Growth of chalcopyrite type magnetic semiconductors

K. Sato, T. Ishibashi, V. Smirnov,H. Yuasa, J. Jogo, T. Nagatsuka,

Y. Kangawa and A. Koukitu

TUAT

Scope of this talk

• Brief summary of previous studies of chalcopyrite type magnetic semiconductors

• Results of in-situ photoelectron spectroscopy

• Suggested existence of chalcopyrite MnGeP2

• Thermodynamic analysis

• MOMBE growth of MnGeP2

• Characterization

Brief summary of previous studies of chalcopyrite type magnetic semiconductors

• We have been working with Mn-substituted chalcopyrite type semiconductors CdGeP2 and ZnGeP2, in which we have confirmed ferromagnetic behavior up to 423 K and 350 K, respectively. Magneto-optical effect was also observed.

• These samples were obtained by deposition and subsequent diffusion of Mn to bulk single crystals of ternary compounds.

• Ab-initio calculation suggests that CdGeP2 system with vacancies or non-stoichiometric composition will lead to ferromagnetism although ferromagnetism is not favored in stoichiometric (Cd, Mn)GeP2.

Chalcopyrite Structure

V

Cd Ge P

III V

IV

IV VII

Si, Ge

GaP

CdGeP2

Diamond structure

Zincblende structure

Chalcopyrite structure

II-IV-V2 chalcopyritesa(Å) c(Å) Tm(°C) Eg(eV) no, ne n, p

ZnSiP2 5.399 10.435 1370 2.96 ~3.1 260, 11

ZnSiAs2 5.606 10.890 1096 2.12 3.355, 3.392 40, 170

ZnGeP2 5.465 10.771 1025 2.34 3.248, 3.295 - , 20

ZnGeAs2 5.672 11.153 850 1.15 ~3.38 - , 23

ZnSnP2 5.651 11.302 930 1.66 ~3.21 - , 55

ZnSnAs2 5.852 11.705 775 0.73 ~3.53 - , 190

CdSiP2 5.678 10.431 1120 2.45 ~2.95 150, 90

CdSiAs2 5.884 10.882 850 1.55 ~3.22 - , 500

CdGeP2 5.741 10.775 790 1.72 3.356, 3.390 1500,80

CdGeAs2 5.943 11.217 670 0.57 3.565, 3.678 4000,1500

CdSnP2 5.900 11.518 570 1.17 ~3.14 2000,150

CdSnAs2 6.094 11.918 596 0.26 ~3.46 11000,190

CdGeP2-MnMagnetization (VSM)

-4000 -2000 0 2000 4000

-0.001

0.000

0

0.000

0.001

H (Oe)

M (

emu)

-4000 -2000 0 2000 4000

-0.001

0.000

0

0.000

0.001

H (Oe)

M (

emu)

-4000 -2000 0 2000 4000

-0.001

0.000

0

0.000

0.001

H (Oe)

M (

emu)

77K

287K

423K

K.Sato et al.: J.Phys.Chem.Solids64(2003)1461

K.Sato, G.Medvedkin, T. Ishibashi: J.Cryst. Growth 236 (2002) 609

ZnGeP2-MnMagnetization (SQUID)

5K

150K

350K

Magneto-Optical Kerr Effect

1 1.5 2 2.5 3 3.5 4 4.5-0.15

-0.10

-0.05

0.00

0.05

Photon Energy [eV]

K K,

K (d

eg)

KK

K. Sato et al.: J. Magn. Soc. Jpn. 25 (2001) 283.

Previous preparation method for chalcopyrite-type magnetic semiconductors

• Mn was deposited on single crystals of CdGeP2 and ZnGeP2 at Tsub400C, by which Mn was diffused into the bulk to substitute group II and IV cations.

• During growth RHEED pattern of chalcopyrite structure seems to remain.

• Mn-diffused crystals show ferromagnetism above room temperature.

