InP structures for laser application on 2 inch wafers by LP-MOVPE at 20 mbar

Journal of Crystal Growth 93 (1988) 285-291 North-Holland, Amsterdam

285

CONTROLLED UNIFORM GROWTH OF G a l n A s P / I n P STRUCTURES FOR LASER APPLICATION ON 2 INCH WAFERS BY LP-MOVPE AT 20 mbar R. MEYER, D, GRI[ITZMACHER, H. JORGENSEN and P. BALK Institute of Semiconductor Electronics, Technical Unioersity Aachen, D-MOOAachen, Ref. Rep. of Germattv

This ~aper reports on the large area growth of G a l n A s P / l n P structures for 1.3 and 1.55 t,tm applications in a LP-MOVPE system at 20 ml'ar, at conditions comparable to those used for GalnAs and InP. i'he effects of the partial pressures and temperatures on growth ! ae and composition were studied. The composition along 2 inch wafers can be adjusted within 2%, the peak emission wavelength of 1,3 and 1.55/~m material within 30 rim. The FWHM of the phomluminescence peak at 300 K amounts to 48 nm and at 4 K to 7 nm. The F W H M of X-ray curves is below 23 arc sec. Thickness uniformity is better than 2%. Mobilities of 45,40b cm2/V.s (1.55/~m GaInAsP) at n =1 xl015 cm -3 were obtained. Intentional n-type doping of GalnAsP in the range 1016 < n < 1019 cm -3 was realized using H 2S. With thin intentionally doped InP layers in G a I n A s P / l n P structures, a two-dimensional electron gas was generated at the heterointerface with a constant mobility P77 = 58.300 cm2/V •s in the GalnAsP between 50 and 4 K.

1. Introduction For production of optical devices, especially laser structures emitting in the 1.3 and 1.55 #m range, controlled growth of GalnAsP of well-defined composition with good homogeneity in thickness and composition across a 2 inch wafer is required [1-4]. The fabrication of layer structures for lasers has been reported for a number of deposition systems. However, the MOCVD technology is attractive for large area deposition and for the preparation of structures with extremely abrupt interfaces like quantum wells (QW). Results of film uniformity of GalnAs and InP layers and on the control in quantum well fabrication [5-7] suggest that low pressure growth would also be advantageous for the deposition of Galr~,sP. Since the standard LP-MOCVD system used in this study had been well characterized for the deposition of GalnAs and InP [5,6] this facilitated the ex_tension to quaternary growth°

2. Experimental Grox.th of GalnAsP was performed at 20 mbar at a flow rate of 1.4 m / s in a LP-MOVPE system with horizontal reactor taking 2 inch wafers [5].

Trimethylgallium (TMG), t,'imethylindium (TMIn), AsH3 (100%) and PH3 (100%) were used as group llI and group V sources in H 2 carrier gas. As the n-type uopant, H2S (1000 ppm in H2) was chosen. A special arrangement of electronic mass flow controllers and pressmc controllers allowed adjustment of the H2S pressure over more than four orders of magnitude. All growth experiments started by first depositing a 0.4 ~m InP buffer layer (n < 5 × 1014 cm --~) at 913 K [5] on top of a 2 inch semi-insulating InP wafer. Using a five-way vent-run switching manifold with minimized dead space [8] extremely short switching times between growth of individual layers can be realized. The hetero and quantum well structures were grown at 913 K with growth rates of 1.2 t~m/h for InP and 1.8 g m / h for GaInAsP. Thickness measurements of the GaInAsP layers were carried out on steps generated by lithography and selective etching, using a surface roughness profiler with resolution better than 3 nm, The film compc ,on was obtained from combined doublecryst,/ X-ray diffraction (DCXD) and room temperature photolurninescence (PL) measarements. Carrier concentrations and mobilities were determined by the standard Van der Pauw technique. Further characterization of more sophisticated structures like quantum ~ells (QWs) or

0022-0248/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

R. Meyer et a L / Controlled uniform growth of GalnAsP / lnP structures

286

layered structures for HEMTs was performed by low temperature PL and low temperature Hall measurements. The detailed results of this study will be reported elsewhere [9].

