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Metalorganic molecular beam epitaxy of strained InAsPInGaAsP multi-quantum-wells for 1.3 μm wavelength laser diodes
N
j. . . . . . . .
ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 147 (1995) 1-7
Metalorganic molecular beam epitaxy of strained InAsP/InGaAsP multi-quantum-wells for 1.3/xm wavelength laser diodes Hideo Sugiura *, Manabu Mitsuhara, Hiromi Oohashi, Takuo Hirono, Kiichi Nakashima NTT Opto-electronicsLaboratories, 3-1 Morinosato-Wakamiya, Atsugi 243-01, Japan Received 16 June 1994; manuscript received in final form 15 August 1994
Abstract
This paper describes the growth of both InAsP single layers and InAsP/InGaAsP multi-quantum-well (MQW) structures by metalorganic molecular beam epitaxy (MOMBE). The A s / P ratio in the InASrPl_y films is proportional to the ratio of the AsH3/PH 3 supply sources. The well-number dependence of the MQWs is characterized by X-ray analysis, photoluminescence (PL), and transmission electron microscopy, revealing that the critical thickness of InAs0.sP0.5 is approximately 70 nm at 520°C. The PL spectrum of an MQW with 8 nm thick InAsP well layers has a full width at half maximum (FWHM) of 4.1 meV at 4 K. The MQW lasers have a threshold current density of 0.74 k A / c m 2 with a cavity length of 300/xm. The maximum operating temperature is as high as 145°C for a 10-well MQW laser with cleaved facets. r
1. I n t r o d u c t i o n
Semiconductor lasers operating at a 1.3 /xm wavelength are of great importance for applications in optical communications such as computer interconnects and subscriber systems. Not only a low threshold current, but also high-temperature operation are required for that purpose [1]. It has b e e n shown that strained multi-quantum-well ( M Q W ) lasers have a lower threshold current density than their lattice-matched counterparts [21. For the active layer materials in these MQWs,
* Corresponding author.
compressively-strained I n G a A s and I n G a A s P were used exclusively. InAsyPl_y covers a luminescence bandgap in the 1.0 to 3.0 /xm wavelength range. Schneider and Wessels [3] reported that the conduction-band offset in the I n A s P / I n P system, A E c / A E g , is greater than that in the I n G a A s P / I n P system, which is conductive to high-temperature operation. In addition, Yam a m o t o et al. [4] reported a compressively strained InAsP M Q W laser operating at 132°C. Imajo et al. [5], however, reported that InAsP layers grown by metalorganic vapor phase epitaxy (MOVPE), even in double quantum wells, fluctuate in thickness owing to both the large lattice mismatch of 1.5% and high M O V P E growth temperature over 600°C. Recently, there has been
0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0248(94)00650-4
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H. Sugiura et aL/Journal of Crystal Growth 147 (1995) 1-7
some attempts to grow t h e m at lower temperatures by gas-source molecular b e a m epitaxy (GSMBE) and metalorganic molecular b e a m epitaxy ( M O M B E ) [6,7]. Those methods provided InAsP M Q W s with 10 wells at 500°C. T h e r e has b e e n no reports of laser applications using M Q W s grown at low temperatures. In this p a p e r , we describe the growth of I n A s P / I n G a A s P M Q W s by M O M B E and their application to laser diodes emitting at 1.3 txm wavelength. The critical thickness of the M Q W s is studied by X-ray analysis, photoluminescence, and transmission electron microscopy. We also study high-temperature laser characteristics of strained MQWs. The maximum operating temperature is shown to be 145°C for the laser with ten wells.
2. Experimental procedure We used the V a c u u m G e n e r a t o r s V-400 CBE system. Triethylgallium ( T E G ) and trimethylindium (TMI) were used as G r o u p I I I sources, A s H 3 and P H 3 as G r o u p V sources. The hydride gases were decomposed in a low-pressure cracker cell heated to 950°C. The flux intensities of the raw materials were controlled with a pressure control system; the pressure in the tube connected to the cracker cell was independently controlled using Baratron gauges and fast response valves. Substrate t e m p e r a t u r e was kept at 505°C for InP and 520°C for I n G a A s and I n G a A s P films. T h e t e m p e r a t u r e was measured with an Accufiber Model-10 optical p y r o m e t e r calibrated with the InSb melting point. Solid Be and Sn were used as p and n-type dopants, respectively. A two-inch substrate m o u n t e d on an In-free holder was rotated at 6 rpm. We used pre-etched substrates purchased from Sumitomo, so no chemical t r e a t m e n t was done before growth. We grew M Q W s consisting of 5.5 nm thick InAsP and 1! nm thick I n G a A s P (equivalent wavelength, h = 1.13 /xm). The thicknesses were nominal values calculated from the growth rates. The n u m b e r of wells in the M Q W was varied from two to fourteen. Double X-ray diffraction m e a s u r e m e n t was carried out to determine the
lattice constants of bulk InAsP films and rocking curves for the MQWs. The spectra obtained from the various reflections were analyzed with a kinematic step model to determine well dimensions and compositions [8]. Cross-sectional transmission electron microscopy ( T E M ) observation was undertaken to determine the well-layer thicknesses and to observe dislocations. Photoluminescence (PL) m e a s u r e m e n t was carried out at room t e m p e r a t u r e (RT) and 4 K. The luminescence was excited with a 15 m W 514.5 nm wavelength Ar laser b e a m with a 1 m spectrometer and detected with a liquid-N2-cooled Ge detector. The M Q W lasers had separate confinement double heterostructures (SCH) with MQWs. W e grew by M O M B E a structure consisting of a 500 n m thick n-type InP buffer layer, 100 nm thick u n d o p e d I n G a A s P (h = 1.13 /zm), undoped MQWs, 100 nm thick undoped I n G a A s P (h = 1.13 /xm), a p-type 0.5 /zm thick InP, and an I n G a A s cap layer. After removing the cap layer, a p-type 2 /xm InP layer and a p-type I n G a A s contact layer were sequentially overgrown at 620°C by liquid phase epitaxy (LPE). The M Q W s had either four, six, or ten periods of 11 n m thick I n G a A s P (h = 1.13 Ixm) barriers and 5.5 nm thick InAsP wells. The S C H - M Q W structure was processed into 40 tzm wide stripe lasers, each with a 300/.~m long cavity. The threshold currents of the laser chips without a facet coating were measured at room t e m p e r a t u r e under pulsed current. To study the high-temperature operation characteristics, the laser chips with cleaved facets were m o u n t e d on a mount header, and the heat-sink t e m p e r a t u r e was calibrated by the t e m p e r a t u r e dependence of the I - V curve of a Si diode.
