InP DH laser wafers under a PH3 ambient

Journal of Crystal Growth 51(1981) 502—508 © North-Holland Publishing Company

HIGH QUALITY LPE GROWTH OF InGaAsP/InP DH LASER WAFERS UNDER A PH

3 AMBIENT

Shin-ichi TAKAHASHI and Haruo NAGAI Mu sashmo Electrical Co,n,nunication Laboratory, Nippon Telegraph and Telephone Public Corporation, Musashino, Toks’o 180, Japan Received 7 July 1980; manuscript received in final form 19 August 1980

Liquid-phase-epitaxial growth of InGaAsP/lnP DH laser weiers under a PH3 partial pressure has been carried out. The addition of PH3 effectively prevents thermal degradation of the loP substrates. To investigate the PH3 influence on the lnGaAs/lnP DH laser wafer quality, the active layer photoluminescence and urn-temperature threshold current for laser diodes fabricated were measured for each wafer. It was found that the PL intensities for wafers grown under a PH3 ambient are relatively stronger than the intensities for wafers grown by the InP covering method. The results also show that laser diodes with lower threshold current are obtained reproducibly from wafers grown under a PH3 ambient.

1. Introduction

One is melt etching of the InP substrate surface by In solution prior to growth [81. However, this method hardly gives reproducibily complete flatness of the substrate surface and its application is rather restricted. The technique involving covering the substrate with another InP wafer [9,10] enables the introduction of a phosphorus vapor pressure into the ambient just above the substrate. This method is effective at relatively lower growth temperatures, such as 600°C, but its effectiveness reduces with increasing growth temperature because of partial decomposition of the substrate. This work reports results of an investigation on a method which adds PH3 partial pressure in the hydrogen ambient to prevent thermal degradation of the InP substrates for the LPE growth of the lnGaAsP/lnP DH laser wafers. The use of PH3 for fabrication of actual devices has been reported by Clawson et al., on lnGaAsP/lnP photodiodes and InP photodiodes and InP MISFETs [11—13]. In the current paper the experimental characterization is described of the Pt-I3 influence on the lnGaAsP/lnP DH laser wafers on photoluminescence (PL) intensity measurements and laser diode performances by roomtemperature pulsed threshold currents. PL intensity values for wafers grown by utilizing a PH3 partial pressure in the hydrogen ambient were dramatically greater than that of wafers grown by the

Recent studies on semiconductor lasers as sources for optical fiber communication systems have been made to assure that it will be available in the near future. Since it has found that fused silica fibers have lower transmission losses at wavelengths near 1.3 pm, compared to the 0.8 pm region [1], the development of lnGaAsP/InP double-heterostructure (DH) lasers has progressed rapidly [2,3]. Development work at present is concentrated towards the reduction of the threshold current density, to overcome its considerable temperature sensitivity, and controlling the emission modes for application to single-mode fiber transmission systems. However, it is rather more difficult to fabricate laser wafers by embedding and/or selective epitaxial growth (including buried-hetero-epitaxial growth) in the lnGaAsP/InP crystal systems than in the GaAlAs/ GaAs systems [4—6]. This problem is attributed to thermal degradation of the InP substrate surface during liquid-phase-epitaxial (LPE) growth because of phosphorus evaporation [7]. This has impeded good reproducibility and uniformity not only for buriedhetero-epitaxial growth but also for conventional DH epitaxial growth. Various techniques have been employed to suppress such thermal degradation of the substrates, 502

S. Takahashi, H Nagai / High quality LPE growth of 1NGaAsP/InP DH laser wafers

covering InP method. DH laser diodes with low threshold currents were obtained with high reproducibility and uniformity. It was found to be quite suitable for reproducible LPE growth of the InGaAsP/InP DH

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Fig. 1

shows a diagram of the experimental

system. 3/min, Pd-diffused H2 flowed at a constant rate of monitored with a mass flow controller, 100 cm The phosphine source was 500 ppm PH3 diluted in H2 and introduced through another mass flow controller at various flow rates. In order to determine the optimum PH3 concentration, heat treatments on the InP substrate were carried out for 2 h at 650°C.The Br—methanol n-type 3) (100)InPpolished, substrates were(Sn-doped, placed in “-‘1 X 1018 cm” the graphite illustrated fig. 2.a A tional sliding boat boat aswas preparedin with fewconvenmillimeter height spacing just above the substrate position for efficient diffusion of phosphorus to the substrate, When an InP cover wafer was used to prevent substrate degradation, it was introduced in this spacing. Each solution chamber was covered by a graphite cap to prevent contamination from the volatile components during LPE growth of the DH laser wafers. Fig. 3 shows the differential-phase-contrast micrographs of the InP substrate surface after heat treatments with various PH3 partial pressures. The InP ~OUThET BOAT PUSH ROD I

