Kim, Kiyoon

Nishizawa, Jun-ichi

Oyama, Yutaka

Suto, Ken

Journal of Applied Physics

English

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Stoichiometry-dependent deep levels in p-type InP

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Stoichiometry-dependent deep levels in p-typeInP著者 小山  裕journal orpublication titleJournal of applied physicsvolume 81number 7page range 3151-3154year 1997URL http://hdl.handle.net/10097/35557doi: 10.1063/1.364349Stoichiometry-dependent deep levels in p-type InPJun-ichi NishizawaTohoku University, Katahira Aoba, Sendai 980, Japan and Semiconductor Research Instituteof Semiconductor Research Foundation, Kawauchi Aoba, Sendai 980, JapanKiyoon KimDepartment of Materials Science and Engineering, Faculty of Engineering, Tohoku University,Aramaki Aoba, Sendai 980, JapanYutaka OyamaDepartment of Materials Science and Engineering, Faculty of Engineering, Tohoku University,Aramaki Aoba, Sendai 980, Japan and Semiconductor Research Institute of Semiconductor ResearchFoundation, Kawauchi Aoba, Sendai 980, JapanKen SutoDepartment of Materials Science and Engineering, Faculty of Engineering, Tohoku University, AramakiAoba, Sendai 980, Japan and Semiconductor Research Institute of Semiconductor ResearchFoundation, Kawauchi Aoba, Sendai 980, Japan~Received 26 August 1996; accepted for publication 17 December 1996!Photocapacitance measurement is applied to investigate the stoichiometry-dependent deep levels inp-type InP crystals doped with Zn prepared by 4 h annealing at 700 °C under controlled phosphorusvapor pressure. Photocapacitance results reveal three dominant deep levels. A dominant deep levelat 1.05 eV above the valence band is detected commonly before and after annealing, and the changeof level densities is shown as a function of the phosphorus vapor pressure. Another two deep levelsare also detected after annealing at 0.74 eV above the valence band and at 0.51 eV below theconduction band. In view of the deviation from the stoichiometric composition of InP, possibleorigins of these levels and their optical transition mechanism are discussed. © 1997 AmericanInstitute of Physics. @S0021-8979~97!00107-2#I. INTRODUCTIONInP crystal is one of the most important semiconductormaterials for the application of ultra-fast electronic devices,opto-electronic devices and so on. However, deviations fromthe stoichiometric composition can be significant comparedto Ga–As based compounds. Therefore, the most importantfactor to be controlled in InP is the deviation from the sto-ichiometric composition.Nishizawa1–3 has previously pointed out the importanceof stoichiometry control of compounds and the systematicresearch on the stoichiometry-dependent defects of III-V andII-VI compounds has been carried out.Up until now, many reports on the deep levels have beenpublished by means of deep level transient spectroscopy~DLTS!,4 photoluminescence ~PL!,5 photo-induced transientspectroscopy ~PITS! method.6 In the case of InP crystal, un-derstanding of the stoichiometry-dependent deep levels hasbeen lacking compared with those of GaAs or GaP crystals.Recently, the authors7 applied the photocapacitance ~PH-CAP! method to investigate the stoichiometry-dependentdeep levels in intentionally-undoped n-type InP crystal pre-pared by 4 h annealing at 700 °C under controlled phos-phorus vapor pressure. Photocapacitance ~PHCAP!8 mea-surement by constant capacitance method is a very usefulmethod to determine the precise level densities and their en-ergy levels. From the photocapacitance results on n-InP, it isshown that three dominant deep levels were detected at 0.63and 1.1 eV below the conduction band and 0.74 eV abovethe valence band, respectively. The phosphorus vapor pres-J. Appl. Phys. 81 (7), 1 April 1997 0021-8979/97/81(7)/3151Downloaded¬11¬Nov¬2008¬to¬130.34.135.83.¬Redistribution¬subjesure dependence of these level densities has also been re-ported.In this article, the 77 K-PHCAP method under constantcapacitance conditions is applied to the intentionally Zn-doped p-type InP prepared by 4 h annealing at 700 °C undercontrolled phosphorus vapor pressure followed by rapidcooling. The phosphorus vapor pressure dependence of thedeep level density is shown, and excitation photocapacitanceresults show the precise optical transition mechanism ofthese deep levels.II. EXPERIMENTThe starting crystals used were liquid-encapsulated-Czochralski ~LEC! grown p-type InP doped with Zn with the$100%-oriented crystal surface. Carrier concentration is331017 cm23. After cleaning and etching by a H2SO4–H2O2solution, a sample crystal is placed in one end of a dumbbell-type quartz ampoule with 6N red phosphorus in the otherend. After sealing in vacuum, the ampoule was put into atwo-temperature zoned electric furnace. Heat