KAPL-P-000 169 (K97053)

C Q h F - 7 7 0 w y - - ELECTRICAL AND OPTICAL PROPERTIES OF DEGENERATELY DOPED

N - t v a e x G h h

G. W. Charache, D. M. DePoy, J. L. Egley, et. al.

May 1997

I* i

KAPL ATOMIC POWER LABOWORY SCHENECTmY,

Operated for the U. S . Department of Energy by W L , Inc. a Lockheed Martin company

TE

NOTICE

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KAPL ATOMIC POWER LABOWORY SCHENECTmY,

Operated for the U. S . Department of Energy by W L , Inc. a Lockheed Martin company

L A O 9 Ld El%lmUm

NEW YORK 10701

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REPRODUCED AT GOV7

Electrical and Optical Properties of Degenerately-Doped N-type InXGal,,As

GW Charache, DM DePoy, JL Egley, RJ Dziendziel, MJ Freeman, PF Baldasaro, and BC Campbell

Lockheed Martin, Inc., Schewctady, NY 12301-1072

PR Sharps, ML Timmons Research Trinngle Institute, Research Triangle Park NC 27709-2194

RE Fahey, K B a n g OFC Corp., Natick, MA 01 760

JM Borrego Rensselaer Polytechnic Imtitute, Troy, NY 12301-3590

Abstract. Degeneratelydoped (> lo1’ ~ r n - ~ ) n-type InxGal-,As (x > 0.53) possesses a number of intriguing electrical and optical propeaies relevant to electro-optic devices and thermophotovoltaic devices in particular. Due to the low electron effective mass of this material (m,* .. 0.1) and the demonstrated ability to incorporate n-type dopants into the mid-10’’ range, both the Moss-Burnstein bandgap shift and plasma reflection characteristics are particularly dramatic. These properties are investigated for In,Gal-,As as a function of doping concentration, dopant type, and growth conditions. For undoped InGaAs with a naming bandgap

of 0.G eV, doping this material to 5 x 1019 increased the effective optical bandgap to 1.1 eV and has a plasma turn-on wavelength of 5 microns. This filter was coupled to a non-absorbing interference fiiter, creating a functional tandem fiiter for thennophotovoltaic applications. .

Introduction

In,Gal-,As (x > 0.53) thermophotovoltaic (TPV) devices have demonstrated the highest performance of any infrared material system utilized for energy conversion of blackbody radiators [ 11. Unique electrical and optical properties of this material system offer to improve its performance for both TPV devices and mid-infrared optoelectronic devices. Due to the low

electron effective mass of this material (m, - 0.1) and the ability to incorporate n-type dopants

into the mid-10” cm-3 range, both the Moss-Burnstein bandgap shift and plasma reflection characteristics are particularly dramatic. These properties are investigated for In,Gal_,As as a function of dopant type, dopant concentration, and growth conditions.

The electron-concentration dependence of the near bandgap absorption has been investigated for a variety of III-V semiconductor materials since the mid-1950’s [Z], and recently in n-InAs and lattice-matched n-InGaAs [3,4]. It has been shown that a theoretical

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description of the optical bandgap shift is due to both band f i n g and bandgap narrowing of the gamma valley. However, when electrically active carrier concentrations approach the rnid-10l9 level, a two-valley model may be required. The effective optical bandgap (Le., the position of the Fermi-level above the valence band) for a parabolic conduction band is given by

where, Ego is the nominal semiconductor bandgap, h is Planck’s constant, m* is the conduction band effective mass, q is the electron charge, N is the net electrically active carrier concentration, and E,,, is the effective bandgap narrowing due to both bandtailing and many- body effects. When adding the effects of a non-parabolic conduction band, the effective mass is modified to take into account the filling of the conduction band as carrier concentration increases [S 1.

