Monolithic stacked InGaN light-emitting diode (LED) connected by a polarization-enhanced GaN/AlN-based tunnel junction is demonstrated experimentally in this study. The typical stacked LEDs exhibit 80% enhancement in output power compared with conventional single LEDs because of the repeated use of electrons and holes for photon generation. The typical operation voltage of stacked LEDs is higher than twice the operation voltage of single LEDs. This high operation voltage can be attributed to the non-optimal tunneling junction in stacked LEDs. In addition to the analyses of experimental results, theoretical analysis of different schemes of tunnel junctions, including diagrams of energy bands, diagrams of electric fields, and current-voltage relation curves, are investigated using numerical simulation. The results shown in this paper demonstrate the feasibility in developing cost-effective and highly efficient tunnel-junction LEDs.
© 2017 Optical Society of America
The III-nitride light-emitting diode (LED) is widely employed in commercial applications from green to ultraviolet spectral range [1, 2]. One of the major issues in the high-power applications of III-nitride LEDs is that of efficiency droop [3, 4] i.e., the reduction of efficiency at high current density. However, although continuous efforts have been exerted to address this problem, an overall solution for efficiency droop remains lacking. Instead of promoting quantum efficiency at high current density, one of the current approaches is to circumvent the efficiency droop issue; this approach is based on the use of monolithic multi-active-region structure cascaded by tunnel junctions (TJs) [5, 6]. In recent years, TJs based on polarization induced charges in the III-nitride materials have been investigated theoretically and experimentally [7–13]. Stacking of multiple LED structures with TJs allows the repeated use of electrons and holes for photon generation in each individual single LED. Thus, the required carrier density in each multi-quantum well (MQW) active region can be reduced effectively for the same level of light output power compared with a single LED. The efficiency profile of the cascaded LED under this approach does not differ significantly from a single LED. However, this approach allows III-nitride LEDs to operate at low current density where internal quantum efficiency is relatively high. The idea is simple, but the manufacturing of cascaded III-nitride LED remains a challenge because of the difficulty in achieving effective III-nitride TJ and the problem in the epitaxial growth of layers subsequent to TJ.
In this study, two tunnel-junction LED (TJLED) structures with different TJs were fabricated. The LED structures are schematically illustrated in Fig. 1.
All epitaxial wafers were grown on 2-inch c-plane sapphire (0001) substrates through metal-organic chemical vapor deposition (MOCVD). To construct the bottom LED, low-temperature(550 °C) GaN nucleation layer with a thickness of 30 nm was initially deposited on the sapphire substrate, followed by a 2-μm-thick unintentionally doped GaN (u-GaN) buffer layer and a 2.5-μm-thick Si-doped GaN (n+-GaN) layer grown at 1000 °C. On top of the n+-GaN layer, three 2.5-nm-thick In0.15Ga0.85N quantum wells (QWs) sandwiched by four 5-nm-thick GaN barriers were grown at 750 °C, followed by a 100-nm-thick Mg-doped GaN (p-GaN) layer grown at 1000 °C. After growing the bottom LED structure, the TJ structure composed of n++-GaN/p++-In0.1Ga0.9N, which is denoted as TJ-1, and the top LED structure were sequentially grown on the bottom LED structure. The growth temperatures of P++-InGaN layer and the n++-GaN layer in the TJ-1 were 850 °C and 1000 °C, respectively. The ambient gases during the growth of P++-InGaN layer and the n++-GaN layer in the TJ-1 were N2 and H2, respectively. Next, a heavily Si-doped InGaN topmost layer with a thickness of 3 nm was grown on the p-GaN layer of top LED to facilitate the formation of transparent ohmic contact . The stacked LEDs with TJ-1 tunnel junction were labeled as TJLED-1. TJ structure made of n++-GaN/AlN/GaN/AlN/p++-GaN, which is denoted as TJ-2, and the top LED structure were grown sequentially on the bottom LED structure. It should be noted that the growth temperatures of AlN and GaN layers in the TJ-2 were all grown at a temperature of 1000 °C. The ambient gas during the growth of AlN and GaN layers in the TJ-2 was H2. Stacked LEDs with TJ-2 tunnel junction were labeled as TJLED-2. Except for the TJ structure between the top and bottom LEDs, the growth conditions and layer scheme of each top and bottom LED in the TJLED-1 and TJLED-2 are the same.