treatment wascarried out at 700 °C under controlled phosphorus vaporpressure. Heat treatment time was kept to be 4 h in thepresent experiments. Phosphorus vapor pressure at the phos-phorus zone, P0 , is determined from the temperature of redphosphorus referring to Honig.9 The phosphorus vapor pres-sure at the crystal zone, P , was obtained from the followingequation:10P5P0~T/T0!1/2, ~1!3151/4/$10.00 © 1997 American Institute of Physicsct¬to¬ASCE¬license¬or¬copyright;¬see¬http://jap.aip.org/jap/copyright.jspwhere T and T0 are the temperature of InP crystals and redphosphorus, respectively. After annealing, the ampoule wasrapidly cooled by dipping into water. Metal–semiconductorcontact diodes were made by Au evaporation as a barriermetal, and AuZn/Au was used for the ohmic contact on theback surface. 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. Thechange of the ion density, DNt , is given by the followingequation:DVph5~e/2C2!DNt , ~2!where C is the predetermined constant capacitance of thesample diode, e is dielectric constant, and DVph is the changeof bias voltage. Precise description of the PHCAP measure-ment and the equipment is reported elsewhere.11The PHCAP measurements were carried out as follows.A sample diode was cooled and the forward bias injectionwas applied in the dark in order to neutralized deep levelsbefore light irradiation. Then, a saturating bias voltage Vdarkwas obtained in the dark. Vdark is attributable for the ther-mally ionized level density in the dark. When monochro-matic light was fed into the depletion region of the samplediode, bias voltage Vph changed according to the ionizationof deep levels and the saturating bias voltage was obtained.Net ion density induced by the light irradiation is obtainedby DVph5Vph2Vdark . Under constant capacitance condi-tions, the depletion layer thickness is kept constant regard-less of the change in the ion density.FIG. 1. Ion density PHCAP spectrum of intentionally Zn-doped p-type InPwith the carrier concentration of 331017 cm23 before annealing.3152 J. Appl. Phys., Vol. 81, No. 7, 1 April 1997Downloaded¬11¬Nov¬2008¬to¬130.34.135.83.¬Redistribution¬subjeIII. RESULT AND DISCUSSIONFigure 1 shows the ion density PHCAP spectrum of in-tentionally Zn-doped p-type liquid-encapsulated crystal~LEC! InP crystal before annealing. It is shown that the iondensity increases sharply at 1.05 eV above the valence band.In the case of p-type semiconductors, the increase of iondensity is induced by the optical transition of electrons fromthe valence band to the neutral deep acceptor levels.Figure 2 shows the ion density PHCAP spectrum of in-tentionally Zn-doped InP crystals prepared by 4 h annealingat 700 °C under various phosphorus vapor pressure. In Fig.2, it is shown that the 1.05 eV1Ev level can be detectedcommonly before and after annealing. In addition, the photo-ionization of another deep level is detected at 0.74 eV abovethe valence band when p-InP crystals are annealed underlower phosphorus vapor pressure in the range of 1–100 Torr.However, 0.74 eV1Ev level cannot be detected, when phos-phorus vapor pressure applied was above ;760 Torr. Inview of the 0.74 eV1Ev deep level, we have already re-FIG. 2. Ion density PHCAP spectra of intentionally Zn-doped p-type InPprepared by 4 h annealing at 700 °C under phosphorous vapor pressure of~a! 1 Torr, ~b! 100 Torr, and ~c! 760 Torr, respectively.Nishizawa et al.ct¬to¬ASCE¬license¬or¬copyright;¬see¬http://jap.aip.org/jap/copyright.jspported in our previous article on n-type InP7 that photoca-pacitance results revealed the electron capture at 0.74 eVabove the valence band even in intentionally undoped n-typeInP sample before and after annealing under various phos-phorus vapor pressure. It is also shown that the 0.74 eV1Evlevel density in n-type InP decreases with increasing phos-phorus vapor pressure in the vapor pressure range below,100 Torr. It means that the photoresponse at 0.74 eV1Evin Zn-doped p-InP corresponds to the electron capture at0.74 eV in n-type InP. Therefore, it is concluded that thephosphorus vapor pressure dependence of the 0.74 eV1Evlevel density shows good correspondence between n- andp-type InP crystals, respectively.Therefore, it is considered that this level is not inducedby the unexpected impurity contamination, but is probablyrelated to phosphorus vacancies or a vacancy. Further, fromthe present PHCAP spectrum, we conclude that the 0.74 eV1Ev level is related to the phosphorus vacancy due to thedeviation from stoichiometry, because this deep level isformed by annealing under a lower phosphorus vapor pres-sure.Figure 3 shows phosphorus vapor pressure dependenceof the 1.05 eV deep level density. It is shown that 1.05 eV1Ev level density increases with increasing phosphorus va-por pressure. Deep