InP lasers and LEDs

Journal of Crystal Growth 54 (1981) 69—75 North-Holland Publishing Company

69

COMPARISON OF SINGLE-AND TWO-PHASE LPE GROWTH METHODS FOR InGaAsP/InP LASERS AND LEDS * I. LADANY and F.Z. HAWRYLO, RCA Laboratories, Princeton. New Jersey 08540, USA

Two well known methods for growth of lnGaAsP/InP material for emitters operating in the 1.3 ~.smrange are the single phase method, in which accurately equilibrated melts used, and the two-phase method in which solid lnP is present in the melt. Both of these methods have been applied at RCA Laboratories in the fabrication of lasers and LEDs. Performance of devices obtained by both methods was similar, and equivalent to the best values published in the literature. Thus the choice of methods must be made on the basis of other criteria and it was the purpose of this work to attempt to provide a basis for this choice. A number of runs were made with each process, using both identical melts, and melts with slight changes in composition. The resultant epitaxial layers were analyzed by X-ray diffraction and photoluminescence, and the uniformity and consistency of growth obtainable from the two methods are reported.

1. Introduction A number of techniques have been used for LPE growth of InGaAsP laser and LED materials. Two of these which we consider in this paper are singlephase-supercooling, which was successfully used by Hsieh [1] in developing quaternary lasers, and twophase-supercooling, adapted to InGaAsP by Yamamoto et al. [21 and further explored by Pollack et al. [3], which differs from the first in having a solid InP phase always present to saturate the melt. Besides the obvious simplification of the latter method, in that phosphorus content is controlled automatically, it is not easily established whether one or the other method has any advantages as far as material growth is concerned. For example, the difficulty of maintaining the required phosphorus concentration in the single-phase method, and the effect of platelets in obtaining good layer growth in the two phase method, are matters one would like to explore, The purpose of the present work was to examine these two processes experimentally, concentrating on reproducibility, a subject not often discussed in *

Work partially supported by National Aeronautics and Space Administration, Langley Air Force Base, Hampton, Virginia, USA.

0022-0248/81/0000—0000/502.50 © North-Holland

the literature, and on the uniformity of the material grown by these two methods.

2. Description of the growth processes Details of these processes are not necessarily the same for different laboratories. Our versions are defined with the aid of fig. 1. As seen, the initial saturation temperature and the degree of supercooling are different, but for most of the runs the growth temperature of 63 5°Cwas the same. In both cases, before data were collected for this study, a number of runs were made to obtain the best melt compositions, and to settle on processing and sequencing of various steps. The growths were carried out in two separate systems, the singlephase method in a semi-transparent “gold” furnace, and the two-phase method in a conventionally wound and insulated tube furnace. Graphite boats similar to those used in A1GaAs work were employed, and the boats were operated in palladium-diffused hydrogen. Substrates used in this work were (100) oriented InP wafers supplied by Metals Research, with a dislocation density on the order of 1 o4 cm2. Wafers were lapped and polished using bromine-methanol solutions and etched before insertion into the furnace. In both processes, InP n-type layers were first grown

70

1. Ladany, F.Z. Ilawrvlo

/ Single- and

two-phase LPE growth methods for InGaAsP/InP

SINGLE PHASE, SUPERCOOLING

TWO PHASE. SUPERCOOLING

~RAT~1T

~

a 63~~ START QUATERNARY—~~

START QUATERNARY

GROWTH

I

I

WAFERS

GROWTH

I~

I

I

I

0 204060 80 100120140160 MINUTES

J~_I________I

I

I

I

I

I

0 20406080100120140160 MINUTES

Fig. 1. Schematic of the single-phase and the two-phase growth methods used in this work.

in order to improve

the

substrate for subsequent

Ga atomic fractions. The actual equations used were:

layers. Single crystal wafers of lnP were used to

supply Pin the

two-phase method,

84y + 0.0130xy

a

0 (A) = 5.8696 + 0.1894x

—

0.41

(I) 3. Measurement techniques Most of the measurements were made on complete laser structures, consisting of an n-type InP layer, a quaternary cavity layer, a p-type layer, and a quaternary cap layer whose purpose was to improve the ohmic contact. The composition of the solid was determined by measuring its lattice constant using X-ray diffractometry, and its bandgap using photoluminescence. In some cases this necessitated the removal of the quaternary cap layer in order to obtain optical excitation of the active layer. For the most part, both quaternary layers could be measured in a single X-ray scan without removing the cap. In a few runs, the active layer bandgap was estimated from the lasing wavelength. Using these two numbers, the bandgap and the

Eg (eV) = 1.35 (0.758

—

x

+ 1 .4y

—

0.33xy

0.28x)y(l —y)

(0.101 + 0.109v)x(l —x)

(2)

where x refers to the arsenic atom fraction, andy to the gallium atom fraction. The bandgap equation is identical to that in Moon et al. [4] , and the lattice constant equation uses values given by Nahory et al. [5]. Recent measurements suggest a further slight modification of the a 0 parameters [61. Given a0 and Eg, these equations yield x andy as a result of an iteration procedure programmed for an HP-41C calculator. Before entering eq. (1), a0 values were corrected for strain (Olsen and Smith [71).