3. Results and discussion We first studied the alloy composition of thick InASyPl_y layers. In Fig. 1, the y / ( 1 - y ) value for the films is plotted against the source supply ratio of A s H 3 / P H 3. The y / ( 1 - y) values correspond to the A s / P ratio in the films. Each y value was determined from the lattice constants of the 500 nm thick films, assuming Vegard's rule. Most of the films were grown on InP sub-
H. Sugiura et aL/Journal of Crystal Growth 147 (1995) 1-7 i
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0
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Fig. 1. Relationship between the value of y/(1 - y) of the 500 nm thick InAsrPl_y films and the source ratio of AsH 3/PH 3.
strates and some on G a A s substrates in order to relax t h e m easily. The y values obtained for the two kinds of substrates coincided within a + 0.01 margin. The source ratio of A s H 3 / P H 3 along the horizontal axis was calculated from the pressure values in each tube connected to the cracker cell. The V / I I I ratio was kept in the range of 3.3 to 4.3 depending on the InAsyPl_y compositions. The figure indicates that the y / ( 1 - y ) value increases linearly with the ratio in a wide range of y, i.e., from 0.2 to 0.65 (see the right-hand vertical axis). F r o m a practical point of view, this linear dependence indicates the capability to precisely control the In_AsP composition by controlling the pressure in our M O M B E system.
I 0 wells
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InP {400)
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F r o m a physical point of view, the linear dependence means that the incorporation efficiency of the cracked A s H 3 and P H 3 are independent, in accordance with the results of M O V P E [5]. The slope of the straight line in the figure is about 5. Since the pressure ratio corresponds to the molecule n u m b e r ratio, the fact that the slope is much greater than one indicates that the cracked A s H 3 is much m o r e easily incorporated into the films than the cracked P H 3 is. This may be due to a greater re-evaporation of phosphorus on the growing surface. The structure p a r a m e t e r s of I n A s P / I n G a A s P M Q W s were determined by X-ray diffraction analysis. Scans were taken around the (400), (422), (333), and (511) reflections. Fig. 2a shows a (400) rocking curve of the M Q W with ten wells. Higher-order satellite peaks are clearly generated around the main superlattice p e a k (n = 0). Curve fitting using the model proposed in Ref. [8] revealed that the InAsP well layer is under compressive strain of 2.95 + 0.05% and is 5.5 nm thick, while the I n G a A s P barrier layer is under compressive strain of 0.15% and is 10.5 nm thick. Curve fitting for a 6-well MQW, not shown here, gave approximately the same values for strain and thickness. The calculated well and barrier thicknesses agree well with those obtained from T E M micrographs of the samples, as will be shown later. T h e lattice mismatch of the well layers was 1.47 + 0.03%, as calculated from the elastic constants given in Ref. [9]. T h e arsenic composition
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Fig. 2. (a) (400) rocking curve of the MQW with 10 wells. (b) Dependence of the full width at half maximum (FWHM) of the satellite peaks on the InAsP/InGaAsP MQWs with from 2 to 14 wells.
4
H. Sugiura et al./Journal of Crystal Growth 147 (1995) 1-7
of the InAsyPl_y corresponding to the mismatch value is therefore concluded to be y = 0.48 + 0.01. Hereafter, we will describe the well-layer composition as InAs0.sP0. 5 for the sake of convenience. Fig. 2b shows the dependence of the full width at half maximum (FWHM) of the satellite peaks for various samples whose well numbers range from two to fourteen. The figure includes two important results; first, the FWHM decreases in-
versely with the number of wells in the 4-10 range, and second, the FWHM of the 14-well sample is more than four times greater than that of the 10-well sample. The first result agrees well with theoretical predictions [10], meaning that the interface between the InAsP/InGaAsP layers must be very flat in the growth direction. The fact that the 14-well MQW has a large FWHM is indicative of crystalline deterioration, since the
InAsP layer
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Fig. 3. Cross-sectional T E M micrographs of M Q W s with (a) 10 and (b) 14 wells. Note that the 10-well sample has no dislocations, but the 14-well sample has a dislocation at the interface between the first I n A s P layer and the buffer layer.
