Silver Doped p-Type ZnS Crystals

phys. stat. sol. (b) 229, No. 1, 365–370 (2002)

Silver Doped p-Type ZnS Crystals T. V. Butkhuzi, T. G. Tchelidze, E. G. Chikoidze1), and N. P. Kekelidze Semiconductive Materials Science Laboratory, Tbilisi State University, Chavchavadze Ave. 3, 380028 Tbilisi, Georgia (Received July 27, 2001; accepted September 30, 2001) Subject classification: 61.72.Vv; 65.40.Gr; 71.55.Gs; 72.80.Ey; 78.55.Et; S8.11 Thermodynamic analyses for the system ZnS:Ag (crystal)–S2(vapor) is performed. It turned out that it is impossible to receive hole conductivity in this system. In our experiment ZnS crystals covered by silver layer are silver doped by the mechanism of atom recoil during S+ ion implantation. The resistivity of obtained p-type layers is 102 –103 W cm. The activation energy of acceptor is determined to be 0.3 eV. In the PL spectra of p-type samples l = 454 and 520 nm bands are observed and identified.

Introduction Zinc sulfide is an n-type semiconductor with the highest band gap of 3.74 eV among II–VI compounds. Such a high band gap makes this material very attractive for optoelectronics, but at the same time it represents one of the main reasons of intensive compensating processes, which results in a strong preference of one type of doping. The difficulties in p-doping of II–VI wide band gap compounds have been studied extensively [1–6]. In spite of these difficulties obtaining of low-Ohmic p-type ZnS samples has been reported [7, 8]. This paper deals with the problem of obtaining p-type ZnS crystals by silver doping. Thermodynamical Analysis of ZnS : Ag System In order to analyze the difficulties of p-type conductivity realization in the system ZnS : Ag the method of quasi-chemical reactions (QCR) [9] is used. The purpose is to obtain the concentration of defects and free charge carriers as a function of treatment temperature and surrounding gas pressure. The basis processes we consider with corresponding mass action law are as follows: The creation of vacancy pairs: O ! VZn þ VS ;

KS ¼ ½VZn ½VS ;

ð1Þ

lattice thermal ionization: O ! e þ h;

Ki ¼ np ;

ð2Þ

sulfur incorporation from gas phase into lattice site creating zinc vacancy: 1 S2 ! VZn ; 2

KVZn ¼

VZn 1=2

ð3Þ

;

PS2

1 ) Corresponding author; Tel.: 995 32 37 59 65; Fax: 995 32 29 47 86; e-mail: [email protected]

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T. V. Butkhuzi et al.: Silver Doped p-Type ZnS Crystals

the first ionization of intrinsic acceptor and donor centers: VZn ! V0Zn þ h ;

½V0Zn p ; ½VZn ½V n ¼ S ; ½VS

KAI ¼

VS ! VS þ e ;

KDI

ð4Þ ð4aÞ

their second ionization: V0Zn ! V00Zn þ h ;

½V00Zn p ; ½V0Zn ½V e ¼ S ; ½VS

KAII ¼

VS ! VS þ e ;

KDII

ð5Þ ð5aÞ

displacement of Agi and Agi from the interstitial to the substitution site: Agi þ VZn ! AgZn ;

K1 ¼

½AgZn ; ½Agi ½VZn

Agi þ V00Zn ! Ag0Zn ;

K2 ¼

½Ag0Zn ; ½Agi ½V00Zn

ionization of Agi and AgZn:

ð6Þ ð6aÞ

Agi ! Ag i + e,

KAgi ¼

AgZn ! Ag0Zn + h ,

½Agi n ; ½Agi ½Ag0Zn p : ½AgZn

KAgZn ¼

(7) (7a)

In these equations VZn, V0Zn , V00Zn , denote neutral, negatively one-charged, negatively double-charged zinc vacancies; VS , VS , VS denote neutral, positively one-charged, positively double-charged sulfur vacancies; Agi ; and Ag i denote neutral and positively onecharged interstitial silver atoms; AgZn and Ag0Zn denote neutral and negatively onecharged substitutional silver atoms, respectively. The same variables in square brackets denote the concentrations of the corresponding species; e and h denote electron and hole; n and p their concentrations; PS2 denotes sulfur pressure. The system of Eqs. (1) to (7) is added by the silver balance equations (SBE) and electro-neutrality condition (ENC) in which only main terms are kept according to Brower’s approximation method [10]. At fixed temperature the choice of dominant defect depends on the partial pressure of compound component and on the total concentration of silver, which is assumed to be constant. At high silver concentration and moderate sulfur pressure Agi and AgZn are taken as dominant species. SBE and ENC have the form: Agi ¼ Ag0Zn ;

½Ag0Zn ¼ ½Agi ¼ 12 ½Agtot :

ð8Þ

In this range an impurity auto-compensation takes place. With increasing sulfur pressure, the concentration of VZn increases and according to ½Ag0Zn ratio increases too. At (6) and (6a) the ½Agi " #2 ½Agtot 2 KAgi 1 PS2 ¼ ; ð9Þ KAII KAI KVZn K2 KAgZn

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phys. stat. sol. (b) 229, No. 1 (2002)

Eq. (8) turns into ½Ag0Zn ¼ ½Agtot :

Ag0Zn ¼ p ;

