dopedGaAs in the structure is the n-doped substrate.

Therefore, we can conclude that the LEIO in thestructures originate from the GaAs substrate. We haveconfirmed this by reflectance simulations, which show thatLEIO arise owing to interference between rays reflectedfrom interface between the nominally undoped 400 nm-thick GaAs buffer and the n-type GaAs substrate, and,similar to the results of [22, 23], this only occurs because ofsmall, but significant, differences in the refractive indices ofthe buffer and substrate layers. These calculations arebeyond the scope of the present work and will be publishedelsewhere [29].

4.2 Laser wavelength dependent room-temperature photomodulated reflectance

Although we can identify the origin of the LEIO, it is clearlyof most importance for the PR to be able to avoid these ifpossible and detect the QW transitions. The above analysissuggests that they are due to both the probe and modulationlight penetrating as deep as the buffer=substrate interface.At first sight this suggests that the LEIO might be avoided ifwe use a laser wavelength long enough such that only theQW would be modulated, but to which all the other layersare transparent. For this experiment we used a 980 nm(1.26 eV) InGaAs semiconductor laser (power �50mW) asthe modulation source. As may be seen from Fig. 8 this doesreveal a PR signal from the QW (corroborated by the PLspectrum also given in Fig. 8), however, it is very weak, andlong signal averaging times were required. Furthermore, theLEIO are not completely eliminated.

The disadvantage of using a long wavelength laser arisesfrom the PR mechanism itself: PR relies on the fact that thechopped laser modulates any in-built electric field byperiodically exciting electron-hole pairs in the structure.The strength of the resulting DR signal depends on howstrongly the dielectric function is modulated via this effect.When a shorter wavelength laser, such as the 633 nm(1.96 eV) HeNe, is used, it is strongly absorbed (though stillpenetrates to the substrate) and all layers are modulated.

Thus, the field throughout the structure is modulated, and soalso is that in the region of the QW. However, in the case ofthe long wavelength 980 nm laser, to which all layers otherthan the QW are transparent, electron-hole pairs can only beexcited in the QW itself. Therefore, the field local to the QWis modulated weakly, resulting in a small PR signal. In fact,optical absorption calculations indicate that < 2% of thepower of the 980 nm laser is absorbed in the whole structure(i.e. in the QW region). This provides a qualitativeexplanation of the poor signal-to-noise ratio of the QWPR signal for this laser.

Alternatively, by judicious choice of laser wavelength, tobelow that of the HeNe, one might be able to reduce thepenetration depth so that the buffer=substrate region ismodulated by much less, so reducing the GaAs PR signaland the associated LEIO, while still modulating the fieldnear the QW. Simple calculations of the penetration depthof the 633 nm HeNe laser, assuming normal incidence, andno multiple internal reflections, show that its incidentintensity is attenuated to �70% and �12% at the depths ofthe QW and buffer=substrate interface, respectively.Following this idea, we used a 514 nm (2.41 eV) Ar-ionion laser (power �10mW) as modulation source. It mightbe noted that using this laser gave stronger signals from theAlGaAs confinement layers compared to the HeNe laser(see Fig. 6). This is due to the fact that these layers are closerto the surface where the Ar-ion laser is now more stronglyabsorbed. More significantly, however, the relative magni-tude of the LEIO has decreased as expected, but the QWsignal is still not clearly revealed, appearing only as a subtleflattening of an interference peak near 1 eV (see Fig. 8). Oursimple penetration-depth calculations show that the incidentAr-ion laser intensity is attenuated to �38% at the QW and�0:6% at the buffer=substrate interface. The latter figure,though clearly much smaller than the corresponding resultfor the HeNe laser, is evidently still enough to cause anappreciable modulation signal from this region.

Consequently, we reduced the excitation wavelengtheven further by using a HeCd laser with lines at 442 nm(2.81 eV, power �4mW) and 325 nm (3.82 eV, power�1mW). As may be seen from Fig. 8, the LEIO are indeednow almost completely suppressed for these two laser lines,and a clear set of signals is revealed near 1 eV, which canonly be due to the QW. The absorption calculations indicatethat, for the blue line, the incident intensity has fallen to�9% at the QW but to < 10�4% at the buffer=substrateinterface. The corresponding figures for the UV line are< 1% and < 10�17%; respectively. For the latter UV line,the fact that so little of the laser power survives to the QWregion suggests that the field modulation in this case isaccomplished predominantly via the strong absorption inthe uppermost layers.

