Key Laboratory for Information Science of Electromagnetic Waves (MoE), School of Information Science and Technology, Fudan University, Shanghai 200438, China
【AIGC One Sentence Reading】:This study achieves low-resistance Ohmic contact for GaN-based lasers using In0.15Ga0.85N layer & Pd/Au electrode, reducing resistance to 10⁻⁵ Ω·cm² level.
【AIGC Short Abstract】:This study introduces an In0.15Ga0.85N contact layer for GaN-based laser diodes, significantly reducing specific contact resistance to 2.57 × 10⁻⁵ Ω·cm². The Pd/Au metal stack further lowers resistance to the 10⁻⁵ Ω·cm² level. Auger electron spectroscopy explains resistance variation under annealing.
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Abstract
Low-resistance Ohmic contact is critical for the high efficiency GaN-based laser diodes. This study investigates the introduction of the In0.15Ga0.85N contact layer on the specific contact resistance. Experimental results reveal that adopting the In0.15Ga0.85N contact layer yields a minimized specific contact resistance of 2.57 × 10?5 Ω·cm2 which is two orders of magnitude lower than the GaN contact layer (7.61 × 10?3 Ω·cm2). A decrease in the specific contact resistance arises from the reduction of the barrier between the metal and p-type In0.15Ga0.85N. To develop an optimal metal electrode combination on the In0.15Ga0.85N contact layer, the Pd/Au and Ni/Au electrode stacks which are most commonly used in the formation of Ohmic contact with p-GaN are investigated. Metal stack of 10/30 nm Pd/Au is demonstrated effective in reducing the specific contact resistance to 10?5 Ω·cm2 level. The mechanism of the variation of the specific contact resistance under different annealing atmospheres is explained by auger electron spectroscopy.
1. Introduction
In the last decade, GaN-based laser diodes have garnered significant attention owing to their potential applications in various fields such as laser displays, laser lighting, laser storage, and visible light communication[1−4]. These applications require higher output power, enhanced thermal stability, and increased modulation bandwidth[5, 6]. One of the critical challenges lies in achieving a balance between higher injection current density and lower series resistance. Effective solutions are crucial for addressing this challenge. During the fabrication of GaN laser diodes, electrode deposition and annealing play crucial roles in achieving the desired low series resistance and optimal wall-plugging efficiency[7−11].
In Ⅲ−nitride materials, there are substantial Schottky barriers between GaN and its corresponding metal electrode. This challenge becomes particularly pronounced when a positive bias is applied to the p−n junction, leading to the negative biasing of two Schottky barriers. To enable efficient current injection from the metal electrode, it becomes essential to establish an Ohmic contact at the interface. Researchers often chose Ti/Al as the n-electrode. Such material combination has demonstrated Ohmic contacts with n-GaN[12−14], resulting in specific contact resistance levels as low as 10−6 to 10−8 Ω·cm2.
However, research about the metallization process with p-GaN has yet to achieve a satisfying contact resistance value. Studies have showcased that high work function materials like Ni, Ti, Pd, among others, can effectively help to reduce the Schottky barrier height at the interface of metal with p-GaN. Some researchers put up with electrode schemes such as Ni/Au[15−18], Pt/Au[19], Cr/Au[20], Pd/Au[20−22], Pt/Ni/Au[23], Ti/Pt/Au[24], etc. Another method to reduce the Schottky barrier height is to change the composition of the contact layer[25]. In this letter, we proposed a distinct method to reduce the specific contact resistance of the p electrode by replacing the GaN contact layer with the In0.15Ga0.85N contact layer. Furthermore, two commonly used electrodes Pd/Au and Ni/Au were chosen as the electrode to interpret the influence of containment of In on the contact resistance.
2. Experimental procedure
The p-GaN epitaxial layer was grown on a sapphire substrate using metal organic chemical vapor deposition (MOCVD) technique. Samples are divided into group Ⅰ and group Ⅱ. The layer structure of both groups, as depicted in Fig. 1(a), comprises a 4 μm unintentionally doped GaN buffer layer at the bottom, followed by a 20 nm AlGaN layer with a doping concentration of 1 × 1020 cm−3, a 500 nm AlGaN/GaN superlattice with a doping concentration of 1 × 1019 cm−3, and a 25 nm p-doping contact layer with a doping concentration of 5 × 1020 cm−3. This epitaxial structure closely resembles the epitaxial layer used in the laser diodes. Differences in these two groups lie in that the contact layer of group Ⅰ is GaN and the contact layer of group Ⅱ is In0.15Ga0.85N. To ensure the quality of the epitaxial material, the In fraction is set at 0.15. To remove the surface oxide layer which hinders the carrier transportation from metal to semiconductor[23], the surface of the p-type GaN/In0.15Ga0.85N was treated with peroxymonosulfuric acid for 10 min, hydrofluoric (1% concentration) for 30 s and hydrochloric acid for 10 min before being rinsed in deionized water. The average sheet resistance of the sample with GaN contact layer and In0.15Ga0.85N contact layer by the non-contact Hall test are 48.90 and 49.13 Ω/□.
