INTRODUCTIONA lot of progress has been made on the fabricationof solid-state light emitters; however, there is a spec-tral gap in the green-yellow region (530–590 nm) forall commercially available devices.1This spectral re-gion is important for many emerging applications,such as the use of plastic-optical ﬁbers, that requiregreen lasers to achieve the lowest attenuation coefﬁ-cient.2Thus, ZnSe-based devices are still of high in-terest because, in principle, they can cover the wholespectral region between 490–590 nm. However, suc-cessful production of bright, “long-lived” laser diodesand light-emitting diodes is tempered by the lack ofgood p-type ZnSe, among other problems.3The useof the ZnSe-Te system for improving the p-type dop-ing has been suggested because ZnTe is easily dopedp-type. The best results so far within such an ap-proach have been obtained by Jung et al.,4who havereported hole concentrations up to 7 ϫ 1018cmϪ3byusing a ␦-doped ZnSe/ZnTe:N superlattice. However,because of a large lattice mismatch between theZnSe and ZnTe (ϳ7.4%), such a structure tends toform defects, for instance, dislocations,5that aredetrimental to device operation.Recently,6the use of a planar (␦-) doping tech-nique has been suggested to reduce the average Tecontent in the material. In such an approach, ZnSeis codoped with Te and N in delta layers (which donot form full ZnTe monolayers), separated by un-doped-ZnSe spacers.6The highest achieved net-ac-ceptor concentration (NA Ϫ ND) is ϳ6 ϫ 1018cmϪ3with an average Te concentration of Ͻ1.8% (TableI).7In this paper, we report results of photolumines-cence (PL) studies of ␦-doped ZnSe with very low Tecontent but otherwise similar to previously reportedsamples. We studied two types of samples: the so-called single ␦-doped (␦-doping) and triple ␦-doped(␦3-doping), both of which have Te concentrations aslow as about 0.5%.GROWTH AND DOPINGGrowth was performed in a Riber 2300 molecularbeam epitaxy system in city college of CUNY.Atomic N was produced by a radio-frequency dis-charge N source. All samples were grown on (001) p-type GaAs substrates. The present samples, prior tothe growth of the delta region, have buffer layers ofuniformly N-doped ZnSe. The growth rate was about0.8 m/h. The shutter control sequence used duringthe growth is described elsewhere.6Two types ofsamples were studied. One was single ␦-doped (sam-ple A), where only a small enhancement in the net-acceptor concentration was obtained; the other was␦3-doped (sample B), and a signiﬁcant increase ofthe net-acceptor concentration was obtained.PHOTOLUMINESCENCE RESULTSIn Figs. 1 and 2, we show the PL from samples Aand B (both have Te content of ϳ0.5%). The PL fromboth samples shifts to red with decreasing excitationJournal of ELECTRONIC MATERIALS, Vol. 31, No. 7, 2002 Special Issue PaperHeavily p-Type Doped ZnSe Using Te and N CodopingY. GU,1,3IGOR L. KUSKOVSKY,1G.F. NEUMARK,1W. LIN,2S.P. GUO,2O. MAKSIMOV,2and M.C. TAMARGO21.—Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY10027. 2.—Chemistry Department, City College of CUNY, New York, NY 10007. 3.—E-mail:email@example.comWe have studied photoluminescence (PL) of ZnSe samples codoped with Te andN in ␦-layers. We have concluded that Te clusters are involved in the PL. Wealso compared the PL data of samples with lower Te concentrations to thosewith higher Te concentrations; the results corroborate the Te-cluster conclu-sion.Key words: ZnSe, Te clusters, photoluminescence, p-type(Received October 3, 2001; accepted March 2, 2002)799
intensity, with such a shift being considered a hall-mark of a donor-acceptor pair (DAP) PL.8The PL ofsample A shows a series of peaks at 2.686 eV, 2.655eV, 2.625 eV, and 2.594 eV. These peaks are sepa-rated by 30–31 meV, which is about the ZnSe longi-tudinal-optical phonon energy.9However, it is clearfrom Fig. 1 that the intensity of the peak at 2.655 eVdoes not scale with the intensity of the 2.686-eVpeak, which indicates that these two peaks are asso-ciated, at least partly, with different centers. Also, wenote that the peak at 2.655 eV is in the 2.63–2.68-eVenergy range, where we have previously observed PLcaused by excitons bound to Te pairs and/or clusters.10Therefore, we suggest that this 2.655-eV peak hassome contributions from the recombination of exci-tons bound to such Te complexes. As to the 2.686-eVpeak, it is very close to the peak position of the “deep”DAP emission in ZnSe:N.11The current value isslightly high, but the present system is expected tohave a relatively strong Coulomb shift because ofpreferential DAP11emission, which could account forthis. Furthermore, it can be seen that, at low-excita-tion intensity, this sample shows a small broadshoulder at around 2.42 eV (Fig. 3), which is in therange of the peak position (2.487 eV) of excitonsbound to TenՆ3 clusters.10Therefore, even though theTe concentration is as low as 0.5% in this sample, itstill shows quite efﬁcient Te-cluster-related PL. Thiscan be understood by considering the geometry ofour sample, which could result in relatively high Teconcentration in the ␦-doped layer.As to sample B, it shows only one broad peak ataround 2.457 eV. As mentioned, this peak shifts tored with decreasing excitation intensity. However,2.457 eV is too deep for the “ordinary” DAP peaks inZnSe. Furthermore, as mentioned previously, thispeak is close to the one assigned to excitons bound toTenՆ3 clusters. Therefore, we believe that, in additionto N, Te clusters are also involved in this transition.Previously, we reported PL spectra of another setof ␦- and ␦3-doped ZnSe:(N ϩ Te) samples12with800 Gu, Kuskovsky, Neumark, Lin, Guo, Maksimov, and TamargoTable I. The Net Acceptor ConcentrationsTeSamples and Spacer NA Ϫ ND ConcentrationDoping (ML) (cm Ϫ3) (%)Uniform ZnSe:N N/A 3.0 ϫ 1017N/A␦-ZnSe:(N) N/A 5.6 ϫ 1017N/A␦-ZnSe:(Te, N) 10 1.5 ϫ 1017ϳ0.5␦3-ZnSe:(Te, N) 12 4.0 ϫ 1017Ͻ3␦3-ZnSe:(Te, N) 7 6.0 ϫ 1017Ͻ1.8Fig. 1. PL spectra of sample A at different excitation intensities atT ϭ 10K.Fig. 2. PL spectra of sample B at different excitation intensities atT ϭ 10K.Fig. 3. The PL spectra of sample A at low-excitation intensity.