4. Compositions

lattice constant, the solid composition was obtained

by inverting the equations of Moon et a!. [4] giving the bandgap the lattice constant from the As and the

As our main interest is to relate the melt compositions to their corresponding solid compositions, it

I. Ladany, F.Z. Hawrylo

/ Single- and two-phase LPE growth methods for InGaAsP/InP

71

ATOM FRACTION Ga IN SOLID 0.2

0.3

3Si~m

XE

9~I.

0.6

XE

0 U,

9 “5

z 0

p

I-

I

4.9k

-j w 5.01— U)

~

°I.3~m 0 4 - 0.5 ~ ~ ~ 0 4

~

I

I

0.7 0.8 0.9 ATOM

/. Ga IN MELT

Fig. 2. Composition plot for two-phasegrowth at 635°C.Results for five runs are shown.

ATOM FRACTION Ga IN SOLID 0.2 ~

50

0.3 I

I

0

XE9” I.35~m

0 0

0

0 0 0

XE9 I.30~rn 06

‘~ 0

XE9~I.25FLm

Z

XE9” I.2O~um

U) 4 z 0

L~J 0 0 U)

4

-

0.5

0

4

~

U.

0

~4O-

0

4

4 0

.7

-

4=0

0

3.5

p 0

0

-

I

I

0.5 1.0 ATOM % Ga IN MELT

Fig. 3. Composition plot for two-phase growth at 631°C.

0.4

ATOM

FRACTION Ga

IN

SOLID

72 02

0.3 I

04 I

3O~rn

//

-

07

XE

-j U, 0

z

/

-oe~ z 0

-J U.

I-

.

0

4

5.1z —

U)

<

U.

50-

.

4.8

-

4.7

0

0

4.90

-~-=-0,25°/=

.

~

a

4

I I I 0.7 0.8 09 1.0 1.1 ATOM “4 Ga IN MELT

Fig. 4. Composition plot for one layer growth by the single-phase method; growth temperature 635°C. ATOM

FRACTION Go

0.2

0.3 I

IN

SOLID

0.4

/

I

a” XE

9 “I 4O~m

XE”I,35~m

07 .

-J 0

0

U)

5,5

0

-

X~ I 30~m (/) 4

0

z -06 0 F—

5.0-

/

0

-

=5

0

a 4,5

/

-

-0.25%

-

I

I

I

I

I

I

I

0.7 0.8 0.9 1.0 II 1.2 13 ATOM % Ga IN MELT

0

0

0)

I 14

Fig. 5. Composition plot for single-phase growth at 635°C.

2

05

/ Single- and

I. Ladany, F.Z. Hawrylo

two-phase LPE growth methods for InGaAsP/InP

is convenient to plot both on a single graph. For the two-phase method, as the phosphorus concentration is maintained automatically, is proves unnecessary

73

runs using same melt compositions, we obtain a spread of 0.04% in a0 and of 1 .07 in Eg. The analysis of complete laser structures grown by

to specify it for the melt,

the single-phase method is shown in fig. 5. Here the

In the case of single-phase runs, optimum phosphorus melt concentrations were determined in separate experiments (typically 0.21 at% for 1.3 pm) and were held constant for a series of runs using identical melt compositions. In any case, the results were not found to be sensitive to small changes in phosphorus concentration, as already reported by Feng et al. [8]. Fig. 2 illustrates the results obtained from five runs using the two-phase method, where all steps were repeated as closely as possible, using, however, new melts for each run. The solid compositions were tightly clustered as shown. The spread of a0 values was 0.12% and of Eg 0.77%. The next series, shown in fig. 3, was grown at 631°C,and the composition of the melts was varied in order to obtain growths with different bandgap values. It is to be noticed that the solid compositions show a random scatter around the desired zeromismatch line. From the liquid composition points it

clustering of the solid compositions is somewhat different. In general, it is observed that the spread in bandgap becomes larger, compared to the previous results, whereas the spread in lattice constants is again small. Thus, for the lower group of five runs using same melt compositions, the range of a0 is zero, whereas the Eg spread is 3.3%. One might be tempted to interpret these results as an example of cornpositional latching, were it not for the upper series in the figure, which shows a similar distribution, however not latched to the zero mismatch line. In this case, lumping together the melts having approximately 5 at% As and 1 at% Ga we obtain a range of 0.11% in a0, and 5.8% in Eg. The results of this section are summarized in table 1.

is possible to deduce an approximate melt composition for lattice matched growth of materials with

method is shown in fig. 6. The layers comprising the

various bandgaps. Four melts having the same cornpositions are also shown, for which case the range is 0.04% in a0, and 1 .7% fri Eg. Next we examine similar runs for the single-phase method. In fig. 4 we see the results of growths at 63 5°C, where, however, only a single quaternary layer was grown (i.e., not a complete laser structure). The results are essentially equivalent to the two phase method. For example, considering the cluster of six

