Ohmic Contacts on N-Face n-Type GaN After Low ...

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Luca Redaelli, Anton Muhin, Sven Einfeldt, Peter Wolter, Leonhard Weixelbaum, and Michael Kneissl. Abstract— The electrical properties of different metal.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 13, JULY 1, 2013

Ohmic Contacts on N-Face n-Type GaN After Low Temperature Annealing Luca Redaelli, Anton Muhin, Sven Einfeldt, Peter Wolter, Leonhard Weixelbaum, and Michael Kneissl

Abstract— The electrical properties of different metal systems for ohmic contacts on the nitrogen-face of c-plane n-type GaN substrates are investigated. The metal contacts are compatible with the fabrication process and the packaging technology for group III-nitride laser diodes. The metal system Ti/Al/Mo/Ti/Ni/Au/Ti/Pt is determined as the best suitable, since it is ohmic already after annealing at a temperature of 450 °C for 60 s. This annealing temperature is high enough to make the contact insensitive against later soldering on a heat-sink at 330 °C. At the same time, the temperature is low enough that the Pd-based p-contact, previously annealed at 530 °C, does not degrade. In addition, the Ti/W/Al and Pd/Ti/Al metal systems form low-resistance ohmic contacts, too, although they require a longer annealing time of several minutes or a higher temperature of 500 °C. Index Terms— GaN laser diodes, contacts N-face GaN, GaN wet-chemical etching, TMAH.

to

n-GaN,

Fig. 1. Current-voltage characteristics of RW laser diodes with a ridge width of 1.5 μm and a resonator length of 600 μm. The foot print of the chip, i.e. the area of the n-contact, is given in parenthesis.

VER the last years, a lot of effort has been spent in the development and optimization of GaN-based laser diodes (LDs), due to applications such as optical storage, sensing and displays [1]. In this letter we report on the optimization of electrical contacts to nitrogen-face, n-type GaN, to be used in c-plane edge-emitting laser diodes with vertical current injection. In this kind of devices, fabricated on bulk GaN substrates, the p-contact is deposited on top of the MOVPE-grown epitaxial layer stack (gallium-face, (0001)) and annealed in the first stages of the chip fabrication process. The n-contact is deposited at the very end of the fabrication process on the backside of the GaN substrate (nitrogen-face, ¯ after wafer substrate thinning. (0001))

Contacts to N-face n-GaN show very different properties from contacts to Ga-face n-GaN. This behavior is usually attributed to the different direction of the internal polarization fields, which influences the surface band-bending and therefore the contact properties [2]–[4]. Several metal systems have been proposed as N-face contacts in light-emitting diodes (LEDs) [5]–[8] or transistors (HEMTs) [9], [10]. In all cases the contacts are fabricated on smooth epitaxial layers or on polished surfaces; here, we study contacts on rough GaN surfaces as provided from substrate lapping. This is our standard for laser diodes, as long as polishing has not been proven to be advantageous for the device performance. On the contrary, our experiments show that a rougher surface improves metal adhesion and makes handling easier during facet cleaving, dicing and mounting.

Manuscript received January 28, 2013; revised March 14, 2013; accepted April 26, 2013. Date of publication May 6, 2013; date of current version June 18, 2013. This work was supported in part by the German Federal Ministry of Education and Research through the innovative regional growth core “Berlin WideBaSe” under Grant 03WKBT03B. L. Redaelli, S. Einfeldt, P. Wolter, and L. Weixelbaum are with the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Berlin 12489, Germany (e-mail: [email protected]; sven.einfeldt@ fbh-berlin.de; [email protected]; leonhard.weixelbaum@ fbh-berlin.de). A. Muhin was with the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Berlin 12489, Germany. He is now with the Technische Universität Ilmenau, Ilmenau 98693, Germany (e-mail: anton.muhin@ tu-ilmenau.de). M. Kneissl is with the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Berlin 12489, Germany, and also with the Institute for Solid State Physics, Technische Universität Berlin, Berlin 10623, Germany (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2013.2261808

