orientation dependent growth behaviour during hydride vpe regrowth of inp:fe around reactive ion...

Journal of Electronic Materials, Vol. 20, No. 7, i991

Orientation Dependent Growth Behaviour During Hydride VPE Regrowth of InP-Fe Around Reactive Ion Etched Mesas

BO HAMMARLUND,* SEBASTIAN LOURDUDOSS and OLLE KJEBON

Swedish Institute of Microelectronics, Box 1084, S-164 21 Kista, Sweden

An investigation on the hydride vapour phase epitaxy (HVPE) growth of SI-InP: Fe on patterned surfaces indicates that iron incorporation does not always synchronize with the rate of growth. This is especially true in certain crystallographic directions, e.g., <1, -1 , 0> where the growth rate is high. This phenomenon has been attributed to the local chemistry which limits the availability of iron or to a simple kinetically limited supply of iron when the growth rate is high. However, it is shown that by a two step procedure such a synchronization can be achieved. The proposed procedure which is applicable to reactive ion etched vertical mesas is tested on a laser mesa and the surrounding SI- InP:Fe is found to provide good current confinement and high modulation performance.

Key words: VPE, regrowth, InP: Fe, semi-insulating compounds and reactive ion etching

1. INTRODUCTION

One important step in fabricating high speed semiconductor lasers is the regrowth of semi-insulating (SI) InP. This is done to embed the laser structure and thereby define the path for the in- jected current into the volume of active, lasing material. It is known for a long time that contrary to the commonly used reversed p-n junctions for current blocking layers, this configuration decreases the parasitic capacitances and the laser chip can be modulated to high bit rates. 1 Besides, the structures embedded with semi-insulating InP:Fe does not ne- cessitate further complicated processing but contacting. For example it has been shown that metal contacts can be deposited on top of the laser mesa as well as on the surrounding SI-InP: Fe without degrading the properties of the overall structure. 2 However, it has been observed that the SI-InP:Fe regrown in a hydride VPE reactor to embed a laser mesa was converted to p-type in the region very close to the mesa wall (near mesa region) as a result of orientation dependent growth behaviour of InP:Fe and the p-type converted region was triangular in shape and pit-filled when the cross-section after regrowth was subjected to stain etchant. 3 Numerous experiments carried out in our laboratory indicate that even when n-type InP mesa formed by reactive ion etching is embedded with InP:Fe, the stained cross-section can exhibit similar characteristic triangles in the near mesa region on both sides of the mesa wall. The material inside such triangles was clearly different from that in the region away from the mesa (far mesa region) at least as much as was revealed by stain etchant. Since such a situation could have deleterious effects on a component, in this investigation we have attempted to study this phenomenon by analysing the orientation dependent

*Presen t address : S a n g u s , Box 5004, S-162 05 V~illingby, Sweden.

(Received December 12, 1990)

0361-5235/1991/1401-52355.00�9 TMS

growth behaviour of InP: Fe around mesas of different relevant shapes as revealed by stain etchant.

A mesa can be formed in different ways. Wet etching is a widely used technique because of its simplicity. Reactive ion etching (RIE), dry etching is a more promising technique for fabrication of, e.g. laser mesas because, unlike wet etching, it enables an accurate prediction of the shape and dimension of the mesa which is advantageous for large scale production. However, the regrowth of SI-InP: Fe has been so far exclusively combined with the wet etched mesas in almost all of the epitaxial techniques, see e.g. t-6 In the case of metal organic vapour phase epitaxy (MOVPE), regrowth of unintentionally doped InP has been achieved on entirely reactive ion etched mesas. 7 But whenever regrowth of SI-InP:Fe has been attempted to bury laser mesas in the case of device fabrication, an extra wet etching step was deemed necessary prior to growth, s-9 Liquid phase epitaxy (LPE) has been employed to bury the entirely RIE etched laser mesa but with a reversed p- n junction. '~ As regards vapour phase epitaxy (VPE), although it has shown a high growth rate, ease of regrowth around different shapes of mesas and grooves, 4-5'n-12 regrowth of SI-InP:Fe has not been reported on dry etched mesas. In the present investigation, we report for the first time the regrowth of SI-InP: Fe by HVPE around mesas fabricated by RIE. The regrowth parameters optimized in accor- dance with the orientation dependent growth behaviour led us to obtain a nearly homogeneous material in between the mesas: the triangles adherent to the mesa were also devoid of pits after staining. The results obtained on an embedded laser pertain- ing to this investigation are also presented.

