Manufacturing of the DFB grating

3.3 Development of the DFB grating 105 The duty cycle has been defined in section1.2.2 asc_d = (Λ−b)/Λ, where Λ is the grating period and b the width of the etched grooves. In Fig. 3.15 (a), the duty cycle is cd = 1/4. Fig. 3.15 (b) shows the DFB grating after overgrowth with AlGaAs.

It has been already mentioned, that the duty cycle of a second-order DFB grating must be c_d = 1/4orc_d= 3/4in order to achieve the maximum coupling coefficient from the structure. The final choice between this two possibilities can be therefore made with respect to the specific duty cycle, which will potentially enable a better charge carrier transport through the grating. From Fig. 3.15 (b) one can derive, that there are two regions with different numbers of hetero-interfaces and different layer thicknesses. Firstly, charge carriers can flow through the regions of the grooves of the grating.

In this case, carriers have to cross only two interfaces (AlGaAs/InGaP and InGaP/AlGaAs) and one thin layer of InGaP. Secondly, charge carriers can also flow through the grating ridges. In contrast to the first possibility, they would have to cross three hetero-interfaces (AlGaAs/GaAs, GaAs/InGaP and InGaP/AlGaAs) and two layers, one from GaAs and one from InGaP.

From the number of hetero-interfaces and from the thickness of the layers which have to be crossed by the carriers in addition to the unperturbed AlGaAs waveguide, it is assumed, that series resistance and series voltage might be lower, if the area of the grooves predominates the area of the grating ridges. In terms of the duty cycle, this means that c_d = 1/4 is expected to be the best choice and this case is in fact shown in Fig. 3.15.

[Ebe07]. We implement this in AlGaAs-based designs [Maa08] and in the GaAs/In_0.49Ga_0.51P-based grating design. The ≈ 290 nm-period photoresist line pattern which is required for the lithographic structuring of the DFB grating is generated by exposure with an ultraviolet line pattern from a krypton ion gas laser. In the following, the fabrication of the DFB grating is explained in detail and advantages and disadvantages of the used method are discussed.

After finishing the first epitaxy with the 25 nm GaAs cap layer of the grating, the wafers are taken out of the reactor. A positive photoresist is spin coated on top of the wafer. In the following process step, the photoresist is exposed with a ultraviolet light line-pattern with a period of≈290 nm. The line pattern is generated by the heterodyne of the split-of beam of a krypton ion gas laser at 351 nm. Afterwards, the photoresist is developed, so that a photoresist mask remains on the wafer surface as shown in Fig.3.16(a). Now, theGaAscap layer is patterned by selective etching with a solution ofC4H6O6

and H₂O₂, that stops on the In_0.49Ga_0.51P layer. When the photoresist is removed, a GaAs line pattern remains on the surface and is used as an etch mask or protection cap layer for the In0.49Ga0.51P as depicted in Fig. 3.16 (b). The photoresist is removed with a suitable photoresist remover (solvent).

Afterwards, the wafers are rinsed with water to complete the cleaning process.

Subsequently, the wafers are inserted back into the MOVPE reactor.

Within the reactor, the last etching step is applied with an in-situ etching technique. At first, the process temperature in the MOVPE reactor is in-creased to600^◦C. During the in-situ etching, the process temperature is kept at this value in order to prevent that theIn_0.49Ga_0.51Pgrating stripes, which are formed gradually during the etching process, can diffluence (indium atoms have a low binding energy) before they are overgrown withAl0.15Ga0.85Asand their shape is fixed. In-situ etching is accomplished with carbontetrabromide (CBr₄), which is conventionally used for p-doping with carbon. During the in-situ etching, the partial pressure of CBr4 is 0.5 Pa, group-V-partial pres-sures are 5 and 25 Pa for arsine (AsH₃) and phosphine (PH₃), respectively [Maa08]. The etching rates ofGaAs,Al_0.15Ga_0.85As[Maa08] andIn_0.49Ga_0.51P were experimentally determined as ≈200, ≈190 and ≈360 nm h⁻¹, respec-tively. The observed selectivity in the etching rates enables GaAs to act as an etch mask for the In_0.49Ga_0.51P. However, this is not a strict selectivity like the selectivity for etching GaAs with a solution of C4H6O6 and H2O2, which does not affect In_0.49Ga_0.51P at all. The shape of the DFB grating after the in-situ etching is depicted in Fig.3.16 (c). In the scheme it can be seen that the corrugation is transferred or molded into theIn0.49Ga0.51Player (≈360 nm h⁻¹). Simultaneously, the thickness of theGaAs cap stripes is re-duced as well (≈200 nm h⁻¹). Three different in-situ etching times (t_is= 90