The spectral reflection and transmission of a semiconductor plasma filter are determined from the complex index of refraction and film thickness. The complex index of refraction, Ti, is given by n-ik, where n is the real part and k is the imaginary part. The parameter k is also referred to as the extinction coefficient and is related to the absorption coefficient, a, at a wavelength, A, by the following: a = 4nk/h. The complex index of refraction for a plasma reflector is given by the Drude relation:

2 2 ( n - i k ) = E ~ - ,

corn* - (o - iyo)

where E, is the high frequency limit of the dielectric constant, N is the free carrier concentration,

eo is the permittivity of free space, m* is the free carrier effective mass, o is the angular frequency, and y is the damping rate. The angular frequency is related to wavelength, h, by the following: o = 2nc/h, where c is the speed of light; and the damping rate is related to the free carrier mobility, p, by the following: y = q / p * . Solution of this equation for the real and imagi- nary parts of the refractive index as a function of wavelength allows the calculation of the reflec- tion, transmission, and absorption for a semiconductor plasma filter for a given thickness.

Due to the high cari-ier concentrations that have been obtained in In,Gal,As [Table lland low electron effective mass, both the Moss-Burstein optical bandgap shift and plasma reflection characteristics are expected to be particularly dramatic. As noted in Table 1, higher electrically active carrier concentrations are observed as the indium fraction increases. Thus, carrier concen- trations approaching lo2’ indium fractions greater than 0.53.

should be readily attained with a variety of dopant sources for

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Indium Fraction (x)

0

0

0

0.53

0.53

0.53

0.53

1

1

1

Highest Electrically Active carrier Concentration

Dopant Source I SiH4, Si2H6, TESiH

DETe

1 x 1O2O

3 x 1019

sn

SiBr4

1 6 x loi9 TESn

Si I 6 ~ 1 0 ' ~

I I 1-2X1O2O j Te

I 8 x lo2' ' Se

I 1 X 1 O 2 l IS I

Reference

6,7,8,9

6,8,10

11

12

13

14

REPRODUCED AT GOv7 UBENSE #*48

Table 1 - Highest reported electrically active carrier concentrations in In,Gal-,As.

EIectrical Properties

For this study single layers of n-In,Gal-,As (0.53 < x < 0.7) were grown using atmospheric pressure organometallic vapor phase epitaxy (AP-OMVPE) on semi-insulating InP wafers. Electrical properties were optimized by varying the following experimental parameters: dopant type (hydrogen selenide, methylallyltelluride), dopant mole fraction, and VmI ratio. Figures 1 and 2 illustrate the Hall Effect results for both Se- and Te-doped samples as a function of dopant mole fraction. Figure 1 also includes the data of Pizone [ 131 for Sn-doping of In0.53Ga0.&is. These samples had substrate orientations of <loo> 2 O -off toward <1 lo>,

growth temperatures of -640°C, and V/Lu ratios greater than 50. Higher carrier concentrations were achieved with Te-doping, which is believed to be due to the lower vapor pressure of Te versus Se. For both 0.6 eV (x = 0.67) and 0.73 eV (x = 0.53) material and Te dopant mole

fractions greater than

5 x 1019 and 2 x 10" ~ m - ~ , respectively; while the mobility remains fmed or decreases. This

the carrier concentration remains fixed at a maximum level of

Page 3

suggests a significant amount of inactive tellurium dopant, which is confi ied by secondary ion mass spectroscopy (SIMS) analysis [Fig. 31. One important feature of this work is the high carrier mobility (> 1000 cm2N-s) achieved for lattice mismatched films with electrically active carrier concentrations greater than 1019 ~ m - ~ . It is these high mobility values that are required to demonstrate high quality (sharp turn-on) plasma filters.

Figure 4 illustrates the effects of varying the V/III ratio on electrically active carrier

Although it was expected that concentrations for tellurium mole fractions greater than decreasing the Vmlr ratio would increase the dopant incorporation, the results show no significant trend.

