Kim, Kiyoon

Nishizawa, Jun-ichi

Oyama, Yutaka

Suto, Ken

Journal of Applied Physics

English

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Stoichiometry-dependent deep levels in n-type InP prepared by annealing under controlled phosphorus vapor pressure

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Stoichiometry-dependent deep levels in n-typeInP prepared by annealing under controlledphosphorus vapor pressure著者 小山  裕journal orpublication titleJournal of applied physicsvolume 80number 3page range 1488-1492year 1996URL http://hdl.handle.net/10097/35556doi: 10.1063/1.362986Stoichiometry-dependent deep levels in n-type InP prepared by annealingunder controlled phosphorus vapor pressureJun-ichi NishizawaTohoku University, Katahira Aoba-ku, Sendai 980, Japan and Semiconductor Research Institute ofSemiconductor Research Foundation, Kawauchi Aoba-ku, Sendai 980, JapanYutaka Oyama and Ken SutoDepartment of Materials Science and Engineering, Faculty of Engineering, Tohoku University, AramakiAoba-ku, Sendai 980-77, Japan and Semiconductor Research Institute of Semiconductor ResearchFoundation, Kawauchi Aoba-ku, Sendai 980, JapanKiyoon KimDepartment of Materials Science and Engineering, Faculty of Engineering, Tohoku University, AramakiAoba-ku, Sendai 980-77, Japan~Received 8 March 1996; accepted for publication 26 April 1996!The photocapacitance method is applied to investigate the stoichiometry-dependent deep levels inintentionally-undoped n-type InP crystals prepared by 4 h annealing at 700 °C under controlledphosphorus vapor pressure. The present photocapacitance measurements reveal three dominant deeplevels at 0.63 and 1.1 eV below the conduction band and 0.74 eV above the valence band,respectively. The phosphorus vapor pressure dependence of these level densities is also shown. Theexcitation photocapacitance results show the precise optical transition mechanism of these deeplevels. In conjunction with the phosphorus vapor pressure dependence of the photocapacitanceresults, the defect formation mechanism is also discussed. © 1996 American Institute of Physics.@S0021-8979~96!07215-5#I. INTRODUCTIONThe most important factor to be controlled in III-V com-pounds is the deviation from the stoichiometric composition.Starting with the study of iron-pyrite in 1951,1 Nishizawaand his co-workers have studied the effects of stoichiometryof III-V2 and II-VI3 compounds systematically and haveshown that the crystal quality is controlled by the applicationof vapor pressure of group V or VI elements.InP is one of the most promising semiconductor materi-als for very fast opto-electronics and transistor application.However, the deviation from stoichiometric composition ismore serious compared with GaAs. In the case of InP crys-tals, it is already reported4 that the zone-melting method un-der controlled phosphorus vapor pressure was applied to ob-tain bulk crystals and it is shown that the maximum Hallmobility and minimum carrier concentration is obtained un-der a specific phosphorus vapor pressure. However, the pre-cise investigation of the stoichiometry-dependent defectshave not been performed yet. At present, many reports on thedeep levels in InP have been published by means of deeplevel transient spectroscopy ~DLTS!,5 photoluminescence~PL!,6 and photo-induced transient spectroscopy ~PITS!method.7 Conventional photocapacitance ~PHCAP!8 methodwas also applied to the n-type InP crystals grown by theliquid encapsulated Czochralski ~LEC! and the liquid phaseepitaxial ~LPE! method. However, the understanding on thestoichiometry-dependent deep levels of InP is far from a cru-cial conclusion at present.In this paper, the PHCAP method9 under constant ca-pacitance condition is applied to the intentionally-undopedn-type InP prepared by 4 h annealing at 700 °C under con-trolled phosphorus vapor pressure. The excitation photoca-1488 J. Appl. Phys. 80 (3), 1 August 1996 0021-8979/Downloaded¬11¬Nov¬2008¬to¬130.34.135.83.