2. EXPERIMENTAL P R O C E D U R E S

A. Mesa Format ion

Silicon nitride mask stripes 1.5-2 ftm wide and separated apart by 200 ~m were formed along the

524

(1,1,0) direction by a conventional lithographic technique on (0,0,1) InP wafers or on a laser structure grown on the same. RIE was achieved by meth- ane and hydrogen plasma at 150 mW and 15 mtorr. The gaseous composition w a s C H 4 : H 2 = 7:35 cm3/ min. This set of parameters yielded an etching time of 35, 60, 70 and 75 m i n / ~ m for InP, GaInAsP (Eg = 0.9 eV), GaInAsP (E~ = 0.85 eV) and GaInAs (E~ = 0.75 eV), respectively. The mesa geometry was rectangular in shape. By virtue of its geometry, this mesa exposes a horizontal ((0,0,1)) and two vertical ((1,-1,0) and (-1,1,0)) planes for regrowth. In this report, the surface which exhibits rectangular walls is denoted as R and shown in Fig. la.

When surface R was subjected to an etchant con- mining equal amounts of concentrated HC1 and water for 3 min at room temperature, a new pat tern was created: the vertical wall became more narrow (and to some extent less vertical) and two new slanting planes identified as (1,-1,1) and (-1,1,1) emerged. The lat ter planes add a t r iangular shape at the base of the mesa of R and hence this surface is denoted as TR and shown in Fig. lb.

B. VPE Regrowth

Regrowth was realized in a HVPE reactor suita- bly designed to grow SI-InP: Fe in an ambient of ni- trogen, the relevant details of which have been published elsewhere. 13 The patterned substrate was treated by oxygen plasma to remove traces of poly- mers caused by RIE and the subsequently formed indium oxide was removed by concentrated sul- phuric acid just before growth. The lat ter step did not etch away the bulk material sufficiently enough to be detected by scanning electron microscope (SEM) and hence the geometry of the mesa was not al- tered. Silicon nitride on top of the mesa stripes was not removed prior to regrowth to avoid complete overgrowth. Two sets of experimental parameters, namely A and B, employed for regrowth are given in Table I. They correspond to high FeC12 concentration and low growth rate and vice versa. Four different regrowth experiments were carried out: 1) one regrowth step for six min on surface R (a laser structure) with parameters B, 2) one regrowth step for 45s on surface TR (InP wafer) with parameters B, 3) a two consecutive step regrowth with parameters A and B for six rain each on surface R (InP wafer) and 4) a two consecutive step regrowth with parameters A for six min and with B for three min

a b Fig. 1 - - Cross-sectional depiction of mesas from a) surface R and b) surface TR. �9 represents silicon nitride mask.

Table I.

Hammarlund, Lourdudoss and Kjebon

Summary of Regrowth Parameters A and B Described in the Text

Parameters

Description A B

Temperature (~ In Source zone 740 740 Mixing zone 730 730 Deposition zone 685 685 Fe source zone 575 425 Preheat zone 690 690 Exhaust zone 675 675

Gas flow (cm3 /min) HCl(In) (100%) 3.5 8 PH3 (30% in H2) 50 110 HC1 (Fe) (100%) 0.15 0.8 N2 (carrier gas) 2000 2000

on surface R (InP wafer). The cleaved cross-sections of the resultant samples were stain etched under illumination for about 5s with a solution containing 1 gm of K3[Fe(CN)6] and 4 gm of KOH in 16 ml of water and thereafter investigated by SEM.

3. R E S U L T S A N D D I S C U S S I O N S

The SEM pictures of stain etched cross-section of the regrown samples corresponding to experiments 1 - 4 are shown in Figs. 2 -5 , respectively. In all the cases it is clearly seen that the mesa stripe is pro- tected from a complete overgrowth by the silicon nitride mask on top. Thus regrowth has taken place selectively on the opened surface. Referring to Fig. 2, one can clearly distinguish a portion of the regrown volume in the near mesa region to be different from that in the far mesa region. The near mesa region exhibits triangles of etched pits whereas no such tendency is observed in the far mesa region. It is well known that the stained cross-sections exhibit different pat terns depending upon the nature of the majority carriers in the material. Strongly stain etched n-type InP is easily recognized by its etched pit pattern. It is observed that the pit-filled triangles in the near mesa region strongly resem- bles stain etched n-type InP. Interestingly, the tri- angle adherent to the mesa is very nearly a right- angled one with its right angle pinned at the bot- tom of the mesa. The hypotenuse was identical with (-1,1,1) or (1,-1,1) plane. This phenomenon is discussed further below.

Figure 3 corresponds to the stained cross-section of surface TR after the regrowth experiment no. 2. Although horizontal, (nearly) vertical and slanting planes had been exposed for regrowth by surface TR, the figure shows that the growth initiated only on the (nearly) vertical planes and continued towards the <1,-1,0> direction; besides, noticeable growth took place neither on the horizontal plane nor on the slanting planes during the growth period of 45 s. The employed growth parameters, B should have yielded a growth rate of 20 /~m/hr on (0,0,1) plane as has been confirmed by a separate investigation.