In ridge waveguide (RW) LDs the n-contact usually covers the whole wafer backside, while the p-contact is only a narrow stripe on the frontside. This implies that the n-contact area is by far larger than the p-contact area, and it may be speculated that its contribution to the overall device resistance is negligible. However, this is not always the case. Fig. 1 shows three current-voltage curves successively measured on the same RW laser diode. In this case a non-alloyed Ti/Al/Mo/Au contact was used as n-contact, which is quite common for ohmic contacts on Ga-face n-GaN [9], [10]. The first measurement was performed on a 6 mm-long laser bar. The bar was diced in 400 μm-wide single chips for the second measurement. A small increase in the voltage is visible, which is attributed to an increased series resistance due to the reduction of the backside contact area. The third measurement was

I. I NTRODUCTION

O

II. E XPERIMENT

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TABLE I M ETAL S YSTEMS IN T EST Metal Layers Ti/Al/Mo/Au Pd/Ti/Al In/Ti/Al Ti/W/Al Ti/Al/Mo/Ti/Ni/Au/Ti/Pt

Respective Thickness (nm)

Source

10/50/20/200 5/30/500 200/50/200 5/10/200 10/50/20/10/50/20/10/50

our study [5] [7] [8] our study

done after soldering the chip p-side-up on a metalized ceramic submount with a AuSn-alloy. The soldering process was performed at a temperature of 330 °C and lasted less than 30 s. Nevertheless, the heating was strong enough to degrade the backside n-contact and further increase the series resistance. In this example the n-contact area of the single chip is about 260 times larger than the p-contact area. The specific resistance of the p-contact is in the range of 10−3 cm2 . For simplicity reasons, we assume large current spreading in the n-type substrate, thus a nearly homogeneous current flow through the whole backside contact. If the specific resistance of the n-contact was in the same order of magnitude as the p-contact or below, its influence on the operating voltage would be negligible. Therefore, the main goal of this letter will be to develop ohmic contacts on N-face GaN with a specific resistance of < 1 × 10−3 cm2 . The precise value of the specific resistance will be of secondary importance, as long as it satisfies this condition. To avoid the voltage increase due to soldering, a contact metal system which does not degrade at temperatures around 330 °C for about 30 s is needed. Therefore, the contact should be annealed before soldering at higher temperature and/or for longer time. On the other hand, n-contact annealing should not degrade the previously fabricated p-contact, which was annealed at 530 °C for 5 min. Thus the target window for the n-contact annealing process is 330 °C < T < 530 °C, 30 s < t < 5 min. Three of the most promising contact systems published recently were selected for this study: Pd/Ti/Al [5], In/Ti/Al [7], and Ti/W/Al [8]. The reference contact Ti/Al/Mo/Au, and Ti/Al/Mo/Ti/Ni/Au/Ti/Pt (abbreviated: Ti/Al...Pt), were investigated as well. In our previous experiments, these two last metal systems have proven to build good ohmic contacts on n-type Ga-face GaN (unpublished). An overview of the contact metals and the respective thicknesses is given in Table 1. The contacts were fabricated on the backside of bulk cplane GaN wafers provided by the company LUMILOG Saint Gobain Crystals, with typical carrier concentrations of 2 × 1018 cm−3 . After wafer thinning by lapping with boron carbide (grain diameter 8–10 μm), two different surface preparation schemes were applied. Five wafers were dry-etched in a short Cl-plasma, which removes the top 400 nm of the surface without significantly changing the topology. Dry etching supposedly generates donorlike surface defects, which improve the contact properties [11]. Another three wafers were wetchemically etched for a few minutes in boiling tetramethylammonium hydroxide (TMAH) solution (at about 90 °C). TMAH is a strong base which selectively etches the N-face

Fig. 2. Scanning electron microscopy images of a lapped N-face GaN surface before (a) and after (b) etching in TMAH solution.