higher Te concentrations (1.8% and 3%, respec-tively). It is instructive to compare those resultswith those of our new samples with less Te concen-trations (0.5%). In Fig. 4, we plot the integrated in-tensity versus the excitation intensity for each ofour new samples, which show one slope for sample A(␦-doped) and two slopes for sample B (␦3-doped). Wenote that with our previous samples, there are twodistinct slopes for each of them. Because the onlydifference between the new ␦-doped sample and theprevious one is the Te concentration, the differencein slopes corroborates our conclusion that Te is in-volved in the PL. Regarding the two-slope case, asdiscussed in Refs. 12 and 13, the plots show twoslopes if two recombination paths are present. We,thus, note that despite the low Te content of the newsample, the ␦3geometry still gives a large number ofTe complexes around the ␦3-layer region, enough togive two paths. We also note that the PL-peak posi-tion of the new ␦3-doped sample is higher than thatof the previous one. This could be due to the fact thatthe previous ␦3-doped sample has more TenՆ3 clus-ters, or alternatively, a sufﬁciently high Te concen-tration to change the bandgap for this effect to takeplace.SUMMARYIn summary, we employed a modulation-dopingtechnique that allowed us to achieve a net-acceptorconcentration as high as 6 ϫ 1018cmϪ3with Te lessthan 1.8%. The low-temperature PL spectra of thesesamples showed dominant Te-related peaks but alsoinvolved N. We also compared the PL spectra of ourpresent samples (with lower Te concentrations) tothat of previous samples (with higher Te concentra-tions). The results are consistent with our conclu-sion that Te clusters are involved in the PL.ACKNOWLEDGEMENTThe authors acknowledge the support of the De-partment of Energy under Grant Nos. DE-FG02-98ER45694 and DE-FG02-98ER45695.REFERENCES1. R. Haitz, F. Kish, J. Tsao, and J. Nelson, Comp. Semicond.6, 34 (2000).2. A. Weinert, Plastic Optical Fibers: Principles, Components,Installation (Erlangen and Munich: Publicis MCD Verlag,1999).3. G.F. Neumark, Mater. Lett. 30, 131 (1997).4. H.D. Jung, C.D. Song, S.Q. Wang, K. Arai, Y.H. Wu, Z. Zhu,T. Yao, and H. Katayama-Yoshida, Appl. Phys. Lett. 70,1143 (1997).5. W. Faschinger, J. Nurnberger, E. Kurtz, R. Schmitt, M.Korn, K. Schull, and M. Ehinger, Semicond. Sci. Technol.12, 1291 (1997).6. W. Lin, S.P. Guo, M.C. Tamargo, I.L. Kuskovsky, C. Tian,and G.F. Neumark, Appl. Phys. Lett. 76, 2205 (2000).7. I.L. Kuskovsky, Y. Gu, C. Tian, G.F. Neumark, S.P. Guo, W.Lin, O. Maksimov, M.C. Tamargo, A.N. Alyoshin, and V.M.Belous, Phys. Status Solidi (b) 229, 385 (2002).8. J.I. Pankove, Optical Process in Semiconductor (New York:Dover, 1974).9. H. Hartman, R. Mach, and B. Selle, “Wide Gap II–VI Com-pounds as Electronic Materials,” Current Topics in Materi-als Science, vol. 9, ed. E. Kaldis (Amsterdam: North-Hol-land, 1982).10. I.L. Kuskovsky, C. Tian, G.F. Neumark, J.E. Spanier, I.P.Herman, W. Lin, S.P. Guo, and M.C. Tamargo, Phys. Rev. B63, 155205 (2001).11. G.F. Neumark, L. Radomsky, and I. Kuskovsky, Proc. SPIE2346, 159 (1994).12. I.L. Kuskovsky, C. Tian, C. Sudbrack, G.F. Neumark, W.-C.Lin, S.P. Guo, and M.C. Tamargo, J. Appl. Phys. 90, 2269(2001).13. V.V. Serdyuk, G.G. Chemeresyuk, and M. Terek, Photoelec-trical Processes in Semiconductors (Kiev-Odessa: VischaShkola, 1982) (in Russian).Heavily p-Type Doped ZnSe Using Te and N Codoping 801Fig. 4. The integrated PL intensity as a function of laser-excitationintensity: (a) ␦-doped ZnSe:Te, N) and (b) ␦3-doped ZnSe:(Te, N).
COPYRIGHT INFORMATIONTITLE: Heavily p-Type Doped ZnSe Using Te and N CodopingSOURCE: J Electron Materns Part A 31 no722 Jl 20021998The magazine publisher is the copyright holder of this article and itis reproduced with permission. Further reproduction of this article inviolation of the copyright is prohibited. To contact the publisher:http://www.springerlink.com/content/0361-5235/