_
1-1.Sugiura et al. /Journal of Crystal Growth 147 (1995) 1-7
total thickness of the fourteen InAsP well layers is far beyond the critical thickness calculated with Matthews and Blakeslee's formula [11]. We studied TEM micrographs for the 14-well MQW to investigate the drastic change in the FWHM of the satellite peaks. Figs. 3a and 3b compare cross-sectional TEM micrographs of samples whose MQWs have ten and fourteen wells, respectively. Fig. 3a indicates that the 10well MQW has very sharp interfaces from the first to the tenth InAsP layer. No fluctuations are observed in the thickness of the well layers, unlike in MOVPE-grown films [5]. Careful examination of the lattice image of the MQW reveals no dislocations; neither at the MQW-buffer layer nor in the MQWs themselves. Fig. 3b shows part of the 14-well MQW, i.e., from the first to the sixth layer. A dislocation line is generated at the interface between the first InAsP layer and the buffer layer, extending into the buffer layer. The density of the dislocation is about 5 x 104 cm-1. Although the figure shows part of the MQW, we have confirmed that there is no thickness fluctuation in the MQW layers, i.e., up to the fourteenth layer. The plan-view TEM micrographs of the MQW reveal that the dislocations observed at the interface are assigned misfit dislocations. When we define the critical thickness for MQWs as the sum of the thicknesses of each strained layer, the critical thickness of the InAs0.sP0. 5 layer is determined to be between 60 and 80 nm, according to the TEM micrographs. We determined the well and barrier thicknesses from the TEM micrograph of the 10-pair MQW to be 5.7 ___0.3 and 11.4 + 0.3 nm, respectively. These values are close to the nominal values estimated from the growth rates and those calculated from the X-ray rocking curve fitting. The 14-pair MQW TEM micrograph shows exactly the same thickness, even though it contains dislocations. This may be due to local relaxation of the lattices in the MQW. Fig. 4 shows the PL intensity dependence of the MQWs on the number of wells. PL measurement was carried out at room temperature for samples with two to fourteen wells. The figure shows that the PL intensity gradually increases with well number up to ten. However, the inten-
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sity for the 14-well sample decreases to less than one-tenth that of the others, which indicates that a lot of nonradiative centers related to the dislocations have been generated in the film [12]. The effects of annealing on the optical properties of the MQWs were studied by annealing at 620°C for 2.5 h in ambient PH 3 in a separate furnace. The annealed samples with from two to eight wells had the same PL intensities as nonannealed ones, while the annealed 10-well MQW had half the intensity of the non-annealed one. The peak wavelengths of all the samples, including the 10-well sample, remained unchanged, i,e., 1.324 ___0.002 /zm both before and after annealing. The deterioration in PL intensity after annealing suggests that the 10-well MQW could reach the critical thickness at 620°C. In other words, critical thickness seems to depend on temperature. Here, we briefly discuss the critical thickness of the InAs0.sP0+5 layer. The experimental results in Figs. 2 to 4 indicate that the critical thickness is about 70 nm at a growth temperature of 520°C. This value is more than twice that of MOVPEgrown InAsP [4]. This difference is due to the fact that the growth temperature for MOMBE is about 100°C lower than that for MOVPE. The annealing results described above suggest that the critical thickness at 620°C may be approximately 60 nm. That is, the critical thickness decreases as temperature increases. This kind of p h e n o m e n a has also been r e p o r t e d for
6
H. Sugiura et aL/Journal of Crystal Growth 147 (1995) 1-7
4K kw = 5.5 nm
Lw = 8 nm
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WAVELENGTH (nm) Fig. 5. Low-temperature PL spectra of two I n A s P / I n G a A s P D Q W s measu red at 4 K. One has an InAsP layer thickness of L w = 5.5 nm and the other 8 nm. The F W H M of the PL p e a k is as small as 4.1 meV for the D Q W with L w = 8 nm.
Sil_xGex/Si heterostructures [13]. Although neither Matthews and Blakeslee's [11] nor People and Bean's [14] model can explain the temperature dependence of the critical thickness, Price [15] proposed a new model which includes a force for dislocation activation with an exponential temperature dependence. This model seems to be more reliable than the other two. We, therefore, can conclude not only theoretically but also experimentally that low-temperature growth has an advantage in that it increases the critical layer thickness for dislocation generation in strained MQWs. Next, we studied the optical properties of InAsP MQWs from the viewpoint of interface sharpness in the MQWs. Fig. 5 shows the PL spectra of two double-quantum wells (DQWs) measured at 4 K: one has a well layer thickness of L w = 5.5 nm and the other L w = 8 nm. The well layers are sandwiched between 11 nm thick InGaAsP (~ = 1.13 /xm). The D Q W with thinner well layers has a PL peak at 1235.6 nm whose F W H M is 7.8 meV. The one with thicker wells has a peak at 1285.5 nm whose F W H M is 4.1 meV. These samples have a F W H M of 20 meV at RT. These F W H M values, both at 4 K and RT, are the smallest ever reported for InAsP MQWs [4,5]. The smaller values indicate that the inter-