GaN surface without attacking the Ga-face surface, in a very similar way as already reported for KOH [12], [13]. By means of TMAH etching the surface of the semiconductor, damaged by lapping and therefore compressively strained, is removed. In this way the wafer bow can be reduced from a wafer radius of 0.7 m to more than 2 m for 200 μm thick wafers. Wetchemical etching of the N-face is also supposed to improve the contact properties, as recently reported [14]. A comparison of the wafer surface topology, immediately after lapping and after TMAH etching, is shown in Fig. 2. Linear TLM structures consisting of six 100 μm by 460 μm metal pads were defined by photoresist lift-off on all wafers. Pad distances were 6 μm, 8 μm, 10 μm, 15 μm, 20 μm. Immediately before metallization, the samples were etched for 30 s in a HCl/H2 O (1:1) solution to remove surface oxides. The metal layers were deposited by e-beam evaporation at a base pressure < 2 × 10−6 mbar. After metal lift-off the contacts were alternately annealed in nitrogen atmosphere and characterized. Annealing was performed in oven for 5 min at 200 °C and 300 °C. For temperatures > 300 °C, a rapid thermal annealing (RTA) equipment was used. The annealing time was 60 s for the In/Ti/Al contacts and 30 s for all other contact systems, according to literature data (when available) [5], [7]. After each annealing step the current-voltage (I-V) characteristics were measured in two-point geometry between two neighboring metal pads prior to the TLM measurements at different currents (0.1 mA, 1 mA, 5 mA). III. R ESULTS AND D ISCUSSION The behavior of the sample group with the dry etched surfaces will be discussed first. Fig. 3 shows typical I-V characteristics after annealing at different temperatures: as-deposited, 300 °C, 450 °C and 500 °C. Comparing the as-deposited contacts, only In/Ti/Al and Ti/W/Al form ohmic contacts, all the other systems are more or less nonlinear. It is worth noting that even the reference contact (Ti/Al/Mo/Au) is not ohmic, which could explain the previously discussed voltage measurements on RW laser diodes (cf. Fig. 1). At 300 °C all contacts are degraded and show non-linear characteristics (Fig. 3b). Significant improvement is only seen at 450 °C and 500 °C, where some of the contact systems become ohmic (Figs. 3c and 3d). As long as the contacts are not ohmic, it is not possible to derive a specific contact resistance. To visualize the evolution of the contact performance with increasing annealing temperature, the voltage at

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Fig. 5. Voltage at 90 mA measured in two-point geometry for different annealing times at 450 °C for the three selected contact systems. Circled points correspond to linear I-V characteristics.

Fig. 3. Typical I-V characteristics of the tested contact systems annealed at different temperatures. (In (d), the apparent non-linearity of the curve between 90 mA and 100 mA is a measurement artifact.)

Fig. 4. Voltage at 90 mA measured in two-point geometry over annealing temperature for the five different contact systems. Circled points correspond to linear I-V characteristics.

a fixed current (90 mA) in two-point measurements is used, as shown in Fig. 4. The voltage drop measured in this way is: Vtot = I (2 × Rcontact + Rsheet ) where Rcontact is the contact resistance and Rsheet the sheet resistance. Since the current and the sheet resistance are in all cases the same, the total voltage drop is a reasonable figure of merit for the contact resistance. From Fig. 3 and Fig. 4 some clear trends can be identified. The reference contact Ti/Al/Mo/Au is not ohmic over the whole annealing temperature span. The In/Ti/Al contact is ohmic in the as-deposited state, as reported in literature [7], but it steadily degrades with increasing temperature. The formation of large (several tens of μm) clusters or bubbles after annealing at temperatures above 350 °C is also visible

