Vol. 132, No. 7 S E E D E D , R E C R Y S T A L L I Z E D S I L I C O N F I L M S 1717

edl-3, 161 (1982). 2. C. I. Drow]ey and T. I. Kamins, Mater. Res. Soc. Sym.

Proc., 13, 511 (1983). 3. J. Sturm and J. F. Gibbons, Paper 3.2 presented at the

IEEE SOS/SOI Workshop, Hilton Head Island, Oct. 2-4, 1984.

4. P. Zorabedian and T. I. Kamins, Mater. Res. Soc. Sym. Proc., 33, 81 (1984).

In,Ga1-,As Photodetector for the 1.7-2.0/ m Spectral Region C. B. Morrison*, 1 and S. M. Bedair

Department of Electrical Engineering, North Carolina State University, Raleigh, North Carolina 27650

K. J. Bachmann*

Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27650

ABSTRACT

InGaAs photodetectors for the 1.7-2.0 t~m region have been fabricated by LPE growth of InGaAs layers on interme- diate layers of InAsP on an InP substrate, p-n Junctions were formed either by Zn diffusion into an n-type In~Ga~_xAs layer or by epitaxial growth of a p-type layer of In~Ga,_~As doped with Mn. The effect of threading dislocations on mi- nority carrier lifetime is analyzed on the basis of minority carrier diffusion length data obtained by fitting a computer model to the experimentally observed quantum efficiency.

In~.Ga, ~.As~P,_~ lattice matched to InP is currently the preferred materials system for the fabrication of light sources and detectors for optical communications. The maximum wavelength accommodated in this system is 1.65 ~m, corresponding to the bandgap of Ga047In0.~3As, and thus it encompasses the wavelength region of mini- mum total loss of silica-based fiber optical waveguides (1). Although for such fibers the position of this mini- mum is not likely to shift much farther into the infrared, small adjustments in the most desirable wavelength of transmission beyond 1.7 t~m may become necessary in the future. This paper reports on the fabrication of a photodetector that covers the 1.7-2.0 t~m region (2). The detector is fabricated in the InxGa~_~As ternary system (0.53 < x < 0.70) on InP substrates by liquid phase epitaxy (LPE). To accommodate the lattice mismatch, InAsyP~_~ step-graded layers have been grown between the substrate and the active region. The In-As-P ternary sys- tem was chosen because it is easy to step grade and can also act as a window on the active region if the detector is illuminated from the substrate side.

Experimental and Results The present work was preceded by the determination

of the phase diagrams of both the In-Ga-As (3) and the In- As-P (4) ternary systems. The respective solidus and liquidus isotherms for several temperatures were gener- ated by the aid of the regular solution approximation in conjuction with extended data from across the solidus composition of each ternary system. The extended soli- dus and liquidus data sets were acquired by growing by LPE multiple epitaxial layers using step grading tech- niques (3). For the present work, saturation temperature for the In-Ga-As melts was 650~ In-As-P melts were sat- urated at slightly higher temperatures.

The possibility of growing by LPE an InGaAs layer on intermediate layers of InAsP on an InP substrate is dem- onstrated in the single-crystal x-ray diffraction pattern de- picted in Fig. I. This pattern shows clearly seven of the eight expected peaks from a single InGaAs epitaxial layer grown on two intermediate, step-graded layers of InAsP on a ( l l l )B InP substrate. The InGaAs layer is approxi- mately 3 t~m thick and the InAsP layers are each approxi- mately 5 t~m thick. The copper K~ 2 peak of the InGaAs layer is overlapping with the copper K~ peak of the top- most InAsP layer. As indicated in Fig. 1, the compositions

* Electrochemical Society Active Member. ~Present address: TRW Electro Optics Research Center,

Redondo Beach, California 90278.

of the InAs,Pl y layers are y = 0.057 and 0.095 and the composition of the In rGa~_=As layer is x = 0.62. The over- all lattice mismatch between the InP substrate and the InGaAs layer shown in Fig. 1 is calculated to be 0.57%. The lattice mismatch between the top InAsP step-graded layer and the InGaAs layer shown in Fig. 1 is calculated to be 0.27%. In later growth runs, the lattice mismatch be- tween the top two epitaxial layers was successfully de- creased to the resolution of our x-ray diffractometer, which is 0.10%.