3.3 Development of the DFB grating 107 ,120 and 150 s) have been applied to different wafers in order to achieve dif-ferent etching depths into the In_0.49Ga_0.51P layer. The expected changes in the shape of the resulting DFB grating will be discussed later in this section in more detail.

After finishing the ex-situ and in-situ structuring, a second epitaxy is applied on top of the grating surface. This epitaxial overgrowth starts with Al_0.15Ga_0.85As in order to finish the p-type waveguide. Subsequently, the epitaxial growth is finish with the cladding and contact layer. The re-growth on top of the patterned surface is a critical process during the manufacturing of buried gratings. In the beginning of the re-growth, the growth temperature must be chosen low enough to ensure that especially indium atoms are not able to migrate too much. Otherwise, the grating ridges would diffluence during the overgrowth. This requirement is fulfilled by starting the re-growth at ≈ 600^◦C, then, the process temperature is increased to the final growth temperature during the first 50 nm above the grating interface. At this low process temperatures, the growth rate must be very low to achieve a defect-free semiconductor crystal. Figure3.16 (d) shows a scheme of the overgrown double layer GaAs / In0.49Ga0.51P DFB grating with a duty cycle of 1/4, embedded in the Al_0.15Ga_0.85As waveguide.

As already mentioned above, three different in-situ etching timest_is = 90, 120 an 150 s have been chosen. This variation will lead to wafers with three different DFB gratings. The differences of these gratings are the thickness of theGaAscap stripesd_GaAs, the etch depth into theIn_0.49Ga_0.51Playerd_InGaP and the thickness of the residual In0.49Ga0.51P layerrInGaP. With increasing etching-time, the etching depth into the In_0.49Ga_0.51P layer increases, while the thickness of the residual In_0.49Ga_0.51P layer decreases. The sum of both values remains constant: dInGaP+rInGaP = 20 nm. Also thickness of theGaAs cap stripes decreased with increasing etching time. From the specific indices of refraction for this materials at 975 nm it follows, that the index contrast between the ridges of the grating and the grooves and thus, the coupling coefficient of the DFB grating, increases with the etching time.

Schemes for DFB gratings, fabricated with the three different in-situ etch-ing times tis= 90 , 120 an150 s, mentioned above, are depicted in Fig. 3.17.

In Fig. 3.17 (a), (b) and (c) the upper scheme shows the pre-structured and cleaned surface before the in-situ etching. In the lower scheme, the shape of the grating after the in-situ etching is depicted. In the small boxes be-low, the specific values for the thickness of the GaAs cap stripes d_GaAs, the etch depth into the In_0.49Ga_0.51P layer d_InGaP and the thickness of the resid-ual In0.49Ga0.51P layer rInGaP is given, simply calculated on the basis of the experimentally determined etching rates.

Additionally, higher material removal with increasing in-situ etching time

Figure 3.16: Schematic illustration of the processing of the DFB grating. (a) A mask of photoresist is structured on top of the grating layers. (b) The mask is transferred via selective etching into the GaAs cap-layer. (c) The GaAs grating is transferred with a non-selective in-situ etching into theIn_0.49Ga_0.51P layer. (d) The waveguide structure is finished with a second epitaxy step.

will presumably lead to a re-growth interface with a lower impurity density.

