Ordering in strained GaxIn1−xP quantum wells grown by metalorganic vapor phase epitaxy

D

CRYSTAL GROWTH Journal of Crystal Growth 145 (1994) 740—745

ELSEVIER

Ordering in strained Ga~In1_~P quantum wells grown by metalorganic vapor phase epitaxy C. Geng, M. Moser, R. Winterhoff, E. Lux, J. Hommel, B. Höhing, H. Schweizer, F. Scholz * 4. Physikalisches Inst itut, Universitàt Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany

Abstract Strained GaInP quantum wells with AIGaInP barriers have been grown by metalorganic vapor phase epitaxy (MOVPE). The influence of strain in single quantum wells on the optical properties is examined by photoluminescence. The emission energy of strongly compressively strained quantum wells shows an anomalous temperature dependence and a shift of 13 meV per decade of excitation power. This is interpreted as an accurate measure of the critical strain where strain relaxation occurs. By varying growth temperature and substrate orientation, the influence of ordering in strained and unstrained quantum wells is examined. Whereas unstrained quantum wells show a similar band gap reduction as bulk GaInP, this is significantly weakened in strained quantum wells. This has two reasons. For one, the degree of ordering is limited due to unequal Ga and In amounts. Additionally, according to Wei et a!. [AppI. Phys. Lett. 64 (1994) 757], a further reduction is expected if the joint influence of strain and ordering is taken into account.

1. Introduction

limited. Thermodynamic calculations [9,10] show that alternating monolayers of GaP and InP along

Ga05In05P lattice matched to GaAs substrates has become an important material for laser diodes emitting visible light around 650 nm. Excellent laser diodes with very low threshold current [1] and high output power [2—41have been reported. Recently, the introduction of strained GaInP quantum wells has brought a drastic improvement on laser performance [5,6]. In spite of this, the knowledge of many fundamental material properties, e.g. the built-up of ordering [4,7,81, is still

(111) or (111) planes are formed when GaInP is grown under certain conditions in metalorganic vapor phase epitaxy (MOVPE). This CuPtB structure is energetically more stable on the surface than any other kind of ordering or the disordered state. According to theory, this leads to a reduction of the band-edge [11,121 and to a splitting of the quartet-degenerated hole states into two doublets [13]. The degree of ordering is mainly controlled by the growth temperature [4,7,81:ordered Ga05In05P is grown in MOVPE at temperatures around 670°C, whereas at higher temperatures fairly disordered material is achieved. A further parameter which influences ordering is the sub-

________

*

Corresponding author,

0022-0248/94/$07.OO © 1994 Elsevier Science By. All rights reserved SSDI 0022-0248(94)00325-G

C. Geng et a!. /Journal of Crystal Growth 145 (1994) 740—745

strate misorientation. The ordered phase is only

metastable but “locked” in the crystal. This is confirmed by annealing experiments, where crysannealing temperature of about 890°C[14].

108

741

-

~ simulation

I°~ ~surement

terized by an ordering parameter z~denoting the

distribution of Ga and In on the respective sublattices (Ga0 5(1 +~)InO,S(l n)P/ Ga0 5(1 In05 1± P) on one hand and the limited size of orde~reJ~domains on the other hand. The latter has been reported to be in the range of a few to several hundred nm [15—17]for bulk GaInP. —

Presently, it is not clear how ordering develops

~

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~ 100

I

30002000

1000

0

1000

2000

Position [arc sec] Fig. 1. HRXRD measurement and simulation of a lox (Ga042In058P/(A105Ga05)05In05P) MQW-structure with 4.5 and 22.4 nm as well and barrier widths, respectively.

during the epitaxial growth process in quantum wells having a thickness which is in the same range as the domain size. In addition, it is not clear to what extent ordering is affected by strain.

The joint influence of ordering and strain on the band structure has recently been calculated by Wei et al. [18] and is investigated in this paper.