The detailed layer structures and growth parameters of TJ-1 and TJ-2 are described in Table 1. After epitaxial growth, LED chip with a mesa area of 300 × 300 μm2 was defined using the standard photolithography and dry etching process. Indium tin oxide (ITO) film was deposited on top of TJLEDs, which acts as a transparent current spreading layer. Ni/Au and Ti/Al were then deposited on the ITO layer and on the exposed n+-GaN layer to serve as p-type and n-type bonding pads, respectively.
3. Results and discussion
The light output power-current (L-I) and current-voltage (I-V) characteristics of TJLED-1 and TJLED-2 were measured with a calibrated Si photodiode and a programmable source meter. Given an injection current of 20 mA, the light output power (operation voltage) of TJLED-1 and TJLED-2 are 31.9 mW (38.04 V) and 31.0 mW (9.51 V), respectively. The typical L-I characteristics obtained from TJLED-1 and TJLED-2 were similar, as depicted in Fig. 2(a). However, a notable difference can be observed in the I-V characteristics between the two LED structures, that is, the operation voltage of TJLED-1 is larger than that of TJLED-2, as depicted in Fig. 2(b). Theoretically, light output power and operation voltage should be approximately double the size of a single LED when two identical single LED structures are cascaded via the connection of an ideal TJ. The typical operation voltages of TJLED are larger than the theoretical number, especially for TJLED-1, thus, the effectiveness of the designed TJ in the aforesaid cascade LEDs should be examined further. The following section discusses the theoretical simulation results performed with the APSYS simulation program to investigate the characteristics of the TJs utilized in TJLEDs .
Carrier transport via band-to-band tunneling could be quite difficult because of the wide bandgap energy of III-nitride materials and the difficulty in achieving degenerate p-type impurity doping [16, 17]. However, the strong polarization-induced electric field provides an opportunity to achieve viable TJs used in GaN-based optoelectronic device structures. The TJ structure (TJ-1) utilized in TJLED-1 is based on the design of Ref. 18 except for the indium composition utilized. Specifically, TJ-1 contains an In0.1Ga0.9N intermediate layer to create polarization-induced surface charges at GaN/In0.1Ga0.9N hetero-interfaces, which provides the polarization field (Ep) in In0.1Ga0.9N layer to induce moderate built-in filed (Ebi). For the TJ structure (TJ-2) utilized in TJLED-2, the multi-layer structure is proposed to adjust the polarization charge profile to enhance the tunneling current. Specifically, the thin AlN/GaN/AlN multi-layers are employed as intermediate region to induce additional polarization charges at the AlN/GaN hetero-interfaces and the polarization field in the thin GaN layer. The heavy p-type doping is moved from the intermediate region to the neighboring GaN layer to facilitate the doping-induced field that covers the entire intermediate region. Figure 3 depicts the calculated energy band diagrams and electric fields of TJ-1 and TJ-2 at equilibrium. The illustrations of the plus/minus signs of the polarization charges at the hetero-interfaces and the direction of built-in and polarization fields can be found in Figs. 3(a) and 3(b). The detailed material parameters employed in the simulation are described elsewhere . The interface charge density induced by spontaneous and piezoelectric polarizations is assumed to be 50% because of the partial compensation of built-in polarization by strain relaxation and imperfect film quality . Figure 3(b) shows that the direction of polarization field for TJ-2 in the intermediate GaN layer is similar to that of the doping-induced built-in field. By contrast, the polarization field in the AlN layers is in the opposite direction of the doping-induced built-in field. Under these circumstances, the total electric field in the GaN intermediate layer of TJ-2 is markedly higher than that in the GaN layer of TJ-1. Thus, the increased electric field in the GaN intermediate layer of TJ-2 results in significant band bending at the AlN/GaN hetero-interfaces to facilitate carrier transport further via tunneling mechanism.
Figure 4 shows that the calculated I-V characteristics of TJ-1 and TJ-2 exhibited a marked difference in current density because the tunneling current density of TJ-2 is almost two orders higher than that of TJ-1 under a given reversed bias. The calculated I-V characteristics of TJ-1 and TJ-2 are consistent with the experimental data showing that TJLED-2 had lower operation voltage than TJLED-1, as shown in Fig. 2(b).
Additional experiments are performed to explore the device characteristics of TJLED-2 further. First, a single LED identical to the bottom LED structure of TJLED-2 was fabricated as reference, which is denoted as SLED. A TJLED-2 wafer was then cut into two equal pieces. One half of the wafer was etched by 0.3 μm to the n-GaN layer of the top LED, which is denoted as TLED, and the other half of the wafer was etched by 1.1 μm to the n-GaN layer of the bottom LED, which is denoted as TJLED. The structure of TJLED is identical to that of TJLED-2. The insets of Figs. 5(a)-5(c) illustrate the schematic device structures of SLED, TLED, and TJLED, respectively.