REPRODUCED AT GOV7 UBENSE # 48

Optical Properties

Reflection and transmission measurements (F’TR) were utilized to extract the effective photonic bandgap as a function of doping level for Ino 67G% 33As [Fig. 51. The measured photonic bandgap along with calculated values for both parabolic and non-parabolic band approximations are included. The deviation of the measured and calculated values at doping levels greater than lo1’ cm-3 may be due to two effects: filling of the satellite L-valley and bandgap narrowing. The position of the L-valley in %.67G% 33As was estimated from published values of the L-valley bandgap in GaAs (Es = 1.70 - 1.73 eV) and InAs (Eg = 1.07 - 1.43 eV) [16-181 and the L-valley bowing parameter (c - 0.5 eV) [15,17]. Depending on the position of the L-valley in InAs, the L-valley bandgap in 67Ga033AS can range from 1.16 - 1.6 eV. The lower end of this range may effect the measured photonic bandgap as shown in Fig. 5. Thus, it is unclear whether filling of the satellite valley or bandgap narrowing is the dominant effect.

Long wavelength (> 2 pm) reflection scans were measured to investigate the plasma reflection characteristics for In0 67G% 33As [Fig. 61. These films demonstrate the characteristic plasma-edge shift toward shorter wavelengths as the free-electron concentration increases. The

sharp turn-on of the sample doped 5 x d9 ~ r n - ~ is due to the high mobility (low effective mass) of Ino.6$ho33As. Modeling the reflection characteristics of these films with the Drude-theory allows the extraction of the below bandgap optical constants. Using the measured Hall carrier concentration and mobility for films greater than 1 micron in thickness, allow the fitting of the reflection characteristics with the effective mass as the only fitting parameter. Figure7 illustrates calculated values of the real (n) and imaginary (k) parts of the refractive index as a function of

wavelength and mobility for a free can-ier concentration of 5 x IO1’ ~ m - ~ . For this calculation

E = 12.5, m = 0.1, and ni = 1 (i.e., air). Figure 8 plots the calculated reflection and absorption as *

a function of wavelength and film thickness for a free carrier concentration of 5 x ~ m - ~ , a

mobility of 1400 cm2N-s, and an effective mass of 0.1. These properties represent the best InGaAs plasma filter fabricated to-date. This figure illustrates that these InGaAs plasma filters demonstrate high below bandgap reflection and negligible above bandgap absorption for films

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REPRODUCED AT GOVT EXPENSE # 48

1-2 microns in thickness.

These films coupled with an appropriate non-absorbing interference filter yield a highly effective tandem filter concept for TPV spectral control. Figure 9 illustrates the reflection versus wavelength for an initial tandem filter concept. For this concept, an interference filter was directly deposited on the InGaAs plasma and an anti-reflection (AR) coating was deposited on the backside of the InP substrate, in order to maximize above bandgap transmission. This filter demonstrates a spectral utilization factor of -65% for a 2000 F blackbody radiator [19].

Conclusions

The low effective mass of degenerately-doped n-In,Gal-,As makes this material an ideal

~ m - ~ . This has tremendous impact on

plasma filter for TPV applications. The effective photonic bandgap nearly doubles as the free

electron concentration increases from 1 x IO the design of many mid-infrared optoelectronic devices, including therrnophotovoltaics. Recently, the Moss-Burstein shift was used to design an improved InGaAs resonant cavity photodiode [20]. For TFV devices, this effect may be utilized to create transparent emitter or cap layers for n-on-p devices, eliminating the need for front surface passivation; transparent graded layers for p-on-n devices that utilize a back surface reflector [21]. In addition, it was found necesssuy to utilize the effective optical bandgap of n+ InGaAs in order to effectively model the performance of SPIRE-fabricated TFV devices. Finally, the low effective mass

ensures high values of mobility even at carrier concentrations exceeding 1 x IO1' cm-3 . This sharp plasma resonance due to these high mobilities ensures very low parasitic absorption in the above bandgap region (1.0 - 2.2 pin wavelength) and very high reflection (>95%) just beyond the plasma turn-on. The InGaAs plasma filter was successfully coupled to a non-absorbing interference filter that yielded a spectral utilization factor of -65% for a 2000 F blackbody radiator.