2. Experimental procedure

In 10 nm SQWs, the strain has been varied in a wide range (+1.76% to —1.06%) by adjusting the Ga and In gas flows accordingly, whereas a lattice mismatch (Lta/a) of less than iO~ has been adjusted for all nominally unstrained layers in the samples. We used GaAs substrates of various misorientations in the same epitaxial run to control the

Strained and unstrained Ga ~In1 —x P single and multi quantum well structures (SOW and MOW, respectively) embedded into (A105Ga05)05In05P barriers lattice matched to GaAs have been grown in a horizontal MOVPE reactor with a rotating gas foil susceptor (Aixtron) at 100 hPa and a V/Ill ratio of about 200. The source materials were trimethylgallium (TMGa), trimethylindium (TMIn), trirnethylaluminum (TMAI), phosphine (PH3) and for the growth of a GaAs buffer also arsine (AsH 3). To influence crystal ordering, the growth temperature has been varied between 600 and 800°C. Composition, thickness, and interface quality have been checked on MOW structures by highresolution X-ray diffraction (HRXRD) using a Philips MRD-HR diffractometer. A large number of satellite peaks resulting from the superlattice structure could be found in all samples (Fig. 1). A perfect match between simulated [19] and measured spectra indicated good homogeneity, excellent interfacial quality, and a perfect agreement between intended and achieved layer properties.

formation of ordering. Misorientations of 2°and 6°off the (100) surface towards the (110> direction (further on denoted 2° and 6° to (110>, respectively) result in moderate crystal ordering. Strongly disordered samples have been grown on substrates with a misorientation of 6°towards the next (111>A direction (6°to (111>A), whereas the highest degree of ordering has been achieved on substrates with a misorientation of 6° towards (111)’B (6°to (111>B). The shift of the main photoluminescence (PL) emission line has been taken as a measure for the formation of ordering. To this end, PL spectra have been recorded on all samples, most of them at low temperatures (T = 2 K). Some quantum well structures have been ternpered by rapid thermal annealing (RTA) under conditions which destroy crystal ordering, but which do not result in intermixing of the wells and barriers (temperature T = 925°C,time t = 60 s). By comparing their PL characteristics before and after RTA, an indication of the degree of ordering could be evaluated.

742

C. Geng et at /Journal of Crystal Growth 145 (1994) 740—745

Misfit ta/a

3. Results and discussion 10°! 2.0

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One of our main goals was the study of the formation of ordering in strongly strained quanturn wells. In order to determine the maximum strain which can be built up in a 10 nm SOW, we have examined a series of Ga~In1.1P quantum wells in the range of x 0.28 to 0.66 (+ 1.76% to 1.06% strain) by photoluminescence. These layers have been grown at 750°C(which results in quite disordered bulk layers) on 6°to (110> substrates. Due to the increasing well depth, the PL intensity of 10 nm single quantum wells at 300 K increases strongly for increasing compressive strain (Fig. 2a). This may be explained by the following equation describing the thermal emission of the electrons out of the quantum well:

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=exp[—(/.lEc2—zlEci)/kTI, (1) where I~is the PL intensity for a given sample i, and kT the thermal energy at 300 K. The conduction band offset ~ of GaInP to the 12/11

AlGaInP barrier has been assumed to be 35% of the total band gap difference [20]. The PL intensity drops strongly when the compressive exceedsindication + 1.4% (see 2a) which is taken strain as a strong forFig. strain relaxation. This is in good agreement with estimations of the critical thickness following the theory of Matthews and Blakeslee [21] or that of Van der Merwe and Jesser [22]. Even the samples having low PL intensity still exhibit mirror-like surfaces. The energetic position of the PL peak at 300 K is in good agreement with the theory of Krijn [23], when the quantization is taken into account. However, the expected energy shift due to strain-release at the critical thickness is not observed, In the following, we focus on the compressive side (Ga 11n1 ~ with x <0.5). A closer look at the strongly strained sample with still high PL intensity (e + 1.3%) reveals an anomalous upshift of 40 meV with temperature increasing from 2 to 150 K (Fig. 3a “as-grown”). At low temperatures, moreover, the peak can be upshifted by =

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0.3

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0.5

0.6

x of Ga~In1...~P Fig. 2. (a) PL intensity at room temperature, (b) observed shift of PL peak for varying excitation power (“yes” or “no”) at 2 . — 300 K+70 meV)) the observed peak energy K, and (c) difference of the and expected PL peak energy at T —in2 K (EPL( dependence of Ga content (strain).

about 13 meV per decade of excitation power (Fig. 4). This anomalous property vanishes and the low temperature PL peak is shifted to higher energies, when the sample undergoes an RTA step as described above (Fig. 3a “after RTA”). The same behavior has been observed for those samples which have low PL intensity due to strain relaxation (~ 1.5%, Figs. 3b and 4), whereas samples with a strain of 1.2% and smaller only show a very slight anomaly over temperature (Fig. =

2c), no PL shift versus excitation power (Fig. 2b) and no PL shift after the RTA step. Owing to the step-like appearance at a certain strain level, we attribute this to be related to strain relaxation processes near the critical thickness. Thus, the maximum strain in a 10 nm SOW is about 1.25%,