P-electrode
Composition
Metal stack-1
Pd/Au: 10/30 nm
Metal stack-2
Pd/Au: 20/30 nm
Metal stack-3
Pd/Au: 30/30 nm
Metal stack-4
Ni/Au: 10/30 nm
Metal stack-5
Ni/Au: 20/30 nm
Metal stack-6
Ni/Au: 30/30 nm
Table 1. Detail of the electrode film composition.
Figure 1.(Color online) (a) Schematic of epitaxial layer structure with GaN contact layer (left) and In0.15Ga0.85N contact layer (right). The composition and doping concentration of each layer were labeled. (b) Microscope photo of the fabricated samples with c-TLM pattern. (c) SEM imagine of the Pd/Au = 10/30 nm electrode. Insert of the figure is the SEM imagine of the etched cross section.
For sample group Ⅰ andⅡ, we fabricated the different metal stacks summarized in Table 1. The Pd thickness is 10, 20, and 30 nm for stack 1, 2, and 3, respectively. The thickness of Ni layers is 10, 20, and 30 nm for stack 4, 5, and 6, respectively. The thickness of Au is kept at 30 nm.
To assess the specific contact resistance, the circular transmission line method (c-TLM) pattern with an inner radius of 100 μm was defined using a negative photoresist. Spacing between the metal electrodes varied from 5 to 50 μm. A schematic of the c-TLM pattern is illustrated in Fig. 1(b). The Pd/Au and Ni/Au metal electrodes were deposited using the MSP-300B automatic magnetron sputtering coater, after being masked with a photoresist. The lift-off process was finalized by subjecting the samples to ultrasonic cleaning in acetone. Fig. 1(c) shows the scanning electron microscope (SEM) imagine of the Pd/Au = 10/30 nm electrode. Insert of the Fig. 1(c) is the SEM imagine of the etched cross section.
After the deposition, the samples were rapidly thermal annealed by AccuThermo AW610 rapid thermal processor at a temperature of 600 °C for 2 min under N2 : O2 = 4 : 1 or only N2 atmosphere to form Ohmic contact based on past experimental experience.
3. Results and discussion
We measured the current−voltage (I−V) characteristic curves by using a Keithley 2450 source measurement unit. The total resistance was obtained through the fitting of the I−V curves using the least square method.
Fig. 2(a) presents the I−V characteristics of the sample with In0.15Ga0.85N contact layer using the Pd/Au = 10/30 nm electrode annealing in N2 : O2 = 4 : 1. Fitting of the I−V curve helps to define the total resistance. Total resistance varied with spacing between the metal electrodes is shown in the insert of Fig. 2(a). The relationship between the total resistance with ln(R/r) is plotted in the inset, where ln means the logarithm, R is the outer circle radius and r is the inner circle radius, as labeled in Fig. 1(b). Series resistance increased along with the increase of distance between two electrodes and can be well fitted into a linear relationship. Fig. 2(b) shows the I−V characteristics of the sample with GaN contact layer using the Pd/Au = 10/30 nm electrode annealing in N2 : O2 = 4 : 1. Total resistance versus the gap distance is shown in the insert of Fig. 2(b).
Figure 2.(Color online) (a) I−V characteristic of the sample with In0.15Ga0.85N contact layer using the Pd/Au = 10/30 nm electrode annealing in N2 : O2 = 4 : 1. Insert of (a) is the total resistance along with the Ln(R/r). R is the outer electrode radius and r is the inner electrode radius. (b) I−V characteristic of the sample with GaN contact layer using the Pd/Au = 10/30 nm electrode annealing in N2 : O2 = 4 : 1. Insert of (b) is the total resistance along with the Ln(R/r).
In the c-TLM, the collective resistance of the circuit can be represented by Eq. (1)[26, 27]. is the total circuit resistance; is the sheet resistance of the sample; is the radius of the outer circle; is the radius of the inner circle; and is the transfer distance. The transfer distance refers to the distance between the points where the current is maximum and where it decreases to 1/e of this maximum. This can be mathematically expressed by Eq. (2).
The specific contact resistance () can be derived using Eq. (3) by treating as an independent variable. The slope of the curve () was determined as . Notably, at , the total resistance () equals . According to the deduction of the above results, when using the same electrode of Pd/Au = 10/30 nm, the specific contact resistance of group Ⅰ and group Ⅱ are 7.61 × 10−3 and 2.57 × 10−5 Ω·cm2, respectively. As a conclusion, replacing the GaN contact layer with In0.15Ga0.85N can help to reduce the specific contact resistance effectively.