faces should be quite sharp, probably because of the lower growth temperatures. We studied the high-temperature characteristics of I n A s P / I n G a A s P MQW-SCH lasers based on the laser structure described in the experimental section. We have prepared three kinds of structures: MQWs with four, six, and ten wells with other conditions such as barrier composition and thickness fixed. Facets of laser chips were as cleaved. The threshold current densities were measured under a pulse bias at RT. The device yield, i.e., the percentage of the chips which oscillated, was 100% for all three structures. The average current density of the thirty chips for each structure was 0.79 + 0.01 k A / c m 2 for the 4-well lasers, 0.74 + 0.02 k A / c m 2 for the 6-well lasers, and 1.1 +_0.17kA/cm 2 for the 10-well lasers. The standard deviation of the current densities is quite small for the 4- and 6-well structures. More than ninety laser chips oscillated at a wavelength of 1.335 ___0.005/zm. These two kinds of uniformities, i.e., the threshold current density and the oscillating wavelength, reflect the excellent uniformity of the two-dimensional PL intensity of the MQWs [16]. For the 10-well laser, the threshold currents were distributed about 15% around the average value. The sample had a cross-hatch pattern on the surface after L P E regrowth. Hence, the rather high deviation of the threshold current is presumably due to dislocations generated in the M Q W active layer. Fig. 6 compares the threshold current dependence of the three lasers on the heat-sink temperature. It can be seen from the figure that the threshold currents gradually increase up to about 90°C but increase rapidly above 100°C. The characteristic temperature T o estimated from the three curves increases with the number of wells in the MQW: from T o = 43 to 60 K in the range of 35 to 80°C. The maximum operating temperature of the lasers, Tmax, also increases with the number of wells; 105°C for the 4-well, 136°C for the 6-well, and 145°C for the 10-well laser. The value of Tmax = 145°C is greater than the highest reported value for a MOVPE-grown InAsP M Q W laser with coated facets, higher by more than 10°C [4]. The increase in Tm~x with an increasing
H. Sugiura et al. /Journal of Crystal Growth 147 (1995) 1-7
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laser operating temperature increases with the number of MQW pairs: Tm~ is 145°C for the 10-well sample. The performance of these MQW lasers is superior to that of MOVPE-grown MQW lasers. We expect a higher Tm~ can be realized by introducing strain-compensated MQWs grown by MOMBE.
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20
40
60
80
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120
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HEAT SINK TEMPERATURE (°C)
Fig. 6. Comparison of the temperature dependence of the threshold current density for the three InAsP/InGaAsP MQW lasers; these lasers have 4, 6, and 10 wells, and oscillating wavelengths of 1.335+0.005 /xm at room temperature. The highest operating temperature is 145°C for the 10-well laser.
number of wells can be explained in terms of the suppression of carrier overflow for larger-wellnumber MQWs [17].
4. Conclusion We have studied the growth of InAsP/InGaAsP MQWs emitting at a 1.3 /zm wavelength and their laser characteristics. The A s / P ratio in single InAsyPl_y films is proportional to the ratio of the A s H 3 / P H 3 supply sources. The wellnumber dependence of the PL intensity, FWHM of the X-ray satellite peak, and TEM cross-section micrograph indicate that the critical thickness of MQWs composed of InAs0.sP0.5/InGaAsP (A = 1.13 ~m) is approximately 70 nm at the growth temperature of 520°C but 60 nm at 620°C. These values are about twice that of MOVPE-grown MQWs. The low-temperature PL spectrum showed the FWHM of the PL peak was as small as 4.1 meV. The larger critical thickness and smaller PL FWHM are attributed to the lower-temperature growth of MOMBE compared to MOVPE. The lasers operated at a threshold current density of 0.74 _+ 0.02 k A / c m 2 for 30 chips with a cavity length of 300/~m. The highest
We would like to thank Drs. M. Yamamoto, J. Nakano, S. Seki and H. Tsuchiya for their continuous encouragement, and Dr. H. Kamada for his technical support in low-temperature PL measurement.
References [1] H. Temkin, D. Coblentz, R.A. Logan, J.M. Vandenberg, R.D. Yadvish and A.M. Sergent, Appl. Phys. Lett. 63 (1993) 2321. [2] P.J.A. Thijs, L.F. Tiemeijer, P.I. Kuindersma, J.J.M. Binsma and T. van Dongen, IEEE J. Quantum Electron. QE-27 (1991) 1426. [3] R.P. Schneider, Jr. and B.W. Wessels, J. Electron. Mater. 20 (1991) 1117. [4] M. Yamamoto, N. Yamamoto and J. Nakano, IEEE J. Quantum Electron., in press. [5] Y. Imajo, A. Kasukawa, T. Namegawa and T. Kikuta, Appl. Phys. Lett. 61 (1992) 2506. [6] H.Q. Hou, A.N. Cheng, H.H. Wieder, W.S.C. Chang and C.W. Tu, Appl. Phys. Lett. 63 (1993) 1833. [7] A. Freundlich, A.H. Bensaoula and A. Bensaoula, J, Crystal Growth 127 (1993) 246. [8] K. Nakashima, J, Appl. Phys. 71 (1992) 1189. [9] S. Muramatsu and M. Kitamura, J. Appl. Phys. 73 (1993) 4270. [10] A. Segmnller and A.E. Blakeslee, J. Appl. Cryst. 6 (1973) 19. [11] J.W. Matthews and A.E. Blakeslee, J. Crystal Growth 27 (1974) 118. [12] T. Taguchi, Y. Takeuchi, K. Matsugatani, "Y. Ueno, T. Hattori, Y. Sugiyama and M. Tacano, J. Crystal Growth 134 (1993) 147. [13] D.C. Houghton, J. Appl. Phys. 70 (1991) 2136. [14] R. People and J.C. Bean, Appl. Phys. Lett. 47 (1985) 322. [15] G.L. Price, Phys. Rev. Lett. 66 (1991) 469. [16] It. Sugiura, M. Mitsuhara, R. Iga and N. Yamamoto, J. Crystal Growth 141 (1994) 299. [17] S. Seki, T. Yamanaka, W. Lui and K. Yokoyama, J. Appl. Phys. 75 (1994) 1299.
j. . . . . . . .
ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 147 (1995) 1-7
Metalorganic molecular beam epitaxy of strained InAsP/InGaAsP multi-quantum-wells for 1.3/xm wavelength laser diodes Hideo Sugiura *, Manabu Mitsuhara, Hiromi Oohashi, Takuo Hirono, Kiichi Nakashima NTT Opto-electronicsLaboratories, 3-1 Morinosato-Wakamiya, Atsugi 243-01, Japan Received 16 June 1994; manuscript received in final form 15 August 1994
Abstract
This paper describes the growth of both InAsP single layers and InAsP/InGaAsP multi-quantum-well (MQW) structures by metalorganic molecular beam epitaxy (MOMBE). The A s / P ratio in the InASrPl_y films is proportional to the ratio of the AsH3/PH 3 supply sources. The well-number dependence of the MQWs is characterized by X-ray analysis, photoluminescence (PL), and transmission electron microscopy, revealing that the critical thickness of InAs0.sP0.5 is approximately 70 nm at 520°C. The PL spectrum of an MQW with 8 nm thick InAsP well layers has a full width at half maximum (FWHM) of 4.1 meV at 4 K. The MQW lasers have a threshold current density of 0.74 k A / c m 2 with a cavity length of 300/xm. The maximum operating temperature is as high as 145°C for a 10-well MQW laser with cleaved facets. r
1. I n t r o d u c t i o n
Semiconductor lasers operating at a 1.3 /xm wavelength are of great importance for applications in optical communications such as computer interconnects and subscriber systems. Not only a low threshold current, but also high-temperature operation are required for that purpose [1]. It has b e e n shown that strained multi-quantum-well ( M Q W ) lasers have a lower threshold current density than their lattice-matched counterparts [21. For the active layer materials in these MQWs,
* Corresponding author.
compressively-strained I n G a A s and I n G a A s P were used exclusively. InAsyPl_y covers a luminescence bandgap in the 1.0 to 3.0 /xm wavelength range. Schneider and Wessels [3] reported that the conduction-band offset in the I n A s P / I n P system, A E c / A E g , is greater than that in the I n G a A s P / I n P system, which is conductive to high-temperature operation. In addition, Yam a m o t o et al. [4] reported a compressively strained InAsP M Q W laser operating at 132°C. Imajo et al. [5], however, reported that InAsP layers grown by metalorganic vapor phase epitaxy (MOVPE), even in double quantum wells, fluctuate in thickness owing to both the large lattice mismatch of 1.5% and high M O V P E growth temperature over 600°C. Recently, there has been
0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0248(94)00650-4
2
H. Sugiura et aL/Journal of Crystal Growth 147 (1995) 1-7
some attempts to grow t h e m at lower temperatures by gas-source molecular b e a m epitaxy (GSMBE) and metalorganic molecular b e a m epitaxy ( M O M B E ) [6,7]. Those methods provided InAsP M Q W s with 10 wells at 500°C. T h e r e has b e e n no reports of laser applications using M Q W s grown at low temperatures. In this p a p e r , we describe the growth of I n A s P / I n G a A s P M Q W s by M O M B E and their application to laser diodes emitting at 1.3 txm wavelength. The critical thickness of the M Q W s is studied by X-ray analysis, photoluminescence, and transmission electron microscopy. We also study high-temperature laser characteristics of strained MQWs. The maximum operating temperature is shown to be 145°C for the laser with ten wells.
2. Experimental procedure We used the V a c u u m G e n e r a t o r s V-400 CBE system. Triethylgallium ( T E G ) and trimethylindium (TMI) were used as G r o u p I I I sources, A s H 3 and P H 3 as G r o u p V sources. The hydride gases were decomposed in a low-pressure cracker cell heated to 950°C. The flux intensities of the raw materials were controlled with a pressure control system; the pressure in the tube connected to the cracker cell was independently controlled using Baratron gauges and fast response valves. Substrate t e m p e r a t u r e was kept at 505°C for InP and 520°C for I n G a A s and I n G a A s P films. T h e t e m p e r a t u r e was measured with an Accufiber Model-10 optical p y r o m e t e r calibrated with the InSb melting point. Solid Be and Sn were used as p and n-type dopants, respectively. A two-inch substrate m o u n t e d on an In-free holder was rotated at 6 rpm. We used pre-etched substrates purchased from Sumitomo, so no chemical t r e a t m e n t was done before growth. We grew M Q W s consisting of 5.5 nm thick InAsP and 1! nm thick I n G a A s P (equivalent wavelength, h = 1.13 /xm). The thicknesses were nominal values calculated from the growth rates. The n u m b e r of wells in the M Q W was varied from two to fourteen. Double X-ray diffraction m e a s u r e m e n t was carried out to determine the
lattice constants of bulk InAsP films and rocking curves for the MQWs. The spectra obtained from the various reflections were analyzed with a kinematic step model to determine well dimensions and compositions [8]. Cross-sectional transmission electron microscopy ( T E M ) observation was undertaken to determine the well-layer thicknesses and to observe dislocations. Photoluminescence (PL) m e a s u r e m e n t was carried out at room t e m p e r a t u r e (RT) and 4 K. The luminescence was excited with a 15 m W 514.5 nm wavelength Ar laser b e a m with a 1 m spectrometer and detected with a liquid-N2-cooled Ge detector. The M Q W lasers had separate confinement double heterostructures (SCH) with MQWs. W e grew by M O M B E a structure consisting of a 500 n m thick n-type InP buffer layer, 100 nm thick u n d o p e d I n G a A s P (h = 1.13 /zm), undoped MQWs, 100 nm thick undoped I n G a A s P (h = 1.13 /xm), a p-type 0.5 /zm thick InP, and an I n G a A s cap layer. After removing the cap layer, a p-type 2 /xm InP layer and a p-type I n G a A s contact layer were sequentially overgrown at 620°C by liquid phase epitaxy (LPE). The M Q W s had either four, six, or ten periods of 11 n m thick I n G a A s P (h = 1.13 Ixm) barriers and 5.5 nm thick InAsP wells. The S C H - M Q W structure was processed into 40 tzm wide stripe lasers, each with a 300/.~m long cavity. The threshold currents of the laser chips without a facet coating were measured at room t e m p e r a t u r e under pulsed current. To study the high-temperature operation characteristics, the laser chips with cleaved facets were m o u n t e d on a mount header, and the heat-sink t e m p e r a t u r e was calibrated by the t e m p e r a t u r e dependence of the I - V curve of a Si diode.