C. Geng et a!. /Journal of Crystal Growth 145 (1994) 740—745

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0

50 100 150 200 250300

Temperature [K]

after RTA

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0

50 100 150 200 250300

Temperature [K]

Fig. 3. Anomalous up-shift of PL peak energy for compressively strained SQWs, (a) s = 1.3% and (b) characterization temperature (upper row) and normal behavior after RTA (lower row).

a somewhat lower value as indicated by the drop in PL intensity, 3.2. Ordering in strained quantum wells In consequence, our further studies concentrated on SOWs with a compressive strain just II

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below 1.25%. First, the dependence of the PL peak energy on growth temperature and substrate orientation for such SOWs has been investigated (Fig. Sc) and compared to similar unstrained bulk (Fig. 5a) and quantum well (Fig. Sb) layers. At a first glance, quantum wells have a similar ordering behavior as bulk material, i.e. the largest shift of the PL peak to lower energies occurs at the same growth temperatures of about 690°Cand for the same type of substrates (6° to (111>B). However, the curve for compressively strained

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meV). Apparently, the variation of ordering versus growth temperature and strained substratesamples. orientation is less pronounced in these

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Fig. 4. Dependence of the PL peak energy on the excitation power for the same SQWs as in Fig. 3.

Obviously, perfect crystal ordering in these compressively strained samples is not possible due to unequal Ga and In amounts, i.e. the ordering parameter is limited. A linear interpolation between unstrained material (strongest or-

744

C. Geng et al. /Journal of Crystal Growth 145 (1994) 740—745

deringeffect)andthebinarylnP(noorderjngat all) results in a weakening of the band gap shrinkage to about 72% for our samples with a Ga content of x = 0.36 (instead of x = 0.5). This, however, would correspond to a reduction from 95 to 68 meV as opposed to the observed 50 meV.beThe further using reduction of thecalculation energy shift can aa reduced recent of Wei et explained al., who predict ordering related band gap shrinkage in strained GaInP [18]. An extrapolation of their data results in a further

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650

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growth temp. [°C]

Fig. 6. PL peak-energy “as-grown” and after RTA for (a) unstrained and (b) strained quantum wells (~ + 1.2%).

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additional of reduction of the shiftcorresponds down to 53 meV. reduction about 22%, which to an This is consistent with the observed 50meV. This deduction is confirmed by RTA experiments on strained and unstrained SQWs (Fig. 6).

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ordering energies, (I~rowth 790°C)end thea disordered state. at 66 similar Strained PL quantum indicating wells show shift ofuponly meV at maximum, whereas unstrained quantum wells shift up to 124 meV. The same calculation as above reduces 124 meV down to 70 meV, which is also in line with the experiment.

1 .80

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This shows that taking both effects into acshrinkage between strained and unstrained orcount, our observed difference of the band gap

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dered quantum wells can be well explained.

600

650

700

750

800

growth temp. [oc] Fig. 5. PL peak energy of samples grown on differently oriented GaAs-substrates at different growth temperatures: (a) bulk material, (b) unstrained, and (c) strained quantum wells ( = 1.2%). The various symbols denote substrate orientations: (0)6° to (111)A, (+) 6°to Kilo), (~)2°to Kilo), ~ 6°to (lli)~.

4. Summary The optical attributes of strained Ga~In1 ~P quantum wells with (A105Ga05)05In05P barriers have been investigated. By introducing compressive strain, the PL intensity at room temperature can be strongly increased. This is explained by a

C. Geng et al. /Journal of Crystal Growth 145 (1994) 740—745

weaker thermal emission of the electrons out of the well. An anomalously low position of the low ternperature PL peak in strongly strained SOWs which is upshifted at higher characterization ternperatures or PL excitation powers is taken as an indication for the pass-over of the critical strain at 1.25% for a quantum well thickness of 10 nm. In contrast to unstrained quantum wells, which behave similar to bulk material, strained quantum wells below the critical strain show less band gap shrinkage when the growth temperature or substrate orientation is varied. This is confirmed by annealing experiments under conditions which destroy ordering. Two effects are responsible for this occurrence. A partial weakening of the band gap shrinkage can be explained by the fact that unequal Ga and In amounts only admit less perfect ordering. A further reduction of the shrinkage is explained by the joint influence of ordering and strain on the band structure.

Acknowledgements The authors would like to thank E. Kohler for his support in the MOVPE growth and M. Pilkuhn for stimulating discussions. Parts of this work have been financially supported by the EEC Ufl der ESPRIT contract no. 6134 (HIRED).

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