II-IV-V2 single crystals

• CdGeP2{112}• Directional freezing of th

e stoichiometric melt in a quartz ampoule or graphite crusible

• Rate: 4deg/h for 48h• Highly compensated n-t

ype• Prepared at Ioffe Inst.

• ZnGeP2(001)• Vertical bridgeman tec

hnique• Bulk ingot of 28mm a

nd 150mm in length• Highly compensated p

-type• Prepared at Siberian P

hysico-Technical Inst.

Preparation of Mn-doped chalcopyrites

Host crystal: CdGeP2, ZnGeP2

Mn depositionTsub=RT to 380-400°C

Mn diffusion@T=300-500°C

II-IV-V2 single crystal

II-IV-V2 single crystalMn

II-IV-V2 single crystalMn-diffused layer

Tsub.= R.T. Tsub. = 400℃

ZnGeP2

During depo.

After depo.

After annealing550 30min.℃

RHEED patterns during Mn deosition

Problems

• Inhomogeneous depth profile of Mn obtained by the deposition-diffusion technique.

• Electrical properties of the surface shows a metallic behavior.

• Preparation of homogeneously Mn-doped layer is necessary.

Effort to obtain CdGeP2:Mn thin films by MBE is proceeding

Inhomogeneous depth profile of Mn in CdGeP2:Mn

Careful preparation necessary

• Synthesis of bulk or powder CdGeP2:Mn from constituent elements was tried. However, It was difficult to prevent formation of second phase compounds.

• In bulk ZnGeP2:Mn prepared at elevated temperature, room-temperature ferromagnetism is suspected as due to MnP precipitated in the material.

• Careful preparation of films with homogeneous distribution of Mn is strongly required.

Magnetic properties of bulk ZnMnGeP2

• Preparation by solid state reaction of Zn+Ge+Mn+P at max 1130C

• Antiferromagnetism below 47K• Ferromagnetism between 47 and 312K

MT curve MH curve 　 Mn3% MH curve Mn5.6%

Cho et al. Phys. Rev. Lett. 88 (2002)257203

NMR studies in ZnMnGeP2

• Very small amount of MnP phase that cannot be found by XRD was detected by NMR in polycrystalline ZnMnGeP2 material prepared by the same method as did by Cho.

Hwang et al.: Appl. Phys. Lett. 83 (2003) 1809

ZnMnGeP2 Mn15%

Mixture of ZnGeP2 and MnP

In-situ photoelectron spectroscopy

• Photoelectron spectrometer with MBE systemWith Ar-ion etching device

• Synchrotron radiation: Photon Factory BL-18A• Specimen: ZnGeP2single crystal, polished and

etched• Deposit Mn and interrupt to measure PES• After deposition of 50nm Mn, sputter-etched

by Ar-ion and at each stage PES was measured

Radiation E~100meV Mg KX-ray E~800meV

Mn evaporator (Omicron EFM-4)99.999%Mn

Thickness monitor

Ion gun 1.5kV Ar+

1cm

Heater

Therm

ocouple

P<5x10-9 Torr (sample growth)P<7x10-10 Torr (PES measurement)

Cleaning the substrate Sputtering out the surface layer

Photoemission Apparatus at Photon Factory BL-18A

132 128 124 120 116

P 2p Ge 3p

650 640

Mn 2p1/2 Mn 2p3/2

1024 1020

Zn 2p3/2

32 28

Ge 3d

d = 510A 260 130 64 32 16 8 4 2 annealed

substrate

T = 400 C 　 (const.)0 < d < 510Å

Inte

nsi

ty (

arb

itra

ry u

nits

)

結合エネルギー (eV)

Mg K

PES during deposition

start

end

ZnGeP2:Mn

MnGeP2?

Core signal intensity

Mn 堆積3.0

2.5

2.0

1.5

1.0

0.5

0.0

12 3 4 5 6 7

102 3 4 5 6 7

1002 3 4 5

Mn 2p

Zn 2p3/2

Ge 3d

P 2p

内殻

電子

放出

強度

(ar

b.un

its)

名目上の Mn 層厚 (A)

0

No Zinc

MnGeP?