level at 1.05 eV was not detected in un-doped InP but can be observed in Zn-doped p-InP. In viewof phosphorus vapor pressure dependence of this deep leveland the effect of impurity doping, the origin of 1.05 eV mayinclude defect-impurity complex with relation to either Invacancies or P interstitials.We performed excitation PHCAP to investigate the op-tical transition mechanism of the ;1.05 eV level above thevalence band. Figure 4~a! shows the excitation PHCAP spec-trum of an intentionally Zn-doped p-InP sample prepared by4 h annealing at 700°C under a phosphorus vapor pressure of760 Torr. After the primary light irradiation at all interval of0.02 eV in the spectral range from 1.0 to 1.4 eV, ion densityFIG. 3. Phosphorous vapor pressure dependence of deep level density of1.05 eV in Zn-doped InP samples prepared by annealing at 700 °C undervarious phosphorous vapor pressure.J. Appl. Phys., Vol. 81, No. 7, 1 April 1997Downloaded¬11¬Nov¬2008¬to¬130.34.135.83.¬Redistribution¬subjePHCAP spectrum was measured repeatedly from the longwavelength by changing the primary excitation light wave-length. As shown in Fig. 4~a!, after 1.08 eV light irradiation,it is shown that the neutralization is induced at 0.51 eV be-low the conduction band. Figure 4~b! shows the change ofion density of Ec20.51 eV level as a function of primaryexcitation light wavelength in the range of 1.08–1.4 eV. Theion density of the Ec20.51 eV level looks to be saturatedabove ;1.20 eV light irradiation.Figure 5 shows schematic drawing of the optical transi-tion process of this level by the simple flat band diagram.This level at 1.08 eV above valence band is ionized after1.08 eV light excitation light. Then, the neutralization of theFIG. 4. ~a! Ion density PHCAP spectra measured from long wavelengthafter the primary excitation light irradiation. ~b! The change of ion densityof Ec–0.51 eV level as a function of the primary excitation light wave-length. Sample used was intentionally Zn-doped p-type InP prepared by 4 hannealing at 700 °C under phosphorous vapor pressure of 760 Torr.FIG. 5. Schematic drawing of the optical transition of Ec–0.51 eV and 1.05eV1Ev levels in p-type InP deduced from the present excitation PHCAPresults.3153Nishizawa et al.ct¬to¬ASCE¬license¬or¬copyright;¬see¬http://jap.aip.org/jap/copyright.jsplevel is induced by the electron transition at 0.51 eV belowconduction band by the Frank–Condon shift of about 0.09eV. Therefore, in view of optical transition mechanism, it isconcluded that the neutral 1.08 eV1Ev level corresponds tothe ionized Ec20.51 eV level.VI. CONCLUSIONThe photocapacitance measurement is applied to inves-tigate the stoichiometry-dependent deep levels in intention-ally Zn-doped p-type InP crystals prepared by 4 h annealingat 700 °C under controlled phosphorus vapor pressure. ThePHCAP spectrum revealed two deep levels at 0.74 and 1.05eV above valence band. The 1.05 eV1Ev level has beendetected in common before and after annealing and the iondensity increased with increasing phosphorus vapor pressure.This level is considered to be a complex defect with relationto the impurity-intrinsic defect. The 0.74 eV1Ev level isformed by annealing under a low phosphorus vapor pressure.0.74 eV1Ev level is attributed to phosphorus vacancy atleast due to the deviation from stoichiometry. In view of the3154 J. Appl. Phys., Vol. 81, No. 7, 1 April 1997Downloaded¬11¬Nov¬2008¬to¬130.34.135.83.¬Redistribution¬subjeoptical transition mechanism of these deep levels, the exci-tation PHCAP results revealed the neutralization at 0.51 eVjust after 1.08 eV light irradiation. Therefore, it is concludedthat the ionized level of the neutral 1.08 eV1Ev level lo-cates at 0.51 eV below the conduction band.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!.3 J. Nishizawa, K. Suto, and Y. Oyama, Mater. Res. Soc. Symp. Proc. 242,179 ~1992!.4Y. Yamazoe, Y. Sasai, and T. Nishino, Jpn. J. Appl. Phys. 20, 347 ~1981!.5V. V. Negresukul, E. V. Russu, S. I. Radanstan, and A. G. Cheban, Sov.Phys. Semicond. 9, 587 ~1975!.6Y. Yamazoe, Y. Sasai, T. Tishino, and Y. Hamakawa, Jpn. J. Appl. Phys.20, 47 ~1981!.7 J. Nishizawa, Y. Oyama, K. Suto, and K. Kim, J. Appl. Phys. 80, 1488~1996!.8A. Ito, T. Sukegawa, and J. Nishizawa, Tech. Rep. Transistor SpecialistCommittee, IEEE Japan, Jan. ~1967!; available on request.9R. E. Honig and D. A. Kramer, RCA Rev. 30, 285 ~1969!.10 J. Nishizawa, Y. Kobayashi, and Y. Okuno, Jpn. J. Appl. Phys. 19, 345~1980!.11 J. Nishizawa, Y. Oyama, and K. Dezaki, J. Appl. Phys. 67, 1884 ~1990!.Nishizawa et al.ct¬to¬ASCE¬license¬or¬copyright;¬see¬http://jap.aip.org/jap/copyright.jsp

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Stoichiometry-dependent deep levels in p-type InP

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