APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 6 6 AUGUST 2001

Direct-to-indirect band gap crossover for the Be xZn1ÀxTe alloyO. Maksimova),c) and M. C. Tamargob),c)

New York State Center for Advanced Technology on Ultrafast Photonics, Center for Analysis of Structuresand Interfaces, City College of New York, New York, New York 10031

~Received 9 April 2001; accepted for publication 5 June 2001!

We have investigated the growth and optical properties of a set of BexZn12xTe epitaxial layershaving different composition, withx ranging from 0–0.7. Comparison of the reflectivity and thephotoluminescence spectra allowed us to locate the direct-to-indirect band gap crossover for thisalloy at x'0.28. TheG→G direct band gap exhibits a linear dependence on composition over theentire compositional range and can be fitted to the equationEG

g(x)52.26* (12x)14.1* x. Itincreases linearly with BeTe content at a rate of 18 meV for a change of 1% in BeTe content. TheG→X indirect band gap for BexZn12xTe can be fitted to the equationEX

g(x)53.05* (12x)12.8* x20.5* x* (12x), suggesting that the energy of the indirectG→X transition for ZnTe isabout 3.05 eV. ©2001 American Institute of Physics.@DOI: 10.1063/1.1390327#

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In the past few years, ZnCdMgSe-based light emittdiodes~LEDs! operating in the visible range of the spectruwere reported by several research groups.1–3 The LED struc-tures, which were grown on InP substrates, utilized lattimatched ZnSeTe or ZnMgSeTe as the topp-type contact lay-ers. However, there are drawbacks involved in the useeach of these materials. In the case of ZnSeTe, which cadoped p-type to carrier concentration levels in excess1019cm23, absorption of the visible light by the top contalayer limits the performance of surface emitting LEDs4

When ZnMgSeTe layers with band gaps of 3.1 eV are usthe maximum free hole concentration is in the lo1018cm23, making the formation of ohmic contacts modifficult.2

A promising alternative material for use as ap-type con-tact layer is BexZn12xTe. Previous studies showed that, wia BeTe mole fraction~x! of '0.48, it can be lattice matcheto the InP substrates, and that it can be dopedp-type to the1019cm23 level.5,6 Furthermore, since BeTe and ZnTe hadirect band gaps of 4.1 eV~Ref. 7! and 2.26 eV, respectivelyit is expected that BexZn12xTe layers lattice matched to Inwill not absorb in the visible range. However, BeTe isindirect semiconductor with an indirectG→X band gap tran-sition at 2.8 eV,7 while ZnTe is a direct band gap semicoductor. Therefore, the band gap of the BexZn12xTe ternaryalloy should undergo a direct-to-indirect crossover at sovalue of x. The position of this crossover is still unknowThe potential development of heterostructures based onmaterial requires an investigation of the band gap properof BexZn12xTe.

In this work, we have investigated the room temperat~RT! reflectivity and low-temperature photoluminescen~PL! of a set of BexZn12xTe epilayers withx varying from0–0.7. We determined the variation of the BexZn12xTe bandgap as a function of BeTe content~x! and estimated that thposition of the direct-to-indirect crossover is atx'0.28. Our

a!Electronic mail: maksimov@netzero.netb!Electronic mail: tamar@sci.ccny.cuny.educ!Also at: Department of Chemistry, City College of New York, New YorNew York 10031.

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results indicate that Be0.48Zn0.52Te, which is lattice matchedto InP, is an indirect semiconductor with aG→X indirectband gap at 2.77 eV and aG→G direct band gap at 3.14 eV

Layers of BexZn12xTe were grown by molecular-beamepitaxy ~MBE! on semi-insulating epiready~001! InP sub-strates using elemental Zn, Be, and Te sources in a R2300 MBE system. This system consists of III–V and II–Vgrowth chambers connected by an ultrahigh vacuum chanThe InP substrates were deoxidized in the III–V chamberheating to 500 °C under an As flux. Then, a lattice matchInGaAs buffer layer~170 nm! was grown. After this, thesamples were transferred in a vacuum to the II–VI chambThe growth of BexZn12xTe was carried out at 270 °C and thgroup VI to group II flux ratio was adjusted at a value>1.5to maintain a Te-stabilized surface as characterized by a31) surface reconstruction.8 The growth rate was around 0.mm/h and the layers were 1–1.5mm thick. Control of thecomposition of the ternary layers was accomplished byjusting the Be and Zn fluxes. Although the layers were ncapped, no surface degradation due to oxidation by atspheric oxygen was observed under a Nomarski microsceven after months of exposure to air.

The composition of the BexZn12xTe layers was determined from the lattice constant measured using single-cryx-ray diffraction. A linear dependence of lattice constantthe alloy composition~Vegard’s Law! was assumed. The RTreflectivity measurements were performed in a Cary 500 Uvisible spectrophotometer with a variable angle specularflectance accessory. For the low-temperature PL measments the samples were mounted on the cold finger oclosed-cycle He cryostat maintained at 6 K. The 325 nm lof a He–Cd laser was used for excitation.

Figure 1 shows reflectivity spectra measured at 298for six BexZn12xTe layers ranging in composition fromx50.06 tox50.52. The end of Fabry–Perot oscillations in treflectivity spectrum corresponds to the onset of interbaabsorption and can be identified as theG→G direct band gaptransition. The position of the absorption edge was takenbe the first minimum of the derivative of the reflectivitspectrum. The reflectivity measurements shown in Fig. 1

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783Appl. Phys. Lett., Vol. 79, No. 6, 6 August 2001 O. Maksimov and M. C. Tamargo

dicate that the band gap continuously shifts to a higherergy as the Be concentration increases. The band gap enincreases linearly with BeTe content at a rate of 18 meVa change of 1% in BeTe content.