3. Results and discussion We first studied the alloy composition of thick InASyPl_y layers. In Fig. 1, the y / ( 1 - y ) value for the films is plotted against the source supply ratio of A s H 3 / P H 3. The y / ( 1 - y) values correspond to the A s / P ratio in the films. Each y value was determined from the lattice constants of the 500 nm thick films, assuming Vegard's rule. Most of the films were grown on InP sub-
H. Sugiura et aL/Journal of Crystal Growth 147 (1995) 1-7 i
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0
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Fig. 1. Relationship between the value of y/(1 - y) of the 500 nm thick InAsrPl_y films and the source ratio of AsH 3/PH 3.
strates and some on G a A s substrates in order to relax t h e m easily. The y values obtained for the two kinds of substrates coincided within a + 0.01 margin. The source ratio of A s H 3 / P H 3 along the horizontal axis was calculated from the pressure values in each tube connected to the cracker cell. The V / I I I ratio was kept in the range of 3.3 to 4.3 depending on the InAsyPl_y compositions. The figure indicates that the y / ( 1 - y ) value increases linearly with the ratio in a wide range of y, i.e., from 0.2 to 0.65 (see the right-hand vertical axis). F r o m a practical point of view, this linear dependence indicates the capability to precisely control the In_AsP composition by controlling the pressure in our M O M B E system.
I 0 wells
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F r o m a physical point of view, the linear dependence means that the incorporation efficiency of the cracked A s H 3 and P H 3 are independent, in accordance with the results of M O V P E [5]. The slope of the straight line in the figure is about 5. Since the pressure ratio corresponds to the molecule n u m b e r ratio, the fact that the slope is much greater than one indicates that the cracked A s H 3 is much m o r e easily incorporated into the films than the cracked P H 3 is. This may be due to a greater re-evaporation of phosphorus on the growing surface. The structure p a r a m e t e r s of I n A s P / I n G a A s P M Q W s were determined by X-ray diffraction analysis. Scans were taken around the (400), (422), (333), and (511) reflections. Fig. 2a shows a (400) rocking curve of the M Q W with ten wells. Higher-order satellite peaks are clearly generated around the main superlattice p e a k (n = 0). Curve fitting using the model proposed in Ref. [8] revealed that the InAsP well layer is under compressive strain of 2.95 + 0.05% and is 5.5 nm thick, while the I n G a A s P barrier layer is under compressive strain of 0.15% and is 10.5 nm thick. Curve fitting for a 6-well MQW, not shown here, gave approximately the same values for strain and thickness. The calculated well and barrier thicknesses agree well with those obtained from T E M micrographs of the samples, as will be shown later. T h e lattice mismatch of the well layers was 1.47 + 0.03%, as calculated from the elastic constants given in Ref. [9]. T h e arsenic composition
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Fig. 2. (a) (400) rocking curve of the MQW with 10 wells. (b) Dependence of the full width at half maximum (FWHM) of the satellite peaks on the InAsP/InGaAsP MQWs with from 2 to 14 wells.
4
H. Sugiura et al./Journal of Crystal Growth 147 (1995) 1-7
of the InAsyPl_y corresponding to the mismatch value is therefore concluded to be y = 0.48 + 0.01. Hereafter, we will describe the well-layer composition as InAs0.sP0. 5 for the sake of convenience. Fig. 2b shows the dependence of the full width at half maximum (FWHM) of the satellite peaks for various samples whose well numbers range from two to fourteen. The figure includes two important results; first, the FWHM decreases in-
versely with the number of wells in the 4-10 range, and second, the FWHM of the 14-well sample is more than four times greater than that of the 10-well sample. The first result agrees well with theoretical predictions [10], meaning that the interface between the InAsP/InGaAsP layers must be very flat in the growth direction. The fact that the 14-well MQW has a large FWHM is indicative of crystalline deterioration, since the
InAsP layer
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(a)
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rowth direction
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Fig. 3. Cross-sectional T E M micrographs of M Q W s with (a) 10 and (b) 14 wells. Note that the 10-well sample has no dislocations, but the 14-well sample has a dislocation at the interface between the first I n A s P layer and the buffer layer.