ka

,

~O

3. Results and discussion Since several parameters (partial pressures, total pressure, deposition temperature, flow velocity) are affecting g r o w t h and c o m p o s i t i o n of quaternary materials, the deposition experiments for GaIm~sP (1.3 and 1.55/~m) in this study were carried out using our experience with the growth of GaInAs [6]. The deposition data for the latter c o m p o u n d were chosen as starting parameters. Adding PH 3 and controlling the growth rate by varying the TMGa and T M I n pressures, the composition of the G a l n A s P layers could be adjusted in the 1.3 and 1.55/~m emission wavelength ranges.

~

, pITMInL 0% IPal

0.33

-5

PPI% : 112Pa PAsH3 : 1.68Pa PTId(;o: 0.106pn Ptot :20 mbctr T :913K

138(~

1360

\° 13z*0t •

., . "0,32

, 0.33

N.

plTMInL O'.3t, [PQI

Fig. 2. Dependence of lattice mismatch and emission wavelength on TMI partial pressures (1.3 p.m region).

~1o i 0

l

20

2'5

l

P (ASH3 L

30

IPoI

-5 o -1(3

PPH 112.3PO pT;~r~0.336 PO pTMr.~0 107 PO Ptot 2f) mbQr

1/-00

/o

T

1350[ 8

1300I~"° ~'

p(AsH3) 2~o

2'.s

3'.o

{POT-"

Fig. 1. Dependenceof lattice mismatch and emission wavelength on AsH 3 partial pressures(1.3/~m region).

A substantial number of experiments were carried out to obtain an optimized set of data in the multi-dimensional field of growth parameters. The following examples demonstrate the influence of some of these parameters. Figs. 1 and 2 represent the dependence of lattice mismatch and emission wavelength of 1.3 # m G a I n A s P on the partial pressures of AsH 3 and TMIn. In both cases the lattice constant increases with increasing partial pressure of these compounds due to the fact that As and In have a larger atomic radius than P and Ga, respectively. Variation of the T M G a and PH 3 pressures shows the opposite behavior. The effect on the emission wavelength (~), which is inversely proportional to the band gap energy, is different. With increasing As concentration, ~. is increasing; with increasing In concentration ~ is decreasing. Fig. 3 shows the effect of the growth temperature. With decreasing temperature the lattice constant increases and so does ~.. This effect is most likely caused by enhanced cracking of PH 3 at higher temperatures.

R. Meyer et al. / Controlled uniform growth of GalnAsP / hiP structures 940

i

920

i

I

g~

%G

gAO

I

I

1

840

820 I

~

~" ~o

~lo 30

i 20

1.10

- -

~.TIK] %0

920

900

1

I

I

I~ 1 7 0 0

1.20

1,15 880

840

860

I

' "

o I

I

0

/

1~,- o,218 Pa ~.: 0,106 ~a

o/o/

Rot: " mbo,r 0,o,3600 mtlrnln

1500 I

1,05

I

1,10

1,15 --lIT

I

1,20 [xlO3K~] -

Fig. 3. Influence of the reciprocal deposition temperature on lattice mismatch and emission wavelength (1.55/tm region).

287

Since the decomposition temperature of AsH 3 is much lower than that of PH 3, the increase in temperature has a less pronounced effect on the cracking of AsH 3, which is dissociated to a considerable extend already at the lower temperatures. This behavior appears to be in agreement with the results reported in ref. [10]. The flow velocity (1.4 m / s ) had been chosen to obtain a uniform growth rate along a 2 inch wafer. In the flow direction, the variations could be reduced to below 2% (fig. 4), comparable to that observed perpendicular to the flow direction. This elucidates the advantage of the low pressure system in comparison to atmospheric pressure growth. Using the latter method, excellent material properties may be obtained and the fabrication of devices using this material has been demonstrated. However, in this case the departure from uniformity is of the order of 10%. Also for a low pressure system fluctuations in the order of 10% have been reported [10]; this relatively high value is probably caused by less than optimum adjustment of the gas flow velocity. Particularly for optoelectronic devices, achievement of lateral and vertical uniformity of the quaternary composition is very important. The combined measurement of lattice constant and emission wavelength determines the optimum composition for a certain wavelength leading to