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substrate showed severe degradation in the absence of PH 3, as shown in fig. 3a. One can see many thermal etch pits and indium droplets on the roughened surface. It was found that the thermal degradation on the substrate surface decreases considerably as the PH3 concentration is increased. Fine thermal etch pits, as shown in fig. 3c, cause formation of stacking faults in with the epitaxial Their pit density decreases increasing layers. PH 3 pits concentration. At a 3/min,no etch and indium dropflow can rate be of seen, 60cm as shown in fig. 3d. The polished lets InP crystal surface shows no change before and after treatment. A typical as-grown surface morphology of a Ge-doped InP layer grown under optimum PH 3 concentration is shown in fig. 4a. The surface is extremely smooth and no growth defects can be seen. For comparison, the surface of an InP layer, grown on a thermally deteriorated InP substrate without introducing a PH3 partial pressure, is shown in fig. 4b. The nonuniform and wavy surface has a number of indium droplets remaining in the depressions due to incomplete wiping. The optimum amount of PH3 necessary to prevent thermal degradation was found to be 2 X i0”~atm at 650°C in the present experiments. This value is considerably higher than the

threshold data at 650°C reported by Clawson et al. This is probably due to the lack of PH3 decomposition and incomplete phosphorus diffusion to the substrate surface. In spite of such an ambient with high phosphorus concentrations, the composition of the InGaAsP grown layer was not noticeably affected.

3. DH laser wafer LPE growth

(500ppm)

Fig. 1. Diagram of experimental system for InGaAsP/lnP DH wafer LPE growth.

Conventional InGaAsP/InP DH laser wafers were grown using the above mentioned growth system. Fig.

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S shows a time—temperature profile used for DH laser wafer growth. The growth melts were maintained for 2 h at about 650°C to assure sufficient mixing of all the constituents. The PH

3 flow condition was also maintained in the above mentioned optimum value throughout the LPE process, including hydrogen purging in the quartz tube, melt saturation, cooling for growth and rapid cooling after growth. The PH3 flow during forced cooling after growth prevents the phosphorus decomposition from the grown surface. LPE growth was carried out by the supercooling technique at 0.8°C/mm cooling rate on the Sn-doped 3) (lOO)InP substrates. The first (n i x 1018 cm”’ 8 cm3) lnP layer and layer is a Ge-doped (n 2The X iD’ is about 5.0 pm thick, second lnGaAsP active

layer is always grown •at 640°C. intentionally undoped. The active layer composition was selected to provide a bandgap corresponding to the peak PL

wavelength of 1.27

±0.01 pm and to adjust the lattice-match wihtin 0.04% to the InP crystals. The supersaturation of the solution for the InGaAsP active layer growth was about 10°.The growth solotion influence on the lnGaAsP/lnP DH laser wafer quality was characterized elsewhere [14]. The active layer thicknesses for each wafer were determined precisely by using the scanning electron microscope solution of (SEM) Fe3” : 1with HNOthe sample ethced with a : 1 H20. The thickness was maintained at 0.20 ±30.05 pm throughout the whole experimental DH wafer growth. It is well

S Takahashi, H Nagai

/ High quality

LPE growth of INGaAsP/InP DH laser wafers

505

—~ Fig. 6. As-grown surface of a typical four-layer DH laser wafer grown under PH 3 ambient.

known that the threshold current density dependence on active layer thickness shows a minimum value at a thickness of around 0.2 pm for InGaAsP/InP DH lasers emitting at 1.3 pm [15,16]. 3) third is a 1.5 Cd-doped (p ‘~thick. 3 X 10t7 cm’of InPThe layer and layer is about to 2.5 pm Control the acceptor concentration in the p-type InP cladding layer is important to reduce the threshold current density for the DH lasers [17]. Because of easy control of the doping levels and lower diffusion coeffiFig. 4. As-grown surface morphology for InP layer: (a) Grown under PH 3/min flow rate. (b) Grown after heat3 treatment ambient at in apure 60 cm H 2 with no preventing InP substrate degradation.

.

cient in InP layers, Cd is a more suitable acceptor dopant compared to Zn. 18

.