_
1-1.Sugiura et al. /Journal of Crystal Growth 147 (1995) 1-7
total thickness of the fourteen InAsP well layers is far beyond the critical thickness calculated with Matthews and Blakeslee's formula [11]. We studied TEM micrographs for the 14-well MQW to investigate the drastic change in the FWHM of the satellite peaks. Figs. 3a and 3b compare cross-sectional TEM micrographs of samples whose MQWs have ten and fourteen wells, respectively. Fig. 3a indicates that the 10well MQW has very sharp interfaces from the first to the tenth InAsP layer. No fluctuations are observed in the thickness of the well layers, unlike in MOVPE-grown films [5]. Careful examination of the lattice image of the MQW reveals no dislocations; neither at the MQW-buffer layer nor in the MQWs themselves. Fig. 3b shows part of the 14-well MQW, i.e., from the first to the sixth layer. A dislocation line is generated at the interface between the first InAsP layer and the buffer layer, extending into the buffer layer. The density of the dislocation is about 5 x 104 cm-1. Although the figure shows part of the MQW, we have confirmed that there is no thickness fluctuation in the MQW layers, i.e., up to the fourteenth layer. The plan-view TEM micrographs of the MQW reveal that the dislocations observed at the interface are assigned misfit dislocations. When we define the critical thickness for MQWs as the sum of the thicknesses of each strained layer, the critical thickness of the InAs0.sP0. 5 layer is determined to be between 60 and 80 nm, according to the TEM micrographs. We determined the well and barrier thicknesses from the TEM micrograph of the 10-pair MQW to be 5.7 ___0.3 and 11.4 + 0.3 nm, respectively. These values are close to the nominal values estimated from the growth rates and those calculated from the X-ray rocking curve fitting. The 14-pair MQW TEM micrograph shows exactly the same thickness, even though it contains dislocations. This may be due to local relaxation of the lattices in the MQW. Fig. 4 shows the PL intensity dependence of the MQWs on the number of wells. PL measurement was carried out at room temperature for samples with two to fourteen wells. The figure shows that the PL intensity gradually increases with well number up to ten. However, the inten-
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sity for the 14-well sample decreases to less than one-tenth that of the others, which indicates that a lot of nonradiative centers related to the dislocations have been generated in the film [12]. The effects of annealing on the optical properties of the MQWs were studied by annealing at 620°C for 2.5 h in ambient PH 3 in a separate furnace. The annealed samples with from two to eight wells had the same PL intensities as nonannealed ones, while the annealed 10-well MQW had half the intensity of the non-annealed one. The peak wavelengths of all the samples, including the 10-well sample, remained unchanged, i,e., 1.324 ___0.002 /zm both before and after annealing. The deterioration in PL intensity after annealing suggests that the 10-well MQW could reach the critical thickness at 620°C. In other words, critical thickness seems to depend on temperature. Here, we briefly discuss the critical thickness of the InAs0.sP0+5 layer. The experimental results in Figs. 2 to 4 indicate that the critical thickness is about 70 nm at a growth temperature of 520°C. This value is more than twice that of MOVPEgrown InAsP [4]. This difference is due to the fact that the growth temperature for MOMBE is about 100°C lower than that for MOVPE. The annealing results described above suggest that the critical thickness at 620°C may be approximately 60 nm. That is, the critical thickness decreases as temperature increases. This kind of p h e n o m e n a has also been r e p o r t e d for
6
H. Sugiura et aL/Journal of Crystal Growth 147 (1995) 1-7
4K kw = 5.5 nm
Lw = 8 nm
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WAVELENGTH (nm) Fig. 5. Low-temperature PL spectra of two I n A s P / I n G a A s P D Q W s measu red at 4 K. One has an InAsP layer thickness of L w = 5.5 nm and the other 8 nm. The F W H M of the PL p e a k is as small as 4.1 meV for the D Q W with L w = 8 nm.
Sil_xGex/Si heterostructures [13]. Although neither Matthews and Blakeslee's [11] nor People and Bean's [14] model can explain the temperature dependence of the critical thickness, Price [15] proposed a new model which includes a force for dislocation activation with an exponential temperature dependence. This model seems to be more reliable than the other two. We, therefore, can conclude not only theoretically but also experimentally that low-temperature growth has an advantage in that it increases the critical layer thickness for dislocation generation in strained MQWs. Next, we studied the optical properties of InAsP MQWs from the viewpoint of interface sharpness in the MQWs. Fig. 5 shows the PL spectra of two double-quantum wells (DQWs) measured at 4 K: one has a well layer thickness of L w = 5.5 nm and the other L w = 8 nm. The well layers are sandwiched between 11 nm thick InGaAsP (~ = 1.13 /xm). The D Q W with thinner well layers has a PL peak at 1235.6 nm whose F W H M is 7.8 meV. The one with thicker wells has a peak at 1285.5 nm whose F W H M is 4.1 meV. These samples have a F W H M of 20 meV at RT. These F W H M values, both at 4 K and RT, are the smallest ever reported for InAsP MQWs [4,5]. The smaller values indicate that the inter-