Etch pit density (EPD) studies were performed on the mismatched InGaAs epitaxial layer by etching for 5-30s in AB etch at room temperature. The revealed defect den- sity in all cases tested exceeded 106 cm -=. This high EPD is attributed to the large mismatch between adjoining epitaxial layers and to some extent to the (111) orientation of the epitaxial layer.

The In.rGal_xAs undoped n-type epitaxial layer was characterized by Hall data from an epitaxial layer grown on a semi-insulating substrate and C-V measurement data on a junction. These data show consistently a carrier con- centration of n = 4 x 10 '~ cm -3. The Hall data result in a mobility of tL = 9100 cm'-'/V-s for x = 0.54. Devices were fabricated either by Zn diffusion into an n-type In~Ga,_~As layer or by epitaxial growth of a p-type layer

[nAsyPl_y InxGal _xAS y = 9.48% K(~2 Kc~ 1 , X = 61.63%

90 K~I IP xGal-xAs V

,o "k A y = 5 .69%

70 InAsP f t | |

lnP

50 K~I Ir~AsPA | ~ I ~. 40 InAsP ~ IK~z ~ V i(

20 ~ ~ [ ~11 10

0 - - - I I 131.5 131 130.5 130 129.5 129 128.5

, 29 ~ Fig. 1. X-ray diffraction pattern from o simple InxGm ~,As epitaxial

layer grown on two intermediate step-graded layers of InAs~Pl_y on on InP substrate.

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1718 J. Electrochem. Soc.: S O L I D - S T A T E S C I E N C E A N D T E C H N O L O G Y July 1985

hp

t / In0.60Ga0-40As

Au-Zn-Au 1 ~ / /DIFFUSION O H M I C C O N T A C T ~ ~ / . ~ BOUNDRY

~ p - ~ j lnAs0.12P0.88 ; 4 . / ~ 5 ~

/ ~ - ~ 1 A 0.06%.94 [ j \ l n-TYPE (Sn)

~ "~ Sn-Ag

OHM IC CONTACT

hv F p-TYPE (Mn)

u o I " / / A H~,Zc'c~uN TACT ~ ~ " ~ / / / / ,nAsa3 ~ m

_/ V - - 0o0%04

/ InP (111) B (~) l Sn- DOPED

~ i ~ Sn-Ag OHMIC CONTACT Fig. 2. Sketches of In~Ga~_,As photodetectors for 1.7-2.0/~m region.

Top: Zn diffused. Bottom: Mn doped.

ni tude-higher doping concent ra t ion of Mn explains the lower mobi l i ty and q u a n t u m efficiencies because of a re- duc t ion of the minor i ty carrier l i fe t ime due to increased scat ter ing effects. With lower Mn doping, h igher q u a n t u m efficiencies can be expected . On the o ther hand, because the electr ical p-n junc t ion is separated f rom the metallur- gical junc t ion in the Zn-diffused devices, q u a n t u m effi- ciencies are i m p r o v e d (5, 6). Fur the rmore , in agreement wi th the h igher mobi l i ty of the Zn-di f fused epi taxial lay- ers and in v i ew of the fact that the field genera ted by the doping gradient guides minor i ty carriers toward the col- lect ing junct ion , the Zn-diffused devices have h igher effi- ciencies.