5. Material uniformity An angle lapped wafer grown by the two-phase DH laser structure are readily identified. The smoothness of the layers and interfaces is considered desirable for laser material. The magnification in this photomicrograph is smaller in the direction along the interfaces, so that one obtains a very sensitive examination of the structure over distances comparable to those used in laser devices. An excellent way of assessing the uniformity in composition of such material is to analyze it by Secondary Ion Mass Spectrometry (SIMS). A SIMS depth profile is shown in fig. 7 for the same sample as in fig. 6. From the composition indicated at the top

T bI 1 Range of a0

of the two quaternary layers, we can estimate the and E

obtained from repeated growths ~a0

~

(%)

(%)

(“C)

0 O 11

3.3 5.8

635 635

0.12 O 04

0.77 1 7

635 631

-

Temperature

-

Single-phase

Two-phase

Smgle-phase, one layer only 0.04 1.07 635 ________________________________________________________

minimum detectable change in composition to be

0.01 in atom fraction for Ga or P. Thus, we condude that the material analyzed in this scan is of constant composition within these limits. The quaternary material at the left end of the figure is a cap ,

layer, whose thickness is 0.4 pm. Except for the rise at the left edge, which we attribute to a surface effect .

the method is not reliable, we see no evidence of compositional grading over the indicated and where

thickness.

74

1. I.adani’, 1~.Z.Hawrvlo

/ Single- and two-phase LPE growth

methods for InGaAsP/InP

I]2pm

_ 1OO~m Fig. 6. 10 angle lap of complete laser structure grown by two-phase m~thod.

In

81Go~gAs41P~g

~ los

—

Fig. 8 shows a SIMS depth profile for a laser struc-

In75Ga25As54P45

Ga

_.....~..__.!!2-.___——.————-.

ture grown by the single-phase method. In this case

—O24~m

-

the cavity, on the right side of the picture, is 900 A in width. The variation in the Ga signal is attributed to irregularities in the layer flatness and the great sensitivity of SIMS to the element Ga. Uniformity of

—

O.4~m

E~

quaternary composition is equal to that in the previous case.

(1)

I-

~

A

As

102

10

Conclusions

Ga

I0~

0

200

400

600

I

800

I

1000

SPUTTER TIME (seconds) Fig. 7. Secondary Ion Mass Spectrometry (SIMS) depth profile of sample shown in fig. 6.

Two popular techniques for the growth of laser and LED material in the InGaAsP system have been compared. Although an attempt was made to eliminate systematic errors and to maintain various processes as nearly alike as possible, it cannot be known whether this effort has been totally successful. The following conclusions have been reached. The reproducibility of lattice constants is essentially the same, whereas the bandgap reproducibility

I. Ladany, F.Z. Hawrylo ~5 ~

/ Single- and two-phase

LPE growth methods for InGaAsP/InP

75

Ga

—f~-

—~

I0~-

__

P

C

o

~

~ I0~I—

p

z ~ 0 0

0

2

-

As (0

I I I I 0________________________ 200 400 600 SPUTTER BOO 1000 1200 TIME Iseconds)

Fig.

8. SIMS depth profile of laser structure grown by

is better for the two-phase method. Both methods are capable of producing a layer thickness of at least 0.4 pm without compositional grading. In repeated runs, using nominally identical melts and processes, a given lattice constant was reproduced within about 0.1%, whereas a desired bandgap value was reproduced within 2% by the two-phase method, and within 6% by the single-phase method.

1400

_

single-phase method.

References [1] J.J. Hsieh, Appl. Phys. Letters 28(1976) 283. [2] T. Yamamoto, K. Sakai and S. Akiba, Japan, J. Appi. Phys. 16 (1977) 1699. [31 M.A. Poilack, R.E.Letters Nahory, J.C. DeWinter man, Appl. Phys. 33(1978) 314. and A.A. Ball[4] R.L. Moon, G.A. Antypas and L.W. James, J. Electron.

Mater. 3 (1974) 635. [5] R.E.

Acknowledgements We gratefully acknowledge the contribution of R.T. Smith in X-ray diffraction, and C.W. Magee in SIMS. A large part of the LPE growths were carried out by T.R. Furman; photoluminescence measurements are due to N. DiGiuseppe, and the InP polishing was done by A.J. Tocci.

I ~0O

_____________________

Nahory, M.A. Poilack, W.D. Johnston, Jr. and R.L. Barns, Appl. Phys. Letters 33 (1978) 659. [6] G.H. Olsen, private communication. [7] G.H. Olsen and R.T. Smith, Phys. Status Solidi (a) (31) (1975) 739. [8] M. Feng, L.W. Cook, M.M. Tashima and G.E. Stifiman, J. Electron. Mater. 9

(1980) 241.