in~J'leS

~d

InP

d I0 0

5~ o

oOOO

ooo ° o

o

a

o00o

~,

o°

o o

o o

~ - 5 ~ -

t"_

i_

A - t'~

I

lO~/@

t. . . . . . . . ,

c o~

• -,. :-,,,->:,,,-/,//Gq

/

,

, ....., -,,/, , / / /

-300-200-100 0 Aw

Fig. 4. Homogeneity of OalnAsP film thickness parallel to gas flow direction.

,100

(orcsl

Fig. 5. Typical X-ray diffraction spectrum.

288

R. Meyer et aL / Controlled uniform growth of GalnAsP / lnP structures

4'K

?,5nm Ltl

1400

22

1360

\

1240

1320 1280 WAVELENGTH (nm)

data from MBE (17 meV at 8 K [3]) and LPMOCVD (28 meV at 20 K [12]) published so far. The distribution of points measured for the emission wavelength along a 2 inch GalnAsP layer in the direction of gas flow has been plotted in fig. 7. It indicates that the uniformity of composition is better than 2%. This value should be compared to published data on the spread in this quantity obtained for atmospheric- and low pressure growth, which amounts to more than 20% in ref [14] and 6% in ref. [10], respectively.

Fig. 6. Photoluminescence spectra of ].3/am GalnAsP layer.

lattice matched, unstrained conditions [11]. The FWHM of the X-ray peaks portrays the vertical homogeneity of composition in the investigated volume. The FWHM of 23 arc sec (fig. 5) is comparable to the best data from LPE [2]. Photoluminescence curves taken at 300 and 77 K demonstrate the high optical quality of the material (fig. 6). The FWHM of 54 nm (38 meV) for GalnAsP (h = 1.3 .um) and 65 nm (35 meV) for GaInAsP (X = 1.55 ~tm) at room temperature are comparable to the best published data from other LP-MOCVD experiments (50 meV (1.55 ~m) [10], 55 meV (!.3 ,am) [12] on Si substrates). An estimation using FWHM = 1.8kT leads to 46.5 meV at 300 K, which again provides evidence of the good optical properties of our material. A FWHM of 5.9 meV at 4 K (;k = 1.3 #m) is among the best

t

x

f:

300 K

i

77 K

10 ~

0

I

I

I

I

1

2

3

/,

S

10s

Eka ],

....

2 in,he5

___,__27K o

A~

o

),

1o %

5% x o

a

o

.

x

,,

x

x

o

30O K ,,,,

--'x~

J

i

- 1o% I

L

Susceptor

I

103 1

Fig. 7. Homogeneity of emission wavelength of GalnAsP parallel to gas flow direction.

2

3

~,

S

[¢m]

Fig. 8. Uniformity of electrical properties of GalnAsP.

289

R. Meyer et a L / Controlled uniform growth of GalnAsP,/ InP structures o,,, LPE~ lit.dQta

105 >

= LP-MOVPE; o • this study

lit d~,tn

77K o

o

=~&.1

10t' O,o

.~

m_~,%...~", i~ - i' - ' -i -

300 K

•

k g

1300

lt~O0

[nmi~

1S00

Fig. 9. Dependenceof Hall mobility on wavelength, i.e. on composition of GalnAsP layers. Values for background doping and electron mobility are shown in fig. 8. In all cases the dominant residual dopant is a donor, giving rise to electron concentrations at a level between 5 × 1014 and 2 x 1015 cm -3, depending on the quarternary composition. The variation along a 2 inch wafer appears to be slight. Mobility values in this sample are also fairly uniform at 6000 cm2/V • s at room temperature and 25,000 cm2/V • s at 77 K.