The dens i ty of misfit dislocations depends to a large ex ten t on the lat t ice misfit strain b e t w e e n adjacent mis- ma t ched epi taxia l layers or be tween a m i sma tched epi- taxial layer and the adjacent substrate. Furthermore, these dislocations tend to bend into the epitaxial layers, creating "inclined" or "threading" dislocations that can transmit the impact of the lattice misfit into the bulk of the epitaxial layer and thus contribute to a degradation of the layer properties (7). The direct relationship between an increased lattice mismatch, an increase in misfit dislo- cation density, and a decrease in device performance has been reported in several lattice-mismatched ternary sys- tems (8-10). In view of these observations, an analysis of the known lattice mismatch and corresponding device

of InxGal xAs doped with Mn, as i l lus t ra ted in Fig. 2a and 2b, respect ively. Zn diffusion was pe r fo rmed ei ther by sealed ampu l diffusion or by plat ing with Zn fol lowed by diffusion on a strip heater. The Zn-di f fused material was character ized by Hall data resul t ing in typical ly p = 5 • 1018 cm -3 and ~ = 73 cm2/V-s. For Mn-doped growth, Mn was added to the mel t at the ratio 1.0-1.5 mg/g-In. For a ra- tio of 1.2 mg/g-In in Mn-doped material , the ne t hole con- centrat ion and Hall mobi l i ty were p = 3 • 1019 cm -3 and = 17 cm~/V-s.

The devices were character ized by I-V characteris t ics and spectral response. Representa t ive I-V characterist ics are shown in Fig. 3 and 4. For the I-V l ight and dark cur- rents, reverse bias b r e a k d o w n b e c a m e increas ingly soft as lat t ice mi sma tch increased. For Zn-di f fused In~Ga,_~As junc t ions of compos i t ions of x = 0.545, 0.58, and 0.625, open-circui t vol tages were 0.21, 0.18, and 0.14V, respec- tively, and peak q u a n t u m efficiencies were 0.662, 0.628, and 0.226, respect ively . The q u a n t u m eff iciency spectra of these three devices are depic ted in Fig. 5. Mn-doping In~.Gal_j.As junc t ions of composi t ions of x = 0.54, 0.59, and 0.69 had peak q u a n t u m efficiencies of 0.265, 0.202, and 0.059. For the Mn-doped In~Gal_xAs pho tode tec to r of composi t ion x = 0.69, the q u a n t u m eff iciency was fairly flat to 1.9 ~ m and diss ipated thereaf te r to 2.0 ~m as v iewed in Fig. 6.

Analysis and Discussion The dif ference in the q u a n t u m efficiencies be tween the

Zn-doped and the Mn-doped In~Gal_~.As photode tec tors may be a t t r ibu ted to several causes. The order-of-mag-

Fig. 3. a: I-V characteristic of a Zn-diffused Ino.58Goo.42As photodetector, b: Alternative view.

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Vol . 132, No. 7 In~Ga~_xAs P H O T O D E T E C T O R 1719

(%) '~176 60.0

50.0 >- c~ z ~40.0 u. Li. LU

30.0 I-.- Z <

20.0

10.0

�9 In0.545Ga0.455As �9 I n0.58G a0.42As

l i I | In0.625Ga0,375As

i i i i i i i i i l i L A R v A l

Q

1.4 115 1 6 1.7

m i i I I d

i �9 A

i

****. , . In| 1.6 1.9

WAVELENGTH (pm)

Fig. 5. Quantum efficiency spectra of three Zn-diffused In~Ga~ ,~As photodetectors.

Fig. 4. a: I-V characteristic of a Zn-diffused Ino,62~Ganm~As photodetector, b: Lattice-matched Mn-doped photodetector.

quantum efficiencies for the In~.Gal_xAs photodetectors described in this paper is appropriate.