The best QW PR signals were obtained with the 442 nmHeCd laser line. Although the LEIO are strongly suppressedhere, it may be seen from the �1:15–1:35 eV region inFig. 8 that there is still a residual contribution from theLEIO. To analyse the QW PR signal accurately it is helpfulto remove this weak oscillatory background, which we didby subtracting the LEIO spectrum obtained with the HeNelaser, suitably scaled to match the 1.15–1.35 eV region ofthe blue-HeCd PR spectrum. The resulting QW PR signal isshown in Fig. 9. This spectrum was then fitted with a sum ofthe following well-known Aspnes oscillatory lineshapefunctions [26]:

DR

R¼ Re

Ceiy

½ðE � EgÞ þ iG�n� �

ð4Þ

Fig. 8 Room temperature PR spectra (at incidence angle of 45)for strain-compensated sample, using different modulating laserswhose wavelengths are indicated to left of each spectrum

The baselines of the spectra are offset vertically for clarity. Although PRsignals from the GaAsSb QW could be obtained with a long-wavelength980 nm laser (uppermost PR spectrum), the most obvious QW signals wereobtained (between �0:90 and �1:15 eV) with the shortest wavelengthHeCd lasers available (two lowermost spectra). In these spectra, the low-energy interference oscillations (LEIO) apparent for the HeNe and Ar-ionlasers (centre plots), are suppressed. The room temperature PL spectrum isalso shown for comparison. Spectra are offset vertically, for clarity

IEE Proc.-Optoelectron., Vol. 152, No. 2, April 2005 115

where Eg is the transition energy, C an amplitude factor, y aphase, G a broadening term, and n a exponent whichdepends on the type of transition, typically being 2 forconfined systems. We found that the sum of three suchfunctions were appropriate to fit the PR spectrum, as can beseen in Fig. 9. The transition energies fitted with (4) are0.993, 1.030 and 1.085 eV (all �5meV) with broadeningfactors of 28, 44 and 42 meV, respectively. These roomtemperature PR broadenings are much smaller than thecorresponding PL FWHM of �150meV (see Fig. 8),though are comparable to the 10 K PL FWHM of �34meV(although no PL spectrum was able to resolve these threeQW transitions). Note that, since these QW transitionenergies range over �90meV; this suggests that the PLfeature appearing at 1.18 eV in Fig. 2 (�140meV above themain PL peak) is unlikely to be associated with any higher-order QW transition, so confirming our interpretation thatthis feature is due to defect=impurity levels in the AlGaAs.Note also that the 300 K PL peak at 0:99 � 0:01 eV (Fig. 8)is in agreement with the above QW ground-state transitionenergy (0.993 eV) obtained from fitting the PR spectrum.

Clearly such PR measurements are potentially very usefulin the study of the GaAsSb QW systems, especially withregard to identifying the origin of higher-order QWtransitions, such as those seen in Fig. 9. The present PRstudy of a single sample represents an initial measurementof a more systematic PR study of such structures in whichthe measured manifold of QW transition energies will becompared with rigorous theoretical modelling in order toelucidate the band alignment in these structures, and itsvariation with structural aspects such as the spacerthickness.

5 Conclusion

We performed a variety of spectroscopic studies on a rangeof GaAsSb=GaAs quantum well (QW) structures grown onGaAs substrates by molecular beam epitaxy. First, weinvestigated their low temperature photoluminescence(PL). In an attempt to address an issue of somecontroversy, whether the GaAsSb=GaAs interface is type Ior type II, we investigated the 10 K PL of a series ofGaAsSb=GaAs=AlGaAs structures, with GaAs spacersbetween the GaAsSb QWs and the AlGaAs confinementlayers, in which the spacer thickness was varied between1 and 9 nm. We observed that the samples with GaAsspacers thicker than � 2 nm showed a clear blue shift intheir low temperature PL peaks, which varied withexcitation laser power, while those with narrower spacers

did not. One possible interpretation of this is an evolutionfrom a situation where the electrons and holes are bothconfined predominantly in the centre of the QW region, inthe samples with narrow spacers, to one where the holes arestill confined in the QW but the electron wave-function ismore strongly localised in the spacer region, in the sampleswith wider spacers. For the samples with thicker spacers,which did show a blue shift, the results are consistent withthe GaAsSb=GaAs interface being weakly type II, at atemperature of 10 K, in agreement with the conclusions ofChiu et al. [12].

We also investigated the room-temperature photomodu-lated reflectivity (PR) of related samples. Here, weencountered strong oscillations, which obscuredthe PR signals from the QW in the low-energy region ofthe PR spectra. However, we found that these featuresmoved as the angle of incidence was altered, while theelectronic transition features in the PR spectra did not. Thisidentified the low energy oscillations as being due to opticalinterference effects. We exploited the angle dependence ofthese oscillations in the PR spectra in order to estimate therefractive index of the layer responsible. From this, we wereable to deduce that the low energy interference oscillationsare due to light rays reflected from the undoped GaAs bufferand its interface with the n-doped GaAs substrate. However,we also showed a way to avoid these interferenceoscillations, by changing the laser excitation wavelengthfrom the red (HeNe) to the blue (HeCd). This successfullyrevealed good room-temperature PR signals from the QWregion. Fitting this spectrum yielded the energies of threesharp QW transitions, which could not be obtained from thecorresponding room temperature or 10 K PL spectra andwill help to model the band-structure of these samples.

6 Acknowledgments

The authors gratefully acknowledge the EPSRC and the EUfor support of GB.

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