17 19 to 5 x 10

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,

References

Ill G.W. Charache, et al, "Current Status of Low-Temperature Radiator Thennophotovoltaic Devices," in

121 E. Burstein, "Anomalous Optical Absorption Limit in InSb," Phys. Rev. 92,632 (1954). 131Y.B. Li et al, "Infrared Reflection and Transmission of Undoped and Si-doped InAs Grown on GaAs by

[4] D. Hahn, et al, "Electron Concentration Dependence of Absorption and Refraction in n-In053G%.47As Near the

151 E.O. Kane, "Band Structure of Indium Antimonide", J. Phys. Chem. Solids, 1,249 (1957). [61 M. Weyers, et al, "Gaseous Dopant Sources in MOMBWCBE', JL Crystal Growth, 105,383 (1990). 171 S.Z. Sun, et al, "Zinc and Tellurium Doping in GaAs and AlGaAs grown by MOCVD," JL Crystal Growth, 113,

103 (1991). [SI J. Musolf, et al, "Doping of GaAs and InP in MOMBE Using DEZn, TESn, and DETe," JL Crystal Growth,

107,1043 (1991). 191 Y.M. Houng and T.S. Low, "Te-doping of GaAs and AlGaAs using DETe in LPOMVPE," Jl. CrystaZ Growfh,

77,272 (1986). [lo] C. R. Abernathy, et al, "Sn Doping of GaAs and AlGaAs by MOMBE Using Tetraethyltin," Jl. Crystal

Growth, 113,412 (1991). 1111 M.B. Panish, et al, "Very High Tin Doping of GaInAs by Molecular Beam Epitaxy," Appl. Phys. Lett., 56,

1137 (1990). 1121 SL. Jackson, et al, "Silicon Doping of InP, GaAs, InGaAs and InGaP Grown by Gas Source and Metalorganic

Molecular Beam Epitaxy Using a SiBr4 Vapor Source," in Proceedings of the InP and Related Materials Cogerence, 1994, pp. 57-60, "High Efficiency Silicon Doping of InF and XnGaAs in Gas Source and Metalorganic Molecular Beam Epitaxy using Silicon Tetrabromide," AppL Phys. Lett., 64,2867 (1994).

1131 CJ. Pizone, et al, "Tin-doped n+-InP and GaInAs Grown by AP-MOCVD," Electron. Left., 25,1315 (1989). CJ. Pizone et al, "Heavily-doped n-type InP and InGaAs Grown by Metalorganic Chemical Vapor Deposition Using Tetraethyltin," Jl. Appl. Phys., 67,6823 (1990).

Proceedings of the 25th IEEE PVSC, 1996, pp. 137-141. I

Molecular Beam Epitaxy," Semicond Sci. Technol., 8,101 (1993).

Bandedge," Jl. Electron. Mat., 24,1357 (1995).

1141 T. Fujii, et al, "Heavily Si-Doped InGaAs Lattice Matched to InP Grown by MBE," Electron. Left., 22, (1986). [15] V.M. Glazov, et al, "Investigation of the Relationship Between the Electron Density and Solubilities of Sulfur,

1161 S. Tiwari and DJ. Frank, "Empirical Fit to Band Discontinuities and Barrier Heights in III-V Alloy System",

1171 M. Levenshtein et al, E&., Handbook Series on Semiconductor Parameters voL I , , New York World

1181 S. Adachi, "Bandgaps and Refractive Indices of AlGaAsSb, GaInAsSb, and InPAsSb: Key Properties for a

1193 P J . Baldam0 et al, "Experimental Assessment of Low-Temperature Voltaic Energy Conversion," AIP

1203 S.S. Murtaza et al, "Resonant Cavity Photodiode at 1.55 Microns with Burstein-Shifted In0.53Ga0.47As/InP

1211 G.W. Charache, et al, "Thennophotovoltaic Devices with a Back Surface Reflector for Spectral Control," AZP

Selenium and Tellurium in Indium Arsenide," Sov. Phys. Semicond., 10,378 (1976).