Fig. 9 compares the mobilities measured on several unintentionally doped 1.3 and 1.55 ~tm GalnAsP samples. Maximum mobilities in 1.55 ~tm material are 45,500 and 7,500 cm2/V • s at 77 and 300 K, respectively. Highest mobilities at these temperatures in 1.3 #m material are 21,900 and 5000 cm2/V • s. These values exceed those reported from MBE grown samples [13]. For device fabrication the controlled introduction of donors during growth is important. We used sulfur from an H2 S source as the n-type dopant. Adjusting the H2S pressure in the reactor between 10-3 and 3 Pa electron concentrations in 1.55/~m GalnAsP between 2 × 1016 and 10 ]9 cm -3 were obtained. The relationship between H2S pressure and electron density is nearly linear up to the 1017 cm -3 range, sublinear at higher densities (fig. 10). The distribution coefficient for 1,3 #m material appears to be higher than in 1.55 ~tm material. Also indicated are data on the dopant incorporation into InP and GalnAs. The total spread between the four curves is approximately one order of magnitude. This makes H2S an attractive doping gas for all compositions of the

1019

/x S

1018

1017 //~ / ~ ' / 1016

/

o GolnAs ° GalnAsP(13Mm} ® 13oli,AsP (I.55Mm)

X

60 In As

p(H~S)

10"3 10"2 10"1 100 10÷1 [Pal Fig. 10. Effectof H2S pressureon S incorporation into lnt', GalnAs and GalnAsP.

R. Meyer et al. / Controlled uniform growth of GalnAsP / InP structures

290

Ei_1

105 77 K

°

10~

"~"-~~_____~

300 K

"~I

|

I

!

I

¢ I ,ill

S

I

i,.l|.

1015

•

I

I

|J|llt

10~

,

.

l

i,.lH

10I'/

I

,

l

,

.....l

1018

t

n

, i,l**,_

1019

km-al

Fig. 11. Hall mobility versus free carder concentration of doped and undoped layers.

G a - l n - A s - P material system investigated and a useful alternative to Sill4 [14]. Fig. 11 shows the change of electron mobility at 77 and 300 K with carrier concentration. The curve shows the typical behavior expected for this dependence. Our values appear to be compatible with the literature data on LPE films [15-17] for the concentration range from 1014 to 1016 cm-3. For high quality devices abrupt transitions between the individual layers in heterostructures have to be achieved. A first indication of the abruptness

x

105~- ' - r - ' '

x x

x

x

xx

' ' I

)(

x

and flat.css of the interface is the formation of a two-dimensional electron gas (2DEG) near the interface. To this end Hall measurements were carded out at ,different temperatures from 6 to 300 K on 1.3 ~m, and 1.55 # m G a l n A s P / I n P structures (fig. 12). For comparison, data from G a I n A s / I n P structures are also indicated. Below 50 K the mobilities remain fairly constant at 58,300 (1.55 # m ) and 31,000 c m 2 / V • s (1.3/tin). To obtain a sufficiently high sheet carder concentration first a highly n-type doped (lO 18 cm --~) InP source

x

~¢x

x

x ~¢ X X x x

GolnAs GalnAsP (1.55~m) GoInAsP (I 3 ~m)

0 ®

x x x

0 (30

000

x 0

O

O0

O

x 0 0

>

x

OOO O@ O@QOQ@Q OQ @ O@OOO@eOO@Q

E

x

O

00

x

o

x 0

0

x

0

GalnAs [P)

•

I lain

0 Q

104 2 DEG. . . . . . . . . . . . . . .

x 0

6

[nP- Spacer

300 /~,

InP n=1.10""; cm )

0

x

@ 0

O

0

100

@00@~

InP-Buffer ,

.

I

l

I

I

i

i

i

,

~

i

tO0

I0

Temperoture [K] Fig. 12. Comparison of Hall mobilities in GalnAs and G a l n A s P / I n P (1.3 and 1.55 ~m) heterostructures,

R. Meyer et aL / Controlled uniform growth of GalnAsP / InP structures

1

350

E

300

I

I

2K

I

GQIn A s P / I n P

LIJ

<3

I

4oo A 250

InP 400~

-~

~-.-~.-~..z_~c_~.~.~ 40A ~

13talnAsP (1.5 ~Jm)

"//////////~1ooo~

100

~oI~A~P

.

_

_

.

_

_

0 I

0

100

We are indebted to AIXTRON for technical support regarding the equipment used.