ð10Þ

In this range p-type conductivity conditioned by silver is realized. The following increase of sulfur pressure causes the appearance of intrinsic defect hole conductivity. In this range VZn becomes the dominant acceptor. The sulfur vapor pressure at the boundary between the latter ranges is " #2 ½Agtot 2 2 : ð11Þ PS2 ¼ KVZn KAI In Fig. 1 the concentration diagram for ZnS:Ag system based on this defect model is presented. The temperature and silver content are taken at T = 1000 K and [Ag]tot = 1018 cm ––3. At low concentration silver solves in crystal mainly in the form of substitutional acceptor AgZn and in the first range it is compensated by intrinsic donor. In this case ENC and SBE have the form ½Ag0Zn ¼ 2½VS ;

½Ag0Zn ¼ ½Agtot :

ð12Þ

In this case the boundary between ranges II and I is at P*S2ð1Þ ¼

½Agtot KDI KDII : Ki2 KVZn

ð13Þ

Dominant defects in ranges I and II are identical to those for high silver concentration. In Fig. 2 the concentration diagram for the latter defect model is shown. Transition from “low” to “high” concentration should occur at KDI KDII KS K1 ; ½Ag* ¼ Ki

ð14Þ ð1Þ

which is obtained by equating PS2 and P S2 ð1Þ. ð1Þ Actually PS2 and P S2ð1Þ always exceed sulfur saturated vapor pressure. This fact makes difficult to obtain p-type conductivity in the ZnS : Ag(crystal)–S2 (vapor) system. ð1Þ PS2 and P S2ð1Þ are large because Ki and KVZn are small. So, the doping problem is caused on the one hand by the electronic

Fig. 1. Concentration diagram for the ZnS : Ag system for Ag0Zn ¼ Agi compensation type

368


Fig. 2. Concentration diagram for ZnS:Ag sys tem. forAg0Zn ¼ 2VS compensation type

structure of compound, which is unable to be regulated, and on the other hand by the peculiarities of the interaction between the two phases ZnS (crystal)–S2 (vapor). The value of KVZn is small because sulfur mainly exists in vapor as S2 molecules with considerable dissociation energy ( 4.2 eV). Before interaction with crystal these molecules must be dissociated. Therefore, the reaction of sulfur incorporation into the crystal has high energetic barrier ( 1.45 eV), whereas the reaction of metal incorporation into crystal, when non-metal vacancies are creating, goes with energy gain. If we use another system, where during the equilibrium settling the creation of metal vacancies is more intensive and formation of non-metal vacancies is suppressed, we should get positive results. Experiment ZnS crystals covered by 100––150 A thick silver layers are implanted by S+ ions. The ion energy is E = 150 keV and implantation doses are 1015 –1016 cm ––2. Silver atoms dope the crystal by the mechanism of atom recoil. The implanted samples are treated in argon atmosphere within the temperatures range T = 400–800 C and according to thermo-EMF measurements, they reveal hole conductivity. Samples with lowest resistivity (r = 102 –103 W cm) are obtained at T = 600–650 C (Fig. 3). The resistivity is decreasing much more rapidly than the dose increases. This increase of hole conductivity and its nonlinear dose dependence can be ex-

Fig. 3. Resistivity heat treatment temperature dependence for ZnS crystals implanted by S+ ions. D = 5 1015 cm ––2, E = 150 keV

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phys. stat. sol. (b) 229, No. 1 (2002)

Fig. 4. PL spectra of ZnS crystals: (1) Before implantation, (2) implanted by S+ ions and treated under silver layer in argon atmosphere at T = 450 C, (3) implanted by S+ ions and treated under silver layer in argon atmosphere at T = 650 C; D = 1016 cm ––2; lext = 365 nm

plained by equilibrium settling conditions for the system ZnS–Ag. During the S+ ion implantation sulfur atoms saturate the silver layer. At D = 1016 cm ––2 there is a sufficient quantity of sulfur atoms in the silver layer for creating the Ag2S compound. Since the silver layer is saturated, the sulfur atom diffusion from the ZnS crystal into the layers is depressed, while zinc atom extraction from the crystal into the silver layer is very intense. That is, the creation of non-metal vacancies is limited and the creation of VZn defects is activated, which leads to an increase in hole conductivity. In our opinion, this is the reason why the conductivity increases more rapidly than the concentration of recoiled silver atoms. From temperature dependence of conductivity for p-type ZnS implanted crystals the activation energy of acceptor center is defined as Ea ¼ ð0:3 0:03Þ eV. The PL spectra of implanted ZnS crystals treated at T = 450 and 650 C is shown in Fig. 4 There is weak emission l = 454 nm and green emission at l = 520 nm with large intensity in the PL spectra of low-Ohmic samples. The l = 454 nm wavelength emission is more intense in the PL spectrum of the samples with high resistivity and heavy compensation. This band should be connected to the compensated center ðAg0Zn VS Þ. With decreasing compensation degree the intensity of this band decreases while the intensity of the l = 520 nm green emission sharply increases. In our opinion this is related with the fact that doped Ag atoms are occupying the VZn sites and reserve the electro-neutrality of the crystal, V S defect transfers into V S defect. Just with this center we connect the l = 520 nm wavelength emission. This band appears in ZnS samples with lowered Fermi level, obtained by various methods [11]. That is why we connect it to the compensating non-metal vacancy, and when the Fermi level is lowering the formation enthalpy of this center decreases [2].

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