T = 400 Cd = 250Å

Binding energy (eV)

Inte

nsi

ty (

arb

itra

ry u

nits

)

Mg K

Start sputter

Endsputter

PES during sputter

Total sputtering time (min.)

Co

re-le

vel i

nte

nsity

rat

ioZn:Ge:P ~ same as substrate composition

Mn2+compounds (DMS phase)

Core signal intensity during sputtering

Mn-rich composition

MPMSMPMS

Magnetization (by SQUID magnetometer)

Suggested existence of chalcopyrite MnGeP2

• Photoemission The surface composition is MnGeP2

• RHEED pattern of initial chalcopyrite structure remained during growth

Is chalcopyrite-type MnGeP2 really exist?

MOMBE growth of MnGeP2

• We applied MOMBE technique to obtain MnGeP2 films on GaAs substrate.

• Mn and Ge are supplied from solid state source using K-cells

• As P source, TBP (tertiary butyl phosphine) MO source is employed.

• TBP is cracked to form P2 and P4 using cracking cell at 813 C

Thermodynamic analysis for MOMBE growth of MnGeP2

• To know whether MnGeP2 can be obtained as a stable compound using the MOMBE technique, thermodynamic analysis is performed.

Driving force for deposition

• In the thermodynamic analysis, we used parameters of driving force for deposition P, Input partial pressure, P0, and equilibrium partial pressure at vapor-solid interface, P.

• Here, driving force for deposition P is the difference between input partial pressure and equilibrium partial pressure :P=P0-P;

where P0 is input partial pressure, and P equilibrium partial pressure

• Using these parameters, we can obtain Input mole ratio, RMn, and solid composition, x, as follows:

GeMn

Mn

GeMn

MnMn PP

Px

PP

PR

,00

0

P0: Input partial pressure

Su

bst

rate

su

rfac

e

P

P0

Par

tial

pre

ssu

re

Distance

Boundary layer

P=P0-P

X

Driving force for deposition, P

Driving force for deposition, P

P: Equilibrium partial pressure

MOMBE

• Mn(g)+1/2 P2(g) = MnP(s)• Ge(g)+1/2 P2(g) = GeP(s)Conservation constraints• PMn+Ge

0-PMn+Ge= 2(PP20-PP2)

Pi = PMn+PGe+PP2

Equilibrium equation for reaction

activity:,constantmEquilibriu:

,2

22

1

aK

PP

aK

PP

aK

PGeP

GeP

PMn

MnP

xxH

RT

xxa

RT

xxa

m

GePMnP

1/

exp1,1

exp22

•Here, we assume P2 molecule as a group-V source, because more than 80% of TBP is cracked and changed to P2 rather than P4 at 813C.

using these equations equilibrium partial pressure, which is unknoun parameter, is calculated.

Ab-initio calculation of enthalpy of mixing

• Enthalpy of mixing Hm

Hm=EMnGeP-{xEMnP+(1-x)EGeP}

• Interaction parameter = Hm/x(1-x)

• Solid composition xx= PMn/(PMn+ PGe) vs Input molar ratio of MnRMn=P0

Mn/(P0Mn+P0

Ge)

enthalpy of mixing of (Mn,Ge)P as a function of solid composition

• The function, Hm, is estimated from the ab initio total energy calculations for structure models.

Ab-initio calculation

• Ab initio calculations 　 using CASTEP code– Task: Geometry optimization– Electron correlation: GGA– Energy cutoff: 240eV

GeP MnPMn0.5Ge0.5P

Mn Ge P

Stable Formation of MnGeP2

• It is found, in the graph, that enthalpy of mixing has negative value.

• This is because the chalcopyrite structure with x=0.5 becomes stable compared with random alloy

Interaction parameter =Hm/x(1-x)From the calculated enthalpy of mixing, Hm, we estimated interaction parameter, , to be 3044x-33726 [cal/mol]. Using this function, we carried out the thermo-dynamic analyses and examined the relationships between input mole ratio and solid composition.