Figure 6 (a) depicts the typical L-I characteristics of the SLED, TLED, and TJLED. In theory, TJLEDs should be able to generate double light output power compared with SLED or TLED. However, the typical output power of TJLED was slightly lower by twice that of the power output of SLED or TLED. Figure 6(a) shows that the light output power of TLED is less than that of SLED. The small difference in light output power between TLED and SLED might be because of the light absorption at the bottom layers, including the MQW layers in the bottom LED structure. TLED exhibited relatively higher operation voltage as displayed in Fig. 6(b), which could be attributed to its higher series resistance than that of SLED. This higher series resistance is due to the effective thickness (teff) of n-GaN layer (~0.3 mm) in the TLED structure being markedly lower than that of SLED with a teff of approximately 1.1 μm. In other words, SLED with a relatively larger teff of n-GaN layer will have a large current spreading length and low series resistance . Near-field light-emissionimages were taken under different driving currents from SLED and TLED to clarify the effects of current spreading. Figures 5(a)-5(c) show the light emission images of SLED, TLED and TJLED, respectively. SLED displays a uniform top light-emission image. However, a relatively bright area around the n-electrode bonding pad was observed at TLED, indicating significant current crowding and high series resistance. This result is consistent with the I-V characteristics, as shown in Fig. 6(b). Figure 6(c) shows that the peak EQE of SLED is higher than that of TLED when the injection current is lower than 5 mA. Shockley–Read–Hall (SRH) recombination generally governs the transition loss of a LED at low current injection . Compared with SLED, the relatively low peak EQE of TLED driven at low current could be because of the TLED structure grown on a heavily doped p++/n++ GaN junction, which may cause high defect density in the epitaxial layers, and hence, high SRH recombination rate. When the injection current is higher than 5 mA, the EQE curves of the SLED and TLED become comparable. However, the EQE of TLED is slightly less than that of SLED as shown in Fig. 6(c). In addition to the potential absorption of the bottom layers, the slightly lower output power than that of SLEDs, which is obtained at injection current higher than 5 mA from TLEDs, could be attributed to significant current crowding, as shown in Figs. 3 5(a) and 5(b). This contention is evidenced indirectly by the relatively small slope (i.e., larger series resistance) of I-V curve obtained from TLEDs, as shown in Fig. 6(b). The comparison between TLEDs and SLEDs shows that the typical light output power of the TJLEDs was less than the sum of two SLEDs (31.0 mW vs. 17.3 × 2 mW at 20 mA), as shown in Fig. 6(a). The typical operation voltage of a TJLED was higher than the sum of two SLEDs (9.5 V vs. 3.4 × 2 V at 20 mA). In the near-field images of the LED structures, as shown in Fig. 5, it is noted that only TJLED emits with bright spots, and some regions in the emitting surface doesn't emit any light especially near the device periphery. However, images taken from the TLED and SLED showed emitting surface without bright spots. The difference could be attributed to the fact that the thickness of AlN layer in the embedded TJ is not uniform to cause the tunneling junction electrically inactive at some local regions. In addition, as shown in the inset of Fig. 6(c), the wall-plug efficiency of TJLED is still far lower than the SLED because the turn-on voltage of TJLED is still far higher than the theoretical number, as shown Fig. 6(b). Therefore, the performances of AlN/GaN/AlN TJ should be further improved to enhance the wall-plug efficiency of the TJLED. Figure 6(d) displays typical electroluminescence (EL) spectra taken from the experimental LEDs driven at a DC currentof 20 mA. It is shown that all the LEDs exhibited a single peak at approximately 450 nm. The single-peak EL spectrum indicates that two unit LEDs connected via TJ in the TJLED emitted nearly the same EL spectra.
In summary, monolithic stacked blue InGaN LEDs connected by polarization-enhanced AlN/GaN/AlN TJs were demonstrated. The experimental results and theoretical simulation indicated that TJ is the key to realizing efficient TJLEDs. In this study, the light output power of TJLED exhibited 80% enhancement in output power compared with conventional SLED. However, electric performance from TJLED was not as good as expected. Given an injection current of 20 mA, the operation voltage of TJLED was 40% higher than twice that of SLED. This result is because of the non-optimal TJ caused by the difficulty in growing high-quality AlN layer and achieving heavily Mg-doped p++-GaN layer.
Ministry of Science and Technology of Taiwan (MOST-105-2112-M-018-005-MY3, MOST 104-2112-M-006-009-MY3 and MOST 104-2221-E-218-011-MY3).
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