Note that with increasing in-situ etching time, the duty-cycle of the grating

3.3 Development of the DFB grating 109

Figure 3.17: Scheme of the DFB grating before and after the in-situ etching for three different in-situ etching times t_is. (a) t_is = 90 s. (b) t_is = 120 s. (c) t_is= 150 s. In the small boxes below, the thickness of theGaAscap stripesd_GaAs, the etch depth into theIn0.49Ga0.51PlayerdInGaPand the thickness of the residual In_0.49Ga_0.51Player r_InGaP is given.

decreases as well. This occurs, because the etching does not only occur perpendicular to the wafer surface. With respect to the in-plane etching, the width of the photoresist stripes must be chosen specifically for every in-situ etching time in order to achieve a duty-cycle of1/4in the final re-grown DFB grating. This is also suggested in the upper schemes of Fig.3.17(a), (b) and (c), where the width of theGaAscap stripes, which depends on the width of the photoresist stripes, increases with increasing target value for the in-situ etching time.

Fig.3.18 shows a scanning tunneling electron microscope (STEM) micro-graph of a buried DFB grating, which has been structured and overgrown as explained above, using an in-situ etching time of t_is = 120 s. Four grating stripes are visible in the shown region, which extends over ≈1150 nm along the grating interface and ≈ 100 nm below and above the grating in growth direction, respectively. The structuredIn_0.49Ga_0.51Player (white) has a

thick-ness of≈ 20 nm at the grating stripes and a etching is ≈13 nm deep in the grooves. A ≈ 7 nm thin residual layer of In_0.49Ga_0.51P is left over between the grating stripes. GaAs cap layer (dark gray) is found to have a thickness of ≈ 15 nm. The layer thicknesses and the angled edge of the grating are depicted in the inset.

Figure 3.18: SSTEM micrograph of a buried DFB grating with ≈ 15 nm GaAs cap-layer,≈20 nm In_0.49Ga_0.51Player and 120 sin-situ etching time. The edge of one grating filament and the corresponding layer thicknesses are depicted in the inset. Variations in the aluminum concentration are visible on top of the grating corrugations as slightly contrasts.

The duty cycle of the shown DFB grating is slightly larger than the targeted value of c_d = 0.25. With respect to the fact, that the side faces of the DFB grating are angled and the width of the grating stripe is broader at the basis and narrower on top, an average duty cycle ofcd ≈0.32can be estimated from the STEM measurements.

In the waveguide directly above the grating, a weak contrast structure provides an indication of variations in the aluminum concentration. Such for-mation of aluminum-rich and aluminum-poor regions which is observed in the regrownAl_0.15Ga_0.85Ashas been already reported in [Hof01] and [Bug11]. Self organized changes in the semiconductor material composition during over-growth of patterned surfaces have been also reported for regrownInGaAs(P) [Koo00]. The self-organized variations in the aluminum concentration result in an additional AlxGa1−xAs grating [Bug11] and therefor alter the overall refractive index contrast of the grating region and hence, they alter the cou-pling coefficient. Therefore, this effect will be discussed in more detail in the

3.3 Development of the DFB grating 111 following section 3.3.3.

The reason, why in-situ etching is applied to the DFB grating is, that this technique is expected to enable the lowest possible contamination with oxygen and other impurities in the grating region after epitaxial overgrowth.

When the ex-situ pre-structured wafers are transferred back into the MOVPE reactor, a thin layer of surface oxides covers the surface which have developed under the presence of atmospheric oxygen. Depending on the used in-situ etching time, the first ≈5- 15 nm(for GaAsand In_0.49Ga_0.51P with t_is = 90 - 150 s) of the semiconductor surface (containing the surface oxides), are removed by in-situ etching under conditions, where nearly no new oxygen atoms are available to be incorporated into the semiconductor crystal. This is done before regrowth begins.

As already mentioned above, wafers with three different in-situ etching times t_is = 90, 120 and 150 s were fabricated for the fabrication of diode lasers, which will be characterized subsequently. A further series of wafers, fabricated with different in-situ etching times t_is = 90, 120, 150 and 180 s has been used to investigate possible differences in the material properties in the grating region. Secondary ion mass spectroscopy (SIMS) has been used for material-analysis, which allows to detect the material composition of a semiconductor layer system and also the existence and percentage of impurities like oxygen. Therefore, a hole is sputtered into the surface of the probe with a focused primary ion beam and secondary ions are then col-lected and analyzed with a mass spectrometer. The information about the etch depth can be used to scale the vertical position in the epitaxy structure.