under an optical microscope. Both metal systems are therefore not suitable for LDs. The three other metal systems form linear, low-resistance contacts after annealing at 450 °C (Ti/Al...Pt) and 500 °C (Pd/Ti/Al and Ti/W/Al), respectively. Both temperatures are inside the previously defined process window. The TLM data of the ohmic contacts were found to scatter heavily across the wafer. This effect is attributed to the very rough, irregular GaN surfaces (cf. Fig. 2a). TLM measurements require a well-defined length of the current path between neighboring contact pads. If the surface is rough or cracked, the path length may vary in an unpredictable manner and the determination of a precise contact resistance becomes difficult. Nevertheless, we identified several TLM-structures on each wafer with a reasonably linear correlation between the measured resistance and the contact spacing (correlation coefficient of the fit: 0.8 to 0.9). From all these structures, specific contact resistance values in the range 10−6 cm2 to 10−5 cm2 were obtained. Similar values were also reported by other groups [5], [8]. Therefore, we consider the evaluation of our limited TLM data as reliable, at least to conclude that the specific contact resistance is below the target value of 1 × 10−3 cm2 . The influence of the annealing time was studied as shown in Fig. 5. The three most promising metal systems were chosen from the previous experiment and the annealing temperature was set to 450 °C. This time the Ti/Al...Pt contact needed 60 s annealing, to become ohmic; this difference is probably caused by the cumulative effect of all annealing steps in the first experiment. Ti/W/Al and Pd/Ti/Al become linear only after 8 min annealing. A detailed analysis of the In/Ti/Al, Ti/W/Al, and Pd/Ti/Al contact systems can be found in literature [5], [7], [8], therefore it will not be further discussed here. Moreover, it has been observed in previous experiments (unpublished) that the best contact system, i.e. Ti/Al...Pt, shows a far smoother surface than the reference system Ti/Al/Mo/Au after high temperature annealing. It is believed that the topmost Pt layer acts like a “capping” layer, hindering the formation of large metal clusters during annealing. Presumably, this effect is important for the formation of good ohmic contacts on N-face n-GaN. The second group of samples, whose GaN surfaces were wet-chemically etched by TMAH, was processed and

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step at temperatures well above 330 °C assures contact stability during device mounting. At the same time, the moderate annealing temperatures allow the n-contact formation at the end of the fabrication process, i.e. after wafer thinning, without degrading the previously annealed p-contact. ACKNOWLEDGMENT The authors would like to thank A. Dounia for technical assistance. Fig. 6. Voltage at 90 mA measured in two-point geometry for different annealing temperatures for the three selected contact systems on wetchemically etched GaN surface. Circled points correspond to linear I-V characteristics.

measured in the same way. In this experiment again only the three most promising contact systems were investigated. Furthermore, the long annealing steps at 200 °C and 300 °C were skipped. The results are summarized in Fig. 6. The contact behavior on the TMAH etched surface is quite different from the one on the dry-etched surface, although some common trends can be identified. In the as-deposited state, the voltage at 90 mA is generally higher for the wetchemically etched samples in comparison to the dry-etched samples. In this experiment, too, the contact performance improves above 450 °C, however ohmic contacts are obtained only with the Ti/Al...Pt and Ti/W/Al metal systems above 530 °C, which is beyond the defined process window. For this reason, it is concluded that TMAH wet-chemical etching is not a viable solution to reduce the wafer bow after thinning, because it is difficult to fabricate ohmic contacts on the etched surfaces. However, further investigations would be necessary to confirm this conclusion. The differences in the performance of contacts on wet-chemically-etched and dry-etched surfaces may result from inhomogeneous metal layer thicknesses on the extremely rough surface due to shadowing effects. IV. C ONCLUSION In summary, the properties of several contact systems on N-face n-GaN after surface lapping were investigated. Plasma etching and wet-chemical etching prior to metallization, which can be used to reduce the wafer bow resulting from lapping, were compared. On wet-chemically etched surfaces ohmic contacts could be fabricated, but annealing temperatures as high as 530 °C were needed. On plasma etched surfaces, instead, three different metal systems were identified that are ohmic and low resistive after annealing at moderate temperatures of 450 °C or 500 °C. These contacts are suitable as n-type backside (N-face) contacts for laser diodes on c-plane GaN substrates. The annealing

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