¬Redistribution¬supacitance method is also applied and shows the precise op-tical transition mechanism of these deep levels. In view ofthe phosphorus vapor pressure dependence of the level den-sity and the effects of impurity doping on the defect forma-tion, the defect formation mechanism is discussed.II. EXPERIMENTA. Sample preparationThe starting crystals used were liquid encapsulatedCzochralski ~LEC! grown n-type InP with the $100%-orientedcrystal surface. Carrier concentration is 1.2–1.531016 cm23.After cleaning and etching by a H2SO4–H2O2 solution, asample crystal is placed in one end of a dumbbell-type quartzampoule and 6N-red phosphorus in the other end. After seal-ing in vacuum, the ampoule was put into a two-temperaturezoned electric furnace. Heat treatment was carried out at700 °C under controlled phosphorus vapor pressure. Heattreatment time was kept to be 4 h in the present experiments.Phosphorus vapor pressure at the phosphorus zone, P0 is de-termined from the temperature of red phosphorus referring toHonig.10 The phosphorus vapor pressure at the crystal zone,P, was determined from the following equation:11P5P0~T/T0!1/2, ~1!where T and T0 are the temperature of InP crystals and redphosphor, respectively. After annealing, the ampoule wasrapidly cooled by dipping into the water at nominal roomtemperature.96/80(3)/1488/5/$10.00 © 1996 American Institute of Physicsbject¬to¬ASCE¬license¬or¬copyright;¬see¬http://jap.aip.org/jap/copyright.jspB. Photocapacitance measurementsPhotocapacitance ~PHCAP!12 measurement by constantcapacitance method9 is performed to determine the level den-sities and their energy levels. Metal–semiconductor contactdiodes were made by Au-evaporation as a barrier metal, andAuGe/Au were fabricated for the ohmic contact on the backsurface. Monochromatic light was fed into the depletionlayer of the sample diodes. PHCAP measurement was car-ried out at 77 K.To determine the level density and their energy levels,the charge state of the deep levels in the depletion layer wasmade to be neutral before each light irradiation at 77 K. Forthis purpose, forward bias injection was carried out in thedark before each light irradiation. After light irradiation, biasvoltage ~DVph! changes to keep the junction capacitance con-stant according to the ionization of the deep levels. However,the depletion layer thickness is kept constant regardless ofthe change of ion density. The change of the ion density,DNt, is given by9DVph5~e/2C2!DNt , ~2!where C is the predetermined constant capacitance of thesample diode, e is the dielectric constant and DVph is thechange of bias voltage. Precise description of the PHCAPmeasurement and the equipment will be referred elsewhere.9III. RESULTS AND DISCUSSIONSFigure 1 shows the ion density PHCAP spectrum ofintentionally-undoped n-type LEC InP crystal before anneal-ing. The PHCAP measurements were carried out as follows.A sample diode was cooled and forward bias injection wasapplied in the dark in order to neutralize every deep levelbefore light irradiation. Then, saturating bias voltage Vdarkwas obtained in the dark. Vdark is attributed to the thermallyFIG. 1. Ion density PHCAP spectrum of intentionally-undoped n-type InPwithout annealing. Forward bias injection was carried out in the dark at eachwavelength before light irradiation to neutralize very deep levels.J. Appl. Phys., Vol. 80, No. 3, 1 August 1996Downloaded¬11¬Nov¬2008¬to¬130.34.135.83.¬Redistribution¬suionized level density in the dark. When monochromatic lightwas irradiated into the depletion region of the sample diode,bias voltage Vph changed according to the ionization of deeplevels and the saturating bias voltage was obtained. Net iondensity induced by the light irradiation is obtained byDVph5Vph2Vdark . Under the constant capacitance condition,the depletion layer thickness is kept constant regardless ofthe change of the ion density. It is shown that almost con-stant Vdark at each wavelength confirms the neutralization ofevery deep levels before each photo-excitation.In Fig. 1 it is shown that the ion density shows gradualincrease at ;0.4 eV and then rapid increase at 0.63 eV. Thedecrease of ion density at 0.74 eV is induced by the photo-neutralization of ionized deep level. In the wavelength regionof 0.9–1.1 eV, another photoionization is observed at 1.1 eV.Figure 2 shows the ion density PHCAP spectrum ofintentionally-undoped n-type InP prepared by 4 h annealingat 700 °C under controlled phosphorus vapor pressure. Deeplevels are observed nearly at the same photon energies asthose for the samples before annealing. However, it is no-ticed that InP crystal annealed under the phosphorus vaporpressure of 100 Torr shows another photo-ionization at 1.01eV1Ev.FIG. 2. Ion density PHCAP spectra of intentionally-undoped n-type InPprepared by 4 h annealing at 700 °C under phosphorus vapor pressure of ~a!631024 Torr, ~b! 100 Torr, and ~c! 3800 Torr, respectively.1489Nishizawa et al.bject¬to¬ASCE¬license¬or¬copyright;¬see¬http://jap.aip.org/jap/copyright.jspFigure 3 shows the change of deep level density as afunction of the applied