The quantum efficiency spectra of the Zn-diffused de- vices were fitted on the basis of a homojunction model for solar ceils (11). A computer algorithm performed the fit by finding opt imum values of the diffusion lengths of the minority carriers and junction depth. An example of this fit is shown in Fig. 7 for the Zn-diffused Ino.~Gao.4~As detector. The fit is not perfect, since the model itself is meant to be only a first approximation of a diffused junc- tion. Furthermore, the model requires estimated values for wavelength-dependent parameters such as the absorp- tion coefficient and the reflectance. The modeling re- suits indicated that the quantum efficiency spectra were fairly insensitive to the minority carrier hole diffusion length on the n-type side of the electrical junction. This insensitivity can be expected if most of the absorption takes place near the surface of the device.

Under these circumstances, the minority carrier life- time, ~., of the electrons in the p-type side of the electrical junction can be calculated on the basis of the modeling results by the relationship of ~, = L,2/D~, where D, is the electron diffusion coefficient and L. is the electron diffu- sion length. Also, the threading dislocation density can be calculated from the derived expression (12)

30.0

~25.0

I-

~20. 0 O u-

~15.0 b- Z <

10.0 u

5.0

1.4

oo iQ~ �9 In 0 54Ga 0 46 As

�9 �9 In0.59Ga0.41As A In0.69Ga0.31As

a ram,mare � 9 1 4 9 at

AAALI, AAAA AAAAh~A AAAA AA �9 AA

�9 �9 A

I I I I m , ~ . * i 1.5 7.6 1.7 1.8 1,9

WAVELENGTH (/.L m)

i

2.0

Fig. 6. Quantum efficiency spectra of three Mn-lnxGal ~As photodetectors.

~ (1 + [I2Lr~2pa) -1

where p~ is the threading dislocation density, Vo is the peak quantum efficiency in the absence of dislocations, and ~ is the peak quantum efficiency in the presence of

0.60.Tr In058Ga04'~As" . - I

0.5 . . . . " . . . . i

~ 0 . 4

uJ 0.3 0.2

0.1

O 0.0 1.05 1.19 1.33 1.47 1.61 1.75 1.89

WAVELENGTH (#m) Fig. 7. Quantum efficiency experimental data and the fitted model for

a Zn-diffused Ino,58GaoA~As photodetector.

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1720 J. Electrochem. Soc.: S O L I D - S T A T E S C I E N C E A N D T E C H N O L O G Y

o I P-TYPE GaAs ..------

~ 10-9 -

J 10_10

PRESENT I n x G a l _ x A S D A T A U N E ~ ' ~

- - . . . _ 10 -11 I I I

10 5 i0 6 10 7 m 8 10 9

D I S L O C A T I O N D E N S I T Y (cm - 2 )

Fig. 8. Minority carrier lifetime vs . misfit dislocation of Zn-diffused In~Ga~_~As photodetectors.

July1985

10.0

9.0 PHOTODIODE WITH 8.0 STEP-GRADED LAYERS "~ ~

. ' ; - " 1 "-:

7.0 ~ .*;J ', o S s ~ |

< ~HOTODIODE WITHOUT �9 ~ , " ~ 1 7 6 TEP-GRAO O LAYERS > ',

> 5 0 J ' " ,,

s ~ .'.,',t." " � 9 | Oo ~,s@.. "~'~ �9 . |

tr ~ 3.0 ~ .~ ....-....-" �9 l f ..~ ..* " " . t|

2,0 ~....,.''" " �9 t

1.0 ...:.........~i . . \ .~ . . . . - r" . n I n "';"i

0.7 0.9 1.1 1.3 1.5 1.7 1.9 pm

Fig. 9. Responsibility of Ino,ssGao.4~As photodetectors with and with- out step-graded layers of InAs~Pl_~:

dislocations. By allowing Vo to be 70% (13, 14), V to be the peak value for each of the Zn-diffused devices, and Ln to be the corresponding values deduced from the fitting procedures, the threading dislocation density can be esti- mated for each device. Figure 8 indicates the relationship between the computed minority carrier lifetimes and their corresponding estimated dislocation densities for the Zn-diffused devices of the present work. Figure 8 thus indicates the estimated trend of the effects of threading dislocation density on the minority carrier lifetime. Simi- lar work with p-type GaAs (7) is also presented in Fig. 8 for comparison.