3) lnGaAsP of the same composition The final layercap is alayer 1.0 pm Ge-doped (n 2 X 10 as ‘-~

cm active layer, allowing restricting current flow by the O.S’C/rnin

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Zn-diffusion. Fig. 6 shows a four layer InGaAsP/InP DH laser wafer after LPE growth by introducing PH 3 partial pressure. The surface has a mirrorlike appearance and shows uniform growth all over the wafer. An SEM photograph of a cleaved and etched cross section of a grown wafer is shown in fig. 7.

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2 3 TIME (hours)

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Fig. 5. Time—temperature profile for LPE growth of typical four-layer InGaAsP/InP DH laser wafer.

4. Evaluation To investigate the effect of PH3 introduction on InGaAsP/InP DH laser wafer growth, a systematic

S. Takahas/ii, H Nagai / lug/i quality LPE growth of INGaAsP/InP DH laser wafers

506

lnGaAsP CAP LAYER

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F p-’InPCLADDJNG LAYER

n-InP CLADDING LAYER 1 pm I’ig. 7. SEM

photograph of the cleaved and etched cross section of the InGaAsP/InP DH laser waler.

study was carried out on the correlation between PL intensities for the InGaAsP active layer and the threshold currents of the DH lasers with planar-stripestructure at room-temperature. For each DH wafer,

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Fig. 8. Typical photoluminescence emission spectra from the

l’,g. 9. Threshold current ‘th as a lunct,on of photolumt’ nescence intensity for an active layer. Each data point

p-InP cladding layer and InGaAsP active layer of the DH wafer under photopumping with a 6328 A beam. The fourth InGaAsP cap layer was removed by selective etching,

represents the minimum ‘th for laser diodes fabricated from each wafer. The filled circle represents a wafer which is used to compare the PL intensity of an active layer for eaeh wafer.

S. Takahashi, H. Nagai / High quality LPE growth of INGaAs.P/InP DH laser wafers

507

room-temperature PL measurements, under excita-

introducing PH

tion from a He—Ne laser beam (6328 A), were performed with a constant excitation density on the p-InP cladding layer after the n-InGaAsP cap layer was removed by selective etching in a 2 H2S04 : 1 HNO3 1 H20 solution, Fig. 8 shows a typical PL emission spectrum from a DH wafer. The PL intensity from the p-InP cladding layer is very weak and is hardly observed. The PL emission at 1 .27 pm from the InGaAsP active layer

circles. The results for wafers grown by covering lnP method are shown as open triangles. The lowest value of the threshold current in the present experiments was 88 mA for the PH3 addition sample. This value is lower than the best values reported so far, for the stripe-geometry InGaAsP/InP DH lasers, except for mode controlled lasers as well as buried-heterostructure laser. Most laser diodes with ‘th ~ 200 mA pulsed threshold current were confirmed to be able to

is obtained by the following two processes,

operate continuously at room-temperature.

(I) The pumping beam is absorbed into the p-InP layer and generates carriers which diffuse into the lnGaAsP active layer and subsequently recombine radiatively. The PL emitted from the active layer passes through the p-InP window layer. (2) The InGaAsP active layer is -probably photopumped by the emission at ~9300 A from the

3 are indicated as open and filled

From the figure, it is seen that there is a strong ‘th dependence on the PL intensity from the active

layer. Threshold current reduces with increasing PL intensity. Data obtained from wafers grown by the covering InP method have large scattering and low reproducibility. The wafers grown by using PH3 have larger PL intensity and have uniformly lower thres-

pumped p-InP layer.

hold current. This indicates that the LPE growth

Process (1) is probably dominant. In both cases, because the InGaAsP—InP interface is not exposed

under PH3 partial pressure increases the PL intensity from the InGaAsP active layer and decreases the

to

threshold current density for DH lasers.

the air during the measurements, it can be

evaluated with high reliability for a preprocessing check of the quality of the InGaAsP/InP DH laser wafers. This evaluation method is applicable to the InGaAsP/InP DH laser wafer with an active layer of