phosphorus vapor pressure. It isshown that the Ec20.63, 1.1 eV and 0.74 eV1Ev level den-sity decreases with increasing phosphorus vapor pressure inthe range below 100 Torr. Then, the level densities increasewhen the applying phosphorus vapor pressure exceeds 1000Torr. These levels are commonly detected in intentionally-undoped n-type LEC InP crystals before and after annealing.Even in p-type InP crystals doped with Zn ~p5331017cm23!, the ion density PHCAP measurements reveal thephoto-ionization at 0.74 and 1.1 eV, respectively, as shownin Fig. 4. The photo-ionization at 0.74 eV in p-type InPcorresponds to the electron capture at 0.74 eV in n-type crys-tal. Therefore, it is considered that these deep levels are notinduced by the unexpected impurity contamination duringannealing, but most probably by the intrinsic defects.On the Ec20.63 eV and 0.74 eV1Ev level, it is shownthat the amphoteric impurity doping annihilates the forma-tion of these deep levels. Figure 5 shows the ion densityPHCAP spectrum of Sn-doped LEC InP crystal with the car-rier concentration of 2.231016 cm23. The carrier concentra-tion is almost the same as that of intentionally-undoped LECInP ~n51.131016 cm23!. As shown in Fig. 5, ionization andneutralization cannot be detected at around 0.63 and 0.74 eV,respectively. It is considered that the doped impurity Sn willoccupy both In and P sublattices. Therefore, In and/or P va-cancies are thought to be annihilated by Sn doping. In addi-ton, ion density PHCAP spectrum of S-doped InP crystalwith the carrier concentration of 531017 cm23 does not showthe ionization at 0.63 eV nor the neutralization at 0.74 eV~Fig. 6!. Group VI impurity S will occupy the P-sublattices.Therefore, it is considered that the doped impurity S will alsoreduce the phosphorus vacancy concentration. In view of thephosphorus vapor pressure dependence of the level density,it is considered that these deep levels are stoichiometry-FIG. 3. Phosphorous vapor pressure dependence of deep level densities ofintentionally-undoped n-type InP prepared by 4 h annealing at 700 °C.1490 J. Appl. Phys., Vol. 80, No. 3, 1 August 1996Downloaded¬11¬Nov¬2008¬to¬130.34.135.83.¬Redistribution¬sudependent and are related most possibly with at least thephosphorus vacancy. It is also considered that the annealingunder extremely high vapor pressure induces non-equilibrium defects in the lattice. Indeed, in the case ofGaAs, it is noticed that high-pressure annealing induces notonly the interstitial arsenic atom-related point defects butFIG. 4. Ion density PHCAP spectrum of Zn-doped p-type InP crystal withthe carrier concentration of 331017 cm23 .0.74 eV1Ev level can be detectedin p-type material.FIG. 5. Ion density PHCAP spectrum of Sn-doped InP crystal with thecarrier concentration of 2.231016 cm23. Ec20.63 eV and 0.74 eV1Ev levelare not shown in the amphoteric impurity doped InP crystals.Nishizawa et al.bject¬to¬ASCE¬license¬or¬copyright;¬see¬http://jap.aip.org/jap/copyright.jspstructural defects like stacking faults and extendeddislocations.13 In such highly degraded lattices, it is consid-ered that both the excess phosphorus composition-related de-fect and the vacancy-related defects are generated.14Next, in order to clarify the optical transition mechanismof Ec20.63 eV level, ion density PHCAP spectrum wasmeasured from long wavelength and short wavelength, re-spectively.Figure 7 shows the ion density PHCAP spectrum ob-tained from ~a! long wavelength and from ~b! short wave-length light irradiation. Sample used was intentionally-undoped n-type InP without any heat-treatment. When themonochromatic light is irradiated from the long wavelength,the ionization is induced at 0.63 eV and the neutralizationFIG. 6. Ion density PHCAP spectrum of S-doped InP crystal with the carrierconcentration of 531017 cm23. Ec20.63 eV and 0.74 eV1Ev level are notdetected in the donor-impurity doped InP crystals.FIG. 7. Ion density PHCAP spectrum measured from ~a! long wavelengthand from ~b! short wavelength, respectively. Sample was intentionally-undoped n-type InP crystal with the carrier concentration of 1.131016 cm23.J. Appl. Phys., Vol. 80, No. 3, 1 August 1996Downloaded¬11¬Nov¬2008¬to¬130.34.135.83.