faces should be quite sharp, probably because of the lower growth temperatures. We studied the high-temperature characteristics of I n A s P / I n G a A s P MQW-SCH lasers based on the laser structure described in the experimental section. We have prepared three kinds of structures: MQWs with four, six, and ten wells with other conditions such as barrier composition and thickness fixed. Facets of laser chips were as cleaved. The threshold current densities were measured under a pulse bias at RT. The device yield, i.e., the percentage of the chips which oscillated, was 100% for all three structures. The average current density of the thirty chips for each structure was 0.79 + 0.01 k A / c m 2 for the 4-well lasers, 0.74 + 0.02 k A / c m 2 for the 6-well lasers, and 1.1 +_0.17kA/cm 2 for the 10-well lasers. The standard deviation of the current densities is quite small for the 4- and 6-well structures. More than ninety laser chips oscillated at a wavelength of 1.335 ___0.005/zm. These two kinds of uniformities, i.e., the threshold current density and the oscillating wavelength, reflect the excellent uniformity of the two-dimensional PL intensity of the MQWs [16]. For the 10-well laser, the threshold currents were distributed about 15% around the average value. The sample had a cross-hatch pattern on the surface after L P E regrowth. Hence, the rather high deviation of the threshold current is presumably due to dislocations generated in the M Q W active layer. Fig. 6 compares the threshold current dependence of the three lasers on the heat-sink temperature. It can be seen from the figure that the threshold currents gradually increase up to about 90°C but increase rapidly above 100°C. The characteristic temperature T o estimated from the three curves increases with the number of wells in the MQW: from T o = 43 to 60 K in the range of 35 to 80°C. The maximum operating temperature of the lasers, Tmax, also increases with the number of wells; 105°C for the 4-well, 136°C for the 6-well, and 145°C for the 10-well laser. The value of Tmax = 145°C is greater than the highest reported value for a MOVPE-grown InAsP M Q W laser with coated facets, higher by more than 10°C [4]. The increase in Tm~x with an increasing
H. Sugiura et al. /Journal of Crystal Growth 147 (1995) 1-7
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laser operating temperature increases with the number of MQW pairs: Tm~ is 145°C for the 10-well sample. The performance of these MQW lasers is superior to that of MOVPE-grown MQW lasers. We expect a higher Tm~ can be realized by introducing strain-compensated MQWs grown by MOMBE.
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60
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Fig. 6. Comparison of the temperature dependence of the threshold current density for the three InAsP/InGaAsP MQW lasers; these lasers have 4, 6, and 10 wells, and oscillating wavelengths of 1.335+0.005 /xm at room temperature. The highest operating temperature is 145°C for the 10-well laser.
number of wells can be explained in terms of the suppression of carrier overflow for larger-wellnumber MQWs [17].
4. Conclusion We have studied the growth of InAsP/InGaAsP MQWs emitting at a 1.3 /zm wavelength and their laser characteristics. The A s / P ratio in single InAsyPl_y films is proportional to the ratio of the A s H 3 / P H 3 supply sources. The wellnumber dependence of the PL intensity, FWHM of the X-ray satellite peak, and TEM cross-section micrograph indicate that the critical thickness of MQWs composed of InAs0.sP0.5/InGaAsP (A = 1.13 ~m) is approximately 70 nm at the growth temperature of 520°C but 60 nm at 620°C. These values are about twice that of MOVPE-grown MQWs. The low-temperature PL spectrum showed the FWHM of the PL peak was as small as 4.1 meV. The larger critical thickness and smaller PL FWHM are attributed to the lower-temperature growth of MOMBE compared to MOVPE. The lasers operated at a threshold current density of 0.74 _+ 0.02 k A / c m 2 for 30 chips with a cavity length of 300/~m. The highest
We would like to thank Drs. M. Yamamoto, J. Nakano, S. Seki and H. Tsuchiya for their continuous encouragement, and Dr. H. Kamada for his technical support in low-temperature PL measurement.
References [1] H. Temkin, D. Coblentz, R.A. Logan, J.M. Vandenberg, R.D. Yadvish and A.M. Sergent, Appl. Phys. Lett. 63 (1993) 2321. [2] P.J.A. Thijs, L.F. Tiemeijer, P.I. Kuindersma, J.J.M. Binsma and T. van Dongen, IEEE J. Quantum Electron. QE-27 (1991) 1426. [3] R.P. Schneider, Jr. and B.W. Wessels, J. Electron. Mater. 20 (1991) 1117. [4] M. Yamamoto, N. Yamamoto and J. Nakano, IEEE J. Quantum Electron., in press. [5] Y. Imajo, A. Kasukawa, T. Namegawa and T. Kikuta, Appl. Phys. Lett. 61 (1992) 2506. [6] H.Q. Hou, A.N. Cheng, H.H. Wieder, W.S.C. Chang and C.W. Tu, Appl. Phys. Lett. 63 (1993) 1833. [7] A. Freundlich, A.H. Bensaoula and A. Bensaoula, J, Crystal Growth 127 (1993) 246. [8] K. Nakashima, J, Appl. Phys. 71 (1992) 1189. [9] S. Muramatsu and M. Kitamura, J. Appl. Phys. 73 (1993) 4270. [10] A. Segmnller and A.E. Blakeslee, J. Appl. Cryst. 6 (1973) 19. [11] J.W. Matthews and A.E. Blakeslee, J. Crystal Growth 27 (1974) 118. [12] T. Taguchi, Y. Takeuchi, K. Matsugatani, "Y. Ueno, T. Hattori, Y. Sugiyama and M. Tacano, J. Crystal Growth 134 (1993) 147. [13] D.C. Houghton, J. Appl. Phys. 70 (1991) 2136. [14] R. People and J.C. Bean, Appl. Phys. Lett. 47 (1985) 322. [15] G.L. Price, Phys. Rev. Lett. 66 (1991) 469. [16] It. Sugiura, M. Mitsuhara, R. Iga and N. Yamamoto, J. Crystal Growth 141 (1994) 299. [17] S. Seki, T. Yamanaka, W. Lui and K. Yokoyama, J. Appl. Phys. 75 (1994) 1299.