On the basis of the analysis resulting in Fig. 8, the de- crease in lifetime due to the increase in misfit disloca- tions in In~Gal_~As appears to be less critical than the de- crease in lifetime due to the increase in defect density in GaAs specifically and other materials (Ge, Si, GaP, and A1GaAs) in general (7). This difference in part reflects the fact that devices with short diffusion lengths are less sen- sitive to a moderate dislocation density than devices with long diffusion lengths. This difference can also be attrib- uted to the differences in the nature of the defects in those other materials and as well to the presence of step- graded layers used in the present study. Improved electri- cal performance of mismatched material due to the pres- ence of step-graded layers included to relieve mismatch strain has been noted in GaAsSb photodiodes (8) and in In~Ga, xAs layers grown on GaAs substrates (9). This ef- fect has also been noted in the present work, as can be ob- served in Fig. 9. This figure shows the responsivities of In0.~sGa0.42As homojunction photodiodes both with and without intermediate step-graded layers of InAs,P,_,. Both devices were Zn diffused and otherwise identical except for the presence of the intermediate step-graded layers. The device with the intermediate step-graded lay- ers has more than twice the responsivity of the device with a single mismatched layer. Thus, although increased threading dislocations due to lattice-mismatched material cause a serious decrease in efficiency of the device de-

scribed in the present work, the introduction of interme- diate step-graded epitaxial layers has made it possible to produce by LPE InGaAs photodetectors for the 1.7-2.0 /~m spectral region.

A c k n o w l e d g m e n t This work was supported by the Army Research

Office, Research Triangle Park, North Carolina.

Manuscript submitted April 4, 1984; revised manuscript received Jan. 31, 1985.

TRW Electro Optics Research Center assisted in meeting the publication costs of this article.

REFERENCES 1. H. Osonai, T. Shioda, T. Moriyama, S. Araki, M. Hori-

gaki, T. Izawa, and H. Taketa, Electron. Lett., 12, 550 (1976).

2. C. B. Morrison, S. M. Bedair, and K. J. Bachmann, in "III-V Opto-Electronics Epitaxy and Device Re- lated Processes," V. G. Keramidas and S. Mahajan, Editors, p. 139, The Electrochemical Society Soft- bound Proceedings Series, Pennington, NJ (1983).

3. S. M. Bedair, C. Morrison, R. Fang, and N. A. E1- Masry, J. Appl. Phys., 51, 5413 (1980).

4. C. B. Morrison and S. M. Bedair, To be published. 5. A. K. Chin, B. V. Dutt, H. Temkin, W. A. Bonner, and

D. D. Roccasecca, Appl. Phys. Lett., 36, 924 (1980). 6. T. P. Lee, C. A. Burrus, Jr., and A. G. Dental, IEEE J.

Quantum Electron., qe-17, 232 (1981). 7. H. Kressel, Semicond. Semimetals, 16, 1 (1981). 8. R. L. Moon, J. Appl. Phys., 51, 556 (1980). 9. M. Ettenberg, C. J. Neuse, J. R. Appert, J. J. Gannon,

and R. E. Enstrom, J. Electron, Mater., 4, 37 (1975). 10. M. L. Timmons and S. M. Bedair, J. Appl. Phys., 52,

1134 (1981). 11. H. J. Hovel, Semicond. Semimetals, 11, (1975). 12. R. J. Roedel, A. R. Van Neida, R. Caruso, and L. R.

Dawson, This Journal, 126, 637 (1979). 13. K. J. Bachmann and J. L. Shay, Appl. Phys. Lett., 32,

446 (1978). 14. T. P. Pearsall, IEEE J. Quantum Electron., qe-16, 709

(1980).

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