¬Redistribution¬suoccurred at 0.74 eV, respectively. However, when the light isirradiated from the short wavelength, the ionization is in-duced at 0.79 eV and then the ion density decreases a little.This decrease of ion density is considered to be due to ther-mal neutralization. Therefore, from these results, it is consid-ered that the Ec20.63 eV level is photo-ionized by the 0.63eV-monochromatic light irradiation and can be neutralizedby the 0.74 eV monochromatic light irradiation via electrontransition from the valence band to be ionized Ec-0.63 eVlevel. And the neutral Ec20.63 eV level is ionized in thewavelength region of 0.63–0.79 eV.Figure 8 shows the excitation PHCAP spectrum of anintentionally-undoped InP crystal before annealing. After1.40 eV light irradiation, the ion density PHCAP spectrumwas measured from long wavelengths at 40 K. In Fig. 7, it isshown that the decrease of ion density is induced at 0.43 and0.45 eV after 1.40 eV light irradiation. This decrease of iondensity is caused by the neutralization of deep levels ionizedby the 1.40 eV light irradiation. By changing the primal ex-citation wavelength, it is shown that the neutralization at0.43 and 0.45 eV is attributable for the ionized level ofEc21.1 eV via electron transition from the valence band. Inaddition, it is noticed that the ionization at 1.1 eV is notcaused by the interband transition from Ec20.63 eV to the Xvalley, because ion density PHCAP spectrum of our LPE InPcrystal shows photo-ionization at 1.1 eV but the Ec20.63 eVlevel cannot be detected in LPE sample ~not shown here!.In view of optical transition mechanism of major deeplevels, it is concluded that the ionized Ec20.63 eV levellocates at 0.74 eV above the valence band at 77 K and thatthe neutral state of 0.43 and 0.45 eV ~at 40 K! 1Ev levellocates at 1.1 eV below the conduction band.IV. CONCLUSIONThe photocapacitance method was applied in the con-stant capacitance condition to investigate the stoichiometry-dependent deep levels in intentionally-undoped n-type InPcrystals. Samples were prepared by 4 h annealing at 700 °Cunder controlled phosphorus vapor pressure. The presentFIG. 8. Ion density PHCAP spectrum of intentionally-undoped n-type InPmeasured from long wavelength after 1.40 eV-light irradiation at 40 K.1491Nishizawa et al.bject¬to¬ASCE¬license¬or¬copyright;¬see¬http://jap.aip.org/jap/copyright.jspphotocapacitance measurements revealed three dominantdeep levels at 0.63 and 1.1 eV below the conduction bandand 0.74 eV above the valence band, respectively. Theselevel densities decreased by increasing phosphorus vaporpressure in the range below 100 Torr. When the phosphorusvapor pressure exceeded 1000 Torr, the deep level densitiesincreased. The excitation photocapacitance results have re-vealed the electron capture at 0.43 and 0.45 eV correspond-ing to the ionized state of Ec21.1 eV level. In view of thephosphorus vapor pressure dependence of the level densityand the effects of impurity doping on the annihilation ofdeep levels, it was suggested at Ec20.63 eV and 0.74eV1Ev levels would relate most probably with the phos-phorus vacancy.1Y. Watanabe, J. Nishizawa, and I. Sunagawa, Kagaku, Iwanami 21, 140~1951!.2H. Otsuka, K. Ishida, and J. Nishizawa, Jpn. J. Appl. Phys. 8, 632 ~1969!.1492 J. Appl. Phys., Vol. 80, No. 3, 1 August 1996Downloaded¬11¬Nov¬2008¬to¬130.34.135.83.¬Redistribution¬sub3 J. Nishizawa, K. Suto, and Y. Oyama, Materials Research Society Sym-posium Proceedings, 1992, edited by T. D. Moustakas, J. I Pankove, andY. Hamakawa, Vol. 242, p. 179.4 J. Nishizawa and Y. Oyama, Mater. Sci. Eng. R12, 273 ~1994!.5Y. Yamazoe, Y. Sasai, and T. Nishino, Jpn. J. Appl. Phys. 20, 347 ~1981!.6V. V. Negresukul, E. V. Russu, S. I Radanstan, and A. G. Cheban, Sov.Phys. Semicond. 9, 587 ~1975!.7Y. Yamazoe, Y. Sasai, T. Nishino, and Y. Hamakawa, Jpn. J. Appl. Phys.20, 47 ~1981!.8S. H. Chiao and G. A. Antypas, J. Appl. Phys. 49, 466 ~1978!.9 J. Nishizawa, Y. Oyama, and K. Dezaki, J. Appl. Phys. 67, 1844 ~1990!.10R. E. Honig and D. A. Kramer, RCA Rev. 30, 285 ~1969!.11 J. Nishizawa, Y. Kobayashi, and Y. Okuno, Jpn. J. Appl. Phys. 19, 345~1980!.12A. Ito, T. Sukegawa, and J. Nishizawa, Tech. Rep. Transistor SpecialistCommittee, IEE Japan, Jan., 1967.13 J. Nishizawa, N. Toyama, Y. Oyama, and K. Inokuchi, Proceedings of theThird International School on Semiconductor Optoelectronics, edited byMarian A. Herman ~Scientific Publishers, Warszawa, 1983!, p. 27.14 J. Nishizawa, Y. Okuno, and K. Suto, Japan Annual Report on ElectricalCommunication ~JARECT! 19, 17 ~1986!.Nishizawa et al.ject¬to¬ASCE¬license¬or¬copyright;¬see¬http://jap.aip.org/jap/copyright.jsp

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Stoichiometry-dependent deep levels in n-type InP prepared by annealing under controlled phosphorus vapor pressure

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