In Fig.3.19, the oxygen concentration (calibrated) and the aluminum concen-tration (un-calibrated, shown to visualize the vertical position in the epitaxy structure) are depicted as functions of the etch depth. For orientation, the p-type cladding layer ends at ≈ 0.60µm(≈ 1·10⁵counts s⁻¹ of aluminum), were thep-type waveguide begins. Thep-type waveguide (≈6·10³counts s⁻¹ of aluminum) extends from ≈0.60µmtill ≈1.35µm, were the DQW active region can be observed as a sharp dip in the aluminum concentration. The DFB grating is visible in the aluminum concentration as a weak variation at ≈ 0.80µm. Please note, that the etch depth, used as x-axis is not very precise (∼100 nm resolution).

The oxygen concentration reaches its background level in the p-type cladding layer and waveguide at. Note, that the background level of the SIMS for oxygen is calibrated only for the waveguide region and depends on the aluminum concentration in Al_xGa1−xAs. With a background level of

≈5·10¹⁵cm⁻³, the detection limit is better than one oxygen atom per four million atoms in the Al_0.15Ga_0.85As crystal.

In the grating region at ≈0.80µm, the peak hight of the oxygen signal is

0 . 6 0 . 8 1 . 0 1 . 2 1 . 4 1 0 ^{1 5}

1 0 ^{1 7} 1 0 ^{1 9} 1 0 ^{2 1}

DQW

in-situ etching time t_is 90s 120s

150s 180s oxygen [atoms cm-3 ]

depth [µm]

(a) DFB grating

1 0 ⁰ 1 0 ¹ 1 0 ² 1 0 ³ 1 0 ⁴ 1 0 ⁵

aluminium [counts s-1 ]

Figure 3.19: SIMS depth profiles of oxygen volume impurity density (calibrated) and aluminum content (un-calibrated) after different in-situ etching timestis= 90, 120,150and 180 s.

found to be a function of the in-situ etching timetis. At tis= 90 s, the peak oxygen contamination is≈1.17·10¹⁹cm⁻³. For an increasing in-situ etching time, the peak oxygen signal decreases to ≈ 4.48·10¹⁷cm⁻³ at t_is = 120 s,

≈9·10¹⁶cm⁻³ attis = 150 sand finally ≈8·10¹⁶cm⁻³ attis = 180 s. Admittedly, the SIMS resolution is too low to determine if the oxygen is distributed over a range along the growth direction or whether it is located at a sharp interface. Therefore it is favorable to quote both the peak volume impurity densityρ_peak and the calculated areal impurity densityρ_areal, which has been obtained by integrating over the SIMS profiles in the grating region.

Both values are summarized in table 3.4. The areal impurity density must be understood as lower limits.

An in-situ etching time of t_is = 120 senables to reduce the peak volume impurity density (or areal impurity density) significantly, compared to the values at t_is = 90 s. In-situ etching times t_is ≥ 150 s are sufficient to reduce the oxygen contamination to ρ_peak ≤ 1·10¹⁷cm⁻³ or ρ_areal ≤ 3·10¹¹cm⁻². This should be preferred for the fabrication of DFB gratings for DFB-BA lasers in later work. If the oxygen contaminations sufficiently reduced to achieve the same internal optical loss as it has been found for the reference FP-BA lasers in section 3.1, must be investigated subsequently. Further answers to the question, if the oxygen contaminations sufficiently reduced, will be obtained by the measurement of P-I characteristics and comparison to the P-I characteristics from reference FP-BA lasers. Certainly, using in-situ etching techniques enabled a remarkable reduction of the oxygen impurity contamination compared to buried DFB gratings, which were fabricated only

3.3 Development of the DFB grating 113

Table 3.4: Peak volume impurity densityρpeakfrom the oxygen signal of the SIMS and calculated areal impurity densityρ_areal for different in-situ etching timestis.

tis ρpeak ρareal

[s] [cm⁻³] [cm⁻²]

90 1.2·10¹⁹ 2.4·10¹³ 120 4.5·10¹⁷ 1.6·10¹² 150 9.2·10¹⁶ 3.0·10¹¹ 180 7.7·10¹⁶ 2.6·10¹¹

with ex-situ etching and have for example, peak volume impurity density of ρ_peak ≈4·10¹⁸cm⁻³ [Bug11].