Orienta t ion Dependent Growth Behaviour Dur ing Hydride VPE Regrowth of I n P : F e 525

Fig. 2 - - SEM micrograph of stained cross-section of surface R after regrowth experiment 1.

However, the (nearly) vertical planes were favoured in preference to the other planes and the growth rate on the former was estimated to be around 110 ~ m / hr. These results indicate that even in the case of surface R, the growth should have initiated on the vertical walls and continued towards the (1,-1,0) direction and presumably the initial growth rate should have been also very high. The regrown volume in Fig. 3 also exhibits a pitted surface after staining resembling n-type InP similar to the near mesa region of mesa R, see Fig. 2. It has been observed by Kuroda et al. that an increase in growth rate of InP:Fe resulted in a decrease in iron concentration and resistivity. ~4 In the light of their findings it can be proposed that the Fe content in

Fig. 4 - - SEM micrograph of stained cross-section of surface R after regrowth experiment 3.

the rapidly grown material (pitted-filled regions in Figs. 2 and 3) is less than that in the rest of the material. This can be understood from a chemical argument based on a qualitative analysis of the fol- lowing reactions:

InCl(g) + PH3(g) = InP(c) + HCl(g) + H2(g) (1)

FeC12(g) + H2(g) = Fe(c) + 2 HCl(g) (2)

(1) denotes the formation of InP in an ambient of N2 and (2) is responsible for Fe incorporation in the InP lattice. It is apparent that the local concentration of HC1 produced from (1) is high if the growth rate is high. Such a local enhancement in HC1 on the reacting surface can counteract (2) since it also involves HC1 as a product, thereby limiting the availability of Fe for lattice incorporation. As a result, the concentration of Fe in the fastly grown volume can be too low to electrically compensate the residual donors and hence the resemblance of this volume with n-type material. Based on this expla-

Fig. 3 - - SEM micrograph of stained cross-section of surface TR Fig. 5 - - SEM micrograph of stained cross-section of surface R after regrowth experiment 2. after regrowth experiment 4.

526 Hammarlund, Lourdudoss and Kjebon

nation, it can be agreed that the far mesa region being devoid of pits, see Fig. 2, its growth should have been slower than that of the near mesa region and slow enough to accommodate enough iron to impart semi-insulation. Indeed the resistivity of S1- InP:Fe grown on plain InP wafer with parameters B was greater than 1"10 s / / . cm. is Thus it is apparent that two growth rates are operative during regrowth around vertical mesas: a) rapid growth rate until the boundary set up by {1,-1,1} and (0,0,1) planes (i.e., the triangles) is attained and b) tran- sition to a slower growth rate across that boundary. This explains the origin of the pit-filled triangles. An alternate explanation for the reduction in Fe concentration in the high growth rate regime near the mesa could be a simply kinetically limited supply of Fe to the growing material. With these conditions, the concentration would be reduced to 20 t t m / h r / l l 0 t tm/hr = 18% of the planar concentration. This reduction could easily prevent InP from becoming semi-insulating. Essentially Fe is "di- luted" in a larger volume of the grown InP.

The above-mentioned facts suggest that the initial growth should be decelerated and/or the FeC12 concentration should be somehow enhanced in order to obtain a pit-free near mesa region. Both these factors have been taken into account in experiment 3. Figure 4 is the SEM micrograph of the stained cross-section of surface R after a two step regrowth experiment no. 3. In this figure, the pit-filled region is absent and only two different grey tones are observed. It is proposed that the grey tones are caused by two different concentrations of iron and may be explained by analyzing the experimental parameters used: six min of growth with A and another six min of growth with B. The sets of parameters A and B differ in the Fe source temperature and the partial pressures of PH3, InC1 and HC1 over Fe. It has been inferred from a separate investigation that pa- rameter A resulted in a growth rate which was five times less than that for parameters B, on the vertical planes. The decrease in growth rate with pa~ rameters A with respect to that of B was caused by the lower partial pressure of InC1 despite the con- stancy of III to V ratio in both A and B. As has been discussed above, the lower growth rate reduces the local HC1 concentration at the wafer surface and the concomitant increase of Fe source temperature in A enhances the partial pressure of FeC12,13 the combined effect of which should have increased the Fe dopant concentration in the near mesa triangular region, unlike in the case of experiments 1 and 2. It has been observed that with parameters A, prog- ress of further growth was virtually zero beyond the boundary of the triangles. This indicates that the initial growth on the vertical walls with parameters A is stopped at the {1,-1,1} planes. However it is evident from Fig. 4 that a total regrowth can be ac- complished with parameters B after the first step with A. It should be remembered that with B, a pit- free far mesa region was obtained even in the case of experiment 1.