MnGeP2

Vapor/solid distribution relationship• Here, the thermodynamic calculations were perfor

med under the following conditions;PMn

0+PGe0=1.0x10-7 torr

PP20=2.0x10-7 torr

• It is found, in the graph, that small input mole ratio is required to make MnGeP2 at higher temperatures.

• This is because that Mn-P bond is easily formed compared with Ge-P bond.

Crystal Growth ConditionsCrystal Growth Conditions

TBP flow rate @1.6 sccm

Flux intensity:

(Cracking temp. @813 )℃

Substrate @SI-GaAs(100) Just

Deposition time @20,60 min

CdMn Ge

TBP (Gas)

REED

Substrate

Ge @1.3,2.1×10 Torr-8

Mn @2.0×10 Torr-8

Base pressure @3.0×10 Torr-8

(Solid)

(K-cell temp. @1035,1060 )℃

(K-cell temp. @725 )℃

Growth temperature @360,415℃

(Etched by H20+H2O2+NH3)

Flux monitor

Screen

Pump C.C gauge

EDXEDX

#1

#2

Mn Ge P

0.99 1.00 2.67

Mn Ge P

2.09 1.00 5.20

Mn flux

[Torr]

Ge flux

[Torr]TBP flow rate [sccm]

Growth Temp. [ ]℃

Depo.time [min]

Sample#1 2.0×10 2.1×10 1.6 360 20

Sample#2 2.0×10 1.3×10 1.6 360 20

-8

-8

-8

-8

#3

Mn Ge P

1.03 1.00 1.89

Mn flux

[Torr]

Ge flux

[Torr]TBP flow rate [sccm]

Growth Temp. [ ]℃

Depo.time [min]

Sample#3 2.0×10 -8 2.0×10 -8 1.6 415 60

SEM ObservationSEM Observation

Sample#1 Sample#2 Sample#3

300nm 400nm400nm

Sample#1 Sample#2 Sample#3

800nm 800nm 800nm

XRD PatternsXRD Patterns

Sample#1

Sample#2

Sample#3

65.6 65.8 66 66.2 66.4102

103

104

105

106

2θ 　 [deg]

Inte

nsi

ty [cp

s]

65.6 65.8 66 66.2 66.4

2θ [deg]

106

105

104

103

102

Inte

nsity

[cp

s]

62 64 66 68 70 72

103

104

105

106

107

108

109

2θ [deg]

Inte

nsi

ty [cp

s]XRD Measurement

MnGeP2 　– narrow scan ① 1.72 ： 1.00 ： 2.99 (1h)

GaA

s(00

4)

GeP?

MnGeP2?

Detailed XRD study is underway

Magnetic PropertiesMagnetic Properties

-550 -450 -350 -250 -150 -50 50 150 250 350 450 550

-550 -450 -350 -250 -150 -50 50 150 250 350 450 550

Parallel

Perpendicular

H (Oe)

H (Oe)

Sample#3

2

4

6

8

10

12

14

-2

-4

-6

-8

-10

-12

-14

2

4

6

8

10

-2

-4

-6

-8

-10

0

0

M (

10

em

u)-5

M (

10

em

u)-5

・ Room Temp.・ VSM

Mn;1.03 Ge;1.00 P;1.89

- 0.01-0.005

00.0050.010.0150.020.0250.030.035

1 2 3 4 5 6

Photon　Energy　(eV)

θK　and　η

K(deg) θ (degree)ｋ

η (degree)ｋ

Polar Magneto-optical Spectra in MnGeP2

Summary

• As one of approaches to elucidate the origin of room temperature ferromagnetism in magnetic chalcopyrites, growth of chalcopyrite type MnGeP2, which is not existing in nature is investigated.

• Thermodynamic study including ab-initio evaluation of Hm confirms stable formation of MnGeP2 by MBE technique.

• MOMBE growth of MnGeP2 films are studied. Nearly stoichiometric compounds are obtained. They show ferromagnetism and weak magneto-optical effect.

• Further careful investigation is necessary to discriminate the effect of possible second phase material.