Appl. Phys. Lett., 60,630 (1992).

Scientific, 1996.

Variety of the 2-4 pin Optoelectronic Device Applications", JE. AppL Phys., 61,4869 (1987).

Conference Proceedings, 358,29 (1995).

Reflectrors," Appl. Phys. Lett., 69,2462 (1996).

Corgference Proceedings, 321,371 (1996).

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lo-;o 1

Dopant Mole Fraction

Figure 1 - Measured carrier concentration versus dopant mole fraction for Se- and Te-doped InXGal-,As.

5000

4000

3 "E 3000 0 v

1000

9

C . .1 . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . Oj3 eV l$G&s vex i Oi6 e~ InGaAs (.$e) i i j .. . . . . . . .

%.QY.!P.qwy . . . e$?) .i.i.: ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . , . . - ... . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0-l Dopant Mole Fraction

Figure 2 - Measured carrier mobility versus dopant mole fraction for Se- and Te-doped Zn,Gal-,h.

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c

10 9 8

7

6

5

4

3

2

1 I IO' 1 o2

SlMS Concentration (1 0 " c m ~

Figure 3 - Electrical activation of tellui-ium dopant in In,Gal-,As.

10

9

S 0 6 2 a 5

c

c c 0 S 0

0 4 - a r

............. ................ .....................

.............

8 .......i................................................. : o ................................................................

...................... j ..................................................................................................... 1 0

...........................

..........................................................................................................................

? i

i o ...........................................................................................................................

REPRODUCED AT GO- EXPENSE # 40

Figure 4 - Dependence of the V/lII ratio on the electrically active carrier concentration in InxGal-,As.

6e tt 3swdx3 LA^ N a m n a o w

20 40 60 80 100

SlMS Concentration (1 0'' cm3)

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b .

1.3

1.2

1.1

- 1.0 3 5 0.9 a .

+-. 0.8

0.7

0.6

z 0) 0

0 - 0 c R

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 ol* io” 1 O2O 0.5

1 oi7 Doping Density (cma)

Figure 5 - Theoretical and measured values of the effective photonic bandgap €or 0.6 eV InGaAs.

1UO

95

90

a5

80

75

70

65

60

55

50

45

40

35

30.

25 < 20 f

15-

10

5

4 6 a 10 12 14 16 18 2

Wavelangth (micrometers)

Figure 6 - Measured plasma reflection characteristics of 0.6 eV InGaAs.

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REPRODUCED

wavelength (microns)

p=30

p= 10 p=3m

14

I p = 1000 p=300

p=100

p=30

p = 10

wavelength (microns)

Figure 7 - Calculated values of the real (n) and imaginary (k) components of the refractive index

as a function of wavelength and mobility for a free carrier concentration of 5 x 1019 ~ r n - ~ , E = 12.5, m* = 0.1, and ni = 1 &e., air).

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t , ~b

Film Thickness (microns)

Figure 8 - Calculated dependence of plasma filter thickness on reflectivity as function photon

wavelength for a plasma filter with a free carrier concentration of 5 x lo1’ ~ m ’ ~ , a mobility

of 1400 cm2N-s, and an effective mass of 0.1. 100

90

80

70

60

50 ‘ 40 s

.- CI 0

30

20

10

0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5

Wavelength (microns)

Figure 9 - Measured reflection versus wavelength for a 0.6 eV InGaAs plasma / non-absorbing interference filter tandem filter.

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