After the p-lnP cladding layer was etched off with a 3 HC1 : 1 H20 solution, PL emission from the InGaAsP active layer surface was also observed under direct excitation by a He—Ne laser beam. The PL

various InGaAsP composition lattice-matched to the

intensity

InP substrate, For measurements of the room-temperature pulsed threshold current ‘th (pulse width of 500 ns, 1 kHz), Zn-diffused planar-stripe lasers (cavity length L = 200 pm) were fabricated by conventinal processing methods [18]. p-Type stripe regions 12pm wide were formed by Zn-diffusion through the n-InGaAsP cap layer into the p-InP cladding layer. The threshold

through the p-InP window layer are about three times higher than that from the directly excited InGaAsP active layer surface. Increasing PL intensity of the DH wafer correspondingly influences the improvements in lasing characteristics, described above, The reasons are not well understood at present. However, the increasing PL intensity from wafers grown under a PH3 partial

current measurements for a number of laser diodes

pressure is believed to be related to the incremental

with differrent stripe widths from each wafer resulted in an optimum width of about 12 pm for fabricating the planar-stripe lnGaAsP/InP DH lasers having minimum threshold current. The relation between PL intensities and threshold currents at room-temperature are summarized in fig. 9. The data points in the figure represent the lowest observed values of threshold currents from the laser diodes from each wafer. The PL intensities of the data points are normalized to the value obtained for one relatively good DH wafer (‘th = 115 mA, mdicated as a filled circle. Results for wafers grown by

emission efficiency of the InGaAsP active layer. It may be due to a reduction in nonradiative recombination centers attributed to defects which originate on the damaged substrate surface and propagate through the epitaxial layers and/or generate in the epitaxial layers. It may also be due to reduction in interfacial recombination velocity in the InP/InGaAsP/ InP heterostructure grown under phosphorus vapor pressure. Further work is necessary to investigate the internal quantum efficiencies, interfacial recombination velocities, and minority carrier lifetimes in the InGaAsP/InP DH laser diodes.

values from the InGaAsP active layer

508

S. Takahashi, H Nagai /High quality LJ-’E growth of IIVGaA sF/loP DHlaser wafers

5. Conclusion The lnP substrate thermal degradation influences reproducibility and uniformity for an InGaAsP/lnP DH laser wafer grown by LPE growth. The authors have investigated the effect of introducing phosphorus vapor pressure by PH 3 into the hydrogen ambient during DH laser wafer growth by measuring the relation between PL intensities of the lnGaAsP active layer and threshold currents for the DH lasers.

It was found that the PL intensity of wafers grown by PH3 addition is stronger than that of wafers grown

by the covering InP method, Laser diodes with lower threshold current were obtained reproducibly from wafers grown under phosphorus vapor pressure. Although no evident reasons can be given here, it is

believed that the LPE growth under PH3 ambient allows the fabrication of high quality lnGaAsP/lnP DH lasers. This technique should be applicalbe to the epitaxial growth, which is performed on the wafer

surface treated by various fabrication processes as well as mesa and/or channel etching.

References

Ill

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N. Chinone and K. Aiki, 37th Annual Device Research Conf., Post Deadline, June 1979. 161 N. Nishi,Appi. M. Yano, Nishitani, Y, Akita sagawa, Phys. Y. Letters 35 (1979) 232.and M. Taku171 K. Pak, T. Nishinaga and S. Uchiyama, Japan. J. Appi. Phys. 14 (1975) 1613. [8] V. Wrick, G.J. Scilla and LI’. Eastman, Electron. Let12 (1976) 394. 191 tars A. Doi, N. Chinone, K. Aiki and R. Ito, AppI. Phys. Letters 34 (1979) 1859.

[101 PD. Wright, Y.G. Chai and G.A. Antypas, IEEE Trans. Electron. Devices ED-26 (1979) 1220. [11] AR. Clawson, W.Y. Lum and G.E. McWilliams, J. Crystal Growth 46 (1979) 300. [12] A.R. Clawson, W.Y. Lum, G.E. McWilliams and 1-1.1-I. Wieder, Appl. Phys. Letters 32 (1978) 549.

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Acknowledgement

1151 RE. Nahory and MA. Pollack, Electron. Letters 14 (1978) 727. [161 M. Yano, H. Nishi and M. Takusagawa, IEEE J. Quan-

The authors wish to thank M. Watanabe, T. Suzuki and M. Fujimoto for their encouragement throughout this work. They are also grateful to G. Iwane and H.

Saito for supplying the laser diodes in this study and to K. Takahei for helpful discussions.

tum Electron. QE-l5 (1979) 571. [17] Y. Itaya, Y. Suematsu, S. Katayama, K. Kishino and S. Japan. I.J. Sakuma, AppI. Phys. (1979) 1795. [18] Arai, H. Yonezu, K. 18 Kobayashi, T. Kamejima, M. Ueno and Y. Nannichi, Japan. J. App!. Phys. 12 (1973) 1585.