The stained cross-section of R after a consecutive two-step regrowth with parameters A and B as described by experiment no. 4 is shown in Fig. 5. In this experiment as a difference from the previous experiment, the second regrowth step was termi- nated at a point where only 50% of the mesa was buried. The figure strongly suggests that if in the first step the triangles adherent to the mesa were formed as a result of growth propagation in the <-1,1,0> direction, in the second step the growth took place only in the <0,0,1> direction.

We have exemplified by this investigation the vi- ability of regrowth of InP:Fe on surface R by experiment 3, which yielded pit-free region around mesas. In order to check whether InP:Fe regrown around the dry etched mesa with such a procedure was actually electrically insulating, the lasing characteristics of a buried heterostructure (BH) laser, i.e. a laser embedded in InP:Fe was investigated. To this end, two configurations were considered: 1) a BH laser with a contact opening smaller than the mesa's width and 2) the same BH laser with a contact opening larger than the mesa's width. The SEM pictures of the stained cross-sections of these configurations are given in Fig. 6. If the regrown InP:Fe is not current blocking in both the cases, by virtue of metal contacting directly on InP:Fe, configuration (2) should be more leaky than configuration (1). On t h e other hand, if the embedding layer is insulating, then both the configurations should lase and their I-P behaviour should be nearly identical. Indeed both of them lased and their I-P curves are depicted in Fig. 7. From the nearly identical nature of their I-P curves, it is evident that no current leakage occurred through the embedding SI-InP: Fe. A high threshold current was expected due to the large magnitude of the mesa's width, the choice of which is justified in view of the aim of the present investigation. We extended our studies to a BH similar to configuration (2) but with a mesa width of 2 ttm and the results are presented in the form of current-voltage curves in Fig. 8. Curves a and b depict the I-V behaviour when experiment 1 and experiment 3 were employed for regrowth, respectively. The shape of b is clearly that of the laser diode in- dicating that the bias current flows through the p- n junction. Contrarily, the shape of a is indicative of the current bypassing the p - n junction to flow through the near-mesa material deprived of suffi- cient insulation. It should be pointed out that the I-V curves obtained as a result of experiment 1 (curves a) were not always identical even within the wafer, i.e. the extent of leakage was found to be varying whereas experiment 3 always resulted in ideal diode characteristics. The latter diode structure exhibited low parasitic capacitance and when modulated its cut-off frequency was around 7.5 GHz. Its more detailed performance data can be found elsewhere. 1~'16 All these results indicate that the S1- InP:Fe regrown by experiment 3 can yield good current confinement and low parasitic capacitance enabling high modulation.

Orientation Dependent Growth Behaviour During Hydride VPE Regrowth of InP:Fe 527

(a)

Fig. 7 -- Current-power (I-P) curves a and b corresponding to the BH-lasers depicted in Fig. 6a and 6b, respectively.

(b)

F i g . 6 - - SEM micrographs of stained cross-section of BH-laser mesa: a) contact opening smaller than the mesa's width and b) contact opening larger than the mesa's width.

4. CONCLUSION

When regrowth of InP: Fe around a dry etched mesa lying in the (110) direction was carried out, the growth rate on the {1,-1,0} planes was found to be about 110 ~m/hr . As revealed by stain etching under illumination, the rapidly grown material re- sembled n-type InP and such a resemblance has been attributed to the preclusion of Fe incorporation as- sociated with too high a growth rate. The fast growth initially propagating towards the (1,-1,0) direction was either virtually stopped at the {1,-1,1} planes or continued to grow with a slower different growth rate after the {1,-1,1} planes depending upon the growth parameters used in this investigation. These effects are proposed to be the origin of the n-type resembling pit-filled triangles adherent to the vertical mesas. However, an increase in the partial pressure of FeC12 with a concomitant decrease in the

Fig. 8 -- I-V curves of a BH-laser with regrowth conditions of experiment i (curve a) and experiment 3 (curve b).

528

growth rate of InP: Fe during the initial stage of growth, virtually eliminated n-type resemblance of these triangles. Such a procedure adopted to grow InP: Fe around RIE etched mesas was found to yield a homogeneous material in between the mesas. A buried heterostructure laser fabricated by employ- ing the above procedure was found to exhibit ex- cellent current confinement properties as well as high modulating performance.

ACKNOWLEDGEMENTS

It is a pleasure to thank Michael Rask and Peter Ojala for providing us with their MOVPE grown laser structure and Gunnar Landgren for valuable sug- gestions.

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orientation dependent growth behaviour during hydride vpe regrowth of inp:fe around reactive ion...

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