Figure 2 shows normalized PL spectra measured atfor the same group of samples. The PL spectra are dominby narrow emission lines. In several spectra a second,ally weak peak is present at lower energy, about 50 mbelow the high-energy peak. It can be seen from the datFig. 2 that the energy of the dominant PL peak initially icreases with Be content and then becomes nearly consta

FIG. 1. RT reflectivity spectra for several BexZn12xTe alloys of differentcompositions are shown.

FIG. 2. Low-temperature~6 K! PL spectra for several BexZn12xTe alloys ofdifferent compositions are shown.

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a value ofx>0.28. It should be noted that the intensity of thPL decreased sharply as the BeTe content became largerthat value, and continued to decrease as the Be contencreased.

Based on their optical properties, the samples candivided into two different groups. For BexZn12xTe with x,0.28, the PL emission energy increases with Be contenthis group of samples, the PL emission energy, which is msured at 6 K, is about 90 meV above the RT band gap vaobtained from the reflectivity measurements, consistent wthe expected variation of the band gap with temperatuThus, the PL emission in this group of samples is assignetheG→G near bandedge emission. For the BexZn12xTe withx>0.28, the PL emission energy is nearly constant whiledirect band gap value continues to increase linearly wcomposition. Therefore, our results suggest thatBexZn12xTe layers withx>0.28 are indirect band gap materials. This conclusion is also supported by the reducedintensity observed for these samples.

In Fig. 3, we have plotted the direct band gaps obtainfrom reflectivity measurements~open circles! and the PLemission energies~crosses! as a function of Be content for alarger set of samples that includes the six shown in Figsand 2. Using the RT band gap measured for ZnTe~2.263 eV!and the reported value for the direct band gap of BeTe~4.1eV!,7 the reflectivity data can be well fitted to the lineequation:

EGg~x!52.263* ~12x!14.1* x for 0<x<1, ~1!

which describes the RT direct band gap dependence on cposition for this alloy. The same equation corrected fortemperature difference describes the behavior of 6 K PL datafor samples withx,0.28.

To analyze the PL data forx>0.28, we must considethe G→X band gap energy for the binary end points. Tvalue for BeTe is well known to be at 2.8 eV.7 However, the

FIG. 3. Direct band gap at 298 K from reflectivity data~open circles! andPL energy at 6 K~crosses! as a function of BeTe content~x! in BexZn12xTe.The direct and indirect band gaps for BeTe are taken from Ref. 7.indirect band gap of ZnTe is taken from Ref. 9. The dashed line is a fitthe G→X transition and the solid line is a fit for theG→G transition ofBexZn12xTe alloys as a function of composition.

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784 Appl. Phys. Lett., Vol. 79, No. 6, 6 August 2001 O. Maksimov and M. C. Tamargo

G→X transition energy for ZnTe is not known preciseRecently, a value of 3.05 eV was reported,9 while a value of3.45 eV was obtained usingab initio calculations.10 The val-ues the same authors report for theX→X transition@5.45 eV~Ref. 9! and 5.63 eV~Ref. 10!# are comparable to the valuemeasured by other group@5.23 eV ~Ref. 11! and 5.30 eV~Ref. 12!#. Assuming that the PL emission energies for tsamples withx above 0.28 correspond to theG→X indirectband gap transition, we have fitted the variation of tG→X indirect band gap with composition~x! to the equa-tion:

EXg~x!53.05* ~12x!12.8* x20.5* x* ~12x!

for 0<x<1, ~2!

where 0.5 eV is a bowing parameter. A good correlationEq. ~2!, with our data was obtained, while attempts to fitan equation that uses 3.45 eV as the ZnTeG→X indirectband gap was not successful. This supports our interpretaof the data and indicates that theG→X band gap transitionfor ZnTe is about 3.05 eV.

Comparison of the reflectivity and PL data shows ththe direct-to-indirect crossover occurs atx'0.28. This resultimplies that Be0.48Zn0.52Te, which is the alloy compositionthat is lattice matched to InP, is an indirect semiconducwith a G→X indirect band gap of 2.77 eV and aG→G directband gap of 3.14 eV. A previous report5 predicted that thedirect-to-indirect crossover for BexZn12xTe alloy would oc-cur at a much higher Be content (x.0.7). This predictionwas based on the assumption that theG→X ZnTe band gapis at 5.97 eV. Our data gives a clear evidence of the posiof the direct-to-indirect crossover and indicates that 5.97is not the correct value for theG→X transition for ZnTe.

In summary, epitaxial layers of BexZn12xTe were grownby MBE on InP substrates and their band structure and ocal properties were investigated. It was shown tBexZn12xTe has a direct band gap for low BeTe concenttions and becomes indirect, exhibiting aG→X character, forx>0.28. TheG→G direct band gap increases linearly wi

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BeTe content at a rate of 18 meV for a change of 1% in Becontent for the entire compositional range. The dependeof the indirect band gap on composition was also establisand a good fit was obtained when a value of 3.05 eV wused for the ZnTeG→X transition. Our results indicate thaBe0.48Zn0.52Te, which is the composition that is latticmatched to InP, is an indirect semiconductor with aG→Xindirect band gap of 2.77 eV, and aG→G direct band gap of3.14 eV. Therefore, it is well suited for use as ap-type con-tact layer in LED structures emitting throughout the visibrange.

The authors would like to acknowledge support from tNational Science Foundation through Grant Nos. DM9805760 and DGE-9972892. The work was performed unthe auspices of the New York State Center for AdvancTechnology on Ultrafast Photonics and the Center for Anasis of Structures and Interfaces.

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