To explore the mechanism for the reduced contact resistance of the In0.15Ga0.85N contact layer compared with the GaN contact layer, we simulate the band structure of the contact material for the interface between the metal electrode and the semiconductor. Schematic of the band diagrams are shown in the Fig. 3. The barrier height () for the transport of holes at the interface of a Pd/p-type semiconductor can be determined by the In composition of the contact layer.
Figure 3.(Color online) Energy band diagrams at the interface of Pd with p-type (a) GaN; (b) In0.15Ga0.85N.
As shown in Fig. 4, the barrier height varies with the In composition of the contact layer. Valence band level increases along with the increase of the In fraction[28]. Work function of Pd is fixed at 5.12 eV from the vacuum energy. An increase of the In composition leads to the reduction of the barrier height. Holes could penetrate the interface with lower barrier height more easily which demonstrated to be with lower specific contact resistance.
Figure 4.Barrier level of the interface versus the In composition in the contact layer.
To develop an optimal metal electrode combination on the In0.15Ga0.85N contact layer, the Pd/Au and Ni/Au electrodes which are most commonly used in the formation of Ohmic contact with p-GaN are investigated. The thickness of Pd and Ni was changed from 10 to 30 nm.
As shown in Fig. 5(a), there is a dramatic increase in specific contact resistance as the Pd thickness increases to 20 nm. As demonstrated in the related studies, a plot of current vs. voltage should produce a straight line if tunneling is dominating condition mechanism[20]. An increase of the Pd thickness to 20 nm leads to a non-linear relationship for the I−V curve. The tunneling effect could have arisen from the acceptor-type defects produced by the reaction of VN with the neighbor host Ga atom. Fermi level position shifts to the energy level of acceptor defects, GaN and VGa, where GaN is the antisite defect and VGa is the Ga vacancy because their energy levels are located at the vicinity of the valence band[29, 30]. An increase in the Pd thickness may block the escape of N atoms along with suppression of the acceptor-type defect. Thus, the tested specific contact resistance is much higher than the sample with 10 nm Pd. Adopting the Pd/Au = 10/30 nm facilitates to achieve the lowest specific contact resistance of 2.57 × 10−5 Ω·cm2.
Figure 5.(Color online) Specific contact resistance of the samples with different contact metal thicknesses including the (a) Pd/Au and (b) Ni/Au electrode groups annealing in N2 : O2 = 4 : 1. Au layer is fixed at 30 nm.
As shown in Fig. 5(b), there is an increase in the specific contact resistance as the Ni thickness changes from 20 to 30 nm. Since the crystalline NiO and the amorphous Ni−Ga−O phases play an important part in low resistance Ohmic contact to p-GaN[15, 31, 32], a 30 nm Ni layer is too thick to be fully oxidized in the 2 min annealing time.
As to the influence of the annealing atmosphere, the results under different conditions for Pd/Au behave little difference but the specific contact resistance of Ni/Au for annealing in N2 is much higher than in N2 : O2 = 4 : 1.
To find out the influence of the annealing atmosphere on the specific contact resistance, we performed the auger electron spectroscopy (AES) measurement.
As shown in Figs. 6(a) and 6(b), the atomic distribution is almost the same in different annealing atmospheres for Pd/Au. As to the Ni/Au electrode, there is large-scale diffusion of Ni through the Au capping layer to the surface of the contact where it oxidized annealing in N2 : O2 = 4 : 1 atmosphere. However, this reaction mechanism might play an insignificant role when annealing in the N2 atmosphere.
Figure 6.(Color online) Auger electron spectroscopy of (a) Pd/Au = 10/30 nm annealing at N2 : O2 = 4 : 1 atmosphere; (b) Pd/Au = 10/30 nm annealing at N2 atmosphere; (c) Ni/Au = 10/30 nm annealing at N2 : O2 = 4 : 1 atmosphere; (d) Ni/Au = 10/30 nm annealing at N2 atmosphere.
In this study, we investigate the effects of substituting In0.15Ga0.85N for GaN as the contact layer on the specific contact resistance of the interface between a p-type semiconductor with a metal electrode. Introduction of In composition leads to the reduction of the barrier height. Furthermore, adopting the Pd/Au = 10/30 nm as the metal electrode, we achieved the specific contact resistance of 2.57 × 10−5 Ω·cm2, which is a comparable value with those from the n-GaN electrode. Effects of the doping level on the depletion width and barrier height are also investigated by simulation. Finally, the Pd/Au and Ni/Au electrodes which are most commonly used in the formation of Ohmic contact with p-GaN are investigated. Variation of the specific contact resistance with metal thickness is studied. AES results help to explain the mechanism of the variation of specific contact resistance under different annealing atmospheres.
[29] J L Lee, J K Kim, J W Lee et al. Transparent Pt ohmic contact on p-type GaN with low resistivity using (NH4)2Sx treatment, electrochem. Solid-State Lett, 3, 53(1999).