References

InP-buffer

SO

Acknowledgement

I00~

~oo L

low defect densities, high carrier mobilities and high optical quality. Using this technique abrupt interfaces with low defect densities and well-defined quantum wells with properly confined heterojunction are obtained.

20A

400 A 200

291

I

,

200

l

300 - -

LZ{~)~

Fig. 13. Energy shift due to quantum size effect versus well width.

layer was grown on an undoped InP layer, followed by a 30 nm undoped (10 is cm -3) InP spacer. These mobility data qualified the material for more intensive studies on 2DEG effects using Shubnikov-De Haas measurements. Results of this study are reported in ref. [9]. QW structures with well thicknesses of 10 and 20 nm revealed excellent PL peaks [9,18]. The energy shift obtained for different well widths is plotted in fig. 13. The behavior is comparable to data from GaAs/A1GaAs [19] and GalnAs/InP [6] MQW structures. The large energy shift and the narrow linewidth demonstrate the abruptness of the interfaces and the good control of film thickness even for very thin layers.

Conclusions

Our study shows that LP-MOVPE is well-suited for controlled growth of 1.3 and 1.55 p.m GaInAsP layers of excellent homogeneity in thickness, composition and backgroung doping. The films exhibit

[1] G.PL Olsen and T.J. Zamerowski, IEEE J. Quantum Electron. QE-17 (1981) 128. [2] M. Razeghi, J.P. Duchemin and J.C. Portal, Appl. Phys. Letters 46 (1985) 46. [3] P.J.A Thijs, W. Nijmann and R. Metselaar, J. Crystal Growth 74 (1986) 625. [4] C. Bocchi, C. Ferrari, P. Franzosi, G. Fornuto, S. Pellegrino and F. Taiariol, J. Electron. Mater, 16 (1987) 245. [5] D. Griitzmacher, D. Schmitz, H. Jnrgensen, M. Heyen and P. Balk, to be published. [6] D. Griitzmacher, K. Wolter, M. Zachau, H. Jtirgensen, H Kurz and P. Balk, in: Proc. 14th Intern. Symp. on GaAs and Related Compounds, Crete, 1987, Inst. Phys. Conf. Ser. 91, Eds. A Christou and H.S. Rupprecht (Inst. Phys., London-Bristol, 1988~. t7] P.J.A. Thijs, J.M. Lagemaat and R, Wohjer, to be published [8] 1, ~Urgensen, German Patent DE 3,537,544 C1 (1985). [91 D. Griitzmacher, R. Meyer, M. Zachau, P. Helgesen, A. Zrenner, K. Wolter, H. Jiirgensen, F, Koch and P. Balk, J. Crystal Growth 93 (1988) 382. [10] H. Nakao, K. Sato, M. Oishi, Y. Haya and Y. lmamura, J. Appl. Phys. 63 (1988) 1722. [111 R.E. Nahory, M.A. Pol]ack, W.P. Johnston, Jr. and R.L. Barus, Appl. Phys. Letters 33 (1978) 659. [12] M. Razeghi, F. Omnes, M. Defour and Ph. Maurel, Appl. Phys. Letters 52 (1988) 109. [13] D. Huet and M. Lambert, J. Electron. Mater. 15 (1986) 37. [14] A. Mircea, R. Mellet, B. Rose, D. Robein, H. Thibierge, G. Leroux, P. Daste, S. Godefroy, P. Ossart and A.M. Pougnet, J. Electron. Mater. 15 (1986) 205. [15] A. Schlachetzki, AE0 40 (1986) 302. [16] E. Kuphal and A. Pi3cker, J. Crystal Growth 58 (1982) 133. [17] J.L Benchimol, M. Quiliec and S. Slempkes, J. Crystal Growth 64 (1983) 96. [18] M.B. Panish, H. Temkin, R.A. Harem and S.N.G. Chu, Appl. Phys. Letters 49 (1986) 164. [19] D. Schmitz, G. Strauch, J. Knauf, H. Jiir~ensen, M, Heyen and K. Woher, J. Crysv,d Growth 93 (19h8) 312.